Nanoreactors comprising a metal precursor in a carrier are provided as well as methods of initiating, methods of preparing, and methods of using nanoreactors. In some embodiments, upon exposure to heat, the metal precursor forms nanoparticles that can be detected, e.g., by detecting a color change in the nanoreactor and/or by detecting the number and/or size and/or size distribution and/or shape of the nanoparticles. The nanoreactors can be used, in some embodiments, as time-temperature indicators for perishable goods.