the spectra of mvpCu (λabs = 676 nm) and t-mvpCu (λabs = 754 nm) were sufficiently resolved, meaning we would be able to distinguish between the two species. Next, we tested the responsiveness of mvpCu to Cu(I) treatment. From a concentration of 0 to 100 µM, we observed a dose-dependent response (Figure ). After 3 hours, probe activation was complete, and the corresponding turn-on response was 25.8 ± 6.3-fold. Finally, we evaluated potential interference from other metal ions including alkali, alkaline earth, and transition metals. Under no circumstance did we observe interference (Figure ). Finally, mvpCu and t-mvpCu (30 µM) were embedded in a tissue-mimicking phantom and imaged at the λPA for the probe (670 nm) and turnover product (765 nm) using PA (Figure ). Additional characterization, including MTT assays can be found in the SI (Figure ).