Introduction

Chloramine-T, also known as sodium N-chloro-4-methylbenzenesulfonamide, is a byproduct of saccharin fabrication. It is cheap, easy to handle and water-tolerant [1–5]. It plays very important roles in chemical engineering, environmental industries, as well as food and clinical medicine. For example, chloramine-T is disinfectant for external use, and is suitable for the disinfection of tableware, drinking water, vegetables, etc. [6]. It is used as a bleach and oxidative desizing agent in dyeing and printing industry [4]. Chloramine-T has been used for the detection of sulfonamides in pharmaceutical industry and tin [7], the preparation of indicator, and as chlorine reagent in the analysis work of laboratory [1, 3–5]. Obviously, the developments of detection methods for chloramine-T are of broad interest in synthetic chemistry and environmental sciences [1–5].

Many analytical methods for detecting chloramine-T in foodstuff, pharmaceutical, environmental and veterinary samples have been reported. These include cyclic voltammetry [2], differential pulse voltammetry [6], flow injection biamperometry [8], liquid chromatography combined with mass spectrometry [9], spectrophotometry [10]. Either expensive equipments and maintenance and time-consuming sample preparations are necessary or the detection limits are not low enough for most of these methods.

We find that chloramine-T can oxidize p-aminophenylboronic acid to p-QI. Since p-QI is a well-known compound that can be easily measured with excellent sensitivity and selectivity by amperometric method [11–14]. Herein, we report a novel strategy for the detection of chloramine-T at a low potential based on the oxidation of p-aminophenylboronic acid by chloramine-T to generate easily detectable p-QI. Figure 1 shows the detection scheme. Chloramine-T oxidizes p-aminophenylboronic acid to yield p-aminophenol. In contrast to hydrogen peroxide which cannot oxidize p-aminophenol to generate p-QI [15, 16], chloramine-T can further oxidize p-aminophenol to generate p-QI [17–22] because of its stronger oxidation capability. Finally, chloramine-T is measured through the detection of the reduction of p-QI at a low potential. The excellent advantage of this method is high sensitivity, wide linear range, mild reaction conditions and operational simplicity.

Fig. 1
figure 1

Schematic mechanism of electrochemical detection of chloramine-T by reaction between chloramine-T and p-aminophenylboronic acid

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Experimental

Chemicals and reagents

p-Aminophenylboronic acid was obtained from Energy Chemical Co., Ltd., Shanghai. (http://www.energy-chemical.com.cn/). Chloramine-T, hydrogen peroxide, sodium chlorate, sodium perchlorate, ammonium sulfate, p-benzoquinone and bromine were purchased from Sinopharm Chemical Reagent Co., Ltd. (http://www.sinoreagent.com/). Chloramine-T medical powder was bought from Tianjin Bo Di Co., Ltd. (http://bdhg.company.lookchem.cn/). Sodium hypochlorite was purchased Aladdin (http://www.aladdin-e.com/). Iodine and potassium iodate were purchased from Beijing Chemical Works. (http://www.pvc123.com/b-beijinghuagong/). Artemisinine was purchased from TCI (Shanghai) Development Co., Ltd. (http://www.tcichemicals.com/zh/cn/). Other chemicals were all of analytical-reagent grade. The double distilled water was used throughout all the electrochemical experiments. All the experiments were carried out at room temperature conditions.

Instruments

A CHI 830B electrochemical Workstation (Shanghai Chenhua, China, http://chi.instrument.com.cn) was employed to carry out electrochemical experiments. The conventional three-electrode cell consisted of glassy carbon working electrode, a gold wire counter electrode and an Ag/AgCl reference electrode (saturated KCl). The working electrode was polished with alumina powder (Al2O3, 0.3 μm, 0.05 μm), sonicated and cleaned with doubly distilled water before measurements.

