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.

Dual-functional DNAzyme powered CRISPR-Cas12a sensor for ultrasensitive and high-throughput detection of Pb2+ in freshwater

Freshwater lead pollution has posed severe threat to the environment and human health, underscoring the urgent necessity for accurate and user-friendly detection methods. Herein, we introduce a novel Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR-Cas) sensor for highly sensitive Pb2+ detection. To accomplish this, we designed a dual-functional deoxyribozyme (df-DNAzyme) probe that functions as an activator for the CRISPR-Cas12a system while also recognizing Pb2+. The df-DNAzyme probe was subsequently combined with gold nanoparticles (AuNPs) to fabricate a DNAzyme/AuNP nanoprobe, facilitating the activation of CRISPR-Cas12a in a one-to-multiple manner. Upon exposure to Pb2+, the df-DNAzyme is cleaved, causing disintegration of the DNAzyme/AuNP nanoprobe from magnetic beads. The degraded DNAzyme/AuNP containing multiple double-stranded DNA activators efficiently triggers CRISPR-Cas12a activity, initiating cleavage of fluorescence-quenched reporter DNA and generating amplified signals accordingly. The amplified fluorescence signal is accurately quantified using a quantitative polymerase chain reaction (qPCR) instrument capable of measuring 96 or 384 samples simultaneously at the microliter scale. This technique demonstrates ultra-sensitive detection capability for Pb2+ at concentrations as low as 1 pg/L within a range from 1 pg/L to 10 μg/L, surpassing limits set by World Health Organization (WHO) and United States Environmental Protection Agency (US EPA) guidelines. This study offers an ultrasensitive and high-throughput method for the detection of Pb2+ in freshwater, thereby advancing a novel approach towards the development of precise and convenient techniques for detecting harmful contaminants.

A sensitive and facile electrochemical paper-based sensor for glucose detection in whole blood using the Pd/CB-Ni@rGO modified electrode

We created novel Pd/CB-Ni@rGO nanomaterials for glucose detection. The as-synthesized nanomaterials were dropped on the electrode surface using the drop casting technique. The prepared electrode was then attached to a paper-based device containing the sample zone and the reaction zone, enabling plasma isolation and an enzymatic reaction for glucose detection in whole blood. The nanomaterials and surfaces of electrodes were characterized by FTIR, TEM, and SEM. The proposed approach is a disposable glucose detection method that is unaffected by protein fouling on the electrode, and it requires only one drop of human blood. Therefore, there is no need for extensive sample preparation, and there is less sample consumption. Under optimal conditions, Pd/CB-Ni@rGO can accurately measure blood glucose levels with a linear range of 7 to 7140 μM (R2 = 0.9986) and a low detection limit of 0.82 μM. Besides, the developed sensor shows excellent anti-interference capacity, stability, and satisfactory reproducibility and repeatability. Importantly, Pd/CB-Ni@rGO was successfully applied for glucose in whole blood from 4 volunteers, with results that correlated well with those obtained using an Accucheck glucometer at a 95% confidence level. Given its low cost, high accuracy, and ease of use, the blood glucose sensor holds significant potential for clinical use and broadens the area of future noninvasive sensor development.

A novel electrochemical sensor for simultaneous detection of Cd2+ and Pb2+ by MXene aerogel-CuO/carbon cloth flexible electrode based on oxygen vacancy and bismuth film

Herein, a novel MXene aerogel-CuO/carbon cloth (MXA-CuO/CC) electrochemical sensor was constructed, and the synergistic adsorption of heavy metal ions by oxygen vacancies and Bi (III) was investigated with Cd²⁺ and Pb²⁺ as detection targets. The oxygen vacancies of CuO have a strong affinity for heavy metal ions, which promoted the adsorption of Cd²⁺ and Pb²⁺ on the electrode surface. In addition, the introduced Bi (III) can form alloys with heavy metal ions, which effectively enhanced the adsorption capacity of sensing electrodes for Cd²⁺ and Pb²⁺. Differential pulse anodic stripping voltammetry (DPASV) was used to study the performance of MXA-CuO/CC sensitive electrode for the detection of Cd²⁺ and Pb²⁺ separately and simultaneously. The constructed sensing electrode has excellent detection performance, and can detect Cd²⁺ (4 μg L^-1- 800 μg L^-1) and Pb²⁺ (4 μg L^-1- 1200 μg L^-1) simultaneously with detection limits of 0.3 μg L^-1 (Cd²⁺) and 0.2 μg L^-1 (Pb²⁺), respectively. The proposed sensor electrode also has good anti-interference performance, excellent stability and reproducibility. It is worth mentioning that the proposed method can accurately detect Cd²⁺ and Pb²⁺ in food and water samples, which is consistent with the detection results of inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS).

