Multifunctional Encoded Particles for High-Throughput Biomolecule Analysis

by Daniel C. Pregibon, Mehmet Toner, Patrick S. Doyle, Science (2007) 315: 1393-1396

Purpose
To build a platform that allows for high-throughput screening of oligonucleotide analytes, with multiplexing capabilities.

Methods
Previous work by Patrick Doyle’s group allowed for the formation of microparticles in a microfluidic system by flowing a polymer/photoinitiator mixture through the chamber, and then pulsing UV irradiation through a photomask (placed on light source between condenser and objective) through a method they called “continuous flow lithography” (CFL).

In this work, the group used CFL (and its ability to rapidly form microparticles, ~100/second) to create multifunctional microparticles. This was done by exploiting laminar flow of multiple polymer chemistries and polymerizing particles the shape of pills that were perpendicular to the direction of the flow. By incorporating oligonucleotide probes into the flowed polymer mixtures, particles were created that displayed multiple probes in well-defined regions. The pattern also allowed for the incorporation of a barcode tag to help identify the oligonucleotide probes that were present on each specific microparticle.

The target sequences for the probes can be easily fluorescently labeled. Incubation of a mixture of microparticles with these fluorescently labeled targets allows the target to attach to their probes on the microparticles with high specificity. Another microfluidic device and a custom computer program was written to be able to analyze the particles for the presence of fluorescent signal at high rates.

Key Findings
-The scheme for microparticle synthesis (bar codes) and the well-written software allows for the formulation of a vast amount of different probes, each of which by themselves have multiplexing capabilities due to the fabrication method.

-Accurate and reproducible detection of analytes with sensitivity on the order of hundreds of attomoles, though this required longer incubation time between microparticles and the target mixture.

Definitions
-Oligonucleotide Probe: a single strand of DNA containing (rough estimate) 10-100 nucleotides, which can be used to probe for the presence of its complementary sequence in solution.

Nanodiamond lateral field emitter devices on thick insulator substrates for reliable high power applications

by K. Subramanian, W.P. Kang*, J.L. Davidson, M. Howell

Diamond & Related Materials 17 (2008) 786-789

Purpose

To fabricate nanocrystalline diamond field emitter devices on thick insulator substrates for high power applications. These monolithic lateral field emitter diodes will be fabricated in comb arrays on 640 μm-thick aluminum nitride insulating substrates that have been integrated with nanodiamond for device electrode isolation.

Fabrication

A 1 μm-thick layer of polysilicon was deposited on the aluminum nitride (ALN) substrates by low-pressure chemical vapor deposition (LPCVD). This layer is needed to enhance adhesion between the AlN substrate and CVD diamond film. The silicon/aluminum nitride sandwich substrates were then pretreated by ultra-sonicating with a 5-20 nm nanodiamond powder/acetone solution to increase diamond nucleation. Microwave plasma enhanced chemical vapor deposition (MPECVD) was used to grow the nanodiamond film on the polysilicon side of the Si/AlN substrates. The nanodiamond film was then lithographically micro-patterned in the lateral emitter diode structure, with aluminum as the mask for diamond etch using oxygen plasma by ICP-RIE (Inductive coupled plasma reactive ion etch) process technique. The polysilicon layer was then etched by SF₆ plasma RIE, isolating the nanodiamond electrodes on the ALN, and yielding the lateral device.

Field Emission Characterization

A 325-fingered nanodiamond lateral comb array was characterized for vacuum field emission. The diode had a turn-on voltage of ~ 30 V and the emission current was found to increase exponentially with the applied voltage. The device then demonstrated a high emission current of 1 mA at an anode voltage of 360 V. A Fowler-Nordheim (F-N) plot of the I-V field emission data indicates that the observed current is due to electron field emission from the nanodiamond emitter-fingers. The emission current was found to be stable over time, around 1 mA at constant applied voltage.

Keywords: Nanocrystalline diamond; Field emission; Lateral vacuum device

Optical Gain and Stimulated Emission in Nanocrystal Quantum Dots

V. I. Klimov, A. A. Mikhailovsky, Su Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. –J. Eisler, M. G. Bawendi.

OCTOBER 2000 VOL 290 SCIENCE

Purpose of the study

Examine the competing dynamical processes involved in optical amplification and lasing in the nanocrystal quantum dots. Demonstrate the feasibility of nanocrystal quantum dot lasers.   

