In this study, we developed a new chlorine gas detection method using anodic oxidation and a photochemical reaction. Chlorine gas was temporarily solvated with an aprotic polar solvent having an extensive potential range in the positive direction, and the solvated chlorine molecule was detected by an anodic oxidation reaction. In addition, when combined with ultraviolet light irradiation, we could detect high sensitivity using the photochemical reaction.

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Electrochemical bio-sensing using paper-based detection systems is the main focus of this review. The different existing designs of 2-dimensional and 3-dimensional sensors, and fabrication techniques are discussed. This review highlights the effect of adopting different sensor designs, distinct fabrication techniques, as well as different modification methods, in order to produce reliable and reproducible reading. The use of various nanomaterials have been demonstrated in order to modify the surface of electrodes during fabrication to further enhance the signal for subsequent analysis. The reviewed sensors were classified into categories based on their applications, such as diagnostics, environmental and food testing. One of the major advantages of using paper-based electrochemical sensors is the potential for miniaturization, which only requires relatively small amount of samples, and the low cost for the purpose of mass production. Additionally, most of the devices reviewed were made to be portable, making them well-suited for on-site detection. Finally, paper-based detection is an ideal platform to fabricate cost-effective, user-friendly and sensitive electrochemical biosensors, with large capacity for customization depending on functional needs.

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Cellulose-nanomaterial hybrid systems are promising platforms for the development of portable devices that can be used for fast and inexpensive analysis in the clinical, environmental and food monitoring fields. By combining the chemical and physical properties of the cellulosic network with the unique optical, electrical and catalytic functions of nanomaterials, it is possible to create versatile devices with engineered sensing functions. This review describes the most commonly used types of nanomaterials, their unique properties and assembly in hybrid structures in conjunction with cellulose paper and provides an overview of the most commonly used detection methodologies and their performance for selected applications. Finally, future perspectives and challenges to the implementation of these devices for real world applications are discussed, with focus on method optimization, validation and regulation in order to reach consumers.

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In this study, we demonstrate the precise control of fluid flow using femtosecond (FS) laser-induced microstructures. A microgroove structure inscribed on a poly(methyl methacrylate) (PMMA) substrate functions as a superhydrophilic membrane similar to paper. We first estimated the flow rate for pure water on microgrooves fabricated at various laser fluences in the range from 9.2 to 100.8 J/cm². The results showed that the flow rate could be tuned in the range from 0.30 to 12.07 μL/s by varying the laser irradiation parameters. The fluid flow was reproducible, with a calculated relative standard deviation (RSD%) of less than 8% in the flow rate. We then fabricated a microfilter for blood separation and estimated its filtration ability using artificial blood containing resin microparticles. This method would be useful in a technology related to a paper-based diagnostic device for precise reagent manipulation.

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We report on the effects of fabrication methods, photolithography, wax printing, screen printing, and craft cutting, on selected properties of microfluidic paper-based analytical devices (μPADs): cost, fabrication precision, wicking rate, and analytical accuracy. Photolithography requires numerous fabrication steps, and an oxygen plasma treatment is necessary when using an aqueous solution. Although the boundary between the hydrophobic and hydrophilic areas in the μPAD is sharpest, the obtained K-scale intensity in measuring of protein concentrations is lower than those of the devices by other methods. Wax printing offers the simplest and fastest fabrication, although solution leakage measures should be taken to improve the wicking rate and to prevent cross-contamination. Screen printing also offers easy fabrication. The screen-printed μPAD has a good wicking performance and shows a high detection intensity. Craft cutting allows automated fabrication of many μPADs at once. The craft cut μPAD has the fastest wicking rate among the four μPADs due to bare cellulose fibers. We consider that the detection intensity of this μPAD can be raised by optimizing the evaporation rate.

