Novel Fluorescein Hierarchical Structures Fabricated by

CRYSTAL GROWTH & DESIGN

Novel Fluorescein Hierarchical Structures Fabricated by Recrystallization under Control of Polyelectrolytes

2007 VOL. 7, NO. 12 2419–2428

Tie-Kai Zhang, Jian-Hua Zhu, Hong-Bin Yao, and Shu-Hong Yu* DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed April 14, 2007; ReVised Manuscript ReceiVed August 15, 2007

ABSTRACT: Complex porous fluorescein microspheres with hierarchical superstructures and relievo-like complex fluorescein crystals can be easily generated by a recrystallization/reprecipitation method using poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) as structure directing agents at room temperature. The polymer-induced liquid precursor (PILP) phase is believed to play a role in the formation of such unusual shapes and structures. The shape and sizes of fluorescein crystals can be well controlled by adjusting the amounts of raw fluorescein precursor and polyelectrolytes. Furthermore, shaking and ultrasonic treatments can provide additional parameters for fabricated of fluorescein crystals with unique superstructures. The UV–vis spectroscopy indicates that the addition of PAH results in shifting of the absorption peak of monomer from 481 nm toward 494 nm, and such shifting is dependent on the PAH concentration.

1. Introduction Recently, one of the major technological challenges in nanoscience and nanotechnology is the self-organization of nanoscale building blocks into hierarchical structures with desirable functionalities because of their unique physical and chemical properties in various fields such as catalysis, medicine, electronics, ceramics, pigments, and cosmetics.1 The ability to control the formation of nanoscale structures of well-defined sizes and shapes is still a hot topic.2 During the past few decades, synthesis of higher-ordered inorganic crystals, hybrid inorganic/ organic materials3 or macromolecules4 has attracted a lot of attention. However, to precisely control the morphology of small organic molecules with new optical and electronic properties is still in its infancy. In contrast with inorganic systems, organic systems offer high flexibility both in their physicochemical and self-assembling properties and are diverse and may exhibit a wide range of electrical and optical properties, and thus are likely to provide a new method for modifying the optical and electronic properties of organic functional materials.5 From this viewpoint, it is of technological interest to fabricate organic nanomaterials with especially desired hierarchical shape and controllable sizes. Among few reported methods,6 a reprecipitation method is found to be simple and effective for preparation of several kinds of organic microcrystals from low-molecular-weight organic molecules with special morphology.7 This facile method is based on solvent displacement. In most cases, an organic compound is dissolved in a quantity of organic solvent which serves as a good solvent, and then this concentrated solution is injected into a volume of water which acts as a bad solvent, resulting in the recrystallization and the formation of microcrystals. Using this method, low-dimensional dye aggregates,8 nanofibers,9 novel mesostructures of dye,10 and submicrotubes11 can be fabricated. Very recently, we demonstrated that the crystals of the organic dye acid green 27 (AG27) with well-defined shape and size can also be crystallized by this approach.12

* To whom correspondence should be addressed. Fax: +86 551 3603040. E-mail: [email protected].

The influences of the parameters such as the concentration of the injected organic solution,13,14 UV-radiation,15 temperature,12,16,17 bad solvents,18 ultrasound,11 and microwave irradiation19 have been discussed. Alternatively, additives including surfactants9 and dendrimers20 have been introduced as inhibitors for controlled crystallization of organic crystals with welldefined shape. Although these factors have been discussed, it is still in its infancy to use this method to prepare other kinds of novel organic crystals. Fluorescein is a highly fluorescent molecule in water,21 and fluorescein dyes have been widely employed to develop useful fluorescence probes for important biomolecules since fluorescence imaging is the most powerful technique currently available for continuous observation of the dynamic intracellular events of living cells.22 Thus, investigation of the possibility of controlling its sizes and shape is of great importance. In this paper, we demonstrate that the small fluorescein organic molecules can be recrystallized into porous microspheres with hierarchical structures and relievo-like crystals in the presence of low molecular weight polyelectrolytes including poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS). The roles of polyelectrolyte concentration as well as the amount of injected fluorescein ethanol solution on the morphology have been investigated. The effects of shaking and ultrasonic treatments on the microstructures of the dye crystals are also discussed.

