Ultrasensitive Fluorescence Detection of Peroxymonosulfate Based on

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Ultrasensitive Fluorescence Detection of Peroxymonosulfate Based on a Sulfate Radical-Mediated Aromatic Hydroxylation Gui-Xiang Huang, Jin-Yan Si, Chen Qian, Wei-Kang Wang, Shu-Chuan Mei, Chu-Ya Wang, and Han-Qing Yu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04047 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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Analytical Chemistry

Ultrasensitive Fluorescence Detection of Peroxymonosulfate Based on a Sulfate Radical-Mediated Aromatic Hydroxylation

Gui-Xiang Huang†, Jin-Yan Si†, Chen Qian, Wei-Kang Wang, Shu-Chuan Mei, Chu-Ya Wang, Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Applied Chemistry, University of Science & Technology of China, Hefei, 230026, China

* Corresponding author: Prof. Han-Qing Yu, Fax: +86 551 63601592; E-mail: [email protected]

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Recently, peroxymonosulfate (PMS)-based advanced oxidation processes have exhibited broad application prospects in the field of environment. Accordingly, a simple, rapid and ultrasensitive method is highly desired for the specific recognition and accurate quantification of PMS in various aqueous solutions. In this work, SO4•−-induced aromatic hydroxylation was explored, and based on that, for the first time, a novel fluorescence method was developed for the PMS determination using Co2+ as a PMS activator and benzoic acid (BA) as a chemical probe. Through a suite of spectral, chromatographic, and mass spectrometric analyses, SO4•− was proven to be the dominant radical species, and salicylic acid was identified as the fluorescent molecule. As a result, a whole radical chain reaction mechanism for the generation of salicylic acid in the BA/PMS/Co2+ system was proposed. This fluorescence method possessed a rapid reaction equilibrium (< 1 min), an ultrahigh sensitivity (detection limit = 10 nM; quantification limit = 33 nM), an excellent specificity and a wide detection range (0–100 μM). Moreover, it performed well in the presence of possible interfering substances, including two other peroxides (i.e., peroxydisulfate and hydrogen peroxide), some common ions and organics. The detection results for real water samples further validated the practical utility of the developed fluorescence method. This work provides a new method for the specific recognition and sensitive determination of PMS in complex aqueous solutions.


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INTRODUCTION

In recent decades, persulfate-based advanced oxidation processes (AOPs) have received great attention due to their high efficiency in organic contaminant removal and the convenient storage and long half-life of persulfate.1,2 The activation of persulfate, including peroxydisulfate (PDS) and peroxymonosulfate (PMS), through thermal treatment,3,4 base injection,5 UV irradiation,6,7 sonication8,9 or catalysis10-14 can produce a sulfate radical (SO4•−), which is a strong oxidant with a high reduction potential (2.5-3.1 V).15 Compared with PDS, PMS has recently attracted increasing interests because it is generally easier to be activated via the catalysis of transition metals (e.g., Fe(II), Co(II), Mn(II), Cu(I), Ti(III), etc.) due to its asymmetrical molecular structure with a lower energy for its lowest unoccupied molecular orbital (LUMO) level.16-20 In addition, PMS has been widely used in many other fields such as in the synthesis of organic chemicals as an oxidizing agent,21,22 in the paper and pulp industry as a bleaching agent,23,24 in pools and spas disinfection as a cleaning agent,12,25 and so on. With the rapidly expanding studies on PMS-related technologies and their promising applications in the above fields, a simple, rapid and sensitive method is highly desired for the accurate measurement of residual PMS in its oxidation processes and the detection of trace PMS when it is released into natural waters. Currently, several methods have been developed for the PMS determination, including thermometric titrimetry,26 liquid chromatography,27 chemiluminescence,28 3 ACS Paragon Plus Environment


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and UV-Vis absorption spectroscopy.29-33 To make a clear comparison among these methods, their linear ranges and detection limits are summarized in Table S1. Thermometric titrimetry and liquid chromatography could be used for the quantification of PMS in a mixed solution containing PDS and H2O2, but they are time-consuming and limited by a low sensitivity.26,27 The chemiluminescence method possesses a relatively low detection limit; however, the linear range is too narrow.28 The KI spectrophotometry shows an advantage because of its simplicity and convenience with regards to both the instrument and operational process, and thus, it has been the most widely adopted method to date.29,30 Nevertheless, it is also feeble with regards to its sensitivity for PMS and fails to work well towards a PDS- or H2O2-containing

sample.

