An ICT-based microenvironment-sensitive fluorescence probe, DCI-MIN demonstrates its utility for the specific recognition and quantification of HSA in human urine and blood samples as well as the serum albumin in living cells.

The reaction of MeO-salox (4-methoxy salicylaldehyde-oxime) with Mn(OAc)₂·4H₂O in a 1 : 1 molar ratio in methanol medium and in the presence of tetrabutylammonium hydroxide yields the hexanuclear complex [Mn₆O₂(4-MeO-salox)₆(N₃)₂(MeOH)₂(H₂O)₂·2H₂O]_n (1). Single crystal X-ray diffraction reveals that complex 1 adopts a 1D chain network facilitated by two EE-azido bridges. BSA binding studies reveal that complex 1 causes fluorescence quenching of BSA through both static and dynamic mechanisms. A 1 : 1 binding mode is established through a Benesi–Hildebrand plot with an intrinsic binding constant of (3.40 ± 0.01) × 10⁴ M⁻¹. The circular dichroism studies indicate variation of the secondary structure of BSA upon binding with complex 1. The site-selective binding and molecular docking studies reveal the probable binding site of complex 1 is at the mouth of site I in subdomain IIA of BSA.

Here, we report the design and synthesis of a D⋯π⋯A-based fluorescent probe, (E)-4-(4-(dibutylamine)-2-hydroxystyryl)-1-methylquinolin-1-ium (DHMQ), which is nonfluorescent in ∼100% PBS buffer medium due to a twisted intra molecular charge transfer (TICT) phenomenon and it becomes highly fluorescent (∼149 fold) in the presence of human serum albumin (HSA), owing to the restriction of its intramolecular free rotation inside the hydrophobic binding cavity of HSA. The site-selective fluorescence displacement assay and molecular docking studies clearly reveal that DHMQ selectively binds at subdomain IB of HSA. The 3σ/slope method was adopted to determine the limit of detection (LOD) value, which was as low as 2.39 nM in ∼100% PBS medium, indicating its high sensitivity towards HSA. The low dissociation constant value [K_d = (1.066 ± 0.017) μM] suggests a strong complexation between the DHMQ and HSA. Importantly, it has been demonstrated that DHMQ is capable of detecting HSA in real human serum and urine samples and was found to be suitable for live cell imaging of HSA.

Fluorescence spectroscopy is a significant bio-analytical technique for specific detection of nitric oxide (NO) and for broadcasting the in vitro and in vivo biological activities of this gasotransmitter. Herein, a benzo-coumarin embedded smart molecular probe (BCM) is employed for NO sensing through detailed fluorescence studies in purely aqueous medium. All the spectroscopic analysis and literature reports clearly validate the mechanistic insight of this sensing strategy i.e., the initial formation of 1,2,3,4-oxatriazole on treatment of the probe with NO which finally converted to its carboxylic acid derivative. This oxatriazole formation results in a drastic enhancement in fluoroscence intensity due to the photoinduced electron transfer (PET) effect. The kinetic investigation unveils the second and first-order dependency on [NO] and [BCM] respectively. The very low detection limit (16 nM), high fluorescence enhancement (123 fold) in aqueous medium and good formation constant (K_f = (4.33 ± 0.48) × 10⁴ M⁻¹) along with pH invariability, non-cytotoxicity, biocompatibility and cell permeability make this probe a very effective one for tracking NO intracellularly.