Cross-validation of analytical methods for citrate ligand quantification on upconversion and iron oxide nanoparticles

Analysis of nanomaterial (NM) surface chemistry is essential for understanding and controlling NM properties relevant for NM functionality, stability, and safety. In this work, we present a multimethod study of the quantification of the frequently used, biocompatible surface ligand citrate for lanthanide-based upconversion nanoparticles (UCNPs) and iron oxide nanoparticles (IONPs). Methods applied that address different yet correlated measurands with varying chemo-selectivity and rely on different sample preparation workflows include chemo-selective reversed-phase high-performance liquid chromatography (RP-HPLC) with photometric detection measuring liquid samples and dissolved NMs as well as non-selective thermogravimetric analysis (TGA) and chemo-selective pyrolysis-gas chromatography-mass spectrometry (PyGC-MS) analyzing dried NMs. Thereby, we demonstrate the applicability of the RP-HPLC method, recently developed for citrate quantification on IONPs, for UCNPs as another application-relevant NM class. In addition, the feasibility of PyGC-MS, emerging for material characterization at the trace level due to the consumption of very small sample amounts and minimum sample preparation, for NM ligand quantification is assessed. This method, which has not been applied before for NM surface analysis, indirectly quantifies citrate via the amount of acetone formed during citrate decomposition. For UCNPs, PyGC-MS measurements led to a good agreement with the TGA and RP-HPLC results within the respective measurement uncertainties. However, quantifying citrate on IONPs with PyGC-MS resulted in an overestimation of citrate, which is tentatively ascribed to IONP catalytic activity. Overall, our method cross-comparison underlines the importance of multimethod characterization schemes to assure a reliable and precise ligand quantification and to identify method- and NM-specific effects.