Droplets are ubiquitous microscopic systems - ranging in size from several nanometers to ~100 micrometers – that undergo abundant environmental interactions. Researchers have shown that droplets can impact both earth climate and air quality through physical and chemical processes. Droplets released from the human respiratory system, either suspended in air or deposited on surfaces, can carry pathogens (e.g., influenza viruses, the SARS-CoV-2 virus), and are thus important for disease transmission. The need to understand the role of droplets in environmental processes requires appropriate tools for droplet characterization. We used Raman and surface-enhanced Raman spectroscopy (SERS) based imaging as such tools due to their capacity for simultaneous collection of abundant molecular information inside droplets and their potential to collect detailed images of droplet component distributions. We imaged pH and chemical moiety distributions inside droplets over a wide range of: 1) droplet compositions; 2) surrounding environmental conditions (relative humidity, temperature); and 3) droplet morphologies. This dissertation describes measurement of droplet pH in droplets containing mixtures of phosphate buffer (PB), one of the most commonly used biological solvents, and ammonium sulfate (AS), arguably the most abundant chemical species in atmospheric droplets, at room temperature. We observed a pH gradient inside PB droplets while a homogeneous pH distribution was found inside AS droplets, thus showing a significant pH effect due to droplet composition. We attributed the contrasting pH distribution in the two droplet systems to different ionic interactions at the air-water interface. In addition, we obtained AS droplet images at 223K to investigate ice nucleation upon freezing. We observed variable nucleation behavior in AS droplets as a function of concentration, a finding with implications for atmospheric cloud nucleation. We also investigated virus deposition during sessile droplet evaporation using gold nanoparticles. SERS imaging enabled development of correlations between virus viability and droplet deposition pattern and related them in terms of the coffee-ring effect. Suppression of the coffee-ring effect can reduce virus infectivity on surfaces during droplet evaporation. These works collectively exhibit the potential of Raman and SERS imaging for droplet characterization. / Doctor of Philosophy / Droplets are ubiquitous in the environment. Small droplets can form clouds and fogs, and are often micro- to nano-scale in size. Droplets can either grow or shrink in the environment when they absorb or lose water. Similarly, reactions may happen when droplets contain various species. Droplets in human breath exhalate may contain pathogens, such as the SARS-CoV-2 virus that is the cause of the COVID-19 pandemic. If the virus stays viable in droplets, no matter where the droplets are located, the virus will remain infectious and may be transmitted to others through contact. The studies in this dissertation were conducted to determine the distributions of soluble and insoluble components inside droplets and to elucidate how the observed distributions correlate with important droplet properties and environmental processes. We used two methods to observe droplets: Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS). Molecules are constantly vibrating, these vibrations result in characteristic Raman signals that can be monitored. Both Raman and SERS provide such measurements, except that SERS has greater sensitivity due to the signal enhancement provided by gold or silver nanoparticles. In this dissertation, we obtained images of droplets with variable compositions at both room temperature and -50 °C. We also examined virus survival inside droplets during droplet drying. Using the collected images, we related the component distribution inside a droplet to its acidity, and evaluated virus survival in terms of droplet drying patterns. The images demonstrate that Raman and SERS imaging are promising tools for the study of droplets.

