Accelerating gold nanorods synthesis with nanomolar concentrations

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Accelerating Gold Nanorods Synthesis with Nanomolar Concentrations of Poly(vinylpyrrolidone) Katherinne I Requejo, Anton Liopo, Paul J. Derry, and Eugene R. Zubarev Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02942 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Accelerating gold nanorods synthesis with nanomolar concentrations of poly(vinylpyrrolidone) Katherinne I. Requejo‡, Anton V. Liopo‡, Paul J. Derry†, and Eugene R. Zubarev*

‡

These authors contributed equally to this work. *Corresponding author: e-mail: [email protected] ORCID: 0000-0002-6401-3287 Department of Chemistry, Rice University, Houston, Texas 77005, USA. † Current Address: Department of Neurology, Baylor College of Medicine, 7200 Cambridge St., Suite 9A, Houston, Texas 77030, USA. [email protected]

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ABSTRACT A novel modification for the seedless synthesis of gold nanorods (AuNRs) has been developed. Nanomolar concentrations of 10 kDa poly(vinylpyrrolidone) (PVP) can be introduced to a growth solution containing 25, 50 or 100 mM CTAB to significantly reduce the dimensions of AuNRs. We found that PVP accelerates the growth rate of AuNRs by more than two times for nanorods grown in 50 and 100 mM CTAB solution. Additionally, there is a time-dependent effect of adding PVP to the nanorod growth solution that can be utilized to tune their aspect ratio. Because the concentration of PVP is far below the concentration of HAuCl4 in the reaction mixture, PVP primarily functions not as a reducing agent, but as a capping or templating ligand to stabilize the growing nanorods. Our reproducible protocol enables the synthesis of AuNRs in high yield with tunable sizes: 45 x 6.7 nm, 28 x 5.5 nm and 12 x 4.5 nm for 100 mM, 50 mM and 25 mM CTAB, respectively. We estimated the number of PVP chains per nanorod in growth solutions to be around 30 which suggests that the effect on aspect ratio is caused by a direct interaction between the AuNRs surface and the PVP.


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INTRODUCTION Gold nanorods (AuNRs) have unique optical properties with many potential applications in imaging, sensing, therapy, catalysis, photovoltaic devices, and light emitting diodes.1–7 AuNRs have attracted significant interest as a novel platform for nanobiotechnology and biomedicine because of convenient surface bioconjugation with molecular probes and remarkable optical properties derived from localized surface plasmon resonances (SPR), or a collective oscillation of surface electrons generated by the absorption of light of the resonance wavelength.8–13 Moreover, the release of heat after irradiation of nanorods with a specific laser wavelength has enabled their biomedical application as photothermal agents for the disruption of cancer cells.12,14–17 The longitudinal SPR (LSPR) band wavelength depends primarily on the aspect ratio (length/width) and can be tuned from the visible to the near infrared (NIR) region by increasing the aspect ratio.14,18 Many AuNRs synthesis procedures have been reported which have distinct dimensions and crystalline structures. However, variations of the seed-mediated synthesis represent the most common and efficient approach.19,20 In 2003, El-Sayed et al. presented an improvement in the seed-mediated protocol by using cetyltrimethylammonium bromide (CTAB)-capped gold seeds and silver nitrate in the growth solution.21 To synthesize high aspect ratio AuNRs, a binary surfactant mixture composed of CTAB and benzyldimethylhexadecylammonium chloride (BDAC) was employed.22 The addition of non-ionic surfactants such as Triton X-100 and Tween, as well as electrolytes such as NaCl and KCl to the growth solution have also been reported.23 Murray and coworkers showed an improvement in monodispersity and aspect ratio tunability by using aromatic additives and binary surfactant mixtures composed of CTAB/sodium oleate or CTAC/sodium oleate.24–26 In binary surfactant mixtures, reduced


