Surface Modified Thread-Based Microfluidic Analytical Device for Selective Potassium Analysis

A nanosilica-coated thread-based analytical device for nitrate and nitrite detection in food samples

A new microfluidic thread-based analytical device (μTAD) for nitrate and nitrite determination in food samples was developed. The cotton thread substrate was coated with nanosilica to increase its hydrophilicity and stability, and polylactic acid was applied to one end of the nanosilica-coated thread to constrain the fluid flow along the thread in one direction. Quantification of nitrate and nitrite was based on the modified Griess reaction, using sulfanilamide and N-(1-naphthyl) ethylenediamine as chromogenic reagents, and utilizing a distance-based detection technique. Linear responses were observed in a range of 4-25 mg L-1 (R2 = 0.9991) for nitrite and a range of 8-50 mg L-1 (R2 = 0.9989) for nitrate. The limits of detection for nitrite and nitrate were 1.5 and 3.1 mg L-1, respectively. The detection time was 5 min for nitrite analysis, and 7 min for nitrate analysis. The new method demonstrated good precision, accuracy, selectivity, and stability. The performance of the proposed μTAD for nitrite and nitrate determination in real food samples was comparable to that of the conventional UV-Vis spectrophotometric method. The proposed μTAD could serve as a simple, low-cost, and portable method for nitrite and nitrate detection in food samples.

Reversible Hydrophobic Deep Eutectic Solvent-Based Uranyl-Sensing Optode Film in Aqueous Streams: Color Transformation and Reusability

A hydrophobic deep eutectic solvent (HDES)-based optode was designed for the preconcentration and determination of the UO22+ ion in aqueous media using spectroscopic techniques [energy-dispersive X-ray fluorescence (EDXRF) and solid-state absorption]. The optode was developed by incorporation of HDES (tri-n-octyl phosphine oxide and decanoic acid in an equimolar ratio), tri-(2-ethylhexyl) phosphate, and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol into a cellulose triacetate matrix. Characterization studies were carried out using different techniques to understand the roles of HDES as a plasticizer, UO22+ extractant, and Br-PADAP immobilizer. Uptake studies revealed that the optimal pH was 3 and sorption followed the type II adsorption isotherm. Uranium in the U-sorbed optode can be directly analyzed over a large concentration range of 0.021 × 10-3-2.1 × 10-3 Mol L-1 using EDXRF. The optode film exhibited a linear dynamic range of 0.84 × 10-6-84 × 10-6 Mol L-1 for uranium, with a lowest limit of detection of 0.084 × 10-6 Mol L-1 by colorimetric analysis. This optode-based method was employed for seawater analysis for its UO22+ concentration without any matrix separation, and the concentration was found to be 1.30 ± 0.06 × 10-8 Mol L-1. The optode exhibited better selectivity for UO22+ in the presence of various cations including Sr2+ and Cs+ in an aqueous medium. Compared to other prevailing optical sensors, this optode performed better in terms of key factors like pH, equilibration time, reusability, and detection limit.

Comparative analysis of a bulk optode based on a valinomycin ionophore and a nano-optode in micelles with pluronic F-127 for the quantification of potassium in aqueous solutions

In this work, two types of optical sensors were prepared for the quantification of potassium: the bulk optode (BO) and nano-optode (NO). The BO was prepared using three main components: the ionophore valinomycin, the ion exchanger tetrakis(4-chlorophenyl) potassium borate (K-TCPB), and the chromoionophore ETH 5294 (CHI). The optimal composition was found to be in a ratio of [1 : 1 : 1]. The NO was prepared by miniaturizing the BO through sonication in surfactant Pluronic F-127. The working range for the linear calibration model of BO was from 10-6 to 1.0 M K+ with a LODBO = 0.31 μM, meanwhile for NO was from 10-4 to 1.0 M K+ with a LODNO = 30.3 μM. Both optodes were tested for selectivity towards K+ in the presence of alkaline and alkaline earth ions, with a selectivity coefficient > 1.0. Furthermore, precision and stability studies of BO and NO were performed for three levels of K+ concentrations, 10-6, 10-3, 1.0 M for BO and 10-4, 10-2, 1.0 M for NO, showing a good homogeneity of the NO in the whole concentration range. However, an excessive variability was obtained for BO at 1.0 M K+. Therefore, the NO represents a potential tool for quantification of K+.

