Water-dispersible poly(thiophene-co-3-thiopheneacetic acid) (PThTA) nanoparticles were successfully prepared by Fe3+-catalyzed oxidative polymerization in the absence of a surfactant. This facile method adopted the recyclable FeCl3/H2O2 initiator couple to yield a high conversion of approximately 97%, and the sodium salt of 3-thiopheneacetic acid (TA) was used for the colloidal stability of the PThTA nanoparticles. The particle formation and growth of PThTA were examined with photoluminescence (PL) spectroscopy and time-evolution variation of the particle number in the early period of polymerization. The final average particle size of PThTA was 320 nm, measured by dynamic light scattering (DLS). The number-average molecular weight of PThTA was about 2 x 10(4) g/mol. The light-emitting properties of the PThTA nanoparticles in an emulsion state were studied with UV-vis absorption and PL spectroscopy, and it was found that the quantum efficiency increased from 1.43 to 3.22 with the polymerization time at an excitation wavelength of 405 nm. Our results provide new insights on surfactant-free oxidative polymerization and may serve as guidelines for the preparation of new conjugated polymer emulsion systems for potential optoelectronic devices. (C) 2014 The Authors. Published by Elsevier Ltd. All rights reserved.