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This paper reviews the development and recent use of everyday communications and IT equipment (mobile phones, digital cameras, scanners, webcams, etc) as detection devices for colorimetric chemistries. Such devices can readily be applied for visible detection using reaction formats such as microfluidic paper based analytical devices (µPADs), indicator papers, and well plate reaction vessels. Their use is highly advantageous with respect to cost, simplicity and portability, and offers many opportunities in the areas of point of care diagnosis, and at-site monitoring of environmental, agricultural, food and beverage parameters. [ABSTRACT FROM AUTHOR]

A method was developed for the fabrication of microfluidic paper-based analytical devices (μPAD). This method was based on the silanisation of cellulose in filter paper using alkyltrimethoxysilane coupled with UV radiation. The filter paper sheet was hydrophobised by immersion in an octadecyltrimethoxysilane/heptane (OTMS/heptane) solution (0.25 vol. %) containing 5 vol. % of ethyl acetate (EtOAc). The hydrophobic-hydrophilic contrast was generated on the filter paper after the hydrophobised paper sheet was exposed to UV light with a metal mask creating the desired pattern on the sheet. The exposed area was oxidised to create a hydrophilic area, while the hydrophobic area was protected by the metal mask. The optimal conditions for the fabrication of μPAD were studied; these included ethyl acetate concentration (CEtOAc), immersion time, octadecyltrimethoxysilane concentration (COTMS) and exposure time. This method is costeffective and simple. In addition, different functional groups could be further grafted for various assay purposes. To demonstrate the feasibility of the μPAD in analytical applications, a flowershaped μPAD with eight channels and eight detection units was fabricated and used to determine the nitrite content in pickled vegetables. The nitrite content (124 μg g−1) in the sample determined by this method compared favourably with that measured using a standard method (137 μg g−1). [ABSTRACT FROM AUTHOR]

A microfluidic paper-based analytical device integrating a chromoreactand – a formylazo dye– has been fabricated and used for a colorimetric assay of sulfites. The chromoreactand was covalently linked to paper by vinyl sulfone chemistry. This work presents two robust capillary microfluidic devices to determine sulfite in wine without any pretreatment. One of them based on thread (µTPAD) useful to determine it in white wine and another based on paper (µPAD) to specifically determine sulfite in red wine as well as in white wine. Both are based on the selective recognition of sulfite by means of a chromoreactand that turns from orange to yellow in the presence of sulfite. The colour information acquired (H coordinate) using a digital camera readout allows for a range of application of the µTPAD from 7.8·10−5 M (8.1 mg L−1) to 2.7·10−3 M (279.3 mg L−1) with a limit of detection (LOD) of 78 µM. The strong interference caused by the dyes present in red wine is eliminated by including a laminated paper channel in the µPAD structure that allows for the separation of colorants from red wine before the recognition of the sulfite. This makes it possible to adjust the µPAD procedure to the usual sulfite concentration in wine, with an LOD of 2.2·10−4 M (22.7 mg L−1) and a CV of 2.6%. [Display omitted] • μPAD based on a novel chromoreactand for fast, easy and selective sulfite determination in wine. • Buffering and decolorization of wine sample is performed in the μPAD. • Red and white wine can be analysed without any kind of sample pre-treatment. [ABSTRACT FROM AUTHOR]

Sample loading, Element enrichment and Visualization processes. [Display omitted] • Novel designed lateral flow strip based on functional Pb (II) MIPs paper chip was successfully fabricated. • A newly designed μ PAD platform and visualization methodology was proposed. • The presented platform demonstrated sensitivity, rapidness and application ruggedness for real sample with 1.375 mg kg−1 (LOD) and 2.5–100 mg kg−1 (linear range) A high-performance lateral flow and paper-based microfluidic device (μ PAD) was constructed to develop a rapid read-out analytical platform for heavy metal elements (Pb (II)) in green tea with complex matrices. The entire analytical device was integrated with several functional parts, including a liquid flow channel and an enrichment visualization zone prepared by the molecularly imprinted polymer (MIP) protocol. Under the optimal conditions, the Pb (II) content in tea products can be sensitively and specifically analyzed using an in situ visualization readout. Herein, a rapid visual read-out analysis of the lead level can be achieved by overcoming the other co-existing interfering elements in the traditional green tea sample. The ionic imprinted paper chip can effectively capture the Pb (II) of the digested sample solution, and a visualization sensing process occurred between the metal elements and a chelating visual reagent with high sensitivity and selectivity in situ. Owing to the stable visualization performance, the constructed chip demonstrated good contrast, with a low background and easily recognizable colored spots. The linear range was 2.5–100 mg kg−1, and the limit of detection for Pb (II) in traditional green tea was 1.375 mg kg−1. In addition, other co-existing metal elements did not interfere with the analysis because of the appropriate pretreatment and selectivity of the paper-based chip. Furthermore, the analytical results for tea samples were verified by atomic absorption spectrophotometer (AAS) tests, which demonstrated the robustness and reliability of the proposed method. Therefore, the high-performance visualization lateral flow- μ PAD is a promising platform for the rapid analysis of metal element levels in food products. [ABSTRACT FROM AUTHOR]

