Fabrication of liquid crystal based sensor for detection of hydrazine vapours

Article abstract of DOI:10.1016/j.cplett.2014.09.020

A novel liquid crystal (LC) based sensor to detect trace level amount of hydrazine vapour has been developed. The LC 4′-pentyl-4-biphenylcarbonitrile (5CB) doped with 0.5 wt% 4-decyloxy benzaldehyde (DBA) shows dark to bright optical texture upon exposure

Full text of DOI:10.1016/j.cplett.2014.09.020

Chemical Physics Letters 614 (2014) 62–66
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Fabrication of liquid crystal based sensor for detection of hydrazine
vapours
Rajib Nandia,1, Sachin Kumar Singhb,1, Hemant Kumar Singhb,
Bachcha Singhb,∗, Ranjan K. Singha,∗
a Department of Physics, Banaras Hindu University, Varanasi 221005, India
b Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2014
In final form 6 September 2014
Available online 16 September 2014
A novel liquid crystal (LC) based sensor to detect trace level amount of hydrazine vapour has been devel-
oped. The LC 4ꢀ-pentyl-4-biphenylcarbonitrile (5CB) doped with 0.5 wt% 4-decyloxy benzaldehyde (DBA)
shows dark to bright optical texture upon exposure of hydrazine vapours as revealed by polarizing opti-
cal microscopy under crossed polarizers. The hydrazine interacts with the doped DBA and form diimine
compound which disrupt the orientation of aligned 5CB. The interaction between DBA and hydrazine has
been also studied by Raman spectroscopy.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
and electrical power. It has long been known that LCs can be used for
real-time detection of hazard organic vapours [14,27–35]. The basic
Hydrazine (H2N NH2) is widely used in many chemical and
pharmaceutical industries as rocket propellant, reactant in military
fuel cells, polymerization catalyst, blowing agent, reducing agent in
organic reaction, oxygen scavenger in boiler feed water to inhibit
corrosion and in photography [1,2]. It is a highly toxic and harmful
environmental pollutant [3,4]. It is desirable to development a reli-
able, sensitive, economical and selective analytical method for the
determination of trace level of hydrazine especially in environmen-
tal samples and industry. There are several effective techniques for
the determination of trace level hydrazine such as titrimetric [5],
chromatography–mass spectrometric [6], electrochemical meth-
ods [7], spectrophotometric methods [8], flow injection analysis [9],
high-performance liquid chromatography (HPLC) [10] and poten-
tiometry [11]. However most of these methods involve complex
instrumentation, time consuming procedures and high cost.
Liquid crystals (LCs) have numerous potential applications
mainly in the field of displays and electro-optics. In last one decade,
another important application of LCs is emerging in the field of
chemical and biological sensors [12–35]. The main advantage of LC
based sensor is that the optical signal produced can be visualized
easily with the naked eye without any need of complex instruments
principle of LC based sensor for organic gas detection with high
sensitivity is orientational transitions of LC thin films supported
on chemically functionalized surfaces upon exposure of gas. Shah
and Abbott et al. [14,27] reported a LC sensor for hexylamine gas
by showing homeotropic to planar (tilted) transition of 4ꢀ-pentyl-
4-biphenylcarbonitrile (5CB) molecules supported on a carboxylic
acid decorated surface when it was exposed to hexylamine vapour.
In this system, the reaction between hexylamine and carboxylic
acid groups breaks the hydrogen bonds between 5CB and carboxylic
acid group that results in the in-plane orientational transition of
5CB. Bi et al. [30] developed a LC based sensor for vapourous glu-
taraldehyde based on orientational transition of 5CB due to the
reaction between glutaraldehyde and amine decorated on surface.
The same group in another study developed a LC sensor for buty-
lamine gas in air [32]. In this system lauric aldehyde doped 5CB
shows bright to dark optical response to butylamine vapour based
on reaction between aldehyde and amine. Xu et al. [34] developed
a LC sensor for detecting vapour thiols based on the orientational
transitions of 5CB driven by the binding of thiol molecules to immo-
bilize copper ion. Optical sensors based on cholesteric LCs for the
detection of amines were also reported [36,37].
