Rapid size characterization of silver nanoparticles by single particle ICP-MS and isotope dilution

The increasing application of silver nanoparticles (AgNP) in consumer products and their potential release into the environment call for intensive investigation of their toxicity, stability, and fate. Analytical methods that are able to detect and characterize AgNPs at low concentrations and in complex matrices are needed. Single particle inductively coupled plasma mass spectrometry (spICP-MS) has in recent years emerged to a reliable technique suitable to quantify and size nanoparticles at low concentrations. However, the size determination by means of spICP-MS depends on external calibration with nanoparticle or element standard solutions. Here, a new approach is introduced using internal calibration with isotope dilution to determine the size of different AgNPs. External calibration becomes unnecessary, leading to a more rapid nanoparticle characterization and more robustness towards matrix effects. The power of the method is shown by determining the size of 50 nm citrate capped and 80 nm PVP capped AgNPs spiked with Ag-109 enriched silver standard. The method is highly reproducible and shows good agreement with results obtained by established methods. The successful size determination of AgNPs in wastewater and river water using this method demonstrates its practicability even in samples with high matrix loads. Sample preparation only requires the addition of an isotope enriched standard, which makes the method interesting for long-term studies when AgNPs have to be characterized on many different days. As an example, the size alteration of AgNPs under different conditions was monitored over a period of four days, employing the developed method of internal calibration with isotope dilution.