Silver(I)-induced anisotropy in the plasmon-driven growth of nanoscale gold

ABSTRACT

The plasmon-mediated synthesis of anisotropic gold nanoparticles (AuNPs) has only recently been investigated and achieved. This process typically utilizes the surface plasmon resonances (SPRs) of the nanoparticles themselves to drive metal deposition and growth. Many of the traditional methods for shape control, including use of functional surfactants and etchants, have yet to be utilized within the context of light-driven synthesis. Here, the underpotential deposition of Ag(I) is utilized as a shape-controlling strategy in a hot electron-driven photochemical synthesis to grow anisotropic gold nanoparticles. The strategy also relies on L-pyroglutamic acid, a natural amino acid metabolite, as a photochemical relay aiding in the transport and localization of plasmon-generated hot electrons. By carefully adjusting reagent concentrations, the morphologies of the resulting nanoparticles can be facilely tuned from spheres to nanostars and nanoflowers. The high extinction in the NIR region of the ‘ideal’ gold nanostars synthesized advocates their use as photothermal therapy agents, supported by a sufficient photothermal conversion efficiency (η = 30.8%) under 808 nm CW laser irradiation. This contribution expands the repertoire of gold nanostructures accessible to visible-light-driven plasmonic photochemistry.

Different gold nanoparticle shapes have been accessed using a plasmon-mediated growth technique. Ag(I) ions induce symmetry breaking, yielding nano-sized gold spheres, stars, and flowers. A specialized photoreactor, equipped with 505 nm LED illumination, facilitates the transfer of hot electrons from a pre-made gold nanocube seed surface. Gold reduction and deposition occur, and anisotropic structures grow with the aid of L-pyroglutamic acid and Triton X-100 as coordinating and shape-directing agents.

GRAPHICAL ABSTRACT

KEYWORDS:

• Gold nanoparticles
• plasmon-mediated
• branched nanostructures
• underpotential deposition
• photothermal therapy

1. Introduction

The synthesis of anisotropic gold nanoparticles has garnered significance interest over the past two decades (1). Due to their unique optical and electronic properties and the applications that such properties allow for, particle morphologies such as stars (2), rods (3) flowers (4) and more are heavily researched. While the field is still relatively young, considerable progress has been made in understanding the synthetic techniques that enable the control of morphological shaping (5). Among the most intriguing methods for creating metal nanostructures is harnessing ultraviolet or visible light to manipulate size and shape during the nucleation and growth processes. While exploiting the distinctive surface plasmon resonance of other noble metal nanostructures has successfully led to the induction of anisotropic growth, the shape-controlled synthesis of gold nanostructures remains greatly limited in such methods (6,7).

Many anisotropic plasmon-mediated gold nanoparticle syntheses focus on the creation of gold prisms through the creation of highly energetic or ‘hot’ electron–hole pairs from the surface of prefabricated gold seeds (7,8). Irradiation of these seeds at wavelengths close to their localized surface plasmon resonance can result in the generation of hot electron/hole pairs. By utilizing a hot hole-sacrificial agent such as methanol, ethanol, formaldehyde, or citrate, recombination of the hot electrons and hot holes is inhibited (9). Meanwhile, the hot electrons generated have been utilized in photocatalytic reactions (10), can be transferred to nearby molecules creating hot carriers (11), and most relevantly, can be used to directly reduce metal salts (12). One of the challenges facing the plasmon-mediated growth of gold nanostructures is the incompatibility between the lifetime of the hot electrons returning to the metal Fermi level and the rate of HAuCl₄ reduction (13). Two possible pathways forward to overcome this barrier are to (i) pre-reduce the Au(III) salt structures to Au(I) and other prenucleation structures (14) or (ii) utilize a chemical species such as polyvinylpyrrolidone (PVP), which under acidic conditions induces an electrical adlayer on the nanocrystal surface capable of not only stabilizing hot electrons but also capturing negatively-charged gold-chloride salts to facilitate gold(0) deposition (8).