Nowadays, developing a cost-effective, easy-to-operate, and efficient signal amplification platform is of important to microfluidic paper-based analytical devices ( μ PAD) for end-use markets of point-of-care (POC) assay applications. Herein, an ultrasensitive, paper-based photoelectrochemical (PEC) bioassay platform is constructed by in situ grown ZnO/ZnIn 2 S 4 heterojunctions onto paper fibers, which acted as photoactive signal amplification probes for enhancing the sensitivity of antibodies-based diagnostic assays, for the sensitive detection of alpha-fetoprotein (AFP) targets. The crystalline flake-like ZnIn 2 S 4 composited with hexagonal nanorods (NRs) morphology of ZnO is an in situ grown, at the first time, onto cellulose fibers surface supported with Au nanoparticle (Au NP) modification to improve conductivity of the device working zone. The obtained composites on paper fibers are implemented as a flexible paper-based photoelectrode to realize remarkable performance of the fabricated μ PAD, resulting from the enhanced PEC activity of heterojunctions with effective electron-hole pair separation for accelerating photoelectric conversion efficiency of the sensing process under light irradiation. Once the target AFP was introduced into the biosensing interface assistant, with a specific recognition interaction of AFP antibody, a drastically photocurrent response was generated, in view of the apparent steric effects. With the concentration increase of AFP targets, more immune conjugates could be confined onto the biosensing interface, eventually leading to the quantitative decrease of photocurrent intensity. Combined with an ingenious origami design and permitting the hydrophobic/hydrophilic conversion procedure in the bioassay process, the ultrasensitive PEC detection of AFP targets was realized. Under the optimized conditions, the level of AFP could be sensitively tracked by the prepared μ PAD with a liner range from 0.1 to 100 ng mL -1 and limit of detection of 0.03 ng mL -1 . This work provides a great potential application for highly selective and sensitive POC testing of AFP, and finally, developments for clinical disease diagnosis.

Current centralised healthcare models pose many challenges, particularly for developing countries such as South Africa, where travel and time costs make it difficult for patients to seek healthcare, even when urgently needed. To address this issue, point-of-care (PoC) tests, which are performed at or near the site of clinical care, have gained popularity and are actively being developed. Microfluidic systems, in which small volumes of fluids can be processed, provide an ideal platform on which to develop PoC diagnostic solutions. Specifically, the emerging field of paper-based microfluidics, with advantages such as low-cost, disposability and minimal external equipment requirements, provides unique opportunities for addressing healthcare issues in developing countries. This work explores the field of paper-based microfluidics, with step-by-step instructions on the design, manufacture and testing processes to realise paper-based devices towards diagnostic applications. Paper-based microfluidic and electronic components are presented, as well as the integration of these components to provide smart paper-based devices. This serves as an educational tool, enabling both beginners and experts in the field to fast-track development of unique paper-based solutions towards PoC diagnostics, with emphasis on the South African context, where both the need for and impact of these solutions are great.

