Realization of Red Plasmon Shifts up to ∼900 nm by AgPd-Tipping

Sep 11, 2017 - Herein we report on a AgPd-tipping process on Au nanobipyramids with the resultant red plasmon shifts reaching up to ∼900 nm. The lar...
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Realization of Red Plasmon Shifts up to ∼900 nm by AgPd-Tipping Elongated Au Nanocrystals Xingzhong Zhu,†,# Hang Kuen Yip,‡,# Xiaolu Zhuo,‡ Ruibin Jiang,§ Jianli Chen,∥ Xiao-Ming Zhu,∥ Zhi Yang,*,† and Jianfang Wang*,‡ †

Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ‡ Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China § Key Shaanxi Engineering Lab for Advanced Energy Technology, School of Material Science and Engineering, Shaanxi Normal University, Xi’an 710119, China ∥ State Key Laboratory of Quality Research in Chinese Medicine, Macau Institute for Applied Research in Medicine and Health, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau SAR, China S Supporting Information *

ABSTRACT: The synthesis of metal nanostructures with plasmon wavelengths beyond ∼1000 nm is strongly desired, especially for those with small sizes. Herein we report on a AgPd-tipping process on Au nanobipyramids with the resultant red plasmon shifts reaching up to ∼900 nm. The large red plasmon shifts are ascribed to the deposition of the metal at the tips of Au nanobipyramids, which is verified by electrodynamic simulations. The method has been successfully applied to Au nanobipyramids and nanorods with different longitudinal dipolar plasmon wavelengths, demonstrating that the plasmon wavelengths of these Au nanocrystals can be extended to the entire near-infrared region. Pt can also induce the tipping on Au nanobipyramids and nanorods to realize red plasmon shifts, suggesting the generality of our approach. We have further shown that the metal-tipped Au nanobipyramids possess a high photothermal conversion efficiency and good photothermal therapy performance. This study opens up a route to the construction of Au nanostructures with plasmon resonance in a broad spectral region for plasmon-enabled technological applications.

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INTRODUCTION Plasmonic metal nanostructures, owing to their attractive optical properties, have great potential for applications in wide areas, such as waveguides,1 optics,2,3 solar energy harvesting,4 photocatalysis,5,6 sensing,7,8 and biomedicine.9,10 A majority of plasmonic metal nanostructures have plasmon resonances in the visible and near-infrared (NIR) regions below ∼1000 nm.11−16 Therefore, most of the applications based on plasmonic metal nanostructures are limited in this spectral range.6,11−18 Recently, metal nanostructures with plasmon wavelengths beyond ∼1000 nm have been receiving increasing attention. For example, optically active nanomaterials are strongly desired for biomedical imaging and diagnostics in the second biological transparency window (from ∼1000 to ∼1400 nm), where tissues exhibit low light scattering and absorption and therefore high transmission, allowing for deep light penetration through tissues.19−24 In addition, the tailoring of the plasmon wavelengths of metal nanostructures beyond ∼1000 nm has also enabled widespread potential applications in plasmon sensing,25 solar energy harvesting,26,27 and surfaceenhanced infrared absorption.28−31 Exploring and extending © 2017 American Chemical Society

such applications require high-quality plasmonic metal nanostructures and effective methods for finely adjusting their plasmon wavelengths beyond ∼1000 nm. So far, only a few types of metal nanostructures have been demonstrated to have plasmon wavelengths beyond ∼1000 nm, including highaspect-ratio metal nanorods, nanobipyramids, and nanoplates.32−37 However, such metal nanostructures are limited and suffer from three drawbacks, making them disadvantageous for many practical applications. First, the extension of the plasmon wavelengths for all of these nanostructures is based on the increase of their particle sizes. The particle volumes of the metal nanostructures with their plasmon wavelengths beyond ∼1000 nm are therefore relatively large (∼105−106 nm3). Such large particle volumes not only lead to the large consumption of the metals in practical applications but also bring disadvantages in biomedical applications, including the difficulty in cellular uptake and low photothermal conversion efficiencies. Second, Received: July 17, 2017 Published: September 11, 2017 13837

