Development of Ultrasensitive Plasmonic Nanosensors
Nanostructures (NSs) based localized surface plasmon resonance (LSPR) sensors have brought a transformation in development of sensing devices due to their ability to detect extremely small changes in surrounding refractive index (R.I.). NS-based LSPR sensing approaches have been employed to enhance the sensitivity for a variety of applications, such as diagnosis of disease, food and environmental analysis, and chemical and biological threat detection. Generally in LSPR spectroscopy, absorption and scattering of light is greatly enhanced at a frequency that excites the NS’s LSPR and results in well-defined LSPR extinction peak (λLSPR). This λLSPR is highly dependent on the size, shape, and surrounding R.I. of NSs. Compositional and confirmational change within the surrounding R.I. near the NS could be detected by monitoring the shifts in λLSPR. This thesis specifically focuses on the rational development of the plasmonic nanosensors for various sensing applications by utilizing the LSPR properties of Au NS with prismatic shape. First the chemical synthetic approach that can produce Au nanoprisms, which displayed λLSPR in 650-850 nm range corresponding to 20-50 nm edge lengths has been developed. The chemically synthesized Au nanoprisms were attached to silanized glass substrate and employed as a solid-state sensing platform for the development of label-free plasmonic nanosensors. The size, shape, and surface of nanoprisms were characterized through transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-visible spectroscopy. Further, the influence of the structure, size and surface ligand chemistry onto the λ LSPR of nanoprisms were investigated in detail. Both bulk and local R.I. sensitivity, and the electromagnetic-field (EM-field) decay length were derived for various edge lengths of nanoprisms through measuring the λ LSPR shifts by UV-visible spectroscopy. Finally, nanoprisms-based LSPR nanosensors (“plasmonic nanosensors”) have been developed for different sensing applications. Specifically, these plasmonic nanosensors displayed capacity to detect streptavidine, glucose, microRNA (cancer biomarker), as well as molecular and stimuli-responsive polymers conformational changes. In this study we found that the plasmonic nanosensors are exceptionally sensitive compared to other NSs and the sensitivity is highly edge length dependent. An ultrasensitive plasmonic nanosensor has been developed for the detection of microRNAs in crude plasma collected from pancreatic cancer patients. It shows that the LSPR-based nanosensor has the ability to detect and quantify the microRNA concentrations in clinical samples without any purification. The results presented here show potential for patients to commence treatment in early stage cancer diagnosis. The effect of various physiological medias and edge length of nanoprisms on the sensitivity of this nanosensor has been discussed. Second, molecular sensors have been developed by functionalization of azobenzene molecule contain alkanethiols onto the nanoprisms surface. Molecular conformational changes basis on a very less dielectric thickness changes have been detected through λLSPR shift of nanoprisms and confirmed through surface enhanced Raman spectroscopy (SERS). In this study, the influence of resonance energy transfer between the molecule and nanoprisms onto the λ LSPR shift and Raman intensity has been investigated by changing the distance between them. Finally, utilization of stimuli-responsive polymers structural change in the development of stimuli-responsive such as pH and temperature-responsive plasmonic nanosensors has been demonstrated. It was found that the stimuli-responsive nanosensors were able to detect very small R.I. change due to the polymers structural change. The enzymatic reaction between glucose and glucose oxidase has been used to detect glucose in bovine plasma using pH-responsive nanosensor. Results of this work displays potential of replacing finger prick methodology in glucose self-monitoring for diabetes patients with use of plasma/urine samples. Overall, the research work demonstrated here provides a significant progress in the development of LSPR-based plasmonic nanosensors and addresses the resolution of many scientific complications, fundamental, chemical, and biological.
SARDAR, Purdue University.
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