Application and mechanism of Co₃O₄/Co(OH)₂ heterojunctions as matrices for small molecules detection by MALDI-TOF MS

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has emerged as a superior technique for detecting small molecules, owing to the strategic utilization of inorganic nanomaterial matrices. Despite the impressive capabilities of various novel matrices, the underlying interaction mechanisms between inorganic matrices and analytes remain largely unexplored. In this study, we synthesized Co₃O₄ nanocubes, Co(OH)₂ nanosheets, Co₃O₄/Co(OH)₂ heterojunctions, and Co₃O₄+Co(OH)₂ composites via a facile one-pot method. Amino acids were selected as model analytes for performance evaluation. Notably, the Co₃O₄/Co(OH)₂ heterojunctions significantly enhanced signal intensity and lowered detection limits to the parts-per-billion (ppb) level, outperforming Co₃O₄ nanocubes, Co(OH)₂ nanosheets, and Co₃O₄+Co(OH)₂ composites. The detection results for environmental pollutants, especially in solutions containing real samples, highlight the outstanding performance of the heterojunction material as a matrix. Further characterization indicates that the formation of the heterojunction enhances nanoparticle dispersion and promotes the separation of photogenerated electron-hole pairs. Consequently, more photogenerated charge carriers can be transferred to the target analytes, facilitating their charging ability and ultimately enhancing signal intensity in MALDI-TOF MS. These results clearly demonstrate that the enhanced photocatalytic performance directly drives the improvement of MALDI-TOF MS performance, excluding interference from other factors. Thus, this study successfully combines photocatalytic mechanisms with MALDI-TOF MS mechanisms, providing new insights into the enhancement of MALDI-TOF MS performance. Additionally, it offers a new detection method for environmental pollutants.