Dye fluorescence quenching by newly synthesized silver nanoparticles

Article abstract of DOI:10.1016/j.jphotochem.2013.11.005

New types of silver nanoparticles (Ag NPs) were prepared by the reduction of silver trifluoroacetate with diamine-sarcophagine (DiAmsar) in DMF solution. Nanoparticles growth, morphology, and chemical composition were determined by ultraviolet-visible spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy, respectively. The effect of prepared Ag NPs on the fluorescence quenching of the dyes rhodamine 6G and 4-hydroxycoumarin were investigated in different temperatures. The results of the quenching experiments were analyzed using Stern-Volmer equation. The Stern-Volmer constant values showed that the quenching efficiencies decreased as the temperature increases indicating that the quenching process was a dynamic-type quenching. Moreover, the energy transfer behavior between Ag NPs and dye molecules were interrogated using F?rster/fluorescence resonance energy transfer and nanomaterial surface energy transfer models. The anti-bacterial activities of Ag NPs were assessed against the Staphylococcus aureus and Escherichia coli bacteria. Results showed that the entitled Ag NPs have excellent antibacterial properties.

Full text of DOI:10.1016/j.jphotochem.2013.11.005

Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Dye fluorescence quenching by newly synthesized silver nanoparticles
∗
Ghasem Rezanejade Bardajee , Zari Hooshyar, Mohaddeseh Khanjari
Department of Chemistry, Payame Noor University, PO Box 19395-3697, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 August 2013
Received in revised form 22 October 2013
Accepted 13 November 2013
Available online 21 November 2013
New types of silver nanoparticles (Ag NPs) were prepared by the reduction of silver trifluoroacetate
with diamine-sarcophagine (DiAmsar) in DMF solution. Nanoparticles growth, morphology, and chemi-
cal composition were determined by ultraviolet–visible spectroscopy, transmission electron microscopy,
and Fourier transform infrared spectroscopy, respectively. The effect of prepared Ag NPs on the fluo-
rescence quenching of the dyes rhodamine 6G and 4-hydroxycoumarin were investigated in different
temperatures. The results of the quenching experiments were analyzed using Stern–Volmer equation.
The Stern–Volmer constant values showed that the quenching efficiencies decreased as the temperature
increases indicating that the quenching process was a dynamic-type quenching. Moreover, the energy
transfer behavior between Ag NPs and dye molecules were interrogated using Förster/fluorescence res-
onance energy transfer and nanomaterial surface energy transfer models. The anti-bacterial activities of
Ag NPs were assessed against the Staphylococcus aureus and Escherichia coli bacteria. Results showed that
the entitled Ag NPs have excellent antibacterial properties.
Keywords:
Fluorescence quenching
Dye
Silver nanoparticle
Energy transfer
DiAmsar.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
in interaction between a dye and a metal NP, metal NP as a universal
energy acceptor can interact with dye as an energy donor, leading
to energy transfer process in a way similar to that occurs dur-
ing Förster/fluorescence resonance energy transfer (FRET) [11,12].
However, several theoretical and experimental studies have been
explained the energy transfer between a dye molecule and a metal
NP to be more appropriate by nanomaterial surface energy trans-
fer (NSET) [13,14]. The main differences between FRET and NSET
mechanisms are in the physical bases and the distance between
the NP and the dye molecule. In comparison to the dipole–dipole
interaction in FRET, the metal–dye interaction has been considered
as the interaction between a dipole and a metal surface in NSET.
Fluorescence quenching is one of the most commonly power-
ful techniques used in chemical, biological and medical sciences
because of its useful information on the partition and mobil-
ity of small molecules in the micro phases of the organized
systems and also on the properties (micropolarity and micro-
viscosity) of the systems in the surroundings of the probe and
the quencher [1–6]. It refers to any process which fluorescence
intensity of the solute decreases by variety of molecular interac-
tions such as excited state reactions, molecular rearrangements,
energy transfer, ground-state complex formation (static quench-
ing) or collisional interactions (dynamic quenching). In this regard,
fluorescence quenching depends on the nature of fluorophore,
quencher molecule, polarity of the solvent medium, and range of
quencher concentration.
