Surface-enhanced Raman scattering (SERS) has long been the interest of researchers in chemistry, physics and engineering, especially since the discovery that SERS can probe into the system down to the single molecule (SM) level. Despite the large number of publications regarding the fabrication of SERS substrates, it has been a challenge in the field to quantify the SERS signal and universally compare substrates. Traditionally, enhancement factor (EF) is used as an indicator of substrate quality, but the EF calculation is hugely dependent on the estimation of the surface coverage and other factors that are determined largely subjectively. Therefore, this thesis aims at discussing other parameters that can also be used to evaluate different substrates. Six different SERS substrates of Ag or Au nanoparticles of different sizes were fabricated by nanosphere lithography (NSL) and characterized by electron microscopy and UV-vis spectroscopy. SERS substrates were mapped for different concentrations of a probe molecule. Through subsequent baseline correction and principle component analysis (PCA), the "intensity" of individual spectrum was obtained and the shapes of intensity histograms of each substrate were acquired. Instead of calculating EF, five criteria (six quantification methods in total) were employed to comprehensively evaluate the six substrates. These were density of hot spots (characterized by the number of zero-intensity events), enhancement (represented by mean intensity), spatial variation (calculated by RSD of intensity), repeatability (realized by cross correlation) and histogram shape (quantified by skewness and kurtosis). These new methods provide insights to the understanding of the properties of SERS substrates in terms of hot spots. Different substrates may exhibit better performance in terms of one criterion but worse in terms of others. Those variations in performance can be explained by their surface morphology. These more elaborated methods are believed to provide a more comprehensive approach to evaluate and compare substrates than the traditional EF values. The thesis also paves the way for future study on SM-SERS and fabricating better SERS substrates. / Graduate

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references. Also available in electronic version.

This thesis focuses on the analytical properties of the acoustic vibrations and applications of metallic nanoparticles. With regard to the analytical properties of the acoustic vibrations, we focus on nanoparticle acoustic resonance enhanced four-wave mixing (FWM) as an in situ characterization technique for characterizing nanoparticles’ shape, size, and size distribution. The nonlinear optical response of metallic nanoparticles is resonantly driven by the electrostriction force which couples to the acoustic vibrations of nanoparticles. Information about nanoparticles’ shape, size, and size distribution can be obtained by analyzing the resonant peak position and linewidth in the FWM signal which carries the information about the vibrational modes. We characterize different nanoparticle solutions of different materials, shapes, and sizes using this FWM technique. Information obtained from the FWM characterization agrees well with the scanning electron microscopic examination, indicating the FWM technique can serve as an in situ nanoparticle characterization tool. We also demonstrate the FWM technique can be used for monitoring nanoparticle growth in situ. iii With regard to the applications of metallic nanoparticles, we focus on quantification of an exogenous cancer biomarker Acetyl Amantadine using surface-enhanced Raman scattering (SERS). Raman spectroscopy can provide unique fingerprint information of molecules, which can be used as a chemical detection and identification technique. The intrinsically weak Raman signal caused by the small scattering cross section presents a barrier for trace chemical detection. Localized surface plasmon resonance of metallic nanoparticles can provide large local field enhancement, which can be utilized to enhance the intrinsically weak Raman signal. In order to achieve higher local field enhancement, we focus on using the gap structures formed between nanoparticles instead of using discrete nanoparticles. Molecules should locate within the hot spots of the gap structures to experience the largest enhancement. This requires that molecules should be extracted from volume onto the metallic surface. Based on these guidelines, two SERS platforms are designed using gold nanoparticles (nanorods and nanospheres) combined with different surface functionalization techniques. The performance of these two platforms are characterized by investigating the sensitivity and limit of detection (LOD). 16 ng/mL and 0.4 ng/mL LODs are achieved for nanorod and nanosphere platforms, respectively. / Graduate