Simultaneous Cross-type Detection of Water Quality Indexes via a Smartphone-App Integrated Microfluidic Paper-Based Platform

Water quality guarantee in remote areas necessitates the development of portable, sensitive, fast, cost-effective, and easy-to-use water quality detection methods. The current work reports on a microfluidic paper-based analytical device (μPAD) integrated with a smartphone app for the simultaneous detection of cross-type water quality parameters including pH, Cu(II), Ni(II), Fe(III), and nitrite. The shapes, baking time, amount, and ratios of reaction reagent mixtures of wax μPAD were optimized to improve the color uniformity and intensity effectively. An easy-to-use smartphone app was established for recording, analyzing, and directly reading the colorimetric signals and target concentrations on μPAD. The results showed that under the optimum conditions, the current analytical platform has reached the detection limits of 0.4, 1.9, 2.9, and 1.1 ppm for nitrite, Cu(II), Ni(II), and Fe(III), respectively, and the liner ranges are 2.3-90 ppm (nitrite), 3.8-400 ppm (Cu(II)), 2.9-1000 ppm (Ni(II)), 2.8-500 ppm (Fe(III)), and 5-9 (pH). The proposed portable smartphone-app integrated μPAD detection system was successfully applied to real industrial wastewater and river water quality monitoring. The proposed method has great potential for field water quality detection.

Paper-Based Electrochemical Biosensors for Food Safety Analysis

Nowadays, foodborne pathogens and other food contaminants are among the major contributors to human illnesses and even deaths worldwide. There is a growing need for improvements in food safety globally. However, it is a challenge to detect and identify these harmful analytes in a rapid, sensitive, portable, and user-friendly manner. Recently, researchers have paid attention to the development of paper-based electrochemical biosensors due to their features and promising potential for food safety analysis. The use of paper in electrochemical biosensors offers several advantages such as device miniaturization, low sample consumption, inexpensive mass production, capillary force-driven fluid flow, and capability to store reagents within the pores of the paper substrate. Various paper-based electrochemical biosensors have been developed to enable the detection of foodborne pathogens and other contaminants that pose health hazards to humans. In this review, we discussed several aspects of the biosensors including different device designs (e.g., 2D and 3D devices), fabrication techniques, and electrode modification approaches that are often optimized to generate measurable signals for sensitive detection of analytes. The utilization of different nanomaterials for the modification of electrode surface to improve the detection of analytes via enzyme-, antigen/antibody-, DNA-, aptamer-, and cell-based bioassays is also described. Next, we discussed the current applications of the sensors to detect food contaminants such as foodborne pathogens, pesticides, veterinary drug residues, allergens, and heavy metals. Most of the electrochemical paper analytical devices (e-PADs) reviewed are small and portable, and therefore are suitable for field applications. Lastly, e-PADs are an excellent platform for food safety analysis owing to their user-friendliness, low cost, sensitivity, and a high potential for customization to meet certain analytical needs.

Recent developments in microfluidic paper-based analytical devices for pharmaceutical analysis

In the last decade, due to the global increase in diseases, drugs for biomedical applications have increased dramatically. Therefore, there is an urgent need for analytical tools to monitor, treat, investigate, and control drug compounds in diverse matrices. The new and challenging task has been looking for simple, low-cost, rapid, and portable analytical platforms. The development of microfluidic paper-based analytical devices (µPADs) has garnered immense attention in many analytical applications due to the benefit of cellulose structure. It can be functionalized and serves as an ideal channel and scaffold for the transportation and immobilization of various substances. Microfluidic technology has been considered an effective tool in pharmaceutical analysis that facilitates the quantitative measurement of several parameters on cells or other biological systems. The µPADs represent unique advantages over conventional microfluidics, such as the self-pumping capability. They have low material costs, are easy to fabricate, and do not require external power sources. This review gives an overview of the current designs in this decade for µPADs and their respective application in pharmaceutical analysis. These include device designs, choice of paper material, and fabrication techniques with their advantages and drawbacks. In addition, the strategies for improving analytical performance in terms of simplicity, high sensitivity, and selectivity are highlighted, followed by the application of µPADs design for the detection of drug compounds for various purposes. Moreover, we present recent advances involving µPAD technologies in the field of pharmaceutical applications. Finally, we discussed the challenges and potential of µPADs for the transition from laboratory to commercialization.

Electric Field-Induced Specific Preconcentration to Enhance DNA-Based Electrochemical Sensing of Hg2+ via the Synergy of Enrichment and Self-Cleaning

Efficient preconcentration is critical for sensitive and selective electrochemical detection of metal ions, but rapid specific enrichment with depressed absorption of interfering ions at the electrode is challenging. Here, we proposed an electric field-induced specific preconcentration to boost the analytical performance of DNA-based electrochemical sensors for Hg²⁺ detection. As for such preconcentration, a positive external electric field was first used to enrich Hg²⁺ at an electrode assembled with T-rich DNA, thus boosting T-Hg²⁺-T recognitions. The following applied inverse electric field strips the nonspecifically absorbed Hg²⁺ and other interfering ions, thus depressing matrix interferences via self-cleaning. Based on this principle, we designed a portable device to realize programmable control of electric fields; a T-Hg²⁺-T recognition-based electrochemical sensor was thus fabricated as a model platform to assess the feasibility of electric field-induced preconcentration. The experimental results revealed that such a strategy decreased the time of T-Hg²⁺-T-based recognition from 60 to 20 min and led to detection with better reproducibility by depressing the influence of free Hg²⁺ as well as interfering ions. This strategy offered Hg²⁺ detection limits of 0.01 pM─three-fold better than that without preconcentration─within 22 min. The proposed preconcentration strategy offers a new way to enhance the analytical performance of sensing at the solid-liquid interface.

