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Forster resonance energy transfer (FRET) with a donor–acceptor system
adsorbed on silver or gold nanoisland films
Emilia Giorgetti,*a Stefano Cicchi,b Maurizio Muniz-Miranda,c
Giancarlo Margheri,dTommaso Del Rosso,
dAnna Giusti,wa Alessio Rindi,
e
Giacomo Ghini,bStefano Sottini,
dAgnese Marcelli
fand Paolo Foggizg
Received 7th May 2009, Accepted 28th July 2009
First published as an Advance Article on the web 19th August 2009
DOI: 10.1039/b909123h
We observed Forster resonance energy transfer (FRET) with a covalently linked donor–acceptor
pair D–A consisting of two naphthalene groups acting as the donors and a benzofurazan group
acting as the acceptor and adsorbed onto Ag or Au nanoisland films. The use of metal nanoisland
films, which caused a strong enhancement of the Raman signal, permitted description of the
adsorption mechanism onto the two metals. The intense fluorescence response of molecular
adsorbates and the different behavior of the antenna on Ag and Au nanoislands are partly
explained in terms of the radiating plasmon model.
Introduction
Bichromophoric donor–acceptor (D–A) systems1 consist of
two kinds of fluorescent chromophores covalently linked to
the same scaffold, of which one acts as energy donor (D) and
the other as acceptor (A). An efficient Forster resonance
energy transfer (FRET) process towards the A moiety takes
place upon irradiation in the absorption region of the D
moiety and depends on the so-called Forster distance between
the two chromophores. The energy transfer results in the
emission of light at a wavelength belonging to the emission
spectrum of the A. These molecules are expected to find
different valuable applications,2,3 for example in the development
of chemosensors, since they combine the high sensitivity of
fluorescence detection with the possibility to control the FRET
process by an analyte.
Such D–A molecules have been tested, mostly, in solutions.1
In particular, some of us have recently developed a new D–A
system where the naphthalene group acts as D and the
benzofurazan group acts as A. Its spectroscopic properties in
solutions were studied in detail, as well as the FRET process,
which takes place between the D and the A upon excitation at
300 nm.4 However, a basic step toward practical utilization of
such molecular systems requires their immobilization onto
solid substrates.5
In this paper, we investigated on the formation of self
assembled monolayers (SAMs) of the same D–A systems
described in ref. 4 onto Ag or Au nanoisland films (NIF)
deposited on quartz substrates6,7 and their FRET behaviour in
the solid state. We performed a study of their properties by
infrared (IR), Raman, SERS, extinction and fluorescence
spectroscopies and by atomic force microscopy (AFM), in
order to obtain a description of the chemisorption mechanism
on the two metals and to correlate the spectroscopic properties of
SAMs and the FRET behavior with the surface morphologies.
In particular, we let the deposited NIFs free to auto-organize
and to form agglomerates by skipping the silanization of the
quartz substrates, which is usually performed to improve
metal adhesion,8 or to observe single particle behaviour.9
Mobility and subsequent rearrangement of the nanoislands
during incubation with pure solvents or with solutions of the
D–A systems were aimed at improving the efficiency of Raman
response and fluorescent emission and to elucidate the role of
surface topography on the final properties of the molecular
adsorbates.
Experimental
Materials
The chemical structure of the D–A system (1) is reported in
Fig. 1a. It was synthesized according to ref. 4 and consists of
two naphthalene groups acting as donors (D) and a benzofurazan
group acting as acceptor (A). Fig. 1b reports the chemical
structure of the sulfur-functionalized isolated A moiety (2). Its
synthesis procedure will be reported elsewhere.
Samples preparation
Gold and silver films were deposited on quartz slides.
