A novel surfactant-like fluorophore and its probing ability to the aggregation of amphiphilic compounds

Article abstract of DOI:10.1016/j.jphotochem.2012.07.001

A novel surfactant-like non-ionic fluorophore, 3-(bis(2-hydroxyethyl)amino) -N-(4-(pyrene-1-sulfonamido)butyl) propanamide (PSDA-DEA), was designed and prepared. Fluorescence and surface tension studies revealed that the fluorophore aggregates in aqueous medium, and its critical aggregation concentration (CAC) is ～3.3 × 10^-5 M. Cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS) and atomic force microscopy (AFM) measurements demonstrated that the diameter of the aggregates as formed is of a few hundred nano-meters. It was also shown that the profile of the emission spectrum of the compound is well dependent upon the polarity of its medium, as indicated by a change up to 53% of its intensity ratio at 399 nm and 380 nm (I₃₉₉/I₃₈₀) when the solvent changed from water to ethanol. It is of the polarity sensitive property that the fluorophore can be used for monitoring micelle formation of several anionic, cationic and non-ionic surfactants. Furthermore, PSDA-DEA is also a valuable probe for sensing transition of different aggregates such as micelles to vesicles. Comparative studies demonstrated that the present probe is more versatile than pyrene when it is used as a fluorescence probe.

Full text of DOI:10.1016/j.jphotochem.2012.07.001

Journal of Photochemistry and Photobiology A: Chemistry 245 (2012) 58–65
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A novel surfactant-like fluorophore and its probing ability to the aggregation of
amphiphilic compounds
Lei Zhanga, Lining Gaob,1, Qiaojun Liua, Feipeng Yangb, Yu Fanga,∗
a Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China
b Engineering Research Center of Transportation Materials, Ministry of Education, School of Materials Science and Engineering, Chang’an University, Xi’an 710064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
novel surfactant-like non-ionic fluorophore, 3-(bis(2-hydroxyethyl)amino)-N-(4-(pyrene-1-
Received 5 March 2012
Received in revised form 11 June 2012
Accepted 2 July 2012
sulfonamido)butyl) propanamide (PSDA-DEA), was designed and prepared. Fluorescence and surface
tension studies revealed that the fluorophore aggregates in aqueous medium, and its critical aggregation
concentration (CAC) is ∼3.3 × 10−5 M. Cryogenic transmission electron microscopy (cryo-TEM), dynamic
light scattering (DLS) and atomic force microscopy (AFM) measurements demonstrated that the
diameter of the aggregates as formed is of a few hundred nano-meters. It was also shown that the
profile of the emission spectrum of the compound is well dependent upon the polarity of its medium, as
indicated by a change up to 53% of its intensity ratio at 399 nm and 380 nm (I399/I380) when the solvent
changed from water to ethanol. It is of the polarity sensitive property that the fluorophore can be used
for monitoring micelle formation of several anionic, cationic and non-ionic surfactants. Furthermore,
PSDA-DEA is also a valuable probe for sensing transition of different aggregates such as micelles to
vesicles. Comparative studies demonstrated that the present probe is more versatile than pyrene when
it is used as a fluorescence probe.
Available online 7 July 2012
Keywords:
Fluorescence probe
Surfactant
Aggregation
Pyrene
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
microscopy and light scattering can only provide limited informa-
tion about the structures of the aggregates at a molecular level.
Abundant information relevant to intra-/inter-molecular inter-
actions and supramolecular aggregation and dis-aggregation of
organic molecules can be obtained by utilization of fluorescence
techniques [1–4]. This is because compared to other techniques,
fluorescence techniques possess a number of superiors including
but not limited to multiples in parameters, which can be selected
for meeting different purposes, abundance in fluorophores, which
are the bases for the techniques to find a wide variety of appli-
cations, and easiness in use, etc. In addition, the techniques are
sensitive to small changes in the micro-environment of the rele-
vant fluorophores. However, selection of a suitable fluorophore is a
priority for the successful utilization of the techniques. Practically,
these fluorophores can be introduced into the systems to be studied
either as probes or as labels [1,5–8]. For a fluorophore to be used
as a label, it must be chemically attached at a specific position of a
system to be studied, but for the one to be used as a probe, it is, as
a general practice, simply introduced in a physical way.
