A Solution NMR Toolbox for Characterizing the Surface Chemistry of

Review pubs.acs.org/cm

A Solution NMR Toolbox for Characterizing the Surface Chemistry of Colloidal Nanocrystals Zeger Hens*,†,‡ and José C. Martins*,¶ †

Physics and Chemistry of Nanostructures, ‡Center for Nano- and Biophotonics (NB-Photonics), and ¶NMR and Structural Analysis Unit, Ghent University, Belgium, ABSTRACT: The possibilities offered by 1H solution NMR for the study of colloidal nanocrystal ligands are reviewed. Using CdSe and PbSe nanocrystals with tightly bound oleate ligands as examples, the solution NMR toolbox for ligand analysis is introduced, highlighting 1D 1H, diffusion ordered (DOSY) and nuclear Overhauser effect (NOESY) spectroscopy as NMR techniques that enable bound ligands to be distinguished from free ligands. For each of the toolbox techniques, it is outlined how dynamic stabilization in the fast exchange regime affects the spectra obtained. Next, it is shown how the perturbation of a purified dispersion by dilution or the addition of excess ligands can be used to analyze the binding of ligands to a nanocrystal. Finally, saturation transfer difference (STD) spectroscopy is presented as an NMR technique that may complement the established toolbox. KEYWORDS: nanotechnology, surface chemistry, nuclear magnetic resonance spectroscopy, quantum dots

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shape, crystal structure, etc.18, and the ligand used where, e.g., the use of phosphonic acids induces the growth of rodlike nanocrystals in the case of CdSe and CdS. However, ligands yielding optimal control over synthesis are rarely of interest from the application point of view. For example, the long aliphatic chains make the NCs insoluble in aqueous or polar solutions and hamper charge transfer to and from NCs. As a result, a variety of ligand exchange procedures has been developed to adjust the interaction between a NC and its surroundings.7 Moreover, by exchanging the original ligands with specific bifunctional ligands, colloidal NCs could be turned into macromolecular building blocks, whereas the functionalization with biomolecules makes possible their use as specific biolabels or biosensors.16,17 A key element to direct research efforts on the role of ligands during synthesis or the development of ligand exchange procedures is the understanding of the nanocrystal−ligand interaction. This requires a methodology to study NC ligands and analyze their properties. Among the various methods used for this purpose, which include infrared spectroscopy,19 photoluminescence spectroscopy20,21 and X-ray photoelectron spectroscopy,22 solution nuclear magnetic resonance spectroscopy (NMR) stands out, where typically 1H, 13C or 31P nuclei are probed. Important assets of solution NMR for NC ligand analysis are (1) the in situ and nondestructive analysis of dispersed NCs, not of NC powders, (2) the possibility to

INTRODUCTION Over the last 20 years, colloidal synthesis in an organic, apolar medium has appeared as a generic approach for the precision synthesis of metal, metal oxide and semiconductor nanocrystals (NCs).1−7 The approach generally results in hybrid nanocrystals consisting of an inorganic core covered by a shell or capping of organic ligands. The case of colloidal semiconductor nanocrystals or quantum dots (QDs) exemplifies well the vast interest in these novel nanomaterials. Because of size quantization, their opto-electronic properties can be tuned by the size and shape of the inorganic core, which means that the properties of colloidal QDs can be fit to specific applications. Moreover, their suitability for solution-based processing allows for the straightforward deposition of QD thin films and enables them to be combined with a wide range of materials processing technologies, such as optical lithography, atomic layer deposition or chemical vapor depostion. As a result, research has focused on both the understanding of quantum-size effects and the exploration of various applications, which now cover very diverse fields such as photovoltaics,8,9 photodetection,10,11 lighting12,13 and displays,14 nanoelectronics15 and bioimaging,16,17 and sensing.17 Opposite from the inorganic core, which determines the nanocrystal physical properties, the role of the nanocrystal ligands is more diverse. Especially with synthesis recipes making use of noncoordinating solvents, surfactants such as carboxylic acids, amines, or phosphonic acids with a long aliphatic chain are used to control the nucleation and growth of NCs.7 As a result, they end up as ligands with their headgroup adsorbed at the NC surface and their aliphatic chain exposed to the surroundings. Various examples have demonstrated a relation between the outcome of a NC synthesis in terms of NC size, © XXXX American Chemical Society

Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: October 17, 2012 Revised: January 31, 2013

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dx.doi.org/10.1021/cm303361s | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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distinguish between free species and species interacting with or bound to NCs, (3) the identification and quantification of bound and free ligands and (4) the direct observation of ligand exchange. Solution 1H NMR has been used for analysis of colloidal NC ligands for more than 20 years. A few initial studies indicated its potential to distinguish free from bound ligands and obtain information on the binding of ligands to nanocrystals.23−27 This work was followed by a more systematic study on various dispersions of metal oxide and semiconductor NCs, focusing on the specific features of bound ligands in NMR spectra. It was shown that especially diffusion ordered spectroscopy (DOSY) makes possible the assignment of specific resonances in 1D 1H spectra to bound ligands,28,29 while nuclear Overhauser effect spectroscopy (NOESY) enabled nanocrystal-ligand interactions to be pinpointed, even with ligands in a fast adsorption/ desorption equilibrium and in complex solvent mixtures.30 By now, the possibilities offered by NMR to analyze bound ligands have been used by various research groups to study the composition of the ligand shell,31−33 to analyze the binding between ligands and nanocrystals,32−42 and to determine the relative binding strength of different ligands.43 Moreover, it is more and more used to support studies that link physicochemical properties of NCs to the NC surface chemistry.41,44−46 Since various nanocrystal synthesis recipes make use of phosphorus containing ligands, such as tri-octylphosphine (TOP), tri-octylphosphine oxide (TOPO) and various alkylphosphonic acids, solution 31P NMR can often complement 1H NMR studies. Here, the interest is not so much the study of bound ligands, which typically show considerable line broadening in 31P NMR, but rather the selective probing of phosphorus containing species. This has been used, for example, to study reaction mechanisms by analyzing crude reaction products using 31P NMR,47 to identify bound ligands after stripping them from the nanocrystal surface,43,48,49 or to study the effect of impurities on the synthesis outcome.50 The aim of this short review is to describe the toolbox of solution NMR techniques that has been established over the last 5−10 years for the study of colloidal nanocrystal ligands, or more general, the colloidal nanocrystal surface chemistry, with a focus on 1H NMR. Using model systems consisting of solvent, nanocrystals and bound ligands only, the different toolbox techniques are first introduced with a focus on the features characterizing bound ligands. Next, the complications brought about by ligands exchanging between a bound and a free state on the NMR spectra are discussed. After establishing the toolbox, it is shown how solution NMR analysis can be used to study ligand exchange processes in situ and obtain new insights in the nanocrystal−ligand interaction. Finally, future prospects and challenges in the study of nanocrystal surface chemistry by solution NMR are addressed, where in particular saturation transfer difference (STD) spectroscopy is proposed as a technique that may complement the existing toolbox.

