•An approach for direct detection of free IAA and SA in tiny plant sample were proposed.•The tiny plant sample was put on the surface of the working electrodes in PADS.•The low sample volume of ∼10 μL allowed detection of IAA and SA at the level of ng.•The obtained results were comparable to previous indirect detection approaches.•The IAA and SA in different location of pea seedlings were useful for further study.It is not easy to directly obtain concentrations of auxin (mainly indole-3-acetic acid, or IAA) and other phytohormones in different locations of individual plants although such information is critical for the study of their functions. Here we developed a paper-based analytical device consisting of a disposable working electrode to provide a platform for direct and simultaneous detection of free IAA and salicylic acid (SA) in different parts of pea seedlings with the weights of several milligrams. The double sided conductive carbon tape modified with carbon nanotube was used as the disposable working electrodes after they were treated with oxygen plasma. Different parts of pea seedlings were applied on the surface of working electrodes for direct electrochemical detection of free IAA and SA in paper-based analytical devices. The requirement of solution volumes with only 10 microliters made it possible to quantify free IAA and SA at the level of ng. Our results suggested that large amounts of inherent IAA and SA could be lost because of complicated and time-consuming preparation of samples in traditional gas/liquid chromatography-mass spectrometry methods Our approach not only avoids such steps but also provides concentrations of IAA and SA in different zones of individual pea seedlings. The obtained results could be applied for the study of individual differences and interactions between IAA and SA in development of peas. This strategy not only paved the foundation for further investigation of regulating mechanisms of free IAA and its interaction with other phytohormones but also might provide an alternative approach for detection of free electroactive biomolecules in other living organs.

To improve electron transfer, a combination of electroactive hemoglobin (Hb) and Au nanoparticles (Au NPs) was designed to obtain nanocomposites (NCs). Cyclic voltammetry and redox probe characterization showed that Hb–Au NCs incorporating the layer-by-layer technique to improve the ration of electroactive Hb, showed a better conjugated structure, lower standard entropy, and faster electron transfer. Au entrapped in Hb on the glassy carbon electrode (GCE) also exhibited a wider linear range, and excellent reproducibility and recovery for analytical applications of hydrogen peroxide, oxygen, and trichloroacetic acid. Therefore, our research could potentially become a promising alternative by combining bioactive molecules with nanoparticles, and broaden the applications of mediator-free biosensors.

The authors demonstrate a multivalent recognition and triple signal amplification strategy for amperometric cytosensing and dynamic evaluation of cell surface sialic acid (SA). The strategy is based on the use of (a) graphene oxide functionalized with 3-aminophenylboronic acid (APBA-CG) for cell capture, and (b) of Sambucus nigra agglutinin and thionine conjugated to gold nanoparticles (SNA-AuNP-Th) acting as a nanoprobe. The use of APBA-CG accelerates electron transfer and warrant a large surface area for cell capture. The AuNPs, in turn, nanoprobe provide an effective matrix for mass loading of thionine molecules and thus dramatically improve sensitivity. The results obtained with the biosensor demonstrate that sialic acid is strongly expressed levels in cancer cells compared to normal ones. The cytosensor was applied to the detection of HepG2 cells. Best operated at a working voltage of typically 0.42 V vs. Ag/AgCl, it displays a wide linear range (that extends from 1.0 × 102 to 1.0 × 107 cells per mL and a low detection limit (50 cells per mL)). The method was successfully applied dynamic monitoring of the changes in cell surface SA expression in response to the addition of the enzyme sialidase. In our perception, the SNA-based biosensor represents a viable tool to study SA-related biological processes and for diagnosis and treatment of early stages of cancer.

