Applications of metal compounds and cholesteric liquid crystals for chemical sensing

APPLICATIONSOFMETALCOMPOUNDS AND
CHOLESTERICLIQUIDCRYSTALS
FORCHEMICALSENSING 
LAURASUTARLIE
NATIONALUNIVERSITYOFSINGAPORE
2012
APPLICATIONSOFMETALCOMPOUNDS AND
CHOLESTERICLIQUIDCRYSTALS
FORCHEMICAL SENSING
LAURASUTARLIE
(B.Eng.,ITB)
ATH ESISSUBMITTED
FORTHEDEGREEOFDOCTOROFPHILOSOPH Y
DEPARTMENTO F
CHEMICALANDBIOMOLECULARENGINEERING
NATIONALUNIVERSITYOFSINGAPORE
2012
i


ACKNOWLEDGEMENTS

First of all, I would like to express my sincere gratitude to my supervisor Dr. Yang
Kun-Lin. He has challenged my way of thinking and provided valuable advice and
guidance for my research. I have learnt many things about research and various skills
from him.

In my PhD study, I am grateful for the graduate student research scholarship from
National University of Singapore (NUS) with support from ASEAN University
Network/ Southeast Asia Engineering Education Development Network
(AUN/SEED-net). Moreover, I am thankful to all members of Dr. Yang’s group for

sharing their knowledge and creating enjoyable lab atmosphere. In addition, I highly
appreciate all suggestions and comments from everyone in the Monday group
meeting. I also would like to thank all lab officers who have kindly assisted me in
various matters, especially Mr. Boey Kok Hong, Ms. Lee Chai Keng, Ms. Alyssa Tay,
and Mr. Ng Kim Poi from workshop.

Furthermore, I would like to express my deepest gratitude to my parents and my
brother for their continuous prayer, support, and encouragement. I am grateful as well
for all the support and encouragement from my housemates and all of my friends in
NUS and outside campus. The last but not the least, I would like to thank Jesus God
for His grace and blessings. He is the help, the strength, and the hope that keeps me
on.
ii

TABLE OF CONTENTS


ACKNOWLEDGEMENTS……………………………………………………
i
TABLE OF CONTENTS………………………………………………………
ii
SUMMARY……………………………………………………………………
vi
LIST OF FIGURES …………………………………………………………….
viii
LIST OF TABLES ……………………………………………………………
xiii
LIST OF SCHEMES ……………………………………………………………
xiv
NOMENCLATURES…………………………………………………… ……

xv


CHAPTER 1. INTRODUCTION ………………………………………… …
1
1.1. Applications of metal compounds for chemical sensing ………………….
1
1.2. Applications of cholesteric liquid crystals (CLCs) for chemical sensing ….
3
1.3. Objectives …………………………………………………………………
4


CHAPTER 2. LITERATURE REVIEW ……… ……………………………
7
2.1. Chemical sensors for VOCs ………………………………………………
7
2.1.1. Electrochemical sensors ……………………………………………
7
2.1.2. Mass sensors ………………………………………………………
10
2.1.3. Optical sensors ………………………………………………………
11
2.1.4. Recent advances in VOCs sensors …………………………………
13
2.1.4.1. Arrays with multiple sensing elements …………………….
13
2.1.4.2. Microfluidic devices ……………………………………….
14
2.2. Interactions between molecular receptors and target analytes ……………

16
2.2.1. Hydrogen bonds ……………………………………………………
17
2.2.2. Electron pair donor acceptor and metal-ligand interactions ………
19
2.3. Metal compounds and their applications for VOCs sensing ……………….
20
2.3.1. Metal compounds as molecular receptors for VOCs sensing ……….
20
2.3.2. Chemical sensitive layers with metal compounds as molecular
receptors …………………………………………………………….

23
2.4. Cholesteric liquid crystals (CLCs) and their applications for VOCs sensing
25
2.4.1. Optical properties of CLCs…………………………………………
25
2.4.2. CLC based VOCs sensors …………………………………………
28


CHAPTER 3. COLORIMETRIC RESPONSES OF TRANSPARENT
POLYMERS DOPED WITH METAL PHTHALOCYANINE FOR
DETECTING VAPOROUS AMINES………….…….……………………… 29
3.1. Introduction…………………………………………………………………
29


iii

3.2. Experimental methods ……………………………………………………
32
3.2.1. Materials …………………………………………………………….
32
3.2.2. Preparation of polymer ……………………………………………
33
3.2.3. UV-Vis and FTIR spectroscopy …………………………………….
33
3.3. Results and discussion ……………………………………………………
34
3.3.1. Screening candidate MPcs for detecting amines ……………………
34
3.3.2. Selectivity for hexylamine ………………………………………….
35
3.3.3. FePc-doped polymers………………………………………………
38
3.4. Conclusions…………………………………………………………………
44


CHAPTER 4. COLORIMETRIC RESPONSES OF COPPER IONS TO
AMMONIA VAPOR: DISPERSION AND SURFACE LIGANDS
EFFECTS


45
4.1. Introduction ………………………………………………………………
45
4.2. Experimental methods ……………………………………………………
48