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concentrations of CTAB or CTAC (37 mM) were used to obtain AuNRs longer than 30 nm or smaller than 25 nm, respectively. In 2013, hydroquinone was reported as a novel reducing agent by Vigderman et al. along with a near quantitative conversion of gold ions to metallic gold and LSPR wavelengths up to 1230 nm.27 A modified, seedless, version was described by Jana et al. in 2005 where NaBH4 was directly injected into a growth solution containing gold ions, silver nitrate, CTAB, and ascorbic acid.28 However, this method produces highly polydisperse AuNRs with a large fraction of spherical particles. In the seedless synthesis with ascorbic acid, the aspect ratio can be tuned from 2 to 5 by varying the concentration of NaBH4.29,30 Later, El-Sayed and coworkers31 showed that decreasing the solution pH with HCl could produce nanorods smaller than those obtained with conventional seeded syntheses. Conversely, another study32 demonstrated the one-pot synthesis of AuNRs with acetylacetone at pH 10. Increasingly, di- and trihydroxybenzene derivatives such as catechol (1,2-dihydroxybenzene), resorcinol (1,3dihydroxybenzene)33, dopamine (3,4-dihydroxyphenethylamine)34, 1,2,4-trihydroxybenzene35, pyrogallol36 and 5-Bromosalicylic acid37 have been used to further improve the monodispersity and control the nanorod dimensions in the seed-mediated and seedless syntheses. Recently, dopamine was used as both a reducing agent and a stabilizer whereby the CTAB concentration could be reduced to 22 mM from 100 mM with tunable LSPR wavelengths between 700 and 1050 nm in high yield.38 The water-soluble polymer poly(vinylpyrrolidone) (PVP) has been utilized as a surface stabilizer, growth modifier, nanoparticle dispersant, and reducing agent amongst its many other applications.39,40 The PVP chain is somewhat amphiphilic with hydrophilic pyrrolidone moieties and a hydrophobic backbone.


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PVP has historically been utilized as a surface protecting agent for metal nanoparticle preparation41–43 and has been demonstrated to be capable of complexing and stabilizing metal nanoparticles (NPs).44–46 The interaction between metal colloids and PVP is an important factor that influences the stability of colloidal nanoparticles and their physicochemical properties.40 Furthermore, PVP is widely used as a stabilizing and shape-directing agent in the polyol synthesis of NPs composed of plasmonic (Ag, Au, Cu), catalytic (Pd, Pt), magnetic (Co, Ni) and bimetallic compositions (Au–M, Pt–M). PVP (40 kDa) has been also shown to function as a weak reducing agent for the synthesis of noble metal nanoplates.47 The most common application of PVP, however, is to function as a reducing agent through high-temperature polyol-type syntheses of Co, Ni, Pb, Ag, Cu, Pd and alloys of these metals to produce particles with narrow size distributions.48–50 Early studies49,50 used a PVP-ethylene glycol blend to synthesize monodisperse spherical gold particles by reduction of tetrachloroauric acid. The interaction of PVP in this case is complex because it serves not only as a reducing agent, but also as a stabilizing agent.49,50 Various gold nanostructures, such as decahedra, icosahedra, and plate-like particles, have been synthesized through the heating of an aqueous PVP/HAuCl4 solution, with the ratio of polymer OH end-groups to Au ions identified as an important parameter for controlling the particles shape.51 PVP is also capable of serving as a reducing agent for metallic salts at low temperature without the addition of any other reducing agent in aqueous solutions to produce gold and silver colloids in a one-step process.39,52 In all of the nanoparticle syntheses that utilize PVP, the PVP:metal salt ratio is typically very large, however, in these cases the role of PVP is primarily to function as a reducing agent. The potential use of PVP as a shape-directing additive has not yet been reported for the aqueous, low-temperature synthesis of AuNRs. Therefore, in this investigation, we report a novel protocol


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that utilizes PVP (10 kDa) as shape-directing additive for the seedless synthesis of AuNRs in a tunable and a controllable fashion.