•A paper-based analytical device based on Au nanoparticles modified indium tin oxide electrode has been designed.•The proposed device exhibited low detection limit for the electrocatalytical reduction of H2O2.•The sensor could be used to detect cellular H2O2 released from living cells and further evaluate drug-induced apoptosis.•The approach is low-cost, portable and promising in biological and biomedical applications.Developing cost-effective and simple analysis tools is of vital importance for practical applications in bioanalysis. Here, a disposable paper-based analytical device based on Au nanoparticles modified indium tin oxide electrode has been designed, which was applied for the reliable and non-enzymatic detection of H2O2. Due to the excellent electrocatalytic activity of Au nanoparticles, the disposable electrode exhibited favorable performance toward H2O2 reduction in the linear concentration range from 0.1 to 15 μM. The detection limit has been estimated to be 0.08 μM, which was lower than certain enzymes and other metal nanomaterials-based sensors. Because of these analytical advantages, the constructed device was used to study the extracellular H2O2 release from NB4 cells and further applied to evaluate sodium selenite induced apoptosis. The results obtained by electrochemical method are correlated well with the results of MTT assays. The developed paper-based sensor is easy-to-fabricate and portable, providing an effective platform for cellular H2O2 sensing and can be used to study the dynamic biological process involving H2O2 in biological and biomedical applications.A disposable paper-based device based on Au nanoparticles modified indium tin oxide electrode has been designed, which was used to study the extracellular H2O2 release from NB4 cells and further applied to evaluate drug-induced apoptosis.

•Three-electrode system was written on the filter paper with graphite pencils.•Fully-drawn pencil-on-paper sensors were easily fabricated.•The proposed devices were applied for the electroanalysis of dopamine.•This approach is extremely simple, low-cost and environmentally friendly.In this paper, a facile and low-cost “pencil-drawing” method is exploited for writing electrodes on filter paper to fabricate an electrochemical paper-based analytical device. Unlike other techniques using inks or pastes, the three-electrode system can be prepared directly by drawing with nothing more than pencils and paper. The optimization using ferricyanide as the redox probe showed that the device fully drawn with 6B grade pencil possesses excellent electrochemical properties and sensor-to-sensor reproducibility in spite of its truly simple fabrication process. Experimental results demonstrated that the proposed device could be utilized for quantification of dopamine with high sensitivity and no interference from ascorbic acid and uric acid, which are normally present in biological fluids. Human urine and blood serum samples were analyzed using the proposed device as proof of concept. Such fully-drawn sensors are flexible, portable and disposable, represent a cheap and environmentally friendly approach that can be rapidly and easily fabricated without any specialized materials, affording great convenience for practical use in resource-limited or emergency situations.A “pencil-drawing” method is used to write electrodes on paper to fabricate an electrochemical paper-based analytical device, which is applied for dopamine detection.

•We fabricate uniform Hb-BSA microsphere using porous CaCO3 and glutaraldehyde.•Electrochemical study shows excellent oxygen-carrying ability of the microsphere.•BSA improves the dispersity of the microspheres and prevents agglomeration.The production of oxygen carriers is currently a topic of paramount interest in nanotechnology. Particularly, there is a focus in the development of microparticles with round and uniform shapes to further expand their potentialities. Here, we prepare homogenous microspheres with diameters of ~ 3 μm using porous calcium carbonate spheres as the template and glutaraldehyde as the cross-linking agent. Also, we present for the first time the use of electrochemical avenue to evaluate the oxygen-carrying ability of these as-prepared samples. Our results demonstrate successful construction of prospective microspheres which perform extraordinary oxygen-carrying capability with protection of bovine serum albumin layer on the surface.The homogenous Hb-BSA microspheres modified electrode exhibited excellent performance with enhanced capacity to store and transport oxygen compared with pure Hb-coated microspheres. The BSA molecule could improve the dispersity of the Hb-BSA microspheres and enhance their oxygen-carrying capacity. Furthermore, the utilization of electrochemistry for the study of our prepared Hb-BSA microspheres might be a novel effective approach for further application of oxygen carriers in electrochemical field.

Hemoglobin-coated microspheres with one layer and five layers were fabricated by layer-by-layer assembly. The electrochemical methods were used to study their capacity for carrying oxygen. The results provided direct evidences that microspheres with five-layer hemoglobin could store and release more oxygen than those with one layer. This study also provided an alternative method for evaluating the oxygen carrying capacity of hemoglobin microspheres.

