Integrated Distance-Based Origami Paper Analytical Device for One-Step Visualized Analysis

In Situ Monitoring of Fluid Shear Stress Enhanced Adherence of Bacteria to Cancer Cells on Microfluidic Chip

Mechanosensing mechanisms for surface recognition by bacteria play an important role in inflammation and phagocytosis. Here, we describe a set of DNA probes for revealing microbe adherence to cancer cells under fluid shear stress. DNA probes modified with a biotin group, an azido group, and hexadecanoic acid were indiscriminately anchored to the cell surface, acting as indicators for the membrane proteins, cell-surface carbohydrate, and phospholipids. When cancer cells were exposed to bacteria in fluid, enhanced accumulation of membrane proteins was indicated by the strong fluorescence aggregation, meanwhile the weakened accumulation of cell-surface carbohydrate and phospholipids indication was indicated by attenuated fluorescence. Further research demonstrates that this mechanosensing strategy was applicable to different bacterial-cancer cell interactions. This study not only uncovered new cellular mechanotransduction mechanisms, but also provided a versatile method that enabled in situ and dynamic indication of cancer cell responses to mechanical stimuli.

Soft and flexible material-based affinity sensors

Recent advances in biosensors and point-of-care (PoC) devices are poised to change and expand the delivery of diagnostics from conventional lateral-flow assays and test strips that dominate the market currently, to newly emerging wearable and implantable devices that can provide continuous monitoring. Soft and flexible materials are playing a key role in propelling these trends towards real-time and remote health monitoring. Affinity biosensors have the capability to provide for diagnosis and monitoring of cancerous, cardiovascular, infectious and genetic diseases by the detection of biomarkers using affinity interactions. This review tracks the evolution of affinity sensors from conventional lateral-flow test strips to wearable/implantable devices enabled by soft and flexible materials. Initially, we highlight conventional affinity sensors exploiting membrane and paper materials which have been so successfully applied in point-of-care tests, such as lateral-flow immunoassay strips and emerging microfluidic paper-based devices. We then turn our attention to the multifarious polymer designs that provide both the base materials for sensor designs, such as PDMS, and more advanced functionalised materials that are capable of both recognition and transduction, such as conducting and molecularly imprinted polymers. The subsequent content discusses wearable soft and flexible material-based affinity sensors, classified as flexible and skin-mountable, textile materials-based and contact lens-based affinity sensors. In the final sections, we explore the possibilities for implantable/injectable soft and flexible material-based affinity sensors, including hydrogels, microencapsulated sensors and optical fibers. This area is truly a work in progress and we trust that this review will help pull together the many technological streams that are contributing to the field.

Control of capillary behavior through target-responsive hydrogel permeability alteration for sensitive visual quantitative detection

DNA hydrogels have received considerable attention in analytical science, however, some limitations still exist in the applications of intelligent hydrogels. In this paper, we describe a way to prepare gel film in a capillary tube based on the thermal reversible principle of DNA hydrogel and the principle of capillary action. Because of the slight change in the internal structure of gel, its permeability can be increased by the addition of some specific targets. The capillary behavior is thus changed due to the different permeability of the hydrogel film. The duration time of the target solution flowing through the capillary tube with a specified length is used to quantify this change. With this proposed method, ultra-trace DNA hydrogel (0.01 μL) is sufficient to realize the sensitive detection of cocaine without the aid of other instruments, which has a low detection limit (1.17 nM) and good selectivity.

DNA hydrogels have received considerable attention in analytical science but limitations still exist in the applications of intelligent hydrogels. Here, the authors describe a DNA hydrogel sensor for quantitative detection of cocaine based on the permeability change in a DNA hydrogel film.

