Engineered nanoflowers, nanotrees, nanostars, nanodendrites, and nanoleaves for biomedical applications

2.1 Nanoflowers

Nanoflowers are a new class of NPs (size of <100 nm) mimicking the shape of flowers [30]. They have been widely applied in medical applications such as cardiovascular disease treatment, sensors and biosensors, and cancer treatment [31]. Their high surface-to-volume ratios make them desirable agents in biomedical applications [32]. Figure 3 shows a schematic representation of the movement of the flower-like NP into the cell to deliver therapeutic genes. Nanoflowers can be fabricated based on carbon, metals, metal oxides, hydroxides, oxosalts, sulfides, selenides, telluride, nitrides, phosphides, organic, and coordination compounds [33]. There are also hybrid nanoflowers based on deoxyribonucleic acid (DNA) [31], protein [34], enzymes [35], and polymers [36].

Figure 3

Schematic presentation of nanoflowers drug delivery through cell: (1) delivery of DNA-loaded nanoflowers to the cell via endosomal delivery, (2) encapsulated nanoflowers in cell membrane, (3) release of nanoflower and DNA to intracellular environment, and (4) disbanding of DNA and delivery of DNA to the cell [40] (Created with BioRender).

There is a relationship between particles catalytic activities and their morphology [37]. The free electrons in metals can generate different surface plasmon resonance bands depending on particle size, shape, and the corresponding medium [37]. Organic–inorganic hybrid nanoflowers have also attracted much attention in recent years. The organic component includes enzymes, protein, and DNA [32]. They are easy to be fabricated and functionalized, and capable of stabilizing enzyme [38]. The organic–inorganic hybrid nanomaterials show many promising applications as biosensors, drug delivery agents, and in catalysis [38]. Nhung et al. [39] developed gold nanoflowers for surface-enhanced resonance scattering-based sensor. In this study gold nanoflowers were fabricated by seeding Au NPs on films made of cross-linked tripolyphosphate (TPP). Chitosan-TPP template films were considered as the nucleation sites and stabilizing agents to form anisotropic gold nanoflowers. Their size and optical properties were correlated with the ratio of gold salts to chitosan-TPP template films.

A higher content of gold salt can loosen the stabilizing capacity of the template films and Au NPs aggregation which would cause the formation of corners, junctions, and edges on the surface. These physical and geometrical properties made them strong near-infrared (NIR) absorbent and potential candidates for surface-enhanced Raman spectroscopy (SERS) applications [39].

Combining the tree-type surfactant – bis(amidoethylcarbamoylethyl) octadecylamine (C18N3) – with multiamine head group has been proposed as a new approach to fabricate hollow gold nanoflowers [41]. This nanostructured material was biologically safe under exposure of visible light and was also highly cytotoxic to HeLa cells when exposed under NIR light irradiation.

The toxicity of Au NPs was also investigated [42]. The fabricated samples consisted of jagged structure with plenty of non-uniform tips. It was assumed that this type of shape leads to non-toxicity in comparison to needle-like carbon nanotubes. The highest toxicity in HeLa cells after 24 h treatment was observed in nanoflowers with a diameter of 340–410 nm at the highest concentration (300 µM). Gold is the one of the most commonly used metallic nanomaterials in biomedical applications.

Moreover, Au NPs with a diameter of 5 nm successfully inhibit proliferation and enhance apoptosis in two lung cancer cell lines. Contrarily Au NPs with dimensions of 10, 20, and 40 nm indicated no cytotoxic effect on lung cancer cells [43].

In some investigations Au NPs reported to be cytotoxic to different cell lines [44–46]. The Au NPs cytotoxic effect depends on the size of NPs and the cell type. The cytotoxic effect in small NPs results from their easy endocytosis into the cells, while no notable cytotoxicity was reported for large particles. Moreover, small NPs would cause upregulation of MMP9 expression [47]. The results of this study indicated that the gold colloid-based monolayers are selectively toxic to certain cells, such as red blood cells (RBCs) [48]. Moreover, the size, composition, and surface properties can also alter the toxicity [49]. The recent studies suggest that diminishing Au NPs size to <2 nm (the size of nanocluster) improves antimicrobial activity against Gram-positive and Gram-negative bacteria [50].

Moreover, the asymmetry shape of nanoflowers enhanced the uptake in cancer cells. The magnetic gold nanoflowers were fabricated with the cancer treatment purpose (Figure 4) [51]. Magnetic NPs have diverse applications as an antimicrobial and anticancer agent and they can also be applied as contrast agents in targeting, imaging, diagnosis, and hyperthermia [52].

