Duyoung Choi

Highly specific SNP detection using 2D graphene electronics and DNA strand displacement

Significance We describe the first, to our knowledge, integrated dynamic DNA nanotechnology and 2D material electronics to overcome current limitations for the detection of DNA single-nucleotide polymorphism (SNP). Electrical detection of DNA has been advancing rapidly to achieve high specificity, sensitivity, and portability. However, the actual implementation of DNA detection is still in infancy because of low specificity, especially for analytically optimal and practically useful length of target DNA strands. Most of the research to date has focused on the enhancement of the sensitivity of DNA biosensors, whereas the specificity problem has remained unsolved. The low specificity is primarily attributed to the nonspecific binding during hybridization of the probe and the target DNA. Here, we have addressed these limitations by designing a functional prototype of electrical biosensors for SNP detection. Single-nucleotide polymorphisms (SNPs) in a gene sequence are markers for a variety of human diseases. Detection of SNPs with high specificity and sensitivity is essential for effective practical implementation of personalized medicine. Current DNA sequencing, including SNP detection, primarily uses enzyme-based methods or fluorophore-labeled assays that are time-consuming, need laboratory-scale settings, and are expensive. Previously reported electrical charge-based SNP detectors have insufficient specificity and accuracy, limiting their effectiveness. Here, we demonstrate the use of a DNA strand displacement-based probe on a graphene field effect transistor (FET) for high-specificity, single-nucleotide mismatch detection. The single mismatch was detected by measuring strand displacement-induced resistance (and hence current) change and Dirac point shift in a graphene FET. SNP detection in large double-helix DNA strands (e.g., 47 nt) minimize false-positive results. Our electrical sensor-based SNP detection technology, without labeling and without apparent cross-hybridization artifacts, would allow fast, sensitive, and portable SNP detection with single-nucleotide resolution. The technology will have a wide range of applications in digital and implantable biosensors and high-throughput DNA genotyping, with transformative implications for personalized medicine.

High-performance flexible hydrogen sensor made of WS2 nanosheet–Pd nanoparticle composite film

We report a flexible hydrogen sensor, composed of WS2 nanosheet-Pd nanoparticle composite film, fabricated on a flexible polyimide substrate. The sensor offers the advantages of light-weight, mechanical durability, room temperature operation, and high sensitivity. The WS2-Pd composite film exhibits sensitivity (R 1/R 2, the ratio of the initial resistance to final resistance of the sensor) of 7.8 to 50,000 ppm hydrogen. Moreover, the WS2-Pd composite film distinctly outperforms the graphene-Pd composite, whose sensitivity is only 1.14. Furthermore, the ease of fabrication holds great potential for scalable and low-cost manufacturing of hydrogen sensors.

Unusually High Optical Transparency in Hexagonal Nanopatterned Graphene with Enhanced Conductivity by Chemical Doping

Graphene has received appreciable attention for its potential applications in flexible conducting film due to its exceptional optical, mechanical, and electrical properties. However increasing transmittance of graphene without sacrificing the electrical conductivity has been difficult. The fabrication of optically highly transparent (≈98%) graphene layer with a reasonable electrical conductivity is demonstrated here by nanopatterning and doping. Anodized aluminium oxide nanomask prepared by facile and simple self-assembly technique is utilized to produce an essentially hexagonally nanopatterned graphene. The electrical resistance of the graphene increases significantly by a factor of ≈15 by removal of substantial graphene regions via nanopatterning into hexagonal array pores. However, the use of chemical doping on the nanopatterned graphene almost completely recovers the lost electrical conductivity, thus leading to a desirably much more optically transparent conductor having ≈6.9 times reduced light blockage by graphene material without much loss of electrical conductivity. It is likely that the availability of large number of edges created in the nanopatterned graphene provides ideal sites for chemical dopant attachment, leading to a significant reduction of the sheet resistance. The results indicate that the nanopatterned graphene approach can be a promising route for simultaneously tuning the optical and electrical properties of graphene to make it more light-transmissible and suitable as a flexible transparent conductor.

