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Ascorbic acid surface modified TiO2-thin layers as a fully integrated analysis system for visual simultaneous detection of organophosphorus pesticides† Shunxing Li,*a,b Wenjie Liang,b Fengying Zheng,a,b Xiaofeng Linb and Jiabai Caib TiO2 photocatalysis and colorimetric detection are coupled with thin layer chromatography (TLC) for the first time to develop a fully integrated analysis system. Titania@polystyrene hybrid microspheres were surface modified with ascorbic acid, denoted AA-TiO2@PS, and used as the stationary phase for TLC. Because the affinity between AA-TiO2@PS and organophosphorus pesticides (OPs) was different for different species of OPs (including chlopyrifos, malathion, parathion, parathion-methyl, and methamidophos), OPs could be separated simultaneously by the mobile phase in 12.0 min with different Rf values. After surface modification, the UV-vis wavelength response range of AA-TiO2@PS was expanded to 650 nm. Under visible-light irradiation, all of the OPs could be photodegraded to PO43− in 25.0 min. Based on the chromogenic reaction between PO43− and chromogenic agents (ammonium molybdate and ascorbic acid), OPs were quantified from color intensity images using a scanner in conjunction with image processing software. So, AA-TiO2@PS was respectively used as the stationary phase of TLC for efficient separ-
Received 3rd August 2014, Accepted 16th September 2014
ation of OPs, as a photocatalyst for species transformation of phosphorus, and as a colorimetric probe for
DOI: 10.1039/c4nr04430d
on-field simultaneous visual detection of OPs in natural water. Linear calibration curves for each OP ranged from 19.3 nmol P L−1 to 2.30 μmol P L−1. This integrated analysis system was simple, inexpensive,
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easy to operate, and sensitive.
1.
Introduction
Organophosphorus pesticides (OPs) are used extensively for controlling insects in order to increase agricultural production. However, their high biochemical activity can also pose unacceptable risks to the environment and human health.1,2 Regulatory limits and guideline values for pesticide residues in drinking water have been set by many countries and are summarized by the International Union of Pure and Applied Chemistry (IUPAC).3 Therefore, efficient, sensitive, and selective analytical methods for the determination of trace OP residues in environmental water are required. Several validated methods are currently available for the identification and quantification of OPs, including capillary electrophoresis, gas chromatography, liquid chromatography, and chromatographic techniques coupled to mass spectrometry.4,5 These methods need expensive instruments, complicated
a Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Zhangzhou, 363000, China b College of Chemistry and Environment, Minnan Normal University, Zhangzhou, 363000, China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr04430d
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and time-consuming pretreatments, and well trained operators (which can limit their applications in developing countries) and on-field monitoring.6 So, enzyme-based biosensors have emerged in the past few years as an alternative technique for on-site detection of pesticides. But the enzyme can be easily inactivated due to the complicated water environment.7,8 Therefore, a simple, rapid, and low cost technology for on-field separation and detection of OPs in water samples is of great practical interest. Thin layer chromatography (TLC) is particularly well suited for simultaneous on-field detection of OPs due to its advantages of simple operation, low cost (including devices and operating costs), rapidity, and valid identification and detection based on chemical color reactions.9–11 Unfortunately, the application of TLC is limited by its poor separation capability and sensitivity. Sample preparation such as elution and digestion can give rise to additional contamination and errors. The properties of TLC media and the interaction between the stationary phase and analytes in sample mixtures are dominated by the stationary phase morphology and surface chemistry.12 When new stationary phases and new sample pretreatment methods are used together, the separation capability of TLC can be increased, the contamination can be avoided,
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the blanks and matrix effects can be reduced, and the performance of TLC in the detection of OPs can be improved. Moreover, the color intensity of the images can be quantified using a camera with image processing software and then the sensitivity and detection limit of TLC are improved. Photochemistry-based sample treatment is a greener approach owing to its subsequent achievements in analytical characteristics (e.g., improved detection limits, low blanks, fewer matrix effects and simpler procedures) and greenness-related issues (environmentally friendly, safe procedures, cost-effective methods, and minimum use of chemicals).13 Herein, visiblelight photocatalysis is coupled with TLC by us for the first time to transform all of OPs into PO43− without using any chemicals. As a stationary phase of TLC, TiO2-based nanocomposites are used for the first time, due to their excellent properties, including high surface area/body weight ratio and affinity with organophosphorus compounds.14 Moreover, nanometer size TiO2 is widely used for the photodegradation of organophosphorus15,16 owing to its advantages such as low cost, longterm photochemical stability, and nontoxicity.17 A commercially available TiO2 based adsorbent has been used for in situ preconcentration of phosphorus in fresh and marine waters, and the affinity between TiO2 and phosphate is high.18 However, such affinity is based on the groups of Ti–O–P,14 and the separation capability of pure TiO2 as the stationary phase is limited by non-selective interaction. Worse still, during the application, the adsorption sites of OPs would be decreased by the aggregation of TiO2 nanoparticles. The photodegradation of organophosphorus should be induced by UV irradiation for 60 min15,16 and can be limited by the extremely low coverage of organic pollutants on the TiO2 photocatalyst.17 Polystyrene (PS) was used as the core to control the size and morphology of TiO2. Ascorbic acid surface modified titania@ polystyrene core–shell hybrid microspheres were used and were denoted AA-TiO2@PS in order to: (a) avoid the aggregation of TiO2, (b) introduce the functional groups of ascorbic acid onto the surface of TiO2, (c) adjust the affinity between AA-TiO2@PS and OPs for the separation of OPs, (d) expand the UV-Vis wavelength response range of TiO2 into the visible-light region, (e) improve the photodegradation capability of TiO2, and (f) bring a reducing agent (i.e., ascorbic acid) to form molybdenum blue for colorimetric detection of OPs. So, AA was used as a surface modification agent for visible-light-driven photocatalysis and as a color development reagent in this study. Herein, the study is focused on the development of a new fully integrated analysis system for OPs, using AA-TiO2@PS as (a) the stationary phase of TLC for efficient separation of OPs, (b) a photocatalyst for species transformation of phosphorus, and (c) a colorimetric probe for on-field simultaneous visual detection of OPs in natural water.
2. Experimental section 2.1.
Materials
Five species of OP standards were produced by the Agroenvironmental Protection Institute, Ministry of Agriculture,
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China, and their structure and molecular weight are shown in Table S1.† All other chemicals were of AR grade and were used directly without further purification. A cut-off filter ZWB2 was produced by Nanjing Stonetech Electric Equipment Co., Ltd (China). Deionized water was purified using a Millipore-Q system (Millipore Co., USA) and were used as a solvent with a resistivity of 18.2 MΩ cm−1. 2.2.
