Nano-aggregate-Fe3+ complex based on benzimidazole-modified calix[4] arene for amplified fluorescence detection of ADP in aqueous media

In this study, a dipodal 2-(2-aminophenyl) benzimidazole scaffold appended to the lower rim of calix[4] arene at 1,3-alternate positions (L1) was synthesized. Organic nanoparticles of receptor L1 (O1) displayed remarkable selectivity and sensitivity for Fe3+ ions via a turn-off fluorescence response in an aqueous medium. The O1 center dot Fe3+ complex was further studied for its binding ability toward biologically prevalent anions, and showed excellent selectivity for ADP in the nanomolar range, even in the presence of structurally similar nucleotides, with a blue shift and four-fold enhancement in the intensity of fluorescence emission. The determination of ADP is crucial because ADP plays significant roles in various biological reactions catalyzed by ATPases and kinases. ATP and ADP are also known to generate energy in cells via phosphate bond cleavage. The fluorescence quenching upon addition of Fe3+ ions could be attributed to the conglomeration of the nano-aggregates, which either separated or further assembled when ADP was added to the ensemble. Consequently, the fluorescence of the ensemble could be recovered. The sensing system was also applied to the detection of ADP in spiked blood serum samples; the excellent recovery of up to 99.3% validated its practicality. The off-on sensing mode, simple fabrication process, and ease of synthetic methodology are some of the advantages for further applications. The sensor was also successfully applied to monitoring of the hydrolysis of ADP to AMP by apyrase, which indicated its potential application for ADP-pertinent biological activities.