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Chapter 4 A study on the optical property-Determination of fluorescence quantum yield The theory of quantum yield measurement using thermal lens technique is briefly described. Transmission electron microscopy, Absorption and Photoluminescence studies of silver nanofluids having different particle sizes have been carried out. Fluorescence quantum yield (FQY) of dye-doped nanofluids were determined by tailoring the parameters such as size and concentration of nanoparticles. The present investigations shown that FQY of the emitter (Rhodamine 6G) decreases with increase in concentration and size of the nanoparticles. 130 Chapter 4 4.1. Introduction Fluorescence quantum yield (FQY) is an important photophysical parameter and is a measure of the rate of nonradiative transitions that compete with the emission of light. It is defined as the fraction of the molecules that emit a photon after direct excitation by a light source [1]. FQY is one of the most important properties of fluorescent materials. FQY of a molecule helps in providing the intrinsic luminescence efficiency of the material. Both from theoretical and practical point of view, fluorescence quantum yield values are important, since they provide information on radiationless processes in molecules and in the assignment of electronic transitions. It is also of use in assessing the potential of the fluorimetric determination of materials. They are necessary for calculating thresholds for laser action and for judging the suitability of materials as wavelength shifters in optical pumping experiments or for use as energy donors [2]. Yields, coupled with luminescence data, also allow evaluation of the purity of materials [2]. Noble metal nanoparticles exhibit unique optical properties due to surface plamon absorption band and can influence the emission behavior of fluorescent dyes. In this context, the absolute values of FQY of dye-nano mixtures are important. There exist a number of methods for the determination of FQY of a substance. Of these, the most popular one is the comparative method, by using a fluorescence standard for which the fluorescence quantum yield is known [2]. Such methods are based on the fact that if two substances 1 and 2 are studied using the same apparatus, and using the same incident light intensity, the integrated areas under their corrected fluorescence spectra (S1 Optical property of pure and dye-doped nanofluids and S2) are simply related as 131 S1 φ 2 A2 = where φ values are quantum S 2 φ1 A1 yields and A values are absorbances for a specific excitation wavelength. Though this is the most popular method for the determination of fluorescence quantum yield, the reliability of such relative determinations is limited both by the accuracy of the standard yield value and by the confidence that can be placed on the comparison technique. Even after making various corrections for system geometry, re-absorption, polarization, etc, the accuracy of the quantum- yield values obtained from photometric measurements is rather poor. More-over, this comparative method requires a series of suitable standard materials if it is to be used over a wide range of wavelengths. There are a number of requirements for a particular standard to be considered ideal. Though most of the criteria can be met by dilute solutions of appropriate compounds, some of the requirements are mutually exclusive. The need of a fluorescence standard can be eliminated if we go for photothermal methods like thermal lens technique. It is a highly sensitive method, which can be used to measure the optical absorption and thermal characteristics of a sample. The advantage of photothermal method is that it can be used to investigate the optical properties of materials that are not possible with traditional spectrophotometry. The thermal and fluorescence spectroscopy which are used here to measure the absolute fluorescence quantum yield are in fact complementary: the former measures the photon energy, which is converted into heat, while fluorescence spectroscopy observes re-emitted photons. The thermal fluctuations produced by non-radiative relaxation may be probed optically, since the resulting density changes also produce a change in the refractive index. The transient refractive index forms an effective lens, which diverges the light as it passes through the sample often called thermal lensing. 132 Chapter 4 Measurements based on photothermal methods are capable of giving absolute values of FQY with high accuracy and reproducibility. 