Gold nanocrystals and nanorods functionalized with protein and polymeric ligands for environmental, energy storage, and diagnostic applications: A review

2.1 Advancements in Au nanocluster synthesis techniques

Advancements in Au nanocluster synthesis techniques have led to the development of a wide range of methods to produce nanoclusters with precise control over their size, shape, and surface properties. These techniques have enabled researchers to tailor the properties of Au nanoclusters for specific applications, including energy storage, catalysis, sensing, and biomedicine. Some of the notable advancements in Au nanocluster synthesis techniques are as follows:

1. Ligand-assisted synthesis : Ligand-assisted synthesis methods involve using specific ligands as stabilizing agents during the Au nanocluster formation. By carefully selecting the ligands, researchers can control the size and shape of the nanoclusters. This approach allows for the production of monodisperse Au nanoclusters with well-defined structures and enhanced stability [19].

2. Seed-mediated growth : Seed-mediated growth is a technique that involves using preformed Au nanocrystals as seeds for the controlled growth of larger nanoclusters. By adjusting the growth conditions, researchers can precisely control the size and morphology of the resulting nanoclusters. This method enables the synthesis of Au nanoclusters with uniform size and shape distributions [20].

3. Microfluidic synthesis : Microfluidic platforms offer a highly controlled and reproducible environment for synthesizing Au nanoclusters. The precisely controlled flow conditions and reaction parameters in microfluidic channels allow for the synthesis of uniform and monodisperse nanoclusters with tuneable properties [21].

4. Bottom-up approaches : Bottom-up approaches involve the reduction in Au ions or Au complexes in the presence of reducing and stabilizing agents. These methods enable the formation of Au nanoclusters from atomic or molecular precursors. The careful selection of reactants and reaction conditions allows for fine-tuning nanocluster properties [20,22].

5. Biogenic synthesis : Biogenic synthesis involves using biological entities, such as microorganisms or plant extracts, as reducing and stabilizing agents for Au nanocluster formation. This environmentally friendly approach offers a sustainable and green route to produce nanoclusters with unique properties [23].

6. Plasma synthesis : Plasma-based methods, such as plasma sputtering or plasma-induced nucleation, provide a controlled environment for synthesizing Au nanoclusters. These techniques allow for the production of nanoclusters with well-defined sizes and structures.

7. Template-assisted synthesis : Template-assisted synthesis involves using templates or scaffolds with specific shapes and sizes to guide the nucleation and growth of Au nanoclusters. This method enables the production of nanoclusters with tailored shapes and arrangements [24].

8. Photochemical synthesis : Photochemical synthesis methods utilize light irradiation to drive the reduction and nucleation of Au ions, leading to the formation of Au nanoclusters. Light-controlled synthesis allows for the precise control of nanocluster properties [25].

9. Gas-phase synthesis : Gas-phase synthesis techniques involve the nucleation and growth of Au nanoclusters in the gas phase, followed by deposition onto a substrate. This approach offers high purity and control over nanocluster size and composition [26].

Advancements in Au nanocluster synthesis techniques have opened up new opportunities for tailoring their properties for various applications. Researchers continue exploring novel approaches and fine-tuning existing methods to produce Au nanoclusters with enhanced performance and functionality in energy storage, catalysis, sensing, and biomedical applications. These advancements in Au nanocluster synthesis techniques offer researchers the ability to tailor the properties of nanoclusters for specific applications in energy storage, catalysis, sensing, and biomedicine. Each method has advantages and disadvantages, and ongoing research aims to optimize these techniques further for improved performance and functionality (Table 1).

2.2 Emerging applications of Au nanoclusters

Emerging applications of Au nanoclusters have garnered significant attention in recent years due to their unique properties and versatile nature. These tiny clusters of a few to several dozens of Au atoms exhibit distinct electronic, optical, and catalytic features that make them attractive for various cutting-edge applications. Some of the emerging applications of Au nanoclusters are as follows:

1. Energy storage : Au nanoclusters have shown promise in energy storage applications, particularly in developing advanced batteries and supercapacitors. Their small size and high surface area-to-volume ratio allow efficient charge storage and fast charge–discharge rates. Incorporating Au nanoclusters into electrode materials can enhance energy storage devices’ electrochemical performance and cycling stability [27].

2. Sensing and detection : Au nanoclusters have exceptional sensing capabilities, making them valuable in environmental monitoring, disease diagnostics, and food safety. Their fluorescence and plasmonic properties enable sensitive and selective detection of target analytes, such as heavy metals, environmental toxins, and biomolecules [28].

3. Catalysis : Au nanoclusters possess immense potential as catalysts for various chemical reactions. Their unique electronic and geometric structures allow for efficient activation of reactants and selective catalytic transformations. These nanoclusters find applications in green chemistry, hydrogen generation, and pollutant degradation [29].

4. Biomedical imaging and therapy : Au nanoclusters have been explored for bioimaging and therapeutic medical applications. Their biocompatibility, tuneable fluorescence, and surface chemistry are ideal candidates for targeted imaging and drug delivery systems. Additionally, their photothermal properties enable localized hyperthermia for cancer therapy [30].

5. Optoelectronics and photonics : Au nanoclusters have shown potential in optoelectronics and photonics, contributing to advancements in light-emitting devices, lasers, and sensors. Their size-dependent emission and tuneable fluorescence allow for the design of novel optical devices with enhanced efficiency and functionality [31].

6. Nanoelectronics : Au nanoclusters are being explored for applications in nanoelectronics, such as single-electron transistors and molecular electronic devices. Their discrete energy levels and quantum confinement effects offer opportunities for developing nanoscale electronic components [32].

7. Environmental remediation : Au nanoclusters can be functionalized to enhance their pollutant removal capabilities. Their high surface area and catalytic activity enable the degradation of organic pollutants and the removal of contaminants from water and air [33].

8. Photovoltaics : Au nanoclusters have the potential to enhance light absorption and charge separation in solar cells. By incorporating these nanoclusters into solar cell materials, researchers aim to improve the efficiency and performance of photovoltaic devices [34].

9. Nanomedicine : Au nanoclusters are being investigated for drug delivery and theranostic applications. Their ability to carry and release therapeutic agents and their imaging capabilities allow personalized medicine and targeted treatments [35].

As research on Au nanoclusters continues to progress, these tiny nanoparticles are expected to find even more innovative applications across various fields, driving advancements in technology, medicine, and environmental sustainability. Table 2 summarizes the emerging applications of Au nanoclusters and their potential advantages and disadvantages.

2.3 Unique characteristics and surface influences of Au nanocrystal

The transition from the atomic or molecular system to like material is considerable when considering nanoscale plans [36,37]. At the nanoscale, materials implement unique characterization and behaviours that vary from their bulk counterparts due to the dominance of surface influences, quantum confinement, and increased surface-to-volume ratio [38,39,40]. Materials and particles possess distinct electronic and optical features at an atomic and molecular scale [41]. Quantum mechanical influences become pronounced. Physiological features like band gaps, energy levels, and optical transitions may be sensitively tuned by monitoring the nano diameter, nanoparticles’ shape, and nanoscale materials’ composition [42]. These unique features have been used in electronics, photonics, catalysis, energy storage, and biomedicine. However, as the diameter of the particles increases, the collective behaviour of a large atom or molecule number becomes more relevant, leading to the emergence of bulk features. The transition from atomic and molecular systems to bulk systems occurs when the number of molecules or atoms reaches a sufficiently large scale [37]. Macroscopic features like density, conductivity, and thermal stability may describe the material’s behaviour. In bulk matter, the influence of individual atoms or molecules becomes less pronounced, and the statistical behaviour of large ensembles becomes more prominent. It involves considering the inter implement among nanoscale influences and bulk features and the scalability of nanoscale phenomena to macroscopic usages.

2.4 Harnessing the potential of Au nanocrystals for enhanced fluorescence imaging and sensing

By harnessing the unique features of nanoscale materials and influencing and integrating them into bulk systems, it is possible to develop innovative technologies with enhanced performance and functionality [43]. Clusters of a little Au atom, often referred to as Au clusters, may implement features that resemble those of molecules. These clusters typically consist of a few Au atoms, ranging from a little to several dozens of atoms. Due to its small diameter, Au clusters may implement discrete electronic energy levels resembling molecular orbitals in molecules. This leads to unique electronic and optical features that vary from individual Au atoms to bulk Au materials [44]. Indeed, in the nanoscale diameter range, there is a fascinating transition zone where Au nanoclusters of approximately 100 metal atoms may be found. The behaviour and features of these nanoclusters are influenced by their electronic structure, which determines their stability and reactivity. When considering the lowest-energy configurations of Au nanoclusters, disordered and ordered structures may be produced on the cluster potential energy surface (PES). The PES represents the potential energy of the cluster as a function of its atomic positions, and finding the lowest-energy configuration corresponds to the most stable structure. A series of mono-cuboctahedral Au nanoclusters has been observed, showing a correlation among its architecture and features. “Mono-cuboctahedral” refers to a specific arrangement of atoms resembling a cube and octahedron combination. This series of nanoclusters implements enhanced features and photoluminescence, which is the emission of light upon excitation. In contrast to nanoclusters, where the interactions among metal atoms dominate, Au complexes demonstrate considerable charge transfer among metal atoms. Au complexes refer to larger assemblies or molecules containing Au atoms bonded to other ligands or atoms. The existence of ligands may influence these complexes’ electronic architecture and charge transfer features. Understanding the electronic architecture and features of Au nanoclusters and Au complexes is pivotal for exploiting their unique characteristics in various usages, including catalysis, sensing, and nanoelectronics. These nanoscale systems offer opportunities for tailoring their features through precisely monitoring their diameter, shape, and surface chemistry, leading to potential advancements in materials science and technology [45].

