Magnesium-Dependent Electrical Actuation and Stability of DNA

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Functional Nanostructured Materials (including low-D carbon)

Magnesium Dependent Electrical Actuation and Stability of DNA Origami Rods Felix Kröner, Lukas Traxler, Andreas Heerwig, Ulrich Rant, and Michael Mertig ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18611 • Publication Date (Web): 25 Dec 2018 Downloaded from http://pubs.acs.org on December 27, 2018

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ACS Applied Materials & Interfaces

Magnesium Dependent Electrical Actuation and Stability of DNA Origami Rods Felix Kroener 1, 2, Lukas Traxler 2, Andreas Heerwig 3, Ulrich Rant *, 2 and Michael Mertig * ,1, 3 1

Professur für Physikalische Chemie, Mess- und Sensortechnik, Technische Universität

Dresden, 01062 Dresden, Germany 2

Dynamic Biosensors GmbH, 82152 Planegg, Germany

3

Kurt-Schwabe-Institut für Mess- und Sensortechnik e.V. Meinsberg, 04736 Waldheim,

Germany Keywords: DNA origami, electrical actuation of DNA, origami stability, switchSENSE, biosensors

ACS Paragon Plus Environment

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ABSTRACT Dynamic methods of biosensing based on electrical actuation of surface-tethered nanolevers require the use of levers whose movement in ionic liquids is well controllable and stable. In particular, mechanical integrity of the nanolevers in a wide range of ionic strength will enable to meet the chemical conditions of a large variety of applications where the specific binding of biomolecular analytes is analyzed. Herein we study the electrically induced switching behavior of different rod-like DNA origami nanolevers and compare that to the actuation of simply double-stranded DNA nanolevers. Our measurements reveal a significantly stronger response of the DNA origami to switching of electrode potential, leading to a smaller potential change necessary to actuate the origami, and subsequently, to a long-term stable movement. Dynamic measurements in buffer solutions with different Mg2+ content show that the levers do not disintegrate even at very low ion concentration and constant switching stress, and thus, provide stable actuation performance. The latter will pave the way for many new applications without largely restricting application-specific environments.


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INTRODUCTION DNA origami

1,2

nanostructures have become a versatile tool in nanoscale applications

3–8

.

Origami structures are being used as lattices for coordinated nanometer precise positioning of molecules 9 as well as for detection of molecules in super-resolution microscopy 10. In recent years the application of DNA origami in the field of biosensing has been expanding rapidly. Because they can be manufactured massively parallel with high precision, the use of DNA origami as electro-mechanical devices bears immense potential for future applications, like electrically controlled nano-machines. A first step towards using DNA origami as electromechanical devices is the ability to control their orientation by means of electric fields, which has been demonstrated recently 11,12. The orientation of DNA origami structures can be manipulated on the milli-second timescale by electric fields. The use of electric fields is, however, limited by the field strength acting on the origami’s charged backbone. This field strength can be drastically increased by the absence of ions, which screen the electric field, in solution.13 Therefore, low ion concentrations favor the successful manipulation of origami structures by electric fields due to the reduced screening effect. Moreover, the use of DNA origami for biophysical or biomedical applications may require low salt environments for the interaction of interest to take place. For many years the stability of origami in solutions containing low Mg2+ ion concentrations was considered to be poor14–16, however, recent research suggests that origami can be stable at Mg2+ concentrations below millimolar.17–21 In the following, we present investigations on the electrical actuation of a 100 nm long sixhelix bundle and a 50 nm long four-helix bundle origami structure at different Mg2+ ion concentrations. We assessed the salt-dependent structural integrity of the structures and examined their stability over extended time periods under permanent electro-mechanical stress.


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MATERIALS AND METHODS

Figure 1. (A) Schematic of the investigated origami and oligodeoxynucleotide (ODN) rods attached to a gold electrode. From left to right: 100 nm long six-helix bundle (100nm-6HB) attached with a 16 nm long 48 bp DNA anchor; 50 nm long four-helix bundle (50nm-4HB) attached with 16 nm long 48 bp DNA anchor; 32 nm long 96 bp DNA (32nm-1H) and 16 nm long 48 bp DNA (16nm-1H). (B) and (C) Electrically induced orientation switching of origami rods: When positive potentials are applied to the electrode, the rods are attracted to the electrode and the fluorescence emission from dyes attached to their top ends is quenched by the metal surface (B). When negative potentials are applied, the rods are repelled from the surface and the fluorescence emission is high (C).

