Environ. Sci. Technol. 2008, 42, 8076–8082
Preparation of DNA-Adsorbed TiO2 Particles with High Performance for Purification of Chemical Pollutants HIROSHI SUZUKI, TAKEHARU AMANO, TATSUSHI TOYOOKA, AND YUKO IBUKI* Institute for Environmental Sciences, Graduate School of Nutritional and Environmental Sciences, University of Shizuoka, 52-1 Yada, Shizuoka-shi 422-8526, Japan
Received April 4, 2008. Revised manuscript received July 18, 2008. Accepted August 13, 2008.
Photocatalysis using semiconductors such as titanium dioxide (TiO2) has been studied and applied to the treatment of wastewater and purification of air, because of its ability to decompose organic contaminants. However, there are still problemsassociatedwiththepracticalapplicationofphotocatalytic reactions, one of which is that contact between the reactants and catalysts is absolutely required, because the reaction occurs at the surface of the catalysts. This restricts the purification of pollutants on a large scale. In this study, we developed novel DNA-adsorbed TiO2 particles (DNA-TiO2) to solve the problem. Because DNA has an unique double-stranded structure and interacts with several chemicals, DNA-TiO2 can accumulate chemicals on the surface of TiO2. DNA intercalators (Methylene Blue and ethidium bromide), small amounts of which exist in large-volume solutions, were instantaneously trapped in DNA-TiO2 and degraded under ultraviolet (UV) light rapidly, compared to nonadsorbed TiO2. The efficiency of removal and photocatalytic degradation was dependent on the amount of DNA adsorbed on the surface of TiO2 and was independent of the size of DNA. Even if the pH (2-10) and temperature (∼56 °C) of the solution were changed, DNA remained stable on TiO2, and the ability to remove intercalators was also maintained. DNA-TiO2 could accumulate other pigments such as Acridine Orange, Orange II, Neutral Red, Brilliant Green, and Crystal Violet. These results suggested that DNA-TiO2 is beneficial for the removal and degradation of chemicals having affinity for DNA and dispersing in a large field.
Introduction Heterogeneous photocatalytic oxidation is a promising and emerging process for the purification of water and air. A semiconductor photocatalytic reaction using titanium dioxide (TiO2) has been widely studied to promote the degradation and total mineralization of various pollutants, for the purification of drinking water and for the cleaning of industrial wastewater, etc. (1, 2). The photocatalytic oxidation of TiO2 offers various advantages, that is, the reaction occurs at room temperature and atmospheric pressure, and the strong power of the oxidation causes the molecules to be destroyed rather than be converted to another form. Widespread persistent organic chemicals, including pharmaceuticals, endocrine-disrupting compounds, and azo dyes have * Author to whom correspondence should be addressed. Tel./ Fax: +81-54-264-5799. E-mail:
[email protected]. 8076
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been subjected to degradation using TiO2 (3–5). However, there are still many problems associated with the application of photocatalytic reactions on a commercial scale. A serious problem is that the photocatalytic reaction is difficult if reactants exist in dispersion and that contact between the reactants and catalysts is absolutely required, because the reaction occurs at the gas/solid and liquid/solid interface (6, 7). Therefore, currently, the photocatalytic oxidation of TiO2 cannot be applied to purification on a large scale without a process of concentration. DNA is one of the most important genetic materials of living organisms. Because of its unique double-stranded structure, DNA has highly specific functions, such as intercalation, groove binding interactions, and electrostatic interactions with several compounds. Almost all environmental compounds that are genotoxic react with DNA in some