Copper- and Cobalt-Codoped CeO2 Nanospheres with Abundant Oxygen Vacancies as Highly Efficient Electrocatalysts for Dual-Mode Electrochemical Sensing of MicroRNA

High performance photoelectrochemical immunosensing platform based on front-illuminated Mo:BiVO4 photoelectrodes for procalcitonin assay

The outstanding photoactive materials are the imperative for the construction of a front-illuminated photoelectrochemical (PEC) biosensor, which is crucial step for improving the detection sensitivity. Yet, the weak and unstable initial PEC signals of the photoelectrodes have limited evidently the detection performance. Herein, a front-illuminated "on-off" PEC immunosensor was constructed based on Mo:BiVO4 as photoactive matrix and Au/CeO2 as signal quencher for sensitive detection of procalcitonin (PCT). Systematic studies reveal that the Mo doped BiVO4 can increase the charge carrier density of BiVO4, leading to much higher initial signal under front illumination than back illumination. Moreover, Mo:BiVO4 was directly grown on conducting substrates, which effectively overcomes the loose combination of sensing substrate ensuring good electrical contact and continuity. Upon coupling with Au/CeO2 as signal quencher, the initial photocurrent signal can be significantly quenched. As a result, the proposed PEC immunosensor presents a wide linear range from 10 fg mL-1 to 50 ng mL-1 with a detection limit of 2.45 fg mL-1. Impressively, this study will open a new avenue for the construction of highly efficient and stable photoelectrode, as well as extend the application of PEC biosensor for biomarkers detection in early disease diagnosis.

Nano-size cobalt-doped cerium oxide particles embedded into graphitic carbon nitride for enhanced electrochemical sensing of insecticide fenitrothion in environmental samples: An experimental study with the theoretical elucidation of redox events

In the present work, a nanocomposite, based on embedding Co-doped CeO2 nanoparticles into graphitic carbon nitride (g-C3N4), was applied to functionalize commercial glassy carbon paste. This is the first application of the electrochemical sensor, developed through the proposed procedure, in electrochemical sensing. The sensor was utilized for the electrochemical determination of organophosphate pesticide fenitrothion (FNT). Cyclic voltammetry identified reversible oxidation of FNT (oxidation at 0.18 V and reduction at 0.13 V) and additional reduction at -0.62 V vs. Ag/AgCl in HCl solution (pH = 1). Theoretical calculations were carried out to model and elucidate experimentally observed redox processes. Special attention was devoted to modeling experimental conditions, and based on the obtained results, a detailed redox mechanism of the investigated analyte was proposed. This represents the first complete and unambiguous elucidation of the FNT redox mechanism, supported by joined experimental and theoretical data. Square wave voltammetry (SWV) was utilized for quantification, whereby the FNT oxidation peak was chosen for monitoring the analyte concentration. The developed sensor provided a nanomolar detection limit (3.2 nmol L-1), a wide linear concentration range (from 0.01 to 13.7 μmol L-1), and good precision, repeatability, and selectivity towards FNT. Practical application possibility was explored by testing the sensor performance for examining tap water and apple samples. Recovery tests, conducted during the FNT-spiked sample assays, showed a great application capability of the developed sensor for real-time monitoring of FNT traces in environmental samples.

Polydopamine on Copper-Doped Cerium Dioxide Nanosheets as Peroxidase Mimics for the Intelligent Detection of Cholesterol

The catalytic ability of nanozymes has become an enzymology hotspot in the field of application. Most nanozymes were characterized to simultaneously have oxidase-like and peroxidase-like activities, but the practical application often focuses on certain activity; other complex activities may cause interference. The peroxidase-like activity (POD-like activity) of nanozymes have been widely used in the colorimetric detection of H2O2 or substances producing H2O2 as an intermediate, such as the detection of small biological molecules with the oxidative reaction of a chromogenic reagent in the presence of POD-like nanozymes. In this work, we used polydopamine (PDA) as the surface coating of Cu-CeO2 nanosheets (PDA@ Cu-CeO2), which enhanced peroxidase-like activity while inhibiting their oxidase-like activity, providing a feasible method for the sensitive determination of cholesterol by integrating visual colorimetric detection and a smartphone application as a readout. The absorbance intensity and RGB values displayed a linear range on cholesterol from 0.05 to 1.2 mM with the LOD (limit of detection) of 42.7 and 99.4 μM. In addition, the method is expected to apply in detecting cholesterol in human serum with acceptable accuracy.

