Hello Omar Wehbe

I suggest you have a look at this forum. In this context, you can check out a related question on RG. Best Wishes

https://www.researchgate.net/post/Help_with_gold_nanoparticle_with_nile_blue_why_sodium_azide

Dear Sir

I think this link is useful for you

https://fnkprddata.blob.core.windows.net/domestic/download/pdf/IBS_antibody_purification_guide.pdf

Omar Wehbe

You're adding sodium azide to the Au colloid to demonstrate the instability effect. Are you adding a solution or a solid? If the former (solution), what occurs when you simply add DI water (without the azide) to the system in the same amount?

Alan F Rawle I added sodium azide as a solid to be used as a preservative. I do not have any of the original gold nanoparticle solution to test the addition of DI water to the system.

Ahmed M. Abu-Dief purifying the gold nanoparticles with a centrifugal spin column (30 kDa) was unsuccessful and lead to the aggregation and entrapment of the gold nanoparticles on the membrane. Interestingly, the usage of a nylon syringe filter membrane (0.22 um) to filter the 40 nm gold nanoparticles lead to the irreversible sedimentation of the gold nanoparticles on the filter membrane, which was wetted with isopropyl alcohol before usage.

Update:

I performed the pH/antibody concentration optimization experiments using the gold nanoparticle solution containing the added 0.1% sodium azide. There was no problem with differentiating the red-blue color range when the complex was challenged with NaCl. Then, the optimal conditions were used for the 1 mL-scale gold-conjugation: successful conjugation was observed and validated using LFIA. Nevertheless, I am still interested in knowing how sodium azide can be separated from the gold nanoparticle solution for future experiments.

Omar Wehbe

My comment related to the probability that just diluting the system in DI water will destabilize it simply due to the dilution of ions and stabilizers in the system. The sodium azide may not be the actual cause and unrelated...

Alan F Rawle

Thank you for the explanation, I should mention that I also tried using PEG4000 as a stabilizing agent during centrifugation. It seems to delay the appearance of an insoluble pellet (especially at higher concentrations 5-10% v/v) for the gold nanoparticles with sodium azide. However, after 3 centrifugation cycles (4500 g and 10 mins), an insoluble black pellet started appearing at the side of the microfuge tube. At the same time, pH-calibrated (0.1 M Na2CO3) gold nanoparticles that were stored without sodium azide were highly stable and had absolutely no aggregation/insoluble pellet formation even after 6 centrifugation cycles. So, I am assuming that the dilution of ions and stabilizers might not be the actual cause. Regardless, the pH calibration required a small volume (~30-50 uL) of Na2CO3 for a 5 mL gold nanoparticle solution, so it might not have really given the dilution force necessary to alter the concentrations of ions/stabilizers. Lastly, the gold nanoparticle solution was only provided in dH20 according to the manufacturer.

Omar Wehbe

Mechanisms of stabilization are different with PEG (steric stabilizer) and ions/pH (charge/electrostatic). In the case of PEG (absorbed on the surface) then dilution may not have any effect on the stabilization. In the case of charge stabilization then the ionic concentration is vital. Some generalities in these webinars (registration required):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

Metal colloids - their preparation, application and characterization

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W160218MetalColloids.html

I spent a lot of time in the late 70's (that's the 1970's not my 70's which are to come) making a variety of Pd, Au, Ag, Pt, and alloy colloidal catalyst materials stabilized on inert (?) supports such as silica.

Hence my original thoughts on whether you were adding a solid to your system (little volume change) or adding a solution (could be significant volume and dilution change)

Perhaps of some interest for your query: https://www.researchgate.net/post/Is_there_any_method_to_separate_the_mixture_of_sodium_azide_and_sodium_chloride