Sensitive, selective, and pretreatment-free detection of ferric ions in different solvents based on organic-soluble carbon dots

Abstract

The presence of ferric ion (Fe³⁺) impurities can reduce the stability and efficiency of organic solvents, which can significantly impact the production of chemicals, food, medical supplies, and daily necessities. Herein we describe the synthesis of organic-soluble carbon dots (CDs) and their first application for Fe³⁺ detection in various organic solvents without requiring pretreatment procedures. The CA-CDs were prepared via a solvothermal method using caffeic acid (CA) as a precursor and anhydrous ethanol as the solvent. The presence of Fe³⁺ triggered changes in the fluorescence intensity of CA-CDs and exhibited good linearity in five protonic and non-protonic solvents with different polarities, including ethanol (3.0–50 μM, R² = 0.9915), methanol (5.0–50 μM, R² = 0.9903), ethyl acetate (5.0–50 μM, R² = 0.9945), acetonitrile (6.0–50 μM, R² = 0.9998), and dichloromethane (4.0–40 μM, R² = 0.9940). The corresponding detection limits were 0.96, 1.66, 1.54, 1.73, and 1.19 μM, respectively. Owing to the formation of an iron hydroxyl complex, CA-CDs demonstrated high selectivity towards Fe³⁺ over other potentially interfering metal ions in both pure solvents and solvents containing 1% (v/v) water. The accuracy of CA-CDs was validated by comparison with results from the Inductively Coupled Plasma-Optical Emission Spectrometer method. With the above outstanding properties, the proposed CA-CDs were successfully employed for Fe³⁺ quantification in automotive ethanol gasoline with a detection limit of 2.82 μM. Compared to the contamination and errors associated with sample pretreatment in most conventional assays, the CA-CD-based platform offers low-cost, high sensitivity, selectivity, operational simplicity, and contamination-free detection.