Philip G. Collins

A simple and robust electron beam source from carbon nanotubes

A simple fabrication method is described to produce reliable and robust (self‐repairing) field emission sources from randomly aligned carbon nanotubes. The sources display stable and reproducible current–voltage characteristics with sharp turn on near 100 V and emission currents up to 400 mA/cm2 at 200 V. Two successful methods are described for patterning the emission source area into emitting and nonemitting regions.

1/f noise in carbon nanotubes

The electrical noise characteristics of single-walled carbon nanotubes have been investigated. For all three cases of individual isolated nanotubes, thin films of interconnected nanotubes, and bulk nanotube mats, anomalously large bias-dependent 1/f noise is found. The noise magnitude greatly exceeds that commonly observed in metal films, carbon resistors, or even carbon fibers with comparable resistances. A single empirical expression describes the noise for all nanotube samples, suggesting a common noise-generating mechanism proportional only to the number of nanotubes in the conductor. We consider likely sources of the fluctuations, and consequences for electronic applications of nanotubes if the excessive noise cannot be suppressed.

Single-Molecule Lysozyme Dynamics Monitored by an Electronic Circuit

Observing Protein Dynamics Following the dynamics of protein conformational changes over the relatively long periods of time typical of enzyme kinetics can be challenging. Choi et al. (p. 319; see the Perspective by Lu) were able to observe changes in lysozyme conformation, which changes its electrostatic potential, by using a carbon-nanotube field-effect transistor. Slower hydrolysis steps were compared with faster, but unproductive, hinge motion, and changes in lysozyme activity that occur with pH were shown to arise from differences in the relative amount of time spent in processive versus nonprocessive states. Changes in protein conformation can be detected via changes in electrostatic potential with a carbon nanotube transistor. Tethering a single lysozyme molecule to a carbon nanotube field-effect transistor produced a stable, high-bandwidth transducer for protein motion. Electronic monitoring during 10-minute periods extended well beyond the limitations of fluorescence techniques to uncover dynamic disorder within a single molecule and establish lysozyme as a processive enzyme. On average, 100 chemical bonds are processively hydrolyzed, at 15-hertz rates, before lysozyme returns to its nonproductive, 330-hertz hinge motion. Statistical analysis differentiated single-step hinge closure from enzyme opening, which requires two steps. Seven independent time scales governing lysozyme’s activity were observed. The pH dependence of lysozyme activity arises not from changes to its processive kinetics but rather from increasing time spent in either nonproductive rapid motions or an inactive, closed conformation.

Materials: Peeling and sharpening multiwall nanotubes

To realize the full potential of multiwall carbon nanotubes in applications such as biological and scanned probes, it is desirable to develop techniques for controlling their shape and geometry. Here we describe a method by which the outer layers of a multiwall nanotube can be successively removed at the end to produce what is effectively a sharpened structure.

Hydrogen Sensing and Sensitivity of Palladium-Decorated Single-Walled Carbon Nanotubes with Defects

Individual single-walled carbon nanotubes (SWCNTs) become sensitive to H(2) gas when their surfaces are decorated with Pd metal, and previous reports measure typical chemoresistive increases to be approximately 2-fold. Here, thousand-fold increases in resistance are demonstrated in the specific case where a Pd cluster decorates a SWCNT sidewall defect site. Measurements on single SWCNTs, performed both before and after defect incorporation, prove that defects have extraordinary consequences on the chemoresistive response, especially in the case of SWCNTs with metallic band structure. Undecorated defects do not contribute to H(2) chemosensitivity, indicating that this amplification is due to a specific but complex interdependence between a defect sites electronic transmission and the chemistry of the defect-Pd-H(2) system. Dosage experiments suggest a primary role is played by spillover of atomic H onto the defect site.

Nanoscale electronic devices on carbon nanotubes

Conductivity measurements were performed on bundles of single-walled carbon nanotubes with the aid of a scanning tunneling microscope (STM). Semimetallic current-voltage (I-V) characteristics generally indicated the bundles to be electronically similar to graphite. However, by moving the STM tip along the length of the nanotubes, sharp deviations in the I-V characteristics could also be observed. Well-defined positions were found at which the nanotube transport current changed abruptly from a graphitic response to one that is highly nonlinear and asymmetric, including near-perfect rectification. This abrupt change in the nanotube transport suggests that the STM tip had passed a region of the nanotube which acts less like a wire than it does a Schottky barrier or other heterojunction. Similar on-tube nanodevices have been theoretically predicted for point defects in individual carbon nanotubes and are consistent with our observations.

