Tim Albrecht

DNA tunneling detector embedded in a nanopore.

We report on the fabrication and characterization of a DNA nanopore detector with integrated tunneling electrodes. Functional tunneling devices were identified by tunneling spectroscopy in different solvents and then used in proof-of-principle experiments demonstrating, for the first time, concurrent tunneling detection and ionic current detection of DNA molecules in a nanopore platform. This is an important step toward ultrafast DNA sequencing by tunneling.

Ultrafast Surface Enhanced Resonance Raman Scattering Detection in Droplet-Based Microfluidic Systems

The development of ultrafast Raman-based detection is one of the most interesting challenges underpinning the application of droplet-based microfluidics. Herein, we describe the use of surface-enhanced resonance Raman spectroscopy (SERRS) with submillisecond time resolution as a powerful detection tool in microdroplet reactors. Individual droplets containing silver nanoparticle aggregates functionalized with Raman reporters are interrogated and characterized by full spectra acquisitions with high spatial resolution in real time. Whereas previous works coupling SERRS with droplet-based microfluidics acquire a single spectrum over single or multiple droplets, we build upon these results by increasing our temporal resolution by 2 orders of magnitude. This allows us to interrogate multiple points within one individual droplet. The SERRS signals emitted from the aggregates are utilized to access the influence of flow rate on droplet size and throughput. Accordingly, our approach allows for high-throughput analysis that facilitates the study of other biological assays or molecular interactions.

In situ scanning tunnelling spectroscopy of inorganic transition metal complexes

Redox molecules with equilibrium potentials suitable for electrochemical control offer perspectives in nanoscale and single-molecule electronics. This applies to molecular but also towards higher sophistication such as transistor or diode function. Most recent nanoscale or single-molecule functional systems are, however, fraught with operational limitations such as cryogenic temperatures and ultra-high vacuum, or lack of electrochemical potential control. We report here cyclic voltammetry (CV) using single-crystal Au(111)- and Pt(111)-electrodes and electrochemical in situ scanning tunnelling microscopy (STM) of a class of Os(II)/(III)- and Co(II)/(III)-complexes, the former novel molecular electronics. The complexes are robust, with ligand groups suitable for linking the complexes to the Au(111)- and Pt(111)-surfaces via N- and S-donor atoms. The data reflect monolayer behaviour. Interfacial ET of the Os-complexes is fast, kET(0) > or = 10(6) s(-1), while the Co-complex reacts much more slowly, kET(0) approximately (1-3) x 10(3) s(-1). In STM of the Os-complexes shows a maximum in the tunnelling current/overpotential relation at constant bias voltage with up to 50-fold current rise. The peak position systematically the bias voltage and equilibrium potential, in keeping with theoretical frames for two-step electron transfer (ET) of in situ STM of redox molecules. The molecular conductivity behaves broadly similarly. The Co-complex also shows a tunnelling spectroscopic feature but much weaker than the Os-complexes. This can be ascribed much smaller interfacial ET rate constant, again caused by large intramolecular nuclear reorganization and weak electronic coupling to the substrate electrode. Overall the has mapped the properties of target molecules needed for stable electronic switching, possible importance in molecular electronics towards the single-molecule level, in room temperature condensed matter environment.

Rapid Ultrasensitive Single Particle Surface-Enhanced Raman Spectroscopy Using Metallic Nanopores

Nanopore sensors embedded within thin dielectric membranes have been gaining significant interest due to their single molecule sensitivity and compatibility of detecting a large range of analytes, from DNA and proteins, to small molecules and particles. Building on this concept we utilize a metallic Au solid-state membrane to translocate and rapidly detect single Au nanoparticles (NPs) functionalized with 589 dye molecules using surface-enhanced resonance Raman spectroscopy (SERRS). We show that, due to the plasmonic coupling between the Au metallic nanopore surface and the NP, signal intensities are enhanced when probing analyte molecules bound to the NP surface. Although not single molecule, this nanopore sensing scheme benefits from the ability of SERRS to provide rich vibrational information on the analyte, improving on current nanopore-based electrical and optical detection techniques. We show that the full vibrational spectrum of the analyte can be detected with ultrahigh spectral sensitivity and a rapid temporal resolution of 880 μs.

Single-molecule studies of intrinsically disordered proteins using solid-state nanopores.

Partially or fully disordered proteins are instrumental for signal-transduction pathways; however, many mechanistic aspects of these proteins are not well-understood. For example, the number and nature of intermediate states along the binding pathway is still a topic of intense debate. To shed light on the conformational heterogeneity of disordered protein domains and their complexes, we performed single-molecule experiments by translocating disordered proteins through a nanopore embedded within a thin dielectric membrane. This platform allows for single-molecule statistics to be generated without the need of fluorescent labels or other modification groups. These studies were performed on two different intrinsically disordered protein domains, a binding domain from activator of thyroid hormone and retinoid receptors (ACTR) and the nuclear coactivator binding domain of CREB-binding protein (NCBD), along with their bimolecular complex. Our results demonstrate that both ACTR and NCBD populate distinct conformations upon translocation through the nanopore. The folded complex of the two disordered domains, on the other hand, translocated as one conformation. Somewhat surprisingly, we found that NCBD undergoes a charge reversal under high salt concentrations. This was verified by both translocation statistics as well as by measuring the ζ-potential. Electrostatic interactions have been previously suggested to play a key role in the association of intrinsically disordered proteins, and the observed behavior adds further complexity to their binding reactions.

