Nail polish and carbon powder: An attractive mixture to prepare paper-based electrodes

Innovative Electrochemical Nano-Robot: Integrating Printed Nanoelectronics with a Remote-Controlled Robotic for On-Site Underwater Electroanalysis

Smart and remote sensing technologies offer significant advantages across various applications. This study introduces an innovative approach integrating printed electrochemical nanosensors with a remotely controllable underwater robot, creating a "Robo-sensor" for underwater and surface water sensing. Unlike conventional underwater sensors, the Robo-sensor operates in three dimensions, both on the surface and underwater, targeting specific locations such as coastal areas and deep-sea environments while transmitting data to an external analytical unit. To achieve this, we developed a conductive nanoink by combining graphite, silver nanorods (AgNRs; diameter: 30 ± 20 nm, length: 2 ± 0.2 μm), nail polish as a cohesive agent, and an organic solvent. This ink was used to fabricate an electroanalytical system on the mini-robot's body. The Robo-sensor, connected to a portable potentiostat, demonstrated linear responses to hydroquinone (HQ) and nitrite ions, with detection ranges of 5.0-1356.0 and 3.0-1200.0 μM, respectively, under artificial seawater conditions (High salinity). Its repeatability (RSD <6%), stability (up to 40 continuous applications with an error < ± 10%), and sensitivity were thoroughly evaluated. The Robo-sensor's practical applications included detecting chemical leaks in underwater pipelines containing HQ, a hazardous chemical relevant to coastal industries such as petrochemicals. Additionally, it analyzed surface water contaminated with NO2- near and far from a wastewater discharge pipeline, providing valuable insights for environmental and ecosystem investigations. In conclusion, the developed Robo-sensor enables on-site analysis both on the surface and underwater, reducing time, costs, and risks in high-hazard environments like deep waters. With further modifications, this strategy could be adapted for diverse applications, including offshore oil and petrochemical operations, corrosion studies, and underwater environmental monitoring, thus expanding the real-world impact of electrochemical science.

Simple graphite/PVC ink-designed paper-based electrodes integrated with a 3D-printed electrochemical device for affordable analyses

A simple and cost-effective methodology for manufacturing a portable electroanalytical device is reported. The device is based on a graphite/polyvinyl chloride (PVC) paper-based electrode coupled to a miniaturized 3D-printed electrochemical cell (3DEC). The 3DEC was designed to ensure the reproducibility of the system by delimitating the paper-based graphite electrode (PGE) area. The disposable PGE was fabricated by paint-brushing a conductive ink based on graphite powder and toluene-free PVC glue, onto a kraft paper. Different weight proportions (wt%) of graphite/PVC were evaluated regarding mechanical stability and electrochemical behavior. Cyclic voltammetric (CV) analysis in the presence of the [Fe(CN)6]3-/4- redox probe has shown that as the wt% of graphite in the ink increased from 50 to 90%, a clear decrease in peak potential separation (ΔEp) and increase in current are observed, indicating an improvement in charge transfer kinetics. However, 90 wt% graphite electrodes have shown poor adhesion to the substrate and easy leaching due to the small amount of PVC (binder). Therefore, the best PGE was achieved using 80:20 wt% graphite/PVC ink (PGE8020). Moreover, scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) mapping revealed a rugous and more uniform deposition of the conductive ink containing 80 wt% graphite. As a proof of concept, the graphite/PVC ink-based disposable electrodes were employed for the detection of 3-nitro-L-tyrosine (3-NLT) in synthetic urine samples, showing a detection limit of 2.85 μmol L-1, and %recovery in synthetic urine between 97 and 109%, highlighting the reliability and applicability of the proposed approach.