2.5 Gold nanoclusters: Bridging the gap between molecules and bulk materials

It is pivotal to comprehend the electronic level of energy architecture of Au nanoclusters to comprehend the molecular or metallic behaviour for energy storage usages. Little-atom Au nanoclusters implement unique features due to the influence of quantum confinement, which arises from the confinement of electrons within the small diameter of the cluster. This influence leads to discrete energy levels, similar to molecular orbitals, rather than a continuous band architecture typically seen in bulk metals. A mono-cub octahedral series of Au nanoclusters implements structure–property correlation, considerable enhancement, and photoluminescence origin. These gaps may reduce and slow the transport of nonradiative electrons among excited states with various features. Au nanoclusters, like the Au156 molecular Au nanocluster, demonstrate distinct energy levels resembling molecular orbitals and metallic bands in their excited state electron dynamics. This behaviour arises from combining molecular-like characteristics and metallic features within the same nanocluster. By gathering a small number of Au ions and stabilizing agents, the molecules with a ligand layer Au, NPs have also been utilized for novel energy storage usages [37]. The Au nanocluster behaves like a molecule because the thermal energy is insufficient to excite the electrons as long as there is a considerable gap among it and the thermal energy. Excitons are formed in Au nanoclusters by photons with energies greater than the HOMO–LUMO gap. Excitons may split apart and produce free electrons and holes. In the matter of Au nanoclusters, the thermal excitation of valence electrons from fully occupied levels to upper levels becomes more dominant when the energy gap among the two bands is small. This thermal excitation is responsible for the absorption of photons and the promotion of electrons to higher energy levels. The existence of energy gaps in Au nanoclusters may considerably influence their electronic and optical features. These energy gaps are pivotal in slowing down the nonradiative electron transport among excited states with various features. Nonradiative electron transport refers to processes where excited electrons lose excess energy through mechanisms other than emitting light. As the diameter of the Au nanoclusters diminishes, the energy gap among distinct energy levels tends to increase. This broadening of the energy gap results in a shift towards higher energy levels and, in optical absorption, a blue shift. In other words, smaller Au nanoclusters tend to absorb light at shorter λ _max, leading to a blue-shifted absorption spectrum. This phenomenon is attributed to the influence of quantum confinement, where the diameter of the nanoclusters restricts the motion of electrons, causing discrete energy levels to emerge. As the nanoclusters become smaller, the quantum confinement influence becomes more pronounced, resulting in considerable alterations in their electronic and optical feature.s Understanding and monitoring the diameter-dependent optical feature of Au nanoclusters is pivotal for various usages, like sensing, imaging, and optoelectronics. Researchers may tailor their absorption and emission features by manipulating the diameter and architecture of Au nanoclusters, opening up possibilities for designing nanoscale devices with desired optical functionalities [46]. Due to their distinctive physical and chemical characteristics, Au and silver NPs are preferred over metallic NPs for various nanotechnology usages. In the matter of AuNPs, as the diameter diminishes and the nanoparticle becomes more “metallic,” the symmetry of the Au core tends to diminish. This symmetry diminishes due to the crystal lattice distortion as the number of Au atoms diminishes and the surface-to-volume ratio increases. In larger AuNPs, the Au core may implement a high degree of symmetry, often approximating a spherical shape. As the diameter diminishes and the number of Au atoms becomes smaller, the surface atoms have a more considerable impact on the overall architecture and features of the nanoparticle. These surface atoms experience various local environments compared to the core atoms, leading to deviations from perfect symmetry. The distortion of the Au core’s symmetry may result in various shapes, like rod-like, triangular, or branched structures, depending on the specific conditions during nanoparticle synthesis and the interactions with stabilizing agents or ligands. These shape deviations from perfect symmetry may considerably influence the electronic, optical, and catalytic features of the AuNPs. The lower symmetry of the Au core in smaller AuNP introduces new electronic states and energy levels, which may affect its chemical reactivity and physical features. These unique electronic states are often responsible for the diameter-dependent feature and enhanced catalytic activity observed in little AuNPs. Understanding the relationship among the diameter, symmetry, and features of AuNP is essential for tailoring its behaviour and designing novel usages. Researchers continue exploring synthesis methods and characterization techniques to precisely monitor the diameter, shape, and symmetry of AuNP for various usages in catalysis, sensing, imaging, and nanoelectronics. The fundamental mechanism results from distributing the NP electronic states near Fermi energy sensitive to NP symmetry. Gold nanoclusters, such as Au144, exhibit molecular-like properties, whilelarger clusters, like Au333, begin to display metallic characteristics, highlighting the size-dependent transition in AuNP behavior. AuNPs may alter from non-metallic to metallic in just 33 atoms [47,48]. AuNPs and silver NPs are the most accepted among other metallic NPs due to their distinctive physical and chemical characteristics. AuNPs have been extensively explored for a variety of uses in nanotechnology. The nanoparticle is more “metallic” with the lower symmetry of the Au centre. The fundamental mechanism results from the symmetry-sensitive distribution of the nanoparticle’s electronic states near Fermi energy [49]. This confirmed that Au atoms have photophysical features [50]. Figure 1 shows that AuNPs were created by collecting a small number of Au atoms. For example, Mustalahti et al. [51,52] used a small number of Au nanoclusters as a temporary energy storage system. An analogous potential would be to store an electric charge for use with molecular electronics, which might be used for bio-imaging in medical research. He and his co-authors employed 102 Au atoms to create the water-soluble cluster Au₁₀₂(pMBA)₄₄, which is stabilized by a layer of para-mercaptobenzoic acid (pMBA) molecules. But when phenylethanethiolate (SC2H4Ph) is employed as a ligand layer, the Au atom becomes 144, resulting in a somewhat bigger cluster of Au₁₄₄(SC2H4Ph)₆₀ that may be categorized as a “metal.” At the molecular level at the nanoscale, molecules behave quite differently from real metal. Transient mid-IR spectroscopy is a technique employed to investigate the energy relaxation dynamics of atomically accurate Au systems. It supplies insights into the behaviour of Au atoms or clusters at the atomic scale and its interaction with light. For example, Au atom at 102 and 144 of Au₁₀₂(pMBA)₄₄ and Au₁₄₄(SC2H4Ph)₆₀ in the molecular and metallic state, respectively. Excitation at 652 nm corresponds to a photon energy of approximately 1.9 electron volts (eV). When AuNP or systems are excited with photons at this energy, it may lead to various electronic and vibrational transitions within the system and it will be used in the experiment to simultaneously examine electronic and vibrational dynamics.

Figure 1

(a) The schematic demonstrates the core–shell architecture of Au NCs representing an NC with a gold core surrounded by a shell made of various materials, like PVP. The core–shell architecture may supply additional functionalities and features to the NC. For example, the shell material may act as a protective layer, preventing oxidation or degradation of the core material. It may also enable surface modifications and enhance the stability, optical feature, or catalytic activity of the nanocrystal, and (b) as the number of the atoms in a nanomaterial diminishes, like in nanoscale materials or NPs, the material’s band gap may alter. In bulk materials, the band gap remains relatively constant. However, quantum confinement influences start to operate when the material is shrunk to the nanoscale. Researchers have discovered Au NCs with the “magic number” of atoms implement exceptional stability and comparable photophysical capabilities. The quantity of Au atoms in atomically accurate Au nanoclusters directly correlates with its photophysical characteristics. Delocalized “super atomic orbitals,” like 1S, 1P, 1D, 2S, 1F, and more, are present in the core of Au NCs. The electronic shell model has been researched and may supply a qualitative explanation for the “magic number” of Au clusters stabilized by the shell of PVP. The atomically precise NCs, often called nanoscale noble metals with magic atom numbers, have a core–shell structure – copyright 2021 American Chemical Society [53].

2.6 Time-dependent density functional perturbation theory (TDDFPT) for studying vibrational dynamics in Au nanocrystal and ligand molecules

In the context of studying the stretching vibration of a ligand molecule and the relaxation processes in the AuNP system, TDDFPT may be a valuable tool. TDDFPT allows a system to calculate electronically excited states and their corresponding transitions, including vibrations and electronic excitations. In the experiment, the sample is electronically activated using visible or near-infrared (NIR) light, which excites the system to higher energy states. This excitation may create electronic states localized on the Au core and induce alterations in the ligand molecule, like stretching vibrations. After the excitation, the relaxation processes may be monitored by observing the transient absorption of a specific vibrational mode in the ligands. Transient absorption spectroscopy is a technique that measures the alteration in the absorption of a sample as a function of time following an initial excitation. The relaxation dynamics, energy transfer, and interactions among the ligand and Au core may be studied by monitoring the transient absorption of the targeted vibrational mode. By combining experimental measurements of transient absorption with theoretical calculations using TDDFPT, researchers may gain insights into the relaxation pathways, electronic states, and vibrational dynamics of the ligand–AuNP system. This approach supplies a detailed understanding of the inter-implement among electronic and vibrational features and may help elucidate the processes governing energy transfer and relaxation in these systems. These studies provide a comprehensive picture of the dynamics of energy relaxation, including the following time constants: (1) 0.5–1.5 ps electronic relaxation; (2) 6.8 ps vibrational cooling; (3) 84 ps intersystem crossing from the lowest triplet state to the ground state; and (4) 3.5 ns internal conversion to the ground state. The supplied search results contain information about the behaviour of AuNPs and the influence of metal-enhanced fluorescence (MEF). Unlike the previously studied bigger Au₁₄₄(SC2H4Ph)₆₀ cluster, which showed relaxation typical of metallic particles, the enormous cluster with 102 metal atoms reportedly behaves like a little molecule. These demonstrate that metallic property results from a molecular behaviour shift among the Au of 102 and 144 species.

