Enhanced catalytic and SERS activities of size-selective Rh NPs on DNA scaffolds

Size-selective, ultra-small Rh nanoparticles (NPs) were synthesized for the first time utilizing a simple UV photo-irradiation route. The synthesis was done by the reaction of Rh(III) salt with alkaline 2,7-dihydroxynaphthalene (2,7-DHN) on DNA scaffolds just within 5 h of reaction at room temperature and under ambient conditions. The average diameter of the Rh NPs is similar to 1-3 nm and the size-selective NPs can be prepared easily just by varying the Rh(III) to DNA molar ratio. The synthesized Rh NPs were examined for two potential applications such as in catalytic reduction and in surface enhanced Raman scattering (SERS) studies. The catalytic reduction of 4-nitroaniline (4-NA) was chosen as a model reaction to evaluate the catalytic activity of the synthesized Rh NPs. The maximum observed catalytic rate was 2.17 x 10(-1) min(-1) which is the highest catalytic rate ever reported for Rh NPs. The size effect on the catalysis reaction was examined and it was observed that large size Rh NPs acted as a better catalyst compared to other size Rh NPs. Methylene blue (MB) was chosen as a Raman probe molecule. The highest enhancement factor (EF) value ever reported for Rh NPs was observed as similar to 1.53 x 10(2) using small size Rh NPs. For the size-selective Rh NPs, the order of EF follows as: small size > medium size > large size. The strong SERS enhancement might be due to the self-assembled structure on DNA which can generate a greater number of surface active 'hot spots'. Other than catalysis and SERS, these self-assembled Rh NPs may find potential applications such as in other organic catalysis reactions, biomolecular detection at trace levels, ultra-violet plasmonics and medical diagnostic purposes.