Nano-Anatase-Enhanced Peroxyoxalate Chemiluminescence and Its

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Nano-Anatase-Enhanced Peroxyoxalate Chemiluminescence and Its Sensing Application Liang Yang,†,‡ Guijian Guan,† Suhua Wang,*,†,‡ and Zhongping Zhang*,†,‡ †

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, China Department of Chemistry, University of Science & Technology of China, Hefei, Anhui 230026, China

‡

S Supporting Information *

ABSTRACT: This paper reports a new nanosized anatase particle enhanced chemiluminescence sensor that utilizes the catalytic surface of anatase for sensitive detection of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). This chemiluminescence sensor was composed of anatase nanoparticles grafted with the nitrobenzoxadiazole (NBD) fluorophore, bis(2,4,6-trichlorophenyl)oxalate (TCPO), and hydrogen peroxide (H2O2). The chemiluminescence efficiency of the sensor has been greatly enhanced by 6 times compared with that in the absence of nano-anatase. However, 2,4-D could greatly suppress the chemiluminescence enhancement of anatase nanoparticles probably by adsorbing and competitively reacting with the activated hydrogen peroxide on the anatase surface. The phenomenon has been used to detect 2,4-D by monitoring the quenching of the chemiluminescence of the system. The limit of detection of the chemiluminescence sensor system was estimated to be as low as 0.33 nmol/L. The simple and sensitive sensor reported herein exhibited an effective combination of traditional chemiluminescence with nano-anatase for sensitive detection, thus promoting the advances of chemiluminescence sensing on the basis of nanomaterials.

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INTRODUCTION Chemiluminescence has been widely used as a rapid analytical technique in many basic research and practical applications, such as biology, pharmacology, environmental chemistry, clinical diagnosis, and food analysis, due to its low background nature, high sensitivity, simplicity, and low cost of instrumentation.1−4 The emission resulting from the reaction of some oxalic acid derivatives is one of the most studied branches of chemiluminescence.5 This group of chemiluminescence reactions, called peroxyoxalate chemiluminescence (PO-CL), shows powerful analytical potential for a wide variety of fluorescent compounds. The detection method on the basis of the PO-CL system offers an advantage in chemiluminescence imaging or integrated determination, ascribed to the virtue of its steady chemiluminescence and relative high quantum yield.6−8 Bis(2,4,6-trichlorophenyl)oxalate (TCPO) is one of the most widely used oxalate esters in the PO-CL system, and the reaction between TCPO and hydrogen peroxide (H2O2) has been shown to be able to excite various fluorescent molecules and yield chemiluminescence.1,9 Recently, the combination of nanomaterials with chemiluminescence has attracted substantial interest because the nanomaterials have been found to possess the ability as an energy acceptor,10 catalyst,11 and chemiluminescence resonance energy transfer platform.12 It has been reported that nanomaterials possess reactive surfaces, high surface areas, and good adsorption characteristics, which could be utilized to improve the chemiluminescence efficiency, and hence have great © 2012 American Chemical Society

significance for the development of new chemiluminescence reaction systems. For example, Cui et al reported that noble metal nanoparticles, such as gold nanoparticles, could be excited by the peroxyoxalate reaction and emitted light.10 A chemiluminescence resonance energy transfer platform for sensing in biomedical applications has also been established based on different-sized water-soluble quantum dots.11 Zhang et al developed a nanomaterials-based sensing system with high sensitivity and selectivity using the chemiluminescence from the catalytic oxidation of organic molecules on the surface of nanosized titanium dioxide (TiO2).12 As one of the most photoactive materials, TiO2 has already been shown to have the ability in the conversion between photoenergy and chemical energy. In addition, TiO2 shows chemical stability and low toxicity, so it could play a more important role in the development of nanomaterial-enhanced chemiluminescence for sensing applications.13 Herein, we demonstrate an effective chemiluminescence sensor composed of TCPO and anatase nanoparticles grafted with fluorophores. The nanosized anatase showed a higher chemiluminescence efficiency than the amorphous TiO2 and SiO2 nanospheres. The enhanced chemiluminescence has been found to be very sensitive to phenoxyacetic acids, and hence, this system has been immobilized in a thin polymer film for Received: October 27, 2011 Revised: January 6, 2012 Published: January 6, 2012 3356

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sensitive and selective detection of such compounds, such as the herbicide 2,4-D.

