Photoconductive Surface Research Articles

Optical Dielectrophoretic (DEP) Manipulation of Oil-Immersed Aqueous Droplets on a Plasmonic-Enhanced Photoconductive Surface.

We present a plasmonic-enhanced dielectrophoretic (DEP) phenomenon to improve optical DEP performance of a floating electrode optoelectronic tweezers (FEOET) device, where aqueous droplets can be effectively manipulated on a light-patterned photoconductive surface immersed in an oil medium. To offer device simplicity and cost-effectiveness, recent studies have utilized a polymer-based photoconductive material such as titanium oxide phthalocyanine (TiOPc). However, the TiOPc has much poorer photoconductivity than that of semiconductors like amorphous silicon (a-Si), significantly limiting optical DEP applications. The study herein focuses on the FEOET device for which optical DEP performance can be greatly enhanced by utilizing plasmonic nanoparticles as light scattering elements to improve light absorption of the low-quality TiOPc. Numerical simulation studies of both plasmonic light scattering and electric field enhancement were conducted to verify wide-angle scattering light rays and an approximately twofold increase in electric field gradient with the presence of nanoparticles. Similarly, a spectrophotometric study conducted on the absorption spectrum of the TiOPc has shown light absorption improvement (nearly twofold) of the TiOPc layer. Additionally, droplet dynamics study experimentally demonstrated a light-actuated droplet speed of 1.90 mm/s, a more than 11-fold improvement due to plasmonic light scattering. This plasmonic-enhanced FEOET technology can considerably improve optical DEP capability even with poor-quality photoconductive materials, thus providing low-cost, easy-fabrication solutions for various droplet-based microfluidic applications.

Paper-Based ZnO Oxygen Sensor

In this paper, we report the development of an oxygen sensor using a simple device made from a film of ZnO crystals dispersed over a paper surface. Due to the high porosity of this kind of device made from paper and ZnO, a large photoconductive effect is observed as result of the fast absorption and desorption of oxygen from the ZnO surface. To detect oxygen, we propose to use ultraviolet (UV) illumination over the photoconductive surface of the sensor, the applied UV light produces oxygen desorption causing current variations through the sensor that are proportional to the oxygen presence nearby the sensor surface. The sensor developed in this paper is attractive because it is remarkably easy to fabricate, employs only very low-cost materials for its construction, and shows a better sensitivity at low concentration of oxygen at partial vacuum environments.

Self-Rotation of Cells in an Irrotational AC E-Field in an Opto-Electrokinetics Chip

The use of optical dielectrophoresis (ODEP) to manipulate microparticles and biological cells has become increasingly popular due to its tremendous flexibility in providing reconfigurable electrode patterns and flow channels. ODEP enables the parallel and free manipulation of small particles on a photoconductive surface on which light is projected, thus eliminating the need for complex electrode design and fabrication processes. In this paper, we demonstrate that mouse cells comprising melan-a cells, RAW 267.4 macrophage cells, peripheral white blood cells and lymphocytes, can be manipulated in an opto-electrokinetics (OEK) device with appropriate DEP parameters. Our OEK device generates a non-rotating electric field and exerts a localized DEP force on optical electrodes. Hitherto, we are the first group to report that among all the cells investigated, melan-a cells, lymphocytes and white blood cells were found to undergo self-rotation in the device in the presence of a DEP force. The rotational speed of the cells depended on the voltage and frequency applied and the cells' distance from the optical center. We discuss a possible mechanism for explaining this new observation of induced self-rotation based on the physical properties of cells. We believe that this rotation phenomenon can be used to identify cell type and to elucidate the dielectric and physical properties of cells.

