ABSTRACTBiosensors for in situ detection of pathogenic bacteria in liquid are developed using magnetostrictive particles (MSP) as the sensor platform. The sensing elements used are phage E2 against Salmonella typhimurium, monoclonal antibody against Listeria monocytogenes, polyclonal antibody against Escherichia coli, and polyclonal antibody against Staphylococcus aureus, respectively. These biosensors were characterized in cultures with different populations ranging from 5 × 101 to 5 × 108 cfu/mL. It is found that the MSP‐based biosensors work well in water and have a rapid response with a response time in minutes, which makes the MSP‐based sensors suitable for in situ and real‐time detection of pathogenic bacteria in liquid. The experimental results show that all MSP‐phage and MSP‐antibody biosensors in size of 1.0 mm × 0.3 mm × 15 µm exhibit a detection limit better than 100 cfu/mL. Based on the Hill plot, it is concluded that each bacterial cell is bound onto the sensor surface through about four‐to‐five sites. When the cultures with low population (<106 cfu/mL) are tested, both MSP‐phage and MSP‐antibody sensors exhibit the similar response. However, the phage‐MSP sensors exhibit a higher capability in the capture of target bacterial cell. Biotechnol. Bioeng. 2014;111: 2229–2238. © 2014 Wiley Periodicals, Inc.

Two types of magnetostrictive resonators – magnetostrictive microcantilever (MSMC) and magnetostrictive particle (MSP) – have been introduced as sensor platforms. Their principles and advantages as sensor platforms are discussed along with the materials selection. A detailed and complete comparison between the MSMC and MSP is given. It is concluded that for the resonators with the same size, an MSP exhibits a higher sensitivity and has a much higher resonant frequency. For the resonators with the same resonant frequency, MSMCs exhibit a much higher sensitivity and have a much smaller size than MSPs. Using antibody as the sensing element, MSP biosensors for in situ detection of Escherichia coli and Listeria monocytogenes are developed and characterized. These biosensors exhibit a high performance. For example, the MSP-antibody biosensors of 1mm×0.3mm×15μm exhibit a detection limit less than 100cfu/ml for in situ detection of bacterial cell in water. A new type actuator is introduced using MSPs. The MSP actuator is operated using AC magnetic field with a frequency close to, but different than, its resonant frequency. The MSP actuator exhibits an unlimited displacement, and its moving direction is controlled by the operating frequency used.

A type of biosensor based on filamentous phage as bioprobe and magnetostricitve milli/micro-cantilever (MSMC) as a sensor platform is developed for in situ detection of Bacillus anthracis spores in water. The phages are immobilized onto the MSMCs' surface through physical absorption. It is found that the immobilized phages are attached onto the MSMCs' surface through their ends and have a highly packed 2-D configuration. The real-time detection of B. anthracis spores in water with different concentrations was conducted. The experimental results indicate that the smaller the MSMC, the better the detection limit. For example, the detection limit is 104 spores/ml and 105 spores/ml for the MSMCs in size of 1.4 mm × 0.8 mm × 35 μm and 2.8 mm × 1.0 mm × 35 μm, respectively. The SEM observations confirm that the shift in the characteristic frequency of the biosensor is due to the binding of spores on the sensor surface. It is also found that the density of the spores binding at the tip of the cantilever is higher than that in the middle. The specificity of the sensor was studied. It is found that the sensor can detect B. anthracis spores with a much higher binding affinity than the spores of B. cereus and B. megaterium.

To ensure the safety of food, a detection device, which can detect/monitor the present of bacteria in a real-time manner and can be easily used for in-field tests, is highly desirable. Recently, magnetostrictive particles (MSPs) as a new type of high-performance biosensor have been developed. The detection of various bacteria and spores in food with high sensitivity has already been experimentally demonstrated. To fully use the technique for food safety, two miniaturized interrogation systems based on frequency-domain and time-domain technique are developed to fabricate a handheld detection device. The detection of Salmonella typhimurium (S. typhimurium) in liquid using a time-domain based interrogation system was demonstrated.

Magnetostrictive microcantilever (MSMC), as a high-performance biosensor platform, was introduced recently. By using physical absorption, an antibody against Escherichia coli is immobilized onto the surface of the MSMC to form a biosensor. The real-time and in situ detection of E. coli in water using the biosensor is reported. The responses of the biosensor in E. coli suspension with different concentrations were studied, which were used to determine the detection limit of the biosensor. It is confirmed by using SEM observation and a control sensor approach, respectively, that the experimentally observed response of the biosensor is due to the binding of E. coli on the surface of the biosensor. It is found that the detection limit is improved by reducing the size of the MSMC, which is consistent with the theoretical calculation. For an MSMC with the size of 1.5mm×0.8mm×35μm, the detection limit is 105cfu/ml.

