Photo-active carbon nitride and gold nanostuctures as autononomous and magnetically augmented entities for therapeutic & imaging applications

Abstract

Nanomaterials in isolation or in combination with unique photophysical & magnetic properties have been explored as multifunctional theranostic applications. The first part delineates the synthesis, characterization and application of graphitic carbon nitride quantum dots (g-CNQDs) as photodynamic agent for both anti-bacterial and anti-cancer applications. Graphitic carbon nitride (g-CN, commonly called as g-C 3 N 4 ) quantum dots exhibiting excellent photoluminescence behavior were synthesized by two-step thermal polymerization and selective dialysis approach yielding particle size in the range of 2 to 7 nm with a large specific surface area of 264 m 2 /g. XPS and FTIR analysis confirmed the presence of active functional groups such as aromatic C-N and C=N-C moieties, unique to carbon nitride. The fluorescent g-CNQDs could efficiently generate superoxide and hydroxyl radicals under visible light irradiation, inhibiting ~99% of Gram negative E.coli and ~90% of Gram positive S. aureus at a concentration of 0.1 mg/mL. A strong correlation between the TEM, SEM and confocal microscopic analysis is observed demonstrating that g- CNQDs cause light induced photo toxicity. The g- CNQDs were found to be non- toxic to fibroblast cells upto a concentration of 3.2 mg/mL. A comprehensive comparison of g-CNQDs with bulk non-porous g-CN, mesoporous CN, silver loaded mesoporous CN and bare silver nanoparticles indicated them to be promising photo induced bactericidal material. Subsequently, the metal-free photocatalytic g-CNQDs were explored for effective killing of cancer cells under visible blue light excitation. As part of in vitro optimization of the photodynamic therapy, various parameters such as light source, time of exposure and cellular uptake were studied. Acute toxicity studies in C57BL/6 mice did not depict any profound histopathological lesions or organ damage 24 hrs post injection via intravenous route at a dose of 20mg/Kg. Towards augmenting the photo-responsive property of g-CN with additional functionality, mesoporous magnetic nanohybrid functionalized with 14 wt% CN and loaded with curcumin was developed as a combination platform for photodynamic therapy and magnetic hyperthermia. CN-Curcumin complexes on the nanohybrid surface facilitate fast charge separation of hole-electrons under blue LED light irradiation and subsequent singlet oxygen generation. Cur release from the nanoparticle was significant only when exposed to both lysosomal pH (pH=5.2) and an alternating current magnetic field (AMF). The mesoporous magnetic carbon nitride (MMCN) caused a 350% increase in the level of intracellular ROS as compared to the light exposed untreated control group. The nanohybrid was non-hemolytic and found to be biocompatible with HUVEC cells at concentrations up to 360 µg/mL. A similar concentration under AMF exposure caused a localized temperature rise of 4.2 °C and resulted in a 60% reduction in C6 glioma cell viability. The cancer cell death further increased up to 80% under sequential exposure to light and AMF. The combinatorial treatment exerted significant cytoskeletal and nuclear damage in the cancer cells as assessed by confocal microscopy. The nanohybrid also exhibited relaxivity of 88 mM -1 s -1 , imparting significant T 2 weighted contrast to the cancer cells. The final two chapters elaborate the evolution of biodegradable water stable nanoassemblies encompassing ultra-small iron oxide nanoclusters and their further surface functionalization with ultra-small gold nanoparticles for photothermal radiosensitization applications. Disintegrable nanoassemblies with gold seed or ICG conjugated gold nanoclusters coating with core having iron oxide crystallites of size 5-10 nm were prepared for photothermal radiotherapy and magnetic resonance & photoacoustic imaging applications. The photo- responsive nanoassemblies possessed strong absorption in near infra red region either due to plasmon coupling between closely positioned gold seeds or because of the dye, ICG. NIR laser triggered temperature rise of ~8.5 o C and ~13.5 o C from physiological temperature was observed for gold seed and ICG functionalized nanoassemblies, respectively. The nanoassemblies were found to disintegrate at lysosomal pH of 5.2 and proteolytic environment. With an acute increase of over 300% to 400 % intracellular reactive oxygen species soon after γ- irradiation and assistance from Fenton reaction mediated supplemental oxidative stress, nanoassemblies in conjunction with radiation killed over 50% of hepatoma cells. The cell viability further decreased to less than 10% when they were sequentially exposed to NIR laser followed by γ-irradiation. The nanoassemblies were also ascertained as contrast agents for both optical and magnetic resonance imaging.