Different from the methods as mentioned, fluorescence probes
are highly sensitive to the changes of the properties of their
micro-environment, such as micro-polarity and micro-viscosity,
and thereby they can provide more detailed information of the
systems under study [1,5,6,9–13].
In the studies of amphiphilic aggregation, positioning of the flu-
orescence probe in the aggregates is a key issue. However, for most
commercially available probes, they are not ideal for this kind of
studies because the fluorescence parameters recorded are com-
binations of the contributions from the molecules of the probes
directly sensing the process and those far away from the process.
As an example, pyrene and 1,6-diphenyl-1,3,5-hexatriene (DPH)
have been widely used to determine the critical micelle concen-
tration (CMC) of various surfactants since they are typical polarity
or mobility probes [4,14–16]. These probes, however, are only
effective for monitoring formation of micelles of surfactants, and
display poor ability to probe transformation of different aggre-
gates of them. To improve the performances of pyrene-like known
fluorescence probes in monitoring formation of complex aggre-
gates of surfactants, various efforts have been made [14,17,18].
One of the strategies to solve this problem is to use fluores-
cence probes which behave as surfactants in aqueous medium.
It is believed that this kind of fluorescence probe may take part
in the formation of the aggregates, and thereby provides reliable
In the studies of various aggregates, such as micelles, vesi-
cles and lamellar phases, traditional methods including electron
∗
Corresponding author. Tel.: +86 29 85300081; fax: +86 29 85310097.
E-mail addresses: yfang@snnu.edu.cn, yufangsnu@gmail.com (Y. Fang).
Equal contribution to the present work as made by the first author.
1
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.07.001

L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 245 (2012) 58–65
59
1,4-Diaminobutane (Acros, 99%) and diethanolamine (DEA, 99%)
were used as received without further purification. Triethylamine
(TEA) was vacuum distilled over CaH2. Sodium dodecyl sulfate
(SDS, 99%) and polyoxyethylene (10) isooctylphenyl ether (Triton
X-100, 99%) were purchased from Acros, and were used as received
without further purification. Dodecyltrimethylammonium bro-
mide (DTAB, 99%) and dodecyltriethylammonium bromide (DEAB,
99%) were synthesized from n-alkyl bromide and correspond-
ing trialkylamine. These two products were re-crystallized from
acetone–ethanol for five times. The purity of all the synthesized
cationic surfactants was examined and no surface tension min-
imum was found in the surface tension curve. Other starting
materials were purchased commercially and used as received. Sol-
vents were of analytical grade, unless otherwise noted. Water used
throughout was de-ionized and then double distilled.
2.2. General instruments
1H NMR and 13C NMR spectra were recorded by a Fourier Digi-
tal NMR spectrometer (AVANCE 300 MHz, 300 MHz) in CDCl3, and
chemical shifts are reported in parts per million relative to TMS
in proton spectra. All 13C spectra were determined with complete
proton decoupling. Dynamic light scattering (DLS) measurements
for aggregate diameters were determined by BI – 90Plus laser par-
ticle size analyzer (Brukerhaven Instrument, USA) equipped with
a 15 mW solid-state laser (659 nm). AFM measurements were con-
ducted on a SOLVER P47 PRO system. The sample was prepared by
drop casting the dilute aqueous solution of PSDA-DEA (5 × 10−5 M)
onto a freshly cleaved, mica surface. For cryogenic transmission
electron microscopy (cryo-TEM) characterization, samples were
prepared in a controlled environment vitrification system (CEVS)
at preset temperature. Typically, a 5 ␮L sample solution was loaded
onto a carbon-supported lacey TEM grid, which was held by tweez-
ers. The excess solution was blotted with a piece of filter paper,
resulting in the formation of thin films suspended on the mesh
holes, and the samples were quickly plunged into a reservoir of
liquid ethane (cooled by liquid nitrogen) at its melting tempera-
ture. The vitrified samples were then stored in liquid nitrogen until
they were transferred to a cryogenic sample holder (Gatan 626)
and examined using a JEM 2200FS TEM (200 keV) at about −174 ◦C.
