Influence of skew rays on the sensitivity and signal-to-noise ratio of a fiber-optic surface-plasmon-resonance sensor: a theoretical study

We have theoretically analyzed the influence of skew rays on the performance of a fiber-optic sensor based on surface plasmon resonance. The performance of the sensor has been evaluated in terms of its sensitivity and signal-to-noise ratio (SNR). The theoretical model for skewness dependence includes the material dispersion in fiber cores and metal layers, simultaneous excitation of skew rays, and meridional rays in the fiber core along with all guided rays launching from a collimated light source. The effect of skew rays on the SNR and the sensitivity of the sensor with two different metals has been compared. The same comparison is carried out for the different values of design parameters such as numerical aperture, fiber core diameter, and the length of the surface-plasmon-resonance (SPR) active sensing region. This detailed analysis for the effect of skewness on the SNR and the sensitivity of the sensor leads us to achieve the best possible performance from a fiber-optic SPR sensor against the skewness in the optical fiber.     

Influence of temperature on the sensitivity and signal-to-noise ratio of a fiber-optic surface-plasmon resonance sensor

We have theoretically analyzed the influence of temperature on the performance of a fiber-optic sensor based on surface-plasmon resonance (SPR). The performance of the sensor has been evaluated in terms of its sensitivity and signal-to-noise ratio (SNR). The theoretical model for temperature dependence includes the thermo-optic effect in the fiber core and sensing layer, and phonon-electron scattering along with electron-electron scattering in the metal layer. The effect of temperature on the SNR and the sensitivity of the sensor with two different metals has been compared. The same comparison is carried out for the sensing layers with positive and negative thermo-optic coefficients. The theoretical model has been analyzed for both the nonremote and remote sensing cases. This detailed analysis of temperature-dependent SNR and sensitivity leads to achieving the best possible performance from a fiber-optic SPR sensor against the temperature variation.     

UV fluorescence lifetime imaging microscopy: a label-free method for detection and quantification of protein interactions

Due to the ability to detect multiple parameters simultaneously, protein microarrays have found widespread applications from basic biological research to diagnosis of diseases. Generally, readout of protein microarrays is performed by fluorescence detection using either dye-labeled detector antibodies or direct labeling of the target proteins. We developed a method for the label-free detection and quantification of proteins based on time-gated, wide-field, camera-based UV fluorescence lifetime imaging microscopy to gain lifetime information from each pixel of a sensitive CCD camera. The method relies on differences in the native fluorescence lifetime of proteins and takes advantage of binding-induced lifetime changes for the unequivocal detection and quantification of target proteins. Since fitting of the fluorescence decay for every pixel in an image using a classical exponential decay model is time-consuming and unstable at very low fluorescence intensities, we used a new, very robust and fast alternative method to generate UV fluorescence lifetime images by calculating the average lifetime of the decay for each pixel in the image stack using a model-free average decay time algorithm.To validate the method, we demonstrate the detection and quantification of p53 antibodies, a tumor marker in cancer diagnosis. Using tryptophan-containing capture peptides, we achieved a detection sensitivity for monoclonal antibodies down to the picomolar concentration range. The obtained affinity constant, Ka, of (1.4 +/- 0.6) x 10(9) M(-1), represents a typical value for antigen/antibody binding and is in agreement with values determined by traditional binding assays.     

Influence of slanted guiding layer on reflection curve and sensitivity for air-gap displacement sensor

We investigate the influence of slanted guiding layer on reflection curves and sensitivity for air-gap optical waveguide structures. The theoretical analysis method is based on the interference between multiple light beams in the slanted guiding layer. The main relative characteristics of reflection curves and sensitivity as a function of inclination angle of slanted air-gap have been demonstrated in detail for symmetrical metal-cladding waveguide (SMCW) and Fabry–Pérot (FP) system. Results show that the sensitivity of SMCW with millimeter air-gap is more influenced by inclination angle than that of FP system. When inclination angle is larger than 10?μrad, the reflection curve shows serious distortion for all waveguide configurations.     

Optical-fiber sensor for simultaneous measurement of pressure and temperature: analysis of cross sensitivity

The use of a highly elliptical core two-mode fiber for simultaneous measurement of pressure (radial pressure or hydrostatic pressure) and temperature is presented. The sources of errors are discussed. Expressions are developed to calculate the cross sensitivities. From the numerical examples, some useful conclusions are given.     