Standard AuNPs do not usually glow, although little AuNP shows prominent surface plasmon resonance (SPR) peaks. The MEF influence is one technique that may be used to make AuNP luminous. The phenomenon was familiar as “metal-enhanced fluorescence” is made possible by the remarkable plasmonic characteristics of AuNPs, which are predicted to help dye fluorescence intensification (MEF). The creation of ultra-bright fluorescent Au core–shell NP is based on the phenomenon of MEF. This approach uses AuNPs as the core and tagged with fluorophores to achieve enhanced fluorescence signals. These fluorescent AuNPs may be employed in multimodal imaging techniques like dark-field scattering, fluorescence, and electron microscopy. The diameter of the metal core, i.e. the AuNPs, plays a pivotal role in MEF. The plasmonic features of the AuNPs, including their diameter and shape, determine the extent of light absorption and scattering by the metal core. Larger AuNPs implement stronger plasmonic influences, enhancing light–matter interactions, including increased absorption and scattering. The separation space among the fluorescent molecules (fluorophores) and the metal core also influences the fluorescence enhancement. When fluorophores are positioned near a metal surface, they experience a significant enhancement in the local electromagnetic field, resulting in increased fluorescence emission through mechanisms such as elevated radiative decay rates and reduced non-radiative losses.

The fluorescence enhancement may be optimized by carefully monitoring the diameter of the Au core and the separation space among the fluorophores and the metal surface. Various factors need to be considered to optimize fluorescence enhancement, including the diameter of the Au core and the separation distance between the fluorophores and the metal surface. Table 3 is a comparison table illustrating how different core diameters and separation distances can affect fluorescence enhancement.

Generally, more minor AuNPs with monitoring diameters and shapes are preferred to achieve higher fluorescence enhancements. The separation space may be monitored by functionalizing the surface of the AuNP with appropriate linkers or spacers, allowing for precise positioning of the fluorophores. By studying the diameter-dependent and space-dependent influences, researchers may gain insights into the fundamental mechanisms underlying MEF and optimize the design of fluorescent AuNPs for various imaging usages. These NPs offer the combined benefits of strong Au nanoparticle absorption, scattering features, and enhanced fluorescence, making them promising candidates for advanced imaging techniques. A straightforward procedure in glycerol makes it possible to label individual 20 nm AuNPs fluorescently. Fluorescent AuNP usage includes radio sensitization, fluorescent nanocomposites, and cancer cell imaging [54].

Regarding its photostability and biocompatibility, metal Au nanoclusters (NCs) are superior to colloidal semiconductor quantum dots (QDs) and organic dyes. Although AuNPs are frequently applied as labelling materials, QDs are more appealing and sensitive than AuNPs. Compared to organic dyes, metal chelates, and semiconductor QDs, luminescent Au nanocrystals, a recently created material that may be utilized for energy transfer sensitization, offer benefits (QDs). Fluorescent Au nanoclusters or nanodots (NDs) are a unique class of Au nanomaterials with diameters typically smaller than 3 nm. These nanoclusters implement fascinating fluorescence features, making them highly valuable for various uses. The small diameter of Au NCs results in quantum confinement influences, giving rise to its distinct optical and electronic features. These nanoclusters often implement diameter-dependent fluorescence emission, with emission λ _max ranging from visible to NIR parts. The tunability of its fluorescence emission makes it suitable for a broad range of usages. A novel technique for creating Au NCs with red emissions uses a popular protein to capture and diminish Au precursors [55].

By customizing their diameter, structure, composition, and surface chemistry, metal Au nanoclusters have undergone numerous attempts to increase fluorescence efficiency. Ligand exchange with electron-rich species has proven to be a successful approach for increasing the fluorescence intensity of gold nanoclusters (Au NCs). Bright NIR fluorescence is produced, and the great coverage of zwitterionic ligands improves surface stiffness. A novel technique for creating Au NCs with red emissions uses a typical protein to adsorb and diminish Au precursors. Broad-spread interest in biological usages has been generated by monitoring the optical features of Au nanoclusters and enhancing their performance [56]. According to the search findings, ligand-to-metal or metal–metal charge transfer may be improved by exchanging capping ligands with a high electron donation capability. The “ligand exaltation” procedure frequently swaps out the native capping ligand for the wanted one. This process is typically prompted by the various binding strengths of the two capping ligands. Electron transfer rates monotonically diminish with longer alkanethiol capping ligands. The efficient removal of many commonly used capping ligands from the surface of noblemetal nanoclusters (NCs) can be achieved through a customizable ligand exchange process. This process enables substitution with a wide range of ligands to tailor the NCs surface properties. Ligand exchange typically occurs during phase transfer, where the NCs are moved from a non-polar to a polar solvent system [57].

3.1 Polymer stabilization of Au clusters and its effects on electronic architecture

The protective ligand implements a pivotal role in the Au nanoclusters’ fluorescence. In fundamental scientific research, the inherent nanoparticle fluorescence, like nanoclusters, unique from its large crystals, has drawn considerable interest for practical use in many disciplines, including electronic and environmental usages [57]. Ligands may passivate the surface of the metal core, preventing aggregation and providing stability to the nanoclusters. Various ligands may interact with the metal core, affecting the nanoclusters’ electronic architecture and energy levels. This, in turn, may impact its fluorescence feature [59,60]. To illustrate this, innovative synthesis techniques should be created to create distinctive structures that would otherwise be challenging to produce using the present, broadly used techniques (mostly Brust–Schiffrin and ligand-exaltation techniques) [59]. Recently discovered anti-galvanic reduction (AGR), originally used to create bimetal nanoclusters (like Au25Ag2), has also been used to create mono-metal nanoparticles, like Au44(SC2H4Ph)32 [59]. Inspired by this, Yao et al. [59] synthesized the fluorescent phenylethanethiolated Au nanoclusters of Au24(SC2H4Ph)20, Au25(SC2H4Ph)18, Au38(SC2H4Ph)24, and Au144(SC2H4Ph)60 by AGR. The fluorescence quantum yield (QY) of Au24, approximately 40 times more than Au25 molecules, is the highest of all samples of phenylethanethiolated Au nanoclusters (including Au24, Au25, Au38, and Au144). Additionally, Au24 has three fluorescence lifetimes, each measuring 1.16, 45.25, and 267.63 ns. The ground state and the first singlet’s lowest vibrational level in the Au24 molecule have significant energy gaps. In Figure 3, the effect of quantum confinement on the optical absorption of gold nanoclusters (Au NCs) is illustrated. As the diameter of the Au NCs decreases, the spacing between discrete energy levels increases, leading to a blue shift in their optical absorption. For example, the Au10-12(SG)10-12, Au15(SG)13, Au18(SG)14, and Au25(SG)18 clusters exhibit absorption onsets at approximately 450 nm, 650 nm, 700 nm, and 900 nm, respectively. cluster, for instance, have absorption onsets that are 450, 650, 700, and 900 nm, respectively. For small-diameter Au NCs, solid quantum confinement influences lead to relaxation dynamics that are greatly influenced by atomic packing, shape, and diameter. The conversion of Au-sulphide (Au₂S) particles to AuNPs may lead to considerable alterations in the optical absorption feature, including a blueshift in the plasmon absorption peak. This phenomenon may be attributed to the quantum diameter influences displayed by the AuNPs. Quantum diameter influences arise from the confinement of electrons within the nanoscale dimensions of the NPs. When the diameter of a metallic NP becomes comparable to or smaller than the characteristic length scale of the electrons, like the Fermi λ _max, its electronic architecture and energy levels undergo quantization. This quantization leads to discrete energy levels and a modification of the electronic density of states. In the matter of AuNPs, as the diameter diminishes, the quantization of electronic states becomes more pronounced. This shifts the plasmon absorption peak to higher energies, leading to a blueshift in the observed optical absorption spectrum. The plasmon absorption peak corresponds to the collective oscillation of conduction electrons in the NP, and its energy is strongly influenced by the diameter and shape of the NP [61].

Figure 3

(a) The structural diagram and variations in the optical absorption spectra of Au NCs of various diameters, like Au12, Au15, Au18, and Au25. The space among distinct energy levels broadens as the diameter of the Au NCs shrinks, and the clusters’ optical absorption turns blue. The number of Au atoms in atomically exact Au nanoclusters is intimately related to its photophysical characteristics; hence, the “magic number” of Au atoms was discovered to be present in Au NCs. The Au nanoclusters with magic numbers show high photophysical stability and resemblance to one another. The molecular Au nanocluster Au156 has a unique electronic structure among the molecular and metallic states, with distinct energy levels like molecular orbitals and metallic bands. Also given are the optical absorption spectra of identical-diameter Au NCs shielded by various ligands and the Tauc plot. Copyright 2014 American Chemical Society [62]. (b) The optical absorption spectra of charge-neutral and anionic Au25 clusters in solution are presented, highlighting the effect of cluster charge on their optical properties Copyright 2008 [63].

4.1 Lysozyme-stabilized Au NCs for bio functions

Lysozyme-stabilized Au NCs have emerged as a fascinating class of nanomaterials with diverse biofunctions. Lysozyme, an enzyme naturally found in various biological systems, is an excellent stabilizer for Au NCs due to its unique structural and functional properties. One prominent bio-function of lysozyme-stabilized Au NCs is their inherent antimicrobial activity. Lysozyme can disrupt bacterial cell walls by hydrolysing peptidoglycan, an essential component of bacterial cell walls. When combined with Au NCs, these nanoclusters can exhibit enhanced antimicrobial properties, making them valuable candidates for applications in antibacterial coatings, disinfectants, and wound dressings, contributing to environmental health and safety.

Moreover, lysozyme-stabilized Au NCs have shown remarkable potential in biomedical imaging. Their strong fluorescence properties make them suitable as fluorescent probes for imaging biological structures and processes, aiding in the visualization and study of cellular and molecular events in environmental samples or living organisms. In addition to their bioimaging applications, lysozyme-stabilized Au NCs have demonstrated promising results in drug delivery. These nanoclusters can be engineered to encapsulate therapeutic agents, protecting them from degradation and facilitating targeted delivery to specific sites in the body. This capability holds the potential for environmentally friendly and precise drug delivery systems, reducing the environmental impact of pharmaceutical compounds.

