Variations in fluorescence quantum yield of basic fuchsin with silver nanoparticles prepared by femtosecond laser ablation

Variations in fluorescence quantum yield of basic fuchsin with silver nanoparticles prepared by femtosecond laser ablation

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 522–526 Contents lists available at ScienceDirect Spectrochimica Acta...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 522–526

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Variations in fluorescence quantum yield of basic fuchsin with silver nanoparticles prepared by femtosecond laser ablation Bini Pathrose a,⇑, H. Sahira b, V.P.N. Nampoori a, P. Radhakrishnan a, A. Mujeeb a,c a

International School of Photonics, Cochin University of Science and Technology, Cochin, Kerala, India Department of Microbiology, Medical College, Thiruvananthapuram, Kerala, India c LBS Centre for Science and Technology, Thiruvananthapuram, Kerala, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Silver sol is prepared by femtosecond

laser.  Absolute fluorescence quantum yield

is calculated using thermal lens technique.  Quantum yield variations of dye with and without silver sol are plotted.  The presence of silver sol decreases the fluorescence quantum efficiency.  The presence of silver sol enhances the thermal lens signal.

a r t i c l e

i n f o

Article history: Received 6 November 2013 Received in revised form 29 January 2014 Accepted 11 February 2014 Available online 28 February 2014 Keywords: Basic fuchsin Thermal lens spectroscopy Fluorescence quantum yield Silver nanoparticles Femtosecond laser Laser ablation

a b s t r a c t Nano structured noble metals have very important applications in diverse fields such as photovoltaics, catalysis, electronic and magnetic devices, etc. In the present work, the application of dual beam thermal lens technique is employed for the determination of the absolute fluorescence quantum yield of the triaminotriphenylmethane dye, basic fuchsin in the presence of silver sol is studied. Silver sol is prepared by femtosecond laser ablation. It is observed that the presence of silver sol decreases the fluorescence quantum efficiency. The observed results are in line with the conclusion that the reduction in quantum yield in the quenching region is essentially due to the non-radiative relaxation of the absorbed energy. It is also observed that the presence of silver sol enhances the thermal lens signal which makes its detection easier at any concentration. Ó 2014 Elsevier B.V. All rights reserved.

Introduction The fluorescence quantum yield (FQY) is defined as the ratio of the number of photons emitted to the number of photons

⇑ Corresponding author. Tel.: +91 4922273080; mobile: +91 9446061126. E-mail addresses: [email protected] (B. Pathrose), [email protected] (H. Sahira), [email protected] (V.P.N. Nampoori), [email protected] (P. Radhakrishnan), [email protected] (A. Mujeeb). http://dx.doi.org/10.1016/j.saa.2014.02.078 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

absorbed. Various methods are reported for the determination of FQY of samples [1–7]. The most popular one is the comparative method, by using a fluorescence standard [8]. Such methods are based on the fact that if two substances are studied using the same apparatus and the same incident light intensity, the integrated areas under their corrected fluorescence spectra (S1 and S2) are related as

