Poly(ethylene glycol)-Based Multidentate Oligomers for Biocompatible Semiconductor and Gold Nanocrystals

We have developed a new set of multifunctional multidentate OligoPEG ligands, each containing a central oligomer on which were laterally grafted several short poly­(ethylene glycol) (PEG) moieties appended with either thioctic acid (TA) or terminally reactive groups. Reduction of the TAs (e.g., in the presence of NaBH₄) provides dihydrolipoic acid (DHLA)-appended oligomers. Here the insertion of PEG segments in the ligand structure promotes water solubility and reduces nonspecific interactions, while TA and DHLA groups provide multidentate anchoring onto Au nanoparticles (AuNPs) and ZnS-overcoated semiconductor quantum dots (QDs), respectively. The synthetic route involves simple coupling chemistry using <i>N</i>,<i>N</i>-dicylohexylcarbodiimide (DCC). Water-soluble QDs and AuNPs capped with these ligands were prepared via cap exchange. As prepared, the nanocrystals dispersions were aggregation-free, homogeneous, and stable for extended periods of time over pH ranging from 2 to 14 and in the presence of excess electrolyte (2 M NaCl). The new OligoPEG ligands also allow easy integration of tunable functional and reactive groups within their structures (e.g., azide or amine), which imparts surface functionalities to the nanocrystals and opens up the possibility of bioconjugation with specific biological molecules. The improved colloidal stability combined with reactivity offer the possibility of using the nanocrystals as biological probes in an array of complex and biologically relevant media

Anti-Galvanic Reduction of Silver Ion on Gold and Its Role in Anisotropic Growth of Gold Nanomaterials

The role of silver ions in the seed-mediated growth of gold nanostructures has been investigated. Silver submonolayer or monolayer on specific facet of gold is assumed in previously suggested mechanism owing to underpotential deposition (UPD) of silver by ascorbic acid having weak reducing power. Silver overpotential deposition by ascorbic acid, however, is confirmed by electrochemical stripping voltammetry, whereas submonolayer of silver on gold is spontaneously formed by anti-galvanic reduction in the absence of ascorbic acid. In the presence of cetyl­trimethyl­ammonium bromide (CTAB), silver overpotential deposition by ascorbic acid does not occur, but submonolayer of silver is formed on gold surface. Adsorption of silver and CTAB on gold dramatically hindered the electron transfer by the oxidation of ascorbic acid on gold, which reduces gold ions to metallic gold in seed-mediated growth. These results provide the evidence to the in-depth observation of mechanism in seed-mediated growth where the blocking effect of CTAB/Ag­(submonolayer)/Au for oxidation of reducing agent determine the shape and facet of gold nanomaterials

Synthesis of Highly Crystalline and Monodisperse Maghemite Nanocrystallites without a Size-Selection Process

The synthesis of highly crystalline and monodisperse γ-Fe2O3 nanocrystallites is reported. High-temperature (300 °C) aging of iron−oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 °C, was found to generate monodisperse iron nanoparticles. The resulting iron nanoparticles were transformed to monodisperse γ-Fe2O3 nanocrystallites by controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Transmission electron microscopic images of the particles showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the γ-Fe2O3 structures. Monodisperse γ-Fe2O3 nanocrystallites with a particle size of 13 nm also can be generated from the direct oxidation of iron pentacarbonyl in the presence of oleic acid with trimethylamine oxide as an oxidant

Multidentate Catechol-Based Polyethylene Glycol Oligomers Provide Enhanced Stability and Biocompatibility to Iron Oxide Nanoparticles

