Rotational Paper-Based Microfluidic-Chip Device for Multiplexed and Simultaneous Fluorescence Detection of Phenolic Pollutants Based on a Molecular-Imprinting Technique

Microfluidic paper analytic device (μPAD) technology for food safety applications

Foodborne pathogens, food adulterants, allergens, and toxic chemicals in food can cause major health hazards to humans and animals. Stringent quality control measures at all stages of food processing are required to ensure food safety. There is, therefore, a global need for affordable, reliable, and rapid tests that can be conducted at different process steps and processing sites, spanning the range from the sourcing of food to the end-product acquired by the consumer. Current laboratory-based food quality control tests are well established, but many are not suitable for rapid on-site investigations and are costly. Microfluidic paper analytical devices (μPADs) are a fast-growing field in medical diagnostics that can fill these gaps. In this review, we describe the latest developments in the applications of microfluidic paper analytic device (μPAD) technology in the food safety sector. State-of-the-art μPAD designs and fabrication methods, microfluidic assay principles, and various types of μPAD devices with food-specific applications are discussed. We have identified the prominent research and development trends and future directions for maximizing the value of microfluidic technology in the food sector and have highlighted key areas for improvement. We conclude that the μPAD technology is promising in food safety applications by using novel materials and improved methods to enhance the sensitivity and specificity of the assays, with low cost.

Distance-based paper analytical device for multiplexed quantification of cytokine biomarkers using carbon dots integrated with molecularly imprinted polymer

This article introduces distance-based paper analytical devices (dPADs) integrated with molecularly imprinted polymers (MIPs) and carbon dots (CDs) for simultaneous quantification of cytokine biomarkers, namely C-reactive protein (CRP), tumor necrosis factor-alpha (TNF-α), and interleukin-6 (IL-6) in human biological samples for diagnosis of cytokine syndrome. Using fluorescent CDs and MIP technology, the dPAD exhibits high selectivity and sensitivity. Detection is based on fluorescence quenching of CDs achieved through the interaction of the target analytes with the MIP layer on the paper substrate. Quantitative analysis is easily accomplished by measuring the distance length of quenched fluorescence with a traditional ruler and naked eye readout enabling rapid diagnosis of cytokine syndrome and the underlying infection. Our sensor demonstrated linear ranges of 2.50-24.0 pg mL-1 (R2 = 0.9974), 0.25-3.20 pg mL-1 (R2 = 0.9985), and 1.50-16.0 pg mL-1 (R2 = 0.9966) with detection limits (LODs) of 2.50, 0.25, and 1.50 pg mL-1 for CRP, TNF-α, and IL-6, respectively. This sensor also demonstrated remarkable selectivity compared to a sensor employing a non-imprinted polymer (NIP), and precision with the highest relative standard deviation (RSD) of 5.14%. The sensor is more accessible compared to prior methods relying on expensive reagents and instruments and complex fabrication methods. Furthermore, the assay provided notable accuracy for monitoring these biomarkers in various human samples with recovery percentages ranging between 99.22% and 103.58%. By integrating microfluidic systems, nanosensing, and MIPs technology, our developed dPADs hold significant potential as a cost-effective and user-friendly analytical method for point-of-care diagnostics (POC) of cytokine-related disorders. This concept can be further extended to developing diagnostic devices for other biomarkers.

Distance-based paper analytical devices integrated with molecular imprinted polymers for Escherichia coli quantification

The development of distance-based paper analytical devices (dPADs) integrated with molecularly imprinted polymers (MIPs) to monitor Escherichia coli (E. coli) levels in food samples is presented. The fluidic workflow on the device is controlled using a designed hydrophilic bridge valve. Dopamine serves as a monomer for the formation of the E. coli-selective MIP layer on the dPADs. The detection principle relies on the inhibition of the E. coli toward copper (II) (Cu2+)-triggered oxidation of o-phenylenediamine (OPD) on the paper substrate. Quantitative detection is simply determined through visual observation of the residual yellow color of the OPD in the detection zone, which is proportional to E. coli concentration. The sensing exhibits a linear range from 25.0 to 1200.0 CFU mL-1 (R2 = 0.9992) and a detection limit (LOD) of 25.0 CFU mL-1 for E. coli detection. Additionally, the technique is highly selective with no interference even from the molecules that have shown to react with OPD to form oxidized OPD. The developed device demonstrates accuracy and precision for E. coli quantification in food samples with recovery percentages between 98.3 and 104.7% and the highest relative standard deviation (RSD) of 4.55%. T-test validation shows no significant difference in E. coli concentration measured between our method and a commercial assay. The proposed dPAD sensor has the potential for selective and affordable E. coli determination  in food samples without requiring sample preparation. Furthermore, this strategy can be extended to monitor other molecules for which MIP can be developed and integrated into paper-microfluidic platform.

Sandwich-type aptamer-based biosensors for thrombin detection

Thrombin, a proteolytic enzyme, plays an essential role in catalyzing many blood clotting reactions. Thrombin can act as a marker for some blood-related diseases, such as leukemia, thrombosis, Alzheimer's disease and liver disease. Therefore, its diagnosis is of great importance in the fields of biological and medical research. Biosensors containing sandwich-type structures have attracted much consideration owing to their superior features such as reproducible and stable responses with easy improvement in the sensitivity of detection. Sandwich-type platforms can be designed using a pair of receptors that are able to bind to diverse locations of the same target. Herein, we investigate recent advances in the progress and applications of thrombin aptasensors containing a sandwich-type structure, in which two thrombin-binding aptamers (TBAs) identify different parts of the thrombin molecule, leading to the formation of a sandwich structure and ultimately signal detection. We also discuss the pros and cons of these approaches and outline the most logical approach in each section.

A solid-phase fluorescence sensor for measuring chemical species in water

In this study, the first of its kind, a solid-phase fluorescence sensing platform was developed to quantify contaminants in water. ZnO quantum dots (QDs) were combined with molecularly imprinted polymers (MIPs) to form fluorescence sensing materials. Solid sensing layers were formed via a straightforward spin-coating method, which demonstrated a strong attachment to the sensor substrate while maintaining the integrity of the sensing materials. The developed sensing platform comprised a portable fluorescence detector to measure fluorescence intensity, instead of traditional fluorescence spectroscopy. The solid sensing platform was first tested with 2,4-dichlorophenoxyacetic acid (2,4-D), demonstrating high sensitivity (0.0233) and a very strong correlation (0.98) between the target molecule concentration and sensor signal. Further, the sensing platform was successfully adapted to measure a substance with a different molecular mass and chemical structure, the algae toxin microcystin-LR (MCLR); this demonstrated the sensor's versatility in quantifying target molecules. Tap water samples spiked with MCLR were also used to test the sensor's practical application. Finally, the working mechanism of the sensing platform was established, and the key information for using the sensor to measure various contaminants was determined. With its high performance, broad applicability, and ease of use, the developed platform provides a suitable basis for lab-on-chip image-based sensing devices for environmental monitoring.

