Improving the Shape Yield and Long-Term Stability of Gold

Jul 10, 2019 - (33) The synthesis of AuNPRs was also reported by using PVP and hydrogen peroxide, leading to nanoprisms with edge length from 70 to 20...
0 downloads 0 Views 2MB Size
Download PDF
Article Cite This: Langmuir XXXX, XXX, XXX−XXX

pubs.acs.org/Langmuir

Improving the Shape Yield and Long-Term Stability of Gold Nanoprisms with Poly(vinylpyrrolidone) Katherinne I. Requejo, Anton V. Liopo,† Paul J. Derry,† and Eugene R. Zubarev* Department of Chemistry, Rice University, 6100 S Main Street Houston, Texas 77005, United States

Downloaded via KEAN UNIV on July 20, 2019 at 07:57:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Gold nanoprisms (AuNPRs) are anisotropic nanostructures that have gained great attention in recent years because of their interesting and unique optical properties that can be tailored for biomedical, energy, and sensing applications. At present, several protocols have reported the high yield synthesis of AuNPRs of different dimensions using a seed-mediated approach. However, there is a need to develop reproducible and scalable methods with the goal of a controllable synthesis. Here, we report an improved seedmediated synthesis of small monodisperse AuNPRs of distinct sizes in high yield using poly(vinylpyrrolidone) (PVP) as an additive in nanomolar concentrations. We show optimal synthetic parameters for a blue-shifting of the surface plasmon resonance band which correlates with the reduction in the edge length (L) of AuNPRs from 75 to 35 nm. Using measured extinction coefficients for AuNPRs of different sizes, a linear equation is proposed to estimate the concentration of unknown samples by using Beer’s law. Interestingly, the use of nanomolar amounts of PVP during the growth of AuNPRs significantly improves the shape yield. The surface chemistry properties of AuNPRs were measured by X-ray photoelectron spectroscopy and attenuated total reflectance infrared spectroscopy and revealed that PVP chains interact with AuNPRs through the carbonyl oxygen. This method is reproducible and scalable and enables the synthesis of AuNPRs with long-term shape stability (1 year) in aqueous solution.

■

INTRODUCTION During the last decade, a considerable amount of effort has been made toward the synthesis of anisotropic gold nanostructures because of their shape- and size-dependent physicochemical properties.1,2 Gold nanoprisms (AuNPRs) have gained attention because of their tunable optical properties that, in addition to morphology, depend on the surface functionalization and refractive index of the solvent.3,4 The optical properties of AuNPRs emerge from the localized surface plasmon resonance (LSPR) with a typical band above 600 nm and a weak shoulder around 530 nm.5−7 The LSPR band can be tuned by varying the ratio of the edge length (L) to thickness (T).5 The edge length is highly tunable and has encouraged the development of new protocols and the improvement of existing ones for potential applications of AuNPRs in sensing and biomedicine for surface-enhanced Raman spectroscopy, photothermal therapy, photoacoustic, and optical coherent tomography imaging.7−14 Recently, Bhattarai et al. reported a large-scale synthesis of gold nanotriangles with 60 nm edge length for in vivo X-ray radiosensitization.13 The team of de la Zerda demonstrated their use as optical coherence tomography contrast agents for enhanced angiography in live animals.4 The synthesis of AuNPRs has been accomplished by chemical reduction, photocatalysis, photochemical reduction, femtosecond laser irradiation, vesicular templating, and thermal reduction.6,15−22 In addition, several studies have © XXXX American Chemical Society

demonstrated the synthesis of AuNPRs using biologically derived compounds such as plant extracts, microorganisms, and bael gum.23−27 Chemical reduction is the most common synthetic approach and is composed of seed-mediated and seedless protocols.28−30 The formation of high-quality AuNPRs in the presence of cetyltrimethylammonium chloride (CTAC) was reported by Chen et al.31 In another publication, the AuNPRs were utilized as seeds for the overgrowth step by the seed-mediated approach. It was found that the tip morphology of AuNPRs is preserved and edge lengths of up to 180 nm and thickness of 80 nm can be achieved.32 Moreover, poly(vinylpyrrolidone) (PVP) has been used as reducing and capping agents in the presence of bromide ions and methanol as the solvent to produce AuNPRs with an edge length of 30 and 90 nm. While large AuNPRs are produced at room temperature, small nanotriangles are obtained by preheating PVP and bromide ion solution before addition of gold ions.33 The synthesis of AuNPRs was also reported by using PVP and hydrogen peroxide, leading to nanoprisms with edge length from 70 to 200 nm.34 Here, we report a significant improvement in the seedmediated synthesis to produce AuNPRs in a controllable Received: March 18, 2019 Revised: June 17, 2019 Published: July 10, 2019 A

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

substrate of cleaved mica, and the thickness was measured by sectioning individual AuNPRs and generating a series of height profiles with NanoScope Analysis software (Bruker). Inductively coupled plasma−optical emission spectrometry (ICP−OES) analysis was performed on a PerkinElmer Optima 8300. Attenuated total reflectance (ATR) infrared spectroscopy was conducted on a Nicolet FTIR infrared microscope. Spectra were collected from 128 scans at a resolution of 32 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI Quantera XPS, equipped with a monochromatic Al Kα X-ray (1486.7 eV) source. For all samples, high-resolution scan spectra were acquired in 0.1 eV steps with a pass energy of 26 eV.

manner with edge lengths ranging from 35 to 75 nm and thicknesses ranging from 20 to 35 nm. We also evaluated the effect of PVP addition at nanomolar concentrations during the first minutes of AuNPR growth, which was found to improve the quality of the nanocrystals. Interestingly, the long-term shape stability of AuNPRs is also enhanced because of the polymer additive as evidenced by transmission electron microscopy (TEM) analysis of aged samples.