Fabrication of a fully integrated analysis system
2.2.1. Preparation of polystyrene microspheres. Polystyrene (PS) microspheres were synthesized by a typical method that was used by Mukesh et al.19 A double-necked flask equipped with a reflux condenser, a mechanical stirrer, a temperature controller, and a nitrogen inlet was used. The air in this flask was replaced by a stream of N2 and then 85.0 g of water, a certain amount of styrene, and 0.5 g of methacrylate were added into the flask. This mixture was kept under N2 until the polymerization was complete. After bubbling N2 gas through the reaction media for 30.0 min, the reaction system was heated to 70.0 °C, and 5.0 mL of potassium persulfate solution (30.0 mg mL−1) was added to start the polymerization process. The polymerization reaction was carried out at 70.0 °C for 24.0 h and then PS microspheres were collected. 2.2.2. Synthesis of titania@polystyrene core–shell hybrid microspheres (TiO2@PS). One milliliter of PS microspheres dispersion (10.0 wt%), 0.15 g of deionized water, and a polyvinyl pyrrolidone K30 solution (i.e., 0.01 g of polyvinyl pyrrolidone K30 surfactant was dissolved in 72.0 mL of ethanol) were mixed in a round-bottomed flask and then sonicated for 15.0 min. With continuous stirring, a certain amount of tetrabutyl titanate was added to the round-bottomed flask. This mixture dispersion was refluxed at 80 °C for 4.0 h. After centrifugation, the precipitates with 40 ml water were transferred into a Teflon-lined stainless steel autoclave (100 mL), heated to 120 °C for 2 h, and centrifuged again. The precipitate was washed with ethanol three times and then dried at 60.0 °C for 8 h. 2.2.3. Preparation of AA-TiO2@PS. In situ surface modification was carried out by stirring TiO2@PS microspheres in ascorbic acid solution (0, 50, 100, 200, 300, and 400 mg L−1, respectively) at pH 4.0 for 30 min in the dark. After surface modification, the mixture was centrifuged at 10 000 rpm and then AA-TiO2@PS with a color of rufous was rinsed with deionized water three times and dried under vacuum at 60.0 °C for 12.0 h. 2.2.4. Preparation of TLC plates. AA-TiO2@PS (5 g), CaSO4·1/2H2O (0.5 g, as a fixing agent), and sodium carboxymethyl cellulose (0.5% m/v, 13.0 mL, as a cross-linking agent) were co-added and stirred until homogeneous slurry was obtained. The slurry was coated onto a clean glass plate using a wire bar coater and the thickness was 200 μm. These TLC plates were dried under vacuum at 60.0 °C and kept in an airtight chamber. After being activated at 105 °C for 30.0 min, TLC plates were used. 2.3.
Colorimetric detection of OPs
In the preliminary TLC studies, 20.0 μL of OPs were spotted by means of a microsyringe at 0.8 cm from the bottom of the TLC
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plate and the spot was dried at 50.0 °C. The above operations were repeated 2 times. In a chromatographic chamber, these TLC plates were placed in 10.0 mL of the mobile phase for 12.0 min in the dark. Using an ascending technique, five species of OPs were separated simultaneously. The TLC plates were taken out and irradiated with visible light (350 W xenon lamp; filtered λ < 385 nm incident using a cut-off filter ZWB2, 23 400 lux) for 25.0 min. Molybdate solution was sprayed onto the TLC plates and developed for 5.0 min in the dark. After drying the plates, five spots with a color of baby blue appeared on the TLC plates and then the Rf values (i.e., distance from the origin to the component spot/distance from the origin to the solvent front) could be calculated. 2.4.
Quantitative image processing
Images of TLC plates were filmed and stored in JPEG format at 300 dpi. The average gray intensity of color at the detection reservoir was measured using the image processing software ImageJ.20 Once optimized, the threshold ranges set for each image were all the same. This process was demonstrated in the ESI.† The interested regions in the image could be automatically selected by the software and the average gray intensity was measured. The data were imported into Origin 8.0 for the determination of OPs.
3. Results and discussion 3.1.
SEM images
The separation ability of TLC was associated with the morphology of the stationary phase. The morphology of PS and TiO2@PS was investigated by SEM. As shown in Fig. 1a, the PS templates were microspheres with a uniform and smooth surface with a mean diameter of 400 nm. After coating with a TiO2 layer, there was no significant variation in the overall morphology of the final products TiO2@PS with a mean diameter of 500 nm (seen in Fig. 1b). The TiO2 layers could be uniformly coated onto the PS templates via catalytic self-condensation of hydroxyl groups.21 The loading amount of TiO2 on PS spheres was investigated by thermogravimetry (TG). As shown in Fig. S2,† the PS spheres could be decomposed at 450 °C and the loading amount of TiO2 on PS spheres was about 54%.
Fig. 1
SEM images of PS (a) and TiO2@PS (b).
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3.2.
Effect of surface modification degree
TiO2@PS nanoparticles were in situ surface modified by the chemisorption of ascorbic acid, i.e., specific binding between ascorbic acid and ortho-substituted hydroxyl groups on the surface of TiO2 22 led to the formation of bidentate surface complexes with five-membered rings.23,24 These surface complexes were stable during the process of photodegradation.25,26 So, in Fig. 2, TiO2@PS failed to absorb light (λ > 380 nm), but the UV-vis absorption wavelength response range of AA-TiO2@PS was shifted toward 650 nm for the influence of surface complexes on AA-TiO2@PS. AA-TiO2@PS could be used as a visible-light-driven photocatalyst. With the increase of concentration of AA, more and more bidentate surface complexes were formed, which could improve the electron transfer and enhance the visible light absorption. But when the concentration of AA reached 300 mg L−1, the light absorption by AA-TiO2@PS was relatively stable, because such visible light absorption was induced by a direct electron transfer from surface complexes to the conduction band of TiO2,27 and the light absorption intensity was controlled by the contents of surface complexes. Because AA could participate in the colorimetric reaction, its amount should be sufficient. The effect of AA concentration on the colorimetric detection of phosphate was studied using 2.45 μmol L−1 of PO43−. As shown in Fig. S3,† the gray scale intensity was linearly increased with increasing the AA concentration from 50 to 200 mg L−1, and thereafter the grayscale intensity was stable even though more AA was used. Thus, the AA concentration of 300 mg L−1 was chosen for further experimental work. OPs could be photodegraded into PO43− by TiO2-based nanocomposites.28 As one species of OPs with the highest molecular weight, chlopyrifos was chosen to compare the photodegradation ability of AA-TiO2@PS and TiO2@PS. According to the concentration of PO43−, the photodegradation ratio was calculated and then the results are shown in Fig. 3.
