Christopher J.
Orendorff
,
Latha
Gearheart†
,
Nikhil R.
Jana‡
and
Catherine J.
Murphy
*
University of South Carolina, Department of Chemistry and Biochemistry, Columbia, SC 29208, USA. E-mail: murphy@mail.chem.sc.edu
First published on 18th October 2005
Silver and gold nanorods with aspect ratios from 1 to 16 have been used as substrates for surface enhanced Raman spectroscopy (SERS) in colloidal solution. The nanorod aspect ratio is varied to give different degrees of overlap between the nanorod longitudinal plasmon band and excitation source in order to determine its effect on overall surface enhancement. Results suggest that enhancement factors are a factor of 10–102 greater for substrates that have plasmon band overlap with the excitation source than for substrates whose plasmon bands do not.
While the use of colloidal SERS substrates is widespread, studies of substrates with different morphologies are limited, despite their shape-dependent optical properties which make them attractive candidates for SERS.24,25 For example, in addition to having transverse plasmon absorption, gold and silver nanorods have longitudinal plasmon bands that can be tuned from the visible to the near-infrared region by varying the nanorod aspect ratio.24,25 Then, Raman scattering enhancements from these materials can be maximized by plasmon resonance with the excitation source, effectively optimizing contributions from the EM enhancement mechanism for a given aspect ratio.
The use of gold and silver nanorods or wires as SERS substrates has been reported by this and other laboratories. SERS on silver nanowire assemblies have recently been reported by Tao et al.26 giving rise to surface enhancement factors of 105 for hexadecanethiol and 109 for rhodamine 6G. Moskovits and coworkers have studied SERS on similar silver nanowire rafts27 and Aroca et al. have used multilayer films of silver nanowires as SERS substrates.28 Nikoobakht et al.29,30 have reported surface enhancement factors of ∼105 for 2-aminothiophenol on separated and aggregated gold nanorods. For separated gold nanorod SERS experiments, there is no overlap between the plasmon absorption and Raman excitation wavelengths and only a slight overlap for the aggregated nanorod experiments, resulting in primarily CHEM enhancement from the Au{110} surface of these substrates.29,30 Recently, we reported the use of gold nanorods and other nanoparticle morphologies immobilized on self-assembled monolayers on planar substrates.31 While enhancement factors of 108 were estimated for 4-mercaptobenzoic acid monolayers using nanorods in this geometry, nanorod aspect ratio dependence on SERS enhancement factors was not observed because of convoluting effects of nanoparticle optical properties and plasmon coupling between the nanoparticles and the gold surface.31
Those previous reports describe SERS on primarily aggregated nanorods and nanowires, where determining the dependence of SERS on nanoparticle optical properties is difficult due to convoluting plasmon coupling contributions known to vastly improve SERS enhancements.5,16,17 In order to determine EM contributions to SERS from the substrate optical properties alone, one would have to use a more homogeneous experimental system; for example, dilute colloidal solutions of nanorods or well dispersed nanorods on planar substrates.
Halas and coworkers studied the effect of tunable plasmon absorption of gold nanoshells as SERS substrates.32 Large Raman scattering enhancement factors of 109–1010 were observed for 4-mercaptoaniline, however, no significant difference in SERS enhancement factors for shell nanoparticles in and off resonance was observed.32
This report describes the use of silver and gold nanorods with varied aspect ratios as SERS substrates in colloidal solution. The optical properties of these anisotropic nanocrystals have been tailored to have variable degrees of plasmon overlap with the excitation source. As a result, EM contributions to the overall enhancement should vary with the nanorod aspect ratio. The goal is to experimentally interrogate the effect of nanorod plasmon resonance on Raman scattering enhancements in the absence of other convoluting effects, primarily plasmon coupling.
