In the preparation and storage of gold nanoparticles (Au-NPs) in colloidal form, the stability of the colloid is of utmost importance. We report a novel strategy for the synthesis of the phosphonic acid-functionalized gold nanoparticles (Au-NPs) with the high colloid stability by using ethylenediamine-tetramethylene phosphonic acid (EDTMP) as the reducing agent and its oxidation product as the stabilizing agent. The resultant phosphonic acid-functionalized Au-NPs show a remarkable colloidal stability, which likely arises from strong electrostatic effect of negatively charged phosphonate groups and the extremely hydrophilic property of phosphonate groups. Through the present method, the scope of reducing and stabilizing agents for preparation of phosphonic acid-functionalized Au-NPs extend from the -PO3H2-terminated thiols to the aminopolyphosphonates. (C) 2010 Elsevier Inc. All rights reserved.