Probing biomolecules at the single-molecule level can provide useful information about molecular interactions, kinetics and motions that is usually hidden in ensemble measurements. Techniques with improved sensitivity and time resolution are required to explore fast biomolecular dynamics. Here, we report the first observation of DNA hybridization at the single-molecule level using a carbon nanotube field-effect transistor. By covalently attaching a single-stranded probe DNA sequence to a point defect in a carbon nanotube, we are able to measure two-level fluctuations in the nanotube conductance due to reversible hybridizing and melting of a complementary DNA target. The kinetics are studied as a function of temperature, allowing the measurement of rate constants, melting curves and activation energies for different sequences and target concentrations. The kinetics show non-Arrhenius behavior, in agreement with DNA hybridization experiments using fluorescence correlation spectroscopy. This technique is label-free and has the potential for studying single-molecule dynamics at sub-microsecond time-scales.

ABSTRACT:The controlled radical polymerization of a variety of acrylate monomers is reported using an Ir-catalyzed visible light mediated process leading to well-defined homo-, random, and block copolymers. The polymerizations could be efficiently activated and deactivated using light while maintaining a linear increase in molecular weight with conversion and first order kinetics. The robust nature of the fac-[Ir(ppy) 3 ] catalyst allows carboxylic acids to be directly introduced at the chain ends through functional initiators or along the backbone of random copolymers (controlled process up to 50 mol % acrylic acid incorporation). In contrast to traditional ATRP procedures, low polydispersity block copolymers, poly(acrylate)-b-(acrylate), poly(methacrylate)-b-(acrylate), and poly(acrylate)-b-(methacrylate), could be prepared with no monomer sequence requirements. These results illustrate the increasing generality and utility of light mediated Ircatalyzed polymerization as a platform for polymer synthesis. have revolutionized the field of polymer chemistry, allowing for the synthesis of well-defined macromolecular structures with excellent functional group tolerance. Perhaps of greater importance is the facile reaction conditions that allow nonexperts access to these materials, enabling significant advances across a number of fields. More recently, additional control over living radical polymerizations has been achieved through regulation of the chain growth process by an external stimulus. 5 For example, electrochemical ATRP has been used to pattern polymer brushes on surfaces, 6− 8 as well as gain control over aqueous polymerizations.9 While the employment of externally regulated polymerizations is in its infancy, the potential for further innovation is significant.In considering the wide range of possible external stimuli, light offers many attractive features such as readily available light sources, tunability, and both spatial and temporal control. On this basis, significant work has been dedicated to the development of photoinitiated 10− 17 and photoregulated radical polymerizations (i.e., photocontrolled RAFT, 29 This approach uses a simple reaction setup with only ppm levels of Ir(ppy) 3 and enables efficient activation and deactivation of polymerization leading to control over molecular weight and molecular weight distributions. A fundamental element of this process is that in the absence of irradiation, the chain end rests as the dormant alkyl bromide, protected from deleterious radical reactions but available for reactivation upon re-exposure to light. Moreover, the spatial and temporal control of Ir-catalyzed photomediated processes has been exploited for patterning polymer brushes on surfaces to give novel, 3-D nanostructures. 30Our previous reports on photomediated radical polymerizations focused exclusively on methacrylates. In order to increase the scope and applicability of this strategy, extension to other monomer families is required. Our attention was therefore drawn to acrylate-based polymer...

Point-functionalized carbon nanotube field-effect transistors can serve as highly sensitive detectors for biomolecules. With a probe molecule covalently bound to a defect in the nanotube sidewall, two-level random telegraph noise (RTN) in the conductance of the device is observed as a result of a charged target biomolecule binding and unbinding at the defect site. Charge in proximity to the defect modulates the potential (and transmission) of the conductance-limiting barrier created by the defect. In this Letter, we study how these single-molecule electronic sensors are affected by ionic screening. Both charge in proximity to the defect site and buffer concentration are found to affect RTN amplitude in a manner that follows from simple Debye length considerations. RTN amplitude is also dependent on the potential of the electrolyte gate as applied to the reference electrode; at high enough repulsive potentials, the target DNA is completely repelled and RTN is suppressed.

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n-Doping of P(BTP-DPP) with the organometallic dimer (RuCp*mes) 2 , processed through sequential casting, is reported. Maximum conductivities of 0.45 S cm −1 were achieved that are relatively high for n-type semiconducting polymers. Electron paramagnetic resonance spectroscopy, ultraviolet visible spectroscopy, and ultraviolet photoemission spectroscopy are consistent with the introduction of high carrier concentrations by sequential processing, leading to bipolaronic, or otherwise spin-paired carriers. P(BTP-DPP) has glassy ordering in thin films, observed using wide angle X-ray scattering, that allows efficient incorporation of the dopant as a function of processing condition. The changes in electrical conductivity as a function of the dopant concentration are proposed to occur by charge percolation through domains with a mixture of polaronic and bipolaronic carriers.