Sensitive and Selective Near-Infrared Fluorescent Off–On Probe and Its Application to Imaging Different Levels of β-Lactamase in Staphylococcus aureus

A highly sensitive and selective near-infrared (NIR) fluorescent probe, (E)-2-(2-(6-((2-carboxy-8-oxo-7-(2-phenylacetamido)-5-thia-1-azabicyclo[4.2.0]oct-2-en-3-yl)methoxy)-2,3-dihydro-1H-xanthen-4-yl)vinyl)-3,3-dimethyl-1-propyl-3H-indol-1-ium (1), is developed for the determination of β-lactamase. The probe is designed and synthesized by incorporating the specific substrate (cephalosporin) of β-lactamase into a stable hemicyanine skeleton. The fluorescence of 1 itself is very weak due to the alkylation of the hydroxyl group of the hemicyanine fluorophore; however, β-lactamase can selectively react with its substrate (β-lactam ring) in the probe, thereby causing a spontaneous fragmentation. This action leads to the release of the fluorophore and a large fluorescence enhancement at 707 nm (λex = 680 nm). Under the optimized conditions, the fluorescence response of probe 1 is directly proportional to the concentration of β-lactamase in the range of 0.05–2 nM, with a detection limit of 0.02 nM. The validity of the probe has been confirmed by determining β-lactamase in human urine samples in comparison with that determined by iodimetry. Moreover, by taking advantage of its high sensitivity and NIR emission feature, the probe has also been utilized to image β-lactamase in three types of Staphylococcus aureus, including methicillin-resistant S. aureus ATCC BAA44, penicillin-resistant strain ATCC 11632, and penicillin-susceptible strain ATCC 29213, which clearly reveals the significantly different expression levels of β-lactamase in these S. aureus.