First of all, in general the rate of chemical kinetics increases with increasing the concentration of reactants. Secondly, at your dilute solution there may be the solvent molecules work as a barrier between reactants due to their absorption on the surface through intermolecular forces. But, this problem will be ceased when the reactant concentrations increased.

Solubility limit of Au on Si is given by eutectic Au-Si phase diagram, seems like at 10 fold dilution you do not have supersaturation, Au particles are under saturation. Look up the kinetics of this process.

There may be a cooperative effect going on. It maybe that extended structures of AuNP are formed on the surface that further increase the kinetics of binding. If it is a cooperative interaction you should see a sigmoidal shaped curve as a function of concentration.

First of all, in general the rate of chemical kinetics increases with increasing the concentration of reactants. Secondly, at your dilute solution there may be the solvent molecules work as a barrier between reactants due to their absorption on the surface through intermolecular forces. But, this problem will be ceased when the reactant concentrations increased.

Thanks for all your help guys. Miroslav, can you elaborate why I'd expect a sigmoidal curve in this case?

Nagaboopathy, this is glass covered in a highly-reactive crosslinking molecule. In this cause, you still think this is a eutectic Au-Si system?

Adam. in cooperative activity the binding of one substrate changes the affinity of the next one. It is possible to have positive or negative cooperativity. I think the Hill equation, usually used for enzyme kinetics, explains why this is sigmoidal. Intuitively a cooperative interaction rate takes off exponentially rather than linearly before flattening off. This is because as the number of interacting substrates increase it causes a chain reaction, the reason it does not continue exponentially is that there is probably a well defined optimum cluster of molecules beyond which additional substrates will not increase the rate.

In my opinion, the driving force of the absorption of your AuNPs from the solution to the surface is the concentration. And lower concentration means lower driving force. Plus there is other effect - the citrate on the surface of your AuNPs will be change for the amine on the bottom, but not at the top of your AuNPs, so there is a repulsion between the AuNPs in the solution and those on the surface. This repulsion might be partially supressed as you mentioned by increasing the AuNPs concentration..

Adam, have you tried mixing of your AuNPs solution? It can be just a case of diffusion limited process...

I totally agree with Rabah - the rate of chemical kinetics increases with increasing the concentration of reactants. They are closer to the surface and it`s easier for them to diffuse and adsorb. That is why I`m suggesting stirring. Good luck :)

Hey Marta,

Yes the AuNPs are deposited under mixing to compensate for diffusion effects.

Adam, If your system follows Rabah's comment then possibly eutectic argument is not applicable. Thatswhy I mentioned to look up the kinetics, at lower concentration the rate at which NPs attaching is lower than rate of detachment. When you have higher concentration the supersaturation leads the reaction in forward direction

Entropy...

Are you sure that is only an effect of AuNP concentration?

What about reproducibility on the formation of aminosilane films from one to the other batch of experiments?

If free amine groups are not properly exposed you will find a poor reproducibilty in the AuNP adsorption/coverage.

A procedure for building such fims for proper cross-linking and a high percentage of free amino exposed groups for AuNP attachment that could help you, would be as follow:

1h from ethanol or metanol + curing step of the film during 1h at 100ºC before immersion in the coloidal solution. (Do not go below 90ºC and beyond 110ºC)

If a covalent attachment by electrostatic interactions with the citrate capped AuNPs is achieved I don´t see a main drawback for a proper adsorption of AuNP even at lower concentrations, although the kinetics play a role in the immersion time for obtaining higher surface coverages close to that found for higher AuNP concentrations.

If it is not the case, because the cross-linking of the film and the percentage of disposable free amine groups is low, they are buried or even other orientations of the molecules are possible, no covalent attachment can play a significant role and the rate of AuNP desorption can also influence dramatically the final surface coverage achieved.

Good luck

Hey guys,

I found a paper that hypothesizes that the concentration dependence is actually due to ions in the solution, not the concentrations of the AuNPs per-se. The more free citrates, and other ions, the more of a screening effect and they describe it nicely. Not sure if this explanation is correct, but worth a look.

See attached.

Rafael, thanks for your help. We use a similar procedure and have had success getting nice layers. I'm pretty confident the aggregates, in some cases at least, are forming on layers that are otherwise "good".