Figure 4

Schematic illustration of developed magnetic nanoflower for cancer treatment. Nanoflowers loaded with Fe₂O₃ NPs conjugated with polyethylene glycol (PEG) and Raman probe can be used for magnetic detection, or NIR detection of tumors via SERS or MRI imaging [51] (Created with BioRender).

To fabricate this structure Au was grown on γFe₂O₃ NPs, thus a rough surface was achieved. The developed structure demonstrated incredible SERS enhancement, strong PA signals, and enhanced reactivity that are essential for cancer treatment [51].

The extensive use of gold is due to its novel physical and optical features. Their nanoscale dimensions make them suitable candidate for studying and manipulating biological systems [53]. They have been applied as diagnostics agents, photothermal therapy (PTT), biosensing, bioimaging, and drug/gene delivery. Au NPs convert the energy of a photon into heat which makes them an effective photothermal agent in cancer treatment [54]; additionally, the generated heat may have an antibacterial effect [55].

ZnO was also applied in a wide variety of biomedical fields such as delivery vehicles, treatment agent for various diseases such as microbial infection, diabetes, ischemic, cardiovascular diseases, and wound healing [51]. ZnO nanoflower was fabricated via microwave irradiation by Patra and Barui [56]. The proangiogenic property of ZnO nanoflowers via in vivo and in vitro experiments was evaluated. The ZnO nanoflowers stimulated endothelial cells (HUVECs) proliferation which improved proangiogenic property of ZnO nanoflowers. Also, an enhancement in wound healing for endothelial cells was observed. It was assumed that these features are caused by H₂O₂, which is a redox signaling molecule. In 2015 a biosensor based on ZnO nanoflowers was developed [57]. To fabricate this structure, Zn nanowires were placed on a silicon substrate and then were spotted by Au (Figure 5). The presence of Au NPs eased the thiolated biomolecules binding.

Figure 5

Schematic illustration and SEM images of fabrication process of biosensor based on Au spotted ZnO nanoflowers by Perumal [57]. Copyright 2015, with permission from Nature Publishing.

The antibacterial features of the ZnO nanoflowers were investigated by Cai et al. [58]. For this purpose, samples were fabricated in three different morphologies: rod flowers, fusiform flowers, and petal flowers via the hydrothermal process. Staphylococcus aureus and Escherichia coli were used to evaluate the antimicrobial activities. Petal flowers, fusiform flowers, and rod flower samples, respectively, demonstrated the highest antimicrobial power.

The differences in anti-microbial power originated from variation in microscopic factors like specific surface area, size of pores, and polar plane of Zn. The mechanisms that result in antimicrobial properties of ZnO can be caused by Zn²⁺ ion release, either adsorption or generation of reactive oxygen species (ROS). It can also damage the cell membrane or DNA breakage and inhibition of the energy metabolism [59]. Furthermore, the neurogenic activity of ZnO nanofibers (NFs) was studied [60]. ZnO NF was fabricated by an advanced microwave irradiation approach and tested in vitro and in vivo. The in vivo investigations in Fischer rats indicated that the neuroprotective efficacy of nanoflowers came from Neurabin-2 and NT-3 upregulation. In in vitro experiments, their study identified the role of PI3K/Akt and MAPK/ERK1/2 in signaling paths behind nanoflowers mediated neurogenesis.

Other metallic elements like Mo have also been fabricated in the form of nanoflowers. For example, bovine serum albumin (BSA)-coated molybdenum disulfide (MoS₂) nanoflowers were developed via a hydrothermal approach [61]. The MoS₂ nanoflower had a layered structure which led to an improvement in its heating effect. Layered MoS₂ nanoflowers displayed a significant antitumor effect under MW irradiation at 1.8 W, which made them a promising agent for cancer MW thermal therapy. A synergistic antitumor platform was developed using MoS₂, azo initiator 2,2′-azobis[2-(2-imidazolin-2-yl) propane] dihydrochloride (AIBI), and phase-change material (PCM) (tetradecanol) (MoS₂@AIBI–PCM nanoflowers) [62]. The nanomaterials were fabricated using polyethylene glycol-functionalized molybdenum disulfide (PEG-MoS₂) nanoflowers in conjunction with azo initiator and PCM. Under the exposure to NIR laser irradiation, free radicals were produced. This process resulted from the AIBI decomposition initiated by photothermal feature of PEGMoS₂. Therefore, MoS₂@AIBI–PCM nanoflowers were applied as ROS-based cancer therapy. BSA-modified Ag nanoflowers for sensing DLD-1 human colon cancer cells were also developed [63]. The fabricated structure displayed a suitable cell-immobilization capacity due to its porous structure. The presence of BSA as a natural layer resulted in the acceptable biocompatibility. BSA-incorporated Ag nanoflowers performed well in quantifying DLD-1 cells and the amount of the expressed sialic acid on the surface of a single cell were estimated. The as-developed agent would ease early diagnosis and treatment in human cancer.

