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This paper presents an innovative application of chitosan material to be used as pH-responsive valves for the precise control of lateral flow in microfluidic paper-based analytical devices (μPADs). The fabrication of μPADs involved wax printing, while pH-responsive valves were created using a solution of chitosan in acetic acid. The valve-forming solution was applied, and ready when dry; by exposure to acidic solutions, the valve opens. Remarkably, the valves exhibited excellent compatibility with alkaline, neutral, and acidic solutions with a pH higher than 4. The valve opening process had no impact on the flow rate and colorimetric analysis. The potential of chitosan valves used for flow control was demonstrated for μPADs employed for nitrate determination. Valves were used to increase the conversion time of nitrate to nitrite, which was further analyzed using the Griess reaction. The μPAD showed a linear response in the concentration range of 10-100 μmol L -1 , with a detection limit of 5.4 μmol L -1 . As a proof of concept, the assay was successfully applied to detect nitrate levels in water samples from artificial lakes of recreational parks. For analyses that require controlled kinetics and involve multiple sequential steps, the use of chitosan pH-responsive valves in μPADs is extremely valuable. This breakthrough holds great potential for the development of simple and high-impact microfluidic platforms that can cater to a wide range of analytical chemistry applications., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

3D printing has revolutionized the manufacturing process of microanalytical devices by enabling the automated production of customized objects. This technology promises to become a fundamental tool, accelerating investigations in critical areas of health, food, and environmental sciences. This microfabrication technology can be easily disseminated among users to produce further and provide analytical data to an interconnected network towards the Internet of Things, as 3D printers enable automated, reproducible, low-cost, and easy fabrication of microanalytical devices in a single step. New functional materials are being investigated for one-step fabrication of highly complex 3D printed parts using photocurable resins. However, they are not yet widely used to fabricate microfluidic devices. This is likely the critical step towards easy and automated fabrication of sophisticated, complex, and functional 3D-printed microchips. Accordingly, this review covers recent advances in the development of 3D-printed microfluidic devices for point-of-care (POC) or bioanalytical applications such as nucleic acid amplification assays, immunoassays, cell and biomarker analysis and organs-on-a-chip. Finally, we discuss the future implications of this technology and highlight the challenges in researching and developing appropriate materials and manufacturing techniques to enable the production of 3D-printed microfluidic analytical devices in a single step., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Inorganic phosphorous (as phosphate (PO 4 3- ), is one of the essential nutrients for all living forms, either terrestrial or marine. In oligotrophic seawaters, this macronutrient is limited (10 -9  M) and its ratio with other elements (nitrogen or carbon) is denoting the health state of the marine environment; a small variation of its concentration can produce eutrophication. The gold standard method used for PO 4 3- detection is based on colorimetric detection of phosphomolybdate. The colored complex is obtained by mixing water-soluble molybdenum salts (Mo(VI)) and reducing agents in acid media, along with the sample containing PO 4 3- . Moreover, the kinetic of complex formation is slow, about 1 h is generally required for color to develop, exposing the assay to the drawbacks of interferences as those from silica. The detection is preferably performed in a controlled environment (i.e. in a laboratory) because several chemicals and steps of preparations are required as well as the optical instrumentation is not intended for in-field use. Electrochemical sensors offer portability and simplicity making them a practical option for on-site detection applications. To gain an analytical alternative in measuring low quantities of PO 4 3- (10 -9  M), and overcome some of the drawbacks of the classical approaches, we optimised a new easy way to produce a plastic electrode decorated with an alkyl Mo-polyoxometalate (Mo 8 O 26 4- ), that is soluble in organic solvents. This tetra-butyl-ammonium octamolybdate powder, [N (C4H9)4]4 Mo8O26, purposely synthetized was identified with FTIR, Raman, MS methods, and the electroactivity and reactivity with PO 4 3- was confirmed in solution with cyclic voltammetry (CV). When the Mo-decorated electrode was in contact with PO 4 3- , an electroactive phosphomolybdate aggregate formed at the electrode surface that was electrochemically detectable with square wave voltammetry (SWV). A remarkably low detection limit of 6.1 nM, to PO 4 3- , as well as good stability and selectivity were obtained also in real samples. In fact, PO 4 3- was measured in saline simulated and real seawater samples at nM concentrations in less than 5 min. The present investigation provides a new alternative to the current standard colorimetric methods to detect low phosphate concentrations, showing the potential to be used for monitoring nutrients in oligotrophic seawater., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments., (Copyright © 2021 Elsevier B.V. All rights reserved.)

This study describes for the first time the development of 3D printed microfluidic devices with integrated electrodes for label-free counting of E. coli cells incorporated inside droplets based on capacitively coupled contactless conductivity detection (C 4 D). Microfluidic devices were fully fabricated by 3D printing in the T-junction shape containing two channels for disperse and continuous phases and two sensing electrodes for C 4 D measurements. The disperse phase containing E. coli K12 cells and the continuous phase containing oil and 1% Span ® 80 were pumped through flow rates fixed at 5 and 60 μL min -1 , respectively. The droplets with incorporated cells were monitored in the C 4 D system applying a 500-kHz sinusoidal wave with 1 V pp amplitude. The generated droplets exhibited a spherical shape with average diameter of 321 ± 9 μm and presented volume of 17.3 ± 0.5 nL. The proposed approach demonstrated ability to detect E. coli cells in the concentration range between 86.5 and 8650 CFU droplet -1 . The number of cells per droplet was quantified through the plate counting method and revealed a good agreement with the Poisson distribution. The limit of detection achieved for counting E. coli cells was 63.66 CFU droplet -1 . The label-free counting method has offered instrumental simplicity, low cost, high sensitivity and compatibility to be integrated on single microfluidic platforms entirely fabricated by 3D printing, thus opening new possibilities of applications in microbiology., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The present study was aimed to test an electrochemical sensing approach for the detection of an active chemolithotrophic metabolism (and therefore the presence of chemolithotrophic microorganisms) by using the corrosion of pyrite by Acidithiobacillus ferrooxidans as a model. Different electrochemical techniques were combined with adhesion studies and scanning electron microscopy (SEM). The experiments were performed in presence or absence of A. ferrooxidans and without or with ferrous iron in the culture medium (0 and 0.5 g L -1 , respectively). Electrochemical parameters were in agreement with voltammetric studies and SEM showing that it is possible to distinguish between an abiotically-induced corrosion process (AIC) and a microbiologically-induced corrosion process (MIC). The results show that our approach not only allows the detection of chemolithotrophic activity of A. ferrooxidans but also can characterize the corrosion process. This may have different kind of applications, from those related to biomining to life searching missions in other planetary bodies., (Copyright © 2018 Elsevier B.V. All rights reserved.)