Figure 1. (a) 1D 1H NMR spectrum of OA in toluene-d8. (b) 1D 1H NMR spectrum of a Q-CdSe dispersion in toluene-d8 (dNC = 3.0 nm, [Q−CdSe] = 40 μM). Labeled resonances are identified as (1−6) different OA protons as indicated in the figure, (†) residual toluene-d8 and ‡ a proton pool related to H2O contamination. See ref 38 for more details.

different OA resonances have been indicated according to literature, with most notably the alkene protons in the range 5.4−5.8 ppm and the methyl resonance around 0.9 ppm.38 Comparing both spectra, one sees that the resonances of the oleic acid protons are broadened and show a downfield shift (i.e., an increased chemical shift). Although resonance broadening depends on the solvent used − with chloroform or THF giving more narrow peaks than toluene,32,43 this appears as a general characteristic of bound ligands throughout the literature.28,31,33,38,43,48,52 Although a variety of relaxation mechanisms may contribute to the NMR line width, the line broadening is generally attributed to the transversal interproton dipolar relaxation mechanism that is rendered more efficient by the restricted rotational mobility of the ligands when bound to the NC surface.23,53 As exemplified by the resonances of the (2) α-CH2, (5) the alkene protons and (6) the CH3 end group in Figure 1b, resonance broadening is typically more pronounced for protons closer to the nanocrystal surface. This is in line with the view that as the distance from the NC surface is increased, additional degrees of freedom become available, thereby increasing local mobility and concomitantly reducing the transverse relaxation rates. When executed under quantitative conditions, 54 the integrated intensity of a resonance is proportional to the concentration of the corresponding protons in the sample. To link this signal intensity to a concentration requires either the addition of a concentration standard such as a known amount of CH2Br2 or the internal calibration of the spectrometer by, e.g., the PULCON method.55 Provided the concentration of nanocrystals is known, the integration of bound ligand resonances thus gives access to the ligand density. Although the ligand density thus obtained depends on the accuracy of the concentration determination, which can be problematic,56,57 typical ligand densities thus obtained amount to a few ligands per square nanometer (see Table 1). Comparable values have been reported using elemental analysis based on X-ray photoelectron spectroscopy,58 inductively coupled plasma atomic emission spectroscopy,59 or thermogravimetric analysis.60 However, opposite from these ex situ techniques, solution NMR can easily distinguish bound from free, residual ligands.

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TIGHTLY BOUND LIGANDS: INTRODUCING THE TOOLBOX Quantitative 1D 1H NMR Spectroscopy. Figure 1 shows the 1D 1H spectrum of oleic acid − a widely used ligand in colloidal nanocrystal synthesis − and of a well-purified dispersion of CdSe QDs (Q-CdSe) synthesized using oleic acid (OA) as the sole ligand to dissolve the Cd precursor and stabilize the nanocrystals formed.51 In both spectra, the B



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field gradients are used to link a translational diffusion coefficient to each resonance.27,62−67 Figure 3 shows a DOSY

Table 1. Densities of Tightly Bound Ligands As Obtained by Quantitative 1D 1H Nmr studies material

ligand

density (nm−2)

ref

CdSe CdSe CdSe PbSe PbS PbS PbS

TOPO oleate phosphonate carboxylate carboxylate carboxylate thiolate

1.3 4.6 3.5 4.2 3.0 5.3 5.0

44 38 43 31 33 46 46

1D 31P NMR Spectroscopy. Figure 2 represents the 1H and 31P spectra of CdSe nanocrystals synthesized in the Figure 3. DOSY NMR spectrum of a Q-CdSe dispersion in toluene-d8 (dNC = 3.5 nm, [Q−CdSe] = 40 μM). Labeled are (3−6) the OA resonances and (†) residual toluene-d8. The diffusion coefficient of (dashed line) free OA is added as a reference. See ref 38 for more details.

spectrum, representing the signal intensity by contour plots as a function of chemical shift and diffusion coefficient, of a QCdSe|OA dispersion.38 It follows that the signals of OA all have a diffusion coefficient of (9.4 ± 0.1) 10−11 m2/s, almost 1 order of magnitude smaller than the value expected for free OA. For spherical particles, the diffusion coefficient D can be linked to a hydrodynamic diameter dH using the Stokes−Einstein relation (kB, Boltzmann constant; T, absolute temperature; η, solvent viscosity):

Figure 2. (a) 1H NMR spectrum of a suspension of CdSe QD (545 μM, 2.9 nm) dissolved in d8-THF. Labeled resonances are identified as (1−18) different ODPA protons, (†) methanol and ‡ residual THFd8. (b) 31P NMR spectrum of (red) the same CdSe QDs and (blue) after treating the CdSe dispersion with bis(trimethysilyl)selenide. The blue lines mark the ppm values where free ODPA, ODPASi, and anhSi have their resonances in THF-d8. See 43 for more details.

dH =

kBT 3πηD

(1)

