Catechols as ligands for CdSe–ZnS quantum dots

Abstract

We introduce catechol-containing molecules as potential ligands for quantum dots (QD), via a ligand exchange method. Hydrophilic ligands were attached to their surface, which resulted in a successful transfer to the aqueous phase. A significant quenching effect on the luminescent properties was observed. The interaction between the QD and the catechols made it possible to sense dopamine in the nanomolar range.

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Introduction

Nanometer-sized colloidal QD have attracted much attention because of their unique properties.^1–4 These chemically robust, size-tunable nanocrystals with narrow emission profiles provide distinct advantages compared to other fluorescent markers like proteins or dyes.⁵ Quantum dots can be more than 20 times brighter and are 100 times more stable which makes them ideal for imaging, ultrasensitive detection and labelling applications.⁶ Advances in synthesis have enabled the production of monodisperse core–shell particles with high quantum yield and sharp emission peaks. After synthesis, their surface is usually coated with apolar molecules like trioctylphosphine (TOP), trioctylphospinoxide (TOPO) or oleic acid.^4,6–8 To modify the properties of the QD, a ligand-exchange method, amongst others, can be performed which replaces the hydrophobic ligands by hydrophilic molecules or shells. Mono- and multilayer surface coatings have been reported, as well as the introduction of silica and polymer-based shells and micelle formation.^9,10 Although very robust, growth of silica shells enlarges the particle and achieving a spherical and monodisperse final product is time consuming.^10,11 Polymer shells and phospholipid double layers also enlarge the particle which can limit intracellular mobility.^11–14 On the other hand, monolayer formation has several advantages including high reproducibility, thin size, close packing and high reaction speeds.¹⁵ Most often, the exchange process is based on the interaction of thiols with the metal atoms (e.g. Zn) and/or thiols on the surface of the particle. However, the colloidal stability of QD coated with a thiol-based self-assembled monolayer (SAM) is often not sufficient.¹⁶ Moreover, thiols are known to reduce the quantum yield of QD, since they act as hole traps at non-ideal concentrations.¹⁷

In this paper we introduce catechols as an alternative class of ligands for coating QD. The concept of using catechols as highly versatile ligands is derived from nature itself, since the molecule is present in the adhesive proteins secreted by mussels for attachment to wet surfaces.¹⁸ They were already successfully introduced as ligands for iron oxide nanoparticles, where they interact strongly with the iron atoms on the surface of the particle.^19–22 Two oxygen atoms of neighbouring hydroxyl groups on the characteristic phenyl ring form a strong chelating bond with the metal atom, one of the strongest known metal chelating bonds (log K_s ≈ 30–40).^23,24 We utilized this strong interaction to introduce these molecules onto the surface of core–shell CdSe–ZnS QD. Two ligands were synthesized to prove the direct catechol interaction with the surface: a hydrophobic version containing an oleylamine alkane chain (C₁₈) and a hydrophilic version with a polyethylene glycol (PEG₁₀) moiety, connected to a catechol end-group. Moreover, several commercially available catechol molecules with different functional groups were introduced via a ligand-exchange method. The photoluminescent properties of the particles were investigated upon addition of different amount of catechols. Very efficient quenching of the luminescence was observed, for which a mechanism was proposed. This effect could have a large impact on imaging applications where QD and catechols are both present.

Experimental

Core–shell quantum dots were purchased from Sigma Aldrich (Lumidot™ CdSe–ZnS 640). To perform the ligand exchange process with hydrophilic molecules, 1 mg of QD was dispersed in 2 ml THF. To this dispersion, 0.02 mmol of the catechol-containing molecule in 200 microliters of MilliQ was added. This mixture was placed in an ultrasonication bath for 4 hours. To separate the particles from the THF–water mixture, 3 ml ethyl acetate was added to induce phase separation between the organic and water layer. Via a separatory funnel, the 2 layers can be separated. The water layer was purified under reduced pressure to remove any remaining THF and ethyl acetate. Afterwards the suspension was washed 3 times with MilliQ and centrifuged for 3 min at 7000 rpm, after which the precipitated particles were redispersed in MilliQ.

