Irreversible visual sensing of humidity using a cholesteric liquid crystal

Article abstract of DOI:10.1039/c2cc16934g

Irreversible optical sensing of humidity by a doped cholesteric liquid crystal is achieved by using a thin film of nematic host E7 with a binaphthylorthosilicate ester as dopant (guest). The film changes its color from blue (to green to orange to red) to colorless when exposed to humidity as the dopant is hydrolyzed. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc16934g

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COMMUNICATION
Irreversible visual sensing of humidity using a cholesteric liquid crystalwz
Abhijit Saha,a Yoko Tanaka,a Yang Han,a Cees M. W. Bastiaansen,b Dirk J. Broerb and
Rint P. Sijbesma*a
Received 8th November 2011, Accepted 15th March 2012
DOI: 10.1039/c2cc16934g
Irreversible optical sensing of humidity by a doped cholesteric
liquid crystal is achieved by using a thin film of nematic host E7
with a binaphthylorthosilicate ester as dopant (guest). The film
changes its color from blue (to green to orange to red) to
colorless when exposed to humidity as the dopant is hydrolyzed.
reacted with the analyte, causing a change in helical twisting
power of the dopant and hence the color. When a thin layer of
CLC with the responsive dopant was exposed to an analyte
(CO2 or O2),19 the initial color changed within minutes. In this
communication, we describe an irreversible humidity sensor
(a so-called time–humidity integrator) based on a doped
cholesteric liquid crystal incorporating a chiral dopant that
reacts with water.
Monitoring the storage conditions of products such as food,
pharmaceuticals and chemicals is important for extending
shelf life, preserving freshness, improving quality, preventing
spoilage and/or avoiding use after spoilage.1 A convenient way
to monitor products is by the use of optical disposable sensors
built into packaging.2 Using optical sensors is advantageous
because color change is easily detected visually, obviating the
use of any electronic circuit or instruments.3,4 Moreover, with
suitable technology, the sensors may be printed on the packaging in
an economical process. Monitoring humidity in a way that records
the complete exposure history of a product with so-called time–
analyte integrators is important for packaging of food, drugs, and
chemicals and a range of other products. Several humidity sensors
have been reported, among which are a sensor based on photonic
ionic liquids,5,6 a colorimetric humidity sensor based on
encapsulation and dissolution of dye in urea,7 as well as one
based on amorphous Al2O3 nanotubes.8 However, these sensors
are reversible and real-time humidity conditions are indicated.
Cholesteric liquid crystals (CLCs)9–14 can be used as optical
sensors as they exhibit a reflection band that can be tuned by
appropriate chiral dopants. The helical structure of the cholesteric
or chiral nematic (n*) liquid crystals is induced by the interaction
between the nematic liquid crystalline host molecules and the
chiral guests (dopants).15–23 Hence, the colour of such materials
can be tuned either by proper selection of the helical twisting
power of the chiral dopant or its concentration. Recently, we have
demonstrated that optical sensors can be fabricated by dissolving
reactive chiral dopants in nematic liquid crystals. A CLC optical
sensor was obtained, for instance, in which the chiral dopant
Binaphthyl derivatives are known for their use as dopants
due to their high capability to induce a helical pitch in nematic
liquid crystals.24–27 This capability is usually expressed as
helical twisting power (HTP or b). Covalently bridged 1,1-
binaphthyl derivatives usually possess a higher HTP than open
chain analogs.27,28 This difference in HTP stimulated us to
synthesize the hydrolytically labile orthosilicate ester
2
(Scheme 1), and to compare its HTP to that of R-BINOL
(1) to establish the suitability of 2 as dopant for a CLC based
humidity sensor.