Scheme 1. Schematic Diagram Illustrating the Synthesis of the NBD−APTS Conjugate and the NBD-MoietyFunctionalized Anatase Nanoparticles

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EXPERIMENTAL SECTION Materials and Solutions. 2,4-Dichlorophenoxyacetic acid (2,4-D), tetrabutyltitanate, 3-aminopropyltriethoxysilane (APTS), tetraethylorthosilicate (TEOS), and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-solution) were used as received from Sigma-Aldrich. Ammonium hydroxide (25%), anhydrous ethanol, acetonitrile, and poly(vinyl alcohol) (PVA) were purchased from Sinopharm Chemical Reagent Co. Ltd. Bis(2,4,6-trichlorophenyl)oxalate (TCPO) was obtained from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). A working solution of TCPO was prepared by dissolving the solids in acetonitrile, and the working solutions of H2O2 were prepared daily by diluting 30% (v/v) H2O2 in acetonitrile. Synthesis of TiO2 Nanoparticles and Anatase Transformation. The amorphous TiO2 nanoparticles were prepared by a sol−gel procedure with tetrabutyltitanate as the starting material. Briefly, 1.56 mL of tetrabutyltitanate was mixed with 36 mL of acetonitrile and 18 mL of anhydrous ethanol in a dry nitrogen atmosphere. The mixture solution was then added dropwise into a mixture containing 0.5 mL of ammonium hydroxide, 36 mL of acetonitrile, and 18 mL of anhydrous ethanol at 4 °C under vigorous stirring. After continuous stirring for 3 h, a milky sol was obtained, which was then allowed to stand for 6 h at room temperature. The assynthesized TiO2 gel is amorphous in structure and ∼250 nm in diameter (am-TiO2). The am-TiO2 gel was treated with 365 nm UV light for 12 h, resulting in the transformation from amorphous to crystalline anatase. For comparison, SiO2 nanoparticles were synthesized by the hydrolysis of TEOS in aqueous ammonia solution according to the method reported previously.14 The as-synthesized SiO2 is amorphous in structure and ∼200 nm in diameter. Grafting of TiO2 Nanoparticles with Nitrobenzofurazan. Typically, 11.8 mg of NBD-Cl was first dissolved in 10 mL of anhydrous ethanol solution containing 0.5 mL of APTS (1:40 in molar ratio) under slow stirring in a dry nitrogen atmosphere. After 12 h of reaction in the dark, NBD-Cl reacted with APTS to form NBD−APTS conjugates by the nucleophilic substitution reaction in the mixture solution.15 The above mixture was then added into 100 mL of an ethanol suspension of anatase (0.6 g) and am-TiO2 nanoparticles (0.6 g), respectively. The surfaces of these TiO2 nanoparticles were then grafted with monolayers of both the NBD−APTS conjugates and the residual APTS monomers due to the hydrolysis of the silane groups and the hydroxyls on TiO2 surfaces (Scheme 1). The product was then separated by centrifugation and purified by washing three times with ethanol. The NBD-moiety-functionalized anatase and amorphous TiO2 nanoparticles were abbreviated as NBD-anatase and NBDamTiO2 from here on for easy communication. Both the two functional TiO2 nanoparticles were redispersed in ethanol (50 mL) for future use. Similarly, SiO2 nanoparticles grafted with the NBD−APTS functional were also prepared following the same procedure (NDB-SiO2). For comparison, the conventional chemiluminescence system composed of free NBD− ATPS, TCPO, and H2O2 was also prepared using the same measurement procedure (NBD-solution). Immobilization of the NBD-Anatase Sensor in Polymer Film. A 6 mg portion of the NBD-anatase was suspended in 1 mL of water solution containing 0.2% PVA. A

