Visual indication of enviromental humidity by using poly(ionic liquid) photonic crystals

Article abstract of DOI:10.1039/c003325a

The combination of poly (ionic liquid) and photonic structure affords a new class of self-reporting humidity sensory materials with excellent reversibility, which are able to rapidly, sensitively and visually indicate environmental humidity with colour change from blue to green, orange, and red, covering the whole visible range.

Full text of DOI:10.1039/c003325a

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Visual indication of enviromental humidity by using poly(ionic liquid)
photonic crystalsw
Jing Huang, Cheng-an Tao, Qi An, Changxu Lin, Xuesong Li, Dan Xu, Yiguang Wu,
Xiaogang Li, Dezhong Shen and Guangtao Li*
Received 17th February 2010, Accepted 14th April 2010
First published as an Advance Article on the web 11th May 2010
DOI: 10.1039/c003325a
The combination of poly (ionic liquid) and photonic structure
affords a new class of self-reporting humidity sensory materials
with excellent reversibility, which are able to rapidly, sensitively
and visually indicate environmental humidity with colour
change from blue to green, orange, and red, covering the whole
visible range.
sophisticated technical equipments. The principle behind the
controlled optical response of such nanostructured PILs is as
follows: the capture of the targeted species within the PIL
porous lattice will cause changes in the lattice constant
of photonic PIL through swelling or shrinkage, and
consequently, induce shifts of the spectral position of the
colored Bragg reflections these structures display. It is
conceivable that the combination of ionic liquids and porous
photonic structure actually provides a useful platform for the
development of organic vapor sensors with a series of desirable
properties. As a demonstration, in this work, a new class of
self-reporting humidity sensors is constructed based on the
above-mentioned strategy. Different from the reported
IL-based sensors, such sensors described here are enable to
rapidly, sensitively and optically detect environmental vapors
with visible colour change (Scheme 1), and at the same time
exhibit excellent stability and reversibility.
Ionic liquids (ILs), a class of compounds consisting of organic
cations and anions, exhibit a series of unique properties
including negligible vapor pressure, nonflammability, high
ionic conductivity, a wide electrochemical window, and good
chemical and thermal stability.1 More importantly, such
favorable properties can be individually manipulated through
modulating the combination of cations and anions, providing
unprecedented tunability.2 Thereby, based on these distinct
properties and features, besides as green solvents for organic
synthesis, ILs have also been explored as various functional
materials like solid-state polyelectrolytes, catalysts, separation
membranes, high CO2 absorbing resins and microwave
absorbing materials, etc.1–4
Colloidal crystal templating method was employed for
preparation of poly(ionic liquid)s with ordered macroporous
structure. Three-step approach was involved:8 fabrication of a
face centered cubic (fcc) SiO2 opaline template; infiltration of
the template with a mixed solution of ionic liquid monomer,
cross-linker and initiator followed by photopolymerization;
and selective dissolution of SiO2 template to afford an ordered
macroporous structure (inverse opal). Uniform SiO2 particles
with diameters ranging from 200 to 400 nm were used in this
work, and for improving the film formability of PILs, methyl
methacrylate (MMA) was utilized as co-monomer in the
Recently, Dai and coworkers found that ionic liquids are
also a new class of sensing materials suitable for detection of
volatile organic compounds (VOCs).5 Besides remarkable
stability at both room and high temperatures, tunable affinities
of ILs toward certain organic vapors through the modification
of cations or anions highlight the superior properties of ILs for
sensor applications. Nevertheless, since ILs usually don’t
exhibit optical or redox properties, signal transducers of most
ILs-based sensors reported so far are limited to quartz crystal
microbalance (QCM) and microcantilever.5–6 Therefore, to
further exploit the distinguished properties of ILs as sensing
materials for fabricating novel chemical sensors, the development
of a new transduction approach, especially optical model, is
highly desirable and important in many cases.
In this Communication, we report the fabrication of
imidazolium-based poly (ionic liquid)s (PILs) with ordered
macroporous structure, and show that the integration of
porous photonic crystal structure with ILs can endow ILs
with optical element,7 by which recognition events of ILs can
be facilely transduced into a readable optical signal without
Key Lab of Organic Optoelectronics and Molecular Engineering,
Department of Chemistry, Tsinghua University, 100084 Beijing,
China. E-mail: lgt@mail.tsinghua.edu.cn; Tel: +86 10 62792905
w Electronic supplementary information (ESI) available: experimental
details, FTIR spectra, UV/vis spectra of PILs with different counter
anions and proposed mechanism. See DOI: 10.1039/c003325a
Scheme
1
Schematic illustration of the visual indication of
environmental humidity by using poly (ionic liquid) photonic crystals.
