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May 17, 2018 - School of Materials Science and Engineering, State Key Laboratory for Power Metallurgy, Central South University, Changsha,. Hunan 4100...
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Elucidating the Growth Mechanism of Plasmonic Gold Nanostars with Tunable Optical and Photothermal Properties Yuhan Pu,† Yawen Zhao,‡ Peng Zheng,§ and Ming Li*,† †

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School of Materials Science and Engineering, State Key Laboratory for Power Metallurgy, Central South University, Changsha, Hunan 410083, China ‡ China Academy of Engineering Physics, Mianyang 621900, China § Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, United States S Supporting Information *

ABSTRACT: Gold nanostars (GNSs) have received considerable attention in surface-enhanced spectroscopies, catalysis, biosensing, photothermal therapy, and photovoltaics because of their unique optical properties arising from the anisotropic structure. GNSs typically consisting of a central core and several protruding tips are usually synthesized by a seed-mediated growth approach, but the growth mechanism and optical properties have yet to be fully understood. Here, we systematically investigate the seed-mediated growth process of GNSs to gain an insight into the growth mechanism and evolution of their optical and photothermal properties. By tailoring the core size, tip length and tip angle, the main localized surface plasmon resonance (LSPR) peak wavelength can be broadly tuned from the visible to near-infrared (NIR) region. Our observations show that the protruding tips grow rapidly away from the central core at the initial growth stage, leading to a redshift of the main LSPR peak. The preferential deposition of gold atoms onto the gold core takes place at the later growth stage, gradually blue-shifting the main LSPR peak. GNSs exhibit a large molar extinction coefficient ranging from 4.0 × 108 M−1 cm−1 to 4.5 × 1010 M−1 cm−1, the log value of which correlates linearly with the main LSPR peak wavelength and accordingly allows for facile determination of the GNS concentration in a suspension. In addition, GNSs are excellent NIR photothermal materials with the LSPR-dependent photothermal conversion efficiency. The maximum photothermal conversion efficiency of GNSs occurs at a LSPR wavelength of 740 nm, blue-shifted from the incident laser wavelength. Our present work suggests that GNSs exhibit excellent optical and photothermal properties that can be optimized by tailoring the dimensional parameters.

1. INTRODUCTION The growing interest in plasmonic metal nanomaterials over the past three decades has largely stemmed from a plethora of applications, including chemical and biological biosensing, photothermal therapy, bioimaging, surface-enhanced spectroscopies, catalysis, photovoltaics, nanolaser and spaser, and others.1−6 These applications are mainly based on the extraordinary localized surface plasmon resonance (LSPR) effect that arises from the excitation of collective oscillations of free conduction electrons within noble-metallic nanomaterials induced by the incident light.4,7,8 Much of the research focus has been currently devoted to those noble metals such as gold, silver, and copper with their tunable LSPR absorption in the visible-near-infrared (NIR) region. Compared with silver and copper, gold nanostructures have attracted more attention because of their versatility in surface modification, bioinertness, and structure stability.9−11 The LSPR properties of gold nanoparticles (GNPs) strongly depend on their size, shape, morphology, surface chemistry, and the dielectric properties of the surrounding medium.4,12 The LSPR excitation induces an electromagnetic near-field enhancement in the vicinity of the © XXXX American Chemical Society

nanoparticle. This enhanced near-field is able to concentrate incident light on a length scale far below the wavelength of light and provides a means to use GNPs as nanoscale optical antennae in various applications. Both experimental and theoretical results show that the increasing particle size of spherical GNPs produces a large particle surface area and extinction cross sections but red-shifts the LSPR wavelength with a limited range of 510−550 nm.4,13,14 Thus, applications of spherical GNPs are obstructed by both the narrow tuning range of LSPR wavelength and low electromagnetic enhancements on the order of ∼102 in isolated spherical GNPs.12 To extend the tuning range of LSPR and maximize the electromagnetic enhancement, clusters such as dimers and trimers of spherical GNPs have been developed to achieve the electromagnetic coupling upon forming “hot spots,” which have been demonstrated with single-molecule detection sensitivity.15−19 However, this requires complicated synthetic protocols with poor stability of plasmonic structures, which Received: May 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

