How did you observed that these nanoparticles are aggregated. ?

Based on the color changed from red to blue immediately after addition of cysteamine.

Are your gold nanoparticles "naked" or is their surface covered or inactivated in any form?

Sulfhydryl groups are well known to bind strongly to free gold surfaces (= naked gold nanoparticles). That binding by itself would change the size and absorption properties of your particles. In addition, the particles would become positively charged (depending on pH, of course) due to the amino group of the cysteamine.

BTW: I don't think the ratio for gold particles vs cysteamine you gave is correct, if the concentrations are as you've written.

Hello Dr. Holger, Thanks for your comments. The gold nanoparticles are unconjugated with citrate stabilized net negative charge.

I agree with your comment that absorption property will change but after 12 hr of RT incubation, I saw black color aggregates settled at the bottom. I was hoping that positively charged NP will repel each other to keep them in dispersed form.

I am following the article (http://npl.csircentral.net/25/1/16.pdf) for the protocol.

I would appreciate if you could suggest an article or suitable concentration for this kind of reaction?

Thanks,

Trupti

Hi Trupti,

Alright, now I understand a bit better what your experiments are about. 

Based on Figure 1 you should expect a color change upon addition of cysteamine, so far so good. But the ratio between cysteamine and the gold nanoparticles was just given as a volumetric ratio, not a molecular ratio (as I had surmised).

Have you tried mixing those two reagents in different ratios? I'm speculating that you didn't have enough cysteamine in your reaction to fully saturate your gold nanoparticles, which would cause the formation of larger aggregates (= precipitate). If my speculation is right, then you should get a stable suspension with a different (higher) cysteamine to gold nanoparticle ratio.

Hello Dr. Holger,

That makes complete sense to me now..Thank you so much !! I appreciated. 

I will try higher concentration of cysteamine in different ratio with gold nanoparticles solution.

Best,

Trupti

 Trupti Terse hi, would you tell me your result capping cys with AuNPs, I am start to work on this conjugation so I also concerning how to avoid aggregation.

On another hand I also concerned that in my case I am working with cysteamine (no the hydrochloride version) so I am not sure if this compound since you have to dissolve it in HCl could affect AuNPs

Hi Carlos,

I tried with different ratios of cysteamine to AuNP but unfortunately it did not work for me. One of my friend also faced the same problem so he suggested me to use long chain linker such as mercaptoundecanoic or decanoic etc. and then use EDC-NHS. or  another approach would be to use amine group linker and using glutaraldehyde you can attach protein.

I hope that helps.