Shuttle-like supramolecular nanostructures formed by self-assembly of a porphyrin via an oil/water system
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In this paper, in terms of the concentration of an aqueous solution of a surfactant, we investigate the self-assembly behavior of a porphyrin, 5, 10, 15, 20-tetra(4-pyridyl)-21H, 23H-porphine [H2TPyP], by using an oil/water system as the medium. We find that when a chloroform solution of H2TPyP is dropwise added into an aqueous solution of cetyltrimethylammonium bromide [CTAB] with a lower concentration, a large amount of irregular nanoarchitectures, together with a small amount of well-defined shuttle-like nanostructures, hollow nanospheres, and nanotubes, could be produced. While a moderate amount of shuttle-like nanostructures accompanied by a few irregular nanoarchitectures, solid nanospheres, and nanorods are produced when a CTAB aqueous solution in moderate concentration is employed, in contrast, a great quantity of shuttle-like nanostructures together with a negligible amount of solid nanospheres, nanofibers, and irregular nanostructures are manufactured when a high-concentration CTAB aqueous solution is involved. An explanation on the basis of the molecular geometry of H2TPyP and in terms of the intermolecular π-π interactions between H2TPyP units, and hydrophobic interactions between CTAB and H2TPyP has been proposed. The investigation gives deep insights into the self-assembly behavior of porphyrins in an oil/water system and provides important clues concerning the design of appropriate porphyrins when related subjects are addressed. Our investigation suggests that an oil/aqueous system might be an efficient medium for producing unique organic-based nanostructures.
KeywordsPorphyrin Aging Time Soret Band Assembly Behavior Hollow Nanospheres
5, 10, 15, 20-tetra(4-pyridyl)-21H, 23H-porphine
zinc 5, 10, 15, 20-tetra(4-pyridyl)-21H, 23H-porphine
low-resolution transmission electron microscopy
high-resolution transmission electron microscopy
fast Fourier transformation
energy-dispersive X-ray spectroscopy
scanning electron microscopy.
Currently, tremendous efforts have been devoted to open up various facile methodologies for the fabrication of nanostructured materials with novel morphologies since the fascinating architectures and the unique physicochemical properties of the produced nanomaterials make them of considerable interest as promising components for chemical/biochemical sensors, opto- and nano-electronic devices, bionanotechnology, and so forth [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. Among various nanostructured materials, those with well-defined discrete architectures, typically exemplified by nanofibers, nanotubes, nanorods, nanospheres, nanocubes, etc., have so far been intensively investigated [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. Besides these sophisticated typical nanomaterials, those with complicated yet well-defined morphologies, for example, cauliflower-like nanostructures, nanopolyhedrons, nanotetrapods, nanosprings, nanospirals, nanospindles, nanoshuttles, etc. [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30], have also attracted much attention owing to their extraordinary physicochemical properties. With respect to this issue, a paramount of inorganic-based systems have been developed [1, 2, 3, 4, 5, 6, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24], while examples in terms of organic-based systems are relatively fewer [4, 5, 6, 10, 11]. As a matter of fact, in contrast to inorganic nanomaterials [1, 2, 3, 4, 5, 6], organic nanostructures have peculiar electronic and optical properties and can render impressive varieties and flexibilities in molecular design and tunability of physicochemical properties [7, 8, 9, 10, 11, 12, 13, 14]. This makes the organic-based nanostructures promising candidates for nanoscience and nanotechnology. Consequently, an exploration on the fabrication of organic nanostructures with unique yet well-defined morphologies should be an important issue to be explored intensively.
When considering the fabrication of organic-based nanostructured materials, supramolecular assembly, which aims at manufacturing sophisticated, organized molecular associations through various non-covalent interactions, including π-π interactions, hydrophobic interactions, electrostatic interactions, etc., has currently been demonstrated to be an excellent strategy. It provides fertile, new, and promising opportunities for nanoscience and nanotechnology, material science, biological science, and so on [7, 8, 9, 10, 11, 12, 13, 14, 31]. Various fascinating organic-based nanomaterials have so far been manufactured in terms of diverse solution-based self-assembly technique, template-induced self-assembly process, physical vapor deposition, etc. [7, 8, 9, 10, 11, 12, 13, 14, 25, 26, 27, 28, 29, 30], where the surfactant-assisted self-assembly [SASA] has been considered to be an important solution-based method owing to its nice adaptability, simplicity, and reproducibility [32, 33].
