Open access peer-reviewed chapter
Abstract
Different synthesis methods such as lithography, physical vapor deposition, layer-by-layer deposition, self-assembly, and laser irradiation are commonly used to deposit and arrange functional nanoparticles on a substrate. The properties of these hierarchically organized materials can be fine-tuned by controlling the shape, size, and crystallinity of the fundamental building blocks. However, achieving controlled organization of the building blocks in a desired architecture on a substrate remains a formidable challenge. This topic will discuss the most recent progress in self-assembly technique and challenges in achieving linear assembly of gold nanorods on a suitable substrate in one, two, or three dimensions and their impact on photonic applications.
Keywords
- self-assembly
- linear array
- gold nanorods
- different templates
- optoelectronics
1. Introduction
Gold nanorods (GNRs) are valuable materials for fundamental studies because of their stability, ease of synthesis using wet-chemical methods, controllable yield, and cost efficiency. Moreover, their shape, size, and crystallinity can be readily adjusted, allowing modification of their plasmonic properties. Consequently, GNRs are preferred for various applications such as sensors, surface-enhanced Raman spectroscopy (SERS), catalysis, and photothermal applications [1, 2, 3, 4, 5].
To harness the unique properties and controllable characteristics of GNRs, these particles are commonly assembled in a host material to create a hybrid material. This organization of particles on un-patterned/pre-patterned surfaces of substrates enables the modification of individual particle properties related to electronics, magnetism, and optics, opening exciting potential applications in sensors, catalysis, and biomedicine. As an illustration, consider the optical properties, such as the longitudinal surface plasmon resonance peaks, which are correlated with the local orientation of the GNRs. The axial interaction leads to a red-shift of the longitudinal plasmon resonance peak, whereas the lateral interaction leads to a blue-shift of the main plasmon band. It has been reported that the lateral orientation yields more pronounced spectral changes than the axial case [6]. Therefore, GNRs play a vital role as key materials and building blocks in the bottom-up fabrication of new materials, shaping the future of novel optoelectronic devices [7]. These devices exploit not only the collective behavior of the ensemble but also preserve the directionality and long-range order while controlling the spacing between them. Thus, it is crucial to understand the properties of GNRs along with their interaction with the supporting material to effectively utilize and enhance the functionality of such an ordered system [8].
Various methods have been employed to generate efficient, reliable, and sustainable micro/nanodevices on predefined geometries through self-assembly [3, 9, 10], layer-by-layer [11, 12], and physical vapor deposition [13, 14]. Here, we limit ourselves to the discussion of the self-assembly technique for the organization of particles in a linear array. The self-assembly is a simple powerful technique due to its easiness, low cost, flexibility, and reproducibility. Therefore, it is considered the most promising approach for organizing particles with the desired properties into higher-order structures [10]. However, the construction of organized arrays of GNRs with tangible functionality on a desired platform presents several challenges. These challenges include the quantum size effect of GNRs, choice of the host material, arrangement of nanoparticles within the pattern, nucleation onto the platform, grain boundaries, and the coupling among the rods [8, 15, 16]. Usually, particle size, shape, and composition can be adjusted during chemical synthesis. The coupling or interparticle spacing can be addressed by selecting capping or cross-linking molecules. The rod arrangement can be controlled via assembly or guiding through a patterned surface. These steps are mutually exclusive and do not affect one another. Thus, the strength of creating hierarchically organized materials through self-assembly lies in the independence of the building blocks and assembly process [8].
