Effect of nano-clay platelet on fluorescence resonance energy transfer

Invertis Journal of Renewable Effect of Energy, nano-clay Vol. platelet 6, No. on 3, 2016 fluorescence ; pp. 1-5 resonance energy DOI transfer No. :... Effect of nano-clay platelet on fluorescence resonance energy transfer SYED ARSHAD HUSSAIN*, ARPAN D. ROY, J. SAHA, DIBYENDU DEY AND D. BHATTACHARJEE Department of Physics, Tripura University, Suryamaninagar - 799130, Tripura India *E-mail: sa_h153@hotmail.com Abstract This communication reports the phenomenon of fluorescence resonance energy transfer observed in between the binary solution of two laser dyes acriflavine (donor) and rhodamine B (acceptor), with and without nano-clay (laponite). It was observed that presence of clay particles increases the energy transfer efficiency. The effect of acceptor concentration on the total energy transfer efficiency has been studied to quantify the energy transfer, which may be used for photon emission and enhancement in lasing efficiency. Key words : Rhodamine, laponite, acriflavine, FRET 1. Introduction Fluorescence resonance energy transfer (FRET) is an electrodynamic phenomenon that occurs through the transfer of exited state energy from donor (D) to acceptor (A) [1-3]. This nonradiative energy transfer occurs as a result of dipole-dipole coupling between the donor and the acceptor, and does not involve the emission and reabsorption of photon. The process can be expressed as : D + hν D* D* + A D + A* A* A + hν where h is the Planck's constant and ν is the frequency of the radiation. The rate of energy transfer depends upon the extent of spectral overlapping area of the emission spectrum of donor with the absorption spectrum of the acceptor, the relative orientation of the donor and acceptor transition dipoles and the distance between these molecules [1-3]. Nonradiative energy transfer is primarily dependent on the D-A distances. Due to its sensitivity to distance, FRET has been used to investigate molecular level interaction [2-4]. FRET mechanisms are also important in other phenomenon, such as photosynthesis kinetics, chemical reactions and Brownian dynamics [1]. FRET has wide applications in biomedical, protein folding, RNA/DNA identifications and their energy transfer process [5-6]. Another important application of FRET phenomenon is in dye lasers. If a dye laser has to be used as an ideal source, its spectral region needs to be extended. The use of such energy transfer in dye lasers is also helpful in minimizing the photo quenching effects and thereby increasing the laser efficiency. In case of dye laser energy transfer process may also be used to extent the spectral range of operation. Therefore, study of energy transfer process using different dyes are very important from the application point of view. On the other hand clay particles are natural nano materials with interesting properties-cation exchange capacity (CEC), layered structure, intercalation property etc [7]. Clay particle adsorb dye molecules onto their surfaces and play an important role in concentrating the dye molecules onto their surfaces [7-8]. Accordingly, clay particle provide a favourable environment to the dye molecules to come close to each other and transfer energy from one molecule to other. There are very few report of energy transfer between dyes adsorbed onto clay surfaces [9-13]. 1

Syed Arshad Hussain et al. The size of the clay platelet is less than 0.05 µm and CEC is 0.739 meq/g determined with CsCl [15.]. The dyes were dissolved in ultrapure Milli-Q water for solution preparation. The clay dispersion of laponite was prepared in Milli-Q ultrapure water (electrical resistivity18.2 MΩ-cm) and stirred for 24 h with a magnetic stirrer. UV-vis absorption and fluorescence spectra of the solutions were recoded by a UV-Vis Spectrophotometer (Lambda-25, Perkin Elmer) and Fluorescence Spectrophotometer (LS-55, Perkin Elmer) respectively. The excitation wavelength was 420 nm. 3. Results and discussions Fig. 1. Normalized fluorescence spectrum of Acf (energy donor) and absorption spectrum of Rhb (energy acceptor). The spectral overlap is shown. In order to investigate the FRET between Acf and Rhb, the fluorescence spectra of the pure and mixed In the present communication, the FRET phenomenon between two dyes has been reported. Here we used two cationic dyes namely, Acriflavine (Acf) and Rhodamine B (Rhb) as donor and acceptor respectively. We have investigated this phenomenon in water and clay dispersion solutions. These two dyes Acf and Rhb are in principle suitable for FRET [1-3]. Both the dyes are highly fluorescent. The fluorescence spectrum of Acf sufficiently overlaps with the absorption spectrum of Rhb [see figure 1]. P. D. Sahare et al [14] observed energy transfer in binary solution mixture of acriflavine and rhodamine 6G and acriflavine and rhodamine B by life time measurement. However, the effect of nano-clay laponite on energy transfer between Acf and Rhb has never been reported. The aim of this study was to investigate the influence of nano-clay platelet laponite on the energy transfer between these two dyes. 2. Experimental Acf and Rhb were purchased from Sigma Chemicals Co., USA and used as received. The dyes used in our studies are positively charged. The clay mineral used in the present work was laponite, obtained from Laponite Inorganic, UK and used as received. Laponite particles are negatively charged. Fig. 2. Fluorescence spectra of aqueous solution of pure Acf (1), Rhb (2) and their mixture (50:50 volume ratio) in absence (3) and presence (4) of clay. Inset shows the excitation spectra of Acf-Rhb mixture. The excitation wavelength were (I) 500 nm (Acf fluorescence maximum) and (I) 575 nm (Rhb fluorescence maximum). 2

