Sudoku-like Lab-on-Paper Cyto-Device with Dual Enhancement of Electrochemiluminescence Intermediates Strategy

Tetrahedral DNA nanostructure enhanced toehold-mediated strand displacement for highly sensitive electrochemiluminescence assay of CA125

Cancer antigen 125 (CA125) is a typical tumor marker of ovarian cancer. Here, a multi-amplified electrochemiluminescence (ECL) aptasensor was developed for efficient recognition of CA125 using tetrahedral DNA nanostructure (TDN) enhanced toehold-mediated strand displacement (TMSD) coupled with gold nanoparticles/Ru(bpy)32+/metal-organic framework (AuNPs/Ru/ZIF-MOF) signal probe. AuNPs and Ru(bpy)32+ modified ZIF-MOF acted as initial ECL signal and further used for the immobilization of TDN, the activated DNA templates on the surface of TDN were firstly hybridized with ferrocene labeled DNA probe (S6) and S5, in which, S6 acted as the energy acceptor of ECL signal from Ru(bpy)32+, making the sensor in a "signal-off" state. After the specific recognition of aptamer (AP) with CA125, DNA initiator (S7) was freed to induce the happen of TMSD by using S8 as the helper DNA, accompanying with the release of S6 from the electrode surface and the recovery of Ru(bpy)32+ ECL signal, making the sensor in a "signal-on" state. Then, S7 was recycled for the next TMSD, making the sensor highly sensitive with a detection limit of 6 × 10-3 pg/mL. Moreover, the proposed aptasensor achieved high performance for CA125 detection in human serum samples, illustrating the reliability of the sensor in clinical analysis.

"Do it yourself" protocol to fabricate dual-detection paper-based analytical device for salivary biomarker analysis

This paper describes the design and construction of dual microfluidic paper-based analytical devices (dual-μPADs) as a lab-on-paper platform involving a "do-it-yourself" fabrication protocol. The device comprises a colorimetric and electrochemical module to obtain a dual-mode signal readout sensing strategy. A 3D pen polymeric resin was used to prepare graphite carbon-based electrodes and hydrophobic barriers on paper substrates. The proposed carbon-based ink was employed to manufacture electrodes on paper based on a stencil-printing approach, which were further characterized by electrochemical and morphological analyses. The analytical performance of the dual-μPADs was simultaneously evaluated for lactate, pH, nitrite, and salivary amylase (sAA) analysis. To demonstrate the proof-of-concept, saliva samples collected from both healthy individuals and those with periodontitis were successfully tested to demonstrate the feasibility of the proposed devices. Samples collected from individuals previously diagnosed with periodontitis showed high levels of nitrite and sAA (> 94 μmol L^-1 and > 610 U mL^-1) in comparison with healthy individuals (≤ 16 μmol L^-1 and 545 U mL^-1). Moreover, periodontitis saliva resulted in acid solution and almost null lactate levels. Notably, this protocol supplies a simple way to manufacture dual-μPADs, a versatile platform for sensitive detecting of biomarkers in saliva playing a crucial role towards the point-of-care diagnosis of periodontal disease.

Microfluidic Paper-Based Device for Medicinal Diagnosis

BACKGROUND

The demand for point-of-care testing (POCT) devices has rapidly grown since they offer immediate test results with ease of use, makingthem suitable for home self-testing patients and caretakers. However, the POCT development has faced the challenges of increased cost and limited resources. Therefore, the paper substrate as a low-cost material has been employed to develop a cost-effective POCT device, known as "Microfluidic paper-based analytical devices (μPADs)". This device is gaining attention as a promising tool for medicinal diagnostic applications owing to its unique features of simple fabrication, low cost, enabling manipulation flow (capillarydriven flow), the ability to store reagents, and accommodating multistep assay requirements.

OBJECTIVE

This review comprehensively examines the fabrication methods and device designs (2D/3D configuration) and their advantages and disadvantages, focusing on updated μPADs applications for motif identification.