DNA nanoflowers are another example of inorganic hybrid materials [64]. They exhibit excellent features such as water solubility, suitable electronegativity, sequence programmability, and automated controllable fabrication approach [65]. These structures are made from long DNA building blocks fabricated via rolling circle replication [66]. The incorporation of functional aptamers, fluorophores, and drug loading capability into DNA-nanoflowers hybrids were investigated [67].

Nano- and micro-scale hybrid nanoflowers carrying various divalent cations such as Mn²⁺, Co²⁺, and Zn²⁺ and DNA were fabricated for drug delivery, sensing, biocatalysis, energy (as super capacitors), and separation technologies. By changing the composition of the enzyme buffers and polymerase enzyme, the as-made structure exhibited versatile properties due to the controllable morphology, size, manipulation of the DNA levels within the particles, and the surface potential [64]. Metal-containing artificial analog-incorporated DNA nanoflowers were developed [66]. These structures were fabricated via three-step approach. In the first step, Ferrocene (Fc)-DNA was fabricated, and then DNA nanoflowers were constructed. In the final step, H₂O₂-responsive Fc DNA with DNA nanoflowers were incorporated. The as-developed structures were bio-inspired, self-degradable, and size-controllable. For drug delivery evaluations, doxorubicin was loaded and in vitro experiments proved selective uptake of system by protein tyrosine kinase 7 (PTK7)-positive cancer cells. The presence of H₂O₂ sensitive Fc resulted in quick release, nuclear accumulation, and enhanced cargo cytotoxicity. In vivo studies on mice bearing the PTK7 + HCT-116 tumors confirmed the improvements in targeted delivery and the therapeutic efficacy of doxorubicin [67].

Hybrid nanoflowers containing metals ions and polymers indicated unique features. In an investigation using Cu and Ca ions and elastin-like polypeptide (ELP), it was observed that due to temperature-dependent conformational behavior in ELPs, controllable resultant morphological patterns were achieved. Moreover, ELPs can alter the nucleation and growth process by formation of ordered crystalline nanostructures [36]. The developed nanoflowers are summarized in Table 1.

2.2 Nanotrees

The synthetic approach for nanotrees can be classified into three main groups: one-step growth, sequential branched growth, and template-assisted growth, which are demonstrated and summarized in Figure 6. The solution-based chemical synthesis and vapor phase methods are the common approaches for synthesizing the nanotree platforms. Additionally, full-solution processed synthesis approaches like hydrothermal and solvothermal synthesis have been broadly applied for constructing 3D branched array structures. Vapor-based techniques include vapor oxidization of metal substrates, laser ablation, chemical vapor deposition (CVD), and plasma approaches [68]. Generally, the full-solution processed fabrication approaches like hydrothermal or solvothermal synthesis protocol displayed great potential in fabrication of 3D branched array configuration [69]. The developed tree-like structures exhibited various unique features such as high surface area, quick transfer of electrons, and ideal crystallinity [68].

Figure 6

(a) Schematic representation of nanotrees synthetic approach [68] (Created with BioRender); Copyright 2017, with permission from Royal Society of Chemistry. (b) SEM image of synthesized nanotrees [135]; Copyright 2014, with permission from Nature Publishing.

ZnO@MnO₂-ramified nanowire arrays were fabricated via hydrothermal and redox approach [70]. In this structure, ZnO and MnO₂ were considered as the core and shell, respectively. Owing to an increase in the surface area the structure can improve the electrical conductivity properties. Also, this core–shell structure displayed an efficient charge–discharge stability.