4.2. Quantum yield - Theory The extremely sensitive thermal blooming measurement allows one to measure exceptionally weak absorption [3-5]. Hu and Whinnery [3] in their landmark contribution suggested that when combined with conventional transmission data, a thermal blooming measurement permits calculation of a luminescence quantum yield. Brannon and Magde [6] presented a detailed theory for the calculation of the luminescence quantum yield and reported successful results with experiments on fluorescein. They pointed out that the thermal blooming method is not merely an alternative approach for measuring luminescence quantum yields, but rather one which offers very significant advantages. After him, this technique is widely used for the quantum yield measurement of fluorescent solutions [7]. However, researchers used singlebeam thermal lens (TL) methods, where an auxillary lens of suitable focal length is used to create a beam waist in the laser beam. Dual beam TL technique is more advantageous than single beam TL technique and so researchers have been used this technique for the determination of fluorescence quantum yield, to detect small absorption, to record thermal lens spectra, and for the determination of various thermo-optic parameters like thermal diffusivity. In the present experimental setup dual beam thermal lens technique is used to find the fluorescence quantum efficiency of dye-nano mixture since this method is more sensitive than single beam configuration [8]. The transition of the molecule from electronically excited singlet state to the ground state resulting in fluorescence emission and the typical emissive rate of such quantum mechanically allowed transitions is about Optical property of pure and dye-doped nanofluids 133 10 8 s − 1 [9]. The process in which the fluorescence intensity of substance decreases, which assuredly increases the non radiative transition, gives the way to find out the fluorescence quantum yield values. The essential idea is very simple. The method of finding the quantum yield is based on the principle of conservation of energy. The energy (power) conservation involved is illustrated in Figure 1. Figure.1. Power conservation for luminescence If P0 is the power of the incident pump beam and Pt the power of the transmitted beam, then the absorbed power is the sum of the power transmitted, thermal power degraded to heat Pth and luminescence emission power Pf , provided that there occurs no photochemical reaction. Hence we can write P0 = Pth + Pf + Pt where it is assumed that the reflection and scattering losses are negligibly small [10] so that the transmission ratio is given by (1) 134 T= Chapter 4 Pt P0 (2) The fractional absorption is given by A = 1−T (3) Thus the absorbed power is given by AP0 = Pth + Pf (4) Then we may write Pf = AP0 − Pth (5) The emission quantum yield is by definition [6] Qf = Pf υf (6) (Po − Pt ) υ 0 Here υ o is the laser frequency and υ f is the mean luminescence emission frequency, evaluated as υ f = ∫ υ f dn (υ f ) ∫ dn (υ f ) (7) The quantity dn (υ f ) in photons/sec is the number of photons emitted in an incremental band width centered at υ f We may rewrite equation (6) in the form Qf = υ0 Pth 1 − υ f AP0 (8) The ratio υ 0 υ f takes account of the Stokes shift, which entails some deposition of heat in the sample even for a 100% luminescence quantum yield. The absorption A may be measured with an ordinary Optical property of pure and dye-doped nanofluids 135 spectrophotometer. The thermal blooming technique [3] offers a novel means of measuring Pth . Brannon and Madge [6] developed a comparison method in which thermal lensing measurements are made on both the fluorescent sample and nonluminescent reference absorber. The fluorescent sample is now designated by a superscript ‘s’ and a nonluminescent sample is used as the reference denoted by ‘r’. It is not necessary for the reference to be strictly nonfluorescent. For the reference sample A r P0r = Pthr (9) Hence we can write, Qf = υ0 Ar P r 1 − υ f A s P s Pths Pthr (10) where As is the absorption coefficient of the luminescent sample, Ar is the absorption coefficient of the nonluminescent reference, Pths is the thermal power generated in the sample and Pthr is the thermal power generated in the reference absorber. The absorption coefficient A may be measured with an ordinary spectrophotometer and Pth can be measured using thermal lens technique. Eqn. (10) assumes that the same solvent and the same excitation laser wavelength are used for the sample and the reference. In using this method, care must be taken to maintain the ratio Ar P r near unity and to As P s work with reasonably dilute solutions. Otherwise a systematic error occurs when the sample and reference solutions are of considerably different optical densities [7]. Another problem encountered in measuring light absorption coefficient As using spectrophotometer is high