you must make optimization steps. For instance, you can change the ratio between bulk solution and plant, algal, fungal extracts 1:1 or 1:2 or 1:9 or etc. I found especially for synthesizing NPs using supernatant of algal culture that the high algal supernatant ratio to low bulk give me stable NPs with low aggregation.

Test the temperature, concentration of bulk (1 mM, 0.6 M, etc.) time of exposure, light intensity, pH, etc.

Nanoparticles are either stabilized electrostatically or sterically by a molecule added or provided by your “green” method. This is not an option, it is a requirement for the nanoparticle to exist.

Mohamad Sofi Abu Hassan

You have the formation of a gel consisting of nanoparticles and extract substances. You take too concentrated solutions of gold precursor and extract for synthesis. Try reducing the concentration of your mixture by diluting with water to reduce sedimentation (gel formation). Look at approximate concentrations in our articles.

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Do change or adjust your solvent polarity to reduce the rate of aggregation. Au particles are usually negatively charged in non polar solvents, electrostatic stabilisation leads to particle aggregation. You need to work on two aspects simultaneously, 1) reduce the aggregation 2) Enhance the growth. So, coating naked Au with some hydrophobic organic molecules can prove effective.

Increase the concentration of the green extract. That will prevent aggregation

I agree with Mr. Yuri Mirgorod . You can either reduce the concentration of the metal solution or you can try adding sodium citrate solution for extra stabilization. Sodium citrate is environmentally safe as it can be found naturally in fruits. Increasing the plant extract concentration and using a low concentration of the metal solution (in mM) may also help in preventing aggregation as metabolites present in plants in themselves are great stabilizers.

Agree with Ramchander Merugu

Dear Mohamad Sofi Abu Hassan this is an important technical question which is of significant general interest to everyone working with green synthesized nanoparticles. There are a number of relevant discussions and research papers available in which this problem is discussed. Please have a look e.g. at the following potentially useful links:

How to minimize the aggregation of nanoparticles?

https://www.researchgate.net/post/How-to-minimize-the-aggregation-of-nanoparticles

(34 answers)

Prevention of the Aggregation of Nanoparticles during the Synthesis of Nanogold-Containing Silica Aerogels

This paper has been published Open Access so that the full text is freely available on the internet (please see attached pdf file).

Concentration Effect of Reducing Agents on Green Synthesis of Gold Nanoparticles: Size, Morphology, and Growth Mechanism

https://nanoscalereslett.springeropen.com/track/pdf/10.1186/s11671-016-1393-x.pdf

(also attached)

I hope this helps. Good luck with your work and please stay safe and healthy!

This is a major issue in green synthesis. You need to optimize the concentration of the extract (plants or microbial extract). In the case of microbial extract, nitrate reductase acts as a reducing agent, and other macromolecules function as capping agents. You will not find many issues regarding aggregation in this case. But the issues come when you are using plant extract. Mostly, flavonoids are the reducing agents here and other biomolecules are stabilizing agents. The ratio of both is not synchronized. Therefore when you increase the extract concentration reduction will be faster but sufficient stabilizing agents will not be available to stabilize and aggregation happens.

Thank you everyone for the insight on my question. It is really helpful.

To avoid formation of hard agglomerates as well as to gain highly dispersed particles, preparing and storing the particles in colloidal slurries form is a better option as compared to dry powder form. Check out section 5 of this article: Article Permanent agglomerates in powdered nanoparticles: Formation ...