LETTER pubs.acs.org/JPCL
Surface-Enhanced Raman Spectroscopy: Investigations at the Nanorod Edges and Dimer Junctions Jatish Kumar† and K. George Thomas*,†,‡ † ‡
Photosciences and Photonics, National Institute for Interdisciplinary Science and Technology (NIIST), CSIR, Thiruvananthapuram, 695 019, India School of Chemistry, Indian Institute of Science Education and Research-Thiruvananthapuram (IISER-TVM), CET Campus, Thiruvananthapuram, 695 016, India
bS Supporting Information ABSTRACT: Raman signal enhancement of two analyte molecules, containing bipyridine and phenyl moieties, were investigated by linking them (i) onto the edges of Au nanorods using monothiol derivatives and (ii) at the junctions of two Au nanorods using dithiol derivatives. Edges of Au nanorods are regions of high electric field, and specific interaction of the thiol molecules on the {111} planes at the edges resulted in an enhanced Raman signal. When two Au nanorods are brought together in a linear fashion through dithiol linkages, their longitudinal plasmon oscillations couple each other, creating regions of enhanced electric field (hot spots) at the junctions. Interestingly, dimerization leads to a spontaneous enhancement in the intensity of Raman signals (enhancement factor of ∼1.4 105) due to the localization of molecules at the junctions of Au nanorod dimers. SECTION: Nanoparticles and Nanostructures regions of enhanced electric field. Raman-active molecules, when trapped between the nanoparticle junctions, experience high electric field, which results in an enhancement of SERS signals upon illumination with light of appropriate wavelength.13,14 Various approaches for the design of nanostructured materials with hot spots include (i) assembling of nanoparticles by chemical methods using dithiols15,16 and DNA molecules,17 (ii) salt-induced aggregation methods,18 (iii) etching of nanocubes,19,20 and (iv) various lithographic methods.21 Most of the chemical methods utilize spherical metal nanoparticles for designing SERS substrate; however random aggregation (and poor reproducibility), due to their isotropic nature, is one of the limiting factors.22 It is reported by our group23-25 and others26-28 that the anisotropic features of Au nanorods allow their linear assembly by adopting electrostatic, supramolecular, and covalent approaches. It was further concluded that the plasmon coupling in Au nanorods proceeds through an incubation step, followed by the dimerization and subsequent oligomerization in a preferential end-to-end fashion.25 SERS studies have been performed using aggregated Au nanorods as the substrate;29 more recently, enhancement of Raman signals has been demonstrated through salt-induced assembly of Au nanorods as linear chains by blocking their lateral sides with polymers.30 As the number of Au nanorods in a chain increases, several possible orientations exist between the nanorods, and such variations can disrupt effective plasmon coupling and enhancement of the electric field at the junctions;31-33 hence, studies based on dimers may be more suited for SERS. Recently,
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here is a growing interest in the development of newer spectroscopic techniques for the enhancement of molecular signals, particularly for the detection and identification of chemically and biologically important molecules.1,2 Among the various techniques, surface-enhanced Raman spectroscopy (SERS), using noble metal nanoparticles as substrates, has emerged as one of the most powerful tools due to the unique ability of the nanoparticles to interact with light.3-5 Light absorption in spherical metal nanoparticles originates from surface plasmon resonance; the surface electrons on metal nanoparticles coherently oscillate upon interaction with light, and the trapping of light at the metal dielectric interface leads to an increase in electric field.6 A Raman-active molecule placed in the vicinity of the nanoparticle’s surface can lead to enhanced signal intensity due to (i) charge-transfer interactions between the metal nanoparticle and the molecular system (chemical enhancement)4 and (ii) a high electric field prevailing on the metal surface (electromagnetic enhancement).7 The electric field is uniformly distributed around the spherical nanoparticles, whereas the edges of the anisotropic nanomaterials such as spheroids, rods, bipyramids, and triangles experience an enhanced electric field, making them promising candidates for SERS studies.8,9 An enhanced electric field at the edges of Au nanorods has been experimentally demonstrated by our group through electrostatic interactions with oppositely charged gold nanoparticles.10 It is reported that the selective excitation of the longitudinal plasmon band of Au nanorods results in a large enhancement in the Raman signal of the adsorbate molecules.11 Another factor that influences the SERS signal of an adsorbed molecule is the presence of hybridized (coupled) plasmon, which arises when metal nanoparticles are brought in close proximity.12 Hybridized plasmon generates hot spots, which are essentially r 2011 American Chemical Society
Received: January 13, 2011 Accepted: February 21, 2011 Published: February 28, 2011 610
dx.doi.org/10.1021/jz2000613 | J. Phys. Chem. Lett. 2011, 2, 610–615
The Journal of Physical Chemistry Letters
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we have demonstrated the tuning of plasmon coupling in Au nanorod dimers by varying the distance and orientation.34 The main objective of the present investigation is to understand the variation in the Raman intensity of a molecule when placed at the (i) edges of an isolated Au nanorod and (ii) junctions of Au nanorod dimers. Monothiol as well as dithiol derivatives of Raman-active molecules (Scheme 1) has been used to investigate these aspects.35,36 The rationale behind this strategy is that the monothiol derivatives (bipy-MT and bz-MT) specifically bind on the edges of an isolated Au nanorod, whereas the dithiol derivatives (bipy-DT and bz-DT) bind on the edges of two nanorods, leading to dimerization and subsequently the localization of molecules at the junctions.25 Any variation in orientation between the nanorods can disrupt the effective plasmon coupling,34 and hence, rigid molecules were used as linkers in the present study.
