Graphene quantum dots (GQDs) have been widely used as fluorescence probes to detect metal ions with satisfactory selectivity. However, the diverse chemical structures of GQDs lead to selectivity for multiple metal ions, and this can lead to trouble in the interpretation of selectivity due to the lack of an in depth and systematic analysis. Herein, bare GQDs were synthesized by oxidizing carbon black with nitric acid and used as fluorescent probes to detect metal ions. We found that the specific ability of GQDs to recognize ferric ions relates to the acidity of the medium. Specifically, we demonstrated that the coordination between GQDs and Fe3+ is regulated by the pH of the aqueous GQDs solution. Dissociative Fe3+ can coordinate with the hydroxyl groups on the surface of the GQDs to form aggregates (such as iron hydroxide), which induces fluorescence quenching. A satisfactory selectivity for Fe3+ ions was achieved under relatively acidic conditions; this is because of the extremely small Ksp of ferric hydroxide compared to those of other common metal hydroxides. To directly survey the key parameter for Fe3+ ion specificity, we performed the detection experiment in an environment free of interference from the buffer solution, noninherent groups, and other complex factors. This study will help researchers understand the selectivity mechanisms of GQDs as fluorescence probes for metal ions, which could guide the design of other GQD-based sensor platforms.

•A PNT–CS composite was prepared by self-assembly of dipeptide in CS solution.•The biocompatible composite combined the advantages of two constituents.•Electrochemical cytosensing for K562 cells was realized based on the composite.•The cytosensor exhibited a sensitive response and its preparation is very simple.A novel peptide nanotube and chitosan composite (PNT–CS) is prepared by self-assembly of dipeptide monomers in a CS aqueous solution. A scanning electron microscopy image showed that the composite had a three-dimensional (3D) network structure. Contact angle analysis indicated good hydrophilicity of the PNT–CS. X-ray diffraction patterns and FT-IR spectra revealed that the basical crystallinity properties and assembled manner of the PNTs in the composite could be maintained. Electrochemical characterization demonstrated that the PNT–CS had good conductivity and excellent stability during the electrochemical measurements. The composite combined the advantages of the two constituent materials and was further employed as an electrochemical cytosensor for the detection of cancer cells (K562). The linear range from 5 × 103 to 5 × 107 cells mL−1 with a detection limit of 630 cells mL−1 was obtained at the PNT–CS-modified electrodes using the electrochemical impedance method. These results are comparable with the previously reported cytosensors based on conventional nanomaterials. Nevertheless, the present method is very simple and suitable for down-up fabrication. The combination of the electrochemical properties, easy preparation, and excellent biocompatibility makes the PNT–CS a promising platform for next generation cytosensors.

Seawater splitting to produce hydrogen and oxygen is compelling but greatly challenging because of the impurity of seawater and the low activity of the existing catalysts. In the present work, Co-Fe layered double hydroxide (Co-Fe LDH) nanoparticles are synthesized by separate nucleation and aging steps, and investigated as electrocatalysts for seawater oxidation at near-neutral pH. Co-Fe LDH loaded onto a Ti-mesh electrode shows high electrocatalytic activity (a current density of 10 mA cm−2 at 530 mV overpotential) and good selectivity for the oxygen evolution reaction at pH 8 in seawater. Furthermore, no additional buffered electrolytes are required, and the catalyst displays an acceptable stability for a diurnal cycle (8 h day−1) under the given operating conditions. The possible mechanism for the enhanced performance is also explored. The synergistic action of Co-Fe LDH and multiple ions of sea salt is proposed. The combination of the electrochemical metrics and the facile and cost-effective synthesis make the Co-Fe LDH as a promising catalyst for a viable renewable energy storage technology.