METHODS

The evolution of paper-based devices, starting from the traditional devices of dipstick and lateral flow assay (LFA) with μPADs, has been described. Patterned structure fabrication of each technique has been compared among the equipment used, benefits, and drawbacks. Microfluidic device designs, including 2D and 3D configurations, have been introduced as well as their modifications. Various designs of μPADs have been integrated with many powerful detection methods such as colorimetry, electrochemistry, fluorescence, chemiluminescence, electrochemiluminescence, and SER-based sensors for medicinal diagnosis applications.

CONCLUSION

The μPADs potential to deal with commercialization in terms of the state-of-the-art of μPADs in medicinal diagnosis has been discussed. A great prototype, which is currently in a reallife application breakthrough, has been updated.

Recent Advances and Applications in Paper-Based Devices for Point-of-Care Testing

Point-of-care testing (POCT), as a portable and user-friendly technology, can obtain accurate test results immediately at the sampling point. Nowadays, microfluidic paper-based analysis devices (μPads) have attracted the eye of the public and accelerated the development of POCT. A variety of detection methods are combined with μPads to realize precise, rapid and sensitive POCT. This article mainly introduced the development of electrochemistry and optical detection methods on μPads for POCT and their applications on disease analysis, environmental monitoring and food control in the past 5 years. Finally, the challenges and future development prospects of μPads for POCT were discussed.

All-sealed paper-based electrochemiluminescence platform for on-site determination of lead ions

Lab-on-paper (LOP) devices are urgently required for the rapid development of point-of-care diagnoses and environmental assays. Herein, an all-sealed paper-based electrochemiluminescence (ECL) platform was developed to achieve lead ions (Pb²⁺) sensitive analysis via incorporating convenient plastic package technology. Benefiting from transparent plastic encapsulation, the sealed devices effectively avoided the interference of O₂. Meanwhile, myrica rubra-like Pt nanomaterials (MPNs) prepared by an economical and easy-to-operate ultrasound method were employed to catalyze H₂O₂ decomposition. With the help of Pb²⁺-specific DNAzymes, the oligonucleotide probe functionalized via MPNs could be detached from the device in the presence of target, resulting in the reduced ECL intensity. Moreover, the combination of modified paper electrode with functional regions separated by multiple layers of wax enhanced the practicability of the LOP device for rapid detection. Under the optimal conditions, the all-sealed platform achieved wide linear relationship ranging from 0.01 nM to 0.05 μM with a low detection limit of 0.004 nM for sensitive detecting Pb²⁺. It is believed that this platform could provide a robust, simple and versatile strategy for sensitive determination of heavy metal ions, and be applied in on-site contamination analysis in the future.

3D DNA Walker-Assisted CRISPR/Cas12a Trans-Cleavage for Ultrasensitive Electrochemiluminescence Detection of miRNA-141

In this study, a CRISPR/Cas12a (LbCpf1)-mediated electrochemiluminescence (ECL) paper-based platform on the basis of a three-dimensional (3D) DNA walker was proposed for the ultrasensitive detection of miRNA-141. Initially, 3D-rGO with a tremendous loading space was modified on the paper working electrode (PWE) to construct an excellent conductive substrate and facilitate the growth of AuPd nanoparticles (NPs). Afterward, the AuPd NPs were introduced as the coreaction emitter medium of the 3D-rGO/PWE to provide convenience for the transformation between S₂O₈^2- and SO₄^2-, amplifying the ECL emission of g-C₃N₄ nanosheets (NSs). Meanwhile, with the help of Nt.BsmAI nicking endonuclease, a 3D DNA walker signal amplifier was designed to convert and magnify the target miRNA-141 into a particular trigger sequence, which could act as activator DNA to motivate the trans-acting deoxyribonuclease activity of CRISPR/Cas12a to further achieve efficient annihilation of the ECL signal. Furthermore, the proposed multimechanism-driven biosensor exhibited excellent sensitivity and specificity, with a relatively low detection limit at 0.331 fM (S/N = 3) in the concentration range between 1 fM and 10 nM. Consequently, the designed strategy not only extended the application scope of CRISPR/Cas12a but also devoted a new approach for the clinical diagnosis of modern medicine.

