Electrochemistry on Paper-based Analytical Devices: A Review

Paper-Based Electrochemical (Bio)Sensors for the Detection of Target Analytes in Liquid, Aerosol, and Solid Samples

The last decade has been incredibly fruitful in proving the multifunctionality of paper for delivering innovative electrochemical (bio)sensors. The paper material exhibits unprecedented versatility to deal with complex liquid matrices and facilitate analytical detection in aerosol and solid phases. Such remarkable capabilities are feasible by exploiting the intrinsic features of paper, including porosity, capillary forces, and its easy modification, which allow for the fine designing of a paper device. In this review, we shed light on the most relevant paper-based electrochemical (bio)sensors published in the literature so far to identify the smart functional roles that paper can play to bridge the gap between academic research and real-world applications in the biomedical, environmental, agrifood, and security fields. Our analysis aims to highlight how paper's multifarious properties can be artfully harnessed for breaking the boundaries of the most classical applications of electrochemical (bio)sensors.

Development of a biodegradable microfluidic paper-based device for blood-plasma separation integrated with non-enzymatic electrochemical detection of ascorbic acid

In the present article, we developed an electrochemical microfluidic paper-based device (EμPAD) for the non-enzymatic detection of Ascorbic Acid (AA) concentration in plasma using whole human blood. We combined LF1 blood plasma separation membrane and Whatman grade 1 filter paper to separate plasma from whole blood through wax printing. A screen-printed electrode (SPE) was modified with spherical-shaped MgFe2O4 nanomaterial (n-MgF) to improve the catalytic properties of SPE. The n-MgF was prepared via hydrothermal method, and its material phase and morphology were confirmed via XRD, FTIR, TEM, SEM, and AFM analysis. The fabricated n-MgF/SPE/EμPAD exhibited detection of AA ranging from 0 to 80 μM. The obtained value of the detection limit, limit of quantification, sensitivity, and response time are 2.44 μM, 8.135 μM, 5.71 × 10-3 mA μM-1 cm-2, and 10 s, respectively. Our developed n-MgF/SPE/EμPAD shows marginal interference with the common analytes present in plasma, such as uric acid, glutamic acid, glucose, urea, lactic acid, and their mixtures. Overall, our low-cost, portable device with its user-friendly design and efficient plasma separation capability offers a practical and effective solution for estimating AA concentration from whole human blood in a single step.

Tuneable Plasmonic Resonances Of A Dynamic Thin Film Of Ultrasmall Nanocrystals Modified In the Anti-Galvanic Reduction Process

Ultrasmall gold nanoparticles (NPs) have revolutionized nanotechnology as they are an excellent starting substrate for the synthesis of organic-inorganic hybrid materials with photonic or energy conversion applications, often with a responsive nature. However, ultrasmall NPs do not sustain plasmonic resonances, preventing their use in plasmon-related applications. In the presented work, we show a method of chemical modification of ultrasmall gold nanoparticles in order to fabricate dynamically controlled plasmonic thin films. For this purpose, we used the Anti-Galvanic Reduction process (AGR) to modify the surface of small gold nanoparticles, inducing plasmonic properties without notable size increases. Au@Ag NPs are then modified with liquid crystal-like organic ligands. The obtained NPs can assemble into densely packed films with long-range order and temperature-dependent structural properties. Namely, we detect two, fully reversible phase transitions between the hexagonal and cubic symmetries. The combination of AGR and organic surface modifications enabled us to demonstrate the possibility of managing plasmonic properties in the thin film of ~2 nm diameter metallic NPs.

Recent advances in the modification of electrodes for trace metal analysis: a review

This review paper summarizes the research published in the last five years on using different compounds and/or materials as modifiers for electrodes employed in trace heavy metal analysis. The main groups of modifiers are identified, and their single or combined application on the surface of the electrodes is discussed. Nanomaterials, film-forming substances, and polymers are among the most used compounds employed mainly in the modification of glassy carbon, screen-printed, and carbon paste electrodes. Composites composed of several compounds and/or materials have also found growing interest in the development of modified electrodes. Environmentally friendly substances and natural products (mainly biopolymers and plant extracts) have continued to be included in the modification of electrodes for trace heavy metal analysis. The main analytical performance parameters of the modified electrodes as well as possible interferences affecting the determination of the target analytes, are discussed. Finally, a critical evaluation of the main findings from these studies and an outlook discussing possible improvements in this area of research are presented.