Using a cotton thread-based colorimetric sensor modified by carboxymethylcellulose and cuprizone with smartphone detection for quantification of copper

In the present work, we report the development of a novel cotton thread-based colorimetric sensor modified by carboxymethylcellulose (CMC) and cuprizone (CPZ) with smartphone detection and its application for the quantitative determination of cupric ions in water and cachaça. The cotton thread/smartphone detection-based colorimetric method is an easily affordable, low-cost technique which allows one to perform real-time and on-field determination analyses, especially with limited financial resources. The method involves the complexation of Cu(II) with CPZ, which causes a change in the coloration of the cotton thread from a shade of white to blue in the detection zone of the colorimetric sensor. The immobilization of CPZ on CMC in the cotton thread leads to the pre-concentration of Cu(II) via a complexation mechanism with colorimetric reaction. The application of the colorimetric sensor allows the quantification of copper in the range from 1 to 12 mg L-1, with a low limit of detection of 0.21 mg L-1. In addition, the recovery assays conducted in samples of water and cachaça resulted in recovery percentages ranging from 84.9% to 107%, which is indicative of a precise method. To validate the precision of the proposed colorimetric method, the values obtained from the quantification analysis were compared with those of the flame atomic absorption spectrometry and a good agreement at the 95% confidence level was obtained.

Threads in tubing: an innovative approach towards improved electrochemical thread-based microfluidic devices

Thread-based microfluidic analytical devices have received growing attention since threads have some advantages over other materials. Compared to paper, threads are also capable of spontaneously transporting fluid due to capillary action, but they have superior mechanical strength and do not require hydrophobic barriers. Therefore, thread-based microfluidic devices can be inexpensively fabricated with no need for external pumps or sophisticated microfabrication apparatus. Despite these outstanding features, achieving a controlled and continuous flow rate is still a challenging task, mainly due to fluid evaporation. Here, we overcome this challenge by inserting a cotton thread into a polyethylene tube aiming to minimize fluid evaporation. Also, a cotton piece was inserted into the outlet reservoir to improve the wicking ability of the device. This strategy enabled the fabrication of an innovative electrochemical thread in a tubing microfluidic device that was capable to hold a consistent flow rate (0.38 μL s^-1) for prolonged periods, allowing up to 100 injections in a single device by simply replacing the cotton piece in the outlet reservoir. The proposed device displayed satisfactory analytical performance for selected model analytes (dopamine, hydrogen peroxide, and tert-butylhydroquinone), in addition to being successfully used for quantification of nitrite in spiked artificial saliva samples. Beyond the flow rate improvement, this "thread-in-tube" strategy ensured the protection of the fluid from external contamination while making it easier to connect the electrode array to the microchannels. Thus, we envision that the thread in a tube strategy could bring interesting improvements to thread-based microfluidic analytical devices.

Thread-based isotachophoresis coupled with desorption electrospray ionization mass spectrometry for clean-up, preconcentration, and determination of alkaloids in biological fluids

A thread-based isotachophoresis method coupled with desorption electrospray ionization mass spectrometry (TB-ITP-DESI-MS) was developed and applied for clean-up, preconcentration, and determination of alkaloids in biological fluids. This simple approach enables the focusing and rapid analysis of analytes of interest in complex matrices that are otherwise challenging using direct ambient mass spectrometry. The TB-ITP platform components were rapidly and reproducibly fabricated at low-cost using 3D printing. A single string of nylon 6 thread was used as the electrophoresis substrate and a cotton knot, tied to the nylon thread, was used as the trapping zone of the ITP focused model analytes (coptisine, berberine and palmatine). The trapping efficiency was evaluated upon different commercially available threads with different chemical properties and cotton was selected as the best material due to its highest trapping efficiency and subsequent DESI-MS ionization efficiency. Up to 11.6-fold increase in signal to noise ratio (S/N) was obtained using the proposed method compared to direct DESI-MS detection, due to the reduced matrix interference and focusing. The results demonstrated that the TB-ITP-DESI-MS approach is a viable solution for the analysis of complicated biological fluid samples.

Nanomaterial-assisted thread-based isotachophoresis with on-thread solute trapping

This research describes a nanomaterial-assisted TB-ITP setup for the clean-up, preconcentration, and trapping of alkaloids in biological fluids, followed by their on-thread DESI-MS determination.

