Surface-enhanced Raman spectroscopy for forensic analysis of human saliva
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Through oral contact, saliva can be transferred to a variety of objects commonly encountered in forensic casework, such as cigarette butts, condoms, drinkware, masks, clothing, and vaginal or penile swabs. As saliva does not require external stimulation for secretion, it is readily available for exposure to the external environment. For this reason, the detection and analysis of saliva are relevant to many types of crime, including but not limited to sexual assaults, kidnappings, homicides, and burglaries. Although the presence of saliva does not serve as a definitive indication of crime, it can link individuals to a crime scene or serve as evidence of physical contact. When sufficient quality and quantity of saliva is present at a crime scene or on an item of evidence, it typically serves as a source of DNA.1 Therefore, detection of saliva is also pertinent to the identification of potential victims and perpetrators. Currently, forensic identification of biological stains is limited to individual body fluids, such that analysis of a mixture containing multiple body fluids will require a separate test for each biological component. In addition, all routine forensic assays for saliva are of a preliminary nature. Due to the lack of confirmation techniques for saliva, surface-enhanced Raman spectroscopy (SERS) emerges as a potential technique for rapid, sensitive, and confirmatory identification of saliva stains. This study is designed to examine the efficacy of a SERS based methodology for the detection, characterization, and identification of human saliva. Using this technology, the major molecular component detected in saliva was salivary thiocyanate. A three-peak thiocyanate SERS signature was observed at 440 cm-1, 740 cm-1, and 2107 cm-1 and is recommended for use as a biomarker in the detection and identification of liquid and dried saliva. Assessment of validation parameters was also performed to determine the robustness, accuracy, and sensitivity of this methodology. Examination of donor variability among and within individuals demonstrates variability in salivary thiocyanate concentrations. As a result, the sensitivity of this technology is limited by the inherent thiocyanate concentrations of individuals. In the samples utilized for this study, SERS could readily detect salivary thiocyanate at concentrations as low as 1/100. Classification of spectral data from 12 donors indicates some donor variability arising from differences in protein content. However, deviations in matrix proteins do not hinder the identification of saliva because this methodology uses a salivary thiocyanate SERS signature for the basis of saliva detection. A preliminary sample aging study demonstrates that saliva can be rapidly identified in 20-month old samples aged at room temperature. This suggests that the salivary thiocyanate SERS signature is stable without the need for controlled environments and, therefore, highly suitable for use in forensic detection of human body fluids. At many crime scenes, biological fluids are present in a mixed sample consisting of human fluids such as saliva, semen, blood, and vaginal fluid. Salivary mixtures with blood and vaginal fluid were respectively analyzed to assess the efficiency of this SERS methodology in detecting and identifying saliva from the mixture. As thiocyanate is also an inherent component of blood, the ability to identify saliva from a salivary mixture with blood is limited to mixtures where saliva is a predominant component. For samples used in this study, saliva identification is limited to mixtures containing 75% saliva and 25% blood. As vaginal fluid does not contain salivary thiocyanate, this SERS methodology can readily identify salivary thiocyanate from mixtures containing 1% saliva and 99% vaginal fluid extract. This study examined two potential substrates and their corresponding efficiencies in body fluid detection for forensic casework. The thiocyanate signature is readily observed on both gold (Au) and silver (Ag) substrates. With respect to sensitivity, Ag substrates are more sensitive and capable of detecting thiocyanate in lower concentrations of saliva. Au substrates exhibit greater selectivity for other salivary components at low thiocyanate concentrations, which can be useful in characterizing the protein content. In view of all experimental results, this study demonstrates that SERS is a rapid and sensitive technique capable of detecting and identifying saliva among complex body mixtures for forensic science applications.