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Micelle-Mediated Chemiluminescence as An Indicator for Micellar Transitions Meiting Xie, Zhuoyong Zhang, Weijiang Guan, Wenjuan Zhou, and Chao Lu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03774 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Analytical Chemistry

Micelle-Mediated Chemiluminescence as An Indicator for Micellar Transitions

Meiting Xie,a Zhuoyong Zhang,a Weijiang Guan,b Wenjuan Zhou,*a and Chao Lub

aDepartment bState

of Chemistry, Capital Normal University, Beijing 100048, China

Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology, Beijing 100029, China

*E-mail:

[email protected]

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ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The structural phase of micelles plays an important role in controlling the micellar performance. Despite the great developments of some advanced characterization techniques, it remains challenging to achieve fast and sensitive determination of micellar transitions in solution. Herein, a novel indicator system for micellar transitions was developed based on the micelle-mediated peroxyoxalate chemiluminescence that showed a sensitive response towards the changes of micellar morphologies. A peroxyoxalate derivative and a fluorophore were firstly co-assembled into the hydrophobic cavities of micelles of the typical cationic surfactant cetyltrimethylammonium bromide (CTAB). A strong and rapidly falling chemiluminescence response was exhibited in spherical micelles as a result of the loose arrangement of CTAB molecules. By contrast, rod- or worm-like micelles transformed from spherical micelles could induce a compact arrangement of CTAB molecules, leading to a weak chemiluminescence emission with a slow decay rate. The practicability and universality of the chemiluminescent indicator were demonstrated by determining the micellar transitions in a variety of surfactant solutions (ionic, non-ionic and polymeric). These findings open attractive perspectives for the practice of chemiluminescence technique in micelle characterization.

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INTRODUCTION Micellar aggregates formed in solution by the self-assembly of surfactant have been attracting increasing attention owing to their extensive applications in daily chemical industry, drug delivery, and nanomaterial synthesis, etcetera.1-3 Typically, spherical micelles are generated above the critical micelle concentration (CMC) of surfactants in aqueous environment, and they can transform into ellipsoidal, rodlike, or wormlike structures under different solvent conditions.4,5 The specific performance of micelles (e.g., the viscoelastic properties and the rheological response) depends on their structural phases which can be changed by controlling some parameters, such as the chemical structure and concentration of surfactants, counterions, surrounding ionic strength, pH and temperature.6-8 Effective characterization of these micellar transitions could provide a practicable guidance for the design and application of functional micelles. Among the numerous characterization techniques, electron microscopy is the most direct way for the structural observation of micelles,9,10 although the dissolution environment of micelles might be significantly changed during the process of sample preparation. The light scattering techniques (e.g., small-angle neutron scattering and small angle X-ray scattering) allow in situ measurement of micellar structures without destroying or disturbing the state of the micellar solution.11-13 However, it often requires complicated data analyses. Recently, fluorescence imaging was used as an alternative for the determination of micellar transitions in aqueous solutions by grafting fluorophores to surfactant molecules,14 whereas the grafted fluorophores might affect the properties of micelles (e.g., the micelle binding constant). Thus, it is eagerly desired to develop a simple method for the rapid and sensitive determination of micellar transitions in solution. Chemiluminescence (CL) technique has been considered as a promising method for rapid and nondestructive analysis of nanostructures with various advantages of low background, 3



high signal-to-noise ratio, and simple instrumentation.15-17 Given the capability of dissolving insoluble CL reagents and stabilizing reactive intermediates, micelles have been commonly used―especially for the peroxyoxalate CL―to greatly improve the CL efficiency.18-22 The hydrophobic peroxyoxalate and fluorophore molecules are assembled into micelles in the aqueous environment, which allowed strong CL emission upon exposure to hydrogen peroxide (H2O2).23-25 Inspired by the unique CL property of micelles, we would suggest a rapid and convenient method for measuring micellar structure transitions. Herein, a peroxyoxalate (bis[3,4,6-trichloro-2-(pentyloxy-carbonyl)phenyl] oxalate, CPPO) and a fluorophore (9,10-bis(phenylethynyl)anthracene, BPEA) were encapsulated into the hydrophobic cavities of the cetyltrimethylammonium bromide (CTAB) micelles through hydrophobic self-assembly. The CL behavior of peroxyoxalate in micelles was found to be closely correlated to the arrangement of CTAB molecules, which would change with micelle morphology (Scheme 1). It was demonstrated that the spherical micelle structure induced a strong CL signal with a high rate of decrease. The rod- or worm-like structure transitions would cause a decrease in the CL intensity, while also resulting in a slow decay rate. These unique micelle-mediated CL behaviors of peroxyoxalate make it a possible indicator for micellar transitions. Its general utility was proven by studying different types of micelles composed of a variety of surfactants (non-ionic, polymeric, and anionic). Accordingly, a novel CL indicator system was developed, with advantages of high sensitivity, fast read-out, and simple operation, which would provide a powerful tool for in situ characterization of micelles in solution.