4.2.1. Materials …………………………………………………………….
48
4.2.2. Preparation of polymer ……………………………………………
49
4.2.3. Detection of ammonia vapor ………………………………………
49
4.2.4. Leaching Cu
2+
from silica gel ….…………………………………
50
4.3. Results and discussion ……………………………………………………
50
4.3.1. Colorimetric responses of Cu
2+
to ammonia vapor… …… ………
50
4.3.2. Effect of anions on colorimetric response of Cu
2+
…………………
53
4.3.3. Surface ligands effect………………………………………………
54
4.3.4. Surface acidity effect………………………………………………
60
4.4. Conclusions…………………………………………………………………
61


CHAPTER 5. POLYMER STABILIZED CHOLESTERIC LIQUID
CRYSTAL ARRAYS FOR DETECTING VAPOROUS AMINES……………

63
5.1. Introduction ………………………………………………………………
64
5.2. Experimental methods ……………………………………………………
66
5.2.1. Materials …………………………………………………………….
66
5.2.2. Preparation of PSCLC cells and PSCLC thin films…………………
66
5.2.3. Vapor detection…………….………………………………………
67
5.2.4. Vis spectroscopy…………… ….…………………………………
68
5.3. Results and discussion ……………………………………………………
69
5.3.1. Dependence of PSCLC colors on temperature…………… ……….
69
5.3.2. Colorimetric responses of PSCLCs to amine vapors………………
72
5.3.3. Specificity of PSCLCs… …………………………………………
73
5.3.4. Correlations between molecular weights and detection limits of
PSCLCs to aliphatic primary amine…………………………………

76


iv

5.3.5. Response time and reversibility of PSCLCs…………………………
79
5.4. Conclusions…………………………………………………………………
81


CHAPTER 6. CHOLESTERIC LIQUID CRYSTALS DOPED WITH
DODECYLAMINE FOR DETECTING ALDEHYDE VAPORS …………….

83
6.1. Introduction ………………………………………………………………
83
6.2. Experimental methods ……………………………………………………
86
6.2.1. Materials …………………………………………………………….
86
6.2.2. Preparation of thin films of PDMS………………………….……….
86
6.2.3. Preparation of CLCs and CLCs doped with dodecylamine………….
87
6.2.4. Exposure of CLC thin films to vapor analytes………………………
88
6.2.5. UV-Vis spectroscopy………………………………………………
89
6.2.6. Fourier transform infrared (FTIR) spectroscopy…………………….
89
6.3. Results and discussion ……………………………………………………
90
6.3.1. Effect of glass on CLCs doped with dodecylamine…………………
90

6.3.2. Effect of dopant concentrations……………………………………
92
6.3.3. Colorimetric responses of CLCs to pentyl aldehyde vapor… ……
94
6.3.4. Response time and reversibility……………………………………
96
6.3.5. Detection limit and sensitivity………………………………….……
98
6.4. Conclusions…………………………………………………………………
100


CHAPTER 7. MONITORING SPATIAL DISTRIBUTION OF ETHANOL IN
MICROFLUIDIC CHANNELS BY USING A THIN LAYER OF
CHOLESTERIC LIQUID CRYSTALS………… ………………


101
7.1. Introduction ………………………………………………………………
101
7.2. Experimental methods ……………………………………………………
104
7.2.1. Materials …………………………………………………………….
104
7.2.2. Preparation of PDMS and thin films of PDMS………………… …
105
7.2.3. Preparation of CLCs …………………………………… …………
105
7.2.4. Preparation of microfluidic devices with embedded PDCLCs or
embedded CLCs……………………………………………………

105
7.2.5. Ethanol detection inside microfluidic channels……………………
107
7.2.6. Visible spectrometry……………………………… ……………….
107
7.2.7. Detection of ethanol produced from fermentation…………………
108
7.3. Results and discussion ……………………………………………………
108
7.3.1. Ethanol detection in microfluidic channels with embedded PDCLCs
108
7.3.2. Reversibility of colorimetric responses in PDCLCs…………………
111
7.3.3. Ethanol detection in microfluidic channels with embedded CLCs….
112
7.3.4. Reversibility of colorimetric responses to ethanol in CLCs…………
113
7.3.5. Detection of ethanol from fermentation in microfluidic channels…
114
7.4. Conclusions…………………………………………………………………
116
v



CHAPTER 8. CONCLUSIONS AND RECOMMENDATIONS………….…
117
8.1. Conclusions…………………………………………… …………………
117

8.2. Recommendations…………………………………………………………
121


REFERENCES………………………………………………………………….
125
LIST OF PUBLICATIONS……………………………………………………
142




















vi

SUMMARY

Chemical sensing of volatile organic compounds (VOCs) has attracted much
attention. However, challenges such as how to produce low-cost, portable VOC
sensors with good sensory performance remain. Motivated by these challenges, in this
thesis we developed VOCs sensors from two potential materials. The first one is metal
compounds because they can be used as molecular receptors for VOCs based on
metal-ligand interactions. Firstly, metal phthalocyanines were employed for
colorimetric detection of amines vapor. To develop portable amines sensors, metal
phthalocyanines were added as dopants in free-standing transparent polymers.
Secondly, metal compounds are often immobilized on surfaces having ligand groups
to construct chemical sensitive layers. However, the effects of surface ligands on the
metal compounds are still unclear. To obtain a better understanding on the effects of
surface ligands, we studied the colorimetric responses of Cu
2+
immobilized on various
surfaces to ammonia.

