A colorimetric chemosensor for hydrogen peroxide (H₂O₂) and biosensor for cholesterol is developed in this work following a ‘label-free and reagentless’ concept. In this strategy, silver nanoparticles (AgNPs) are used as a bifunctional probe, namely, a chemical capture probe for H₂O₂ recognition and a signal probe for displaying detection results. The working principle of these colorimetric sensors is based on the redox reaction between AgNPs and H₂O₂. This reaction converts AgNPs into Ag⁺ ions, which can induce a visible color change from yellow to pinkish or colorless depending on the amount of H₂O₂. Therefore, H₂O₂ concentration can be measured. The developed colorimetric H₂O₂ chemosensor exhibits a highly selective and sensitive property towards H₂O₂ with a limit of detection (LOD) of 3.5 μM. Subsequently, a colorimetric biosensor is developed for cholesterol detection by combining AgNPs and cholesterol oxidase (ChOx) enzyme. Here, ChOx oxidizes cholesterol into cholest-4-en-3-one and H₂O₂. Then, the detected as-generated H₂O₂ concentration reflects the cholesterol level in the sample. This cholesterol biosensor exhibits high selectivity and sensitivity for cholesterol detection with a LOD of 40 μM. The developed sensor is successfully applied to the detection of cholesterol levels in serum samples. Our results show that the AgNP–based colorimetric method is cost-effective, sensitive and selective. It has potential applications in H₂O₂ and cholesterol detection and in clinical and medical diagnoses.