Detection Of Biothiols And Folic Acid Based On MIL-n Metal-organic Frameworks

Metal-organic framewoks(MOFs) with tunable pore size, high surface area, luminescence property as well as catalytic activity has been wildly utilized in gas adsorption, organic catalysis or photocatalysis, chromatographic separation, drug delivery, chemical sensing, etc. As one of the important members in the metal–organic framework family, MIL- n are a fascinating class of MOFs originating through the self-assembly of carboxyl containing organic linkers coordinated by transition metal(Fe, Cr, Al, V, etc.), some of them show unique properties such as structural flexibility, high chemical stability, catalytic activity and excellent biocompatibility. In our paper, we mainly studied three kinds of MOFs: Fe-MIL-88, Fe-MIL-88NH2 and MIL-101(Cr), and used them for detection of b iological thiol compounds as well as folic acid, further expand the application of MOFs in the field of analysis. The research contents as follows:(1) Selective detection of biothiols: The nonfluorescent Fe-MIL-88 gives weak fluorescence in the presence of H2O2. It was found that biothiols such as glutathione(GSH), cysteine(Cys) or homocysteine(Hcy) could induce fluorescence turn-on of the Fe-MIL-88/H2O2 system. Under optimal conditions, the relative fluorescence intensity exhibited a good linear relationship in the range from 50 nM~10 μM. Based on this, we present a novel strategy for detecting biothiols in human serum.Mechanism investigation reveals that biothiols could associate with Fe-MIL-88 via hydrogen bonding and electrostatic interaction followed by redox reaction between biothiols and Fe3+present in the Fe-MIL-88, Fe3+was thus reduced to Fe2+, and then Fe2+could efficiently catalyze the decomposition of H2O2 to yield.OH radicals through the Fenton reaction. Besides, biothiols were able to reduce H2O2 to produce.OH radicals directly. Thus the Fe-MIL-88 as well as biothiols could cooperatively contribute to the activation of H2O2 to generate higher amounts of.OH radicals, which in turn oxidize the free ligand terephthalic acid(BDC) outside or within the Fe-MIL-88 structure to strongly fluorescent hydroxylated terephthalic acid(OHBDC), thereby turning on the fluorescence.(2) Selective recognition of 6-mercaptopurine: A novel and rapid spectrofluorometry method for the recognition of 6-mercaptopurine(6-MP) has been developed based on luminescent Fe-MIL-88NH2 as fluorescent probe. The strong fluorescence of Fe-MIL-88NH2 at 430 nm could be quenched by 6-MP directly, and the Fe-MIL-88NH2 shows high selectivity for 6-MP compared to other thiol-containing amino acids such as homocysteine(Hcy), cysteine(Cys), glutathione(GSH), etc. Under optimal conditions, the relative fluorescence intensity was linearly proportional to the concentration of 6-MP in the range of 5~600μM with the detection limit at 1.17μM. Furthermore, the present approach has been successfully applied to the determination of 6-MP in human serum samples. The possible fluorescence quenching mechanism has also been investigated, where it is revealed that the quenching was attributed to competition of absorption of the light source energy as well as electron transfer between Fe-MIL-88NH2 and 6-MP.(3) Synthesis of AgNPs/MIL-101(Cr) and its application: In Tris-HC l buffer solution(pH = 8.5), PDA could be formed by self-polymerization of dopamine, and then effectively attach onto the surface of MIL-101(Cr) for further Ag+reduction. By this strategy, Ag NPs could be efficiently anchored on MIL-101(Cr) to form AgNPs/MIL-101(Cr) hybrids, which exhibited strong enrichment of folic acid due to the electrostatic interaction, and the anchored Ag NPs greatly enhanced the SERS spectra of folic acid. Thus, the as-prepared hybrids have been used as excellent surface enhanced Raman scattering(SERS) substrates for the sensitive and selective detection of FA.In conclusion, we have successfully developed the methods with high sensitivity as well as high selectivity for the detection of biothiols, 6- mercaptopurine and folic acid based on Fe-MIL-88, Fe-MIL-88NH2; AgNPs/MIL-101(Cr); further expanded the application of MOFs in biochemical analysis.