Using nanostructured conductive carbon tape modified with bismuth as the disposable working electrode for stripping analysis in paper-based analytical devices

In situ deposition of bismuth on pre-anodized screen-printed electrode for sensitive determination of Cd2+ in water and rice with a portable device

Electrochemical detection is favorable for the rapid and sensitive determination of heavy metal cadmium. However, the detection sensitivity needs to be further improved, and a portable, low-cost device is needed for on-site detection. Herein, an in-situ bismuth modified pre-anodized screen-printed carbon electrode (SPCE) was developed for Cd2+ determination by square wave anodic stripping voltammetry (SWASV). The in-situ bismuth modification enhances the enrichment of Cd2+, and together with pre-anodization improve the electron transfer rate of electrode, thus enhancing the detection sensitivity. The electrode modification method combines pre-anodization and in-situ bismuth deposition, which is very easy and effective. Furthermore, a self-made PSoC Stat potentiostat coupled with a stirring device was fabricated for portable and low-cost electrochemical detection. After comprehensive optimization, the developed method can reach a testing time of 3 min, a detection limit of 3.55 μg/L, a linear range of 5-100 μg/L, and a recovery rate of 91.7-107.1% in water and rice samples for Cd2+ determination. Therefore, our method holds great promise for the rapid, sensitive and on-site determination of Cd2+ in food samples.

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.

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.

Application of electrochemical methods for the detection of abiotic stress biomarkers in plants

Abiotic stress is the main cause of low productivity in plants. Therefore, it is important to detect stress and respond to it in a timely manner to avoid irreversible damage to plant productivity and health. The application of traditional methods in agriculture is limited by expensive equipment and cumbersome sample processing. More effective detection methods are urgently needed due to the trace amounts and low stabilities of plant biomarkers. Electrochemical detection methods have the unique advantages of high accuracy, a low detection limit, fast response and easy integration with systems. In this review, the application of three types of electrochemical methods to phytohormone assessment is highlighted including direct electrochemical, immunoelectrochemical, and photoelectrochemical methods. Research on electrochemical methods for detecting abiotic stress biomarkers, including various phytohormones, is also summarized with examples. To date, the detection limit of exogenous plant hormones can reach pg/mL or even lower. Nevertheless, more efforts need to be made to develop a portable instrument for in situ online detection if electrochemical sensors are to be applied to the detection of the endogenous hormones or the physiological state of plants. Additionally, plant-wearable sensors that can be directly attached to or implanted into plants for continuous, noninvasive and real-time monitoring are emphasized. Finally, rational summaries of the considered methods and present challenges and future prospects in the field of abiotic stress detection-based electrochemical biosensors are thoroughly discussed.

Paper-based electroanalytical devices for stripping analysis of lead and cadmium in children's shoes

Children's shoes are potential sources of toxic heavy metals, especially for younger children. Electrochemical detection could be applied for effective stripping analysis of heavy metals (such as Cd and Pb). However, the substrates of working electrodes are still limited and it is not well known which property is critical. Herein ITO glass was used as the substrate and the working electrode was modified with carbon cement for stripping analysis of Cd and Pb. The electrochemical impedance spectra of the ITO modified electrodes suggested the connection between the resistance and the electrochemical responses of heavy metals in stripping analysis, depending on the dilution ratio of the carbon cement. After optimization, the ITO modified electrodes in paper-based analytical devices could be used to sensitively quantify Cd and Pb with the concentration ranging from 10 to 1000 ppb. The detection limit of Pb2+ could reach less than 1 ppb while that of Cd2+ could reach 5 ppb, depending on the pH value of the sample solution. The paper-based electroanalytical devices could be used to quantify the concentration of Cd and Pb in children's shoes. This study implied the impact of the electric conductivity of the electrode substrates on stripping analysis, which might help to find more materials for the fabrication of the working electrodes.

Paper-Based Working Electrodes Coated with Mercury or Bismuth Films for Heavy Metals Determination

Paper-based carbon working electrodes were modified with mercury or bismuth films for the determination of trace metals in aqueous solutions. Both modification procedures were optimized in terms of selectivity and sensitivity for the determination of different heavy metals, aiming their simultaneous determination. Cd (II), Pb (II) and In (III) could be quantified with both films. However, Cu (II) could not be determined with bismuth films. The modification with mercury films led to the most sensitive method, with linear ranges between 0.1 and 10 µg/mL and limits of detection of 0.4, 0.1, 0.04 and 0.2 µg/mL for Cd (II), Pb (II), In (III) and Cu (II), respectively. Nevertheless, the bismuth film was a more sustainable alternative to mercury. Tap-water samples were analyzed for the determination of metals by standard addition methodology with good accuracy, by using a low-cost and easily disposable paper-based electrochemical platform. This system demonstrated its usefulness for monitoring heavy metals in water.

Electrochemical paper-based devices: sensing approaches and progress toward practical applications

Paper-based sensors offer an affordable yet powerful platform for field and point-of-care (POC) testing due to their self-pumping ability and utility for many different analytical measurements. When combined with electrochemical detection using small and portable electronics, sensitivity and selectivity of the paper devices can be improved over naked eye detection without sacrificing portability. Herein, we review how the field of electrochemical paper-based analytical devices (ePADs) has grown since it was introduced a decade ago. We start by reviewing fabrication methods relevant to ePADs with more focus given to the electrode fabrication, which is fundamental for electrochemical sensing. Multiple sensing approaches applicable to ePADs are then discussed and evaluated to present applicability, advantages and challenges associated with each approach. Recent applications of ePADs in the fields of clinical diagnostics, environmental testing, and food analysis are also presented. Finally, we discuss how the current ePAD technologies have progressed to meet the analytical and practical specifications required for field and/or POC applications, as well as challenges and outlook.

