Ultraminiaturized mass spectrometers are highly sought-after tools, with numerous applications in areas such as environmental protection, exploration, and drug development. We realize atomic scale mass sensing using doubly clamped suspended carbon nanotube nanomechanical resonators, in which their single-electron transistor properties allows self-detection of the nanotube vibration. We use the detection of shifts in the resonance frequency of the nanotubes to sense and determine the inertial mass of atoms as well as the mass of the nanotube. This highly sensitive mass detection capability may eventually enable applications such as on-chip detection, analysis, and identification of compounds.Because of their small size, nanoscale systems are highly sensitive to their environment, affording great potential for sensing applications. Chemical and biological charge-based sensors using carbon nanotubes 1,2 and nanowires 3 have already been demonstrated. Nanoelectromechanical systems have been proposed for highly sensitive mass detection of neutral species, 4,5 and significant progress has been made in using nanofabricated resonators 6-8 and carbon nanotubes 9-12 for mass sensing. However, the ultimate limit of atomic resolution mass sensitivity remains unrealized. Here we demonstrate that individual double-clamped single-walled carbon nanotube resonators are capable of atomic-scale mass sensing and determining the inertial mass of atomic species. Analysis of our data also yields the nanotube mass, giving a general approach to determining the mass of molecular nanostructures. Our results open the door to ultraminiaturized mass spectroscopy or arrays, potentially enabling on-chip detection and identification of unknown analytes, as well as the study of single atom adsorption and desorption.We fabricate our devices by first growing nanotubes from catalyst islands 13 under either pure CH 4 13 or a CH 4 /H 2 mixture 14 on an oxidized Si wafer. We then attach Pd/Au source/drain electrodes and a side gate using electron beam lithography (EBL). In a second step, a layer of poly(methyl methacrylate) (PMMA) is spun on the chip, and a ∼450 nm window is opened over a segment of the nanotube between the electrodes, again using EBL. The SiO 2 is etched using buffered HF to suspend the carbon nanotube within the window. Following a critical point drying step, the samples are electrically tested at room temperature; we then load selected devices into a custom-built cryostat, which is maintained at a high vacuum. Figure 1a shows a scanning electron microscope (SEM) image of a completed suspended nanotube transistor device.At the temperature T ≈ 6 K of our experiment, the nanotubes act as single-electron transistors (SET) in which the charge on the nanotube is an integer multiple of the electron charge e. 15 As the gate voltage V g is swept, electrons enter the SET one at a time, with each transition between discrete charge states producing a Coulomb peak in the source-drain conductance (see e.g. ref 16). Figure 1b shows this b...
Electrostatic forces play a key role in mediating interactions between proteins. However, gaining quantitative insights into the complex effects of electrostatics on protein behavior has proved challenging, due to the wide palette of scenarios through which both cations and anions can interact with polypeptide molecules in a specific manner or can result in screening in solution. In this article, we have used a variety of biophysical methods to probe the steady-state kinetics of fibrillar protein self-assembly in a highly quantitative manner to detect how it is modulated by changes in solution ionic strength. Due to the exponential modulation of the reaction rate by electrostatic forces, this reaction represents an exquisitely sensitive probe of these effects in protein-protein interactions. Our approach, which involves a combination of experimental kinetic measurements and theoretical analysis, reveals a hierarchy of electrostatic effects that control protein aggregation. Furthermore, our results provide a highly sensitive method for the estimation of the magnitude of binding of a variety of ions to protein molecules.
A two-dimensional variational data assimilation (2DVAR) method for blending sea surface temperature (SST) data from multiple observing platforms is presented. This method produces continuous fields and has the capability of blending multiple satellite and in situ observations. In addition, it allows specification of inhomogeneous and anisotropic background correlations, which are common features of coastal ocean flows. High-resolution (6 km in space and 6 h in time) blended SST fields for August 2003 are produced for a region off the California coast to demonstrate and evaluate the methodology. A comparison of these fields with independent observations showed root-mean-square errors of less than 18C, comparable to the errors in conventional SST observations. The blended SST fields also clearly reveal the finescale spatial and temporal structures associated with coastal upwelling, demonstrating their utility in the analysis of finescale flows. With the high temporal resolution, the blended SST fields are also used to describe the diurnal cycle. Potential applications of this SST blending methodology in other coastal regions are discussed.
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