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.

Chemiluminescence-Derived Self-Powered Photoelectrochemical Immunoassay for Detecting a Low-Abundance Disease-Related Protein

Early diagnosis of cancers relies on the sensitive detection of specific biomarkers, but most of the current testing methods are inaccessible to home healthcare due to cumbersome steps, prolonged testing time, and utilization of toxic and hazardous substances. Herein, we developed a portable self-powered photoelectrochemical (PEC) sensing platform for rapid detection of prostate-specific antigen (PSA, as a model disease-related protein) by integrating a self-powered photoelectric signal output system catalyzed with chemiluminescence-functionalized Au nanoparticles (AuNPs) and a phosphomolybdic acid (PMA)-based photochromic visualization platform. TiO₂-g-C₃N₄-PMA photosensitive materials were first synthesized and functionalized on a sensor chip. The sensor consisted of filter paper modified with a photocatalytic material and a regional laser-etched FTO electrode as an alternative to a conventional PEC sensor with a glass-based electrode. The targeting system involved a monoclonal anti-PSA capture antibody-functionalized Fe₃O₄ magnetic bead (mAb₁-MB) and a polyclonal anti-PSA antibody (pAb₂)-N-(4-aminobutyl)-N-ethylisoluminol-AuNP (ABEI-AuNP). Based on the signal intensity of the chemiluminescent system, the photochromic device color changed from light yellow to heteropoly blue through the PMA photoelectric materials integrated into the electrode for visualization of the signal output. In addition, the electrical signal in the PEC system was amplified by a sandwich-type capacitor and readout on a handheld digital multimeter. Under optimum conditions, the sensor exhibited high sensitivity relative to PSA in the range of 0.01-50 ng mL^-1 with a low detection limit of 6.25 pg mL^-1. The flow-through chemiluminescence reactor with a semiautomatic injection device and magnetic separation was avoid of unstable light source intensity inherent in the chemiluminescence process. Therefore, our strategy provides a new horizon for point-of-care analysis and rapid cost-effective clinical diagnosis.

Logarithmic Data Processing Can Be Used Justifiably in the Plotting of a Calibration Curve

The article is a response to a recent opinion piece that log concentration values should not be applied in analytical chemistry. An essential aim in the development of analytical chemistry methods is to obtain more sensitive and accurate detection values. For the application of chemical analysis methods, the obtained experiment data need to fit with the mathematical functions in the first place. As influenced by different detection principles and analytical methods, data can be displayed in a coordinate system with two linear axes for linear function fitting, or the data can first be taken through a logarithmic transformation and then for function fitting. Using raw data or data after logarithmic transformation primarily depends on analytical principles, without special rules of data formats. For example, ultraviolet-visible spectrophotometric data are more suitable for direct linear fitting. However, enzyme-catalyzed reaction or electrochemical data in logarithmic form are more appropriate for function fitting. This transformation of data form will not affect the soundness of fit statistics; rather, it simplifies the complexity of function analysis and calculation, which are the essence of analytical chemistry. In this brief article, we provide justification and legitimacy of the application of logarithmic processing in various fields of quantitative analytical chemistry.

A Simple Distance Paper-Based Analytical Device for the Screening of Lead in Food Matrices

A simple and rapid distance paper-based analytical device (dPAD) for the detection of lead (Pb) in foods is proposed herein. The assay principle is based on competitive binding between carminic acid (CA) and polyethyleneimine (PEI) to Pb in a food sample. The paper channels were pre-immobilized with PEI, before reacting with a mixture of the sample and CA. Pb can strongly bind to the CA; hence, the length of the red color deposition on the flow channel decreased as a lower amount of free CA bound to PEI. The dPAD exhibited good linear correlation, with ranges of 5-100 µg·mL-1 (R2 = 0.974) of Pb. Although, the limit of detection (LOD) of this platform was rather high, at 12.3 µg·mL-1, a series of standard additions (8.0, 9.0, and 10.0 µg·mL-1) can be used to interpret the cutoff of Pb concentrations at higher or lower than 2 µg·mL-1. The presence of common metal ions such as calcium, magnesium, nickel, and zinc did not interfere with the color distance readout. The validity of the developed dPAD was demonstrated by its applicability to screen the contamination of Pb in century egg samples. The results obtained from the dPAD are in accordance with the concentration measured by atomic absorption spectroscopy (AAS) (n = 9). In conclusion, this proposed dPAD, combined with the standard addition method, could be applied for screening Pb contamination in food matrices. This platform is, therefore, potentially applicable for field measurements of Pb in developing countries, because it is cheap and rapid, and it requires no significant laborious instruments.