■

EXPERIMENTAL SECTION

■

Seed Solution. The preparation of the seed solution was carried out by following the procedure reported by Bhattarai et al.13 with slight modifications. First, 4.7 mL of aqueous CTAC solution (100 mM) was added to a 25 mL vial followed by 25 μL of HAuCl4 (50 mM). Next, 300 μL of NaBH4 (10 mM, ice cold) was injected at once under vigorous stirring (1000 rpm). The seed solution was aged for 2 h at 25 °C before use. After that, the seed solution was diluted 10 times (10×) in 100 mM CTAC. Growth Solutions. The two growth solutions (GS1 and GS2) were prepared as follows. In GS1, 1.6 mL of CTAC solution (100 mM) was added to 8 mL of Milli-Q water in a 50 mL Erlenmeyer flask. To this mixture, 40 μL of HAuCl4 (50 mM) was added followed by 15 μL of NaI solution (10 mM). GS1 was utilized for the overgrowth step of the diluted seed solution. GS2 was prepared in a 125 mL Erlenmeyer flask by adding 500 μL of HAuCl4 (50 mM) and 40 mL of CTAC solution (50 mM) and mixed to form a transparent solution. To GS2, 300, 400, 600, or 800 μL of NaI solution (10 mM) was added. Next, 40 and 400 μL of ascorbic acid (100 mM) were added to GS1 and GS2, respectively. The growth solutions were handstirred until they became colorless. Last, 100, 200, 400, or 600 μL of diluted seed solution was injected to GS1 and hand-stirred for 2−3 s. Immediately after that, 3.2 mL of this solution was added to GS2 and hand-stirred for 10 s. GS2 turned red-pink followed by violet-purple and finally blue during the first 3 min of the synthesis. The reaction mixture was kept at 25 °C for 2 h. Purification. For the purification step, a certain volume of CTAC 25% w/w solution was added to the 125 mL Erlenmeyer flask to precipitate the AuNPRs by depletion flocculation. The final concentration of CTAC varied from 0.135 to 0.35 M and depended on the size of the AuNPRs (estimated by the SPR position). Large AuNPRs require a lower concentration of the surfactant to flocculate. After 16 h at 25 °C, the supernatant was removed and the precipitated AuNPRs were redispersed in 10 mM CTAC solution. For TEM analysis, the sample was purified by one round of centrifugation at 4000−6000 rpm for 15 min. Seed-Mediated Synthesis of AuNPRs with Different Molecular Weights of PVP. The seed and growth solutions GS1 and GS2 were prepared as described above. High-range concentrations from 0.05 to 5 mg/mL of 10, 55, and 360 kDa PVP were tested during the nanoprism growth. In this case, the polymer was added 5 min after the growth initiation. Stock solutions of PVP (5, 10, 55, and 360 kDa) were prepared and added to GS2 during the growth stage of the synthesis (1, 5, 10, 15 min) to obtain 0.1 μg/mL as the final concentration of the polymer additive. In a different set of experiments, stock solutions of 55 and 360 kDa PVP were used to achieve the same molar concentration as 10 kDa PVP. The purification step for all samples was performed as previously described. Characterization. UV−vis spectra were measured with an Evolution 220 UV−vis spectrophotometer (Thermo Fisher Scientific). TEM was performed on a JEOL 1230 Field Emission Gun (JEOL) instrument operating at 80 and 200 kV, respectively. Scanning electron microscopy (SEM) images were collected on an FEI ESEM instrument operating at 15 kV. AuNPR size distribution calculations (200 nanoprisms) were obtained using ImageJ (National Institutes of Health). Atomic force microscopy (AFM) was conducted on a Bruker MultiMode 8 microscope operating in contact mode with a silicon tip on nitride cantilever (ScanAsyst-Air, resonance frequency 70 kHz and spring constant 0.4 N/m). AuNPRs were deposited on a

RESULTS AND DISCUSSION We followed the previously reported seed-mediated protocols described by Bhattarai et al.13 to synthesize monodisperse AuNPRs of various edge lengths. Similar volumes of seed solution were tested (100−600 μL), and the NaI concentration was varied between 50 and 300 μM in the second growth solution (GS2) for each seed amount (Figure 1). During the

Figure 1. Schematic representation of the synthesis and purification of AuNPRs. The seed solution is aged for 2 h and diluted 10× before use. Distinct amounts of seed particles are added to the GS1. NaI concentration is changed in GS2 and PVP is introduced at different points of time (1−15 min) during the growth of AuNPRs (GS2).

purification, specific volumes of CTAC 25% aqueous solution were utilized depending on the AuNPR size estimated by the SPR band position. For instance, the largest size AuNPRs require a lower CTAC concentration (0.135 M) to flocculate as compared to the smallest size (0.35 M). For the synthesis with the smallest volume of seed solution (100 μL), the optimal iodide concentration was found to be 75 μM (Figure S1A), and the shape yield obtained for AuNPRs was 80% (Figure S1D). At the lowest concentration of iodide tested (50 μM), the purity of the product was reduced because of the formation of spherical particles indicated by an increase in the absorbance around 530 nm. When 100−125 μM sodium iodide was used, the absorbance at 530 nm is considerably less intense, and the edge length of AuNPRs is largely unchanged (Table S1B). The AuNPR polydispersity (standard deviation of edge length) correlates positively with iodide concentration. In agreement with previous protocols,6,35 the purification step decreases the absorbance around 530 nm (Figure S1C), which indicates the removal of spherical impurities. When the seed volume was increased to 200 μL, the iodide concentration that produced monodisperse AuNPRs was 100 μM (Figure S2A), B