For the example shown in Figure 3, this yields dH = 7.8 ± 0.1 nm. Because this number corresponds to the sum of the nanocrystal diameter and twice the ligand shell thickness, this indicates that the broadened OA resonances come from ligands bound to the CdSe QDs. More systematic studies in the case of InP and PbSe have confirmed that dH increases linearly with dNC.28,31 The possibility DOSY offers to distinguish in situ free from bound ligands is a unique characteristic of solution NMR spectroscopy. It is crucial from a practical purpose, e.g., the analysis of sample purity, and it forms the basis for the quantitative study of the colloidal nanocrystal surface chemistry. A concern when considering DOSY measurements is that the encoding of diffusion into the resonances requires minimal delays for the gradient pulses during which the magnetization evolves in the transverse plane. As a result, resonances with a short T2 relaxation time are filtered out in a DOSY measurement as can be seen in Figure 3, where no diffusion coefficient is retrieved for the α-CH2 resonance. Nuclear Overhauser Effect Spectroscopy. Nuclear Overhauser effect spectroscopy (NOESY) offers a second way to distinguish free from bound ligands by NMR spectroscopy.30 It exploits the dependence of dipolar spin relaxation pathways between two sets of protons (called Hα and Hβ) on the rotational correlation time τc of the molecule they are part off.68,69 More particularly the cross-relaxation pathways are of importance here, as they each cause a transfer of spin polarization between Hα and Hβ, albeit with opposite effect. For slowly tumbling molecules, i.e., molecules where τc ≪ 1/ω0 with ω0 the angular frequency of the spectrometer, the zero quantum cross-relaxation pathway dominates, while for rapidly tumbling molecules, the double quantum cross relaxation

presence of TOPO and octadecylphosphonic acid (ODPA).43 The 1H resonances have been attributed to the different protons of the ODPA alkyl chain, yet especially the CH2 and CH3 resonances can overlap with other possible ligands such as TOPO. In the 31P spectrum, a broad resonance with little features is obtained, in line with several other studies.24,48,61 To identify the phosphorus containing species, the bound ligands can be stripped by adding, e.g., deprotonated propionic acid49,61 or bis(trimethysilyl)selenide43,48 followed by 31P NMR analysis of the resulting species. For the example shown in Figure 2b, ligand stripping with bis(trimethysilyl)selenide leads to the appearance of 31P resonances that can be assigned to O,O′-bis(trimethylsilyl)octadecylphosphonic acid (ODPASi, 13.8 ppm) and the racemic and meso forms of O,O′bis(trimethylsilyl)octadecylphosphonic acid anhydride (anhSi, 14.3 and 13.7 ppm). It thus follows that the CdSe nanocrystals are stabilized by a mixture of octadecylphosphonate (ODPA−) and its anhydride.43 A similar approach was followed to elucidate the composition of the ligand shell in CdSe nanocrystals synthesized using TOPO as a coordinating solvent. Opposite from previous studies,24 it was found that the majority of the ligands are not TOPO molecules yet various acidic phosphorus containing species such as di-n-octylphosphinate, octylphosphonate and its corresponding anhydride.61 Diffusion-Ordered NMR Spectroscopy. Although resonance broadening appears as a general feature of tightly bound ligands, it does not give conclusive evidence that species are bound to a nanocrystal. This information can be provided by diffusion-ordered NMR spectroscopy (DOSY), where pulsed C



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Figure 4. (a) NOESY NMR spectrum of OA in toluene-d8, (b) NOESY NMR spectrum of a Q-CdSe|OA dispersion and (c) ROESY NMR spectrum of the same dispersion. Labels refer to the different cross peaks involving the OA alkene protons. Red and blue contours denote negative and positive signal intensity, respectively. It should be noted that negative NOE cross-peaks have the same sign and positive NOE cross-peaks appear with opposite sign to the peaks on the diagonal. See ref 38 for more details.

pathway is most important.69 In general, cross-relaxation between neighboring protons is measured using transient NOESY, where spins at either a specific frequency (1D NOESY) or all protons (2D NOESY) are inverted or saturated and the effect on the signal intensity of all other spins is measured. In the case of 2D NOESY, zero quantum crossrelaxation leads to the rapid buildup of intense and negative Hα−Hβ cross peaks (also called NOEs) between dipolar coupled protons, while double quantum cross relaxation results in the slow buildup of weak and positive cross peaks.69 To avoid confusion, it should be pointed out that a positive NOE effect gives rise to a cross-peak with oppositely signed intensity to the diagonal peaks or negative cross-peak, whereas a negative NOE gives rise to an equally signed or positive cross-peak. Given the link with overall molecular tumbling, the buildup of negative NOEs enables bound ligand molecules, which will tumble with the characteristics of the QD, to be distinguished from small molecules such as free ligand molecules. This idea is confirmed by Figure 4. Figure 4a shows that the 2D NOESY spectrum of oleic acid features weak NOEs, if any, where the cross-peaks only show zero-quantum coherence artifacts, typically recognized from their antiphase (positive and negative) contribution. Opposite from this, Figure 4b demonstrates that in a Q-CdSe|OA dispersion intense and negative NOE cross peaks appear between all resolved resonances.38 For instance, the alkene protons develop pronounced NOEs toward (5−4) the neighboring methylenes and (5−3) the unresolved methylene groups in the chain and even weak NOEs appear to (5−6) the end chain methyl group and (5−2) the α-CH2. This crossover from weak and positive to strong and negative NOEs when comparing free ligands with ligands bound to a colloidal nanocrystal is in line with the increased τc for bound ligands. Assuming rigid spherical objects in toluene, these have been estimated at 70 ps for d = 1 nm, and 15 ns for d = 6 nm, numbers that indeed lie at either side of 1/ω0 for a 500 MHz spectrometer (300 ps).30 Negative NOEs have been observed for various nanocrystal/ligand systems, including early work on gold nanocrystals27 followed by studies on PbSe,30 InP,32 PbS,33 and CdSe either stabilized by carboxylate 38 or phosphonate ligands.43 Based on this, a number of recent studies have used the occurrence of negative NOEs as an indication of molecule-nanocrystal interactions and relate that to the presence or absence of, e.g., luminescence quenching45 or the photoinduced transfer of electrons or holes, where 1DNOESY was used.46,70 A technique closely related to NOESY is rotating frame nuclear Overhauser effect spectroscopy or ROESY.69 ROESY