The second material exploited here is cholesteric liquid crystals (CLCs) which are
colorful to the naked eyes and can show colorimetric responses to VOCs.
Nevertheless, development of useful VOCs sensors based on CLCs face challenges
such as limited portability, small temperature range, and selectivity of CLCs that
remains unclear. Firstly, to improve portability of CLCs, polymer stabilized
cholesteric liquid crystals (PSCLCs) were developed. PSCLCs were made into an
array of PSCLCs with different polymer concentration to expand the working
vii

temperature range. Their colorimetric responses to amines vapor were studied and
their selectivity to any particular VOCs was investigated. Furthermore, to improve the

selectivity of CLCs, we incorporated suitable molecular receptors as dopants in CLCs.
We demonstrated addition of dodecylamine as a dopant in CLCs for aldehyde vapor
detection. The colorimetric responses of the CLCs doped with dodecylamine to
aldehyde vapor were studied in term of their selectivity, sensitivity, and reusability.

Finally, because our results show that CLCs give colorimetric responses with fast
response time, reusability, and detection limit at low VOCs concentration, we
explored the possibility of utilizing CLCs to develop integrated VOCs sensors with
microfluidic channels. Here, a thin layer of CLCs or polymer dispersed cholesteric
liquid crystals (PDCLCs) was embedded to microfluidic channels for monitoring
ethanol inside the channels. Furthermore, we also demonstrated their utilization for
monitoring ethanol production from fermentation in microfluidic channels. Overall,
the VOCs sensors based on metal compounds and CLCs developed in this thesis show
colorimetric responses to VOCs without complex instrumentation. They are suitable
to be used as portable and low-cost sensors.








viii


LIST OF FIGURES

Figure 2.1.

Orientational and positional order of solid, liquid crystal, and
liquid…………………………………………………………….

12

Figure 2.2.

Molecular arrangement of CLC molecules forming helical
pattern…………………………………………………………

26



Figure 3.1.


Molecular structure of metal phthalocyanine (MPc). M = H
+
,
Cu
2+
, Ni
2+
, Pb
2+
, Zn
2+
, Fe
2+

.


32
Figure 3.2.
Photographs of various metal phthalocyanines in toluene
solution (50 µM) showing colorimetric responses (a) before and
(b) after the addition of 5 mmol hexylamine……………………



34
Figure 3.3.
FTIR spectra of dodecylamine (dotted line) and mixture of
FePc and dodecylamine with a 1:1 molar ratio (solid line)……


35
Figure 3.4.
FePc solution in toluene (5 µM) and its colorimetric responses
(a) before and (b) after the addition of 10 mM of hexane,
heptanol, DMMP, DIPEA, DIPA, hexylamine, and EDA (from
left to right)……………………………………………………



36

Figure 3.5.
Absorbance of FePc solution (50 µM in toluene) at 663 nm
when aliquots of hexylamine (solid circle) and EDA (hollow
circle) were added to the FePc solution ………………………


37

Figure 3.6.
Images of several FePc-doped (0.03 wt%) transparent polymers
before (first column) and after (second column) being exposed
to 11,600 ppmv of hexylamine vapor. These polymers are (a)
PVA (b) NOA65 (c) PDMS……………………… … … …



38

Figure 3.7

Time resolved UV/Vis absorption spectra of three FePc doped
(0.03 wt%) polymers upon exposure to 11,600 ppmv
hexylamine vapor. (a) PVA (b) NOA65 (c) PDMS………….….


39

Figure 3.8.
Colorimetric responses of NOA65 and PDMS, either doped

with FePc (0.03 wt%) or without FePc, after being (a) stored in
clean air, (b) exposed to 11,600 ppmv hexylamine vapor, (c)
exposed to 15,000 ppmv EDA vapor………………… … ……



40





ix

Figure 3.9.
Colorimetric responses of FePc-doped PDMS to various
concentration of hexylamine vapor……………………… …….

42

Figure 3.10.
Reversibility of colorimetric responses of PDMS (doped with
FePc) (a) original, (b) after exposure to 11,600 ppmv of
hexylamine vapor for 3 h, (c) left in open air for 1 week, (d)
second exposure to 11,600 ppmv of hexylamine vapor for 3 h,
(e) left in open air again for 1 week, (f) third exposure to 11,600
ppmv of hexylamine vapor for 3 h ……………………………

42

Figure 4.1.
Colorimetric response of Cu
2+
to 50,000 ppmv of ammonia
vapor. Cu
2+
is dispersed on (a) no dispersion (b) sand (c) silica
gel (d) aluminum oxide. The surface density of Cu
2+
on the
substrate is 0.1 mmol/ g substrate………….………… ……….



