High-Efficiency FRET Processes in BODIPY-Functionalized Quantum Dot Architectures

Article abstract of DOI:10.1002/chem.202003574

Efficient FRET systems are developed combining colloidal CdSe quantum dots (QDs) donors and BODIPY acceptors. To promote effective energy transfer in FRET architectures, the distance between the organic fluorophore and the QDs needs to be optimized by a c

Full text of DOI:10.1002/chem.202003574

Accepted Article
Accepted Article
Title: High Efficiency FRET Processes in BODIPY Functionalized
Quantum Dot Architectures
Authors: Marinella Striccoli, Annamaria Panniello, Mariachiara Trapani,
Massimiliano Cordaro, Carlo Nazareno Dibenedetto, Raffaele
Tommasi, Chiara Ingrosso, Elisabetta Fanizza, Roberto
Grisorio, Elisabetta Collini, Angela Agostiano, Maria Lucia
Curri, and Maria Angela Castriciano
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003574
Link to VoR: https://doi.org/10.1002/chem.202003574
01/2020

10.1002/chem.202003574
Chemistry - A European Journal
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High Efficiency FRET Processes in BODIPY Functionalized
Quantum Dot Architectures
Annamaria Panniello, *[a] Mariachiara Trapani,[b] Massimiliano Cordaro,[c] Carlo Nazareno
Dibenedetto,[d,a] Raffaele Tommasi,[e] Chiara Ingrosso,[a] Elisabetta Fanizza,[d,a] Roberto Grisorio,[f]
Elisabetta Collini,[g] Angela Agostiano,[d,a] Maria Lucia Curri,[d,a] Maria Angela Castriciano,[b] and
Marinella Striccoli*[a]
[a]
Dr. A.Panniello, C.N. Dibenedetto, Dr. C. Ingrosso, Prof. E. Fanizza, Prof. A. Agostiano, Prof. M.L. Curri, Dr. M. Striccoli
Istituto per i Processi Chimico Fisici del CNR (IPCF-CNR)
c/o Dipartimento di Chimica, Università degli Studi di Bari “Aldo Moro”
Via Orabona, 4 70126 Bari, Italy
E-mail: a.panniello@ba.ipcf.cnr.it; m.striccoli@ba.ipcf.cnr.it
Dr. M. Trapani, Dr. M.A. Castriciano
Istituto per lo Studio dei Materiali Nanostrutturati del CNR (ISMN-CNR)
c/o Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, Università degli Studi di Messina
Viale F. Stagno D'Alcontres31, 98166 Messina, Italy
Dr. M. Cordaro
Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali
Università degli Studi di Messina
Viale F. Stagno D'Alcontres31, 98166 Messina, Italy
C.N. Dibenedetto, Prof. E. Fanizza, Prof. A. Agostiano, Prof. M.L. Curri
Dipartimento Chimica
Università degli Studi di Bari “Aldo Moro”
Via Orabona, 4 70126 Bari, Italy
Prof. R. Tommasi
Dipartimento di Scienze Mediche di Base, Neuroscienze e Organi di Senso
Università degli Studi di Bari “Aldo Moro”
Piazza G. Cesare, 11 70124 Bari, Italy
[b]
[c]
[d]
[e]
[f]
Dr. R. Grisorio
Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e di Chimica (DICATECh)
Politecnico di Bari,
Via Orabona, 4 70126 Bari, Italy
[g]
Prof. E. Collini
Dipartimento Scienze Chimiche
Università di Padova
via Marzolo 1, 35131 Padova, Italy
Supporting information for this article is given via a link at the end of the document.
Abstract: Efficient FRET systems are developed combining colloidal
CdSe quantum dots (QDs) donors and BODIPY acceptors. To
promote effective energy transfer in FRET architectures, the distance
between the organic fluorophore and the QDs needs to be optimized
by a careful system engineering. In this context, BODIPY dyes
bearing amino-terminated functionalities are used in virtue of the high
affinity of amine groups in coordinating the QD surface. A preliminary
QD surface treatment with a short amine ligand is performed to favour
the interaction with the organic fluorophores in solution. The
successful coordination of the dye to the QD surface, accomplishing
a short donor-acceptor distance, provides effective energy transfer
already in solution, with efficiency of 76%. The efficiency further
increases in solid state where the QDs and the dye are deposited as
single coordinated units from solution, with a distance between the
fluorophores down to 2.2 nm, demonstrating the effectiveness of the
coupling strategy.