2. Experimental Section Materials. The compound fluorescein (analytical grade) was used as received. The polycation, PAH, Mw 15 000 g mol-1, and the polyanion, PSS, Mw 70 000 g mol-1, were obtained from Aldrich without further purification. Ethanol (analytical grade) and purified water (18.2 MΩ cm, Millipore-Q) were used as good and bad solvents, respectively. Preparation Procedures. All glassware (glass bottles and small pieces of glass substrates) was cleaned and sonicated in ethanol for 5 min, then rinsed with distilled water, further soaked with a H2O-HNO3 (65%)H2O2 (30%)(1:1:1, v/v/v) solution, then rinsed with purified water, and finally dried with acetone. In a typical procedure, 0.01 g of PAH was dissolved in 10 mL of pure water in a glass bottle. Then, 2 mL of the 4.0 × 10-3 mol L-1 ethanol solution of fluorescein was dispersed dropwise into above solution under vigorously stirring at room

10.1021/cg0703644 CCC: $37.00  2007 American Chemical Society Published on Web 11/03/2007

2420 Crystal Growth & Design, Vol. 7, No. 12, 2007

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Figure 1. The morphological and structural characterization of fluorescein microparticles prepared by a reprecipitation reaction without additives. Two milliliters of ethanol solution of fluorescein. (a, b) A general and typical image of fluorescein particles; (c) the detailed surface structure of the crystal; (d) the XRD pattern of the sample. temperature. After adding the fluorescein ethanol solution, stirring was still kept for another 1 min. Finally, some small pieces of silica wafers were put at the bottom of this container. The vial was kept at room temperature for several hours. The fluorescein crystals precipitated on the surface of the glass substrate at the bottom of the solution. The glass substrates were then taken out and dried in air when the precipitation process finished or when potential intermediates were to be characterized. To investigate the role of preparation conditions in determining the crystal morphologies, the concentrations of polyelectrolytes and the starting materials were varied. In addition, both shaking and ultrasonic treatments were applied in a similar system to examine their influences on the shape and structures of fluorescein crystals. Characterization. For morphology observation of the particles on the glass slide, a JEOL JSM-6700F scanning electron microscopy (SEM) with an acceleration voltage of 5 kV was employed. The coverslips described above were coated with a layer of platinum directly to improve the sample conductivity. The solid powders were also characterized by X-ray diffraction (XRD) pattern, which was recorded on a Philips X’Pert Pro Super X-ray diffractometer with monochromatized Cu KR radiation (λ ) 1.541874 Å). UV–vis absorption spectra of the aqueous dispersion of fluorescein particles were measured on a Shimadzu UV-2450 UV–vis spectrophotometer with a scanning speed of 480 nm/min and a slit width of 1 nm. FT-IR spectra were recorded on an EQUIVOX55 Fourier Transform spectrometer using KBr pellets. The emission images of the crystals were acquired on a fluorescence microscope (Olympus B202) with a spot-enhanced charge-coupled device (CCD, Olympus DP70).

3. Results and Discussion 3.1. Recrystallization of Fluorescein Crystals by Addition of Fluorescein Ethanol Solution to Water. A first insight into the microscopic structure of fluorescein crystals was observed by scanning electron miscroscopy (SEM). Figure 1 shows the SEM images of the morphologies of flower-like fluorescein crystals prepared by the reprecipitation method without any additives (default experiment). In this experiment, a small quantity of precursor was dissolved in ethanol to prepare 4.0 × 10-3 mol L-1 fluorescein solution. Then the above 2 mL ethanol solution of fluorescein was injected into 10 mL of distilled water and aged for 48 h at room temperature. Figure 1a,b show that the shape of the particles is irregular and the size of the particles is in the range of 100–200 µm. The detailed surface structure of a representative particle was shown in Figure 1c, displaying a layered structure. The XRD pattern obviously indicates that the particles are well crystallized (Figure 1d). All the peaks can be indexed to an orthornombic space group Pc21n with a unit cell parameter a ) 10.53 Å, which is in good agreement with the literature value (JCPDS card 51-2219, a ) 10.56 Å). No other impurities could be detected from the XRD pattern.