51

Figure 4.2.
Effect of surface density of Cu
2+
(dispersed on unmodified
silica gel) on the colorimetric response to ammonia vapor. The
surface density of Cu
2+
is 0.01, 0.05, 0.1, 0.5, 5, and 50 mmol/ g
silica gel (from left to right)………………………………….….

52

Figure 4.3.
Anions effect on the colorimetric response of Cu
2+
deposited on
unmodified silica gel with Cu
2+
density of 0.5 mmol/ g silica
gel to ammonia vapor. Copper salts used in this experiment are
copper nitrate, copper sulfate, copper perchlorate, copper
bromide, and copper acetate (from left to right)………… …….




54

Figure 4.4.
Surface ligand effect on the colorimetric response of Cu
2+
with
density of 0.5 mmol / g silica gel to ammonia vapor. The
chemical functional groups on the surface of silica gel (from
left to right) are: silanol (unmodified silica gel), primary amine,
carboxylate, and methyl………… …………………………….

55

Figure 4.5.
The change of Cu
2+
density on various silica gel surfaces
(unmodified silica gel, silica gel modified with primary amine
group, and silica gel modified with carboxylate group) after
Cu
2+
leaching experiment…………… ………………… ……



57

Figure 4.6.
Effect of the acidity of the surface ligand on the colorimetric
response of Cu
2+
with density of 0.5 mmol/ g silica gel to
ammonia vapor. The chemical functional groups on the surface
of silica gel (from left to right) are: sulfonate (pKa ~ 1), silanol
(pKa ~ 2), carboxylate (pKa ~ 4.5), and primary amine (pKa ~
10)……………………………………………………………….

61

Figure 5.1.
Effect of temperature on visible spectra of cholesteric liquid
crystals with inset showing cholesteric liquid crystals color
change from red (35°C) to green (37°C) and to blue (39°C)……


69




x

Figure 5.2.
Effect of temperature on (a) optical appearance and (b) peak
wavelength in visible spectra of PSCLCs with different rations
of NOA61 (0 – 30% w/w)……………………………… ……


71

Figure 5.3.
Effect of temperature on the optical responses of PSCLCs (with
5%, 10%, and 20% of NOA61) to 400 ppmv of octylamine. The
exposure time was 2 h……………………………………… ….

73

Figure 5.4.
Response of PSCLCs (20% NOA61) to a variety of organic
vapors at 400 ppmv including (a) alkane and various amines;
(b) esters, aldehyde, and primary alcohol vapors. (c)
Comparison of colorimetric response of PSCLCs (20%
NOA61) to primary amines and primary alcohols vapor of
similar molecular weight at 400 ppmv……… ………………





74

Figure 5.5.
(a) Comparison of the colorimetric responses of PSCLCs (20%
NOA61) at 25°C for butylamine, hexylamine, and octylamine.
(b) Shift in peak wavelength for PSCLCs (20% NOA61) at 25
ºC after the exposure to butylamine (solid triangle up),
hexylamine (solid square), and octylamine vapor (solid triangle
down) at different concentrations………….…………………





77

Figure 5.6.
Thermodynamic activities of primary amines having a carbon
chain length from C
4
to C
10
. The thermodynamic activity was
calculated as the vapor pressure (at the detection limits) divided
by its saturated vapor pressure.…………………………………



79

Figure 5.7.
Time series of optical responses of (a) a droplet and (b) thin
film of PSCLCs (20% NOA61) upon exposure to 200 ppmv of
octylamine vapor. The left part of the thin film is covered with
a glass slide as a control for temperature changes and the
uncovered right part shows responses……………………….….




80

Figure 6.1.
Images of various CLC thin films with various covers on the
half part of the thin films (top side of each image). CLCs doped

with 2 wt% dodecylamine with (a) a clean glass slides cover
and (d) a clean glass slide cover on top of PDMS thin film as
prepared in Scheme 6.1.b. Undoped CLCs with (b) a clean glass
slide cover and (c) a DMOAP-coated glass slide cover. The
surrounding temperature when the image is taken is indicated
below each image…………………….………………………….







91

Figure 6.2.
Dodecylamine dopant effect on the optical property of CLCs.
(a) Effect of increasing dodecylamine concentration (in wt%)
on the color uniformity of CLC thin films. (b) CLC thin film
doped with 2 wt% dodecylamine kept at 34 °C after 1 week
showing color stability without dodecylamine phase separation
or evaporation. The surrounding temperature when the image is
taken is indicated below each image…………………………….