As mentioned earlier, the ensemble of GNRs displays distinct properties arising from the coupling of surface plasmons, and the interaction of electric or magnetic moments among particles collectively, or through external fields. Organizing GNRs into self-assemblies shows potential applications as electrodes for light-harvesting devices, electrochemical cells, conductive platforms for plasmon-enhanced imaging, solar cells, photodetectors, and plasmonic waveguides [3, 17, 18]. The bottom-up self-assembly approach miniaturizes modern optoelectronic devices while offering an alternative to conventional top-down nanodevice fabrication. Currently, self-assembled GNR arrays are primarily used for sensing biological or chemical agents and identifying molecules through SERS. Detection of biochemical components of these sensors relies on changes in UV–Vis or photoluminescence spectroscopy of plasmon wavelength peaks [19, 20]. Limited applications exist in the area of photothermal and cancer treatments. Compared to conventional sensors, GNR-assembled sensors demonstrate enhanced selectivity, sensitivity, and improved charge dynamics. In SERS detection, the array shows several times enhancement in Raman scattering compared to discrete gold nanorods [21]. The intensity of scattering and hot spot locations (points with the highest electromagnetic field concentration) is found to be sensitive to the shape and size of the GNRs [22]. Overall, the self-assembled GNR arrays hold great promise for advancing nanotechnology and finding practical applications in fields such as sensing, molecular detection, imaging, and medical treatments.
In the following section, we begin our discussion with the common host materials used in self-assembly. We then explore the strategies for GNR self-assembly, followed by its effect on optoelectronic properties, and potential applications of such ensemble.
2. Self-assembly across various platforms
2.1 Self-assembly on glass substrates
Glass is a popular host material due to its rigidity, low cost, and abundance of exposed surface OH groups. The optical transparency of glass substrates is another major benefit. To prepare the glass surface to bind the GNRs suspended in colloids, the substrates must first be treated with an acidic solution and then rinsed in siloxane. Thus, functionalized glass introduces chemical groups that can easily tie GNRs either covalently or electrostatically. Typically, a monolayer of GNRs is deposited using this method [23]. Thiosilane is a widely used molecule that can create uniform high-quality monolayers of GNR on glass substrates [24]. Similarly, a glass substrate modified with polyanionic polymers can facilitate the end-to-end assembly of GNRs [25]. An efficient, reusable, thermal, and photochemical GNR-catalyst assembly was created on glass through electrostatic interactions by treating glass with (3-aminopropyl) trimethoxysilane and polystyrene sulfonate [26]. Additionally, the self-assembly of GNRs can be performed directly in a bulk solution or on a glass surface. Despite the high yield of GNR dimers in both cases, assembling particles directly on the substrate was reported to be more flexible in controlling the shape and size of NR within the dimer. In addition, these GNR dimers can be used to enhance two-photon-excited fluorescence signals at the single-molecule level [22]. The evaporative self-assembly of noble metal nanorods into ordered structures on glass holds great promise for fabricating optical and plasmonic devices [10]. These studies suggest that glass plays a crucial role in the fabrication of hybrid nanostructures for plasmonic nanophotonics.
2.2 Self-assembly on metal
The assembly of GNRs on metal surfaces was similar to that on glass substrates. In both cases, the surface must first be functionalized for the adsorption of GNRs. The organization of GNRs on metals has shown superior quality in optoelectrical applications. For example, the assembly of a GNR core in Zr-based metal–organic framework composites exhibited a remarkable enhancement of SERS by a factor of two and eight times higher than that of ordered bare GNR arrays and random stacking of bare GNRs, respectively [21]. The self-assembly of GNRs on a Cu substrate has shown that it is possible to create thin films of one-, two-, and three-dimensional structures. Some of the three-dimensional assemblies can be extended to the superlattices of GNRs [9].
2.3 Self-assembly on silicon (Si) wafers
Si is a common material used for fabricating micro/nanostructured devices. The self-assembly technique, which is based on convective and capillary forces, was used to create standing arrays of GNRs on lithographically predefined areas of functionalized Si wafers. The hexagonal close-packed structure of GNRs thus creates an ideal substrate for SERS [27]. A wire-like superstructure was built by evaporative self-assembly of GNRs onto a functionalized 3-mercaptopropyl trimethoxysilane Si substrate. This procedure enabled the generation of single wire-like superstructures together with double and triple repeating wire-like arrays [28]. In another report, controlled self-assembly of gold nanoarrows was achieved through the bulk solution evaporation method on a Si wafer, resulting in nanoparticles with distinct packing and interlocking capabilities in two- and three-dimensional supercrystals. This approach enables the creation of tunable self-assembled complex superstructures on configurable architecture, offering unusual functionality for nanophotonics and metamaterials [29].