•An origami paper-based electrochemical analytical device was designed.•Three-electrode system was written directly on paper by pencils.•The proposed device was applied for the electroanalysis of glucose.•This approach is extremely simple, low-cost and environmentally friendly.In this work, an origami paper-based analytical device for glucose biosensor by employing fully-drawn pencil electrodes has been reported. The three-electrode system was prepared on paper directly by drawing with nothing more than pencils. By simple printing, two separated zones on paper were designed for the immobilization of the mediator and glucose oxidase (GOx), respectively. The used paper provides a favorable and biocompatible support for maintaining the bioactivities of GOx. With a sandwich-type scheme, the origami biosensor exhibited great analytical performance for glucose sensing including acceptable reproducibility and favorable selectivity against common interferents in physiological fluids. The limit of detection and linear range achieved with the approach was 0.05 mM and 1–12 mM, respectively. Its analytical performance was also demonstrated in the analysis of human blood samples. Such fully-drawn paper-based device is cheap, flexible, portable, disposable, and environmentally friendly, affording great convenience for practical use under resource-limited conditions. We therefore envision that this approach can be extended to generate other functional paper-based devices.An origami paper-based analytical device for glucose biosensor by employing fully-drawn pencil electrodes has been designed. The proposed device is cheap, flexible, portable, disposable, and environmentally friendly.

This study describes an amperometric sensor for hydrogen peroxide (H2O2) that uses an ITO glass electrode which was modified with a nanocomposite consisting of electrochemically reduced graphene oxide and gold nanoclusters (AuNCs). The sensor was used to quantify extracellular H2O2 released from human neuroblastoma cells of type SH-SY5Y. The calibration plot, established best at a working voltage of −0.4 V (vs. Ag/AgCl) is linear in the 40 nmol⋅L−1 to 2 μmol⋅L−1 concentration range, and the detection limit is 20 nmol⋅L−1 (at a signal-to-noise ratio of 3). The method was further applied to study bupivacaine-induced cell damage and the protective effects of α-lipoic acid. The study indicated that pretreatment of the cells with lipoic acid retards cell damage induced by bupivacaine. The sensor can be easily fabricated, is disposable and highly sensitive. The sensor is perceived to represent an alternative for studying the interactions of drugs with cells, and as an effective tool to quantify cell-secreted H2O2.

A hemoglobin (Hb)-modified electrode based on chitosan/Fe3O4 nanocomposite coated glassy carbon has been constructed for trichloroacetic acid (TCA) detection. The structure of chitosan/Fe3O4 nanocomposite was investigated using energy-dispersive X-ray analysis (EDS) and X-ray diffraction (XRD) patterns. The electron transfer rate constant (ks) of Hb was estimated for as high as 3.12 s−1. The immobilized Hb exhibited excellent electro-catalytic activity toward the reduction of TCA. The response current regressed to the concentration of TCA within the range of 5.70 μM to 205 μM with a detection limit of 1.9 μM (S/N = 3).

•Paper could physically trap red blood cells on the surface of working electrodes.•The permeability of paper allows oxygen or nitrogen to reach cells without disturbance.•The breathing process of red blood cells could be electrochemically studied.•Electrochemical responses might be ascribed to reduction of oxyhemoglobin.Herein we utilized the filter paper to physically trap red blood cells (RBC) to observe the breathing process of red blood cells based on the permeability of the filter paper. By integrating double-sided conductive carbon tape as the working electrodes, the device could be applied to monitor electrochemical responses of RBC for up to hundreds of minutes. The differential pulse voltammetry (DPV) peak currents increased under oxygen while decreased under nitrogen, indicating that RBC could take in and release oxygen. Further studies demonstrated that the RBC suspension could more effectively take in oxygen than the solution of hemoglobin and the supernatant of RBC, suggesting the natural advantage of RBC on oxygen transportation. This study implied that simple paper-based analytical devices might be effectively applied in the study of gas-participating reactions and biochemical detections.

The increasing use of modified Fe3O4 magnetic microparticles has raised safety concerns regarding their use and effect on human health. This study assessed the in vivo biosafety, DNA, and chromosome damage of modified Fe3O4 microparticles such as Au@Fe3O4, Ag@Fe3O4, Cs@Fe3O4, Pt@Fe3O4, and CdS@Fe3O4, using spleen-deficient rats. Spleen-deficient rats treated with naked and modified (Au, Cs, Pt) Fe3O4 microparticles (5000 mg/kg) displayed low toxicity. Only treatment with Cds@Fe3O4 resulted in elevated toxicity and death in rats. Au-, Ag-, and Pt-modified Fe3O4 increased the rate of hemolysis in rats relative to treatment with naked Fe3O4. Despite this, Au- and Pt-modified Fe3O4 increased the biocompatibility and reduced DNA and chromosome damage in rats relative to naked Fe3O4. While Cs@Fe3O4 microparticles displayed a higher biocompatibility than naked Fe3O4, they displayed no significant reduction in DNA and chromosome damage. In summary, Au and Pt surface-modified Fe3O4 microparticles display elevated in vivo biosafety compared to unmodified particles. The precious metal material, with good biological compatibility, surface modification of Fe3O4 is an effective strategy to improve the overall safety and potential therapeutic utility of these magnetic materials.

