Detection of pathogen Escherichia coli O157:H7 using self-excited PZT-glass microcantilevers

Water-based technologies for improving water quality at the point of use: A review

Water safety concerns are increasing tremendously as a result of the rising population and environmental pollution. As a result, viable water treatment approaches need to be designed to meet the water consumption demands of the population, particularly in developing countries. The recent technological advances in water treatment and purification are well articulated in this review. The efficiency of the materials used for purification and their affordability for people living in rural and remote settlements in various parts of the world have been discussed. Water treatment techniques prior to the rapid advancement of science and technology included a variety of strategies such as coagulation/flocculation, filtration, disinfection, flotation and pH correction. The use of nanotechnology in water treatment and purification has modernized the purification process. Therefore, efficient removal of microbes such as bacteria and viruses are exquisitely accomplished. These technologies may include membrane filtration, ultraviolet irradiation, advanced oxidation ion-exchange and biological filtration technologies. Thus, nanotechnology allows for the fabrication of less expensive systems, allowing even low-income people to benefit from it. Most developing countries find these technologies particularly valuable because access to clean and safe water for drinking and residential needs is critical. This is because access to municipal water supplies is also difficult. This article is categorized under: Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Particle Diffusometry: An Optical Detection Method for Vibrio cholerae Presence in Environmental Water Samples

There is a need for a rapid, robust, and sensitive biosensor to identify low concentrations of pathogens in their native sample matrix without enrichment or purification. Nucleic acid-based detection methods are widely accepted as the gold standard in diagnostics, but robust detection of low concentrations of pathogens remains challenging. Amplified nucleic acids produce more viscous solutions, which can be measured by combining these products with fluorescent particles and measuring the change in the particle diffusion coefficient using a technique known as particle diffusometry. Here, we utilize Vibrio cholerae (V. cholerae) as a proof-of-concept for our detection system due to its inherently low concentration in environmental water samples. We demonstrate that particle diffusometry can be used to detect down to 1 V. cholerae cell in molecular-grade water in 20 minutes and 10 V. cholerae cells in pond water in just 35 minutes in 25 µL reaction volumes. The detection limit in pond water is environmentally relevant and does not require any enrichment or sample preparation steps. Particle diffusometry is 10-fold more sensitive than current gold standard fluorescence detection of nucleic acid amplification. Therefore, this novel measurement technique is a promising approach to detect low levels of pathogens in their native environments.

Rapid identification of spinal ventral and dorsal roots using a quartz crystal microbalance

The fast and accurate identification of nerve tracts is critical for successful nerve anastomosis. Taking advantage of differences in acetylcholinesterase content between the spinal ventral and dorsal roots, we developed a novel quartz crystal microbalance method to distinguish between these nerves based on acetylcholinesterase antibody reactivity. The acetylcholinesterase antibody was immobilized on the electrode surface of a quartz crystal microbalance and reacted with the acetylcholinesterase in sample solution. The formed antigen and antibody complexes added to the mass of the electrode inducing a change in frequency of the electrode. The spinal ventral and dorsal roots were distinguished by the change in frequency. The ventral and dorsal roots were cut into 1 to 2-mm long segments and then soaked in 250 μL PBS. Acetylcholinesterase antibody was immobilized on the quartz crystal microbalance gold electrode surface. The results revealed that in 10 minutes, both spinal ventral and dorsal roots induced a frequency change; however, the frequency change induced by the ventral roots was notably higher than that induced by the dorsal roots. No change was induced by bovine serum albumin or PBS. These results clearly demonstrate that a quartz crystal microbalance sensor can be used as a rapid, highly sensitive and accurate detection tool for the quick identification of spinal nerve roots intraoperatively.