Advances in Multiplexed Paper-Based Analytical Devices for Cancer Diagnosis: A Review of Technological Developments

Cancer is one of the leading causes of death worldwide producing estimated cost of $161.2 billion in the US in 2017 only. Early detection of cancer would not only reduce cancer mortality rates but also dramatically reduce healthcare costs given that the 17 million new cancer cases in 2018 are estimated to grow 27.5 million new cases by 2040. Analytical devices based upon paper substrates could provide effective, rapid, and extremely low cost alternatives for early cancer detection compared to existing testing methods. However, low concentrations of biomarkers in body fluids as well as the possible association of any given biomarker with multiple diseases remain as one of the greatest challenges to widespread adoption of these paper-based devices. However, recent advances have opened the possibility of detecting multiple biomarkers within the same device, which could be predictive of a patient's condition with unprecedented cost-effectiveness. Accordingly, this review highlights the recent advancements in paper-based analytical devices with a multiplexing focus. The primary areas of interest include lateral flow assay and microfluidic paper-based assay formats, signal amplification approaches to enhance the sensitivity for a specific cancer type, along with current challenges and future outlook for the detection of multiple cancer biomarkers.

Portable electrochemical carbon cloth analysis device for differential pulse anodic stripping voltammetry determination of Pb2

A novel electrochemical carbon cloth (CC) analysis device (eCAD) is proposed for the determination of Pb²⁺ in environmental water samples, which was assembled using a single-step functional CC as both the sensing and the substrate material. The modified CC was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectra, and electrochemical impedance spectroscopy. The increase in electrochemical activity is due to the increased defective extent and excellent electrochemical activity of CC. Under optimum conditions (viz. a pH value of 4.5, deposition time of 160 s), the sensor is capable of determining Pb²⁺ by differential pulse anodic stripping voltammetry (DPASV) at a typical working potential of - 1.0 V (vs. Ag/AgCl). Response is linear from 5.0 × 10^-9 to 3.0 × 10^-6 M Pb²⁺, and the detection limit is 4.8 nM (at S/N = 3). The sensor was successfully applied to the determination of Pb²⁺ in real samples, with apparent recoveries from 96.0 to 102.0% and a relative standard deviation of less than 3.4%. In addition, the integration of the sensor with signal collection components has enabled us to realize on-site analysis of Pb²⁺, which is highlighted as a new generation of electrode platform for the development of a portable analysis device.Graphical abstract.

Paper-based electrochemiluminescence determination of streptavidin using reticular DNA-functionalized PtCu nanoframes and analyte-triggered DNA walker

A paper-based electrochemiluminescence (ECL) biosensor characterized by the signal amplification of reticular DNA-functionalized PtCu nanoframes (DNA-PtCuTNFs) and analyte-triggered DNA walker was developed for sensitive streptavidin assay. Silver microflower functionalized paper-based sensing platform was prepared to fix the hairpin strand (S1). With addition of the streptavidin, plenty of DNA walkers consisting of the walking strands (S2) labeled with biotin and streptavidin were established, which protected S2 from digestion via the terminal protection mechanism. The sequential introduction of the DNA walker and capture probe initiated the hairpin structure opening of S1 and strand displacement reaction (SDR) happening, causing the S2 release. Subsequently, S1 hybridized with S3. The free S2 further hybridized with adjacent S1 to trigger the next cycle. After multiple cycles, the DNA-PtCuTNFs, the fire-new signal enhancer, with remarkable peroxidase activity, were successfully attached onto the paper electrode via metal-catalyst-free click chemistry. Based on the SDR of the DNA walker and the catalysis of DNA-PtCuTNFs, a significantly boosted ECL signal of luminol was obtained. Under the optimal conditions, the developed sensor for streptavidin assay exhibited a low detection limit of 33.4 fM with a linear range from 0.1 pM to 0.1 μM. Graphical abstract.

