Wireless excitation of quartz crystals immersed in an aqueous fluid

A novel toroidal coil geometry able to induce remote acoustic waves in quartz crystals has been evaluated for the development of (bio)sensors. Remote acoustic generation in air was obtained for two alternative toroidal coils, with corresponding electrical impedance changes of 40 Omega for a PDMS- and 140 Omega for a ferrite-supported toroid respectively. It was found that the range of remote acoustic generation relative to the spiral coil standard was much improved, increasing the axial separation of their resonant sensing element from 0.1 mm to 20 mm, thereby allowing electromagnetic wave penetration across glass walls and fluid media to be utilised. Consideration of the transduction mechanism, along with measured cyclic changes in acoustic signal as a function of rotation, indicated that the large PDMS toroidal coil produced an asymmetric electric field. It was shown for the first time that a quartz crystal blank fully immersed in an aqueous fluid could support chemically sensitive shear acoustic standing waves that were excited and detected remotely. A signal to noise ratio of 30 ratio 1 at 20.13 MHz was achieved by placing a ferrite supported toroidal coil on the lower side of a glass beaker containing a 12 x 0.25 mm AT crystal blank and 1 mL of water. This discovery allows wireless shear acoustic wave measurements to be performed with total separation between the electronic detection system and assays undertaken in fluidic systems.     

Adsorption behavior of cytochrome c, myoglobin and hemoglobin in a quartz surface probed using slab optical waveguide (SOWG) spectroscopy.

Slab optical waveguide (SOWG) spectroscopy was used to observe the adsorption behavior of three important heme proteins, namely cytochrome c, myoglobin and hemoglobin, in a quartz surface. Using prism-coupled polychromatic visible light propagated into a quartz waveguide by internal total reflection, the real-time monitoring of evanescent wave absorption revealed a strong dependence of the protein-surface interaction on the protein concentration, the solution pH and the ionic strength. For the three proteins studied, the absorbance-bulk concentration ratio was higher at low bulk concentrations, and decreased at higher concentrations. For cytochrome c and myoglobin, the absorbance approached a limiting value, but buffered hemoglobin surprisingly did not show any indication of forming a signal plateau. Moreover, the slow introduction of protein into the solution lessened the total adsorbed amount per unit area. These observations suggested a possible conformational transition of the protein molecules at the quartz surface after adsorption. For a bulkier protein, hemoglobin, adsorption onto the quartz surface was enhanced in the presence of a phosphate buffer, while the opposite effect was observed for the smaller cytochrome c and myoglobin molecules. The results of pH studies concurred with the electrostatic interactions predicted from the isoelectric data of proteins and the quartz surface.     

Two-layered metallic film-induced surface plasmon polariton for fluorescence emission enhancement in on-chip waveguide

We demonstrate an enhancement of fluorescence emission due to bimetallic silver-gold film-induced surface plasmon wave extension. Rhodamine B (RhB) dyes were excited by the evanescent wave field produced from surface plasmon polaritons excited on metal-deposited sections along an embedded strip waveguide. Various silver-gold combinations were used to quantify for the evanescent field enhancement. The underlying silver yields better evanescent field enhancement, while the overlying gold ensures that the stability of the sensing surface is not compromised. In comparison to the conventional single gold film surface plasmon resonance (SPR) configuration, the two-layered metallic structure is capable of enhancing the surface plasmon polariton (SPP) evanescent field considerably, as verified experimentally by the ca. 4.0 times improvement in the RhB fluorescence emission. The compact waveguide structure and improved electric field probing depth can potentially be exploited for on-chip SPR--fluorescence excitation of less concentrated fluorophore-labelled biological and chemical analytes, with a capability of massively parallel processing for high throughput screening.     

Thin films sensitive to pH changes prepared by sol-gel method

The light absorption change of phenolphthalein entrapped in a silica or titania matrix coated as a thin film onto a quartz optical fibre or on planar substrate by the sol-gel technique has been studied. The possibility to use this effect together with evanescent wave principle for the monitoring of pH changes has been tested. The film drying conditions necessary for the film to be sensitive to pH changes were determined experimentally. The stability and reversibility of the colour change was found to be better in the case of titanium matrix.     