way. For example, ethidium bromide (EtBr) and propidium iodide, which are used as DNA-staining dyes for the detection of nucleic acids in laboratories, are known to be powerful mutagens and carcinogens, because of their binding to DNA (8–10). Polycyclic aromatic hydrocarbons (PAHs) formed in the environment during incomplete combustion are relatively nontoxic and noncarcinogenic themselves; however, their metabolic products such as diolepoxides and diones cause DNA-covalent adducts, resulting in mutagenic and carcinogenic effects (11, 12). Therefore, the unique property of DNA is useful for the accumulation of genotoxic compounds in the environment. DNA filters, DNA beads, and DNA gels have been used in the purification of environmental chemicals (13–17). DNAimmobilized glass beads and filters can selectively remove harmful compounds, such as dioxins, polychlorobiphenyl derivatives, and DNA intercalators. Chemicals that have a planar structure were removed most effectively (15). DNA is purified from salmon milts, but large amounts of DNAenriched materials have been discarded as waste in the industry. Using this discarded DNA as a material for the purification of environmental chemicals is doubly valuablefrom the environmental point of view: first is the purification of environmental pollutants and second is the beneficial use of the waste. One problem is how to handle the DNA materials that are bound to toxic chemicals. Then, we devised DNA-adsorbed TiO2 particles (henceforth referred to as DNA-TiO2), which might selectively bind genotoxic environmental chemicals and destroy them via a photocatalytic reaction under ultraviolet (UV) light. The complex of DNA with nanoparticles has a wide range of biotechnological applications as transfection agents (18, 19) or biosensors linked with sequence-specific nucleic acid interactions (20, 21). Synthesized TiO2-oligonucleotide nanocomposites were developed as a new tool for gene therapy (22, 23). However, an approach to environmental purification with a DNA-TiO2 hybrid has not been established, although much research concerning the degradation of environmental chemicals has been performed using TiO2 particles. In addition, many studies that have involved the chemical synthesis of complexes of DNA with several types of particles have been reported, whereas the DNA-TiO2 hybrid that we developed could be made without a chemical reaction, because TiO2 particles attracted DNA only by mixing. This made it easy for the hybrid to attach and remove DNA. In this paper, we evaluated a performance of DNA-TiO2, in terms of the removal of chemicals and the efficiency of degradation by photocatalytic reaction. Typical DNAintercalating compounds, such as Methylene Blue (MB) and EtBr, were mainly used for the evaluation. MB is a model 10.1021/es800948d CCC: $40.75
2008 American Chemical Society
Published on Web 10/03/2008
pollutant used to determine the photocatalytic efficiency (24). EtBr, which is a powerful mutagen, is widely used in biomedical laboratories as an agent for visualizing nucleic acid and recently has been degraded using a photocatalyst (25). DNA-TiO2 effectively trapped these chemicals, and the ability to degrade under UV light was much higher in DNA-TiO2 than in nonadsorbed TiO2. This hybrid might serve as a new photocatalyst having additional functions for the purification of chemical pollutants.
