A sedimentation study of graphene oxide in aqueous solution using gradient differential centrifugation.

This work involved the study of sedimentation of graphene oxide (GO) in aqueous solution by gradient differential centrifugation. GO sheets of size varying from 400 nm to 1100 nm were separated with layer numbers ranging from 2-17. Semiquantitative analysis of FT-IR spectra was conducted based on statistical variance, in which relative oxidation and hydration degrees were numeralized. Combining XRD, optical microscopy and particle size measurements, the dominant effects of hydration and d-spacing on GO sediment in aqueous solution were observed. However, lateral particle shifting showed a relatively insignificant influence even though it has much larger effects on the GO weight compared with the GO thickness. Highly oxidized GO sheets were highly hydrated and had the tendency to face more severe resistance in aqueous sedimentation. Larger d-spacing allowed more water molecules to enter into interlayers and thus improved the degree of hydration, the lower density and the lower sedimentation velocity of GO sheets. However, fast sedimentation could be found in both large and small GO sheets due to the relatively non-dominant effect from lateral size shifting. The underlying mechanism was further probed by the mathematical modeling process. Our study reveals the potential limitations of current theory for explaining GO sedimentation and also demonstrates the effectiveness of gradient differential centrifugation for sorting GO sheets varying in hydration degree and thickness.

Physisorption and chemisorption of a self-assembled monolayer by the quartz crystal microbalance.

A new adsorption model was developed to investigate the adsorption process of SAMs in the gaseous scenario, and the results were verified by using a prototype quartz crystal microbalance (QCM) sensor. In the experimental study, the observed properties in the gaseous scenario did not conform to the conventional theories well but matched the proposed adsorption model better. Hence, an optimal methodology for the theoretical study of adsorption process of SAMs was developed. The conventional adsorption model of Langmuir is suitable only for the fast initial adsorption step (i.e., physisorption) in the process of forming SAMs in the liquid scenario. Here, the rates of adsorption and desorption (ka, kd) at different temperatures were investigated. The activation barrier Ea = 59.738 kJ/mol was obtained by the Arrhenius equation. The result agreed well with that obtained experimentally. More importantly, this study has established a new avenue of QCM chip applications.

Study on non-bioparticles and Staphylococcus aureus by dielectrophoresis.

This article demonstrated a chip device with alternating current (AC) dielectrophoresis (DEP) for separation of non-biological micro-particle and bacteria mixtures. The DEP separation was achieved by a pair of metal electrodes with the shape of radal-interdigital to generate a localized non-uniform AC electric field. The electric field and DEP force were firstly investigated by finite element methods (FEM). The mixed microparticles such as different scaled polystyrene (PS) beads, PS beads with inorganic micro-particles (e.g., ZnO and silica beads) and non-bioparticles with bacterial Staphylococcus aureus (S. aureus) were successfully separated at DEP-on-a-chip by an AC electric field of 20 kHz, 10 kHz and 1 MHz, respectively. The results indicated that DEP trapping can be considered as a potential candidate method for investigating the separation of biological mixtures, and may well prove to have a great impact on in situ monitoring of environmental and/or biological samples by DEP-on-a-chip.

A review of polystyrene bead manipulation by dielectrophoresis.

Exploitation of the intrinsic electrical properties of particles has recently emerged as an appealing approach for trapping and separating various scaled particles. Initiative particle manipulation by dielectrophoresis (DEP) showed remarkable advantages including high speed, ease of handling, high precision and being label-free. Herein, we provide a general overview of the manipulation of polystyrene (PS) beads and related particles via DEP; especially, the wide applications of these manipulated PS beads in the quantitative evaluation of device performance for model validation and standardization have been discussed. The motion and polarizability of the PS beads induced by DEP were analyzed and classified into two categories as positive and negative DEP within the time and space domains. The DEP techniques used for bioparticle manipulation were demonstrated, and their applications were conducted in four fields: trapping of single-sized PS beads, separation of multiple-sized PS beads by size, separation of PS beads and non-bioparticles, and separation of PS beads and bioparticles. Finally, future perspectives on DEP-on-a-chip have been proposed to discriminate bio-targets in the network of microfluidic channels.

