Self-referenced SPR Sensing Using Multiple Surface-Plasmon Modes

Project Summary Surface-plasmon resonance (SPR) has become a widely used, label free technique to detect and study biological and chemical interactions. Nevertheless, a fundamental challenge remains unresolved: How does one differentiate between non-specific effects (temperature fluctuations, solution refractive index changes, non-specific binding of interferents, etc.) and detection of a target analyte? This problem currently limits the effectiveness of SPR in complex biological samples and for medical, environmental, food safety, and defense applications that require field deployable sensors. The proposed research effort seeks to address this challenge by developing a selfreferencing SPR platform based on simultaneous coupling to multiple surface-plasmon modes. The resulting platform will be used to develop a sensor that selectively detects the presence of pathogenic bacteria such as E. Coli in complex solutions. The first objective of the proposed effort is to understand how to optimally distinguish surface and bulk effects using long- and short- range surface-plasmon modes of thin metal films. Numerical and analytical modeling will be used to explore the impact of different materials, geometries, and operation wavelengths on the proposed sensor's sensitivity and limit-of-detection. Optimal designs will be fabricated and experimentally evaluated through alkanethiol-gold and avidin-biotin binding experiments. The second objective is to understand how to design a completely self-referencing sensor by using multiple surface-plasmon modes of patterned microstructures. By measuring the response of three or more modes, one can separately quantify three critical effects: specific binding of a target analyte, non-specific binding of interferents, and solution refractive index changes. Finite element modeling will be used to establish an initial design, and then multi-moded sensors will be fabricated and experimentally evaluated to explore the tradeoffs between materials, geometries, and interrogation parameters. The final objective is to produce a self-referencing device that detects small concentrations of bacterial pathogens. Combining the self-referencing sensor platform with oriented, instead of random, immobilization of antibodies using self-assembled monolayers (SAMs) will increase sensitivity for large analytes such as bacteria. The sensors will be experimentally optimized by varying SAM composition and antibody loading, and their performance will be evaluated using environmental samples, cell culture media, and food samples containing safe simulants for Escherichia coli and Bacillus anthracis. The intellectual merit of the proposed research rests on the first use of multiple surfaceplasmon modes to differentiate the presence of a target analyte from non-specific effects. For the first time, the electromagnetic field distributions and dispersion relations of surface plasmon waves supported by microstructures will be engineered to sense multiple biological and chemical processes. This new understanding of the response of various surface-plasmon modes to surface and bulk refractive index changes will allow one to design sensors that optimally distinguish specific and nonspecific effects. Combining this self-referencing approach with oriented immobilization of antibodies on the sensor surface will permit SPR to detect small concentrations of bacterial pathogens in real-world solutions. The project's broader impact begins with developing a new sensor platform that may better serve society's needs in drug discovery, medical diagnosis, food quality assurance, and bio-chemical defense. From an educational perspective, the project will provide graduate training in an inherently interdisciplinary field while strengthening the multi-disciplinary partnerships in the University of Kentucky's Center for Nanoscale Science and Engineering. The project will also serve as a demonstration platform for a diversity outreach effort targeting high-school students from geographically and economically underrepresented groups and for a new Micro- and Nano-Photonics undergraduate course offered in connection with the University of Kentucky's Nanoscale Engineering Certificate Program (NECP).