Piezoelectric-excited Millimeter-sized Cantilever Sensors Research Articles

Performance of I-shaped piezoelectric-excited millimeter-sized cantilever in sensing applications: modeling, simulation and experiment

Detection sensitivity has a key role in the realm of piezoelectric-excited millimeter-sized cantilever (PEMC) sensors. Operating principle of these sensors is based on measuring the electrical impedance of piezoelectric layer attached to the base cantilever. While most of studies in this area have focused on sensitivity improvement by minimizing the dimensions of the base cantilever, in this paper, effect of modifying the cross-section shape of the cantilever on enhancing the detection sensitivity of these sensors has been studied. To this end, a new PEMC sensor with I-shaped cross-section was proposed. Performance of this sensor in detecting the tiny masses added on the cantilever tip and also detecting the liquid level changes has been explored by theoretical modelling, finite element simulations and via experiments. The achieved results were compared with the detection sensitivities of the sensor with the same dimensions but rectangular-shaped cross section. Theoretical and simulation results were in good agreement with experimental outcomes. By changing the cross section of the base cantilever from rectangular-shaped to I-shaped, mass detection and liquid level detection sensitivities achieved via experiments, increased up to 2.04 and 3.75 times, respectively.

Fabrication of a resonant PEMC sensor using hybrid $$\upvarepsilon $$ ε -constraint lexicographical ranking DE-MSBA with Jacobean approximation

In the current research, a novel algorithm is proposed for the optimal fabrication of a resonant piezoelectric excited millimeter-sized cantilever (PEMC) sensor. PEMC sensor is one of the most well-known types of sensors with a wide range of applications in today’s industry. The proposed design mechanism is general, and can be used for the fabrication of PEMCs for different applications. A physics-based analytical model based on Euler–Bernoulli theory is adopted for performance analysis. A novel hybrid nature-inspired optimizer, called $$\upvarepsilon $$ -constraint lexicographical ranking differential evolution mutable smart bee algorithm ( $$\upvarepsilon $$ lr-DEMSBA), is developed which can effectively search the non-convex and multimodal solution domain. The optimal values of PEMC design parameters are obtained using $$\upvarepsilon $$ lr-DEMSBA. To ensure the veracity of the parameters suggested by $$\upvarepsilon $$ lr-DEMSBA, a PEMC is fabricated using the suggested optimal values, and some post analysis are carried out. By investigating the relation between the frequency shift of natural frequency and increasing of immersion depth, it is observed that the linear performance zone of optimized PEMC sensor is increased by 2.5 mm which is a favorable property. Also, from numerical viewpoint, it was observed that $$\upvarepsilon $$ lr-DEMSBA surpasses different variants of optimization techniques for the current case study.

Piezoelectric Cantilever Biosensors for Label-free, Real-time Detection of DNA and RNA.

This chapter reviews the design, fabrication, characterization, and application of piezoelectric-excited millimeter-sized cantilever (PEMC) sensors. The sensor transduction mechanism, sensing principle, and mode of operation are discussed. Bio-recognition strategies and surface functionalization methods for detection of DNA and RNA are discussed with a focus on self-assembly-based approaches. Methods for the verification of biosensor response via secondary binding assays, reversible binding assays, and the integration of complementary transduction mechanisms are presented. Sensing applications for medical diagnostics, food safety, and environmental monitoring are provided. PEMC sensor technology provides a robust platform for the real-time, label-free detection of DNA and RNA in complex matrices over nanomolar (nM) to attomolar (aM) concentration ranges.

Piezoelectric excited millimeter sized cantilever sensors for measuring gas density changes

Rapid, in-situ measurement of gas density is valuable for monitoring industrial processes. We show that piezoelectric-excited millimeter-sized cantilever (PEMC) sensors exhibit precision for measuring gas density changes as small as 0.088g/l with sensitivity equivalent to 0.049g/(lHz). PEMC sensors were fabricated by bonding 127μm thick piezoelectric layer (lead zirconate titanate) to a base 160μm thick silica layer. Gases (He, N2, Ar) were passed continuously through an isothermally maintained flow cell, in which the PEMC sensors were mounted and the steady state resonant frequency values were monitored. The changes in observed resonant frequencies were collected and compared in multiple experiments with well-established theory for the frequency response of a dynamic cantilever surrounded by an inviscid fluid and were found to be in reasonable agreement. A finite element model of the PEMC sensor showed good agreement with the measured resonant frequency values.

