Liquid-Phase Surface Enhanced Raman Spectroscopic Detection of Nerve Agent Motifs using Gold Nanostars

Organophosphates and their derivatives are deadly compounds owing to their ability to disrupt key regulators of biological activity and neuronal transmission. As such, typical uses for synthetic and natural organophosphates include nerve agents and toxic pesticides. Organophosphate pesticides alone are estimated to make up around 3-10% of the international pesticide poisoning deaths, or approximately 5-15,000 deaths annually. Mechanistically, they irreversibly bind to acetylcholinesterase, a crucial enzyme dealing with motor control that has minimal structural differences across all walks of life explaining the universal toxicity of organophosphates. In this study, we present the fabrication of a nanoparticle-based detection method to detect two commonly used organophosphates: dimethyl methylphosphonate (DMMP) which is used as the starting material for many chemical warfare agent syntheses, and dimethyl chlorophosphate (DMCP) which is a potent acetylcholinesterase inhibitor and readily harmful to the body through dermal absorption or ingestion. We further demonstrate accurate and precise micromolar detection of these organophosphates with a simple portable Raman spectrometer, and additionally demonstrate the ability to easily detect and distinguish between both analytes in a heterogenous solution using this methodology. The nanoparticle-based detection relies on a method of Surface-enhanced Raman spectroscopy (SERS) in liquid phase. Throughout the duration of the experiment, liquid solutions and a portable Raman spectrometer were utilized to demonstrate efficacy of the detection method even with instruments and conditions that would be typically seen in the environment of organophosphate use, such as in a less technology developed warfare or rural setting. Through this detection method, we found a reliable fingerprint peak for DMMP (710.63cm^-1) with a limit of detection (LOD) of 9.11mM and a limit of quantitation (LOQ) of 27.6mM, with 1.5 ± 0.11 SERS. Similarly, for DMCP (755.97cm^-1), a LOD of 17.79mM, a LOQ of 53.90mM, and 1.3 ± 0.01 SERS enhancement were observed. Our theoretical studies predict possible SERS enhancement of >10⁴, with significantly stronger signal compared with spherical nanoparticles. Time and pH studies show that our detection method is both fast and facile, without any required field modifications for baseline detection.