apparent transmittance, which results from the low fluorescence, received by the 136 Chapter 4 detector from measurements on a fluorescent sample. This causes uncertainty in the quantum yield values. The problem can be solved if a quenched luminescent sample is used as the reference absorber because the quenched sample has the same light absorption coefficient as the luminescent sample. In the case of a totally fluorescence quenched sample we can consider that the entire excitation energy is converted into nonradiative relaxation process. Hence the fluorescence quantum yield Q f is given by [6], Qf = λ f Pth λ f = 1 − AP0 λ Pα λ Pf (11) where Pα = AP0 (12) Here λ f is the peak fluorescence wavelength and λ is the excitation wavelength. Pth is directly proportional to η , the TL signal measured for each sample and Pα is proportional to TL signal ηα corresponding to the sample (particle size and / concentration) at which the fluorescence intensity is quenched completely. By knowing λ f , η and ηα , we can evaluate the quantum efficiency Q f from the equation given by Qf = λf λ η 1 − ηα (13) The thermal lens signal η has been measured as the as the variation of light intensity at far field at the centre of the probe beam [11]. Optical property of pure and dye-doped nanofluids 137 4.3. Size dependence of silver nanoparticle on the quantum yield of rhodamine 6G 4.3.1. Introduction Fluorescence quantum yield, a measure of conversion efficiency of absorbed photons into emitted photons, is one of the fundamental properties of emitters such as organic dyes, quantum dots etc [12-14]. The optical responses of organic and inorganic materials are strongly influenced by metallic nanoparticles and rough metallic surfaces. Recent studies reported the spectral dependence of fluorescence quenching in molecular systems composed of organic dyes and metal nanoparticles [15]. During last decades, noble metal nanoparticles (NPs) have been attracting a great deal of attention due to the capability of tuning the electrical, chemical or optical properties by tailoring the intrinsic particle parameters such as shape, size or morphology and their potential applications in diverse fields [16-19]. The fluorescence of an emitter can be tailored when the fluorophore placed in the vicinity of an entity possessing an electromagnetic (plasmon) field [20]. Recently, metal nanoparticles (NPs) that exhibit plasmonic absorption band in the visible range have been employed to tailor the fluorescence quantum yield of the dye molecules via plasmonic field created by the particle [21-22]. In close proximity of a nanoparticle, depending upon the dyenanoparticle distance, the fluorescence emission efficiency of a dye molecule can either be enhanced or decreased [23-24]. The plasmonic field generated around the particle by the incident light can increase excitation decay rate of the fluorophore which in turn enhances the level of fluorescence emission. On the other hand, the dipole energy around the nanoparticle reduces the ratio of the radiative to non-radiative decay rate and the quantum yield of the fluorophore, resulting in fluorescence quenching. In general, the energy 138 Chapter 4 transfer efficiency between the NP and the dye molecule depends upon three factors i) Coulombic spectral overlap integral ii) The position of the absorption spectrum of the nanoparticle (surface plasmon frequency) and iii) Width of the absorption spectrum (inverse of plasmon life time). However, for a given metal NP, the plasmonic absorption band that normally lies in the visible wavelength range of the electromagnetic spectrum and it is a function of particle size and shape. As a consequence, the size of the NP can be used to tailor the emission behavior of a nearby dye molecule. Normally, the energy transfer mechanism between the dye molecule and NP can be either radiative or nonradiative. The radiative energy transfer involves the emission of photon by the emitter and subsequent re-absorption by the absorber. The efficient and rapid nonradiative energy transfer occurs via dipole-nanoparticle interaction. Recent studies showed that, compared to dipole-dipole interaction based on Forster Resonance Energy Transfer (FRET) mechanism that follows R-6 dependence for the emitter-acceptor distance, dipole – metal surface based Nanometerial Surface Energy Transfer (NSET) mechanism that follows R-4 scaling is more appropriate in explaining energy transfer mechanism between dye-NP mixtures [25-27]. In general, for large NPs (a ~ 7 nm) the plasmon frequencies can be considered to be independent of particle size. In such cases, the energy transfer rate is mainly determined by the Coulombic interactions and the spectral overlap between the plasmonic absorption