Mechanochemical versus sol–gel silica loading of phenolate- and acetate-bridged dizinc complexes: toward instant and inexpensive hybrids for controlled binding and release of Zn2+ in pure water

Abstract

Herein, we have explored expeditious and lowered expense SiO₂-loading in the context of fluorescent “off–on” and “on–off” Zn²⁺ ion detection/controlled release. A simple racemic ligand (H₂rlys) (1) prepared in situ can be mechanically SiO₂-loaded so as to induce surface sites that allow for “off–on” aqueous fluorescence (λ_em,max = ∼450 nm) detection of Zn²⁺; with Cu²⁺ as a competing input, “inhibit” logic gate behaviour is present. Oppositely, dimeric, chiral fluorescent zinc complexes (Φ_F = ∼0.20) prepared in a “one pot” method, and bearing different Zn⋯Zn internuclear spacings, can be incorporated into silica by two methods. Specifically, the chiral Zn₂(slysH)₂Cl₂ (2) and racemic Zn₂(rslysH)₂(μ-OAc)₂ species (3) [slys or rslys = 6-amino-2-{(2-hydroxybenzylidene)amino}hexanoate] were mechanochemically loaded. An adsorption constant was 1.99 × 10⁻⁴ (molecules of 2/units SiO₂)(min)⁻¹ for times of 5–20 min, as determined by extensive SEM-EDS data. These confirmed the highly selective “on–off” rapid aqueous fluorescent Cu²⁺ detection (versus, Li⁺, Na⁺, Cs⁺, Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Hg²⁺, Pb²⁺) through direct Zn²⁺ displacement under bulk neat water flow (pH = 7.4) as verified by an authentic sample of [Cu₂(slysH)₂(NO₃)₂]–silica (4–silica). With the use of 2·SiO₂, Na₄EDTA and Cu²⁺ “inhibit” logic gate behaviour was also found. Sol–gel techniques allowed for loadings of 2 (20–890 mg) up to 1 ∶ 1 (by wt). The preparation of these inorganic hybrids are extremely inexpensive and rapid to prepare from commercial starting materials and can be extended to variegated forms of SiO₂; alumina gel was also functionalized with 2. Cu²⁺ and Zn²⁺ are metals pertinent to human neurological health promotion and in neurodegenerative diseases.