Size Selection and Concentration of Silver Nanoparticles by Tangential Flow Filtration for SERS-Based Biosensors

Abstract

<p>Silver nanoparticles (AgNPs) have received tremendous attention for their unique properties as ultrasensitive surface-enhanced Raman spectroscopy (SERS)-based biosensors. One of the main challenges in SERS-based biosensing is to fabricate colloidal AgNPs that are non-hazardous, non-toxic, reproducible, stable, and at low energy costs. AgNP size and aggregation state are typically controlled during synthesis through the use of chemically aggressive surfactants, stabilizers, and capping agents, which are not suitable for biological applications. To determine a method for overcoming these disadvantages, a slightly modified Creighton colloid was subjected to two methods of isolation: ultracentrifugation and tangential flow filtration. The resulting colloids were examined by transmission electron microscopy, UV-Vis absorption spectrophotometry, SERS, fluorescence and flame atomic absorption spectroscopy (FAAS). Ultracentrifugation yielded a suspension containing both AgNPs and AgNP-aggregates, which were 2.5 fold larger in size and had a 2.0 fold greater size distribution than the monomers in the filtration method. The UV-Vis absorption spectra confirmed these differences. FAAS showed that the concentration of AgNPs obtained by filtration and ultracentrifugation were 198.7 μg mL-1 and 77.4 μg mL-1, respectively. The analytical (8.1x107) and surface enhancement factors (7.9x104) corresponding to the final AgNP filtration were similar to those determined for AgNP ultracentrifugation. These factors were calculated based on the SERS and fluorescence spectra for an analyte (rhodamine 6G) concentration of 10-6 M. Both methods had a 1,000 fold improvement in SERS-sensitivity as compared to the original colloid. However, the filtration method provides size-specificity and minimal aggregation in a highly concentrated colloidal suspension of AgNPs.</p><p>This presentation occurred at the Wright State University Campus-Wide Celebration of Research, Scholarship and Creative Activities on April 16, 2010</p>