Hemoglobin (Hb) has been demonstrated to endow electrochemical sensors with pH-switchable response because of the presence of carboxyl and amino groups. Hb was deposited in a chitosan matrix on a glassy carbon electrode (GCE) that was previously coated with clustered gold nanoparticles (Au-NPs) by electrodeposition. The switching behavior is active (“on”) to the negatively charged probe [Fe(CN)63−] at pH 4.0, but inactive (“off”) to the probe at pH 8.0. This switch is fully reversible by simply changing the pH value of the solution and can be applied for pH-controlled reversible electrochemical reduction of H2O2 catalyzed by Hb. The modified electrode was tested for its response to the different electroactive probes. The response to these species strongly depends on pH which was cycled between 4 and 8. The effect is also attributed to the presence of pH dependent charges on the surface of the electrode which resulted in either electrostatic attraction or repulsion of the electroactive probes. The presence of Hb, in turn, enhances the pH-controllable response, and the electrodeposited Au-NPs improve the capability of switching. This study reveals the potential of protein based pH-switchable materials and also provides a simple and effective strategy for fabrication of switchable chemical sensors as exemplified in a pH-controllable electrode for hydrogen peroxide.

Integrating a disposable ITO electrode modified by gold nanorod–chitosan nanocomposites, a paper-based electroanalytical device for the real-time detection of H2O2 released from cancer cells has been developed. It provided a portable platform for biological and biomedical studies dealing with living cells.

•A new disposable electrochemical sensor using ITO glass as the substrate of working electrodes and paper as the electrolytic cell was proposed.•The sensor can be used to study the electrochemical behavior of K562 cells and the effect of anticancer drugs on cell viability.•The proposed method is cost-effective, simple and rapid.•The sensor is suitable for experimental study with tumor cells or other types of pathogens for disease diagnosis and drug selection.Developing cost-effective and simple analysis tools is of vital importance for practical applications in bioanalysis. In this work, a new disposable electrochemical cell sensor with low cost and simple fabrication was proposed to study the electrochemical behavior of leukemia K562 cells and the effect of anticancer drugs on cell viability. The analytical device was integrated by using ITO glass as the substrate of working electrodes and paper as the electrolytic cell. The cyclic voltammetry of the K562 cells at the disposable electrode exhibited an irreversible anodic peak and the peak current is proportional to the cell number. This anodic peak is attributed to the oxidation of guanine in cells involving two protons per transfer of two electrons. For the drug sensitivity tests, arsenic trioxide and cyclophosphamide were added to cell culture media. As a result, the electrochemical responses of the K562 cells decreased significantly. The cytotoxicity curves and results obtained corresponded well with the results of CCK-8 assays. In comparison to conventional methods, the proposed method is simple, rapid and inexpensive. More importantly, the developed sensor is supposed to be a single-use disposable device and electrodes were prepared “as new” for each experiment. We think that such disposable electrodes with these characteristics are suitable for experimental study with cancer cells or other types of pathogens for disease diagnosis, drug selection and on-site monitoring.

We have studied the trans-membrane electron transfer in human red blood cells (RBCs) immobilized in a chitosan film on a glassy carbon electrode (GCE). Electron transfer results from the presence of hemoglobin (Hb) in the RBCs. The electron transfer rate (ks) of Hb in RBCs is 0.42 s−1, and <1.13 s−1 for Hb directly immobilized in the chitosan film. Only Hb molecules in RBCs that are closest to the plasma membrane and the surface of the electrode can undergo electron transfer to the electrode. The immobilized RBCs displayed sensitive electrocatalytic response to oxygen and hydrogen peroxide. It is believed that this cellular biosensor is of potential significance in studies on the physiological status of RBCs based on observing their electron transfer on the modified electrode.

