Naked-eye quantitative aptamer-based assay on paper device

A Label-Free Colorimetric AuNP-Aptasensor for the Rapid Detection of Vibrio cholerae O139

Purpose

Waterborne pathogens pose a significant threat to public health, emphasizing the continuous necessity for advancing robust detection techniques, particularly in preventing outbreaks associated with these pathogens. This study focuses on cholera, an infectious disease caused by Vibrio cholerae, serogroups O1 and O139, often transmitted through contaminated water and food, raising significant public health concerns in areas with poor sanitation and limited access to clean water.

Methods

We developed a colorimetric biosensor using aptamer-functionalized gold nanoparticles to identify Vibrio cholerae O139 and address this issue. The detection mechanism relies on the color change of gold nanoparticles (AuNPs) from red to blue-purple induced by NaCl after the pathogen incubation and aptamer-target binding. Initial steps involved synthesizing and characterizing AuNPs, then exploring the impact of aptamer and NaCl concentrations on AuNP agglomeration. Optimization procedures for aptamer concentration and salt addition identified the optimal conditions for detection as 120 pM aptamers and 1 M NaCl.

Results

The aptasensor demonstrated a robust linear relationship, detecting V. cholerae concentrations from 103 to 108 CFU/mL, with a limit of detection (LOD) of 587 CFU/mL. Specificity tests and accurate sample analyses confirmed the efficiency of the AuNPs aptasensor, showcasing its reliability and speed compared to traditional culture examination methods. Moreover, we extended the aptasensor to a paper-based sensing platform with similar detection principles.

Conclusion

The change in color upon target binding was captured with a smartphone and analyzed using image processing software. The paper-based device detected the target in less than 2 min, demonstrating its convenience for on-field applications.

Paper-based multiplex biosensors for inexpensive healthcare diagnostics: a comprehensive review

Multiplex detection is a smart and an emerging approach in point-of-care testing as it reduces analysis time and testing cost by detecting multiple analytes or biomarkers simultaneously which are crucial for disease detection at an early stage. Application of inexpensive substrate such as paper has immense potential and matter of research interest in the area of point of care testing for multiplexed analysis as it possesses several unique advantages. This study presents the use of paper, strategies adopted to refine the design created on paper and lateral flow strips to enhance the signal, increase the sensitivity and specificity of multiplexed biosensors. An overview of different multiplexed detection studies performed using biological samples has also been reviewed along with the challenges and advantages offered by multiplexed analysis.

One-step enzyme-free detection of the miRNA let-7a via twin-stage signal amplification

MicroRNAs (miRNAs) play a significant role in diverse biological processes. The abnormal expression of miRNAs is related to the development of cancers and various diseases. It is of great importance to sensitively and accurately detect miRNAs for early disease diagnosis and treatment. Here, a new fluorescence strategy was initially proposed for the enzyme-free sensing of let-7a by combining the strand displacement reaction (SDR) with the hybridization chain reaction (HCR). The sensor was successfully applied to the detection of the let-7a gene with a wide linear range from 25 pM to 250 nM and a limit of detection (LOD) of 9.01 pM. The fluorescence intensity has a good linear relationship with the logarithm of the target concentration. In addition, the biosensor allowed for the highly sensitive detection of the target genes even in complex human serum samples. With simple operation yet improved detection capability for let-7a, the developed fluorescent biosensor thus shows great potential for early clinical diagnosis as well as biological research.

A fluorimetric testing strip for the visual evaluation of mercury in blood using copper nanoclusters with DMSO-enhanced fluorescence and stability

A fluorimetric analytical method using test strips has been fabricated for detecting Hg²⁺ ions in blood by using copper nanoclusters (Cu NCs) prepared via a biomineralization route. Unexpectedly, the as-prepared Cu NCs displayed greatly amplified red fluorescence once dispersed in DMSO, the intensity of which decreased specifically in the presence of Hg²⁺. Moreover, the resultant Cu NCs were deposited onto test strips to be further fast dried on superhydrophobic substrates in vacuum. The test strip-based fluorimetry can allow for the direct analysis of Hg²⁺ in blood in the linear concentration range of 0.10-1000 nM. Importantly, this solvent-enhanced fluorescence protocol for different metal probes such as Cu NCs promises extensive analysis applications for designing numerous fluorimetric platforms such as test strips.

