The use of facemasks by the general population is recommended worldwide to prevent the spread of SARS-CoV-2. Despite the evidence in favour of facemasks to reduce community transmission, there is also agreement on the potential adverse effects of their prolonged usage, mainly caused by CO2 rebreathing. Herein we report the development of a sensing platform for gaseous CO2 real-time determination inside FFP2 facemasks. The system consists of an opto-chemical sensor combined with a flexible, battery-less, near-field-enabled tag with resolution and limit of detection of 103 and 140 ppm respectively, and sensor lifetime of 8 h, which is comparable with recommended FFP2 facemask usage times. We include a custom smartphone application for wireless powering, data processing, alert management, results displaying and sharing. Through performance tests during daily activity and exercise monitoring, we demonstrate its utility for non-invasive, wearable health assessment and its potential applicability for preclinical research and diagnostics.

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A new system for CO2 measurement (0-100%) by based on a paired emitter-detector diode arrangement as a colorimetric detection system is described. Two different configurations were tested: configuration 1 (an opposite side configuration) where a secondary inner-filter effect accounts for CO2 sensitivity. This configuration involves the absorption of the phosphorescence emitted from a CO2-insensitive luminophore by an acid-base indicator and configuration 2 wherein the membrane containing the luminophore is removed, simplifying the sensing membrane that now only contains the acid-base indicator. In addition, two different instrumental configurations have been studied, using a paired emitter-detector diode system, consisting of two LEDs wherein one is used as the light source (emitter) and the other is used in reverse bias mode as the light detector. The first configuration uses a green LED as emitter and a red LED as detector, whereas in the second case two identical red LEDs are used as emitter and detector. The system was characterised in terms of sensitivity, dynamic response, reproducibility, stability and temperature influence. We found that configuration 2 presented a better CO2 response in terms of sensitivity.Key words. Carbon dioxide sensor; Gas sensor; Optical sensor; Paired emitter detector-diode sensor; Portable instrumentation. INTRODUCTIONCO2 is an important industrial gas for many different uses that include production of chemicals (e.g. urea ), inert agent for food packaging (to extend the shelf-life of meat, cheese, etc.), beverages, refrigeration systems, welding systems, fire extinguishers, water treatment processes, and many other smaller scale applications [1;2]. In the agro-food industry CO2 is widely used in modified-atmosphere packaging where its task is to inhibit growth of spoilage bacteria [3;4]. For instance, in active packaging technologies a CO2 generating system can be considered as a technique complimentary to oxygen scavenging [5]. High CO2 levels (10-80 %) are desirable for foods such as meat and poultry in order to inhibit surface microbial growth and to extend their life time. The maintenance of CO2 concentration within packages in food [6], as instance using inserted sachets [7;8], must be carefully monitored since CO2 permeability is 3-5 times higher than that of oxygen for most of the plastic films and because CO2 is absorbed by many foods like meat and poultry. Real-time process monitoring is fundamental for effective process control. The rapid development of bioprocess applications together with the agro-food industry have led to an intensive search for new sensors capable of providing real-time information about the state of the processes. Conventional methods for CO2 determination include, among others, infrared spectrometry [9] or electrochemical sensors based on liquid (Severinghaus-type electrodes) or solid electrolytes [10;11]. The most popular sensors used for CO2 gas sensing in biotechnological applications are based on electrochemical measurements that can...

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