On-off QD switch that memorizes past recovery from quenching by diazonium salts

Article abstract of DOI:10.1039/c003490h

The understanding of the interaction of CdSe/ZnS semiconductor quantum dots (QD) with their chemical environment is fundamental, yet far from being fully understood. p-Methylphenyldiazonium tetrafluoroborate has been used to get some insight into the effect of diazonium salts on the spectroscopy of QD. Our study reveals that the surface of CdSe/ZnS quantum dots can be modified by diazonium salts (although not functionalized), showing and on-off fluorescence behaviour that memorizes past quenching recoveries. Facile modification of the surface confers protection against quenching by new molecules of diazonium salt and other known quenchers such as 4-amino-TEMPO. The reaction mechanism has been explored in detail by using different spectroscopic techniques. At the first time after addition of diazonium salt over QD the fluorescent is turned off with Stern-Volmer behaviour; the fluorescence recovers following irradiation. Subsequent additions of diazonium salts do not cause the same degree of quenching. We have noted that the third addition (following two cycles of addition and irradiation) is unable to quench the fluorescence. Monitoring the process using NMR techniques reveals the formation of p-difluoroborane toluene as a result of the irradiation of diazonium-treated QD; the treatment leads to the fluorination of the QD surface.

Full text of DOI:10.1039/c003490h

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
On-off QD switch that memorizes past recovery from quenching
by diazonium saltsw
Marta Liras,* Marıa Gonzalez-Bejar and J. C. Scaiano*
´
´ ´
Received 23rd February 2010, Accepted 14th May 2010
DOI: 10.1039/c003490h
The understanding of the interaction of CdSe/ZnS semiconductor quantum dots (QD) with
their chemical environment is fundamental, yet far from being fully understood.
p-Methylphenyldiazonium tetrafluoroborate has been used to get some insight into the effect
of diazonium salts on the spectroscopy of QD. Our study reveals that the surface of CdSe/ZnS
quantum dots can be modified by diazonium salts (although not functionalized), showing and
on-off fluorescence behaviour that memorizes past quenching recoveries. Facile modification of
the surface confers protection against quenching by new molecules of diazonium salt and other
known quenchers such as 4-amino-TEMPO. The reaction mechanism has been explored in detail
by using different spectroscopic techniques. At the first time after addition of diazonium salt over
QD the fluorescent is turned off with Stern–Volmer behaviour; the fluorescence recovers following
irradiation. Subsequent additions of diazonium salts do not cause the same degree of quenching.
We have noted that the third addition (following two cycles of addition and irradiation) is unable
to quench the fluorescence. Monitoring the process using NMR techniques reveals the formation
of p-difluoroborane toluene as a result of the irradiation of diazonium-treated QD; the treatment
leads to the fluorination of the QD surface.
are quite sensitive to their capping ligand, highlighting the
Introduction
importance of surface phenomena. The presence of different
molecules close to the QD can change their luminescent
properties and these changes can be used for different applica-
tions. Fluorescence quenching or enhancement can be observed
as a result of small molecule-QD interactions.
Semiconductor nanocrystals or quantum dots (QD) have
attracted much attention due to their size-dependent physical
properties originating from quantum confinement.1,2 A wide
range of applications such as QD laser,3 biological markers,4
light emiting diodes (LEDs)5,6 have been investigated. These
semiconductor nanoparticles exhibit a broad excitation spectra
coupled with a narrow size-tunable luminescence emission,
high brightness and good photostability.1 QD are formed by
atoms from group II (alkyl metals, metal oxides or organic
salts) and group VI (Se, S and Te). The addition of an
inorganic shell (e.g., ZnS) leads to core–shell QD, which have
high fluorescence quantum yield and stability.7,8
Diazonium salts readily lose N2 by either direct photo-
excitation or through electron transfer processes.13 Direct
photoexcitation yields highly reactive aryl carbocations, while
the (frequently favored) electron transfer process tends to yield
aryl radicals also highly reactive towards a wide range of
substrates, both organic and inorganic (Scheme 1).
