Marc Tornow

Electrical manipulation of oligonucleotides grafted to charged surfaces

The electrical manipulation of short DNA molecules on surfaces offers novel functionalities with fascinating possibilities in the field of bio-interfaces. Here we present systematic investigations of the electrical interactions which govern the structure of oligonucleotides on charged gold surfaces. Successively, we address influences of the applied field strength, the role of DC electrode potentials, in particular for polycrystalline surfaces, as well as screening effects of the surrounding electrolyte solution. Data obtained for single and double stranded DNA exhibit differences which can be attributed to the dissimilar flexibility of the different molecular conformations. A comparison of the experimental results with a basic model shows how the alignment of the molecules adjusts according to a balance between electrically induced ordering and stochastic thermal motions. The presented conclusions are expected to be of general relevance for the behaviour of polyelectrolytes exposed to localized electric fields at interfaces.

Nanometre spaced electrodes on a cleaved AlGaAs surface

We present a novel technique for fabricating nanometre spaced metal electrodes on a smooth crystal cleavage plane with precisely predetermined spacing. Our method does not require any high-resolution nanolithography tools, all lateral patterning being based on conventional optical lithography. Using molecular beam epitaxy we embedded a thin gallium arsenide (GaAs) layer in between two aluminium gallium arsenide (AlGaAs) layers with monolayer precision. By cleaving the substrate an atomically flat surface is obtained exposing the AlGaAs–GaAs sandwich structure. After selectively etching the GaAs layer, the remaining AlGaAs layers are used as a support for deposited thin film metal electrodes. We characterized these coplanar electrodes by atomic force microscopy and scanning electron microscopy; this revealed clean, symmetric and macroscopically flat surfaces with a maximum corrugation of less than 1.2 nm. In the case of a device with a 20 nm thick GaAs layer the measured electrode distance was 22.5 nm with a maximum deviation of less than 2.1 nm. To demonstrate the electrical functionality of our device we positioned single colloidal gold nanoparticles between the electrodes by an alternating voltage trapping method; this resulted in a drop of electrical resistance from ~11 G Ω to ~1.5 k Ω at 4.2 K. The device structure has large potential for the manipulation of nanosized objects like molecules or more complex aggregates on flat surfaces and the investigation of their electrical properties in a freely suspended configuration.

Observation of electrostatically released DNA from gold electrodes with controlled threshold voltages

DNA oligo-nucleotides, localized at Au metal electrodes in aqueous solution, are found to be released when applying a negative bias voltage to the electrode. The release was confirmed by monitoring the intensity of the fluorescence of cyanine dyes (Cy3) linked to the 5 end of the DNA. The threshold voltage of the release changes depending on the kind of linker added to the DNA 3-terminal. The amount of released DNA depends on the duration of the voltage pulse. Using this technique, we can retain DNA at Au electrodes or Au needles, and release the desired amount of DNA at a precise location in a target. The results suggest that DNA injection into living cells is possible with this method.

Excessive Counterion Condensation on Immobilized ssDNA in Solutions of High Ionic Strength

We present experiments on the bias-induced release of immobilized, single-stranded (ss) 24-mer oligonucleotides from Au-surfaces into electrolyte solutions of varying ionic strength. Desorption is evidenced by fluorescence measurements of dye-labeled ssDNA. Electrostatic interactions between adsorbed ssDNA and the Au-surface are investigated with respect to 1), a variation of the bias potential applied to the Au-electrode; and 2), the screening effect of the electrolyte solution. For the latter, the concentration of monovalent salt in solution is varied from 3 to 1600 mM. We find that the strength of electric interaction is predominantly determined by the effective charge of the ssDNA itself and that the release of DNA mainly occurs before the electrochemical double layer has been established at the electrolyte/Au interface. In agreement with Mannings condensation theory, the measured desorption efficiency (etarel) stays constant over a wide range of salt concentrations; however, as the Debye length is reduced below a value comparable to the axial charge spacing of the DNA, etarel decreases substantially. We assign this effect to excessive counterion condensation on the DNA in solutions of high ionic strength. In addition, the relative translational diffusion coefficient of ssDNA in solution is evaluated for different salt concentrations.

A silicon-on-insulator vertical nanogap device for electrical transport measurements in aqueous electrolyte solution

A novel concept for metal electrodes with few 10 nm separation for electrical conductance measurements in an aqueous electrolyte environment is presented. Silicon-on-insulator (SOI) material with 10 nm buried silicon dioxide serves as a base substrate for the formation of SOI plateau structures which, after recess-etching the thin oxide layer, thermal oxidation and subsequent metal thin film evaporation, feature vertically oriented nanogap electrodes at their exposed sidewalls. During fabrication only standard silicon process technology without any high-resolution nanolithographic techniques is employed. The vertical concept allows an array-like parallel processing of many individual devices on the same substrate chip. As analysed by cross-sectional TEM analysis the devices exhibit a well-defined material layer architecture, determined by the chosen material thicknesses and process parameters. To investigate the device in aqueous solution, we passivated the sample surface by a polymer layer, leaving a micrometre-size fluid access window to the nanogap region only. First current–voltage characteristics of a 65 nm gap device measured in 60 mM buffer solution reveal excellent electrical isolation behaviour which suggests applications in the field of biomolecular electronics in a natural environment.

