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Materials Chemistry and Physics 148 (2014) 964e967

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Fluorescence resonance energy transfer of sulforhodamine B attachedon silica spheres

Byoung-Ju Kim, Kwang-Sun Kang*

Department of New and Renewable Energy, Kyungil University, 50 Gamasilgil Hayangup Gyeongsan, Gyeongbuk 712-701, South Korea

h i g h l i g h t s

� Sulforhodamine B was attached to the photonic crystal by two step synthesis.� The SRH and ICPTES were covalently connected with sulfonate and isocyante groups.� The synthesized molecules were attached to the silica spheres.� Although the SRH shows hypsochromic shift, the ICPSRH exhibits bathochromic shift.� The SRH, ICPSRH and ICPSRHS show different Stokes shifts.

a r t i c l e i n f o

Article history:Received 15 April 2014Received in revised form1 September 2014Accepted 6 September 2014Available online 17 September 2014

Keywords:Chemical synthesisPhotoluminescenceLuminescenceOptical properties

* Corresponding author.E-mail address: kkang@kiu.ac.kr (K.-S. Kang).

http://dx.doi.org/10.1016/j.matchemphys.2014.09.0060254-0584/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Fluorescence resonance energy transfer (FRET) of sulforhodamine B (SRH) has been investigated. TheSRH was attached to silica spheres by two step synthetic processes including a urethane bond formationbetween a 3-isocyanatopropyl triethoxysilane (ICPTES, eN]C]O) and an SRH (SieOH) in pyridine at50 �C and hydrolysisecondensation reaction between synthesized ICPTES/SRH (ICPSRH) and silicaspheres. The disappearance of FTIR absorption peak at 2270 cm�1 representing asymmetric stretchingvibration of eN]C]O indicated the progress of the reaction, and a new absorption peak at 1712 cm�1

characterizing eC]O stretching vibration implied the formation of the urethane bond. The UVevisibleabsorption spectra of SRH and the mixture of ICPTES and SRH in methanol showed the same spectralprofile. However, the synthesized ICPSRH/silica-spheres (ICPSRHSS) exhibited about 13 nm bathochromicshift. The photoluminescence (PL) peak of the SRH in methanol manifested a hypsochromic shift with theincrease of the excitation wavelength. However, the PL peak for the ICPSRH had a bathochromic shiftwith the increase of the excitationwavelength. Approximately 13 nm Stokes shift was observed for the PLof the SRH in methanol compared with the absorption spectrum. Approximately 43 and 70 nm Stokesshifts were observed for the ICPSRHSS and ICPSRH films, respectively, which might be due to the self-FRET between SRH molecules without significant PL quenching.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The fluorescent probes for wide range of applications includingbiological probes and clinical diagnosis have been paid enormousattention [1e3]. Fluorescent quantum dots (QDs) have been amajorresearch field for the biological probes and clinical diagnosis due totheir advantageous properties in this research field including sta-bility against photobleaching and compatibility with aqueousenvironment [4,5]. However, QD toxicity has become a hot topic inrecent years due to the most useful semiconductor QDs contain

elements that are detrimental to health and the environment [6].Fluorescent organic dyes have major drawbacks including blinkingcharacteristics, photobleaching, low signal intensities, hydropho-bicity, self-aggregation in aqueous solution, small Stokes shiftcausing the self-quenching and measurement error due to theoverlap with excitation and emission spectra. Nevertheless, com-posite nano-/micro-particles with incorporation of organic dyemolecules by solvent evaporation technique and dye encapsulationinside the spheres have been successfully used for fluorescentprobes [7e9].

Fluorescence imaging in biological field requires long emissionwavelength (longer than 500 nm), and large Stokes shift (more than50 nm) to avoid spectral overlap between excitation light and

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emission light. Fluorescence resonance energy transfer (FRET) is amechanism relating energy transfer between two light-sensitivedonor-acceptor molecules. A donor molecule becomes electronicexcited state by absorbing a photon and the excited electrontransfer the energy to an acceptor molecule by nonradiative dipo-leedipole coupling. The efficiency of the energy transfer isinversely proportional to the sixth power of the distance betweentwo molecules, which makes FRET extremely sensitive to changesin distance. FRET between electron acceptors and electron donors isa good method to achieve large Stokes shift. In this case, the exci-tation wavelength excites an electron of the donor molecule, andthe excited electron jumps to the excitation state of the acceptormolecule and emits the fluorescing light. Large Stokes shift wasachieved by FRET with donor and acceptor pairs [9e11]. Anothermethod achieving the large Stokes shift is photoinduced intra-molecular electron transfer by careful design and synthesis of thefluorophores [12e14].