The typical methods for shaping and refining gold nanostructures into branching morphology involve the use of facet-selective growth inhibitors or etchants (5). One commonly employed technique is the underpotential deposition (UPD) of silver, which has been successfully used to create a wide variety of gold nanomorphologies. However, such a technique has not yet been demonstrated in the context of plasmon-mediated synthesis. In this study, we present the application of symmetry-breaking techniques, commonly utilized in traditional chemical reduction methods for gold nanoparticle synthesis, within a plasmon-mediated nanoscale synthesis. Specifically, gold nanocubes were prepared and used as seed templates for the photochemical reduction of free Au(III) ions by hot electrons generated through excitation near the nanocube surface plasmon resonance (SPR). This reduction was enabled by oxidative scavenging of the resulting hot hole by a short-chain alcohol (methanol) with use of L-pyroglutamic acid as an electrochemical relay. Stable, anisotropic gold colloids were successfully formed through the stabilization and shape-directing action of Triton X-100 (TX-100) and AgNO₃, respectively. By judiciously adjusting the concentrations of the reagents, we achieved a range of morphologies, including quasispherical nanoparticles, nanostars, and nanoflowers (Scheme 1). The gold nanostars were then examined for their potential as photothermal therapy agents.

Scheme 1. Summary of the shape dependence for the final products of plasmon-driven synthesis of gold nanostructures on the L-pyroglutamic acid (PGA), Triton X-100 (TX-100) and AgNO₃ concentrations in the aqueous growth solution. The product morphologies shown were obtained via irradiation of the growth solution with 505 nm LED light for 1 h. The central multipodal star was prepared using 2.44 mm PGA, 24 mm TX-100, and 1.28 µm AgNO₃ for a growth solution containing 0.2 nm Au nanocube seeds and 0.5 mm HAuCl₄.

2. Results and discussion

In seminal work, the Wei group conducted a significant study on the plasmon-mediated growth of gold nanoprisms (8). In that work, it was proposed that the γ-lactam ring (a five-membered cyclic amide) of polyvinylpyrrolidone (PVP) played a critical role in coordinating AuCl₄^– structures near sites of hot electron propagation as well as inducing anisotropic growth. In support of this, its monomer unit N-vinylpyrrolidone (VP) showed a similar function. Based on those findings, we hypothesized that other γ-lactam-containing molecules, including biomolecules more suitable for biomedical applications, might fulfill a similar role. L-Pyroglutamic acid (PGA), a naturally occurring cyclized amino acid derivative and a metabolite in the glutathione cycle, is one such molecule having biocompatible functionality. In fact, salts of PGA are in use as non-irritant humectants for skin and hair products and are also sold as nootropic dietary supplements. Initial syntheses using PGA instead of the previously used PVP reported by the Wei group (8) affirm the original hypothesis regarding the significance of the γ-lactam ring in the growth of anisotropic prisms (Figure S1), although further refinement in the synthetic procedure will be necessary to achieve uniform prisms. Interestingly, many prisms (200.2 ± 37.1 nm) synthesized with PGA are found to have triangular or polygonal shapes, in contrast to the hexagonal shape observed in the original PVP-based synthesis. This finding suggests that not only does the γ-lactam ring function as hypothesized, but the choice of reagent may have interesting morphological shape-directing capacities as well. Additional experiments demonstrated that L-pyroglutamic acid does not reduce nor form stable colloidal gold nanoparticles under any pH, temperature, or concentration conditions. This indicates that, like previous studies, these prisms are formed through the reduction of gold salt by hot electrons. To fully understand the role PGA plays in this synthesis, future work will include electrochemical and NanoSIMS experiments. Nevertheless, the use of PGA in such syntheses shows promise, and its exploration is further discussed in subsequent sections of this manuscript.

While there have been successful attempts at plasmon-mediated anisotropic growth of gold nanostructures, most of the research has been focused onmaterials with 2D-like characteristics. Other anisotropic structures, such as 3D rods, stars, flowers, andurchins,have yet to be achieved without the use ofchemical reducing agents. Consequently, the currentexperiments were designed to explore the plasmon-mediated growth of 3D anisotropic gold nanoparticles by employing commonly used shape-directing agents and surfactants. Each of the synthetic additions was carefully selected for its compatibility in either facet blocking of the cubic seed structure, its compatibility with the plasmon-mediated synthesis of gold nanoparticles, or both. Gold nanocubes were selected as seed templates instead of the more commonly used single crystalline or dodecahedral seeds, specifically due to their potential for facilitating anisotropic growth. Firstly, the presence of sharp convex corners results in significantly higher surface energy considerations compared to the rest of the structure, potentially making these morphological features more susceptible to targeted gold deposition. Secondly, the presence of low-energy facets on the faces of a cube allows for the utilization of reagents that specifically target these facets to block growth on those sites. To induce symmetry breaking, silver nitrate was added to the growth solution. It was discovered that the addition of Ag(I) alone led to unstable colloids; TX-100 was introduced as an additional surfactant to address this instability. Finally, a small-chain alcohol, methanol in this case, was incorporated as a hot hole scavenging agent.

