Gold Trials

Most recently I’ve been working with colloidal gold nanoparticles (referred to as AuNP from here on). We chose to use AuNP for separations because it is an ideal hard sphere particle, unlike a floppy protein. We thought we could achieve more information about the cutoffs or the sieving characteristics of the membrane by using such a model species.

AuNP are made by reacting gold chloride with citrate. By changing the stoichiometry of this reaction you can achieve different sized particles. Each nanoparticle is a cluster of neutral gold surrounded by negatively charged citrate and chloride. AuNP remain in solution by repelling each other, and if this negative charge was reduced or shielded they will flocculate, which includes both transient agglomeration and irreversible aggregation.

The AuNP are dissolved in DI water in the stock solution. I observed no separation of the stock solution by our membrane. We believe that in a low ionic strength solution, pnc-Si membranes are negatively charged. This means that in DI water, the membrane is repelling the gold. We naively thought that we could achieve our separation if the AuNP were dissolved in a salt containing solution. In this case, the membrane charge is shielded by the mobile salt ions, but so are the AuNP and flocculation immediately occurs. In order to perform hard sphere separations, we needed to try something a little different.

I successfully coated AuNP with BSA and was able to suspend the BSA-AuNP in PBS and perform separations. We don’t believe that we achieved the results we wanted though, as the BSA-AuNP were not simple hard spheres any longer. My most recent work has concerned applying a small ligand to the surface of the AuNP that will enable the dissolution of AuNP in a salt buffer. I have tried to attach beta-mercaptoethanol (BME) to the surface in several ways:

• Simply adding 1% BME to the stock – this results in flocculation in all but a few trials (unrepeatable)
• Raising pH to 10-11 and then adding 1%BME – changing the pH of AuNP results in flocculation
• Adding a pH11 1% BME solution – AuNP are stable and were incubated overnight, but crashed once introduced to salts

BME may be too small of a ligand. It seems that it is possible to add your ligand immediately during the formation of AuNP, but we are not equipped to make our own AuNP. Other solutions may exist, but I think we’re considering a slightly different approach. If the AuNP cannot be in salts, maybe we can modify the membrane so that separations can occur in DI water. I will try to modify the membrane by aminosilanization and link aminopropyltriethoxysilane to the surface. This will terminate the surface in an amino group, and possibly allow diffusion of AuNP through the membrane.