Estimating the SERS-based sensing capapbilities of colloidal silver nanoparticles: A novel physical chemistry and nanotechnology laboratory experiment

Abstract

<p>Surface-enhanced Raman spectroscopy (SERS) is an embodiment of Raman spectroscopy that hat all molecular fingerprint capabilities of Raman and extremely high sensitivity. Theory predicted and experiments confirmed that exceptionally large Raman cross-section increases (i.e., single-molecule detection events) are associated with targeted molecules located in the nano-sized interstitial sites of aggregates of interacting silver nanoparticles (AgNPs). Recently, a novel laboratory experiment was successfully implemented for undergraduate and graduate students in physical chemistry and nanotechnology classes. The main scientific goal of this experiment was to demonstrate the SERS-based sensing capabilities of colloidal AgNPs by estimating the analytical (AEF) and surface (SEF) enhancement factors. AEF and SEF are the most important values for characterizing the SERS effect. Additionally, most SERS-based cutting-edge applications (e.g., cellular imaging, chemical and biological warfare agent detection, protein and DNA recognition, etc.) require an accurate determination of the magnitude of the signal enhancement (i.e., AEF and SEF). To achieve this, students synthesized a Creighton colloid and characterized its optical properties by UVVIS absorption spectrophotometry. Rhodamine 6G (R6G), a fluorescent dye, was added to the colloidal AgNPs as a test probe to determine the AgNP enhancement efficiency. Raman, SERS, and fluorescence measurements were then performed to estimate the AEF (7.4 x 10A4) and SEF (5.2 x 10A1) values. Although these factors do not correspond to singlemolecule detection events (maximum enhancement of 10A5 and 7 x 10A9 for colloids), the R6G concentration (1.0 x 10A-6 M) was three orders of magnitude less than in previous laboratory experiments and facilitated the rapid acquisition of SERS spectra with very good signal-to-noise ratio. This laboratory experiment successfully introduced students to the fundamentals of SERS spectroscopy and to concepts related to light scattering, surface chemistry and resonance effects. Furthermore, students acquired new instrumental and nanotechnology-related skills that will benefit them in technologically-demanding careers.</p><p>This presentation occurred at the Wright State University Campus-Wide Celebration of Research, Scholarship and Creative Activities on April 8, 2011</p>