This application claims the benefit of Japanese Application No. JP 2013-28481 filed, in Japan on Feb. 16, 2013, of which is hereby incorporated by reference in this entirely.
The present invention relates to a resonant type mass sensor.
A resonant type mass sensor has been used for a method of measuring the concentration of a trace amount of chemical substances contained in gas or liquid. This sensor utilizes the fact that when another material attaches to a vibrator that is vibrating at a certain frequency, the resonant frequency also changes in accordance with the slight change in mass. As vibrators, a self excitation type vibrators such as a quartz crystal microbalance (QCM) and those generated by connecting an oscillator, such as a piezoelectric element, to the vibrator. The vibrator such as a cantilever made of ceramics has been used. The former can be called the self excitation type mass sensor, whereas the latter can be called a separate excitation type mass sensor.
The self excitation type mass sensor equipped with a thin-film resonator made of a piezoelectric material as the vibrator is disclosed in JP 2005-533265 A, for example. Furthermore, the mass sensor wherein the vibration is caused by a driving section electrostatically coupling to a Si oscillator is disclosed in JPB 4638281. The separate excitation type mass sensor coupling to the oscillator made of the piezoelectric element connected to the vibrator is disclosed in JPB 3298897, for example.
The absolute value of resonant frequency of the vibrator decreases in proportion to the decreasing of the mass of the target material. Consequently, a method of fastening the vibrator, a power feeding method, and environmental factors such as temperature and humidity affect the sensitivity limit of measurement, which is common to all these resonant type mass sensors.
However, with mass sensors based on the prior art, the power is fed to the vibrator itself (See the self excitation type mass sensor, Tama Device Co., Ltd. Japan, http://tamadevice.co.jp/9mhz.htm), or to the piezoelectric element mechanically fastened (by adhesion) to the vibrator (the separate excitation type mass sensor). Owing to these reasons, the maintaining these electrical supply lines inhibits resonant phenomena, thereby disturbing improvement in sensitivity.
In addition, the separate excitation type mass sensors include those that excite the vibrator magnetically and not electrically (See J. Teva, et al, “A femtogram resolution mass sensor platform, based on SOI electrostatically driven resonant cantilever,” Ultramicroscopy, Vol. 106, pp. 800-807, 2006). However, in this type, it is necessary that the gap between the vibrator and a magnetic head must be positioned very accurately.
As described above, in the conventional separate excitation type mass sensor, it is necessary to fasten the vibrator to a housing, which inhibits resonant phenomena, thus disturbing improvement in sensitivity.
In consideration of the above mentioned problem, the object of the present invention is to provide a separate excitation type resonant type mass sensor having a high-sensitivity.
The present inventor has thought upon the present invention by discovering that by exciting the resonant frequency of a microbeam, namely an vibrator whose both ends are free, in the transverse vibration mode excited by a piezoelectric element, the resonant type mass sensor having 38.8 ng/Hz measurement sensitivity can be achieved.
In order to realize the above-mentioned objectives, the present invention provides a resonant type mass sensor comprising: an oscillator; an vibrator placed on the oscillator; and a detecting unit for detecting the resonant frequency of the vibrator, and characterized in that the vibrator and the oscillator are not coupled mechanically, and the vibrator is not coupled mechanically to any of other members.
In the above mentioned aspect, the vibration of the vibrator is preferably represented by a standing wave.
The vibrator is preferably equipped with a molecular recognition means for recognizing the molecules of a substance to be measured.
The molecular recognition means preferably uses an antigen-antibody complex reaction.
The vibrator preferably includes at least a magnetizable part. To the magnetizable part, magnetic beads, to which an antibody or antigen is immobilized, are preferably adsorbed magnetically.
The detecting unit preferably includes a light-emitting device and a photo-sensitive device, and is equipped with a means to detect any one of oscillation, displacement, velocity, and acceleration.
According to the resonant type mass sensor of the present invention, since there is no need to fasten the vibrator to the oscillator, resonant phenomena are not prohibited by power supply or fastening, limitation in design of shape and dimensions of the vibrator can be eliminated, and the change in mass can be measured highly sensitively. Furthermore, since the vibrator is not fastened to the oscillator, the vibrator can be replaced quite easily.
