"smart" materials-based near-infrared light-responsive drug delivery systems for cancer treatment: A review

To overcome drawbacks of conventional chemotherapy for cancer treatment, stimuli-responsive drug delivery systems (DDSs) including internal and external stimuli-based cues are the potential candidates. Both internal and external stimuli-responsive behavior can be utilized for engineering of the so-called 'smart' DDSs. Drug release triggered by external stimuli is more controllable and avoid individual variability. Among them, light-responsive DDSs are more promising because of spatiotemporal control. With considerable penetration features, near-infrared (NIR) light is potential stimulus with clinical implication. There are three types of NIR-responsive DDSs (NIRDDSs) based on the mechanism, i.e., (1) photothermal effect, (2) two-photon absorption, and (3) up-converting nanoparticles (UCNPs). Photothermal effect-based DDS has been extensively studied because of their tunable optical properties and flexible surface chemistry. Carbon nanomaterials, gold nanomaterials, indocyanine green, and metallic sulfides/oxides are the commonly employed photothermal agents. Two-photon absorption-based DDSs provide higher excitation and overcome the drawback of UV/Visible light-sensitive DDSs of poor penetration. UCNPs are inorganic crystalline nanoscale particles (1-100 nm) that exhibit photon up-conversion, i.e., conversion of NIR excitation light into UV/Visible emission light and empower deeper penetration into biological samples due to reduced light scattering. In this review, we discussed different NIRDDSs. The emphasis was also given to their drug release mechanisms and applications in the treatment of cancer.