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A chip that creates microscale vortices in water and mimics biochemistry

A prototype chemical device can perform micro-stirring, pH measurement and control, and even the sensing of single molecules.
1 May 2006, SPIE Newsroom. DOI: 10.1117/2.1200603.0120

An electrolyte transistor—essentially a chemical device that controls the flow of electricity—could help scientists understand and explore the chemical activity between small numbers of molecules. The device could be used in demanding areas such as the sensing of substances from the human body, applications where basic and applied scientists need smart, sensitive, and selective devices that operate in real time. In wet chemistry, for instance, our electrolyte transistor allows micro- and nanoscale observations of reactants forming products. In addition, the platform described here can control dynamic conditions, stirring aqueous reactants by creating microscale vortices for example.

Long ago, scientists proposed and demonstrated electrolyte transistors as electrochemical sensors. These devices also proved to be biocompatible, which means that they could be used in physiological environments. Nevertheless, no practical device was demonstrated. Our prototype combines an appealing design with advanced nanolithography: the technology used to make computer chips.

The prototype transistor was processed on a low-doped, n-type, oxidized-silicon substrate. Conventional electron-beam lithography was used.1 On the same substrate, we fabricated different cells with active linear dimensions ranging from few hundred micrometers down to few nanometers. A drop of clean-room, deionized water with a resistivity of 18.3MΩ was sprayed onto the device's active area. Figure 1 shows a photomicrograph of the device in action, creating vortices. Our team used different cells for the three experiments described here.

Figure 1. A photomicrograph of an electrolyte transistor reveals the formation of vortices (rings) at the anode. This cell is 100μm × 100μm with 3.2 volts (dc) of applied voltage. (Copyright 2005 American Institute of Physics)

The first deals with microscale vortices stirring water.2 These experiments took place in a micro-cell (100 μm × 100 μm, see Figure 1 and Figure 2). Applying an external electric field between the anode and cathode of such an electrochemical cell breaks up the water molecules, which forms both negative hydroxide ions (OH-) and protons (hydrogen ions, H+).2 The protons move in a spiral path in the water, and vortex rings form. Vortices with diameters ranging from 10 to 50μm have been observed, as shown in Figure 1. These vortex rings can noninvasively stir aqueous reactants. Moreover, adjusting the applied external field can create vortices at both the anode and cathode.2

Figure 2. This photomicrograph shows measurements being taken from a water transistor. The insert provides a magnified picture of the water drop and the active device area in the middle.

This device can also work on the nanoscale. For example, it can precisely control pH and convert this chemical parameter into electrical signal variation (i.e., a transistor with no amplification). Figure 3 displays the controlling and sensing components. Large or small active elements control the pH, and closely spaced nano-electrodes sense this chemical signal. We have demonstrated nano-gaps with a spacing of 20–200nm. An example of the observed current-voltage characteristics is shown in Figure 3(d).

Figure 3. The transistor can be designed to measure or modify pH in in various ways: (a) large pH electrodes make up the transistor base nano-electrode, (b) a configuration of small pH electrodes, with the emitter and collector nano-electrodes in the middle, and (c) pH nano-electrodes with electrical insulation. The red arrows show the position of the nano-active device area. (d) An example of the current-voltage characteristics (common-emitter convention) when employing electrode configurations shown in (a) and (b).

Last, we can also design this device to detect single molecules. In this case it would operate in a fluorescent aqueous environment.3

In conclusion, our prototype electrolyte transistor combines nanolithography with an appealing design and process. This device can operate in physiological environments, and it provides small-scale control of the conditions of a desired experiment, such as controlling pH, noninvasivly stirring reactants, and so on. The device demonstrated provides a step toward mimicking the conditions of chemical processes in living cells and other biocompatible environments.

Magnus Willander
Physical Electronics and Photonics, Gothenburg University
Gothenburg, Sweden
Department of Science and Technology, Linkoping University
Norrkoping, Sweden
Magnus Willander holds two chaired full professorships in the Physics Department at Göteborg University and the Department of Science and Technology at Linköping University. His research focuses on materials and devices, and he combines experimental and theoretical research in these areas. He has published around 800 papers and seven books.
Zakaria Chiragwandi and Omer Nur
Physics Deaprtment, Gothenburg Univeristy
Gothenburg, Sweden