Riboflavin chelated luminescent metal-organic framework: Identified by liquid-assisted grinding for large-molecule sensing via chromaticity coordinates

Liquid-assisted grinding protocol has been successfully employed to identify (1) the use of riboflavin as a sensing molecule for melamine and acetoguanamine and (2) the chelation of riboflavin on a silver atom. Riboflavin is then chelated onto a visible light-emitting, silver-containing metal-organic framework, AgL {i.e., [AgL] _n•nH ₂O (L = 4-cyanobenzoate)} to produce (AgL)-(riboflavin) chelation crystals. These chelation crystals can be used for the detection of melamine and acetoguanamine by forming binding crystals of (AgL)-(riboflavin)-(melamine) and (AgL)-(riboflavin)-(acetoguanamine). The Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates of AgL crystals, (AgL)-(riboflavin) chelation crystals, (AgL)-(riboflavin)-(melamine) binding crystals, and (AgL)-(riboflavin)-(acetoguanamine) binding crystals based on their solid-state photoluminescence (PL) emission spectra are calculated to be approximately (0.16, 0.16), (0.32, 0.44), (0.25, 0.37), and (0.23, 0.15), respectively. The original PL emission of AgL may be attributed to ligand-centered luminescence. However, the chelation with riboflavin may cause (1) a bathochromatic spectral shift (i.e., red shift), (2) an emission broadening, and (3) a quenching effect through the mode of Förster resonance energy transfer. The binding with the amine analytes (i.e., good electron donors), such as melamine and acetoguanamine, may alter the redox potential of riboflavin inhibiting quenching and enhancing luminescence of binding crystals of (AgL)-(riboflavin)-(melamine) and (AgL)-(riboflavin)- (acetoguanamine). π → π* energy in the riboflavin-melamine or riboflavin-acetoguanamine binding complex is then enhanced. Consequently, hypsochromic spectral shifts (i.e., blue shift) are observed in their PL emission responses.