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Abstract: In this work, the magnetic core–shell Fe3O4@Au nanoparticles attached to the surface of a magnetic glassy carbon electrode (MGCE) were applied to the immobilization/adsorption of myoglobin (Mb) for fabricating Mb/Fe3O4@Au biofilm. The morphology, structure, and electrochemistry of the nanocomposite were characterized by transmission electron microscope, UV–vis spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry, respectively. The resultant Fe3O4@Au NPs not only have the magnetism of Fe3O4 NPs that make them easily manipulated by an external magnetic field, but also have the good conductivity and excellent biocompatibility of Au layer which can maintain the bioactivity and facilitate the direct electrochemistry of Mb in the biofilm. The modified electrode based on this Mb/Fe3O4@Au biofilm displayed good electrocatalytic activity to the reduction of H2O2 with a linear range from 1.28 to 283μM. The proposed method simplified the immobilization methodology of proteins and showed potential application for fabricating novel biosensors and bioelectronic devices. [Copyright &y& Elsevier]

Abstract: A novel experimental methodology based on the unique characteristics of chitosan-branched ferrocene (CS-Fc) and gold nanoparticles (Au NPs) was developed to design a label-free amperometric immunosensor for the sensitive detection of hepatitis B surface antigen (HBsAg) as a model protein. The controllable electrodeposition of CS-Fc solution formed a three-dimensional robust film with good biocompatibility and large surface area for the assembly of Au NPs and further immobilization of hepatitis B surface antibody (HBsAb) on an electrode. The morphologies and electrochemistry of the formed nanocomposite biofilm were investigated by using scanning electron microscopy and electrochemical techniques including cyclic voltammetry and electrochemical impedance spectroscopy, respectively. The HBsAg concentration was measured through the decrease of amperometric responses in the corresponding specific binding of antigen and antibody. The decreased differential pulse voltametric values were proportional to the HBsAg concentration in the range of 0.05–305ngmL−1 with a detection limit 0.016ng. This would provide an approach for the application of mediator in immunoassays. [Copyright &y& Elsevier]

Abstract: A simple and controllable electrodeposition method is described to fabricate a homogeneous chitosan-ferrocene/Au nanoparticles/glucose oxidase (CS-Fc/Au NPs/GOx) nanocomposite film. The morphologies and electrochemistry of the nanocomposite film were investigated by using scanning electron microscopy (SEM) and electrochemical techniques including electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), respectively. The ferrocene group (Fc) covalently bounded chitosan hybrid (CS-Fc) can effectively avoid the leakage of Fc and retain its electrochemical activity. Further immobilization of Au NPs into the CS-Fc matrix enhances both the charge-transport properties of the composite and bioaffinity to enzyme. Biosensor based on this CS-Fc/Au NPs/GOx film has advantages of fast response, excellent reproducibility and high stability. This biosensor shows a linear response to glucose in the concentration range from 0.02 to 8.66mM with a detection limit of 5.6μM at a signal-to-noise ratio of 3. The present method offers a facile way to fabricate biosensors and bioelectronic devices. [Copyright &y& Elsevier]

Abstract: A novel dopamine sensor was fabricated by forming the 6-ferrocenylhexanethiol (HS(CH2)6Fc) functionalized Fe3O4@Au nanoparticles (NPs) films on the surface of a carbon paste electrode with the aid of a permanent magnet. HS(CH2)6Fc, which acted as the redox mediator, was self-assembled to Fe3O4@Au NPs via Au–S bond. Transmission electron microscopy, UV–visible absorption spectroscopy, Fourier transform infrared spectra, and cyclic voltammetry were used to characterize the properties of the Fe3O4@Au NPs/HS(CH2)6Fc nanocomposite. It is shown that the prepared ferrocene-functionalized Fe3O4@Au NPs composite shuttled electrons between analyte and electrode, increased the mediator loading, and more importantly prevented the leakage of the mediator during measurements, which resulted in the substantially enhanced stability and reproducibility of the modified electrode. The electrooxidation of dopamine could be catalyzed by Fc/Fc+ couple as a mediator and had a higher electrochemical response due to the unique performance of Fe3O4@Au NPs. The nanocomposite modified electrode exhibited fast response (3s) and the linear range was from 2.0×10−6 to 9.2×10−4 M with a detection limit of 0.64μM. This immobilization approach effectively improved the stability of the electron transfer mediator and is promising for construction of other sensors and bioelectronic devices. [Copyright &y& Elsevier]