Methods

This report uses femtosecond (fs) transient absorption (TA) and time-resolved PL to investigate the dynamical processes leading to buildup and decay of the optical gain. It shows that there are intrinsic mechanisms that complicate the development of stimulated emission in strongly confined QDs but do not inherently prevent it.. 

Key findings

1. In very small QDs, the spacing of the electronic states is much greater than the available thermal energy (strong confinement), which result in a lasing threshold that is temperature-insensitive at an excitation level of only one electron-hole (e-h) pair per dot on average..
2. Strong quantum confinement in nanocrystal QDs results in a large splitting of band-edge states and in an enhancement of instrinsic nonradiative Auger recombination, which dominates optical gain loss in QDs.
3. Both electron and hole intraband relaxations in nanocrystal QDs are sufficiently fast to successfully compete with the Auger effect.
4. Spectral energy can be controlled by facile manipulation of QD size and semiconductor composition.
5. The stimulated emission can only be observed if its buildup time is faster than the gain relaxation.

Useful terms: ( from http://en.wikipedia.org/wiki)

Quantum dot: a semiconductor whose excitations are confined in all three spatial dimensions.

Auger recombination: An electron and electron hole (electron-hole pair) can recombine giving up their energy to an electron in the conduction band, increasing its energy.

XPS study of the surface chemistry of Ag-covered L-CVD SnO2 thin films

M. Kwoka, L. Ottaviano, M. Passacantando, G. Czempik, S. Santucci, J. Szuber

Applied Surface Science 254 (2008) 8089-8092

 

Purpose of Study:

To produce and characterize the surface chemistry of silver covered SnO₂ films prepared by laser chemical vapor deposition and characterized by x-ray photoelectron spectroscopy. 

 

Methods:

Tin dioxide thin films were prepared using laser chemical vapor deposition (L-CVD).  Si (1 0 0) wafers were used as substrates to grow the thin films of SnO₂, which were grown from a mixture of TMT and O₂.  The resulting films had an average thickness of 20 nm after 1 hour.   One monolayer of silver was then deposited in a thin layer on the SnO₂ films using a thermal source under UHV in the same CVD chamber.  Some of the resulting samples were then equilibrated with dry air and then UHV annealed.  The control samples were UHV annealed.   All samples were then characterized using an XPS spectrometer (PHI 5700 model with an Al Kα 1486.6 eV x-ray source).  Notably, depth profiling was performed using the XPS to determine how far into the SnO₂ film the Ag penetrated.  Atomic force microscopy was also used to determine the surface topography and morphology of the samples.

 

Results:

1.    XPS analysis of the control showed that the SnO2 films were mixtures of SnO and SnO₂.  The [Ag]/[Sn] ratio was found to be 0.50 ± 0.05, which corresponds to approximately a 0.5 nm layer of Ag on the SnO₂ films (1 monolayer coverage).

2.    The samples aged in dry air prior to annealing oxidized (their [O]/[Sn] ratio reached 1.7 ± 0.05).  These samples also showed carbon contamination via an C 1s XPS peak (>2 monolayers of carbon contamination).

3.    UHV annealed samples showed less carbon contamination and the XPS depth profile analysis showed that Ag migrated throughout the SnO₂ films.  Specifically, the depth profile analysis confirmed that the Ag atoms diffused up to a depth of 20 nm, which corresponds to the SnO₂ film thickness.

4.    This analysis showed that the different methods for producing Ag-doped SnO₂ thin films produce films with different physical characteristics.  Specifically, exposing the films to dry air causes oxidation and carbon contamination to occure.  Annealing the films with UHV at 400C decreased the carbon contamination and oxidation, but causes the silver atoms to diffuse throughout the SnO₂ film.

The UV-nanoimprint lithography equipment with multi-head imprinting unit for sub-50 nm half-pitch patterns

JaeJong Lee, KeeBong Choi, GeeHong Kim, SeungWoo Lee. The UV-nanoimprint lithography equipment with multi-head imprinting unit for sub 50-nm half pitch patterns. Microelectronic Engineering 84 (2007) p.963-966.