By following a photochemical method, gold nanorods with an average aspect ratio of ∼2.5 were synthesized, and excess capping agents such as CTAB were removed by centrifugation.37 Gold nanorods possess a large positive zeta potential in water (ζ = 28.5 mV), and a reversal in ζ was observed in acetonitrile-rich solvents.34 For Raman and absorption spectroscopic studies, we have used a solvent mixture (1:4) of water and acetonitrile (ζ = -28.8 mV) wherein Au nanorods are stable for several hours. Au nanorods, having an aspect ratio of ∼2.5, possess two distinct surface plasmon absorption bands (Figure 1), a short wavelength band at 520 nm corresponding to the transverse mode of plasmon oscillation and a long wavelength band at 635 nm corresponding to the longitudinal mode.23-27 Raman spectra were recorded using a confocal Raman spectrometer using a He-Ne laser source having an excitation wavelength of 633 nm and an acquisition time of 10 s. CTAB-capped Au nanorods suspended in a mixture (1:4) of water and acetonitrile showed Raman signals corresponding to the solvent molecules; however, no spectroscopic signals corresponding to the CTAB molecules were observed. The Raman intensity of a molecule placed at the edges of an isolated Au nanorod (edge effect) was investigated by adding monothiol derivatives (0.5-3.0 μM) to a solution of Au nanorods (0.12 nM). After the addition of monothiol derivatives, solutions were kept for 10 min, and the Raman and absorption spectra were recorded. It is reported that the thiol derivatives preferentially bind onto the {111} planes of the Au nanorods, and this specific interaction leads to the localization of monothiol molecules more at the edges.26,27 Both of the plasmon absorption bands of Au nanorods remain more or less unaffected upon increasing the concentration of bipy-MT (1.0-3.0 μM), except for a small dampening (Figure 1A). The Raman spectrum of bipy-MT bound to the edges of Au nanorods showed a few
Scheme 1. Schematic Representation of Au Nanorods with Molecules at the Edges of (A) an Isolated Au Nanorod and (B) Junctions of Au Nanorod Dimers along with Structures of Thiol Derivatives Used in the Study, (C) Monothiols and (D) Dithiols
Figure 1. Absorption spectral changes of Au nanorods (a) in the absence of monothiols and (b-e) upon addition of varying amounts of bipy-MT (A) and bz-MT (C) to a solvent mixture (1:4) of water and acetonitrile and the corresponding changes in the Raman spectrum (B,D) upon excitation with He-Ne laser (633 nm; acquisition time of 10 s). The concentrations of monothiols (bipy-MT and bz-MT) correspond to b = 0.5 μM, c = 1 μM, d = 2 μM, and e = 3 μM. 611
dx.doi.org/10.1021/jz2000613 |J. Phys. Chem. Lett. 2011, 2, 610–615
The Journal of Physical Chemistry Letters
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Figure 2. Absorption and Raman spectral changes of Au nanorods (0.12 nM) upon addition of dithiols bipy-DT (A,B) and bz-DT (C,D) in a solvent mixture (1:4) of water and acetonitrile. Plasmon absorption (A,C) of Au nanorods (a) in the absence of dithiols, (b) at a lower concentration of dithiol (0.5 μM for both bipy-DT and bz-DT; no change in the spectrum with time), and (c-g) at a higher concentration of dithiols (0.8 μM of bipy-DT and 1.0 μM of bz-DT) recorded successively at a time interval of 3 min. The corresponding Raman spectra of bipy-DT (B) and bz-DT (D) upon exciting with a He-Ne laser (633 nm; acquisition time of 10 s) are presented as traces b-g.
peaks; however, their intensities were low. The peaks observed at 1600, 1499, 1324, and 1237 cm-1 are attributed to the ring stretching (C—C and C—N), in-plane C—H bending, interring stretching, and ring deformation modes of bipy-MT (Figure 1B and Supporting Information).35 Intensities of various peaks were almost doubled upon increasing the concentration of bipy-MT to 1.0 μM and leveled off with further increase in concentration. The nanorod solution underwent precipitation upon addition of bipy-MT above 3.0 μM. Raman and absorption spectral studies were also carried out by varying the concentration of bz-MT; solutions were quite stable at a higher concentration of bz-MT, and both of the plasmon absorption bands remained unaffected. Raman peaks observed at 1600, 1222, and 655 cm-1 are attributed to the ring stretching, CH2 wagging, and C—S stretching modes of bz-MT, respectively (Figure 1D and Supporting Information).36 In both cases, a gradual increase in the intensity of Raman peaks was observed with the increase in the concentration of monothiol. It is interesting to note that Raman signals corresponding to bipy-MT/bz-MT were not observed in the absence of Au nanorods, even upon increasing the concentration by 104 fold (5 mM). These results clearly indicate that specific interaction and localization of monothiol molecules at the edges of Au nanorods, which are domains of high electric field, lead to the gradual enhancement in signals.
Further, we have investigated the Raman spectrum of the dithiol derivatives (bipy-DT and bz-DT) during dimerization of Au nanorods. Both of the plasmon absorption bands of Au nanorods remain more or less unaffected at lower concentrations (