Single-sided continuous optoelectrowetting (SCOEW) for droplet manipulation with light patterns

Electrowetting-on-dielectric (EWOD) promises to be an important lab-on-a-chip approach for effectively manipulating droplets with electric field-controlled surface tension. Droplets manipulated in electrowetting-based devices are typically sandwiched between two parallel plates and actuated by digital electrodes. The size of pixilated electrodes limits the minimum droplet size that can be manipulated. Here, we report on a single-sided continuous optoelectrowetting (SCOEW) mechanism that enables light-patterned electrowetting modulation for continuous droplet manipulation on an open, featureless, and photoconductive surface. SCOEW overcomes the size limitation of physical pixilated electrodes by utilizing dynamic and reconfigurable optical patterns and enables the continuous transport, splitting, merging, and mixing of droplets with volumes ranging from 50 microL to 250 pL, over 5-orders of magnitude. This single-sided open configuration provides a flexible interface for integration with other microfluidic components, such as sample reservoirs through simple tubing. Light-triggered, parallel, and volume-tunable droplet injection with volume variation less than 1% has been demonstrated with SCOEW. The unique lateral field-driven optoelectrowetting mechanism also enables extremely low light intensity actuation, and droplet manipulation can be achieved by directly positioning the SCOEW chip on a LCD screen used in a laptop or portable cellular phone.

A light-induced dielectrophoretic droplet manipulation platform

We report on a light-actuated, droplet based microfluidic platform enabling two-dimensional (2D) droplet manipulation on an open chamber with a single-side, featureless photoconductive surface. The droplet actuation mechanism is based on recently demonstrated floating electrode optoelectronic tweezers (FEOET), which enable light-induced dielectrophoretic forces to manipulate aqueous droplets immersed in electrically nonconductive oil, with a light intensity as low as 400 microW/cm2. In this paper, we study the shape effect of optical patterns for 2D droplet actuation, and demonstrate light-actuated droplet manipulation functions including 2D droplet transport, merging, mixing, and multidroplet processing, for up to 16 droplets in parallel. Such an open chamber platform also permits easy interfacing and integration with other microfluidic structures, such as wells and close-channel based droplet devices to increase its versatility for biochemical analyses.

Light-Actuated AC Electroosmosis for Nanoparticle Manipulation

We present a novel light-actuated ac electroosmosis (LACE) mechanism that allows the concentration and transportation of micro- and nanoscopic particles using light-patterned dynamically reconfigured microfluidic vortices on a photoconductive surface. LACE is realized by sandwiching an aqueous liquid medium between a featureless photoconductive surface and a transparent indium tin oxide electrode. By applying an ac electrical bias with a frequency that is close to the electric double-layer relaxation frequency, a light-patterned virtual electrode can induce ac electroosmotic flow to concentrate and transport nanoscopic particles on the photoconductive surface. By integrating with a spatial light modulator such as a digital micromirror device microdisplay, we can create 31000 microfluidic vortices on a 1.3 times 1-mm2 area for massively parallel trapping of 2- and 1-mum polystyrene beads. We have also demonstrated LACE concentration and transportation of nanoscopic particles including 200- and 50-nm polystyrene beads, lambda-phage DNA molecules, and quantum dots.

Massively parallel manipulation of single cells and microparticles using optical images

The ability to manipulate biological cells and micrometre-scale particles plays an important role in many biological and colloidal science applications. However, conventional manipulation techniques--including optical tweezers, electrokinetic forces (electrophoresis, dielectrophoresis, travelling-wave dielectrophoresis), magnetic tweezers, acoustic traps and hydrodynamic flows--cannot achieve high resolution and high throughput at the same time. Optical tweezers offer high resolution for trapping single particles, but have a limited manipulation area owing to tight focusing requirements; on the other hand, electrokinetic forces and other mechanisms provide high throughput, but lack the flexibility or the spatial resolution necessary for controlling individual cells. Here we present an optical image-driven dielectrophoresis technique that permits high-resolution patterning of electric fields on a photoconductive surface for manipulating single particles. It requires 100,000 times less optical intensity than optical tweezers. Using an incoherent light source (a light-emitting diode or a halogen lamp) and a digital micromirror spatial light modulator, we have demonstrated parallel manipulation of 15,000 particle traps on a 1.3 x 1.0 mm2 area. With direct optical imaging control, multiple manipulation functions are combined to achieve complex, multi-step manipulation protocols.

