Graphitic carbon nitride (g-C3N4) has emerged as a promising material in biomedical applications due to its unique physicochemical properties, including excellent chemical stability, tunable electronic structure, and biocompatibility. The material’s layered structure, akin to graphene but with a bandgap, allows for versatile functionalization, making it suitable for drug delivery, photodynamic therapy, and biosensing. Its ability to absorb visible light and generate reactive oxygen species (ROS) under irradiation further enhances its utility in therapeutic applications. This article explores the use of g-C3N4 in these biomedical domains, focusing on its biocompatibility, surface modification strategies, and performance in in vitro and in vivo studies.

In drug delivery, g-C3N4 has been investigated as a nanocarrier due to its high surface area and ease of functionalization. The material’s porous structure enables efficient loading of therapeutic agents, including chemotherapeutic drugs and nucleic acids. Surface modifications with polymers such as polyethylene glycol (PEG) improve its dispersibility in physiological environments and prolong circulation time. Studies have demonstrated that g-C3N4-based systems can achieve pH-responsive drug release, leveraging the acidic microenvironment of tumor tissues. For instance, doxorubicin-loaded g-C3N4 nanosheets exhibited sustained release at pH 5.0, mimicking tumor conditions, while remaining stable at physiological pH. In vitro cytotoxicity assays using human cancer cell lines showed enhanced therapeutic efficacy compared to free drug formulations, with cell viability reductions exceeding 60% in certain cases. In vivo studies in murine models further confirmed tumor growth inhibition, with minimal systemic toxicity observed at therapeutic doses.

Photodynamic therapy (PDT) represents another major application of g-C3N4, capitalizing on its photocatalytic properties. Under visible light irradiation, g-C3N4 generates ROS, such as singlet oxygen and hydroxyl radicals, which induce oxidative stress in target cells. The material’s bandgap of approximately 2.7 eV allows activation by wavelengths up to 460 nm, making it compatible with biological windows. Researchers have optimized g-C3N4 nanostructures for PDT by engineering defect sites or doping with heteroatoms to enhance ROS generation efficiency. For example, sulfur-doped g-C3N4 demonstrated a 2.5-fold increase in ROS production compared to pristine g-C3N4. In vitro studies using breast cancer cells revealed significant light-dependent cytotoxicity, with cell death rates exceeding 80% under optimal conditions. In vivo PDT experiments in tumor-bearing mice showed marked regression of solid tumors following localized irradiation, with histological analysis confirming apoptosis in treated tissues. The lack of dark toxicity further underscores its potential for safe clinical translation.

Biosensing applications of g-C3N4 leverage its fluorescence properties and electron transfer capabilities. The material’s intrinsic fluorescence can be quenched or enhanced upon interaction with target analytes, enabling detection of biomolecules such as glucose, DNA, and proteins. Functionalization with recognition elements like aptamers or antibodies enhances selectivity. For instance, a g-C3N4-based biosensor for glucose detection exhibited a linear response range of 0.1 to 10 mM, covering clinically relevant concentrations. The limit of detection was reported as 0.05 mM, with high specificity against interfering substances. Similarly, DNA hybridization assays using g-C3N4 nanosheets achieved picomolar sensitivity, facilitating early diagnosis of genetic disorders. Electrochemical biosensors incorporating g-C3N4 have also been developed, capitalizing on the material’s catalytic activity toward redox reactions. Hemoglobin-modified g-C3N4 electrodes demonstrated efficient hydrogen peroxide detection, with a sensitivity of 0.32 μA/μM and a detection limit of 0.1 μM.

Biocompatibility is a critical factor for biomedical applications, and g-C3N4 has shown favorable profiles in both in vitro and in vivo settings. Cytotoxicity assays across various cell lines, including fibroblasts and epithelial cells, revealed no significant reduction in viability at concentrations below 100 μg/mL. Hemocompatibility studies confirmed minimal hemolysis, with rates below 5% at therapeutic doses. Long-term in vivo toxicity assessments in rodent models indicated no adverse effects on major organs, as evidenced by normal serum biochemical markers and histopathological findings. However, surface modifications remain essential to mitigate potential immune responses. PEGylation, for example, has been shown to reduce macrophage uptake and prolong blood circulation half-life to over 8 hours in murine models.

Surface modification strategies for g-C3N4 focus on enhancing colloidal stability, targeting specificity, and therapeutic efficacy. Covalent functionalization with carboxyl or amine groups facilitates conjugation with biomolecules, while non-covalent coatings with polymers improve physiological compatibility. Hybrid structures combining g-C3N4 with metals or other carbon materials have been explored to augment functionality. Gold nanoparticle-decorated g-C3N4, for instance, exhibited enhanced photothermal effects alongside PDT capabilities. Similarly, graphene oxide hybrids improved drug loading capacity by 40% compared to pure g-C3N4. These modifications are tailored to specific applications, balancing performance with safety.

In vitro and in vivo studies collectively support the potential of g-C3N4 in biomedical applications. Standardized protocols for assessing its performance include cell uptake studies, ROS quantification, and therapeutic outcome measurements. In vivo models typically evaluate biodistribution, pharmacokinetics, and treatment efficacy, with results indicating preferential accumulation in tumor tissues due to enhanced permeability and retention effects. Real-time imaging using g-C3N4’s intrinsic fluorescence has further enabled tracking of material distribution in living systems.

Despite these advances, challenges remain in scaling up production and ensuring batch-to-batch consistency. The synthesis of g-C3N4 with controlled morphology and defect density is critical for reproducible performance. Current efforts focus on optimizing thermal polymerization methods to achieve uniform nanosheets or quantum dots. Sterilization techniques compatible with g-C3N4, such as gamma irradiation, are also under investigation to meet clinical standards.

In summary, graphitic carbon nitride exhibits considerable promise in drug delivery, photodynamic therapy, and biosensing, supported by its biocompatibility and adaptability to surface modifications. Continued research into its functionalization and scalable synthesis will be pivotal for advancing its clinical applications. The material’s multifunctionality positions it as a versatile platform for next-generation biomedical technologies.