Nanopatterned Graphene Field Effect Transistor Fabricated Using Block Co-polymer Lithography

We demonstrate a successful fabrication of Nanopatterned Graphene (NPG) using a PS-b-P4VP polymer, which was never used previously for the graphene patterning. The NPG exhibits homogeneous mesh structures with an average neck width of ∼19 nm. Electronic characterization of single and few layers NPG FETs (field effect transistors) were performed at room temperature. We found that the sub-20 nm neck width creates a quantum confinement in NPG, which has led to a bandgap opening of ∼0.08 eV. This work also demonstrates that BCP (block co-polymer) lithography is a pathway for low-cost, high throughput large-scale production of NPG with critical dimensions down to the nanometer regime.

Vertical Si nanowire arrays fabricated by magnetically guided metal-assisted chemical etching.

In this work, vertically aligned Si nanowire arrays were fabricated by magnetically guided metal-assisted directional chemical etching. Using an anodized aluminum oxide template as a shadow mask, nanoscale Ni dot arrays were fabricated on an Si wafer to serve as a mask to protect the Si during the etching. For the magnetically guided chemical etching, we deposited a tri-layer metal catalyst (Au/Fe/Au) in a Swiss-cheese configuration and etched the sample under the magnetic field to improve the directionality of the Si nanowire etching and increase the etching rate along the vertical direction. After the etching, the nanowires were dried with minimal surface-tension-induced aggregation by utilizing a supercritical CO2 drying procedure. High-resolution transmission electron microscopy (HR-TEM) analysis confirmed the formation of single-crystal Si nanowires. The method developed here for producing vertically aligned Si nanowire arrays could find a wide range of applications in electrochemical and electronic devices.

Geometrically Planar Ion-Implant Patterned Magnetic Recording Media Using Block Copolymer Aided Gold Nanoisland Masks

We have developed patterned media via ion implantation using Au nano mask approach for local control of coercivity of magnetically hard [Co/Pd]n multilayer film. Au nano-islands produced through a di-block copolymer templated technique is used as the mask for ion implantation. The sputter deposited [Co 0.3 nm/Pd 0.8 nm]8/Pd 3 nm/Ta 3 nm multilayer film having vertical magnetic anisotropy is coated with a diblock copolymer layer, two phase decomposed into vertically pored nanostructure, then chemically processed to nucleate gold nanoislands corresponding to the diblock copolymer nanostructures. Subsequent nitrogen (N) ion implantation, using these Au islands as implantation-blocking masks, allows a patterned penetration of implanted ions into unmasked portion of the [Co/Pd]n multilayer film, thus creating invisible but magnetically isolated bit island geometry while maintaining the overall flat configuration of the patterned media.

Deformation and electrical properties of magnetic and vertically conductive composites with a chain-of-spheres structure

Vertically anisotropically conductive composites with aligned chain-of-spheres of 20–75 mm Ni particles in an elastomer matrix have been prepared by curing the mixture at 100°C–150°C under an applied magnetic field of ∼300–1000 Oe. The particles are coated with a ∼120 nm thick Au layer for enhanced electrical conductivity. The resultant vertically aligned but laterally isolated columns of conductive particles extend through the whole composite thickness and the end of the Ni columns protrude from the surface, contributing to enhanced electrical contact on the composite surface. The stress-strain curve on compressive deformation exhibits a nonlinear behavior with a rapidly increasing Young’s modulus with stress (or pressure). The electrical contact resistance Rc decreases rapidly when the applied pressure is small and then more gradually after the applied pressure reaches 500 psi (∼3.4 MPa), corresponding to a 30% deformation. The directionally conductive elastomer composite material with metal pads and conductive electrodes on the substrate surface can be used as a convenient tactile shear sensor for applications involving artificial limbs, robotic devices, and other visual communication devices such as touch sensitive screens.