Oxytetracycline (OTC) is a member of the most common broad-spectrum tetracycline (TCs) group of antibiotics. It has been widely used as veterinary drug and feed additive in livestock production for the treatment of infectious diseases and promote growth, due to its effective antimicrobial properties and low cost  . However, the inappropriate and illegal use of OTC has led to the OTC residues in some food and groundwater    . Long-term intake of food and drinking water containing OTC will cause serious threats to human health   . Therefore, there have been extensive efforts to develop the sensitive and selective system for OTC detection in food products and water.
To date, various analytical methods have been proposed for the detection of OTC, such as chromatography methods   , fluorescence method   , and electrochemistry   , etc. Though most of these methods have high sensitivity, they suffer from the disadvantages of high costs, time-consuming, complicated sample pretreatment and sophisticated instrument manipulation, which limit their application for rapid, on-site and real-time determination. Therefore, the development of a simple, rapid and cheap method for detecting OTC has become increasingly attractive.
Compared with some other analytical methods, colorimetric analysis is simple and can be read out by the naked eye without the aid of sophisticated instruments, thus realizing the visual on-site analysis     . There have been only a few reports about colorimetric OTC detection methods so far, which based on the formation of gold nanoparticles  or aggregation of gold nanoparticles   . However, these sensing systems required synthesis and modifying gold nanoparticles for OTC detection. Thus, it is still a great challenge to establish more convenient and reliable label-free colorimetric strategies for the sensitive OTC detection. The chromogenic reaction between 3,3’,5,5’-tetramethylbenzidine (TMB) and an oxidizing agent has been widely applied to the construction of label-free colorimetric sensing system   . It has been reported that some ion possessing ability to oxidise TMB, such as Fe3+,  Ag+,  ClO−,  and produce a blue oxidation product. In this work, it was observed that the addition of OTC could remarkably enhance the Fe(III)-oxidized TMB reaction, which provided a colorimetric method for OTC detection. To the best of our knowledge, this is the first report for detecting OTC based on the nonenzymatic colorimetric reaction using the intrinsic oxidized activity of OTC/Fe3+ complex. Particularly, the present approach has been successfully applied to determine OTC in honey samples.
2.1. Chemicals and Instrument
3,3’,5,5’-tetramethylbenzidine (TMB), OTC and other antibiotics were obtained from Aladdin Industrial Inc. (
2.2. OTC Detection
A stock solution of OTC (1 mM) was prepared in water and various concentrations of OTC were obtained by serial dilution of the stock solution. For the detection of OTC, 250 μM of TMB, 250 μM of FeCl3, and OTC with different concentrations were added sequentially in 1 mL HAc-NaAc buffer solutions (0.1 M, pH 3.5). After that, the mixture was vortex mixed thoroughly and transferred for UV-vis scanning after incubating for 5 min at 20˚C.
To elevate the selectivity of the proposed method, 250 μM of TMB, 250 Μm of FeCl3, and 2 μM of kanamycin, streptomycin, chloramphenicol, doxitard, penicillin, tetracycline, vancomycin, neomycin, and OTC were added sequentially in 1 mL of 0.1 M NaAc buffer solutions (pH 3.5). UV-vis spectroscopy was recorded after incubating for 5 min at 20˚C. Absorbance at 652 nm was used for quantitative analysis.
2.3. Pretreatment for the Analysis in Honey Samples
Honey samples were purchased from local supermarket and stored at room temperature before use. For removal of matrix effect, honey samples (1 g) were mixed with 5 mL of Mc Ilvaine buffer containing 20 mM EDTA (pH 5.0) and 0.5% (v/v) trifluoroacetic acid for 5 min using a vortex mixer, then homogenized by ultrasonic for 5 min, and centrifugated at 4˚C for 20 min at 4000 rpm. Then 4 M NaOH was added drop wisely to the supernatant to adjust the pH to 7.6. For the final test, the above sample was diluted 2000 times.
3. Results and Discussion
3.1. OTC Enhanced the Fe(III)-TMB Reaction
To demonstrate the feasibility of our approach, the TMB oxidation was first studied in different systems (Figure 1(a)). As illustrated in Figure 1(a), the Fe3+ can oxidize TMB to produce a blue color with a maximum absorbance at 652 nm (Figure 1(a)(2)). After the addition of OTC to Fe3+-TMB system, a dramatic
Figure 1. (a) UV-vis spectra of obtained solutions under different conditions (followed by bottom-up): TMB, TMB + OTC, TMB + Fe3+, and TMB + OTC + Fe3+ in pH 3.5 NaAc-HAc buffer at 20˚C. Inset: corresponding photographs. (b) Effect of Fe salts on absorbance change in the presence or absence of 20 μM OTC. The concentrations of Fe salts were 250 μM.
increase of absorbance is observed (Figure 1(a)(4)). However, OTC has no appreciable effect on the oxidation of TMB. Therefore, we speculate that a OTC・Fe3+ complex could be formed after the addition of OTC to Fe3+, which would enhance the Fe3+-TMB reaction. In order to optimize the reaction system, we have chosen different Fe salts to oxidize TMB (Figure 1(b)). It is found that FeSO4 cannot oxidize TMB, even after OTC was added into the Fe2+-TMB solution (Figure 2). Adding OTC to FeCl3-TMB or Fe(NO3)3-TMB mixtures lead to the close absorbance increase, but the ferric acetylacetonate (Fe(acac)3)-TMB system shows a remarkable decrease in peak absorbance after adding OTC (Figure 1(b)). Thus, FeCl3 was used as Fe3+ source in the following studies.
