A Composite Catalytic Oxidation-fluorescence Sensing System for 2,4-dichlorophenol Analysis based on Fe(III)PcTs-BuOOH-CdTe QDs

TONG Yilin, ZHANG Yu, YU Kan＊, BAO Jiaqi, YIN Juanjuan, ZENG Zhihong

(1.Information Science and Technology Department, Wenhua College, Wuhan 430074, China; 2. Hubei Institute of Measurement and Testing Technology, Wuhan 430223, China)

Abstract: The active oxygen species in the catalytic oxidation system of Fe(III)PcTs-t-BuOOH were identified, and the mechanism of the catalytic oxidation of phenolic substrates was proposed. Quinone imine molecules, the main products of catalytic oxidation reaction, can be adsorbed on the surface of CdTe QDs, resulting in their fluorescence quenching. A dual function of catalytic oxidation and fluorescence sensing was developed for the determination of dichlorophenol (DCP) based on the Fe(III)PcTs-BuOOH-CdTe QDs system. The linear detection range of DCP was 1×10-6-1.3×10-4 mol/L, and the detection limit 2.4×10-7 mol/L. This method was characterized by high selectivity, good repeatability and desirable stability, presenting promising potentials for analyzing DCP concentration in real water samples.

Key words: dichlorophenol (DCP); Fe(III)PcTs-BuOOH-CdTe QDs system; quinone imine; catalytic oxidation; fluorescence sensing

1 Introduction

The catalytic oxidation of chlorinated phenols based on natural enzyme-oxidant systems has been widely explored for the biodegradation and optical detection of phenolic pollutants[1-4]. There have been a number of reports on the analysis and detection of phenolic pollutants using chemical sensors (or biosensors) designed according to this principle[5-7]. For example, it has been found that peroxidase can catalyze the oxidation of chlorinated phenols into quinone intermediates and oligomers[8-10], and that quinone intermediates can quench the fluorescence of quantum dots (QDs)[11-13]. It has also been reported that the peroxidase-H2O2system can be used to detect chlorinated phenols by fluorescence analysis[14]. However, the use of natural enzyme-oxidant systems has been restricted by the poor stability, difficult preparation, and high cost of natural enzymes. In this context, replacing natural enzymes with metal phthalocyanine biomimetic enzymes is of vital significance for the optical detection of chlorinated phenols, and has a bright application prospect[15]. QDs are compatible with many biological systems (such as various natural enzymes), and a range of analytes can interact with the surface of QDs, causing changes in their fluorescence[16-18]. An analyte can be detected by leveraging the law of fluorescence enhancement or weakening. Therefore, the exploration of phthalocyanine metal and QDs system with green oxidant for catalytic oxidation of phenolic pollutant can overcome the defects of traditional analysis methods. Furthermore, understanding the details of reaction mechanism and performance is important for the development of detection of phenolic pollutants.

In this study, a sensitive novel biomimetic method was developed for dichlorophenol (DCP) detection based on a composite biomimetic catalysisfluorescence sensing system, in which tetrasulfonate iron (III) phthalocyanine(Fe(III)PcTs) was used as the biomimetic enzyme. The chromogenic reaction of Fe(III)PcTs-catalyzed oxidation of DCP into quinoneimine dye under the action of tert-butyl hydroperoxide (BuOOH) was investigated. The active substances of the system were verified based on a capture experiment, and the mechanism of the catalytic reaction was proposed. Considering that quinoneimine dye has a deep quenching effect on the fluorescence of L-cysteine CdTe QDs, we introduced QDs into the tetrasulfonate iron (III) phthalocyanine biomimetic catalysis system following this principle. A fluorescence detection method for chlorophenols using the composite Fe(III)PcTs-BuOOH-CdTe QDs system was established, and the linear detection range and detection limit of DCP were obtained. The linear detection range was 1.0×10-5-1.3×10-4mol/L, and the equation of linear regression wasy=1.038+0.286x,R2=0.996. The system showed good repeatability, selectivity, stability, and the results of real sample detection based on this method suggested that this method is promising for the application of fast and accurate measurement of phenolic pollutants.

2 Experimental

2.1 Reagents and apparatus

UV-Vis spectra were recorded on a Shimadzu UV-2450 spectrophotometer. Fluorescence spectra were recorded by a Hitachi F-4500 fluorescence spectrometer.

4-sulfophthalic acid (C8H6O7S), urea (CO(NH2)2), ammonium molybdate (NH4)6Mo7O24·6H2O), ammonium chloride (NH4Cl), ferrous sulfate (Fe2(SO4)3), and 4-Aminoantipytine were obtained from Sinopharm Chemical Reagent Co., Ltd. Fe(III)PcTs was synthesized and purified according to the Ref.[19]. CdCl2, sodium citrate (C6H5Na3O7·2H2O), sodium tellurateIV (Na2TeO3), L-cysteine (C3H7NO2S), and sodium borohydride (NaBH4) were obtained from Shanghai Chemical Reagent Co., Ltd.