Procedure for the determination of chloramine-T

Chloramine-T was detected by amperometry. Firstly, the working electrode, counter electrode, and reference electrode were immersed in 5 mL of 0.1 M pH 5.5 acetate buffer containing 400 μM p-aminophenylboronic acid. Then the amperometric detection of chloramine-T was performed in stirring solution at the potential of 0 V by the successive addition of given volume of high concentrations of chloramine-T every 30 s. Specifically, 2.5, 2.5, 2.5, 2.5, 10, 10, 10, and 10 μL of 100 μM chloramine-T were injected sequently to the stirring solution for the detection of 50 nM, 100 nM, 150 nM, 200 nM, 400 nM, 600 nM, 800 nM, 1 μM chloramine-T, respectively. And then 1, 2, 2, 5, 5, 5, 5, 5, 5, 5, 5, and 5 μL of 10 mM chloramine-T were injected consecutively to the stirring solution for the detection of 2 μM, 6 μM, 10 μM, 20 μM, 30 μM, 40 μM, 50 μM, 60 μM, 70 μM, 80 μM, 90 μM, and 100 μM of chloramine-T, respectively.

Detection of chloramine-T in pharmaceutical samples

400 μL of 5.0 mM p-aminophenylboronic acid, 10 μL of 5.0 mM medical chloramine-T sample and 40 μL the double distilled water were pipetted into 4.55 L of 0.1 M acetate buffer (pH 5.5) , vortex-mixed and used for amperometry detection. To detect recoveries, a given amount of standard chloramine-T solution was added into the resulting solutions and used for amperometry measurements. The experiments were performed in triplicate.

Detection of chloramine-T in water samples

400 μL of 5.0 mM p-aminophenylboronic acid and 50 μL of tap water were pipetted into 4.55 L of 0.1 M acetate buffer (pH 5.5) , vortex-mixed and used for amperometry detection. To detect recoveries, a given amount of standard chloramine-T solution was added into tap water samples and used for amperometry measurements. The experiments were performed in triplicate. Similarly, the concentration of chloramine-T in lake water samples and the recoveries were determined by amperometric according to the procedures as mentioned above.

Results and discussion

Cyclic voltammograms of p-aminophenylboronic acid in the absence and presence of chloramine-T

Figure 2a and b shows cyclic voltammograms of 1 mM p-aminophenylboronic acid and 10 μM chloramine-T in 0.1 M pH 5.0 acetate buffer solutions, respectively. No redox peak is observed for either p-aminophenylboronic acid or chloramine-T. A pair of well-defined redox peaks around 0 V appears when chloramine-T is added into p-aminophenylboronic acid solution. The potential differences between cathodic peak and anodic peak were determined to be 33 mV, which shows the good reversible feature of this system. The redox peaks result from p-QI which is generated from the oxidation of p-aminophenylboronic acid by chloramine-T.

Fig. 2
figure 2

Cyclic voltammograms (a) 1 mM p-aminophenylboronic acid (b) 10 μM chloramine-T (c) 1 mM p-aminophenylboronic acid and 10 μM chloramine-T. Supporting electrolyte, 0.1 M pH 5.0 acetate buffer solutions. Reaction time: 5.0 min. Scan rate: 100 mV⋅s−1

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Choice of the method and optimization of method

Amperometry, cyclic voltammetry, linear sweep voltammetry, differential pulse voltammetry and square wave voltammetry are frequently used as the electrochemical detection methods. In comparison with other electrochemical methods in which potentials are scanned during measurements, amperometry use a constant potential. By using a suitable potential, it is possible to achieve high selectivity with amperometry. Therefore, amperometry is selected for the detection of chloramine-T.

The following parameters were optimized: (a) Sample pH value; (b) operating potential; (c) reaction time; (d) p-aminophenylboronic acid concentration. Respective data and Figures are given in the Electronic Supporting Material. We found the following experimental conditions to give best results: (a) A sample pH value of 5.5; (b) an operating potential of 0 V; (c) a reaction time of 90 s; (d) a p-aminophenylboronic acid concentration of 400 μM.