10 μL portion of the suspensions was applied to the well bottoms of polypropylene microtiter plates (96 wells) and dried at 70 °C for 30 min, resulting in the formation of a transparent thin film with immobilized NBD-anatase at the bottom of the wells. The chemiluminescence sensing experiments were performed in such functionalized microplate wells. Chemiluminescence Measurements. For the sensor immobilized in PVA films, each well was incubated for 2 h with 50 μL of the target solutions containing different concentrations of the analyte. The wells were then thoroughly washed three times with ethanol and dried in air. Subsequently, 50 μL of TCPO acetonitrile solution and 50 μL of H2O2 acetonitrile solution were added simultaneously into the wells, followed by collecting the chemiluminescence signals. For the blank experiment, the same volumes of blank solvent were applied to the immobilized NBD-anatase sensor in PVA films, followed by recording the chemiluminescence. The control experiments with the sensor suspended in the solution phase were also performed by the following procedure. A 10 μL portion of the NBD-anatase ethanol suspensions (6 mg/mL) was added into the wells of the plate. A 50 μL portion of the analyte ethanol solutions, e.g., 2,4-D, with different concentrations was then added into the wells, followed by incubation for 2 h. After that, 50 μL of TCPO acetonitrile solutions (3 mmol/L) and 50 μL of H2O2 acetonitrile solutions (4 mol/L) were then injected into the wells at the same time. The chemiluminescence signals were monitored. Instrumentation. Chemiluminescence signals were recorded in a 96-well polypropylene microtiter plate using a Berthold LB 960 microplate luminometer. The formation of the NBD−APTS conjugates was determined with a Proteome XLTQ mass spectrometer employing a regular ESI source setup. The UV−visible absorption spectra were obtained with a Shimadzu UV-2550 spectrometer. The FT-IR spectra were obtained with a Thermo Scientific Nicolet iS10 spectrometer. The morphologies of the TiO2 nanoparticles were examined by a FEI Sirion-200 field emission scanning electron microscope (FE-SEM). The crystalline structure of anatase TiO2 nanoparticles was examined by X-ray diffraction on a Philips X’Pert Pro with Cu Kα radiation (1.5418 Å). Thermogravimetric analysis (TGA) experiments were carried out on a TGA Q5000 under the condition of 10 °C/min with a nitrogen flow at 75 mL/min.

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RESULTS AND DISCUSSION Functionalization of Nano-Anatase with NBD Fluorophores. To functionalize the amorphous and anatase TiO2 nanoparticles with a hybrid monolayer of the NBD fluorophore 3357

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Figure 1. (A) ESI-MS spectra of the NBD−APTS conjugates. (B) Fluorescence spectrum of NBD−APTS conjugates obtained under 470 nm excitation (inset shows the photographs of NBD-Cl and NBD−APTS conjugate ethanol solutions under a UV lamp).

Figure 2. (A) SEM images of NBD-APTS grafted anatase nanoparticles (NBD-anatase) show a monodispersed spherical morphology. (B) FT-IR spectra of (a) pure anatase and (b) NBD-anatase nanoparticles. (C) Thermogravimetric analysis curves of anatase nanoparticles (a) before and (b) after functionalization with NBD moieties. (D) UV−vis spectra of (a) pure anatase, (b) NBD−APTS conjugates, and (c) NBD-anatase nanoparticles.

conjugates and the excess APTS monomers in the mixture were then anchored onto the surface of the TiO2 nanoparticles through a condensation reaction between the alkoxysilane residues of APTS and the hydroxyl groups on the TiO2 surface,16 which led to the formation of a hybrid monolayer of NBD fluorophores with secondary amino groups, as illustrated in Scheme 1. The unreacted NBD-Cl and APTS in the solution were then removed by centrifugation. The TiO2 nanoparticles remained monodispersed after surface grafting with the NBD-APTS fluorophores, as shown in the SEM image of Figure 2A. By comparing the emission spectra of NBD−APTS conjugates with the functionalized TiO2 nanoparticles, it can be concluded that the NBD−APTS conjugates grafted onto the surface of the TiO2 nanoparticles and had the same fluorescence properties