Nature of sensitive high-order resonant modes in piezoelectric excited millimeter sized cantilever (PEMC) sensors

High-order resonant modes of piezoelectric-excited cantilever (PEMC) sensors were previously shown to be very highly sensitive at 0.3–2 fg/Hz for in-liquid applications. The purpose of this work is to show experimentally and with finite element model (FEM) simulations that such sensitive modes are strongly influenced by the sensor width, suggesting that the sensitive modes are torsional or buckling modes. From experimental observations the resonant frequency of high-order modes had a strong dependence on width, where a sensor with a smaller width had resonant modes at a higher frequency. Also the FEM simulations indicate that in this frequency range there are resonant modes with a buckling nature that change for a decrease in width, consistent with experimental observations. In order to establish that the width-dependent modes are mass-sensitive in liquids, resonant frequency change to density changes in flow experiments under fully liquid-immersed conditions were determined. Average frequency shifts of 475 ± 49 Hz ( n = 5), 533 ± 31 Hz ( n = 5), 715 ± 103 Hz ( n = 5) and 725 ± 37 Hz ( n = 5) were obtained for the four designs investigated in response to a density change of 0.0118 g/cm 3. The results show that the resonant frequency response to variations in the geometry provides insightful data on the role of width in PEMC sensor design.

Detection of Cryptosporidium parvum in buffer and in complex matrix using PEMC sensors at 5 oocysts mL −1

We show for the first time that a piezoelectric-excited millimeter-sized cantilever (PEMC) sensor can detect five Cryptosporidium parvum oocysts in 25% milk in PBS background in a flow format (1 mL min −1). To improve sensitivity, a secondary antibody (murine IgM) was used to confirm the attachment of oocysts to the sensor. PEMC sensor is a resonant-mode cantilever sensor whose resonant frequency decreases when target analyte binds to its surface. The sensor was functionalized with Protein G, and then immobilized with goat polyclonal IgG anti- C. parvum for oocysts detection. In the dynamic range of 50–10,000 oocysts mL −1 the sensor response is characterized by a semi-log relationship between resonant frequency response and C. parvum oocysts concentration. In 25% milk background, binding kinetics was slower and total sensor response was lower (∼45%) than in water-like buffer.

Rapid and Sensitive Detection of Giardia lamblia Using a Piezoelectric Cantilever Biosensor in Finished and Source Waters

The current method for detecting the waterborne parasite Giardia lamblia is tedious and requires a preconcentration step. We show for the first time a piezoelectric-excited millimeter-sized cantilever (PEMC) biosensor immobilized with a monoclonal antibody against G. lamblia that exhibits selective and sensitive detection of G. lamblia cysts in several water matrixes (buffer, tap, and river water) at a detection limit of 1-10 cysts/mL without a preconcentration step. The PEMC sensor is a resonance-based device that functions at a high-order mode near 1 MHz. The antibody-immobilized sensor was exposed to 1-10,000 G. lamblia cysts/mL samples in a flow arrangement. When the cysts bind to the antibody on the sensor, the resonant frequency of the cantilever sensor decreases and is recorded continuously. Positive confirmation of sensor detection responses was obtained by environmental scanning electron microscope of sensor surface after detection experiments. Higher sample flow rates (0.5-5.0 mL/min) gave higher sensor detection response. Detecting as few as 10 cysts per mL was achieved in all three water matrixes tested, and significant sensor response was obtained in 15 min. We also show the feasibility of analyzing at a low concentration of 1 cyst/mL in a one liter sample at a high flow rate of 5 mL/min.