Similar

drawbacks

exist

in

the

N,N-diethyl-p-phenylenediamine (DPD) spectrophotometry.31 In addition to the coloration methods mentioned above, the oxidative decolorization of some dye molecules, such as methyl orange, methyl violet, rhodamine B, etc., has also been applied in the spectrophotometric determination of PMS.32,33 Although these decolorization methods are rapid and sensitive, they suffer from interference from some other colored substances. The molecular fluorescence methods have recently received considerable attention due to their excellent properties of great sensitivity, high specificity, wide linear range, and real-time monitoring.34-37 In the pure •OH-based AOPs, aromatic compounds, such as benzoate, coumarin and phenoxazinone, could be hydroxylated, and the products, in which the -OH is added to a curtain site of the aromatic ring, are 4 ACS Paragon Plus Environment

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highly fluorescent.38 Based on this property, some assay methods have been developed for the qualitative or quantitative analysis of •OH.38-40 Similarly, in the SO4•− system, the aromatic hydroxylation can also occur in the degradation process of some aromatic compounds.41-44 Thus, we hypothesize that some hydroxylated products generated in the SO4•− system may also possess a fluorescent property, which could be used for the determination of PMS. However, to the best of our knowledge, there is no such a study reported for PMS fluorometry so far. In this work, a fluorescence method was developed for the first time for the rapid and sensitive determination of PMS in aqueous solutions. In this method, benzoic acid (BA) was adopted as a chemical probe, and a homogeneous catalyst, Co2+, was used to initiate the BA hydroxylation via activating PMS to generate the free radicals. First, the specific fluorescent molecule and the dominant radical species were identified, based on which a reaction mechanism for the fluorescence analysis was proposed. Then, the impacts of the reaction conditions, including the initial BA concentration, Co2+ dosage and solution pH, on the PMS detection were examined. Next, the linear range, sensitivity, reproducibility, robustness, selectivity and interference rejection of this method (named as BA fluorometry hereinafter) were comprehensively evaluated. Finally, the practical utility of BA fluorometry was tested in several types of real water samples.

EXPERIMENTAL SECTION



Chemicals and Reagents. PMS (KHSO5·0.5KHSO4·0.5K2SO4) and sodium benzoate were purchased from Beijing J&K Co., China. Acetonitrile, methanol (gradient grade) and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were purchased from Sigma-Aldrich Co., USA. Suwannee River fulvic acid (FA) was purchased from International Humic Substance Society. Bovine serum albumin (BSA, molecular biology grade) was purchased from Aladdin Co., China. Dextran (reagent grade) was purchased from Sangon Biotech (Shanghai) Co., China. Other reagents were purchased from Shanghai Chemical Reagent Co., China. Unless otherwise specified, all chemicals and reagents were of analytical grade and used without further purification. Experimental Procedures. Unless otherwise specified, the fluorescence analytic experiments were conducted in a 10-mL polypropylene plastic tube with a total reaction solution volume of 3.0 mL at room temperature (25 ± 2 ºC); the pH value of the reaction solutions was adjusted with 0.1 M NaOH or HClO4 when needed. Typically, the sample solution of PMS and the stock solution of sodium benzoate were mixed, and CoCl2 solution was then dosed to initiate the reaction. After reacting for a given time, the solution was transferred to a cuvette for immediate fluorescence analysis. The KI spectrophotometry analytic experiments were conducted according to a protocol reported previously.30 All experiments were carried out in duplicate or triplicate, and the average data with their standard deviations are presented in the figures. The reproducibility of BA fluorometry was examined by replicating measurements on two different days and with two different cuvettes. To assess the 6 ACS Paragon Plus Environment