Effect of nano-clay platelet on fluorescence resonance energy transfer dyes in solution were measured. Figure 2 shows the fluorescence spectra of pure Acf, pure Rhb, Acf & Rhb mixture. The concentration of dyes in aqueous solution was 10-6 M. The excitation wavelength was 420 nm (Acf absorption maximum) and was selected approximately to excite the Acf molecules directly and to avoid or minimize the direct excitation of the Rhb molecules. The Acf fluorescence spectrum (curve 1) possesses a prominent band at about 500 nm. A less intense weak emission band at about 575 nm is obtained for pure Rhb (curve 2). Both the spectra show the characteristics of monomer fluorescence. Under these circumstances we can say that prominent emission from Rhb may be possible only after excitation via energy transfer from the Acf molecule. The Acf & Rhb mixture (50:50 volume ratio) fluorescence spectrum (curve 3) possesses bands at 500 nm and 575 nm which are due to the Acf & Rhb monomer respectively. Here the intensity of Acf fluorescence band is lower and Rhb fluorescence band is higher compared to their pure counterparts. The interesting thing is that here Acf fluorescence decreases in favour of Rhb fluorescence. In this case the energy is transferred from Acf (donor) to Rhb (acceptor). This transferred energy excites more Rhb molecules to its excited state followed by light emission from the Rhb, which is added to the original Rhb fluorescence. As a result the Rhb fluorescence intensity gets sensitized. In order to confirm this we have measured the excitation spectra of Acf - Rhb mixture with excitation wavelength 500 nm (Acf fluorescence maximum) and 575 nm (Rhb fluorescence maximum). The corresponding spectra are shown in inset of figure 2. Both the spectra possess the characteristic band of Acf absorption spectrum. This concludes that the observed Rhb fluorescence in Acf - Rhb mixture is mainly due to the light absorption by Acf monomer and corresponding transfer of the energy to Rhb. In presence of clay (curve 4, fig. 2) the donor fluorescence intensity decreases further in favour of acceptor fluorescence intensity resulting an increase in energy transfer efficiency. It is interesting to mention in this context that Fluorescence resonance energy transfer (FRET) process is very sensitive to distances between the Fig. 3. Schematic diagram showing the dyes in aqueous solution and in clay dispersion. 3

