Novel turn-on fluorescent biosensors for selective detection of cellular Fe3+ in lysosomes: Thiophene as a selectivity-tuning handle for Fe3+ sensors

Iron-selective turn-on sensors are indispensable tools for understanding iron-related cell death processes and human diseases. In this study, we report a novel class of fluorescent sensors derived from an indolizino[3,2-c]quinoline scaffold that exhibit high selectivity for Fe3+ over other biologically abundant cations in cells, including Fe2+, Al3+, Zn2+, and Mn2+. IQ18 works as a ratiometric sensor with a K-d value of 7.1 x 10(-7)M and a detection limit of 5.2 nM in ethanol, whereas IQ44 displays fluorescence enhancement upon binding with Fe3+ in both ethanol and water. In aqueous solution, IQ44 exists as 150-nm nanoparticles. The suppressed fluorescent emission of IQ44 nanoparticles in water is switched on in response to Fe3+, working as a turn-on nanoparticle sensor. Structure-property relationship analysis with IQ derivatives revealed that the thiophene ring confers selectivity for Fe3+. By installing thiophene in IQ44 as a selectivity-tuning handle, fluorescence in the presence of Fe3+ resulting from restriction of intramolecular rotation (RIR) and increased torsion angle induced by iron demonstrated that IQ44 is specifically localized in lysosotiies, where it recognizes cellular Fe3+ in live cells, as determined using confocal microscopy. In addition, the increased fluorescent puncta of IQ44 in the presence of Fe3+ colocalized well with the RFP-tagged LC3 proteins (pmRFP-LC3), enabling the detection of the autophagy process.