Nowadays, with the development of nanotechnology, fluores-
cence quenching of laser dyes (e.g. coumarins and rhodamines) by
noble metal nanoparticles (NPs) have attracted great attention in
biophotonics as well as in material science [7–10]. In the close prox-
imity of metal NPs, it has been reported that the quenching of the
fluorescence intensity of dye can be attributed to the reduction in
ratio of the radiative to nonradiative decay rate and the quantum
yield of the dye due to the dipole energy around the NPs. In general,
−
6
The distance in FRET mechanism follows R (<100 A˚ ), whereas in
−
4
NSET mechanism follows R (∼220 A˚ ). Thus, these mechanisms
can act as a spectroscopic ruler around the NPs. The efficiency of
these mechanisms strongly depends on the several factors such as
the spectral overlap between the spectrum of dye emission and
the absorption spectrum of the NP, the desirable orientation of the
dye and NP’ dipoles, size and shape of the NPs. Therefore, the syn-
thesis of NPs and investigation on their interaction with dyes are
of great importance for both fundamental research and technical
applications [8,10].
Among noble metal NPs, silver (Ag) NPs are extensively used
to alter the emission behavior of a dye molecule because their
plasmon resonance bands are located in the visible range of
the spectrum and match the absorption and emission bands of
dyes [15–17,11]. Moreover, their high compatibility and stability
with biomoloecules, strong affinity for thiols, antibacterial activ-
ity toward germs without release of toxic biocides and exceptional
∗ Corresponding author. Tel.: +98 281 3336366; fax: +98 281 3344081.
E-mail addresses: rezanejad@pnu.ac.ir, grezanejad@ymail.com
(
G. Rezanejade Bardajee).
1
010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.11.005

1
14
G. Rezanejade Bardajee et al. / Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
(Rh 6G) and 4-hydroxycoumarin (4HC) were investigated. The
HN
HN
observed results and the probable explanations are detailed in this
paper.
NH
NH
R 1
R 2
2. Experimental
NH
HN
2.1. 2.1. Materials
1
2
R
R
Name
Sar
Double-distilled water was used when necessary. Silver triflu-
H
H
oroacetate, acetonitrile, N,N-dimethylformamide (DMF), tetrahy-
drofuran (THF), ethanol, tri (ethylenediamine) cobalt (III) chloride
dehydrate, formaldehyde, sodium hydroxide, hydrochloric acid,
methanol, stannous chloride dihydrate, hydrochloric acid, sodium
hydroxide, cobalt (II) chloride, sodium cyanide, rhodamine 6G (Rh
6G), 4-hydroxycoumarin (4HC), and acetonitrile were obtained
from Acros, Merck and Sigma–Aldrich Chemicals and used without
further purification.
NH2
NH2
DiAmsar
NH2
NH2
SarAr
NitrobenzylDiAmsar
Bis(nitrobenzylDiAmsar)
2.2. Instrumentation
Fig. 1. Molecular structure of some sarcophagine derivatives.
TEM observation was performed with a Hitachi H-700 CTEM.
FT-IR spectra were recorded on KBr pellets by a Jasco 4200 FT-
IR spectrophotometer. UV–vis spectra of samples were collected
by a Shimadzu UV–visible 1650 PC spectrophotometer from 200
to 800 nm with 1.00 cm path length quartz cuvettes. Fluorescence
measurements were performed on a SCINCO’s fluorescence spec-
trometer FluoroMate FS-2. 1H and C NMR spectra were recorded
on a Varian Mercury 400 spectrometer and a Bruker Avance III
spectrometer, respectively with deuterium oxide as solvent. Chem-
ical shifts reported are referenced against the deuterated solvent
used.
physico-chemical properties offer excellent prospects for biolog-
ical sensing and various medical applications [18,19]. The most
popular approach for the preparation of Ag NPs has been chemical
reduction using variety of organic and inorganic reducing agents,
electrochemical techniques, physicochemical reduction, and radi-
olysis [20–26]. In these methods, to control the size of Ag NPs,
stabilizers such as alkylthiols, alkylamines, fatty acids, carbon disul-
fide, and polymers have been generally required to prepare Ag NPs
with appropriate sizes. However, most of these methods have dif-
ficulty to apply to large-scale synthesis, owing to its highly diluted
and exothermic condition. Moreover, the derived Ag NPs with the
above methods are hard to be dissolved in organic solvents, which
limit their applications.