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method robustness, the experiments were conducted at different reaction temperatures ranging from 15 to 35 ºC. The impacts of two other commonly used peroxides (i.e., PDS and H2O2) and some common ions, which were selected according to the literature (the concentrations are listed in Table S2),45 on the PMS determination by BA fluorometry and KI spectrometry were evaluated. FA, BSA and dextran, which are the model substances of the natural organic matter (NOM), as well as phenol, a model organic pollutant, were used to evaluate the influence of organic substances. The real water samples from natural and artificial water sources were collected and tested in October, 2018. Samples from Chaohu Lake (31°32.081′ N, 117°38.845′ E, Anhui Province, China) and Binhu Wetland Park (31°71.803′ N, 117°40.102′ E, Hefei City, China) were used as the natural waters. The tap water samples were obtained from the drinking water supply system in our campus. The effluent samples of the Jingkaiqu Municipal Wastewater Treatment Plant (WWTP) (Hefei City, China) were also collected. These samples were filtered twice through 0.20 μm hydrophilic PTFE filters (produced by Tianjin Fuji Science & Technology Co., China) to remove the solid suspensions. The water samples were then prepared by spiking with known concentrations of PMS for recovery tests and triplicate measurements were conducted using BA fluorometry for each sample. The real PMS-containing sample was collected from a demonstration base of an ecological remediation project in Songjiang District, Shanghai City, China. The sample was filtered twice through 0.20 μm hydrophilic PTFE filters before the measurement using BA fluorometry. To improve the anti-interference ability and the pH buffer capacity of the fluorescence method, a 7 ACS Paragon Plus Environment


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higher BA concentration (50 mM) was used in the experiments of this section. Analytical Methods. The fluorescence emission spectra and excitation-emission matrix spectra (EEMS) were obtained using a fluorescence spectrophotometer (Aqualog, Horiba Co., Japan), and the spectral integration time was 0.1 s unless otherwise specified. The absorbance at 352 nm was measured using an UV-vis spectrophotometer (model 2450, Shimadzu Co., Japan). The pH values of the reaction solutions were recorded with a pH meter (model PHS-3E, INESA Co., China). Free radicals were detected using an electron paramagnetic resonance (EPR) spectrometer (JES-FA200, JEOL Co., Japan). Reaction products of the BA/PMS/Co2+ system were determined using an ultra-high-performance liquid chromatography (UHPLC) system (1290 Infinity II, Agilent Co., USA) with a C18 column (ZORBAX Eclipse Plus, P.N. 959757-902) and a gas chromatography-mass spectrometry (GC-MS) system (Agilent Co., USA), which consists of an Agilent 7890B GC system with an HP-5MS column (P.N. 19091S-433UI) and an Agilent 5977B single quadrupole mass spectrometric detector. The detection or quantification limit (𝑐L) was calculated according to the following equation:46 𝑐L = 𝑘 ∙ 𝑆𝐵 𝑚

(1)

where k is the signal-to-noise ratio (the value of 3 and 10 are commonly used for detection and quantification limit, respectively), SB is the standard deviation of the blank samples and m is the slope of the regression equation in the low-concentration range of the substance. 8 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION

Identification of the Fluorescent Molecule and Radical Species in the BA/PMS/Co2+ system. After the hydroxylation of BA by Co2+-activated PMS, a fluorescence peak with a maximum excitation wavelength at 295 nm and a maximum emission wavelength at 405 nm was observed in the excitation-emission matrix spectrum (EEMS) (Figure 1a). Based on the position of this EEMS peak, all of the two-dimensional fluorescence emission spectra hereinafter were collected with excitation at 295 nm. In the emission spectrum of the pure BA solution, there was no recognizable fluorescence signal, and the same results appeared in the combination of any two of the BA, PMS and Co2+ solutions (Figure 1b), which indicates the indispensable role of both PMS and Co2+ in the generation of the fluorescent product. It is reported that the hydroxylation of BA could lead to three constitutional isomers (i.e., ortho-, meta-, and para-hydroxybenzoic acids).47,48 To identify the specific fluorescent molecule in the BA/PMS/Co2+ reaction system, the EEMS of the standard substances of the three hydroxybenzoic acids were also examined. As shown in Figure S1, only in the EEMS of o-hydroxybenzoic acid a strong fluorescence peak was observed, the shape and position of which were both identical with those of the one in Figure 1a; in contrast, almost no fluorescence signal appeared in the case of isomolar meta and para-hydroxybenzoic acids. Therefore, o-hydroxybenzoic acid [i.e., salicylic acid (SA)] was identified as the fluorescent molecule in the 9 ACS Paragon Plus Environment