Furthermore, lysozyme-stabilized Au NCs have attracted attention as potential sensors for detecting various analytes in environmental samples. Their unique fluorescence properties can be exploited to create sensitive and selective sensing platforms for monitoring environmental pollutants or detecting biological molecules. Lysozyme (Lys) has been widely used by various researchers to stabilize gold nanoclusters (Au NCs). Lysozyme-stabilized Au nanoclusters (Lys-Au NCs) have been developed and applied as fluorescent probes for selective cyanide detection. The search results highlight the versatility of Au nanoclusters as functional nanomaterials in sensing and analytical chemistry. The average Lys-Au NC diameter was 4 nm, and they emitted a red light at 650 nm. Lys-Au NCs have also been used as a nanomedicine to induce osteogenic variolation and diminish osteoclast activity. The enzymatic activities of lysozyme may be modulated by the existence of surface ligands on ultra-low Au nanoclusters. This opens up possibilities for regulating enzymatic functions using these nanoclusters as molecular tools. In the context of Lys-Au NCs, these nanoclusters have been employed for label-free ratiometric fluorescent pH sensing. pH is a pivotal parameter in various biological and chemical processes, and the ability to monitor pH alterations in real-time is of great significance in many usages. Lys-Au NCs have also been used to develop photoactivated multifunctional nanoplatforms with improved antibacterial abilities compared to pure curcumin and Lys-Au NCs. Lysozyme NP-encapsulated Au nanoclusters (LysNP-Au NCs) have been designed as a dual-emission probe for ratiometric fluorescent detection of cyanide (CN⁻) in various environmental samples. Cyanide is a toxic substance that may be found in contaminated water sources, certain plants containing cyanogenic glycosides, and soil [91].

The lysozyme-capped Au NCs, which have a nano diameter of about 4 nm and were created in a previous study, have much potential for use in cyanide ion diagnostics. Cyanide ions linearly reduced the lysozyme-Au NCs’ emissive qualities. Combining NIR fluorescence with CT imaging in vivo makes bimodal bioimaging easier to distinguish between cancerous and healthy tissues. They found that when folic acid was added as a targeting agent, the lysozyme-capped Au NCs gathered in the tumour site after being delivered intravenously to HeLa tumour-bearing mice. When the folic acid alteration was not applied, the Au NCs’ fluorescence did not show up at the tumour site. The liver and kidney showed positive signal increases an hour after receiving the Au NCs injections for use in CT imaging, showing that the Au NCs mostly gather in these organs without tumour tissue [92].

4.2 Lactotransferrin (Lf) and horseradish peroxidase (HRP) gold nanoclusters in biomedical applications

Au NCs Lf refers to Lf protein that has been conjugated or encapsulated with Au nanoclusters. This hybrid nano material combines the unique feature of Lf and Au nanoclusters, opening up potential usages in various fields, including biomedical and biotechnological research [93]. HRP Au NCs refer to the conjugation or encapsulation of HRP enzyme with Au nanoclusters. HRP is an enzyme popularly found in horseradish roots and is broadly used in various biochemical and analytical usages due to its catalytic activity [94]. Human serum albumin (HSA) Au NCs refer to the conjugation or encapsulation of HSA protein with Au nanoclusters. HSA is a highly abundant protein found in human blood plasma. It plays important roles in maintaining osmotic pressure, transporting various molecules, and regulating the distribution of nutrients and drugs in the body [95]. Combining pepsin with Au NCs, the resulting nanomaterials may possess fluorescence features, enabling their use as fluorescent probes or markers in various usages [96]. Trypsin Au NCs with fluorescence capabilities may be applied for cellular or tissue imaging. The nanoclusters may be specifically targeted to certain cells or tissues by conjugating them with targeting ligands or antibodies. This allows for the visualization and tracking of specific biological targets in real-time [97] and egg white [98]. Production of photoluminescent Au NCs – several methods may produce photoluminescent Au nanoclusters. One popular approach is the reduction of Au ions in the existence of stabilizing ligands or capping agents. Here is a general outline of the process: Preparation of Au NCs precursor: Start with a suitable Au precursor, like Au salts (e.g. Au chloride, Au acetate, or AuNPs. These precursors supply the source of Au ions to construct Au NCs. Reducing agents: Introduce a reducing agent to reduce Au ions into Au NCs. Popular reducing agents include sodium borohydride (NaBH₄), ascorbic acid, or other suitable reducing agents. The choice of reducing agent depends on the specific synthesis method and conditions. Stabilization with ligands: To monitor the diameter, shape, and stability of the Au NCs, add stabilizing ligands or capping agents. These ligands may supply surface passivation, prevent aggregation, and impart photoluminescent feature to the Au NCs. Popular ligands used for Au NC synthesis include thiols (e.g. thiolated organic molecules), polymers, proteins, or other surface-active molecules. Reaction and growth: Allow the reduction reaction to proceed under suitable conditions like temperature and reaction time. The reduction process leads to the growth and construction of photoluminescent Au NCs stabilized by the ligands. The specific reaction conditions may influence the diameter, shape, and photoluminescent feature of the resulting Au NCs. Purification and characterization: After the synthesis, the Au NCs need to be purified to remove any unreacted precursors, by-products, or excess ligands. Purification techniques may include centrifugation, dialysis, or other methods based on the features of the Au NCs. The purified Au NCs may then be characterized using UV-Vis spectroscopy, fluorescence spectroscopy, transmission electron microscopy (TEM), or other analytical methods to assess their diameter, morphology, and photoluminescent feature. Generally speaking, proteins with plenty of cysteines and tyrosines are thought to be excellent candidates for producing protein-Au NCs [91]. For example, due to their catalytic and optical properties, AuNP clusters like HRP-Au NCs (horseradish peroxidase-AuNP clusters) have potential applications as biocatalysts and biosensors. Biosensors for hydrogen peroxide detection (HRP-Au NCs) can be used as catalysts for reducing hydrogen peroxide, generating an optical or electrochemical signal that can indicate the presence of H₂O₂. This could be used to develop sensitive biosensors for hydrogen peroxide [99]. Further, HRP-Au NCs could act as catalytic labels in immunoassays, providing signal amplification through their peroxidase-like catalytic activity. This could enhance the sensitivity of assays for detecting biomarkers, proteins, etc. [100]. Additionally, the combination of enzymatic activity and photoluminescence opens up possibilities for advanced catalysis and sensing usages (HRP is an enzyme familiar for its peroxidase activity, which involves the catalytic reduction/oxidation of hydrogen peroxide (H₂O₂) [91]. In an in vivo study, insulin-stabilized Au NCs were employed to maintain their bioactivity in monitoring blood glucose like natural insulin [101]. No extra reducing agent is required because some proteins may serve as capping and reducing agents [102]. Alternately, the creation of Au NCs may benefit from the etching of larger Au NPs (typically 2–4 nm) by thiol compounds in an alkaline solution. The etching of larger Au NP (typically 2–4 nm) by thiol compounds in an alkaline solution is a popularly used method for the creation of smallest Au nanoclusters. This process is often called “etching-induced synthesis” or “ligand-mediated etching.” Here is a general outline of the etching-induced synthesis process: Preparation of larger Au nanoparticles: Initially, larger Au NPs are prepared through conventional synthesis methods, like chemical reduction or colloidal synthesis. These NP typically have diameters in the range of 2–4 nm. Thiol compounds: Thiol compounds, like thiols or thiolated ligands, are introduced to the solution containing the larger AuNPs. The thiol compounds act as etchants and attach to the surface of the AuNPs. Etching process: The thiol compounds in the alkaline solution undergo a redox reaction with Au atoms on the surface of the larger NPs. This reaction results in removing Au atoms from the surface and constructing Au-thiol complexes in the solution. Construction of Au nanoclusters: As the etching process continues, the Au NP gradually transforms into smaller-diameter Au nanoclusters. The diameter and shape of the resulting Au NCs may be monitored by adjusting the etching time, the concentration of thiol compounds, and reaction conditions. Purification and characterization: The Au NCs are typically purified by centrifugation or other separation techniques to remove excess thiol compounds and by-products after the etching process. The purified Au NCs may then be characterized using techniques like UV-Vis spectroscopy, TEM, or X-ray diffraction (XRD) to determine their diameter, morphology, and optical features [68].

Manjunath et al. [103] developed an electrochemical sensor using a nickel-activated carbon/PEDOT composite derived from coffee silver skin for detecting glyphosate (GLY) and hexaconazole (HEXA). The composite leverages activated carbon’s porosity, nickel’s electron-transfer enhancement, and PEDOT’s redox activity, achieving ultra-low detection limits of 0.8 ppt for GLY and 0.613 ppt for HEXA. Real-sample analysis in coffee beans validated the sensor’s practicality, with liquid chromatography–mass spectrometry corroborating its potential for precise pesticide monitoring in environmental and agricultural settings. On comparison, the AC/Ni/PEDOT sensor outperforms methods like Sadhu et al.’s [104] fluorescent gold nanocluster-based approach for HEXA detection, which reported limit of detection (LOD) of 21.94 nM (∼6,760 ppt). The composite’s ppt-level sensitivity demonstrates superior trace detection capabilities, making it more suitable for analysing complex agricultural matrices. The study by Nguyen Thi Nhat et al. [105] investigates the application of gold nanospheres (AuNSps) as a SPR-based sensor for in situ detection of residual fungicides, focusing on thiophanate methyl. AuNSps were synthesized via a seed-mediated method, where gold nanoseeds, formed by reducing chloroauric acid with trisodium citrate dihydrate (TSC), were grown into 53 nm spherical NPs using HAuCl₄, TSC, and ethylenediaminetetraacetic acid. UV-vis spectroscopy, XRD, SEM-EDX, and TEM characterized their uniform size, crystallinity, and plasmonic properties with a resonance peak at 560 nm. When deposited on a glass substrate and coated with thiophanate methyl, the AuNSps demonstrated significant surface-enhanced Raman spectroscopy (SERS) effects, amplifying the fungicide’s Raman signal intensity compared to bare glass. This enhancement highlights the nanoparticles’ ability to improve trace-level detection sensitivity. The research underscores the potential of AuNSps-based sensors for rapid, on-site monitoring of fungicide residues in agricultural products, offering a practical solution to ensure food safety and reduce human exposure to harmful chemicals through cost-effective, field-deployable technology.