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A universal method for the detection, quantification, and characterization of polyquaterniums (PQs) in a simple background electrolyte solution and in more complex recreational swimming pool water samples is presented. This method involves the application of polycation-sensitive ion-selective optodes (ISOs) prepared by inkjet printing dinonylnaphthalenesulfonic acid (H⁺DNNS⁻) and chromoionophore I directly onto Whatman^TM qualitative filter paper. No plasticizer or added polymer matrix is required for the fabrication of the sensing layer which is coated on the cellulose fibers of the filter paper. PQ-6, PQ-2, PQ-10, and poly(2-methacryloxyethyltrimethylammonium) chloride (PMETAC) are used as model PQ species for direct optical detection at ppm levels. We further demonstrate that PQ-6 can be detected in recreational swimming pool water samples using this new type of sensor, and that detection of polyanions is also possible using an indirect detection method. Lastly, to circumvent the challenge of polyion-sensitive ISOs exhibiting a pH dependence, the sensors were soaked in buffer and dried to provide local buffering for applied liquid samples and an optical signal independent of sample pH.

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Lateral flow tests (LFTs) are widely used analytical tools characterized by portability, operator simplicity and short analysis times. A remaining challenge is their limited analytical sensitivity, which in classical immunoassay formats is overcome by enzyme-linked immunosorbent assay (ELISA) formats. The implementation of ELISA to an LFT format however, is hampered by the complexity of the procedure requiring the enzyme substrate addition after sample addition. In this work, a simple method for automation of this procedure without user interference is presented. Originally used sample pads of LFTs have been replaced by hydrophobic wax-modified filter paper-based sample pads to realize a delayed flow a pre-deposited colorimetric ELISA substrate without other alterations to the classical lateral-flow immunoassay format. The performance of the system has been characterized by visualizing flow behavior and final proof-of-concept is provided by a model mouse IgG assay, achieving a limit of detection of 15.8 ng mL⁻¹ from just a single application of the sample solution.

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Although paper-based microfluidic devices are an ideal platform for point-of-care (POC) diagnostics, it is difficult to achieve microfluidic control required for sensitive analyses such as ELISA on a paper substrate. Here, we present a novel lateral-flow test chip that can perform operations similar to a pump, such as flowing, stopping, and replacing a solution, just by adding the solution onto an inlet port. The chip was fabricated by laminating paper, film, and adhesive tape. For sensitive and accurate detection in an immunoassay, the transparency and flatness of the substrate is crucial for precise analysis of weak light generated by a specific antigen-antibody reaction; however, paper is not flat and is opaque. Therefore, transparent film was applied to the detection area of the chip in this study. The chip showed a good correlation at 0.1 – 100 ng ml⁻¹ concentrations of C-reactive protein, demonstrating high quantitative analysis of CRP in serum suitable for clinical trials. The signal intensity of the novel chip was higher than that of a chip made of nitrocellulose membrane, and the variation was smaller. The limit of detection of the chip was 0.1 ng ml⁻¹, whereas that of the nitrocellulose membrane was 100 ng ml⁻¹. This novel chip can be used for sensitive sandwich immunoassays just by adding solutions.

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The implementation of continuous flow in paper-based analytical devices (PADs) was challenging because of the large-volume introduction that was created; but this allowed for the development of novel types of PADs for preconcentration, separation, and sensitive detection. In this study, pump-free continuous flow was applied to a distance-based PAD for the determination of iron ions. Continuous flow enabled the introduction of a volume that exceeded what was necessary to fill the hydrophilic channel of a PAD. Thus, this continuous-flow method significantly improved both the limits of detection (LOD) and the limits of quantification (LOQ) for a distance-based PAD by increasing the sample volume that could be introduced into the PAD. The values for LOD and LOQ were 20 and 26 ppb, respectively, which were more than 150-times lower than that obtained using a small sample volume (50 μL), and were comparable to those of inductively coupled plasma–atomic emission spectrometry. The continuous-flow technique was applicable to the determination of iron ions at levels of several tens of ppb in natural water without preconcentration.