Biosensors for whole-cell bacterial detection

Bacterial pathogens are important targets for detection and identification in medicine, food safety, public health, and security. Bacterial infection is a common cause of morbidity and mortality worldwide. In spite of the availability of antibiotics, these infections are often misdiagnosed or there is an unacceptable delay in diagnosis. Current methods of bacterial detection rely upon laboratory-based techniques such as cell culture, microscopic analysis, and biochemical assays. These procedures are time-consuming and costly and require specialist equipment and trained users. Portable stand-alone biosensors can facilitate rapid detection and diagnosis at the point of care. Biosensors will be particularly useful where a clear diagnosis informs treatment, in critical illness (e.g., meningitis) or to prevent further disease spread (e.g., in case of food-borne pathogens or sexually transmitted diseases). Detection of bacteria is also becoming increasingly important in antibioterrorism measures (e.g., anthrax detection). In this review, we discuss recent progress in the use of biosensors for the detection of whole bacterial cells for sensitive and earlier identification of bacteria without the need for sample processing. There is a particular focus on electrochemical biosensors, especially impedance-based systems, as these present key advantages in terms of ease of miniaturization, lack of reagents, sensitivity, and low cost.

Hybrid integrated label-free chemical and biological sensors

Label-free sensors based on electrical, mechanical and optical transduction methods have potential applications in numerous areas of society, ranging from healthcare to environmental monitoring. Initial research in the field focused on the development and optimization of various sensor platforms fabricated from a single material system, such as fiber-based optical sensors and silicon nanowire-based electrical sensors. However, more recent research efforts have explored designing sensors fabricated from multiple materials. For example, synthetic materials and/or biomaterials can also be added to the sensor to improve its response toward analytes of interest. By leveraging the properties of the different material systems, these hybrid sensing devices can have significantly improved performance over their single-material counterparts (better sensitivity, specificity, signal to noise, and/or detection limits). This review will briefly discuss some of the methods for creating these multi-material sensor platforms and the advances enabled by this design approach.

Biosensing using dynamic-mode cantilever sensors: a review

Current progress on the use of dynamic-mode cantilever sensors for biosensing applications is critically reviewed. We summarize their use in biosensing applications to date with focus given to: cantilever size (milli-, micro-, and nano-cantilevers), their geometry, and material used in fabrication. The review also addresses techniques investigated for both exciting and measuring cantilever resonance in various environments (vacuum, air, and liquid). Biological targets that have been detected to date are summarized with attention to bio-recognition chemistry, surface functionalization method, limit of detection, resonant frequency mode type, and resonant frequency measurement scheme. Applications published to date are summarized in a comprehensive table with description of the aforementioned details including comparison of sensitivities. Further, the general theory of cantilever resonance is discussed including fluid-structure interaction and its dependence on the Reynolds number for Newtonian fluids. The review covers designs with frequencies ranging from ∼1 kHz to 10 MHz and cantilever size ranging from millimeters to nanometers. We conclude by identifying areas that require further investigation.

Organosilane deposition for microfluidic applications

Treatment of surfaces to change the interaction of fluids with them is a critical step in constructing useful microfluidics devices, especially those used in biological applications. Silanization, the generic term applied to the formation of organosilane monolayers on substrates, is both widely reported in the literature and troublesome in actual application for the uninitiated. These monolayers can be subsequently modified to produce a surface of a specific functionality. Here various organosilane deposition protocols and some application notes are provided as a basis for the novice reader to construct their own silanization procedures, and as a practical resource to a broader range of techniques even for the experienced user.

Microcantilever biosensors for chemicals and bioorganisms

In the last fifteen years, microcantilevers (MCLs) have been emerging as a sensitive tool for the detection of chemicals and bioorganisms. Because of their small size, lightweight, and high surface-to-volume ratio, MCL-based sensors improve our capability to detect and identify biological agents by orders of magnitude. A biosensor is a device for the detection of an analyte that combines a biological component with a physicochemical detector component. The MCL biosensors have recently been reviewed in several papers. All of these papers were organized based on the sensing biological elements (antibody, enzyme, proteins, etc.) for recognition of analytes. In this review, we intend to summarize the microcantilever biosensors in a format of each specific chemical and bioorganism species to make information on individual biosensors easily accessible. We did this to aid researchers to locate relevant references.