Optofluidic sensing utilizes the advantages of both microfluidic and optical science to achieve tunable and reconfigurable high-performance sensing purpose, which has established itself as a new and dynamic research field for exciting developments at the interface of photonics, microfluidics, and the life sciences. With the trend of developing miniaturized electronic devices and integrating multi-functional units on lab-on-a-chip instruments, more and more desires request for novel and powerful approaches to integrating optical elements and fluids on the same chip-scale system in recent years. By taking advantage of the electrowetting phenomenon, the wettability of liquid droplet on micro/nano-structured surfaces and the Leidenfrost effect, this doctoral research focuses on developing high-performance optofluidic sensing systems, including optical beam adaptive steering, whispering gallery mode (WGM) optical sensing, and surface-enhanced Raman spectroscopy (SERS) sensing. A watermill-like beam steering system is developed that can adaptively guide concentrating optical beam to targeted receivers. The system comprises a liquid droplet actuation mechanism based on electrowetting-on-dielectric, a superlattice-structured rotation hub, and an enhanced optical reflecting membrane. The specular reflector can be adaptively tuned within the lateral orientation of 360°, and the steering speed can reach ~353.5°/s. This work demonstrates the feasibility of driving a macro-size solid structure with liquid microdroplets, opening a new avenue for developing reconfigurable components such as optical switches in next-generation sensor network. Furthermore, the WGM sensing system is demonstrated to be stimulated along the meridian plane of a liquid microdroplet, instead of equatorial plane, resting on a properly designed nanostructured chip surface. The unavoidable deformation along the meridian rim of the sessile microdroplet can be controlled and regulated by tailoring the nanopillar structures and their associated hydrophobicity. The nanostructured superhydrophobic chip surface and its impact on the microdroplet morphology are modeled by Surface Evolver (SE), which is subsequently validated by the Cassie-Wenzel theory of wetting. The influence of the microdroplet morphology on the optical characteristics of WGMs is further numerically studied using the Finite-Difference Time-Domain method (FDTD) and it is found that meridian WGMs with intrinsic quality factor Q exceeding 104 can exist. Importantly, such meridian WGMs can be efficiently excited by a waveguiding structure embedded in the planar chip, which could significantly reduce the overall system complexity by eliminating conventional mechanical coupling parts. Our simulation results also demonstrate that this optofluidic resonator can achieve a sensitivity as high as 530 nm/RIU. This on-chip coupling scheme could pave the way for developing lab-on-a-chip resonators for high-resolution sensing of trace analytes in various applications ranging from chemical detections, biological reaction processes to environmental protection. Lastly, this research reports a new type of high-performance SERS substrate with nanolaminated plasmonic nanostructures patterned on a hierarchical micro/nanostructured surface, which demonstrates SERS enhancement factor as high as 1.8 x 107. Different from the current SERS substrates which heavily relies on durability-poor surface structure modifications and various chemical coatings on the platform surfaces which can deteriorate the SERS enhancement factor (EF) as the coating materials may block hot spots, the Leidenfrost effect-inspired evaporation approach is proposed to minimize the analyte deposition area and maximize the analyte concentration on the SERS sensing substrate. By intentionally regulating the temperature of the SERS substrate during evaporation process, the Rhodamine 6G (R6G) molecules inside a droplet with an initial concentration of 10-9 M is deposited within an area of 450 μm2, and can be successfully detected with a practical detection time of 0.1 s and a low excitation power of 1.3 mW. / Doctor of Philosophy / Over the past two decades, optofluidics has emerged and established itself as a new and exciting research field for novel sensing technique development at the intersection of photonics, microfluidics and the life sciences. The strong desire for developing miniaturized lab-on-a-chip devices and instruments has led to novel and powerful approaches to integrating optical elements and fluids on the same chip-scale systems. By taking advantage of the electrowetting phenomenon, the wettability of liquid droplet on micro/nano-structured surfaces and the Leidenfrost effect, this doctoral program focuses on developing high-performance optofluidic sensing systems, including optical beam adaptive steering, whispering gallery mode (WGM) optical sensing, and surface-enhanced Raman spectroscopy (SERS) sensing. During this doctoral program, a rotary electrowetting-on-dielectric (EWOD) beam steering system was first fabricated and developed with a wide lateral steering range of 360° and a fast steering speed of 353.5°/s, which can be applied in telecommunication systems or lidar systems. Next, the meridian WGM optical sensing system was optically simulated using finite difference time domain (FDTD) method and was numerically validated to achieve a high quality-factor Q exceeding 104 and a high refractive index sensitivity of 530 nm/RIU, which can be applied to the broad areas of liquid identification or single molecule detection. Lastly, a SERS sensing platform based on a hierarchical micro/nano-structured surface was accomplished to exhibit a decent SERS enhancement factor (EF) of 1.81 x 107. The contact angle of water droplet on the SERS substrate is 134° with contact angle hysteresis of ~32°. Therefore, by carefully controlling the SERS surface temperature, we employed Leidenfrost evaporation to concentrate the analytes within an extremely small region, enabling the high-resolution detection of analytes with an ultra-low concentration of ~10-9 M.

