Synthesis of Absorption-Dominant Small Gold Nanorods and Their

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Synthesis of Absorption-Dominant Small Gold Nanorods and Their Plasmonic Properties Henglei Jia, Caihong Fang, Xiao-Ming Zhu, Qifeng Ruan, Yi-Xiang J. Wang, and Jianfang Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01444 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 19, 2015

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Synthesis of Absorption-Dominant Small Gold Nanorods and Their Plasmonic Properties Henglei Jia,† Caihong Fang,† Xiao-Ming Zhu,‡ Qifeng Ruan,† Yi-Xiang J. Wang,‡ and Jianfang Wang*,† †

Department of Physics and ‡Department of Imaging and Interventional Radiology, Prince of

Wales Hospital, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

ABSTRACT:

Absorption-dominant small Au nanorods with diameters less than 10 nm are

prepared using a facile seed-mediated growth method. The diameters of the small gold nanorods range from 6 nm to 9 nm and their lengths vary from 16 nm to 45 nm. Their aspect ratios can be tailored from 2.7 to 4.7. As a result, the longitudinal plasmon resonance wavelengths are readily tunable from ~720 nm to ~830 nm by changing the seed-to-Au(III) molar ratio in the growth solution. The fractions of the scattering in the total extinction of the small Au nanorods are found to be in the range of 0.005 to 0.025 with finite-difference time-domain simulations, confirming that the extinction values of these small Au nanorods are dominantly contributed to by the light absorption. Moreover, the small Au nanorod sample is coated with dense silica layer for photothermal therapy with three cell lines. It shows an improved photothermal therapy

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performance compared to a big Au nanorod sample at the same cellular Au contents. Our study suggests that small Au nanorods are promising light absorbers and photothermal therapy agents.

INTRODUCTION Gold nanorods have attracted much attention in many fields, such as optics,1−3 biotechnology,4−7 and catalysis,8−10 owing to their attractive localized plasmon resonance properties. They have two localized plasmon modes, the longitudinal and transverse ones, depending on the collective electron oscillation direction.2 For the former one, the electron oscillation is along the length axis, while that for the latter one is perpendicular to the length axis. The transverse mode is relatively insensitive to the nanorod size, whereas the longitudinal plasmon wavelength is approximately linearly dependent on the length-to-diameter ratio, which is usually called the aspect ratio.11,12 Another important property of Au nanorods is their large extinction crosssections, which are orders of magnitude larger than those of conventional nanoscale optical species, such as organic fluorophores, semiconductor nanocrystals, and atoms/ions.13 Moreover, the relative contributions of the absorption and scattering to the total extinction of Au nanorods at the longitudinal plasmon resonance wavelength are dependent on the diameter if the aspect ratio is fixed.11,14 The unique properties of Au nanorods, including large absorption cross-sections, efficient photothermal conversion, tunable longitudinal plasmon wavelengths, high chemical stability and low cytotoxicity, have enabled their applications in functioning as optically active agents for various biomedical applications.15−17 Photothermal therapy (PTT), where malignant cells are killed by local temperature rise through photothermal conversion of introduced therapy agents,


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holds great promise for cancer treatment.18−20 Gold nanorod-based PTT was first reported in 2006. In that work,18 antibody-conjugated Au nanorods were applied to selectively kill cancer cells through efficient photothermal conversion. Since then, great efforts have been made on this aspect, including both in vitro and in vivo photothermal heating and theoretical modeling to study the PTT performance and design effective PTT agents.21−23 For example, a computationally guided photothermal tumor therapeutic method has been developed by combining the quantitative biodistribution data of Au nanorods with computational modeling. With this method, photothermal temperature gradients in three-dimensional tissues can be computed.23 In addition, owing to the advantage of facile conjugation with targeting molecules and drugs, Au nanorods have been widely used in precise control of drug/gene delivery and release in live cells.24−27 Drug or gene, usually attached with mercapto or amino groups, can anchor themselves to the surface of Au nanorods through the formation of Au−S or Au−N bonds.4 The processes of drug loading and release can also be monitored by fluorescence microscopy if drugs are fluorescent. Recently, coating mesoporous silica on Au nanorods or incorporating Au nanorods into composite materials have been employed to increase the drug loading amount or control the drug release by switching on and off laser irradiaton.24,25 Moreover, on the basis of the photothermal property, Au nanorods have been used in photothermal and photoacoustic imaging.28−30 Silica-coated Au nanorods utilized as contrast agents for photoacoustic imaging exhibit attractive advantages over the traditional cell imaging techniques. Its resolution is at least an order of magnitude better than those of the traditional approaches.28 Besides the aforementioned applications, uncommon biomedical investigations based on the photothermal properties of Au nanorods have also been emerging.31−34 For instance, Au nanorods have been employed in surgery operations for cutting the carotid arteries of