2.4 Self-assembly on carbon
The electrodeposition of gold structures onto carbon electrodes has potential applications in electrocatalysis. For instance, direct electrodeposition of Au nanostructures onto glassy carbon electrodes can be used for the non-enzymatic detection of glucose [30]. GNRs can be self-assembled into long fibers by inserting graphene quantum dots between rods, which is useful for optical properties. This is due to electrostatic interactions between the functional groups of CTAB and the amine groups of graphene [19]. Combining the plasmonic properties of self-guided GNRs with the superior electronic, chemical, and mechanical properties of single-walled carbon nanotubes (SWCNTs) creates multifunctional hybrid nanocomposites for optoelectronic applications. This was supported by drop-cast deposition of GNRs on SWCNT films/fibers, resulting in macroscopic arrays of ordered composites [17]. Layer-by-layer self-assembly of GNRs and glucose oxidase onto an SWCNT-functionalized three-dimensional sol-gel matrix for biosensor applications [31]. Similarly, the self-assembly of GNRs can form a ring-like structure on polystyrene microspheres at the fluid–fluid interface via dip coating followed by solvent evaporation [32].
Spontaneous self-assembly of GNRs functionalized with polyethylene glycol (PEG) has shown tremendous potential for bioimaging and photothermal [33]. A thermoplastic polymer mixed with a small amount of glycerol can be used as a matrix material for making GNR-composite films, which behave like a dichroic polarizer with a transmission axis perpendicular to the stretch direction [34]. The self-assembly of polymer nanocomposite films containing GNRs and poly (2-vinyl pyridine) has shown that, depending on the nanorod volume and thickness of the film, the particles are aligned side-by-side or end-to-end. This research demonstrated a strong correlation between nanoparticle dispersion and optical absorption, showing that side-by-side and end-to-end alignments induce blue and red shifts, respectively [35, 36, 37].
2.5 Self-assembly at metal/dielectric interfaces
Interface plasmons, which propagate along metal-dielectric film boundaries, have the potential to guide waves along patterned films, stripes, and nanowires. Additionally, this can enhance Raman scattering and nonlinear processes [38]. The self-assembly of polymer end-tethered GNRs into two-dimensional arrays with tilted structures has been shown to have tunable physicochemical properties. These properties are useful in photonics, electronics, plasmonics, and other applications [39].
The functionalization of GNRs with (11-mercaptoundecyl)hexa(ethylene glycol) and self-assembly using ethanol/water resulted in highly regular side-by-side arrays. This hierarchically structured material has sizes ranging from 200 nm to 10 μm and periodicities ranging from 400 nm to 20 μm. This structure has been demonstrated as a SERS sensor for biomolecule detection in water and complex biological media [12].
Incorporating octadecyltrimethoxysilane -functionalized, gold/silica (Au/SiO2) core/shell nanospheres and nanorods into the active layer of an organic photovoltaic device increased the photoconversion efficiency [40]. Surface plasmon-enhanced photoluminescence was also demonstrated in monolayer MoS2 in the presence of GNRs. However, the enhancement did not increase monotonically with an increase in the metal plasmonic treatment. Instead, emissions dropped after a certain coverage [12].
Once the functionalized surface of a suitable host material is prepared, appropriate assembly methods can be selected to create highly ordered materials. In this section, we explore different methods for organizing noble metal particles.