13
2.3. Preparation of DiAmsar
Multi-step processes are needed for the synthesis of DiAmsar
which are outlined as followed (shown in Scheme 1a–c) [27,29]:
To overcome these problems, the choice of suitable stabilizer is
a key step for synthesis of Ag NPs. It is thought that the cage-type
ligands derived from 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane
3+
(A) [Co-(NO ) -sar] ,Cl : the complex of tri (ethylenediamine)
2
2
3
(
commonly called sarcophagines, Fig. 1) can be a good choice to this
cobalt (III) chloride dihydrate (1 eqiv., 2 g) was dissolved in
a mixture of water (3 mL) and aqueous formaldehyde 37%
(10.4 eqiv., 4.5 mL). Then, nitromethane (3.45 eqiv., 1.06 mL)
was added to the solution. The resulting solution was cooled
aim. Main advantage of these cage-like frameworks is the preser-
vation of chromophore and redox characteristics of the metal in a
range of chemical environments [27,28]. This cage was first syn-
thesized by Sargeson and co-workers in the 1970 using the cobalt
◦
at 4 C in an ice-water bath. Aqueous sodium hydroxide (4
◦
(
III) ion as a template to hold the reacting organic fragments [29].
M, 3.45 eqiv., 5 mL) was cooled to 4 C and mixed with the
After that, considerable efforts have been made to encapsulate tran-
sition metal ions into sarcophagines. However, the obtained Co
resulting solution above. The combined solution was stirred
magnetically for 90 min in ice-water bath while maintain-
ing the temperature at 4 C. The mixture rapidly turned deep
◦
(
III) complexes are extremely stable both kinetically and thermo-
dynamically. Therefore, the metal ion may be effectively removed
with either cyanide or concentrated HBr. After the isolation of the
free ligand, other metal ions including Cu(II), Ni(II), Co(II), Fe(II),
Mn(II), Mg(II), Zn(II), Cd(II), Hg(II), Cr(III), Ga(III), In(III), V(II), Ag(II),
Re(I), Ru(II), Rh(II), Ir(III) and Pt(II) have been encapsulated within
the sarcophagine cage [30–32]. The transition metal complexes of
sarcophagines are expected to find various specific applications in
molecular sensors, electron transfer agents, and radiopharmaceu-
ticals [33,34].
brown from the initially orange color. After 90 min, the reac-
tion temperature was allowed to come to room temperature.
At the end, the reaction was quenched by the addition of
concentrated hydrochloric acid (HCl 37%, 60 mmol, 5 mL). The
orange precipitate was collected by filtration after cooling on
ice for 90 min, and washed with methanol. Then, the orange
1
powder was dried at room temperature. ( H NMR (400 MHz,
D O) ı (ppm): 2.94–2.96 (d, J = 8 Hz, 6H, NH CH2 CH2 NH ),
2
3.36–3.40 (d, J = 11 Hz, 6H, NH CH2
C NO ), 3.58–3.60 (d,
2
In this work, for the first time, we use diamine-sarcophagine
DiAmsar) for the synthesis of Ag NPs at room temperature in
J = 8 Hz, 6H, NH CH CH2 NH ), 3.89–3.93 (d, J = 11 Hz, 6H,
NH CH2 C NO ). C NMR (400 MHz, D O) ı (ppm): 51.28,
2 2
2
1
3
(
organic medium. DiAmsar was chosen as a reducing agent as
well as capping agent because of the presence of six nitrogen
atom donors into its cage-like framework. The resulting Ag NPs
were characterized by ultraviolet–visible (UV–vis) spectroscopy,
transmission electron microscopy (TEM), and Fourier transform
infrared spectroscopy (FT-IR). Furthermore, the effects of Ag NPs on
the bacterial activity of Staphylococcus aureus and Escherichia coli
were examined. Also, fluorescence intensity of rhodamine 6G
54.62, 87.53. HRMS (EI, m/z) calcd. for C14H31CoN O 431.3565
8
4
+
[M−3 HCl] , Found 431.2689).