The phase contrast was enhanced by under focus. The images were
recorded on a Gatan multiscan CCD and processed with Digital
Micrograph.
All of the fluorescence measurements were conducted with a
time-correlated single photon counting Edinburgh FLS 920 fluores-
cence spectrometer at room temperature (25 ◦C). The path length
of the quartz cell is 1 cm. The samples containing PSDA-DEA and
pyrene were both excited at 345 nm and determined at 380 nm.
Fluorescence lifetimes were calculated from time-resolved fluores-
cence intensity decays using an FLS 920 fluorescence spectrometer
in the time-correlated single-photon-counting mode. FLS 920 is
equipped with a thyratron-gated nanosecond flash lamp filled with
hydrogen as the plasma gas (0.38–0.40 bar) and is operated at
40 kHz. For each measurement, at least 5000 photon counts were
collected in the peak channel to ensure the decay quality. The value
of ꢀ2, which is an indicator of the goodness of fit to a decay curve,
is close to 1, and generally it is less than 1.2.
Scheme 1. (a) Structure of the PSDA-DEA and (b) molecular dynamics modeling of
the PSDA-DEA.
information which is directly relevant to the formation and struc-
tures of the aggregates. For example, Huang and co-workers [19]
designed an anionic surfactant-like fluorescence probe, sodium 12-
(N-dansyl)aminododecanate (12-DAN-ADA), which well remains
the polarity and viscosity sensitive properties of dansyl, a key
structure of the probe, in aqueous medium. It was shown that
the emission maximum and fluorescence anisotropy of the probe
changes greatly along with transformation of the aggregates from
micelles to vesicles, a change hardly observed by employing pyrene
or DPH as a probe [14,16]. However, the probe reported by them
cannot be used for monitoring aggregation of cationic surfactants in
aqueous medium due to electrostatic attraction between the probe
and the surfactants. Even so, Huang’s probe is still distinctive. This
is because in the design Huang and co-workers put intentionally
the dansyl group at the far end of the surfactant tail. It is believed
that this design may report changes occurring in the most interior
of the aggregates under study.
It is well known that the aggregation behavior of surfactants
in aqueous medium is usually complex and depends strongly on
their structure and solution conditions. Therefore, interrogation of
the processes and the details of the structures of the assemblies
need various approaches. For fluorescence techniques, there must
be no universal probes, and thereby a probe library needs to be built.
Based upon these considerations, a novel non-ionic surfactant-like
fluorophore, PSDA-DEA (Scheme 1a), was designed and synthesized
in the present work. Its aggregation behavior has been investi-
gated, and its application in probing formation of micelles and
their transformation to vesicles of some surfactant systems was
also studied. As expected, this probe is more adaptive and displays
superior probing abilities to the aggregation of various surfactants
and to the transformation of different aggregates of some surfac-
tant systems. It is believed that this finding is of values for exploring
details of aggregation and aggregate structures. This paper reports
the details.
2. Materials and methods
2.3. Experimental method
2.1. Materials
Stock solutions (1.0 × 10−3 M) of the fluorophores, PSDA-DEA
and pyrene, were prepared in water and ethanol, respectively. A
certain amount of pyrene stock solution was added to a volu-
metric flask and heated slightly to remove the solvent. Then the
final concentration of pyrene solution was adjusted by adding an
Acryloyl chloride (AC, Sinopharm Chemical Reagent Co., 98%)
was distilled at reduced pressure before use. Pyrenesulfonyl chlo-
ride (PSC) was synthesized by adopting a literature method [20].

