The long-term objectives of this research are to focus on the development of nanobiosensor devices based on bacteriorhodopsin (bR)suitable for applications in the areas of environmental monitoring and testing. Such biosensors may be interfaced directly to mobile, autonomous or semi autonomous robotic devices or be applied as part of integrated "bio-chips" for field analysis applications. The specific and immediate goals of this research are to investigate the application of bR as an optical transducer and also investigate specific applications such as biosensors for toxicity monitoring based on bio-luminescent systems coupled to bateriorhodopsin which is used as the biological signal transducer. A biosensor is defined as a compact analytical device incorporating a biological or biologically-derived sensing element either integrated within or intimately associated with a physico-chemical transducer. The usual aim of a biosensor is to produce either discrete or continuous electronic signals which are proportional to a single analyte or a related group of analytes [1]. The biological sensing element can be an enzyme, antibody,nucleic acid, antibody fragment, or a whole microbial cell, or plant or animal tissue.
Recent advances in materials engineering have enabled photovoltaic (PV) cells to be fabricated from solid state semiconductors,
photosensitive organic dyes, and photoactive proteins. One type of organic PV cell is based on the natural
light-harvesting protein bacteriorhodopsin (bR) found in the plasma membrane of a salt marsh archaebacteria. When
exposed to sunlight, each bR molecule acts as a simple proton pump which transports hydrogen ions from the
cytoplasmic to the extracellular side through a transmembrane ion channel. Two types of bR-PV cells comprised of
photosensitive dry and aqueous (wet) bR thin films are described in this paper. The self-assembled monolayer of
oriented purple membrane (PM) patches from the bR protein is created on a bio-functionalized gold (Au) surface using a
biotin molecular recognition technique. The dry bR monolayer is covered with an optically transparent Indium Tin
Oxide (ITO) electrode to complete the dry bR-PV device. In contrast, the aqueous bR-PV cell is created by
immobilizing the bR monolayer on an Au-coated porous substrate and then inserting the assembly between two micro-reservoirs
filled with KCl solutions. Platinum wire probes are then inserted in the opposing liquid reserviors near the
porous bR monolayer. The dry bR-PV cell generated a photo-electric response of 9.73 mV/cm2, while the aqueous bR-PV
produced 41.7 mV/cm2 and 33.3 μA/cm2. Although the generated voltages appear small, it may be sufficient to
power various microelectromechanical systems (MEMS) and microfluidic devices.
Glass is often used as a substrate material for developing microfluidic chips because it is hydrophilic (attracts and holds moisture), chemically inert, stable over time, optically clear, non-porous, and can be fabricated at low cost. However, the size and geometry of the various components, flow channels and fluid reservoirs are all fixed on the substrate material at the time of microfabrication. Recent advances on the development of a light driven microactuator for actively changing the size and geometry of micro features, based on a photo-responsive hydrogel, are described in this paper. Each discrete microactuator in the platform structure is a bi-layered hydrogel that exploits the ionic nature of the pH sensitive polymer blend of polyethylenimine (PEI) and poly(vinyl alcohol) (PVA), and the proton pumping ability of the retinal protein bacteriorhodopsin (bR). When irradiated by a light source with a peak response of 568 nm the bR molecules in the (bR-PVA) layer undergo a complex photocycle that causes protons to be pumped into the adjoining pH sensitive (PEI-PVA) layer. The diffusion of similarly charged ions through the second actuating layer generates electrostatic repulsive and attractive forces which alter the osmotic pressure within the cross-linked polymer network. Depending upon the type of electrostatic forces generated, the pH sensitive hydrogel layer will swell or, alternatively, collapse. The fabrication of the (bR-PVA)-(PEI-PVA) hydrogel microactuator is described and the experimental results from preliminary tests are presented. The application of the light sensitive hydrogels to developing a reconfigurable microchip platform is briefly discussed.
KEYWORDS: Sensors, Eye, Imaging systems, Imaging arrays, Electrodes, Signal detection, Signal to noise ratio, Spherical lenses, Retina, Chemical elements
The performance of wide field-of-view (FOV) and omni-directional sensors are often limited by the complex optics used to project three-dimensional world points onto the planar surface of a charged-couple device (CCD) or CMOS array. Recent advances in the design and development of a spherical imaging system that exploits the fast photoelectric signals generated by dried bacteriorhodopsin (bR) films are described in this paper. The bendable, lightweight and durable bR-based photocell array is manufactured on an indium-tin-oxide (ITO) coated plastic film using Electrophoretic Sedimentation technique (EPS). The effective sensing area of each pixel in the preliminary prototype is 2x2 mm2, separated by 1mm and arranged in a 4x4 array. When exposed to light, the differential response characteristic is attributed to charge displacement and recombination within the bR molecule, as well as loading effects of the attached amplifier. The peak spectral response occurs at 568nm and is linear over the tested light power range of 200μW to 12mW. Response remains linear at the other tested wavelengths, but at reduced signal amplitude. Excess material between the bR sensing elements can be cut from the plastic substrate to increase structure flexibility and permit the array of photodetectors to be wrapped around the exterior, or adhered to the interior, of a sphere.
