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A novel turn-on fluorescent sensor for the detection of ascorbic acid (AA) has been developed based on the fluorescence resonance energy transfer (FRET) between graphene quantum dots (GQDs) and squaric acid (SQA)-iron(iii). In this assay, iron(iii) can rapidly coordinate with the SQA to produce SQA-iron(iii). The absorbance band of SQA-iron(iii) could be largely overlapped with the emission band of GQDs, thus resulting in the fluorescence resonance energy transfer (FRET)-induced fluorescence quenching of GQDs. Moreover, the fluorescence of GQDs can be sensitively turned on by AA through the oxidation-reduction between iron(iii) and AA. After the transformation of SQA-iron(iii) to SQA-iron(ii), because the absorbance of the solution reduces, the FRET decreases. This FRET-based nanosensor shows high selective and sensitive response in the concentration of AA ranging from 1.0 to 95 μM with the detection limit as lower as 200 nM, which is lower than that of other fluorescent assays. Finally, the proposed sensing system was successfully applied to the direct analysis of AA in the real samples with satisfactory results.

In this contribution, DNA-templated silver nanoclusters (DNA-AgNCs) were successfully utilized as novel fluorescent probes for the detection of quinolones for the first time. The as-prepared DNA-AgNCs exhibited strong emission at 596 nm, which could be quenched by Cu2+ions. The addition of quinolones caused restoration of fluorescence intensity of the DNA-AgNCs–Cu2+system. This detection protocol exhibited excellent sensitivities for four kinds of quinolones. For instance, the linear ranges were from 20 to 100 nM for nalidixic acid with the limit of detection (LOD) 2.0 nM, from 5 to 120 nM for cinoxacin with the LOD 1.2 nM, from 5 to 100 nM for ciprofloxacin with the LOD 1.0 nM, and from 1 to 60 nM for moxifloxacin with the LOD 96 pM, respectively. Benefiting from high sensitivity, satisfactory recoveries were obtained for trace detection of quinolones in real samples, such as tablets and human urine.