Analyte-sensitive hydrogels that incorporate optical structures have emerged as sensing platforms for point-of-care diagnostics. The optical properties of the hydrogel sensors can be rationally designed and fabricated through self-assembly, microfabrication or laser writing. The advantages of photonic hydrogel sensors over conventional assay formats include label-free, quantitative, reusable, and continuous measurement capability that can be integrated with equipment-free text or image display. This Review explains the operation principles of photonic hydrogel sensors, presents syntheses of stimuli-responsive polymers, and provides an overview of qualitative and quantitative readout technologies. Applications in clinical samples are discussed, and potential future directions are identified.

Owing to their structural variability, metal− organic frameworks (MOFs) lend themselves well as chemical sensing materials by providing both high sensitivity and selectivity. Here, we integrate different types of MOFs (ZIF-8, HKUST-1, CAU-1-NH 2 ) into photonic multilayers referred to as Bragg stacks (BSs), which report on adsorption events through changes in their effective refractive index (RI). The fabrication of photonic multilayers is accomplished by spincoating colloidal suspensions of MOF nanoparticles and/or the high RI-material TiO 2 . While their incorporation in BSs allows for the label-free readout of host−guest interactions, the choice of particular types of MOFs determines the sensing properties of the BS. Here, we present MOF-based BSs with enhanced specificity toward molecular analytes by combining two different MOFs in a single platform. The sensing performance of our BSs is demonstrated by a combined spectroscopic and principal component analysis of their vapor response. Time-dependent measurements reveal fast response times and good recoverability of the multilayers. Moreover, we demonstrate that combinatorial sensing is feasible by arranging different MOF BSs in a basic color pattern, which highlights the potential of MOF-based multilayers in arrayed sensor devices.