Figure 1 schematically depicts the two origami rods and two oligodeoxynucleotides (ODNs) which were investigated in this work: The oligodeoxynucleotides were mixed sequences of 48 bp and 96 bp (MPC-48 and MPC-96 chips from Dynamic Biosensors). The 100 nm long sixhelix bundle origami rod and the 50 nm long four-helix bundle rod were folded from truncated,


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single-stranded M13mp18 virus scaffold DNA as described previously. (cf. Supporting Information).22 The diameters of the six-helix and four-helix bundles are 7.8 and 6.3 nm, respectively. Both origami designs feature a single-stranded anchor strand of 48 bases protruding from one end to allow immobilization of the rods onto the surface of a gold electrode via DNA hybridization. The anchor sequence is complementary to single-stranded capture DNAs, which were preimmobilized via sulfur-gold chemistry on the electrodes of a switchSENSE® biochip (MPC-48 chip, Dynamic Biosensors). The origami rods were hybridized to the single-stranded capture DNA onto the surface through potential-enhanced immobilization as described previously.12 Therefore, no purification step was required after origami folding but the origami levers were immobilized from a crude folding solution.12 Approximately 105 – 106 capture DNAs were present on an electrode area of 0.01 mm2. For fluorescence detection, the ODNs and the origami rods were labeled with green fluorophores at their surface-distal ends. The origami rods were labeled with three fluorophores, positioned 5 nm below their surface-distal ends (see Figure 1 A). They slightly protrude from the origami and are separated from the staple strands by two adenine nucleotides. The ODNs were labeled with single fluorophores at their surface-distal ends. The labeling site at the surface opposite ends was chosen so that the orientation of the rods with respect to the electrode surface could be monitored by fluorescence energy transfer. The dye fluorescence is quenched in the proximity of the metal electrode surface: Low fluorescence intensities indicate a lying origami/ODN orientation, while high fluorescence intensities indicate standing orientations.23 All experiments were performed with a DRX² biosensor instrument (Dynamic Biosensors), which features a time-correlated single-photon counting unit for fluorescence detection, a flow system to exchange the solution in the microfluidic channels of the chips, and electronics to control the voltages which are applied to the gold electrodes on the chips with respect to indiumtin-oxide (ITO) quasi-reference electrodes.


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If not stated otherwise, all experiments were conducted in origami folding buffer containing 20 mM Tris (pH 8.0) and 12.5 mM MgCl2, at a flow rate of 20 µl/min. To investigate the influence of different magnesium concentrations on the stability and switching behavior of the origami structures, 20 mM Tris-buffers (pH 8.0) with different Mg2+ concentrations varying from 0 mM to 20 mM were prepared (see Supporting Information).

RESULTS Origami and ODN switching behavior At first, we characterized the response of the 100 nm and 50 nm long six-helix and four-helix bundle origami rods to applied electrical potentials in voltage-resolved and time-resolved measurements. The origami orientation switching as a function applied electrode potentials is depicted in Figure 2. At negative voltages, high fluorescence intensities are observed, which indicates upright origami and ODN orientations. The origami rods and ODNs, which are both intrinsically negatively charged at their phosphate backbones, are repelled from the negatively charged electrode and extend away from the surface. Upon sweeping the electrode potentials to positive voltages, the fluorescence intensities decrease, which indicates a lying orientation of


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the origami rods and ODNs, because the fluorescence emission is quenched when the dyes are in close proximity to the metal surface.

Figure 2. (A) Orientations of the origami and ODN rods as a function of applied electrode voltage. High fluorescence intensities indicate standing orientations, low fluorescence intensities indicate lying orientations. Potentials are swept in 25 mV steps every three seconds. All curves represent averages over n = 4 individual measurements. For comparison, fluorescence values are normalized and voltages are off-set so that the curves’ inflection points align at 0 V. (B) Maximum slopes of the voltage-resolved fluorescence curves depicted in (A).