Possible electron donors include metal and semiconductor
surfaces, where electrochemistry has proven a valuable tool for
surface derivatization.14 In this case electron transfer from the
cathode leads to rapid nitrogen loss following one electron
reduction; aryl radicals are thus generated at the electrode
surface, where they can react in competition with diffusion
into the bulk liquid phase. There are also examples of sponta-
neous reactions in different materials such as nanotubes,15–17
metals such as Au, Pb and Pt,18 and semiconductors such as
silicon and GaAs.19 We wondered what effect diazonium salts
could have on the spectroscopy of semiconductor nanocrystals,
and whether photoinduced electron transfer could be used to
QD are usually synthesized using trioctylphosphine oxide
(TOPO) and trioctylphosphine (TOP) as ligands, which make
them soluble in organic solvents. Understanding ligand exchange
dynamics and mechanisms in which the TOPO ligands on the
QD surface can be replaced with the ligand of choice has
become crucial since one could obtain, for example, quantum
dots soluble in water or make them amenable to chemical
reactions as desired. Thiols have proven useful in ligand
exchange due to their high affinity for the particle surface.9
Alternatives to thiols include carbodithioates,10 peptides,11
and oligomeric phosphines.12 The optical properties of QDs
Department of Chemistry and Centre for Catalysis Research and
Innovation, University of Ottawa, Ottawa, Ontario, K1N 5N6.
E-mail: tito@photo.chem.uottawa.ca; Fax: +1 613 562 5633
w Electronic supplementary information (ESI) available: Pictures of
QD solutions after additions of compound 1, lifetime distribution for
QD610 and QD546, fluorescence recovery comparison for QD610 under
UVB and UVA illumination, evolution of QD546 fluorescence and
data for core QD558. See DOI: 10.1039/c003490h
Scheme 1 Electron transfer processes of diazonium salts.
Phys. Chem. Chem. Phys., 2010, 12, 9757–9762 | 9757
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modify these particles. With these ideas in mind, we explored
the interaction of core–shell CdSe/ZnS quantum dots (QD)
with a simple diazonium salt (R = CH3 in Scheme 1).
was prepared in a precision quartz cuvette. Added to the
cuvette were varying amounts of a 2.02 mM or 20.2 mM fresh
solutions of 1 in acetonitrile, such that the total volume added
to each cuvette in each addition never exceeded 50 mL. In all
UV-Visible and fluorescence experiments, 1 was added in the
dark (and kept in the dark). Samples were exposed to illumi-
nation exclusively for the fluorescence recovery studies. For
the experiments with 4-amino-TEMPO, aliquots of 2 mL of
stock solutions (20 mM for high concentrations and 2 mM for
lower ones) were added.
Experimental
General details
p-Toluidine and 4-amino-TEMPO (4aT) were purchased from
Aldrich and used without further purification. CdSe/ZnS QD
were purchased from Evident Technologies and the experi-
ments were carried out in toluene solution at room tempera-
ture. In order to remove excess TOPO free in solution, the
QD610 and QD546 (where the subscript identifies the maximum
emission wavelength) were precipitated with methanol, centri-
fuged and redissolved in the same amount of toluene. The
concentration of the new solution was determinated using a
literature protocol.20
Core CdSe QD558 were synthesized as previously reported.21
Absorption spectra were recorded using a Cary-50 UV-Visible
spectrometer. Fluorescence was recorded with a Photon Techno-
logy International (PTI) spectrofluorometer. Fluorescence life-
time measurements were recorded using an Easylife LS (PTI)
using a 490 nm pulsed LED and the appropriate filters for
each case. In all cases the experimental error was r5% (unless
otherwise indicated). The irradiation of the samples was
performed with Luzchem LZC-ORG photoreactor using
6 visible or UVA lamps. Dynamic light scattering measure-
ments were recorded using Zetasizer Nano (ZS) apparatus
from Malvern with the 4 mW, 633 nm He–Ne laser.