Microfluidic devices based on textile threads for analytical applications: state of the art and prospects

Microfluidic devices based on textile threads have interesting advantages when compared to systems made with traditional materials, such as polymers and inorganic substrates (especially silicon and glass). One of these significant advantages is the device fabrication process, made more cheap and simple, with little or no microfabrication apparatus. This review describes the fundamentals, applications, challenges, and prospects of microfluidic devices fabricated with textile threads. A wide range of applications is discussed, integrated with several analysis methods, such as electrochemical, colorimetric, electrophoretic, chromatographic, and fluorescence. Additionally, the integration of these devices with different substrates (e.g., 3D printed components or fabrics), other devices (e.g., smartphones), and microelectronics is described. These combinations have allowed the construction of fully portable devices and consequently the development of point-of-care and wearable analytical systems.

A colorimetric paper-based optode sensor for highly sensitive and selective determination of thiocyanate in urine sample using cobalt porphyrin derivative

In this work, a highly sensitive colorimetric paper-based optode for the determination of thiocyanate in urine samples was developed for the first time. The cocktail solution of the optode was composed of 5,10,15,20-tetrakis(4-octyloxyphenyl)porphyrin cobalt(II) complex (L), tridodecylmethylammonium chloride (TDMACl), 2-nitrophenyl octyl ether, and polyvinyl chloride as an ionophore, an ion exchanger, a plasticizer, and a polymer, respectively. The paper-based optode responded to thiocyanate by increasing the blue component in the RGB index and a visible change, with the naked-eye, of the optode color from pink to green was observed. From the central composite design, the optimized conditions that yielded the highest sensitivity were 4.70 mmol/kg TDMACl and 13.75 mmol/kg L. The developed optode sensor was highly selective and responded to thiocyanate over other anions, with a working range of 0.001-5 mM and with a coefficient of determination (R²) of 0.9915. The limits of detection using naked-eye and camera were determined to be 50.0 μM and 1.26 μM, respectively. In addition, the LOD and LOQ estimated from the standard deviation of the blank were 0.65 and 1.87 μM, respectively. Furthermore, this sensor was successfully applied to the detection of thiocyanate in urine samples from non-smokers and smokers. The results were in good agreement with the standard ion chromatography (IC) technique. This developed paper-based optode sensor was simple, low-cost, portable, and easy to use as a sensing device without any complicated instrument.

Electrophoresis on a polyester thread coupled with an end-channel pencil electrode detector

We present the design and characterization of a low cost, thread-based electrophoretic device with integrated electrochemical detection. The device has an end-channel pencil graphite electrode placement system for performing electrochemical detection on the thread electrophoresis platform with direct sample pipetting onto the thread. We also established the use of methylene blue and neutral red as a pair of reference migration markers for separation techniques coupled with electrochemical detection, as they have different colors for visual analysis and are both electroactive. Importantly, neutral red was also found to migrate at a similar rate to the EOF, indicating that it can be used as a visual identifier of EOF. The utility of our system was demonstrated by electrophoretic separation and electrochemical detection of physiologically relevant concentrations of pyocyanin in a solution containing multiple electroactive compounds. Pyocyanin is a biomarker for the detection of pathogenic Pseudomonas aeruginosa and has a redox potential that is similar to that of methylene blue. The system was able to effectively resolve methylene blue, neutral red, and pyocyanin in less than 7 min of electrophoretic separation. The theoretical limit of detection for pyocyanin was determined to be 559 nM. The electrophoretic mobilities of methylene blue (0.0236 ± 0.0007 mm² /V·s), neutral red (0.0149 ± 0.0007 mm² /V·s), and pyocyanin (0.0107 ± 0.0003 mm² /V·s) were also determined.

Low-Cost Optical Assays for Point-of-Care Diagnosis in Resource-Limited Settings

Readily deployable, low-cost point-of-care medical devices such as lateral flow assays (LFAs), microfluidic paper-based analytical devices (μPADs), and microfluidic thread-based analytical devices (μTADs) are urgently needed in resource-poor settings. Governed by the ASSURED criteria (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free, and deliverability) set by the World Health Organization, these reliable platforms can screen a myriad of chemical and biological analytes including viruses, bacteria, proteins, electrolytes, and narcotics. The Ebola epidemic in 2014 and the ongoing pandemic of SARS-CoV-2 have exemplified the ever-increasing importance of timely diagnostics to limit the spread of diseases. This review provides a comprehensive survey of LFAs, μPADs, and μTADs that can be deployed in resource-limited settings. The subsequent commercialization of these technologies will benefit the public health, especially in areas where access to healthcare is limited.