Cathode Photoelectrochemical Paper Device for microRNA Detection Based on Cascaded Photoactive Structures and Hemin/Pt Nanoparticle-Decorated DNA Dendrimers

In this work, a lab-on-paper cathode photoelectrochemical (PEC) sensing platform was constructed for ultrasensitive microRNA-141 (miRNA-141) assay using cascaded multiple photo-active structures as signal generators and hemin/Pt nanoparticle (Pt NP) trunk-branching-decorated DNA dendrimers as signal reinforcers. Specifically, pyramid-like Cu₂O was first in situ grown on the Au nanoparticle-functionalized tangled cellulose fibers network, followed by the sensitization of trepang-like BiVO₄-Bi₂S₃ heterostructures, forming the cascaded sensitization structures. Then, the DNA dendrimer was introduced into the photocathode sensing interface by coupling the duplex-specific-nuclease (DSN)-induced target recycling reaction with multiple-branched hybridization chain reaction (MHCR). The programmed target recycling procedures propelled using DSN guaranteed the highly amplified transduction of miRNA-141 to the exposed initiator strand, which triggered the cascaded MHCR accompanied by the formation of the DNA dendrimer with unique trunk-branching structures. Finally, the hemin/Pt NP trunk-branching-decorated DNA dendrimer (HPTD) was acquired by the assembly of Pt NPs and hemin on the trunk and branch, respectively. The resulting HPTD with the synergy catalysis of Pt NPs and hemin could efficiently catalyze the decomposition of H₂O₂ for in situ generation of O₂ as the electron acceptor, leading to an enhanced photocurrent response. Based on the target-dependent photocurrent enhancement, ultrasensitive determination of miRNA-141 was realized with persuasive selectivity, high stability, and excellent reproducibility. Thus, the proposed paper-based cathode PEC sensing platform possessed promising application prospect in clinical miRNA diagnosis.

Electrochemical paper-based devices: sensing approaches and progress toward practical applications

Paper-based sensors offer an affordable yet powerful platform for field and point-of-care (POC) testing due to their self-pumping ability and utility for many different analytical measurements. When combined with electrochemical detection using small and portable electronics, sensitivity and selectivity of the paper devices can be improved over naked eye detection without sacrificing portability. Herein, we review how the field of electrochemical paper-based analytical devices (ePADs) has grown since it was introduced a decade ago. We start by reviewing fabrication methods relevant to ePADs with more focus given to the electrode fabrication, which is fundamental for electrochemical sensing. Multiple sensing approaches applicable to ePADs are then discussed and evaluated to present applicability, advantages and challenges associated with each approach. Recent applications of ePADs in the fields of clinical diagnostics, environmental testing, and food analysis are also presented. Finally, we discuss how the current ePAD technologies have progressed to meet the analytical and practical specifications required for field and/or POC applications, as well as challenges and outlook.

Donor/Acceptor-Induced Ratiometric Photoelectrochemical Paper Analytical Device with a Hollow Double-Hydrophilic-Walls Channel for microRNA Quantification

Integrating ratiometric photoelectrochemical (PEC) techniques with paper microfluidics to construct a ratiometric PEC paper analytical device for practical application is often restricted by the grave dependence of ratiometric assay on photoactive materials and low mass-transfer rates of the paper channel. Herein, a universal donor/acceptor-induced ratiometric PEC paper analytical device with a hollow double-hydrophilic-walls channel (HDHC) was fabricated for high-performance microRNA-141 (miRNA-141) quantification. Concretely, a photoanode and photocathode were integrated on the paper-based sensing platform in which the photocathode served as a biosensing site for the pursuit of higher selectivity. For formulation of a cascading signal amplification strategy, a unique duplex-specific nuclease-induced target recycling reaction was engineered for the output of a double amount of all useful DNA linkers instead of conventional output of only one available DNA product, which could guarantee the output of abundant DNA linkers with the initiation of a cascade of hybridization chain reaction on both the trunk and branch in the presence of miRNA-141. Then the formed dendriform polymeric DNA duplex structures were further decorated with glucose oxidase (GOx)-mimicking gold nanoparticles by the electrostatic interaction to form a branchy gold tree (BGT). Profiting from the perfect GOx-mimicking activity of BGT and high mass-transfer rates of HDHC, the cathodic photocurrent from Ag₂S/Cu₂O hybrid structure was in a "signal off" state while the anodic photocurrent from graphene quantum dots (GQDs) and Ag₂Se QDs cosensitized ZnO nanosheets was in a "signal on" state because BGT-catalyzed glucose oxidation reaction evoked the consumption of dissolved O₂ as an electron acceptor and the generation of H₂O₂ as an electron donor. With calculation of the ratio of two photocurrent intensities, the quantitative detection of miRNA-141 was achieved with high sensitivity, accuracy, and reliability.

