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Conjugated polymers (CPs) for photothermal therapy

PTT utilizes photothermal transduction agents (PTAs) which can transform light energy to heat through photothermal effect to raise the temperature of tumor area and thus cause the ablation of tumor cells. [1, 5] Specifically, ideal PTAs should have high photothermal conversion efficiency (PCE), excellent optical stability and biocompatibility, and strong light adsorption in the near-infrared (NIR) region (750-1350nm) due to the deep-tissue penetration and minimal absorption of NIR light in the biological tissues. [1, 5] PTAs mainly include inorganic materials and organic materials. [5] Inorganic PTAs, such as noble metal materials, carbon-based nanomaterials, and other 2D materials, have high PCE and excellent photostability [5], but they are not biodegradable and thus have potential long-term toxicity in vivo [6]. Organic PTAs including small molecule dyes and conjugated polymers (CPs) have good biocompatibility and biodegradability, but poor photostability [5]. Among them, small molecule dyes, such as cyanine, porphyrin, phthalocyanine, croconaine, are limited in the field of cancer treatment because of their susceptibility to photobleaching and poor tumor enrichment ability [5].

In the recent few years, researchers have found that conjugated polymers with large π−π conjugated skeleton and a high electron delocalization structure, if appropriately functionalized, could serve as encouraging organic PTAs for PTT due to their strong NIR absorption, excellent photostability, low cytotoxicity, outstanding PCE, good dispersibility in aqueous medium, increased accumulation at tumor site, and long blood circulation time. [1, 5, 6, 7] Moreover, conjugated polymers can be easily combined with other imaging agents and drugs to construct multifunctional nanomaterials for selective and synergistic cancer therapy. [1] To date, the CPs used for tumor PTT mainly include polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), polydopamine (PDA), donor−acceptor (D-A) conjugated polymers, and poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS). [1, 5]. These CPs are introduced below.