Properties Evaluation of Different Graphite Brands through the Development of Electrochemical Sensors for Phenolic Compounds Detection

Graphite is one of the most utilized materials for producing conductive inks for screen-printed electrodes (SPEs). Because of their wide range of applications, several brands are available in the market. Consequently, it is possible to find particles with different properties. In this context, these differences can generate variations in the device performance. Based on this, the present work compares paper analytical devices (PADs), which were prepared with three different graphite brands and photographic paper, to evaluate how these possible distinctions can affect the devices' performance. CA detection was carried out with square wave voltammetry, where the devices produced with Fisher Chemical and Sigma-Aldrich graphite stood out for their high stability and current magnitude. Under the optimized parameters, analytical curves were constructed for each PAD, resulting in 5.0 × 10-6 to 1.0 × 10-4 mol L-1 and 5.0 × 10-7 to 1.0 × 10-4 mol L-1 for that manufactured with Fisher Chemical and Sigma-Aldrich graphite, respectively. In addition, the limits of detection were 1.39 μmol L-1 for the one produced with Fisher Chemical graphite and 0.288 μmol L-1 for the one produced with Sigma-Aldrich graphite.

Kraft-Based Femtosecond Laser-Induced Graphene for Electrochemical Dopamine Sensing

In this work, laser-induced graphene from kraft paper (kraft paper-LIG) was employed for the nonenzymatic electrochemical sensing of dopamine (DA). We reported the fabrication and characterization of a disposable, cost-effective, kraft-based electrochemical dopamine sensor with the sensing electrode consisting of laser-induced graphene derived from kraft paper. Kraft paper-LIG was formed by the femtosecond laser modification of kraft paper into a three-dimensional (3D) graphene arrangement. Our study labeled that the electrochemical activity of kraft paper-LIG can be improved via a dual pass (defocused, followed by focused lasing). Kinetic analysis showed that the effective heterogeneous electron transfer rate constant of kraft paper-LIG was 2.8 × 10-3 cm s-1 using Fe(CN)63-/4- as a redox probe. The kraft paper-LIG electrodes successfully detected and quantified dopamine in phosphate-buffered solution (PBS) within the 1-100 μM range. A linear response (R2 = 0.9986) was observed, with a sensitivity of 37.381 μA cm-2 μM-1. Furthermore, the kraft paper-LIG electrodes denoted sufficient selectivity for DA in electrolytes containing ascorbic acid (AA) and uric acid (UA). Kraft paper-LIG electrodes also had good stability at ordinary temperature.

Development of an Electrochemical Paper-Based Device Modified with Functionalized Biochar for the Screening of Paracetamol in Substandard Medicines

The global prevalence of counterfeit and low-quality pharmaceuticals poses significant health risks and challenges in medical treatments, creating a need for rapid and reliable drug screening technologies. This study introduces a cost-effective electrochemical paper-based device (ePAD) modified with functionalized bamboo-derived biochar (BCF) for the detection of paracetamol in substandard medicines. The sensor was fabricated using a custom 3D-printed stencil in PLA, designed for efficient production, and a 60:40 (m/m) graphite (GR) and glass varnish (GV) conductive ink, resulting in a robust and sensitive platform. The electroactive area of the ePAD/BCF sensor was determined as 0.37 cm2. Characterization via SEM and cyclic voltammetry (CV) verified its structural and electrochemical stability. The sensor demonstrated linear detection of paracetamol from 5.0 to 60.0 µmol L-1 with a detection limit of 3.50 µmol L-1. Interference studies showed high selectivity, with recoveries of over 90%, and the sensor successfully quantified paracetamol in commercial analgesic and anti-flu samples. This sustainable, bamboo-based ePAD offers a promising solution for rapid on-site pharmaceutical quality control, with significant potential to enhance drug screening accuracy.

Electroanalysis of Statin Drugs: A Review on the Electrochemical Sensor Architectures Ranging from Classical to Modern Systems

An overview of the latest advances in the design of electrochemical sensor architectures dedicated to the determination of drugs from the statin class is presented in this review. Statins are drugs widely consumed for cholesterol control, and their determination in different matrices through the application of electroanalysis is growing considering advantages such as operational simplicity, lower cost and ease of sample preparation. Within the context of statins, electrochemical sensor architectures can be subdivided into conventional/classical electrodes such as glassy carbon electrodes, carbon paste electrodes, pencil graphite electrodes, boron-doped diamond electrodes and metallic electrodes, and more modern electrode systems, including the screen-printed electrodes and 3D-printed electrodes. Thus, different aspects related to the preparation of these electrochemical sensors and analytical performance are presented, also reflecting advances in terms of designs of new architectures and possible improvements not previously reviewed. Analyzed samples, advantages and disadvantages of different implemented sensor's modification strategies and perspectives for the electroanalysis of statins are also included throughout the work.

