Hybrid Microgel–MOF (M&M) Etalon Sensors for In Situ Detection of Dissolved CO 2

The continuous rise of atmospheric CO 2 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)-NH 2 metal–organic framework (MOF) nanoparticles for in situ sensing of dissolved CO 2 (dCO 2 ). Using an in situ hybridization strategy, MIL-53(Al)-NH 2 is uniformly embedded within the polymeric matrix, yielding microgel–MOF (M&M) hybrids with tunable porosity, optical responsiveness to dCO 2, 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 CO 2 uptake compared to pristine MIL-53(Al)-NH 2 MOF due to partial pore blocking by the polymer, yet the hybrid beads consistently outperform MOF-free ones, confirming that MIL-53(Al)-NH 2 remains functionally active within the microgel network. Ultraviolet–visible (UV–Vis) absorbance experiments highlight distinct hybrid behavior with dCO 2, 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 dCO 2 through characteristic red shifts driven by MOF-mediated CO 2 adsorption and pore expansion, while MOF-free microgels show purely pH-driven blue shifts in response to dCO 2 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 CO 2 uptake and deliver the most stable optical performance, maintaining reproducible responses over three consecutive CO 2 cycling tests. Notably, even the relatively low MOF loadings retained within the polymer matrix are sufficient to impart measurable CO 2 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 dCO 2 monitoring with potential applications in environmental and carbon capture technologies.