Effects of Environmental Factors and Metallic Electrodes on AC Electrical Conduction Through DNA Molecule

Abstract

Background: Deoxyribonucleic acid (DNA) is one of the best candidate materials for various device applications such as in electrodes for rechargeable batteries, biosensors, molecular electronics, medical- and biomedical-applications etc. Hence, it is worthwhile to examine the mechanism of charge transport in the DNA molecule, however, still a question without a clear answer is DNA a molecular conducting material (wire), semiconductor, or insulator? The answer, after the published data, is still ambiguous without any confirmed and clear scientific answer. DNA is found to be always surrounded with different electric charges, ions, and dipoles. These surrounding charges and electric barrier(s) due to metallic electrodes (as environmental factors (EFs)) play a substantial role when measuring the electrical conductivity through λ-double helix (DNA) molecule suspended between metallic electrodes. We found that strong frequency dependence of AC-complex conductivity comes from the electrical conduction of EFs. This leads to superimposing serious incorrect experimental data to measured ones.

Methods: At 1 MHz, we carried out a first control experiment on electrical conductivity with and without the presence of DNA molecule. If there are possible electrical conduction due to stray ions and contribution of substrate, we will detected them. This control experiment revealed that there is an important role played by the environmental-charges around DNA molecule and any experiment should consider this role.

Results and discussion: We have succeeded to measure both electrical conductivity due to EFs (σ _ENV) and electrical conductivity due to DNA molecule (σ _DNA) independently by carrying the measurements at different DNA-lengths and subtracting the data. We carried out measurements as a function of frequency (f) and temperature (T) in the ranges 0.1 Hz < f < 1 MHz and 288 K < T < 343 K. The measured conductivity (σ _MES) portrays a metal-like behavior at high frequencies near 1 MHz. However, we found that σ _DNA was far from this behavior because the conduction due to EFs superimposes σ _DNA, in particular at low frequencies. By measuring the electrical conductivity at different lengths: 40, 60, 80, and 100 nm, we have succeeded not only to separate the electrical conduction of the DNA molecule from all EFs effects that surround the molecule, but also to present accurate values of σ _DNA and the dielectric constant of the molecule ε'_DNA as a function of temperature and frequency. Furthermore, in order to explain these data, we present a model describing the electrical conduction through DNA molecule: DNA is a classical semiconductor with charges, dipoles and ions that result in creation of localized energy-states (LESs) in the extended bands and in the energy gap of the DNA molecule.

Conclusions: This model explains clearly the mechanism of charge transfer mechanism in the DNA, and it sheds light on why the charge transfer through the DNA can lead to insulating, semiconducting, or metallic behavior on the same time. The model considers charges on DNA, in the extended bands, either could be free to move under electric field or localized in potential wells/hills. Localization of charges in DNA is an intrinsic structural-property of this solitaire molecule. At all temperatures, the expected increase in thermal-induced charge is attributed to the delocalization of holes (or/and electrons) in potential hills (or/and potential wells) which accurately accounts for the total electric and dielectric behavior through DNA molecule. We succeeded to fit the experimental data to the proposed model with reasonable magnitudes of potential hills/wells that are in the energy range from 0.068 eV.

Keywords: AC-complex conductivity; DNA molecule; Dielectric constant; Environmental and surrounding charges; Localized charges; Potential hills/wells.

Fig. 1

a Ideal DNA sample without disorder factors and without surrounding charges. b Illustrates an ideal n-type semiconductor DNA sample without disorder factors but it has surrounding charges. c represents a real n-type semiconductor DNA sample with both disorder factors and surrounding charges. They are five trapping levels lying above the valence band. One will denote them H1, H2, H3, H4, and H5. Note that this figure represents ideal n-type semiconductor, and the same assumptions are valid for p-type semiconductor

Fig. 2

Equivalent electronic circuits of DNA molecule at different frequency ranges

Fig. 3

a Two sets of eight Au contacts with interspacing 20 nm are shown. DNA molecule is placed on one set of these electrical Au-contacts. The other set of Au-contacts connect a standard material. b shows image of fluorescent microscope due to point-contacts before pipetting Epicenter-solution. c Shows the same image after pipetting that solution. The thiol contact points on DNA serve to main jobs: (1) they carry out good metallic-DNA contacts and (2) they fix the molecule through eight fixing points which enable constant effective surface area between the metal and DNA d for I-V-L measurements at T = 298 K, we put DC-potential polarization between the points 10, 15, and we get the corresponding DC-currents from points: 0 and 2, 0 and 3, 0 and 4, and 0 and 5 for lengths 40, 60, 80, and 100 nm, respectively (Fig. 3c). While for I-V-T measurements at L = 60 nm, we put DC-potential polarization between the points 10 and 15, and we get the corresponding DC-current from points: 0 and 5. For AC-measurements at constant temperature (at T = 298 K), we put AC-leads of Solartron between the points: 0 and 2, 0 and 3, 0 and 4, and 0 and 5 for lengths 40, 60, 80, and 100 nm, respectively. While for AC-measurements at constant length (at L = 60 nm), we put AC-leads of Solartron between the points: 0 and 3 for all temperatures. We repeat these same experimental details on a standard material to have some dimension information about the effective surface area between the metal and standard sample which gives the effective surface area between the metal and DNA. The thiol contact points on DNA serve to main jobs: (1) they carry out good metallic-DNA contacts and (2) they fix the molecule through eight fixing points which enable constant effective surface area between the metal and DNA

