Online monitoring of dopamine particle formation via continuous light scattering intensity measurement

Using time resolved total intensity light scattering a two phase process was found in the conversion of 2-(3,4-dihyroxyphenyl)ethylamine, dopamine (DP), into microparticles. Phase 1 appears to involve oxidation of the DP, and may also include oligomerization below the light scattering threshold of detectability. After a certain lag time, the ‘Phase 1 Period’ (P1P), dependent upon reaction conditions, the light scattering increases sharply, heralding the onset of Phase 2. Precipitating particles are eventually formed in latter Phase 2. The solution goes through a series of color changes throughout Phase 1 and 2, starting as pink in Phase 1, and culminating in black particles in Phase 2. The reaction proceeds under virtually any conditions; purged with O2 or N2, unpurged, stirred or unstirred, under different pH conditions, etc. Stirring, increasing temperature, and adding potassium persulfate (KPS), all accelerate the reaction. P1P varied over nearly four orders of magnitude, from 10 s (pHinitial = 9.5) to 8 × 104 s (T = 25 °C, no pH control). Arrhenius behavior is found for P1P with low activation energies in the range of 10–25 Kcal/Mole. The precipitating particulates rapidly formed in Phase 2 suggest that they may involve non-covalent associations of oligomers formed in Phase 1, possibly due to loss of oligomeric solubility, in addition to possibly involving covalent polymer branching and cross-linking. Non-covalent aggregation of oligomers formed in Phase 1 seems most likely.