Near infrared light-driven liquid crystal phase transition enabled by hydrophobic mesogen grafted plasmonic gold nanorods

Article abstract of DOI:10.1039/c5cc02127h

Light-driven phase transition in liquid crystals is a fascinating endeavour from both scientific and technological points of view. Here we demonstrate the proof-of-principle that the photothermal effect of organo-soluble plasmonic gold nanorods can introd

Full text of DOI:10.1039/c5cc02127h

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Near infrared light-driven liquid crystal phase
transition enabled by hydrophobic mesogen
grafted plasmonic gold nanorods†
Cite this: Chem. Commun., 2015,
51, 9845
Received 12th March 2015,
Accepted 7th May 2015
Karla G. Gutierrez-Cuevas,‡a Ling Wang,‡a Chenming Xue,a Gautam Singh,b
Satyendra Kumar,b Augustine Urbasc and Quan Li*a
DOI: 10.1039/c5cc02127h
www.rsc.org/chemcomm
Light-driven phase transition in liquid crystals is a fascinating application as ‘‘nanoheaters’’ by dispersing them in different
endeavour from both scientific and technological points of view. organic media and irradiation using an appropriate light source.
Here we demonstrate the proof-of-principle that the photothermal
In recent years, dedicated research has focused on the study
effect of organo-soluble plasmonic gold nanorods can introduce the of photoinduced phase transitions stimulated by UV, visible or
phase transition of thermotropic liquid crystals upon near infrared NIR light in soft materials by the incorporation of different types
laser irradiation. Interestingly, the reverse process occurs when the of nanoparticles or light-driven molecular additives as dopants.
laser is switched off.
For example, single-walled carbon nanotubes (SWNTs) have been
used to photoinduce a phase transition in poly(N-isopropyl
Gold nanorods (GNRs) are a promising class of metallic anisotropic acrylamide) using 1064 nm NIR laser irradiation.5 Similarly,
nanoparticles, which have been widely investigated in different thermotropic liquid crystalline phases (e.g. nematic, cholesteric,
areas such as nanoscience and nanotechnology, biological and smectic, and blue phases)6 have been reported to undergo phase
biomedical science, sensing, photonics, and metamaterials.1 transitions upon UV or visible light irradiation with the incorpora-
Among the unique properties of GNRs, the fascinating ‘‘photo- tion of photoswitchable moieties such as azobenzene. Nevertheless,
thermal effect’’ resulting from the longitudinal surface plasmon the use of UV or visible radiation implies higher energy that could
resonance (LSPR) has been recently in the limelight. When GNRs cause material degradation or poor penetration; therefore the use
are exposed to the laser light resonant with their surface plasmon of NIR light is attractive due to its low energy and deeper penetra-
oscillation, they can strongly absorb the light and rapidly convert tion. Recently, we have demonstrated the use of upconversion
it into heat via a series of photophysical processes. Since the nanoparticles as NIR nanotransducers to tune the self-organized
longitudinal absorption of GNRs can be tuned to the near helical superstructures of thermotropic chiral liquid crystals by the
infrared (NIR) and even the infrared region, the appealing in situ generation of UV and visible light.7 However, the use of
photothermal effect of GNRs has been utilized in biological NIR-light to induce phase transitions in thermotropic liquid
and biomedical applications. GNRs can be made organo-soluble crystals remains an unexplored arena. Here we present the
and compatible with different organic matrices by the incorpora- synthesis and characterization of hydrophobic mesogenic thiol
tion of different kinds of organic molecules onto their surface monolayer-protected GNRs (M6S-GNRs, Scheme 1). The resulting
such as polymers,2 chromophores,3 and mesogens,4 which confer hybrid GNRs were found to be soluble in organic solvents and
special properties and enable applications like self-assembly, displayed the typical transverse and longitudinal surface plasmon
fluorescence quenching, solubility, and drug delivery among resonances (SPRs) of GNRs. The protected GNRs were homo-
others. The combination of organo-solubility and efficient photo- geneously dispersible in a commercially available thermotropic
thermal effects of GNRs provides the necessary thrust for their nematic liquid crystal host E7. Interestingly, the resultant liquid
crystalline nanocomposites were found to exhibit a nematic–
isotropic phase transition upon irradiation using an 808 nm
NIR laser, caused by the photothermal effect of the plasmonic
M6S-GNRs. To the best of our knowledge, study of such phase
transitions in thermotropic liquid crystal nanocomposites has
so far not been reported.
The new mesogenic thiol surfactant M6SH was synthesized
in a straightforward manner (see the ESI†). The mesogenic thiol
exhibited a wide temperature range of the liquid crystal phase
a Liquid Crystal Institute, Kent State University, Kent, Ohio, 44242, USA.