An electrochemical microfluidic device for non-enzymatic cholesterol determination using a lab-made disposable electrode

Cholesterol is an important steroid and hormone precursor, and its levels in the blood are associated with risk factors for cardiovascular diseases. In this work, a non-enzymatic methodology for cholesterol determination in serum samples is described. First, a working electrode was constructed using homemade ink and a plastic substrate by a simple dunking process. Next, the dunked electrode (DWE) was modified with nickel ions (Ni-DWE) and combined with a low-cost microfluidic platform, resulting in a thread-based electroanalytical device (μTED). The arrangement of μTED consists of two coupled electrodes (one reference in the inlet reservoir and an auxiliary electrode against the outlet reservoir) and a mobile support for facile working electrode exchange. After optimization of construction parameters, the system was applied for non-enzymatic determination of cholesterol under alkaline conditions using the redox pair Ni(II)/Ni(III) as a mediator. Under the best analytical conditions, a calibration curve was constructed with a linear dynamic range (LDR) from 0.25 to 25.0 μmol L^-1, and the calculated limits of detection (LOD) and quantification (LOQ) were 0.074 and 0.24 μmol L^-1, respectively. No effects of possible interferents on electrochemical response were found in the presence of ascorbic acid, uric acid, dopamine, cysteine, and glucose, suggesting that the proposed device can be used for the determination of cholesterol without significant matrix effects of human plasma. Finally, cholesterol analysis was carried out using spiked plasma samples, and good recovery values were achieved.

Paper-based electrochemical biosensors for the diagnosis of viral diseases

Paper-based electrochemical analytical devices (ePADs) have gained significant interest as promising analytical units in recent years because they can be fabricated in simple ways, are low-cost, portable, and disposable platforms that can be applied in various fields. In this sense, paper-based electrochemical biosensors are attractive analytical devices since they can promote diagnose several diseases and potentially allow decentralized analysis. Electrochemical biosensors are versatile, as the measured signal can be improved by using mainly molecular technologies and nanomaterials to attach biomolecules, resulting in an increase in their sensitivity and selectivity. Additionally, they can be implemented in microfluidic devices that drive and control the flow without external pumping and store reagents, and improve the mass transport of analytes, increasing sensor sensitivity. In this review, we focus on the recent developments in electrochemical paper-based devices for viruses' detection, including COVID-19, Dengue, Zika, Hepatitis, Ebola, AIDS, and Influenza, among others, which have caused impacts on people's health, especially in places with scarce resources. Also, we discuss the advantages and disadvantages of the main electrode's fabrication methods, device designs, and biomolecule immobilization strategies. Finally, the perspectives and challenges that need to be overcome to further advance paper-based electrochemical biosensors' applications are critically presented.

Recycling 3D Printed Residues for the Development of Disposable Paper-Based Electrochemical Sensors

Here, we propose a recyclable approach using acrylonitrile-butadiene-styrene (ABS) residues from additive manufacturing in combination with low-cost and accessible graphite flakes as a novel and potential mixture for creating a conductive paste. The graphite particles were successfully incorporated in the recycled thermoplastic composite when solubilized with acetone and the mixture demonstrated greater adherence to different substrates, among which cellulose-based material made possible the construction of a paper-based electrochemical sensor (PES). The morphological, structural, and electrochemical characterizations of the recycled electrode material were demonstrated to be similar to those of the traditional carbon-based surfaces. Faradaic responses based on redox probe activity ([Fe(CN)6]3-/4-) exhibited well-defined peak currents and diffusional mass transfer as a quasi-reversible system (96 ± 5 mV) with a fast heterogeneous rate constant value of 2 × 10-3 cm s-1. To improve the electrode electrochemical properties, both the PES and the classical 3D-printed electrode surfaces were modified with a combination of multiwalled carbon nanotubes (MWCNTs), graphene oxide (GO), and copper. Both electrode surfaces demonstrated the suitable oxidation of nitrite at 0.6 and 0.5 V vs Ag, respectively. The calculated analytical sensitivities for PES and 3D-printed electrodes were 0.005 and 0.002 μA/(μmol L-1), respectively. The proposed PES was applied for the indirect amperometric analysis of S-nitroso-cysteine (CysNO) in serum samples via nitrite quantitation, demonstrating a limit of detection of 4.1 μmol L-1, with statistically similar values when compared to quantitative analysis of the same samples by spectrophotometry (paired t test, 95% confidence limit). The evaluated electroanalytical approach exhibited linear behavior for nitrite in the concentration range between 10 and 125 μmol L-1, which is suitable for realizing clinical diagnosis involving Parkinson's disease, for example. This proof of concept shows the great promise of this recyclable strategy combining ABS residues and conductive particles in the context of green chemical protocols for constructing disposable sensors.