Signal-switchable lab-on-paper photoelectrochemical aptasensing system integrated triple-helix molecular switch with charge separation and recombination regime of type-II CdTe@CdSe core-shell quantum dots

Herein, a new "on-off-on" signal switch system combined triple helix molecular switch with efficient charge separation and transfer between different sensitization units was designed for the ultrasensitive photoelectrochemical (PEC) determination of prostate-specific antigen (PSA). Concretely, the initial "signal-on" state was obtained via the cascaded sensitization structure consisting of type-II CdTe@CdSe core-shell quantum dots (QDs), CdS QDs, and ZnO nanotubes, which were assembled on Au nanoparticles modified paper fibers with the aid of signal transduction probe (STP). Thereinto, the type-II CdTe@CdSe QDs with hole-localizing core and electron-localizing shell could enable the ultrafast charge transfer and retard the charge recombination, magnifying the initial photocurrent response and preserving the high efficiency of signal-switchable PEC aptasensing system. Subsequently, the PSA aptamer (PSA-Apt) modified with gold nanoparticles (GNPs) was introduced by the hybridization of PSA-Apt with STP and the hairpin configuration of STP changed from closed to open state, forming a triple-helix structure. Hence, the CdTe@CdSe QDs labeled on the terminal of STP moved away from the electrode surface while the GNPs kept attached close to it. The proposed aptasensor turned to "signal-off" state because of the dual inhibition of vanished cosensitization effect and signal quenching effect of GNPs. Upon the target recognition, the triple-helix structure was perturbed with the formation of DNA-protein complex and the recovery of STP hairpin structure, resulting in the second "switch-on" state. Based on the target-induced photocurrent enhancement, the proposed PEC aptasensor was utilized for the determination of PSA with high sensitivity, persuasive selectivity, and excellent stability.

Dual Enhanced Electrochemiluminescence of Aminated Au@SiO2/CdS Quantum Dot Superstructures: Electromagnetic Field Enhancement and Chemical Enhancement

This paper reports dual enhanced electrochemiluminescence (ECL) of CdS quantum dot (QD)-decorated aminated Au@SiO₂ core/shell (Au@SiO₂-NH₂/CdS) superstructures. A maximum ECL emission of the Au@SiO₂-NH₂/CdS superstructures (Au core, ca. 55 nm) with a silica shell of 38 nm was 35-fold stronger than that of the counterparts (containing neither Au cores nor amino groups) with H₂O₂ as a coreactant. The fold of ECL enhancement is the largest, and the optical path of maximum ECL enhancement is the longest reported so far. The larger the Au cores in the superstructures, the stronger the ECL emission of CdS QDs was. Two types of ECL enhancement mechanisms were clearly proposed for the dual enhanced ECL of the Au@SiO₂-NH₂/CdS superstructures. One was the electromagnetic field enhancement induced by localized surface plasmon resonance of Au cores, and the other was the chemical enhancement from amino groups modified on the silica surface involved in the ECL process in the assistance of H₂O₂. It is the first time to put forward the new concept of chemical enhanced ECL that was directly related to the participation of other chemicals, which caused a decrease in the difference in the redox potential between emitters and coreactants for the increase of their redox currents. The constructed ECL platform was demonstrated to have promising applications in highly sensitive detection of glutathione (GSH), and the response mechanism of GSH was also explored.