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

Article

Journal of the American Chemical Society

Synthesis of the Au NR Samples. The Au NR samples were prepared as described in previous works with minor modification. Typically, a freshly prepared, ice-cold NaBH4 solution (0.6 mL, 0.01 M) was injected quickly into a mixture solution that was premade by mixing together HAuCl4 (0.25 mL, 0.01 M) and CTAB (10 mL, 0.1 M). The seed solution was kept at room temperature for 2 h before use. The seed solution (0.2 mL) was injected into the growth solution that was made in advance by mixing together a CTAB solution (9.5 mL, 0.1 M), HAuCl4 (0.5 mL, 0.01 M), AgNO3 (0.15 mL, 0.01 M), and hydroquinone (0.5 mL, 0.1 M), followed by gentle inversion mixing for 10 s. The reaction solution was left undisturbed overnight at room temperature. The longitudinal plasmon wavelength of the obtained Au NR sample was ∼798 nm. For Au NRs with longer longitudinal plasmon wavelengths, the added amounts of the seed solution/AgNO3 were 0.6/0.25 mL and 1/0.35 mL for the growth of the 996 and 1116 nm Au NR samples, respectively. Synthesis of the Metal-Tipped Au Nanostructures. The metal-tipped Au nanostructures with different compositions and shapes were produced following the same procedure, with the amounts of the involved metal precursors varied. In a typical synthesis, the purified Au NBPs (2 mL, the longitudinal dipolar plasmon peak extinction value is ∼1 when measured with a 0.5 cm cuvette) were centrifuged at 7000 rpm for 10 min. The precipitate was redispersed into a CTAC solution (2 mL, 0.08 M), followed by the subsequent addition and mixing of AgNO3 (10 μL, 0.01 M) and ascorbic acid (5 μL, 0.1 M) under gentle shaking. The mixture solution was placed in an air-bath shaker (60 °C, 100 rpm) and kept for 4.5 h, during which Ag was overgrown on the Au NBPs to form Au NBP@Ag nanostructures. The resultant sample was centrifuged twice at 6000 rpm for 10 min. The precipitate was redispersed into a CTAB solution (2 mL, 0.003 M). The formation of metal nanoparticles at the tips of the Au NBPs was carried out by the sequential addition of H2PdCl4 (0.001 M) and ascorbic acid (0.01 M) under gentle shaking. The volume of the H2PdCl4 solution was varied from 0.5 to 15 μL. The volume of the ascorbic acid solution was the same as that of the H2PdCl4 solution for each overgrowth experiment. The reaction was left undisturbed overnight at room temperature. The resultant sample was centrifuged at 3500−5000 rpm for 10 min. The precipitate was redispersed into water (2 mL) for further use. For the Pt- and Auinduced metal tipping on the Au NBPs, the procedure was the same as that for the preparation of the Pd-induced metal-tipped Au NBPs except that H2PdCl4 was replaced with H2PtCl6 and HAuCl4, respectively. For the synthesis of the metal-tipped Au NRs, the procedure was the same as that for the synthesis of the metal-tipped Au NBPs except that the Au NBPs were replaced with the Au NRs. Coating of Dense Silica on the Tipped Au NBPs. The coating of dense silica on the tipped Au NBPs followed previous works with minor modification. Typically, thiol-terminated methoxy poly(ethylene glycol) (mPEG-SH, molecular weight: 5000, 0.2 mL, polymer chain concentration 1 mM) was added to the metal-tipped Au NBP sample (2 mL). The resultant solution was kept undisturbed at room temperature overnight and then centrifuged at 4000 rpm for 10 min. The precipitate was redispersed into ethanol (1.5 mL), followed by the sequential addition of water (0.45 mL) and NH3·H2O (0.03 mL, 30 wt %). Tetraethyl orthosilicate (10 μL, 5 vol % in ethanol) was added twice at a 2 h interval under continuous ultrasonication. The obtained silica-coated metal-tipped Au NBPs were centrifuged at 3000 rpm for 10 min and then redispersed into water (2 mL). Electrodynamic Simulations. Finite-difference time-domain (FDTD) simulations were performed on the metal nanostructures using FDTD Solutions 8.7 (Lumerical Solutions). During the simulations, an electromagnetic pulse in the spectral range from 300 to 1300 nm was launched into a box containing a target nanostructure. A mesh size of 1 nm was employed in calculating the absorption, scattering, and extinction spectra of the Au NBPs tipped with Au, Ag, Pd, Pt, and AgPd alloy and the Au NRs tipped with Au, Ag, Pd, and Pt. The refractive index of the surrounding medium was set at 1.33, the refractive index of water. The dielectric function of Au was obtained by fitting the measured data of Johnson and Christy, and that of Ag, Pd, and Pt was fitted from Palik’s data. For simplification, the Au NBP was

the synthetic methods are not general. The plasmon wavelengths are difficult to vary for different needs. Third, the plasmon line widths of these nanostructures are broad owing to inhomogeneous size distributions. These three obstacles have severely hindered the realization of the potential of these metal nanostructures in many plasmonic applications in the region beyond ∼1000 nm. Therefore, the synthesis of metal nanostructures with plasmon wavelengths in the NIR region is still an incipient area and in strong demand. In this work, we demonstrate the Pd-induced metal deposition at the tips of Au nanobipyramids (NBPs) with the assistance of Ag. The deposition of metal nanoparticles at the tips of Au NBPs results in red plasmon shifts up to ∼900 nm, while the volume of the entire nanostructure is increased only slightly. The dependence of the red plasmon shift of the Pdinduced metal-tipped Au NBPs has been systematically examined as a function of the structure and composition. The metal-tipping approach is also applied to Au NBPs and nanorods (NRs) with different plasmon wavelengths, where the plasmon wavelengths can be extended to ∼2200 nm, covering the entire NIR region. The generality of this method is also verified by the successful Pt-induced metal deposition at the tips of Au NBPs and NRs. Moreover, the superior photothermal conversion and therapy characteristics of the Pdinduced metal-tipped Au NBPs are demonstrated by the photothermal studies with cancer cells in the NIR region at 1064 nm.