In this letter we are reporting a novel LC-based sensor to detect
hydrazine vapour by studying optical response of 5CB doped with
4-decyloxy benzaldehyde (DBA). The hydrazine vapour interacts
with the DBA doped in 5CB to form diimine compound that is
observed as change in texture of 5CB from dark to bright by
polarizing optical microscopy (POM). We further concentrated to
∗
Corresponding author.
E-mail addresses: bsinghbhu@rediffmail.com (B. Singh),
ranjanksingh65@rediffmail.com (R.K. Singh).
1
Main workers with equal contribution.
http://dx.doi.org/10.1016/j.cplett.2014.09.020
0009-2614/© 2014 Elsevier B.V. All rights reserved.

R. Nandi et al. / Chemical Physics Letters 614 (2014) 62–66
63
understand the interaction between DBA and hydrazine by Raman
spectroscopy, and how it causes orientational transition in 5CB. This
study provides a principle for developing a real-time simple, fast
and inexpensive chemical sensor for hazardous chemical vapours
like hydrazine.
was kept in an oven at 125 ◦C to evaporate hydrazine into vapour.
The syringe was cooled down to room temperature (28 ◦C) before
exposing to LC sensors. The vapour concentration of hydrazine was
controlled by changing the hydrazine concentration in the solution
[34].
2. Experimental
2.6. Synthesis of diimine product
2.1. Materials
N,Nꢀ-bis(4-decyloxybenzylidene)azine: The diimine product
was prepared by shaking together absolute ethanolic solution of
4-decyloxy benzaldehyde (DBA) (2 equiv.) and hydrazine monohy-
drate (1 equiv.) for 10 min and leaving solution for 30 min in a flask
closed with guard tube. The microcrystalline pale yellow solid was
filtered off by suction, thoroughly washed with cold ethanol and
recrystallized from absolute ethanol and dried at room tempera-
ture.
A polarized light microscope (HT 30.01 NTT 268 Lomo) was used
to capture image of optical textures of nematic 5CB. The LC samples
were placed on a circular stage between two crossed polarizers. All
images were obtained using a 4× objective lens between crossed
polarizers. Images of the optical appearance of each liquid crystal
cell were captured with a digital camera (DS-2Mv, Nikon, Tokyo,
Japan) that was attached with the polarized light microscope. All
images were captured with an exposure time of 40 ms at a resolu-
tion of 1600 × 1200 pixels and a shutter speed of 1/10 s. Textures
were quantified by interpreting them to a grey scale of intensities,
and their luminosities were analyzed using image processing soft-
ware Adobe Photoshop [38]. Raman spectra of all samples were
recorded on a micro-Raman setup from Renishaw, UK equipped
with a grating of 2400 lines/mm and a peltier cooled charge cou-
pled device (CCD). The excitation source was 514.5 nm line of Ar+
laser. The spectral resolution of the spectrometer with 50 ␮m slit
opening was ∼1 cm−1. Other details of spectrometer were given
elsewhere [39].
Glass microslides were obtained from Blue Star, Mumbai, India.
Nematic liquid crystal 5CB, dimethyloctadecyl[3-(trimethoxysilyl)-
propyl] ammonium chloride (DMOAP), 4-hydroxy benzaldehyde
and hydrazine monohydrate were purchased from Sigma–Aldrich
(India) and were used as received. Transmission electron micro-
scope grids of copper (100 square mesh) were purchased from
Thorlabs GMBH, Munich, Germany. All solvents used in this study
were purchased from Merck chemicals and are HPLC grade.
2.2. Synthesis of 4-decyloxy benzaldehyde (DBA)
4-Decyloxy benzaldehyde (DBA) was prepared by reaction of
4-hydroxy benzaldehyde with decyl bromide in the presence of
mild base (potassium carbonate) in refluxing 2-butanone for 24 h.