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir and the average edge length of AuNPRs obtained was 52 nm. Although the edge length standard deviation for the synthesis with 50 μM NaI is the lowest, shape yield is reduced (Figure S2A, blue spectrum). The increase in the NaI concentration to 150−200 μM resulted in the formation of AuNPRs with an overall longer edge length and higher polydispersity (Table S2B). After the purification step, the intensity of the shoulder around 530 nm was greatly decreased, and the shape yield was found to be 85% (Figure S2C,D). The highest shape yield (90%) of AuNPRs was achieved by using 400 μL of seed solution and 150 μM of NaI (Figure S3A). Similar to the experiments with 200 μL of seeds, there is little effect on the edge length when the iodide concentration is varied from 100 to 250 μM (Table S3B). TEM images of the 40 nm AuNPRs synthesized with 150 μM NaI (Figure S3C) show sharp tips, whereas those obtained with 250 μM NaI (Figure S3D) are truncated and have rounded tips. These results are in agreement with other investigations36−38 in which the higher iodide concentration favored nonpreferential adsorption of iodide ions to the side facets instead of the top and bottom (111) facets. For the largest volume of seed solution (600 μL), the observed edge length is around 36 nm and is independent of the iodide concentration (Table S4B). The UV−vis spectra with different NaI concentrations ranging from 150 to 300 μM are shown in Figure S4A, and only the highest concentration of NaI has different and the lowest SPR band. TEM images (Figure S4C) showed that AuNPRs maintained their sharp tip morphology when synthesized with 200 μM NaI, but further increment in NaI concentration induced tip rounding and the formation of hexagonal particles (Figure S4D). As seen in Figure S4E, AuNPR monodispersity is reduced when 300 μM NaI is used as indicated by the presence of large and truncated nanoprisms. A comparison of the UV−vis spectra for AuNPR samples synthesized with 100−600 μL of seed solution shows a blueshift of the SPR band as the amount of seeds increases (Figure 2). The similar effect was described for big gold nanotriangles

Table 1. Edge Length, Thickness, AR, and Extinction Coefficient for AuNPRsa seed (μL) 100 200 400 600

edge length (nm) 72 52 40 33

± ± ± ±

3.9 2.2 1.8 1.9

thickness (nm) 35 28 23 20

± ± ± ±

2.7 2.1 2.7 2.3

extinction coefficient (ε, M−1 cm−1)

AR 2.04 1.85 1.70 1.64

± ± ± ±

0.13 0.11 0.14 0.14

3.04 1.39 0.76 0.41

± ± ± ±

0.19 0.08 0.06 0.04

× × × ×

1010 1010 1010 1010

a Edge length of n = 200 nanoprisms was obtained from the TEM image analysis and thickness of 25 AuNPRs was measured by AFM for the syntheses with specific volumes of seed solution.

AuNPRs synthesized with 200 μL of seed solution possess an average thickness of 28 nm as determined by the AFM height profile and show a flat top surface (Figure 3). Because the thickness of the AuNPRs differs for each seed condition, the aspect ratio (AR) can be tuned from 1.64 to 2.04.

Figure 3. AFM height profile for AuNPR synthesized with 200 μL of seeds. The inset at the right shows the sectioning line of a scanned AuNPR.

We determined the extinction coefficients (ε) at the SPR wavelengths to be around 0.41 to 3.04 × 1010 M−1 cm−1 for AuNPRs produced using our synthesis (Table 1). The average extinction coefficient was calculated from the values of the molar concentration of Au derived from ICP-OES for each AuNPR sample, optical density (UV−vis spectra), and data obtained from TEM (volume of AuNPRs, and the corresponding number of Au atoms). Figure 4 shows a linear relationship between ε and the AR of AuNPRs, indicating that the average value of ε serves as a basis for the estimation of the molar concentration of AuNPRs in solution by using Beer’s law. A summary scheme is presented in Figure S5 for the syntheses with different seed solution volumes (100−400 μL).

Figure 2. Normalized UV−vis spectra of AuNPRs synthesized with different amounts of seed solution ranging from 100 to 600 μL.

with sizes from 147 to 59 nm and thickness around 30 nm.6 However, our improved protocol enables the synthesis of AuNPRs with edge length from 75 to 35 nm and thickness from 35 to 20 nm and is correlated to the volume of seed solution. Although there is a small difference in the wavelength values for 400 and 600 μL seeds, the dimensions obtained by the TEM/AFM analysis show smaller edge lengths and thicknesses for 600 μL seeds. as illustrated in the Table 1.

Figure 4. Linear relationship between the extinction coefficient and the AR of AuNPRs obtained by the seed-mediated synthesis. C

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 5. Normalized UV−vis spectra of AuNPRs synthesized with 200 μL of seeds and 10PVP. AuNPRs with 0.1 μg/mL 10PVP added during 1− 15 min of growth (A). A control sample is shown in purple. The right spectra (B) show the enlarge region of the shoulder for the control and sample with 10PVP added at 15 min of growth. TEM images of AuNPRs (C) without and (D) with polymers.

thus indicating the formation of fewer spherical particles. The largest difference in absorbance around 550 nm is observed for AuNPRs synthesized with 200 μL of seed solution and 10PVP added at 15 min of growth (Figure 5B). In this case, the shape yield of AuNPRs calculated from TEM images was 94% for the sample with 10PVP, which was significantly higher than the control (Figure 5C,D).The UV−vis spectra of AuNPRs synthesized with 100−600 μL of seed solution and 0.1 μg/ mL polymer show that the SPR band blue-shifts as the amount of seeds increases (Figure S7). A comparison of the UV−vis spectra for the other seed volumes indicates that the AuNPR quality is also improved for the syntheses with 400 and 600 μL of seed solution and polymer added at 15 min of growth (Figure S8). However, the enlarged region of the shoulder for the synthesis with 100 μL of seed solution reveals a stronger absorbance for the sample treated with 10PVP (Figure S8A). Thus, the stabilization by the polymer additive during the growth may depend on the AuNPR size. The histograms of edge length distribution for AuNPRs synthesized with 100−600 μL seeds are presented in Figure S9. The largest size of AuNPRs obtained is 72 nm, although the edge length distribution appears to be positively skewed (Figure S9A). However, the edge length histogram for the synthesis with 200 μL of seeds shows a Gaussian distribution for AuNPRs of 52 nm (Figure S9B). Further increment of seed volumes in the range from 400 to 600 μL produces similar edge length distributions, but their skewness is reduced because of their smaller size (Figure S9C,D). Interestingly, the histograms of edge length for AuNPRs synthesized with 10PVP have a very uniform edge length distribution. Because the polymer was introduced at 15 min of growth, it could not modify the edge length for the synthesis with 200−600 μL of seed solution. In addition, the histogram shows a narrow edge length distribution for AuNPRs of 80 nm prepared with a 10PVP additive. This observation agrees with the studies of silver nanoplates and PVP as the stabilizing agent in which the

