Surface-enhanced Raman spectroscopy as a rapid detection method for pathogenic bacteria using highly sensitive silver nanorod array substrates
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A portable SERS probe based on silver nanorod array has been developed. The substrate consists of a base layer of 500 nm Ag film and a layer of Ag nanorod array with length of ~1 m deposited by oblique angle deposition method at a vapor incident angle of 86°. The incoming laser beam was designed to focus onto the SERS substrate at 45° incident angle to maximize -17SERS signal. With a fiber Raman system, a detection sensitivity of 10 moles for trans-1,2-bis(4-pyridyl)ethane molecules has been demonstrated. SERS spectra of whole cell bacteria from several species and strains, including Gram positive and Gram negative have been obtained from this novel SERS substrate. Spectral difference between Gram types, species and strains were observed. Viable and noviable cells were also examined and significantly reduced SERS responses at major Raman bands were observed for noviable cells. The observation of SERS spectra of bacteria on single cell level excited at low incident powers (12 W) and short collection time (10 s) was also demonstrated. SERS spectra of two different mixed cultures consisting of E.coli O157:H7 and S. aureus; E.coli O157:H7 and S. typhimurium, as well as their respective pure culture were measured. PCA was applied to group these microorganisms based on their spectral fingerprints. The resultant PCs score plots showed correct grouping of these microorganisms, including discrimination between two pure cultures and the mixed culture in each case. These results show that the highly sensitive silver nanorod arrays substrate is a potential analytical sensor for rapid identification of microorganisms with minimum sample preparation. In addition, SERS spectra can be utilized to identify a pure culture in a mixed cell populations via its spectral fingerprint as demonstrated by principle component analysis. This study shows that the integrated OAD silver nanorod arrays substrates and fiber Raman system has great potential as a portable and remote sensor for on-site biological or chemical detection.