Localized pH Pulses in PBS Buffer Repeatedly Induced by Visible Light

Article abstract of DOI:10.1021/acs.jpcb.8b11677

The pH of biological systems is important for the activity of enzymes, and abnormal cellular pH is related to many diseases. Spatial and temporal modulation of pH with light will be useful for studying the pH effects on enzymatic functions and disease mec

Full text of DOI:10.1021/acs.jpcb.8b11677

Article
pubs.acs.org/JPCB
Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX
Localized pH Pulses in PBS Buffer Repeatedly Induced by Visible
Light
Adnan Elgattar, Nawodi Abeyrathna, and Yi Liao*
Florida Institute of Technology, Melbourne, Florida 32901, United States
ABSTRACT: The pH of biological systems is important for the
activity of enzymes, and abnormal cellular pH is related to many
diseases. Spatial and temporal modulation of pH with light will be
useful for studying the pH effects on enzymatic functions and disease
mechanisms and may lead to new drug delivery and therapeutic
methods. However, the pH of biological systems is maintained by pH
buffers, which implies that only temporary pH change (pH pulse) can
be induced in an open system. A key fundamental problem is whether
a photoinduced pH pulse can be strong and long enough to generate a
significant effect. In this work, a photoinduced pH pulse in a
micrometer hydrophilic film in PBS buffer has been demonstrated.
The thin film was made of an metastable-state photoacid (mPAH)
polymer. It is an open system that allows exchange of protons. A quick
release of the protons from the mPAHs and the proton exchange between the film and PBS resulted in a pH pulse generated by
moderate visible-light irradiation. The magnitude of the pulse is 1.4−1.9 units with maximum pH change occurring after ∼18 s
of the irradiation. Since the mPAH is a reversible photoacid, the pH pulse could be repeatedly generated after the photoacid
recovered in the dark. This work shows that photochemical modulation of pH is possible even in buffered solutions.
collaboration with our group showed that the photoacidity of
an mPAH was enough to kill drug-resistant bacteria and assist
the antibacterial activity of colistin.24
1. INTRODUCTION
A suitable pH of a biological system is critical for the activity of
enzymes, and abnormal cellular pH is related to many diseases
including cancer, cardiovascular diseases, Alzheimer’s disease,
etc. A method that allows spatial and temporal modulation of
pH with light will be useful for studying the pH effects on
enzymatic functions and disease mechanisms and may lead to
new drug delivery and therapeutic methods. It is worth
mentioning that photoinduced proton transfer at the molecular
level has been extensively studied using ultrafast spectrosco-
py.1,2 Our goal is to controllably produce a large proton
concentration with measurable pH change and significant
macroscopic effects.
Metastable-state photoacids (mPAHs) can produce a large
proton concentration with high efficiency and good reversi-
bility under visible light.3 Reversible pH change over two units
has been demonstrated previously.4 Over the past several years,
mPAH has become a useful tool for controlling various proton
transfer processes with light. Applications of mPAHs in energy
conversion,5−7 sensor,8,9 polymerization,10 patterning,11−13
nanomaterials,9,14,15 molecular machines,16−18 photochromic
materials,13,19−21 fragrance release,22 and biomedical23,24 areas
have been reported by our group and other groups. For
example, Gray, Dougherty, and co-workers showed that ion
channels associated with vision and pain can be reversibly
activated with light using an mPAH.23 The Li group
demonstrated that a chloroplast entrapped with an mPAH
produced 2.9 times more ATP than natural chloroplast.5
Chumbimuni-Torres’ group developed a calcium biosensor by
incorporating mPAHs in polymer thin films.8 The Su group in
Although the potential of mPAHs is clearly demonstrated in
these works, no photoinduced pH change in a pH buffer has
been reported. Nonbuffered solutions of mPAHs were often
used for modulating pH with light. The challenge is apparent
for the buffer is against the pH change. However, since all
biological systems are in pH buffers, it is expected that a
localized pH change in a pH buffer is required for most
biomedical applications. In an open system that allows proton
exchange, the photoinduced pH drop will be neutralized by the
buffer, which will result in a pH pulse. Since mPAH is
reversible, another pulse can be generated after the photoacid
recovers in the dark. The key is that the pH pulse must be large
and long enough to generate a useful effect. In this work, we
show that pH pulses of 1.4−1.9 units can be repeatedly
generated in a micrometer film of a hydrophilic mPAH
polymer in PBS.