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EXPERIMENTAL SECTION

Chemicals. HAuCl4·3H2O (99%), PdCl2 (99%), NaBH4 (98%), trisodium citrate (99%), ascorbic acid (99%), and AgNO3 (99%) were purchased from Sigma-Aldrich. Cetyltrimethylammonium bromide (CTAB, 98%) was obtained from Alfa Aesar. NH3·H2O solution (25 wt %) and HCl solution (5 M) were obtained from E. Merck and Scharlab, respectively. H2O2 solution (30 wt %) and cetyltrimethylammonium chloride (CTAC, 97%) were purchased from Aladdin Reagent. Deionized water with a resistivity of 18.2 MΩ cm and produced by a Direct-Q 5 ultraviolet water purification system was used in all experiments. Synthesis of the Au NBP Samples. The Au NBP samples were prepared using the seed-mediated growth method, as described in previous works. Briefly, a freshly prepared, ice-cold NaBH4 solution (0.15 mL, 0.01 M) was injected quickly into an aqueous solution that was premade by mixing together HAuCl4 (0.125 mL, 0.01 M), trisodium citrate (0.25 mL, 0.01 M), and water (9.625 mL). The resultant seed solution was kept at room temperature for 2 h before use. The seed solution (0.2 mL) was injected into the growth solution that was made in advance by mixing together CTAB solution (40 mL, 0.1 M), HAuCl4 (2 mL, 0.01 M), AgNO3 (0.4 mL, 0.01 M), HCl (0.8 mL, 1 M), and ascorbic acid (0.32 mL, 0.1 M), followed by gentle inversion mixing for 10 s. The reaction solution was left undisturbed overnight at room temperature. The longitudinal plasmon wavelength of the obtained Au NBP sample was ∼804 nm. For Au NBPs with longer longitudinal dipolar plasmon wavelengths, a cetyltributylammonium bromide (CTBAB) growth solution was prepared by the sequential addition of HAuCl4 (1.2 mL, 0.01 M), AgNO3 (0.6 mL, 0.01 M), and ascorbic acid (0.4 mL, 0.1 M) into an aqueous CTBAB solution (28.5 mL, 0.01 M). The seed solution was then added. The reaction solution was mixed by gentle inversion for 10 s and left undisturbed overnight in an oven at 60 °C. The added amounts of the seed solution were 0.17, 0.13, and 0.09 mL for the growth of the 934, 1132, and 1330 nm Au NBP samples, respectively. The purification of the as-prepared Au NBPs was conducted using a depletion-induced separation method. The number percentage of the purified Au NBPs was found from transmission electron microscopy (TEM) imaging to be 99%. 13838

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

Article

Journal of the American Chemical Society

Figure 1. AgPd tipping on the 804 nm Au NBPs. (a) Schematic illustrating the synthetic process. (b, c) TEM (top) and SEM (bottom) images of the Au NBP sample and the Au NBP@Ag nanostructure samples, respectively. (d) TEM (top) and SEM (bottom) images of the Pd-induced metaltipped Au NBP samples produced with 1, 4, and 12 μL of H2PdCl4 (1 mM), respectively. (e) Extinction spectra of the Au NBP sample, the Au NBP@Ag sample, and the Pd-induced metal-tipped Au NBP samples produced with varying amounts of H2PdCl4. (f) Variations of the longitudinal dipolar plasmon wavelength and peak intensity as functions of the added volume of H2PdCl4. (g) Dependences of the Pd:Au and Ag:Au molar ratios of the AgPd-tipped Au NBP samples on the added volume of H2PdCl4. The point at 0 μL represents the Au NBP@Ag nanostructure sample. Photothermal Conversion Study. The setup for the measurement of the photothermal conversion efficiency was composed of a 1 cm path length quartz cuvette that was covered with a foam cap, a Ktype thermocouple connected to a digital thermometer (CIE 305), a 1064 nm laser (MDL-N-1064−5 W, Changchun New Industries Optoelectronics Tech., China), and a magnetic stirrer. The laser power densities during the measurements were 1.18, 1.96, and 2.74 W cm−2, respectively. Cell Viability Assay. The cell viability was determined by the 3(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. Five thousand U-87 MG cells (American Type Culture Collection, Manassas, VA, USA) were seeded into each well on a 96-well plate. After 24 h of incubation, the medium in the wells was replaced with a fresh medium containing 15, 30, and 60 μg Au mL−1 of the Pd-induced metal-tipped Au NBPs. After 48 h of incubation, the cells were washed with the fresh medium, followed by the addition of a fresh medium (100 μL) containing MTT (0.5 mg mL−1) to each well. After 3 h of incubation, the medium was removed and the purple formazan product in the live cells was dissolved with dimethyl sulfoxide (150 μL). The resultant mixture was centrifuged at 5000 rpm for 10 min, and the supernatant was transferred into the wells on

modeled as two back-to-back-stacked circular cones with spherically rounded tips. The Au NR was modeled as a cylinder capped with a hemisphere at each end. The size was set according to the average diameter and length measured from TEM images. For the tipped, 804 nm Au NBP, the diameter and length were set at 35 and 100 nm, respectively, with the tip angle and radius adjusted to be 34° and 4 nm. The tipping metal nanosphere was concentric with the rounding sphere at the end of the Au NBP. The radius of the tipping metal nanosphere was increased from 5 nm to 15 nm at 1 nm intervals. The overlapped region between the tipping metal nanosphere and the Au NBP was set to be Au. For the tipped, 798 nm Au NR, the diameter and length were set at 14 and 51 nm, respectively, with the radius of the hemisphere set at 7 nm. The tipping metal nanosphere was concentric with the hemisphere at the end of the Au NR. The radius of the tipping metal nanosphere was increased from 5 nm to 15 nm at 1 nm intervals. The overlapped region between the tipping metal and Au was set to be Au as well. Because we are interested in the variation behaviors of the longitudinal plasmon peak positions, the excitation light direction was set to be perpendicular to the length axis, with the electric field aligned along the length axis. 13839

