Metal Organic Frameworks as Potential Candidates in Photocatalysis and Chemical Sensing Applications

Abstract

This work demonstrates a systematic investigation on aqueous phase applications of contemporary Metal Organic Frameworks (or MOFs). The proposed areas of research in this study were photocatalysis and chemical sensing. The investigated MOFs were primarily MIL-125(Ti), NH2-MIL-125(Ti), and MIL-53(Fe). Different variants of MIL-53 (Fe) were synthesized via cation (Li+, Na+, K+), anion (S2-), and metal (Au, Ag) doping. Similarly, Polyaniline (PA) functionalization of NH2-MIL-125(Ti) was successfully carried out. All parent MOFs and their variants were systematically synthesized via facile microwave routes in a high-pressure microwave reactor. Syntheses recipes were thoroughly optimized for reaction temperature and time for consistency in product formation. Optical band gap energy of hybrid MOFs from UV-vis diffused reflectance analysis was estimated using both Kubelka-Munk (K-M) and dielectric models and compared. A host of conventional inorganic photocatalysts (semiconductors), conductors, and insulators were successively analyzed for comparison. The dielectric model was found to be equally effective when compared with Kubelka-Munk (K-M)) method and the variations were within 0.16 - 7.07% for different sets of conventional and hybrid materials. Additionally, the dielectric model was found to be more definitive when multiple absorption peaks/unresolved peaks were present in the diffused reflectance spectra (a common feature with hybrid MOF or poly-functional materials). The dispersion-dissipation plot based on the dielectric model could rightly predict the material class (semi-conductor/conductor/insulator) accordingly and was shown in the study. MIL-125(Ti) showed remarkable aqueous phase stability with measured BET surface area of ca. 112 m2g-1. The product was found to be photo-catalytically active with a band gap energy of 3.14 eV and successfully degraded methylene blue dye (ca. 96.77% in 360 minutes). Water stable NH2-MIL-125(Ti) was successfully functionalized with Polyaniline (PA) with an increase in electron conductivity (as reflected in various intrinsic parameters such as fluorescence intensities, quantum yield, and dielectric functions) leading to successful detection of dopamine (DA) with excellent sensitivity. The corresponding LOD was estimated to be 10 nM and 5 nM in the case of NH2-MIL-125 (Ti) and PA-NH2-MIL-125 (Ti) respectively via Stern Volmer model at Signal-to-Noise (S/N) = 3.0. Band gap tuning of synthesized MIL-53(Fe) through cation (Li+, Na+, K+), anion (S2-), and metal (Au, Ag) doping (structural/interstitial) and their influences in photocatalytic applications viz. dye degradation and peroxidase activity were investigated. The influence of cation doping showed a decrease in band gap energy: Li+ (1.81 eV), Na+ (1.875 eV) and K+ (1.89 eV) when compared with the parent MIL-53(Fe) (1.95 eV) whereas the trend was reversed for anion (S2-) doping: 1% S (2.224 eV); 4% S (2.376 eV), 7% S (2.448 eV) and 10% S (2.579 eV). Cation doping, leading to the distortion of the octahedral geometry of Fe3+-Fe2+ in MIL-53(Fe) played a significant part in influencing the performance of doped variants of MIL-53 (Fe). The IVCT band intensity was highest for 1% S when compared with MIL-53(Fe) and its cation doped counterparts which were reflected on the photoactivity. The intrinsic material properties viz. band gap energy, electron conductivity, and surface area had a combinatorial influence on the photocatalytic performances. The incorporation of Au/Ag nanoparticles (NPs) into MIL-53(Fe) matrix occurred interstitially and it could be observed in HR-TEM analysis. Similarly, the plasmonic activity of gold was observed at 540 nm with no change in band gap for MIL-53(Fe). The study showed marked improvement in H2O2 detection with LOD of 0.0623 μM within a linear range (1- 10 μM) (S/N = 3). The presence of H2O2 in spiked milk samples were analyzed and modeled through linear regression and the resulting model was validated with an accuracy of 0.465% and 0.45% with RSD % lying within 1.93% and 3.81% for two different models falling within linear ranges (1-10 μM) and (10-100 μM) respectively. The interference/sensitivity studies showed inherent pitfalls in the assay due to the presence of certain common milk constituents. Overall, Au@MIL-53(Fe) demonstrated as an excellent colorimetric sensor for the detection of H2O2 both in model solution as well as in processed milk and could trace possible adulteration of milk. The material holds promise in the fabrication of various diagnostic prototypes for specific applications in food processing, medical and environmental remediation.