Research:

Cantilever Based Lab on a Chip for Detection of Biological Entities

J. Jang, A. Gupta, D. Akin, S. Broyles *, M. Ladisch^, R. Bashir,

School of Electrical and Computer Engineering, Weldon School of Biomedical Engineering, *Department of Biochemistry, ^ Department of Agriculture and Biological Engineering, Purdue University.

In this NIH funded project, we are developing integrated devices with cantilever sensors for detection of viruses. The virus particles we used in the study were vaccinia virus, which is a member of the Poxviridae family and forms the basis of the smallpox vaccine. We have demonstrated the detection of a single vaccinia virus particle with an average mass of 9.5 fg (Gupta, et al. 2005). The frequency spectra of the cantilever beams, due to thermal and ambient noise, were measured using a laser Doppler vibrometer under ambient conditions. The change in resonant frequency as a function of the virus particle mass binding on the cantilever beam surface forms the basis of the detection scheme. We have also recently demonstrated the use of ultra thin cantilevers (~20nm thickness) and the behaviour of protein adsorption on these cantilevers. We have shown that the response of a nanomechanical biosensor is far more complex than previously anticipated.

Figure 2: (a) vaccinia virus on nano-scale thickness cantilevers, (b) frequency shift downwards as expected after the antibody attachment and virus capture, (c) Frequency shift upwards upon antibody attachment and then downwards upon virus capture.

Figure 4: (a) Schematic describing use of cantilevers as DEP electrodes, (b) Top view image of cantilever array with fluorescenctly labeled particles, with DEP on, the particles are capture on cantilevers, whereas with DEP off, the particles are not captured.

Indeed, in contrast to classical microscale sensors, the resonant frequencies of the nanosensor may actually decrease or increase after attachment of protein molecules. We demonstrate theoretically and experimentally that the direction of the frequency change arises from a size-specific modification of diffusion and attachment kinetics of biomolecules on the cantilevers (Gupta, et al. 2006). This work may have broad impact on microscale and nanoscale biosensor design, especially when predicting the characteristics of bio-nanoelectromechanical sensors functionalized with biological capture molecules.

We are also developing integrated devices where the cantilevers are used as DEP concentration electrodes and as mass sensors, since even though cantilevers can be designed for single particle sensitivity, the key challenge remains to get the particles to the sensor sites. As shown in Figure 4, we can use positive DEP to attact the particles to the sensor site, i.e. the cantilever edges where the field gradient is the maximum. Such antibody coated, integrated DEP/cantilever devices can be used for rapid detection of biological entities in fluids.
We have also used the cantilevers to begin integration of these structures with mammalian cells for cell biology and biomimitic applications. Figure 5 shows 8E5LAV lymphocyte cells attached on cantilevers and the measurement of the mass of these cells in fluid. We have initiated new efforts in developing microsystems for cell biology. A specific effort on-going is the interface of microcantilever sensors for characterization of physical properties of cells such as its mass and study of single cell growth and differentiation in the cardiac myocyte cells.