In the drawings:
The embodiments of the present invention will hereafter be described in detail by referring to drawings.
Meanwhile, the vibration mode of the vibrator 2 is set not to be in a single-axis vibration mode where the transfer is made in a certain direction as in the case of a standing wave type ultrasonic motor. Namely, since the vibration mode of the vibrator 2 is not the vibration mode where transfer is made in a certain direction and the amplitude is as small as on nanometer (nm) order, the vibrator 2 never comes off the oscillator 3.
When the vibrator 2 is exposed to a measurement sample in a liquid or gas form, or when the measurement sample attaches to the vibrator 2, the mass of the vibrator 2 increases. Consequently, the resonant frequency of the vibrator 2 changes, and then the concentration of a specific chemical substance in the sample can be estimated by the change rate of the resonant frequency. The change in the resonant frequency of the vibrator 2 is detected by the detecting unit 5. As a means to measure the resonant frequency of the vibrator 2, the detecting unit 5 can use an optical detecting means made up of a light-emitting device and a photo-sensitive device. The optical detecting means may measure any one of the frequency, displacement, velocity, and acceleration of the vibrator 2. As the detecting means 5, a laser displacement gauge, etc. may be used. By using a laser displacement gauge, non-contact measurement of the change in the resonant frequency of the vibrator 2 can be made.
(Vibration Mode of the Vibrator)
The vibration mode of the vibrator 2 of the present invention will be described.
The symbols in the formula (1) as shown above represent the following.
In addition to the transverse vibration mode used for the vibrator 2 of the present invention, the longitudinal vibration mode, which is the stretching vibration of the microbeam, and the torsional vibration mode caused by torsion of the microbeam are generated. The frequency in the transverse vibration mode used for the vibrator 2 of the present invention is set so as to be different from those in the longitudinal vibration mode and in the torsional vibration mode of the vibrator 2.
The resonant frequency (fn) of the longitudinal vibration, namely stretching vibration of the microbeam, is represented by the formula (2) as shown below.
In the above formula, λn is as follows under the condition where both ends are free: λ1=π, λ2=2π, and λ3=3π.
The resonant frequency (fn) of the torsional vibration of the microbeam is represented by the formula (3) as shown below.
In the above formula, λ1=π, λ2=2π, and λ3=3π under the condition where both ends are free, and G represents modulus of transverse elasticity (g/cm·s2). Here, G of crystal is 2.95×1011 (g/cm·s2).
(Mode Order of transverse Vibration of the Vibrator)
The mode order when the vibrator 2 of the present invention is in the transverse vibration mode will be described.
As shown in
(Resonant Frequency of the Vibrator in the Transverse Vibration Mode)
The crystal was selected as the material of the vibrator 2, and the resonant frequencies in the primary mode to the third-order mode of the transverse vibration mode were calculated using the formula (1) as shown above when the length of the microbeam was 4 mm, its width was 0.4 mm, and its height was 0.4 mm. Table 1 summarizes the calculation results of transverse vibration mode with the values of the longitudinal vibration mode and the torsional vibration mode.
As shown in Table 1, the resonant frequency of the vibrator 2 in the transverse vibration mode increases with the increase of the mode order, and its value is different from the values in the longitudinal vibration mode and the torsional vibration mode.
According to the resonant type mass sensor 1 of the present invention, by be unnecessary mechanical fixing the vibrator 2 to the oscillator 3, the inhibition on resonant phenomena due to supply of power or fastening can be eliminated along with the restrictions on the design of shape and dimensions of the vibrator 2. Consequently, change of the mass can be measured with high sensitivity. Furthermore, since the vibrator 2 is not fastened to the oscillator 3 mechanically, the vibrator 2 can be replaced quite easily.
(Method of Immobilizing the Antibody)
A method of immobilizing the antibody 16a to the vibrator 12 will be described. The antibody 16a can be immobilized to the vibrator 12 by the method comprising:
forming a thin metallic film on the surface of the vibrator 12;
forming a molecular film on the order of monomolecular layer on this metal film by sputtering, etc.; and
modifying the antibody 16a to this molecular layer.
