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Sulfide Sensor Based on Room-Temperature Phosphorescence of PbO/SiO2 Nanocomposite Ting Zhou, Na Wang, Chenhuan Li, Hongyan Yuan,* and Dan Xiao* Key laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry and College of Chemical Engineering, Sichuan University, Chengdu, 610065 People’s Republic of China A new strategy for the fabrication of a sulfide sensor based on room-temperature phosphorescence (RTP) of PbO/ SiO2 nanocomposite is proposed. The PbO/SiO2 phosphor is prepared by a sol-gel method, and it produces highly emissive broad-band RTP under the irradiation of UV light. The phosphorescence intensity of PbO/ SiO2 nanocomposite could be quenched by sulfide, and the response behavior of the sensor is dependent on the value of pH of the solution. At pH 11.0, the sensor exhibits a linear response toward sulfide at the concentration range from 2.67 to 596 µM. The detection limit for the sensor is estimated to be 0.138 µM (3 σ), and the precision for five replication detections of 6 µM sulfide is 1.82% (relative standard deviation). The color of the sensor and its phosphorescence intensity change obviously and could be observed with the naked eye when there was continuous addition of sulfide from the concentration of 50 µM. The phosphorescence intensity of quenched PbO/SiO2 phosphor can be recovered when dipping it into H2O2 solution, which demonstrates a good sulfide response characteristic of reusability. Furthermore, the sensor is easy to apply for trace hydrogen sulfide determination in the gas phase. Room-temperature phosphors have drawn much attention because they are important for display and lighting technologies;1,2 however, much less attention has been paid to their potential as a phosphorescence sensor. Doped silicon dioxide has also been employed as phosphor powders for color display,3,4 and some of them have acted as room-temperature phosphorescence (RTP) sensors for the trace analysis of chemical species, such as TiO2/ SiO2 and Mn-ZnS/SiO2.5-8 Nevertheless, they have seldom * To whom correspondence should be addressed. E-mail:
[email protected] (D.X.),
[email protected] (H.Y.). Phone: +86-028-85415029. Fax: +86-02885416029. (1) Ozawa, L.; Makimura, M.; Itoh, M. Mater. Chem. Phys. 2005, 93, 481– 486. (2) Chen, S. H.; Greeff, A. P.; Swart, H. C. J. Lumin. 2005, 113, 191–198. (3) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor, M. J. Science 1997, 276, 1826–1828. (4) Qiu, J. R.; Gaeta, A. L.; Hirao, K. Chem. Phys. Lett. 2001, 333, 236–241. (5) He, Y.; Wang, H. F.; Yan, X. P. Anal. Chem. 2008, 80, 3832–3837. (6) Wang, H. F.; He, Y.; Ji, T. R.; Yan, X. P. Anal. Chem. 2009, 81, 1615–1621. (7) Li, Y.; Liu, X. Y.; Yuan, H. Y.; Xiao, D. Biosens. Bioelectron. 2009, 24, 3706– 3710. (8) Shu, X. H.; Chen, Y.; Yuan, H. Y.; Gao, S. F.; Xiao, D. Anal. Chem. 2007, 79, 3695–3702. 10.1021/ac902121t 2010 American Chemical Society Published on Web 01/28/2010
been explored for sulfide determination in the solution and gas phase. The long lifetime of phosphorescence allows an appropriate delay time so that any fluorescence emission and scattering light can be easily avoided.5,6,9 In the present study, we report a PbO/ SiO2 nanocomposite prepared by the sol-gel method, which displays a stably and highly broad visible RTP that appears green to the eye under irradiation of UV light; the phosphorescence intensity can be quenched by sulfide; thus, the material can act as a sulfide sensor. Sulfide is widely existent in nature water and wastewater where it is either used as a reactant or is produced as a byproduct of manufacturing or industrial processes, such as in the paper, petrochemical, and leather industries.10-12 And the interactions between ubiquitous sulfate-reducing bacteria and exogenous sulfate as well as the bacterial degradation of sulfur-containing proteins and amino acids are also a major contribution to the release of hydrogen sulfide gas.13 Due to the toxicity of sulfide, the detection of sulfide has been paid much attention from the biological and industrial point of view, and the development of new methods for detecting sulfide captures great interest. Several techniques have