Fig. 2 UV-vis absorption spectra of TiO2@PS microspheres surface modified by ascorbic acid (AA) with different concentrations (0 mg L−1, 50 mg L−1, 100 mg L−1, 200 mg L−1, and 300 mg L−1).
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the complex into the conduction band of TiO2 in a coherent one-step manner (Fig. 4). Such a photoexcitation and injection process was often referred to as ligand to metal charge transfer.29 This spectral extension was able to make the colorless organic compounds undergo photoinduced reactions on the surface of TiO2 under visible light irradiation. 3.3. Influence of the stationary phase and the mobile phase on separation performance
Fig. 3 Photodegradation ratio of chlopyrifos irradiated by visible light (1.0 mg L−1 chlopyrifos and 1.0 g L−1 photocatalyst).
After surface modification with ascorbic acid, i.e., AA-TiO2@PS was used, the photodegradation could be induced by visible light and the photodegradation performance of TiO2@PS on OPs could be significantly improved. The time-dependent photodegradation curves for the other four OPs are included in Fig. S4.† Chlopyrifos, malathion, parathion, parathionmethyl, and methamidophos could be photodegraded into PO43− in 25.0 min. The photooxidation pathway of OPs on AA-TiO2@PS is proposed in Fig. 4. When there was a strong electronic coupling between the π orbital of organic compounds and the 3d orbital of Ti4+, a surface charge-transfer complex might be formed between organic compounds and coordinatively unsaturated surface Ti(IV) atoms.29 Upon excitation of the charge-transfer complex, the electron was promoted from the ground state of
The separation performance of TLC was controlled by the properties of the mobile and stationary phases. AA-TiO2@PS with different diameter levels (300, 400, 500, 600, and 700 nm, respectively) and mixed solvent (hexane–acetone–methanol– water, v/v = 5 : 2 : 1.5 : 1.5) were used as the stationary phase and the mobile phase, respectively, and the Rf values of OPs are listed in Table 1. With the increase of the AA-TiO2@PS microsphere size from 300 nm to 500 nm, the specific surface area of AA-TiO2@PS was decreased, its adsorption capacity of OPs was weakened obviously, and the Rf values of OPs were increased. When the microsphere size of AA-TiO2@PS was beyond 500 nm, the separation performance was decreased rapidly. Consequently, 500 nm was selected as the microspheres size for further experiments. In Table S2,† we found that the influence of the mobile phase on the separation performance of TLC could not be ignored. The polarity of OPs was different with the difference of the species of OPs, the solubility of different species of OPs in same solvent or same species of OPs in different solvent is different, and then the interaction between OPs and the mobile phase was different. Therefore, using different mobile phases and the same TLC plate, the separation performance was different. The optimum mobile phase was the mixed solvent of hexane, acetone, methanol, and water (v/v = 5 : 2 : 1.5 : 1.5). Five species of OPs were dissolved respectively in the above optimum mobile phase. Using AA-TiO2@PS (or TiO2@PS, 500 nm) as an adsorbent, the adsorption kinetics study was used to explain the influence of the stationary phase, especially the surface modification of TiO2 with ascorbic acid, on the separation performance. The kinetic data were fitted to the following eqn (1): lnðqe qt Þ ¼ ln qe kt
Table 1
ð1Þ
Rf value of OPs
Diameter of ascorbic acid-surface modified polystyrene/titania hybrid microspheres 300 nm
Fig. 4 Schematic illustration of the photooxidation pathway of OPs on AA-TiO2@PS.
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OPs
Rf value
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
0.45 0.29 0.19 0.28 0.45
400 nm
500 nm
600 nm
700 nm
0.72 0.56 0.33 0.48 0.73
0.86 0.79 0.49 0.69 0.90
— 0.90 0.68 0.89 —
— — 0.84 0.98 —
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Table 2
Pseudo first order kinetic equations and parameters (temperature 303 K)
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AA-TiO2@PS
TiO2@PS
OPs
Equation
R2
qe,exp (mg g−1)
Equation
R2
qe,exp (mg g−1)
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
y = −0.0603x + 3.7868 y = −0.0537x + 3.6577 y = −0.0754x + 4.0827 y = −0.0598x + 3.8102 y = −0.051x + 3.527
0.9707 0.9748 0.9482 0.9543 0.9302
21.04 20.04 19.45 19.89 18.89
y = −0.0532x + 3.5504 y = −0.0557x + 3.6902 y = −0.0677x + 3.9175 y = −0.0621x + 3.8102 y = −0.0707x + 3.9424
0.9969 0.9848 0.9718 0.9753 0.9295
20.11 20.44 19.85 20.01 19.45
where qe is the equilibrium adsorption capacity, qt is the adsorbed amount of OPs at time t, and qe and qt were calculated by HPLC. The equation, qe and R2 are shown in Table 2 (y = ln(qe − qt), x = t ). The plots of ln(qe – qt) versus t showed a certain linear relationship (i.e., R2 was in the range of 0.9295–0.9969) for TiO2@PS. However, the value of R2 for each species of OPs on AA-TiO2@PS was less than that on TiO2@PS. A comparison of kinetic models applied to the adsorption of OPs on AA-TiO2@PS was evaluated for the pseudo first-order and the pseudo second-order, respectively. According to the results (shown in Table S3†), the pseudo first-order kinetic model was found to correlate well with the experimental data, suggesting physical adsorption as the dominant process. But their R2 values were less than 0.98, which indicated the existence of chemical adsorption between OPs and surface complexes from AA-TiO2@PS. Furthermore, the affinity was different for different species of OPs, so their R2 were different. After adsorption for 12 min, the adsorption capacity of OPs by AA-TiO2@PS was increased in the order: methamidophos < chlopyrifos < malathion < phoxim < parathion-methyl (seen in Table 3) and this order was opposite to the Rf value. As the affinity between AA-TiO2@PS and different species of OPs was different, OPs could be separated simultaneously by the mobile phase in 12 min with different Rf values. 3.4.
Degradation kinetic studies
The dependence of photodegradation rates on the concentration of organic pollutants could be described well by the Langmuir–Hinshelwood (L–H) kinetic model.30 The modified L–H equation is given by: r¼
dC kr KC ¼ dt 1 þ KC
ð2Þ
But during the photodegradation, the intermediates were formed and their competitive adsorption and degradation against OPs might interfere in the determination of kinetics.