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Scheme 1. Schematic illustration for the CL response of the CPPO−BPEA assembled in CTAB micelles towards micellar transitions.

EXPERIMENTAL SECTION Chemicals and Materials. Cetyltrimethyl ammonium bromide (CTAB, 99%) and Pluronic P123 (EO20PO70PEO20) were supplied by Shanghai Macklin Biochemical Co., Ltd. Bis(2,4,5-trichloro-6-(pentyloxycarbonyl)phenyl) oxalate (CPPO, >98%), sodium oleate (NaOA, 97%), and Triton X-100 were purchased from Tokyo Chemical Industry Co., Ltd. 9,10-Bis(phenylethynyl)anthracene (BPEA, 97%) was obtained from Sigma-Aldrich. 1-Naphthol were purchased from Tianjin Heowns Biochemical Technology Co., Ltd. Sodium chloride (NaCl, AR grade), acetonitrile (AR grade), absolute ethyl alcohol (AR grade), and hydrogen peroxide (H2O2, AR grade, 30% v/v) were purchased from Tianjin Fuchen Chemical Reagents Factory. All materials were used as received without further purification. Deionized water (18.2 MΩ.cm) from a Millipore Milli-Q system was used throughout all experiments. Assembly of the CPPO−BPEA Probe into Micelles. Stock solution of CPPO (10 mg/mL) and BPEA (25 μg/mL) were prepared by dissolving CPPO and BPEA in acetonitrile solvent, respectively. Work solution of CPPO−BPEA was obtained by mixing the stock solution of CPPO and BPEA at an equal volume ratio. The CPPO−BPEA probe-loaded CTAB micelles were freshly prepared by adding the work solution of CPPO−BPEA into aqueous micellar solutions of CTAB under stirring at a volume ratio of 1:1000. 5



CL Measurements for Salt-Induced Micellar Transitions. NaCl was applied to induce the morphology changes of CTAB micelles. CL measurements were performed using a static injection setup (Figure S1). CPPO−BPEA probe-loaded CTAB micellar solution (200 μL) was mixed with different concentration of NaCl solution (200 μL) in the quartz vial. The final concentration of CTAB was 10 mM. The final concentrations of NaCl were 0 M, 0.01 M, 0.1 M, 1.0 M, 2.0 M, 4.0 M, respectively. For the CL determination, 100 μL of H2O2 solution (5.0 mM) was injected into the above mixed solution with a final concentration of 1.0 mM. The CL signals were detected by a PMT adjacent to the quartz vial with a work voltage of ‒1000 V and data integration time of 0.5 s. Instruments and Methods. The fluorescence spectra of the samples were obtained by using Hitachi F−7000 fluorescence spectrophotometer (Tokyo, Japan). The morphology of the CTAB micelles was observed by a transmission electron microscope (TEM, Hitachi-7650, Japan). Dynamic light scattering (DLS) and zeta potential measurements of CTAB micelles were performed using a Zetasizer from Nano ZS (Malvern Instruments Ltd., UK). CL measurements were performed on an ultra-weak biophysics chemiluminescence (BPCL) analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). CL spectra were detected by a Hitachi F−7000 fluorescence spectrophotometer upon shutting off the excitation light. Shear viscosity measurements were carried out on a rheometer (TA AR2000ex) with a cone-plate sensor at 25.0 ± 0.1 °C.

RESULTS AND DISCUSSION CL Behavior of CPPO in Micelles. CPPO and BPEA could be self-assembled into the CTAB micelles through the hydrophobic interactions. As shown in Figure 1A, a strong fluorescence emission band with a peak at about 470 nm was observed for the solution of BPEA in acetonitrile. By contrast, almost no fluorescence signals were recorded in aqueous 6


