Internal Light Source-Driven Photoelectrochemical 3D-rGO/Cellulose Device Based on Cascade DNA Amplification Strategy Integrating Target Analog Chain and DNA Mimic Enzyme

A novel conductive nanocomposite-based biosensor for ultrasensitive detection of microRNA-21 in serum, using methylene blue as mediator

MicroRNA-21 (miRNA-21) is a common biomarker with high expression in breast tumors. Therefore, sensitive detection of miRNA-21 is of great significance for clinical breast tumor diagnosis. A TH/rGO/CMK-3/AuNPs nanocomposite is composed of thionine (TH), reduced graphene oxide (rGO), ordered mesoporous carbon (CMK-3), and gold nanoparticles (AuNPs), which help to increase the specific surface area of a glassy carbon electrode (GCE) and to amplify the DPV signal. Meanwhile, methylene blue (MB) was combined with the capture probe guanine and absorbed by the composite material to mediate the differential pulse voltammetry (DPV) of the obtained miRNA biosensor. The current response decreased with increasing miRNA-21 concentration under optimal conditions. The biosensor responds to miRNA-21 in the 0.1fM-1 pM concentration range, and the detection limit (LOD) was 0.046 fM. Moreover, human serum samples were effectively detected utilizing the miRNA-21 biosensor with satisfactory results.

Recent Advances of Nanostructured Materials for Photoelectrochemical Bioanalysis

Nowadays, the emerging photoelectrochemical (PEC) bioanalysis has drawn intensive interest due to its numerous merits. As one of its core elements, functional nanostructured materials play a crucial role during the construction of PEC biosensors, which can not only be employed as transducers but also act as signal probes. Although both chemical composition and morphology control of nanostructured materials contribute to the excellent analytical performance of PEC bioassay, surveys addressing nanostructures with different dimensionality have rarely been reported. In this review, according to classification based on dimensionality, zero-dimensional, one-dimensional, two-dimensional, and three-dimensional nanostructures used in PEC bioanalysis are evaluated, with an emphasis on the effect of morphology on the detection performances. Furthermore, using the illustration of recent works, related novel PEC biosensing patterns with promising applications are also discussed. Finally, the current challenges and some future perspectives in this field are addressed based on our opinions.

Enzyme-Free Molecularly Imprinted and Graphene-Functionalized Photoelectrochemical Sensor Platform for Pollutants

In this work, a label-free nonenzymatic photoelectrochemical (PEC) sensor is successfully developed for the detection of a typical pollutant, microcystin-LR (MC-LR), based on a visible-light-responsive alloy oxide, with highly ordered and vertically aligned Ti-Fe-O nanotubes (NTs) as substrates. Ti-Fe-O NTs consisting mainly of TiO₂ and atomically doped Fe₂O₃ are in situ prepared on a Ti-Fe alloy by electrochemical anodic oxidation. Using a simple electrochemical deposition technique, reduced graphene oxide (RGO) could be grown onto Ti-Fe-O NTs, exhibiting significant bifunctions. It not only provides an ideal microenvironment for functionalization of molecularly imprinted polymers (MIPs) on the surface but also serves as the PEC signal amplification element because of its outstanding conductivity for photons and electrons. The designed MIP/RGO/Ti-Fe-O NT PEC sensor exhibits high sensitivity toward MC-LR with a limit of detection as low as 10 pM. High selectivity toward MC-LR is also proven for the sensor. A promising detection platform not only for MC-LR but also for other pollutants has therefore been provided.

Hybridization chain reaction and its applications in biosensing

To pursue the sensitive and efficient detection of informative biomolecules for bioanalysis and disease diagnosis, a series of signal amplification techniques have been put forward. Among them, hybridization chain reaction (HCR) is an isothermal and enzyme-free process where the cascade reaction of hybridization events is initiated by a target analyte, yielding a long nicked dsDNA molecule analogous to alternating copolymers. Compared with conventional polymerase chain reaction (PCR) that can proceed only with the aid of polymerases and complicated thermal cycling, HCR has attracted increasing attention because it can occur under mild conditions without using enzymes. As a powerful signal amplification tool, HCR has been employed to construct various simple, sensitive and economic biosensors for detecting nucleic acids, small molecules, cells, and proteins. Moreover, HCR has also been applied to assemble complex nanostructures, some of which even act as the carriers to execute the targeted delivery of anticancer drugs. Recently, HCR has engendered tremendous progress in RNA imaging applications, which can not only achieve endogenous RNA imaging in living cells or even living animals but also implement imaging-guided photodynamic therapy, paving a promising path to promote the development of theranostics. In this review, we begin with the fundamentals of HCR and then focus on summarizing the recent advances in HCR-based biosensors for biosensing and RNA imaging strategies. Further, the challenges and future perspective of HCR-based signal amplification in biosensing and theranostic application are discussed.