Novel Fluorescein Hierarchical Structures

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Figure 2. The morphological and structural characterization of porous fluorescein microspheres prepared by a reprecipitation process at a PAH concentration of 1 g L-1, pH 3.94, 2 mL of ethanol solution of fluorescein. (a) Low magnification SEM image; (b) high magnification SEM image; (c) the detailed surface structure of the sphere; (d) XRD pattern of the sample.

3.2. Morphogenesis of Fluorescein Crystals under Control of PAH. A totally different morphology is observed in the presence of PAH serving as a structure directing agent. Figure 2 shows typical SEM images of the sample obtained in the presence of PAH at room temperature. The samples were prepared by adding 2 mL of ethanol solution of fluorescein (4.0 × 10-3 mol L-1) into 10 mL of 1 g L-1PAH aqueous solution and aging for 7 h. The pH value of the initial PAH aqueous solution is 3.94 and decreases to 3.76 after injecting fluorescein ethanol solution. SEM images in Figures 2a,b indicate that the obtained particles are almost identically spherical, with a diameter of about 16 µm. A magnified SEM image in Figure 2c shows that the spheres were constructed by nanoflakes with a thickness ranging from 100 to 400 nm. Figure 2d displays the XRD pattern of the porous fluorescein microspheres. The pattern displays a different phase (JCPDS card 36-1916), showing the structure of fluorescein–methanol. The two strongest diffraction peaks in fluorescein are indexed to the (032) and (113) facets, with corresponding d values of 3.37 and 3.21 Å, respectively. Those two strongest diffraction peaks in fluorescein-methanol can be indexed to the (220) and (202) facets, with corresponding d values of 3.37 and 3.20 Å, respectively. Therefore, the primary diffraction peaks of the fluorescein and fluorescein–methanol are overlapped. It is reported that fluorescein can exist in four different pH dependent states: cationic, neutral, anionic, and dianionic as illustrated in Scheme 1.21,23 At lower pH, it exists in a neutral state and mainly in a lactone form. When the pH comes to 4.31 and 6.43, the fluorescein will exist in the anionic state with a quinoid

structure and dianionic state, respectively. At pH 4, a considerable population of fluorescein is in a neutral state and exists in a lactone form. Lactone may form complexes with methanol (1:1), and the complexes are stable.24 For these complexes, each fluorescein molecule appears to participate in hydrogen bonds with methanol molecule and determines the special crystal structure of fluorescein-methanol.25 In this reaction system, PAH adjusted the pH value to around 4, and the solvent was a mixture of ethanol and water. So the fluorescein molecule is mainly in a lactone form and also tends to participate in hydrogen bonds with ethanol molecule. It can be found the υc)o of the lactone structure at 1734 cm-1 26 in the FT-IR spectrum (see Supporting Information Figure S1c). The vibrational peak with frequency of 1700 cm-1 (see Supporting Information Figure S1a) is attributed to the carboxyl of fluorescein molecule. We think that ethanol can also form a certain crystal structure with the fluorescein, which is similar to that of fluorescein–methanol, although the fluorescein–ethanol phase is not reported yet. Therefore, it is easy to explain the phase of fluorescein–methanol in the XRD pattern of Figure 2d. In addition, the experimental results indicated that the structure of the crystal obtained in the mixture of methanol and water is the same as that obtained in ethanol (see Supporting Information Figure S2); thus, it is reasonable to name this phase as a fluorescein–methanol-like phase. In this system, PAH plays a main role that has the effect of adjusting the pH. In the controlled experiment (1 g L-1 PAH), no precipitate or crystals were obtained when the pH value of the solution was adjusted to 2.0 (by HCl), 6.5 and 8.0 (by


