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

• New molecule from condensation of tryptophan and salcylaldehyde synthesised.
• The molecule acts as fluorescent and electrochemical sensor for Zn2+ ion.
• The interaction between the sensor and Zn2+ ion is proved by DFT calculations.

Single molecule acting as both fluorescent and electrochemical sensor for Zn2+ 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. Zn2+ 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 Zn2+ ion and L is 1:1 with log β = 4.55. Binding of Zn2+ 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 Zn2+ ion makes the first redox couple irreversible while the second couple undergoes a 0.089 V positive shift in redox potential. Metal ions – Cd2+, Cu2+, Co2+, Hg2+, Ag+, Ni2+, Fe2+, Mn2+, Mg2+, Ca2+and Pb2+, individually or all together, has no effect on the fluorescent as well as electrochemical property of L. DFT calculations showed that Zn2+ ion binds to L to form a stable complex. The detection limit for both fluorescence as well as electrochemical detection was 10−6 M.

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