Nanoreactors offer a great platform for the onsite generation of functional products. However, the production of the desired compound is often limited by either the availability of the reagents or their diffusion across the nanoreactor shell. To overcome this issue, we synthesized self-sustaining nanoreactors carrying the required reagents with them. They are composed of active enzymes crosslinked as nanocapsules and the inner core serves as a reservoir for reagents. Upon trigger, the enzymatic shell catalyzes the conversion of the encapsulated payload. This concept was demonstrated by the preparation of nanoreactors loaded with sensing molecules for the detection of glucose in biological media. More importantly, the system introduced here serves as an adaptable platform for biomedical applications, since the nanoreactors display good cellular uptake and high activity within cells. Consequently, they could act as nanofactories for the in situ generation of functional molecules.