- Email: [email protected]
Microcantilever Sensors
Microcantilever Sensors In the past 15 years, fueled by advances in micromachining techniques, there has been a considerable amount of work devoted to developing chemical and biological sensors based on microcantilever platform. A natural extension has been the microfabrication of cantilever arrays for multiplexed detection of chemical and biological analytes (Boisen and Thundat 2009). Microcantilevers are normally approximately 100–200 mm long, 20–40 mm wide, and 0.5–1 mm thick and are fabricated from silicon or silicon nitride. For comparison, the diameter of a human hair is approximately 80 mm. An electron micrograph of a cantilever array chip showing four piezoresistive cantilevers in a microfluidic well is shown in Fig. 1. The interest in microcantilever-based sensing drives primarily from the need to develop highly sensitive miniature sensors using small volumes of samples as well as the realization that multiple analytes in a complex medium need to be simultaneously detected for increasing the chemical specificity. Detection of multiple target molecules in a small volume of sample has immediate relevance in many areas ranging from early detection of diseases to environmental monitoring to security. Often multiple signals are needed to positively identify the target molecules. At present, chemical and biomedical sensing methods rely on analyses performed by a variety of instruments that utilize a number of different sensing technologies. Most of these technologies are expensive and suffer from intrinsic limitations in volume, weight, and power that impede their practical implementation as cost-effective, widespread sensors. Currently used technologies, therefore, are not well suited for either integration into a single device or adaptation as a basis for the development of a universal platform for multiplexed detection of chemical and biological species. Microfabricated cantilever sensors can measure the extremely small forces caused by molecular adsorption as well as very small
Figure 1 Electron micrograph of a cantilever array chip showing four piezoresistive cantilevers in a microfluidic well (Cantion, Inc.).
mass variations due to adsorbed mass and offer an exciting and unparalleled opportunity for the development of highly sensitive chemical and biological sensors (Datar et al. 2009). Because of recent advances in microlithographic technologies and microfabrication techniques, these sensors can be mass-manufactured on silicon wafers and other materials in a cost-effective and modular fashion. Microcantilevers are, however, not chemical or biological sensors. They are physical sensors that can detect displacement with subnanometer resolution (Finot et al. 2008). The deformation of a cantilever can be either static or dynamic. The static displacement of the cantilever can be used for measuring extremely small forces with unprecedented sensitivity, whereas using the dynamic response, the resonance frequency can be used for measuring extremely small masses (Naik et al. 2009). Both these signals, therefore, can be used for physical, chemical, and biological sensing. In a typical static measurement, molecular adsorption can result in cantilever bending, which is monitored (see Fig. 2). Similarly, in the dynamic mode, from an accurate measurement of the cantilever resonance frequency, it is possible to measure changes in mass due to molecular adsorption on cantilever surface. The microcantilever sensors are capable of measuring the extremely small molecular forces caused by the adsorption events and although sensitive to other stimuli such as temperature and pressure, they offer an exciting and unparalleled opportunity for the development of highly sensitive chemical and biological sensors based on the specific channel of adsorption-induced forces.
Figure 2 A cartoon showing cantilever bending caused by adsorption of molecules on one of its surfaces due to changes in surface free energy that is related to surface stress. Adsorption of molecules on both sides of the cantilever results in no deflection because the stress variations on both sides are canceled. 1
Microcantilever Sensors 1.
Modes of Cantilever Operation
A properly designed cantilever sensor, being essentially a physical sensor, can measure extremely small displacements, generally in the subnanometer range. By using optimal design, it is possible to measure both bending and resonance frequency simultaneously. Alternately, it is possible to design two different cantilevers on the same chip where one can be used for mass detection, whereas the other one can be used for detection of adsorption-induced bending. We now consider the two operation modes of the sensor: the dynamic mode and the static mode.