Surface enhanced Raman scattering (SERS) has achieved much attention since its conception in 1974. The analytical technique overcomes many difficulties associated with conventional Raman whilst also increasing sensitivity. However, the increased interest and work in the field has also identified flaws, many of which are centred on the irreproducibility of the SERS enhancement effect. The majority of the work described in this thesis focusses on the ‘optimisation’ of solid-state and solution based SERS systems. Optimisation plays a crucial role in maximising both enhancement effects and reproducibility. Here criteria are outlined for the synthesis of high performance solid-state SERS substrates and the synthesis of a range of substrates is assessed, each with associated pros and cons. The most successful substrate was synthesised by exploiting redox potentials which allow for the direct deposition of silver onto copper foil. The deposition times and temperatures were optimised sequentially to generate a high performance substrate capable of detecting Rhodamine 6G at trace levels. Reproducibility comparisons of the silver on copper (SoC) substrate were carried out against commercial substrates: Klarite and QSERS, multiple univariate and multivariate methods were used to assess the substrates performance. The results confirmed that the SoC substrate performed better than both the commercial substrates. The work also highlights the importance of using multiple data analysis methods in order to assess the performance of a solid-state SERS substrate. Deposition of the silver surface was also successful on British 2p coins allowing the for the detection and discrimination of illegal and legal drugs when coupled with multivariate data analysis methods such as PCA and PLS. Solution based SERS analyses were also carried out successfully using different optimisation strategies. The initial investigation involved careful control of the individual components of a SERS system (nanoparticles, aggregating agents and analyte) in order to establish a low limit of detection for the increasingly abused ‘legal high’ MDAI. The use of a reduced factorial design was then successfully employed to explore a greater number of SERS variables and define a low limit of detection for the class B drug mephedrone. The robust experimental design also allowed an insight into the importance of each of the individual components within a solution based SERS system. The final piece of work carried out was the SERS discrimination of antibiotics: ampicillin, ticarcillin and carbenicillin. Optimisation of the solution based experiment allowed the in-situ hydrolysis of the β-lactam moiety present in ampicillin rendering it pharmacologically inactive to be followed under acidic conditions at concentrations of 10 ppm.

Thesis (Ph.D.)--Boston University / The large electromagnetic field enhancement provided by nanostructured noble metal surfaces forms the foundation for a series of enabling optical analytical techniques, such as surface enhanced Raman spectroscopy (SERS), surface enhanced IR absorption spectroscopy (SEIRA), surface enhanced fluorescent microscopy (SEF), to name only a few. Critical sensing applications have, however, other substrate requirements than mere peak signal enhancement. The substrate needs to be reliable, provide reproducible signal enhancements, and be amenable to a combination with microfluidic chips or other integrated sensor platforms. These needs motivate the development of engineerable SERS substrate "chips" with defined near- and far-field responses. In this dissertation, two types of rationally designed SERS substrates - nanoparticle cluster arrays (NCAs) and SERS stamp - will be introduced and characterized. NCAs were fabricated through a newly developed template guided self-assembly fabrication approach, in which chemically synthesized nanoparticles are integrated into predefined patterns using a hybrid top-down/bottom-up approach. Since this method relies on chemically defined building blocks, it can overcome the resolution limit of conventional lithographical methods and facilitates higher structural complexity. NCAs sustain near-field interactions within individual clusters as well as between entire neighboring clusters and create a multi-scale cascaded E-field enhancement throughout the entire array. SERS stamps were generated using an oblique angle metal deposition on a lithographically defined piston. When mounted on a nanopositioning stage, the SERS stamps were enabled to contact biological surfaces with pristine nanostructured metal surfaces for a label-free spectroscopic characterization. The developed engineered substrates were applied and tested in critical sensing applications, including the ultratrace detection of explosive vapors, the rapid discrimination of bacterial pathogens, and the label-free monitoring of the enzymatic degradation of pericellular matrices of cancer cells.