Design and analysis of a PZT-based micromachined acoustic sensor with increased sensitivity

The ever-growing applications of lead zirconate titanate (PZT) thin films to sensing devices have given birth to a variety of microsensors. This paper presents the design and theoretical analysis of a PZT-based micro acoustic sensor that uses interdigital electrodes (IDE) and in-plane polarization (IPP) instead of commonly used parallel plate-electrodes (PPE) and through-thickness polarization (TTP). The sensitivity of IDE-based sensors is increased due to the small capacitance of the interdigital capacitor and the large and adjustable electrode spacing. In addition, the sensitivity takes advantage of a large piezoelectric coefficient d33 rather than d31, which is used in PPE-based sensors, resulting in a further improvement in the sensitivity. Laminated beam theory is used to analyze the laminated piezoelectric sensors, and the capacitance of the IDE is deduced by using conformal mapping and partial capacitance techniques. Analytical formulations for predicting the sensitivity of both PPE- and IDE-based microsensors are presented, and factors that influence sensitivity are discussed in detail. Results show that the IDE and IPP can improve the sensitivity significantly.     

Cytoplasmic RNA in undifferentiated neural stem cells: a potential label-free Raman spectral marker for assessing the undifferentiated status

Raman microspectroscopy (rms) was used to identify, image, and quantify potential molecular markers for label-free monitoring the differentiation status of live neural stem cells (NSCs) in vitro. Label-free noninvasive techniques for characterization of NCSs in vitro are needed as they can be developed for real-time monitoring of live cells. Principal component analysis (PCA) and linear discriminant analysis (LDA) models based on Raman spectra of undifferentiated NSCs and NSC-derived glial cells enabled discrimination of NSCs with 89.4% sensitivity and 96.4% specificity. The differences between Raman spectra of NSCs and glial cells indicated that the discrimination of the NSCs was based on higher concentration of nucleic acids in NSCs. Spectral images corresponding to Raman bands assigned to nucleic acids for individual NSCs and glial cells were compared with fluorescence staining of cell nuclei and cytoplasm to show that the origin of the spectral differences were related to cytoplasmic RNA. On the basis of calibration models, the concentration of the RNA was quantified and mapped in individual cells at a resolution of ~700 nm. The spectral maps revealed cytoplasmic regions with concentrations of RNA as high as 4 mg/mL for NSCs while the RNA concentration in the cytoplasm of the glial cells was below the detection limit of our instrument (~1 mg/mL). In the light of recent reports describing the importance of the RNAs in stem cell populations, we propose that the observed high concentration of cytoplasmic RNAs in NSCs compared to glial cells is related to the repressed translation of mRNAs, higher concentrations of large noncoding RNAs in the cytoplasm as well as their lower cytoplasm volume. While this study demonstrates the potential of using rms for label-free assessment of live NSCs in vitro, further studies are required to establish the exact origin of the increased contribution of the cytoplasmic RNA.     

Surface plasmon resonance imaging of biomolecular interactions on a grating-based sensor array

A surface plasmon resonance sensor array based upon a grating substrate was developed for the detection of biomolecular interactions. The substrate consisted of a gold grating prepared by wet chemical treatment of a commercial recordable compact disk. A custom-built floating pin microspotter was constructed to deliver solutions containing omega-functionalized linear alkanethiols to the grating surface and produce an array of sensor elements with different exposed functional end groups. This array platform can be used to study biomolecular interactions in a label-free, sensitive, and high-throughput format. To illustrate the performance of this device, a test protein (bovine serum albumin) was exposed to sensor elements containing an array of functionalized alkanethiols possessing either activated carboxylic acid-, amine-, or hydroxyl-terminated regions. Local changes in plasmon resonance were monitored in a fixed-angle imaging configuration. Plasmon images clearly distinguish the degree of protein attachment at the various surfaces. The molecular binding events on the grating were also confirmed by ellipsometry. This grating-based SPR imaging platform represents a simple and robust method for performing label-free, high-sensitivity, and high-throughput detection of biomolecular interactions.     

Development trends in the sensor technology: a new BCG matrix analysis as a potential tool of technology selection for a sensor suite

This paper presents a new variant of the Boston Consulting Group matrix analysis and applies it on sensor technologies. This classifies technologies in four classes on the basis of application growth rate of technology on one axis and competence available on the other. The quadrant, with high competence and increasing application growth rate, will be the most favored one and called star. The star technologies need application-oriented research with low risk, but they need money for design and development. A new sensor suite can select these technologies if the product development time available is roughly two to three years. The quadrant with low competence but high application growth rate is the question mark. The question mark technologies need intense R&D efforts, and they involve money and high to moderate risk. These technologies can be selected if product development time available is roughly more than three years. The technologies with low or falling application growth rate, but high competence available, are cash cows. These technologies are readily available (sometimes with production agencies), need absolutely no research, and a little bit of design or application orientation. In the cases where products are immediately needed, they have to be cheaper and do not need to last long technology wise; these can be selected. Finally, the quadrant with low competence and low application growth rate is called dogs, and such technologies can be left during consideration. Time and cost frames have also been well discussed. The role of development trends in prediction of technological evolution through a new BCG matrix is a new aspect introduced in this paper. As an example, this paper applies the above analysis in the area of night vision. This analysis can broadly guide the design of future sensor suites.     

