Covalent Organic Frameworks: A Novel Class of Metal-Free Catalysts for Efficient Photochemical Vapor Generation

Photochemical vapor generation (PVG) is an attractive yet under-exploited sample-introduction strategy for atomic spectrometry. There remain challenges in improving the efficiency of PVG of elements and expanding the range of PVG applications. Here, these issues have been tackled by introducing a metal-free porphyrinic covalent organic framework (SU-3), cross-linked by robust polyimide bonds and exhibiting strong ultraviolet light absorption and photocatalytic activity. Liquid-phase exfoliation converts the bulk framework into ultrathin SU-3 nanosheets (SU-3 CONs), further amplifying its photocatalytic capacity. Both SU-3 COF and SU-3 CONs were harnessed as photocatalysts to boost PVG of rhenium (Re) prior to inductively coupled plasma mass spectrometry (ICP-MS), yielding 27-fold and 71-fold signal enhancements, respectively. Compared to bulk COF, the superior performance of nanosheets stems from their more exposed catalytic sites and amplified photocatalytic activity. The method minimizes low-molecular-weight acid consumption, precludes extraneous transition-metal sensitizers, and affords linearity from 0.05 to 100 ng mL^-1 with a 0.7 ng L^-1 limit of detection─superior to that of traditional PVG methods sensitized by transition metal ions. Accurate Re quantification in river, lake, and tap water samples confirms the robustness and practicality of the method. This work thus establishes metal-free COF nanosheets as a versatile platform for advancing photochemical vapor generation, opening new avenues for ultrasensitive elemental analysis.