Introduction
Förster resonance energy transfer (FRET) is one of the key
mechanisms taking place between two interacting fluorophores,
affecting numerous advanced applications, including (bio)-
sensors,[1] energy harvesting[2] and light emitting diodes.[3] The
FRET process is an excitation energy transfer occurring via a non-
radiative dipole-dipole interaction between an excited donor (D)
and an acceptor (A). The FRET efficiency depends on the spectral
overlap between the donor fluorescence and the acceptor
extinction, on the D-A separation distance, typically ranging from
few Å to 10 nm, and on the relative orientation between the
transition dipole moments of donor and acceptor.[4] Moreover,
according to the classical FRET theory, a high quantum yield (QY)
of the donor is required to maximize the process efficiency.[5]
Inorganic quantum dots (QDs) are being increasingly exploited as
donors in FRET systems, thanks to their broad absorption spectra,
the size-tuneable narrow emission, the high QY and the excellent
1
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10.1002/chem.202003574
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chemical and photostability. Besides, QDs offer the benefit of
minimizing the acceptor direct excitation, as their broad
absorption band allows excitation in a large portion of the UV-Vis
spectrum, and to tune the FRET efficiency by hosting multiple
acceptors per QD.[6] QDs can be also employed as acceptors in
FRET systems, as they exhibit large spectral overlap integrals,
show high FRET-sensitization rate, since enable interfacing of
each QD with multiple donors, and allow to have one donor for
multiplexed FRET processes, with simultaneous participation of
two or more D-A pairs to the same system.[6]
to dye molecules, through the conjugation with the organic shell
surrounding the nanoparticles,[10a, 14] or involving the formation of
bioconjugates, also via non-specific interactions (electrostatic
interactions, hydrogen bonds) with biological molecules, as
proteins or oligonucleotides.[15]
Here, the purpose of the work is to fabricate highly efficient FRET
systems, in both solution and solid state, coupling colloidal CdSe
QDs and BODIPY molecules, properly derivatized with
aminostyril groups to allow their direct anchoring at the QD
surface.
Several examples are present in the recent literature concerning
the fabrication of FRET systems combining QDs-QDs of different
sizes or QDs and dyes. A variety of FRET studies performed on
thin films made of close-packed CdSe QDs with mixed sizes
demonstrate that the smaller QDs behave as exciton donors and
the larger ones as acceptors in the ET process.[7] Recently,
Hoffman et al. have demonstrated the key role of the QD surface
chemistry in the design and engineering of functional devices
based on FRET processes.[7b] In the application of the Förster
In particular, monoaminostyryl substituted BODIPY (MASB), with
one amino moiety, and diaminostyryl functionalized BODIPY
(DASB), a bidentate dye bearing two amino groups, have been
selected and synthesized.[16] A preliminary treatment of the QD
surface with ethanolamine (EA) has been performed to favour the
interaction with the organic fluorophores in solution. While the
DASB, due to its high steric hindrance and its poor spectral
overlap, provides
a limited FRET efficiency (29%), the
MASB/QDs hybrid system demonstrates very effective transfer
properties already in solution with an efficiency of 76% and a
calculated center to center D-A distance of 2.6 nm, consistent with
the effective coordination of the dye at the QD surface. The
coupling procedure results successful also when the QDs and the
organic chromophores are deposited on a substrate as single
combined units, with respect to the conventional layer-by-layer
approach. Indeed, the FRET efficiency of QDs/DASB and
QDs/MASB systems enhances up to 58% and 80%, respectively,
thus demonstrating the effectiveness of the coupling strategy
even in solid state.
formalism to
a homogeneously blended solid-state films
composed by two differently sized CdSe QDs, discrepancies
between the calculated and the measured FRET parameters
have been observed. Such incongruities have been ascribed to
several factors as QD shape, transition dipole orientation,
enhanced absorption cross-section or dipole-multipole coupling,
and highlight the limits of the Förster formalism when applied to
solid-state systems.[8]
FRET systems have been obtained by sensitizing the QDs with
organic dyes, combining them by means of hydrophobic
interactions, or by using an amphiphilic polymeric coating layer
able to interdigitate its chains with the pristine ligands.[9] On the
other hand, an effective interaction between the donor and
acceptor can be achieved by the (bio)conjugation of the QD
surface, obtained by directly replacing the pristine ligands with
organic dye molecules or by using functional linkers to mediate
the conjugation. In the first case, the dye is required to bear
chemical groups suitable to bind the QD surface (thiols, amines
or carboxylates). In the second case, a bi-functional linker can be
used to connect QD and dye, allowing the modulation of the
distance between donor and acceptor.
Among the organic dyes, compounds containing rigid aromatic
units or macrocyclic structures, such as phthalocyanines and
porphyrins, are largely used as FRET acceptors. Indeed, these
molecules present a relatively simple synthesis and can be
possibly further derivatized to insert specific functional groups
able to bind the donor surface.[5] In addition, a proper setting of
the number of functional moieties and their arrangement on the
dye aromatic ring may lead to the formation of polydentate ligands
that are expected to bind more strongly to the QD surface, due to
the chelating effect, thus, ultimately, envisaging higher FRET
efficiencies.
Results and Discussion
To design an efficient FRET system based on BODIPYs
successfully coupled to the QD surface, several issues need to
be considered. Firstly, QD size should be selected to confer to the
donor the emission spectrum well overlapping the absorption of
the dye. Then, both the fluorophores need to be dispersed in the
same solvent and, finally, the QD surface has to be sterically
available for the dye approach and its effective anchorage, to
minimize the D-A distance. While QDs prepared by means of hot
injection synthesis are generally highly dispersible in poorly polar
organic solvents, such as hexane, toluene or chloroform, the
aminostyryl substituted BODIPYs are soluble in polar organic
solvents, such as methanol or tetrahydrofuran. Therefore, to
disperse the chromophores in the same medium, the chemical
structure of the BODIPY and/or QD surface chemistry needs to
be suitably modified. For this purpose, an exchange of the QD
pristine capping layer with a new ligand has been performed.