Hollow SnO2/CdS QDs/CdCO3 heterostructured nanocubes coupled with hollow PtPd/MnCo-CeO2 nanozyme-mediated synergistic amplification for ultrasensitive PEC immunoanalysis of lung cancer biomarker

Nowadays, lung cancer is one of the most dangerous cancers threatening human life all over the world. As a crucial biomarker, cytokeratin 19 fragment 21-1 (CYFRA 21-1) is extraordinary important for diagnosis of non-small cell lung cancer (NSCLC). In this work, we synthesized hollow SnO₂/CdS QDs/CdCO₃ heterostructured nanocubes with high and stable photocurrents, which applied to construction of a sandwich-typed photoelectrochemical (PEC) immunosensor for detection of CYFRA 21-1, integrated by in-situ catalytic precipitation strategy with home-built PtPd alloy anchored MnCo-CeO₂ (PtPd/MnCo-CeO₂) nanozyme for synergistic amplification. The interfacial electron transfer mechanism upon visible-light irradiation was investigated in details. Further, the PEC responses were seriously quenched by the specific immunoreaction and precipitation catalyzed by the PtPd/MnCo-CeO₂ nanozyme. The established biosensor showed a wider linear range of 0.001-200 ng mL^-1 and a lower limit of detection (LOD = 0.2 pg mL^-1, S/N = 3), coupled by exploring such analysis even in diluted human serum sample. This work opens a constructive avenue to develop ultrasensitive PEC sensing platforms for detecting diverse cancer biomarkers in clinic.

Dual-mode biosensor platform based on synergistic effects of dual-functional hybrid nanomaterials

Detection of biomarkers is very vital in the prevention, diagnosis and treatment of diseases. However, due to the poor accuracy and sensitivity of the constructed biosensors, we are now facing great challenges. In addressing these problems, nanohybrid-based dual mode biosensors including optical-optical, optical-electrochemical and electrochemical-electrochemical have been developed to detect various biomarkers. Integrating the merits of nanomaterials with abundant active sites, synergy and excellent physicochemical properties, many bi-functional nanohybrids have been reasonable designed and controllable preparation, which applied to the construction dual mode biosensors. Despite the significant progress, further efforts are still needed to develop dual mode biosensors and ensure their practical application by using portable digital devices. Therefore, the present review summarizes an in-depth evaluation of the bi-functional nanohybrids assisted dual mode biosensing platform of biomarkers. We are hoping this review could inspire further concepts in developing novel dual mode biosensors for possible detection application.

A dual-modal electrochemical aptasensor based on intelligent DNA Walker with cascade signal amplification powered by Nb.BbvCI for Pb2

As a unique nanomachine, DNA Walker can move continuously along a specific orbit to amplify signal. Therefore, based on DNA Walker and endonuclease assisted signal amplification strategy, a novel dual-mode visual electrochemical aptasensor was constructed for the detection of Pb²⁺. Ceric dioxide@mesoporous carbon (CeO₂/CS)@AuNPs not only could improve the conductivity of sensing interface but also could fix the aptamer. DNA Walker moved on the surface of the electrode to realize the pairing with the Ag-γFe₂O₃/cDNA probe, forming a special base sequence that could be spliced by the Nb.BbvCI. Under the action of endonuclease Nb.BbvCI, the Ag-γFe₂O₃/cDNA probe was continuously sheared and the amount on the electrode was decreased to amplify the signal. Besides, the nanoenzyme of Ag-γFe₂O₃ could catalyze 3'3'5'5'-tetramethylbenzidine (TMB) to blue color realizing the visual detection of Pb²⁺. The sensor has been successfully applied to the visual and accurate rapid detection of Pb²⁺ in aquatic products. The fabricated method of the sensor open up a new way for visual and accurate the detection of environmental pollutants.