1.1
Dynamic Mode
The dynamics of microcantilevers can be studied within the framework of continuum mechanics. The mathematical description of the dynamics of a cantilever-shaped material may be written as a partial differential equation, which predicts an infinite number of eigenmodes for the resonances of the oscillator. In practice, only the few first modes can be measurably excited and used in measurements. The resonance frequencies of the sensor depend on the material and the geometry of the cantilever. For microcantilevers made of silicon, in various geometries such as triangular, rectangular, and V-shaped, the first few resonance frequencies appear in a spectrum from a few kilohertz up to several tens of megahertz. When modeling the cantilever dynamics, appropriate account of dissipative processes must be taken so that an accurate relationship can be established between the resonance frequency and the adsorbed mass on the sensor. Therefore, the interaction of the cantilever with its environment is of paramount importance in the quantitative determination of the adsorbed mass from frequency shift measurements. This is of particular importance when the sensor is operated in a fluid where the oscillations are severely damped, making precise frequency shift measurements difficult. The dynamic motion of the cantilever, either driven by an external oscillator, for example, a piezoelectric crystal, or driven by the ambient temperature fluctuations (Brownian motion), can be used for detecting mass variation due to molecular adsorption. The mass resolution of a cantilever sensor increases with its resonance frequency. A cantilever’s resonance frequency varies as a function of adsorbed mass. Cantilevers with higher resonance frequency have higher mass resolution. In the simplest model of a cantilever, that is, when modeled as a point mass in harmonic motion, the resonance frequency, f, of a cantilever can be shown to vary as a function of mass 2
loading (Dm), according to 1 f ¼ 2p
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k m þ aDm
ð1Þ
where k is the force constant of the cantilever, m is the effective mass of the cantilever, and a is a numerical constant. Under equilibrium in a medium of a given viscosity, a Brownian microcantilever oscillator with an effective mass m satisfies the Langevin equation. Using the fluctuation–dissipation theorem, when modeled as a point mass, one can show that the averaged square of the position of the cantilever equals bQ2, where b ¼ 2m2kBTDo/pg3 and the quality factor Q ¼ mo/g with o and g being the resonance frequency and the damping of the oscillations, respectively, kB is the Boltzmann constant, and Do is the measurement bandwidth. When the cantilever is modeled as an extended elastic body, the averaged square of the position of a point on the cantilever equals bQn2 for the nth resonance mode of the cantilever, where Qn is proportional to the eigenfrequency on. Mass detection using variations in the cantilever resonance frequencies is well suited for measurements in vacuum or in air. It is possible to increase the adsorbed mass by increasing the surface area of a cantilever by nanopatterning. Cantilevers with small dimensions and, therefore, higher resonance frequencies can produce mass resolution in the femtogram and attogram ranges. The mass resolution, however, is very poor when operated in solution due to damping. The resonance frequency variations, therefore, are generally not used for highly sensitive detection of adsorbed mass in liquid environments. Recently, this challenge has been overcome by using a hollow cantilever concept called suspended microchannel resonator (SMR), where the fluid is confined inside the cantilever vibrating in vacuum (Burg et al. 2007). This clever design avoids liquid damping and increases the sensitivity of the cantilever mass sensor. Similarly, techniques originating from control theory, such as self-excited feedback loop, may be invoked to improve the performance of the cantilever in liquid. The resonance response (frequency) of a cantilever is rich in information, and many other physical variables can be deduced from the dynamic response (Craighead 2007). In addition to the analysis of the resonance frequencies, the phase and the amplitude of the dynamic response can be used. The spectral changes can also be used for measuring changes in viscosity and density of the medium. 1.2
Static Mode
As mentioned earlier, cantilever beams undergo bending due to surface stresses created by molecular adsorption when adsorption is confined to a single
Microcantilever Sensors side of the cantilever. The bending of the beam is linearly related to surface coverage of the adsorbed molecules. Adsorption of molecules on a solid surface is primarily driven by a need to minimize energy; the surface free energy of a solid decreases during molecular adsorption. If the adsorption of molecules on the surface of a thin material is restricted mostly to one side, for example, by making the opposite surface inert, a differential surface stress is generated between the two surfaces. This differential surface stress causes the material to deform. This effect can be easily observed with a microcantilever beam. Surface stress, s, and surface free energy, g, can be related using the Shuttleworth equation: s¼gþ
dg de
ð2Þ
where the surface strain de is defined as the ratio of the change in surface area to the total area, de ¼ dA/ A. For liquids, the second term is zero. When the differential surface stress, created by molecular adsorption, reaches a threshold, the cantilever bending becomes measurable (discernible from noise). Stoney’s equation relates the difference in surface stress, Ds, between the chemically modified surface and the untreated surface with the cantilever deflection, Dh: Dh ¼
3ð1 nÞL2 ðDs1 Ds2 Þ Et2
ð3Þ
where n is the Poisson ratio of the material, E is Young’s (elastic) modulus of the cantilever material, and t and L are the thickness and the length of the cantilever, respectively. By optimizing the length and thickness, it is possible to make a cantilever very sensitive to small changes in surface free energy. Unlike a clean surface in vacuum conditions, a surface exposed to ambient chemicals has adsorbed molecules occupying adsorption sites. When exposed to target molecules, the target molecules either adsorb on to the molecules that are already adsorbed on the surface or replace the molecules that are already adsorbed on the surface. Therefore, the surface energy change determined from experiment may not readily match the theoretical values. The cantilever bending can also be caused by other effects, for example, thermal expansion, bimaterial effect, swelling of cantilever constituent layers such as polymer layers in response to changes in the temperature or the ambient chemicals. Interaction of light with a cantilever or the interaction of (both reactive and inert) gas molecules with the surfaces of the cantilever under certain conditions can also result in a measurable response. In the case of the light–cantilever interaction, a number of processes including stress-optical, photon pressure, optical absorption, and thermo-optic can all cause cantilever displacement. In the presence of temperature
gradients (inert, reactive, or both), rest gas molecules may interact with the surfaces of the cantilever to cause spurious or even intentional large net forces, the so-called radiometric (or Knudsen) forces. These effects can be used for designing cantilevers to provide additional signals. It is possible to eliminate these effects by using reference cantilevers for obtaining differential signals.
2.