EXPERIMENTAL SECTION Seedless synthesis of gold nanorods (AuNRs). In a typical synthesis, 25 mL of CTAB aqueous solution (50, 100 or 200 mM) and 25 mL HAuCl4 (1 mM) were transferred to a 250 mL Erlenmeyer flask and the mixture was hand-stirred. After 15 min, 250 µL of AgNO3 solution (100 mM) was added to the flask and hand-stirred followed by the addition of 2.50 mL of hydroquinone (50 mM). Next, the stirring rate was set to 600 rpm and 75 µL of aqueous NaBH4 (10 mM, ice cold) was added in a single injection to the flask. After 1-2 min, the stirring was stopped and the reaction mixture was kept at 27 °C overnight. The as-synthesized AuNRs were purified twice by centrifugation at 15,000 rpm for 30 min and redispersed into 10 mM CTAB. Seedless synthesis of AuNRs with poly(vinylpyrrolidone) (PVP). The effect of 10 kDa PVP on the growth of AuNRs was investigated using the protocol described above but with aqueous solutions of PVP introduced before and after the addition of NaBH4 to give concentrations of 0.05, 0.25, 1.25 and 5.0 mg/mL PVP in growth solutions with a 50 mM CTAB concentration. For the course time experiment, each growth solution containing CTAB (25, 50 or 100 mM), HAuCl4, AgNO3, and HQ was split into equal volumes (10.5 mL per batch) and 15 µL of NaBH4 was added in a single injection to each flask and stirred at 600 rpm for 1-2 min. After stirring, an aqueous solution of PVP (0.05 mg/mL) was added to each flask at 1, 30, 60, and 90 min to achieve final PVP concentrations of 0.05, 0.1 and 0.2 µg/mL for each concentration of CTAB (25, 50 and 100 mM). A set of control samples was prepared with the monomer vinyl pyrrolidone (VP) for 100 mM CTAB. An aqueous solution of VP (1.8 µM) was added to each


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AuNR growth solution at 1, 30, 60, and 90 min to create a mixture containing an equivalent number of VP monomer units as the solutions with 10 kDa PVP. The reaction mixtures were hand-stirred for 30 s after addition of PVP or VP and kept at 27 °C overnight. The as-synthesized AuNRs were purified twice by centrifugation at 15,000 rpm for 30 min and redispersed into necessary volume of 10 mM CTAB. Kinetics of hydroquinone synthesis of AuNRs without and with PVP. The growth kinetics of AuNRs synthesized with different concentrations of CTAB (25, 50 and 100 mM) were monitored using a Cary 5000 UV-vis-NIR spectrophotometer (Varian) equipped with a temperature-controlled cuvette holder set to 27 °C. The AuNR growth solution was prepared as described above and an aliquot (1 mL) was placed in the spectrophotometer immediately after initiation of the reaction by NaBH4. A similar set of experiments was performed for solutions containing PVP (0.2 µg/mL for 25, 50 and 100 mM CTAB). Spectra were collected between 400 and 1300 nm every 10 min for 12 h with 1 nm resolution. Two-dimensional time-wavelengthintensity plots were produced using OriginPro (Origin Lab) with interpolated values between each time interval over the course of 720 min with each spectrum collected every 10 min. Characterization. UV-vis spectra were obtained using poly(methylmethacrylate) cuvettes (1 x 4 cm) on a Cary 5000 UV-vis-NIR spectrophotometer (Varian). Transmission electron microscopy (TEM) was performed with a JEOL-1230 (JEOL) instrument operating at 80 kV. AuNRs size distribution calculations (200 nanorods) and TEM analysis were performed using ImageJ (National Institutes of Health). Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES) Analysis. A final volume of 2.5 mL of AuNRs solution prepared from 5.0×10-6 mol HAuCl4 was used after synthesis and centrifugation. 100 µL of the sample was diluted in Milli Q water (up to 1 mL) and


