The phase transfer of fluorescent CdSe based quantum dots (QDs) while retaining their properties and offering some advantages concerning the stability and functionalization characteristics is an important and intensively investigated field of research. Here we report how to tune and control the properties of CdSe/CdS/ZnS core-shell-shell QDs in water, using poly(isoprene-block-ethylene oxide) (PI-b-PEO) as a versatile system of amphiphilic diblock copolymers for the micellular encapsulation of nanoparticles (NPs). We show the synthesis of a novel PI-b-(PEO)2 miktoarm star polymer and how this different architecture besides the variation of the polymers' molecular weight gives us the opportunity to control the size of the built constructs in water between 24 and 53 nm. Because of this size control, an upper limit of the construct's diameter for the cellular uptake could be determined by a systemic study with human alveolar epithelial cells (A549) and murine macrophage leukemia cell (RAW-264.7). Furthermore, fluorescence quenching experiments with copper(II) and iron(III) ions show a strong influence of the used polymer on the shielding against these ions. This enables us to control the permeability of the polymer shell from very porous shells, which allow an almost complete cation exchange up to very dense shells. These even offer the possibility to perform copper(I) catalyzed click reactions while keeping the fluorescence of the QDs. All these results underline the huge variability and controllability of the PI-b-PEO diblock copolymer system for the encapsulation and functionalization of nanoparticles for biological applications. As a general trend, it can be stated that those coatings, which were most stable against quenchers, also showed the best resistivity with respect to unspecific cellular uptake.
Semiconductor nanocrystals (NCs) are robust inorganic chromophores that combine an efficient broadband absorption with a narrow-band fluorescence spectrum. Hence, they have a great potential as photoactive systems in attractive applications such as biolabeling, [1] solar cells, [2,3] light-emitting diodes, [4] and even in sensor systems. [5][6][7] Whereas most of these applications rely on the tunability of the fluorescence wavelength and thus on the size of the particles, sensor devices require a change of the fluorescence intensity, which strongly depends on surface effects in NCs. As the fluorescence arises from the recombination of photoexcited electron-hole pairs (excitons) within the inorganic NC core, its intensity is lowered if charge carriers are transferred to surface-bound ligands. This process, involving a photoinduced electron transfer (PET), strongly depends on the energetic position of the electronic levels of the NC with respect to those of the molecular orbitals (MOs) of the ligands.It has been known for many years that ligands may either increase or decrease the fluorescence intensity of NCs. For example, for the well-known CdSe NCs, prepared by standard methods in trioctylphosphine oxide (TOPO), [8] the replacement of TOPO ligands by amines increases the fluorescence intensity, [9] whilst that by thiols leads to a complete fluorescence quenching.[10] For CdTe NCs, prepared by similar methods, the situation is reversed: ligand exchange of TOPO by thiols leads to an increase of the fluorescence intensity.[10]The fluorescence quenching is assumed to be due to a transfer of the photoexcited hole from the top of the NC valence band (VB) to the highest occupied molecular orbital (HOMO) of the attached ligand. [10] We have investigated the electronic interaction between NC and ligand in detail by optical and electrochemical measurements of similarly sized CdSe and CdTe NCs in combination with different surface ligands, including the phenanthroline ligand 1 (Figure 1). Specifically, we compared the emission intensity of NC-ligand combinations with their respective energy levels of isolated NCs and ligands as determined by cyclic voltammetry (CV). Using the ligand 1, the oxidation potential of which can be adjusted by ion complexation, we could demonstrate that the addition of metal ions modulates the PET between the NC core and the attached ligands. As a consequence, the fluorescence of the NC-1 hybrids is made sensitive to the presence of metal ions.As depicted in Figure 2 a,b, the attachment of octadecylamine (ODA) leads to an enhancement of the CdSe-NC fluorescence and to a decrease of the CdTe-NC emission. For thiol ligands, such as dodecylthiol (DT) or mercaptopropionic acid (MPA), the fluorescence of CdSe NCs is quenched whilst that of the CdTe NCs is enhanced. This result can be understood by comparison with Figure 2 c, which shows CV measurements from the NCs and also from the respective ligands. The anodic peaks (positive values) are related to the VB of the NCs or the HOMO of the ligands, whi...
Seeded emulsion polymerization is a powerful universal method to produce ultrasmall multifunctional magnetic nanohybrids. In a two-step procedure, iron oxide nanocrystals were initially encapsulated in a polystyrene (PS) shell and subsequently used as beads for a controlled assembly of elongated quantum dots/quantum rods (QDQRs). The synthesis of a continuous PS shell allows the whole construct to be fixed and the composition of the nanohybrid to be tuned. The fluorescence of the QDQRs and magnetism of iron oxide were perfectly preserved, as confirmed by single-particle investigation, fluorescence decay measurements, and relaxometry. Bio-functionalization of the hybrids was straightforward, involving copolymerization of appropriate affinity ligands as shown by immunoblot analysis. Additionally, the universality of this method was shown by the embedment of a broad scale of NPs.
Herein, we present a general route towards defined nanohybrids, comprised of a fluorescent quantum dot (QD) or superparamagnetic iron oxide (Fe2O3) nanocrystal core and a tuneable corona of plasmonic gold or silver nanoparticles (NPs), adhered by a cross-linked poly(isoprene)-b-poly(ethylene glycol) diblock copolymer (PI-b-PEG) matrix. To this end, the PEG-terminus of the amphiphilic polymer was acylated with lipoic acid (LA), which, as is known, forms quasi-covalent Au-thiol- or Ag-thiol-bonds. Surprisingly, by variation of the ratio of the different NPs, inverse core/satellite structures bearing QDs or Fe2O3 around a metallic NP core were obtained. Furthermore, gold NPs or even closed gold shells were grown by in situ reductive deposition of Au(3+) ions on Fe2O3 NP seeds. Finally, in order to demonstrate the scope of the method, ternary nanohybrids, composed of QDs, Fe2O3 and Au NPs, were accomplished. All magneto-plasmonic and fluorescent-plasmonic materials were thoroughly characterized by absorption and emission spectroscopy, TEM and TEM-EDX. Antibody conjugation to these novel nanohybrids proved their practical utility in a prototype immunoassay.
Detailed steady-state and time-resolved fluorescence quenching measurements give deep insight into ion transport through nanometer thick diblock copolymer membranes, which were assembled as biocompatible shell material around CdSe/CdS quantum dot in quantum rods. We discuss the role of polymer chain length, intermolecular cross-linking and nanopore formation by analysing electron transfer processes from the photoexcited QDQRs to Cu(II) ions, which accumulate in the polymer membrane. Fluorescence investigations on single particle level additionally allow identifying ensemble inhomogeneities.
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