We have designed, prepared, and tested a new set of multidentate catechol- and polyethylene glycol (PEG)-derivatized oligomers, OligoPEG-Dopa, as ligands that exhibit strong affinity to iron oxide nanocrystals. The ligands consist of a short poly(acrylic acid) backbone laterally appended with several catechol anchoring groups and several terminally functionalized PEG moieties to promote affinity to aqueous media and to allow further coupling to target molecules (bio and others). These multicoordinating PEGylated oligomers were prepared using a relatively simple chemical strategy based on <i>N,N</i>′-dicyclohexylcarbodiimide (DCC) and <i>N</i>-(3-dimethylaminopropyl)-<i>N</i>′-ethylcarbodiimide (EDC) condensation. The ability of these catechol-functionalized oligomers to impart long-term colloidal stability to the nanoparticles is compared to other control ligands, namely, oligomers presenting several carboxyl groups and monodentate ligands presenting either one catechol or one carboxyl group. We found that the OligoPEG-Dopa ligands provide rapid ligand exchange, and the resulting nanoparticles exhibit greatly enhanced colloidal stability over a broad pH range and in the presence of excess electrolytes; stability is notably improved compared to non-catechol presenting molecular or oligomer ligands. By inserting controllable fractions of azide-terminated PEG moieties, the nanoparticles (NPs) become reactive to complementary functionalities <i>via</i> azide–alkyne cycloaddition (Click), which opens up the possibility of biological targeting of such stable NPs. In particular, we tested the Click coupling of azide-functionalized nanoparticles to an alkyne-modified dye. We also measured the MRI <i>T</i>₂ contrast of the OligoPEG-capped Fe₃O₄ nanoparticles and applied MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay to test the potential cytotoxicity of these NPs to live cells; we found no measurable toxicity to live cells

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

Generalized and Facile Synthesis of Semiconducting Metal Sulfide Nanocrystals

We report on the synthesis of semiconductor nanocrystals of PbS, ZnS, CdS, and MnS through a facile and inexpensive synthetic process. Metal−oleylamine complexes, which were obtained from the reaction of metal chloride and oleylamine, were mixed with sulfur. The reaction mixture was heated under appropriate experimental conditions to produce metal sulfide nanocrystals. Uniform cube-shaped PbS nanocrystals with particle sizes of 6, 8, 9, and 13 nm were synthesized. The particle size was controlled by changing the relative amount of PbCl2 and sulfur. Uniform 11 nm sized spherical ZnS nanocrystals were synthesized from the reaction of zinc chloride and sulfur, followed by one cycle of size-selective precipitation. CdS nanocrystals that consist of rods, bipods, and tripods were synthesized from a reaction mixture containing a 1:6 molar ratio of cadmium to sulfur. Spherical CdS nanocrystals (5.1 nm sized) were obtained from a reaction mixture with a cadmium to sulfur molar ratio of 2:1. MnS nanocrystals with various sizes and shapes were synthesized from the reaction of MnCl2 and sulfur in oleylamine. Rod-shaped MnS nanocrystals with an average size of 20 nm (thickness) × 37 nm (length) were synthesized from a 1:1 molar ratio of MnCl2 and sulfur at 240 °C. Novel bullet-shaped MnS nanocrystals with an average size of 17 nm (thickness) × 44 nm (length) were synthesized from the reaction of 4 mmol of MnCl2 and 2 mmol of sulfur at 280 °C for 2 h. Shorter bullet-shaped MnS nanocrystals were synthesized from a 3:1 molar ratio of MnCl2 and sulfur. Hexagon-shaped MnS nanocrystals were also obtained. All of the synthesized nanocrystals were highly crystalline

Designed Synthesis of Atom-Economical Pd/Ni Bimetallic Nanoparticle-Based Catalysts for Sonogashira Coupling Reactions

We synthesized Ni/Pd core/shell nanoparticles from the consecutive thermal decomposition of metal−surfactant complexes. The nanoparticle catalyst was atom-economically applied for various Sonogashira coupling reactions

Design of a Multi-Dopamine-Modified Polymer Ligand Optimally Suited for Interfacing Magnetic Nanoparticles with Biological Systems

