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Storing highly concentrated reagents and releasing them with high efficiency is crucial for developing highly sensitive biosensors. However, the performance of reservoirs in biosensors made of filter paper is limited by the intrinsic physicochemical properties of untreated cellulose. In this article, the impact of 9 different polymer modifications on the storage and release of enzymes and nanoparticles in paper-based biosensors is studied for the first time. Carboxymethylcellulose, polyvinyl alcohol, carrageenan, and chitosan, which contain hydroxyl groups, yielded paper-based reservoirs with the highest concentration of reagents in them. Polymer reservoirs made of carboxymethylcellulose release enzymes rapidly and at high concentration, which results in colorimetric glucose biosensors with an ultralow limit of detection of 0.005 mM. Reservoirs made with a blend of polymers concentrate antibody-decorated nanoparticles and decrease 10 times the limit of detection of a model immunosensor. An adaption of this design was used for detecting the pathogen Klebsiella pneumoniae in urine samples from hospital patients at the infectious dose threshold rapidly and with high specificity. The fabrication of the reservoirs only requires drop-casting a polymer solution on the paper and drying. This procedure is compatible with the fabrication of origami biosensors, which are made from a single piece of filter paper. • Dry polymer drops in paper substrates concentrate reagents and release them with high efficacy. • Carboxymethylcellulose (CMC) enhances the release of highly concentrated enzymes. • A colorimetric glucose biosensor using CMC reservoirs had a low limit of detection of 0.005 mM. • Using CMC and PSS enhanced the release of antibody-decorated nanoparticles and decreased 10 times the LOD the immunosensor. • Immunosensors for detecting urinary tract infections by K.pneumoniae were developed. [ABSTRACT FROM AUTHOR]

• Detection of cytokine storm biomarkers with mobile biosensors. • Immunosensors are made solely of cellulose modified with antibody-decorated nanoparticles. • Turnaround time under 10 min. • Detection of biomarkers in respiratory samples allows monitoring local inflammation. • Compatible with decentralized health care schemes. Decentralizing COVID-19 care reduces contagions and affords a better use of hospital resources. We introduce biosensors aimed at detecting severe cases of COVID-19 in decentralized healthcare settings. They consist of a paper immunosensor interfaced with a smartphone. The immunosensors have been designed to generate intense colorimetric signals when the sample contains ultralow concentrations of IL-6, which has been proposed as a prognosis biomarker of COVID-19. This is achieved by combining a paper-based signal amplification mechanism with polymer-filled reservoirs for dispensing antibody-decorated nanoparticles and a bespoken app for color quantification. With this design we achieved a low limit of detection (LOD) of 10−3 pg mL-1 and semi-quantitative measurements in a wide dynamic range between 10−3 and 102 pg mL-1 in PBS. The assay time is under 10 min. The low LOD allowed us to dilute blood samples and detect IL-6 with an LOD of 1.3 pg mL-1 and a dynamic range up to 102 pg mL-1. Following this protocol, we were able to stratify COVID-19 patients according to different blood levels of IL-6. We also report on the detection of IL-6 in respiratory samples (bronchial aspirate, BAS) from COVID-19 patients. The test could be easily adapted to detect other cytokines such as TNF-α and IL-8 by changing the antibodies decorating the nanoparticles accordingly. The ability of detecting cytokines in blood and respiratory samples paves the way for monitoring local inflammation in the lungs as well as systemic inflammation levels in the body. [ABSTRACT FROM AUTHOR]