Improving the performance of a pyramid wavefront sensor with modal sensitivity compensation

We describe a solution to increase the performance of a pyramid wavefront sensor (P-WFS) under bad seeing conditions. We show that most of the issues involve a reduced sensitivity that depends on the magnitude of the high frequency atmospheric distortions. We demonstrate in end-to-end closed loop adaptive optics simulations that with a modal sensitivity compensation method a high-order system with a nonmodulated P-WFS is robust in conditions with the Fried parameter r 0 at 0.5 microm in the range of 0.05-0.10 m. We also show that the method makes it possible to use a modal predictive control system to reach a total performance improvement of 0.06-0.45 in Strehl ratio at 1.6 microm. Especially at r 0=0.05 m the gain is dramatic.     

New modal wave-front sensor: a theoretical analysis

We present a new design of a modal wave-front sensor capable of measuring directly the Zernike components of an aberrated wave front. The sensor shows good linearity for small aberration amplitudes and is particularly suitable for integration in a closed-loop adaptive system. We introduce a sensitivity matrix and show that it is sparse, and we derive conditions specifying which elements are necessarily zero. The sensor may be temporally or spatially multiplexed, the former using a reconfigurable optical element, the latter using a numerically optimized binary optical element. Different optimization schemes are discussed, and their performance is compared.     

The limit sensitivity of a transformer variable electric field sensor in the sea

An analytical expression for the limit sensitivity of a transformer sensor is obtained. It is shown that at frequencies higher than a certain limit the sensitivity is determined solely by the geometrical dimensions of the sensor and is independent of the number of turns in its coil and the magnetic permeability of the core. __________ Translated from Izmeritel’naya Tekhnika, No. 7, pp. 51–53, July, 2008.     

Time-resolved surface-enhanced ellipsometric contrast imaging for label-free analysis of biomolecular recognition reactions on glycolipid domains

We have applied surface-enhanced ellipsometry contrast (SEEC) imaging for time-resolved label-free visualization of biomolecular recognition events on spatially heterogeneous supported lipid bilayers (SLB). Using a conventional inverted microscope equipped with total internal reflection (TIR) illumination, biomolecular binding events were monitored with a lateral resolution near the optical diffraction limit at an acquisition rate of ～1 Hz with a sensitivity in terms of surface coverage of ～1 ng/cm(2). Despite the significant improvement in spatial resolution compared to alternative label-free surface-based imaging technologies, the sensitivity remains competitive with surface plasmon resonance (SPR) imaging and imaging ellipsometry. The potential of the technique to discriminate local differences in protein binding kinetics was demonstrated by time-resolved imaging of anti-GalCer antibodies binding to phase-separated lipid bilayers consisting of phosphatidylcholine (POPC) and galactosylceramide (GalCer). A higher antibody binding capacity was observed on the GalCer-diluted fluid region in comparison to the GalCer-rich gel phase domains. This observation is tentatively attributed to differences in the presentation of the GalCer epitope in the two phases, resulting in differences in availability of the ligand for antibody binding. The complementary information obtained by swiftly switching between SEEC and fluorescence (including TIR fluorescence) imaging modes was used to support the data interpretation. The simplicity and generic applicability of the concept is discussed in terms of microfluidic applications.     

Improved sensitivity of a histamine sensor using an engineered methylamine dehydrogenase

Methylamine dehydrogenase (MADH) may be immobilized in a polypyrrole (PPy) film on an electrode surface and used as an amperometric sensor for the determination of histamine. Using site-directed mutagenesis, phenylalanine 55 on the alpha subunit of MADH was converted to alanine. This alphaF55A MADH exhibits a 400-fold lower Km value for histamine than does native MADH when assayed in solution. An alphaF55A MADH-PPy sensor was constructed, and its properties were compared to that of the native MADH-PPy sensor. The alphaF55A MADH immobilized on the electrode exhibited Michaelis-Menten behavior in response to varied concentrations of histamine with an approximately 3-fold lower Km value than that exhibited by the immobilized native MADH. The detection limit for the native MADH-PPy sensor was approximately 20 microM while the alphaF55A MADH-PPy sensor exhibited a detection limit of approximately 5 microM, a 4-fold increase compared to the native MADH-PPy sensor. This work highlights the potential value of using site-directed mutagenesis to engineer enzymes to alter and improve biosensor performance.     

