Spectral investigations on the influence of silver nanoparticles on the fluorescence quenching of 1,4-dimethoxy-2,3-dibromomethylanthracene-9,10-dione

Article abstract of DOI:10.1140/epjd/e2014-50257-5

Silver nanoparticles (Ag NPs) of different sizes from 9 to 17 nm were synthesized by Creighton method and characterized using UV-vis spectroscopy and high resolution transmission electron microscopy (HRTEM). Fluorescence quenching of 1,4-dimethoxy-2,3-dibromomethylanthracene-9,10-dione (DMDBMAD) in methanol has been studied by fluorescence spectroscopy combined with UV-vis absorption spectroscopic techniques. It has been observed that the fluorescence intensity of DMDBMAD decrease with increase in the size of the Ag NPs. The quenching rate constant and association constant were determined using Stern-Volmer and Benesi-Hildebrand plots. The Stern-Volmer plot suggested that the quenching of DMDBMAD fluorescence by silver NPs was a dynamic process. The obtained value of the association constant infers that there is an association between DMDBMAD and the Ag NPs. Using F?rster resonance energy transfer (FRET) theory, the distance between the donor (DMDBMAD) to acceptor (Ag NPs) and the critical energy transfer distance were obtained. Long range dipole-dipole interaction between the excited donor and ground state acceptor molecules is the dominant mechanism responsible for the energy transfer.

Full text of DOI:10.1140/epjd/e2014-50257-5

Eur. Phys. J. D (2014) 68: 308
DOI: 10.1140/epjd/e2014-50257-5
THE EUROPEAN
PHYSICAL JOURNAL D
Regular Article
Spectral investigations on the influence of silver nanoparticles
on the fluorescence quenching
of 1,4-dimethoxy-2,3-dibromomethylanthracene-9,10-dione
Singlepatti Ramasamy Kavitha1, Mahalingam Umadevi1,a, Patrice Vanelle2,3
Thierry Terme2,3, and Omar Khoumeri2,3
,
1
Department of Physics, Mother Teresa Women’s University, 624101 Kodaikanal, Tamil Nadu, India
Laboratoire de Pharmaco-Chimie Radicalaire, Facult´e de Pharmacie, Aix-Marseille University, CNRS,
Institut de Chimie Radicalaire ICR, UMR 7273, 27 Boulevard Jean Moulin, 13385 Marseille Cedex 05, France
2
3
Received 30 March 2014 / Received in final form 29 July 2014
c
Published online 21 October 2014 – ꢀ EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2014
Abstract. Silver nanoparticles (Ag NPs) of different sizes from 9 to 17 nm were synthesized by Creighton
method and characterized using UV-vis spectroscopy and high resolution transmission electron microscopy
(HRTEM). Fluorescence quenching of 1,4-dimethoxy-2,3-dibromomethylanthracene-9,10-dione (DMDB-
MAD) in methanol has been studied by fluorescence spectroscopy combined with UV-vis absorption spec-
troscopic techniques. It has been observed that the fluorescence intensity of DMDBMAD decrease with
increase in the size of the Ag NPs. The quenching rate constant and association constant were determined
using Stern-Volmer and Benesi-Hildebrand plots. The Stern-Volmer plot suggested that the quenching of
DMDBMAD fluorescence by silver NPs was a dynamic process. The obtained value of the association
constant infers that there is an association between DMDBMAD and the Ag NPs. Using Fo¨rster resonance
energy transfer (FRET) theory, the distance between the donor (DMDBMAD) to acceptor (Ag NPs) and
the critical energy transfer distance were obtained. Long range dipole-dipole interaction between the ex-
cited donor and ground state acceptor molecules is the dominant mechanism responsible for the energy
transfer.
1 Introduction
protein-ligand interactions by fluorescence spectroscopic
techniques [3]. These molecules have important applica-
tions as a prominent family of pharmaceutically active
and biologically relevant chromophores, as an analytical
tool for the determination of metals, and in many aspects
of electrochemistry. The quinone structural fragment is of-
ten found in naturally occurring compounds which exhibit
antitumour, antibacterial, malaricidal and fungicidal ac-
tions. These are important class of a quinone system that
absorbs light in the visible region [4].
