Probing Mineral-Organic\nInterfaces in Soils and Sediments\nUsing Optical Photothermal Infrared Microscopy

Interactions among microbes, minerals, and organic matter\nare key\ncontrols on carbon, nutrient, and contaminant dynamics in soils and\nsediments. However, probing these interactions at relevant scales\nand through time remains an analytical challenge due to both their\ncomplex nature and the need for tools permitting nondestructive and\nreal-time analysis at sufficient spatial resolution. Here, we demonstrate\nthe ability and provide analytical recommendations for the submicron-scale\ncharacterization of complex mineral-organic microstructures using\noptical photothermal infrared (O-PTIR) microscopy. Compared to conventional\ninfrared techniques, O-PTIR spectra collected at submicron resolution\nof environmentally relevant mineral and organic reference compounds\ndemonstrated similar spectral quality and sensitivity. O-PTIR detection\nsensitivity was greatest for highly crystalline minerals and potentially\nfor low molecular weight organic compounds. Due to photothermal effects,\nO-PTIR was more sensitive toward organics than minerals compared to\nconventional IR approaches, even when organics were mineral-bound.\nMoreover, O-PTIR resolved mineral-bound and unbound organics in a\ncomplex mixture at submicron (<500 nm) resolution. Finally, we\nprovide best practices for artifact-free analysis of organic and mineral\nsamples by determining the appropriate laser power using damage thresholds.\nOur results highlight the potential of O-PTIR microscopy for nondestructive\nand time-resolved analysis of dynamic microbe-mineral-organic matter\ninteractions in soils and sediments.