Jong-in Hahm

Ultrasensitive DNA sequence detection using nanoscale ZnO sensor arrays

We report that engineered nanoscale zinc oxide structures can be effectively used for the identification of the biothreat agent, Bacillus anthracis by successfully discriminating its DNA sequence from other genetically related species. We explore both covalent and non-covalent linking schemes in order to couple probe DNA strands to the zinc oxide nanostructures. Hybridization reactions are performed with various concentrations of target DNA strands whose sequence is unique to Bacillus anthracis. The use of zinc oxide nanomaterials greatly enhances the fluorescence signal collected after carrying out duplex formation reaction. Specifically, the covalent strategy allows detection of the target species at sample concentrations at a level as low as a few femtomolar as compared to the detection sensitivity in the tens of nanomolar range when using the non-covalent scheme. The presence of the underlying zinc oxide nanomaterials is critical in achieving increased fluorescence detection of hybridized DNA and, therefore, accomplishing rapid and extremely sensitive identification of the biothreat agent. We also demonstrate the easy integration potential of nanoscale zinc oxide into high density arrays by using various types of zinc oxide sensor prototypes in the DNA sequence detection. When combined with conventional automatic sample handling apparatus and computerized fluorescence detection equipment, our approach can greatly promote the use of zinc oxide nanomaterials as signal enhancing platforms for rapid, multiplexed, high-throughput, highly sensitive, DNA sensor arrays.

Low-threshold two-photon pumped ZnO nanowire lasers

We report in this communication the two-photon absorption (TPA)-induced room-temperature lasing performance of ZnO nanowires. Under femtosecond pulse-excitation at lambda = 700 nm in the infrared regime, a remarkably low threshold of 160 microJ/cm(2) was observed for the TPA-induced lasing action, which is of the same order of magnitude as that measured for the linear lasing process. Time-resolved photoluminescence characterization of two-photon pumped ZnO nanowires reveals the presence of a fast decay (3-4 ps) in the stimulated emission as compared to the slow decay (50-70 ps) for the spontaneous emission. The TPA process in ZnO nanowires was characterized with the nonlinear transmission measurement, which uncovers an enhanced TPA coefficient, about 14.7 times larger than that of bulk ZnO samples. The observed TPA enhancement in ZnO nanowires accounts for the low threshold lasing behavior, and has been attributed to the intensified optical field confined within the nanowire waveguides.

Ultrasensitive detection of cytokines enabled by nanoscale ZnO arrays.

Early detection of disease markers can provide higher diagnostic power and improve disease prognosis. We demonstrate the use of zinc oxide nanorod (ZnO NR) arrays in a straightforward, reliable, and ultrasensitive detection of the cytokines interleukin-18 and tumor necrosis factor-alpha. Specifically, we exploit the fluorescence-enhancing properties of ZnO NR platforms in cytokine assays involving both a pure buffer and urine. The detection sensitivity achieved using this ZnO NR method is in the subfemtogram per milliliter level, which is 3-4 orders of magnitude more sensitive than conventional assay detection limits. This unparalleled detection sensitivity is achieved without the need for indirect enzyme reactions or specialized instrumentation. We highlight various advantages of using ZnO NR arrays in the ultrasensitive profiling of cytokine levels. Key advantages include robustness of NR arrays, simple and direct assay schemes, high-throughput and multiplexing capabilities, and the ability to correlate directly measured signals to cytokine levels. In conjunction with the extremely high sensitivity demonstrated in this work, our ZnO NR array-based approach may be highly beneficial in early detection of many cytokine-implicated diseases.

Integration of planar and bulk heterojunctions in polymer/nanocrystal hybrid photovoltaic cells

We present in this communication a hybrid polymer/nanocrystal photovoltaic device architecture wherein a net poly(3-hexylthiophene) (P3HT) light-absorbing film is inserted underneath the blended layer of P3HT and PbSe nanocrystal quantum dots in the active region. Such a design features the vertical integration of planar and bulk heterojunctions, which allows for the employment of a thinner bulk heterojunction for more efficient carrier collection without an excessive reduction of the overall light absorption by the photovoltaic cell. The measured device performance represents a significant improvement over previously reported hybrid cells containing bulk heterojunctions of P3HT and Pb(S,Se) nanocrystal quantum dots.