Knitting Thread Devices: Detecting Candida albicans Using Napkins and Tampons

Reproducible and in situ microbial detection, particularly of microbes significant in urinary tract infections (UTIs) such as Candida albicans, provides a unique opportunity to bring equity in the healthcare outcomes of disenfranchised groups like women in low-resource settings. Here, we demonstrate a system to potentially detect vulvovaginal candidiasis by leveraging the properties of multifilament cotton threads in the form of microfluidic-thread-based analytical devices (μTADs) to develop a frugal microbial identification assay. A facile mercerization method using heptane wash to boost reagent absorption and penetration is also performed and is shown to be robust compared to other existing conventional mercerization methods. Furthermore, the twisted mercerized fibers are drop-cast with media consisting of l-proline β-naphthylamide, which undergoes hydrolysis by the enzyme l-proline aminopeptidase secreted by C. albicans, hence signaling the presence of the pathogen via simple color change with a limit of detection of 0.58 × 106 cfu/mL. The flexible and easily disposable thread-based detection device when integrated with menstrual hygiene products showed a detection time of 10 min using spiked vaginal discharge. The developed method boasts a long shelf life and high stability, making it a discreet detection device for testing, which provides new vistas for self-testing multiple diseases that are considered taboo in certain societies.

A thread/fabric-based band as a flexible and wearable microfluidic device for sweat sensing and monitoring

Flexible biosensors for monitoring systems have emerged as a promising portable diagnostics platform due to their potential for in situ point-of-care (POC) analytic devices. Assessment of biological analytes in sweat can provide essential information for human physiology. Conventional measurements rely on laboratory equipment. This work exploits an alternative approach for epidermal sweat sensing and detection through a wearable microfluidic thread/fabric-based analytical device (μTFAD). This μTFAD is a flexible and skin-mounted band that integrates hydrophilic dot-patterns with a hydrophobic surface via embroidering thread into fabric. After chromogenic reaction treatment, the thread-embroidered patterns serve as the detection zones for sweat transferred by the hydrophilic threads, enabling precise analysis of local sweat loss, pH and concentrations of chloride and glucose in sweat. Colorimetric reference markers embroidered surrounding the working dots provide accurate data readout either by apparent color comparison or by digital acquirement through smartphone-assisted calibration plots. On-body tests were conducted on five healthy volunteers. Detection results of pH, chloride and glucose in sweat from the volunteers were 5.0-6.0, 25-80 mM and 50-200 μM by apparent color comparison with reference markers through direct visual observation. Similar results of 5.47-6.30, 50-77 mM and 47-66 μM for pH, chloride and glucose were obtained through calibration plots based on the RGB values from the smartphone app Lanse®. The limit of detection (LOD) is 10 mM for chloride concentration, 4.0-9.0 for pH and 10 μM for glucose concentration, respectively. For local sweat loss, it is found that the forehead is the region of heavy sweat loss. Sweat secretion is a cumulating process with a lower sweat rate at the beginning which increases as body movement continues along with increased heat production. These results demonstrate the capability and availability of our sensing device for quantitative detection of multiple biomarkers in sweat, suggesting the great potential for development of feasible non-invasive biosensors, with a similar performance to conventional measurements.