Silver nanorods with aspect ratios of 3.5 ± 0.7 (length = 43 ± 3, width = 12 ± 2 nm) and 10 ± 5 (length = 214 ± 37, width = 20 ± 4 nm) and gold nanorods with aspect ratios of 1.7 ± 0.2 (length = 41 ± 3, width = 24 ± 3 nm), 4.5 ± 0.2 (length = 57 ± 8, width = 13 ± 2 nm), and 16 ± 5 (length = 372 ± 119, width = 23 ± 4 nm) were prepared via the seed-mediated techniques in aqueous surfactants described previously.33–35 After preparation, solutions of silver and gold nanorods were separated from spheres and excess surfactant by two centrifugation and re-dispersion steps.33–35 Silver spheres 33 ± 7 nm in diameter were prepared according to conventional citrate reduction for comparison.36 Gold spheres of 29 ± 6 nm diameter were prepared using a seeding procedure in aqueous surfactant.37 Excess surfactant was removed from these gold spheres by two successive centrifugation and re-dispersions steps.37 With the primary focus of this work on rod-shaped nanoparticles, both gold and silver spheres will be referred to as aspect ratio 1 nanorods throughout, in order to simplify the discussion.
Fig. 1 Absorption spectra of (a) silver nanorods with aspect ratios 1 (trace I), 3.5 (trace II), and 10 (trace III) and (b) gold nanorods with aspect ratios 1 (trace I), 1.7 (trace II), 4.5 (trace III), and 16 (trace IV). The vertical dashed line represents the excitation wavelength for SERS measurements at 632.8 nm. |
Fig. 2 shows TEMs of nanorods. Aspect ratio 3.5 silver nanorods (∼95%) were homogeneous in size and often formed ordered, self-assembled layers when dried, as shown in Fig. 2b. However, the aspect ratio 10 silver nanorods were polydisperse (10 ± 5), and significant amounts of platelets and spheres (50 ± 10 nm, ∼50%) were still present (Fig. 2c). Gold nanorods with aspect ratios 2 and 16 are generally free from other shapes, but more polydisperse than aspect ratio 4 nanorods. It is important to note that for aspect ratio 10 rods with the presence of significant numbers of other shapes in the SERS samples, the resulting SERS spectra will be a convolution of those for analytes adsorbed to nanorods and to the fraction of other shapes. Since EM enhancement of aspect ratio 10 rods is expected to be greater than that for nanospheres, resulting SERS enhancement factors for these samples could be an underestimate relative to samples containing pure nanorods.
Fig. 2 Transmission electron microscopy images of silver (a–c) and gold (d–g) nanorods with aspect ratios (a) 1, (b) 3.5, (c) 10, (d) 1, (e) 1.7, (f) 4.5, and (g) 16. The scale bars represent 100 nm for each image. |
Nanoparticle | Average particle sizeb/nm | Dimensionsc/nm |
---|---|---|
a Aspect ratio 1.0 = spheres. b Measured by light scattering. c Measured from TEM images (l = length, w = width). | ||
Aspect ratio 10 silver nanorods | 90 ± 17 | l = 214 ± 37, w = 20 ± 4 |
Aspect ratio 3.5 silver nanorods | 31 ± 7 | l = 43 ± 3, w = 12 ± 2 |
Aspect ratio 1.0 silver nanorodsa | 34 ± 5 | Diameter = 33 ± 7 |
Aspect ratio 1.7 gold nanorods | 34 ± 6 | l = 41 ± 3, w = 24 ± 3 |
Aspect ratio 4.5 gold nanorods | 38 ± 6 | l = 57 ± 8, w = 13 ± 2 |
Aspect ratio 16 gold nanorods | 105 ± 24 | l = 372 ± 119, w = 23 ± 4 |
Aspect ratio 1.0 gold nanorodsa | 31 ± 3 | Diameter = 29 ± 6 |
SERS spectra of 2,2′-bipyridine, 4-aminothiophenol, and 4-mercaptopyridine adsorbed to aspect ratio 1, 3.5, and 10 silver nanorods between 700 and 1400 cm−1 are shown in Fig. 3. SERS spectra of these same analytes on aspect ratio 1, 1.7, 4.5, and 16 gold nanorods are shown in Fig. 4. Peak frequencies and assignments for each of these analytes on gold and silver substrates are given in Table 2. It is important to note that in aqueous solution nanorods are randomly oriented and these SERS spectra are representative of all possible nanorod orientations averaged over the entire acquisition time. The ideal geometry would be to fix the nanorod orientation with the long axis parallel to the excitation source polarization, in order to have maximum longitudinal plasmon overlap. However, colloidal solution samples are used to eliminate nanoparticle aggregation effects. SERS spectra for these analytes are comparable to those acquired previously for these analytes adsorbed to colloidal substrates.39–41 Qualitatively, the vibrational mode intensity and signal-to-noise are generally better for analytes adsorbed to silver than to gold nanoparticles, in fact, no vibrational modes are observed for 2,2′-bipyridine on any gold substrate or for 4-aminothiophenol on aspect ratio 16 gold nanorods. Moreover, the signal-to-noise is generally better for analytes on aspect ratio 10 nanorods than other silver nanorods and for molecules adsorbed to aspect ratio 1 and 1.7 nanorods than aspect ratio 4.5 or 16 nanorods. Even though all of these nanorods are capped with CTAB,33–35 no characteristic vibrational modes for CTAB are observed in these spectra. This is likely to be due to the fact that the ν(C–C), ν(C–N), or δ(C–H) CTAB modes are significantly weaker than the δ(C–H) or ν(C–C)ring modes of these aromatic analytes and are simply below the detection limit of the spectrometer. The dependence of the nanorod aspect ratio on surface enhancement is quantified by calculating surface enhancement factors for the analytes on each substrate.