was produced with a Millipore Direct-Q3 UV system (Millipore Corporation, Molshein, France) and used throughout the experiments. All chemicals and solvents were of analytical grade and used as received. Glassware was cleaned with aqua regia, rinsed with 18.2 MΩ·cm−1 water, and air-dried before use in experiments. 2.2. Synthesis of PVP-Coated Gold Seeds. Gold seeds were synthesized according to previously published protocols.12,31 In a typical synthesis, 1 mL of 1 wt % HAuCl4·4H2O aqueous solution was diluted to 90 mL with the addition of ultrapure water, and then 2 mL of 38.8 mM sodium citrate aqueous solution was quickly injected. A total of 1 mL of freshly prepared NaBH4 solution (0.075 wt % in 38.8 mM sodium citrate solution) was then slowly added. After being kept overnight with constant stirring at room temperature, the gold seed solution was prepared. The UV−visible extinction spectrum showed that the gold seed solution had an extinction maximum at 519 nm. Subsequently, 5.0 g of PVP-10 was dissolved in 50 mL of the gold seed solution. After being stirred at room temperature for 24 h, PVPcoated gold seeds were prepared. 2.3. Synthesis of Gold Nanostars. GNSs were synthesized using a seed-mediated growth method as described in our previous reports.12,31 In a typical synthesis, 1.5 g of PVP-10 was dissolved in 15 mL of DMF. Then, 82 μL of 50 mM HAuCl4·4H2O aqueous solution was added, followed by the addition of the PVP-coated gold seeds. To tune the LSPR wavelength of GNSs, the quantity of PVPcoated gold seeds added was varied. After being stirred at room temperature for 3 h, the reaction solution was successively centrifuged and washed twice with ethanol and water, respectively. The pellets were finally redispersed into 1 mL of ethanol for further use. 2.4. Characterization. Extinction spectra were collected using an Agilent Cary 5000 UV−vis−NIR spectrophotometer (Agilent Technologies, USA). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were taken on an FEI Titan G2 80−200 transmission electron microscope operating at an acceleration voltage of 300 kV, which was equipped with an aberration corrector and allowed to reach a spatial resolution of ∼0.7 Å. STEM samples were prepared by dropping the gold colloidal solution on an ultrathin Formvar-coated 200-mesh copper grid (Ted Pella, Inc.) and then being dried in the air. Particle size (core, tip length and tip angle) distributions were determined from the STEM data using the ImageJ analysis software. At least 100 particles were measured for each type of particle. Quantitative analysis of gold was performed with an Agilent 5100 inductively coupled plasma−optical emission spectrometer (ICP-OES, Agilent Technologies, USA). For the sample preparation of ICP-OES measurements, GNSs were centrifuged and redispersed into ultrapure water and then digested by aqua regia to obtain a clear solution. Gold standard solution was diluted with ultrapure water to achieve a series of solutions of various concentrations, followed by the ICP-OES measurement to establish the calibration curve for gold quantification. The temperature of GNS suspensions was measured by an FLIR A65 infrared thermal imaging instrument (FLIR Systems, USA) under the irradiation of an 808 nm laser. Molar Extinction Coefficient and Photothermal Performance of GNSs. To evaluate the molar extinction coefficient and photothermal conversion efficiency of GNSs, GNS suspensions of various LSPR wavelengths were investigated. Measurement of the Molar Extinction Coefficient. The molar extinction coefficient (ε(λ)) of GNSs was determined according to the Lambert−Beer law:

limits broad applications of spherical GNPs, especially in robust biological environments. Previous studies demonstrated that anisotropic structures with sharp edges or vertices offer a broad degree of freedom in tuning the LSPR wavelength into the NIR region.4,20,21 Among those anisotropic gold nanostructures, gold nanostars (GNSs) typically consisting of a central core and several protruding arms with sharp tips have received particular interest due to their unique morphology and optical properties.22−28 In GNSs, the core acts as an antenna dramatically producing electromagnetic field enhancements of protruding tips, and the geometry of the protruding tips allows for facile tuning of LSPR wavelengths and extinction cross sections through hybridization with the core plasmon.29,30 Thus, GNSs have been considered as a class of excellent plasmonic materials with intrinsic “hot-spots” available at protruding tips. For these reasons, tremendous efforts have been devoted to developing synthetic strategies for controllable synthesis of GNSs.11,27,31−35 Several methods have been developed for synthesizing GNSs of tunable LSPR wavelengths from the visible to NIR region.12,27,31−35 Although a few seed-free methods are reported in the literature,33,36 the seed-mediated growth method using gold seeds of various sizes and shapes is currently the mainstream in the research community to produce GNSs because of its merits, such as high yield and operation simplicity. Typically, the seed-mediated growth method is performed in a solution containing N,Ndimethylformamide (DMF) and poly(vinylpyrrolidone) (PVP).12,31,32 PVP serves as both a reducing agent and stabilizing agent for formation of GNSs in the presence of DMF.37,38 In spite of tremendous efforts on GNS related studies, the growth mechanism of GNSs in the seed-mediated growth approach and their optical/photothermal properties have yet to be fully understood. In our previous work, we revealed the excellent optical properties of GNSs prepared by the seed-mediated method in the presence of PVP and DMF and demonstrated the effect of the LSPR peak wavelength on the surface-enhanced Raman scattering (SERS) enhancement.4,12,22−26,31 Here, we further investigate the structural evolution of GNSs during the seedmediated growth process to gain an insight into the growth mechanism of GNSs. We tailor the size and morphology of GNSs through simply changing the gold seed concentration, and thus tune the LSPR wavelength from the visible to NIR region. The correlation of LSPR peak wavelengths with the structural parameters will be elucidated. In addition, we investigate the effect of LSPR peak wavelength on molar extinction coefficients and photothermal conversion efficiency since these parameters are of significance for the design of GNS-based plasmonic materials and their LSPR-related applications.