3. Approaches to self-assemble GNRs
3.1 Drop-casting (spin-coating)
Drop-casting is a versatile, cost-effective technique that enables nanoparticles to spread on a solid substrate for an ordered structure because by precisely controlling the evaporation of droplets on hydrophobic substrates, a large-scale ordered assembly of GNRs can be achieved (Figure 1). Upon solvent evaporation, dispersion forces between particles and the substrate or between particles facilitate the self-organization of materials into desired mono- or multilayer structures [8]. As an illustration, hydrophobic substrates were created by spin-coating silicone oil onto glass slides followed by annealing to create the desired arrays. These arrays exhibit immense potential for SERS in rhodamine detection [41].
Figure 1.
Influence of pre-evaporation and correlation between droplet appearance and the resulting assembly structure. Partial evaporation of EtOH leads to the pre-assembly of the AuNRs at the liquid-air interface, causing a golden shimmer at the droplet surface due to interparticle coupling (a and inset droplet photos). For identical initial composition and volume of the dispersion, contact with the target substrate at different times leads to distinct substrate qualities (b–d). The degree of pre-assembly directly correlates with the amount of particles deposited on the structured substrate area, where the underfilled surface is marked with a red frame, the optimally prepared sample with a green frame, and a vastly overfilled array with a blue one. Reprinted with permission from Ref. [
3.2 Langmuir–Blodgett technique
The Langmuir–Blodgett (LB) technique involves the self-assembly of molecules or other hydrophobic materials into an ordered monolayer at an air-water interface, followed by transferring the monolayer onto a solid substrate [42]. This method has been successful in creating highly packed two-dimensional arrays of GNRs with various structures by adjusting the surface pressure in the LB trough [34].
3.3 Cross-linking through molecules
Accurate particle linking is essential for bottom-up device fabrication, which can be achieved through molecular linkers [43]. These cross-linked molecules facilitate the formation of transport channels between individual GNRs, enabling collective optoelectronic behavior as a single entity. This technique generates not only GNR dimers but also trimers or higher-order alignments, allowing control over the shape and size of each rod in the final product [22]. Moreover, sandwiched molecules, nanoparticles, and molecules have been reported to exhibit superior charge transport. This is because of the combination of the electrical properties of nanoparticles and organic ligands, indicating the potential for large-area devices [44]. Electrical measurements conducted on connected GNRs via thiol end-capped oligophenylenevinylene solution for both before and after immersion in the solution, demonstrated a three-order-of-magnitude increase in conductance, highlighting molecular-mediated transport for nanoelectronics, and photonics [45]. GNRs synthesized by radiolysis were selectively end-functionalized by a fully conjugated thiol bearing a pendant terpyridine group; the addition of ferrous ions led to end-to-end one-dimensional self-assembly of the nanorods [46], and the linkers can create three-dimensional assemblies of covalently linked particles [47]. Varying the concentrations of these ligands can alter the organization of the group, as shown by Zhou et al. [48], who observed a change in the orientation of rods from end-to-end to side-by-side by adjusting the linker density.
3.4 External perturbation-assisted assembly
External stimuli, such as pH, temperature, light, magnetic field, and electric field, are commonly utilized to control the self-assembly of plasmonic particles. While pH, temperature, and light indirectly impact nanoparticles by primarily affecting linker molecules, electric and magnetic field-driven assembly directly influences particles through thermal gradients, magnetic dipole–dipole forces, and dielectrophoretic forces. In the case of field-guided self-assembly, surface modification is not necessary, making it easier to control GNRs. Field-driven assembly offers the potential for reversible control over particle arrangement, which may not apply in the former case [49].