Indeed, ligand exchange represents a simpler and more versatile
approach than the modification of the BODIPY chemical structure
that would require more demanding synthetic and purification
efforts, possibly inducing a substantial alteration of dye optical
properties. The new ligand needs to be a bifunctional molecule
able to coordinate QDs and, at the same time, expose polar end
groups to facilitate their dispersion in polar solvents. In addition,
to favour the dye approach at the QD surface, the ligand should
be endowed with a short and linear aliphatic chain. In fact, the QD
pristine capping layer, composed of long chain amines,
Few studies report FRET systems based on QD-linker-dye
architecture, where the rigidity of the linker plays also a
fundamental role in the energy transfer process, enabling a
systematic experimental validation of FRET theory.[10]
A coupled D-A FRET system has been developed by covalently
binding an ATTO-dye to the CdSe QD surface.[11] QDs-dye
coupled systems have been reported as effective pH-responsive
sensors, for example for measuring the cellular pH,[12] or for the
selective sensing of metal cations.[13] A wealth of examples
reports the application of FRET systems relaying on QDs coupled
phosphines or carboxylic acids, is characterized by
a
considerable steric hindrance, detrimental for the dye molecule
2
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penetration and anchoring to the QD surface. Then, the new
ligand should help thin out the QD surface. However, a capping
agent with a short alkyl chain cannot provide the QDs with
adequate colloidal stability in solution. On the other hand, the
original capping layer is effective in passivating the QD surface,
preserving the optical properties, while its complete exchange can
lead to a dramatic deterioration of the emission. Therefore, only a
partial replacement of the capping layer with the new ligand can
conveniently overcome these critical issues.
Here, a two-step procedure has been proposed for the fabrication
of the FRET system. Firstly, a preliminary partial capping
exchange of the QD surface by using a short chain ligand, namely
ethanolamine (EA), has been carried out. Such pre-treatment
ensures the dispersion of QDs in a solvent suitable to solubilize
also the dye molecules, reducing, at the same time, the steric
hindrance of the ligand at QD surface. Then, the second step
consisted in the functionalization of the QDs with the dye
molecules, by taking advantage of the aminostyril groups
available for the coordination to the CdSe surface.
Figure 1. (A) UV-Vis absorption (solid lines) and PL emission (dashed lines)
spectra of CdSe QDs capped by pristine (black, hexane solvent) and EA (red,
DCM/MeOH solvent) ligands. The inset shows a TEM picture of EA-QDs (scale
bar 50 nm). (B) TR-PL decays of the CdSe QDs before (black squares) and
after the capping exchange with EA (red squares), detected at the PL peak
(λExc=485 nm). The corresponding data fitting are reported as solid lines.
=6%) (Figure S2 in the SI), without any relevant nanoparticle
aggregation upon EA treatment (inset of Figure 1A).
The PL spectra (Figure 1A) show a decrease of the emission
intensity of EA- QDs. Figure 1B reports the PL decays of the QDs
before and after the EA treatment. The “as synthesized” CdSe
QDs have a multi-exponential fluorescence decay that has been
best-fitted by a 3-exponential function with an average lifetime of
18.7 0.5 ns (Table S1 in the SI), in accordance with the
literature.[20] Upon the EA introduction into the organic shell, a
substantial shortening of the recombination dynamics is observed,
with an average lifetime of 10.6 0.3 ns. Both the variations can
be ascribed to the partial deprotection of the QD surface, caused
by the limited desorption and displacement of the pristine ligands
by the short bifunctional EA molecules whose surface passivation
results less efficient. Moreover, the solvent composition and its
different polarity also affects the QD emission and the exciton
relaxation dynamics.[20a, 21]
Ligand exchange with EA
EA has been selected as bifunctional ligand to coordinate the QD
surface, being characterized by a short alkyl chain (~0.4 nm long),
and an amino functionality that has a good affinity with the CdSe
QD surface.[17] According to a recent classification,[18] TOPO and
HDA are L-type ligands, as possess a lone electron pair, able to
coordinate surface metal atoms, while OLEA is an X-type ligand
that can donate one electron. Any ligand exchange reaction at the
surface of the QDs has to ensure their electro-neutrality. The
addition of the short EA ligand can provide, by mass action, the
removal of some of the pristine amphiphilic long alkyl chain TOPO
and HDA molecules from the QD surface, opening available
space in the capping layer, which is advantageous for penetration
of new solvent or new molecules.
FT-IR-ATR analysis has been carried out in order to investigate
the chemistry at the CdSe QD surface before and after the
capping exchange with EA. The FT-IR-ATR measurements of the
EA-CdSe QDs (Figure S1 in the SI) clearly demonstrate that EA
binds the CdSe QDs through the amino groups, as discussed in
detail in the SI. Due to the -OH group of the EA ligand, the CdSe
QDs can be, in principle, dispersed in polar solvents. However,
the presence of residual pristine OLEA molecules, along with the
retention of TOPO and HDA ligands, only provides their sufficient
dispersion in a DCM/MeOH 70/30 % v/v mixture. Such a binary
mixture, able also to solubilize the BODIPYs, has been used as
solvent for the fabrication of the BODIPY/QDs FRET architectures,
as reported in the experimental section.
The UV-Vis absorption and PL spectra of the QDs, before and
after the ligand exchange, are reported in Figure 1A. The “as
synthesized” QDs show the typical absorption profile[19]
characterized by a well-defined first excitonic peak, which is also
preserved after the organic shell modification with EA (Figure 1A).