Table 3
Therefore, the rate of formation of PO43− was used to represent the rate of reaction: r¼
1 dCi kr KCi ¼ vi dt 1 þ KCi
ð3Þ
where kr is the reaction rate constant, K is the reactant adsorption constant, vi is the stoichiometric coefficient of PO43−, and Ci is the concentration of PO43− at any time t. Because the OPs used were not more than 2.45 μmol P L−1, after photodegradation, Ci was lower, and so KCi ≪ 1, eqn (3) could be simplified as: r¼
1 dCi ¼ kKCi vi dt
ð4Þ
When t = 0, the concentration of PO43− was zero (Ci0 = 0); when t = t, the concentration of PO43− was Ci. Integrating eqn (3): t¼
1 1 ln Ci vi kK
ð5Þ
ln Ci − t shows a linear relationship. So the photodegradation reaction was a first order kinetics reaction (seen in Table 4, y = ln Ci, x = t ). When the initial concentrations of OPs were 2.0 μmol P L−1, all OPs could be photodegraded into PO43− after irradiation for 25.0 min. According to the results of Tables 3 and 5, a positive correlation was found between the decomposition rate and the adsorption rate, so the degradation rate depended on the diffusion rate (i.e., adsorption rate) of OPs from the bulk solution onto the surface of AA-TiO2@PS. The diffusion rate was controlled by the affinity between AA-TiO2@PS and OPs, which were different for different species of OPs. 3.5.
Colorimetric assay of OPs
The most widely recommended method for phosphate determination is the phosphomolybdenum blue method (PMB), because of its higher sensitivity and lower susceptibility to
Adsorption capacity of OPs by AA-TiO2@PS for 12 min
OPs
Chlopyrifos
Malathion
Parathion-methyl
Phoxim
Methamidophos
Adsorption capacity (μmol g−1)
9.68
11.24
16.44
13.18
9.57
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Table 4 First order kinetic equations of photodegradation (initial concentration of OPs 2.0 μmol P L−1)
OPs
Equation
R2
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
y = 0.2318x − 3.8169 y = 0.1994x − 3.2398 y = 0.2321x − 3.8166 y = 0.2208x − 3.617 y = 0.1797x − 2.908
0.9911 0.9974 0.9925 0.9906 0.9996
Table 5
Five regression equations of OPs
OPs
Equation
R2
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
I = 0.1054C + 77.962 I = 0.1001C + 92.871 I = 0.1153C + 100.53 I = 0.1066C + 95.574 I = 0.1202C + 104.4
R2 = 0.9943 R2 = 0.9831 R2 = 0.9798 R2 = 0.9842 R2 = 0.9905
interference.31 Using this PMB technique, orthophosphate and a small amount of condensed phosphorus compounds could be measured.31 Under visible-light irradiation for 25 min, all OPs could be photodegraded by AA-TiO2@PS, the phosphate was transformed into PO43−, which was determined by the PMB technique. The conversion ratio of phosphate was in the range of 96.8–98.8%, as shown in Table S4.† Under the optimum experimental conditions described above, the working curve for OPs determination was obtained by establishing a linear correlation between the concentration of OPs and the color intensity, which is shown in Fig. 5. Five regression equations are listed in Table 5. The lowest amount of OPs on the TLC plate that could be detected reproducibly was 19.3 nmol P L−1 and the linear calibration curves for each OP ranged from 19.3 nmol P L−1 to 2.30 μmol P L−1. According to the European Union directive on water quality (98/83/CE), the maximum admissible concentration for pesticides was 0.1 mg L−1 for each individual
Fig. 5 Working curve for OPs determination obtained under optimum experimental conditions (the concentration of P from the bottom to the top was 5, 10, 20, 50, 100, 200, 300, 400, 500, and 600 μg L−1, respectively).
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Table 6
Analytical results for the Jiulong river water
Jiulong river water Organophosphorus pesticides
Added (μg L−1)
Found (μg L−1)
Recovery (%)
Certified values (μg L−1)
This work (μg L−1)
HPLC (μg L−1)
Chlopyrifos
0 20.0 0 20.0 0 15.0 0 15.0 0 10.0
7.5 26.2 — 18.7 — 16.2 — 16.5 — 10.9
— 93.5 — 93.5 — 108.0 — 110.0 — 109.0
—
—
—
50.0
51.8
50.7
50.0
51.9
50.6
50.0
48.1
50.4
40.0
41.7
40.8
Malathion
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Mixed standard sample (SB05-069-2008)
Parathion-methyl Phoxim Methamidophos
substance, and 0.5 mg L−1 was the maximum total concentration of all organophosphorus pesticides. In China, the maximum admissible concentration of chlopyrifos, malathion, and parathion-methyl for drinking water standards (GB57492006) was 0.03 mg L−1, 0.25 mg L−1, and 0.02 mg L−1, respectively. So the sensitivity of our proposed method was sufficient for the detection of OPs in natural water. 3.6.
Real sample analysis
Our proposed method was applied for the analysis of OPs in natural water and a certified reference material (CRM, SB05069-2008, produced by the Agro-environmental Protection Institute, Ministry of Agriculture, China), and the results are listed in Table 6. There was good agreement for OPs in CRM between experimental values and certified values. The recoveries of OPs from spiked water samples were in the range of 93.5–110.5%. So, the precision and accuracy of our proposed method were good.
4.
Conclusions
An inexpensive fully integrated analysis system for simultaneous on-field detection of OPs in water samples was proposed, based on TLC, visible light assisted photocatalysis, and colorimetric detection. Compared to conventional TLC, our proposed method showed excellent performance in the separation and detection limit of OPs. In addition, this method can overcome the problems arising from the direct use of TiO2 in the sample treatment of OPs, including UV irradiation, aggregation, and low oxidation percentage. Moreover, using a camera with image processing software, the determination of OPs was visualized. Although only five kinds of OPs were discussed here, the device could easily be modified for the detection of other OPs. More importantly, in this experiment, the photocatalysis and colorimetric detection were combined with TLC for the first time, providing a novel idea for TLC development, giving TLC a new start.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (20977074, 21175115, and 21475055, S.X.L.), the Program for New Century Excellent Talents in University (NCET-11 0904, S.X.L.), Outstanding Youth Science Foundation of Fujian Province, China (2010J06005, S.X.L.), and the Science & Technology Committee of Fujian Province, China (2012Y0065, F.Y.Z.).
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14 G. Guerrero, P. Mutin and A. Vioux, Chem. Mater., 2001, 13, 4367–4373. 15 N. Negishi, T. Sano, T. Hirakawa, F. Koiwa, C. Chawengkijwanich, N. Pimpha and G.-R. M. Echavia, Appl. Catal., B, 2012, 128, 105–118. 16 G. R. M. Echavia, F. Matzusawa and N. Negishi, Chemosphere, 2009, 76, 595–600. 17 S. X. Li, F. Y. Zheng, S. J. Cai, W. J. Liang and Y. C. Li, Sens. Actuators, B, 2013, 188, 280–285. 18 J. G. Panther, P. R. Teasdale, W. W. Bennett, D. T. Welsh and H. Zhao, Environ. Sci. Technol., 2010, 44, 9419–9424. 19 A. Mukesh, G. Smrati, P. Andrij, E. Z. Nikolaos and S. Manfred, Chem. Mater., 2009, 21, 5343–5348. 20 M. M. Mentele, J. Cunningham, K. Koehler, J. Volckens and C. S. Henry, Anal. Chem., 2012, 84, 4474–4480. 21 X. Yu, S. Liu and J. Yu, Appl. Catal., B, 2011, 104, 12–20. 22 T. Kamegawa, S. Matsuura, H. Seto and H. Yamashita, Angew. Chem., Int. Ed., 2013, 52, 916–919.