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

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Journal of the American Chemical Society another plate. The absorbance of the solution was measured at 570 nm using a plate reader (Tecan Infinite M200 PRO, Tecan Group, Mannedorf, Switzerland). The cell viability (%) relative to the control was then calculated. Photothermal Therapy Study. Five thousand U-87 MG cells were seeded into each well of a 96-well plate. After incubation for 24 h, the medium was removed, and a fresh phenol-red-free medium containing 30 μg Au mL−1 of the tipped Au NBPs was added. After 24 h of incubation, the cells were irradiated with a 1064 nm laser for 3 min at the laser power density of 1.18, 1.96, and 2.74 W cm−2, respectively. After the treatment, the culture medium was replaced with α-MEM (100 μL) containing 10% fetal bovine serum. The MTT assay and calcein AM staining were separately performed to determine the cell viability. After incubation with calcein AM (2 μM, SigmaAldrich) for 30 min, the cells were washed with a fresh culture medium, and the green fluorescence emitted from the cells was observed on an Olympus IX71 fluorescence microscope. Characterization. Extinction spectra were measured on a Lambda 950 ultraviolet/visible/near-infrared spectrophotometer with plastic cuvettes of 0.5 cm optical path length. For the measurements beyond 1400 nm, water was replaced by deuterium oxide. The samples were centrifuged and washed twice by deuterium oxide and then redispersed in deuterium oxide. The cuvette with 1 mm optical path length was used to reduce the overall extinction. Scanning electron microscopy (SEM) images were acquired on an FEI Quanta 400 FEG microscope operated at 20 kV. TEM imaging was carried out on an FEI Tecnai Spirit microscope operated at 120 kV. High-resolution transmission electron microscopy (HRTEM) imaging, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization, and elemental mapping were performed on an FEI Tecnai F20 microscope operated at 200 kV and equipped with an Oxford energy-dispersive X-ray (EDX) analysis system.

(Figure S1b−e) shows that negligible Pd nanoparticles are deposited when only a small amount of the Ag precursor is supplied, while Pd nanoparticles appear on the side surface of the Au NBPs in the presence of excessive Ag atoms. As the CTAB concentration is increased, the longitudinal plasmon peak first red-shifts and then blue-shifts, with the turning point located at 0.003 M, in the range between its first and second critical micelle concentrations (Figure S2).50 The addition of CTAC reduces the magnitude of the red-shift and makes the entire surface of the Au NBPs covered with discrete Pd nanoparticles (Figure S3).51 Moreover, neither the absence of Ag shell nor the replacement of Ag atoms deposited on the Au NBPs with Ag+ ions in the solution can induce the metaltipping process on the Au NBPs (Figure S4). Taken together, the optimal conditions for realizing large red plasmon shifts by metal-tipping the 804 nm Au NBPs were found to be 10 μL of AgNO3 and 0.003 M CTAB. For Au NBPs and NRs with different longitudinal plasmon wavelengths, the optimal volume of AgNO3 is around 10 μL with small deviations, while the optimal CTAB concentration is always 0.003 M. TEM and SEM imaging of the Au NBP sample, the Au NBP@Ag sample, and the Pd-induced metal-tipped Au NBP samples reveals the uniform morphologies and narrow size distributions for all samples (Figure 1b−d). The purified Au NBP sample has an average length of 99 ± 3 nm and waist diameter of 34 ± 1 nm. The deposition of Ag causes the transformation from the bipyramid shape of the Au NBPs to a rice shape for the Au NBP@Ag nanostructures, with Ag atoms preferentially deposited on the side surface of the Au NBPs.40 The size of the metal at the tips can be controlled by the volume of the added H2PdCl4 solution (Figure 1d and Figure S5). All of the metal atoms are preferentially deposited at the tips of the Au NBPs, while no metals are seen on the side surface. Such a deposition behavior is not affected by the amount of the added Pd precursor. The increase in the supplied H2PdCl4 amount causes a gradual red-shift of the longitudinal plasmon peak, which is accompanied by peak broadening (Figure 1e). The transverse plasmon peak, however, shows very small shifts. The longitudinal plasmon wavelengths and peak intensities are plotted in Figure 1f as functions of the added H2PdCl4 volume. The wavelength and peak intensity change dramatically with the increase in the amount of H2PdCl4 when the supplied amount of H2PdCl4 is small. They become saturated as the amount of H2PdCl4 is further increased. The peak broadening and peak intensity reduction arise mainly from the increase of the size distribution and the plasmon damping caused by the large imaginary part of the Pd dielectric function.51 The increase in the size distribution results mainly from the metal components deposited at the tips, because the starting Au NBPs are highly uniform in shape and size.39 The time-dependent growth experiments carried out on the 15 μL H2PdCl4 sample further verified the mechanism of the metaltipping process (Figure S6). The deposition of Pd is almost completed after the first 60 min, while Ostwald ripening occurs in the last 11 h. In order to find out the composition of the metal components deposited at the tips of the Au NBPs, we carried out EDX analysis. Figure 1g displays the elemental composition changes with respect to the added volume of H2PdCl4. The results show that the deposited metals are composed of Pd and Ag atoms. On one hand, the Pd:Au molar ratios of the nanostructures are nearly linearly dependent on the H2PdCl4 volume. On the other hand, the Ag:Au molar ratios show an