Fig. 3 Surface enhanced Raman spectra of 2,2′-bipyridine, 4-aminothiophenol, and 4-mercaptopyridine at 10−6 M using silver nanorods with aspect ratios (a,d, and g) 10, (b,e, and h) 3.5, and (c, f, and i) 1. Acquisition times are (a) 30 s, (b) 60 s, (c) 120 s, (d) 30 s, (e) 60 s, (f) 60 s, (g) 10 s, (h) 10 s, and (i) 60 s. |
Fig. 4 Surface enhanced Raman spectra of 2,2′-bipyridine, 4-aminothiophenol, and 4-mercaptopyridine at 10−6 M using gold nanorods with aspect ratios (a,e, and i) 1.7, (b,f, and j) 4.5, (c, g, and k) 16 and (d, h, and l) 1. Acquisition times are 120 s for all spectra. |
Peak frequency/cm−1 | Assignmentbc | |||||
---|---|---|---|---|---|---|
4-ATP on gold NRse | 4-ATP on silver NRs | 4-mpyr on gold NRs | 4-mpyr on silver NRs | 2,2′-bipy on gold NRs | 2,2′-bipy on silver NRs | |
a 4-ATP = 4-aminothiophenol, 4-mpyr = 4-mercaptopyridine, 2,2′-bipy = 2,2′-bipyridine. b Assignments from refs. 37–39. c δ = bend or deformation; ν = stretch; ring = ring breathing mode; ip = in-plane mode. d Not observed. e NRs = nanorods. | ||||||
d | 760 | δ ip(C–C) | ||||
1000 | δ(C–H) | |||||
1002 | 1006 | ν(C–C)ring | ||||
d | 1004 | ν(C–C)ring | ||||
d | 1056 | δ ip(C–H) | ||||
1058 | 1058 | δ(C–H) | ||||
1071 | 1073 | ν(C–C)ring | ||||
1087 | 1091 | ν(C–C)ring | ||||
d | 1165 | δ ip(C–H) | ||||
1167 | 1174 | δ ip(C–H) | ||||
1211 | 1217 | δ(C–H), δ(N–H) | ||||
d | 1298 | ν(C–C), ν(C–N) |
Estimated surface enhancement factors (EF) for each of these substrates are shown in Table 3. EFs were determined using the following expression:29
EF = [ISERS]/[IRaman] × [Mbulk]/[Mads] | (1) |
Analyte/vibrational mode | ||||
---|---|---|---|---|
Substrate | 4-Mercaptopyridine/ν(C–C) | 4-Aminothiophenol/ν(C–S) | 2,2′-Bipyridine/ν(C–C) | 2,2′-Bipyridine/δip(C–H) |
a Aspect ratio 1.0 = spheres. b No observed vibrational modes. | ||||
Aspect ratio 10 silver nanorods | 2.3 ± 0.1 × 107 | 2.3 ± 0.05 × 106 | 8.5 ± 0.07 × 105 | 1.1 ± 0.03 × 106 |
Aspect ratio 3.5 silver nanorods | 2.5 ± 0.2 × 106 | 2.7 ± 0.1 × 105 | 2.1 ± 0.2 × 104 | 8.3 ± 0.08 × 104 |
Aspect ratio 1.0 silver nanorodsa | 4.8 ± 0.5 × 106 | 3.9 ± 0.3 × 105 | 2.6 ± 0.2 × 104 | 1.4 ± 0.1 × 105 |
Aspect ratio 1.7 gold nanorods | 1.4 ± 0.37 × 105 | 2.3 ± 0.07 × 104 | b | b |
Aspect ratio 4.5 gold nanorods | 6.2 ± 1.84 × 104 | 4.3 ± 0.3 × 103 | b | b |
Aspect ratio 16 gold nanorods | 1.8 ± 0.2 × 104 | b | b | b |
Aspect ratio 1.0 gold nanorodsa | 1.2 ± 0.2 × 104 | 2.0 ± 0.4 × 103 | b | b |
In general, enhancement factors for aspect ratio 10 silver nanorods are 101–102 times greater than for other silver nanorods and range from 105–107. Enhancement factors for aspect ratio 1.7 gold nanorods are a factor of 10 greater than for other gold substrates and range from 103–105. This is attributed to greater EM enhancement for the aspect ratio 10 silver nanorods and aspect ratio 1.7 gold nanorods, which have plasmon absorption overlap with the excitation source, relative to the other nanorod substrates, as shown in Fig. 1. As described above, the samples containing aspect ratio 10 silver nanorods also contain a large fraction of spheres and other shapes (∼50%). The EF values calculated for these nanorods are actually an underestimate of the EF anticipated for pure nanorods.