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Scheme 1. pH Value Dependence of the Fluorescein States21,23

NH3 · H2O). When the pH value decreased to 2, PAH and fluorescein molecules were both in the protonated form, which will break the equilibrium of ionization and will be not favorable for the self-assembly. This explanation is suitable for the condition of pH 6.5 and 8.0, too. Therefore, the appropriate pH value for the system to produce the novel hierarchical structure is about 4. 3.2.1. Influence of Polymer Concentration on the Morphogenesis of Fluorescein Crystals. To clarify the role of PAH on the morphogenesis of fluorescein crystals, controlled experiments were performed by varying the PAH concentration while leaving the other parameters unchanged. At a very low PAH concentration (0.1 g L-1, pH 4.37), the particles with different

shapes were generated (Figure 3a). When the concentration of PAH increased to 0.2 g L-1 (pH 4.28), the crystals turn out to be uniform double-cone structures constructed by nanoflakes (Figure 3b). The double-cone structure is highly symmetric, and the length is almost equal to the diameter (about 20 µm). The size is similar to that of the rhombus in Figure 3a, and the cone is composed of many regular nanoflakes with a thickness of about 900 nm. With further increase of the PAH concentration to 0.5 g L-1 (pH 4.11), the morphology of the obtained crystals was changed to dumbell-like aggregates with netlike surface structure (Figure 3c). A nearly spherical particle shown in Figure 3d indicated

Figure 3. SEM images of fluorescein crystals prepared in the presence of PAH at low concentration. Two milliliters of ethanol solution of fluorescein. (a) 0.1 g L-1, pH 4.37; (b) 0.2 g L-1, pH 4.28; (c, d) a general SEM image of the particles and a typical image of an individual fluorescein crystal. 0.5 g L-1, pH 4.11.



Figure 4. High-magnification SEM images and the detailed surface structure of the fluorescein crystals prepared in the presence of PAH at high concentration, 2 mL ethanol solution of fluorescein. (a, b) 5 g L-1, pH 3.57; (c, d) 10 g L-1, pH 3.39.

that the spherical particles consist of nanoflakes as subunits which grow into netlike porous structures. The employment of higher PAH concentration (5 g L-1, pH 3.57) can produce welldefined porous spheres with three-dimensional (3D) hierarchical structures, which were constructed by nanoflakes with an average thickness of about 600 nm (Figure 4a,b). The diameter of the sphere is about 140 µm, which is much larger than that of the particles prepared in the presence of 1 g L-1 PAH. More interestingly, beautiful coral-like hierarchical spheres with netlike 3D network structures were obtained when the PAH concentration increased up to 10 g L-1. The diameter of the sphere is up to 170 µm, and the porous particles are constructed by 3D connected nanosheets with a thickness of around 100 nm (Figure 4c,d). Most of the particles are rhombus with layered structures. Fluorescein molecules are prone to assemble into crystals spontaneously on the surfaces of the silicon wafer. The wideangle X-ray scattering (WAXS) is shown in Figure 5. The curve clearly reveals the preferential orientation of the (020) plane, indicating that these exposed faces of the crystals would be derived from the selective absorption of the polymer on the (020) plane. The above results provide a clue about the growth process of the microspheres under control of PAH. These special structures clearly indicate that the growth of the 3D superstructures involves a multistage process with nanoflakes as their subunits. Eventually, spontaneous but ordered aggregation of these nanoflakes comes into play to generate such hierarchical superstructures. This is similar to the process of morphogenesis commonly observed in nature.1

Figure 5. XRD pattern of fluorescein crystals formed spontaneously on the surfaces of the silicon wafers in the presence of 0.1 g L-1 PAH, 2 mL ethanol solution of fluorescein.