Advances in paper-based electrochemical immunosensors: review of fabrication strategies and biomedical applications

Cellulose paper-based sensing devices have shown promise in addressing the accuracy, sensitivity, selectivity, analysis time and cost of current disease diagnostic tools owing to their excellent physical and physiochemical properties, high surface-area-to-volume ratio, strong adsorption capabilities, ease of chemical functionalization for immobilization, biodegradability, biocompatibility and liquid transport by simple capillary action. This review provides a comprehensive overview of recent advancements in the field of electrochemical immunosensing for various diseases, particularly in underdeveloped regions and globally. It highlights the significant progress in fabrication techniques, fluid control, signal transduction and paper substrates, shedding light on their respective advantages and disadvantages. The primary objective of this review article is to compile recent advances in the field of electrochemical immunosensing for the early detection of diseases prevalent in underdeveloped regions and globally, including cancer biomarkers, bacteria, proteins and viruses. Herein, the critical need for new, simplistic early detection strategies to combat future disease outbreaks and prevent global pandemics is addressed. Moreover, recent advancements in fabrication techniques, including lithography, printing and electrodeposition as well as device orientation, substrate type and electrode modification, have highlighted their potential for enhancing sensitivity and accuracy.

Trends in Paper-Based Sensing Devices for Clinical and Environmental Monitoring

Environmental toxic pollutants and pathogens that enter the ecosystem are major global issues. Detection of these toxic chemicals/pollutants and the diagnosis of a disease is a first step in efficiently controlling their contamination and spread, respectively. Various analytical techniques are available to detect and determine toxic chemicals/pathogens, including liquid chromatography, HPLC, mass spectroscopy, and enzyme-linked immunosorbent assays. However, these sensing strategies have some drawbacks such as tedious sample pretreatment and preparation, the requirement for skilled technicians, and dependence on large laboratory-based instruments. Alternatively, biosensors, especially paper-based sensors, could be used extensively and are a cost-effective alternative to conventional laboratory testing. They can improve accessibility to testing to identify chemicals and pollutants, especially in developing countries. Due to its low cost, abundance, easy disposal (by incineration, for example) and biocompatible nature, paper is considered a versatile material for the development of environmentally friendly electrochemical/optical (bio) sensor devices. This review presents an overview of sensing platforms constructed from paper, pointing out the main merits and demerits of paper-based sensing systems, their fabrication techniques, and the different optical/electrochemical detection techniques that they exploit.

Paper-Based Humidity Sensors as Promising Flexible Devices: State of the Art: Part 1. General Consideration

In the first part of the review article "General considerations" we give information about conventional flexible platforms and consider the advantages and disadvantages of paper when used in humidity sensors, both as a substrate and as a humidity-sensitive material. This consideration shows that paper, especially nanopaper, is a very promising material for the development of low-cost flexible humidity sensors suitable for a wide range of applications. Various humidity-sensitive materials suitable for use in paper-based sensors are analyzed and the humidity-sensitive characteristics of paper and other humidity-sensitive materials are compared. Various configurations of humidity sensors that can be developed on the basis of paper are considered, and a description of the mechanisms of their operation is given. Next, we discuss the manufacturing features of paper-based humidity sensors. The main attention is paid to the consideration of such problems as patterning and electrode formation. It is shown that printing technologies are the most suitable for mass production of paper-based flexible humidity sensors. At the same time, these technologies are effective both in the formation of a humidity-sensitive layer and in the manufacture of electrodes.

Molecular testing devices for on-site detection of E. coli in water samples

Escherichia coli (E. coli) cells are present in fecal materials that can be the main source for disease-causing agents in water. As a result, E. coli is recommended as a water quality indicator. We have developed an innovative platform to detect E. coli for monitoring water quality on-site by integrating paper-based sample preparation with nucleic acid isothermal amplification. The platform carries out bacterial lysis and DNA enrichment onto a paper pad through ball-based valves for fluid control, with no need of laboratory equipment, followed by loop-mediated isothermal amplification (LAMP) in a battery-operated coffee mug, and colorimetric detection. We have used the platform to detect E. coli in environmental water samples in about 1 h, with a limit of quantitation of 0.2 CFU/mL, and 3 copies per reaction. The platform was confirmed for detecting multiple E. coli strains, and for water samples of different salt concentrations. We validated the functions of the platform by analyzing recreational water samples collected near the Atlantic Ocean that contain different concentrations of salt and bacteria.

Advances on microfluidic paper-based electroanalytical devices

Since the inception of the first electrochemical devices on paper substrates, many different reports of microfluidic paper-based electroanalytical devices (μPEDs), innovative hydrophobic barriers and electrode fabrication processes have allowed the incorporation of diverse materials, resulting in different applications and a boost in performance. These advancements have led to the creation of paper-based devices with comparable performance to many standard conventional devices, with the added benefits of pumpless fluidic transport, component separation and reagent storage that can be exploited to automate and handle sample preprocessing. Herein, we review μPEDs, summarize the characteristics and functionalities of μPEDs, such as separation, fluid flow control and storage, and outline the conventional and emerging fabrication and modification approaches for μPEDs. We also examine the recent application of μPEDs in biomedicine, the environment, and food and water safety, as well as some limitations and challenges that must be addressed.

