Fig. 7.

Probes that are activated by bioorthogonal chemistries in cell-based settings. (a, b) Staudinger ligation-based fluorogenic reporters are activated by reaction with azides to yield fluorescent products. The probe in (a) is initially quenched by the phosphine lone pair electrons¹⁴⁷, whereas the probe in (b) utilizes a FRET quencher that is released upon Staudinger ligation⁸³. (c) Staudinger ligation-based bioluminescent reporter that yields luciferin upon reaction with azides.¹⁴⁸ Once luciferin is released, luciferase-expressing cells catalyse the conversion of luciferin to ocyluciferin in a bioluminescent reaction that yields visible light. (d) Cu-free click chemistry-based reporter becomes fluorescent upon reaction with azides.¹⁴⁹ (e-h) Several Cu(I)-catalysed azide alkyne cycloaddition-based reporters become fluorescent when the azide or alkyne-based probe reacts with alkynes or azides, respectively.^{70, 150, 151} (i) Photoinduced tetrazole-based reporter fluoresces blue upon reaction with alkenes.¹¹⁵ (j) Tetrazine quenches BODIPY fluorescence; the conjugate becomes fluorescent when the tetrazine reacts with cyclopropene to yield a cycloadduct.¹⁵²