Synthesizing Nano-Silver with Honey and Ultrasonics

Nano-silver is use for its anti-bacterial properties to reinforce materials in medicine and material science. Ultrasonication allows for a rapid, efficacious, safe, and environmental-friendly synthesis of spherical silver nanoparticles in water. Ultrasonic nanoparticle synthesis can be easily scaled from small to large production.

Ultrasonically-Assisted Synthesis of Colloidal Nano-Silver

Sonochemical synthesis, which refers to chemical reactions facilitated by ultrasonic irradiation, is a widely applied method for producing nanoparticles. These include silver, gold, magnetite, hydroxyapatite, chloroquine, perovskite, latex and many other nano-materials.

Ultrasonic Wet-Chemical Synthesis

Multiple ultrasonically-assisted synthesis routes have been developed for producing silver nanoparticles. One notable method employs honey as both a reducing and capping agent. Components in honey, such as glucose and fructose, act synergistically in these roles during the synthesis process.
Similar to many nanoparticle synthesis techniques, ultrasonic nano-silver synthesis falls under the category of wet chemistry. The process begins with the nucleation of silver nanoparticles within a solution. During sonication, a silver precursor (e.g., silver nitrate (AgNO₃), or silver perchlorate (AgClO₄)) is reduced in the presence of a reducing agent, such as honey, to produce colloidal silver.

Mechanism of Ultrasonic Silver Nucleation and Growth

Initial Nucleation Phase: As the concentration of dissolved silver ions increases, metallic silver ions begin binding to form small clusters. At this stage, these clusters are energetically unstable due to a negative energy balance. The energy required to create new surfaces exceeds the energy gained by reducing the dissolved silver concentration.

• Critical Radius: When a cluster reaches a specific size (the critical radius), the process becomes energetically favorable, stabilizing the cluster. This stability allows the cluster to act as a nucleus for further growth.
• Growth Phase: During growth, additional silver atoms diffuse through the solution and attach to the growing nanoparticle surface. The growth continues until the concentration of dissolved silver drops below the nucleation threshold, halting the formation of new nuclei.
• Diffusion and Completion: The remaining dissolved silver is incorporated into existing nanoparticles, completing the process.

Sonication accelerates mass transfer, particularly the wetting and diffusion processes, which leads to faster nucleation and controlled growth. By precisely adjusting sonication parameters, such as intensity and duration, the size, growth rate, and shape of the nanoparticles can be finely tuned. This precise control ensures consistent nanoparticle structures tailored for specific applications.

Ultrasonically-assisted synthesis stands out as an effective, scalable, and green chemistry approach to producing nano-silver with well-defined properties, offering significant advantages for diverse applications in research and industry.

Click here to read more about another green method to ultrasonically synthesize nano-silver using carrageenan!

Sonication facilitates the rapid, green synthesis of small silver nanoparticles with a narrow size distribution.

Case Study of Ultrasonic Nano-Silver Synthesis

The study titled “Honey-Based and Ultrasonic-Assisted Synthesis of Silver Nanoparticles and Their Antibacterial Activities” by Oskuee et al. (2016) explores a simple and eco-friendly method for synthesizing silver nanoparticles (Ag-NPs) using natural honey as both a reducing and stabilizing agent. The process, which involves the reduction of silver nitrate (AgNO₃) under ultrasonic irradiation, is characterized by various parameters including silver ion concentration, honey concentration, and sonication time. The resulting Ag-NPs have an average size of about 11.8 nm and exhibit antibacterial properties against pathogenic bacteria like Staphylococcus aureus, Pseudomonas aeruginosa, and E. coli.
The study highlights the benefits of using honey in nanoparticle synthesis, emphasizing its green, low-cost, and non-toxic nature. The authors demonstrate that the size and yield of Ag-NPs can be controlled by adjusting reaction parameters such as silver concentration, honey content, and sonication duration. The synthesized Ag-NPs were shown to possess effective antibacterial activity, particularly against E. coli and S. aureus, with minimum inhibitory concentrations (MIC) of approximately 19.46 ppm. This method presents a potential application for Ag-NPs in medical fields, including wound healing and infection control.

