Isotropic silver nanoparticles (iAg NPs) can be easily prepared at low costs, have a low electrochemical potential and high extinction coefficient. An effective colorimetric assay for HO is reported here based on the finding that HO can induce the shape transformation of citrate-capped iAg NPs with the help of citrate. The substantial shape variation affords an apparent surface plasmon resonance (SPR) shift, accompanied by a vivid color change from light yellow to mauve. The color change can be observed visually if the concentration of HO is 2 μM or higher. A good linear relationship was obtained over the concentration range of 0.2-32 μM with a limit of detection of 90 nM. By making use of glucose oxidase, the method is further extended to glucose detection. Glucose at a concentration as low as 10 μM can be well determined with bare eyes. Benefitting from the high selectivity, the detection of glucose in human serum is realized, and the results are in good agreement with those provided by a clinical analyzer. Graphical abstract Schematic presentation of a colorimetric assay for HO and glucose based on citrate-promoted HO-mediated shape transformation of the isotropic silver nanoparticles (Ag NPs). The shape variation of isotropic Ag NPs induces a color change from light yellow to mauve.