The Effect of Paper on the Detection Limit of Paper-Based Potentiometric Chloride Sensors

While paper is an excellent material for use in many other portable sensors, potentiometric paper-based sensors have been reported to perform worse than conventional rod-shaped electrodes, in particular in view of limits of detection (LODs). Reported here is an in-depth study of the lower LOD for Cl^- measurements with paper-based devices comprising AgCl/Ag transducers. Contamination by Cl^- from two commonly used device materials─a AgCl/Ag ink and so-called ashless filter paper─was found to increase the concentration of Cl^- in paper-contained samples far above what is expected for the spontaneous dissolution of the transducer's AgCl, thereby worsening lower LODs. In addition, for the case of Ag⁺, the commonly hypothesized adsorption of metal cations onto filter paper was found not to significantly affect the performance of AgCl/Ag transducers. We note that in the context of chemical analysis, metal impurities of paper are often mentioned in the literature, but Cl^- contamination of paper has been overlooked.

Paper-Based Enzymatic Electrochemical Sensors for Glucose Determination

The general objective of Analytical Chemistry, nowadays, is to obtain best-quality information in the shortest time to contribute to the resolution of real problems. In this regard, electrochemical biosensors are interesting alternatives to conventional methods thanks to their great characteristics, both those intrinsically analytical (precision, sensitivity, selectivity, etc.) and those more related to productivity (simplicity, low costs, and fast response, among others). For many years, the scientific community has made continuous progress in improving glucose biosensors, being this analyte the most important in the biosensor market, due to the large amount of people who suffer from diabetes mellitus. The sensitivity of the electrochemical techniques combined with the selectivity of the enzymatic methodologies have positioned electrochemical enzymatic sensors as the first option. This review, focusing on the electrochemical determination of glucose using paper-based analytical devices, shows recent approaches in the use of paper as a substrate for low-cost biosensing. General considerations on the principles of enzymatic detection and the design of paper-based analytical devices are given. Finally, the use of paper in enzymatic electrochemical biosensors for glucose detection, including analytical characteristics of the methodologies reported in relevant articles over the last years, is also covered.

Advancements in Testing Strategies for COVID-19

The SARS-CoV-2 coronavirus, also known as the disease-causing agent for COVID-19, is a virulent pathogen that may infect people and certain animals. The global spread of COVID-19 and its emerging variation necessitates the development of rapid, reliable, simple, and low-cost diagnostic tools. Many methodologies and devices have been developed for the highly sensitive, selective, cost-effective, and rapid diagnosis of COVID-19. This review organizes the diagnosis platforms into four groups: imaging, molecular-based detection, serological testing, and biosensors. Each platform's principle, advancement, utilization, and challenges for monitoring SARS-CoV-2 are discussed in detail. In addition, an overview of the impact of variants on detection, commercially available kits, and readout signal analysis has been presented. This review will expand our understanding of developing advanced diagnostic approaches to evolve into susceptible, precise, and reproducible technologies to combat any future outbreak.

Plot-on-demand integrated paper-based sensors for drop-volume voltammetric monitoring of Pb(II) and Cd(II) using a bismuth nanoparticle-modified electrode

The fabrication of fully ink-drawn fluidic electrochemical paper-based analytical devices (ePADs) is reported for the determination of trace Pb(II) and Cd(II) by differential pulse anodic stripping voltammetry (DPASV). The fluidic pattern was formed on the paper substrate using an inexpensive computer-controlled x-y plotter and a commercial hydrophobic marker pen. Then, electrodes were deposited on the devices using a second x-y plotting step with a commercial technical pen filled with a graphite-based conductive ink prepared in house. The fabrication parameters of the ePADs were studied by cyclic voltammetry using the ferro/ferri couple as a probe and by scanning electron microscopy. The ePADs, featuring a bismuth nanoparticle-modified working electrode, were applied to the determination of Pb(II) and Cd(II) by DPASV. The chemical and instrumental conditions were studied. The limits of detection were 3.1 μg L^-1 for Cd(II) and 4.5 μg L^-1 for Pb(II) whereas the between-device reproducibility (expressed as the % relative standard deviation of the response at 6 different ePADs) was < 14%. Each ePAD requires 120 s to fabricate and costs less than 0.15 € in terms of consumables. The ePADs are suitable for the on-site determination of Pb(II) and Cd(II) in environmental and food samples.

