NMR Determination of a Solution-Phase
Binding Isotherm for Para-Substituted
Anilines on CdSe Quantum Dots
Undergraduate Researcher
Jacqueline M. Godbe
Northwestern University, Evanston, IL
Faculty Mentor
Emily A. Weiss
Department of Chemistry
Northwestern University
Graduate Student Mentor
Martin D. Donakowski
Department of Chemistry
Northwestern University
Abstract
A systematic nuclear magnetic resonance (NMR) study of parasubstituted anilines (R-An) and cadmium selenide (CdSe) quantum
dots (QDs) provides insight into the solution-phase equilibrium of these
two compounds. For different ratios of CdSe QDs to R-An, different
chemical shifts of the R-An ortho protons are observed in their NMR
spectra. These shifts represent a weighted average of the bound and
unbound (free) R-An species in solution. By establishing the chemical
shifts of the free and bound R-An species, the ratio of free R-An to bound
R-An in a sample can be determined. Fitting this data with a Langmuir
isotherm yields equilibrium constants for each system.
Introduction
Dye-sensitized solar cells (DSSCs) are a new and exciting avenue in
alternative energy research. These low-cost solar cells have conversion
efficiencies greater than 10%—higher than many other types of solar
cells recently introduced.1,2 DSSCs convert sunlight into energy by
means of an excitation cascade. When DSSCs are exposed to sunlight,
photons from the light excite dye molecules bound to the surface of
TiO2 nanoparticles. The excited dye then injects an electron into the
conduction band of the TiO2 nanoparticles.1 The dye is then regenerated via a redox reaction to return the system to its starting state to
undergo further energetic conversions.
Quantum dot–sensitized solar cells (QDSSCs) offer a new avenue
to improve on the existing technology. QDSSCs can produce multiple
hole/electron pairs from a single photon, and they have superior
tenability—the ability to custom-design the range over which the
cells can absorb photons.3
Many of these desired properties are dependent on the surface
chemistry of the QDs. Due to the size of these particles, a significant
portion of the crystal is on the surface. It has been shown that the
surface chemistry—namely, the ligand types that attach to the surface
sites—can effectively control many QD properties, from their
fluorescence and absorbance to solubility.4,5 QD tunability and how
successfully QDs bind to the desired substrate depend on what occurs
at the surface. Both factors are influential in the design of QDSSCs.
However, it is difficult to quantitatively determine how many ligands
attach to the surface of a QD under certain conditions and how strongly
they do so.
Recent inquiries have provided some insight into the binding
constants for various compounds. For example, certain interactions
between QDs and ligands are visible in NMR.6,7 However, the relative
binding strength of weak ligands to CdSe QDs is not known. These
QDs are most useful because they can be tuned to cover virtually the
entire visible range and have been used in the QD/ZnO nanowire solar
cell produced by Leschkies et al.3,8 For this reason, determination of the
binding affinity of ligands to CdSe QDs is important and necessary for
further developments in this field.
Background
The quest to determine binding constants for QDs is not new. Many
methods have been used, the most prevalent being fluorescence
spectroscopy.9,10 However, it has been shown that fluorescence is not
linearly correlated to surface coverage.11,12 An effective method is to
use NMR spectroscopy instead because it can identify the binding of
a particular compound to colloidal nanoparticles.
In 1H-NMR spectroscopy, magnetic pulses are used to probe
molecules by examining the electron density around their hydrogen
atoms. When hit with a magnetic pulse, the spin of each proton is
momentarily perturbed and responds with a characteristic resonance
that depends on its electron density. This can be useful in identifying
compounds, as many molecules and organic functional groups have
known NMR signatures. This type of spectroscopy can also be used to
study smaller-scale differences that affect electron distribution within
a molecule, such as solution effects.
The possibility of NMR use with nanoparticles was raised when
Moreels et al. used 1H-NMR spectroscopy to find the equilibrium
constant for indium phosphide (InP) QDs with trioctylphosphine oxide
(TOPO).6 In this system, NMR revealed two separate peaks for TOPO
corresponding to TOPO molecules that were bound to the surface of
the QDs and those that remained free in solution. This distinction is
possible because the InP-TOPO equilibrium exchange occurs slowly
with respect to the relative NMR timescale, thus allowing the two
peaks to remain distinct from one another.6 This is not the case with
fast-exchanging ligands on nanoparticles.