2. MATERIALS AND METHODS 2.1. Chemicals and Materials. Chloroauric acid (HAuCl4·4H2O, 99% trace metals basis) was purchased from Shanghai Civi Chemical Technology Co., Ltd. (Shanghai, China). Trisodium citrate dihydrate (HOC(COONa)(CH2COONa)2·2H2O, analytical reagent), sodium borohydride (≥98%), and N,N-dimethyformamide (DMF, anhydrous 99.8%) were purchased from Sinopharm (Beijing, China). Poly(vinylpyrrolidone) (PVP-10, (C6H9NO)n; molecular weight, 10 kg/ mol) was purchased from Sigma-Aldrich (Sigma-Aldrich Shanghai Trading Co Ltd., Shanghai, China). Gold standard solution (1000 μg/ mL) containing 1.5 M HCl was obtained from Guobiao (Beijing) Testing and Certification Co., Ltd. Ultrapure water (18.2 MΩ·cm−1)

A(λ) = ε(λ)LC

(1)

where A(λ) is the optical density (extinction intensity) at a wavelength of λ, L is the path length (usually 1 cm for the standard cuvette) of the incident light passing through the suspension, and C is the concentration of GNSs (in mol/L). To quantify the molar extinction coefficient, both the optical density and the concentration of GNSs in suspensions must be obtained from the UV−vis−NIR extinction spectra and the ICP-OES measurement, respectively. Details for conversion of the atomic concentration of gold measured B

DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry by ICP-OES into the particulate concentration of GNSs can be found in the Supporting Information. Measurement of the Photothermal Conversion Efficiency. To evaluate the photothermal conversion efficiency, the temperature of GNS suspensions (0.45 g/L) of various LSPR peak wavelengths was recorded as a function of time under continuous irradiation of the 808 nm laser with a power density of 1.22 W/cm2, and after reaching the steady-state temperature, the laser was then switched off to naturally cool down to room temperature. The temperature was monitored by a FLIR A65 infrared thermal imaging instrument. The photothermal conversion efficiency (η) was calculated using the following equation:41

η=

hS(Tmax,GNS − Tmax,H2O) − Q 0 I(1 − 10−A(λ))

(2)

where A(λ) is the optical density of GNSs at 808 nm (λ is the laser wavelength, here 808 nm), h is the heat transfer coefficient, S is the surface area of the quartz cuvette covered by the sample, I is the incident laser power, Tmax,GNS and Tmax,H2O represent the steady-state temperatures for GNS suspensions and water, respectively, and Q0 represents the heat dissipated due to the absorption of incident light (808 nm laser) by the quartz cuvette only containing the solvent (water here). Q0 can be independently calculated by the equation Q 0 = 5.4 × 10−4 × I (J/s)

(3) Figure 1. (A) Schematic illustration of preparation of gold nanostars (GNSs) by the seed-mediated growth method. Gold seeds serve as the nucleation site for the anisotropic growth of GNSs through the reduction of AuCl4− in the presence of PVP and DMF. (B) Extinction spectra of gold seeds and GNSs prepared with various concentrations of gold seeds (left to right: gold seeds, 0.84 nM, 0.58 nM, 0.29 nM, 0.218 nM, 0.145 nM, 72.50 pM, 36.25 pM, 7.25 pM, and 1.45 pM). HAADF-STEM images of (C) gold seeds and GNSs prepared with various concentrations of gold seeds: (D) 0.84 nM, (E) 0.29 nM, (F) 0.218 nM, (G) 0.145 nM, and (H) 36.25 pM. Insets in D−H show STEM images of GNSs at a higher magnification.

