A hydrogel-based nanosensor reported online in Nature Materials this week furthers our understanding of the process, and most importantly the timing, of cell death following the transplantation of cells into a living organism.

Current methods to evaluate the fate of transplanted cells rely on either cell-labelling approaches, which may continue to give a signal following cell death, or the use of reporter genes, which has the clinical disadvantage of introducing foreign genetic material.

Now, Michael McMahon and colleagues report a pH-based sensor that, using magnetic resonance imaging (MRI), can non-invasively monitor cell death in vivo. By incorporating a pH-sensitive contrast agent into a liposomal microcapsule, changes in the local pH that accompany cell death are detected using MRI. This method allows the monitoring of cell death without the concerns of optical imaging techniques, for example, limited signal penetration of biological tissue. Furthermore, the information obtained can be combined with high-resolution anatomical images to allow easier clinical evaluation.