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RESULTS AND DISCUSSION AgPd Tipping on Au NBPs. The Au NBP sample was prepared through seed-mediated growth and subsequent depletion force-induced purification.38,39 The overgrowth of Ag and Pd on the Au NBPs followed the procedure described in our previous works.40,41 A silver shell is found to be crucial for the anisotropic deposition of a second metal on pregrown nanocrystals.42−45 Figure 1a shows the schematic illustration of the experimental procedure for the synthesis of the Pd-induced metal-tipped Au NBPs. It involves the overgrowth of Ag and the preferential deposition of Pd at the tips. Ag atoms are mainly deposited on the side surface of the Au NBPs, as mentioned previously.40 The Pd-induced metal tipping is realized through the synergetic action of the galvanic replacement and co-reduction of the metal ions. It is different from Pt tipping on CdS nanorods reported previously.46 When H2PdCl4 is added, the Pd complex ions are preferentially reduced at the tips by electrons from Au, because Au has a larger electronegativity than Ag and electrons are partially transferred from Ag to Au. At the same time, the Ag atoms on the side surface are oxidized, with the Ag+ ions released into the solution. Subsequently, the Ag(I) and Pd(II) complex ions in the solution are co-reduced by ascorbic acid.47,48 The preferential deposition of Ag and Pd atoms at the tips of the Au NBPs can be ascribed to the nonuniform distribution of CTAB molecules on elongated Au nanocrystals.49 As shown in Figures S1−S3 of the Supporting Information, the amount of the Ag precursor, the surfactant concentration, and the composition of the surfactant mixture between CTAB and CTAC are all crucial for the successful preparation of the Pd-induced metal-tipped Au NBPs with large red plasmon shifts. The red-shift of the longitudinal dipolar plasmon mode is the largest with 10 μL of AgNO3 (Figure S1a). TEM imaging 13840

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

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Journal of the American Chemical Society abrupt reduction upon the supply of H2PdCl4 and tend to remain nearly constant with fluctuations as the amount of H2PdCl4 is increased. The reduction of the Ag:Au molar ratio suggests that there are still Ag species dissolved in the solution during the galvanic replacement and co-reduction processes. To further reveal the composition of the metal components at the tips of the Au NBPs, we performed HAADF-STEM imaging and elemental mapping on the 4 μL AgPd-tipped Au NBPs (Figure 2a). Elemental mapping clearly shows the

Figure 2. HAADF and HRTEM imaging of the AgPd-tipped Au NBPs. (a) HAADF-STEM and elemental mapping images of a single 4 μL metal-tipped Au NBP. The rightmost image is the overlapped image of the Au, Ag, and Pd elemental maps. The elemental mapping images have the same size scale as the HAADF-STEM image. (b) HRTEM image of a single 4 μL metal-tipped Au NBP. (c) HRTEM image recorded in the region indicated with the white box in (b).

presence of Ag and Pd atoms at the tips, while the map of Au atoms maintains the bipyramidal shape with neither Ag nor Pd observable on the side surface. The metal components at the tips are AgPd alloy, as evidenced by the complete overlap of the elemental distributions of Ag and Pd on the EDX maps.52 HRTEM imaging (Figure 2b,c) revealed that the lattice spacing of the metal components at the tips of the Au NBPs is 0.23 nm, which is between those of the {111} planes of Ag (0.235 nm) and Pd (0.225 nm). The measured spacing for the {111} lattice fringes further confirms the formation of AgPd alloy at the tips. The random orientation of the lattice fringes indicates the polycrystalline nature of the AgPd alloy. In order to further understand the unique large red plasmon shifts caused by AgPd tipping, FDTD simulations were performed to ascertain the plasmonic properties of the AgPdtipped Au NBPs (Figure 3). For simplification, the Pd-tipped Au NBPs were used to represent the AgPd-tipped Au NBPs. The geometrical models utilized in the simulations were set according to the shapes observed on the TEM images. The length and waist diameter of the 804 nm Au NBP were set to be 100 and 35 nm, respectively. The radius of the Pd nanosphere was varied from 5 to 15 nm at an interval of 1 nm (Figure 3a). The excitation light direction was perpendicular to the length axis, with the electric field polarized along the length axis. Therefore, only the longitudinal plasmon mode was excited. The longitudinal plasmon peak exhibits gradual redshifts as the radius of the Pd nanosphere is increased (Figure

Figure 3. FDTD simulations. The longitudinal dipolar plasmon wavelength of the starting Au NBP is 804 nm. Two identical metal nanospheres are attached at the ends of the Au NBP, with their radii gradually increased. (a) Schematic models utilized in the simulations. (b) Simulated extinction spectra for the case of Pd nanospheres. (c) Variations of the longitudinal dipolar plasmon wavelengths as functions of the radius of the metal nanosphere made of Au, Ag, Pd, and Pt, respectively.

3b). The extinction peak intensities of the Pd-tipped Au NBPs are greatly reduced in comparison with that of the starting Au NBP, which is ascribed to the plasmon damping by Pd.51 The slight increase in the peak intensity of the Pd-tipped Au NBPs can be attributed to the increase in the overall particle volume due to the enlargement of the two tipping Pd nanospheres. In comparison, the peak intensities of the AgPd-tipped Au NBP samples decrease with increasing amounts of AgPd deposited at 13841

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

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Journal of the American Chemical Society

Figure 4. AgPd tipping on the 934, 1132, and 1330 nm Au NBPs. (a) Extinction spectra of the Au NBP sample and the corresponding AgPd-tipped Au NBP samples. The longitudinal plasmon wavelengths of the AgPd-tipped Au NBP samples 1, 2, and 3 are 1112, 1462, and 2216 nm, respectively. (b, c) TEM images of the 934 nm Au NBP sample and the corresponding AgPd-tipped Au NBP sample, respectively. The supplied volumes of AgNO3 (0.01 M) and H2PdCl4 (1 mM) are 7 and 2 μL, respectively. (d, e) TEM images of the 1132 nm Au NBP sample and the corresponding AgPd-tipped Au NBP sample, respectively. The supplied volumes of AgNO3 (0.01 M) and H2PdCl4 (1 mM) are 5 and 2 μL, respectively. (f, g) TEM images of the 1330 nm Au NBP sample and the corresponding AgPd-tipped Au NBP sample, respectively. The supplied volumes of AgNO3 (0.01 M) and H2PdCl4 (1 mM) are 4 and 2 μL, respectively.