A 3D nanotree-like ZnO/Si nanocomposite was fabricated by hydrothermal method [71] (Figure 7). In this method, systematically patterned silicon nanorods (SiNRs) were synthesized by merging of polystyrene sphere template with metal-catalyzed etching. The ZnO branches were formed on SiNRs through hydrothermal deposition approach. ZnO nanowires (NWs) improved the electric field of SiNRs, boosted the emitting dots number, and reduced the screening effect due to their high ARs, which made the 3D ZnO/Si a great candidate for application in field emission devices. The 3D jagged ZnO nanotrees (ZNTs) on the ITO–PET substrates were also investigated [72]. Two-step hydrothermal technique was applied for construction of ZNTs. Furthermore, a self-powered piezoelectric nanogenerators based on ZNT was developed. In this structure, nanowire branches affected the electric output and resulted in an enhancement in performance. The highest output of current measurement is about 300 nA. ZnO NPs are biocompatible, non-toxic, self-cleansing, compatible with skin, and antimicrobial. It should be noted that features such as the size, shape, and morphology can affect thermal and electrical properties [73].

Figure 7

Schematic illustration and FE-SEM images of synthesis of ZnO NWs/SiNRs by Lv et al. [71] (Created with BioRender); Copyright 2015, with permission from ACS Publishing.

The 3D branched and hyper-branched nanotree structure can be achieved via vapor phase based fabrication approaches [68]. A novel 3D symmetric micro-super capacitor made of polypyrrole (PPy)-coated silicon nanotree (SiNTr) hybrid electrodes were developed via CVD and electrochemical technique [74]. This hybrid structure can be applied as electronic biomedical implants such as neuro-stimulators due to their portability. It was synthesized via precipitating PPy on the surface of SiNTrs. The developed hybrid indicated significant electrochemical features due to electroactive pseudo-capacitive and electric double layer capacitive property of PPy and branched SiNWs, respectively. PPy-coated SiNTr-based micro-supercapacitors displayed an excellent performances in comparison to silicon nanowire hybrid micro-supercapacitors concerning specific capacitance and energy density [75].

Additionally, the Si nanoneedles were grown onto ZnO nanorod arrays and the AgNPs decorations were fabricated as a substrate for SERS detection. In the first step, the Si nanoneedles were implanted onto the ZnO nanorod surface via catalyst-assisted vapor–liquid–solid (VLS) process. This method is based on catalyst droplet losing in plasma-enhanced CVD. In comparison to other techniques such as hydrothermal growth or CVD approach, this method is economical and eco-friendly. In the second step, AgNPs were placed on the surface of ZnO/Si nanomace by a galvanic displacement reaction. Rhodamine 6G (R6G) was applied to evaluate the potential of developed 3D SERS substrate. The Raman enhancement factors are shown to be up to 8.7 × 10⁷, which confirmed the acceptable performance of ZnO/Si heterostructured nanomace arrays as SERS substrate. Other techniques such as thermal evaporation have also been applied for fabrication of nano -tree structures. The PbS-doped ZNTs were investigated as the promising agent for LED, photovoltaic cell, piezoelectric apparatus, and biosensor and gas sensor application [76]. The samples were fabricated via thermal evaporation on silicon and glass substrates. Furthermore, nanotrees have been investigated based on other elements and compounds. Novel treelike two-level Pd _x Ag _y nanostructures were developed in bifunctional fuel cell electro-catalysis application [77]. The nanostructure made of a tiny Pd₁Ag₁ alloy NDs and Pd₁Ag₂ alloy nanobranches provided various active sites and improved the stability of the structure. The Pd₃Ag₁ NTs were more operative and steadier in comparison to other Pd/Ag ratios. In this investigation 1-naphthol was applied as reductant and structure directing agent. Findings confirmed that the suitable feeding ratio of Pd/Ag forerunner and the introduction of 1-naphthol play key roles in successful fabrication of well-defined Pd _x Ag _y . The developed nanotrees are summarized in Table 2.

2.3 Nanostars

Gold nanostars, with unique structural feature, have been applied in various biomedical applications such as chemical sensing [78], medical imaging [79], cancer treatment, and drug [43] and gene delivery. They have gained much attention owing to their spectacular optical properties and their versatility in surface modification (Figure 8) [80]. They display magnificent features such as high X-ray absorption coefficient, ease of synthetic manipulation, adjustable physicochemical properties of particles, and high affinity to form bonds with thiols, disulfides, and amines [13].

Figure 8

Schematic representation of two types of polymeric nanostars’ structure and their medical application (Created with BioRender).

