Efficient and Controlled Synthesis of Gold Nanoparticles

Gold nanoparticles of uniform shape and morphology can be efficiently synthesized via sonochemical route. The ultrasonically promoted chemical reaction of gold nanoparticle synthesis can be precisely controlled for particle size, shape (e.g., nanospheres, nanorods, nanobelts etc.) and morphology. The efficacious, simple, rapid and green chemical procedure allows for reliable production of gold nanostructures on industrial scale.

Gold Nanoparticles and Nanostructures

Gold nanoparticles and nano-sized structures are widely implemented in R&D and industrial processes due to the unique properties of nano-sized gold including electronic, magnetic, and optical characteristics, quantum size effects, surface plasmon resonance, high catalytic activity, self-assembly amongst other properties. The fields of application for gold nano-particles (Au-NPs) range from the use as catalyst to the manufacturing of nanoelectronic devices, as well as the use in imaging, nano-photonics, nanomagnetic, biosensors, chemical sensors, for optical and theranostic applications, drug delivery as well as other utilizations.

Methods of Gold Nanoparticle Synthesis

Nano-structured gold particles can be synthesized via various routes using high-performance ultrasonication. Ultrasonication is not only a simple, efficient and reliable technique, furthermore sonication creates conditions for the chemical reduction of gold ions without toxic or harsh chemical agents and enables for the formation of noble metal nanoparticles of different morphologies.The choice of route and sonochemical treatment (also known as sonosynthesis) allows to produce gold nanostructures such as gold nanosheres, nanorods, nanobelts etc. with uniform size and morphology.
Below you can find selected sonochemical paths for the preparation of gold nanoparticles.

Ultrasonically Improved Turkevich Method

Sonication is used to intensify the Turkevich citrate-reduction reaction as well as modified Turkevich procedures.
The Turkevich method produces modestly monodisperse spherical gold nanoparticles of around 10–20nm in diameter. Larger particles can be produced, but at the cost of monodispersity and shape. In this method, hot chloroauric acid is treated with sodium citrate solution, producing colloidal gold. The Turkevich reaction proceeds via formation of transient gold nanowires. These gold nanowires are responsible for the dark appearance of the reaction solution before it turns ruby-red.
Fuentes-García et al. (2020), who sonochemically synthesized gold nanoparticles, report that it is feasible to manufacture gold nanoparticles with high absorption interaction using ultrasonication as an only energy source, reducing laboratory requirements and controlling properties modifying simple parameters.
Lee et al. (2012) demonstrated that ultrasonic energy is a key parameter for producing spherical gold nanoparticles (AuNPs) of tunable sizes of 20 to 50 nm. The sonosynthesis via sodium citrate reduction produces monodisperse spherical gold nanoparticles in aqueous solution under atmospheric conditions.

The Turkevich-Frens Method using Ultrasound

A modification of the above described reaction path is the Turkevich-Frens method, which is a simple multiple-step process for the synthesis of gold nanoparticles. Ultrasonication promotes the Turkevich-Frens reaction pathway in the same manner as the Turkevich route. The initial step of Turkevich-Frens multiple-step process, where reactions occur in series and in parallel, is the oxidation of citrate that yields dicarboxy acetone. Then, the auric salt is reduced to aurous salt and Au⁰, and the aurous salt is assembled on the Au⁰ atoms to form the AuNP (see scheme below).
 

Gold nanoparticle synthesis via Turkevich method.
scheme and study: ©Zhao et al., 2013

This means that dicarboxy acetone resulting from the oxidation of citrate rather than citrate itself is acting as the actual AuNP stabilizer in the Turkevich-Frens reaction. The citrate salt additionally modifies the pH of the system, which influences the size and size distribution of the gold nanoparticles (AuNPs). These conditions of the Turkevich-Frens reaction produce nearly monodisperse gold nanoparticles with particle sizes between 20 to 40nm. The exact particle size can be modified upon variation of the pH of the solution as well as by the ultrasonic parameters. Citrate-stabilized AuNPs are always larger than 10 nm, due to the limited reducing ability of trisodium citrate dihydrate. However, using D₂O as the solvent instead of H2O during the synthesis of AuNPs allows to synthesize AuNPs with a particle size of 5 nm. As the addition of D2O increase the reducing strength of citrate, the combination of D2O and C₆H₉Na₃O₉. (cf. Zhao et al., 2013)

Protocol for the Sonochemical Turkevich-Frens Route

In order to synthesize gold nanoparticles in a bottom-up procedure via Turkevich-Frens method, 50mL of chloroauric acid (HAuCl₄), 0.025 mM is poured into a 100 mL glass beaker, into which 1 mL of 1.5% (w/v) aqueous solution of trisodium citrate (Na₃Ct) is added under ultrasonication at room temperature. Ultrasonication was performed at 60W, 150W, and 210W. The Na₃Ct/HAuCl₄ ratio used in the samples is 3:1 (w/v). After ultrasonication, the colloidal solutions showed different colours, violet for 60 W and ruby-red for 150 and 210 W samples. Smaller sizes and more spherical clusters of gold nanoparticles were produced by increasing sonication power, according with the structural characterization. Fuentes-García et al. (2021) show in their investigations the strong influence of increasing sonication on particle size, polyhedral structure and optical properties of the sonochemically synthesized gold nanoparticles and the reaction kinetics for their formation. Both, gold nanoparticles with the size of 16nm and 12nm can be produced with a tailored sonochemical procedure. (Fuentes-García et al., 2021)
 

(a,b) TEM image (scale of 50 nm) and (c) size distribution of AuNPs synthesized at a
sonication power 17.9 Wcm2.
Picture and study: © Dheyab et al., 2020.