Quantification of Staphylococcus aureus using surface acoustic wave sensors.

Quartz crystal microbalance (QCM), surface acoustic wave (SAW)-Rayleigh and ZnO based SAW-Love sensors were fabricated and their sensitivity was comparatively analyzed for the quantification of Staphylococcus aureus (S. aureus). The SAW based sensors showed response magnitudes up to three times greater than that of the QCM sensor for the same mass loading of S. aureus. The ZnO nanoparticle-based SAW-Love sensor has a maximum mass loading sensitivity of 328.75 Hz ng-1. The SAW-Love sensor achieved a lower limit of detection of 2 × 103 CFU mL-1 compared to the QCM counterpart (2 × 105 to 2 × 106 CFU mL-1) under the same conditions. The SAW-Love sensor could be used as a disposable chip in micro or ultra-trace accurate diagnosis methods.

Bond-rupture immunosensors--a review.

It has long been the goal of researchers to develop fast and reliable point-of-care alternatives to existing lab-based tests. A viable point-of-care biosensor is fast, reliable, simple, cost-effective, and detects low concentrations of the target analyte. The target of biosensors is biological such as bacteria or virus and as such, the antibody-antigen bond derived from the real immune response is used. Biosensor applications include lab-based tests for the purposes of diagnostics, drug discovery, and research. Additional applications include environmental, food, and agricultural monitoring. The main merits of the bond-rupture method are quick, simple, and capable of discriminating between specific and non-specific interactions. The separation of specific and non-specific bonds is important for working in real-life complex serums such as blood. The bond-rupture technique can provide both qualitative results, the detection of a target, and quantitative results, the concentration of target. Bond-rupture achieves this by a label-free method requiring no pre-processing of the analyte. A piezoelectric transducer such as the quartz crystal microbalance (QCM) shakes the bound particles free from the surface. Other transducers such as Surface Acoustic Wave (SAW) are also considered. The rupture of the bonds is detected as electronic noise. This review article links diverse research areas to build a picture of a field still in development.

Implementation of guiding layers of surface acoustic wave devices: A review.

The purpose of overviewing research and development status of dependable, efficient, and portable and miniaturized surface acoustic wave (SAW) is to propose practical devices for biosensing and medical diagnosis. SAW Love-mode sensors fortunately have a great deal of attention during last two decades. Several periodic structure models of SAW devices were reviewed, especially interdigital transducers (IDTs), wave guiding layers, patterned-ZnO. SAW devices based on such periodic wave guiding layers and patterned-ZnO were demonstrated with superior performance, much better than conventional SAW devices. Both 2D and 3D models of phononic-crystal-based SAW devices can be respectively fabricated by an array of periodic cylindrical holes and pillars, which allowed SAW devices to have both higher Q-factor and GHz-level frequency. Ring waveguide and spherical SAW devices would have potential applications and implementation in biosensing. ZnO is one of attractive guiding-layer materials. Its nanostructures, such as nanowires, nanorods and nanofibers provided with excellent properties, will make nanoscaled SAW devices contribute to be much more sensitive in biosensors. A range of applications based on SAW and ZnO guiding-layer would be therefore expected among of immunochemical analysis, in-situ virus or bacteria determination, microfluidic automation, and cell manipulation.

Application of Vertical Electrodes in Microfluidic Channels for Impedance Analysis.

This paper presents a microfluidic device with electroplated vertical electrodes in the side walls for impedance measurement. Based on the proposed device, the impedance of NaCl solutions with different concentrations and polystyrene microspheres with different sizes was measured and analyzed. The electroplating and SU-8-PDMS (SU-8-poly(dimethylsiloxane)) bonding technologies were firstly integrated for the fabrication of the proposed microfluidic device, resulting in a tightly three-dimensional structure for practical application. The magnitude of impedance of the tested solutions in the frequency range of 1 Hz to 100 kHz was analyzed by the Zennium electrochemical workstation. The results show that the newly designed microfluidic device has potential for impedance analysis with the advantages of ease of fabrication and the integration of 3D electrodes in the side walls. The newly designed impedance sensor can distinguish different concentrations of polystyrene microspheres and may have potential for cell counting in biological areas. By integrating with other techniques such as dielectrophoresis (DEP) and biological recognition technology, the proposed device may have potential for the assay to identify foodborne pathogen bacteria.

