Fluorescence Enhancement of Rhodamine B in the Presence of

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Fluorescence Enhancement of Rhodamine B in the Presence of Photonic Crystal Heterostructures Edina Rusen,* Alexandra Mocanu, Aurel Diacon, and Bogdan Marculescu University Politehnica of Bucharest, Department of Polymer Science, 149 Calea Victoriei, RO-010072 Bucharest, Romania ABSTRACT: Photonic crystal heterostructures have been obtained by mixing a styrenehydroxyethylmethacrylate (ST-HEMA) colloidal dispersion with Au colloidal solution (1:5 volume ratio). To modify the heterogeneous photonic crystal with Rh, the first step has been the chemical attachment of HO-R-SH to the surface of Au particles. The next reaction was the esterification of hydroxyl modified photonic crystal with Rh. In UVvis spectra the peak at 600 nm is possible due to the Bragg diffraction of dye molecule aggregates. In the case of ST-HEMA-Au-SHRh, the same signal has been shifted to 625 nm, induced by the existence of preformed particles. A decrease in the reflection response of Rh is noticed, and this corresponds to the absorption domain of dye, namely, the Q-band. The large decrease in area for particles ST-HEMA-Au-SH-Rh could be attributed to the overlapping of the Rh reflection band with the photonic crystal stop band. In the case of fluorescence, to the same excitation wavelength, for the STHEMA-Au-SH-Rh film, the emissions have been amplified compared with the signal of Rh film. A preliminary test has been performed for putting into evidence the chemosensor properties of ST-HEMA-Au-SH-Rh film modified with triethylenetetramine.

1. INTRODUCTION Nanostructured materials are attracting much attention due to their application in photonics, electronics, biology, and catalysis. Photonic crystals display many properties, including the appearance of pass bands, a photonic band gap, and a complex dispersion relation, opening up the possibility of many optical device applications, for example, all optical switches, lasers, fibers, and filters.19 Introducing defects into photonic crystals in a controlled manner has attracted great interest. One of the possibilities to introduce defects consists in the photonic crystal heterostructures, combining two or more photonic crystals composed of particles of different sizes.1015 Thus, photonic crystals heterostructures can exhibit a broader band gaps,16,17 which may offer functionality for engineered photonic behavior. Therefore, it is very important to improve the quality of photonic crystal heterostructures and to reduce the space of the interface. One of the possibilities to decrease the space occupied by air could consist in the surface modification, for a type of particles which form photonic crystal heterostructures. In addition, selfassembled monolayers (SAM) are a versatile tool for the modification of surfaces allowing for the creation of well-ordered molecular assemblies. These SAM on the surface of gold offer an attractive means for building chemical functionality onto a surface with the eventual goal of defining and controlling interfacial activity. The possible arrangement of photonic crystal heterostructures consisted by two type of particles, polymeric and gold nanoparticles with SAM on the surface presented in scheme 1. To improve the optical properties of photonic crystal heterostructures, SAM was functionalized with a dye. Thus, Rhodamine B is an important laser dye with excellent photophysical properties, such as a long wavelength absorption and r 2011 American Chemical Society

Scheme 1. Possible Arrangement of Photonic Crystal Heterostructures Modified with SAMa

a

(A) View from above. (B) View from cross section.

emission, high fluorescence quantum yield, large extinction coefficient, and high stability against light.1820 There are various methods to fabricate organic dyes thin films, but each method has some disadvantages. The photonic crystal heterostructures could be considered as a physical support for Rhodamine film. Because of the increasing interest of optical properties of photonic crystal heterostructures, the current investigation first attempt has been to prepare high quality fluorescent film. The film has been obtained by covalent attachment of Rhodamine to the SAM, contained in the photonic crystal heterostructures. Received: May 4, 2011 Revised: June 16, 2011 Published: June 23, 2011 14947

dx.doi.org/10.1021/jp2041422 | J. Phys. Chem. C 2011, 115, 14947–14953

The Journal of Physical Chemistry C

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Figure 1. Optical microscope image at various magnifications: (a) 20; (b) 50; (c) 100; (d) photo image.

Figure 3. UVvis spectrum for ST-HEMA film.

Figure 2. SEM image of the film obtained from the ST-HEMA dispersion (a) and the AFM results for the same system (b). Insertion gives the FFT data.