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This article describes a simple and inexpensive microfluidic paper-based analytical device (μPAD) for the determination of hexavalent chromium (Cr^VI) in water samples. The μPADs were fabricated on paper by photolithography using a photomask printed with a 3D printer and functionalized with reagents for a colorimetric assay. In the μPAD, Cr^VI reacts with 1,5-diphenylcarbazide to form a violet-colored complex. Images of μPADs were captured with a digital camera; then the red, green, and blue color intensity of each detection zone were measured using images processing software. The green intensity analysis was the best sensitive among the RGB color. A linear working range (40 – 400 ppm; R² = 0.981) between the Cr^VI and green intensity was obtained with a detection limit of 30 ppm. All of the recoveries were between 94 and 109% in recovery studies on water samples, and good results were obtained.

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A smartphone application, called CAnal, was developed as a colorimetric analyzer in paper-based devices for sensitive and selective determination of mercury(II) in water samples. Measurement on the double layer of a microfluidic paper-based analytical device (μPAD) fabricated by alkyl ketene dimer (AKD)-inkjet printing technique with special design doped with unmodified silver nanoparticles (AgNPs) onto the detection zones was performed by monitoring the gray intensity in the blue channel of AgNPs, which disintegrated when exposed to mercury(II) on μPAD. Under the optimized conditions, the developed approach showed high sensitivity, low limit of detection (0.003 mg L⁻¹, 3SD blank/slope of the calibration curve), small sample volume uptake (two times of 2 μL), and short analysis time. The linearity range of this technique ranged from 0.01 to 10 mg L⁻¹ (r² = 0.993). Furthermore, practical analysis of various water samples was also demonstrated to have acceptable performance that was in agreement with the data from cold vapor atomic absorption spectrophotometry (CV-AAS), a conventional method. The proposed technique allows for a rapid, simple (instant report of the final mercury(II) concentration in water samples via smartphone display), sensitive, selective, and on-site analysis with high sample throughput (48 samples h⁻¹, n = 3) of trace mercury(II) in water samples, which is suitable for end users who are unskilled in analyzing mercury(II) in water samples.

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A low-cost and user-friendly microfluidic paper-based analytical device (μPAD) was developed for identification of bullet hole from gunshot residue (GSR) on cotton fabric target. The device (25 × 82 mm) is made of filter paper with a printed pattern consisting of a circular sample loading reservoir (6 mm i.d.), a circular waste reservoir (4 mm i.d.) and a straight flow channel (3 mm wide and 60 mm long). A sticker with a ruler scale in millimeters was mounted alongside the channel. The straight channel is first impregnated with rhodizonate and dried at ambient temperature. Tartrate extract of the target fabric is loaded on the sample reservoir. If Pb(II) ions are present in the extract, pink streak of Pb(II)-rhodizonate precipitate is formed as the sample solution flows from the reservoir along the channel. The length of the pink strip is employed to estimate the firing distance.

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This study describes the use of electrochemical paper-based analytical devices (ePADs) drawn with graphite pencil for the determination of ascorbic acid (AA) in commercial tablets. ePADs were fabricated using vegetal paper and graphite pencil. First, the three-electrode electrochemical cell drawn using a graphical software and toner lines were laser printed on the vegetal paper surface to delimit the electrode areas. Then, the electrode regions were manually painted with graphite pencil. Afterwards, the pseudo-reference electrode was defined with the deposition of silver ink over the graphite surface. Cyclic voltammetry and square wave voltammetry (SWV) experiments were performed to optimize the electroanalytical parameters as well as to quantitatively determine the AA concentration in two commercial tables. ePADs exhibited linear behavior for a concentration range between 0.5 and 3.0 mmol L⁻¹. The achieved limit of detection and sensitivity were 70 μmol L⁻¹ and 0.47 μA/mmol L⁻¹, respectively. The AA concentration levels found by SWV experiments in both Cenevit^TM and Energil C^TM were 2.80 ± 0.02 and 3.10 ± 0.01 mmol L⁻¹, respectively. The accuracy of the proposed devices was investigated through recovery experiments in three concentration levels and it presented values between 95 and 115%.