S1 /2 A2 ¼ S2 /1 A1

ð1Þ

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where /1 and /2 are the corresponding Quantum Yields, A1 and A2 are the corresponding absorbance of the two samples, for a particular excitation. The limitation of this method is that it requires a series of suitable standard materials if it is to be used over a wide range of wavelengths. The need of a fluorescence standard can be eliminated if photothermal method like thermal lens technique is adopted [6,7]. Thermal lens technique is a versatile and viable technique for exploring nonlinear processes taking place in organic materials [9], dyes [10], dye mixtures [11] and metallic colloids [12]. It is a highly sensitive method, capable of giving absolute values of FQY with high accuracy and reproducibility. The absolute values of FQY of laser dyes are important for the calculation of thresholds of laser action. The thermal and fluorescence spectroscopy used to measure FQY are complementary to one another: the former measures the photon energy, which is converted into heat, and the latter observes re-emitted photons. The thermal fluctuations produced by the non-radiative relaxation results in density variations which lead to refractive index variations. A lens is created through the temperature dependence of the sample refractive index and the phenomenon is called thermal lensing. For most of the liquids, the temperature coefficient of refractive index is negative and hence the thermal lens signal generated is divergent. Noble metal nanoparticles have unique size-dependent optical, magnetic and catalytic properties [13–15]. Because of these properties silver nanoparticles are used for applications in various areas such as catalytic, optical and antibacterial applications [16,17]. In order to produce pure nanoparticles, and to eliminate surfactant for capping of colloidal nanoparticles laser ablation method is commonly used [18–20]. The FQY and photoluminescence spectra can be altered in the presence of nanoparticles. Association of metal nanoparticles with dye molecules results in enhancement or quenching of fluorescence of dye molecules [21]. The quenching of fluorescence is due to the increase in the nonradiative decay and enhancement of fluorescence is due to the increase in excitation decay rate caused by the plasmon field created around the nanoparticles by the incident radiation [22]. Association of silver nanoparticles with Rhodamine6G increases the photoluminescence quenching and hence FQY is reduced [17,23]. It is reported that quantum yield and photoluminescence spectra of CdSe/ZnS core–shell quantum dots suspended in toluene and tetrahydrofuran solvents are independent of the excitation wavelength [24]. In the present study, a femtosecond laser is utilized to generate silver nanoparticles in ethanol. Basic fuchsin (BF) is a triaminotriphenylmethane dye with molecular formula C20H20ClN3 (Fig. 1). It is a mixture of three dyes Pararosaniline, Rosaniline, and Magenta II and is known as Magenta II. This dye is inflammable in nature and possesses anesthetic, bactericidal (gram positive), and fungicidal properties. It is widely used as coloring agent for textile and leather materials, staining of collagen, muscle, mitochondria, and tubercle bacillus. The present paper describes the effect of silver nanoparticles on the FQY of fuchsin dye.

523

Experimental The experimental set up (Fig. 2) used is similar to the one reported by Fang and Swofford [25]. A diode pumped solid state (DPSS) laser emitting at 532 nm having a maximum power of 100 mW is used as the pumping source and a low power Helium–Neon laser (Spectra Physics-5 mW, 632 nm) is used as the probe beam. The pump beam is intensity modulated using a chopper (frequency 3 Hz) and the probe beam is made collinear and passed through a quartz cuvette containing sample solution through an assembly of dichroic mirror and convex lens (focal length 20 cm). The absorption coefficient of fuchsin basic at 632 nm is very narrow as compared to that of the pump beam and hence the perturbation in refractive index due to probe beam can be neglected. The thermal lens signal generated is filtered to allow the passage of signal at 632 nm only. After filtering, the thermal lens signal is then collected with the help of an optical fiber which in turn is connected to a monochromator–PMT–Digital Storage Oscilloscope assembly. Optical density filters were used in between the pumping source and sample to vary the laser intensity. In the present study a power output of 80 mW from a DPSS laser is used for heating the sample. Silver nanoparticles were prepared by pulsed laser ablation of a silver plate of thickness 1 mm having purity of 99.99%. The plate was polished, and then washed with deionized water several times to remove impurity from the surface. The silver plate was placed at the bottom of a culturing dish filled with 15 ml of ethanol. The laser system was a femtosecond pulsed laser which consists of a femtosecond laser seed (Tsunami (Mode-locked Ti: sapphire Laser), 700 mW, 800 nm, about 100 fs, 80 MHz) and a regeneration amplifier (Spitfire Pro). The output laser had a repetition at 1 kHz with pulse width of 120 fs and wavelength of 800 nm. During the procedure of laser ablation, the target was rotated manually to ensure uniform ablation and to avoid texturing effect. The Gaussian laser beam was focused by a biconvex lens with a focal length of 5 cm to the target, which was immersed in ethanol. The amplitude and frequency of the surface Plasmon resonance (SPR) peak depends on the nature of metal, shape and size of the particles, nature of the solvent and particle density [26]. The size and shape of nanoparticles can be qualitatively described by the peak position and shape of the absorption spectrum [27]. It is observed that the absorption peak of silver nanoparticles is having a single peak is and is located around 404 nm (Fig. 3). The sample solution is prepared by an accurately weighed amount of dye and dissolving it in ethanol to obtain samples of various concentrations from 102 mol/L to 105 mol/L. To these samples an appropriate amount of 104 mol/L silver sol is added. Theory The fluorescence quantum yield Qf is the ratio of the number of photons emitted as fluorescence to the number of photons absorbed. The method is based on the principle of energy conservation. Let P0 be the power of the incident beam, Pt be the power of the transmitted beam, Pf be the emission power and Pth be the power dissipated as heat. In the absence of any photochemical reaction the absorbed power can be written as a sum of transmitted power, thermal power degraded to heat and the emission power as given by Eq. (2).