Investigations of metal NPs have attracted significant
attention because of their unusual size dependent optical
and electronic properties. Presently there is intense inter-
est in the optical properties of Au and Ag colloids. These
colloids display bright colors due to the surface Plasmon
resonance (SPR) absorption, which is in fact due to a
combination of absorption and scatter [5]. The color of
the NP is found to depend on the shape and size of the
nanoparticle and dielectric constant of the surrounding
medium, leading to many studies on their synthesis and
applications. The color of the metal NPs are mainly due
to the collective oscillation of the electrons in the con-
duction band, known as the surface plasmon oscillation.
The oscillation frequency is usually in the visible region
for Au and Ag giving rise to the strong SPR absorption.
Anthraquinone is the most important quinone deriva-
tive of anthracene. It is a group of aromatic compound
containing two opposite carbonyl groups (C = O) and
the other two pairs of carbon atoms linked by Vinylene
group (–CH = CH) in a six membered unsaturated ring.
Quinones are one of such compounds which occur nat-
urally in some plants (aloe latex, senna and rhubarb),
fungi, lichens, and insects, where they serve as a basic
skeleton for their pigments. Quinones and their deriva-
tives comprise an important class of redox mediators and
electron-transfer quenchers. Anthraquinones are naturally
colored and therefore have been extensively used as an an-
tioxidant in medicine, a catalyst in paper industry and a
coloring material in textile industry. They are considered
together with other traditional colorants for application
in technologies such as optical data transmission storage
and solar energy conversion [1]. These derivatives are of
major importance in obtaining luminescent materials with
potential applications as materials for lasers, paints, lumi-
nescent photolayers and light-emitting devices [2]. They
are also used as fluorescent probes in order to investigate
a
e-mail: ums10@yahoo.com

Page 2 of 8
Eur. Phys. J. D (2014) 68: 308
MERCK. Spectral grade methanol (CH3OH) was pur-
chased from NICE. All the chemicals were of Analytical
Grade and used as purchased without further purification.
Doubly distilled water was used throughout the experi-
ment. All glasswares were properly washed with distilled
water and dried in hot air oven before use.
2.2 Preparation of Ag NPs
Silver colloidal sol was prepared by Creighton method [15]
at different AgNO3 concentration ratios. In brief, AgNO3
solution and NaBH4 solution were individually prepared
by dissolving 0.0545 mM of AgNO3 in 50 ml of distilled
water and 3 mM of NaBH4 in 100 ml of distilled water with
vigorous stirring (1 h). Both the solutions are kept sepa-
◦
Fig. 1. Structure of 1,4-dimethoxy-2,3-dibromomethylanthra-
cene-9,10-dione (DMDBMAD).
rately in ice cooled temperature (–1 C). 50 ml of AgNO3
solution was added dropwise to 100 ml of NaBH4 solution
and the mixture was shaken vigorously until glassy yellow
solution was obtained (S1). It was repeated for different
concentration of silver nitrate solution (0.1, 0.1384 and
0.1714 mM) (S2, S3 and S4) at constant concentration of
NaBH4 solution (3 mM). The silver colloid was stored in
These optical properties make metallic colloids potentially
useful in sensing and imaging techniques [6].
The interactions of fluorophores with metal nanopar-
ticles results in the fluorescence enhancement/quenching,
increased photostability, decreased lifetime because of in-
creased rates of system radiative decay and increased
transfer distances for FRET [7]. The enhancement or
quenching of the fluorescence emission of molecules near
a metal surface arises from interactions with surface
plasmon resonance in the metal particles. These interac-
tions may also result in shortening of the excited-state
lifetime thus improving the photostability of the dye. The
optical properties of a fluorescent molecule located near
the metal nanoparticles are affected by the near-field elec-
tro dynamical environment [8]. This can cause an enhance-
ment or quenching of the fluorescence depending on the
distance between the molecule and the metal surface. In
the case of fluorescent molecules located at very short dis-
tances from a metal surface, non-radiative energy trans-
fer to surface plasmon in the metal takes place [3]. The
quenching of fluorescence provides useful information on
the nature of interaction between the fluorophore and the
quencher. Our group has studied the spectroscopic char-
acteristics of anthraquinone derivatives in Ag NPs envi-
ronment [9–14]. The dye doped Ag NPs have captured re-
cent interest in medical diagnostic, proteins, membranes,
sensing, labeling, optical data transmission storage, bio-
analytical probes, solar energy conversion and dye synthe-
sized solar cells. The biological applications of dye doped
quenchers and fluorescence quenching property motivated
the group to conduct the study. In the present case, we
got interested in the effect of silver nanoparticles on the
fluorescence properties of DMDBMAD (Fig. 1).
