Ion-Step Method for Surface Potential Sensing of Silicon Nanowires.

This paper presents a novel stimulus-response method for surface potential sensing of silicon nanowire (Si-NW) field-effect transistors. When an "ion-step" from low to high ionic strength is given as a stimulus to the gate oxide surface, an increase of double layer capacitance is therefore expected. Thus, a change of conductance through the Si-NWs is measured. The surface potential on the Si-NW gate is changed from negative for a bare SiO2 surface to neutral/positive when there is poly-l-lysine adsorption at certain pH, which also indicates a shift of point-of-zero charge pH after surface modification. This change is measured by a drop of current variation at the ion-step. The ion-step is performed to the Si-NW through a polydimethylsiloxane microfluidic chip with automatic sample switching. A reduction of the ion-step response from 2 nA to almost zero at pH 5.0 is observed by increasing the potassium ion concentration from 10 mM to 50 mM, which corresponds to a surface potential change of ∼12 mV. We show that this method can be used as an alternative method for surface potential sensing, making it less sensitive to drift.

Spatial Site-Patterning of Wettability in a Microcapillary Tube.

Substrate functionalization is of great importance in successfully manipulating flows and liquid interfaces in microdevices. Herein, we propose an alternative approach for spatial patterning of wettability in a microcapillary tube. The method combines a photolithography process with self-assembled monolayer formation. The modified microcapillaries show very sharp boundaries between the alternating hydrophilic/hydrophobic segments with an achieved smallest domain dimension down to 60 µm inside a 580 µm inner diameter capillary. Our two-step method allows us to pattern multiple types of functional groups in an enclosed channel. Such structures are promising regarding the manipulation of segmented flows inside capillaries.

Design of turbulent tangential micro-mixers that mix liquids on the nanosecond time scale.

Unravelling (bio)chemical reaction mechanisms and macromolecular folding pathways on the (sub)microsecond time scale is limited by the time resolution of kinetic instruments for mixing reactants and observation of the progress of the reaction. To improve the mixing time resolution, turbulent four- and two-jet tangential micro-mixers were designed and characterized for their mixing and (unwanted) premixing performances employing acid-base reactions monitored by a pH-sensitive fluorescent dye. The mixing performances of the micro-mixers were determined after the mixing chamber in a free-flowing jet. The premixing behavior in the vortex chamber was assessed in an optically transparent glass-silicon replica of a previously well-characterized stainless-steel four-jet tangential micro-mixer. At the highest flow rates, complete mixing was achieved in 160ns with only approximately 9% premixing of the reactants. The mixing time of 160ns is at least 50 times shorter than estimated for other fast mixing devices. Key aspects to the design of ultrafast turbulent micro-mixers are discussed. The integration of these micro-mixers with an optical flow cell would enable the study of the very onset of chemical reactions in general and of enzyme catalytic reactions in particular.

Electrokinetic DNA transport in 20 nm-high nanoslits: evidence for movement through a wall-adsorbed.

The electrokinetic transport behavior of λ-DNA (48 kbp) in 20 nm-high fused-silica nanoslits in the presence of short-chain PVP is investigated. Mobility and video data show a number of phenomena that are typical of DNA transport through gels or polymer solutions, thus indicative of rigid migration obstacles in the DNA pathway. Calculations show that a several nanometer thin layer of wall-adsorbed PVP ('nano-gel') can provide such a rigid obstacle matrix to the DNA. Such ultrathin wall-adsorbed polymer layers represent a new type of matrix for electrokinetic DNA separation.

Field-effect based attomole titrations in nanoconfinement.

This paper describes a novel capacitive method to change the pH in micro- and nanofluidic channels. A device with two metal gate electrodes outside an insulating channel wall is used for this purpose. The device is operated at high ionic strength with thin double layers. We demonstrate that gate potentials applied between the electrodes cause a release or uptake of protons from the silicon nitride surface groups, resulting in a pH shift in the channel and a titration of solution compounds present. Due to the high quality silicon nitride insulating layer, the effect is purely capacitive and electrolysis can be neglected. Fluorescein was employed as a fluorescent pH indicator to quantify the induced pH changes, and a maximum change of 1.6 pH units was calculated. A linear relationship was found between applied potential and fluorescein intensity change, indicating a linear relation between actuated proton amount and applied voltage. Since this pH actuation method avoids redox reactions and can be operated at physiological ionic strength, it can be very useful as a "soft" way to change the pH in very small volumes e.g. in bioassays or cell-based research. The sensitivity of the optical detection method poses the only limit to the detectable amount of substance and the observed volume. In a preliminary measurement we show one possible application, namely titration of 100 attomol of TRIS in a 7 pL detection volume. It is important to stress that this pH actuation principle fundamentally differs from the pH changes occurring in ionic transistors which are due to counterion enrichment and coion exclusion, because it does not rely on double-layer overlap. As a result it can be operated at high ionic strength and in channels of up to at least 1 microm height.