The iodide concentration can be varied to produce monodisperse AuNPRs. Size tunability of the nanoprisms can be achieved by changing the volume of seed solution used in the reaction. In the work reported by Scarabelli et al.,6 the resultant nanotriangles had edge lengths between 150 and 60 nm with a constant thickness of 30 nm for seed solution volumes ranging from 100 to 1200 μL. Unlike the initial report, we found that the thickness of AuNPRs, as measured by AFM and by TEM of vertically aligned AuNPRs, actually decreases from 35 to 20 nm as the seed solution volume is increased. The difference in thickness and nanoprism size can be attributed to the available supply of Au+ ions relative to the concentration of seed particles. In other words, in solutions with fewer seed particles there are more Au+ ions per seed particle to form triangular nanoprisms. We previously reported that the shape yield of gold nanorods is enhanced by using 10 kDa PVP (10PVP) at nanomolar concentrations when compared to growth solutions containing higher molecular weights of PVP added during the first minutes of nanoparticle growth.39,40 Therefore, we evaluated the effect of different molecular weights of PVP (5−360 kDa) on the shape yield, tip morphology, and dimensions of AuNPRs. Initially, the effect of the 10PVP concentration between 0.05 and 5 mg/mL was assessed on the synthesis with 100 μL of seeds for the polymer added at 5 min into the growth stage. The UV−vis spectra in Figure S6A show that the SPR band becomes broad, red-shifts, and has lower intensity in the 10PVP concentration range from 0.5 to 5 mg/ mL. We also observed that low-quality AuNPRs with a much larger proportion of decahedral and octahedral nanoparticles were obtained with 5 mg/mL PVP when compared to the control (Figure S6B,C). Next, we grew AuNPRs in solutions containing 0.1 μg/mL 10PVP added at 1, 5, 10, and 15 min following the addition of 200 μL of seed particles. We found that the intensity of the shoulder around 550 nm was markedly decreased (Figure 5A), D

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

the AuNPR surface (Figure 7). The N 1s peak present in AuNPRs with and without the polymer has the same binding energy as the N 1s peak in CTAC; thus, the nitrogen atom of PVP does not interact with the surface of AuNPRs. In the case of the O 1s peak of AuNPRs prepared with the polymer additive, it is deconvoluted in two peaks. The peak at lower binding energy matches that of the 10PVP, and the peak at higher binding energy indicates the adsorption of the oxygen atom to the AuNPR surface, which is in agreement with previous investigations.39,47,48 In addition to the improved shape yield of AuNPRs because of the presence of 10PVP during the synthesis, we assessed the long-term shape stability of AuNPRs prepared with and without the polymer additive after they have been stored at room temperature for 1 year. A representative TEM image of the AuNPR control shows more shape impurities (octahedra) as compared to AuNPRs synthesized with 10PVP (Figure 8A,C). We found that after 1 year of shelf-life, the content of other morphologies (octahedra, hexagons) is higher for the control, and the AuNPRs prepared with the PVP polymer maintain their shape stability with fewer contaminations (Figure 8B,D). It is conceivable that some AuNPRs underwent either reshaping to more thermodynamically stable structures or oxidative etching on the tips to produce hexagonal structures.49−51 The effect of other molecular weights of PVP was also evaluated during the growth of AuNPRs at nanomolar concentration of PVP (0.1 μg/mL). As seen in the UV−vis spectra for the synthesis with 200 μL of seed solution (Figure S13A), the use of 5PVP produced a spectrum similar to that of 10PVP (Figure 5A). Although the addition of 55PVP and 360PVP at 15 min of the growth reduced the intensity of the shoulder at 550 nm, the absorbance at longer wavelengths increased (black arrow). Therefore, the polydispersity of the AuNPR samples prepared with higher molecular weight of PVP increases when compared to the control. TEM images of nanoprisms produced with 55PVP and 360PVP show that the tip morphology is not affected, but the amount of shape impurities increases (Figure S14A,C). The histograms of these samples indicate similar edge lengths but higher value of standard deviation when compared to the control (Figure S14B,D). For the synthesis with 400 μL of seed solution (Figure S13B) and specific molecular weights of PVP, AuNPRs with 360PVP present a broad SPR band with an increase in absorbance at shorter and longer wavelengths because of a higher content of impurities and higher polydispersity, respectively. Overall, modifying the chain length of the polymer has little or no effect on AuNPR dimensions with respect to the control when the macromolecular additive is introduced at 15 min of reaction. Hence, small sizes of nanoprisms in higher yield can be prepared by utilizing low molecular weights of PVP such as 5 and 10 kDa during the growth reaction. Mechanism of AuNPR Formation and PVP Stabilization during the Growth Process. Based on previous investigations of the growth mechanisms for plate-like morphologies, the crystallographic structure of the seeds and preferential adsorption of ligands to certain crystal facets influence the final nanoparticle shape.5,8,41,52 It has been widely accepted that the high yield of nanoprism formation is a kinetically controlled process that requires the presence of twinned seeds.31,34,37,53,54 The seed particles exhibit {100} and {111} facets, where the latter have the lowest surface