◦
a dark place (20 C).
2.3 Synthesis of DMDBMAD
1,4-dimethoxy-2,3-dibromomethyl anthracene-9,10-dione
were prepared according to the literature [16]. In brief,
to a solution of 1,4-dimethoxy-2-methylanthracene-9,10-
dione (1 g, 3.52 mmol) in carbon tetrachloride (100 mL)
was added N-bromosuccinimide (1.89 g, 10.62 mmol) and
catalytic amount of benzoyl peroxide. The reaction mix-
ture was heated to 80 ◦C and irradiated with a 300 W sun
lamp for 7 h. After cooling and evaporation, the residue
was dissolved in chloroform and washed with water. The
organic layer was dried over magnesium sulfate and the
solvent was removed under vacuum. Purification on sil-
ica gel eluting with ethyl acetate/petroleum ether (1/2)
gave 1.34 g (87%) of DMDBMAD as yellow solid. This
synthetic method was based on the following chemical
reaction,
The concentration of DMDBMAD in methanol was main-
tained at 0.02 mM throughout the experiment. To investi-
gate the influence of Ag NPs on DMDBMAD, the dye and
quencher solution have been taken in 1:1 volume ratio.
2 Experimental
2.4 Methods of characterization
2.1 Materials
Optical absorption spectra were recorded using
a
Silver nitrate (AgNO3, 99.5%), sodium borohydride Shimadzu UV-1700 pharmaspec UV-vis spectrophotome-
(NaBH4, 95%) and Rhodamine 6G were purchased from ter. Jobin Yvon Flurolog-3-11 Spectrofluorometer was

Eur. Phys. J. D (2014) 68: 308
Page 3 of 8
Table 1. Photo-physical properties of Ag NPs and Energy transfer parameters of DMDBMAD in Ag NPs.
Concentration SPR peak
of AgNO3
(mM)
Fluorescence
quantum
yield (ϕ)
FWHM
(nm)
J (λ)
(cm3 L mol−1
R0
(nm) (nm)
r0
S.No
of Ag NPs
(nm)
E (%)
)
1.
2.
3.
4.
0.0545 (S1)
0.1 (S2)
0.1384(S3)
0.1714 (S4)
393
396
399
401
40
56
64
66
0.0059
0.0022
0.0013
0.0005
2.11 × 10−14
1.72 × 10−14
1.37 × 10−14
1.25 × 10−14
198
191
184
181
104
97
90
87
100
96
95
92
the observed surface plasmon resonance (SPR) peak and
full width at half maximum (FWHM) values of the pre-
pared Ag NPs. The maximum absorption peak appears
around 400 nm, which is due to the characteristic surface
plasmon resonance of Ag NPs [17]. Conduction electrons
on the surface of silver NPs undergo a collective oscilla-
tion known as SPR due to its strong interaction with the
incident light. For spherical NPs much smaller than the
wavelength of light, the electromagnetic field is uniform
across the particle causing all the conduction electrons
to move in-phase producing only dipole-type oscillations
manifested by a single, narrow peak in the SPR spec-
trum. As the size increases and deviates from the spherical
shape, the field across the particle becomes non-uniform.
Phase retardation then broadens the dipole resonance and
excites higher multi-pole resonances. Further, the oscillat-
ing electrons induce polarisation of the opposite direction
in the surrounding medium which reduces the restoring
force for the electrons resulting in a shift in the SPR [18].
Fig. 2. Optical absorption spectra of Ag NPs.
An absorption peak at around 400 nm observed gen-
erally is attributed to the SPR excitation of Ag NPs.
Mie [19] and Wang et al. [20] have carried out theoret-
ical studies on the dependence of UV absorption maxi-
mum on the size of metal NPs. The spectrum can exhibit
a shift towards the red end or the blue wavelength depend-
ing upon the particle size, shape, state of aggregation and
the surrounding dielectric medium [21]. Absorption spec-
tra obtained for the sample S4 indicates that the size of
the particles must be large than that of S1. This result is
in consistent with the TEM images of the colloidal silver.
