Detection of 3,3’-Dihexyloxacarbocyanine iodide by SERS in Silica Sol-gels

The term sol-gel can be derived from a process in which the hydrolysis of precursor molecules in solution react to form a three-dimensional coherent network. In this investigation, tetramethyl orthosilicate (TMOS) was used as the precursor compound to form silica (SiO2) sol-gels because of its ability to form porous, highly structured matrices. In recent years, silica sol-gels (and sol-gels in general) have drawn increasing interest for their applications as possible biosensors and coatings because of their high porosity and emissivity, making them an energy-saving material used commonly in spacecraft and industrial furnaces. Furthermore, the sol-gels can trap other species within their matrix such as silver colloids, creating a unique class of materials that can serve as substrates for Surface-Enhanced Raman Spectroscopy (SERS) measurements on probe molecules either trapped within the silica matrix or that diffuse into the matrix from solution. An organic fluorescent dye, 3,3’-dihexyloxacarbocyanine iodide (DiOC6(3)), was chosen as a target molecule for detecting distinguishable SERS signals in base-catalyzed and acid-catalyzed sol-gels containing silver colloid because of its facilitating ability to stain and identify mitochondrial changes during early apoptosis in animal and plant cells, and for its capability to stain and visualize thrombus formation in red blood samples. The sol-gel formation was also monitored by Ultraviolet-visible (UV/vis) Spectroscopy to determine the stability of the silver colloids and dye over time. The results suggested that SERS spectra of DiOC6(3) were experimentally obtainable and that there was a high degree of stability present for DiOC6(3) within acid-catalyzed silica sol-gels, but quickly disappeared within a 24-hour period in base-catalyzed sol-gels. This high level of stability was correlated with the stability of silver nanoparticles as shown through UV/Vis spectrophotometry. Thus, it is likely that the observed DiOC6(3) SERS signal stability of the positively charged dye led to the stabilization of the silver nanoparticles in the acid-catalyzed silica sol-gel matrix. It was ultimately concluded that acid-catalyzed sol-gels containing silver nanoparticles were viable substrates for SERS of DiOC6(3) and that these materials could lead to the development of new techniques for detecting mitochondrial DNA and characterizing other biological, physical, and electrochemical matter as SERS-based sensors.