Ligase detection reaction generation of reverse molecular beacons for near real-time analysis of bacterial pathogens using single-pair fluorescence resonance energy transfer and a cyclic olefin copolymer microfluidic chip

Detection of pathogenic bacteria and viruses require strategies that can signal the presence of these targets in near real-time due to the potential threats created by rapid dissemination into water and/or food supplies. In this paper, we report an innovative strategy that can rapidly detect bacterial pathogens using reporter sequences found in their genome without requiring polymerase chain reaction (PCR). A pair of strain-specific primers was designed based on the 16S rRNA gene and were end-labeled with a donor (Cy5) or acceptor (Cy5.5) dye. In the presence of the target bacterium, the primers were joined using a ligase detection reaction (LDR) only when the primers were completely complementary to the target sequence to form a reverse molecular beacon (rMB), thus bringing Cy5 (donor) and Cy5.5 (acceptor) into close proximity to allow fluorescence resonance energy transfer (FRET) to occur. These rMBs were subsequently analyzed using single-molecule detection of the FRET pairs (single-pair FRET; spFRET). The LDR was performed using a continuous flow thermal cycling process configured in a cyclic olefin copolymer (COC) microfluidic device using either 2 or 20 thermal cycles. Single-molecule photon bursts from the resulting rMBs were detected on-chip and registered using a simple laser-induced fluorescence (LIF) instrument. The spFRET signatures from the target pathogens were reported in as little as 2.6 min using spFRET.

Expression of picogram sensitive bending modes in piezoelectric cantilever sensors with nonuniform electric fields generated by asymmetric electrodes

Single-layer uniform cross-sectioned piezoelectric macro-cantilevers fabricated with an asymmetric electrode configuration enabled electrical measurement of picogram-sensitive resonant bending modes in liquids. Bending modes were otherwise not electrically measurable without excitation by a nonuniform electric field created by the geometric asymmetry in electrode design used. Electrode modification was confirmed by energy-dispersive X-ray spectroscopy (EDS). Mass-change sensitivity was tested using both bulk density changes and surface chemisorption experiments in a continuous flow apparatus. Significant response to density changes as small as 0.004 g/mL was measured. A sensitivity limit of ∼1 picogram in liquid was determined from 1-dodecanethiol chemisorption experiments. The sensitivity decreased with chemisorbed mass and was log-linear over five orders of magnitude. The observed resonance responses were in agreement with previously reported models of resonating cantilever sensors. This work demonstrates experimentally for the first time that introducing electrode asymmetry enables measurement of bending modes in cantilevers containing only a single piezoelectric layer.

Sensor to detect endothelialization on an active coronary stent

Background

A serious complication with drug-eluting coronary stents is late thrombosis, caused by exposed stent struts not covered by endothelial cells in the healing process. Real-time detection of this healing process could guide physicians for more individualized anti-platelet therapy. Here we present work towards developing a sensor to detect this healing process. Sensors on several stent struts could give information about the heterogeneity of healing across the stent.

Methods

A piezoelectric microcantilever was insulated with parylene and demonstrated as an endothelialization detector for incorporation within an active coronary stent. After initial characterization, endothelial cells were plated onto the cantilever surface. After they attached to the surface, they caused an increase in mass, and thus a decrease in the resonant frequencies of the cantilever. This shift was then detected electrically with an LCR meter. The self-sensing, self-actuating cantilever does not require an external, optical detection system, thus allowing for implanted applications.

Results

A cell density of 1300 cells/mm² on the cantilever surface is detected.

Conclusions

We have developed a self-actuating, self-sensing device for detecting the presence of endothelial cells on a surface. The device is biocompatible and functions reliably in ionic liquids, making it appropriate for implantable applications. This sensor can be placed along the struts of a coronary stent to detect when the struts have been covered with a layer of endothelial cells and are no longer available surfaces for clot formation. Anti-platelet therapy can be adjusted in real-time with respect to a patient's level of healing and hemorrhaging risks.