Eco-friendly electrochemical lab-on-paper for heavy metal detection

A disposable electrochemical lateral flow paper-based sensing device for heavy metal detection is proposed. The quantification of lead and cadmium in aqueous samples is demonstrated in a range from 10 to 100 ppb with a limit of detection of 7 and 11 ppb respectively. Moreover, the platform itself, which is made of paper, served as a sample pretreatment material due to its filtering properties. Real samples, especially in which the matrix is usually turbid and would in principle need a previous filtration, are successfully analyzed. This lab-on-paper device is simple, low cost, easy to fabricate, and portable, being a promising tool for new point-of-care applications in environmental monitoring, public health, and food safety.

Highly Sensitive Detection and Removal of Lead Ions in Water Using Cysteine-Functionalized Graphene Oxide/Polypyrrole Nanocomposite Film Electrode

We synthesized cysteine-functionalized graphene oxide (sGO) using carbonyldiimidazole as a cross-linker via amide and carbamate linkages. The sGO/polypyrrole (PPy) nanocomposite film was grown on the working electrode surface of a screen-printed electrode (SPE) via controlled one-step electrochemical deposition. The sGO/PPy-SPE was used to detect lead ions (Pb(2+)) in water by first depositing Pb(2+) on the working electrode surface for 10 min at -1.2 V, and then anodic stripping by differential pulse voltammetry (DPV). The DPV signals were linear in the ranges of 1.4-28 ppb (R(2) = 0.994), 28-280 ppb (R(2) = 0.997), and 280-14 000 ppb (R(2) = 0.990) Pb(2+). The measurable detection limit of the sensor is 0.07 ppb (S/N = 3), which is more than 2 orders of magnitude below the 10 ppb threshold for drinking water set by the World Health Organization. The average removal efficiency of Pb(2+) deposited on the electrode was 99.2% (S/N = 3), with relative standard deviation (RSD) of 3.8%. Our results indicate good affinity of sGO/PPy nanocomposite to Pb(2+), which can be used to effectively adsorb and remove Pb(2+) in water samples. Therefore, sGO/PPy nanocomposite we synthesized is useful for highly sensitive on-site and real-time monitoring of heavy metal ions and water treatment.

Fabrication of multiwalled carbon nanotube-surfactant modified sensor for the direct determination of toxic drug 4-aminoantipyrine

A multi-walled carbon nanotube (MWCNT)-cetyltrimethylammonium bromide (CTAB) surfactant composite modified glassy carbon electrode (GCE) was developed as a novel system for the determination of 4-aminoantipyrine(AAP). The oxidation process was irreversible over the pH range studied and exhibited a diffusion controlled behavior. All experimental parameters were optimized. The combination of MWCNT-CTAB endows the biosensor with large surface area, good biological compatibility, electricity and stability, high selectivity and sensitivity. MWCNT-CTAB/GCE electrode gave a linear response for AAP from 5.0×10⁻⁹ to 4.0×10⁻⁸ M with a detection limit of 1.63×10⁻¹⁰ M. The modified electrode showed good selectivity against interfering species and also exhibited good reproducibility. The present electrochemical sensor based on the MWCNT-CTAB/GCE electrode was applied to the determination of AAP in real samples.

One-step synthesis of isoreticular metal–organic framework-8 derived hierarchical porous carbon and its application in differential pulse anodic stripping voltammetric determination of Pb(ii)

A novel electrochemical sensor for Pb(ii) was constructed based on an IRMOF-8-derived NPC with three-dimensional hierarchy of micro-, meso-, and macropores (BET surface area = 1715 m² g⁻¹).

Microfabricated three-electrode on-chip PDMS device with a vibration motor for stripping voltammetric detection of heavy metal ions

A microfabricated three-electrode on-chip device with a vibration motor for stripping voltammetric detection of Cd(2+) and Pb(2+) ions is introduced. Gold electrodes on a glass substrate are utilized as the counter electrode directly, as well as the working electrode with bismuth coating. The reference electrode is achieved by painting silver/silver chloride layer on the gold layer. Polydimethylsiloxane (PDMS) cover is bonded onto the glass substrate, organizing a compact module. A vibration motor in the device provides vibration as the alternate for stirring to improve detection limits and decrease deposition time. Cyclic voltammetry (CV) is performed with the on-chip reference electrode and a commercial liquid electrolyte Ag/AgCl electrode, showing that the potential shift is less than 20 mV. Differential pulse stripping voltammetry (DPSV) is used to detect Cd(2+) and Pb(2+) ions. Linearity is well defined from 10 µg/L to 80 µg/L for both ions. Detection limits for Cd(2+) and Pb(2+) ions are 0.7 µg/L and 1.2 µg/L, respectively, at 325 s deposition without vibration; the limits are 0.11 µg/L and 0.25 µg/L at 120 s deposition with vibration. The reliability is verified by detecting Cd(2+) and Pb(2+) concentrations of natural water samples and comparing with the results obtained from inductively coupled plasma mass spectrometry (ICP-MS).