A water-stable metal–organic framework (MOF) [PCN-333(Fe)] with ultra-large cavities and ultra-high porosity was synthesized and employed to encapsulate horseradish peroxidases (HRP) for the fabrication of electrochemical biosensors. Due to the size-match of a HRP and large cage of PCN-333(Fe), encapsulation of enzyme into the PCN-333(Fe) was achieved. The prepared HRP@PCN-333(Fe) was characterized by XRD, SEM, confocal microscopy, N2 adsorption isotherms, UV-vis spectroscopy and circular dichroism. The analytical performance of the electrochemical biosensor based on the HRP@PCN-333(Fe) for the detection of H2O2 was investigated by cyclic voltammetry and amperometry. Encapsulation of HRP in PCN-333(Fe) presented a high enzyme loading and excellent electrocatalytic activity toward H2O2 reduction. An extended linear range from 0.5 μM to 1.5 mM with a low detection limit of 0.09 μM (S/N = 3) was obtained based on the biosensor. More importantly, the operational acid and thermal stabilities of the biosensor were significantly improved due to the HRP adsorbed on the surface of the support. These good properties are mainly attributed to the confinement of HRP in the cage of PCN-333(Fe), which not only essentially eliminates enzyme aggregation and leaching, and improves the catalytic efficiency, but also effectively hinders the conformational change of immobilized HRP when continuously used, or heated under acidic condition. As a result, the encapsulation of enzymes in the mesopore PCN-333(Fe) provides a new and excellent platform for the development of highly stable and sensitive electrochemical biosensors.

A water-stable metal–organic framework (MOF) [PCN-333(Fe)] with ultra-large cavities and ultra-high porosity was synthesized and employed to encapsulate horseradish peroxidases (HRP) for the fabrication of electrochemical biosensors. Due to the size-match of a HRP and large cage of PCN-333(Fe), encapsulation of enzyme into the PCN-333(Fe) was achieved. The prepared HRP@PCN-333(Fe) was characterized by XRD, SEM, confocal microscopy, N2 adsorption isotherms, UV-vis spectroscopy and circular dichroism. The analytical performance of the electrochemical biosensor based on the HRP@PCN-333(Fe) for the detection of H2O2 was investigated by cyclic voltammetry and amperometry. Encapsulation of HRP in PCN-333(Fe) presented a high enzyme loading and excellent electrocatalytic activity toward H2O2 reduction. An extended linear range from 0.5 μM to 1.5 mM with a low detection limit of 0.09 μM (S/N = 3) was obtained based on the biosensor. More importantly, the operational acid and thermal stabilities of the biosensor were significantly improved due to the HRP adsorbed on the surface of the support. These good properties are mainly attributed to the confinement of HRP in the cage of PCN-333(Fe), which not only essentially eliminates enzyme aggregation and leaching, and improves the catalytic efficiency, but also effectively hinders the conformational change of immobilized HRP when continuously used, or heated under acidic condition. As a result, the encapsulation of enzymes in the mesopore PCN-333(Fe) provides a new and excellent platform for the development of highly stable and sensitive electrochemical biosensors.

Rational design and fabrication of electrocatalytic nanomaterials is an effective approach for improving the sensitivity and stability of electrochemical sensors. Here, we report a novel CoAl-layered double hydroxide nanowall-supported Pd nanoparticle (PdNP/CoAl-LDHNW) composite as an electrochemical sensing platform for highly sensitive and durable detection of hydrazine. CoAl-LDHNWs, composed of vertical and interconnected two-dimensional LDH nanosheets, were synthesized on the surface of indium tin oxide (ITO) electrodes via a hydrothermal method. Next, highly dispersed PdNPs (∼2.4 nm) were anchored on the surface of CoAl-LDHNWs through a facile in situ reduction reaction between the PdCl42− in solution and Co2+ in the support. The resultant PdNP/CoAl-LDHNW-modified ITO electrode exhibited excellent electrocatalytic activity for the electrochemical oxidation of hydrazine in a neutral solution. The electrochemical sensor showed a linear dynamic range of 0.1 to 655 μM with a low detection limit of 10 nM at −0.1 V; these values are superior to those of most previously reported PdNP-based composite-modified electrodes. The outstanding electroanalytical performance is mainly attributed to sufficient exposure of active sites due to the fine dispersion and clean surfaces of small size PdNPs on three-dimensional CoAl-LDHNWs. Additionally, the sensor exhibited good stability. In our perception, this sensor represents a valuable tool for advanced sensing of hydrazine.

