Simultaneous dual detection of total chlorine in oil matrices via quantum dot-based LSPR

Abstract

Chlorine compounds including inorganic chloride ions and organic chlorine species are major precursors in the formation of 3-monochloropropane-1,2-diol (3-MCPD) during edible oil refining, raising significant food-safety concerns. This study presents a dual-mode localized surface plasmon resonance (LSPR) sensor integrating silver nanotriangles (AgNT) with succinyl-functionalized nitrogen-doped carbon quantum dots (succinyl-NCQD) for simultaneous detection of inorganic and organic chlorine in oil matrices. Hydrothermally synthesized NCQDs were modified with succinic anhydride to introduce amide (–CONH–) and carboxyl (–COOH) groups while retaining a fraction of reactive amines, as confirmed through FTIR, PL, and AFM analysis. Mechanistic evaluation shows that inorganic chloride ions interact via electrostatic pairing with protonated amines (–NH₃⁺), whereas organic chlorine represented by 1-chloro-2,4-dinitrobenzene (DNCB) is recognized through hydrogen bonding with succinyl groups and π–π stacking with NCQD sp² domains, followed by nucleophilic aromatic substitution (SNAr) forming a stabilized Meisenheimer intermediate. The AgNT–succinyl-NCQD sensor exhibited a sensitivity of 1.69 nm ppm⁻¹ for organic chlorine (0.5–5 ppm), with LOD = 0.56 ppm, LOQ = 1.85 ppm, and R² = 0.9068, while inorganic chloride was detected with 0.81 nm ppm⁻¹ sensitivity (0–10 ppm) and LOD = 2.08 ppm. Repeatability analysis showed good consistency (RSD = 4.97%) and selectivity evaluation yielded a selectivity factor of 3.3, indicating that the sensor's response to organic chlorine was more than three times higher than the strongest signal produced by any non-target interferent. Validation in edible oil samples confirmed clear and stable LSPR red-shift responses with minimal matrix interference. Compared to GC–MS and total chlorine analyzers (TCA) which require laborious sample preparation and cannot differentiate chlorine classes, this LSPR platform provides a rapid, label-free, and cost-effective approach for identifying both inorganic chloride and organic chlorine precursors. These advantages position the AgNT–succinyl-NCQD sensor as a promising early-warning tool for real-time monitoring and mitigation of 3-MCPD formation risks in the edible oil industry.