and amino group, the reactive NBD-Cl fluorophore was first covalently linked with APTS through a nucleophile reaction, as shown in Scheme 1. The formation of NBD−APTS conjugates can be proved with positive-mode ESI-MS spectra (Figure 1A). It can be seen that the predominant peak has a mass-to-charge ratio (m/z) of 385.08, consistent with the chemical structure of the NBD−APTS conjugate. The result suggests the successful covalent linkage between NBD and APTS. Figure 1B shows the fluorescence properties of the NBD−APTS conjugates, and the inset images were taken from the NBD-Cl fluorophore and NBD−APTS conjugates under UV lamp illumination. Clearly, the NBD−APTS conjugates have a green fluorescence, whereas the NBD-Cl before conjugation with APTS does not show any fluorescence. This result also indicates the bond formation between NBD and APTS. The resultant NBD−APTS 3358

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Figure 3. (A) Chemiluminescence profiles of the (a) NBD-anatase, (b) NBD-amTiO2, (c) NBD-SiO2, and (d) free NBD-solution. The inset shows the initial maximum chemiluminescence intensities of the four different systems. Final concentrations: TCPO, 1.0 mmol/L; H2O2, 1.3 mol/L. (B) Powder XRD pattern of (a) TiO2 nanoparticles treated with UV light for 12 h and (b) as-synthesized TiO2 nanoparticles without UV treatment. The vertical lines indicate the standard XRD stick pattern of crystalline anatase.

subsequently oxidizes the TCPO readily to produce the energyrich intermediate.17 It has been reported that the anatase surface could catalyze H2O2 to generate OOH, so the boosted production of OOH on nano-anatase could be the first reason for the chemiluminescence enhancement.18−20 The more the OOH species were generated, the faster the oxidation of TCPO and the more energy-rich intermediates were then produced. Further experiments show that the chemiluminescence enhancement of NBD-anatase is proportional to the UV light treatment time for the phase transformation of amorphous TiO2 within 12 h (Figure S2, Supporting Information). After 12 h of UV light treatment, the chemiluminescence intensity keeps constant, indicating that 12 h of UV illumination is enough to transform the amorphous TiO2 to crystalline anatase, as evidenced by powder X-ray diffraction (XRD) results (Figure 3B). For the positive control experiment, the as-synthesized amorphous TiO2 was also transformed into crystalline anatase by calcination overnight under 200 °C. Such anatase nanoparticle samples also have the same ability to enhance the chemiluminescence intensity (Figure S3, Supporting Information). Therefore, the enhancement of the chemiluminescence intensity can be attributed to the surface catalytic properties of nano-anatase. Optimization of the TCPO Concentrations. As one of the chemiluminescence significant variables, the concentration of TCPO was optimized when the concentration of hydrogen peroxide was fixed at 4 mol/L. A 10 μL portion of NBD-anatase (6 mg/mL) nanoparticles, 50 μL of 2,4-D (10 μmol/L), and 50 μL of TCPO solutions were added into a polypropylene microtiter plate. Upon the addition of 50 μL of H2O2, the relationship between the chemiluminescence intensities and the TCPO concentration was obtained. The TCPO concentration was varied in the range of 3.3 μmol/L to 2.7 mmol/L at a constant concentration of H2O2 in the presence and absence of 2,4-D, respectively (Figure 4). The chemiluminescence intensity increased gradually with the increasing of TCPO concentrations and reached the maximum when the concentration of TCPO was at about 1 mmol/L, whether in the presence or in the absence of 2,4-D. These results indicate that the optimized concentration of TCPO for the chemiluminescence system is about 1 mmol/L in acetronitrile. Effect of 2,4-D on the Chemiluminescence of the NBD-Anatase Sensor. Scheme 2 illustrates the reaction mechanism of the TCPO-H2O2 chemiluminescence system and