band and the emission band of the dye molecule. In this work, the dependence of silver NP(Ag NP) sizes of the range between 12 nm to 31 nm on the quantum yield of Rhodamine 6G (Rh 6G) using dual beam thermal lens technique is investigated. In addition to being a complementary approach to conventional fluorescence technique, the TL technique offer the advantage of direct probing of nonradiative path of Optical property of pure and dye-doped nanofluids 139 de-excitation of the excited molecule. TL technique is successfully employed for the evaluation of quenching efficiency of dye-nano mixtures [28], thermal diffusivity measurement of noble metallic nanofluids[29], energy transfer study in organic dye mixtures, quantum yield study etc [30-31]. Rhodamine 6G dye molecules in presence of metallic nanoparticles exhibit changes in optical properties [32-34]. The present study on Ag NP size dependent quantum efficiency measurement of Rh 6G is extremely important as Rh 6G is a commonly employed laser medium. Rh 6G-AgNP combination is prominent among various dye-nano mixtures, since Rh 6G is readily adsorbed to AgNP surfaces. It has been reported that in the presence of aggregated Ag NPs, Rh 6G exhibit emission band with a peak wavelength around 612 nm [35]. The aggregated Ag NPs can efficiently act as substrate for Surface Enhanced Raman Spectroscopic studies. The chemical nature of the absorbed molecule and the bonding to the metal surface also plays important role in SERS. The surface plasmon resonance causes enhanced Raman scattering, which is due to the enhancement of the electromagnetic fields close to the nanoparticle surface [36-37]. The nanoparticles can therefore be regarded as nanoamplifiers and SERS spectroscopy has been studied intensively. It is also reported that Ag NPs can either enhance or decrease the intrinsic fluorescence of Rh 6G depending upon the relative distance between them [38]. This has been exploited in surface enhanced fluorescence studies as well as in many biosensors. The quenching of fluorophore emission may be due to energy transfer [26], electron transfer [39] or by decreasing the radiative rate of the fluorophores [20] and enhancement in emission is attributed to the increase in the radiative rate or absorbance of dye. Recently, it is reported that Rh 6G adsorbed on Ag NP assembly substrates show obvious fluorescence quenching [40]. Researchers 140 Chapter 4 carried out detailed studies based on both fluorescence emission and enhancement of Rh 6G on silver island films [41]. It is seen that the fluorescence enhancement depends on the morphology of the substrate surface and the distance between fluorophore molecules and the substrate surface. Fluorescence quenching of laser dyes in the presence of AgNPs in different media has also been studied [42]. In a recent work published, random laser emission characteristics of Polymethylmethaacrylate films containing Rh 6G has been varied with the incorporation of silver NPs [43]. Recently, surface enhanced fluorescence of Rh 6G monolayer molecules deposited on metal substrates with silver fractal like structures and nanoparticles (NPs) is studied [44]. Shalaev et al. also demonstrated that the enhancement in the nonlinear optical properties when metallic nanoparticles form fractal-structured aggregates [34]. The optical properties of metallic nanoparticles providing novel opportunities for bioimaging and sensing [4547]. Nam et al. reported the application of metal nanoparticle in biology as biosensors in protein detection [48]. Owing to the wide spread applications, the present study wherein the focus is made on the non radiative probing of Ag NP size dependent luminescence behavior of Rh 6G has great physical significance as well as practical importance. 4.3.2. Experimental The dual beam TL set up implemented for quantum yield study is same as that of energy transfer study as explained in chapter 3. A continuous wave DPSS laser (Coherent) at 532 nm with a maximum power of 10 mW is used as the pump beam and 632.8 nm laser radiations is used as the probe beam to generate thermal lens in the sample. Optical property of pure and dye-doped nanofluids 141 4.3.2.1. Sample preparation (a) Rhodamine 6G (Rh 6G) Rhodamine 6G is an important xanthene derivative that cover the wavelength region from 460-700 nm and are generally very efficient. The Rh 6G dye is prominent among laser dyes and has remarkably high photostability, high fluorescence quantum yield (0.95), low cost, and its lasing range has close proximity to its absorption maximum (approximately 530 nm). Rh 6G dyes, with emission in the yellow-red region of the spectrum, are known for their excellent lasing performance in liquid solution. It is a common dye used in colloidal silver SERS studies, generally adsorbed to silver surfaces. It was one of the first molecules used for single molecule SERS studies with enhancement as large as 1014 to 1015 being observed [49-51]. For the present study, Rh 6G chloride, a bronze/red powder with the chemical formula C28H31ClN2O3 is used. Accurately weighed amount of rhodamine 6G was dispersed uniformly in double distilled water to give various concentrations. The structure of rhodamine 6G is given in figure. 