A dual-mode assay kit using a portable potentiostat connected to a smartphone via Bluetooth communication and a potential-power angle-based paper device susceptible for low-cost point-of-care testing of iodide and dopamine

BACKGROUND

Considering that the brain controls most of the body's activities, it is very important to measure the factors affecting its function, such as dopamine and iodide. Due to the growing population in the world, it is necessary to provide fast, cheap and accurate methods with the capability of on-site analysis and without the need for invasive sampling and operator skill. As a result, there is a strong desire to replace laboratory instruments with small sensors for point-of-care testing. Paper-based analytical devices (PADs) are one of the popular zero-cost approaches to achieve this goal.

RESULTS

We developed a simple and disposable diagnostic paper system based on electroanalytical and potential-power angle-based methods. First, we prepared an angle-based analytical system capable of performing semi-quantitative iodide analysis simply by reading the colored angle traveled. This system design is based on a channel containing complex reagents and two pencil-drawn electrodes to apply a constant voltage accelerating the anions migration. Meanwhile, a three-electrode system based on conductive pencil graphite is developed to measure dopamine concentration based on linear sweep voltammetry. For the quantitative analysis, the voltammetric data was wirelessly transmitted to a mobile device via Bluetooth communication. In this context, a power supply providing the required voltage for the migration of iodide ions, a portable potentiostat system, and a mobile application for measuring dopamine were developed. The calibration curves for I- and dopamine range from 3.5 × 10-4-47.0 × 10-4 and 10.0 × 10-6-1000.0 × 10-6 mol L-1 with LODs of 2.3 × 10-4 and 5.0 × 10-6 mol L-1, respectively.

SIGNIFICANCE AND NOVELTY

A new portable dual-mode voltage-assisted integrated PAD platform was designed for iodide and dopamine analysis. The characteristics of this device allow non-experts to carry out in-field analysis using sub-100 μL saliva sample with a time-to-result of <10 min along with reducing the overall cost and operational complexity.

Post-imprinting modification of molecularly imprinted polymer for proteins detection: A review

Inspired by protein post-translational modification (PTM), post-imprinting modification (PIM) has been proposed and developed to prepare novel molecularly imprinted polymers (MIPs), which are similar to functionalized biosynthetic proteins. The PIM involves site-directed modifications in the imprinted cavity of the MIP, such as introducing high-affinity binding sites and introducing fluorescent signal molecules. This modification makes the MIP further functionalized and improves the shortcomings of general molecular imprinting, such as single function, low selectivity, low sensitivity, and inability to fully restore the complex function of natural antibodies. This paper describes the characteristics of PIM strategies, reviews the latest research progress in the recognition and detection of protein biomarkers such as lysozyme, prostate-specific antigen, alpha-fetoprotein, human serum albumin, and peptides, and further discusses the importance, main challenges, and development prospects of PIM. The PIM technology has the potential to develop a new generation of biomimetic recognition materials beyond natural antibodies. It can be used in bioanalysis and other multitudinous fields for its unique features in molecule recognition.

Gold nanoparticles embedded Fe-BTC (AuNPs@Fe-BTC) metal-organic framework as a fluorescence sensor for the selective detection of As(III) in contaminated waters

In this article, a new off-mode fluorescent platform based on the metal-organic framework (MOF) is introduced as a highly selective and rapid chemical sensor for the detection of As(III) in water and wastewater samples. A typical Fe-BTC (BTC = 1,3,5-benzenetricarboxylate or trimesic acid) MOF was used as a porous template for loading gold nanoparticles (AuNPs@Fe-BTC MOF). The physicochemical properties of AuNPs@Fe-BTC MOF were characterized by Fourier-transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EAX), element mapping (MAP) and X-ray diffraction (XRD) analysis. This sensing method for As(III) ions is based on the fact that the fluorescence intensity of AuNPs@Fe-BTC MOF sensor decreases in proportion to the increase in As(III) concentration. The main effective factors on the performance of the sensor signal such as MOF dosage, sonication time, pH and reaction time were optimized. Under optimized conditions, the calibration graph was linear in the concentration range of 0.5-380 ng mL-1 of As(III) and the limit of detection was 0.2 ng mL-1. The proposed method was successfully validated by addition/recovery experiments by the determination of As(III) in four river water and two wastewater effluent samples.

Selective and Rapid Detection of 4-Nitrophenol in River and Treated Industrial Wastewater by a Luminescent Lanthanide Metal-Organic Framework Sensor

Phenolic organic compounds are widely used industrial chemicals that exist extensively in the environment and have a significant impact on human health. 4-Nitrophenol (4-NP) is a typical phenolic organic compound found in aqueous environments. Efficient detection of 4-NP in wastewater is highly challenging due to the complexity of testing environmental samples. Herein, a luminescent lanthanide metal-organic framework (MOF) sensor based on the Eu3+ ion {[Eu(HL)(L)(H2O)]·2H2O}n (EuMOF; H2L = 5-(4H-1,2,4-triazol-4-yl)benzene-1,3-dicarboxylic acid) was successfully synthesized for efficient 4-NP detection in wastewater. Fluorescence sensing experiments revealed that 4-NP could greatly quench the EuMOF fluorescence. Subsequently, EuMOF was applied to 4-NP detection in distilled water, tap water, river water, and treated industrial wastewater, exhibiting high sensitivity, a fast response within 30 s, high selectivity, excellent reusability, and a low detection limit. Finally, the fluorescence quenching mechanism was explored and attributed to competitive absorption of irradiated light between 4-NP and the ligand.

Molecular imprinting-based indirect fluorescence detection strategy implemented on paper chip for non-fluorescent microcystin

Fluorescence analysis is a fast and sensitive method, and has great potential application in trace detection of environmental toxins. However, many important environmental toxins are non-fluorescent substances, and it is still a challenge to construct a fluorescence detection method for non-fluorescent substances. Here, by means of charge transfer effect and smart molecular imprinting technology, we report a sensitive indirect fluorescent sensing mechanism (IFSM) and microcystin (MC-RR) is selected as a model target. A molecular imprinted thin film is immobilized on the surface of zinc ferrite nanoparticles (ZnFe2O4 NPs) by using arginine, a dummy fragment of MC-RR. By implementation of IFSM on the paper-based microfluidic chip, a versatile platform for the quantitative assay of MC-RR is developed at trace level (the limit of detection of 0.43 μg/L and time of 20 min) in real water samples without any pretreatment. Importantly, the proposed IFSM can be easily modified and extended for the wide variety of species which lack direct interaction with the fluorescent substrate. This work offers the potential possibility to meet the requirements for the on-site analysis and may explore potential applications of molecularly imprinted fluorescent sensors.

