Conjugation-induced fluorescence labelling of mesoporous silica nanoparticles for the sensitive and selective detection of copper ions in aqueous solution

Although the development of sensors for the detection of Cu²⁺ ions has significant importance, sensitive and selective recognition systems are still limited. Herein, the non-fluorescent small molecule of 2-bromomaleimide (BM) was firstly converted to 2-aminomaleimide (NM) by a simple nucleophilic addition-elimination reaction with pre-grafted amino groups in the meso-space of mesoporous silica nanoparticles (MSNs), and then was designed as fluorescent "on-off" probes for Cu²⁺ ion recognition. The strong green fluorescence emitted from the pendent NM was induced by the interaction between the lone electron pairs of the N atoms in amino groups and the conjugated maleimide rings. The fluorescence of MSNs labelled NM (MSNs-NM) could be selectively quenched by Cu²⁺ ions in aqueous solution and consequently exhibited sensitive fluorescence "on-off" switching effects for Cu²⁺ ions. The detection limit was as low as 0.28 μM Cu²⁺, which was far below the U.S. EPA limit of 20 μM. This prefigured the potential applications of the current chemosensor for the detection of Cu²⁺ ions in aqueous solution and in living cells. The small-angle XRD, TEM, BET, FT-IR, UV-vis absorption measurements all agreed on the fact that the sensitive probes of BM had been successfully labelled at the pore surface of MSNs.