3.4.1 Self-assembly by variation of pH
By changing the pH of the solution, GNRs can be self-assembled in an end-to-end fashion. Sethi et al. [50] demonstrated the self-assembly of GNRs into linear structures using cysteine. Cysteine controlled the assembly through thiol and amine attached to the tips of the individual NRs. End-to-end assemblies of nanorods were formed through cysteine mediation by varying the pH. The activity of cysteine was highest below the p
3.4.2 Self-assembly by laser irradiation
By irradiating a linearly polarized laser and utilizing the geometrically oriented attachment and self-assembly of GNRs single-crystalline Au nanowires can be synthesized [18] (Figure 2). Reinhardt et al. [52] employed laser simultaneously to reorganize and fuse GNRs to build nanowire arrays through directed assembly or self-organization on a Si-wafer. This process involves the initial creation of laser-induced periodic surface structures, followed by concomitant melting facilitated by optical heating, which promotes the formation of continuous structures, such as periodic gold nanowire arrays.
Figure 2.
A hypothesis for the plasmonic-mediated synthesis of single-crystalline Au NWs. A schematic illustration for the light-driven formation of Au NWs including the plasmonic-mediated oriented attachment of Au NRs (primary building blocks) and 3D self-assembly of the secondary, tertiary, and even higher-order building units, as well as the last stage of plasmonic heat treatment for recrystallization. Reprinted with permission from Ref. [
3.4.3 Self-assembly by variation of temperature and humidity
Temperature is a critical factor in the self-assembly of GNRs as observed in a climate chamber. According to Liang et al. [53], the optimal temperature for achieving nematic, curved end-to-end assembly and transition mode is 30° C– 50° C on a wrinkled template. Moreover, GNRs can be molded at room temperature using Au-Si alloy as feedstock [54]. By thermal annealing the GNRs after the evaporation of the solvent and gradually increasing the temperature from 20°C to 140°C, a quasi-continuous wire-like structure was obtained [28]. Rey et al. [45] also discovered that a higher yield of linear arrays for broader impact in nano-electronics, photonics, medicine, and sensors can be attained at the optimum temperature difference between the assembly and the dew point of the solvent. Similarly, humidity also plays a role in the arrangement of GNRs. In low humidity, GNRs tend to form the nematic arrangement, while in high humidity, GNRs prefer the curved end-to-end fashion as mentioned in ref. [54].
3.4.4 Self-assembly under magnetic field
Self-assembled arrays of GNRs can be created using a magnetic field by exploiting the magnetic behavior of bare plasmonic particles or by reincorporating superparamagnetic materials. An external magnetic field must be used to manipulate or reorient the assembly. For instance, in the case of pristine GNRs, the assembly process involves partial capping with either laterally or terminally attached nematic liquid crystals. Subsequently, the incorporation of organic matter on TEM grids was performed, which caused the orientation of rods to be quasi-planar to vertical under the influence of an external magnetic field [55]. Similarly, in another study, dispersed GNRs were self-aligned through cylindrical micelle self-assembly in nematic and hexagonal liquid crystalline phases of anisotropic fluids (Figure 3). In this case, the external magnetic field along with shearing is responsible for the alignment and realignment of the liquid [56].
Figure 3.
(a) Schematic illustration of GNR alignment in the nematic liquid crystal (LC). (b) GNR alignment in a columnar hexagonal lyotropic LC phase. The diameter of micelles is 2–3 nm. GNRs have diameter within 15–25 nm and length of ˜ 50 nm. Reprinted with permission from Ref. [
Theoretical [57] and experimental [58, 59] studies have shown that magnetic-field-assisted self-assembly of one-dimensional chains of colloidal Fe3O4@Au is possible for optical, photothermal, and SERS detection. The volume fraction and surface charge of plasmonic particles play a significant role in the formation and distribution of chains. As a chain forms, its absorption peak redshifts and approaches that of an infinite chain, while photothermal transduction concentrates at its center. Rizvi et al. [60] demonstrated the magnetic alignment of GNRs covered with Fe3O4 nanoparticles carbon-coated Cu substrate and colloidal solution. The magnetic dipolar interactions among the neighboring core-shell structures give rise to magnetic anisotropy, which causes the composite material to align parallel to the applied magnetic field, thereby modifying the surface plasmon resonances. These composite materials exhibit useful magnetic properties while preserving their optical properties. The magnetic-field-induced alignment was sensitive to the core-shell size, and saturation could be achieved under a low magnetic field.
