I have been successfully depositing home-made, 30nm citrate-capped gold nanoparticles onto amine-functionalized glass surfaces. Recently, I noticed that a couple other batches of particles (one is citrate-capped, one is tannic-acid capped) will not appreciably self-assemble. I'm positive the surface functionalization is not the problem, as I can put the same surface back into the original particle batch and monitor self-assembly.
I realize that the capping agent in one of the batches may be an issue; however, am perplexed as to why the other citrate-capped batch does not self-assemble. I noticed that adjusting the PH seems to help a bit, and wondered if anyone know of optimal ph (and/or other relevant conditions) for self-assembly in this system.
If you have cleaned your particle via dialysis or centrifuge cleaning, the pH value and the ionic strength of the solution must have changed and might cause the problems. The main parameters that affect the assembly are 1. the pH value which affects the electrostatic repulsion between gold nanoparticles and the electrostatic attraction between the gold and the substrate surfaces; and 2. the ionic strength ( salt concentration) that screens the Debye length of the interactions. Since citrate is always largely excessive in the synthetic solution, I don't think the covering of the surfactant on the gold surface is a big problem.
Usually I assemble my citrate-capped nanoparticles (silver, not gold) on amino terminated SAMs at their native pH. In my conditions the NP sol has a sligthly acidic pH (around 5) and I never had problems with the particle self-assembly. I never tried to change the pH, though, so I can say if it is a critical parameter.
If you have cleaned your particle via dialysis or centrifuge cleaning, the pH value and the ionic strength of the solution must have changed and might cause the problems. The main parameters that affect the assembly are 1. the pH value which affects the electrostatic repulsion between gold nanoparticles and the electrostatic attraction between the gold and the substrate surfaces; and 2. the ionic strength ( salt concentration) that screens the Debye length of the interactions. Since citrate is always largely excessive in the synthetic solution, I don't think the covering of the surfactant on the gold surface is a big problem.
Depends on if the amine functionalities are secondary, tertiary or quaternary. If the latter, pH does not matter for charging of the amine surface. Citrate is a tri-carboxylate and. the primary and secondary acid constants are between pH 4 and 5, so you would want to keep the pH at 5 or above to maximize the attractve electrostatic interaction with the amine surface. If the amine is not quaternary, then it will require some protonation to carry a positive charge. So there is a happy medium between the acid-base properties of the two species...probably between pH 5 and 7. The other factor is mutual repulsion between citrate coated gold particles on the substrate surface. If this repulsion is significant, it could reduce the deposition density. To prevent such repulsion, a small level of electrolyte (of order 10 mmolar) would be sufficient to reduce (screen) the repulsive interactions without leading to loss of stability. One more factor...if you are using commercial citrate gold (e.g., from Pella, BBI, etc) we have found that the free citrate concentration is nearly zero...that is, virtually all the citrate is on the particle surface. If you are making your own, and not removing the excess, that free citrate will bind to the amine surface blocking sites for particle adsorption/deposition. One last suggestion...if deposition is lower, try increasing the deposition time.
Thanks for your reply Vincent. I'm glad to hear from you and hope your work is going well. Some of the papers you shared with me a while ago are proving invaluable. In fact we are trying to see if the triangular shell filling model you used to quantify BSA binding will yield results consistent with predictions of Mie Theory in relation to the UVVis spectra of BSA-coated AuNP's in free solution.
The amines we are using are from 3-Aminopropyltrimethoxysilane and, as far as I understand, are protonated for most ph values.
In fact, I am finding that the deposition I'm getting is quite high, and for ph > 5, it seems that it is too high. In other words, I am seeing a great deal of particle clustering instead of a monolayer. Your idea of adding an electrolyte may help reduce np-np aggregates on the fiber surface, if indeed this behavior is mostly due to electrostatics (I fear it is more involved based on paper...).
I'm hoping to have a 2-prong attack to optimize my deposition. I will try to improve monlayer quality with some new tricks, and also find the ideal absorption conditions. You raise a good point about excess citrates in our NP's and I will certainly take that into consideration.
Hope all is well and I hope to stay in touch.
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