Parameters such as the size of NP, the nature of the ligand, molecular weight, and grafting density can alter the cellular uptake. Some studies indicated that cellular uptake is not significantly affected by Au NPs size although in multicellular tumor spheroids, Au NPs with smaller size indicated higher cellular uptake and advantages [81].

The cellular uptake is dependent on the shape of Au NPs and the type of cell. In previous studies on Au NPs, Au nanostars exhibited the lowest cellular uptake in RAW264.7 cells in comparison to triangle and rod-shaped NPs [82,83]. In another study, gold nanostars indicated the highest cellular uptake in MCF7 cells compared to other morphology (hollow, rods, and cages) [84].

Furthermore, it was confirmed that size-dependent interaction between sub-nanometer Au NPs and the glycocalyx can affect the cellular uptake. Observations confirmed the higher cytotoxicity of the smaller Au NPs (4–5 nm) in comparison with larger particles (18–20 nm). Besides the size, other factors affecting cytotoxicity include the cell type, tissue distribution, tissue absorption, and penetration capacity [83].

The shape of the NPs on surface-enhanced Raman scattering (SERS) has also been examined [85]. The gold nanostars displayed the highest magnitude of the SERS response in comparison to other shapes such as nanospheres and nanotriangles. It was assumed that the existence of five or more vertices at 30° increased the local field enhancement and sequentially higher SERS. Nanostars are also widely used as drug delivery agents. For example, mitoxantrone (MTX)-loaded gold nanostars were applied as an anti-cancer drug delivery system [86]. MTX was placed on the gold nanostars surface by thiolated and carboxylated PEG spacer. The accumulation of the drug in a healthy mouse heart and lung tumor bearing mice was evaluated. The observations confirmed enrichment in Raman signal from MTX molecules by the plasmonic field of the NPs. The peptide-functionalized gold nanostars were proposed as an agent for cancer treatment [87]. The Au NPs were functionalized by a pH (low) insertion peptide (pHLIPs), which can pass through the pH-dependent transmembrane. This feature eased the crossing of lipid bilayer into tumor cells due to acidic activation. The samples were evaluated via in vitro and in vivo experiments and outcomes showed an improvement in cellular internalization (at pH 6.4), tumor accumulation (after intravenous injection), CT signals, and photoacoustic (PA) imaging compared with GNS-mPEG. The in vivo experiments on tumor-bearing mice indicated an increase in CT signals of the tumors in both GNS-mPEG- and GNS-pHLIP-treated mice which confirmed the accumulation of NPs in tumor. Specifically, CT values in GNS-pHLIP-injected mice were notably higher than that of GNS-mPEG-injected ones.

Coated gold nanostars (AuNSts) were fabricated with the purpose of drug delivery. In this study, MS-coated gold nanostars (AuNSts) that consist of paraffin as a thermo-sensitive factor was used [88]. The AuNSts were synthesized by seed growth technique and then coated with a MS shell and capped with paraffin. Doxorubicin (DOX) was conjugated into porous silica and then covered with thermo-sensitive paraffin. It was found that un-irradiated AuNSt@mSiO₂@Dox@paraffin NPs indicate no cytotoxicity to HeLa cells. Under exposure of 808 nm laser, the paraffin layer melted and resulted in cargo release. As-designed drug photo-delivery AuNSts systems demonstrated minimum leakage and low laser power requirements.

The nanostars have also been applied with other coatings. For instance, temperature sensitive liposomes coated nanostar was fabricated as a promising drug delivery agent [89]. The gold composite including siRNA of cyclooxygenase-2 (siCOX-2) that was modified by 2-deoxyglucose (DG) (a tumor targeting ligand) and transmembrane peptide 9-poly-d-arginine (9R) were applied to the siCOX-2(9R/DG-GNS) structure. Gold nanostars were synthesized using the seed growth approach. After successful conjugation of siCOX-2, a paclitaxel (PTX)-TSL-siCOX-2(9R/DG-GNS) co-delivery system was developed. This agent was taken rapidly in high temperatures by tumor. It was found that this system not only defeated PTX resistance, but also improved drug delivery to the cancer cells by photothermal conversion effect.

RBC and platelet membrane-coated gold nanostars loaded by curcumin were also proposed as a promising drug delivery system [90]. The macrophage phagocytosis is avoided in this system. Curcumin was conjugated to the constructed system. Observations proved controlled drug release at high temperature and tumor growth inhibition under exposure of NIR laser irradiation.