The role of methanol in solution was crucial in demonstrating the hot-electron-driven reduction and deposition of free gold ions. While methanol has been demonstrated to act as a weak reducing agent in the case of the other noble metals for the formation of colloidal nanoparticles, it’s role in the synthesis of gold nanoparticles formed from HAuCl₄ reduction has been restricted to solvation and electron hole scavenging (14). In the absence of methanol, gold colloids were not formed, Au deposition was not observed, and HAuCl₄ remained in the Au(III) state, as evidenced by the intensity of the Au(III) UV peak observed in the growth solution both before and after 2 h of irradiation by 505 nm LED light (Figure S2). Although there was a slight decrease in the peak observed at 314 nm, indicating some reduction of Au(III), likely assisted by the hot hole scavenging ability of the citrate on the nanocube seed surface, the absence of a visible range plasmon band indicates the necessity for an additional scavenger to achieve successful colloid synthesis (12). It is possible that visible light irradiation under these conditions causes rearrangements to the gold nanocube precursors that cannot be identified through traditional UV-vis spectroscopy or transmission electron microscopy; as such, further advanced characterization of the seeds under these conditions will be necessary in future work. We examined the influence of incident light wavelength using a range of LEDs emitting between 448 and 720 nm, but effective plasmon-mediated colloidal growth was only observed for synthesis under 505 nm irradiation, a wavelength chosen to coincide with the SPR band of the gold nanocube seeds. The plasmon-mediated synthesis was further confirmed through water-suppressed ¹H-NMR, which revealed the formation of formaldehyde, the oxidation photochemical by-product of methanol, in the reaction solution after 1 h of irradiation using a 505 nm LED (Figure S3). Taken together, these results confirm that SPR excitation of the Au seeds is required to initiate the photochemical reaction and to ensure significant hot electron generation and that methanol acts as a sacrificial hot hole scavenger.

To help induce anisotropic growth, one of the most common shape-directing strategies in the synthesis of gold nanoparticles, the underpotential deposition (UPD) of Ag(I) ions, was utilized. Although the exact mechanism by which silver induces anisotropic growth is debated, it is understood that the addition of Ag(I) to seed-mediated nanoparticle syntheses in acidic media can cause symmetry breaking at nucleation sites, resulting in branch formation (15). Previous studies have demonstrated that even small adjustments to the amount of Ag(I) in a nanostar synthesis can significantly impact length and sharpness of the branches. Silver not only acts to inhibit isotropic growth but also to stabilize the base of branching structures (16). However, adding small amounts of silver nitrate (0–4 µm) to both PVP and PGA-based prism syntheses results in instability of the gold colloids, at all PVP or PGA concentrations tested (0–5 mM) (Figure S4). This suggests that the facet selectivity of the underpotential deposition of silver and the γ-lactam-containing surfactants are competing factors, although it should be noted that it is possible that this disruptive competition could be avoided at significant excesses of the PVP or PGA. At high enough surfactant concentrations, the PGA or PVP will kinetically outcompete the UPD of Ag(I) on the surface of the gold colloidal precursors. In these cases, the lack of observed colloids is not due to unstable formations caused by the addition of Ag(I) but by the extreme excess of γ-lactam-containing surfactants. However, we found that the introduction of an additional surfactant, TX-100, leads to the formation of stable colloids. To investigate the impact of silver concentration on the plasmon-mediated synthesis of gold nanoparticles, all other reactant concentrations were held constant. Briefly, 5 mL growth solutions containing nanocube seed (0.2 nm), HAuCl₄ (0.5 mm), PGA (2.44 mm), MeOH (0.5 mL), and TX-100 (24 mm) were supplemented with AgNO₃ (0.0–5.12 µm), resulting in a growth solution pH of 4.8. Notably, stable nanoparticles did not form in the absence of AgNO₃. While Ag(I) is commonly employed as a facet-selective symmetry breaking agent in the formation of anisotropic nanoparticles, it has been reported that silver deposition on the gold nanoparticle core helps to stabilize branching growth (16,17). With an increase in Ag(I) concentration, there is a corresponding increase in the UV-vis extinction of the particles in the NIR, highly suggestive of effective branching or anisotropic growth (Figure S5). Correspondingly, TEM images of these particles reveal the emergence of small-branched growths as the Ag(I) concentration increases, demonstrating the selective deposition of gold onto high energy facets during growth (Figure 1, Table S1). The continued irradiation by 505 nm light during this growth phase, resulting in continued hot electron generation at the highly energetic anisotropic features, additionally helps drive further gold deposition resulting in branched morphologies (18).

