Low-Cost Paper Electrode Fabricated by Direct Writing with Silver Nanoparticle-Based Ink for Detection of Hydrogen Peroxide in Wastewater

Development and application of a CNT-Ag-Cu-Al/PS-based paper electrode for detecting diverse analytes in complex matrices

Electrochemical sensors play a crucial role in the detection of different analytes in complex matrices, and their performance is highly dependent on the electrode capacity. However, most of the available electrodes can only be used for single-component detection, so it is urgent to develop electrodes with high sensitivity and selectivity for different components. Herein, we report an amphiprotic amino-bonded carbon nanotube-Ag/Cu/Al nanoparticle/polystyrene-coated paper electrode (CNT-Ag-Cu-Al/PS electrode), which can be used for the measurement of glucose (Glc), oxytetracycline (OTC), and hydroquinone (HQ), respectively. The results showed that the analytical sensitivity and selectivity of the CNT-Ag-Cu-Al/PS electrode were comparable to those of single metal-coated paper substrate. The developed electrode also exhibited excellent linear responses for Glc, OTC, and HQ in the ranges of 1.0-1000.0 μM, 1.0 × 10-2 to 10.0 μM, and 5.0 × 10-3 to 50.0 μM, and the limits of detection (LODs) were 0.2055 μM (Glc), 0.0074 μM (OTC), and 0.0048 μM (HQ). Owing to the characteristics of good selectivity, anti-interference, stability, and reproducibility, the CNT-Ag-Cu-Al/PS paper electrode has been successfully applied to the detection of these analytes in complex human body fluids, food, and environmental waters. The paper electrode is promising for the detection of target compounds in complex matrices.

Pen direct writing of SERRS-based lateral flow assays for detection of penicillin G in milk

Direct pen writing offers versatile opportunities for development of low-cost tests for point-of-care applications. In this work a lateral flow immunoassay (LFIA) test was fabricated by hand "writing" immunoprobes onto hand-cut nitrocellulose strips with a commercial fountain pen. The qualitative capabilities of the test were extended by addition of a Raman reporter and consequent design and fabrication of a Surface Enhanced Resonant Raman Scattering (SERRS)-LFIA test. As proof-of-concept, dual detection of penicillin G was achieved in milk with a visual LOD of 20 ppm and a dynamic range of 0.03-97.5 ppm. Evaluation against equivalent tests performed with conventionally prepared LFIA strips showed comparable results, thus demonstrating the validity of the test. These results demonstrate the potential for further decrease in cost and consequent broader use of LFIA tests in remote regions and resource-limited environments.

Inkjet-printed flexible graphene paper electrode for the electrochemical determination of mercury

Here, we report an inkjet-printed graphene paper electrode (IP-GPE) for the electrochemical analysis of mercuric ions (Hg(ii)) in industrial wastewater samples. Graphene (Gr) fabricated on a paper substrate was prepared by a facile solution-phase exfoliation method in which ethyl cellulose (EC) behaves as a stabilizing agent. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to determine the shape and multiple layers of Gr. The crystalline structure and ordered lattice carbon of Gr were confirmed by X-ray diffraction (XRD) and Raman spectroscopy. The nano-ink of Gr-EC was fabricated on the paper substance via an inkjet printer (HP-1112) and IP-GPE was exploited as a working electrode in linear sweep voltammetry (LSV) and cyclic voltammetry (CV) for the electrochemical detection of Hg(ii). The electrochemical detection is found to be diffusion-controlled illustrated by obtaining a correlation coefficient of 0.95 in CV. The present method exhibits a better linear range of 2-100 μM with a limit of detection (LOD) of 0.862 μM for the determination of Hg(ii). The application of IP-GPE in electrochemical analysis shows a user-friendly, facile, and economical method for the quantitative determination of Hg(ii) in municipal wastewater samples.

Nanobiosensors Design Using 2D Materials: Implementation in Infectious and Fatal Disease Diagnosis

