Stefan Cular

Removal of Nonspecifically Bound Proteins on Microarrays Using Surface Acoustic Waves

Nonspecific binding of proteins is an ongoing problem that dramatically reduces the sensitivity and selectivity of biosensors. We demonstrate that ultrasonic waves generated by surface acoustic wave (SAW) devices remove nonspecifically bound proteins from the sensing and nonsensing regions of the microarrays. We demonstrate our approach for controllably and nondestructively cleaning the microarray interface. In this work, SAWs were generated using 128 YX lithium niobate, chosen for its high coupling coefficient and efficient power transfer to mechanical motion. These waves propagating along the surface were coupled into specifically bound and nonspecifically bound proteins on a patterned surface of 40 mum feature size. Fluorescence intensity was used to quantify cleaning efficacy of the microarrays. Our results have shown that excess protein layers and aggregates are removed leaving highly uniform films as evidenced by fluorescence intensity profiles. Selected antigen-receptor interactions remained bound during the acoustic cleaning process when subjected to 11.25 mW of power and retained their efficacy for subsequent antigen capture. Results demonstrate near-complete fluorescence signal recovery for both the sensing and nonsensing regions of the microarrays. Of significance is that our approach can be integrated into existing array technologies where sensing and nonsensing regions are extensively fouled. We believe that this technology will be pivotal in the development and advancement of microsensors and their biological applications.

Enhancing effects of microcavities on shear-horizontal surface acoustic wave sensors: A finite element simulation study

Shear-horizontal surface acoustic wave (SAW) sensors with microcavities in the delay paths of 36° YX-LiTaO3 substrate were studied using finite element methods. Microcavities of square cross sections of sizes λ∕4 and λ∕2 and of different depths were located in the middle of the delay path. Simulation results for nonfilled and polystyrene-filled microcavity devices were compared with standard delay line shear-horizontal SAW, optimized Love-wave, and etched grating sensors. We found that the best case microcavities studied reduce insertion loss by 19.25dB from 33.28dB and exhibit velocity sensitivity 4.83 times larger than that of the standard SAW sensor simulated.

Acoustoelectric Effect in Hydrogen Surface Acoustic Wave Sensors with Phthalocyanine-Palladium Sensing Bi-layers

Hydrogen has a role as an important chemical commodity that will continue to increase with the developments of the hydrogen economy. With a lower explosive limit of 4.73% by volume fast and accurate sensors are needed. Our recent work has shown that bilayers of phthalocyanine-palladium structures can be optimized to construct effective SAW sensors for hydrogen. In a bilayer sensing film structure, we can use the much stronger acoustoelectric effect in the SAW sensor response as the main detection mechanism more effectively than with a single sensing layer. This effect can be many times greater than the mass effect which can be dominant in nonconductive polymer films and simple metal and dielectric films typically employed in SAW gas sensors. The “work point” of such a structure must be shifted to the high sensitivity region, where small variations in conductivity (under the influence of gas molecules) cause remarkable changes in the wave velocity (see Figure 1). Thus, to take full advantage of the high sensitivity offered by the SAW sensor, the conductivity of sensing film must be tailored to a particular range.

Acoustically determined linear piezoelectric response of lithium niobate up to 1100 V

We present a method to measure high voltages using the piezoelectric crystal lithium niobate without using voltage dividers. A 36° Y-X cut lithium niobate crystal was coupled to two acoustic transducers, where direct current voltages were applied from 128–1100 V. The time-of-flight through the crystal was determined to be linearly dependent on the applied voltage. A model was developed to predict the time-delay in response to the applied voltage. The results show a sensitivity of 17 fs/V with a measurement error of 1 fs/V was achievable using this method. The sensitivity of this method can be increased by measuring the acoustic wave after multiple passes through the crystal. This method has many advantages over traditional techniques such as: favorable scalability for larger voltages, ease of use, cost effectiveness, and compactness.

Hexagonal surface acoustic wave devices for enhanced sensing and materials characterizaton

We present the design, fabrication and testing of a hexagonal surface acoustic wave (SAW) array device fabricated in YZ lithium niobate for non-destructive evaluation of thin organic, inorganic and biological films. Propagation along the Y-axis generates a Rayleigh mode wave where off-axis propagation excites a mixture of other SAWs. Our approach permits rapid and simultaneous extraction of multiple film parameters (film material density or thickness, Lame and shear moduli, sheet conductivity) of a thin film material to achieve a more complete characterization than when a single SAW device is utilized. In sensor applications, this capability translates to better discrimination of the analyte and possibly more accurate quantification. The device is based on a double split finger delay-line design with a line width of 4 µm and a delay path of 197 λ. The individual delay paths of each hexagonal device intersect in the center of the die producing a single region for sensor analysis. Additionally, the central region where the acoustic waves intersect is shorted to reduce the number of modes of waves traversing the surface. Initial testing has shown the pass band frequency of the individual delay paths to be centered around 97 MHz. The acoustic velocities of the rotated device have been measured to be 3593 m/s, 3721 m/s, and 3620 m/s, which correspond to the theoretical values of 3542 m/s, 3646 m/s, and 3622 m/s, respectively. Vapor sensing tests were conducted by exposing a poly(isobutylene)-coated device to various concentrations of benzene, chloroform, and n-hexane in the range of 0.8 to 16.6 volume percent. Measured attenuation and phase angle shifts at a fixed, near-center frequency revealed significant, signature-type differences for the three delay-paths at each exposure concentration. These responses can be exploited in constructing better sensors and sensor arrays utilizing these hexagonal SAW devices.