Abstract: In this study, magnetic core–shell Fe3O4@Al2O3 nanoparticles (NPs) attached to the surface of a magnetic glassy carbon electrode (MGCE) were used as a functional interface to immobilize several heme proteins including hemoglobin (Hb), myoglobin (Mb) and horseradish peroxidase (HRP) for fabricating protein/Fe3O4@Al2O3 film. Transmission electron microscope, UV–vis spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry were used to characterize the films. With the advantages of the magnetism and the excellent biocompatibility of the Fe3O4@Al2O3 NPs, the protein/Fe3O4@Al2O3 film could be easily fabricated in the present of external magnetic field, and well retained the bioactivity of the immobilized proteins, hence dramatically facilitated direct electron transfer of heme proteins and excellent electrocatalytic behaviors towards H2O2 were demonstrated. The presented system avoids the complex synthesis for protecting Fe3O4 NPs, supplies a facile, low cost and universal way to immobilize proteins, and is promising for construction of third-generation biosensors and other bio-magnetic induction devices. [ABSTRACT FROM AUTHOR]

We propose a ratiometric electrochemical assay for detecting microRNA (miRNA) on the basis of dual-amplification mechanism by using distinguishable electrochemical signals from thionine (Thi) and ferrocene (Fc). The thiol-modified and ferrocene-labeled hairpin capture probes (CP) are first immobilized on an Au electrode via Au-S reaction. The target miRNA hybridizes with CP and unfolding the hairpin structure of CP to form miRNA-DNA duplexes. Then, kamchatka crab duplex specific nuclease (DSN) specifically cleaves the DNA in miRNA-DNA duplexes, leading to the release of miRNA and another cleaves cycle, meanwhile, numerous Fc leaves away from the electrode surface and leads to the signal-off of Fc. The residual fragment on electrode surface acts as a HCR primer to form dsDNA polymers through in situ HCR with the presence of the primer and two probes (HDNA and HDNA’), resulting in the capture of numerous DNA/Au NPs/Thi and the signal-on of Thi. The dual-amplification mechanism significantly amplifies the decrease of Fc signal and the increase of Thi signal for ratiometric readout (I Thi /I Fc ), thus providing a sensitive method for the selective detection of miR-141 with a detection limit down to 11 aM. The dual-signal ratiometric outputs have an intrinsic self-calibration to the effects from system, which is promising to be applied in biosensing and clinical diagnosis. [ABSTRACT FROM AUTHOR]

We report a sensor combining two distinguishable magnetic nanoprobes (DNA1/Fe 3 O 4 NPs/Thi and DNA2/Fe 3 O 4 NPs/Fc) with target-triggered hybridization chain reaction (HCR) strategy for the simultaneous detection of microRNA-141 (miR-141) and microRNA-21 (miR-21). In the presence of targets, the thiol-modified hairpin capture probes (HCP1 and HCP2) specifically hybridize with miR-141 and miR-21 on a gold electrode, leading to the conformation change of HCP1 and HCP2, respectively. The conformation change subsequently triggers HCR to generate plentiful bonding sequences of magnetic nanoprobes. Thus, numerous thionine (Thi) modified DNA1/Fe 3 O 4 NPs/Thi and ferrocene carboxaldehyde (Fc-CHO) modified DNA2/Fe 3 O 4 NPs/Fc are captured by the well-designed HCR, via DNA hybridization respectively, giving rise to the dual magnified response of currents. The increase in the electrochemical currents at different potentials of the two magnetic nanoprobes enables us to simultaneously and quantitatively detect miR-141 and miR-21. Target-triggered HCR increases the amount of captured nanoprobes due to the increasing number of bonding sequences, greatly amplifying the currents of the two magnetic nanoprobes in the presence of targets, and ultimately realizing the dual signal amplification with increased sensitivity. The sensor can be applied for detecting miRNAs in cell lysates, thus, promising to be a clinic diagnosis of cancers by means of simultaneous detection of a variety of miRNA biomarkers. [ABSTRACT FROM AUTHOR]

