Micro-cantilevers with end-grafted stimulus-responsive polymer brushes for actuation and sensing

doi:10.1016/j.snb.2005.06.003

Sensors and Actuators B: Chemical

Volume 114, Issue 1, 30 March 2006, Pages 371-378

https://doi.org/10.1016/j.snb.2005.06.003 Get rights and content

Abstract

Stimulus-responsive macromolecules have attracted significant interest due to their potential applications in molecular motors, drug delivery, sensors, and actuation devices. Poly(N-isopropylacrylamide) (pNIPAAM) alone or as a copolymer is a stimulus-responsive polymer that undergoes an inverse phase transition triggered by changes in the solvent quality, such as temperature, ionic strength, pH, or co-solvent concentration. Associated with this phase transition is a significant conformational change. We show that micro-cantilevers, decorated on one side with a pNIPAAM brush or poly(N-isopropylacrylamide-co-N-vinylimidazole) (pNIPAAM-VI) (7:3) brush, can be used to detect and transduce this phase transition behavior. Changes in the conformational state of the brush, induced by the phase transition or changes in osmotic pressure, cause significant changes in the surface stress in the brush that leads to detectable changes in cantilever deflection. We show that the use of pNIPAAM and its copolymers is exciting for cantilever actuation and sensing because commonly available micro-fabricated cantilever springs offer a simple and non-intrusive way to detect changes in solvent type, temperature, and pH, promising great potential for sensing applications in micro-fluidic devices.

Introduction

In recent years, there has been a proliferation of micro-cantilever based sensors (biosensors [1], [2], [3], [4], [5], chemical sensors [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], and physico-chemical sensors [16], [17], [18], [19], [20]) because of their simplicity and ability to function in small sample volumes. In most cases, these cantilever sensors are coated on one side with a sensing film or receptor molecules where analyte binding causes a surface stress that leads to measurable cantilever deflections (DC detection) [1], [21]. Binding events can also change cantilever mass which can be detected as a change in cantilever resonant frequency (AC detection) [22]. While the use of the AC detection circumvents the problem of long-term thermal drift (inherent to DC detection), it requires the additional use of Q-control instrumentation to increase the effective quality factors of the micro-cantilevers in liquids [22].

While many micro-cantilever sensors rely on thin film coatings with specific recognition properties or binding capabilities, stimuli-responsive polymer (SRP) brushes and stimulus-responsive bio-molecular thin films have not been used to date for micro-cantilever based sensing and actuation applications. However, SRP and stimulus-responsive thin bio-molecule films offer a means to amplify changes in the solvent environment (such as changes in pH, temperature, or ionic strength) by a change in their molecular conformation [23]. Copolymerization offers a convenient route to tailor the SRP response to specific environmental conditions. Micro-cantilevers decorated with self-assembled monolayers (SAMs) [14] and stimulus-responsive hydrogels [9], [10] have been previously used as sensitive pH sensors. However, SAMs develop only minor surface stresses and hydrogel layers are orders of magnitude thicker than SRP brushes and thus much slower in their conformational response.

In this article, we show that micro-cantilevers, decorated with a SRP brush layer, can be used as sensitive pH sensors, and as actuators, when cycling the solvent type. The use of SRP-coated micro-cantilevers promises great potential for sensing and actuation applications in micro-fluidic and Bio-MEMS devices because of the unique way by which SRP amplify changes in the solvent environment and the availability of off-the-shelf micro-cantilevers.

Section snippets

Materials

N-Isopropylacrylamide (NIPAAM, 97%) and N-vinylimidazole (VI, 97%) monomers, copper(I) bromide (Cu(I)Br, 99.9%), copper(I) chloride (Cu(I)Cl, 99.9%), and methanol (MeOH, 99.9%) were obtained from Sigma–Aldrich (Milwaukee, WI). NIPAAM was purified by recrystallization from toluene–hexane before use. Milli-Q™ (Millipore, Billerica, MA) water (18 MΩ/cm) and methanol were used as polymerization solvents. N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA) was used as received from Acros Organics

Effect of thermal drift

An AFM cantilever is typically constructed from an unbalanced laminate of a thick layer of silicon nitride and a gold thin film. Thermal stresses, caused by the mismatch between the coefficients of thermal expansion, cause the cantilever to bend in much the same fashion as a bimetallic strip in a thermostat [18]. In the AFM, localized heating from the laser raises the temperature of the cantilever locally and causes thermally induced drift effects. However, a steady-state deflection is reached

Conclusions

We showed that stimulus-responsive polymer brush-coated micro-cantilevers can be used to sense and transduce changes in solvent conditions. The change in micro-cantilever bending results from changes in the interfacial surface stress acting at the two opposite faces of the micro-cantilever. With respect to cantilevers decorated with a pNIPAAM brush alone, deflections were greater for brushes in a good solvent (swollen state) than for brushes in a poor solvent (collapsed state). We found that

Acknowledgments

We gratefully acknowledge the National Science Foundation for support through grants NSF EEC-021059, NSF 0324459, NSF DMR-0239769 CAREER AWARD, and the U.S. Army Research Office through grant ARO DAADG55-98-D-0002. We thank Mr. Hongwei Ma (Department of Biomedical Engineering, Duke University) for the synthesis of the thiol initiator.