Sensitive and selective detection of mycoplasma in cell culture samples using cantilever sensors

In this article we report a new biosensor-based method that is more sensitive and rapid than the current approach for detecting mycoplasma in cell culture samples. Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors respond to mass change via resonant frequency change. They are sensitive at femtogram level and can be used directly in liquid for label-free detection. Common cell culture contaminant, Acholeplasma laidlawii was detected in both buffer and cell culture medium. Two different sources (positive control from a commercial kit and ATCC 23206) were analyzed using antibody-immobilized PEMC sensor. Resonant frequency decrease caused by binding of A. laidlawii was monitored in real-time using an impedance analyzer. Positive detection was confirmed by a second antibody binding. The limit of detection (LOD) was lower than 10(3) CFU/mL in cell culture medium using PEMC sensor while parallel ELISA assays showed LOD as 10(7) CFU/mL. This study shows that PEMC sensor can be used for sensitive and rapid mycoplasma detection in cell culture samples.

Mass-change sensitivity of high-order mode of piezoelectric-excited millimeter-sized cantilever (PEMC) sensors: Theory and experiments

We show that mass-change sensitivity predicted from first principles model of piezoelectric-excited millimeter-sized cantilever (PEMC) sensors is in reasonable agreement with previously reported experimental values. We introduce a new attogram-level mass addition calibration method using 11-mercaptoundeconoic acid that enables easy measurement of mass-change sensitivity of PEMC sensors. The finite element model of PEMC sensors predicts resonant frequency values that agree within 2% of experimental values. However, the model gives mass-change sensitivities that are within one order of magnitude of experimentally measured values. Simulations show two interesting properties of PEMC sensors that are observed experimentally. First, the sensor response is log-linear and thus it is most sensitive at low attached mass or low analyte concentration. Second, the sensor becomes less sensitive with increased attached mass or at high analyte concentration which gives the sensor the highly desirable property of wide dynamic range of 6-logs.

A novel method for monitoring mass-change response of piezoelectric-excited millimeter-sized cantilever (PEMC) sensors

We show that monitoring impedance at a fixed frequency near resonant frequency of a piezoelectric-excited millimeter-sized cantilever (PEMC) sensor provides equivalent measurement as the conventional approach of monitoring resonant frequency. Two sensing experiments are used to validate the proposed approach: density change and detection of a pathogen ( E. coli O157:H7). The impedance approach is feasible because PEMC sensors exhibit modest Q-values of 30–60 at ∼900 kHz and typical biosensing response falls well below 5 kHz, a frequency band near resonant frequency where impedance is a linear function of frequency. The simpler impedance approach lends itself to high throughput applications where large number of sensor responses is to be monitored simultaneously.

Mass-change sensitivity of piezoelectric-excited millimeter-sized cantilever (PEMC) sensors: Model and experiments

Abstract We show for the first time that approximate analytical models do not predict mass-change sensitivity of cantilever sensors of non-uniform cross-section. On the other hand, finite element model (FEM) is a better approach for determining the performance of such sensors. Specifically, FEM of piezoelectric excited millimeter-sized cantilever sensors (PEMC) give sensitivity values that are in reasonable agreement with experimentally determined values of 20 pg/Hz, reported earlier. The approximate analytical model predicts resonance frequency of PEMC within 10% error, but it under predicts mass-change sensitivity by three orders of magnitude. FEM analysis of first and second resonance mode frequencies of a sample of ten PEMC sensors of various geometries were found to compare favorably (∼2–4%) with experimental values. Mass sensitivity determined using FEM is within a factor of three of experimental values. Through simulations we discover two important properties of the PEMC sensors: (1) The sensitivity is a strong function of attached mass. Specifically, sensitivity is the highest at low attached mass. (2) PEMC sensors become less sensitive with increasing attached mass, which results in sensors that exhibit wide dynamic range of six logs.