Rational design and fabrication of electrocatalytic nanomaterials is an effective approach for improving the sensitivity and stability of electrochemical sensors. Here, we report a novel CoAl-layered double hydroxide nanowall-supported Pd nanoparticle (PdNP/CoAl-LDHNW) composite as an electrochemical sensing platform for highly sensitive and durable detection of hydrazine. CoAl-LDHNWs, composed of vertical and interconnected two-dimensional LDH nanosheets, were synthesized on the surface of indium tin oxide (ITO) electrodes via a hydrothermal method. Next, highly dispersed PdNPs (∼2.4 nm) were anchored on the surface of CoAl-LDHNWs through a facile in situ reduction reaction between the PdCl42− in solution and Co2+ in the support. The resultant PdNP/CoAl-LDHNW-modified ITO electrode exhibited excellent electrocatalytic activity for the electrochemical oxidation of hydrazine in a neutral solution. The electrochemical sensor showed a linear dynamic range of 0.1 to 655 μM with a low detection limit of 10 nM at −0.1 V; these values are superior to those of most previously reported PdNP-based composite-modified electrodes. The outstanding electroanalytical performance is mainly attributed to sufficient exposure of active sites due to the fine dispersion and clean surfaces of small size PdNPs on three-dimensional CoAl-LDHNWs. Additionally, the sensor exhibited good stability. In our perception, this sensor represents a valuable tool for advanced sensing of hydrazine.

An electrochemical sensor for hydrogen peroxide (H2O2) has been fabricated by electrodepositing gold nanoparticles (AuNPs) on an indium tin oxide (ITO) electrode modified with cobalt and manganese-layered double hydroxides (CoMn-LDHs). Scanning electron microscopy images reveal well-dispersed AuNPs with a typical size of 12 nm on the surface of the layered double hydroxides. Electrochemical characterizations show that the introduction of the LDH support significantly enhances the voltammetric response to H2O2. This is underpinned by a comparative study that uses single transition metal-LDHs (CoAl-LDHs or MgMn-LDHs) as supports. The results indicate that the presence of binary transition metals in the LDH layer may result in multiple interaction and interfacial effects between AuNPs and the support. At an optimal working voltage of +0.55 V (vs. Ag/AgCl), the sensor displays a wide linear range (0.1 μM to 1.27 mM), low detection limit (0.06 μM) and high sensitivity (125.0 μA∙mM−1∙cm−2), which are characteristics superior to those of most previously reported AuNP-based composite-modified electrodes. The sensor has good reproducibility and storage stability. In our view, this sensor represents a valuable tool for advanced sensing of H2O2.

A self-assembled peptide nanofibrous hydrogel composed of N-fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) was used to construct a smart biointerface. This biointerface was then used for enzyme-based electrochemical biosensing and cell monitoring. The Fmoc-FF hydrogel had two functions. One was as a matrix to embed an enzyme model, horseradish peroxidase (HRP), during the self-assembly of Fmoc-FF peptides. The other was use as a robust substrate for cell adhesion. Experimental data demonstrated that HRP was immobilized in a stable manner within the peptide hydrogel, and that HRP retained its inherent bioactivity toward H2O2. The HRP also can realize direct electron transfer in the Fmoc-FF hydrogel. The resulting third-generation electrochemical H2O2 biosensor exhibited good analytical performance, including a low limit of detection of 18 nM, satisfactory reproducibility, and high stability and selectivity. HeLa cells were then adhered to the HRP/Fmoc-FF hydrogel-modified electrode. The sensitive in situ monitoring of H2O2 released from HeLa cells was realized. This biointerface based on the Fmoc-FF hydrogel was easily prepared, environmentally friendly, and also versatile for integration of other cells and recognized molecules for the monitoring of various cellular biomolecules. The smart biointerface has potential application in broad physiological and pathological investigations.Keywords: cell monitoring; electrochemical biosensing; H2O2; peptide hydrogel; self-assembly

Anodic electrogenerated chemiluminescence (ECL) of the self-assembled peptide nanotube (PNT) modified electrode in an aqueous system was observed for the first time using tri-n-propylamine (TPrA) as the coreactant. The potential application of ECL PNTs in analytical chemistry was also demonstrated using Cu2+ as an example.