Notably, the standing-to-lying transition in Fig. 2A gets steeper the longer and bigger the DNA structures become. The steepest transition is observed for the longest rod, the 100nm6HB origami with 1,700 bp, the flattest transition is observed for the shortest 16 nm ODN with 48 bp. In other words, the origami structures react more abruptly than short ODNs when the electrode voltage is being reversed. To illustrate this further, the maximal slopes of the transitions are plotted in Fig. 2B. Therefore, the origami can be displaced by lower electric potentials compared to the ODNs. This behavior can be understood considering that the


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orientation of surface-tethered rod-like charged molecules in an electric field depends on their number of charges per unit length along the helix axis (line-charge density) and their total charge.24 For comparison, the line-charge density of a single DNA helix is -2e per bp (0.34 nm)25 while the line-charge density of the 50nm-4HB origami is -8e per 0.34 nm and -12e per 0.34 nm for the 100nm-6HB origami. The total charges are -96e, -192e, -988e, and -3400e for the 16nm-1H, 32nm-1H, 50nm-4HB, and 100nm-6HB molecules, respectively. The results of Figure 2 A also show that – compared to ODNs – one can use a narrower potential window to electrically manipulate the origami orientation, which is an advantage from an electrochemical point of view. When the voltages that are required to actuate the origami structures on the electrode can be kept low, the structures will remain more stable on the long term. For the 100nm-6HB and 50nm-4HB rods, an orientation switching can be induced by voltage amplitudes as low as ±100 mV and ±150 mV, respectively. The time-resolved switching response of the origami rods upon application of high-frequency square-wave voltages was analyzed using the time-correlated single photon counting capabilities of the DRX² biosensor instrument.


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Figure 3. Time-resolved origami orientation switching. Black: 16 nm long 48bp DNA helix; red: 32 nm long 96bp DNA helix; green: 50 nm long 4HB origami; blue: 100 nm long 6HB origami. (A) Upward orientation switching upon a voltage step from -0.4 to 0.4 V (vs. ITO) at t = 0. Switching frequencies were f = 5 kHz for 48bp and 96bp, f = 1 kHz for 4HB, and f = 200 Hz for 6HB origami . All curves are averages of n = 4 measurements. The inset shows a magnified view of the upward motion on a time scale of 25 µs. (B) Rise time (10 - 90%) for origami and ODN rods, analyzed from the time-resolved curves shown in (A).

Figure 3 A shows normalized fluorescence traces of the upward motions upon switching the electrode from attractive to repulsive potentials for the two origami and two ODN rods. Before t = 0 all molecules are in their lying orientations and the fluorescence is low (quenched by the metal surface). At t = 0 the surface potential is switched from attractive (+0.5 V) to repulsive (0.3 V) voltages and the upward motion of the molecules is measured as a gradual increase in the fluorescence intensity. One can clearly observe that the longer and larger the structures are, the longer it takes them to reach their upright orientations. The fact that the rod size significantly influences its rotational speed is further illustrated by a quantitative analysis of the 10-to-90% signal rise times (see Figure 3 B). While the short and thin ODNs feature rise times under 10 µs, the 50nm-4HB takes 40 µs and the 100nm-6HB close


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to 100 µs to reach the upright orientations. Overall, we observe a clear decrease in the mobility of the structures with increasing length and diameter. This is expected because the molecules’ motions are overdamped and limited by their diffusion coefficients. The rotational diffusion coefficient of rod-like objects such as origami and ODNs depends on the length and diameterto-length ratio of the cylindrical structures.26,27 Additionally, the line-charge density and total number of charges influence the magnitude of the electrostatic force that pushes the molecules upward. As a first approximation, we may assume that the rise time trise scales inversely with a molecules’ rotational diffusion coefficient Dr and the electric force acting on it, which, in turn, depends on its total charge q. Hence, we can estimate the ratio of rise times of two origami rods as 𝐷𝑟,1 ⋅𝑞1 𝐷𝑟,2 ⋅𝑞2

≈

𝑡𝑟𝑖𝑠𝑒,2 𝑡𝑟𝑖𝑠𝑒,1

(1)

When we calculate the diffusion coefficient for the origami rods by approximating them as cylinders27, and insert their total charge (cf. Figure 1), we find a rise time ratio for the two origami structures of trise, 50nm-4HB / trise, 100nm-6HB = 0.40, which fits the experimentally determined ratio of 0.41 (cf. Figure 3 B). While Equation 1 can be considered only a rough estimation, it predicts the rise time relation between the two origami rods remarkably accurately.


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Figure 4. Time-resolved origami switching in different Mg2+ concentrations. (A) 50nm-4HB switched at f = 1 kHz; (B) 100nm-6HB switched at f = 100 Hz; (C) 16nm-1H and (D) 32nm1H switched at f = 5 kHz. Electrode potentials were ±0.4 V. The Mg2+ ion concentration was varied from 0 to 20 mM with constant 20 mM Tris buffer. (E) 10-90% rise time over Mg2+ concentration for 16nm-1H (black), 32nm-1H (red), 50nm-4HB (green), and 100nm-6HB (blue).