Near real-time detection of Cryptosporidium parvum oocyst by IgM-functionalized piezoelectric-excited millimeter-sized cantilever biosensor

Piezoelectric-excited millimeter-sized cantilever (PEMC) biosensors were fabricated and functionalized with immunoglobulin M (IgM) for the detection of Cryptosporidium parvum oocyst in a flow configuration at 1 mL/min. The detection of 100, 1000, and 10,000 oocysts/mL was achieved with a positive sensor response in less than 1 min. Bovine serum albumin (BSA) was used as a blocking agent in each experiment and was shown to eliminate non-specific binding. The sensor's resonance frequency response correlates with C. parvum oocyst concentration logarithmically. The oocyst attachment rate was found to increase by an order of magnitude in increasing concentration from 100 to 10,000 oocysts/mL. The significance of these results is that IgM-functionalized PEMC sensors are highly selective and sensitive to C. parvum oocyst and therefore, have the potential to accurately identify and quantify C. parvum oocyst in drinking water.

Preparation-Free Method for Detecting Escherichia coli O157:H7 in the Presence of Spinach, Spring Lettuce Mix, and Ground Beef Particulates

We show the detection of 100 cells per ml of Escherichia coli O157:H7 in the presence of spinach, spring lettuce mix, and ground beef washes and particulate matter with piezoelectric-excited millimeter-sized cantilever (PEMC) sensors. The PEMC sensors (sensing area, 2 mm2) were immobilized with polyclonal antibody specific to E. coli O157:H7 (EC) and were exposed to 10 aqueous washes of locally purchased spinach, spring lettuce mix, and ground beef for testing if EC was present. Absence of resonance frequency shift indicated that EC was not present in the 30 samples tested. Following the last sample in each food matrix, 1,000 cells per ml of EC were spiked into the sample container, and resonance frequency change was monitored. The total resonance frequency change was 880 ± 5, 1,875 ± 8, and 1,417 ± 4 Hz for spinach, spring lettuce mix, and ground beef, respectively. A mixture of the three food matrices spiked with 100 cells per ml of EC gave a sensor response of 260 ± 15 Hz. The resonance frequency changes are approximately 40% lower than our previously reported study on ground beef. It is suggested that the reduction in sensitivity is due to differences in pathogen adherence to food matrices, which affects target binding to the sensor surface. We conclude that detection selectivity is conserved in the three food matrices examined and that the magnitude of sensor response is a function of the food matrix.

Detection and Confirmation of Staphylococcal Enterotoxin B in Apple Juice and Milk Using Piezoelectric-Excited Millimeter-Sized Cantilever Sensors at 2.5 fg/mL

A sensitive and reliable method for the detection of a model toxin, staphylococcal enterotoxin B (SEB) in buffer, apple juice, and milk is shown using piezoelectric-excited, millimeter-sized cantilever (PEMC) sensors. Limit of detection in spiked milk and apple juice samples is 10 and 100 fg, respectively. PEMC sensors (2 mm(2)) are prepared by immobilizing a polyclonal antibody specific to SEB, which was exposed to 1 mL of 1% milk and apple juice containing 10 fg-10 ng. Sensor response to 100 fg, 1 pg, and 10 pg of SEB in apple juice resulted in resonance frequency decreases of 113 +/- 18 (n = 4), 308 +/- 24 (n = 4), and 521 +/- 20 (n = 2) Hz, respectively. In milk, 10 fg, 100 fg, 1 pg, and 10 pg of SEB resulted in resonance frequency decreases of 126 +/- 18 (n = 2), 143 +/- 35 (n = 4), 310 +/- 32 (n = 5), and 557 +/- 25 (n = 2) Hz, respectively. Positive detection of SEB in the sample solution was observed within the first 20 min. The responses of the sensor to positive (SEB present, but no antibody on sensor), negative (SEB absent, antibody on sensor), and buffer (SEB absent, antibody on sensor) controls were -17 +/- 10 (n = 3), -9 +/- 5 (n = 3), and -6 +/- 12 (n = 18) Hz, respectively. Positive verification of SEB detection was confirmed by two methods: (1) low-pH buffer release caused increase in resonance frequency, and (2) second antibody binding to SEB attached to sensor that caused further resonance frequency decrease. The significance of these results is that PEMC sensors can reliably detect SEB at 10-100 fg (effective concentration of 2.5 and 25 fg/mL) in complex fluids without sample preparation or the use of labeled reagents.

