Preparation of DNA-adsorbed TiO₂ particles — Augmentation of performance for environmental purification by increasing DNA adsorption by external pH regulation

Abstract

We have previously developed a novel photocatalyst, DNA-attached titanium dioxide (DNA–TiO₂), useful for the recovery and decomposition of chemicals [Suzuki et al. Environ. Sci. Technol. 42, 8076, 2008]. Chemicals accumulated in DNA near the surface of TiO₂ and were degraded under UV light. The efficiency of their removal was dependent on the amount of DNA adsorbed on TiO₂, indicating the attachment of larger amounts of DNA to result in higher efficiency. In this study, we succeeded in improving the performance of DNA–TiO₂ by increasing the amount of DNA adsorbed by regulating the external pH. The adsorption of DNA by TiO₂ dramatically increased at pH2, to about fourfold that at other pH values (pH4–10). Repeating the process of DNA addition increased the adsorption further. The attached DNA was stable on the surface of TiO₂ at pH2–10 and 4–56 °C, the same as DNA–TiO₂ prepared at pH7. As the DNA–TiO₂ prepared at pH2 retained much DNA on its surface, chemicals (methylene blue, ethidium bromide, etc.) which could intercalate or react with DNA were effectively removed from solutions. The photocatalytic degradation was slow at first, but the final degradation rate was higher than for non-adsorbed TiO₂ and DNA–TiO₂ prepared at pH7. These results indicated that preparation of DNA–TiO₂ at pH2 has advantages in that much DNA can be attached and large amounts of chemicals can be concentrated in the DNA, resulting in extensive decomposition under UV light.

Introduction

Titanium dioxide (TiO₂) is focused as a photocatalyst for environmental purification including wastewater treatment and indoor air cleaning (Carp et al., 2004, Kwon et al., 2008). As a result of a photocatalytic reaction (oxidation) by TiO₂, organic chemicals, such as endocrine disruptors, agrochemicals, etc., were finally decomposed into carbon dioxide and water (Lhomme et al., 2008, Ohko et al., 2001, Vautier et al., 2001). The advantage of environmental purification using TiO₂ is that the reaction can occur at room temperature and pressure, and requires only solar energy. However, there are still problems associated with the application of photocatalytic reactions on a commercial scale. One problem is that contact between the reactants and catalysts is absolutely required because the reaction occurs at the surface of the catalysts (Linsebigler et al., 1995). To solve this problem, attempts to form a complex between catalysts and adsorbents have been made (El-Sheikh et al., 2007, Mogyorósi et al., 2002, Takeda et al., 1995). Combinations of inert materials such as zeolite, silica, and activated carbon, and TiO₂ enhanced the photodegradation of organic chemicals.

Previously, we prepared DNA-adsorbed TiO₂ particles (DNA–TiO₂) to solve the problem of contact between the reactants and catalysts (Suzuki et al., 2008). DNA could be adsorbed on the surface of TiO₂ particles strongly by mixing only, and the hybrid was stable under actual environmental conditions (pH 2–10, temperature 4–56 °C). DNA–TiO₂ could accumulate several chemicals on the surface of TiO₂ because DNA interacts with some chemicals. The activity of photocatalytic degradation with DNA–TiO₂ was more remarkable than that with non-adsorbed TiO₂, which might be attributable to contact between the reactants and TiO₂.

Many environmental mutagens and carcinogens are highly reactive to DNA. For example, ethidium bromide 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 (Ferguson and Denny, 2007, Fukunaga and Yielding, 1983, McCann et al., 1975, Waring, 1965). Using this characteristic, attempts have been made to use DNA as filters and films for removing environmental pollutants. The group of Nishi reported that DNA films and DNA-immobilized glass beads could be useful to remove planar structural harmful chemicals like ethidium bromide, dioxin-derivatives, polychlorobisphenol (PCB)-derivatives, and polycyclic aromatic hydrocarbons (PAHs) from solutions (Liu et al., 2005, Yamada et al., 2001, Yamada et al., 2002, Zhao et al., 2004). Compared with those DNA materials, DNA–TiO₂ is beneficial in that toxic chemicals are completely decomposed by the photocatalytic reaction.

The efficiency with which chemicals were removed by DNA–TiO₂ was dependent on the amount of DNA attached to TiO₂ (Suzuki et al., 2008). This indicated that larger amounts of DNA attached to TiO₂ might be desirable for the more effective removal of chemicals. However, the method for additional attachment of DNA is not known because exactly how DNA hybridizes to TiO₂ has not yet been clarified. Hybridization of DNA with silver, gold, silica, etc. has been studied (Basu et al., 2008, Ganachaud et al., 1997, Mao et al., 1994, Melzak et al., 1996, Storhoff et al., 2002, Zinchenko et al., 2007). DNA, negatively charged, makes a complex with cationic particles through electrostatic attraction (Ganachaud et al., 1997, Zinchenko et al., 2007). Hydrogen bonds are formed between the phosphate group in DNA and the silanol group on the surface of silica (Mao et al., 1994, Melzak et al., 1996). Functional groups such as amines and carbonyls in nucleosides could act as ligands for gold particles as well as phosphate groups (Basu et al., 2008, Storhoff et al., 2002). Although previous studies about the affinity of DNA for particles are limited, some studies of DNA adsorption to silica showed that decreasing the pH increased the adsorption (Geng et al., 2009, Isailovic et al., 2007, Melzak et al., 1996). This indicated that the amount of DNA adsorbed to TiO₂ particles could be controlled by changing an external parameter, pH. Furthermore, adsorption by soil minerals has been well examined. In the actual environment, DNA may be released into soil and aquatic ecosystems from dead and living organisms. The DNA could be adsorbed to soil, sand and clay (Ogram et al., 1988). More than forty years ago, Greaves and Wilson (1969) reported that the adsorption of DNA by minerals is dependent on pH. The adsorption increased at pH values below the isoelectric point of DNA (Cai et al., 2006, Greaves and Wilson, 1969, Khanna and Stotzky, 1992).

For the purpose of developing effective photocatalysts, we tried to manufacture DNA-hybridized TiO₂ by controlling pH. Large amounts of DNA were adsorbed by TiO₂ particles under strong acidic conditions. The DNA-adsorbed TiO₂ hybrid effectively trapped chemicals, and the final rate of photocatalytic degradation under ultraviolet (UV) irradiation was also augmented.