Nanoplastics prepared with uniformly distributed metal-tags: a novel approach to quantify size distribution and particle number concentration of polydisperse nanoplastics by single particle ICP-MS

Nanoplastics (NPs, <1 mu m), the smallest size fraction of environmental microplastics, are a contaminant of emerging concern due their high environmental concentrations, enhanced environmental mobility, and greater bioavailability compared to microplastics. Due to their majority carbon composition, diversity in size, polymer type, surface properties, and shape, NPs are difficult to detect and quantify, hindering our ability to understand NP behavior. To overcome this challenge, we have created irregularly shaped metal-tagged NPs with continuous sub-micron size distributions by cryo-milling lab-generated plastics containing 1% w/w concentrations of an organometallic additive. These metal-tagged NPs are detectable by single particle ICP-MS (spICP-MS) which is capable of measuring NP size distributions (PSD) and particle numbers (PNC) at low mu g L-1 concentrations. The ease of synthesis and flexibility of this method has enabled a suite of metal-tagged NPs to be created for a range of commercially important polymers (PS, PMMA, PVC, LDPE, PVP). By using unique metal additive-polymer combinations (e.g. PS tagged with Sn, PMMA tagged with Ta) the influence of polymer composition on NP environmental behavior can be studied using NP mixtures. Due to the sensitivity of the spICP-MS, we are able to use low metal-loadings to ensure the NPs surface properties remain unchanged compared to unmodified NPs. Advantages of this approach compared to existing NP labelling approaches are discussed along with illustrative examples in laboratory-based studies of NP production from macroscopic plastics (e.g. abrasion), photochemical NP degradation, and NP uptake into biological organisms.