A New Class of Electronic Devices Based on Flexible Porous Substrates

With the advent of the Internet of Things era, the connection between electronic devices and humans is getting closer and closer. New-concept electronic devices including e-skins, nanogenerators, brain-machine interfaces, and implantable medical devices, can work on or inside human bodies, calling for wearing comfort, super flexibility, biodegradability, and stability under complex deformations. However, conventional electronics based on metal and plastic substrates cannot effectively meet these new application requirements. Therefore, a series of advanced electronic devices based on flexible porous substrates (e.g., paper, fabric, electrospun nanofibers, wood, and elastic polymer sponge) is being developed to address these challenges by virtue of their superior biocompatibility, breathability, deformability, and robustness. The porous structure of these substrates can not only improve device performance but also enable new functions, but due to their wide variety, choosing the right porous substrate is crucial for preparing high-performance electronics for specific applications. Herein, the properties of different flexible porous substrates are summarized and their basic principles of design, manufacture, and use are highlighted. Subsequently, various functionalization methods of these porous substrates are briefly introduced and compared. Then, the latest advances in flexible porous substrate-based electronics are demonstrated. Finally, the remaining challenges and future directions are discussed.

An electrochemical paper-based analytical sensor for one-step latex protein detection

A proposed simple electrochemical paper-based analytical sensor offered one-step latex protein detection by measuring remaining copper after online protein complexation

Low-cost electrochemical paper-based device for exosome detection

A low-cost electrochemical paper-based analytical device was developed to quantify cancer cell-derived exosomes.

Couple batch-injection analysis and microfluidic paper-based analytical device: A simple and disposable alternative to conventional BIA apparatus

Under controlled dispersion conditions, sample injection towards a detector opened essential fields for the Analytical Chemistry fast development methods. Flow injection analysis (FIA) and batch injection analysis (BIA) systems are crucial for injecting the sample in these analytical methods. The BIA system eliminated the flow manifold, with samples injected directly onto the detector inside the batch injection cell. Paper was slightly evaluated coupled to FIA, and no reports were found associated with BIA. Still, it can potentially reduce the BIA manifold by removing the batch injection cell based on the capillarity properties to disperse the injected solution over the detection system. Hence, this article reported the first work coupling batch-injection analysis and microfluidic paper-based analytical device (BIA-μPAD) with pencil-drawn electrodes directly attached to the paper using a CO₂ laser pre-treated chromatographic paper. The laser pretreatment of the paper (optimized conditions: 6.5% laser power, 12 mm s^-1 scan rate, and 12 mm output distance) was essential to enhance the electrochemical response for ferri/ferrocyanide redox couple and paracetamol (PAR), as shown by spectroscopic and electrochemical techniques. The proposed BIA-μPAD was evaluated using pharmaceutical paracetamol samples as proof-of-concept (optimized conditions: 15 μL injected volume and 6.4 μL s^-1 dispensing rate), obtaining good linearity (R = 0.9961) and recovery values ranging from 95 to 103%. Repeatability (n = 16) and reproducibility (n = 9) tests with 1 mmol L^-1 PAR also presented well relative standard deviation (RSD) results of 5.1% and 6.6%, respectively. A sampling frequency of 76 h^-1 was obtained, which is a similar value compared with conventional BIA apparatus. Limits of detection and quantification were estimated in 0.046 and 0.154 mmol L^-1, respectively. Additionally, an improvement in the current response and the sample throughput was observed when comparing FIA and BIA-μPADs, attesting the applicability of the proposed device and opening for new possibilities related to paper-based devices coupled with flow techniques.

Electrochemical Capillary-Flow Immunoassay for Detecting Anti-SARS-CoV-2 Nucleocapsid Protein Antibodies at the Point of Care

Rapid and inexpensive serological tests for SARS-CoV-2 antibodies are needed to conduct population-level seroprevalence surveillance studies and can improve diagnostic reliability when used in combination with viral tests. Here, we report a novel low-cost electrochemical capillary-flow device to quantify IgG antibodies targeting SARS-CoV-2 nucleocapsid proteins (anti-N antibody) down to 5 ng/mL in low-volume (10 μL) human whole blood samples in under 20 min. No sample preparation is needed as the device integrates a blood-filtration membrane for on-board plasma extraction. The device is made of stacked layers of a hydrophilic polyester and double-sided adhesive films, which create a passive microfluidic circuit that automates the steps of an enzyme-linked immunosorbent assay (ELISA). The sample and reagents are sequentially delivered to a nitrocellulose membrane that is modified with a recombinant SARS-CoV-2 nucleocapsid protein. When present in the sample, anti-N antibodies are captured on the nitrocellulose membrane and detected via chronoamperometry performed on a screen-printed carbon electrode. As a result of this quantitative electrochemical readout, no result interpretation is required, making the device ideal for point-of-care (POC) use by non-trained users. Moreover, we show that the device can be coupled to a near-field communication potentiostat operated from a smartphone, confirming its true POC potential. The novelty of this work resides in the integration of sensitive electrochemical detection with capillary-flow immunoassay, providing accuracy at the point of care. This novel electrochemical capillary-flow device has the potential to aid the diagnosis of infectious diseases at the point of care.

