Suiqiong Li

Direct detection of Salmonella typhimurium on fresh produce using phage-based magnetoelastic biosensors.

Current bacterial detection methods require the collection of samples followed by preparation and analysis in the laboratory, both time and labour consuming steps. More importantly, because of cost, only a limited number of samples can be taken and analyzed. This paper presents the results of an investigation to directly detect Salmonella typhimurium on fresh tomato surfaces using phage-based magnetoelastic (ME) biosensors. The biosensor is composed of a ME resonator platform coated with filamentous E2 phage, engineered to bind with S. typhimurium. The ME biosensors are wireless sensors, whose resonance oscillation and resonance frequency are actuated and detected through magnetic fields. The sensors used in this study were 0.028 mm×0.2 mm×1 mm in size. In this study, the tomato surface was spiked with S. typhimurium suspensions with concentrations ranging from 5×10(1) to 5×10(8)CFU/ml and then allowed to dry in air. The detection was conducted by directly placing ME measurement biosensors and control sensors on the spiked surface for 30 min in a humid environment. The control sensors were identical to the measurement biosensors, but without phage. Both measurement and control sensors were blocked with BSA to reduce non-specific binding. The resonance frequencies of both measurement and control sensors were measured prior to and after the placement of the sensors on the tomato. Shifts in the resonance frequency of the measurement biosensors were observed, while the control sensors showed negligible change. Scanning electron microscopy (SEM) was used to verify the specific binding of S. typhimurium to the biosensor. Results of multiple biosensor detection and corresponding analyzes showed statistically different responses between the measurement and control sensors for tomatoes spiked with S. typhimurium suspensions with concentrations of 5×10(2)CFU/ml and greater. This study demonstrates the direct detection of food-borne bacteria on fresh produce.

Biosensor based on magnetostrictive microcantilever

Magnetostrictive microcantilever (MSMC) as remote biosensor platform is reported. The mass sensitivity of the MSMCs is simulated and compared with the other microcantilevers. MSMCs with a thickness of 30–35μm and different lengths and widths were fabricated from the magnetostrictive metal glass coated with a copper layer by sputtering. The resonance behavior of the MSMCs was experimentally determined. It is experimentally found that the MSMCs work well in either air or liquid. For MSMCs operated in air, a Q value of more than 500 was obtained. For MSMCs operated in water, the Q value reaches more than 30. The application of a MSMC as a biosensor platform is demonstrated by in situ detection of the yeast cells in water using the MSMC sensor.

Rapid and sensitive detection of Salmonella Typhimurium on eggshells by using wireless biosensors.

This article presents rapid, sensitive, direct detection of Salmonella Typhimurium on eggshells by using wireless magnetoelastic (ME) biosensors. The biosensor consists of a freestanding, strip-shaped ME resonator as the signal transducer and the E2 phage as the biomolecular recognition element that selectively binds with Salmonella Typhimurium. This ME biosensor is a type of mass-sensitive biosensor that can be wirelessly actuated into mechanical resonance by an externally applied timevarying magnetic field. When the biosensor binds with Salmonella Typhimurium, the mass of the sensor increases, resulting in a decrease in the sensors resonant frequency. Multiple E2 phage-coated biosensors (measurement sensors) were placed on eggshells spiked with Salmonella Typhimurium of various concentrations (1.6 to 1.6 × 10(7) CFU/cm(2)). Control sensors without phage were also used to compensate for environmental effects and nonspecific binding. After 20 min in a humidity-controlled chamber (95%) to allow binding of the bacteria to the sensors to occur, the resonant frequency of the sensors was wirelessly measured and compared with their initial resonant frequency. The resonant frequency change of the measurement sensors was found to be statistically different from that of the control sensors down to 1.6 × 10(2) CFU/cm(2), the detection limit for this work. In addition, scanning electron microscopy imaging verified that the measured resonant frequency changes were directly related to the number of bound cells on the sensor surface. The total assay time of the presented methodology was approximately 30 min, facilitating rapid detection of Salmonella Typhimurium without any preceding sampling procedures.