Figure 1. Transmission electron microscopy images of gold nanostructures synthesized through 1 h of plasmon-mediation under 505 nm light at various concentrations of AgNO₃ (A) 0.16 µm, (B) 0.32 µm, (C) 0.64 µm, (D) 1.28 µm, (E) 2.56 µm, (F) 5.12 µm in a 5 mL growth solution containing 0.2 nm nanocube seed, 0.5 mm HAuCl₄, 2.44 mm L-pyroglutamic acid, 0.5 mL methanol, and 24 mm Triton X-100.

At Ag(I) concentrations of 1.28 µm and higher, definitive gold nanostars are formed (Figure 1E,F). When the silver concentration is doubled from 1.28 to 2.56 µm, the core size of each nanoparticle stays relatively consistent at 48.7 ± 16.1 and 49.2 ± 18.7 nm, respectively. However, there is a corresponding increase in both branch length (37.6 ± 15.8 vs 46.6 ± 13.3) and branch tip width (9.3 ± 1.9 and 13.3 ± 3.2 nm), an effect of Ag(I) concentration observed in previous literature (19). The observed blue shift in the UV-vis spectra observed at Ag(I) concentrations higher than 1.28 µm may be caused by silver overgrowth, a plasmonic phenomenon observed in cases of transition metal-doping of nanoparticles (20). This is further supported by a significant increase in the core size of the particles synthesized at the highest concentration of Ag(I), 5.12 µm (55.9 ± 17.3 nm).

The resulting nanoparticle morphology was greatly influenced by the concentration of Ag(I), but stable colloids were not formed in the absence of TX-100. Previous studies on the synthesis of six- and seven-branched gold nanostars have demonstrated the gold-coordinating capabilities of TX-100 (16,21). Micellar TX-100 is capable of binding gold ions, while also controlling reduction rate, allowing for kinetic-dependent morphology control as well as potentially serving as a coordinator for supramolecular gold salt structures (22). Therefore, the impact of TX-100 concentration on the plasmon-mediated synthesis of anisotropic gold nanostructures was investigated. With concentrations ranging from 0 to 48 mm, the amount of TX-100 used well exceeded the critical micelle concentration (∼0.23 mm), ensuring that the surfactant is capable of encapsulating gold salt species. Keeping all other variables constant, the effect of TX-100 on the resulting structures was studied. Briefly, a growth solution of 5 mL total was created containing the cube seeds (0.2 nM), HAuCl₄ (0.5 mM), AgNO₃ (1.28 µm), PGA (2.44 mm), and methanol (0.5 mL), with 0–150 mg (0–48 mM) of TX-100. The samples were placed in a custom-built cooling chamber under 505 nm LED illumination for 60 min (Figure S6, Figure 2).

Figure 2. Transmission electron microscopy images of gold nanostructures synthesized through 1 h of plasmon-mediation under 505 nm light at various concentrations of Triton X-100, (A) 4 mm, (B) 8 mm, (C) 16 mm, (D) 24 mm, (E) 36 mm, (F) 48 mm in a 5 mL growth solution containing 0.2 nm Au nanocube seeds, 0.5 mm HAuCl₄, 2.44 mm L-pyroglutamic acid, 0.5 mL methanol, and 1.28 µm AgNO₃.

