Bifunctional manganese-doped silicon quantum dot-responsive smartphone-integrated paper sensor for visual multicolor/multifluorescence dual-mode detection of nitrite

Considering great threats of nitrite in food toward public health, it is significant to develop portable, dependable, and cost-effective methods to detect the target. In comparison with traditional colorimetric or fluorometric strategies relying on the intensity variation at a single wavelength, multimodal measurement with notable color changes is a more attractive paradigm because of its stronger resistance against external conditions, expanded application scenario, improved reliability, and naked-eye distinguishable readouts. However, it remains a challenge on how to attain this goal. Here we proposed a smartphone-integrated paper sensor based on manganese-doped silicon quantum dots (Mn-SiQDs) for the visual dual-mode multicolor/multifluorescence determination of nitrite. The Mn-SiQDs featured both photoluminescence and oxidase-mimetic catalytic activity, and could stimulate the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation to a blue species TMB+. When nitrite existed, the generated TMB+ selectively reacted with the analyte to produce yellow diazotized TMB+. Meanwhile, the photoluminescence of Mn-SiQDs could be suppressed by the diazotized TMB+ via inner filter effect. As a result, with a facile electrostatic assembly of Mn-SiQDs and [Ru(bpy)3]2+, the formed complex Mn-SiQDs/[Ru(bpy)3]2+ led to the variations of both color and photoluminescence upon different levels of nitrite. By combining the principle with paper strips and smartphone sensing, the fabricated sensor offered naked-eye visible multicolor/multifluorescence dual-mode response toward nitrite, exhibiting excellent specificity, performance stability, and practicability in food analysis. Our work provides an effective tool to achieve the multicolor/multifluorescence bimodal measurement of nitrite, and it will inspire future effort on designing versatile materials for advanced sensing applications.