Photodynamic therapy (PDT) is a clinical treatment in which a light-absorbing drug called a photosensitizer is combined with light and molecular oxygen to generate cytotoxic singlet oxygen. PDT provides additional tissue selectivity versus conventional chemotherapy in that singlet oxygen is generated only in areas with both accumulated PS and simultaneous illumination by a light source with sufficient irradiance and dose. Early photodynamic therapy beacons built on this concept by adding an analyte-responsive element that simultaneously "turns-on" PDT and fluorescence, providing both an additional layer of selectivity and real-time feedback of the PS's activation state. More recent photodynamic therapy beacons have expanded this idea, with new methods now available for sensing analytes, generating singlet oxygen, and reporting treatment status. In this review, we consider developments in advanced activation strategies implemented in therapeutic and theranostic beacons.

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