93

xi


Figure 6.3.
Colorimetric responses of CLCs doped with 2 wt%
dodecylamine at 34°C after exposure to (a) water, (b) ethanol,
(c) acetone, (d) pentylamine, (e) pentyl alcohol, and (f) pentyl
aldehyde at the same concentration of 200 ppmv for 15 min…



94

Figure 6.4.
FTIR spectra showing the change of N-H stretching peak at
3334 cm
-1
, N-H bending peak at 1652 cm
-1
, and formation of
C=N peak at 1670 cm
-1
of CLCs with dodecylamine dopant in
the following conditions: (a) initial, (b) after 1.5 min exposure
to saturated pentyl aldehyde vapor, (c) after 5 min exposure to
pentyl alcohol vapor, and (d) after 5 min exposure to pentyl
amine vapor

95

Figure 6.5.
Dynamic colorimetric response and reversibility of a thin film
of CLCs doped with 2 wt% dodecylamine after exposing to (a)
300 ppmv of pentyl aldehyde vapor and (b) 2,000 ppmv of
pentyl aldehyde vapor at 34°C. (c) Colorimetric responses of a
thin film of CLCs doped with 2 wt% dodecylamine subjected to
four cycles of exposure to 2000 ppmv of pentyl aldehyde for 2
min followed by air for 90 min…………………….…… ……






97

Figure 6.6.
Shifts in peak wavelength of CLCs doped with 2 wt%
dodecylamine after exposure to pentyl aldehyde (solid
triangles), butyl aldehyde (solid squares), methyl aldehyde
(solid circles), and undoped CLCs to pentyl aldehyde (hollow
triangles) and butyl aldehyde (hollow squares) (a) for 0 - 1,000
ppmv of aldehyde vapor. The dashed square region is enlarged
in part b. (b) for 0 – 10 ppmv of aldehyde vapor………………

99

Figure 7.1.
Images of microfluidic channels with embedded PDCLCs after
the channels were filled with (a) ethanol, (c) water, and (d)
aqueous solution containing 100 g/ L sucrose inside the
channels for 1 min. (b) Reflectance visible spectra of PDCLCs
before and after dropping ethanol liquid on the PDCLC surface
with insets showing the PDCLCs respective optical appearance.





109

Figure 7.2.
Images of microfluidic channels with embedded PDCLCs when
the channels are filled with aqueous solution with different
ethanol concentrations………………………………… ………


110

Figure 7.3.

Images of microfluidic channels with embedded PDCLCs after
the channel was: (a) filled with 50% ethanol for 1 min, (b)
purged with nitrogen for 5 min and left in open air for 1 h, (c)
filled again with 50% ethanol for 1 min, (d) purged with
nitrogen again for 5 min and left in open air for 1 h…………….




111





xii


Figure 7.4.
Images of microfluidic channels with embedded CLCs after the
channels were filled for 1 min with (a) ethanol, (b) water, and
(c) aqueous solution containing 100 g/ L sucrose inside the
channels…………………………………………………………



112

Figure 7.5.
Images of microfluidic channels with embedded CLCs when

the channels are filled with various concentration (% v/v)
aqueous solution containing ethanol inside the channels……….


113

Figure 7.6.
Images of microfluidic channels with embedded CLCs after: (a)
filled with 50% ethanol for 1 min, (b) purged with nitrogen for
5 min and left in open air for 1 h, (c) filled again with 50%
ethanol for 1 min, (d) purged with nitrogen again for 5 min and
left in open air for 1 h…………………………………….……




114

Figure 7.7.
Images of microfluidic channels with embedded CLCs when
the channel was filled with fermentation solution containing
sucrose and yeast (a) at initial time and (b) after 12 h. (c) Shifts
in peak wavelength of CLC visible spectra by time when the
CLCs were immersed in the fermentation solution (solid
triangle facing up), sucrose solution without yeast (hollow
circle), and yeast solution without sugar (hollow triangle facing
down)……………………………………………………… …

115

Figure 8.1
(a) Quantification of an image of Cu
2+
on silica gel sample
before ammonia exposure. (b) Luminosity decreases with the
increasing ammonia vapor concentration……………………….


123











xiii


LIST OF TABLES

Table 5.1.

Amount of liquid analytes added inside bottles and their vapor
concentration………………… ………………………………….

68

Table 6.1.
Amount of liquid analytes added and concentration of the vapor
generated inside bottles……………………………………………

88

Table 8.1.
Summary of VOCs sensors described in this thesis.……….……
120

xiv

LIST OF SCHEMES


Scheme 4.1.

Complex formation of Cu
2+
to its surface ligand and ammonia
after vapor exposure. The Cu
2+
is deposited on (a) unmodified
silica gel, (b) modified with primary amine group, and (c)
modified with carboxylate group………………………………



56

Scheme 5.1.
Molecular structures of (a) cholesteryl benzoate, (b) cholesteryl
nonanoate, (c) cholesteryl oleyl carbonate………………… …

67

Scheme 5.2.
Mechanism of vapor diffusion into PSCLCs; (a) vapor diffuses
through the gap between glass slides into PSCLC droplet, (b)
vapor contacts the uncovered part of PSCLC thin film…………


72

Scheme 6.1.
Configurations of CLC-based gas sensors. A thin film of CLCs
is supported on a clean glass slide with two pieces of plastic
spacer placed on the side to control the thickness of the CLC
thin film. Half of the CLC thin film is protected by using a
cover glass slide to serve as an internal control (not permeable
to gas). (a) No PDMS film between CLC thin film and the
cover slide, (b) a PDMS film is used to separate the CLC thin
film and the cover slide………………………………………….







