Influence of silver nanoparticles on 2,3-Bis(Chloromethyl)anthracene-1,4,9, 10-tetraone

Article abstract of DOI:10.1007/s10895-009-0533-4

Size effect of silver nano particles on the photophysical properties of 2,3-bis(chloromethyl)anthracene-1,4,9,10-tetraone (BCMAT) have been investigated using optical absorption and fluorescence emission techniques. Silver NPs of different sizes have been prepared by two different methods. Quenching of fluorescence of BCMAT has been found to increase with decrease in the size of the silver NPs. Stern-Volmer quenching constants have also been calculated.

Full text of DOI:10.1007/s10895-009-0533-4

J Fluoresc (2010) 20:153–161
DOI 10.1007/s10895-009-0533-4
ORIGINAL PAPER
Influence of Silver Nanoparticles on 2,3-Bis(Chloromethyl)
Anthracene-1,4,9,10-Tetraone
Mahalingam Umadevi & N. A. Sridevi & A. S. Sharmila &
Beulah J. M. Rajkumar & M. Briget Mary & P. Vanelle &
T. Terme & O. Khoumeri
Received: 30 April 2009 /Accepted: 7 August 2009 /Published online: 25 August 2009
#
Springer Science + Business Media, LLC 2009
Abstract Size effect of silver nano particles on the photo-
physical properties of 2,3-bis(chloromethyl)anthracene-
1,4,9,10-tetraone (BCMAT) have been investigated using
optical absorption and fluorescence emission techniques.
Silver NPs of different sizes have been prepared by two
different methods. Quenching of fluorescence of BCMAT
has been found to increase with decrease in the size of the
silver NPs. Stern–Volmer quenching constants have also
been calculated.
activities) and have the wide application for medicines,
pesticides, etc. [2]. They are used principally in photo-
graphic dye chemicals, in medicine, as an antioxidant, in
paints, varnishes, motor fuels, oils, pigments and in organic
inhibitor. They play a vital role in paper industry as a
catalyst to increase the pulp production yield and to
improve the fiber strength through reduction reaction of
cellulose to carboxylic acid [3, 4]. The anthraquinone
derivative rhein is significantly antiseptic and is especially
toxic to the enteric pathogen Shigella dysenteriae as well as
to Staphylococci [5]. Moreover, many anthraquinones have
been investigated for their anticancer potential [6].
Keyword 2,3-bis(chloromethyl)anthracene-1,4,9,
.
.
.
10-tetraone Optical absorption Fluorescence emission
.
Silver NP Fluorescence quantum yield
The integration of nanotechnology with biology and
medicine is expected to produce major advances in
molecular diagnostics, therapeutics and bioengineering
[7, 8]. Nanoscale metal particles have attracted significant
attention because of their unusual size dependent optical
and electronic properties. These nanomaterials have found
potential applications in electronics, nonlinear optics,
chemical and biochemical sensors, and in catalysis [9].
Due to surface plasmon excitation, NPs of silver, gold and
copper may exhibit sharp electronic absorption bands in the
visible region [10]. The size, shape and structure of NPs
define their properties. Further the development of new
procedures for the synthesis of nanomaterials with a well
defined morphology is a very important research target
[11]. The fluorescence of fluorophore might be enhanced or
quenched due to the presence of nearby metallic NPs. The
strength of the enhancement/quenching is influenced by
many factors such as size and shape of the metal NPs, the
orientation of the fluorophore dipole moments relative to
the NPs, the radiative decay rate and quantum yield of the
fluorophore. Lakowicz et al. reported that the quenching is
usually observed if the fluorescence is located at a very
Introduction
Anthraquinone is the most important quinone derivative of
anthracene. Plants containing anthraquinones have been
used as dyestuffs and purgatives [1]. Anthraquinone
derivatives have various biochemical characteristics (bio-
:
:
M. Umadevi (*) N. A. Sridevi A. S. Sharmila
Department of Physics, Mother Teresa Women’s University,
Kodaikanal 624 102 Tamil Nadu, India
e-mail: ums10@yahoo.com
:
B. J. M. Rajkumar M. B. Mary
Lady Doak College,
Madurai, Tamil Nadu, India
:
:
P. Vanelle T. Terme O. Khoumeri
Laboratory of Radical Pharmaco-Chemistry, Faculty of Pharmacy,
UMR CNRS 6264, University of the Méditerranée,
27 Bd Jean Moulin,
13385 Marseille Cedex 5, France

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J Fluoresc (2010) 20:153–161
short distance (<5 nm) from the metal surface. When the
fluorophore-metal distance is increased, both fluorescence
quenching and enhancement have been observed [12].
