Microfluidic synthesis of carbon quantum dots and detection of iron ions using paper-based chip devices

Accurate detection of iron ions (Fe 3+ ) is crucial for environmental and health monitoring. Traditional methods mainly rely on larger and complex instruments, making it difficult to meet the demand for rapid or on-site analysis. This study aims to develop a novel method integrating microfluidic synthesis, paper-based sensing, and smartphone-based detection. Through simulation and experimental verification, a three-dimensional micromixer capable of inducing vortex flow was studied. Based on orthogonal experiments in the microfluidic system, the synthesis process of nitrogen-doped carbon quantum dots (N-CQDs) was optimized, increasing the fluorescence quantum yield to 44.2%. For the detection of Fe 3+ , a microfluidic paper-based analytical device (μPAD) was designed and fabricated using a commonly available laser printer, with the N-CQDs immobilized as the probes afterwards. By integrating a portable device with a self-developed mobile phone application, quantitative Fe 3+ analysis was achieved within minutes. The method exhibits a linear detection range of 0.01–3 mg/L, a limit of detection of 8 μg/L and excellent selectivity for Fe 3+ , with recovery rates of 98.0%–110.0% in spiked recovery experiments for water and serum samples. This work provides a feasible solution for on-site and low-cost detection of heavy metal ions in environmental monitoring and point-of-care diagnostics.