As the concentration of silver nitrate increases, the sur-
face plasmon band shifts towards the red (longer) wave-
length and its FWHM increases. The plasmon peak and
full width at half maxima depends on the extent of col-
loid aggregation [22]. In the present case, the size of pre-
pared silver nanoparticles increases as the concentration
of silver nitrate increases. The observed plasmon band
observed around 400 nm suggests that the Ag NPs are
spherical in shape [23]. For a small particle, the maximum
absorbance intensity and bandwidth of the SPR peak rely
on the surroundings of the medium. When the Ag NPs
strongly dominate the matrices condition, a large broad-
ening of the SPR occurs. This would result in blue shift
characteristics. Similarly, a red shift with small broaden-
ing would take place when low interactions occur between
the Ag NPs and matrices. This would result in red shift
characteristics [24].
used to record the emission spectra. The excitation wave-
length of DMDBMAD is 330 nm. The morphology and the
size of the nanoparticles were determined using a JEOL
JEM 2100 high resolution transmission electron micro-
scope operating at 200 kV. All the measurements were
performed at room temperature.
2.5 Fluorescence quantum yield
The relative fluorescence quantum yield (ϕrel) [3] of the
sample in terms of reference sample (ϕ0) is
ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ
F
OD0
OD
n
(ϕrel) =
ϕ0
F0
n0
where F and F0 are the integrated fluorescence intensities,
OD and OD0 are the optical densities and n and n0 are
the refractive indexes for Rhodamine 6G in methanol. The
relative fluorescence quantum yield (ϕrel) in which Rho-
damine 6G was used as fluorescence standard (ϕ = 0.94).
In the present case, the fluorescence quantum yield of
DMDBMAD in methanol was found to be 0.0154.
3 Results and discussion
3.1 Optical absorption spectral studies: Ag NPs
UV-vis absorption spectra of prepared Ag NPs at differ-
Henglein [25] reported the substantial decrease in red
ent concentrations are shown in Figure 2. Table 1 shows shift of Ag NPs SPR band, while studying the effect of

Page 4 of 8
Eur. Phys. J. D (2014) 68: 308
Fig. 3. TEM micrographs of the silver nanoparticles. (a) S1
(the scale bar corresponds to 50 nm) and (b) S4 (the scale bar
corresponds to 50 nm).
excess Ag+ ion on Ag NPs. It has been explained in terms Fig. 4. Optical absorption and fluorescence emission spectra
of DMDBMAD in methanol.
of decrease of electron density on the nanoparticles surface
upon chemisorption of silver ions. Since the wavelength of
the absorption maximum is inversely proportional to the
density of free electrons in the metal for particles of a few
nanometers, it is independent of size. In the present case,
as the concentration of AgNO3 increases density of free
electrons in the silver decreases which are chemisorbed on
the surface of nanoparticle and thus the plasmon respon-
sible for SPR shift.
3.2 Morphological analysis of Ag NPs
Transmission electron micrographs in Figures 3a and 3b
display the silver nanoparticles obtained under different
concentrations S1 and S4, respectively. The average size
of these nanoparticles is approximately ranged from 9 nm
(S1) to 17 nm (S4) and this result was also confirmed
by UV-vis spectra of Ag NPs. The average size increased
with elevated concentration which was consistent to the Fig. 5. Optical absorption spectra of DMDBMAD in Ag NPs.
red shift (longer wavelength) of the absorption peaks in
the UV-vis spectra. The degree of size dispersity of silver
When the particles are smaller in size, the multiple twin-
ning is the favorable structural configuration. It may be
due to the smaller surface and volume energies [28].
NPs increased with increasing Ag concentration (Tab. 1).
The formation of large particles with higher size disper-
sity could be attributed to the silver migration and ag-
gregation. The migration and aggregation of the Ag par-
ticles might be driven by the instability of silver atoms
due to their high surface free energy. Their aggregation
would produce thermodynamically stable clusters [26,27].