The quartz slides were pre-treated by ultrasonic baths in
a ISC-CNR and INSTM, Via Madonna del Piano 11,Sesto Fiorentino, Firenze 50019, Italy.E-mail: [email protected]
bDipartimento di Chimica Organica and INSTM, Universita di Firenze,Sesto Fiorentino, Firenze, Italy
cDipartimento di Chimica and INSTM, Universita di Firenze,Sesto Fiorentino, Firenze, Italy
d ISC-CNR, Sesto Fiorentino, Firenze, ItalyeDipartimento di Chimica e Chimica Industriale, Universita di Genova,Genova, Italy
f Laboratorio Europeo di Spettroscopia Nonlineare (LENS),Sesto Fiorentino, Firenze, Italy
gDipartimento di Chimica, Universita di Perugia, Perugia, Italyw Present address: Institut de Chimie Moleculaire et de Materiauxd’Orsay, Universite0 Paris-Sud, Orsay (France).z Also at Laboratorio Europeo di Spettroscopia Nonlineare (LENS),Sesto Fiorentino, Firenze (Italy) and INOA-CNR, Firenze (Italy).
9798 | Phys. Chem. Chem. Phys., 2009, 11, 9798–9803 This journal is �c the Owner Societies 2009
PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics
trichloroethylene, acetone and deionized water (resistivity
418.0 MO cm) purified using a Millipore Direct-Qt system.
The metallic depositions were made with an electron gun
assisted High Vacuum equipment at a pressure of 10�6 mbar.
The rate was varied by adjusting the electron beam current
and the thickness was measured with a quartz crystal micro-
balance system. The rate of deposition was 3 A s�1 for silver
and 0.6 A s�1 for gold.
The SAMs were prepared by immersion of the metal
coated substrates into a 0.5 g l�1 solution of 1 in chloroform.
We used 48 h incubation time, after which the samples were
extracted from the solutions and gently rinsed with flowing
chloroform. After complete evaporation of chloroform, SAMs
were rinsed again with flowing pure water and dried with
nitrogen gas.
AFM analysis
The AFM analysis was performed with a Nanosurf Easyscan
instrument in tapping mode, using silicon cantilevers. The
images resolution was 256 � 256 pixels2. The analysis was
carried out by ImageJ10 and by WSxM11 softwares.
Raman and IR spectroscopy
Raman spectra of powder samples and SERS spectra of
SAMs deposited on Au NIFs were collected using a
Renishaw RM2000 micro-Raman apparatus, coupled with
a diode laser source emitting at 785 nm. Sample irradiation
was accomplished using the 50� microscope objective of a
Leica Microscope DMLM. The beam power was B3 mW,
the laser spot size was adjusted between 1 and 3 mm.
Raman scattering was filtered by a double holographic
notch filters system and collected by an air-cooled CCD
detector. The acquisition time for each measurement was
10 s. All spectra were calibrated with respect to a silicon
wafer at 520 cm�1. SERS spectra of samples deposited as
SAMs on AgNIFs were recorded using the 514.5 nm line
of an Ar+ laser, a Jobin-Yvon HG2S double monochromator
equipped with a cooled RCA-C31034A photomultiplier
and a data acquisition facility. A defocused laser beam
(30 mW incident power) was used to impair thermal effects.
Power density measurements were performed with a power
meter instrument (model 362; Scientech, Boulder, CO, USA)
giving B5% accuracy in the 300–1000 nm spectral range.
Infrared spectra of samples in KBr pellet were obtained
in the 4000–450 cm�1 region by using a Perkin-Elmer FT-IR
RX/I spectrometer.
Absorption, extinction and fluorescence spectroscopy
Absorption and fluorescence spectra of the solutions were
recorded with a Cary 5 Varian spectrophotometer and with
a Jasco FP-750 spectrofluorimeter, respectively.
In the case of metal NIFs-based samples we measured
extinction spectra, due to the presence of both absorption
and strong scattering bands associated to the large dimensions,
shape irregularities and particle interactions. Such spectra
were recorded with a Cary 5 Varian spectrophotometer in
transmission mode and with the impinging beam perpendicular
to the sample/air interface. The effect of the quartz substrate
was negligible in the optical region of interest, so that
the results only depend on size, dimension and long range
arrangement of the nanostructure of the functionalized
(or non functionalized) silver and gold NIFs.
