Beyond these excellent multimodal properties, it is essential for SWIR-HD to exhibit exceptional photoand chemo-stability to serve as a viable dye for in vivo applications. First, because the body consists of various acidic and alkaline gradients, we measured the absorbance across a pH range of 6.0 to 8.0 (Figure ). The intensity did not change substantially, indicating excellent tolerance to potential pH fluxes in the body. Next, we tested the likelihood of photobleaching by irradiating SWIR-HD at 988 nm using a pulsed laser set (1.25 MW). After 900 s, more than 95% of the dye remained (Figure ), indicating SWIR-HD would be sufficiently photostable during a typical imaging session. Lastly, we treated SWIR-HD with a panel of biologically relevant reactive oxygen and nitrogen species, as well as redox active metal ions (Figure ). Although the parent HD is prone to rapid oxidative decomposition, SWIR-HD was remarkably resistant against strong oxidants such as peroxynitrite even at levels 100,000-fold higher than the reported cellular concentration of 5-10 nM. Likewise, we observed no significant loss of SWIR-HD in the presence of redox-active metals (Fe and Cu) capable of generating ROS via Fenton and Fenton-like chemistry, respectively.