Finite Element modeling of hexagonal surface acoustic wave biosensor based on LiTaO 3

The main advantage of numerical methods such as Finite Elements (FE) in modeling surface acoustic wave (SAW) devices lies in their ability to model devices involving complicated transducer geometries. We present a 3-D finite element model of a novel hexagonal SAW biosensor based on LiTaO3 substrate. This SAW biosensor involves the use of one delay path for biological species detection whereas the other delay paths are used to simultaneously remove the non-specifically bound proteins using the acoustic streaming phenomenon, thus eliminating biofouling issues associated with other biosensors. Prior to this biosensor fabrication on any piezoelectric substrate, it is important to establish the type of waves that are generated along the various delay paths. The choice of a delay path for sensing and simultaneous cleaning application depends on the propagation characteristics of the wave generated along the crystal cut and orientation corresponding to that delay path. The frequency response as well as wave propagation characteristics along the delay path corresponding to crystal orientation with on-axis propagation along 36deg YX LiTaO3 substrate are analyzed using a coupled field FE model. Similar analysis is extended to the off-axis propagation directions corresponding to Euler rotations by 60deg and -60deg along the x-z plane. Our findings indicate that the on-axis direction with a significant surface shear horizontal (SH) component should be employed for biological species detection whereas the off-axis directions having mixed modes with a dominant Rayleigh wave component are most suitable for simultaneous cleaning or removal application.

Predicting the mechanism of removal of nonspecifically bound proteins in a surface acoustic wave biosensor: A fluid-solid interaction study

Biosensors typically operate in liquid media for measurement of biomarkers and suffer from fouling mechanisms such as nonspecific binding of protein molecules to the device surface. In the current work, using a novel numerical technique as well as experiments, we have identified that fluid motion induced by high intensity sound waves, such as those propagating in these sensors, can lead to the removal of the nonspecifically bound proteins, thereby eliminating sensor fouling. We present a computational and experimental study of the acoustic-streaming phenomenon induced biofouling elimination by surface acoustic waves (SAWs). The transient solutions generated from the developed coupled field fluid solid interaction (FSI) model were utilized to predict trends in acoustic-streaming velocity for various design parameters such as voltage intensity, device frequency, fluid viscosity and density. The model predictions were utilized to compute the various interaction forces involved and thereby identify the possible mechanisms for removal of nonspecifically-bound proteins. Our study indicates that the SAW body force overcomes the adhesive forces of the fouling proteins to the device surface and the fluid-induced drag and lift forces prevent its re-attachment. The streaming velocity fields computed using the finite-element models in conjunction with the proposed mechanism were used to identify the conditions leading to improved removal efficiency. Our research findings have significant implications in designing reusable and highly sensitive biosensors.

High Frequency Surface Acoustic Wave Devices Based on Multilayered LiNbO3/Diamond/AlN Substrates: A Finite Element Study

We have used 3-D coupled field structural finite element models to study the acoustic wave propagation characteristics of diamond/AlN/LiNbO3 multilayered piezoelectric surface acoustic wave devices for applications in chemical and biological sensing. These devices were studied as a method to increase device frequency and sensitivity, and maintain standard fabrication procedures. Although recent experimental investigations have realized GHz frequency devices based on such multilayered substrates, very little is known about the acoustic wave propagation characteristics in these devices. Identifying the optimum configuration and thickness of the various layers involved still represents a challenge which is addressed in this work.

Experimental validation of a high voltage pulse measurement method.

This report describes X-cut lithium niobates (LiNbO3) utilization for voltage sensing by monitoring the acoustic wave propagation changes through LiNbO3 resulting from applied voltage. Direct current (DC), alternating current (AC) and pulsed voltage signals were applied to the crystal. Voltage induced shift in acoustic wave propagation time scaled quadratically for DC and AC voltages and linearly for pulsed voltages. The measured values ranged from 10 - 273 ps and 189 ps - 2 ns for DC and non-DC voltages, respectively. Data suggests LiNbO3 has a frequency sensitive response to voltage. If voltage source error is eliminated through physical modeling from the uncertainty budget, the sensors U95 estimated combined uncertainty could decrease to ~0.025% for DC, AC, and pulsed voltage measurements.

Organic vapor sensing and discrimination using enhanced sensitivity thickness shear mode devices

Thin film coatings of poly-isobutylene (PIB) on 96 MHz TSM resonators were utilized as high sensitivity organic vapor sensors. Commercially available AT-quartz TSM devices were milled to 17 µm, leading to a thin quartz membrane surrounded by a 50 µm thick outer ring. The organic vapors studied were benzene, toluene, hexane, cyclohexane, heptane, dichloroethane, dichloromethane, and chloroform at levels ranging from 0.2 to over 13.7 volume percentage in the vapor phase. The Butterworth-VanDyke (BVD) equivalent circuit model was used to model both the perturbed and unperturbed TSM resonators. Monitoring the sensor response through the equivalent circuit model allowed for discriminating between the organic vapors. Detailed results for various sensor parameters such as sensitivity, baseline noise and drift, limit of detection, response and recovery times, dynamic range, and repeatability for the 96 MHz device were compared with those for 10 and 20 MHz devices.