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Enhancement of the fluorescence spectra of Rhodamine 101 dye using silver aggregate nanoparticles

I.

I.  Introduction

The most powerful propriety of laser dyes is their tunability band laser emission which gives a varity of applications in many fields. Laser dyes are used efficiently in the isotope separation field. They also have applications in biology such that they modify the DNA by irradiation with ultraviolet light to prevent genetic mutations. The most successful, advanced and widely used application of laser in medicine is certainly eye surgery by photocoagulation, the laser dyes can be also used to coagulate blood vessels in order to obstruct the flow of blood (1). They also can be used in monitoring industrial pollution by means of differential absorption lidar (DIAL) technique (2).

According to these importances there are different methods used for increasing the laser fluorescence intensity with specific wavelength. For example by using the laser induced fluorescence (LIF) technique, the effect of concentration on the laser dye Rhodamine B dissolved in ethanol can be studied such that the fluorescence emission intensity of Rhodamine B gets broad and shifts to higher wavelength with concentration (3). Also a dye fluorescence spectrum shift can be obtained without fluorescence intensity enhancement under the influence of the solvent used (4).

Corresponding authors:

+ Lotfi Z. Ismail: Department of Physics, Faculty of Science, Cairo University.

e-mail: Lotfizaki@hotmail.com

* Mohamed A. El Shaer: Department of Physics and Mathematics, Faculty of Engineering, Zagazig University.

e-mail: Melshaer@link.net

Recently nanoparticles (NPs) have a very interesting role on the laser dyes emission fluorescence such that adding (NPs) to a dye solution enhances the fluorescence emission of the dye. That enhancement of the fluorescence emission of molecules near a metal surface arises from interactions with surface plasmons (sp) resonance in the metal particles; these interactions may also result in shortening of the excited-state lifetime thus improving the photostability of the dye (5).

In the spots, where local fields are concentrated, both linear and nonlinear optical responses of molecules and atoms are gigantically enhanced which will lead to a number of important applications, the most important one is the surface enhanced Raman scattering (SERS) (6) . The presence of the nanoparticles (NPs) will increase the fluorescence quantum efficiency which is expected to have a very important consequence in (SERS) (7).

For example when silver (NPs) are added to the dye solution, dye molecules will be adsorbed on islands and films of the metallic (NPs) and when the surface plasmon resonance (SPR) of the metallic (NPs) coincides with the dye absorption band that will modify the intensity of the electromagnetic field (EMF) around the molecules which will increase the emitted fluorescence intensity (8). That modification of the (EMF) is due to the very high field gradient near the metallic surfaces (9).

Adding Ag (NPs) to the dye solution can cause either an enhancement or a quenching of the fluorescence intensity of the dye depending on the distance between the dye molecules and the metal surfaces. When the metallic (NPs) are in close proximity to the fluorophores, quenching of the luminescence occurs because the non-radiative decay of the excited molecules will increase due to the energy transfer from the dye molecules to silver (NPs), whereas when the metallic (NPs) are located at certain distance, enhancement in the luminescence is observed due to the decrease of the non-radiative decay (10). Silver and gold (NPs) are used in most popular dyes because their plasmon resonance frequency is located in the visible spectrum which matches with the absorption and the emission bands of these dyes. There are important factors affecting the strength of the fluorescence intensity which are the size and shape of the (NPs), the orientation of the dye dipole moments relative to the (NPs) surface normal, the overlap of the absorption and emission bands of the dye with the plasmon band of the metal and the radiative decay rate and the quantum yield (Q) of the fluorescent molecule (5, 11).

 Since Rhodamine dyes are the most and wieldy used dyes in many fields such that they can be used in nonphotochemical hole burning experiments on the mitochondrial dye Rhodamine 800 incubated with two human ovarian surface epithetical cell lines. This dye is believed to be selective for the plasma and inner membranes of the mitochondria (12). Gold nanoparticles can be used as a colorimetric sensor for protein conformational change (13).

So in this work we study the effect of adding silver (NPs) to Rhodamine 101 dye of concentration 10-4 M/L with different weighting factors between the Ag (NPs) and the dye solution.

