Plasmon-Enhanced Optical Sensing by Engineering Metallic Nanostructures

The world’s booming population projected to reach 10 billion by 2050 causes enormous stresses on environmental safety, food supply, and healthcare, which in return threatens human civilizations. One of the most promising solutions lies at innovating point-of-care (POC) sensing technologies to conduct detection of environmental hazards, monitoring of food safety, and early diagnosis of diseases in a timely and accurate manner. The discovery of surface-enhanced spectroscopy in the 1970s has significantly stimulated research on light-matter interaction which gives rise to enhanced optical phenomena such as surface-enhanced Raman scattering (SERS), plasmon-enhanced fluorescence (PEF), and particularly, they have found enormous applications in optical sensing. To fully exploit surface-enhanced spectroscopy to advance sensing technologies, it requires innovations in the sensor design as well as the plasmonic metallic nanostructures, which is exactly the focus of this dissertation. Owing to their strong capabilities of revealing molecular fingerprints and conducting single molecule analysis, both SERS and PEF have received extensive research interests. Since SERS directly correlates with the local electromagnetic (EM) field enhancement, it is featured by the simplicity in signal amplification. However, high SERS spectral resolution cannot be achieved without a tightly focused laser beam, which compromises the design of SERS-based POC sensing platforms. In contrast, the emission nature of fluorescence makes PEF easily coupled with POC readers, but optimal PEF requires a delicate control of the separation distance between the fluorophore and the nanostructure to minimize fluorescence quenching. SERS and PEF are essentially two complementary techniques and both hold great promise for POC sensing technologies. In the dissertation, in the first place, two label-free SERS sensors have been developed aiming to reduce the number of elements used in a sensor, which could potentially minimize interference, reduce the cost, and enhance the performance. In this regard, a label-free SERS sensor for mercury ions (Hg2+) detection has been developed based on functionalized gold nanoparticles, which employs a small molecule 4-mercaptobenzoic acid to capture mercury ions. A coordination bond formed only in the presence of mercury ions produces a new SERS peak at 374 cm-1, allowing unique detection of mercury ions. The other label-free SERS sensor has been developed for nitrite (NO2-) detection following the mechanism of Griess reaction based on the plasmonic coupling between gold nanostars and silver nanopyramid arrays. A newly formed azo compound produces at least three characteristic SERS peaks at 1140 cm-1, 1389 cm-1, and 1434 cm-1, which allow a highly specific detection of nitrite. While label-free SERS sensing has proved effective to enhance the performance, the need for a tightly focused laser beam hinders SERS from being easily coupled with POC readers for rapid signal readout. To address this limitation of SERS, on-chip PEF sensors have been developed, which can be inserted into POC readers for rapid signal readout. Optimizing PEF usually requires a delicate control of the separation distance between the fluorophore and the nanostructure to balance the excitation and emission enhancement which have different distance dependence. In addition to the separation distance, scattering has been found to be strongly correlated with quantum efficiency enhancement, which has been established as another tuning parameter in optimizing PEF. By making PEF work in the near-infrared (NIR) biological transparency window, the strength of PEF is further manifested by its compatibility with biological matrix featured as low background interference and high penetration depth. As a proof of concept, a NIR fluorescent biosensor has been developed for detection of traumatic brain injury biomarker in the blood plasma. The selection of a gold nanopyramid array pattern as the sensing platform not only generates intense localized EM field for the excitation enhancement, but also allows all the tests to be conducted using a POC fluorescence reader. While noble metals such as gold and silver are often used in developing sensing technologies as they support strong localized surface plasmon resonance (LSPR), it remains an open question as whether they could be replaced by alternative inexpensive metals such as copper and aluminum without compromising the performance. The discovery of a strong and sharp LSPR on copper nanoparticles when the shape is made cubic strongly suggests this possibility. By means of a numeric and theoretical study, it is found that the observed LSPR on copper nanocubes originates from the corner mode which survives damping as it is spectrally separated from the interband transitions. Compared to the dipole mode of a gold nanosphere of the same volume, a copper nanocube displays a comparable extinction coefficient but a local EM field enhancement 7.2 times larger. Furthermore, a film-coupled copper nanocube system has been designed for plasmon-enhanced NIR fluorescence. Because of the coupling between the copper nanocube and the underlying film, a plasmonic cavity mode is generated and featured as a spectrally tunable LSPR and an intense local EM field. By tailoring the resonance to the NIR wavelength region, the film-coupled copper nanocube system has been demonstrated to support a large NIR fluorescence enhancement owing to the strong excitation enhancement and the quantum efficiency enhancement.