•Dipeptide–AuNP 3D hybrid spheres were prepared by a simple self-assembled process.•The hybrid sphere has excellent biocompatibility and fast charge-transport ability.•A high-performance amperometric H2O2 biosensor was obtained based on the hybrid.•The 3D hybrid spheres provide a promising electrochemical biosensing platform.Novel self-assembled dipeptide–gold nanoparticle (DP–AuNP) hybrid microspheres with a hollow structure have been prepared in aqueous solution by a simple one-step method. Diphenylalanine (FF) dipeptide was used as a precursor to form simultaneously peptide spheres and a reducing agent to reduce gold ions to gold nanoparticles in water at 60 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that formed AuNPs were localized both inside and on the surface of the dipeptide spheres. Horseradish peroxidase (HRP) as a model enzyme was further immobilized on the dipeptide–AuNP hybrid spheres to construct a mediate H2O2 amperometric biosensor. UV–vis spectroscopy showed that the immobilized HRP retained its original structure. Cyclic voltammetry characterization demonstrated that the HRP/dipeptide–AuNP hybrid spheres modified glassy carbon electrode showed high electrocatalytic activity to H2O2. The proposed biosensor exhibited a wide linear response in the range from 5.0×10−7 to 9.7×10−4 M with a high sensitivity of 28.3 µA mM−1. A low detection limit of 1.0×10−7 M was estimated at S/N=3. In addition, the biosensor possessed satisfactory reproducibility and long-term stability. These results indicated that the dipeptide–AuNP hybrid sphere is a promising matrix for application in the fabrication of electrochemical biosensors due to its excellent biocompatibility and good charge-transfer ability.

•The ECL behavior of peptide nanovesicles was firstly observed.•The possible ECL reaction mechanism was explored.•Based on the ECL response, highly sensitive detection of dopamine was realized.The electrogenerated chemiluminescence (ECL) behavior of the bioinspired peptide nanovesicles (PNVs) was reported for the first time. The PNVs modified glassy carbon electrodes have shown a stable and efficient cathodic ECL signal with K2S2O8 as coreactant in aqueous solution. The possible ECL reaction mechanism was proposed. Dopamine (DA) was chosen as a model analyte to study the potential of the PNVs in the ECL analytical application. It was found that the ECL intensity of the PNVs was effectively increased by trace amounts of DA. The limit of detection was estimated to be 3.15 pM (S/N=3). These results suggest that the PNVs could be a new class of promising materials for the ECL design and bioassays in the future due to their fascinating features, such as excellent biocompatibility, tunable composition as well as capability of molecular recognition.

Co3O4/Co2MnO4 nanocomposites, derived from a single-source CoMn-layered double hydroxide precursor, exhibit excellent bifunctional oxygen electrode activities for both oxygen reduction and evolution reactions, which can be attributed to the large specific surface area and well-dispersed heterogeneous structure of the nanocomposites.

Controlling the macroscopic organization of self-assembled peptide nanostructures on a solid surface is a key challenge in enabling their technological applications. Here, we report a simple approach to achieve the horizontally organized self-assembly of dipeptides by introducing graphene sheets. We show at the first time the formation of a macroscopic-scale, high-density, and ordered interlaced array of peptide nanowires and graphene composite (PNWs-G) on a silicon surface under mild conditions. The action of graphene sheets in the formation of the organized bionanostructure was preliminarily investigated. Furthermore, due to the introduction of graphene, the electronic conductivity of the bionanostructures was greatly improved, which is very beneficial for their applications in bioelectrochemical and nanoelectronic devices. As an applied example, the significantly enhanced electrochemical sensing performance for dihydronicotinamide adenine dinucleotide (NADH) was also demonstrated at the PNWs-G modified electrode relative to the alone component and unordered composite modified electrodes. The simple and mild approach described here opens a new avenue for the fabrication of macroscopic-scale organized self-assembled peptide bionanostructures on a solid surface, which should be capable of being extended to other biosystems based on graphite surface-template assembly, allowing a variety of functional bionanostructures to be fabricated and used in practical applications.

Core–shell structure Fe3O4@graphene oxide (GO) submicron particles have been prepared via a simple electrostatic self-assembly process. The Fe3O4@GO particles had good dispersibility in water, high saturation magnetization and sensitive magnetic response. Bovine serum albumin (BSA) was chosen as model protein to study the efficacy of the Fe3O4@GO particles for protein adsorption. By virtue of the combined benefits of GO and Fe3O4, the Fe3O4@GO particles exhibited large adsorption capacity (181.8 mg/g) and fast adsorption kinetics for BSA. The performance of GO as a shell material for core–shell magnetic composites was found to be superior to that of conventional shell materials such as polymers and silicon, thus demonstrating the great potential of the Fe3O4@GO particles for application in magnetic bioseparation.Highlights► Core-shell structural Fe3O4@graphene oxide (GO) submicron particles were prepared. ► The Fe3O4@GO particles had high saturation magnetization. ► The Fe3O4@GO particles showed large adsorption capacity for BSA. ► A new strategy to apply GO in magnetic bioseparation.

