http://bio.ph.tum.de/uploads/media/Colloids_Hydration_and_internal_Schoenhoff.pdf

Hydration and internal properties of polyelectrolyte multilayers

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time-resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material-dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.

Exciton Dynamics in Semiconductor Nanocrystals

Silicon nanowire films have been modified with boron and used as sensors to measure glucose in aqueous solution. These sensors have a wide linear range (0–10 mM glucose), high sensitivity (172 nA mmol–1), good reproducibility, and long-term stability. Silicon nanowire films have also been modified with magnesium and shown to perform as sensors for detecting hydrogen peroxide in aqueous solution.

Silicon Nanowire Sensors for Bioanalytical Applications: Glucose and Hydrogen Peroxide Detection†

The present invention provides biomimetic sensor devices that utilize proteins—such as G-protein coupled receptors—and are useful in high-sensitivity analysis of analyte-containing samples. These sensors may be used to determine the presence or concentration of one or more analytes in a sample. The invention also includes methods of fabricating the devices and methods of using the devices to assay samples.

Biomimetic Chemical Sensors Using Nanoelectronic Readout of Olfactory Receptors

Photonic crystal nanostructures for optical biosensing applications.

A Semiconductor sensor device for the detection of target molecules and molecular interactions, based on Silicon-on-Insulator (SOI) technology.

Silicon-on-insulator biosensor device

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.

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

Efficient detection of specific or nonspecific interactions at solid ± liquid interfaces is becoming increasingly important for a wide variety of applications and technologies, which range from industrial and biomedical applications to basic research. 2] The most prominent example of applications in basic research is the label-free detection of biomolecular interactions. There is growing need for new detection schemes, as the common fluorescent labeling techniques are limiting the applicability of DNA or protein arrays. Increasingly sophisticated techniques are being developed to provide versatile techniques for labelfree detection methods. 5] The main requirement for new technologies is the highly sensitive and specific detection of molecules. At the same time, for the applicability of the technique towards screening of biomolecular interactions, the potential of simultaneous parallel detection is mandatory. Several different approaches have been developed to address these requirements. For the electrical detection of molecular interactions, physical principles of field-effect transistors are used in most cases: Variations of the surface potential induce changes in the charge carrier concentration and thus in the conductivity of the semiconductor. The most extensively studied technique to date is the technology of ion-sensitive field-effect transistors (ISFETs), which have been shown to be versatile tools for detecting chemical and enzymatic reactions. The direct electrical detection of cell signaling was realized with recently developed semiconductor chips. 8] The capacitive detection of inversion layers in silicon structures has successfully been used to monitor the hybridization of DNA molecules. A similar approach with a biofunctionalized, porous silicon electrolyte ± insulator ± semiconductor (EIS) structure as a sensing device was used to detect biomolecules, such as penicillin, with a sensitivity down to 10 nM. Here, we present a device based on silicon-on-insulator (SOI) substrates that enables the detection of changes of electrolyte concentrations and of small numbers of charged biomolecules. In the SOI substrates the conducting layer is limited to a thin surface layer, covered by a thin native oxide layer. Hence, the conductivity of this thin conductive layer is strongly dependent on variations of the surface potential and the distribution of the surface states, which results in variations of the space charge region. We utilized a four-point resistance measurement in a hallbar geometry to determine the conductivity of the sensing layer. A variation of the salt concentration of the electrolyte could be detected over five magnitudes and an excellent agreement with the Grahame equation was found. The unspecific adsorption of poly-L-lysine was detectable at concentrations of 1 nM (80 ngmL ). The measurements showed that variations of surface charges of 0.01 e per nm, or one electronic charge per 100 nm, could be detected by the device. For the experiments, commercially available SOI wafers (ELTRAN, Canon) were used. The silicon layer in these wafers was 30 nm thick and was slightly doped with boron (10 cm ). To control the concentration of charge carriers in this layer, a voltage was applied to the backgate of the substrate. For some experiments, this upper silicon layer was overgrown by molecular beam epitaxy (MBE) with a 50 nm thick, single crystalline layer of doped silicon. The p-doping concentration of boron was 10 cm 3 or 10 cm . A sketch of the SOI device is given in Figure 1. In the subsequent steps, the silicon layer was patterned

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Silicon-on-Insulator Based Thin-Film Resistor for Chemical and Biological Sensor Applications

We present the fabrication and characterization of nanopore microcavities for the investigation of transport processes in suspended lipid membranes. The cavities are situated below the surface of silicon-on-insulator (SOI) substrates. Single cavities and large area arrays were prepared using high resolution electron-beam lithography in combination with reactive ion etching (RIE) and wet chemical sacrificial underetching. The locally separated compartments have a circular shape and allow the enclosure of picoliter volume aqueous solutions. They are sealed at their top by a 250 nm thin Si membrane featuring pores with diameters from 2 µm down to 220 nm. The Si surface exhibits excellent smoothness and homogeneity as verified by AFM analysis. As biophysical test system we deposited lipid membranes by vesicle fusion, and demonstrated their fluid-like properties by fluorescence recovery after photobleaching. As clearly indicated by AFM measurements in aqueous buffer solution, intact lipid membranes successfully spanned the pores. The nanopore cavity arrays have potential applications in diagnostics and pharmaceutical research on transmembrane proteins.

Silicon-on-insulator based nanopore cavity arrays for lipid membrane investigation.

A novel concept for the integration of liquid phase charge sensors into microfluidic devices based on silicon-on-insulator (SOI) technology is reported. Utilizing standard silicon processing we fabricated basic microfluidic cross geometries comprising of 5-10-mm-long and 55-/spl mu/m-wide channels of 3 /spl mu/m depth by wet sacrificial etching of the buried oxide of an SOI substrate. To demonstrate the feasibility of fluid manipulation along the channel we performed electroosmotic pumping of a dye-labeled buffer solution. At selected positions along the channel we patterned the 205-nm thin top silicon layer into freely suspended, 10-/spl mu/m wide bars bridging the channel. We demonstrate how these monolithically integrated bars work as thin-film resistors that sensitively probe changes of the surface potential via the field effect. In this way, a combination of electrokinetic manipulation and separation of charged analytes together with an on-chip electronic detection can provide a new basis for the label-free analysis of, for example, biomolecular species as envisaged in the concept of micrototal analysis systems (/spl mu/TAS) or Lab-on-Chip (LOC).

Karin Buchholz

Papers

Silicon-on-insulator biosensor device

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

Silicon-on-Insulator Based Thin-Film Resistor for Chemical and Biological Sensor Applications

Silicon-on-insulator based nanopore cavity arrays for lipid membrane investigation.

Silicon-on-insulator microfluidic device with monolithic sensor integration for /spl mu/TAS applications