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Ascorbic acid surface modified TiO2-thin layers as a fully integrated analysis system for visual simultaneous detection of organophosphorus pesticides† Shunxing Li,*a,b Wenjie Liang,b Fengying Zheng,a,b Xiaofeng Linb and Jiabai Caib TiO2 photocatalysis and colorimetric detection are coupled with thin layer chromatography (TLC) for the first time to develop a fully integrated analysis system. Titania@polystyrene hybrid microspheres were surface modified with ascorbic acid, denoted AA-TiO2@PS, and used as the stationary phase for TLC. Because the affinity between AA-TiO2@PS and organophosphorus pesticides (OPs) was different for different species of OPs (including chlopyrifos, malathion, parathion, parathion-methyl, and methamidophos), OPs could be separated simultaneously by the mobile phase in 12.0 min with different Rf values. After surface modification, the UV-vis wavelength response range of AA-TiO2@PS was expanded to 650 nm. Under visible-light irradiation, all of the OPs could be photodegraded to PO43− in 25.0 min. Based on the chromogenic reaction between PO43− and chromogenic agents (ammonium molybdate and ascorbic acid), OPs were quantified from color intensity images using a scanner in conjunction with image processing software. So, AA-TiO2@PS was respectively used as the stationary phase of TLC for efficient separ-
Received 3rd August 2014, Accepted 16th September 2014
ation of OPs, as a photocatalyst for species transformation of phosphorus, and as a colorimetric probe for
DOI: 10.1039/c4nr04430d
on-field simultaneous visual detection of OPs in natural water. Linear calibration curves for each OP ranged from 19.3 nmol P L−1 to 2.30 μmol P L−1. This integrated analysis system was simple, inexpensive,
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easy to operate, and sensitive.
1.
Introduction
Organophosphorus pesticides (OPs) are used extensively for controlling insects in order to increase agricultural production. However, their high biochemical activity can also pose unacceptable risks to the environment and human health.1,2 Regulatory limits and guideline values for pesticide residues in drinking water have been set by many countries and are summarized by the International Union of Pure and Applied Chemistry (IUPAC).3 Therefore, efficient, sensitive, and selective analytical methods for the determination of trace OP residues in environmental water are required. Several validated methods are currently available for the identification and quantification of OPs, including capillary electrophoresis, gas chromatography, liquid chromatography, and chromatographic techniques coupled to mass spectrometry.4,5 These methods need expensive instruments, complicated
a Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Zhangzhou, 363000, China b College of Chemistry and Environment, Minnan Normal University, Zhangzhou, 363000, China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr04430d
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and time-consuming pretreatments, and well trained operators (which can limit their applications in developing countries) and on-field monitoring.6 So, enzyme-based biosensors have emerged in the past few years as an alternative technique for on-site detection of pesticides. But the enzyme can be easily inactivated due to the complicated water environment.7,8 Therefore, a simple, rapid, and low cost technology for on-field separation and detection of OPs in water samples is of great practical interest. Thin layer chromatography (TLC) is particularly well suited for simultaneous on-field detection of OPs due to its advantages of simple operation, low cost (including devices and operating costs), rapidity, and valid identification and detection based on chemical color reactions.9–11 Unfortunately, the application of TLC is limited by its poor separation capability and sensitivity. Sample preparation such as elution and digestion can give rise to additional contamination and errors. The properties of TLC media and the interaction between the stationary phase and analytes in sample mixtures are dominated by the stationary phase morphology and surface chemistry.12 When new stationary phases and new sample pretreatment methods are used together, the separation capability of TLC can be increased, the contamination can be avoided,
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the blanks and matrix effects can be reduced, and the performance of TLC in the detection of OPs can be improved. Moreover, the color intensity of the images can be quantified using a camera with image processing software and then the sensitivity and detection limit of TLC are improved. Photochemistry-based sample treatment is a greener approach owing to its subsequent achievements in analytical characteristics (e.g., improved detection limits, low blanks, fewer matrix effects and simpler procedures) and greenness-related issues (environmentally friendly, safe procedures, cost-effective methods, and minimum use of chemicals).13 Herein, visiblelight photocatalysis is coupled with TLC by us for the first time to transform all of OPs into PO43− without using any chemicals. As a stationary phase of TLC, TiO2-based nanocomposites are used for the first time, due to their excellent properties, including high surface area/body weight ratio and affinity with organophosphorus compounds.14 Moreover, nanometer size TiO2 is widely used for the photodegradation of organophosphorus15,16 owing to its advantages such as low cost, longterm photochemical stability, and nontoxicity.17 A commercially available TiO2 based adsorbent has been used for in situ preconcentration of phosphorus in fresh and marine waters, and the affinity between TiO2 and phosphate is high.18 However, such affinity is based on the groups of Ti–O–P,14 and the separation capability of pure TiO2 as the stationary phase is limited by non-selective interaction. Worse still, during the application, the adsorption sites of OPs would be decreased by the aggregation of TiO2 nanoparticles. The photodegradation of organophosphorus should be induced by UV irradiation for 60 min15,16 and can be limited by the extremely low coverage of organic pollutants on the TiO2 photocatalyst.17 Polystyrene (PS) was used as the core to control the size and morphology of TiO2. Ascorbic acid surface modified titania@ polystyrene core–shell hybrid microspheres were used and were denoted AA-TiO2@PS in order to: (a) avoid the aggregation of TiO2, (b) introduce the functional groups of ascorbic acid onto the surface of TiO2, (c) adjust the affinity between AA-TiO2@PS and OPs for the separation of OPs, (d) expand the UV-Vis wavelength response range of TiO2 into the visible-light region, (e) improve the photodegradation capability of TiO2, and (f) bring a reducing agent (i.e., ascorbic acid) to form molybdenum blue for colorimetric detection of OPs. So, AA was used as a surface modification agent for visible-light-driven photocatalysis and as a color development reagent in this study. Herein, the study is focused on the development of a new fully integrated analysis system for OPs, using AA-TiO2@PS as (a) the stationary phase of TLC for efficient separation of OPs, (b) a photocatalyst for species transformation of phosphorus, and (c) a colorimetric probe for on-field simultaneous visual detection of OPs in natural water.
