Polymer Encapsulated Self-Assemblies of Ultrasmall Rhenium Nanoparticles: Catalysis and SERS Applications

Abstract

Ultrafast formation of stable and self-assembled rhenium (Re) nanoparticles (NPs) using a poly allylamine hydrochloride (PAH) scaffold within 120 s of wet-chemical reaction at room temperature in aqueous solution has been reported. The average diameters of the two different sets of Re NPs synthesized are ∼0.7 ± 0.25 and ∼1.7 ± 0.3 nm, which can be easily achieved by controlling the polymer to Re7+ molar ratio. The small-size Re NPs are formed in solution, self-assembled together to form the chain-like or necklace-like structure. The synthesized Re NPs were used in two different potential applications, such as in catalysis and in surface-enhanced Raman scattering (SERS) studies. Catalysis study was done for 4-nitroaniline (4-NA) reduction with excess NaBH4 taking two different sets of Re NPs as catalyst. The highest catalytic rate for nitroaromatics reduction ever reported of ∼1.52 × 10–1 min–1 has been observed with large-size Re NPs as catalyst. In SERS, methylene blue (MB) was used as a Raman probe molecule. Strong SERS enhancements were observed with both sets of Re NPs due to their ultrasmall size, narrow interparticle gap, and self-assembled structure in PAH scaffold. These closely tethered and self-assembled Re NPs generated more surface active “hot spots” that resulted in good SERS enhancement. The present synthesis route is easy, cost effective, and fast and can generate stable Re NPs which could further be applied in interdisciplinary fields other than catalysis and SERS in the near future.