Biomedical subjects

Nicholas A Kotov

Publications and source records attributed to Nicholas A Kotov.

6 recordsLinked to original sources

PubMed2003

Multicolor luminescence patterning by photoactivation of semiconductor nanoparticle films.

Indiscriminate adsorption of nanoparticles (NPs) significantly complicates the preparation of mesoscale NP patterns considered as enabling technology for many devices and processes. Instead of selected chemical functionalization of the substrate surface prior to the assembly of nanocolloids, the required optical properties - in our case, high quantum yield luminescence - are imparted to the layer-by-layer assembled films by spatially selected photoactivation. The films are made by sequential adsorption of a positively charged polyelectrolyte and a negatively charged CdSe/CdS aqueous dispersion with an initial quantum yield of 0.5-2%. The photoactivation process takes place in the presence of oxygen and may be accompanied by photoetching. A 50-500-fold increase in the luminescence intensity of CdSe/CdS citrate-stabilized particles (quantum yield 25-45%) after visible light illumination provides excellent pattern contrast. Micron scale luminescence patterns were produced from NPs of various CdSe core diameters with red, yellow, and green emission. It was also demonstrated that different emission colors such as orange and green can be combined in one image by taking advantage of spatially selective photoetching. The presented optical patterning technique significantly simplifies the preparation of luminescence patterns as compared to conventional methods. The high signal-to-noise ratio associated with it is essential for optical devices, information processing, and biophotonics. The most immediate use of this approach is expected in cryptography and cell monitoring.

Cadmium Compounds↗

PubMed2003

Nanostructured artificial nacre.

Finding a synthetic pathway to artificial analogs of nacre and bones represents a fundamental milestone in the development of composite materials. The ordered brick-and-mortar arrangement of organic and inorganic layers is believed to be the most essential strength- and toughness-determining structural feature of nacre. It has also been found that the ionic crosslinking of tightly folded macromolecules is equally important. Here, we demonstrate that both structural features can be reproduced by sequential deposition of polyelectrolytes and clays. This simple process results in a nanoscale version of nacre with alternating organic and inorganic layers. The macromolecular folding effect reveals itself in the unique saw-tooth pattern of differential stretching curves attributed to the gradual breakage of ionic crosslinks in polyelectrolyte chains. The tensile strength of the prepared multilayers approached that of nacre, whereas their ultimate Young modulus was similar to that of lamellar bones. Structural and functional resemblance makes clay- polyelectrolyte multilayers a close replica of natural biocomposites. Their nanoscale nature enables elucidation of molecular processes occurring under stress.

Aluminum Silicates↗

PubMed2002

Spontaneous organization of single CdTe nanoparticles into luminescent nanowires.

Nanoparticles of CdTe were found to spontaneously reorganize into crystalline nanowires upon controlled removal of the protective shell of organic stabilizer. The intermediate step in the nanowire formation was found to be pearl-necklace aggregates. Strong dipole-dipole interaction is believed to be the driving force of nanoparticle self-organization. The linear aggregates subsequently recrystallized into nanowires whose diameter was determined by the diameter of the nanoparticles. The produced nanowires have high aspect ratio, uniformity, and optical activity. These findings demonstrate the collective behavior of nanoparticles as well as a convenient, simple technique for production of one-dimensional semiconductor colloids suitable for subsequent processing into quantum-confined superstructures, materials, and devices.

Journal Article↗

PubMed2002

CdS nanoparticles modified to chalcogen sites: new supramolecular complexes, butterfly bridging, and related optical effects.