Self-Powered Electrochromic Readout of Potentiometric pH Electrodes

An absorbance-based colorimetric sensor array that is self-powered by an ion-selective electrode (ISE) in a short-circuited cell is presented. As the cell voltage is maintained at zero, the potential at the ISE serves as the power generator to directly transfer its power to a potential-dependent Prussian blue (PB) film in contact with an electrolyte solution in a separate detection compartment. This allows one to activate the color change of the PB film without the need for an external power supply. The potential of the PB detection element is optimized to change color between 50 and 250 mV (vs Ag/AgCl). Because the potential originates at the ISE, it is proportional to the ion activity in the sample in agreement with the Nernst equation. In this way, a higher cation activity in the sample generates a more positive potential, which enhances the PB absorbance that serves as the analytical signal. A self-powered optical sensor array coupled to poly(vinyl-chloride)-based pH electrodes based on two different ionophores is utilized here as a model. The measuring range is tuned chemically by varying the pH of the inner filling solution of each ISE, giving a measuring range from pH 2 to 10.5. As the optical sensor is driven by a potentiometric probe, the sensor output is independent of solution ionic strength. It is successfully applied for quantitative analysis in unmodified turbid/colored samples that included red wine, coke, coffee, baking soda, and antacid. The colorimetric output correlates well with the reference method, a calibrated pH electrode. Compared to earlier systems where the cell potential is dictated by an external power source, the PB film exhibits excellent reproducibility and a rapid response time of about 44 s.

Towards smart personalized perspiration analysis: An IoT-integrated cellulose-based microfluidic wearable patch for smartphone fluorimetric multi-sensing of sweat biomarkers

Practical obstacles, such as intricate designs and expensive equipment/materials, in the fabrication of wearable sweat sensors, have limited their feasibility as a personalized healthcare device. Herein, we have fabricated a cellulose-based wearable patch, which further paired with a smartphone-based fluorescence imaging module and a self-developed smartphone app for non-invasive and in situ multi-sensing of sweat biomarkers including glucose, lactate, pH, chloride, and volume. The developed Smart Wearable Sweat Patch (SWSP) sensor comprises highly fluorescent sensing probes embedded in paper substrates, and microfluidic channels consisted of cotton threads to harvest sweat from the skin surface and to transport it to the paper-based sensing probes. The imaging module was fabricated by a 3D printer, equipped with UV-LED lamps and an optical filter to provide the in situ capability of capturing digital images of the sensors via a smartphone. A smartphone app was also designed to quantify the concentration of the biomarkers via a detection algorithm. Additionally, we have recommended an Internet of Things (IoT)-based model for our developed SWSP sensor to promote its potential application for the future. The field studies on human subjects were also conducted to investigate the feasibility of our developed SWSP sensor for the analysis of sweat biomarkers. Our findings convincingly demonstrated the applicability of our developed SWSP sensor as a smart, user-friendly, ultra-low-cost (~0.03 $ per sweat patch), portable, selective, rapid, and non-invasive healthcare monitoring device for immense applications in health personalization, sports performance monitoring, and medical diagnostics.

Thread integrated smart-phone imaging facilitates early turning point colorimetric assay for microbes

This study employs a commercial multifilament cotton thread as a low-cost microbial identification assay integrated with smartphone-based imaging for high throughput and rapid detection of pathogens. The thread device with inter-twined fibers was drop-cast with test media and a pH indicator. The target pathogens scavenge the media components with different sugars and release acidic by-products, which in turn act as markers for pH-based color change. The developed thread-based proof-of-concept was demonstrated for the visual color detection (red to yellow) of Candida albicans (≈16 hours) and Escherichia coli (≈5 hours). Besides that, using a smart-phone to capture images of the thread-based colorimetric assay facilitates early detection of turning point of the pH-based color change and further reduces the detection time of pathogens viz. Candida albicans (≈10 hours) and Escherichia coli (≈1.5 hours). The reported thread and smartphone integrated image analysis works towards identifying the turning point of the colorimetric change rather than the end-point analysis. Using this approach, the interpretation time can be significantly reduced compared to the existing conventional microbial methods (≈24 hours). The thread-based colorimetric microbial assay represents a ready-to-use, low-cost and straightforward technology with applicability in resource-constrained environments, surpassing the need for frequent fresh media preparation, expensive instrumentation, complex fabrication techniques and expert intervention. The proposed method possesses high scalability and reproducibility, which can be further extended to bio(chemical) assays.