The acoustic spectrophonometer: a novel bioanalytical technique based on multifrequency acoustic devices

A measurement technique similar to optical absorption spectroscopy but based on evanescent acoustic waves is described in this paper. This format employs a planar spiral coil to vibrate a single crystal of quartz from 6 to 400 MHz, in order to measure multifrequency acoustic spectra. Consistency with the defined Sauerbrey and Kanazawa terms K1 and K2 when applied to multiple frequencies was found for these specific operating conditions in terms of a significant fit between the measured and calculated values: For an IgG surface density of 13.5 ng mm(-2) the measured value of K1 is 22.5 x 10(-6) and the calculated value is 20.4 x 10(-6), whilst for glycerol viscous loadings of 5.131 cP the measured value of K2 is 0.47 and the calculated value is 0.54. Thus for these specific surface loadings the multifrequency data fits to the predictions of the Sauerbrey model to within 10% and to Kanazawa model within 13%. However collective frequency shifts for 5.131 cP solutions of sucrose, dextran and glucose were found to exhibit an unanticipated additional variability (R2 < 0.4) with frequency, but retained a square root of frequency dependency within a factor 2 of the interpolated K2 values. The response to the 5.131 cP dextran solution was found to be significantly below the other isoviscous solutions, with a substantially reduced frequency shift and K2 value than would be expected from its bulk viscosity. In comparison with these viscous solutions, IgG protein films consistently produced linear frequency shifts with little scatter (R2 > 0.96) that were proportional to the operating frequency, and fully consistent with the Sauerbrey model under these specific conditions. A t-test value of 14.52 was calculated from the variance and mean of the two groups, and demonstrates that the acoustic spectrophonometer can be used to distinguish between the acoustic impedance characteristics of two chemical systems that are not clearly differentiable at a single operating frequency.     

Broadband excitation pulses for high‐field solid‐state nuclear magnetic resonance spectroscopy

In nuclear magnetic resonance spectroscopy, experimental limits due to the radiofrequency transmitter and/or coil means that conventional radiofrequency pulses (“hard pulses”) are sometimes not sufficiently powerful to excite magnetization uniformly over a desired range of frequencies. Effects due to nonuniform excitation are most frequently encountered at high magnetic fields for nuclei with a large range of chemical shifts. Using optimal control theory, we have designed broadband excitation pulses that are suitable for solid‐state samples under magic‐angle‐spinning conditions. These pulses are easy to implement, robust to spinning frequency variations, and radiofrequency inhomogeneities, and only four times as long as a corresponding hard pulse. The utility of these pulses for uniformly exciting ¹³C nuclei is demonstrated on a 900 MHz (21.1 T) spectrometer. Copyright © 2012 John Wiley & Sons, Ltd.     

Graphene-Based Waveguides: Novel Method for Detecting Biological Activity

We demonstrate the fabrication of a biosensor based on graphene coupled with polydimethylsiloxane (PDMS) waveguide. Biosensors work on the principle of local evanescent graphene-coupled wave sensor. It is observed that the evanescent field shifts in the presence of chemical or biological species as evanescent waves are extremely sensitive to a change in refractive index. This method helps to monitor the target analyte by attaching the selective receptor molecules to the surface of the PDMS optical waveguide resulting in its optical intensity distribution shift. We monitor the electrical properties of graphene in the dark and under illumination of PDMS waveguide. The changes in photocurrent through the graphene film were monitored for blue, green, and red light. We observed that the fabricated graphene-coupled PDMS optical waveguide sensor is sensitive to visible light for the used bioanalytes.     

Magic angle spinning NMR with metallized rotors as cylindrical microwave resonators

We introduce a novel design for millimeter wave electromagnetic structures within magic angle spinning (MAS) rotors. In this demonstration, a copper coating is vacuum deposited onto the outside surface of a sapphire rotor at a thickness of 50 nm. This thickness is sufficient to reflect 197‐GHz microwaves, yet not too thick as to interfere with radiofrequency fields at 300 MHz or prevent sample spinning due to eddy currents. Electromagnetic simulations of an idealized rotor geometry show a microwave quality factor of 148. MAS experiments with sample rotation frequencies of ω_r/2π = 5.4 kHz demonstrate that the drag force due to eddy currents within the copper does not prevent sample spinning. Spectra of sodium acetate show resolved ¹³C J‐couplings of 60 Hz and no appreciable broadening between coated and uncoated sapphire rotors, demonstrating that the copper coating does not prevent shimming and high‐resolution nuclear magnetic resonance spectroscopy. Additionally, ¹³C Rabi nutation curves of ω₁/2π = 103 kHz for both coated and uncoated rotors indicate no detrimental impact of the copper coating on radio frequency coupling of the nuclear spins to the sample coil. We present this metal coated rotor as a first step towards an MAS resonator. MAS resonators are expected to have a significant impact on developments in electron decoupling, pulsed dynamic nuclear polarization (DNP), room temperature DNP, DNP with low‐power microwave sources, and electron paramagnetic resonance detection.     