The residue was filtered off and the solvent was removed from the
filtrate under reduced pressure. The compound was obtained as
yellow oil after purification by column chromatography. The com-
pound was fully characterized by various spectroscopic techniques,
and satisfactory analytical data were obtained (supplementary
information).
2.3. Preparation of DMOAP-coated glass slides
At first, glass slides were immersed in a 5% (v/v) Decon-90
(a commercially available detergent) solution for 2 h. Then, they
were rinsed with copious amounts of deionized water and cleaned
in an ultrasonic bath twice, each time for 15 min. Subsequently,
the slides were etched with a 4.0 M sodium hydroxide solution
for 30 min and rinsed thoroughly with deionized water, ethanol
and methanol and then dried under a stream of nitrogen. The
cleaned substrates were then stored overnight in an oven at 120 ◦C.
The glass slides were functionalized with DMOAP by dipping the
cleaned glass slides in 0.5% (v/v) DMOAP solution at room tempera-
ture for 20 min, followed by drying in a vacuum chamber according
to the reported procedures [23]. Copper grids (100 square meshes,
Electron Microscopy Sciences, Hatfield, PA) were cleaned sequen-
tially in ethanol and methanol, dried under nitrogen, and heated at
100 ◦C for 24 h.
3. Results and discussions
Copper grids placed on DMOAP coated glass slides were filled
with pure 5CB and 0.5 wt% DBA doped 5CB to study optical appear-
ance by POM. The optical micrographs are shown in Fig. 1a–b.
Fig. 1(a) shows a dark texture, signifying a homeotropic alignment
of 5CB molecules on DMOAP coated glass slide. In this configura-
tion LC has an air–LC interface and a LC–DMOAP coated glass slide
interface. The dark appearance of LC is caused due to perpendicular
orientation of 5CB molecules at both air and DMOAP coated slide
interfaces as well as throughout the LC film due to the hydropho-
bic interactions between DMOAP and LCs [40]. When the LC film
is placed between the crossed polarizer of POM, the director of
the LC is parallel to the direction of incident light and therefore
incident light experiences only one refractive index. The plane of
polarization of the light does not change on passing through the
LC. Therefore it appears dark in crossed polarizer [40]. On doping
with 0.5 wt% DBA in 5CB there is no effect on the alignment of 5CB
molecules as revealed by the dark texture as shown in Fig. 1(b) and
also there is no significant change in melting and clearing points.
On increasing the doping of DBA in 5CB the LC phase range reduces.
We have carried out all experiments with 0.5 wt% DBA doped 5CB.
In the next step we exposed the grid containing 5CB doped
with DBA by hydrazine vapour at different concentrations. All tex-
tures were taken after exposure of hydrazine vapour after 30 min.
Fig. 1(c) shows that upon exposure of 500 ppm of hydrazine vapour
on LC grid a fully bright texture is observed. On reducing the con-
centration of hydrazine vapour to 300 ppm, 200 ppm and 100 ppm,
brightness of LC textures are reduced as shown in Fig. 1d–f. On
further lowering the concentration of hydrazine vapour to 50 and
2.4. Preparation of DBA doped 5CB and fabrication of LC sensors
The nematic LC 5CB doped with different concentrations of DBA
was prepared by diluting 10 wt% of DBA doped 5CB with pure 5CB
at different ratios. Clean glass slide was cut into small squares
(1.0 cm × 1.0 cm each). Then, an empty copper grid was placed on
the top of the slide and approximately 0.5 ␮L of DBA doped 5CB was
dispensed into the copper grid. The excess mixture was removed
by touching the grid surface with a capillary tube. The sensor thus
prepared was then placed inside a small laboratory-made chamber
linked to the gas generator.
