Development of a methodology for analyzing nanometer-sized iron oxide by the single particle ICP-MS technique†

The nanomaterial field is experiencing exponential growth due to exceptional properties of nano-objects. Among them, iron oxide nanoparticles (IONPs) are of particular interest due to their outstanding magnetic properties which can be applied in the biomedical area. Therefore, their complete characterization is crucial. This work presents the development of a methodology using the single particle inductively coupled plasma mass spectrometry (SP ICP-MS) technique to characterize IONPs. For this purpose, a study of instrumental parameters has been carried out to obtain the best signal to background ratio for the iron element. This best ratio was reached with high energy helium as collision gas and a forward power of 1200 W. An improvement in the mathematical treatment to precisely find the threshold between background and IONP signals has also been achieved. This optimization of instrumental conditions and mathematical treatment was first set up on a commercial 30 nm IONP suspension showing a limit of detection in size of 24.1 ± 5.0 nm with a dwell time of 3 ms and 11.5 ± 0.4 nm with a lower one of 0.1 ms. The optimized methodology was then applied to the characterization of other IONP suspensions: a size-polydisperse (50 to 100 nm) and a monodisperse (centred at 20 nm) commercial suspension, as well as a laboratory-synthesized one (with an average size of 10 nm). The size results using our optimized methodology were found to be in good correlation with the expected size of all the analysed IONP suspensions. This demonstrates that nanometer-sized IONP suspensions can be characterized with reliability by the SP ICP-MS technique by following the methodology optimized herein.