4.3 Fluorescent gold nanoclusters stabilized by BSA for biomedical applications

Numerous multifunctional nanocomposites have been used to construct numerous protein-stabilized Au NCs that may be used for medicinal, sensing, and targeting usages. Based on a one-pot synthetic route, Xie et al. used BSA to prepare Au NCs (37°C) with red emission λ _em max and QY of 640 nm and 6%, respectively [106]. The protein molecules absorbed and sequestered the Au ions when Au(iii) ions were introduced to the aqueous BSA solution [107]. When the pH of the reaction was raised to 12 to activate the reduction ability of BSA molecules, the trapped ions gradually reduced to form Au NCs in situ. The 25 Au atoms in the as-prepared Au NCs were stabilized inside BSA molecules as BSA-Au NC bioconjugates. The use of BSA coating layer on Au nanoclusters supplies several advantages, including the ability to make post-synthesis surface modifications using functional ligands and the economic and environmental benefits associated with BSA as a biocompatible and cost-influence protein. When exposed to ultraviolet (UV) light (365 nm), the dark brown solution of BSA-coated Au NCs implements red fluorescence (Figure 4a, item no. 3 in the lower image). The mild blue fluorescence observed in the monitoring BSA solution under UV light may be attributed to the existence of aromatic side groups in the amino acid residues of BSA. BSA contains several aromatic amino acids, like tryptophan, tyrosine, and phenylalanine, which are familiar to implement intrinsic fluorescence (Figure 4a, item no. 2 in the lower image). The monitoring BSA solution was a pale-yellow colour when viewed with the naked eye (Figure 4a, item no. 2 in the upper image). Excitation and emission peaks for the fluorescent Au NCs were seen at 480 and 640 nm, respectively (Figure 4b). The QY for red photoluminescence was 6%.

Figure 4

(a) Photographs of BSA and BSA-Au NCs (gold nanoclusters): 1. BSA powder: A sample of dry BSA powder, a common form of storage for proteins like BSA. 2. BSA aqueous solution: A clear or slightly cloudy liquid containing BSA dissolved in water. BSA is soluble in water and other aqueous solutions. 3. BSA-Au NCs aqueous solution under visible light: An aqueous solution containing BSA-Au NCs (gold nanoclusters) under regular visible light. The colour of the solution may vary depending on the size and concentration of the gold nanoclusters. 4. BSA-Au NCs powder under visible light: A dry powder or solid form of BSA-Au NCs under visible light. (b) Optical absorption and photoemission spectra: The graph represents the optical absorption and photoemission (fluorescence) spectra of two different samples: Blue curve represents the aqueous solution of BSA. The dashed line indicates the absorption spectrum, which shows the wavelengths of light absorbed by BSA. The solid line represents the photoemission spectrum, which shows the wavelengths of light emitted by BSA after photoexcitation. Red curve represents the aqueous solution of BSA-Au NCs. The dashed line indicates the absorption spectrum of the BSA-Au NCs, showing the wavelengths of light absorbed by the gold nanoclusters. The solid line represents the photoemission spectrum of BSA-Au NCs, showing the wavelengths of light emitted by the gold nanoclusters after photoexcitation at a wavelength of 470 nm (λ _ex = 470 nm). The inset in the graph likely shows the photoexcitation spectrum of BSA-Au NCs. This spectrum reveals the light absorption efficiency of the gold nanoclusters at different excitation wavelengths. Overall, the graph provides valuable information about the optical properties of BSA and BSA-Au NCs, demonstrating how adding gold nanoclusters can alter BSA’s absorption and photoemission properties. Copyright 2009 American Chemical Society [106].

Ivleva et al. [108] produced and stabilized bimetallic Au-Cd NCs with diameters less than 1.6 nm using BSA. In Figure 5, the maximum fluorescence emission λ _max observed for various types of nanoclusters indicates its unique optical feature and behaviour. Figure 5 shows some characteristics of the maximum fluorescence emissions for Au NCs, Cd NCs, and mixed Au-Cd NCs.

Figure 5

The TEM images and mass spectra of (a) Au NCs, (b) Cd NCs under UV light at 365 nm, and (c) fluorescent emission spectra of Au NCs. (d) Fluorescent emission spectra of Cd NCs. (e) Fluorescent emission spectra of Au/Cd NCs.

For Au NCs, maximum emission λ _max was observed at 325, 410, and 650 nm. This emission λ _max indicates that Au NCs implement fluorescence in the spectrum’s UV, blue, and red parts. The excitation at 325 nm suggests the existence of specific electronic transitions or energy levels in the Au NCs that result in emission at shorter λ _max (UV part). The 410 and 650 nm emissions indicate fluorescence in the blue and red parts, respectively. For Cd NCs, the maximum emission λ _max was observed at 370 and 500 nm. These emission λ _max values suggest that Cd NCs implement fluorescence in the UV and green parts of the spectrum. The excitation at 370 nm indicates electronic transitions or energy levels in the Cd NCs, resulting in emission at shorter λ _max (UV part). The emission at 500 nm suggests fluorescence in the green part.

For mixed Au-Cd NCs, two emission peaks were observed at 440 and 640 nm. These dual emission peaks indicate the existence of various chromophores or energy levels within the mixed Au-Cd NCs. The excitation at 365–380 nm results in the simultaneous emission at 440 and 640 nm, suggesting the existence of distinct emission mechanisms or transitions. Compared to conventional organic fluorophores and QDs, the stabilized fluorescent NCs offer several advantages, such as minor diameter: NCs are typically on the nanometre scale, allowing for better penetration and interaction with biological systems. Considerable Stokes shift: The difference among the excitation and emission λ _max, familiar as the Stokes shift, is often more prominent in NCs. This feature helps reduce self-absorption and improves detection sensitivity. Biocompatibility: NCs may be engineered biocompatible, making them suitable for various biological and biomedical usages. High stability: NCs may implement excellent chemical and photostability, ensuring long-term fluorescence performance. Prolonged fluorescence lifetime: NCs often have longer than organic fluorophores, which may be advantageous for time-resolved measurements and imaging techniques. These advantages make stabilized fluorescent NCs promising candidates for various usages, including bioimaging, biosensing, and optoelectronics.

4.4 Glutathione (GSH)-coated gold nanoclusters with enhanced fluorescence

The tripeptide GSH, which has a low affinity for and interaction with biological proteins, has frequently been used as an Au NC surface ligand [109].

GSH-coated AU NCs (GSH-Au NCs) have emerged as a remarkable class of nanomaterials with enhanced fluorescence properties, making them highly valuable for various applications. The unique combination of gold nanoclusters with GSH, a naturally occurring tripeptide found in living organisms, enhances fluorescence emission. GSH is an excellent stabilizer for gold nanoclusters, preventing aggregation and enhancing their fluorescence intensity.

One of the key advantages of GSH-Au NCs is their excellent biocompatibility. GSH is a vital antioxidant in cells, and its presence in the nanoclusters makes them biologically compatible and safe for use in various biological applications, including bioimaging and biosensing.

In bioimaging, GSH-Au NCs have shown great promise as fluorescent probes for cellular imaging. Their enhanced fluorescence emission allows for high-resolution imaging of cellular structures and processes, enabling researchers to gain insights into cellular behaviour and functions in environmental studies.

Moreover, GSH-Au NCs have demonstrated potential in biosensing applications. Their strong and stable fluorescence can be exploited to design sensitive and selective sensors for detecting various analytes in environmental samples, such as heavy metals, pollutants, or biomolecules. This capability offers a powerful tool for environmental monitoring and detecting contaminants in different environmental settings.

Another advantage of GSH-Au NCs is their potential as drug delivery vehicles. The biocompatibility and stability of GSH-Au NCs make them suitable for loading therapeutic agents and delivering them to specific targets in the body. This could lead to environmentally friendly and targeted drug delivery systems with reduced side effects. GSH has been observed to improve renal clearance and diminish the buildup of Au NCs in the liver and spleen, causing at least 50% of GSH-Au NCs to be successfully eliminated from the body via the urinary systems within 24 h following IV injection [110]. Zhou et al. observed a QY of 3.5% and photoluminescence at about 560 nm in the NIR band [110]. Further to this, Luo et al. [111] found that aggregation by solvent mixing and a lower thiol-to-Au ratio (1.5:1 instead of 2:1) during the synthesis of Au NCs may result in a higher QY of about 15% in the setting of GSH-Au complexes [68,111]. A now-promising and well-recognized phenomenon is that metal nanocluster aggregation-induced emission enables the influence synthesis of extremely luminous nanoclusters. The fluorescence QYs of the Au nanocrystals stabilized by GSH-Au NCs dispersed in an aqueous solution are only 1%. Still, when Zn NP is added, the QYs of the Zn-GSH-Au NCs go up to 40%. A now-promising and well-recognized phenomenon is that metal nanocluster aggregation-induced emission enables the influence synthesis of extremely luminous nanoclusters. The fluorescence QYs of the Au nanocrystals stabilized by GSH-Au NCs dispersed in an aqueous solution are only 1%. Still, when Zn NP added, the QYs of the Zn-GSH-Au NCs go up to 40% [112]. Poly(amidoamine) is a dendrimer with a sphere-like form that has been the subject of the most research due to its robust production, availability, dendritic structure, and protein and peptide mimic feature [113].

