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. This enhanced the applications of these particles, which may now be effectively dispersed in both aqueous media and various organic solvents, as well. The fine dispersion state would be expected to increase the applications of PTs in various electrical and electro-optical fields without introducing substituted thiophene monomers with side chains.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). And PThTA nanoparticles with various sizes were directly prepared by Fe3+-catalyzed oxidative emulsion polymerization of thiophene with varying hydrogen peroxide (H2O2) concentrations. In the polymerization, FeCl3/H2O2 (catalyst/oxidant) combination system was used as an initiator couple. At the optimized reaction condition, percentage monomer conversions were above 95%. With controlled H2O2 molar concentration, the average sizes of the PT nanoparticles can be controlled in nanoscale, and their photo-emission wavelengths shifted from red to blue color at the maximum excitation wavelength (λUVmax = 400 nm). This was confirmed by SEM, TEM, UV, and PL analysis. As a result, we could tune the emitting colors which resulted from variations of the effective conjugation chain length by manipulating the size of PThTA nanoparticles. The number-average molecular weight of PThTA was about 2×104 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. This resulted from the increased total surface area of PT nanoparticles and weak self-absorption effect due to the particle size reduction. PT nanoparticles prepared directly used to make film at 110°C without any additional solvent casting procedure. The poly(ethylene terephthalate) (PET) film coated with PThTA nanoparticle was high transparent in a dried state. This size reduction induced the enhancement of PL property and quantum yield, which was characterized by PL spectra, due to the increase of surface area. And the wavelength of each maximum PL intensity shifted a little toward short wavelength. Their morphology, determined by SEM, is almost like sphere. The molecular weights of PThTA nanoparticles were measured by GPC after nitration process.Our results provide new insights on surfactant-free oxidative polymerization and may serve as guidelines for the preparation of new conducting polymer emulsion systems for practical applications. The physical characterizations of the synthesized PT, PT/PSS, and PS/PT nanoparticles were studied by SEM, TEM, GPC, CHDF, UV-vis, PL, In-situ FT-IR, TGA and electrical conductivity & optical studies.