5+
(B) [Co-(NH ) -sar] ,Cl5:The
stannous chloride dihydrate
3
2
(13 eqiv., 3 g) was introduced into a round bottom flask
under nitrogen atmosphere. Then, concentrated hydrochlo-
ric acid (37%, 145 eqiv., 12 mL) was added followed by the
addition of ethanol (6 mL). The resulting solution was stirred
◦
magnetically and heated to 70 C under nitrogen until the

G. Rezanejade Bardajee et al. / Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
115
(
A)
(
B)
(
(
C)
D)
Scheme 1. (A), (B), and (C) synthesis of DiAmsar and (D) synthesis of Ag NPs.
solution became colorless. Then, the [Co-(NO ) -sar]3+,Cl3
2.4. Preparation of Ag NP (Scheme 1d)
2
2
complex (1 eqiv., 600 mg) was added to the solution under
◦
strong stirring. The resulting solution was heated at 70 C for
h. The color of the solution turned green from orange/brown
In a typical procedure, an appropriate amount of purified
DiAmsar (0.01 g, 0.032 mmol) was dissolved in 10 mL DMF at room
temperature. Then, silver trifluoroacetate (0.0069 g, 0.032 mmol) in
another 5 mL DMF was added dropwise into the above solution and
the reaction was allowed to proceed for 2 h at room temperature,
until it turned from colorless to brown which indicated the forma-
tion of Ag NPs. In all the times, the reaction flask was covered with
aluminum foil.
4
and then, became darker. After that, water (6 mL) was added to
the solution, and it turned orange. The solution was cooled at
room temperature for 15 min before the flask was immersed in
an ice-water bath. The orange precipitate formed was filtered
and dried. The [Co-(NH ) -sar] ,Cl5 complex was obtained
with a yield around 90%. ( H NMR (400 MHz, D O) ı (ppm):
5+
3
2
1
2
2
.93–2.97 (m, 12H, NH CH2 CH2 NH ), 3.51–3.57 (m, 12H,
NH3+). C NMR (400 MHz, D O) ı (ppm): 51.4,
13
NH CH2
C
2.5. Anti-bacterial activity test
2
5
4.7, 58.5).
(
C) DiAmsar: Sodium hydroxide (2.11 eqiv., 77 mg) was dissolved
in deoxygenated water (7 mL) and the solution was placed
under nitrogen to eliminate oxygen. The bubbling was con-
The antimicrobial activities of the Ag NPs were determined by
disk diffusion method. The bacterial strains used in this study were
S. aureus (ATCC 6538) and E. coli (ATCC 8739) which were provided
by the NIGEB Bacterial Bank (Tehran, Iran). Cultures of Gram-
positive bacteria, S. aureus and Gram-negative bacteria, E. coli,
5+
tinued for 30 min. Then, [Co-(NH ) -sar] ,Cl5 complex (1 eqiv.,
3
2
5
00 mg) and cobalt (II) chloride (1.01 eqiv., 118 mg) were dis-
◦
solved in the basic solution. After homogenization of the
solution, sodium cyanide (17.7 eqiv., 783 mg) was added to
the resulting solution under strong stirring. The mixture was
were grown and maintained on Luria Bertani (LB) agar at 37 C for
18–24 h prior to place test samples onto it. Following incubation at
◦
37 C for 24 h, antibacterial activity was measured by the diameter
◦
heated to 70 C and vigorously stirred under nitrogen until
of the inhibition zone under and around the tested samples.
