Regulation of enzyme activity is fundamentally challenging but practically meaningful for biology and medicine. However, noninvasive remote control of enzyme activity in living systems has been rarely demonstrated and exploited for therapy. Herein we synthesize a semiconducting polymer nanoenzyme with photothermic activity to digest collagen for enhanced cancer therapy. Upon near-infrared (NIR) light irradiation, the activity of the nanoenzyme can be enhanced by 3.5-fold to efficiently digest collagen in tumor extracellular matrix (ECM), leading to enhanced nanoparticle accumulation in tumors and consequently improved photothermal therapy (PTT). This study thus provides a promising strategy to remotely regulate enzyme activity for cancer therapy. Controlling of enzyme activity in living systems at designated locations and times helps understand their underlying physiological functions and potentially could lead to new medicines. However, there are only a few approaches to remotely regulate enzyme activity. For instance, chemical modification of enzyme using biomarker-responsive segments presents a way to reversibly deactivate and restore enzyme activity. The use of conditional protein splicing system is another way to generate functional enzymes from inactive fragments in the presence of ligands. Local hyperthermia under an alternating magnetic field has also been used to increase the activity of thermophilic enzymes for biocatalysis. In contrast to these approaches, light activation seems to be a more ideal noninvasive method to control enzyme activity because of its simpler operability, better controllability and higher spatiotemporal resolution. Currently, light activation of enzymes is generally relied on ultraviolet (UV) and visible light, which has shallow tissue penetration and thus limited in vivo applications. Semiconducting polymer nanoparticles (SPNs) with controllable optical properties have been exploited for optical imaging and phototherapy. Particularly, they can efficiently convert light into heat for photothermal therapy (PTT) and photoacoustic (PA) imaging. In addition, the high photothermal ability of SPNs allows them to serve as nanotransducers to remotely control gene expression and thermosensitive ion channels in living systems. However, such photothermal feature of SPNs has not be exploited to control enzyme activity so far. In this study, we report the synthesis of semiconducting polymer nanoenzymes with near-infrared (NIR) photothermic activity and demonstrate their proof-of-concept application in insitu and on-demand activation for enhanced cancer therapy in living mice. Such nanoenzyme contains two key components: semiconducting polymer amphiphile and bromelain (Bro), serving as photothermal nanotransducer and temperaturesensitive enzyme, respectively. Bro (Mw = 33000) is a protease that proficiently digests collagen, and its optimal activity is ~45 oC. Thus, under NIR laser irradiation, the semiconducting polymer nanoenzymes can undergo efficient photothermal conversion (Figure 1), leading to the local temperature increase and thus the photothermic activation of collagen digestion. As collagen is the most abundant tumor extracellular matrix (ECM) protein, such a photothermally-triggered collagen digestion can enhance the accumulation of nanoparticles in the tumor, enabling improved PTT. Figure 1. Mechanistic illustration of photothermally triggered enzyme activation of PCB1-Bro towards collagen digestion for enhanced accumulation of nanoparticles in tumor. To construct the semiconducting polymer nanoenzymes, two semiconducting polymer amphiphiles (PCB1 and PCB2) were synthesized via a grafting-on approach (Table S1 and Figures S1-2, Supporting Information). PCB1 was grafted with the short chain methoxy-polyethylene glycol (PEG) (Mw, 1000) and the long chain carboxyl-PEG (Mw, 2000); whereas, PCB2 was grafted with both long chain methoxy-PEG (Mw, 2000) and long chain carboxyl-PEG (Mw, 2000). The percentage of carboxyl-PEG in both PCB1 and PCB2 was optimized to 20%. The resulted semiconducting polymer amphiphiles were further covalently linked to Bro (Figure 2a), affording PCB-Bro. Bradford protein assay revealed that each PCB1 particle had approximately 4 enzyme molecules with the enzyme/PCB weight ratio of 73.2% (the coupling efficiency of 14.6%); in contrast, the weight ratio was much low (