2.2 UV-Vis spectra of DCP catalyzed by the Fe(III)PcTs-t-BuOOH system

Fe(III)PcTs and t-BuOOH are both soluble in water, so the catalyzation of the oxidation of DCP by the Fe(III)PcTs-t-BuOOH system is a homogeneous reaction. The operation is shown as follows: 30 mL deionized water, 10 mL 4-AAP aqueous solution (1.0×10-3mol/L), 5 mL DCP solution (1.0×10-3mol/L), 5 mL Fe(III)PcTs aqueous solution, and a proper amount of t-BuOOH solution were dissolved in a 60 mL beaker, and well stirred. 3 mL filtrate was collected at regular intervals, and detected with a UV-Vis spectrophotometer. (The catalytic oxidation reaction is shown in Fig.1)

Fig.1 Catalytic oxidation of DCP to antipyrilquinoneimine dyes by Fe(III)PcTs-t-BuOOH system

2.3 Fluorescence detection of DCP by the Fe(III)PcTs-BuOOH-CdTe QDs system

First 1.0 mL DCP solution was added into a 25 mL glass beaker. Then 2.0 mL 4-AAP aqueous solution, 5 mL Fe(III) PcTs aqueous solution (1.0×10-4mol/L),2.0 mL CdTe QDs aqueous solution, and 5μL t-BuOOH solution were successively added into the mixture, and stirred at room temperature. Finally, the fluorescence intensity was measured by taking 1 mL reaction solution at regular intervals. The excitation wavelength of the fluorescence spectrometer was set as 400 nm, and the slit width as 10.0 nm.

3 Results and discussion

3.1 Catalytic oxidation system of Fe(III)PcTs-t-BuOOH

3.1.1 Identification of active oxygen species in the Fe(III)PcTs-t-BuOOH system

The absorption peak of Fe(III)PcTs-catalyzed DCP oxidation at 510 nm basically remained unchanged after 180 min under the optimal conditions, suggesting that the catalytic reaction was basically over (DCP concentration=1.0×10-4mol/L). However, the absorption value of the reaction at 510 nm was only 0.37, meaning that the DCP in the system had not been completely oxidized. When the Fe(III)PcTs-t-BuOOH system was used to catalyze DCP oxidation, the absorption value at 510 nm exceeded 1.0 under the same conditions, indicating that more DCP was oxidized. In addition to the dissolved oxygen which served as an oxidant, t-BuOOH also played an important role in the oxidation of DCP[20].

To identify the active oxygen species in the system, we carried out the reaction with continuous nitrogen-bubbling to eliminate the dissolved oxygen in solution. Fig.2 shows the time courses of the dye formation process in the DCP system. Under anaerobic conditions (curve f), dye formation was largely inhibited compared to typical catalysis (curve a), suggesting that oxygen was crucial for the oxidation of chlorophenol. Generally, O2molecules can trap electron to produce active oxygen radicals (O2●-and OH●) that can initiate oxidation reaction[21,22]. To further confirm the presence of specific reactive species, several scavengers (iso-propanol for OH●, K2Cr2O7and NaN3for O2●-) were employed. The addition of isopropanol exhibited little influence on dye formation compared to typical catalysis (curve b versus curve a), suggesting that it was unlikely that OH●had taken part in the catalytic oxidation reaction. However, when K2Cr2O7or NaN3was added into the reaction solution, dye formation was significantly inhibited (curves d, g and e). Especially, when the concentration of K2Cr2O7was increased to 0.001 M, the catalytic reaction could hardly proceed (curve g). Therefore, the generation of O2●-is crucial for the chromogenic reaction, and an O2●--mediated mechanism of Fe(III)PcTs catalysis is hereby confirmed. Moreover, typical catalysis could also proceed efficiently in the dark (curve a versus curve c), which indicates that sunlight is not essential for chlorophenol oxidation in the presence of Fe(III)PcTs catalyst.