2.5. Preparation of hydrazine vapours from its solutions
To prepare hydrazine vapour, 0.5 ␮L hydrazine in aqueous solu-
tion was taken inside a 100 mL syringe. To avoid leakage the
opening of the syringe was sealed with a Teflon® plug. The syringe

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R. Nandi et al. / Chemical Physics Letters 614 (2014) 62–66
Fig. 1. Polarizing optical images of (a) pure 5CB and (b) 5CB doped with 0.5 wt% DBA. Optical images of 5CB doped with 0.5 wt% DBA when they were exposed to hydrazine
vapours at concentrations (c) 500 ppm, (d) 300 ppm, (e) 200 ppm, (f) 100 ppm, (g) 50 ppm and (h) 30 ppm. All images were taken on stabilization after 30 min at temperature
28 ◦C.
30 ppm, the area and intensity of bright texture are decreased.
The luminosity of 5CB doped DBA upon exposure of hydrazine
vapour at different concentration has been calculated consider-
ing the luminance of the image of the 5CB doped with 0.5 wt%
DBA without hydrazine vapour as minimum (0%) and the lumi-
nance of the same upon exposure by 500 ppm hydrazine vapours
as maximum (100%) as shown in Fig. 2. Based on thrice repeated
measurements the mean value of luminosity and standard error at
different concentration of hydrazine vapours are reported in Fig. 2.
The lowest detected limit of hydrazine concentration for this sen-
sor is 30 ppm. The bright optical appearance of 5CB indicates either
planar or tilted orientation of 5CB molecules at LC–air interface.
In this condition the tilt angle of 5CB relative to the surface normal
varies continuously (through splay and bend deformations) from 0◦
at the LC–DMOAP interface to 90◦ at the air–LC interface. The bire-
fringence of liquid crystal rotates the polarization of the incident
light and produces in general elliptically polarized light, resulting in
a bright optical appearance between two crossed polarizers. Thus,
we can conclude that these bright textures in Fig. 1c–h were due
to exposure of hydrazine vapour. From these experiments, we can
identify the presence of hydrazine.
After exposure of hydrazine vapours the bright optical texture
of LC cell was caused by the disruption of 5CB due to reac-
tion between DBA and hydrazine. The LC sensor mechanism is
illustrated in Fig. 3. The reaction between DBA and hydrazine is
very fast and sensitive, which leads to formation of diimine com-
pound (N,Nꢀ-bis(4-decyloxybenzylidene)azine). It is worth noting
that hydrazine is a symmetrical molecule with two amine groups.
Therefore, both the amine groups have equal reactivity towards
aldehydes and the reaction always favours the formation of diimine
products at normal condition of temperature. The calculated dipole
moment of optimized structure of DBA molecule is 5.00 D and
that of diimine molecule is almost zero (supplementary informa-
tion). The 5CB doped with DBA gives the dark texture because DBA
molecule has a significant dipole moment to align it perpendicu-
larly to the surface along with the 5CB molecules. But the diimine
molecule having long hydrocarbon chains at both terminals has
negligible dipole moment, enough to disrupt the alignment of 5CB
molecule at LC/air surface. The change in surface energy after expo-
sure to hydrazine leads to parallel/tilt alignment of 5CB orientation.
In this process, the main factors that are responsible to detect the
hydrazine vapours are; the reaction kinetics between aldehyde and
hydrazine, and the reorientations of 5CB molecules. To understand
further details of the reaction between DBA and hydrazine, we have
performed Raman spectroscopic measurements. The diimine prod-
uct was also synthesized separately to identify the Raman marker
bands of the product formed due to interaction of hydrazine vapour
with doped DBA when exposed to the LC sensors. The Raman
spectra of pure 5CB, DBA doped 5CB and DBA doped 5CB after
exposure of hydrazine along with the prepared pure diimine doped
Fig. 2. Luminosity (L) of the optical images of 0.5 wt% DBA doped 5CB upon expo-
sure of hydrazine vapours at different concentrations calculated using the equation L
(%) = (S − Smin/Smax − Smin) × 100, where S is the raw luminance of each image, Smin is
the luminance of the image of the 5CB doped with 0.5 wt% DBA and Smax is the lumi-
nance of images of 5CB doped with 0.5 wt% DBA and exposed to 500 ppm hydrazine
vapours. In every set Smax was taken as 100%, therefore we do not report error bar
for this data point.