To calculate hS, a dimensionless driving force temperature (θ) is introduced, scaled using the maximum system temperature (Tmax): θ=

T − Tmax,H2O Tmax,GNS − Tmax,H2O

(4)

and a time constant (τs) of the GNS suspension system is introduced: hS =

mH2OC H2O τs

(5)

t = − τs ln θ

(6)

N

εsim =

where T is the temperature of GNS suspensions at a cooling time (t) after the laser is turned off, mH2O and CH2O are the mass and heat capacity (4.2 J/g) of water used as the solvent here. τs can be determined by applying the linear time data from the cooling period vs −ln θ after the laser is switched off. 2.5. Finite-Difference Time-Domain (FDTD) Simulations. Theoretical extinction spectra for GNSs were calculated by commercial FDTD Solutions 8.6 (Lumerical Solutions, Inc., Canada). The dielectric function of gold was taken from Johnson and Christy.40 The background refractive index of the surrounding medium (water) was set to be 1.33. A total-field scattered field was used as the input source. The GNS model was created with a mesh size of 1 nm. Although the core size, tip length, and tip angle are all the tuning parameters, the wide LSPR tuning range was dominated by the contribution from the tip length. Therefore, variation of the tip lengths leads to a broadened LSPR peak observed experimentally (Figure 1). To account for the tip length variation-induced LSPR peak broadening in FDTD simulations, the GNS model was constructed using the mean values of the core size and tip angle while varying the tip length, following the statistics in Figure S2. Each extinction spectrum calculated at a tip length of x was normalized by the probability function f(x|μ,σ2) given by

f (x|μ , σ 2) =

1 2πσ

2

2

e−(x − μ)

∑ εi(λ)f (xi|μ , σ 2) i=1

(8)

where N is the number of tip lengths studied and εi(λ) is the molar extinction coefficient of GNSs with a tip length of xi at λ.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of GNSs. Gold seeds were prepared through the reduction of AuCl4− in the presence of both NaBH4 and sodium citrate. The strong reduction capability of NaBH4 facilitated the rapid nucleation process and produced small sized gold seeds with a particle size of 6.3 nm in diameter (Figure 1A,C and Figures S1 and S2). In the seed-mediated preparation of GNSs, DMF serves as a weak reducing agent for the incomplete reduction of Au3+ to Au+,37,38 as demonstrated in the color transformation of the reaction solution from the light-yellow to colorless within minutes. Disappearance of the light-yellow color originates from the ligand−metal charge transfer in the Au+−DMF complex.37 PVP plays multiple roles in the seed-mediated synthesis of GNSs. For example, PVP is the stabilizing agent of the resulting GNSs and serves as a reducing agent for further reduction of Au+ to Au0 and the stabilizing agent of the resulting Au0 as well. More importantly, PVP guides the anisotropic deposition of Au0 onto the gold seed surface to form the GNS structure. Thus, the growth of GNSs undergoes a series of color changes from the light-yellow to colorless and finally to dark blue. We did not observe the appearance of dark

/2σ 2

(7)

where μ and σ2 are the mean value of the tip length and the standard variation, respectively. The final extinction coefficient (εsim) for a GNS was obtained using C

DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (A) Schematic structure of GNSs, illustrating the central core, protruding tips, and tip angle. (B) Distributions of (i) core size, (ii) tip length, and (iii) tip angle of GNSs statistically analyzed from STEM images. (C) Core size, tip length, and tip angle of GNSs as a function of the gold seed concentration.

overall size (core plus protruding tips), where the core size becomes 16.3 nm but the tip is very short. As the gold seed concentration decreases, both core size and tip length further increase (Figure 1D−H and Figure S2). The rationale is that more gold atoms from the reduction of AuCl4− are available per gold seed when the lower concentration of gold seeds was used. STEM images reveal that GNSs prepared in this work have five protruding conical tips with uniform distribution on the central core. Figure 2A shows the schematic model of a GNS with the three well-defined dimensional parameters: core size, tip length, and tip angle. We statistically investigated the distribution of the core size, tip length, and tip angle of GNSs based on STEM images to evaluate the shape heterogeneity (Figure 2B and Figure S2). At the high gold seed concentration, the core size possesses a quite homogeneous size distribution similar to that of gold seeds but a little broadening with GNSs becoming much larger (Figure 2B,i). However, GNSs from the low gold seed concentration are much more heterogeneous for both tip length and tip angle (Figure 2B,i,ii). The increase in the shape heterogeneity of GNSs leads to the broadening of the main LSPR bands (Figure 1B). To better evaluate the evolution of the dimensional parameters of GNSs, we further correlated these structural parameters with the gold seed concentration (Figure 2C). It can be clearly seen that both core size and tip length decrease with the increasing gold seed concentration. In particular, as the gold seed concentration increases, both of them decrease faster at the lower gold seed concentration region (0.1 nM). We suggest that the deposition of Au0 is a diffusion-controlled process depending on the reduction of Au+ in the reaction solution. At the lower gold seed concentration, there are more gold atoms available for each gold seed, and thus the resulting GNSs become much larger. Furthermore, the growth of secondary structure on the protruding tips was observed at the relatively high AuCl4−/ gold seed ratio (low gold seed concentration; Figure S1I,J). In addition, it was observed that the tip angle increases as the gold seed concentration decreases (Figure 2C). The growth of