BA/PMS/Co2+ system. This inference is further confirmed by the UHPLC and GC-MS test results (Figures 1c, S2 and S3). Moreover, the conversion factor of SA based on PMS was determined via UHPLC quantitative analysis, and the calculated value was 0.11 (Table S3). This result means that approximately one mole of SA molecules could be generated from ten moles of PMS in the BA/PMS/Co2+ system. To identify the radical species involved in the process of BA hydroxylation by Co2+-activated PMS, EPR tests were carried out using DMPO as the spin-trapping agent. As shown in Figure S4, both the characteristic peaks of DMPO•-OH (with the hyperfine coupling constants of aN = 15.01 and aβ-H = 14.72) and DMPO•-SO4− (with the hyperfine coupling constants of aN = 13.67, aβ-H = 10.24, aγ-H1 = 1.52 and aγ-H2 = 0.79) were observed in the EPR spectrum of the BA/PMS/Co2+ reaction sample, indicating the coexistence of •OH and SO4•−.9,49,50 To further distinguish the contribution ratio of the two radicals, three radical scavengers, i.e., methanol (MeOH), ethanol (EtOH) and tert-butyl alcohol (tBuOH), were used to inhibit the generation of the fluorescent molecule. It is reported that alcohols with α-hydrogens (e.g., MeOH and EtOH) possess a high reactivity with both •OH and SO4•−, while tBuOH that possesses no α-hydrogen has a good reactivity with •OH but poor reactivity with SO4•−.51,52 In the tests for the BA/PMS/Co2+ system (Figure 1d), both MeOH and EtOH almost completely suppressed the formation of the fluorescent product, which could be attributed to the competitive consumption of both •OH and SO4•− by the alcohols; in the case of tBuOH, however, 85% of the fluorescence intensity was still detected. Therefore, SO4•− was recognized as the predominant 10 ACS Paragon Plus Environment

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radical, which directly participated in the conversion from BA to SA. This result is consistent with the results in previous studies on the degradation of organics by the PMS/Co2+ oxidation process.12,42 Based on the above analyses, a reaction mechanism, through which the fluorescent molecule (i.e., SA) was generated in the BA/PMS/Co2+ system, is proposed by concurrently considering previous studies involving aromatics, SO4•− and •OH (Scheme 1). First, Co2+ donates an electron to PMS and thus initiates its decomposition to generate a SO4•− (Reaction 1).12 Then, SO4•− attacks the aromatic ring of BA and captures an electron from it, through which a carbon-centered radical cation (HOOCC6H5•+) is formed (Reaction 2).41 The subsequent reaction between HOOCC6H5•+ and H2O leads to the formation of an OH-adduct radical product (Reaction 3),41 and the same product could also be obtained by the attack of •OH, which is generated through the reaction between SO4•− and H2O (Reaction 4),53 via direct addition to the BA aromatic ring (Reaction 5).54 This reaction, however, has been proven to be a minor pathway in this work. Finally, the OH-adduct radical is oxidized by the high-valence metal ions (i.e., Co3+),41,54 through which an SA molecule is obtained, accompanied with the accomplishment of a Co2+/Co3+ cycle (Reaction 6). According to previous studies, O2 may also act as an oxidizing agent in Reaction 6.42-44,55 To explore this contribution, additional experiments under different atmospheric conditions were conducted. As shown in Figure S5, there was no obvious change (< 5%) in the fluorescence intensity whether in N2- or O2-saturated solution, indicating that O2 was not the main acceptor of the unpaired electron in the 11 ACS Paragon Plus Environment