Optical Sensing with a Potentiometric Sensing Array by Prussian Blue Film Integrated Closed Bipolar Electrodes

The simultaneous optical readout of a potentiometric sensor array of ion-selective electrodes (ISEs) based on PVC membranes is described here for the first time. The optical array consists of electrochromic Prussian Blue (PB) films in multiple closed ion-selective bipolar electrodes (BPEs), which gives a physical separation between the optical detection and sample compartments. The potential-dependent turnover of PB generates Prussian White (PW). A near-Nernstian response of the PB film is confirmed by colorimetric absorbance experiments as a function of applied potential. In the combined bipolar electrode cell, the overall potential is kept constant with a single potentiostat over the entire array where each PB spot indicates the potential change of an individual connected potentiometric probe. For cation-selective electrodes, the absorbance or blue intensity of the connected PB film is enhanced with increasing target cation activity. The colorimetric absorbance changes are simultaneously followed by a digital camera and analyzed by Mathematica software. A multiple cation-BPE array allows one to achieve simultaneous quantitative analysis of potassium, sodium, and calcium ions, demonstrated here in highly colored fruit juices. Mass transport at the PB thin film is shown not to be rate-limiting. The measuring ranges can be tuned in a wide range by potential control. The PB film exhibits greatly improved reproducibility and stability as compared to previous work with a ferroin redox probe confined in a thin solution layer.

Paper-based cation-selective optode sensor containing benzothiazole calix[4]arene for dual colorimetric Ag+ and Hg2+ detection

A new paper-based analytical device based on bulk ion-selective optodes (ISOs) for dual Ag⁺ and Hg²⁺ detection has been developed. A plasticized PVC hydrophobic phase composed of 25,27-di(benzothiazolyl)-26,28-hydroxycalix[4]arene (CU1) as an ion-selective ionophore, potassium tetrakis(4-chlorophenyl)borate as an ion-exchanger and chromoionophore XIV as a lipophilic pH indicator was entrapped in the pores of cellulose paper. This paper strip showed higher selectivity for Ag⁺ and Hg²⁺ over common alkali, alkaline earth and some transition metal ions with a color change from blue to yellow. With the proposed sensor, Ag⁺ and Hg²⁺ can be measured with the range of 1.92 × 10^-6 to 5.00 × 10^-3 M for Ag⁺ and 5.74 × 10^-7 to 5.00 × 10^-5 M for Hg²⁺ with a limit of detection of 1.92 × 10^-6 M for Ag⁺ and 5.74 × 10^-7 M for Hg²⁺. The proposed sensor was successfully applied to determine the amount of mercury in various water sources and the amount of silver in cleaning product samples containing silver nanoparticles (AgNPs). The results were in good agreement with inductively couple plasma-optical emission spectrometric measurements (ICP-OES).

Thread-based isoelectric focusing coupled with desorption electrospray ionization mass spectrometry

Efficient ‘on-thread’ isoelectric focusing of proteins, with direct on-thread detection using desorption electrospray ionisation mass spectrometry.

A simple platform with moving drops for downscaling chemical analysis incorporating smartphone detection

A simple cost-effective device for downscaling chemical analysis with microliter-scale dropwise handing and smartphone detection is proposed. The platform was made from easily available materials. Drops of constant volume of analyte and reagent were placed onto a channel then moved by gravity force to mix and react with each other. The color change due to reaction was monitored with a smartphone. Assays of iron, copper and phosphate employing the platform were demonstrated with 1,10-phenanthroline, bicinchoninic acid and molybdate reagents, respectively. Kinetic study was also demonstrated for this simple device.

Open Microfluidic Capillary Systems

Open microfluidic capillary systems are a rapidly evolving branch of microfluidics where fluids are manipulated by capillary forces in channels lacking physical walls on all sides. Typical channel geometries include grooves, rails, or beams and complex systems with multiple air-liquid interfaces. Removing channel walls allows access for retrieval (fluid sampling) and addition (pipetting reagents or adding objects like tissue scaffolds) at any point in the channel; the entire channel becomes a "device-to-world" interface, whereas such interfaces are limited to device inlets and outlets in traditional closed-channel microfluidics. Open microfluidic capillary systems are simple to fabricate and reliable to operate. Prototyping methods (e.g., 3D printing) and manufacturing methods (e.g., injection molding) can be used seamlessly, accelerating development. This Perspective highlights fundamentals of open microfluidic capillary systems including unique advantages, design considerations, fabrication methods, and analytical considerations for flow; device features that can be combined to create a "toolbox" for fluid manipulation; and applications in biology, diagnostics, chemistry, sensing, and biphasic applications.

