Chemosensing of neurotransmitters with selectivity and naked eye detection of l-DOPA based on fluorescent Zn(ii)-terpyridine bearing boronic acid complexes

Abstract

Biological catecholamines such as L-DOPA and dopamine play vital physiological roles in the brain and are chemical indicators of human diseases. A new range of fluorescent Zn(II)-terpyridine complexes are described and studied in-depth as chemosensors for catecholamine-based neurotransmitters and nucleosides in pure water. The new Zn-terpyridine-based chemosensors contain a cationic N-isoquinolinium nucleus as the optical indicator covalently linked to three different isomers of strongly acidified phenylboronic acids (ortho-, 2.Zn; meta-, 3.Zn and para-, 4.Zn, substituted derivatives) as catechol binding sites. The addition of L-DOPA, dopamine, epinephrine, L-tyrosine and nucleosides to Zn(II)-boronic acid chemosensors at physiological pH quenches their blue emission with a pronounced selectivity and an unprecedented high affinity towards L-DOPA (log K = 6.01). This efficient response by L-DOPA was also observed in the presence of coexisting species in blood plasma and urine with a detection limit of 3.0 μmol L⁻¹. A photoinduced electron transfer quenching mechanism with simultaneous chemosensor^–L-DOPA complexation in both the excited and ground states is proposed. The fluorescence experimental observations show that the 2.Zn·eosin-Y adduct can be used as a selective naked-eye chemosensing ensemble for L-DOPA with a fast turn-on fluorescent response and color change from blue to green under UV light at the micromolar level. On the basis of multiple spectroscopic techniques (¹H, ¹¹B NMR, UV-Vis, and fluorescence), MS-ESI experiments, crystal structures, and DFT calculations, the binding mode between Zn(II)-chemosensors and L-DOPA is proposed in a 1 : 1 model through a cooperative two-point recognition involving the reversible esterification of the boronic acid moiety with the aromatic diol fragment of L-DOPA together with the coordination of the carboxylate anion to the Zn(II) atom with strong electrostatic contribution.