implements a pulse sequence that is designed such that both cross-relaxation pathways produce equally signed effects. As a result, cross-relaxation gives rise to positive NOEs and therefore negative cross-peaks, regardless of the molecular tumbling rate.71 From a practical point of view, there are no obvious differences between the ROESY and NOESY spectrum of tightly bound ligands, apart from the change in sign of the cross peaks (see Figure 4c). However, because NOESY involves pure longitudinal relaxation effects while ROESY involves an important transversal relaxation component, the kinetics of the buildup and dissipation of the cross-peak intensity is not the same and may lead to apparent differences.69 Especially for resonances with a high transverse relaxation rates, NOE crosspeaks may be quickly extinguished, requiring the spinlock mixing time for the cross-relaxation buildup in the ROESY experiment to be reduced compared to the mixing time in the NOESY experiment, where generally slower, longitudinal relaxation occurs. In addition, when performing a ROESY experiment a pulse sequence that suppresses TOCSY type cross-peaks should preferably be used, as these will obscure the intramolecular NOE intensities.72 Hence, NOESY spectroscopy is in general the more robust technique to determine NOE contacts. On the other hand, NOESY does not allow to distinguish negative NOE cross-peaks, such as those connecting resonances of bound ligands, from those arising from chemical exchange between two distinct chemical environments, e.g. a ligand exchanging between a bound and a free state, since both have the same sign. As ROESY always produces positive NOEs, their sign is opposite to that of chemical exchange cross-peaks enabling this distinction to be made. Thus, to assess the presence of chemical exchange between two or more spectrally distinct environments, ROESY shows a clear benefit. This is of particular use when investigating in situ ligand exchange (vide infra).38 Relaxation Analysis. For the organic ligands of interest, the relaxation of the nuclear magnetization to its equilibrium value both along the direction of the applied magnetic field (longitudinal, spin−lattice, or T1 relaxation) and in a plane perpendicular to this field (transverse, spin−spin, or T2 relaxation) is determined by dipole−dipole interactions that fluctuate according to the overall and local molecular rotational mobility.73 Here, T2, which determines the homogeneous line width of a resonance, typically goes down with increasing correlation time, whereas T1 attains a minimum when 1/τc matches the angular frequency ω of the spectrometer. It was already recognized by Sachleben et al. that for a bound ligand, various types of motion can contribute to τc, including D



Review

Figure 5. NMR toolbox analysis of Q-PbSe synthesized using Pb(OA)2 and TOP-Se and dispersed in toluene-d8. (a) 1H NMR spectrum. (b) DOSY spectrum. (c) NOESY spectrum. Labeled are the resonances of (2−6) OA, (†) residual toluene-d8 and (‡) CH2Br2, which was added as a concentration standard. See ref 30 and 31 for more details.

NMR with the concentration and the diameter of the Q-PbSe yields a ligand density of 4.2 nm−2 (see Table 1).

the isotropic tumbling of the entire nanocrystal, the motion of a ligand with respect to the nanocrystal and the motion of ligands with respect to each other.23 Opposite from free ligands, isotropic tumbling of an entire nanocrystal leads to transversal relaxation in the slow motion regime (ωτc ≪ 1).23 This results in a broadening of the resonances of bound ligands, which as demonstrated with gold nanocrystals is more pronounced for larger nanocrystals.53 Although local rotational freedom within the ligand shell can reduce the correlation time,23 faster relaxation rates and broader resonances compared to freely tumbling molecules appear as a typical characteristic of bound ligands.31 When interpreting linewidths in terms of mobility, distinction should be made between homogeneous and heterogeneous line broadening. In the latter case, the resonance envelope is composed of a distribution of sharper resonances, enabling the signal to remain visible in DOSY and NOESY/ ROESY experiments even though the lines appear prohibitively broad. More recently, the reduction of spin−lattice relaxation has been used as an indication of nanocrystal−ligand interaction,46 although some care is needed here due to the more complex dependence of T1 on the rotational correlation time. A Case Study: Q-PbSe|OA. In colloidal nanocrystal research, the question as to what species used in a synthesis end up in the ligand shell is highly relevant if only because ligands also influence the nanocrystal’s physical properties. This type of question can be answered by the solution NMR toolbox as introduced here. As an example, Figure 5 shows a toolbox analysis of dispersions of PbSe nanocrystals synthesized using lead oleate (Pb(OA)2) and tri-octylphosphine selenium (TOPSe) in a mixture of diphenylether (DPE), TOP, and OA. The 1D 1H spectrum (Figure 5a) shows a combination of broad and sharp resonances, where the broad resonances closely resemble those of bound OA in Figure 1a while the sharp resonances have been identified as TOP-Se.31 According to DOSY and NOESY, only the broad resonances have the slow diffusion coefficient and the negative NOEs that characterize bound ligands (Figure 5b, c). Hence, NMR does not give an indication of interaction between TOP-Se and the PbSe NCs. Moreover, the 2:3 ratio between the broad alkene (5) and CH3 resonance (6) agrees with what is expected for OA. Hence, no other moieties with possibly overlapping CH2 and CH3 resonances, such as TOP, are present in the ligand shell.31 Finally, combining the ligand concentration as obtained from 1D 1H

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DYNAMIC LIGANDS: CHEMICAL EXCHANGE IN NMR SPECTROSCOPY Chemical Exchange in NMR Spectroscopy. For nanocrystal synthesis, where ligands should adsorb at the NC surface to stabilize the NC dispersion without blocking further growth of the NC, the concept of dynamic ligands that constantly adsorb at and desorb from the NC is essential. This is an example of what is called chemical exchange in NMR spectroscopy, where a species can switch between two or more states (or pools) during the NMR measurements.74 Talking of chemical exchange, distinction is made between slow, intermediate and fast exchange depending on the relation between the on/off rate kex and the inverse of the characteristic time constant τNMR of the NMR measurement. For 1D 1H experiments, the relevant rate 1/τNMR is the frequency difference between the free and the bound resonance, for DOSY, it is the inverse of the diffusion delay. In the case of slow exchange (kex ≪ 1/τNMR), the NMR spectrum is merely the sum of the spectra of the species in their two distinct states. Hence, separate NMR resonances, diffusion coefficients or NOEs will show up in 1D 1H, DOSY or NOESY, respectively. This changes when the exchange rate strongly exceeds 1/τNMR (fast exchange kex ≫ 1/τNMR). In this regime, each 1H spin that contributes to the spectrum of the exchanging species yields a single signal in 1D 1H or DOSY, which is a population average of the bound and the free signal in the case of dynamic ligands. The NOE intensity on the other hand is not a simple bound/free population average because of the different buildup rates and opposite sign of the NOE cross-peaks associated with each state. By tuning the mixing time in the 2D NOESY to short values (50−100 ms), only the bound state will effectively contribute to the development of the NOE cross-peak intensity. Because the spin polarization transferred between neighboring protons when the ligand is bound is not lost upon desorption, but decays over a time on the order of seconds, the strongly negative NOE built up in the bound state is retained by the desorbed ligand in the free state. In addition several ligand molecules may become bound and desorb within the mixing time, allowing further enhancement of the cross-peak intensity. This give rise to the transfer NOE effect, where in the case of chemical exchange strongly negative NOEs can be measured at E