Plasmonics can achieve the tight optical confinement and localization in the subwavelength domain. Surface plasmon polaritons (SPPs) are closely related to coupling to emitters in excitation and emission, waveguiding, and active modulating on the nanoscale. Due to these phenomenon, plasmonic nanostructures can be used for applications, such as light emission, photodetection, optical sensing, and spectroscopy. Conventional plasmonic nanostructures can support plasmonic modes, and it is typically optimized for a single wavelength window with planar plasmonic structures. Recent studies have reported some in-plane composite nanostructures and core-shell geometries can induce multiple plasmonic responses. However, it is challenging to achieve the control of individual plasmonic response due to the interdependent spectral tunability with changes in their in-plane geometries. In this dissertation, the concept of out-of-plane engineered nanoantenna structures is introduced, numerically calculated, and experimentally demonstrated. The nanolaminated MIM plasmonic structures show multiresonant plasmonic responses in the same antenna and each wavelength band can be tunable individually with different thicknesses of dielectric layers. The nanolaminated plasmonic structures has been reported for a scalable Surface-enhanced Raman spectroscopy (SERS) substrate for single-molecule sensitive and label-free chemical analysis. Due to the strong optical field confinement, the nanolaminated SERS substrates achieve increased SERS enhancement factor (EF) up to 1.6 x 108 with proper partial etching of dielectric layers. Furthermore, the nanolaminated MIM plasmonic structures have been successfully integrated with micro-scale pillar arrays to control the surface wettability for ultrasensitive SERS measurements. The hierarchical micro/nano plasmonic surface has densely packed intrinsic SERS-active hot spots that give rise to SERS EFs exceeding 107. This platform can take full advantage of low surface energy to control and measure the analyte in water droplets. Leidenfrost evaporation-assisted SERS sensing on the hierarchical substrates provides the way for ultrafast and ultrasensitive biochemical detections without a need for additional surface modifications and chemical treatments. / Doctor of Philosophy / The life in the 21th century has benefited from the technical revolutions of computational power that is based on the manipulation/storage of electrons. As predicted in Moore's law, the size of electronic microchip would go down, and the computational power has been enhanced due to the increase of transistor integration density. However, the two major factors, such as energy dissipation of electrons and signal delay of electronic circuit, limit the communication speed of electronics. These barriers have caused slowdown in the performance of computational power. Photonic solutions have been offered to solve the limitations based on the larger bandwidth and a rare energy dissipation, compared to electronic counterparts. Moreover, optical communications typically demand much lighter channel to deliver similar power/information than practical electrical cables do. Thus, light manipulation/enhancement techniques are envisioned to overcome the limitations and guide to the methodology of interconnections between the electronic circuits and optical platforms. Plasmonics can achieve the nanoscale light confinement and localization in the subwavelength domain. This strong confinement is originated from the coupling between the photons and the electron gas on the metal that results in surface plasmon polariton (SPP). SPPs are closely related to coupling to emitters in excitation and emission, waveguiding, and active modulating on the nanoscale. Due to these phenomenon, plasmonic nanostructures can be used for applications, such as light emission, photodetection, optical sensing, and spectroscopy. In this dissertation, the concept of out-of-plane engineered nanoantenna structures is introduced, numerically calculated, and experimentally demonstrated. This vertically stacked nanoantenna structure is composed of metal-insulator-metal (MIM) laminates fabricated by physical vapor deposition techniques. Although conventional plasmonic nanostructures can support plasmonic modes, it is typically optimized for a single wavelength window. The nanolaminated MIM nanostructures, by contrast, can induce multiresonant plasmonic response in the same antenna with several advantages: (1) reduced individual footprint size and volume of nanoantenna, (2) accurate control of layer thicknesses by thin film deposition technique for resonance tuning, (3) easier integration with other functional materials as gap layers, and (4) efficient transport of charge carriers or heat in nanolaminated layers. As a result of the tight optical field confinement, the nanolaminated plasmonic structures can be used for sensing application called Surface-enhanced Raman spectroscopy (SERS), which is a promising sensing platform for label-free biochemical analysis at the single-molecule level. Partial oxide etching process enables the analyte molecules to accommodate in strong enhancement region of the nanolaminated structures, resulting in amplified unique Raman features of molecular compounds as a finger print. The SERS enhancement factor is increased by one order of magnitude achieving 1.6x108. Furthermore, the nanolaminated plasmonic structures have been integrated with micro-scale pillar arrays to control the surface wettability for ultrasensitive SERS measurements.

