Evaluating self-segregation of seminal fluid components via surface-enhanced Raman spectroscopy

In recent years, Raman spectroscopy has proven to be a promising confirmatory test for seminal fluid found in sexual assault kits. Perhaps the main disadvantage to using Raman spectroscopy in this context is low signal, which results from both the low probability of Raman-scattered photons and the common scenario of trace amounts of analytes in forensic casework. Signal quality can be dramatically increased by way of surface-enhanced Raman spectroscopy (SERS), which can be done by growing silver nanoparticles (AgNPs) on nylon swab fibers. In order to establish this method as a confirmatory test, the nature of the adsorption of seminal fluid on silver needs to be understood. The following details a study involving fabrication of severalAgNP-coated substrates, whose extents of percolation and surface wettabilities were determined. Findings from those experiments were used to explain trends in SERS signals and stain shapes from semen simulant and true semen samples deposited on these substrates. The percolation threshold was achieved at 15 nm thicknesses when grown thermally, and 61-minute reaction time when grown chemically. The thermally-grown layers were consistently more hydrophobic than the chemically-grown layers, which had a significant influence on stain spreading and shape. SERS data showed that all droplets containing choline exhibited a drastic and statistically significant increase in droplet spreading on AgNP-coated surfaces, which may have been the result of a chemical reaction between choline and silver. SERS semen spectra were also compared to a previous study conducted in this lab wherein SERS spectra of semen were collected on AgNP-coated swabs. By comparing residuals between semen spectra from the previous study and the current study, the SERS spectra collected on the swabs were assigned tentative locations along semen stains prepared on substrates fabricated for this study