Herein, a simple microfluidic paper-based analytical device (μPAD) by using platinum nanoparticles (Pt NPs) as highly active peroxidase mimic for simultaneous determination of glucose and uric acid was fabricated. The μPAD consisted of one sample transportation layer, four paper-based detection chips, and two layers of hydrophobic polyethylene terephthalate (PET) films. The four detection chips were immobilized with various chromogenic reagents, Pt NPs, and specific oxidase (glucose oxidase or uricase). H 2 O 2 generated by specific enzymatic reactions could oxidize co-immobilized chromogenic reagents to produce colored products by using Pt NPs as efficient catalyst. The multi-layered structure of μPAD could effectively improve the color uniformity and color intensity. Total color intensity from each two detection chips modified with distinct chromogenic reagents were used for quantitative analysis of glucose and uric acid, respectively, resulting in significantly improved sensitivity. The linear range for glucose and uric acid detection was 0.01–5.0 mM and 0.01–2.5 mM, respectively. Satisfied results were obtained for glucose and uric acid detection in real serum samples. An easy-to-use smartphone APP was developed for convenient and intelligent detection. The developed μPAD integrated with smartphone as detector holds great applicability for simple and portable on-site analysis. [Display omitted] • A multi-layered μPAD by using Pt NPs as highly active peroxidase mimic was fabricated. • The multi-layered structure of μPAD could effectively improve color uniformity and intensity. • Simultaneous determination of glucose and uric acid in mixture was achieved by using smartphone as detector. • Improved sensitivity was obtained by using total colorimetric responses from each two detection chips as quantitative signal. • Easy-to-use smartphone APP was developed for convenient and intelligent detection. [ABSTRACT FROM AUTHOR]

This work presents two simple methods for estimating the firing distance from the gunshot residues (GSRs) on fabric targets. Four types of fabric targets, namely twill weave denim cotton-polyester (80/20), jersey knitting 100% cotton, plain weave cotton-polyester (80/20) and plain weave cotton-polyester (60/40), were employed. The firing tests were carried out using these white fabrics as targets at distances of 5-100 cm, respectively. In the first method, digital images of the black GSRs on fabric materials were recorded inside an illuminated box and the inverted gray intensity values were plotted against the firing distances. Since the plots of all fabrics are not significantly different, the estimation of firing distance employs the same exponential curve for all test fabrics. Although simple, the imaging method is not suitable for dark-colored materials. A chemical-based method was therefore developed as an alternative method. In the second method, a small disposable microfluidic paper-based analytical device (μPAD) was employed for detecting Pb(II) extracted from the GSRs. The μPAD method uses the measurement of the length of a narrow band of a pink color resulting from reaction between rhodizonate reagent and the Pb(II) extract. The plots indicated that the data of thick denim material are significantly different to other test fabrics which are much thinner. These three fabrics share the same estimation curve. However, it is recommended that the separate estimation curve for denim materials must be used. Both methods are suitable for short range firing distance, no further than 60 cm, since at greater distances the inverted gray intensity and the 'band-length' methods are unable to detect the GSRs. [ABSTRACT FROM AUTHOR]

Microfluidic paper-based analytical devices (μPADs) have experienced an unprecedented story of success. In particular, as of today, most people have likely come into contact with one of their two most famous examples─the pregnancy or the SARS-CoV-2 antigen test. However, their sensing performance is constrained by the optical readout of nanoparticle agglomeration, which typically allows only qualitative measurements. In contrast, single-impact electrochemistry offers the possibility to quantify species concentrations beyond the pM range by resolving collisions of individual species on a microelectrode. Within this work, we investigate the integration of stochastic sensing into a μPAD design by combining a wax-patterned microchannel with a microelectrode array to detect silver nanoparticles (AgNPs) by their oxidative dissolution. In doing so, we demonstrate the possibility to resolve individual nanoparticle collisions in a reference-on-chip configuration. To simulate a lateral flow architecture, we flush previously dried AgNPs along a microchannel toward the electrode array, where we are able to record nanoparticle impacts. Consequently, single-impact electrochemistry poses a promising candidate to extend the limits of lateral flow-based sensors beyond current applications toward a fast and reliable detection of very dilute species on site.

Microfluidic paper combines pump-free water transport at low cost with a high degree of sustainability, as well as good availability of the paper-forming cellulosic material, thus making it an attractive candidate for point-of-care (POC) analytics and diagnostics. Although a number of interesting demonstrators for such paper devices have been reported to date, a number of challenges still exist, which limit a successful transfer into marketable applications. A strong limitation in this respect is the (unspecific) adsorption of protein analytes to the paper fibers during the lateral flow assay. This interaction may significantly reduce the amount of analyte that reaches the detection zone of the microfluidic paper-based analytical device (µPAD), thereby reducing its overall sensitivity. Here, we introduce a novel approach on reducing the nonspecific adsorption of proteins to lab-made paper sheets for the use in µPADs. To this, cotton linter fibers in lab-formed additive-free paper sheets are modified with a surrounding thin hydrogel layer generated from photo-crosslinked, benzophenone functionalized copolymers based on poly-(oligo-ethylene glycol methacrylate) (POEGMA) and poly-dimethyl acrylamide (PDMAA). This, as we show in tests similar to lateral flow assays, significantly reduces unspecific binding of model proteins. Furthermore, by evaporating the transport fluid during the microfluidic run at the end of the paper strip through local heating, model proteins can almost quantitatively be accumulated in that zone. The possibility of complete, almost quantitative protein transport in a µPAD opens up new opportunities to significantly improve the signal-to-noise (S/N) ratio of paper-based lateral flow assays. [ABSTRACT FROM AUTHOR]