been developed to determine sulfide, such as titrimetric,14 spectrophotometic,15 electrochemical,16 fluorescence,17,18 and chromatographic methods.19 The titrimetric detection method uses simple apparatus, but it is less sensitive; it is only suitable for analyzing samples of high sulfide concentrations (millimolar levels). The spectrophotometic and fluorescence methods often need complex compounds to react to the sulfide.15,17,18 There are numbers of electrochemical approaches to the measurement of sulfide, but they are difficult to maintain an accurate, properly functioning ion-selective electrode.20 Chromatographic methods offer alternative ways of developing the (9) Sa´nchez-Barraga´n, I.; Costa-Ferna´ndez, J. M.; Valledor, M.; Campo, J. C.; Sanz-Medel, A. TrAC, Trends Anal. Chem. 2006, 25, 958–967. (10) Font, J.; Gutierrez, J.; Lalueza, J.; Perez, X. J. Chromatogr., A. 1996, 740, 125–132. (11) Bhattacharjee, S.; Datta, S.; Bhattacharjee, C. Desalination 2007, 212, 92– 102. (12) Vaiopoulou, E.; Melidis, P.; Aivasidis, A. Water Res. 2005, 39, 4101–4109. (13) Lawrence, N. S.; Davis, J.; Compton, R. G. Talanta 2000, 52, 771–784. (14) Pawlak, Z.; Pawlak, A. S. Talanta 1999, 48, 347–353. (15) Davidson, J. M.; Pikramenou, Z.; Ponce, A.; Winpenny, R. E. P. Anal. Chem. 2009, 81, 3669–3675. (16) Huang, R. F.; Zheng, X. W.; Qu, Y. J. Anal. Chim. Acta 2007, 582, 267– 274. (17) Yang, X. F.; Wang, L. P.; Xu, H. M.; Zhao, M. L. Anal. Chim. Acta 2009, 631, 91–95. (18) Choi, M. F. Analyst 1998, 123, 1631–1634. (19) Han, K.; Koch, W. F. Anal. Chem. 1987, 59, 1016. (20) Howard, A. G.; Yeh, C. Y. Anal. Chem. 1998, 70, 4868–4872.
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determination of sulfide; however, the chromatographic equipment is expensive and depends on other detection.19 In this study, we proposed a new method of room-temperature phosphorimetry for sulfide determination. The phosphorescence material of PbO/SiO2 composite was easily prepared by the sol-gel method, and its phosphorescence intensity was selectively quenched by sulfide because of the strong and specific reaction between the sulfide and PbO/SiO2 phosphor. Moreover, trace hydrogen sulfide in the gas phase can be indirectly detected by the PbO/SiO2 phosphor. To the best of our knowledge, a sulfide sensor based on RTP material has not been reported. It is interesting to note that the phosphorescence intensity of PbO/SiO2 could be recovered by simply rinsing it with a solution of H2O2. The proposed sulfide sensor combining both the advantages of the colorimetric and the luminescence is simple, visual, cheap, sensitive, selective, and reproducible, which can be practically applied for the determination of sulfide in solution and hydrogen sulfide in the gas phase. EXPERIMENTAL SECTION Chemicals. Tetraethyl orthosilicate (TEOS), disodium hydrogen phosphate (Na2HPO4), acetic acid (HAc), and sodium hydroxide (NaOH) were purchased from Tianjin Chemicals (Tianjin, China). Glycerol (C3H8O3), sodium sulfide (Na2S), nitrate acid (HNO3), hydrochloric acid (HCl), 30% hydrogen peroxide (H2O2) aqueous solution, and nitrogen gas (N2) were obtained from Chengdu Chemicals (Sichuan, China). Lead acetate (Pb(Ac)2) was purchased from Chongqing Chemicals (Chongqing, China). Poly(tetrafluoroethylene) (PTFE) film (only gas could pass through the PTFE film, but the water cannot) was purchased from Dupont Co. (U.S.A.). All chemicals were of analytical reagent grade and used without further purification. Doubly distilled water was used throughout all experiments. Instrumentation. Phosphorescence measurements were performed at room temperature using a fluorescence-phosphorescence spectrophotometer (Hitachi, F-4500) and a Varioskan Flash (Thermo Scientific). Infrared (IR) spectra were performed on an infrared spectrophotometer (Nicolet, NEXUS, FT-IR 670). The microstructure of the nanocomposite oxide was characterized by scanning electron microscopy (SEM) (Hitachi, S-4800) and transmission electron microscopy (TEM) (Hitachi, H-800). The chemical composition of the material was analyzed by energydispersive X-ray (EDS) (Hitachi, S-4800). The elementary distributions of the material were carried out by EDS (Czech, TESCAN VEGA II LMU). The X-ray diffraction patterns of the materials were measured with an X-ray diffractometer (XRD) (Dandong Fuyuan instrument, DX-1000). X-ray photoelectron spectroscopy (XPS) analysis was performed on a photoelectron spectroscopy instrument (Kratos, XSAM 800). The element analyses of the material samples were determined by X-ray fluorescence (XRF) (Shimadzu, 1800). The rate of N2 was controlled by mass flow controllers (Brooks Instrument, 0154). A pH meter (Thermo, Orion 920A) was used to measure the pH of the solution. The tablet press machine (Tianjin Gangdong Science and Technology, DF-4) was used to press the PbO/SiO2 powder into a disk; compressive stress was set at 10 MPa. 1706
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Preparation of the PbO/SiO2 Nanoparticles. The nanometer particles of PbO/SiO2 were prepared by the sol-gel method.21,22 TEOS was hydrolyzed at room temperature with a mixed solution of H2O, C3H8O3, HNO3, HAc, and Pb(AC)2 in molar ratios of 20:1:0.02:1:0.04 per mole of TEOS. (Warning: lead and its compounds are highly toxic when eaten or inhaled; please wear a respirator and gloves to prevent being poisoned, and take back the waste residue.) The mixed sol was stirred until a transparent sol formed and gelled at room temperature. Finally, the nanometer-sized PbO/SiO2 particles were obtained after calcinations were performed at 550 °C for 2 h in the air; then, they were kept in desiccators before use. Measurement Procedure. The phosphorescence intensity and lifetime measurements of the sensor were carried out on a fluorescence-phosphorescence spectrophotometer (F-4500) in the absence and presence of a series of sulfide solutions when the spectrophotometer was set in the phosphorescence mode. The slit widths of excitation and emission were 10 and 10 nm, respectively. The measurement delay time of the spectrophotometer of the Hitachi F-4500 can be set from 0 to 99 s. All the phosphorescence measurements of the materials were measured immediately without waiting, and the measurement delay time was set at 0 s. The photomultiplier tube (PMT) voltage was set at 750 V. The powder of PbO/SiO2 nanoparticles was pressed into disks 12 mm in diameter and 1 mm in thickness. Then, the disk was placed in a usual quartz cell with the aid of a thin glass plate. The response of the sensor to sulfide of different concentrations was detected in the configuration (Supporting Information Figure S1). The angle formed between the excitation and emission beams was 90°, with an incident angle of 45°.23 Each phosphorescence disk was placed in a usual quartz cell for 30 min of equilibration with buffer before subsequent addition of the freshly prepared sulfide solution. In order to equably diffuse sulfide ions to the buffer solution and react to the PbO/SiO2 phosphor as soon as possible, nitrogen gas was pumped into the buffer solution at the rate of 10 mL/min for stirring. After PbO/SiO2 interacted with sulfide for 600 s, the phosphorescence spectra were obtained with the maximum excitation wavelength at 290 nm and the maximum emission wavelength at 510 nm. All measurements were carried out at room temperature. The standard Na2S solutions were freshly prepared in 50 mM phosphate buffers (PBS) from pH 7.0 to pH 11.0, and the Na2S solutions were standardized by iodometric titration. H2S in the gas phase with various concentrations was generated by dropping HCl into 100 mL of different Na2S solutions. RESULTS AND DISCUSSION Preparation and Characterization of the PbO/SiO2 Composite Oxide before and after Adding Sulfide. The nanometer particles of PbO/SiO2 were prepared by a sol-gel method according to the above-mentioned procedure. The phosphorescence intensities and experimental conditions are summarized in Supporting Information Table S1. When the C3H8O3 aqueous solution acted as solvent, the Pb/Si molar ratio was (21) Dhlamini, M. S.; Terblans, J. J.; Ntwaeaborwa, O. M.; Joubert, H. D.; Swart, H. C. Phys. Status Solidi 2008, 2, 598–601. (22) Grandi, S.; Tomasi, C.; Mustarelli, P.; Dossena, A.; Cecchet, G. Thermochim. Acta 2004, 418, 117–122. (23) Orrea, R. A.; Escandar, G. M. Anal. Chim. Acta 2006, 571, 58–65.