Assembling fluorophore-metal NPs superstructures as a
two or three dimensional architecture provides routes to
design novel materials with tailored electrical, optical,
lithographic, sensing and photochemical properties [13,
14]. Resonant energy transfer systems consisting of organic
dye molecules and noble metal NPs have recently gained
considerable interest in biophotonics as well as in materials
science [15].
The biological importance of BCMAT molecule and
silver NPs has prompted to the study the photo physical
properties and the influence of size effect of silver NPs on
this molecule. Interaction of a dye with the medium at the
molecular level is reflected in its visible and fluorescence
spectra. Fluorescence quenching is a technique to under-
stand the interaction within the medium in view of the
special role of surfaces of the nanoclusters in guiding and
modifying physicochemical processes.
Procedure
The silver NPs used in this study were synthesized by two
different methods.
In the first method silver sol were prepared according to
the following method. In brief 4 ml of silver nitrate solution
(0.6 mM) was added drop wise to 25 ml of sodium
borohydride solution (1.2 mM) with vigorous stirring. It
was repeated for different volume of sodium borohydride
(8, 12 and 16 ml) at constant volume of silver nitrate
solution (25 ml). Both the solutions were chilled to ice
temperature.
In the second method, the reduction of silver nitrate in
boiling solution by addition of sodium citrate was
employed. This method simply consists of addition of
1 ml of 1% sodium citrate solution to a 50 ml of boiling
solution of silver nitrate and keeping it boiling for different
times 90, 110, 130, 150 and 170 min.
The concentration of BCMAT in methanol was
0.01 mM. For the effect of silver NPs on BCMAT, BCMAT
in methanol and silver sol have been taken in 1:1 volume
ratio.
Recently our group has studied the effect of Ag NPs on
1,4-dihydroxy-2,3-dichloromethyl-9,10-anthraquinone [16].
C.D. Geddes et al. described the effect of Ag NPs on near
by fluorophores [17, 18]. In the present study, optical
absorption and fluorescence emission spectroscopic techni-
ques have been employed to investigate the effect of silver
NPs on 2,3-bis(chloromethyl)anthracene-1,4,9,10-tetraone
(BCMAT) (Fig. 1).
Fluorescence quantum yield
The relative fluorescence quantum yield (frel) [20] of the
sample in terms of the reference sample (fo) is
ꢀ ꢁ
F
ꢀ ꢁ
2f where F and Fo
À
Á
ODo
n
are the integrated
frel
¼
o
Fo
fluorescenceOiDntennsoities, OD and ODo are the optical densities
and n and no are the refractive indexes for BCMAT in
methanol and for 1,4-dihydroxy-2,3-dimethyl-9,10-anthraqui-
none (fo=0.0283) [21] in methanol respectively. In the
present case the fluorescence quantum yield of BCMAT in
methanol was found to be 0.0039.
Experimental
Reagents
Spectral grade methanol was obtained from SISCO labora-
tory and was used without further purification. Silver nitrate
and sodium borohydride, purchased from Aldrich, were
used as received. 2,3-bis(chloromethyl)anthracene-1,4,9,10-
tetraone (BCMAT) was prepared in an original four step
synthesis procedure from previously studied 1,4-dihydroxy-
2,3-dimethylanthracene-9,10-dione. The 2,3-bis(chloro-
methyl)anthracene-1,4,9,10-tetraone was prepared in a
four-step synthesis procedure from previously studied 1,4-
dihydroxy-2,3-dimethylanthracene-9,10-dione [16]. After
the oxidation of this last compound using lead acetate in
toluene [19], the 2,3-dimethylanthracene-1,4,9,10-tetraone
was brominated to give the unexpected 2,3-bis(bromo-
methyl)-1,4-dihydroxyanthracene-9,10-dione. An halogen
exchange reaction using lithium chloride in THF, followed
by another oxidation step with lead acetate in toluene [19]
were necessary to form the desired 2,3-bis(chloromethyl)
anthracene-1,4,9,10-tetraone. Doubly distilled water was
used throughout.