2. RESULTS AND DISCUSSION
PBS buffer (1×, pH = 7.4) was chosen since it is commonly
used for biological study. To produce a localized pH change, a
high local concentration of mPAH is required. Therefore, a
copolymer of a indazole mPAH and hydroxyethyl methacrylate
Received: December 4, 2018
Revised: January 4, 2019
Published: January 7, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jpcb.8b11677
J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B
Article
Scheme 1. (a) Photoreaction of an Indazole mPAH, (b) Synthesis of the Photoacid Polymer P-mPAH, and (c) Synthesis of the
Indicator Polymer P-MR
(HEMA) was synthesized as Scheme 1. Poly(hydroxyethyl
methacrylate) is a well-known biocompatible hydrophilic
polymer. It has been used as a host material for the preparation
of mPAH thin films in previous work.25 Although phenolic
mPAHs are most commonly used for different applications,
they release a significant amount of protons without irradiation
in PBS due to relatively high dark acidity.26 Our group
reported previously that an mPAH with indazole and
benzothiazolium moieties released its proton in PBS under
visible light but not in the dark.26 (Scheme 1) Therefore, the
indazole mPAH monomer 1 was synthesized from 6-amino-2-
methylbenzothiazole and then copolymerized with hydrox-
yethyl methacrylate to yield the mPAH polymer (P-mPAH).
As shown in Scheme 1, 6-amino-2-methylbenzothiazole was
reacted with acryloyl chloride to yield the acrylamide 2, which
was then reacted with propane sultone in THF to yield
compound 3. Compound 3 was coupled with indazole-7-
carboxylaldehyde via a Knoevenagel reaction using ammonium
acetate as the catalyst, which gave the monomer 1 as an orange
precipitate. P-mPAH was then synthesized by copolymerizing
1 (5 wt %) and HEMA via an AIBN initiated radical
polymerization. The stable amide linker between the polymer
and the mPAH avoids hydrolysis in either basic or acidic
conditions. The loading of the mPAH on the polymer was
about 5 wt %. We found it was difficult to achieve a higher
loading using this monomer. When 10 wt % of the monomer 1
was used in the polymerization, the resultant polymer
contained less than 5 wt % of the photoacid. It is likely that
the bulky photoacid sterically hinders the polymerization since
it is close to the reactive acryloyl group.
A thin film of P-mPAH was prepared by drop coating a
solution of the polymer in methanol on a glass substrate. The
polymer is insoluble in PBS buffer and, thus, does not require
cross-linking. The film was dried on a heat plate (plate
temperature 70 °C) for 15 min and then kept in a ventilation
hood overnight. It was soaked in water for 15 min and then in
PBS for 30 min. The sample was then put in a quartz cell filled
with PBS and studied by UV−vis spectroscopy. The UV−vis
spectrum of the film showed a strong absorption peak at 438
nm (Figure 1). Upon irradiation with a 470 nm LED from the
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DOI: 10.1021/acs.jpcb.8b11677
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Figure 1. UV−vis spectra of a P-mPAH film in PBS before and after irradiation (a) and UV−vis spectra of a P-MR film in water before after
protonation with HCl (b).
top of the cell, the 438 nm peak was diminished and
absorption near 320 nm substantially increased indicating the
formation of the acidic product26 (Scheme 1).
To demonstrate the photoinduced pH change, a copolymer
of methyl red (MR 5 wt %) and HEMA was synthesized
(Scheme 1). MR was chosen as the indicator because it has a
pKa of 5.1 and is suitable for measuring pH between 6 and 4.
As described below, the photoinduced pH in the photoacid
film fell in this range. Another reason is that part of the
absorption band of the protonated MR (MRH+) does not
overlap with the absorption band of the indazole mPAH. As
shown in Figure 1, a thin film of the MR polymer (P-MR) in
water has a strong absorption band centered at 420 nm. After
addition of HCl, the absorption peak of the protonated P-MR
shifted to 501 nm. While P-mPAH has no significant
absorption above 550 nm, the protonated P-MR substantially
absorbs light between 550 and 600 nm, which allows us to
Figure 2. UV−vis spectra of a film of P-mPAH and P-MR in PBS
before irradiation and after 6, 18, and 72 s under irradiation.
Experimental setup for monitoring the absorption change during the
irradiation (inset).
observe the protonation of MR by the photoacid. It is worth
mentioning that it is necessary to use the polymers instead of
molecules of the photoacid and the indicator to avoid leakage
during the tests.