Use of Rhodamine B derivatives has recently gained much interest in obtaining fluorescent chemosensors since the dye framework offers selectivity based on its particular structural property.2124 Because of the roughness of the surface (presented in Scheme 1), Rhodamine B has been adsorbed on the surface in a certain amount. The unreacted carboxylic group of adsorbed Rhodamine B makes possible the reaction with substances containing multifunctional amine groups. Thus, the presence of heavy metal ions induce ring-opening of the spirolactam and gives rise to strong fluorescence emission and a strong pink color. The aim of this study consisted in obtaining high fluorescent chemosensor film.

Figure 4. TEM image of Au nanoparticles.

2. MATERIALS AND METHODS 2.1. Materials. Styrene (ST) (Merck) has been purified through vacuum distillation. Hydroxyethylmethacrylate (HEMA) (Aldrich) has been passed through separation columns filled with Al2O3 to remove inhibitors. Potassium persulphate (KPS) 14948

dx.doi.org/10.1021/jp2041422 |J. Phys. Chem. C 2011, 115, 14947–14953


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(Merck) has been recrystallized from an ethanol/water mixture and then vacuum dried. Tetrachloroauric acid trihydrate 99.5% (HAuCl4 3 3H2O) (Merck), trisodium citrate dehydrate (Na3C6O7 3 2H2O) (Fluka), 2-mercaptoethanol (HO-R-SH) (Aldrich), Rhodamine B (Rh) (Merck), N,N0 -dicyclohexylcarbodiimide (DCC) (Merck), 4-dimethylaminopyridine (DMAP) (Aldrich), triethylenetetramine (TETA) (Merck), and dichloromethane (Merck) have been used with no previous purification. 2.2. Soap-Free Emulsion Polymerization. ST (6.5 mL) and HEMA (2 mL) have been added in 100 mL of distilled water together with 0.0625 g of KPS. The reaction mixture has been nitrogen purged and then maintained for 8 h at 75 °C under continuous stirring. The final dispersion has been dialyzed in distilled water for 7 days, using cellulose dialysis membranes (molecular weight cutoff: 12,00014,000) to remove the unreacted monomer and initiator. 2.3. Preparation of Gold (Au) Nanoparticles. Au colloids were prepared by Na3C6O7 3 2H2O reduction of HAuCl4. HAuCl4 (90 mL of 3 104 M aqueous solution) was allowed to boil, at which point 3.6 mL of 6.8 102 M Na3C6O7 3 2H2O was added dropwise with stirring. After the addition of Na3C6O7 3 2H2O, the solution began to darken and turn bluish-gray or purple. After approximately 10 min, the reaction was completed and the final color of solution was a deep wine red. The solution was cooled to room temperature with continued stirring. 2.4. Chemical Attachment of HO-R-SH to Au Particles/ SAM Preparation. The ST-HEMA colloidal dispersion has been mixed with an Au colloidal solution (1:5 volume ratio) and has been deposed on glass substrate (drying at 60 °C in an air current). The obtained film has been immersed in 20 mL of an aqueous HO-R-SH solution (1%). The reaction has been kept 24 h at room temperature. The modified film has been washed with distillate water and dried to vacuum.

2.5. Esterifcation with Rh (ST-HEMA-Au-SH-Rh). The thiolmodified film/SAM has been immersed in a dichloromethane solution. The 20 mL of solution of dichloromethane contained 0.02 g of Rh, 0.03 g of DCC, and 0.008 g of DMAP. The reaction has been kept 24 h at room temperature with stirring. The modified film has been washed with dichloromethane and dried to vacuum. 2.6. Reaction of ST-HEMA-Au-SH-Rh with TETA. The STHEMA-Au-SH-Rh film has been immersed in a 20% aqueous solution of TETA. The reaction has been kept for 5 h at 70 °C with stirring. The modified film has been washed with water and dried to vacuum. 2.7. Characterization. The morphologies of polymer particles have been investigated through XL-30-ESEM TMP scanning electron microscopy (SEM). The samples were sputtered with a thin layer of gold prior to imaging. The particle size measurement, through dynamic light scattering (DLS) and the Z potential, have been obtained with a Nani ZS device (red badge). Transmission electron microscopy (TEM) images were recorded on a Philips CM 120 ST equipment using an acceleration voltage of 100 kV. The infrared absorption spectra have been recorded at room temperature with a Nicolet 6700 FTIR spectrometer in the range of 4000400 cm1. Structural characterization has been performed on an atomic force microscopy (AFM) NT-MDTP47H apparatus. All samples analyzed were particle films obtained by gravitational sedimentation, in an air current (oven drying at 60 °C). Microphotographs have been taken with an optical microscope (Olympus, BX-41) with a CCD

Figure 5. The DLS measurement for the colloidal mixture.