Fig.2 Dye formation process in DCP system versus reaction time under different conditions (λabs=510 nm) (a, ■) Typical catalytic reaction; (b, ▼) Iso-Propanol 0.01 M; (c, ♦) In the dark; (d, ◄) K2Cr2O7 0.1 mM; (e, ▲) NaN3 0.01 M; (f, ●) N2 bubbled; (g, ►) K2Cr2O7 1 mM

3.1.2 Proposed mechanism of the catalytic oxidation of phenolic substrates

From the point of view of Meunier and Sorokin[21,22], the proposed active oxygen species were responsible for DCP oxidation, and the fast oxidation of chlorophenol could explain the succeeding generation of p-quinoid radical (possible chlorophenol oxidation product). In turn, the active oxygen species could further oxidize the amino group of 4-AAP to produce the Antipyrine-NH• (radical form of 4-AAP). Immediately, p-quinoid radical and Antipyrine-NH• could combine to generate the final product of pink chloro-substituted antipyrilquinoneimine dye. This constitutes the possible mechanism of the generation of O2●-and dye formation via radical coupling. As for DCP oxidation, the resulting pink dye should be antipyrilquinoneimine (Fig.3).

Fig.3 Proposed mechanism for O2 oxidation of DCP and dye generation via radical coupling in the presence of Fe(III)PcTs-t-BuOOH system

When t-BuOOH was added to the system, Fe(III)PcTs interacted with t-BuOOH, and the initial product might have been TsPcFeIII-OOBu[23,24]. The O-O bonds of this product have two ways of breaking, that is, homolysis and heterolysis[25], which generate PcSFeIV=O and BuO●radicals, and PcSFeV=O and BuO-anions, respectively. Due to FeVis commonly found in dimers of metal phthalocyanine, the main active species should be PcSFeIV=O and BuO●when Fe(III)PcTs interacts with t-BuOOH. Therefore, the possible active oxygen species should bePcTsFeIV=O, and BuO●in the Fe(III)PcTs-t-BuOOH system. More active oxygen species were found in the Fe(III)PcTs-t-BuOOH system, so the Fe(III)PcTs-t-BuOOH system has a stronger oxidation ability and a faster oxidation rate for the substrate DCP.

3.2 Fluorescence quenching of CdTe QDs by antipyrilquinoneimine dye

We introduced CdTe QDs into the Fe(III)PcTs-t-BuOOH system, and took DCP as the research object to observe changes in the fluorescence intensity of CdTe during the chromogenic reaction. CdTe QDs were synthesized according to a method previously reported by Er-Kang Wang[26]. The fluorescence intensity of CdTe gradually declined after t-BuOOH and 4-AAP were introduced into the system, as shown in Fig.4. If t-BuOOH was not added to the system, the fluorescence intensity of CdTe would hardly change. It is known that the antipyrilquinoneimine generation reaction is the dominant reaction in the Fe(III)PcTs-t-BuOOH system, so the fluorescence quenching of CdTe is closely related to the rapid generation of antipyrilquinoneimine dye in the system. This is because quinone imine molecules can be adsorbed on the surface of CdTe, giving rise to the fluorescence quenching of CdTe QDs.

Fig.4 (a)The PL spectra representing the quenching without 4-AAP and BuOOH (b) The PL spectra representing the quenching without 4-AAP (c)The PL spectra representing the quenching without/with BuOOH (d) The realationship of different PL Intensity at 2 min

Fig.5 shows the change in the fluorescence intensity of CdTe QDs with the progress of the oxidation reaction. Because of the fluorescence quenching effect of the oxidation product quinone imine on CdTe, the fluorescence intensity of CdTe gradually declined, and finally reached a stable value.

Fig.5 The fluorescence intensity change of CdTe QDs with the progress of DCP oxidation. [DCP]=1.0×10mol/L；[4-AAP]=2.0×10mol/L, [Fe(III)PcTs] =7.0×10-4 mol/L, and λabs=510 nm

The fluorescence intensity ratio (P0/P) at different time was used to indicate the degree of fluorescence quenching in Fig.5. TheP0/Pvalue reached 1.30 in the first 30 seconds, and 6.94, 65.32, and 92.70 after 2, 4, and 6 minutes, respectively (DCP concentration =1×10-4mol/L).

The principle of the Fe(III)PcTs-BuOOHCdTe QDs system is that the degree of quantum dot fluorescence quenching has a quantitative relationship with the concentration of DCP in a certain concentration range. When the concentration of DCP is higher, more antipyrilquinoneimine dye is generated, and the degree of fluorescence quenching is raised. If the initial fluorescence intensity of QDs is set toP0, the fluorescence intensity at a given point of time after the QDs interact with the quenching agent (antipyrilquinoneimine) is set toP, theP0/Pvalue will increase as the concentration of chlorophenol increases in a certain concentration range. There is a quantitative relationship between them. By measuring the initial fluorescence intensityP0of the QDs and the fluorescence intensityPat different DCP concentrations, we can obtain accurate concentrations of DCP.