E-mail: qli1@kent.edu; Tel: +1-330-6721537
b Department of Physics, Kent State University, Kent, Ohio, 44242, USA
c Materials and Manufacturing Directorate, Air Force Research laboratory WPAFB,
Ohio 45433, USA
† Electronic supplementary information (ESI) available: Synthesis of M6SH,
M6S-GNRs, nanocomposite preparation, and their characterization. See DOI:
10.1039/c5cc02127h
‡ These authors contributed equally to this research.
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B115 1C (Fig. S5, ESI†), suggesting the liquid crystal phase to be
highly ordered smectic B phase (SmB). Furthermore, lowering
the temperature to 50 1C resulted in the same X-ray pattern.
The synthesized liquid crystal thiol M6SH was reacted with the
as-prepared water-soluble cetyltrimethylammonium bromide
(CTAB) coated GNRs (CTAB-GNR) through the thiol exchange
reaction in order to obtain organo-soluble hybrid M6S-GNRs.
Note that the ionic surfactant CTAB forms a densely packed
dynamic surface layer around the side wall of the GNRs which
renders them water-soluble, but the CTAB surface layer is
dynamically unstable which makes thiol surfactant exchange
possible. During the thiol exchange process, the mesogenic
thiol monolayer covers the surface of the GNR through strong
Au–S covalent linkages. Due to the hydrophobic properties of
the mesogenic surfactant, the resulting hybrid M6S-GNRs were
found to be soluble in organic solvents such as chloroform,
dichloromethane, and tetrahydrofuran (Fig. S8, ESI†). Both the
initial CTAB-GNR and the hybrid M6S-GNR show two characteristic
plasmonic resonance peaks of GNRs (Fig. 2a). No significant
broadening of the absorption peaks in the spectrum of the M6S-
GNR compared to the CTAB-GNR suggests the absence of aggrega-
tion during the ligand exchange procedure. The mesogenic hybrid
M6S-GNRs were further characterized using TEM (Fig. 2b). The
GNRs had an average size of 44.26 Æ 3.74 nm  14.75 Æ 1.75 nm
and an aspect ratio of 3.04 Æ 0.42 nm based on the calculation of
the size of 500 GNRs. The length, width and aspect ratio distribu-
tion curves are presented in the ESI,† Fig. S10.
Scheme 1 Synthesis of the organo-soluble hybrid M6S-GNRs by the new
hydrophobic mesogenic thiol M6SH grafted on the GNR via the covalent
Au–S linkage starting from the as-prepared water-soluble CTAB-GNRs.
The dispersion quality and compatibility of the mesogen
functionalized M6S-GNRs were explored in the commercially
available liquid crystal host E7, which is a eutectic mixture of
liquid crystal components designed for display applications
and has a wide nematic liquid crystal phase and the nematic–
isotropic phase transition (TN–I) around 60 1C. Conventional
water-soluble CTAB-GNRs showed poor solubility in E7, leading
to phase separation, whereas M6S-GNRs exhibited good solubility
in E7 due to the similarity of the molecular structure of meso-
genic thiol M6SH with the components of E7. The nanocomposites
with 0.01%, 0.02%, 0.04%, 0.08%, 0.10%, 0.15%, 0.20%, 0.35%
and 0.50 wt% M6S-GNRs in E7 were investigated using the liquid
crystal cells of 25 mm thickness with no alignment layer in order to
study their corresponding texture, phase transition temperature as
well as the effect of NIR irradiation. The optical textures of their
mixtures at 55 1C are shown in the ESI,† Fig. S12, presenting the
typical Schlieren texture of the nematic liquid crystal. The absence
as revealed using polarizing optical microscopy (POM) and
X-ray diffraction studies (Fig. 1 and ESI†). Upon cooling from the
isotropic phase, the mesogenic thiol exhibited a liquid crystal
phase from about 116 1C to room temperature. To identify the
mesophase structure, we undertook a variable temperature
X-ray diffraction study. The sample was heated above the isotropic
temperature and cooled down slowly in the presence of an
external magnetic field in a 1 mm quartz capillary. In the
isotropic phase (120 1C), both small angle and wide angle reflec-
tions are diffuse rings as shown in Fig. 1d. These diffuse peaks
observed in the isotropic phase change to sharp reflections at
Fig. 1 (a–c) Textures of M6SH under polarizing optical microscopy at
different temperatures in the cell, (a) isotropic phase, (b and c) smectic B
phase; (d–f) XRD patterns of M6SH, (d) isotropic phase, (e and f) smectic B phase.
Fig. 2 (a) UV-visible spectra of CTAB-GNRs in H2O (black line) and M6S-GNRs
in CHCl3 (red line); (b) the TEM micrograph of M6S-GNR.
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of observable GNR aggregates in the textures indicates that
M6S-GNRs are well dispersed in the liquid crystal matrix.
