We proposed a facile method to prepare the nitrogen-doped graphene quantum dots (NGQDs) doped silica (NGQDs@SiO2) nanoparticles (NPs). The NGQDs@SiO2 NPs were further explored as a versatile signal indicator for ochratoxin A (OTA) aptasensing by combination with electrochemiluminescence (ECL) and fluorescence (FL) detection. In this strategy, the core-shell Fe3O4@Au magnetic beads (MBs) acted as a nanocarrier to immobilize the thiolated aptamer specific for OTA, and the amino modified capture DNA (cDNA) was efficiently tagged with NGQDs@SiO2 NPs. The multifunctional aptasensor was thus fabricated by assembly of the NGQDs@SiO2 NPs onto the surface of Fe3O4@Au MBs through the high specific DNA hybridization between aptamer and cDNA. Upon OTA incubation, the aptamer linked with Fe3O4@Au MBs preferred to form an aptamer-OTA complex, which resulted in the partial release of the preloaded NGQDs@SiO2 NPs. The more OTA molecules in the detection system, the more NGQDs@SiO2 NPs were released into the bulk solution and the less preloaded NGQDs@SiO2 NPs were accumulated on the magnetic electrode surface. This provided a dual channel for OTA detection by combination with the enriched solid-state ECL and homogeneous FL detection. The FL assay exhibits a wide dynamic range and is more reproducible due to the homogeneous detection while the ECL assay possesses a lower detection limit and is preferable by using a cheaper instrument. One can obtain a preliminary screen from FL assay and a more accurate result from ECL assay. Integrating the virtues of dual analytical modality, this aptasensing strategy well-balanced the rapidity, sensitivity, and dynamic range, making it promising to other targets with aptamer sequences.

A color change observable by the naked eye to indicate the content of an analyte is considered to be the most conceivable way of various sensing protocols. By taking advantage of the Förster resonance energy transfer (FRET) principles, we herein designed a dual-emission ratiometric fluorescent aptasensor for ochratoxin A (OTA) detection via a dual mode of fluorescent sensing and onsite visual screening. Amino group-modified OTA's aptamer was firstly labeled with the green-emitting CdTe quantum dots (gQDs) donor. The red-emitting CdTe QDs (rQDs) which were wrapped in the silica sphere could serve as the reference signal, while the gold nanoparticle (AuNP) acceptors were attached on the silica surface to bind with the thiolated complementary DNA (cDNA). The hybridization reaction between the aptamer and the cDNA brought gQD-AuNP pair close enough, thereby making the FRET occur in the aptasensor fabrication, while the subsequent fluorescence recovery induced by OTA was obtained in the detection procedure. Based on the red background of the wrapped rQDs, the aptasensor in response to increasing OTA displayed a distinguishable color change from red to yellow-green, which could be conveniently readout in solution even by the naked eye. Since the bioconjugations used as the aptasensor can be produced at large scale, this method can be used for in situ, rapid, or high-throughput OTA detection after only an incubation step in a homogeneous mode. We believe that this novel aptasensing strategy provides not only a promising method for OTA detection but also a universal model for detecting diverse targets by changing the corresponding aptamer.

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