The objective of this research is to develop arrays of nanoparticle-coated microfabricated opto-fluidic ring resonator sensors with unprecedented capabilities to discriminate among different volatile organic compounds, to use such an array as the detector in a gas chromatographic microsystem, and to demonstrate the quantitative analysis of targeted compounds in arbitrarily complex mixtures. The approach entails micromachining high-aspect-ratio resonators with wave confinement contours, demonstrating enhanced vapor discrimination by use of vapor-sorptive interface layers comprising thiolate-monolayer-protected gold nanoparticles, and integrating arrays of such micro-resonators into the microsystem.
Intellectual Merit This research entails a new approach to vapor sensor array design, which will lead to the first microfabricated optofluidic ring resonator detectors made using scalable, precision microfabrication techniques. The proposed device structures promise inherently greater vapor sensitivity. Since vapor induced changes in refractive index will vary with the nanoparticle size, thiolate-ligand functionality, and probing wavelength, arrays of such devices promise response diversity inherently greater than that produced by current sensor arrays, which will enhance vapor discrimination.
Broader Impacts The successful project will fill a need for small, inexpensive, turn-key instrumentation for the determination of multiple vapors at trace levels in complex mixtures, with direct application to the detection of disease biomarkers, explosives, and airborne toxicants. Dissemination of project results to representatives of technology-oriented companies and leading microsystem research centers throughout the world will be facilitated through extant affiliations of the investigators. This will promote the commercialization of the developed technologies and expand the scope of training provided to the students on the project.
PI: E. T. Zellers, University of Michigan This project was highly successful and the broad goals set forth at the outset were met: we have developed a new microsensor, the micro-optofluidic ring resonator (µOFRR), and demonstrated its capabilities; we have shown that multi-wavelength optical vapor sensing with certain plasmonic-nanoparticle films provides a greater degree of response diversity than is possible with arrays of current microsensors employing the same nanoparticles; and we have integrated polymer-coated and nanoparticle-coated µOFRRs into single- and two-dimensional micro-scale gas chromatographic (µGC) systems and shown that, when included in such microsystems, this sensing technology is capable of quantitatively analyzing the components of volatile organic compound (VOC) mixtures. The specific aims were to 1) microfabricate rugged, high-aspect-ratio µOFRRs with wave confinement contours and high Q factors; 2) demonstrate that using plasmonic thiolate-monolayer-protected Au nanoparticles (MPNs) having different core sizes and ligand functionalities as vapor-sorptive OFRR interface layers that are probed at multiple wavelengths can enhance vapor discrimination; 3) assemble arrays of µOFRRs with optimized subsets of MPN interface layers, and assess their performance in analyzing simple mixtures of VOCs; and 4) integrate them into a µGC-µOFRR prototype instrument capable of rapid in-situ analysis of targeted VOCs in complex mixtures in air or breath. Most of the specific aims were met. Intellectual Merit: Current stand-alone microsensor arrays (aka "electronic noses") are not capable of quantitatively analyzing mixtures of more than two or three VOCs due to the limited diversity in the composite response patterns generated, regardless of the transducer or interface layers employed. The use of µOFRRs with plasmonic MPN interface layers represents a new approach to vapor sensor array design that should overcome this limitation, while also being suitable for integration into µGC platforms. The use of individual, polymer-coated (non-microfabricated) OFRRs as sensitive GC detectors had been demonstrated through prior NSF support. However, such devices are made by cumbersome drawn-capillary techniques that are not amenable to mass production or to arrayed integration with upstream µGC components. The research performed in this project yielded the first µOFRR vapor sensors ever made, using precise, scalable, batch microfabrication techniques. The thinner, more uniform walls of the µOFRR afford inherently greater sensitivity than in non-microfabricated predecessor devices because a greater fraction of the evanescent wave energy resides in the polymeric or MPN sensing layer. The µOFRRs developed here were wall-coated with isotropic polymers and plasmonic MPNs. By probing films of the latter materials at three wavelengths simultaneously, certain MPNs exhibited unprecedented selectivities toward target vapors; selectivities that varied with Au-core size (from 4-40 nm), thiolate-ligand functionality, and probing wavelength. Several new MPN materials with different core sizes and ligand functionalities were designed/synthesized. We proved the principle that the diversity of responses afforded by an optimized µOFRR array would enhance analyte recognition/discrimination. Although we fell short of creating an integrated array of µOFRRs, we succeeded in integrating both a polymer-coated µOFRR and a MPN-coated µOFRR into a µGC system and a more sophisticated 2-dimensional (µGC×µGC) system, and demonstrating their performance. Supporting hardware and control software required to create working prototypes were also developed. As the very first examples of such microsystems, these represent a significant advancement in µGC instrumentation. Broader Impacts: This successful project has contributed greatly toward meeting the need for small, inexpensive, turn-key instrumentation for identifying/quantifying trace-level VOCs in complex mixtures. Such instrumentation is critical to numerous applications requiring such analytical capabilities in field or clinical settings, e.g., breath biomarker determinations, explosives detection, personal exposure monitoring, and military surveillance. Several fundamental aspects of the processes, devices, and materials developed here were elucidated, while others that were revealed in the course of study remain to be resolved in the future. This project comprised a natural extension of previous work by each of the co-PIs as well as a natural collaboration. The students and faculty on this team represented several disciplines/ perspectives (i.e., biomedical engineering, physics, chemistry, and environmental health science), which promoted interdisciplinary learning in optics, nano/microfabrication, analytical chemistry, materials science, and environmental (public) health. Dissemination of project results to representatives of technology-oriented companies and leading research centers in microsystems was facilitated through the Michigan Center for Wireless Integrated Microsensing and Systems (WIMS2) and the Michigan-Freiburg-Kyoto MicroAlliance. These affiliations promoted the transfer of the developed technologies toward practical application and further expanded the scope and value of training provided to the students. A certificate degree program in Environmental Monitoring Microsystems created in parallel with the research performed in this project, which is pending formal approval, provides courses on microfluidics, sensors, and relevant applications and will be open to graduate students across multiple departments at the University of Michigan. This will fill a niche in cross-disciplinary education that is highly relevant to the careers of many science and engineering students.