Three-dimensional origami paper-based device for portable immunoassay applications

In this study, we demonstrate a three-dimensional surface-modified origami-paper-based analytical device (3D-soPAD) for immunoassay applications. The platform enables the sequential steps of immunoassays to be easily performed using a folded, sliding paper design featuring multiple pre-stored reagents, allowing us to take advantage of the vertical diffusion of the analyte through the different paper layers. The cellulose substrate is composed of carboxymethyl cellulose modified with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide, which provide covalent bonding sites for bio-recognition molecules. After the optimization of the operation parameters, we determined the detection limit of the 3D-soPAD for human immunoglobulin G (HIgG) which can be as low as 0.01 ng mL-1, with a total turnaround time of 7 min. In order to study the long-term storage of the platform, anti-HIgG horseradish peroxidase (aHIgG-HRP) conjugates were stored by freeze-drying in sugar matrices composed of 10% sucrose/10% trehalose (w/w%) on the paper device, retaining 80% of their activity after 75 days of storage at 4 °C. To evaluate the performance of the paper device using real samples, we demonstrated the detection of protein A (a biomarker for Staphylococcus aureus infection) in highly viscous human synovial fluid. These results show that the proposed 3D-soPAD platform can provide sensitive, high-throughput, and on-site prognosis of infection in resource-limited settings.

Flexible Electronics Based on Micro/Nanostructured Paper

Over the past several years, a new surge of interest in paper electronics has arisen due to the numerous merits of simple micro/nanostructured substrates. Herein, the latest advances and principal issues in the design and fabrication of paper-based flexible electronics are highlighted. Following an introduction of the fascinating properties of paper matrixes, the construction of paper substrates from diverse functional materials for flexible electronics and their underlying principles are described. Then, notable progress related to the development of versatile electronic devices is discussed. Finally, future opportunities and the remaining challenges are examined. It is envisioned that more design concepts, working principles, and advanced papermaking techniques will be developed in the near future for the advanced functionalization of paper, paving the way for the mass production and commercial applications of flexible paper-based electronic devices.

Fast naked-eye detection of zinc ions by molecular assembly-assisted polymerization of diacetylene

Portable visual detection systems for environmental monitoring or diagnostic purposes are eagerly anticipated in low-resource settings. Inexpensive device requirements and visualization are key challenges for the development of any portable analysis system. We report herein a new strategy for developing portable rapid ion detection technology by the coupling of topochemical polymerization and supramolecular (SM) self-assembly. The rapid sol-gel or gel-sol phase transition of SM hydrogels has been widely applied for the detection of many important analytes including metal ions. However, one problem that remains is the difficulty inaccurately quantifying the degree of self-assembly with the naked eye. To address this problem, we designed a diacetylene-grafted peptide that can be polymerized following self-assembly into a hydrogel triggered by zinc ions. Before adding zinc ions, the molecules dissolved well in aqueous solution and arranged randomly, and were unable to be polymerized through UV light irradiation. After mixing with zinc ions, the peptide chelated with zinc ions immediately and self-assembled into a SM hydrogel. The molecules arranged orderly and could be easily polymerized through irradiation of a hand-held UV lamp in less than 2 minutes. The hydrogel showed a quick and sharp chromatic change from colorless to dark blue, which allowed the quantification of self-assembly (i.e. concentration of zinc ions) with the naked eye. In addition, the monomers were insensitive to light, pH and temperature changes, which is a highly desired characteristic in practical applications.

High-throughput deposition of chemical reagents via pen-plotting technique for microfluidic paper-based analytical devices

The deposition of chemical reagent inks on paper is a crucial step in the development and fabrication of microfluidic paper-based analytical devices (μPADs). A pen-plotting approach, delivering chemical ink deposition using technical pens filled with reagents and inserted into a desktop electronic plotter, is shown herein to be a versatile, low-cost, simple, rapid, reproducible, and high-throughput solution. The volume of the deposited ink was quantified gravimetrically, confirming that nanoliter volumes of reagents can be deposited reproducibly (e.g. 7.55 ± 0.37 nL/mm for a plotting speed of 10 cm/s) in detection zones of μPADs, typically spatially defined using wax printing. This approach was further investigated with regard to deposition of reagents in different geometrical forms (circular and linear), so demonstrating its applicability for preparation of μPADs with flexible design and application. By adjusting the plotting speed for linear deposition, lines with a relatively large range of widths (≈628-1192 μm) were created. Circular deposition was also demonstrated via delivery of reagents within wax printed circular fluidic barriers of a range of diameters (inner diameter = 1.5-7 mm). These capabilities were practically demonstrated via the fabrication of μPADs, based upon differing detection principles for determination of aluminum in natural waters using Morin as the fluorescent reagent. Traditional μPADs based on digital image colorimetry (DIC) were produced using circular deposition, whilst distance-based μPADs exploited linear deposition. Both types of μPADs developed using this method showed excellent precision for determination of trace concentrations of aluminium (average RSDs = 3.38% and 6.45%, and LODs = 0.5 ng (0.25 ppm) and 2 ng (0.5 ppm), for traditional and distance-based detection, respectively).