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Figure 6. NMR toolbox analysis of Q-CdSe stabilized by octylamine. (a) 1H NMR spectra of (red) OctA and (blue) Q-CdSe/OctA in chloroform-d. Resonances in the chemical shift range shown are attributed to (†) methanol and (1−5) Octa. (b) DOSY spectrum of (red) free OctA and (blue) QCdSe/OctA in chloroform-d measured using a diffusion delay of 50 ms. (c). NOESY spectrum of Q-CdSe/OctA in chloroform-d measured using a 600 ms mixing time. NOEs starting from the α-CH2 are indicated in the spectrum, where the numbers refer to the labeling introduced in Figure 6a. See ref 37 for more details.

or by diluting a nanocrystal dispersion, the possibilities that solution NMR offers to identify and quantify bound and free ligands can also here lead to new insights. A first example in this respect is the Q-InP/TOPO system,34 where it was found that dilution leads to the desorption of bound TOPO. Since TOPO proved to be in slow exchange, quantitative 1D 1 H NMR enabled to determine the concentration of free and bound TOPO and thus, the adsorption isotherm (see Figure 7). Provided van der Waals

short mixing time, even if most of the exchanging species are in the free state, because the NOEs build up in the bound state are transferred to the free state, where they dissipate slowly because of T1 relaxation.30,75 Even when only a few percent of the ligands are bound to the surface at any one time, the rapid buildup combined with continuous exchange with the free pool, and slow dissipation in the free state, leads to clear and visible transfer NOE intensities. Dynamic Ligands. Figure 6 shows a typical toolbox analysis of Q-CdSe QDs stabilized through post-synthesis ligand exchange by octylamine (OctA).37 Opposite from the OA resonances (Figure 1), Figure 6a demonstrates that the OctA resonances for the Q-CdSe dispersion (blue) show little broadening and almost no chemical shift difference with respect to free OctA (red). A similar conclusion holds for the diffusion coefficient, which is hardly reduced as compared to free OctA (Figure 6b). Both observations could be interpreted as the result of chemical exchange, where a free and a bound pool continuously swap ligands. If so, the close correspondence between free OctA and OctA in the Q-CdSe dispersion would mean that most of the ligands are part of the free pool. The assumption of chemical exchange is confirmed by NOESY. As shown in Figure 6c, the OctA resonances in a Q-CdSe dispersion, although strongly resembling free OctA, still develop strongly negative NOEs. This points toward a transfer NOE where ligands are exchanged between a pool of free and a pool of bound ligands where the strongly negative NOEs developed in the bound state are conserved when a ligand is transferred to the free state. The same dynamic stabilization in the fast exchange regime has been observed with amines interacting with CdTe,30 CdSe,37 PbS,33 and ZnO30,42 and with para-substituted aniline ligands on CdSe QDs.39 In the case of CdSe and CdTe, it could be concluded that the observation of fast exchange sets a lower limit on the desorption rate constant of 50 s−1.

Figure 7. (a) Concentration of bound TOPO ligands normalized with respect to the dilution − a quantity proportional to the fractional occupation of the adsorption sites − as a function of the concentration of free TOPO together with fits to (dashed line) a Langmuir and (full lines) a Fowler isotherm. (b) representation of ligand/nanocrystal and ligand/ligand interaction free energy obtained from fitting the experimental data to a Fowler isotherm.

interactions between adjacent bound ligands were taken into account (Fowler isotherm), the experimental data could be modeled well using a simple L-type bond formation between TOPO (L in general) and a free InP surface site (NC): NC + L ⇌ NC − L

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(2)

The same binding scheme has been used by Donakowski et al. to analyze the adsorption of para-substituted aniline ligands on CdSe QDs, with the additional complexity that with these ligands, the bound and the free state are in fast exchange.39 By interpreting experimental chemical shifts as weighted averages of a bound and a free value, these authors could obtain an equilibrium constant and thus a standard adsorption free energy from the NMR data. Binding according to eq 2 has also been proposed for amines, TOP, and TOPO, an interpretation in line with the observed dynamic stabilization of several QDs

IN SITU LIGAND EXCHANGE: PERTURBING SYSTEMS TO LEARN MORE Analyzing the Ligand/Nanocrystal Bond. The example of Q-PbSe/OA shows how solution NMR can be used to identify tightly bound ligands and quantify their surface density. What it does not provide is direct information on the bond between the ligand and the nanocrystal. However, by provoking ligand exchange, e.g., by exposing nanocrystals to new ligands F



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by amines;30,33,37,42 the observed loss of TOP, TOPO, and hexadecylamine during successive purification of CdSe QDs;49 and the absence of TOP as a ligand in PbSe QDs synthesized using TOP-Se.31 Opposite from the Q-InP/TOPO system, dilution does not induce ligand desorption in the case of Q-CdSe|OA.38 Addition of excess oleic acid (OAe) to a Q-CdSe|OA dispersion on the other hand leads to the appearance of new resonances next to those of bound (OAb). As shown in Figure 8a for the example

Figure 9. (a) Cartoon image of the stabilization of CdSe QDs by Xtype oleate ligands and the oleic acid self-exchange involving proton transfer between bound oleate (RCOO-NC) and free oleic acid (R′COOH). See ref 38 for more details. (b) Cartoon image of a CdSe NC stabilized by a mixture of alkylphosphonate and its anhydride and the replacement of this ligands by a thiolate involving the transfer of a SiMe3 moiety. See ref 48 for more details.

