Self-Cleaning Catalytic Filters Against Pesticides and Chemical Agents

Introduction: Heightened awareness of the hazards of chemical pollutants and pesticides, coupled with a growing threat of chemical exposure due to accidental spills or terrorist action, poses a challenge to develop countermeasures. Current protection gear (e.g., gloves, masks, and clothing) is based on the removal of environmental toxins using efficient adsorption materials, and/or the use of impermeable barriers to separate toxins from protected targets. Both approaches suffer from problems such as weight, cost, bulkiness, regeneration capabilities, and disposal safety concerns. Therefore, there is an urgent need to develop noncorrosive, environmentally benign, cost-effective, lightweight, robust, self-decontaminating, hazardous material-free systems for handling and neutralizing pesticides and toxins present in air or water.

Focus of Our Research: We are focusing on developing multifunctional thin-film (~8 nm) coatings containing existing catalysts1 capable of neutralizing toxins on contact. These catalysts include biological species such as enzymes and chemical compounds like metal-amine complexes, both of which can deactivate toxins like pesticides or nerve agents. Both catalyst types have deficiencies and merits. Metal complexes are long lasting, but exhibit low activity. For example, the pesticide methyl parathion (MPT) is slowly hydrolyzed (k ~2.6 X 10-2s-1) by the stable bis-ethylenediamine Cu(II) complex. In contrast, enzymes such as organophosphorus hydrolase (OPH)1 rapidly hydrolyze MPT (k ~ 50 s-1), but lose activity quickly as they denature in the environment. Despite improvements in enzyme stability via genetic alterations, an effective system having both high catalytic activity and stability for protection and threat reduction applications does not yet exist.

Enzyme Stability: Our approach for developing such systems addresses the key question of enzyme stability via enzyme encapsulation in polyelectrolyte (PE) multilayer films. Such films are readily fabricated by sequential adsorption of oppositely charged PEs, such as branched polyethylenimine (BPEI) and polyacrylic acid (PAA), onto substrates like beads or cloth from H₂O via dipcoating or spraycoating methods. This layer-by-layer approach is well established2 and, by adjusting PE chemistries, provides an environment that stabilizes enzymes against denaturation while preserving activity. Figure 9(a) shows a structural cross-section of a generalized film of this type. Occasional replacement of a PE layer by a layer of charged nanoparticle adsorbents, metal complexes, or enzymes during film fabrication creates a film with a tunable sandwich structure. Toxins entering such a film initially encounter enzyme layers that rapidly hydrolyze most of the toxin. Underlying layers of metal complexes provide a backup to the enzymes. The innermost adsorbent layers prevent toxin by-products from reaching the protected target. The versatility of the method is shown by the use of beads (e.g., glass or polycyclodextrin (PCD)) in Fig. 9(b) or glass fabrics (e.g., electrospun fiber) in Fig. 9(c) as substrates for PE film deposition.

FIGURE 9
Self-cleaning filters against chemical agents: (a) Components and their placement; (b) Filter cartridge schematic; (c) A bioactive surface (electrospun glass fabric).

Figure 10 illustrates an application of our PE-coated beads for filter technologies. PCD microbeads, which adsorb and concentrate MPT for more efficient enzyme hydrolysis, were initially dipcoated with three bilayers of BPEI and PAA, then five bilayers of BPEI and OPH enzyme, and finally a capping BPEI layer to form a film of structure PCD-[BPEI/PAA]₃[BPEI/OPH]₅BPEI. These modified beads were packed into the column shown in Fig. 10 and tested as a filter for removal of MPT from water. Under continuous solution flow with a filter residence time of ~114 s, the filter removed >99% of MPT from a 100-µM MPT input stream for 60 days.

FIGURE 10
Set up for testing catalytic efficiency of beads in flow-through filter system.

Figure 11 shows the application of the thin PE film layers onto a fiberglass (FG) cloth using a spraycoating method. In this case, a film of structure FG-BPEI/OPH/BPEI containing a single OPH layer was formed. This cloth (~0.1 g piece) was used to hydrolyze 12 fresh 100-µM MPT solutions during 19 days. The OPH sustained 100% activity during the first two cycles, dropping to ~72% of its initial activity by the third cycle. Activity dropped to ~50% during the second week and ~33% during the third week, which is still excellent for most practical applications. In total, the 12 catalytic cycles led to hydrolysis of ~390-µM MPT. In contrast, an aqueous solution of OPH denatured and lost >95% activity within ~96 h. Initial results using cotton cloths coated with our PE films suggest that OPH stabilities are comparable or greater than those in FG cloths.

FIGURE 11
Fabrication of bioactive multilayers by spraycoating on cloth for individual protection.

Demonstrated Ability: We have now demonstrated the ability to stabilize enzymes in thin, multilayer PE assemblies while sustaining activity under harsh working environments. This accomplishment, coupled with the ability to deposit films on nearly any substrate, opens the possibility for developing user-friendly, cost-effective, efficient means for chemical agent protection. We are continuing our research with the goals of incorporating materials capable of both sensing (e.g., indicators of film activity or impending threats) and decontaminating a wider range of threats (e.g., biological), as well as integrating our films into currently used military technologies (e.g., clothes or water filters).

[Sponsored by ONR]