P0 ¼ P th þ Pf þ Pt

ð2Þ

Assuming reflection and scattering losses to be negligibly small, the transmittance can be written as

T¼ Fig. 1. Molecular structure of basic fuchsin.

Pt P0

ð3Þ

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Fig. 2. Schematic representation of the experimental set-up. PL1: Pump Laser (DPSS, 532 nm); C: Chopper; L: Lens; DM: Dichroic Mirror; S: Sample cell; OF: Optical Fiber; PL2: Probe Laser (He–Ne, 632 nm); MPA: Monochromator and PMT assembly; DSO: Digital Storage Oscilloscope.

having absorbance, A as

A¼1T

ð4Þ

Hence the absorbed power, AP0 is

AP 0 ¼ P th þ Pf

ð5Þ

Re-arranging Eq. (5)

Pf ¼ AP 0  Pth

ð6Þ

In the case of completely fluorescence quenched sample the quantum efficiency can be calculated by [28]

Qf ¼

      kf Pf kf g ¼ 1 ga k AP 0 k

ð7Þ

where Pa = AP0 The ratio of the fluorescence wavelength kf to the excitation wavelength k corresponds to the Stokes shift. The thermal power degraded to heat, Pth is directly proportional to g, the thermal lens signal measured for each sample and Pa is proportional to thermal lens signal ga corresponding to the concentration at which the fluorescence intensity is quenched completely [3]. The thermal lens signal g has been measured as the variation of intensity at the center of the probe beam at a far field as a result of the thermal lens effect in the medium [28].

Fig. 3. Absorption spectrum of silver sol produced by femtosecond laser ablation at a concentration of 104 M showing SPR peak around 404 nm.

Results and discussions The BF aggregates in ethanol, resulting in reduction of fluorescence with increase in concentration. The fluorescence intensity in a wide range of concentrations is measured and the specific concentration at which fluorescence is quenched completely is determined. Fluorescence and thermal lensing studies were extended for the dye dissolved in ethanol with and without the presence of silver sol, for a wide range of concentrations. The absorption peak of BF locates around 550 nm and hence perturbations due to probe beam can be neglected. The absorption spectrum of dye with and without silver sol (Fig. 4) was taken using a Jasco U-570 UV/VIS/NIR spectrophotometer. The suppression of the absorption peak by the addition of silver sol is due to the aggregation of dye molecules around the metal nanoparticles which prevents plasmonic oscillations. Absorption spectroscopy provides information on the average ground state of the molecules that absorb light whereas fluorescence spectroscopy provides the dynamic properties of solutions. It is also observed that the peak fluorescence wavelength of the dye is blue shifted by the addition of silver nanoparticles. The observed blue shift can be because of the local enhancement of the optical fields near the dye molecules by interactions with silver nanoparticles. The fluorescence spectrum (excitation wavelength of 532 nm) of BF with and without silver sol (Fig. 5) for a

Fig. 4. Absorption spectrum at a concentration of 105 M showing peak around 550 nm.

concentration of 105 mol/L was taken using a Varian Cary Eclipse fluorescence spectrophotometer. The fluorescence is significantly affected by the nature of the solvent or, by the type of the environment of the fluorophore. In the present study, the observations showed the usual concentration dependant red shift of the fluorescence emission both in the case of dye with and without the presence of silver sol (Fig. 6) for an excitation wavelength of 532 nm. The red shift in the peak fluorescence wavelength implies

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An increase in n will decrease the energy loss, whereas an increase  F . The refracin e increases the energy difference between v A and v tive index is a high frequency response which depends on the movement of electrons within the solvent molecules and can occur during light absorption. The dielectric constant (e) is a static property, which depends on both electronic and molecular motions. The ground and excited states gets easily stabilized by the movement of electrons within the solvent molecules by an increase in the refractive index resulting in a decrease in the energy difference between the two states. An increase in e also results in stabilization of the ground and excited states. However, the reduction in