The gold nanostar composites have also been applied as a drug delivery agent. Au nanostar@metal–organic framework was evaluated as a stimuli-triggered synergic approach for chemo-photothermal therapy [91]. In this structure, ZIF and Au nanostars were shell and yolk, respectively. The ZIF shell underwent degradation in tumor acidic environment and resulted in cargo release; on the other hand, Au nanostar performed as second near-infrared (NIR-II) responsive photothermal conversion. NIR-II laser penetrated deeply within the tissue and a higher resolution was observed. Furthermore, combining chemo-photothermal therapy achieved an astonishing synergistic effect for preventing tumor growth. Another nanocomposite consisting Au nanostar and ZIF-8 was developed as stimuli responsive active cargo in living cells [92]. In this structure, NSs were considered as seeding sites for the ZIF-8 shell. Developed structure entered into the intracellular environment successfully and was stable in aqueous environment. Nanostars can also be used in antibacterial applications. Poly(vinyl alcohol) (PVA) functionalized gold nanostar (GNS) was proposed as a potential agent for protective antibacterial films and coatings [55]. In this experiment, the photo-thermally-induced local high temperature resulted in antibacterial feature. The temperature increase was contributed to GNS because blank PVA films did not indicate any change in temperature under NIR exposure. Bacterial growth, viability, and proliferation were decreased by about 50% in GNS containing samples under exposure of NIR. Au nanostars and 2D reduced graphene oxide nanocomposite were introduced as cooperative killing multidrug resistant bacteria [93]. Samples were made via seed-mediated growth technique and antibacterial effect was evaluated using methicillin-resistant S. aureus (MRSA) bacteria. The results indicated an enhancement in antibacterial feature due to the limited hyperthermal effect of rGO/AuNS. The disintegration of cell walls or membranes caused by the nanocomposite’s prickly and sharp edges can be considered as another factor in the antibacterial capabilities that were obtained. Wang et al. [94] developed a unique agent for Gram-positive bacteria elimination. This system consisted of gold nanostars (AuNSs) and vancomycin (Van). The MRSA was recognized and eliminated by this system under exposure of NIR laser irradiation, but this system demonstrates less efficiency against Van-resistant Gram-positive bacteria so further investigations are needed. PEG-functionalized gold nanostars with multiple sharp branches demonstrated an increase in NIR photothermal conversion efficiency in comparison to other shapes of Au NPs. Surfactant-free fabrication methods resulted in more biocompatible systems. This agent was fabricated via seed-mediated growth method which resulted in ultrasmall dimension, effective metabolizability, and high computed tomography (CT) value [95]. Generally, it has been shown that Au NPs can be incorporated into the Gram-negative bacteria more effectively compared to Gram-positive ones. Also Au NPs are considered safer for mammalian cells due to their ROS-independent mechanism [96].

Apart from metals, nanostars can be made from other materials such as polymers. Star polymers are tree-shaped polymers which are made of a central core and attached linear chains. They are classified based on branches, molecular weights, or topology into three different groups including symmetric stars, asymmetric stars, and miktoarm stars. In symmetric stars all branches are like linear chains, while asymmetric stars referred to NPs consisting of branches differ in molecular weights or topology. In miktoarm stars, branches differ in chemical composition [97].

The star-shaped polymers have attracted much attention. They are synthesized through three different approaches: “arm-first,” “core-first,” and the grafting-onto [98]. A star-shaped hermos responsive amphiphilic copolymer nano-assembly was developed using reversible-addition fragmentation chain transfer (RAFT) dispersion polymerization approach [99]. This indicates that the hermos responsive property was highly affected by tethered PNIPAM chains topology. Also gold nanostars with poly dopamine coating were proposed as a candidate for multidrug resistance breast cancer treatment [100]. This agent can be applied as an antitumor and antiangiogenic agent. The methods of synthesizing nanostars are summarized in Table 3.

2.4 NDs

The dendrimers are generally in the shape of connected trees by a common core. The word dendrimers have originated the Greek word “dendron” which means tree (Figure 9). These structures displayed significant bioactive properties. Nanodendrimers have been applied widely in different fields such as drug delivery, gene transfection, material modification, clinical diagnosis, sensors, and MRI [101]. Monometallic, bimetallic, and trimetallic NDs are examples of different ND assemblies [102]. The gold nanodendrites (AuNDs) via novel seed-mediated techniques were fabricated [103]. Observations confirmed that the degree of branching (DB) can influence NPs performance. The higher DB in nanostructure led to the more effective performance in photothermal elimination of tumors at lower NIR wavelengths. On the other hand, NPs with a lower DB functioned well under exposure at a higher wavelength NIR irradiation. Therefore, this versatility of options in applied NIR range results in the selective determination of a laser wavelength, accordingly the best performance in cancer therapeutic is achieved. The correlation between optical and AuNDs DB were also investigated. [104]. The in vivo and in vitro experiments confirmed that the manageable DB led to a wide variety of optical features in a comprehensive range of NIR radiation. The dependence of photothermal features of AuNDs on the applied wavelength was observed.