3.2. Optimization of Detection Conditions
To improve the sensitivity of the established colorimetric system, experimental conditions for OTC detection were carefully optimized at first. The reduction of absorbance (ΔA = A − A0) was used as a criterion to optimize the detection conditions, where A and A0 represent the absorbance at 652 nm in the presence and absence of OTC, respectively.
3.2.1. Effect of pH
pH is an crucial factor for almost every sensing system. The pH effect for OTC detection was firstly studied as shown in Figure 3(a). It is shown that ΔA increase with increasing pH value in the range from 2.0 to 3.5. Further increase of pH value results in the decrease of ΔA. Therefore, pH 3.5 was adopted in the following experiments.
3.2.2. Effect of Reaction Temperature and Reaction Time
Figure 3(b) and Figure 3(c) show the effect of reaction temperature and reaction time on the Fe3+-TMB sensing system. Different incubation times ranging from 5 min to 30 min and temperature ranging from 20˚C to 50˚C were investigated to
Figure 2. UV-vis spectra of 3,3’,5,5’-tetramethylbenzidine solution with diffreent Fe salts in the presence or absence of 20 μM oxytetracycline (OTC). The concentrations of Fe salts were 250 μM.
Figure 3. Effect of pH (a), reaction temperature (b), reaction time (c), Fe3+ concentration (d) and TMB concentration (e) on the ΔA. (f) is the effect of metal ions on absorbance in the presence or absence of OTC.
study their effects on the ΔA. The results indicated that the highest ΔA was obtained at 5 min when the reaction temperature was fixed at 20˚C.
3.2.3. Effect of Fe3+ and TMB Concentrations
The effects of TMB and Fe3+ concentrations on the ΔA were displayed in Figure 3(d) and Figure 3(e), respectively. The experiments show that the maximum of ΔA was obtained when the TMB concentration was fixed at 250 μM and the Fe3+ concentration was 250 μM.
3.2.4. Effect of Different Metal Ions
An experiment was also carried out to examine the effect of different metal ions on OTC detection. According to the comparison of absorbance with or without 0.5 μM OTC under different 50 μM metal ions (Figure 3(f)), Fe3+ shows the best catalytic activity towards the TMB solution to produce an absorbance increase and the other metal ions have no obvious catalytic activities. This result indicates that Fe3+ is the favored metal ion for constructing OTC sensing system.
3.3. Colorimetric Detection for OTC
As shown in Figure 4(a), the ΔA gradually increased with increasing OTC concentration and a good linear relationship was is found between ΔA and OTC concentration under the optimum conditions. The linear correlation of ΔA = 0.023 + 0.469 × C [OTC] (R2 = 0.985) was obtained over the tested concentration range of OTC from 20 - 1000 nM. The limit of detection (LOD) was calculated to be 7.97 nM (3.96 μg/L) at a signal-to-noise ratio of 3. The LOD is better than those of some reported methods,    such as colorimetric aptasensor (25 nM)  , electrochemiluminescence sensor (0.1 μM)  , colorimetric detection (0.0838 μg/mL)  , and aptasensor (12.3 μg/L)  .
3.4. Selectivity of Sensor
To further examine the selectivity of this proposed sensor for OTC, the responses of system to eight other antibiotics including kanamycin, streptomycin, chloramphenicol, doxitard (DOX), penicillin, tetracycline (TC), oxytetracycline, vancomycin, and neomycin were studied. With 20 μM analytes, OTC causes the maximum ΔA in the sensing system as shown in Figure 5. However, the addition of TC and DOX also leads to slightly increase of absorbance comparing with other antibiotics. We assume that TC, DOX and OTC are antibiotics of the tetracycline group and have similar structures. The results demonstrate that our sensor is highly selective toward OTC over other classes of antibiotics.
3.5. OTC Detection in Honey Sample
To investigate whether the colorimetric method can be used in real samples analysis, the OTC detection in honey sample was studied (Table 1). It can be seen that the recoveries vary from 89.93% to 100.02%, indicating the proposed colorimetric assay is highly reproducible and accurate for rapid detection of OTC.
In summary, based on the Fe3+-TMB system, a facile and rapid colorimetric
Figure 4. Dependence of the absorbance change at 652 nm, temperature 20˚C and pH 3.5 on the concentration of OTC (a) and the corresponding linear calibration plots (b).
Table 1. Colorimetric determination of OTC in honey samples (n = 3) by Fe(III)-TMB) system.
Figure 5. The ΔA response of the sensing system toward OTC and other antibiotics.
sensing system for OTC detection was successfully fabricated. The linear dynamic range for OTC was found from 20 nM to 1000 nM with the detection limit of 7.97 nM. The sensor for OTC detection showed high sensitivity under optimal conditions. When used the system to honey sample, recoveries varying from 89.93% to 100.02% were obtained. This assay is lowcost and convenient. Furthermore, the proposed method is promising for the analysis of OTC in foods.
This work was financially supported by the National Natural Science Foundation of China (21878134), the Natural Science Foundation of Jiangsu Province (BK20161363) and the Jiangsu Postdoctoral Foundation (1601091B).