Visual/Photoelectrochemical Off-On Sensor Based on Cu/Mn Double-Doped CeO2 and Branched Sheet Embedded Cu2O/CuO Nanocubes

An integrated dual-signal bioassay was devised to fulfil thrombin (TB) ultrasensitive detection by integrating visualization with the photoelectrochemical technique based on G-quadruplex/hemin. During the process, branched sheet embedded copper-based oxides prepared with illumination and alkaline condition play a vital role in obtaining the desirable photocurrent. The switchover of photoelectrochemical signal was realized by the adjustable distance between electron acceptor G-quadruplex/hemin and interface materials due to dissociation of the Cu/Mn double-doped cerium dioxide (CuMn@CeO₂)/DNA caused by the addition of TB. Then, CuMn@CeO₂ transferred onto visual zones triggered catalytic reactions under the existence of 3,3',5,5'-tetramethylbenzidine and hydrogen peroxide, making a variation in color recognized by the naked eye and providing visual prediction. Under optimized conditions, this bioassay protocol demonstrated wide linear ranges (0.0001-50 nM), high selectivity, stability, and reproducibility. More importantly, the proposed visual/photoelectrochemical transduction mechanism platform exhibits a lower background signal and more reliable detection results, which also offers an effective way for detecting other proteins and nucleic acids.

Ultrasenstive SERS biosensor based on Zn2+ from ZnO nanoparticle assisted DNA enzyme amplification for detection of miRNA

In this work, a simple and sensitive SERS biosensor was proposed for ultrasensitive detecting miRNA 122 based on ZnO nanoparticle amplification strategy and the full utilization of DNA chain. Firstly, ZnO@S1/S2 and CoFe₂O₄@S3 complexes can flock together with the assistance of target miRNA. Accompanied with the incremental amount of miRNA, the quantity of ZnO@S1/S2 would increase. Therefore, a significant amplification capability can be obtained by converting ZnO complexes into Zn²⁺ with the assistance of HCl. In this case, the DNA chain S2 can be obtained by the ZnO dissolving. In addition, through a clever design, the obtained Zn²⁺ can be further utilized to induce DNA enzyme cycle amplification to cleave S5 into DNA chain which was similar with DNA S2. This step greatly avoided the waste of DNA chains and improved the utilization efficiency of DNA chains. The S2 and abundant S2 analogues can complement with S4 on the Raman sensing interface to imbed lots of Raman probe DOX for obtaining strong Raman signal. By this way, with the increased number of miRNA, the S2 and abundant S2 analogues would increase, so the amount of DOX would increase to produce strong Raman signal to quantitatively detect target miRNA. As a result, this SERS biosensor based on Zn⁺ amplification and high utilization efficiency of DNA chain can obtain a low detection limit of 6.82 aM and wide linear range from 10 aM to 10 pM, which shown great potential in the clinical application and medical diagnosis.

Modulating anion defect in La0.6Sr0.4Co0.8Fe0.2O3-δ for enhanced catalytic performance on peroxymonosulfate activation: Importance of hydrated electrons and metal-oxygen covalency

