MOF-on-MOF nanomaterial (Fe-MOF@UiO-66)-based ratiometric fluorescence/colorimetric dual-mode sensor for ultrasensitive detection of uric acid.

Background: Uric acid (UA) is a key metabolite of purine metabolism, and its elevated concentration in the blood is closely associated with various health conditions, particularly gout. It is of great significance to develop a biosensor to detect UA in human body fluids for the diagnosis and prevention of related diseases. The hybrid "MOF-on-MOF" structure offers unprecedented tunability, synergistic effects, and superior performance compared to single MOFs. Moreover, the cooperative effects between the two MOF materials enhanced charge separation and light absorption, which is valuable for applications in analytical sensing.

Results: A MOF-on-MOF nanomaterial (Fe-MOF@UiO-66) with both excellent peroxidase-like activity and fluorescence property is synthesized, where UiO-66 is first synthesized as a core material, and the Fe-MOF layer is then grown on its surface. Based on the remarkable properties, a dual-mode UA sensor combining colorimetric and ratiometric fluorescence detection was developed. Both modes feature a wide linear range (0.2-250 and 0.2-200 μM) and lower detection limits (0.096 and 0.038 μM). The EPR detection revealed that •OH and O are major and minor reactive oxygen species (ROS) intermediates, respectively. The quenching mechanism was studied in detail by Stern-Volmer equation, Parker equation, the overlap integral J (λ) and density functional theory (DFT) calculations. The biosensor was successfully applied to the detection of UA in human serum and urine. Furthermore, principal component analysis was employed to construct a gout risk prediction model, which can accurately differentiate between safe and high-risk patients.

Significance: While various applications of MOF-on-MOF structures have been reported in prior studies, their utilization for biomarker detection in human body remains scarcely explored. Furthermore, the unique heterostructure of MOF-on-MOF endows the composite with dual functionalities: nanozyme-like catalytic activity and intrinsic fluorescence properties. Fe-MOF@UiO-66 as nanozymes and fluorescence probe were explored toward the sensing platforms for efficient detection of UA in real samples and analytical results have been testified by comparison the results with standard method. The nanozymes-based sensing platforms show great prospects in colorimetry/fluorescence dual-model sensing in biological samples. And this study promotes the development and application of MOF-on-MOF materials in multi-mode biomarker sensing.