Electronic Monitoring of Cantilever Deflection
Current microfabrication techniques allow incorporation of electronic detection of cantilever motion, such as variation in piezoresistivity or incorporation of embedded MOSFET (see Metal Oxide–Semiconductor Field Effect Transistors). In the piezoresistive technique, the resistance of an asymmetrically doped cantilever varies sensitively as a function of bending. In the embedded MOSFET readout method, a field-effect transistor is embedded at the base of the cantilever. The stress from the bending of the cantilever changes the carrier mobility and drain current. Both these techniques are compatible with microfabrication and miniaturization. The piezoresistive method has been well investigated. Piezoresistivity (see Stress Coupled Phenomena: Piezoelectric Effect) is where the resistance of certain conductive material, such as doped silicon, varies as a function of applied strain. In piezoresistive cantilever deflection monitoring technique, the cantilever is fabricated with an integrated resistor with piezoresistive properties. Because of this piezoresistive property, the resistance of the cantilever changes as a function of cantilever bending. Thus, the extent of cantilever bending can be sensitively measured by a simple electrical measurement of the resistance variation of the cantilever. In general, cantilever resistance variation is measured with respect to a reference cantilever fabricated adjacent to the sensor cantilever. The reference cantilever is inert and is used to eliminate noise, such as temperature changes, in the system. A piezoresistive cantilever is defined in microcrystalline or single crystal silicon by boron doping, followed by etching to release the freestanding cantilever. The thickness of the deposited silicon nitride on the cantilever on either side of the cantilever is adjusted in such a way that the neutral axis of the cantilever lies inside the silicon nitride layer rather than in the silicon layer. This asymmetry in material composition reduces the electronic noise as well as the drift in the cantilever deflection. In addition, the silicon nitride serves as an efficient electrical insulation of the resistor and ensures that the device can be operated in liquids.
3.
Chemical and Biological Selectivity
Although cantilever sensors are extremely sensitive, they are physical sensors and do not offer any 3
Microcantilever Sensors −0.00
Stress (V)
DNA −0.02
−0.04 Buffer −0.06 0
1000
2000
Time (s)
Figure 3 Cantilever bending as a function of time in flow condition. Hybridization of DNA causes the cantilever bending (measured as voltage due to change in resistance of the piezoresistive cantilever). Changing the flow to buffer alone (at 2000 s) removes the physisorbed DNA.
intrinsic chemical or biological selectivity. Chemical and biological selectivity are achieved by immobilizing biological receptors or chemical interfaces on cantilever surfaces (Raiteri et al. 1999). Chemically selective interfaces can be selective polymers, selfassembled monolayers, or metal or oxide coatings. Binding of target molecules to the selective layer immobilized on one side of the cantilever results in free energy change causing the cantilever to bend. The cantilever bending has been shown to be proportional to the analyte concentration (see Fig. 3). The selective layer can bind the target irreversibly or reversibly depending on the analyte–receptor/chemical interface interaction. For example, cations, such as Cs, have been successfully detected using special calixarenes. However, these bindings are irreversible under normal conditions. Selective and sensitive detection of many biomolecules have been demonstrated using cantilever arrays immobilized with biological receptors (McKendry et al. 2002). Examples of biological interfaces include immobilized antibodies, peptides, DNA, or enzymes. The selective interfaces are immobilized on the cantilever surface using linker groups that can tether the receptors strongly to cantilever surfaces. For example, thiol group has been extensively used for immobilizing bioreceptors on cantilever surfaces. Microcantilevers have been used to demonstrate the detection of a number of chemical species, such as chemical warfare agents, volatile 4
organic compounds, and toxic industrial compounds using immobilized selective layers. Selective and sensitive detection of a number of biomolecules, organisms, and biochemicals have been demonstrated using cantilever platform. Examples of successful demonstration include markers for cancer and cardiac diseases, DNA markers, biotoxins, biowarfare pathogens, glucose, and Ca ions. Although the approach of biomolecular detection using selective receptors for biological analytes has been very selective, the same cannot be said about small molecule detection, especially in vapor phase. Although there are reports of detection of chemical species, such as chemical warfare agents, explosives, volatile organic compounds, and toxic industrial compounds, they seem to suffer from interference problems. The selectivity challenge encountered with small molecules is mainly due to lack of specificity of the chemically selective layers. Often room temperature reversible interactions, such as hydrogen bonds, are utilized in designing receptors for small molecules. Although room temperature reversible chemistry is highly attractive, it is very unspecific. It is possible to improve the selectivity using an array of cantilever sensors where each cantilever element is modified by a different selective layer. The array response to the same analyte can improve the selectivity when pattern recognition algorithms are used. However, the system will still have false positives because the signals are not orthogonal.
3.1
Selectivity – Based on Physical Principles
The cantilever platform, however, is so versatile that it offers many novel ideas for achieving chemical selectivity to be incorporated into the sensor platform. For example, a bimaterial cantilever is extremely sensitive to temperature. It is possible to exploit this sensitivity for molecular detection. A bimaterial cantilever with adsorbed molecules can deflect differently depending on the optical absorption characteristics of the adsorbed molecules. This technique is called photothermal spectroscopy (PDS) and allows the detection of monolayers of molecules with high selectivity without loosing sensitivity (Krause et al. 2008). In PDS, a bimaterial cantilever with adsorbed molecules is exposed to mid infrared (IR) light from a monochromator. The cantilever bending as a function of illuminating wavelength resembles IR spectrum of the adsorbed molecules. The PDS is based on the extremely high thermal sensitivity of a bimaterial cantilever. The PDS, therefore, combines the extreme high sensitivity of a cantilever beam and the selectivity of optical spectroscopy. Because the cantilever also allows the measurement of adsorbed mass using resonance frequency, the mass of the adsorbed molecules can be detected simultaneously.