To increase milk production, antibiotics are administered to animals to provide weight gain and to prevent or treat diseases. The inappropriate use of these substances can lead to antibiotic resistance and allergic reactions and toxic effects to milk consumers. We describe the development of a simple, fast, portable, and low-cost microfluidic paper-based analytical device (μPAD) to quantify sulfonamides in milk using the inhibition of the colorimetric reaction between carbonic anhydrase (CA) and 4-nitrophenyl acetate. The main advantages presented by the μPAD include reproducible batch production, simple application, and precise analysis without previous treatment. The µPAD displayed good linearity (R2 ≥ 0.986) in a wide range of sulfonamides in milk (2.5 to 40.0 µmol L−1), being selective for the drugs even in a highly complex matrix. We expect that this device allows in situ monitoring of milk quality, reducing the prejudicial conditions associated with high concentrations of sulfonamides in milk. [ABSTRACT FROM AUTHOR]

We report a novel three-dimensional microfluidic paper-based analytical device (3D-μPAD) with colorimetric detection, using Mn–ZnS quantum dot embedded molecularly imprinted polymer (Mn–ZnS QD-MIP), for selective glyphosate determination in whole grain samples. Detection is based on the catalytic activity of Mn–ZnS QD-MIP in the H 2 O 2 oxidation of ABTS. Glyphosate imprinted polymer is successfully synthesized on the Mn–ZnS QD surface using a poly (N-isopropylacrylamide) (NIPAM) and N, N′-Methylenebisacrylamide (MBA) as the functional monomers. The catalytic activity depends on binding or non-binding of glyphosate molecules on the synthetic recognition sites of the Mn–ZnS QD-MIP. Glyphosate selectively binds to the cavities embedded on the Mn–ZnS QD surface, and subsequently turns-off or inhibits the ABTS oxidation and color change to light green. The change of reaction color from dark green to light green depends on the concentration of glyphosate. We report, for the first time, using the relatively new penguard enamel colour to create a hydrophobic barrier. The foldable 3D-μPAD comprises three layers (top/center/bottom), named as the detection zone, immobilized Mn–ZnS QD-MIP disc, and sample loading. Assay on the 3D-μPAD can determine glyphosate by ImageJ detection, over an operating range of 0.005–50 μg mL−1 and with a detection limit of 0.002 μg mL−1. Our 3D-μPAD exhibits high accuracy, with a 0.4% (intra-day) and 0.7% (inter-day) relative difference from the certified CRM value. Moreover, the fabricated 3D-μPAD provides good reproducibility (1.7% RSD for ten devices). The developed 3D-μPAD was successfully applied to determine the glyphosate concentration in whole grain samples and shows great promise as an alternative highly selective and sensitive colorimetric method. The 3D-μPAD is well suited to food-quality control and onsite environmental-monitoring applications, without sophisticated instrumentation. Image 1 • Mn–ZnS QD-MIP was synthesized for selective and sensitive glyphosate determination. • Colorimetric detection is based on glyphosate inhibit activity of Mn–ZnS QD-MIP in the H 2 O 2 oxidation of ABTS. • 3D-μPAD fabricated from Penguard enamel colour was firstly present. • The proposed 3D-μPAD exhibits high accuracy, sensitivity, selectivity and suitable for whole-grain samples. [ABSTRACT FROM AUTHOR]

