Inner-filter effect of nitrogen-doped carbon quantum dots-MnO₂ nanotubes for smartphone-integrated dual-mode sensing...

Nitrogen-doped carbon quantum dots (N-CQDs) exhibit unique fluorescence properties and are considered one of the best candidates for the development of fluorescence-based sensors for the detection of many analytes. In this work, a smartphone-assisted fluorescent sensor has been developed using N-CQDs and MnO₂ nanotubes (MnO₂ NTs) for the detection of glutathione (GSH) and captopril (CAP). N-CQDs were facilely synthesized via the solvothermal method, where o-phenylenediamine (o-PD) and urea were used as nitrogen precursors. Likewise, MnO₂ NTs were synthesized using the hydrothermal method. Relying on the excellent fluorescence quenching ability of MnO₂ NTs, a nanocomposite of N-CQDs and MnO₂ NTs is prepared, wherein the fluorescence intensity of N-CQDs was effectively quenched in the presence of MnO₂ NTs via the inner-filter effect (IFE). The addition of thiolated compounds (GSH and CAP) helped in the recovery of the fluorescence of N-CQDs by triggering the redox reaction and decomposing the MnO₂ NTs. An investigation of fluorescence along with smartphone-based studies by evaluating the gray measurement using Image J software showed a great response towards GSH and CAP providing LODs of 4.70 μM and 5.22 μM (fluorometrically) and 5.76 μM and 2.81 μM (smartphone-based), respectively. The practical applicability of the sensing system has been verified using human blood plasma samples.