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rabbits.31 Another interesting example is photothermal inhibition of the neural activity with Au nanorods as photothermal transducers on cellular membranes.32 In all of these applications, the used Au nanorods typically possess large scattering cross-sections that are comparable to their absorption cross-sections, or their scattering contributions to the total extinction have not been judiciously minimized. Large scattering contribution will reduce the photothermal conversion efficiency of Au nanorods, and therefore degrade the application performances based on plasmonic photothermal conversion. In this regard, Au nanorods with their absorption dominant in the total extinction will be highly beneficial for photothermal conversion-based plasmonic applications. We have previously found from both electrodynamic simulations and experiments that the absorption-to-scattering ratio for Au nanorods increases as the diameter is reduced.11,35 Therefore, the synthesis of Au nanorods with small diameters is highly desired to obtain the absorption-dominant property at the longitudinal plasmon resonance. Although small Au nanospheres, with diameters less than ~3 nm, can be readily prepared, their plasmon wavelengths are located around 520 nm in aqueous solutions.36 Since the first biological transparency window is in the spectral range from ~650 nm to ~950 nm, the plasmon resonance wavelengths of small Au nanospheres limit their applications in biotechnology. Our previous studies have shown that Au nanorods become absorption-dominant if their diameters are smaller than ~10 nm.11,35 Efforts have been made to synthesize such small Au nanorods. A seedless method has been developed,37 but the synthetic tunability of the longitudinal plasmon wavelength has not been mentioned. The seedless method has been further modified by use of aromatic compounds as the reducing agent for the preparation of small Au nanorods.38 In the modified method, the concentration of Ag+ ions in the growth solution is adjusted. As the concentration of Ag+ ions is increased, the longitudinal plasmon wavelength gets longer, the


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diameter becomes smaller, and the number yield decreases for the Au nanorod product. As a result, the longitudinal plasmon wavelength of the Au nanorods produced by this method with diameters less than 10 nm is limited to be longer than ~850 nm. On the other hand, the seedmediated method has also been employed to grow small Au nanorods with diameters less than 10 nm, but the number yield, size uniformity and longitudinal plasmon wavelength have not been specifically controlled.39‒41 Small Au nanorods are only mentioned as side products in these previous works. In addition, in seed-mediated growth, only a very small fraction of the seed solution is usually consumed. A majority of the seed solution is simply discarded. Therefore, the synthesis of small Au nanorods in high yields with good size uniformity and adjustable longitudinal plasmon wavelength is still highly desirable. We have shown that high-indexfacetted Au nanocrystals with diameters larger than ~120 nm can be obtained when a small amount of the seed solution is added in the growth solution.42 In this work, we describe the growth of absorption-dominant small Au nanorods using the seed-mediated method by dramatically increasing the amount of the seed solution. The diameters of the obtained small Au nanorods are below 10 nm, with the longitudinal plasmon resonance wavelengths synthetically tunable from ~720 nm to ~830 nm by changing the seed-to-Au(III) molar ratio in the growth solution. Finite-difference time-domain (FDTD) simulations show that the fractions of the scattering in the total extinction of small Au nanorods are in the range of 0.005 to 0.025, confirming that the extinction of these small Au nanorods is dominantly contributed to by the light absorption. In addition, one small and one big Au nanorod sample, the latter of which has a diameter of 21.3 ± 2.6 nm, are coated with dense silica and employed in PTT with three cell lines. The small Au nanorod sample shows an improved PTT performance compared to the corresponding big Au nanorod sample at the same intracellular Au content.


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EXPERIMENTAL SECTION Preparation of the Small Au Nanorods. The small Au nanorod samples were prepared by the seed-mediated growth method in aqueous solutions through changing the seed-to-Au(III) molar ratio in the growth solution. They are named with GmSn, where G denotes the growth solution, S refers to the seed solution, m is the volume of the surfactant solution used in preparing the growth solution with the volumes of the other ingredient solutions kept unchanged, and n is the volume of the seed solution. The units of m and n are both mL, with m varied from 9 mL to 1 mL at a step of 1 mL and n varied from 1 mL to 9 mL accordingly. During the preparation, cetyltrimethylammonium bromide (CTAB) or cetyltripropylammonium bromide (CTPAB) was employed as the stabilizing surfactant. Take G9S1 as an example. The seed solution was made by adding a freshly prepared, ice-cold NaBH4 solution (0.6 mL, 0.01 M) into a mixture solution composed of HAuCl4 (0.25 mL, 0.01 M) and CTAB (9.75 mL, 0.1 M) under vigorous stirring. The resultant solution was stirred for 2 min and then kept at room temperature for at least 2 h before use. The growth solution was made by sequential addition of HAuCl4 (0.5 mL, 0.01 M), AgNO3 (0.1 mL, 0.01 M) and HCl (0.2 mL, 1.0 M) into CTAB (9 mL, 0.1 M). A freshly prepared ascorbic acid solution (0.08 mL, 0.1 M) was then added under rapid stirring. Once the resultant solution became colorless, the seed solution (1 mL) was then added into the growth solution under vigorous stirring. If the volume of the seed solution was larger than that of growth solution, the growth solution was added into the seed solution. The resultant solution was kept under stirring for 2 min and then left undisturbed overnight. The preparation of the samples with CTPAB followed the same procedure except that CTAB (0.1 M) was changed to CTPAB (0.01 M) and that the volume of the ascorbic acid solution was changed to 0.3 mL.


