Binding BSA to silver surface via large difference in isoelectric points of the two materials

Question

I would like to bind BSA to a silver surface so that I can utilize plasmonic sensing to detect the BSA. There seems to be two methods of doing this: 1) to rely on electrostatic forces or 2) to form a covalent bond. The former seems to be the easier route, especially for someone like me who does not have a background in chemistry/biology.

So I found a paper (https://doi.org/10.1016/0021-9797(91)90121-N), which has been cited 54 times, that determined a silver (oxide) surface has an isoelectric point (iP) of ~10 pH. So this should mean that it has a positive charge when a neutral pH solution covers the surface. In other literature BSA has been shown to have an iP of pH 4-5, so it should become negatively charged in a neutral pH solution. So the BSA should bond to the silver surface in a neutral solution, correct?

The reason I ask is two-fold. First, I found a more recent paper (https://doi.org/10.1021/am400909g) that has a conflicting value for the iP of a silver film (~3 pH). Second, I found a paper (https://doi.org/10.1016/j.vibspec.2003.09.003) were the authors adsorbed BSA onto silver films, but with the intermediate step of applying 2-mercaptoethanesulphonate to the silver surface. Why the need for this if the silver should attract the BSA electrostatically?

In addition, if BSA bonding to silver were to work in this way, am I correct in thinking that a neutral solvent such as phosphate buffered saline would hinder the electrostatic bonding process because the positive and negative ions from the salt solution would bond to the respectively charged molecules and neutralize the charge?

Answer 1

Concerning the apparent discrepancy between your first and second citations, the difference in values is addressed by the authors in the more recent work:

Perusal of the literature yields few IEP values for silver metal. Chau and Porter reported an IEP of 10.4 for an evaporated silver film as determined by contact angle titration.(6) The method employed in their investigation used only neutral and basic liquids, and the maximum contact angle achieved on the silver films was ∼35°, in contrast to the maximum we observe of 52° at pH 3.2 (Figure 1d). This dichotomy may be indicative of the amphoteric behavior of the native oxide at the surface.

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Concerning your third citation where the authors bound BSA to silver via a 2-mercaptoethanesulphonate linkage monolayer, the authors give their reasoning in the abstract:

The direct adsorption of proteins on metal surfaces usually leads to loss of their enzymatic activity

So, to answer one of your questions

Why the need for this if the silver should attract the BSA electrostatically?

BSA here is a proxy for other proteins. The purpose of the research was to examine how the structure of the linkage monolayer changed in response to protein adsorption, not how BSA specifically binds a silver substrate.