表面等离子体波共振与常规检测技术的联用

表面等离子体波共振(SPR)是被入射电磁波所激发、存在于金属和电介质界面上电荷密度振动的谐振波.SPR是一种消逝场光学成功应用的典范,它具有体积小、分辨率高、无需标记、抗电磁干扰能力强等特点.本文介绍了SPR与电化学方法(循环伏安法、溶出伏安法)、光学方法(荧光光谱、红外光谱)、质谱和石英晶体微天平等其它常规检测技术联用的研究进展,与其它常规方法联用能进一步提高分析能力,可弥补彼此的不足.本文还特别详细说明了部分电化学方法、干涉测量法与表面等离子体波共振联用的优势及不足.     

Effect of the Coil Angle in an Inductively Coupled Plasma Torch: A Novel Two-Dimensional Model

A two-dimensional model has been developed for the calculation of the electromagnetic (EM) fields generated by spiral coil currents, in order to obtain a better representation of the actual configuration used in a typical inductively coupled plasma (ICP) torch. In order to obtain the EM fields in a two-dimensional model, the change of EM field in tangential direction is neglected and the coil is assumed to be a concentric cylinder. In order to justify our assumption, the EM, flow and temperature fields resulting from five-ring coil and concentric cylinder coil are compared and the results are almost the same except for the EM field in the vicinity of the coil. In the case of the spiral coil, the coil current is inclined with respect to the horizontal plane. Therefore current in the cylinder coil is assumed to have the same inclined angle, which is split into tangential and axial components. The axial electric field and hence an axial current in plasma is induced by the axial component of the spiral coil current. Charge density is accumulated in the plasma, since the axial current cannot form a loop. In order to obtain the EM field and the charge distribution in the plasma generated by the spiral coil, the equations of axial vector potential and electrostatic potential have been derived. Due to the swirling Lorentz force (J_z×B_r) an axisymmetrical swirling fluid model is used to simulate the plasma flow in an axisymetrical configuration. With an inclined angle of the coil current being 3.7° and the frequency being 3 MHz, computational results show that the swirling Lorentz force causes plasma swirling with a maximum speed of 3.41 m/s near the plasma center when the injected sheath gas and central gas are not swirling. In these conditions, the real and imaginary parts of the maximum electrostatic potential are 0.95 V and 1.66 V, respectively. When the electrostatic field is neglected, the swirling velocity of the plasma is 3.95 m/s.     

The Synthesis, Characterization and Microwave Properties of ZnCo-Substituted W-Type Barium Hexaferrite, from a Sol-Gel Precursor

The preparation of BaZn_{2 – x} Co _x Fe₁₆O₂₇ W-type hexaferrites powders by a citrate sol-gel method has been investigated. The samples were characterized by TG-DSC, X-ray diffraction (XRD), scanning electron microscopy (SEM). The complex dielectric constant and complex permeability of hexaferrite-paraffin wax composites were measured by the transmission/reflection coaxial line method in the range from 50 MHz to 3 GHz. The dependence of complex dielectric constant and permeability on annealing temperature, composition and measuring frequency was presented.     

中空结构聚苯胺/Fe_3O_4/炭黑复合材料的制备及吸波性能

采用界面聚合和Pickering乳液聚合相结合的方法制备了具有微米尺寸的中空聚苯胺/Fe_3O_4/炭黑微球复合材料,研究了其形貌、电磁性能和吸波性能.结果表明,中空聚苯胺/Fe_3O_4/炭黑微球的平均粒径约为2.0μm;在2~4 GHz范围内的磁损耗主要是自然共振和交换共振,而在4~18 GHz范围内的磁损耗主要是涡流损耗;在2~18 GHz范围内,随着涂层厚度增加,反射损耗峰向低频方向移动,当涂层厚度增大到5.0mm时,反射损耗曲线出现2个反射损耗峰,分别位于C波段(4~8 GHz)和Ku波段(12~18 GHz),说明中空聚苯胺/Fe_3O_4/炭黑微球复合材料可作为特定频段的吸波材料.     

