I have used dopamine hydrochloride solution: 2g/L of 10 mM Tris-HCL (pH=8.5) buffer for dip coating certain surfaces such as PDMS for around 24 hours while stirring slowly. The solution changes color to dark brown signifying the oxidation of catechols and the AFM scan shows the formation of a thin layer on ODMS film. The problem is I am getting zero adhesion in the dry state as well underwater. I do not understand what I am missing and why the surface is not sticky at all. I followed a couple of papers and the following is one of them. Can anybody please help me out in this regard?
H. Lee et al. Mussel-inspired surface chemistry for multifunctional coatings. Science 318, 426–430 (2007)
I assume PDMS is poly(dimethyl siloxane). This is a material with a very low surface free energy. It only forms very week interaction with other materials. If you want to have a good adhesion of anything on PDMS you have to alter the surface properties by treating it e.g. with oxidizing agents or electrical discharge plasmas.
Yes, Andreas Holländer, It is poly(dimethyl siloxane) and I performed plasma oxidation of the PDMS surface before coating it for its surface modification. In that case, the polydopamine film had a good bonding with it but not showing any adhesion under water as claimed by many authors. I am getting the polydopamine coating, which I confirmed by Raman spectroscopy, that thing is why this layer is not showing any adhesion?
In my opinion there are 2 things to consider: 1) The interaction between the dopamine structures and the surface are largely based on donor acceptor interactions (hydrogen bonds); there are no covalent bonds. 2) Dopamine and its oligomers are water soluble. Considerable cross-linking is required to prevent the dissolution in water.
The small molecules of water strongly interfere with 1) forming stronger hydrogen bonds with the surface and with dopamine. Cross-links in the dopamine coating can slow down the disruption of the dopamine-surface interaction.
I suggest to analyze the surface of your PDMS e.g. with XPS. It is quite possible that there is a monolayer or so left which you will not be able to detect with Raman spectroscopy.
Okay, I'll surely follow that. Thank you so much for your valuable time and suggestions Andreas Holländer.
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