Determination of chlorpyrifos residues by needle filter solid phase microextraction-enzyme inhibition-digital image colorimetry

Abstract: Pesticides have been widely used to ensure the yield and quality of crops in modern agriculture, such as insecticides, herbicides, fungicides, and plant growth regulators. Among them, chlorpyrifos can be one of the best-selling organophosphorus pesticides in the world. However, the irrational use of chlorpyrifos can lead to excessive pesticide residues in food products, event harm to human health. The persistence and toxicity of chlorpyrifos have received ever-increasing attention in recent years. Furthermore, chlorpyrifos can be restricted or banned in a few countries, due to the residue risks. Therefore, the high-enrichment factor needle filter solid-phase microextraction and a highly sensitive enzyme inhibition-digital image colorimetry have been used for the rapid detection of chlorpyrifos residues in food samples, in order to avoid the traditional cumbersome steps and low sensitivity. However, there are still some challenges in both the pre-treatment and detection. In the pre-treatment process, the carbon materials as the solid phase adsorbents were added to a needle filter, and then the enrichment of chlorpyrifos in the food samples was achieved to repeat pumping of the needle filter by a syringe. The needle filter solid phase microextraction has the advantages of less adsorbent, less solvent, shorter time, and larger enrichment factors, compared with traditional solid phase extraction. The carbon material type and sample volume were optimized to achieve better recovery. In the detection, the tyrosinase can be used to oxidize the aminophenol and ferric chloride system into the yellow color. But chlorpyrifos can inhibit the activity of tyrosinase, even indirectly affect the color of the aminophenol and ferric chloride system. Particularly, digital image colorimetry can be used to analyze the color data after capturing the picture with a smartphone. The linear relationship can be quantitatively analyzed between chlorpyrifos concentration and green intensity. In this study, a highly efficient tyrosinase was selected, instead of acetylcholinesterase in the traditional enzyme inhibition. Digital image colorimetry was achieved in the qualitative and quantitative analysis, compared with traditional colorimetry. A better sensitivity was obtained to optimize the aminophenol concentration, tyrosinase concentration, reaction time, and incubation time. The results showed that the better linearity of chlorpyrifos was in the range of 0.005-0.1 mg/L with the determination coefficient (R2) ≥ 0.99. The limit of detection using a signal-to-noise ratio of 3 was 0.001 5 mg/L in mineral water, tea, and fruit juice. The limit of quantification using a signal-to-noise ratio of 10 was 0.005 mg/L in the mineral water, tea, and fruit juice. The spiked recoveries in the samples were 86.6%-92.5% with a relative standard deviation of 3.2%-6.9%, indicating higher accuracy and precision. The further application can be expected in real sample analysis. Consequently, the developed pre-treatment can be applied to extract different hydrophobic pesticides in food samples, together with the different organophosphorus and carbamate pesticides, indicating better inhibitory effects on tyrosinase. In conclusion, the finding can provide a strong reference to develop rapid pretreatment and detection techniques for pesticides.