Nanobiosensors are devices that utilize a very small probe and any form of electrical, optical, or magnetic technology to detect and analyze a biochemical or biological process. With an increasing population today, nanobiosensors have become the broadly used electroanalytical tools for the timely detection of many infectious (dengue, hepatitis, tuberculosis, leukemia, etc.) and other fatal diseases, such as prostate cancer, breast cancer, etc., at their early stage. Compared to classical or traditional analytical methods, nanobiosensors have significant benefits, including low detection limit, high selectivity and sensitivity, shorter analysis duration, easier portability, biocompatibility, and ease of miniaturization for on-site monitoring. Very similar to biosensors, nanobiosensors can also be classified in numerous ways, either depending on biological molecules, such as enzymes, antibodies, and aptamer, or by working principles, such as optical and electrochemical. Various nanobiosensors, such as cyclic voltametric, amperometric, impedimetric, etc., have been discussed for the timely monitoring of the infectious and fatal diseases at their early stage. Nanobiosensors performance and efficiency can be enhanced by using a variety of engineered nanostructures, which include nanotubes, nanoparticles, nanopores, self-adhesive monolayers, nanowires, and nanocomposites. Here, this mini review recaps the application of two-dimensional (2D) materials, especially graphitic carbon nitride (g-C₃N₄), graphene oxide, black phosphorous, and MXenes, for the construction of the nanobiosensors and their application for the diagnosis of various infectious diseases at very early stage.

Electrodes and electrocatalysts for electrochemical hydrogen peroxide sensors: a review of design strategies

H₂O₂ sensing is required in various biological and industrial applications, for which electrochemical sensing is a promising choice among various sensing technologies. Electrodes and electrocatalysts strongly influence the performance of electrochemical H₂O₂ sensors. Significant efforts have been devoted to electrode nanostructural designs and nanomaterial-based electrocatalysts. Here, we review the design strategies for electrodes and electrocatalysts used in electrochemical H₂O₂ sensors. We first summarize electrodes in different structures, including rotation disc electrodes, freestanding electrodes, all-in-one electrodes, and representative commercial H₂O₂ probes. Next, we discuss the design strategies used in recent studies to increase the number of active sites and intrinsic activities of electrocatalysts for H₂O₂ redox reactions, including nanoscale pore structuring, conductive supports, reducing the catalyst size, alloying, doping, and tuning the crystal facets. Finally, we provide our perspectives on the future research directions in creating nanoscale structures and nanomaterials to enable advanced electrochemical H₂O₂ sensors in practical applications.

Inkjet Printing: A Viable Technology for Biosensor Fabrication

Printing technology promises a viable solution for the low-cost, rapid, flexible, and mass fabrication of biosensors. Among the vast number of printing techniques, screen printing and inkjet printing have been widely adopted for the fabrication of biosensors. Screen printing provides ease of operation and rapid processing; however, it is bound by the effects of viscous inks, high material waste, and the requirement for masks, to name a few. Inkjet printing, on the other hand, is well suited for mass fabrication that takes advantage of computer-aided design software for pattern modifications. Furthermore, being drop-on-demand, it prevents precious material waste and offers high-resolution patterning. To exploit the features of inkjet printing technology, scientists have been keen to use it for the development of biosensors since 1988. A vast number of fully and partially inkjet-printed biosensors have been developed ever since. This study presents a short introduction on the printing technology used for biosensor fabrication in general, and a brief review of the recent reports related to virus, enzymatic, and non-enzymatic biosensor fabrication, via inkjet printing technology in particular.

A graphene-printed paper electrode for determination of H2O2 in municipal wastewater during the COVID-19 pandemic

Graphene prepared through exfoliation process was printed on paper substrate using inkjet-printer and then printed paper electrode was used as an electrochemical sensor for analysis of H2O2 in cyclic voltammetry.

Development of a surface-modified paper-based colorimetric sensor using synthesized Ag NPs-alginate composite

There has been an increase in the discovery and usage of sensors for the detection of chemical compounds in the field of analytical chemistry since the last several years. This has led to progressive research in nanotechnology for developing efficient nanomaterials for bio-chemical sensing applications. Thereby, a deft synthesis of silver nanoparticles (Ag NPs) under microwave irradiation was achieved using sodium alginate as a reducing and capping agent in a fast and cost-effective approach. As per the X-ray diffraction analysis, the average particle size of Ag NPs was found to be 10 nm. X-ray photoelectron spectroscpopy analysis showed characteristic peaks at binding energies of 368.10 and 374.11 eV indicating the formation of Ag NPs. The synthesized Ag NPs-alginate composite was further used to develop a paper-based sensor for the detection of H₂O₂. Detection of H₂O₂ is based on the discolouration of the Ag NPs-alginate composite modified paper sensor as a function of H₂O₂ concentration. The analysis of the decoloured paper strips was done by a smartphone camera and an RGB Colour Reader application (app) to measure colour intensity. The sensing characteristics were found in the range of 0.1-10 mM. The colour analysis revealed piecewise linear relationship of intensity of RGB to H₂O₂ concentration in the range of 0.1-1.5 and 2-10 mM with R² values of 0.97 and 0.9778, respectively. Owing to the high sensitivity, selectivity, and cost-effectiveness, the developed paper sensor can be a potential tool for real-time analysis of H₂O₂.