Extensive biomedical applications of nanoparticles are mainly determined by their safety and compatibility in biological systems. The aim of this study was to compare the biosafety and biocompatibility of gold nanoparticles (GNPs) prepared with HEPES buffer, which is popular for cell culture, and sodium citrate, a frequent reducing agent. From experimental results on the body weight and organ coefficients of acute oral toxicity tests, it could be observed that HEPES-prepared GNPs are biologically safer than citric-prepared GNPs at the same dose of 500 μg/kg. The in vitro cell viability was higher for HEPES-prepared GNPs than citric-prepared GNPs at 5.0- and 10.0-ug/mL concentrations. More reactive oxygen species (ROS) were generated in the cell suspension when supplemented with citric-prepared GNPs than HEPES-prepared GNPs when their concentrations were higher than 20 μg/mL. The results stated that HEPES-prepared GNPs had better biosafety and biocompatibility than citric-prepared GNPs. This study not only revealed the influence of reducing agent on biosafety and biocompatibility of nanomaterials but also provided accumulative evidence for nanomaterials in biomedical applications.

•Au@Fe3O4 modified electrodes were applied for detection of salicylic acid.•Buffer solution with pH 7.0 was used for determination.•Linear range for salicylic acid is 1.0 μM to 1.2 mM and detection limit is 0.1 μM.•Time-dependent variation of salicylic acid extracted from tomato leaves could be differentiated.Salicylic acid (SA) is one of essential phytohormones in the regulation of many physiological processes, especially for the defense of plants under various biotic stresses. Although SA could be electrochemically detected with various ways, their application was limited by unsatisfactory detection performance, or the requirement of acid/alkaline solutions. In this study, glassy carbon electrodes (GCEs) were coated with chitosan (CS) and then modified using the nanocomposite of Au@Fe3O4 for the detection of SA. It was found that the Au@Fe3O4-CS modified GCEs could be applied for the electrochemical determination of SA in the Britton–Robinson buffer solution at pH 7.0. After the optimization of the concentration of Au@Fe3O4 and the pH value of the buffer solution for detection, the concentration of SA could be measured with the linear range from 1.0 μM to 1.2 mM and the lowest detection limit at 0.10 μM based on differential pulse voltammetry. The electrochemical response of SA at the Au@Fe3O4 modified GCEs was stable and reproducible. Moreover, the modified electrodes were applied for monitoring time-dependent variation of SA extracted from normal leaves of tomatoes and those infected by fungal pathogen. Our investigation revealed that the Au@Fe3O4 modified GCE could effectively determinate SA in mild conditions with a wide linear range, which can be potentially applied for the study of defense mechanisms of plants.

We report on a novel electrochemical biosensor that was fabricated by immobilizing hemoglobin (Hb) onto the surface of a gold electrode modified with a chitosan@Fe3O4 nano-composite. The Fe3O4 nanoparticles were prepared by co-precipitation and have an average size of 25 nm. They were dispersed in chitosan solution to obtain the chitosan@Fe3O4 nano-composite particles with an average diameter of 35 nm as verified by transmission electron microscopy. X-ray diffraction patterns and Fourier transform IR spectroscopy confirmed that the crystallite structure of the Fe3O4 particles in the nano-composite has remained unchanged. At pH 7.0, Hb gives a pair of redox peaks with a potential of about −0.21 V and −0.36 V. The Hb on the film maintained its biological activity and displays good electrocatalytic reduction activity towards hydrogen peroxide. The linear range for the determination of H2O2 is from 2.3 μM to 9.6 mM, with a detection limit at 1.1 μM concentration (at S/N = 3). The apparent Michaelis-Menten constant is 3.7 mM and indicates the high affinity of Hb for H2O2. This biosensor also exhibits good reproducibility and long-term stability. Thus, it is expected to possess potential applications in the development of the third-generation electrochemical biosensors.

