RESUMEN
One of the most important methods for selective and repeatable assembly of carbon nanotubes (CNTs) is alternating current dielectrophoresis (DEP). This method has been demonstrated experimentally as a viable technique for nano-scale manufacturing of novel CNT based devices. Previous numerical analyses have studied the motion of nanotubes, the volume from which they are assembled, and the rate of assembly, but have been restricted by various simplifying assumptions. In this paper we present a method for simulating the motion and behavior of CNTs subjected to dielectrophoresis using a three-dimensional electrostatic finite element analysis. By including the CNT in the finite element model, we can accurately predict the effect of the CNT on the electric field and the resulting force distribution across the CNT can be determined. We have used this information to calculate the motion of CNTs assembling onto the electrodes, and show how they tend to move towards the center of an electrode and come into contact at highly skewed angles. Our analysis suggests that the CNTs move to the electrode gap only after initially contacting the electrodes. We have also developed a model of the elastic deformation of CNTs as they approach the electrodes demonstrating how the induced forces can significantly alter the CNT shape during assembly. These results show that the CNT does not behave as a rigid body when in close proximity to the electrodes. In the future this method can be applied to a variety of real electrode geometries on a case-by-case basis and will provide more detailed insight into the specific motion and assembly parameters necessary for effective DEP assembly.
RESUMEN
Microfabricated multiple-channel glass chips were successfully interfaced to an electrospray ionization mass spectrometer (ESI-MS). The microchip device was fabricated by standard photolithographic, wet chemical etching, and thermal bonding procedures. A high voltage was applied individually from each buffer reservoir for spraying sample sequentially from each channel. With the sampling orifice of the MS grounded, it was found that a liquid flow of 100-200 nL/min was necessary to maintain a stable electrospray. The detection limit of the microchip MS experiment for myoglobin was found to be lower than 6 x 10(-8) M. Samples in 75% methanol were successfully analyzed with good sensitivity, as were aqueous samples. The parallel mutliple-channel microchip system allowed ESI-MS analysis of different samples of standard peptides and proteins in one chip.