Xanthene-based near-infrared chromophores for high-contrast fluorescence and photoacoustic imaging of dipeptidyl peptidase 4

Near-infrared (NIR) chromophores with analyte tunable emission and absorption properties are highly desirable for developing activatable fluorescence and photoacoustic (PA) probes for bioimaging and disease diagnosis. Here we engineer a class of new chromophores by extending the π-conjugation system of a xanthene scaffold at position 7 with different electron withdrawing groups. It is demonstrated that these chromophores exhibit pH-dependent transition from a spirocyclic "closed" form to a xanthene "open" form with remarkable changes in spectral properties. We further develop fluorescence and PA probes by caging the NIR xanthene chromophores with a dipeptidyl peptidase 4 (DPPIV) substrate. In vitro and live cell studies show that these probes allow activatable fluorescence and PA detection and imaging of DPPIV activity with high sensitivity, high specificity and fast response. Moreover, these two probes allow high-contrast and highly specific imaging of DPPIV activity in a tumour-bearing mouse model in vivo via systemic administration. This study highlights the potential of a xanthene scaffold as a versatile platform for developing high-contrast fluorescence and PA molecular probes.