The continuous rise of atmospheric CO2 and its associated impacts, including ocean acidification, demand innovative strategies for capture, detection, and monitoring. Here, we report a class of hybrid optical sensing platforms based on poly(N-isopropylacrylamide-co-methacrylic acid) (pNIPAm-co-MAA) microgels interlaced with MIL-53(Al)-NH2 metal–organic framework (MOF) nanoparticles for in situ sensing of dissolved CO2 (dCO2). Using an in situ hybridization strategy, MIL-53(Al)-NH2 is uniformly embedded within the polymeric matrix, yielding microgel–MOF (M&M) hybrids with tunable porosity, optical responsiveness to dCO2, and structural integrity controlled by the cross-linker. N,N’-Methylene-bis-acrylamide (BIS)-cross-linked M&M hybrid beads display greater rigidity, while poly(ethylene glycol)diacrylate (PEGDA)-cross-linked beads exhibit higher swelling ratios. Gas adsorption studies reveal reduced CO2 uptake compared to pristine MIL-53(Al)-NH2 MOF due to partial pore blocking by the polymer, yet the hybrid beads consistently outperform MOF-free ones, confirming that MIL-53(Al)-NH2 remains functionally active within the microgel network. Ultraviolet–visible (UV–Vis) absorbance experiments highlight distinct hybrid behavior with dCO2, inducing clear decreases in absorbance linked to MOF-mediated swelling, in contrast to the negligible response of pristine microgels. Reflectance spectroscopy further demonstrates that M&M-based etalons respond to dCO2 through characteristic red shifts driven by MOF-mediated CO2 adsorption and pore expansion, while MOF-free microgels show purely pH-driven blue shifts in response to dCO2 due to the acidic environment. M&M beads with three different mass ratios between the microgel and the MOF are synthesized, among which the 5:1 hybrids with the BIS cross-linker exhibit the highest CO2 uptake and deliver the most stable optical performance, maintaining reproducible responses over three consecutive CO2 cycling tests. Notably, even the relatively low MOF loadings retained within the polymer matrix are sufficient to impart measurable CO2 adsorption capacity and distinct optical responsiveness, underscoring the efficiency of the hybrid design. Our study establishes M&M etalons as dynamic, tunable platforms for real-time dCO2 monitoring with potential applications in environmental and carbon capture technologies.

Stimuli-responsive microgel-based etalons are promising optical and bio-sensors. These sensors play a pivotal role in modern healthcare by enabling rapid biomolecule detection, contributing to organ-on-chip applications and early disease diagnosis. This study investigates the suitability of poly(N-isopropylacrylamide) (pNIPAm)-based microgels for inkjet printing, focusing on optimizing their properties for effective deposition. Key parameters, including surface tension, viscosity, and particle size, are characterized to ensure compatibility with inkjet-printing requirements. The addition of surfactants tunes the suspensions’ properties to be in line with the requirements of inkjet printing. Jetting of pNIPAm-based microgels on gold-coated substrates forms a cohesive drop in a range of a few millimeters. Optical and scanning electron microscopy confirm the formation of a uniform microgel layer. The optical reflectance spectroscopy results indicate that inkjet-printed microgel-based etalons can effectively respond to changes in temperature and glucose concentration. In-liquid atomic force microscopy demonstrates the swelling dynamics of the microgels in different glucose concentrations, shedding light on their response dynamics. Our work demonstrates, for the first time, the feasibility of printing microgels in a controlled way, fabricating biocompatible inkjet-printed microgel-based etalon sensors with precise dimensions. The size precision and the sensitive monitoring capabilities of biomolecules hold great promise for in situ and continuous sensing in a wide range of biological and organ-on-chip applications.

We introduce Microgel-based Etalon Membranes (MEMs), based on the combination of stimuli-responsive microgels with an etalon, which is an optical device consisting of two reflecting plates and is used to filter specific wavelengths of light. The microgels are sandwiched between two reflective layers and, in response to a stimulus (e.g., temperature, pH, or biomarker concentration), swell or de-swell, thereby changing the distance between the two reflective layers and generating multiple peaks in the reflectance spectra. This property gives a MEM the unique capability of simultaneous separation and tunable responses to environmental changes and/or biomarker concentrations. We propose a design based on gold layers on a silicon nitride wafer membrane. Our comprehensive characterization, employing permeability experiments, in situ optical reflectance spectroscopy, in-liquid atomic force microscopy (AFM) analysis, and captive bubble contact angle measurements, elucidates the dynamic response of MEM to pH, temperature, and glucose stimuli and the corresponding effect of microgel swelling/de-swelling on the membrane properties, e.g., permeability. The AFM results confirm the dynamic changes of the microgel layer’s thickness on the membrane surface in response to the stimuli. Although the microgel’s swelling/de-swelling influences the effective pore radius, the decrease in the membrane’s permeance is limited to less than 10%. In the swollen state of the microgels, the etalon membranes show a prominent hydrophilic behavior, while they become less hydrophilic in the microgels’ de-swollen state. This work introduces MEM and provides novel insights into their behavior. The fundamental understanding that we reveal opens the way to applications ranging from point-of-care testing to continuous environmental monitoring.