Coffee ring effect assisted improved S. aureus screening on a physically restrained gold nanoflower enriched SERS substrate

Proactive water pathogen identification process holds significance in view of increasing bioterrorism prospects, quantification of virulence properties and water supply regulation for human welfare. Strikingly, pathogen concentration is rather low in water bodies to be identified using conventional methods and usually involves three typical steps: 1) concentration 2) purification and 3) characterization and assay. Surface enhanced Raman scattering (SERS) analysis on other hand could easily surpass these steps to give a rather robust identification owing to its exceptionally high specificity and sensing abilities. In this article we use SERS as a diagnostic tool to identify low concentrations of noxious pathogens from water, using a two level confinement that results in a superlative SERS analytical assay, capable of analyte identification down to single molecule level. Unlike the regular dip coat process, we restrict the nanostructure deposition only to a very small area of the larger substrate, by restricting its application volume. In this way, we limit our data processing time, increase our probability of target interaction and also reduce nanostructure/analyte sample quantities for analysis. To enhance the signal intensities further, we use natural three phase pinning or the coffee ring effect to pre-concentrate the analytes on our spatially confined nanostructure pocket, allowing maximal deposition at the edges and subsequently higher signal intensities. The methodology was employed to measure the presence of S. aureus and R6 G in spiked drinking water. LOD achieved for R6 G and S. aureus is 10^-12 M and 10³ CFU/ml respectively. The increasing signal intensities share a linear relationship with analyte concentration. The acquired signals were reproducible and non-degenerate, overcoming the drawback of irregular data patterns associated with conventional metal nanostructure based SERS systems. Estimated RSD value for physical confinement and coffee ring effect measured was 11.6% and 8.37% respectively. This simple, low cost, robust and reproducible method can be useful for increasing the sensitivity of SERS based detection devices with capability to be developed into a micro total analysis system (μTAS).