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centrifuged at 15 000 rpm followed by the addition of 1 mL of acetone. The precipitated AuNRs were rinsed carefully with Milli-Q H2O (18 MΩ) to remove residual CTAB and acetone and were then dried under a nitrogen stream. The resulting AuNRs were digested with aqua regia (1:3 HNO3:HCl) and diluted to 10 mL as a stock solution. ICP-OES analysis was performed on a 10:1 dilution of the stock solution with a Perkin-Elmer Optima 8300. Confirmation of surface chemistry of AuNRs with attenuated total reflectance (ATR). All solutions of AuNRs were first purified of excess CTAB and PVP by centrifugation and the pellet was redispersed in 50 µL water. ATR sample analysis was performed by placing one drop of concentrated AuNRs solution (without and with PVP) onto the diamond surface of the instrument (Nicolet FTIR Infrared Microscope). The samples were dried before the measurement. CTAB and 10 kDa PVP (10PVP) powders were used directly without further treatment as control samples. Each spectrum was collected after 128 scans at the highest resolution (32 cm-1).

RESULTS AND DISCUSSION A series of control experiments were performed to determine the effect of reducing the concentration of CTAB for AuNRs seedless synthesis on the reaction kinetics, particle morphology, and light absorption. These reactions were performed by preparing growth solutions containing 0.5 mM HAuCl4, 0.5 mM AgNO3, 2.5 mM hydroquinone, and either 100, 50, or 25 mM CTAB. The reactions were initiated by direct addition of 75 µL of 10 mM NaBH4. Immediately, after initializing the reaction, a cuvette containing the active growth solution was placed in a thermostatic UV-vis spectrophotometer and full spectra were collected every 10 min. Figure 1 shows the evolution of the absorption spectra of the nanorod solutions grown with


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hydroquinone and 100, 50, or 25 mM CTAB. Two distinct growth phases were observed as indicated by the arrows in Figure 1A, B, and C. The two phases are most pronounced with 100 mM CTAB. First, the LSPR band undergoes a red-shift from 600 to 1030 nm during the first 4-5 h. The second phase is a marked increase in absorbance with little shift of the plasmon wavelength followed by a slight blue shift in the very late stages of the reaction. The gradual redshift in LSPR band was also observed in the growth solutions prepared with 50 mM CTAB (Figure 1B), however, the maximum LSPR wavelength is reached in a substantially shorter duration than with the 100 mM CTAB growth solution. The LSPR band remains constant after 4 h or approximately half the time of the 100 mM CTAB growth solution. The reaction kinetics of AuNRs synthesized with 25 mM CTAB is faster than 50 mM and the intensity of the LSPR band is stable at 90 min (Figure 1C, S1). The concentration dependent kinetics and final LSPR wavelength can be attributed to a decrease in the concentration of CTAB. Reducing the concentration of CTAB increases the diffusion constant and decreases the templating effect of the CTAB on the growing nanorods. These observations are in agreement with previously reported kinetics experiments for seed-mediated and seedless AuNRs syntheses.27,38,53 The use of PVP as an additive has not been reported, but it has been widely utilized as a capping agent for the one-pot and seed-mediated syntheses of anisotropic gold and silver nanostructures at high temperature.40,54–57 In these protocols, a high concentration of 10 kDa PVP or higher molecular weight is employed to direct anisotropic growth. PVP has been reported to act as a weak reductant due to the hydroxyl end groups.56 In the case of the synthesis of gold nanostructures, the molar ratio of PVP to gold ions, in terms of repeating unit, is usually higher than 5 and is several thousand between PVP and the number of gold nanoparticles.