For hydrophobic ligands, the exchange can be performed in toluene or THF. To separate the particles from the solvent, 3 ml of acetone was added to induce instability, after which the particles can be collected and washed (toluene–acetone mixture, ratio 1/5) by centrifugation. Full experimental details of the ligand synthesis can be found in the ESI.†

All dopamine detection data was measured in Hellma OS quartz cuvettes. For these experiments, 0.1 mg of QD (20 microliters from a 5 mg ml⁻¹ stock solution in toluene) was diluted in 460 microliters of 1,4-dioxane. To this dispersion, dopamine hydrochloride was added, dissolved in 20 microliters of MilliQ.

These mixtures were allowed to react for 1 hour, before the fluorescence was measured. The Lumidot™ CdSe–ZnS core–shell QD, having an emission maximum at 640 nm, were excited at 520 nm, while their photoluminescence spectra were recorded with a Photon Technology International Quantamaster™ 60. Spectra were corrected for wavelength dependence of the source and the spectral response of the detector.

Results and discussion

Efficient surface modification of QD is crucial for their use in various applications. We preferred interaction with catechol-containing molecules, since they would form a functional monolayer, which is almost irreversibly bound to the surface. A chelating bond can be formed via a bidentate or a bridging bidentate interaction, as is shown in Fig. 1.²⁴ The large variety of available catechols allows the direct introduction of various functional groups like carboxylic acids, amines, sugars or amino acids. These basic catechols can also be used as a starting point for further design of complex organic ligands. Amstad et al. already modified them with polyethylene glycol chains for optimal stability in complex aqueous environments.²⁵ Apolar moieties like polymers or alkanes can be introduced in a similar way. In this paper, we synthesized two molecules that have either an apolar (catechol–C₁₈) or a polar chain (catechol–PEG) attached to the catechol moiety, without any extra functional groups present. Since only the catechol group can bind to the surface, a successful ligand exchange proves that the bond between the catechol and the zinc sulphide surface is formed.

Fig. 1 The core–shell CdSe–ZnS quantum dot interacts with the catechol molecules via bidentate or bridged bidentate chelating interactions on the zinc atoms on the surface. When a hydrophilic ligand is introduced, a phase transfer of the QD from the upper organic layer to the aqueous bottom layer will occur (shown on the right).

The presence of these ligands was evidenced by Fourier transform infrared measurements (FTIR). Fig. 2 shows the spectra of the QD after ligand exchange with the synthesized hydrophilic and hydrophobic catechol (see ESI† for all other FTIR spectra of modified QD). Several characteristic peaks of the introduced ligands can be found in the spectra, like the carboxylate stretch at 1548 cm⁻¹, several aromatic carbon–carbon vibrations in the 1400–1600 cm⁻¹ region and the typical PEG ether vibrations around 1000–1500 cm⁻¹.¹⁵ A broad band between 2500 and 3500 cm⁻¹ can be attributed to OH stretching vibrations. Note that residual peaks remain at 2850 and 2922 cm⁻¹ in the spectrum of the modified QD. Their origin cannot be determined clearly since C–H vibrations are present in both the original and the new ligand. Since a ligand exchange is often not complete, a small amount of original ligand can remain on the surface.²⁶ This effect is well known for ligand exchange methods, based on non-covalent interactions.¹⁰ Nevertheless, the presence of the aromatic vibrations confirms the presence of the catechol ligands. The formation of a double layer by the catechol–C₁₈ molecule through hydrophobic interaction is in principle also possible, but is ruled out because of the excellent colloidal stability in toluene. If a double layer were present, the catechol groups would point outwards and this would result in a polar surface, which is incompatible with an apolar solvent. The introduction of a hydrophilic ligand (like the catechol–PEG), on the other hand, induces a phase transfer to the aqueous phase. This is a strong extra indication of a successful ligand exchange (see Fig. 1). The QD were modified with several commercially available water-soluble catechols as well, to underline the generic character of the anchor group (see ESI†).

Fig. 2 The characteristic vibrations of the catechol anchor groups in the 1400–1600 cm⁻¹ region (aromatic ring) are clearly visible in the FTIR spectrum after the ligand exchange procedure.