Paper microfluidics with deep learning for portable intelligent nucleic acid amplification tests

During global outbreaks such as COVID-19, regular nucleic acid amplification tests (NAATs) have posed unprecedented burden on hospital resources. Data of traditional NAATs are manually analyzed post assay. Integration of artificial intelligence (AI) with on-chip assays give rise to novel analytical platforms via data-driven models. Here, we combined paper microfluidics, portable optoelectronic system with deep learning for SARS-CoV-2 detection. The system was quite streamlined with low power dissipation. Pixel by pixel signals reflecting amplification of synthesized SARS-CoV-2 templates (containing ORF1ab, N and E genes) can be real-time processed. Then, the data were synchronously fed to the neural networks for early prediction analysis. Instead of the quantification cycle (Cq) based analytics, reaction dynamics hidden at the early stage of amplification curve were utilized by neural networks for predicting subsequent data. Qualitative and quantitative analysis of the 40-cycle NAATs can be achieved at the end of 22nd cycle, reducing time cost by 45%. In particular, the attention mechanism based deep learning model trained by microfluidics-generated data can be seamlessly adapted to multiple clinical datasets including readouts of SARS-CoV-2 detection. Accuracy, sensitivity and specificity of the prediction can reach up to 98.1%, 97.6% and 98.6%, respectively. The approach can be compatible with the most advanced sensing technologies and AI algorithms to inspire ample innovations in fields of fundamental research and clinical settings.

Synthesis of fluorescent artificial receptors with high specificity for simultaneous detection of non-steroidal anti-inflammatory drugs

Development of multiple detection methods to monitor non-steroidal anti-inflammatory drugs (NSAIDs) in food is an effective way to protect human health. Here, we aimed to synthesize fluorescent artificial receptors by molecular imprinting technique to construct a simultaneous detection system targeting NSAIDs. Rhodamine B and fluorescein-functionalized silanes were employed as the fluorescence signal reporters for naproxen and ketoprofen, respectively. Two fluorescent molecularly imprinted polymers (FMIPs) were obtained with high specificity, giving cross-reactivity factors of 6.4-15.8 (naproxen) and 2.6-25.6 (ketoprofen). Both FMIPs also displayed rapid response time (5 min) and high sensitivity (detection limit at ∼ nM level). A simultaneous detection system was constructed based on the FMIPs and applied for sensing the spiked NSAIDs in real samples, showing recoveries of 71-119 %, comparable with the HPLC methods (70-113 %). In summary, use of different FMIPs to construct simultaneous detection systems is practicable, and provides a flexible way for sensing multiple hazards in food samples.

Lanthanide bimetallic MOF-based fluorescent sensor for sensitive and visual detection of sulfamerazine and malachite

A lanthanide terbium/europium metal-organic framework (Tb_0.6Eu_0.4-MOF) was prepared by one-step solvothermal method at room temperature. A series of characterizations including scanning electron microscopy, powder X-ray diffraction spectra, Fourier transform infrared spectra and X-ray photoelectron spectroscopy were carried out to clarify the physical characteristics of the synthesized material. The data clarified that the prepared Tb_0.6Eu_0.4-MOF possessed rod-like morphology with a width of 1-2 μm, and had good crystal structure, good stability, response speed and excitation-independent emission feature. The bunchy Tb_0.6Eu_0.4-MOF was then used to construct fluorescent sensors for rapid identification of malachite green and sulfamerazine. It was revealed that the detection mechanism was inner filter effect. The effects of different parameters such as excitation wavelength and incubation times were investigated on the fluorescence analysis performance. The data clarified that the optimal excitation wavelength and incubation time was 240 nm and 3 min, respectively. The detection platform exhibited the high sensitivity and selectivity toward malachite green in the linear range of 2-180 μM and determined limit of detection was 1.12 μM. Besides, the proposed sensor allowed sensitive detection of sulfamerazine in the linear range of 2-140 μM with a low detection limit of 0.1 μM. Meaningfully, a smartphone application was designed to assist the proposed sensor to realize visual, intelligent and rapid detection of malachite green and sulfamerazine. Furthermore, the practical application of the proposed sensor has been also verified by high performance liquid chromatography, showing good accuracy, sensitivity and satisfactory recoveries. The results suggested that the Tb_0.6Eu_0.4-MOF-based ratiometric fluorescent sensor had the potential to become a promising technique for rapid detection of malachite green or sulfamerazine with smartphone application. Therefore, the prepared Tb_0.6Eu_0.4-MOF is one kind of efficient and cost-effective potential materials for developing fluorescent sensor for rapid, sensitive and selective detection of sulfamerazine and malachite.

A Paper-Based Fluorescent Sensor for Rapid Early Screening of Oral Squamous Cell Carcinoma

Various types of sensors play an irreplaceable role in the detection of biomarkers, but their high cost and complicated operation make it difficult to benefit ordinary people. Herein, we develop a low-cost, double-layered, paper-based fluorescent sensor (CP/HQ) structurally consisting of the upper reaction layer loaded with two oxidases (lactate oxidase and choline oxidase) and the bottom fluorescent layer that physically associates with the porphine-grafted composite fluorescent polymer colloids (PF-PDMTP/HQ). Based on the dramatic and rapid fluorescence decrease of porphine induced by the oxidation between saliva and oxidases and subsequent fluorescence resonance energy transfer from oxidized hydroquinone, the resultant fluorescent paper sensor enables us to achieve visual detection of OSCC, which was further recognized by smartphone scanning as the grayscale variation. It was found that the linear sensing range of grayscale value are 10-200 μM for lactic acid and 10-100 μM for choline, with LODs of 5.7 and 8.9 μM, respectively. More importantly, the sensor can achieve a powerful detection capability comparable to that of high-performance liquid chromatography (HPLC) in clinical settings with simple operation, demonstrating its great application potential. Our proposed sensor not only improves the accuracy of OSCC diagnosis but also provides a valuable attempt for the device modification of polymer-sensing systems and the development of non-invasive and easy-to-operate disease screening methods.