A thin film containing 80 wt % of P-mPAH and 20 wt % of
P-MR was prepared by drop coating a methanol solution of the
mixture on a glass slide and dried on a heat plate. The
thickness of the film was measured to be 8 μm. After being
soaked in water and then PBS, the sample was put in a quartz
cell filled with PBS and studied by UV−vis spectroscopy. The
setup is illustrated in Figure 2. This setup allows us to monitor
the change of UV−vis absorption while the sample is under
irradiation. The sample in PBS was irradiated by a 470 nm
LED from the top of the cell. The photon flux at the center of
the sample was ∼425 μmol s−1 m−2 (11 mW/cm−2) measured
by a quantum meter. UV−vis spectra were recorded every 6 s.
As shown in Figures 2 and 3, upon irradiation the absorption at
438 nm quickly decreased due to the photoreaction of the
mPAH. In the meantime, the absorption between 500 and 600
nm increased indicating the formation of MRH+. The
absorption of MRH+ at 550 nm maximized at ∼18 s and
then gradually decreased with time even though the sample
was still under irradiation (Figure 3). The absorption at 438
nm kept decreasing after 18 s until the irradiation was stopped
at 72 s. The results show that the proton concentration in the
thin film increased upon irradiation and reached its maximum
at ∼18 s, after which the proton concentration decreased
although more acids were generated from mPAHs.
Upon irradiation, the mPAHs were quickly converted to
their acidic state and released protons. The released protons
acidified the film and protonated MR. In the meantime, the
H+/OH− exchange between the film and PBS basified the film.
In the first 18 s, the acidification is faster than basification,
which resulted in an increase of MRH+. After 18 s, the
basification was faster than acidification even though more
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Figure 3. Absorption of the P-mPAH/P-MR film at 550 nm (a) and 438 nm (b) during and after irradiation.
acids were generated from the mPAHs. This is likely due to
two reasons. The rate of acid generation decreased with time
due to a decreased amount of the mPAHs. In addition, when
the protons were transferred from the photoacids to MR or
PBS, the photoacids became anions and formed salts with
MRH+ or the cations from PBS, which increased the
hydrophilicity of the film and consequently accelerated the
H+/OH− exchange. The two factors led to the decrease of
MRH+ in the film after 18 s. On the basis of the absorbance
change at 438 nm (Figure 3), we estimated that ∼3 × 10−2 M
of mPAHs had changed to the acidic form and released their
protons in the ∼8 μm film during the first 18 s. As described
below, the pH in the film was ∼5.5 at 18 s. The proton
concentration was ∼3 × 10−6 M, which was 4 orders of
magnitude lower than the concentration of the acidic form of
the mPAH. This indicates that most of the protons were
quickly neutralized by the buffer soaked in the film before
irradiation and diffused into the film during irradiation.
Since the mPAH is reversible, the pH pulse can be
reproduced after the mPAH recovers in the dark. To test
Figure 4. Changes of the absorption of the P-mPAH/P-MR film at
550 nm (up) and 438 nm (down) during the irradiation and recovery
cycles.
this, the sample was kept in the dark for 8 h in PBS. The
absorption at 438 nm grew back from 0.546 to 1.223
confirming the recovery of most of the photoacid (Figure 4).
The sample was then irradiated for a minute. The maximum
absorption at 550 nm (MRH+ absorption) during the
irradiation was 0.112, which was lower than that of the first
irradiation (0.143). The absorption at 438 nm decreased to
0.529 after irradiation. The sample was kept in the dark again
for 8 h. The absorption at 438 nm went back to 1.152, and the
absorption at 550 nm was 0.035, which was close to the value
after the first irradiation (0.036). The sample was then
irradiated for the third time. The maximum absorption
observed at 550 nm was 0.108, and absorption at 438 nm
went down to 0.535 after irradiation. To finish the third cycle,
the sample was kept in the dark for 8 h. The absorption at 438
nm went back to 1.100, and the absorption at 550 nm was
0.035. The results of the second and the third irradiation were
very close showing the reversibility of the system. It is also
worth mentioning that the maximum absorption at 550 nm
was observed at the third (18 s) or fourth scan (24 s) during
the irradiation in all three tests. (As shown in Figure 3, the
absorption values at the third and the fourth are very close.)
To quantify the pH change induced by irradiation, the very
same sample that was tested in the above experiments was
immersed in a pH 6 buffer and kept in the dark for 15 min.