Fast exchange occurs between CdSe and R-An. The R-An adsorb
and desorb (Figure 1) from the surface of the QD so quickly that only
one broad chemical shift is observed on the NMR spectrum. This shift
corresponds to the weighted average of the bound and free peaks
observed in other systems.7 Different ratios of bound and free ligands
therefore display different shifts. This has been demonstrated in alkyl
www.nanoscape.northwestern.edu Volume 8, Issue 1, Summer 2011 Nanoscape 45
NMR Determination of a Solution-Phase Binding Isotherm for Para-Substituted Anilines on CdSe Quantum Dots
(continued)
Figure 1. A schematic of the equilibrium-binding scheme of MeO-An to the cadmium
surface of a QD.
Figure 3. The ortho-proton peak of MeO-An is the feature on the right-hand side. The
meta-proton feature appears on the left. Due to the resonance structure of the
aromatic ring, the ortho proton is more susceptible to shifting as a result of electron
withdrawal by the quantum dot. This shift was larger and thus was used in this study.
The two peaks in the ortho-proton doublet were averaged to obtain δtotal .
In order to vary the amounts of ligand bound on CdSe QDs and
free ligand in solution, the ratio of R-An to QD in solution was varied.
At low concentrations of R-An, nearly all of the R-An will be bound
to the surface of the QD, enabling the bound peak’s location to be
estimated. Conversely, at high concentrations of R-An, the observed
shift will approach that of the entirely free R-An.
Before this model can be used, however, it is necessary to prove
that the ligands are indeed binding to the surface of the QDs. To do
this, nuclear Overhauser effect spectroscopy (NOESY) NMR was
employed. In this type of spectroscopy, spatially proximal molecules
are able to “talk” with each other and exchange spin information via
the nuclear Overhauser effect. In the case of R-An/CdSe systems, only
R-An that were bound to the surface of the QD would be close enough
together for a long enough time to communicate. A signal here provides
proof of binding.7
Results
Figure 2. The unadjusted data for Br-An showing the averaged shift of the two
ortho-proton peaks compared with the Br-An:QD ratio in solution. All solutions
were kept at 0.750 mL to ensure that volume considerations were reduced.
amine/CdSe systems, though the observed shift in these systems was
too small to produce quantifiable results.7
Approach
Anilines are a logical choice to study the bound/free equilibrium
described above. The aromatic nature of the benzene ring produces a
larger shift between the bound and free states compared with aliphatic
amines. Additionally, by substituting different groups in the para
position, a wide range of pKa’s can be achieved.13 This allows the
exploration of electronically different ligands while minimizing
concerns such as steric differences. The aniline derivatives 4-bromoaniline (Br-An) and 4-methoxyaniline (MeO-An) were chosen
for this series as representative electron-donating and electronwithdrawing ligands, respectively. Additionally, both ligands have
been shown to have different effects on the photoluminescence of
QD based on concentration.8
46 Nanoscape Volume 8, Issue 1, Summer 2011 www.nanoscape.northwestern.edu
In order to establish that the R-An ligands bind to the nanoparticles,
NOESY 2-D NMR spectroscopy was used to examine the QD/R-An
mixture.7 Cross-peaks between R-An and octylphosphonic acid (OPA)
on the surface of the dot showed that the R-An were indeed binding
to the dot.
In 1H-NMR, downfield shifts of 20–30 Hz (0.04–0.06 ppm)
were observed between R-An and QD mixtures and R-An standards,
a tenfold increase over the 2–3 Hz observed by Fritzinger et al.7 The
chemical shift of MeO-An with respect to the MeO-An:QD ratio is
shown in Figure 3. This shift is caused by electron withdrawal by the
Cd2+ ions on the surface of the QD or the bond between the QD and
the ligand. This donation removes electron density from the R-An and
moves the chemical shift of the bound R-An (δbound) upfield relative to
the chemical shift of the free R-An (δfree).
The total amounts of free and bound ligand in solution can be
obtained from the following equation:
The values for Br-An δfree were obtained from the chemical shift of
Br-An in dichloromethane without the presence of QDs. However, it
was noted that the chemical shift of MeO-An shifted in response to
concentration. Thus, the values for MeO-An δfree were obtained from
the fit of the calibration curve given in Figure 4. Although this
contribution was significant to the observed signal change based on
concentration, it did not account for all of it.
The values for δbound were obtained by assuming 50% of the R-An
population was bound to the QD at the lowest added concentration.
An estimate of K was obtained from this approximation and used to
recalculate the amount of R-An bound to the surface at the lowest added
concentration of R-An. This process was repeated until the iterations
converged to yield δbound.