Enhanced functional DNA biosensor for distance-based read-by-eye quantification of various analytes based on starch-hydrolysis-adjusted wettability change in paper devices

Low-cost, equipment-free and quantitative detection of a wide range of analytes of interest at home and in the field holds the potential to revolutionize disease diagnosis, environmental pollution monitoring, and food safety analysis. Herein, we describe a functional DNA biosensor for the first time that integrates analyte-directed assembly of enzyme-coated microbead probes for robust yet efficient signal amplification with a simple quantitative detection motif of distance measurement on portable paper devices based on starch-hydrolysis-adjusted wettability change of paper. Its utility is well demonstrated with highly sensitive and specific detection of model analytes ranging from adenosine (an important small biomolecule; 1.6 μM detection limit) to interferon-γ (a protein marker; 0.3 nM detection limit) and Pb2+ (a highly toxic metal ion; 0.5 nM detection limit) by simply using an inexpensive, ubiquitous ruler. The developed general method with the distance-measuring readout should be easily tailored for the portable, read-by-eye, quantitative detection of many other types of analytical targets by taking advantage of their specific functional DNA partners like aptamers and DNAzymes.

Use of an ether-derived 3-hydroxy-4-pyridinone chelator as a new chromogenic reagent in the development of a microfluidic paper-based analytical device for Fe(III) determination in natural waters

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.

Ready-to-use, functionalized paper test strip used with a smartphone for the simultaneous on-site detection of free chlorine, hydrogen sulfide and formaldehyde in wastewater

The simultaneous detection of free chlorine, hydrogen sulfide and formaldehyde in wastewater samples was performed. In this report, we designed and fabricated functionalized paper test strips featuring detection zones that use 3-aminopropyltriethoxysilane (APTES) for the immobilization of chromogenic substrates to detect free chlorine, hydrogen sulfide and formaldehyde. After multiple chromogenic reactions, red, blue and purple colors were obtained on the detection zones and analyzed using a smartphone. Under optimum conditions, the paper test strips showed 1.7, 1.8 and 1.7 orders of magnitude for free chlorine, hydrogen sulfide and formaldehyde, respectively. This sensitivity is caused by the formation of homogeneous complexes on detection zones resulting from the chromogenic reagents immobilized on the detection zone via APTES. Through this strategy, free chlorine, hydrogen sulfide and formaldehyde analysis was achieved within 5 min with detection limits of 0.08, 0.14 and 0.13 mg L^-1, respectively. The developed paper test strip was able to selective detection of free chlorine, hydrogen sulfide and formaldehyde even in the presence of common interfering agents therefore, the test strip was highly selective. In a further demonstration, the developed functionalized paper test strip was successfully used for simultaneous detection of free chlorine, hydrogen sulfide and formaldehyde in wastewater in the field and exhibited with high precision and accuracy in detecting free chlorine, hydrogen sulfide and formaldehyde in wastewater samples. Compared to other methods, this assay was advantageous in terms of its low detection limit, time savings, good stability and highly portable format, which facilitates rapid on-site environmental monitoring with a smartphone.