2. Figure. 2. Structure of Rhodamine 6G 142 Chapter 4 (b) Silver nanofluid Silver nanofluids having different particle sizes were synthesized using chemical reduction method more specifically citrate-reduction method as described in chapter 2. Chemical reduction method is adopted for preparation due to high yield, low preparation cost and yielding nanoparticles without agglomeration [52-54]. Particle size can be varied by varying the amount of citrate concentration. The prepared silver nanoparticles are highly stable since a repulsive force acting along the particles which develops due to the negative charge of adsorbed citrate ions. This prevents aggregation of the particles for several weeks. Rhodamine 6G (0.001mM concentration) is mixed with silver nanofluid having varying particle sizes. 4.3.3. Transmission Electron Microscopic studies (TEM) The size and morphology of silver nanoparticles were determined using Transmission Electron Microscope (HITACHI H-7650). Suspension of the sample were dropped on formvar grids, dried and viewed under TEM at an accelerating voltage of 80KV. The spherical particles having size ranging from 12 nm to 31 nm is shown in Figure 3(a), (b), (c) and (d) respectively. Optical property of pure and dye-doped nanofluids 143 Figure. 3 (a-d): Transmission Electron Microscope images of silver nanoparticles (a) 12nm, (b) 15nm, (c) 22nm and (d) 31nm. 4.3.4. Absorption studies The absorption spectra of silver nanofluids are measured using UV-VIS absorption spectrophotometer (SHIMADZU UV-2401 PC) and shown in Figure 4. 144 Chapter 4 Figure.4. Absorption spectra of silver nanofluids. The LSPR (Localized Surface Plasmon Resonance) wavelengths varied from 428 to 440 nm and the variation of LSPR wavelength with particle size is shown in figure 5. The LSPR absorption band of silver nanoparticles strongly depends upon the particle size, shape, state of agglomeration and the surrounding dielectric media [55-57]. Dependence of LSPR bands on the above parameters led the metal nanoparticles to be useful in a variety of biodiognostic applications [58-59]. When the particle size is less than mean free path of ‘d’ electrons in Ag atoms (50 nm), extinction is dominated by absorption rather than scattering. The position of the peak depends upon the dielectric constant of the particle and the surrounding medium. In the case of spherical NPs whose size is within a few percentage of the incident light, a single localized plasmon resonance corresponding to dipolar resonance is Optical property of pure and dye-doped nanofluids 145 observed. In the present case, the particles are nearly spherical and the red shift can be attributed mainly to the increase in particle size which is consistent with Mie theory [60]. Figure.5. Variation of SPR wavelength with particle size. 4.3.5. Fluorescence and thermal lens studies of Ag nanofluids alone First report on luminescence from metals has been given by Mooradian (1969) [61]. The band structure of metal has been represented by a simplified model which includes an s–p conduction band and occupied d bands. The synthesized silver nanofluids having different particle sizes are found to be photoluminiscent. It is observed that the intensity decreases with increase in particle size while the emission wavelength is independent of the particle size. The emission spectra is taken using spectrofluorophotometer (SHIMADZU RF-5301 PC) and the observed emission peak for all the nanofluids was at 146 Chapter 4 360 nm for an excitation wavelength of 250 nm and shown in figure 6. Zhao et al. [62] also reported photoluminescence peak from Ag colloidal nanoparticles at 340 nm, when the particles were subjected to excitation using a laser at 248 nm. Figure.6. Fluorescence spectra of silver nanofluids. The origin of this visible photoluminescence of Ag nanofluids was attributed to the radiative recombination of an electron-hole pair. The incident photons are initially absorbed by the d-band electrons, leading to interband transitions. The photon energy promotes these electrons from the filled d band to electronic states above the Fermi level in the sp conduction band. Even if the excitation is carried out in the plasmon band, rapid relaxation to the interband transition must