60
L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 245 (2012) 58–65
Scheme 2. Synthetic route of PSDA-DEA.
appropriate amount of the analyte solution. The certain concentra-
tion of PSDA-DEA was directly diluted with water.
washed with 150 mL secondary water and 5% NaCl aqueous
solution for 3 times, the organic phase was separated and dried
by using anhydrous MgSO4, the mixture was filtered, and the fil-
trate was evaporated in vacuum to dryness. The solid as obtained
was further dried in vacuum. A desired product (PSDAB-AC) was
obtained as a yellow powder in 81% yield. 1H NMR (DMSO-
d6/Me4Si, 300 MHz): ı (ppm) 9.03 (1H, d, ArH), 8.61 (1H, d, ArH),
8.12–8.48 (7H, m, ArH), 7.95 (1H, s, CONH), 6.03–6.17 (2H, m,
CH2 CH), 5.54 (1H, d, CH CH2), 2.97 (2H, t, CH2), 2.82 (2H, t, CH2),
1.22 (4H, t, CH2). 13C NMR (DMSO-d6/Me4Si, 76 MHz): ı (ppm)
165.02 (1C, CO), 134.36, 132.24, 132.19, 131.03, 130.18, 129.87,
127.41, 127.24, 127.12, 125.14, 124.78, 124.66, 123.92, 123.69 (14H,
ArC), 130.34, 127.57 (2C, CH2 CH), 42.62, 38.23, 27.10, 26.60 (4C,
CH2). MS (ESI): m/z Calcd for [(M+Na)]: 429.1351, Found: 429.1243.
2.4. Synthesis
2.4.1. Synthesis of N-(4-aminobutyl) pyrene-1-sulfonamide
(PSDAB)
1,4-Diaminobutane (0.5 mL, 5 mmol) was dissolved in 50 mL
CHCl3, under a N2 atmosphere, 50 mL CHCl3 solution of PSC (0.15 g,
0.5 mmol) was added drop-wise under stirring in an ice-bath (drop-
ping process generally last 10 h). After the addition, the mixture was
stirred for 2 h, then the mixture was washed with 150 mL secondary
water and 5% NaCl aqueous solution for at least 10 times. The
organic phase was separated and dried by using anhydrous MgSO4,
the mixture was filtered, and the filtrate was evaporated in vacuum
to dryness, the obtained solid was purified with Soxhlet extraction
by water for 24 h. The dried solid was concentrated in vacuum to
dryness for 18 h. A desired product (PSDAB) was obtained as a yel-
low powder in 73% yield. 1H NMR (CDCl3/Me4Si, 300 MHz): ı (ppm)
9.04 (1H, d, ArH), 8.72 (1H, d, ArH), 8.11–8.30 (7H, m, ArH), 3.04
(2H, t, CH2), 2.56 (2H, t, CH2), 1.45 (2H, t, CH2), 1.31 (2H, t, CH2).
13C NMR (CDCl3/Me4Si, 76 MHz): ı (ppm) 134.63, 130.99, 130.18,
129.99, 129.71, 127.44, 127.10, 126.85, 126.74, 123.86, 123.41 (11C,
ArC), 43.22, 41.26, 30.52, 27.69 (4C, CH2). MS (ESI): m/z Calcd for
[(M+H)]: 353.1245, Found: 353.1221.
2.4.3. Synthesis of 3-(bis(2-hydroxyethyl)amino)-N-(4-(pyrene-
1-sulfonamido)butyl) propanamide
(PSDA-DEA)
PSDAB-AC (4.2118 g, 0.01 mol) and diethanolamine (3.1542 g,
0.03 mol) was dissolved in 10 mL DMF, the mixture was stirred vig-
orously at 90 ◦C for 48 h (protect from light). The reaction solution
was distilled at reduced pressure to remove DMF at 100 ◦C. Then
the solution was dissolved with 150 mL chloroform, and the chlo-
roform solution was washed with water, which was pre-saturated
with salt, for several times. The organic phase was separated and
dried by using anhydrous MgSO4, the mixture was filtered, and the
filtrate was evaporated in vacuum to dryness. TLC of PSDA-DEA
(4:1 CHCl3/methanol mixture) exhibited the product at Rf = 0.45.
