Holographic pH Sensors - Holographic Sensors - page 55

Fig. 3.3 Optical images of the surface gratings of pHEMA + Ag 0 NP system, showing three spots
of different holograms. Scale bars = 10

µ m

3.3.1 Microscopic Imaging of Holographic pH Sensors

Optical images of the surface topography of holographic sensors were taken using a
Nikon OPTIPHOT-2 microscope in bright field mode. The surface gratings of the

holographic pH sensors were analysed by studying the images of pHEMA + Ag 0 NP
system (Fig. 3.3 ). Using the grating equation and measuring the position of the order
from the images, the grating spacing was calculated as 3.01

m. These gratings are
transmission holograms caused by the interference of the object beam and internally
re

μ

ection mode [ 25 , 26 ].
Samples before (control) and after photochemical patterning were imaged.
HELIOS 600 SEM/FIB was used for scanning electron microscopy (SEM). Squares
were cut out of the matrices and stuck to a SEM Al stub using carbon tab/tape. An
Au 0

fl ected of light in Denisyuk re fl

10 nm) was sputtered on the top surface to improve conductivity.
A trench was prepared using a focused gallium ion beam milling. The cleaning was
achieved through low current milling. Figure 3.4 a, b illustrates cross sectional
images of the pHEMA-co-MAA + Ag 0 NP system before laser-induced photo-
chemical patterning (control samples), and Fig. 3.4 c, d shows the system after
patterning. The Ag 0 NPs are situated up to

film ( *

s
surface (Fig. 3.4 a). However, SEM was not suitable for polymer imaging since the
electron beam generated heat as it hit the specimen, and this damaged the polymer
and generated bubbles (Fig. 3.4 c, d). Additionally, the melting temperature (T m )is
size dependent. Ag 0 NPs with diameters of 5, 11, 14 and 20 nm melt at 480, 550,
580 and 650

5 6 µ m below the polymer matrix '

* -

C, respectively [ 27 ]. Since laser-induced photochemical patterning
attenuates the size of the Ag 0 NPs [ 28 ], Fig. 3.4 c

°

d have smaller Ag 0 NPs, hence
subject to melting as compared to the polymer matrices before patterning.
Environmental Scanning Electron Microscopy (ESEM) was used to image the
cross section of the pHEMA-co-MAA + Ag 0 NP system. Holographic sensors on
PMMAwere cross-sectioned using a microtome. Images were obtained with a Philips
XL30 FEG ESEM, acceleration voltage = 6 kV with a working distance (WD) of
13.5 mm. An advantage of ESEM is that images can be obtained at room temperature
(24

-

° C). Using ESEM, dynamic swelling experiments were performed in wet mode.