Single-Use Fluidic Electrochemical Paper-Based Analytical Devices Fabricated by Pen Plotting and Screen-Printing for On-Site Rapid Voltammetric Monitoring of Pb(II) and Cd(II)

This work reports the fabrication of integrated electrochemical fluidic paper-based analytical devices (ePADs) using a marker pen drawing and screen-printing. Electrodes were deposited on paper using screen-printing with conductive carbon ink. Then, the desired fluidic patterns were formed on the paper substrate by drawing with a commercial hydrophobic marker pen using an inexpensive computer-controlled x-y plotter. The working electrode was characterized by cyclic voltammetry and scanning electron microscopy. The analytical utility of the electrochemical PADs is demonstrated through electrochemical determination of Pb(II) and Cd(II) by anodic stripping voltammetry. For this purpose, the sample was mixed with a buffer solution and a Bi(III) solution, applied to the test zone of the PAD, the metals were preconcentrated as a bismuth alloy on the electrode surface and oxidized by applying an anodic potential scan. The proposed manufacturing approach enables the large-scale fabrication of fit-for-purpose disposable PADs at low cost which can be used for rapid on-site environmental monitoring.

Origami Paper-Based Electrochemical (Bio)Sensors: State of the Art and Perspective

In the last 10 years, paper-based electrochemical biosensors have gathered attention from the scientific community for their unique advantages and sustainability vision. The use of papers in the design the electrochemical biosensors confers to these analytical tools several interesting features such as the management of the solution flow without external equipment, the fabrication of reagent-free devices exploiting the porosity of the paper to store the reagents, and the unprecedented capability to detect the target analyte in gas phase without any sampling system. Furthermore, cost-effective fabrication using printing technologies, including wax and screen-printing, combined with the use of this eco-friendly substrate and the possibility of reducing waste management after measuring by the incineration of the sensor, designate these type of sensors as eco-designed analytical tools. Additionally, the foldability feature of the paper has been recently exploited to design and fabricate 3D multifarious biosensors, which are able to detect different target analytes by using enzymes, antibodies, DNA, molecularly imprinted polymers, and cells as biocomponents. Interestingly, the 3D structure has recently boosted the self-powered paper-based biosensors, opening new frontiers in origami devices. This review aims to give an overview of the current state origami paper-based biosensors, pointing out how the foldability of the paper allows for the development of sensitive, selective, and easy-to-use smart and sustainable analytical devices.

Development and Application of Mobile Apps for Molecular Sensing: A Review

Modern smartphone-based sensing devices are generally standalone detection platforms that can transduce signals (via the built-in USB port, audio jack, or camera), perform analysis through mobile applications (apps), and display results on the screen/user interface. The advancement toward this ultimate form of on-site chemical analysis and point-of-care diagnosis is tied closely with the evolution of mobile technology. Previous reviews in the field mainly focused on the physical platforms while overlooking the role of mobile apps in such devices. There exist three general stages throughout the development: (1) early generation telemedicine, (2) mobile phone-assisted clinical diagnosis (without apps), and (3) mobile app-based sensing devices for various analytes. This review presents the key breakthroughs during each stage, recent development, remaining challenges, and future perspectives of the field. Representative examples, spanning from the pioneering point-of-care testing to the latest devices with integrated mobile apps, are classified by their sensing mechanisms. The review also discusses the scarcity of open-source apps dedicated to molecular sensing. With the introduction of more open-source and commercial apps, the mobile app-based detection system is anticipated to dominate point-of-care diagnosis and on-site molecular sensing in our opinion.

Cost-effective paper-based electrochemical immunosensor using a label-free assay for sensitive detection of ferritin

Ferritin, a blood cell protein containing iron, is a crucial biomarker that is used to estimate the risk assessment of iron deficiency anemia. For point-of-care analysis, a reliable, cost-effective, selective, sensitive, and portable tool is extremely necessary. In this study, a label-free electrochemical immunosensor for detecting ferritin using a paper-based analytical device (ePAD) was created. The device pattern was custom designed onto filter paper to successfully fabricate a deliverable immunosensor. Graphene oxide was first modified onto the working electrode using an inkjet printing technique. An activation step of the electrode surface was then performed using standard 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)/N-hydroxysulfosuccinimide (sulfo-NHS) chemistry. Anti-ferritin antibodies were covalently immobilized onto the amine-reactive ester surface. The amount of ferritin was monitored by observing the electrochemical signal of the selected redox couple by differential pulse voltammetry (DPV). In the presence of ferritin, the sensor showed a considerable decrease in electrochemical response in a concentration-dependent manner. In contrast, there was no observable change in current response detected in the absence of ferritin. The current response provided a good correlation with ferritin concentrations in the range of 1 to 1000 ng mL-1, and the limit of detection (3SD/slope) was found to be 0.19 ng mL-1. This fabricated immunosensor offered good selectivity, reproducibility, and long-term storage stability. In addition, this proposed immunosensor was successfully applied to detect ferritin in human serum with satisfactory results. The promising results suggested that this handmade paper-based immunosensor may be an alternative device for the diagnosis of iron deficiency anemia.

