A device and system for measuring and/or monitoring an analyte present on the skin is provided. The system includes a skin-mountable device that may be attached to an external skin surface and a reader device. The skin-mountable device includes a substrate, a plurality of micro-needles, and nanosensors encapsulated in the micro-needles. The micro-needles are attached to the substrate such that attachment of the substrate to an external skin surface causes to the micro-needles to penetrate into the skin to contact interstitial fluid. The micro-needles can include a sacrificial agent and are configured to become porous on contact with a solvent, e.g., interstitial fluid, which dissolves at least a portion of the sacrificial agent. The nanosensors encapsulated in the micro-needles include a detectable label and are configured to interact with a target analyte present in the interstitial fluid. The reader device is configured to detect the analyte in interstitial fluid via interaction with the skin-mountable device.
