Chemistry Reference
In-Depth Information
X-ray wavelengths can be used to probe even sub-nanometer length scales and
the specific advantages of X-rays concern (i) the resolution, (ii) the kinematic nature
of the scattering process enabling quantitative image analysis, (iii) element specific
contrast variation and (iv) the potential of high time resolution. While on the one
hand, scattering techniques such as grazing incident diffraction/small angle X-ray
scattering (GID/GISAXS), X-ray reflectometry (XRR) and anomalous scattering
have been widely used to study lipid membranes, they are not suitable for structure
resolution in a functional context. This must be probed
in situ
via direct imaging
techniques and all direct X-ray microscopy techniques are limited in spatial reso-
lution to a few 10 nm (
2 orders of magnitude away from the classical diffraction
barrier) [
7
,
8
] due to aberratons and the limited numerical aperture of today's X-ray
optics. The fabrication of high resolution lenses currently poses severe technolog-
ical challenges. Lensless coherent X-ray diffractive imaging (CXDI) [
9
] has been
proposed to circumvent these practical limitations, but its full potential is yet to be
realized.
Recently, a technique based on hard X-ray Fresnel diffraction (propagation imag-
ing) was used to image free standing lipid bilayers, with phase contrast arising from
free space propagation of the beam traversing through the lipid bilayer [
10
]. A sim-
ple, yet extendable model was used to quantitate the information from the phase
contrast image and a resolution down to
∼
5nm in one dimension, could be obtained
[
11
] by using a focussed beam instead of a parallel beam. In this chapter, we extend
the scope of this technique by imaging the lipid membranes formed in microfluidic
channels, where the depth and planarity of the membrane are of potential importance
in increasing the phase contrast, thus improving resolution.
∼
4.2 Experimental Techniques
4.2.1 Synchrotron Setup
Divergent beam imaging experiments were performed at the insertion device 22NI
undulator station at ESRF (Grenoble, France). The beamline includes a U42 undula-
tor, which is operated at the third harmonic to deliver a so called 'pink X-ray beam'
with photons of 17.5keV energy and a corresponding wavelength of 0
.
708 Å. The
source size at the undulator is 700
µ
m(h)
×
30
µ
m (v) (FWHM) with a divergence
of 28
min
the horizontal direction. For additional focusing a Kirkpatrick-Baez (KB) mirror-pair
is located in the experimental hutch [
12
,
13
]. The microfluidic device is placed in
the focus of the KB system that coincides with the focus of the on-axis microscope
(OAM). The focus is located 30mm downstream the KB mirrors and has a diameter
of 130nm (v) and 140nm (h), which was measured by performing knife edge scans
in both directions. A drilled mirror allows for the simultaneous detection of light
microscope and X-ray images and thus a faster alignment of the sample in the beam.
µ
rad (h)
×
5
µ
rad (v). A set of slits defines a secondary source size of 25
µ
