Expanding the Toolbox of Rare Earth Quantification for Bioaccumulation Studies: Selective Lanthanide Determination Using ICP-OES and Sensitized Luminescence for High Throughput Microbial Screening

Abstract

Demand for rare earth elements (REEs) is surging, with several REEs deemed “critical” due to supply chain vulnerability and ubiquitous industrial use. Thus, there is growing interest in biological separation of REEs from waste products such as coal ash, e-waste, and mine tailings, which requires accurate screening of microbial candidates for metal sequestration. Current REE analysis methods include Inductively Coupled Plasma Mass Spectrometry (ICP−MS) and the colorimetric arsenazo III assay; both however possess limitations for high throughput analysis of cell cultures. ICP−MS incurs significant operational costs while arsenazo III is indiscriminate across the lanthanide series, contributing to an analytical bottleneck for strain ranking. Developing complementary metal quantitation methods is vital for scalability and automation of microbial screening. Hydroxypyridinone chelators, 5-LIO(Me-3,2-HOPO) and 3,4,3-LI(1,2-HOPO), were examined as spectroscopic antennas for Nd3+ and Eu3+ within microbial supernatant. Notably, 5-LIO(Me-3,2-HOPO) sensitized the near-IR luminescence of Nd3+ for sub-μM quantitation. Nd3+ and Eu3+ luminescence assays exhibited strong calibration linearity (R2 = 0.99) with limits of 0.14 μM for Nd3+ and 40 nM for Eu3+, outperforming arsenazo III. This work also validated Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) to quantify REEs for bioaccumulation experiments. ICP-OES quantitation limits (excluding Sm) were ≤33 ng mL−1 and ≤24 μg g−1 cell dry weight for supernatant and biomass respectively, with matrix effects and spike recovery documented. Bioaccumulation trials with Methylobacterium extorquens (AM1 and ΔmxaF) were used to compare ICP-OES and luminescence measurements. Overall, sensitized lanthanide luminescence and ICP-OES offer reliable, complementary approaches to existing analytical methods for REE determination.