2. Experimental section 2.1.
Materials
Five species of OP standards were produced by the Agroenvironmental Protection Institute, Ministry of Agriculture,
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China, and their structure and molecular weight are shown in Table S1.† All other chemicals were of AR grade and were used directly without further purification. A cut-off filter ZWB2 was produced by Nanjing Stonetech Electric Equipment Co., Ltd (China). Deionized water was purified using a Millipore-Q system (Millipore Co., USA) and were used as a solvent with a resistivity of 18.2 MΩ cm−1. 2.2.
Fabrication of a fully integrated analysis system
2.2.1. Preparation of polystyrene microspheres. Polystyrene (PS) microspheres were synthesized by a typical method that was used by Mukesh et al.19 A double-necked flask equipped with a reflux condenser, a mechanical stirrer, a temperature controller, and a nitrogen inlet was used. The air in this flask was replaced by a stream of N2 and then 85.0 g of water, a certain amount of styrene, and 0.5 g of methacrylate were added into the flask. This mixture was kept under N2 until the polymerization was complete. After bubbling N2 gas through the reaction media for 30.0 min, the reaction system was heated to 70.0 °C, and 5.0 mL of potassium persulfate solution (30.0 mg mL−1) was added to start the polymerization process. The polymerization reaction was carried out at 70.0 °C for 24.0 h and then PS microspheres were collected. 2.2.2. Synthesis of titania@polystyrene core–shell hybrid microspheres (TiO2@PS). One milliliter of PS microspheres dispersion (10.0 wt%), 0.15 g of deionized water, and a polyvinyl pyrrolidone K30 solution (i.e., 0.01 g of polyvinyl pyrrolidone K30 surfactant was dissolved in 72.0 mL of ethanol) were mixed in a round-bottomed flask and then sonicated for 15.0 min. With continuous stirring, a certain amount of tetrabutyl titanate was added to the round-bottomed flask. This mixture dispersion was refluxed at 80 °C for 4.0 h. After centrifugation, the precipitates with 40 ml water were transferred into a Teflon-lined stainless steel autoclave (100 mL), heated to 120 °C for 2 h, and centrifuged again. The precipitate was washed with ethanol three times and then dried at 60.0 °C for 8 h. 2.2.3. Preparation of AA-TiO2@PS. In situ surface modification was carried out by stirring TiO2@PS microspheres in ascorbic acid solution (0, 50, 100, 200, 300, and 400 mg L−1, respectively) at pH 4.0 for 30 min in the dark. After surface modification, the mixture was centrifuged at 10 000 rpm and then AA-TiO2@PS with a color of rufous was rinsed with deionized water three times and dried under vacuum at 60.0 °C for 12.0 h. 2.2.4. Preparation of TLC plates. AA-TiO2@PS (5 g), CaSO4·1/2H2O (0.5 g, as a fixing agent), and sodium carboxymethyl cellulose (0.5% m/v, 13.0 mL, as a cross-linking agent) were co-added and stirred until homogeneous slurry was obtained. The slurry was coated onto a clean glass plate using a wire bar coater and the thickness was 200 μm. These TLC plates were dried under vacuum at 60.0 °C and kept in an airtight chamber. After being activated at 105 °C for 30.0 min, TLC plates were used. 2.3.
Colorimetric detection of OPs
In the preliminary TLC studies, 20.0 μL of OPs were spotted by means of a microsyringe at 0.8 cm from the bottom of the TLC
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plate and the spot was dried at 50.0 °C. The above operations were repeated 2 times. In a chromatographic chamber, these TLC plates were placed in 10.0 mL of the mobile phase for 12.0 min in the dark. Using an ascending technique, five species of OPs were separated simultaneously. The TLC plates were taken out and irradiated with visible light (350 W xenon lamp; filtered λ < 385 nm incident using a cut-off filter ZWB2, 23 400 lux) for 25.0 min. Molybdate solution was sprayed onto the TLC plates and developed for 5.0 min in the dark. After drying the plates, five spots with a color of baby blue appeared on the TLC plates and then the Rf values (i.e., distance from the origin to the component spot/distance from the origin to the solvent front) could be calculated. 2.4.
Quantitative image processing
Images of TLC plates were filmed and stored in JPEG format at 300 dpi. The average gray intensity of color at the detection reservoir was measured using the image processing software ImageJ.20 Once optimized, the threshold ranges set for each image were all the same. This process was demonstrated in the ESI.† The interested regions in the image could be automatically selected by the software and the average gray intensity was measured. The data were imported into Origin 8.0 for the determination of OPs.
3. Results and discussion 3.1.
SEM images
The separation ability of TLC was associated with the morphology of the stationary phase. The morphology of PS and TiO2@PS was investigated by SEM. As shown in Fig. 1a, the PS templates were microspheres with a uniform and smooth surface with a mean diameter of 400 nm. After coating with a TiO2 layer, there was no significant variation in the overall morphology of the final products TiO2@PS with a mean diameter of 500 nm (seen in Fig. 1b). The TiO2 layers could be uniformly coated onto the PS templates via catalytic self-condensation of hydroxyl groups.21 The loading amount of TiO2 on PS spheres was investigated by thermogravimetry (TG). As shown in Fig. S2,† the PS spheres could be decomposed at 450 °C and the loading amount of TiO2 on PS spheres was about 54%.
Fig. 1
SEM images of PS (a) and TiO2@PS (b).
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3.2.
Effect of surface modification degree
TiO2@PS nanoparticles were in situ surface modified by the chemisorption of ascorbic acid, i.e., specific binding between ascorbic acid and ortho-substituted hydroxyl groups on the surface of TiO2 22 led to the formation of bidentate surface complexes with five-membered rings.23,24 These surface complexes were stable during the process of photodegradation.25,26 So, in Fig. 2, TiO2@PS failed to absorb light (λ > 380 nm), but the UV-vis absorption wavelength response range of AA-TiO2@PS was shifted toward 650 nm for the influence of surface complexes on AA-TiO2@PS. AA-TiO2@PS could be used as a visible-light-driven photocatalyst. With the increase of concentration of AA, more and more bidentate surface complexes were formed, which could improve the electron transfer and enhance the visible light absorption. But when the concentration of AA reached 300 mg L−1, the light absorption by AA-TiO2@PS was relatively stable, because such visible light absorption was induced by a direct electron transfer from surface complexes to the conduction band of TiO2,27 and the light absorption intensity was controlled by the contents of surface complexes. Because AA could participate in the colorimetric reaction, its amount should be sufficient. The effect of AA concentration on the colorimetric detection of phosphate was studied using 2.45 μmol L−1 of PO43−. As shown in Fig. S3,† the gray scale intensity was linearly increased with increasing the AA concentration from 50 to 200 mg L−1, and thereafter the grayscale intensity was stable even though more AA was used. Thus, the AA concentration of 300 mg L−1 was chosen for further experimental work. OPs could be photodegraded into PO43− by TiO2-based nanocomposites.28 As one species of OPs with the highest molecular weight, chlopyrifos was chosen to compare the photodegradation ability of AA-TiO2@PS and TiO2@PS. According to the concentration of PO43−, the photodegradation ratio was calculated and then the results are shown in Fig. 3.