II. Experimental samples and setups

Experimentally, we use Rh 101 dye of molecular weight of 591.06 gm, which appears as a green solid with maximum absorption wavelength of 568 nm when it is dissolved in absolute ethanol (99.9 %). The selected concentration of the dye solution is 10-4 M/L.

The aggregated nanoparticles are prepared by vinylpyrrolidone reduction of AgNO3 in ethanol solution. AgNO3 (5 mg) is dissolved in EtOH (100 ml), put in a reflux condenser and heated while stirring for 15 min. Then poly (vinylpyrrolidone) (molecular weight, 40 000; 1.2 g) was added, with stirring, and kept in a reflux condenser for about 10 min. After that NaOH (1 wt%; 0.5 ml) is added and kept at boiling while stirring for 30 min. Then the mixture was kept stirred until it cooled to room temperature. That colloid prepared this way contains mostly isolated Ag particles.

In the preparation of Ag aggregate, ethanol solution of AgNO3 (25 mg in 25 ml of EtOH) is mixed with 25 ml of initial colloid at boiling temperature while stirring. Then, 4 ml of NaOH (1 wt %) is added to the mixture, which was kept stirred at boiling temperature for 30 min and then until it cooled to room temperature. The estimated concentration of Ag particles in the mixture was 8.8*1013  cm-3 (9). Interaction of Rhodamine 101 dye molecule with silver nanoparticles is schematically shown in figure 1.

Fig.1 Schematic representation of the formation of dye-Ag nanoparticles complex

This molecule has two nitrogen atoms with which it can bind to silver nanoparticles. Among these two nitrogen atoms one of them is more electropositive and can bind to silver nanoparticles preferentially to form dye silver nanoparticles complex. Due to the affinity of dye molecule with silver nanoparticles, electron transfer mechanism becomes easier and enhancement is obtained.

The pure dye solution and the dye aggregate mixtures are pumped by argon (Ar) ion laser of output wavelength 488 nm generated from Lexel 95 laser generator and the fluorescence spectrum is detected by SPEX 750M monochromator. An Acquisition card is used to analyze the fluorescence spectra with pc-computer software specially written to scan the wavelength as shown in figure 2.  

Fig.2 Schematic diagram of the spectroscopic system and data acquisition part of the experiment.

III. Results

First, a pure Rh 101 dye solution of concentration 10-4 M/L is pumped with Ar ion laser, and then Ag (NPs) is added to the dye solution with ratios from 1:7 to 1:3 of the Ag (NPs) to the dye in the solution, the fluorescence intensities are recorded as shown in figure 3 which presents a comparison between the fluorescence emission curves of the four cases such that the lowest fluorescence intensity belongs to the pure dye solution (black curve) of concentration 10-4 M/L and equals to 1.32 a.u. and its peak at ? = 627.5 nm, (green curve) the fluorescence intensity of the Ag aggregate to dye solution ratio is 1:7 increases to be 1.55 a.u. and its peak is shifted to be at ? = 617.5 nm, (red curve) the fluorescence intensity of the Ag aggregate to dye solution ratio is 1:3.5  increases to be 1.6 a.u. and its peak is shifted to be at ? = 615 nm, (blue curve) the  fluorescence intensity of the Ag aggregate to dye solution ratio is 1:3 increases to be 2.05 a.u.- the highest value -  and its peak is shifted to be at ? = 614.5 nm.

Fig. 3 A comparison between the fluorescence intensity versus wavelength of different cases (Black) pure dye of concentration 10-4 M/L, (Green) Ag aggregate to dye ratio 1:7, (Red) Ag aggregate to dye ratio 1:3.5, and (Blue) Ag aggregate to dye ratio 1:3.

Figures 4, 5 represent the relation between the dye fluorescence intensity and its peak spectral wavelength shift versus the Ag aggregate percentage in the aggregate-dye solution respectively. They showed that there is a linear relationship between the Ag aggregate percentage in the mixture and the fluorescence intensity and its peak spectral wavelength shift. As the Ag aggregate nanoparticles percentage increased in the mixture, the fluorescence intensity increased and the peak spectral shift also increased.

Fig. 4 The relation between Ag aggregate percentages and the fluorescence intensity.