(Figure S1, Supporting Information). Figure 2B represents the FT-IR spectra of TiO2 nanoparticles before and after functionalization with the NBD moieties. Clearly, a new vibration band around 943 cm−1 appeared in the NBD-anatase sample compared with that of the pure anatase nanoparticle, which can be assigned to the vibration of Si−O−Ti groups,16 indicating the successful covalent grafting. The stretching vibration bands of the aromatic ring skeleton of the NBD moiety can also be observed around the frequency of 1500 cm−1 in the sample. The bands in the range of 1250−1460 cm−1 can be ascribed to the characteristic vibrational frequencies of propylamine groups of APTS grafted on the surface of anatase nanoparticles.15 Thermogravimetric analysis (TGA) was performed to estimate the loading of NBD moieties in the functionalized anatase nanoparticles, as shown in Figure 2C. It can be seen that the NBD-functionalized anatase anaoparticles exhibited a 4.9% greater mass loss than that before NBD loading. This excess mass loss could be attributed to the thermal degradation of the surface-grafted NBD moieties during the TGA process. On the basis of the result, it was figured out that there were about 3700 NBD molecules at the surface of one anatase nanoparticle. The result indicates that a barely monolayer of NBD fluorophores formed on the surface of anatase nanoparticles. UV−visible absorption of the NBDfunctionalized anatase shows a maximum at 506 nm, as indicated with a black arrow in Figure 1D, also confirming the successful modification of the fluorophores. Enhanced Chemiluminescence of the NBD-Anatase System. Figure 3A represents the chemiluminescence profiles obtained from four different chemiluminescence systems, as described in the Experimental Section, namely, the NBDanatase, NBD-amTiO2, NBD-SiO2, and NBD-solution, respectively. The NBD-anatase system is much different and the chemiluminescence intensity is about 6 times higher than the other three systems. The other three systems, however, have nearly the same chemiluminescence intensities. The results imply that the nano-anatase nanoparticles greatly enhanced the chemiluminescence efficiency due to their catalytic surface, whereas the amorphous TiO2 and SiO2 do not have the capability to enhance the chemiluminescence efficiency. The PO-CL chemiluminescence system generally involves the formation of an energy-rich intermediate 1,2-dioxetanedione via the oxidation of TCPO by H2O2. The first step of the oxidation is the decomposition of H2O2 to OOH, which 3359

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Figure 5. Evolution of the chemiluminescence intensity (I/I0) of different systems versus the concentration of 2,4-D: (a) NBD-SiO2, (b) NBD-amTiO2, (c) NBD-solution, and (d) NBD-anatase. I0 and I are the chemiluminescence intensities before and after adding 2,4-D, respectively. Final concentrations: TCPO, 1.0 mmol/L; H2O2, 1.3 mol/L.

Figure 4. Influence of TCPO concentration on the chemiluminescence intensity of the NBD-anatase system at a constant H2O2 concentration: the chemiluminescence intensity at different TCPO concentrations (a) in the presence of 2,4-D and (b) in the absence of 2,4-D. H2O2, 1.3 mol/L; NBD-anatase, 6 mg/mL; 2,4-D, 33 μmol/L.

Scheme 2. Schematic Illustration of the Chemiluminescence Enhancement Mechanism and the Surface Structure of NBD-Anatase Nanoparticles

energy-rich intermediate, and hence the decrease of chemiluminescence. The assumption of 2,4-D oxidation by H2O2 on anatase has been supported by ESI-MS experiments, as shown in Figure 6, which clearly revealed the presence of the oxidation

