Adjustable synthesis of polydopamine nanospheres and their nucleation and growth

The growing interest in polydopamine (PDA) nanospheres leads to a strong increase in demand for size-controllable synthesis strategies. Understanding the formation pathways of PDA nanosphere is of utmost importance in order to predictably synthesize these nanoparticles with the desired diameters. In the present study we synthesized PDA nanospheres with various diameters by the self-oxidative polymerization of dopamine (DA) in an alkaline solution. The diverse reaction conditions including the DA:NaOH molar ratio, the DA concentrations, the reaction time and temperature as well as the oxidant types were extensively investigated, and the results of the TEM and DLS analysis clearly showed the dependence of the diameters of the PDA nanospheres on reaction conditions. In addition, the relationships between the polymerization kinetics of DA and the particle size of PDA nanospheres were examined by UV–vis spectroscopy. The polymerization kinetics of DA were significantly accelerated by the presence of more hydroxide ions (OH−) or a strong oxidizing agent, which increased the nucleation rate of PDA, resulting in PDA nanospheres of smaller diameter. The increasing DA concentrations cause the PDA cores to grow, resulting in an increase in diameter. Moreover, the size of the PDA nanospheres increased with the prolonged reaction until a certain time and as long as sufficient precursors are available. It was also found that higher temperatures not only accelerate the nucleation of PDA particles but also promote the diffusion of DA monomers. Based on the results we suggest the DA:NaOH molar ratio as the most promising tool to affect the diameter and the size distribution of PDA spheres due to its influence at the very beginning of the reaction pathway.