Advanced Signal-Amplification Strategies for Paper-Based Analytical Devices: A Comprehensive Review

Paper-based analytical devices (PADs) have emerged as a promising approach to point-of-care (POC) detection applications in biomedical and clinical diagnosis owing to their advantages, including cost-effectiveness, ease of use, and rapid responses as well as for being equipment-free, disposable, and user-friendly. However, the overall sensitivity of PADs still remains weak, posing a challenge for biosensing scientists exploiting them in clinical applications. This review comprehensively summarizes the current applicable potential of PADs, focusing on total signal-amplification strategies that have been applied widely in PADs involving colorimetry, luminescence, surface-enhanced Raman scattering, photoacoustic, photothermal, and photoelectrochemical methods as well as nucleic acid-mediated PAD modifications. The advances in signal-amplification strategies in terms of signal-enhancing principles, sensitivity, and time reactions are discussed in detail to provide an overview of these approaches to using PADs in biosensing applications. Furthermore, a comparison of these methods summarizes the potential for scientists to develop superior PADs. This review serves as a useful inside look at the current progress and prospective directions in using PADs for clinical diagnostics and provides a better source of reference for further investigations, as well as innovations, in the POC diagnostics field.

Battery-free fully integrated microfluidic light source for portable lab-on-a-chip applications

Integrating a light source inside a Lab-on-a-Chip (LOC) platform has always been as challenging as much as an appealing task. Besides the manufacturing issues, one of the most limiting aspects is due to the need for an energy source to feed the light emission. A solution independent of external energy sources can be given by Chemiluminescence (CL): a well-known chemical phenomenon in which light emission is achieved because of a chemical reaction. Here we present the fabrication and the characterization of a chemiluminescent light source, fully integrated on a microfluidic platform by means of the direct writing technique known as Femtosecond Laser Micromachining. The key advantage is the possibility to insert within LOC devices light sources with complete placement freedom in 3D, wide flexibility of the emitting source geometry and no external feeding energy. The characterization is carried out by investigating the effect of confining a chemiluminescent rubrene-based reaction in small volumes and the inject pressures impact on the emission spectra. Moreover, exploiting microfluidics principles, it's possible to move from the typical flash-type CL emission to a prolonged one (several hours). This allows to disengage bulky, external light sources, adding an extra step on the road to real device portability.

Advances in functional nucleic acid based paper sensors

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

A self-powered origami paper analytical device with a pop-up structure for dual-mode electrochemical sensing of ATP assisted by glucose oxidase-triggered reaction

A self-powered origami paper-based analytical device (oPAD), being with a pop-up structure as mechanical valve to first realize dual-mode of differential pulse voltammery (DPV)/supercapacitor amplified signal read out systems, was designed for detecting adenosine 5'-triphosphate (ATP) assisted by glucose oxidase (GOx)-triggered reaction. In order to accommodate the alternative step for dual-mode detection, a pop-up structure inspired by pop-up greeting cards was developed, making it possible to change the fluidic path with good registration and repeatability. To realize supercapacitor detection mode, a sandwich structure of a DNA sequence (DNA1), aptamer and a DNA sequence modified with GOx (GOx-DNA2) was formed on detection zone by hybridization reaction. With the addition of ATP, the GOx-DNA2 could be released with the specific binding between ATP and aptamer, and flowed into the reaction zone to catalyze the oxidation of glucose. Due to the difference in concentrations of [Fe(CN)₆]^3- and [Fe(CN)₆]^4- caused by the GOx-triggered reaction, a voltage could be produced to charge a paper supercapacitor which could provide a high instantaneous current with a digital multimeter to transduce the result of the assay, and realize the self-generation of an amplified electrical signal. By simply varying the direction of pop-up structure, the electrochemical signal from DPV read out mode could be achieved through catalytic oxidation of glucose by the remaining GOx-DNA2 on the detection zone. The proposed self-powered oPAD enabled the sensitive diagnosis of ATP in a linear range of 10-5000 nM with a limit of detection of 3 nM and 1.4 nM, respectively.

Microfluidic paper-based photoelectrochemical sensing platform with electron-transfer tunneling distance regulation strategy for thrombin detection

This work reports a microfluidic paper-based photoelectrochemical (μ-PEC) sensing platform for thrombin (TB) detection with electron-transfer tunneling distance regulation (ETTDR) and aptamer target-triggering nicking enzyme signaling amplification (NESA) dual strategies. Specifically, paper-based TiO₂ nanosheets (PTNs) were prepared with an efficient hydrothermal process, serving as the direct pathway for the charge carriers transfer. When CeO₂-labeled hairpin DNA 3 (HP3) was closely located at the PTNs, the CeO₂-PTNs heterostructure was formed, which could great facilitate the photogenerated carries separation of CeO₂. In addition, with the aid of aptamer target-triggering NESA strategy, the input TB could be transducted to numerous output target of DNA (tDNA), achieving the goal of desirable signal amplification. In the presence of TB, the output tDNA could be further hybridized with HP3 and unfold its hairpin loop, which forced the CeO₂ away from the surface of PTNs and vanished the CeO₂-PTNs heterostructure, resulting in the obviously reducing of photocurrent signal. The as-designed sensing platform exhibited a linear range from 0.02 pM to 100 pM with a detection limit of 6.7 fM. Importantly, this μ-PEC sensing platform could not only realize the highly efficient TB detection, but also pave a luciferous way for the detection of other protein in bioanalysis.