Microcantilever Sensors The PDS has been demonstrated for detection of picograms of adsorbed molecules with very high selectivity. Another example of physical sensing would be temperature programmed heating of the cantilever (Senesac et al. 2009). One of the most underexploited properties of cantilever sensors is their extreme low thermal mass that allows achieving controllable temperature–time gradients, dT/dt, where T is the temperature and t is the time. It is possible to achieve temperature–time gradients up to 108 1C s1 using microcantilevers. The tunable temperature gradient offers an exciting opportunity for obtaining thermodynamic properties of adsorbed molecules. It can also be used for temperature programmed desorption (TPD), capable of achieving a higher dT/dt of 104 1C s1, which opens up the possibility of investigating the thermal behavior of adsorbates including thermally induced decomposition of subnanogram quantities of materials in milliseconds. The measured signal as a function of heating time (or temperature) is the rate of change of thermal mass, which is related rate of change of adsorbed mass with temperature (dM/dT). Because the observed signal is with respect to a reference cantilever, the mechanical buckling of the bridge under thermal stress does not play a role. Melting, sublimation, evaporation, and deflagration (for energetic molecules) can be detected using thermal techniques. Cantilever platform also offers ideal methods for incorporating electrochemical detection of electroactive species. In this case, the cantilever with a metal coating acts as a microelectrode in a three-electrode electrochemical cell arrangement. Electrochemical reactions happening on the cantilever surface result in cantilever bending. Selectivity of the ion detection comes from the electrode potential at which electrode reactions occur. Many ions have been successfully detected using microcantilever electrochemical sensors. However, this method works only for electroactive species.
4.
Conclusions
Microcantilevers are universal platforms for measuring a multitude of physical, chemical, and even biochemical factors, depending on the type of coating. Microcantilever array-based detection offers increased reliability by several sensors operating in parallel. Such increased reliability is especially important for practical applications. Microcantilever arrays fabricated in groups can be used for simultaneous detection of multiple chemical or biological analytes. Potential applications are widespread within the consumer, military, industrial, and clinical markets. This will be especially true when mixtures of multiple target molecules can be analyzed and screened on a single, miniature chip. Telemetry will
enable the use of mobile units worn or carried by personnel and the deployment of fieldable devices to relay pertinent data to central collection stations and may even replace wired sensors in some applications. However, a number of challenges remain to be overcome before microcantilever sensors can find widespread applications. The technology for designing and fabricating cantilever arrays with electronic readout is well advanced. Integration of cantilever arrays and microfluidic channel networks is still under development. Receptor immobilization on individual cantilevers in an array in a reproducible fashion from chip to chip still remains as a challenge. Novel robust receptor immobilization techniques that can work under solution in a reproducible manner need to be developed. Although detection of many different chemical and biological analytes has been demonstrated in the laboratory conditions, multianalyte detection from a raw sample still remains as a challenge. Therefore, integration of some rudimentary on-chip sample processing is required before detection by cantilever arrays. Such on-chip processing can be incorporated without increasing the chip size or power requirements using available techniques. See also: Magnetic Sensors: Principles and Applications; Multifrequency Atomic Force Microscopy; Oxide Gas Sensors; Solid-state Gas Sensors: Design and Fabrication
Bibliography Boisen A, Thundat T 2009 Design and fabrication of cantilever array sensors. Mater. Today 12, 32–8 Burg T P, Godin M, Knudsen S M, Shen W, Carlson G, Foster J S, Babcock K, Manalis S R 2007 Weighing of biomolecules, single cells and single nanoparticles in fluid. Nature 446, 1066–9 Craighead H 2007 Nanomechanical systems: measuring more than mass. Nat. Nanotechnol. 2, 18–19 Datar D, Kim S, Jeon S, Manalis S, Hesketh P, Boisen A, Thundat T 2009 Cantilever sensors: nanomechanical tools for diagnostics. MRS Bull. 34, 449–54 Finot E, Passian A, Thundat T 2008 Measurement of mechanical properties of cantilever shaped materials. Sensors 8, 3497–541 Krause A R, Van Neste C, Senesac L R, Thundat T, Finot E 2008 Trace explosive detection using photothermal deflection spectroscopy. J. Appl. Phys. 103, 094906 McKendry R, Zhang J, Arntz Y, Strunz T, Hegner M, Lang H P, Baller M K, Certa U, Meyer E, Guntherodt H, Gerber C 2002 Multiple label-free biodetection and quantitative DNA-binding assays on a nanomechanical cantilever array. Proc. Natl. Acad. Sci. USA 99(15), 9783–8 Naik A K, Hanay M S, Hiebert W K, Feng X L, Roukes M L 2009 Towards single-molecule nanomechanical mass spectrometry. Nat. Nanotechnol. 4, 445–50
5
Microcantilever Sensors Raiteri R, Nelles G, Butt H J 1999 Sensing of biological substances based on the bending of microfabricated cantilevers. Sens. Actuators B Chem. 61(1–3), 213–17 Senesac L R, Yi D, Grave A, Hales J H, Davis Z J, Nicholson D M, Boisen A, Thundat T 2009 Microdifferential thermal analysis and detection of adsorbed explosive
molecules using microfabricated bridges. Rev. Sci. Instrum. 80, 035102
A. Passian and T. Thundat
Copyright r 2011 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials: Science and Technology ISBN: 978-0-08-043152-9 pp. 1–6 6
Figure 1 Electron micrograph of a cantilever array chip showing four piezoresistive cantilevers in a microfluidic well (Cantion, Inc.).
