Highly-dispersed 17 +/- 2-nm SiO2 nanospheres were obtained by a modified StOber method and then used as material precursors in a green chemistry process for obtaining SiO2-Ag core-shell nanocomposites with the shell thickness of 1.5 nm. In addition, an enzymeless SiO2-Ag core-shell modified electrode was fabricated and used for the first time to detect H2O2. The modified electrode has a fast amperometric response of <2 s at low detection limit of 0.1 mu M. The linear range of H2O2 detection is estimated to be from 0.25 mu M to 6.7 mM with a determination coefficient R-2 of >0.99. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.015201esl] All rights reserved.