Research progress of mild photothermal therapy in cancer treatment

Cancer incidence and mortality are rapidly growing around the world. As a noninvasive strategy for cancer treatment, photothermal therapy (PTT) utilizes near-infrared (NIR) light absorbents to produce local hyperpyrexia from light energy, resulting in the thermal elimination of tumors. Different nanostructures that possess strong absorbance of NIR light have been exploited as photothermal agents (PTAs) that need to be biodegradable and have strong optical absorbance in the NIR optical window. However, it remains highly challenging to identify the small molecule-based photothermal agents with a high photothermal conversion efficiency (PCE). Therefore, various types of PTAs have been developed in the last decades. These PTAs usually absorb NIR light where the living cells exhibit no absorption and autofluorescence and are able to transit absorption light to heat energy. Based on previous reports, the synthesized PTAs can be classified into two types: Inorganic nanomaterials and organic small molecular nanomaterials. Recently, for the first time, we demonstrated the synthesis of two dimensional (2D) monoelemental germanene quantum dots (GeQDs) through a facile liquid exfoliation method for photonic cancer nanomedicine. Our research can solve the major problem that 2D materials are difficultly synthetized and traditional materials have low photothermal conversion efficiency. Currently, photothermal therapy is a new type of highly effective, non-invasive and highly targeted therapy for cancer, however, the high temperature induced by PTT may also cause serious damage to healthy tissues. Therefore, mild photothermal therapy (MPTT) with relatively low temperature has attracted more and more attention. Due to the limited antitumor effect of MPTT, it is usually combined with other strategies to synergistically eliminate cancer cells. MPTT nanoplatform with enhanced curative efficacy and lower side effect is promising to apply in the field of drug controlled release, imaging-guided cancer treatment, and integrated therapy with other anticancer approaches. In this review, we briefly introduce the types and characteristics of PTAs, their anticancer function and the combined strategies for cancer therapy. Finally, we discuss the role of MPTT as a promising new therapeutic strategy for cancer therapy.