Hydroquinone Based Synthesis of Gold Nanorods

Summary

This paper describes a protocol for the synthesis of gold nanorods, based on the use of hydroquinone as reducing agent, plus the different mechanisms for controlling their size and aspect ratio.

Abstract

Gold nanorods are an important kind of nanoparticles characterized by peculiar plasmonic properties. Despite their widespread use in nanotechnology, the synthetic methods for the preparation of gold nanorods are still not fully optimized. In this paper we describe a new, highly efficient, two-step protocol based on the use of hydroquinone as a mild reducing agent. Our approach allows the preparation of nanorods with a good control of size and aspect ratio (AR) simply by varying the amount of hexadecyl trimethylammonium bromide (CTAB) and silver ions (Ag⁺) present in the "growth solution". By using this method, it is possible to markedly reduce the amount of CTAB, an expensive and cytotoxic reagent, necessary to obtain the elongated shape. Gold nanorods with an aspect ratio of about 3 can be obtained in the presence of just 50 mM of CTAB (versus 100 mM used in the standard protocol based on the use of ascorbic acid), while shorter gold nanorods are obtained using a concentration as low as 10 mM.

Introduction

Gold nanoparticles (AuNPs) are one of the most widespread and promising nanostructures to be used in biomedical applications. Their use is essential in many point-of-care in vitro diagnostics products.¹ They have been proposed as an effective tool for a number of other different applications: as a contrast agent in imaging studies,² as a drug delivery system³ and as drugs for light-induced thermotherapy (or photothermal therapy).⁴ The great potential of AuNPs has driven, in the last twenty years, intense research on the development of new synthesis that is able to increase the control on the size and shape obtained.....

Protocol

1. Synthesis of Gold Nanorods

Note: Use highly purified water throughout.

1. Preparation of the gold seeds
   1. Dissolve 364.4 mg of hexadecyltrimethylammonium bromide (CTAB) in 5 ml water, under ultrasonication at 40 °C until the solution becomes clear. Let the CTAB solution cool down to room temperature.
   2. Separately, prepare 5 ml of tetrachloroauric acid (HAuCl₄) in water (0.5 mM).
   3. Add the HAuCl₄ solution to the CTAB solution under vigorous magnetic stirring, maintaining the temperature constant at 27 °C.
   4. Prepare 600 µl of sodium borohydride (NaBH₄....

Results

UV visible spectra of the gold seeds can be seen in Figure 1. UV visible spectra acquired at different times after the injection of the gold seeds are presented in Figure 2. UV visible spectra and transmission electron microscopic (TEM) images of the obtained gold nanorods are shown in Figure 3. UV visible spectra and transmission electron microscopic (TEM) images of gold nanorods with different aspect ratio obtained by varying the amount.......

Discussion

The protocol presented here applies hydroquinone, an aromatic molecule characterized by a weak reduction potential, to produce gold nanorods. There are two main advantages of the present protocol toward the most commonly employed synthetic route based on the use of ascorbic acid: the first is that hydroquinone is able to almost quantitatively reduce the gold ions allowing the production of higher amount of gold nanorods.¹¹ The latter is given by the fact that it requires a lower amount of CTAB and a subsequent.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Gold(III) chloride trihydrate	Sigma Aldrich	520918
Hydroquinone	Sigma Aldrich	H17902
Silver Nitrate	Sigma Aldrich	209139	toxic
Sodium Borohydride	Sigma Aldrich	480886
Hexadecyltrimethylammonium bromide (CTAB)	Sigma Aldrich	H5882	Acute Tox. (oral). In this study we tested three different batches of CTAB (H5882) from Sigma Aldrich. Two of them were marked as made in China while one as made in India. In our experience only the batches marked as made in China were effective for the preparation of AuNR.
Spectrophotometer	Thermo scientific	Nanodrop 2000C
TEM	JEOL	2100