Fig. 4

I-V characteristics at different lengths of DNA molecule: L _DNA 40, 60, 80, 100 nm

Fig. 5

The saturation current estimated after the fitting processes of experimental data in Fig. 4 to Eqs. (5, 6, and 7)

Fig. 6

The disorder energy of DNA molecule for different lengths estimated after fitting of experimental data to Eqs. (5–11)

Fig. 7

DC-current as a function of temperature for different lengths of DNA molecule. Symbols represent experimental data, and continuous lines are calculated data after Eqs. (5–7)

Fig. 8

Variation of the disorder energy γ as a function of temperature for different DNA lengths

Fig. 9

Dependence of the Gaussian factor (g) as a function of temperature for different DNA lengths

Fig. 10

The DC-electric current, calculated from Eq. (6), as a function of temperature for different disorder energy

Fig. 11

I-V characteristics for DNA molecule at different temperatures, lines represent calculated values after Eqs. (5–7) and symbols represent experimental values

Fig. 12

DC-electrical conductivity, calculated from Eq. (6-a), as a function of temperature for different DNA-lengths

Fig. 13

At 298 K, the measured AC-conductivity as a function of frequency for DNA molecule as a function of applied frequency, lines represent calculated values after Eq. (12) with the data in Figs. 8 and 9. Symbols represent experimental values. There are five humps marked as H1, H2, H3, H4, and H5

Fig. 14

AC-conductivity as a function of frequency for DNA molecule as a function of frequency, for different temperatures. Lines represent calculated values after Eq. (12) with the data in Figs. 8 and 9. Symbols represent experimental values. Lines on figure illustrate the effect of temperature on four humps (H2, H3, H4, and H5). Hump H1 is independently represented in Fig. 15 because the scale, here, is not suitable to represent all H1 data

Fig. 15

Low frequency dependence of the AC-conductivity for DNA molecule and for different temperatures (H1). Lines represent calculated values after Eq. (12) with the data in Figs. 8 and 9. Symbols represent experimental values. One can see the effect of temperature on H1

Fig. 16

At 298 K, the electric losses ε” as a function of frequency for DNA molecule. The relaxation times of H2, H3, H4, and H5 correspond to the maximum frequencies as arrows point out. One can better see the effect of frequency on H1 at lower frequencies in next figure

Fig. 17

a Electric losses as a function of frequency calculated after AC-conductivity through DNA molecule for different temperature humps (H1). Lines represent calculated values after Eq. (12) with the data in Figs. 8 and 9. b Electric losses as a function of frequency calculated after AC-conductivity through DNA molecule for different temperature humps (H2). Lines represent calculated values after Eq. (12) with the data in Figs. 8 and 9. c Electric losses as a function of frequency calculated after AC-conductivity through DNA molecule for different temperature humps (H3). Lines represent calculated values after Eq. (12) with the data in Figs. 8 and 9. d Electric losses as a function of frequency calculated after AC-conductivity through DNA molecule for different temperature humps (H4). Lines represent calculated values after Eq. (2) with the data in Figs. 8 and 9. e Electric losses as a function of frequency calculated after AC-conductivity through DNA molecule for different temperature humps (H5). Lines represent calculated values after Eq. (12) with the data in Figs. 8 and 9

Fig. 18

Relaxation time of electric charges on H1, H2, H3, H4, and H5 as a function of 1000/T (K ⁻¹). The slopes of semilog lines are 0.72, 0.58, 0.33, 0.24, and 0.05 eV for charges on H1, H2, H3, H4, and H5, respectively

Fig. 19

The frequency dependence of measured dielectric constant ε‘at 298 K. One notices the presence of H1, H2, H3, H4, and H5. H1 is presented in the inset of the figure

Fig. 20

The frequency dependence of measured dielectric constant ε‘at different temperatures. One notices the presence of H1, H2, H3, H4, and H5

Fig. 21

At low frequency range, for H1: dielectric constant dielectric constant, ε’ measured as a function of frequency at different temperatures. Symbols represent experimental values, and lines represent calculated values after Eq. (11); for the temperatures 288, 293, 298, 303, 303, 308, 313, 318, 323, 328, 333, 338, and 343 K, where red rectangles represent values of ε’ at 288 K, and yellow circles represent value of ε’ at 293 K, respectively

Fig. 22

Cole-Cole graph: dependence of electric losses as a function of dielectric constant at temperature = 298 K. One notices that H1, H2, H3, H4, and H5 have different behaviors

Fig. 23

Cole-Cole graph: dependence of electric losses as a function of dielectric constant for the charges at the interface between electrode and DNA (H1) at different temperatures. One notices that presence of nearly complete semicircle

Fig. 24

Cole-Cole curves for the charges at DNA (H4 and H5) at different 298 K. One notices that there is complete departure from the “semi-circle behavior”