Graphene Oxide Nanocarriers for Fluorescent Sensing of Calcium Ion Accumulation and Direct Assessment of Ion-Induced Enzymatic Activities in Cells

Graphene oxide (GO) is a nanomaterial with attractive chemo/physical properties that make it highly adaptable to the fields of sensing, energy devices (battery electrodes), and biotechnology. We report here the development of GO-PEGylated (PEG-GOs; PEG = poly(ethylene glycol)) nanocarriers for characterization of enzyme activities in cells. The capacity of GO nanocarrier as an intracellular sensor was first evaluated through fluorescent detection of cell uptake and compatibility study. Zeta-potential and dynamic light scattering measurements confirmed desirable physiochemical properties of GO nanocarriers for in vitro work, which include ease of bioconjugation, high cell permeability, and compatibility. The application of the nanocarriers to cell activity monitoring was further characterized by using a calpain-cleavable peptide conjugate, where fluorescence measurement was utilized to trace calcium-dependent cleavage of the caged probes in cancer cells. Using this nanosensor, we determined that exposing cancer cells to glutamate, hydrogen peroxide, and camptothecin resulted in varying apoptotic pathways, which each had a unique effect on concentration-dependent activation. Glutamate appears to be a strong reagent for generating apoptosis through a calcium buildup pathway. The results show that GO nanosensors are effective for investigating enzyme activity by fluorescence imaging and for diagnostic evaluation of apoptotic pathways. The combination of fluorescence-based tracking with the PEG-GOs nanocarrier system offers a new technical platform for intracellular sensing.