take place. Subsequently electrons and holes Optical property of pure and dye-doped nanofluids 147 relax by phonon scattering processes that leads to an energy loss. Finally radiative recombination of an electron from an occupied sp band with the hole give rise to the observed luminescence [61, 63-64]. The dependence of particle size on the fluorescence intensity of silver nanofluid is also shown in figure 7. Figure.7. Dependence of particle size on the fluorescence intensity. Thermal lens measurements and fluorescence measurements are complementary to each other. To check the complementary nature we have also taken the thermal lens signals of different particle sizes and observed a linear behavior with increase in particle size. Correspondingly we have noticed decrease in luminescence intensity with increase in particle size as shown in figure 7. Thermal lens signal variation with particle size is given in figure 8. 148 Chapter 4 Figure.8. Thermal lens signal variation with Ag nanoparticle size. 4.3.6. Fluorescence and thermal lens measurement studies in Rh 6G-Ag NP mixtures The fluorescence spectra of Rh 6G with varying molar concentrations are taken as shown in figure 9. The fluorescence spectra of Rh 6G ( 1× 10 −6 M ) alone and in the presence of silver nanofluid having different particle sizes were taken with an excitation wavelength of 510 nm as shown in Figure 10. Most of the previous studies showed that the direct mixing of fluorophores with metal nanoparticles in solution causes efficient quenching of the fluorescence [20, 65]. Optical property of pure and dye-doped nanofluids 149 Figure.9. Fluorescence spectra of rhodamine 6G It is clear from figure 10 that the nanoparticles in the mixture quench the fluorescence emission of the Rh 6G and quenching efficiency increases with the increase in particle size. This can be attributed to the difference in plasmon field strength around a nanoparticle that influences the fluorescence of the dye molecule. The plasmon field strength around a nanoparticle depends upon its particle size and its strength decreases rapidly with the increase in distance from the surface. For small particles, the field strength decreases rapidly compared to larger particles. Thus, at a given distance from the particle, the interactive field experienced by the dye molecule is smaller for small particle and the corresponding quenching efficiency is smaller. In addition, the increase in absorption coefficient as well as red shift in the plasmon absorption band of the Ag NPs with particle size increases the spectral overlap between 150 Chapter 4 emission spectrum of Rh 6G and LSPR band of Ag NPs. This enhanced spectral overlap enables more efficient nonradiative energy transfer and subsequently enhance the quenching of dye molecule fluorescence by the NP. It is also observed that the peak fluorescence emission wavelength of Rh 6G is slightly blue shifted with the addition of nanoparticles. The observed blue shift may be due to the local enhancement of the optical fields near the dye molecules by interactions with silver plasmons. Figure.10. Fluorescence spectra of Rh 6G-Ag nanoparticle mixture. The size of Ag NP varies from 12 to 31nm. (a) dye (1× 10−6 M ) alone, (b) 12nm, (c) 15nm,(d) 22nm and (e) 31nm. In order to probe that the fluorescence quenching is happening via nonradiative energy transfer rather than electronic transfer, the thermal lens signal of Rh 6G in the presence and absence of NPs is measured. The Optical property of pure and dye-doped nanofluids 151 variation in thermal lens signal as a function of particle size is shown in figure 11. Figure.11. Thermal lens signal variation of Rh 6G with silver nanoparticle size. The value corresponds to zero particle size is the thermal lens signal of Rh 6G in the absence of nanoparticle. It is observed that in the presence of NPs, the thermal lens signal enhances with NP size. The concentration of Rh 6G employed here is very low so that the possibility of self-quenching by aggregate (H or J type) dimer formation is excluded. Moreover, the NP size dependence of thermal lens signal indicate that spectral overlap between the Rh 6G fluorescence emission spectrum and plasmonic absorption spectrum of the NP is the main contributing factor in determining the experimentally observed fluorescence quenching or enhancement in thermal lens signal. With the increase in particle size, the height and the width of the plasmonic absorption band 152 Chapter 4 increases. For particle sizes employed here, the absorption properties of the particles dominate over the scattering. In the proximity of these particles, the dyes transfer their energy nonradiatively to the nanoparticles and exhibit fluorescence quenching or increase