A graphene quantum dots based fluorescent sensor for anthrax biomarker detection and its size dependence

Graphene quantum dots (GQDs) with two different diameters were modified with a Eu^III-macromolecule complex and applied in dual emission fluorescent sensors for detection of Bacillus anthracis spores. The Eu-GQD sensors exhibited a morphology of ultrafine particles, increased surface-to-volume ratio, enhanced dispersibility, and extraordinary sensitivity. The 3 nm Eu-GQDs showed three emission bands, which are ascribed to the emission from the blue GQDs (435 nm) and the red [(Eu)-(DPA)] complex (593 nm and 616 nm). Accordingly, incorporation of the GQDs as a non-interfering internal calibration makes it possible for use as a ratiometric sensor. The time dependent fluorescence response study revealed that the reaction was complete within 8 s, thus enabling rapid detection of B. anthracis spores. It is noteworthy that the Eu-GQD sensors exhibited an extraordinary limit of detection (LOD) of ca. 10 pM towards B. anthracis, which is six orders of magnitude smaller than the infectious dose of the spores (60 μM). Furthermore, the selectivity study indicates that Eu-GQD sensors have an outstanding selectivity of 103-fold for DPA over competing aromatic ligands.