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Signal Amplification: A Sharp Impermeable‐Permeable Transition for Highly Sensitive Low‐Cost Detection

This manuscript describes a signal‐amplifying mechanism for achieving highly sensitive yet low‐cost detection. The main component of the device is a hypersensitive material that consists of a stimuli‐responsive hydrogel coated with a thin layer of superhydrophobic nanoparticles. Initially, the superhydrophobic nanoparticles block the outflow of a dye (i.e., the impermeable state). When the stimuli‐responsive hydrogel expands after coming into contact with an appropriate stimulus, the dye diffuses through the gaps between the nanoparticles (i.e., the permeable state) and spread outward. The large outflow of dye (i.e., a clear visible readout) due to the small concentration or small amount (<1 µL) of chemicals added to the device produces the effect of signal amplification. Highly sensitive detection of concentration of as low as ∼10 nm is demonstrated. Fundamentally, the thin layer (<5 µm) of the superhydrophobic nanoparticles gives rise to the sharp impermeable‐permeable transition for the highly sensitive detection. This hypersensitive material provides the opportunity for the detection to be highly sensitive and yet has many other desirable features, including low cost, portable, simple to fabricate and operate, capable of quantifying concentration (i.e., by measuring time), and able to perform complex analysis (e.g., via logic gates by detecting multiple signals).