Dissecting Single-Molecule Signal Transduction in Carbon Nanotube Circuits with Protein Engineering

Single-molecule experimental methods have provided new insights into biomolecular function, dynamic disorder, and transient states that are all invisible to conventional measurements. A novel, nonfluorescent single-molecule technique involves attaching single molecules to single-walled carbon nanotube field-effective transistors (SWNT FETs). These ultrasensitive electronic devices provide long-duration, label-free monitoring of biomolecules and their dynamic motions. However, generalization of the SWNT FET technique first requires design rules that can predict the success and applicability of these devices. Here, we report on the transduction mechanism linking enzymatic processivity to electrical signal generation by a SWNT FET. The interaction between SWNT FETs and the enzyme lysozyme was systematically dissected using eight different lysozyme variants synthesized by protein engineering. The data prove that effective signal generation can be accomplished using a single charged amino acid, when appropriately located, providing a foundation to widely apply SWNT FET sensitivity to other biomolecular systems.

Electronic Measurements of Single-Molecule Catalysis by cAMP- Dependent Protein Kinase A

Single-molecule studies of enzymes open a window into their dynamics and kinetics. A single molecule of the catalytic domain of cAMP-dependent protein kinase A (PKA) was attached to a single-walled carbon nanotube device for long-duration monitoring. The electronic recording clearly resolves substrate binding, ATP binding, and cooperative formation of PKAs catalytically functional, ternary complex. Using recordings of a single PKA molecule extending over 10 min and tens of thousands of binding events, we determine the full transition probability matrix and conversion rates governing formation of the apo, intermediate, and closed enzyme configurations. We also observe kinetic rates varying over 2 orders of magnitude from one second to another. Anti-correlation of the on and off rates for PKA binding to the peptide substrate, but not ATP, demonstrates that regulation of enzyme activity results from altering the stability of the PKA-substrate complex, not its binding to ATP. The results depict a highly dynamic enzyme offering dramatic possibilities for regulated activity, an attribute useful for an enzyme with crucial roles in cell signaling.

Nonlinear Optical Imaging of Individual Carbon Nanotubes with Four-Wave-Mixing Microscopy

Dual color four-wave-mixing (FWM) microscopy is used to spatially resolve the third-order optical response from individual carbon nanotubes. Good signal-to-noise is obtained from single-walled carbon nanotubes (SWNT) sitting on substrates, when the excitation beams are resonant with electronic transitions of the nanotube, by detecting the FWM response at the anti-Stokes frequency. Whereas the coherent anti-Stokes (CAS) signal is sensitive to both electronic and vibrational resonances of the material, it is shown that the signal from individual SWNTs is dominated by the electronic response. The CAS signal is strongly polarization dependent, with the highest signals found parallel with the enhanced electronic polarizibility along the long axis of the SWNT.

Single Molecule Dynamics of Lysozyme Processing Distinguishes Linear and Cross-linked Peptidoglycan Substrates

The dynamic processivity of individual T4 lysozyme molecules was monitored in the presence of either linear or cross-linked peptidoglycan substrates. Single-molecule monitoring was accomplished using a novel electronic technique in which lysozyme molecules were tethered to single-walled carbon nanotube field-effect transistors through pyrene linker molecules. The substrate-driven hinge-bending motions of lysozyme induced dynamic electronic signals in the underlying transistor, allowing long-term monitoring of the same molecule without the limitations of optical quenching or bleaching. For both substrates, lysozyme exhibited processive low turnover rates of 20-50 s(-1) and rapid (200-400 s(-1)) nonproductive motions. The latter nonproductive binding events occupied 43% of the enzymes time in the presence of the cross-linked peptidoglycan but only 7% with the linear substrate. Furthermore, lysozyme catalyzed the hydrolysis of glycosidic bonds to the end of the linear substrate but appeared to sidestep the peptide cross-links to zigzag through the wild-type substrate.