UNLABELLED: Photothermal therapy (PTT), a rapidly advancing non-invasive cancer treatment modality, utilizes photothermal agents to convert light energy into thermal energy, enabling precise and localized destruction of cancer cells. Recent developments have focused on photothermal agents operating in the second near-infrared (NIR-II) biological window (1000–1350 nm), which offer enhanced tissue penetration depth and improved therapeutic precision for deep-seated tumors while minimizing collateral damage to healthy tissues. In this study, we developed a novel class of nitrogen-doped carbon dots (N-CDs) through a facile one-pot hydrothermal synthesis approach. The synthesized N-CDs demonstrate remarkable dual functionality, exhibiting both superior photothermal performance and fluorescence imaging capabilities within the NIR region, thereby enabling simultaneous tumor diagnosis and therapy. These N-CDs display exceptional biocompatibility and achieve impressive photothermal conversion efficiencies of 31.25% and 27.12% under 808 nm and 1060 nm laser irradiation, respectively, with corresponding temperature changes of 42.8 ℃ and 39.7 ℃ in vitro. Notably, the N-CDs exhibit a strong fluorescence emission peak at 660 nm, approaching the NIR-I window, which facilitates high-contrast bioimaging. In vivo studies confirmed the therapeutic efficacy of N-CDs, demonstrating cancer cell ablation under both 808 nm and 1060 nm laser irradiation, coupled with accurate tumor localization capabilities. The unique combination of intense fluorescence emission, exceptional photothermal conversion efficiency, and outstanding biocompatibility positions these N-CDs as a highly promising theranostic platform for integrated cancer diagnosis and treatment. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12951-025-03497-6.