in thermal lens signal. This quenching in fluorescence or increase in thermal lens signal is normally reflected in the fluorescence quantum yield (FQY) of the dye molecule. Corresponding to each particle size, the FQY is measured using the equation Qf = λf λ η 1 − ηα where η is the TL signal measured for each particle size and ηα is the TL signal corresponding to the particle size at which the fluorescence intensity is quenched completely. By knowing λ f , η and ηα , we can evaluate the quantum efficiency Q f as shown in figure 12. The FQY of the pristine Rh 6G (0.93) agrees well with the earlier reported values [66]. However, the enhanced nonradiative energy transfer with increase in particle size reduces the intrinsic quantum yield of the Rh 6G molecules. Recent studies shows that the energy transfer between a dye molecule and a metal NP can be better explained in terms of Nanomaterial Surface Energy Transfer (NSET) [27, 67-69]. Optical property of pure and dye-doped nanofluids 153 Figure.12. Variation of quantum yield of Rh 6G vs size of nanoparticle. The value corresponds to zero particle size is the quantum yield of pristine Rh 6G. 4.3.7. Dependence of particle size on the quenching efficiency of fluorophore. Effect of silver nanoparticle on the quenching efficiency is also studied. The quenching efficiency can be evaluated from the photoluminescence (PL) measurement using the equation φET = 1− F F0 (14) where F0 and F are the relative fluorescence intensity in the absence and presence of acceptor respectively. In terms of thermal lens signal, the quenching efficiency can be written as [27-28] 154 φ ET = 1 − Chapter 4 η L0 (15) η LA where η L0 and η LA are the thermal lens signals measured by the fractional change in the detected power for donor alone and with the acceptor respectively. The quenching efficiency of Rh 6G ( 1× 10 −6 M ) in presence of AgNPs of different sizes is evaluated using TL and PL method and the calculated efficiencies are given in table 1. Table 1. Calculated quenching efficiency of Rh 6G for various particle sizes. Particle size (nm) Φ% TL measurement Φ% PL measurement 12 60.07 59.06 15 66.10 65.44 22 72.88 71.07 31 77.94 76.23 From the table 1, it is clear that the evaluated quenching efficiency values are nearly the same for TL as well as steady state fluorescence technique, indicating the complementary nature of both techniques. Analysis suggests that the quenching efficiency of Rh 6G increases with increase in AgNP size. The increase in quenching efficiency of Rh 6G with increase in particle size can be understood in terms of more efficient nonradiative energy transfer between the particle and the dye molecule due to enhanced spectral overlap. From the present study it is evident that there is an increase in quenching efficiency as well as decrease in FQY of Ag NP-Rh 6G mixture with increase in nanoparticle size. Optical property of pure and dye-doped nanofluids 4.4. 155 Concentration dependence of silver nanoparticle on the quantum yield of Rh 6G 4.4.1. Introduction In this section the photoluminescence and quantum yield of Rh6G dye dispersed in double distilled water in the presence of different aqueous amounts of citrate stabilized silver nanoparticles of average size around 11 nm has been studied. Organic dyes have been studied extensively due to their luminescence properties and possibility to tune their properties by localizing near metal surfaces [70-71]. Tuning the luminescence intensity of dye molecules near a metal surface was studied by Drexhage in as early as 1970, leading to various reports on interaction of fluorophores with metal nanoparticles or surfaces (mostly silver) displaying various spectral changes [72]. It has been found that when metal nanoparticles are in close proximity to the fluorophores, quenching of luminescence occurs [73]. These effects have been explained by coupling of surface plasmon resonance from metal particles and fluorophore. Theoretical studies have been carried out on absorption and luminescence of dye molecules adsorbed on silver and gold nanoparticles or surfaces. Quenching of the luminescence of dye molecules adsorbed on a smooth Ag surface was observed by Ritchie and Burstein (1981) [74]. 4.4.2. Sample preparation Silver nanofluid is prepared as described in chapter 3. The corresponding TEM image and absorption spectrum of silver nanoparticles (inset) is given in figure 13. Rh 6G dye (C28H31N2O3Cl) was dispersed uniformly in double distilled water to prepare a lower concentration and to this solution measured amount of aqueous dispersion of silver nanoparticles was added viz. 0⋅5 ml, 1ml, 2ml and 3ml respectively. 