Rise of polycatecholamine ultrathin films: From synthesis to smart applications

2.1 Biogenic CAs: Neurophysiological functions and chemical characteristics

CAs are involved in a wide range of essential biological processes and are best known for their critical roles as hormones and neurotransmitters in the regulation of physiological functions [58] (Figure 2). They are central to the regulation of stress responses and mood balance [59]. Among the CAs, l-DOPA has been shown to mediate the release of neurotrophic factors within the brain and spinal cord, thereby exerting systemic effects on the central nervous system [60]. Other important CAs, such as DA, NE, and epinephrine (EP) function as adrenal medullary hormones and are integral to sympathetic nervous system activity [58].

Figure 2

Schematic representation of CAs and their role as neurotransmitters. Illustration of the biochemical pathway for the synthesis of the different CAs starting from l-tyrosine. Image of the brain regions is taken from Ref. [60] and licensed under CC BY-NC.

These three molecules are biosynthetically derived from l-DOPA via enzymatic transformations involving the enzymes DOPA decarboxylase (DDC), dopamine β-hydroxylase (DBH), and methyl transferase [61]. The initial l-DOPA precursor is synthesized from the amino acid l-tyrosine through the action of tyrosine hydroxylase enzyme (Figure 2). Due to their vital regulatory functions, CAs have been extensively studied for decades and are also being investigated as potential biomarkers for various medical conditions, including Parkinson’s disease [62] and neuroblastoma, a rare pediatric tumor [63,64] or hypertension [65]. Beyond their biological significance, CAs possess reactive functional groups, making them attractive building blocks in chemical synthesis and valuable components in the design and functionalization of advanced polymeric biomaterials [66].

2.2 Eumelanin: A prototype for functional PCA materials

The naturally occurring CA-based pigment melanin is a negatively charged, hydrophobic, polymeric material [67,68,69,70,71]. It exists primarily in two major classes: eumelanin and pheomelanin, both of which are present in the skin, hair, and eyes, where they function as essential photo-protectants. Eumelanin, composed of indole units, exhibits a black to brown coloration [72], while pheomelanin, a sulfur-containing polymer made up of benzothiazine units, appears in shades ranging from red to yellow [73,74,75,76]. It is important to note that in the literature, pheomelanin, eumelanin, and melanin in general are not always precisely distinguished; the boundaries between these pigments can be blurred, and their physicochemical properties often overlap [77]. However, because eumelanin more closely resembles the synthetic PCAs discussed in the later part of this study, and given the similarity of properties among melanin types, this discussion will focus primarily on eumelanin.

Melanin polymers have attracted significant interest due to their diverse biological functions, which include metal biosorption, radioprotection, and radical scavenging [78,79,80]. Notably, melanin can neutralize high-energy radiation, a feature exploited during radiation therapy for cancer [81]. Its broad absorption across the electromagnetic spectrum also underlies its protective effects against UV radiation, making it valuable in cosmetics, photo-protective creams [82], and optical filters such as eyeglasses [83]. These advantageous properties have driven growing interest in the controlled synthesis of melanin and the development of melanin-inspired materials.

Eumelanin is biosynthesized from tyrosine within specialized cells called melanocytes [84], via a cascade of enzyme-catalyzed reactions. Key precursors, 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), undergo oxidation and cross-linking, facilitated by tyrosinase and other tyrosinase-related proteins, to form the eumelanin polymer (Figure 3) [70,85]. Although its precise chemical structure remains elusive, high-resolution transmission electron microscopy and X-ray diffraction studies suggest that eumelanin is an amorphous material composed of lamellar structures with an interlayer spacing of approximately 3–4 Å [86]. This amorphous character, combined with its insolubility in most solvents and exceptional chemical stability, makes eumelanin highly attractive for applications in detoxification [87], bioremediation [88], radiation shielding [89–91], photovoltaics [92], light-emitting diodes (LEDs) [92], and organic electrochemical transistors [92].

Figure 3

Reaction scheme of the biosynthesis pathway of eumelanin catalyzed by tyrosinase (TYR) and different tyrosinase-related proteins (Tyrp1 and Tyrp2) adapted from literature [70]. Adopted with permission from Ref. [70]. Copyright 2002 WILEY VCH.

Despite its utility, the structural elucidation of eumelanin remains a significant scientific challenge [33]. Fourier-transform infrared (FT-IR) spectroscopy has provided some insight: Eumelanin exhibits a broad absorption band near 3,400 cm⁻¹, characteristic of –OH and –NH₂ stretching vibrations [93]. Additional peaks around 2,900 cm⁻¹ arise from CH₂ vibrations, while bands at approximately 1,600 cm⁻¹ correspond to C═O stretching. The presence of C–S stretching vibrations in the 700–600 cm⁻¹ region typically indicates pheomelanin content in natural samples [93]. Due to eumelanin’s broadband optical absorption, it is proposed to consist of disordered aggregates of heterogeneous aromatic oligomers, with overlapping absorption bands contributing to its UV-visible absorbance profile [94]. Recent advancements, such as ultrafast vibrational spectroscopy, have been employed to explore the electronic structure of eumelanin. One study used this approach to observe the ultrafast recombination of polaron pairs, providing insight into its photoprotective properties [95]. Polaron pairs that fail to recombine may lead to long-lived radical species, detectable via electron paramagnetic resonance (EPR) in photoexcited natural and synthetic eumelanins [96]. These trapped radicals exhibit paramagnetic behavior [97].

Environmental factors, including pH, temperature, and hydration state, influence radical presence and stability in eumelanin, reinforcing its function as a potent scavenger of both oxidizing and reducing radicals [96,98–100]. The efficiency of radical scavenging is linked to eumelanin’s electronic structure, while its chemical composition and supramolecular organization also contribute significantly to this activity [101]. In particular, the presence of non-covalent π–π stacking interactions among indole units plays a crucial role in defining eumelanin’s antioxidant capacity [101]. Notably, depending on its redox state, eumelanin may function either as an antioxidant or a pro-oxidant, highlighting its complex role in both cellular protection and oxidative stress [102].

In addition to its well-established absorption, radical scavenging, and redox properties, eumelanin exhibits several unique features that expand its functional versatility. One of these is its remarkable ability to bind metal ions, which arises from the presence of catechol, quinone, and carboxylic acid groups [103–107]. These reactive functionalities involved in metal coordination vary depending on the type of metal ion, as confirmed by EPR [103,104], infrared (IR) spectroscopy [105,106], and Raman spectroscopy [107].

Eumelanin demonstrates strong chelating abilities for heavy metal ions, including Fe(iii) [108,109], Cu(ii) [103,104], Cd(ii) [106], and Pb(ii) [106], whereas lighter metal ions such as Ca(ii) and Mg(ii) exhibit weaker interactions [105,106]. These selective binding features play a crucial role in biological systems, contributing to the regulation of metal ion homeostasis [110] and the detoxification of harmful metals [87]. Beyond its physiological relevance, eumelanin’s metal-binding capability is of increasing interest in bioremediation strategies aimed at removing toxic metal pollutants from the environment [88].

Eumelanin also provides effective protection against ionizing radiation, a property attributed to the Compton effect [111,112]. This scattering phenomenon is influenced by the energy of the incident radiation, with forward scattering dominating at higher energies (e.g., 662 keV) and a combination of forward and backward scattering occurring at lower energies. The spatial organization of melanin particles further modulates their radiation-absorbing capacity [111,112]. This photoprotective property has significant implications for radiation shielding, particularly in medical contexts, where eumelanin-based materials could protect healthy tissues during radiotherapy or in the aftermath of nuclear accidents [89–91].

Another exceptional feature of eumelanin is its electrical conductivity, which is unusual among biological polymers. Studies have demonstrated that under conditions where hydration effects are minimized, ion conduction is not the dominant mechanism of charge transport in eumelanin [113]. Instead, the material exhibits a temperature-dependent increase in conductivity, a characteristic typical of semiconductors, and the transport has been attributed to small polaron hopping mechanisms [114]. Nevertheless, humidity significantly influences the conductive behavior of eumelanin, underscoring its environmentally responsive properties [114]. These characteristics open promising avenues for applications in electronic sensors, biocompatible electronics, and optoelectronic devices, including photovoltaics, LEDs, and organic electrochemical transistors, topics reviewed by Vahidzadeh et al. [92].

The unique physicochemical properties and multifunctionality of eumelanin have inspired the development of synthetic analogues, most notably PDA, which has been shown to mimic and in some aspects even enhance eumelanin's diverse properties [115,116]. PDA, also referred to as synthetic melanin or synthetic eumelanin, shares key structural and functional features with its natural counterpart, including strong adhesion, redox activity, and metal-binding capacity.

The properties and potential applications of eumelanin have been comprehensively reviewed, with recent reviews focusing on its structure, biosynthesis, photoprotective functions, and roles as a sustainable resource for advanced material applications [85,111,117–119]. Building upon these foundations, PDA and its related derivatives are explored here, focusing on their formation mechanisms, physicochemical characteristics, and expanding utility across a wide range of application areas, including catalysis, biosensing, biomedicine, membrane technologies, and emerging fields such as nanorobotics and actuator systems.

3.1 PDA

Synthetic PCAs such as PDA exhibit a complex and heterogeneous structures that share notable similarities with melanin, particularly in terms of functional group composition and polymerization mechanisms. Despite extensive research efforts [120–129], the distinct molecular structure of PDA remains a subject of active debate. Both PDA and melanin are derived from DA or structurally related CA monomers, and both contain key functional motifs such as catechol, quinone, and amine groups. These structural features arise through oxidative polymerization, a process that endows the resulting materials with adhesive and surface-modifying properties [30].

Although the complete mechanism of PDA and melanin-like polymer formation remains under investigation, it is widely accepted that the initial oxidation step involves the removal of two electrons and two protons from the catechol ring. This step can be initiated by chemical oxidants (under acidic or basic conditions) [130,131], UV light irradiation (at ∼260 nm) [132,133], or electrochemical methods [134,135].

While the initial stages of DA polymerization, namely, monomer oxidation and oligomer formation, are relatively well understood [125], the subsequent cross-linking reactions that generate the extended polymer network are significantly more complex and challenging to map out [125]. Numerous synthetic strategies, including auto-oxidation, electropolymerization, spray coating, and interfacial polymerization, have been employed to produce PDA, with the choice of method and experimental conditions (e.g., pH, buffer composition, temperature) strongly influencing the degree of polymerization, cross-linking density, and functional group availability [120,121,136].

Two primary structural models for PDA have been proposed. The first model suggests that PDA is a supramolecular aggregate composed of covalently cross-linked oligomers formed via oxidative polymerization, stabilized through a combination of covalent and non-covalent interactions (e.g., hydrogen bonding, π–π stacking) [122]. A more recent alternative model describes PDA as a linear polymeric structure, consisting of covalently linked oxidized and cyclized DA monomers, with residual unreacted monomers incorporated within the matrix [123]. However, the latter model is based solely on single-molecule force spectroscopy (SMFS), which, while informative, only probes molecular interactions at the individual molecule level. In contrast, the former model is supported by a broader range of characterization techniques, including X-ray photoelectron spectroscopy and UV-Vis spectroscopy.