3.2.2. Influence of the Precursor Concentration on Morphogenesis of Fluorescein Crystals. The employment of different amounts of fluorescein ethanol solution does not change the shape and microstructures so much while the concentration of the PAH is kept constant (1 g L-1). When 1 mL of fluorescein solution was added, uniform spheres can be obtained (Figure 6a). The diameter of the sphere is about 30 µm, and the thickness of the nanoflakes ranges from 60 to 100 nm. Figure 6b exhibits the surface structure of the sample, which displays more clearly porous structure than the samples shown in Figure 2a, indicating that the spherical structures can be controlled by changing the ratio of raw material to PAH. Furthermore, by increasing


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Figure 6. High-magnification SEM images and the detailed surface structure of the fluorescein crystals prepared in the presence of 1 g L-1 PAH with different amounts of fluorescein ethanol solution. (a, b) 1 mL of fluorescein ethanol solution; (c, d) 4 mL of fluorescein ethanol solution.

Figure 7. SEM images of fluorescein crystals obtained in the presence of 1 g L-1 PAH mediated by shaking. Two milliliters of ethanol solution of fluorescein. (a) Low-magnification SEM image; (b) high-magnification SEM image; (c) the surface structure of a typical sphere.

Figure 8. SEM images of fluorescein crystals obtained in the presence of 1 g L-1 PAH mediated by ultrasonic treatment. Two milliliters of ethanol solution of fluorescein. (a) Low-magnification SEM image; (b) high- magnification SEM image; (c) the surface structure of a typical sphere.

the volume of the fluorescein ethanol solution up to 4 mL, the particles are rare and not spherical. It can be seen from Figure 6d that the surface structure of the hemispheroid is also constructed by flakes, and the surface of the flake is not smooth and composed of many small particles. 3.2.3. Shaking and Ultrasonic Treatment Assisted Morphogenesis of Fluorescein Crystals. For comparative investigation, shaking and ultrasonic treatment were both employed, and novel spherical superstructures were synthesized. In the synthesis process assisted by shaking, 8 mL of ethanol solution of fluorescein was injected into 40 mL of distilled water under stirring for 1 min and then placed in an incubator shaker (model innova 40) at the speed of 300 rpm for 18 min at room temperature. After the vial was kept for 7 h at room temperature,

the crystals were collected, and the morphology is shown in Figure 7. Each sphere is composed of loosely branched nanoflakes. The spheres have a mean diameter of 15 µm, which is similar to that shown in Figure 2. The detailed surface structure is shown in Figure 7c, and the thickness of the flakes is about 400 nm. Uniform and compact fluorescein crystals were synthesized in the presence of 1 g L-1 PAH under sonication conditions (AS 3120, 120 W) as shown in Figure 8. In the synthesis process, 2 mL of ethanol solution of fluorescein was injected to 10 mL of distilled water under stirring and then the sample was ultrasonically irradiated for 18 min. Then the samples were collected after 12 h. The spheres obtained under this condition are much bigger and compacter than those prepared in the



Figure 9. Time-dependent shape evolution process of fluorescein crystals. 1 gL-1 PAH, 2 mL ethanol solution of fluorescein. (a, b) Low and high magnification SEM images and surface structure, 0 min; (c) 5 min; (d) 1 h.