MODs vs. NPs: Vying for the Future of Printed Electronics

This Minireview compares two distinct ink types, namely metal-organic decomposition (MOD) and nanoparticle (NP) formulations, for use in the printing of some of the most conductive elements: silver, copper and aluminium. Printing of highly conductive features has found purpose across a broad array of electronics and as processing times and temperatures reduce, the avenues of application expand to low-cost flexible substrates, materials for wearable devices and beyond. Printing techniques such as screen, aerosol jet and inkjet printing are scalable, solution-based processes that historically have employed NP formulations to achieve low resistivity coatings printed at high resolution. Since the turn of the century, the rise in MOD inks has vastly extended the range of potentially applicable compounds that can be printed, whilst simultaneously addressing shelf life and sintering issues. A brief introduction to the field and requirements of an ink will be presented followed by a detailed discussion of a wide array of synthetic routes to both MOD and NP inks. Unindustrialized materials will be discussed, with the challenges and outlook considered for the market leaders: silver and copper, in comparison with the emerging field of aluminium inks.

Inkjet-printed paper-based electrochemical sensor with gold nano-ink for detection of glucose in blood serum

An inkjet-printed paper electrode with gold nanoparticle-ink as a non-enzymatic electrochemical sensor for detection of glucose in blood serum is reported.

Simple Electrochemical Synthesis of Polyethylenimine-Encapsulated Ag Nanoparticles from Solid AgCl Applied in Catalytic Reduction of H2O2

We report a simple and environmentally friendly synthesis of polyethylenimine (PEI)-encapsulated Ag nanoparticles (AgNPs) by a direct electroreduction of solid AgCl. The AgNPs, characterized by field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectroscopy (EDS), revealed that AgNPs diameters (100–500 nm) depended on the loading of the AgCl precursor. Using cyclic voltammetry (CV), it was confirmed that the AgNPs had a catalytic effect on the electrochemical reduction of H2O2. The obtained AgNPs were subsequently used to construct an electrochemical H2O2 sensor exhibiting a low detection limit (1.66 μM) and a wide linear response range, with real-life tests indicating an insensitivity to common interferents and confirming the potential use of the developed technique in diverse applications.

Disposable 3D GNAs/AuNPs DNA-Circuit Strip for miRNAs Dynamic Quantification

Real-time quantitative monitoring of miRNAs plays an essential role in diagnosis and therapeutics. Herein, a DSN-coupled graphene nanoarray/gold nanoparticles (GNAs/AuNPs) carbon paper (CP) electrode for the dynamic, sensitive, and real-time analysis of miRNAs is reported. GNAs are vertically grown on the conductive CP by radio frequency plasma enhanced chemical vapor deposition, and AuNPs are electrodeposited on CP/GNAs to build a 3D ultrasensitive sensing interface with large specific surface area, good conductivity and biocompatibility. The dynamic quantitative monitoring of microRNA-21 (miR-21) is realized by cyclic voltammetry with a series of different concentrations within 16 min, and this 3D GNAs/AuNPs DNA-circuit strip shows good performance for the simultaneous detection of miR-21 and miR-155, and the detection limits are as low as 21.4 and 30.3 am, respectively. Moreover, comparable detection results are achieved for clinical samples between the proposed sensor and qRT-PCR, suggesting the reliability of the constructed sensor. This ultrasensitive sensing and disposable DNA-circuit strip with 3D structure can efficiently shorten the diffusion distance between reactive biomolecules and the sensing interface, enhance the hybridization of probes and improve the sensitivity of the biosensor, holding great promise for the rapid, quantitative and dynamic monitoring of multiple low concentrations of biomolecules in point-of-care clinical analysis.

A low-cost paper-based flexible energy storage device using a conducting polymer nanocomposite

Herein, a simple approach is demonstrated for the fabrication of a paper-based flexible symmetrical supercapacitor as an energy saving device with composite functional materials of nickel nanoparticles (Ni NPs) and polypyrrole (PPy).

A KBr-impregnated paper substrate as a sample probe for the enhanced ATR-FTIR signal strength of anionic and non-ionic surfactants in an aqueous medium