Experimental Section Materials. TiO2 particles (anatase) were purchased from Wako Chemical Co. (Japan). The zeta potential (ζ) of TiO2 particles exhibited a positive value in acidic media (pH e6.5). On the other hand, negative potential value was observed under alkaline conditions (see Figure S1A in the Supporting Information). The average particle size (pH 6.5) was 460 nm (see Figure S1B in the Supporting Information). DNA obtained from salmon testis was purchased from Sigma-Aldrich (St. Louis, MO). MB, EtBr, and other pigments were purchased from Wako Chemical Co. EcoRI was purchased from New England Biolabs, Inc. (Ipswich, MA). Preparation of DNA-TiO2. TiO2 particles (20 mg) were suspended in 1000 µL of DNA solution (∼1 mg/mL) and sonicated for 1 min twice using a bath-type sonicator (Bioruptor; CosmoBio Co. Ltd., Japan). They were centrifuged at 12 000 rpm for 10 min and the supernatant was removed. The pellet was resuspended with 1 mL of water and further sonicated for 1 min. These procedures for washing were repeated twice. Determination of Amount of DNA Adsorbed onto TiO2 Particles. The adsorbed amount of DNA was examined based on the recovery of attached DNA, using alkali and heat extraction. DNA-TiO2 was suspended in 120 µL of NaOH solution (pH 14) and sonicated once for 1 min using a bathtype sonicator. The suspended DNA-TiO2 was heated in a water bath at 98 °C for 10 min, sonicated for 1 min, then centrifuged at 12 000 rpm for 10 min. The upper solution was recovered and absorbance at 260 nm was determined using a spectrophotometer (DNA(µg/mL) ) 50 × Abs(260 nm) × dilution factor) (model DU-640, Beckman Instruments, Inc., Fullerton, CA). Determination of Removal Efficiency of DNA Intercalators. MB (2-80 µg/400 µL) and EtBr (5-100 µg/400 µL) were added to TiO2 or DNA-TiO2 (20 mg) and sonicated for 1 min. They were centrifuged at 12 000 rpm for 10 min, and the upper solution was diluted and transferred to a 96-well microplate (100 µL). Absorbance was measured at 664 nm for MB and at 478 nm for EtBr, respectively, using a microplate-reader (Power wave XS, BIO-TEK Instruments, Inc., Winooski, VT). The removal of MB from solutions with a large volume was examined as follows: several volumes (1, 10, 100, and 500 mL) of solution containing the same amount of MB (4 µg) and DNA (400 µg)-TiO2 (20 mg) were mixed for 30 min and centrifuged. The supernatants were freeze-dried and resolved again in a definite volume of water. The absorbance at 664 nm for MB was determined. To examine the influence of pH and temperature on the removal, MB (2 µg/400 µL) and EtBr (20 µg/400 µL) prepared at several different pH values (2, 4, 6, 8, 10, 12, and 14) and temperatures (4, 37, and 56 °C) were added to TiO2 or DNA (400 µg)-TiO2 (20 mg). They were immediately centrifuged and the absorbance of the supernatant was determined. Preparation of Different Sizes of DNA and Application to DNA-TiO2. The DNA solution (1 mg/mL) was sonicated several times (1-100 times) for 30 s each time, using a bathtype sonicator. To avoid excess heating by sonication, the water bath was kept at 20 °C with ice. While the DNA (1 mg/ mL) was digested by EcoRI (600 U/mL) for 6 h at 37 °C. These
FIGURE 1. Adsorption of DNA to TiO2 particles. TiO2 particles (20 mg) were mixed with 1000 µL of DNA solution (∼1 mg/mL). The amount of DNA adsorbed was determined by the recovery of DNA, using alkali and heat extraction. were electophoresed in a 1.5% agarose gel. The gel was stained with EtBr and visualized using a transilluminator. The fragmented DNA (1 mg/mL) was mixed with 20 mg of TiO2 and washed as described previously. The adsorbed DNA and efficiency of removal of MB were determined. Evaluation of Photocatalytic Degradation. TiO2 or DNA (400 µg)-TiO2 (20 mg) were suspended in 500 mL of distilled water, and 2 mL of MB solution (20 µg/mL) was added. Irradiation was provided by a UVA lamp (model FL20S-BLB, Hitachi (Japan)) with major emission at 365 nm (1.4 mW/ cm2). A magnetic stirrer was equipped at the bottom of the vessel to achieve effective dispersion. After stirring for 30 min and UVA irradiation, the suspension was centrifuged and the supernatant was collected. HCl solution (pH 2, 300 µL) was added and the suspended DNA-TiO2 was heated at 98 °C for 10 min, by which MB trapped in DNA was extracted. After centrifugation at 12 000 rpm for 10 min, the absorbance of the supernatant was determined using a microplate reader.