Characterization of p-difluoroborane toluene (2)
To a solution of 2.25 mL QD610 (23 mM) in CDCl3, 21 mL of 1
(480 mM) were added in a NMR tube (final solution of 1
was 4.6 mM). The sample was irradiated for 3 h. The proccess
(addition and irradiation) was repeated twice. Then,
the sample was precipitated with deuterated methanol and
centrifuged. The methanolic part was analyzed by NMR
1
(Fig. 1). H NMR (MeOD, 400 MHz): d 7.18 (J = 8.4 Hz,
d, 2H, ArH), 7.10 (J = 8.4 Hz, d, 2H, ArH), 2.21 (s, 3H, CH3).
11B NMR (MeOD, 128.4 MHz): d 20.3 ppm. 19F NMR
(MeOD, 376.5 MHz): ꢁ152.46 ppm.
Result and discussion
The choice of core–shell CdSe/ZnS QD and the diazonium salt
1 for this study deserves some comment: core–shell QD of this
type have proven quite stable,1 highly luminescent, leading to
numerous applications.3–6 The synthesis of the diazonium salts
is generally straightforward if the corresponding aromatic
amine is available.22 Thus, the para-CH3 group in the diazonium
salt selected is a representative substituent that can be easy
detectable by 1H-NMR. In our case p-CH3C6H5N2+, BF4ꢁ (1)
was synthesized from 4-aminotoluene as described in the
NMR spectra were obtained on a Bruker Avance-400
spectrometer at 400 MHz using CDCl3 or methanol-d4 as
solvent without an internal standard.
Synthesis of p-tolyldiazonium tetrafluoroborate salt (1)22
To 1.98 g of boron trifluoride etherate (1.7 mL, 13.9 mmol)
contained in a three-neck round-bottom flask fitted with two
addition funnels and a reflux condenser connected to a gas
buret was added 1 g (9.3 mmol) of p-toluidine in 10 mL of
anhydrous THF. Prior to amine addition, the boron trifluoride
etherate was cooled at ꢁ15 1C in an ice-acetone bath. If a solid
amine-BF3 complex formed, additional solvent or ethyl ether
were added to produce a homogeneous solution. tert-Butyl
nitrite 1.3 mL (11.6 mmol) in 5 mL of the same solvent was
added dropwise to the rapidly stirred solution over a 10 min
period. Following complete addition, the temperature of the
reaction solution was maintained at ꢁ15 1C for 10 min and
then allowed to warm to 5 1C in an ice-water bath over a
20 min period. A crystalline precipitate usually formed during
the addition of tert-butyl nitrite, and following the 20 min
period at 5 1C, precipitation was completed. The solid was
filtered, washed with cold diethyl ether, air-dried, and weighed
to afford 1.6 g (92% yield) of white needles. 1H NMR (MeOD,
400 MHz): d 8.47 (J = 8.4 Hz, d, 2H, ArH), 7.78 (J = 8.4 Hz,
d, 2H, ArH), 2.63 (s, 3H, CH3).
UV-Visible or steady state fluorescence quenching experiments
In a typical experiment, 2 mL of a toluene solution of QD
0.5 mM (for both UV-Visible and fluorescence spectroscopy)
Fig. 1 Characterization of p-difluoroborane toluene 2 by NMR.
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Fig. 3 Stern–Volmer plot of a QD610 solution (0.5 mM) in toluene (top)
and QD546 solution (0.5 mM) in toluene (bottom). Insets: QD610 (top)
and QD546 (bottom) emission spectra (top) upon incremental additions
of 1. The ratio is represented F0/F by circles and t0/t by squares.
Fig. 2 Changes in absorption spectra for QD610 (top) and QD546
(bottom), both 0.5 mM in toluene upon addition of incremental
amounts of 1, and recorded 30 min after addition. Inset (top): A)
QD610 absorption in the first exciton peak at different concentrations
of 1 monitored until 1 h after addition. Inset: Absorbance changes in
the first exciton peak for QD610 (top B) and QD546 (bottom) with
incremental amounts of 1.
The absorbance changes of Fig.
