Smartphone-based fluorescence detection of 2,4-dichlorophenoxyacetic acid using ImageJ analysis on a molecularly imprinted membrane/carbon-dot system <i>via</i> a Schiff-base method

The persistent presence of 2,4-dichlorophenoxyacetic acid (2,4-D) in aquatic ecosystems poses a significant risk to both the environment and human health. This study presents a novel fluorescence-based sensor designed to detect ultra-trace amounts of the pesticide 2,4-D. The sensor utilizes a dual-functional approach by combining nitrogen-doped carbon dots (N-CDs) and molecularly imprinted polymers (MIPs) integrated into a PVDF membrane. N-CDs were synthesized using a hydrothermal method with citric acid and ethylenediamine (EDA). The N-CDs were then modified with 3-aminopropyltriethoxysilane (APTES) to enhance their photoluminescence stability and their ability to recognize specific molecules. These functionalized nanomaterials were then covalently attached to a Piranha-activated polyvinylidene fluoride (PVDF) membrane. For the first time, this modified membrane was used as a substrate for 2,4-D molecular imprinting and exploited to reveal its physicochemical resilience, high porosity, and modifiable surface chemistry. A Schiff-base sol-gel imprinting approach enabled the fabrication of a robust molecularly imprinted polymer (MIP) layer with high affinity and selectivity toward 2,4-D. The dual-functional N-CDs simultaneously served as optical transducers and recognition elements. Fluorescence quenching, specifically in the context of binding 2,4-D, was detected using smartphone imaging and ImageJ analysis. The proposed sensor exhibited satisfactory linearity in the 2,4-D concentration range of 10-150 μg L^-1 with the limit of detection (LOD) being 1.97 μg L^-1 and the limit of quantification (LOQ) being 5.97 μg L^-1. This sensor also exhibited photostability for 23 days at 4 °C, recoveries ranging from 96% to 103% and RSD value of less than 6. This work presents a smartphone-compatible MIP/N-CDs membrane designed for the detection of 2,4-D in water samples.