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Preparation of the Big Au Nanorods. The big Au nanorod samples were obtained from NanoSeedz. They were also prepared by the seed-mediated growth method following our reported procedure.11 Coating of Dense Silica on the Au Nanorods. The coating of dense silica on the Au nanorod samples followed a reported procedure, with slight modification.43,44 The stabilizing surfactant molecules on the Au nanorods were first replaced with thiol-terminated methoxy-poly(ethylene glycol) (mPEG-SH, molecular weight: 5,000, RAPP polymere, Germany). Typically, the solution (10 mL) of an as-grown Au nanorod sample was washed once by centrifugation to remove the excess surfactant molecules and then redispersed into deionized water (5 mL). The extinction value at the longitudinal plasmon peak was 4.28 for the big Au nanorod sample. mPEG-SH (0.5 mL, polymer chain concentration: 1 mM) was added into the washed Au nanorod solution. The resultant solution was kept undisturbed at room temperature for more than 6 h. After the excess mPEG-SH molecules were removed by centrifugation twice at 36,000 g for 10 min, the mPEG-SH-encapsulated Au nanorods were redispersed in absolute ethanol (7.5 mL), followed by the addition of deionized water (2.25 mL) and NH3⋅H2O (0.15 mL, 30 wt%). The silica precursor tetraethylorthosilicate (TEOS, each at 20 µL, 10 vol% in ethanol) was injected at a 1.5-h interval time under continuous ultrasonication for three times. The obtained silica-coated Au nanorods were washed by centrifugation and redispersed in deionized water (10 mL). FDTD Simulations. The FDTD simulations of the Au nanorods were performed using FDTD Solutions 8.5, which was developed by Lumerical Solutions, Inc. The dielectric function of gold was represented by fitting the experimental data points of Johnson and Christy. Both of the small and big Au nanorods were modeled as a cylinder capped with a hemisphere at each end, using average diameters and lengths measured from the TEM images. The individual Au nanorod was


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surrounded by a virtual boundary at an appropriate size. The Au nanorod and its surrounding medium inside the boundary were divided into meshes of 0.25 nm and 1 nm in size for the small and big nanorods, respectively. The refractive index of the surrounding medium was set to be 1.33, the refractive index of water. An electromagnetic pulse was launched into the boundary to simulate a propagating plane wave interacting with the nanorod. The longitudinal and transverse plasmon resonance modes of the Au nanorods were simulated by setting the electric field of the plane wave parallel and perpendicular to the length direction of the Au nanorods, respectively. Considering that the nanorods were dispersed in water with random orientations, the spectra simulated under the longitudinal and transverse excitation polarization states were added together with weighting factors of 1/3 and 2/3, respectively.45 Cell Culture. Human glioblastoma cells (U-87 MG), human breast cancer cells (MDA-MB231 and MDA-MB-435S) and murine fibroblasts (L929) were provided by American Type Culture Collection. U-87 MG was cultured in alpha-modified Minimum Essential Medium (αMEM), which contained 10% fetal bovine serum (FBS), 100 unit mL−1 penicillin and 100 µg mL−1 streptomycin at 37 °C in a humidified 5% CO2 atmosphere. MDA-MB-231, MDA-MB435S and L929 cells were cultured in Dulbecco's Modified Eagle Medium. Cell Viability Assay. The cell viability was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide (MTT) assay. Ten thousand cells were seeded into each well on a 96-well plate. After 12-h incubation, the medium in the wells was replaced with a fresh medium containing the Au nanorod sample at various concentrations (10−80 µg-Au mL−1). After 24-h incubation, the cells were washed with phosphate-buffered saline (PBS) solution, followed with the addition of a fresh medium (100 µL) into each well. MTT solution (10 µL, 5 mg mL−1) was subsequently added into each well. After 3-h incubation, the medium was removed and the