default experiment. The spheres are uniform and have an average diameter of 85 µm and a mean thickness of 600 nm (Figure 8a,b). 3.2.4. Time-Dependent Shape Evolution Process of Fluorescein Crystals. To understand how the fluorescein superstructures were formed, shape evolution process of the fluorescein crystals in the early stages of crystal growth were carefully followed. Figure 9a shows the SEM image of the particles obtained as soon as the stirring is finished. An organic film can be observed, and there are many droplets with a chapped flake structure. The detailed surface structure of the flake is revealed by the magnified SEM image shown in Figure 9b. It is obvious that the surface is coated with many nanoparticles. After the samples continued to grow for 5 min, nearly spherical particles with a diameter of about 3 µm were observed (Figure 9c). These particles are relatively dense, and no obvious porous structures are observed. If the process was prolonged to 1 h, the spheres with clearly porous structure were formed (Figure 9d). The spheres are of a size of about 4 µm, and the porous structure is similar to that shown in Figure 2a. 3.3. Morphogenesis of Fluorescein Crystals under Control of PSS. The sample prepared by adding 2 mL of fluorescein ethanol solution into 10 mL 1 g L-1 PSS aqueous solution and aging for 1 h is composed of complex but not welldefined particles, including irregular particles with incomplete shuttle-like surface and rhombic-like parts that we called relievolike (Figure 10a). Furthermore, both kinds of particles have very smooth surfaces and the average size of the relievo-like crystals is about 60–95 µm. If more PSS is used, the particles are shuttlelike and have a mean length of 25 µm and an average width of 16 µm. By increasing the amount of fluorescein ethanol solution

up to 4 mL, flower-like crystals are generated (Figure 10c,d). Figure 10d shows a typical flower-like particle with a size of about 100 µm, which is constructed by many nanoplates. 3.4. Formation Mechanism of Fluorescein Superstructures under Control of PAH. On the basis of the studies on the formation process of crystals, it is believed that the formation of the microspherical superstructures mainly results from the directing effect provided by PAH. It has been established that polyelectrolytes stabilized amorphous nanoparticles act as the precursor for the follow-up mesoscale selfassembly27 just like in inorganic systems. Sebastian et al. described the fine droplets of a polymer-induced liquid precursor (PILP) phase28 for an amino acid/polymer system to produce complex structures from amino acids.29 PILP phase allows facile morphosythesis of novel minerals with unique structures through shaping of a liquid on micrometer scale, and it is usually applied in synthesis of novel inorganic material with complex structures.28 Herein, the PILP phase consists of fluorescein dyes and charged polyelectrolyte in ethanol/water mixture. After the addition of fluorescein ethanol solution to an aqueous solution containing polyelectrolyte, PILP microdroplets are formed spontaneously, and these precursor phases can subsequently be crystallized to build the highly ordered spherical structures of the component fluorescein dyes. The PILP plays a very important role in the process of the synthesis for the porous microspheres. Additionally, if 0.1 M HCl is used to adjust the pH value to 3–4, no such structure can be formed. According to the above experimental results, a possible formation mechanism of the fluorescein microspheres is proposed as shown schematically in Figure 11.


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Figure 10. SEM images of fluorescein crystals prepared in the presence of PSS. (a) 2 mL ethanol solution of fluorescein, 1 g L-1 PSS; (b) 2 mL ethanol solution of fluorescein, 10 g L-1 PSS; (c, d) a general and typical image of fluorescein flower-like crystals, 4 mL of ethanol solution of fluorescein, 1 g L-1 PSS.

Figure 11. Proposed growth mechanism for the formation of microspheres using PAH as additive. (a) Initial polymer solution; (b) recrystallization and aggregation of amorphous nanopaticles in the presence of PAH; (c) formation of fine droplets of a polymer-induced liquid precursor (PILP) phase; (d) crystallization from the PILPs.

Furthermore, the growth modes of the superstructures obtained in the presence of PAH with different concentrations are schematically presented in Figure 12. The inhabiting effect caused by PAH leads to the formation of a platelike fluorescein crystal, mainly exhibiting (020) faces (see section 3.2.1). As the PAH concentration increases, the flakes assemble into spherical superstructures. 3.5. Optical property of Fluorescein Superstructures. Figure 13 displays the absorption spectra of the fluorescein particles dispersed in the solution with and without PAH. In ethanol solution, fluorescein dyes present as an equilibrated mixture of monomers (M) and dimers (D).30 For the diluted ethanol solution of 0.7 × 10-3 mol L-1 (2 mL of 4 mM fluorescein ethanol solution dilute in 10 mL of ethanol solution),

