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We have successfully developed a homogeneous colorimetric assay using DNA aptamer and unmodified gold nanoparticles (AuNPs) for sensitive and specific detection of oxytetracycline, one of most commonly used antibacterial agents.1 A highly specific DNA aptamer that bind to oxytetracycline with high affinity was employed to discriminate other structurally similar antibacterial agents in tetracycline's group, such as tetracycline and doxycycline. Oxytetracycline binding DNA aptamers protected the aggregation of AuNPs at high salt concentration by the adsorption on the surface of AuNPs in the absence of oxytetracycline.2 The aggregation of AuNPs was induced by desorption of oxytetracycline binding aptamers on the AuNPs as a result of the interaction between aptamers and oxytetracycline, leading to the color change from red to purple.
The color change of mixture was observed as low as 1 uM oxytetracycline with naked eyes, but other antibacterial agents didn't induced the color change. The detection limit of oxytetracyline was enhanced up to 25 nM by UV spectrometer analysis, which was 20-fold low than the limit USA-EPA regulated, with two orders of magnitudes in its linear dynamic range by successful optimization on the salt and AuNPs, and the molar ratio of aptamers to AuNPs. This colorimetric assay is advantageous over the other conventional methods in terms of its simple signal generation and detection with naked eyes, which can be realized in on-site detection of antibacterial oxytetracycline.