energy of the excited state due to e occurs only after reorientation of the solvent dipoles. This process requires movement of the entire solvent molecule, not just its electrons. The stabilization of the ground and excited states of the fluorophore depends on the dielectric constant (e) and is time dependent. The rate of solvent relaxation depends on the temperature and viscosity of the solvent [29]. The excited state shifts to lower energy on a timescale comparable to the solvent reorientation time. The terms h, c and a in Eq. (8) corresponds to Plank’s constant (=6.6256  1027 ergs), speed of light (=2.9979  1010 cm/s) and  A and v  F are radius of the cavity where the fluorophore resides. v the wavenumbers (cm1) of the absorption and emission respectively. The term inside the large parentheses in Eq. (8) is called the orientation polarizability (Df). The first term (e  1)/(2e + 1) accounts for the spectral shifts due to both the reorientation of the solvent dipoles and to the redistribution of the electrons in the solvent molecules. The second term (n2  1)/(2n2 + 1) accounts for only the redistribution of electrons. The difference of these two terms accounts for the spectral shifts due to reorientation of the solvent molecules. According to this simple model, only solvent reorientation is expected to result in substantial Stokes shifts. The redistribution of electrons occurs instantaneously, and both the ground and excited states are approximately equally stabilized by this process. As a result, the refractive index and electronic redistribution has a comparatively minor effect on the Stokes shift. lE and lG represent the dipole moments in the excited state and ground state respectively. Typically, the fluorophore has a larger dipole moment in the excited state (lE) than in the ground state (lG). Upon excitation the solvent dipoles reorient or relax around lE, which lowers the energy of the excited state. As the solvent polarity is increased, this effect becomes larger, resulting in emission at lower energies or longer wavelengths. Only fluorophore that are polar display a large sensitivity to solvent polarity [29]. The experiment was done under identical conditions for a wide range of dye concentrations with and without the presence of silver sol and the FQY values are calculated. The addition of the sol enhances the thermal lens signal which makes the detection easier at any concentration of the sample under study. The FQY variations with dye concentrations are as shown in Fig. 7. The additional decay mechanism because of the silver nanoparticles arises from the very rapid transfer of the excitation from the dye molecule to the silver particles via the excitation of the plasma resonance and can be radiative or non-radiative. Since the excitation wavelength used in the present experiment (532 nm) is far from the plasma resonance wavelength (404 nm) it can be assumed that

Fig. 6. Variation of peak fluorescence wavelength with concentration for an excitation wavelength of 532 nm.

Fig. 7. Variation of FQY with concentration for an excitation source having wavelength of 532 nm.

Fig. 5. Fluorescence spectrum at a concentration of 105 M for an excitation wavelength of 532 nm.

a loss of energy of the emitted photons or an increase in Stokes shift with concentration. This is because of the energy losses due to dissipation of vibrational energy, redistribution of electrons in the surrounding solvent molecules, reorientation of the solvent molecules, and interactions between the fluorophore and the solvent or the solute. According to Lippert equation (Eq. (8)), the energy difference between the ground and excited states is dependent on the refractive index (n) and the dielectric constant (e) of the solvent.

v A  v F ¼

  2 e1 n2  1 ðlE  lG Þ2  2 þ constant hc 2e þ 1 2n  1 a3

ð8Þ

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fluorescence is not much affected by this additional decay mechanism induced by resonance. The greater optical field for the excited fluorophore which is produced because of the silver nanoparticles induces the formation of charge transfer complexes resulting in a non-radiative decay channel for the excited fluorophore and its fluorescence quantum yield is reduced. Reduction in reduced fluorescence can be because of the scattering by the silver nanoparticles. Reduction in absorbed light quanta also results in reduced fluorescence. The presence of nano particles like silver and gold can enhance the Raman scattering and resonance Raman scattering [30]. There is no reduction in the FQY at low dye concentration when the intensity of the pump laser is increased. This means that the additional de-excitation process in the presence of silver sol is intensity dependant. The basic fuchsin is having same FQY characteristics as compared to Rhodamine6G. But in the case of Rhodamine6G the FQY is reduced to about 50% by the association of silver sol prepared by chemical method [23]. In the present case the silver nanoparticles is prepared by ultrafast laser ablation of silver target and the reduction in FQY is relatively small. Detailed studies are in progress. It is expected that the FQY can be enhanced by varying the concentration, shape and size of silver sol. Conclusion In the present work, it is proved that the presence of silver sol reduces the quantum yield of basic fuchsin dye. Its possible favorable outcome is the enhancement of Raman scattering signal. A discussion is presented on the possible reasons for this reduction in FQY. It is also observed that the presence of silver sol enhances the thermal lens signal which makes the detection of signals easier at any concentration. Acknowledgments The author is grateful to the University Grant Commission, New Delhi for the research fellowships. DST is also acknowledged for the financial assistance.

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