Effect of pH and Ca2+-induced associations of soybean proteins.

An experimental procedure was developed to characterize the solubility of the soybean protein fractions close to the isoelectric point. The results show that the 7S fraction is precipitated in a much narrower range of pH values than the 11S fraction. Surprisingly, the addition of salt to the solutions leads to increased solubility of proteins, unlike the common "salting out" effect generally expected for proteins in solution in this range of salt concentrations. The precipitation equilibria of both soybean fractions in the presence of calcium ions and electrolyte were characterized. The amount of calcium ions required to precipitate a mole of the 7S fraction is much larger than that required for the 11S fractions. The precipitation pattern can be correlated to the charge density per surface area of the proteins.

Study on pivot-point vibration of molecular bond-rupture events by quartz crystal microbalance for biomedical diagnostics.

Bond-rupture scanning for biomedical diagnostics is examined using quartz crystal microbalance (QCM) experiments and microparticle mechanics modeling calculations. Specific and nonspecific interactions between a microparticle and its binding QCM surface can be distinguished by gradually increasing the amplitude of driving voltage applied to QCM and monitoring its frequency changes. This research proposes a mechanical model of interactions between biological molecules and a QCM substrate surface. The mechanical force required to break a biotin-streptavidin bond was calculated through a one-pivot-point bottom-up vibration model. The bond-rupture force increases with an increase of the microparticle radius, the QCM resonant frequency, and the amplitude of driving voltage applied to the QCM. The significance of the research on biological molecular bond rupture is extremely important in characterizing microbial (such as cells and virus) specificity, due to the force magnitude needed to break bonds using a transducer.

Characterization of molecular interactions of an immobilized biotinylated monolayer and streptavidin-coated microspheres by bond-rupture scanning.

Bond-rupture approach has been used in the understanding of biomolecular interactions of highly specific recognition, e.g., an antibody and its antigen, by a functionalized and self-assembled monolayer (SAM). One of the most challenging issues of diagnostics is to distinguish between true binding and the ever-present non-specific binding in which a species gives false results in conventional affinity methods. In this study, bond-rupture scanning was proposed to characterize bindings by introducing energy mechanically through displacement of a resonant quartz crystal. This system was able to measure the resonant frequency difference, due to mass changes and bond breakages between supramolecular interaction of biotinylated SAM and streptavidin-coated polystyrene microsphere (SCPM). Both 2-µm and 4-µm of SCPMs revealed two recognized desorption patterns at 4 V and 2 V amplitudes respectively. It rapidly provided confirmation of the presence of a target analyte. From this study, it can be shown that an established approach of dynamic bond-rupture scanning can be adopted as a promising diagnostic tool for investigating various interactions of bacteria or virus on an immobilized biomolecular surface by measuring the characteristic level of mechanical energy required to break bonds.

Bond rupture of biomolecular interactions by resonant quartz crystal.

Significant progress has been achieved in understanding affinity-based diagnostics, which use the highly specific "lock and key" recognition and binding between biomolecules, for example, an antibody and its antigen. These are the most specific of analytical tests. One of the most challenging issues is to distinguish between true binding and ever-present nonspecific binding in which more loosely bound proteinaceous material gives false results in conventional affinity methods. We have used bond-rupture scanning to eliminate nonspecific binding by introducing energy mechanically through displacement of a resonant quartz crystal. The removal of the analyte was recorded with a simple all-electronic detection system quickly providing confirmation of the presence of the target molecule. The system can measure the resonant frequency difference and detect noise signals, respectively, due to mass changes and bond breaks between biotinylated self-assembled monolayer (SAM) and streptavidin-coated polystyrene microspheres (SCPM). Both static and dynamic scanning modes can reveal previously unrecognized desorption of streptavidin-coated polystyrene microspheres. An established framework of bond-rupture scanning is a promising diagnostic tool for investigating the specific and nonspecific interactions by measuring the characteristic level of mechanical energy required to break the bond.