Among various building blocks for the fabrication of supramolecular nanostructures, porphyrins have been reported to be one of the most useful units. This is promoted by their multifunctionality, biocompatibility, flexible and tunable molecular structure by chemical modification, rigid and planar molecular skeleton, which make porphyrin-involved nanomaterials have rich assembly, physicochemical and biochemical properties [26, 27, 29, 32, 33, 34, 35, 36, 37, 38, 39]. For example, we have recently demonstrated that a metal porphyrin, zinc 5, 10, 15, 20-tetra(4-pyridyl)-21H, 23H-porphine [ZnTPyP], could be organized to form various well-defined nanostructures in a controllable manner via a SASA method through an oil/water system . In that case, we showed that depending on the aging time and the concentration of the aqueous solution of the surfactant, cetyltrimethylammonium bromide [CTAB], hollow or solid nanospheres, nanotubes, nanorods, and nanofibers could be facilely obtained. Therein, we suggested that the intermolecular π-π interactions between ZnTPyP units together with the hydrophobic interactions between ZnTPyP and CTAB played an important role during the assembly process.
On the basis of the aforementioned brief review, we herein investigate the assembly behavior of H2TPyP via a process similar to that for ZnTPyP . We find that a large amount of irregular nanoarchitectures, together with a small amount of shuttle-like nanostructures, hollow nanospheres, and nanotubes, could be produced using a lower concentration of a CTAB aqueous solution. When a CTAB aqueous solution with a moderate concentration is used, a moderate amount of shuttle-like nanostructures accompanied by a few irregular nanoarchitectures, solid nanospheres, and nanorods could be produced, while a good quantity of shuttle-like nanostructures, together with a very small amount of solid nanospheres, nanofibers, and a negligible amount of irregular nanostructures could be predominantly manufactured when a high-concentration CTAB aqueous solution is employed. An explanation on the basis of the molecular geometry of H2TPyP and in terms of the intermolecular π-π interactions between H2TPyP molecules and the hydrophobic interactions between CTAB and H2TPyP has been proposed. Although various porphyrin-involved organic nanostructures [26, 27, 29, 32, 33, 34, 35, 36, 37, 38, 39] and inorganic shuttle-like nanostructures have been previously produced [22, 23, 24], this might be the first example showing that porphyrin-based shuttle-like nanoarchitectures could be facilely produced via an oil/water medium. The study gives deep insights into the self-assembly behavior of porphyrins in an oil/water system and provides important clues to the designing of appropriate porphyrins when related subjects are addressed.
Chemicals and reagents
CTAB (99%, Alfa Aesar) and H2TPyP (97%, Aldrich) were used as received without further purification. Milli-Q water (18 MΩ cm) and distilled chloroform were used as the solvents for CTAB and H2TPyP, respectively.
Methods and procedures
The process for the synthesis of H2TPyP-based nanostructures via the oil/water medium is similar to those described for ZnTPyP . Typically, 400 μL of a chloroform solution of H2TPyP (2 × 10-4 M) was added dropwise into a 10-mL aqueous solution of CTAB under vigorous magnetic stirring within 1 to 2 min. The concentrations of the CTAB aqueous solutions were 0.225, 0.9, and 4.5 mM for the samples named as I, II, and III, respectively. Soon after adding H2TPyP chloroform solution, an opaque solution was obtained; a transparent yellowish solution was obtained after vigorous stirring was maintained for 15 min, as shown in Figure 1. The solution was then kept at room temperature without disturbing for a desired time for aging, after which the UV/vis spectra of the solutions were measured. The formed nanostructures were dramatically washed with Milli-Q water several times via repeating filtration or centrifugation, after which they were subjected to various characterizations. The Millipore filter (Whatman) used for the filtration has a pore size of 200 nm. In the case of centrifugation, a rotation speed of 10, 000 rpm is adopted. We found that similar results could be obtained for the samples washed by these two methods. Subsequently, the nanostructures were characterized by low-resolution transmission electron microscopy [LRTEM], high-resolution transmission electron microscopy [HRTEM], fast Fourier transformation [FFT], energy-dispersive X-ray spectroscopy [EDX], and scanning electron microscopy [SEM].