In the absence of TX-100, stable colloidal nanoparticles were not formed, which parallels the results observed when silver nitrate was added to the prism synthesis. Without either TX-100 or AgNO₃, gold prisms can form with PGA as the sole surfactant; however, to achieve stable gold colloids in their presence, both additional surfactants are necessary. When TX-100 is present at low concentrations (4 mm), the amount of free gold ions for PGA coordination is increased, leading to faster reduction kinetics and quasispherical gold nanoparticles (40.1 ± 10.1 nm) (Figure 2A, Table S2) (21). As the concentration of TX-100 increases, the availability of free gold ions decreases, and the {111} facets become passivated, leading to slower gold reduction and increased branch formation (Figure 2D). However, at significantly high concentrations of TX-100 (36 mm), the excess surfactant triggers secondary nucleation events (23), resulting in hyperbranching with a mean branch length of 52.4 ± 16.5 nm (Figure 2E). In the presence of a huge excess of TX-100 (48 mm, ∼200 × CMC) the high micelle concentrations force the assembly of the hyperbranched particles into a nanoflower-type morphology, comprised of aggregates of smaller nanostructures, approaching microparticle size (378.5 ± 53.4 nm) (Figure 2F). These findings demonstrate that the tuning of morphology in plasmon-mediated syntheses is influenced by not only traditional symmetry-breaking elements like AgNO₃ but can also be steered by surfactant self-assembly.

Similarly, the effect of L-pyroglutamic acid (PGA) concentration must be carefully examined. PGA, much like PVP in the synthesis of anisotropic gold prisms, is believed to function as a coordinator for gold salt and an electrochemical relay, enabling photochemical reduction by hot electrons. Attempts to synthesize gold nanoparticles using PGA as a chemical reducing agent under various experimental conditions of growth solution concentration, pH, and temperature proved unsuccessful in the absence of plasmon mediation, indicating that PGA itself is incapable of directly reducing the HAuCl₄ precursor under normal conditions. On the other hand, the current plasmon-mediated synthesis does not proceed in the absence of PGA. At lower concentrations of PGA, UV-vis analysis of the colloids shows weak extinction in the visible range and some intensity in the NIR, tentatively suggestive of anisotropic particles (Figure S7). As the concentration of PGA increases, so too does the extinction. Indeed, the role of PGA is speculated to involve coordinative enrichment of gold at the gold nanocrystal surface via PGA/AuCl₄^– complexation, facilitating reduction at the gold nanocrystal surface. A similar role was previously suggested for PVP used to photochemically grow gold nanoprisms, with the adsorbed PVP proposed to assist in the accumulation of hot electrons as a photochemical relay, arbitrating the incommensurate rates of hot electron decay (10¹²–10¹⁵ s⁻¹) and gold reduction (10³–10⁶ s⁻¹) (8). The protonated γ-lactam of PGA appears to offer a similar role as functional capping agent in the current photochemical synthesis. However, when the PGA concentration exceeds a critical concentration (>2.44 mM), the UV-vis spectra show reduced NIR extinction resembling that of more isotropic particles. This spectroscopic observation is correlated with the evolution in Au nanocrystal shape observed in TEM images (Figure 3, Table S3), where higher concentrations of PGA result in the formation of quasispherical particles, suggesting faster reaction kinetics due to the increased ability of PGA to concentrate gold salt near the seed surface.

Figure 3. Transmission electron microscopy images of gold nanostructures synthesized through 1 h of plasmon-mediation under 505 nm light at various concentrations of L-pyroglutamic acid, (A) 0.61 mm, (B) 1.22 mm, (C) 2.44 mm, (D) 3.66 mm, (E) 4.90 mm synthesized in a 5 mL growth solution containing 0.2 nm Au nanocube seeds, 0.5 mm HAuCl₄, 0.5 mL methanol, 1.28 µm AgNO₃, and 24 mm TX-100.

In contrast, lower and moderate concentrations of PGA yield highly anisotropic particles. These morphologies indicate a competitive reaction between PGA and TX-100. While coordination of the gold-salt by PGA is necessary for gold reduction onto the cube seed surface, as evidenced by the lack of gold reduction in its absence, the branching morphologies are formed through facet-selective coordination and slower kinetics afforded by micelle-coordination of the gold salt species.

