Enhancement of sensitivity of paper-based sensor array for the identification of heavy-metal ions

Paper-based microfluidic devices have been widely investigated in recent years. Among various detection techniques, colorimetric method plays a very important role in paper-based microfluidic devices. The limitation, however, is also clear: they generally require highly sensitive indicators. In this work, we have developed a novel enrichment-based paper test for the discrimination of heavy-metal ions. Comparing to regular paper-based microfluidic devices, enrichment-based technique showed largely improved sensitivity. Combining with eight pyridylazo compounds and array technologies-based pattern-recognition, we have obtained the discrimination capability of eight different heavy-metal ions at same concentration as low as 50 μM using our enrichment-based pyridylazo compounds array paper. Identification of the heavy-metal ions was readily achieved using a standard chemometric approach. This method can be, of course, used for other analytes as well.     

Distance versus Capillary Flow Dynamics-Based Detection Methods on a Microfluidic Paper-Based Analytical Device (μPAD)

In recent years, there has been high interest in paper-based microfluidic sensors or microfluidic paper-based analytical devices (μPADs) towards low-cost, portable, and easy-to-use sensing for chemical and biological targets. μPAD allows spontaneous liquid flow without any external or internal pumping, as well as an innate filtration capability. Although both optical (colorimetric and fluorescent) and electrochemical detection have been demonstrated on μPADs, several limitations still remain, such as the need for additional equipment, vulnerability to ambient lighting perturbation, and inferior sensitivity. Herein, alternative detection methods on μPADs are reviewed to resolve these issues, including relatively well studied distance-based measurements and the newer capillary flow dynamics-based method. Detection principles, assay performance, strengths, and weaknesses are explained for these methods, along with their potential future applications towards point-of-care medical diagnostics and other field-based applications.     

Recent development in nanomaterials fabricated paper-based colorimetric and fluorescent sensors: A review

The paper-based sensing devices have drawn a broad interest in analytical chemistry for colorimetric and fluorescent-based analysis of biological, environmental, clinical, and food samples. It is due to the simple, rapid, biodegradable, user-friendly, less expensive, and low waste generation into the environment. Here, the recent development of paper-based sensors fabricated with different noble metal nanoparticles (NPs) and semiconductor and carbon quantum dots (QDs) is demonstrated to analyze several chemical substances from various samples. User-friendly and portable recording devices such as digital cameras, smartphones, scanners, etc. along with color detecting softwares are employed to measure the color intensity of nanomaterials fabricated paper devices after the deposition of a sample solution containing various chemical substances. The advantages and disadvantages of incorporating nanomaterials in the paper substrate (direct deposition, inkjet printing, screen printing and wax printing) are illustrated. The mechanism for colorimetric, fluorescence, phosphorescence, and chemiluminescence sensing using noble metal NPs (Ag, Cu, and Au), semiconductors, and carbon QDs for the determination of metal ions, anions, pesticides, biomolecules, and other toxic chemical substances are discussed. Thus, this review article would be highly useful for scientists and researchers to design colorimetric sensors to monitor chemical toxicants in clinical, environment, foods, and many other related samples.     

Characterization and Validation of a Platinum Paper-based Potentiometric Sensor for Glucose Detection in Saliva

A paper-based potentiometric sensor was constructed and characterized for the detection of glucose in saliva. Dilution of the samples was optimized to afford the optimum experimental conditions of measurements. The performance allows for detecting abnormal high glucose concentrations observed in diabetes patients. Repeatability data were presented and the performance of the sensor compared to literature examples showing suitable characteristics at a much lower cost. The validation in real saliva samples was performed against a commercial colorimetric kit showing that glucose could be effectively determined in the 4–10 mM range based on the comparison with a reference method.     