Figure 9

Schematic illustration of ND structure [105] (Created with BioRender).

A ND was developed in the core–shell structure. It consisted of gold as a core, palladium as a shell, and PEG as the modifier [106]. The negative charge of PEG and positive charge of DOX created an electrostatic interaction which led to DOX loading. The results confirmed that the DOX desorption depends on various factors such as pH, solvents, temperature, and enzymes; hence, an accurate and controllable stimuli-responsive system was achieved. NDs are also applied as sensors and diagnostic tools. Dendritic NPs made of graphene oxide-based magnetic and bimetallic (Fe/Ag) and modified by vinyl were fabricated [107]. This structure was applied for trace level detection of pyrazinamide (PZA). The nanodendritic shape indicated higher electrocatalytic activity in comparison to NPs with the exact same components. RGO@BMNDs also successfully detected PZA in pharmaceutical, human blood serum, plasma, and urine samples.

The AuND-modified electrode was applied as substrate for preparing hemoglobin (Hb)-imprinted poly(ionic liquid)s (HIPILs) [108]. In this experiment the NDs were applied on the electrode surface as a factor to enhance accessible surface area. The samples were fabricated via electrodeposition approach. HIPILs were applied as a modification agent. Applying ionic liquid as functional monomer decreased the detection limits of developed Hb sensors in comparison with other sensors. In another study [109], platinum NDs were loaded on poly (diallyldimethylammonium chloride) modified and MoS₂ nanosheet hybridized PPy nanotubes. Results indicated that the catalytic activity was enhanced by applying Pt NDs and MoS₂ nanosheet [109]. Developed nanocomposite was highly active in H₂O₂ reduction and displayed an appropriate potential to inactivate antibodies, and low detection range of AFP.

Another biosensor was fabricated based on nanodendritic gold/graphene which can be applied for fluorescence, SERS, and electrochemistry tri-mode miRNA detection [110]. This biosensor was fabricated through electrochemical deposition, patterning, and graphene assembly. The microwell is capable of conducting sensitively electrochemical sensing and SERS detection owing to both high surface area and ND hotspot density. The trimetallic AgPtCo NDs and magnetic AgPtCo NDs were also studied as electrochemical signal readout [111]. The AgPtCo NDs were fabricated through convenient one-pot synthesis approach. The NDs with high surface area supplied enough surface for secondary antibodies immobilization. The potential of this electrochemical immune sensor as a POCT platform for sensitive analysis of different biomarkers in clinical samples was confirmed.

Furthermore, a novel multimodal enzyme-linked immunosorbent assay (M-ELISA) was developed based on Au@Pt NDs [112]. Cardiac troponin I (cTnI) was tested as biomarker. Due to the properties of Au@Pt NDs such as peroxidase-like activity and photothermal effect, the concentration of cTnI can be successfully quantified by temperature measurement, colorimetric, and ratiometric fluorescent signal responses. These evaluations were precise and well-founded which ensure their usability for clinical assay protein markers. An electrochemical sensing approach for recognition of chiral enantiomers was developed based on AuNDs [113]. This approach was simple, stable, highly efficient, and sophisticated; also the electrode pretreatment in composites synthesis and surface modification was not required. NDs have also been considered as an anti-inflammatory and an antibacterial agent. Bewersdorff et al. [114] in 2017 proposed sulfated dendritic polyglycerol (dPGS)-coated Au NPs as an anti-inflammatory agent and tested in vitro and in vivo. dPGS NPs were highly propended to interact with serum proteins. In 2017 Kienzle et al. [115] proposed dendritic structure as a drug delivery system. A pH triggered block copolymer with dendritic mesoporous silica nanoparticles (DMSNs) coating was developed with the aim of delivering tumor necrosis factor-alpha (TNF-α) to cancer cell lines and dendritic cells. Polyethylenimine (PEI)-hydrophilic PEG copolymer was applied for coating. It was observed that applying this delivery system reduced systematic toxicity of TNF-α while retaining the pleiotropic antitumor activity.