Development of New Simple Compositions of Silver Inks for the Preparation of Pseudo-Reference Electrodes

Silver materials are known to present excellent properties, such as high electrical and thermal conductivity as well as chemical stability. Silver-based inks have drawn a lot of attention for being compatible with various substrates, which can be used in the production uniform and stable pseudo-reference electrodes with low curing temperatures. Furthermore, the interest in the use of disposable electrodes has been increasing due to the low cost and the possibility of their use in point-of-care and point-of-need situations. Thus, in this work, two new inks were developed using Ag as conductive material and colorless polymers (nail polish (NP) and shellac (SL)), and applied to different substrates (screen-printed electrodes, acetate sheets, and 3D-printed electrodes) to verify the performance of the proposed inks. Measurements attained with open circuit potential (OCP) attested to the stability of the potential of the pseudo-reference proposed for 1 h. Analytical curves for β-estradiol were also obtained using the devices prepared with the proposed inks as pseudo-references electrodes, which presented satisfactory results concerning the potential stability (RSD < 2.6%). These inks are simple to prepare and present great alternatives for the development of pseudo-reference electrodes useful in the construction of disposable electrochemical systems.

Fully handwritten electrodes on paper substrate using rollerball pen with silver nanoparticle ink, marker pen with carbon nanotube ink and graphite pencil

Herein, a so-called carbon nanotube (CNT) electrode was printed in on a paper substrate using the handwriting technique and carbon nanotube ink in a marker pen to print the working electrode, graphite pencil to print the counter electrode and graphite/silver nanoparticle (AgNP) ink in a rollerball pen to print the quasi-reference electrode. The carbon nanotube electrode was characterized via scanning electron microscopy. The electrode was optimized based on the type of paper, hydrophobic barrier and number of layers. In summary, the optimized parameters included the use of matte paper with a mineral spirit layer. The number of carbon nanotube layers to achieve the best electrochemical performance was 25. The final graphite electrode was a miniaturized and flexible paper-based electrochemical electrode. To evaluate the electrical properties of the electrodes, the ohmic resistance of each ink was tested using a multimeter and the obtained values were 18.62 kΩ for the CNT ink, 1.53 Ω for the AgNP ink and 3.53 kΩ for the graphite trace. These results indicate the good conductivity of each synthesized ink used in the fabrication of the CNT electrode. Finally, the electrode was used to measure the electrochemical response of different concentrations of K₄[Fe(CN)₆]. Then, a calibration curve was obtained from the voltammograms and linearity was observed in the range of 0.5-3.5 mM. This suggests that the CNT electrode has the potential to be used as an amperometric electrode.

Fabrication of paper-based analytical devices using a PLA 3D-printed stencil for electrochemical determination of chloroquine and escitalopram

In recent years, the use of prescribed and non-prescribed drugs has increased. Therefore, advances in new technologies and sensors for detecting molecules in natural environments are required. In this work, a 3D-printed polylactic acid stencil is used to fabricate paper-based analytical devices (ePADs). Herein, we report the use of carbon-based lab-manufactured conductive ink for the fabrication of sensors towards the detection of chloroquine and escitalopram. For each batch, eight ePADs were successfully fabricated. Firstly, the fabricated sensors were evaluated morphologically by scanning electron microscopy and electrochemically by cyclic voltammetry and electrochemical impedance spectroscopy experiments. The sensors displayed a well-defined voltammetric profile in the presence of the redox couple, when compared to a commercial carbon screen-printed electrode. Differential pulse voltammetry conducted the detection of chloroquine and escitalopram with detection limits of 4.0 and 0.5 µmol L⁻¹, respectively. The ePADs fabricated using the 3D stencil are here presented as alternatives for the fabrication of electrochemical analytical devices.

