Molecular assembly of a squaraine dye with cationic surfactant and nucleotides: Its impact on aggregation and fluorescence response

Article abstract of DOI:10.1039/c0ob01061h

A novel fluorescent system has been assembled by using ATP, surfactant, and a squaraine dye in an aqueous buffer solution. In the system, a cationic surfactant such as cetyl trimethyl ammonium bromide (CTAB) forms a sphere-like micelle, whose positive charge at the surface of the micelle attracts the negatively charged ATP to form a unique organized nanostructure. Such an organized system is shown to interact with the squaraine dye (SQ) to perturb its aggregate structure, thereby generating the optical response. The nanostructure of the assembly has been characterized by dynamic light-scattering (DLS) and atomic force microscopy (AFM). The unique feature of the developed sensing system is that the analytes ATP form part of the assembly structure. The system utilizes forces such as electrostatic interaction and π-π stacking of the aromatic segment of ATP and SQ to achieve the selective detection of ATP.

Full text of DOI:10.1039/c0ob01061h

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Organic &
Biomolecular
Chemistry
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PAPER
Molecular assembly of a squaraine dye with cationic surfactant and
nucleotides: its impact on aggregation and fluorescence response†
a
b
c
a
Yongqian Xu, Andrey Malkovskiy, Qiuming Wang and Yi Pang*
Received 22nd November 2010, Accepted 1st February 2011
DOI: 10.1039/c0ob01061h
A novel fluorescent system has been assembled by using ATP, surfactant, and a squaraine dye in an
aqueous buffer solution. In the system, a cationic surfactant such as cetyl trimethyl ammonium
bromide (CTAB) forms a sphere-like micelle, whose positive charge at the surface of the micelle attracts
the negatively charged ATP to form a unique organized nanostructure. Such an organized system is
shown to interact with the squaraine dye (SQ) to perturb its aggregate structure, thereby generating the
optical response. The nanostructure of the assembly has been characterized by dynamic light-scattering
(
DLS) and atomic force microscopy (AFM). The unique feature of the developed sensing system is that
the analytes ATP form part of the assembly structure. The system utilizes forces such as electrostatic
interaction and p–p stacking of the aromatic segment of ATP and SQ to achieve the selective detection
of ATP.
Introduction
ATP, however, few of these sensors show turn-on emission upon
ATP binding in aqueous medium over PPi.
5
e–g
The design and construction of chemical sensors that selectively
recognize anions has received ever-increasing attention in recent
years, because anions are ubiquitous in biological systems.
Squaraines form a class of novel dyes possessing sharp and
intense absorption and fluorescence in the red to near-infrared
1
6
region. In the solution, squaraine dyes can be spontaneously
Among various anions, detection of phosphates, pyrophosphate
assembled with chromophores in a parallel-oriented fashion (H-
aggregates), or in a head-to-tail arrangement (J-aggregates) by
varying the solvents or adding ionic species. The J-aggregates
give red-shifted absorption bands (as compared with monomer
absorption), and H-aggregates give blue shifted absorption bands.
The different form of the aggregate also affects the emission
properties, with H-aggregates usually being poor emitters while
2
(
PPi), and nucleotides is of special interest. As a multifunctional
nucleotide, adenosine triphosphate (ATP) functions as an energy
shuttle to power nearly all forms of cellular work, from the
generation of light by fireflies to the movement of muscle cells.
It also provides an energy source for biosynthetic reaction,
3
consumption of many enzymes and cell division. Adenosine
diphosphate (ADP) is another nucleotide, which is the product of
ATP dephosphorylation by ATPases and can be converted back
to ATP by ATP synthases. The conversion of these two molecules
plays a critical role in supplying energy for many processes of
7,8
J-aggregates often give efficient luminescence. The distinction
in optical characteristics between J- and H-aggregates can be
used as an effective means to monitor the molecular assembly.