Perovskite oxides are promising catalysts in peroxymonosulfate (PMS) activation for wastewater treatment, attributed to their flexible structures. In this study, halogen anion (F^- or Cl^-) was doped in La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) for PMS activation, showing that appropriate anion doping enhances the catalytic performances. La_0.6Sr_0.4Co_0.8Fe_0.2O_2.75-δCl_0.25 (LSCFCl_0.25) exhibits a superior catalytic activity to pristine LSCF and La_0.6Sr_0.4Co_0.8Fe_0.2O_2.75-δF_0.25 (LSCFF_0.25), attributed to the strong surface acidity, sufficient oxygen vacancies, and improved B-site metal-oxygen bonding. The rich acidic sites favor PMS adsorption on the catalyst surface. The sufficient hydrated electrons (e_aq^-) in the oxygen vacancies participated in the generation of free radicals (SO₄^•- and O₂^•-) and singlet oxygen (¹O₂). The enlarged B-site metal-oxygen covalency could boost the electron transfer between PMS and Co^(III)/Fe^(III), and thus accelerate the redox reaction. SO₄^•- and ¹O₂ are the dominating species for the degradation. This study deepens the catalytic mechanism and uncovers the active sites of perovskite catalysts for PMS activation, providing an inspiring modification strategy to improve the catalytic performances.

Dual-readout immunosensor based on multifunctional MXene probe triggers the signal amplification for detection of autoimmune hepatitis marker

A dual-readout immunosensor coupled with electrochemical impedance and temperature signal was successfully proposed to detect autoimmune hepatitis markers (ASGPR). Nb₂C MXene with excellent conductivity, abundant surface functional groups, and extraordinary photothermal conversion efficiency, was designed to be a multifunctional biological probe, whose specific binding with antigen enhanced steric hindrance to generate electrochemical impedance signal, and at the same time, it had a strong optical response in the near-infrared band to achieve temperature output. In addition, poly(N-isopropyl acrylamide) (PNIPAM) was a temperature-sensitive polymer, which was adopted as the sensing matrix. When the multifunctional probe was specifically bound to the antigen, under 808-nm laser irradiation, the captured Nb₂C MXene achieved photothermal conversion to increase the electrode surface temperature, and the conformation of PNIPAM changed from a free spiral to a spherical shape, further realizing double amplification of the EIS signal. Under the optimized experimental conditions, the impedance values and the temperature changes increased proportionally with the increase of the ASGPR concentration from 10^-5 to 1 ng/mL, and the detection limit of the immunosensor was 3.3 × 10^-6 ng/mL. The established dual-readout immunosensor exhibited good selectivity and acceptable stability and provided an effective detection method for autoimmune hepatitis marker detection.

Recent Progresses in Electrochemical DNA Biosensors for MicroRNA Detection

MicroRNAs (miRNAs), as the small, non-coding, evolutionary conserved, and post-transcriptional gene regulators of the genome, have been highly associated with various diseases such as cancers, viral infections, and cardiovascular diseases. Several techniques have been established to detect miRNAs, including northern blotting, real-time polymerase chain reaction (RT-PCR), and fluorescent microarray platform. However, it remains a significant challenge to develop sensitive, accurate, rapid, and cost-effective methods to detect miRNAs due to their short size, high similarity, and low abundance. The electrochemical biosensors exhibit tremendous potential in miRNA detection because they satisfy feature integration, portability, mass production, short response time, and minimal sample consumption. This article reviewed the working principles and signal amplification strategies of electrochemical DNA biosensors summarized the recent improvements. With the development of DNA nanotechnology, nanomaterials and biotechnology, electrochemical DNA biosensors of high sensitivity and specificity for microRNA detection will shortly be commercially accessible.

MOF-derivated MnO@C nanocomposite with bidirectional electrocatalytic ability as signal amplification for dual-signal electrochemical sensing of cancer biomarker