3.4.5 Self-assembly under electric field
An alternating electric-field-assisted assembly can be used to position individual nanowires suspended in a dielectric medium between two electrodes defined lithographically on a SiO2 substrate [61]. An external electric field can control the orientation and direct the self-assembly of anisotropic Au nanoparticles into highly organized two- and three-dimensional assemblies for optical applications [62, 63]. The assembly of polystyrene-tethered AuNRs was confined and reoriented in a one-dimensional cylindrical system with the assistance of an electric field for dual applications in optoelectronics and magneto-storage [64] as shown in Figure 4. Droplet evaporation and application of dielectrophoretic forces have been employed to self-organize GNRs into ordered arrays into end-to-end lines of one-dimensional, side-to-side fashion for two-dimensional or hexagonal arrangements for three-dimensional [65].
Figure 4.
Illustration showing the strategy for confined assembly of polymer-tethered AuNRs in anodic aluminum oxide Cylindrical Channel with the assistance of electric field. Reprinted with permission from Ref. [
3.4.6 Capillary self-assembly or template-assisted self-assembly
An alternative pathway to position and orient nanoscale particles on a target substrate involves directed pathways, such as capillary or template-assisted methods, either individually or in combination. Capillary assembly involves casting a controlled drop of a colloidal solution onto a structured template to organize particles at predetermined sites. Microscopically, particles at the assembly sites experience capillary forces from the liquid-air interface. These forces apply for a moment as the contact line is pinned at the assembly site. Therefore, for successful assembly, particles must be present at the assembly site during a capillary breakup [66] (Figure 5). Evaporative self-assembly of noble metal nanoparticles into ordered structures by strong unidirectional microflow in the capillary of a monolayer membrane in glass cells [10]. Similarly, capillary-force-assisted self-assembly of AuNPs into highly ordered plasmonic thin films could be useful for ultrasensitive SERS [67].
Figure 5.
Capillary assembly and experimental setup. Nanorods are assembled in holes at the three-phase contact line by capillary forces. The silicon slide and the colloidal drop are stationary while the PDMS template is moved to the right. Reprinted with permission from Ref. [
In template-assisted assembly, GNRs are assembled into restructured surfaces, particularly polymers, to preserve their positions and orientations [68]. Hierarchical self-assembly of GNRs into patterned nanostructures, thereby achieving high control over supercrystal morphology and tuning the topography of the patterned substrate on the millimeter scale for SERS [69].
4. Optoelectronic applications of self-assembled GNR-arrays
4.1 Sensors
The organized ensemble of GNRs has enormous potential for fabricating high-performance enzyme-based biosensors. For instance, in ref. [31], a novel amperometric glucose biosensor was fabricated using layer-by-layer self-assembly of GNRs and glucose oxidase onto SWCNTs functionalized three-dimensional sol-gel matrix. These sensors demonstrated high sensitivity, fast recovery, and response time as compared to conventional biosensors. Moreover, another study [50] showed that this assembly approach could be applied to larger biological species like peptides and proteins.
4.2 Luminescence
A localized surface plasmon of GNRs around the monolayer of MoS2 resonates with MoS2 gain spectrum. The spatial and spectral overlapping of the localized surface plasmon polariton and waves emitted from MoS2 thus further enhance the light emission from the MoS2 monolayer. This approach couples GNRs with two-dimensional nanomaterials to boost emission efficiency for future optoelectronic devices [20]. In another photoluminescence study, long fibers of GNRs self-assembled on graphene demonstrate modification of the absorption peak along with photoluminescence quenching over time. This unique arrangement of GNRs on graphene shows potential for biomedical-based nanoscale devices [19].
