Alkyl Chain Appended Fe(III) Catecholate Complex as a Dual-Modal T1 MRI–NIR Fluorescence Imaging Agent via Second Sphere Water Interactions

The C 12 -alkyl chain-conjugated Fe(III) catecholate complex [Fe(C 12 CAT) 3 ] 3–, Fe ( C 12 CAT ) 3 [C 12 CAT = N -(3,4-dihydroxyphenethyl)dodecanamide], was synthesized and characterized, reported as a dual-modal T 1 -MRI and an optical imaging probe. The DFT-optimized structure of Fe(C 12 CAT) 3 reveals a distorted octahedral coordination geometry around the high spin Fe(III) center. The formation constant (−log K) of Fe(C 12 CAT) 3 was calculated as 45.4. The complex exhibited r 1 -relaxivity values of 2.31 ± 0.12 and 1.52 ± 0.06 mM –1 s –1 at 25 and 37 °C, respectively, on 1.41 T at pH 7.3 via second-sphere water interactions. The interaction of Fe(C 12 CAT) 3 with human serum albumin showed concomitant enhancement of r 1 -relaxivity to 6.44 ± 0.15 mM –1 s –1 . The MR phantom images are significantly brighter and directly correlate to the concentration of Fe(C 12 CAT) 3 . Adding an external fluorescent marker IR780 dye to Fe(C 12 CAT) 3 leads to the formation of self-assembly by C 12 -alkyl chains. It resulted in the fluorescence quenching of the dye, and its critical aggregation concentration was calculated as 70 μM. The aggregated matrix of Fe(C 12 CAT) 3 and IR780 dye is spherical, with an average hydrodynamic diameter of 189.5 nm. This self-assembled supramolecular system is found to be non-fluorescent and was “turn-on” under acidic pH via dissociation of aggregates. The r 1 -relaxivity is found to be unchanged during the matrix aggregation and disaggregation. The probe showed MRI ON and fluorescent OFF under physiological conditions and MRI ON and fluorescent ON under acidic pH. The cell viability experiments showed that the cells are 80% viable at 1 mM probe concentration. Fluorescence experiments and MR phantom images showed that Fe(C 12 CAT) 3 is a potential dual model imaging probe to visualize the acidic pH environment of the cells.