F. Zörgiebel

Schottky barrier-based silicon nanowire pH sensor with live sensitivity control

We demonstrate a pH sensor based on ultrasensitive nanosize Schottky junctions formed within bottom-up grown dopant-free arrays of assembled silicon nanowires. A new measurement concept relying on a continuous gate sweep is presented, which allows the straightforward determination of the point of maximum sensitivity of the device and allows sensing experiments to be performed in the optimum regime. Integration of devices into a portable fluidic system and an electrode isolation strategy affords a stable environment and enables long time robust FET sensing measurements in a liquid environment to be carried out. Investigations of the physical and chemical sensitivity of our devices at different pH values and a comparison with theoretical limits are also discussed. We believe that such a combination of nanofabrication and engineering advances make this Schottky barrier-powered silicon nanowire lab-on-a-chip platform suitable for efficient biodetection and even for more complex biochemical analysis.

Patterned biochemical functionalization improves aptamer-based detection of unlabeled thrombin in a sandwich assay.

Here we propose a platform for the detection of unlabeled human α-thrombin down to the picomolar range in a fluorescence-based aptamer assay. In this concept, thrombin is captured between two different thrombin binding aptamers, TBA1 (15mer) and TBA2 (29mer), each labeled with a specific fluorescent dye. One aptamer is attached to the surface, the second one is in solution and recognizes surface-captured thrombin. To improve the limit of detection and the comparability of measurements, we employed and compared two approaches to pattern the chip substrate-microcontact printing of organosilanes onto bare glass slides, and controlled printing of the capture aptamer TBA1 in arrays onto functionalized glass substrates using a nanoplotter device. The parallel presence of functionalized and control areas acts as an internal reference. We demonstrate that both techniques enable the detection of thrombin concentrations in a wide range from 0.02 to 200 nM with a detection limit at 20 pM. Finally, the developed method could be transferred to any substrate to probe different targets that have two distinct possible receptors without the need for direct target labeling.

Compact Nanowire Sensors Probe Microdroplets

The conjunction of miniature nanosensors and droplet-based microfluidic systems conceptually opens a new route toward sensitive, optics-less analysis of biochemical processes with high throughput, where a single device can be employed for probing of thousands of independent reactors. Here we combine droplet microfluidics with the compact silicon nanowire based field effect transistor (SiNW FET) for in-flow electrical detection of aqueous droplets one by one. We chemically probe the content of numerous (∼10(4)) droplets as independent events and resolve the pH values and ionic strengths of the encapsulated solution, resulting in a change of the source-drain current ISD through the nanowires. Further, we discuss the specificities of emulsion sensing using ion sensitive FETs and study the effect of droplet sizes with respect to the sensor area, as well as its role on the ability to sense the interior of the aqueous reservoir. Finally, we demonstrate the capability of the novel droplets based nanowire platform for bioassay applications and carry out a glucose oxidase (GOx) enzymatic test for glucose detection, providing also the reference readout with an integrated parallel optical detector.

Ionic effects on the transport characteristics of nanowire-based FETs in a liquid environment

AbstractFor the development of ultra-sensitive electrical bio/chemical sensors based on nanowire field effect transistors (FETs), the influence of the ions in the solution on the electron transport has to be understood. For this purpose we establish a simulation platform for nanowire FETs in the liquid environment by implementing the modified Poisson-Boltzmann model into Landauer transport theory. We investigate the changes of the electric potential and the transport characteristics due to the ions. The reduction of sensitivity of the sensors due to the screening effect from the electrolyte could be successfully reproduced. We also fabricated silicon nanowire Schottky-barrier FETs and our model could capture the observed reduction of the current with increasing ionic concentration. This shows that our simulation platform can be used to interpret ongoing experiments, to design nanowire FETs, and it also gives insight into controversial issues such as whether ions in the buffer solution affect the transport characteristics or not.

Multiscale Modeling of nanowire-based Schottky-barrier field-effect transistors for sensor applications

We present a theoretical framework for the calculation of charge transport through nanowire-based Schottky-barrier field-effect transistors that is conceptually simple but still captures the relevant physical mechanisms of the transport process. Our approach combines two approaches on different length scales: (1) the finite element method is used to model realistic device geometries and to calculate the electrostatic potential across the Schottky barrier by solving the Poisson equation, and (2) the Landauer-Büttiker approach combined with the method of non-equilibrium Greens functions is employed to calculate the charge transport through the device. Our model correctly reproduces typical I-V characteristics of field-effect transistors, and the dependence of the saturated drain current on the gate field and the device geometry are in good agreement with experiments. Our approach is suitable for one-dimensional Schottky-barrier field-effect transistors of arbitrary device geometry and it is intended to be a simulation platform for the development of nanowire-based sensors.