Bis-[binaphthylen-(2.20)-dioxy]-silan (2) (Scheme 1) was
synthesized from (R)-(+)-1,10-bi(2-naphthol) (R-BINOL; 1)
by stirring its solution in THF with SiCl4 in an inert atmosphere
at 0 1C.29
The hydrolytic lability of 2 was confirmed by exposing a
CDCl3 solution to humid air. A peak at d 5.1 ppm corres-
ponding to the phenolic proton of 1 was observed after 1 h
exposure of the solution to humid air (Scheme 1 and ESIz,
Fig. S1). The favorable thermodynamics of the hydrolysis and
the insolubility of the SiO2 produced guarantee irreversibility of
humidity sensors based on this reaction. Kinetics of hydrolysis
a Laboratory for Macromolecular and Organic Chemistry, Eindhoven
University of Technology, P.O. Box 513, 5600 MB, Eindhoven,
The Netherlands. E-mail: r.p.sijbesma@tue.nl
b Functional Materials and Devices, Eindhoven University of
Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands
w This article is part of the ChemComm ‘Chirality’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
details for CLC preparation, exposure to humidity, determination of
HTP, handedness, 1HNMR spectra of the dopant (before and after
exposure to humidity) and R-BINOL in CDCl3 and kinetics of
hydrolysis of the dopant. See DOI: 10.1039/c2cc16934g
Scheme 1 Synthesis of bis-[binaphthylen-(2.20)-dioxy]-silan (2) from
(R)-(+)-1,10-bi(2-naphthol) (1) and reaction of bis-[binaphthylen-
(2.20)-dioxy]-silan (2) with water to form (R)-(+)-1,10-bi(2-naphthol)
(1) and SiO2.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4579–4581 4579

of the dopant in THF was investigated by monitoring optical
rotation with time using a polarimeter (Fig. S2, ESIz). After
addition of water, first a sharp increase in specific rotation
from +52.12 at 1 min to +55.39 at 3.8 min was observed, then
a gradual decrease was noticed to +25.09 at 195 min. This
indicates that the dopant is hydrolysed in the presence of water.
The helical twisting power (HTP or b) of a dopant is defined
as b = (p ꢀ ee ꢀ c)ꢁ1, where p is the pitch, ee the enantiomeric
excess of the dopant, and c the concentration of the dopant.
HTPs of 1 and 2 in E7 were determined from the position of
the reflection band from the relationship l = p ꢀ n, where l is
the centered wavelength of reflection and n is the average
in-plane refractive index. Assuming a refractive index of 1.6,
b was calculated to be ꢁ34.9 mmꢁ1 using a 10.1 wt% solution
of 2 in E7, which has a reflection band at 454 nm and forms a
blue film. Compound 1 has a much lower solubility than 2 in
E7, and the HTP was determined by a modified Grandjean
Cano method30 on a 0.98% solution, which was prepared by
dissolving the dopant above the clearing point of E7 and then
measured at room temperature. The pitch was 3.22 mm and
the HTP of the sample was found to be ꢁ31.7 mmꢁ1 in E7.
At higher concentrations, 1 was not soluble in E7 and
precipitated. Hence, upon hydrolysis of 2 to 1, the helical pitch
in films of E7 is expected to change significantly. However, this
will only to a small extent be caused by the small change in
HTP, but predominantly by the low solubility of 1, which
results in precipitation of 1 from its host material.
Fig. 1 (top) Photographs indicating color changes upon exposure to
air (relative humidity of 75%) of a CLC film of E7 containing 10.1%
of 2. (bottom) Reflection spectrum of this film as a function of time.
The numbers in the graph are minutes of exposure time.
In order to establish whether diffusion of water through the
film is a rate limiting factor, kinetics of the pitch change were also
studied as a function of film thickness. Films with 10.1 wt%
dopant in E7 and thicknesses of 6 mm and 12 mm were prepared.
The position of the reflection band was monitored during
exposure to N2 with 75% RH. The time dependence of 1/lmax
is plotted in Fig. 2b. Both films showed a similar response to
exposure, which indicates that in this thickness range, the kinetics
of response is not a diffusion limited process, such as the diffusion
of water into the film.