Dual-signal strategy has great potential in improving the accuracy and sensitivity of cancer biomarker determination. However, most sensors based on nanomaterials as signal amplification usually output single detectable signal. It is still a challenge to achieve dual-signal sensing of biomarkers with nanomaterials as signal amplification. Herein, MnO@C nanocomposite was prepared with Mn-MOF-74 as precursor by pyrolysis. It possesses bidirectional electrocatalytic ability toward both oxidation and reduction of H₂O₂ for its fully exposed crystal facets. After loading AuNPs, MnO@C@AuNPs can connect aptamer (Apt) via Au-S and then as a signal amplification for the construction of sandwich-type aptasensor for dual-signal electrochemical sensing of cancer biomarker. Thus, taking mucin 1 (MUC1) as a model system. The aptasensor has the parallel output of differential pulse voltammetry (DPV) and chronoamperometry responses based on oxidation and reduction of H₂O₂, respectively, which implemented sensitive and accurate measurements to avoid false results. The linear response ranges of 0.001 nM-100 nM (detection limit of 0.31 pM) for DPV technique and 0.001 nM-10 nM (detection limit of 0.25 pM) for chronoamperometry technique were obtained. It opens up a new way to design elegant dual-signal aptasensors with potential applications in early disease diagnosis.

A Photoresponsive and Metal-Organic Framework Encapsulated DNA Tetrahedral Entropy-Driven Amplifier for High-Performance Imaging Intracellular MicroRNA

The further development of high-performance fluorescent biosensors to image intracellular microRNAs is beneficial to cancer medicine. By virtue of the need for enzymes and hairpin DNA probes, the entropy-driven reaction-assisted signal amplification strategy has shown an enormous potential to accomplish this task. Nevertheless, this good option still meets with poor biostability, low cell uptake efficiency, and unsatisfactory accuracy. On the basis of these challenges, we put forward here a battery of solving pathways. First, the straight DNA probes are anchored onto the vertexes of dual DNA tetrahedrons, and thus the enzyme resistance of the whole sensing system is observably enhanced. A metal-organic framework (ZIF-8 nanoparticle), which can be effectively dissociated into a weakly acidic environment, then is employed as an additional delivery vehicle to encapsulate such a DNA tetrahedron sustained biosensor and finally bring about a more efficient endocytosis. Last, a kind of photocleavage-linker triggered photoresponsive manner is incorporated to achieve an exceptional precise target identification, by which the biosensor can only be initiated under the irradiation of an externally mild 365 nm ultraviolet light source. In accordance with the above efforts, worthy assay performance toward microRNA-196a has given rise to this newly constructed biosensor, whose sensitivity is down to 2.7 pM and also able to distinguish single-base variation. Beyond that, the amplifier can work as a powerful imaging toolbox to accurately determine the targets in living cells, providing a promising intracellular sensing platform.

A Comprehensive Assessment of Catalytic Performances of Mn2O3 Nanoparticles for Peroxymonosulfate Activation during Bisphenol A Degradation

Catalytic performances of Mn2O3 nanoparticles for peroxymonosulfate (PMS) activation in bisphenol A (BPA) degradation were comprehensively investigated in this study. Experimental results showed that 10 mg/L BPA could be 100% degraded within 20 min with the dosages of 0.2 g/L Mn2O3 and 0.1 mM PMS. Moreover, Mn2O3 showed remarkable activity in activation of PMS and excellent adaptability in various real water matrices, including river water, tap water and secondary effluents. Based on the radical detection and scavenging experiments, it was found that both radical and non-radical oxidation contributed to the degradation of BPA and 1O2 was the dominant species in the degradation compared to •OH, SO4•− and O2•−. A total of 15 transformation products were identified by LC/MS-MS during BPA degradation in the Mn2O3/PMS system, and degradation pathways via three routes are proposed. Compared with lab-made catalysts reported in the literature, the Mn2O3 catalyst demonstrated its superiority in terms of its high TOC removal, low PMS consumption and fast degradation rate for BPA.

