*Department of Environmental Health Sciences Preface

In medical and biochemical research, when the domain of the sample is reduced to micrometer regimes, e.g. living cells or their subcompartments, the real-time measurement of chemical and physical parameters with high spatial resolution and negligible perturbation of the sample becomes extremely challenging. A traditional strength of chemical sensors (optical, electrochemical, etc.) is the minimization of chemical interference between sensor and sample, achieved with the use of inert, “biofriendly ” matrices or interfaces. However, when it comes to penetrating individual live cells, even the introduction of a sub-micron sensor tip can cause biological damage and resultant biochemical consequences. In contrast, individual molecular probes (free sensing dyes) are physically small enough but usually suffer from chemical interference between probe and cellular components. Our recently developed PEBBLE sensors (Probes Encapsulated By Biologically Localized Embedding) are nano-scale spherical devices consisting of sensor molecules entrapped in a chemically inert matrix. This protective coating eliminates interferences such as protein binding and/or membrane/organelle sequestration, which alter dye response. Conversely, the nanosensor matrix also provides protection to the cellular contents, enabling dyes that would usually be toxic to cells to be used for intracellular sensing. In addition, the inclusion of reference dyes allows quantitative, ratiometric fluorescence techniques to be used. Furthermore, the matrix phase allows the implementation of synergistic sensing schemes. PEBBLEs have been used to measure analytes such as calcium, potassium, nitric oxide, oxygen, chloride, sodium and glucose