To investigate the influence of Mg2+ on the switching dynamics of the origami and ODN rods we analyzed their motions in different Mg2+ concentrations using time-resolved measurements, cf. Figure 4. To this end, the rods were subsequently exposed to test buffers containing Mg2+ concentrations from 0 to 20 mM. We observe opposite Mg2+-induced effects for the origami and the ODN rods, respectively. With increasing Mg+2 concentrations the upward motion of the ODNs becomes faster, while


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the origami rods move slower. This opposing behavior is highlighted in Figure 4 E, which shows the rise times of origami and ODN rods as a function of the Mg2+ concentration. These opposing trends can be explained by the different time scales of the origami and ODN rods’ motions and how they are influenced by the electrode charging time: A polarized electrolyteelectrode interface constitutes an electrical RC element, where the ionic double-layer (Gouy Chapman Stern layer) forming at the solution side of the interface represents a capacitance C, which is charged through the solution resistance R. The charging time of the interface depends on the ionic strength of the solution, and thus, decreases with increasing ionic strength. For the electrodes used here, the charging time is approximately 2 µs (depending on the Mg2+ concentration). The motion of the ODNs takes a little longer but occurs on a similar time scale of 3 to 8 µs. This means that the ODN motion is not completely decoupled from the doublelayer charging time, but that the ODNs move while the double-layer is forming. Thus, the ODN motion becomes faster when the double-layer is charged more quickly at high ionic strengths.28,29 On the contrary, the origami rods move slowly compared to the electrode charging time and their dynamics are decoupled from the dynamics of the double-layer formation. However, the strength of the electrical interaction that the double-layer exerts on the origami is crucial. As the double-layer extension shrinks with increasing ionic strength, the electrical force acting on the origami weakens. Consequently, the origami rods move slowly at high Mg2+ concentrations.


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Origami stability

Figure 5. Origami stability at different Mg2+ concentrations. The fluorescence amplitude represents the difference between the fluorescence signals in standing and lying orientations when switching the origami orientations between -0.4 and 0.4 V at a frequency of f = 100 Hz. Fluorescence switching amplitude of (A) 50nm-4HB and (B) 100nm-6HB during and after injection of different Mg2+ concentrations. At first, the baseline was measured in folding buffer containing 12.5 mM Mg2+ for 30 s, followed by 10 minutes in test buffer with 0, 1, 5 or 20 mM Mg2+, followed by 10 minutes in folding buffer again. (C) and (D) Two-hour stability measurement upon exposure of 50nm-4HB and 100nm-6HB to 20 mM Tris buffer with added 1, 5 or 20 mM Mg2+. Scale bars represent a loss in amplitude of 20%, traces are offset in ydirection for better comparison.


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The possibility to increase the motion speed of the origami by reducing the Mg2+ concentration in solution raises the question about the origami rods’ stability at low concentrations of Mg2+. Therefore, we investigated the short- and long-term stability of the origami under conditions relevant for most biophysical experiments. As a means to evaluate the integrity of the origami structures, we analyzed the fluorescence modulation amplitude (ΔF = difference in fluorescence intensities measured for lying and standing orientations) under continuous electrical actuation. The ΔF signal originating from attached and functional (i.e. switchable) origami structures, does not depend on background fluorescence, and thus, is well suited to assess the number of intact origami rods on the surface. First we tested the short-term stability of the origami rods upon exposure to solutions with Mg2+ concentrations that were lower and higher than that of the folding/storage buffer, see Figure 5. The origami orientations were electrically switched constantly and a baseline was recorded for 30 s. Then, the folding/storage buffer (12.5 mM Mg2+) was exchanged with test buffers and the origami were exposed to 0, 1, 5 or 20 mM of Mg2+ in 20 mM Tris buffer for 10 minutes. Finally, the solution was changed back to folding/storage buffer, and the switching amplitude was monitored for another 10 minutes to detect the loss of fluorescence during the test-buffer step. A continuous flow of 20 µl/min was maintained at all times, so as to exchange the cell volume of the microfluidic channel covering the sensor electrodes every 1 second throughout the measurement. Both origami rods proved to be stable in all test buffers over the tested 10 min period. Most remarkably, even when no Mg2+ was present at all, the origami rods did not show signs of degradation. A slight, quasi-linear decrease of approximately -15% over 20 min can be observed for all traces, which does not depend on the used Mg2+ concentration and most likely is caused by photobleaching of the dyes under the used illumination conditions.