7.1 Multimodal imaging using Au nanoclusters: Advantages and applications

Researchers have tested the feasibility of Au NCs and hybrid materials, including Au NCs as an imaging agent in vivo, using X-ray computed tomography (X-ray CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and NIR fluorescence imaging [128]. Each imaging technique has advantages and disadvantages of its own (Table 5). Multimodal imaging integrates many imaging techniques, fusing the best features of each [129]. As a result, employing more than one imaging technique with a single imaging agent is beneficial. More imaging agents should be collected in the tumour than other organs during imaging tests to diagnose cancer. Therefore, the platform should target the tumour actively or inactively [130].

7.2 Passive and active targeting strategies for Au nanoclusters in cancer imaging

Passive targeting is made possible by using the enhanced permeability and retention influence in body areas where there is a lot of hypoxia and/or inflammation, both of which are features of the tumour microenvironment [131]. Extravasation of Au NC-containing NP platforms from 10 to 500 nm in the blood serum into the tissue is conceivable because the tumour’s unpredictable growth and holes in the endothelium cause irregularities in the vasculature [132]. In vivo, FA- or HA-coated BSA-stabilized Au NCs displayed similar fluorescent features and gathered in HeLa or Hep-2 tumours, respectively. MRI typically uses gadolinium-functionalized Au NCs [128]. Au MRI multimodal imaging studies have also used silica quantum rattles and mesoporous silica NP packed with Au NCs and AuNPs. NIR fluorescence imaging and CT imaging may be done depending on the various modifications or conjugations made to the Au NC. Shortwave infrared emitting Au NCs have demonstrated outstanding potential for in vivo imaging with more excellent contrast than current NIR imaging and support for PET [128]. Additionally, the acquired system could be exploited for PET and fluorescence dual-imaging in lung cancer by coupling iodine-124 to a peptide-protected Au NC, as done by Daems et al. [133]. The use of radiolabelled Au for image-guided therapy is also discussed, as well as the improvement of targeted radionuclide therapy and nano brachytherapy through an increased dose deposition and radio sensitization, as demonstrated by numerous Monte Carlo studies and experimental in vitro and in vivo studies [133]. In 2023, Borse et al. [134] prepared bimetallic nanoclusters of copper and Au with papain as a ligand for fluorescence detection of cortisone in vitro biological samples.

Its investigation discovered that the hybrid nanoclusters displayed outstanding signals for NIR fluorescence with an impressive detection limit of 1.89 nM at a concentration as low as 0.031–1.00 M. Bimetallic Au and copper nanoclusters (Au-Cu NCs) were produced using papain as a ligand. The papain-Au-Cu NCs displayed red fluorescence at 365 nm of UV light. The papain-Au-Cu NCs, as they are generated, show a high fluorescence at 656 nm when activated at 390 nm. Red fluorescence “switches off” in papain-encapsulated Au-Cu NCs due to a particular corticosteroid interaction site. When cortisone was added as a biomarker, papain-Au-Cu NCs demonstrated rapid response and outstanding fluorescence quenching. The constructed probe is revealed to be capable of quantifying cortisone by creating the calibration graph among the fluorescence ratio (I ₀/I) and cortisone concentration (0.031–1.00 M) with an astounding detection limit of 1.89 nM.

Interestingly, papain-Au-Cu NCs are highly selective for cortisone detection, as seen by their relatively poor responsiveness to frequent interfering species and other biomarkers. The probe was successfully used to check for the cortisone biomarker in plasma and urine samples. Furthermore, by assessing the analytical validity of the probe by assaying cortisone in intra- and inter-day, it was demonstrated that papain-Au-Cu NCs might be a promising probe for the fast detection of cortisone in biological materials.

Another triple-modal imaging platform was described in 2023 by Wang et al. [135]. The development of a non-invasive multimodal contrast agent (aminoglycoside-polyglutamate [AGGP]) coordinated with a high-affinity prostate-specific membrane antigen (PSMA) ligand (PSMA1) offers promising opportunities for the accurate detection and characterization of prostate cancer (PCa). The AGGP agent combines MR, CT, and near-infrared fluorescence (NIRF) imaging modalities, allowing for the comprehensive and precise imaging of PSMA expression in PCa lesions. Existing contrast agents, particularly Gd-containing NPs, have encountered limitations like nonspecific redistribution in the mononuclear phagocyte system (MPS) and insufficient perfusion to target locations. Moreover, the intrinsic limitations of MR tools have made it challenging to depict PSMA features accurately using these techniques alone. By utilizing AGGP, researchers have successfully demonstrated potent signal augmentations in PCa lesions by integrating MR, CT, and NIRF imaging. The AGGP agent implements specific targeting to PSMA, efficient evasion of the MPS, and beneficial renal-clearable behaviour in living mice, thus overcoming the limitations of previous contrast agents. Biocompatibility and histopathology tests have also supported the high safety of AGGP in vivo, making it a promising tool for more accurate early-stage PCa detection and the development of influence multifunctional nanotherapeutics.

7.3 Temperature-sensitive fluorescence properties of Au nanoclusters for biological thermometry

Furthermore, the AGGP agent allows for active targeting by coupling targeting ligands to the nanoclusters, enhancing its specificity towards PSMA-expressing cells or tissues. This multimodal approach enables the use of a single agent for NIRF, MR, and photoacoustic imaging in both in vitro and in vivo settings, providing a comprehensive assessment of PSMA expression and distribution. Au nanoclusters also possess temperature-sensitive features besides their imaging capabilities.

The fluorescence lifetime and emission intensity of Au nanoclusters undergo significant changes within the physiologically relevant temperature range of 15–45°C. This temperature-dependent behaviour can be attributed to the influence of temperature on the electronic structure and optical properties of the Au NCs.

At lower temperatures, such as 15°C, the fluorescence lifetime of Au NCs may increase, leading to longer emission decay times. This could be due to changes in the excited-state relaxation processes or energy transfer mechanisms within the nanoclusters. Additionally, the emission intensity of Au NCs may be lower at lower temperatures, potentially caused by decreased radiative transition rates or increased nonradiative pathways.

As the temperature increases within the physiological range, the fluorescence lifetime of Au NCs tends to decrease. This decrease in lifetime is often associated with increased thermal energy, which promotes faster relaxation of the excited states and shorter emission decay times. Moreover, the emission intensity of Au NCs may also increase at higher temperatures, indicating enhanced radiative transition rates and potentially reduced nonradiative processes.

These temperature-dependent alterations in the fluorescence properties of Au NCs can be leveraged for various applications, including temperature sensing or imaging in biological systems. By monitoring the changes in fluorescence lifetime and emission intensity of Au NCs within the physiological temperature range, it is possible to obtain valuable information about local temperature variations in biological environments or during specific processes.

It is worth noting that the exact temperature sensitivity and behaviour of Au NCs may vary depending on factors such as their specific size, composition, surface chemistry, and surrounding environment. Therefore, comprehensive studies and characterization are necessary to understand and optimize the temperature-responsive properties of Au NCs for specific applications in multimodal nuclear medicine imaging or other fields.

Fluorescence lifetime imaging microscopy (FLIM) has indeed been applied to investigate the thermometric features of Au NCs, enabling temperature mapping and monitoring of biological processes. FLIM is a powerful imaging technique that measures the fluorescence lifetime of fluorophores, which is the average time the fluorophore spends in the excited state before returning to the ground state.

In the context of Au NCs, FLIM can be utilized to probe their temperature-dependent fluorescence lifetime changes. A calibration curve relating fluorescence lifetime to temperature can be established by measuring the fluorescence lifetime at different temperatures. This calibration curve can then map and quantify temperature variations within biological samples.

The principle behind this technique is that temperature-induced changes influence the fluorescence lifetime of Au NCs in their electronic and vibrational states. As the temperature increases, the increased thermal energy affects the relaxation dynamics of the excited states, leading to changes in the fluorescence lifetime. By quantifying these changes through FLIM, temperature variations can be visualized and measured with high spatial resolution.

FLIM-based thermometry using Au NCs has several advantages. First, Au NCs exhibit excellent photostability and brightness, making them suitable for long-term imaging and monitoring of temperature changes in biological systems. Second, Au NCs can be functionalized and targeted to specific cellular or subcellular locations, allowing for precise temperature measurements in localized regions of interest. Additionally, the non-invasive nature of FLIM imaging makes it compatible with live-cell and in vivo experiments, enabling the study of dynamic temperature changes during various biological processes.

The combination of FLIM and Au NCs as a thermometric tool holds promise for studying temperature gradients in cells and tissues, monitoring heat generation during treatments, and investigating temperature-dependent biological reactions. This approach provides valuable insights into the spatiotemporal dynamics of temperature within biological systems and contributes to our understanding of temperature-sensitive processes.

In summary, the development of AGGP as a noninvasive multimodal contrast agent offers excellent potential for accurately detecting and characterizing PSMA expression in PCa lesions. Its integration of MR, CT, and NIRF imaging modalities, along with the temperature-sensitive feature of Au NCs, opens up new avenues for more precise early-stage PCa detection, imaging-guided therapy, and the development of multifunctional nanotherapeutics [135]. Fluorescence imaging is broadly used in human clinical bioimaging due to its high sensitivity, multidirectional detection, and relatively low cost compared to other imaging modalities. Traditional fluorescent probes, like semiconductor QDs and organic dye-doped NPs, have been extensively studied for bioimaging usages. However, these probes have some limitations, including potential toxicity and photobleaching.

In contrast, Au nanoclusters offer several advantages that make them promising candidates for fluorescent imaging probes in clinical bioimaging. First, Au NCs have an ultra-little diameter, typically ranging from a little to tens of nanometres, which allows them to easily penetrate biological barriers and reach the target sites in the body. This minor diameter also contributes to enhanced tissue penetration depth and improved biodistribution.