Sensors-integrated organ-on-a-chip for biomedical applications

As a promising new micro-physiological system, organ-on-a-chip has been widely utilized for in vitro pharmaceutical study and tissues engineering based on the three-dimensional constructions of tissues/organs and delicate replication of in vivo-like microenvironment. To better observe the biological processes, a variety of sensors have been integrated to realize in-situ, real-time, and sensitive monitoring of critical signals for organs development and disease modeling. Herein, we discuss the recent research advances made with respect to sensors-integrated organ-on-a-chip in this overall review. Firstly, we briefly explore the underlying fabrication procedures of sensors within microfluidic platforms and several classifications of sensory principles. Then, emphasis is put on the highlighted applications of different types of organ-on-a-chip incorporated with various sensors. Last but not least, perspective on the remaining challenges and future development of sensors-integrated organ-on-a-chip are presented.

Development of Pipetteless Paper-Based Analytical Devices with a Volume Gauge

In this work, we propose a new design for paper-based analytical devices (PADs) that eliminate the need to use a micropipette for sample introduction. With this design, a PAD is equipped with a distance-based detection channel that is connected to a storage channel that indicates the volume of a sample introduced into the PAD. The analyte in the sample solution reacts with a colorimetric reagent deposited into the distance-based detection channel as the sample solution flows into the storage channel where the volume is measured. The ratio of the lengths of the detection channel and that of the storage channel (D/S ratio) are constant for a sample containing a certain concentration, which is independent of the introduced volume. Therefore, the PADs permit volume-independent quantification using a dropper instead of a micropipette because the length of the storage channel plays the role of a volume gauge to estimate the introduced sample volume. In this study, the D/S ratios obtained with a dropper were comparable to those obtained with a micropipette, which confirmed that precise volume control is unnecessary for this PAD system. The proposed PADs were applied to the determinations of iron and bovine serum albumin using bathophenanthroline and tetrabromophenol blue as colorimetric reagents, respectively. The calibration curves showed good linear relationships with coefficients of 0.989 for iron and 0.994 for bovine serum albumin, respectively.

A Simple and Effective Visual Fluorescent Sensing Paper-Based Chip for the Ultrasensitive Detection of Mercury Ions in Environmental Water

Traces of mercury ions in environmental water can harm humans and animals. Paper-based visual detection methods have been widely developed for the rapid detection of mercury ions; however, existing methods are not sensitive enough to be used in real environments. Here, we developed a novel, simple and effective visual fluorescent sensing paper-based chip for the ultrasensitive detection of mercury ions in environmental water. CdTe-quantum-dots-modified silica nanospheres were firmly absorbed by and anchored to the fiber interspaces on the paper's surface to effectively avoid the unevenness caused by liquid evaporation. The fluorescence of quantum dots emitted at 525 nm can be selectively and efficiently quenched with mercury ions, and the ultrasensitive visual fluorescence sensing results attained using this principle can be captured using a smartphone camera. This method has a detection limit of 2.83 µg/L and a fast response time (90 s). We successfully achieved the trace spiking detection of seawater (from three regions), lake water, river water and tap water with recoveries in the range of 96.8-105.4% using this method. This method is effective, low-cost, user-friendly and has good prospects for commercial application. Additionally, the work is expected to be utilized in the automated big data collection of large numbers of environmental samples.

Threaded 3D microfluidic paper analytical device-based ratiometric fluorescent sensor for background-free and visual detection of organophosphorus pesticides

With the increasing concerns of food safety and environmental protection, it is desirable to develop reliable, effective, and portable sensors for detection of organophosphorus pesticides (OPs). Here, a cascade reaction system integrated with threaded 3D microfluidic paper analytical device (3D μPAD) was firstly developed for background-free and visual detection of OPs in agricultural samples. Butyrylcholinesterase (BChE) hydrolyzed acetylcholine into thiocholine (TCh), which reduced MnO₂ nanosheets into Mn²⁺. With addition of OPs, BChE activity was irreversibly inhibited, and the generation of TCh and the reduction of MnO₂ nanosheets were prevented. Then the remaining MnO₂ nanosheets oxidized o-phenylenediamine into 2,3-diaminophenazine with yellow-emission fluorescence, which quenched the fluorescence intensity of red-emission carbon dots (RCDs) via inner-filter effect. Based on above mechanism, a ratiometric fluorescent system was established for OPs detection. Threaded 3D μPAD consisted of 4 layers, which allowed to load and/or add reagents to trigger the cascade reaction system for OPs detection. The fluorescent images presented distinguishable color variations from red to yellow with dichlorvos concentrations ranging from 2.5 to 120 μg L^-1, and the limit of detection was 1.0 μg L^-1. In the practical samples testing, threaded 3D μPAD can eliminate background influence on fluorescent signal for OPs detection. Threaded 3D μPAD integrated with ratiometric sensing platform has merits of accuracy response, facile operation, and background-free detection, which supplies a new alternative approach for on-site pesticide detection.

Optimization of a silver-nanoprism conjugated with 3,3',5,5'-tetramethylbenzidine towards easy-to-make colorimetric analysis of acetaldehyde: a new platform towards rapid analysis of carcinogenic agents and environmental technology

Acetaldehyde acts as an important mediator in the metabolism of plants and animals; however, its abnormal level can cause problems in biological processes. Although acetaldehyde is found naturally in many organisms, exposure to high concentrations can have effects on the eyes, respiratory system, etc. Due to the importance of detecting acetaldehyde in environmental samples and biofluids, determination of its concentration is highly demanded. There are some reports showing exposure to high concentrations of acetaldehyde for a long time can increase the risk of cancer by reacting with DNA. In this work, we presented a novel colorimetric method for rapid and sensitive detection of acetaldehyde with high reproducibility using different AgNPs with various morphologies. The redox reaction between AgNPs, 3,3',5,5'-tetramethylbenzidine (TMB) solution, and analytes endows a color change in 15 minutes that is detectable by the naked eye. UV spectrophotometry was further used for quantitative analysis. An iron mold with a hexagonal pattern and liquid paraffin were also used to prepare the paper-based microfluidic substrate, as a low cost, accessible, and rapid detection tool. Different types of AgNPs showed different lower limits of quantification (LLOQ). The AgNPs-Cit and AgNPrs could identify acetaldehyde with linear range of 10^-7 to 10 M and an LLOQ of 10^-7 M. The AgNWs showed the best color change activity with a linear range 10^-5 to 10 M and the lowest diagnostic limit is 10^-5 M. Finally, analysis of human biofluids as real samples were successfully performed using this system.