However, if the concentration of M6S-GNRs is over 0.5 wt%, the
resultant liquid crystalline nanocomposite appears inhomogeneous,
i.e. there is phase separation. Moreover, it was observed that the
nematic–isotropic transition temperature of the prepared mixtures
was lower than that in pure E7 when the concentration of the
M6S-GNRs reached 0.15 wt% and higher (Table S1, ESI†).
The photothermal effect of the mesogen functionalized M6S-
GNRs to induce the phase transition in the liquid crystalline
nanocomposites via NIR laser irradiation was investigated. DT
value was introduced such that DT = T À TI–N where T is the
temperature of the liquid crystalline nanocomposite at which
the NIR laser irradiation of the sample was performed. This
value was used as a standard parameter for each sample because
Fig. 4 Photoinduced phase transition of (A) the pure liquid crystal host E7,
and (B) the 0.2 wt% M6S-GNR/E7 composite in a 25 mm unaligned cell at
DT = 4 1C from TI–N at different exposure times.
at each concentration of the prepared composites the TI–N differs of the effects of the concentration and temperature on the NIR
and decreases when the concentration of nanorods is increased. irradiation process, we systematically monitored different areas
The irradiation process was performed using an 808 nm NIR in the prepared liquid crystal cells (25 measurements in different
laser and the progressive phase transition process was captured regions of the cell), and averaged the response time which is
using POM. For example, 0.5 wt% M6S-GNRs in E7 exhibited defined as the time taken by the sample to go from the nematic
the typical Schlieren texture of the nematic phase (Fig. 3). Upon phase to the isotropic state. The time-dependence curve of M6S-
irradiation using the 808 nm NIR laser for approximately GNR/E7 composites with different concentrations of the M6S-
2 seconds, the liquid crystalline nanocomposites underwent GNRs is shown in the ESI,† Fig. S13 and S14 at distinct DT values.
the nematic to isotropic phase transition. Interestingly, the In these experiments, a clear dependence of the phase transition
phase transition was found to be reversible when the NIR laser time on the M6S-GNR concentration is observed and the nano-
was turned off. The reverse phase transition was observed to composites containing 0.5 wt% M6S-GNR change rapidly from
occur in approximately 1 second. It is important to note that the one phase to the other (0.8 seconds) at DT = 1 1C. These results, as
pure E7 host did not present a noticeable change under the anticipated, suggest that the presence of more GNRs contributes
POM when it was irradiated using the NIR laser under identical to higher efficiency of the photothermal effect in the nano-
conditions. These observations imply that it is the photother- composites. It is also evident from the results that when the
mal effect of the embedded GNRs in M6S-GNRs, resulting in the irradiation is performed further from the TI–N, for example at
phase transitions in the nanocomposites. Since the longitudinal DT = 14 1C, the phase transition process will occur slowly as
surface plasmon resonance of M6S-GNRs overlaps with the NIR observed in Fig. S15 (ESI†) while at DT = 15 1C, the complete
laser frequency, they can absorb the NIR light and convert it into phase change was not observed. For lower concentrations
heat (photothermal effect) and convey the thermal energy to the (0.01%) at DT o 3 1C, the complete photodriven phase
surrounding media, thereby inducing a phase transition. Fig. 4 transition was observed at longer exposure times (Fig. S16,
shows the evolution of the phase transition process for pure ESI†); however, at higher values of DT this process was not
E7 and 0.20 wt% M6S-GNR/E7 with same exposure times at observed.
DT = 4 1C for comparison. The nanocomposites underwent
phase transition in about 7 seconds while the pure E7 mixture and its liquid crystalline behaviour was characterized via the
did not undergo a phase transition even after 5 minutes. combination of POM and XRD studies. The terminal thiol was
In conclusion, the new liquid crystal thiol was synthesized
Furthermore, the time required to change from one phase to grafted onto the surface of the GNR through a strong covalent
another was highly dependent on the concentration of the Au–S linkage. The resultant organo-soluble M6S-GNRs were
mesogen coated GNRs. In order to have a better understanding incorporated into a commercially available nematic liquid crystal
E7 at different concentrations. The homogeneous liquid crystalline
nanocomposites were found to undergo a reversible nematic–
isotropic phase transition within a few seconds upon NIR laser
irradiation, resulting from the photothermal effect of the embedded
plasmonic GNRs. The nanocomposites with higher concentration of
GNRs exhibited faster phase transitions. The NIR light-induced
phase transition demonstrated here might provide impetus in
developing new functional stimuli responsive materials where direct
heating is not favored.
Fig. 3 Phase transition of the 0.5 wt% M6S-GNR/E7 composite at DT = 3 1C
with NIR laser irradiation took approximately 2 seconds from the nematic to
The support of this research by the Air Force Office of
Scientific Research (FA9950-09-1-0254), the National Science
Foundation award#DMR-1410649 under the U.S. Ireland R&D
the isotropic phase whereas it took approximately 1 second to return from
the isotropic to the nematic phase when the NIR light was turned off.
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