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medium due to the insolubility of BPEA in water. With the help of CTAB micelles, BPEA was soluble in water and thus has the similar fluorescence behavior to that in acetonitrile, along with a slight red-shift in emission wavelength and decrease in intensity. This could be attributed to the aggregation of the BPEA molecules in the internal hydrophobic cavities of CTAB micelles, also demonstrating the successful package of the BPEA molecules in micelles. The CL behaviors of peroxyoxalate systems were investigated in a static injection CL setup (Figure S1). In acetonitrile medium, the CL intensity of the CPPO−H2O2 system can be greatly enhanced in the presence of BPEA as the energy acceptor (Figure 1B and Figure S2). It was found that a remarkable enhancement effect was obtained after assembling CPPO and BPEA (CPPO−BPEA) into the CTAB micelles at the same concentration (Figure 1C and Figure S3). In addition, the micelle-amplified CPPO−BPEA−H2O2 system exhibited a maximum CL emission wavelength at 478 nm in micelles (Figure 1D), which was corresponded to the emission band of excited BPEA, indicating effective energy transfer between the CL donor CPPO and the fluorophore BPEA coexisted in micelles.26 These results further confirm the feasibility of encapsulating CPPO and BPEA into CTAB micelles, while maintaining enough CL reactivity.

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Figure 1. (A) Fluorescence spectra of BPEA in different solvents. (B) CL kinetic curves for the CPPO (10 μg/mL) and CPPO−BPEA in acetonitrile medium after injection of H2O2 (4 mM). The concentration of BPEA was 0.05 μg/mL. (C) CL kinetic curves for the CPPO and CPPO−BPEA in CTAB (10 mM) micelles after injection of H2O2 (1.0 mM). The concentrations of CPPO and BPEA were 2.5 μg/mL and 0.05 μg/mL, respectively. (D) CL spectra of the CPPO−H2O2 (a) and CPPO−BPEA−H2O2 (b) systems in CTAB micelles.

Effects of the CL Probe on Micelle Structure. Given the excellent CL properties in micelles, CPPO−BPEA was expected as a CL probe for the determination of micelle structures. It was observed that no change in fluorescence properties of BPEA was presented after the CL reaction (Figure S4), indicating high stability of the fluorophore in micelles. In order to verify the capability of the CL probe for micelle characterization, the effects of the added CL probe on micelle structures were investigated firstly. The hydrodynamic diameter of CTAB micelles (10 mM) in aqueous medium was 2~5 nm based on the dynamic light scattering (DLS) results (Figure 2A). No change in the hydrodynamic diameter of CTAB micelles was observed after loading the CPPO−BPEA probe into the micelles (Figure 2B). Moreover, the transmission electron microscopy (TEM) images of CTAB micelles shown in Figure S5 also indicated that the encapsulated CPPO−BPEA probe had no effect on the 8


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morphology and size distribution of CTAB micelles.

Figure 2. DLS results for (A) the CTAB micelles and (B) the CPPO−BPEA probe-loaded CTAB micelles.

CL Response of the CPPO‒BPEA Probe towards Micellar Transitions. The feasibility of the CPPO−BPEA probe for the characterization of micellar transition was investigated using NaCl as the ionic strength adjustor.27-29 Figure 3A showed the CL kinetic response curves of the CPPO−BPEA probe in micelles under different concentrations of NaCl. CL signals immediately appeared after injecting H2O2 into the CPPO−BPEA probe-loaded CTAB micellar solutions. A gradual decrease in CL intensity was observed with increasing the NaCl concentration. At the same time, the rate of CL decay was also dependent on the NaCl concentration, which showed a slowing trend with the salt concentration increasing. In addition, the size distributions of CTAB micelles at different ionic strengths were estimated by DLS measurements. As shown in Figure 3B, the hydrodynamic diameter of CTAB micelles gradually increased with increasing the NaCl concentration, indicating the morphology transitions of CTAB micelles at high salt concentrations. Taking advantage of the unique micelle-mediated CL behaviors of the CPPO−BPEA probe, 9



we then evaluated its analytical performance in micellar transition analysis. We fixed the concentration of CTAB and carried out a series of experiments to draw a plot of CL intensity versus NaCl concentration (Figure 3C and S6). At a very low concentration, NaCl had little effects on the structure of CTAB micelles, resulting in little change in CL intensity. When the NaCl concentration was gradually increased to 0.01 M, there was a sudden drop in CL intensity, indicative of the start of micellar transition. As NaCl concentration continued to increase, an inflection point appeared at around 0.1 M, suggesting the formation of typical rodlike micelles. Once the concentration of NaCl was higher than 1.0 M, a slower decline in the CL intensity was observed owing to the formation of worm-like micelles in solutions of high ionic strength. Given the above results, the phase transition points of CTAB micelles can be accurately detected through the plot of CL intensity versus NaCl concentration. The micellar transitions were further investigated with rheology measurements. The zero-shear viscosities (η0) of CTAB/NaCl system were shown in Figure 3D. The CTAB solution with a low salt content had an extremely low viscosity. When the NaCl concentration increased to 0.01 M, a sharp increase in the viscosity was observed, indicating the morphology changes of the CTAB micelles.30,31 However, the viscosity of the CTAB solution began to decrease as the NaCl concentration increased to 0.1 M due to the cross-linking of micelles.32 Once the NaCl concentration was greater than 1.0 M, a further decrease of viscosity was observed, resulting from the formation of the entangled network of worm-like micelles.33 These results are in good agreement with those from CL response of the CPPO−BPEA probe.