All present approaches to surface modification of nanoparticles (NPs) with organic ligands exploit metal (cadmium) sites as anchor points. To obtain efficient interaction of NP surface with p-orbitals of organic chromophores, we utilize the chalcogen (sulfur) sites on the NP surface. These sites present several advantages stemming from a stronger interaction of their atomic orbitals with both modifier and NP core. The chalcogen modification of CdS was achieved by using a mixed ligand (2,2'-bipyridyl-N,N')(malonato-O,O')-copper(II) monohydrate complex. The weak monodentate ligands (water) are replaced by a copper-sulfur bond during the modification reaction. The structure of the product was investigated by optical spectroscopy, electron spin resonance, and nuclear magnetic resonance. The modified NP can be described as a few tens (<40) of (2,2'-bipyridyl-N,N')(malonato-O,O')-copper units attached to the CdS core. Steady-state and time-resolved luminescence measurements, molecular orbital calculations, and UPS data indicate that delocalized surface states enveloping the surface chalcogen atoms of NP, transition metal, and p-orbitals of the bipyridine ligand are present in the synthesized species. The delocalized states are made possible due to the bridging of p-levels of sulfur and pi-orbitals of bipyridine by butterfly d-orbitals of the transition metal atom placed between them. Chalcogen-modified NP can be considered as a new member of the family of supramolecular compounds based on transition metal complexes. Both NP and metal complex parts of the prepared supramolecules are very versatile structural units, and new molecular constructs of similar design, in which quantum effects of NPs are combined with optical properties of transition metal complexes, can be obtained with different NPs and metal complexes.

Journal Article↗

PubMed2002

Quantum dot on a rope.

The conjugation of nanoparticles (NPs) typically yields supramolecular materials which are fairly rigid, and the electronic coupling between the NP and other structural units of these compounds is fixed by covalent bonds. Here, we report on a novel bichromophor system constructed from a quantum dot tethered to a semiconducting polymer, which demonstrates the possibility of the dynamic interunit coupling in the NP supramolecules. The NP bichromophoric system was made on the basis of the layer-by-layer assembled (LBL) films of an anionic polyelectrolyte with poly(p-phenylene ethynylene) backbone, aPPE, and poly(allylamine hydrochloride) PAH polycation. To conjugate CdTe NPs to the (aPPE/PAH)(m) LBL film, we took advantage of the reactive groups of NP stabilizer, that is, -COOH, and the aminogroups on PAH. Tethering of CdTe was accomplished by using poly(ethyleneglycole), PEG, chains with two reactive terminals such as t-BOC-NH-PEG-COO-NHS. The evidence for successful conjugation of NPs to the LBL films can be seen both in AFM images and in optical data. The latter also indicate that the light quanta emitted by the NPs originate from the light absorption of the polymer film, which proves the presence of the aPPE-->NP energy-transfer process. The average separation distance between the NPs tethered to the LBL films can be changed by altering the dielectric properties of the solvent affecting PEG tether coiling (water/alcohol mixture). The reduced emission intensity of aPPE was found to follow the extension of the PEG tether. The quenching of aPPE is reversible when the original composition of the solvent mixture is restored. Thus, CdTe-PEG-aPPE is an example of an organized NP system with tunable optical coupling. Variable electronic coupling offers a convenient structural platform for new nanotechnological devices for which spatial control translates into a higher level of sophistication. PEG molecules afford a wide variety of polymer chain configurations with different reactive terminals, which makes possible the preparation of diverse NP superstructures.

Journal Article↗

PubMed2002

Molecular design of strong single-wall carbon nanotube/polyelectrolyte multilayer composites.

The mechanical failure of hybrid materials made from polymers and single-wall carbon nanotubes (SWNT) is primarily attributed to poor matrix-SWNT connectivity and severe phase segregation. Both problems can be successfully mitigated when the SWNT composite is made following the protocol of layer-by-layer assembly. This deposition technique prevents phase segregation of the polymer/SWNT binary system, and after subsequent crosslinking, the nanometre-scale uniform composite with SWNT loading as high as 50 wt% can be obtained. The free-standing SWNT/polyelectrolyte membranes delaminated from the substrate were found to be exceptionally strong with a tensile strength approaching that of hard ceramics. Because of the lightweight nature of SWNT composites, the prepared free-standing membranes can serve as components for a variety of long-lifetime devices.

Acrylic Resins↗