Since the total amount of R-An in the system is known, equation (1)
can be solved to yield both [R-An:QD] and [R-An]. [R-An:QD] is then
taken and divided by the number of QD surface sites.14 This calculation yields the total surface coverage of the QD by R-An. Plotting these
concentrations against one another shows how fractional surface
coverage of the QD corresponds to the concentration of R-An in
solution. This graph can be fitted to a Langmuir curve of the form:
The Langmuir isotherm is an appropriate choice for modeling this
system because (a) it fits the data collected, (b) it is the simplest binding
model for surface adsorption with the least variables, and (c) it allows for
physical interpretation of the observed trends. The K obtained from the
Langmuir fit is the equilibrium constant that describes the affinity
between the surface of the QD and the ligand. For Me-OAn, K = 282
+/- 31. For Br-An, K = 459 +/- 106.
Discussion
The ratio of R-An:QD to available binding sites on the dot consistently
approached values between 0.3 and 0.4. Because the Langmuir equation
approaches 1, the scaling constant, A, was used to reflect this value.
Assuming two ligands attach per Cd 2+ atom, these numbers match the
percentage of free space left by OPA surface coverage for QDs purified
with the purification procedure used.15 OPA coverage can be modeled
as a constant for two reasons. First, it is a much stronger binding agent
than R-An, so competitive binding effects are negligible. Second, OPA
requires a proton to detach from the surface of the QD.15 Dichloromethane (the solvent used in the experiment) does not provide this
proton, nor were there acidic protons available in the R-An.
The Langmuir fit, however, requires several assumptions. First, it
assumes all binding sites are equivalent. The first ligand binds just as
strongly as the final ligand. Second, it assumes that all interactions
between the ligand on the surface of the QD are negligible. In reality,
concentration data from this study show that MeO-An in solution is
highly sensitive to concentration and interacts with itself even in the
absence of QD. This may occur on the surface as well. This would
cause δbound to shift as more MeO-An bound to the dot. In this
experiment δbound was assumed to be constant.
Comparing the K values obtained from the Langmuir fits, it is
interesting to note that MeO-An and Br-An have similar binding
affinity for the QDs. Compared with the equilibrium constant of
TOPO on InP (3.3 x 105), they are almost identical even though they
have different basicities due to their para-substituents.6,13 The electron-
Figure 4. 1H-NMR shows a clear concentration dependence for the ortho proton of
Me-OAn. The first regime is due to simple intermolecular interactions; the second
regime may indicate higher-order structure such as dimerization.
withdrawing nature of the methoxy group in MeO-An suggests that it
will have a higher equilibrium constant than Br-An. Previous experiments studying photoluminescence behavior with R-An have suggested
this as well.8 From this data, it can be concluded that basicity alone is
not responsible for the binding affinity of R-An. It can also be deduced
that the monolayer R-An formation observed in this experiment does
not correlate with the photoluminescence response of the QD. Some
other mechanism must be responsible for the photoluminescencequenching properties of R-An on QDs.
The error in this experiment can be attributed mainly to the fact
that multiple experiments were performed over an extended period.
Different NMR shimming, differences in the glassware, volume
changes, incorrect R-An additions, and QD loss during transfer all
contributed to the wide differences in measurements.
Conclusions
This paper has demonstrated an in situ solution-phase method to
determine binding constants for R-An and CdSe QDs. This method
can also be used to determine OPA surface coverage by examining
asymptotic surface coverage behavior. Equilibrium constants for both
MeO-An and Br-An on the surface of CdSe quantum dots were
obtained via H-NMR. The fact that the K values obtained for each
ligand were so similar suggests that basicity alone is not responsible for
binding affinity. Additionally, it was shown that photoluminescence
does not correspond to surface coverage.
Acknowledgments
This research was supported primarily by the Nanoscale Science and
Engineering Research Experience for Undergraduates Program under
National Science Foundation award number EEC – 0647560. Any
opinions, findings, conclusions, or recommendations expressed in this
material are those of the authors and do not necessarily reflect those of
the NSF.
www.nanoscape.northwestern.edu Volume 8, Issue 1, Summer 2011 Nanoscape 47
NMR Determination of a Solution-Phase Binding Isotherm for Para-Substituted Anilines on CdSe Quantum Dots
(continued)
Figure 5. Br-An Langmuir fit; K = 459 +/- 106; A = 32.9 +/- 1.37%
Figure 6. Me-OAn Langmuir fit; K = 282 +/- 31; A = 38.5 +/- .95%
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