• Paper-based microfluidic device combined with a graphene heater for silicon nanodots synthesis. • Total carbohydrates determination based on fluorescence intensity of the silicon nanodots. • Heater temperature controlled by portable electronic system with wireless communication. We report a simple, rapid, low-resource and one-step method for formation of fluorescent silicon nanodots (SNDs) in a microfluidic paper-based analytical device (μPAD) incorporated in a reusable, portable and flexible heater for determination of total carbohydrates. The synthesis of SNDs is based on the redox reaction between (3-aminopropyl) triethoxysilane (APTS) reagent and carbohydrates, which act as a reducer. The graphene-based heater was fabricated by laser ablation of Kapton polyimide. Thereby, the developed system heat the μPAD during the synthesis of SNDs at 80 °C for 30 min. The blue emitting SNDs formed have an emission peak wavelength at 475 nm. We used a digital camera and smartphone for the quantitative analysis of total carbohydrates expressed such as index of glucose or fructose with grey scale value as the analytical parameter. Under the optimal conditions, the method has a low detection limit (0.80 μM for glucose and 0.51 μM for fructose, respectively), and a linear response (10–200 μM for glucose and 10–100 μM for fructose). The method has been applied to the determination of glucose in biological fluids (serum and urine samples). In addition, determination of total carbohydrates in commercial juices and teas have been carried out. [ABSTRACT FROM AUTHOR]

• A machine learning based platform is proposed for glucose determination with a μ PAD. • A smartphone app was developed to perform machine learning in colorimetric analysis. • Over 98% accuracy was obtained in glucose determination with TMB. • First study to link machine learning, μ PAD and smartphone for glucose determination. • The platform has great potential for non-invasive measurement of glucose. Potassium iodide (KI) and 3,3 ′ ,5,5 ′ -tetramethylbenzidine (TMB) are frequently used as chromogenic agents in μ PADs for glucose determination. Chitosan (Chi) has peroxidase like activity and improves the analytic performance of μ PADs when used in combination with a chromogenic agent. Here, a portable platform incorporating a μ PAD with a smartphone application based on machine learning was developed to quantify glucose concentration in artificial saliva. The detection zones of the μ PAD were modified with three different detection mixtures containing; (i) KI, (ii) KI+Chi and (iii) TMB. After the color change, the images of the μ PADs were taken with four different smartphones under seven different illumination conditions. The images were first processed for feature extraction and then used to train machine learning classifiers, resulting in a more robust and adaptive platform against illumination variation and camera optics. Different machine learning classifiers were tested and the best machine learning classifier for each detection mixture was obtained. Next, a special application called " GlucoSensing " capable of image capture, cropping and processing was developed to make the system more user-friendly. A cloud system was used in the application to communicate with a remote server running machine learning classifiers. Among the three different detection mixtures, the mixture with TMB demonstrated the highest classification accuracy (98.24%) with inter-phone repeatability under versatile illumination. [ABSTRACT FROM AUTHOR]

The main problem for the expansion of the use of microfluidic paper-based analytical devices and, thus, their mass production is their inherent lack of fluid flow control due to its uncontrolled fabrication protocols. To address this issue, the first step is the generation of uniform and reliable microfluidic channels. The most common paper microfluidic fabrication method is wax printing, which consists of two parts, printing and heating, where heating is a critical step for the fabrication of reproducible device dimensions. In order to bring paper-based devices to success, it is essential to optimize the fabrication process in order to always get a reproducible device. Therefore, the optimization of the heating process and the analysis of the parameters that could affect the final dimensions of the device, such as its shape, the width of the wax barrier and the internal area of the device, were performed. Moreover, we present a method to predict reproducible devices with controlled working areas in a simple manner. [ABSTRACT FROM AUTHOR]

• A practical concept of a solid reference electrode was introduced. • The reference electrode was integrated with paper-based microfluidics and ISEs. • The paper substrates defined the solution wicking dynamics and sample filtration. • The developed device was used for direct analysis of wastewater sludge and sweat. A new concept of a solid reference electrode integrated with microfluidic paper-based sampling was introduced and applied in potentiometric ion sensing. The new reference electrode consisted of an Ag/AgCl reference element (reusable) that is in contact with a disposable paper substrate (DPS) containing solid KCl. Thus, no KCl reference solution was applied during the analysis step, but the applied sample solution dissolved the solid KCl in the paper providing the electrolyte for the Ag/AgCl reference element. Such reference electrode was found to give a relatively constant potential after less than ca 1 min of equilibration. Moreover, the response of the reference electrode was not affected by concentrations of background electrolytes in the sample. Thus the proposed reference electrode was integrated with a paper-based microfluidic device and solid-contact ion-selective electrodes (ISEs) sensitive to K+, Na+ and Cl−. The optimized systems were characterized with near-Nernstian sensitivities (59.1 ± 1.5, 57.5 ± 0.5 and –56.4 ± 0.6 mV dec−1) and detection limits (10−4.1 ± 0.1, 10−3.3 ± 0.1 and 10−4.1 ± 0.1 mol dm−3) for K+, Na+ and Cl− ions, respectively. The paper-based measurement setup was favourably used for potentiometric determination of ions in environmental (wastewater sludge) and clinically relevant (sweat) samples. [ABSTRACT FROM AUTHOR]