Organophosphonates as model system for studying electronic transport through monolayers on SiO2/Si surfaces

We report electrical transport measurements made on alkylphosphonate self-assembled monolayers grown on nanometer-thin SiO2 on top of highly p-doped silicon. At small bias direct tunneling is characterized by a decay constant of βu2009≈u20090.7/carbon. At larger positive bias to the silicon (1.1–1.5u2009V) the current-voltage traces feature a prominent shoulder, reminiscent of a negative differential resistance. We attribute this feature to a significant reduction in trap-assisted tunneling, as supported by a simulation. Hence, organophosphonate monolayers are excellent model systems to study electrical transport through ordered structures; they also provide highly efficient electrical passivation of the SiO2/Si surface.

Technology Assessment of a Novel High-Yield Lithographic Technique for Sub-15-nm Direct Nanotransfer Printing of Nanogap Electrodes

We have demonstrated direct nanoscale transfer printing (nTP) of PdAu lines from a hard mold onto a hard substrate at room temperature without employing any flexible buffer layers or organic adhesion promoters or release agent layers. PdAu was evaporated onto the mold surface, and a Ti layer was deposited on top of the PdAu layer. By pressing the mold against a Si/SiO2 substrate, the PdAu/Ti sandwich structure was directly transferred onto the SiO2 surface. The molds used in these experiments were GaAs/AlGaAs sandwich structures fabricated by molecular beam epitaxy that we cleaved and selectively etched afterwards in order to generate 3-D grating structures with nanometer resolution on their edges. We fabricated positive multiline molds with different aspect ratios, linewidths between 15 and 100 nm, and spacings between lines ranging from 5 to 70 nm. We also fabricated negative single-line molds with a positive supporting structure comprising a single 16-nm-wide groove feature. The experiments revealed that direct hard-on-hard transfer of nanoscale structures from a mold onto a substrate can be used to fabricate PdAu gaps with widths down to 9 nm. We also performed electronic measurements on transfer patterns and demonstrated that transferred structures can be used as electrodes, which are electrically isolated by these gaps. Since isolation characteristics of gaps improved with decreasing gap length, we partitioned longer gap segments into multiple shorter ones by focused ion beam lithography and conventional optical lithography in combination with wet chemical or plasma etching of the mold or the substrate, respectively. In this paper, we give a detailed description of all technological aspects of the developed direct nTP technique, including mold preparation, patterning efficiency, short reduction techniques, and yield.

High-yield metal transfer printing on alkyl bis-phosphonate monolayers

The successful transfer printing of thin metal films onto monolayers of aliphatic bisphosphonic acids (bisPAs) is reported. These monolayers were prepared from solution on plasma-grown aluminum oxide, and were compared with analogous monolayers of alkyl monophosphonic acids (monoPAs) for transfer printing efficacy. Water contact angle and AFM measurements indicated uniform films formed from both classes of phosphonic acids. Evaporated Au/Ti films were prepared using patterned polymeric stamps, and were transfer printed onto the various phosphonate monolayers; this process resulted in close to 100% yield for bisPA monolayers, but only poor results were measured for the monoPAs. We attribute efficient printing onto bisPA monolayers to the formation of strong chemical bonds between distal phosphonic acid groups and the stamp-adhered metal film via its native Ti-oxide termination.

Horizontal γ-PNA immobilization through organophosphonate chemistry for biosensing applications

Silicon-based field effect devices have been widely investigated in recent years for the label-free detection of DNA hybridization. The devices rely on detecting changes in the electrical surface potential that occur as a result of adsorbing charged DNA. To provide surface-immobilized affinity receptors for DNA hybridization, a suitable organic interface is obligatory that has a high density of receptor binding sites and a short distance between surface and probe DNA or its analogue, peptidic nucleic acid (PNA), to minimize electrolyte screening effects. In this work, we report on the bio-functionalization and characterization of silicon oxide-terminated surfaces with γ-PNA through organophosphonate interfacial chemistry. Functionalizing via attachment groups at the γ-points along the PNA backbone allows for multidentate binding of the PNA receptor in a lying configuration on the device surface, with potential application in label-free biosensing device optimization.

Synthesis and optimization of organic sensing platforms for label-free DNA detection

Label-free DNA detection by silicon-based field effect devices has been widely studied in recent years. DNA recognition is based on the detection of changes in the electrical surface potential and thus requires reliable and selective immobilization of charged biomolecules on the device surface. Self-assembled monolayers of phosphonic acids (SAMPs) can be used as an underlying platform to prepare well-defined organic interfaces with a high density of receptor binding sites close to the sensing surface. In this work, we report the functionalization and characterization of a silicon surfaces covered by a thin native oxide layer with different types of peptide nucleic acid (PNA), a synthetic analogue to DNA. To investigate the impact of receptor density and morphology on DNA hybridization, PNA molecules are covalently bound either in a multidentate or monodentate fashion to the underlying SAMPs. Multidentate binding of the receptor via attachment groups at the γ-points along the PNA backbone results in a rigid, lying configuration on the device surface, whereas a monodentate binding provides more flexible and more accessible receptor binding sites.