Electric yo-yo centrifugation combining with paper-based microfluidic immunoassay chip for inflammatory biomarkers detection in whole blood

At present, most countries or regions use commercial centrifuges for centrifugation, but this is out of reaching for limited-resource areas. To overcome this problem, a portable electric yo-yo as centrifuge was firstly proposed to obtain serum, and this device can be combined with paper-based analytical devices for enzyme-linked immunosorbent assay (ELISA) analysis from human whole blood. In this study, inflammatory biomarkers C-reactive protein (CRP) and serum amyloid A (SAA) were used as target biomarker to verify the performance of the proposed method. The results shows good performance and their detection limits were determined to be 580 pg/mL for CRP and 800 pg/mL for SAA, respectively. We believe this method provides a new platform of low cost and fast detection for inflammatory biomarkers in the limited-resource settings.

An electrochemical paper-based hydrogel immunosensor to monitor serum cytokine for predicting the severity of COVID-19 patients

Analysis of cytokines levels in human serum is critical as it can be a "symptom diagnostic biomarker" in COVID-19, giving real-time information about human health status. Here, we present the construction and performance of a low-price immunosensor (∼US$0.428 per test) based on microfluidic paper-based system to detect cytokine for predicting the health status of COVID-19 patients. Interleukin-6 (IL-6) was selected as the detection model for the close relationship between IL-6 and COVID-19. The assay, which we integrated into foldable paper system, leverages the magnetic immunoassay, the streptavidin-horseradish peroxidase (HRP) associated with tetramethyl benzidine/hydrogen peroxide (TMB/H₂O₂) to amplify the signal for electrochemical readout. To improve the sensitivity of cytokine detection, a hybrid of gold nanoparticles (AuNPs) and polypyrrole (PPy) hydrogel was modified on the working electrode to increase the conductivity and improve the electron transfer rate. With our prototypic origami paper-based immunosensor operated in differential pulse voltammetry (DPV) mode, we achieved excellent results with a dynamic range from 5 to 1000 pg/mL and a lower detection limit (LOD) of 0.654 pg/mL. Furthermore, we evaluated the capability of the clinical application of the proposed immunosensor using human serum samples from a hospital. The results indicate that our proposed immunosensor has great potential in early diagnosing high-risk COVID-19 patients.

Colorimetric detection of heavy metal ions with various chromogenic materials: Strategies and applications

Detection of heavy metal ions has drawn significant attention in environmental and food area due to their threats to the human health and ecosystem. Colorimetry is one of the most frequently-used methods for the detection of heavy metal ions owing to its simplicity, easy operation and rapid on-site detection. The development of chromogenic materials and their sensing mechanisms are the key research direction in the area of colorimetric method. Since each chromogenic material has their unique optical and chemical properties, they have totally different colorimetric sensing mechanisms. This review focuses on the chromogenic materials and their sensing strategies for the colorimetric detection of heavy metal ions. We divide the chromogenic materials into three types, including organic materials, inorganic materials, and other materials. As for each type of chromogenic material, we discuss their detailed sensing strategies, sensing performance, and real sample applications. Moreover, current challenges and perspectives related to the colorimetry of heavy metal ions are also discussed in this review. The aim of this review is to help readers to better understand the principles of colorimetric methods for heavy metal ions and push the development of rapid detection of heavy metal ions.

Recent progress of microfluidic chips in immunoassay

Microfluidic chip technology is a technology platform that integrates basic operation units such as processing, separation, reaction and detection into microchannel chip to realize low consumption, fast and efficient analysis of samples. It has the characteristics of small volume need of samples and reagents, fast analysis, low cost, automation, portability, high throughout, and good compatibility with other techniques. In this review, the concept, preparation materials and fabrication technology of microfluidic chip are described. The applications of microfluidic chip in immunoassay, including fluorescent, chemiluminescent, surface-enhanced Raman spectroscopy (SERS), and electrochemical immunoassay are reviewed. Look into the future, the development of microfluidic chips lies in point-of-care testing and high throughput equipment, and there are still some challenges in the design and the integration of microfluidic chips, as well as the analysis of actual sample by microfluidic chips.

Simultaneous Cross-type Detection of Water Quality Indexes via a Smartphone-App Integrated Microfluidic Paper-Based Platform

Water quality guarantee in remote areas necessitates the development of portable, sensitive, fast, cost-effective, and easy-to-use water quality detection methods. The current work reports on a microfluidic paper-based analytical device (μPAD) integrated with a smartphone app for the simultaneous detection of cross-type water quality parameters including pH, Cu(II), Ni(II), Fe(III), and nitrite. The shapes, baking time, amount, and ratios of reaction reagent mixtures of wax μPAD were optimized to improve the color uniformity and intensity effectively. An easy-to-use smartphone app was established for recording, analyzing, and directly reading the colorimetric signals and target concentrations on μPAD. The results showed that under the optimum conditions, the current analytical platform has reached the detection limits of 0.4, 1.9, 2.9, and 1.1 ppm for nitrite, Cu(II), Ni(II), and Fe(III), respectively, and the liner ranges are 2.3-90 ppm (nitrite), 3.8-400 ppm (Cu(II)), 2.9-1000 ppm (Ni(II)), 2.8-500 ppm (Fe(III)), and 5-9 (pH). The proposed portable smartphone-app integrated μPAD detection system was successfully applied to real industrial wastewater and river water quality monitoring. The proposed method has great potential for field water quality detection.

Molecularly Imprinted Ratiometric Fluorescence Nanosensors

Molecularly imprinted ratiometric fluorescence (MIR-FL) nanosensors feature recognition selectivity, detection sensitivity, application universality, visualization accuracy, and device portability, and have gained popularity. However, the fluorescence intensity, nanostructure, color range, and practical application of the sensor still face severe difficulties to be solved. New strategies combined with various technologies have been developed to construct MIR-FL nanosensors for expanded applications. This Perspective highlights current resarch challenges and future prospects involving constructions and applications of MIR-FL nanosensors including dual-emission and triple-emission modes. The postimprinting mixing/modification strategies, microdevices, and multitarget detection are focused, and technology synergy, sensitivity/reproducibility improvement, application diversity/portability, etc. are proposed.