88

Scheme 6.2.
Arrangement of layers of CLC molecules doped with
dodecylamine near the boundary with (a) air and (b) a clean

glass slide. The presence of dodecylamine adsorbed on the
clean glass slide decreases the tilt angle of CLC layers (
α
) and
reduces the effective pitch (P’) for light reflectance. The
effective pitch is a function of CLC pitch (P) as P’ = P cos
α






91

Scheme 7.1.
Preparation of (a) the first configuration, microfluidic channels
with embedded PDCLCs and (b) the second configuration,
microfluidic channels with embedded CLCs……………………


107

xv

NOMENCLATURES
Abbreviations
APES
3-aminopropyltriethoxysilane
CES
carboxyethylsilanetriol
CLCs
cholesteric liquid crystals
DIMP
diisopropyl methyl phosphonate
DIPA
diisopropylamine
DIPEA
diisopropyl ethylamine
DMMP
dimethyl methylphosponate
DMOAP
N, N Dimethyl- N-octadecyl-3-aminopropyltrimethoxysilylchloride
EDA
ethylene diamine
FET
field effect transistors
FTIR
Fourier Transform Infrared

HSPS
3-hydroxysilyl-1-propane sulfonic acid
ICP-OES
inductively coupled plasma – optical emission spectroscopy
LED
light emitting diodes
LIF
laser induced fluorescence
MOS
metal oxide semiconductors
MPcs
metal phthalocyanines
MW
molecular weight
NOA
Norland Optical Adhesive
OTS
octadecyltrichlorosilane
PDCLCs
polymer dispersed cholesteric liquid crystals
PDMS
poly(dimethylsiloxane)
ppb
parts per billion
ppmv
parts per million by volume
PSCLCs
polymer stabilized cholesteric liquid crystals
PVA
poly(vinylalcohol)

QCM
quartz crystal microbalance
SAMs
self-assembled monolayers
SAW
surface acoustic wave
UV
ultra violet
Vis
visible

Notations
α

tilt angle of CLC layers
λ
wavelength
θ
light angle of incidence
K
binding constant
n
eff

effective refractive index of cholesteric liquid crystals
P
pitch of cholesteric liquid crystals
P’
effective pitch of cholesteric liquid crystals
T

temperature

1
CHAPTER 1
INTRODUCTION
Volatileorganiccompounds(VOCs)suchasalcohols,organoamines,andaldehydes
appear in many chemical products and are widely used as solvents or reactants in
industries. Many VOCs are considered as hazardous chemicals for health and
environment and it is important to detect and monitor VOCs. Because of its
importance, chemical sensing of VOCs has attracted much attention and many
chemicalsensorsforVOCshavebeendeveloped.ThechemicalsensorsforVOCsare
often fabricated as combination of chemical sensitive layers containing molecular
receptors with transducers. In VOCs detection, interactions between chemical
sensitivelayersandVOCscreatechemicalsignalsthatareconvertedintoobservable
signalsthroughthetransducers(Edmonds,1988,SchultzandTaylor,1996,Jameset
al.,2005).Nevertheless,challengessuchashowtoproducelowcost,portableVOCs
sensorswith goodsensoryperformanceremain. Thesechallengesmotivateresearch
and development of VOCssensors described in this thesis. Below, an overview of
how tousemetalcompoundsand cholesteric liquid crystals(CLCs) for developing
VOCs sensors is provided. Furthermore, a more detailed review of VOCs sensors,
metalcompoundsandCLCsapplicationsforchemicalsensingispresentedinChapter
2.
1.1. Application sofmetalcompoun dsforchemicalsensing
InthedevelopmentofVOCssensors,molecularreceptorswhichbindspecificallyto
VOCsare needed.Metalcompoundsareoneclassofmolecularreceptorsthat have
2
received great interest in recent years. Theyare capable of binding with N or S
containing VOCs such as organoamines, thiols, ammonia through metalligand
interactions (Hierlemann et al., 1999, D'Amico et al., 2000). In addition, metal
compounds are available in a large number and theyoffer a wide range of metal

