Green Sonochemical Route to Silver Nanoparticles

Silver nanoparticles (AgNPs) are frequently utilized nanomaterials due to their anti-microbial properties, optical properties and high electrical conductivity. The sonochemical route using kappa carrageenan is a simple, convenient and environmental-friendly synthesis method for the preparation of silver nano particles. κ-carrageenan is used as a natural eco-friendly stabilizer, whilst power ultrasound acts as a green reducing agent.

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Sonochemical synthesis of silver nanoparticles (AgNPs) with the ultrasonicator UP400S

Ag+/κ-carrageenan (left) and sonicated Ag/κ-carrageenan (right). Sonication was performed with the UP400S for 90min.
[Elsupikhe et al. 2015]

Green Ultrasonic Synthesis of Silver Nanoparticles

Elsupikhe et al. (2015) have developed a green ultrasonically-assisted synthesis route for the preparation of silver nanoparticles (AgNPs). Sonochemistry is well known to promote many wet-chemical reactions. Sonication enables to synthsize AgNPs with κ-carrageenan as natural stabilizer. The reaction runs at room temperature and produces silver nanoparticles with fcc crystal structure without any impurities. The particle size distribution of the AgNPs can be influenced by the concentration of κ-carrageenan.

Green sonochemical synthesis of silver NPs. (Click to enlarge!)

Scheme of interaction between the Ag-NPs charged groups that are capped with κ-carrageenan under sonication. [Elsupikhe et al. 2015]

Procedure: Sustainable Synthesis of Silver Nanoparticles

The Ag-NPs were synthesized by reducing AgNO₃ using ultrasonication in the presence of κ-carrageenan. To obtain different samples, five suspensions were prepared, by adding 10 mL of 0.1 M AgNO₃ to 40-mL κ-carrageenan. The κ-carrageenan solutions used were 0.1, 0.15, 0.20, 0.25, and 0.3 wt%, respectively.
The solutions were stirred for 1h to obtain AgNO₃/κ-carrageenan.
Then, the samples were exposed to intense ultrasonic irradiation: The amplitude of the ultrasonic device UP400S (400W, 24kHz) was set to 50%. Sonication was applied for 90min at room temperature. The sonotrode of the ultrasonic liquid processors UP400S was immersed directly into the reaction solution.
After sonication, the suspensions were centrifuged for 15min and washed with double distilled water four times to remove the silver ion residue. The precipitated nanoparticles were dried at 40°C under vacuum overnight to obtain the Ag-NPs.

Equation

nH₂O —sonication–> +H + OH
OH + RH –> R + H₂O
AgNo₃–hydrolysis–> Ag+ + NO₃^–
R + Ag⁺ —> Ag° + R’ + H⁺
Ag⁺ + H –reductions–> Ag°
Ag⁺ + H₂O —> Ag° + OH + H⁺

Analysis and Results: Ultrasonically Synthesized Silver Nanoparticles

To evaluate the results, the samples were analyzed by UV-visible spectroscopic analysis, X-ray diffraction, FT-IR chemical analysis, TEM and SEM images.
The number of Ag-NPs increased with increasing κ-carrageenan concentrations. The formation of Ag/κ-carrageenan was determined by UV-visible spectroscopy where the surface plasmon absorption maximum was observed at 402 to 420nm. The X-ray diffraction (XRD) analysis showed that the Ag-NPs are of a face-centered cubic structure. The Fourier transform infrared (FT-IR) spectrum indicated the presence of Ag-NPs in κ-carrageenan. Transmission electron microscopy (TEM) image for the highest concentration of κ-carrageenan showed the distribution of Ag-NPs with an average particle size near to 4.21nm. Scan electron microscopy (SEM) images illustrated the spherical shape of the Ag-NPs. The SEM analysis shows that with increasing κ-carrageenan concentration, changes in the surface of Ag/κ-carrageenan occurred, so that small-sized Ag-NPs with spherical shape were obtained.

TEM images of sonochemically synthesized Ag/κ-carrageenan. (Click to enlarge!)

TEM images and corresponding size distributions for sonochemically synthesized Ag/κ-carrageenan at different concentrations of κ-carrageenan. [0.1%, 0.2%, and 0.3%, respectively (a, b, c)].

UP400S ultrasonic homogenizer (Click to enlarge!)

UP400S – the ultrasonic device used for the sonochemical synthesis of Ag nanoparticles

Literature/References

Elsupikhe, Randa Fawzi; Shameli, Kamyar; Ahmad, Mansor B; Ibrahim, Nor Azowa; Zainudin, Norhazlin (2015): Green sonochemical synthesis of silver nanoparticles at varying concentrations of κ-carrageenan. Nanoscale Research Letters 10. 2015.
Reza Kazemi Oskuee, Azhar Banikamali, Bibi Sedigheh Fazly Bazzaz, Hasan Ali Hosseini, and Majid Darroudi (2016): Honey-Based and Ultrasonic-Assisted Synthesis of Silver Nanoparticles and Their Antibacterial Activities. Journal of Nanoscience and Nanotechnology Vol. 16, 7989–7993, 2016.
Bose, Rahul & Roychoudhury, Piya & Pal, Ruma (2021): In-situ green synthesis of fluorescent silica–silver conjugate nanodendrites using nanoporous frustules of diatoms: an unprecedented approach. Bioprocess and Biosystems Engineering 44, 2021
I. De La Calle, N. Cabaleiro, M. Costas, F. Pena, S. Gil, I. Lavilla, C. Bendicho (2011): Ultrasound-assisted extraction of gold and silver from environmental samples using different extractants followed by electrothermal-atomic absorption spectrometry. Microchemical Journal, Volume 97, Issue 2, 2011. 93-100.