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Figure 3. (A) CL kinetic curves of the CPPO−BPEA probe in CTAB micelles after injection of H2O2 (1.0 mM) under different concentrations of NaCl. (B) DLS results for the CTAB micelles in different concentrations of NaCl. (C) Plot of CL intensity versus concentration of NaCl. (D) Zero-shear viscosity (η0) of 10 mM CTAB solutions as a function of the concentration of NaCl.

Furthermore, the morphologies of CTAB micelles were directly observed in TEM images. The micelles exhibited a nearly spherical shape at low salt concentrations, whose size increased with the increase of the salt concentration (Figure 4A-C). Apparent transitions to micellar morphology occurred when the salt concentration increased to 1.0 M (Figure 4D). When the salt concentration was further increased, entangled network structures of micelles were formed (Figure 4E and 4F). These results and the rheology data were in accordance with the CL response of the CPPO−BPEA probe towards the morphology transitions of CTAB micelles. Thus, we could conclude that the CPPO−BPEA probe was perfectly capable of detecting and characterizing micellar transitions with a high sensitivity to the tiny changes in the micelle morphology at low salt concentrations (0.01-0.1 M).

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Figure 4. TEM images of the CTAB micelles in different concentrations of NaCl: (A) 0 M, (B) 0.01 M, (C) 0.1 M, (D) 1.0 M, (E) 2.0 M, (F) 4.0 M.

Mechanism of CL Response for Micellar Transitions. To examine the effects of the addition of salt on the optical properties of BPEA in CTAB micelles, its fluorescence spectra were investigated in the aqueous solution with different concentrations of NaCl. As shown in Figure S7, BPEA maintains the same fluorescence emission behavior, indicating that the CL behavior change of the CPPO−BPEA probe in micelles was attributed to the ionic strength-induced transitions of micelle structures. Moreover, the remarkable alterations of surface charges were observed upon salt treatments as shown by zeta potential measurements (Figure S8). A highly positive zeta potential at low salt concentration resulted in a strong electrostatic repulsion between the cationic head groups. The loose arrangement of CTAB molecules and strong hydration of the charged head groups allowed a quick permeation of H2O2 into the inner cores of CTAB micelles, leading to a strong and rapidly falling CL response of the CPPO−BPEA probe in micelles. An increase in the ionic strength can effectively shield the charge of the cationic head groups, which could induce a compact arrangement of CTAB molecules and a weaker hydration of the head groups. As a result, the H2O2 permeation was inhibited and the CPPO−BPEA probe in micelles exhibited a weak CL 12


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emission and a slow decay rate. Universality of the CPPO‒BPEA Probe. It is also well known that spherical micelles of CTAB can aggregate to form rod-like micelles above the second critical micellar concentration (20 mM).34-36 We have further investigated CL response of the CPPO−BPEA probe for the morphology transition of micelles by altering the CTAB concentration. As expected, CL intensity of the CPPO−BPEA probe in micelles decreased with increasing CTAB concentration (Figure S9), demonstrating the formation of rod-like micelles at high concentrations of CTAB. Except NaCl and CTAB concentration, the addition of 1-naphthol that can form hydrogen-bonds with interfacial water molecules could also induce the micellar growth.37,38 The response ability of the proposed CL probe for the hydrogen-bond-induced morphology transition of CTAB micelles was studied in the presence of 1-naphthol. As depicted in Figure 5A, CL intensity of the CPPO−BPEA in CTAB micelles decreased with increasing 1-naphthol concentration, indicating the changes of packing arrangements of micelles due to their growth behavior. This result is consistent with those from changing NaCl and CTAB concentration. In consideration of the impact of stability on our sensing platform, we also investigated the stability of the CPPO−BPEA probe in CTAB micelles. As depicted in Figure S10, the CL signals of the CPPO−BPEA probe decreased sharply during the first 15 minutes and then became steady. Such a decrease might be caused by the rapid hydrolysis of the CPPO close to the hydration layer of CTAB micelles. Moreover, the CPPO−BPEA probe in CTAB micelles after 20 minutes of storage was chosen to study in detail the CL behavior at different NaCl concentrations (Figure S11). Compared with the freshly prepared CPPO−BPEA probe, its CL intensity has the same trend that can also accurately indicate the phase transition points of CTAB micelles. Although the probe stored for a while could be used for sensing micellar 13



transitions, we still recommended using the freshly prepared one.