blue in the absence of PVP, indicating that PVP is critical for formation of the GNS structure. Previous studies showed that PVP preferentially adsorbed onto {111} facets of gold nanocrystals,43 which inhibited the further growth of gold atoms at these faces and facilitated the anisotropic growth of gold nanostructures. We tailored the morphology of GNSs by varying the gold seed concentration. It is obvious that the more gold seeds present in the reaction solution, the less Au0 initially from AuCl4− there is available per gold seed since the AuCl4− concentration keeps constant, and thus the smaller the resulting GNS is. Figure 1B shows extinction spectra of gold seeds and GNSs produced with various gold seed concentrations. GNSs typically possess two characteristic LSPR bands in their extinction spectra that are assigned to the central core- and protruding tip-associated LSPR modes, respectively.29,30 Similar to gold nanorods, the LSPR band (main LSPR band) localized at the long wavelength is for the longitudinal resonance mode due to the plasmon hybridization of the protruding tips with the central core, and the LSPR band associated with the central core appears at the left (low wavelength) shoulder of the main LSPR band, considered as the transversal mode of the structure consisting of the central core and a protruding tip (Figure S3).29,44 As the gold seed concentration increases from 1.45 pM to 0.84 nM, the main LSPR peak blue-shifts from 886 to 535 nm while the core related LSPR peak blue-shifts less as well with a relative intensity increase. The main LSPR intensity is dominant in the GNSs prepared with low gold seed concentrations, and the core-associated LSPR absorption becomes dominant in the GNSs prepared with high gold seed concentrations. To study the structural evolution of GNSs as a function of the gold seed concentration, STEM images were presented for GNSs synthesized with different gold seed concentrations (Figure 1C−H and Figure S1). As stated above, GNSs possess a three-dimensional structure consisting of a central core and several protruding tips. The protruding tips possess a conical shape away from the central core with a rounded end. At the gold seed concentration of 0.84 nM, the GNS has a small D

DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

blue-shifts is a very interesting phenomenon. We attribute this change to the rapid growth of the protruding tips at the initial reaction stage, leading to a much higher aspect ratio similar to the gold nanorods.44 The blue-shift of GNSs at the later stage indicates the preferential deposition of Au0 onto the central core so that the aspect ratio decreases. The optical density of the growth solution increases with the GNSs becoming bigger, indicating much larger extinction cross-section for large sized GNSs (Figure 3B). On the basis of all aforementioned observations, we proposed the growth mechanism of GNSs, as illustrated in Scheme 1. First, AuCl4− is reduced by the weak reducing agent

GNSs becomes more random as the GNSs grow much bigger, so that GNSs become increasingly heterogeneous for both tip length and tip angle. 3.2. Growth Mechanism of GNSs. The growth of GNSs was real-time monitored by the extinction spectra of the reaction solution, as shown in Figure 3. It can be observed that

Scheme 1. Proposed Growth Mechanism of GNSs Prepared by the Seed-Mediated Growth Approach in the Presence of DMF and PVP

DMF to form the Au+−DMF complex, as demonstrated by the color change from the light-yellow of the growth solution to colorless. Following this, the further reduction of Au+ by PVP to the Au0 atoms allows the nucleation of Au0 with the PVPcoated gold seed as the nucleation site. Here, PVP serves as both a stabilizing agent and reducing agent during the growth process and induces the anisotropic growth of gold atoms, eventually leading to the formation of GNSs. Both central core and protruding tips simultaneously grow over the entire course of synthesis, but the protruding tips rapidly grow at the initial stage, leading to the high aspect ratio. At the later stage, the gold atoms are further deposited onto the central core to fill the space between protruding tips, which decreases the aspect ratio. The main LSPR wavelength is due to hybridization of the tip and core plasmons, and the LSPR mode at the left shoulder of the extinction spectrum is associated with the central core of the GNS. Thus, the main LSPR band first rapidly red-shifts at the initial growth stage of the GNS synthesis and then blueshifts to the steady LSPR wavelength as the reaction progresses. 3.3. Molar Extinction Coefficients of GNSs. The molar extinction coefficient (ε) is an important parameter for both evaluation of optical properties of GNSs and determination of the GNP concentration. Although extinction coefficients of gold nanospheres and gold nanorods have been widely studied and well documented in the literature,8 few efforts have been devoted to understanding the ε of GNSs due to their complicated geometry. This work further evaluated the molar