for example reagents with reactive silicon-chalcogen and siliconchlorine bonds.48 Although the in situ information about the ligand exchange regime is lost, the analysis of stripped-off ligands is easier since the line broadening typical of bound ligands is absent. As already discussed before, using bis(trimethylsilyl)-sulfide on CdSe NCs synthesized in the presence of alkylphosphonic acids, it could thus be shown that the actual ligands are a mixture of the alkylphosphonate and its anhydride. Moreover, it was found that the ligand desorption required the exchange of a SiMe3 moiety,48 which again indicated binding of the original ligands as X-type alkyl phosphonates (see Figure 9b). Metal selenide nanocrystals such as CdSe, CdTe, PbS, and PbSe are often found to be cation rich, which is typically interpreted as a surface excess of cations.31 In the case of CdSe| OA, it was found that the surface density of oleate ligands is twice that of cadmium cations. This finding supports an NC model where a layer of excess cations bind to the Lewis basic chalcogenide surface sites of the NC core and are charge balanced by the X-type carboxylate ligands.38,48 In the case of PbS|OA, a similar picture emerged although the cation charge is in that case balanced by a combination of oleate ligands and chloride ions.33 On the other hand, with PbSe|OA, only a 1:1 ratio between excess lead and oleate was found. This possibly indicates that additional X-type binding moieties are present at the surface which are however not accessible to NMR or elemental analysis techniques such as Rutherford backscattering spectrometry. It should be stressed that the NC model outlined here applies to NCs in well purified dispersions. Various examples show the importance of L-type ligands during synthesis and their presence prior to NC purification. However, when the L-type bond is too weak, these ligands are easily lost in the purification process.49 Direct Demonstration of Chemical Exchange Using ROESY. The example of CdSe|OA shows that addition of

Figure 8. (a) Change of the alkene resonance of a Q-CdSe|OA dispersion upon progressive addition of free OA. The gray background is the alkene resonance measured in the original Q-CdSe|OA suspension (Figure 1b), while the vertical black line indicates the position of the alkene resonance of free OA. The different colors correspond to different [OAe]/[OAb] ratios. (b) Change of the diffusion coefficient related to the (○) OAe and (•) OAb resonance as a function of the [OAe]/[OAb] ratio. The dashed line gives the free OA value, whereas the blue line is a guide to the eye. (c) Scheme showing the interpretation of the two resonances in terms of consecutive exchange processes.

of the alkene resonance, the additional signals have the features typical of chemical exchange, with a chemical shift and a diffusion coefficient moving toward the free OA value when the concentration of free OA is raised (see Figure 8a, b).38 This was interpreted in terms of a two step exchange process combining (Figure 8c, 1 → 2) a physisorption process in rapid exchange with (Figure 8c, 2 → 3) a chemisorption process in slow exchange. Moreover, using deuterated OA, where the carboxylic acid hydrogen has been replaced by deuterium, it was demonstrated that this OA self-exchange involves a proton transfer between the free and the bound OA. Hence, the bound moiety is an oleate ion binding via an X-type bond instead of an oleic acid molecule and the exchange can be written as (see Figure 9a) follows38 R′COOH + RCOO − NC ⇌ RCOOH + R′COO − NC

(3)

A similar stabilization by carboxylates was demonstrated for QPbS33 and Q-InP.32 A more drastic example of identifying NC ligands and analyzing the NC-ligand bond is complete ligand stripping by cleaving the NC-ligand bond. This proved possible by adding G



Review

Figure 10. (a) NOESY spectrum of a Q-CdSe|OA suspension with excess OA. Negative NOE cross-peaks (red) involving the excess OA resonances are clearly visible. The inset shows a zoom of the alkene region, with an indication of the excess-bound cross-peaks 5e5b and 5b5e between the alkene protons. (b) Zoom on the NOE cross-peaks of the alkene protons with an indication of the different cross-peaks, where the numbers refer to the different OA protons as indicated in Figure 1. (c) ROESY spectrum of the same suspension. Here the positive sign (blue) of all cross-peaks except 5e5b and 5b5e confirms that these are true NOE cross-peaks, while the negative sign (red) of the 5e5b and 5b5e cross-peaks conclusively establishes them as resulting from chemical exchange.

excess free ligands induces an exchange between free ligands in solution and bound ligands on the NC surface. When the resonances of free and bound species are spectrally resolved, chemical exchange will generate an exchange cross-peak connecting the resonances of the same spin in both environments. However, an unambiguous demonstration of chemical exchange is only possible using ROESY. In NOESY, a negative NOE cross peak between two sets of protons Hα and Hβ occurs when spin polarization created in Hα is transferred by cross relaxation to Hβ. However, if a species switches between two states (called a and b) during the NMR measurements, the exchange process in itself will transfer spin polarization initially created in Hα in state a (Hα,a) to Hα in state b (Hα,b). Thus, chemical exchange between bound and free ligands will lead to a negative αa−αb cross peak, provided that both states are spectrally resolved, which cannot, however, be distinguished from spin polarization transfer by crossrelaxation. In ROESY, cross-relaxation through dipolar coupling leads to positive cross peaks, regardless of the molecular tumbling rate. As a result, true NOE contacts can be distinguished from chemical exchange, which still results in negative cross peaks. As an example, Figure 10 presents the NOESY and ROESY spectra of CdSe|OA dispersions with excess OA.38 The overview NOESY spectrum (Figure 10a) and the zoom on the alkene region (Figure 10b) shows the occurrence of strongly negative cross peaks, in particular between the alkene protons of OAb and OAe (5b − 5e cross peak, inset of Figure 10a). In ROESY, this cross peak keeps its negative sign (Figure 10c), which demonstrates that the transfer of spin-polarization between the alkene protons of bound and excess OA is dominated by chemical exchange and not by cross-relaxation. Thus, OA moieties are truly swapped between a bound and a free state in this case. Mapping Relative Binding Strengths. Under conditions of slow exchange, 1D 1H NMR allows for the direct determination of the concentration of bound and free ligands. This has been exploited to determine adsorption isotherms,34 yet it can also be used to determine the equilibrium in a true ligand exchange process where, e.g., an acidic ligand (AH) could replace an X-type bound moiety (B) by proton transfer: BH + A − NC ⇌ AH + B − NC