Although ATP has been demonstrated to be a building block
4
life. Thus, selective detection of ATP from ATP/ADP in aqueous
9
to aid the H-aggregate formation of a cyanine dye, no study
media has emerged as an important area of research. Although
several studies have employed colorimetric and fluorescent probes,
electrostatic and hydrogen bonding interactions to recognize ATP,
it remains a challenge to find new approaches to simplify ATP
detection while keeping higher selectivity over other nucleoside
triphosphates such as GTP. Fluorescent probes as a conventional
and highly sensitive analytical method have been used to detect
has been found utilizing the electrostatic interaction (between the
negatively charged ATP and positively charged squaraine dye)
to influence the aggregate structures of squaraine dye (SQ) in
aqueous solution. An interesting question is whether such an
aggregate structure change, if induced, could lead to a useful
spectroscopic response to detect ATP.
5
Molecular self-assembly offers an excellent tool to construct, via
a “bottom-up” approach, diverse supermolecular architectures in
which the organization of individual molecules can be controlled
a
Department of Chemistry & Maurice Morton Institute of Polymer Science,
10
The University of Akron, Akron, OH, 44325. E-mail: yp5@uakron.edu
by proper selection of different self-assembling components.
b
Department of Polymer Science, The University of Akron, Akron, OH,
For example, surfactants are well known to self-assemble in
4
4325
aqueous solutions to give organized nanostructures (e.g., mi-
c
Department of Chemical and Biomolecular Engineering, The University of
Akron, Akron, OH, 44325
11
celles and vesicles). Utilization of surfactants in the chemical
sensor ensemble has led to detection of several transition-metal
†
Electronic supplementary information (ESI) available: Synthetic proce-
12
13
-
2+
dures, additional spectroscopic data. See DOI: 10.1039/c0ob01061h
cations, proteins and other analytes such as Cl , pH, Ba and
2
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3 3
Scheme 1 Structures of SQ, ATP and schematic illustration of the working micellar system. In the SQ structure, the counter ion CF is not shown
SO -
for clarity.
1
4
lipophilicity. The current strategy is limited to encapsulating a
fluorophore in the micelle, whose fluorescence is then quenched by
an analyte (e.g., by using heavy atom effect or electron transfer).
The well-defined nanostructure of the surfactant micelle raises
the possibility that it can be used as an assembly building block.
To demonstrate its feasibility, we have examined a ternary system
that consists of ATP, squaraine dye, and cationic surfactant (cetyl
trimethyl ammonium bromide (CTAB)) as illustrated in Scheme 1.
The basis for our hypothesis is as follows: in the system, CTAB
monomeric form (Fig. 1). Solvent polarity only slightly affects the
absorption peak, with lmax = 646 nm in the nonpolar toluene and
l
max = 631 nm in methanol. To assess the feasibility of detecting
ATP at physiological pH values, the sensing ability of SQ toward
ATP was examined in an aqueous solution containing phosphate
buffer (10 mM, pH 7.20) and 1% EtOH. Poor water solubility
requires us to first dissolve SQ dye in EtOH, which was then
diluted in water.
15
can form a sphere-like micelle, whose positive charge at the
surface of the sphere attracts the ATP to form a unique organized
nanostructure through electrostatic interaction. It is known that
squaraine dye tends to form dimers or H-aggregates through p–p
7
stacking, owing to its poor solubility in aqueous media. Upon
interaction with ATP at the outer surface of the nanostructured
sphere, the squaraine dyes in the H-aggregate form are found to
be changed to the J-aggregate form (head-to-tail arrangement), as
a consequence of electrostatic interaction between the squaraine
16
and the phosphate group of ATP. In this study, we report the
formation of a nanostructured assembly by using CTAB, ATP
and squaraine SQ. The optical responses of SQ, induced by the
assembly structure, exhibit potential for desirable ATP detection
in aqueous solution.
Fig. 1 UV–vis absorption spectra of SQ (5 mM) in different solvents.
In the aqueous buffer solution, SQ showed characteristic
absorption maxima at 547, 569 and 627 nm (Fig. 2a). The
absorption band at 627 nm was assigned to the monomeric species.
