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Label-free electrical quantification of the dielectrophoretic response of DNA.

Henning A, Henkel J, Bier FF, Hölzel R - PMC Biophys (2008)

Bottom Line: The method has been applied to the characterisation of the dielectrophoretic response of DNA without the need for any chemical modifications.The results are in good agreement with data from dielectrophoretic studies on fluorescently labelled DNA.Extension of the method down to the single molecule level appears feasible.PACS: 87.50.ch, 87.80.Fe, 87.85.fK.

View Article: PubMed Central - HTML - PubMed

Affiliation: Fraunhofer Institute for Biomedical Engineering, Am Mühlenberg 13, 14476 Potsdam-Golm, Germany. ralph.hoelzel@ibmt.fraunhofer.de.

ABSTRACT
A purely electrical sensing scheme is presented that determines the concentration of macromolecules in solution by measuring the capacitance between planar microelectrodes. Concentrations of DNA in the ng/mL range have been used in samples of 1 muL volume. The method has been applied to the characterisation of the dielectrophoretic response of DNA without the need for any chemical modifications. The influence of electrical parameters like duty cycle, voltage and frequency has been investigated. The results are in good agreement with data from dielectrophoretic studies on fluorescently labelled DNA. Extension of the method down to the single molecule level appears feasible.PACS: 87.50.ch, 87.80.Fe, 87.85.fK.

No MeSH data available.


Capacitance changes as a function of DEP voltage. Dielectrophoretic excitation was performed at a frequency of 1 MHz. pBluescript DNA concentration was 18 nM. The inset shows the data in a double logarithmic plot.
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Figure 5: Capacitance changes as a function of DEP voltage. Dielectrophoretic excitation was performed at a frequency of 1 MHz. pBluescript DNA concentration was 18 nM. The inset shows the data in a double logarithmic plot.

Mentions: In order to quantify the dependence of molecular dielectrophoresis on field strength DEP voltage was varied from 2 VRMS to 4.5 VRMS at a fixed frequency of 1 MHz (Fig. 5). From the slope of the double logarithmic plot clearly a cubic relation follows, which is in contradiction to generally accepted DEP theory [37,38]. DEP action usually is explained by dipoles that are induced by the DEP field and interact with this field, resulting in a square dependence on field strength. Similar experiments on fluorescently labelled pBluescript DNA also showed a cubic relation at 1 MHz [18], whilst the data of Asbury et al. [39] at 30 Hz are more consistent with a square dependence. Tuukkanen et al. [31] have investigated the trapping efficiency for DNA samples of various lengths as a function of applied DEP voltage. They found a deviation from a purely quadratic relation and interpreted this by assuming a threshold resulting from thermal drag force with which the dielectrophoretic force competes. They also mentioned the well known fact that DNA longer than a few hundreds of basepairs is in a globular shape as long as there are no external forces present, and that DEP leads to elongation of the molecules and, hence, to an enhanced polarisability. In our view this mechanism of enhanced polarisability alone is sufficient to account for the observed more-than-quadratic DEP-voltage dependence. It means that in the equation for the dielectrophoretic force FDEP the polarisability α is no longer a constant but a function of the electric field E itself: FDEP = α(E)·▽ (E2). Bakewell and Morgan [19] reported DEP collection data from fluorescently labelled supercoiled plasmid DNA that deviated from a purely square relation. They, too, considered a change in plasmid shape under the action of a high DEP force and, additionally, discussed the action of fluid flow that is caused by electrohydrodynamic (EHD) forces, in particular AC-electroosmosis [40,41]. When re-evaluating the data of Tuukkanen et al. [31] by plotting their voltage-fluorescence relation in a double-logarithmic plot (data not shown) we got slopes between 2.9 and 3.8 (r2 = 0.723...0.994) for DNA lengths ranging from 27 bp to 8461 bp. Therefore the origin of the deviation from a purely quadratic dependence between electric field and molecular DEP response remains ambiguous. For a further clarification the DEP response of globular molecules like proteins should be quantified or, even better, stable compact DNA constructs like origami structures [42,43] should be studied. Variation of the DEP field's duty cycle especially in the region of small capacitance changes would surely clarify the influence of threshold effects.


