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Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance Ryo Takahata, Seiji Yamazoe, Kiichirou Koyasu, Kohei Imura, and Tatsuya Tsukuda J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02884 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018
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Journal of the American Chemical Society
Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance Ryo Takahata,1 Seiji Yamazoe,1,2,3 Kiichirou Koyasu,1,2 Kohei Imura,4 Tatsuya Tsukuda1,2* 1
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan 3 CREST, JST, Tokyo 102-0076, Japan 4 Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Shinjuku, Tokyo 169-8555, Japan 2
ABSTRACT: We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5–20 nm while keeping the diameter constant (~2 nm) by changing the relative concentration of OA and Au(I). It is proposed based on the timeresolved optical spectroscopy that AuUNRs are formed via the formation of small (< 2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is redshifted with the AR. The NIR bands of AuUNRs with AR < 5 were redshifted upon the ligand exchange from OA to thiolates, in sharp contrast to the blueshift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ~2 nm significantly affects the optical response. The redshift of the LSPR band can be ascribed to the increase in effective mass of electrons in AuUNRs.
that these rod-shaped Au cores exhibit distinct absorption bands in the near-infrared (NIR) region. These bands are assigned to a single-electron excitation from highest-occupied to lowestunoccupied molecular orbitals.13–18 Namely, the rod-shaped small Au clusters do not exhibit the longitudinal mode of LSPRs although they correspond to thinner analogues of AuNRs with comparable ARs and their lengths exceed the critical value (~2 nm) for the appearance of LSPR bands in Au nanospheres (AuNSs).18, 19 Optical properties of bare Au rods with pentagonal or facecentered cubic (fcc) motifs have been studied theoretically. The theory predicted the redshift of LSPR wavelengths with the AR26– 29 and a threshold in the AR for the appearance of the LSPR:26,29 pentagonal rods constructed by stacking slabs of a five-membered Au5 ring and a central Au atom along the longitudinal axis of an Au13 decahedron exhibit the LSPR band when the number of the constituent atoms exceeds 103 (length > ~5 nm). The appearance of the LSPR band is due to the resonance energy becoming smaller than that for interband transitions from the d-bands, which are responsible for the damping of LSPR in small clusters. Importantly, the resonance energy cannot be determined solely by the AR, but also by the absolute size. Recently, we developed a new class of one-dimensional Au nanostructures with the diameter in the range of 1.5–2.0 nm: gold ultrathin nanorods (AuUNRs).20 AuUNRs occupy a unique
1. INTRODUCTION Gold nanorods (AuNRs) have been studied extensively as one of the representative anisotropic nanostructures. A unique feature of AuNRs is that they exhibit transverse and longitudinal modes of the localized surface plasmon resonance (LSPR) upon photoillumination. These LSPR modes originate from the collective oscillation of conduction-band electrons along the transverse and longitudinal axes, respectively, in response to the electric field of the incoming light. Most notably, the wavelength of the longitudinal LSPR mode can be tuned from the visible to the infrared (IR) region by increasing the aspect ratio (AR) from ~2 to ~20.1–3 The LSPR wavelengths are affected not only by the AR of the rods, but also by their surface modification and surrounding environment. By taking advantage of such tunable LSPR bands, AuNRs have found potential application in various fields such as imaging, photothermal therapy, photovoltaics and biomolecule sensing.4–8 In contrast, rod-shaped Au clusters with diameters smaller than 1 nm protected by organic ligands (thiolates, phosphines, alkynes and/or halides) have gained growing attention. Single-crystal Xray diffraction studies have demonstrated the formation of linear oligomers of icosahedral Au13 units, such as Au23 (Ref. 9), Au25 (Refs. 10, 11), and Au37 (Ref. 12), and those of cuboctahedral Au13 units, such as Au15 (Ref. 13), Au20 (Ref. 14), Au22 (Ref. 15), Au44 (Ref. 16), and Au49 (Ref. 17). Optical spectroscopy revealed
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ferred to as AuUNR:OA (A–D), respectively. 2.3 Preparation of Glutathionate-Protected AuUNRs (AuUNR:SG). AuUNR:OA (10 mg) was dispersed in CHCl3 (10 mL) containing OA (1 g). To this was added a methanol solution (10 mL) containing OA (1 g) and GSH (100 mg), and the mixture was stirred for ~2 hrs. The solution turned from transparent brown to clouded black. The mixture was rotary evaporated to 90% based on a GC analysis. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4×4H2O) was obtained from Tanaka Precious Metal. 1-Dodecanethiol (C12SH), glutathione (GSH), triisopropylsilane (TIPS), cyclohexane, methanol, acetone, chloroform, and hexane were purchased from Wako Pure Chemical Industries. Deionized water with a resistivity of >18.2 MW cm was used. 2.2 Preparation of Oleylamine-Stabilized AuUNRs (AuUNR:OA). OA-stabilized AuUNRs (AuUNR:OA) with monodispersed morphologies were synthesized using a previously reported method with slight modifications.20 First, 175, 350, 500, or 700 mg of OA and 10 mg of HAuCl4·4H2O were dissolved in 10 mL of cyclohexane to obtain mixed solutions a–d, respectively. The solutions were stirred for 2 h until the color of the solution changed from orange to yellow. To each solution, 500 mL of cyclohexane was added, and the mixture was stirred for several minutes. After the stirring, 1 mL of TIPS was added to the solution and the mixture was left to stand. After 30 h, the color turned to dark brown. The solution was condensed to