ligand interaction strength. These metal compounds can be utilized as molecular
receptorsanddevelopedasarraysofmultiplesensing elementsforVOCssensing.
Recent studies have demonstrated the utilization of metalloporphyrins spotted on
reversephasesilicagelplatesascolorimetricarraysforVOCsdetection(Rakowand
Suslick, 2000, Rakow et al., 2005). Metalloporphyrins are macrocyclic compounds
with metal ions inside theircavities and the interactions of metalloporphyrins with
VOCs are accompanied by colorimetric responses. By utilizing metalloporphyrins,
thesestudiesbringforththepossibilityofmakingmicroarrayswithmultiplesensing
elements.Thesemicroarrays givedirectcolorimetricsignalswithoutdependencyon
electric power, whichreducesoperational cost and improvesthe sensorportability.
These studies were focused on metalloporphyrins, but colorimetric responses from
othertypesofmetalcompoundssuchasmetalphthalocyanines,which havesimilar
macrocyclic structure, have not received the same level of attention as
metalloporphyrins.
Furthermore, metal compounds need to be immobilized on sensor surfaces to
constructsensitivelayers.Thisisoftenaccomplishedbymodifyingthesurfaceswith
ligands such as carboxylates or amines (Kepley et al., 1992, Zhang et al., 2000).
Although metal compounds immobilized on surfaces through metalligand
interactionshave high stability, it isstill unclear howthesurface ligands affectthe
3
performanceofmetalcompoundsasmolecularreceptorsforbindingwithVOCs.
1.2. Applications of cholesteric liquid crystals (CLCs) for chemic al
sensing
ChemicalsensorsforVOCsalsocanbedevelopedby usingcholestericliquidcrystals
(CLCs).CLCmoleculesarrangeinlayerswithorientationalorderofeachsubsequent
layerrotatesasmallangleandformsahelicalpattern(Collings,2002,Oswaldand
Pieranski,2005).Thedistancetocreate360°rotationoronehelicalpatterniscalleda
pitch.BecauseCLCshaverepeatedhelicalstructureswithperiodicdistanceofhalfof
the pitch, CLCs exhibit constructive interference and selectively reflects light with
wavelength depending on the pitch (Fergason, 1964). When the reflected light has

wavelengthinvisiblelightrange,theCLCsappearcolorfultothenakedeye.
SeveralstudieshavereportedtheapplicationsofCLCsforchemicalsensing.Inthese
studies, CLCs show colorimetric responses upon exposure to VOCs because the
VOCscan dissolveinCLCstocreateatorqueonCLCs.Thiseventisknowntocause
changes in the pitch and color of CLCs (Dickert et al., 1994, Rey, 1997). The
colorimetricresponsesofCLCsdependonthemolecularsizeofVOCs.LargerVOCs
molecules have stronger influence on CLCs, and that results in more significant
colorimetricresponses(Dickertetal.,1992).Nevertheless,theselectivityofCLCto
anyparticularVOCsisstillunclear.
In addition, there are other challenges in the development of useful CLCbased
sensorsfordetectingVOCs.First,althoughCLCsarerelativelyviscous,they stillcan
4
flowfreelylikeliquid.ThisfeaturemakesitdifficulttoprepareportableCLCsensors
with good stability and durability. Second, the pitch and the color of CLCs are
affected by temperature (deGennes, 1974, Kelker and Hatz, 1980), and CLC only
showsvisiblecoloroverasmalltemperaturewindow.Thisrequiresthemaintenance
ofaconstanttemperaturewhenCLCsensorsareusedtomonitorVOCs.Sofar,there
isstillnostrategytoaddressthetemperaturesensitiveissue.
1.3. Objectives
The overall objective of this thesis is to develop VOCs sensors by using metal
compoundsandCLCs.Furthermore,thisthesiscanbeoutlinedasfollows:
1. The first objective of this thesis is to develop portable sensors based on metal
compounds for the detection of primary amines vapor. In chapter 3, metal
phthalocyanines (MPcs), which have a similar macrocyclic structure to
metalloporphyrins,arestudied.WeinvestigatecolorimetricresponsesofMPcsto
hexylamineandidentify suitableMPcsasmolecularreceptorsfordetectionofthe
vaporofhexylamine.Then,thepossibilityofincorporatingMPcsinthinpolymer
filmsasportablegassensorsisstudied.
2. The second objective of this thesis is to develop better understanding on the
effectsofsurfaceligandson theperformanceofmetalionsasmolecularreceptors.

InChapter4, theeffects ofthesurfaceligandsonthecolorimetricresponses of
Cu
2+
to ammonia vapor are studied. In addition, several factors which may
influencethecolorimetricresponsesofCu
2+
toammonia,includingtheeffectsof
metalionsdensityon thesurfaceandanionsofmetalsaltsarestudied.
3. ThethirdobjectiveistodevelopVOCssensorsbasedonCLCs.First,weaddress
5
existing issues in CLCbased gas sensors including the fluidity of CLCs and
limitedworkingtemperaturerangeofCLCs.Inchapter5,weshowthatby adding
asmallamountofpolymertoCLCs,polymerstabilizedcholestericliquidcrystals
(PSCLCs)canbeformed.Theeffectofpolymeradditiontotheopticalpropertyof
CLCs and the temperature range of CLCs are studied. Then, utilization of
PSCLCs as a sensor arrayto address the issue of limited working temperature
rangeisstudied.Inaddition,thecolorimetricresponsesofCLCstovariousVOCs
arestudiedinordertoassesstheselectivityofCLCstotargetedVOCs.
4. Next, to improve the selectivity of CLCs to aldehyde vapor, dodecylamine as
suitablemolecularreceptorsareincorporatedintoCLCsasdopants.Thisstudyis
describedinChapter6.Inthischapter,wealsostudiedeffectsofthedopantson
thestability ofCLCs,colorimetricresponsesofthedopedCLCstovariousVOCs,
and molecular interactions between the dopants in CLCs and VOCs by using
infraredspectrometry.
5. Recently, microfluidic devices have attracted a lot of attention for various
applicationsincluding nanomaterialssynthesis, organiccompounds synthesis,or
bioprocess. In these applications, the microfluidic devices require integrated
detection and monitoring of chemicals inside the microfluidic channels.
Nevertheless,monitoringVOCsinsidemicrofluidicchannelsarestillchallenging
tasksbecause common analytical instrumentscannot be easily miniaturized and

