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.

Colloidal plasmonic nanomaterials, consisting of metals such as gold and silver, are excellent candidates for advanced optical probes and devices, but precise control over surface chemistry is essential for realizing their full potential. Coupling thiolated (R-SH) molecules to nanoprobe surfaces is a convenient and established route to tailor surface properties. The ability to dynamically probe and monitor the surface chemistry of nanoparticles in solution is essential for rapidly manufacturing spectroscopically tunable nanoparticles. In this study, we report the development of surface-enhanced Raman spectroscopy (SERS) as a method to monitor the kinetics of gold-thiolate bond formation on colloidal gold nanoparticles. A theoretical model combining SERS enhancement with the Beer-Lambert law is proposed to explain ensemble scattering and absorption effects in colloids during chemisorption. In order to maximize biological relevance and signal reproducibility, experiments used to validate the model focused on maintaining nanoparticle stability after the addition of water-soluble aromatic thiolated molecules. Our results indicate that ligand exchange on gold nanoparticles follow a first-order Langmuir adsorption model with rate constants on the order of 0.01 min 1. This study demonstrates an experimental spectroscopic method and theoretical model for monitoring binding kinetics that may prove useful for designing novel probes.

Conjugated metallic nanoparticles are a promising means to achieve ultrasensitive and multiplexed sensing in intact three-dimensional samples, especially for biological applications, via surface enhanced Raman scattering (SERS). We show that enhancement and extinction are linked and compete in a collection of metallic nanoparticles. Counterintuitively, the Raman signal vanishes when nanoparticles are excited at their plasmon resonance, while increasing nanoparticle concentrations at off-resonance excitation sometimes leads to decreased signal. We develop an effective medium theory that explains both phenomena. Optimal choices of excitation wavelength, individual particle enhancement factor and concentrations are indicated. The same processes which give rise to enhancement also lead to increased extinction of both the illumination and the Raman scattered light. Nanoparticles attenuate the incident field (blue) and at the same time provide local enhancement for SERS. Likewise the radiation of the Raman-scattered field (green) is enhanced by the near-by sphere but extinguished by the rest of the spheres in the suspension on propagation.

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