Carbon Quantum Dot-Based Label-Free Fluorescent Biosensor to Detect E. coli

We prepared carbon quantum dots (CQDs) by hydrothermal method using celery juice as carbon source. And the fluorescence of CQDs was burst by silver nanoparticles (Ag NPs) through inner filter effect (IFE), using Ag NPs as absorbers to construct label-free sensor. This strategy relies on the absorption spectra of Ag NPs overlapping with the fluorescence excitation or emission of fluorophores and the specific binding ability of $\textit {Escherichia coli (E. coli)}$ aptamers with target. The free aptamer sequence is first depleted to form complexes, leading to Ag NPs aggregation at high salt concentrations when adding E. coli to system. At this point, the absorption spectrum of Ag NPs changes and no longer overlaps with the fluorescence emission spectrum of CQDs, leading to a significant fluorescence recovery of the system. The excitation wavelength of the prepared CQDs was 348 nm and the maximum emission wavelength was 514 nm. The average particle size was about 8.84 nm with uniform dispersion. The linear range of this sensor was $2\times 10^{\mathbf {{2}}}\sim ~2\times 10^{\mathbf {{7}}}$ CFU $\cdot $ mL $^{\mathbf {-{1}}}$ , and the detection limit for E. coli was as low as 185 CFU $\cdot $ mL $^{\mathbf {-{1}}}$ .