Elsevier

Ultramicroscopy

Volume 97, Issues 1–4, October–November 2003, Pages 417-424
Ultramicroscopy

Enhancing chemi-mechanical transduction in microcantilever chemical sensing by surface modification

https://doi.org/10.1016/S0304-3991(03)00069-XGet rights and content

Abstract

The use of chemically selective thin-film coatings has been shown to enhance both the chemical selectivity and sensitivity of microcantilever (MC) chemical sensors. As an analyte absorbs into the coating, the coating can swell or contract causing an in-plane stress at the associated MC surface. However, much of the stress upon absorption of an analyte may be lost through slippage of the chemical coatings on the MC surface, or through relaxation of the coating in a manner that minimizes stress to the cantilever. Structural modification of MC chemical sensors can improve the stress transduction between the chemical coating and the MC. Surfaces of silicon MC were modified with focused ion beam milling. Sub-micron channels were milled across the width of the MC. Responses of the nanostructured, coated MCs to 2,3-dihydroxynaphthalene and a series of volatile organic compounds (VOCs) were compared to smooth, coated MCs. The analytical figures of merit for the nanostructured, coated MCs in the sensing of VOCs were found to be better than the unstructured MCs. A comparison is made with a previously reported method of creating disordered nanostructured MC surfaces.

Introduction

The use of microcantilevers (MCs) as transducers in physical, chemical, and biological sensing systems has increased in recent years [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. The increase in their use stems from certain inherent advantages over other transducers such as surface acoustic wave (SAW) and quartz crystal microbalance (QCM) devices. For example, MCs are well suited for miniaturization and can be used as elements in sensor arrays [16], [17], [18]. These arrays provide a higher degree of selectivity than can be achieved using a single MC due to the use of multiple chemical layers. Another unique feature of ultra-thin MCs is that they possess an extremely high surface-to-volume ratio. Thus, changes in the conditions on or near the surface resulting from the interaction with chemical species present in the environment of the MC can significantly modulate surface stress. This can result in bending and resonance frequency shifts of the MC if opposite sides are modulated by different degrees. When a uniform surface stress is applied to an isotropic material it either increases (compressive stress) or decreases (tensile stress) the surface area. If this stress is not compensated by an equal stress on the opposite side of the MC, it will cause a bending of the entire structure with a constant radius of curvature. The deflection of the tip of the MC, zmax, resulting from differential stress on its opposite sides, Δσ, is described by the Stoney equation (Eq. (1)) [19]zmax=3l2(1−υ)Et2Δσ,where υ and E are, respectively, Poisson's ratio and Young's modulus for the MC, and l and t are its length and thickness, respectively. When MCs are used as chemical sensors, this differential stress can be generally assured by using asymmetric MCs where one side of the MC interacts preferentially with the target analyte(s) through absorption into a thin chemical film on that surface. The untreated, opposite side is essentially passive with respect to the chemically treated side creating a situation that produces a differential stress.

Recent work from several research groups [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [16], [17], [18] confirms that sensors based on MCs have substantial potential for various analytical applications. In order to fully realize this potential, however, further optimization of MC designs and chemical coating selections may be required. A clean smooth solid surface generally exhibits a tensile (positive) surface stress due to the electronic arrangement of the atoms composing the surface. Significant changes in stress on a surface can occur when surface atoms are caused to rearrange due to adsorption by a chemical species [20]. The change in stress can be either compressive or tensile depending upon the nature of the adsorbed species. The surface stress and surface free energy are related by the Shuttleworth equation (Eq. (2))σ=γ+dγdεe,where σ is the surface stress, γ is the surface free energy per unit area of the strained surface, and εe is the elastic surface strain that is defined as dA/A where A is the surface area and dA is the elastic increase in surface area [20], [21], [22]. In principle, the second term can be comparable to the surface free energy and assume a positive or negative value [22]. However, a general trend is that if the initial surface Gibbs free energy is large, then modulation in surface stress and, hence, MC response can be large. For example, pure gold surfaces in contact with air have large surface free energies, typically exceeding 1 N m−1. Not surprisingly, when MCs coated on one side with gold are exposed to alkylthiols in the gas phase very large total responses are observed as the thiol compounds covalently bond to the gold [23], [24].