Fig. 2 UV-vis absorption spectra of TiO2@PS microspheres surface modified by ascorbic acid (AA) with different concentrations (0 mg L−1, 50 mg L−1, 100 mg L−1, 200 mg L−1, and 300 mg L−1).
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the complex into the conduction band of TiO2 in a coherent one-step manner (Fig. 4). Such a photoexcitation and injection process was often referred to as ligand to metal charge transfer.29 This spectral extension was able to make the colorless organic compounds undergo photoinduced reactions on the surface of TiO2 under visible light irradiation. 3.3. Influence of the stationary phase and the mobile phase on separation performance
Fig. 3 Photodegradation ratio of chlopyrifos irradiated by visible light (1.0 mg L−1 chlopyrifos and 1.0 g L−1 photocatalyst).
After surface modification with ascorbic acid, i.e., AA-TiO2@PS was used, the photodegradation could be induced by visible light and the photodegradation performance of TiO2@PS on OPs could be significantly improved. The time-dependent photodegradation curves for the other four OPs are included in Fig. S4.† Chlopyrifos, malathion, parathion, parathionmethyl, and methamidophos could be photodegraded into PO43− in 25.0 min. The photooxidation pathway of OPs on AA-TiO2@PS is proposed in Fig. 4. When there was a strong electronic coupling between the π orbital of organic compounds and the 3d orbital of Ti4+, a surface charge-transfer complex might be formed between organic compounds and coordinatively unsaturated surface Ti(IV) atoms.29 Upon excitation of the charge-transfer complex, the electron was promoted from the ground state of
The separation performance of TLC was controlled by the properties of the mobile and stationary phases. AA-TiO2@PS with different diameter levels (300, 400, 500, 600, and 700 nm, respectively) and mixed solvent (hexane–acetone–methanol– water, v/v = 5 : 2 : 1.5 : 1.5) were used as the stationary phase and the mobile phase, respectively, and the Rf values of OPs are listed in Table 1. With the increase of the AA-TiO2@PS microsphere size from 300 nm to 500 nm, the specific surface area of AA-TiO2@PS was decreased, its adsorption capacity of OPs was weakened obviously, and the Rf values of OPs were increased. When the microsphere size of AA-TiO2@PS was beyond 500 nm, the separation performance was decreased rapidly. Consequently, 500 nm was selected as the microspheres size for further experiments. In Table S2,† we found that the influence of the mobile phase on the separation performance of TLC could not be ignored. The polarity of OPs was different with the difference of the species of OPs, the solubility of different species of OPs in same solvent or same species of OPs in different solvent is different, and then the interaction between OPs and the mobile phase was different. Therefore, using different mobile phases and the same TLC plate, the separation performance was different. The optimum mobile phase was the mixed solvent of hexane, acetone, methanol, and water (v/v = 5 : 2 : 1.5 : 1.5). Five species of OPs were dissolved respectively in the above optimum mobile phase. Using AA-TiO2@PS (or TiO2@PS, 500 nm) as an adsorbent, the adsorption kinetics study was used to explain the influence of the stationary phase, especially the surface modification of TiO2 with ascorbic acid, on the separation performance. The kinetic data were fitted to the following eqn (1): lnðqe qt Þ ¼ ln qe kt
Table 1
ð1Þ
Rf value of OPs
Diameter of ascorbic acid-surface modified polystyrene/titania hybrid microspheres 300 nm
Fig. 4 Schematic illustration of the photooxidation pathway of OPs on AA-TiO2@PS.
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OPs
Rf value
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
0.45 0.29 0.19 0.28 0.45
400 nm
500 nm
600 nm
700 nm
0.72 0.56 0.33 0.48 0.73
0.86 0.79 0.49 0.69 0.90
— 0.90 0.68 0.89 —
— — 0.84 0.98 —
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Table 2
Pseudo first order kinetic equations and parameters (temperature 303 K)
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AA-TiO2@PS
TiO2@PS
OPs
Equation
R2
qe,exp (mg g−1)
Equation
R2
qe,exp (mg g−1)
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
y = −0.0603x + 3.7868 y = −0.0537x + 3.6577 y = −0.0754x + 4.0827 y = −0.0598x + 3.8102 y = −0.051x + 3.527
0.9707 0.9748 0.9482 0.9543 0.9302
21.04 20.04 19.45 19.89 18.89
y = −0.0532x + 3.5504 y = −0.0557x + 3.6902 y = −0.0677x + 3.9175 y = −0.0621x + 3.8102 y = −0.0707x + 3.9424
0.9969 0.9848 0.9718 0.9753 0.9295
20.11 20.44 19.85 20.01 19.45
where qe is the equilibrium adsorption capacity, qt is the adsorbed amount of OPs at time t, and qe and qt were calculated by HPLC. The equation, qe and R2 are shown in Table 2 (y = ln(qe − qt), x = t ). The plots of ln(qe – qt) versus t showed a certain linear relationship (i.e., R2 was in the range of 0.9295–0.9969) for TiO2@PS. However, the value of R2 for each species of OPs on AA-TiO2@PS was less than that on TiO2@PS. A comparison of kinetic models applied to the adsorption of OPs on AA-TiO2@PS was evaluated for the pseudo first-order and the pseudo second-order, respectively. According to the results (shown in Table S3†), the pseudo first-order kinetic model was found to correlate well with the experimental data, suggesting physical adsorption as the dominant process. But their R2 values were less than 0.98, which indicated the existence of chemical adsorption between OPs and surface complexes from AA-TiO2@PS. Furthermore, the affinity was different for different species of OPs, so their R2 were different. After adsorption for 12 min, the adsorption capacity of OPs by AA-TiO2@PS was increased in the order: methamidophos < chlopyrifos < malathion < phoxim < parathion-methyl (seen in Table 3) and this order was opposite to the Rf value. As the affinity between AA-TiO2@PS and different species of OPs was different, OPs could be separated simultaneously by the mobile phase in 12 min with different Rf values. 3.4.
