Coupling of optical data-processing devices with microelectronics, telecocommunication and sensory functions, is
among the biggest challenges in molecular electronics. Intensive research is going on to find suitable nonlinear optical
materials that could meet the demanding requirements of optoelectronic applications, especially regarding high
sensitivity and stability. In addition to inorganic and organic crystals, biological molecules have also been considered
for use in integrated optics, among which the bacterial chromoprotein, bacteriorhodopsin (bR) generated the most
interest. bR undergoes enormous absorption and concomitant refractive index changes upon initiation of a cyclic
series of photoreactions by a burst of actinic light. This effect can be exploited to create highly versatile all-optical
logical elements. We demonstrate the potential of this approach by investigating the static and dynamic response of
several basic elements of integrated optical devices. Our results show that, due to its relatively high refractive index
changes, bR can be used as an active nonlinear optical material to produce a variety of integrated optical switching and
modulation effects.
Electro-osmosis is an efficient means to move fluid in microfluidic channels. The flow is driven by the interaction of the electrical double layer at the channel wall with an electric field along the channel. The flow can be controlled by modifying the electrical &muparameters, either the charge of the channel wall or the electric field. If the surface chagre or the surface rsistance of the channel wall is sensitive to light, the flow can be modulated by light. We have demonstrated this effect by using photoconductive surfaces. The resistance change due to the illumination changes the electric field above the photoconductive layer and consequently changing the rate of fluid flow. By using channels where upon a photoresistive CdS surface a linear PDMS channel was placed, flow rate changes of an order of magnitude were achieved. This gives serious possibilities for optical control of flow. We further developed the method by building channel structures of more complicated patterns, e.g. Y-junctions. By appropriate illumination of the arms the flow direction could be selected between the arms optically. This unit is the basis of more complex flow patterns, it demonstrates the feasibility of optical control of such devices.
Electro-osmotic pumping is an efficient way to move fluids in microfluidic systems. It is driven by the interaction of the Debye layer formed in the vicinity of the charged channel wall with a tangential electric field. The key parameters that determine the flow properties are the zeta potential of the surface and the electric field that drives the flow. Consequently, the flow can be controlled by appropriately modifying these parameters. Controlling the charge on the channel wall makes it possible to modify fluid flow. Likewise, the electric field close to the surface can be modified by changing the conductivity of the surface. The surface charge of appropriate materials can be changed by light illumination: the application of this phenomenon offers the possibility to optically control flow parameters. We have tested this possibility with several light sensitive surfaces. In the class of materials that change their charge upon illumination TiO2, a well known photoactive material was investigated. Experiments were also performed with the protein bacteriorhodopsin, known to change its surface charge following the release of protons into the solvent upon illumination. CdS was tested as the photoconductive material to modify the electric field by light. Linear microfluidic channels were prepared by soft lithography: a PDMS mold was placed upon a planar glass surface so that a rectangular cross section channel was formed upon the glass. The photosensitive materials covered the bottom glass surface.
The experiments show that the flow can be readily modulated by illumination. The results demonstrate that it is possible to dynamically control microfluidic flow, opening up the prospect to create optically controlled complex microfluidic networks.
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