CSEM Scientific and Technical Report 2008

BioTool – an Integrated Parallel Atomic Force Microscopy Platform for the Life Sciences
A. Meister, M. Favre, J. Polesel-Maris • , J. Przybylska, R. Ischer, T. Overstolz, P. Niedermann, A. Hoogerwerf, M. Schnieper,
S. Dasen, G. Gruener, J. Auerswald, S. Generelli, M. Liley, P. Vettiger, H. Heinzelmann
An instrument that allows the acquisition in parallel of several atomic force microscopy images or force curves has been developed. Data
acquisition uses one- or two-dimensional arrays of cantilevers, and can be performed in air or in liquid. Different kinds of probe arrays have been
developed to address specific biological applications.
Atomic force microscopy (AFM) techniques have made
enormous progress in recent years, especially in the field of
nanobiotechnology [ 1] . These techniques allow scientists to
observe, characterize and manipulate biological and
molecular systems in their native environment, for example,
determining the mechanical properties of living cells or
measuring interaction forces between ligand-receptor pairs.
AFM manufacturers have developed special instruments for
bio-applications, integrating temperature-controlled fluidics
chambers to reproduce the native environment of biological
samples (usually 37°C in aqueous media), and mounting
them on an inverted optical microscope for concurrent
fluorescence imaging. However, one major limitation of these
‘bioAFMs’ still remains to be addressed, which is the
extremely slow rate of data acquisition caused by slow serial
measurement using a single AFM cantilever.
The BioTool platform has been developed with the aim of
overcoming this limitation. BioTool uses arrays of cantilevers
in one- or two-dimensions. The 3-dim positions of these
cantilevers in the array can be measured in parallel using a
Michelson interferometer and imaging optics. The resulting
interferogram is captured by a CMOS camera and analysed
using dedicated software in order to determine the deflection
of each cantilever.
Figure 1 shows the setup of the BioTool platform. The sample
is placed on top of a piezoelectric nanopositioning stage that
can be scanned in three dimensions. Two data acquisition
modes can be used. In the first mode, the probe array is
scanned parallel to the sample surface and the deflection of
the cantilevers is measured as a function of their location in
order to determine the topography of the sample. In the
second mode, known as force spectroscopy, the probe array
is moved perpendicularly to the sample surface (z-axis). Here
the deflections of the cantilevers are determined as a function
of the distance to the sample. This mode is used to precisely
measure the local mechanical properties of the sample, or to
measure adhesion forces.
Figure 1: Conceptual sketch of the parallel detection scheme and
picture of the BioTool platform
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Different types of cantilever arrays have been developed in
order to address diverse bio-applications: (i) Two-dimensional
polymeric probe arrays are being developed for force
spectroscopy, to determine the mechanical properties of cells
in parallel. These arrays are made by a sol-gel replication
process (Figure 2a) and have tips with a well-defined radius of
curvature. The arrays have been designed to be cheap and
disposable. (ii) Two-dimensional arrays of silicon nitride
cantilevers (Figure 2b). These probes have sharp tips and will
be used for high resolution topographical investigations e.g.
determining the presence of a target molecule on top of
capture molecules in an affinity assay. (iii) One-dimensional
arrays of hollow silicon dioxide cantilevers with an aperture at
the tip (Figure 2c). When connected to a probe holder
equipped with microfluidic delivery channels, these probes
can be used for nanoscale dispensing (NADIS) of liquids, for
example, to write high density microarrays.
Figure 2: Examples of probes used on the BioTool platform. Optical
images of (a) a polymeric and (b) a silicon nitride probe array (scale
bar 500 µm). (c) SEM picture of a NADIS probe array (scale bar
50 µm).
Figure 3: Force-distance curves measured on a hard sample in
parallel in a liquid environment
First experimental measurements have demonstrated the use
of the platform to operate 2D cantilever arrays in a liquid
environment. Figure 3 shows the first force-distance curves
obtained in parallel in water.
•
Current address: CEA Saclay, 91191 Gif-sur-Yvette, France
[1] D.J. Müller, Y.F. Dufrêne, "Atomic force microscopy as a
multifunctional molecular toolbox in nanobiotechnology", Nature
Nanotech., 3 (2008), 261