Nanoparticles of antimony doped tin dioxide as a liquid petroleum gas sensor: effect of size on sensitivity

The gas sensitivity exhibited by nanoparticles of 1 wt% Pd catalysed antimony doped tin dioxide (ATO) prepared by a citrate-nitrate process is reported here. The reduction of particle size to <3 nm, a dimension smaller than double the thickness of the charge depletion layer, has resulted in an exceptionally high butane sensitivity and selectivity. The sensitivity and selectivity of ATO particles of different sizes unequivocally proved that reducing the size of particles to below twice the Debye length dimension produces materials with exceptionally high sensitivity and selectivity for sensor applications. The sensitivity of the samples towards 1000 ppm butane varied in the order 98%>55%>47%, for CNP>SP>CP samples having crystallite sizes of the order of 2.4 nm to 18 nm to 25 nm, respectively. The ATO nanoparticles exhibited not only a remarkable increase in gas sensitivity of around 98% towards 1000 ppm butane at 350?°C, but also a preferential selectivity to butane compared to other gases such as CO, CO2, SO2, CH4 and H2. In addition to the exceptionally high sensitivity and selectivity, the developed sensors also exhibited an improved response time and long term stability, which are of paramount importance for practical device development.     

Analysis of biomolecular interactions using a miniaturized surface plasmon resonance sensor

A commercially available miniaturized surface plasmon resonance sensor has been investigated for its applicability to biological interaction analysis. The sensor was found to exhibit excellent repeatability and linearity for high-refractive index solutions and good reproducibility for the binding of proteins. Its detection limit for the monoclonal antibody M1 was found to be 2.1 fmol, which corresponds to a surface concentration of 21 pg/mm2. Simple surface immobilization procedures relying on biotin/avidin or glycoprotein/lectin chemistry have been explored. Equilibrium dissociation constants for the binding of the FLAG peptide to its monoclonal antibody (M1) and for the binding of concanavalin A to a glycoprotein have been determined. The close agreement of these measurements with values obtained by surface fluorescence microscopy and fluorescence correlation spectroscopy helps to validate the use of this device. Thus, this sensor shows promise as an inexpensive, portable, and accurate tool for bioanalytical applications in laboratory and clinical settings.     

A new special element for stress concentration analysis of a plate with elliptical holes

Special hole elements are presented for analyzing the stress behavior of an isotropic elastic solidcontaining an elliptical hole. The special hole elements are constructed using the special fundamental solutions for an infinite domain containing a single elliptical hole, which are derived based on complex conformal mapping and Cauchy integrals. During the construction of the special elements, the interior displacement and stress fields are assumed to be the combination of fundamental solutions at a number of source points, and the frame displacement field defined over the element boundary is independently approximated with conventional shape functions. The hybrid finite element model is formulated based on a hybrid functional that provides a link between the two assumed independent fields. Because the fundamental solutions used exactly satisfy both the traction-free boundary conditions of the elliptical hole under consideration and the governing equations of the problems of interest, all integrals can be converted into integrals along the element boundary and there is no need to model the elliptical hole boundary. Thus, the mesh effort near the elliptical hole is significantly reduced. Finally, the numerical model is verified through three examples, and the numerical results obtained for the prediction of stress concentration factors caused by elliptical holes are extremely accurate.     

Effect of film thickness and curing temperature on the sensitivity of ZnO:Sb thick-film hydrogen sensor

This paper reports the effect of film thickness and curing temperature on the sensitivity of the screen-printed thick-film ZnO (incorporated with 7 wt%Sb) H₂, CO and CH₄ sensors. The sensitivity of the sensors increases up to 60 μm and decreases for higher film thickness. Increase in the curing temperature of the films, decreases the sensitivity of the sensors because of the increase in the average grain size, as observed by scanning electron microscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solid-mechanics finite element simulations of the draping of fabrics: a sensitivity analysis

This paper covers numerical investigations of the draping of woven fabrics into a “hat” shape, combining a hemispherical cup with a wide flat rim. A mechanical approach is adopted using finite element analysis (FEA) methodology. In this, the fabric is considered as a solid sheet with mechanical properties and friction properties. In this study, a linear elastic anisotropic material model describes the deformation of fabrics. An explicit dynamic finite element analysis is applied and systematic parametric numerical studies are presented, which incorporate investigations of the effects of numerical parameters, material properties and processing conditions on the draping of fabrics. More specifically, the effects of the following variables and parameters are included: number of elements, number of time increments in the dynamic FEA analysis, punch speed, shear and tensile moduli of fabric, coefficient of friction for all interfaces and level of load on the fabric holder.