A large influence on the photoluminescence was observed when the QD were modified. The catechol molecules are efficient quenchers of the particles' fluorescence. Ji et al. and Zhao et al. observed this effect before when catechols were added to a respectively, thiol and (3-aminopropyl)-triethoxysilane functionalized, QD solution in water.^27,28 In our case the interaction between the surface of the quantum dot and the ligand is not via an intermediate surface layer, but via a direct chelating bond. This results in a very efficient energy transfer, since the path between the particle and the benzene ring is fully conjugated. Catechol-containing molecules are known for their fast and reversible oxidation processes. We hypothesize that when an electron of the QD is excited to the conduction band, an electron from one of the catechol ligands can easily fill the valence band hole, thereby quenching the fluorescence. In our case, the quantum yield of the original QD (24%) was dramatically lowered when their surface was covered with the catechol–C₁₈ (1.1%) and catechol–PEG (0.5%) ligands.

For biomedical applications, this efficient fluorescence quenching could be an important issue. Many hormones (e.g. adrenaline, norepinephrine, …) contain catechol moieties in their structure, hence they could interact directly with the QD's surface if the used surface coating is not covalently bonded. This could largely influence imaging experiments with QD in serum or blood. We demonstrated next the possibility to sense very low levels of catechol-containing molecules through the direct interaction of the catechol-moieties with the QD surface. A dispersion of unmodified QD (coated with TOPO and 1-hexadecylamine) in 1,4-dioxane was spiked with dopamine, an important neurotransmitter in the brain, after which the fluorescence intensity was measured (Fig. 3). The catechols interacted with the surface of the nanoparticle, thereby serving as an electron donor, which quenches the fluorescence. A correlation between the neurotransmitter's concentration and the reduction of luminescence was observed. Using these conditions we were able to detect dopamine levels of 20 nanomolar. Compared to earlier reported detection of catechols with QD, this is a significant improvement.²⁷ Since no additional organic layer separates the QD and catechol, the molecules can interact directly with the surface thereby increasing the sensitivity. The solvent was chosen for its miscibility with both water and organic solvents, so it would stabilize the hydrophobically unmodified QD as well as the partially hydrophilically coated QD. This approach eliminates the need of water-dispersible QD. However, it cannot discriminate between different catechol containing molecules as they interact similarly. Further research will be necessary to investigate the exact nature of the chelating bond as well as the influence of the electron donating character of the ligands on the luminescent properties.

Fig. 3 The addition of dopamine to a quantum dot dispersion clearly influences the photoluminescent behaviour, even at very low concentrations. The inset shows the corresponding photoluminescence spectra.

The surface functionalization of QD by catechols could also be very valuable for solar cell development, where efficient charge transfer is crucial for improved performance. Kim et al. showed that QD can be introduced on zinc oxide nanowires and a polymer can act as a hole conductor.²⁹ Khetubol et al. also pointed out the importance of the interaction between the ligand and the surrounding polymer matrix.³⁰ Commercially available catechols have versatile functional groups (e.g. amines or carboxylic acids) which can easily be coupled to other organic molecules or polymers to improve the dispersibility of QD in a variety of environments.³¹ Similarly an aliphatic apolar ligand can be useful for depositing QD on surfaces from suspension, where extra functionality is important.³² This ability of catechols to act as an anchor point, while forming a strongly bonded monolayer, is very promising for future QD ligand design.

Conclusions

By using a straightforward ligand exchange method, successful modification of the quantum dots' surface was achieved. Interaction of catechols with the surface of CdSe–ZnS QD results in a strong chelating bond, which was investigated with a hydrophobic and hydrophilic ligand. FTIR measurements and phase transfer (in case of hydrophilic ligands) proved their presence on the quantum dots' surface. This approach can also serve as a direct detection method for these ligands since it strongly influences the photoluminescent behaviour of the nanocrystals. The electron donating character of the ligand, in combination with the fully conjugated path it contains, could serve as an important future ligand in solar cell development.

Acknowledgements

We express our thanks to Prof. Dr M. Van Der Auweraer, S. Vandendriessche, M. K. Vanbel, and W. Brullot for the useful and inspiring discussions. This work was financially supported by Grant G.0618.11 N of the Fund for scientific research, Flanders (FWO-V); the Agency for Innovation by Science and Technology in Flanders (IWT), and the KU Leuven (GOA, OT). M. B. is grateful for support from the IWT. P.-J. D. is grateful to the KU Leuven for his FLOF scholarship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Extra experimental information, photoluminescence data and additional structural and FTIR data. See DOI: 10.1039/c3ra47844k

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