A desired product (PSDA-DEA) was obtained as a yellow powder
in 41% yield. 1H NMR (CDCl3/Me4Si, 300 MHz): ı (ppm) 9.00 (1H,
d, ArH), 8.65 (1H, d, ArH), 8.03–8.27 (7H, m, ArH), 7.09 (1H, s,
CONH), 6.43 (1H, s, SO2NH), 3.65 (4H, t, CH2), 3.13 (2H, t, CH2),
2.90 (2H, t, CH2), 2.74 (2H, t, CH2), 2.61 (4H, t, CH2), 2.35 (2H, t,
CH2), 1.47 (4H, t, CH2). 13C NMR (CDCl3/Me4Si, 76 MHz): ı (ppm)
2.4.2. Synthesis of N-[(1-pyrenylsulfonamido) butyl]acrylamide
(PSDAB-AC)
CH2Cl2 solution of acryloyl chloride (0.488 mL, 0.006 mol) was
added to the 50 mL CH2Cl2 solution containing PSDAB (1.5 g,
0.005 mol) and triethylamine (0.867 mL, 0.006 mol) drop-wise
under stirring in an ice-bath (dropping process is generally last-
ing for 1 h). After the addition, the mixture was stirred at 0 ◦C
for 2 h. Then, the reaction mixture was filtered, the filtrate was

L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 245 (2012) 58–65
61
Fig. 1. I399/I380 of PSDA-DEA in ethanol/water and propanol/water binary systems
Fig. 2. Fluorescence spectra of PSDA-DEA recorded at different concentrations.
(CPSDA-DEA = 2.5 × 10−7 M).
emission is also medium-polarity sensitive [27]. It is to be noted
that with exception of acetone, for other systems contribution from
excimer emission to the profile of the spectrum is negligible (cf.
Fig. S2).
As a surface-active fluorophore, PSDA-DEA may be incorporated
into the aggregated structure of a surfactant, and thereby sense the
structural changes of the system. Accordingly, the sensing ability of
PSDA-DEA to the changes of organizations of common surfactants
in aqueous phase was conducted. Prior to the application stud-
ies of PSDA-DEA, its aggregation behavior in aqueous phase was
investigated.
173.07 (1C, CO), 134.68, 131.43, 130.91, 130.06, 129.97, 127.98,
127.22, 127.04, 126.82, 125.18, 123.84, 123.27 (12H, ArC), 59.32,
55.96, 50.86, 42.79, 38.76, 34.05, 26.78, 26.39 (8C, CH2). MS (ESI):
m/z Calcd for [(M+H)]: 512.2141, Found: 512.2220. The synthesis
route of PSDA-DEA was shown in Scheme 2.
3. Results and discussion
3.1. Steady-state fluorescence measurements and
polarity-dependence of PSDA-DEA
The fluorescence excitation and emission spectra of PSDA-DEA
in aqueous solution (1.0 × 10−5 M) were measured, and the results
are shown in Fig. S1 (Supplementary materials). It is seen that
the emission is composed of three sharp peaks around 380 nm,
399 nm and 419 nm, respectively. All the emissions can be assigned
to pyrene monomer (M) emission, and it is to be noted that there is
no pyrene excimer (E) emission. Similar results were reported for
other water soluble pyrene derivatives as reported in literatures
[21,22].
