An efficient method for the surface functionalization of luminescent quantum dots with lipoic acid-based ligands

We describe an operationally advantageous general methodology to efficiently activate lipoic acid-based compounds - a family of popular surface ligands for semiconductor nanocrystals - by the use of a borohydride exchange resin, and the use of the activated species to replace the native surface ligands of quantum dots. The procedure enables the phase transfer of the nanocrystals between polar and aqueous media and, if unsubstituted lipoic acid is used, a facile adjustment of their solubility in a wide range of solvents with varying polarity (from hexane to water). We show that the protocol is applicable to different types of nanocrystals and a variety of lipoic acid-based ligands, and that the resulting quantum dots maintain their optical properties - in particular, an intense luminescence - and long term colloidal stability

Effect of Protons on CdSe and CdSe–ZnS Nanocrystals in Organic Solution

Core and core–shell quantum dots are covered with a layer of organic ligands which prevents aggregation and eliminates surface defects, thus enhancing the photophysical properties and stability of the material. These ligands are usually Lewis bases and can therefore be affected by the presence of acid in the surrounding environment. We synthesized core CdSe and core–shell CdSe–ZnS quantum dots with various shell thicknesses and different organic ligands, and we investigated the effect of acid and base on their photophysical properties. In dilute CHCl₃ solution, the organic ligands can be protonated upon addition of acid and detached from the surface of the nanoparticles. As a consequence, the nanoparticles aggregate and their luminescence is quenched. Aggregated particles can be partly disgregated and the luminescence restored by deprotonation of the free ligands with a base. Since the presence of organic ligands on the surface is an essential characteristic of quantum dots, these effects should be taken into consideration when designing quantum dot-based sensors

Photoinduced Phase Transfer of Luminescent Quantum Dots to Polar and Aqueous Media

We report a new strategy for the photomediated phase transfer of luminescent quantum dots, QDs, and potentially other inorganic nanocrystals, from hydrophobic to polar and hydrophilic media. In particular, we demonstrate that UV-irradiation (λ < 400 nm) promotes the in situ ligand exchange on hydrophobic CdSe QDs with lipoic acid (LA)-based ligands and their facile QD transfer to polar solvents and to buffer media. This convenient method obviates the need to use highly reactive agents for chemical reduction of the dithiolane groups on the ligands. It maintains the optical and spectroscopic properties of the QDs, while providing high photoluminescence yield and robust colloidal stability in various biologically relevant conditions. Furthermore, development of this technique significantly simplifies the preparation and purification of QDs with sensitive functionalities. Application of these QDs to imaging the brain of live mice provides detailed information about the brain vasculature over the period of a few hours. This straightforward approach offers exciting possibilities for expanded functional compatibilities and reaction orthogonality on the surface of inorganic nanocrystals

On the pH-Dependent Quenching of Quantum Dot Photoluminescence by Redox Active Dopamine

We investigated the charge transfer interactions between luminescent quantum dots (QDs) and redox active dopamine. For this, we used pH-insensitive ZnS-overcoated CdSe QDs rendered water-compatible using poly (ethylene glycol)-appended dihydrolipoic acid (DHLA-PEG), where a fraction of the ligands was amine-terminated to allow for controlled coupling of dopamine–isothiocyanate onto the nanocrystal. Using this sample configuration, we probed the effects of changing the density of dopamine and the buffer pH on the fluorescence properties of these conjugates. Using steady-state and time-resolved fluorescence, we measured a pronounced pH-dependent photoluminescence (PL) quenching for all QD-dopamine assemblies. Several parameters affect the PL loss. First, the quenching efficiency strongly depends on the number of dopamines per QD-conjugate. Second, the quenching efficiency is substantially increased in alkaline buffers. Third, this pH-dependent PL loss can be completely eliminated when oxygen-depleted buffers are used, indicating that oxygen plays a crucial role in the redox activity of dopamine. We attribute these findings to charge transfer interactions between QDs and mainly two forms of dopamine: the reduced catechol and oxidized quinone. As the pH of the dispersions is changed from acidic to basic, oxygen-catalyzed transformation progressively reduces the dopamine potential for oxidation and shifts the equilibrium toward increased concentration of quinones. Thus, in a conjugate, a QD can simultaneously interact with quinones (electron acceptors) and catechols (electron donors), producing pH-dependent PL quenching combined with shortening of the exciton lifetime. This also alters the recombination kinetics of the electron and hole of photoexcited QDs. Transient absorption measurements that probed intraband transitions supported those findings where a simultaneous pronounced change in the electron and hole relaxation rates was measured when the pH was changed from acidic to alkaline