Gold nanoclusters possess both theoretical and practical importance in the development of ultrasensitive biosensors based on surface-enhanced Raman spectroscopy (SERS). Manipulation of gold nanoclusters in a predictable and reproducible manner for the application of refined biochemical analysis still remains challenging. In this study, high-purity gold nanoclusters are isolated via a simple method based on density gradient centrifugation. Three distinct bands including monomers, small aggregates (2-4 nanospheres), and large aggregates (>5 nanospheres) can be separated via density gradient centrifugation. The isolated gold nanoclusters greatly enhance the Raman intensity of the trapped dye molecules such that single nanocluster detection is feasible. To develop a gold nanoparticle-based biosensor for influenza virus, effort was also made to modify recognition moieties such as aptamers to gold nanoparticles via distinct approaches. The increase of hydraulic diameter and the shift of optical absorbance spectrum indicate the success of surface modification to gold nanoparticles. / Master of Science

Pathogenic microbial contamination of the environment poses a significant threat to human health. Accordingly, microbial surveillance is needed to ensure safe drinking water and air quality. Current analytical methods for microbes are generally either culture-based, gene amplification-based, or sequencing-based. However, these approaches require centralized facilities, well-trained personnel, and specialized instruments that result in high costs and long turnaround times. Surface-enhanced Raman spectroscopy (SERS)-based techniques have been proposed to overcome these limitations. In this dissertation, we discuss work conducted to develop novel SERS-based methods to enable both sensitive microbial quantification and analysis of the interactions of pathogens, their hosts, and the surrounding environment. We first developed a labeled SERS-based lateral flow test for virus quantification. Optimization of the lateral flow design and digital signal analysis enabled high sensitivity towards SARS-CoV-2. To elicit a comprehensive understanding of pathogen infection, label-free living-cell SERS sensors were engineered by incubating host cells with nanoparticles. SERS spectral changes in host cellular components and metabolites during infection were used for viral quantification and offered inherent insights into the temporal and spatial molecular-level mechanisms of infection. These biosensors were validated using bacteriophage Phi6 and then developed for infectious H1N1 influenza. To understand microbial survival in the environment, living-cell SERS methods were applied under various conditions. Results showed cell inactivation and antibiotic treatment induced significant cellular and metabolic responses in the living whole-cell sensors, implying their potential applicability to various environmental conditions. Our research achieves rapid and on-site pathogen quantification and infection mechanism identification. / Doctor of Philosophy / Pathogenic microbes, such as the SARS-CoV-2 virus, can spread through air and water and are potentially harmful to human health. Monitoring the concentrations of these microbes in the environment is crucial to track their presence and provide an early warning of their spread. Unfortunately, current microbial detection methods are often expensive and take a long time since they typically require professional facilities and expert elicitation. Our research relies on a technique called surface-enhanced Raman spectroscopy (SERS) to address these challenges. SERS enables identification and quantification of microbes by analyzing specific features (i.e., peak position, peak intensity) in the spectra. We first applied this technique by modifying a commercial SARS-CoV-2 antigen test kit with a label molecule that provides SERS signals. We achieve accurate and sensitive quantification, even in the presence of high levels of environmental interference. To better understand how these harmful microbes interact with our bodies, we developed sensors that can measure SERS signal changes in host cells before and after infection. These sensors were tested using the bacteriophage virus Phi6 that infects bacteria and infectious H1N1 influenza virus. Furthermore, we applied these sensors to study how bacteria respond to different environmental conditions, providing valuable insights into their survival and behavior under various conditions. In summary, our research introduces methods that are more accessible to identify and quantify harmful microbes that can be potentially used by the general public. The methods provide us with molecular level understanding of pathogen interactions with humans and the environment.

