3. NIR materials for MPTT
According to different properties, PTAs used in MPTT can be divided into organic agents and inorganic agents. Organic PTAs include NIR cyanine dyes, donor-acceptor (D-A) structured small molecules, semiconductor polymer NPs (SPNs).[23, 78-80] Inorganic PTAs include noble metal materials, metal sulfide materials, metal oxide materials, quantum dots (QDs), carbon-based nanomaterials (such as graphene and carbon nanotubes) and other two-dimensional (2D) materials (such as black phosphorus, boron nitride and graphite carbon nitride, Mxenes), etc.[81-83] Organic PTAs may have the advantages in biodegradability and biocompatibility, while inorganic PTAs usually present the higher PCE and better photothermal stability.
3.1. Organic agents
3.1.1. NIR cyanine dyes
Cyanine dyes, anthocyanin series of fluorescent dyes, are a class of synthetic fluorescent dyes with polymethyl bridge chain chemical structure. Cyanine dyes have a very high extinction coefficient, and the length of the bridge chain and the chromophores at both ends directly control the absorption peak and emission peak of the dyes.[84-86] There are many kinds of cyanine dyes, which can be divided into cyanine, partial cyanine and multinucleated cyanine according to the conjugated system of chromophore. According to the types of heterocyclic nuclei they contain, they can be divided into sulfur carbocyanine, oxygen carbocyanine and indocarbocyanine. According to the length of multiple methyl chains, it can be divided into first methyl cyanine, second methyl cyanine (carbon cyanine), fifth methyl cyanine (two carbon cyanine), seventh methyl cyanine (three carbon cyanine), etc. By adjusting the length of the conjugated chain, the spectrum of cyanine can extend from the visible region to the near infrared region.[30, 87-90] Background interference can be effectively eliminated by using the red region measurement, and more ideal analytical sensitivity and selectivity can be obtained. Cyanine dye spectrum is very sensitive to the change of external microenvironment, which is very suitable for the analysis of biological and environmental samples. Yang et al. developed a straightforward approach for creating modified one-dimensional nanoscale coordination polymers (1D-NCPs) using poly(ethylene glycol) (PEG), which exhibit inherent biodegradability, a substantial surface area, pH-responsive behaviors, and versatile theranostic capabilities. Taking the instance of NCPs containing Mn2+ and indocyanine green (ICG), the researchers demonstrated the fabrication of Mn-ICG@pHis-PEG as a carrier for Gambogic acid (GA), a natural inhibitor of HSP90 that plays a crucial role in cellular resistance to heat-induced damage. Upon NIR triggered heating, the Mn-ICG@pHis-PEG/GA nanocomplex achieved MPTT at around 43 °C, demonstrating exceptional efficacy in tumor ablation.[91] In another study, Wu et al. introduced a versatile and biodegradable hollow mesoporous organosilica nanocapsule platform (HMONs), which was ingeniously capped with gemcitabine (Gem) molecules using a pH-sensitive acetal covalent linkage. HMONs showcased a favorable release of small molecules specifically at tumor tissue/cellular sites, thereby mitigating the risk of drug accumulation. Upon encapsulating ICG and the HSP90 inhibitor 17AAG, the resulting construct denoted as ICG−17AAG@HMONs−Gem−PEG demonstrated MPTT with an operating temperature of around 41 °C, accompanied by exceptional efficacy in tumor ablation.[92] Jiang et al. presented a groundbreaking study featuring a novel nanoscale coordination polymer centered around a heptamethine cyanine dye, termed Hf-HI-4COOH. This compound emerged as an exceptionally potent photosensitizer, particularly tailored for nucleus-targeted MPTT. Notably, Hf-HI-4COOH exhibited robust NIR absorption characteristics and boasted an efficient PCE of 39.51%.[93] The formulation’s prolonged circulation within the bloodstream, augmented accumulation at tumor sites, and its nucleus-targeting attributes collectively contribute to a substantial suppression of breast tumors in murine models subjected to MPTT, all without apparent toxicity. Shi et al. employed poly(lactic-co-glycolic acid) (PLGA) as a carrier, loaded with a NIR photothermal agent (IR820), a fluorescence imaging agent (ZnCdSe/ZnS QDs), and an autophagy inhibitor (chloroquine, CQ) to form PLGA/IR820/ZnCdSe/ZnS co-loading NPs (PIFC NPs). The effective intracellular delivery of CQ molecules via PIFC NPs plays a pivotal role in attenuating the degradation of autophagic lysosomes within tumor cells (Figure 1).[94] This, in turn, effectively inhibits the autophagy-mediated repair process of cells damaged by MPTT. Notably, the modulation of autophagy sensitizes MPTT, presenting a novel avenue for future cancer treatment research.