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purple formazan product in the live cells was dissolved with dimethyl sulfoxide (150 µL). The resultant mixture was centrifuged at 4,000 g for 10 min, and the supernatant was transferred into the wells on another plate. The absorbance of each well was measured using a Thermo Fisher Multiskan GO ultraviolet/visible microplate spectrophotometer at a wavelength of 540 nm. The cell viability (%) for each sample relative to control was calculated. Cellular Uptake Observation. Five to ten thousand cells were seeded into each well of a 24well plate. After 12-h incubation, the culture medium was replaced with a fresh medium containing the different Au nanorod samples (0.5 mL, 75 µg-Au mL−1). After further incubation for 24 h, the cells were washed with PBS buffer to remove any Au nanorods loosely adsorbed on the cell surface. The cells were subsequently fixed for 40 min using 4% paraformaldehyde. The Au nanorod cellular uptake in the different cell lines was observed using a Nikon TE2000 fluorescence microscope under the bright-field mode. Intracellular Au Content Assay. U-87 MG, MDA-MB-231 and MDA-MB-435S cells were cultured in 60-mm Petri dishes. When the cells reached 80% confluence, each culture medium was replaced with a fresh medium containing the Au nanorod sample at the concentration of 75 µg-Au mL−1. After incubation for 24 h, the cells were washed with PBS, collected, counted and then subsequently centrifuged at 4,000 g for 5 min. The obtained cell pellets were redispersed in NaOH (50 µL, 10 M) to dissolve the silica coating of the Au nanorods. Aqua regia (200 µL; caution: highly corrosive and extremely dangerous) was then added to digest the Au nanorods. The volume of the digested solution was subsequently adjusted to 3.0 mL by adding deionized water. The Au concentration of the resultant solution was determined with a PerkinElmer Optima 4300 DV inductively coupled plasma optical emission spectroscopy (ICP-OES) system. The Au amount per cell was then calculated.


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Photothermal Therapy. Ten thousand U-87 MG, MDA-MB-231 or MDA-MB-435S cells were seeded into each well of a 96-well plate. After incubation for 12 h, the cells were treated with the different Au nanorod samples (75 µg-Au mL−1), followed with further incubation for 24 h. The cells were washed with PBS buffer repeatedly to remove the extracellular Au nanorods. Each culture medium was then changed with phenol red-free Roswell Park Memorial Institute medium (RPMI) 1640 (100 µL) containing 10% FBS to avoid light absorption by phenol red. For photothermal ablation, the plastic cover of the 96-well plate was removed to avoid laser light reflection. The designated wells were exposed to a JENOPTIK continuous-wave 809-nm semiconductor diode laser for 3 min. The laser power density was 12 W cm−2 with a spot diameter of 5 mm, which was equal to that of the bottom of the well. After the treatment, the culture medium was replaced with α-MEM (100 µL) containing 10% FBS. After incubation for 24 h, the MTT assay was performed to determine the cell viability. Statistical Analysis. The data were expressed as mean ± standard deviation. The statistical difference was evaluated with one-way ANOVA analysis. A P value of < 0.05 was considered statistically significant. Characterization. Transmission electron microscopy (TEM) imaging was performed on an FEI Tecnai Spirit microscope operating at 120 kV. The sizes and number percentages of the Au nanorods were measured from their TEM images, with ~150 particles measured for size and ~300 particles counted for number percentage per sample. The extinction spectra of the solution samples were measured on a Hitachi U-3501 ultraviolet/visible/near-infrared (NIR) spectrophotometer with quartz cuvettes of 1-cm optical path length.

RESULTS AND DISCUSION


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The most fascinating feature of Au nanorods is that their plasmonic properties can be tailored by synthetically tuning their sizes and shapes. The synthetic parameters play an important role on the morphology and therefore the plasmonic properties of Au nanorods. Of all parameters involved during the growth process, the seed-to-Au(III) molar ratio in the growth solution has a large effect on the size.40 At a given concentration of Au(III) ions, the size of grown Au nanorods usually decreases as the seed concentration is increased, because a decreased amount of Au(III) ions per seed particle is available for growth. Our previous studies have shown that Au nanorods grow thicker as the seed volume is reduced. High-index-facetted Au nanocrystals can be produced when the supplied seed amount is much smaller than that used for the preparation of commonly-sized Au nanorods.42 Contrary to that work, in this study, we dramatically increased the seed amount relative to the Au(III) amount in the growth solution in order to reduce the diameter of Au nanorods. If we consider the molar ratio of the Au amount contained in the seed solution to that supplied in the growth solution, the value in our work is estimated to be ~40‒3600 times that in the traditional seed-mediated approach for the growth of commonly-sized Au nanorods. CTAB or CTPAB were utilized as the stabilizing surfactant in the growth solution during the synthesis of small Au nanorods. The growth solutions were made by mixing together the preprepared HAuCl4, AgNO3, HCl, the surfactant and ascorbic acid solutions. To maintain the concentrations of HAuCl4, AgNO3, HCl and ascorbic acid unchanged among the different growth experiments, we varied the volumes of the surfactant solution and the seed solution while keeping their sum unchanged. With CTAB, the longitudinal plasmon peak blue shifts as the seed-to-Au(III) molar ratio is increased. The yield of Au nanorods gradually decreases, as revealed by the extinction spectra (Supporting Information, Figure S1). The extinction ratio of