in the simulations is valid, and the deposition of metals on Au NBPs provides a powerful method for tailoring the longitudinal plasmon resonance toward lower energies in a wide spectral region. The longest plasmon wavelength achieved by the AgPdtipping process for the 804 nm Au NBPs is ∼1180 nm (Figure 1e). To obtain nanostructures with even longer plasmon wavelengths, three Au NBP samples with longitudinal plasmon wavelengths at 934, 1132, and 1330 nm, respectively, were further synthesized for the AgPd-tipping process. Figure 4 displays the extinction spectra and TEM images of the three Au NBP samples, together with their corresponding AgPd-tipped Au NBP samples. All of the three AgPd-tipped Au NBP samples show very large red-shifts after the AgPd-tipping process (Figure 4a). The 1330 nm Au NBPs have a maximal red-shift of ∼900 nm, giving the longest longitudinal plasmon resonance wavelength of ∼2216 nm. Such a large plasmon shift arises from the high refractive index sensitivity of the 1330 nm Au NBP sample and the large size of the deposited AgPd alloy nanospheres, as revealed by FDTD simulations (Figure S9). In general, the refractive index sensitivities of plasmonic metal nanoparticles in similar shapes increase with their plasmon wavelengths.11 In addition, the weak peak at ∼870 nm in the spectrum of the 1330 nm Au NBP sample after metal tipping can be ascribed to a higher-order longitudinal plasmon mode.39 The as-prepared Au NBPs possess uniform sizes and shapes, with their average lengths/waist diameters determined from TEM imaging (Figure 4b−g) to be 120 ± 7/30 ± 2 nm, 197 ± 8/42 ± 2 nm, and 267 ± 8/49 ± 2 nm, respectively. The metal components were also seen to be deposited at the tips of the Au NBPs, irrelevant of the sizes and longitudinal plasmon wavelengths of the Au NBPs. The variation of the added Pd amount can precisely control the size of the metal nanospheres at the tips of the Au NBPs and the red plasmon shift (Figures S10 and S11), similar to the results for the 804 nm Au NBPs (Figure 1). As a result, to produce the tipped Au NBPs with a desired longitudinal plasmon wavelength, we can start from Au NBPs with an appropriate longitudinal plasmon wavelength

the two tips (Figure 1e). The reduction observed experimentally can be ascribed to the inhomogeneous distributions in size and shape for the metal-tipped Au NBP samples. The inhomogeneous distributions increase as more metals are deposited at the two tips and therefore cause the weakening and broadening of the extinction peak. The separation of the extinction into the absorption and scattering contributions in the simulations shows that the absorption and scattering peak intensities have a similar tendency, while the percentage of the intensity reduction for scattering is much larger (Figure S7). This result indicates that the Pd-tipped Au NBPs can function as ideal candidates for light absorption-based applications, especially in the spectral region beyond ∼1000 nm. In general, the simulated variation trend is in agreement with the experimental one for the longitudinal plasmon wavelength. We also performed FDTD simulations by replacing the Pd nanospheres at the tips of the Au NBP with Au, Ag, and Pt nanospheres. The obtained longitudinal plasmon wavelengths of the Au/Ag/Pd/Pt-tipped Au NBPs are plotted against the radius of the metal nanosphere at the tips (Figure 3c). Intriguingly, the longitudinal plasmon wavelength variations of the four types of the metal-tipped Au NBPs remain approximately the same, with that of the Pt-tipped ones showing a little deviation. In addition, FDTD simulations were also performed on the Au NBPs that were tipped with AgPd nanospheres, where the molar fraction of Pd was varied from 0 to 1 at a step of 0.1 and the radius of the alloy nanospheres was fixed at 10 nm. The dielectric function of the AgPd alloy was taken as the sum of those of Ag and Pd according to their molar fractions in the alloy.53 As the molar fraction of Pd is increased, the longitudinal plasmon wavelength remains nearly unchanged and the peak weakens gradually (Figure S8). The weakening of the peak results from Pd-induced plasmon damping. These simulation results unambiguously indicate that the red-shifts observed in the AgPd-tipped Au NBPs are mainly caused by the change in shape when the metals are deposited at the tips. The type of the metal deposited at the tips plays a minor role. Hence, our simplification by replacing the AgPd alloy with Pd 13842

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

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Figure 5. AgPd tipping on the 798 nm Au NRs. The supplied volume of AgNO3 (0.01 M) is 20 μL. (a) Extinction spectra of the Au NR sample, Au NR@Ag sample, and AgPd-tipped Au NR samples produced with varying amounts of H2PdCl4. (b) Variations of the longitudinal dipolar plasmon wavelength and peak intensity as functions of the added volume of H2PdCl4. (c) TEM images of the Au NR sample, Au NR@Ag sample, and AgPdtipped Au NR samples produced with 2 and 8 μL of H2PdCl4, respectively.

