Novel CO2-responsive conductive polymer particles based on poly(N-(3-amidino)-aniline) (or PNAAN) are reported in this work. A CO2-responsive N-(3-amidino)-aniline (NAAN) monomer is firstly synthesized with the pendant amidine group at the meta-position of aniline (AN) and subsequently polymerized into the PNAAN polymer by chemical oxidation. Self-assembly of PNAAN in turn forms the polymer particles. In the strong or weak acid media, the amidine group protonates into cationic amidinium and self-stabilizes the PNAAN particles without the use of any stabilizers. The reaction media are found to affect the polymerization rate and self-assembly of particles, and hence the size and size distribution of the resultant particles. The particles synthesized in strong basic media show CO2-responsvie properties since the H+ released by dissolved CO2 (dCO(2)) can protonate the amidine group into hydrophilic amidinium group and result in swelling of the PNAAN particles. Zeta-potential measurements show the reversible change of particle surface charges in the presence and absence of dCO(2). Dynamic light scattering (DLS) measurements show the particle size linearly changed with dCO(2) concentration in the range of 5 x 10(-4) and 2.5 x 10(-2) atm. This is the first reported CO2-responsive polyaniline (PANI) particles for dCO(2) sensing or reversible fixation of CO2