In order to impart selectivity to MCs used in analytical sensing, chemically selective receptor phases need to be immobilized on one of the sides of the MC. Ideally, the interaction of the analyte with the receptor phase, while being selective, is reversible and exhibits reasonable kinetics for sensing applications. The use of MCs with reversible receptor phases for measurements in liquids (e.g., aqueous solutions) has not received a great deal of attention. In part, this is because organic receptor phases in water possess surface free energies that are substantially smaller than the gold–gas phase case mentioned above. Therefore, modulation of surface stress is smaller and often within an order of magnitude of the inherent noise of MCs mounted in aqueous environments [1]. This gives rise to low signal-to-noise levels and somewhat limited dynamic range.

Recently, we reported on the use of nanostructured MCs in aqueous environments to improve the transduction of molecular recognition events into cantilever responses [25]. The limitations of smooth surface MCs were overcome by creating a highly disordered nanostructured surface on one side of the MC and further modifying the surface with a self-assembled monolayer (SAM). The nanostructuring process increased the available surface for SAM phases and analyte binding and creates a quasi-3D structure that is colloidal in nature. Importantly, the short-range forces associated with intermolecular interactions in the tight interstitial spaces of colloidal systems can be very large [26]. The in-plane component of these forces can serve to efficiently convert the chemical energy associated with analyte–receptor binding into mechanical energy manifested as MC static bending. The same disordered nanostructured MC was used to stabilize thin films of a vapor-deposited chemically selective phase. It was shown that limits of detection for both modes of operation, SAMs and thin films could be improved by orders of magnitude over MCs with smooth surfaces [25].

Herein, we report on another approach to improve sensitivity of MCs in liquid-phase measurements using patterned modification of the MC surface. The surfaces in our present work have larger features than the nanostructured surfaces reported previously and are created by controllably milling the silicon surface using a focused ion beam (FIB). Nanoscale channels are milled across the width of MCs and then filled with an organic receptor phase. As analytes absorb into the receptor phase the stress created in the phase can be more efficiently transferred to the surface creating a greater differential stress and thus more bending.

Section snippets

Experimental

The silicon MCs that were used in this study are commercially available, approximately 1.5 μm thick, beam shaped and coated with a layer of aluminum (MikroMasch, Tallinn, Estonia). Each chip carried two sizes of MCs, 400 nm long by 100 nm wide, and 200 nm long by 50 nm wide, the latter size was used for these studies. The aluminum layer was removed from the MCs by immersing them in aqua regia (75% HCl, 25% HNO3) for 5 min. The MCs were then rinsed in deionized (DI) water for 10 min, and dried under a

Results and discussion

When an MC is modified with an analyte permeable coating that is much thicker than a monolayer, a dominant mechanism of MC deflection is analyte-induced swelling of the coating. Such swelling processes may be quantified by evaluating the molecular forces acting within the coating and between the coating and the analyte species. The absorption of analytes into a coating can alter dispersion, electrostatic, steric, osmotic, and solvation forces acting within the coating [26]. The absorption of

Conclusions

Surface modification of MCs has shown the potential to increase sensitivity of these devices as chemical sensors. This increase in sensitivity is produced by improvement of the transduction of stress created in a chemical receptor phase, by analyte adsorption or absorption, to surface stress on the MC. With an asymmetric MC this leads to a larger differential stress, which results in larger deflections of the MC tip. It should be noted that previous MC work with disordered nanostructured

Acknowledgements

This research was supported by the US Department of Energy, Environmental Management Program under Grant DOE DE-FG07-98ER62718 and by DOE Basic Energy Sciences under Grant DE-FG02-96ER14609. Oak Ridge National Laboratory is operated for the US Department of Energy by UT-Battelle under Contract DE-AC05-96OR22464.

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