title: Off-resonance surface-enhanced Raman spectroscopy from gold nanorod suspensions as a function of aspect ratio: not what we thought.
abstract: 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.
authors: Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
authors: Sean T
authors: ST
authors: Sivapalan
authors: Brent M
authors: BM
authors: Devetter
authors: Timothy K
authors: TK
authors: Yang
authors: Thomas
authors: T
authors: van Dijk
authors: Matthew V
authors: MV
authors: Schulmerich
authors: P Scott
authors: PS
authors: Carney
authors: Rohit
authors: R
authors: Bhargava
authors: Catherine J
authors: CJ
authors: Murphy
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry; D020099:Coated Materials, Biocompatible / Q000737:chemistry; D006046:Gold / Q000737:chemistry*; D053768:Metal Nanoparticles / Q000737:chemistry*; D008751:Methylene Blue / Q000737:chemistry; D036103:Nanotechnology; D043942:Nanotubes / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D020349:Surface Plasmon Resonance; D013499:Surface Properties; D013535:Suspensions
chemical_list: D000180:Acrylic Resins; D020099:Coated Materials, Biocompatible; D013535:Suspensions; C006903:carbopol 940; D006046:Gold; D008751:Methylene Blue