mass variations due to adsorbed mass and offer an exciting and unparalleled opportunity for the development of highly sensitive chemical and biological sensors (Datar et al. 2009). Because of recent advances in microlithographic technologies and microfabrication techniques, these sensors can be mass-manufactured on silicon wafers and other materials in a cost-effective and modular fashion. Microcantilevers are, however, not chemical or biological sensors. They are physical sensors that can detect displacement with subnanometer resolution (Finot et al. 2008). The deformation of a cantilever can be either static or dynamic. The static displacement of the cantilever can be used for measuring extremely small forces with unprecedented sensitivity, whereas using the dynamic response, the resonance frequency can be used for measuring extremely small masses (Naik et al. 2009). Both these signals, therefore, can be used for physical, chemical, and biological sensing. In a typical static measurement, molecular adsorption can result in cantilever bending, which is monitored (see Fig. 2). Similarly, in the dynamic mode, from an accurate measurement of the cantilever resonance frequency, it is possible to measure changes in mass due to molecular adsorption on cantilever surface. The microcantilever sensors are capable of measuring the extremely small molecular forces caused by the adsorption events and although sensitive to other stimuli such as temperature and pressure, they offer an exciting and unparalleled opportunity for the development of highly sensitive chemical and biological sensors based on the specific channel of adsorption-induced forces.
Figure 2 A cartoon showing cantilever bending caused by adsorption of molecules on one of its surfaces due to changes in surface free energy that is related to surface stress. Adsorption of molecules on both sides of the cantilever results in no deflection because the stress variations on both sides are canceled. 1
Microcantilever Sensors 1.
Modes of Cantilever Operation
A properly designed cantilever sensor, being essentially a physical sensor, can measure extremely small displacements, generally in the subnanometer range. By using optimal design, it is possible to measure both bending and resonance frequency simultaneously. Alternately, it is possible to design two different cantilevers on the same chip where one can be used for mass detection, whereas the other one can be used for detection of adsorption-induced bending. We now consider the two operation modes of the sensor: the dynamic mode and the static mode.
1.1
Dynamic Mode
The dynamics of microcantilevers can be studied within the framework of continuum mechanics. The mathematical description of the dynamics of a cantilever-shaped material may be written as a partial differential equation, which predicts an infinite number of eigenmodes for the resonances of the oscillator. In practice, only the few first modes can be measurably excited and used in measurements. The resonance frequencies of the sensor depend on the material and the geometry of the cantilever. For microcantilevers made of silicon, in various geometries such as triangular, rectangular, and V-shaped, the first few resonance frequencies appear in a spectrum from a few kilohertz up to several tens of megahertz. When modeling the cantilever dynamics, appropriate account of dissipative processes must be taken so that an accurate relationship can be established between the resonance frequency and the adsorbed mass on the sensor. Therefore, the interaction of the cantilever with its environment is of paramount importance in the quantitative determination of the adsorbed mass from frequency shift measurements. This is of particular importance when the sensor is operated in a fluid where the oscillations are severely damped, making precise frequency shift measurements difficult. The dynamic motion of the cantilever, either driven by an external oscillator, for example, a piezoelectric crystal, or driven by the ambient temperature fluctuations (Brownian motion), can be used for detecting mass variation due to molecular adsorption. The mass resolution of a cantilever sensor increases with its resonance frequency. A cantilever’s resonance frequency varies as a function of adsorbed mass. Cantilevers with higher resonance frequency have higher mass resolution. In the simplest model of a cantilever, that is, when modeled as a point mass in harmonic motion, the resonance frequency, f, of a cantilever can be shown to vary as a function of mass 2
loading (Dm), according to 1 f ¼ 2p
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k m þ aDm
ð1Þ
where k is the force constant of the cantilever, m is the effective mass of the cantilever, and a is a numerical constant. Under equilibrium in a medium of a given viscosity, a Brownian microcantilever oscillator with an effective mass m satisfies the Langevin equation. Using the fluctuation–dissipation theorem, when modeled as a point mass, one can show that the averaged square of the position of the cantilever equals bQ2, where b ¼ 2m2kBTDo/pg3 and the quality factor Q ¼ mo/g with o and g being the resonance frequency and the damping of the oscillations, respectively, kB is the Boltzmann constant, and Do is the measurement bandwidth. When the cantilever is modeled as an extended elastic body, the averaged square of the position of a point on the cantilever equals bQn2 for the nth resonance mode of the cantilever, where Qn is proportional to the eigenfrequency on. Mass detection using variations in the cantilever resonance frequencies is well suited for measurements in vacuum or in air. It is possible to increase the adsorbed mass by increasing the surface area of a cantilever by nanopatterning. Cantilevers with small dimensions and, therefore, higher resonance frequencies can produce mass resolution in the femtogram and attogram ranges. The mass resolution, however, is very poor when operated in solution due to damping. The resonance frequency variations, therefore, are generally not used for highly sensitive detection of adsorbed mass in liquid environments. Recently, this challenge has been overcome by using a hollow cantilever concept called suspended microchannel resonator (SMR), where the fluid is confined inside the cantilever vibrating in vacuum (Burg et al. 2007). This clever design avoids liquid damping and increases the sensitivity of the cantilever mass sensor. Similarly, techniques originating from control theory, such as self-excited feedback loop, may be invoked to improve the performance of the cantilever in liquid. The resonance response (frequency) of a cantilever is rich in information, and many other physical variables can be deduced from the dynamic response (Craighead 2007). In addition to the analysis of the resonance frequencies, the phase and the amplitude of the dynamic response can be used. The spectral changes can also be used for measuring changes in viscosity and density of the medium. 1.2