The prepared silver NPs shows twinning in the crystalline
3.3 Optical absorption spectral studies: DMDBMAD
in Ag NPs
structure. Twinning is one of the most commonly noticed Figures 4 and 5 represent the absorption spectrum of
planar defects in nanocrystals, and is frequently observed DMDBMAD in methanol and absorption spectra of
for metallic nanocrystals of face centered cubic (fcc) struc- DMDBMAD in different sizes of Ag NPs, respectively. Ab-
ture. It occurs when two subgrains share a common crys- sorption spectrum of DMDBMAD in the methanol shows
tallographic plane. The structure of one subgrain is then a broad band in the visible region from 300 to 400 nm with
a mirror image of the other through the twinning plane. one strong peak observed at 370 nm. The absorption spec-
The silver nanoparticles were found to be more or less tra of DMDBMAD in different sizes of Ag NPs show broad
spherical, elongated in shape and the nanoparticles are band in the region 300 to 500 nm. When donor molecule
poly-dispersed. It was also observed that the majority of is added into silver sol, the original features of the spectra
the particles in this case exhibit internal structure, char- have been changed and the silver plasmon peak is shifted
acterized by strain effects and stacking faults (multiple- towards the longer wavelengths and its intensity decreases,
twinned nanoparticles) with five-fold symmetry, i.e. deca- i.e., damping and broadening of the plasmon band is ob-
hedral and icosahedral particles. Because of this five-fold served. The major reason for SPR broadening is elec-
axis, strain was induced in the particle to fill the gap. tron surface scattering which may be enhanced for very

Eur. Phys. J. D (2014) 68: 308
Page 5 of 8
Fig. 7. Fluorescence emission spectra of DMDBMAD in Ag
NPs.
Fig. 6. Benesi-Hildebrand plot.
small clusters [29]. This change in spectral shift and band
broadening of the surface plasmon band reveals that these
DMDBMAD may bind on the silver surface. The damp-
ing of the silver plasmon band indicates the attachment
of the DMDBMAD molecule on the nanoparticles surface.
The interaction of silver nanoparticles with DMDBMAD
alters the electron density of the Ag NPs, thereby directly
affecting the absorption of the surface bound DMDBMAD
as well as surface plasmon absorption band.
in Table 1. Fluorescence emission spectrum of DMDB-
MAD in methanol shows a broad band in the visible re-
gion 350–550 nm with strong peak observed at 433 nm.
The fluorescence spectra of DMDBMAD in different sizes
of Ag NPs show broad band in the region 390–530 nm. The
DMDBMAD molecules added with the nanoparticles are
in thermodynamical equilibrium and the only fluorescent
component arises from the free DMDBMAD molecules in
the solution. And hence the fluorescence is due to the un-
bound probe molecules in the system. With increase in the
concentration of silver nanoparticles, more dye molecules
were adsorbed on silver surface and hence the fluorescence
of the dye was quenched. This leads to a situation that the
existence of less number of free probe dye molecules and so
the number of free fluorophores decreases and thus the de-
crease in fluorescence intensity. In the presence of Ag NPs,
the fluorescence intensity of the SPR band and quantum
yield of DMDBMAD decreases (Tab. 1) without any new
broad band in the fluorescence emission spectra which re-
veal that the DMDBMAD bound to Ag NPs failed to ex-
hibit intermolecular interaction [9–14]. The lowered inten-
sities show that Ag NPs are responsible for this quenching
effect. The smaller Ag NPs of S1 showed greatest quench-
ing, while the larger NPs like S4 quenched to a lesser ex-
tent. This quenching in the fluorescence intensity is mainly
due to energy transfer from the DMDBMAD to the Ag
NPs. When the donor molecules are in close vicinity of
the acceptor molecule, resonance energy transfer occurs
resulting in reduced fluorescence intensity [31].
3.4 Benesi-Hildebrand approach
The interaction between DMDBMAD and Ag NPs was in-
vestigated by determining the association constant (Kass).
The association constant can be measured by various
spectroscopic methods. Among those methods, Benesi-
Hildebrand approach [30] was used. Using these spectral
shifts the binding constant of DMDBMAD with Ag NPs
in the ground state can be calculated. By this approach,
the association constant for the complex formed between
Ag NPs and DMDBMAD is obtained by analyzing the
optical absorption spectral changes. The association con-
stant of Kass = 2055 M−1 was obtained from the plot
of [DMDBMAD]/ΔA330 vs. 1/[Ag] as shown in Figure 6,
where ΔA330 is the changes in absorbance of DMDBMAD
with and without Ag NPs at 330 nm and [Ag] is the con-
centration of silver. The value of association constant ob-
served in this experiment suggests that there is an associ-
ation between DMDBMAD and Ag NPs [9–14].
