Silver Nanoparticle Arrays on Track Etch Membrane Support as Flow-Through Optical Sensors for Water Quality Control
Principal Investigator:	Dr. Volodymyr V. Tarabara, Department of Civil & Environmental Engineering
Research Assistant:	Julian S. Taurozzi
Funding Agency:	National Science Foundation

Timely detection of water supply contamination depends on the availability of reliable water quality monitoring technologies. At the core of these technologies are deployable environmental sensors subject to such criteria as fingerprinting capability, low fouling characteristics, reproducibility, and low detection limits. The detection process can be viewed as consisting of two consecutive stages: (1) selective preconcentration/separation of pollutants and (2) measurement of pollutants’ concentration. Preconcentration allows not only to separate pollutants from the system matrix, but also to enhance detection sensitivity, which is especially important for the detection of water pollutants in extremely low concentrations. Measurement methods that are sensitive and selective are required to quantify the concentration of target pollutants in the sample.

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 main difficulty in fabricating SERS-active substrates that reproducibly yield high enhancements is the single most significant factor that hampers further applications of the SERS method for sensing. Development of highly enhancing, but also stable, reproducible, and robust SERS-active substrates is key for SERS to mature into an “off-the-shelf” detection method.

Our project focuses on the development of flow-through SERS-active substrates wherein arrays of silver nanoparticles are assembled on the surface of track etch polycarbonate membranes. To induce chemisorption of silver nanoparticles on the membrane supports, 3-aminopropyltrimethoxysilane was introduced on the surface of polycarbonate filters. Nanoparticle-modified membranes were then characterized with respect to their hydraulic and optical properties. Deposition patterns and distribution of silver nanoparticles on the membrane surface were characterized qualitatively using scanning electron microscopy (SEM). Filtration performance of the modified membranes was studied in a series of clean water flow tests using a normal flow filtration cell. Optical and SERS properties of the membranes were analyzed using UV-vis absorption as well as Raman and SERS spectroscopies. SERS enhancing properties 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 the sensitivity of the modified membranes in a broader context that included membrane-enabled preconcentration as the factor that augments the inherently high sensitivity of nanoparticle-based SERS substrates.

The assembled arrays were established to be SERS-active with reproducible Raman enhancements. The hydraulic control of analyte transport to the permeable SERS-active surface was demonstrated to dramatically improve the detection limit of the novel sensors. The findings indicate the potential benefit of combining high specification SERS-active systems and flow-through designs for the development of analytical sensors for the trace detection of pollutants in water.

• Taurozzi, J.S., Tarabara, V.V. (2007). Silver nanoparticle arrays on track etch membrane support as flow-through optical sensors for water quality control. Environ. Eng. Sci. 24 (1), 122-137.