Degradation kinetic studies
The dependence of photodegradation rates on the concentration of organic pollutants could be described well by the Langmuir–Hinshelwood (L–H) kinetic model.30 The modified L–H equation is given by: r¼
dC kr KC ¼ dt 1 þ KC
ð2Þ
But during the photodegradation, the intermediates were formed and their competitive adsorption and degradation against OPs might interfere in the determination of kinetics.
Table 3
Therefore, the rate of formation of PO43− was used to represent the rate of reaction: r¼
1 dCi kr KCi ¼ vi dt 1 þ KCi
ð3Þ
where kr is the reaction rate constant, K is the reactant adsorption constant, vi is the stoichiometric coefficient of PO43−, and Ci is the concentration of PO43− at any time t. Because the OPs used were not more than 2.45 μmol P L−1, after photodegradation, Ci was lower, and so KCi ≪ 1, eqn (3) could be simplified as: r¼
1 dCi ¼ kKCi vi dt
ð4Þ
When t = 0, the concentration of PO43− was zero (Ci0 = 0); when t = t, the concentration of PO43− was Ci. Integrating eqn (3): t¼
1 1 ln Ci vi kK
ð5Þ
ln Ci − t shows a linear relationship. So the photodegradation reaction was a first order kinetics reaction (seen in Table 4, y = ln Ci, x = t ). When the initial concentrations of OPs were 2.0 μmol P L−1, all OPs could be photodegraded into PO43− after irradiation for 25.0 min. According to the results of Tables 3 and 5, a positive correlation was found between the decomposition rate and the adsorption rate, so the degradation rate depended on the diffusion rate (i.e., adsorption rate) of OPs from the bulk solution onto the surface of AA-TiO2@PS. The diffusion rate was controlled by the affinity between AA-TiO2@PS and OPs, which were different for different species of OPs. 3.5.
Colorimetric assay of OPs
The most widely recommended method for phosphate determination is the phosphomolybdenum blue method (PMB), because of its higher sensitivity and lower susceptibility to
Adsorption capacity of OPs by AA-TiO2@PS for 12 min
OPs
Chlopyrifos
Malathion
Parathion-methyl
Phoxim
Methamidophos
Adsorption capacity (μmol g−1)
9.68
11.24
16.44
13.18
9.57
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Table 4 First order kinetic equations of photodegradation (initial concentration of OPs 2.0 μmol P L−1)
OPs
Equation
R2
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
y = 0.2318x − 3.8169 y = 0.1994x − 3.2398 y = 0.2321x − 3.8166 y = 0.2208x − 3.617 y = 0.1797x − 2.908
0.9911 0.9974 0.9925 0.9906 0.9996
Table 5
Five regression equations of OPs
OPs
Equation
R2
Chlopyrifos Malathion Parathion-methyl Phoxim Methamidophos
I = 0.1054C + 77.962 I = 0.1001C + 92.871 I = 0.1153C + 100.53 I = 0.1066C + 95.574 I = 0.1202C + 104.4
R2 = 0.9943 R2 = 0.9831 R2 = 0.9798 R2 = 0.9842 R2 = 0.9905
interference.31 Using this PMB technique, orthophosphate and a small amount of condensed phosphorus compounds could be measured.31 Under visible-light irradiation for 25 min, all OPs could be photodegraded by AA-TiO2@PS, the phosphate was transformed into PO43−, which was determined by the PMB technique. The conversion ratio of phosphate was in the range of 96.8–98.8%, as shown in Table S4.† Under the optimum experimental conditions described above, the working curve for OPs determination was obtained by establishing a linear correlation between the concentration of OPs and the color intensity, which is shown in Fig. 5. Five regression equations are listed in Table 5. The lowest amount of OPs on the TLC plate that could be detected reproducibly was 19.3 nmol P L−1 and the linear calibration curves for each OP ranged from 19.3 nmol P L−1 to 2.30 μmol P L−1. According to the European Union directive on water quality (98/83/CE), the maximum admissible concentration for pesticides was 0.1 mg L−1 for each individual
Fig. 5 Working curve for OPs determination obtained under optimum experimental conditions (the concentration of P from the bottom to the top was 5, 10, 20, 50, 100, 200, 300, 400, 500, and 600 μg L−1, respectively).
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Table 6
Analytical results for the Jiulong river water
Jiulong river water Organophosphorus pesticides
Added (μg L−1)
Found (μg L−1)
Recovery (%)
Certified values (μg L−1)
This work (μg L−1)
HPLC (μg L−1)
Chlopyrifos
0 20.0 0 20.0 0 15.0 0 15.0 0 10.0
7.5 26.2 — 18.7 — 16.2 — 16.5 — 10.9
— 93.5 — 93.5 — 108.0 — 110.0 — 109.0
—
—
—
50.0
51.8
50.7
50.0
51.9
50.6
50.0
48.1
50.4
40.0
41.7
40.8
Malathion
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Mixed standard sample (SB05-069-2008)
Parathion-methyl Phoxim Methamidophos
substance, and 0.5 mg L−1 was the maximum total concentration of all organophosphorus pesticides. In China, the maximum admissible concentration of chlopyrifos, malathion, and parathion-methyl for drinking water standards (GB57492006) was 0.03 mg L−1, 0.25 mg L−1, and 0.02 mg L−1, respectively. So the sensitivity of our proposed method was sufficient for the detection of OPs in natural water. 3.6.
Real sample analysis
Our proposed method was applied for the analysis of OPs in natural water and a certified reference material (CRM, SB05069-2008, produced by the Agro-environmental Protection Institute, Ministry of Agriculture, China), and the results are listed in Table 6. There was good agreement for OPs in CRM between experimental values and certified values. The recoveries of OPs from spiked water samples were in the range of 93.5–110.5%. So, the precision and accuracy of our proposed method were good.
4.
Conclusions
An inexpensive fully integrated analysis system for simultaneous on-field detection of OPs in water samples was proposed, based on TLC, visible light assisted photocatalysis, and colorimetric detection. Compared to conventional TLC, our proposed method showed excellent performance in the separation and detection limit of OPs. In addition, this method can overcome the problems arising from the direct use of TiO2 in the sample treatment of OPs, including UV irradiation, aggregation, and low oxidation percentage. Moreover, using a camera with image processing software, the determination of OPs was visualized. Although only five kinds of OPs were discussed here, the device could easily be modified for the detection of other OPs. More importantly, in this experiment, the photocatalysis and colorimetric detection were combined with TLC for the first time, providing a novel idea for TLC development, giving TLC a new start.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (20977074, 21175115, and 21475055, S.X.L.), the Program for New Century Excellent Talents in University (NCET-11 0904, S.X.L.), Outstanding Youth Science Foundation of Fujian Province, China (2010J06005, S.X.L.), and the Science & Technology Committee of Fujian Province, China (2012Y0065, F.Y.Z.).
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