We reported a facile strategy to assemble a ratiometric nanosensor for the ovalbumin (OVA) fluorescence determination and meanwhile it can be utilized for selective visual identification by naked eyes with fluorescent test papers under 365 nm UV lamp. The nanosensor was prepared through simply mixing blue color carbon dots (CDs) and green color core-shell imprinted polymers. Blue CDs were used directly as the internal reference without participating in the imprinting process and modified molecularly imprinted polymers (MIPs) were synthesized by post-imprinting, using fluorescein isothiocyanate (FITC) as fluorescence enhanced signal. Upon the addition of different concentrations of OVA, the fluorescence intensity of FITC was enhanced, while the fluorescence intensity of CDs was almost unchanged, leading to a detection limit as low as 15.4 nM. Accordingly, the fluorescence color was gradually changed from blue to dark olive green to green with naked eyes observation. Moreover, the ratiometric nanosensor was successfully applied to detect OVA in the human urine samples with satisfactory recoveries attaining of 92.0-104.0% with relative standard deviation (RSD) of 3.3-3.9% and 93.3-101.0% with RSDs of 2.7-3.8% for the spiked chicken egg white samples. This strategy reported here opens a novel pathway for biomacromolecule detection in real applications and can realize the visual observation on fluorescent test papers.

Manganese-doped ZnS quantum dots (ZnS:Mn(II) QDs) were synthesized and modified with L-cysteine (L-Cys) and 6-mercaptonicotinic acid (MNA). This prevents the aggregation of the QDs and makes them available for the interaction with Cu(II) ions via Cu(II)-S interaction. As a result, the fluorescence of the QDs is quenched by Cu(II) due to an electron transfer mechanism. The QDs display two emission peaks under 325 nm excitation, one (being red) peaking at 593 nm, the other (blue) at 412 nm. The red fluorescence is strongly quenched, while the blue fluorescence is not affected. An easily distinguishable color change from orange red to purple can be observed in fluorescence as the concentration of Cu(II) is increased. The probe is selective over commonly encountered other ions. The ratio of fluorescence intensities at 593 and 412 nm increases linearly in the 5 to 500 nM Cu(II) concentration range, and the detection limit is 1.2 nM. Graphical abstract Schematic of the preparation of manganese-doped quantum dots functionalized with L-cysteine and 6-mercaptonicotinic acid for selective and sensitive visual detection of copper ions.

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