On the basis of the observed spectral blue shift, the bands at 547
and 569 nm were attributed to the H-aggregates (in the dimer
Results and discussion
17
The SQ dye was synthesized as reported previously. UV–vis
absorption spectra of SQ in various organic solvents gives one
band at about 635 nm with similar absorbance, attributed to the
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Fig. 2 Absorption change (a) and fluorescence response (b) of SQ (5 mM) in phosphate buffer (10 mM, pH 7.2) upon addition of CTAB (0–333 mM),
followed by ATP (0–133 mM). The emission spectra were acquired when the solutions were excited at 620 nm.
7,8
or oligomeric form). The absorption intensity of these three
peaks increased slightly with increasing CTAB concentration (Fig.
S2, ESI†). After the CTAB concentration in the solution reached
0
.33 mM, ATP was added. When the ATP concentration was
increased, the aggregate absorption peaks at 547 and 569 nm
weakened, while the monomeric absorption at 627 nm went up.
No change in the absorption spectra was observed upon addition
2
-
of other ions, including AMP, PPi, phosphate, adenosine, CO ,
3
-
2-
-
-
-
-
+
+
2+
2+
AcO , SO
4
, NO , Cl , Br , I , Na , K , Ca , Mg . The specific
3
optical response from ATP, which was visible (Fig. 3), indicated
that the SQ could be used as colorimetric probe for ATP in
aqueous buffer solution. Addition of ATP also induced a shoulder
at ~665 nm, suggesting the formation of a new chemical species-
Fig. 4 Percentage change of SQ (5 mM) in phosphate buffer (10 mM,
pH 7.2) upon addition of CTAB (0–333 mM), followed by ATP (0–200 mM).
Since the concentration of each SQ species is proportional to the
18
“SQ-surfactant” complex, although formation of J-aggregate
remained a possibility.
i
corresponding absorbance A , the percentage change is calculated to be
A
i
/(A547 + A569 + A627). Here, the absorption at 547, 569, and 627 nm was
used to approximate the enriched H-dimer, oligomer H-aggregate, and
monomer, respectively.
Fig. 3 Color changes induced in SQ (5 mM) in the presence of CTAB
(
333 mM) in 3 mL phosphate buffer (10 mM, pH 7.2) upon addition of
different anions and adenosine (0.2 mM).
In order to understand the sensing process, the percentage
change of each species (dimer and higher H-aggregate, and
monomer) was correlated with the addition of CTAB, followed by
ATP. As seen in Fig. 4, the critical micellar concentration (CMC)
19
of CTAB can be obtained at 133.3 mM. In the micellar solution
CTAB concentration = 133–333 mM), ATP created a favorable
(
environment to transfer squaraine dye from H-aggregates to
monomer to some extent. Since CTAB could disperse squaraine
aggregates like other surfactants (Fig. S6, ESI†), CTAB concen-
tration in the solution was controlled below 0.33 mM.
Fig. 5 Comparison of the fluorescence response of SQ (5 mM) in
phosphate buffer (10 mM, pH 7.2) upon addition of various concentrations
of CTAB and ATP.
20
SQ exhibited a fluorescence maximum at ~650 nm in aqueous
buffer solution, which can be attributed to the monomer since
squaraine dye typically has a small Stokes’ shift (about 10–30 nm).
Although the anionic phosphate ATP could be associated with
the cationic SQ species, addition of ATP (up to 2 mM) to the SQ
solution induced nearly no fluorescence response in the absence
of CTAB (Fig. 5 and Fig. S7†). The control experiment clearly
indicated that an adequate amount of CTAB was necessary for
the assembly to work. As illustrated in Fig. 2b, CTAB had a
negligible effect on the fluorescence spectra of SQ when CTAB
concentration was low (<0.33 mM). Subsequent addition of ATP
to the SQ solution containing CTAB (0.33 mM concentration),
however, resulted in a new fluorescence peak at 676 nm (Fig. 2b).
2
880 | Org. Biomol. Chem., 2011, 9, 2878–2884
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The current assembly design utilized 333 m M CTAB to form
just sufficient micelles, which were used as the building blocks
for further assembly with the ATP and SQ. In other words, the
strategy provided the optimum condition to incorporate the ATP
and SQ into the desirable assembly, while avoiding the interruption
of the SQ aggregate structure by excessive CTAB.