The formation of PDA typically occurs under mild alkaline conditions (pH > 7.5), where DA undergoes air oxidation, isomerization, and intramolecular cyclization to initiate self-polymerization [124]. The initial oxidation of DA yields dopamine quinone, which undergoes Michael addition to form leucodopaminechrome [35,137]. This intermediate is further oxidized and rearranged into DHI, and subsequently into 5,6-indolequinone. Both of these indole derivatives are reactive at multiple positions, allowing for the formation of a variety of isomeric dimers, oligomers, and larger polymeric assemblies (Figure 4) [137]. These oligomers can participate in cross-linking reactions, particularly between catechol and o-quinone moieties, and may also form non-covalent aggregates, such as trimers involving DA and DHI units, which are incorporated into the final polymer structure (Figure 4).

Figure 4

Suggested PDA synthesis pathways via covalent conjugation reactions and physical interactions. Adapted with permission from Ref. [137], copyright 2012 John Wiley and Sons.

The reaction conditions during PDA formation play a pivotal role in determining polymer structure and properties. For example, the use of tris buffer instead of phosphate buffer alters the reaction kinetics of PDA, as the amine groups in tris can react with quinone groups, thereby inhibiting further polymerization [120]. Likewise, solution pH significantly influences polymerization efficiency. Acidic conditions reduce the oxidation potential of the catechol group by shifting the equilibrium toward its protonated form, thereby limiting PDA deposition [120]. In contrast, the addition of oxidizing agents such as sodium periodate (NaIO₄) and an increase in temperature have been shown to accelerate PDA formation [130]. In addition to chemical oxidants, light-induced polymerization has been explored as a means to control PDA deposition. Due to its aromatic structure, DA absorbs in the UV range, and irradiation at around 260 nm can promote catechol deprotonation, initiating polymerization via the generation of reactive oxygen species (ROS) in solution [132]. This approach offers a tunable and non-invasive strategy for initiating or enhancing PDA film formation.

Despite the complex underlying reaction mechanisms, the self-polymerization of DA is remarkably straightforward and does not require elaborate equipment or procedures. This simplicity has led to the widespread adoption of the method for fabricating functional coatings and materials. Substantial efforts have been devoted to elucidating the reaction mechanism and identifying intermediate species through various analytical and spectroscopic techniques [125,126]. Evidence for covalent bonding between PDA oligomers and the presence of non-cyclized DA chains has been obtained using ¹³C and ¹H solid-state NMR with magic angle spinning, as well as high-resolution electrospray mass spectrometry [125]. EPR studies have revealed the presence of persistent radicals within PDA, although the spin distribution was found to be uniform across the sample, with no evidence of unpaired spins. A three-dimensional EPR image reconstruction of the sample was performed, showing a strong correlation with the corresponding photographic representation. This suggests that PDA could be used as an EPR imaging marker for biomedical applications, such as visualizing PDA-coated materials or composites [127]. FT-IR spectroscopy of PDA powders has revealed peaks at 1,515 and 1,605 cm⁻¹, consistent with indole or indoline structures. A broad absorption band between 3,200 and 3,500 cm⁻¹ has been attributed to hydroxyl and aromatic amine groups, although intercalated water molecules may also contribute to this region [128]. Notably, distinct carbonyl signals were not observed, likely masked by the broad, overlapping peaks within the 800–1,750 cm⁻¹ range [128].

Recent studies have highlighted the role of cation intercalation, particularly cation–π interactions, as a major driving force in PDA formation [129]. Based on this, PDA is now understood as a heterogeneous assembly of DA derivatives and oligomers formed via multiple reaction pathways. Contrary to earlier assumptions emphasizing π–π interactions, evidence suggests that cation–π interactions, especially between protonated amine groups and the π-system of indole units, play a more dominant role in PDA assembly (Figure 5a) [129].

Figure 5

Interactions of PDA with different cations and the determination of the intermolecular interactions by applying atomic force microoscpy (AFM). (a) Schematic representation of cation–π interactions in PDA assemblies. Strong cation–π interactions are observed between protonated amines and the π-system under neutral conditions. Upon pH-induced deprotonation, weaker cation–π interactions with Na⁺ dominate. Reintroduction of stronger interactions occur with the addition of K⁺ under deprotonated conditions. Adapted with permission from Ref. 124. Copyright 2018 American Association for the Advancement of Science. Licensed under CC BY-NC 4.0. (b) Schematic of AFM experiments probing intermolecular interactions between a PDA-coated tip and a PDA-coated surface. Adapted with permission from Ref. [123]. Copyright 2019 WILEY-VCH.

Environmental conditions strongly influence the structural integrity of PDA films. For instance, high sodium ion concentrations and alkaline pH weaken cation–π interactions, leading to film delamination. However, this process can be reversed by adding cations with stronger binding affinities, such as potassium (K⁺), which restore film stability under basic conditions (Figure 5a) [129]. The valence and type of cation also affect the degree of polymerization and surface characteristics. Among various tested cations (e.g., Li⁺, Na⁺, K⁺, R– NH 3 + NR 4 + ), quaternary ammonium ions (e.g., RNMe 3 + ) exhibited particularly strong cation–π interactions, resulting in enhanced affinity for hydrophobic aromatic domains [129]. Additionally, ion incorporation during polymerization significantly alters film surface properties, such as wettability, providing a route to tailor material properties through controlled synthesis.

Another key finding is the influence of the polymerization environment, specifically, whether PDA forms in solution or on surfaces. These differences are attributed to local oxygen concentrations, which affect reaction kinetics and product composition [122,138]. MALDI-MS analysis of PDA formed in bulk vs on surfaces revealed only one shared signal at 402 m/z, indicating a marked difference in molecular composition [138]. Further structural insight was obtained through SMFS using atomic force microscopy (AFM). Retraction curves of PDA-coated cantilevers from surfaces displayed behavior characteristic of polymer chains, including contour lengths up to 200 nm and intermittent “sticky” interaction points, consistent with covalent chain formation and supporting the extended chain model of PDA structure (Figure 5b) [123].

This chapter has shown that the structure of PDA is highly dependent on the synthesis method and reaction conditions, with different protocols yielding distinct structural features and properties [120,121,129]. As a result, no single universal mechanism or structure can fully describe PDA, and comparative studies are essential for understanding its complexity. Therefore, interpretations of PDA structure and function must always consider the specific synthesis parameters used.

Due to their high degree of cross-linking, synthetic PCAs such as PDA exhibit outstanding chemical and mechanical stability, with elastic moduli in the gigapascal (GPa) range [44,139]. The extensive π-electron system within these films also accounts for their broadband absorbance, extending from the UV-visible to the near-infrared (NIR) region. Notably, PDA, like natural eumelanin, demonstrates high photothermal conversion efficiency, making it an attractive material for NIR-triggered photothermal therapy and the development of photoactuated devices [140].

3.2 Poly-l-Dopa

l-DOPA is an early intermediate in the biosynthesis of black eumelanin pigments, formed from l-tyrosine via the action of the copper-containing enzyme tyrosinase [70]. Structurally, l-DOPA differs from DA by the presence of an additional carboxylic acid group, alongside its catechol and amine functionalities. This carboxylic group, which has a relatively low pKa, imparts zwitterionic properties to l-DOPA. Like DA, l-DOPA undergoes oxidative polymerization under mildly basic conditions to form PDOPA. The polymerization pathway begins with the oxidation of l-DOPA to form dopaquinone, which then rearranges to leucodopachrome, following a mechanism similar to that of PDA formation. Beyond this point, the reaction branches into two parallel routes (Figure 6a) [35].

Figure 6

(a) and (b) Suggested mechanism of poly-l-dopa (PDOPA) and PNE formation, respectively. Adapted and reprinted with permission from Ref. [35], copyright 2020 Elsevier B.V. licensed under CC BY-NC-ND 4.0.

The first route involves the decarboxylation of the intermediate, yielding DHI, while the second leads to the formation of DHICA. The latter pathway results in structures that closely resemble naturally occurring eumelanin, consistent with l-DOPA’s role as a precursor in melanogenesis in human skin [141]. Interestingly, while enzymatic oxidation via tyrosinase in biological systems favors DHICA formation, synthetic methods tend to yield polymers with a greater abundance of DHI-type units. Despite its structural similarity to PDA and its relevance to natural melanin, PDOPA has received comparatively little attention in both fundamental research and technological applications, even though its films exhibit distinctive and potentially useful properties.

3.3 PNE

The synthesis of PNE follows a similar initiation pathway to PDA, beginning with the oxidation of catechol groups to form quinones. However, unlike PDA, a secondary polymerization pathway is initiated by the formation of dihydroxybenzylaldehyde, as outlined in Figure 6b [35,142,143]. In the first detailed investigation of PNE synthesis, high-performance liquid chromatography mass spectrometry (HPLC-MS), and NMR spectroscopy confirmed the formation of dihydroxybenzylaldehyde. Specifically, the NMR spectrum of an intermediate at 138.9 m/z revealed two distinct signals near 9.8 ppm, corresponding to aldehyde protons, thereby supporting the presence of the aldehyde intermediate.

Further experiments investigated the formation of a dimer through the interaction between dihydroxybenzylaldehyde and its quinone form, which was found to exist in equilibrium in aqueous media. This dimer can dissociate into two protonated dihydroxybenzylaldehyde units, which are proposed to react with NE via a Schiff-base (C═N) formation, followed by reductive imine bond cleavage (Figure 6b). Two potential reduction mechanisms of the imine bond were proposed. The first involves intramolecular reduction by the catechol group of the dihydroxybenzylaldehyde itself (but not by NE), yielding a dihydroxybenzylaldehyde-NE product. However, the yield of this product was low, likely due to its simultaneous immobilization into the forming surface coating, which limits its accumulation in solution.

Interestingly, dihydroxybenzylaldehyde was shown to modulate film morphology by reducing the surface roughness of PNE-coated substrates. This effect was attributed to the alternative polymerization pathway, which inhibits excessive aggregation. In the initial study that identified this mechanism, adding dihydroxybenzylaldehyde during PDA film formation resulted in a marked reduction in surface roughness [142]. Specifically, adding just a few molar equivalents of the aldehyde compound led to the disappearance of large PDA aggregates, which are known to contribute significantly to film inhomogeneity.

4.1 Dip coating

Dip coating is one of the simplest and most widely adopted techniques for the fabrication of CA-based films, including ultrathin coatings. The method involves immersing a substrate into a solution containing either the monomer, which polymerizes in situ, or a pre-formed polymer, followed by controlled withdrawal. Key parameters such as immersion time and withdrawal rate directly influence the resulting film thickness, surface morphology, and uniformity. Additionally, the viscosity, polarity, and vapor pressure of the coating solution, as well as the molecular weight of the polymer, play important roles in determining film characteristics [144].