presence of the polymer produces a narrow size distribution of nanocrystals.41 Regarding the thickness of AuNPRs synthesized in the presence of 10PVP, the same average values (20− 40 nm) as compared to the controls were obtained when measured by AFM. Further confirmation of the same average thicknesses for the control and sample synthesized with the polymer additive (∼27 nm) was obtained by SEM imaging of vertically aligned AuNPRs prepared with 200 μL of seed solution (Figure S10). In agreement with previous reported studies of seedless synthesis of AuNPRs of similar dimensions,42 the sides of our AuNPRs are smaller and flat. In order to evaluate any changes in the tip morphology of AuNPRs synthesized in the presence of the polymer additive, HR-TEM images were acquired (Figure S11A−H). For the syntheses with 100−600 μL seeds, the tip morphology was preserved for the samples with 10PVP as compared to the controls. As observed in all of the HR-TEM images, the AuNPR tips are not atomically sharp as reported by others.30,31 In the case of the crystallographic structure, fast Fourier transform (FFT) images show hexagonal patterns characteristic of the (111) facets of AuNPRs29,33,36,38,43 (Figure S11I− P). Thus, the AuNPRs synthesized with and without 10PVP are single crystalline particles and the lattice plane (111) seen in FFT patterns confirms that AuNPRs have planar surfaces.28,31 Because small amounts of 10PVP added during AuNPR growth enhanced their shape yield, we performed surface characterization by ATR and XPS to determine any interaction of the polymer with the surface of nanoprisms. Figure 6

Figure 6. ATR spectra of CTAC and 10 kDa PVP powders, AuNPRs coated with CTAC (AuNPR control) and AuNPRs with 0.1 μg/mL PVP added during their synthesis (AuNPRs 10PVP).

presents the ATR spectra of CTAC, 10PVP, and AuNPRs with and without the polymer additive. The polymer shows a strong peak around 1660 cm−1 because of the carbonyl stretching vibration that is absent in the surfactant CTAC. Different from the AuNPR control that lacks the carbonyl peak, the AuNPRs with 10PVP show a peak at lower intensity (dotted line, black arrow). In order to corroborate the adsorption of the polymer to the AuNPR surface, ATR spectra were collected after several rounds of centrifugation. As seen in Figure S12, the peak around 1660 cm−1 presents a shift to lower frequencies and decreases its intensity for 2× and 3× rounds of centrifugation as compared to one round (dotted line). These outcomes are in accordance with previous studies on the interaction of PVP with gold and silver nanocrystals.33,39,44−46 Further investigation of the type of surface interaction by high-resolution XPS confirmed the adsorption of the carbonyl group of PVP to E

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 7. High-resolution XPS spectra of N 1s and O 1s for CTAC, PVP, AuNPR control (without polymer), and AuNPR sample synthesized with 0.1 μg/mL 10PVP.

growth rate along the {100} direction enabling the formation of AuNPRs in high yield.65 Because iodide ions are incorporated into the growth solution before PVP, the adsorption of iodide ions onto growing Au{111} facets occurs first. In our work, PVP is introduced in nanomolar concentration during the growth step; thus, the polymer is acting only as a stabilizer of the growing AuNPRs. Although the overall size of AuNPRs cannot be directly controlled by the polymer, the presence of the PVP chains enhances their formation and results in a higher shape yield of the nanoprisms.

■

CONCLUSIONS An improved seed-mediated protocol for the synthesis of AuNPRs has been developed by using the PVP additive as a stabilizing agent during the growth of nanoprisms. Near monodisperse small AuNPRs with edge length between 35 and 75 nm and thickness from 20 to 35 nm were synthesized by modifying the amount of seed particles and the iodide concentration. A linear equation is presented to derive the extinction coefficient (0.41 to 3.04 × 1010 M−1 cm−1) based on the AuNPR AR. Interestingly, the use of nanomolar concentration of 10PVP improves the AuNPR shape yield without affecting their dimensions. Because the PVP polymer adsorbs on the surface of AuNPRs, it stabilizes the nanocrystals and ensures their long-term stability even after 1 year of storage when compared to control samples.

Figure 8. TEM images of AuNPRs synthesized with 200 μL of seeds (A) without and (C) with 10PVP. TEM images after 1 year of shelflife of AuNPRs (B) without and (D) with 10PVP.

energy.37,55−57 In order to break the symmetry of the cubic lattice and reduce the surface energy, defects such as stacking faults and planar twins are created during the nucleation step because of coalescence of two {111} crystal facets or adsorption of iodide ions to gold {111} facets.1,5,6,54,58,59 However, controlling the crystal habit of seeds is challenging because of the simultaneous formation of multiply twinned, untwinned, or single-crystalline seeds during the nucleation step.19,41,46,60 Considering the adsorption of iodide ions to gold {111} facets, the growth is favored uniformly for {110} facets of single-twinned seeds to form hexagonal nuclei.52,53,61 The hexagonal nuclei have a twinned plane with concave and convex side facets, and a two-dimensional lateral growth is favored at the concave side with reentrant groves.5,53,62 Further anisotropic growth of AuNPRs is facilitated due to the iodide ions and PVP adsorption on Au{111} facets.19,36,56,63 Based on computational studies such as density functional theory and molecular dynamics, PVP monomers can interact more favorably with Au{111} facets through the oxygen atom.64 This facet selective interaction reduces the growth rate along the {111} direction while it enhances the

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00794. Normalized UV−vis spectra and TEM images of AuNPRs with varying seed amounts, normalized UV− vis spectra and histograms of edge length distribution of AuNPRs with and without PVP, SEM and HR-TEM images of nanoprisms, ATR spectra of AuNPRs after several rounds of centrifugation, normalized UV−vis F

