Paper-based sensors offer an affordable yet powerful platform for field and point-of-care (POC) testing due to their self-pumping ability and utility for many different analytical measurements.

Demonstration of a simple melt based process to overcome the issue of poorly integrated, low activity and complicated methods of carbon electrode integration into microfluidics.

The COVID-19 pandemic focused attention on a pressing need for fast, accurate, and low-cost diagnostic tests. This work presents an electrochemical capillary driven immunoassay (eCaDI) developed to detect SARS-CoV-2 nucleocapsid (N) protein. The low-cost flow device is made of polyethylene terephthalate (PET) and adhesive films. Upon addition of a sample, reagents and washes are sequentially delivered to an integrated screen-printed carbon electrode for detection, thus automating a full sandwich immunoassay with a single end-user step. The modified electrodes are sensitive and selective for SARS-CoV-2 N protein and stable for over 7 weeks. The eCaDI was tested with influenza A and Sindbis virus and proved to be selective. The eCaDI was also successfully applied to detect nine different SARS-CoV-2 variants, including Omicron.

Point-of-care (POC) methods currently available for detecting SARS-CoV-2 infections still lack accuracy. Here, we report the development of a highly sensitive electrochemical immunoassay capable of quantitatively detecting the presence of the SARS-CoV-2 virus in patient nasopharyngeal samples using stencil-printed carbon electrodes (SPCEs) functionalized with capture antibodies targeting the SARS-CoV-2 nucleocapsid protein (N protein). Samples are added to the electrode surface, followed by horseradish peroxidase (HRP)-conjugated detection antibodies also targeting the SARS-CoV-2 N protein. The concentration of the virus in samples is quantified using chronoamperometry in the presence of 3,3′5,5′-tetramethylbenzidine. Limits of detection equivalent to less than 50 plaque forming units/mL (PFU/mL) were determined with virus sample volumes of 20 μL. No cross-reactivity was detected with the influenza virus and other coronavirus N proteins. Patient nasopharyngeal samples were tested as part of a proof-of-concept clinical study where samples were also tested using the gold-standard real-time quantitative polymerase chain reaction (RT-qPCR) method. Preliminary results from a data set of 22 samples demonstrated a clinical specificity of 100% ( n = 9 negative samples according to RT-qPCR) and a clinical sensitivity of 70% for samples with RT-PCR cycle threshold (Ct) values under 30 ( n = 10) and 100% for samples with Ct values under 25 ( n = 5), which complies with the World Health Organization (WHO) criteria for POC COVID-19 diagnostic tests. Our functionalized SPCEs were also validated against standard plaque assays, and very good agreement was found between both methods ( R 2 = 0.9993, n = 6), suggesting that our assay could be used to assess patient infectivity. The assay currently takes 70 min from sampling to results.

Carbon composite electrodes are frequently used for enzymatic electrodes but suffer from poor electrocatalytic activity and/or are single use. Our group has recently developed thermoplastic electrodes (TPEs) that use a thermopolymer binder. The electrodes can be easily molded without extensive use of organic solvents enabling catalysts and enzymes to be added directly to the electrode material. The TPE material is bulk-modified with cobalt phthalocyanine and glucose oxidase, resulting in a robust sensor that is stable for over 100 minutes. These sensors can be molded into intricate shapes and sanded for surface renewal, allowing the sensors to be continuously reused.

Correction for ‘Electrochemical paper-based devices: sensing approaches and progress toward practical applications’ by Eka Noviana et al., Lab Chip, 2019, DOI: 10.1039/c9lc00903e.

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Thermal treatment of paper leads to the formation of aromatic structures, allowing patterning the substrates and their use as sensors.