Anomalous resonance in a nanomechanical biosensor

Abstract

The decrease in resonant frequency (؊⌬ r) of a classical cantilever provides a sensitive measure of the mass of entities attached on its surface. This elementary phenomenon has been the basis of a new class of bio-nanomechanical devices as sensing components of integrated microsystems that can perform rapid, sensitive, and selective detection of biological and biochemical entities. Based on classical analysis, there is a widespread perception that smaller sensors are more sensitive (sensitivity Ϸ ؊0.5 r/mC, where mC is the mass of the cantilever), and this notion has motivated scaling of biosensors to nanoscale dimensions. In this work, we show that the response of a nanomechanical biosensor is far more complex than previously anticipated. 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. 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.nanocantilevers ͉ nanotechnology ͉ protein adsorption ͉ virus detection V ibrating cantilever beams have been used to detect ultrasmall masses, ranging from femtogram down to the zeptogram range (1-6). This principle of detection is simple: The first natural frequency of a unloaded cantilever is r ϭ (k C ͞m C ) 1/2 , where k C and m C are the linear spring constant and effective mass of the cantilever, respectively (see Table 1 Classical theory of resonators suggests that scaling down the area (A C ) of a cantilever would reduce m C and m A and therefore allow detection of smaller concentrations or amounts of biomolecules, whereas scaling down the thickness (t C ) decreases the resonant frequencies to be within measurable range. Specific capture of antigens requires attachment of receptor molecules such as antibodies (Abs) (4, 7) whose thickness (t A ) may be comparable with nanoscale cantilevers (t A ϳ t C ). As such, the mechanical properties of nano-cantilevers, coated with receptor molecules, can be significantly different from their bulk counterparts. Earlier works have described the alteration of the spring constant, k C , of microscalethick cantilever beams due to changes in stress on one side of the cantilever (8, 9), but the effect of the mass of the adsorbed layer was negligible in those experiments because the mass of cantilever, m C , was much greater than mass of the adsorbed layer, m A . The influence of the mass and the spatial distribution of proteins on nanoscale sensor have never been studied explicitly and remains a poorly understood phenomenon (10). In this work, we use detailed vibration measurements and fluorescence microscopy to establish that the protein attachment increases wit...

Flexure-FET biosensor to break the fundamental sensitivity limits of nanobiosensors using nonlinear electromechanical coupling

“…In general, however, capture of target molecules increases stiffness of the membrane. If this increase in stiffness compensates the corresponding increase in the mass, there might be no change in resonance frequency at all (12,28), and the sensitivity could be vanishingly small. One must independently measure the change in the stiffness (29,30) to decouple the mass effect from stiffness effect so that the mass of the adsorbed molecule can be correctly estimated.…”

“…1C) do not require biomolecules to be charged for detection. Here, the capture of target molecules on the cantilever surface modulates its mass, stiffness, and/or surface stress (5,11,12). This change in the mechanical properties of the cantilever can then be observed as a change in its resonance frequency (dynamic mode), mechanical deflection, or change in the resistance of a piezoresistive material (static mode) attached to the cantilever (6, 13).…”

“…For transduction, the proposed Flexure-FET biosensor utilizes the change in suspended gate stiffness from k to k þ Δk, (12,24,(27)(28)(29) due to the capture of biomolecules. The change in stiffness due to the capture of biomolecules has been demonstrated by several recent experiments of mass sensing using nanocantilever-based resonators (12,27,28) (Fig. S2).…”

Experimental study of aerodynamic damping in arrays of vibrating cantilevers

“…Micro-and nano-scale applications of cantilevers include biosensors (Gupta et al 2006 andIlic et al 2001), atomic force microscope cantilevers (Binnig 1986), and rheological measurement devices (Boskovic et al 2002). At larger length scales, applications such as piezoelectric fans for electronics cooling (Açıkalın et al 2004) and Kimber et al 2007) and flapping wings for propulsion (Shyy et al 1999) involve vibrating cantilevers.…”

Advances in the application of nanotechnology in the diagnosis and treatment of gastrointestinal tumors

“…Cantilevers are able to conduct biosensing through the principle that these tiny probes naturally vibrate at a certain frequency dictated by mechanical and mass properties. When a biological molecule binds to this nanoscale probe, it alters the baseline probe frequency, which is typically measured by a difference in the characteristics of the light deflection pattern of the probe or through electrical means (14). Cantilever vibrations are mainly deflected in atomic force microscopy (AFM) force feedback mode to obtain the real-time imaging (15).…”