To date, 55% of inhabitants of the developing world still live in rural regions and they have a very urgent need to improve the level of disease diagnosis, especially in resource-limited settings. Herein, we present a microfluidic system that centrifuges whole blood and quantifies contained biomarkers in a rapid, feasible and integrated way. Such a device provides a blood-in/answer-out capability and is small enough to be carried by any individual in any environment. The successful integration a hand-powered centrifuge and immunoassay unit within a rotational paper-based device allows for diagnostic application by untrained users and in environments where access to electricity cannot be assumed. In addition, the low cost (~$ 0.5), light weight and small instrumental footprint make the device ideally suited for rapid on-site detection. To validate the applicability of the system in a clinical diagnostic testing, we successfully perform enzyme-linked immunosorbent assay (ELISA) analysis of carcinoembryonic antigen and alpha fetoprotein from human blood samples. We expect that this powerful platform technology will provide the opportunities for point-of-care diagnosis in resource-limited settings. • Integrated hand-power blood centrifugation and ELISA analysis was firstly proposed on paper based device. • The device provides a rapid, low-cost and robust diagnostic test with blood-in/answer-out capability. • Two typical tumor markers CEA and AFP are successfully detected on this platform. [ABSTRACT FROM AUTHOR]

This article reports on the development and validation of a disposable microfluidic paper-based analytical device (μPAD) for on-hand, in-situ, and cheap Fe(III) determination in natural waters complying with World Health Organization guidelines. The developed μPAD used 3-hydroxy-4-pyridinone (3,4-HPO) as a colour reagent due to its considerably lower toxicity than traditionally used iron analytical reagents. It was selected among a group of hydrophilic 3,4-HPO chelators containing ether-derived chains in their structure which were prepared using green methods. The relatively high water solubility of these chelators improved the detection limit and applicability as μPAD reagents. Under optimal conditions, the μPAD is characterised by a quantification range between 0.25 and 2.0 mg/L, a detection limit of 55 μg/L and 15 min of analysis time. The signal stability extends up to 4 h and the device is stable for at least one month. The reagent consumption is below 0.2 mg per analysis and the μPAD method was validated by analysis certified reference materials and by comparison with atomic absorption results (RD < 10%). The newly developed μPAD was successfully applied to the determination of iron in river, well and tap waters with no need of any prior sample pre-treatment. Image 1 • First μPAD for iron analysis using specially designed hydrophilic 3,4-HPO chelator. • Arrangement of three staked hydrophilic layers for improving the limit of detection. • Successful application to natural water samples with no need of sample pretreatment. • Dynamic range complying with WHO guidelines for Fe(III) content in drinking waters. [ABSTRACT FROM AUTHOR]

• A novel analytical method based on origami paper-based microfluidic device as a platform for paper chromatography. • Device fabrication processes is a simple single side printing and curing paper. • Consumption low amount of reagents/solvents and cost-effective analysis. • The method does not require any sample pretreatment step, applying a raid analytical procedure by the green aqueous solvent Herein, we report three-dimensional paper chromatography (3D-PC) as a micro-chromatographic platform. The method was based on applying the origami microfluidic device for separation, coupled by colorimetric methods for simultaneous determination. The microfluidic device fabrication was a facile printing approach. Two azo food dyes, Tartrazine (E102) and Indigo carmine (E132), were selected as a model analyte, while carbonate-bicarbonate buffer was used as the mobile phase. Our micro-chromatographic device is associated with two big advantages including needing very small volume of mobile phase (~12 µL) and ultrafast separation time (~35 s). Under the optimal conditions, the method provided acceptable linear ranges of 0. 0 g L 1–18.0 g L 1 (R2 = 0.997) for E102 and 0.070 g L 1–10.0 g L 1 for E132 and the limits of detection (3σ/slope) were evaluated as 0.620 and 0.060 g L 1, respectively. The proposed method was successfully applied in the separation and quantification of these dyes in commercial food products such as jelly, candy, and four kinds of drink samples without any sample preparation prior to analysis. The mean recovery values for the real sample analysis were in the range of 100.14%–102.38% for E132 and E102 respectively. The inter-device relative standard deviations were in the ranges of 1.5%–11.8%. In total, our chromatographic μPAD is small (1.0 cm × 1.0 cm × 0.5 cm), portable, inexpensive, no need of specialized user, requires low volumes of sample (0.5 μL), and can perform separation using 12 μL of aqueous mobile phase in very short time. [ABSTRACT FROM AUTHOR]

