Reversible photorheological fluids based on spiropyran-doped reverse micelles

Article abstract of DOI:10.1021/ja202412z

We describe a new class of photorheological (PR) fluids whose rheological properties can be reversibly tuned by light. The fluids were obtained by doping lecithin/sodium deoxycholate (SDC) reverse micelles with a photochromic spiropyran (SP) compound. Initially, the lecithin/SDC/SP mixtures formed highly viscoelastic fluids, reflecting the presence of long, wormlike reverse micelles. Under UV irradiation, the SP was isomerized to the open merocyanine (MC) form, causing the fluid viscosity to decrease 10-fold. When the UV irradiation was switched off, the MC reverted to the SP form, and the viscosity recovered its initial value. This cycle could be repeated several times without loss of response. The rheological transitions are believed to reflect changes in the lengths of the reverse worms. To our knowledge, this is the first example of a simple, reversible PR fluid that can be made entirely from commercially available components.

Full text of DOI:10.1021/ja202412z

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pubs.acs.org/JACS
Reversible Photorheological Fluids Based on Spiropyran-Doped
Reverse Micelles
†
†
†
‡
,†
Hee-Young Lee, Kevin K. Diehn, Kunshan Sun, Tianhong Chen, and Srinivasa R. Raghavan*
†
Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742-2111, United States
TA Instruments-Waters LLC, 159 Lukens Drive, New Castle, Delaware 19720, United States
‡
S Supporting Information
b
In this paper, we report a simple, photoreversible PR fluid
based on reverse micelles doped with a photosensitive additive. We
began with a formulation developed in our laboratory that gives rise
ABSTRACT: We describe a new class of photorheological
PR) fluids whose rheological properties can be reversibly
(
13
tuned by light. The fluids were obtained by doping lecithin/
sodium deoxycholate (SDC) reverse micelles with a photo-
chromic spiropyran (SP) compound. Initially, the lecithin/
SDC/SP mixtures formed highly viscoelastic fluids, reflect-
ing the presence of long, wormlike reverse micelles. Under
UV irradiation, the SP was isomerized to the open mer-
ocyanine (MC) form, causing the fluid viscosity to decrease
to wormlike reverse micelles (“worms” for short), which are long,
flexible, cylindrical chains. The formulation combines the phospho-
lipid lecithin and the bile salt sodium deoxycholate (SDC) in a
14,15
nonpolar organic solvent such as cyclohexane (Figure 1).
While
lecithin alone forms discrete, spherical reverse micelles in cyclohex-
ane, the addition of SDC promotes the axial growth of lecithin
micelles until these become long worms (diameter ∼4.4 nm and
contour length >140 nm, as determined by small-angle neutron
1
0-fold. When the UV irradiation was switched off, the MC
14
13
reverted to the SP form, and the viscosity recovered its initial
value. This cycle could be repeated several times without
loss of response. The rheological transitions are believed to
reflect changes in the lengths of the reverse worms. To our
knowledge, this is the first example of a simple, reversible PR
fluid that can be made entirely from commercially available
components.
scattering). These worms entangle to form a transient network
and thereby impart a high viscosity and viscoelasticity to the fluid.
To endow these reverse worms with photoresponsive properties, we
added a small concentration of the spiropyran (SP) derivative
0
0
0
0
1 ,3 ,3 -trimethyl-6-nitrospiro[1(2H)-benzopyran-2,2 -indoline].
SPs are well-known photochromic compounds that can be rever-
sibly photoisomerized between the colorless SP form and the
colored merocyanine (MC) form by irradiation at different wave-
16,17
lengths of light.
As shown in Figure 1, the closed SP form is
hydrophobic and nonionic, whereas the open MC form is zwitter-
ionic and hydrophilic. These two photoisomers are known to
luids whose rheological properties, such as viscosity, can be
18ꢀ20
interact differently with the headgroups of lecithin.
As we will
F
controlled by light are of interest to scientists and engine-
show below, they thus have different effects on the assembly of
lecithin/SDC reverse worms. This leads to reversible light-
induced changes in the rheological properties of the solutions.
Photorheological results for a typical formulation are shown
below. The sample contained 100 mM lecithin, 35 mM SDC, and
1
,
2
3
,
4
ers. Such fluids can be termed photorheological (PR) fluids.
Because light can be directed at a precise point with microscale
accuracy, PR fluids may be useful in microfluidic devices as the
basis for valves or flow sensors. Other applications have also been
envisioned for PR fluids, including their use as drag-reducing
fluids in recirculating systems for district heating and cooling, in
microrobotics, and as patternable materials.
1
5 mM SP in cyclohexane. It should be noted that the SP
concentration is close to the maximum that can be dissolved in
cyclohexane at ambient temperature. A photograph of this
sample is shown in the left panel of Figure 2. The sample held
its weight in the inverted vial, which is indicative of its viscoelastic
character. Next, we subjected this sample to broad-band UV light
from a 200 W mercury arc lamp. Within ∼5 min, the sample
became noticeably less viscous and also changed color from
yellow to red. This is shown by the photograph in the middle
panel of Figure 2, which shows the red liquid flowing down the
sides of the inverted vial. Such a color change is typical for the
Our approach to PR fluids has differed from that of other
research groups in one important aspect. Typically, researchers
have focused on developing PR fluids using new, original types of
5
ꢀ10
photosensitive organic molecules.
However, we have em-
phasized the design of PR fluids using only commercially avail-
4,11,12
able molecules or particles.
The rationale for simpler PR
fluids is that these could easily be recreated in the laboratory by
scientists in both academia and industry without investing time
and effort in organic synthesis. In turn, it is hoped that the greater
availability and awareness of such PR systems will help advance
new applications. Several examples of simple PR fluids have been
1
6,17
conversion of the closed SP form into the open MC form.
Thereafter, we switched off the UV light, and within ∼10 min,
the sample recovered its initial viscosity and color. This is shown
by the photograph in the right panel of Figure 2; it should be
noted that the sample was again able to hold its weight in the
4,11,12
published recently by our laboratory;
however, all of these
fluids permit only one-way changes in rheological properties.
Typically, the changes in rheology were induced by UV light, but
these could not be subsequently reversed by irradiation at other
wavelengths.
Received: March 16, 2011
Published: May 12, 2011
r 2011 American Chemical Society
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dx.doi.org/10.1021/ja202412z
|J. Am. Chem. Soc. 2011, 133, 8461–8463