Fluorescence spectra of films were recorded with a Perkin-
Elmer LS 55 fluorescence spectrometer supplied with a Xenon
discharge lamp (equivalent to 20 kW for 8 ms duration), a
Monk–Gillieson monochromator and a gated red-sensitive
(R928) photomultiplier.
Results
Absorption and fluorescence spectra in solution
The molecular extinction coefficients e of the isolated D and A
moieties of compound 1 and the Forster radius were evaluated
in ref. 4 for acetonitrile solutions. The molecular extinction
coefficients in the absorption maxima were e(298 nm) =
5200 M�1 cm�1 for D and e(343 nm) = 9800 M�1 cm�1 and
e(482 nm) = 27 900 M�1 cm�1 for A. The Forster radius was
calculated in the hypothesis of random chromophore orientation
and was 12.4 A 4.
Fig. 2 reports the absorption and fluorescence spectra of 1 in
ethanol solutions. The bands at 297 nm and at 471 nm in the
absorption spectrum correspond to the D and A groups,
respectively. The D emission, peaked at 344 nm, favours the
A emission in the visible range at 528 nm, due to a strong
spectral overlap with the A absorption band centred at 343 nm.4
Correlation between extinction spectroscopy and atomic force
microscopy
It is well known that continuous and reflecting Ag and Au
films cause a strong quenching of the fluorescence signal,12,13
which can impair the FRET effect. In contrast, metal
Fig. 1 Chemical structure of the covalently linked donor–acceptor
pair D–A 1 (a) and of the sulfur-functionalized acceptor group 2 (b).
Fig. 2 Absorption (black curves) and fluorescence (grey curves)
spectra of 1 in ethanol solution. Excitation wavelength: 300 nm.
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nanoisland films have been recognized to enhance the
Raman and, in some cases, the fluorescence signal of molecular
adsorbates.6–7,14 Therefore, we studied adsorption of 1 onto
thin Ag and Au layers, namely strongly coloured, non reflecting
and non conducting films, which resulted to consist of islands
separated by voids. In order to check the reproducibility of
the functionalized metal NIFs, we prepared different samples
by following the same procedure and by using the same
nominal metal thickness. All samples exhibited well-reproducible
spectral features with extinction bands positioned within a
couple of nm.
Fig. 3 shows the extinction spectra and the corresponding
morphology of a typical Ag-based sample. The solid line in
Fig. 3a represents the extinction spectrum of a fresh AgNIF,
while the dotted and dashed lines refer, respectively, to the
same NIF after 48 h incubation in pure chloroform and in a
solution of 1 in chloroform. The corresponding morphologies
of the samples are reported in Fig. 3b–d.
In the case of the fresh AgNIF (Fig. 3b), the morphology is
characterized by hemispheroids with average diameter of
15 nm and average height of 6.5 nm. The degree of substrate
coverage is high and, consequently, the interparticle distance is
small. This fact allows the electromagnetic coupling among
particles, resulting in a single extinction peak at 497 nm
(Fig. 3a, solid line), which is significantly red shifted with
respect to that pertaining to the isolated absorbing Ag nano-
particle, typically around 390 nm.15 The incubation in chloroform
leads to a first change in the surface morphology (Fig. 3c). The
silver particles coalesce into larger ones, with mean diameter
of B28 nm, average height of 11 nm and increased inter-
particle distance. We found also particles having diameters as
large as 100 nm. The effect of coalescence causes formation of
large metal clusters with high scattering cross section, resulting
in the increased near infrared (NIR) extinction observed in
Fig. 3a, dotted line. In contrast, the broad plasmon band
at 430 nm appearing in the same spectrum can be attributed to
residual sparse small particles.15 An even stronger modification
of the particles morphology is observed after the incubation of
the freshly evaporated silver film in a solution of compound 1
in chloroform (Fig. 3d). In this case, a huge increase in average
particle size occurs. Indeed, the particles aggregate into
ellipsoids, with average major axis of 120 nm, minor axis of
75 nm and height 55 nm, respectively. However, the AFM
analysis reveals also the presence of sparse and clustered small
particles. These sparse particles are weakly interacting. The
corresponding extinction spectrum is showed in Fig. 3a,
dashed curve. It shows two bands. The one at 424 nm is due
to small sparse particles, while the one at 518 nm is ascribed to
large metal particles with high scattering cross section.