Figure 1. (A) EDS spectra of PbO/SiO2 nanocomposite before (a) and after interaction with Na2S (b). (B) XRF spectra of PbO/SiO2 nanocomposite before (a) and after interaction with Na2S (b). (C) XRD spectra of PbO/SiO2 nanocomposite before (a) and after interaction with Na2S (b).
1:25, and the PbO/SiO2 was calcined at 550 °C for 2 h, the maximum phosphorescence intensity of PbO/SiO2 was obtained. If the calcination temperature exceeds 650 °C, the PbO/ SiO2 composite oxides have no phosphorescence. The excitation and emission phosphorescence spectra of the nanocomposite oxides PbO/SiO2 are shown in Supporting Information Figure S2. The maximum excitation and emission wavelengths of the PbO/SiO2 composite oxide are 290 and 510 nm, respectively. The morphological studies and elementary distributions of the material studies were carried out by SEM, TEM, and EDS (Supporting Information Figures S3 and S4). The measurement results show that the particle size distributions are relatively broad; the particle diameters range from 50 to 100 nm. It is clear from the images that some of the larger particles are made up of an agglomeration of smaller particles, which makes it difficult to determine a mean particle size as shown in Supporting Information Figure S3. Furthermore, elements including O, Si, and Pb are equably distributed in the material (Supporting Information Figure S4); however, the sulfide is detected only in the PbO/SiO2 after interaction with Na2S (Supporting Information Figure S4b5). The characterizations of the chemical composition of the PbO/ SiO2 were further carried out by EDS, XRD, and XRF. As shown in Figure 1A, EDS on the prepared material may illustrate the existence of PbO in the SiO2 matrix (Figure 1A, part a), and after
sulfide was added, PbO seems to turn to PbS (Figure 1A, part b).22,24 In Figure 1B, XRF spectra further illustrate that the sulfide is present in the quenched PbO/SiO2 composite oxides (Figure 1B, part b). XRD analysis is performed on the PbO/SiO2 nanocomposite before and after adding Na2S as shown in Figure 1C, parts a and b. Due to the presence of amorphous SiO2 spectra, there are only weak peaks observed for PbO,25,26 which exhibits the peaks for (101), (111), (112), (211) of PbO in Figure 1C, part a. However, after PbO/SiO2 interaction with sulfide, the apparent peaks of crystallization PbS in the SiO2 matrix are shown in the XRD spectrum. Figure 1C, part b, shows the five diffraction peaks corresponding to (111), (200), (220), (311), and (222) planes in the cubic phase of PbS.22,24 The Fourier transform infrared (FT-IR) of SiO2, PbO/SiO2, and PbS/SiO2 are shown in Supporting Information Figure S5. Because the bonds of Pb-O and Pb-S are mainly electrovalent bonds, the FT-IR spectra of doped samples do not show the strong bands associated with Pb-O and Pb-S stretching and bending (24) Dhlamini, M. S.; Terblans, J. J.; Ntwaeaborwa, O. M.; Ngaruiya, J. M.; Hillie, K. T.; Botha, J. R.; Swart, H. C. J. Lumin. 2008, 6, 16–23. (25) Li, S. Y.; Yang, W.; Chen, M.; Gao, J. Z.; Kang, J. W.; Qi, Y. L. Mater. Chem. Phys. 2005, 90, 262–269. (26) Konstantinov, K.; Ng, S. H.; Wang, J. Z.; Wang, G. X.; Wexler, D.; Liu, H. K. J. Power Sources 2006, 159, 241–244.
