Synthesis of orange-emissive silicon nanoparticles as "off-on" fluorescence probe for sensitive and selective detection of l-methionine and copper.

Fluorescent silicon nanoparticles (Si NPs) are of great interest as they are free of heavy ions. However, most of Si NPs exhibit blue or green emission, while orange or red-emitting Si NPs are required for an extensive range of applications. Copper ion (Cu2+) and l-methionine (L-Met) detection is critically valuable point since their abnormal level is an indicator of various diseases. In this work, we illustrate an "off-on" method for sensitively and selectively determination of Cu2+ and L-Met using Si NPs as fluorescent probe. The Si NPs emitting orange fluorescence with the quantum yield of 2.23% were prepared via one and easy step of hydrothermal treatment of 3(2-aminoethylamino) propyl (dimethoxymethylsilane) (AEAPDMMS) and 2-aminophenol as precursors. The fluorescence of Si NPs was quenched in the presence of Cu2+ due to the strong metal-ligand coordination and electrostatic interactions between the large amount of amino and hydroxyl groups on the surface of Si NPs and Cu2+. Surprisingly, the resulted non-fluorescent Si NPs-Cu2+ complex displayed a fluorescence "turn-on" toward L-Met, due to the competitive coordination of Cu2+ between L-Met and Si NPs which leads to the unique "off-on" response to L-Met after the release of free Si NPs. The as-proposed approach is fast, simple, low cost and environmental-friendly. More importantly, it has been applied in the determination of Cu2+ and L-Met in water and urine samples, respectively with satisfactory recoveries. Furthermore, the approach could detect Cu2+ and L-Met with detection limit of 0.012 μM and 0.07 μM, which are lower than the level of Cu2+ in drinking water and of L-Met in human urine sample (maximum ~20 μM and ~5.9 μM, respectively).