Paper-based electrochemical sensor for on-site detection of the sulphur mustard

Herein, we report a novel paper-based electrochemical sensor for on-site detection of sulphur mustards. This sensor was conceived combining office paper-based electrochemical sensor with choline oxidase enzyme to deliver a sustainable sensing tool. The mustard agent detection relies on the evaluation of inhibition degree of choline oxidase, which is reversibly inhibited by sulphur mustards, by measuring the enzymatic by-product H₂O₂ in chronoamperometric mode. A nanocomposite constituted of Prussian Blue nanoparticles and Carbon Black was used as working electrode modifier to improve the electroanalytical performances. This bioassay was successfully applied for the measurement of a sulphur mustard, Yprite, obtaining a detection limit in the millimolar range (LOD = 0.9 mM). The developed sensor, combined with a portable and easy-to-use instrumentation, can be applied for a fast and cost-effective detection of sulphur mustards.

Aptamer-Integrated Multianalyte-Detecting Paper Electrochemical Device

On-site detection of multiple small-molecule analytes in complex sample matrixes would be highly valuable for diverse biosensing applications. Paper electrochemical devices (PEDs) offer an especially appealing sensing platform for such applications due to their low cost, portability, and ease of use. Using oligonucleotide-based aptamers as biorecognition elements, we here for the first time have developed a simple, inexpensive procedure for the fabrication of aptamer-modified multiplex PEDs (mPEDs), which can robustly and specifically detect multiple small molecules in complex samples. These devices are prepared via an ambient vacuum filtration technique using carbon and metal nanomaterials that yields precisely patterned sensing architecture featuring a silver pseudo-reference electrode, a gold counter electrode, and three gold working electrodes. The devices are user-friendly, and the fabrication procedure is highly reproducible. Each working electrode can be readily modified with different aptamers for sensitive and accurate detection of multiple small-molecule analytes in a single sample within seconds. We further demonstrate that the addition of a PDMS chamber allows us to achieve detection in microliter volumes of biological samples. We believe this approach should be highly generalizable, and given the rapid development of small-molecule aptamers, we envision that facile on-site multi-analyte detection of diverse targets in a drop of sample should be readily achievable in the near future.

Nanomaterial-enhanced 3D-printed sensor platform for simultaneous detection of atrazine and acetochlor

The widespread use of herbicides in agriculture and gardening causes environmental and safety issues such as water pollution. Thus, efficient and convenient analysis of the levels of herbicide residues is of significant importance. Here, we employed 3D-printing to design a multiplex immunosensor for simultaneous detection of two widely used herbicides, atrazine and acetochlor. Multiplexing was achieved through customization of a lateral flow immunoassay, and then integrated with an electrochemical analyzer for ultrasensitive detection. Quantification of herbicide residues was realized through the detection of a novel nanomaterial label, the mesoporous core-shell palladium@platium nanoparticle (Pd@Pt NP), for its outstanding peroxidase-like property. During the electrochemical analysis, the catalytic activity of Pd@Pt NPs on the redox reaction between thionin acetate and hydrogen peroxide provided an electrochemically driven signal that accurately indicated the level of herbicide residues. Using this Nanomaterial-enhanced multiplex electrochemical immunosensing (NEMEIS) system, simultaneous detection of atrazine and acetochlor was realized with a limit of detection of 0.24 ppb and 3.2 ppb, respectively. To further evaluate the feasibility, the optimized NEMEIS was employed for detection in atrazine and acetochlor residue-containing spiked samples, and an overall recovery with 90.8% - 117% range was obtained. The NEMEIS constructed with the aid of 3D-printing provides a rapid, precise, economical, and portable detection device for herbicides, and its success suggests potential broad applications in chemical analysis, biosensors and point-of-care monitoring.

