Brian A. Mazzeo

Paper‐Based, Capacitive Touch Pads

Metallized paper is patterned to create touch pads of arrayed buttons that are sensitive to contact with both bare and gloved fingers. The paper-based keypad detects the change in capacitance associated with the touch of a finger to one of its buttons. Mounted on an alarmed cardboard box, the keypad requires the appropriate sequence of touches to disarm the system.

Method for Estimating the Internal Permittivity of Proteins Using Dielectric Spectroscopy

Protein charge organization is dependent on the low-permittivity region in the hydrophobic core of the molecule. We suggest a novel approach to estimate the dielectric constant of this region by comparing measured and simulated first- and second-order charge moments. Here, the dipole moment is measured as a function of pH using dielectric spectroscopy. The results are compared to dipole moments based on Poisson-Boltzmann estimates of pK(a) shifts calculated from structures in the Protein Data Bank. Structures are additionally refined using CHARMM molecular dynamics simulations. The best estimate for the internal permittivity is found by minimizing the root-mean-square residual between measured and predicted charge moments. Using the protein β-lactoglobulin, a core dielectric constant in the range of 6-7 is estimated.

Transient deflection response in microcantilever array integrated with polydimethylsiloxane (PDMS) microfluidics

We report the integration of a nanomechanical sensor consisting of 16 silicon microcantilevers with polydimethylsiloxane (PDMS) microfluidics. For microcantilevers positioned near the bottom of a microfluidic flow channel, a transient differential analyte concentration for the top versus bottom surface of each microcantilever is created when an analyte-bearing fluid is introduced into the flow channel (which is initially filled with a non-analyte containing solution). We use this effect to characterize a bare (nonfunctionalized) microcantilever array in which the microcantilevers are simultaneously read out with our recently developed high sensitivity in-plane photonic transduction method. We first examine the case of non-specific binding of bovine serum albumin (BSA) to silicon. The average maximum transient microcantilever deflection in the array is -1.6 nm, which corresponds to a differential surface stress of only -0.23 mN m(-1). This is in excellent agreement with the maximum differential surface stress calculated based on a modified rate equation in conjunction with finite element simulation. Following BSA adsorption, buffer solutions with different pH are introduced to further study microcantilever array transient response. Deflections of 20-100 nm are observed (2-14 mN m(-1) differential surface stress). At a flow rate of 5 μL min(-1), the average measured temporal width (FWHM) of the transient response is 5.3 s for BSA non-specific binding and 0.74 s for pH changes.

Correlation of chromatographic performance with morphological features of organic polymer monoliths.

Monoliths are considered to be a low pressure alternative to particle packed columns for liquid chromatography (LC). However, the chromatographic performance of organic monoliths, in particular, has still not reached the level of particle packed columns. Since chromatographic performance can be attributed to morphological features of the monoliths, in-situ characterization of the monolith structure in three dimensions would provide valuable information that could be used to help improve performance. In this work, serial sectioning and imaging were performed with a dual-beam scanning electron microscope for reconstruction and quantitative characterization of poly(ethylene glycol) diacrylate (PEGDA) monoliths inside a capillary column. Chord lengths, homogeneity factors, porosities and tortuosities were calculated from three-dimensional (3D) reconstructions of two PEGDA monoliths. Chromatographic efficiency was better for the monolith with smaller mean chord length (i.e., 5.23μm), porosity (i.e., 0.49) and tortuosity (i.e., 1.50) compared to values of 5.90μm, 0.59 and 2.34, respectively, for the other monolithic column. Computational prediction of tortuosity (2.32) was found to be in agreement with the experimentally measured value (2.34) for the same column. The monoliths were found to have significant radial heterogeneity since the homogeneity factor decreased from 5.39 to 4.89 (from center to edge) along the column radius.

Parasitic capacitance influence of potential-sensing electrodes on four-electrode liquid impedance measurements

Four-electrode techniques are used to minimize the effects of electrode polarization on measurements of the frequency-dependent impedance of conductive solutions. Basic circuit analysis confirms that an ideal Schwan-type four-electrode measurement can theoretically determine the polarization-free impedance of a sample. However, circuit analysis including parasitic effects indicates that the input impedance of the potential-sensing electrodes will be a significant influence on the measurement. This is verified in experiments on conductive solutions employing measurements of the gain and phase between potential-sensing electrodes in a four-electrode arrangement. The electrode impedance is varied by adding capacitors in parallel with the probes. The combined theory and experimental data indicate that parasitic capacitance of the potential-sensing electrodes is non-negligible and will alter the measured capacitance at all frequencies and even distort conductivity measurements at high-frequencies. This result ...

