Sensor Technology Page 3. 3 The nitrocellulose manufacturer may also control the amount of nitro groups, making the substrate more affined for proteins.As greater is the amount of nitro groups in the nitrocellulose, lower would be the affinity of proteins for the substrate, so they may flow easily across the paper. However in this last case it would be harder to immobilize biomolecules (e.g. antibodies) in the test and control lines.The affinity of paper can be also modified by treating the paper substrate with other proteins or surfactants. Another important element on a LFBs is the transducer, the materi al which produces the colour that can be observed on the test and control lines. Nowadays, nanomaterials are the standard transducer on LFBs, replacing enzymes (e.g. horseradish peroxidase bound antibodies) and dyed microparticles (e.g. latex beads).The different properties of the nanomaterials increase the sensitivity of the assay,3 allowing the upgrade of the tests from a qualitative measurement (positive vs. negative) to semiquantitative (the colour intensity of test line can be related with the amount on nanoconjugate stopped in the area and, therefore, with the concentration of analyte). Gold nanoparticles (AuNPs) are the most used nanomaterial on LFBs, espe cially in commercial tests.AuNPs of around 20 nm diameter exhibit a strong red colour, good stability and are very easy to conjugate with biomolecules such as antibodies, DNA strains or aptamers. Other nanomaterials such as graphene, metal oxide nanoparticles and quantum dots (the latter with fluorescent properties) start to emerge in academic reports, but not yet in the market. Limitations of LFBs must be also considered.The detection limit and sensitivity of a LFBs will not (or will hardly) surpass the levels achieved by an ELISA test, even when employing the same antibodies. LFBs are an affordable, robust and portable tool that can provide a fast response, being easy-to-use by even non-trained users, but as the response is read without any equipment, by naked eye, it is hard to identify low concentrations of analyte and discern between close concentration values. Future perspectives Nowadays, several LFBs tests for diagnostic applications can be found in the market: pregnancy prediction (human chorionic gonadotropin hormone), Covid-19, Human Immunodeficiency Virus (HIV), parasites (e.g. Malaria), allergens (in food, drink and rooms), bacteria (e.g. Clostridium difficile Toxin A+B), etc. It is important to highlight the fact that, being able to use these biosensors at home (at point-of-care), LFBs grant the patient with a plus of privacy.After pandemics, LFBs have demonstrated their usefulness, so it is to expect many LFBs (rapid diagnostic tests) being launched for uncovered medical needs and new biomarkers (e.g. miRNA or extracellular vesicles, be tween others), but also for applications out of the diagnostic field, as environmental sensing (e.g. detection of heavy metals and pollutants in water).
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