A homemade wide-field cryo-imaging testing system was employed to simultaneously measure the redox ratio and sample surface temperature, as shown in Fig. 1. An illumination unit combined light from 365- and 455-nm light-emitting diodes (Edmund Optics, Barrington) together to provide shadow-free, cool, uniform, and direct illumination of the sample surface. NADH and FAD fluorescence signals were collected with two emission filters (, , Edmund Optics) and detected by a CCD camera (Retiga 4000DC, Surrey and Canada, Qimaging, mounted on an Olympus SZX10 microscope). The sample was mounted vertically in a customized sample box filled with liquid nitrogen. The block face of the sample was obtained by horizontal cutting and kept above the liquid nitrogen level to ensure image quality during the experiment. A temperature sensor (Pt100, Beijing and China, BJZYHS) directly contacted the sample surface to record the surface temperature in real time. As liquid nitrogen in the sample box vaporized, the sample temperature gradually increased. NADH and FAD images were captured alternately at different temperatures and the real-time temperature of the sample surface was simultaneously recorded.