We have gold nanoparticle monolayers on silanized glass surfaces, and have been trying to coat the particles on the surface with mixed monolayers of OH- and COO- terminated alkanethiols. We've tried many different protocols, but our surfaces do not seem to be resistant to protein adsorption, even though the OH- terminated alkanethiols have demonstrable resistance in free solutions of gold particles.
In any case, we did a run recently where the mixed monolayer was formed after 12 hours immersion, then rinsed for an additional day, and dried in air an additional day. Surprisingly, these samples did show the properties we were looking for, in terms of resistance and COO- activation. We're double-checking the results, but do you guys think that the long rinse and drying phase really can explain the veritable results? Is drying a very crucial step to alkanethiol-gold monolayers?
Hi Adam!
Profs. Andrews and Weiss at UCLA have been exploring molecular SAM systems for prevention of nonspecific adsorption of proteins on surfaces; are you using oligo ethyleneglycol molecules as your chains? The OEG functionality prevents such adsorption, but I do not think simple alkyl chains show the same behavior.
perhaps the following links will help point your efforts in a positive direction!
http://pubs.acs.org/doi/pdf/10.1021/jp402921y
http://pubs.acs.org/doi/pdf/10.1021/jp207396m
http://pubs.acs.org/doi/pdf/10.1021/cn1000205
It is important to note that depositing mixed monolayers on flat gold substrates will often yield mixed monolayer compositions that do not always reflect solution composition-- and phase separation is common. I expect deposition onto gold-nanoparticle films to be largely analogous to flat surfaces.
For carboxylic acid terminated molecules, hydrogen bound dimers have been noted to be very important in depositing carboxy-terminated thin films. By depositing in an excess of acetic acid, film quality can be improved, as the molecules are able to adsorb and assemble as single molecules rather than bound pairs. the following link is probably a great place to start for modifying conditions.
http://www.ncbi.nlm.nih.gov/pubmed/15835147
In my personal experience, I have found that elevated temperature is substantially more effective at improving SAM quality than any concentration, time, purity, or solvent control. I tend to avoid "soaks" and always dry by using compressed gasses to blast solvent off the surface rather than allowing anything to dry onto the substrate-- any contaminant present in the solvent will be deposited in an air-dry. However, for functional films, this can be made difficult because of hydrogen bonding interactions with the surface. Perhaps an ethanolic acetic acid rinse will help remove any bound molecules?
Thanks. We did finally switch to ethylene glycols and found that the non-specific binding was mostly removed. It seemed to be a matter of hydrophobic interactions afterall. Thank you for this information, I will certainly look into it to improve my layers!