Frequently Asked Questions: Green Sonochemical Synthesis of Silver Nanoparticles

How does ultrasound improve the green synthesis of silver nanoparticles?

Ultrasound drives acoustic cavitation, creating localized hotspots that rapidly reduce silver ions using eco-friendly agents. This sonoechemical process accelerates reaction kinetics, promotes uniform nucleation, and enables precise control over particle size and shape–all without harsh chemicals or high thermal energy.

What are the main advantages of a green sonochemical route for AgNPs?

Green sonochemical synthesis combines ultrasound with biocompatible reducing and capping agents (like plant extracts, sugars, or polymers) to produce non-toxic, environmentally benign silver nanoparticles. The method reduces energy use, shortens synthesis time, eliminates hazardous byproducts, and yields highly stable nanoparticles with consistent size distribution.

Can ultrasound control the size and morphology of silver nanoparticles?

Yes. By adjusting ultrasonic amplitude, frequency, power density, and sonication time, manufacturers can finely tune nucleation and growth rates. This enables reproducible production of AgNPs from 5–100 nm in various shapes (spheres, cubes, triangular plates), tailored to specific biomedical, electronic, or industrial applications.

Is ultrasonic green synthesis scalable for industrial AgNP production?

Absolutely. Industrial ultrasonic processors support continuous flow-through reactors that seamlessly scale from lab batches to full manufacturing. Inline sonication ensures uniform cavitation distribution, real-time process monitoring, and batch-to-batch consistency, making it ideal for high-volume, eco-friendly AgNP manufacturing.

What green reducing agents work best with ultrasonic silver nanoparticle synthesis?

Effective eco-friendly agents include botanical extracts (e.g., aloe, green tea, citrus peels), carbohydrates, amino acids, proteins, and biodegradable polymers. Ultrasound enhances their reducing and stabilizing efficiency, allowing full replacement of synthetic chemicals while maintaining high yield, colloidal stability, and functional performance.

How does sonochemical synthesis compare to traditional chemical methods for AgNPs?

Conventional chemical routes often rely on toxic reducers (e.g., sodium borohydride), extreme pH, and generate hazardous waste. Green sonochemical synthesis uses safe, renewable materials, operates at ambient or mild temperatures, cuts processing time significantly, and produces cleaner, more biocompatible nanoparticles with superior dispersion and shelf life.

Which industries benefit from green ultrasonic silver nanoparticles?

AgNPs synthesized via green sonochemical methods are widely used in antimicrobial coatings, medical devices, wound care, water filtration, flexible electronics, and sustainable packaging. Their eco-friendly production ensures compliance with strict environmental and safety standards while delivering high performance and regulatory acceptance.

What are Silver Nanoparticles?

Silver nano-particles are characterized by a size of between 1nm and 100nm. While frequently described as being ‘silver’ some are composed of a large percentage of silver oxide due to their large ratio of surface-to-bulk silver atoms. Silver nanoparticles can appear with different structures. Most commonly, spherical silver nanoparticles are synthesized, but diamond, octagonal and thin sheets are also utilized.
Silver nanoparticles are highly frequented in medical applications. The silver ions are bioactive and have strong antimicrobial and germicidal effects. Their extremely large surface area allows for the coordination of numerous ligands. Other important characteristics are conductivity and unique optical properties.
For their conductive features, silver nanoparticles often incorporated in composites, plastics, epoxies and adhesives. The silver particles increase the electrical conductivity; therefore silver pastes and inks are frequently used in the manufacturing of electronics. Since silver nanoparticles support surface plasmons, AgNPs have outstanding optical properties. Plasmonic silver nanoparticles are used for sensors, detectors and analytical equipment such as Surface Enhanced Raman Spectroscopy (SERS) and Surface Plasmon Field-enhanced Fluorescence Spectroscopy (SPFS).

What is Carrageenan?

Carrageenan is a cheap natural polymer, which is found in various species of red seaweeds. Carrageenans are linear sulphated polysaccharides that are widely used in the food industry, for their gelling, thickening, and stabilizing properties. Their main application is in dairy and meat products, due to their strong binding to food proteins. There are three main varieties of carrageenan, which differ in their degree of sulphation. Kappa-carrageenan has one sulphate group per disaccharide. Iota-carrageenan (ι-carrageenen) has two sulphates per disaccharide. Lambda carrageenan (λ-carrageenen) has three sulphates per disaccharide.
Kappa carrageenan (κ-carrageenan) has a linear structure of sulfated polysaccharide of D-galactose and 3,6-anhydro-D-galactose.
κ- carrageenan is widely used in the food industry, e.g. as gelling agent and for texture modification. It can be found as additive in ice cream, cream, cottage cheese, milkshakes, salad dressings, sweetened condensed milks, soy milk & other plant milks, and sauces to increase the product viscosity.
Furthermore, κ-carrageenan can be found in non-food products such as thickener in shampoo and cosmetic creams, in toothpaste (as stabilizer to prevent constituents separating), fire fighting foam (as thickener to cause foam to become sticky), air freshener gels, shoe polish (to increase viscosity), in the biotechnology to immobilize cells/enzymes, in pharmaceuticals (as an inactive excipient in pills/tablets), in pet food etc.

SEM images of ultrasonically synthesized silver nanoparticles (Click to enlarge!)

SEM images for Ag/κ-carrageenan at different concentrations of κ-carrageenan. [0.1%, 0.2%, and 0.3 %, respectively (a, b, c)]. [Elsupikhe et al. 2015]