Table 1.

Selected activatable ligands toward endogenous/exogenous stimuli

Stimulus	Activatable ligand and descriptions	Chemical structure or transition process	Ref
pH	Amide bond or bridge: cleavage		68
	Acetal bond: cleavage		69
	Poly(β-amino esters): amine protonation		70
GSH	Disulfide bond: cleavage		71
	Diselenide bond: cleavage		72
ROS	1O2-activatable thioketals: cleavage		73
	H2O2-activatable ferrocene: hydrophobic to hydrophilic transformation		74
	H2O2-sensitive phenylboronic acid		75
Enzyme	MMP2-cleavable peptide PLGIAG		76
	Caspase 3/7-cleavage DEVD peptide (Asp-Glu-Val-Asp)		27
	Cathepsin B-cleavage GFLG tetrapeptide (Gly-Phe-Leu-Gly)		77
	GGT-cleavage γ-glutamyl moieties		78
Hypoxia	A cleavable p-nitrobenzyl group by nitroreductase		79
	Azobenzenes (AZO): reduction		80
	2-nitroimidazole: hydrophobic to hydrophilic transformation		81
Light	Photocleavable linker (PCL)		82
	Photolysis of O-nitrobenzyl ester		83
	Light-cleavable coumarin ester		84
US	pMEMA: poly(methoxyethyl methacrylate)		85
	Indirect breakage of thioketal bonds with the aid of a sonosensitizer		86
	Cleavage of ACVA C-N bonds		87
Ionizing radiation	Reduction of N-oxide		88
	Diselenid bond: cleavage		89
	Geometrical structure transformation: cis-GdAzo to trans-GdAzo		90
Thermal	Poly(N-isopropylacrylamide) (PNIPAAm)		91
	Poly[(N-N-diethyl)acrylamide] (pDEA)		92
	Dipalmitoyl phosphatidylcholine (DPPC): gel to liquid-crystalline phase transition		93

MMP2: matrix metalloproteinase 2; GGT: gamma-glutamyl transpeptidase; US: ultrasound; ACVA: 4,4'-Azobis(4-cyanovaleric acid).