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Figure 2. XPS spectra of Pb4f (A) and Si2p (B) of PbO/SiO2 nanocomposite before (a) and after interaction with Na2S (b) and after being regenerated by H2O2 (c).
vibrations. In Supporting Information Figure S5a-c, we suppose that the shifts in wavenumber are ascribed to the change in the chemical environment of the material. In order to obtain additional formation of the phosphor and quenched phosphor supporting the existence of PbO and PbS domains, an XPS study has been carried out on the PbO/SiO2 and PbS/SiO2 nanocomposite. XRD experiments did not apparently detect the existence of PbO in the nanocomposite oxide; however, XPS results proved the existence of PbO in the nanocomposite oxide. The Pb4f photoelectron peak is shown in Figure 2A; the binding energy (BE) of the Pb4f in the PbO/SiO2 composite is 0.9 eV higher than that in pure PbO.27 Due to the higher electronegativity of Si than that of Pb, the valence electron density of Pb in the Pb-O-Si bond is lower than that in the Pb-O-Pb bond. We proposed that the bonds of Pb-O-Si were formed at the interface between PbO particles and the pore walls of SiO2 which would result in an increase in covalence with respect to bulk PbO.28-30 It is consistent with the reports that lead is partially connected to the silica network.22,31,32 In Figure 2A, the spectrum of Pb4f is shifted to lower BE after adding sulfide. This shift in BE is attributed to the change in the chemical (27) Chen, S.; Liu, W.; Yu, L. G. Wear 1998, 218, 153–158.
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environment of the PbO/SiO2 nanocomposite. In Figure 2B, the shift spectrum is also observed. This change also illustrates that the chemical environment of the PbO/SiO2 nanocomposite is different from the environment of PbO/SiO2 nanocomposite after interaction with sulfide.8 And in Figure 2, the BE values of regenerated PbO/SiO2 are the same as that of PbO/SiO2; it illustrates the structure of regenerated PbO/ SiO2 might be identical with the PbO/SiO2. The lead impurity creates luminescence centers in many materials;33,34 silicate glasses are no exception to the rule. Supporting Information Figure S2 shows the excitation and emission spectra of the pure SiO2 and PbO-doped samples. The relative phosphorescence intensity of the doped samples increases remarkably than that of the pure SiO2, and the excitation and emission spectra of the pure SiO2 are 245 and 430 nm, respectively. Therefore, the excitation (290 nm) and emission (510 nm) peaks of the doped samples are assigned to the luminescence of PbO in the PbO/SiO2 nanocomposite. The Pb2+ luminescence band is always a composite because of the incorporation of lead ion with certain defects. Usually, they are described by the 1S0-3P0.1 transitions which originate from the 6S2-6S6P configurational transition.35-37 The luminescence of the SiO2 is well-known and has been ascribed to the defects (such as oxygen vacancy, carbon impurity).3 The structure defects of the sol-gel SiO2 can be affected by the impurity of PbO. In the phosphorescence spectra of PbO/SiO2 nanocomposite, the emission peak at around 450 nm is ascribed to the defect luminescence in the SiO2; the emission peak at around 510 nm is ascribed to the centers involving PbO. Thus, the PbO/SiO2 phosphor is mainly due to the interaction between PbO nanoparticles and SiO2. Effect of pH on the Performance of the Sulfide Sensor Based on PbO/SiO2 Nanocomposite. The pH value of the determination condition is selected considering the following two aspects: (1) the stability of the PbO/SiO2 nanocomposite; (2) the suitable determination conditions for the sensor