R. Nandi et al. / Chemical Physics Letters 614 (2014) 62–66
65
Fig. 3. Schematic illustration of orientational transitions of the 5CB molecules doped with DBA after exposure of hydrazine vapour into the optical cell from homeotropic
orientation to planar/tilted orientation.
(0.25 wt%) in 5CB are shown in Fig. 4. In Raman spectrum of DBA
doped 5CB the band at 1686 cm−1 is due to the stretching vibration
of C O group associated with DBA molecule. Complete disappear-
ance of the 1686 cm−1 band and appearance of a new weak band
at 1548 cm−1 are observed in the DBA doped 5CB on exposure with
hydrazine (Fig. 4). The absence of C O band (∼1686 cm−1) in doped
5CB after exposure of hydrazine indicates condensation of aldehyde
to hydrazine and therefore diimine product formation (Fig. 3). The
Raman spectrum of pure diimine (0.25 wt%) doped in 5CB is similar
as the spectrum of the DBA doped 5CB on exposure with hydrazine.
In both spectrums a new band at 1548 cm−1 is observed which is
due the diimine product as a result of reaction between DBA and
hydrazine. It is also to be noted that the intensity of the 1548 cm−1
band for pure diimine (0.25 wt%) doped in 5CB is greater by a very
small amount than the spectrum of the DBA doped 5CB on expo-
sure with hydrazine because the doped percent of DBA is only 0.5%
in 5CB and the yield of diimine derivatives are ∼70–80% at room
temperature conditions. Therefore very low amount (≤0.25%) of
diimine product is formed during hydrazine sensing experiments.
We have exposed decylamine vapours and secondary diethy-
lamine on 5CB doped with 0.5 wt% DBA to study selectiveness of
the sensor. From this study we have found no significant optical
appearance at concentration 1000 ppm under crossed polarizers as
shown in Fig. S2 (supplementary information). Upon exposure of
decylamine and diethylamine vapours on LC cell, there is a possibil-
ity of formation of imine products with the reaction of DBA doped
in 5CB. These imine molecules have a significant dipole moment
and therefore have less capability to disturb the alignment of 5CB
molecules.
4. Conclusions
First time, a LC based sensor is developed for detecting hydrazine
vapours with high specificity. The principle behind the sensor is
based on the fact that DBA doping in 5CB react with hydrazine
to form diimine compound which disturb the orientation of uni-
formly aligned 5CB molecules and consequently dark to bright
optical response are observed under crossed polarizers in POM. This
is a convenient method of detecting hydrazine vapours in air and
also distinguishing from other volatile amine compounds and the
results can be observed with the naked eye without using addi-
tional instrumentation. The lowest detection limit for this sensor is
∼30 ppm and further work will be done to achieve more sensitivity.
Acknowledgements
RN is grateful to the University Grants Commission (UGC), New
Delhi, India for providing fellowship under Research Fellowship
in Sciences for Meritorious Students. SKS, HS and RKS are grate-
ful to Council of Scientific and Industrial Research (CSIR), New
Delhi, India, for financial support. SKS and RKS are grateful to DFG,
Germany. BS is grateful to DST, India (Project No. SR/S1/IC-08/2012)
for providing financial assistance.
Fig. 4. The Raman spectra of pure 5CB, 5CB doped with DBA, 5CB doped with
DBA after exposure of hydrazine vapours (500 ppm) and externally prepared pure
diimine product doped (0.25 wt%) in 5CB.

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R. Nandi et al. / Chemical Physics Letters 614 (2014) 62–66
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