The tail at long wavelengths in the EA-QD absorption spectrum
can be ascribed to Rayleigh light scattering, due to their reduced
dispersibility in the binary solvent mixture that induce an
aggregation, though limited, of the nanoparticles, with respect to
that of the “as synthesized” QDs in hexane.
FRET systems in solution
Hybrid systems for ET have been fabricated by properly
functionalizing the EA-QDs with BODIPYs molecules. Namely,
two organic fluorophores, MASB and DASB, have been
specifically synthesized to promote the formation of the
supramolecular adducts by means the high affinity of the amino
group for the QD surface. Furthermore, the DASB, bearing two
amino functionalities, is expected to bring the additional
advantage of a bidentate coordination to the QD surface, although
its higher steric hindrance can be detrimental.
On the other hand, the introduction of substituent groups on
BODIPY skeleton at the meso and pyrrolic positions can
significantly affect the photophysical properties, and, accordingly,
turns into a different FRET behaviour. The MASB and DASB have
optical features in the red region of the electromagnetic
spectrum,[22] with main absorption peaks at 596 nm and 677 nm,
respectively, attributed to the S0→S1 transition[23] and
corresponding fluorescence emission bands at 673 nm and 713
nm (Figure S3 in the SI). The functionalization of the EA-QDs with
the BODIPYs has been performed according to the procedure
reported in the experimental section, using an excess of dye with
respect to EA-QDs.
The absorption line-shape of the EA- QDs/MASB system is shown
in Figure 2A. The close spectral proximity of the MASB signal to
the main excitonic peak of QDs hinders a clear identification of
the dye absorption, while the tail at long wavelengths reflects the
TEM micrographs and the statistical analyses, acquired on the “as
synthesized” and the EA-QDs, indicate that the QDs do not
significantly differ in terms of shape, size and size distribution,
presenting, in both cases, a mean particle size of 3.3 nm (σ%
3
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Figure 2. (A) UV-Vis absorption spectra of EA-QDs/MASB system (yellow line), bare EA-QDs (red circles) and MASB (blue circles). In inset: magnification of the
absorption peaks. (B) FT-IR-ATR spectra of EA-QDs/MASB system (yellow line), EA-QDs (red line) and MASB (blue line). (C) PL emission spectra of EA-QDs/MASB
system (yellow line), EA-QDs (red circles) and MASB (blue circles) (λExc 485 nm). For sake of clarity, the absorption and PL intensities of QDs and MASB are
displayed as normalized to the corresponding peaks of the coupled system. (D) TR-PL intensity decays of the EA-QDs (red squares) and EA-QDs/MASB system
(yellow squares), detected at the QD emission peak with the corresponding data fitting (solid line). (In inset) Decay profile of the MASB before (blue squares) and
after EA-QD functionalization (dark orange squares) (λExc 485 nm).
spectroscopic characteristics of the EA-QDs. However, a faint
variation in the spectral position of the main absorption peak of
the coupled system (Inset of Figure 3A) suggests a change in the
EA-QD. surface chemistry and environment that can be ascribed
to the interaction between the two fluorophores.
Figure 2B shows the FT-IR-ATR measurements of EA-
QDs/MASB functionalized system (Figure 2B, orange line), neat
EA-QDs (Figure 2B, red line) and MASB (Figure 2B, blue line).
The comparison of the spectra, which is discussed in detail in the
SI, assesses the coordination of the organic chromophore to the
QD surface in the hybrid sample, together with the presence of
residual EA capping molecules.
The fluorescence line-shape of the EA-QDs/MASB (Figure 2C)
clearly shows the characteristic contributes of the QD excitonic
emission and of the dye PL peak, respectively, that appear slightly
shifted with respect to the spectra of the single components. The
shift provides an evidence of the interaction taking place between
the two chromophores in solution. It is worth to note that the
spectra have been obtained by exciting the samples at 485 nm,
in the spectral range where the acceptor has a negligible
absorption, while the QD extinction coefficient is very intense. In
agreement with the results of the UV-Vis, FT-IR and PL
measurements, it can be reasonably assumed that the MASB
coordinates the QDs through supramolecular interactions,
involving weak forces between the two fluorophores.
Instead, in case of the EA-QDs/ DASB, the overall line-shape
resembles the sum of the single components and no shift in the
absorption peak is observed (Figure S5A in the SI). Only a slight
broadening of the QD emission on the high-energy side and a red
shift of the dye emission peak are detected (Figure S5B in the SI).
FT-IR-ATR investigation on the coupled system does not highlight
any significant variation in the vibrational spectra of the EA-QDs
in the presence of the dye (data not shown). In this case, in spite
of the possible, favourable, bidentate binding of DASB, a direct
coordination of the dye to the QD surface could not be assessed.
Such an evidence can be reasonably ascribed to the higher steric
hindrance and to a higher rigidity of DASB with respect to MASB
that hamper the approach to the QD surface through the capping
layer. However, the DASB and the QDs can interact through
FRET mechanisms in solution, if they are sufficiently close to each
other.
The distinctive mark of a FRET process between two fluorophores
is the quenching of donor emission together with the concomitant
enhancement of the acceptor fluorescence, which also reflects in
a shorter donor lifetime. The PL decays of the EA-QDs and of the
MASB, together with those of the coupled system, are reported in
Figure 2D, while the analogous measurements for the DASB
system are in Figure S5 C-D of the SI.
