A facile, one-step self-assembly of polymer nanoreactors is reported for the fabrication of uniform PEGylated gold nanoparticles. Nanoreactor assembly occurs within milliseconds and gold nanoparticles are produced within minutes at room temperature. The presented approach enables continuous synthesis of size tunable gold nanoparticles with tailorable surface chemistry. Figure 2. (a) Initial nanoreactor size distributions measured by DLS and (b) UV absorbance of the dialyzed dispersions synthesized at a range of pHs by diluting the effluent from the mixer into PBS, pH 7.4, water (MQ) or 0.01M HCl.The nanoreactor assembly occurs on the order of milliseconds (time scale of mixing). The reduction to Au(0) occurs in the nanoreactor where TA is localized. The reduction is evident by the transition from the light amber dispersion to strongly red or purple colored dispersions associated with colloidal gold (Fig. 2, Supporting Information Figure S2) that occurs within ~20 seconds (at pH 6) to ~7 minutes (pH 2). The effect of pH on the rate of reduction has been used to tune nanoparticle size over a relatively narrow range 17-20 nm 20 . We demonstrate this strategy