Electrochemical tunnelling sensors and their potential applications

The quantum-mechanical tunnelling effect allows charge transport across nanometre-scale gaps between conducting electrodes. Application of a voltage between these electrodes leads to a measurable tunnelling current, which is highly sensitive to the gap size, the voltage applied and the medium in the gap. Applied to liquid environments, this offers interesting prospects of using tunnelling currents as a sensitive tool to study fundamental interfacial processes, to probe chemical reactions at the single-molecule level and to analyse the composition of biopolymers such as DNA, RNA or proteins. This offers the possibility of a new class of sensor devices with unique capabilities.

Transistor effects and in situ STM of redox molecules at room temperature

Inorganic transition metal complexes were identified as potential candidates for transistor-like behavior in an electrochemical scanning tunnelling microscope (STM) configuration at room temperature. The theoretical background has been established based on condensed matter charge transfer theory. It predicts a distinct increase of the tunnelling current close to the equilibrium potential, i.e., if molecular bridge states are tuned into resonance with the Fermi levels of the enclosing electrodes. The complexes display robust electrochemistry on Au(111) electrode surfaces. STM images at molecular resolution reveal detailed information on their surface structure and scanning tunnelling spectroscopy experiments have shown clear evidence of transistor-like behavior.

Synchronized Optical and Electronic Detection of Biomolecules Using a Low Noise Nanopore Platform

In the past two decades there has been a tremendous amount of research into the use of nanopores as single molecule sensors, which has been inspired by the Coulter counter and molecular transport across biological pores. Recently, the desire to increase structural resolution and analytical throughput has led to the integration of additional detection methods such as fluorescence spectroscopy. For structural information to be probed electronically high bandwidth measurements are crucial due to the high translocation velocity of molecules. The most commonly used solid-state nanopore sensors consist of a silicon nitride membrane and bulk silicon substrate. Unfortunately, the photoinduced noise associated with illumination of these platforms limits their applicability to high-bandwidth, high-laser-power synchronized optical and electronic measurements. Here we present a unique low-noise nanopore platform, composed of a predominately Pyrex substrate and silicon nitride membrane, for synchronized optical and electronic detection of biomolecules. Proof of principle experiments are conducted showing that the Pyrex substrates have substantially lowers ionic current noise arising from both laser illumination and platform capacitance. Furthermore, using confocal microscopy and a partially metallic pore we demonstrate high signal-to-noise synchronized optical and electronic detection of dsDNA.

High Precision Fabrication and Positioning of Nanoelectrodes in a Nanopore

A simple and versatile method for the direct fabrication of tunneling electrodes with controllable gap distance by using electron-beam-induced deposition (EBID) is presented. We show that tunneling nanogaps smaller than the minimum feature size realizable by conventional EBID can be achieved with a standard scanning electron microscope. These gaps can easily be embedded in nanopores with high accuracy. The controllability of this fabrication method and the nanogap geometry was verified by SEM and TEM imaging. Furthermore, tunneling spectroscopy in a group of solvents with different barrier heights was used to determine the nanogap functionality. Ultimately, the presented fabrication method can be further applied for the fabrication of arrays of nanogap/nanopores or nanogap electrodes with tunable electrode materials. Additionally, this method can also offer direct fabrication of nanoscale electrode systems with tunable spacing for redox cycling and plasmonic applications, which represents an important step in the development of tunneling nanopore structures and in enhancing the capabilities of nanopore sensors.

Label-free Pb(II) whispering gallery mode sensing using self-assembled glutathione-modified gold nanoparticles on an optical microcavity.

An ultrasensitive assay for the detection of Pb(II) has been developed using whispering gallery mode (WGM) sensing. In this technique a photonic microcavity was decorated with glutathione (GSH)-modified gold nanoparticles (Au NPs). The resonator was functionalized using an aminosilane to promote adhesion of the GSH-modified NPs creating a highly sensitive sensor specific to Pb(II). Upon introduction of Pb(II) solutions via a fluidic cell, Pb(II) ions bind to the GSH-Au NP complex and induce a shift of the resonant wavelength. Using this detection strategy we show that we are able to detect Pb(II) concentrations down to 0.05 nM in the presence of alkaline and heavy metal interferences such as Mg(II), Mn(II), Ca(II), Ni(II), Cd(II), Cr(II), Fe(II), and Hg(II). The signal was found to be proportional to the Pb(II) concentration within the range of 2.40-48.26 nM and was found to have an association constant of 2.15 × 10(5) M(-1) s(-1). The sensitivity obtained shows unparalleled advantages over currently available technology and satisfies the exposure thresholds set out by world organizations such as International Agency for Research on Cancer (IARC) and the Environmental Protection Agency (EPA). We believe that this sensor has the potential to be made portable for applications in environmental monitoring and in-field applications.