Elucidation of protein adsorption behavior on polymeric surfaces: toward high-density, high-payload protein templates.

The elucidation of protein adsorption behavior on polymeric surfaces is very important, since their use as arrays and carriers of biomolecules is ever growing for a wide variety of bioapplications. We evaluate protein adsorption characteristics on chemically homogeneous and heterogeneous polymeric surfaces by employing polystyrene-block-polymethylmethacrylate (PS-b-PMMA) diblock copolymer, PS homopolymer, PMMA homopolymer, and PS/PMMA blend as protein templates. We also investigate distance-dependent protein adsorption behavior on the interfacial region between PS and PMMA. We observe selective protein adsorption exclusively onto PS areas for the chemically heterogeneous PS-b-PMMA and PS/PMMA blend templates. On blend films, protein adsorption is highly favored on the PS regions located near the PS:PMMA interface over that on the PS areas situated away from the interface. Protein density on PS domains is inversely proportional to the separation distance between two neighboring PS:PMMA interfaces. We also observe a higher protein density on the PS-b-PMMA than on the PS or PMMA homopolymer templates. This effect is due to the fact that chemically heterogeneous PS-b-PMMA presents periodically spaced PS:PMMA interfaces on the nanometer scale, whereas no such interfaces are present on homopolymer films. The density of protein molecules on the heterogeneous PS-b-PMMA surface is approximately 3-4-fold higher than on the homogeneous PS surface for the identical experimental conditions. These results demonstrate that self-assembling, chemically heterogeneous, nanoscale domains in PS-b-PMMA diblock copolymers can be used as excellent, high-payload, high-density protein templates. The unique advantages of the diblock copolymer may prove the spontaneously constructed protein nanotemplates to be highly suitable as functional substrates in many proteomics applications.

Functional Polymers in Protein Detection Platforms: Optical, Electrochemical, Electrical, Mass-Sensitive, and Magnetic Biosensors

The rapidly growing field of proteomics and related applied sectors in the life sciences demands convenient methodologies for detecting and measuring the levels of specific proteins as well as for screening and analyzing for interacting protein systems. Materials utilized for such protein detection and measurement platforms should meet particular specifications which include ease-of-mass manufacture, biological stability, chemical functionality, cost effectiveness, and portability. Polymers can satisfy many of these requirements and are often considered as choice materials in various biological detection platforms. Therefore, tremendous research efforts have been made for developing new polymers both in macroscopic and nanoscopic length scales as well as applying existing polymeric materials for protein measurements. In this review article, both conventional and alternative techniques for protein detection are overviewed while focusing on the use of various polymeric materials in different protein sensing technologies. Among many available detection mechanisms, most common approaches such as optical, electrochemical, electrical, mass-sensitive, and magnetic methods are comprehensively discussed in this article. Desired properties of polymers exploited for each type of protein detection approach are summarized. Current challenges associated with the application of polymeric materials are examined in each protein detection category. Difficulties facing both quantitative and qualitative protein measurements are also identified. The latest efforts on the development and evaluation of nanoscale polymeric systems for improved protein detection are also discussed from the standpoint of quantitative and qualitative measurements. Finally, future research directions towards further advancements in the field are considered.

Multiphoton absorption induced amplified spontaneous emission from biocatalyst-synthesized ZnO nanorods