3.2. Aggregate formation of PSDA-DEA
As mentioned already, PSDA-DEA is a fluorescent active and
surfactant-like compound; it should form aggregates provided its
concentration exceeds its own critical aggregation concentration
(CAC) [28]. Both determination of CAC and estimation of micro-
polarity were carried out by conducting steady-state fluorescence
measurements. It should be no difficult to understand that prior
to the aggregation, the molecules of the compound may exist in
mono-molecular state, and upon formation of aggregate, the micro-
environment of the probe should change significantly. Fig. 2 depicts
a few emission spectra of the system recorded at various fluo-
rophore concentrations. Reference to the spectra, it is seen that
the profile of the emission changes significantly along with chang-
ing its concentration, as indicated by the increase in the intensity of
the middle peak. As a comparison, surface tension method was also
applied to determine the CAC of the compound. Fig. 3 shows both
plots of I399/I380 and ꢁ as functions of the concentration of PSDA-
DEA. Clearly, both plots change significantly at about 3.3 × 10−5 M.
To be clear, secondary derivative curves of the two plots are shown
in Fig. S4 (Supplementary materials), and from these plots, more
accurate CAC can be obtained.
To have a deeper understanding of the aggregation of the fluo-
rophore, cryogenic transmission electron microscopy (cryo-TEM),
dynamic light scattering (DLS) and atomic force microscopy (AFM)
were employed to characterize the aggregation of the compound
in aqueous phase. The results are shown in Fig. 4 and Fig. S5,
respectively. From Fig. 4a, it is found that the average diameter
of the aggregates is around 337.4 nm, a value very close to that
observed by AFM measurement, which reveals an average diam-
eter of about 400 nm (Fig. 4b). This result is further supported by
the result from cryo-TEM observation (Fig. S5). From the observa-
tion, 150–1000 nm is the range of the diameters of the aggregates.
As it is well known, pyrene is a popular and excellent polarity
probe [15,23,24]. The so called and widely used “pyrene scale” is the
using of intensity ratio of peak 3 and peak 1 (I384/I373) of its emis-
sion spectrum, which is largely dependent upon the polarity of its
micro-environment. It was expected that this polarity-dependence
character of pyrene should be remained when it is modified
into a surfactant-like structure, such as PSDA-DEA. Based upon
this consideration, the fluorescence emission spectra of PSDA-
DEA in binary systems of ethanol/water and propanol/water were
recorded. As expected, the I399/I380 of PSDA-DEA increases obvi-
ously along with increasing solvent polarity, that is increasing
water content in the mixtures (cf. Fig. 1) [25]. Reference to the fig-
ure reveals that the I399/I380 of PSDA-DEA increased from 0.51 to
0.71 as the solvent changed from ethanol to water. Similar result
was obtained for propanol/water mixture solvents. These results
show clearly that PSDA-DEA, still remains pyrene’s polarity sensi-
tive property. To further confirm this property, a number of typical
solvents of different polarities [26] were chosen to inspect the fluo-
rescence behavior of PSDA-DEA in them. During the measurements,
the PSDA-DEA was fixed at 3.0 × 10−5 M. The emission spectra as
obtained (normalized at peak 1, Fig. S2) are characterized by three
peaks, which appear at similar wavelengths as observed in aqueous
phase. The value of I399/I380 was found to increase from nearly 0.51
to 0.78 in going from nonpolar cyclohexane to polar water (Fig. S3),
in support of the observation that the profile of the PSDA-DEA

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L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 245 (2012) 58–65
Table 1
Critical micelle concentrations (CMCs) of the selected surfactants.
Surfactant
CMC (mM)
Our work
Literature values
Refs.
SDS
DTAB
Triton X-100
8.1
16.0
1.5
8.1–8.4
15.8–16.3
1.3–1.5
[17,29,30]
[17,31]
[17,31,32]
of the probe has been optimized from 9.0 × 10−8 M to 5.0 × 10−7 M
with SDS as a sample surfactant solution, and finally 2.5 × 10−7 M
was chosen as an ideal probe concentration throughout the mea-
surements (cf. Fig. S6, Supplementary materials).