The interfacial properties of the substrate, including surface charge, roughness, and hydrophilicity, must also be considered, as they strongly affect polymer, substrate interactions, film adhesion, and homogeneity. As such, dip-coating protocols typically require case-by-case optimization to ensure reproducibility and performance. A unique advantage of dip coating for PCA films lies in the spontaneous oxidative self-polymerization and auto-deposition of CA monomers (e.g., DA, NE, l-DOPA) under mildly basic conditions. This process is largely independent of substrate properties, allowing uniform coating of materials regardless of their charge, roughness, or hydrophilicity [30]. Consequently, dip coating was the first and remains one of the most commonly employed methods for preparing biomimetic PDA films. A typical example involves simply immersing a substrate in a basic DA solution for a defined period to yield a conformal PDA coating (Figure 7a) [145]. The method does not require heavy equipment, it is compatible with substrates of various shapes and geometries, and has been applied across numerous fields. Examples include antifouling coatings for membranes [146–148], ultrasmooth polymer films on silicon and gold surfaces [148], and functional coatings for non-woven, porous materials, even within their internal pore structures [145].

Figure 7

Overview of different methods for producing PDA coatings by autooxidation of DA in solution. (a) Schematic illustration of DA oxidative polymerization process during dip-coating. Reprinted with permission from [145]. Copyright 2016 American Chemical Society. (b) Schematic procedure of spin-coating system for PDA with different aging time. Reprinted with permission from Ref. [149]. Copyright 2019 American Chemical Society. (c) Schematic representation of tilted dip coating procedure for gradient PDA film formation, reprinted with permission from Ref. [150], Copyrights 2013 The Royal Society of Chemistry.

Despite its advantages, dip coating has limitations. It generally requires large volumes of solution, leading to waste generation, and handling challenges may arise at the air–solution interface during immersion or withdrawal. Additionally, while dip coating can theoretically achieve coatings of unlimited thickness, in practice this is constrained by the simultaneous polymerization in solution, which consumes monomers to form PDA nanoparticles, limiting substrate-bound film growth to tens of nanometers per cycle. Achieving thicker films typically requires multiple sequential deposition steps, similar to layer-by-layer assembly.

Another limitation is the slow reaction kinetics of PDA formation under standard conditions, which can affect both film homogeneity and throughput. However, these challenges can be addressed by incorporating oxidizing agents such as NaIO₄ and elevating the reaction temperature to around 50°C. These adjustments significantly accelerate deposition, reduce aggregation, and improve film uniformity, especially in the initial nanometers of growth [130].

In addition to sodium periodate, other oxidizing agents such as ammonium peroxodisulfate and copper sulfate have been explored for the controlled deposition of PDA. However, these agents were found to be less effective compared to sodium periodate, as they resulted in slower polymerization rates and increased aggregate formation at similar film thicknesses. These observations highlight sodium periodate as a superior oxidant for DA polymerization [130]. Dip coating has also been extended to fabricate composite materials, including functional fibers and blended nanoparticles. A related and more controlled technique is spin coating, in which centripetal force is used to spread the solution evenly across a substrate, enabling the formation of highly homogeneous polymer films. Recently, spin coating was employed to study the solution aging time required to reach the maximum adhesiveness of PDA coatings, revealing important kinetic insights into film development (Figure 7b) [149].

While dip coating is also applicable to the fabrication of PDOPA and PNE films, PNE offers specific advantages over PDA. PNE coatings exhibit significantly lower surface roughness and a higher aliphatic hydroxyl group content, which enables subsequent ring-opening polymerization of ε-caprolactone. This approach allows for surface hydrophobization and the fabrication of advanced materials such as Janus membranes [143]. More complex film architectures, such as gradient coatings, can also be achieved through modifications to the dip-coating procedure. One such strategy involves tilting the substrate during immersion, thereby exploiting depth-dependent oxygen concentration gradients that arise from air diffusion into the solution (Figure 7c) [150,151]. The upper portion of the substrate, closer to the air–liquid interface, is exposed to higher local oxygen levels, resulting in thicker coatings that gradually taper with immersion depth. Furthermore, freestanding nanomembranes of PDA have been fabricated using dip coating on ultrathin silicon oxide substrates. These films, with thicknesses around 60 nm, can be lifted off by immersion in solutions of high pH and high salt concentration [44]. The resulting membranes demonstrated excellent mechanical properties, with a measured very high Young’s modulus of approximately 2 GPa, which increased to 8 GPa upon cross-linking with genipin, a natural and low-toxic agent derived from the gardenia fruit.

4.2 Spray coating

Spray coating is another well-established technique for depositing controlled amounts of polymer onto a wide variety of substrates and is particularly advantageous for coating large surface areas using minimal solution volumes. Unlike dip coating, spray coating does not require immersion vessels, making it well-suited for DA-based coatings, where minimizing exposure time to air is critical due to rapid oxidation of DA in solution. Although the technique offers several advantages, literature reports on PDA spray coating remain limited, likely due to the need for specialized equipment, including pressurized nozzles and controlled spraying systems. In this approach, the polymerizing solution is delivered through a nozzle under pressure and sprayed onto the target surface [144]. Notably, polymerization begins within the reservoir, prior to deposition, offering precise control over film thickness and surface coverage.

To accelerate polymerization, oxidizing agents are often introduced immediately prior to spraying by mixing the DA and oxidant solutions through dual-channel nozzles, allowing for instantaneous reaction and deposition (Figure 8a) [152]. This rapid and controllable process makes spray coating particularly attractive for large-scale applications and irregularly shaped substrates. Moreover, it can be readily adapted to layer-by-layer (LbL) assembly, further increasing its versatility. Spray coating has been successfully applied in diverse contexts, including corrosion protection, catalytically active surfaces, and melanin-based coatings, where it offers a much faster deposition rate compared to traditional dip coating [153]. Additionally, the ability to co-spray monomer and oxidant solutions allows fine control over the polymerization degree and resulting material properties. This method is especially useful for depositing hybrid materials, such as nanoparticles or fibers, onto functional surfaces (Figure 8b) [154].

Figure 8

Applying PDA coatings using spray coating. (a) Representation of spray coating mechanism for PDA film formation, reprinted with permission from Ref. [152]. Copyright 2016 WILEY-VCH. (b) Schematic LbL assembly of a hybrid material containing PDA-polyethylenimine (PEI) and polyacrylic acid (PAA) as components. Reprinted with permission from Ref. [154]. Copyright 2020 Elsevier Ltd.

4.3 Interfacial polymerization

Interfacial polymerization utilizes the boundary between two immiscible phases, commonly liquid–liquid or air–liquid interfaces, as a reaction platform for step-growth polymerization [155]. In liquid–liquid systems, two separate monomer solutions meet at the interface, as demonstrated in classical examples such as Nylon synthesis. In contrast, air–liquid interfaces are often employed to enhance oxidation at the air-exposed side or to leverage the amphiphilic nature of certain monomers for directional self-assembly [156]. PDA exhibits amphiphilic behavior, particularly after the initial oxidative polymerization steps, resembling the self-organization observed in phospholipid assemblies. This amphiphilic character favors interfacial polymerization at the air–water boundary, where oxygen availability is high [156]. Under these conditions, spontaneous film formation leads to the development of freestanding PDA microfilms with Janus morphology, attributed to monomer pre-orientation at the interface.

Film formation at the air–water interface proceeds via homogeneous nucleation, initiated by the oxidation of highly polar and water-soluble DA into amphiphilic intermediates [157]. In the absence of stirring, PDA films form spontaneously at the interface (Figure 9a). However, these films tend to be brittle and difficult to transfer. More recently, transferable, two-dimensional layered PDA films were obtained under optimized interfacial conditions [158].

Figure 9

Various methodologies for the interfacial polymerization of PDA films. (a) PDA film formation at the air–water interface without stirring and subsequent transfer of the film onto a PTFE substrate, adapted with permission from Ref. [157]. Copyright 2014 American Chemical Society. (b) Scheme of the process to achieve the formation of PDA/PEI free-standing composite films. Reprinted with permission from Ref. [160]. Copyright 2014 Royal Society of Chemistry. (c) Schematic illustration of the formation of cross-links between catechol alginate and PDA aggregates. Adapted with permission from Ref. [161]. Copyright 2017 American Chemical Society.

To enhance mechanical robustness and transferability, various cross-linking agents have been employed during interfacial polymerization. For instance, PEI of different high molecular weights (Mw = 600 and 750 kDa, respectively) was introduced during PDA polymerization to yield stable, freestanding composite films (Figure 9b) [159,160]. In this case, covalent interactions between PDA quinones and PEI amines, via Schiff base formation or Michael addition, occur without interfering with DA oxidation. The resulting films display an asymmetric internal structure: a PDA-rich, solid-like domain at the air-exposed side, and a polymer-rich, porous region at the water-exposed surface. Notably, films formed with high-MW PEI exhibited self-healing behavior when damaged on the water surface, enabled by strong adhesion between the regenerated and original film [156]. This occurs autonomously through physical chain interdiffusion and re-entanglement at the interface, driven by the viscoelasticity and high chain mobility of the PEI-rich porous layer. This mechanism is exclusive to high-MW PEI due to its enhanced chain flexibility and supramolecular entanglements. These freestanding PDA–PEI films have been successfully applied as templates for metallization and mineralization. Beyond PEI, other cross-linkers have also been explored. For example, alginate-catechol has been utilized to fabricate biocompatible ultrathin PDA films, also via air–water interfacial polymerization (Figure 9c) [161]. In addition to organic cross-linkers, the presence of metallic cations, such as Na⁺, Ca²⁺, Mg²⁺, and Co²⁺ has been shown to significantly influence the oxidation pathway of DA and the properties of the resulting films [162]. Among these, Co²⁺ was particularly effective in promoting rapid film formation at the air–solution interface, although the resulting films exhibited high surface roughness and a porous internal structure. Conversely, slower polymerization in the presence of Ca²⁺ and Mg²⁺ produced smoother, denser films. Furthermore, significant differences were observed in the surface wettability of the air- and water-exposed sides of the films, as indicated by distinct water contact angles of 62°–66° and 47°–52°, respectively. These disparities suggest the formation of chemically distinct surfaces, likely resulting from asymmetric polymerization dynamics and cation-specific complexation kinetics.

4.4 Electropolymerization

While the self-polymerization of CAs such as DA offers a simple and substrate-independent approach for surface modification, it also presents certain limitations. In particular, the polymerization process is relatively slow, often requiring several hours to achieve significant material deposition. Although this can be accelerated with the addition of chemical oxidants, the method commonly results in the formation of solution-phase aggregates, which can deposit on the substrate and lead to inhomogeneous and rough coatings [163].