Recent advances in thread-based microfluidics for diagnostic applications

Over the past decades, researchers have been seeking attractive substrate materials to keep microfluidics improving to outbalance the drawbacks and issues. Cellulose substrates, including thread, paper and hydrogels are alternatives due to their distinct structural and mechanical properties for a number of applications. Thread have gained considerable attention and become promising powerful tool due to its advantages over paper-based systems thus finds numerous applications in the development of diagnostic systems, smart bandages and tissue engineering. To the best of our knowledge, no comprehensive review articles on the topic of thread-based microfluidics have been published and it is of significance for many scientific communities working on Microfluidics, Biosensors and Lab-on-Chip. This review gives an overview of the advances of thread-based microfluidic diagnostic devices in a variety of applications. It begins with an overall introduction of the fabrication followed by an in-depth review on the detection techniques in such devices and various applications with respect to effort and performance to date. A few perspective directions of thread-based microfluidics in its development are also discussed. Thread-based microfluidics are still at an early development stage and further improvements in terms of fabrication, analytical strategies, and function to become low-cost, low-volume and easy-to-use point-of-care (POC) diagnostic devices that can be adapted or commercialized for real world applications.

Tunable Optical Sensing with PVC-Membrane-Based Ion-Selective Bipolar Electrodes

We show here that the response of ion-selective membrane electrodes (ISEs) based on traditional PVC membranes can be directly translated to a colorimetric readout by a closed bipolar electrode (BPE) arrangement. Because the resulting optical response is based on the turnover of the redox probe, ferroin, dissolved in a thin layer compartment, it directly indicates the potential change at the ISE in combination with a reference electrode. This class of probes measures ion activity, analogous to their ISE counterparts. Unlike other ion optodes, the response is also fully tunable over a wide concentration range by the application of an external potential and occurs in a compartment that is physically separate from the sample. To allow for the electrical charge to pass across the ion-selective electrodes, the membranes are doped with inert lipophilic electrolyte, ETH 500, but otherwise have an established composition. The observed response behavior correlates well with theory. A wide range of ion-selective membranes are confirmed to work with this readout principle, demonstrating the detection of potassium, sodium, calcium, and carbonate ions. The corresponding sigmodal calibration curve is used for quantitative analysis in a range of samples including commercial beverages and river and lake samples. The data are successfully correlated with atomic emission spectroscopy and direct potentiometry.

Ionophore-Based Optical Sensor for Urine Creatinine Determination

Creatinine is a metabolite present in urine, and its concentration is used to diagnose and monitor kidney performance. For that reason, the development of new sensors to analyze this metabolite and obtain accurate results in a short period of time is necessary. An optical disposable sensor for monitoring creatinine levels in urine is described. The system, based on a new aryl-substituted calix[4]pyrrole synthetic receptor, has an unusual coextraction scheme. Due to the low p K_a values of creatininium (p K_a 4.8), a careful selection of a lipophilic pH indicator that works in acid medium is required. The sensor components were optimized, and the new sensor displays a good response time to creatinine (approximately 3 min) over a wide dynamic range (from 1 × 10^-5 to 1 × 10^-2 M). Moreover, the optical selectivity coefficients obtained for creatinine over common cations present in urine meet the requirements for real sample measurements. With a good sensor-to-sensor reproducibility (RSD, 5.1-6.9% in the middle of the range), this method provides a simple, quick, cost-effective, and selective alternative to the conventional methodology based on Jaffé's reaction.

A rapid point-of-care optical ion sensing platform based on target-induced dye release from smart hydrogels

We report here a rapid and versatile metal ion analytical platform based on the dye release from hydrogels entrapping ion-selective microdroplets.

Fully inkjet-printed distance-based paper microfluidic devices for colorimetric calcium determination using ion-selective optodes

Although the determination of calcium ions (Ca²⁺) is of high importance to monitor water hardness, currently available devices for on-site analysis suffer from a lack of user-friendliness and sensitivity. This work demonstrates fully inkjet-printed and low-cost microfluidic paper-based analytical devices (μPADs) for the simple naked-eye colorimetric determination of calcium ions (Ca²⁺) in drinking and tap water samples. The quantification of Ca²⁺ relies on visual readout of the length of a colour-changed detection channel modified with ionophore-doped ion-selective optode nanospheres (nano-optodes), eliminating the requirement of a scanner or a camera. All fabrication steps for deposition of assay reagents have been performed by means of a simple desktop thermal inkjet printer, which is expected to contribute to highly batch-to-batch reproducible device preparation. The detectable Ca²⁺ concentrations between 0.05 mmol L^-1 and 5 mmol L^-1 cover the range recommended by the International Organization for Standardization (0.05-2.5 mmol L^-1) and the World Health Organization (WHO) guideline for Ca²⁺ quantification in drinking water (less than 5 mmol L^-1). The lowest concentration of Ca²⁺ detectable by the naked eye was found to be 0.05 mmol L^-1, which is below the value achieved with previously reported paper-based devices. μPAD quantified Ca²⁺ concentrations in tap or drinking waters were within 15% error of the results obtained with a classical complexometric titration. Hence, distance-based μPADs relying on nano-optodes are sensitive and reproducible tools for equipment-free on-site assaying of Ca²⁺ in real samples.