Analogous structural modifications were observed for
Au-based samples. Extinction spectra and morphologies are
reported in Fig. 4. A typical AFM image of freshly evaporated
gold is shown in Fig. 4b. During the deposition, gold
condenses into discs with mean diameter of B30 nm. The
corresponding extinction spectrum (Fig. 4a, solid line) is
peaked at 542 nm. Again, the red shift with respect to the
absorption expected for the isolated single Au particle is
consistent with the presence of some degree of interaction
among particles.15 Incubation in chloroform favours the clustering
of many particles into big aggregates with average
diameter 120 nm and height 6 nm, while many others remain
isolated (Fig. 4c). As for the case of Ag, this can explain the
presence of the slight blue shift of the extinction band, now
peaked at 534 nm (Fig. 4a, dotted line), and the small increase
of NIR extinction, which is assigned to large, efficient
scatterers. The incubation with solution of compound 1 caused
a dramatic modification of the metal morphology. The AFM
inspection put in evidence the presence of big aggregates, with
dimensions ranging from 100 to more than 200 nm and a
substantial absence of isolated nanoparticles (Fig. 4d). The
only measurable topographic parameter is the average height,
which is of the order of 17 nm. This change in morphology is
accompanied by a parallel change in the extinction spectrum
(Fig. 4a, dashed line), which is characterized by a single
broad band peaked at 698 nm. This band can be reasonably
Fig. 3 (a) Extinction spectra of AgNIF (solid line), AgNIF dipped
for 48 h in chloroform (dotted line) and 1 SAMs on AgNIF (dashed
line); (b) AFM images of freshly evaporated AgNIF on quartz;
(c) AgNIF on quartz after the incubation in chloroform; (d) AgNIF
on quartz after the incubation in chloroform–1 solution. The nominal
thickness of the metal layer is 2.4 nm. The x–y scales are 350 nm� 350 nm
in (b) and 2 mm � 2 mm in (c) and (d).
Fig. 4 (a) Extinction spectra of AuNIF (solid line), AuNIF dipped
for 48 h in chloroform (dotted line) and 1 SAMs on AuNIF (dashed
line); (b) AFM images of freshly evaporated AuNIF on quartz;
(c) AuNIF on quartz after the incubation in chloroform; (d) AuNIF
on quartz after the incubation in chloroform–1 solution. The nominal
thickness of the metal layer is 2.5 nm. The x–y scales are 2 mm � 2 mm.
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attributed to both the presence of large aggregates and the
strong interaction among small particles.
IR, Raman and SERS analysis
Although not specifically functionalized, compound 1 contains
several groups which can provide efficient chemisorption on
metal NIFs. The nature of such adsorption is fundamental
for any application of SAMs and can be investigated by
Raman spectroscopy. We studied compound 1, either as
powder and as SAM on Ag or Au NIFs. The results are
reported in Fig. 5, along with the IR spectrum of powders in
KBr pellet.
The Raman spectrum was obtained by excitation at 785 nm,
in order to avoid fluorescence overlapping. It is relatively
simple, although the molecule is complicated, consisting of
4-amino-7-nitrobenzofurazan chromophore (A) and two
substituted naphthalene units (D). The most intense Raman
bands are attributable to vibrational modes of nitro group and
benzofurazan, according to the assignment proposed in ref. 16
for the infrared spectrum of 4-amino-7-nitrobenzofurazan.