To address these challenges, electropolymerization has emerged as a controlled alternative for PDA film formation. This technique enables the deposition of films with precise thickness control (ranging from a few nanometers) and uniform surface morphology [164]. Electropolymerization is applicable to a wide range of conductive substrates, including gold [164,165], indium tin oxide (ITO) [166,167], and glassy carbon [168], and can also be used to coat non-planar electrode geometries [169]. Mechanistically, the electropolymerization of DA closely resembles its spontaneous oxidation in solution, differing primarily in the driving force, namely, the applied electrochemical potential [168]. Upon application of a potential, electrons are withdrawn from the catechol group, forming dopamine quinone (Figure 10), and generating a measurable current. The overall process comprises two distinct parts: an electrochemical phase, driven by electron removal, and a chemical phase, involving intramolecular reactions such as cyclization and isomerization. The initial steps follow an electrochemical–chemical–electrochemical mechanism, in which DA is oxidized to dopamine quinone, undergoes intramolecular cyclization to form leucodopaminechrome, and is further oxidized to dopaminechrome [138,165] (Figure 10). Isomerization then yields DHI, which is oxidized to 5,6-indolequinone, initiating further coupling reactions that produce insoluble PDA deposited directly onto the electrode surface [170].

Figure 10

Proposed mechanism for the electropolymerization of DA, driven by electron transfer from the electrode. The final polymer will contain different proportions of type (i) DA in the open form, (ii) indoline type, and (iii) chains (indole type), where (i) > (ii) > (iii). Adopted and reprinted with permission from Ref. [170]. Copyright 2019 Elsevier B.V.

When atmospheric oxygen is excluded and degassed solutions are used, the oxidative polymerization of DA proceeds much more slowly, since dissolved oxygen is no longer available as an oxidant. Under these deoxygenated conditions, DA oxidation occurs almost exclusively at the working electrode, driven by the applied electrochemical potential rather than spontaneous reaction with ambient oxygen. This confines PDA formation to the electrode surface, effectively preventing the generation of PDA particles or aggregates in the bulk solution. As a result, a clean, homogeneous, and highly reproducible coating is achieved, which is particularly advantageous for applications requiring precise surface functionalization, such as biomedical implants (e.g., stents) or the development of electrochemical biosensors [135,169]. Various electropolymerization modes can be employed, including cyclic voltammetry (CV), potentiostatic, and pulsed deposition (PD) techniques. In CV, the potential is cyclically swept between two limits (e.g., +0.5 to –0.5 V vs Ag/AgCl), inducing oxidation and reduction processes at the respective potentials [168]. Oxidized DA species polymerize upon deposition, while any non-polymerized intermediates are subsequently reduced, contributing to stepwise film growth. However, due to the low conductivity of PDA, film growth slows over time, resulting in self-limiting behavior as further electron transfer becomes impeded by the insulating film. Alternatively, potentiostatic deposition involves holding the working electrode at a constant potential sufficient to induce oxidation for a defined time [135]. This approach allows for more straightforward control over film thickness, though the rate of film formation diminishes rapidly as monomer concentration near the electrode becomes depleted.

4.5 PD

A third electropolymerization strategy, PD, can be considered a hybrid of CV and potentiostatic deposition. In PD, the applied potential is alternated between positive, negative, and zero voltages for defined time intervals, enabling precise control over PDA film thickness through the number of pulse cycles. A detailed study employing scanning electrochemical microscopy (SECM) in direct mode revealed the formation of microstructured PDA films via PD [171]. The film thickness increased with the number of pulse cycles, reaching a maximum of approximately 70 nm. Similar to other electrochemical techniques, the deposition rate decreased with increasing film thickness due to the insulating nature of PDA, which restricts further electron transfer. SECM was also used to investigate the electrochemical behavior of the resulting films. Upon application of a bias potential, hydroquinone groups in the film were oxidized to quinones, and the charge transfer rate constants for potassium ferrocyanide and ruthenium hexamine were found to vary depending on both the oxidation state and film thickness [171]. These findings demonstrate that the electrochemical properties of PDA films can be precisely tuned by controlling their redox state and thickness. This tunability provides valuable design principles for tailoring the films to specific applications, such as sensors or catalytic surfaces, where optimized electron transfer characteristics are essential. Moreover, the results highlight the importance of both chemical composition and structural parameters in determining the electron transport properties of the films, enabling the rational design of functional coatings for advanced electrochemical devices.

Overall, the effectiveness and precision of electropolymerization techniques for depositing PDA have been demonstrated across multiple studies. Compared to dip coating, electropolymerized films exhibit greater smoothness, controlled thickness, and benefit from the self-limiting growth behavior of PDA [163]. However, a key limitation of this approach is the requirement for a conductive substrate, restricting the range of materials that can be directly coated. This constraint has recently been addressed through a method developed by our group, allowing PDA films to be detached from conductive substrates and transferred onto non-conductive materials [172]. The resulting freestanding films displayed exceptional mechanical performance, with a measured extremely high Young’s modulus of ∼12 GPa for films just 12 nm thick.

Electropolymerization also offers a versatile platform for the formation of copolymer films from multiple monomers, enabling the incorporation of additional functions. For example, copolymerization of DA and pyrrole yields PDA/polypyrrole composite films with both electrical conductivity and mechanical robustness [166]. In this case, DA and pyrrole monomers were co-dissolved, and upon initiation of electropolymerization, a uniform conductive film was deposited. Electropolymerization has also been applied to DA analogs, including l-DOPA, for applications in biosensing and immunosensor development [167]. Despite widespread interest in CA analogs, only two systematic studies on the electropolymerization of NE have been reported, along with a single example of co-deposition with GO [173,174].

5.1 Catalysis

PCAs possess several features that make them highly attractive for catalytic applications. Their rich surface chemistry, including amine, phenol, and quinone groups, allows for the immobilization of molecular catalysts and metal nanoparticles. Furthermore, PCA films are inherently redox-active, offering a degree of protection for immobilized catalysts by mitigating oxidative damage. Their broad absorption spectrum also makes them promising for photocatalysis, while their biocompatibility enables integration into biocatalytic systems. PDA, the most studied PCA, even exhibits intrinsic catalytic activity. Although the mechanism is not fully understood, PDA has been reported to act as an organocatalyst in aldol reactions via imine/enamine intermediates, with the catechol unit contributing through hydrogen bonding (Figure 11a) [175]. Blocking of the amine group inhibits catalysis, while catechol blocking only reduces efficiency, confirming the synergistic roles of both functional groups. PDA has also been shown to mimic phosphate group activity in the hydrolysis of 4-nitrophenyl phosphate [176], and PDA nanospheres have catalyzed the cycloaddition of CO₂ to oxiranes to form cyclic carbonates, especially in the presence of alkali salts [177]. These findings highlight the potential of PDA as a low-cost, metal-free organocatalyst.

Figure 11

Different PDA-catalyzed reactions. (a) Scheme of the rationalized mechanistic pathway and transition state of the PDA-catalyzed aldol reaction adapted from Ref. [175]. Copyright 2014 John Wiley and Sons. (b) Illustration of the strategy to achieve patterned metal films using PDA as a UV-sensitive catalytic layer. Adapted with permission from Ref. [183]. Copyright 2016 American Chemical Society. (c) Degradation/removal of MB in a NADH₄ solution catalyzed by a PDA-coated polyurethane foam (OCPUF). Adapted with permission from Ref. [186]. Copyright 2017 Elsevier B.V.

In environmental remediation, catalytic degradation is a key strategy for removing organic pollutants from water. Numerous bioinspired composites have been developed using PDA to anchor catalytic agents. Inorganic materials such as TiO₂ [178], ZnO [179], and C₃N₄ [180], as well as noble metal nanoparticles (e.g., Au, Ag, Pt) [181,182], have been immobilized on PDA films or particles to catalyze redox transformations, including the reduction of nitroaromatics. DA and its derivatives also serve as mild reducing agents to facilitate the in situ formation of metal nanoparticles on PDA-coated surfaces. Photopatterning techniques can further localize metal deposition: by partially oxidizing PDA through UV irradiation under a photomask, only masked areas remain catalytically active for metal ion reduction (Figure 11b) [183]. This approach enables spatial control of catalytic sites on diverse substrates, including glass, silicon, polymers, and even plant leaves. Beyond metals, PCAs have also been used to reduce graphene oxide (GO) during polymerization. For example, NE was used to coat and reduce GO in a one-step process, producing PNE/reduced GO composites with enhanced performance in photoinduced charge separation using [Ru(bpy)₃]²⁺ photosensitizers [184]. These hybrid materials can be further modified, for instance, by metal nanoparticle functionalization, and have been applied in the form of membranes or catalytic nanoparticles [181,185]. One notable example involves a PDA-coated polyurethane membrane, which catalyzes the reduction of methylene blue (MB) in the presence of NaBH₄ (Figure 11c) [186]. Although the reduction of MB by NaBH₄ is thermodynamically favorable, it is kinetically restricted and requires a catalyst. In this system, the PDA coating acts as an electron relay, facilitating electron transfer from BH 4 − to the dye.

Building on this catalytic function, the integration of nanoparticles further expands the potential of such systems. Nanoparticles are widely employed in catalysis due to their high surface area and unique physicochemical properties, which can significantly enhance reaction rates and selectivity. However, these advantages are often compromised by the tendency of nanoparticles to aggregate, leading to loss of active surface area and diminished catalytic performance. PDA readily forms conformal shells around a wide variety of inorganic nanoparticles, enabling the fabrication of hybrid materials with enhanced and tunable catalytic properties. The combination of PDA with inorganic substrates imparts multiple advantages, including biocompatibility, chemical stability, and a reactive surface for further functionalization, while also acting as a structural template for nano-constructs. For instance, PDA has been used to template the in situ growth of TiO₂ nanoparticles on cotton fabrics, creating photocatalytic surfaces via simultaneous synthesis and immobilization [178]. Metallic nanoparticles, valued for their high surface area, catalytic activity, and durability, have been widely combined with PCAs to facilitate the model reduction of 4-nitrophenol to 4-aminophenol, an established benchmark reaction that proceeds only in the presence of a catalyst [187,188]. Across these systems, PCAs serve as both reductive scaffolds and stabilizing agents for metal nanoparticles (e.g., Ag, Au, Pt, Pd), while substrates such as cotton, silica, or GO serve as structural supports (Figure 12a) [181,182,185,189].

Figure 12

Catalysis with PCA-containing materials. (a) Schematic illustration of the synthesis of Au nanoparticles on PDA-functionalized GO as catalyst for 4-nitrophenol reduction. Reprinted with permission from Ref. [191]. Copyright 2014 Royal Society of Chemistry. (b) Schematic illustration of a variety of reactions that can be catalyzed by silver nanoparticles on PDA-coated hydrated silica. Reprinted with permission from Ref. [182]. Copyright 2018 Elsevier B. V. (c) PDA as a stabilizer located between the cobalt-based oxygen-evolving catalyst (Co-OEC) and the ITO electrode for water oxidation. Reprinted with permission from Ref. [194]. Copyright 2018 Elsevier B. V.

Notably, PDA coating of GO results in partial reduction to conductive reduced GO (RGO) via electron release during DA self-polymerization [190], yielding hybrid materials functionalized with noble metal NPs (Ag, Au, Pt) [181,185,191]. These hybrids exhibit robust catalytic performance and reusability. For example, Pt NPs immobilized on PDA-coated RGO showed a nearly fourfold higher turnover frequency compared to Pt on plain RGO and a ninefold enhancement over commercial Pt/C catalysts [181].