the solution had become almost yellow (∼7 h). The final solu-
tion was dried under air flow, and the residue was extracted
with boiling acetonitrile (3 fold, 10 mL). The total extract was
3
. Results and discussions
◦
reduced under vacuum (10 mL), and cooled to -10 C to precipi-
3.1. Characterization of Ag NPs
tate white crystals of the product. The product was recovered by
1
filtration and dried for further experiments. ( H NMR (400 MHz,
It is well known that when metal NPs are exposed to light,
D O) ı (ppm): 2.80 (s, 12H, NH CH2 CH2 NH ), 2.88 (s, 12H,
2
the oscillating electromagnetic field of the light makes a collective
coherent oscillation of free electrons (conduction band electrons)
of the metal NPs [35,36]. Therefore a charge separation with respect
to the ionic lattice is provided by this electron oscillation around the
13
NH CH2
C NH ). C NMR (400 MHz, D O) ı (ppm): 48.1,
2
2
+
5
1.7, 57.4. HRMS (EI, m/z) calcd. for C14H34N 315.4813 [M+H] ,
8
Found 315.3858).

1
16
G. Rezanejade Bardajee et al. / Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
Fig. 2. UV–vis extinction spectra of Ag NPs at different reaction times (a), TEM images and related histograms of Ag NPs after 2 h (b and c) and 5 days (d and e) in DMF.
particle surface. This phenomenon causes to forming a dipole oscil-
lation along the ion of the electric field of the light. The maximum
amplitude of the oscillation is called surface plasmon resonance
of the Ag NPs with DiAmsar as a reducing agent as well as cap-
ping agent which were taken from different reaction times between
5 min and 5 days. It can be seen that the absorption peaks at nearly
410 nm appear after 2 h which corresponds to the formation of Ag
NP. This spectrum shows single peak when the reaction time is 2 h
and the colorless solution becomes dark brown solution. Also, the
red shift of main peak is occurred with the increasing of reaction
(
SPR) which induces a strong absorption of the incident light and
thus can be measured using a UV–vis spectrophotometer. Based on
this, UV–vis absorption spectra are quite sensitive to the formation
of metal NPs [37,38]. Fig. 2a shows the UV–vis absorption spectra

G. Rezanejade Bardajee et al. / Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
117
Fig. 3. TEM image and related histogram of Ag NP after 5 days in (a) and (b) water, (c) and (d) THF, and (e) and (f) ethanol.
time while the color of solution remains dark brown. Some groups
have investigated the preparation of Ag NPs by optical properties
Ag NPs-capped DiAmsar in water, ethanol, DMF, and THF solutions
(Fig. 4). For DiAmsar spectrum shown in Fig. 4a, the broad band
−
1
[
39,40]. According to their results, the observed spectral and visible
in the region of 3600–3000 cm
is attributed to the stretching
+
color changes confirm the reduction of Ag ions and the subsequent
formation of Ag NPs. Moreover, the position of the SPR absorption
peak depends on the particle size, shape, and adsorption of nucle-
ophile or electrophile to the particle surface. Usually, a red shift is
associated with an increase in particle size or with the withdrawal
of electron density from the surface. It is proved that adsorption of
the nucleophile to the particle surface enhances the Fermi level of
the Ag NP owing to its donation of electron density to the particles.
Size and shape of these Ag NPs were determined with the help of
TEM (Fig. 2b). The particles are spherical or sphere-like in morphol-
ogy and have an average size of 7 and 12.5 nm after 2 h and 5 days,
respectively. These data confirmed the high stability of Ag NPs after
vibration absorption of NH and NH . The bands at 2895 and
2
−
1
2810 cm are ascribed to the asymmetric and symmetric stretch-
ing vibrations of CH2 , respectively. The bands between 1700
and 1550 cm are attributed to the stretching vibration absorp-
tion of NH groups. The bands between 1500 and 1400 cm are
three groups’ bond absorption peaks of . The bands at 1095
and 1075 cm are assigned to the C N asymmetric and symmetric
stretching vibrations, respectively. The FT-IR spectra of Ag NPs in
−
1
−
1
N
C
−
1
5
days.