Excitation spectra obtained by monitoring the emission at
6
45 and 676 nm revealed maxima at 630 and 643 nm (Fig. 6),
respectively. In the excitation spectra, a shoulder at ~585 nm
was also visible, which was attributed to the H-aggregate (in
consistency with its absorption at 569 nm). The emission band at
the longer wavelength (676 nm), therefore, indicated the formation
of a new species, which was likely to be the J-aggregate from
21
SQ on the basis of the spectral red-shift. Although the control
experiment by using excess CTAB also exhibited an emission at
6
76 nm, a similar signal intensity at ~676 nm required the use
of a higher concentration of CTAB (in comparison with ATP)
Fig. 5). In summary, the fluorescence at 645 nm was attributed to
(
Fig. 7 Solvodynamic diameters of CTAB (333 mM) aggregates, CTAB
with SQ, and CTAB with SQ and ATP in water determined by dynamic
light scattering.
SQ monomer and the emissison at 676 nm to its J-aggregate.
in addition to a significantly larger particle at ~3000 nm. Lack
of characteristic sizes from either SQ or SQ–CTAB in solutions
suggested that the well-defined assembly was not occurring in the
absence of ATP. Interestingly, addition of ATP (to the solution
including CTAB and SQ) led to a defined nanostructure with a
diameter of about 570 nm, clearly indicating that a self-assembly
process took place in the solution. A significant increase in
the dimension of the CTAB–SQ–ATP assembly (570 nm), in
comparison with that of CTAB micelle (~220 nm), supported
the assumption that the ATP and SQ were interacting with
CTAB micelle at its outer perimeter. Light scattering data also
revealed a second species (size at ~5200 nm) for CTAB–SQ–ATP
assembly, suggesting that the SQ molecule could link multiple
CTAB micelles during the assembly process.
Fig. 6 Excitation spectra of SQ (5 mM) in the presence of CTAB (333 mM)
and ATP (133 mM).
Dynamic light scattering (DLS) techniques were employed
to probe the size of the self-assembled structures in solution
quantitatively. CTAB (333 mM) exhibited average solvodynamic
diameters of 220 nm, corresponding to its micelle formation
Atomic force microscopy (AMF) further confirmed the inter-
action pattern. The CTAB micelle was detected to be slightly less
than 0.1 mm (Fig. 8). Addition of SQ to CTAB did not alter
the micelle size. From the CTAB+SQ+ATP sample, however, two
distinctive sizes were observed. One species had a dimension of
about 0.1–0.15 nm, in consistency with the proposed assembly
pattern. A second species with significant larger size (~0.4–1 nm)
was also observed, in agreement with the two distinct species
(
Fig. 7). The solution of SQ (5 mM) revealed a broad distribution of
species (with average size of about 430 nm), attributed to H- and J-
aggregates of various sizes. The solution of CTAB and SQ revealed
a species at ~400 nm (similar to the aggregate of SQ alone),
Fig. 8 AFM images of films spin-cast from the CTAB, CTAB+SQ, and CTAB+SQ+ATP solutions. The image of CTAB+SQ+ATP contains the single
species of dimension ~0.1 mm (small arrow) and connected species (thick arrow).
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detected in the light scattering experiment. The consistent trend,
which was revealed from both light scattering and AFM images,
supports the assumption that the ATP was assembled on the
surface of CTAB micelles, resulting in increased particle sizes
(
Scheme 1). It should be noted that the particle sizes from AFM
are different from those detected from DLS. This is because DLS
is measured in solution where solvation results in the larger sizes,
while AFM measures the sizes of dried particles which are laid on
the AFM grid surface. Correlation of AFM and DLS data (Fig. 7
&
8) showed that the CTAB alone existed as spheric micelles
in solution, with an average size of 220 nm. The predominant
particle size in the solution of “SQ+CTAB+ATP” assembly was
about twice as large (~570 nm), and had similar size distribution
to CTAB. The results pointed to the fact that the particles in
the “SQ+CTAB+ATP” solution were likely to be spherical. Both
DLS and AFM data also showed a larger species that existed as a
minor component (Fig. 7).
