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

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.