The presence of high NIR extinction in certain morphologies of these particles suggests their potential use as photothermal (PT) therapy agents. To evaluate the PT effect, the temperature change of colloids with the branching morphology shown in Figure 2D (0.5 mL, 80 µg mL⁻¹) was measured over time during irradiation by an NIR laser (808 nm, 1.6 W cm⁻²) (Figure S8, Figure S9). Upon irradiation with the 808 nm laser, the aqueous colloids indeed exhibited a rapid increase in temperature, reaching a total temperature change (ΔT_max) close to 40°C. This degree of temperature change has been previously shown to be significant for inducing hyperthermia and cell death (6). After each on–off irradiation cycle, the maximum temperature achieved by the photothermal effect slightly decreased due to small amounts of particle deposition on the cuvette inner walls, attributed to the nature of the TX-100 surfactant. However, UV-vis analysis of the irradiated colloid solution showed no significant difference in the normalized spectra of the particles before and after three cycles of irradiation. These results indicate that these nanostructures exhibit efficient and robust PT conversion (η = 30.8%), making them promising candidates for photothermal therapy.

3. Conclusion

In summary, this study demonstrates the successful application of a symmetry-breaking technique for plasmon-mediated growth, resulting in the synthesis of 3D anisotropic gold nanoparticles utilizing gold nanocubes as seed templates. The addition of silver nitrate and Triton X-100 as shape-directing agents and surfactants, along with methanol as a hot hole scavenging agent, enabled the controlled growth of gold nanoparticles with different morphologies, most notably nanostars and nanoflowers. Such morphologies were facilely tuned through adjustments in the concentrations of reagents. Importantly, this investigation also introduces a novel application of L-pyroglutamic acid, a γ-lactam-containing molecule, as a biologically-compatible alternative to polyvinylpyrrolidone as a photochemical relay and structure-directing agent in plasmon-mediated methods. This discovery opens enticing possibilities for exploring other benign γ-lactam-containing molecules in similar roles. The ability to tailor the morphology of the gold nanoparticles to enhance near-infrared extinction highlights their potential as promising agents for photothermal therapy and possibly surface-enhanced Raman scattering (SERS). The use of a cubic precursor seed in this study was proposed due to the specific physicochemical advantages of the nanocube morphology, however, it is possible that similar anisotropy could have been achieved with quasispherical dodecahedral or bipyramidal seeds. To further determine the effects of seed precursor choice, additional work examining the effect of plasmon-mediated growth methods as a function of precursor morphology and crystallinity is planned. While further research is needed to fully elucidate the underlying mechanisms and to optimize procedures for achieving further tailored and uniform shapes, our findings expand the range of noble metal nanostructures available from plasmon-mediated nanocrystal growth and more generally showcase the merits of applying traditional wet colloidal synthetic strategies (e.g. crystal-face-blocking ligands) for manipulating the photochemical growth of nanocrystals.

4. Experimental section/methods

4.1. Materials

Gold(III) chloride trihydrate (≥99.9%), cetrimonium (cetyltrimethylammonium) chloride (CTAC), sodium borohydride, sodium bromide, L-ascorbic acid, sodium citrate dihydrate, Triton X-100 (TX-100), silver nitrate, L-pyroglutamic acid (PGA), polyvinylpyrrolidone (PVP, 40 kDa) were received from Sigma Aldrich, St. Louis, MO USA. Water used was obtained from an ultrapure ELGA Purelab flex water purifier, purified to a resistivity of 18.2 MΩ cm. Rebel 7 LED Round Modules were purchased from Luxeon StarLEDs by QUADICA, Lethbridge, Alberta, Canada. LEDs were controlled by a proprietary, custom-built control board. A cyan (505 nm) LED (122 lm @ 700 mA) was purchased for illumination. A Polymer Optics 7 LED Cluster Concentrator Optic (Part No. 263, LuxeonStar, Lethbridge, Alberta Canada) was utilized to focus output of the 7 Rebel LED into a single beam diameter that is 12 mm wide, 25 mm in front of the lens with an efficiency of 85%. A custom, ethylene glycol-cooled temperature control block was utilized to offset any heating effects from the LEDs themselves, in conjunction with a 130 × 70 mm Rectangular 40 mm high Alpha Heat Sink with a natural convection thermal resistance rating of 1.8°C/W (LuxeonStar) (Figure S10). 20 mL borosilicate glass scintillation vials (Fisher) were washed with aqua regia (3:1 HCl:HNO₃) overnight, followed by washing with ultrapure water and being fully dried before use.