OH-adduct radical. Thus, the reaction where Co3+ involved is the termination of the entire set of radical chain reactions. Development of BA Fluorometry for PMS Detection. As shown in Figure S6, a significant linear correlation (R2=0.9954) exists between the fluorescence intensity and PMS concentration in the range of 0-100 μM. Based on this relationship, a fluorescence assay method was developed for the quantitative determination of PMS in aqueous solutions, in which it takes only tens of seconds to reach the reaction equilibrium in the tested range. To obtain the optimal test conditions, the impacts of solution pH, the initial BA concentration and Co2+ concentration on the determination were subsequently examined. Since pH might affect not only the yield but also the fluorescence property of the fluorescent product (i.e., SA) in the BA/PMS/Co2+ system,56,57 the impact of the solution pH on the fluorescence intensity of an SA standard substance was first evaluated. As shown in Figure S7, the fluorescence intensity of the SA solution increased rapidly as the pH increased from 2.0 to 4.5, and then peaked at pH 4.5-7.5. Therefore, pH values higher than 4.0 were selected to evaluate the impact of pH on the determination of PMS in the BA/PMS/Co2+ system. Since the benzoate itself could act as an effective pH buffer salt,58,59 the solution pH remained stable (with a slight change < 0.3) in the reaction (Figure S8). With the variation of solution pH, both the reaction equilibrium time and the final fluorescence intensity changed (Figure 2a). In the testing range, a higher solution pH resulted in a shorter reaction time needed, while the final fluorescence intensity presented a trend of initial 12 ACS Paragon Plus Environment

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increasing and later decreasing, with a pH of 5.5 as the turning point. Taking these two factors into account, a solution pH of 5.0-6.0 was appropriate for PMS determination by BA fluorometry, and the pH of 5.5 was used in the subsequent experiments. To ensure sufficient probe molecules for the one-to-one trapping of all the radicals derived from PMS, an excess dosage of BA is first needed. As shown in Figure 2b, with the increase in the initial BA concentration, the measured fluorescence intensity first increased and then reached a maximum value that was nearly constant with the BA concentrations ranging from 20 to 50 mM. Therefore, an initial BA concentration of 20 mM was used for the PMS determination in the subsequent experiments. A different phenomenon was observed when examining the effect of the initial Co2+ concentration (Figure 2c). On the one hand, a rapid analytical method could only be established when the initial Co2+ concentration was no lower than 50 μM, below which the reaction could not reach the endpoint within 5 min or an even longer time. On the other hand, a larger Co2+ dosage could result in a lower final fluorescence intensity, which could be attributed to the competitive consumption of SO4•− by the excess Co2+.12,60 In consideration of both the reaction equilibrium time and the final fluorescence intensity that has a positive correlation with the analytical sensitivity, a moderate dosage of Co2+ is optimum for the BA/PMS/Co2+ system. Unless otherwise specified, an initial Co2+ concentration of 50 μM was used for the determination of PMS in the subsequent experiments. It is noteworthy that the Co2+ dosage in this work is only one fortieth and one twentieth of 13 ACS Paragon Plus Environment