It is also interesting to note the variations in EF values among the different analytes, where the thiol molecules give larger surface enhancements than 2,2′-bipyridine on all substrates. Recently, Alvarez-Puebla et al.42 studied the effect of nanoparticle surface charge (ζ potential) on SERS enhancement. The authors observed larger SERS enhancement for analytes that are electrostatically attracted to nanoparticle substrates than those samples without electrostatic interactions.42 In our case, adsorption of negatively charged thiols should be facilitated on CTAB-protected nanoparticles with a positive ζ potential, leading to greater overall enhancement for thiol analytes over 2,2′-bipyridine. However, this is a limited data set and more molecules of varying analyte functionality need to be tested in order to fully describe these observations.
With limited experimental reports on SERS using substrates with tunable plasmon resonance,32 and no reports on the aspect ratio dependence on SERS using nanorods, observed differences in EF values for nanorod substrates can be compared to previous theoretical or experimental studies of wavelength-dependent SERS on spherical metallic substrates.8,9,43–46 In these theoretical calculations, the plasmon bands of the substrates are fixed and the incident wavelengths are variable. However, these can be used as models for wavelength-dependent EM contributions to enhancement using gold and silver nanorods. According to Xu et al.,8 the magnitude of EM enhancement, MEM, is approximated by
MEM = [EL (ωI)/EI (ωI)]4 | (2) |
EL (ωI) = EI (ωI) + Eind(ωI) | (3) |
Eind α (ω/c)2 (ε2 − ε1) | (4) |
Using the model for analyte molecules adsorbed to the surface of 30 nm diameter silver nanoparticles, maximum calculated MEM values are ∼104 for incident photon energies at the plasmon resonant wavelength (∼400 nm), while minimum MEM values are ∼102 for incident wavelengths of 600–1200 nm.8 For gold nanoparticles, calculated MEM values reach a maximum of ∼103 for excitation energies at the plasmon resonant wavelength (∼520 nm), and are <10 at 400 nm, and ∼102 for incident wavelengths of 700–1200 nm.8 Based on these theoretical calculations we can estimate that EF values for silver and gold substrates with plasmon bands in resonance with the incident radiation should be 101−102 times greater than those without overlapping bands. While this simple approximation does not consider nanoparticle shape effects known to affect localized electromagnetic fields,46 this theoretical model for wavelength-dependent EM enhancement is consistent with our observations for experimentally determined enhancement factors for gold and silver nanorods.
Footnotes |
† Present address: Presbyterian College, Department of Chemistry, 503 South Broad St., Clinton, SC 29325, USA. |
‡ Present address: Institute of Bioengineering and Nanotechnology, 51 Science Park Rd., Singapore Science Park II, Singapore, 117586. |
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