A proposed tangential flow ultrafiltration method was compared to the widely used ultracentrifugation method for efficiency and efficacy in concentrating, size selecting, and minimizing the aggregation state of a silver nanoparticle (AgNP) colloid while probing the AgNPs' SERS-based sensing capabilities. The ultrafiltration method proved to be more efficient and more effective and was found to tremendously boost the SERS-based sensing capabilities of these AgNPs through the increased number of homogeneous SERS hot spots available for a biotarget molecule within a minimal focal volume. Future research studies and applications addressing the physiochemical properties or biological impact of AgNPs would greatly benefit from ultrafiltration for its ability to generate monodisperse colloidal nanoparticles, to eliminate excess toxic chemicals from nanoparticle synthesis, and to obtain minimum levels of aggregation during nanoparticle concentration.

R ecently, the National Science Foundation projected that the current 10-billion dollar nanotechnology sector will employ 2 million workers, including as many as 1 million workers in the United States. 1 It is expected that over 80% of the jobs created in this sector will require trained individuals in nanoscience. However, little training at the undergraduate level has been initiated to provide highly specialized scientists to this rapidly developing field. The proposed laboratory experiment, which was implemented for both undergraduate and graduate student laboratories in physical chemistry and nanotechnology, addresses the future projected demand.In 1998, G. C. Weaver and K. Norrod proposed an undergraduate laboratory to introduce the surface-enhanced Raman scattering (SERS) effect and to extend the scope of the Raman theory normally covered in physical chemistry courses. 2 A Raman-active molecule, pyridine, was adsorbed on colloidal silver nanoparticles (AgNPs) to demonstrate the large increase in Raman signal. Although successful, the SERS experiment did not estimate the analytical enhancement factor (AEF) and surface enhancement factor (SEF), the most important values for characterizing the SERS effect. 3,4 The Raman signal enhancement of 100À300 times was simply determined by calculating the ratio of integrated areas for specific vibrational modes of pyridine adsorbed on AgNPs and in solution. However, the pyridine concentrations (1.0 Â 10 À1 M for normal Raman and 6.25 Â 10 À3 M for SERS measurements) were extremely large when compared with the trace amounts of analyte that are now detected via SERS. In the following years, theoretical and experimental studies have demonstrated that single-molecule SERS-based detection and identification can be achieved under favorable circumstances. 5,6 Because of the enormous enhancement, SERS found numerous cutting-edge applications in medical, biological, chemical, military defense, homeland security, pharmacological, and environmental settings. 7À9 Most SERSbased detection and identification applications require an accurate determination of the magnitude of the signal enhancement.In this experiment, Raman and fluorescence spectrophotometers were employed to estimate the analytical and surface enhancement factors for rhodamine 6G adsorbed on a Creighton colloid. 10 Among the many kinds of SERS-active substrates, silver colloids are known to lead to huge enhancement factors and to enable single-molecule SERS experiments. 5À7,11 The Creighton method has been widely used for its simplicity, relative low cost, accessibility, and time efficiency. These parameters were critical in designing a feasible experiment for a laboratory. Not surprisingly, in 2007, Solomon et al. implemented the Creighton procedure for the synthesis of colloidal AgNPs as a new laboratory experiment for a general chemistry class. 12

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