Development of Optical Sensors for Chemical Detection

Detection of biodiesel at low and high concentrations in diesel is highly desired in the aviation and fuel industries. Cross contamination of jet fuel with biodiesel may impact the thermal stability and freezing point which can cause deposits in the fuel system or cause the fuel to gel, leading to jet engine operability problems and possible engine flameout. A dye doped optical sensor utilizing the dye Nile Blue Chloride has been developed for quick and direct detection of biodiesel which mainly contains fatty acid methyl esters (FAME). The sensing mechanism relies on the solvatochromatic properties of the dye which undergoes a color change from blue to pink. A detection limit of 0.250 ppm (parts per million) and quantification limit of 0.750 ppm is obtained with a dynamic range from 0.5–200,000 ppm (20% v/v) FAME. This sensor is a viable alternative to compliment more sophisticated and expensive bench top techniques in current use. The detection of chloroform in aqueous and non-aqueous has direct environmental and pharmaceutical applications, due to its well documented toxicity. A sensor has been developed based on a modified Fujiwara reaction for detecting chloroform, a halogenated hydrocarbon, in the visible spectrum. 2,2’-dipyridyl and tetra-n-butyl ammonium hydroxide are the modified Fujiwara reagents encapsulated within a sensing film. Upon exposure to chloroform in non-aqueous solution, a colored product is produced within the film which can be analyzed spectroscopically yielding a detection limit of 0.830 ppm (v/v) and quantification limit of 2.77 ppm. Monitoring and detection of gas plume constituents is a useful diagnostic tool in evaluating combustion efficiency, ensuring safe testing conditions, and in quantifying greenhouse gas emissions. Rocket engine ground tests are vital to ensure the performance of the rocket engines during critical space missions. Optical sensors were developed for remote sensing applications to detect isopropyl alcohol utilizing the dye Chromoionophore IX. This sensor gave a detection limit of 9, 13, 21 ppm and quantification limits of 32, 43, and 70 ppm for methanol, ethanol, and isopropyl alcohol respectively. Also a fingerprinting method was developed utilizing several indicator dyes in order to detect kerosene vapor

Transparent and Conductive Carbon Nanotube Multilayer Thin Films Suitable as an Indium Tin Oxide Replacement

Transparent electrodes made from metal oxides suffer from poor flexibility and durability. Highly transparent and electrically conductive thin films based on carbon nanotubes (CNTs) were assembled as a potential indium tin oxide (ITO) replacement using layer-by-layer (LbL) assembly. The ultimate objective of this dissertation work is to produce CNT-based assemblies with sheet resistance below 100 Omega/sq and visible light transmission greater than 85 percent. The alternate deposition of positively charged poly(diallyldimethylammonium chloride) [PDDA] and CNTs stabilized with negatively charged deoxycholate (DOC) exhibit linear film growth and thin film properties can be precisely tuned. Ellipsometry, quartz crystal microbalance, and UV-vis were used to measure the growth of these films as a function of PDDA-CNT bilayers deposited, while TEM, SEM, and AFM were used to visualize the nanostructure of these films. Following a literature review describing potential ITO substitutes and LbL technology, the influence of CNT type on optoelectronic performance of LbL assemblies is described. Three different types of nanotubes were investigated: (1) multiwalled carbon nanotubes (MWNTs), (2) few-walled carbon nanotubes (FWNT), and (3) purified single-walled carbon nanotubes (SWNTs). SWNTs produced the most transparent (>85 percent visible light transmittance) and electrically conductive (148 S/cm, 1.62 kOmega/sq) 20-bilayer films with a 41.6 nm thickness, while MWNT-based films are much thicker and more opaque. A 20-bilayer PDDA/(MWNT DOC) film is approximately 103 nm thick, with a conductivity of 36 S/cm and a transmittance of 30 percent. In an effort to improve both transparency and electrical conductivity, heat and acid treatments were studied. Heating films to 300 degree C reduced sheet resistance to 701 Omega/sq (618 S/cm conductivity, 38.4 nm thickness), with no change in transparency, owing to the removal of insulating component in the film. Despite improving conductivity, heating is not compatible with most plastic substrates, so acid doping was investigated as an alternate means to enhance properties. Exposing SWNT-based assemblies to HNO3 vapor reduced sheet resistance of a 10 BL film to 227 Omega/sq. Replacing SWNTs with double walled carbon nanotubes (DWNTs) provided further reduction in sheet resistance due to the greater metallic of DWNT. A 5 BL DWNT film exhibited the lowest 104 Omega/sq sheet resistance (4200 S/cm conductivity, 22.9 nm thickness) with 84 percent transmittance after nitric acid treatment. DWNT-based assemblies maintained their low sheet resistance after repeated bending and also showed electrochemical stability relative to ITO. This work demonstrates the excellent optoelectronic performance, mechanical flexibility, and electrochemical stability of CNT-based assemblies, which are potentially useful as flexible transparent electrodes for a variety of flexible electronics