Figure 7. UVvis spectra.

Figure 6. Optical microscope image at various magnifications: (a) 20; (b) 50; (c) 100; (d) photo image. 14949



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Figure 9. UVvis spectra (insertion Rh optical image).

camera. The UV/vis spectra have been recorded using a V-500 Able Jasco spectrophotometer. The 3D fluorescence spectra have been registered using a FP-6500 Able Jasco spectrofluometer.

3. RESULTS AND DISCUSSION The first stage in our study consisted in preliminary characterization of the ST-HEMA film using an optical microscope. The distance between the defaults (resulted from the water evaporation) was around 200 μm, evidence of the good quality of the final opal film (Figure 1). To obtain more information about the ST-HEMA film and the particles' size, SEM analysis was performed. The SEM image (Figure 2a) shows a regular lattice structure, characteristic for photonic crystals, with particle size of 200 nm, also confirmed by AFM (Figure 2b). Fast Fourier transform (FFT) data (insertion in Figure 2b) shows that particles crystallize/arrange in a compact hexagonal lattice (hcp). Photonic crystals possessing photonic band gaps are ranges of frequency in which light cannot propagate through the structure. The enhancement of the intensity and the far stronger interaction of the canned light with any kind of material make photonic crystals ideal candidates for optical sensing devices. To put in evidence the band gap in our case, the UVvis spectra has been recorded. The characteristic band gap is not well-defined

Figure 10. 3D fluorescence spectra: (a) Rh film; (b) ST-HEMA-AuSH-Rh film.

(Figure 3), because of the compact structure (fractions occupied by spheres and air for a hcp lattice is 74%, respectively 26%).25,26 Photonic crystal heterostructures can exhibit a broader band gap by manipulating the sphere sizes of the two constitutional components, which may offer functionality for engineered photonic behavior. Taking into account this reasoning, the STHEMA colloidal dispersion has been mixed with Au colloidal solution (1:5 volume ratio). The size of the Au particles and the stability of colloidal mixture have been investigated by TEM and DLS. The diameter of metallic particles has been 10 nm (Figure 4) and the ζ potential of the mixture has been 39 mV. This value indicated the colloidal stability of the mixture and the size of the hybrid particles becomes 260 nm (Figure 5). The monodispersity of the hybrid particle has been observed in the same figure. The size of the aggregates corresponds to a cluster of 7 particles 14950



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Scheme 2. Modification Reaction of Rh

Figure 11. Fluorescence spectra at 250-nm excitation wavelength.

(6 Au particles around the 1 ST-HEMA particle). The possible structure is inserted in Figure 5. The film obtained by mixing the two colloidal dispersions has been primary/initially investigated using optical microscope (Figure 6). The distance between the defaults (resulted from the water evaporation) was around 100 μm, proof of the good quality of the final opal film. The next step consisted in the UVvis characterization of the heterostructures photonic crystal. In Figure 7 a broader and shifted band gap has been noticed for the system ST-HEMA-Au, because of the increase of the particles size and due to the volume occupied by the air (insertion in the Figure 7 of the arrangement of the particle). To modify the heterogeneous photonic crystal with Rh, the first step has been the chemical attachment of HO-R-SH to the surface of Au particles. The FT-IR spectra show the characteristic peak of the hydroxyl group at 3300 cm1. The next reaction was the esterification of hydroxyl modified photonic crystal with Rh. In FT-IR spectrum appear new peaks specific to Rh (aromatic CdC 15931595 cm1, CdO ester band at 1705 cm1, the 3100-cm1 absorption band is assigned to CH aromatic). In addition, the 1709.9-cm1 absorption band is assigned to CdN stretching vibration, and the 2963-cm1 absorption can be assigned to C—H asymmetric stretching. The aromatic skeletal C—C stretch and C—O—C stretch were observed at 1339 and 1220 cm1, respectively. The aromatic C—H in-plane bending was observed at 1115.5 cm1. The C—H out of plane bending was observed at 824 cm1, and the band at 680 cm1 can be assigned to the aromatic C—H wagging vibrations.27,28 The values found for the ester band are in agreement with several Rh ester bands at 17071728 cm1. As shown in Figure 8, the hybrid material modified with Rh did not maintain the good quality of the opal film. To get more information about the optical properties of hybrid material modified with Rh, it has been compared with Rh film.