Figure 3. (A) Extinction spectra of the reaction solution with the gold seed concentration of 36.25 pM for real-time monitoring the growth process of GNSs with a time interval of 1 min starting from the original reaction solution (0 min). The inset shows the optical images of the reaction solution for certain time intervals (0, 1, 5, 10, and 180 min). (B) LSPR peak wavelength and extinction intensity at their LSPR peak wavelengths of the reaction solution for the growth of GNSs shown in A as a function of reaction time.

the initial reaction solution has no characteristic absorption band, but an absorption band around 706 nm appears after reacting for 1 min. The LSPR bands shows up and becomes intense as the reaction time is prolonged. We note that the LSPR band around 520−550 nm has a slight blue-shift as the reaction progresses, but the main LSPR band first shows up around 706 nm and rapidly red-shifts to 830 nm within 4 min after the gold seeds were added and then gradually blue-shifts back to a steady LSPR peak wavelength of ∼780 nm (Figure 3). The growth process can be visually examined by the color change of the reaction solution from the light-yellow color to colorless within 1 min, and finally to light blue and dark blue (the inset in Figure 3A). The dark blue color indicates the formation of GNSs with a LSPR peak in the NIR region. The observation that the main LSPR band first red-shifts and then E

DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Parameters of GNSs for Calculation of Their Molar Extinction Coefficients [gold seed] (nM)

LSPRa (nm)

O.D.

gold seed stock solution 0.84 0.58 0.29 0.218 0.145 0.0725 0.03625 0.00725 0.00145

519 535 608 620 657 730 740 775 864 886

0.388 7.420 8.200 5.980 13.150 17.390 16.130 24.270 19.924 17.530

CAu‑atom by ICP-OES (M) 9.829 1.501 2.187 1.686 2.892 3.075 3.440 3.582 3.259 3.253

× × × × × × × × × ×

10−5 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

VGNSb (nm3)

NAu‑atomc

130.924 1372.644 1916.585 3621.124 5510.349 6786.769 12741.339 24656.840 87059.417 141935.451

7768 81438 113710 214840 326927 402656 755938 1462880 5165198 8420970

CNPd (M) 1.265 1.843 1.923 7.846 8.845 7.636 4.551 2.449 6.310 3.863

× × × × × × × × × ×

10−8 10−8 10−8 10−9 10−9 10−9 10−9 10−9 10−10 10−10

εexp (M−1· cm−1) 3.066 4.026 4.263 7.622 1.487 2.277 3.544 9.911 3.157 4.538

× × × × × × × × × ×

107 108 108 108 109 109 109 109 1010 1010

εFDTD (M−1· cm−1) 3.749 4.129 5.873 2.944 5.294 6.498 1.902 6.176 1.169 1.415

× × × × × × × × × ×

107 108 108 109 109 109 1010 1010 1011 1011

a Note: LSPR is the main LSPR wavelength of gold seeds and GNSs. bVGNS is the volume of a single gold seed or GNS directly from the GNS model in the AutoCAD software created with the dimensional parameters statistically obtained from STEM images. cNAu‑atom is the total number of gold atoms in a single gold seed or GNS. dCNP is the concentration of gold seeds or GNSs.