Enhanced Catalytic Activity Induced by the Nanostructuring Effect in Pd Decoration onto Doped Ceria Enabling an Origami Paper Analytical Device for High Performance of Amyloid-β Bioassay

In this work, we fabricated a novel origami paper-based analytical device (oPAD) assisted by the nanostructuring effect of in situ Pd decoration of Cu/Co-doped CeO₂ (CuCo-CeO₂-Pd) nanospheres, functionalized with their strongly enhanced electrocatalytic properties to realize an electrochemical and visual signal readout system in oPAD, for highly sensitive detection of amyloid-β (Aβ). The CuCo-CeO₂-Pd nanospheres were introduced as an enhanced "signal transducer layer" on account of the electron transfer acceleration caused by catalyzing glucose to produce H₂O₂ for differential pulse voltammetry signal readout and further 3,3'5,5'-tetramethylbenzidine (TMB) oxidation for colorimetric analysis. Meanwhile, for achieving superior performance of the proposed oPAD, in situ growth of urchin-like gold nanoparticles (Au NPs) onto cellulose fibers was adopted to improve "the recognition layer" in favor of immobilizing antibodies for targeting Aβ through specific antigen-antibody interactions. Combined with the delicate design of oPAD, exhibiting actuation of the conversion procedure between hydrophobicity and hydrophilicity on paper tabs in the assay process, the oPAD successfully enabled sensitive diagnosis of Aβ in a linear range from 1.0 pM to 100 nM with a limit of detection of 0.05 pM (S/N = 3) for electrochemical detection, providing a reliable strategy for quantifying the Aβ protein in clinical applications.

Dual-Mode SERS and Electrochemical Detection of miRNA Based on Popcorn-like Gold Nanofilms and Toehold-Mediated Strand Displacement Amplification Reaction

MicroRNA (miRNA) has emerged as one of the ideal target biomarker analytes for cancer detection because its abnormal expression is closely related to the occurrence of many cancers. In this work, we combined three-dimensional (3D) popcorn-like gold nanofilms as novel surface-enhanced Raman scattering (SERS)-electrochemistry active substrates with toehold-mediated strand displacement reactions (TSDRs) to construct a DNA molecular machine for SERS-electrochemistry dual-mode detection of miRNA. 3D popcorn-like spatial structures generated more active "hot spots" and thus enhanced the sensitivity of SERS and electrochemical signals. Besides, the TSDRs showed high sequence-dependence and high specificity. The addition of target miRNA will trigger the molecular machine to perform two TSDRs in the presence of signal DNA strands modified by R6G (R6G-DNA), thus achieving an enzyme-free amplification detection of miRNA with a low limit of detection of 0.12 fM (for the SERS method) and 2.2 fM (for the electrochemical method). This biosensor can also serve as a universally amplified and sensitive detection platform for monitoring different biomarkers, such as cancer-related DNA, messenger RNA, or miRNA molecules, with high selectivity by changing the corresponding probe sequence.

Electrons in Oxygen Vacancies and Oxygen Atoms Activated by Ce3+/Ce4+ Promote High-Sensitive Electrochemical Detection of Pb(II) over Ce-Doped α-MoO3 Catalysts

Modulating the active sites of oxygen vacancies (OVs) to enhance the catalytic properties of nanomaterials has attracted much research interest in various fields, but its intrinsic catalytic mechanism is always neglected. Herein, we establish an efficient strategy to promote the electrochemical detection of Pb(II) by regulating the concentration of OVs in α-MoO₃ nanorods via doping Ce³⁺/Ce⁴⁺ ions. α-MoO₃ with the Ce-doped content of 9% (C9M) exhibited the highest detection sensitivity of 106.64 μM μA^-1 for Pb(II), which is higher than that achieved by other metal oxides and most precious metal nanomaterials. It is found that C9M possessed the highest concentration of OVs, which trapped some electrons for strong affinity interaction with Pb(II) and provided numerous atomic level interfaces of high surface free energy for catalysis reactions. X-ray absorption fine structure spectra and density functional theory calculation indicate that Pb(II) was bonded with the surface-activated oxygen atoms (Os) around Ce ions and obtained some electrons from Os. Besides, the longer Pb-O bonds on C9M were easier to break, causing a low desorption energy barrier to effectively accelerate Pb(II) desorbing to the electrode surface. This study helps to understand the changes in electronic structure and catalytic performance with heteroatom doping and OVs in chemically inert oxides and provide a reference for designing high-active electrocatalytic interfaces to realize ultrasensitive analysis of environmental contaminants.

