It is challenging to obtain a highly sensitive enzyme-linked sorbent immunoassay (ELISA) method for the rapid screening of veterinary drug residue in animal tissues. Here we present that a simple and sensitive detection for sulfadimethoxine (SDM) residue in animal tissues was achieved by utilizing a bioconjugate of gold nanoparticles and enzyme-labeled antibody as signal probe in enzyme-linked sorbent immunoassay (ELISA). The developed nano-ELISA has increased the sensitivity of a traditional ELISA for SDM by 20-fold. The sensitivity of this ELISA was 5？pg/mL in buffer, and the detection limit (LOD) of 0.2？μg/kg can be obtained after chicken liver was simply extracted by buffer. This simple and sensitive method can be used to improve the sensitivity of ELISA methods for various small molecule contaminants. 1. Introduction Sulfonamides are widely used to treat bacterial and protozoan infections in food animals. Their pharmacological activity is due to their ability to mimic p-aminobenzoic acid (PABA) and to inhibit the early stages of folic acid synthesis in bacteria and in various protozoans [1]. However, sulfonamides can be accumulated in the body of people who consumed products from those excessively drug-treated animals, resulting in the development of antibiotic-resistant pathogenic bacteria. To minimize this risk, a maximum residue limit of 0.1？mg？kg？1 has been established for total or individual sulfonamides in food samples, including meat and milk, by Codex Alimentarius Commission in European and American countries [2]. Sulfadimethoxine (SDM) is a typical example of sulfonamides which are widely used antibiotics worldwide. Many cases about administrating SDM excessively have been reported for the prevention and treatment of infections in chicken, swine, and cattle [2–5]. Conventional residue methods for the detection of sulfonamides in animal tissues include bioassays, thin layer chromatography (TLC), high-performance liquid chromatography (HPLC), and liquid chromatography-tandem mass spectrometry (LC-MS/MS) [1, 5–8]. In an effort to increase both method sensitivity and sample throughput, enzyme-linked immunosorbent assays (ELISAs) have been developed for several of the sulfonamides [1, 9–11]. Although high sensitivity has been achieved in the above-mentioned immunoassays, accomplishing the detection of veterinary drug residue in animal tissues simply and sensitively by ELISA method is still challenging due to serious matrix interference [12, 13]. Nevertheless, developing highly sensitive ELISA method can avoid this problem by diluting the sample
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