Self-quenching of the fluorescence emission should be reducedto a minimum in order to measure the emission spectra in bio-logical imaging research and any other application fields of thefluorophores. In many cases, the self-quenching of the luminescentdyes becomes a significant problem for the specific requirement ofhigh concentration of the fluorescent dyes and is a complicatedprocess requiring extremely important to investigate the exact ef-fect of the self-quenching process of the luminescent dyes. Theluminescent self-quenching is mainly due to the reabsorption effectand fluorescence homoresonance energy transfer (FHRET) process[15,16]. The fluorescence self-quenching is greatly reduced byintroducing the silver nanoparticles in silica substrate [17]. In thispaper, we report the FHRET of the sulforhodamine B (SRH) bysynthesis of 3-isocyanatopropyl triethoxysilane (ICPTES) and SRH(ICPSRH) and attaching ICPSRH to silica spheres (ICPSRHSS).

2. Experimental

Tetraethylorthosilicate (TEOS, 98%), methanol (HPLC grade), 2-propanol (99%), ammonium hydroxide (28%), ICPTES (95%) andSRH (95%) were purchased from Sigma Aldrich Co LTD. The SRH(60 mg) was dissolved in pyridine, and dry nitrogen was pursed inthe reaction flask for 2 h. The ICPTES (280 mg) was charged to thereaction flask with stirring. The reaction temperature maintainedapproximately 50 �C for 21 h and cooled down to room temper-ature. The monodisperse silica spheres were synthesized withSt€ober synthetic method with the following processes. Approxi-mately 50 ml of 2-propanol was charged to the 250 ml roundbottom flask. The TEOS (1.58 g) was added to the flask, and 50 mlof NH4OH was added to the reaction flask with vigorous stirring.The synthesized spheres were separated and washed with meth-anol by repeated washing-centrifuge processes. The spheres weredispersed in acetone (50 ml), and the sphere solution was addedto the ICPSRH solution. The mixtures were stirred for 17 h at roomtemperature. The final spheres attached the ICPSRH were purifiedby repeated centrifuge and washing processes with acetone. Thespheres were dried in the dry oven overnight. For FTIR spectra ofsilica spheres and SRH, the silica spheres or SRH was dissolved inmethanol. The silica spheres or SRH solution was dropped anddried to a KBr plate and obtained FTIR transmission spectra. ForFTIR spectrum of the ICPTES, the ICPTES was directly dropped tothe KBr plate and obtained FTIR transmission spectrum. The FTIRtransmission spectra for the synthesized ICPSRH and ICPSRHSSwere obtained by the following step. The synthesized materialswere purified and dissolved in methanol. The solution was drop-ped and dried to the KBr plate and obtained the FTIR transmissionspectrum. UVevisible spectra for SRH, SRH/ICPTES, ICPSRH wereobtained by Thermo-Scientific Genesys 10S UVevisible

spectrometer. The field emission scanning electron microscope(FESEM) images were obtained by Hitachi JEOL ISM 7401F scan-ning electron microscope with the acceleration voltage of 5 kV forthe pure silica spheres and ICPSRHSS. The PL spectra for the SRHand ICPSRH in methanol were obtained by dissolving the SRH andICHSRH in methanol using Hitachi F-4500 fluorescencespectrometer.

3. Results and discussion

Colloid crystals comprised of fluorescing dyes have been utilizedas optical data storages and developed nanostructured next gen-eration protecting secure documents such as identification docu-ments. Strongly fluorescing silica spheres attached the SRH on thesurface of the spheres has been fabricated with the followingsynthetic processes. Firstly, a urethane bond is formed by the re-action of a sulfonate (ReS(OO)eO�) of SRH and an isocyanate(eN]C]O) of ICPTES in pyridine at 50 �C. The second reaction is acondensation reaction with ICPTES (eSieOH) and silica spheres(SieOH) at room temperature with base (pyridine) catalyst. Theschematic representation of the synthetic processes is shown inFig. 1.

The reaction product was quenched to the aqueous sodiumcarbonate to remove the unreacted SRH and pyridine. The finalproduct was filtered with the nylon filter having 200 nm pores andwashed with deionized water. Fig. 2 shows a characteristic infraredabsorption peak at 2270 cm�1 representing the asymmetric-stretching vibration of the isocyanate. The disappearance of theisocyanate absorption peak indicates the formation of the urethaneband. The new absorption peaks at 3340, 1714, 1540 and 1260 cm�1

represent the eNH stretching, eC]O stretching, eCHN deforma-tion and eCeN stretching vibration, respectively, which indicatethe formation of the urethane bond. Additionally, the 779 cm�1 canbe assigned the NeH wag band.