In order to testify the function of DMF in the formation of Ag
NP, some control experiments were carried out by altering the
solvent while keeping the total volume unchanged. In these exper-
iments, water, THF, and ethanol were used instead of DMF as the
solvent. The gradually changed of colorless solution to dark brown
were observed after standing for 2 h in the all these solvents. Fig. 3
shows the TEM images of prepared samples in the aforementioned
solvents. The average particle sizes were 30, 62.5, and 71.8 nm cor-
responding to the TEM images of Ag NPs in water, THF, and ethanol,
respectively. Also, compared with the DMF solvent (Fig. 2b), a few
NPs with other shaped appeared in these three solvents. These
results confirmed that the DiAmsar acted as the good reducing
agent in DMF. Moreover, we did not observe the formation of
any type of prefect transparent color in the experiment without
DiAmsar while the other conditions were constant.
The presence of DiAmsar on the surface of the Ag NPs was con-
firmed by investigating the FT-IR spectra of the pure DiAmsar and
Fig. 4. FT-IR spectra of (a) pure DiAmsar, and Ag NPs-capped DiAmsar in (b) water,
(c) ethanol, (d) DMF, and (e) THF.

1
18
G. Rezanejade Bardajee et al. / Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
7
6
6
5
5
4
4
3
3
2
2
1
1
000
500
000
500
000
500
000
500
000
500
000
500
000
3000
a
b
O
O
Cl
H
N
H
N
O
2500
2000
1500
O
OH
O
0 Ag NPs
-4
0
0
Ag NPs-4
0.65 10 M Ag NPs
-4
.65 10 M Ag NPs
1.45 10 M Ag NPs
-4
1000
-4
1
.45 10 M Ag NPs
2
2
.05 10 M Ag NPs
-4
2
.05 10 M Ag NPs
-4
-4
.78 10 M Ag NPs
2
.78 10 M Ag NPs
-4
500
-
4
3.12 10 M Ag NPs
3
.12 10 M Ag NPs
5
00
0
0
550
600
650
700
350
400
450
500
550
600
Wavelength ( nm)
Wavelength ( nm)
2
.0
.9
.8
.7
.6
.5
.4
.3
.2
.1
c
1.9
d
1
1
1
1
1
1
1
1
1
0
0
40 C
0
4
3
3
0 C
1.8
0
35 C
5 C
0
3
0 C
0
1.7
0 C
0
2
5 C
1.6
1.5
1.4
1.3
0
2
5 C
1.2
1.0
1.5
2.0
2.5
3.0
1.0
1.5
2.0
2.5
3.0
-4
-4
[
Q] 10
[Q] 10
−
5
−5
◦
Fig. 5. (a) and (b) fluorescence titration spectra of Rh 6G (8.0 × 10 M, ꢀex = 430 nm) and 4HC (8.0 × 10 M, ꢀex = 303 nm) upon gradual addition of Ag NPs at T = 25 C, and
(
c) and (d) the Stern–Volmer plots of Rh 6G and 4HC fluorescence quenching caused by the addition of Ag-NPs at different temperatures, respectively.