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the longitudinal plasmon peak to the transverse one decreases from 2.9 for G9S1 to 1.3 for G1S9. We have found previously that cationic alkyl ammonium surfactants with larger head groups are stronger in stabilizing Au nanorods.41 Very thin Au nanorods, with diameters as small as 3 nm, can be obtained by use of CTPAB and cetyltributylammonium bromide, whose head groups are larger than that of CTAB.41 We therefore in this study also utilized CTPAB as the stabilizing surfactant for the growth of small Au nanorods. As the seed-to-Au(III) molar ratio in the growth solution is increased, the longitudinal plasmon peak of the product also blue-shifts, but the extinction ratio of the longitudinal plasmon peak to the transverse one increases from 0.8 for G9S1 to 2.3 for G1S9 (Supporting Information, Figure S2). The opposite trends observed for the variation of the extinction ratio of the longitudinal plasmon peak to the transverse one are believed to be mainly caused by the different stabilizing abilities between CTAB and CTPAB. For the purpose of growing small Au nanorods in high number yields, we can then combine together the growths with CTAB and CTPAB, where CTAB is employed for the growths of GmSn (m:n ≥ 1:1), and CTPAB is employed for the growths of GmSn (m:n < 1:1) (Supporting Information, Figure S3). Figure 1a shows the normalized extinction spectra of the small Au nanorod samples prepared with CTAB or CTPAB under the optimal conditions. G1S9, G2S8 and G4S6 were prepared with CTPAB, while G9S1, G8S2 and G6S4 were prepared with CTAB. Each spectrum clearly exhibits two plasmon resonance peaks. We extracted the plasmonic properties of these nanorod samples and collected them in Table 1. As the seed-to-Au(III) molar ratio in the growth solution is increased, the longitudinal plasmon wavelength of the Au nanorod sample is gradually shortened from 829 nm to 726 nm. The transverse plasmon wavelength of the small Au nanorod sample is insensitive to the amount of the seed solution. This behavior is similar to that observed


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for relatively big, commonly-sized Au nanorods.40,46,47 Moreover, the full width at half maximum (FWHM) values of these small Au nanorod samples are comparable or even slightly smaller than those of common big Au nanorods.2,11 The large extinction ratios of the longitudinal plasmon peak to the transverse one suggest that the number yields of Au nanorods in these samples are high. The relatively small FWHM values indicate that the shapes and sizes of these small Au nanorod samples are uniform. In order to further examine the number yields and size uniformity, TEM imaging was performed on these small Au nanorod samples. The TEM images (Figure 1b−g) confirm that the small nanorods are in high number yields despite the existence of small number percentages of spherical Au nanoparticles. We determined the number yields and measured the lengths and diameters from the TEM images. The number yields of all samples are higher than 85%, with those of G8S2 and G9S1 reaching 97%. Both of their diameters and lengths are relatively uniform. Their average diameters range from 6 nm to 9 nm, their average lengths vary from 16 nm to 45 nm, and their average aspect ratios change from 2.7 to 4.7. Because the average diameters of all small Au nanorod samples are smaller than 10 nm, their extinction should be contributed to dominantly by the light absorption.11 The absorptiondominant property is vividly displayed by the photograph (inset of Figure 1a) of these samples. The photograph was taken on the as-prepared small Au nanorod samples that were not concentrated or subjected to any other treatment. Different from commonly big Au nanorod samples showing rich colors,48 the small Au nanorod samples appear much darker due to their dominant absorption in the total extinction. The colors of the G1S9 and G2S8 samples are even black, suggesting that light illuminated on these small Au nanorod samples is almost completely absorbed. The absorption-dominant property, in conjunction with the synthetic tunability of the longitudinal plasmon wavelength over the range from ~720 nm to ~830 nm, endows our small


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Au nanorods with promising potentials in plasmonic applications that demand large absorption and minimized scattering.

Figure 1. (a) Normalized extinction spectra of the small Au nanorod samples grown by varying the seed-to-Au(III) molar ratio in the growth solution. G1S9, G2S8 and G4S6 were grown with CTPAB, while G6S4, G8S2 and G9S1 were grown with CTAB. The inset is the photograph of the as-prepared small Au nanorod samples, which correspond to G1S9 to G9S1 from left to right. (b−g) TEM images of the small Au nanorod samples corresponding to G1S9 to G9S1 shown in (a), respectively.

