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Title: Feasibility of colloidal silver SERS for rapid bacterial screening

Author
item LIU, YONGLIANG - VISITING SCI., UMCP
item Chao, Kuanglin - Kevin Chao
item Nou, Xiangwu
item CHEN, YUD-REN - RETIRED EMFSL EMPLOYEE

Submitted to: Sensing and Instrumentation for Food Quality and Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2008
Publication Date: 5/12/2009
Citation: Liu, Y., Chao, K., Nou, X., Chen, Y. 2009. Feasibility of colloidal silver SERS for rapid bacterial screening. Sensing and Instrumentation for Food Quality and Safety. 3(2):100-107.

Interpretive Summary: Surface-enhanced Raman scattering (SERS) spectroscopy has been developed as an extremely powerful analytical tool for a wide variety of applications over the past three decades. In order to achieve SERS signal enhancement, the analyte must be attached (adsorbed) or in close proximity to a specially prepared surface of noble metals such gold (Au) and silver (Ag), in the forms of metal colloids, roughened electrodes, nanorod arrays, and vacuum deposited metal / nanopartical island films. Among them, silver nanoparticle colloids have been extensively used, because of following advantages: (1) silver colloids can be easily and inexpensively prepared by reduction of aqueous solution of silver salts; (2) silver nanoparticles are suspended in solutions and can slow their oxidation process, meaning that a fresh surface is available for each analysis and lifetime could be long; (3) silver colloids provide homogeneous binding sites between nanoparticles and analyte; and (4) sampling method of silver colloids directly mixed with incubated cultures can reduce the possibility of bacterial cross-contamination. However, like other substrates, there are still major problems associated with the reproducibility and stability of silver colloids. In this study, we applied both relative standard deviation (RSD) of SERS spectra and ratios of SERS peaks from K3PO4 to evaluate the reproducibility, stability, and binding effectiveness of citrate-reduced silver colloids over batch and storage process. Also, we examined the effect of aggregation agent on the stability and binding effectiveness of colloids. This result provides SERS researchers a new sight in developing low-cost silver colloidal nanoparticles for rapid, specific, and routine detection of interest.

Technical Abstract: Citrate-reduced silver colloids have been used extensively for surface-enhanced Raman scattering (SERS) study and are commonly characterized by UV-visible spectroscopy. In this work, relative standard deviation (RSD) of SERS spectra from silver colloidal suspensions and ratios of SERS peaks from small molecule (K3PO4) were used to evaluate the reproducibility, stability, and binding effectiveness of citrate-reduced silver colloids over batch and storage process. The results suggested the reproducibility of silver colloids over batch process and also stability and consistent binding effectiveness over 60-day storage period. Notably, although silver colloidal nanoparticles were stable for at least 90 days, their binding effectiveness began to decrease slightly after 60-day storage, with a binding reduction of about 12% at 90th day. It indicated that the lifetime of silver colloidal nanoparticles is much longer than that of silver nanoparticle films reported by others. Moreover, addition of appropriate amount of Na2SO4, as a aggregation agent, to colloids not only further improved the binding effectiveness but also made duplicate measurements repeatable with less deviation.