The surface functionalization with BODIPYs speeds up the QD
average lifetime from 10.6 0.3 ns to 2.53 0.03 ns for MASB and
to 7.51 0.01 ns for DASB, respectively (Table S1 in the SI). In
4
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addition, the single-exponential PL decay of the BODIPYs is fitted
by a bi-exponential function in the coupled system and gets
slower in time, passing from 1.75 0.02 ns to 2.06 0.01 ns for
MASB. The DASB shows an analogous behaviour, indicating the
occurrence of a FRET process, although the FT-IR-ATR analysis
does not highlight an effective coordination between the
fluorophore and the QDs. To further clarify this point and assess
the possible formation of supramolecular structures between QDs
and DASB, the rotational diffusion of the fluorophores has been
investigated by fluorescence anisotropy decay [r(t)], carried out
on the EA-QDs/DASB system (Figure 3). The rotational
correlation time, detected at the QD excitonic emission, appears
unchanged within the limits of the experimental error, by the
presence of the dye in solution (Figure 3A), whereas a significant
change is observed at the DASB emission (Figure 3B). The
anisotropy decay time for the neat DASB is fitted as a single
exponential with a rotational correlation time of about 0.5 ns
(Figure 3B, green squares), while the hybrid system shows two
rotational correlation times, one of about 1.5 ns and the other
longer than 20 ns (Figure 3B, orange squares). Such results
suggest that, in the hybrid system, the free rotation of DASB in
the solvent is inhibited and the dye molecules move together with
the QDs. This can be possibly ascribed to the entrapping of the
fluorophore in the organic shell surrounding the QD surface, due
to the intercalation of the aminostyryl functional groups of the dye
within the long alkyl chain tails of the QD ligands, without,
however, the occurrence of an effective surface coordination.
From our experimental findings, the occurrence of a FRET
process between the components of the hybrid systems in
solution can be reasonably assumed. The FRET efficiency can be
calculated by experimental parameters, namely by either the
fluorescence peak intensities or the amplitude-weighted average
lifetimes obtained by the multi-exponential fitting of the
Figure 3. PL Anisotropy decays of: (A) the EA-QDs/DASB system (dark orange
squares) and of the neat EA-QDs (red squares), detected at QD emission; (B)
the EA-QDs/DASB system (light orange squares) and the DASB molecules in
solution (green squares), detected at the dye main emission peak. λExc 390 nm.
The corresponding data fittings are represented as solid lines.
To calculate the energy transfer parameters for processes
involving QD donors, the classical ET theory is generally well
accepted, considering valid the point dipole approximation for D-
A distances larger than their respective sizes.[6] The equation
expressing the efficiency of the process can be recast as a
function of the donor-acceptor distance, d, and of R0, the Förster
radius, representing the distance where the energy transfer has a
50% efficiency:
ꢍ
ꢋꢂ
ꢌ
ꢀꢁꢂꢃꢄ
ꢅ
(2)
ꢍ
ꢍ
ꢋꢂ ꢎꢏ
ꢌ
where N is the number of acceptors per donor. The spectral
overlap between the donor emission and the acceptor absorption
contributes to the FRET efficiency, as the Förster radius can be
experimentally calculated by:[24]
ꢣ
ꢍ
ꢘ
ꢔ
ꢖ
ꢙ
ꢐꢑ ꢅ ꢒꢓln ꢕꢑ ꢗ ꢚꢔꢛꢖꢢ
(3)
ꢈ
fluorescence decays[24]
:
ꢟ
ꢡ
ꢋ ꢠ
ꢉ
ꢕꢜꢝ ꢞ
where ꢤꢜ is the orientation factor that can be considered equal to
2/3 for isotropic and dynamically randomly orientated donor and
acceptor molecules,[26-27] Φꢥ is the fluorescence quantum yield of
the donor, ꢦꢧ is the Avogadro’s number, ꢨ is the refractive index
of the dispersing medium and ꢩꢔꢪꢖ is the spectral overlap integral
that has been calculated by:[24]
〈
〉
ꢈꢉ
ꢊ
ꢀꢁꢂꢃꢄ ꢅ 1 ꢆ ꢇ ꢅ 1 ꢆ
(1)
ꢈꢉ
〈
〉
ꢊ
ꢈ
ꢇ
ꢈ
From the calculations, the FRET efficiency results of 76% for EA-
QDs/MASB and only of 29% for DASB hybrid system in solution,
respectively. The difference accounts for the stronger spectral
overlap between the QD donor and the dye acceptor (Figure 4)
and the lower steric hindrance of MASB with respect to DASB. In
agreement with the FT-IR measurements, MASB results to more
effectively coordinate the QD surface, thus reducing the D-A
distance, while DASB amino groups can only reasonably
interdigitate the QD ligands.
