Citric acid was used as the carbon source, and three phenylenediamine isomers — o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine — served as nitrogen sources to synthesize carbon quantum dots (CQDs) via a hydrothermal method. The resulting CQDs exhibited distinct colors: orange-yellow (o-phenylenediamine), light yellow (m-phenylenediamine), and purple (p-phenylenediamine). The CQDs were subsequently modified with oleylamine to introduce hydrophobic surface properties. Structural and optical properties before and after modification were characterized using Fourier transform infrared spectroscopy (FT-IR), fluorescence spectroscopy (FS), and ultraviolet-visible spectroscopy (UV-vis). The results indicate that the CQDs derived from m-phenylenediamine showed the most enhanced hydrophobicity after oleylamine modification, with significantly improved solubility in toluene, which is attributed to strengthened hydrocarbon interactions.