Fast, sensitive, quantitative, and low-cost analysis of environmental pollutants is highly valuable for environmental monitoring. Due to its single-molecule sensitivity and fingerprint specificity, surface-enhanced Raman spectroscopy (SERS) has been widely employed for heavy metal, organic compound, and pathogen detection. However, SERS quantitation is challenging because 1) analytes do not stay in the strongest enhancing region ("hot spots") and 2) SERS reproducibility is poor. In this dissertation, gold nanoparticle/bacterial cellulose (AuNP/BC) substrates were developed to improve SERS sensitivity by increasing hot spot density within the laser excitation volume. Environmentally relevant organic amines were fixed at "hot spots" by lowering solution pH below the analyte pKa and thus enabling SERS quantitation. In addition, a new SERS internal standard was developed based upon the electromagnetic enhancement mechanism that relates Rayleigh (elastic) and Raman (in-elastic) scattering. Rayleigh scattering arising from the amplified spontaneous emission of the excitation laser was employed as a normalization factor to minimize the inherent SERS signal variation caused by the heterogeneous distribution of "hot spots" across a SERS substrate. This highly novel technique, hot spot-normalized SERS (HSNSERS), was subsequently applied to evaluate the efficiency of SERS substrates, provide in situ monitoring of ligand exchange kinetics on the AuNP surface, and to reveal the relationship between the pKa of aromatic amines and their affinity to citrate-coated AuNPs (cit-AuNPs). Finally, colloidally stable stable pH nanoprobes were synthesized using co-solvent mediated AuNP aggregation and subsequent coating of poly(ethylene) glycol (PEG). These nanoprobes were applied for pH detection in cancer cells and in phosphate buffered aerosol droplets. The latter experiments suggest that stable pH gradients exist in aerosol droplets. / PHD / Traditional analytical methods, such as gas chromatography/mass spectroscopy, liquid chromatography/mass spectroscopy, etc., cannot meet the demand for rapid screening of target environmental pollutants in drinking water. This issue arises due to the requirements for time-consuming sample pre-treatment, well-trained experts, complex instrumental parameter optimization, and scale challenges that limit onsite measurement. Surface-enhanced Raman spectroscopy is a promising approach to overcome these limitations. To improve SERS quantitation, surface-enhanced elastic scattering was developed as a novel internal standard to account for the SERS signal variation caused by substrate heterogeneity (“hot spot” normalization). Compared with traditional SERS internal standards, using scattered light as an internal standard reduces cost, time, interference, and experimental complexity for SERS detection. With this novel approach, the kinetics of adsorption/desorption of guest ligands/citrate onto/from the AuNP surface were quantified in situ and in real time. In addition, the SERS intensities of organic amines acquired at different solution pH values were differentiated using “hot spot” normalization, which revealed the relationship between aromatic amine pK_a and their affinity to the AuNP surface. Finally, the chemistry in confined aqueous environments, such as aerosol droplets, membrane channels, and cells, is challenging to probe using conventional analytical tools due to their inaccessible small volumes. To address this problem, SERS pH nanoprobes were synthesized and used to detect the pH inside cancer cells and micrometer-sized aerosol droplets.

Many biological processes, pathogens, and pharmaceuticals act upon, cellular membranes. Accordingly, cell membrane mimics are attractive targets for biosensing, with research, pathology, and pharmacology applications. Lipid bilayers represent a versatile sensor functionalization platform providing antifouling properties, and many receptor integration options, uniquely including transmembrane proteins. Bilayer-coated sensors enable the kinetic characterization of membrane/analyte interactions. Addressed theoretically and experimentally is the self-assembly of model membranes on plasmonic sensors. Two categories of plasmonic sensors are studied in two parts. Part I aims to deposit raft-forming bilayers on planar nanoaperture arrays suitable for multiplexing and device integration. By vesicle fusion, planar bilayers are self-assembled on thiol-acid modified flame-annealed gold without the need for specific lipid head-group requirements. Identification of coexisting lipid phases is accomplished by AFM imaging and force spectroscopy mapping. These methods are successfully extended to metallic, plasmon-active nanohole arrays, nanoslit arrays and annular aperture arrays, with coexisting phases observed among the holes. Vis-NIR transmission spectra of the arrays are measured before and after deposition, indicating bilayer detection. Finally, the extraction of membrane proteins from cell cultures and incorporation into model supported bilayers is demonstrated. These natural membrane proteins potentially act as lipid-bound surface receptors. Part II aims to encapsulate in model lipid bilayers, metallic nanoparticles, which are used as probes in surface enhanced Raman spectroscopy. Three strategies of encapsulating particles, and incorporating Raman-active dyes are demonstrated, each using a different dye: malachite green, rhodamine-PE, and Tryptophan. Dye incorporation is verified by SERS and the bilayer is visualized and measured by TEM, with support from DLS and UV-Vis spectroscopy. In both parts, lipid-coated sensors are successfully fabricated and characterized. These results represent important and novel solutions to the functionalization of plasmonic surfaces with biologically relevant cell membrane mimics.

