Off-Resonance Surface-Enhanced Raman Spectroscopy from Gold Nanorod Suspensions as a Function of Aspect Ratio: Not What We Thought

Abstract

The design of nanoparticles for surface enhanced Raman scattering (SERS) within suspensions is more involved than simply maximizing the local field enhancement. The enhancement at the nanoparticle surface and the extinction of both the incident and scattered light during propagation act in concert to determine the observed signal intensity. Here we explore these critical aspects of signal generation and propagation through experiment and theory. We synthesized gold nanorods of six different aspect ratios in order to obtain longitudinal surface plasmon resonances that incrementally spanned 600-800 nm. The Raman reporter molecule methylene blue was trap-coated near the surface of each nanorod sample, generating SERS spectra, which were used to compare Raman signals. The average number of reporter molecules per nanorod was quantified against known standards using electrospray ionization liquid chromatography mass spectrometry. The magnitude of the observed Raman signal is reported for each aspect ratio along with the attenuation due to extinction in suspension. The highest Raman signal was obtained from the nanorod suspension with a plasmon resonance blue-shifted from the laser excitation wavelength. This finding is in contrast to SERS measurements obtained from molecules dried onto the surface of roughened or patterned metal substrates where the maximum observed signal is near or red-shifted from the laser excitation wavelength. We explain these results as a competition between SERS enhancement and extinction, at the excitation and scattered wavelengths, on propagation through the sample.

SERS-fluorescence dual mode nanotags for cervical cancer detection using aptamers conjugated to gold-silver nanorods

“…As a result, the NRs with an aspect ratio of 2.5, which has a longitudinal band overlapping with the laser excitation wavelength at 633 nm, produces the highest SERS signal. Similar phenomena were also observed previously for 4-mercaptopyridine and methylene blue on NRs with different aspect ratios [16,27]. Therefore, NRs with the aspect ratio of 2.5 were further employed for the aptamer conjugation and target cell detection.…”

Ultrahigh affinity Raman probe for targeted live cell imaging of prostate cancer

“…30,31 The experimental loadings could, though, be lower as reported recently for a nanorod-derived SERS agent. 32 …”

“Elastic” Property of Mesoporous Silica Shell: For Dynamic Surface Enhanced Raman Scattering Ability Monitoring of Growing Noble Metal Nanostructures via a Simplified Spatially Confined Growth Method

“…For the samples in which 50 to 100 μL of AgNO 3 solution was added, a decreased intensity of the peaks was observed because the longitudinal SPR bands gradually shifted away from the wavelength of the 632.8 nm excitation. 36 Thereafter, the SERS intensity began to abruptly increase for samples with thicker Ag shells (Figure 3c). For the sample obtained by adding 150 μL of AgNO 3 , the Raman intensity at 1619 cm −1 was 28 times higher than that for AuNR@mSiO 2 ( Figure S8, Supporting Information), which was attributed to the much greater Raman enhancing ability of Ag than Au.…”