Table 1. Plasmonic Properties, Sizes and Number Yields of the Small Au Nanorod Samples G1S9

G2S8

G4S6

G6S4

G8S2

G9S1


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λL (nm)a

726

745

759

775

812

829

λT (nm)b

514

514

512

514

513

511

EL/ETc

2.3

2.2

1.7

2.0

2.6

2.9

FWHM (nm)d

133.3

137.5

166.6

169.9

170.2

178.3

diameter (nm)

6.0 ± 0.6

6.8 ± 0.7

6.9 ± 0.9

6.9 ± 1.0

8.8 ± 0.6

9.1 ± 0.6

length (nm)

16.3 ± 3.7

21.3 ± 3.5

20.0 ± 5.6

21.8 ± 5.1

36.6 ± 4.0

45.3 ± 6.7

aspect ratioe

2.7 ± 0.4

3.1 ± 0.5

3.1 ± 0.6

3.5 ± 0.5

4.3 ± 0.7

4.7 ± 0.7

number yield (%)f

91

92

85

90

97

97

a

Longitudinal plasmon wavelength. bTransverse plasmon wavelength. cExtinction ratio of the longitudinal plasmon peak to the transverse one. dFWHM refers to the full width at halfmaximum of the longitudinal plasmon peak. The values were determined by fitting the peak with Gaussian function. eAspect ratio refers to the ratio between the length and diameter. fNumber yield refers to the percentage of the number of nanorods in the total number of particles.

FDTD simulations were further performed to ascertain the contributions of the light scattering and absorption to the total extinction of the small Au nanorods. The FDTD technique, based on solving Maxwell's equations, has been reckoned as a powerful tool for studying the plasmonic properties of metal nanostructures. It can provide a full range of plasmonic properties, such as extinction, scattering, absorption cross-sections, and local electric field enhancement contours, some of which are difficult to be measured experimentally. As representative examples, we carried out the simulations on the averagely-sized G1S9 and G8S2 samples and two commonlysized, big Au nanorod samples with correspondingly similar longitudinal plasmon wavelengths for comparison. The ensemble extinction spectra and TEM images of the big Au nanorod samples used for the FDTD simulations are provided in Figure S4 in the Supporting Information. Their longitudinal plasmon wavelengths are 727 nm and 808 nm, respectively. The measured average diameters/lengths are 40.2 ± 2.8 nm/104.3 ± 7.0 nm and 16.6 ± 1.3 nm/62.2 ± 5.4 nm, correspondingly. In comparison, the longitudinal plasmon wavelengths of G1S9 and G8S2 are


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726 nm and 812 nm, and their average diameters/lengths are 6.0 ± 0.6 nm/16.3 ± 3.7 nm and 8.8 ± 0.6 nm/36.6 ± 4.0 nm, respectively. Figure 2 shows the simulated spectra of the small and big Au nanorods. For each sample, the calculated extinction, absorption and scattering spectra are plotted. The simulation results show that the scattering contribution to the extinction is very small for the small Au nanorods. The scattering-to-extinction fraction calculated for G1S9 at the longitudinal plasmon peak wavelength of 726 nm is 0.005. In comparison, the corresponding value for the 727-nm big Au nanorod is 0.6, which is 120 times that of the small Au nanorod. For the 812-nm G8S2 sample and the 808-nm big Au nanorod, the calculated scattering-to-extinction fractions are 0.025 and 0.12, respectively. The value of the big nanorod is 4.8 times that of the small nanorod. These simulation results clearly show that the small Au nanorods are absorption-dominant at their longitudinal plasmon resonance.


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Figure 2. Simulated extinction (blue), absorption (red) and scattering (green) spectra. (a) G1S9. (b) G8S2. (c) Big Au nanorod sample with a longitudinal plasmon wavelength of 727 nm. (d) Big Au nanorod sample with a longitudinal plasmon wavelength of 808 nm.

Nanoparticles with sizes in the range of 10−100 nm have the ability to transport and accumulate in tumor tissues to high extent than in normal tissues owing to the leaky nature of tumor blood vasculatures,49 which is known as the enhanced permeability and retention effect.5 When plasmonic metal nanoparticles are accumulated in a tumor region, owing to their large light absorption ability at the plasmon resonance, they can efficiently absorb light energy and convert it into heat, which causes a local temperature rise in the tumor region and therefore kills