Disposable and low-cost electrochemical sensor based on the colorless nail polish and graphite composite material for tartrazine detection

A new method to manufacture electrochemical devices based on the graphite and colorless nail polish (N-grap) film was developed for tartrazine (Tz) detection. Scanning Electron Microscopy (SEM) demonstrates that the composite material presents a high porous carbon structure. Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were employed to electrochemically characterize the electrode material, which corroborates the porous structure of the N-graph due to the enhanced electroactive area (5.4-fold increase) and presented a heterogeneous electron transfer rate constant (k0) of 5.82 × 10-3 cm s-1 for potassium ferricyanide. The electrochemical determination of the Tz was carried out using square-wave voltammetry (SWV), under the optimized experimental conditions, which showed high sensitivity (0.793 A L mol-1) and a lower limit of detection (LOD) of 2.10 × 10-8 mol L-1 with a linear concentration ranging from 2.0 to 50.0 μmol L-1. The developed sensor was applied for the analysis of Tz in sports drink samples and the result obtained by N-grap device was statistically compared with a spectrophotometric method demonstrating good accordance and the accuracy of the proposed method. Based on these results, we believe that this new fabrication method to produce disposable and low-cost electrochemical devices can be an alternative method for in-field analysis of dye in commercial sport drink samples and other relevant applications.

Plug-and-play assembly of paper-based colorimetric and electrochemical devices for multiplexed detection of metals

We propose a “plug-and-play” (PnP) assembly for coupling paper-based colorimetric and electrochemical devices for multiplexed detection of metals.

Electrochemical analysis of anionic analytes in weakly supported media using electron transfer promotion effect: a case study on nitrite

In this study, a simple technique was developed for the electrochemical detection of anionic analytes in weakly supported media. This was conducted by the use of electrochemical paper-based analytical devices (ePADs). A sensing platform was modified with nereistoxin and used to determine nitrite as a case study. The electrochemical response was improved due to the accelerated electron transfer between the sensing platform and the nitrite through the electrostatic interaction of the amino group of nereistoxin and the nitrite. The electrocatalytic current of the nitrite in the presence of nereistoxin was enhanced in the weakly supported media. By using nereistoxin as a signal enhancer, 97% of the electrochemical signal was obtained at the low ionic strength of the electrolyte, while less than 35% of this signal was obtained in the absence of nereistoxin. The limit of detection was as low as 20 nM using an ePAD. Generally, the proposed ePAD serves as a promising, efficient and low-cost device for sensing applications in weakly supported media.

Stone Paper as a New Substrate to Fabricate Flexible Screen-Printed Electrodes for the Electrochemical Detection of Dopamine

Flexible screen-printed electrodes (HP) were fabricated on stone paper substrate and amperometrically modified with gold nanoparticles (HP-AuNPs). The modified electrode displayed improved electronic transport properties, reflected in a low charge-transfer resistance (1220 Ω) and high apparent heterogeneous electron transfer rate constant (1.94 × 10⁻³ cm/s). The voltammetric detection of dopamine (DA) was tested with HP and HP-AuNPs electrodes in standard laboratory solutions (pH 6 phosphate-buffered saline (PBS)) containing various concentrations of analyte (10⁻⁷–10⁻³ M). As expected, the modified electrode exhibits superior performances in terms of linear range (10⁻⁷–10⁻³ M) and limit of detection (3 × 10⁻⁸ M), in comparison with bare HP. The determination of DA was tested with HP-AuNPs in spiked artificial urine and in pharmaceutical drug solution (ZENTIVA) that contained dopamine hydrochloride (5 mg/mL). The results obtained indicated a very good DA determination in artificial urine without significant matrix effects. In the case of the pharmaceutical drug solution, the DA determination was affected by the interfering species present in the vial, such as sodium metabisulfite, maleic acid, sodium chloride, and propylene glycol.

