Anion recognition and electrochemical characteristics of the self-assembled monolayer of nickel(II) azamacrocyclic complex

Anionic recognition of the self-assembled monolayer of dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate (1) was studied electrochemically. The dinickel(II) complex 1 adsorbs on gold electrodes from methanol solutions and yields stable, self-assembled electroactive monolayers (SEMs); the SEM of 1 shows a reversible redox wave at 0.82 V in aqueous 0.1 M NaNO3 corresponding to the Ni3+/2+ redox reaction. The surface coverage, Gamma, of the self-assembly of 1 determined by cyclic voltammetry is constant (Gamma = (1.4 +/- 0.08) x 10(-10) mol cm(-2)) with change in the deposition time (2-36 h) and the concentration of 1 in methanol solution (0.2-5 mM) and is equivalent to a monolayer coverage of the nickel macrocyclic complex. The capacitance of the monolayer of 1 was determined from the double-layer capacitance measurements by chronoamperometry; the monolayer of 1 is assigned to be well-solvated by observing that the dielectric constant of the self-assembly domain (epsilon(film) = 74) is nearly equal to that of water (epsilon(water) = 78). Electrochemical investigations reveal that the monolayer of 1 can sense electrochemically various non-electroactive anions, NO3-, CF3COO-, SO42-, H2PO4-, HPO42-, ClO4-, PF6- and SCN-, from the variation of the formal potential, E degrees', in aqueous solutions of different anions. The E degrees' of the monolayer of 1 is 0.82 V in aqueous 0.1 M NaNO3 and shifts to a less positive potential, 0.55 V, in aqueous 0.1 M Na2SO4; the shift in the E degrees' was reversible on exchanging the monolayer of 1 between 0.1 M NaNO3 and 0.1 M Na2SO4. The shift in the E degrees' of the monolayer has been explained by an axial coordination of electrolyte anions with the trivalent nickel ion. The redox reaction of the SEM of 1 is not observed in aqueous solutions of 0.1 M NaClO4 and 0.1 M NaSCN; but the redox activity was retained on changing the monolayer electrode to an aqueous solution of 0.1 M Na2SO4 or 0.1 M NaNO3. The monolayer of 1 could detect electrochemically the biologically important phosphate anion, adenosine triphosphate (ATP), at submillimolar concentrations; on addition of 1 mM ATP, the formal potential of the monolayer shifts towards the less positive potential region by about 250 mV. The CVs of the SEM of 1 were recorded in aqueous solutions containing different concentrations of NaH2PO4 or Na2SO4, keeping the ionic strength of the electrolyte solution constant with added NaNO3. The E degrees' of the monolayer shifts to the less positive potential region with an increase in the concentration of H2PO4- or SO42- anion in solution phase, and the analysis of cyclic voltammetric results reveals that the nickel(III) complex forms a 1:1 complex with SO42- anion but a 1:2 complex with H2PO4- anion. (C) 2000 Elsevier Science S.A. All rights reserved.