Environ. Sci. Technol. 2010, 44, 7884–7889
Sensitive Detection of Polycyclic Aromatic Hydrocarbons Using CdTe Quantum Dot-Modified TiO2 Nanotube Array through Fluorescence Resonance Energy Transfer L I X I A Y A N G , †,‡ B E I B E I C H E N , † S H E N G L I A N L U O , * ,†,‡ J U A N X I U L I , † RONGHUA LIU,† AND QINGYUN CAI* State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, People’s Republic of China, and College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China
Received May 25, 2010. Revised manuscript received July 13, 2010. Accepted September 7, 2010.
CdTe quantum dots (QDs) are prepared on TiO2 nanotubes (TiO2 NTs), for the first time, with pulse electrodeposition. A novel single-drop optical sensor is prepared with the CdTe QDsmodified TiO2 NTs, and applied for the detection of polycyclic aromatic hydrocarbons (PAHs) based on fluorescence resonance energy transfer (FRET). Excited at 270 nm, the sensor shows fluorescence emission at around 370 nm. As PAHs are with absorption/fluorescence emission at around 364/ 410 nm, FRET happens between the CdTe QDs and PAHs with the CdTe QDs as donors and PAHs as receptors. The sensitivity is dependent on the number of rings of the PAHs, with the highest sensitivity observed in the response to benzo(a)pyrene (BaP). Using FRET, the sensitivity to BaP is enhanced by about 2 orders with respect to the direct fluorescent spectrometry. The proposed sensor shows a linear response to the logarithm of BaP concentration in the range of 400 nM to 40 pM, with a detection limit of 15 pM, which is much close to the quality criteria (15.1 pM) in drinking water set by U.S. Environment Protection, suggesting that the proposed sensor can be used for quick scanning of PAHs. The achieved sensitivity is much higher than that of the published sensor-based methods. As PAHs are quantified based on the relative fluorescence intensity at 410-370 nm, the sensor need no calibration with a standard sensor, avoiding the influence from the sensor-to-sensor difference. The practicability of the sensor is tested by analyzing PAHs in Xiangjiang River water, the PAHs contents ranges from 0.045 to 2.847 ng/L based on the sampling spots.
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are globally distributed environmental contaminants which attract considerable concern because of their high toxicities and * Address correspondence to either. E-mail:
[email protected] (Q.C.);
[email protected] (S.L.). † State Key Laboratory of Chemo/Biosensing and Chemometrics. ‡ College of Environmental Science and Engineering. 7884
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bioaccumulative properties (1-3). The identification and monitoring of them are of great interests (4-7). During the past decades, many approaches have been proposed for the qualitative and quantitative analysis of PAHs, such as HPLC, LC-MS, and GC-MS (8-10), capillary electrophoresis (11), quartz crystal microbalance (12), surface-enhanced Raman scattering (13, 14), and immunoassay combined with various detection techniques (15-17). As most PAHs with fused aromatic ring structures have special fluorescent properties, fluorescent spectrometry is used for the direct detection of PAHs (18-20). Although the classic HPLC, LC-MS, and GCMS are powerful analytical methods for PAHs, the pretreatment of samples is time-consuming and the equipment is expensive. The fluorescent spectrometry and Raman scattering is limited by the background interferences and the relatively high limit of detection (LOD). Consequently, it is valuable to develop simple, fast and sensitive methods for the analysis of PAHs in quick scanning purpose. Among PAHs, BaP is the most toxic one and often regarded as a key indicator compound of PAHs because it almost occurs in all mixtures where PAHs are present (1, 8). Therefore, evaluating total PAH contents by referring to the calibration curve of BaP can be viable in fast scanning techniques. Recently, anodic TiO2 nanotube (NT) arrays have attracted increasing interests due to the tunable pore sizes and highly oriented growth characteristics (21-23). The TiO2 NTs offer large free space in their interior and outer space that can be modified with active materials such as semiconductors (24), carbon (25), and heavy metals (26), giving them significant advantages over TiO2 materials in powder form. Herein CdTe quantum dots (QDs) sensitized TiO2 NTs were prepared, for the first time by employing a simple pulsed electrodeposition technique (PET). The highly orientated growth and uniform open-top characteristics of anodic TiO2 NTs provide a homogeneous environment for the fast nucleation and growth of CdTe crystals. Furthermore, the PET is an efficient way to construct nanopariticle with small sizes and good crystallization. The technique is of low-cost, convenience and environmental compatibility to prepare CdTe QDs on TiO2 NTs, exhibiting significant advantages over CdTe, CdSe nanoparticles fabricated by other methods requiring long reaction time from the elements in bulk form (27, 28). Moreover, to the best of our knowledge, there is no report in the utilization of QDs-TiO2 NTs for the detection of organic environmental contaminants based on FRET. In this study, the fluorescence intensity of BaP obtained on CdTe QDs-modified TiO2 NTs sensor was enhanced by 15 times by employing FRET-based fluorescence spectroscopy as compared with the direct liquid fluorescent spectrometry. The resulting CdTe/TiO2 NTs can be applied in the fast detection of PAHs based on the FRET between CdTe QDs and PAHs.
