Ionic Liquid-Responsive Phase Transfer of Gold Nanoparticles: Anionic Metathesis

In this work, a fresh approach has been proposed for the efficient transfer of gold nanoparticles (AuNPs) from an aqueous to organic phase by the metathesis reaction or anion exchange reaction. Here, we synthesized ionic liquid 1-butyl 3-hexadecyl imidazolium bromide [C₄C₁₆Im]Br-stabilized AuNPs which exhibit excellent stability in solution. Transfer of Au@[C₄C₁₆Im]Br from an aqueous to organic phase was investigated by the metathesis reaction with different hydrophobic ionic liquid-forming salts such as LiNTf₂, LiClO₄, and KPF₆. The anionic exchange process in ionic liquids at the AuNP surface to make hydrophilic to hydrophobic AuNPs is demonstrated. It was found that hydrophobic ionic liquids provide the most effective transfer of AuNPs from the aqueous to organic phase. Interestingly, we have noticed no change in color, size, and shape of AuNPs for more than a month, indicating more efficient transfer of AuNPs in organic solvents, which remained stable for over a month. The ionic liquids with anions NTf₂^-, ClO₄^-, and PF₆^- make the AuNP surface hydrophobic, indicating their good dispersibility in nonpolar solvents. Finally, these AuNPs exhibit excellent sensitivity toward the refractive index of organic solvents, which is correlated with the surface plasmon resonance (SPR) λ_SPR bands.

Kinetic investigation for the catalytic reduction of nitrophenol using ionic liquid stabilized gold nanoparticles

We demonstrate the synthesis of gold nanoparticles (AuNP) stabilized by 1-butyl-3-hexadecyl imidazolium bromide (Au@[C₄C₁₆Im]Br) and their use as a catalyst for the reduction of nitrophenol. The AuNPs show excellent stability in presence of [C₄C₁₆Im]Br ionic liquids for the reduction of 4-nitrophenol and 2-nitrophenol using NaBH₄ as a reducing agent. The detailed kinetics for the reduction of 4-nitrophenol and 2-nitrophenol were investigated and the catalytic activity of Au@[C₄C₁₆Im]Br was evaluated. The pseudo first-order rate constant (k_app) values for 4-nitrophenol was observed to be greater than that of 2-nitrophenol and explained on the basis of hydrogen bonding present in 2-nitrophenol. Au@[C₄C₁₆Im]Br showed good separability and reusability and hence, it can be used for the complete reduction of nitrophenols in multiple cycles. The Langmuir–Hinshelwood reaction mechanism is elucidated for reduction of 4-nitrophenol by Au@[C₄C₁₆Im]Br nanocatalyst on the basis of the k_app values. The thermodynamic activation parameters such as activation energy, enthalpy of activation and entropy of activation were determined and explained using the temperature dependent kinetics for the reduction of nitrophenol using Au@[C₄C₁₆Im]Br. The above results reveal that the Au@[C₄C₁₆Im]Br nanocatalyst demonstrates excellent catalytic performance for the reduction of nitrophenol by NaBH₄ at room temperature.

Catalytic reduction of nitrophenol using ionic liquid stabilized AuNPs.