Selective recognition of CdTe QDs and strand displacement signal amplification-assisted label-free and homogeneous fluorescence assay of nucleic acid and protein

Due to their simplicity of design and operation, homogeneous bioassays have been of great interest to researchers. Herein, a label-free and free separation fluorescence sensing platform was constructed for the determination of nucleic acid and prostate specific antigen (PSA) using CdTe QDs as the signal molecule. In our previous work, we surprisingly found that the CdTe QDs can selectively distinguish Ag+ and the C-Ag+-C complex, which was the basis of the sensor. On the basis of the selective cation exchange reaction (CER), combined with the signal amplification of the strand displacement reaction (SDR), this work was first applied for the sensitive analysis of DNA. There are two types of hairpin structures in this sensing system, including the recognition probe (HP) and Ag+, which formed the C-Ag+-C structure, and the hairpin structure formed by the helper DNA itself. In this work, target DNA can trigger the SDR that generates lots of HP-helper double-stranded DNA (dsDNA) and recycles the target DNA while releasing a large amount of Ag+, thus quenching the fluorescence signal of CdTe QDs to achieve the highly sensitive detection of DNA. In order to verify the versatility of this system using DNA as a bridge and aptamers as recognition probes, we extended the system to the detection of PSA. After examining its experimental performance, it was determined that this method displayed good analytical capability for DNA in the range of 10-13-10-10 M and PSA in the range of 10-13-10-10 g mL-1 with low 25 fM and 30 fg mL-1 limits of detection (LODs), respectively; high selectivity for both the target sequence and protein was shown. In addition, this platform was successfully used for the analysis of PSA in serum samples.

New trends in the development of electrochemical biosensors for the quantification of microRNAs

MicroRNAs (miRNAs) are non-coding regulatory RNAs that play an important role in RNA silencing and post-transcriptional gene expression regulation. Since their dysregulation has been associated with Alzheimer disease, cardiovascular diseases and different types of cancer, among others, miRNAs can be used as biomarkers for early diagnosis and prognosis of these diseases. The methods commonly used to quantify miRNAs are, in general, complex, costly, with limited application for point-of-care devices or resource-limited facilities. Electrochemical biosensors, mainly those based on nanomaterials, have emerged as a promising alternative to the conventional miRNA detection methods and have paved the way to the development of sensitive, fast, and low-cost detection systems. This review is focused on the most relevant contributions performed in the field of electrochemical miRNAs biosensors between 2017 and the beginning of 2020. The main contribution of this article is the critical discussion of the different amplification strategies and the comparative analysis between amplified and non-amplified miRNA electrochemical biosensing and between the different amplification schemes. Particular emphasis was given to the importance of the nanostructures, enzymes, labelling molecules, and special sequences of nucleic acids or analogues on the organization of the different bioanalytical platforms, the transduction of the hybridization event and the generation the analytical signal.

Efficient Fenton-like Process for Pollutant Removal in Electron-Rich/Poor Reaction Sites Induced by Surface Oxygen Vacancy over Cobalt-Zinc Oxides

