Abstract
The amplification of Raman signals of the heteroaromatic cation 1-(N-methylpyrid-4-yl)-2-(N-methylpyrrol-2-yl)ethylene (PEP+)) bound to Au nanorods (NRs) was investigated at different excitation wavelengths to study the effect of the laser resonance with the absorption band of the PEP+ moiety and with the two plasmon oscillation modes of the NR. Two different PEP+ derivatives, differing in the length of the alkyl chain bearing the anchoring group, were used as target molecules. Raman spectra obtained exciting at 514 or at 785 nm (i.e., exciting the transverse or the longitudinal plasmon band) present a higher intensity than that at 488 nm suggesting a higher Raman amplification when the laser excitation wavelength is resonant with one of the two plasmon modes. Moreover, considering results of Discrete Dipole Approximation (DDA) calculations of the local field generated at the NR surface when either the transverse or the longitudinal plasmon modes are excited, we deduced that the resonance condition of the 514-nm laser excitation with the absorption band of the dye strongly contributes to the amplification of the Raman signal.
Highlights
Raman spectroscopy, owing to its ability to recognize typical molecular fingerprints, provides a unique approach for solving analytical problems [1]
The normalized extinction spectrum of cetyltrimethylammonium bromide (CTAB)-stabilized Au NRs in water is shown in Fig. 2a, the dotted line corresponds to the orientationally averaged extinction spectrum obtained via the Discrete Dipole Approximation (DDA) method
This translates into an inhomogenous broadening of the longitudinal plasmon that alters the peak intensity ratio of the experimental spectrum compared to the calculated one
Summary
Raman spectroscopy, owing to its ability to recognize typical molecular fingerprints, provides a unique approach for solving analytical problems [1]. Raman scattering literally appears in a new light when it takes place in the local optical field of metal nanostructures [2] These systems, thanks to the ability of sustaining strong localized surface plasmon resonances (LSPR), provide the key effect for the observation of enhanced Raman signals from molecules attached to them (Surface Enhanced Raman Spectroscopy—SERS). Rod-shaped metal nanoparticles possess two plasmon resonances, a transverse mode perpendicular to the long axis of the rod and a longitudinal mode parallel to the long rod axis [11] The latter depends linearly on the aspect ratio, i.e., the length divided by the width of the nanorods (NRs), and it Plasmonics (2014) 9:581–593 is widely tunable in the visible and in the near infrared region of the spectrum. The possibility to tune the resonance wavelength can be exploited to amplify both laser and scattered field in SERS measurements at the desired wavelength
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