In the UVvis spectra showed in Figure 9, for Rh film the reflection response has been noticed at 600 nm. According to recent studies,29 the refractive index n of Rh thin film shows anomalous dispersion in the spectral range 400900 nm. This anomalous behavior is due to the resonance effect between the incident electromagnetic radiation and the electron’s polarization, which leads to the coupling of electrons in Rh films to the oscillating electric field. Moreover, the peak in the refractive index corresponds to the fundamental energy gap of Rh film. The peak at 600 nm is possible due to the Bragg diffraction of dye molecules aggregates (insertion of Rh optical image). In the case of ST-HEMA-Au-SH-Rh, the same signal has been shifted to 625 nm, induced by the existence of preformed particles. A decrease in the reflection response of Rh is noticed to 500 nm that correspond to the absorption domain of dye, namely Q-band, assigned to the first ππ* transition.30,31 The large decrease area for particles ST-HEMA-Au-SH-Rh could be attributed to the overlapping of Rh reflection band to photonic crystal stop band. The films have been further characterized by fluorescence spectroscopy. In Figure 10 are presented the 3D fluorescence spectra which consist of emission intensity vs emission wavelength and excitation wavelength. According to previous articles3234 the photonic crystal may act as a Bragg mirror and can effectively increase fluorescence intensity of organic dyes because they enhance excitation and/or emission light. A condition for fluorescence enhancement is that the excitation wavelength is in the stop-band of the photonic crystal. The excitation light reflected by a Bragg mirror can stimulate more dye molecules, which is also in favor of fluorescence enhancement. In our case, the emission response for Rh film is observed at 500 and 750 nm for a 250 nm excitation wavelength (Figure 10a). To the same excitation wavelength, in the case of ST-HEMA-Au-SH-Rh film, the emissions have been amplified (Figure 10b). Although, the excitation wavelength is not in the stop-band region of the photonic crystal, a fluorescent enhancement was noticed. To put into evidence the chemosensor properties of photonic crystal heterostructures, the free amine groups provided from TETA has been reacted with Rh adsorbed on the surface. These are potentially available to interact with specific metal ions.3538 A preliminary test has been performed putting in contact an aqueous solution (20%) of Pb(CH3COO)2 with the ST-HEMAAu-SH-Rh film. The reactions are presented in Scheme 2. The surface exposed to Pb2+ showed fluorescence, indicating a Pb2+-induced ring-opening of the spirolactam. Comparing the fluorescent emission of ST-HEMA-Au-SH-Rh (580 nm Rh emission) film with the ST-HEMA-Au-SH-Rh-TETA exposed to Pb2+ could notice the appearance of a new band situated to 520 nm.3941 This peak has been attributed to ring-opening of the spirolactam (Figure 11). 14951



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The film obtained after the contact with Pb2+ has been analyzed to the optical microscope (Figure 12). It has been noticed a strong pink color, specific to identification reaction. A detailed study about the sensitivity and selectivity of chemosensor will be the topic of future work.

4. CONCLUSION Photonic crystal heterostructures have been obtained by mixing a ST-HEMA colloidal dispersion42 with Au solution (1:5 volume ratio). The size of the aggregates corresponds to a cluster of 7 particles (6 Au particles around the 1 ST-HEMA particle). In UVvis analysis a broader and shifted band gap has been noticed for the ST-HEMA-Au system, because of the increase of the particles size and because of the volume occupied by the air. To modify the heterogeneous photonic crystal with Rh, the first step has been the chemical attachment of HO-R-SH to the surface of Au particles. The next reaction was the esterification of hydroxyl modified photonic crystal with Rh. In UVvis spectra the peak at 600 nm is possible due to the Bragg diffraction of dye molecules aggregates. In the case of STHEMA-Au-SH-Rh, the same signal has been shifted to 625 nm, induced by the existence of preformed particles. A decrease in the reflection response of Rh is noticed that corresponds to the absorption domain of dye, namely, the Q-band. The large decrease in area for particles ST-HEMA-Au-SH-Rh could be attributed to the overlapping of Rh reflection band to photonic crystal stop band. In the case of fluorescence, to the same excitation wavelength, for the ST-HEMA-Au-SH-Rh film, the emissions have been amplified compared with the signal of Rh film. Preliminary tests have been performed for putting into evidence the chemosensor properties of ST-HEMA-Au-SH-Rh film modified with TETA. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Authors recognize financial support from the European Social Fund through POSDRU/89/1.5/S/54785 project “Postdoctoral Program for Advanced Research in the Field of Nanomaterials”.

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Fluorescence Enhancement of Rhodamine B in the Presence of

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