Biosensor for human IgE detection using shear-mode FBAR devices
© Chen et al.; licensee Springer. 2015
Received: 30 June 2014
Accepted: 6 January 2015
Published: 18 February 2015
Film bulk acoustic resonators (FBARs) have been evaluated for use as biosensors because of their high sensitivity and small size. This study fabricated a novel human IgE biosensor using shear-mode FBAR devices with c-axis 23°-tilted AlN thin films. Off-axis radio frequency (RF) magnetron sputtering method was used for deposition of c-axis 23°-tilted AlN thin films. The deposition parameters were adopted as working pressure of 5 mTorr, substrate temperature of 300°C, sputtering power of 250 W, and 50 mm distance between off-axis and on-axis. The characteristics of the AlN thin films were investigated by X-ray diffraction and scanning electron microscopy. The frequency response was measured with an HP8720 network analyzer with a CASCADE probe station. The X-ray diffraction revealed (002) preferred wurtzite structure, and the cross-sectional image showed columnar structure with 23°-tilted AlN thin films. In the biosensor, an Au/Cr layer in the FBAR backside cavity was used as the detection layer and the Au surface was modified using self-assembly monolayers (SAMs) method. Then, the antigen and antibody were coated on biosensor through their high specificity property. Finally, the shear-mode FBAR device with k t 2 of 3.18% was obtained, and the average sensitivity for human IgE detection of about 1.425 × 105 cm2/g was achieved.
In recent years, piezoelectric materials have been used in surface acoustic wave (SAW) resonators [1-5] and film bulk acoustic wave resonators (FBARs) [6-10] because of their low cost, low weight, and good reproducibility. However, the SAW resonator has high insertion loss and poor power handling capability. Hence, this study evaluated the potential applications of FBARs for biosensors because of their advantages, including low insertion loss, good power handling, and small size. The FBAR devices were constructed by a piezoelectric layer sandwiched between two electrodes and attached to substrate with backside cavity. Piezoelectric materials such as zinc oxide (ZnO) and aluminum nitride (AlN) have been used in FBAR devices for various applications [11-13] owing to their high acoustic velocity, better quality factor, and high electromechanical coupling coefficient. Besides, the piezoelectric materials of ZnO and AlN can be combined with silicon technologies in semiconductor fabrication processes [14,15]. Moreover, the acoustic velocity of AlN is 10,400 m/s, and it suits application for FBAR devices.
The acoustic wave of a FBAR has two transmittance modes: longitudinal mode and shear mode. In shear mode, acoustic wave energy does not dissipate in a liquid environment . The backside cavity of FBAR can be used as the detection area for adsorbent matter. Under a mass loading, a frequency shift would be resulted in the frequency response of a FBAR . The analysis methods were used for biosensor in liquid and tiny mass detection in air through the shear mode and longitudinal mode, respectively. Thus, FBAR devices were fabricated and constructed to evaluate their potential use in biosensors.
Fabrication of FBAR devices
The characteristics of AlN thin films, including crystalline properties, preferred orientation, and cross-sectional morphologies were examined. The crystalline properties and preferred orientation of the AlN thin films were determined by X-ray diffraction scanning between 20° and 60° using a Siemens D5000 (Munich, Germany) with CuKα radiation. The surface morphologies and cross sections of AlN thin films were observed by field-emission scanning electron microscope (FESEM, JEOL-6700; JEOL Ltd., Akishima-shi, Japan). Finally, the frequency responses of FBAR devices with the biosensors were measured by HP8720 network analyzer.
FBAR devices for biosensor applications
Step (1): Use oxygen plasma process for Au surface cleaning.
Step (2): Inject cysteine solution (R.T., 1 h).
Step (3): Inject deionized (DI) water and dry using N2 gas.
Step (4): Inject glutaraldehyde solution (2.5%, R. T., 1 h).
Step (5): Inject DI water and dry with N2 gas.
Step (1): Wash with 200 μl phosphate-buffered saline (PBS) solution three times.
Step (2): Dip 200 μl diluted mouse anti-human IgE antibody (37°C, 2 h).
Step (3): Inject 200 μl, Tween-20 wash buffer three times.
Step (4): Inject 200 μl, 10 wt.% bovine serum albumin (BSA) solution (37°C, 0.5 h).
Step (5): Inject 200 μl, Tween-20 wash buffer three times.
Step (6): Inject 200 μl, diluted human IgE antigen with 0.707 μg/ml concentration.
where δm is the loading mass (9.1875 ng/cm2) and δf is the variation of the resonate frequency. Finally, the sensitivities of FBAR devices for human IgE detection were investigated.
Results and discussion
Structural and morphological properties of AlN thin films
Frequency responses of shear-mode FBAR devices
where V L and V S are the acoustic velocity, C 33 and C 44 are an elastic constant, and ρ is density of the wurtzite AlN. In this study, the practical acoustic velocity of longitudinal mode is 1.76 times than that of the shear mode, which is still consistent with the literature [25,27]. The electromechanical coupling coefficient (k t 2) of shear mode is a numerical measurement of the conversion efficiency between electrical and acoustic energy in piezoelectric materials. The k t 2 of the shear mode of the FBAR was calculated to be about 3.18%.
Frequency responses of biosensors for human IgE detection
The Au/Cr thin films were adopted as detection layer using a dual-gun DC sputtering system, the oxygen plasma process was used to clean the surface of the Au layer in order to improve the hydrophilic properties of the contact area between the bio-drop and Au layer [28-32].
The frequency shift and sensitivity of biosensors
Frequency shift, δf (MHz)
Sensitivity, S m (cm2/g)
1.41 × 105
1.44 × 105
The results of this study demonstrate that the proposed shear-mode FBAR device is highly promising for use in human IgE detection because of its high sensitivity, small size, low-cost, and rapid reaction process than conventional quartz crystal micro-balance (QCM) [37-41].
This study fabricated shear-mode FBAR devices for biosensor applications. The AlN thin films and Pt/Ti were adopted as the piezoelectric and electrode layers, respectively, in FBAR devices. The AlN thin films were fabricated at a working pressure of 5 mTorr, substrate temperature of 300°C, sputtering power of 250 W, and off-axis of 50 mm. The resulted AlN thin films exhibited a strong c-axis orientation and 23°-tilted. The obtained shear-mode FBAR devices had a frequency response of 1.175 GHz and a k t 2 of about 3.18%. For biosensor applications, the Au/Cr thin films were deposited on backside cavity of FBAR as bio-detection layer. The SAMs method was used for surface modification of Au thin films. Human IgE was detected by using a coating process to detect antibody with antigen. The average sensitivity for the shear-mode FBAR devices for human IgE detection was about 1.425 × 105 cm2/g.
The authors gratefully acknowledge the financial support from the National Science Council, the Republic of China (NSC Grant Numbers: No. NSC 102-2221-E-366-002, and NSC102-2221-E-110-029) and from the National Sun Yat-sen University (The Aim for the Top University Project, NSYSU).
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