Dielectric spectroscopy on W/O emulsions under influence of shear forces

The dielectric properties of concentrated w/o-emulsions have been investigated, both at rest and during shear. The volume fraction water ranged from 0.50 to 0.95. The time domain dielectric spectroscopy techniques (TDS) was used to record the dielectric spectra, which covered the frequency region from 25 MHz to 2 GHz. In order to simultaneously record rheological and dielectric data a modified viscometer of the coaxial cylinder type was applied.A close connection between the viscosity and the dielectric properties of w/o emulsions is demonstrated. The very large effects of shear both on the static permittivity and the dielectric relaxation time for the emulsion can partially be ascribed to the degree of flocculation in the system. At high shear rates, at which the emulsions are expected to have a low degree of flocculation, the observed dielectric properties differ from those expected from a theoretical model for spherical emulsion droplets.     

Dielectric spectroscopy at microwave frequencies

A method is described which is capable of measuring the dielectric properties of liquids as a continuous function of frequency over the range 10 MHz to 18 GHz; an evaluation is made of its performance over the frequency range up to 4.2 GHz. The method described has substantial advantages in ease of operation and evaluation of results over the conventional “spot” frequency and time-domain spectrometer techniques.     

Temperature‐Induced Swelling and De‐swelling of Thin Poly(N‐Isopropylacrylamide) Gels in Water: Combined Acoustic and Optical Measurements

The temperature‐induced swelling and deswelling of thin layers of poly(N‐isopropylacrylamide) gels in water was measured as a function of cross‐link density and thickness. The collapse behavior was probed via an in situ combination of a quartz‐crystal microbalance (QCM) and a surface plasmon resonance (SPR) spectrometer. The shifts in the SPR coupling angle are explained in terms of decrease of the refractive index inside the film. The evanescent optical wave mostly probes the film's interior properties. The acoustic shear wave emanating from the quartz resonator, on the other hand, propagates to the outer surface of the film, unless the film is very dilute. The acoustic data are dominated by the changes in thickness, rather than in its viscosity. The combination of acoustic and optical measurements, therefore, provides complementary information on the film that can be exploited for sensing applications.     

Study of the dynamics of water/aerosol OT/n-heptane system by dielectric relaxation measurements

Most of the previous dielectric studies on microemulsions and related systems have been conducted at relatively low frequencies where the dielectric response is sensitive to dynamic percolation phenomena. There is a lack of experimental studies at microwave frequencies where water plays a central role in dielectric relaxation. In this paper the dielectric complex permittivity of water/Aerosol OT/n-heptane microemulsions has been measured by a frequency domain coaxial technique in the range 0.02–20 GHz as a function of molar ratioW = [water]/[AOT] at low volume fraction of the dispersed phase (=0.1 and =0.2). The data show two dielectric dispersions: The first located in the 100 MHz frequency region and the second at frequencies higher than 20 GHz. The evolution of the dielectric parameters of these relaxations has been studied as a function of the molar ratioW in the range 0<W<28.     

Immobilization of liposomes onto quartz crystal microbalance to detect interaction between liposomes and proteins

To study the interaction between liposomes and proteins, intact liposomes were immobilized on a metal planar support by chemical binding and/or bioaffinity using a quartz crystal microbalance (QCM). A large decrease in the resonance frequency of quartz crystal was observed when the QCM, modified by a self-assembled monolayer (SAM) of carboxythiol, was added to liposome solutions. The stable chemical immobilization of intact liposomes onto SAM was judged according to the degree with which adsorbed mass depended on the prepared size of liposomes, as well as on the activation time of SAMs when amino-coupling was introduced, where the liposome coverage of electrodes was 69+/-8% in optimal conditions. When avidin-biotin binding was used on amino-coupling liposome layers, liposome immobilization finally reached 168% coverage of the electrode surface. Denatured protein was also successfully detected according to the change in the frequency of the liposome-immobilized QCM. The adsorbed mass of denatured carbonic anhydrase from bovine onto immobilized liposomes showed a characteristic peak at a concentration of guanidine hydrochloride that corresponded to a molten globule-like state of the protein, although the mass adsorbed onto deactivated SAM increased monotonously.     