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Figure 3. Real-time photorheology of a sample containing 100 mM
lecithin, 35 mM SDC, and 15 mM SP in cyclohexane at ambient
temperature. Upon UV irradiation for 150 s, the complex viscosity η*
decreased 10-fold relative to its initial value. The UV light was switched
off at the 200 s mark, whereupon the sample recovered to nearly its
original viscosity over the next 400 s.
Figure 1. Components of a reversible PR fluid: the phospholipid
lecithin is combined with the bile salt SDC and an SP derivative. The
SP is initially in a closed form but can be photoisomerized to the open
(MC) form by UV irradiation. This can be reversed by visible-light
irradiation or heating.
Figure 2. (left) Sample containing 100 mM lecithin, 35 mM SDC, and
1
5 mM SP in cyclohexane. (middle) After UV irradiation, the viscosity of
Figure 4. UVꢀvis spectra of a sample containing 25 mM lecithin,
the sample decreased, and its color changed from yellow to red. (right)
When the UV irradiation was stopped, the sample viscosity and color
reverted to their initial states. This cycle could be repeated several times.
8
5
.75 mM SDC, and 3.75 mM SP sample in cyclohexane. A peak at
65 nm developed upon UV irradiation, indicating the conversion of SP
to MC. The UV light was then switched off, and spectra were recorded
after different wait times (at 0, 2, 6, 10, and 20 min top to bottom). The
peak was observed to decrease, indicating the reversion of the MC to SP.
inverted vial. The recovery of the sample viscosity and color
indicates the inverse conversion of MC into SP. These data were
reproducible, and the sample could be cycled more than 10 times
without any deterioration of its response.
We quantified the above photorheological changes in real time
using a rheometer with a built-in UV apparatus. The sample was
placed between transparent parallel plates and irradiated with UV
0
Dynamic frequency spectra (elastic modulus G and viscous
0
0
modulus G as functions of frequency ω) of the above sample
were also measured [Figure S1 in the Supporting Information
(
SI)]. In all cases, the data were fit well by a Maxwell model with a
2
single relaxation time (eq S1 in the SI), as is typical of wormlike
light at 365 nm with an intensity of 150 mW/cm . The sample
13,14
micelles.
Initially (Figure S1a), the data revealed a typical
was monitored under oscillatory shear at a frequency of 7 rad/s.
At this frequency, significant rheological changes were observed
and the measurements could be made rapidly (i.e., the time per
measurement was low). The results are plotted in Figure 3 in
terms of the complex viscosity η*. Initially, the sample was
viscous with η* = 2.5 Pa s. Upon irradiation with UV light, the
viscosity dropped quickly and reached a plateau value of ∼0.2 Pa
s (10-fold lower) within ∼100 s. At the 200 s mark, the UV light
was switched off, at which point the viscosity began to grow back.
It recovered to nearly its initial value over the next 400 s.
viscoelastic response. That is, at high ω or short time scales, the
0
sample showed elastic behavior with G tending to a plateau and
0
0
dominating over G . On the other hand, at low ω or long time
scales, the sample showed viscous behavior, with G exceeding
G . The parameters in the Maxwell model are the plateau
0
0
0
modulus G
value of G
and the relaxation time t
was 455 Pa and the initial t
. From the fits, the initial
value was 77 ms. We then
p
R
p
R
ran the same frequency sweep under UV irradiation after the
sample had been exposed to UV light for 180 s. The frequency
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dx.doi.org/10.1021/ja202412z |J. Am. Chem. Soc. 2011, 133, 8461–8463

