Two-stage hierarchical clustering for analysis and classification of mineral sunscreen and naturally occurring nanoparticles in river water using single-particle ICP-TOFMS†

Abstract

Titanium dioxide (TiO₂) and zinc oxide (ZnO) engineered nanoparticles (NPs) are used in mineral-based sunscreens due to their excellent ultraviolet light protection abilities. Over time, surface water can become contaminated with these particles because of human recreational activities such as bathing, swimming, and other water sports. Thus, there is a need to measure these engineered particles present in surface waters to gain better understanding of anthropogenic inputs. In this study, we measure natural stream water spiked with mineral sunscreen along with naturally occurring NPs and microparticles (μPs) at the single-particle level using single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS). We use two-stage hierarchical clustering analysis (HCA) to identify distinct multi-elemental compositions that are characteristic of sunscreen-derived particles. Specifically, sunscreen NPs can be isolated from naturally occurring NPs and μPs based on elevated Ti and Zn mass fractions in individual particles compared to natural particles that are rich in Fe, Al, Mn, Ti, Mg, Zn, Ce, La, and/or Pb. Based on clusters assigned by HCA, we demonstrate classification of sunscreen-derived Ti and Zn NPs across more than two orders of magnitude and at number concentrations up to 50 times lower than those of naturally occurring Ti- and Zn-containing particles. This study demonstrates the accurate class assignment of sunscreen released and naturally occurring particles in river water.