Flexible metallosalen complex for pyrophosphate sensing in biological milieu: Use disassembly strategy for signal amplification

Abstract

In the present work, we have designed and synthesized two dimeric fluorescein derivatives with flexible salen (1) or a rigid salophen (2) linker unit. The Fe(III) complexes of these compounds ([1.Fe]+ and [2.Fe]+) showed changes in solution color specifically upon addition of inorganic pyrophosphate (PPi) at pH 7.0 in PBS buffer. Along this, we also observe appearance of green fluorescence (turn-on) in the prescence of PPi. The mechanistic studies suggested PPi-induced demetallation process, which led to disassembly for molecular structure in the aqueous medium and release of highly fluorescent fluorescein aldehyde. The disassembly process was found to be dependent on electronic structure, rigidity of the linker unit, temperature, and local pH etc. The selectivity as well as sensitivity of [1.Fe]+ towards PPi was also found to be superior than that observed with [2.Fe]+. Since each disassembly process released two signalling unit (fluorescein), we could observe a huge increment in FL intensity even with small quantity of PPi (<10 µM). Finally, the sensory system was utilized for detection of PPi in diluted blood serum samples and inside living mammalian cells (HeLa, cervical cancer cells). Chemically-modified paper strips have been developed for the detection of PPi in water samples at outside laboratory conditions.