Purpose of the Study: Nanoimprinting lithography (NIL) is a promising technology to produce sub-50 nm half pitch features on silicon and/or quartz-based substrates. It is considered the next generation lithography.  The fundamental procedure is replicating the patterns defined on the stamp to any deformable materials such as photo resist. In this paper, chip-size multi-head imprinting unit with compliance stage adn overlay/alignment system with moire and dual grating unit are presented to fabricate 50 nanometer half-pitch patterns.

Methods:

To fabricate nanoscale patterns by UV-NIL tool there are two methods: single-step and step and repeat imprinting approach. Single-step can fabricate nanoscale patterns using a large size stamp that is the same size as the wafer. Step and repeat can fabricate nanoscale patterns using the small size stamp one inch square. Each approach has its disadvantages and advantages. However, the single-step has the advantages of short imprinting time and high throughput because of the large stamp size that is the same as the wafer.  Using single-step nanoimprinting lithography tool (ANT-4) with a multi-head imprinting unit, the passive compliant stage, UV system with wavelength of 365 nm, and fine stage with 3 nm resolution.

Key Findings:

1. The key issues of  uniform contact method between the stamp and the substrate, the overlay and alignment unit between the stamp and the substrate for fabrication are resolved by using the chip-size multi-head imprinting unit for above 4 inch wafer.

2. 50 nm dot adn 100 nm half-pitch channels for biosensor and 50 nm grating patterns for optical tele-communication device are clearly transferred to the Si substrate using the UV-NIL tool.

Controlled Electrophoretic Deposition of Uniquely Nanostructured Star Polymer Films

Suseela Somarajan, Saad A. Hasan, Chinessa T. Adkins, Eva Harth, James H. Dickerson. Controlled Electrophoretic Deposition of Uniquely Nanostructured Star Polymer Films. J. Phys. Chem. B 112 (2008) 23-28.

Purpose of the study
Fabrication and characterization of polystyrene/divinylbenzene (PS/DVB) star polymer nanocrystalline thin films using controlled electrophoretic deposition (EPD) techniques

Methods

The polystyrene/divinylbenzene NMP star polymers were prepared by first degassing a mixture of polystyrene, styrene, and divinylbenzene in chlorobenzene via four freeze-pump-thaw cycles under argon and then heating/stirring the reaction mixture at 124°C for 24 hours. Methanol was used to precipitate the PS/DVB polymers. Nanocrystalline thin films were prepared by applying EDP techniques to a colloidal suspension of star polymer in a stratified hexane:dichloromethane immiscible mixture with a 9:1 volume ratio. This suspension was used in both a mixed and unmixed state. The EPD runs were performed using: silicon dioxide passivated and indium tin oxide coated polished float glass electrodes, 5min deposition time, and an applied voltage of 100V. The resulting thin films were characterized via Atomic Force Microscopy (AFM), Reflectance-Absorption Infrared Spectroscopy (RAIRS), and Scanning Electron Microscopy (SEM).

Key findings

1. EPD of the unmixed PS/DVB solution showed deposition of a translucent film on the (+) electrode while an opaque film deposited onto the (-) electrode. EDP of the mixed PS/DVB solution showed deposition of an opaque film on the (-) electrode [no (+) electrode deposition was observed].
2. Optical microscopy studies showed that micrometer-sized aggregates were dispersed within the tightly packed polymer films. Aggregate diameter lengths ranged from (0.5-5 um) and (> or = to 1 um) for the unmixed negative and positive electrode films (respectively) and (0.5-3um) for the mixed negative electrode film. SEM analysis confirmed these approximations
   
3. AFM studies showed that each of the three collected films possessed a different nanostructure/polymer conformation. The unmixed positive electrode film was found to have the PS/DVB polymers packed in such a way that the individual “arms” of the polymer where absorbed completely to the surface. The unmixed negative electrode film showed intermediate adsorption of the polymer to the surface, resulting in a conformation where only some of the polymer “arms” were absorbed to the surface. The mixed negative electrode film showed weak polymer absorption, resulting in a conformation were only a few of the polymer “arms” were partially absorbed to the surface.
   
4. In general, the above discoveries provide a methodology that permits the synthesis of both positive and negatively charged thin films starting from a neutrally charged polymer.

Glossary

star polymer: macromolecules that have a three-dimensional, compact shape where several polymer chains act as arms linked   to a single core