Novel, simple and low-cost alternative method for fabrication of paper-based microfluidics by wax dipping

Paper-based microfluidic devices are an alternative technology for fabricating simple, low-cost, portable and disposable platforms for clinical diagnosis. Hereby, a novel wax dipping method for fabricating paper-based microfluidic devices (μPADs) is reported. The iron mould for wax dipping was created by a laser cutting technique. The designed pattern was transferred onto paper by dipping an assembly mould into melted wax. The optimal melting temperature and dipping time were investigated. The optimal melting temperature was in the range of 120-130 °C, and the optimal dipping time was 1 s. The whole fabrication process could be finished within 1 min without the use of complicated instruments or organic solvents. The smallest hydrophilic channel that could be created by the wax dipping method was 639 ± 7 μm in size. The reproducibility of the μPAD fabrication for hydrophilic channel width of the test zone and sample zone was 1.48% and 6.30%, respectively. To verify the performance of the μPAD, multiple colorimetric assays for simultaneous detection of glucose and protein in real samples were performed. An enzymatic assay and the bromocresol green (BCG) method were conducted on the paper device to determine the presence of glucose and protein in a test solution. The results of the assays were not significantly different from those of the conventional methods (p > 0.05, pair t-test and one-way ANOVA method). The wax dipping provides a new alternative method for fabricating lab-on-paper devices for multiple clinical diagnostics and will be very beneficial for developing countries.     

Bioactive paper provides a low-cost platform for diagnostics

Bioactive paper includes a range of potential paper-based materials that can perform analytical functions normally reserved for multi-well plates in the laboratory or for portable electronic devices. Pathogen detection is the most compelling application. Simple paper-based detection, not requiring hardware, has the potential to have impacts in society, ranging from the kitchen to disasters in the developing world. Bioactive-paper research is an emerging field with significant efforts in Canada, USA (Harvard), Finland and Australia.Following a brief introduction to the material and surface properties of paper, I review the literature. Some of the early work exploits the porosity of paper to generate paper-based microfluidics (“paperfluidics”) devices. I exclude from this review printed electronic devices and plastics-supported devices.     

Paper-based fluorescent devices for multifunctional assays: Biomarkers detection,inhibitors screening and chiral recognition

The development of a single analytical platform with different functions is highly desirable but remains a challenge at present. Here, a paper-based device based on fluorescent carbon dots (CDs) functionalized paper/MnO₂ nanosheets (MnO₂ NS) hybrid devices (PCD/NS) was proposed for single-device multi-function applications. MnO₂ NS functioned as a fluorescence quencher of CDs and recognizer of H₂O₂ released from the oxidase catalyzed system. Fluorescence recovery would occur after the decomposition of MnO₂ NS induced by H₂O₂, by which a simple and effective strategy could be developed for fluorescence monitoring multiplex biological events. Xanthine (XA) sensing, xanthine oxidase (XOD) inhibitors screening analysis and chiral recognition of glucose enantiomers were performed on PCD/NS to investigate the multifunctional application of the paper-based device. By means of PCD/NS, XA could be determined in the range of 0.1–40 µmol/L with a low detection of limit of 0.06 µmol/L. The IC₅₀ value of allopurinol, the model inhibitor of XOD, was sensitively detected to be 7.4 µmol/L. Glucose enantiomers were also recognized in terms of the specific fluorescence response to d-glucose. This work firstly presented a paper-based device capable of biomarkers detection, inhibitors screening and chiral recognition, which enlightened a promising strategy for the construction of multifunctional devices and hold the great potential application in clinical diagnosis and drug discovery.     

Quantitative biomarker assay with microfluidic paper-based analytical devices

This article describes the use of microfluidic paper-based analytical devices (μPADs) to perform quantitative chemical assays with internal standards. MicroPADs are well-suited for colorimetric biochemical assays; however, errors can be introduced from the background color of the paper due to batch difference and age, and from color measurement devices. To reduce errors from these sources, a series of standard analyte solutions and the sample solution are assayed on a single device with multiple detection zones simultaneously; an analyte concentration calibration curve can thus be established from the standards. Since the μPAD design allows the colorimetric measurements of the standards and the sample to be conducted simultaneously and under the same condition, errors from the above sources can be minimized. The analytical approach reported in this work shows that μPADs can perform quantitative chemical analysis at very low cost.      

Lab-on-paper-based devices using chemiluminescence and electrogenerated chemiluminescence detection

As an analytical support, paper, being low cost, highly abundant, of high porosity, disposable or biodegradable, and easy to use, store, transport, and print, has excellent chemical compatibility with many applications. Since the first microfluidic paper-based analytical device (μ-PAD or lab-on-paper) was proposed, the paper-based assay has never attracted as much attention as it does now. There has recently been rapidly increasing interest in using sensitive luminescence methods, for example chemiluminescence (CL) and electrogenerated chemiluminescence (ECL), as the detection strategy for lab-on-paper devices. Because of their intrinsic characteristics, CL and ECL provide outstanding performance while retaining the simplicity, low cost, multifunctionality, versatility, flexibility, and disposability of μ-PADs. The objective of this review is to cover the development of lab-on-paper-based devices using CL and ECL detection, including fabrication of paper devices, construction of sensing interfaces, signal amplification strategies, external instruments used, and applications. We believe that lab-on-paper devices with CL and ECL detection methods will meet the diverse requirements of point-of-care diagnosis.     

