The intertransformation of silver nanoparticles (AgNPs) and ionic silver (Ag(I)) in the environment determines their transport, uptake, and toxicity, demanding methods to simultaneously separate and quantify AgNPs and Ag(I). For the first time, hollow fiber flow field-flow fractionation (HF5) and minicolumn concentration were on-line coupled together with multiple detectors (including UV-vis spectrometry, dynamic light scattering, and inductively coupled plasma mass spectrometry) for full spectrum separation, characterization, and quantification of various Ag(I) species (i.e., free Ag(I), weak and strong Ag(I) complexes) and differently sized AgNPs. While HF5 was employed for filtration and fractionation of AgNPs (>2 nm), the minicolumn packed with Amberlite IR120 resin functioned to trap free Ag(I) or weak Ag(I) complexes coming from the radial flow of HF5 together with the strong Ag(I) complexes and tiny AgNPs (<2 nm), which were further discriminated in a second run of focusing by oxidizing >90% of tiny AgNPs to free Ag(I) and trapped in the minicolumn. The excellent performance was verified by the good agreement of the characterization results of AgNPs determined by this method with that by transmission electron microscopy, and the satisfactory recoveries (70.7-108%) for seven Ag species, including Ag(I), the adduct of Ag(I) and cysteine, and five AgNPs with nominal diameters of 1.4 nm, 10 nm, 20 nm, 40 nm, and 60 nm in surface water samples.
It is a great challenge to monitor the physical and chemical transformation of nanoparticles at environmentally relevant concentration levels, mainly because the commonly used techniques like dynamic light scattering and transmission electron microscopy are unable to characterize and quantify trace level nanoparticles in complex matrices. Herein, we demonstrate the on-line coupled system of hollow fiber flow field-flow fractionation (HF5), minicolumn concentration, and inductively coupled plasma mass spectrometry (ICPMS) detection as an efficient approach to study the aggregation and chemical transformation of silver nanoparticles (AgNPs) and ionic Ag species in the aqueous environment at ng/mL levels. Taking advantage of the in-line dialysis of HF5, the selective capture of Ag(I) species by the resin in minicolumn, and the high selectivity and sensitivity of ICPMS detection, we recorded the aggregation of 10 ng/mL AgNPs in complex matrices (e.g., NOM, Na/Ca), revealing an interesting tiny AgNPs formation process of photoreduction of trace level Ag(I) that is different from larger AgNPs generated at high concentration of Ag(I) by accurate characterization and respectively identifying and quantifying new thiol-complexed Ag(I) and residual Ag(I) in the intertransformation of Ag(I) and AgNPs in domestic wastewater by simultaneously detecting the S and Ag signals via ICPMS.
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