Biocompatibility is another pivotal aspect of Au NCs, meaning they are well-tolerated by biological systems and do not cause considerable cytotoxicity or adverse influences. This characteristic is significant for clinical usages where safety is a primary concern. Au NCs also possess remarkable brightness and photostability, enabling long-term imaging without the considerable signal intensity or photobleaching loss. This stability ensures reliable and accurate imaging results over extended periods. Furthermore, Au NCs may be easily functionalized with targeting ligands, like antibodies or peptides, allowing specific binding to cellular or molecular targets. This capability enables selective imaging of specific tissues, cells, or biomarkers, facilitating precise diagnostic and therapeutic usage. Overall, the unique combination of ultra-little diameter, biocompatibility, brightness, and photostability make Au NCs highly attractive as fluorescent imaging probes for clinical bioimaging of humans. Ongoing research and development in this field aim to optimize its features further and expand its usage in medical diagnostics, imaging-guided therapy, and personalized medicine [136]. Au nanoclusters are one of the newest materials because of their luminosity. However, the internal vibration and rotation of the capping ligands considerably lower the fluorescence intensity of the Au NCs in an aqueous media. Bera et al. [137] described the behaviour and characteristics of capped Au nanoclusters in various conditions and by adding certain compounds. Au NCs are capped with ligands like 16-mercapto hexadecanoic acid (MHDA) and 11-mercaptoundecanoic acid (MUA), which allow them to be dispersed in aqueous solutions. These capped Au NCs have QYs of 3.01 and 6.27%, respectively. Zeta potential measurements indicate that these Au NCs carry a negative charge. When a positively charged long hydrocarbon chain containing an imidazolium surface-active ionic liquid called 1-hexadecyl-3-methylimidazolium chloride (C16mimCl) is added to these Au NC solutions, the fluorescence intensities increase. This enhancement is attributed to forming rigid Au NC aggregates induced by adding C16mimCl. The construction of these aggregates minimizes the intramolecular vibration and rotation of the MHDA and MUA capping ligands, leading to increased QYs of the Au NCs to 5.78 and 6.87%, respectively. The concentration of additional C16mimCl in the solution plays a pivotal role in the aggregation-induced emission. The solution undergoes three distinct zones: first turbid, precipitate, and second turbid. The aggregation-induced emission enhancement is pronounced in MHDA-capped Au NCs compared to MUA-capped Au NCs due to stronger hydrophobic interactions with MHDA. Furthermore, as the pH of the solution diminishes, the carboxylate (−COO⁻) groups of the capping ligands get protonated, losing their ability to interact electrostatically with Au NCs and C16mimCl. This pH-dependent behaviour results in a linear relationship among emission intensity and solution pH in the range of 7.5–1.5, with a progressive decline in emission intensity. Overall, adding C16mimCl to the capped Au NCs induces the construction of rigid aggregates, leading to enhanced fluorescence intensities. The pH of the solution also affects the emission intensity due to the protonation of carboxylate groups on the capping ligands. These findings supply valuable insights into the behaviour and potential usages of capped Au NCs in various fields, like sensing, imaging, and optoelectronics.

8.1 Laser interaction with AuNPs for nanotechnology and medical applications

Indeed, the interaction between lasers and AuNPs is a fascinating area of research with broad implications for nanotechnology, engineering, and medical usage. One of the key factors in this interaction is the efficiency of absorption, scattering, and extinction of laser light by AuNPs. The efficiency factors of absorption, scattering, and extinction describe how much of the incident laser light is absorbed, scattered, or combined in both processes. These factors are based on several parameters, including the AuNPs’ diameter and shape and the laser light’s λ _max.

The efficiency factors may be estimated for specific laser λ _max for spherical Au with radii between 5 and 100 nm. As the diameter of the AuNP changes, its optical features, including absorption, scattering, and extinction, also vary. AuNP implements LSPR arising from the collective oscillation of conduction electrons in response to the incident electromagnetic field. The LSPR strongly depends on the NPs’ diameter, shape, composition, and surrounding medium. For example, resonance occurs when the diameter of the AuNP matches the λ _max of the incident laser light, leading to enhanced absorption and scattering. By adjusting the size and shape of the nanoparticles, the resonance behavior can be manipulated to achieve desired optical characteristics.

By studying the absorption, scattering, and extinction efficiency factors at various laser λ _max, researchers may optimize the interaction among Au and laser light for various usages. These usages include targeted photothermal therapy for cancer treatment, enhanced imaging techniques, optical sensing, and plasmonic-enhanced catalysis. Understanding AuNPs’ optical features and interaction mechanisms with lasers is pivotal for designing and developing efficient and precise nanoscale systems for various disciplines. Ongoing research continues to advance our knowledge and open up new possibilities for technological advancements in nanotechnology, engineering, and medicine. Figure 8 estimates the maximum temperatures and the amount of laser energy that Au NP may absorb. A wealth of physics and chemistry follows the findings. For NPs, it is critical to use the correct optical parameter values. It is possible to achieve laser action by using laser pulses with a λ _max considerably absorbed by AuNP but not by the media around them and a pulse duration that is brief enough to minimize heat transfer away from the absorbing particles.

Figure 8

Pulsed lasers interacted with plasmonic AuNP and related influences.

For instance, the SPR of AuNP at roughly 530 nm causes them to absorb laser light at 532 nm. The absorbed energy is then released as heat into the environment. Utilizing lasers to modify and possibly monitor NP diameter might be revolutionary. Studies have shown that laser-induced heating may modify NPs’ diameter, shape, and structure. Kurita et al. [138] irradiated AuNP by laser irradiation (532 nm Nd:Yag laser), and it was found that the diameter of AuNP was reduced. This is considered a photothermal process, with thermal energy driving it instead of the laser energy initially absorbed by the particles.

According to Figure 8, four possibilities have been identified for consideration when AuNP interacts with laser light. The creation of optical recording media used this approach [139]. The simultaneous occurrence of particle photochemical growth and laser-induced diameter reduction (308 nm excimer laser) is a novel diameter-monitoring-led production technique published in numerous works of literature [140]. There has been much interest in studying nanometre-diameter particles’ vibrational modes over the past few years [141,142]. The symmetric breathing mode of Au particles in an aqueous solution was investigated using laser ultrafast transient absorption spectroscopy (400, 780–800 nm, Ti-sapphire laser) [139] – the existence of an external energy source, like a laser pulse. The ultrafast pump laser pulse transfers energy to the electrons within the Au colloid, increasing its temperature. The lattice temperature gradually rises over a little picosecond as the heated electrons equilibrate with the phonon modes. During this process, the particles experience expansion due to the increased lattice temperature. The expansion of the Au colloid particles is a consequence of the thermal energy being transferred from the excited electrons to the surrounding lattice. This expansion may be observed and studied through various experimental techniques, including transient absorption spectroscopy. In systems with Au colloid, the transient absorption signal primarily arises from the excitation of the symmetric breathing mode. The symmetric breathing mode is a collective oscillation of the Au colloid particles, where the entire particle expands and contracts symmetrically. This mode is particularly pronounced in spherical particles due to its symmetric shape. By analysing the transient absorption signal, researchers may gain insights into the dynamics of the energy transfer processes and the behaviour of the Au colloid system. This understanding is pivotal for optimizing the use of Au colloids in various usages, including energy storage, catalysis, and biomedical imaging.

Overall, the ultrafast pump laser pulse selectively deposits energy into the electronic degrees of freedom of the Au colloid, leading to equilibration among electrons and phonons and subsequent lattice heating.

The expansion of particles and the observation of transient absorption signals can provide valuable information about the behaviour of Au colloids, leading to further studies and applications. Transient absorption spectroscopy is a powerful technique to investigate the dynamics of photo-excited states in materials.

When Au colloids are photoexcited, the absorbed energy generates excited states within the particles. This excitation can result in various processes, such as electron transfer, energy transfer, or structural changes. The expansion of particles is one of the possible outcomes, where the absorbed energy causes the particles to undergo an expansion or contraction on a transient timescale. By monitoring the transient absorption signals, which correspond to the changes in the absorption spectrum of the Au colloids over time after excitation, valuable insights can be gained about the behaviour of the particles. The temporal evolution of the transient absorption signal provides information about the dynamics of the excited states, relaxation pathways, and interparticle interactions.

The expansion of particles observed in the transient absorption signal can be related to the structural changes occurring within the Au colloids. This expansion could indicate interatomic distances, bond lengths, or particle morphology changes. By studying and analysing these expansion dynamics, researchers can better understand the underlying processes and properties of Au colloids.

The information obtained from transient absorption studies of Au colloids can be applied in various areas. For example, it can contribute to developing and optimizing Au colloid-based materials for sensing, catalysis, and energy conversion applications. Understanding the expansion behaviour can help design and engineer colloidal systems with enhanced properties and functionalities.

Furthermore, transient absorption spectroscopy can investigate the interactions between Au colloids and other materials or molecules. By studying the transient absorption signals in the presence of different environments or additives, researchers can gain insights into the processes of charge transfer, energy transfer, or surface interactions in these systems. In summary, observing particle expansion and analysing transient absorption signals provide valuable information about the behaviour of Au colloids. This knowledge can be utilized for further studies and applications, contributing to the advancement of colloidal science and the development of new materials and technologies. The transient absorption signal in systems with Au colloids is mainly attributed to the symmetric breathing mode, which is excited for spherical particles in Figure 8. In laser medicine, AuNPs may generate highly targeted cell damage. These NPs may absorb light energy from short laser pulses, which leads to localized heating. When the AuNPs absorb the laser energy, they convert it into thermal energy, increasing the temperature in its vicinity. This localized heating may be precisely monitored to induce thermal damage to specific cells or tissues while minimizing harm to the surrounding healthy cells. AuNPs are also engaging in the context of TES systems. TES systems are designed to store heat or cold for later use at various temperatures, locations, or power levels. While AuNPs may not serve as TES materials, they may be employed with other TES technologies. For example, in sensible heat storage, AuNP may be incorporated into heat transfer fluids or storage materials to enhance its thermal feature. The high thermal conductivity and efficient heat emission of AuNP may contribute to improving the overall performance and efficiency of the TES system.