PEMC-based Method of Measuring DNA Hybridization at Femtomolar Concentration Directly in Human Serum and in the Presence of Copious Noncomplementary Strands

Piezoelectric-excited, millimeter-sized cantilever (PEMC) sensors having high-mode resonance near 1 MHz are shown to exhibit mass change sensitivity of 1-300 ag/Hz. Gold-coated PEMC sensors immobilized with 15-mer single-stranded DNA (ssDNA) were exposed to 10-mer complementary strands at concentrations of 1 fM, 1 pM, and 1 microM, both separately and sequentially at 0.6 mL/min in a sample flow cell housing the sensor. Decrease in resonance frequency occurred as complementary strands hybridized to the immobilized probe DNA on the sensor surface. Hybridization in three background matrixes--buffer, buffer containing 10,000 times higher noncomplementary strands, and 50% human plasma--were successfully tested. Sensor hybridization responses to 1 fM, 1 pM, and 1 microM complementary strand were nearly the same in magnitude in all three matrixes, but the hybridization rates were different. In each case, the sensor detected the presence of 2 amol of complementary 10-mer strand. The extent of hybridization calculated from resonance frequency change did not decrease in serum. The findings suggest ssDNA can be detected at 2 amol without a sample preparation step and without the use of labeled reagents.

Detection of airborne Bacillus anthracis spores by an integrated system of an air sampler and a cantilever immunosensor

Abstract A near real-time, sensitive and reliable method of detecting airborne Bacillus anthracis spores by air sampling (wet mode) and then exposing the collected sample to an antibody-functionalized piezoelectric-excited millimeter-sized cantilever (PEMC) sensor is presented. Using a commercial air sampler, a 10 min air sample at 267 L/min captured the airborne particulates containing Bacillus anthracis (BA) spores and concentrated them into 5 mL of phosphate buffered saline (PBS). This sample was then injected into a flow cell containing an antibody-functionalized PEMC sensor. The resonant frequency of the PEMC sensor at 925.1 kHz decreased exponentially as the BA spores attached to the sensor surface producing a positive response well beyond the noise level in 2 min and reached a steady state value in 20 min. In liquid phase, the sensor response correlated well (R2 = 0.99) with spore concentration and was shown to follow: [Response in Hz] = (0.0637) × (spore concentration in #/mL). Our results show that detection of 38 BA spores/L of air is achievable in near real-time with an estimated lower limit of detection of ∼5 spores/L of air in the configuration tested.

Detection of Staphylococcus enterotoxin B at picogram levels using piezoelectric-excited millimeter-sized cantilever sensors

We report a highly sensitive and rapid method for the detection of Staphylococcus aureus enterotoxin B (SEB) at picogram levels using a piezoelectric-excited millimeter-sized cantilever (PEMC) sensor. Affinity purified polyclonal antibody to Staphylococcal enterotoxin B (anti-SEB) was immobilized on the aminated cantilever glass surface, followed by exposure of SEB containing samples flowing at a rate of 1 mL/min. The binding of SEB was measured by monitoring the sensor's fundamental resonance frequency. The fundamental frequencies in air and under liquid immersion were 17.50 ± 0.01 kHz and 10.37 ± 0.01 kHz, respectively. Binding of SEB to the sensor surface resulted in an exponential decrease of the resonance frequency and reached a steady state frequency change of 31 ± 1 Hz ( n = 2) and 208 ± 1 Hz ( n = 2) corresponding to SEB concentrations of 50 pg/mL and 12.5 ng/mL, respectively. In each experiment, the bound SEB was released by a pH 2.0 HCl/PBS solution and the sensor response was nearly identical to the frequency change caused by attachment. Full antibody surface regeneration was obtained after the release by rinsing the sensor with PBS pH 7.4. We report the detection limit of PEMC sensors to SEB is between 12.5 pg/mL and 50 pg/mL.