Herein, we report a KBr-impregnated paper substrate as a sample probe to enhance the attenuated total reflection-Fourier transform infrared (ATR-FTIR) signal strength of anionic surfactants (AS) and non-ionic surfactants (NS) in an aqueous solution. The mechanism for the sensing of AS and NS is based on the strong interaction of surfactants with the silicate groups (SiO₄⁴⁻) of the KBr-impregnated paper substrate. The role of SiO₄⁴⁻ on the surface of the paper is to enhance the adsorption of AS and NS, resulting in improved IR signal intensities for the target analytes. The improved signal intensity at 1253 cm⁻¹ (SO₄²⁻, symmetric stretching) for AS and 1114 cm⁻¹ (C–O–C, stretching vibration) for NS were selected for quantification. SEM-EDX was employed to determine the elemental compositions of pre- and post-adsorbed AS and NS on glass fibre filter paper (GFF). The linear range for the determination of AS and NS was 10–100 μg L⁻¹ with a method detection limit (MDL) of 4 μg L⁻¹ and method quantification limit (MQL) of 12 μg L⁻¹. The good relative recovery of 71.4–109.7% and the interference studies showed the selectivity of the method for the determination of AS and NS in environmental water and commodity samples. The advantages of this method include its cost-effectiveness, enhanced sensitivity, disposability and accessibility of the paper substrate.

Flow diagram of the procedures for the analysis of surfactants using modified GFF paper substrate.

Reactive Conductive Ink Capable of In Situ and Rapid Synthesis of Conductive Patterns Suitable for Inkjet Printing

We report a fabrication method of the conductive pattern based on in situ reactive silver precursor inks by inkjet printing. The reactive silver precursor inks were prepared with ethylene glycol and deionized water mixture as the solvent, and silver nitrate as silver source. Sodium borohydride solution as the reducing agent was first coated on photographic paper by screen printing process, and then dried at 50 °C for 4 h. Furthermore, the reactive silver precursor inks were printed on a photographic paper coated with sodium borohydride using inkjet printing to form silver nanoparticles in situ due to redox reaction, and thus a conductive pattern was obtained. The effects of the reactive silver precursor ink concentration and printing layer number and treatment temperature on the electrical properties and microstructures of the printed patterns were investigated systematically. The size range of in situ-formed silver nanoparticles was 50–90 nm. When the reactive silver precursor ink concentration was 0.13 g/mL, the five-layer printed pattern exhibited a sheet resistance of 4.6 Ω/γ after drying at room temperature for 2 h; furthermore, the sheet resistance of the printed pattern decreased to 1.4 Ω/γ after drying at 130 °C for 2 h. In addition, the display function circuit was printed on the photographic paper to realize the display of the numbers 0–99. It provides new research ideas for the development of environmentally friendly and low-cost flexible paper-based circuits.

The direct-writing of low cost paper based flexible electrodes and touch pad devices using silver nano-ink and ZnO nanoparticles

We report a novel and simple approach for the synthesis of silver nanoparticles capped with inositol (Ag NPs/Ino) by the reduction of silver salt with ascorbic acid under basic conditions. UV-vis, TEM, FTIR and TGA techniques were used to characterize the Ag NPs/Ino to determine the size, shape and surface modification of the NPs. Stable silver nano-ink was prepared in aqueous solution containing 1% PVP (stabilizer) and glycerol (cosolvent) and was used for the direct-writing of a paper electrode with a roller ball-point pen for electrochemical applications. The solvent, stabilizing agents, concentration of NPs (10%), paper substrate, sintering temperature (40 °C) and sintering time (15 min) were optimized to obtain a uniform coating of Ag NPs on the paper substrate. Further, the synthesis and fabrication of ZnO NPs on a paper substrate was put forward to design a touch pad device based on the piezoelectric effect. The preparation of paper based devices suggests a direction for the development of a simple, low cost and compatible approach for the direct-writing of paper based flexible electrodes and electronics for future applications.

An anti-passivation ink for the preparation of electrodes for use in electrochemical immunoassays

p-Nitrophenylphosphate (PNPP) is usually employed as the substrate for enzyme-linked immunosorbent assays. p-Nitrophenol (PNP), the product of PNPP, with the catalyst alkaline phosphatase (ALP), will passivate an electrode, which limits applications in electrochemical analysis. A novel anti-passivation ink used in the preparation of a graphene/ionic liquid/chitosan composited (rGO/IL/Chi) electrode is proposed to solve the problem. The anti-passivation electrode was fabricated by directly writing the graphene-ionic liquid-chitosan composite on a single-side conductive gold strip. A glassy carbon electrode, a screen-printed electrode, and a graphene-chitosan composite-modified screen-printed electrode were investigated for comparison. Scanning electron microscopy was used to characterize the surface structure of the four different electrodes and cyclic voltammetry was carried out to compare their performance. The results showed that the rGO/IL/Chi electrode had the best performance according to its low peak potential and large peak current. Amperometric responses of the different electrodes to PNP proved that only the rGO/IL/Chi electrode was capable of anti-passivation. The detection of cardiac troponin I was used as a test example for electrochemical immunoassay. Differential pulse voltammetry was performed to detect cardiac troponin I and obtain a calibration curve. The limit of detection was 0.05 ng/ml.