The Effect of Relative Humidity on the Toner in the Developing Process

In the image developing process, the latent electrostatic image on the photoconductive surface is converted into visible by the deposition of the toner particles on the photoconductive surface. The toner is either dual components or mono-component developers.In the mono-component development system, the magnetic toner is held by the magnetic forces on the sleeve, the toner is moved around the sleeve spaced several hundred micrometer from the photoreceptor, under the electric field the magnetic toners jump over the gap onto the photoconductor.It is found that, the optical density and the background of the printed image is dependent on the relative humidity of the toner. Hence, we investigate the causes by the measurement of Q/M or Q/A of the toner under different humidity. As the humidity increases, the water film will induce the instability of the electric charge, so the Q/M will decreases, but Q/A. increases.

Process for producing laser-formed video calibration markers

A process for producing calibration markers directly on the photoconductive surface of video camera tubes has been developed. This process includes the use of a Nd:YAG laser operating at 1.06 microm with a 9.5-nsec pulse width (full width at half-maximum). The laser was constrained to operate in the TEM(00) spatial mode by intracavity aperturing. The use of this technology has produced an increase of up to 50 times the accuracy of geometric measurement. This is accomplished by a decrease in geometric distortion and an increase in geometric scaling. The process by which these laser-formed video calibrations are made will be discussed.

Procedure for the Characterization of Zinc Oxide Photocopy Papers

Abstract A method is presented for the characterization of zinc oxide photocopy papers. The tests include nondestructive physical examinations of weight, pH, and color under ultraviolet light. Chemical tests, consisting of thin-layer chromatographic examination of the sensitizing dyes and infrared spectrophotometry of the binder resins, are carried out on extractions of the photoconductive surface layer. A semiqualitative flow chart has been prepared that permits differentiation of most of the zinc oxide photocopy papers on the basis of the composite of characteristics of 50 papers supplied by eleven manufacturers.

Xerography of the breast.

Xerography of the breast has many advantages over conventional mammography done with Eastman Kodak “M” Film. There is greater detail on the xerograms, and they are easier to produce and interpret. Xerography utilizes an electrostatic charge placed on a photoconductive surface (a selenium-coated aluminum plate) which is then exposed to radiant energy in the form of x-rays. The residual electrostatic charge is developed by spraying the plate with negatively charged powder. The image is then transferred to paper by pressure for permanence. A better image is produced by xerography with less radiation than is needed with conventional film mammograpy. With further development of the equipment, results of this new technique can be even more encouraging.

A Lens Measuring Method Using Photoconductive Cells

Many characteristics of a lens can be evaluated using a method based on changes in the light distribution which take place in the image plane as the image of high-contrast target is moved through focus. A photoconductive surface in the image plane measures this information in the image, without having to resort to conventional photographic techniques or the human eye. Basically, the output of the instrument indicates the degree of sharpness of image. Thus, by locating the plane of best focus by instrument very exactly, the characteristics that can be measured are: the optical distances, and variations thereof at various wavelengths, in the visible and in part of the infrared region; and particular aberrations of a lens. Indirectly, the instrument indicates the resolving ability of the lens; thus qualitative comparisons between lenses can be made with this method.

Phenomena of Image Sharpness Recognition of CdS and CdSe Photoconductors

A theory is presented to account for the fact that a photoconductive cell can detect when an image projected on it is in sharpest focus. Each of the small particles in the photoconductive surface is treated as an individual photoconductor in series–parallel connection with all the other particles. When the distribution of light on the surface of such a photoconductive cell is varied (total incident flux being constant) the total conductance across the whole surface of the cell depends upon detail contrast, acutance, and other factors of the projected image. The observed dependence of conductance on sharpness of focus can be explained by this theory.