The UVevisible absorption spectra of the SRH and mixture ofSRH and ICPTES in methanol and ICPSRH are shown in Fig. 3. Thespectral profiles of the SRH and the mixture of SRH and ICPTESexactly match with the maximum peak at 555 nm. However, thesynthesized ICPSRH shows bathochromic shift approximately13 nmwith the maximum absorption peak at 568 nm, which is dueto the intermolecular interaction between the SRH molecules.

The FESEM images of as-synthesized silica spheres and theICPSRHSS are shown in Fig. 4(a) and (b), respectively. The surface ofpure silica spheres is smooth, but the surface of the ICPSRHSS haslarge number of tens nanometers particles on the surface of thespheres. The diameter of the spheres is approximately 360 nm.

Fig. 5(a) and (b) show the excitation wavelength dependent PLspectra of the SRH and the ICPSRH in methanol, respectively. Theemission peak of the SRH in methanol is at 568 nm for the exci-tation wavelength at 450 nm and shows slight blue shift with theincrease of the excitation wavelength. This blue shift is due to therapid relaxation process of a red emitting contribution. However,the emission peak for the ICPSRH is at 598 nm for the excitationwavelength at 460 nm and shows red shift as the excitationwavelength increases, which can be interpreted as a neighboringenvironmental effect. In this case, the dipolar relaxation time issimilar or longer than the emission time. Therefore, the emissionprocess is relatively slow with respect to the neighboring dipolarreorganization.

The absorption and luminescence spectra of SRH in methanoland emission spectra for ICPSRH and ICPSRHSS films with theexcitation wavelength at 480 nm are shown in Fig. 6. The absorp-tion maximum is at 555 nm. The emission maxima of the SRH inmethanol and ICPSRH and ICPSRHSS films are at 568, 598 and625 nm, respectively. The full width at half maximum (FWHM) of

Fig. 1. Schematic representation of urethane bond formation between ICPTES and SRH and covalent bond formation between ICPSRH and silica spheres.

Fig. 2. (a) FTIR spectra of ICPTES, SRH and ICPSRH and (b) pristine silica spheres and ICPSRHSS.

Fig. 3. UVevisible absorption spectra of SRH, mixture of SRH and ICPTES and ICPSRHin methanol.

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the luminescence profile of the ICPSRHSS is much wider than theemission profiles of SRH in methanol and ICPSRH film, which im-plies the large intermolecular interaction between the fluorophoresfor the ICPSRHSS film. The Stokes shifts of the SRH in methanol andthe ICPSRHSS and the ICPSRH films are 13, 43 and 70 nm, respec-tively. The relatively large Stokes shift of the ICPSRHSS and ICPSRHmay be due to the FHRET. The cause of the bigger Stokes shift ofICPSRH film than that of ICPSRHSS film is due to the more inter-molecular interaction. These results indicate that the intermolec-ular charge transfer in the solid state can be achieved by propersynthetic process.

4. Conclusions

The SRH is attached on the surface of silica spheres by two stepsynthetic processes including a urethane bond formation betweenan ICPTES (eN]C]O) and an SRH with elevated temperature inpyridine and hydrolysisecondensation reactions between

Fig. 4. FESEM images of (a) as-synthesized silica spheres and (b) ICPSRHSS.

Fig. 5. Excitation wavelength dependent PL spectra of (a) the SRH in methanol and (b) the ICPSRH film.

Fig. 6. Comparison of the absorption spectra of SRH in methanol, PL spectra of the SRHin methanol, and PL spectra of ICPSRH and ICPSRHSS films.

B.-J. Kim, K.-S. Kang / Materials Chemistry and Physics 148 (2014) 964e967 967

synthesized ICPSRH and silica spheres. The reduction of the ab-sorption peak at 2270 cm�1 and new absorption peak at 1712 cm�1

indicate the formation of urethane bond. The UVevisible absorp-tion peak of SRH in methanol is at 555 nm. The FWHM of theICPSRHSS shows the strong intermolecular interaction of theluminescent chromophores. The Stokes shifts of the SRH in

methanol, ICPSRHSS and ICPSRH films are 13, 43 and 70 nm,respectively.

Acknowledgments

This work has been supported by Kyungil University in Korea.

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