the surface structure of the NBD-anatase chemiluminescence sensor. TCPO can be oxidized by H2O2 to generate 1,2dioxetanedione, an energy-rich intermediate.10,17,21,22 The 1,2dioxetanedione interacts with a fluorescent molecule, such as NBD, to form a transient charge-transfer complex. Upon rapid decomposition of the complex, an excited state of the fluorescent molecule generates and subsequently decays to a ground state through emitting chemiluminescence. The chemiluminescence efficiency could be enhanced by improving the activity of H2O2; this has been achieved on the surface of nano-anatase, as demonstrated in this work. 2,4-D, a compound widely used as a herbicide,23 showed the ability to reduce the enhanced chemiluminescence intensity, as shown in Figure 5. Clearly, the chemiluminescence intensity decreases gradually as 2,4-D was added into the system. In contrast, the chemiluminescences of NBD-amTiO2, NBD-SiO2, and NBD-solution were not affected very much by the herbicide presence. The result shows that 2,4-D could be involved in the surface catalytic process of anatase during the chemiluminescence reaction. First, 2,4-D could be adsorbed and enriched around the anatase surface through interacting with the primary amine groups. This has been supported by the formation of the 2,4-D−APTS conjugate in the following experiment, in which 2,4-D was incubated with APTS in the dark, followed by the performance of ESI-MS and ESI-MS2 measurements (Figure S4, Supporting Information). Second, the enriched 2,4-D around anatase could competitively react with OOH, which subsequently reduces the production of the

Figure 6. (A) Negative-mode ESI-MS spectra of 2,4-D in the presence of H2O2 and NBD-anatase nanoparticles in ethanol solution. (B) Negative-mode ESI-MS spectra of 2,4-D in the presence of only H2O2 in ethanol solution. (C) The oxidation reaction of 2,4-D by H2O2 in the presence of NBD-anatase nanoparticles.

product around m/z 161 (Figure 6A,C). However, no new species were observed for the mixture of 2,4-D and H2O2 without anatase (Figure 6B). The results suggest that 2,4-D reacted rapidly with the activated H2O2 on the anatase surface, resulting in the decrease of chemiluminescence. Sensitive Detection of Herbicide 2,4-D. The NBDanatase system immobilized in PVA film was first incubated with 2,4-D solution. The chemiluminescence substrate of TCPO and H2O2 was then added into the plates, followed by recording the chemiluminescence intensity. The chemiluminescence signal intensity was plotted as a function 2,4-D concentration to yield a calibration curve. As Figure 7A shows, the chemiluminescence intensity decreased gradually when the 3360

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Figure 7. (A) The dependence of the chemiluminescence profiles of the NBD-anatase system on the 2,4-D concentrations. (B) The plot of the chemiluminescence decrement (I/I0) versus the concentration of 2,4-D. I0 and I are the chemiluminescence intensities before and after adding 2,4-D, respectively. The inset in (B) shows the linear correlations of the data included in the box. Final concentrations: TCPO, 1.0 mmol/L; H2O2, 1.3 mol/L.

concentration of 2,4-D was increased. When the concentration of 2,4-D reached 3.3 mmol/L, the chemiluminescence intensity nearly decreased by ∼93%. The chemiluminescence signal versus the concentration of 2,4-D could be fitted to a linear regression equation with a linear coefficient R2 = 0.9968 in the concentration range from 33 nmol/L to 330 μmol/L (Figure 7B). The LOD of the chemiluminescence sensor was calculated to be 0.33 nmol/L based on the result. The LOD value of the immobilized sensor is somewhat higher than that of the sensor suspended in solution phase, maybe due to the sheltering effect of the PVA matrix. The latter has an LOD of about 0.03 nmol/ L for 2,4-D detection (Figure S5, Supporting Information). The immobilized NBD-anatase sensor shows a wider linear range and lower detection limit than other chemiluminescence methods reported earlier, which reported the detection limits of 34 nmol/L and 3 ng/mL (13.6 nmol/L).24,25 Moreover, the chemiluminescence sensor immobilized in PVA film shows a good selectivity for phenoxyacetic acid (PAA) derivative compounds over other similar compounds. It can be seen in Figure 8 that all phenoxyacetic acid compounds,

selectivity of the chemiluminescence sensors. The phenoxyacetic derivatives have a much higher reactivity than other tested compounds to coordinate on the anatase surface and subsequently react with the activated hydrogen peroxide, OOH, as discussed in the previous section. The other types compounds tested do not react with H2O2 on anatase, as suggested by ESI-MS experiments (Figure S6, Supporting Information). The acidity of the phenoxyacetic derivatives could also contribute to the selectivity; for example, they have values of pKa less than 4.0, whereas the values of pKa of other compounds are higher than 4.0. The lower the pKa of the analytes are, the easier the compounds combine with the amino groups of nano-anatase chemiluminescence sensors. Therefore, the difference in pKa values could also play some roles in the enrichment of the analytes around anatase nanoparticles.

