Turn-off/on fluorescent sensor for SARS-CoV-2 detection using waste-derived carbon dots

In this work, a fluorescence-based biosensor was developed for detecting SARS-CoV-2 oligonucleotide sequences using waste-derived carbon dots (CDs). The CDs, synthesized through a green hydrothermal route using sawdust as a carbon precursor, exhibited a high quantum yield of 35.9%. The fluorescence of CDs was effectively quenched by Fe³⁺ and subsequently recovered upon addition of the target ORF1ab-SARS-CoV-2 oligonucleotide. This behaviour was attributed to the stronger affinity of Fe³⁺ toward the negatively charged phosphate backbone of the viral sequence, which prevented quenching of the CDs. The system exhibited a linear response between 0.10 and 1.5 µM, with a correlation coefficient (R) of 0.9994, a limit of detection (LOD) of 0.0092 µM, and a limit of quantification (LOQ) of 0.031 µM. The sensor demonstrated excellent repeatability (RSD = 1.68%) and reproducibility across different days (RSD = 3.61%), along with selectivity against potential interferents, including various ions and biomolecules. Moreover, it showed selective behaviour toward the target sequence over other sequences related to either SARS-CoV-2 or SARS-CoV, with hybridization with complementary strands providing an additional layer of selectivity. Matrix effect studies confirmed that detection performance depends on pH and ionic strength, with optimal results obtained in near-neutral conditions (pH 7.4) and in media of moderate conductivity. These findings highlight the potential of Fe³ ⁺@CDs as a simple, cost-effective, and environmentally friendly fluorescent sensing platform for the rapid detection of viral oligonucleotides.