2 are accompanied
by extensive fluorescence changes for QD610 and QD546
(Insets Fig. 3). This type of quenching is reminiscent of what
occurs with non-binding nitroxides such as TEMPO, with a
characteristic upward curvature that can be fitted to a Perrin
Model (based on the concept of the sphere of action) which in
experimental section. Commercial QD610 and QD546, with
TOPO (trioctylphosphine oxide) as stabilizer, were used
in toluene. Addition of 1 to solutions containing QD
(0.5 mM) results in a linear increase in absorbance for both,
QD610 and QD546, that is proportional to the concentra-
tion of 1 and is complete in less than an hour. The process
follows first order kinetics with k (QD610) = (1.3 ꢂ 0.1) ꢃ
a
simplistic representation assumes that only immobile
quenchers with a certain radius are effective.26 This model
predicts that quenching should follow eqn (1), rather than the
conventional Stern–Volmer equation.
I0 F0
¼ ea½Quencherꢄ
ð1Þ
10ꢁ3 ꢁ1, as is illustrated in inset A and inset B in Fig. 2A,
s
¼
I
F
respectively.
Absorption spectra changes for QD upon addition of
ligands has been mainly attributed to the QD high sensitivivity
to local changes in the environmental charge distribu-
tion.21,23,24 Thus, Bawendi et al. have shown how J-aggregated
cyanine dyes donors are electrostatically associated to the QD
in solution via surface ligands producing a narrow-band
absorption enhancement of colloidal quantum dots (QD)
where I0 and I are the observable emission intensities. Thus,
the exponential fitting for the ratio between areas in the
absence or the presence of 1 gives a = 32.4 Mꢁ1 for QD610
and a = 103 Mꢁ1 for QD546
.
It is well known that QD quenching efficiency is size-
dependent, being stronger for smaller QD.27 Therefore, lower
concentrations of 1 are needed to achieve the fluorescence
quenching for QD546 than for QD610. As expected, a is much
through efficient Forster resonance energy transfer (FRET).24
¨
bigger (103 Mꢁ1) in this case due to the smaller size of QD546
.
We have recently shown that QD UV-Visible spectra undergo
significant changes upon 7-mercapto-4-methylcumarin addi-
tion due to charge transfer interactions.21 A possible origin
for spectral changes would be Rayleigh scattering as a con-
sequence of QD aggregation. However, all the solutions
remained clear and stable (See Supporting Information
Fig. S1–S3) at the concentrations used for this study. Further,
it has been recently shown for PbSe that the rising absorption
of the Q-PbSe suspension at short wavelengths is not due to
Rayleigh scattering, but it reflects a strong increase in the local
field factor at these energies.25
Time resolved fluorescence studies suggest that there is a
mixture of static and dynamic quenching, as illustrated in
Fig. 4 where the decay traces show both a decrease of the
initial maximum intensity (static) and a shortening of the
lifetime (dynamic). It is common for the fluorescence decay
of QD to show complex behavior and our system is no
exception. We show the kinetic time profiles obtained for
QD610 (Fig. 4A) and QD546 (Fig. 4B) in the absence and
presence of different amounts of 1. At higher concentrations
the signal is much weaker and the analysis less reproducible.
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Fig. 5 Dynamic light scattering of QD610 (0.5 mM) in three indepen-
dent measurements (dash lines) and QD610 to 0.3 mM of 1 (solid line).
Fig. 4 (A) Kinetic time profiles of QD610 (0.5 mM) in toluene with
different amounts of 1 (0.02 to 0.1 mM). (B) Kinetic time profiles of
QD546 (0.5 mM) in toluene with different amounts of 1 (0.01 to
0.04 mM). Red traces are IRF.
involve the short lived diazenyl radical (ArN2ꢅ). However,
once N2 has been eliminated there is no opportunity for
reversal and the tolyl radical could bind to a Zn site right on
the surface.29 Indeed, as illustrated below, our results show
that the quenching can be completely reversed upon con-
tinuous illumination of the ‘‘quenched’’ QD with either
UVA (centered at 350 nm) or visible light (Fig. S5) after 2 h
(Fig. S6). The fluorescence recovery is a first order kinetic
process either under nitrogen (k = 0.034 ꢂ 0.001 minꢁ1 or air
k = 0.020 ꢂ 0.001 minꢁ1).
