Near-Infrared Probes Designed with Hemicyanine Fluorophores Featuring Rhodamine and 1,8-Naphthalic Derivatives for Viscosity and HSA Detection in Live Cells.

This paper presents the development of near-infrared (NIR) fluorescent probes, and , engineered from hemicyanine dyes with 1,8-naphthalic and rhodamine derivatives for optimized photophysical properties and precise mitochondrial targeting. Probes and exhibit absorption peaks at 737 nm and low fluorescence in phosphate-buffered saline (PBS) buffer. Notably, their fluorescence intensities, peaking at 684 () and 702 nm (), increase significantly with viscosity, as demonstrated through glycerol-to-PBS ratio experiments. This increase is attributed to restricted rotational freedom in the fluorophore and its linkages to rhodamine or 1,8-naphthalic groups. Theoretical modeling suggests nonplanar configurations for both probes, with primary absorptions in the rhodamine and hemicyanine cores (: 543; : 536 nm), and additional transitions to 1,8-naphthalic (: 478 nm) and rhodamine (: 626 nm) groups. Probe is also responsive to human serum albumin (HSA), a key biomarker, with fluorescence increasing in HeLa cells as HSA concentrations rise. In contrast, probe shows no response to HSA, likely due to steric hindrance from its bulky rhodamine group, illustrating a selectivity difference between the probes. Probe , however, excels in mitochondrial imaging, confirmed through cellular and in vivo studies. In HeLa cells, it tracked viscosity changes following treatment with monensin, nystatin, and lipopolysaccharide (LPS), with fluorescence increasing in a dose-dependent manner. In fruit flies, probe effectively detected monensin-induced viscosity changes, demonstrating its stability and applicability. These findings highlight the versatility and sensitivity of probes and as tools in biological research, with potential applications in monitoring mitochondrial health, detecting biomarkers like HSA, and investigating mitochondrial dynamics in disease.