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A splitting of the curves can be observed for different Mg2+ concentrations. In solutions of lower ionic strength than the folding/storage buffer, the observable switching (fluorescence modulation) amplitude increases, whereas in solutions of higher ionic strength the switching amplitudes are lower (see Figure 5 A). As before, this can be explained by the strength of the electrical interactions: At low ionic strength, the electric field emanating from the surface reaches far into solution and the origami structures can be manipulated efficiently. At high ionic strengths, screening effects weaken the electrical forces acting on the origami, and thus, the switching amplitudes are smaller. It is interesting that the effect of the Mg2+ ion concentration (the observed splitting) is significantly more pronounced for the smaller of the two origami rods (cf. Fig. 5A and B). We assume that due to its higher charge the larger origami features almost maximal switching amplitudes even at 12.5 mM Mg2+, and hence, the reduction of ionic strength does not lead to a substantial increase in amplitude in Fig. 5B anymore. Further, we examined the long-term stability of the origami rods in different Mg2+ conditions. Low single-digit Mg2+ concentrations are of particular importance for applications because these conditions resemble physiological conditions in cells. Switching the origami rods for two hours in test buffers with different Mg2+ concentrations of 1 mM, 2 mM, and 10 mM showed that the 50nm-4HB and 100nm-6HB structures were long-term stable under low Mg+2 conditions. Figure 5 C and D depict the fluorescence amplitude traces for the two origami rods. Again, we observe a small constant loss of fluorescence amplitude over the course of the experiment (-30% for 50nm-4HB, -40% for 100nm-6HB). However, the fluorescence amplitude loss does not depend on the Mg2+ concentration as the traces of all measurements are parallel. Thus, we attribute the decrease in fluorescence to photobleaching and believe that the origami structures stayed intact. We presumed that the origami stability is facilitated by Mg2+ ions which are retained within the origami structure after Mg2+ ions had been removed from the surrounding test buffer. To ACS Paragon Plus Environment

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test this assumption, we conducted the previously described 10-minute exposure experiment with test buffers containing 0.5, 1.0 and 1.5 mM of EDTA (see Supporting Information, Figure S1). Being a chelating agent for divalent ions, EDTA is expected to actively remove Mg2+ from the origami structures. Even at a low EDTA concentration of 0.5 mM, we observed an additional loss of -10% fluorescence amplitude compared to a blank run in folding buffer. Increasing the EDTA concentration to 1.0 mM and 1.5 mM produced signal losses of -60 to 90% within 10 min and resulted in a substantial destruction of the origami structures. This significantly decreased stability of the origami can be attributed to EDTA’s ability to chelate Mg2+ ions which are bound to the DNA backbones inside the origami structure. We conclude that the 50nm-4HB and 100nm-6HB origami structures are stable in the absence of Mg2+ ions for at least 2 hours, if Mg2+ ions bound to the internal DNA backbones are not removed actively by scavengers such as EDTA.

DISCUSSION The ability to electrically manipulate the orientation and motion of differently sized origami rods under various Mg2+ conditions extends the potential applications for DNA origami. The behavior of the origami with regard to their increased sensitivity to an external electrical potential agrees with previous results.12 The immediate responsiveness of the origami to electric potentials allows the use of lower potentials than required for ODNs to induce orientation changes. The use of lower potentials can significantly increase electrode lifetime which makes DNA origami a valuable tool for electro-switchable biosurfaces.30 The independence of the origami rods’ motion from the electrode charging time simplifies the description of their motion, and thus, also simplifies the description and analysis of dynamic