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cancer cells. This has been known as plasmonic PTT. In order to minimize light scattering and absorption by organs and normal tissues and maximize light penetration through tissues, the laser wavelength used in PTT treatments is often preferred to be in the NIR biological transparency window. Au nanorods have proven to be promising PTT agents due to their synthetic tunability of the longitudinal plasmon wavelength, extremely large light absorption cross-sections and attractive photothermal conversion efficiencies as high as 100%.2,35 Our small Au nanorods, with dominant absorption at their longitudinal plasmon wavelengths, will be attractive for functioning as PTT agents. Recently, the effect of Au nanorod size on the PTT performance has been studied by comparing the photothermal therapeutic efficacies of three differently sized Au nanorod samples.50 The most effective size has been identified, providing a guidance for further developing effective PTT agents based on Au nanorods. However, in this previous work, the actual intracellular uptake amount of Au nanorods has not been taken into account for evaluating the photothermal therapeutic efficacy. The intracellular uptake of Au nanorods is expected to play an important role in PTT, because a radial temperature distribution is usually generated spatially around each Au nanorod under laser light illumination. Considering this point, we examined and compared the intracellular uptake behaviors as well as the PTT performances between the small Au nanorods and commonly-sized big Au nanorods. As PTT agents, Au nanorods should be biocompatible with little cytotoxicity. Cationic alkyl ammonium surfactants have been known to be cytotoxic.51 Replacing the cationic surfactant molecules with PEG-thiol molecules and then coating dense silica on PEG-capped Au nanorods has proven to be an effective approach for increasing their biocompatibility.44 Figure 3 displays the extinction spectra and TEM images of one small (G8S2) and one big Au nanorod sample before and after coating with dense silica. The two samples were chosen to match their


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longitudinal plasmon wavelengths after silica coating as close as possible with the wavelength of the laser used in the PTT experiments. The longitudinal plasmon peak is slightly red-shifted for the big Au nanorod sample while blue-shifted for the small Au nanorod sample. After silica coating, the longitudinal plasmon wavelengths of the small and big nanorod samples are 802 nm and 809 nm, respectively. The measured average diameter and length of the big Au nanorod sample are 21.3 ± 2.6 nm and 73.9 ± 5.9 nm, respectively. The sizes of this big nanorod sample are slightly different from those of the 808-nm one used in the FDTD simulations, because silica coating causes a shift in the longitudinal plasmon peak. The thicknesses of the silica layers were measured from the TEM images to be 9.5 ± 1.7 nm and 9.3 ± 1.5 nm for the small and big nanorod samples, respectively.

Figure 3. (a) Normalized extinction spectra of the as-prepared, PEG-capped and silica-coated small Au nanorod sample (G8S2). (b,c) TEM images of the silica-coated small Au nanorod sample in low and high magnifications, respectively. The TEM image of the as-prepared small Au nanorod sample is shown in Figure 1f. (d) Normalized extinction spectra of the as-prepared,


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PEG-capped and silica-coated big Au nanorod sample. (e,f) TEM images of the as-prepared and silica-coated big Au nanorod sample, respectively.

L929 fibroblast cells were chosen for the biocompatibility evaluation.52,53 The silica-coated Au nanorod samples at different concentrations were applied to the cell culture. The viability of the cells was determined by MTT assay. The result (Figure 4a) indicates that the cytotoxicity of both small and big Au nanorod samples is very small. The cell viability remained to be above 95% even when the cells were exposed to the Au nanorods at concentrations as high as 80 µg-Au mL−1 for 24 h.

Figure 4. (a) Viability of L929 cells after 24-h exposure to the small (white) and big (pink) silica-coated Au nanorod samples. The concentrations of the nanorod samples were varied in the range of 10−80 µg-Au mL−1. Each measurement was repeated five times for each sample. (b) Optical images showing the internalization of the small (upper) and big (bottom) silica-coated Au nanorod samples into U-87 MG, MDA-MB-231 and MDA-MB-435S cells. The cells were incubated with both samples at a concentration of 75 µg-Au mL−1 for 24 h.


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Cellular uptake of Au nanoparticles is crucial for effective PTT. Optical microscopy was employed to qualitatively observe the cellular uptake efficiencies of the two nanorod samples in three cell lines (Figure 4b). Before cellular uptake, the concentrations of both small and big Au nanorod samples were adjusted with ICP-OES to 75 µg-Au mL‒1. Then, for both samples, 0.5 mL was utilized for the cellular uptake experiments. Maximal cellular uptake capacities were reached for both samples after incubation for 24 h at the concentration of 75 µg-Au mL−1. In MDA-MB-231 and MDA-MB-435S cells, both samples show similar cellular uptake ability. However, in U-87 MG cells, the internalized amount of the small Au nanorods is much smaller than that of the big nanorods. We further quantitatively measured the intracellular uptake of the two Au nanorod samples in these three cell lines by ICP-OES (Figure 5a). The result verifies what is observed from the optical images. There are no statistically significant differences for the two samples in MDA-MB-231 and MDA-MB-435S cells, while a significant difference (P < 0.01) is present between the two samples in U-87 MG cells. These results indicate that the cellular uptake of Au nanorods depends on both particle size and cell type. Our results are also in agreement with previous ones on that cellular uptake kinetics and saturation intracellular concentrations are correlated with the physical dimensions of Au nanoparticles and that a suitable size is necessary for uptake by a particular cell line. The different behaviors can be ascribed to the uptake mechanisms of receptor-mediated endocytosis, such as serum protein binding to Au nanoparticles, the contact area of Au nanoparticles with cell membranes and Au nanoparticle sedimentation.51,54,55 The big Au nanorods were found to sediment in a much larger extent at the bottom of the well than the small Au nanorods. The actual concentration of the big Au nanorods on the cell surface was therefore higher than that of the small Au nanorods. The amount of the big Au nanorods internalized by U-87 MG cell was larger than that of the small