Static Mode
As mentioned earlier, cantilever beams undergo bending due to surface stresses created by molecular adsorption when adsorption is confined to a single
Microcantilever Sensors side of the cantilever. The bending of the beam is linearly related to surface coverage of the adsorbed molecules. Adsorption of molecules on a solid surface is primarily driven by a need to minimize energy; the surface free energy of a solid decreases during molecular adsorption. If the adsorption of molecules on the surface of a thin material is restricted mostly to one side, for example, by making the opposite surface inert, a differential surface stress is generated between the two surfaces. This differential surface stress causes the material to deform. This effect can be easily observed with a microcantilever beam. Surface stress, s, and surface free energy, g, can be related using the Shuttleworth equation: s¼gþ
dg de
ð2Þ
where the surface strain de is defined as the ratio of the change in surface area to the total area, de ¼ dA/ A. For liquids, the second term is zero. When the differential surface stress, created by molecular adsorption, reaches a threshold, the cantilever bending becomes measurable (discernible from noise). Stoney’s equation relates the difference in surface stress, Ds, between the chemically modified surface and the untreated surface with the cantilever deflection, Dh: Dh ¼
3ð1 nÞL2 ðDs1 Ds2 Þ Et2
ð3Þ
where n is the Poisson ratio of the material, E is Young’s (elastic) modulus of the cantilever material, and t and L are the thickness and the length of the cantilever, respectively. By optimizing the length and thickness, it is possible to make a cantilever very sensitive to small changes in surface free energy. Unlike a clean surface in vacuum conditions, a surface exposed to ambient chemicals has adsorbed molecules occupying adsorption sites. When exposed to target molecules, the target molecules either adsorb on to the molecules that are already adsorbed on the surface or replace the molecules that are already adsorbed on the surface. Therefore, the surface energy change determined from experiment may not readily match the theoretical values. The cantilever bending can also be caused by other effects, for example, thermal expansion, bimaterial effect, swelling of cantilever constituent layers such as polymer layers in response to changes in the temperature or the ambient chemicals. Interaction of light with a cantilever or the interaction of (both reactive and inert) gas molecules with the surfaces of the cantilever under certain conditions can also result in a measurable response. In the case of the light–cantilever interaction, a number of processes including stress-optical, photon pressure, optical absorption, and thermo-optic can all cause cantilever displacement. In the presence of temperature
gradients (inert, reactive, or both), rest gas molecules may interact with the surfaces of the cantilever to cause spurious or even intentional large net forces, the so-called radiometric (or Knudsen) forces. These effects can be used for designing cantilevers to provide additional signals. It is possible to eliminate these effects by using reference cantilevers for obtaining differential signals.
2.
Electronic Monitoring of Cantilever Deflection
Current microfabrication techniques allow incorporation of electronic detection of cantilever motion, such as variation in piezoresistivity or incorporation of embedded MOSFET (see Metal Oxide–Semiconductor Field Effect Transistors). In the piezoresistive technique, the resistance of an asymmetrically doped cantilever varies sensitively as a function of bending. In the embedded MOSFET readout method, a field-effect transistor is embedded at the base of the cantilever. The stress from the bending of the cantilever changes the carrier mobility and drain current. Both these techniques are compatible with microfabrication and miniaturization. The piezoresistive method has been well investigated. Piezoresistivity (see Stress Coupled Phenomena: Piezoelectric Effect) is where the resistance of certain conductive material, such as doped silicon, varies as a function of applied strain. In piezoresistive cantilever deflection monitoring technique, the cantilever is fabricated with an integrated resistor with piezoresistive properties. Because of this piezoresistive property, the resistance of the cantilever changes as a function of cantilever bending. Thus, the extent of cantilever bending can be sensitively measured by a simple electrical measurement of the resistance variation of the cantilever. In general, cantilever resistance variation is measured with respect to a reference cantilever fabricated adjacent to the sensor cantilever. The reference cantilever is inert and is used to eliminate noise, such as temperature changes, in the system. A piezoresistive cantilever is defined in microcrystalline or single crystal silicon by boron doping, followed by etching to release the freestanding cantilever. The thickness of the deposited silicon nitride on the cantilever on either side of the cantilever is adjusted in such a way that the neutral axis of the cantilever lies inside the silicon nitride layer rather than in the silicon layer. This asymmetry in material composition reduces the electronic noise as well as the drift in the cantilever deflection. In addition, the silicon nitride serves as an efficient electrical insulation of the resistor and ensures that the device can be operated in liquids.
