One‑Step Synthesis of Nitrogen‑Doped Hydrophilic Mesoporous Carbon Spheres from Chitosan for Enhanced Hydroxycamptothecin Delivery

Introduction

Mesoporous materials—including silica, zeolites, and carbon—have become indispensable in biomedicine due to their high surface area, robust pore structures, and chemical stability. Among them, mesoporous carbon stands out for its superior surface area, pore volume, and thermal resistance, making it an attractive drug carrier for hydrophobic antitumor agents such as camptothecin, doxorubicin, and paclitaxel.

Conventional carbon precursors like phenolic resins and sucrose raise environmental concerns or entail costly, multi‑step syntheses. Moreover, carbon from these sources often exhibits poor hydrophilicity, limiting their application in injectable or systemic delivery. Post‑synthetic nitrogen doping or acid oxidation can enhance hydrophilicity but typically damages the pore structure or adds complexity.

Chitosan—a naturally abundant, biocompatible polysaccharide rich in –OH and –NH₂ groups—offers a dual role as both carbon and nitrogen source. Previous studies using chitosan and F127 have produced mesoporous carbons with wide pore distributions and low surface areas. To address these limitations, we introduce TEOS as a secondary template that, through hydrolysis and polycondensation, forms Si–OH groups that interact with chitosan’s functional groups, stabilizing the porous network during carbonization.

This study details a spray‑drying, one‑step synthesis of nitrogen‑doped, hydrophilic mesoporous carbon spheres (NMCs‑x/3). We systematically vary the carbon‑to‑silicon (C/Si) ratio to tune pore structure, nitrogen content, and hydrophilicity, and we evaluate their capacity to load and release the poorly soluble anticancer drug hydroxycamptothecin.

Materials and Methods

Raw Materials and Reagents

Triblock copolymer F127 (Mw ≈ 12 600, Sigma‑Aldrich), TEOS (Aladdin), chitosan (≥ 95 % deacetylation, Aladdin), HCPT (Chengdu Yuancheng), and standard laboratory reagents were used as received. Deionized water was employed throughout.

Preparation of NMCs

Chitosan solutions (2.1 % w/w) were prepared by dissolving 7.0, 8.4, 9.8, or 11.2 g of chitosan in 5 % acetic acid at 40 °C. In parallel, 2.1 g of F127 was dissolved in 50 mL ethanol, to which 15.6 mL TEOS and 15 mL 0.2 M HCl were added. After 10 min, the mixture was combined with the chitosan solution and stirred for 60 min, then rested for 60 min. Spray drying (inlet 170 °C, flow 3.5 mL min⁻¹) yielded a composite powder (CS/SiO₂/F127). The powder was calcined in a nitrogen atmosphere: 400 °C (2 h) to remove F127, followed by 900 °C (2 h) to carbonize the matrix. Subsequent immersion in 1 M NaOH at 85 °C removed silica, producing the final NMCs‑5/3, ‑6/3, ‑7/3, and ‑8/3.

Characterization

BET surface areas and pore size distributions were measured by N₂ adsorption (Micrometrics ASAP2020). Elemental composition (C, H, O, N) was determined by elemental analysis. Thermal evolution was monitored via TGA (Netzsch STA 449C). XRD patterns (Bruker D8, CuKα) revealed amorphous carbon signatures. TEM (FEI Tecnai G2) visualized morphology. XPS (Thermo Scientific Escalab 250XI) identified nitrogen species. Water contact angles were recorded with a Dataphysics OCA25.

HCPT Loading and Release

HCPT was dissolved in ethanol (0.2–1.2 mg mL⁻¹) and mixed with 20 mg of each NMC at 37 °C for 24 h. The drug content was quantified by UV–Vis (λ = 385 nm). Release studies employed dialysis against PBS (pH 7.4 or 5.0, 0.1 % Tween‑80) at 37 °C, sampling every 1–12 h. Release kinetics were fitted to the Ritger–Peppas model.

Results and Discussion

Thermal Behavior and Calcination Strategy

TGA of the CS/SiO₂/F127 composite shows ~55 % weight loss below 500 °C, mainly from F127 and chitosan pyrolysis, with a plateau above 800 °C confirming complete carbonization. Holding at 400 °C for 2 h efficiently removes F127, while a 900 °C final step preserves nitrogen and enhances graphitization.

Pore Structure and Morphology

BET analysis reveals that the NMCs‑x/3 series display a maximum surface area (2061.6 m² g⁻¹) and pore volume (0.77 cm³ g⁻¹) at a C/Si ratio of 7:3. The pore size distribution (2.01–3.65 nm) aligns well with the dimensions of HCPT, facilitating efficient loading. TEM images confirm spherical particles < 1 µm with worm‑like mesoporosity.

Composition and Nitrogen Doping

FTIR and XPS confirm the presence of pyridinic, pyrrolic, and quaternary nitrogen species. Nitrogen content decreases from 6.04 % (NMC‑5/3) to 4.75 % (NMC‑8/3), correlating with increasing C/Si ratio. The D/G Raman ratio follows the same trend, indicating higher disorder at higher nitrogen loadings.

Hydrophilicity

Dynamic contact‑angle measurements show that NMC‑5/3 reaches < 20° within 0.45 s, markedly faster than NMC‑8/3 (2.71 s). Enhanced surface nitrogen and roughness promote hydrogen bonding with water, improving wettability—a critical feature for injectable formulations.

HCPT Adsorption and Release

Adsorption isotherms fit the Langmuir model, yielding a maximum loading of 1013.51 mg g⁻¹ for NMC‑5/3. The loading capacity order matches nitrogen content. XRD of HCPT‑loaded samples shows loss of crystalline peaks, indicating an amorphous drug state that accelerates dissolution. Release studies demonstrate that 86.7–93.8 % of HCPT is released within 12 h at pH 7.4, with a slower release at pH 5.0, suitable for tumor microenvironments. Higher nitrogen content slows release due to stronger drug–carrier interactions, offering a tunable release profile.

Conclusion

We have established a scalable, one‑step synthesis of nitrogen‑doped, hydrophilic mesoporous carbon spheres from chitosan, F127, and TEOS. The optimal C/Si ratio (7:3) yields the highest surface area, pore volume, and nitrogen content, translating into superior HCPT loading and controllable release. The materials’ enhanced hydrophilicity and tunable drug kinetics position them as promising carriers for poorly soluble antitumor drugs.