Temperature-stable parallel-plate dielectric cell for broadband liquid impedance measurements.

A liquid impedance cell for broadband impedance measurements up to 110 MHz is presented. The design incorporates temperature control and minimizes parasitic capacitance and inductance. The cell is simple to fabricate and uses chemically resistant materials, stainless steel, and Teflon. This dielectric cell can be used in a variety of liquid measurements, particularly those related to impedance measurements of biological objects in solution. Temperature control is illustrated in measurements of the permittivity of deionized water from 5 to 55  °C. Numerical fitting procedures employed on the relaxation curves indicate good agreement with previous studies on beta-lactoglobulin and hen lysozyme. Titration capability is demonstrated through dielectric titration of hen lysozyme and beta-lactoglobulin.

Increased bandwidth for dielectric spectroscopy of proteins through electrode surface preparation

Dielectric spectroscopy measurements of liquids are often limited by electrode polarization. The influence of surface polishing and deposition of the conducting polymer polypyrrole/polystyrenesulfonate (PPy/PSS) on the polarization impedance is investigated. A quantitative description of the electrode polarization contribution to the real-valued permittivity spectrum is derived. This description explains the origin of the ω(-const). (const.>1) dependency commonly observed in permittivity measurements. Electrode surface roughness is correlated with both the magnitude and phase of the constant phase element. Generally, rougher electrodes have better performance, and an order of magnitude bandwidth improvement is achieved using PPy/PSS electrodes.

Note: Electrode polarization of Galinstan electrodes for liquid impedance spectroscopy

Electrode polarization is a significant obstacle in the impedance measurements of ionic liquids. An atomically smooth electrode surface could potentially reduce unwanted impedance contributions from electrode polarization. Liquid metal electrodes were formed by adhering Galinstan to acrylic plates in a parallel-plate capacitor arrangement. Electrode polarization was compared to a similar cell with stainless steel electrodes. The impedance of salt and protein solutions (β-lactoglobulin) was measured from 40 Hz to 110 MHz. Because of oxide layer formation, the performance of the Galinstan electrode is significantly different than the theoretical ideal.

Contactless conductivity detection of small ions in a surface micro-machined CE chip

A microchip is presented which is capable of CE separations and is built using exclusively thin film deposition techniques, fully compatible with microelectronics batch processing. Standard photolithography provides control of the spacing between electrodes used in conductivity measurement and overall channel geometry. Fluid channels are arranged as a double‐T injector with a 50 μm offset at the arm intersection. The chips performance was tested using concentrations of sodium chloride and calcium chloride ranging from 1 μM to 1 mM in a 5‐mM MES/histidine buffer. Separations were performed by applying different voltages to reservoirs positioned at the four fluid channel openings. Conductivity detection was performed by applying a small AC voltage (1 Vrms) to the insulated electrodes positioned inside the fluid channels. A computer running LabVIEW controlled the AC signal generation, data acquisition and storage. Measurements indicated that the chips detection limit was below 1 μM for both sodium and calcium cations.

Weak Adsorption-Induced Surface Stress for Streptavidin Binding to Biotin Tethered to Silicon Microcantilever Arrays

We investigate the performance and limitations of silicon microcantilever arrays as biosensors in which receptor molecules are directly tethered to the native oxide without an intermediate metal adhesion layer such as gold. We use our recently developed in-plane photonic microcantilever (PMCL) readout method to simultaneously measure deflection of individual microcantilevers in a 16-PMCL array integrated with polydimethylsiloxane microfluidics. We demonstrate label-free biosensing of streptavidin using biotin that has been immobilized to only the top side of a subset of PMCLs within the array. The maximum surface stress that we measure is rather small (6 mN/m) for relatively high streptavidin concentration (4.7 μM). We attribute this result to lack of a metal adhesion layer, which eliminates strong surface stress generation from change in metal electronic charge density upon uptake of streptavidin by surface-bound biotin.