3.
Chemical and Biological Selectivity
Although cantilever sensors are extremely sensitive, they are physical sensors and do not offer any 3
Microcantilever Sensors −0.00
Stress (V)
DNA −0.02
−0.04 Buffer −0.06 0
1000
2000
Time (s)
Figure 3 Cantilever bending as a function of time in flow condition. Hybridization of DNA causes the cantilever bending (measured as voltage due to change in resistance of the piezoresistive cantilever). Changing the flow to buffer alone (at 2000 s) removes the physisorbed DNA.
intrinsic chemical or biological selectivity. Chemical and biological selectivity are achieved by immobilizing biological receptors or chemical interfaces on cantilever surfaces (Raiteri et al. 1999). Chemically selective interfaces can be selective polymers, selfassembled monolayers, or metal or oxide coatings. Binding of target molecules to the selective layer immobilized on one side of the cantilever results in free energy change causing the cantilever to bend. The cantilever bending has been shown to be proportional to the analyte concentration (see Fig. 3). The selective layer can bind the target irreversibly or reversibly depending on the analyte–receptor/chemical interface interaction. For example, cations, such as Cs, have been successfully detected using special calixarenes. However, these bindings are irreversible under normal conditions. Selective and sensitive detection of many biomolecules have been demonstrated using cantilever arrays immobilized with biological receptors (McKendry et al. 2002). Examples of biological interfaces include immobilized antibodies, peptides, DNA, or enzymes. The selective interfaces are immobilized on the cantilever surface using linker groups that can tether the receptors strongly to cantilever surfaces. For example, thiol group has been extensively used for immobilizing bioreceptors on cantilever surfaces. Microcantilevers have been used to demonstrate the detection of a number of chemical species, such as chemical warfare agents, volatile 4
organic compounds, and toxic industrial compounds using immobilized selective layers. Selective and sensitive detection of a number of biomolecules, organisms, and biochemicals have been demonstrated using cantilever platform. Examples of successful demonstration include markers for cancer and cardiac diseases, DNA markers, biotoxins, biowarfare pathogens, glucose, and Ca ions. Although the approach of biomolecular detection using selective receptors for biological analytes has been very selective, the same cannot be said about small molecule detection, especially in vapor phase. Although there are reports of detection of chemical species, such as chemical warfare agents, explosives, volatile organic compounds, and toxic industrial compounds, they seem to suffer from interference problems. The selectivity challenge encountered with small molecules is mainly due to lack of specificity of the chemically selective layers. Often room temperature reversible interactions, such as hydrogen bonds, are utilized in designing receptors for small molecules. Although room temperature reversible chemistry is highly attractive, it is very unspecific. It is possible to improve the selectivity using an array of cantilever sensors where each cantilever element is modified by a different selective layer. The array response to the same analyte can improve the selectivity when pattern recognition algorithms are used. However, the system will still have false positives because the signals are not orthogonal.
3.1
Selectivity – Based on Physical Principles
The cantilever platform, however, is so versatile that it offers many novel ideas for achieving chemical selectivity to be incorporated into the sensor platform. For example, a bimaterial cantilever is extremely sensitive to temperature. It is possible to exploit this sensitivity for molecular detection. A bimaterial cantilever with adsorbed molecules can deflect differently depending on the optical absorption characteristics of the adsorbed molecules. This technique is called photothermal spectroscopy (PDS) and allows the detection of monolayers of molecules with high selectivity without loosing sensitivity (Krause et al. 2008). In PDS, a bimaterial cantilever with adsorbed molecules is exposed to mid infrared (IR) light from a monochromator. The cantilever bending as a function of illuminating wavelength resembles IR spectrum of the adsorbed molecules. The PDS is based on the extremely high thermal sensitivity of a bimaterial cantilever. The PDS, therefore, combines the extreme high sensitivity of a cantilever beam and the selectivity of optical spectroscopy. Because the cantilever also allows the measurement of adsorbed mass using resonance frequency, the mass of the adsorbed molecules can be detected simultaneously.