Apparatus
Optical absorption spectra were recorded using a
(Shimadzu UV-1700 pharmaspec) UV-Visible spectropho-
tometer. Elico Spectrofluorimeter SL174 was used to record
fluorescence spectra. For fluorescence measurements the
excitation wavelength was 500 nm. The spectra were
recorded at room temperature.
Results and discussion
a) Silver nanoparticle size determination
Gustav Mie was the first to provide an explanation on
the dependence of color on the metal particle size [22].
Surface plasmon resonance can be thought of as the
collective oscillation of the conduction band electrons in

J Fluoresc (2010) 20:153–161
155
Fig. 1 In-plane (a) and out-of-
plane (b) structure of BCMAT
(a) in-plane
(b) out-of-plane
the metals. This is due to the small size of the particle and
surface property and is not exhibited by individual atoms or
bulk materials. When the size of the particle is less than one
tenth of the wavelength of the incident light, the electric
fields can be assumed as spatially constant and only the
temporal variation is taken into account. This state is called
the quasi-static regime. Conduction bands electrons of
alkali and noble metals interact strongly with the visible
range of the electromagnetic waves. The incoming electric
field of the electromagnetic wave induces a polarization of
the electron with respect to the heavy ionic nucleus. The net
charge difference is felt only at the surface (surface as
defined by depth of penetration of the electromagnetic
wave), which, it compensates with a restoring force in the
form of a dipolar oscillation of the surface electrons in the
same phase [23]. However when the size of the particle is
comparable to their skin depth, all the electrons in the
particle resonates, resulting in strong absorption of
the particular wavelength. Since skin depth is dependent
on the wavelength of the incident wave, particles of
different sizes resonate at different wavelengths. This gives
rise to different colors of silver colloid. The color of the
colloid depends not just on the particle size, but also on
the shape, the refractive index of the surrounding media and
the separation between the particles. A change in any of
these parameters will result in the quantifiable shift in the
surface plasmon resonance absorption peak [24].
The optical properties of dispersions of spherical
particles with a radius R can be predicted by Mie theory
[22], through expression for the extinction cross section
C
ext. For very small particles with a frequency dependent,
complex dielectric function, " ¼ "0 þ i"00, embedded in a
medium of dielectric constant εm this can be expressed as
3
24p2R3"m
"
=
00
2
Cext
¼
ð1Þ
2
ð"0 þ 2"mÞ þ "002
l
The origin of the strong color changes displayed by
small particles lies in the denominator of equation which
predicts the existence of an absorption peak when
"0 ¼ ꢀ2"m
ð2Þ
In a small metal particle, the dipole created by the
electric field of light induces a surface polarization charge
which effectively acts as a restoring force for the free
electrons. The net result is that, when condition (2) is
fulfilled, the long wavelength absorption by the bulk metal

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J Fluoresc (2010) 20:153–161
is condensed into a small surface plasmon band. Using this
Mie theory the radius of the silver NPs have been
calculated (Table 1).
the surface plasmon band was evident as these molecules
complexed with the silver surface. The damping of the
silver plasmon band indicates the attachment the BCMAT
molecule on the particle surface. The interaction of silver
nano particles with BCMAT alters the electron density of
the silver NPs, thereby directly affecting the absorption of
the surface bound BCMAT as well as surface plasmon
absorption band.
Using these spectral shifts we can measure the binding
constants of BCMAT with silver NPs in the ground state.
The association constant for the complexation between
silver NPs and BCMAT were obtained by analyzing the
absorption changes using Benesi–Hildebrand approach.