Plasmonics techniques, such as surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS), have been widely used for chemical and biochemical sensing applications. One approach to excite surface plasmons is through the coupling of light into metallic grating nanostructures. Those grating nanostructures can be fabricated using state-of-the-art nanofabrication methods. Laser interference lithography (LIL) is one of those methods that allow the rapid fabrication of nanostructures with a high-throughput. In this thesis, LIL was combined with other microfabrication techniques, such as photolithography and template stripping, to fabricate different types of plasmonic sensors. Firstly, template stripping was applied to transfer LIL-fabricated patterns of one-dimensional nanogratings onto planar supports (e.g., glass slides and plane-cut optical fiber tips). A thin adhesive layer of epoxy resin was used to facilitate the transfer. The UV-Vis spectroscopic response of the nanogratings supported on glass slides demonstrated a strong dependency on the polarization of the incident light. The bulk refractive index sensitivities of the glass-supported nanogratings were dependent on the type of metal (Ag or Au) and the thickness of the metal film. The described methodology provided an efficient low-cost fabrication alternative to produce metallic nanostructures for plasmonic chemical sensing applications. Secondly, we demonstrated a versatile procedure (LIL either alone or combined with traditional laser photolithography) to prepare both large area (i.e. one inch2) and microarrays (μarrays) of metallic gratings structures capable of supporting SPR excitation (and SERS). The fabrication procedure was simple, high-throughput, and reproducible, with less than 5 % array-to-array variations in geometrical properties. The nanostructured gold μarrays were integrated on a chip for SERS detection of ppm-level of 8-quinolinol, an emerging water-borne pharmaceutical contaminant. Lastly, the LIL-fabricated large area nanogratings have been applied for SERS detection of the mixtures of quinolone antibiotics, enrofloxacin, an approved veterinary antibiotic, and one of its active metabolite, ciprofloxacin. The quantification of these analytes (enrofloxacin and ciprofloxacin) in aqueous mixtures were achieved by employing chemometric analysis. The limit of quantification of the method described in this work is in the ppm-level, with <10 % SERS spatial variation. Isotope-edited internal calibration method was attempted to improve the accuracy and reproducibility of the SERS methodology. / Graduate / 2018-02-17

Corantes orgânicos são uma importante classe de poluentes ambientais, sendo que a natureza da interação de tais espécies com ácidos húmicos determina seu comportamento e destino ambiental. Este trabalho investiga abordagens diferentes para a síntese de nanopartículas de ouro-ácido húmico (NPs Au/AH), o desempenho destas partículas como sensores para a determinação de traço de corantes orgânicos e da natureza da sua interação química com ácidos húmicos por técnicas de espectroscopia Raman intensificada por superfície (surface-enhanced Raman scattering, SERS). As nanopartículas de ouro foram obtidas de forma direta em meio aquoso. A síntese foi realizada na presença de ácido húmico em vários valores de pH e na presença e na ausência de citrato de sódio. Nesta abordagem, o ácido húmico apresenta propósitos diferentes. Na síntese das nanopartículas, ele serve como agente redutor e para a estabilização da superfície, impedindo a coalescência das nanopartículas em meio aquoso. Considerando-se a utilização de nanopartículas como substratos SERS-ativos, o ácido húmico serve como uma fase de extração associada com a plataforma SERS, favorecendo a interação dos corantes orgânicos com as nanopartículas metálicas. Esta abordagem pode ser utilizada para aumentar a sensibilidade e seletividade da técnica SERS e evitar a interferência de outras espécies em solução. É também importante mencionar que o ácido húmico dá origem a um espectro Raman muito fraco e, portanto, não interfere de forma significativa na detecção espectroscópica das espécies de interesse. As NPs Au/AH obtidas foram caracterizadas por espectroscopia eletrônica, microscopia eletrônica de varredura e SERS, a fim de estabelecer uma correlação entre a sua morfologia, plasmon superficial, e seu uso potencial como substratos SERS. As NPs Au/AH foram utilizadas como substratos SERS no estudo de corantes têxteis aniônicos e catiônicos. Os espectros Raman, nestes casos, foram atribuídos ao complexo formado pelo ácido húmico e cada uma das espécies de interesse. Os resultados obtidos revelaram uma dependência significativa entre as propriedades morfológicas e eletrônicas das NPs Au/AH com o procedimento de síntese, e também