In this note we describe a four-channel paper microfluidic device (μPAD) that utilizes gold nanoparticles (AuNPs) as colorimetric sensors and specifically tailored aptamers to capture a set of targeted drugs, including cocaine, codeine and methamphetamine. The design utilizes salt-induced aggregation of AuNPs to produce a color change that indicates the presence of target analytes. In the absence of the targets, the aptamers bind to AuNPs, preventing aggregation. This μPAD was developed by optimizing the concentrations of aggregating agents, AuNPs and aptamers. The resultant device is sensitive and specific, producing a positive result upon detection of the target analyte. • A four-channel paper microfluidic device with gold nanoparticles and aptamers has been developed for siezed drugs. • The design is optimized for length and width of channels, moving solution, quantity of gold nanoparticles, and aptamers. • The runtime of this μPAD is under 10 min and the LOD is 4 μg for cocaine, codeine and methamphetamine. [ABSTRACT FROM AUTHOR]

The main problem for the expansion of the use of microfluidic paper-based analytical devices and, thus, their mass production is their inherent lack of fluid flow control due to its uncontrolled fabrication protocols. To address this issue, the first step is the generation of uniform and reliable microfluidic channels. The most common paper microfluidic fabrication method is wax printing, which consists of two parts, printing and heating, where heating is a critical step for the fabrication of reproducible device dimensions. In order to bring paper-based devices to success, it is essential to optimize the fabrication process in order to always get a reproducible device. Therefore, the optimization of the heating process and the analysis of the parameters that could affect the final dimensions of the device, such as its shape, the width of the wax barrier and the internal area of the device, were performed. Moreover, we present a method to predict reproducible devices with controlled working areas in a simple manner.

In human history, the paper has long been used as a platform to record and preserve information. However, over the decades, paper has found its application in Biomedical Sciences, too. Both paper-based microfluidic devices (μPADs) and paper-based cultures and scaffolds have shown immense potential to be used as a sensor as well as a supporting material for in vitro tissue engineering. μPADs can be used to perform low-cost and fast biomolecular assays at Point-Of-Care (POC). They are being used to detect various biomarkers like viral proteins, metabolites, oncogenes, and antigens; and conditions like Venous Thromboembolism (VTE). On the other hand, the paper has also been used to develop paper-based 3D cultures and scaffolds to test drugs, and monitor cytotoxic effects in vitro cell microenvironments and also as implantable tissues. In this review, we intend to enumerate the development in the field of μPADs, paper-based cell cultures, and paper-based scaffolds and their plethora of applications over the last decade., Competing Interests: No conflicts of interest., (© 2020 The Authors.)

In this work, a double-layered three-dimensional (3D) microfluidic paper-based analytical device (μPAD) with high resolution temporally resolved chemiluminescence (CL) emissions were designed for multiplexed CL analysis. The temporally resolved CL emissions were obtained by virtue of the 3D branched microfluidic channel design, which create time delays for luminol transported from one detection zone to another. The peak intensity and peak shape of the temporally resolved CL emissions were quite stable and base-line separated with resolution as high as 21.2–24.4. Then, the fabricated μPAD was applied to multiplexed determination of glucose, lactate, cholesterol, and choline as model analytes. The sample was added to four detection zone modified with CL catalyst cobalt ion and different oxidase by virtue of chitosan. When luminol flowed to μPAD, four temporally resolved CL peaks were successively generated from the cobalt ion catalyzed CL reactions between luminol and generated H 2 O 2 from the specific enzymatic reactions between the oxidase and the analytes. The generated four CL emission peaks in the CL kinetic curve increased in proportion to the concentrations of glucose, lactate, cholesterol, and choline, respectively. Finally, four linear calibration curves were obtained for the detection of glucose (0.01–1.0 mmol/L), lactate (0.02–5.0 mmol/L), cholesterol (0.01–0.4 mmol/L), and choline (0.001–1.0 mmol/L). The detection limits were as low as 8 μmol/L, 15 μmol/L, 6 μmol/L, and 0.07 μmol/L for glucose, lactate, choline, and cholesterol detection, respectively. The present work provides a new strategy for the fabrication of simple and sensitive 3D μPAD with high resolution temporally resolved CL emissions for multiplexed CL analysis, which holds great application potential for point-of-care diagnosis. • A double-layered 3D μPAD with temporally resolved CL emissions was fabricated. • Stable, steep and base-line separated temporally resolved CL emissions with resolution higher than 21.2 were obtained. • Sensitive, selective and simultaneous determination of glucose, lactate, cholesterol, and choline was achieved. • The first example of μPAD based CL biosensor for lactate, cholesterol and choline detection. [ABSTRACT FROM AUTHOR]