4.3 Electrical applications, photovoltaics and charge dynamics
Gold nanowires, formed on a Si-substrate using an electric-field assisted assembly technique, exhibit room-temperature resistivities of approximately 2.9 and 4.5 × 10−6 ohm-cm [61]. This indicates the suitability of the composite for electrical devices. Additionally, the coupling of external fields and cylindrically confined self-assembly holds promise for generating ordered hybrid assemblies with hierarchical structures. These structures have potential applications in photoelectric devices, biosensors, and data storage devices [64]. If the self-assembled GNRs are sandwiched between a double layer of alkanedithiol linkers, this could enhance conductance, as measured by current–voltage curves. The presence of GNRs offers more efficient electron transport pathways in this complex, hierarchically arranged material. Therefore, the self-assembly of gold nanoparticles can be beneficial for photovoltaic devices by controlling the charge dynamics between different layers of constituent materials [44]. Similarly in ref. [40], the incorporation of self-assembled GNRs with different aspect ratios into organic photovoltaic active layers enhances LSPR near-field absorption over a wide range of wavelengths. This adjustment has shown enhancement in photoconversion efficiency and short-circuit current of two organic photovoltaic polymer systems.
4.4 Photothermal/cancer treatment
Despite limited research on self-assembly of GNRs, the complex arrangement has demonstrated its potential for photothermal cancer treatment [33]. A self-assembled DNA origami-GNR complex has also shown promise for cancer theranostics [70]. Additionally, self-assembled thermal GNR-loaded thermosensitive liposome-encapsulated ganoderic acid has proved effective in cancer photochemotherapy [71].
4.5 SERS
Self-organization of GNRs has significant potential in SERS. Hybrid nanostructures of plasmonic nanocrystals caged in a metal–organic framework exhibit superior SERS sensitivity [21]. The orientation of the GNRs within the arrays plays a critical role in SERS performance. For instance, vertically ordered GNR arrays show higher SERS signals than tilted arrays [39]. Moreover, the face-face stacked orientation yields larger SERS enhancements for Rhodamine 6G detection [48]. The lattice periodicity also influences molecule detection. Decreasing the lattice periodicity can enhance the signal significantly [12].
To broaden the application of SERS, controlling the chemical kinetics of droplet evaporation might be useful to alter packing arrangements while improving the mechanism behind SERS [41, 63]. Additionally, an optimum surface coverage of more than 60% ensures higher efficiency of SERS and superior accumulation of hot spots. Too much particle aggregation deteriorates SERS performance [42, 43]. The electromagnetic hot spots localized in linear assemblies of GNRs can be controlled by monitoring the pH of the assembly in the presence of L-cysteine for minutes to hours [51]. Preparation methods of the assembly could also influence SERS-based performance as in probe molecule rhodamine 6G [63, 67, 69]. Coating GNRs with magnetic materials and supplying an external magnetic field can improve the efficiency of SERS by two orders of magnitude [58].
5. Conclusion
In summary, a bottom-up approach for GNR assembly on patterned surfaces can potentially yield a highly organized, multifunctional nanostructure with tunable properties. In this chapter, we highlighted recent studies on creating GNR-ensemble to exploit their collective behavior for optoelectronic devices. Surface functionalization of rods or host materials and coupling of nanorods through physical and chemical processes are crucial for generating elegant devices. Despite progress in the self-assembly of GNRs and understanding the underlying mechanisms, large-scale production with reproducible results and controllable nanogaps between the rods remain significant challenges in this field. Furthermore, investigating solvent evaporation and drying kinetics is also essential for a better understanding of the fabrication mechanism of hierarchically arranged nanostructures [3, 72, 73]. Efforts should be made to assemble structures comprising anisotropic shapes, such as triangles, plates, stars, or multipods to explore the broader implication of the anisotropic properties of gold particles.
Acknowledgments
P.C. likes to acknowledge the financial support from the NASA EPSCoR prime award # NNX15AR71H (SAU2908 and SAU27014) to initiate the gold nanoparticles research at SAU.
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