those in the reported MO spectrophotometry32 and RhB spectrophotometry,33 respectively. Under the optimized conditions (i.e., [BA] = 20 mM, [Co2+] = 50 μM, and solution pH = 5.5), the sensitivity of BA fluorometry for PMS determination was evaluated. As shown in Figure 2d, the calibration curve showed a good linearity (R2=0.9998) and a high precision (the narrow intervals at 95% confidence) in the low PMS concentration range from 0 to 2.0 μM. The resulting regression equation was F = 6893.6[PMS] - 2.7. The limit of detection (LOD) at a signal-to-noise ratio of 3 was 10 nM, and the limit of quantification (LOQ) at a signal-to-noise ratio of 10 was 33 nM. The regression curves obtained on different days and with different cuvettes are nearly coincident (Figure S9), indicating a good reproducibility of BA fluorometry. In addition, a fluctuation in the reaction temperature within ±10 ºC had no obvious impact on the fluorescence intensity (relative error < 5%, Figure S10), showing the robustness of BA fluorometry. It should be noted that the LOD value obtained in this work is the lowest one when compared with those of the other methods in previous studies (Table S1). Therefore, BA fluorometry is the most sensitive analytical method for PMS determination in aqueous solutions to date. Selectivity for PMS and Anti-Interference Performance with Coexisting Substances. The selectivity of the BA fluorometry for PMS detection was evaluated by recording the fluorescence emission spectrum of the PMS solution in comparison with those of two other commonly used peroxides (i.e., PDS and H2O2). As shown in Figure 3a, except for a strong fluorescence peak in the spectrum of the PMS solution, 14 ACS Paragon Plus Environment

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no fluorescence signals could be detected in the cases of PDS and H2O2, which was in accordance with the EPR results, where neither the characteristic peaks of DMPO•-OH or DMPO•-SO4− appeared in the spectra of PDS and H2O2 (Figure S4). These results suggest that BA fluorometry possesses an excellent specificity for PMS. To further assess the potential interferences caused by the coexistence of PDS or H2O2 for PMS determination, a comparative test was carried out between BA fluorometry and KI spectrophotometry, which is currently the most widely used PMS determination method. As shown in Figure 3b, in BA fluorometry, the presence of PDS, H2O2 and their mixture all had a negligible impact on the fluorescence intensity of the PMS solution; in KI spectrophotometry however, the presence of H2O2, whether alone or mixed with PDS, completely eliminated the absorbance of the PMS solution at 352 nm, making the analytical method inoperational. These results suggest that BA fluorometry has a promising potential for the specific recognition and quantitative determination of PMS in complex aqueous solutions. The interferences of some common ions that may coexist with PMS in practical samples were also investigated, and all the tested anions and metal cations had a negligible influence on the fluorescence intensity of the PMS solution (Figure 4a), indicating the good resistibility of BA fluorometry. Compared with other transition metals such as Co, Ag, Cu, etc., the Fe-based catalysts are considered more environmentally friendly and applicable for PMS activation.7,61 Therefore, the impact of high-content Fe3+ on PMS determination should be evaluated, and the Fe3+ concentration of 0-2 mg L–1 was used to examine its interference effect in BA 15 ACS Paragon Plus Environment


fluorometry and KI spectrophotometry. As shown in Figure 4b, for BA fluorometry, the fluorescence intensity of the PMS solution showed a downward trend with an increase in the content of the coexisting Fe3+. This phenomenon could be attributed to the reduction in the electron cloud density of the p-π conjugated structure of the SA molecules, which is caused by the coordination with Fe3+.62,63 According to the test results, the maximum tolerated concentration of Fe3+ in BA fluorometry was ca. 1 mg L–1 (with a relative error < 5%). In contrast, due to the direct redox reaction between Fe3+ and I–,64 the presence of Fe3+ substantially increased the UV absorbance of the PMS solution in KI spectrophotometry, and the resulting relative error exceeded 5% at 0.5 mg L–1 Fe3+, which increased further with an increase in the Fe3+ concentration. To examine the impact of some organics that may commonly exist in real water or wastewater, the model substances of NOM (i.e., FA, BSA and dextran) and a typical organic pollutant (i.e., phenol) were used to examine their interference effects on BA fluorometry and KI spectrophotometry. As shown in Figure 4c, the maximum tolerated concentration of FA was ca. 1 mg L–1 for the two methods, and a higher concentration led to a relative error over 5%. For the other tested organics (all with a concentration of 20 mg L–1), no obvious influence was observed for BA fluorometry. However, for KI spectrophotometry, the presence of BSA resulted in a substantial relative error (> 20%). Furthermore, the presence of phenol completely eliminated the absorbance and led to the failure of this analytical method. These results suggest that, compared with KI spectrophotometry, BA fluorometry has a superior anti-interference performance toward the potentially coexisting substances. 16 ACS Paragon Plus Environment