uma interação preferencial entre as nanopartículas e os corantes catiônicos. Conclui-se que as NPs Au/AH podem proporcionar uma abordagem útil para a caracterização espectroscópica de espécies relevantes para o ambiente e sua interação química com os ácidos húmicos, através do uso da espectroscopia Raman intensificada pela superfície / Organic dyes are an important class on environmental pollutants, and the nature of the interaction of such species with humic acid strongly determines their environmental behavior and fate. This work investigates different approaches for the synthesis of humic acid-gold nanoparticles (HA-AuNP), the performance of these particles as sensors for trace determination of organic dyes and the nature of their chemical interaction with humic acids by surface-enhanced Raman scattering (SERS). The Au nanoparticles were obtained by direct fabrication in aqueous media.The synthesis were carried out in the presence of humic acid at various pH values, and in the presence and absence of sodium citrate. In this approach, humic acid serves different purposes. In the synthesis of the nanoparticles, they serve as reducing agent and for surface stabilization, preventing coalescence of the nanoparticles in aqueous media. Considering the use of the nanoparticles as SERSactive substrates, the humic acid serves as an extraction phase associated with the SERS platform, favoring the interaction of the organic dyes with the metallic nanoparticles. This approach can be used to enhance the sensitivity and selectivity of SERS technique and avoid interference from other species in solution. It is also important to mention that the humic acid gives rise to a very weak Raman spectrum, and therefore does not interfere significantly in the spectroscopic detection of the species of interest. The obtained HA-AuNPs were characterized by electronic spectroscopy, scanning electron microscopy and SERS, in order to establish a correlation between their morphology, surface plasmon, and their potential use as SERS platforms. The HA-AuNPs have been applied as SERS substrates for anionic and cationic textile dyes. The recorded Raman spectra are, in these cases, assigned to the complex formed by the humic acid and each of the species of interest. The obtained results revealed a significant dependence on the morphological and electronic properties of the HA-AuNPs with the synthesis procedure, and also a strong interaction between the nanoparticles and cationic dies. We conclude that HA-AuNP may provide a valuable approach for the spectroscopic characterization of environmentally relevant species and their chemical interaction with humic acids, through the use of surface enhanced Raman spectroscopy

Corantes orgânicos são uma importante classe de poluentes ambientais, sendo que a natureza da interação de tais espécies com ácidos húmicos determina seu comportamento e destino ambiental. Este trabalho investiga abordagens diferentes para a síntese de nanopartículas de ouro-ácido húmico (NPs Au/AH), o desempenho destas partículas como sensores para a determinação de traço de corantes orgânicos e da natureza da sua interação química com ácidos húmicos por técnicas de espectroscopia Raman intensificada por superfície (surface-enhanced Raman scattering, SERS). As nanopartículas de ouro foram obtidas de forma direta em meio aquoso. A síntese foi realizada na presença de ácido húmico em vários valores de pH e na presença e na ausência de citrato de sódio. Nesta abordagem, o ácido húmico apresenta propósitos diferentes. Na síntese das nanopartículas, ele serve como agente redutor e para a estabilização da superfície, impedindo a coalescência das nanopartículas em meio aquoso. Considerando-se a utilização de nanopartículas como substratos SERS-ativos, o ácido húmico serve como uma fase de extração associada com a plataforma SERS, favorecendo a interação dos corantes orgânicos com as nanopartículas metálicas. Esta abordagem pode ser utilizada para aumentar a sensibilidade e seletividade da técnica SERS e evitar a interferência de outras espécies em solução. É também importante mencionar que o ácido húmico dá origem a um espectro Raman muito fraco e, portanto, não interfere de forma significativa na detecção espectroscópica das espécies de interesse. As NPs Au/AH obtidas foram caracterizadas por espectroscopia eletrônica, microscopia eletrônica de varredura e SERS, a fim de estabelecer uma correlação entre a sua morfologia, plasmon superficial, e seu uso potencial como substratos SERS. As NPs Au/AH foram utilizadas como substratos SERS no estudo de corantes têxteis aniônicos e catiônicos. Os espectros Raman, nestes casos, foram atribuídos ao complexo formado pelo ácido húmico e cada uma das espécies de interesse. Os resultados obtidos revelaram uma dependência significativa