Effects of surface functionalization on the surface phage coverage and the subsequent performance of phage-immobilized magnetoelastic biosensors

One of the important applications for which phage-immobilized magnetoelastic (ME) biosensors are being developed is the wireless, on-site detection of pathogenic bacteria for food safety and bio-security. Until now, such biosensors have been constructed by immobilizing a landscape phage probe on gold-coated ME resonators via physical adsorption. Although the physical adsorption method is simple, the immobilization stability and surface coverage of phage probes on differently functionalized sensor surfaces need to be evaluated as a potential way to enhance the detection capabilities of the biosensors. As a model study, a filamentous fd-tet phage that specifically binds streptavidin was adsorbed on either bare or surface-functionalized gold-coated ME resonators. The surface functionalization was performed through the formation of three self-assembled monolayers with a different terminator, based on the sulfur-gold chemistry: AC (activated carboxy-terminated), ALD (aldehyde-terminated), and MT (methyl-terminated). The results, obtained by atomic force microscopy, showed that surface functionalization has a large effect on the surface phage coverage (46.8%, 49.4%, 4.2%, and 5.2% for bare, AC-, ALD-, and MT-functionalized resonators, respectively). In addition, a direct correlation of the observed surface phage coverage with the quantity of subsequently captured streptavidin-coated microbeads was found by scanning electron microscopy and by resonance frequency measurements of the biosensors. The differences in surface phage coverage on the differently functionalized surfaces may then be used to pattern the phage probe layer onto desired parts of the sensor surface to enhance the detection capabilities of ME biosensors.

A surface-scanning coil detector for real-time, in-situ detection of bacteria on fresh food surfaces

Proof-in-principle of a new surface-scanning coil detector has been demonstrated. This new coil detector excites and measures the resonant frequency of free-standing magnetoelastic (ME) biosensors that may now be placed outside the coil boundaries. With this coil design, the biosensors are no longer required to be placed inside the coil before frequency measurement. Hence, this new coil enables bacterial pathogens to be detected on fresh food surfaces in real-time and in-situ. The new coil measurement technique was demonstrated using an E2 phage-coated ME biosensor to detect Salmonella typhimurium on tomato surfaces. Real-time, in-situ detection was achieved with a limit of detection (LOD) statistically determined to be lower than 1.5×10(3) CFU/mm(2) with a confidence level of difference higher than 95% (p<0.05).

Magnetostrictive Microcantilever as an Advanced Transducer for Biosensors

The magnetostrictive microcantilever (MSMC) as a high-performance transducer was introduced for the development of biosensors. The principle and characterization of MSMC are presented. The MSMC is wireless and can be easily actuated and sensed using magnetic field/signal. More importantly, the MSMC exhibits a high Q value and works well in liquid. The resonance behavior of MSMC is characterized in air at different pressures and in different liquids, respectively. It is found that the Q value of the MSMC in water reaches about 40. Although the density and viscosity of the surrounding media affect the resonance frequency and the Q value of MSMC, the density has a stronger influence on the resonance frequency and the viscosity has a stronger influence on the Q value, which result in that, for MSMC in air at pressure of less than 100 Pa, the resonance frequency of MSMC is almost independent of the pressure, while the Q value increases with decreasing pressure. MSMC array was developed and characterized. It is experimentally demonstrated that the characterization of an MSMC array is as simple as the characterization of a single MSMC. A filamentous phage against Salmonella typhimurium was utilized as bio-recognition unit to develop an MSMC based biosensor. The detection of S. typhimurium in water demonstrated that the MSMC works well in liquid.