The absorption at 550 nm increased from 0.035 to 0.104,
which is close and slightly lower than the maximum absorption
at 550 nm observed during the second and third irradiation
and is significantly lower than that of the first irradiation. The
pH 6 buffer was then replaced by a pH 5.5 buffer. The 550 nm
absorption reached 0.143, which is about the same as the
maximum absorption during the first irradiation. These results
show that the first irradiation produced a pH pulse with a
magnitude of ∼1.9 units and a minimum pH at 5.5, and the
following irradiations produced smaller but consistent pH
pulses of ∼1.4 unit with a minimum at ∼6.0.
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3. CONCLUSION
Article
10.75 (s, 1H), 8.99 (s, 1H), 8.55 (d, 1H, J = 15.6 Hz), 8.37 (d,
1H, J = 9.2 Hz), 8.30 (d, 1H, J = 7.2 Hz), 8.29 (s 1H), 8.25 (d,
1H, J = 16 Hz), 8.05 (d, 1H, J = 8 Hz), 7.86 (d, 1H, J = 9.2
Hz), 7.32 (t, 1H, J = 7.6 Hz), 6.50 (dd, 1H, J = 10 Hz), 6.36
(d, 1H, J = 15.2 Hz), 5.88 (d, 1H, J = 10 Hz), 5.13 (d, 2H, J =
7.6 Hz), 2.69 (d, 2H, J = 6 Hz), 2.25 (m, 2H, J = 6.4 Hz).
Synthesis of Photoacid Polymer P-mPAH. Photoacid
monomer 1 (20 mg), 2-hydroxyethyl methacrylate (380 mg),
and AIBN (2 mg) were dissolved in 1.5 mL of DMSO. After
the mixture was thoroughly purged with nitrogen, it was heated
at 60 °C overnight. After it cooled to room temperature, the
resultant viscous solution was added dropwise to diethyl ether
to precipitate out the polymer product (0.35 g). The loading
percentage of mPAH was measured by UV−vis spectroscopy.
A methanol solution of the polymer with known weight/vol
concentration was prepared, and its UV−vis spectrum was
taken. The molar concentration of the mPAH on the polymer
was calculated by dividing the absorbance at 438 nm by the
extinction coefficient of the photoacid (2.9 × 10−4 L mol−1
cm−1). The weight of the mPAH unit was then calculated from
the molar concentration, the solution volume, and the
molecular weight of the mPAH monomer. The wt % was the
weight of the mPAH unit divided by the weight of the polymer
In this work, a photoinduced pH change in an mPAH polymer
film in PBS has been demonstrated. The hydrophilic polymer
film is an open system that allows exchange of protons. A quick
release of the protons from the mPAH and proton exchange
between the PBS and the film resulted in a pH pulse. The
magnitude of the pulse is 1.4−1.9 units with maximum pH
change occurring after 18 s of the irradiation. Since the mPAH
is a reversible photoacid, the pH pulse can be repeatedly
generated after the photoacid recovered in the dark. This work
shows that photoinduced pH change based on mPAH could be
applied to biological systems even though their pH is
maintained by pH buffers.
4. EXPERIMENTAL SECTION
Unless otherwise noted, reagents and solvents were commer-
cially available and used as received without any further
purification. UV−vis spectra were obtained from a Varian Cary
60 Scan UV−vis spectrophotometer. NMR spectra were
determined in deuterated solvents on a Bruker av400 NMR
spectrometer. Chemical shifts were reported in delta (δ) units,
parts per million (ppm) downfield from TMS. The light
sources for irradiation were 470 nm LED arrays with 120 LEDs
purchased from www.theledman.com. Photon flux was
measured by an apogee quantum meter.
1
used for preparing the solution. H NMR (400 MHz, d6-
DMSO, δ ppm): δ = 7−9 (m, mPAH peaks, weak and broad,
difficult to integrate), 4.79 (b, 1H), 3.86 (b, 2H), 3.54 (b, 2H),
1.75 (b, 2H), 0.90 and 0.73 (b, 3H).
4.1. Synthesis of P-mPAH and P-MR (Scheme 1).
Synthesis of 2 (N-(2-Methylbenzothiazol-6-yl)acrylamide).
The starting material 6-methyl-2-aminobenzothiazole (1.50 g,
9.1 mmol) was dissolved in dry dichloromethane (18 mL).
