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Nanoengineered fluorescent sensor coatings on colloidal carriers have been developed for use intracellularly. These nanosensors are fabricated via the electrostatic layer-by-layer self-assembly technique to form ultrathin polyelectrolyte films containing indicators on fluorescent nanoparticles. The fluorescent nanoparticle templates and the fluorescent indicator are chosen such that their optical properties are complementary, enabling the inert nanoparticle templates to serve as internal intensity references for the fluorescent probe. In this work, the potassium ion indicator, potassium-binding benzofuran isophthalate potassium-binding benzofuran isophthalate was immobilized within poly(styrene sulfonate)/poly(allylamine hydrochloride) films assembled on the surface of fluorescent europium nanoparticles. The indicator retains its sensitivity to potassium ions after immobilization within the films and exhibits sensitivity to increases in potassium concentration over a broad range. In addition, the sensors demonstrate excellent leaching stability, with less than 1% of loaded indicator leached after 14 days of wet storage. The core-referenced nanosensor scheme described here is a simple and elegant way to co-immobilize fluorescent indicator and intensity reference within a single nanoscale package, which may be deployed intracellularly; furthermore, the separation of fluorescent indicator from the cellular environment is attractive, as it may prevent complications due to use of liquid-phase fluorescent sensors intracellularly, such as cytotoxicity and probe compartmentalization.