A uniform three-dimensional (3D) gold nanoparticle (AuNP)–embedded porous graphene (AuEPG) thin film has been fabricated by electrostatic layer-by-layer assembly of AuNPs and graphene nanosheets functionalized with bovine serum albumin and subsequent thermal annealing in air at 340 °C for 2 h. Scanning electron microscopy (SEM) investigations for the AuEPG film indicate that an AuNP was embedded in every pore of the porous graphene film, something that was difficult to achieve with previously reported methods. The mechanism of formation of the AuEPG film was initially explored. Application of the AuEPG film in electrochemical sensing was further demonstrated by use of H2O2 as a model analyte. The AuEPG film-modified electrode showed improved electrochemical performance in H2O2 detection compared with nonporous graphene–AuNP composite film-modified electrodes, which is mainly attributed to the porous structure of the AuEPG film. This work opens up a new and facile way for direct preparation of metal or metal oxide nanoparticle–embedded porous graphene composite films, which will enable exciting opportunities in highly sensitive electrochemical sensors and other advanced applications based on graphene–metal composites.

A high performance electrochemical sensor for the detection of acetaminophen (APAP) at the single-walled carbon nanotube (SWCNT)–graphene nanosheet (GNS) hybrid film modified electrode is reported. The morphology and pore structure of the SWCNT–GNS hybrid were characterized by field emission scanning electron microscopy (SEM) and nitrogen isothermal adsorption–desorption technique, respectively. The electrochemical behavior of APAP at the SWCNT–GNS modified glassy carbon (GC) electrode was investigated by cyclic voltammetry. The results showed significantly enhanced electrochemical reactivity and voltammetric response of APAP. The electrochemical sensor based on the hybrid modified electrode exhibited excellent analytical performance for APAP detection in neutral solution with a low detection limit of 38 nM and a wide linear range of 0.05–64.5 μM. Moreover, the SWCNT–GNS/GC electrode also showed good selectivity and stability. The high performances of the novel APAP sensor are mainly attributed to high surface area and multi-modal pore structure of the SWCNT–GNS hybrid, which provide an increased sensing area and effective mass transportation pathway.

Layered double hydroxides (LDHs), also known as hydrotalcite-like anionic clays, have been investigated widely as promising electrochemical active materials. Due to the inherently weak conductivity, the electrochemical properties of LDHs were improved typically by utilization of either functional molecules intercalated between LDH interlayer galleries, or proteins confined between exfoliated LDH nanosheets. Here, we report a facile protocol to prepare NiAl-LDH/graphene (NiAl-LDH/G) nanocomposites using a conventional coprecipitation process under low-temperature conditions and subsequent reduction of the supporting graphene oxide. Electrochemical tests showed that the NiAl-LDH/G modified electrode exhibited highly enhanced electrochemical performance of dopamine electrooxidation in comparison with the pristine NiAl-LDH modified electrode. Results of high-resolution transmission electron microscopy and Raman spectra provide convincing information on the nanostructure and composition underlying the enhancement. Our results of the NiAl-LDH/G modified electrodes with the enhanced electrochemical performance may allow designing a variety of promising hybrid sensors via a simple and feasible approach.

The direct electrochemistry of glucose oxidase (GOD) immobilized in a modified electrode based on a composite film of exfoliated graphite nanosheets (GNSs) and Nafion has been investigated for the first time. Direct electron communication between GOD and the electrode was achieved with a fast electron transfer rate (12.6 s−1). In addition, the bioactivity of GOD was retained after immobilization in the composite film and glucose could be determined based on the decrease of the electrocatalytic response of the reduced form of GOD to dissolved oxygen. The resulting biosensor exhibited higher sensitivity (3.4 μA mM−1). Considering much lower cost of GNSs and ready preparation from graphite, the GNSs-based modified electrode described here is superior to the carbon nanotubes (CNTs)-based modified electrodes and should have wide applications in third-generation biosensors, bioelectronics and electrocatalysis.

Anodic electrogenerated chemiluminescence (ECL) of the self-assembled peptide nanotube (PNT) modified electrode in an aqueous system was observed for the first time using tri-n-propylamine (TPrA) as the coreactant. The potential application of ECL PNTs in analytical chemistry was also demonstrated using Cu2+ as an example.