Novel fluorescent DNA quantum dots (QDs) were synthesized by hydrothermal treatment of G-/T-rich ssDNA at relatively low reaction temperature. The obtained DNA QDs demonstrate unique optical properties, maintain the basic structure and biological activities of ssDNA precursors, which makes the DNA QDs able to specifically bind with arsenite, driving the (GT) 29 region suffer conformation evolution and form well-ordered assembly rather than random aggregations. We speculate that the strong inter-molecule interaction and efficient stacking of base pairs stiffen the assembly structure, block the nonradiative relaxation channels, populate the radiative decay, and thus making the assembly be highly emissive as a new fluorescence center. The arsenite-induced specific fluorescence enhancement facilitates DNA QDs as light-up probes for arsenite sensing. Under optimal conditions, a linear relationship between the increased fluorescence intensity of DNA QDs and the logarithmic values of arsenite concentration in the range of 1–150 ppb with a detection limit of 0.2 ppb (3σ) was obtained. The nanosensor shows excellent selectivity for “turn on” arsenite determination and arsenate does not show any interference, facilitating its application in complex real water analysis. [ABSTRACT FROM AUTHOR]

A bimetal lanthanide coordination polymer nanoparticle (ATP-Ce/Tb-Tris CPNs) with good biocompatibility was synthesized in Tris–HCl buffer using adenosine triphosphate (ATP) molecules as the bridge ligands. The large absorption cross section and suitable emission energy of Ce 3+ matching to the adsorption energy of Tb 3+ ( 4 f n ) results in the efficient energy transfer from Ce 3+ to Tb 3+ , thus the synthesized ATP-Ce/Tb-Tris CPNs exhibit the characteristic green emission of Tb 3+ . Such energy transfer from metal to metal in fluorescent lanthanide coordination polymer nanoparticles (Ln-CPNs) has been demonstrated. It is found that the oxidation of Ce 3+ in ATP-Ce/Tb-Tris CNPs to Ce 4+ would interrupt the energy transfer from Ce 3+ to Tb 3+ , leading to fluorescence quenching of Tb 3+ . On the basis of this quenching mechanism, ATP-Ce/Tb-Tris CPNs has been successfully used to detect reactive oxygen H 2 O 2 with detection limit as low as 2 nM. If glucose oxidase is present in the system, glucose can be determined using the ATP-Ce/Tb-Tris CNPs nanosensor. [ABSTRACT FROM AUTHOR]

A new strategy based on enzymatically engineered primer extension poly-thymine (EPEPT) and nanomaterials in situ generation technology is reported for direct detection of microRNA (miRNA) in a fluorescence turn-on format using the sequential and complementary reactions catalyzed by Klenow Fragment exo − (KFexo − ) and terminal deoxynucleotidyl transferase (TdTase). The short miRNA can be efficiently converted into long poly-thymine (polyT) sequences, which function as template for in situ formation of fluorescence copper nanoparticles (CuNPs) as nano-dye for detecting miRNA. The polyT-CuNPs can effectively form and emit intense red fluorescence under the 340 nm excitation. For the proof of concept, microRNA-21 (miR-21) was selected as the model target to testify this strategy as a versatile assay platform. By directly using miR-21 as the primer, the simple, rapid and sensitive miRNA detection was successfully achieved with a good linearity between 1 pM and 1 nM and a detection limit of 100 fM. Thus, the EPEPT strategy holds great potential in biochemical sensing research as an efficient and universal platform. [ABSTRACT FROM AUTHOR]

Thrombin is a crucial multifunctional enzyme involved in many physiological and pathological processes. Its detection is of great importance. In this work, a novel bio-dots nanosensor for detection of thrombin with colorimetric, fluorometric and light-scattering signals is developed. This nanosensor is composed of thrombin-binding aptamer bio-dots (TBA-dots) and gold nanoparticles (AuNPs), where TBA-dots serve as fluorometric reporter and AuNPs function as multiple roles as colorimetric reporter, light scattering reporter and fluorescence quencher. TBA-dots retain inherent structures of aptamer to specifically interact with thrombin and simultaneously induce the aggregation of AuNPs. The mechanism of the sensing system is based on distance-dependent aggregation of AuNPs and fluorescence resonance energy transfer (FRET). The nanosensor needs no further surface functionalization required for the as-prepared bio-dots and AuNPs, which provides a sensitive method for the selective detection of thrombin with a detection limit as low as 0.59 nM. In addition, it provides a brand new perspective for bio-dots and its potential use in bioanalysis. [ABSTRACT FROM AUTHOR]

