In this project, hydrothermal methods have been employed to synthesize carbon dots with different chemical composition from citric acid and amino acid. Oxygen-modified carbon dots (O-CDs), oxygen and nitrogen-modified carbon dots (N-CDs), and oxygen, nitrogen and sulfur modified carbon dots (N,S-CDs) have been successfully achieved. These carbon dots achieve strong blue-green luminescence. transmission electron microscopy (TEM) and dynamic light scattering (DLS) are used to characterize the structure and size of these carbon dots, which shows similar structure and size. The chemical composition of these carbon dots are obtained via Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which depicts the oxygen, nitrogen, and sulfur are well modified into carbon dots. These carbon dots could be used as artificial peroxidase for the detection of H2O2. The comprehensive study is performed to investigate the relationship between peroxidase-like catalytic activity and chemical composition in carbon dots.