Absolutely, AuNPs can be utilized in latent heat storage systems by integrating them into phase change materials (PCMs). PCMs are substances that undergo a phase transition, such as melting or solidification, during TES and release. By incorporating AuNPs into PCMs, the thermal properties and performance of the system can be enhanced.

The presence of AuNPs in PCMs can significantly impact their phase transition behaviour and TES capacity. The NPs act as nucleation sites during the phase transition process, facilitating the formation of stable and uniform crystal structures. This nucleation effect can lead to improved heat transfer and reduced supercooling or superheating, enhancing the efficiency of the TES system.

Moreover, the high surface-to-volume ratio of AuNPs provides many active sites for heat exchange. This increased surface area allows for enhanced heat transfer between the NPs and the PCM, enabling faster and more efficient TES and release.

In addition to their role in promoting nucleation and facilitating heat transfer, AuNPs can also contribute to the stability and durability of the PCM. The NPs can act as stabilizers, preventing phase separation or agglomeration of the PCM during repeated thermal cycling.

Furthermore, the plasmonic properties of AuNPs can be harnessed in latent heat storage systems. The NPs’ localized SPR (LSPR) can be tuned to match the solar spectrum, allowing for efficient solar energy absorption. This absorbed solar energy can then be stored in the PCM during the phase transition, providing a sustainable and renewable approach to TES.

Integrating AuNPs into PCMs offers latent heat storage systems several advantages. It improves nucleation, enhances heat transfer, increases stability, and enables efficient solar energy utilization. These advancements contribute to developing advanced TES technologies with improved performance and sustainability.

Further, AuNP may enhance heat transfer and improve the heat storage/retrieval rates of the PCM, thereby optimizing the efficiency of the latent heat storage system. In summary, AuNPs are valuable in various electrochemical energy storage usages and may enhance TES systems’ thermal features and performance. Its exceptional chemical stability, high electronic conductivity, and efficient heat emission make it well-suited for these purposes. Ongoing research and development in this area aim to explore further the potential of AuNP in energy storage and related usages [143,144].

Furthermore, AuNPs may be optimized by reducing their quantity and evenly spreading them over various electrode materials. This approach helps to maximize its efficiency and reduce the overall cost of the system. Conducting polymers, like polyaniline or poly(3,4-ethylene dioxythiophene) (PEDOT), are broadly used as electrode materials due to their low cost, ease of synthesis, and good electrical conductivity. Incorporating a small amount of AuNP into these conducting polymers may enhance its electrical feature, improving energy storage usage performance [145,146], transition metal oxides, and nano carbons.

8.2 Exploiting the longitudinal SPR of Au NRs for therapies, sensing, and data storage

Indeed, the unique feature of Au NRs, particularly its longitudinal surface plasmon resonance (LSPR), may be harnessed for various usages, including heat-based therapies, SERS, and optical data storage.

1. Heat-based therapies: When absorption energy is increased in Au NRs, the longitudinal LSPR is enhanced, releasing considerable energy. This energy may rapidly and nonradiatively transfer to the surrounding environment, generating localized heat. This property may be applied in heat-based therapies, like cancer treatment. Targeting and selectively heating cancer cells using Au NRs makes it possible to induce thermal damage and destroy the malignant cells while minimizing damage to healthy tissues.

2. SERS: The solid electric fields generated by the longitudinal LSPR in Au NRs may significantly enhance the Raman scattering signals of nearby molecules. This influence, known as SERS, enables highly sensitive molecular detection and imaging. Au NRs may serve as excellent substrate materials for SERS, providing enhanced Raman signals that allow for detecting and analysing trace amounts of molecules. This has usages in various fields, including biological sensing, environmental monitoring, and chemical analysis.

3. Optical data storage: Au NRs’ tunability of LSPR λ _max and polarization dependency may be applied in constructing high-capacity information storage devices. Structural deformation may be recorded and encoded by exploiting the sensitivity of Au NRs’ aspect ratios to its longitudinal LSPR λ _max. Laser irradiation of Au NRs achieves high-density optical storage, and alterations in the NRs’ dimensions may be detected by modulating its longitudinal LSPR spectra. This enables the encoding and retrieval of information at a high storage density.

These usages demonstrate the versatility of Au NRs in harnessing its unique feature, particularly the longitudinal LSPR, for various technological advancements. Ongoing research and development in this field aim to optimize further and explore the potential of Au NRs in biomedical usages, optical sensing, and data storage systems [147]. Pérez-Juste et al. [148], with the use of a PVA-Au nanorod composite film, which may be used for optical printing, found that when exposed to laser radiation, the irradiated and un-irradiated sections differed noticeably depending on the polarization angle of the incident light. In the study by Qu et al. [149], silver NP and Au NRs were joined, and the composite was then cross-linked to the Pt/n-Si/Ag photon-electrode pipeline. The spectrum dependence demonstrated that adding Au NRs to the longitudinal LSPR improved photocatalytic activity. Wang et al. [150] placed a Pt submono layer on Au NRs to improve formic acid oxidation catalysis and AgPt alloy on Au NRs to reduce CO poisoning [151].

9.1 Enhancing solar energy storage with AuNPs

1. Plasmonic enhancement: AuNP implements a phenomenon familiar as LSPR, where the collective oscillation of conduction electrons in the NP generates intense and localized electromagnetic fields. These enhanced electromagnetic fields may enhance light absorption in solar cells. By incorporating AuNP into the active layer of solar cells, the LSPR influence may increase light absorption and improve the overall energy conversion efficiency of the device.

2. SERS: AuNPs, particularly those with star-shaped configurations, have been broadly used in SERS usages. SERS is a sensitive spectroscopic technique that detects and identifies molecules based on their unique vibrational fingerprint. The existence of Au NP enhances the Raman scattering signal of molecules adsorbed on its surfaces, improving detection sensitivity. In the context of solar energy storage, SERS may be applied for real-time monitoring of the chemical processes occurring in the solar cell, providing valuable insights for optimization and performance enhancement.

3. Catalytic activity: AuNP implements excellent catalytic activity that may be leveraged for solar energy storage usage. For example, Au may be the catalyst for converting solar energy into chemical fuels, like hydrogen production through water splitting. The existence of AuNP may considerably enhance the efficiency of these catalytic reactions, enabling more efficient energy storage and utilization.

4. Optical feature: AuNP possesses unique optical features, including tuneable plasmon resonances and strong light absorption and scattering capabilities. These features may be harnessed to improve light management in solar cells. By strategically designing and incorporating Au into the solar cell structure, the absorption of sunlight may be optimized, allowing for increased energy generation and storage.

5. Biocompatibility and stability: AuNPs are biocompatible and chemically stable, making them suitable for long-term use in solar energy storage systems. They may withstand harsh environmental conditions and maintain their catalytic and optical feature over time. This stability and biocompatibility make AuNP promising for integration into practical and durable solar energy storage devices.

In conclusion, AuNP offers exciting opportunities to enhance solar energy storage. Its plasmonic feature, catalytic activity, optical capabilities, and stability make them valuable tools for improving the efficiency and performance of solar cells. Continued research and development in this field hold great potential for advancing renewable energy technologies and addressing the challenges of climate alteration.

9.2 Intriguing phenomena and effects of pulsed laser interactions with plasmonic AuNPs

Then, pulsed lasers interact with plasmonic AuNPs, and several intriguing phenomena and effects can occur. The plasmonic properties of AuNPs arise from the collective oscillation of conduction electrons in response to incident electromagnetic radiation. This collective oscillation, known as LSPR, can be excited and manipulated using pulsed laser sources, leading to various interesting outcomes. Here are some of the key interactions and influences observed when pulsed lasers are applied to plasmonic AuNPs:

1. Enhanced optical fields: Plasmonic AuNPs can concentrate electromagnetic fields around them due to their resonant response. When pulsed lasers with appropriate wavelengths are incident on the NPs, the LSPR can significantly enhance the local optical field. This enhanced field can be utilized for SERS or enhanced fluorescence applications [152].

2. Nonlinear optical effects: The high intensity of pulsed lasers can induce nonlinear optical effects in plasmonic Au NPs. These effects can include multiphoton absorption, harmonic generation, or even the generation of optical pulses at new frequencies. Nonlinear optical processes can be exploited for imaging, sensing, and optical signal processing applications [153].

3. Photoacoustic effect: When plasmonic AuNPs absorb laser energy, they can rapidly convert it into heat, leading to localized heating of the surrounding medium. This localized heating can induce a rapid expansion and subsequent medium contraction, generating acoustic waves. This phenomenon, known as the photoacoustic effect, has applications in imaging, sensing, and therapy [154].

4. Photothermal therapy: The ability of plasmonic AuNPs to efficiently convert light into heat makes them promising agents for photothermal therapy. Pulsed lasers can heat the NPs, leading to selectively localized hyperthermia and targeted destruction of cancer cells or diseased tissues [155].

5. Optoacoustic imaging: Plasmonic NPs can also be employed as contrast agents in optoacoustic imaging. Pulsed laser irradiation of the NPs results in rapid expansion and contraction, generating acoustic waves that can be detected and used to reconstruct high-resolution images of biological tissues [156].

6. Nanosurgery and manipulation: Pulsed lasers can be focused to a diffraction-limited spot size, allowing for precise nanosurgery and manipulation of plasmonic gold NPs. Controlling the laser parameters makes it possible to selectively ablate or modify individual NPs or nearby structures [140].

These are just a few examples of the interactions and influences that occur when pulsed lasers interact with plasmonic AuNPs. The precise outcomes depend on the laser parameters, NP properties, and the surrounding environment. The ability to control and harness these interactions opens up exciting possibilities for various applications in biomedicine, sensing, imaging, and nanophotonics.