The association constant of Kass ¼ 8:86 ꢁ 103 Mꢀ1 in
silver NPs prepared by first method, was obtained from
the plot of [BCMAT]/ΔA500 vs 1/[Ag] (Fig. 7) where
ΔA500 is the changes in absorbance of BCMAT with and
without silver NPs at 500 nm and [Ag] is the concentration
of silver. The high value of Kass observed in these
experiments suggests a strong association between the
silver colloid and BCMAT.
Figures 4, 8 and 9 depict the fluorescence emission
spectrum of BCMAT in methanol and the fluorescence
emission spectra of BCMAT in different sizes of silver NPs
prepared by two different methods respectively. The
observed fluorescence quantum yield of BCMAT in
different sizes of NPs has been reported in Table 1.
Fluorescence emission spectrum of BCMAT in methanol
shows a broad band in the visible region with vibrational
features. It shows two peaks at 588 and 683 nm and two
shoulders around 553 and 639 nm. The absence of new
broad band in the fluorescence emission spectra reveals that
the BCMAT bound to silver NPS failed to exhibit
intermolecular interaction such as excimer/exciplex forma-
tion or if there is the formation of any trace of the excimer,
it is also quenched by the metal.
In the present case the absorption spectrum has a
maximum in the range 390–450 nm, which is related to
the plasmon band of the formed nanosized silver particles
(Figs. 2 and 3). This absorption band results from
interactions of free electrons confined to small metallic
spherical objects with incident electromagnetic radiation.
Electronic modes in silver NPs are particularly sensitive to
their shape and size, leading to pronounced effects in the
visible part of the spectrum. The observed plasmon band
shows that the silver NPs are spherical in shape [25].
b) Optical absorption and fluorescence emission studies
Quenching of fluorescence
Figures 4, 5 and 6 show the absorption spectrum of
BCMAT in methanol and absorption spectra of BCMAT
in different sizes of silver NPs respectively. Absorption
spectrum of BCMAT in methanol shows a broad band in
the visible region from 400 to 600 nm. It also indicates
vibrational features with peals at 471, 488, 529 and 580 nm.
The spacing between the peaks are 737, 1,575 and
1,659 cm−1 respectively.
The surface plasmon of silver sol shows a maximum in
the range 390–450 nm for different sizes of silver NPs.
When BCMAT is added to the silver sol, the plasmon band
of silver shifts towards higher wavelength except for 8:25
(volume of AgNO3:NaBH4) and 110 m (heating time)
shifts to shorter wavelength and its absorbance value
decreases. In all the cases the vibrational features of
BCMAT were disappeared and the feature of surface
plasmon peak dominates. Dampening and broadening of
Table 1 Photophysical properties of BCMAT in silver NPs
First Method
S. No.
Volume of aqueous (ml) AgNO3:NaBH4
Particle size (diameter) (nm)
Quantum yield (ϕrel)×10−4
5.51
1
4:25
36
27
22
22
2
8:25
3.74
2.96
1.70
3
12:25
16:25
4
Second Method
S. No.
Heating time (minutes)
Particle size (diameter) (nm)
Quantum yield (ϕrel)×10−4
1
2
3
4
5
90
68.4
9.80
1.58
8.51
2.22
1.47
110
130
150
170
69.2
70.4
71.2
72

J Fluoresc (2010) 20:153–161
157
40
0.08
0.07
0.06
0.05
0.04
0.03
0.02
4ml
0.85
0.75
0.65
0.55
0.45
0.35
0.25
0.15
0.05
8ml
35
30
25
20
15
10
5
12ml
16ml
0
400
450
500
550
600
650
700
Wavelength / nm
Fig. 4 Optical absorption and fluorescence emission spectra of
BCMAT in methanol
325
375
425
475
525
575
adsorbed on the metal surface which break the intermolec-
ular hydrogen bond. This strong intermolecular hydrogen
bond mask the vibrational features of BCMAT which was
absent in silver NP environment and leads to the observed
features.
