Prior to testing SWIR-HD in vivo, we performed two crucial experiments to assess biocompatibility. The substitution of oxygen with other group 16 elements (S and Se) or the installation of halides have been shown to facilitate intersystem crossing, which leads to singlet oxygen production upon exposure to light. This property would be a detriment to any multimodal contrast agent owing to unwanted phototoxicity. When SWIR-HD was irradiated in the presence of DPBF, a colorimetric singlet oxygen sensor, the absorbance from 410-420 nm remained unchanged after 300 and 600 seconds (Figure ). This result indicates the inclusion of the SiMe2 group does not result in singlet oxygen production, which is important to avoid ROS-mediated damage. In addition, we performed a standard MTT assay in HT29, HCT116, and A549 cancer cells to evaluate potential cytotoxicity when imaging with SWIR-HD. No loss of cell viability was observed when cells were incubated with SWIR-HD for 4 hours up to a concentration of 6 µM (Figure ).