DNA Hydrogel as a Template for Synthesis of Ultrasmall Gold Nanoparticles for Catalytic Applications Anatoly Zinchenko,* ,† Yasuyuki Miwa, † Larisa I. Lopatina, ‡ Vladimir G. Sergeyev, § and Shizuaki Murata † † Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan ‡ Department of Colloid Science, Faculty of Chemistry, Moscow State University, Moscow, 119899, Russia § Department of Polymer Science, Faculty of Chemistry, Moscow State University, Moscow, 119899, Russia * S Supporting Information ABSTRACT: DNA cross-linked hydrogel was used as a matrix for synthesis of gold nanoparticles. DNA possesses a strong affinity to transition metals such as gold, which allows for the concentration of Au precursor inside a hydrogel. Further reduction of HAuCl 4 inside DNA hydrogel yields well dispersed, non-aggregated spherical Au nanoparticles of 2-3 nm size. The average size of these Au nanoparticles synthesized in DNA hydrogel is the smallest reported so far for in-gel metal nanoparticles synthesis. DNA hybrid hydrogel containing gold nanoparticles showed high catalytic activity in the hydro- genation reaction of nitrophenol to aminophenol. The proposed soft hybrid material is promising as environmentally friendly and sustainable material for catalytic applications. KEYWORDS: DNA hydrogel, gold nanoparticles, catalysis, nitrophenol reduction ■ INTRODUCTION The affinity of nucleic acids to transition metals, in particular to gold, is well known from earlier biochemical studies originated in the 1970s. 1,2 Au(III) coordinates with DNA bases and forms stable complexes of the compositions [Au]/[nucleotide] = 0.5 and higher. 3,4 The strong DNA affinity to transition metals favored its choice as a template for synthesis of inorganic nanowires, 5-9 metallization of surface-absorbed 10,11 and folded DNA condensates, 12-14 metallization of self-assembled “DNA origami”, 15,16 and other DNA molecular architectures. 17,18 However, currently available DNA metallization protocols are limited by those based on one-dimensional (DNA macro- molecule) or two-dimensional (DNA adsorbed on surfaces) templates. It is promising to further extend the dimensionality of the DNA templates to three-dimensional molecular architectures in order to construct a bulk material containing nanosized metal structures distributed within its volume that can find potential applications in catalysts, sensors, etc. One suitable example of the three-dimensional matrix composed of the DNA is a hydrogel, which can be employed as a platform for templating of a material formed in various chemical reactions. DNA hydrogel was first prepared by DNA crosslinking with epoxide by Tanaka et al., 19 who studied DNA hydrogels in relation to phase transition in low-polar solvents. The understanding of physico-chemical properties of DNA hydrogel was significantly deepened in recent works of Costa et al. 20-22 and Okay et al. 23-25 Another robust biological method based on the ligase-mediated construction of a DNA hydrogel from branched DNA was also reported. 26-28 DNA is a particularly suitable natural polymer for concentrating transition metal ions because purine and pyrimidine bases and, to some extent, phosphate groups of DNA act as efficient chelation sites. Very recently, we reported the utilization of DNA hydrogel as an absorbent for extraction of transition and rare-earth metals from aqueous solutions. 29 Therefore, DNA hydrogel containing transition metal ions is promising as a “reactor” for controllable synthesis of metal nanoparticles, where strong interaction of DNA with transition metals must play an important role to control the growth of inorganic nanoparticles. It is worth noting that the industrial scale of low-cost DNA extracted from fish milt, the waste product of the marine industry, is considered for use in various material applications. 30-33 In the present study, we focused on gold nanoparticles synthesis inside DNA hydrogel by performing a reduction of hydrogel-absorbed Au(III) directly inside the hydrogel matrix. We report the formation of ultrasmall, non-aggregated, and kinetically stable gold nanoparticles of 2-3 nm size inside DNA hydrogel and demonstrate the application of the resulted soft hybrid material in a catalytic hydrogenation of nitrophenol. Received: November 5, 2013 Accepted: February 17, 2014 Published: February 17, 2014 Research Article www.acsami.org © 2014 American Chemical Society 3226 dx.doi.org/10.1021/am5008886 | ACS Appl. Mater. Interfaces 2014, 6, 3226-3232