Nehal Abu-Lail was born in Irbid, Jordan. She received both her B.S. and M.S. degrees in Chemical Engineering from Jordan University of Science and Technology. She received her Ph.D. in chemical engineering for Worcester Polytechnic Institute. She is currently working as a research associate at the Center for Biologically Inspired Materials and Material Systems at Duke University. Her research interests include single molecule spectroscopy of stimulus-responsive polymers and biopolymers, single

References (42)

H.-J. Butt
J. Colloid Interface Sci.
(1996)
P.G. Datskos et al.
Sens. Actuators B, Chem.
(2001)
T. Lalinsky et al.
Sens. Actuators B, Chem.
(1999)
J. Tamayo et al.
Ultramicroscopy
(2001)
H.G. Schild
Progr. Polym. Sci.
(1992)
X. Chen et al.
J. Colloid Interface Sci.
(2003)
G. Wu et al.
Nat. Biotechnol.
(2001)
J. Pei et al.
Anal. Chem.
(2004)
M. Calleja et al.
Sens. Lett.
(2003)
J. Fritz et al.
Science
(2000)

B. Rogers et al.

Rev. Sci. Instrum.

(2003)

H.-F. Ji et al.

Chem. Commun.

(2000)

H.-F. Ji et al.

Anal. Chem.

(2001)

R. Bashir et al.

Appl. Phys. Lett.

(2002)

J.Z. Hilt et al.

Biomed. Microdevices

(2003)

R. Berger et al.

Appl. Phys. Lett.

(1996)

L.A. Pinnaduwage et al.

Nature

(2003)

J. Fritz et al.

Langmuir

(2000)

M.-Y. Jung et al.

Jpn. J. Appl. Phys.

(1999)

T. Thundat et al.

Appl. Phys. Lett.

(1994)

T. Thundat et al.

Anal. Chem.

(1995)

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Marian Kaholek was born in Martin, Slovakia. He received his M.S. degree in Physical Chemistry from Comenius University, Slovakia. He received his Ph.D. degree in Macromolecular Chemistry from Polymer Institute, Slovak Academy of Sciences, Slovakia. He worked as a postdoctoral fellow at the Polytechnic University in New York for three years, and is currently working as a research associate at the Center for Biologically Inspired Materials and Material Systems at Duke University. His research interests include polymer brush synthesis, nanopatterning, preparation of nanogels, and materials science.

Bruce LaMattina received both his B.S. and M.S. degree in Mechanical Engineering from the University of Delaware. He received his Ph.D. degree in Mechanical Engineering from North Carolina State University. He is currently working as Branch Chief, Solid Mechanics Research Program at the Army Research Office. His research interests lie in general in solid mechanics.

Robert Clark received all his degrees in Mechanical Engineering from Virginia Polytechnic Institute and State University. He is the Thomas Lord Professor in the Department of Mechanical Engineering and Materials Science at Duke University, and Senior Dean of Research for the Pratt School of Engineering at Duke University. Work within his research group is devoted to the continued advancement of adaptive structures and a number of new directions in nanoscience/nanoengineering. For the latter, his work addresses issues in instrumentation for the nanoscience community that allows for the characterization of single molecules as well as the deposition and manipulation of single molecules for nanomanufacturing processes. His most recent research activity has been devoted to biodefense and the development of nano-electromechanical transducers capable of identifying the presence of class-A agents.

Stefan Zauscher was born in Starnberg, Germany. He received his M.S. in Materials Science from Oregon State University, and his Ph.D. in Materials Science from the University of Wisconsin at Madison. He is currently working as an assistant professor in the Department of Mechanical Engineering and Materials Science at Duke University. His research interests span the range from fabrication of polymeric and biomolecular nanostructures on surfaces using scanning probe lithography, their characterization and development of new nano- and micro sensors to measurement of the strength of receptor–ligand interactions relevant to the diagnostics of infectious diseases and drug development.

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Micro-cantilevers with end-grafted stimulus-responsive polymer brushes for actuation and sensing

Abstract

Introduction

Section snippets

Materials

Effect of thermal drift

Conclusions

Acknowledgments

J. Colloid Interface Sci.

Sens. Actuators B, Chem.

Sens. Actuators B, Chem.

Ultramicroscopy

Progr. Polym. Sci.

J. Colloid Interface Sci.

Nat. Biotechnol.

Anal. Chem.

Sens. Lett.

Science

Rev. Sci. Instrum.

Chem. Commun.

Anal. Chem.

Appl. Phys. Lett.

Biomed. Microdevices

Appl. Phys. Lett.

Nature

Langmuir

Jpn. J. Appl. Phys.

Appl. Phys. Lett.

Anal. Chem.