Development of aptasensors for the determination of DDT in milk and dairy products

Milk and dairy products are fundamental components of human nutrition and, due to their widespread consumption, must be carefully evaluated in terms of food safety. However, the persistence of pesticides used in agricultural production, their ability to be transported over long distances, and their accumulation in fatty tissues increase the risk of residues in dairy products. Among these risk groups, one of the most concerning is organochlorine pesticides. These compounds pose a significant threat to public health due to their bioaccumulation, biomagnification, and toxicological effects. Reliable determination of organochlorine pesticides in milk and dairy products is essential for ensuring food safety and monitoring compliance with legal limits. Although conventional analytical methods (GC-MS, HPLC, LC-MS/MS, etc.) provide high accuracy, they present limitations in routine analyses because of their cost, long analysis time, requirement for specialized personnel, and complex sample preparation steps. Therefore, there is a need for faster, more selective, cost-effective, and on-site applicable alternative detection platforms. Recent advances in biosensor technology have highlighted aptasensors, particularly due to their high specificity, stability, ease of synthesis, and modifiability. Aptamers, which are oligonucleotide-based molecules, can form unique three-dimensional structures specific to target molecules and are considered strong alternatives to antibodies. Aptasensor-based systems offer rapid and selective analytical capabilities even in complex matrices such as milk. In this thesis study, a novel electrochemical aptasensor was developed for the reliable, rapid, and sensitive determination of DDT residues—one of the organochlorine pesticides found in milk and dairy products. For this purpose, a single-stranded DNA aptamer previously selected specifically for DDT through the Systematic Evolution of Ligands by Exponential Enrichment (SELEX) method was immobilized onto a gold electrode surface as reported in the literature. The performance of the aptasensor was monitored by optimizing various parameters including aptamer incubation times, blocking durations with milk, and interaction times with DDT-containing milk samples. The aptasensor response was monitored using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results demonstrated that the most distinct sensor response was obtained after 2 hours of aptamer immobilization, followed by 2 hours of blocking and 1 hour of interaction with DDT-containing milk. The aptasensor was capable of detecting DDT at levels below the maximum residue limit (0.04 mg/kg milk) specified in the Turkish Food Safety Regulation.