Effect of Phenolic Compounds on the Synthesis of Gold Nanoparticles and its Catalytic Activity in the Reduction of Nitro Compounds.

Elisabete Alegria,Ana Ferraria,Ana Ribeiro,Ana Do Rego,Armando Pombeiro,Marta Mendes

doi:10.3390/nano8050320

Abstract

Gold nanoparticles (AuNPs) were prepared using an eco-friendly approach in a single step by reduction of HAuCl4 with polyphenols from tea extracts, which act as both reducing and capping agents. The obtained AuNPs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible spectroscopy (UV–vis), and X-ray photoelectron spectroscopy (XPS). They act as highly efficient catalysts in the reduction of various aromatic nitro compounds in aqueous solution. The effects of a variety of factors (e.g., reaction time, type and amount of reducing agent, shape, size, or amount of AuNPs) were studied towards the optimization of the processes. The total polyphenol content (TPC) was determined before and after the catalytic reaction and the results are discussed in terms of the tea extract percentage, the size of the AuNPs, and their catalytic activity. The reusability of the AuNP catalyst in the reduction of 4-nitrophenol was also tested. The reactions follow pseudo first-order kinetics.

Highlights

Metallic nanoparticles (NPs) are often synthesized using chemical methodologies involving organic and inorganic reducing agents, which can act as stabilizing agents to avoid the coalescence of the NPs, such as hydrazine, sodium borohydride (NaBH4), or N,N-dimethylformamide [1]
The catalytic activity was quantified in terms of conversion of the substrate and turnover frequency (TOF), considering that 2- or 4-aminophenol; 2, 3, or 4-phenylenediamine; and aniline are the only reduction products
The effect of different concentrations of tea extract (1, 5, and 10% w/v) solutions on the size and dispersity of the Au nanoparticles (AuNPs) was analyzed

Summary

Introduction

Metallic nanoparticles (NPs) are often synthesized using chemical methodologies involving organic and inorganic reducing agents, which can act as stabilizing agents to avoid the coalescence of the NPs, such as hydrazine, sodium borohydride (NaBH4), or N,N-dimethylformamide [1]. Gold nanoparticles (AuNPs) are recently gaining a great interest due to their potential application in many fields, such as catalysis, biosensing, photonics, drug-delivery systems, and antimicrobial agents [13,14,15,16,17,18,19] They have been used to catalyze electron-transfer and oxidation reactions, glucose oxidation [20], aerobic alcohol oxidation [21], reduction of nitroarenes in aqueous media [22], or CO oxidation and propylene epoxidation [23]. The existence of a well-defined absorption band characteristic of this substrate allows to easy screening of the reaction by UV–vis spectroscopy [34,35,36]

Reagents and Instrumentation

Preparation of Au Nanoparticles Using Tea Extracts

Reduction of Nitro Compounds in Aqueous Solution

Au Nanoparticle Identification

Reduction of Other Nitro Compounds

Recycling Studies for AuNPs in 4-NP Reduction