Fig. 5 The relation between Ag aggregate percentages and the shift of the maximum peak wavelength.

As we dilute the dye solution to be of a concentration of 5×10-5 M/L and pump it with Ar ion laser, and then adding Ag (NPs) to dye solution such that the Ag aggregate to dye ratio in the mixture is 1:7, their fluorescence emission intensities are recorded as shown in figure 6 which represents a comparison between the two cases such that  the pure dye solution (red curve) has the minimum fluorescence intensity with its peak  at ?= 612.5 nm, and the fluorescence intensity of the aggregate dye mixture of ratio 1:7 of the Ag (NPs) to dye (black curve) increases with percentage increase in the fluorescence intensity by only 5% with its maximum peak at ?=610 nm, so we can say that the intensity increased and the shift is obtained but with weak dependence on the Ag aggregate percentage in the solution.

Fig.6 A comparison between the fluorescence intensity of two cases, (red curve) the fluorescence intensity of a pure dye of concentration of 5*10-5 M/L, (black curve) the fluorescence intensity of an Ag aggregate to dye (5*10-5 M/L) mixture of ratio 1:7.

IV. Discussion and Conclusion

In this work, adding silver (NPs) to Rhodamine 101 dye solution is investigated. The measured values reveal that adding Ag (NPs) to the dye solution will enhance the dye fluorescence to significant values accompanied by wavelength shift to higher values as the Ag (NPs) percentage increases in the dye aggregate mixture. The fluorescence enhancement could be due to the field enhancement in metallic nanostructures associated with the surface plasmons, whereas the fluorescence spectrum shift may be obtained due to the overlapping between the electronic transition of the metal nanoparticles and the molecular transition of the dye molecules.

 At the same time adding Ag (NPs) to diluted Rh 101 dye solution of concentration 5*10-5 M/L with ratio 1:7 of the aggregated silver to the dye in the mixture will give an intensity increase in the fluorescence of the dye by only 5%, and a spectrum wavelength shift is obtained by only 2.5 nm which gives a weak dependence on the Ag percentage in the solution because dye dilution decreases fluorophores density  required to make bonds to the Ag nanoparticles to enhance the dye fluorescence intensity as seen in figure 1,  accordingly the intensity enhancement is limited.

V. References

1. F. J. Duaarte, L. W. Hillman, (1990), Academic press.

2. M. L. Paascu, N. Moise, A. Staicu, (2001),  Journal of molecular structure, Vol. 598, pp. 57-64.

3. M. Fikry, M. M. Omar, Lotfi Z. Ismail, (2009) , Journal of Fluorescence, Vol. 19, No. 4, pp. 741-746.

4. B. R. Gayathri, J. R. Mannekutla, S.R. Inamdar, (2008), Journal of molecular structure, Vol. 889, pp. 383-393.

5. O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, (2006) , Advanced materials, Vol. 18, pp. 91-95.

6. S. R. Emory, (2000), the international society of optical engineering, pp. 1-2.

7. A. Santhi, M. Umadevi, V. Ramakrishnan, (2004), Spectrochimica Acta part A, Vol. 60, pp. 1077-1083.

8. M. A. Noginov, G. Zhu, V. P. Drachev, (2006), Physical review B, Vol. 74, No. 18, pp. 184203(1-8).

9. M. A. Noginov, G. Zhu, C. Davison, A. K. Pradhan, (2005), Journal of modern optics, Vol. 52, Issue 16, pp. 2331-2341.

10. S. Kalele, A. C. Deshpande, S. Bhushan Singh, S. K. Kulkarni, (2008), Bull. Mater. Sci., Vol. 31, No. 3, pp. 541-544.

11. R. J. Walsh, T. Reinot, J. M. Hayes, K.  R. Kalli, L. C. Hartmann, G. J. Small, (2002), Journal of Luminescence, Vol. 98, pp. 115-121.

12. S. Chah, M. R. Hammond, R. N. Zare, (2005), Chemistry & Biology, Vol. 12, pp. 323-328.

13.  O. Stranik, R. Nooney, C. McDonagh, B. D. MacCaraith, (2007), Plasmonics, Vol. 2, pp. 15-22.

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