Journal of the American Chemical Society
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spectra (Figure S1b) were shifted to higher frequencies (i.e.,
shorter time scales) as well as lower values of the moduli. From
altered in a way that disfavors growth of worms (i.e., the micelles
14
are induced to shorten). The shorter worms evidently entangle
13
the fits, the new G was determined to be 122 Pa, and the
less and relax faster, which explains the reduction in viscosity.
In conclusion, we have demonstrated a PR system in which
reversible viscosity changes can be induced by light. The system is
based on mixtures of lecithin, the bile salt SDC, and a spiropyran;
notably, all of these components are commercially available. The
viscosity of the solution decreased 10-fold when UV light was
switched on and recovered its initial value when the UV light was
switched off. We suggest that these results are caused by the
differential interaction of the zwitterionic lipid lecithin with the SP
(nonpolar) and MC (zwitterionic) forms of the spiropyran.
p
corresponding t was 22 ms. Finally, we switched off the UV light
R
and collected a frequency spectrum after 30 min, which was
ample time to allow full recovery of the sample. The spectra
(
Figure S1c) reverted to close to their initial ones (G = 380 Pa
p
and t ≈ 65 ms). All in all, the sample was observed to become
R
less viscoelastic upon irradiation with UV and to recover its initial
viscoelasticity after the UV light was switched off. The decrease in
viscoelasticity suggests a UV-induced shortening of the reverse
worms, allowing the worms to become less entangled and relax
1
3,14
faster.
When the UV light was switched off, the worms ap-
peared to regain their initial lengths.
’ ASSOCIATED CONTENT
UVꢀvis spectra (Figure 4) confirmed that the rheological
changes were accompanied by the photoconversion of SP to MC
and back. We used a diluted sample (25 mM lecithin, 8.75 mM
SDC, and 3.75 mM SP) to ensure the proper levels of absor-
bance. The initial sample exhibited weak absorption over the
range of wavelengths shown in Figure 4, which correlates with
the colorless nature of the SP form. When irradiated with UV
light for 3 min, the sample transformed to a red liquid, and
correspondingly, a strong absorption peak appeared at 565 nm.
This indicates the UV-induced conversion of the closed SP form
into the open MC form. The UV light was then switched off, and
we collected spectra after different wait times. Figure 4 reveals
that the peak at 565 nm decreased as time progressed, and the
sample correspondingly showed a transition in color from red to
yellow. This occurred because the MC form in nonpolar solvents
often is thermally unstable and thus reverts to the SP form even
S
Supporting Information. Details of experimental proce-
b
dures, additional rheological and physicochemical data, and a sche-
matic of the proposed mechanism for the PR effect. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
sraghava@umd.edu
’
ACKNOWLEDGMENT
This work was partially funded by a CAREER Award from
NSF-CBET.
1
6,17
’
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to the lecithin headgroups,
displacing some of the SDC in
the process (Figure S3). This causes the net geometry to be
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dx.doi.org/10.1021/ja202412z |J. Am. Chem. Soc. 2011, 133, 8461–8463