Electrochemical detection of chloramine-T

Figure 3a displays a typical current-time plots [23, 24] at the bare glassy carbon electrode at the potential of 0 V upon the successive addition of chloramine-T into 0.1 M (pH = 5.5) acetate buffer solution containing 400 μM p-aminophenylboronic acid. The currents increase with increasing chloramine-T concentrations. As shown in Fig. 3b and c, two linear ranges between chloramine-T concentrations and currents (from 50 nM to 30 μM and from 40 μM to 100 μM) are obtained. The detection limit is 6 nM. A comparison of several analytical parameters of the method with those of previously reported papers is summarized in Table 1. It can be observed that the dynamic range of the method is wider and the detection limit of the method is about three orders of magnitude lower than that of other published electrochemical methods [2, 6, 8]. However, the present method needs the addition of p-aminophenylboronic acid which is more expensive than potassium iodide used in previous studies [8]. Considering its high sensitivity, the present electrochemical approach is a promising detection method for chloramine-T.

Fig. 3
figure 3

a Current-time response obtained at glassy carbon electrode upon successive addition of different concentrations of chloramine-T under stirring condition, 2 μM, 6 μM, 10 μM, 20 μM, 30 μM, 40 μM, 50 μM, 60 μM, 70 μM, 80 μM, 90 μM, and 100 μM, respectively. (Inset, 50 nM, 100 nM, 150 nM, 200 nM, 400 nM, 600 nM, 800 nM, and 1 μM, respectively.) b Corresponding calibration plot from 50 nM to 30 μM (Inset, enlarged plot from 50 nM to 1 μM ) and (c) Corresponding calibration plot from 40 μM to 100 μM. Applied potential, 0 V; 0.1 M pH 5.5 acetate buffer solutions containing 400 μM p-aminophenylboronic acid

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Table 1 Comparison of determination of chloramine-T with published methods
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Interference analysis and sample detection

An interference investigation was performed with the solution containing 100 μM of species, including hydrogen peroxide, sodium hypochlorite, sodium chlorate, sodium perchlorate, bromine, iodine, potassium iodate, ammonium sulfate, artemisinin and p-benzoquinone. As shown in Fig. 4, a remarkable signal increment was observed in the presence of chloramine-T. In contrast, the currents of hydrogen peroxide, sodium hypochlorite, sodium chlorate, sodium perchlorate, bromine, iodine, potassium iodate, ammonium sulfate, artemisinin and p-benzoquinone are nearly negligible except the current of p-benzoquinone. The above results suggest the excellent selectivity of our method for the determination of chloramine-T. Some compounds investigated react with p-aminophenylboronic acid, but do not generate p-quinone imine, and some may not react with p-aminophenylboronic acid (e.g. sodium perchlorate), which may result in good selectivity.

Fig. 4
figure 4

Selectivity for the detection of chloramine-T against different interfering species. a hydrogen peroxide, b sodium hypochlorite, c sodium chlorate, d sodium perchlorate, e bromine, f iodine, g potassium iodate, h ammonium sulfate, i artemisinin, j p-benzoquinone, k chloramine-T. The concentrations of all species are 100 μM. Applied potential, 0 V; 0.1 M pH 5.5 acetate buffer solutions containing 400 μM p-aminophenylboronic acid

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To test its feasibility for practical applications, the present method was used to detect chloramine-T recoveries in commercially available pharmaceutical samples, lake water samples and tap water samples (in Table 2). Favorable recoveries are obtained. The results show that this study offers a promising method for the measurement of chloramine-T in real samples.

Table 2 Determination of chloramine-T in pharmaceutical and water samples
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Conclusion

A novel amperometric method based on the oxidation of p-aminophenylboronic acid by chloramine-T was developed for the determination of chloramine-T. The method is more sensitive than other electrochemical analysis methods by about three orders of magnitude. The method does not need the modification of electrode. The simple method displays satisfactory analytical performance such as outstanding sensitivity, excellent selectivity, wide linear range and low detection limit. This method may be extended to detect other oxidants.