An ultrasound activated oxygen generation nanosystem specifically alleviates myocardial hypoxemia and promotes cell survival following acute myocardial infarction

Hypoxemia after acute myocardial infarction (AMI) causes severe damage to cardiac cells and induces cardiac dysfunction. Protection of cardiac cells and reconstruction of cardiac functions by re-introducing oxygen into the infarcted myocardium represents an efficient approach for the treatment of AMI. However, the established methods for oxygen supplementation mainly focus on systemic oxygen delivery, which always results in inevitable oxidative stress on normal tissues. In this work, an ultrasound (US) activated oxygen generation nanosystem has been developed, which specifically releases oxygen in the infarcted myocardium and alleviates the hypoxemic myocardial microenvironment to protect cardiac cells after AMI. The nanosystem was constructed through the formation of calcium peroxide in the mesopores of biocompatible mesoporous silica nanoplatforms, followed by the assembly of the thermosensitive material heneicosane and polyethyleneglycol. The mild hyperthermia induced by US irradiation triggered the phase change of heneicosane, thus achieving US responsive diffusion of water and release of oxygen. The US-activated oxygen release significantly alleviated the hypoxia and facilitated the mitigation of oxidative stress after AMI. Consequently, the survival of cardiac cells under hypoxic conditions was substantially improved and the damage in the infarcted myocardial tissue was minimized. This US-activated oxygen generation nanosystem may provide an efficient modality for the treatment of AMI.

Lab-on-a-Chip Systems for Aptamer-Based Biosensing

Aptamers are oligonucleotides or peptides that are selected from a pool of random sequences that exhibit high affinity toward a specific biomolecular species of interest. Therefore, they are ideal for use as recognition elements and ligands for binding to the target. In recent years, aptamers have gained a great deal of attention in the field of biosensing as the next-generation target receptors that could potentially replace the functions of antibodies. Consequently, it is increasingly becoming popular to integrate aptamers into a variety of sensing platforms to enhance specificity and selectivity in analyte detection. Simultaneously, as the fields of lab-on-a-chip (LOC) technology, point-of-care (POC) diagnostics, and personal medicine become topics of great interest, integration of such aptamer-based sensors with LOC devices are showing promising results as evidenced by the recent growth of literature in this area. The focus of this review article is to highlight the recent progress in aptamer-based biosensor development with emphasis on the integration between aptamers and the various forms of LOC devices including microfluidic chips and paper-based microfluidics. As aptamers are extremely versatile in terms of their utilization in different detection principles, a broad range of techniques are covered including electrochemical, optical, colorimetric, and gravimetric sensing as well as surface acoustics waves and transistor-based detection.

Advances in functional nucleic acid based paper sensors

This article provides an extensive review of paper-based sensors that utilize functional nucleic acids, particularly DNA aptamers and DNAzymes, as recognition elements.

Fluorescence detection of 2,4-dichlorophenoxyacetic acid by ratiometric fluorescence imaging on paper-based microfluidic chips

Novel microfluidic ratiometric fluorescent paper chips for rapid and visual detection of 2,4-D through a fluorescence resonance energy transfer sensing mechanism.

Paper-based point-of-care test with xeno nucleic acid probes

Bridging the unmet need of efficient point-of-care testing (POCT) in biomedical engineering research and practice with the emerging development in artificial synthetic xeno nucleic acids (XNAs), this review summarized the recent development in paper-based POCT using XNAs as sensing probes. Alongside the signal transducing mode and immobilization methods of XNA probes, a detailed evaluation of probe performance was disclosed. With these new aspects, both researchers in synthetic chemistry / biomedical engineering and physicians in clinical practice could gain new insights in designing, manufacturing and choosing suitable reagents and techniques for POCT.