A bendable photocell array that exploits bioelectronic photoreceptors based on bacteriorhodopsin (bR) is described in this paper. Fabricating such a sensor array on a flexible plastic substrate introduces a new design approach that enables lightweight and durable non-planar sensing devices to be created with curved or spherical geometries. In this research, purple membrane patches obtained from wild-type bR are deposited onto a polyethylene terephthalate (PET) substrate coated with a patterned ITO layer using Electrophoretic Sedimentation (EPS) technique. The current prototype consists of a flexible 4x4 pixel array and an amplification circuit that magnifies the small electrical signal arising from the charge displacement and recombination within the dried bR film. Each individual pixel is a 2mm x 2mm square separated by a 1mm distance between neighboring elements. The measured photoelectric response of an individual pixel is approximately linear over the light power range between 200μW and 12mW. These bR photocells respond primarily to visible light with a spectral peak response at 568nm. The response times of the photoelectric signals can reach up to the microsecond range. Preliminary tests have demonstrated that photoresponse characteristics are maintained while the flexible substrate is deformed up to a 10mm bending radius. Unfortunately, dried bR photocells are inherently susceptible to electrical noise because of their extremely high film resistance, necessitating the employment of a noise-filtering amplifier. The image processing capabilities of bR are demonstrated in a motion detection application. Specifically, Reichardt's delay-and-correlate algorithm is implemented and is used to detect both the speed and direction of a moving light spot.
One technological challenge in microfluidic system design has been controlling the directional flow of minute amounts of fluid through various narrow channels. Stimuli-responsive polymers can be used as micro control devices such as valves because these materials significantly change their volumetric properties in response to small environmental changes in pH, temperature, solvent composition, or electric field. In this paper, a bi-layered hydrogel structure is introduced as a light activated microactuator. The first layer of the device is a light sensitive polymer network containing poly(vinyl alcohol) (PVA) and the retinal protein bacteriorhodopsin (bR). The second layer is a blend of PVA hydrogel and a pH sensitive polymer polyethylenimine (PEI). When exposed to a light source with a peak response at 568 nm, the bR molecules in the first layer undergo a multistage photocycle that cause protons to be pumped into the surrounding medium. The diffusion of these similarly charged ions through the adjoining pH responsive hydrogel generates electrostatic repulsive and attractive forces which alter the osmotic pressure within the cross-linked polymer network. Depending upon the type of electrostatic forces generated, the pH sensitive hydrogel layer will swell or, alternatively, collapse. The multi-layered structure can be fabricated and inserted into the microchannel. The expanding volume of the actuating hydrogel is used to regulate flow or control leakage. Preliminary experiments on a 625mm3 optical actuating device are presented to identify key response characteristics and illustrate the mechanism for actuation
The time and frequency response behavior of a new class of photodetectors based on a light-sensitive protein, known as bacteriorhodopsin (bR), is described. Each bR-based detector consists of an indium tin oxide (ITO) electrode/bR thin film/indium tin oxide (ITO) electrode structure. The response of the photodetector to square-wave and transient pulse illumination are both simulated using an equivalent resistor-capacitor (RC) circuit and experimentally observed. The investigative study demonstrates that the physical dimensions of the sensor surface, load resistance and capacitance, and the illumination conditions all have an impact on the transient response and gain-bandwidth characteristics. It is observed that changing the sensing area of the detector only affects the amplitude of the response, but not the bandwidth. Increasing the load resistance produces a higher gain, but reduces bandwidth. Increasing the load capacitance has the effect of dramatically reducing both gain and bandwidth. The observations and conclusions derived from this research provide design guidelines for developing hybrid photoelectric sensors and imaging arrays using bacteriorhodopsin thin films.