Results and Discussion Adsorption of DNA to TiO2 Particles. DNA was strongly adsorbed to TiO2 particles without chemical synthesis. The amount adsorbed was determined by recovery using alkali (0.1 N NaOH) and heat (98 °C) extraction, and the data were plotted as a function of the amount of DNA added (see Figure 1). The largest amount adsorbed was about 400 µg DNA/20 mg TiO2. Detachment of DNA from TiO2 particles was difficult, and treatment with both a strong alkali solution (pH 14) and heat (98 °C) were needed for complete recovery of the attached DNA (see Table S1 in the Supporting Information). The hybrid was stable under several conditions (heat (∼56 °C) or pH (2-10)). The exact mechanism that pulls DNA onto the surface of TiO2 particles is still not understood. We suspect several possibilities: (1) Electrical adsorption: In water (pH 6.5), the surface of TiO2 has a slightly positive charge (see Figure S1A in the Supporting Information, as well as refs 26 and 27). With DNA, which has a negative charge, making a complex with positively charged particles, because of electrostatic attraction (27–29), DNA might be attracted to TiO2. Long duplex DNA was reported to be compacted by cationic nanoparticles, similar to the nucleosome in the chromatin (28). (2) Sharing counterions: The adsorption of negatively charged DNA to negatively charged mica was mediated by the sharing of the DNA and mica counterions (30). When TiO2 is in an alkali solution, which has a negative charge, the DNA-TiO2 hybrid might be structured by the sharing of counterions, because the electrostatic attraction due to the sharing of counterions is particularly effective if the polyelectrolyte and the surface have almost the same surface charge density. (3) Binding of DNA with TiO2: TiO2 is reported to bind with DNA via phosphate groups (31). The clarification of interaction mechanism between DNA and TiO2 particles would be a future subject. VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Accumulation of intercalators by DNA-TiO2. (A) Images of accumulation of intercalators by DNA-TiO2; MB (2 µg/400 µL) (panel a) or EtBr (20 µg/400 µL) (panel b) was mixed with nonadsorbed TiO2 or DNA (400 µg)-TiO2 (20 mg) and centrifuged. Left tube, without TiO2; middle tube, nonadsorbed TiO2; right tube, DNA-TiO2. (B) Time-dependent removal of intercalators by TiO2 or DNA-TiO2. MB (40 µg/400 µL) or EtBr (20 µg/400 µL) was mixed with nonadsorbed TiO2 or DNA (400 µg)-TiO2 (20 mg) for predetermined times. The supernatant after centrifugation was diluted and the absorbance was determined. (C) Removal of intercalators, which is dependent on the amount of DNA adsorbed onto TiO2. Several concentrations of DNA (100-400 µg/mL) were added to TiO2 (20 mg). After washing, they were mixed with MB (20 µg/400 µL) or EtBr (20 µg/400 µL). (D) Concentration-dependent removal efficiency of intercalators; several concentrations of MB (2-80 µg/400 µL) and EtBr (5-100 µg/400 µL) were mixed with TiO2 or DNA (400 µg)-TiO2 (20 mg). The removal efficiency (expressed as a percentage) was calculated based on the absorbance of the supernatant after centrifugation. Efficiency with Which Intercalators Are Removed Using DNA-TiO2. MB and EtBr solutions were treated with 20 mg of DNA-TiO2 that had been prepared with several amounts of DNA (100, 200, 300, and 400 µg/mL), as described in Figure 1. Figure 2A shows photographs of the removal of intercalators by DNA-TiO2. MB and EtBr solutions were mixed with TiO2 or DNA-TiO2 and centrifuged. DNA-TiO2 was precipitated with MB and EtBr with a blue and pink color, respectively, whereas nonadsorbed TiO2 was not. The supernatant was colorless in the DNA-TiO2 system. Next, to clarify the efficiency with which the intercalators were removed, the absorbance to which each was adjusted was measured. In Figure 2B, the contact time between DNA-TiO2 (or TiO2) and intercalators that was needed for removal of the intercalators was examined. The removal by DNA-TiO2 occurred instantaneously. Both intercalators were removed, depending on the amount of DNA adsorbed to TiO2 (see 8078
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Figure 2C). DNA (400 µg)-TiO2 (20 mg) could perfectly trap up to 20 µg of MB and 10 µg of EtBr, respectively (see Figure 2D). In the aforementioned doses, excess intercalators, which could not be trapped, were detected. Even if the reaction time was extended, the amount that was accumulated by DNA-TiO2 did not change (data not shown). In Figure 2, we used typical intercalators (MB and EtBr) to evaluate the removal performance. Because these chemicals were instantaneously removed from solution, other chemicals were also predicted to be effectively removed from water. Table 1 shows the structure of seven pigments and data regarding their removal from solution. The DNA intercalator Acridine Orange was effectively removed, in a manner similar to MB and EtBr. Yamada et al. (15) reported that DNA-immobilized glass beads or films selectively bind environmental pollutants with a planar structure (for example, dibenzo-p-dioxin, dibenzofran, and biphenyl). We
TABLE 1. Removal Efficiency of Pigments by TiO2 or DNA-TiO2 Particlesa
a TiO2 or DNA (400 µg)-TiO2 particles (20 mg) were treated with several pigments (10 µg/400 µL) immediately; they were centrifuged and the absorbance of the supernatant, adjusted to each pigment, was determined.