• Materials: silver nitrate (AgNO₃) as silver precursor; honey as capping / reducing agent; water
• Ultrasonic device: Probe-Type Sonicator UP400St

Ultrasonic Synthesis Protocol

Best conditions to synthesise colloidal silver nanoparticles were found to be the following: Reducing silver nitrate under ultrasonication mediated by natural honey. Briefly, 20 ml of silver nitrate solution (0.3 M) containing honey (20 wt%) was exposed to high-intensity ultrasound irradiation under ambient conditions for 30 min. Ultrasonication was carried out with the probe-type ultrasonicator UP400S (400W, 24 kHz) immersed directly into the reaction solution.
Food-grade honey is used as capping / stabilizing and reducing agent, which makes the aqueous nucleation solution and the precipitated nanoparticles clean and safe for manifold applications.
As ultrasonication time increases, the silver nanoparticles become smaller and their concentration is enhanced.
In the aqueous honey solution, ultrasonication is a key factor that influences the formation of silver nano-particles. Sonication parameters such as amplitude, time and continuous vs pulsating ultrasound are major factors that allow for controlling size and quantity of the silver nano-particles.

Particle size distribution of Ag-NPs synthesized in optimum conditions; silver concentrations (0.3 M), honey concentrations (20 wt%), and ultrasonic irradiation times (30 min)
picture source: ©Oskuee et al. 2016

Result of Ultrasonic Synthesis of Silver Nanoparticles

The ultrasonically promoted, honey-mediated synthesis with the sonicator UP400St resulted in spherical silver nano-particles (Ag-NPs) with an average particle size of about 11.8nm. The ultrasonic synthesis of the silver nano-particles is a simple and rapid one-pot method. The use of water and honey as materials, makes the reaction cost-effective and exceptionally environmental-friendly.
The presented technique of ultrasonic synthesis using honey as reducing and capping agent can be extended to other noble metals, such as gold, palladium, and copper, which offers various additional application from medicine to industry.

Particle size distribution of Ag-NPs synthesized in optimum conditions; silver concentrations (0.3 M), honey concentrations (20 wt%), and ultrasonic irradiation time (30 min)
Study and picture: ©Oskuee et al. 2016

Influencing Nucleation and Particle Size by Sonication

Ultrasound enables for the production of nano-particles such as silver nano-particles tailored to requirements. Three general options of sonication have important effects on the output:
Initial Sonication: The short application of ultrasound waves to a supersaturated solution can initiate the seeding and formation of nuclei. As sonication is only applied during the initial stage, the subsequent crystal growth proceeds unimpeded resulting in larger crystals.
Continuous Sonication: The continuous irradiation of the supersaturated solution results in small crystals since the unpaused ultrasonication creates a lot of nuclei resulting in the growth of many small crystals.
Pulsed Sonication: Pulsed ultrasound means the application of ultrasound in determined intervals. A precisely controlled input of ultrasonic energy allows to influence the crystal growth in order to obtain a tailored crystal size.

High-Performance Ultrasonicators for Nanoparticle Synthesis

Hielscher Ultrasonics offers high-powered, reliable ultrasonic processors designed for advanced sonochemical applications, including sono-synthesis and sono-catalysis. Ultrasonic mixing and dispersing significantly enhance mass transfer, promote the wetting of atom clusters, and facilitate their subsequent nucleation, leading to the efficient precipitation of nanoparticles. Ultrasonic synthesis is recognized as a simple, cost-effective, biocompatible, reproducible, rapid, and safe method for producing high-quality nanomaterials. (Read more about the sonochemical synthesis of Perovskite and ZnO nanostructures!)

Hielscher ultrasonicators are engineered for precise control, enabling optimal conditions for the nucleation and growth of nanomaterials. These digital devices feature intelligent software, a color touch display, and an intuitive menu for safe and user-friendly operation. Additionally, they come with automatic data recording on a built-in SD card, ensuring seamless process documentation.

With a comprehensive range of systems — from compact 50-watt handheld ultrasonicators for laboratory use to robust 16,000-watt industrial systems — Hielscher provides the ideal ultrasonic solution for every application. Designed for durability, Hielscher ultrasonic equipment is built to operate continuously under heavy-duty conditions, even in demanding environments, ensuring reliable performance 24/7.
The table below gives you an indication of the approximate processing capacity of our ultrasonicators:

Comparison of conventional methods and green synthesis methods of nanoparticle synthesis.