2. Experimental Section 2.1. Experimental Materials. Tiannium foil (99.8%, 0.127 mm thick) was purchased from Aldrich (Milwaukee, WI). BaP was purchased from Sigma Aldrich. Sodium hydrogen sulfate, sodium fluoride, cadmium sulfate, sodium tellurite, and methanol of analytical reagent grade were purchased from commercial sources and used as supplied. Ultrapure water was used throughout the experiments. 2.2. Fabrication of CdTe QDs-Modified TiO2 NTs. Prior to anodization, a titanium ribbon was ultrasonically cleaned in HF solution and ultrapure water for 5 min in turn. Anodization was performed in a two-electrode configuration with Ti as the anode and platinum foil as the cathode in an 10.1021/es101760c
2010 American Chemical Society
Published on Web 09/17/2010
FIGURE 1. (A) SEM image showing the lateral view and top view (inset) of TiO2 NTs, (B) SEM image showing the lateral view and top view (inset) of CdTe-TiO2 NTs, (C) EDS spectrum of the CdTe-TiO2 NTs. XPS survey spectrum (D) and Te 3d spectrum (E) of the CdTe-TiO2 NTs sample. electrolyte containing 0.1 M NaF and 0.5 M NaHSO4 at 20 V for 1 h (21). CdTe QDs were pulse electrodeposited on the as-prepared TiO2 NTs with an electrochemical workstation (IM6ex, Zahner Elektrik, German) in a conventional three-electrode system with TiO2 NTs (on Ti foil) as work electrode, a platinum wire counter electrode, and a saturated calomel electrode (SCE) reference electrode in an electrolyte solution (pH 2 adjusted by 1 M H2SO4) containing 0.05 M CdSO4 and 0.02 M NaTeO3. The pulse on-off time ratio was 0.02:1, the running voltage was -2 V, and the efficient electrodepositing time was 800 sequences. 2.3. Characterization Methods. Morphologies of CdTeTiO2 NTs were analyzed by a scanning electron microscopy (SEM, JSM 6700F; JEOL, Tokyo, Japan). Energy dispersive X-ray spectrometers (EDS) fitted to electron microscopes were used for elemental analysis. X-ray photoelectron spectroscopy (XPS) analyses of the specimen were carried out in an ultrahigh vacuumchamber with a pressure of 2 × 10-9 mbar at room temperature (Thermo Fisher Scientific,
ESCALAB 250). The binding energy (BE) scale was calibrated by measuring a C 1s peak at 284.8 eV from the surface contamination. 2.4. Fluorescence Detection. Fluorescence measurements were performed using a fluorescence spectrometer (F-2500, HITACHI, Japan). The slot widths of the excitation and emission were both 10 nm. All measurements were performed under ambient condition. Fluorescence spectra of TiO2 NT array and CdTe-TiO2 NT array were obtained by directly scanning their surfaces. The excitation wavelength was fixed at 270 nm. For the detection of PAHs, a drop of 20 µL sample solution was placed on the surface of TiO2 NTs or CdTe-TiO2 NTs with a geometrical size of 0.5 × 1 cm2. After drying in air, the emission fluorescence at 370 nm was recorded.
3. Results and Discussion 3.1. Morphologies and Spectral Characterization. SEM images in Figure 1A show that the as-anodized TiO2 NTs are of 320 nm in length, and a close-packed structure with an average pore size of 90 nm (inset in Figure 1A). Figure 1B VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Fluorescence spectra of (1) TiO2 NTs; (2) TiO2 NTs loaded with 4 nM BaP; (3) CdTe-TiO2 NTs; (4) CdTe-TiO2 NTs loaded with 4 nM BaP. Inset shows the FRET principle. shows the SEM image of the CdTe-TiO2 NTs. Compact CdTe QDs in a diameter of about 10 nm are uniformly dispersed on the inner and outer walls of the TiO2 NTs without blocking the channels (inset in Figure 1B). The anodized titania NTs are amorphous (21, 22). In an externally applied electric field, polarization of amorphous titania results in electron hopping between neighboring chains as described by Ti3+-O-Ti4+ f Ti4+-O-Ti3+, leading to an enhanced conductivity of the as-anodized titania (21). Furthermore, the uniform morphology and high orientated growth property of the TiO2 NTs provide a homogeneous environment and abundant active sites for the CdTe nucleation during the electrodeposition. EDS spectrum (Figure 1D) exhibits the characteristic peaks of Cd, Te, Ti, and O. Elemental analysis shows that the electrodeposited CdTe QDs are with an atom ratio of approximately 1.33:1 (Te:Cd), not strictly in accordance with 1:1 due to the low pH value of the plating solution. XPS survey spectrum for the sample depicted in Figure 1D shows the presence of Te, Cd, and O characteristic peaks. The characteristic XPS region spectra of Te 3d5/2 at 572.5 eV and Te 3d3/2 at 583.6 eV in Figure 1E depict the appearance of additional peaks of them. The Te 3d5/2 peak at 572.5 eV corresponds to TesCd bond, and the additional peak at 575.8 eV corresponds to TesO bond which comes from CdTeO3 (29). The same phenomenon happens on Te 3d3/2 peak at 583.6 eV. The peak area analysis shows that the content ratio of CdTe to CdTeO3 is about 2:1. Since the CdTe electrodeposition is performed under ambient condition and the precursor is Na2TeO3, the formation of CdTeO3 is inevitable. The crystallization of CdTe QDs can be attributed to the high nucleation rate resulting from the overpotential phenomena on the high oriented TiO2 NTs by applying PET, which is favorable for the construction of the FRET-based fluorescence sensors. 3.2. Fluorescence Responses to BaP. Fluorescence spectra of TiO2 NTs and CdTe-TiO2 NTs were obtained by directly scanning their surfaces with fixing the excitation wavelength at 270 nm. There is only an insignificant and broad fluorescence emission background ranging from 350 to 500 nm in the spectrum of TiO2 NTs (Figure 2, curve 1), which is defined as the direct solid fluorescence spectrum. Such a weak background emission is not enough to excite the BaP, and no FRET is observed on the BaP-loaded TiO2 NTs (Figure 2, curve 2). With the modification of CdTe QDs, a notable fluorescence emission at around 370 nm is observed in the spectrum of CdTe-TiO2 NTs (Figure 2, curve 3). Such a shortwave fluorescent emission is essential to the FRET because the absorption of most PAHs is around 364 nm, as shown in Figure 3A. Excited at 364 nm, BaP in methanol solution (4 7886
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FIGURE 3. (A) Excitation and emission spectra of 4 nM BaP in methanol solution. (B) Fluorescence spectra of CdTe-TiO2 NTs (7) and CdTe-TiO2 NTs loaded with BaP at concentrations of (1) f (6): 400 nM, 40 nM, 4 nM, 400 pM, 40 pM, 4 pM. Inset: relationship between the relative fluorescence intensity and the logarithm of concentrations of BaP. nM) emits a light at 410 nm. While BaP is loaded on the CdTe-TiO2 NTs, FRET happens between CdTe QDs and BaP as illustrated in the inset of Figure 2, resulting in an increase in the 410 nm emission intensity of BaP and a decrease in the 370 nm emission intensity of CdTe QDs (Figure 2, curve 4). As compared with the fluorescence intensity of BaP loaded on the TiO2 NTs, the fluorescence intensity of BaP loaded on CdTe-TiO2 NTs is enhanced by about 3 times (Figure 2, curves 2 and 4). The fluorescence enhancement is attributed to the fluorescence energy transfer from