In situ and real time determination of metallic and semiconducting single-walled carbon nanotubes in suspension via dielectrophoresis

Abstract

We report an alternative method combining dielectrophoresis and impedance spectroscopy to provide rapid, accurate measurement of dielectrophoretic collection of single-walled carbon nanotubes ͑SWNTs͒ in real time. We analyzed a Triton X-100 suspension of mixed SWNTs to measure their precise dielectrophoretic collection behavior. Results indicate that our sample contains 21.5% of metallic and 78.5% of semiconducting carbon nanotubes before separation, and that the range of 1 -15 MHz is ideal to collect only the metallic ones. Optical absorption was used to confirm these proportions. We discuss the possible errors associated with using purely 2 Depending on their diameter and chirality, single-walled carbon nanotubes ͑SWNTs͒ can behave either as a metal or a semiconductor. Dielectrophoresis 3 ͑DEP͒, the movement of particles in nonuniform electric fields, has been used to separate mixtures of semiconducting and metallic SWNTs.4-6 Measurement of the frequency-dependent collection of DEP has thus far been based on Raman spectroscopy, which requires some complex interpretation of results. [7][8][9] In this letter, we report the application of combined dielectrophoresis and impedance measurements to provide real time rapid, accurate measurement of dielectrophoretic collection of SWNTs, and hence their dielectric properties and chiral proportions.SWNTs were made by laser ablation and suspended in a Triton X-100 solution. Measurements were performed on needle-shaped electrodes with a 10 m gap, energized with a sinusoidal 10 V pk-pk , at five or more frequencies per decade between 10 kHz and 20 MHz. A resistance was connected in series, and the voltage measured across the resistor and resistor/electrode combination using a digital oscilloscope. The oscilloscope output to a computer, which calculated the resistance of the interelectrode gap at 1 s intervals. The recording process began prior to the SWNTs suspension being placed on the electrodes, and continued for typically 5 min. The time constant of the change of resistance as a function of time was then obtained. Figure 1͑a͒ shows a representative selection of impedance versus time data which has been normalized to the electrode impedance. The solution conductivity was measured at 1.3 mS m −1 , causing a 30% decrease in resistance after application. Two types of behavior were observed in nanotube solutions. At high frequencies, an initial resistance drop at the application of the sample was followed by an exponential decrease to a stable value at approximately 200 s. The decrease in resistance ͑final with respect to initial values͒ was 53% for 1 MHz, 39% for 2 MHz, and 48% for 5 MHz. At lower frequencies, the reduction was more significant, e.g., 88% for 20 kHz, 79% for 50 kHz, and 67% for 100 kHz. At 20 MHz, the response indicated that the impedance change was due to the medium alone, that is, the result was identical to the control measurement and no collection occurred.If we hypothesize that the time taken for the impedance to change due to the collection of nanotub...

A dielectrophoresis-impedance method for protein detection and analysis

“…By mapping the variation of τ as a function of frequency (but with constant particle concentration) it is possible to determine the DEP collection rate of the particles. This technique was originally developed for carbon nanotubes 16 and nanowires, 19 where it was found that conduction around the electrical double layer 20 dominated the DEP response of particles in this size regime.…”

“…16,19 The inverse of the time constant was then used to approximate the DEP force, as collection rate is inversely proportional to the force causing the particles to collect. Figure 2 shows the inverse time constant for two medium conductivities, 1.1 and 100 mS/m.…”

“…BSA contains of Asparagine, Glutamine, Alanine, Leucine and Lysine as well as the four amino acid residues in the sequence of Glycine-Phenylalanine-Glycine-Asparagine. 15 Typically, experiments where DEP has been used to analyse the medium conductivity-dependent behaviour of biological macromolecules have been performed using fluorescent microscopy, 5 whereas characterisation of nanoscale particles such as carbon nanotubes 16 and zinc oxide nanowires 17,18 has employed impedance measuring techniques; impedance methods have been used for the study of larger particles such as cells, 19 but have never previously been applied to biological nanoparticles. In this paper, we present for the first time and impedance-based method for the determination of the electrical properties of dissolved biomolecules, using serum albumin as a model.…”

Rapid determination of antibiotic resistance inE. coliusing dielectrophoresis

“…The model used here is based on an advancement of the work of Irimajiri (1979) by Broche (2005) and considers a cell to be a homogenous sphere (the cytoplasm) suspended in a second sphere (the cell wall) which is in turn surrounded by the suspending medium. Other models for non-spherical geometry such as ellipsoids or rods are reported in the literature and can be used to model particles such as red blood cells (Jones 1995), elongated bacteria (Sanchis et al, 2004;Castellarnau et al, 2006) or carbon nanotubes (Mureau et al, 2006). Various algorithms and programs for computer models are published as well (Hughes 2002;Morgan and Green 2003).…”

Dielectrophoretic Deposition and Alignment of Carbon Nanotubes

“…Therefore, their applications are relatively limited. By comparison, dielectrophoresis, a simple but versatile method, has proven to be effective in aligning CNTs in small and large scales (Gultepe et al, 2008;Mureau et al, 2006). This method can be conducted at room temperature with low voltages.…”