In order to establish the use of the hydrolytic reaction as the
basis for a CLC sensor, a blue colored CLC film was prepared
by spreading E7 containing 10.1 wt% dopant (2) on a rubbed
triacetyl cellulose (TAC) foil using a wire-bar coater because
TAC foil is widely used in LCD as a protective layer31a and an
optical sensor for nitrites in water and food based on the use of
brilliant cresyl blue immobilized on a triacetyl cellulose
membrane was also reported.31b This film was exposed to
nitrogen (N2) with a 75% relative humidity (RH) (at 22 1C,
100% humid air contains 19.416 g mꢁ3 of water) and the change
in the visible reflection band was continuously monitored. The
initial blue reflection band (peak maximum at 454 nm) of the
CLC film turned to orange-red and finally to colorless within a
few minutes. After 1 h, the reflection band of the CLC had
completely disappeared from the visible wavelength region i.e. the
reflection shifted to the near infrared and its intensity decreased to
some extent. When a CLC film was exposed to dry N2 (i.e., N2
with r0% RH) for 1 h, no change in the position or intensity of
the reflection band was observed, indicating that the reaction
requires an external source of moisture to proceed.
The results indicate that thin films based on a nematic liquid
crystal and a humidity responsive chiral dopant act as an
irreversible humidity sensor with a limited cross-sensitivity. At
room temperature, the thin films are in the chiral-nematic
phase and, consequently, the films tend to exhibit flow and a
limited mechanical coherence. Several routes were developed
in the past to circumvent such issues by incorporating a
network or gel via solvent, temperature or polymerization
induced phase separation.32
In conclusion, we have demonstrated an optical humidity
sensor based on a cholesteric liquid crystal with a dopant
designed to hydrolyze irreversibly. Under humid conditions,
hydrolysis of dopant 2 to the sparingly soluble product 1 leads
to an increase in the pitch of the CLC. As a result, the color of
the films changed from blue to colorless. It was also found that
sensing behavior was independent of film thickness which
Next, the influence of relative humidity (RH) on response time
of the CLC sensor was investigated. A CLC film with 10.1 wt%
dopant was exposed to N2 with RHs of 85, 75, 65, 47, 27 and
7%. This resulted in a continuous shift of the reflection band of
the films and ultimately all films became colorless (Fig. 1).
However, the rate of the shift is strongly dependent on the value
of RH. At a relative humidity of 75%, the initial reflection peak
at 454 nm (0 min) shifted to 821 nm within 10 minutes, whereas
at RH = 7%, this took nearly 60 min. In Fig. 2, the reciprocal
reflection peak maximum; 1/lmax is plotted against time, since
this directly gives a measure of change in HTP (b) with time. At
higher RHs, a faster change of color was observed.
Fig. 2 (a) Plot of the reciprocal reflection peak maximum (1/lmax) vs.
time for 10.1 wt% dopant in E7 upon exposure to six different RHs.
(b) Plot of the reciprocal reflection peak maximum (1/lmax) vs. time for
films of 10.1 wt% dopant in E7 with two different thicknesses upon
exposure to 75% RH.
c
4580 Chem. Commun., 2012, 48, 4579–4581
This journal is The Royal Society of Chemistry 2012

indicates that the kinetics are chemistry controlled and not
limited by diffusion of water vapor into the film.
This research forms part of the research program of the
Dutch Polymer Institute (DPI), project 625.
14 T. J. White, M. E. McConney and T. J. Bunning, J. Mater. Chem.,
2010, 10, 9832.
15 S. Pieraccini, A. Ferrarini and G. P. Spada, Chirality, 2008, 20, 749.
16 (a) R. Eelkema and B. L. Feringa, Org. Biomol. Chem., 2006,
4, 3729; (b) S. Pieraccini, S. Masiero, A. Ferrarini and G. P. Spada,
Chem. Soc. Rev., 2011, 40, 258.
17 K. Akagi, Chem. Rev., 2009, 109, 5354.
18 G. Solladie and R. G. Zimmermann, Angew. Chem., Int. Ed. Engl.,
´
1984, 23, 348.
19 Y. Han, K. Pacheco, C. W. M. Bastiaansen, D. J. Broer and
R. P. Sijbesma, J. Am. Chem. Soc., 2010, 132, 2961.
20 P. V. Shibaev, D. Chiappetta, R. L. Sanford, P. Palffy-Muhoray,
M. Moreira, W. Cao and M. M. Green, Macromolecules, 2006, 39, 3986.
21 J. Ma, Y. Li, T. White, A. Urbas and Q. Li, Chem. Commun., 2010,
46, 3463.