speculate that polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofrans, and polychlorinated biphenyls (referred to as dioxins), which are known to have mutagenic and carcinogenic activity, would be selectively removed using this system, because of their planar structures. Cellular DNA is a target of the reaction of endogeneous and exogeneous mutagens that bind chemically to DNA bases. Several researchers have reported that some mutagenic chemicals form covalent DNA adducts. In addition, a positive correlation between DNA adduct levels and a susceptibility to cancer has been documented (11, 12). In this study, intercalators were captured instantaneously by DNA-TiO2. If DNA-TiO2 makes contact with chemicals for a longer time, some molecules that are apt to adduct to DNA could be removed. The azo dye, Orange II, whose removal efficiency was lower than that of the chemicals with a planar structure, was also effectively removed from solution by DNA-TiO2, compared with nonadsorbed TiO2. Because the textile industry generates large amounts of wastewater that contain several azo dyes, DNA-TiO2 might be valuable for treatments. Effect of DNA Size on the Removal of Intercalators. The size of the DNA did not affect the adsorption to TiO2 and the efficiency of removal of chemicals (see Figure 3). DNA with over 2000 bp, which was used throughout this study, was sonicated several times (1, 10, 20, 50, or 100 times) and shortened to about 300 bp (50 times) (see Figure 3A, lane 2). The restricted enzyme EcoRI digested the DNA to about 200 bp (see Figure 3A, lane 3). Even if the size of DNA was different, the amount of DNA that adsorbed onto TiO2 (see Figure 3B) and the removal efficiency of MB (see Figure 3C) were equal. These results suggested that DNA, which retains a long duplex, was not necessarily required and that poor-quality
DNA that was fragmented in the process of extraction of industrial DNA-enriched waste was usable. Effect of pH and Temperature on Removal of Intercalators. To clarify whether intercalators could be effectively removed by DNA-TiO2 under several conditions, the pH (2, 4, 6, 8, 10, 12, and 14) and temperature (4, 37, and 56 °C) were varied (see Figure 4). Both MB and EtBr could be removed by DNA-TiO2 independent of pH (pH ∼10) and temperature, suggesting that the DNA-TiO2 hybrid retained its form and function under these conditions. However, the removal efficiency of EtBr was decreased in alkali solutions (pH 12 and 14) (because the color of MB changed in the alkali solution, the exact efficiency of removal of MB in alkali solution could not be evaluated). The reason for this is that the hybrid could not form under the conditions, as shown in Table S1 of the Supporting Information. Because we confirmed that DNA could not be unfastened without serious treatment with alkali solution (pH 12-14) or acid solution (pH 2) and/or heat (98 °C), this hybrid is considered to be practical. Removal of MB from Large Volume Solutions and Photocatalytic Degradation Using Adsorbed TiO2 Particles. It is important to clarify whether DNA-TiO2 has large-scale applications; that is, small amounts of chemicals could be removed from large volume solutions. To identify whether MB could be effectively removed from large-volume solutions, the removal efficiency was investigated in several volumes of solution (1, 10, 100, and 500 mL) (see Figure 5A). Even if the volume of solution increased, complete removal was observed equally under every condition. The photocatalytic degradation of MB by DNA-TiO2 was examined in a large volume of water (20 mg DNA-TiO2/500 VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Effect of DNA size on adsorption to TiO2 and removal of MB. (A) Agarose gel electrophoresis of fragmented DNA. DNA was fragmented by sonication or restriction enzyme (EcoRI) and electophoresed in a 1.5% agarose gel. Lane 1, untreated DNA; lane 2, DNA sonicated 50 times; lane 3, DNA fragmentized by EcoRI. (B) Adsorption of fragmented DNA to TiO2. The DNA that was sonicated 1-100 times (1 mg/mL) was mixed with TiO2 (20 mg), and the amount of adsorbed DNA was determined. (C) Removal of MB (2 µg/400 µL) by fragmented DNA-TiO2 (20 mg).