Quenching is a process which reduces the lifetime of
the excited state. A reduction in the lifetime usually
implies a decrease in the quantum yield. Some of the
processes which reduce the quantum yield are: dynamic
(collisional) quenching, static quenching and energy trans-
fer. Both quenching requires contact between the fluo-
3.5 Fluorescence spectral studies
The fluorescence emission spectrum of DMDBMAD in rophore and the quencher. Thus, these methods are useful
methanol and fluorescence spectra of DMDBMAD in dif- to measure rates of diffusion and exposure of fluorescent
ferent sizes of Ag NPs is shown in Figures 4 and 7, re- species to the quencher. Static quenching is observed when
spectively. The observed fluorescence quantum yield of a fluorophore forms a complex with the quencher. It is
DMDBMAD in different sizes of NPs has been reported normally assumed that complex formation will prevent

Page 6 of 8
Eur. Phys. J. D (2014) 68: 308
the fluorophore from emitting a fluorescent photon, how-
ever those fluorophores which are not complexed with the
quencher emit normally with the same lifetime as in the
absence of the quencher [3].
Fluorescence quenching may also result from a photo
induced electron transfer process between the excited
DMDBMAD and the Ag NPs. The interaction with the
dye and the excited state surface reactions may lead to
morphological changes of the Ag NPs. The surface plas-
mon efficiently acts as energy acceptor even at a distance
of 1 nm between the probe molecules and the metal sur-
face. Ag NPs also have a continuum of electronic states
and exhibit energy transfer behavior as excited state
quenchers. The observed changes in the absorbance of
the absorption spectrum and fluorescence intensity reflects
the alteration of the electronic properties of the DMDB-
MAD chromophore as it binds to the Ag NPs which acts
as a excited state quencher. The DMDBMAD has a high
emission quantum yield in the absence of NPs, the dom-
inant effect for the quenching of excited state of DMDB-
MAD in Ag NPs may be due to radiative energy trans-
fer to the Ag surface. Geddes et al. [32] described the
effect of Ag NPs on near the fluorophores. When the
DMDBMAD fluorophore-Ag distance is increased, both
fluorescence quenching and enhancement have been ob-
served [33]. Ditlbacher et al. [34] explained the changes of
molecular fluorescence near Ag NP. When using DMDB-
MAD fluorophore as local probes of the surface plasmon
field of metallic NPs, in the close proximity of an Ag, the
fluorescence rate of the molecules is a function of the dis-
tance between the probe molecule and the metal surface.
When in direct contact with the metal, the fluorescence
of molecules is completely quenched.
Fig. 8. Stern-Volmer plot.
In the present case, non-radiative decay due to joule
heating and surface plasmon absorption takes the dom-
inating effect which in turn decreases the fluorescence
emission and fluorescence quantum yield of DMDBMAD.
This non-radiative decay can also be theoretically stud-
ied using the Fo¨rster energy transfer theory [37]. When
Ag NPs is added into the DMDBMAD, the DMDBMAD
molecules would tend to adsorb on the silver surface due
to physisorption. As the size of the particle increases, the
surface area decreases which in turn decreases the number
of DMDBMAD molecules that would adsorb on the metal
surface. The probability of this energy transfer depends
on the overlap of the donor molecules with the absorp-
tion spectrum of the acceptor molecules. The excitation
of the electronic plasma resonance increases the absorp-
tion rate. Moreover the molecular emission dipole excites
the plasma resonance which increases the rate of radia-
tive decay. In addition, the non-radiative decay provides
damping effect when latter process increases, the quench-
ing of fluorescence is observed. The metallic surface in-
duces a strong quenching of molecular fluorescence due
to the electromagnetic coupling between the Ag and the
fluorophore [38].
The energy transfer and electron-transfer processes are
the major deactivation processes for quenching of excited
state DMDBMAD fluorophores on the Ag surfaces. Ex-
cited state fluorophore behaves as an oscillating dipole.
