Sulfuric acid-mediated PET-derived carbon quantum dots for smartphone-assisted Fe³⁺ screening in biological samples

Recycling reduces environmental pollution, conserves natural resources, and minimizes waste, while monitoring ferric ions is important for evaluating iron-related biological and environmental conditions. The present work addresses two important challenges: recycling plastic waste and developing a simple sensing platform for Fe³⁺ detection. In this study, post-consumer PET bottles were converted into highly fluorescent carbon quantum dots (PET-CQDs) with an average diameter of 5 ± 2 nm for the detection of ferric ions (Fe³⁺). The PET-CQDs exhibited Fe³⁺ detection through an ON-OFF fluorescence sensing platform. A linear range of 10-100 µM was achieved, with a correlation coefficient of R ² ≥ 0.9886. The detection and quantification limits were 3.66 µM and 11.1 µM, respectively. Additionally, a smartphone-based visual detection method achieved a linear range of 10-70 µM for Fe³⁺, with a correlation coefficient of R ² ≥ 0.9877. Using smartphone-assisted detection, the detection and quantification limits were recorded as 4.4 µM and 13.4 µM, respectively. The ON-OFF sensing platform was further examined in a deproteinized human serum as a proof-of-concept matrix study, showing that the PET-CQDs retained Fe³⁺-responsive fluorescence behavior under the tested conditions. Furthermore, the greenness of the proposed method was assessed using various green metrics, including ComplexMoGAPI, AGREEprep, and BAGI tools.