from 535 to 886 nm. We also note that the ε values of GNSs are at least 1 order of magnitude higher than those of gold nanorods with a similar LSPR peak wavelength,40 which is mainly attributed to the sharp protruding tips of GNSs and their coupling with the central core. Not only do the tip−tip and tip−core couplings red-shift the main LSPR wavelength but also increase the extinction cross-section and thereby the ε value of GNSs.12,25 Therefore, GNSs exhibit excellent optical properties with a large molar extinction coefficient. It is further revealed that the log value of the ε of GNSs exhibits a linear relationship with the LSPR peak wavelength over the range from 608 to 886 nm. The linear equation for the calibration curve can be expressed as log(ε) = 0.00695 × λLSPR + 4.488 with an R2 value of 0.97, where λLSPR is the main LSPR peak wavelength. Thus, the molar extinction coefficients of GNSs can be estimated if the LSPR peak wavelength is measured, which can be used for facile determination of the GNS concentration in a suspension. Since the LSPR peak wavelength depends on the structure of GNSs, the structural correlation of the ε value of GNSs was further verified by the FDTD simulation. Similarly, the dimensional parameters statistically obtained from the STEM images were used to create the model for the FDTD simulation. Considering the complicated geometric configuration, we simplified the GNS model with five protruding tips uniformly distributed on the equatorial plane of the central core. It is well-known that the experimental extinction spectra result from GNS ensembles in suspensions. The core size of GNSs displays a Gaussian distribution with the center position ranging from 14.3 to 64.6 nm, the LSPR wavelength of which varies slightly from 520 to 550 nm (Figure 1B and Figure S5). Thus, we used the center value of the Gaussian distribution to represent the core size in the FDTD simulation. To simplify the simulation, we only considered the Gaussian distribution of the GNS tip length with the tip angle fixed to the center position of the Gaussian distribution of the tip angle. We calculated the extinction spectra of each type of GNS, 8−11 tip length values of which were selected from the fitted Gaussian distribution curve of the tip length, Then, all of these extinction spectra were fitted with the Gaussian model to produce the simulated extinction spectra of GNSs (Figure S6). The simulated molar extinction coefficients for all GNS samples are a little higher than those obtained from the Lambert−Beer law, which may be ascribed to the idealized model of GNSs. The simulated molar extinction coefficient

extinction coefficient of GNSs with various LSPR wavelengths and subsequently investigated its structure dependence. First, we created the geometric model of GNSs using the dimensional parameters obtained from STEM images using AutoCAD (Figure S2); it should be noted that the mean values of the Gaussian function for core size, tip length, and tip angle of GNSs were used to create the GNS model so that the total volume of a single GNS was directly obtained using the AutoCAD software. Assuming that GNSs prepared in this work have a face-centered cubic (FCC) crystal structure (Figure S4 and Table S1), we thus are able to calculate the total number of gold atoms in a single GNS (Table 1). We then converted the atomic concentration of gold for each type of GNS determined by ICP-OES to the particulate concentration of GNSs (see the Supporting Information and Table 1). Then, according to the Lambert−Beer law, the molar extinction coefficient of each type of GNS was calculated, as shown in Figure 4 and Table 1. We determined an ε value of 3.1 × 107

Figure 4. Experimental and FDTD simulated molar extinction coefficients of GNSs as a function of LSPR wavelength. Both experimental and FDTD simulated molar extinction coefficients exhibit a linear dependence on the main LSPR wavelength.

M−1·cm−1 at 519 nm for the spherical gold seeds (6.3 nm in diameter) as well, which is in good agreement with the value in the literature.39 The ε value at the main LSPR wavelength for all GNS samples in this work is listed in Table 1. It can be seen that the ε value at the main LSPR wavelength increases by 2 orders of magnitude from 4.0 × 108 M−1·cm−1 to 4.5 × 1010 M−1·cm−1 as the increasing LSPR peak wavelength of GNSs F

DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

decreases with the increasing LSPR peak wavelength from 535 to 886 nm. Surprisingly, the optimal photothermal performance of GNSs occurs with an η value as high as 80% at the LSPR peak wavelength of 740 nm, blue-shifted from the laser wavelength, consistent with the LSPR effect on the SERS enhancement shown in our previous work (Figure 5C).31 The rationale is that both optical scattering and absorption coexist in a GNS suspension, and the optical scattering increases as the light path length across the GNS suspension increases. As is well-known, the optical absorption is responsible for the photothermal conversion, so that the absorption fraction decreases as the penetration of incident light increases. Thus, the highest photothermal efficiency occurs at the LSPR wavelength blue-shifted from the incident laser. Furthermore, the cycling experiment demonstrates the high photothermal stability of GNSs as well (Figure 5D). All these results suggest that the photothermal performance of GNSs can be tuned and optimized by controlling the LSPR wavelength.

follows a linear dependence on the main LSPR peak wavelength as well. Therefore, our results reveal that the LSPR dependence of the ε of GNSs originates from the structural evolution, and GNSs exhibit a high ε value beneficial to broad applications. 3.4. Photothermal Conversion Efficiency. GNSs exhibit extraordinary optical properties with tunable visible−NIR LSPR wavelengths and large ε values. Recent studies revealed that GNSs could be used as photothermal conversion materials for photothermal therapy.45,46 In general, photothermal conversion materials require a large ε value and high photothermal conversion efficiency (η). To evaluate the photothermal performance of GNSs, GNS suspensions of various LSPR wavelengths were exposed to an 808 nm laser at a power density of 1.22 W/cm2. Exposure to the 808 nm laser boosts the suspension temperature to a steady temperature within less than 5 min, which dramatically cools down once the laser is switched off (Figure 5). We can obtain a temperature