responding to sulfide. From the results of the experiments (the data not given), it was observed that PbO/SiO2 nanocomposite was unstable at either pH below 5.0 or pH above 12.0. It may be due to the PbO/SiO2 nanocomposite reaction to acid or alkali at pH below 7.0 or pH above 11.0. In order to maintain the stabilization of PbO/SiO2 nanocomposite, the response behaviors of the PbO/SiO2 nanocomposite at pH 7.0-11.0 were chosen for further studies. If the phosphorescence intensity is proportional to the molar weight of the phosphor, then the (28) Jiang, Q.; Wu, Z. Y.; Wang, Y. M.; Cao, Y.; Zhou, C. F.; Zhu, J. H. J. Mater. Chem. 2006, 16, 1536–1542. (29) Fu, Z. P.; Yang, B. F.; Li, L.; Dong, W. W.; Jia, C.; Wu, W. J. Phys.: Condens. Matter 2003, 15, 2867–2873. (30) Yao, B. D.; Shi, H. Z.; Bi, H. J.; Zhang, L. D. J. Phys.: Condens. Matter 2000, 12, 6265–6270. (31) Fayon, F.; Farnan, I.; Bessada, C.; Coutures, J.; Massiot, D.; Coutures, J. P. J. Am. Chem. Soc. 1997, 119, 6837–6843. (32) Fayon, F.; Bessada, C.; Massiot, D.; Farnan, I.; Coutures, J. P. J. Non-Cryst. Solids 1998, 232-234, 403–408. (33) Folkerts, H. F.; Blasse, G. Chem. Mater. 1994, 6, 969–972. (34) Groenink, J. A.; Blasse, G. J. Solid State Chem. 1980, 32, 9–20. (35) Yang, P.; Song, C. F.; Lu, M. K.; Yin, X.; Zhou, G. J.; Xu, D.; Yuan, D. R. Chem. Phys. Lett. 2001, 345, 429–434. (36) Ragfagni, A.; Mugnai, D.; Bacci, M. Adv. Phys. 1983, 32, 823. (37) Meijerink, A.; Jetten, H.; Blasse, G. J. Solid State Chem. 1988, 76, 115– 123.
Figure 3. Response behavior of the PbO/SiO2 nanocomposite with the addition of different concentrations of Na2S at varying pH values.
quenching efficiency of phosphorescence intensity (R) can be represented as (P0 - P)/P0, where P0 and P are the phosphorescence intensities in the absence and in the presence of sulfide, respectively. Figure 3 shows the response behavior of the PbO/SiO2 nanocomposite with the addition of different concentrations of Na2S at varying pH values. The response behavior of the PbO/SiO2 nanocomposite is pH-dependent. The R value of PbO/SiO2 nanocomposite is lower at pH 7.0-9.0 than that at pH 10.0-11.0. The hydrosulfide ions are the dominant species in a solution at pH 7.0-11.0. When the value of pH is decreased, the concentration of hydrosulfide ions is decreased with their partial conversion into hydrogen sulfide molecules.38 At pH 11.0, the value of R is maximum and the useful detection range obtained is 2.67-596 µM; thus, pH 11.0 is chosen as the working condition. Performance of PbO/SiO2 Nanocomposite for Sulfide Determination. To evaluate the sensitivity of the PbO/SiO2 nanocomposite for the sulfide determination, the change of the PbO/SiO2 phosphorescence intensity was measured at 510 nm by the fluorometer F-4500. As shown in Figure 4, the phosphorescence intensity of the material decreases upon the addition of different concentrations of Na2S. The phosphorescence intensity of PbO/SiO2 nanocomposite and the concentration of the Na2S accord with the logarithmic quantitative equation, lg[(P0 - P)/ P] ) a lg C + b, where P0 and P are the phosphorescence intensity of PbO/SiO2 in the absence and presence of sulfide, respectively. C is the concentration of the Na2S solution. From Figure 4, when the added Na2S concentrations are changed from 2.67 to 596 µM, the linear equation of y ) 1.4938x - 6.8627 is obtained, where y is -lg[(P0 - P)/P], x is -lg C. The linear relative coefficient is R2 ) 0.9941, the precision for five replicate detections of 6 µM sulfide is 1.82% (RSD), and the detection limit (3 σ) is estimated to be 0.138 µM. As shown in Supporting Information Figure S6, the t95 values (time to reach 95% of maximum response) were