To evaluate the probing ability of PSDA-DEA, some surfactants,
including SDS, DTAB and Triton X-100, which belong to anionic,
cationic and nonionic surfactants, respectively, were chosen as
sample systems to study micelle formation in them by using the
fluorophore as a probe. Fig. 5 presents the results from the studies.
As evidenced from the figure, it is seen that a sigmoidal function
of Boltzmann type was used to describe the plots, and the critical
micelle concentrations (CMCs) are determined at the sharp infec-
tion points of the curves. The CMC values obtained are summarized
in Table 1, and the corresponding data from literatures are also
presented [17,30–33]. It shows clearly that PSDA-DEA exhibits out-
standing ability in probing the CMC of the sample surfactants, no
matter they are anionic, cationic or non-ionic in nature.
Fig. 3. I399/I380 and surface tension of PSDA-DEA in water.
Considering the fact that the length of the fluorophore is less than
0.3 nm (molecular dynamics modeling, Scheme 1b), the structure
of the aggregates must be complicated, at least not as simple as a
micelle or a vesicle. In fact, this is not a very surprising result. This
is because aggregates formed via self-assembly of suitable building
blocks can be of different levels of complexity: as simple as a dimer
or as complicated as a cell membrane, a remarkable supramolecular
architecture created by a bilayer of phospholipids embedded with
cholesterol and functional proteins. The details of the complexity
of self-assembly have been discussed by Zhang and co-workers in
a review paper published recently [29]. For PSDA-DEA, the struc-
ture of the sulfonyl amide contained in the hydrophobic tail of
the compound must make it behave more complicated than that
of a routine surfactant, and thereby, the details of the structures
of the aggregates can be only revealed by conducting much more
research, which is out of the scope of present study.
3.3.2. Probing transition between different aggregates of mixed
surfactant solution
Probing transition between micelles to vesicles in mixed
cationic and anionic surfactant system is a challenging problem
in colloid and interface chemistry. Besides DLS and TEM, fluores-
cence probe technique has been found useful in the interrogation of
the transition provided a suitable fluorescence probe is employed.
As mentioned, Huang and co-workers [19] designed and prepared
a dansyl-labeled anionic surfactant, and the fluorescent surfac-
tant was successfully used for the determination of the transition
between micelles and vesicles of a sample of mixed surfactant solu-
tion. In the present study, SDS/DEAB was chosen as a sample of
mixed surfactants, of which the transformation between different
aggregates is hard to be monitored by using an anionic fluores-
cence probe developed by Huang and co-workers [19,32,34]. The
changes of the values of I399/I380 and the fluorescence lifetimes of
PSDA-DEA along with changes in the ratio of the concentrations
3.3. Probing abilities to the aggregation of amphiphiles
3.3.1. Determination of CMC of amphiphiles
In order to achieve better probing ability, the probe concentra-
tion must be optimized according to a few principles, such as, first
the concentration, there is no detectable aggregation of the probe
itself, and second the fluorescence emission of the system should be
strong enough for measurements. Accordingly, the concentration
Fig. 4. (a) Dynamic light scattering result of PSDA-DEA in aqueous solution. (b) AFM height image of the aggregates formed in the aqueous solution of PSDA-DEA
(CPSDA-DEA = 1 × 10−4 M).

L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 245 (2012) 58–65
63
Fig. 5. Plot of I399/I380 of PSDA-DEA as a function of surfactant concentration. The surfactants employed are (a) SDS, (b) DTAB and (c) Triton X-100, respectively.
along with this transition. In contrast, the value of the ratio is
back to 0.65 and the lifetime back to nearly 13 ns when the aggre-
gates change from vesicles to micelles due to increase of DEAB
concentration.