Then, triethylamine (1.00 g, 10.0 mmol) was added, and the
mixture was stirred for 5 min. Next, the mixture was cooled in
an ice bath, and acryloyl chloride (0.87g, 9.60 mmol) was
added dropwise. The reaction mixture was stirred for 24 h at
ambient temperature and then poured into water. The product
was extracted with dichloromethane, and the organic layers
were dried with MgSO4. After the solvent was evaporated
under reduced pressure, the resultant crude product was
purified by silica gel column chromatography using 5:3
hexane/ethyl acetate as the eluent to obtain a light-yellow
Synthesis of 4 (2-((4-((2-Hydroxyethyl)(methyl)amino)-
phenyl)diazenyl)benzoic acid). A mixture of anthranilic acid
(0.5 g, 3.6 mmol), concentrated HCl (0.5 mL), and water (1
mL) was placed in an ice bath and then was diazotized by
adding a cold solution of sodium nitrite (0.25 g, 3.6 mmol, 0.5
mL water) dropwise; then, a cold solution of 2-(N-
methylanilino)ethanol (0.82 g, 5.4 mmol, 1 mL water) was
added to the cold diazonium salt. After that, the reaction
mixture was stirred for 15 min in an ice bath; then, a solution
of sodium acetate (0.52 g in 1 mL water) was added, and
stirring was continued for 1 h. The mixture was kept in a
refrigerator for 12 h, and then, it was left for 2 h at room
temperature. A solution of NaOH (2 mL, 20%) was added into
the reaction mixture. Consequently, it was left at room
temperature for 1 h. The product was filtered and washed
many times with water and acetic acid (3 mL, 10%) to remove
unreacted 2-(N-methylanilino)ethanol. The dried product
material was collected to give a dark-red powder in 65%
1
solid (1.67 g, 84% yield). H NMR (400 MHz, DMSO, δ
ppm): δ = 10.37 (s, 1H), 8.50 (s, 1H), 7.85 (d, 1H, J = 8.8
Hz), 7.56 (d, 1H, J = 8.8 Hz), 6.46 (dd, 1H, J = 10 Hz), 6.28
(d, 1H, J = 15.2 Hz), 5.78 (d, 1H, J = 10 Hz), 2.72 (s, 3H).
Synthesis of 3 ((6-Acrylamido-2-methylbenzothiazol-3-
ium-3-yl)propane-1-sulfonate). A mixture of compound 2
(0.2 g, 1.00 mmol), 1,3-propane sultone (0.16 g, 1.40 mmol),
and 1.00 mg of butylated hydroxytoluene (BHT) in ∼1 mL of
THF was stirred and heated (plate temperature 90 °C) in a
sealed vial for 4 days. The precipitate was washed with THF to
yield the product (0.24 g, 78% yield). 1H NMR (400 MHz, d6-
DMSO, δ ppm): δ = 10.72 (s, 1H), 8.96 (s, 1H), 8.37 (d, 1H, J
= 9.2 Hz), 7.86 (d, 1H, J = 9.2 Hz), 6.46 (dd, 1H, J = 10 Hz),
6.35 (d, 1H, J = 15.2 Hz), 5.86 (d, 1H, J = 10 Hz), 4.86 (t, 2H,
J = 8.4 Hz), 3.15 (s, 3H), 2.63 (t, 2H, J = 6.4 Hz), 2.14 (m,
2H, J = 6.8 Hz).
Synthesis of mPAH Monomer 1. Compound 2 (90 mg,
0.26 mmol), 1H-indazole-7-benzaldehyde (0.44 g, 3.0 mmol),
a catalytic amount of ammonium acetate, and 1 mg of BHT
were stirred in 1.5 mL of ethylene glycol and heated at 75 °C
overnight. The orange precipitate was washed with THF and
acetone to obtain a dark reddish orange solid (69 mg, 56%).
1H NMR (400 MHz, d6-DMSO, δ ppm): δ = 13.95 (s, 1H),
1
yield. H NMR (400 MHz, d6-DMSO, δ ppm): δ = 13.17 (s,
1H), 7.75 (d, 2H, J = 8.8 Hz), 7.71 (d, 1H, J = 2.4 Hz), 7.61
(d, 1H, J = 2.4 Hz), (t, 1H, J = 2.4 Hz), 7.48 (m, 1H), 6.88 (d,
2H, J = 9.2 Hz), 4.80 (t, 1H, J = 5.2 Hz), 3.60 (q, 2H, J = 5.6
Hz), 3.56 (t, 2H, J = 5.2 Hz), 3.09 (s, 3H).