Excited state fluorophore behaves as an oscillating
dipole, when these fluorophores are in close proximity to
the metal surface, the rate of emission and the spatial
distribution of the radiating energy will be modified. The
electric field felt by the fluorophores are affect by the
Wavelength / nm
Fig. 2 Optical absorption spectra of silver NPs prepared by first
method (4 ml–4 ml of NaBH4 in 25 ml of AgNO3)
When silver NPs are added to the BCMAT, the
vibrational features of BCMAT are well resolved and the
intensity was reduced. In the case of BCMAT in methanol,
a strong intermolecular hydrogen bond is formed between
OH group of methanol and C=O groups of BCMAT. But in
silver NP environment the C=O groups of BCMAT
0.6
1.85
4ml+BCMAT
90min.
8ml+BCMAT
110min.
0.55
1.65
12ml+BCMAT
130min.
16ml+BCMAT
150min.
0.5
1.45
1.25
1.05
0.85
0.65
0.45
0.25
0.05
170min.
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
325
375
425
475
525
575
350
400
450
500
550
600
Wavelength / nm
Wavelength / nm
Fig. 3 Optical absorption spectra of silver NPs prepared by second
method (90 min. (90 min)—heating time)
Fig. 5 Optical absorption spectra of BCMAT in silver NPs prepared
by first method

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J Fluoresc (2010) 20:153–161
4ml+BCMAT
8ml+BCMAT
12ml+BCMAT
16ml+BCMAT
90min.+BCMAT
110min.+BCMAT
130min.+BCMAT
150min.+BCMAT
170min.+BCMAT
0.85
0.75
0.65
0.55
0.45
0.35
0.25
0.15
0.05
20
15
10
5
0
520
570
620
670
720
Wavelength / nm
Fig. 8 Fluorescence emission spectra of BCMAT in silver NPs
prepared by first method
350
400
450
500
550
600
Wavelength / nm
In the present case quenching of fluorescence was
observed for particles of different size. BCMAT molecules
are attached to silver NPs via carbonyl groups. BCMAT is a
non planar molecule that belongs to C1 point group
symmetry. The radii of NPs vary between 11 and 36 nm.
Fig. 6 Optical absorption spectra of BCMAT in silver NPs prepared
by second method
interaction of the incident light with the nearby metal
surface. These interactions can increase/decrease the field
felt by the fluorophore and the increase/decrease the
radiative decay rate [20] resulting in many desirable effects
such as increased quantum yield and decreased lifetime.
The radiative and non radiative rates depend on the size and
shape of the NPs, the distance between the dye and the NPs
the orientation of the molecular dipole with respect to the
dye-NP axis and the overlap between the molecular
emission spectrum with the NPs absorption spectrum.
90min.+BCMAT
30
110min.+BCMAT
130min.+BCMAT
150min.+BCMAT
170min.+BCMAT
25
20
15
10
5
0.12
0.08
0.04
0
0
520
570
620
670
720
3
5
7
9
11
13
Wavelength / nm
1/[Ag] x 103 M-1
Fig. 9 Fluorescence emission spectra of BCMAT in silver NPs
Fig. 7 Variation of [BCMAT]/ΔA500 versus 1/[Ag]
prepared by second method

J Fluoresc (2010) 20:153–161
159
The dipole moment of the π conjugated part of the
molecule is oriented parallel to the silver NP axis and
situated approximately 0.12 nm from the particles
surface. The dipole moment of the C=O is oriented
perpendicular to the silver NP axis (Scheme 1).
The energy transfer rate from NPs to the dye is governed
by three factors, the Coulombic overlap integral, the
position (surface plasmon frequency) and width (inverse
surface plasmon lifetime) of the absorption spectrum of the
NPs relative to those of dye. The size dependence of the
energy transfer rate at large distanced for large NPs is
determined entirely by the Coulombic overlap integral
which in turn proportional to the quantum unit of dipole
moment.
In the case of energy transfer from fluorophor to NPs,
initially donor state of the system consists of an excited dye
molecule and a NP in its ground state. The non radiative
energy transfer from the dye to NP involves simultaneous
de-excitation of molecule and collective excitation of
compressible electron gas in the NP. The final acceptor
state of the system consists of ground state dye molecule
and a NP in its excited state. The rate of excitation energy
transfer depends on the strength of Coulombic interactions
and the spectral overlap between the emission spectrum of
dye and absorption spectrum of NPs.