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CONCLUSIONS In summary, we have prepared a chemiluminescence sensor based on the surface grafting of anatase nanoparticles with the NBD fluorophore for sensitive and selective detection of phenoxyacetic acid compounds, such as the herbicide 2,4dichlorophenoxyacetic acid. The nano-anatase nanoparticles could enhance the chemiluminescence efficiency by surface catalytic activation of H2O2. The phenoxyacetic compounds competitively react with hydrogen peroxide on the anatase surface and thus decrease the chemiluminescence efficiency. This feature has been utilized to detect 2,4-D by monitoring the chemiluminescence signals in the presence of 2,4-D molecules. The detection limit of the chemiluminescence sensor can be achieved as low as 0.33 nmol/L.

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Figure 8. The chemiluminescence sensor of NBD-anatase immobilized in PVA film shows a selectivity for phenoxyacetic acid compounds, including 2,4-D, POAC, and 4-CPA. The PAA concentrations are (a) 3.3 × 10−4 mol/L and (b) 3.3 × 10−6 mol/L. I0 and I are the chemiluminescence intensities before and after the addition of the analytes. Final concentrations: TCPO, 1.0 mmol/L; H2O2, 1.3 mol/L.

ASSOCIATED CONTENT

* Supporting Information S

Fluorescence excitation spectra and emission spectra of NBD− APTS conjugates and fluorescence emission spectra of NBDanatase systems; the dependence of the chemiluminescence intensity of the NBD-anatase system on the time of UV light illumination for TiO2 nanoparticles and the stability of the chemiluminescence of the NBD-anatase system for 12 h illumination; the chemiluminescence profiles of the TCPOH2O2 system on TiO2 nanoparticles treated under 200 °C for 12 h; the ESI-MS spectra of the oxidation produce of phenoxyacetic acid compounds and other three compounds by H2O2 with NBD-anatase nanoparticles in ethanol solution; optimization of the PVA concentration; detection of the

including 2,4-D, 4-chlorophenoxyacetic acid (4-CPA), and phenoxyacetic acid (POAC), exhibit great influence on the chemiluminescence signal, whereas other compounds, such as the 2,4-dichlorophenol (2,4-DCP), 1,4-dichlorobenzene (PDB), and benzoic acid (BA), do not have much effect on the chemiluminescence signal. The result suggests that it is the phenoxyacetic moieties of the analytes that differentiate the 3361

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(23) Lahav, M.; Kharitonov, A. B.; Katz, O.; Kunitake, T.; Willner, I. Anal. Chem. 2001, 73, 720−723. (24) Surugiu, I.; Danielsson, B.; Ye, L.; Mosbach, K.; Haupt, K. Anal. Chem. 2001, 73, 487−491. (25) Boro, R. C.; Kaushal, J.; Nangia, Y.; Wangoo, N.; Bhasin, A.; Suri, C. R. Analyst 2011, 136, 2125−2130.

herbicide 2,4-D in solution; the variation in the chemiluminescence intensity with H2O2 and without H2O2; FT-IR spectra of NBD-anatase and NBD-anatase incubated with 2,4-D and 4CPA after 2 h; the ESI-MS and ESI-MS2 spectra of the mixtures of APTS with 4-CPA, POAC, 2,4-DCP, BA, and PDB in ethanol solution, respectively; and SEM images and fluorescence and UV−vis spectra of the anatase system after the sensing applications. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.Z.), [email protected] (S.W.).

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2011CB933700), the Natural Science Foundation of China (Nos. 20925518, 21075123, 30901008), and the Innovation Project of the Chinese Academy of Sciences (KJCX2-YW-H29).

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dx.doi.org/10.1021/jp210316s | J. Phys. Chem. C 2012, 116, 3356−3362