4.2. Characterization

The UV-Vis extinction spectra for the gold colloids were measured at room temperature with a Cary 60 UV-vis spectrophotometer using a quartz cuvette. The transmission electron microscopy (TEM) images were collected from a JEOL JEM-1400 instrument operating at 120 kV. The sample was drop-casted onto carbon-coated copper grids (Ted Pella, Inc., Redding, CA) and allowed to dry for 24 h before analysis. The particle size distribution was calculated using ImageJ software. For each sample, 300 particles were measured to determine particle size distributions.

4.3. Synthesis of gold prisms

Gold nanoprisms were synthesized through a procedure modified from previously published literature (23). 6 mg of L-pyroglutamic acid or 5 mg of polyvinylpyrrolidone was added to 10 mL of fresh, ultrapure water and 1 mL methanol. 0.8 mL of 10 mL HAuCl₄ and 2 µL of gold seeds synthesized according to literature (23) were added and gently mixed. The sample was placed under 505 nm light for 2 h in a custom cooling block set to 20°C.

4.4. Synthesis of gold nanocube seeds

All particles were prepared in fresh, pure 18.2 MΩ cm water. Gold nanocubes of nominally 34 nm edge size were synthesized through adaptation of a previously published seed-mediated method (24). Quasispherical gold nanoparticle seeds were synthesized by the reduction of 10 mL of 0.25 mm HAuCl₄ in 0.1 m cetyltrimethylammonium chloride (CTAC) by 0.45 mL of 0.02 m ice-cold NaBH₄ under stirring (600 rpm) in a 20 mL borosilicate glass scintillation vial. Following borohydride addition, the solution was gently stirred (100 rpm) for 1 h at 30°C to remove excess borohydride. Using freshly prepared seeds, gold nanocubes were prepared (34.3 ± 4.9 nm). Two identical growth solutions, A and B, were prepared containing 0.32 g of CTAC (0.1 m final concentration) in 9.625 mL of deionized water. To each of the growth solutions, 0.25 mL of 10 mm HAuCl₄, 0.01 mL of 10 mM NaBr, and 0.09 mL of 40 mm ascorbic acid were added under stirring at 30°C. To vial A only, 100 μL of seed solution was added followed by 5 s of handshaking. A 25 μL aliquot of vial A was transferred to vial B under mixing for 10 s. Vial B then sat undisturbed at RT for 15 min before the resulting particles were centrifuged for 10 min at 8000 rpm. The supernatant was disposed of, and the particles were redispersed in 10 mL of 0.1 mm sodium citrate for 24 h before analysis by UV-vis spectroscopy and transmission electron microscopy (Figure S11). The concentration of nanocube seeds in synthetic procedures was determined from the initial gold salt concentration under the assumption of quantitative conversion.

4.5. Plasmon-mediated synthesis of anisotropic particles

All solutions were synthesized with a final volume of 5 mL. 0–100 µL of a 1000× dilution of the cube seed solution, 0–200 mg of TX-100, 0–50 µL of 0.1 mm AgNO₃, 245 µL of 10 mm HAuCl₄, 0.5 mL of methanol and 0–5 mm L-pyroglutamic acid were gently vortexed until TX-100 dissolved, then filled to the 5 mL mark with water. The scintillation vial was placed in front of a 505 nm LED for 1 h.

4.6. Photothermal conversion measurements

The photothermal conversion efficiency of the synthesized gold nanoparticles was measured by tracking the temperature change of a dilution of the aqueous colloids (0.5 mL, 80 µg/mL) over time during exposure to a near infrared laser (808 nm, 1.6 W cm⁻²) at a distance of 3 cm. The laser and cuvette containing the samples were contained in a custom 3D-printed holder to ensure experimental precision. A thermocouple was guided to the bottom of the cuvette, outside the reach of the laser beam itself. Temperature was recorded every 100 ms. Samples were irradiated with the 808 nm laser for 13 min, followed by an 18-min cooling period, repeated three times.

Disclosure statement

No potential conflict of interest was reported by the author(s).