Bioconjugation strategies for microtoroidal optical resonators

The development of label-free biosensors with high sensitivity and specificity is of significant interest for medical diagnostics and environmental monitoring, where rapid and real-time detection of antigens, bacteria, viruses, etc., is necessary. Optical resonant devices, which have very high sensitivity resulting from their low optical loss, are uniquely suited to sensing applications. However, previous research efforts in this area have focused on the development of the sensor itself. While device sensitivity is an important feature of a sensor, specificity is an equally, if not more, important performance parameter. Therefore, it is crucial to develop a covalent surface functionalization process, which also maintains the device’s sensing capabilities or optical qualities. Here, we demonstrate a facile method to impart specificity to optical microcavities, without adversely impacting their optical performance. In this approach, we selectively functionalize the surface of the silica microtoroids with biotin, using amine-terminated silane coupling agents as linkers. The surface chemistry of these devices is demonstrated using X-ray photoelectron spectroscopy, and fluorescent and optical microscopy. The quality factors of the surface functionalized devices are also characterized to determine the impact of the chemistry methods on the device sensitivity. The resulting devices show uniform surface coverage, with no microstructural damage. This work represents one of the first examples of non-physisorption-based bioconjugation of microtoroidal optical resonators.

Piezoelectric-excited millimeter-sized cantilever biosensors

In this chapter, method of fabricating a cantilever biosensors and their use in measuring the presence of a protein is described. There are many variations in construction of a cantilever sensor. A simple and an easy version is described in this chapter. The specificity of the sensor is obtained by immobilizing an antibody specific to the antigen of interest. The piezoelectric cantilever sensors are very sensitive and can easily detect a 60 kDa protein at 100 fg/mL concentration. Experimental procedure for carrying out detection of a target analyte is outlined and a sample set of results is included.

Micro- and nanocantilever devices and systems for biomolecule detection

Recent research trends in biosensing have been geared toward developing bioanalytical devices that are label free, small in size, and portable and that can operate in a rapid manner. The performance of these devices has been dramatically improved through the advent of new materials and micro-/nanofabrication technologies. This is especially true for micro-/nanosized cantilever sensors, which undergo a change in mechanical properties upon the specific binding of biomolecules. In this review, we introduce the basic principles of cantilever biosensors in static and dynamic modes. We also summarize a range of approaches to cantilever design, fabrication, and instrumentation according to their applications. More specifically, we describe cantilever-based detections of proteins, DNA molecules, bacteria, and viruses and discuss current challenges related to the targets' biophysical characteristics.

Real-Time, Label-Free, All-Electrical Detection of Salmonella typhimurium Using Lead Zirconate Titanate/Gold-Coated Glass Cantilevers at any Relative Humidity

We have examined non-insulated PZT/gold-coated glass cantilevers for real-time, label-free detection of Salmonella t. by partial dipping at any relative humidity. The PZT/gold-coated glass cantilevers were consisted of a 0.127 mm thick PZT layer about 0.8 mm long, 2 mm wide bonded to a 0.15 mm thick gold-coated glass layer with a 3.0 mm long gold-coated glass tip for detection. We showed that by placing the water level at the nodal point, about 0.8 mm from the free end of the gold-glass tip, there was a 1-hr window in which the resonance frequency was stable despite the water level change by evaporation at 20% relative humidity or higher. By dipping the cantilevers to their nodal point, we were able to do real-time, label-free detection without background resonance frequency corrections at any relative humidity. The partially dipped PZT/gold-coated glass cantilever exhibited mass detection sensitivity, Δm/Δf = -5×10(-11)g/Hz, and a detection concentration sensitivity, 5×10(3) cells/ml in 2 ml of liquid, which was about two orders of magnitude lower than that of a 5 MHz QCM. It was also about two orders of magnitude lower than the infection dosage and one order of magnitude lower that the detection limit of a commercial Raptor sensor.

10-minute assay for detecting Escherichia coli O157:H7 in ground beef samples using piezoelectric-excited millimeter-size cantilever sensors

We detected Escherichia coli O157:H7 (EC) at approximately 10 cells per ml in spiked ground beef samples in 10 min using piezoelectric-excited millimeter-size cantilever (PEMC) sensors. The composite PEMC sensors have a sensing area of 2 mm2 and are prepared by immobilizing a polyclonal antibody specific to EC on the sensing surface. Ground beef (2.5 g) was spiked with EC at 10 to 10,000 cells per ml in phosphate-buffered saline (PBS). One milliliter of supernatant was removed from the blended samples and used to perform the detection experiments. The total resonant frequency change obtained for the inoculated samples was 138 +/- 9, 735 +/- 23, 2,603 +/- 51, and 7,184 +/- 606 Hz, corresponding to EC concentrations of 10, 100, 1,000, and 10,000 cells per ml, respectively. EC was detected in the sample solution within the first 10 min. The responses of the sensor to positive, negative, and buffer controls were 36 +/- 6, 27 +/- 2, and 2 +/- 7 Hz, respectively. Verification of EC attachment was confirmed by low-pH buffer release (PBS-HCl, pH 2.2), microscopy, and second antibody EC binding postdetection. The results indicate that PEMC sensors can reliably detect EC at less than 10 cells per ml in 10 min without sample preparation and with label-free reagents.