To validate the specificity of the system toward the anionic
guests in practice, the fluorescence response of SQ in aqueous
buffer solution was examined upon addition of various biolog-
ically important anions and cations. No apparent fluorescence
enhancement was observed for AMP, PPi, phosphate, adenosine,
2
-
-
2-
-
-
-
-
+
+
2+
2+
CO
3
, AcO , SO
4
, NO
3
, Cl , Br , I , Na , K , Ca , Mg (Fig. 9,
(herein I is the fluorescence
Fig. S11, ESI†). The ratio of I670/I
0
0
intensity of free SQ at 670 nm; I670 represents the fluorescence
intensity at 670 nm upon addition of different ions) is plotted
against the concentration of various ions (Fig. 9). The I670/I
value for ATP reached up to 3.1-fold, in sharp contrast to I670/I
0
0
<
1.1-fold for other ions. Therefore, this system could be used as a
selective sensor for ATP. A good linear correlation was observed
between the relative fluorescence intensity and ATP concentration
(
1
0–140 mM) (Fig. 10), and the detection limit was in the order of
-
6
0
M when the ratio of signal to noise was 3. The Hill’s plot
gave a Hill coefficient of 0.78 (Fig. S1†), which indicates negative
22
cooperative interaction.
The selective fluorescence response to ATP appeared to be
associated with the electrostatic interaction between SQ and
ATP. As illustrated in Scheme 1, a cationic surfactant was
necessary to form the spherical micelle core with a positive
charged surface. When the cationic surfactant CTAB was replaced
by an anionic surfactant such as SDS (sodium dodecyl sulfate)
or Triton X-100 (isooctylphenolicpolyglycol ether), the resulting
system showed no fluorescence response to ATP. The assembly
of the ordered system (Scheme 1) appeared to favor addition in
the following sequence: CTAB, then ATP to the SQ solution. If
the addition sequence is reversed (i.e. ATP first, then CTAB), the
resulting system exhibited a much weaker fluorescence response
0
Fig. 9 (a) Structure of phosphates. (b) Relative fluorescence ratio (I670/I )
of SQ (5 mM) in the presence of CTAB (333 mM) in phosphate buffer
(10 mM, pH 7.2) upon addition of different ions (0.2 mM).
showed that SQ gave the optimum fluorescence response at a
concentration of 5 mM. Higher SQ concentration resulted in
decreased fluorescence response, possibly attributing to a higher
degree of aggregation.
The experimental data were consistent with the proposed model
for the assembly of the ordered system from CTAB, SQ and
ATP (Scheme 1). Driven by the assembly forces, part of the
SQ was changed from the H-aggregate to the more fluorescent
monomer and J-aggregate form, resulting in a spectral red-
shift and fluorescence enhancement. In the absence of ATP, the
surfactant CTAB initially formed the micelles, which also allowed
the SQ to be partially dissolved in the hydrophobic cavity of the
micelles as monomeric or aggregated form (Fig. 11, assemblies I
and II). CTAB concentration in the solution was thus controlled
to below 0.33 mM, thereby creating a favorable environment
for the CTAB–SQ–ATP assembly III while avoiding excessive
solubilization of SQ by CTAB surfactant. Formation of assembly
III was energetically more favorable, which was enhanced by the
(
Fig. S10, ESI†) than that observed in Fig. 2. If no CTAB existed
in the solution, there was no response to ATP (Fig. 5). Therefore,
formation of spherical micelles with a positively charged surface
was a necessary step. The result clearly pointed to the fact that an
assembly with an ordered structure might play an important role
in the observed fluorescence response. The results also support
the assumption that, in the second step, electrostatic interaction
guided the phosphate terminals of ATP to the positively charged
surface of the spherical micelle.
Based on the design, the concentration of the squaraine reporter
is expected to affect the performance of the sensor assembly.
Screening of the dye concentration (between 1–20 mM, Fig. S17†)
2
882 | Org. Biomol. Chem., 2011, 9, 2878–2884
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p stacking interaction could be used to differentiate different
triphosphates. Owing to steric hindrance, SQ molecules had a
tendency to exist in J-aggregate form rather than H-aggregate
form in the cavity of the SQ–ATP–micelle assembly (Scheme 1).