The pH-responsive DMSNs were developed and loaded with folic acid [115]. The in vitro experiments proved the sustain release of cargo and the developed structure displayed a successful performance.

A two-step method for fabrication of platinum (Pt) NDs was developed [116]. In this technique Pd–Pt core-frame NDs including a compact array of Pt branches and a Pd core was fabricated. Pd cores were removed by selective wet etching and turned into vacant Pt NDs. Developed structure demonstrated remarkable antibacterial features against Gram-negative and Gram-positive bacteria and also hastens wound healing process in H₂O₂ low concentration. Balb/c mice, with wounds on their backs were applied for in vivo experiments. A complete cure of infected wounds was achieved in 6 days after injection.

Furthermore, as the improving theranostic agents, PEGylated Au@Pt NDs was introduced in unique X-ray CT and photothermal PTT/radiation therapy (RT) in cancer treatment [117]. It was observed that with Au and Pt, nanomaterials increased regional radiation dose which resulted in less damage in normal tissue. The nanocomposite consisted of Au as nanocore and Pt as nanobranches. This structure improved PTT and RT synergistically due to vast absorbance of NIR light and strong concentrate of X-ray. The developed NDs are summarized in Table 4.

Azizi et al. investigated the cytotoxicity of a folic acid functionalized terbium-doped dendritic fibrous NP (Tb@KCC-1-NH₂-FA) on various cell lines such as MDA breast cancer, HT 29 colon cancer, and HEK 293 normal cell lines. Even in concentration higher than 900 µg/mL no cytotoxicity was observed [118].

2.5 Nanoleaves

In the past few decades, growing interests have emerged in developing and studying two-dimensional nanostructures such as nanoleaves [119–124] due to their high anisotropy structure and unique properties. For example, Xu et al. [120] converted one-dimensional (1D) Cu(OH)₂ nanowires into 2D CuO nanoleaves. In their study, polycrystalline Cu(OH)₂ nanowires were converted to single crystalline Cu(OH)₂ nanoleaves using an oriented attachment. This process consisted of rotation, orientation, and attachment of Cu(OH)₂ NPs with a diameter of 3 nm to produce single crystalline Cu(OH)₂ nanoleaves. After that single crystalline CuO nanoleaves were obtained from single crystalline Cu(OH)₂ nanoleaves via a reconstructive transformation including the nucleation of CuO followed by a two-step oriented attachment of the CuO particles.

In 2019, Warren and LaJeunesse [121] proposed another approach to fabricate CuO nanoleaves. In this study bacterial cellulose (BC) nanofibers were applied as a substrate for deposition of CuO. Obtained CuO nanoleaves were non-uniform and unlike the solution phase CuO nanomaterial synthesis, the nanostructures synthesized by this method were stably integrated in the BC matrix and resist removal even with significant agitation. Another method proposed for fabricating nanoleaves is conventional hydrothermal method. In 2021 Ahmad et al. [122] synthesized hierarchical CuO nanoleaves via the hydrothermal method and developed a nonenzymatic glucose biosensor using engineered hierarchical CuO nanoleaves. The developed biosensor indicated acceptable sensitivity, and detection limit (12 nM) resulted from its excellent electrocatalytic properties. Moreover, it indicated great features such as excellent selectivity, reproducibility, stability, and repeatability in detecting glucose in low glucose level samples. Solid-state ionics method is another method proposed for fabrication of nanoleaves. Xu et al. [123] applied this method to synthesize nanoleaves based on copper. The obtained leaf had the width ranging from 2 to 5 µm. Moreover, numerous NPs with the approximate size of 10–70 nm arranged on the prepared copper nanoleaves resulted in high surface roughness. Also as a consequence of existence of Cu, many hot spots were formed; hence, the developed biosensor based on Cu nanoleaves was significantly accurate with the low limit of detection for CV and R6G. Li et al. [124] studied a novel method to synthesize the leaf-like CuO. In this method the CuO nanoleaves were grown uniformly on the FTO electrode to increase the contact area and decrease the transfer resistance. Moreover, NiO NPs dispersed in the gaps between the leaves enhanced the electron transfer and reduced the diffusion resistance. The results of the electrochemical evaluations confirmed the high sensitivity, broad linear range, and low detection limit of this sensor. Moreover, it indicated not only good reproducibility and stability, but also reflected anti-interference in the detection of actual samples.