微流控纸芯片在环境分析检测中的应用

近年来,微流控纸芯片由于低成本、便携化、检测快等优点,在需要快速检测的环境分析领域中展现出了巨大的应用前景.该综述从微流控纸芯片在环境分析中的应用角度,总结归纳了微流控纸芯片在环境分析中的最新研究进展,并展望了其在未来的发展趋势与挑战.论文内容引用150余篇源于科学引文索引(SCI)与中文核心期刊中的相关论文.该综述包...     

Novel reagentless paper-based screen-printed electrochemical sensor to detect phosphate

Herein we describe a novel reagentless paper-based electrochemical phosphate sensor, manufactured with a simple and inexpensive approach. By following three easy steps, consisting of wax patterning, paper chemical modification, and electrode screen-printing, the filter paper provides an effective electroanalytical platform to sense phosphate ions in standard solutions and real samples (river water). The electrochemical properties of the paper-based platform were evaluated, firstly, by using ferricyanide as a redox mediator, proving no analyte-entrapment due to the cellulose lattice. Then, the reference colorimetric method for phosphate ions, which is based on the formation of phosphomolybdic complex, was successfully adapted to a reagentless electrochemically paper-based platform. This novel and highly sustainable configuration readily allows for the determination of phosphate ions with high reproducibility and long storage stability, achieving a detection limit of 4 μM over a wide linear range up to 300 μM. This in-house approach would be able to generically develop an affordable in situ and user-friendly sensing device without the addition of any reagent, to be applied for a broad range of analytes.     

Paper-based assay of antioxidant activity using analyte-mediated on-paper nucleation of gold nanoparticles as colorimetric probes

With the increasing interest in the health benefits arising from the consumption of dietary products rich in antioxidants, there exists a clear demand for easy-to-use and cost-effective tests that can be used for the identification of the antioxidant power of food products. Paper-based analytical devices constitute a remarkable platform for such expedient and low-cost assays with minimal external resources but efforts in this direction are still scarce. In this work we introduce a new paper-based device in the form of a sensor patch that enables the determination of antioxidant activity through analyte-driven on-paper formation of gold nanoparticles. The principle of detection capitalizes, for the first time, on the on-paper nucleation of gold ions to its respective nanoparticles, upon reduction by antioxidant compounds present in an aqueous sample. The ensuing chromatic transitions, induced on the paper surface, are used as an optical “signature” of the antioxidant strength of the solution. The response of the paper-based sensor was evaluated against a large variety of antioxidant species and the respective dose response curves were constructed. On the basis of these data, the contribution of each species according to its chemical structure was elucidated. For the analysis of real samples, a concentration-dependent colorimetric response was established against Gallic acid equivalents over a linear range of 10 μM–1.0 mM, with detection limits at the low and ultra-low μM levels (i.e. <1.0 μM) and satisfactory precision (RSD = 3.6–12.6%). The sensor has been tested for the assessment of antioxidant activity in real samples (teas and wines) and the results correlated well with commonly used antioxidant detection methods. Importantly, the sensor performed favorably for long periods of time when stored at moisture-free and low temperature conditions without losing its activity thus posing as an attractive alternative to the assessment of antioxidant activity without specialized equipment. The use of the sensor by non-experts for a rapid assessment of natural products in field testing is envisioned. Importantly, we demonstrate for the first time that analyte-mediated growth of nanomaterials directly on the paper surface could open new opportunities in paper-based analytical devices.     

