FRET-Mediated Zn²⁺Sensing in Aqueous Micellar Solution: Application in Cellular Imaging and Molecular Logic Gate

Förster (fluorescence) resonance energy transfer (FRET) is an excellent method in generating characteristic fluorescence signals between two light sensitive molecules in the 1–10 nm range. On the other hand, studies of Zn²⁺detection and imaging are also important, as zinc is the second most abundant transition metal ion in the human body and is involved in many biological activities and neurological functions. This work is a superposition between these two realities, which demonstrates the excellent performance of 4-(pyridine-2-yl)-3H-pyrrolo [2, 3-c] quinoline (PPQ) as a FRET sensor for Zn²⁺, entrapped in aqueous micellar solution, in presence of Rhodamine B (RB)/Rhodamine 6G (R6G). We show that easy, superior detection and molecular imaging of Zn²⁺becomes possible via FRET, as PPQ nonradiatively transfers its energy to R6G/RB, only when it binds with Zn²⁺, over important biological cations including Na⁺, K⁺, Ca²⁺, Mg²⁺. During FRET, the Förster radius (R₀) between the donor (PPQ) and the acceptor (RB/R6G) is calculated to be ≈ 4 nm, when they are encapsulated in sodium dodecyl sulphate (SDS) micelles at pH 6. Moreover, on careful observation of the fluorescence behavior of PPQ, we also demonstrate that PPQ functions like a monomolecular circuit in an INHIBIT logic gate, based upon two chemical inputs from Zn²⁺and Rhodamine.