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This work reports a portable distance-based detection paper device that has a thermometer-like shape for rapid, instrument-free determination of antioxidant activity using a nanoceria assay. The assay is based on partial reduction of cerium ion from Ce⁴⁺ to Ce³⁺ on nanoceria deposited along the detection channel by antioxidants present in food, giving highly reactive oxidation products. Either these products or the parent antioxidant compounds could then bind to the OH-rich ceria nanoparticles and generate charge transfer ceria–antioxidant complexes resulting in a yellow to brown color change. The distance of the brown color on the detection channel is directly proportional to antioxidant activity, and can be easily measured using an integrated ruler without the need of any external sophisticated instrument for detection. The paper sensor has been studied for the analysis of common antioxidants and its performance was validated against traditional antioxidant assays for 11 tea sample analyses. Using the Spearman rank correlation coefficient method, the antioxidant activity of tea samples obtained from the paper device correlated with the traditional assay at the 95% confidence level. The developed sensor provided a high recovery and tolerance limit and was stable for 50 days both when stored at ambient and low temperature (6 and −20°C). The results demonstrated that the developed paper device is an alternative to allow for fast, simple, instrument-free, cheap, portable and high-throughput screening of antioxidant activity analysis in real samples.

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In this work, a simple and inexpensive paper-based colorimetric device (cPAD) was developed for oxalate measurements. The colorimetric assay is based on the formation of formazan via the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) by oxalate decarboxylase and formate dehydrogenase on the paper device. After the sample was spotted on the device, MTT changed color from yellow to purple, and the purple color intensity correlated with the oxalate concentration. The quantitative detection of oxalate ranged from 10 – 1000 μM, with a linear equation, y = 0.0086x + 34.978, and a correlation coefficient (R²) = 0.9994. The detection limit was 10 μM by the naked eye. The recoveries of oxalate spiked in artificial urine samples evaluated by our cPADs were in the range of 81 – 92%. This simple cPAD for rapid and sensitive oxalate determination should be useful for diagnostic urinary oxalate measurements for point-of-care monitoring.

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Simple, low-cost and portable microfluidic paper-based analytical devices (μPADs) for determination of creatinine in urine samples were developed. The methodology was based on Jaffé reaction between the creatinine and picric acid in alkaline conditions, generating a colorimetric creatinine-alkaline picrate complex. The product exhibits an orange color that is clearly visible on the μPADs. The color intensity of the complex, which is indicative of the concentration of creatinine, is then quantitatively determined using ImageJ software. Various experimental parameters were optimized to achieve the best performance of the μPADs. Under the optimum conditions, a wide linear range was obtained in the range of 0.2 – 1 mM with a limit of detection and limit of quantitation of 0.08 and 0.26 mM, respectively. The accuracy of the proposed method was in good agreement with the standard Jaffé method. Finally, the developed devices were successfully applied for the determination of creatinine in urine samples.

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Microfluidics has served as a technology for the design and development of a myriad of devices owing to their reduced reagent consumption rate and short sampling-to-result time. Chemotaxis is the movement of materials, particularly biological species, in response to the influence of chemical stimulation. Herein, we describe, for the first time, chemotactic behavior on a microfluidic paper-based analytical device (μPAD) to afford a distribution of products not obtainable under other (non-μPAD) experimental conditions using as a model enzyme-substrate system glucose oxidase (GOx) and glucose. μPADs are easily fabricated by patterning hydrophobic materials in hydrophilic paper. They are low cost, compatible with biological samples, and have shown promise as platforms for various applications and in resource-limited settings.

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