A new fluorescent and electrochemical Zn2+ ion sensor based on Schiff base derived from benzil and L-tryptophan

Single molecule acting as both fluorescent and electrochemical sensor for Zn(2+) ion is rare. The product (L) obtained on condensation between benzil and L-tryptophan has been characterized by H NMR, ESI-MS and FT-IR spectroscopy. L in 1:1 (v/v) CH3OH:H2O solution shows fluorescence emission in the range 300 nm to 600 nm with λmax at 350 nm when is excited with 295 nm photon. Zn(2+) ion could induce a 10-fold enhancement in fluorescent intensity of L. Fluorescence and UV/Visible spectral data analysis shows that the binding ratio between Zn(2+) ion and L is 1:1 with log β=4.55. Binding of Zn(2+) ion disrupts the photoinduced electron transfer (PET) process in L and causes the fluorescence intensity enhancement. When cyclic voltammogram is recorded for L in 1:1 (v/v) CH3OH:H2O using glassy carbon (GC) electrode, two quasi reversible redox couples at redox potential values -0.630±0.005 V and -1.007±0.005 V are obtained (Ag-AgCl as reference, scan rate 0.1 V s(-1)). Interaction with Zn(2+) ion makes the first redox couple irreversible while the second couple undergoes a 0.089 V positive shift in redox potential. Metal ions - Cd(2+), Cu(2+), Co(2+), Hg(2+), Ag(+), Ni(2+), Fe(2+), Mn(2+), Mg(2+), Ca(2+)and Pb(2+), individually or all together, has no effect on the fluorescent as well as electrochemical property of L. DFT calculations showed that Zn(2+) ion binds to L to form a stable complex. The detection limit for both fluorescence as well as electrochemical detection was 10(-6) M.