Determination of Concentrated Hydrogen Peroxide Free from Oxygen Interference at Stainless Steel Electrode

H₂O₂ is frequently used at high concentrations in various applications. It is very challenging to detect high concentrations of H₂O₂ and to eliminate oxygen interference for H₂O₂ detection through electrochemical reduction. In the present investigation, the electrochemistry of H₂O₂ at stainless steel electrode has been carried out for the first time. A cathodic peak for H₂O₂ reduction was observed at about -0.40 V, and no cathodic peak for dissolved oxygen reduction was observed on type 304 stainless steel electrode. Amperometric determination of H₂O₂ on type 304 stainless steel electrode displayed a linear range from 0.05 up to 733 mM with a detection limit of 0.02 mM (S/N = 3) and a sensitivity of 16.7 μA mM^-1 cm^-2. The type 304 stainless steel electrode not only shows much higher upper limit than other reported electrodes for the detection of concentrated H₂O₂ but also is free from oxygen interference, which is of great importance for practical applications. This method could detect H₂O₂ in wound wash and lake water with excellent recoveries. Moreover, we successfully applied the stainless steel electrode to determine glucose using glucose oxidase to catalyze the oxidation of glucose to generate hydrogen peroxide. The linear range for glucose is between 0.5 and 25 mM, which covers clinically important blood glucose concentrations well.

Yellow-Emissive Carbon Dot-Based Optical Sensing Platforms: Cell Imaging and Analytical Applications for Biocatalytic Reactions

Carbon dots (CDs) have attracted increasing interest in bioimaging and sensing recently. Herein, we present a simple synthetic strategy to prepare yellow-emissive CDs (λ_em = 535 nm) by one-pot hydrothermal treatment of p-phenylenediamine and aspartic acid. The as-prepared CDs possess outstanding optical features, excellent biocompatibility, and low cytotoxicity, especially for fluorescence (FL) cellular imaging. Interestingly, by combining the quenching and recognition ability of silver nanoparticles (AgNPs) with the optical capacity of CDs, a label-free strategy for specifically monitoring H₂O₂-generated biocatalytic processes was proposed, such as glucose oxidase-induced conversion of glucose, cholesterol oxidase-catalyzed oxidization of cholesterol, and bienzyme of acetylcholinesterase and choline oxidase-mediated reaction of acetylcholine. In this process, AgNPs act as a "nanoquencher" to decrease the FL intensity of CDs via surface plasmon-enhanced energy-transfer mechanism. The enzymatic oxidation product (H₂O₂) subsequently etches the AgNPs to silver ions, thus recovering the FL of CDs, which enabled this proposed nanosensor to sensitively detect H₂O₂-generated biocatalytic processes. The above results pave the way to implement CDs as FL labels for biosensors and biological imaging.

Experimental and theoretical approaches for the selective detection of thymine in real samples using gold nanoparticles as a biochemical sensor

We report a simple, selective and cost effective method for the qualitative and quantitative determination of thymine in a DNA standard and urine samples using gold nanoparticles (AuNPs) as a label-free colorimetric biochemical sensor. The mechanism for the detection of thymine is demonstrated via the color change of the AuNPs from pink to blue, followed by the shift of the localized surface plasmon resonance (LSPR) absorption band to a higher wavelength with the introduction of an analyte. The selective detection of thymine was experimentally verified by performing a control experiment with nucleobases, other biomolecules, metal ions and anions. In addition, the computation density functional theory (DFT) and time dependent density functional theory (TD-DFT) using the Gaussian (C.01) program highlighted that the electrostatic potential behavior of the thymine molecule facilitated a non-covalent interaction toward gold for the selective detection of analytes, and the computation was also used to calculate a UV-Vis absorption band as well. The calculated absorption band of the AuNPs with thymine, obtained using TD-DFT, was found to be very close to the experimental data. The omnicapped truncated tetrahedral (ν₃-tetrahedral) Au₂₀ cluster structure was considered as the model for the AuNP optimization. The linear range obtained for the quantitative determination of thymine was found to be 10–1200 ng mL⁻¹ with a limit of detection of 3 ng mL⁻¹. The advantages of using the AuNPs as a biochemical sensor are that they provide a facile and low cost method and are selective for the qualitative and quantitative determination of thymine in a DNA standard and in urine samples in comparison to chromatographic and electrochemical methods.

We report a simple, selective and cost effective method for the qualitative and quantitative determination of thymine in a DNA standard and in urine samples using gold nanoparticles (AuNPs) as a label-free colorimetric biochemical sensor.