Twisted-Intramolecular-Charge-Transfer-Based Turn-On Fluorogenic Nanoprobe for Real-Time Detection of Serum Albumin in Physiological Conditions

Two cyanine-based fluorescent probes, (<i>E</i>)-2-(4-(diethylamino)-2-hydroxystyryl)-3-ethyl-1,1-dimethyl-1<i>H</i>-benzo­[<i>e</i>]­indol-3-ium iodide (<b>L</b>) and (<i>E</i>)-3-ethyl-1,1-dimethyl-2-(4-nitrostyryl)-1<i>H</i>-benzo­[<i>e</i>]­indol-3-ium iodide (<b>L</b>_<b>1</b>), have been designed and synthesized. Of these two probes, the twisted-intramolecular-charge-transfer (TICT)-based probe, <b>L</b>, can preferentially self-assemble to form nanoaggregates. <b>L</b> displayed a selective turn-on fluorescence response toward human and bovine serum albumin (HSA and BSA) in ∼100% aqueous PBS medium, which is noticeable with the naked eye, whereas <b>L</b>_<b>1</b> failed to sense these albumin proteins. The selective turn-on fluorescence response of <b>L</b> toward HSA and BSA can be attributed to the selective binding of probe <b>L</b> with HSA and BSA without its interfering with known drug-binding sites. The specific binding of <b>L</b> with HSA led to the disassembly of the self-assembled nanoaggregates of <b>L</b>, which was corroborated by dynamic-light-scattering (DLS) and transmission-electron-microscopy (TEM) analysis. Probe <b>L</b> has a limit of detection as low as ∼6.5 nM. The sensing aptitude of probe <b>L</b> to detect HSA in body fluid and an artificial-urine sample has been demonstrated.