Exponential distribution analysis (ESM) shows that the behavior
is largely trimodal and this can be approximated with a
triexponential function, from which average lifetimes can be
calculated (Fig. S4).
A Stern–Volmer plot based on lifetimes (Fig. 3) showed a
linear dependence of t0/t ratio with concentration, while F0/F
has a marked positive curvature; this behaviour has been
attributed to a model of quenching by a sphere of action.28
This model interprets non-linear quenching as a mixture of
dynamic and static quenching, with the latter reflecting near-
contact at the time of excitation, and tends to dominate at high
quencher concentrations, as it does in this case. From this
plots the Stern–Volmer constants were deduced giving a
KSV = 4 ꢃ 104 Mꢁ1 and KSV = 1 ꢃ 104 Mꢁ1 for QD610
and QD546, respectively. Taking into account the fluorescence
average lifetimes for QD610 and QD546 (11.9 ns and 2.3 ns
respectively) the values of bimolecular quenching constants
A second addition of fresh diazonium salt leads to quenching,
but the level of quenching is roughly one half of that achieved
with fresh QD. Fig. 6 illustrates these changes for QD610
whereas QD546 are shown in Fig. S7. When these QD are
irradiated, 100% fluorescence recovery is again achieved,
while a third addition of 1 results in no further quenching.
Examination of Fig. 6 leads to several important conclusions.
First, the diazonium salt must modify the QD surface following
fluorescence recovery, since otherwise a second addition of
quenching would lead to similar quenching as the initial
addition; clearly this is not the case, and the particles have
‘‘memory’’ of past quenching-recovering processes. Second,
the fact that 0.02 mM concentrations of 1 are sufficient to
completely prevent quenching when the third addition of 1 is
made suggest that 0.04 mM of 1 (in two additions) is sufficient
to completely protect all quenchable sites on the QD surface.
Recovery of the emission and modification of the QD surface
as in Fig. 6 leads to a minor (3 nm) blue shift in the
fluorescence spectrum of QD610, but suggests a minor decrease
in particle size or reorganization of the surface. In any event
this effect is very minor.
are ca. B1012 Mꢁ1 ꢁ1; which, even allowing for the large
s
dimensions of QD is significantly faster than diffusion controlled
kinetics. This analysis suggests that it has to be a non-random
distribution of diazonium salts in the ground state. Therefore,
the diazonium salt could be in the proximity of the QD surface
(whether on the actual surface or in the TOPO layer), thus
altering the QD environmental charge distribution, in agree-
ment with the absorption changes.
QD610 nanoparticles have a diameter of 5.2 nm (as indicated
by Evident), although dynamic light scattering (DLS) gives
dimensions closer to 8–14 nm of the QD emission with the
instrument laser (633 nm), a rather common problem with
strongly fluorescent nanomaterials. Addition of 0.3 mM 1
caused extensive fluorescence quenching, thus overcoming
the problem and enabling the determination of a hydrodynamic
diameter of 5.7 nm consistent with known dimensions (Fig. 5).
This useful tool could be used to determinate the sizes of
fluorescent particles using DLS techniques.
We note that the fact that fluorescence recovery occurs both
with UV and visible light confirms that the process is initiated
by QD excitation and not direct diazonium salt photolysis,
since the diazonium salt is transparent in the visible region. If
the fluorescence reversal is carried out in presence of a DC
magnetic field the recovery is faster, which supports a radical
mediated process (see Fig. S8).
We asked ourselves about the possible mechanism for
quenching and suspected that given the redox properties of
both QD and diazonium salt, electron transfer would likely be
involved;29 that is, excited QD could transfer an electron from
their conduction band leading with similar consequences as
those in Scheme 1.