•Double sided carbon tape was used to fabricate disposable working electrodes.•The carbon tape electrodes were coated with bismuth for stripping analysis of heavy metals.•The bismuth modified carbon tape electrode was integrated with a paper-based analytical device.•The lead migrated from toys could be quantified with the bismuth modified carbon tape electrodes.Low cost disposable working electrodes are specifically desired for practical applications of electrochemical detection considering maturity of electrochemical stations and data collection protocols. In this paper double-sided conductive adhesive carbon tape with nanostructure was applied to fabricate disposable working electrodes. Being supported by indium tin oxide glass, the prepared carbon tape electrodes were coated with bismuth film for stripping analysis of heavy metal ions. By integrating the bismuth modified electrodes with paper-based analytical devices, we were able to differentiate Zn, Cd and Pb ions with the sample volume of around 15 μL. After the optimization of parameters, including modification of bismuth film and the area of the electrodes, etc., Pb ions could be measured in the linear range from 10 to 500 μg/L with the detection limit of 2 μg/L. Our experimental results revealed that the disposable modified electrodes could be used to quantify migrated lead from toys with the results agreed well with that using atomic absorption spectrometry. Although bismuth modification and stripping analysis could be influenced by the low conductivity of the carbon tape, the low cost disposable carbon tape electrodes take the advantages of large-scaled produced double-sided carbon tape, including its reproducible nanostructure and scaled-up fabrication process. In addition, the preparation of disposable electrodes avoids time-consuming pretreatment and experienced operation. This study implied that the carbon tape might be an alternative candidate for practical applications of electrochemical detection.

Stable and sensitive electrochemiluminescence (ECL) detection relies on successful immobilization of quantum dots (QDs) on working electrodes. Herein, we report a new technique to apply double-sided carbon adhesive tape as the working electrode to improve the stability and reproducibility of QD-based ECL emission. CdS QD-modified electrodes were prepared by dropping and drying CdS QD suspension on the carbon adhesive tape supported by indium tin oxide (ITO) glass. The ECL detection was performed with the prepared electrode on a paper-based platform. We tested our system using H2O2 of various concentrations and demonstrated that consistent ECL emission could be obtained. We attribute stable and reproducible ECL emission to the robust attachment of CdS QDs on the carbon adhesive tape. The proposed method could be used to quantify the concentration of dopamine from 1 μM to 10 mM based on the quenching effect of dopamine on ECL emission of CdS QD system using H2O2 as the coreactant. Our approach addressed the problem in the integration of stable QD-based ECL detection with portable paper-based analytical devices. The similar design offers great potential for low-cost electrochemical and ECL analytical instruments.

This paper reports the application of chitosan–Fe3O4 (CS–Fe3O4) nanocomposite modified glassy carbon electrodes for the amperometric determination of bisphenol A (BPA). We observed that the CS–Fe3O4 nanocomposite could remarkably enhance the current response and decrease its oxidation overpotential in the electrochemical detection. Experimental parameters, such as the amount of the CS–Fe3O4, the accumulation potential and time, the pH value of buffer solution etc. were optimized. Under the optimized conditions, the oxidation peak current was proportional to BPA concentration in the range between 5.0 × 10−8 and 3.0 × 10−5 mol dm−3 with the correlation coefficient of 0.9992 and the limit of detection of 8.0 × 10−9 mol dm−3 (S/N = 3). The proposed sensors were successfully employed to determine BPA in real plastic products and the recoveries were between 92.0% and 06.2%. This strategy might open more opportunities for the electrochemical determination of BPA in practical applications. Additionally, the leaching studies of BPA on incubation time using the as-prepared modified electrode were successfully carried out.

A switchable layer-by-layer assembly film was constructed by alternate adsorption of negatively charged Ag nanoparticles (AgNPs) and positively charged hemoglobin (Hb) with appropriate pH value. The {Hb/AgNPs}n films showed strong pH-sensitive on–off responses toward the electroactive probe Fe(CN)63 −. The {Hb/AgNPs}4 film is active (“on”) to the negatively charged probe, Fe(CN)63 −, at pH 3.0, but inactive (“off”) to the probe at pH 9.0. The switching behavior is fully reversible by simply changing the pH value of the solution and can be applied for pH-controlled reversible electrochemical reduction of H2O2 catalyzed by Hb. The pH-sensitive behavior may be due to the electrostatic interaction between the films and the probe. The modified electrode with the pH-controlled switchable redox activity would possibly become a “smart” interface for the construction of specific electrochemical biosensors with a signal-controlled activity.Highlights► The LBL films of AgNPs and Hb showed strong pH-sensitive on-off responses. ► The switching behavior is reversible and reproducible by changing the solution pH. ► The film could be used for pH-controlled electrochemical determination of H2O2. ► It provides another approach to prepare tunable electrochemical biosensors.

