Exfoliated Graphene for Photothermal Biomedical Applications

Abstract

When a material’s size is reduced to the nanoscale, it is well understood that in addition to the dramatically increased surface area to volume ratio, new properties emerge distinct from those present in the bulk material. When graphite is exfoliated down to graphene, several altered and highly attractive properties emerge which may enable new photothermal roles as well as allowing significant improvements to existing strategies. One such nanosize tuning method that is highly suitable for layered materials is surfactant assisted liquid exfoliation of the bulk material to single and few layered sheets. Successfully isolated graphene presents a range of remarkable properties such as an unsurpassed thermal conductivity of ≈ 5000 W m-1 K-1,1-5 a very high mechanical strength with a Young’s modulus of ≈ 1 TPa,6-7 a broad optical absorptivity with a transmission of 97.7 % independent of wavelength in the visible spectrum (and a large portion of the infrared),8-9 as well as an ultrahigh electron mobility of ≈ 200,000 cm2 V-1 s-1.10-12 The development of photothermal agents for a range of biomedical applications is an area of huge interest and promise with high impact outcomes possible. In this study, single and few layer graphene has been explored for use as a photothermal agent for a range of biomedical roles such as thermal ablation of cancerous cells and photothermally controllable drug release. With this focus, several biomedical photothermal applications were explored including the thermal ablation of cancerous glioma-neuroblastoma cells through photothermal conversion at the target site by the graphene microplates. By exploiting the significant absorption of graphene in the near-infrared, substantial amounts of energy can be delivered deep within biological tissue allowing a highly-localized region of dramatic heating to be achieved resulting in tissue ablation and cell death. 7 This study also explores the use of graphene as a photothermal trigger to activate the controlled release of drug payloads in three different carrier systems. These systems include a graphene loaded (and stabilised) Pickering emulsion carrier with both oil in water and water in oil types possible. A photothermal coring of the emulsions was successfully achieved demonstrating the photothermally induced collapse of the emulsion. Several graphene entrained lipid nanocarrier systems were explored with near-infrared activation inducing phase transitions. Small angle X-ray scattering was used to dynamically monitor photo-activated, reversible phase transitions. An injectable hybrid graphene-surfactant-α-cyclodextrin thermoreversible gel system with graphene as an intrinsic component was also explored within this study. Photothermal drug release switching and controllable release rates were demonstrated with this biocompatible carrier system showing a highly versatile photothermally activatable drug depot. Graphene is a material well suited for photothermal biomedical applications particularly when prepared via liquid exfoliation. This study explores the interactions of specific light energies with surfactant assisted liquid exfoliated graphene for photothermal applications and shows that graphene has a high transduction efficiency, is thermally stable and is intrinsically suitable towards stealth strategies, suggesting that graphene could be a significant addition to a range of photothermal biomedical applications.