The strongest Raman band occurring at 1300 cm�1, intense
also in the infrared spectrum, is assigned to the symmetric
stretching mode of the nitro group; the antisymmetric
stretching is observed in the infrared spectrum at 1500 cm�1.
The stretching vibrations of benzofurazan are, instead,
observed as intense bands in both Raman and IR spectra
around 1280 and 1540 cm�1. The Raman band at 695 cm�1 is
attributable to the ring deformation mode. The IR spectrum
of 1 allows identification of the CQO stretching bands of the
esteric groups between 1740 and 1760 cm�1. These vibrations,
related to large dipole moment changes, are, instead, absent in
the Raman spectrum.
The vibrational assignment of 1 powders provides useful
information for interpreting the SERS spectra of SAMs of
compound 1 deposited on Ag or Au NIFs, which are also
reported in Fig. 5. In the first case we used 514.5 nm excitation,
in order to obtain enhancement of the Raman effect. The most
intense SERS bands of 1 on AgNIFs occur in the 500–1100 cm�1
region, whereas the corresponding Raman bands are observed
with significantly weaker intensities. In particular, the weakness
of the SERS band related to the symmetric stretching mode
of nitro group, observed around 1300 cm�1, suggests that
chemisorption does not take place through this group, but
more likely through the heteroatoms of furazan. Actually, the
formation of Ag–N or Ag–O bonds should be evidenced by
the occurrence of broad bands in the spectral region between
200 and 400 cm�1, which are not visible in our SERS
spectrum, probably masked by the fluorescence background.
However, we can exclude an interaction between the metal and
O atoms belonging to esteric groups, because, if present,
it would originate an intensification of CQO stretching bands,
that, instead, are completely absent in both Raman and SERS
spectra.
In the case of 1 SAMs on AuNIFs, the morphology
results in a close interaction among metal clusters, which
has been recognized to introduce an absorption of electro-
magnetic energy, stored in the so-called hot spots.17 In our
case, this mechanism causes a shift of the absorption band
towards the near IR (Fig. 4a) with an expected strong
enhancement of the Raman response in this spectral region.
Therefore, we chose to record SERS spectra by exciting at
785 nm. In order to stress that the spectrum of Fig. 5 is true
SERS and not resonance Raman spectrum, it is worth
noting that no Raman response could be observed from
the same sample under excitation with 514.5 nm, despite the
remarkable material absorption at this wavelength. The
overall scarce intensity of Raman response is mainly due to
the lack of Raman resonance at 785 nm. Nevertheless, the
SERS effect permitted recognition of several interesting
features. In particular, the SERS of 1 on AuNIFs does not
differ much from the normal Raman spectrum, although, in
this case, some frequency shifts are observed (for example 695,
700 cm�1; 1280, 1270 cm�1, 1378, 1395 cm�1; 1540,
1534 cm�1). The absence of low frequency bands that could
be assigned to Au–N bonds (i.e. around 250 cm�1) and the
high intensity of the band at 1300 cm�1 suggest that, in this
case, the interaction takes place between Au and the
nitro group.
In conclusion, the vibrational analysis permitted to
attribute the chemisorption of compound 1 to the interaction
between the metal and the lone pairs of the nitrogen of
furazan, in the case of AgNIFs and to the nitro group in the
case of AuNIFs.
Fluorescence and excitation spectra of SAMs on Ag and Au
NIFs
The behaviour of SAMs of compound 1 grown on Au and
Ag NIFs and excited at 300 nm can be inferred from Fig. 6,
which shows fluorescence (Fig. 6a) and excitation spectra
(Fig. 6b). In all cases, fluorescence spectra of the metal-coated
substrates before SAM growth were below the noise level of
our equipment and are not reported. In contrast, both Au and
Ag NIFs permit observation of a well-resolved FRET effect,
with intense emission at 535 nm (AgNIFs) and 532 nm
(AuNIFs). The final A fluorescence is more than twice in the
Fig. 5 Raman spectrum of 1 powder (excitation: 785 nm); IR
spectrum of 1 in KBr pellet; SERS spectrum of 1 on AgNIFs
(excitation: 514.5 nm); SERS spectrum of 1 on AuNIFs (excitation:
785 nm). Intensities in arbitrary units.