Sonolysis of Gold Nanoparticles

Another method for the experimental generation of gold particles is by sonolysis, where ultrasound is applied for the synthesis of gold particles with a diameter of under 10 nm. Depending on the reagents, the sonolytic reaction can be run in various manners. For instance, sonication of an aqueous solution of HAuCl₄ with glucose, hydroxyl radicals and sugar pyrolysis radicals act as the reducing agents. These radicals form at the interfacial region between the collapsing cavities created by intense ultrasound and the bulk water. The morphology of the gold nanostructures are nanoribbons with width 30–50 nm and length of several micrometers. These ribbons are very flexible and can bend with angles larger than 90°. When glucose is replaced by cyclodextrin, a glucose oligomer, only spherical gold particles are obtained, suggesting that glucose is essential in directing the morphology toward a ribbon.

Exemplary Protocol for Sonochemical Nano-Gold Synthesis

Precursor materials used to synthesize citrate-coated AuNPs include HAuCl4, sodium citrate and distilled water. In order to prepare the sample, the first step involved the dissolution of HAuCl4 in distilled water with a concentration of 0.03 M. Subsequently, the solution of HAuCl4 (2 mL) was added dropwise to 20 mL of aqueous 0.03 M sodium citrate solution. During the mixing phase, a high-density ultrasonic probe (20 kHz) with an ultrasonic horn was inserted into the solution for 5 min at a sounding power of 17.9 W·cm²
(cf. Dhabey at al. 2020)

Gold Nanobelt Synthesis using Sonication

Single cristalline nanobelts (see TEM image left) can be synthesized via sonication of an aqueous solution of HAuCl4 in presence of α-D-Glucose as reagens. The sonochemically synthesized gold nanobelts show an average width of 30 to 50 nm and several micrometers length. The ultrasonic reaction for the production of gold nanobelts is simple, rapid and avoids the use of toxic substances. (cf. Zhang et al, 2006)

Surfactants to Influence Sonochemical Synthesis of Gold NPs

The application of intense ultrasound on chemical reactions initiates and promotes conversion and yields. In order to obtain uniform particle size and certain targeted shapes / morphologies, the choice of surfactants is a critical factor. The addition of alcohols also helps to control the particle shape and size. For example, in the presence of a-d-glucose, the major reactions in the sonolysis process of aqueous HAuCl₄ as depicted in the following equations (1-4):
(1) H₂ O —> H∙ + OH∙
(2) sugar —> pyrolysis radicals
(3) AIII + reducing radicals —> Au⁰
(4) nAu⁰ —> AuNP (nanobelts)
(cf. Zhao et al., 2014)

The Power of Probe-type Ultrasonicators


Ultrasonic probes or sonotrodes (also called ultrasonic horns) deliver high-intensity ultrasound and acoustic cavitation in very focused form into chemical solutions. This precisely controllable and efficient transmission of power ultrasound allows for reliable, precisely controllable and reproducible conditions, where chemical reaction pathways can be initiated, intensified and switched. In contrast, an ultrasonic bath (also known as ultrasonic cleaner or tank) delivers ultrasound with very low power density and randomly occurring cavitation spots into a large liquid volume. This makes ultrasonic baths unreliable for any sonochemical reactions.
“Ultrasonic cleaning baths have a power density that corresponds to a small percentage of that generated by an ultrasonic horn. The use of cleaning baths in sonochemistry is limited, considering that fully homogeneous particle size and morphology is not always reached. This is due to the physical effects of ultrasound over nucleation and growing processes.” (González-Mendoza et al. 2015)

High-Performance Ultrasonicators for the Synthesis of Gold Nanoparticles

Hielscher Ultrasonics supplies powerful and reliable ultrasonic processors for sonochemical synthesis (sono-synthesis) of nanoparticles such as gold and other noble metal nanostructures. Ultrasonic agitation and dispersion increases the mass transfer in heterogeneous systems and promotes the wetting and subsequent nucleation of atom clusters in order to precipitate nano-particles. Ultrasonic synthesis of nano-particles is a simple, cost-effective, biocompatible, reproducible, rapid, and safe method.
Hielscher Ultrasonics supplies powerful and precisely controllable ultrasonic processors for the formation of nano-sized structures such as nanosheres, nanorods, nanobelts, nano-ribbons, nanoclusters, core-shell particles etc.
Read more about the ultrasonic synthesis of magnetic nanoparticles!
Our customers value the smart features of Hielscher digital devices, which are equipped with intelligent software, coloured touch display, automatic data protocolling on a built-in SD-card and feature an intuitive menu for user-friendly and safe operation.
Covering the complete power range from 50 watts hand-held ultrasonicators for the lab up to 16,000 watts powerful industrial ultrasonic systems, Hielscher has the ideal ultrasonic setup for your application. Sonochemical equipment for batch and continuous inline production in flow-through reactors is readily available at any bench-top and industrial size. The robustness of Hielscher sonicators allow for 24/7 operation at heavy duty and in demanding environments.

The table below gives you an indication of the approximate processing capacity of our ultrasonicators: