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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
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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
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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
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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.
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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. It
should also be possible to enhance it, if fluorescence could be reduced by
some method. It has been observed in our study that both the increase in
concentration and particle size of silver nanoparticle decreases the
fluorescence quantum efficiency. This is expected to have a very important
consequence in enhancing Raman scattering which is an important
spectrochemical tool that provides information on molecular structures.
Optical property of pure and dye-doped nanofluids
161
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