suitable miniaturized VOCs sensors may require complex fabrication and
alignmentonthemicrofluidicdevices.Therefore,thelastobjectiveofthisthesisis
tostudythepossibilityofintegratingCLCsfordetectionandmonitoringofVOCs
inside microfluidic channels. In chapter 7, we utilize thin layers of CLCs and
integrated them to microfluidic devices.We study thecolorimetricresponsesof
6
the CLCs toethanolas amodelofVOCs inside thechannels.Furthermore, we
studythefeasibilityofusingCLCsformonitoringethanolfromrealfermentation
occurringinsidethechannels.
7
CHAPTER 2
LITERATUREREVI EW
Inchapter 1, we briefly introduce metalcompounds and cholesteric liquid crystals
(CLCs)fordevelopingVOCssensors.Inthischapter,weprovideliteraturereviewon
chemicalsensorsforVOCs,utilizationofmetalcompoundsasmolecularreceptors,
andutilizationof CLCsforVOCssensing.
2.1. ChemicalsensorsforVOCs
Chemicalsensorshavemanycivilandmilitaryapplicationssuchaspollutioncontrol
and environmental safety monitoring. Chemical sensors are defined by IUPAC as
devices that transform chemical information into analytically useful signals
(Hulanicki et al., 1991). They have two major parts which are chemical sensitive
layersandtransducers.Interactionsofchemicalsensitive layerswithtargetanalytes
generatechemicalsignalswhichareconvertedintosomemeasureablesignalsbythe
transducers(Edmonds,1988,SchultzandTaylor,1996,Jamesetal.,2005).Basedon
theirtransductionmechanism,chemicalsensorscanbeclassified aselectrochemical
sensors,masssensors,or opticalsensors(SchultzandTaylor,1996).
2.1.1.El ectrochemicalsensors
Electrochemical sensors report changes in electrical properties caused by target
analytes interactions. Common types of electrochemical sensors for VOCs are
chemiresistors and potentiometric sensors. Chemiresistors are commonly made of

8
metal oxide semiconductors (MOS) or conducting polymers. First, MOS based
sensorscommonlyutilizentypesemiconductorssuchasSnO
2
,ZnO,andFe
2
O
3
.They
changetheir conductivity whenreduciblegasessuchas H
2
, CH
4
, CO, or H
2
Sreact
withoxygenadsorbedonthesurfaceofthemetaloxidesat200–500°C(Nantoand
Stetter,2003).TheMOSsensorshavebeendemonstratedtodetectcombustibleVOCs
suchasalkanesandalcohols(Getinoetal.,1999,Leeetal.,2001,Maekawaetal.,
2001,Tomchenko et al.,2003).TheresponsesofMOSsensorstovapordependon
the operating temperature and the activity of metal oxide. In order to improve the
activityofmetaloxide,catalyticmetaladditives havebeenutilizedin somestudies
(Maekawa et al., 2001, Lee et al., 2002) . However, MOS sensors require high
temperaturetoavoidwateradsorptiononthesurfaceofthemetaloxides.Inaddition,
MOS sensors still have poor selectivity because they respond to many types of
combustiblegases(NantoandStetter,2003).
Second,conductingpolymersbasedchemiresistorscommonlyutilize polymerswith
conjugatedπelectronbackbonesuchaspolypyrrole,polyaniline,polythiophene,and
polyacetylene. Conducting polymers change their conductivity upon exposure to
gasesorVOCs(Jamesetal.,2005).Interactionsofconductingpolymerswithgases

canbeclassifiedaschemicalreactionornonreactivephysisorption.Inthefirsttype
ofinteraction,chemicalreactionoccursasoxidationontheconductingpolymersby
electronacceptorsgases(NO
2
,O
2
,O
3
)orreductionontheconductingpolymersby
electron donating gases (H
2
S, NH
3
). The oxidation or reduction on the conducting
polymerschangestheconductivityoftheconductingpolymers(Lange etal.,2008).
In the second type of interaction, physisorption of VOCs creates several effects
leading to conductivity changes in the conducting polymers. For example,