Novel Magnetostrictive Microcantilever and Magnetostrictive Nanobars for High Performance Biological Detection

High performance biosensors are urgently needed from medical diagnosis, to food safety/security, to the war on bio-terrorist. Recently, magnetostrictive microcantilever (MSMC) and magnetostrictive particle (MSP) have been developed as high performance biosensor platform. Both MSMC and MSP are wireless sensors and exhibit advantages over current acoustic wave biosensor platforms. Theoretical analysis and experimental results indicate that micro/nano scale MSMC and MSP have ultra-high sensitivities. However, in real detection, there is a challenge faces all micro/nano scale sensors because of their small size. That is, a long time is required for the tiny sensors to react with the target species. Due to the magnetic and wireless nature, MSP provides a unique way to bring the nanosensors to target species. The fabrication of bar-like MSPs in nanoscale is reported. Amorphous Fe-B alloy was selected as target magnetostrictive materials for fabrication. The properties of these nanobars were determined. The morphology and magnetic properties of the nanobars were characterized. The results ware analyzed and the size effect on the microstructure and properties is discussed.

Development of biosensor based on microdiaphragm

There is an urgent need for real-time bio-detectors with high performance, such as high sensitivity, small size, easy deployment. Sensor platforms based on MEMS, such as microcantilevers (including piezoelectric and silicon-based cantilevers), have been studied. Piezoelectric-based micro-diaphragm, micro-electromechanical diaphragm (MEMD), used as micro-sensor platform, is reported in this article. It is found that the sensitivity of the sensor based on micro-diaphragm is much higher than that based on the micro-cantilever. Since a lower density of material used in the diaphragm results in a better sensitivity, PVDF-based piezoelectric polymer was chosen to fabricate the devices. Both cantilevers and diaphragms made of the same piezoelectric polymer were characterized in order to compare the difference of quality merit factor (Q-value) between the cantilever and diaphragm. It is experimentally found that the Q-value of the diaphragm is higher than that of the cantilever. More importantly, the damping effect of liquid media on diaphragm is much smaller than that that on cantilever. All these indicate that as a sensor platform the micro-diaphragm is much better than the micro-cantilever.

Characterization of Microstructure and Composition of Fe-B Nanobars as Biosensor Platform

Individual magnetostrictive nanobars and arrays comprised of magnetostrictive nanobars were recently introduced as a high performance biosensor platform. In this paper, we report the fabrication and characterization of magnetostrictive nanobars based on Fe-B alloy. The nanobars were synthesized using a template-based electrochemical deposition method. The composition and microstructure of the Fe-B nanobars are directly related to their performance as a biosensor platform. The Fe-B nanobar arrays and individual nanobar were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), as well as Auger electron spectroscopy (AES). Morphologically, nanobars have a very flat top and a smooth cylindrical surface, which are critical factors for obtaining high performance as sensor platforms. Structurally, electron diffraction reveals that the Fe-B nanobars are amorphous. AES analysis indicates that Fe-B nanobars show no significant compositional variation along the length direction. It is found that the nanobars were covered by an oxidation layer of a typical thickness of ∼ 10 nm. It is believed that this oxidation layer is related to the passivation of nanobars in air. High temperature annealing and subsequent structural analysis indicate that the Fe-B nanobars possess a good thermal stability.

Detection of Bacillus anthracis spores in water using biosensors based on magnetostrictive microcantilever coated with phage

Microcantilevers (MCs) as state-of-art sensor platforms have been widely investigated. We recently introduced a new type of MC, magnetostrictive microcantilever (MSMC), as high performance sensor platform. The MSMC is a remote/wireless sensor platform and exhibits a high quality merit factor in liquid. In this paper, a MSMC-based biosensor is developed for detecting B. anthracis spores in liquid, a potential biothreaten agent. The results demonstrated the advantages of MSMCs as a sensor platform. MSMCs with different sizes were fabricated and utilized in the experiments. The MSMCs were coated with the filamentous phage as a bio-recognition element to capture the B. anthracis spores. The phage-coated MSMCs as biosensors were exposed to cultures containing target spores with concentrations ranging from 5 * 104 spores/mL to 5 * 108 spores/mL. The resonance frequency of the MSMC sensors in cultures was monitored in a real-time manner. The results showed that for MSMCs of 2.8 mm * 1.0 mm * 35 &mgr;m and with 1.4 mm * 0.8 mm * 35 &mgr;m have a detection limit of 105 and 104 spores/mL, respectively.