Design and Fabrication of Smart SERS Substrates for Forensic Science Applications

Abstract

In the field of forensics and toxicology, it is crucial for analytical techniques to be practical, highly sensitive, and extremely accurate (specific and selective), especially when incorporating acquired data as evidence in a court case. To limit the breadth of this dissertation, the main forensic focuses are to assay (detection and quantification) drugs in patient biofluid specimens and to detect trace explosives. While currently there are a number of analytical tools such as LC/MS, GC/MS, ELISA immunoassays, and electrochemical and aptamer techniques utilized for these two applications, each one introduces its own unique drawback that hinders their accuracy, and therefore applicability, to be used in a legal environment. To overcome these disadvantages, surface enhanced Raman spectroscopy (SERS) has become increasingly popular in the forensic and toxicology field as it provides high sensitivity and specificity data while remaining flexible and efficient in critical situations such as an emergency department and/or an explosion site. In this dissertation, two different, novel SERS substrates are developed, each one designed to tackle a specific forensic application. While the fabrication method and materials of the substrates are significantly different from one another, both display ideal SERS properties due to their unique localized surface plasmon resonance (LSPR) properties at the nanoscale. LSPR is a phenomenon in which the free carriers (electrons or holes) on the surface of nanoparticles collectively oscillate upon light irradiation. In this current dissertation, we selectively focus on two different nanoparticle compositions where LSPR properties originate from the collective oscillation of free electrons. These oscillations of free carriers allow for an electromagnetic (EM) field enhancement of the incident laser which leads to an increase in the SERS enhancement. Particularly, the area in which the SERS signal is the most enhanced is the nanometer-sized gap between nanoparticles called “hot spots.” While primarily noble metal (e.g., Au and Ag) nanoparticles are heavily used for SERS substrate fabrication, this dissertation expands beyond that and focuses on both gold nanorods and oxygen deficient tungsten oxide (metal oxide semiconductor)-based SERS substrates. This dissertation is organized in three chapters, (1) Introduction, (2) Fabrication of a polymer microneedle-based, multimodal SERS and mass spectrometry substrate for the ultrasensitive detection of illicit drugs in human blood plasma, and (3) LSPR active WO3-x-based SERS substrate for the detection of explosives. In chapter 2, a multimodal substrate was strategically designed to serve as a SERS and electrospray ionization- mass spectrometry substrate by using a novel microneedle platform. The microneedles underwent a surface modification and gold nanorods were subsequently adsorbed onto the surface. Illicit drug analytes could then be drop-casted on the tip of the microneedles and left to dry for further SERS and mass spectrometry analysis. This novel platform detected two types of synthetic opioids, alprazolam and fentanyl, down to at least a picomolar limit of detection and successfully distinguished between the two when analyzing 10 patient blood plasma samples. Furthermore, the multimodal approach was confirmed through the detection of both drugs in patient plasma samples down to the ppb limit using mass spectrometry. Chapter 3 introduces an entirely new SERS substrate, which is fabricated using oxygen deficient tungsten oxide (WO3-x) nanoparticles for the detection of the target explosives, e.g., tetryl, TNT, and DNT. The oxygen deficiency in WO3-x nanoparticle lattice introduces free electrons in the conduction band that introduce LSPR properties into the nanoparticles giving rise to the EM field mechanism for SERS enhancement upon incident laser irradiation. Much like with noble metals, the oscillation of these free carriers at nanoparticle hot spots aid in the development of a functional and cheaper SERS substrate. The SERS enhancement factor (EF) of three different morphologies of WO3-x nanoparticles, i.e., nanorods, nanowires, and nanoplatelets are characterized and deemed comparable to noble metal nanoparticles. A Janowsky complex was formed using the explosive, tetryl, as the target analyte. Using our oxygen deficient WO3-x substrate, tetryl was successfully detected down to the nanomolar level. To our knowledge, this is the first time tetryl has been detected utilizing a non-noble metal-based SERS substrate. Taken together, this dissertation presents the unique aspect of nanotechnology for the development of (1) a multimodal MN-based ultrasensitive and (2) an inexpensive noble-metal comparable WO3-x-based SERS substrates, both of which that can be applied to forensic science research, specifically in forensic toxicology and explosive detection to better, and even save, the lives of individuals and improve their quality of life.