An example of this is the replacement of carboxylic by phosphonic acids at the surface of CdSe QDs.43 Figure 11a shows the alkene region of the 1H NMR spectrum of CdSe QDs synthesized in the presence of octadecylphosphonic acid (ODPA), with the actual ligands being octadecylphosphonate and its anhydride before and after the addition of different amounts of oleic acid. Because the addition of OA results only in a sharp, well-resolved resonance, the spectrum gives no evidence of binding of OA to the CdSe QDs. This is confirmed by a DOSY measurement (Figure 11b), which shows that the OA resonances have a diffusion coefficient equal to that of free OA. On the other hand, the addition of ODPA to CdSe QDs partially capped with oleate ligands leads to the gradual reduction of the bound OA resonances, which is mirrored by the appearance of free OA resonances (see Figure 11c). The deconvolution of this resonance into a contribution of bound and free OA shows an almost quantitative replacement of bound OA by the added ODPA. According to the DOSY spectrum in Figure 11d, only free OA is observed when the ratio between the ODPA added and the original OA is raised to 1.7. Hence, addition of ODPA completely releases bound OA from the nanocrystals. This indicates that the exchange equilibrium eq 4 is strongly shifted toward bound phosphonate in this case, to an extend that the equilibrium constant cannot be determined by NMR. Moreover, the one-to-one replacement of oleate by ODPA shows that ODPA is in fact bound as hydrogen octadecylphosphonate, i.e., ODPA gives up only one of its two acidic protons.

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FUTURE PROSPECTS Currently, the measurement techniques and interpretation methods collected in the NMR toolbox are becoming established as an integral part of the characterization portfolio for colloidal nanocrystals, especially since it provides a complementary insight into QD surface chemistry from the ligand’s point of view and it enables ligand exchange procedures to be verified in detail.32,33,38−43 Its development was largely inspired by the development of NMR within supramolecular chemistry and biomolecular science as a powerful technique for the in situ characterization of intermolecular interactions. Indeed, intermolecular interactions manifest themselves through effects on the chemical shift, relaxation times, and translational diffusion coefficient of the various species and

(4) H



Review

aqueous colloidal disperions of organic nanocrystallites of pigment red PR122.86 The organic core, which is invisible in the NMR spectrum due to excessive line broadening caused by its solid and rigid nature, is key in this experiment as it provides an extensive network of dipolar-coupled protons. When a ligand such as SDS binds to the nanocrystal surface, part of the saturation is transferred via cross-relaxation onto its protons at a rate that is determined by the dissociation rate koff, the complexation constant Kd and the binding geometry. The ligand, being dynamic, is exchanged with other, unsaturated ligands during the saturation period, allowing a sizable fraction of the ligands to be affected. This leads to a reduction of the ligand resonance intensity to Isat,on. This loss is most easily characterized by subtracting a reference, so-called off resonance experiment wherein saturation is applied outside the frequency range where the organic crystalline 1H background and ligand resonances occur, leaving the resonance intensities unperturbed at Isat,off. The difference spectrum yields nonzero intensities ISTD = (Isat,off − Isat,on) only for resonances of binding ligands (see Figure 12a). Thus this technique may also be used as a method to screen for binding ligands in complicated mixtures. As ligand protons in close contact with the surface will receive more saturation than more distant ones, the relative ISTD values of the various ligand resonances can be interpreted in terms of a binding epitope.83,87 For the case of the SDS pigment red system, this allowed us to demonstrate that as the concentration of SDS is increased with respect to the available organic crystalline surface, the difference intensity ISTD, which was uniform for all 1H resonances along the SDS chain, becomes dependent on position of the chain, with the end methylgroep showing the largest effect and the α-CH2 the lowest one. This evolution could be linked to a reorganization of the SDS molecules on the pigment surface. At low concentrations, the available hydrophobic surface area on the pigment is large and flat-on adsorption of SDS molecules occurs. With increasing concentration the SDS molecules reorganize into hemicylindric shaped micellar structures, with sulfonate directed toward the solvent.(see Figure 12b) Thus, the methyl chain ends obtain their saturation mostly from direct contact with the surface while most methylene chains protons receive their saturation indirectly via intra- and interchain spin diffusion pathways, causing the observed variation in ISTD.86 Although the potential of this method for colloidal nanocrystals is clear, the requirement for an organic core cannot be met in most systems of interest. Although NMR active spins are generally present in the inorganic core, their much reduced gyromagnetic ratio limits efficient spin diffusion through cross-relaxation while additional relaxation effects such as quadrupolar relaxation for spin I > 1/2 nuclei may quench the buildup of saturation. Nevertheless, in a variety of systems, the presence of small, hydrogen containing species such as hydroxides has been suspected,88 which could turn out to be sufficient to tap into as a source of saturation to be transferred to the ligands. As such, this could be used to study the presence of these small moieties on the nanocrystal surface, which typically are not seen in 1H NMR because of excessive line broadening, and the packing of ligands in the ligand shell.

Figure 11. (a) 1H NMR spectrum of a suspension of 2.9 nm CdSe QDs (545 μM) dissolved in d8-THF (light gray) before and after the addition of (gray) 46 and (black) 221 mM of oleic acid (yielding OA:ODPA ratios of 1:1 and 5:1, respectively). Only the alkene region is shown. (b) DOSY of a suspension of CdSe QD (545 μM, 2.9 nm) dissolved in d8-THF after the addition of 221 mM OA (yielding an OA:ODPA ratio of 5:1). (c) 1H NMR spectrum zoomed in the alkene resonance of a suspension of CdSe QDs (435 μM, 2.8 nm) dissolved in d8-THF capped with OA and ODPA ligands upon the stepwise addition of ODPA from 0 to 14.3 μM final concentration (ODPA:OA ratio from 0 to 1.5). Inset: bound OA mole fraction as a function of the equivalents of added ODPA. (d) DOSY of a suspension of CdSe QD (435 μM, 2.8 nm) dissolved in d8-THF capped with OA and ODPA ligands after the addition of 1.7 equiv of ODPA (15.8 μM). Only free OA is observed. See ref 43 for more details.

