Foodborne illnesses are a growing concern for the food industry and consumers, with millions of cases reported every year. Consequently, there is a critical need to develop rapid, sensitive, and inexpensive techniques for pathogen detection in order to mitigate this problem. However, current pathogen detection strategies mainly include time-consuming laboratory methods and highly trained personnel. Electrochemical in-field biosensors offer a rapid, low-cost alternative to laboratory techniques, but the electrodes used in these biosensors require expensive nanomaterials to increase their sensitivity, such as noble metals (e.g., platinum, gold) or carbon nanomaterials (e.g., carbon nanotubes, or graphene). Herein, we report the fabrication of a highly sensitive and label-free laser-induced graphene (LIG) electrode that is subsequently functionalized with antibodies to electrochemically quantify the foodborne pathogen Salmonella enterica serovar Typhimurium. The LIG electrodes were produced by laser induction on polyimide film in ambient conditions, and hence circumvent the need for hightemperature, vacuum environment, and metal seed catalysts commonly associated with graphene-based electrodes fabricated via chemical vapor deposition processes. After functionalization with Salmonella-antibodies, the LIG biosensors were able to detect live Salmonella in chicken broth across a wide linear range (25 to 10 5 CFU mL -1 ) and with a low detection limit (13 ± 7 CFU mL -1 ; n = 3, mean ± standard deviation). These results were acquired with an average response time of 22 minutes without the need for sample preconcentration or redox labeling techniques. Moreover, these LIG immunosensors displayed high selectivity as demonstrated by non-significant response to other bacteria strains. These results demonstrate how LIG-based electrodes can be used for electrochemical immunosensing in general and, more specifically, could be used as a viable option for rapid, low-cost pathogen detection in food processing facilities before contaminated foods reach the consumer.

Graphene-based inks are becoming increasingly attractive for printing low-cost and flexible electrical circuits due to their high electrical conductivity, biocompatibility, and manufacturing scalability. Conventional graphene printing techniques, such as screen and inkjet printing, are limited by stringent ink viscosity requirements properties and large as-printed line width that impedes the performance of printed biosensors. Here, we report an aerosol-jet-printed (AJP) graphene-based immunosensor capable of monitoring two distinct cytokines: interferon gamma (IFN-γ) and interleukin 10 (IL-10). Interdigitated electrodes (IDEs) with 40 μm finger widths were printed from graphene-nitrocellulose ink on a polyimide substrate. The IDEs were annealed in CO2 to introduce reactive oxygen species on the graphene surface that act as chemical handles to covalently link IFN-γ and IL-10 antibodies to the graphene surfaces. The resultant AJP electrochemical immunosensors are capable of monitoring cytokines in serum with wide sensing range (IFN-γ: 0.1–5 ng/mL; IL-10: 0.1–2 ng/mL), low detection limit (IFN-γ: 25 pg/ml and IL-10: 46 pg/ml) and high selectivity (antibodies exhibited minimal cross-reactivity with each other and IL-6) without the need for sample prelabeling or preconcentration. Moreover, these biosensors are mechanically flexible with minimal change in signal output after 250 bending cycles over a high curvature (Φ = 5 mm). Hence, this technology could be applied to numerous electrochemical applications that require low-cost electroactive circuits that are disposable and/or flexible.

Carbon nanomaterials such as graphene exhibit unique material properties such as high electrical conductivity, surface area, and biocompatibility that have the potential to significantly improve the performance of electrochemical sensors. Since in-field electrochemical sensors are typically disposable, they require materials that are amenable to low-cost, high-throughput, and scalable manufacturing. Conventional graphene devices based on low-yield chemical vapor deposition techniques are too expensive for such applications, while low-cost alternatives such as screen and inkjet printing do not possess sufficient control over electrode geometry to achieve favorable electrochemical sensor performance. In this work, aerosol jet printing (AJP) is used to create high-resolution (~40 µm line width) interdigitated electrodes (IDEs) on flexible substrates, which are then converted into histamine sensors by covalently linking monoclonal antibodies to oxygen moieties created on the graphene surface through a CO2 thermal annealing process. The resulting electrochemical sensors exhibit a wide histamine sensing range of 6. 25-200 ppm (56.25 µM -1.8 mM) and a low detection limit of 3.41 ppm (30.7 µM) within actual tuna broth samples. These sensor metrics are significant since histamine levels over 50 ppm in fish induce adverse health effects including severe allergic reactions (e.g., Scombroid food poisoning).Beyond the histamine case study presented here, the AJP and functionalization process can likely be generalized to a diverse range of sensing applications including environmental toxin detection, foodborne pathogen detection, wearable health monitoring, and health diagnostics.

Vertically aligned carbon nanotube array (VANTA) coatings have recently garnered much attention due in part to their unique material properties including light absorption, chemical inertness, and electrical conductivity. Herein we report the first use of VANTAs grown via chemical vapor deposition in a 2D interdigitated electrode (IDE) footprint with a high heightto-width aspect ratio (3:1 or 75:25 µm). The VANTA-IDE is functionalized with an antibody (Ab) specific to the human cancerous inhibitor PP2A (CIP2A)-a salivary oncoprotein that is associated with a variety of malignancies such as oral, breast, and multiple myeloma cancers.The resultant immunosensor is capable of detecting CIP2A label-free across a wide linear sensing range (1 -100 pg/mL) with a detection limit of 0.24 pg/mL within saliva supernatant-a range that is more sensitive than the corresponding CIP2A enzyme linked immunosorbent assay (ELISA). These results help pave the way for rapid cancer screening tests at the point-of-care (POC) such as for the early-stage diagnosis of oral cancer at a dentist's office. Table of Content (ToC) Image Description: (Left) Schematic diagram showing antibody functionalized (anti-CIP2A) vertically aligned carbon nanotubes (VANTAs) arrayed in an interdigitated electrode (IDE) footprint. (Left Inset) An optical image of a VANTA IDE immunosensor fabricated on silicon wafer. (Right) Electrochemical impedance sensing of CIP2A antigen concentrations with the biofunctionalized VANTA IDEs.

Rapid, inexpensive, and easy-to-use coronavirus disease 2019 (COVID-19) home tests are key tools in addition to vaccines in the world-wide fight to eliminate national and local shutdowns. However, currently available tests for SARS-CoV-2, the virus that causes COVID-19, are too expensive, painful, and irritating, or not sufficiently sensitive for routine, accurate home testing. Herein, we employ custom-formulated graphene inks and aerosol jet printing (AJP) to create a rapid electrochemical immunosensor for direct detection of SARS-CoV-2 Spike Receptor-Binding Domain (RBD) in saliva samples acquired non-invasively. This sensor demonstrated limits of detection that are considerably lower than most commercial SARS-CoV-2 antigen tests (22.91 ± 4.72 pg/mL for Spike RBD and 110.38 ± 9.00 pg/mL for Spike S1) as well as fast response time (~30 mins), which was facilitated by the functionalization of printed graphene electrodes in a single-step with SARS-CoV-2 polyclonal antibody through the carbodiimide reaction without the need for nanoparticle functionalization or secondary antibody or metallic nanoparticle labels. This immunosensor presents a wide linear sensing range from 1 to 1000 ng/mL and does not react with other coexisting influenza viruses such as H1N1 hemagglutinin. By combining high-yield graphene ink synthesis, automated printing, high antigen selectivity, and rapid testing capability, this work offers a promising alternative to current SARS-CoV-2 antigen tests.

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