Table 2 Types of stimulus responses in the tumor microenvironment

pH

poly(histidine-co-phenylalanine), 2-(diisopropyl amino) ethyl methacrylate

Ref. [45, 46]

Enzyme

MT1-MMP, MMP7, secretory phospholipase A2

Ref. [62, 63, 67]

Hypoxia and oxidative stress

4-nitrobenzyl group (hypoxic trigger), o-hydroxyl E-cinnamic ester (photo-activated group), MnO (glutathione), quaternized chlormethine (H₂O₂)

Ref. [52–61]

Nucleic acid based

molecular beacon

Ref. [47–51]