ꢡ
ꢔ
ꢖ
ꢉ
ꢁ
ꢈ
ꢛ ꢬ ꢔꢛꢖꢛ dꢛ
ꢫ
ꢔ ꢖ
ꢩ ꢪ ꢅ
(4)
ꢔ
ꢖ
ꢛ dꢛ
ꢁ
ꢈ
ꢫ
In equation (4), ꢭꢥꢔꢪꢖ is the donor fluorescence intensity, while
ꢮꢧꢔꢪꢖ is the acceptor molar absorption coefficient, both dependent
on λ. In order to obtain ꢐꢑ values in units of nm, the spectral
However, due to the large surface area of the QDs, the interaction
of more than one dye molecule on the same QD donor can be
expected.[4, 10a, 11, 25]
-1
-1
4
ꢔ ꢖ
overlap integral ꢩ ꢪ has been calculated in M cm nm , by using
nm and M-1 cm-1 as units for λ and ꢮꢧꢔꢪꢖ, respectively, in Equation
The direct assessment of the number of dye molecules for each
QD, based on the comparison of the extinction coefficient of
isolated and coupled components cannot be easily applied here,
due to the close overlap of the spectral bands of the fluorophores,
particularly for MASB, that does not allow a precise deconvolution
of the contributes, and to the possible presence in solution of free
dye. A more precise calculation can be obtained by a method
based on the Poisson distribution of the dye molecules around the
QDs. Such a method,[4, 10a, 25g, 26] allows to determine the average
number of acceptors for each QD, based on the fitting of the time
resolved decay of the FRET systems with a function that takes
into account the Poisson distribution. The calculations, reported
in the SI, indicate an average number of about 1.5 molecules of
MASB and 1.1 of DASB, respectively, per QD.
(4).
ꢔ ꢖ
Figure 4 shows the overlap integrals ꢩ ꢪ between the PL of the
QD donor and the extinction spectra of the dye acceptor and
highlights the presence of a strong resonance for MASB (A),
significantly weaker for DASB (B). Therefore, it is also expected
that fainter the spectral overlap, lower the FRET efficiency. From
equation (2) and calculating EFRET by equation (1) and the
experimental R0 values according to Equation (3) and
reference[7b], the distance d between the two fluorophores can be
obtained, as reported in Table 1.
It is worth noting that, for estimating the distance between the QD
and the dye molecule, the PL emission of the donor and the
absorption of the acceptor can be reasonably assumed to
originate from states positioned at the geometrical centre of
5
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Chemistry - A European Journal
FULL PAPER
Table 1. FRET parameters calculated by applying the ET[6] theory for the CdSe
QDs coupled to MASB or to DASB, both in solution and in solid-state films.
]
EFRET (%)
Solution
R0 (nm)
Solution
d (nm)
Solid
State
Solid
State
Solution
Solid
State
EA-QDs/
MASB
76
29
80[a]
3.2
3.5
2.5[a]
2.8
2.2[a]
55[b]
58[a]
3.3[b]
3.1[a]
3.2[b]
3.0[a]
EA-QDs/
DASB
4.1
Figure 4. Normalized PL emission spectrum of EA-QDs (red line, left axis)
superimposed to the extinction spectrum of (A) MASB (blue line, right axis) and
(B) DASB (green line, right axis). The resulting spectral overlap integrals, J(λ),
are represented as shaded areas.
[a] “Coupled” films; [b] “Bilayer” films. The “Coupled” and “Bilayer” labels refer to
the procedure used for fabricating the solid D-A systems, as described in the
experimental section and schematically summarized in Figure S7A and S8A of
the SI. The FRET efficiency has been calculated by using the average lifetimes
(Equation (1)).
Figure 5. Sketch illustrating the MASB (I, upper panel) and DASB (II, bottom panel) interaction with the QD surface, as emerging from the experimental results.
chromophores. In agreement with the spectroscopic results, the
presence of one aminostyryl group in the fluorophore structure
induces a shortening of the calculated centre to centre distance,
being 2.8 nm for MASB and 4.1 nm for DASB, respectively. The
D-A distance for EA-QDs/MASB system in solution is consistent
with the sum of the CdSe QD mean radius (1.7 nm) and the
expected mean size of the MASB portion bringing the amino
moiety (about 1.2 nm), thus confirming that MASB effectively
coordinates the CdSe QD surface, as also indicated by FT-IR-
ATR measurements and illustrated in the sketch in Figure 5.
Instead, in solution, the estimated d value of 4.1 nm for DASB
results larger than the simple sum of the QD mean radius and the
expected mean size of the DASB region carrying the amino
groups. Here, an accurate evaluation of such a distance needs to
take into account the presence of the organic ligand shell formed
by the ligand molecules that still partially coordinate the QD
surface, which is few Å thick for the EA and up to 2 nm thick for
the longer alkyl chain HDA. Therefore, in this case, the calculated
value of d, can be rationalized with the possible intercalation of
the dye into the alkyl chains of the native ligands for few tenths of
nm (Figure 5), as confirmed by the fluorescence anisotropy
measurements. Then, we can reasonably assume that, in spite of
possible multivalent interaction of the DASB at the QD surface,
6
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Chemistry - A European Journal
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the lower steric hindrance and the larger spectral overlap of
MASB allow for a shorter distance and a higher ET efficiency.
to solution (76%) (Table 1). Conversely, when FRET efficiency is
calculated for the EA-QDs/DASB system, values of about 55%
and 58%, for the “bilayer” and the “coupled” film, respectively
result (Table 1). These values have to be compared with the
efficiency of the same system in solution, that is only 29%. Such
a significant enhancement can be ascribed to an increased
proximity between the fluorophores in solid state, due to the
absence of solvent. Moreover, the lack of solvent induces a coiling
of the longer alkyl chains of the molecules capping the QDs, as
they cannot extend their structure, as it would happen in the
presence of a “good” solvent. As a result, a thinner shell is
expected, and concomitantly, leads to a decrease of the D-A
distance.