Beyond model reactions, PDA-based systems, where PDA primarily acts as an anchoring layer, have been employed in broader catalytic contexts. PDA-coated silica, functionalized with Ag NPs, catalyzed the degradation of Congo Red and facilitated redox reactions involving potassium hexacyanoferrate(iii), overcoming interfacial electrostatic barriers, as depicted in Figure 12b [182]. In another example, cotton microfibers precoated with PDA enabled the in situ formation of Pd-nanoparticles, which served as heterogeneous catalysts in Suzuki coupling reactions [189]. These were further integrated into fixed-bed continuous-flow reactors. Similarly, cluster-like ZnO nanorods grown on PDA-coated polyethylene terephthalate films were further functionalized with Ag NPs, yielding materials with dual functionality: catalytic reduction of 4-nitrophenol and surface-enhanced Raman scattering capability [179].

PDA also offers unique advantages in electrocatalysis and membrane-based systems. Its intrinsic proton conductivity reduces mechanical failure in composite membranes, extending operational lifetimes, key for proton-exchange membrane fuel cells. Spray-coating techniques have been used to deposit PDA–Pt nanoparticle composites onto porous substrates without requiring post-deposition rinsing, enabling scalable fabrication of large-area catalytic surfaces [192].

In solid oxide fuel cells, PNE, a less-explored PCA, was employed to coat electrodes prior to CeO₂ NP deposition. The PNE layer enhanced nanoparticle dispersion and loading, resulting in improved catalytic activity and operational stability [193]. Similarly, PDA has been applied as an interfacial adhesive and electron/proton mediator in water oxidation catalysts (Figure 12c), where it suppresses delamination and Co catalyst dissolution while enhancing electrochemical performance [194].

PDA-coated carbon nanosheets functionalized with Pd nanoparticles have also been evaluated as cathodes in Swagelok-type Li–O² batteries, where the PDA layer increased discharge capacity and facilitated charge transfer by modulating the bandgap and promoting decomposition of discharge products via its nitrogen-containing groups [195].

Recent work has explored PDA-assisted metallization of solid supports with in situ–generated Au nanoparticles [196]. However, PDA’s reductive capacity is limited by the redox potential of the target metal; only cations with standard reduction potentials above ∼0.3 V vs NHE are readily reduced. Lower-potential metals such as Cu²⁺, Ni²⁺, or Fe³⁺ require an external reducing agent [183] or thermal annealing for successful metallization [196]. For instance, Cu–Ni alloy NPs were synthesized on PDA nanosheets via annealing, yielding catalytic materials with preliminary hydrogen evolution reaction (HER) activity, albeit with higher overpotentials than directly reduced Pt–PDA systems [196]. Altogether, these studies highlight the versatility of PCAs, particularly PDA, as platforms for engineering functional nanohybrids with tailored catalytic properties across a range of chemical and electrochemical transformations.

5.2 Photocatalysis

Photocatalysis offers a sustainable route to environmental remediation and clean energy conversion, using solar energy to drive redox reactions across a broad spectrum of applications, from pollutant degradation to fuel generation [197,198]. Both homogeneous and heterogeneous systems are employed for diverse reaction types, including oxidation, reduction, and organic transformation. Effective photocatalysts must simultaneously achieve efficient light absorption, high catalytic activity, and long-term stability against photocorrosion [199]. However, many current materials fail to meet all three benchmarks, prompting efforts to enhance performance, for example, by incorporating PCA coatings. As matrices, PCAs enable the fabrication of nanocomposite photocatalysts with improved performance. Their broad absorption spectrum enhances light harvesting, while their redox-active functional groups modulate conductivity and promote electron transfer. Most commonly, PCAs serve to improve photocatalytic activity or photostability in hybrid materials. For example, PDA has been combined with semiconductor materials such as TiO₂ [178], ZnO [179], and ZnS [200] facilitating charge separation at the interface and suppressing electron-hole recombination by introducing additional trap states. In some cases, PDA extends the light absorption range from the UV to the visible region, enabling utilization of lower-energy photons [180] and enhancing the overall quantum efficiency of the system [201,202].

PDA can function analogously to a polymeric semiconductor, forming heterojunctions with semiconductors of dissimilar bandgaps. The resulting photocatalytic behavior depends strongly on energy band alignment at the interface and the specific architecture of the composite [200]. For example, in a CdS/PDA/TiO₂ core, shell ternary hybrid, both CdS and PDA are photoexcited, with electrons transferred from PDA to the conduction bands of CdS and TiO₂, while holes migrate from CdS to PDA, enabling efficient charge separation [203]. In contrast, PDA in a BiVO₄@PDA/TiO₂/Ti photoanode acts primarily as an electron transfer mediator due to its π-conjugated structure [204]. Similarly, in an Ag/PDA@Ag₂S heterojunction, Nekouei et al. proposed an S-scheme charge transfer mechanism in which low-energy electrons from the conduction band of PDA recombine with Ag₂S holes, leaving long-lived charge carriers for redox reactions [205]. While the exact charge transfer mechanisms in these systems remain under debate, their ability to improve charge separation and photocatalytic performance is well established. PDA also acts as a protective layer, shielding photocatalysts from stoichiometric drift, environmental degradation, and photooxidation, thereby extending their operational lifetime [206,207]. A notable example is the stabilization of CdS quantum dots by PDA, which quenches photogenerated holes and prevents sensitizer degradation. In CdS/PDA composites, photocurrent was increased and long-term stability enhanced [208], as shown in Figure 13a.

Figure 13

Role of PDA in different photocatalytic processes. (a) Photostable PDA as a semiconductor can boost the photoelectrochemical water splitting of CdS on a fluorine-doped tin oxide (FTO) electrode. Reprinted with permission from Ref. [208]. Copyright 2003 Royal Society of Chemistry. (b) Synthetic eumelanin ultrathin films usable for photocatalytic hydrogen peroxide production. Reprinted with permission from Ref. [210] and licensed under CC BY-NC 3.0. (c) Efficient hydrogen production due to better charge separation by the PDA coating on ZnS electrodes. Reprinted with permission from Ref. [200]. Copyright 2020 Elsevier B.V.

PCAs have also been used to tune electron transfer in hybrid organic–inorganic systems. For example, GO coated with NE to yield PNE/RGO served as a biomimetic redox shuttle in solar water oxidation [184]. In this system, cobalt phosphate functioned as the catalyst and [Ru(bpy)₃]²⁺ as the photosensitizer. The presence of PNE/RGO more than doubled the catalytic efficiency, attributed to two synergistic interactions: (1) quinone groups in PNE rapidly accepted electrons from the excited photosensitizer, and (2) the RGO backbone facilitated long-range electron transfer [184]. A similar concept was demonstrated by Park et al., who employed PDA as a redox-active matrix to shuttle electrons between [Ru(bpy)₃]²⁺ and cobalt phosphate, mimicking quinone/catechol redox behavior [209].

Beyond their supporting role, PCA derivatives can themselves be photoactive. Spin-coated films of DHI-derived eumelanin on PET showed photocatalytic activity for O₂ reduction, producing H₂O₂ with turnover numbers 2–4× higher upon addition of electron donors [210], confirming true photocatalytic function (Figure 13b). When used as photoelectrodes, these films exhibited strong photocathodic currents and a 90% Faradaic yield under cycling illumination, with enhanced performance under photoelectrochemical conditions [210]. Photothermal contributions have also been linked to PDA-containing photocatalysts. In g-C₃N₄/PDA composites, PDA was proposed to improve catalytic efficiency through localized heating. This effect was amplified by incorporating plasmonic Ag NPs, which generated nanoscale thermal hotspots. PDA aided in both light-to-heat conversion [211] and in localizing thermal energy at reactive sites [212]. The chemical versatility of PCAs also facilitates covalent functionalization of nanostructures with catalytic components. In other examples, CdSe@CdS dot-in-rod structures were functionalized via PDA to immobilize either a Rh-based molecular catalyst, enabling photocatalytic NAD⁺ reduction [213] or with Co-based molecular catalysts for hydrogen evolution [214]. Here the PDA layer provided both a stable interface and reactive sites for covalent coupling.

Together, these studies demonstrate the multifunctional role of PCA coatings, enhancing light absorption, mediating electron flow, stabilizing photocatalysts, enabling hybrid interfaces, and in some cases, directly participating in photochemical transformations.

The adaptability of PCAs, particularly PDA, continues to drive innovation in photocatalysis. A notable example is the replacement of an Rh-based molecular catalyst with a cobaloxime catalyst capable of catalyzing the HER. In this system, PDA enabled hydrogen production in pure aqueous media – an environment typically incompatible with such molecular catalysts [214]. Further highlighting its multifunctionality, PDA was used to modify ZnS nanorods by forming a conformal coating that introduced oxygen atoms during the oxidative polymerization of DA, leading to the formation of a partial ZnO or ZnS₁₋ₓOₓ shell. The resulting ZnS/ZnSO/PDA structure exhibited a staggered double band alignment, which significantly improved photostability and facilitated efficient photocatalytic HER under visible light irradiation (Figure 13c) [200].

PDA’s role in heterojunction design is exemplified in the construction of a C₃N₄/PDA S-scheme photocatalyst, synthesized via in situ polymerization of DA hydrochloride on the U-CN surface [215]. This hybrid demonstrated excellent photocatalytic performance for H₂O₂ generation, attributed to its extended optical absorption and the efficient charge separation afforded by the S-scheme heterojunction architecture. In a related system, PDA/g-C₃N₄ composites were developed for visible-light-driven degradation of organic pollutants. Acting simultaneously as a light absorber, electron acceptor, and interfacial adhesive, PDA significantly enhanced photocatalytic performance. A composite containing 10 wt% PDA achieved 98% degradation of MB within 3 h under visible light, and maintained >90% efficiency after four cycles, demonstrating long-term durability and reusability (Figure 14a) [216].

Figure 14

Charge carrier transfer in PDA-based photocatalytic materials. (a) Schematic representation of electron transfer and the proposed photocatalytic dye degradation mechanism by PDA/g-C3N4 under visible-light irradiation. Reprinted with permission from Ref. [216]. Copyright 2017 American Chemical Society. (b) Schematic diagram illustrating the photocatalytic reaction mechanism of PDA@Ni _x Co_100−x nanotubes in methyl orange. Reprinted with permission from Ref. [217]. Copyright 2023 Elsevier Ltd. (c) Schematic illustration of the band structure of TiO₂ and PDA with the S-scheme charge transfer mechanism. Reprinted with permission from Ref. [226]. Copyright 2022, American Chemical Society (d) Diagram of the found electron transfer in the g-C3N4@PDA system. Reprinted with permission from Ref. [223]. Copyright 2019 WILEY‐VCH Verlag GmbH & Co. KGaA.

The versatility of PDA is further demonstrated in PDA@NixCo₁₀₀₋ ₓ nanotubes, which were synthesized as magnetic, recyclable photocatalysts for environmental remediation. These nanotubes showed high degradation efficiency toward azo dyes and nitroaromatic compounds, with strong saturation magnetization enabling easy recovery under an external magnetic field. Their structural stability and catalytic performance across multiple cycles highlight their suitability for wastewater treatment applications (Figure 14b) [217].