Syed Arshad Hussain et al. fluorophore (donor-acceptor) and occurs only when the distance is of the order of 1-10 nm [1-4]. In the present case clay particles play an important role in determining the concentration of the dyes on their surfaces or to make possible close interaction between energy donor and acceptor components in contrast to inactive system based on homogeneous solution. A red shift of Rhb fluorescence band of the order of 8 nm occurred in clay dispersion. Such smaller shift in Rhb fluorescence in montmorillonite and hectorite were reported and attributed to monomer fluorescence of Rhb adsorbed on the external clay surface or in the interlamellar regions of clay sheets [16]. A schematic diagram showing the dyes in aqueous solution and clay dispersion are presented in figure 3. In aqueous solution both the dyes are randomly oriented and the distance between the dyes are large resulting a decrease in energy transfer efficiency (figure 3a). Both the dyes used in the present study are cationic. Clay particles are negatively charged having cation exchange capacity and layered structure. In the clay dispersion strong electrostatic interaction occurred between the cationic dyes and laponite. As a result the dyes are adsorbed onto the laponite surface through cation exchange reaction. This enhances the closer association of Acf and Rhb in clay dispersion (figure 3b) resulting an increase in energy transfer efficiency. The fluorescence spectra of Acf & Rhb mixture in presence of clay for different mixing ratio are shown in figure 4. From the figure it is observed that the Rhb fluorescence increases with the increasing amount of Acf (donor). The energy transfer efficiencies (E) have been calculated for various dye concentrations using the equation [17] F E = DA 1 FD where F DA is the fluorescence intensity of donor in presence of acceptor and F D is the fluorescence intensity of the donor in absence of acceptor. The plot of FRET efficiency as a function of acceptor concentration in presence of clay has been shown in the inset of figure 4. The FRET efficiency increases with increasing acceptor concentration and Fig. 4. Fluorescence spectra of ACF and RhB mixture for different mixing ratio in presence of clay. Inset shows the plot of FRET efficiency as a function of acceptor (Rhb) concentration. was maximum at 99% of RhB concentration. When the RhB concentration was less than 50%, the FRET was inconsistent. 3. Conclusion FRET between two laser dyes Acf and Rhb has been demonstrated successfully in presence and absence of nano clay platelets laponite. Efficient Energy transfer does not normally takes place in dilute dye solution due to their large distance between molecules. However, energy transfer efficiency increases in presence of clay. The introduction of clay actually reduces the intermolecular separation between the donoracceptor pair, which helps in maximum energy transfer. It has been observed that the FRET efficiency increases with increasing acceptor concentration in the mixed systems. The FRET efficiency was maximum for Rhb concentration of the order of 99%. This findings may be useful in order to enhance the lasing efficiency using the dyes. 4

Effect of nano-clay platelet on fluorescence resonance energy transfer Acknowledgements The authors are grateful to DST, CSIR and DAE, Government of India for providing financial assistance through DST Project No: SR/S2/LOP-19/ 07, Ref. No. SE/FTP/PS-54/2007 and CSIR Project Ref. No. 03 (1146)/09/EMR-II and DAE Young Scientist Research Award No. 2009/20/37/8/BRNS/ 3328. References [1] Th. Förster and Z. Naturforsch., 4A (1949) 321. [2] R. Selvin Paul, Nature Structural Biology., 7(9) (2000) 730. [3] S. A. Hussain, S. Chakraborty, D. Bhattacharjee and R. A. Schoonheydt, Spectrochemica Acta Part A, 75 (2010) 664. [4] Th. Förster, Discuss. Faraday Soc., 27 (1959) 7. [5] G. Haran, J. Phys.: Condens. Matter., 15 (2003) 1291. [6] M. S. Csele and P. Engs, Fundamentals of Light and Lasers, Wiley, New York, USA (2004). [7] F. Bergaya, B.K.G. Theng and G. Lagaly, in: Handbook of Clay Science, Elsevier, Amsterdam, (2006). [8] S. A. Hussain and R. A. Schoonheydt, Langmuir, 26(14) (2010) 11870. [9] R. A. Schoonheydt, Clay Clay Miner., 50 (2002) 411. [10] D. Mahadhavan and K. Pitchumane, Tetrahedran, 58 (2002) 9041. [11] M.G. Neumann, H.P.M. Oliveira and A.P.P. Cione, Adsorption, 8 (2002) 141. [12] Y. Ishida, T. Shimada, D. Masui, H. Tachibana, H. Inoue and S. Takagi, J. Am. Chem. Soc., 133 (2011) 14280. [13] A. Czímerová, J. Bujdak and N. Iyi, J. Photochem. Photobiol. A, 187 (2007) 160. [14] P.D. Sahare, V.K. Sharma, D. Mohan and A.A. Rupasov, Spectrochim. Acta Part A, 69 (2008) 1257. [15] T. Szabo, R. Mitea, H. Leeman, G.S. Premachandra, C.T. Johnston, I.M. Szekeres and R.A. Schoonheydt, Clays Clay Miner., 56 (2008) 494. [16] Z. Grauer, A.B. Malter, S. Yariv and D. Avnir, Colloid Surf. 25 (1987) 41-65. [17] D. Seth, D. Chakrabarty, A. Chakraborty and N.S. Sarkar, Chem. Phys. Lett., 401 (2005) 546. 5