CdTe QDs to BaP. As compared with the fluorescence emission of 4 nM BaP in methanol solution (Figure 3A), the fluorescence emission of BaP (20 µL 4 nM was loaded) on the CdTe-TiO2 NTs is magnified by 15 times (Figure 2, curve 4). The FRET-based method achieves a much higher sensitivity of BaP than both the direct solid and liquid fluorescence spectroscopy. With increasing BaP concentration, the fluorescence emission of CdTe-TiO2 NTs at 370 nm is successively quenched and the sensitized fluorescence emission of BaP at around 410 nm increases substantially (Figure 3B). As the fluorescence intensity at both 410 and 370 nm is dependent on the BaP concentration, the target is quantified based on the relative fluorescence intensity change as defined as (I410 nm I370 nm)/I370 nm (∆I′). The ∆I’ of the CdTe-TiO2 NTs without BaP loading is -0.09. With increasing the BaP loading, the ∆I’ increases, and is linear dependent on the logarithm of BaP concentration in the range of 400 nM to 40 pM as shown in the inset of Figure 3B, with a LOD of 15 pM calculated based on 3 times noise. As the quantification is based on the relative fluorescence intensity, the sensor does not need any
FIGURE 4. Fluorescence spectra of (5) CdTe-TiO2 NTs and CdTeTiO2 NTs loaded with different kinds of PAHs (1) naphthalene (NAP), (2) anthracene (ANT), (3) pyrene (PYR), (4) benzo(a)pyrene (BaP) at the concentration of 100 µg/L. calibration with a standard sensor, and the sensor-to-sensor difference does not affect the detection, which is much important for practical applications since in most cases the sensor-to-sensor difference makes the practical detection impossible if the sensor reproducibility is not satisfied (12, 30, 31). The fluorescence properties of PAHs depend mainly on their fused aromatic structures. PAHs with different number of phenyl rings exhibit different responses, as illustrated in Figure 4, the fluorescence intensity of 100 µg/L naphthalene (NAP), anthracene (ANT), pyrene (PYR), and BaP increases with the increase in the number of phenyl rings; in the order of NAP, ANT, PYR, and BaP with a ∆I’ value of 0.98, 1.94, 3.16, and 8.03, respectively. The PAH with more phenyl rings has more fluorescence efficiency, and therefore higher FRET efficiency. The quality criteria of ANT, PYR and BaP in water set by U.S. Environment Protection (EPA) is 8300 µg/L (46.57 µM), 830 µg/L (4.10 µM), and 3.8 ng/L (∼15.1 pM) (32), respectively. The ∆I’ corresponding to the BaP criteria (3.8 ng/L) obtained on CdTe/TiO2 NTs is 0.27. Consequently, if the measured ∆I’ is lower than 0.27, the PAH contamination meets the EPA criteria. Otherwise, further analysis is needed to confirm the contamination of PAHs. 3.3. Interference Investigation. The FRET happens only when there is appreciable overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. Basically as for those compounds whose absorption spectrum is not overlapped with the emission spectrum of CdTe QDs, no FRET happens, and therefore these compounds do not interfer the detection of PAHs. To validate this hypothesis, the proposed sensor was applied to the detection of organic compounds including pentachlorophenol (PCP), polybrominated diphenyl ether (PBDE), fluorescein, and Rhodamine B. PCP and PBDE are nonfluorescent, while fluorescein and rhodamine B are fluorescent substances with absorption at 480 and 520 nm, respectively. Figure 5 shows the fluorescence spectra of these compounds loaded on the CdTe-TiO2 NTs. Under the same detection condition, no FRET happens on these compounds. Herein we can conclude those compounds without fluorescence emission or with fluorescence emission but whose absorption spectrum is not overlapped with the emission spectrum of CdTe QDs do not interfere the detection of PAHs. 3.4. Analysis of Water Samples. The low LOD and high selectivity of the proposed sensor suggest that it can be applied for the quick scanning of PAHs. The sensor was first validated in a recovery study. BaP in the concentrations of 5, 10, and 30 µg/L was spiked in different water samples,
FIGURE 5. The fluorescence spectra of CdTe-TiO2 NTs (a) and CdTe-TiO2 NTs loaded with PCP (b), Rhodamine B (c), PBDE (d), and fluorescein (e) at concentration of 10 nM.