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Applicability in Real Water Samples. To assess the utility of BA fluorometry in practical applications, the detection of PMS in real water samples was demonstrated. The recovery tests were first conducted with the real samples collected from tap water, Chaohu Lake, Binhu Wetland and WWTP effluent. The average recovery and the relative standard deviation (RSD) were obtained based on triplicate measurements. As shown in Table 1, the recoveries of 95-105% of spiked PMS at a concentration of 10-40 μM were obtained for all the tested samples, implying that the measured concentrations were statistically close to those values added. Also, the RSDs below 3% suggest a high precision of BA fluorometry. In addition, a real PMS-containing sample collected from a demonstration base of an ecological remediation project in Shanghai City, China, was also tested using BA fluorometry, and the PMS content was determined to be 4.94 μM with the external standard method. To further validate this result, the internal standard method was used for the elimination of the internal interferences from the sample. As shown in Figure 4d, with the addition of PMS standard substance in the sample, the detected fluorescence intensity of the solution increased, resulting in a good linearity in the added PMS concentration range from 10 to 40 μM. The original PMS content in the sample (i.e., the absolute value of the x-intercept in Figure 4d) was calculated to be 4.85 μM, which is close to that obtained by the external standard method. These results suggest that BA fluorometry possesses a potential to quantify PMS in practical applications.

CONCLUSIONS 17 ACS Paragon Plus Environment


In this work, a novel fluorescence method for PMS determination was established based on an SO4•−-induced aromatic hydroxylation by using BA as a chemical probe. This method possesses a rapid reaction equilibrium (< 1 min), an ultrahigh sensitivity (LOD = 10 nM; LOQ =33 nM), a great selectivity (among PMS, PDS and H2O2), a good reproducibility, an excellent robustness, and a wide detection range (0-100 μM). Due to the above advantages, BA fluorometry exhibits a promising potential for practical applications and performs well in several types of real water samples. Since the Fe3+ content in some practical situations (e.g., samples from Fe-catalyzed PMS oxidation processes for wastewater treatment or in situ chemical oxidation for the remediation of the contaminated groundwater and aquifer solids) could probably reach a level beyond 1 mg L–1, future works need to be performed to improve the performance of this fluorescence method for its anti-interference towards Fe3+ via some approaches, such as the introduction of an appropriate masking agent. Although the specific recognition of PMS has been realized in BA/peroxide/Co2+ systems due to the higher reduction potential of the Co3+/Co2+ redox pair (1.92 V) than that of S2O82−/SO4•− (1.39 V) and H2O2/•OH (0.80 V),64,65 how to simultaneously determine PMS, PDS, and H2O2 in a complex solution remains a challenge. Moreover, the question of whether the isomer distribution of the hydroxylated products formed in the SO4•−-mediated aromatic hydroxylation differs from that in the •OH-mediated aromatic hydroxylation and, if it does, whether this difference could be used for the simultaneous quantification of SO4•− and •OH in aqueous solutions, also warrant 18 ACS Paragon Plus Environment

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further investigations.

ASSOCIATED CONTENT Supporting Information Available. Tables showing the comparison of linear range and detection limit between BA fluorometry and other PMS determination methods; concentrations of the tested cations and anions; and conversion factor of BA and SA in the BA/PMS/Co2+ system. Figures showing the EEMS of BA, ortho-, meta-, and para-hydroxybenzoic acids; GC-MS chromatogram of the BA/PMS/Co2+ reaction sample; MS spectra of BA and SA; EPR spectra in activation of PMS under different conditions; impact of atmosphere conditions on the fluorescence intensity; fluorescence emission spectra under different PMS concentrations, relationship between fluorescence intensity and PMS concentration and the corresponding calibration curve; effect of solution pH on the fluorescence intensity of SA solution; change of solution pH during the BA/PMS/Co2+ reaction; calibration curves of PMS obtained on different dates and with different cuvettes; and impact of the reaction temperature on PMS determination in BA fluorometry. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION † These authors contributed equally to this work. *Corresponding author. 19 ACS Paragon Plus Environment


Prof. Han-Qing Yu, Fax: +86 551 63601592; E-mail: [email protected] ORCID Gui-Xiang Huang: 0000-0003-1223-0164 Han-Qing Yu: 0000-0001-5247-6244 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors thank the National Natural Science Foundation of China (21590812, 51538011 and 51821006), the Collaborative Innovation Center of Suzhou Nano Science and Technology of the Ministry of Education of China for supporting.

