Kinetic study of polydopamine sphere synthesis using TRIS: relationship between synthesis conditions and final properties

Abstract

The synthesis and characterization of polydopamine (PDA) using dopamine (DA) as the monomer and (hydroxymethyl)aminomethane (TRIS) as the oxidant is studied. The effect of temperature and TRIS concentration on the kinetics of dopamine polymerization is evaluated, and the kinetic parameters are also calculated. Three TRIS concentrations are used to assess their effect on DA polymerization kinetics. The reaction at 1.5 mmol of TRIS shows a sustained increase of the rate constant with temperature from 2.38 × 10^-4 to 5.10 × 10^-4 when the temperature is increased from 25 to 55 °C; however, not all reactions follow an Arrhenius law. In addition, the correlation between the synthesis parameters and morphological, structural, and thermal properties of polydopamine is established. The morphology of the PDA particles is evaluated by Scanning Electron Microscopy (SEM), the relationships between the diameter, distribution size, and the rate constant. Thermal characterization by Differential Scanning Calorimetry (DSC) shows an endothermic transition around 130 °C associated with the melting of PDA's regular structure. It is supported by structural studies, such as infrared and Raman spectroscopy and X-ray Diffraction (XRD), by observing a broad peak at 23.1° (2θ) that fits with a graphitic-like structure of PDA.

Fig. 1. (A) UV-vis spectra of the DA solution at different reaction times with 1.5 mmol of TRIS and 25 °C. (B) PDA spectrum in water solution. Reaction solution color change with 1.5 mmol TRIS, (C-1) at 25 °C, (C-2) at 40 °C, and (C-3) at 55 °C.

Fig. 2. (A) Chemical route of the DA auto-oxidation, (B) PDA formation through the different oligomers.

Fig. 3. Change of dopamine concentration against time and exponential decline fitting for (A) 1.5 mmol TRIS, (B) 4.5 mmol TRIS, (C) 7.5 mmol TRIS, and (D) zoom of the reaction onset at 7.5 mmol TRIS.

Fig. 4. Effect of the TRIS solution on DA consumption in the solution immediately after its addition (one minute after).

Fig. 5. E _a vs. Ln(A) in the range of temperatures of 25 to 55 °C.

Fig. 6. Micrographics of PDA synthesized at different conditions and particle diameter distribution; (A) 1.5 mmol, 25 °C, (B) 1.5 mmol, 40 °C, (C) 1.5 mmol, 55 °C, (D) 4.5 mmol, 25 °C, (E) 4.5 mmol, 40 °C, (F) 4.5 mmol, 55 °C, (G) 7.5 mmol, 25 °C, (H) 7.5 mmol, 40 °C and, (I) 7.5 mmol, 55 °C. The bar represents 2 μm.

Fig. 7. Thermogravimetric analysis of (A) dopamine hydrochloride (DA·HCl) and (B) synthesized PDA at 25 °C.

Fig. 8. Thermograms obtained from PDA synthesized at (A) 1.5 mmol, (B) 4.5 mmol, and (C) 7.5 mmol for each temperature.

Fig. 9. Relationship between (A) rate constant and enthalpy of fusion, and (B) synthesis and melting temperature.

Fig. 10. FT-IR spectra of PDA at each synthesis condition from 550 to 4000 cm⁻¹.

Fig. 11. Raman spectra of PDA at each synthesis condition.

Fig. 12. Relationship between the temperature of the reaction and the G band position.

Fig. 13. X-ray diffraction spectra of PDA for each synthesis. condition.

Fig. 14. Representation of monomer proposed by (A) Meng and Karixas, (B) Liebscher, and (C) three-layer stacked model for PDA. Atoms color: black, blue, red, and white for carbon, nitrogen, oxygen, and hydrogen, respectively.