The identification, fabrication and evaluation of biomimetic templates hold great potential for a variety of applications. The generation of templates on these substrates is achieved using an appropriately modified atomic force microscope (AFM) tip. The fabrication process of tissue and cell based templates reported in this paper is accomplished using Dip-Pen Nanolithography. This report identifies critical parameters necessary for the patterning process: initial surface quality, temperature and humidity conditions. The results of this proof of concept experiment can be utilized in future in vitro and in vivo studies to test the templates bio-compatibility and specificity.
We present a new methodology to fabricate surface structures based on coated magnetic particles and charged polymers. We control the surface architecture via a nanolithographic approach, Dip-Pen Nanolithography, which utilizes Atomic Force Microscopy (AFM) to delivery molecular "inks" on soid substrates. This process results in surfaces with a pre-programmed architecture composed of well-characterized negatively and postively charged polymeric regions. Charged magnetic particles coated via the Layer-by-Layer method can be assembled onto the pre-programmed surface blue-print using electrostatic interactions. The possibility of incorporating our templates in simple device strategies is currently being explored.
The presence of surface states, which pins the Fermi level within the bandgap, contributes to the degradation in the performance of light-emitting diodes (LEDs) and p-i-n photodiodes due to an increase in the non-radiative recombination rate. Chemical modification on the facet or device surface can greatly enhance the output power of LED's and photocurrent of p-i-n photodiodes. Adsorbing molecules can change either the density or energy distribution of surface states. This effect leads to changes in surface recombination which result in systematic variations in light output, and thus the effect can be used for detection of analytes. This mechanism was used to realize a compact chemical sensor based on III/V LED/Detector structures. Initially, we fabricated InGaAlP/InGaP/InGaAlP double heterostructure (DH) LEDs (400 X 1000 micrometer<SUP>2</SUP>) with three different active region thicknesses: 50, 250 and 500 nm. In constant current mode, the DH LED exhibits electroluminescence (EL) at approximately 670 nm for an InGaP active region. The EL intensity changes of the LED in various gaseous ambients (NH<SUB>3</SUB>, NH<SUB>2</SUB>(CH<SUB>3</SUB>), NH(CH<SUB>3</SUB>)<SUB>2</SUB>, N(CH<SUB>3</SUB>)<SUB>3</SUB>, and SO<SUB>2</SUB>) are measured. The data show reproducible trends: DH LED structures with thicker active regions result in larger emission intensity changes due to analyte adsorption. Our findings are consistent with active-layer surface area dependence. Thicker active layer devices have larger carrier losses due to nonradiative surface recombination, and thus show a stronger sensitivity to the surface chemistry. Furthermore, we used this DH LED design to build a highly versatile compact sensor. The MOCVD-grown LED wafer is patterned and chemically etched to fabricate integrated GaAs/AlGaAs edge-emitting LEDs and p-i-n photodiode units. The light emitted from the edge-emitting LEDs is absorbed at the sidewall of the adjacent photodiode, and the resulting photocurrent is measured. The device design concept is based on increasing the ratio of analyte-accessible facet area to the volume of the active region. This integrated LED- photodiode device can serve as an on-line chemical sensor for a variety of analytes.