Green Synthesis of Sulfur- and Nitrogen-Doped Carbon Quantum Dots for Determination of L-DOPA Using Fluorescence Spectroscopy and a Smartphone-Based Fluorimeter

Sulfur- and nitrogen-doped carbon quantum dots (S,N-CQDs)weresynthesized using feijoa leaves as a green precursor via a novel route.Spectroscopic and microscopic methods such as X-ray photoelectronspectroscopy, fluorescence spectroscopy, and high-resolution transmissionelectron microscopy were used to analyze the synthesized materials.The blue emissive S,N-CQDs were applied for qualitative and quantitativedetermination of levodopa (L-DOPA) in aqueous environmental and realsamples. Human blood serum and urine were used as real samples withgood recovery of 98.4-104.6 and 97.3-104.3%, respectively.A smartphone-based fluorimeter device was employed as a novel anduser-friendly self-product device for pictorial determination of L-DOPA.Bacterial cellulose nanopaper (BC) was used as a substrate for S,N-CQDsto make an optical nanopaper-based sensor for L-DOPA determination.The S,N-CQDs demonstrated good selectivity and sensitivity. The interactionof L-DOPA with the functional groups of the S,N-CQDs via the photo-inducedelectron transfer (PET) mechanism quenched the fluorescence of S,N-CQDs.The PET process was studied using fluorescence lifetime decay, whichconfirmed the dynamic quenching of S,N-CQD fluorescence. The limitof detection (LOD) of S,N-CQDs in aqueous solution and the nanopaper-basedsensor was 0.45 mu M in the concentration range of 1-50 mu M and 31.05 mu M in the concentration range of 1-250 mu M, respectively.