Piezoelectric-excited millimeter-sized cantilever sensors detect density differences of a few micrograms/mL in liquid medium

We show that piezoelectric-excited millimeter-sized cantilever sensors operating in the inviscid region (Reynolds number ∼10 5) detect density differences of a few μg/mL. The self-sensing mechanism of the sensor provides a sensitive method and enables detection of solute concentration differences of 10 ppm. We show detection sensitivity using aqueous solutions of glycerol, sodium chloride and 1-propanol at mass fractions 0–0.2 in an isothermal (20.3 °C) measurement system. While there is good agreement with theory (∼5%) over the entire density range examined (0.9698–1.0786), considerable deviations occur at low mass fraction (∼0.001). Observation of the deviations has been possible because of the high sensitivity of the sensor used in this study and the use of continuous flow measurements. We show for the first time with continuous flow experiments that surface adsorption of solute (glycerol) contributes to the observed deviation.

Detect of Escherichia coli O157:H7 in ground beef samples using piezoelectric excited millimeter-sized cantilever (PEMC) sensors

Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors consisting of a piezoelectric and a borosilicate glass layer with a sensing area of 4 mm2 were fabricated. An antibody specific to Escherichia coli (anti-E. coli) O157:H7 was immobilized on PEMC sensors, and exposed to samples containing E. coli O157:H7 (EC) prepared in various matrices: (1) broth, broth plus raw ground beef, and broth plus sterile ground beef without inoculation of E. coli O157:H7 served as controls, (2) 100 mL of broth inoculated with 25 EC cells, (3) 100 mL of broth containing 25 g of raw ground beef and (4) 100 mL of broth with 25 g of sterile ground beef inoculated with 25 EC cells. The total resonant frequency change obtained for the broth plus EC samples were 16+/-2 Hz (n=2), 30 Hz (n=1), and 54+/-2 Hz (n=2) corresponding to 2, 4, and 6h growth at 37 degrees C, respectively. The response to the broth plus 25 g of sterile ground beef plus EC cells were 21+/-2 Hz (n=2), 37 Hz (n=1), and 70+/-2 Hz (n=2) corresponding to 2, 4, and 6 h, respectively. In all cases, the three different control samples yielded a frequency change of 0+/-2 Hz (n=6). The E. coli O157:H7 concentration in each broth and beef samples was determined by both plating and by pathogen modeling program. The results indicate that the PEMC sensor detects E. coli O157:H7 reliably at 50-100 cells/mL with a 3 mL sample.

Use of Piezoelectric-Excited Millimeter-Sized Cantilever Sensors To Measure Albumin Interaction with Self-Assembled Monolayers of Alkanethiols Having Different Functional Headgroups

In this paper, we describe a new modality of measuring human serum albumin (HSA) adsorption continuously on CH3-, COOH-, and OH-terminated self-assembled monolayers (SAMs) of C11-alkanethiols and the direct quantification of the adsorbed amount. A gold-coated piezoelectric-excited millimeter-sized cantilever (PEMC) sensor of 6-mm2 sensing area was fabricated, where resonant frequency decreases upon mass increase. The resonant frequency in air of the detection peak was 45.5 +/- 0.01 kHz. SAMs of C11-thiols (in absolute ethanol) with different end groups was prepared on the PEMC sensor and then exposed to buffer solution containing HSA at 10 microg/mL. The resonant frequency decreased exponentially and reached a steady-state value within 30 min. The decrease in resonant frequency indicates that the mass of the sensor increased due to HSA adsorption onto the SAM layer. The frequency change obtained for the HSA adsorption on CH3-, COOH-, and OH-terminated SAM were 520.8 +/- 8.6 (n = 3), 290.4 +/- 6.1 (n = 2), and 210.6 +/- 8.1 Hz (n = 3), respectively. These results confirm prior conclusions that albumin adsorption decreased in the order, CH(3) > COOH > OH. Observed binding rate constants were 0.163 +/- 0.003, 0.248 +/- 0.006, and 0.381 +/- 0.001 min(-1), for methyl, carboxylic, and hydroxyl end groups, respectively. The significance of the results reported here is that both the formation of self-assembled monolayers and adsorption of serum protein onto the formed layer can be measured continuously, and quantification of the adsorbed amount can be determined directly.