156 Chapter 4 Figure.13. TEM image of silver nanoparticle of size around 11nm (inset shows absorption spectra) 4.4.3. Fluorescence measurements Fluorescence spectra measurements were recorded using spectrofluorophotometer SHIMADZU RF-5301 PC. The fluorescence spectra at an excitation wavelength of 510 nm are shown in figure 14. Here also we seen that the emission peaks are slightly blue shifted with respect to the fluorophore. It is clear from fluorescence spectra that as the amount of aqueous dispersion of silver nanoparticle increases, the luminescence intensity goes on decreasing. The interaction between dye molecules and silver nanoparticle can leads to such interesting optical property. Optical property of pure and dye-doped nanofluids 157 Figure.14. Fluorescence spectra of Rh 6G alone and in the presence of different aqueous amount of silver nanofluid. 4.4.4. Measurement of Quantum yield Here we calculated the quantum yield of various samples using equation (13). The sample with 3ml nanofluid is taken as the quenched sample. The thermal lens signal for various samples is depicted in figure 15. It is seen that thermal lens signal increases with increase in the amount of nanofluid. A graph of FQY as a function of amount of silver nanofluid added is given in figure 16. From the analysis it is also clear that the quantum yield of Rh 6G decreases with the increase in the amount of nanofluid. There are many factors that leads to decrease in fluorescence intensity or inturn in FQY [15]. The relative orientation of the fluorophore and the metallic nanoparticle to the laser 158 Chapter 4 excitation could contribute much effect in the fluorescence of dye molecule for an assembly comprising fluorophores and metallic nanoparticles. Figure.15. Thermal lens signal variation of Rh 6G in presence of Ag nanofluid. Another reason for the quenching of fluorescence intensity is the random movement of nanoparticles and fluorophores in the solution. Last but not least, efficient energy transfer from fluorophore to metal nanoparticle. Among these, the decrease in quantum yield with increase in amount of silver nanofluid can be explained on the basis of energy transfer between rhodamine 6G (donor) and silver nanoparticle (acceptor). Recently researchers have reported that quenching of fluorescence intensity of Rh 6G with increase in acceptor concentration is due to the surface energy transfer process between Rh 6G and Ag nanoparticle [27]. Optical property of pure and dye-doped nanofluids 159 Figure.16. FQY of Rh 6G as a function of amount of Ag nanofluid. 4.5. Conclusion Dual beam thermal lens technique was successfully employed to probe the dependence of silver nanoparticle size on the quantum efficiency of rhodamine 6G. It is observed that, in the Rhodamine 6G-Ag nanoparticle mixture, the intrinsic fluorescence of Rh 6G decreases with increase in particle size due to the more efficient spectral overlap between the emitter and absorber. The enhancement of absolute intensity and spectral width of the absorption spectrum of the nanoparticle size and consequent more efficient spectral overlap enables more efficient nonradiative energy transfer between the nanoparticle and the dye molecule which reflected in corresponding thermal lens as well as static fluorescence signal. Moreover, the quenching efficiency values measured using both approaches clearly 160 Chapter 4 show that thermal lens technique that probe nonradiative path of deexcitation can be effectively utilized to measure the quantum yield of dye-NP mixtures. Thus thermal lens technique can be used as an alternate technique where direct fluorescence measurements are practically difficult. This size dependent increased nonradiative energy transfer between the dye molecule and the nanoparticle can be exploited to develop future biological sensors. The present study clearly shows that by tuning the nanoparticle size it is possible to decrease the fluorescence of fluorophore (or increase the photothermal signal). Another study describes the effect of nanoparticle concentration on the quantum yield of Rh 6G. The absolute values of FQY of laser dyes are necessary for the calculation of thresholds of laser action and hence describes the effect of nanoparticles on the FQY of Rh 6G. The size and concentration dependent optical properties of Ag nanoparticles have received considerable attention and expected to play important role in enhancing the Raman scattering and thus to provide important contribution towards sensing and bio-medical applications. Raman Scattering also helps in determining the molecular structure of specific analyte molecules. However, it is inherently weak effect often masked by fluorescence. Raman scattering can be enhanced by resonance or by surface enhancement. 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