Smartphone-Assisted Sensing of Trinitrotoluene by Optical Array

Here we report the design and fabrication of an array-based sensor, containing functionalized Carbon Dots, Bodipy's and Naphthalimide probes, that shows high fluorescence emissions and sensitivity in the presence of low amounts of TNT explosive. In particular, we have fabricated the first sensor device based on an optical array for the detection of TNT in real samples by using a smartphone as detector. The possibility to use a common smartphone as detector leads to a prototype that can be also used in a real-life field application. The key benefit lies in the possibility of even a nonspecialist operator in the field to simply collect and send data (photos) to the trained artificial intelligence server for rapid diagnosis but also directly to the bomb disposal unit for expert evaluation. This new array sensor contains seven different fluorescent probes that are able to interact via noncovalent interactions with TNT. The interaction of each probe with TNT has been tested in solution by fluorescence titrations. The solid device has been tested in terms of selectivity and linearity toward TNT concentration. Tests performed with other explosives and other nitrogen-based analytes demonstrate the high selectivity for TNT molecules, thus supporting the reliability of this sensor. In addition, TNT can be detected in the range of 98 ng∼985 μg, with a clear different response of each probe to the different amounts of TNT.

A mesoporous silica-based probe with a molecularly imprinted polymer recognition and Mn:ZnS QDs@rhodamine B ratiometric fluorescence sensing strategy for the analysis of 4-nitrophenol

In this study, a mesoporous silica fluorescence probe co-doped with manganese-doped zinc sulfide quantum dots (Mn:ZnS QDs) and rhodamine B (RB) and coated with molecularly imprinted polymer (MIP) has been prepared by sol-gel methods. The morphology and structure were characterized in detail by transmission electron microscopy (TEM), Fourier transform-infrared absorption spectroscopy (FT-IR) and ultraviolet-visible absorption spectroscopy (UV-vis). The probe exhibited two characteristic emission peaks at 411 nm and 582 nm, and the synchronous ratiometric fluorescence responses F₄₁₁/F₅₈₂ to different concentrations of 4-nitrophenol (4-NP) showed a good linear correlation in the range of 0.01-10 μmol L^-1 besides achieving the sensitive detection of 4-NP with a detection limit as low as 3.0 nmol L^-1 (3σ). The probe possesses the advantages of selectivity toward the target molecular structure, self-stability in the detection time domain and anti-interference ability, exhibiting excellent potential for application in 4-NP detection in different water environments.

Auto-fluorescence of cellulose paper with spatial solid phrase dispersion-induced fluorescence enhancement behavior for three heavy metal ions detection

Auto-fluorescence of cellulose paper is often considered as an interfering fluorescence, which directly impedes the cellulose paper as a substrate material. This paper creatively explored the composition and properties of auto-fluorescence, and lignosulfonate was primarily speculated as the main source of auto-fluorescence. Surprisingly, its spatial solid phrase dispersion-induced fluorescence enhancement behavior was found. Then, cellulose paper was modified with Mn-doped ZnS quantum dots, and the prepared ratiometric fluorescent paper chip has good performances on morphology, stability, and fluorescence properties. Besides, the paper chip exhibited different fluorescence responses to three heavy metal ions in water sample. The limit of detection for Cd²⁺, Hg²⁺ and Pb²⁺ reached 1.61 nM, 0.01 nM, and 0.02 nM, respectively. In short, the molecular simulation results theoretically proved that heavy metal ions owned substitution affinity with lignosulfonate. Ultimately, this study was the first attempt to utilize paper-based auto-fluorescence, which could better accelerate the development of paper-based chips.

Molecularly-Imprinted SERS: A Potential Method for Bioanalysis

The most challenging step in developing bioanalytical methods is finding the best sample preparation method. The matrix interference effect of biological sample become a reason of that. Molecularly imprinted SERS become a potential analytical method to be developed to answer this challenge. In this article, we review recent progress in MIP SERS application particularly in bioanalysis. Begin with the explanation about molecular imprinting technique and component, SERS principle, the combination of MIP SERS, and follow by various application of MIP SERS for analysis. Finally, the conclusion and future perspective were also discussed.

Quadruple analyte responsive platform: Point-of-care testing and multi-coding logic computation based on metal ions recognition and selective response

While it is recognized that instrumentation techniques can provide precise and sensitive solutions to heavy metal ion monitoring, it remains challenging to transform laboratory testing into a convenient, on-site, and quantitative sensing platform for point-of-care testing (POCT) in a resource-constrained setting. To address these limitations, an affordable and user-friendly colorimetric POCT sensing system is proposed here for selectively monitoring four metal ions (Fe³⁺, Co²⁺, Pb²⁺, and Cd²⁺) based on the sulfur quantum dots (S dots). Quadruple distinct visual signals (green, brown, precipitation, and bright yellow) are presented on the fabricated paper-based analytical devices (PADs) when mixing S dots and metal ions. The high-quality photographs of the PADs are captured by a scanner, while a smartphone App converts visual signals to HSV values. The quantitative analysis relies on the digital colorimetric reading, and the limits of detection are 0.59, 0.47, 0.82, and 0.53 μM for Fe³⁺, Co²⁺, Cd²⁺, and Pb²⁺, respectively. This metal ions-responsive platform is engineered as a smart strategy for multiple logic operations (YES, NOT, AND, INHIBIT, and NOR) by integrating multi-responsive blocks into the S dots with encoded patterns, which improves the computing capability. Accordingly, this strategy demonstrates its potential for on-site environmental testing and sophisticated molecular computation.

Enzyme-free and rapid visual quantitative detection for pesticide residues utilizing portable smartphone integrated paper sensor

Serious toxicity for organisms from pesticide glyphosate (Gly) residues to the ecosystem and human health has become a consensus. Rapid and selective detection of glyphosate, especially using a simple and portable instrument, is highly desired. In this work, we develop a novel enzyme-free rapid and visual ratiometric fluorescence sensor for selectively quantitative detecting glyphosate by integrating the designed blue carbon nanodots (CDs) and gold nanoclusters (Au NCs). The fluorescence of CDs can be quickly quenched via aggregation-caused quenching (ACQ) within 2 s after introducing glyphosate, resulting from the formation of CDs-Gly-CDs complex aggregation. While the Au NCs serve as the reference signal without any change, therefore leading to obvious and instant ratiometric fluorescence variation from blue to pink to orange. The broad linear range was obtained from 0 to 180 nM with a satisfactory detection limit of 4.19 nM. Furthermore, this approach was successfully applied to detect glyphosate in real samples and a portable smartphone platform integrated paper sensor was developed for in-site visual quantitative glyphosate detection, offering a promising strategy for the construction of enzyme-free trace hazard detection system.