Bacteriorhodopsin (bR) thin films have been investigated in recent years as a viable biomaterial for constructing micro- or nanoscale optical devices. During illumination, the bR molecules in the thin film undergo a photocycle that is followed by a proton transport from the cytoplasmatic side to the extracellular side of the cell membrane. The photoelectric response induced by the charge displacement can be influenced by both the wavelength and intensity of the impinging light sources. A photocell based on the photoelectric properties of a thin bR film is described in this paper. The bR-based photocell is built as a sandwich-structural device with an ITO (Indium Tin Oxide) electrode/bR film/ITO electrode configuration. The photocell is fabricated by depositing the oriented bR film onto the grounded ITO electrode. The cytoplasmic side of the bR membrane is attached to the ITO conductive surface and the extracelluar side is placed in contact with the second ITO electrode that provides the signal input to the instrumentation circuit. A polyester thin film was used as the spacer separating the two ITO electrodes. The size of the active area of the photocell is about 10×10 mm. A HeNe laser coupled with an acoustic-optical scanning system is used as the light source. Experimental results confirm that the photoelectric response generated by the bR-photocell prototype is durable, stable, and highly sensitive to changes in light intensity. The sensitivity of the proposed signal transducer is 10.25mV/mW. The wavelength dependence of the photoelectric responses is similar to the optical absorption spectrum of bR membrane.
Light activated optical circuits have several key advantages over conventional electronics because they are free from electrical current losses, resistive heat dissipation, and friction forces that greatly diminish system performance and efficiency. The effects of current leakage and power loss are also crucial design constaints in developing micro-electromechanical (MEMS) technology. An essential device for creating viable micro-optical circuitry is a robust photonic transistor that can act as a small signal switch and amplifier. The proposed photonic transistor is based on the complementary suppression-modulated light transmission properties of thin bacteriorhodopsin (bR) films. The light transmission properties exhibited by the thin film are controlled using the variable wavelength and intensity of the impinging light soruces. The light transmisison properties of the bR film are illustrated using a mathematical model for the two-state photoreaction system. The two-state model represents the longest lifetime in the bR photocycle, largest change in absorption maxima, and high photochemical stability. The optical response is proportional to changes in the light transmission properties of the biometrical, and therefore represents a viable material for creating optoelectronic devices.
An optical transducer based on the light modulated transmission properties of bacteriorhodopsin (bR) film is described in this paper. The bR protein molecules undergo a complex photocycle when absorbing light energy that is characterized by several measurable states. The most relevant states in the photocycle for this application are the initial B state (λmax= 570 nm) and the longest lived M intermediate state (λmax= 410 nm). If a yellow light source with a wavelength of approximately 570 nm and a second deep blue source at 410 nm illuminate the same region of the thin bR film, the two beams will mutually suppress the optical transmission properties of the thin film and reduce the intensity of the light output. The suppression-modulated transmission mechanism of the bR polymeric film is, therefore, controlled by the intensity and wavelength of the two light sources. Based on this simple mechanism, a number of different protein-based optical devices have been proposed in the literature for optical signal and information processing. The focus of this research is to exploit the light transmission properties of the bR film to develop efficient optical transducers that can be easily interfaced with micro-electro-mechanical systems for mechatronic applications. The proposed transducer design is activated by an external light source and free from electrical noise. Illustrations of how thin bR film can be used for the modulation of light intensity, optical switches, and logic gates are presented.
12 A biosensor telemetry system for the on-line remote monitoring of toxic sites is described in this paper. The device is a self-contained field measurement system that employs immobilized luminescent. Vibrio fisheri bacteria to detect airborne contaminants. The presence of toxic chemicals in the air will lead to a measurable decrease in the intensity of light produced by the bacteria population. Both cellular and environmental factors control the level of bioluminescence exhibited by the bacteria. The biological sensing element is placed inside a miniature airflow chamber that houses a light-to-frequency transducer, power supply, and Radio-Frequency (RF) transmitter to convert the intensity of bioluminescence exhibited by the bacteria population into a radio signal that is picked up by a RF receiver at a safe location. The miniature biosensor can be transported to the investigated on either a terrestrial or airborne robotic vehicle. Furthermore, numerous spatially distributed biosensors can be used to both map the extent and the rate-of-change in the dispersion of the hazardous contaminants over a large geographical area.
The rapid detection of toxic contaminants released into the air by chemical processing facilities is a high priority for many manufacturers. This paper describes a novel biosensor for the remote monitoring of toxic sites. The proposed biosensor is a measurement system that employs immobilized luminescent Vibrio fisheri bacteria to detect airborne contaminants. The presence of toxic chemicals will lead to a detectable decrease in the intensity of light produced by the bacteria. Both cellular and environmental factors control the bioluminescence of these bacteria. Important design factors are the appropriate cell growth media, environmental toxicity, oxygen and cell concentrations. The luminescent bacteria are immobilized on polyvinyl alcohol (PVA) gels and placed inside a specially constructed, miniature flow cell which houses a transducer, power source, and transmitter to convert the light signal information into radio frequencies that are picked up by a receiver at a remote location. The biosensor prototype is designed to function either as a single unit mounted on an exploratory robot or numerous units spatially distributed throughout a contaminated environment for remote sensing applications.
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