Coating Steel With Polymers Derived From the Neurotransmitter Dopamine.

[NNadir]

The paper I'll discuss in this post is this one: Anodic Coating of 1.4622 Stainless Steel with Polydopamine by Repetitive Cyclic Voltammetry and Galvanostatic Deposition (Sabrina Schindler,† Noemí Aguiló-Aguayo,† Urs Dornbierer,‡ and Thomas Bechtold, Ind. Eng. Chem. Res. 2020, 59, 1, 236-244)

Because of my interest in the anodic dissolution of used nuclear fuels in molten salts as part of a process known as pyroprocessing, I am always interested in anodes, which are the place where oxidation takes place in an electrochemical cell, batteries being a subset of such cells. Most times when I see the word anode when scanning a table of contents in a scientific journal, I pause to look into it a little more deeply, since, like it or not, the behavior and sustainability of anodes is, albeit very subtle, a key technical issue in addressing climate change.

Dopamine is an important neurotransmitter which is derived from the endogenous compound L-DOPA, itself an oxidative metabolite of the amino acid tyrosine. L-DOPA is administered to Parkinson's disease patients because, being an amino acid, it is subject to transport across cell membranes. As such, it can localize in brain tissue to increase the levels of dopamine, which is not permeable to important membranes. By doing so, it addresses one of the physiological biomarkers associated with Parkinson's, specifically, low dopamine levels.

The paper under discussion has nothing at all, however to do with brain physiology, but is instead about materials science, specifically about preventing corrosion, an important issue where steel is in contact with corrosive environments, notably seawater.

From the introductory text, touching on some of what I just said:

Properties like good corrosion resistance and biocompatibility combined with chemical inertness and relatively low costs render stainless steel unique and thus frequently used for medical devices as well as material in industrial installations. Duplex stainless steel (type 1.4622, X2CrNiMoN22-5-3; Duplex 2205) is a preferred steel type for applications where the combination of high strength and corrosion resistance is a requirement. Representative examples are stainless steel-based fish-farming nets, which are used in the corrosive marine environment. However, if in contact with aqueous solutions, being under physiological conditions inside the human body or in any aquatic environment, steel tends to suffer from biological fouling, which renders its limited use. To prevent stainless steel from fouling, different coating and treatment methods have been developed. Besides paints, which contain incorporated biocides (e.g., copper oxides), coatings with heterobifunctional silane-poly(ethylene glycol) overlayers or introduction of binary polymer brushes on polydopamine anchors have been proposed as antifouling and antimicrobial coatings.(1,2)
Dopamine is an important neurotransmitter and an impairment in dopamine metabolism manifests in Parkinson’s disease.(3) Dopamine also is the anchor through which mussels are able to attach to surfaces.(4) In 2007, dopamine has been identified as a highly interesting material in surface science due to its unique reactivity and coating properties.(5) From then on, this easy-to-do coating procedure has been applied extensively to coat nearly any kind of solid material, among them stainless steel as a widely used corrosion-resistant material.

Dopamine has been used to coat different types of stainless steels either to serve as a coating itself,(6) to serve as an anchor for grafting PEG-based antifouling coatings onto it,(7) or to further functionalize it with nanostructured hydroxyapatite.(8)...

...Dopamine can be triggered oxidatively to form polydopamine, a melanin-like black aggregate, which is known to coat surfaces and thus to cover electrodes and to render them insensitive to electronic stimuli. If repetitive cyclic voltammograms are recorded consecutively in dopamine-containing solutions and the working electrode is not cleaned between the cycles, oxidation and reduction peak heights will decline due to the well-known phenomenon of “electrode fouling”.(12) Recently, the importance of pH on the reaction mechanism of electrochemical coating on gold electrodes has been highlighted.(13) Different dissociated species of dopamine are present in solution dependent on the solution pH, which also determine the pathway of anodic polydopamine formation. Below pH 7, the amino group of dopamine is present in its protonated ammonium form; thus, participation of the amino group in the anodic formation of polydopamine is expected to gain relevance above pH 7–8. The electrochemical deposition of polydopamine has been studied in detail mostly on noble metals like gold so far.(13−15) At neutral pH, the rate of dopamine auto-oxidation is reduced to such an extent that polydopamine formation is initiated at the anode surface exclusively...

.... The presented results indicate an efficient procedure to achieve polydopamine coatings under well-controlled electrochemical conditions. The selective formation of an electrically insulating layer on conductive elements embedded in textile structures also is of high interest for localized development of insulating and protective layers in smart textile applications.(16)

Some pictures from the text:




The caption:

Figure 1. Scheme of the pouch cell used for anodic coating of stainless steel plates. Left: top and side view. Two stainless steel plates form anode and counter electrode (1), (Ag/AgCl, 3 M KCl) reference electrode (2). Right: Photograph of the experimental setup (stainless steel plates (1), reference electrode (2)).

The caption:

Figure 2. Structure of dopamine, corresponding dissociated forms (I–IV) and the respective pKa values.