An adhesion layer and a platinum (Pt) film formed on the adhesion layer can be used as the metallic thin film. Ti, Cr, etc. may be used as the adhesion layer. As the molecular film on the order of monomolecular layer, a self-assembled monolayer (hereinafter called as the SAM layer) can be used.
(Specific Example of the Method of Immobilizing the Antibody)
An example of the method of immobilizing the antibody 16a will be described.
Titanium (Ti) and platinum (Pt, 500 Å) are deposited by the sputtering method to form a film on one of the surfaces of the microbeam-like vibrator 12. To immobilize the antibody 16a on the surface of the platinum, the SAM film is formed and the antibody 16a is modified on the SAM film. The amine coupling process was used to couple the SAM film and the antibody 16a. In addition, to prevent unnecessary protein from attaching, thus adversely affecting the measurement, the blocking was performed by using an ethanolamine solution, and the blocking using a bovine serum albumin (BSA) solution was also performed.
The typical process of forming the SAM film on the microbeam-like vibrator 12 and modifying the antibody 16a will hereafter be described further in detail.
As shown in
In the above description, the SAM film was formed on the microbeam-like vibrator 12 and then the antibody 16a was modified on the SAM film. However, the antigen 16b may be modified on the microbeam-like vibrator 12.
According to the resonant type mass sensor 10 of the present invention, after the mass of specific molecules contained in the above measurement sample is measured, the vibrator 12, to which the antibody 16a is immobilized, can be reused. For example, the antigen 16b, namely molecules to be measured, can be removed in several minutes by immersing the used antibody-immobilized vibrator 12 in a dissociation solution of antigenic agent of a given concentration. Then, the separate excitation resonant type mass sensor 10 can be reused. The concentration of the dissociation solution of antigenic agent can be determined depending on the type of the antigen 16b to be measured.
The vibrator 22 can be constructed by including at least a magnetizable part. Magnetic beads 26, to which the antibody 16a or antigen 16b is adsorbed, may be adsorbed magnetically to this magnetizable part. The vibrator 22 made of a ferromagnetic material may be used as the vibrator. The ferromagnetic materials that can be used for the vibrator 22 include soft iron, silicon steel, ferrite, cobalt, nickel, and alnico magnet.
(Method of Fabricating the Ferromagnetic Vibrator)
(Another Method of Fabricating the Ferromagnetic Vibrator)
a) to (d) is a chart describing a method of use of the resonant type mass sensor 20 according to the third embodiment of the present invention.
First, the magnetic measurement sample solution 31 described previously is prepared to examine the antigen-antibody complex reaction (See
Then, the magnetic measurement sample solution 31 is dropped on the vibrator 22, allowing it to be adsorbed onto the surface of the vibrator 22 and dried thoroughly, to make the magnetic measurement sample to be adsorbed onto the surface of the vibrator 22 (See
The change in the resonant frequency of the measurement sample adsorbed onto the vibrator 22 (See
The magnetized measurement sample adsorbed onto the vibrator 22 can be removed from the vibrator 22 by demagnetizing the vibrator 22 (demagnetization), (See
In a biosensor using the antigen-antibody complex reaction, the antibody 16a is immobilized onto the sensing part (free end) of the vibrator 22 to collect specific molecules by the antigen-antibody complex reaction. Once the substance to be measured (antigen 16b) is immobilized onto the vibrator 22, the resonant frequency decreases in accordance with the mass (See
According to the third embodiment of the resonant type mass sensor 20, the magnetic measurement sample attaching to the vibrator 22 can be removed from the vibrator 22 by demagnetizing the magnetic property of the vibrator 22. For this reason, it is more easily to reuse the vibrator 22 (See
The present invention will hereafter be described further in detail by referring to examples.
(Fabrication of the Vibrator)
First, the vibrator 2 as shown in
A sponge 8 is inserted between the oscillator 3 made of the piezoelectric element and the XYZ table 6 (TSD-805S, Sigma Koki Co., Ltd., Japan) not to obstruct the vibration of the oscillator 3. The following description assumes that the oscillator 3 is made of the piezoelectric element.
