Proton-assisted iron(III) recognition with 4,4′-(4,5-diaminonaphthalen-1,8-yldiazo)-bisbenzenesulfonate

Azo compounds, including 4,4′-(4,5-diaminonaphthalen-1,8-yldiazo)-bisbenzenesulfonate (DBS), 4,5-bis-(4-nitro-phenylazo)-naphthlene-1,8-diamine (NND), 4-(4-aminophen-1-ylazo)-benzenesulfonic acid (ABS), 4-(4-aminonaphthalen-1-ylazo)-benzenesulfonic acid (ANBS) and 1,1′-(4,5-diaminonaphthalen-1,8-diylazo)-bisbenzene (DNB) were synthesized and characterized for their potential in Fe³⁺ recognition in acidic solutions. DBS interacts specifically with Fe³⁺ in HCl solution, resulting in a significant alteration of the absorption and emission spectra. According to the spectral changes, DBS probably forms a 1:1 complex with Fe³⁺; the equilibrium constant was estimated to be ∼10⁵ M^-1. Replacing Fe³⁺ with Fe²⁺, Ni²⁺, Al³⁺ and Cr³⁺ or substituting NND, ABS, ANBS and DNB for DBS did not yield similar results, indicating that a configuration of two sulfonate groups is essential for Fe³⁺ chelation. DBS can be modified on glassy carbon and Au-sputtered quartz crystal electrodes (Au-QCM, f_o = 9 MHz), via azotization and in situ electrochemical deazotization processes in HCl solution. When immobilized on QCM, DBS showed ability to attract Fe³⁺ from the solution phase, leading to a linear relationship between log{divides}Δf{divides} and log[Fe³⁺] over the range 10^-4-10^-3 M in 0.01 M HCl, with little interference from Fe²⁺ and Cr³⁺. Accordingly, DBS is a promising receptor for Fe³⁺ in acidic conditions.