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bioanalytical binding experiments. To describe the motion of surface-tethered DNA layers upon electrode potential change, it is necessary to describe the electrode potential as a function of time.24 For the origami, the reversal of the electrode potential and build-up of the ionic doublelayer can be assumed to be instantaneous, and origami rods show no significant motion until the double-layer is fully charged. Using DNA origami with low concentrations of Mg2+ ions in solution qualifies them for a broad range of biomedical and biophysical applications; for instance, 2 mM Mg2+ would emulate physiological conditions and the absence of Mg2+ could be desired to modulate the activity of Mg2+-dependent enzymes. The observation that six-helix bundle origami maintain their structure in Mg2+ down to 10 µM was recently published by Keller et al..20 Our results extend their findings. Tubular four-helix and six-helix bundle origami rods are not only stable at very low Mg ion concentrations, which is an important prerequisite for persistent electrical actuation, but they maintain their structure in low Mg2+ conditions even under the stress of an electrically induced oscillation between two orientational states in a constant buffer flow. In other words, both structure and electrical switching behavior are stable. The major part of the fluorescence amplitude losses we observed during our stability measurements are most likely due to bleaching of the origami dyes. Using EDTA to intentionally destabilize the origami by scavenging Mg2+ ions from their volumes, we find that the six-helix bundle was significantly more stable than the smaller four-helix bundle (60% loss vs 90% loss). During two-hour stability measurements, the six-helix bundles showed a significantly higher loss of fluorescence over the experiments’ duration. These two observations seem contradictory at first but can be explained: Due to the increased distance from the surface of the dyes attached to the 100 nm long six-helix bundle compared to the ones attached to the 50 nm long four-helix bundle, their energy transfer to the surface is relatively


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low. Therefore, the lifetime of their excited state is higher than the lifetime of the shorter origami’s dyes, which increases their chance of bleaching. The ability to control the orientation of a rod-like origami in the presence or absence of Mg2+ ions opens the door for many new applications without restricting the required operation conditions.

CONCLUSIONS In conclusion, the comparative study of the electrically stimulated actuation of two surfacetethered rod-like DNA origami structures of different lengths and diameters to the movement of two double-stranded DNA rods with different numbers of base pairs revealed a long-term stable, periodic up-and-down motion of the mechanically more stable origami-based nanolevers. Both origami levers respond stronger to changes of the electrode potential, which can be attributed to their higher line-charge density compared to that of single double-stranded DNA helices. Because the surface-potential change needed for a complete standing-up-lying-down orientational change is primarily determined by the overall charge and entropic properties of the levers, it takes smaller potential changes to efficiently manipulate the stiff, rod-like origami levers. This is a clear advantage over double-stranded DNA levers, since lower electrical potentials significantly increase the electrode lifetime, making origami levers a valuable tool for electro-switchable biosurfaces. Although in general the movement is slowed down with increasing lever size, the origami structures do show advantages in their dynamic behavior. Because the origami lever motion is much slower than the electrode charging time, the origami dynamics are decoupled from


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capacitive charging effects. This considerably simplifies the interpretation of alterations of the origami switching dynamics in the context of biosensing applications. In view of future practical applications of origami, the most important finding of this study is that is possible to electrically actuate origami structures over hours under various Mg2+ concentrations. In particular, we have shown that six-helix and four-helix bundle origami structures maintain their structural integrity in solutions containing low single-digit mM Mg2+ concentrations, and are stable for at least several minutes even when no Mg2+ is present. The experiments with EDTA demonstrate that the origami retain Mg2+ ions within their volume even when the outside buffer does not contain Mg2+, and that only when the Mg2+ is actively being scavenged by EDTA, they can be disintegrated. In summary, the investigated origami rods are robust and can be operated well under conditions of low Mg2+ concentrations. Using recently reported covalent linking of the origami staples would increase this stability even further.31 This will enable new biophysical and biomedical applications, which are not compatible with high Mg2+ concentrations.


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ASSOCIATED CONTENT Supporting Information. The supporting information contains information about origami synthesis including strand sequences, cleaning and folding protocols, sequences for ODNs used as rods, compositions for all used test buffers and their Mg2+concentrations and data for origami stability measurements with varying EDTA concentrations.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Funding Sources We gratefully acknowledge financial support by the German Ministry of Economy and Energy (BMWi) through the ZIM grants No. ZF4131001MD5 and ZF4088702MD5; the German research Foundation (DFG) through the Cluster of Excellence “Center for Advancing Electronics Dresden” (cfaed, EXC1056/1), and the ESF through the project MindNano (100226937).


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ACKNOWLEDGMENT We are thankful to Wolfgang Kaiser and Kristin Woerner for preparation of the switchSENSE biochips. The authors gratefully acknowledge financial support by the Bundesministerium für Wirtschaft und Energie (BMWi/ZIM): ZF4131001MD5 and ZF4088702MD5), the German Excellence Initiative via the DFG Cluster of Excellence EXC “Center for Advancing Electronics Dresden” (cfaed, EXC1056/1), and the ESF project MindNano (100226937).

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Magnesium-Dependent Electrical Actuation and Stability of DNA

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