Flexible Electronics Based on Micro/Nanostructured Paper

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

Amplified electrochemiluminescence detection of CEA based on magnetic Fe3O4@Au nanoparticles-assembled Ru@SiO2 nanocomposites combined with multiple cycling amplification strategy

In this work, we designed a new strategy for ultrasensitive detection of CEA based on efficient electrochemiluminescence (ECL) quenching of Ru(bpy)₃²⁺-doped SiO₂ nanocomposite by ferrocene using target recycling amplification technique. A large number of Ru@SiO₂ ECL signal probe were firstly assembled on the novel magnetic core-shell Fe₃O₄@Au nanoparticles (NPs), then the ferrocene-labeled ECL quenching probe (Fc-probe) was linked to the magnetic NPs. Finally, numerous DNA1 sequences were produced by target CEA-triggered multiple recycling amplification and displaced the Fc-probe on the magnetic NPs, leading to significantly enhanced ECL signal for CEA detection. Because of the designed cascade signal amplification strategy, the newly developed method achieved a wide linear range of 10 fg/mL to 10 ng/mL with a low detection limit of  3.5 fg/mL. Furthermore, taking advantages of the magnetic Fe₃O₄@Au NPs for carring abundant signal probes, sensing target and ECL detection, the developed ECL strategy is convenient, rapid and displayed high sensitivity for CEA detection, which has great potential for analyzing the clinical samples in practical disease diagnosis applications.

Stackable Lab-on-Paper Device with All-in-One Au Electrode for High-Efficiency Photoelectrochemical Cyto-Sensing

Highly conductive, robust, and multifunctional integrated paper-supported electrodes are requisite to fulfill the promise of paper-based analytical application. Herein, an all-in-one Au electrode comprising of detection zone, waterproof electronic bridge, and signal output contactor was engineered via combining the double-sided growth method with the secondary wax-printing. Benefiting from the strongly omnidirectional conductivity and desirably mechanical robustness of the as-prepared electrode, a stackable lab-on-paper cyto-device integrated with high-efficiency photoelectrochemical strategy was developed for the MCF-7 cells assay. Specifically, the detection zone of the electrode, serving as the signal generator, was functionalized with a low-toxic cosensitized structure composed of corn-like ZnO nanorods, graphene quantum dots (GQDs), and Ag₂Se QDs. With the proximity control of DNA hairpin-based aptamer probe (DHAP), a strong photocurrent could be promoted by the activated cosensitization effect and collected on the signal output contactor via the electron transport of waterproof electronic bridge. Upon the MCF-7 cells recognition, the DHAP switched from closed to open state with the formation of DNA-cell bioconjugates and the spatial separation of Ag₂Se QDs linked on the terminal of DHAP from the electrode surface. The photocurrent was noticeably decreased due to the double inhibition of steric hindrance effect and vanished cosensitization effect. Based on the target-triggered photocurrent attenuation, the sensitive detection of target cells was achieved. This work not only provided a unique method for paper-based electrode preparation but also offered a powerful platform for the highly sensitive photoelectrochemical bioanalysis.

Strong Electrochemiluminescence from MOF Accelerator Enriched Quantum Dots for Enhanced Sensing of Trace cTnI

The development of a sensitive and practical electrochemiluminescence (ECL) bioassay relies on the use of ECL signal tags whose signal intensity is high and stable. In this work, strong ECL emission was achieved from metal organic framework (MOF) accelerator enriched quantum dots (CdTe), which were applied as an efficient ECL signal tag for trace biomarker detection. It is particularly noteworthy that a novel mechanism to drastically enhance the ECL intensity of CdTe is established because isoreticular metal organic framework-3 (IRMOF-3) with 2-amino terephthalic acid (2-NH₂-BDC) as the organic ligand not only allows for loading a large amount of CdTe via the encapsulating effect and internal/external decoration but also functions as a novel coreactant accelerator for promoting the conversion of coreactant S₂O₈^2- into the sulfate radical anion (SO₄^•-), further boosting the ECL emission of CdTe. On the basis of the simple sandwich immunoreaction approach, cardiac troponin-I antigen (cTnI), a kind of biomarker related with myocardial infarction, was chosen as a detection model using an IRMOF-3-enriched CdTe labeled antibody as the signal probe. This immunosensor demonstrated desirable assay performance for cTnI with a wide response range from 1.1 fg mL^-1 to 11 ng mL^-1 and a very low detection limit (0.46 fg mL^-1). This suggested that the IRMOF-3-enriched CdTe nanocomposite strategy can integrate the coreactant accelerator and luminophore to significantly enhance the ECL intensity and stability, providing a direction for promising ECL tag preparation with broad applications.