Collagen fibrils are the main constituent of the extracellular matrix surrounding eukaryotic cells. Even though the assembly and structure of collagen fibrils is well characterized, very little is known about the physico-chemical properties of their surface which is one of the key determinants of their biological functions. One way to obtain surface sensitive structural and chemical data is to take advantage of the near field nature of surface and tip-enhanced Raman spectroscopy. Using Ag and Au nanoparticles bound to collagen type I fibrils, as well as tips coated with a Ag nanoparticles and a thin layer of Ag, we obtained Raman spectra characteristic of the first layer of collagen molecules at the surface of the fibrils. The most frequent Raman peaks were attributed to aromatic residues such as phenylalanine and tyrosine. We also observed in several instances Amide I bands with a full width at half maximum of 10-30 cm-1. The assignment of these Amide I bands positions suggests the presence of collagen-helices as well as alpha-helices and beta-sheets at the fibril’s surface. As a step towards in vivo characterization of collagen fibrils, fascicles removed from tendons were also examined with surface-enhanced Raman spectroscopy.

Detecting lung cancer is di cult as it is hidden in the body, and current clinical methods are not elective at an early stage; the one-year survival rate after diagnosis in the World is just 29-33%. Acetyl amantadine (AcAm) is recognised as an exogeneous cancer biomarker because it is the product of a metabolic process known to be significantly up-regulated in cancerous cells. After ingestion, the an-tiparkinson and antiviral drug amantadine is acetylated in the body by the enzyme spermidine/spermine N1 acetyltransferase to give AcAm, which can be detected in patient’s urine. However, techniques previously used to quantify AcAm in urine, such as liquid chromatography-mass spectrometry (LC-MS), are undesirable for clin- ical adoption due to high costs and long run times. Further costs and delays result from the requirement for solid phase extraction (SPE). Therefore, it is highly desired to lower the costs and delays in processing by exploring different quantification approaches, ideally without the need for SPE processing. In this thesis, I investigate the use of surface enhanced Raman spectroscopy (SERS) to quantify AcAm in urinalysis. I prepare two kinds of Raman substrates with hydrophobic pocket surface capture agents beta -cyclodextrin (beta -CD) that work to extract the AcAm from the urine, followed by the surface enhanced Raman measurement using two kinds of Raman systems. The detection strategy is more economical than the currently used LC-MS approach, and enables development of an easy-to-use point-of-care tool that should provide a more rapid turnaround to the health care provider. The next step will be to use real samples. If it is achieved, it will be a promising step in early cancer diagnostics. / Graduate

Due to a lack of fundamental knowledge about the role of molecular structures in molecular electronic devices, this research is focused on the development of instruments to understand the relation between device design and the electronic properties of electroactive components. The overall goal is to apply this insight to obtain a more efficient and reliable scheme and greater functional control over each component. This work developed a fabrication method for porphyrinoids on graphene-based field effect transistors (FETs), and a chemical sensing platform under an ambient environment by integrating a tip-enhanced Raman spectroscope (TERS), atomic force microscope (AFM), and electronic testing circuit. The study is divided into three aspects. The first is aimed at demonstrating fabrication processes of nanoscale FETs of graphene and porphyrinoid composites based entirely on scanning probe lithography (SPL). A nanoshaving mechanism was used to define patterns on octadecanethiol self-assembled monolayers on gold film evaporated on graphene flakes, followed by metal wet etching and/or oxygen plasma etching to develop patterns on Au films and graphene, respectively. The integrity and optoelectronic properties were examined to validate the processes. The second area of study focused on the development of the chemical sensing platform, enabling chemical changes to be monitored during charge transports under an ambient environment. The localized Raman enhancement was induced by exciting surface plasmon resonance in nanoscale silver enhancing probes made by thermal silver evaporation on sharp AFM tips. As the system was designed along an off-axis illumination/collection scheme, it was demonstrated that it was capable of observing molecular decomposition on opaque and conductive substrates induced by an electric bias. The third line of work proposed a novel TERS system and a probe preparation method. Silver nanowires mounted on AFM tips were used to locally enhance the Raman scattering. The observed Raman enhancement allows quick chemical analysis from a nanoscale region, and thus enables chemical mapping beyond the diffraction limit. Compared with other TERS geometries, the new optical design not only allows analysis on large or opaque samples, but also simplifies the design of the optical components and the alignment processes of the setup.