In the present case the overlap is between tail end of the
absorption spectrum of NP and raising end of fluorescence
emission spectrum of BCMAT and which is negligible.
Therefore the rate of excitation energy transfer depends on
spectral overlap is very weak. In this case the rate of
excitation energy transfer depends mainly on the strength of
Coulombic interactions between BCMAT and silver nano
particles.
The influence of Coulombic interactions on the energy
transfer has been studied in two different ways. First, the
full Coulombic interactions between dye and NPs depends
on the charge densities of dye and NPs. Second, the
interaction within the dipole approximation, where the
Coulombic interaction is approximated by the dipole
interactions among the charge distribution of the dye and
NPs. This dipole interaction mainly depends on the dipole
moments of the dye and NPs and the distance between the
dye and the NPs. The higher charge densities of BCMAT
and the dipole moments of BCMAT and silver NPs are
responsible for the energy transfer between BCMAT and
silver NPs which leads to lower quantum yield.
Due to the electromagnetic coupling between the metal
and the fluorophore, the metallic surface induces a strong
quenching of molecular fluorescence. Weitz et al. [26]
proposed a model to explain the NPs induced fluorescence
quenching. The excitation of the electronic plasma reso-
nance increases the absorption rate. Again, the molecular
emission dipole excites the plasma resonance which
increases the rate of radiative decay. In addition, the non-
radiative decay provides damping effect when latter process
increases, the quenching of fluorescence is observed. In the
present case the excitation wavelength is 500 nm which
does not coincide with the plasmon resonance peak of
silver NPs and the silver NPs do not exhibit any
fluorescence. Therefore the change in BCMAT fluorescence
mediated by the electric near field due to silver NPs
plasmon resonance is weak.
Dipole
moment
derivative unit
vector of π
conjugated
system
0.28nm
0.12nm
Dipole moment derivative
unit vector of C=O system
22nm
Scheme Orientation of BCMAT on silver NPs
Scheme 1 Orientation of BCMAT on silver NPs

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J Fluoresc (2010) 20:153–161
Ditlbacher et al. [27] explained the changes of molecular
of the NPs driven by the dipole field of the adjacent NPs
and the dipole of the NPs driven by the dipole field of the
molecule. The resonant energy transfer of the molecular
excitation is treated by calculating the absorption of the
molecular dipole field at the position of the metal NPs. The
observed decreased emission rate (lower quantum yield) is
due to the phase shift between the BCMAT and the silver
NP dipole leading to a destructive interference effect. This
phase shift is caused by the complex nature of the silver
dielectrics.
Both electron transfer and energy transfer processes are
considered to be the deactivation pathways for excited
fluorophores on the metal surface. The electron transfer
mechanism is predominant for particle sizes of <5 nm as
the particles do not exhibit any surface plasmon band in the
visible region [30, 31]. In the present case silver NPs are
larger than 5 nm, energy transfer dominates the quenching
mechanism.
fluorescence near a silver NP. When using fluorophore as
local probes of the surface plasmon field of metallic NPs, in
the close proximity of a metal, the fluorescence rate of the
molecules is a function of the distance between the probe
molecule and the metal surface. When in direct contact with
the metal, the fluorescence of molecules is completely
quenched.
The distance of closest approach of the probe molecules
to the nano particles will change with the particle size. Thus
the origin of the NP size dependence on the quenching
process related to the absorption spectrum of the electronic
states of the silver NPs. Due to the chemisorption of the
BCMAT with the silver NP surface, the molecular orbitals
of the BCMAT molecules are mixed with the metallic band
states. Certain orbitals of BCMAT interact strongly with the
silver NPs and are responsible for the chemisorption bond,
while others are little affected by adsorption [28].
The size regime dependence of the silver NPs on the
fluorescence quenching process can be explained by the
structures of the NPs in terms of the size and surface
chemistry. By changing the size of the NP using the same
fluorophore, the interplay of size and surface effect on the
structure of NPs can be realized.
The medium/adsorbed molecules (electron donor or
acceptor) can change markedly the effective concentration
of electrons in the particle, resulting in a shift in the
resonance and change in the width. Apart from this static
dynamic effects related to the transfer of electrons from the
Fermi surface to the adjacent levels of adsorbed molecules
or the environment is also possible.