Colorimetric and Electrochemiluminescence Dual-Mode Sensing of Lead Ion Based on Integrated Lab-on-Paper Device

A highly selective two-point separation strategy was designed based on a cross-like all-in-one lab-on-paper analytical device. The stable and cleavable enzyme-coated reduced graphene oxide (rGO)-PdAu probe was fabricated as the signal reporter to enable the visualization and electrochemiluminescence (ECL) dual-mode sensing of Pb²⁺. Concretely, the experimental workflow consists of the following process: (i) fabrication of the lab-on-paper device and growth of Au nanoparticles on ECL detection zone, (ii) immobilization of Pb²⁺-specific DNAzyme, and (iii) hybridization between DNAzyme and rGO-PdAu-glucose oxidase (GOx) labeled oligonucleotide to form the double-stranded DNA. Upon addition of Pb²⁺ into the prepared system, the double-helix structure of the DNA was destroyed, resulting in the release of cleaved rGO-PdAu-GOx probe to visualization bar to promote the effective oxidation and color change of 3,3',5,5'-tetramethylbenzidine. As a consequence, the color change can be recognized by naked eye, meanwhile GOx on an uncleaved signal probe can oxidize glucose along with the H₂O₂ production. As a co-reaction reagent for luminol ECL system, the concentration of H₂O₂ is proportional to the ECL intensity, which constitutes a new mechanism for colorimetric and ECL dual mode to detect Pb²⁺. With the method developed here, the concentration of Pb²⁺ could be easily determined by the naked eye within a linear range from 5 to 2000 nM, as well as by monitoring the decreased ECL intensity of luminol in a linear range of 0.5-2000 nM. This work not only constructs a simple and versatile platform for on-site visible monitoring of Pb²⁺ in tap water and river water but also furnishes a strategy for designing a dual-mode sensing toward different heavy metal ions based on specific DNAzyme in the fields of environmental monitoring-related technologies.

Ultrasensitive microfluidic paper-based electrochemical/visual biosensor based on spherical-like cerium dioxide catalyst for miR-21 detection

In this work, an electrochemical biosensor based on Au nanorods (NRs) modified microfluidic paper-based analytical devices (μPADs) were constructed for sensitive detection of microRNA (miRNA) by using cerium dioxide - Au@glucose oxidase (CeO₂-Au@GOx) as an electrochemical probe for signal amplification. Au NRs were synthesized by in-situ growth method in μPADs surface to enhance the conductivity and modified hairpin probe through Au-S bonds. The construction of "the signal transducer layer" was carried out by GOx catalyzing glucose to produce H₂O₂, which was further electrocatalyzed by CeO₂. After the biosensor was constructed, an obvious electrochemical signal was observed from the reduction of H₂O₂. In order to make the detection more convincing, the visual detection was performed based on the oxidation of 3,3',5,5'-tetramethylbenzidine by H₂O₂ with the help of Exonuclease I. The electrochemical biosensor provided a wide linear range of 1.0fM to 1000fM with a relatively low detection limit of 0.434fM by the electrochemical measurement. Linear range of 10fM to 1000fM with a relatively low detection limit of 7.382fM was obtained by visual detection. The results indicated the proposed platform has potential utility for detection of miRNA.

An electrochemiluminescent sensor based on functionalized conjugated polymer dots for the ultrasensitive detection of Cu2+

An ultrasensitive electrochemiluminescence (ECL) detection for Cu²⁺ was explored using the carboxyl functionalized poly(9,9-dioctylfluorenyl-2,7-diyl) (PS-COOH-co-PFO) dots as the signal label without adding any coreactant.