Fabrication and modification of homemade paper-based electrode systems

This work reports the simple and inexpensive fabrication of homemade paper-based carbon-printed electrodes (HP C-PEs), aiming to produce an alternative way to generate electrochemical biosensors to all and promoting their wide use. This is especially important in times of pandemics, considering the excellent features of electrochemical biosensing, which may ensure portability, low-cost and quick responses. HP C-PEs were fabricated using a standard cellulose filter paper that was first modified with wax, to make it hydrophobic. Then, the electrodes were manually printed on top of this cellulose/wax substrate. The electrodes were designed by having standard configurations for potentiometric and electrochemical readings, combining two or three electrodes. In general, both electrode systems showed excellent electrochemical and mechanical features, which were better in specific cases than commercial devices. The 3-electrode system displayed high current levels with low peak-to-peak potential separation, yielding highly stable signals after consecutive electrode bending that corresponded to high active areas. The possibility of modifying the devices with polymers produced in-situ was also explored and proven successful, providing also advantageous features when compared to other devices. The 2-electrode system was also proven highly stable and capable of subsequent use in potentiometric sensing development. Overall, the fabrication process of the 2- and 3-electode systems described herein may be employed in laboratories to produce successful electrochemical biosensors, with the final devices displaying excellent electrochemical and mechanical features. This procedure offers the advantages of being simple and inexpensive, when compared to other commercial devices, while using materials that are promptly available and that may undergo a worldwide use.

Paper-Based Screen-Printed Electrodes: A New Generation of Low-Cost Electroanalytical Platforms

Screen-printed technology has helped considerably to the development of portable electrochemical sensors since it provides miniaturized but robust and user-friendly electrodes. Moreover, this technology allows to obtain very versatile transducers, not only regarding their design, but also their ease of modification. Therefore, in the last decades, the use of screen-printed electrodes (SPEs) has exponentially increased, with ceramic as the main substrate. However, with the growing interest in the use of cheap and widely available materials as the basis of analytical devices, paper or other low-cost flat materials have become common substrates for SPEs. Thus, in this revision, a comprehensive overview on paper-based SPEs used for analytical proposes is provided. A great variety of designs is reported, together with several examples to illustrate the main applications.

Fabrication of Aptamer-Modified Paper Electrochemical Devices for On-Site Biosensing

Electrochemical aptamer-based (E-AB) sensors offer a powerful and general means for analyte detection in complex samples for various applications. Paper-based E-AB sensors could enable portable, low-cost, and rapid detection of a broad range of targets, but it has proven challenging to fabricate suitable three-electrode systems on paper. Here, we demonstrate a simple, economic, and environmentally friendly strategy for fabricating aptamer-modified paper electrochemical devices (PEDs) via ambient vacuum filtration. The material, shape, size, and thickness of the three-electrode PED system can be fully customized. We developed aptamer-modified PEDs that enable sensitive and specific detection of small molecules in minimally processed biosamples. The sensitivity and stability of the PEDs are comparable to E-AB sensors based on commercial gold electrodes. We believe our strategy can lead to the development of high performance PEDs for the on-site detection of a variety of analytes.

An origami paper-based nanoformulated immunosensor detects picograms of VEGF-C per milliliter of blood

Detecting vascular endothelial growth factor C (VEGF-C), a kind of tumor biomarker, is of significant clinical importance in evaluating the prognosis of patients with cancer. However, laboratory analyses are usually not suitable for point-of-care testing because they are expensive and time consuming. In response to these challenges, we fabricated an origami paper-based microfluidic electrochemical device. To improve the specificity of VEGF-C detection, nanocomposites, synthesized by new methylene blue (NMB), amino-functional single-walled carbon nanotubes (NH₂-SWCNTs), and gold nanoparticles (AuNPs), were used to modify the surface of working electrodes. Results of electrochemical detection showed that the immunosensor had excellent linearity, ranging from 0.01 to 100 ng mL⁻¹ (R² = 0.988), and the limit of detection was 10 pg mL⁻¹. To confirm the high specificity of the device under real-world conditions, we evaluated the device using clinical serum samples from our hospital. The results demonstrated that the device had an excellent performance and could provide a platform for real-time detection of cancers.

Sun, Wang et al. report an origami paper-based immunosensor for the electrochemical detection of the VEGF-C biomarker in blood serum. The immunosensor is made by modifying the surface of working electrodes with new methylene blue, amino-functional single-walled carbon nanotubes, and gold nanoparticles and demonstrates excellent performance with a limit of detection in the range of picograms per milliliter.