Inspired by the function of kerosene oil lamps, a PDA-coated, carbonized cotton strand was employed as a solar wick for interfacial solar evaporation [218]. This simple yet effective device achieved a solar-thermal conversion efficiency of 88.8% under 1 sun irradiation, enabling high-performance desalination with clean water production. The system leveraged naturally derived, low-cost, and biodegradable materials, reinforcing the potential of PDA-based materials for sustainable water purification. Beyond these photocatalytic and solar-thermal systems, the functional versatility of PDA allows for straightforward integration with various substrates. For instance, PDA-coated open-cell polyurethane foam was further functionalized with Eosin Y through surface silanization [219]. The catechol groups of PDA served as anchoring sites for the photosensitizer, extending the application range of PCA-based materials to solar-driven chemical transformations. Together, these examples underscore PDA’s multifunctional contributions, as a structural matrix, electronic mediator, interfacial stabilizer, and light harvester, in enabling high-efficiency and durable photocatalytic systems across diverse environmental and energy-related applications.

The growing body of research highlights the promise of PCAs as multifunctional components in photocatalytic systems, capable of contributing to performance enhancements through various roles, including photostabilization, charge carrier separation, electron transfer mediation, and surface modification. For example, a PCA-modified photocatalyst was successfully applied in the aerobic oxidation of N-methylisoquinolinium salt to the corresponding isoquinolone, demonstrating its efficacy in organic photoredox catalysis [219]. This further supports the utility of PCAs as adaptable and tunable building blocks in the design of photocatalytic hybrid materials.

Despite extensive empirical evidence supporting the functional benefits of PDA in composite systems, a comprehensive understanding of the underlying mechanisms remains elusive. Numerous studies ascribe different roles to PDA, ranging from light-harvesting and exciton generation to charge transfer facilitation or passive physical stabilization, yet few offer mechanistic clarity. Readers interested in detailed system comparisons and further discussion of PDA-based photocatalysis are referred to dedicated reviews [220,221].

A significant challenge in the field lies in the divergent interpretations of PDA’s function in hybrid systems. For example, in a g-C₃N₄/PDA system, Guo et al. described PDA solely as a charge transfer mediator enabled by its conjugated π-structure, without attributing any photoactivity to the material [222]. In contrast, Wang et al. proposed that PDA acts as a photoactive species capable of generating excitons and donating electrons to g-C₃N₄ under visible light irradiation (Figure 14d) [223]. In many other studies, PDA is primarily discussed as a polymer matrix providing environmental shielding, a structural scaffold, or a surface modifier [213,214,219–221,224].

This dichotomy, between PDA as a passive polymer and as an active photo-responsive component, is especially pronounced in complex, multi-component systems, where mechanistic insight is often limited by the experimental complexity and the lack of suitable characterization tools. Conventional methods, including steady-state optical spectroscopy, electrochemical techniques (e.g., impedance spectroscopy, linear sweep voltammetry, photocurrent measurements), and catalytic performance evaluations, provide valuable but often indirect evidence.

To address this gap, time-resolved optical spectroscopy, particularly transient absorption (TA) spectroscopy, has emerged as a powerful tool for disentangling charge transfer dynamics in PCA-based hybrid materials. For instance, TA studies on TiO₂/PDA composites revealed a significantly faster decay component (<10 ps) in comparison to bare TiO₂, indicative of ultrafast electron transfer from TiO₂ to PDA [225]. This observation supports the formation of a functional heterojunction, confirming PDA's role in promoting charge separation. Similarly, in a TiO₂/PDA system studied by Wang et al., a rapid electron transfer event (∼5 ps) from TiO₂ to PDA was observed under visible light, followed by a slower back transfer to TiO₂ upon cessation of illumination [226]. The faster electron transfer relative to TiO₂ recombination (∼10 ns) validates an S-scheme heterojunction mechanism (Figure 14c), which has implications for the design of artificial photosynthesis platforms and light-driven energy conversion systems.

TA spectroscopy has also been applied to PDA-coated plasmonic systems. In one example, coating Au nanoparticles with a PDA shell introduced a new ultrafast exciton decay pathway, with evidence of successful electron transfer from Au to PDA occurring on a competitive timescale [227]. More recently, Petropoulos et al. resolved sub-100 fs excitonic dynamics in PDA itself, confirming the formation of short-lived charge transfer states [228]. Their work demonstrated that UV excitation leads to faster formation and slower decay of these states compared to visible light, providing direct insight into the excited-state deactivation mechanisms intrinsic to PDA.

Together, these findings highlight the critical importance of advanced spectroscopic tools in resolving the functional complexity of PCAs in photocatalytic systems. The multifunctionality of PDA, as a light harvester, charge mediator, and structural modifier, depends strongly on the context of the hybrid material and its interface dynamics. Continued efforts to unravel these mechanisms will be essential to fully exploit the design potential of PCAs in high-performance photocatalytic and photoelectrochemical devices.

5.3 Biocatalysis

Biocatalytic materials are designed to replicate the catalytic efficiency and specificity of enzymes in biological systems. PCAs, due to their structural resemblance to eumelanin and their versatile surface chemistry, offer bio-derived matrices that can support the incorporation of enzymes into artificial catalytic systems. Their ability to stabilize enzymes enhances both catalytic efficiency and operational lifetime. Additionally, PCAs provide reactive functional groups for covalent coupling of biomolecules through Michael addition or Schiff base reactions with nucleophilic residues such as amines and thiols. As discussed above, these functional groups – particularly the redox-active catechol and quinone moieties – not only enable bioconjugation but also play a crucial role in facilitating charge transport within the material, supporting efficient electron transfer processes essential for electrochemical and catalytic applications

In one example, PDA was used to prepare enzyme-functionalized cellulose films via a two-step approach. A monolayer of PDA was electrochemically deposited on gold using a thiol-modified DA derivative, followed by immobilization of horseradish peroxidase (HRP). The resulting films exhibited low electrochemical impedance and retained catalytic activity for H₂O₂ oxidation [229]. Beyond HRP, PDA has enabled the chemical immobilization of various enzymes, including lipase from Candida sp. [230], Candida rugosa [231], alkaline phosphatase (ALP) [176], and catalase from bovine liver [232], on diverse substrates. For example, lipase from Candida sp. was immobilized at the oil–water interface of Pickering emulsions stabilized by SiO₂ nanoparticles. PDA formed in situ at the interface enhanced emulsion stability through hydrogen bonding and simultaneously served as a robust platform for enzyme anchoring (Figure 15a) [230]. Similarly, immobilization of ALP and catalase onto PDA films preserved enzymatic activity during storage at 4°C in Tris buffer [232]. Notably, PDA-immobilized catalase exhibited enhanced thermal tolerance, maintaining activity at 50°C, substantially higher than that of the free enzyme. This improved thermal resilience is attributed to the multiple anchoring points provided by the PDA matrix, a feature potentially generalizable to other enzyme classes.

Figure 15

PCA-containing materials for biocatalysis. (a) Interfacial polymerization of PDA in a Pickering solution leads to higher stability and possible enzyme functionalization for catalysis. Adapted with permission from Ref. [230]. Copyright 2015 American Chemical Society. (b) Comparison of enzyme immobilization on either PDA or PNE, leading to higher amounts of enzymes on PDA while more active sites are available on PNE. Adapted with permission from Ref. [232]. Copyright 2015 Royal Society of Chemistry. The tube structures were redrawn with the assistance of ChatGPT (OpenAI). The authors take full responsibility for the accuracy and representation of the figure.

PDA has also been applied to engineer recyclable catalytic systems. Lipase from Candida rugosa was immobilized on hollow Fe₃O₄ microspheres coated with mesoporous, flower-like PDA shells [231]. These materials, with tunable pore sizes, were used for biodiesel synthesis and demonstrated excellent reusability, maintaining catalytic activity over six cycles with facile magnetic recovery. Enzymes can also be directly integrated into PCA matrices during film formation. For instance, tyrosinase was incorporated into PDA-based films to create p-LDopa/enzyme composites for phenol and atrazine sensing [233]. In this case, the enzymatic reaction is harnessed to produce an electrochemical signal, enabling sensitive and selective sensing of these analytes. Similarly, in the case of catalase, both PDA and PNE were investigated as immobilization matrices. While PDA facilitated higher enzyme loading, PNE coatings resulted in greater relative enzymatic activity [232]. The enhanced performance of PNE was attributed to its smoother surface morphology, which likely provided more accessible active sites (Figure 15b).

In addition to serving as enzyme immobilization matrices, PCAs have also been introduced to synthesize enzyme-mimicking catalytic materials. For example, synthetic melanin nanoparticles with peroxidase-like activity were prepared by chelating iron ions into hollow particles derived from l-DOPA [234]. These materials showed higher catalytic rates under mild acidic conditions (pH 5.4–6) than natural HRP under its optimal conditions (pH 3.5–4.5). Catalysis was more effective when Fe(ii) was used instead of Fe(iii), likely due to the formation of a mixed-valence state during synthesis, which enhanced the overall redox activity. Importantly, the system showed minimal Fe leaching despite the redox cycling and radical species involved, which is highly desirable because iron leaching would decrease catalytic activity.

PCAs have also been employed in catalytic systems mimicking glutathione peroxidase to achieve biologically relevant nitric oxide (NO) release. In one example, stainless steel substrates were coated with PDA and functionalized with Cu²⁺ ions via catechol chelation. In the presence of glutathione and SNAP, this system achieved NO release rates comparable to native endothelium. In vivo evaluation demonstrated promotion of healthy endothelial regeneration and suppression of intimal hyperplasia [235]. Similarly, PDA-coated TiO₂ films were modified with selenocystamine, a component of the glutathione peroxidase active site, resulting in comparable NO release behavior and effective prevention of neointimal hyperplasia in stent applications [236].

These examples collectively highlight the functional versatility of PCAs in constructing catalytic systems for both enzyme-based and enzyme-mimetic applications. PCAs not only enable enzyme immobilization but also enhance catalytic efficiency, thermal stability, and operational durability, underscoring their potential in industrial biocatalysis, biosensing, and therapeutic device development.

5.4 Biosensors

The interface between a biosensor and its external environment plays a critical role in determining sensor performance, as it governs both analyte recognition and signal transduction. In typical biosensors, a chemical recognition event is transduced into an electrical or optical signal, with the surface, often composed of metals or semiconductors, directly interfacing with the biological sample. Effective surface functionalization must enable high-density immobilization of biorecognition elements while supporting reliable signal transduction and minimizing nonspecific interactions. Thin films with abundant functional groups and antifouling characteristics are particularly desirable for achieving high sensitivity and specificity. PCAs have emerged as promising materials for biosensor platforms due to their robust adhesion to diverse substrates and the presence of catechol and amine functional groups suitable for bioconjugation. Their use spans a range of biosensing modalities, including molecularly imprinted sensors, immunoassays, and aptamer-based devices.