their sensitivity to variables such as concentration, solvent and temperature,76−81 much of which is now also exploited within the NMR toolbox. Not surprisingly, these fields may serve as further sources of inspiration. For instance, a host of specific NMR approaches is routinely applied to screen for ligand binding to biomolecular targets, such as protein receptors, providing detailed information about the binding process and conformation.79 Saturation Transfer Difference (STD) spectroscopy stands out in this respect as a possible addition in the study of systems involving dynamic ligands. It is a powerful technique to investigate ligand binding to biomacromolecules ranging from proteins to viral assemblies or membrane embedded receptors.82−85 STD involves saturation of the invisible 1H background of the nanocrystalline core in a so-called on resonance experiment, taking care not to affect the ligand resonances. This selective saturation subsequently spreads throughout the entire network of dipolar-coupled protons in the core via highly efficient multistep cross-relaxation, a process known as spin diffusion. Recently, the technique was adapted to investigate the interaction of sodiumdodecylsulfate (SDS) as stabilizer of

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CONCLUSION A solution NMR toolbox is available to study the surface chemistry of sterically stabilized colloidal nanocrystals. In this toolbox, techniques such as 1D 1H, diffusion-ordered (DOSY), I



Review

Figure 12. (a) (Isat,off) Reference 1H NMR spectrum of an SDS-PR122 dispersion containing 7% w/w SDS and 5% w/w PR122 in D2O. The SDS resonances occur as indicated. The presence of micellar SDS is most easily recognized from the sharp α−CH2 triplet riding atop a much broader peak from the same moiety in SDS molecules that interact with the pigment surface. (ISTD) STD NMR spectrum obtained by alternate 2 s selective irradiation at 20 ppm (on-resonance) and 300 ppm (off-resonance). Only the broadened resonances appear, simultaneously demonstrating the adsorption of SDS molecules on the pigment and the lack of interaction with micellar SDS and citric acid. (b) Schematic representation of the reorganization of the SDS molecules at the pigment surface, proposed to explain the differentiation observed in STD amplification factors. See ref 86 for more details. (8) Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2008, 8, 3488−3492. (9) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.; Asbury, J. B.; Sargent, E. H. Nat. Mater. 2011, 10, 765−771. (10) Rauch, T.; Boeberl, M.; Tedde, S. F.; Fuerst, J.; Kovalenko, M. V.; Hesser, G.; Lemmer, U.; Heiss, W.; Hayden, O. Nat. Photonics 2009, 3, 332−336. (11) Konstantatos, G.; Sargent, E. H. Nat. Nanotechnol. 2010, 5, 391−400. (12) Coe, S.; Woo, W.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800−803. (13) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V. Nano Lett. 2009, 9, 2532−2536. (14) Kim, T.-H.; Cho, K.-S.; Lee, E. K.; Lee, S. J.; Chae, J.; Kim, J. W.; Kim, D. H.; Kwon, J.-Y.; Amaratunga, G.; Lee, S. Y.; Choi, B. L.; Kuk, Y.; Kim, J. M.; Kim, K. Nat. Photonics 2011, 5, 176−182. (15) Chung, D. S.; Lee, J.-S.; Huang, J.; Nag, A.; Ithurria, S.; Talapin, D. V. Nano Lett. 2012, 12, 1813−1820. (16) Gao, X.; Yang, L.; Petros, J.; Marshal, F.; Simons, J.; Nie, S. Curr. Opin. Biotechnol. 2005, 16, 63−72. (17) Medintz, I.; Uyeda, H.; Goldman, E.; Mattoussi, H. Nat. Mater. 2005, 4, 435−446. (18) Cozzoli, P. D.; Manna, L. In Bio-Applications of Nanoparticles; Chan, W. C. W., Ed.; Advances in Experimental Medicine and Biology; Springer: New York, 2007; Vol. 620; pp 1−17. (19) von Holt, B.; Kudera, S.; Weiss, A.; Schrader, T. E.; Manna, L.; Parak, W. J.; Braun, M. J. Mater. Chem. 2008, 18, 2728−2732. (20) Jasieniak, J.; Mulvaney, P. J. Am. Chem. Soc. 2007, 129, 2841− 2848. (21) Koole, R.; Schapotschnikow, P.; Donega, C. d. M.; Vlugt, T. J. H.; Meijerink, A. ACS Nano 2008, 2, 1703−1714. (22) Lobo, A.; Moller, T.; Nagel, M.; Borchert, H.; Hickey, S.; Weller, H. J. Phys. Chem. B 2005, 109, 17422−17428. (23) Sachleben, J.; Wooten, E.; Emsley, L.; Pines, A.; Colvin, V.; Alivisatos, A. Chem. Phys. Lett. 1992, 198, 431−436. (24) Becerra, L.; Murray, C.; Griffin, R.; Bawendi, M. J. Chem. Phys. 1994, 100, 3297−3300. (25) Majetich, S.; Carter, A.; Belot, J.; Mccullough, R. J. Phys. Chem. 1994, 98, 13705−13710. (26) Sachleben, J.; Colvin, V.; Emsley, L.; Wooten, E.; Alivisatos, A. J. Phys. Chem. B 1998, 102, 10117−10128.

and nuclear Overhauser effect spectroscopy stand out because they enable bound ligands to be distinguished from free ligands and tightly bound ligands from dynamic ligands in a rapid adsorption/desorption equilibrium. This opens the way to identify and quantify nanocrystal ligands and, in the case of dynamic ligands, to estimate a lower limit to the exchange rate and determine the adsorption isotherm. Moreover, the perturbation of a purified dispersion by successive dilutions or additions of excess free ligands can give a detailed view on the binding between a ligand and a nanocrystal. Inspired by developments of NMR applications within supramolecular chemistry and biomolecular science, saturation transfer difference spectroscopy is proposed as a technique that may complement the existing toolbox in future nanocrystal ligand studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the FWO-Vlaanderen for funding this research (project nr. G.0794.10).

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REFERENCES

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A Solution NMR Toolbox for Characterizing the Surface Chemistry of

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