"On-Off-On" Photoelectrochemical/Visual Lab-on-Paper Sensing via Signal Amplification of CdS Quantum Dots@Leaf-Shape ZnO and Quenching of Au-Modified Prism-Anchored Octahedral CeO2 Nanoparticles

An effective "on-off-on" photoelectrochemical (PEC)/visual sensing system based on cleaning-switchable lab-on-paper device was designed to achieve ultrasensitive detection of analytes. The first amplified "signal-on" PEC state was gained by CdS quantum dots sensitized leaf-shape ZnO (CdS QDs/leaf-shape ZnO) structure, which was assembled on reduced graphene oxide (rGO) modified paper electrode. Then Au-modified prism-anchored octahedral CeO₂ nanoparticles (Au@PO-CeO₂ NPs), as an efficient signal quencher, were immobilized on the CdS QDs/leaf-shape ZnO with the assistance of DNA hybridization, resulting in a noticeable photocurrent response decrement with the "signal-off" PEC state. With the addition of analytes, the quencher Au@PO-CeO₂ NPs were immediately released from the sensing surface and robust PEC response was recovered to the signal-on state again. Meanwhile, the disengaged quencher in electrolyte solution flowed to the colorimetric detection area of lab-on-paper device and catalyzed oxidation of the chromogenic substrate 3,3',5,5'-tetramethylbenzidine in the presence of H₂O₂ to form the colored product, making the analytes detection more convincing with the visual discrimination. Under optimal conditions, the proposed PEC/visual lab-on-paper device possessed the detection limits toward adenosine and potassium ion as low as 0.15 and 0.06 nM, respectively. With ingenious design of actuating conversion process between hydrophilicity and hydrophobicity by slipping paper tab to solve cleaning issue in the assay procedures, the cleaning-switchable lab-on-paper device was constructed for high-performance biosensing applications. It provides an unambiguous simplicity and portable operation for exploring high reliability and sensitivity of novel point-of-care diagnostic tool with dual-signal readout.

Wettability alteration in a functional capillary tube for visual quantitative point of care testing

Capillarity is an extremely common physical-chemical phenomenon related to wettability in nature, which has wide theoretical and practical interest. Herein, we reported a facile sensing device based on capillary force change in a vertical capillary tube. In this height-based capillary sensor (HCS), the inner surface of the capillary tube was modified with a layer of molecules with wetting responsibility based on the well-known simple surface chemistry. With targets in different concentrations, the wettability of the surface modified with responsive molecules would produce different changes. The responsive surfaces would change the capillary force of the vertical capillary tube, and result in different column heights. Like a thermometer, H⁺ and phenol have been quantified visually based on the height of the liquid inside the capillary tube.

"Dip-and-read" paper-based analytical devices using distance-based detection with color screening

An improved paper-based analytical device (PAD) using color screening to enhance device performance is described. Current detection methods for PADs relying on the distance-based signalling motif can be slow due to the assay time being limited by capillary flow rates that wick fluid through the detection zone. For traditional distance-based detection motifs, analysis can take up to 45 min for a channel length of 5 cm. By using a color screening method, quantification with a distance-based PAD can be achieved in minutes through a "dip-and-read" approach. A colorimetric indicator line deposited onto a paper substrate using inkjet-printing undergoes a concentration-dependent colorimetric response for a given analyte. This color intensity-based response has been converted to a distance-based signal by overlaying a color filter with a continuous color intensity gradient matching the color of the developed indicator line. As a proof-of-concept, Ni quantification in welding fume was performed as a model assay. The results of multiple independent user testing gave mean absolute percentage error and average relative standard deviations of 10.5% and 11.2% respectively, which were an improvement over analysis based on simple visual color comparison with a read guide (12.2%, 14.9%). In addition to the analytical performance comparison, an interference study and a shelf life investigation were performed to further demonstrate practical utility. The developed system demonstrates an alternative detection approach for distance-based PADs enabling fast (∼10 min), quantitative, and straightforward assays.