A paper-based microfluidic sensor array combining molecular imprinting technology and carbon quantum dots for the discrimination of nitrophenol isomers

Paper-based microfluidic analytical devices (μPADs) have recently attracted attention as a rapid test kit owing to their low cost and nonrequirement for external driving pump. However, low accuracy and poor anti-interference ability of μPADs under complex detection condition limit their practical applications. Here, we present a facile way to prepare a novel fluorescence sensor-array μPAD for multi-analyte discrimination based on molecular imprinting technology, and its sensing behavior was studied by using three nitrophenol (NP) isomers (2-, 3-, and 4-NP) as the testing models. Carbon quantum dots (CQDs) emitting blue light were grafted on glass-fiber paper, followed by in-situ modification of three types of molecularly imprinted polymers (MIPs) with 2-, 3-, and 4-NP as template. Each sensing unit on the array showed differential yet cross-reactive binding affinity to NP isomers, resulting in distinct fluorescence quenching efficiency. Thus, precise distinguishment of the three NPs was realized with the MIPs/CQDs/paper-based sensor array. Furthermore, the discrimination ability of the platform was evaluated in mixtures of the NP isomers. Practicability of this apparatus was validated by identification of blind samples and 100% accuracy was achieved. The μPAD has proven to be highly sensitive and accurate, which will serve as an ideal analytical tool in the fields of environment monitoring, disease prognosis, food safety and so on.

Paper-Based Molecular-Imprinting Technology and Its Application

Paper-based analytical devices (PADs) are highly effective tools due to their low cost, portability, low reagent accumulation, and ease of use. Molecularly imprinted polymers (MIP) are also extensively used as biomimetic receptors and specific adsorption materials for capturing target analytes in various complex matrices due to their excellent recognition ability and structural stability. The integration of MIP and PADs (MIP-PADs) realizes the rapid, convenient, and low-cost application of molecular-imprinting analysis technology. This review introduces the characteristics of MIP-PAD technology and discusses its application in the fields of on-site environmental analysis, food-safety monitoring, point-of-care detection, biomarker detection, and exposure assessment. The problems and future development of MIP-PAD technology in practical application are also prospected.

Glass-patternable notch-shaped microwave architecture for on-chip spin detection in biological samples

We report a notch-shaped coplanar microwave waveguide antenna on a glass plate designed for on-chip detection of optically detected magnetic resonance (ODMR) of fluorescent nanodiamonds (NDs). A lithographically patterned thin wire at the center of the notch area in the coplanar waveguide realizes a millimeter-scale ODMR detection area (1.5 × 2.0 mm²) and gigahertz-broadband characteristics with low reflection (∼8%). The ODMR signal intensity in the detection area is quantitatively predictable by numerical simulation. Using this chip device, we demonstrate a uniform ODMR signal intensity over the detection area for cells, tissue, and worms. The present demonstration of a chip-based microwave architecture will enable scalable chip integration of ODMR-based quantum sensing technology into various bioassay platforms.

A Portable Sensing Platform Using an Upconversion-Based Nanosensor for Visual Quantitative Monitoring of Mesna

Mesna is an important regional antidote for protecting the urinary system of chemotherapy patients, which requires monitoring its level in real time to ensure the curative effect. The fluorescence method is a powerful tool in real-time detection with the advantages of fast response and visualization. However, the background interference limits its application in biological sensing. Here, we developed a portable sensing platform using an upconversion-based nanosensor for visual quantitative monitoring of mesna in real-time/on-site conditions. The nanosensor was constructed by upconversion nanoparticles (UCNPs) and ethyl violet (EV), in which the UCNPs emitted red and green light, while EV quenched the green light due to the inner filter effect (IFE). The reaction of mesna with EV caused its fading and broke the IFE process, leading to the recovery of green light. By the fluorescence and colorimetric chromaticity variations, the nanosensor achieved a dual-readout detection for mesna with low limits of detection (LODs) of 26 and 48 nM, respectively. Furthermore, a highly compatible sensing platform was fabricated for facile determination of mesna with an LOD of 56 nM, realizing visual quantitative monitoring of the mesna level to ensure the curative effect and providing a new strategy for point-of-care testing of drugs in clinical settings.

A rotary multi-positioned cloth/paper hybrid microfluidic device for simultaneous fluorescence sensing of mercury and lead ions by using ion imprinted technologies

A novel rotary cloth/paper hybrid microfluidic analytical device (μCPAD) was proposed via the synergy of the fluorescence sensing cloth-based component and rotary paper-based microfluidic analytical device (μPAD) for simultaneous detection of mercury (Hg²⁺) and lead (Pb²⁺) ions. Fluorescence sensing cloth-based component was prepared by grafting quantum dots onto cotton cloth and then modifying with ion imprinted polymers (IIP). Because the cloth has good ductility and durability, it can bear strong oscillation during the fabrication of grafting quantum dots and IIP, and brings a lot of convenience to the production process. At the same time, because rotary μCPAD was stacked by three-layer papers with designed hydrophilic channels and hydrophobic barriers, it could realize simultaneous detection of Hg²⁺ and Pb²⁺ ions by rotating top layer counterclockwise or clockwise. The fluorescence signals were obtained through quantum dots' electron transfer fluorescence quenching effect with the limits of detection were 0.18 and 0.07 μg/L, respectively. This method successfully realized the transference of specific and sensitive fluorescence sensing materials (quantum dots) onto the microfluidic device to improve the portability and expanded applications. Moreover, the novel microfluidic device may have great potential in point-of-care testing of heavy metal ions in environmental monitoring fields.

Hand-Operated, Paper-Based Rotational Vertical-Flow Immunosensor for the Impedimetric Detection of α-Fetoprotein

This study demonstrates a hand-operated, paper-based rotational vertical-flow immunosensor (rotational VFI) platform requiring fewer pipetting steps, designed for the electrochemical detection of α-fetoprotein with multiple and time-sequenced steps. The platform allows users to perform electrochemical measurements without interference from the convective component of fluid motion, which is unfavorable in most techniques. Users can freely transfer-switch-stop fluid flows by manually rotating the paper disk, evidencing the superior flexibility of this sensor compared to other biosensors. Furthermore, the overall assay duration can be considerably shortened to 9 min. The linear range (LR) is determined to be 0.01-500 ng/mL, with a limit of detection (LOD) of 1.65 pg/mL, and the sensitivity can be significantly enhanced simply by switching off the sample stream to ensure detention at the binding zone (for up to 30 min). This additional step can widen the LR to 0.5 pg/mL, with a LOD of 3.54 fg/mL, which is the lowest detectable level ever reported among paper-based sensors. The advantages of the designed rotational VFI qualify it as a suitable alternative to various biosensors.