Paper-based enzyme-free immunoassay for rapid detection and subtyping of influenza A H1N1 and H3N2 viruses

Development of rapid screening in the ambulatory environment is the most pressing needs for the control of spread of infectious disease. Despite there are many methods to detect the immunoassay results, quantitative measurement in rapid disease screening is still a great challenge for point-of-care applications. In this work, based on the internal structural protein, i.e., nucleoprotein (NP), and outer surface glycoproteins, i.e., H1 and H3, of the influenza viruses, specific and sensitive immunoassay on paper-based platform was evaluated and confirmed. Detection and subtyping of influenza A H1N1 and H3N2 viruses found in people were demonstrated by colorimetric paper-based sandwich immunoassay. Concentration-dependent response to influenza viruses was shown and the detection limits could achieve 2.7 × 10³ pfu/assay for H1 detection and 2.7 × 10⁴ pfu/assay for H3 detection, which are within the clinical relevant level. Moreover, detection of influenza virus from infected cell lysate and clinical samples was demonstrated to further confirm the reliability of the paper-based immunoassay. The use of paper for the development of diagnostic devices has the advantages of lightweight, ease-of-use, and low cost and paper-based immunoassay is appropriate to apply for rapid screening in point-of-care applications.     

Hysteresis controlled water droplet splitting on superhydrophobic paper

Fabrication of surfaces with heterogeneous contact angle hysteresis enables extraction of droplet samples from bulk liquid volumes. These surfaces are created by printing high hysteresis wax islands onto low hysteresis superhydrophobic paper. The volume of the sampled droplets depends on the hysteresis of the printed islands, which can be controlled through both physical and chemical means. Physically, hysteresis is modified through the addition of surface roughness. Chemical hysteresis is tuned by changing the active chemical groups present on the wax surface. The observed control of the volume of sampled droplets, which is necessary for quantitative biochemical or chemical assays, extends to scenarios in which multiple droplet samples are extracted simultaneously from a single bulk droplet. Demonstration of the capacity of this technique to perform colorimetric glucose immunoassays is described. The ability to obtain well-defined microliter sample volumes and to extract several samples simultaneously from the same source enables the development of two-dimensional paper-based microfluidic devices for biomedical testing.     

Determination of nitrite in saliva using microfluidic paper-based analytical devices

Point-of-care platforms can provide fast responses, decrease the overall cost of the treatment, allow for in-home determinations with or without a trained specialist, and improve the success of the treatment. This is especially true for microfluidic paper-based analytical devices (μPAD), which can enable the development of highly efficient and versatile analytical tools with applications in a variety of biomedical fields. The objective of this work was the development of μPADs to identify and quantify levels of nitrite in saliva, which has been proposed as a potential marker of periodontitis. The devices were fabricated by wax printing and allowed the detection of nitrite by a colorimetric reaction based on a modified version of the Griess reaction. The presented modifications, along with the implementation of a paper-based platform, address many of the common drawbacks (color development, stability, etc.) associated with the Griess reaction and are supported by results related to the design, characterization, and application of the proposed devices. Under the optimized conditions, the proposed devices enable the determination of nitrite in the 10–1000 μmol L⁻¹ range with a limit of detection of 10 μmol L⁻¹ and a sensitivity of 47.5 AU [log (μmol L⁻¹)]⁻¹. In order to demonstrate the potential impact of this technology in the healthcare industry, the devices were applied to the analysis of a series of real samples, covering the relevant clinical range.     

Paper-based electrochemiluminescent 3D immunodevice for lab-on-paper, specific, and sensitive point-of-care testing

Recent research on microfluidic paper-based analytical devices (μPADs) has shown that paper has great potential for the fabrication of low-cost diagnostic devices for healthcare and environmental monitoring applications. Herein, electrochemiluminescence (ECL) was introduced for the first time into μPADs that were based on screen-printed paper-electrodes. To further perform high-specificity, high-performance, and high-sensitivity ECL on μPADs for point-of-care testing (POCT), ECL immunoassay capabilities were introduced into a wax-patterned 3D paper-based ECL device, which was characterized by SEM, contact-angle measurement, and electrochemical impedance spectroscopy. With the aid of a home-made device-holder, the ECL reaction was triggered at room temperature. By using a typical tris(bipyridine)ruthenium-tri-n-propylamine ECL system, this paper-based ECL 3D immunodevice was applied to the diagnosis of carcinoembryonic antigens in real clinical serum samples. This contribution further expands the number of sensitive and specific detection modes of μPADs.     