To achieve high efficiency and low consumption for water treatment in the Fenton reaction, we use the surface oxygen vacancies (OVs) as the electron temporary residences to construct a dual-reaction-center (RDC) Fenton-like catalyst with abundant surface electron-rich/poor areas consisting of OV-rich Co-ZnO microparticles (OV-CoZnO MPs). The lattice-doping of Co into ZnO wurtzite results in the formation of OVs with unpaired electrons (electron-rich OVs) and electron-deficient Co³⁺ sites according to the structural and electronic characterizations. Both experimental and theoretical calculations prove that the electron-rich OVs are responsible for the capture and reduction of H₂O₂ to generate hydroxyl radicals, which quickly degrades pollutants, while a large amount of pollutants are adsorbed at the electron-deficient Co³⁺ sites and act as electron donors for the system, accompanied by their own oxidative degradation. The electrons obtained from the pollutants in the electron-deficient sites are transferred to the OVs through the internal bond bridge to achieve the balance of electron gain/loss. Through this process, pollutants are efficiently converted and degraded by multiple pathways in a wide range of pH (4.5-9.5). The reaction rate of the OV-CoZnO MPs/H₂O₂ system is increased by ∼17 times compared with the non-DRC system. This discovery provides a sustainable strategy for pollutant utilization, which shows new implications for solving the troublesome issues of the Fenton reaction and for developing novel environmental remediation technologies.

Ultrasensitive Photoelectrochemical Detection of MicroRNA on Paper by Combining a Cascade Nanozyme-Engineered Biocatalytic Precipitation Reaction and Target-Triggerable DNA Motor

Developing efficient strategies for sensitive detection of microRNAs, the noncoding bioactive molecules and well-established biomarkers, has aroused great interests due to its great potential values in genetic and pathological analyses. Herein, a highly selective and disposable paper-based photoelectrochemical (PEC) sensor was rationally designed for sensing microRNA based on simple self-assembly of a target-triggerable DNA motor and nanozyme-catalyzed multistage biocatalytic precipitation reaction. Specifically, a brand-new type II heterojunction of TiO₂-CeO₂ nanotubes decorated with carbon fiber paper (CFP) was first prepared, which gave an enhanced photoreactive surface and realized fast electron transport and extraction, markedly accelerating photoelectric conversion efficiency of the sensor. For achieving target detection, cascade nanozyme centers of the CeO₂ and Au nanoparticles modified by cyclodextrin were drafted, greatly decreasing the photocurrent intensity and achieving an ultralow background signal. With target introduction, the DNA motor was activated and automatically moved along the predesigned route driven by an endonuclease cleavage reaction, resulting in more substrate probe digestion and nanozyme release from CFP. Consequently, the repressive inner enhancement mechanism was gradually renewed with constant advancement of the enzymatic reaction and walker probe walking progressively, eventually allowing multiple enzymatic factor output in each target import. As a proof-of-concept application, the developed PEC sensor successfully performed detection of miRNA-141, showing a low detection limit of 0.6 fM, and was further applied to real sample bioassays with satisfying results. This work proposes promising strategies to boost the catalytic cascade DNA-motor adhibition in biological samples analysis and also exhibits potential capability in detection of other targets.

Rapid and sensitive electrochemical detection of microRNAs by gold nanoparticle-catalyzed silver enhancement

Using the proposed method the microRNA assay was successfully carried out in less than 70 min and the detection limit was as low as 15 fM.

Rapid evaluation of oxygen vacancies-enhanced photogeneration of the superoxide radical in nano-TiO2 suspensions

Reactive oxygen species (ROS) play an important role in the photocatalytic degradation of pollutants and are closely related to the surface defects of a semiconductor.

Avenues Toward microRNA Detection In Vitro: A Review of Technical Advances and Challenges

Over the decades, the biological role of microRNAs (miRNAs) in the post-transcriptional regulation of gene expression has been discovered in many cancer types, thus initiating the tremendous expectation of their application as biomarkers in the diagnosis, prognosis, and treatment of cancer. Hence, the development of efficient miRNA detection methods in vitro is in high demand. Extensive efforts have been made based on the intrinsic properties of miRNAs, such as low expression levels, high sequence homology, and short length, to develop novel in vitro miRNA detection methods with high accuracy, low cost, practicality, and multiplexity at point-of-care settings. In this review, we mainly summarized the newly developed in vitro miRNA detection methods classified by three key elements, including biological recognition elements, additional micro-/nano-materials and signal transduction/readout elements, their current challenges and further applications are also discussed.