The caption:

Figure 3. Repetitive cyclic voltammograms recorded on an Au electrode recorded in the potential interval between −100 and +800 mV (5 mM dopamine in 0.1 M phosphate buffer, scan rate of 50 mV/s) at pH values of (a) 6.1, (b) 6.5 (inset, 20 mV/s scan rate), (c) 7.0, and (d) 8.0. Each figure shows seven consecutive cyclic voltammograms recorded without intermediate cleaning of the electrode surface.

The caption:

Figure 4. Decrease of anodic peak current (Ip)a in repetitive CV experiments as function of scan in the potential interval of −100 to +800 mV at pH values 6.1, 6.5, 7.0, and 8.0 including experiments in the presence of 0.5 M NaCl (Au electrode area, 3.14 mm2; 5 mM dopamine solution in 0.1 M phosphate buffer; scan rate, 50 mV/s).

Here is the structure of the polymers produced at two different pHs, shown with the mechanism of their formation:



The caption:

Figure 5. Mechanism of anodic polydopamine formation based on different reaction pathways as proposed in the literature.(20,21)

One would think, naively, that the polydopamine formed under basic conditions might well be an electrical conductor, given the highly conjugated pi system, but this is not the case: The polymer is insulating. The authors remark on this:

Thus, the structural elements present in polydopamine will differ dependent on the conditions applied during polymerization.(20,21) Remarkably, despite the proposed presence of the quinone–hydroquinone structures in polydopamine, no corresponding anodic or cathodic currents are observed at the coated electrode. This can be explained with both a low electrical conductivity of the polydopamine on the surface of the electrode and low electrochemical activity of the deposited polydopamine. Thus, no electrochemical response of the polymeric products deposited at the electrode surface is observed during the reverse scan in the CV.

One can certainly imagine modifications to this system that might make the polymer conducting, but whether such polymer modifications as may affect this would be stable to anodic oxidation, at least long term oxidation is certainly open to question, albeit interesting questions.

Here's what the polymer looks like:



The caption:

Figure 6. (a) Anodic oxidation of 5 mM dopamine in 0.05 M phosphate buffer pH 6.6 during five repetitive cyclic voltammetry cycles with 50 mV/s scan rate on a stainless steel wire electrode. (b) Stainless steel electrode (2 mm diameter, area 3.14 mm2) showing the coated cylindric barrel of the wire.

The graphic here shows the effect of increasing electrical insulation during the process:



The caption:

Figure 7. Repetitive cyclic voltammograms on stainless steel plate (active area 45 × 60 mm2, 5 mM dopamine in 0.05 M phosphate buffer pH 6.6, 10 cycles of anodic deposition of dopamine, scan rate 50 mV/s). (a) Cyclic voltammograms

The caption:

Figure 8. (a) Galvanostatic coating of stainless steel plates. Anode potential as function of total charge flow in 5 mM dopamine solution at pH 5.0 (black, 250 s, Q = −3.75 C, 0.05 M acetate buffer) and at pH 6.5 (red, 280 s, Q = – 4.20 C, 0.1 M phosphate buffer) and anode potential during repetitive galvanostatic coating of stainless steel (2.6 mM dopamine in pH 6.5 phosphate buffer) first coating 35 mA current (green dotted line) and second coating 20 mA (blue dotted line). (b) Photograph of a polydopamine coated stainless steel plate type 1.4462 (2.6 mM dopamine, pH 6.5, 0.1 M phosphate buffer, total charge flow 2.44 C). Arrows indicates the level of dopamine containing the electrolyte.

The caption:

Figure 9. Diffuse reflectance of uncoated steel type 1.4462 and of polydopamine-coated steel surface (2.6 mM dopamine, pH 6.5, 0.1 M phosphate solution, 2.44 C charge flow) (curves represent the mean of two measurements).

From the paper's conclusion:

Stainless steel is a conductive material, thus anodic oxidation permits electrochemical deposition of dopamine to form homogeneous coatings within a few seconds. The rate of anodic polydopamine formation is strongly dependent on pH. At pH 6.1, only minor deposition is observed during seven repetitive CV cycles; however, at pH 8, the electrode becomes covered rapidly with polydopamine. With increasing pH, also uncontrolled auto-oxidation gains relevance, which leads to increased losses of dopamine through auto-oxidation in solution. At pH 6.5, rapid anodic deposition of polydopamine on the electrode surface could be obtained, while auto-oxidation of dopamine in solution still could be suppressed.

Galvanostatic conditions at a current density of 7–13 μA/mm2 permit controlled deposition of polydopamine on 1.4462 stainless steel. A threshold value for an anode potential of +1100 mV permits controlled galvanostatic deposition without significant side reactions due to water electrolysis or steel corrosion. The formation of a polydopamine layer can be recognized in development of an electrically insulating colored coating and in a reduction of the water contact angle.
Compared to the dip coating with spontaneous oxidation and polydopamine formation, the process of electrochemical deposition allows use of a lower solution pH and thus prevents uncontrolled auto-oxidation of dopamine and retards the formation of polydopamine aggregates in solution. As a result, a more efficient use of the expensive substance dopamine is possible, which opens technical routes for large-scale applications of polydopamine coating for medical devices, antifouling coating, and corrosion prevention.

It's a nice little paper, I think. These very subtle things, albeit not generally perceived in the public, can make a huge difference in the sustainability of our living conditions.

Have a pleasant Sunday.