When these fluorophores are in close proximity to the
metal Ag NPs, the rate of emission and the spatial dis-
tribution of the radiating energy are modified. The elec-
tric field felt by the DMDBMAD fluorophores are affected
by the interaction of the incident light with the nearby
Ag surface. These interactions can increase/decrease in
the field felt by the DMDBMAD fluorophore and the in-
crease/decrease in the radiative decay rate [3] will result
in many desirable effects such as increased quantum yield
3.6 Stern-Volmer approach
and decreased lifetime. When a metal NPs has been ex- The quenching rate constant has been calculated from
cited and is oscillating in the incident electromagnetic Stern-Volmer plot. The Stern-Volmer equation accounting
field, the excited DMDBMAD fluorophore molecules may for both static and dynamic quenching is generally writ-
have a fluctuating electric dipole moment and causes the ten as ϕ0/ϕ = 1 + KSV [Ag] [3] where KSV = KS + KD
radiation [35]. This radiation from dipole moment of ex- where KS and KD are the static and dynamic quench-
cited fluorophore molecule provides the path for radiative ing constants, respectively, ϕ0 and ϕ are the fluorescence
decay. On the other hand, the joule heating plasmon ab- quantum yield of DMDBMAD in the absence and pres-
sorption caused by these fields provides the path for non- ence of quencher [Ag], respectively. Figure 8 shows the
radiative decay channels [36]. The competition between plot of ϕ0/ϕ vs. silver concentration at constant DMDB-
radiative and non-radiative decay energy transfer affects MAD concentration. The value of the quenching constant
the fluorescence emission of the DMDBMAD fluorophore is found as KSV = 99×103 M−1. The linearity of the Stern-
molecules located near the Ag NPs due to joule heating Volmer plot indicates that only one type of quenching oc-
and surface plasmon absorption.
curs in the system. There is no possibility of formation

Eur. Phys. J. D (2014) 68: 308
Page 7 of 8
of static quenching complexes via attractive electrostatic
interaction between fluorophore-quencher pairs collisional
(dynamic) quenching is effectively possible. In the pres-
ence of Ag NPs there is no change in the peak position in
the absorption spectra of the DMDBMAD, the possibility
of static quenching is absent. The absence a new broad
band indicates that DMDBMAD has been bound to sil-
ver nanoparticles and so has failed to exhibit excimer and
exciplex formation due to intermolecular interactions or
if there is the formation of any trace of the excimer, it
is also quenched by the metal. The Stern-Volmer quench-
ing constant by silver nanoparticles in the solution is high
indicating the presence of adsorption of the DMDBMAD
molecule on the Ag surface. The high quenching efficiency
of the solution results from the larger surface area pro-
vided by the smaller nanoparticles, which enhances their
DMDBMAD adsorption capability.
Fig. 9. Optical absorption spectra of Ag NPs and fluorescence
emission spectrum of DMDBMAD in methanol.
of the fluorescence emission spectrum of the donor and
absorption spectra of the acceptor was given by:
3.7 FRET between acceptor and donor
Fo¨rster resonance energy transfer is a distance dependent
interaction between the different electronic excited states
of dye molecules in which excitation energy is transferred
from one molecule (donor) to another molecule (acceptor)
at the cost of emission from the former molecular system.
According to F¨orster theory, the rate of energy transfer is
based on the spectral overlap between emission spectrum
of the fluorophore and absorption spectra of the Ag NPs,
the relative orientation of the donor and acceptor transi-
tion dipoles, the distance between the donor and acceptor
transition dipoles and the fluorescence quantum yield of
the donor. It results in long range dipole-dipole interac-
tions between the donor and acceptor [3,39].
In order to understand energy transfer, we have
calculated the distance between NPs and DMDBMAD
molecules using the F¨orster mechanism of non-radiative
energy transfer. According to Fo¨rster theory, the energy
transfer efficiency is related not only to the distance be-
tween the acceptor and donor but also to the F¨orster en-
ergy transfer distance, is given by:
∞
ꢂ
F (λ) ε (λ) λ4dλ
0
J (λ) =
(3)
∞
ꢂ
F (λ) dλ
0
where F (λ) is the fluorescence intensity of the fluorescence
donor at wavelength (λ) and ε(λ) is the molar absorptivity
of the acceptor at wavelength (λ).