Figure 5. (A) CL kinetic curves of the CPPO−BPEA probe in CTAB micelles under different concentrations of 1-naphthol (PMT voltage: -1000 V). (B−D) CL response of the CPPO−BPEA probe towards micellar transitions in different kinds of surfactant systems: (B) 0.25 g/L of Triton X-100 solution (PMT voltage: -1100 V); (C) 5.0 g/L of Pluronic P123 (PMT voltage: -1100 V); (D) 50 mM of NaOA solution (PMT voltage: -1100 V).

In addition, to validate the substantial promise for the indication of micellar transitions, we have further studied the general utility of the proposed CPPO−BPEA probe by detecting the morphology transitions of non-ionic (Triton X-100), polymeric (Pluronic P123), and anionic (NaOA) surfactants.39-42 In the case of Triton X-100 and Pluronic P123 systems, the CL intensity of the CPPO−BPEA probe also gradually decreased as a result of the salt-induced spherical-to-wormlike micelle transition (Figure 5B and 5C), showing a similar response to the cationic CTAB micelles. Unlike the cationic, non-ionic, and polymeric systems, anionic surfactant solutions used in the micelle transition are usually alkaline, which could facilitate the hydrolysis of the peroxyoxalate inside the micelles. As a result, very weak CL signals were obtained in NaOA systems without NaCl (Figure 5D), owing to the limited protection of 14


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the spherical micelles. Then, a gradually increased CL signal was observed as NaCl concentration increased. The addition of NaCl would induce the formation of rodlike or wormlike micelles with a compact arrangement of anionic surfactant molecules, inhibiting the base hydrolysis of the peroxyoxalate. Based on the above results, we could confirm the universality of the CPPO−BPEA probe as an indicator for micellar transitions in different kinds of surfactant systems.

CONCLUSIONS In summary, a sensitive CL probe for determining the micellar transitions in solution was developed through the self-assembly of the peroxyoxalate reagents and fluorophores into the internal hydrophobic cavities of micelles. By increasing the NaCl, CTAB, and 1-naphthol concentrations in CTAB solutions, the CTAB micelles experienced a morphologic transition from spherical to rodlike or wormlike, along with the decrease in CL intensity and decay rate. The changes of CL signals towards the micellar transitions are attributed to the different arrangements of surfactant molecules in the spherical, rodlike, and wormlike micelles. Its practicability and reliability were further demonstrated by evaluating the phase transition points during the salt-induced spherical-to-wormlike morphologic transition of the CTAB micelles. In addition, the micellar transitions in other kinds of surfactants (non-ionic, polymeric, and anionic) were also successfully determinized. This success develops a simple and promising CL strategy for nondestructive characterization of micelles in solution.

ASSOCIATED CONTENT Supporting Information Schematic diagram of a static injection CL setup; CL kinetic curves for the CPPO and CPPO−BPEA in acetonitrile medium after injection of H2O2; fluorescence spectra of the 15



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CTAB micelle-encapsulated BPEA before and after the CL reaction; TEM images of the CTAB micelles and CPPO−BPEA probe-loaded CTAB micelles; CL signals of the CPPO−BPEA probe in CTAB micelles under different concentrations of NaCl; effects of different

concentrations

of

NaCl

on

the

fluorescence

emission

of

the

CTAB

micelle-encapsulated BPEA; zeta potential of the CPPO−BPEA probe-loaded CTAB micelles at different NaCl concentrations; CL kinetic curves of CPPO−BPEA in different concentrations of CTAB micelles after injection of H2O2; CL intensity of the CPPO−BPEA probe in CTAB micelles over different time of storage; plot of CL intensity of the CPPO−BPEA probe versus concentration of NaCl after 20 minutes of storage. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Wenjuan Zhou: 0000-0002-1681-9198 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21804094), the National Instrumentation Program (2012YQ140005), and Free Topic Program of BUCT (ZY1820).

REFERENCES 16


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