In situ, in-liquid, all-electrical detection of Salmonella typhimurium using lead titanate zirconate/gold-coated glass cantilevers at any dipping depth

Most biosensing techniques are indirect, slow, and require labeling. Even though silicon-based microcantilever sensors are sensitive and label-free, they are not suitable for in-liquid detection. More recently lead zirconate titanate (PZT) thin-film-based microcantilevers are shown to be sensitive and in situ. However, they require microfabrication and must be electrically insulated. In this study, we show that highly sensitive, in situ, Salmonella typhimurium detection can be achieved at 90% relative humidity using a lead zirconate titanate (PZT)/gold-coated glass cantilever 0.7 mm long with a non-piezoelectric 2.7 mm long gold-coated glass tip by partially dipping the gold-coated glass tip in the suspension at any depth without electrically insulating the PZT. In particular, we showed that at 90% relative humidity and with a dipping depth larger than 0.8mm the PZT/gold-coated glass cantilever showed virtually no background resonance frequency up-shift due to water evaporation and exhibited a mass detection sensitivity of Deltam/Deltaf=-5 x 10(-11)g/Hz. The concentration sensitivities of this PZT/gold-coated glass cantilever were 1 x 10(3) and 500 cells/ml in 2 ml of liquid with a 1 and 1.5mm dipping depth, respectively, both more than two orders of magnitude lower than the infectious dose and more than one order of magnitude lower than the detection limit of a commercial Raptor sensor.

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.

Real-time detection of Escherichia coli O157:H7 sequences using a circulating-flow system of quartz crystal microbalance

A DNA piezoelectric biosensing method for real-time detection of Escherichia coli O157:H7 in a circulating-flow system was developed in this study. Specific probes [a 30-mer oligonucleotide with or without additional 12 deoxythymidine 5'-monophosphate (12-dT)] for the detection of E. coli O157:H7 gene eaeA, synthetic oligonucleotide targets (30 and 104 mer) and PCR-amplified DNA fragments from the E. coli O157:H7 eaeA gene (104 bp), were used to evaluate the efficiency of the probe immobilization and hybridization with target DNA in the circulating-flow quartz crystal microbalance (QCM) device. It was found that thiol modification on the 5'-end of the probes was essential for probe immobilization on the gold surface of the QCM device. The addition of 12-dT to the probes as a spacer, significantly enhanced (P<0.05) the hybridization efficiency (H%). The results indicate that the spacer enhanced the H% by 1.4- and 2-fold when the probes were hybridized with 30- and 104-mer targets, respectively. The spacer reduced steric interference of the support on the hybridization behavior of immobilized oligonucleotides, especially when the probes hybridized with relatively long oligonucleotide targets. The QCM system was also applied in the detection of PCR-amplified DNA from real samples of E. coli O157:H7. The resultant H% of the PCR-amplified double-strand DNA was comparable to that of the synthetic target T-104AS, a single-strand DNA. The piezoelectric biosensing system has potential for further applications. This approach lays the groundwork for incorporating the method into an integrated system for rapid PCR-based DNA analysis.