4. CONCLUSIONS In summary, we systematically investigated the growth process of GNSs in the seed-mediated approach in the presence of PVP and DMF. The dimensional parameters including core size, tip length, and tip angle and optical/photothermal properties of resulting GNSs can be manipulated by the gold seed concentration. In general, the core and protruding tips of GNSs grow simultaneously over the entire growth course. It is found that GNSs of large core sizes and tip lengths are produced at a low concentration of gold seeds. The nucleationcontrolled growth of protruding tips leads to a high aspect ratio at the initial growth stage, but the preferential deposition of gold onto the central core takes place at the later stage. PVP induces the anisotropic growth of gold atoms on the surface of gold seeds to form the GNS structure. The two-stage growth mechanism is further confirmed by the observation that the main LSPR peak wavelength rapidly red-shifts at the initial stage and then gradually blue-shifts to a steady LSPR wavelength at a later stage. We are able to tune the LSPR peak wavelength of GNSs from the visible to NIR region by changing the gold seed concentration. The molar extinction coefficient of GNSs increases by 2 orders of magnitude from 4.0 × 108 M−1·cm−1 to 4.5 × 1010 M−1·cm−1 as the main LSPR peak wavelength increases from 535 to 886 nm. The quantitative correlation reveals that the logarithm of the molar extinction coefficient of GNSs has a linear dependence on the LSPR peak wavelength. Thus, the GNS concentration can be determined if the LSPR peak wavelength is achieved. It is found that the photothermal conversion efficiency of GNSs first increases and then decreases with the increasing LSPR peak wavelength, so that the optimized photothermal conversion efficiency can be achieved at an LSPR wavelength of 740 nm under irradiation of an 808 nm laser, blue-shifted from the incident laser wavelength. Therefore, GNSs exhibit excellent optical properties with both a high molar extinction coefficient and extraordinary photothermal performance. The present work strengthens our understanding on the growth of GNSs in the seed-mediated synthetic method and provides us with important information for broad applications based on their excellent optical and photothermal properties.

Figure 5. (A) Photothermal heating curves of gold seed and GNS aqueous suspensions (0.45 g/L) prepared with 0.0725, 0.218, and 0.84 nM gold seeds added under the irradiation of 808 nm laser (power density: 1.22 W/cm2). The 808 nm laser was turned on to heat the suspensions until a steady temperature was reached, and then the laser was switched off. (B) Plot of time vs −ln(θ) taken from the photothermal heating curves at the cooling period after the laser was switched off (light yellow region in A). Time constant (τs) is determined from the slope of the time vs −ln(θ) curve. (C) Effect of the LSPR wavelength of GNSs on the photothermal conversion efficiency (η). The red line indicates the laser wavelength (808 nm) used for the present photothermal experiment in this work. (D) Photothermal heating curve of the GNS aqueous suspension prepared with 0.0725 nM gold seeds over three laser on/off cycles under the irradiation of an 808 nm laser (1.22 W/cm2, [GNS]: 0.45 g/L).

increase by more than 46 °C for the GNS suspension with a LSPR peak wavelength of 740 nm after a laser exposure less than 5 min. Gold seeds used for the GNS synthesis exhibit a negligible photothermal effect (Figure 5A). To calculate the photothermal conversion efficiency, we first obtained the time constant (τs) by determining the slope of the time vs −ln θ curve from the cooling period of the photothermal heating curve (Figure 5B). It can be seen that the photothermal conversion efficiency of GNSs first increases and then G

DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01354. HAADF-STEM images of gold seeds and gold nanostars prepared with various concentrations of gold seeds; core size distribution, tip length distribution, and tip angle distribution of gold nanostars prepared with various concentrations of gold seeds; schematic illustration of plasmon resonance modes; calculation of the concentration of gold nanostars (GNSs); crystallographic parameters of the FCC unit cell of gold taken from Crystallography Open Database; effect of particle size (core size) on the LSPR peak wavelength of spherical gold nanoparticles; FDTD simulated extinction spectra of gold seeds and gold nanostars (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected]. ORCID

Peng Zheng: 0000-0001-5907-8505 Ming Li: 0000-0002-2289-0222 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.L. acknowledges financial support by the National Thousand Young Talents Program of China, InnovationDriven Project of Central South University (No. 2018CX002), and Hunan Provincial Science and Technology Program (No. 2017XK2027).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01354 Inorg. Chem. XXXX, XXX, XXX−XXX