REFERENCES

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(64) Bratsch, S. G. Standard electrode potentials and temperature coefficients in water at 298.15 K. J. Phys. Chem. Ref. Data 1989, 18, 1-21. (65) Brandt, C.; van Eldik, R. Transition Metal-Catalyzed Oxidation of Sulfur(IV) Oxides. Atmospheric-Relevant Processes and Mechanisms. Chem. Rev. 1995, 95, 119-190.



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Table 1. Detection of PMS in Spiked Real Water Samples with BA Fluorometry. sample

spiked concn. (μM)

found (μM)

recovery (%)

RSD (n = 3; %)

tap water

10

9.97

99.7

1.5

20

20.21

101.0

1.0

40

41.16

102.9

0.4

10

10.32

103.2

2.0

20

19.88

99.4

1.3

40

39.18

98.0

0.9

10

10.06

100.6

2.2

20

19.47

97.3

2.6

40

38.65

96.6

0.7

10

10.00

100.0

2.7

20

19.19

95.9

2.1

40

38.44

96.1

2.2

Chaohu Lake

Binhu Wetland

WWTP effluent


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Scheme 1. Proposed Radical Chain Reactions for the Generation of SA in the BA/PMS/Co2+ System.



Figure captions

Figure 1. EEMS of the BA/PMS/Co2+ reaction system (a); fluorescence emission spectra of different systems (b); UHPLC chromatograms of the BA/PMS/Co2+ system and two standard substances (c); and effect of radical scavengers on the fluorescence intensity of the BA/PMS/Co2+ system (d). Reaction conditions: [PMS] = 80 μM, [BA] = 10 mM, [Co2+] = 50 μM, [alcohols] = 1 M (for d) and solution pH = 6.5.

Figure 2. Effects of the solution pH (a), the BA (b) and Co2+ (c) dosages on the fluorescence intensity of the BA/PMS/Co2+ system; and calibration curve of PMS in a low concentration range with the spectral integration time set as 4 s (d). The dotted magenta curves in (d) represent the bounds of 95% confidence intervals of the regression line (n = 3). Reaction conditions: [PMS] = 80 μM (for a-c), [BA] = 20 mM (for c-d; 10 mM for a), [Co2+] = 50 μM (for a, b and d) and solution pH = 5.5 (for b-d).

Figure 3. Fluorescence emission spectra of different BA/peroxide/Co2+ systems (a) and the impacts of PDS and H2O2 on the determination of PMS in BA fluorometry and KI spectrophotometry (b). Reaction conditions: [peroxides] = 80 μM, [BA] = 20 mM, [Co2+] = 50 μM and solution pH = 5.5.

Figure 4. Impact of common ions on the fluorescence intensity of the BA/PMS/Co2+ system (a); interferences of Fe3+ (b) and organics (c) on PMS determination in BA fluorometry and KI spectrophotometry; and calibration curve of the added PMS in the pond sample with the spectral integration time set as 1 s (d). Reaction conditions: [FA1] = 1 mg L–1, [FA2] = 2 mg L–1, [BSA] = [dextran] = [phenol] = 20 mg L–1, [PMS] = 80 μM (for a-c), [BA] = 20 mM (for a-c; 50 mM for d), [Co2+] = 50 μM and solution pH = 5.5.


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Figure 1



Figure 2


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Figure 3



Figure 4


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For TOC Only


Ultrasensitive Fluorescence Detection of Peroxymonosulfate Based on

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