Detection of Bacillus anthracis spores and a model protein using PEMC sensors in a flow cell at 1mL/min

Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors of 4mm(2) sensing area were immobilized with antibody specific to Bacillus anthracis (anti-BA) spores or bovine serum albumin (anti-BSA). Detection of pathogen (Bacillus anthracis (BA) at 300 spores/mL) and BSA (1 mg/mL) were investigated under both stagnant and flow conditions. Two flow cell designs were evaluated by characterizing flow-induced resonant frequency shifts. One of the flow cells labeled SFC-2 (hold-up volume of 0.3 mL), showed small fluctuations (+/-20 Hz) around a common resonant frequency response of 217 Hz in the flow rate range of 1-17 mL/min. The total resonant frequency change obtained for the binding of 300 spores/mL in 1h was 90+/-5 Hz (n=2), and 162+/-10 Hz (n=2) under stagnant and flow conditions, respectively. Binding of antibodies, anti-BA and anti-BSA, were more rapid under flow than under stagnant conditions. The sensor was repeatedly exposed to BSA with an intermediate release step. The first and second responses to BSA were nearly identical. The total resonant frequency response to BSA was 388+/-10 (n=2) Hz under flow conditions. Kinetic analysis is carried out to quantify the effect of flow rate on antibody immobilization and the two types of detection experiments.

PEMC sensor's mass change sensitivity is 20pg/Hz under liquid immersion

To enhance the mass change sensitivity of the resonating piezoelectric-excited millimeter-sized cantilever (PEMC) sensors, we reduced its length and eliminated one layer of its composite structure. As a result the mass sensitivity of the second flexural mode increased by two orders of magnitude (from 10(-9) to 10(-11)g/Hz) and the resonant frequency increased by more than 5 kHz. We demonstrate the effects of modification by detecting a model pathogen Group A Streptococcus (GAS) at 700 cells/mL. The resonant frequency change of the second mode at concentrations of 700, 7 x 10(3), 7 x 10(5), 7 x 10(6), 7 x 10(7), and 7 x 10(9)cells/mL resulted in, respectively, 3.1+/-0.5, 11.6+/-1, 15.7+/-1, 25.7+/-0.15, 28.5+/-2, and 40.5+/-3 ng (n=3 for all) of pathogen attachment. A kinetic model for the binding is proposed and verified. The observed binding rate constant was found to be in the range of 0.051-0.166 min(-1). The significance of the results we report is that the modified PEMC sensors have high mass sensitivity that pathogens can be detected at very low concentration under liquid immersion conditions.

Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors detect Bacillus anthracis at 300spores/mL

Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors consisting of a piezoelectric and a borosilicate glass layer with a sensing area of 2.48 mm2 were fabricated. Antibody specific to Bacillus anthracis (BA, Sterne strain 7702) spores was immobilized on PEMC sensors, and exposed to spores (300 to 3x10(6) spores/mL). The resonant frequency decreased at a rate proportional to the spore concentration and reached a steady state frequency change of 5+/-5 Hz (n=3), 92+/-7 Hz (n=3), 500+/-10 Hz (n=3), 1030+/-10 Hz (n=2), and 2696+/-6 Hz (n=2) corresponding to 0, 3x10(2), 3x10(3), 3x10(4), and 3x10(6) spores/mL, respectively. The reduction in resonant frequency is proportional to the change in cantilever mass, and thus the observed changes are due to the attachment of spores on the sensor surface. Selectivity of the antibody-functionalized sensor was determined with samples of BA (3x10(6)/mL) mixed with Bacillus thuringiensis (BT; 1.5x10(9)/mL) in various volume ratios that yielded BA:BT ratios of 1:0, 1:125, 1:250, 1:500 and 0:1. The corresponding resonance frequency decreases were, respectively, 2345, 1980, 1310, 704 and 10 Hz. Sample containing 100% BT spores (1.5x10(9)/mL and no BA) gave a steady state frequency decrease of 10 Hz, which is within noise level of the sensor, indicating excellent selectivity. The observed binding rate constant for the pure BA and BT-containing samples ranged from 0.105 to 0.043 min-1 in the spore concentration range 300 to 3x10(6)/mL. These results show that detection of B. anthracis spore at a very low concentration (300 spores/mL) and with high selectivity in presence of another Bacillus spore (BT) can be accomplished using piezoelectric-excited millimeter-sized cantilever sensors.

