Detection and surface reactivity of engineered nanoparticles in water

Engineered nanoparticles (NPs) pose risk potentials, if they exist in water systems at significant concentrations and if they remain reactive to cause toxicity. Three goals guided this study: (1) establishing NP detecting methods with high sensitivity to tackle low concentration and small sizes, (2) achieving assays capable of measuring NP surface reactivity and identifying surface reaction mechanisms, and (3) understanding the impact of surface adsorption of ions on surface reactivity of NPs in water.

The size detection limit of single particle inductively coupled plasma spectrometry (spICP-MS) was determined for 40 elements, demonstrating the feasibility of spICP-MS to different NP species in water. The K-means Clustering Algorithm was used to process the spICP-MS signals, and achieved precise particle-noise differentiation and quantitative particle size resolution. A dry powder assay based on NP-catalyzed methylene blue (MB) reduction was developed to rapidly and sensitively detect metallic NPs in water by measuring their catalytic reactivity.

Four different wet-chemical-based NP surface reactivity assays were demonstrated: “borohydride reducing methylene blue (BHMB)”, “ferric reducing ability of nanoparticles (FRAN)”, “electron paramagnetic resonance detection of hydroxyl radical (EPR)”, and “UV-illuminated methylene blue degradation (UVMB)”. They gave different reactivity ranking among five NP species, because they targeted for different surface reactivity types (catalytic, redox and photo reactivity) via different reaction mechanisms. Kinetic modeling frameworks on the assay outcomes revealed two surface electron transfer schemes, namely the “sacrificial reducing” and the “electrode discharging”, and separated interfering side reactions from the intended surface reaction.

The application of NPs in chemical mechanical polishing (CMP) was investigated as an industrial case to understand NP surface transformation via adsorbing ions in water. Simulation of wastewater treatment showed CMP NPs were effectively removed (>90%) by lime softening at high pH and high calcium dosage, but 20-40% of them remained in water after biomass adsorption process. III/V ions (InIII, GaIII, and AsIII/V) derived from semiconductor materials showed adsorption potentials to common CMP NPs (SiO2, CeO2 and Al2O3), and a surface complexation model was developed to determine their intrinsic complexation constants for different NP species. The adsorption of AsIII and AsV ions onto CeO2 NPs mitigated the surface reactivity of CeO2 NPs suggested by the FRAN and EPR assays. The impact of the ion adsorption on the surface reactivity of CeO2 NPs was related to the redox state of Ce and As on the surface, but varied with ion species and surface reaction mechanisms.