As a control, pyrene was employed as fluorescence probe to
monitor the transition from micelles to vesicles of the SDS/DEAB
system. The result is shown in Fig. 7. Reference to the figure, it is
clearly seen that different from PSDA-DEA, pyrene cannot sense
the transition between the two different aggregates. Furthermore,
for the present mixture surfactant system, it is also more sensi-
tive than 12-DAN-ADA, another surfactant-like fluorescence probe,
which was reported by Huang and co-workers [19]. This is because
the probe reported by them is negatively charged and possesses
specific interactions with the component surfactants, which may
disturb the probing process. Therefore, it may be concluded that
PSDA-DEA is superior to traditional fluorescence probes, such as
pyrene and anionic dansyl derivatives in probing aggregate changes
occurring in mixed surfactant solutions.
For future work, we are currently using the fluorescence probe
reported in this work and those under development to study the
micellization of a wider range ionic and non-ionic surfactants, and
the micelle growth [35] in typical surfactant solutions, such as SDS
system since morphologically the micelles formed in the system
could change from spheres to rods, and then to worms when NaBr
is introduced [36].
Fig. 6. I399/I380 and fluorescence lifetime of PSDA-DEA in SDS/DEAB system
(CSDS+DEAB = 1.0 × 10−2 M and CPSDA-DEA = 2.5 × 10−7 M). M: micelle; V: vesicle; P: pre-
cipitate.
of the two surfactants (Note: the total concentration of the two
surfactants was kept at 1.0 × 10−2 M) are shown in Fig. 6. It can
be seen that the value of I399/I380 changes from 0.6 to 0.52 with
the aggregates transform from micelles to vesicles. At the same
time, the fluorescence lifetime increases sharply from 20 ns to 30 ns
Fig. 7. I399/I380 of PSDA-DEA and pyrene in SDS/DEAB system (CSDS+DEAB = 1.0 × 10−2 M and CPSDA-DEA = 2.5 × 10−7 M). M: micelle; V: vesicle; P: precipitate.

64
L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 245 (2012) 58–65
4. Conclusion
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In summary, a novel non-ionic surface-active fluorescence
probe, PSDA-DEA has been designed and prepared. The aggregation
behavior of the probe in aqueous medium has been studied by using
a steady state fluorescent method and a surface tension method.
It was found that the CAC of the probe is about 3.0 × 10−5 M.
DLS, AFM and cryo-TEM measurements revealed that the aver-
age diameter of the aggregates of the surfactant-like fluorescence
probe in aqueous phase is about a few hundred nanometers. Flu-
orescence measurements showed that the ratio of I399/I380 of
PSDA-DEA is highly sensitive to the changes in the polarity of
its micro-environment as evidenced by the observations obtained
from ethanol/water or propanol/water mixture system studies.
Furthermore, the surfactant-like fluorescence probe has also been
successfully used for monitoring aggregation in the aqueous solu-
tion of SDS, DTAB or Triton X-100, and the transformation between
micelles and vesicles in SDS/DEAB system, a system very hard to
study by using routine fluorescence probes, including surfactant-
like fluorescence probes reported in literatures.
It is interesting to note that compared with the results from
similar studies conducted by Huang and co-workers [19], where an
anionic surfactant-like fluorophore was developed and utilized as
a probe, the finding from present study has proven that PSDA-DEA,
a typical neutral surfactant-like fluorophore, is a more powerful
and adaptive probe for monitoring formation and transformation
of aggregates of surfactants in aqueous phase no matter they are
anionic, cationic or neutral in nature. From a broader perspective,
the present study contributes not only a new and precious mem-
ber to the library of surfactant-like fluorescence probes, but also
an operationally simple tool to aid researchers working on col-
loid and interface science and relevant fields to explore details of
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We gratefully thank the Natural Science Foundation of China
(20902055, 20927001, 91027017, 20903015) and the 13115 Project
of Shaanxi Province (2010ZDKG-89) for financial support. This work
is also supported by “Program for Chang Jiang Scholars and Inno-
vative Research Team in University” of China (IRT1070).
Appendix A. Supplementary data
Supplementary data associated with this article can be
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