Microfluidic thread-based electrode system to detect glucose and acetylthiocholine

A reusable and simple to fabricate electrochemical sensor for the detection of glucose and acetylthiocholine using thread-based electrodes and nylon thread is described. The fabrication of the device consisted of two steps. First, three nylon-based electrodes (reference, working, and counter) were painted with one layer of conductive inks (silver and carbon ink, or silver/silver chloride ink). The electrodes were taped onto parafilm, and a piece of white nylon thread was wrapped around each electrode connecting the three electrodes. For the glucose system, a PBS solution containing glucose oxidase (GOx) (10 mg/mL), and potassium ferricyanide (K₃ [Fe(CN)₆ ]) (10 mg/mL) as mediator, was dried onto the thread, and increasing concentrations of glucose (0-15 mM) was added to the thread and measured by cyclic voltammetry (CV). The current output from the glucose oxidation was proportional to the concentration of glucose. For the second system, a solution of acetylcholinesterase (AChE) (0.08 U/mL) in PBS was added to the nylon thread, and increasing concentrations of acetylthiocholine (ATC) (0-9.84 mg/mL) was added and measured by CV. The current output from the oxidation of thiocholine (produced by AChE reacting with ATC) was proportional to the concentrations of ATC added to the thread. From both systems, a graph of current output versus substrate concentration was produced and fitted with a linear regression line that gave R² values of 0.985 (GO_X /glucose) and 0.995 (AChE/ATC).

Implementation of a plasticized PVC-based cation-selective optode system into a paper-based analytical device for colorimetric sodium detection

On the example of a colorimetric sodium assay, this work demonstrates the implementation of a classical cation-exchange optode relying on an ionophore-doped plasticized PVC membrane into a paper-based analytical device (PAD). An ion-selective optode (ISO) system has been arranged into a vertically-assembled PAD (vPAD) integrating a pH-buffering function. Capillary force-driven sample liquid transportation through the paper matrix enabled pH-adjustment prior to the optical detection of the analyte cation. Functionalized paper layers with inkjet-deposited ISO membranes were combined with whole device lamination to attain a stable ion-exchange equilibrium required for the theoretical behavior of ISOs. Whole device lamination limited rapid evaporation of sample liquid on vPADs to avoid an increase of target concentration. Sigmoidal response curves between 10^-5 and 1 M of Na⁺ at pH 5.0-7.0 have been confirmed on vPADs, following the theory defined by the cation-exchange equilibrium reaction. Finally, the influence of the cellulosic paper substrate matrix acting as a cation-exchanger on the optode response behavior has been evaluated and compared with conventional plastic film optodes.

Thread-based microfluidics: Flow patterns in homogeneous and heterogeneous microfiber bundles

Thread-based microfluidics has recently seen considerable developments in the domain of portable diagnostic systems, smart bandages and tissue engineering. Similarly to paper-based microfluidics, thread-based microfluidics uses the wicking of fibers to move fluids. It has the advantage of confining and guiding the fluid along the yarns in a one, two or three dimensional space. A global approach to the motion of fluids in yarns and fiber bundles has already been reported in the literature based on the Lucas-Washburn-Rideal law. However no detailed investigation of the flow pattern inside the bundle has been conducted, depending on the internal structure of the bundle. Especially when the bundle possesses heterogeneous wetting properties, such as two different wetting regions interior and exterior, different flow patterns may exist. In this work, we perform a theoretical and numerical analysis of the different flow regimes for homogenous and heterogeneous fiber bundles. It is demonstrated that a limited number of fibers is sufficient for thread-based capillary flows, and that a caging of the flow can be achieved by realizing a lyophobic envelope.