A novel platform, which hemoglobin (Hb) was immobilized on core–shell structurally Fe3O4/Au nanoparticles (simplified as Fe3O4@Au NPs) modified glassy carbon electrode (GCE), has been developed for fabricating the third biosensors. Fe3O4@Au NPs, characterized using transmission electron microscope (TEM), scanning electron microscope (SEM) and energy dispersive spectra (EDS), were coated onto GCE mediated by chitosan so as to provide larger surface area for anchoring Hb. The thermodynamics, dynamics and catalysis properties of Hb immobilized on Fe3O4@Au NPs were discussed by UV–visible spectrum (UV–vis), electrochemical impedance spectroscopy (EIS), electrochemical quartz crystal microbalance technique (EQCM) and cyclic voltammetry (CV). The electrochemical parameters of Hb on Fe3O4@Au NPs modified GCE were further carefully calculated with the results of the effective working area as 3.61 cm2, the surface coverage concentration (Γ) as 1.07 × 10−12 mol cm−2, the electron-transfer rate constant (Ks) as 1.03 s−1, the number of electron transferred (n) as 1.20 and the standard entropy of the immobilized Hb (ΔS0′) as calculated to be −104.1 J mol−1 K−1. The electrocatalytic behaviors of the immobilized Hb on Fe3O4@Au NPs were applied for the determination of hydrogen peroxide (H2O2), oxygen (O2) and trichloroacetic acid (TCA). The possible functions of Fe3O4 core and Au shell as a novel platform for achieving Hb direct electrochemistry were discussed, respectively.Research highlights► In recent years, immobilization of biomolecule onto nanomaterials, which could be utilized in the investigation of biomolecule reactions and the preparations of the biosensors, has attracted much research attention. A novel platform, which hemoglobin (Hb) was immobilized on core–shell structurally Fe3O4/Au nanoparticles (simplified as Fe3O4@Au NPs) modified glassy carbon electrode (GCE), has been developed for fabricating the third biosensors in this paper. ► Magnetic NPs stand out because of their added properties. However, naked Fe3O4 NPs are very sensitive to oxidation because of their high chemical reactivity and being prone to aggregate. Those defects limit their further applications. We presented a simple approach to synthesize Au modified Fe3O4 NPs with core–shell structure, which was characterized by transmission electron microscopy, scanning electron microscope, energy dispersive spectra and UV–vis spectroscopy. ► The thermodynamics, dynamics and catalysis properties of Hb immobilized on Fe3O4@Au NPs were discussed by UV–visible spectrum, electrochemical impedance spectroscopy, electrochemical quartz crystal microbalance technique and cyclic voltammetry. The electrocatalytic behaviors of the immobilized Hb on Fe3O4@Au NPs were applied for the determination of hydrogen peroxide, oxygen and trichloroacetic acid. The possible functions of Fe3O4 core and Au shell as a novel platform for achieving Hb direct electrochemistry were also discussed, respectively.

Hemoglobin (Hb) was immobilized on CdS:Mn nanoparticles (NPs) capped with citrate to obtain an amperometric biosensor for determination of hydrogen peroxide (H2O2). The NPs were prepared from aqueous solution via a precipitation method. The UV-vis spectra of the film with Hb on the NPs and of free Hb solutions suggested that Hb retained its bioactivity and native conformation in the film. In buffer of pH 7.0, the sensor film exhibited a pair of quasi-reversible redox peaks originating from the Hb-heme Fe(III)/Fe(II) redox couple. The formal potential of Hb varied linearly with the increase of pH from 3.0 to 9.0 with a slope of −46 mV·pH−1, this indicating that one proton participates in the redox reaction. The electron transfer rate constant (ks) is 2.2 s−1, suggesting that the NPs provide a benign microenvironment for Hb to undergo direct electron transfer. Despite the absence of an electron mediator, the biosensor displays an analytical performance which is better than any others described in previous reports. Hence, the NPs may provide a simple and alternative approach towards electrochemical biosensors.