Anomalous diffusion in an electrolyte saturated paper matrix

Diffusion of colored dye on water saturated paper substrates has been traditionally exploited with great skill by renowned water color artists. The same physics finds more recent practical applications in paper-based diagnostic devices deploying chemicals that react with a bodily fluid yielding colorimetric signals for disease detection. During spontaneous imbibition through the tortuous pathways of a porous electrolyte saturated paper matrix, a dye molecule undergoes diffusion in a complex network of pores. The advancing front forms a strongly correlated interface that propagates diffusively but with an enhanced effective diffusivity. We measure this effective diffusivity and show that it is several orders of magnitude greater than the free solution diffusivity and has a significant dependence on the solution pH and salt concentration in the background electrolyte. We attribute this to electrically mediated interfacial interactions between the ionic species in the liquid dye and spontaneous surface charges developed at porous interfaces, and introduce a simple theory to explain this phenomenon.     

Fabrication of paper-based microfluidic sensors by printing

A novel method for the fabrication of paper-based microfluidic diagnostic devices is reported; it consists of selectively hydrophobizing paper using cellulose reactive hydrophobization agents. The hydrophilic–hydrophobic contrast of patterns so created has excellent ability to control capillary penetration of aqueous liquids in paper channels. Incorporating this idea with digital ink jet printing techniques, a new fabrication method of paper-based microfluidic devices is established. Ink jet printing can deliver biomolecules and indicator reagents with precision into the microfluidic patterns to form bio-chemical sensing zones within the device. This method thus allows the complete sensor, i.e. channel patterns and the detecting chemistries, to be fabricated only by two printing steps. This fabrication method can be scaled up and adapted to use high speed, high volume and low cost commercial printing technology. Sensors can be fabricated for specific tests, or they can be made as general devices to perform on-demand quantitative analytical tasks by incorporating the required detection chemistries for the required tasks.     

Colloids engineering and filtration to enhance the sensitivity of paper-based biosensors

Paper-based biosensors represent a disruptive technology by providing instantaneous and low-cost diagnostics for health and environmental applications. The lack of sensitivity can be an obstacle for this technology to compete with traditional analytical instrumentations. Aiming to improve the sensitivity of a paper-based colorimetric biosensor, we have applied colloids engineering in combination with filtration to lower the paper substrate backgrounds and optimize the immobilization of bio-molecules on paper. A model system consisting of an enzyme, alkaline phosphatase (ALP), and an inorganic colloid, calcium carbonate (CC), flocculated by a cationic dimethylamino-ethyl-methacrylate polyacrylamide (CPAM), demonstrated that the optimized CC flocs are best for enhancing the detecting sensitivity of ALP. The CC floc structure on paper was optimized by modulating its structure in suspension. Subsequently, the filtration process and the wicking ability of paper enabled to freeze the deposited CC structure inherited from the suspension. The incorporation of biomolecules into the CC before immobilizing on paper through filtration provided not only a better microenvironment, but also a higher surface density of immobilized biomolecules. The ALP detection limit of 117 fmol per zone (5 mm circle) in the current study was fifty times lower than that of the common soaking method for biomolecule immobilization. The minimum amount of biomolecules per unit substrate area required for detection was lowered by over an order of magnitude, compared with spotting methods (i.e. inkjet printing). The improvement was also demonstrated by the steepest slope of standard curve, the lowest background, and the highest activity of the bioactive paper probed with the diluted BCIP/NBT liquid substrates.     

A Colorimetric Biosensing Platform with Aptamers,Rolling Circle Amplification and Urease-Mediated Litmus Test

Here we report on an ultra-sensitive colorimetric sensing platform that takes advantage of both the strong amplification power of rolling circle amplification (RCA) and the high efficiency of a simple urease-mediated litmus test. The presence of a target triggers the RCA reaction, and urease-labelled DNA can hybridize to the biotinylated RCA products and be immobilized onto streptavidin-coated magnetic beads. The urease-laden beads are then used to hydrolyze urea, leading to an increase in pH that can be detected by a simple litmus test. We show this sensing platform can be easily integrated with aptamers for sensing diverse targets via the detection of human thrombin and platelet-derived growth factor (PDGF) utilizing structure-switching aptamers as well as SARS-CoV-2 in human saliva using a spike-binding trimeric DNA aptamer. Furthermore, we demonstrate that this colorimetric sensing platform can be integrated into a simple paper-based device for sensing applications.