Water Detoxification by a Substrate-Bound Catecholamine Adsorbent.

Ensuring an adequate supply of clean water is an urgent global issue. In 1992, the United Nations (UN) designated March 22 of every year as World Water Day (WWD) to increase awareness of the importance of clean water conservation. Also, Population Action International (PAI) has reported a shortage of fresh water in developing and under-developed countries. The demand for clean water will continue to increase because of industrialization and population growth. Thus, the development of technologies that can effectively purify contaminated water has been an emerging area of research. Adsorption-based technologies have been used to remove a variety of toxic chemicals from contaminated water through batch or continuous flow processes. The carboxyl and amine groups of activated carbon and polysaccharides such as alginate and chitosan are the most widely implemented adsorbents owing to their ability to chelate toxic heavy metals. However, several limitations of existing adsorbents can be identified. First, the attachment of polysaccharides onto solid phases is essential, yet these adsorbents lack inherent adhesive properties to facilitate their immobilization onto substrates. Second, the generation of secondary pollutants during chemical processing of adsorbents is a serious environmental issue. In the case of activated carbon adsorbent, a strongly acidic solution—typically 10–50% (v/v) HNO3—has been used. [5] Third, the variety of toxic chemicals that can be removed by existing adsorbents is limited—they often show excellent performance in the removal of heavy metals but perform poorly in the removal of toxic organic molecules, particularly in the case of polysaccharide adsorbents. Fourth, methods for regenerating adsorbents and isolating adsorbed toxic chemical complexes have not been adequately developed. Finally, the cost of carbon materials is rapidly increasing, a particular concern for developing and resource-poor settings. Thus, novel approaches to overcome the aforementioned limitations, in whole or in part, may lead to improved and more cost-effective water detoxification processes. Adaptations of chemical and physical principles found in the biological world can offer good alternative approaches to water detoxification. In this case, biological strategies that combine surface adhesion with the ability to isolate, bind, and sequester heavy metals and other toxins are of great interest. The proteins comprising the byssal attachments of marine mussels share many of these qualities. The byssal adhesive pads are effective at attaching to substrates, and have been reported to be enriched in various metals (Fe, Mn, Zn, Cu, Ni, etc.). As an indicator of the protein’s metal binding ability, the concentration of iron in dried byssus is approximately a million-fold higher than the typical concentration in seawater. Furthermore, the strong metal binding property of mussel byssus has led to investigations of byssal tissue as a sensitive biomonitoring organ for heavy metals in the marine environment. Thus, we hypothesize that a mussel-inspired approach combining the substrate adhesion and metal binding affinity of catechol and amine functional groups would be useful to remove toxic metals from contaminated water. Furthermore, quinones formed by catechol oxidation may provide additional capability of removing selected toxic organic compounds, as they are known to react with amines and thiols. In fact, we have demonstrated that this chemical reaction occurs at interfaces at the single-molecule level. Polydopamine is a synthetic mimic of mussel adhesive proteins that deposits as a thin (monolayer to 50 nm or more) coating on virtually any material by spontaneous oxidation of dopamine in an alkaline aqueous solution (Figure 1b). Compared with other methods of coating substrates, polydopamine has the advantage of being inexpensive, adherent, and simple to deposit onto substrates without the need for surface pretreatment. Polydopamine nanolayers form on virtually any material surface, including noble metals, oxides, semiconductors, ceramics, synthetic polymers, and graphene oxide, as well as on superhydrophobic surfaces. The strength of catechol adhesion on TiO2 and gold surfaces was reported to be stronger than the well-known avidin–biotin interaction at a singlemolecule level, thus explaining the robustness of the polydopamine coating. The binding force on glass substrates, however, has not been reported, but it has been observed that the coating remained stable under vigorous mechanical stirring conditions ( 1000 rpm). Also, the catecholamines that do not participate in surface binding can perform a variety of chemical reactions, resulting in water detoxification. [a] M. Lee, S. Hong, Prof. H. Lee Department of Chemistry The Graduate School of Nanoscience and Technology Korea Advanced Institute of Science and Technology (KAIST) 335 Science Rd. Daejeon, 305-701 (South Korea) Fax: (+82)42-350-2810 E-mail : [email protected] [b] J. Rho, Prof. P. B. Messersmith Biomedical Engineering, Materials Science and Engineering Chemical and Biological Engineering Department Northwestern University, Evanston, IL 60208 (USA) Fax: (+1)847-491-4928 E-mail : [email protected] [c] Dr. D.-E. Lee, S.-J. Choi Radioisotope Research Division Research Reactor Utilization and Development KAERI, Daejeon, 305-353 (South Korea) [] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201200209.