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The development of devices for sensing and monitoring CO2 levels is crucial for many fields such as food packaging and for human safety indoors. Herein the fabrication of an optical CO2 sensor by integration of a metal–organic framework (MOF) onto bimodal optical waveguides is reported. This sensor is constructed via self-assembly of a transparent film of zeolitic imidazolate framework-8 (ZIF-8) nanoparticles (size: 32 ± 5 nm) on the waveguides. The nanoZIF-8-based sensor exhibits a broad linear response, with limits of detection of 3130 ppm at room temperature and 774 ppm at 278 K. Furthermore, it is robust, selective, fast and reusable, and can be stored under humid conditions with no loss in performance., This work was supported by a grant from the Fundación Domingo Martínez (Ayuda a la Investigación 2016. Área de Aplicaciones Materiales), the Spanish MINECO (projects PN MAT2015-65354-C2-1-R and MAT2016-79053-P), the Catalan AGAUR (project 2014 SGR 80 and 2014 SGR 624), and the ERC, under the EU FP7 (ERC-Co 615954). It was also funded by the CERCA Program/Generalitat de Catalunya. This work also made use of the Spanish ICTS Network MICRONANOFABS, which is partially supported by MEINCOM. ICN2 and ICMAB acknowledge the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program, under Grants SEV-2013-0295 and SEV-2015-0496.