Piezoelectric membrane was recently introduced as a high performance platform for the development of biosensor. The sensor principle is based on the resonance frequency change due to mass load. Therefore, determination of the resonance frequency is the key. In this paper, the fabrication and characterization of square membranes made of piezo-PVDF thin film are reported. The thickness of the PVDF is about 30 pin, while the size of the membrane is from 2 mm to 6 mm. The resonance behavior of these membranes was characterized under different conditions: two sides with air in different pressure, one side in air and another side in different liquids, It is experimentally found that the membrane based devices, working in the bend mode, could keep a nearly same Q value in water than in air, which makes the piezoelectric membrane a stronger candidate for biosensor platform used in liquid environment. Furthermore, the influences of size, pressure difference, density and viscosity of the environments on the resonance frequency and the Q value are experimentally determined and the results are discussed.

Magnetostrictive micro/milli-cantilever (MSMC) was recently introduced as a promised biosensor platform due to its high performance in liquid and wireless nature. To better understand the resonance behavior of the MSMC, unimorph-type MSMCs with thickness about 30μm and different lengths and widths were fabricated, and their resonance behaviors were investigated in air and in different liquids. The influence of the driving magnetic fields and the surrounding media on their resonance behavior was also studied. It is found that the amplitude of the driving ac magnetic field has very weak influence on the resonance frequency. On the basis of the damping effect of different liquids on the resonance behavior of the MSMCs, it is found that the characteristic frequency of the MSMC is linearly dependent on the density of the liquid media, while the Q value is inversely proportional to the square root of the product of the density and viscosity of the liquid media. It is also found that the damping effect of liquid on the MSMC can be treated as a damping string of sphere and the effect radius of the oscillating sphere for an MSMC is a constant. The value of the effect radius for different MSMCs was experimentally determined. Additionally, the resonance frequency of the MSMC is very stable. Due to their wireless nature, MSMCs are suitable for the development of a cantilever array. It is experimentally demonstrated that the characterization of an MSMC array is as simple as a single MSMC. The detection of Bacillus anthracis spores in water was performed using MSMC biosensors in a real-time manner to demonstrate the in situ detection capability.

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 * 10(4) spores/mL to 5 * 10(8) 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 min 1.0 mm * 35 mu n and with 1.4 mm * 0.8 mm * 35 mu m have a detection limit of 10(5) and 10(4) spores/mL, respectively.

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.

To develop biosensors with the capability of detecting very small mount of biological agents, such as single or several cells, magnetostrictive bars or stripes in size from nanometer to micrometer are required. In this paper, magnetostrictive nanobars and nanobar arrays, with a diameter from 50 to 200 nm and a length of 2~5 μm, were fabricated based on template-based synthesis. The amorphous Fe-B alloy was selected as the magnetostrictive material to fabricate the nanobars. The study on resonance behavior and magnetic properties of plated Fe-B thin films indicate that amorphous Fe-B alloy is a good candidate for fabricating high performance sensor platform. The magnetization hysteresis loop of Fe-B nanobars was characterized. It is found that all the nanobar arrays exhibit easy axis of magnetization along bar length direction but with smaller coercivity, which is different with bulk materials. The physics behind the phenomena is discussed.

High sensitivity and high Q value, as well as working well in liquid, make the newly developed magnetostrictive microcantilevers (MSMCs) a great candidate for developing a high performance biosensor. In this paper, blood cell identification by the MSMCs was demonstrated. The MSMCs were fabricated and their surface was functionalized by immobilizing anti-B antibody as the bioreceptor for blood cells inspection. By immersing the MSMCs into different type blood cells and monitoring the resonance frequency shift, due to blood cell binding, the blood cells A and B were distinguished.

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.

Recently, the magnetostrictive microcantilever (MSMC) as a high performance biosensor platform was introduced. The MSMC is a wireless acoustic wave (AW) sensor and exhibits a high Q value. More importantly, the MSMC works well in liquid. In this paper, the detection of Bacillus anthracis spores using MSMCs with filamentous phage as the bioprobe is reported. The phage-coated MSMC biosensors were exposed to cultures containing target spores with increasing concentrations ranging from 5 × 10 4 to 5 × 10 8 spores/mL. By monitoring the shift in the resonance frequency of the MSMCs, the spores were detected in a real-time manner and a detection limit of 10 5 spores/mL was obtained for the MSMCs used in this research. Higher sensitivity is expected for the MSMCs with smaller size.

Magnetostrictive nanobars as sensor platform were induced. Based on the resonance behavior of strips made from thin films, it is identified that the amorphous Fe-B alloy is a good candidate for fabricating high performance sensor platform. The fabrication process of amorphous Fe-B nanobars using electrochemical deposition is reported. The magnetization hysteresis loop of Fe-B nanobars with the diameters of 50, 100 and 200 nm, respectively, was characterized. It is found that, for all nanobars, the coercive field measured along length direction is smaller than the coercive field measured perpendicular to length direction. The physics behind the phenomena is discussed.