Chemistry Reference
In-Depth Information
The detector is a Fast-Readout, Low-Noise (FRELON 2000; ESRF, Grenoble) CCD
camera [
14
]. It is combined with a 24
µ
m thick LSO:Tb (Terbium doped Lutetium-
Oxoorthosilikat, Lu
2
SiO
5
) scintillator, optimized for high resolution imaging, that
converts the X-rays into visible light photons of 550nm wavelength and is supported
by a 170
m YSO (Ytterbium-Oxoorthosilicate, Yb
2
SiO
5
) substrate [
15
,
16
]. It is
placed 526mm behind the KB focus. The image is magnified by a 10
µ
×
objective and
a2
eyepiece to fit the dimension of the 14-bit Kodak KAF4320 CCD chip with a
physical pixel size of 24
.
5
×
µ
m and a resolution of 2048
×
2048 pixels. Consequently,
the theoretical pixel size of the images is 0.96
m. The effective pixel size depends
on the position
z
of the sample and was varied in a range of defocusing distances
3mm
µ
≤
z
≤
200 mm, with respect to the KB mirror focus.
4.2.2 Microfluidic Devices and Membrane Preparation
Microfluidic devices made out of PDMS (Sylgard 184, Dow Chemicals) are not
suitable for X-ray studies due to the scattering from the crosslinked PDMS matrix.
We developed a new technique for use in the X-ray imaging studies. The design
of the SU8-silicon masters (positive) simply consists of two crossed channels in
different widths of 300
m or 1mm. To reproducibly prepare a large amount
of microfluidic channels a positive PDMS master is created for the preparation of
microfluidic chips made from UV-curable glue (Norland Optical Adhesive No. 81,
Edmund Optics; Karlsruhe, Germany). This fast curing UV glue is used due to
its transparency, weak X-rays scattering properties and its stiffness (compared to
PDMS) after intense curing. Therefore the SU8-silicon master (Fig.
4.2
a) is placed
in a Petri dish and the PDMS-crosslinker solution is poured in (Fig.
4.2
b). Degassing
is performed in a vacuum chamber to get rid of the air that is trapped underneath the
silicon wafer. Subsequently, the PDMS is cured at 85
◦
C for 1h or more and then a
scalpel is used to cut out the negative copy. This part is put in a sealed glass beaker,
together with a vial filled with hexamethyldisilazane (HMDS, Fluka; Germany), for
approximately 15min. The negative PDMS copy is placed in a Petri dish lined with
aluminium foil and PDMS solution is poured onto it (Fig.
4.2
c). After curing the
PDMS, as described previously, everything is peeled of the aluminium foil and both
PDMS parts are separated by cutting with a scalpel—leaving the negative copy and
a positive secondary master of PDMS (Fig.
4.2
d). The latter one is cut in a way that
smooth edges are left and is subsequently laid on top of a very flat stamp of PDMS
(Fig.
4.2
e). A drop of UV glue is applied that crawls towards the channel cross by
capillary forces. Glue is added for as long as the whole space between PDMS master
and stamp is filled and no air bubble is left. The UV glue is exposed for 5-10min
to UV light (PolyluxPT, Dreve; Unna, Germany) and the final MF chip is carefully
peeled off the PDMS under addition of Isopropanol (Fig.
4.2
f). These devices offer
the advantage that they are inert, very robust and can be used several times.
The membranes are prepared by running a pump (Fig.
4.1
) which is connected
to the oil channel for approximately 10min to flush the channels with the lipid
solution. This procedure has an additional hydrophobizing effect helping to promote
µ
m, 500
µ
