A novel surfactant-like fluorophore and its probing ability to the aggregation of amphiphilic compounds
Graphical abstract
The applications of a newly synthesized surfactant-like fluorophore in the transition of the aggregates of amphiphilic compounds, especially between micelles and vesicles, were investigated systematically by using the compound as a probe.
Highlights
► Prepared a novel surfactant-like non-ionic fluorophore. ► Studied the aggregation behavior of the fluorophore in aqueous phase. ► Monitored the transition between different surfactant aggregates. ► The compound is more powerful fluorescent probe than known probes.
Introduction
Abundant information relevant to intra-/inter-molecular interactions and supramolecular aggregation and dis-aggregation of organic molecules can be obtained by utilization of fluorescence techniques [1], [2], [3], [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 applications, and easiness in use, etc. In addition, the techniques are sensitive to small changes in the micro-environment of the relevant 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], [6], [7], [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.
In the studies of various aggregates, such as micelles, vesicles and lamellar phases, traditional methods including electron microscopy and light scattering can only provide limited information about the structures of the aggregates at a molecular level. 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], [10], [11], [12], [13].
In the studies of amphiphilic aggregation, positioning of the fluorescence 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 combinations 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 concentration (CMC) of various surfactants since they are typical polarity or mobility probes [4], [14], [15], [16]. These probes, however, are only effective for monitoring formation of micelles of surfactants, and display poor ability to probe transformation of different aggregates of them. To improve the performances of pyrene-like known fluorescence probes in monitoring formation of complex aggregates of surfactants, various efforts have been made [14], [17], [18]. One of the strategies to solve this problem is to use fluorescence 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 information which is directly relevant to the formation and structures 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 investigated, 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 surfactant systems. It is believed that this finding is of values for exploring details of aggregation and aggregate structures. This paper reports the details.
Section snippets
Materials
Acryloyl chloride (AC, Sinopharm Chemical Reagent Co., 98%) was distilled at reduced pressure before use. Pyrenesulfonyl chloride (PSC) was synthesized by adopting a literature method [20]. 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
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
Conclusion
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 average diameter of the aggregates of the surfactant-like fluorescence probe in aqueous phase is about a few hundred nanometers.
Acknowledgments
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 Innovative Research Team in University” of China (IRT1070).
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Equal contribution to the present work as made by the first author.