Synthesis of Highly Fluorescent Copper Clusters Using Living Polymer Chains as Combined Reducing Agents and Ligands

We present the synthesis of colloidally stable ultrasmall (diameter of 1.5 ± 0.6 nm) and fluorescent copper clusters (Cu-clusters) exhibiting outstanding quantum efficiencies (up to 67% in THF and approximately 30% in water). For this purpose, an amphiphilic block copolymer poly(ethylene glycol)-<i>block</i>-poly(propylene sulfide) (MPEG-<i>b</i>-PPS) was synthesized by living anionic ring-opening polymerization. When CuBr is mixed with the living polymer chains in THF, the formation of Cu-clusters is detected by the appearance of the fluorescence. The cluster growth is quenched by the addition of water, followed by THF removal. The structural features of the MPEG-<i>b</i>-PPS copolymer control the cluster formation and the stabilization: the poly(propylene sulfide) segment acts as coordinating and reducing agent for the copper ions in THF, and imparts a hydrophobic character. This hydrophobic block protects the Cu-clusters from water exposure, thus allowing to obtain a stable emission in water. The PEG segment instead provides the hydrophilicity, rendering the Cu-clusters water-soluble. To obtain fluorescent and stable Cu-clusters exhibiting outstanding quantum efficiencies, the removal of the excess of free polymer and copper salt was crucial. The Cu-clusters are also colloidally and optically stable in physiological media and showed bright fluorescence even when taken up by HeLa cells, being noncytotoxic when administered at a Cu dose between 10 nM and 1.6 μM. Given the very small size of the Cu-clusters, localization and fluorescent staining of cell nucleus is achieved, as demonstrated by confocal cell imaging performed at different Cu-cluster doses and at different incubation temperatures

Modulation of the solubility of luminescent semiconductor nanocrystals through facile surface functionalization

The solubility of luminescent quantum dots in solvents from hexane to water can be finely tuned by the choice of the countercations associated with carboxylate residues present on the nanocrystal surface. The resulting nanocrystals exhibit long term colloidal and chemical stability and maintain their photophysical properties. This journal is © the Partner Organisations 2014

Colloidal CuFeS₂ Nanocrystals: Intermediate Fe d‑Band Leads to High Photothermal Conversion Efficiency

We describe the colloidal hot-injection synthesis of phase-pure nanocrystals (NCs) of a highly abundant mineral, chalcopyrite (CuFeS₂). Absorption bands centered at around 480 and 950 nm, spanning almost the entire visible and near-infrared regions, encompass their optical extinction characteristics. These peaks are ascribable to electronic transitions from the valence band (VB) to the empty intermediate band (IB), located in the fundamental gap and mainly composed of Fe 3d orbitals. Laser-irradiation (at 808 nm) of an aqueous suspension of CuFeS₂ NCs exhibited significant heating, with a photothermal conversion efficiency of 49%. Such efficient heating is ascribable to the carrier relaxation within the broad IB band (owing to the indirect VB–IB gap), as corroborated by transient absorption measurements. The intense absorption and high photothermal transduction efficiency (PTE) of these NCs in the so-called biological window (650–900 nm) make them suitable for photothermal therapy as demonstrated by tumor cell annihilation upon laser irradiation. The otherwise harmless nature of these NCs in dark conditions was confirmed by in vitro toxicity tests on two different cell lines. The presence of the deep Fe levels constituting the IB is the origin of such enhanced PTE, which can be used to design other high performing NC photothermal agents