Label-free electrical quantification of the dielectrophoretic response of DNA.

Henning A, Henkel J, Bier FF, Hölzel R - PMC Biophys (2008)

Capacitance changes as a function of DEP voltage. Dielectrophoretic excitation was performed at a frequency of 1 MHz. pBluescript DNA concentration was 18 nM. The inset shows the data in a double logarithmic plot.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2666632&req=5

Figure 5: Capacitance changes as a function of DEP voltage. Dielectrophoretic excitation was performed at a frequency of 1 MHz. pBluescript DNA concentration was 18 nM. The inset shows the data in a double logarithmic plot.
Mentions: In order to quantify the dependence of molecular dielectrophoresis on field strength DEP voltage was varied from 2 VRMS to 4.5 VRMS at a fixed frequency of 1 MHz (Fig. 5). From the slope of the double logarithmic plot clearly a cubic relation follows, which is in contradiction to generally accepted DEP theory [37,38]. DEP action usually is explained by dipoles that are induced by the DEP field and interact with this field, resulting in a square dependence on field strength. Similar experiments on fluorescently labelled pBluescript DNA also showed a cubic relation at 1 MHz [18], whilst the data of Asbury et al. [39] at 30 Hz are more consistent with a square dependence. Tuukkanen et al. [31] have investigated the trapping efficiency for DNA samples of various lengths as a function of applied DEP voltage. They found a deviation from a purely quadratic relation and interpreted this by assuming a threshold resulting from thermal drag force with which the dielectrophoretic force competes. They also mentioned the well known fact that DNA longer than a few hundreds of basepairs is in a globular shape as long as there are no external forces present, and that DEP leads to elongation of the molecules and, hence, to an enhanced polarisability. In our view this mechanism of enhanced polarisability alone is sufficient to account for the observed more-than-quadratic DEP-voltage dependence. It means that in the equation for the dielectrophoretic force FDEP the polarisability α is no longer a constant but a function of the electric field E itself: FDEP = α(E)·▽ (E2). Bakewell and Morgan [19] reported DEP collection data from fluorescently labelled supercoiled plasmid DNA that deviated from a purely square relation. They, too, considered a change in plasmid shape under the action of a high DEP force and, additionally, discussed the action of fluid flow that is caused by electrohydrodynamic (EHD) forces, in particular AC-electroosmosis [40,41]. When re-evaluating the data of Tuukkanen et al. [31] by plotting their voltage-fluorescence relation in a double-logarithmic plot (data not shown) we got slopes between 2.9 and 3.8 (r2 = 0.723...0.994) for DNA lengths ranging from 27 bp to 8461 bp. Therefore the origin of the deviation from a purely quadratic dependence between electric field and molecular DEP response remains ambiguous. For a further clarification the DEP response of globular molecules like proteins should be quantified or, even better, stable compact DNA constructs like origami structures [42,43] should be studied. Variation of the DEP field's duty cycle especially in the region of small capacitance changes would surely clarify the influence of threshold effects.

Bottom Line: The method has been applied to the characterisation of the dielectrophoretic response of DNA without the need for any chemical modifications.The results are in good agreement with data from dielectrophoretic studies on fluorescently labelled DNA.Extension of the method down to the single molecule level appears feasible.PACS: 87.50.ch, 87.80.Fe, 87.85.fK.

View Article: PubMed Central - HTML - PubMed

Affiliation: Fraunhofer Institute for Biomedical Engineering, Am Mühlenberg 13, 14476 Potsdam-Golm, Germany. ralph.hoelzel@ibmt.fraunhofer.de.

ABSTRACT
A purely electrical sensing scheme is presented that determines the concentration of macromolecules in solution by measuring the capacitance between planar microelectrodes. Concentrations of DNA in the ng/mL range have been used in samples of 1 muL volume. The method has been applied to the characterisation of the dielectrophoretic response of DNA without the need for any chemical modifications. The influence of electrical parameters like duty cycle, voltage and frequency has been investigated. The results are in good agreement with data from dielectrophoretic studies on fluorescently labelled DNA. Extension of the method down to the single molecule level appears feasible.PACS: 87.50.ch, 87.80.Fe, 87.85.fK.

No MeSH data available.