Enhanced 3D paper-based devices with a personal glucose meter for highly sensitive and portable biosensing of silver ion

A variety of routine methods are available for the detection of silver (I) (Ag⁺) ions, but most of them rely on expensive, sophisticated and desktop instruments. Herein, a low-cost, instrument-free and portable Ag⁺ biosensor was described by initially designing a new class of 3D origami microfluidic paper-based analytical devices (μPADs) into each of which one piece of reagent-loaded nanoporous membrane was integrated. It combines analyte-triggered self-growing of silver nanoparticles to block the membrane's pores in situ for rapid yet efficient signal amplification with a handheld personal glucose meter for a portable and sensitive quantitative readout based on the biocatalytic reactions between the glucose oxidase and glucose. Its utility is well demonstrated with the specific detection of the analyte with a limit of detection as low as ∼58.1 pM (3σ), which makes this new biosensing method one of the most sensitive Ag⁺ assays in comparison with many other typical methods recently reported. Moreover, the satisfactory recovery of analyzing several types of real water examples, i.e., tap water, drinking water, pond water and soil water, additionally validates its feasibility for practical applications.

Microfluidic Technology for Nucleic Acid Aptamer Evolution and Application

The intersection of microfluidics and aptamer technologies holds particular promise for rapid progress in a plethora of applications across biomedical science and other areas. Here, the influence of microfluidics on the field of aptamers, from traditional capillary electrophoresis approaches through innovative modern-day approaches using micromagnetic beads and emulsion droplets, is reviewed. Miniaturizing aptamer-based bioassays through microfluidics has the potential to transform diagnostics and embedded biosensing in the coming years.

Target-triggered three-way junction in conjugation with catalytic concatemers-functionalized nanocomposites provides a highly sensitive colorimetric method for miR-21 detection

With the great advances in DNA nanotechnology, scientists have shown interest in developing dynamic nanostructures for theranostic applications, analyte sensing and cargo delivery. Here, we present a specific enzyme-free ultrasensitive platform based on a multilayer coupled signal amplification strategy to quantify miR-21 molecule. The biosensor was integrated based on three signal amplification gadgets, namely a translator-mediated catalytic hairpin assembly (CHA), a multilayer DNA concatemer on the surface of gold decorated magnetic nanoparticle (GMNP), and a DNAzyme-mediated catalytic signal amplification. MiR-21 mediates the release of a DNA translator from an immobilized duplex to engage in a CHA reaction using three hairpins, including a GMNP-conjugated hairpin 1 (H1), biotin-labeled hairpin 2 (H2) and a GMNP-conjugated hairpin 3 (H3) to form a three-way junction (3WJ). Meanwhile, a plenty of initiator strand 0 (S0) on GMNPs - each of which has been bifunctionalized with S0/H1 or S0/H3 - drive several multilayer peroxidase-mimicking DNAzyme concatemers in the presence of two accessory oligonucleotides; strand 1 (S1) and strand 2 (S2). Since a G-rich sequence was attached at the 5'-end of S1 strand, in the presence of hemin cofactor, an active G-quadruplex DNAzyme with peroxidase activity was formed. The concatemers on the surface of GMNPs can convert a colorless substrate to a green product. The biosensor can detect as low as 1 aM of miR-21 and provide an excellent capability to discriminate single-base mismatches. The required time for the formulation of the assay reagents is about three days and the reaction time for the detection of miR-21 takes place in less than four hours.

"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.

Recent Microdevice-Based Aptamer Sensors

Since the systematic evolution of ligands by exponential enrichment (SELEX) method was developed, aptamers have made significant contributions as bio-recognition sensors. Microdevice systems allow for low reagent consumption, high-throughput of samples, and disposability. Due to these advantages, there has been an increasing demand to develop microfluidic-based aptasensors for analytical technique applications. This review introduces the principal concepts of aptasensors and then presents some advanced applications of microdevice-based aptasensors on several platforms. Highly sensitive detection techniques, such as electrochemical and optical detection, have been integrated into lab-on-a-chip devices and researchers have moved towards the goal of establishing point-of-care diagnoses for target analyses.