The spot diameter of the microscope type laser Doppler vibrometer 5a is 40 μm. The positioning was performed by adjusting the position of the XYZ table 6 while observing the position using the CCD camera 7a so that the vibration velocity at the free ends of the vibrator 2 is measured by the microscope type laser Doppler vibrometer 5a.
The alternating voltage was applied to the piezoelectric element 3 by using the signal generator 4a (SG-4104, IWATSU TEST INSTRUMENTS) to vibrate the piezoelectric element 3. The microbeam-like vibrator 2 placed on the piezoelectric element 3 is resonates by receiving the vibration from the piezoelectric element 3. In other words, when the alternating voltage having a frequency close to the frequency of vibration of the microbeam-like vibrator 2 in the normal mode is applied to the piezoelectric element 3, the vibrator resonates at the frequency of vibration in the normal mode of itself. The vibration-velocity characteristics of this vibrator 2 were measured using the microscope type laser Doppler vibrometer 5a.
The output signal of the signal generator 4 and the vibration velocity measured by the microscope type laser Doppler vibrometer 5a were observed by using an oscilloscope (DS-5120B, IWATSU TEST INSTRUMENTS CORPORATION).
(Frequency Characteristics of the resonant Type Mass Sensor)
The relation between the resonant frequency and the mass of load was measured to evaluate the basic performance of the resonant type mass sensor 1. The frequency characteristics were measured by fastening a plate weight (KAGEYAMA Corp., Thickness; 0.1 mm) to the end of the vibrator 2 with adhesion as a load. Table 2 summarizes measurement conditions.
As shown in
(Measurement of Mass Using Protein)
BSA, a protein obtained by purifying bovine serum albumin, was attached to the vibrator 2, and the change of the resonant frequency at that time was measured.
The BSA was diluted with phosphate buffered saline (PBS) to prepare 0.5%, 3.0%, and 5.0% BSA solutions. For the analysis of total protein of the BSA solution, the calorimetric total protein assay kit (DC protein assay, Bio-Rad Laboratories, Inc., CA) and the microplate reader (ARVO MX, Perkin Elmer Inc.) were used.
As shown in Table 4, when the PBS solution was attached, the change of the mass was 0.03 mg and the change of the resonant frequency was 1.27 kHz.
When the 0.5% BSA solution 43 was attached, the change of the mass was 0.04 mg and the change of the resonant frequency was 1.59 kHz.
When the 3.0% BSA solution 43 was attached, the change of the mass was 0.07 mg and the change of the resonant frequency was 2.88 kHz.
When the 5.0% BSA solution 43 was attached, the change of the mass was 0.09 mg and the change of the resonant frequency was 3.42 kHz.
From the above measurement results, when the BSA solution 43 was attached, the measurement sensitivity to the mass was found to be 31.25 ng/Hz. Meanwhile, when weights of the same mass were respectively placed at both ends of the vibrator 2, the measurement sensitivity to the mass was measured to be 38.88 ng/Hz.
(Measuring the Concentration of Total Protein Contained in Saliva)
The measurement of concentrations of total protein contained in human saliva will be described below. Saliva was collected as follows.
f=13.127×TP (4)
In the above formula, TP represents total protein concentrations (g/ml).
As shown in
From the results described above, the resonant type mass sensor 1 of the present invention was found to have measurement sensitivity sufficient to measure the concentration of a trace amount of protein contained in saliva.
The present invention is not limited to the examples described above, but various variations are allowed within the scope of the claims of the present invention. Needless to say, they are all included in the scope of the present invention.
Number | Date | Country | Kind |
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2013-028481 | Feb 2013 | JP | national |
Number | Date | Country |
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3298897 | Jul 2002 | JP |
2005-533265 | Nov 2005 | JP |
4638281 | Feb 2011 | JP |
Entry |
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“Quarts Crystal Microbalance”, Nov. 12, 2013, http://www.tamadevice.co.jp/9mhz.htm. |
Number | Date | Country | |
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20140234171 A1 | Aug 2014 | US |