The overlap of the optical absorption spectra
of Ag NPs and the fluorescence emission spectrum of
DMDBMAD were shown in Figure 9. The small spec-
tral overlap between donor emission and acceptor absorp-
tion in the DMDBMAD/Ag NPs blends indicates that en-
ergy transfer is inefficient, and that a F¨orster-type energy
transfer is unfeasible from the molecules of the DMDB-
MAD to the Ag NPs. Figure 9 also shows that acceptor
weakly absorbed the excitation photon (330 nm), which
may also contribute to the fluorescence intensity of Ag
NPs. Despite the possibility of electron energy transfer,
this mechanism, however is considered significant because
of weak overlap between emission of donor and absorbance
of acceptor. The calculated values of the energy transfer
parameters are presented in Table 1. From the overlapping
spectrum of the absorption spectra of acceptor and the
fluorescence spectrum of DMDBMAD, J (λ) can be eval-
uated by integrating the spectra from 350 to 600 nm. By
adding the donor molecule into acceptor molecule, the flu-
orescence spectra shifts toward shorter wavelength provid-
ing a larger value of spectral overlap integral. The decrease
in J (λ) in turn causes a decrease in FRET efficiency. The
value of J (λ) changes from 2.11 × 10−14 cm3 L mol−1
to 1.25 × 10−14 cm3 L mol−1 with increase in the size
of the Ag NPs from S1 to S4. Accordingly the energy
transfer efficiency (E) between Ag NPs and DMDBMAD
varies from 100% to 92%. It is evident that the efficiency
F
R06
R06 + r06
E = 1 −
=
(1)
F0
where F is the fluorescence intensity of the DMDBMAD
in the presence of the acceptor, F0 is the fluorescence in-
tensity of the DMDBMAD in the absence of the acceptor.
R0 is the Fo¨rster radius or critical distance, is the charac-
teristic distance at a FRET efficiency of 50%, which varies
for different FRET pairs and r0 is the distance between
the acceptor and donor. FRET efficiency is close to maxi-
mum at distances less than R0 and minimum for distances
greater than R0.
R06 = 8.8 × 10−25k2n−4ϕ J (λ)
(2)
where k2 is the spatial orientation factor of the dipole, n of energy transfer is higher in S1 than that in S4. The
is the refractive index of the medium, ϕ is the fluorescence critical distance between Ag NPs and DMDBMAD for S1
quantum yield of the donor and J is the overlap integral is 198 nm and 181 nm for S4, respectively. This high value

Page 8 of 8
Eur. Phys. J. D (2014) 68: 308
of R0 is responsible for r0 and energy efficiency values. The
large values of R0 indicate that the dominant mechanism
responsible for energy transfer is long range dipole-dipole
energy transfer (Fo¨rster-type) [40]. The distance between
Ag NPs and DMDBMAD for S1 is 104 nm and 87 nm
for S4, respectively. Interestingly it was observed that the
FRET efficiency decreases with increasing AgNO3 con-
centration. In present system FRET parameters mainly
depend on size of the Ag NPs.
9. M. Umadevi, N.A. Sridevi, A.S. Sharmila, B.J.M.
Rajkumar, M.B. Mary, P. Vanelle, O. Khoumeri, J.
Fluoresc. 20, 153 (2010)
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Ramakrishnan, J. Fluoresc. 19, 3 (2009)
11. M. Umadevi, S.R. Kavitha, P. Vanelle, T. Terme,
Plasmonics 8, 859 (2013)
12. M. Umadevi, S.R. Kavitha, P. Vanelle, T. Terme, O.
Khoumeri, J. Lumin. 142, 1 (2013)
13. S.R. Kavitha, M. Umadevi, P. Vanelle, T. Terme, O.
Khoumeri, Plasmonics 9, 443 (2014)
14. S.R. Kavitha, M. Umadevi, P. Vanelle, T. Terme, O.
Khoumeri, B. Sridhar, J. Spectrochim. Acta Part A 133,
472 (2014)
4 Conclusion
Silver NPs of different sizes from 9 nm to 17 nm were
synthesized by Creighton method. The optical and mor-
phological properties of the prepared nanoparticles were
characterized by absorption spectroscopy and transmis-
sion microscopy. Fluorescence quenching of DMDBMAD
molecule was analyzed by emission spectroscopy, it shows
that the attachment of DMDBMAD molecules on the sil-
ver surface which leads to quenching of fluorescence. As
the concentration of silver nitrate increases, it affects the
fluorescence intensity of DMDBMAD. The quenching and
association constants were calculated from Stern-Volmer
and Benesi-Hildebrand plots. Ag NPs have dynamic
quenching nature on the fluorescence of DMDBMAD. The
distance between DMDBMAD to Ag NPs and the criti-
cal energy transfer distance were calculated using F¨orster
theory and it depends on size of the Ag NPs. The study
on interaction of DMDBMAD molecules with Ag NPs is
a useful phenomenon and this system could open up the
possible applications in integration of nanoscience with
biology and medicine. Quenching property of the Ag NPs
can be used for textile dyeing waste water treatments.
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We gratefully acknowledge the financial assistance received
from UGC-MRP and DST-CURIE, New Delhi, India for car-
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