Naked-Eye Detection of Glucose in Saliva with Bienzymatic Paper-Based Sensor

The high incidence of Diabetes Mellitus in low-income regions has promoted the development of low-cost alternatives to replace blood-based procedures. In this work, we present a bienzymatic paper-based sensor suitable for the naked-eye detection of glucose in saliva samples. The sensor was obtained by a stamping procedure and modified with chitosan to improve the colorimetric readout. The bienzymatic reaction of GOx-HRP coupled with 2,4,6-tribromo-3-hydroxy benzoic acid was applied for the detection of glucose within a range from 0 to 180 mgdL⁻¹ in buffer and artificial saliva solutions. The visual readout was perceived by the naked eye and registered with an office scanner to evaluate the analytical performance. The results showed a limit of detection of 0.37 mgdL⁻¹ (S/N = 3) with an R.S.D. of 1.69% and a linear range from 1 to 22.5 mgdL⁻¹ with an R² of 0.99235. The analysis of human saliva samples was performed without pre-processing, achieving recoveries from 92 to 114%. The naked-eye detection was evaluated under two different light settings, showing average recoveries of 108.58 and 90.65% for standard and low illumination. The proposed device showed potential for easy-to-use, sensitive, low-cost, fast, and device-free detection of salivary glucose suitable for untrained personnel operation and limited facilities.

A Paper Sensor Printed with Multifunctional Bio/Nano Materials

We report a paper-based aptasensor platform that uses two reaction zones and a connecting bridge along with printed multifunctional bio/nano materials to achieve molecular recognition and signal amplification. Upon addition of analyte to the first zone, a fluorescently labelled DNA or RNA aptamer is desorbed from printed graphene oxide, rapidly producing an initial fluorescence signal. The released aptamer then flows to the second zone where it reacts with printed reagents to initiate rolling circle amplification, generating DNA amplicons containing a peroxidase-mimicking DNAzyme, which produces a colorimetric readout that can be read in an equipment-free manner or with a smartphone. The sensor was demonstrated using an RNA aptamer for adenosine triphosphate (a bacterial marker) and a DNA aptamer for glutamate dehydrogenase (Clostridium difficile marker) with excellent sensitivity and specificity. These targets could be detected in spiked serum or feacal samples, demonstrating the potential for testing clinical samples.

Mechano-Based Transductive Sensing for Wearable Healthcare

Wearable healthcare presents exciting opportunities for continuous, real-time, and noninvasive monitoring of health status. Even though electrochemical and optical sensing have already made great advances, there is still an urgent demand for alternative signal transformation in terms of miniaturization, wearability, conformability, and stretchability. Mechano-based transductive sensing, referred to the efficient transformation of biosignals into measureable mechanical signals, is claimed to exhibit the aforementioned desirable properties, and ultrasensitivity. In this Concept, a focus on pressure, strain, deflection, and swelling transductive principles based on micro-/nanostructures for wearable healthcare is presented. Special attention is paid to biophysical sensors based on pressure/strain, and biochemical sensors based on microfluidic pressure, microcantilever, and photonic crystals. There are still many challenges to be confronted in terms of sample collection, miniaturization, and wireless data readout. With continuing efforts toward solving those problems, it is anticipated that mechano-based transduction will provide an accessible route for multimode wearable healthcare systems integrated with physical, electrophysiological, and biochemical sensors.

Equipment-Free Quantitative Aptamer-Based Colorimetric Assay Based on Target-Mediated Viscosity Change

In this paper, we describe an aptamer-based colorimetric assay (ABCA), which integrates enzyme-loaded microparticles for signal amplification with distance measurement for equipment-free quantitative readout. The distance measurement readout is on the basis of target-induced selective reduction in viscosity of reaction solution. Its utility is well demonstrated with inexpensive, sensitive, and selective detection of adenosine (model analyte) in buffer samples and real samples of human serum and urine with the naked eye. This ABCA method just requires operators to simply count the number of colored distance-relevant marked bars on the calibrated glass microsyringes (testing containers) to provide quantitative results. It thus holds great promise for wide applications particularly in limited-resource settings.