TABLE 1. Recovery Study for BaP using FRET with Various Water Samples benzo(a)pyrene added (µg/L)
found ((µg/L)
recovery (%)
RSD (%)
tap water
5 10 30
4.76 10.13 29.40
95.3 101.3 98.0
4.12 2.60 5.11
spring water
5 10 30
4.60 9.78 27.84
92.1 97.80 92.8
10.17 5.32 7.93
river water
5 10 30
5.16 9.69 35.67
103.2 96.90 118.9
3.03 1.29 3.34
sample
including tap water, spring water collected from YueLu mountain, and river water collected from Xiangjiang river. All the water samples were filtered by 0.22 µm cellulose membranes before measurement. 20 µL of the solution was applied directly on the sensor, and the fluorescence spectrum was measured after drying. The recovery results are summarized in Table 1. The recovery rate for different water samples ranges from 92.1% to 118.9%, indicating that the proposed method is stable and can be applied for analysis of real samples. Then the water samples were analyzed by measuring the fluorescence spectroscopy of CdTe-TiO2 NTs loaded with 20 µL of the sample. The ∆I’ of these samples are -0.15 (tap water), -0.13 (spring water), and 0.07 (river water). As the river water exhibits a higher PAHs content compared with those of tap water and spring water, it was further analyzed with an enrichment process of a simple liquid-liquid extraction to achieve a more stable and reliable result. The recovery rate was first measured to validate the process. One mL n-hexane was added to 100 mL tap water spiked with 0.2 µg/L BaP. After stirring at room temperature for 1 h, the organic phase was collected. Twenty µL of the organic solution was applied directly on the sensor, and the fluorescence spectrum was measured after the organic chemical volatilized. The recovery rate was 97.18%, 97.46%, and 95.07%, respectively, for three parallel samples. The water in Xiangjiang River at different spots as shown in Figure 6A was sampled. After being filtered with 0.22 µm cellulose membranes, the samples were enriched and analyzed. Figure 6B shows the fluorescence spectra of 100-folds enriched river water samples with the PAH contents shown in Figure 6A. The content ranges from 0.045 to 2.847 ng/L, with the highest concentration detected at the outlets of wastewater (spots i, j, h), suggesting PAHs mainly come from VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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CdTe QDs-modified TiO2 NT array was prepared, for the first time, with pulse electrodeposition technique, and applied for the detection of PAHs. The high oriented TiO2 NT structure favors the formation of CdTe QDs, and the large surface area favors a good dispersion of sample. The FRET betweeen CdTe QDs and PAHs results in a significant enhancement in the emission of PAHs. The emitted fluorescence intensity of BaP is enhanced by 15 times using CdTe QDs-modified TiO2 NTs as compared with the direct fluorescence spectrum of BaP in solution. The sensitivity is dependent on the number of rings of the PAHs. The highest sensitivity is observed in response to BaP, as low as 15 pM BaP can be detected by the proposed sensor. Organic compounds without fluorescence or whose fluorescence absorption spectrum does not overlapped with the emission spectrum CdTe QDs show no interference to the detection of PAHs. As PAHs are quantified based on the relative fluorescence intensity at 410-370 nm, the sensor can be directly applied for the detection of PAHs needing no calibration with a standard sensor, avoiding the influence from the sensor-to-sensor difference.
Acknowledgments This work was supported by the National Basic Research Program of China (2009CB421601), the National Science Foundation for Distinguished Young Scholars (No.50725825) and the National Science Foundation of China (No.20878079). L.Y., B.C., and S.L. contribued equally to this work. FIGURE 6. (A) The content of PAHs contained in Xiangjiang River measured by the proposed method; among the different spots, (i), (g), and (h) are outlets of wastewater. The concentration unit is ng/L. (B) The fluorescence spectra of the 100folds enriched Xiangjiang river water sampled from spots shown in (A). the wastewater. Since the concentrations of PAHs were estimated based on the calibration curve of BaP which is with a relatively high sensitivity, the actual concentrations of PAHs should be higher than the estimated values. Due to the high complexity and diversity of PAHs in contaminated water, it is difficult to achieve accurate value of every PAH by applying a fast scanning detection technique. As Bap, the most toxic one of PAHs, is regarded as a key indicator of PAHs and occurs in all mixtures of PAHs, evaluating total PAHs contents by referring to the calibration curve of Bap is viable in fast scanning detection techniques. The described sensor shows specific response to PAHs with sensitivities significantly higher than the recently published results. A few examples are cited here: Stanley and co-workers (12) developed a QCM sensor for preferentially detection of anthracene in liquid phase with a LOD of 2 ppb (11.2 nM). Boujday and colleagues (16) built up an immunosensor for direct detection of BaP based on surface IR technology with a LOD of 5 µM. Spier and co-workers (17) described an enzyme-linked immunosorbent assay for detection of 3- to 5-ring PAHs with a LOD of 0.1 µg/L (0.56-0.40 nM). Lin et al (33) developed a wireless sensor to detect PAHs using humic acid-coated magnetic Fe3O4 nanoparticles as signal-amplifying tags, with the lowest LOD to BaP of 3 nM. In this work, the highest sensitivity was also obtained when responding BaP with a LOD of 15 pM. In addition to the high sensitivity, a significant merit over the above-mentioned methods is that the proposed sensor needs no calibration with a standard sensor. The quantification is performed based on the relative fluorescence intensity at 410-370 nm, exhibiting stable and reproducible detection results and avoiding the influence from the sensor-to-sensor difference. 7888
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