Intercellular chemical communication proceeds with both spatial and temporal components. A very prominent example is neurotransmitter release from neurons. But so far there are no analytical tools available to detect release and propagation of signaling molecules on small length scales and fast time scales. We have developed fluorescent nanosensors to monitor interactions of cells with their environment e.g. for chemical imaging of small molecules released by cells.

For that purpose we used near infrared (nIR) fluorescent single-walled carbon nanotubes (SWCNTs) and chemically rendered them selective for small signaling molecules such as the neurotransmitter dopamine. When dopamine binds the quantum yield of the sensors increased by up to 400 % and we were able to report single dopamine binding events. Parallel imaging of many of those sensors provides a nIR image (900 nm -1300 nm) of the dopamine concentration around cells. Furthermore, we used a stochastic kinetic Monte-Carlo simulation to predict the collective image of many sensors and how on and off rate constants affect the spatial and temporal resolution of this approach. This technique was used to image hot spots and directionality of dopamine release from neurons and other cells. We observed localized, unlabeled release sites that correlate with membrane curvature and cell morphology.

We furthermore tailored SWCNTs to bind cell surface receptors such as integrins to monitor and investigate cell adhesion and migration on surfaces. These results illustrate how to use fluorescent nanosensors for quantitative chemical imaging of complex biological processes.