Microcantilever Sensors The PDS has been demonstrated for detection of picograms of adsorbed molecules with very high selectivity. Another example of physical sensing would be temperature programmed heating of the cantilever (Senesac et al. 2009). One of the most underexploited properties of cantilever sensors is their extreme low thermal mass that allows achieving controllable temperature–time gradients, dT/dt, where T is the temperature and t is the time. It is possible to achieve temperature–time gradients up to 108 1C s1 using microcantilevers. The tunable temperature gradient offers an exciting opportunity for obtaining thermodynamic properties of adsorbed molecules. It can also be used for temperature programmed desorption (TPD), capable of achieving a higher dT/dt of 104 1C s1, which opens up the possibility of investigating the thermal behavior of adsorbates including thermally induced decomposition of subnanogram quantities of materials in milliseconds. The measured signal as a function of heating time (or temperature) is the rate of change of thermal mass, which is related rate of change of adsorbed mass with temperature (dM/dT). Because the observed signal is with respect to a reference cantilever, the mechanical buckling of the bridge under thermal stress does not play a role. Melting, sublimation, evaporation, and deflagration (for energetic molecules) can be detected using thermal techniques. Cantilever platform also offers ideal methods for incorporating electrochemical detection of electroactive species. In this case, the cantilever with a metal coating acts as a microelectrode in a three-electrode electrochemical cell arrangement. Electrochemical reactions happening on the cantilever surface result in cantilever bending. Selectivity of the ion detection comes from the electrode potential at which electrode reactions occur. Many ions have been successfully detected using microcantilever electrochemical sensors. However, this method works only for electroactive species.
4.
Conclusions
Microcantilevers are universal platforms for measuring a multitude of physical, chemical, and even biochemical factors, depending on the type of coating. Microcantilever array-based detection offers increased reliability by several sensors operating in parallel. Such increased reliability is especially important for practical applications. Microcantilever arrays fabricated in groups can be used for simultaneous detection of multiple chemical or biological analytes. Potential applications are widespread within the consumer, military, industrial, and clinical markets. This will be especially true when mixtures of multiple target molecules can be analyzed and screened on a single, miniature chip. Telemetry will
enable the use of mobile units worn or carried by personnel and the deployment of fieldable devices to relay pertinent data to central collection stations and may even replace wired sensors in some applications. However, a number of challenges remain to be overcome before microcantilever sensors can find widespread applications. The technology for designing and fabricating cantilever arrays with electronic readout is well advanced. Integration of cantilever arrays and microfluidic channel networks is still under development. Receptor immobilization on individual cantilevers in an array in a reproducible fashion from chip to chip still remains as a challenge. Novel robust receptor immobilization techniques that can work under solution in a reproducible manner need to be developed. Although detection of many different chemical and biological analytes has been demonstrated in the laboratory conditions, multianalyte detection from a raw sample still remains as a challenge. Therefore, integration of some rudimentary on-chip sample processing is required before detection by cantilever arrays. Such on-chip processing can be incorporated without increasing the chip size or power requirements using available techniques. See also: Magnetic Sensors: Principles and Applications; Multifrequency Atomic Force Microscopy; Oxide Gas Sensors; Solid-state Gas Sensors: Design and Fabrication
Bibliography Boisen A, Thundat T 2009 Design and fabrication of cantilever array sensors. Mater. Today 12, 32–8 Burg T P, Godin M, Knudsen S M, Shen W, Carlson G, Foster J S, Babcock K, Manalis S R 2007 Weighing of biomolecules, single cells and single nanoparticles in fluid. Nature 446, 1066–9 Craighead H 2007 Nanomechanical systems: measuring more than mass. Nat. Nanotechnol. 2, 18–19 Datar D, Kim S, Jeon S, Manalis S, Hesketh P, Boisen A, Thundat T 2009 Cantilever sensors: nanomechanical tools for diagnostics. MRS Bull. 34, 449–54 Finot E, Passian A, Thundat T 2008 Measurement of mechanical properties of cantilever shaped materials. Sensors 8, 3497–541 Krause A R, Van Neste C, Senesac L R, Thundat T, Finot E 2008 Trace explosive detection using photothermal deflection spectroscopy. J. Appl. Phys. 103, 094906 McKendry R, Zhang J, Arntz Y, Strunz T, Hegner M, Lang H P, Baller M K, Certa U, Meyer E, Guntherodt H, Gerber C 2002 Multiple label-free biodetection and quantitative DNA-binding assays on a nanomechanical cantilever array. Proc. Natl. Acad. Sci. USA 99(15), 9783–8 Naik A K, Hanay M S, Hiebert W K, Feng X L, Roukes M L 2009 Towards single-molecule nanomechanical mass spectrometry. Nat. Nanotechnol. 4, 445–50
5
Microcantilever Sensors Raiteri R, Nelles G, Butt H J 1999 Sensing of biological substances based on the bending of microfabricated cantilevers. Sens. Actuators B Chem. 61(1–3), 213–17 Senesac L R, Yi D, Grave A, Hales J H, Davis Z J, Nicholson D M, Boisen A, Thundat T 2009 Microdifferential thermal analysis and detection of adsorbed explosive
molecules using microfabricated bridges. Rev. Sci. Instrum. 80, 035102
A. Passian and T. Thundat
Copyright r 2011 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials: Science and Technology ISBN: 978-0-08-043152-9 pp. 1–6 6