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We investigated the possibility of PVP serving only as a stabilizing agent during the growth of AuNRs by using low PVP:HAuCl4 ratios. Our initial experiments involved the study of PVP concentrations between 0.05-5.0 mg/mL in 50 mM CTAB while using hydroquinone as a reducing agent. These initial experiments gave extremely poor yield as shown in the UV-vis spectra in Figure S2. The large abundance of spherical particles (as shown by the UV-vis spectrum in Figure 2) may be attributed to the 4.5 molar excess of PVP to HAuCl4. However, when the concentration of PVP is reduced to 0.1 µg/mL, AuNRs are produced but have a lower LSPR wavelength (804 nm) than the control batch (50 mM CTAB without PVP) (Figure 2). TEM images of AuNRs after overnight growth in 50 mM CTAB show that the addition of PVP does not markedly reduce the quality of the nanorods (Figure S3, B and C). Despite reducing the overall aspect ratio of the nanorods, 5.0 ± 0.82 to 4.2 ± 0.51, the percent standard deviation decreases from 16.4% to 12.1% and thus is a narrower size distribution. The aspect ratio modulation effect observed with PVP is likely not due to reduction of Au(III) to Au(I) because the concentration ratio of PVP: Au(III) is very small (< 0.02). The effect of PVP (100 mM CTAB and 0.2 µg/mL PVP) on the growth dynamics and shift of the LSPR band are shown in Figures 3 and 4. To better visualize the differences between AuNRs grown in 100 mM CTAB with and without 0.2 µg/mL PVP, we produced twodimensional time-wavelength-intensity plots with the normalized intensity plotted as color (Figure 3). The absorbance of the reaction mixture containing PVP increases faster than the control, but the LSPR band is at a lower wavelength and remains at a lower absolute absorbance through the course of the experiment when compared to the control. The wavelengths of the LSPR for both samples are shown in Figure S4. The evolution of the LSPR band in the PVPcontaining solution (blue line) is faster, but does not reach the same maximum wavelength as the


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control solution (100 mM CTAB without PVP, red line). The change in absorbance of the LSPR band for the PVP-containing solution has a shorter lag phase than the control solution, but reaches a lower maximum absorbance in approximately half the time of the control (Figure 4, S4). This study was continued with 50 mM and 25 mM solutions containing PVP (0.2 µg/mL) and a similar rapid growth with lower overall absorbance was observed. ATR analysis confirmed that PVP was present after the synthesis on the surface of AuNRs (Figure 5). The ATR spectrum of the PVP sample (10PVP) showed a strong peak around 1660 cm-1 while the CTAB sample lacked this distinctive feature. The ATR spectrum of AuNRs sample grown with 0.2 µg/mL PVP confirms that PVP interacts with the gold surface through the carbonyl group. AuNRs control does not have any absorbance at 1660 cm-1. An additional sample of AuNRs that were synthesized with PVP in the growth solution was measured to determine the presence of PVP added after the synthesis (AuNRs 50 µg/mL PVP). The intensity of the peak at around 1660 cm-1 was significantly higher compared to AuNRs grown with PVP included as an additive to the growth reaction (AuNRs 0.2 µg/mL PVP). Our results suggest an interaction between the PVP and the surface of the AuNRs and are in agreement with previous investigations for gold and silver nanoparticles coated with PVP.58,59 Confirmation that PVP stays on the surface of AuNRs is also shown in Figure S5. As seen in the XPS spectra in Figure S5, the peak of N 1s of PVP cannot be seen in AuNRs sample, but the peak of O 1s of PVP is present at 530.3 eV. In addition, another peak at 532.0 eV was observed by deconvolution. This latter O 1s peak can be attributed to the interaction between oxygen atom of PVP and AuNRs surface. For instance, as reported previously for silver nanowires,60 the interaction with oxygen atom can shift the binding energy to higher values due to the reduction in electron density.