Highly Sensitive Paper-based Analytical Devices with the Introduction of a Large-Volume Sample via Continuous Flow

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

Selection of an aptamer against Muscovy duck parvovirus for highly sensitive rapid visual detection by label-free aptasensor

Muscovy duck parvovirus (MDPV) causes high mortality and morbidity in ducks. This study investigated a novel aptamer-based, label-free aptasensor detection of MDPV. In this study, we developed an ssDNA aptamer using the filtration partition and lambda exonuclease method with an affinity-based monitor and counter-screening process. After 15 rounds of SELEX (systematic evolution of ligands by exponential enrichment), the ssDNA aptamer Apt-10, which specifically bound to MDPV with high affinity (K_d = 467nM) was successfully screened, and the aptamer was also found to be good specific to MDPV. The selected Apt-10 aptamer can be used to distinguish MDPV and goose parvovirus (GPV). Three-dimensional structural analysis of the Apt-10 aptamer indicated that it folded into a compact stem-loop motif, which was related to its high affinity. Finally, a label-free detection method based on unmodified gold nanoparticles and Apt-10 aptamer was developed for MDPV determination. The concentration of Apt-10 aptamer at 5μM was optimal for MDPV determination in the label-free aptasensor. Excellent linearity was acquired and the lowest detection limit was 1.5 or 3 EID₅₀ (50% egg infection dose) of MDPV, respectively, depending upon spectrophotometry or the naked eye were used. These results show the potential of the aptamer for the rapid detection of MDPV and antiviral research.

Integrated ion imprinted polymers-paper composites for selective and sensitive detection of Cd(II) ions

Paper-based sensor is a new alternative technology to develop a portable, low-cost, and rapid analysis system in environmental chemistry. In this study, ion imprinted polymers (IIPs) using cadmium ions as the template were directly grafted on the surface of low-cost print paper based on the reversible addition-fragmentation chain transfer polymerization. It can be applied as a recognition element to selectively capture the target ions in the complex samples. The maximum adsorption capacity of IIPs composites was 155.2mgg^-1 and the imprinted factor was more than 3.0. Then, IIPs-paper platform could be also applied as a detection element for highly selective and sensitive detection of Cd(II) ions without complex sample pretreatment and expensive instrument, due to the selective recognition, formation of dithizone-cadmium complexes and light transmission ability. Under the optimized condition, the linear range was changed from 1 to 100ngmL^-1 and the limit of detection was 0.4ngmL^-1. The results were in good agreement with the classic ICP-MS method. Furthermore, the proposed method can also be developed for detection of other heavy metals by designing of new IIPs.

Controlling Capillary-Driven Fluid Transport in Paper-Based Microfluidic Devices Using a Movable Valve

This paper describes a novel strategy for fabricating the movable valve on paper-based microfluidic devices to manipulate capillary-driven fluids. The movable valve fabrication is first realized using hollow rivets as the holding center to control the paper channel in different layer movement that results in the channel's connection or disconnection. The relatively simple valve fabrication procedure is robust, versatile, and compatible with microfluidic paper-based analytical devices (μPADs) with differing levels of complexity. It is remarkable that the movable valve can be convenient and free to control fluid without the timing setting, advantages that make it user-friendly for untrained users to carry out the complex multistep operations. For the performance of the movable valve to be verified, several different designs of μPADs were tested and obtained with satisfactory results. In addition, in the proof-of-concept enzyme-linked immunosorbent assay experiments, we demonstrate the use of these valves in μPADs for the successful analysis of samples of carcino-embryonic antigen, showing good sensitivity and reproducibility. We hope this technique will open new avenues for the fabrication of paper-based valves in an easily adoptable and widely available way on μPADs and provide potential point-of-care applications in the future.

