Tetracarboxy porphyrins can be polymerized with polyethylene glycol (PEG) diamines to generate hydrogels with intense, near-infrared, and transdermal fluorescence following subcutaneous implantation. Here, we show that the high density porphyrins of the preformed polymer can be chelated with tin via simple incubation. Tin porphyrin hydrogels exhibited increasing emission intensities, ratios, and lifetimes from pH 1 to 10. Tin porphyrin hydrogel emission was strongly reversible and pH responsiveness was observed in the physiological range between pH 6 and pH 8. pH-sensitive emission was detected via noninvasive transdermal implantation in mice. fluorescence imaging in vivo following subcutaneous