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case of AgNIFs with respect to AuNIFs. Notice that, in the
case of AuNIFs, Fig. 6a shows a weak band around 425 nm.
This emission, which is also observed in the fluorescence
spectra of 1 in ethanol (Fig. 2), is absent from the spectrum
of 1 on AgNIFs, due to the strong absorption of AgNIFs in
this spectral region (Fig. 3a). This effect gives evidence of the
strong interaction between 1 and Ag.
The excitation spectra of Fig. 6b exhibit a bifurcation of the
A band around 470 nm. This can be reasonably attributed to
the formation of disordered molecular aggregates, which
absorb at slightly different frequencies due to inhomogeneous
anchoring to the nanoislands.
In order to give further confirm that the fluorescence and
excitation behavior illustrated in Fig. 6 is true FRET, we
repeated the same tests by using the isolated, sulfur
functionalized A moiety, namely compound 2, whose chemical
structure is reported in Fig. 1b. We chemisorbed 2 onto Ag
and Au NIFs obtained in the same conditions as those used
for antenna SAMs and performed fluorescence measurements.
Fig. 7 reports the fluorescence of 2 on both metals for direct
excitation at 474 nm (open triangles) and for excitation at
300 nm (solid lines). The negligible emission obtained by
excitation at 300 nm confirms that the bands around 530 nm
observed in Fig. 6a are due to FRET between D and A groups.
As for 1, the final fluorescence of compound 2 is more than
twice in the case of AgNIF.
Discussion
The valuable amount of published investigations on nano-
particle aggregates, metal colloids, SERS effects, metal
enhanced fluorescence and plasmon emission7,18–22 can greatly
contribute to get an insight into the behaviour of our NIFs,
the chemisorption of our compounds and, lastly, the role of
metal nanoislands in the Raman and fluorescence response.
From our study, we found that the fluorescence of our
compounds is accompanied by a red-shift of the emission
bands. Such a red-shift can be attributed to the different
chemisorption evidenced by Raman spectroscopy, which
showed that 1 interacts with AgNIFs through the lone pairs
of N atom and with AuNIFs through the nitro group. As a
consequence, the different surface complexes exhibit some
differences in the spectroscopic properties.14 Moreover, the
fluorescence is more intense when SAMs of compound 1 are
adsorbed on Ag substrates with respect to Au substrates. In
general, the overall output fluorescence of Ag and Au-based
samples can depend:
(a) on the molecular coverage of NIFs and on the possible
presence of disordered molecular aggregates which could
contribute unpredictably to the final emission;
(b) on the metal-induced amplification or quenching of the
FRET effect;
(c) on the role played by plasmonic excitations of the metals.
A quantitative evaluation of contribution (a) would require
dedicated experiments, whose results could be impaired by the
difficult evaluation of the role of molecular aggregates.
