Key mechanistic details of paraoxon decomposition by polyoxometalates: Critical role of para-nitro substitution

Capture and Decomposition of the Nerve Agent Simulant, DMCP, Using the Zeolitic Imidazolate Framework (ZIF-8)

Understanding mechanisms of decontamination of chemical warfare agents (CWA) is an area of intense research aimed at developing new filtration materials to protect soldiers and civilians in case of state-sponsored or terrorist attack. In this study, we employed complementary structural, chemical, and dynamic probes and in situ data collection, to elucidate the complex chemistry, capture, and decomposition of the CWA simulant, dimethyl chlorophosphonate (DMCP). Our work reveals key details of the reactive adsorption of DMCP and demonstrates the versatility of zeolitic imidazolate framework (ZIF-8) as a plausible material for CWA capture and decomposition. The in situ synchrotron-based powder X-ray diffraction (PXRD) and pair distribution function (PDF) studies, combined with Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), zinc K-edge X-ray absorption near edge structure (XANES), and Raman spectroscopies, showed that the unique structure, chemical state, and topology of ZIF-8 enable accessibility, adsorption, and hydrolysis of DMCP into the pores and revealed the importance of linker chemistry and Zn²⁺ sites for nerve agent decomposition. DMCP decontamination and decomposition product(s) formation were observed by thermogravimetric analysis, FT-IR spectroscopy, and phosphorus (P) K-edge XANES studies. Differential PDF analysis indicated that the average structure of ZIF-8 (at the 30 Å scale) remains unchanged after DMCP dosing and provided information on the dynamics of interactions of DMCP with the ZIF-8 framework. Using in situ PXRD and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), we showed that nearly 90% regeneration of the ZIF-8 structure and complete liberation of DMCP and decomposition products occur upon heating.

In Situ Synthesis of SERS-Active Au@POM Nanostructures in a Microfluidic Device for Real-Time Detection of Water Pollutants

We present a simple, versatile, and low-cost approach for the preparation of surface-enhanced Raman spectroscopy (SERS)-active regions within a microfluidic channel 50 cm in length. The approach involves the UV-light-driven formation of polyoxometalate-decorated gold nanostructures, Au@POM (POM: H₃PW₁₂O₄₀ (PW) and H₃PMo₁₂O₄₀ (PMo)), that self-assemble in situ on the surface of the polydimethylsiloxane (PDMS) microchannels without any extra functionalization procedure. The fabricated LoCs were characterized by scanning electron microscopy (SEM), UV-vis, Raman, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. The SERS activity of the resulting Au@POM-coated lab-on-a-chip (LoC) devices was evaluated in both static and flow conditions using rhodamine R6G. The SERS response of Au@PW-based LoCs was found to be superior to Au@PMo counterparts and outstanding when compared to reported data on metal@POM nanocomposites. We demonstrate the potentialities of both Au@POM-coated LoCs as analytical platforms for real-time detection of the organophosphorous pesticide paraoxon-methyl at 10^-6 M concentration level.

Multimodal Characterization of Materials and Decontamination Processes for Chemical Warfare Protection

This Review summarizes the recent progress made in the field of chemical threat reduction by utilizing new in situ analytical techniques and combinations thereof to study multifunctional materials designed for capture and decomposition of nerve gases and their simulants. The emphasis is on the use of in situ experiments that simulate realistic operating conditions (solid-gas interface, ambient pressures and temperatures, time-resolved measurements) and advanced synchrotron methods, such as in situ X-ray absorption and scattering methods, a combination thereof with other complementary measurements (e.g., XPS, Raman, DRIFTS, NMR), and theoretical modeling. The examples presented in this Review range from studies of the adsorption and decomposition of nerve agents and their simulants on Zr-based metal organic frameworks to Nb and Zr-based polyoxometalates and metal (hydro)oxide materials. The approaches employed in these studies ultimately demonstrate how advanced synchrotron-based in situ X-ray absorption spectroscopy and diffraction can be exploited to develop an atomic- level understanding of interfacial binding and reaction of chemical warfare agents, which impacts the development of novel filtration media and other protective materials.

Metal-Organic Framework- and Polyoxometalate-Based Sorbents for the Uptake and Destruction of Chemical Warfare Agents

The threat of chemical warfare agents (CWAs), assured by their ease of synthesis and effectiveness as a terrorizing weapon, will persist long after the once-tremendous stockpiles in the U.S. and elsewhere are finally destroyed. As such, soldier and civilian protection, battlefield decontamination, and environmental remediation from CWAs remain top national security priorities. New chemical approaches for the fast and complete destruction of CWAs have been an active field of research for many decades, and new technologies have generated immense interest. In particular, our research team and others have shown metal-organic frameworks (MOFs) and polyoxometalates (POMs) to be active for sequestering CWAs and even catalyzing the rapid hydrolysis of agents. In this Forum Article, we highlight recent advancements made in the understanding and evaluation of POMs and Zr-based MOFs as CWA decontamination materials. Specifically, our aim is to bridge the gap between controlled, solution-phase laboratory studies and real-world or battlefield-like conditions by examining agent-material interactions at the gas-solid interface utilizing a multimodal experimental and computational approach. Herein, we report our progress in addressing the following research goals: (1) elucidating molecular-level mechanisms of the adsorption, diffusion, and reaction of CWA and CWA simulants within a series of Zr-based MOFs, such as UiO-66, MOF-808, and NU-1000, and POMs, including Cs₈Nb₆O₁₉ and (Et₂NH₂)₈[(α-PW₁₁O₃₉Zr(μ-OH)(H₂O))₂]·7H₂O, (2) probing the effects that common ambient gases, such as CO₂, SO₂, and NO₂, have on the efficacy of the MOF and POM materials for CWA destruction, and (3) using CWA simulant results to develop hypotheses for live agent chemistry. Key hypotheses are then tested with targeted live agent studies. Overall, our collaborative effort has provided insight into the fundamental aspects of agent-material interactions and revealed strategies for new catalyst development.

Preparation of Peelable Coating Films with a Metal Organic Framework (UiO-66) and Self-Crosslinkable Polyurethane for the Decomposition of Methyl Paraoxon

For the fabrication of a peelable coating material that decomposes methyl paraoxone (MPO), a nerve agent simulant, self-crosslinkable waterborne polyurethanes (PUs) containing silane groups at the ends and a metal organic framework (UiO-66) were synthesized. UiO-66 dispersed PU solutions for spray coating were prepared by controlling the amount of silane in PU and the content of UiO-66. PUs with a large amount of silane (more than 7.2 wt.%) were easily gelated by adding UiO-66 because the solution was changed from neutral (pH = 7.3) to strongly acidic (pH = 2.5). Therefore, the silane content in PUs should be carefully controlled for the fabrication of composite films. When UiO-66 was added to the PU with a silane content of 2.7 wt.%, the reinforcing effect by UiO-66 was observed up to 15.3 wt.%, but a further increase in UiO-66 content decreased both the tensile strength and the elongation. The peel strength of the PU composite films on polyethylene (PET) and glass substrates decreased with increasing UiO-66 content, but their MPO conversion increased with increasing UiO-66 content. The PU composite film with 49.5 wt.% of added UiO-66 showed the MPO conversion of 63.2% and was easily peeled off from PET and glass substrates.