different solvents are shown in Fig. 4b–e. As one see from this fig-
ure, the FT-IR of Ag NPs in different solvents (with different sizes)
are almost the same. Furthermore, the CH2 vibration bands of
DiAmsar are almost unchanged, but the asymmetric and symmetric
to understand the quenching mechanism, fluorescence quenching
was analyzed as follows by the Stern–Volmer equation [41]:
F0
= 1 + Ksv[Q]
(1)
F
−
1
stretching vibration peaks of C N shift from 1045 and 1012 cm
−1
where F0 and F are the fluorescence intensity in the absence
and presence of quencher, respectively. Ksv and [Q] are the
Stern–Volmer quenching constant and quencher concentration,
respectively. According to this equation, there are two types of fluo-
rescence quenching mechanisms, dynamic and static. For dynamic
quenching mechanism, the quenching rate constant increases with
increasing temperature [42]. In contrast, for static quenching mech-
anism, the quenching rate constant decreases with the increasing
temperature. Stern–Volmer plots of Rh 6G-Ag NPs and 4HC-Ag NPs
systems at different temperatures are shown in Fig. 5c and d. Within
the error limits, both the plots are linear and show the correla-
tion coefficient of above 0.98. From the slopes of the linear plots,
the Stern–Volmer quenching constants (Ksv) were calculated and
listed in Table 1. Table 1 indicates that the probable quenching pro-
cess of Rh 6G and 4HC interacting with Ag NPs are dynamic rather
than static quenching, because the Ksv increased with increasing
temperature.
to 1098 and 1066 cm , respectively. These shifted peaks show that
the N atoms of the DiAmsar molecules interact with the surface of
Ag NPs by chemical absorption.
3.2. Fluorescence quenching by Ag NPs
Fig. 5 shows the obtained fluorescence emission spectra of Rh 6G
−
5
−5
(
8.0 × 10 M, ꢀex = 430 nm) and 4HC (8.0 × 10 M, ꢀex = 303 nm)
◦
solutions with different concentration of Ag NPs at 25 C in DMF
solutions. The maximum emission wavelengths were near 543 and
3
95 nm for Rh 6G and 4HC, respectively. The same trends were
observed at other temperatures. With increasing concentration of
Ag NPs (Fig. 5a and b), the fluorescence intensity of Rh 6G and
4
HC decrease progressively, but the emission wavelength at max-
imum emission remains constant. One can conclude that there
was no observable photochemical reaction between Ag NPs and
dyes. Similar behavior was observed by Rh 6G and 4HC with vari-
able concentrations of Ag NPs at different temperatures. In order
To investigate the size effect of prepared Ag NPs on the fluores-
cence quenching mechanisms, the above experiments were also

G. Rezanejade Bardajee et al. / Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
119
Table 1
KSV and R2 values of Rh 6G-Ag NPs and 4HC-Ag-NPs systems at different
temperatures.
Ksv (L mol 1
−
)
R2
Dyes
T (K)
298
303
308
313
1812
2512
2715
2858
0.988
0.991
0.994
0.991
Rh 6G
298
303
308
313
2185
2234
2359
2390
0.999
0.999
0.995
0.998
4
HC
Table 2
−1
−5
Stern–Volmer constants (Ksv, L mol ) of Rh 6G (8.0 × 10 M, ꢀex = 430 nm) and 4HC
−
5
(
8.0 × 10 M, ꢀex = 303 nm) solutions with Ag NPs prepared in water, ethanol, and
◦
THF solutions at 25 C.
Solutions
Dyes
Rh 6G
4HC
DMF
Water
THF
1812
2036
2351
2394
2185
2417
2598
2638
Ethanol
done by Ag NPs prepared in water, ethanol, and THF solutions.
−
5
Table 2 shows the Stern–Volmer constants of Rh 6G (8.0 × 10 M,
−
5
ꢀ
ex = 430 nm) and 4HC (8.0 × 10 M, ꢀex = 303 nm) solutions with
◦
Ag NPs prepared in water, ethanol, and THF solutions at 25 C. These
KSV values reveal that the quenching efficiency of Ag NPs increases
in water, ethanol, and THF solutions which contain Ag NPs with dif-
ferent sizes. Therefore, we can realize the role of size and surface
effect on the quenching. This is probably due to the fact that the
surface area of the particles is larger in the bigger particles. There-
fore, the larger particles can accommodate more number of dye
molecules and cause more quenching. The results suggest that the
collisional quenching plays main role in the studied photophysical
process in the presence of nanoparticles [43,44].
Fig. 6. (a) Spectral overlap of fluorescence spectra of Rh 6G and 4HC with absorption
spectrum of Ag NPs and (b) the quenching efficiency plotted as a function of distance
−
4
−6
for R and R models.
