N-doped carbon dots for dual-modality NIR fluorescence imaging and photothermal therapy

Abstract

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.

Keywords: Bioluminescence imaging; Carbon Dots; Near-infrared II window; Photothermal therapy; Tumor elimination.

Fig. 1

Schematically illustration of preparation and tumor ablation of N-CDs

Fig. 2

(a) Transmission Electron Microscopy (TEM) image of N-CDs. (b) Particle size distribution curve for N-CDs. (c) Fourier Transform Infrared (FTIR) spectrum of N-CDs. (d-f) XPS spectra of N-CDs, C element (d), O element (e), N element (f). (g) UV-vis-NIR spectrum of N-CDs. (h) Fluorescence emission spectrum of N-CDs. (i) Fluorescence decay curve of N-CDs in aqueous solution

Fig. 3

(a-d) Photothermal heating curves of N-CDs, showing the effect of different concentrations (a-b), different powers (c-d). (e) Linear relationship between -lnθ and time. (f) Effect of heating of N-CDs under the thermal imaging camera. (g) Ten photothermal cycles of N-CDs (1.0 mg·mL^− 1) under 1060 nm laser illumination (1.0 W·cm^− 2)

Fig. 4

(a) Viability of DU145 cells incubated with solutions containing different concentrations of N-CDs under 808 nm and 1060 nm laser irradiation. (b) Uptake of N-CDs by DU145 cells incubated in PBS (control group) and N-CDs solution (experimental group). (c) Apoptosis imaging of DU145 cells incubated in PBS (control group) and N-CDs solution (experimental group) under dark conditions, with 1060 nm and 808 nm laser irradiation. (d) Live/dead imaging of DU145 cells incubated in PBS (control group) and N-CDs solution (experimental group) under dark conditions, with 1060 nm and 808 nm laser irradiation. (e) H&E staining of major organs (heart, lung, kidney, liver, and spleen) from healthy mice 1, 7, and 14 days after intravenous injection of PBS and N-CDs. (f)-(j) Values of major functional factors (alanine aminotransferase, ALT; alkaline phosphatase, ALP; aspartate aminotransferase, AST) in the liver, serum creatinine (CR), and urea (UREA) in healthy mice 1, 7, and 14 days after intravenous injection of PBS and N-CDs

Fig. 5

(a) Fluorescence visualization of DU145 tumor-bearing nude mice subsequent to the administration of the injection of N-CDs at corresponding time points, with the red circle indicating the tumor area. (b) Fluorescence photographs of the major organs and tumor in DU145 tumor-bearing nude mice taken 24 h post-injection with N-CDs, along with the fluorescence intensities. (c) Fluorescence intensity values at the tumor site of Nude mice with DU145 tumors at various time intervals following the injection of N-CDs. (d) Infrared thermograms of the tumor sites in DU145 tumor-bearing nude mice from the control and experimental groups under 808–1060 nm (1.0 W·cm^− 2) irradiation at different time intervals, and (e) the resultant temperature increase. (f) Weight change curves of tumor-bearing nude mice over 14 days under different treatments. (g) Tumor volume change curves of tumor-bearing nude mice over 14 days under different treatments. (h) Schematic illustrations of tumors in different groups of tumor-bearing nude mice after 14 days