We present a general thermodynamic top-down analysis of the effects of oxidants and pH on dopamine oxidation and cyclization, supplemented with UV–vis and electrochemical studies. The model is applicable to other catecholamines and various experimental conditions. The results show that the decisive physicochemical parameters in autoxidation are the pK values of the semiquinone and the amino group in the oxidized quinone. Addition of Ce(IV) or Fe(III) enhances dopamine oxidation in acidic media in aerobic and anaerobic conditions by the direct oxidation of dopamine and, in the presence of oxygen, also by the autoxidation of the formed semiquinone. At pH 4.5, the enhancement of the one-electron oxidation of dopamine explains the overall reaction enhancement, but at a lower pH, cyclization becomes rate-determining. Oxidation by Cu(II) at reasonable rates requires the presence of oxygen or chloride ions.

Autoxidation of dopamine to polydopamine by dissolved oxygen is a slow process that requires highly alkaline conditions. Polydopamine can be formed rapidly also in mildly acidic and neutral solutions by using redox-active transition-metal ions. We present a comparative study of polydopamine nanoparticles formed by autoxidation and aerobic or anaerobic oxidation in the presence of Ce(IV), Fe(III), Cu(II), and Mn(VII). The UV–vis spectra of the purified nanoparticles are similar, and dopaminechrome is an early intermediate species. At low pH, Cu(II) requires the presence of oxygen and chloride ions to produce polydopamine at a reasonable rate. The changes in dispersibility and surface charge take place at around pH 4, which indicates the presence of ionizable groups, especially carboxylic acids, on their surface. X-ray photoelectron spectroscopy shows the presence of three different classes of carbons, and the carbonyl/carboxylate carbons amount to 5–15 atom %. The N 1s spectra show the presence of protonated free amino groups, suggesting that these groups may interact with the π-electrons of the intact aromatic dihydroxyindole moieties, especially in the metal-induced samples. The autoxidized and Mn(VII)-induced samples do not contain metals, but the metal content is 1–2 atom % in samples prepared with Ce(IV) or Cu(II), and ca. 20 atom % in polydopamine prepared in the presence of Fe(III). These differences in the metal content can be explained by the oxidation and complexation properties of the metals using the general model developed. In addition, the nitrogen content is lower in the metal-induced samples. All of the metal oxidants studied can be used to rapidly prepare polydopamine at room temperature, but the possible influence of the metal content and nitrogen loss should be taken into account.

The spin-spray-assisted layer-by-layer (LbL) assembly technique was used to prepare coordinative oxidative multilayers from Ce(IV), inorganic polyphosphate (PP), and graphene oxide (GO). The films consist of successive tetralayers and have a general structure (PP/Ce/GO/Ce) n . Such oxidative multilayers have been shown to be a general platform for the electrodeless generation of conducting polymer and melanin-type films. Although the incorporation of GO enhances the film growth, the conventional dip LbL method is very time consuming. We show that the spin-spray method reduces the time required to grow thick multilayers by the order of magnitude and the film growth is linear from the beginning, which implies a stratified structure. We have deposited poly(3,4-ethylenedioxothiophene), PEDOT, on the oxidative multilayers and studied these redox-active films as models for melanin-type capacitive layers for supercapacitors to be used in biodegradable electronics, both before and after the electrochemical reduction of GO to rGO. The amount of oxidant and PEDOT scales linearly with the film thickness, and the charge transfer kinetics is not mass transfer-limited, especially after the reduction of GO. The areal capacitance of the films grows linearly with the film thickness, reaching a value of ca . 1.6 mF cm –2 with 20 tetralayers, and the specific volumetric (per film volume) and mass (per mass of PEDOT) capacitances are ca . 130 F cm –3 and 65 F g –1 , respectively. 5,6-Dihydroxyindole can also be polymerized to a redox-active melanin-type film on these oxidative multilayers, with even higher areal capacitance values.

Oxidative layer-by-layer multilayers, consisting of polyphosphate (PP), Ce(IV), and graphene oxide, are a general platform for the electrodeless, spatially resolved deposition of redoxactive and capacitive films of conducting polymers and melanin-type materials. However, the film formation process has not been closely examined. We show that PP plays a crucial role in the structure, stability, and function of the multilayers. Random P−O−P bond cleavage in PP at low pH rapidly decreases the effective chain length and, together with the lower complexing capacity of the Ce(III) species, leads to the dissolution of the oxidative multilayer during the polymer film deposition. The multilayer dissolution takes place during, e.g., poly(3,4-ethylenedioxythiophene) (PEDOT) film formation, and produces a homogeneous polymer film on the substrate. On the other hand, polymerization of 5,6-dihydroxyindole (DHI), an analogue of polydopamine, is carried out at higher pH, and the DHI-melanin film forms only on the outer surface, leaving the bulk of the multilayer intact. This leads to a poor electrical contact between the substrate electrode and the redox-active polymer film. Low pH and long deposition times are, therefore, beneficial for the formation of good-quality redox-active polymer films. PEDOT films prepared using oxidative multilayers have good specific volume and mass capacitance, and good retention of their capacitance. Capacitance spectroscopy revealed the contribution of different dynamic processes and showed that the redox processes limit their capacitance in the 100 ms timescale, restricting the power density of the film. The capacitance of the DHI-melanin films decreases drastically in the same timescale while the capacitance and charge storage capacity values remain higher than those of the PEDOT films. Improving the electrical connection to the substrate using alternative deposition techniques and increasing the film conductivity will make the DHI-melanin films promising components for biodegradable supercapacitors.