This paper reports on the fabrication and characterization of hemoglobin (Hb)-colloidal silver nanoparticles (CSNs)-chitosan film on the glassy carbon electrode and its application on electrochemical biosensing. CSNs could greatly enhance the electron transfer reactivity of Hb as a bridge. In the phosphate buffer solution with pH value of 7.0, Hb showed a pair of well-defined redox peaks with the formal potential (E0′) of −0.325 V (vs. SCE). The immobilized Hb in the film maintained its biological activity, showing a surface-controlled process with the heterogeneous electron transfer rate constant (ks) of 1.83 s−1 and displayed the same features of a peroxidase in the electrocatalytic reduction of oxygen and hydrogen peroxide (H2O2). The linear range for the determination of H2O2 was from 0.75 μM to 0.216 mM with a detection limit of 0.5 μM (S/N = 3). Such a simple assemble method could offer a promising platform for further study on the direct electrochemistry of other redox proteins and the development of the third-generation electrochemical biosensors.

34 nm gold nanoparticles with good stability were synthesized and characterized and their effect (as a function of concentration) on the proliferation of keratinocytes was evaluated by means of MTT and nucleolar organizer region (AgNOR) count (silver staining). The cell morphology was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that a low concentration of gold nanoparticles enhances the proliferation of keratinocytes. Specifically, a concentration of 5.0 ppm gold nanoparticle has the best effect on the promotion of cell growth. In the experiment group, the AgNOR-positive areas and AgNOR area/nuclear area ratios of keratinocytes co-cultured with 5.0 ppm gold nanoparticles were greater than those in the control group (p < 0.01). At a level greater than10.0 ppm, gold nanoparticles were found to have a cytotoxic effect on keratinocytes. It is concluded that a low concentration of gold nanoparticles may be used as a biomedical material in skin tissue engineering.

Direct electron transfer is demonstrated to occur between an electrode and hemoglobin that was immobilized on a film of Fe3O4@Pt-chitosan (Fe3O4@Pt-CS). Magnetic nanoparticles composed of Fe3O4 were prepared by a chemical coprecipitation method, and platinum nanoparticles were deposited on the Fe3O4 surface to form novel core-shell nanocomposites. In phosphate buffer solution of pH 7.0, the hemoglobin-Fe3O4@Pt-CS assembly on a modified glassy carbon electrode exhibited a couple of well-defined and quasi-reversible redox peaks. The formal potential E0′ was about −0.35 V. The electrode displayed excellent electrocatalytic activity towards oxygen and hydrogen peroxide reduction without the need for an electron mediator.

Nanoparticles (NPs) consisting of an Fe3O4 core and a thin gold shell (referred to as Au@Fe3O4 NPs) were self-assembled on the surface of a glassy carbon electrode modified with ethylenediamine. Following adsorption of hemoglobin, its interaction with the NPs was studied by UV–Vis spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry. Stable and well-defined redox peaks were observed at about −350 mV and −130 mV in pH 6.0 buffer. The modified electrode was used as a mediator-free sensor for hydrogen peroxide (H2O2), with a linear range from 3.4 µM to 4.0 mM of H2O2, and with a 0.67 µM detection limit (at an S/N of 3). The apparent Michaelis-Menten constant is 2.3 mM.

A novel hydrogen peroxide (H2O2) biosensor was developed by immobilizing hemoglobin on the gold colloid modified electrochemical pretreated glassy carbon electrode (PGCE) via the bridging of an ethylenediamine monolayer. This biosensor was characterized by UV-vis reflection spectroscopy (UV-vis), electrochemical impendence spectroscopy (EIS) and cyclic voltammetry (CV). The immobilized Hb exhibited excellent electrocatalytic activity for hydrogen peroxide. The Michaelis–Menten constant (Km) was 3.6 mM. The currents were proportional to the H2O2 concentration from 2.6 × 10−7 to 7.0 × 10−3 M, and the detection limit was as low as 1.0 × 10−7 M (S/N = 3).

We have developed a novel nitrite sensor based on the immobilization of nanometer-sized gold colloid attached to an ethylenediamine monolayer-modified electrochemically pretreated glassy carbon electrode. The electrochemical behavior and mechanism of the modified electrode towards the reduction of nitrite was studied. The electrode exhibited sensitive response to nitrite with a detection limit of 4.5 × 10−5 M (S/N = 3) and a linear range from 1.3 × 10−4 M to 4.4 × 10−2 M. The method was successfully applied to the detection of nitrite in real samples.

Integrating a disposable ITO electrode modified by gold nanorod–chitosan nanocomposites, a paper-based electroanalytical device for the real-time detection of H2O2 released from cancer cells has been developed. It provided a portable platform for biological and biomedical studies dealing with living cells.