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Based on the kinetic experiments with the control reactions, we studied time-dependent effects of PVP on AuNRs growth. We expected the first phase of the growth process to be the most susceptible to the addition of PVP because the LSPR band undergoes the largest red-shift. We limited the time range to 1-90 min and used a constant CTAB/PVP weight ratio (1.8x105) with 0.05, 0.1 and 0.2 µg/mL PVP for 25, 50 and 100 mM CTAB, respectively. We also investigated the potential role of the N-vinylpyrrolidone monomer (VP) on the growth of AuNRs by treating growth solutions of 100 mM CTAB with VP at the same concentration as PVP in terms of the number of monomeric units. The size distribution and optical properties of AuNRs with 0.2 µg/mL PVP or a monomer-normalized equivalent concentration of VP (right panel, Nvinylpyrrolidone) were collected (Figure 6, S6) and compared to the aspect ratio distribution of AuNRs grown with PVP (left panel). Histograms of aspect ratio distributions (Figure 6) are presented for AuNRs grown without PVP or VP (Row A); with PVP or VP added immediately following the initiation (Row B), and 30 min after the initiation (Row C). We observed a significant shift in the aspect ratio distribution with PVP that was not present with VP. This further suggests that the structure of the PVP chain affects the growth of the nanorods through a templating effect as opposed to the binding of individual monomeric units, which is entropically much less favorable process. UV-vis spectra of growth solutions containing PVP (0.2 µg/mL) and VP added at 1, 30, and 90 min were collected and compared to the original untreated solutions. A direct relationship between the final LSPR and the addition time of PVP was observed (Figure S6A), whereas no effect was detected upon the addition of a monomeric equivalent concentration of VP except for the 90 min time point (Figure S6B). Because there is a time-dependent effect on the LSPR


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wavelength correlated to the addition of PVP, it suggests that the surface of the AuNRs becomes blocked by PVP, thus preventing further growth. Reducing the concentration of CTAB effectively eliminates the time-dependent wavelength shift as shown with 50 mM CTAB solution in Figure S3A despite reducing the LSPR wavelength at all time points. Perhaps, the most intriguing fact is the inversely related, timedependent reduction in the absorbance of the LSPR band, which further suggests a PVP-AuNR interaction that prevents the overgrowth of the nanorods during the second phase of the growth. For 25 mM CTAB, there is no observed difference between adding PVP (0.05 µg/mL) to the solution at 30 or 90 min (Figure S7A). TEM images of isolated AuNRs after overnight growth (Figure S7, B and C) show nanorods with average dimensions of 12.8 × 4.5 nm (AR=2.7 ± 0.38) and a small increase in the yield of AuNRs with PVP addition, 66% with PVP as opposed to 50% in the control sample. At the same time, we have not found any morphological changes for small AuNRs compared to the control reactions (Figures S7, D and E). One important optical parameter of these small gold nanorods is the extinction coefficient (ε) as calculated through Beer’s Law (A/ml = ε, where A is absorbance, l is the path length (cm) and m is molar concentration). We obtained the absorbance of the longitudinal SPR and molar concentration of a solution of purified AuNRs via UV-vis spectrophotometry and ICP-OES, respectively. The average mass of the AuNRs was calculated by determining the average volume of the particles using the length and width obtained through TEM. After factoring out the effect of spherical particles on the molar concentration of the nanorods obtained through ICP-OES, we found that the AuNRs extinction coefficient depended on the CTAB concentration: for 100 mM CTAB, ε is 2.9 × 109 M-1 cm-1; at 25 mM, the extinction coefficient is 3.0 × 108 M-1 cm-1. These values of extinction coefficient are in agreement with the reported by El-Sayed where ε was


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estimated to be approximately 1.9 x 108 M-1 cm-1 for small nanorods prepared by the seedless method using ascorbic acid.31 Finally, the average number of PVP chains per nanorod was calculated by dividing the molar concentration of PVP (using 10 kDa as the molar mass) by the molar concentration of AuNRs. It was determined that the average number of PVP chains per nanorod is approximately 40 and 15 (theoretical maximum) for the synthesis with 100 and 50 mM CTAB, respectively. Our experiments suggest that PVP in nanomolar concentrations can template AuNRs growth. It is known that AuNRs are synthesized through the disproportionation of Au(I) into 2Au(0) and Au(III) in the presence of CTAB.38 The role of silver nitrate in the synthesis of AuNRs has been extensively studied and has been found to be critical in directing the growth of the AuNRs by causing a symmetry breaking event early in the growth of the nanocrystal.4,61,62 It was recently reported with X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy combined with inductively coupled plasma−mass spectrometry (ICP-MS) that AuNRs are approximately 9% silver and that it is located preferentially near the surface without preference for a specific face or axis.4,61 A recent computational study63 using density functional theory method (DFT) and molecular dynamics (MD) simulations of PVP monomer analogs adsorbing onto Au nanocrystals40 suggested that the pyrrolidone units bind to Au surfaces through the oxygen atom to Ag on the surface of the gold nanocrystals.59 The orientation of the pyrrolidone moiety is electronically controlled by its distal N and O atoms. Nitrogenous heteroaromatic groups coordinate easily with silver ions.64 Several published reports concluded that PVP interacts with silver through the oxygen atom of the carbonyl functional group with chemical bonding N:Ag:O on the surface of PVP stabilized nanoparticles.45,60 The lone pair of electrons from the nucleophilic nitrogen and oxygen atoms of the PVP repeating unit can be donated into