entre as propriedades morfológicas e eletrônicas das NPs Au/AH com o procedimento de síntese, e também uma interação preferencial entre as nanopartículas e os corantes catiônicos. Conclui-se que as NPs Au/AH podem proporcionar uma abordagem útil para a caracterização espectroscópica de espécies relevantes para o ambiente e sua interação química com os ácidos húmicos, através do uso da espectroscopia Raman intensificada pela superfície / Organic dyes are an important class on environmental pollutants, and the nature of the interaction of such species with humic acid strongly determines their environmental behavior and fate. This work investigates different approaches for the synthesis of humic acid-gold nanoparticles (HA-AuNP), the performance of these particles as sensors for trace determination of organic dyes and the nature of their chemical interaction with humic acids by surface-enhanced Raman scattering (SERS). The Au nanoparticles were obtained by direct fabrication in aqueous media.The synthesis were carried out in the presence of humic acid at various pH values, and in the presence and absence of sodium citrate. In this approach, humic acid serves different purposes. In the synthesis of the nanoparticles, they serve as reducing agent and for surface stabilization, preventing coalescence of the nanoparticles in aqueous media. Considering the use of the nanoparticles as SERSactive substrates, the humic acid serves as an extraction phase associated with the SERS platform, favoring the interaction of the organic dyes with the metallic nanoparticles. This approach can be used to enhance the sensitivity and selectivity of SERS technique and avoid interference from other species in solution. It is also important to mention that the humic acid gives rise to a very weak Raman spectrum, and therefore does not interfere significantly in the spectroscopic detection of the species of interest. The obtained HA-AuNPs were characterized by electronic spectroscopy, scanning electron microscopy and SERS, in order to establish a correlation between their morphology, surface plasmon, and their potential use as SERS platforms. The HA-AuNPs have been applied as SERS substrates for anionic and cationic textile dyes. The recorded Raman spectra are, in these cases, assigned to the complex formed by the humic acid and each of the species of interest. The obtained results revealed a significant dependence on the morphological and electronic properties of the HA-AuNPs with the synthesis procedure, and also a strong interaction between the nanoparticles and cationic dies. We conclude that HA-AuNP may provide a valuable approach for the spectroscopic characterization of environmentally relevant species and their chemical interaction with humic acids, through the use of surface enhanced Raman spectroscopy

In the pursuit of economical and rapid fabrication solutions on the micro and nano scale, polymer replication has proven itself to be a formidable technique, which despite zealous development by the research community, remains full of promise. This thesis explores the potential of elastomers in what is a distinctly multidisciplinary field. The focus is on developing innovative fabrication solutions for planar photonic devices and for nanoscale devices in general. Innovations are derived from treatments of master structures, imprintable substrates and device applications. Major contributions made by this work include fully replicated planar integrated optical devices, nanoscale applications for photolithographic standing wave corrugations (SWC), and a biologically templated, optical fiber based, surface-enhanced Raman scattering (SERS) sensor. The planar devices take the form of dielectric rib waveguides which for the first time, have been integrated with long-period gratings by replication. The heretofore unemployed SWC is used to demonstrate two innovations. The first is a novel demonstration of elastomeric sidewall photolithographic mask, which exploits the capacity of elastomers to cast undercut structures. The second demonstrates that the corrugations themselves in the absence of elastomers, can be employed as shadow masks in a directional flux to produce vertical stacks of straight lines and circles of nanowires and nanoribbons. The thesis then closes by conceptually combining the preceding demonstrations of waveguides and nanostructures. An optical fiber endface is em ployed for the first time as a substrate for patterning by replication, wherein the pattern is a nanostructure derived from a biological template. This replicated nanostructure is used to impart a SERS capability to the optical fiber, demonstrating an ultra-sensitive, integrated photonic device realized at great economy of both time and money, with very real potential for mass fabrication.