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Au nanorods. In contrast, for MDA-MB-231 and MDA-MB-435S cells, the intracellular Au nanorod contents were insensitive to the nanorod size. These results can be explained by referring to our previous finding44 that the internalization capacities of MDA-MB-231 and MDA-MB-435S for Au nanorods are low, while U-87 MG cells exhibit a much larger internalization capacity. In addition, our results with U-87 MG cells are also consistent with a previous finding that nanoparticle sedimentation plays an important role in nanoparticle intracellular uptake.55


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Figure 5. (a) Intracellular Au contents of the small (white) and big (pink) silica-coated Au nanorod samples in U-87 MG, MDA-MB-231 and MDA-MB-435S cells. Each measurement was repeated three times for each sample and each cell line. (b) Cell viability upon the photothermal therapy with the small (white) and big (pink) silica-coated Au nanorod samples in U-87 MG, MDA-MB-231 and MDA-MB-435S cells. The shown data represent the mean ± standard deviation. # and ## indicate significant difference at P < 0.05 and P < 0.01 between the treatments with the small and big Au nanorod samples, respectively.

The photothermal therapeutic effects of the small and big Au nanorod samples in the three cancer cells were evaluated and compared by MTT assay. The PTT study was performed by irradiation with an 809-nm laser at a power density of 12 W cm−2 for 3 min. The sole laser irradiation has no effect on the cell viability.44 The cell viability values of U-87 MG, MDA-MB231 and MDA-MB-435S cells upon the laser irradiation in the presence of the internalized small Au nanorod sample are (41 ± 6)%, (36 ± 6)% and (52 ± 18)%, respectively. In comparison, the corresponding values in the case of the big Au nanorod sample are (19 ± 4)%, (56 ± 9)% and (81 ± 10)%, respectively (Figure 5b). We calculated the PTT efficiency per unit amount of the internalized Au nanorods, which is defined as the cell viability reduction divided by the intracellular Au content in each cell line. The values for the small Au nanorods are 1.7%, 3.0% and 2.4% per pg-Au-in-cell in U-87 MG, MDA-MB-231 and MDA-MB-435S cells. However, the values for the big Au nanorods are 0.95%, 1.7% and 1.2% per pg-Au-in-cell. Therefore, these results reveal that the small Au nanorods show a higher photothermal therapeutic efficacy towards these cancer cells than the big Au nanorods at the same internalized amount. The small Au nanorod sample, G8S2, was chosen for the PTT test to match the wavelength of our NIR


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laser. Table 1 above shows that the average diameters of our small Au nanorod samples become smaller as their longitudinal plasmon resonance is blue-shifted. Because the scattering-toextinction fraction decreases for Au nanorods with smaller diameters,11 we expect that our small Au nanorod samples with shorter longitudinal plasmon wavelengths will have better PTT performance.

CONCLUSIONS We have described a facile seed-mediated growth method for the preparation of absorptiondominant small gold nanorods by utilizing CTAB or CTPAB as the stabilizing surfactant. The dependence of the number yield on the seed-to-Au(III) molar ratio in the growth solution with CTPAB displays an opposite trend in comparison with that with CTAB. The optimal number yields can be realized by combining the growths with CTAB and CTPAB. The diameters of the small gold nanorods are all below 10 nm. Their longitudinal plasmon resonance wavelengths are variable from ~720 nm to ~830 nm by changing the seed-to-Au(III) molar ratio. The absorptiondominant feature of these small Au nanorods is illustrated vividly by the deep colors of the solution samples and verified by FDTD simulations. The cellular uptake and PTT studies with the silica-coated Au nanorods in three cell lines show that the small Au nanorod sample possesses a better PTT performance than a commonly-sized, big Au nanorod sample at the same intracellular Au content. Our results suggest that our absorption-dominant small Au nanorods are promising for plasmonic photothermal conversion-based biomedical applications.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Hong Kong RGC GRF (Ref. No.: CUHK403312) and NNSFC (Ref. No.: 21229101). The FDTD simulations were conducted in the High Performance Cluster Computing Centre, Hong Kong Baptist University, which is supported by Hong Kong RGC and Hong Kong Baptist University.

ASSOCIATED CONTENT Supporting Information Extinction spectra of the small Au nanorod samples grown with CTAB or CTPAB as the surfactant; experimental extinction spectra and TEM images of the big Au nanorod samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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Synthesis of Absorption-Dominant Small Gold Nanorods and Their

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