Sers-Active Silver Nanoparticle Arrays on Track Etch Membrane Support as Flow-through Water Quality Sensors
Julian S. Taurozzi, Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI 48824 and Volodymyr V. Tarabara, Civil and Environmental Engineering, Michigan State University, A132 Research Complex Engineering, East Lansing, MI 48824.
Timely detection of water supply contamination, assessment of related risks and development of remediation strategies depend on the availability of reliable water quality monitoring technologies. At the core of these technologies are environmental sensors, subject to such criteria as fingerprinting capability, low fouling characteristics, reproducibility and low detection limits. The lack of sensing devices that meet these demands is the main limitation of pollution detection methods in use today. The remarkable sensitivity and molecular specificity afforded by surface enhanced Raman scattering (SERS) makes this spectroscopic technique especially attractive for sensing applications, where both low detection limits and fingerprinting capability are needed. The SERS effect can be observed when a light-scattering molecule is adsorbed on a SERS-active substrate. A wide variety of SERS substrates have been proposed, typically roughened surfaces or nanoparticle suspensions of certain coinage metals, gold and silver being primary examples. What all SERS-active systems have in common, however, is that the morphology of substrates has a profound impact on their optical properties. The difficulty in fabricating SERS-active substrates that reproducibly yield high enhancements is the single most significant factor that has hampered further applications of the SERS method for sensing. One approach to obtain reproducible SERS substrates is the immobilization of nanoparticles on stable supports. Permeable supports such as membranes are especially attractive as they may allow for the added advantage of preconcentration. In this work, we report on the development of flow-through SERS active substrates, wherein arrays of silver nanoparticles were assembled on the surface of track etch polycarbonate membranes. These high-specification microfilters are advantageous as the nanoparticle support platform due to such characteristics as narrow pore distribution, well defined cylindrical pore geometry and flat surface - factors that should contribute to improve the reproducibility of the nanoparticle assembly. To induce chemisorption of silver nanoparticles on the membrane supports, 3-aminopropyltrimethoxysilane was introduced on the membrane surface as the chemical linker that would allow for membrane-nanoparticle attachment. Nanoparticle-modified membranes were characterized with respect to their hydraulic and optical properties. Nanoparticle deposition patterns and distribution on the membrane surface were qualitatively characterized using scanning electron microscopy. Filtration performance of the modified membranes was studied in a series of clean water flux tests. Optical properties of the modified membranes were analyzed using UV-vis absorption, and Raman and SERS spectroscopies. SERS enhancing properties of the treated membranes were tested and compared with those of source nanoparticle suspensions, using methylene blue (MB) as an analyte relevant for pathogen detection and inactivation techniques. Finally, we considered sensitivity in a broader context that included membrane-enabled preconcentration to augment the inherently high sensitivity of the SERS substrate. Modified membranes showed SERS activity for MB and preconcentration proved to significantly increase detection sensitivity of the targeted analyte. No major impact on the membranes' inherent resistance was observed upon nanoparticle attachment. The improved reproducibility was attributed to the well defined geometry of track etch membranes. The hydrodynamic control of analyte transport to the permeable SERS-active surface was demonstrated to allow for a dramatic improvement of the detection limit of the sensor. The reported findings indicate the potential benefit of combining high specification SERS-active systems and the flow-through design for the development of analytical sensors for the trace detection of pollutants in water.