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Fig. 1 SEM images of (A) the used SiO2 opaline template and (B) the
resulting 3D-ordered macroporous PIL film. The scale bar is
1 micrometre.
Fig. 2 (A) UV/vis spectral shifts of the ordered macroporous PIL
film under different humidity conditions; (B) induced color changes of
the PIL film after exposure to various humidity. All spectra have been
normalized.
polymerization process. Details of the related preparation are
described in the ESI.w In our case, highly hydrophilic
imidazolium units with BrÀ as counteranions were used for
the make-up of the nanostructured PILs. Fig. 1A–B displays
the SEM images of the used SiO2 opaline template and the
resulting PIL film. This film has a thickness of ca. 2 mm and
possesses a three-dimensional ordered and interconnected
macroporous structure (inverse opal structure).
of the parameter d in PIL film by 30%, assuming that neff
remains unchanged. In fact, as the structure absorbs more
water, the average refractive index (neff) of the PIL will
decrease, but only less than 1% was estimated by using a
method in the literature.10a Thus, more than 30% increase of
the parameter d should occur in this system, indicating that the
shift of l is mainly determined by the change of parameter d.
The stability and repeatability of the prepared humidity
sensor over a large RH range is demonstrated in a multi-cycle
experiment (Fig. 3). When alternately exposed to increasing
and decreasing humidity between 20% and 100%, the Bragg
peak position changes repeatedly and reversibly. At the same
time, even at high RH (e.g. 60%), the peak position remains
almost unchanged for a long exposure period only with a small
fluctuation (Fig. S1, ESI).w These results clearly show that the
PIL-based sensor has excellent durability and reversibility.
The response time is different for swelling and shrinking
events. In the case of swelling, the response time is about 8 s.
But, for the case of the shrinking process, the response became
relative slower and about 20 s was required.
The presence of an ordered macroporous structure gives the
prepared ionic liquid materials interesting optical properties
through Bragg diffraction with visible light, showing bright
colour. Remarkably, when exposed to environmental vapor,
these films can selectively proceed rapid and reversible
swelling/shrinkage, and thus exhibit specific optical responses.
Fig. 2A shows reflection spectra of the original PIL film and its
response upon exposure to different relative humidity
(from 20% RH to 100% RH). The as-prepared PIL with
BrÀ as counteranion in dry state (20% RH) displays a
reflectance peak at 498 nm and shows blue color in appearance.
With increased humidity, the moisture swells the volume of the
PIL film and, as a consequence, induce continual shift of
the diffraction peak of the photonic PIL film, depending on
the amount of water adsorbed. In the present work, a large
shift of over 148 nm from 498 nm to 646 nm is achieved as the
relative humidity varies from 20% to 80%. More importantly,
the sensing of the photonic PIL film accompanies distinct
color changes ranging from blue to red (Fig. 2B), covering
whole visible range, and can be easily discriminated by the
naked eye. It should be noted that the interconnected and
highly ordered porous structure can not only directly generate
optical signals due to Bragg diffraction, but also be favorable
for rapid diffusions of guest molecules within PIL film. When
the sample is moving from lower to higher humidity, most of
the reflection shift completes in several seconds, exhibiting a
very rapid response.
While the position of the peak shifts with varying humidity,
the range of that shift can be adjusted by controlling the elastic
modulus of the photonic structure. Usually, this can be done
by varying the initial ratio of cross-linker to monomer before
For the normal incidence geometry, Bragg reflection wave-
length l of inverse opal is determined by the lattice constant
and the refractive index of PIL network in the inverse opal
structure as predicted by Bragg’s equation, l = 2dneff, where
d is the characteristic spacing, and neff is the effective refractive
index of the structure.7 In our system, the change of the
reflection peak is resulted from the absorption of water, which
causes the swelling of the prepared PIL film, leading to the
increase of parameter d. It is estimated that the humidity
change from 20% RH to 100% RH can induce the increase
Fig. 3 Reversible conversion of the diffraction peak position of the
photonic PIL film by alternately exposing to increasing and decreasing
relative humidity (between 20% and 100%), where the solid and
dashed lines are the ascending and descending humidity procedures,
respectively.