A new class of fluorescent bio-dots have been successfully prepared by hydrothermal treatment of poly-cytosine DNA at low temperature down to 80 °C. The inter-molecule interaction of aromatic cytosine bases forms sp 2 carbon-like centers as the luminescence centers or chromophores. In the presence of Ag + , the formation of C–Ag + –C base pairs results in the destroying of the luminescence centers and thus the fluorescence (FL) quenching of the bio-dots. However, with the addition of biothiols, Ag + prefers to react with biothiols to form the Ag + –S bond, hence a restoration of FL emission can be observed. Moreover, by employing the classic glutathione reductase (GR) catalyzed enzymatic reaction, this concept can be readily applied to the selective quantification of oxidized glutathione (GSSG) as well as the activity of GR with a very robust, simple, and rapid procedure. It is worth looking forward to design DNAs with a specific function to prepare fluorescent bio-dots for simple biological applications. [ABSTRACT FROM AUTHOR]

Herein, a sensitive and selective sensor for biothiols based on colorimetric assay is reported. S-adenosyl- l -methionine (SAM) could induce the selective aggregation of unmodified gold nanoparticles (AuNPs) by electrostatic interaction. In the presence of biothiols, such as glutathione (GSH), homocysteine (Hcy), and cysteine (Cys), AuNPs prefer to react with thiols of biothiols rather than SAM due to the formation of Au–S bond. Thus, the AuNPs turn from the aggregation to the dispersion state, and the corresponding color variation in the process of anti-aggregation of AuNPs can be used for the quantitative screening of biothiols through UV–vis spectroscopy or by the naked eye. Under optimized conditions, a good linear relationship in the range of 0.4–1.2 µM is obtained for Cys, 0.2–0.9 µM for GSH, and 0.6–3.0 µM for Hcys. The detection limits of this assay for GSH, Cys and Hcys are 35.8 nM, 21.7 nM, and 62.4 nM, respectively. This colorimetric assay exhibits rapid operation (within 5 min), high selectivity and sensitivity towards biothiols with tunable dynamic ranges. [ABSTRACT FROM AUTHOR]

DNA methyltransferases catalyze the transfer of a methyl group from S-adenosylmethionine to the target adenine or cytosine, eventually inducing the DNA methylation in both prokaryotes and eukaryotes. Herein, we developed a novel electrochemiluminescence biosensor to quantify DNA adenine methylation (Dam) methyltransferase (MTase) employing signal amplification of GO/AgNPs/luminol composites to enhance the assay sensitivity. The method was developed by designing a capture probe DNA, which was immobilized on gold electrode surface, to hybridize with azide complementary DNA to form the azide-terminated dsDNA. Then, alkynyl functionalized GO/AgNPs/luminol composites as the signal probe were immobilized to azide-terminated dsDNA modified electrode via click chemistry, resulting in a high electrochemiluminescence (ECL) signal. Once the DNA hybrid was methylated (under catalysis of Dam MTase) and further cleaved by Dpn I endonuclease (a site-specific endonuclease recognizing the duplex symmetrical sequence of 5′-G-Am-T-C-3′), GO/AgNPs/luminol composites release from the electrode surface to the solution, leading to significant reduction of the ECL signal. The change of the ECL intensity is related to the methylation status and MTase activity, which forms the basis of MTase activity assay and site-specific methylation determination. This novel strategy can be further used as a universal method for other transferase determination by designing various transferase-specific DNA sequences. In addition, this method can be used for the screening of antimicrobial drugs and has a great potential to be further applied in early clinical diagnosis. [ABSTRACT FROM AUTHOR]