As discussed in Section 5.3 on biocatalysis, PCAs not only provide a biocompatible and stabilizing matrix for enzyme immobilization, enhancing catalytic efficiency and operational lifetime, but also offer reactive functional groups for covalent coupling of biomolecules. In the context of biosensors, these same features are leveraged to create sensitive and specific detection platforms. Here the focus shifts from preparative catalysis to the use of catalytic or binding events as transduction mechanisms, where the resulting signal, rather than the product itself is of primary analytical interest. This dual functionality underscores the value of PCAs in enabling both robust biocatalytic systems and high-performance biosensors.

Immunosensors, which rely on the selective binding of antibodies to antigens, critically depend on the controlled immobilization of antibodies on the sensor surface. Strategies for antibody immobilization aim to maximize orientation, density, and stability of the antibodies to enhance binding efficiency and signal fidelity [237,238]. Traditional approaches include thiol-based monolayers on gold substrates, followed by antibody conjugation. More recently, alternative conductive materials such as PEDOT:PSS, glassy carbon, and reduced GO have been explored [239]. However, antibody immobilization must be tailored to each material’s surface chemistry.

PCA coatings, particularly PDA, offer a universal platform for antibody immobilization. For example, a homogeneous PDA film electrodeposited on gold-coated glass was functionalized with antibodies via Michael addition between the antibody amines and the PDA catechol moieties at basic pH, resulting in a 27% increase in antibody density as measured by surface plasmon resonance (SPR) spectroscopy (Figure 16a) [239]. In another approach, a one-pot method was developed for the direct incorporation of G protein into PDA films, which facilitated oriented antibody attachment. The resulting sensor exhibited enhanced sensitivity toward influenza virus due to the optimized availability of antigen-binding sites (Figure 16c) [240,241].

Figure 16

Various methods for preparing and functionalizing PCA-based films for antibody binding and molecular recognition applications. (a) Preparation of electropolymerized PDA films on gold-coated glass and its activation for antibody binding. Reprinted with permission from Ref. [239]. Copyright 2018 Elsevier B. V. (b) Procedure for the molecular imprinting of electrodeposited PDA for IgG recognition. Reprinted with permission from Ref. [244]. Copyright 2013 Springer Nature. (c) One step G protein PDA film deposition for directional antibody conjugation. Reprinted with permission from Ref. [240]. Copyright 2019 WILEY-VCH.

Beyond PDA, PNE has also been explored for biosensing applications. A PNE film containing a peptide imprinting template was fabricated on a gold SPR sensor chip and combined with magnetic beads encoded with myostatin for the detection of the therapeutic mAb Anti-MYO-029 in human serum [242]. PDA has also been used in the development of aptasensors – devices that use aptamers as selective recognition elements due to their high specificity and lower cost compared to antibodies [243]. In one study, PDA films were used to immobilize aptamers at high density, followed by gold nanoparticle attachment and silver metallization to amplify amperometric signals (Figure 16b) [244]. In another strategy, a self-assembled mixed interface was constructed by co-depositing polyethylene glycol (PEG) and ATP aptamer onto PDA-modified electrodes. The PEG reduced nonspecific adsorption, while the aptamer provided high binding affinity [243].

Molecularly imprinted sensors have also benefited from the properties of DA-derived coatings. For example, a peptide-imprinted PDA film was polymerized directly on a QCM chip for HIV sensing. The resulting biosensor demonstrated a detection limit of 2 ng/mL for HIV-1 gp41, comparable to conventional ELISA assays [245]. In another case, a PDA-based surface-imprinted polymer for IgG detection was constructed via co-electropolymerization of DA and IgG. After template removal, the sensor could detect IgG at concentrations as low as 296 nM [244].

A simple yet effective strategy for biosensor construction involved immersing a QCM chip in a PDA solution followed by conjugation with IgG1 anti-myoglobin antibodies. The resulting device exhibited high affinity and stability during kinetic assays and showed partial regeneration capability, indicating potential for cost-effective reuse [246].

PCAs have also shown great promise in biocatalytic sensors. Their ability to stabilize enzymes and maintain activity under challenging conditions expands their utility across a broad range of biosensing platforms. Overall, the multifunctionality, substrate-independence, and rich surface chemistry of PCAs, particularly PDA and PNE, make them attractive candidates for next-generation biosensors. Their ability to support controlled anchoring of biorecognition elements, suppress nonspecific binding, and enhance signal output positions them as key materials in the development of sensitive, robust, and scalable biointerfaces.

PCAs have attracted significant attention for use in biosensors and bio-catalytic materials due to their capacity to overcome limitations associated with entrapped biocatalysts, which often suffer from poor stability and limited reusability. In particular, enzyme immobilization on solid supports remains a critical challenge in biosensor development, especially for biorecognition-based electrochemical assays targeting clinical biomarkers. Electrochemical biosensing platforms are favored for their rapid response, minimal sample preparation, and low cost. However, effective sensor fabrication requires functional coatings that preserve biological activity, provide reactive groups for conjugation, and ensure strong attachment to avoid leaching.

Laccase is an oxidoreductase enzyme widely found in plants, fungi, and some bacteria, where it plays a key role in lignin degradation and the detoxification of phenolic compounds [247–249]. Its broad substrate specificity and ability to catalyze the oxidation of a variety of aromatic and phenolic compounds make laccase an attractive biorecognition element for biosensor applications [248,250–252]. In many biosensor setups, laccase has been incorporated into PDA-based films, leveraging the adhesive and functional properties of PDA to create stable, sensitive, and efficient enzyme-based sensing interfaces [135,252,253]. For example, laccase was incorporated into a PDA matrix through potentiostatic electropolymerization, resulting in uniform enzyme distribution and strong immobilization without compromising the redox activity of PDA. The resulting sensors exhibited catalytic activity for phenol detection as a proof-of-concept application. In this sensor, laccase catalyzes the oxidation of phenol to its corresponding quinone, using molecular oxygen as the electron acceptor. This enzymatic reaction generates electrons, which are transferred through the redox-active PDA matrix to the electrode, producing a measurable current. The magnitude of this current is directly proportional to the concentration of phenol in the sample, enabling sensitive and selective electrochemical detection (Figure 17a) [135]. Further investigation into the electropolymerization conditions revealed that the scan rate during potentiodynamic deposition significantly affected film performance. A scan rate of 200 mV/s produced more efficient laccase-based sensors compared to those prepared at 20 mV/s, as faster deposition yields thinner, more permeable PDA films that better preserve enzyme activity and facilitate substrate diffusion. In contrast, slower scan rates result in thicker, denser films that can hinder enzyme function and reduce sensor sensitivity. Highlighting the importance of deposition kinetics, a parameter not extensively studied in the literature [170].

Figure 17

Different biosensor designs utilizing PCAs. (a) Schematic illustration of the electrochemical synthesis of the PDA-Laccase biosensor with the mechanism of phenol detection. Adapted from Ref. [135]. Copyright 2019 Elsevier Ltd. (b) Schematic representation of two routes of PDA-ETA film formation for IgG immobilization. Reprinted from Ref. [254], licensed under CC BY 4. (c) Illustration of the photoelectrochemical biosensor consisted of an ITO electrode and a TiO₂ layer coated with PDA-PEI for the detection of ascorbic acid (AA). Reprinted from Ref. [255] and licensed under CC BY-NC-ND 4.0.

While PDA remains the most commonly employed PCA for biosensor applications, other derivatives such as PNE offer unique advantages. PNE provides a cost-effective and versatile alternative that preserves enzyme activity during immobilization [174]. In a comparative study, PDA and PNE films were evaluated for immobilization of a model enzyme on titanate nanotubes [225]. Although PDA’s rougher surface promoted higher enzyme loading, the smoother, more hydrophilic PNE surface led to enhanced enzyme activity and stability, improving overall sensor performance. PNE films have also been utilized for antibody immobilization on graphene-based electrodes, yielding immunosensors with high sensitivity and reproducibility [173].

Electrochemical strategies have also enabled the development of functional copolymer films, such as PDA–ethanolamine (ePDA–ETA), for immunosensing interfaces (Figure 17b) [254]. During potentiodynamic deposition, oxidized PDA quinone groups react with aliphatic amines from ETA, forming covalently bonded copolymers in a one-step process. These films exhibited enhanced antifouling properties and strong protein immobilization, as confirmed by real-time SPR and ellipsometry. While the ePDA–ETA films displayed lower IgG loading compared to post-modified PDA/ETA films, they showed improved antigen-binding affinity, demonstrating the impact of ETA incorporation on biorecognition performance.

In another application, the synergistic combination of PDA and inorganic semiconductors was exploited to fabricate a visible-light-responsive photoelectrochemical biosensor. PDA was copolymerized with polyethyleneimine on an ITO/TiO₂ electrode, leveraging PDA’s visible-light absorption and redox capability along with the electron-attracting nature of polyethyleneimine. The resulting interface exhibited faster electron transfer rates, reduced series resistance, improved suppression of recombination, and enhanced photocurrent output (Figure 17c) [255]. The biosensor enabled sensitive and broad-range detection of ascorbic acid in vitamin C tablets and various beverages, illustrating the system’s analytical robustness. Taken together, these studies underscore the potential of PCAs, particularly PDA and PNE, as versatile platforms for the construction of next-generation biosensors. Their ability to facilitate efficient immobilization of recognition elements, stabilize enzymatic activity, and enhance sensor performance makes them valuable components in bioanalytical device engineering. Continued exploration of their surface chemistry and electropolymerization behavior will further expand their utility across a wide range of biosensing technologies.

5.5 Biomedical applications

Despite the biological origin of their monomers, polymeric PCAs exhibit diverse cellular interactions depending on their chemical structure and processing conditions. This variability is particularly evident in comparative studies evaluating PDA, PDOPA, and PNE coatings on stainless steel for blood-contacting biomedical applications. Among the tested coatings, PNE demonstrated high hemocompatibility, histocompatibility, and vascular cell selectivity. In contrast, PDA induced significant platelet adhesion and activation, while PDOPA exhibited the poorest performance, with high inflammatory potential and defective coating morphology [139].

The widespread use of PCAs in biomaterials and medical devices stems from their robust surface adhesion, ease of functionalization, and compatibility with a variety of substrates. For example, PDA coatings have been functionalized via Aza–Michael addition using Michael acceptors such as sulfobetaine methacrylate, sulfobetaine acrylamide, and methyl ether methacrylate to generate bacteria-repellent surfaces. These modified coatings showed up to 98% reduction in bacterial adhesion on TiO₂ surfaces [256]. In addition, in situ reduction of Ag⁺ on PDA films afforded dual-function antibacterial and fouling-resistant surfaces. Photothermal bactericidal effects of PNE coatings have also been demonstrated, further expanding their antibacterial utility [257].

Beyond coatings, PCAs have been incorporated into more complex biomaterial systems. PDA and PNE were used as functional shells in core–shell granular hydrogels composed of norbornene-functionalized hyaluronic acid cross-linked with Irgacure. The PNE-coated hydrogels exhibited enhanced ionic conductivity and cytocompatibility, showing no skin irritation in human models. These materials were implemented as flexible stress sensors in human–machine interface applications, including a haptic response platform for virtual reality [258]. Another example is a PDA/carboxymethyl cellulose/PAA hydrogel, which showed good biocompatibility and strong UV-shielding performance, making it suitable for applications in UV protection membranes and skincare [259] (Figure 18a).