Using the Rubik's Cube to directly produce paper analytical devices for quantitative point-of-care aptamer-based assays

In this article, we describe a facile method named as Rubik's Cube stamping (RCS) for equipment-free fabrication of microfluidic paper-based analytical devices (μPADs). RCS is inspired by the worldwide ubiquitous RC toy and requires no specialized electric equipment other than a classical six-faced RC that is assembled with home-made small iron components. It can pattern various rosin microstructures in paper simply by either using different functional faces of the modified RC or applying its internal pivot mechanism to adjust the components' patterning forms on one functional face. Such a versatile stamping method is quite simple and inexpensive, and thus holds potential for producing rosin-patterned μPADs by untrained users in resource-limited environments such as small laboratories and private clinics, or even at home and in the field. Moreover, a set of one-channel devices are fabricated to design a point-of-care aptamer-based assay with near sample-in-answer-out capability that integrates enzymatic reactions for robust yet efficient signal amplification and a personal glucometer for portable, user-friendly, rapid and quantitative readout. Its utility is well demonstrated with the sensitive and specific detection of adenosine as a model target in buffer samples and undiluted human urine within several minutes. With the advantages of low cost, simplicity, portability, rapidity, and aptamer variety, this general point-of-care assay system reported here may find broad applications including home healthcare, field-based environmental monitoring or food analysis and emergency situations.

Paper based diagnostics for personalized health care: Emerging technologies and commercial aspects

Personalized health care (PHC) is being appreciated globally to combat clinical complexities underlying various metabolic or infectious disorders including diabetes, cardiovascular, communicable diseases etc. Effective diagnoses majorly depend on initial identification of the causes which are nowadays being practiced in disease-oriented approach, where personal health profile is often overlooked. The adoption of PHC has shown significantly improved diagnoses in various conditions including emergency, ambulatory, and remote area. PHC includes personalized health monitoring (PHM), which is its integral part and may provide valuable information's on various clinical conditions. In PHC, bio-fluids are analyzed using various diagnostic devices including lab based equipment and biosensors. Among all types of biosensing systems, paper based biosensors are commercially attracted due to its portability, easy availability, cheaper manufacturing cost, and transportability. Not only these, various intrinsic properties of paper has facilitated the development of paper based miniaturized sensors, which has recently gained ASSURED (Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment free, Deliverable to all end-users) status for point of care diagnosis in miniaturized settings. In this review, importance of paper based biosensors and their compatibility for affordable and low cost diagnostics has been elaborated with various examples. Limitations and strategies to overcome the challenges of paper biosensor have also been discussed. We have provided elaborated tables which describe the types, model specifications, sensing mechanisms, target biomarkers, and analytical performance of the paper biosensors with their respective applications in real sample matrices. Different commercial aspects of paper biosensor have also been explained using SWOT (Strength, Weakness, Opportunities, Threats) analysis.

Toward practical application of paper-based microfluidics for medical diagnostics: state-of-the-art and challenges

Microfluidic paper-based analytical devices (μPADs) have emerged as a promising diagnostic platform a decade ago. In contrast to highly active academic developments, their entry into real-life applications is still very limited. This discrepancy is attributed to the gap between research developments and their practical utility, particularly in the aspects of operational simplicity, long-term stability of devices, and associated equipment. On the basis of these backgrounds, this review attempts to: 1) identify the reasons for success of paper-based devices already in the market, 2) describe the current status and remaining issues of μPADs in terms of operational complexity, signal interpretation approaches, and storage stability, and 3) discuss the possibility of mass production based on established manufacturing technologies. Finally, the state-of-the-art in commercialisation of μPADs is discussed, and the "upgrades" required from a laboratory-based prototype to an end user device are demonstrated on a specific example.