Confining Metal-Halide Perovskites in Nanoporous Thin Films

Controlling size and shape of semiconducting nanocrystals advances nanoelectronics and photonics. Quantum confined, inexpensive, solution derived metal halide perovskites offer narrow band, color-pure emitters as integral parts of next-generation displays and optoelectronic devices. We use nanoporous silicon and alumina thin films as templates for the growth of perovskite nanocrystallites directly within device-relevant architectures without the use of colloidal stabilization. We find significantly blue shifted photoluminescence emission by reducing the pore size; normally infrared-emitting materials become visibly red, green-emitting materials cyan and blue. Confining perovskite nanocrystals within porous oxide thin films drastically increases photoluminescence stability as the templates auspiciously serve as encapsulation. We quantify the template-induced size of the perovskite crystals in nanoporous silicon with microfocus high-energy X-ray depth profiling in transmission geometry, verifying the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films. Low-voltage electroluminescent diodes with narrow, blue-shifted emission fabricated from nanocrystalline perovskites grown in embedded nanoporous alumina thin films substantiate our general concept for next generation photonic devices

Direct printing of polymer microstructures on flat and spherical surfaces using a letterpress technique

We have developed a letterpress technique capable of printing polymer films with micrometer scale feature sizes onto flat or spherically shaped nonporous substrates. This printing technique deposits polymer only in desired regions thereby eliminating subsequent developing and subtraction steps. Flat or curved printing plates, which are fabricated from either rigid or deformable materials, are used to transfer thin molten polymer films onto flat target substrates. By deforming the printing plates into a spherical shape, it is also possible to print patterned films onto the concave side of a spherically deformed target substrate. These printed films serve as good resists for both wet chemical etching and reactive ion etching. Interferometric measurements of the polymer film thickness are used to probe physical mechanisms affecting printing instabilities, pattern fidelity, and edge resolution. Our experimental study indicates that this letterpress technique may prove suitable for high-throughput device fabrication involving large-area microelectronics

Sputtered Indium Tin Oxide Films for Optoelectronic Applications

High‐quality indium tin oxide (ITO) films have been fabricated using a DC sputtering technique in a pure argon atmosphere with a postannealing in an oxygen environment at atmosphere pressure. Structural, morphological, and electro‐optical parameters of the ITO films were studied at different annealing temperatures for the films fabricated on two types of glass substrates, soda lime and alkali‐free substrates. A comparative analysis shows that low‐cost soda lime substrates are suitable for the fabrication of high‐quality nanocrystalline ITO films after annealing them at 300°C. This result is of great importance for reducing the cost of thin film solar cells, in which ITO films serve as transparent conducting electrodes. We present a comparison of the properties of sputtered ITO films with those fabricated using a spray pyrolysis deposition technique, which is useful for some optoelectronic applications

Carbon nanodots (CNDs): a comprehensive study of the photoluminescence, antioxidation and cellular uptake

Carbon Nanodots (CNDs) have attracted increasing attention owing to their excellent photoluminescence properties, antioxidation activity, lower toxicity and superior water solubility. These physiochemical properties make CNDs promising candidates for biomedical applications including bioimaging, antioxidative agent and drug delivery, etc. However, the lack of consistency of understanding the fluorescence emission and antioxidation reactions in CNDs impedes their practical development in biomedical applications. Moreover, for CNDs cellular uptake study, there are rare reports on the nucleus penetrability of CNDs. Most uptake mechanisms focus on different types of nanoparticles (e.g. metal NPs or quantum dots). Little is known about the cellular uptake of CNDs especially the concentration influence. This dissertation research attempts to address these issues focusing on three aspects. (1) A new approach combining fluorescence spectroscopy and electrochemistry and energy gap analysis are used to investigate the photoluminescence mechanism of CNDs. (2) Electrochemistry and spectroscopy measurement are performed to investigate the antioxidative activities of CNDs against free radical DPPH• and reactive oxygen species (ROS) generated by xanthine/xanthine oxidase (XO) reaction. (3) nucleus penetrability of CNDs cell uptake is investigated with respect to CNDs’ concentration. We found that the excitation dependent fluorescence of the CNDs may be attributed to multiple energy gap levels arising from a combination of the Sp2 carbon core, surface functionalities (C=O, C–O, and COOH), and surface electronic state transitions. Regarding the antioxidative property, the result ofelectrochemical study for DPPH• scavenging is consistent with UV-Vis absorbance dose dependent manner following a coupled hydrogen atom transfer (HAT) reaction mechanism by the CNDs. For the cellular uptake of CNDs, a three-stage mechanism is proposed to explain the observation of concentration-dependent behavior of the nucleus penetrability. Overall, this comprehensive study of the photoluminescence, antioxidation and cellular uptake of CNDs and the findings should aid future development of practical utilization in bioimaging, antioxidation and drug delivery, such as in-vitro and in-vivo studies

Versatile Optochemical Quantification with Optical Mouse

There is an ever increasing need for simple, low-cost instruments for ubiquitous medical and environmental measurements in conjunction with networks and Internet-of-things. This work demonstrates that the optical mouse, one of the most common optoelectronic computer peripherals, can be used for chemical quantification. Particularly, we explore the feasibility of using the preassembled optical platform of mouse for oxygen and pH quantification. The image sensor and the light-emitting diode (LED) serve as photodetector and excitation/illumination light source, respectively, while the preinstalled microoptics (e.g., lens and waveguide) provide a fixed optical arrangement convenient for sample analysis. This novel, cost-effective approach demonstrates the potential application of optical mouse for bioanalytical devices in conjunction with commercial sensor strips or simple microfluidic elements. This is one viable option for seamless integration of bioanalytical capability into existing personal computers and associate networks without significant additional hardware

Large-Area (over 50 cm × 50 cm) Freestanding Films of Colloidal InP/ZnS Quantum Dots

Cataloged from PDF version of article.We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm x 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton-exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surface-emitting light engines