Nanoactuators: Rapid and sensitive detection

Nano Lett. doi: 10.1021/nl200384p (2011)

Superparamagnetic nanoparticles have been used in affinity assays to detect biomolecules. Magnetic nanoparticles that have captured target molecules in a fluid sample form clusters that can be monitored through changes either in the optical transmittance of the sample or in the transverse relaxation time of the clusters in nuclear magnetic resonance. However, these measurements cannot resolve few-particle clusters against a large background of single nanoparticles and therefore limit the sensitivity and speed of the assay. Menno Prins and colleagues at Philips Research and Eindhoven University of Technology have now developed a technique to selectively actuate, characterize and detect clusters of magnetic nanoparticles for highly sensitive and rapid detection of biomolecules.

The Dutch team focused a laser beam on the centre of a cuvette containing magnetic nanoparticles and target biomolecules. They used four electromagnets to drive nanoparticles that have captured target molecules to form chains through interparticle bonding. When the magnetic field is removed to allow unbound nanoparticles to disperse, optical scattering of two-particle clusters is measured and the intensity of the scattered light is correlated to the number, size distribution and magnetic properties of the clusters. Sensitive and selective detection of two-particle clusters is possible because their magnetic shape anisotropy enables frequency controlled rotation and their optical anisotropy generates scattered light. The technique is able to detect biomolecules down to 400 fm in buffer and 5 pm in human plasma in less than three minutes.

This method, which integrates magnetic actuation with optical detection, is potentially useful for various studies in biochemistry and colloidal interactions.

Silver nanoparticles: Spike detection

Angew. Chem. Int. Ed. doi: 10.1002/anie.201100885 (2011)

Silver nanoparticles are capable of killing bacteria and are used in a variety of commercial products including textiles and wound dressings. However, the nanoparticles can be released into the environment during the products' manufacture, use or disposal, and have been found to be toxic to some animals. Their release into river systems is of particular concern and, therefore, detection in such environments is essential. Richard Compton and colleagues at the University of Oxford have now shown that silver nanoparticles can be detected and characterized in aqueous solution by examining the impact between the nanoparticles and an electrode surface.

The researchers carried out a series of electrochemical experiments using a glassy carbon electrode and silver nanoparticles of various sizes dispersed in a citrate solution. When nanoparticles collide with the electrode they are instantly oxidized, generating current spikes. By comparing the onset potential of the spikes and the known anodic stripping voltammetry of silver nanoparticles, the nanoparticles can be identified. Furthermore, by analysing the charge passed per current spike, the size range of the nanoparticles can also be determined.

The Oxford team expect that the approach could be used to characterize other metal nanoparticles, and also mixtures of different nanoparticles.

Covalent organic frameworks: Networks on graphene

Science 332, 228–231 (2011)

Credit: © 2011 AAAS

Creating extended porous networks from molecular building blocks is typically achieved with the help of noncovalent interactions such as hydrogen bonding or metal–ligand interactions. However, much more robust structures can be achieved if organic building blocks are held together by strong covalent bonds. Such covalent organic frameworks (COFs) have been synthesized before as microcrystalline powders or as submonolayers on single-crystal metal surfaces, but these can be difficult to process into thin films, which limits their use in applications such as optoelectronic devices. William Dichtel and colleagues at Cornell University have now synthesized two-dimensional COF films on single-layer graphene surfaces.

The researchers created a framework known as COF-5, which has a hexagonal lattice, using a solvothermal condensation reaction between PBBA (a compound composed of a benzene ring with two boronic acid groups (R–B(OH)2) attached at opposite ends) and HTTP (a flat trigonal building block composed of four fused benzene rings with two OH groups at each corner). The reaction is carried out in the presence of substrate-supported graphene and forms a layered film of COF-5 on the graphene. The layers are vertically aligned and have a crystallinity superior to the equivalent powdered samples.

The Cornell team show that the synthesis can be carried out using graphene supported on a range of substrates such as copper and transparent fused silica. Moreover, they fabricate two other COFs just as effectively with the technique, including one with a two-dimensional square lattice.

Quantum dots: Performance doping

Science 332, 77–81 (2011)

The ability to control the electronic properties of semiconductors is a foundation of the electronics industry. Such control can be achieved by doping impurity atoms into bulk crystalline semiconductors, as well as by controlling the size of reduced-dimension semiconductors such as quantum dots. However, combining these methods — doping quantum dots — has proved to be experimentally difficult. Now, Oded Millo, Eran Rabani, Uri Banin and colleagues at Hebrew University and Tel Aviv University have demonstrated a simple method of doping metal atoms into semiconducting quantum dots.

The researchers added a solution of metal salts to a solution of indium arsenide quantum dots (both dissolved in toluene) at room temperature. Solid-state diffusion then carried some metal atoms into the quantum dot crystal lattices, as confirmed by photoelectron spectroscopy. Different metal salts introduced different metal atoms, with distinct effects: copper atoms were incorporated into the crystal interstices, and added negative charges to the quantum dot, whereas silver atoms replaced indium atoms, and added positive charges. Absorption and emission wavelengths were shifted, reflecting changes to the dot's density of states and Fermi level, and the formation of impurity-derived energy bands.