and adjust the amount of the Pd precursor. Taken together, the successful tunability of the longitudinal plasmon wavelength spanning the NIR region makes our approach very attractive in the development of plasmonic applications in the NIR region. AgPd Tipping on Au NRs. To verify the growth mechanism and improve the generality of our approach, we further investigated the AgPd-tipping process on Au NRs (Figure 5). The extinction spectra as well as the extracted longitudinal plasmon wavelength and peak intensity variations (Figure 5a,b) are similar to the results for the Au NBPs. The smaller overall red-shifts of the longitudinal plasmon peak for the Au NRs can be ascribed to the lower refractive index sensitivities of Au NRs compared with Au NBPs at similar wavelengths.39 The TEM images of the Au NR sample, Au NR@Ag sample, and AgPd-tipped Au NR samples are shown in Figure 5c and Figure S12. The average length and diameter of the starting Au NRs are 49 ± 4 nm and 14 ± 1 nm, respectively. The Ag coating layer is so thin that the Au NR@ Ag nanostructures remain in the cylindrical shape. Similar to the Au NBPs, AgPd alloy nanoparticles were also seen to grow at the ends of the Au NRs. The simulated results of the Pdtipped Au NRs are in agreement with the experimental observations (Figure S13). Moreover, the plots of the longitudinal plasmon wavelength as a function of the radius of the tipping metal nanospheres for Au/Ag/Pd/Pt-tipped Au NRs nearly overlap with each other (Figure S13e). These results further confirm that the red plasmon shifts are mainly determined by the shape change instead of the composition of the tipping metal nanospheres. Besides the 798 nm Au NRs, the AgPd-tipping method can also be applied to Au NRs with different longitudinal plasmon wavelengths (Figure 6 and Figure S14). The longitudinal

plasmon modes of the two Au NR samples are located at 996 and 1116 nm, with their average lengths/diameters being 99 ± 6/17 ± 1 nm and 70 ± 6/10 ± 1 nm, respectively. The wavelength of ∼1250 nm has been the upper limit of CTABstabilized Au NRs so far.54−56 The maximal red-shift achieved with the Au NRs is ∼250 nm, which is far less than that of the Au NBPs (∼900 nm). The smaller red-shift with the Au NRs can be ascribed to the limitation in the synthesis of starting Au NRs with long longitudinal plasmon wavelengths and the lower refractive index sensitivities of Au NRs in comparison with those of Au NBPs. TEM imaging on the tipped Au NRs reveals that all of the metal atoms are deposited at the ends rather than on the side surface of the Au NRs (Figure 6b−e). All of the results with the Au NRs indicate that the AgPd-tipping process is generally applicable for elongated Au nanocrystals in different shapes. AgPt Tipping on Au NBPs and NRs. We further extended our synthetic approach to the use of other metals, such as Pt or Au, to demonstrate its versatility in producing long-plasmonwavelength metal nanostructures with different metal compositions. The procedure was the same as that for the preparation of the AgPd-tipped Au nanocrystals except that H2PdCl4 was replaced with H2PtCl6 (Figure 7). By choosing an appropriate amount of H2PtCl6, large red-shifts of the longitudinal plasmon resonances can be realized for both Au NBPs (Figure 7a,b) and Au NRs (Figure 7c,d). The representative TEM images of the products show the preferential deposition of metal nanoparticles at the tips of the Au NBPs and Au NRs. However, the metal nanoparticles at the tips appear to be loose, unlike the case of the AgPd-tipping process. The difference in the morphology can be attributed to the much higher bonding energy of Pt−Pt (307 kJ mol−1) compared with that of Pt−Ag 13843

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

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Figure 7. AgPt tipping on the Au NBPs and Au NRs. (a) Extinction spectra of the Au NBP sample, AgPt-tipped Au NBP sample produced with 50 μL of H2PtCl6 (1 mM), and AgPd-tipped Au NBP sample produced with 3 μL of H2PdCl4 (1 mM). (b) TEM image of the AgPttipped Au NBP sample. (c) Extinction spectra of the Au NR sample, AgPt-tipped Au NR sample produced with 100 μL of H2PtCl6, and AgPd-tipped Au NR sample produced with 12 μL of H2PdCl4. (d) TEM image of the AgPt-tipped Au NR sample.

transparency window. Previous studies have shown that illumination of a metal nanocrystal solution at the laser wavelength equal to the plasmon wavelength can photothermally heat the solution to the highest steady-state temperature.61 We therefore chose the 4 μL AgPd-tipped Au NBP sample as an example because its longitudinal plasmon wavelength is around 1064 nm, which is the wavelength of the NIR laser used in our experiments (Figure 8a). The temperature rise traces under the laser illumination at three different power densities for the 4 μL AgPd-tipped Au NBP sample are provided in Figure 8b. The corresponding temperature decay curves recorded by switching off the laser after the solution reached the steady state during the photothermal heating process are shown in Figure S16. The average photothermal conversion efficiency determined according to the thermal balance61 is 0.79 ± 0.02, which is a little smaller than the theoretical value (0.88) calculated from Figure S7. The discrepancy between the experiments and simulations results from the inhomogeneous size distribution of the ensemble sample used in the experiments.61,62 To increase the biocompatibility, we replaced the CTAB molecules on the AgPd-tipped Au NBPs with mPEG-SH and subsequently coated the Au NBPs with dense silica. Coating dense silica on PEGylated Au nanocrystals has proved to be an effective approach for increasing their biocompatibility.63,64 After silica coating, the longitudinal plasmon peak of the tipped Au NBPs is slightly red-shifted to 1074 nm (Figure 8a). The coated silica layer is seen to be relatively uniform around the entire surface of each tipped Au NBP, and the overall morphology of the metal nanostructures remains unchanged after silica coating (Figure 8c). The cell viability results obtained by applying the silica-coated AgPd-tipped Au NBP sample at different Au concentrations indicate that the nanostructures have no cytotoxic effect on U-87 MG cells (Figure 8d). Sole irradiation with a 1064 nm laser at 2.74 W cm−2 has no effect on the cell

