Wendong Zhang

Progress of new label-free techniques for biosensors: a review

Abstract The detection techniques used in biosensors can be broadly classified into label-based and label-free. Label-based detection relies on the specific properties of labels for detecting a particular target. In contrast, label-free detection is suitable for the target molecules that are not labeled or the screening of analytes which are not easy to tag. Also, more types of label-free biosensors have emerged with developments in biotechnology. The latest developed techniques in label-free biosensors, such as field-effect transistors-based biosensors including carbon nanotube field-effect transistor biosensors, graphene field-effect transistor biosensors and silicon nanowire field-effect transistor biosensors, magnetoelastic biosensors, optical-based biosensors, surface stress-based biosensors and other type of biosensors based on the nanotechnology are discussed. The sensing principles, configurations, sensing performance, applications, advantages and restriction of different label-free based biosensors are considered and discussed in this review. Most concepts included in this survey could certainly be applied to the development of this kind of biosensor in the future.

Surface stress-based biosensors.

Surface stress-based biosensors, as one kind of label-free biosensors, have attracted lots of attention in the process of information gathering and measurement for the biological, chemical and medical application with the development of technology and society. This kind of biosensors offers many advantages such as short response time (less than milliseconds) and a typical sensitivity at nanogram, picoliter, femtojoule and attomolar level. Furthermore, it simplifies sample preparation and testing procedures. In this work, progress made towards the use of surface stress-based biosensors for achieving better performance is critically reviewed, including our recent achievement, the optimally circular membrane-based biosensors and biosensor array. The further scientific and technological challenges in this field are also summarized. Critical remark and future steps towards the ultimate surface stress-based biosensors are addressed.

One‐Dimensional Nano‐Interconnection Formation

Interconnection of one-dimensional nanomaterials such as nanowires and carbon nanotubes with other parts or components is crucial for nanodevices to realize electrical contacts and mechanical fixings. Interconnection has been being gradually paid great attention since it is as significant as nanomaterials properties, and determines nanodevices performance in some cases. This paper provides an overview of recent progress on techniques that are commonly used for one-dimensional interconnection formation. In this review, these techniques could be categorized into two different types: two-step and one-step methods according to their established process. The two-step method is constituted by assembly and pinning processes, while the one-step method is a direct formation process of nano-interconnections. In both methods, the electrodeposition approach is illustrated in detail, and its potential mechanism is emphasized.

Preparation and characterization of AuNPs/CNTs-ErGO electrochemical sensors for highly sensitive detection of hydrazine

A highly sensitive electrochemical sensor of hydrazine has been fabricated by Au nanoparticles (AuNPs) coating of carbon nanotubes-electrochemical reduced graphene oxide composite film (CNTs-ErGO) on glassy carbon electrode (GCE). Cyclic voltammetry and potential amperometry have been used to investigate the electrochemical properties of the fabricated sensors for hydrazine detection. The performances of the sensors were optimized by varying the CNTs to ErGO ratio and the quantity of Au nanoparticles. The results show that under optimal conditions, a sensitivity of 9.73μAμM(-1)cm(-2), a short response time of 3s, and a low detection limit of 0.065μM could be achieved with a linear concentration response range from 0.3μM to 319μM. The enhanced electrochemical performances could be attributed to the synergistic effect between AuNPs and CNTs-ErGO film and the outstanding catalytic effect of the Au nanoparticles. Finally, the sensor was successfully used to analyse the tap water, showing high potential for practical applications.

Crystal-facet-controllable synthesis of Cu2O microcrystals, shape evolution and their comparative photocatalytic activity

Single crystal Cu2O materials with morphologies of cube, truncated cube, truncated octahedron, octahedron etc. have been synthesized by a facile glucose reduction method. Polyvinylpyrrolidone concentration dependent, crystal-facet-controllable growth mechanism of the synthesized Cu2O crystals has been proposed in the article. The photocatalytic test results show that the octahedral Cu2O crystals with dominant {111} planes have super photocatalytic activity than the cubic ones packaged by {100} surfaces, which could be explained by that the polar, highly active {111} facets with unsaturated “Cu” have more excellent activity on the methyl orange dye degradation. This article will contribute to promote the morphology construction and crystal-dependent catalytic properties of metal oxide semiconductor materials.

