Nanomechanical protein detectors based on the mechanical property measured by nano-gap actuators

https://doi.org/10.1016/j.cap.2006.01.016Get rights and content

Abstract

This paper presents a new method and an associated device for detecting the presence and size of protein based on a protein-induced stiffness change. Compared to the conventional mechanical method, the present nanomechanical method shows higher precision. The present method also offers simple and inexpensive protein detection by removing labeling process and optical components. We design and fabricate the nanomechanical protein detector using an electrothermal actuator and a nano-gap. In experimental study, we measure two stiffness changing points and their coordinate shift between two cases (without/with target proteins). The fabricated device detects the protein presence and size of 14.0 ± 7.4 nm from this coordinate shift of stiffness changing points. Therefore, we experimentally verify the high-precision protein sensing capability of the nanomechanical protein detector.

Introduction

There have been active researches on high-precision protein detectors for diagnosis and prognosis [1]. The previous protein detectors are based on mechanical (resonant) [2], [3], electrochemical [4] and optical [5], [6] detection methods. Compared to the mechanical (resonant) method [2], [3] based on the mass change, the present nanomechanical method based on the protein-induced stiffness change shows higher precision. In the case of a protein molecule (mass of 150 kDa, stiffness of 5 × 10−3 N/m [7]) and a silicon beam (mass of 3.5 × 10−12 kg, stiffness of 6.4 N/m, dimension of 100 μm × 3 μm × 5 μm), the stiffness ratio of the protein and the beam shows the much larger value of 1 × 10−4 than the mass ratio of 1 × 10−11, thus resulting in maximum 107 order of improvement in precision. Compared to the electrochemical and optical methods [4], [5], [6], the present method offers simple and inexpensive detection by removing labeling process and optical components.

Fig. 1 illustrates a simple model of the nanomechanical protein detector, composed of an actuator, a beam spring and the nano-gap electrodes, on which receptors for target proteins are immobilized. The actuator, m1, supplies the motion, x1, to the push bar, m2, through the beam spring, k1. Fig. 2, Fig. 3 illustrate the working principle of the nanomechanical protein detector. The push bar, m2, in Fig. 2 is actuated to x2 direction and touches receptors or target proteins, which act as added springs (krp in Fig. 1). These added springs changes the slopes of the relationship between the input actuation, x1, and a nano-gap reduction, x2, as shown in Fig. 3. We can detect the presence of the target proteins from the coordinate shift of stiffness changing point, P in Fig. 3, because the target proteins reduce the free distance in Fig. 2. We can also detect the size of target protein from this coordinate shift.

Section snippets

Design and theoretical analysis

We design the nanomechanical protein detector as shown in Fig. 4 and Table 1. We use an electrothermal actuator and a nano-gap for the actuation and motion detection in buffer solution. As shown in Fig. 5, the nano-gap, gn, is formed from a fabricated initial gap, gi. We obtain the nano-scale distance between the push bar, m2, and the end plate after attaching the end plate to stoppers.

Fabrication process

This chapter presents the fabrication process, composed of the two-mask microfabrication process and the biotinylation process. The microfabrication process is shown in Fig. 6, representing the cross-section along A–B–C in Fig. 5a. As shown in Fig. 6b, the prototypes are defined by the deep RIE etching of the top silicon layer (5 μm thickness) of the SOI wafer (Fig. 6a). We smooth the nano-gap surface using oxidation (Fig. 6c) and oxide-etching (Fig. 6d). We form the electrical pads and the

Experimental results

We detect and compare an identical device in two cases (without/with target proteins). First, we capture the fluorescence microscope image of the fabricated device in order to check the absence of target proteins. We detect the push bar motion, x2, with actuation, x1, of the electrothermal actuator. The fabricated device is immersed in a 0.5 μm solution of FITC tagged streptavidin for 2 h and washed in 10 mM PBS (phosphate-buffered saline), pH 7.4. After the rinse of the fabricated device, we

Conclusions

This paper presented a new method and device for protein presence and size detection based on the coordinate shift of the stiffness changing points due to proteins. Compared to the conventional resonant method, the present nanomechanical method shows higher precision. The present method also offers simple and inexpensive protein detection by removing labeling process and optical components. We designed and fabricated the nanomechanical protein detector using the electrothermal actuator and the

Acknowledgement

This work has been supported by the National Creative Research Initiative Program of the Ministry of Science and Technology (MOST) under the project title of “Realization of Bio-Inspired Digital Nanoactuators.”

References (8)

  • J. Homola et al.

    Surface plasmon resonance sensors: review

    Sensor. Actuator B

    (1999)
  • J. Rickert et al.

    Sensors for Biomolecular Studies

  • J. Zhang and E.S. Kim, Vapor and Liquid Mass Sensing by Micromachined Acoustic Resonator, in: Proc. 16th IEEE Inter....
  • J. Rickert et al.

    QCM operation in liquids: constant sensitivity during formation of extended protein multilayers by affinity

    Anal. Chem.

    (1997)
There are more references available in the full text version of this article.

Cited by (0)

View full text
Copyright © 2006 Elsevier B.V. All rights reserved.