Therefore, no conclusion can be given on this point. Referring
to contribution (b), we can notice that the ratio of the output
fluorescence in the case of Ag and Au NIFs is almost the same
(D2.2) both in the case of the bichromophoric system 1 and in
the case of isolated acceptor 2 (Fig. 6 and 7). This suggests that
the FRET mechanism is not coupled to plasmons, even if, also
in this case, an exhaustive conclusion would require a better
insight into point (a). In contrast, the contribution of (c) to the
overall output fluoresecence of our samples can be explained
in the frame of the so called radiating plasmon model.7
According to this model, the fluorescence of molecules
adsorbed on nanostructured metals is enhanced when the
scattering part of the extinction spectrum of the nanocomposite
overlaps with the emission band of the fluorophore. In
contrast, the fluorescence is quenched when the emission band
of the fluorophore overlaps with the absorption band of the
nanocomposite. For example, ref. 7 shows that, in the case of
colloids consisting of 200 nm-sized silver particles, a remarkable
fluorescence enhancement is predicted in the spectral range
between 500 and 700 nm. On the contrary, in the case of gold
particles having the same dimensions, a similar enhancement
can be achieved only above 600–650 nm. In our case,
AgNIFs exhibit a very efficient electromagnetic interaction
with compound 1. This effect is clearly illustrated by Fig. 8,
which compares the extinction spectra of our samples
obtained on Ag (dotted line) and Au NIFs (dashed line) with
the emission band of the antenna in solution for excitation
Fig. 6 Fluorescence (a) and excitation (b) spectra of 1 SAMs on
AuNIFs (black lines) and AgNIFs (grey lines). Excitation wavelength:
300 nm. Emission wavelength: 535 nm.
Fig. 7 Fluorescence spectra of 2 SAMs on Ag (grey curves) and Au
(black curves) NIFs. Excitation wavelengths: 300 nm (solid lines) and
474 nm (open triangles).
9802 | Phys. Chem. Chem. Phys., 2009, 11, 9798–9803 This journal is �c the Owner Societies 2009
with 300 nm (solid line). The overlap between the fluorescent
emission and the scattering band of the Ag-coated substrate is
excellent. In contrast, the spectrum of the Au-coated
substrate is completely detuned from fluorophore emission.22
Therefore, silver appears the best choice, while gold is
liable to exhibit a detrimental impact on the operation of
compound 1.
Conclusions
We studied the FRET properties of SAMs of a novel
covalently linked donor–acceptor pair D–A consisting of
two naphthalene groups acting as donors and a benzofurazan
group acting as the acceptor and adsorbed onto Ag and Au
NIFs. SERS spectroscopy confirmed its adsorption on metal
even without any specific functionalization and provided
information on the nature of chemisorption. With both
metals, we observed a well resolved FRET effect, characterized
by an intense fluorescence signal in the green region of the
spectrum, which was twice more intense in the case of Ag
NIFs. Although our tests cannot help to give a conclusive
explanation of this result, however, our AFM analysis and
extinction spectroscopy suggest that a valuable contribution to
the fluorescence emission of the antenna on silver can be
attributed to scattering resonances, in agreement with the
radiating plasmon model. In this frame, we can conclude that
the auto-organizing process used to create the metallic nano-
structures is more favourable to radiative decay paths on
AgNIFs with respect to AuNIFs.
Such findings can help the engineering of proper metal
substrates that efficiently transform the energy transfer, which
occurs within a molecule, into radiative emission. A promising
application is the development of fluorescent chemosensors.23
For example, preliminary tests have shown that the contact of
the SAMs of compound 1 on Ag with aqueous solutions
containing traces of Cu2+ causes a net and selective quenching
of the A fluorescence. In this sense, the future work will focus
on the development of SAMs of new D–A systems containing
D groups absorbing in the visible region of the electromagnetic
spectrum, where cheap sources as laser diodes are available.
The possibility of operating with visible wavelengths would
also open the way to the use of surface plasmon propagation
at silver/chromophore interfaces, which represents a powerful
alternative to the use of localized surface plasmons for efficient
amplification of the final fluorescence.24
Acknowledgements
Funding from the Italian FIRB 2004 ‘‘Molecular compounds
and hybrid nanostructured materials with resonant and non
resonant optical properties for photonic devices’’ (contract no.
RBNE033KMA) and from PRIN 2007 ‘‘Metal–organic plasmonic
nanostructures for sensors’’ contract n1 2007LN873M_002 is
acknowledged.
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Fig. 8 Extinction spectra of 1-coated AgNIFs (dotted line), 1-coated
AuNIFs (dashed line) and fluorescence emission of an ethanol solution
of 1 for excitation with 300 nm (solid line).
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