The effect of a distributed mass loading on the frequency response of a MEMS mesh resonator

This paper reports on the development of an acoustic-wave biosensor based on integrated MEMS technology that promises high sensitivity and selectively without the need for molecular tagging or external optical equipment. The device works by detecting frequency shifts resulting from the selective binding of target molecules to the surface of a functionalized resonating polymer MEMS-composite membrane. Here, we characterize the frequency response of our metal-oxide MEMS resonators. We show that the structural topology, which includes the amount of void area spacing, total mass of the resonator, and how the mass is distributed on the surface, affects the resonant frequency response in a measurable way. Using a multimodal electrostatic drive, we can either excite or suppress higher order harmonic frequencies. The excitation of higher order harmonics is important for multiple analyte detection or redundancy testing. We use a finite element model to demonstrate how a distributed mass loading affect the frequency responses of our MEMS structures.

Acoustic microsensors—the challenge behind microgravimetry

Acoustic microsensors are commonly known as high-resolution mass-sensitive devices. This is a restricted view in many chemical and biosensor applications, especially in liquids. Sensitivity to non-gravimetric effects is a challenging feature of acoustic sensors. In this review we give an overview of recent developments in resonant sensors including micromachined devices and also list recent activity relating to the (bio)chemical interface of acoustic sensors. Major results from theoretical analysis of quartz crystal resonators, descriptive for all acoustic microsensors are summarized, and non-gravimetric contributions to the sensor signal from viscoelasticity and interfacial effects are discussed. We finally conclude with some future perspectives.

Detection of pathogen Escherichia coli O157:H7 AT 70cells/mL using antibody-immobilized biconical tapered fiber sensors

Optical fibers (core diameter 8 microm, cladding diameter 125 microm) was tapered to a waist diameter in the range of 8-12 microm, and then a monoclonal antibody to the pathogen, Escherichia coli O157:H7 was covalently bonded to the surface of the tapered region. Using 470 nm light, the taper was exposed to various concentrations (7 x 10(7), 7 x 10(5), 7 x 10(3), and 70 cells/mL) of the pathogen, and the sensor showed changes in transmitted light as the antigen attached to the antibody on the taper surface. The response was equal and opposite when the pathogen was released from the surface using a low pH buffer. The magnitude of the change was inversely proportional to the concentration of the pathogen. The sensor showed good sensitivity at as low a concentration as 70 cells/mL. The antibody-immobilized taper sensor was also exposed to a mixture of the pathogen and a non-pathogenic variant (JM101) at 0%, 50% and 70% by concentration. The sensor showed good selectivity to the pathogenic antigen. A first order attachment kinetic model is proposed to quantify the rate of attachment of pathogen to the sensor surface. The kinetic rate constant (k) of E. coli O157:H7 to the fiber was found to vary in the range of (2.5-6.1) x 10(-9) min(-1) (cells/mL)(-1).

Detection and quantification of proteins using self-excited PZT-glass millimeter-sized cantilever

A composite self-excited PZT-glass cantilever (4mm in length and 2mm wide) was fabricated and used to measure the binding and unbinding of model proteins. A key feature of the cantilever is that its resonant frequency is dependent on its mass. The fabricated cantilever has mass change sensitivity in liquid of 7.2 x 10(-11)g/Hz. Resonant frequency change was measured as protein reacted or bound with the sensing glass cantilever surface. Protein concentrations, 0.1 and 1.0mg/mL, which resulted in nanogram mass change were successfully detected. The mass change sensitivity gave a total mass change of 54+/-0.45 ng for the binding of anti-rabbit IgG (biotin conjugated) to rabbit IgG immobilized cantilever and the subsequent binding of captavidin. The unbinding of anti-rabbit IgG and captavidin gave a total mass change of 54+/-1.70 ng. Fluorescence based assays showed the combined mass of both proteins in the released samples was 54+/-2.24 ng. The binding kinetics of the model proteins is modeled as first order. The initial binding rate constant of anti-rabbit IgG to rabbit IgG was 1.36+/-0.02(min(mg/mL))(-1). The initial binding rate constant of captavidin to biotinylated anti-rabbit IgG was (2.57 x 10(-1))+/-0.003(min(mg/mL))(-1). The significance of the results we report here is that millimeter-sized PZT-actuated glass cantilevers have the sensitivity to measure in real-time protein-protein binding, and the binding rate constant.