Capping of supramolecular curcubit[7]uril facilitates formation of Au nanorods during pre-reduction by curcumin

Curcumin being biologically available and non-toxic natural compound is an attractive and alternative green reagent for the reduction of gold salts to elemental gold. Here, we bring new insight that shape and size of the Au nanoparticles (NPs) can be tuned by using curcumin in combination with supramolecular host molecule such as curcubit[7]uril (CB[7]) as stabilizing agent. It is found that AuNPs prepared using curcumin alone in aqueous environment gives spherical and smaller size, 10-20 nm, particles with its surface Plasmon resonance peak at similar to 536 nm. M the same time when CB[7] is used as a capping agent along with curcumin, larger size NPs and at higher concentration (similar to 50 mu M) of CB[7] Au nanorods (NRs) are formed with a broad surface Plasmon resonance spectrum peaked at similar to 610 nm. Thus, CB[7] M higher concentration ultimately helps to form Au NRs. Formation of AuNPs and AuNRs has been confirmed by XRD and conjugation of CB[7] and curcumin is established by FT-IR and TGA data. However, these particles aggregate in solution to form particles between 200 nm and 300 nm with a polydispersity of 0.25 as observed in DLS. Zeta potential measurement indicates formation of stable nanoparticles/nanorods without further precipitation within time. Kinetic study shows that the half-life for the formation of AuNP/NRs is similar to 36 min, whereas degradation of curcumin during synthesis has a half-life of similar to 10 min, this discrepancy suggests some amount of curcumin degrades on its own during this process besides acting as reducing agent.