An Fe(III)-Based Fluorescent Probe for Carbon Monoxide only Senses the “CO Donor” Used, CORM-3, but Not CO

Because of the increasing interests in carbon monoxide (CO) as an endogenous signaling molecule, there have been extensive efforts in developing fluorescent probes for CO. In doing so, metal-carbonyl complexes named "CO-releasing molecules" (CORMs) are often used as CO surrogates. The most widely used CORM-2 and CORM-3 are chemically reactive Ru(II) complexes; release minimal or no CO unless in the presence of a strong nucleophile or a reducing agent; and do not function as reliable CO donors. As a result, some reported CO fluorescent probes only detect the CORM used, not CO. Recently, an Fe(III)-fluorophore complex, RBF-Fe(III), has been reported to sense CO using CORM-3 as a CO surrogate. The proposed mechanism involves CO binding to Fe(III). Because of the known affinity of CO for only Fe(II), but not Fe(III), we were intrigued by the report. Re-evaluation work found fluorescence changes of RBF-Fe(III) by CORM-3, but not CO itself. Furthermore, sodium ascorbate and cysteine were found to induce fluorescent changes of the RBF-Fe(III) system. Moreover, RBF-Fe(III) was found to be unstable and to change fluorescence with time or agitation. Regardless of whether it was under N₂, CO, or vacuum, vigorous stirring induced the same level of fluorescence changes, presumably due to precipitation or aggregation of Fe(III) species, which is consistent with literature findings of Fe(III) behaviors. Such results mean that the RBF-Fe(III) system does not sense CO and underscore the need to exercise extra cautions when chemically reactive CO donors are used in developing CO probes.