Observation of a new band, 664 nm in absorption and 676 nm
in fluorescence, is consistent with the proposed ternary structure.
The moderate selectivity between ATP and ADP was associated
with the length of phosphates, as the triphosphate provided more
space to accommodate the SQ aggregates, which can simultane-
ously interact with the phosphate (electrostatic interaction) and
adenosine segments (p–p interaction).
To detect ATP in the biological system, it is necessary to
Fig. 10 Relative fluorescence ratio (I670/I ) of SQ (5 mM) in the
0
assess the selectivity in the presence of alkali and alkali-earth
2
+
presence of CTAB (333 mM) in phosphate buffer (10 mM, pH 7.2) upon
addition of different ions (0–0.2 mM). The broken line on the ATP curve
shows the linear correlation between the fluorescence response and ATP
concentration (0–140 mM).
metal cations. It is known that ATP can bind cations Mg (at
-
1
2+
-1
+
-1
+
-1 24
9554 M ), Ca (at 3722 M ), Na (at 13 M ) and K (at 8 M ).
The influence of various cations on the fluorescence of SQ was
+
+
2+
2+
examined by addition of Na , K , Ca and Mg (Fig. S12,
+
+
ESI†). In the presence of 0.2 mM of various cations, Na and K
exhibit little influence on the fluorescence enhancement, whereas
the presence of Ca and Mg suppressed the fluorescence increase
to some extent. However, these results do not interfere with the
sensing process.
electrostatic interaction between the oppositely-charged species
CTAB–ATP and ATP–SQ, in contrast to the assemblies I and
II that involved the interaction of two positively charged species.
Excessive CTAB could turn on the fluorescence of SQ (Fig. 5),
since the increasing micelle concentration makes the formation of
I and II a more competitive process.
2
+
2+
Conclusions
It was assumed that the SQ was attracted to the phosphate
group of ATP through electrostatic interaction. In this step, the
two phosphate groups appeared to be necessary (one attaching
to positively charged surface of the spherical micelle, the other
to SQ) for the assembly. This was likely to be the reason for the
selective recognition of ATP over AMP, adenosine, phosphate and
other anions. The p–p stacking interaction between SQ backbone
and the aromatic segment of ATP could also play an important
In summary, we have demonstrated a new ternary system, which
incorporates ATP and SQ into an ordered molecular self-assembly
to induce fluorescence turn-on. The molecular assembly is driven
by a balanced electrostatic interaction and p–p stacking of the aro-
matic segment of ATP and SQ. The sensor operates by attenuating
the aggregate structure of squaraine dyes (from H- to monomer
and J-aggregates) under physiological pH conditions. The study
provides a simple approach for the detection of ATP. Due to the
specific requirements for the analyte to enter the nano-structured
molecular assembly, the ternary system exhibits good selectivity to
sense ATP. The new sensor system provides an additional example
23
role in the stability of the SQ–ATP–micelle assembly, which
accounted for the selective recognition of ATP over PPi. The
different response between the nucleotide triphosphates ATP and
GTP (Fig. 9 and 10) further pointed to the fact that the p–
Fig. 11 Micelle-based equilibrium to redistribute the H-aggregate SQ into monomeric and aggregate form.
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that operates by recognizing a specific pattern transformation
12 (a) P. Grandini, F. Mancin, P. Tecilla, P. Scrimin and U. Tonellato,
Angew. Chem., Int. Ed., 1999, 38, 3061–3064; (b) F. Mancin, P. Tecilla,
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(
i.e. here from H- to J-aggregate) that has the advantage of
25
simpler molecular design in comparison with typical sensors.
(
c) P. Pallavicini, Y. A. Diaz-Fernandez and L. Pasotti, Coord. Chem.
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involves a lengthy synthetic procedure. Supramolecular assembly
using easily accessible components, therefore, could provide a cost-
effective solution.
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13 (a) M. A. Azagarsamy, V. Yesilyurt and S. Thayumanavan, J. Am.
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