However, in solid state, possible HOMO FRET transfer
phenomena can also occur among the QDs in the close packed
films, as the inhomogeneous size distribution of the QDs and their
close proximity, together with the exciton diffusion towards
quenching sites, strongly affect the intrinsic decay rate.[7a, 28] The
calculated HOMO FRET contribution results lower than that of the
coupled dye/QD system, while it has a greater influence in the
case of DASB, as reported in the SI (Figure S9 and table S2).
The determination of the theoretical separation distance between
donor and acceptor can be obtained in solid state by applying the
FRET theory, as described above (Equations 2-4). According to
[7b], the calculation of R0 for solid state takes into account a
different value of the refractive index experimented by the QDs,
determined as a volume-weighted average of the refractive index
of CdSe and the organic components, and the change in the
overlap integral. In particular, the curves of the FRET efficiency
FRET systems in solid state
In view of their application in functional materials and sensing
devices, the design of FRET systems with enhanced efficiency[26,
27b] in solid state, where the distance between the fluorophores is
further decreased, is required. In literature, several examples
report efficient FRET systems based on QDs and/or QDs and
organic dye composites, obtained by casting or spin coating
techniques.[7b, 7c] Here, to demonstrate the effectiveness of the
coupling in ET processes the FRET efficiency of solid-state films
obtained depositing the previously coupled QD/dye architecture
directly from the solution has been evaluated. Two different
preparative methods for the FRET system fabrication as a film
onto a quartz substrate have been used: i) a conventional “layer
by layer” approach, relaying on depositing, by casting, a layer of
QDs followed by the deposition of a layer of acceptor molecules
(“bilayer” film) and ii) the direct deposition by drop casting of the
EA-QDs/MASB or DASB systems, previously coupled in solution
(“coupled” film). In all the investigated cases, spectroscopic
investigations show the fluorescence quenching of CdSe QDs
and the enhancement of the BODIPYs PL (Figures 6; S7 and S8
in the SI), indicating the occurrence of the FRET process. In the
calculation of the FRET efficiency in the solid films, the average
fluorescence lifetimes of QDs in solution have been used,
according to Kamat and co-authors[7b]. In the case of EA-
QDs/MASB coordinated system, a weak improvement of the
FRET efficiency has been obtained in the film (80%) with respect
Figure 6. (A) PL spectra of the EA-QDs/MASB coordinated system (yellow line) and of the EA-QDs (red line) deposited on quartz substrates (λExc=485 nm). (B)
The corresponding PL decays with data fitting (solid line), in correspondence of the QD emission peak. (C-D) Theoretical ET efficiency as a function of the D-A
distance, d, for (C) EA-QDs/MASB system in solid state (blue solid line) and in solution (red dashed line) and for (D) EA-QDs/DASB systems, in the two deposition
procedures, namely “coupled” (orange line) and “bilayer” (purple line) and in solution (green dashed line), calculated by Eq. (2). The solid points represent the
experimental values.
7
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10.1002/chem.202003574
Chemistry - A European Journal
FULL PAPER
as a function of the D-A distances can be plotted by equation (2),
imposing the experimental R0 value previously obtained.
In Figure 6C, the curves of the FRET efficiency for the EA-
QDs/MASB coordinated system in solution and in solid film (solid
and dashed
surface. A partial exchange of the native QD capping layer with a
short alkyl chain bifunctional ligand has been preliminary
performed to make QDs dispersible in the same medium of the
dyes and, concomitantly, to leave available space at the QD
surface, for the dye to approach and anchor. The proper
derivatization of the dye with one aminostyril group in MASB has
allowed the successful coordination of the dye at the QD surface
in solution, minimizing the D-A distance and resulting in high
FRET efficiency in solution. Conversely, the DASB demonstrates
a limited efficiency due to the higher steric hindrance and the only
partial spectral overlapping. The coupling procedures result
effective also in solid state, when the hybrid systems are
deposited onto substrate as single coupled units. The illustrated
strategy can effectively impact the design and preparation of
hybrid systems that can find application as FRET sensors both in
solution and in solid state. In addition, the very short D-A distance
achieved in the coupled systems allows for advanced
investigations of possible coherent interactions between
chromophores at such close distances in energy transfer
processes.
line, respectively) are compared to the experimental EFRET values
determined by the time resolved measurements (symbols). While
in solution a d value of 2.8 nm is obtained for MASB coordinated
at the QD surface, in solid state the D-A calculated distance
decreases down to 2.2 nm, thus suggesting a possible change in
the dye molecule geometry and a closer packing occurring in
absence of the solvent that reflects in a moderate increase of the
FRET efficiency. Furthermore, it has been demonstrated that,
when the acceptors are in close proximity to the QD surface,
complex interactions with surface states may provide some
deviations from the Förster formalism, including aberration in the
rigorous calculation of the D-A separation distance.[29] For the
QD/DASB systems, the curves of the FRET efficiency in solution
and for the “coupled” and “bilayer” films are reported in Figure 6D.
