In particular, we evaluated iridium catalyst 9 since it exhibited a 2.7-fold faster rate of reduction of F-1 with sodium formate compared to catalyst 19 (Figure and 7D), while also exhibiting the highest relative ratio of formate to NADH reactivity in the pyridine amide complex series (Figure ), presaging its potential for formate-dependent imaging in living cells using confocal microscopy (vide infra). Indeed, while promising in vitro, catalyst 19 exhibited low sensitivity to exogenously added sodium formate in preliminary live-cell confocal microscopy experiments (Figure SI-16), consistent with an in vitro assay showing strong inhibition of sulfonamide amine-type catalyst 19 to cellular thiols such as cysteine and glutathione (GSH) (Table ). Overcoming the inhibitory activity of cellular thiols on transition metal catalysts is a challenging issue, and while Ward and co-workers have designed protein-encapsulated iridium complexes that can sterically guard a sulfonamide amine iridium complex from inhibitory thiol chelation, demonstration of this strategy in live cells remains elusive. On the other hand, Do and colleagues have shown the capacity of half-sandwich pyridine amide complexes such as Catalyst 9 to operate in live cells, and in vitro assays showed that while aldehyde reduction in the presence of cellular thiols (GSH or cysteine) is slowed, TH catalysis with sodium formate, complex 9, and fluorophore F-1 is still operative (Table and taking selectivity for formate versus NADH into account, we therefore elected to use complex 9 over 19 for subsequent live-cell imaging experiments for formate detection. Although complex 9 is slightly more reactive to NAD(P)H over formate in vitro (Figure ), bioavailable formate concentrations far exceed those of bioavailable NAD(P)H. Indeed, formate accumulates in mammalian blood in the range of 10-100 µM under physiological conditions 1 and reaches above 1 mM intracellularly. In contrast, although estimates of the cellular NADPH (50-250 µM) and NADH (100-200 µM) levels are lower or comparable to formate, the bioavailable pool is much lower, as 80% of mitochondrial NADH is estimated to be protein-bound rather than bioavailable. Moreover, formate itself is a biochemical precursor to NAD(P)H, because formate is directly consumed by the enzymes ALDH1L1/2 to generate NADPH from NADP + . Given these large differences in bioavailable biological concentrations between these hydride donors and that formate is a direct upstream source of NAD(P)H, we hypothesized that a combination of catalyst 9 and ratiometric fluorophore F-7 would provide a suitable two-component system for activity-based imaging of formate pools in living cells.