The surface plasmon efficiently acts as energy
acceptor even at a distance of 1 nm between the probe
molecules and the metal surface [32]. Metal NPs also
have a continuum of electronic states and exhibit energy
transfer behavior as excited state quenchers [28]. The
observed changes in absorbance of the absorption spec-
trum and fluorescence intensity reflects the alteration of
the electronic properties of the BCMAT chromophore as it
binds to the silver NPs which act as a excited state
quencher. The BCMAT has a high emission quantum yield
in the absence of NPs, the dominant effect for the
quenching of excited state of BCMAT in silver NPs may
The adsorption of the probe molecules on the NP surface
is a function of the chemical composition, structure of the
metal particle and nature of the solution. For a fixed set of
solution parameters, the adsorption process is only influ-
enced by the particle size. As the size of the particles
decreases, there is an increasing contribution of the surface
atoms. The increased surface area to volume ratio with
progressive decrease in particle size can accommodate large
number of fluorophore around silver NPs. Therefore small
particles become efficient quenchers of molecular fluores-
cence than that of the larger ones. Silver NPs prepared by
the first method shows that the fluorescence quantum yield
decreases as the particle size increases. Since the small NPs
adopt large number of BCMAT on the surface which leads
to quenching of fluorescence.
When a donor molecule is placed in the vicinity of
the conducting metal surface, resonant energy transfer
takes place between the donor and acceptor. The
probability of this Forster energy transfer depends on
the overlap of the emission spectrum of the fluorophore
with the absorption spectrum of the NPs. In the present
case, this spectral overlap is negligible which leads to
decrease in Forster radius and hence the rate of Forster
energy transfers.
In the Gersten–Nitzan model [29] the radiative rate is
derived by assuming Hertzian dipole emitters. Here the
dipole for the entire system is considered by taking into
account the intrinsic molecular dipole, the molecular dipole
induced by the dipole field of the adjacent NPs, the dipole
25
20
15
ϕ
10
5
0
0.05
0.1
0.15
0.2
0.25
[Ag]/ mM
Fig. 10 Variation of 8o/8 versus [Ag]

J Fluoresc (2010) 20:153–161
161
be due to radiative energy transfer to the metal surface.
Fluorescence quenching may also result from a photo
induced electron transfer process between the excited
BCMAT and the silver NPs. The interaction with the dye
and excited state surface reactions may lead to morpho-
logical changes of the silver NPs.
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The Stern–Volmer equation accounting for both static
and dynamic quenching is generally written as
ϕo=ϕ ¼ 1 þ Ksv½Agꢂ [20] where Ksv ¼ Ks þ KD where Ks
and KD are the static and dynamic quenching constants
respectively, 8o and 8 are the fluorescence quantum yield of
BCMAT in the absence and presence of quencher [Ag],
respectively. There is no possibility of formation of static
quenching complexes via attractive electrostatic interaction
between fluorophore-quencher pairs collisional quenching
is effectively possible. Figure 10 shows the plot of 8o/8 vs
silver concentration at constant BCMAT cꢀo1ncentration. The
plot is linear with Ksv ¼ 9:65 ꢁ 104 M in silver NPs
prepared by first method. The linearity of the Stern–Volmer
plot indicates that only one type of quenching occurs in the
system. In the presence of quencher the absorption
spectrum of BCMAT remains unaltered in frequency. This
indicates that static quenching does not occur. The Stern–
Volmer quenching constant by silver NPs in the solution is
very high indicating the presence of adsorption of the
BCMAT on the quencher surface. The high quenching
efficiency of the solution results from the larger surface
area provided by the smaller NPs, which enhances their
BCMAT adsorption capability.
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Conclusion
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Optical absorption and fluorescence emission techniques
have been employed to study the photophysical properties
of BCMAT on silver NPs. The process of fluorescence
quenching of BCMAT by silver NPs indicate that the
collisional quenching occurs and the quenching depends on
the particle size of the silver NPs. The BCMAT molecules
were adsorbed on the surface of the silver NP which leads
to quenching of fluorescence.