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

■

(12) Xie, X.; Liao, J.; Shao, X.; Li, Q.; Lin, Y. The Effect of shape on Cellular Uptake of Gold Nanoparticles in the forms of Stars, Rods, and Triangles. Sci. Rep. 2017, 7, 3827. (13) Bhattarai, S. R.; Derry, P. J.; Aziz, K.; Singh, P. K.; Khoo, A. M.; Chadha, A. S.; Liopo, A.; Zubarev, E. R.; Krishnan, S. Gold nanotriangles: scale up and X-ray radiosensitization effects in mice. Nanoscale 2017, 9, 5085−5093. (14) Zhao, Y.; Liu, W.; Tian, Y.; Yang, Z.; Wang, X.; Zhang, Y.; Tang, Y.; Zhao, S.; Wang, C.; Liu, Y.; Sun, J.; Teng, Z.; Wang, S.; Lu, G. Anti-EGFR Peptide-Conjugated Triangular Gold Nanoplates for Computed Tomography/Photoacoustic Imaging-Guided Photothermal Therapy of Non-Small Cell Lung Cancer. ACS Appl. Mater. Interfaces 2018, 10, 16992−17003. (15) Singh, C.; Hu, Y.; Khanal, B. P.; Zubarev, E. R.; Stellacci, F.; Glotzer, S. C. Striped Nanowires and Nanorods from Mixed SAMS. Nanoscale 2011, 3, 3244−3250. (16) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin, C. A. Observation of a Quadrupole Plasmon Mode for a Colloidal Solution of Gold Nanoprisms. J. Am. Chem. Soc. 2005, 127, 5312−5313. (17) Chu, H.-C.; Kuo, C.-H.; Huang, M. H. Thermal Aqueous Solution Approach for the Synthesis of Triangular and Hexagonal Gold Nanoplates with Three Different Size Ranges. Inorg. Chem. 2006, 45, 808−813. (18) Miranda, A.; Malheiro, E.; Skiba, E.; Quaresma, P.; Carvalho, P. A.; Eaton, P.; de Castro, B.; Shelnutt, J. A.; Pereira, E. One-pot synthesis of triangular gold nanoplates allowing broad and fine tuning of edge length. Nanoscale 2010, 2, 2209−2216. (19) Zhai, Y.; DuChene, J. S.; Wang, Y.-C.; Qiu, J.; Johnston-Peck, A. C.; You, B.; Guo, W.; DiCiaccio, B.; Qian, K.; Zhao, E. W.; Ooi, F.; Hu, D.; Su, D.; Stach, E. A.; Zhu, Z. Polyvinylpyrrolidone-induced anisotropic growth of gold nanoprisms in plasmon-driven synthesis. Nat. Mater. 2016, 15, 889−895. (20) Liebig, F.; Thünemann, A. F.; Koetz, J. Ostwald Ripening Growth Mechanism of Gold Nanotriangles in Vesicular Template Phases. Langmuir 2016, 32, 10928−10935. (21) Tangeysh, B.; Tibbetts, K. M.; Odhner, J. H.; Wayland, B. B.; Levis, R. J. Gold Nanotriangle Formation through Strong Field Laser Processing of Aqueous KAuCl4 and Postirradiation by Hydrogen Peroxide. Langmuir 2017, 33, 243−252. (22) Schulze, N.; Koetz, J. Kinetically controlled growth of gold nanotriangles in a vesicular template phase by adding a strongly alternating polyampholyte. J. Dispersion Sci. Technol. 2017, 38, 1073− 1078. (23) Chandran, S. P.; Chaudhary, M.; Pasricha, R.; Ahmad, A.; Sastry, M. Synthesis of Gold Nanotriangles and Silver Nanoparticles Using Aloe vera Plant Extract. Biotechnol. Prog. 2006, 22, 577−583. (24) Nadagouda, M. N.; Hoag, G.; Collins, J.; Varma, R. S. Green Synthesis of Au Nanostructures at Room Temperature Using Biodegradable Plant Surfactants. Cryst. Growth Des. 2009, 9, 4979− 4983. (25) Verma, V. C.; Singh, S. K.; Solanki, R.; Prakash, S. Biofabrication of Anisotropic Gold Nanotriangles Using Extract of Endophytic Aspergillus clavatus as a Dual Functional Reductant and Stabilizer. Nanoscale Res. Lett. 2011, 6, 16. (26) Gaware, U.; Kamble, V.; Ankamwar, B. Ecofriendly Synthesis of Anisotropic Gold Nanoparticles: A Potential Candidate of SERS Studies. Int. J. Electrochem. 2012, 2012, 1−6. (27) Balasubramanian, S.; Bezawada, S. R.; Raghavachari, D. Green, Selective, Seedless and One-Pot Synthesis of Triangular Au Nanoplates of Controlled Size Using Bael Gum and Mechanistic Study. ACS Sustainable Chem. Eng. 2016, 4, 3830−3839. (28) Ah, C. S.; Yun, Y. J.; Park, H. J.; Kim, W.-J.; Ha, D. H.; Yun, W. S. Size-Controlled Synthesis of Machinable Single Crystalline Gold Nanoplates. Chem. Mater. 2005, 17, 5558−5561. (29) Pelaz, B.; Grazu, V.; Ibarra, A.; Magen, C.; del Pino, P.; de la Fuente, J. M. Tailoring the Synthesis and Heating Ability of Gold Nanoprisms for Bioapplications. Langmuir 2012, 28, 8965−8970.

spectra, and TEM images and histograms of AuNPRs with PVP of different molecular weights (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Eugene R. Zubarev: 0000-0002-6401-3287 Present Address †

Texas A&M Health Science Center, 2121 W. Holcombe Blvd., Houston, Texas 77030. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Science Foundation (DMR1105878).