In addition, the effect of DiAmsar, as the capping agent, on the
fluorescence of dye molecules was examined. As one can see from
Fig. S1 (see Supporting Information), the DiAmsar does not have a
pronounced effect on the quenching of the dyes.
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
7.35
R(Å ) =
(5)
1/3
(
C )
0
2
013.11.005.
where F0 and F are the fluorescence intensities of donor in the
absence and presence of acceptor, R is the distance between the
dye and metallic NP, R0 is the critical distance when the efficiency
of transfer is 50%, ꢁ2 is the spatial orientation factor related to the
In continue the energy transfer between dyes and Ag NPs was
investigated by FRET and NSET models. As mentioned in the intro-
duction section, energy transfer occurs when the fluorescence
spectrum of the donor overlaps with the absorption spectrum of
the acceptor [12,13]. Fig. 6a shows spectral overlap between fluo-
rescence spectra of Rh 6G and 4HC with absorption spectrum of Ag
NPs. Based on the reasonable overlapping of the fluorescence spec-
tra of Rh 6G and 4HC with absorption spectrum of Ag NPs, it can
be conclude that the energy transfer can occur between aforemen-
tioned dyes and Ag NPs. Therefore, the energy transfer efficiency
2
geometry of the donor and acceptor dipoles (ꢁ = 2/3 for random
orientation as in fluid solution), N is the refractive index of the
medium, ˚D is the fluorescence quantum yield of the donor, J is the
spectral overlap (Fig. 6a) between the emission spectrum of donor
and the absorption spectrum of acceptor which could be calculated
by Eq. (4), F(ꢀ) is the corrected fluorescence intensity of the donor
in the wavelength range from ꢀ to (ꢀ + ꢂꢀ) with the total inten-
sity normalized to unity, ε(ꢀ) is the molar extinction coefficient of
the acceptor at ꢀ, and C0 is concentration of NPs. Majority of the
recent studies on dye–metallic NP mixtures showed that n having
a value of 6 for FRET and 4 for NSET models. The critical distance
R0, corresponding to 50% energy transfer from Rh 6G and 4HC to Ag
NPs, was calculated via Eq. (3) to be 6.10 and 4.45 nm, respectively.
(
E) was calculated from the following equations [11–14]:
ꢀ
ꢁ
F
1
E = 1 −
=
(2)
(3)
(4)
n
F0
1 + (R/R0)
6
0
= 8.8 × 10−25[ꢁ N ˚ ]
2
−4
J
D
R
By the R values, the experimentally obtained quenching efficiency
0
were fitted to Eq. (2) by choosing n = 4 and n = 6 (Fig. 6b). Compar-
ison of fitted results with direct measurements obviously indicate
that the experimental values fit well when n = 4 rather than n = 6.
These results strongly suggest that the energy transfer between
ꢂ
∞
4
F(ꢀ)ε(ꢀ)ꢀ dꢀ
0
J =
ꢂ
∞
F(ꢀ)dꢀ
0

1
20
G. Rezanejade Bardajee et al. / Journal of Photochemistry and Photobiology A: Chemistry 276 (2013) 113–121
Fig. 7. Antibacterial test results for (a) S. aureus and (b) E. coli after 24 h incubation (1, 2, 3, and 4, AgNO3, ampicillin, Ag NPs, and DMF, respectively).
dyes and Ag NPs can be considered by NSET model. The same results
were obtained by Ghosh and co worker for the interaction between
coumarin 153 and Au NPs [13]. As one can see in Fig. 6a, the over-
lap between fluoresce spectra of 4HC and absorption spectra of Ag
NPs is more than overlap of fluoresce spectra of Rh 6G and absorp-
tion spectra of Ag NPs, while the energy transfer between these
dyes and Ag NPs are similar. The discrepancy may come from the
fact that dimensionality of the NPs and the spatial distribution of
the acceptors strongly affects on the energy transfer characteristics
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Acknowledgment
We are grateful to the PNU for funding this work.

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