Figure 18

Different uses of PCA-based materials in biomedical applications. (A) Schematic of dorsal mouse skin covered by PAA and PDA/CMC/PAA hydrogels with photos of skin after 20 min of UV irradiation and a schematic demonstration of UV shielding on (a) naked skin, (b) PAA hydrogel-covered skin, and (c) PDA/CMC/PAA hydrogel-covered skin. Reprinted with permission from Ref. [259]. Copyright 2021 Springer Nature B.V. (B) Schematic illustration of the Met@TA-ZIF-PSG patch, which promotes cell adhesion and migration due to PDA, scavenges excessive ROS by phenolic hydroxy groups from PDA, reduces inflammation, and enhances osteogenic differentiation of PDLSCs. Reprinted with permission from Ref. [261]. Copyright 2023 American Chemical Society.

PCA coatings have also been applied to electrospun polycaprolactone (PCL) microfibers to create bioactive scaffolds for muscle tissue engineering. PNE-coated membranes with fiber diameters of ∼2 μm provided optimal conditions for myoblast adhesion and proliferation, both in vitro and in vivo. Improved wettability and higher surface area were key contributing factors. Mechanistically, the enhanced muscle regeneration was linked to suppression of myostatin, resulting in tissue integration and mechanical strength comparable to native muscle [260]. A recent development includes a multifunctional PDA-based silk fibroin/gelatin patch for treating periodontitis in diabetic patients. This patch integrates ultralong PDA-silk microfibers with tannic acid-modified zeolitic imidazolate framework (ZIF) components, affording mechanical flexibility and sustained drug release. The composite provides anti-inflammatory, antioxidant, and immunomodulatory properties, while also promoting anti-aging and osteogenic responses. These features position the patch as a promising candidate for periodontal therapy under diabetic conditions [261] (Figure 18b).

In wound healing applications, PDA has been widely employed to enhance hydrogel performance through its antioxidant, antibacterial, photothermal, and drug-loading capabilities [262–266]. These multifunctional effects contribute to accelerated tissue repair and reduced infection risk. A detailed overview of PDA and PCA use in wound healing is available in several recent reviews [267–271]. In conclusion, the biomedical potential of PCA-based materials, particularly PDA, PDOPA, and PNE, underscores the importance of tailoring surface chemistry and structure to meet specific biological requirements. While PNE exhibits high hemocompatibility and vascular cell selectivity, both PDA and PNE enable the design of multifunctional hydrogels, antibacterial surfaces, tissue engineering scaffolds, and therapeutic devices. These advances highlight the versatility and promise of PCAs in biomedical engineering and healthcare applications.

5.6 Membranes

Separation membranes serve as selective barriers that enable the controlled permeation of target species while maintaining phase separation, typically between liquids or gases. The interfacial properties of the membrane material with the feed solution are critical in determining permeability and selectivity, both of which are essential for achieving efficient separation performance [12]. Key design parameters for such membranes include surface charge, hydrophilicity, and surface functionality. In this context, PCA coatings, particularly PDA, offer unique advantages due to their substrate-independent adhesion and versatile surface chemistry. PDA has been widely employed as a coating for membrane modification. Simple dip-coating procedures enable surface functionalization of both the outer surface and internal pore structures, making it particularly attractive for porous membrane systems. PDA coatings have been utilized to tune surface hydrophilicity, enhancing water flux and antifouling performance. Moreover, PDA’s ability to reduce metal ions in situ has been exploited to fabricate organic–inorganic hybrid films containing Au and Ag nanoparticles for catalytic applications such as the reduction of MB.

Although dip-coating remains the most commonly employed method for PDA deposition, limitations such as coating uniformity and deposition time have led to the exploration of alternative techniques. Electrospraying has emerged as an efficient approach to form more compact and robust PDA coatings (Figure 19a). Electrosprayed PDA-coated membranes have demonstrated effective dye removal from aqueous solutions, offering improved stability and processing efficiency [272]. Additionally, the self-polymerization of PDA can be controlled by tuning the redox environment. For instance, PDA coatings formed in the presence of NaBH₄ on mixed cellulose ester membranes produced ultrathin films with small particulate features [273]. These membranes exhibited high water permeability (∼400 L m⁻² h⁻¹ bar⁻¹), which, while lower than that of pristine membranes (∼500 L m⁻² h⁻¹ bar⁻¹), was significantly higher than PDA coatings formed under oxidative conditions (∼190 L m⁻² h⁻¹ bar⁻¹). Notably, these coatings also provided excellent resistance to organic fouling, achieving ∼90% bovine serum albumin rejection, and exhibited antimicrobial activity [273].

Figure 19

PCA-based membranes for filtration applications. (a) PEI membrane coated with PDA for oil-water emulsion separation. Reprinted with permission from Ref. [281]. Copyright 2014 Royal Society of Chemistry. (b) Electrospraying method for PDA deposition on a membrane surface. Reprinted and adopted with permission from Ref. [272]. Copyright 2020 Elsevier B. V. (c) PTFE membrane coated with PDA for improved oil-water separation. Reprinted with permission from Ref. [276]. Copyright 2020 Elsevier B. V.

PDA has also been applied to polyethersulfone (PES) membranes to improve oil/water emulsion separation performance [274–276] (Figure 19b). PDA coatings increased the surface hydrophilicity, leading to an ∼800% increase in flux and a separation efficiency of 98% compared to pristine PES membranes. Similarly, co-deposition of PDA and PEI on polypropylene microfiltration membranes further enhanced hydrophilicity and reduced deposition time relative to PDA alone. The PDA-PEI coating solution could be reused with negligible loss in performance, offering a more sustainable alternative to conventional modification approaches [277–279]. The resulting membranes demonstrated ultrahigh water permeability suitable for oil-in-water microfiltration under atmospheric pressure [276] (Figure 19c).

In situ polymerization of DA on polybenzimidazole (PBI) supports yielded membranes with exceptional solvent resistance and permeability to polar aprotic solvents. This strategy enabled the formation of an interpenetrating polymer network between PDA and PBI without the need for covalent cross-linking. Membrane properties, such as molecular weight cutoff and solvent permeability, were tunable via polymerization time. These membranes operated reliably across a broad temperature range (−10 to 100°C) in both conventional and green polar aprotic solvents, demonstrating the viability of PDA coatings as a green alternative to conventional cross-linking strategies [280].

Beyond coatings, PDA has also been investigated as a freestanding membrane material [8]. These membranes feature sub-2 nm pores and strong hydrogen-bonding networks, enabling effective ion separation. Significant selectivities were observed, for example, in the separation of K⁺ over Mg²⁺, [Fe(CN)₆]³⁻, and rhodamine B. In addition, pH-responsive gating behavior was demonstrated, allowing for the separation of dye mixtures with differing charges [8]. These results underscore the potential of PDA not only as a surface modifier but also as a functional membrane material in its own right.

5.7 Nanorobots and actuators

One of the most compelling features of PDA is its strong photothermal response to NIR light, a property that has been extensively exploited in areas such as photothermal therapy. This behavior is not exclusive to PDA; other PCAs, including PDOPA and PNE, also exhibit NIR-responsive characteristics. Beyond biomedical applications, these photothermal properties are increasingly utilized in the development of soft actuators and nanorobots, where light-triggered deformation, programmable shape-memory behavior, and dynamic actuation are highly desirable features [151,282]. These materials serve as platforms for responsive systems capable of converting external stimuli into controlled mechanical motion.

PDA has been successfully integrated into thermo-responsive polymer matrices to create composite systems that exhibit shape-morphing capabilities under NIR irradiation. Early examples include PDA nanoparticles embedded within a polymeric matrix, where the localized heating effect from PDA serves as the actuation trigger. The system was optimized to enhance interfacial contact between PDA and the surrounding polymer, thereby maximizing heat transfer efficiency.

In addition to nanoparticles, PDA thin films have also been employed to fabricate light-responsive actuators. In one study, PDA was selectively deposited on a spray-aligned liquid crystalline network (LCN) via dip-coating [283] (Figure 20a). Upon NIR light exposure, the PDA-coated region heated the underlying LCN, inducing anisotropic bending that was reversible upon cooling. A similar strategy was applied to liquid crystalline elastomers, where PDA-coated surfaces produced rapid and reversible bending motions under light exposure [282]. Notably, the PDA coating not only enabled photothermal response but also allowed fine control of film thickness through pH-dependent degradation in alkaline conditions.

Figure 20

Photoactive films based on PCA composites. (a) NIR light triggered DA-coated liquid-crystal films for photo-actuated movement. Reprinted with permission from Ref. [283]. Copyright 2020 WILEY-VCH. (b) Shape memory polymers regionally coated with PDA. The shape is recovered by irradiating PDA patches with NIR light to heat up the system. Reprinted and adopted with permission from Ref. [282], licensed under CC BY 3.0. (c) Freestanding PDA membrane actuated with laser light, temperature, and vacuum. Reprinted with permission from Ref. [286], licensed under CC BY 4.0.

PDA coatings have also been used to trigger shape-memory effects in pre-patterned polymers. For instance, PDA-coated diphenyl-based LCNs were actuated by NIR-induced heating, enabling recovery of programmed shapes upon illumination [282] (Figure 20b). Additionally, PDA has been incorporated into poly(urea-urethane nanocomposites, where it enhanced mechanical strength, fracture toughness, and self-healing behavior upon NIR irradiation. The composite’s thermal properties, including glass transition temperature and thermal stability, were strongly influenced by the PDA content [284].

Advanced bilayer hydrogel actuators have also utilized PDA-modified MXene nanosheets to overcome aggregation issues and introduce NIR-responsiveness. A bilayer composed of PNIPAm/PDA-MXene/Ca²⁺ and PAA/SBMA was engineered to achieve rapid, reversible actuation, capable of 360° bending in 10 s (Figure 20c) [285]. This enabled the construction of microscale soft robots and light-driven microswimmers, demonstrating the power of PDA-based systems in complex soft robotic architectures.

Recently, a different approach to photothermal actuation was reported using electropolymerized PDA films. These freestanding nanomembranes, fabricated via deposition on gold and release using a PVA sacrificial layer, exhibited ultrafast humidity-responsive actuation, with contraction and expansion times of 20 ms and 140 µs, respectively [286]. The motion was driven by temperature or light-induced changes in humidity levels, causing reversible shrinkage due to lamellar rearrangements within the PDA structure. Unlike previous examples that relied on composites, this behavior was observed in pure PDA films, and was later extended to PDOPA and PNE, indicating that this actuation mechanism is an inherent property of PCAs [37].

Collectively, these findings highlight the broad potential of PCA-based materials as building blocks for light-responsive and programmable soft devices. Their tunable mechanical, thermal, and photothermal properties, combined with facile fabrication techniques, position them as promising candidates for future applications in smart materials, soft robotics, and dynamic biointerfaces.