■

REFERENCES

(1) Langille, M. R.; Personick, M. L.; Zhang, J.; Mirkin, C. A. Defining Rules for the Shape Evolution of Gold Nanoparticles. J. Am. Chem. Soc. 2012, 134, 14542−14554. (2) Lohse, S. E.; Murphy, C. J. The Quest for Shape Control: A History of Gold Nanorod Synthesis. Chem. Mater. 2013, 25, 1250− 1261. (3) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem., Int. Ed. 2009, 48, 60−103. (4) Si, P.; Yuan, E.; Liba, O.; Winetraub, Y.; Yousefi, S.; SoRelle, E. D.; Yecies, D. W.; Dutta, R. Gold Nanoprisms as Optical Coherence Tomography Contrast Agents in the Second Near-Infrared Window for Enhanced Angiography in Live Animals. ACS Nano 2018, 12, 11986−11994. (5) Millstone, J. E.; Hurst, S. J.; Métraux, G. S.; Cutler, J. I.; Mirkin, C. A. Colloidal Gold and Silver Triangular Nanoprisms. Small 2009, 5, 646−664. (6) Scarabelli, L.; Coronado-Puchau, M.; Giner-Casares, J. J.; Langer, J.; Liz-Marzán, L. M. Monodisperse Gold Nanotriangles: Size Control, Large-Scale Self-Assembly, and Performance in SurfaceEnhanced Raman Scattering. ACS Nano 2014, 8, 5833−5842. (7) Reguera, J.; Langer, J.; Jiménez de Aberasturi, D.; Liz-Marzán, L. M. Anisotropic metal nanoparticles for surface enhanced Raman scattering. Chem. Soc. Rev. 2017, 46, 3866−3885. (8) Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Shape control in gold nanoparticle synthesis. Chem. Soc. Rev. 2008, 37, 1783−1791. (9) Pérez-Hernández, M.; del Pino, P.; Mitchell, S. G.; Moros, M.; Stepien, G.; Pelaz, B.; Parak, W. J.; Gálvez, E. M.; Pardo, J.; de la Fuente, J. M. Dissecting the Molecular Mechanism of Apoptosis during Photothermal Therapy Using Gold Nanoprisms. ACS Nano 2015, 9, 52−61. (10) Liopo, A.; Oraevsky, A. A. Nanoparticles as Contrast Agents for Optoacoustic Imaging. In Nanotechnology for Biomedical Imaging and Diagnostics: From Nanoparticle Design to Clinical Applications; Berezin, M. Y., Ed.; Wiley: New Jersey, 2015; pp 111−149. (11) Ma, X.; Cheng, Y.; Huang, Y.; Tian, Y.; Wang, S.; Chen, Y. PEGylated gold nanoprisms for photothermal therapy at low laser power density. RSC Adv. 2015, 5, 81682−81688. G

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (30) Straney, P. J.; Andolina, C. M.; Millstone, J. E. Seedless Initiation as an Efficient, Sustainable Route to Anisotropic Gold Nanoparticles. Langmuir 2013, 29, 4396−4403. (31) Chen, L.; Ji, F.; Xu, Y.; He, L.; Mi, Y.; Bao, F.; Sun, B.; Zhang, X.; Zhang, Q. High-Yield Seedless Synthesis of Triangular Gold Nanoplates through Oxidative Etching. Nano Lett. 2014, 14, 7201− 7206. (32) Kuttner, C.; Mayer, M.; Dulle, M.; Moscoso, A.; LópezRomero, J. M.; Förster, S.; Fery, A.; Pérez-Juste, J.; Contreras-Cáceres, R. Seeded Growth Synthesis of Gold Nanotriangles: Size Control, SAXS Analysis, and SERS Performance. ACS Appl. Mater. Interfaces 2018, 10, 11152−11163. (33) Verma, M.; Kedia, A.; Newmai, M. B.; Kumar, P. S. Differential role of PVP on the synthesis of plasmonic gold nanostructures and their catalytic and SERS properties. RSC Adv. 2016, 6, 80342−80353. (34) Wang, G.; Tao, S.; Liu, Y.; Guo, L.; Qin, G.; Ijiro, K.; Maeda, M.; Yin, Y. High-yield halide-fee synthesis of biocompatible Au nanoplates. Chem. Commun. 2016, 52, 398−401. (35) Ha, T. H.; Kim, Y. J.; Park, S. H. Complete separation of triangular gold nanoplates through selective precipitation under CTAB micelles in aqueous solution. Chem. Commun. 2010, 46, 3164−3166. (36) Ha, T. H.; Koo, H.-J.; Chung, B. H. Shape-Controlled Syntheses of Gold Nanoprisms and Nanorods Influenced by Specific Adsorption of Halide Ions. J. Phys. Chem. C 2007, 111, 1123−1130. (37) DuChene, J. S.; Niu, W.; Abendroth, J. M.; Sun, Q.; Zhao, W.; Huo, F.; Wei, W. D. Halide Anions as Shape-Directing Agents for Obtaining High-Quality Anisotropic Gold Nanostructures. Chem. Mater. 2013, 25, 1392−1399. (38) Sun, M.; Ran, G.; Fu, Q.; Xu, W. The effect of iodide on the synthesis of gold nanoprisms. J. Exp. Nanosci. 2015, 10, 1309−1318. (39) Requejo, K. I.; Liopo, A. V.; Derry, P. J.; Zubarev, E. R. Accelerating gold nanorod synthesis with nanomolar concentrations of poly(vinylpyrrolidone). Langmuir 2017, 33, 12681−12688. (40) Requejo, K. I.; Liopo, A. V.; Zubarev, E. R. Synthesis of gold nanorods using poly(vinylpyrrolidone) of different molecular weight as an additive. ChemistrySelect 2018, 3, 12192−12197. (41) Zhang, Q.; Li, N.; Goebl, J.; Lu, Z.; Yin, Y. A systematic Study of the Synthesis of Silver Nanoplates: Is citrate a “Magic” Reagent? J. Am. Chem. Soc. 2011, 133, 18931−18939. (42) Kim, J.; Song, X.; Ji, F.; Luo, B.; Ice, N. F.; Liu, Q.; Zhang, Q.; Chen, Q. Polymorphic Assembly from Beveled Gold Triangular Nanoprisms. Nano Lett. 2017, 17, 3270−3275. (43) Yan, Z.; Pelton, M.; Vigderman, L.; Zubarev, E. R.; Scherer, N. F. Why Single-Beam Optical Tweezers Trap Gold Nanowires in Three Dimensions. ACS Nano 2013, 7, 8794−8800. (44) Behera, M.; Ram, S. Spectroscopy-Based Study on the Interaction between Gold Nanoparticles and Poly(vinylpyrrolidone) Molecules in a Non-Hydrocolloid. Int. Nano Lett. 2013, 3, 17. (45) Ren, M.; Jin, Y.; Chen, W.; Huang, W. Rich Capping Ligand-Ag Colloid Interactions. J. Phys. Chem. C 2015, 119, 27588−27593. (46) Thomas, N.; Mani, E. Mechanism and modeling of poly[vinylpyrrolidone] (PVP) facilitated synthesis of silver nanoplates. Phys. Chem. Chem. Phys. 2018, 20, 15507−15517. (47) Jiang, P.; Li, S.-Y.; Xie, S.-S.; Gao, Y.; Song, L. Machinable Long PVP-Stabilized Silver Nanowires. Chem.Eur. J. 2004, 10, 4817−4821. (48) Elechiguerra, J. L.; Larios-Lopez, L.; Liu, C.; Garcia-Gutierrez, D.; Camacho-Bragado, A.; Yacaman, M. J. Corrosion at the Nanoscale: The Case of Silver Nanowires and Nanoparticles. Chem. Mater. 2005, 17, 6042−6052. (49) Hong, S.; Shuford, K. L.; Park, S. Shape Transformation of Gold Nanoplates and their Surface Plasmon Characterization: Triangular to Hexagonal Nanoplates. Chem. Mater. 2011, 23, 2011−2013. (50) Hong, S.; Acapulco, J. A. I., Jr.; Jang, H.-J.; Kulkarni, A. S.; Park, S. Kinetically Controlled Growth of Gold Nanoplates and Nanorods via a One-Step Seed-Mediated Method. Bull. Korean Chem. Soc. 2014, 35, 1737−1742.