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polymerization. Fig. S2 in the ESIw displays the optical
response of the films prepared under different ratios. In this
case, where a higher ratio of cross-linker to monomer
(e.g. 2 : 1) is used, more rigid matrix will be formed for the
photonic PIL structure, and the swelling/shrinking capability
of the film will be strongly reduced. In contrast, if too little
cross-linker is used, a well-defined and ordered porous
polymer film is hardly formed, due to insufficient mechanical
strength. Thus, it is possible to achieve a sensing material with
optimized swelling and reflection properties through balancing
different influence factors. Interestingly, in our case the
distinct capability of ion-exchange of IL provides another
convenient and attractive way to tune sensing properties.
Fig. 4 Influence of counteranion (BrÀ and SDSÀ) on the sensing
selectivity of the photonic PIL sensors.
3À
The replacement of BrÀ with the more hydrophilic PO4
Based on the concept of ‘‘task-specific’’ ionic liquids,3
functional groups as anions can also be introduced into IL
units, leading to photonic PILs with specific sensing functions.
In summary, based on the combination of the unique
properties of both ILs and photonic structure, a new concept
was developed to further extend the application scope of ILs
as sensing materials. The integration of ordered macropores
with poly(ionic liquid)s can not only afford a new class of
self-reporting humidity sensors with excellent reversibility,
which are able to rapidly, sensitively and visually indicate
environmental humidity. Because both ionic liquids and
photonic structures offer virtually unlimited tunability, their
combination also provides a unique platform with tremendous
opportunities for the design of new sensing materials or
chemical systems.
anion (Fig. S3, ESI)w facilely affords more sensitive humidity
sensors with a shift range approaching 180 nm (Fig. S4, ESI).w
In recent years, the same principle of using optical response
of photonic structure has been employed for the development
of various stimuli-responsive polymers.9 In this respect, two
studies were described for the fabrication of humidity sensors.
Barry and Wiltzius first reported the fabrication of a humidity
sensing inverse opal hydrogel of polyacryamide (PAAm),10a
which could be operated outside of a supporting solvent with a
response time of the order of seconds. But, due to the
insufficient mechanical strength of PAAm, the sample was
hardly repeatedly used under high humidity conditions, and
otherwise, its maximum peak shift was only 31 nm, which
made it difficult to detect the humidity change with the naked
eye. More recently, Song and his co-workers developed a
humidity sensing material by combining the intrinsic humidity
sensitivity of PAAm and the structural color of PS/PMM/PAA
colloidal crystal array.10b Although the reported composites
could be used to realize colorimetric detection of humidity
with excellent stability and reversibility, the response seemed
too slow (1.5 h). In comparison, the PIL-based sensor in this
study overcomes the mentioned drawbacks in previous work,
and is enable to rapidly, sensitively and visually indicate
environmental humidity. To the best of our knowledge, this
is the first report of producing humidity sensors based on
photonic structured ionic liquid materials.
Notes and references
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Wiley-VCH, Weinheim, 2008.
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The successful fabrication of an optical humidity sensor
demonstrates that the introduction of ordered macroporous
structures is an effective way to further extend the application
scope of ILs as sensing materials. More significantly, we
believe that the association of unique optical properties of
photonic structure with the superior sensing features of ILs
mentioned in introduction section actually provides a very
useful platform for development of novel self-reporting VOCs
chemical sensors. Recently, systematic studies of Brennecke’s
group showed that the nature of the anion has the most
significant influence on the gas solubilities,11 and the anion-
exchange can dramatically change the affinities of selected gas
species in ILs. The preliminary experiments in our present
work also confirmed this result. The photonic PIL film with
BrÀ as counteranions shows a preferred affinity of water
over ethanol vapor. After the anion-exchange with SDSÀ
(Fig. S5, ESI);w however, the resulting PIL film exhibits
completely different sensing selectivity (Fig. 4 and Fig. S6, ESI).w
5 C. Liang, C. Yuan, R. J. Warmack, C. E. Barnes and S. Dai, Anal.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4103–4105 | 4105