the absorption spectrum of an ethanol solution of the dye has two distinct peaks at λD ) 453 and λM ) 481 nm (Figure 13, curve s), which are assigned to the absorption of dimers and monomers, respectively. In the spectra for the conventional reprecipitation method (without PAH), the bands at 481 and 454 nm show a slight blue shift centered at approximately 473 and 450 nm (Figure 13, curve m), respectively. The absorption maximum at λ ∼ 450 nm increases, while the absorption maximum at 473 nm decreases, testifying the predominant formation of dimers and suggesting the formation of Haggregates.31 It is well known that dyes are capable of forming ordered aggregates, in particular, H- and J-aggregates. The aggregates were characterized by high stability, light emission



Figure 12. The schematic illustration of the growth modes for the formation of the fluorescein superstructures, in the presence of PAH at different concentration. (a) 2D growth with preferential adsorption of PAH; (b) self-assembly of plates caused by a large amount of PAH in the medium; (c) 3D growth of the spheres controlled by an excess amount of PAH.

Figure 13. The influence of PAH concentration on the crystallization process of fluorescein crystals, 5 min: absorption spectra of (a) 0.5 g L-1, (b) 1 g L-1, (c) 5 g L-1, and (d) 10 g L-1 PAH. Two milliliters of ethanol solution of fluorescein. (s) Raw materials in ethanol solution; (m) default experiment (without PAH additive).

in a certain spectral region, and nonlinear optical properties.32 The addition of PAH results in a loss of dimers, and the short wavelength peak at λD is less pronounced and is presented as a shoulder, indicating the formation of J-aggregates.33 The spectra of the particles obtained in the presence of PAH experience a red shift by a maximum of 13 nm, appearing at λM ) 494 nm when 0.5 g L-1 PAH is used. As the concentration of PAH increases from 5 g L-1 to 10 g L-1, the peak of monomers shifts to shorter wavelength appearing from λM ) 491 to 488 nm respectively (Figure 13, curves c,d). The results showed that the control of shape and sizes of fluorescein crystals could vary the optical properties. Figure 14 shows the emission color of the obtained crystals and the raw precursor exposed under UV light. It is well known that fluorescein is widespread used as molecular probes and for selective labeling of biomlecules. Further investigation of the optical characteristics of fluorescein crystals with unique shape and structures is still needed in future.

4. Conclusions In summary, a recrystallization/reprecipitation method using PAH and PSS as structure directing agents at room temperature has been developed for the formation of fluorescein crystals with unusual complex morphologies. The results demonstrate

Figure 14. Fluorescence microscopy images of raw materials and asprepared crystals prepared in the presence of PAH that deposited onto slides and excited using UV light with a wavelength range from 340 to 380 nm. (a) Raw materials; (b) flower-like crystals prepared without additives; (c, d) the crystals prepared in the presence of 1 g L-1 and 10 g L-1 PAH, respectively. Two milliliters of ethanol solution of fluorescein was used.

that the shape and sizes of the fluorescein crystals can be finely tuned by addition of the polyelectrolytes. The polymer-induced liquid precursor (PILP) phase is believed to play a role in the formation of such special shapes and structures. In addition, the combination of shaking and ultrasonic treatments with this crystallization method can provide additional parameters for shape controlling of the fluorescein crystals with unique hierarchical structures. Acknowledgment. S.H.Y. acknowledges the special funding support from the Centurial Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China (NSFC, Nos. 20325104, 50732006, 20621061, 20671085), the 973 project (2005CB623601), the Anhui Development Fund for Talent Personnel (2006Z027), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the PartnerGroup of the Chinese Academy of Sciences-The Max Planck Society. Supporting Information Available: FTIR spectra and XRD pattern of fluorescein crystals. This material is available free of charge via the Internet at http://pubs.acs.org.


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