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two sp2 hybrid orbitals of the metal ions and form complex ions and stabilized nanoparticles.57 It has also been reported that PVP can react with the metal ions and donate a pair of electrons from the –N and C=O in the pyrrolidone ring and reduce the ions to their metallic state.65 The interaction between the pyrrolidone groups in the PVP chains and the silver atoms on the surface of AuNRs may block access to the surface of the growing nanorod along specific axes, thus leading to a preferential growth on certain facets. The reduction in the average aspect ratio, however, also suggests that PVP can be blocking the growth along the long axis of the nanorods in the later stages of the reaction. The blockade of certain facets may also be supported by the fact that high concentrations of PVP result in the exclusive formation of spherical particles.

CONCLUSIONS Nanomolar concentrations of 10 kDa poly(vinylpyrrolidone) have been shown to affect the seedless growth of hydroquinone-reduced gold nanorods. Two notable effects are largely conserved across different concentrations of CTAB: greatly accelerated growth, and a tunable longitudinal SPR that depends on the time of PVP addition. Most importantly, the concentration of PVP necessary to impart this effect is far smaller than the concentration of the other constituents in the reaction. Thus, the extremely low concentration may cause a direct PVPAuNR interaction that the individual monomer cannot affect, possibly by a templating or entropic effect. These findings suggest that the growth of AuNRs can be manipulated further by the presence of functional additives.


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Figure 1. Growth dynamics of AuNRs in CTAB solutions of different concentrations. UV-vis spectra of AuNRs growth performed in (A) 100, (B) 50 and (C) 25 mM CTAB. Each colored line corresponds to the same time interval (30 min).


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Figure 2. Normalized UV-vis spectra of AuNRs synthesized without PVP (red), with 0.1 µg/mL (green) and 50 µg/mL (blue) PVP added after NaBH4 addition for 50 mM CTAB. TEM images of AuNRs with (A) 50 µg/mL, (B) 0.1 µg/mL PVP and (C) without PVP. Scale bars are 50 nm.


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Figure 3. Two-dimensional time-wavelength-intensity plots of AuNRs without (left) and with (right) PVP. Contour lines are increments of 0.2 absorbance. Color indicates intensity of solution absorbance.


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Figure 4. Comparison of colloid solution absorbance as a function of time for AuNRs synthesized in 100 mM CTAB without (red) and with (blue) PVP, 0.2 µg/mL.


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Figure 5. ATR spectra of CTAB and 10 kDa PVP (10PVP) powders, AuNRs coated with CTAB (AuNRs control), AuNRs-PVP as additive during synthesis (AuNRs 0.2 µg/mL PVP) and PVP coating synthesized nanorods (AuNRs 50 µg/mL PVP).


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Figure 6. Aspect ratio distribution (AR) of AuNRs grown with 0.2 µg/mL PVP (left panel) or monomer-normalized equivalent concentration of vinylpyrrolidone (right panel). Histograms of AR distributions for AuNR synthesized (A) without PVP or VP, with PVP or VP (B) immediately after initiation of reaction and (C) at 30 min after initiation. Histograms were calculated from TEM images of AuNRs after overnight growth in 100 mM CTAB for each condition. Length (L) and width (W) are in nm.


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Accelerating gold nanorods synthesis with nanomolar concentrations

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