We have designed a set of multifunctional and multicoordinating polymer ligands that are optimally suited for surface functionalizing iron oxide and potentially other magnetic nanoparticles (NPs) and promoting their integration into biological systems. The amphiphilic polymers are prepared by coupling (via nucleophilic addition) several amine-terminated dopamine anchoring groups, poly­(ethylene glycol) moieties, and reactive groups onto a poly­(isobutylene-<i>alt</i>-maleic anhydride) (PIMA) chain. This design greatly benefits from the highly efficient and reagent-free one-step reaction of maleic anhydride groups with amine-containing molecules. The availability of several dopamine groups in the same ligand greatly enhances the ligand affinity, via multiple coordination, to the magnetic NPs, while the hydrophilic and reactive groups promote colloidal stability in buffer media and allow subsequent conjugation with target biomolecules. Iron oxide nanoparticles ligand exchanged with these polymer ligands have a compact hydrodynamic size and exhibit enhanced long-term colloidal stability over the pH range of 4–12 and in the presence of excess electrolytes. Nanoparticles ligated with terminally reactive polymers have been easily coupled to target dyes and tested in live cell imaging with no measurable cytotoxicity. Finally, the resulting hydrophilic nanoparticles exhibit large and size-dependent <i>r</i>₂ relaxivity values

Various-Shaped Uniform Mn₃O₄ Nanocrystals Synthesized at Low Temperature in Air Atmosphere

We report a novel and facile method for the synthesis of manganese oxide (Mn3O4) nanocrystals with various sizes and shapes. Mn3O4 nanocrystals were synthesized via a reaction of manganese(II) acetate with water in xylene in the presence of surfactants at the temperature of as low as 90 °C in air atmosphere. Structural characterizations revealed that the synthesized nanocrystals were tetragonal Mn3O4 structure and that they were highly crystalline in spite of the low reaction temperature. The size and shape of the nanocrystals were readily controlled by varying the experimental conditions such as precursors, surfactants, and injection temperature of water. Nanoplates with a thickness of 5 nm and side dimensions of 9, 15, and 22 nm were synthesized using oleylamine as the surfactant. When carboxylic acid was used as the cosurfactant along with oleylamine, spherical nanocrystals were obtained with sizes of 5.5, 6.2, 7.2, 8.5, and 15 nm. Interesting anisotropic nanostructures including nanowires and nanokites were also prepared by changing the injection temperature of water. Mechanistic studies revealed that in situ generated manganese hydroxide (Mn(OH)2) mainly contributes to the nucleation, whereas the manganese−oleylamine complex contributes to the shape-controlled growth process. The current procedure can be readily applicable to large-scale synthesis because of their facile and mild reaction conditions including low reaction temperature and air environment and the use of nontoxic and inexpensive reagents. For example, under optimized reaction conditions, we were able to synthesize as much as 4.5 g of 15 nm sized Mn3O4 nanoplates using a 1 L reactor. Water-dispersible 9 nm sized Mn3O4 nanoplates exhibited specific relaxivity (r1) value of 0.13 mM−1 s−1, demonstrating the potential application of the nanocrystals to T1 contrast agent for magnetic resonance imaging (MRI)

Simple and Generalized Synthesis of Oxide−Metal Heterostructured Nanoparticles and their Applications in Multimodal Biomedical Probes

Heterostructured nanoparticles composed of metals and Fe3O4 or MnO were synthesized by thermal decomposition of mixtures of metal−oleate complexes (for the oxide component) and metal−oleylamine complexes (for the metal component). The products included flowerlike-shaped nanoparticles of Pt−Fe3O4 and Ni−Fe3O4 and snowmanlike-shaped nanoparticles of Ag−MnO and Au−MnO. Powder X-ray diffraction patterns showed that these nanoparticles were composed of face-centered cubic (fcc)-structured Fe3O4 or MnO and fcc-structured metals. The relaxivity values of the Au−MnO and Au−Fe3O4 nanoparticles were similar to those of the MnO and Fe3O4 nanoparticles, respectively. Au−Fe3O4 heterostructured nanoparticles conjugated with two kinds of 12-base oligonucleotide sequences were able to sense a complementary 24-mer sequence, causing nanoparticle aggregation. This hybridization-mediated aggregation was detected by the overall size increase indicated by dynamic light scattering data, the red shift of the surface plasmon band of the Au component, and the enhancement of the signal intensity of the Fe3O4 component in T2-weighted magnetic resonance imaging