A ZnFe2O4-catalyzed segment imprinted polymer on a three-dimensional origami paper-based microfluidic chip for the detection of microcystin

Microcystin has been causing serious environmental pollution; however, the recognition of such compounds is still challenging because of low abundance and coexisting interfering species. In this contribution, we develop a novel microfluidic paper-based colorimetric sensor by exploiting molecular imprinting technology and Fenton reaction for on-site microcystin-RR determination in complex water samples using a smartphone.

Recent Advances in Quartz Crystal Microbalance Biosensors Based on the Molecular Imprinting Technique for Disease-Related Biomarkers

The molecular imprinting technique is a quickly developing field of interest regarding the synthesis of artificial recognition elements that enable the specific determination of target molecule/analyte from a matrix. Recently, these smart materials can be successfully applied to biomolecule detection in biomimetic biosensors. These biosensors contain a biorecognition element (a bioreceptor) and a transducer, like their biosensor analogs. Here, the basic difference is that molecular imprinting-based biosensors use a synthetic recognition element. Molecular imprinting polymers used as the artificial recognition elements in biosensor platforms are complementary in shape, size, specific binding sites, and functionality to their template analytes. Recent progress in biomolecular recognition has supplied extra diagnostic and treatment methods for various diseases. Cost-effective, more robust, and high-throughput assays are needed for monitoring biomarkers in clinical settings. Quartz crystal microbalance (QCM) biosensors are promising tools for the real-time and quick detection of biomolecules in the past two decades A quick, simple-to-use, and cheap biomarkers detection technology based on biosensors has been developed. This critical review presents current applications in molecular imprinting-based quartz crystal microbalance biosensors for the quantification of biomarkers for disease monitoring and diagnostic results.

Ratiometric fluorescence sensor for sensitive detection of inorganic phosphate in environmental samples

Fast, simple, and low-cost on-site visualized detection of inorganic phosphate (Pi) is in great demand since phosphate is the major reason of eutrophication. In this work, a ratiometric fluorescent probe composed by green carbon dots (GCDs) and red carbon dots (RCDs) has been established for high-sensitivity and selective sensing of Pi. A trend of color change from red to green is observed for the detection of Pi under ultraviolet light and the detection limit is 0.09 μM in the range of 0 to 55 μM. Fluorescent test paper prepared from the probe solution was successfully applied to semi-quantitative visual detection of Pi in real-world water and soil samples, which shows great real-world application potentials.

Multiplex optical bioassays for food safety analysis: Toward on-site detection

Food safety analysis plays a significant role in controlling food contamination and supervision. In recent years, multiplex optical bioassays (MOBAs) have been widely applied to analyze multiple hazards due to their efficiency and low cost. However, due to the challenges such as multiplexing capacity, poor sensitivity, and bulky instrumentation, the further application of traditional MOBAs in food screening has been limited. In this review, effective strategies regarding food safety MOBAs are summarized, such as spatial-resolution modes performed in multi-T lines/dots strips or arrays of strip/microplate/microfluidic chip/SPR chip and signal-resolution modes employing distinguishable colorimetric/luminescence/fluorescence/surface plasma resonance/surface-enhanced Raman spectrum as signal tags. Following this, new trends on how to design engineered sensor architecture and exploit distinguishable signal reporters, how to improve both multiplexing capacity and sensitivity, and how to integrate these formats into smartphones so as to be mobile are summarized systematically. Typically, in the case of enhancing multiplexing capacity and detection throughput, microfluidic array chips with multichannel architecture would be a favorable approach to overcome the spatial and physical limitations of immunochromatographic assay (ICA) test strips. Moreover, noble metal nanoparticles and single-excitation, multiple-emission luminescence nanomaterials hold great potential in developing ultrasensitive MOBAs. Finally, the exploitation of innovative multiplexing strategy hybridized with powerful and widely available smartphones opens new perspectives to MOBAs. In future, the MOBAs should be more sensitive, have higher multiplexing capacity, and easier instrumentation.

Recent advances in synthesis and modification of carbon dots for optical sensing of pesticides

Serious threat from pesticide residues to the ecosystem and human health has become a global concern. Developing reliable methods for monitoring pesticides is a world-wide research hotspot. Carbon dots (CDs) with excellent photostability, low toxicity, and good biocompatibility have been regarded as the potential substitutes in fabricating various optical sensors for pesticide detection. Based on the relevant high-quality publications, this paper first summarizes the current state-of-the-art of the synthetic and modification approaches of CDs. Then, a comprehensive overview is given on the recent advances of CDs-based optical sensors for pesticides over the past five years, with a particular focus on photoluminescent, electrochemiluminescent and colorimetric sensors regarding the sensing mechanisms and design principles by integrating with various recognition elements including antibodies, aptamers, enzymes, molecularly imprinted polymers, and some nanoparticles. Novel functions and extended applications of CDs as signal indicators, catalyst, co-reactants, and electrode surface modifiers, in constructing optical sensors are specially highlighted. Beyond an assessment of the performances of the real-world application of these proposed optical sensors, the existing inadequacies and current challenges, as well as future perspectives for pesticide monitoring are discussed in detail. It is hoped to provide powerful insights for the development of novel CDs-based sensing strategies with their wide application in different fields for pesticide supervision.

A novel polymer-based nitrocellulose platform for implementing a multiplexed microfluidic paper-based enzyme-linked immunosorbent assay

Nitrocellulose (NC) membranes, as porous paper-like substrates with high protein-binding capabilities, are very popular in the field of point-of-care immunoassays. However, generating robust hydrophobic structures in NC membranes to fabricate microfluidic paper-based analytical devices (μPADs) remains a great challenge. At present, the main method relies on an expensive wax printer. In addition, NC membranes very easy to adhere during the printing process due to electrostatic adsorption. Herein, we developed a facile, fast and low-cost strategy to fabricate μPADs in NC membranes by screen-printing polyurethane acrylate (PUA) as a barrier material for defining flow channels and reaction zones. Moreover, hydrophobic barriers based on UV-curable PUA can resist various surfactant solutions and organic solvents that are generally used in immunoassays and biochemical reactions. To validate the feasibility of this PUA-based NC membrane for immunoassays in point-of-care testing (POCT), we further designed and assembled a rotational paper-based analytical device for implementing a multiplexed enzyme-linked immunosorbent assay (ELISA) in a simple manner. Using the proposed device under the optimal conditions, alpha fetoprotein (AFP) and carcinoembryonic antigen (CEA) could be identified, with limits of detection of 136 pg/mL and 174 pg/mL, respectively, which are below the threshold values of these two cancer biomarkers for clinical diagnosis. We believe that this reliable device provides a promising platform for the diagnosis of disease based on ELISA or other related bioassays in limited settings or remote regions.