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Novel nanoscale carriers for fluorescent probes are being developed as optical sensors for intracellular monitoring of sodium, potassium, and oxygen. These carriers are constructed using the electrostatic layer-by-layer (LbL) assembly technique to create polyelectrolyte thin films on the surface of nanoparticles. The thin film surrounding the nanoparticles is then used as a "sponge" to absorb the charged analyte-specific fluorescent indicators. Fluorescent nanoparticles are used as templates, and are chosen such that their optical properties allow them to be used simultaneously with the fluorescent indicators as internal intensity references. The probes embedded in the polyion films retain their sensitivity to sodium, potassium, and oxygen over a broad range, and are stable to leaching for at least one week, much longer than the intended application. In addition, the small size of the devices (>300 nm) makes them amenable to intracellular deployment. The versatile scheme presented here provides an elegant alternative when carriers for probes that require intensity references are needed, and when high precision, control, and repeatability in fabrication are required.