Rapid Analysis in Continuous-Flow Electrochemical Paper-Based Analytical Devices

A simple and low-cost continuous-flow (CF) electrochemical paper-based analytical device (ePAD) coupled with thermoplastic electrodes (TPEs) was developed. The fast, continuous flow combined with flow injection analysis was made possible by adding two inlet reservoirs to the same paper-based hollow channel flowing over detection electrodes, terminating in a fan-shaped pumping reservoir. The upstream inlet reservoir was filled with buffer and provided constant flow through the device. Sample injections were performed by adding 2 μL of the sample to the downstream sample inlet. Differences in flow resistance resulted in sample plugs displacing buffer as the solution flowed over the working electrodes. The electrodes were fabricated by mixing carbon black and polycaprolactone (50% w/w). CF-TPE-ePADs were characterized with chronoamperometry using ferrocenylmethyl trimethylammonium as the electrochemical probe. Optimized flow rates and injection volumes gave analysis times roughly an order of magnitude faster than those of previously reported flow injection analysis ePADs. To demonstrate applicability, the CF-TPE-ePADs were used to quantify caffeic acid in three different tea samples. The proposed method had a linear range from 10 to 500 μmol L^-1 and limits of detection and quantification of 2.5 and 8.3 μmol L^-1, respectively. Our approach is promising for fabricating simple, inexpensive, yet high-performance, flow injection analysis devices using paper substrates and easy-to-make electrodes that do not require external mechanical pumping systems or complicated valves.

Disposable electrodes from waste materials and renewable sources for (bio)electroanalytical applications

The numerous advantages of disposable and screen-printed electrodes (SPEs) particularly in terms of portability, sensibility, sensitivity and low-cost led to the massive application of these electroanalytical devices. To limit the electronic waste and recover precious materials, new recycling processes were developed together with alternative SPEs fabrication procedures based on renewable, biocompatible sources or waste materials, such as paper, agricultural byproducts or spent batteries. The increased interest in the use of eco-friendly materials for electronics has given rise to a new generation of highly performing green modifiers. From paper based electrodes to disposable electrodes obtained from CD/DVD, in the last decades considerable efforts were devoted to reuse and recycle in the field of electrochemistry. Here an overview of recycled and recyclable disposable electrodes, sustainable electrode modifiers and alternative fabrication processes is proposed aiming to provide meaningful examples to redesign the world of disposable electrodes.

Fast and flexible strategy to produce electrochemical paper-based analytical devices using a craft cutter printer to create wax barrier and screen-printed electrodes

This paper describes a simple, low-cost, and highly flexible rapid prototyping method to construct electrochemical paper-based analytical device (ePAD) for multiplexed analyte determinations. The ePAD was composed of two electrochemical cell (EC) compartments, separated by hydrophobic barriers of wax, and screen-printed electrodes (SPEs) deposited directly over the surface of the filter paper. The ePAD was entirely constructed using an inexpensive craft cutter printer with no needed of a wax printer. The rapid prototyping method involves two steps: the deposition of the SPEs and the creation of the wax barriers. In this case, the SPEs were screen-printed on filter paper by using adhesive tape as mask by cutting the electrodes pattern with the cutter printer. Following, the wax barriers were created using stamps made of filter paper also cut with the printer and impregnated with wax. In the ePAD, each ECs containing an array of 4-working electrodes, allowing up to 4 replicates in a single measurement. Both ECs shared one counter and one reference electrodes, permitting the simultaneous multianalysis. The ePAD was successfully applied to simultaneous detection of paracetamol (PAR), caffeine (CAF), and ascorbic acid (AA) in drugs. PAR and CAF were detected in a sample using one EC and AA was detected, in a different sample, on the other EC, both with no chemical modifications in the working electrodes. Limits of detection of 0.04 mmol L^-1 for PAR, 0.22 mmol L^-1 for CAF, and 0.40 mmol L^-1 for AA were obtained. The construction process proposed provide an easy way to implement screen-printing electrodes and wax barriers in filter paper to create electrochemical devices for fast and simultaneous multianalysis.