PDE5A and erectile dysfunction: Currently, several PDE5 inhibitors have been approved for several diseases, such as erectile dysfunction and pulmonary arterial hypertension (PAH).13,14 And recently, the role of PDE5 in idiopathic pulmonary fibrosis (IPF) has attracted much attention.15,16 It is highly desirable to develop fluorescent probes that can detect and image PDE5 in living biosystems accurately and selectively.