Doctoral Dissertations

Date of Award

5-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

MICHAEL J. SEPANIAK

Committee Members

CHARLES FEIGERLE, ZILING XUE, THOMAS L. FERRELL

Abstract

Raman spectroscopy is normally a non-destructive, highly selective technique that has become an ubiquitous tool for analytical chemists. One of the primary limitations of Raman spectroscopy, however, is the relatively low cross-section of the technique. With signal enhancements relative to normal Raman scattering as high as 1011, the ultra-trace detection of adsorbates down to the single molecule level has been achieved with SERS. Despite the dramatic improvement in the sensitivity and the high selectivity afforded by the SERS method, the acceptance of SERS as a general analytical tool has been hindered by a lack of stability and reproducibility in the substrates. This lack of stability has been particularly troublesome because unstable substrates exhibit reduced shelf lives as well as a reduced ability to monitor processes that occur under non-ideal conditions such as high temperature or harsh chemical environments. In this thesis, two different works are reported that address the two major hurdles facing the SERS field in the development of a stable and reproducible SERS substrate. First, the development of a SERS-active substrate that exhibits improved temporal and thermal stability and is capable of in-situ high temperature measurement of analytes adsorbed on the surface is presented. The substrates are prepared by depositing an ultra-thin layer of alumina by Atomic Layer Deposition (ALD) onto silver island films grown by thermal evaporation. We demonstrate the application of alumina-coated substrates to the measurement of the dehydration of trace amounts of calcium nitrate tetrahydrate as a function of temperature. As a development of the above mentioned work, the combination of a silver/gold layered architecture obtained by thermal evaporation with an ultra-thin alumina overlayer to generate a re-usable SERS substrate that is simple, relatively inexpensive and stable is reported. The relative thicknesses of the silver and gold and the alumina overlayer was optimized to deliver the maximum SERS enhancement and optimal stability when the substrate was subjected to high temperature. Utilizing the method of thermal desorption of the analyte, the substrate surface is regenerated and able to be reused multiple times with little reduction in SERS activity. Second, in the development of a reproducible SERS substrate, the application of monodisperse silver nanocube colloidal substrate in microfluidic SERS is demonstrated. In static SERS experiments, one often has to search for “hot spots”, which are positions of a drastically increased SERS signal compared to the rest of the probe volume, in an inhomogeneous solution. To overcome this problem and prevent the decomposition and or fragmentation of SERS substrate and analyte, respectively, the implementation of flow cell is a promising way. At the beginning analyte, colloidal solution and aggregation agent were brought into a mixing chamber, where they were thoroughly mixed before being directed to a sample cell for detection. With this method, a relatively high amount of sample volume is necessary. In addition to the advantages listed above, efforts have been made to reduce the required amount of the sample solution by the design of low-cost poly (dimethylsiloxane) chips via soft lithography technique. The sample solution is passively pumped through the microfluidic channel, where an optical detection window is implemented for acquisition of a SERS spectrum.

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