Designing Sensing Devices Using Porous Composite Materials

The need for portable and inexpensive analytical devices for various critical issues has led researchers to seek novel materials to construct them. Soft porous materials, such as paper and sponges, are ideal candidates for fabricating such devices due to their light weight and high availability. More importantly, their great compatibility toward modifications and add-ons allows them to be customized to match different objectives. As a result, porous material-based composites have been extensively used to construct sensing devices applied in various fields, such as point-of-care testing, environmental sensing, and human motion detection. In this article, we present fundamental thoughts on how to design a sensing device based on these interesting composite materials and provide correlated examples for reader’s references. First, a rundown of devices made with porous composite materials starting from their fabrication techniques and compatible detection methods is given. Thereafter, illustrations are provided on how device function and property improvements are achieved with a delicate use of composite materials. This includes extending device lifetime by using polymer films to protect the base material, while signal readout can be enhanced by a careful selection of protective cover and the application of advanced photo image analysis techniques. In addition to chemical sensors, mechanical responsive devices based on conductive composite materials are also discussed with a focus on base material selection and platform design. We hope the ideas and discussions presented in this article can help researchers interested in designing sensing devices understand the importance and usefulness of composite materials.

Aspects of Point-of-Care Diagnostics for Personalized Health Wellness

Advancements in analytical diagnostic systems for point-of-care (POC) application have gained considerable attention because of their rapid operation at the site required to manage severe diseases, even in a personalized manner. The POC diagnostic devices offer easy operation, fast analytical outcome, and affordable cost, which promote their advanced research and versatile adoptability. Keeping advantages in view, considerable efforts are being made to design and develop smart sensing components such as miniaturized transduction, interdigitated electrodes-based sensing chips, selective detection at low level, portable packaging, and sustainable durability to promote POC diagnostics according to the needs of patient care. Such effective diagnostics systems are in demand, which creates the challenge to make them more efficient in every aspect to generate a desired bio-informatic needed for better health access and management. Keeping advantages and scope in view, this mini review focuses on practical scenarios associated with miniaturized analytical diagnostic devices at POC application for targeted disease diagnostics smartly and efficiently. Moreover, advancements in technologies, such as smartphone-based operation, paper-based sensing assays, and lab-on-a-chip (LOC) which made POC more sensitive, informative, and suitable for major infectious disease diagnosis, are the main focus here. Besides, POC diagnostics based on automated patient sample integration with a sensing platform is continuously improving therapeutics interventions against specific infectious disease. This review also discussed challenges associated with state-of-the-art technology along with future research opportunities to design and develop next generation POC diagnostic systems needed to manage infectious diseases in a personalized manner.

Enhanced performance of pencil-drawn paper-based electrodes by laser-scribing treatment

Electrochemical Paper-based Analytical Devices (ePADs) are an alternative to traditional portable analytical techniques due to features such as low-cost, easy surface modification with different materials, and high sensitivity. A fast and simple method to fabricate enhanced ePADs using pencil-drawing which involves the CO2 laser treatment of the carbon surface deposited on paper is described. The electrochemical performances of the devices were evaluated using cyclic voltammetry (CV) with different redox probes and electrochemical impedance spectroscopy (EIS). The electrochemical results show that a treated surface presents a lower resistance to charge transfer and changes the approach of the probe and the overlap of its orbitals with the electrode. To investigate the effects of the laser treatment process, chemical and structural characteristics were evaluated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. These results indicated that laser treatment promoted the restoration of carbon-carbon double bonds and removed a thin layer of nanodebris present in commercial pencils, resulting in an improvement of the electrochemical kinetics. As a proof-of-concept, the Pencil-Drawing Electrode (PDE) was used for the detection and quantification of furosemide (FUR) in a sample of synthetic urine, exhibiting a limit of detection (LOD) of 2.4 × 10-7 mol L-1. The percentages of recovery of the FUR added to the samples A and B were 95% and 110%, respectively. The analysis using CO2 laser-treated PDE resulted in a fast, simple, and reliable method for this doping agent.

Thermoplastic electrodes as a new electrochemical platform coupled to microfluidic devices for tryptamine determination

This study reports a new electrochemical method for tryptamine determination using a paper-based microfluidic device and a thermoplastic electrode (TPE) as an amperometric detector. Tryptamine (Tryp) is a biogenic amine present in drinks and foods. Even though this compound has some beneficial effects on human health, the ingestion of foods with high concentrations of Tryp may be detrimental, which justifies the need for monitoring the Tryp levels. The TPEs were made from 50% carbon black and 50% polycaprolactone and characterized by cyclic voltammetry, demonstrating enhancement in the analytical response compared to other carbon composites. TPEs also showed a better antifouling effect for Tryp compared to conventional glassy carbon electrodes. Once characterized, the electrodes were incorporated into the microfluidic device to determine Tryp in water and cheese samples using amperometry. A linear range was achieved from 10 to 75 μmol L^-1 with limits of detection and quantification of 3.2 and 10.5 μmol L^-1, respectively. Therefore, this work shows promising findings of the electrochemical determination of Tryp, bringing valuable results regarding the electrochemical properties of thermoplastic composites.