Investigations of the optical properties of thin, highly absorbing films under attenuated total reflection conditions: Leaky waveguide mode distortions

Spectra of thin highly absorbing Nafion films doped with Ru(bpy)₃²⁺ on SF11 glass substrates were studied by internal reflection spectroscopy using a single reflection configuration. For the system under study, two modes of light interaction with the film are available: attenuation due to evanescent wave penetration and light propagation within the absorbing film. Unlike evanescent wave spectroscopy, light propagation within the film causes distortions in the measured spectra due to leaky waveguide propagation modes. Upon light propagation in a film doped with Ru(bpy)₃²⁺ spectral shifts up to 50 nm to longer wavelengths can occur and additional absorbance peaks can appear in the spectra. These film-based distortions depend on the complex refractive index, the thickness of the film and the angle of incidence. These effects become significant for an extinction coefficient above 0.01 and a film thickness above 200 nm. It is shown that spectral distortions can lead to quite complex dynamics in the internal reflection spectra upon analyte preconcentration in the film. Ru(bpy)₃²⁺ partitioning into the Nafion film causes significant refractive index changes that in turn alter leaky waveguide mode conditions in the film and, can even lead to a reduction of measured absorbance despite the increase in the extinction coefficient of the film.     

Design of new integrated optical substrates for immuno-analytical applications

Integrated optical Mach-Zehnder interferometers supply information on changes in refractive index and/or thickness of a film placed as a superstrate on top of one of its surface wave-guides. The internal propagation of light is influenced by the evanescent field reaching into the superstrate. This propagating light interferes with an uninfluenced wave in the second arm after recombination. The result is an intensity modulation depending on the refractive index parameters of the substrate, the waveguide itself and the properties of the superstrate. Taking an antigen layer as the superstrate, its interaction with antibodies changes its thickness by several nanometers. This can be observed by recording the change in intensity of the signal of the interferometer. The sensitivity of such a device depends on particular values of the optical parameters of substrate and waveguide with respect to the given superstrate properties. Computer calculations help to select optimum glass and waveguide fabrication conditions. The numerical results of a variety of assumed conditions have been tested experimentally. The application to the improved detection of triazines is discussed.     

Effects of isothermal 2.45 GHz microwave radiation on the mammalian cell cycle: comparison with effects of isothermal 27 MHz radiofrequency radiation exposure

Synchronized Chinese hamster ovary (CHO) cells were exposed to continuous wave (CH) 2.45 GHz microwave radiation (MWR) or CW 27 MHz radiofrequency radiation (RFR) under isothermal conditions (37±0.2°) to test the following hypotheses: (1) high frequency electromagnetic radiation exposure directly affects the mammalian cell cycle in the absence of radiation-induced heating; and (2) the magnitude of the cell cycle alteration is frequency dependent. CHO cells in either G₀/G₁-, S−, or G₂/M-phase of the cell cycle were simultaneously exposed to CW 27 MHz RFR or CW 2.45 GHz MWR at specific absorption rates (SARs) of 5 or 25 W kg⁻¹, or sham exposed, at 37±0.2°C. Cell cycle alterations were determined by flow cytofluorometry over a 4 d period after exposure. The DNA distributions of RFR, MWR, and sham exposed cells were compared to detect qualitative effects on the cell cycle. Quantitative measures of the effects of isothermal radiation exposure were determined from differences in the number of exposed and sham exposed cells in various cell cycle phases as well as comparison of the mean DNA content of exposed and sham exposed cell samples. Flow cytofluorometric assay precision and accuracy were determined by comparison of DNA distributions of replicate CHO control cell samples and by the use of internal DNA standards.Exposure to 27 MHz RFR or 2.45 GHz MWR altered the CHO cell cycle for periods of up to 4 d following exposure at SARs of 5 or 25 W kg⁻¹. There were significant differences in temporal responses, cell cycle phase sensitivity, and overall degree of cell cycle alteration for 27 MHz compared with 2.45 GHz radiation exposure. In contrast to the effect of 27 MHz RFR, which did not affect G₂/M-phase CHO cells, 2.45 GHz MWR altered all cell cycle phases to varying degrees. Exposure to 2.45 GHz MWR at 5 or 25 W kg⁻¹ was twice as effective as 27 MHz RFR in inducing cell cycle alterations as determined by differences in the number of exposed versus sham-exposed cells in various cell cycle phases.