A JASCO UV-550 spectropolarimeter was used for the UV/vis spectral investigation. The SEM measurements were performed using a Hitachi S-4800 system. The LRTEM and HRTEM images of the nanostructures were obtained with a FEI Tecnai G2 F20 U-TWIN, where the accelerating voltages were set as 200 and 80 kV, respectively. Elemental analysis was achieved using energy-dispersive X-ray spectroscopy on the FEI Tecnai G2 F20 U-TWIN.
Results and discussions
SEM images of H2TPyP-involved nanostructures
UV/vis spectra of H2TPyP-involved nanostructures
TEM images of H2TPyP-involved nanostructures
Moreover, in order to reveal the internal structure of our shuttle-like nanoarchitectures, their HRTEM and FFT images were investigated, respectively. During the experiments, we found that our organic nanostructures suffer a fast amorphization under the strong electron beam (with an accelerating voltage of 200 kV) illumination. Therefore, the HRTEM was carried out with an accelerating voltage of 80 kV, where it was find that amorphization could be decelerated to some extent. As shown in the middle and bottom panels of Figure 5, distinct lattice fringes could be detected from all of the formulated shuttle-like nanoarchitectures. An interlattic spacing of about 1 nm could be obtained, as indicated by the FFT analysis. This value is very close to the side length of H2TPyP square, which was calculated to be approximately 1.1 nm. Accompanied by information deduced from the UV/vis spectra, these results suggest that our shuttle-like nanostructures are essentially composed of H-type molecule associations.
An explanation for the formation of the shuttle-like supramolecular nanostructures
It has been demonstrated that the formation of a porphyrin/surfactant complex is facilitated primarily by the electrostatic or hydrophobic interactions between these two components [46, 47, 48, 49]. In the case of ionic porphyrin/nonionic surfactant or nonionic porphyrin/ionic surfactant systems, the hydrophobic interactions between the alkyl chain of the surfactant and the π system of porphyrin play an essential role [32, 46, 47]. In our present case, the employed porphyrin is a nonionic compound. Consequently, the electrostatic interactions between porphyrin and the surfactant are negligible. Thus, we could simply propose that three kinds of intermolecular interactions, i.e., π-π interactions between porphyrin molecules (H2TPyP/H2TPyP), hydrophobic interactions between porphyrin and CTAB molecules (H2TPyP/CTAB), and CTAB/CTAB interactions, compete with each other during the SASA process.
As has been demonstrated, for zinc porphyrins, owing to the large size of the zinc cation, Zn generally stands out from the mean macrocyclic plane of porphyrin, and instead of a planar geometry, zinc porphyrin always exhibits a bent molecular geometry [50, 51]. This could cause a somewhat steric hindrance between zinc porphyrin molecules when they are assembled through intermolecular π-π interactions . Previously, our experimental facts suggested that the ZnTPyP molecule formed a ZnTPyP/CTAB complex with CTAB through the intermolecular hydrophobic interactions between the alkyl chain of CTAB and the π system of ZnTPyP upon coming into contact with CTAB . This was a result of the dynamic competition between the aforementioned three types of intermolecular interactions. After this, owing to their thermodynamic superiority, ZnTPyP units formed assemblies via π-π interactions gradually during the aging, resulting in the formation of various well-defined nanostructures .
At the same time, it should also be pointed out that, still, a small amount of H2TPyP molecules could act as ZnTPyP during the SASA since small amounts of hollow nanospheres and nanotubes, solid nanospheres and nanorods, and solid nanospheres and nanofibers, which are predominately formed in the corresponding samples of ZnTPyP systems, could after all be observed from samples I, II, and III of H2TPyP, respectively.
In summary, we have shown that when a surfactant aqueous solution with an appropriate concentration is selected, a free-base porphyrin, H2TPyP, could be assembled to predominately form unique yet well-defined shuttle-like nanoarchitectures via SASA in an oil/water medium. This might be the first report of porphyrin-based shuttle-like nanoarchitectures which could be facilely produced. By taking into account the molecular geometry of H2TPyP, the intermolecular non-covalent interactions, as well as the role of the surfactant, a possible explanation has been proposed. It gives deep insights into the self-assembly behavior of porphyrins in an oil/water system and provides important clues concerning the design of appropriate porphyrins when related subjects are addressed. Our investigation suggests that an oil/aqueous system might be an efficient medium for producing organic-based nanostructures with unique yet well-defined morphologies.
We thank the National Natural Science Foundation of China (20773141, 20873159 and 21021003), National Key Basic Research Project of China (2007CB808005 and 2011CB932301), and the Chinese Academy of Sciences for financial support.
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