Colorimetric biosensor for the assay of paraoxon in environmental water samples based on the iodine-starch color reaction

In this work, a new colorimetric biosensor for the assay of paraoxon was developed via the conventional iodine-starch color reaction and multi-enzyme cascade catalytic reactions. In the presence of acetylcholine chloride, acetylcholinesterase (AChE) and choline oxidase (ChO) catalyzed the formation of H₂O₂, which then activated horseradish peroxidase (HRP) to catalyze the oxidation of KI to produce an iodine-starch color reaction. Upon exposure to paraoxon, the catalytic activity of AChE was inhibited and less H₂O₂ generated, resulting in a decrease in the production of I₂ and a drop in the intensity of solution color. This colorimetric biosensor showed high sensitivity for the assay of paraoxon with a limit of detection 4.7 ppb and was applied for the assay of paraoxon in spiked real samples. By employing the conventional iodine-starch color reaction, this biosensor has the potential of on-site assay of OPs residues in environmental samples.

In Situ Ion-Transmission Mass Spectrometry for Paper-Based Analytical Devices

Current detection methods for paper-based analytical devices (PADs) rely on spectroscopic and electrochemical properties, which place special requirements on the analyte or need analyte labeling. Here, ion-transmission mass spectrometry (MS) was proposed for coupling with PADs to enable rapid in situ MS analysis of the sample on paper. The sample was analyzed directly on paper via analyte ionization by ions transmitted through the paper, generated by a low-temperature plasma probe. Prior to MS analysis, the sample can be separated by paper electrophoresis or by paper chromatography, among a variety of other features offered by PADs. The versatility of this technique was demonstrated by MS analysis of a paper microarray, a mixture of amino acids, and whole blood doped with drugs on PADs.

A triple-amplification SPR electrochemiluminescence assay for chloramphenicol based on polymer enzyme-linked nanotracers and exonuclease-assisted target recycling

The present study aimed to explore a novel triple-amplification electrochemiluminescence (ECL) assay for detecting of chloramphenicol (CAP). This strategy was based on single-stranded DNA-binding protein (SSB) and horseradish peroxidase (HRP) enzyme-linked polymer (EnVision reagent, EV) labeled on Au nanoparticles (EV-Au-SSB) as nanotracer and exonuclease-assisted target recycling. The composite probes were prepared via immunoreactions between the CdS nanocrystal (CdS NC)-functionalized partial complementary DNA and aptamer (CdSNCs/Apt-ssDNA1) as capture probes, and EV-Au-SSB as nanotracer. When the composite probe solution co-existed with CAP and Exo I, the aptamer on the capture probes preferentially combined with CAP, and then CAP-Apt and nanotracer complex were released into the solution. Subsequently, Exo I in the solution could further digest the CAP-Apt from the 3'-end of the aptamer and release CAP, which could participate in further reaction with the probes. It was worth mentioning that EV contained a large number of HRPs on its dendritic chain. In the EV-Au-SSB, Au could enhance ECL intensity of CdS NCs by surface plasmon resonance. What's more, HRPs on EV could catalyze the reaction of H2O2, which could obviously enhance ECL intensity of CdS NCs. This study demonstrated excellent performance of the triple-amplification ECL assay, which makes this aptasensor system suitable and promising for the practical application of CAP residues in fish samples. Moreover, the assay might provide a promising avenue to develop efficient aptasensors to determine small-molecule harmful substances in environmental monitoring and food safety.

Nanobiosensors in diagnostics

Medical diagnosis has been greatly improved thanks to the development of new techniques capable of performing very sensitive detection and quantifying certain parameters. These parameters can be correlated with the presence of specific molecules and their quantity. Unfortunately, these techniques are demanding, expensive, and often complicated. On the other side, progress in other fields of science and technology has contributed to the rapid growth of nanotechnology. Although being an emerging discipline, nanotechnology has raised huge interest and expectations. Most of the enthusiasm comes from new possibilities and properties of nanomaterials. Biosensors (simple, robust, sensitive, cost-effective) combined with nanomaterials, also called nanobiosensors, are serving as bridge between advanced detection/diagnostics and daily/routine tests. Here we review some of the latest applications of nanobiosensors in diagnostics field.