Abstract: Carbohydrate–protein interactions mediate the important physiological and pathophysiological processes in living organism. Their study has attracted great attention due to its importance in understanding these biological processes and in fabricating biosensors for diagnostics and drug development. Here, by using concanavalin A (ConA) as a model protein, a novel surface plasmon resonance (SPR) sensor was developed for sensitive detection ConA. In this sensing platform, dextran (Dex) capped gold nanoparticles (Dex-Au NPs) were initially synthesized in one-pot and utilized as amplification reagent. After deposition of graphene oxide (GO) on the SPR gold film, phenoxy-derivatized dextran (DexP) was assembled onto the GO-modified gold chip surface through π–π interaction. The resultant GO/DexP sensing interface could specifically capture ConA which could further react with Dex-Au NPs through the specific interaction between ConA and Dex, forming a sandwich configuration. The morphologies and the electrochemistry of the formed sensing surface were investigated by using scanning electron microscopy and electrochemical techniques including electrochemical impedance spectroscopy and cyclic voltammogram. Owing to the high surface area of GO and the excellent amplification of Dex-Au NPs, the developed sandwich SPR sensor successfully fulfilled the sensitive detection of ConA in the range of 1.0–20.0μgmL−1 with a detection limit of 0.39μgmL−1. Compared to the direct assay format, the prepared sandwich SPR sensor led to an improvement of 28.7-fold in the sensitivity. The results demonstrated that the proposed method might provide a new direction in designing high-performance SPR biosensors for sensitive and selective detection of a wide spectrum of biomolecules. [Copyright &y& Elsevier]

Abstract: In this study, a novel biomolecule immobilization approach has been proposed to the synthesis of multi-functional core–shell glucose oxidase–Au–polydopamine–Fe3O4 magnetic bionanoparticles (GOx–Au–PDA–Fe3O4 MBNPs) using the one-pot chemical polymerization method. Then, a high performance biosensor has been constructed by effectively attaching the proposed GOx–Au–PDA–Fe3O4 MBNPs to the surface of the magnetic glassy carbon electrode. Scanning electron microscope, energy dispersive x-ray spectrometer, UV–vis spectroscopy, and electrochemical methods were used to characterize the GOx–Au–PDA–Fe3O4 MBNPs. The resultant GOx–Au–PDA–Fe3O4 MBNPs not only have the magnetism of Fe3O4 nanoparticles which makes them easily manipulated by an external magnetic field, but also have the excellent biocompatibility of PDA to maintain the native structure of the GOx, and good conductivity of Au nanoparticles which can facilitate the direct electrochemistry of GOx in the biofilm. Hence, the present GOx–Au–PDA–Fe3O4 biofilm displays good linear amperometric response to glucose concentration ranging from 0.02 to 1.875mM. This efficient biomolecule immobilization platform is recommended for the preparation of many other MBNPs with interesting properties and application potentials in many fields, such as biosensing, biocatalysis, biofuel cells, and bioaffinity separation. [Copyright &y& Elsevier]

Abstract: A novel “smart” electrochemical affinity nanobiosensor with “on-off” switchable property was designed for the ultrasensitive determination of glucose. The sensing approach was based on the glucose-ConA-dextran competitive system induced charge evolution in the use of graphene oxide (GO) as transducer element, resulting in the enhancement of interfacial electron transfer kinetics between the redox probe and the electrode. As concanavalin A (ConA) constituent was pH-sensitive, when the ConA-DexP/GO film electrode switched in probe Fe(CN)6 3−/4− solution between pH 4.0 and 8.0, the film was cycled between the “on” and “off” states by the electrostatic attraction and repulsion of Fe(CN)6 3−/4− to and from the electrode surface. Upon introduction of glucose into the ConA-DexP/GO complex at the “off” state, glucose competed with DexP for ConA and displaced ConA from the GO platform, resulting in gradual decrease of the surface negative charge as well as the resistance of probe for electron communication on the sensor surface, and making the switching from “off” state to “on” state simultaneously. This ultrasensitive glucose nanobiosensor had a broad linearity between the decrease in electron transfer resistance (ΔR) and the glucose concentration over a range from 5.0μM to 9.0mM with a detection limit as low as 0.34μM. The proposed method showed potential application for fabricating novel biosensors and bioelectronic devices. [Copyright &y& Elsevier]