Functionalized carbon nanotubes have emerged as versatile platforms for near-infrared photoacoustic imaging and photodynamic therapy, combining diagnostic and therapeutic capabilities in a single nanomaterial system. Their unique structural and electronic properties enable efficient light absorption, drug loading, and surface modification for biomedical applications.
Carbon nanotubes exhibit strong optical absorption in the near-infrared region, particularly between 700 and 1100 nm, a biological transparency window where tissue penetration is maximized. This absorption arises from their extended π-conjugated systems, which facilitate electronic transitions that generate heat upon laser excitation. The heat produces ultrasonic waves detectable via photoacoustic imaging, allowing high-resolution visualization of deep tissues. Compared to organic dyes, CNTs offer superior photostability, resisting photobleaching under prolonged irradiation, and their absorption spectra can be tuned through functionalization or diameter adjustments.
A critical advantage of CNTs is their high surface area, enabling substantial drug-loading capacity through non-covalent interactions such as π-π stacking. Hydrophobic therapeutic agents, including chemotherapeutics like doxorubicin or photosensitizers such as chlorin e6, adsorb onto the nanotube surface without requiring chemical conjugation. This preserves drug activity while allowing controlled release triggered by pH changes or near-infrared light. Loading efficiencies often exceed 70%, with some studies reporting up to 400 mg of drug per gram of CNTs, depending on surface modifications.
Surface passivation is essential to mitigate potential toxicity and improve biocompatibility. Polyethylene glycol (PEG) coating reduces nonspecific protein adsorption and prolongs circulation time by minimizing immune clearance. Additional functionalization with targeting ligands like folic acid or peptides enhances tumor accumulation through receptor-mediated uptake. Covalent modifications, such as carboxylation or amidation, further improve dispersibility in aqueous media while providing anchor points for further bioconjugation.
Signal amplification strategies leverage the intrinsic properties of CNTs. Their high absorption cross-section enhances photoacoustic contrast, enabling detection at lower concentrations than conventional dyes. Hybrid systems incorporating gold nanoparticles or quantum dots further amplify signals through plasmonic coupling or fluorescence resonance energy transfer. Multiwavelength imaging exploits the broad absorption profile of CNTs to differentiate them from background tissue signals, improving specificity.
In photodynamic therapy, functionalized CNTs serve as both carriers for photosensitizers and enhancers of reactive oxygen species generation. The nanotubes themselves can produce singlet oxygen under laser irradiation, but their primary role is to deliver and retain photosensitizers at the target site. The large surface area prevents aggregation-induced quenching, maintaining photosensitizer efficacy. Combined with photoacoustic imaging, this enables real-time monitoring of therapeutic delivery and response.
Comparative advantages over organic dyes include longer circulation half-lives, higher payload capacity, and multifunctionality. While dyes often suffer from rapid renal clearance and limited loading options, CNTs provide stable platforms for combination therapies. Their mechanical strength and flexibility also permit additional functionalization without structural compromise. However, rigorous purification and surface engineering remain necessary to ensure reproducibility and minimize batch-to-batch variability.
Functionalized carbon nanotubes thus represent a promising theranostic tool, integrating imaging and therapy with precise control over biodistribution and activation. Continued optimization of their surface chemistry and evaluation in preclinical models will further establish their potential for clinical translation.