Multiphoton absorption-induced photoluminescence and amplified spontaneous emission (ASE) have been observed from ZnO nanorod arrays upon near-infrared excitation (λexc∼800nm). A low threshold of ∼12mJ∕cm2 was demonstrated for the ASE process. The ultraviolet emission can be attributed to the recombination of carriers coexcited by the processes of three-photon and off-resonant two-photon absorption, which was confirmed by the excitation wavelength-dependent experiments. Additionally, it has been observed that the processes of ASE and second harmonic generation in ZnO nanorods appear to enhance each other when the excitation wavelength approaches the resonant wavelength for two-photon absorption.Multiphoton absorption-induced photoluminescence and amplified spontaneous emission (ASE) have been observed from ZnO nanorod arrays upon near-infrared excitation (λexc∼800nm). A low threshold of ∼12mJ∕cm2 was demonstrated for the ASE process. The ultraviolet emission can be attributed to the recombination of carriers coexcited by the processes of three-photon and off-resonant two-photon absorption, which was confirmed by the excitation wavelength-dependent experiments. Additionally, it has been observed that the processes of ASE and second harmonic generation in ZnO nanorods appear to enhance each other when the excitation wavelength approaches the resonant wavelength for two-photon absorption.

Fundamentals of Nanoscale Polymer–Protein Interactions and Potential Contributions to Solid-State Nanobioarrays

Protein adsorption onto polymer surfaces is a very complex, ubiquitous, and integrated process, impacting essential areas of food processing and packaging, health devices, diagnostic tools, and medical products. The nature of protein–surface interactions is becoming much more complicated with continuous efforts toward miniaturization, especially for the development of highly compact protein detection and diagnostic devices. A large body of literature reports on protein adsorption from the perspective of ensemble-averaged behavior on macroscopic, chemically homogeneous, polymeric surfaces. However, protein–surface interactions governing the nanoscale size regime may not be effectively inferred from their macroscopic and microscopic characteristics. Recently, research efforts have been made to produce periodically arranged, nanoscopic protein patterns on diblock copolymer surfaces solely through self-assembly. Intriguing protein adsorption phenomena are directly probed on the individual biomolecule level for a fundamental understanding of protein adsorption on nanoscale surfaces exhibiting varying degrees of chemical heterogeneity. Insight gained from protein assembly on diblock copolymers can be effectively used to control the surface density, conformation, orientation, and biofunctionality of prebound proteins in highly miniaturized applications, now approaching the nanoscale. This feature article will highlight recent experimental and theoretical advances made on these fronts while focusing on single-biomolecule-level investigations of protein adsorption behavior combined with surface chemical heterogeneity on the length scale commensurate with a single protein. This article will also address advantages and challenges of the self-assembly-driven patterning technology used to produce protein nanoarrays and its implications for ultrahigh density, functional, and quantifiable protein detection in a highly miniaturized format.

Indium Tin Oxide Nanowire Networks as Effective UV/Vis Photodetection Platforms

We demonstrate that indium tin oxide nanowires (ITO NWs) and cationic polymer-modified ITO NWs configured in a network format can be used as high performing UV/vis photodetectors. The photovoltage response of ITO NWs is much higher than similarly constructed devices made from tin oxide, zinc tin oxide, and zinc oxide nanostructures. The ITO NW mesh-based devices exhibit a substantial photovoltage (31–100 mV under illumination with a 1.14 mW 543 nm laser) and photocurrent (225–325 μA at 3 V). The response time of the devices is fast with a rise time of 20–30 μs and a decay time of 1.5–3.7 ms when probed with a 355 nm pulsed laser. The photoresponsivity of the ITO NW devices ranges from 0.07 to 0.2 A/W at a 3 V bias, whose values are in the performance range of most commercial UV/vis photodetectors. Such useful photodetector characteristics from our ITO NW mesh devices are attained straightforwardly without the need for complicated fabrication procedures involving highly specialized lithographic tools. Therefore, our approach of ITO NW network-based photodetectors can serve as a convenient alternative to commercial or single NW-based devices.

Carbon nanotube cantilevers on self-aligned copper silicide nanobeams

In this letter, the authors describe both a growth method for self-aligning copper silicide (Cu3Si) nanobeams and their use as active catalysts for carbon nanotube (CNT) synthesis via chemical vapor deposition. In the unique geometry of these useful structures, CNT cantilevers are anchored firmly to the Cu3Si nanobeams. The resulting CNT-Cu3Si structures may improve accuracy and reliability of CNT applications in nanoelectromechanical systems.