22 R. Eelkema and B. L. Feringa, J. Am. Chem. Soc., 2005, 127, 13480.
23 T. J. White, S. A. Cazzell, A. S. Freer, D.-K. Yang,
L. Sukhomlinova, L. Su, T. Kosa, B. Taheri and T. J. Bunning,
Adv. Mater., 2011, 23, 1389.
24 T. Mori, M. Kyotani and K. Akagi, Macromolecules, 2008,
41, 607.
25 M. Goh and K. Akagi, Liq. Cryst., 2008, 35, 953.
Notes and references
1 P. J. Louis, Packag. Technol. Sci., 1999, 12, 1.
2 D. B. Papkovsky, in Optical Chemical Sensors, NATO Science
Series, 2006, vol. 224, Part 2, pp. 501–514.
3 S. Neethirajan, D. S. Jayas and S. Sadistap, Food Bioprocess
Technol., 2009, 2, 115.
4 A. Mills, Chem. Soc. Rev., 2005, 34, 1003.
5 E. Tian, J. Wang, Y. Zheng, L. Jiang and D. Zhu, J. Mater. Chem.,
2008, 18, 1116.
6 J. Huang, C.-A. Tao, Q. An, C. Lin, X. Li, D. Xu, Y. Wu, X. Li,
D. Shen and G. Li, Chem. Commun., 2010, 46, 4103.
7 A. Mills, P. Grosshans and D. Hazafy, Analyst, 2010, 135, 33.
8 B. Cheng, B. Tian, C. Xie, Y. Xiao and S. Lei, J. Mater. Chem.,
2011, 21, 1907.
9 (a) S.-T. Wu and D.-K. Yang, in Reflective Liquid Crystal Displays,
Wiley, West Sussex, UK, 2001; (b) H.-S. Kitzerow and C. Bahr, in
Chirality in Liquid Crystals, Springer-Verlag, New York, 2001.
10 P. J. Collings and M. Hird, in Introduction to Liquid Crystals:
Chemistry and Physics, ed. G. W. Gray, J. W. Goodby and
A. Fukuda, Taylor & Francis, London, 1997.
´
26 G. Gottarelli, M. Hibert, B. Samoni, G. Solladie, G. P. Spada and
R. Zimmermann, J. Am. Chem. Soc., 1983, 105, 7318.
27 G. Gottarelli, G. P. Spada, R. Bartsch, G. Solladie and
´
R. Zimmermann, J. Org. Chem., 1986, 51, 589.
28 A. Ferarrini, G. J. Moro and P. L. Nordio, Phys. Rev. E: Stat.
Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 1996, 58, 1748.
11 (a) K. Ichimura, Chem. Rev., 2000, 100, 1847; (b) N. Tamaoki, Adv.
Mater., 2001, 13, 1135.
29 R. Muller and L. Heinrich, Chem. Ber., 1961, 94, 2225.
¨
12 T. Iketa and A. Kanazawa, in Molecular Switches, ed. B. L. Feringa,
Wiley-VCH, Weinheim, Germany, 2001.
30 I. Dierking, in Textures of Liquid Crystals, Wiley-VCH, Weinheim,
Germany, 2003.
13 (a) J. Vicario, N. Katsonis, B. S. Ramon, C. W. M. Bastiaansen,
D. J. Broer and B. L. Feringa, Nature, 2006, 440, 163; (b) B. L. Feringa,
N. P. M. Huck and A. M. Schoevaars, Adv. Mater., 1996, 8, 681;
(c) C. Denekamp and B. L. Feringa, Adv. Mater., 1998, 10, 1080.
31 (a) H. Mori, High performance TAC film for LCDs, Proc. SPIE,
2006, 6135, 613503; (b) A. A. Ensafi and M. Amini, Sens. Actuators,
B, 2010, 147, 61.
32 D. Coates, J. Mater. Chem., 1995, 5, 2063.
c
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Chem. Commun., 2012, 48, 4579–4581 4581