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.

Ultrasensitive Photoelectrochemical Assay with PTB7-Th/CdTe Quantum Dots Sensitized Structure as Signal Tag and Benzo-4-chlorohexadienone Precipitate as Efficient Quencher

Herein, an ultrasensitive photoelectrochemical (PEC) assay was developed for the monitoring of microRNA-141 (miRNA-141) based on poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2- b:4,5- b']dithiophene-2,6-diyl- alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophene-4,6-diyl}/CdTe quantum dots ( PTB7-Th/CdTe QDs) sensitized structure as signal tag and the benzo-4-chlorohexadienone (4-CD) precipitate as efficient quencher. PTB7-Th and CdTe QDs were successively modified on the electrode surface to form a novel sensitized structure with eminent photovoltaic performances and surpassing film-forming ability. The PTB7-Th/CdTe QDs sensitized structure served as signal tag for achieving a strong initial PEC signal. Besides, a tiny amount of target (miRNA-141) could be transformed into numerous DNA products via enzyme-assisted target cycling procedure, which further triggered the formation of a DNA supersandwich structure on the electrode surface for loading abundant manganese porphyrin (MnPP). Thereafter, MnPP as mimetic enzyme could catalyze 4-chloro-1-naphthol (4-CN) to generate 4-CD precipitate on the sensing interface in the presence of H₂O₂, which could efficiently block electron transfer, leading to a significantly quenched PEC signal for determination of miRNA-141. The designed PEC biosensor performed a wide detection range from 0.1 fM to 1 nM with a low detection limit of 33 aM for miRNA-141, which paved a new avenue for highly accurate and ultrasensitive monitoring of multifarious analytes in bioanalysis and clinical diagnosis.

A chemiluminescence aptasensor for thrombin detection based on aptamer-conjugated and hemin/G-quadruplex DNAzyme signal-amplified carbon fiber composite

In this work, a highly sensitive and selective chemiluminescence (CL) aptasensor was prepared for thrombin (THR) detection based on aptamer-conjugated and hemin/G-quadruplex DNAzyme signal-amplified carbon fiber composite (HG-DNAzyme/T-Apt/SiO₂@GO@CF). Initially, SiO₂@GO@CF was successfully prepared and characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FT-IR). Thrombin aptamer (T-Apt) as an identification element and simulated enzyme - hemin/G-quadruplex DNAzyme (HG-DNAzyme) as a signal-amplified material, were applied in the CL aptasensor. Then, the immobilization properties of SiO₂@GO@CF and adsorption properties of T-Apt/SiO₂@GO@CF were studied. Lastly, HG-DNAzyme/T-Apt/SiO₂@GO@CF was applied in construction of the CL aptasensor. When THR existed, HG-DNAzyme was desorbed from the surface of T-Apt/SiO₂@GO@CF and catalyzed the CL system of luminol-H₂O₂. Under optimized CL conditions, THR was measured with the linear concentration range of 1.5 × 10^-14 to 2.5 × 10^-11 moL/L and the detection limit of 6.3 × 10^-15 moL/L (3δ). The proposed CL aptasensor was used to the determination of THR in human serum samples and recoveries ranged from 99.0% to 102.4%. Those satisfactory results illustrated the CL aptasensor could achieve highly sensitive and selective detection of THR and revealed potential application in practical samples.

A novel electrochemical cytosensor for selective and highly sensitive detection of cancer cells using binding-induced dual catalytic hairpin assembly

Rare cancer cells in body fluid could be useful biomarkers for noninvasive diagnosis of cancer. However, detection of these rare cells is currently challenging. In this work, a binding-induced dual catalytic hairpin assembly (DCHA) electrochemical cytosensor was developed for highly selective and sensitive detection of cancer cells. The fuel probe, released by hybridization between the capture probe and catalytic hairpin assembly (CHA) products of target cell-responsive reaction, initiated dual CHA recycling, leading to multiple CHA products. Furthermore, the hybridization between fuel probe and capture probe decreased non-specific CHA products, improving the signal-to-noise ratio and detection sensitivity. Under the optimal conditions, the developed cytosensor was able to detect cells down to 30 cells mL^-1 (S/N = 3) with a linear range from 50 to 100,000 cells mL^-1 and was capable of distinguishing target cells from normal cells in clinical blood samples.