3.3 Detection of DCP based on the Fe(III)PcTs-BuOOH-CdTe QDs system

The Fe(III)PcTs-BuOOH-CdTe QDs system was used to detect DCP of different concentrations. The reaction time was 4 min, and theP0/Pvalue was used as a function. The standard curves for detecting DCP in different concentration ranges were obtained (Fig.6). Fig.6 depicts a good linear relationship betweenP0/Pand DCP concentration in the range of 1.0×10-6-9.0×10-6mol/L when the concentration of Fe(III)PcTs was 1.0×10-5mol/L and the amount oft-BuOOH 10 µL. The equation of linear regression isY=1.054+0.032x, and the correlation coefficientR2=0.995. Measurement was conducted three times under each experimental condition. The relative standard deviation (R.S.D) is 3.2%.

Fig.6 Relationship of P0/P with DCP concentration in the range of 1.0×10-6 -9.0×10-6 mol/L

As shown in Fig.7, there is a good linear relationshipP0/Pand DCP concentration in the range of 1.0×10-5-1.3×10-4mol/L, and the equation of linear regression isy=1.038+0.286x,R2=0.996. The lower detection limit of the method is 2.4×10-7mol/L (S/N=3). Measurement was conducted three times under each experimental condition in Fig.8. R.S.D is 4.1%. Since the integrated wastewater discharge standard of China for DCP is 3.68×10-6mol/L[27], the Fe(III)PcTs-BuOOH-CdTe QDs system can meet the demand of practical applications.

Fig.7 Relationship of P0/P with DCP concentration in the range of 1.0×10-5 -1.3×10-4 mol/L

3.4 Properties of the Fe(III)PcTs-BuOOHCdTe QDs system

3.4.1 Repeatability and reversibility

The reversibility of the Fe(III)PcTs-BuOOH-CdTe QDs system was assessed using low-concentration DCP (5.0×10-5mol/L) and high-concentration DCP (1.0×10-4mol/L) in five repetitions, respectively. It was found that the system had a highly reproducible and reversible response to DCP solution, with R.S.Ds of 2.1% and 3.4%, which testified to the good reversibility of the detection system. The repeatability of the response of the system was also investigated by immersing the sensor in a 3.0×10-5mol/L DCP solution. An R.S.D. of 3.81% after 20 successive measurements demonstrated the good repeatability of the detection system.

3.4.2 Selectivity against interference

The actual concentration of DCP was set as 5.0×10-5mol/L, and the interference rates in the presence of potential interferents with different concentrations were used as indicators for the selectivity of the Fe(III)PcTs-BuOOH-CdTe QDs system. The results are summarized in Table 1.

Table 1 Effect of interfering ions on DCP detection with Fe(III)PcTs-BuOOH-CdTe QDs system (DCP concentration is 5×10-5 M)

We can see that none of the interferent caused any significant interference with the response of the detection system, which demonstrated the good selectivity of the system.

3.4.3 Detection of DCP in real water samples

The Fe(III)PcTs-BuOOH-CdTe QDs system was validated by using it to determine DCP in samples collected from the East Lake and the Yangtze River in Wuhan, China. The DCP concentrations in two samples were determined, in triplicate. The results are shown in Table 2. It can be concluded that this method is suitable for the determination of DCP in real samples.

Table 2 Detecting results of practical samples using Fe(III)PcTs-BuOOH-CdTe QDs system

We determined the DCP concentrations in three samples, in triplicate, using the HPLC, the electrochemical sensor and the Fe(III)PcTs-BuOOHCdTe QDs system. The results are shown in Table 3. Sample preparation for HPLC is much more complicated, and the detection process is timeconsuming. Besides its high cost, HPLC is also unavailable for real-time, online detection. Compared with the Fe(III)PcTs-BuOOH-CdTe QDs system, the electrochemical sensor has a detection limit of 5.6×10-6mol/L, which is one order lower. The Fe(III)PcTs-BuOOH-CdTe QDs system is obviously superior to the electrochemical sensor in both repeatability and reversibility.

Table 3 Detection of practical samples using HPLC and electrochemical sensor

4 Conclusions

In summary, Fe(III)PcTs exhibits its best catalytic activity in the presence of t-BuOOH, resulting in the active species of PcOCFeIV=O. 2,4-DCP can be rapidly oxidized by Fe(III)PcTs-t-BuOOH to produce pink quinone imine dye, which can be adsorbed on the surface of CdTe QDs and cause their fluorescence quenching. The Fe(III)PcTs-BuOOH-CdTe QDs system has the dual function of substrate catalysis and fluorescence sensing. In this study, the linear detection range of DCP was 1×10-6-1.3×10-4mol/L, and the detection limit 2.4×10-7mol/L. This method produced good repeatability, selectivity, stability, and was also successfully applied to detection of DCP in real water samples. It presents a bright application prospect for fast and accurate measurement of phenolic pollutants.