Paper-based microfluidic devices have advanced significantly in recent years as they are affordable, automated with capillary action, portable, and biodegradable diagnostic platforms for a variety of health, environmental, and food quality applications. In terms of commercialization, however, paper-based microfluidics still have to overcome significant challenges to become an authentic point-of-care testing format with the advanced capabilities of analyte purification, multiplex analysis, quantification, and detection with high sensitivity and selectivity. Moreover, fluid flow manipulation for multistep integration, which involves valving and flow velocity control, is also a critical parameter to achieve high-performance devices. Considering these limitations, the aim of this review is to (i) comprehensively analyze the fabrication techniques of microfluidic paper-based analytical devices, (ii) provide a theoretical background and various methods for fluid flow manipulation, and (iii) highlight the recent detection techniques developed for various applications, including their advantages and disadvantages. [ABSTRACT FROM AUTHOR]

This method describes a rapid ecofriendly and affordable method for detecting arsenic in the water sample. The system designed works on the principle that involves generation of arsine due to reduction of arsenic by bare and cysteine capped iron oxide nanoparticles and its further reaction with silver nitrate present on the microfluidic paper analytical device (μPAD). Change in the color of μPAD from colorless to reddish brown is a result of reaction between arsine gas and silver nitrate, and is the detection criteria. The sample solution of arsenic was prepared in lemon juice to provide the required acidic environment for hydride generation. This proposed method has detection limit of 0.01 ppm (10 ppb) and 1 ppm for cysteine capped and bare iron oxide nanoparticles respectively. This is for the first time that iron oxide nanoparticles are being used for detection and reduction arsenic species in environmental sample. The same device can be used for on-site detection in an ecofriendly manner.

Norovirus (NoV) is one of the leading causes of acute gastroenteritis, affecting 685 million people per year around the world. The best preventive measure is to screen water for possible NoV contamination, not from infected humans, preferably using rapid and field-deployable diagnostic methods. While enzyme immunoassays (EIAs) can be used for such detection, the low infectious dose as well as the generally inferior sensitivity and low titer of available NoV antibodies render critical challenges in using EIAs toward NoV detection. In this work, we demonstrated smartphone-based Mie scatter detection of NoV with immunoagglutinated latex particles on paper microfluidic chips. Using only three different concentrations of anti-NoV-conjugated particles, we were able to construct a single standard curve that covered seven orders of magnitude of NoV antigen concentrations. Multiple normalization steps and interpolation procedures were developed to estimate the optimum amount of antibody-conjugated particles that matched to the target NoV concentration. A very low detection limit of 10 pg/mL was achieved without using any concentration or enrichment steps. This method can also be adapted for detection of any other virus pathogens whose antibodies possess low sensitivity and low antibody titer.

This work demonstrates the fabrication of microfluidic paper-based analytical devices (µPADs) suitable for the analysis of sub-microliter sample volumes. The wax-printing approach widely used for the patterning of paper substrates has been adapted to obtain high-resolution microfluidic structures patterned in filter paper. This has been achieved by replacing the hot plate heating method conventionally used to melt printed wax features into paper by simple hot lamination. This patterning technique, in combination with the consideration of device geometry and the influence of cellulose fiber direction in filter paper, led to a model µPAD design with four microfluidic channels that can be filled with as low as 0.5 µL of liquid. Finally, the application to a colorimetric model assay targeting total protein concentrations is shown. Calibration curves for human serum albumin (HSA) were recorded from sub-microliter samples (0.8 µL), with tolerance against ±0.1 µL variations in the applied liquid volume.