If you use place exchange to cap your GNPs with a positively charged ligand, I suggest you pay attention to the addition order. Adding particles dropwise to a concentrate of the capping agent (replacement ligand) will yield better results than just mixing the two solutions. The latter typically produces aggregates, apparent from a change in the red color of the solution to purple or clear.

We just published a paper in Langmuir (http://pubs.acs.org/doi/abs/10.1021/la5002013) where we used a 3-part ligand (alkane-thiol/PEG chain/quarternary amine functional group) with reduction of chloroauric acid to yield very stable positively charged GNPs. We're also using PEI polymer to perform place exchange on citrate stabilized GNPs, but here the MW, branching and order of addition are very important...this work has not yet been published.

If you use anything other than a quarternary amine, the positive charge will be pH dependent.

Also avoid using glass containers, as the positively charged GNPs will plate out on the negatively charged glass. Someone mentioned using PAMAM dendimers...I would suggest you check on the chemical stability of PAMAM once it has been solubulized in water. There are more chemically stable dendrimers, but not sure of their commercial availability.

Dear Aishwarya, we faced the same problem with Ag nanoparticles.

The best we found was coating them with poly-L-lysine (see Marsich et al. Poly-L-lysine-coated silver nanoparticles as positively charged substrates for surface-enhanced Raman scattering (2012) Langmuir, 28 (37), pp. 13166-13171).

You have to be careful with the cleaning, the system is very sensitive to impurities, but it works: the coated nanoparticles will be stable for few days before aggregating.

We also tried with other capping agents, like cysteamine (a thiol with a -NH2 ending group), but it was much more difficult to get stable coated particles.

Hope this helps, if you have further questions, just ask.

Hi ...

First to all, I understand that you have synthesized GNPs by microbial, and for some reason you needs those GNPs positively charged. If that is the reason, you can try to use 11-amino undecanethiol as capping agent. This is a very common amino functionalized alcanethiol, but is not a cheaper one.

You need to keep in mind that a complete lingand exchange reaction need a high concentration of tiol and NPs, to avoid a partial exchange and get an unstable system.

But if you need GNPs positively charged and you don't care about the synthetic route, may be that another way is better to obtain GNP well tunned in size, shape, and capping agent type.

Regards

Julio

Adding to Julio's answer. You could also use the trimethylammonium undecanethiol, whose charge would be positive regardless of the pH.

Dear Aishwarya,

As Mr Azcárate already told you, one of the easiest ways to functionalize your nanoparticles would be using thiol or disulfide ligands. Gold has a high affinity for sulfure compounds and depending on the lenght of the spacer between your thiol function and your positively charge functional group, the ligand should be soluble in water or buffer.

By mixing your GNPs and the ligands for one hour or more, a ligand exchange should occur and the excess of ligands can be removed by dialysis in buffer or water.

Regards.

Marion

Hi Aishwarya,

you can try amino silane (APTES; see Liz-Marzan or Mulvaney group for that) or positively charged polymer (PDADMAC; see Caruso group for that). The polymer approach worked best for me. Quite simple mix and shake.

Good luck.

Andrea

Our particles are stabilized by CTAB. To make them applicable for layer-by-layer deposition, we add PSS solution (c = 0.2% wt, 0.1 M NaCl) drop-wise.

In your case, the same procedure should work, if you use positively charged PAH instead of the PSS.

Hi Aishwarya,

I also made the experience, that it is difficult to change the particles charge by changing the capping agent, because often during this process irreversible aggregation occurs. We overcame this problem by immobilizing negatively charged citrate stabilized AuNP on aminosilanized surfaces followed by thorough rinsing of the adsorbed AuNP. After that, we added 1,8-aminooctanethiol as positively charged ligand and removed the particles by ultrasonication from the solid support. We just published this procedure in Langmuir 2014, Issue 2, pp 574.

Good luck with your further experiments!

Aishwarya, there can be several methods

1. Use cysteine at a pH 9 in carbonate buffer

2. APTES in ethanol (DOI:10.1021/la202364c)

3. PAMAM dendrimer (amino terminated from sigma; 412449-2.5G)

Using 3rd option will solve the problem of solution stability due to to huge branching providing sufficiently huge charge to keep it from flocculating.

Cheers

Why your used microbes for Positive or negative charged nanoparticles just you can use any aminoacids for synthesis, stabilization and capping of nanoparticles

I think CTAB is better agent for this porpose. Do you have bare gold nanorods? they must be with some caping agent i suppose? Ho do they have negative charge and with which camping agent they are prepared. could you able send me some publications links regarding with charge issue on gold nanorods?

We modified gold surface with N-(6-mercapto)hexylpyridinium salt. Pyridinium group has advantages over ammonium group. You can see "Potential dependence of SERS spectra of reduced graphene oxide adsorbed on self-assembled monolayer at gold electrode" Chemical Physics Letters 12/2013; 590:141-145

Regards

Olegas

Thank you so much everyone. Will try few few you all suggested. Thanks again.

Mr. Santosh, I have not used any capping agents for synthesis, since microbial produced nanoparticles are very stable as such. The process of nanoparticle formation in bacteria is due to the Surface plasmon vibration. The reduction of Au3+ ions to Au0- ions leads to the formation of negatively charged nanoparticles. This is facilitaed by the enzyme reductase.

If you use place exchange to cap your GNPs with a positively charged ligand, I suggest you pay attention to the addition order. Adding particles dropwise to a concentrate of the capping agent (replacement ligand) will yield better results than just mixing the two solutions. The latter typically produces aggregates, apparent from a change in the red color of the solution to purple or clear.

We just published a paper in Langmuir (http://pubs.acs.org/doi/abs/10.1021/la5002013) where we used a 3-part ligand (alkane-thiol/PEG chain/quarternary amine functional group) with reduction of chloroauric acid to yield very stable positively charged GNPs. We're also using PEI polymer to perform place exchange on citrate stabilized GNPs, but here the MW, branching and order of addition are very important...this work has not yet been published.

If you use anything other than a quarternary amine, the positive charge will be pH dependent.

Also avoid using glass containers, as the positively charged GNPs will plate out on the negatively charged glass. Someone mentioned using PAMAM dendimers...I would suggest you check on the chemical stability of PAMAM once it has been solubulized in water. There are more chemically stable dendrimers, but not sure of their commercial availability.

Treat particles with cysteamine hydrochloride, i.e., HSCH2CH2NH3Cl. Available from Aldrich (Cat. No. M6500). It should chemisorb to the Au via the thiol and produce a surface decorated with protonatable terminal amine groups to maintain water solubility and positive charge. May not work if the Au particles are too big, however. In that case the idea presented by Dr. Mitzscherling may be the better way to go.

Thank you Mr. Dressick.

Thank you very much Mr. Hackley, That was very useful and informative.

Thanks again Sir.

Dear 

Aishwarya Shivashankarappa

Please study article send for you

I hope can help you