Portable measurement system for silicon nanowire field-effect transistor-based biosensors

The aim of this study is the development of a portable measurement system that simplifies the analysis of bio-sensitive silicon nanowire field effect transistors (SiNW - FETs). In order to obtain the pH - dependent transfer characteristics of the Schottky barrier (SB) FETs, Id = f (Vg), a measurement unit and two biochip adapters are proposed. For the parallel measurement of multiple SiNW - FETs, an integrated CMOS analog multiplexer has been designed, allowing to connect of up to 32 transistors individually. In order to determine the influence of the measurement structure on signal integrity, overall size and system cost, an additional discrete analog multiplexer will be compared with the integrated solution.

Channel length dependent sensor response of Schottky-barrier FET pH sensors

We present a multi channel Schottky-barrier (SB) field effect transistor (FET) based platform for chemical sensor applications and investigate its sensitivity on channel length. Designed transistors consist of parallel assembled bottom up grown silicon nanowires with a mean diameter of 20 nm. Focusing on investigations of devices with different channel lengths, we demonstrate that different optimum sensing regimes exist and they are determined by the device geometry. These target at different realizations and operation schemes. The sensitivities of the SB-FETs in linear and subthreshold regime are extracted from analysis of the pH response of silicon nanowire sensor devices.

Package characterization of FET-based biochemical sensors

In this study we present packaging techniques for SD-card form factor test adapters. Our test adapters consist of FET-based biochemical sensors and a CMOS integrated analog multiplexer connected to a printed circuit board (PCB). Protection of the test adapter is required for long term stability and reliable measurements of the biochemical sensor in a liquid environment. Two different encapsulation techniques and biocompatible materials with different thermo-mechanical properties were used for this purpose. Adhesive strength measurements of these encapsulants on different substrates were carried out. Finally, the package was integrated into a portable measurement system for multiplexed measurements. Measured gate characteristics were compared between blank and packaged test adapters. FET characteristics were measured in dry as well as in liquid environment. Our packaging technique is mechanically stable, suitable for measurements in liquid environments influences neither the electrical characteristics of the FET devices as nor data acquisition.

Lab on a Wire: Application of Silicon Nanowires for Nanoscience and Biotechnology

Synergy between biochemistry, medicine, and material science during last decade has led to a tremendous scientific progress in the fields of biodetection and nanomedicine. This tight interaction led to the emergence of a new class of bioinspired systems. These systems are based upon utilizing nanomaterials such as nanoparticles, carbon nanotubes, or nanowires as transducers for producing novel sensor devices, or sophisticated drug delivery agents. This chapter focuses on the developments made in the area of silicon nanowire-based devices and their applications in the diverse areas of nano- and biotechnologies. Firstly, the incorporation of silicon nanowires into the electrical circuits is discussed, together with the sensing mechanism of the devices. In particular, the discussion is directed toward the most important aspects of the fabrication and functioning of the sensors, as well as the issues regarding the organic molecules interfacing with the silicon surface. Moreover, the complex interactions of organic species with nanoscale matter are addressed to as well as the need for sophisticated integration and packaging of the subsystems on a single chip. Finally, the perspectives of the potential applications of the silicon nanowires for biodetection and drug delivery are presented. Thus, the concept of “lab on a wire” is introduced as a set of approaches to engineer the nanowires and to enrich their functionality and potential applications in nanoscience and biotechnology.

Human α-thrombin detection platform using aptamers on a silicon nanowire field-effect transistor

We present a silicon nanowire-based field-effect transistor biosensor with Schottky barriers for highly specific and sensitive human α-thrombin detection. The active sensor area is decorated with thrombin-binding aptamers as receptor molecules. Each sensor chip is integrated into a microfluidic device for flow-through measurements. Instantaneous detection is provided by real-time monitoring of FET transfer curves. With this approach, thrombin concentrations between 200 pM and 200 nM are detected in a label-free, real-time manner, covering a wide dynamic range and enabling to distinguish between healthy and pathological levels. The development of simple, miniaturized devices for blood protein detection is highly interesting for medical diagnostics.