1
A detailed H-NMR study together with the spectroscopic
results shows that prior to irradiation, there are different
signals in the aromatic region that can be attributed to
different environments for 1 after adsorption (see Fig. 7b).
However, upon illumination all the aromatic signals collapsed
to one that could be easily assigned to single para aromatic
system (Fig. 7c). We had hoped that the aryl group would
Such a process would involve a sacrificial role for the
diazonium salt, since the only opportunity for reversal would
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Fig. 6 Bottom: Evolution of QD610 (0.5 mM) fluorescence in toluene
with different concentrations of 1 after the first addition , first
irradiation , second addition , second irradiation
and third
addition ’. Top: detail for QD610 with 0.06 mM of 1; the horizontal
axis corresponds to subsequent stages of the experiment. Irradiations
correspond to 2h of UVA exposure.
1
remain bound to the surface. However, H-NMR, 19F-NMR
and 11B-NMR results were consistent with the presence of
p-difluoroboranetoluene 2 in the reaction mixture (see Fig. 1).
Aryl-Zn in presence of Lewis acids, such as BF3, have been
reported to produce 2.30,31 Our results indicate that as soon as
the aryl group is bound to the QD surface, it leads to 2, which
could possible form a complex with the S present in the QD
shell (Scheme 2).
In fact, 2 can be easily removed from the media if the QD
are precipitated with methanol due to the higher affinity of
boron for oxygen than for sulfur. A possible explanation for
the QD protection against subsequent quenching would
involve surface modification due to the F group attached to
the zinc atoms.
On the other hand, the treatment of QD610 with 1 (additions
and subsequent irradiations) changes the response against
Fig.
7
1H-NMR spectral evolution in the aromatic region
(8.5–6.8 ppm) for (a) QD610, (b) QD610 with 0.1 mM of 1, (c) QD610
with 0.1 mM of 1 after irradiation and (d) QD610 after second addi-
tion of 0.1 mM of 1, second irradiation and then addition of 0.2 mM
of 1.
other quenchers such as 4-amino TEMPO (4aT).27 Quenching
of TOPO-capped core shell QD by 4aT has a characteristic
bimodal behaviour. The first type of quenching, at low
quencher concentrations, is due to a presence of vacancies in
the ZnS shell and, at high concentrations is characteristic of
the exchange of TOPO. Fig. 8 shows the comparative
Stern–Volmer plot for the quenching of QD with and without
treatment with 1 by 4aT. Our result cleary shows a change in
both types of quenching, which also argues for surface
modification.
While illustrated with core–shell QD, a similar quenching
behavior is also observed in the case of core-only CdSe, QD558
(synthesized by Peng’s method).32 In this case much lower
concentrations of 1 are required, as already observed in other
quenching studies for core-only particles (see Fig. S9–S10).
That is, the shell offers considerable protection toward
quenching by molecules in solution. However, the quenching
reversal does not take place in this case due to the unstability
of core CdSe QD upon irradiation.33
Scheme 2 Proposed mechanism for the interaction between QD610
and 1.
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Overall our results lead us to conclude that diazonium salt
provides an easy route for the modification and protection
against quenching for core–shell CdSe/ZnS QD. However, this
method does not lead to aromatic surface modification under
our conditions. Beyond this conclusion, we have learned that
diazonium salt treatment offers a simple experimental ‘‘trick’’
to facilitate DLS measurements where under normal condi-
tions these measurements are sometimes prevented by fluores-
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combination of static and dynamic processes that can be fitted
with a Perrin model in a similar manner as non-binding
nitroxides. Upon photoexcitation an electron transfer proccess
from the QD to the diazonium salt leads to the generation of
aryl-Zn species which in the presence of BF3 gives rise to
p-difluoroboranetoluene. Once all the quencheable sites on the
QD surface have been used, diazonium salt cannot quench the
QD anymore and this new protected surface has attenuated
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We thank the Natural Sciences and Engineering Research
Council of Canada, the Spanish Ministry of Science and
Innovation, FECYT, CFI and the Government of Ontario
for support. We also thank Dr Natalia Pacioni for her helpful
comments.
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