Synthesis of 5 (2-((4-((2-(Acryloyloxy)ethyl)(methyl)-
amino)phenyl)diazenyl)benzoic acid). The mixture of 4
(0.45 g, 1.50 mmol) and triethylamine (0.3 g, 3.00 mmol)
was dissolved in anhydrous THF (10 mL). A solution of
acryloyl chloride (0.27 g, 3.00 mmol) in anhydrous THF (1.5
mL) was added dropwise to the above solution with
temperature kept below 10 °C. The reaction was then stirred
overnight at ambient temperature. The solvent was evaporated
under reduced pressure, and the crude product was purified by
column chromatography using dichloromethane and ethyl
acetate (9:1) as the eluent to obtain a dark red product, yield
1
0.10 g (20%). H NMR (400 MHz, d6-DMSO, δ ppm): δ =
13.12 (s, 1H), 7.76 (d, 2H, J = 9.2 Hz), 7.74 (d, 1H, J = 2 Hz),
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7.61 (d, 1H, and t, 1H, J = 3.2 Hz), 7.48 (m, 1H, J = 3.2 Hz),
6.97 (d, 2H, J = 9.2 Hz), 6.28 (d, 1H, J = 16 Hz), 6.13 (dd,
1H, J = 10.4 Hz), 6.95 (d, 1H, J = 10.4 Hz), 4.33 (t, 2H, J =
5.2 Hz), 3.81 (t, 2H, J = 5.6 Hz), 3.08 (s, 3H).
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δ ppm): δ 7.71, 7.58, 7.46, and 6.87 (b, MR peaks, 0.14 H),
4.78 (b, 1H), 3.86 (b, 2H), 3.53 (b, 2H), 1.75 (b, 2H), 0.90
and 0.73 (b, 3H).
4.2. Test of the Photoinduced pH Pulse in the P-
mPAH/P-MR Film. A mixture of 20 mg of P-mPAH and 5 mg
of P-MR was dissolved in 0.5 mL of methanol assisted by
ultrasonication. The solution was drop casted on glass slides
and dried in a ventilation hood for 15 min at room
temperature and then on a heat plate (plate temperature 70
°C) for 15 min. A reddish thin film was obtained and kept in
the dark until it was tested. The thickness of the film was
measured by a micrometer gauge with a resolution of 1 μm.
Before the test, the thin film on a glass substrate was cut into
small pieces that can fit into a regular quartz cell for a UV−vis
spectrometer. The sample was soaked in water for 15 min and
then in PBS for 30 min, after which the color of the film
changed to orange. It was then put into a quartz cell filled with
PBS with a tilt angle (Figure 2). The sample was irradiated
from the top by a 470 nm LED array, and the UV−vis
absorption change was monitored by a UV−vis spectrometer
during and after irradiation. The photon flux of the irradiation
(∼425 μmol s−1 m−2, 11 mW/cm−2) was measured by putting
the detector of a hand-held quantum meter at the same
distance to the LED as the center of the sample. It is worth
mentioning that although PBS does not absorb 470 nm light,
the photon flux on the sample in PBS could be a little lower
than the measured value due to reflection and scattering. As
described above, the sample was irradiated three times. Each
irradiation lasted about 1 min, and the interval between the
irradiation times was ∼8 h. To quantify the pH change, PBS
was removed from the cell 8 h after the last irradiation. A pH 6
buffer (citric acid/Na2HPO4) was added slowly to the cell
without moving the position of the sample. UV−vis spectrum
was collected after the sample was kept in the new buffer for
∼15 min. The pH 6 buffer was then substituted by a pH 5.5
buffer, and the UV−vis spectrum was collected after the
sample was kept in the new buffer for ∼15 min.
AUTHOR INFORMATION
■
(18) Tatum, L. A.; Foy, J. T.; Aprahamian, I. Waste Management of
Chemically Activated Switches: Using a Photoacid to Eliminate
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Corresponding Author
*E-mail: yliao@fit.edu.
ORCID
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Transfer. J. Photochem. Photobiol., A 2015, 300, 22−26.
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(21) Samanta, D.; Galaktionova, D.; Gemen, J.; Shimon, L.; Diskin-
Posner, Y.; Avram, L.; Kral, P.; Klajn, R. Reversible Chromism of
Spiropyran in the Cavity of a Flexible Coordination Cage. Nat.
Commun. 2018, 9, 641.
Yi Liao: 0000-0002-8217-8202
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support from the National Science Foundation (1565613) is
gratefully acknowledged.
F
DOI: 10.1021/acs.jpcb.8b11677
J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcb.8b11677
J. Phys. Chem. B XXXX, XXX, XXX−XXX