Investigation of Charge Injection and Relaxation in Multilayer Dielectric Stacks for Capacitive RF MEMS Switch Application

This paper proposes a new approach to the problem of irreversible stiction of capacitive radio frequency (RF) microelectromechanical (MEMS) switch attributed to the dielectric charging. We investigate how charge accumulates in multi- and single-layer dielectric structures for a capacitive RF MEMS switch using metal-insulator-semiconductor (MIS) capacitor structure. Two multidielectric-layers are structured, which are SiO2+Si3N4 and SiO2+Si3N4+SiO2 stack films. Meanwhile, Si3N4 single-layer dielectric structure is also fabricated for comparison. In the experiments, the space charges are first injected into the dielectric layers by stressing MIS devices with a dc bias; then the injected charge kinetics are monitored by capacitance-voltage measurement before and after charge injection. We found that the polarity of charge accumulated in the dielectric is strongly influenced by the dielectric structure. When the metal electrode is positively biased, a negative charge accumulates in the single and triple-layer devices, while a positive charge accumulates in the double-layer devices. Furthermore, the experiment results also show that the lowest charge accumulation can be achieved using double-layer dielectric structure even though the fastest relaxation process takes place in triple-layer dielectric structure.

α-Fe2O3 concave and hollow nanocrystals: top-down etching synthesis and their comparative photocatalytic activities

In this work, uniform single-crystalline α-Fe2O3 concave and hollow nanocrystals have been synthesized by a facile top-down etching method. Phosphate ions were employed as an etching agent to etch pristine icositetrahedra along their [006] direction in a top-down manner. The etched Fe2O3 concave and hollow nanocrystals exhibit a much larger specific surface area (SSA) and more edges, corners and rough structures (hot spots) on the crystal surface compared to their mother crystal, thus serving as the possible origin of their comparative photocatalytic activities in the degradation of cationic dye Rhodamine B. These characteristics make them promising candidates as catalysts and sensing materials.

Low-frequency and wideband vibration energy harvester with flexible frame and interdigital structure

As an alternative to traditional cantilever beam structures and their evolutions, a flexible beam based, interdigital structure, vibration energy harvester has been presented and investigated. The proposed interdigital-shaped oscillator consists of a rectangular flexible frame and series of cantilever beams interdigitally bonded to it. In order to achieve low frequency and wide-bandwidth harvesting, Young’s modulus of materials, frame size and the amount of the cantilevers have been studied systematically. The measured frequency responses of the designed device (PDMS frame, quintuple piezoelectric cantilever beams) show a 460% increase in bandwidth below 80Hz. When excited at an acceleration of 1.0 g, the energy harvester achieves to a maximum open-circuit voltage of 65V, and the maximum output power 4.5 mW.

Fabrication and characterization of silicon nanostructures based on metal-assisted chemical etching

We present a facile method to fabricate one-dimensional Si nanostructures based on Ag-induced selective etching of silicon wafers. To obtain evenly distributed Si nanowires (SiNWs), the fabrication parameters have been optimized. As a result, a maximum of average growth rate of 0.15 μm/min could be reached. Then, the fabricated samples were characterized by water contact angle (CA) experiments. As expected, the as-etched silicon samples exhibited a contact angle in the range of 132°–136.5°, whereas a higher contact angle (145°) could be obtained by chemical modification of the SiNWs with octadecyltrichlorosilane (OTS). Additionally, Raman spectra experiments have been carried out on as-prepared nanostructures, showing a typical decreasing from 520.9 cm−1 to 512.4 cm−1 and an asymmetric broadening, which might be associated with the phonon quantum confinement effect of Si nanostructures.

Fe2O3 icositetrahedra: evolution and their comparative photocatalytic activities

Single crystal Fe2O3 icositetrahedra have been synthesized by a facile hydrothermal synthesis method in the presence of dihydrogen phosphate ions. The icositetrahedron particles are of hexagonal shape and enclosed by 24 quadrilaterals. It could be seen as a particle with six equivalent {110} facets added to the truncated hexagonal bipyramidal structure, that is, it is enclosed by six {110} planes, six {104} planes and twelve {113} planes. Dihydrogen phosphate ions play an important role in the formation of α-Fe2O3 icositetrahedron particles, because high concentration of [H2PO4]− drives the ferric oxide nanocrystals growth along the [006] zone axis. Furthermore, the as-synthesized Fe2O3 icositetrahedron single crystals show comparative photocatalytic activities on the degradation of cationic dye Rhodamine B, which make them promising candidates for catalysts and sensing materials.