(51) Jiang, X.; Liu, R.; Tang, P.; Li, W.; Zhong, H.; Zhou, Z.; Zhou, J. Controllably tuning the near-infrared plasmonic modes of gold nanoplates for enhanced optical coherence imaging and photothermal therapy. RSC Adv. 2015, 5, 80709−80718. (52) Alloyeau, D.; Dachraoui, W.; Javed, Y.; Belkahla, H.; Wang, G.; Lecoq, H.; Ammar, S.; Ersen, O.; Wisnet, A.; Gazeau, F.; Ricolleau, C. Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy. Nano Lett. 2015, 15, 2574−2581. (53) Nauert, S.; Paul, A.; Zhen, Y.-R.; Solis, D.; Vigderman, L.; Chang, W.-S.; Zubarev, E. R.; Nordlander, P.; Link, S. Influence of Cross Sectional Geometry on Surface Plasmon Polariton Propagation in Gold Nanowires. ACS Nano 2014, 8, 572−580. (54) González-Rubio, G.; de Oliveira, T. M.; Altantzis, T.; La Porta, A.; Guerrero-Martínez, A.; Bals, S.; Scarabelli, L.; Liz-Marzán, L. M. Disentangling the effect of seed size and crystal habit on gold nanoparticle seeded growth. Chem. Commun. 2017, 53, 11360− 11363. (55) Wang, Z. L. Transmission Electron Microscopy of ShapeControlled Nanocrystals and Their Assemblies. J. Phys. Chem. B 2000, 104, 1153−1175. (56) Umar, A. A.; Oyama, M.; Salleh, M. M.; Majlis, B. Y. Formation of High-Yield Gold Nanoplates on the Surface: Effective TwoDimensional Crystal Growth of Nanoseed in the Presence of Poly(vinylpyrrolidone) and Cetyltrimethylammonium Bromide. Cryst. Growth Des. 2009, 9, 2835−2840. (57) Chantry, R. L.; Atanasov, I.; Siriwatcharapiboon, W.; Khanal, B. P.; Zubarev, E. R.; Horswell, S. L.; Johnston, R. L.; Li, Z. Y. An Atomistic View of the Interfacial Structures of AuRh and AuPd Nanorods. Nanoscale 2013, 5, 7452−7457. (58) Hosoya, Y.; Urabe, S. A study on the mechanism of nucleation and growth of twin tabular AgBr crystals. J. Imaging Sci. Technol. 1998, 42, 487−494. (59) Liebig, F.; Sarhan, R. M.; Prietzel, C.; Reinecke, A.; Koetz, J. “Green” gold nanotriangles: synthesis, purification by polyelectrolyte/ micelle depletion flocculation and performance in surface-enhanced Raman scattering. RSC Adv. 2016, 6, 33561−33568. (60) Zhan, G.; Ke, L.; Li, Q.; Huang, J.; Hua, D.; Ibrahim, A.-R.; Sun, D. Synthesis of Gold Nanoplates with Bioreducing Agent Using Syringe Pumps: A Kinetic Control. Ind. Eng. Chem. Res. 2012, 51, 15753−15762. (61) Kaur, P.; Chudasama, B. Single step synthesis of pluronic stabilized IR responsive gold nanoplates. RSC Adv. 2014, 4, 36006− 36011. (62) Gentry, S. T.; Levit, S. D. Stochastic Control: Transition from Differentiated to Undifferentiated Kinetic Growth in Ag Nanoprisms. J. Phys. Chem. C 2009, 113, 12007−12015. (63) Millstone, J. E.; Wei, W.; Jones, M. R.; Yoo, H.; Mirkin, C. A. Iodide Ions Control Seed-Mediated Growth of Anisotropic Gold Nanoparticles. Nano Lett. 2008, 8, 2526−2529. (64) Liu, S.-H.; Saidi, W. A.; Zhou, Y.; Fichthorn, K. A. Synthesis of {111}-Faceted Au Nanocrystals Mediated by Polyvinylpyrrolidone: Insights from Density-Functional Theory and Molecular Dynamics. J. Phys. Chem. C 2015, 119, 11982−11990. (65) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Platonic Gold Nanocrystals. Angew. Chem., Int. Ed. 2004, 43, 3673− 3677.

H

DOI: 10.1021/acs.langmuir.9b00794 Langmuir XXXX, XXX, XXX−XXX