FIGURE 4. Removal efficiency of intercalators using DNA-TiO2 under several conditions: (A) pH ) 2-14 and (B) temperature ) 4, 37, and 56 °C. mL water) (see Figure 5B). The degradation of MB after UV exposure was measured under the following three conditions: (i) in the absence of TiO2 particles, (ii) in the presence of nonadsorbed TiO2, and (iii) in the presence of DNA-TiO2. MB has been reported to be degraded by TiO2 photocatalyst under UV exposure (24). No apparent degradation was observed when TiO2 particles were not present. The initial 8080
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rate of degradation of MB using DNA-TiO2 was fast, and 54% of the MB was decomposed under UV exposure for 0.5 min, while nonadsorbed TiO2 showed only 17% of degradation at that point. DNA-TiO2 was more effective in adsorbing and decomposing MB than untreated TiO2. The degradation efficiency increased depending on the amount of DNA that was adsorbed onto TiO2 (see Figure 5C). These results
Acknowledgments We thank Dr. Shimizu and Dr. Oku (University of Shizuoka) for their technical support in regard to the use of Zetasizer Nano.
Supporting Information Available Characteristic of TiO2 particles used in this study is provided in Figure S1. Stability of DNA-TiO2 under several conditions is provided in Table S1. (PDF) This information is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited
FIGURE 5. Removal of intercalators from large volume of water and photocatalytic degradation. (A) Removal efficiency of MB; several volumes (1, 10, 100, and 500 mL) of solution containing the same amount of MB (4 µg) were mixed with DNA-TiO2 (20 mg) and centrifuged, and the absorbance of intercalators left in solution was determined. (B) Photocatalytic degradation of MB by DNA-TiO2; TiO2 or DNA-TiO2 (20 mg) were suspended in 500 mL of distilled water and 2 mL of solution of MB (20 µg/mL) was added. After UVA irradiation, the amounts of undegraded MB both in solution and in DNA-TiO2 were determined. (C) DNA concentration-dependent photodegradation. DNA (50-400 µg)-TiO2 (20 mg) were suspended in 500 mL of distilled water and 2 mL of solution of MB (20 µg/mL) was added. They were exposed to UVA for 30 s. indicated that the DNA-TiO2 hybrid accumulated intercalators instantaneously from a large volume of water and then effectively decomposed those with DNA trapped on the surface of the TiO2 particles, under UV light.
Conclusion We have prepared novel DNA-TiO2 particles using a simple method: only mixing. DNA-TiO2 could selectively remove DNA intercalators, which exist in large-volume solutions and degrade quickly under UV irradiation, compared to untreated TiO2, indicating that DNA-TiO2 is beneficial for the purification of environmental chemicals that have an affinity for DNA. Although UV exposure would decompose DNA with trapped chemicals on the surface of TiO2, it would be possible to attach DNA again only by blowing and mixing the TiO2 with the DNA solution. Because DNA is generally discarded as waste, this reattachment would be realized at a low price and would be valuable in the recycling of waste.
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