Highly sensitive microfluidic paper-based photoelectrochemical sensing platform based on reversible photo-oxidation products and morphology-preferable multi-plate ZnO nanoflowers

A microfluidic paper-based analytical device (μPAD) was simply constructed for highly sensitive detection of L-glutamic acid and L-cysteine. The μPAD featured with two functional zones on one strip of paper achieved by preferable multi-plate ZnO nanoflowers (ZnO NFs) and molecularly imprinting polymer (MIP) membranes. The as-designed μPAD was established based on the inherent relation between the photo-oxidation products and photoelectrochemical (PEC) performance with the highly sensitive detection of biomolecules. The ZnO NFs were utilized to produce photo-oxidation products by driving the reaction between ferrocenemethanol and photogenerated holes under ultraviolet light. The photo-oxidation products easily flowed to MIP membranes along the hydrophilic channel via capillary action. MIP membranes as the receptors specifically recognized the analytes as well as decreased the electron loss by blocking the reduction reaction between electrons and photo-oxidation products. The PEC response was obtained in the processes of electrons transfer and exhibited the direct relationships corresponding to the concentrations of target analytes. The μPAD showed the detection limits toward L-glutamic acid and L-cysteine as low as 9.6 pM and 24 pM, respectively. Moreover, it is interesting to point out that ZnO NFs nanostructure shows superior PEC signal compared with those of ZnO nanospheres, nanosheets, and nanorod arrays. In current work, photo-oxidation products are utilized to achieve highly sensitive PEC detection for biomolecules under ultraviolet light as well as avoid the effects of multiple modifications in the same region on the reproducibility, which is beneficial for opening up rich possibility for designing more efficient analytical strategy.

Ultrasensitive Enzyme-free Biosensor by Coupling Cyclodextrin Functionalized Au Nanoparticles and High-Performance Au-Paper Electrode

Microfluidic paper-based analytical device (μPAD), originally developed for improving healthcare in developing countries, presents a simple yet powerful platform for performing low-cost and portable diagnostic devices. Here, we report an enzyme-free μPAD for the detection of two tumor markers. First, a porous structure of gold nanoparticle (AuNP)-modified paper working electrode (Au-PWE), with a feature of all-round conductivity and plenty of active sites favoring biological ligand attachment, was fabricated as a sensor substrate. Next, cyclodextrin functionalized AuNPs (CD@AuNPs) as dual mimicking enzyme were prepared to load secondary antibodies or peptide. On one sample zone, in the presence of carcinoembryonic antigen (CEA), CD@AuNPs could be introduced into the Au-PWE through a sandwich immunoreaction, boosting the electrochemical signal of o-phenylenediamine (o-PD) via the trigger of a cascade catalysis reaction toward glucose and o-PD, eventually resulting in the sensitive detection of CEA. On another working zone, with the introduction of another target prostate-specific antigen (PSA), peptide cleavage took place, which further led to CD@AuNPs being released from Au-PWE, and then, the variation of electrochemical signals was recorded for the detection of PSA. We demonstrated, using the device, that the detection of CEA and PSA clinically had high sensitivity, wide linear ranges, and low detection limits. We believe that our work provides a promising platform for point-of-care testing, especially in resource-limited regions.

Paper-Based DNA Reader for Visualized Quantification of Soil-Transmitted Helminth Infections

Soil-transmitted helminth (STH) infections are a global health issue affecting nearly one-third of the world's population. As most endemic areas of STH are impoverished countries or regions with limited healthcare resources, the accurate diagnosis of STH requires analytical tools that are not only quantitative, but also portable, inexpensive, and with no or minimal demand for external instrument. Herein, we introduce a novel paper-based diagnostic device, termed quantitative paper-based DNA reader (qPDR), capable of quantifying STH at the molecular level by measuring distance as readout, thus eliminating the need for external readers. On the basis of the unique interfacial interaction of a DNA intercalating dye, SYBR Green I, with native cellulose on a chromatographic paper, qPDR allows the distance-based quantification of minute amounts of double-stranded DNA as short as 6 min. By integrating qPDR with polymerase chain reactions that were performed using a smartphone-controlled portable thermal cycler, we were able to quantify minute amount of genetic markers from adult worms of an STH (Trichuris trichiura) that were expelled post-treatment by infected children living in the rural areas of Honduras.