In solid state, the d values and the FRET efficiency calculated for
the two deposition modes described above, namely for the
“bilayer” and the “coupled” films, differ remarkably from the values
measured in solution (Table 1). These results can be justified
considering that, regardless the film fabrication procedure, in both
cases the absence of the solvent reduces the distance, forcing
the fluorophores to be in closer proximity, with a possible deeper
penetration of the dye in the organic shell covering the QD surface,
thus considerably increasing the FRET efficiency. The slight
difference in efficiency and d values calculated for the diverse
deposition modes still confirms that, even if the DASB does not
coordinate the QD surface, the deposition of the two fluorophores
as a single unit from solution provides higher FRET efficiency with
respect to the layer-by-layer approach.
Experimental Section
Materials and experimental methods of the syntheses of CdSe QDs,
mono-aminostyryl phenyl BODIPY (MASB) and di-aminostyryl phenyl
BODIPY (DASB) are described in detail in the Supporting Information.
CdSe QDs surface functionalization with EA The QD surface
functionalization was performed by diluting the as-synthesized QD hexane
solution (2×10-4 M) in 1 mL of DCM/MeOH solvent mixture (70/30 % v/v),
and quickly adding 20 μL (2×10-2 mmol) of a DCM/MeOH stock solution of
EA ligand (1.1 M). The final EA to pristine ligand-capped CdSe QDs molar
ratio was 2×103. The dispersion was vigorously stirred overnight, and then
the CdSe QDs were collected by repeated cycles of centrifugation at 9000
rpm and re-dispersion in the solvent mixture, allowing the removal of the
free ligand excess from the solution.
The overall results demonstrate that the MASB is able to
effectively coordinate the QD surface, thus providing a highly
efficient FRET architecture, both in solution and in solid state, due
to the limited steric hindrance of the dye and the strong spectral
overlap of its absorption spectrum with the QD emission.
Beside the wealth of applications that can be envisioned for this
system in the field of the FRET-based devices, the very short
distance achieved between the donor and the acceptor moieties
is highly promising in terms of the investigation of possible
coherent effects in the energy transfer process. Recent
experimental and theoretical findings have confirmed the
Surface functionalization by MASB and DASB The surface
functionalization of CdSe QDs with the BODIPY dyes was performed by
adding to 1 mL of EA-CdSe QDs solution (10-5 M in DCM/MeOH), 2×10-4
mmol of BODIPY dye (MASB or DASB, 5 ×10-3 M), so as to obtain a molar
ratio BODIPY to EA-CdSe QDs of 20. The resulting solution was left under
vigorous stirring overnight, and then the dye-functionalized CdSe QDs
were washed and collected by several cycles of centrifugation at 9000 rpm
and re-dispersed in DCM/MeOH 70/30 % v/v.
possibility of reliably measuring coherent dynamics processes in
30]
the relaxation dynamics of isolated QDs in solutions[16c,
or
CdSe QD film fabrication QDs films, used as references, were prepared
by simple drop-casting 50 μL of a 2×10-6 M of pristine or EA capped QDs
dispersion, respectively in hexane and DCM/MeOH, onto properly cleaned
quartz substrates. Similarly, the reference MASB and DASB films were
fabricated by drop-casting 50 μL of a 2×10-6 M dye solution in DCM/MeOH.
Bilayer QDs/DASB films were prepared by drop-casting 50 μL of a 2×10-6
M dye solution onto a previously deposited layer of pristine CdSe QDs at
the same concentration used to prepare the reference sample. Coupled
QDs/DASB based films were obtained by drop-casting 50 μL of coupled
EA-QD/ DASB systems in DCM/MeOH solution at a concentration of~
2×10-6 M onto cleaned quartz slides. Coupled QDs/MASB films were
prepared by drop-casting 50 μL of EA-QD/ MASB functionalized system in
DCM/MeOH solution at a concentration of~ 2×10-6 M onto cleaned quartz
slides.
interacting QDs in solid state films.[31] Thus, the QD/MASB hybrids
appear as ideal systems to investigate the presence of coherent
mechanisms of energy redistribution between the inorganic and
the organic moieties in the ultrafast timescales, opening up a
completely new niche of possible applications.
Conclusion
An interesting approach, based on the effective coupling between
CdSe QDs and BODIPY’dyes, has been proposed to realize
highly efficient FRET systems both in solution and in solid state.
For this purpose, two BODIPY based fluorophores have been
designed and synthesized, bearing, respectively, one or two
aminostyril functional groups, having a good affinity with the QD
Spectroscopic and morphologic characterization methods, and details
concerning the determination of the average number of dye acceptors for
8
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Chemistry - A European Journal
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each QD donor and possible contribution of HOMO FRET between QDs
are thoroughly described in the Supporting Information.
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Acknowledgements
This work is financially supported by the MIUR PRIN 2015 Project
n. 2015XBZ5YA. The INSTM and the EU funded H2020 FET
project COPAC (Contract agreement n.766563) are kindly
acknowledged.
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Keywords: BODIPY functionalization • Energy Transfer • FRET
QD-Dye • Luminescence Decay Dynamics • Quantum Dots
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Entry for the Table of Contents
Fabrication of FRET platforms, formed by colloidal CdSe QDs and fluorescent BODIPY dyes. The successful functionalization of the
QD surface by the dye allows to obtain very short distances and high FRET efficiency, both in solution and in solid state, where the
donor and acceptor are deposited in single coupled units.
11
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