Figure 6. AgPd tipping on the 996 and 1116 nm Au NRs. (a) Extinction spectra of the Au NR samples and the corresponding AgPdtipped Au NR samples. The longitudinal plasmon wavelengths of the AgPd-tipped Au NR samples 1 and 2 are 1142 and 1356 nm, respectively. (b, c) TEM images of the 996 nm Au NR sample and the corresponding AgPd-tipped Au NR sample, respectively. The supplied volumes of AgNO3 (0.01 M) and H2PdCl4 (1 mM) are 10 and 4 μL, respectively. (d, e) TEM images of the 1116 nm Au NR sample and the corresponding AgPd-tipped Au NR sample, respectively. The supplied volumes of AgNO3 (0.01 M) and H2PdCl4 (1 mM) are 10 and 4 μL, respectively.

(218 kJ mol−1). In contrast, the bonding energy of Pd−Pd (100 kJ mol−1) is lower than that of Pd−Ag (137 kJ mol−1).57 The loose packing density of the metal nanoparticles at the tips in the case of AgPt tipping is the reason that more H2PtCl6 precursor is required for reaching the same red-shifts in comparison with the case for AgPd tipping on the Au NBPs and Au NRs (Figure 7a,c). On the other hand, Au-induced tipping under the similar conditions was unsuccessful (Figure S15). One probable reason is the high standard reduction potential of the involved Au(III) species,58,59 because the deposited metals are seen under TEM imaging to be distributed irregularly all over the surface of the Au NBPs. Photothermal Performance of the Tipped Au NBPs. Photothermal cancer therapy using NIR laser irradiation is an emerging field due to the deep penetration of NIR light in blood and tissues.20,23,60 In the NIR region, the second biological transparency window has been found to allow for deeper light penetration in tissues than the first biological 13844

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

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Figure 8. Photothermal conversion and therapy studies. (a) Extinction spectra of the 4 μL AgPd-tipped Au NBP sample before and after silica coating. (b) Temperature rise traces of the 4 μL AgPd-tipped Au NBP sample under the 1064 nm laser illumination at different power densities. (c) TEM image of the silica-coated AgPd-tipped Au NBP sample. (d) Viability of U-87 MG cells after 48 h of exposure to the silica-coated AgPd-tipped Au NBP sample at different Au concentrations. The measurement at each concentration was repeated three times. (e, f) Viability of U-87 MG cells upon photothermal therapy with the silica-coated AgPd-tipped Au NBP sample at the concentration of 30 μg Au mL−1. The cells were first incubated for 24 h after the addition of the metal nanostructure sample and then irradiated under a 1064 nm laser at different power densities for 3 min. The cell viability was determined by MTT assay and calcein AM staining, respectively. The live cells were stained with green fluorescence by calcein AM. The measurement at each concentration was repeated three times.

thermal therapy performance in the NIR region around the second biological transparency window. Our work offers a versatile route to the synthesis of Au nanocrystal-based nanostructures with long plasmon wavelengths, which will be very attractive for many plasmon-based applications in photonics and nanobiomedicine.

viability of U-87 MG cells. In comparison, the cell viability values of U-87 MG cells containing the AgPd-tipped Au NBP sample become (91 ± 3), (58 ± 15), and (19 ± 2)% after the laser irradiation at the power densities of 1.18, 1.96, and 2.74 W cm−2, respectively (Figure 8e). In addition, calcein AM staining was also performed to confirm the result of the MTT assay (Figure 8f). The green fluorescence is undetectable after the laser irradiation at 2.74 W cm−2 for most cells. These results reveal that the AgPd-tipped Au NBPs exhibit a high photothermal therapy efficacy toward cancer cells.

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07462. Measured extinction spectra, simulated absorption/ scattering/extinction spectra and plasmon shifts, schematic models, TEM images, time-dependent plasmon peak wavelength and intensity variations, and temperature decay traces (Figures S1−S16) (PDF)

CONCLUSIONS We have achieved red plasmon shifts up to ∼900 nm by AgPd tipping on Au NBPs and NRs. With the assistance of a silver layer that is pregrown on the surface of Au NBPs and NRs, the supplied Pd species can be directed to selectively nucleate and then grow with Ag at the tips of Au NBPs and NRs. It is the unique geometrical structure rather than the composition of the deposited metals that results in the large red-shifts of the longitudinal plasmon peak. Such red-shifts have been verified by FDTD simulations. The successful utilization of the curvature-induced site-selective metal deposition method on Au NBPs and NRs with different longitudinal plasmon wavelengths suggests that nanocrystals with plasmon wavelengths spanning the entire NIR region can be produced readily by the AgPd-tipping process. This method can be further extended to Pt. The applicability of our method to different elongated Au nanocrystals (NBPs and NRs) and with different metals (Pd and Pt) suggests the generality of the method. Moreover, the unique metal-tipped nanostructures exhibit high photothermal conversion efficiencies and excellent photo-

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Ruibin Jiang: 0000-0001-6977-3421 Xiao-Ming Zhu: 0000-0001-8545-1211 Jianfang Wang: 0000-0002-2467-8751 Author Contributions #

X.-Z. Zhu and H. K. Yip contributed equally.

Notes

The authors declare no competing financial interest. 13845

DOI: 10.1021/jacs.7b07462 J. Am. Chem. Soc. 2017, 139, 13837−13846

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NNSFC, 61671299) and the Research Grants Council (RGC) of Hong Kong (NNSFC/RGC Joint Research Scheme, N_CUHK440/14).

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