Novel Approaches to Prepare and Utilize SERS Substrates: Multiplex Microfluidics and Nanotransfer Printing

Over the past few decades, surface enhanced Raman spectroscopy (SERS) has garnered respect as an analytical technique with significant chemical and biological applications. SERS is important for the life sciences because it can provide trace level detection and a high level of molecular structure information. The development of quantitative, highly sensitive substrates requires control over size, shape, and position of metal nanoparticles which function as the SERS active medium. Thus, creating and successfully implementing a sensitive, reproducible, and robust SERS active substrate continues to be a challenging ask. Its future development depends critically on techniques for lithography and nanofabrication. Herein, we report a novel method for SERS that is based upon using colloidal silver nanoparticles in a multiplexed microfluidics (MMFs) platform. The MMF is created in polydimethylsiloxane (PDMS) polymer material and used to perform parallel, high throughput, and sensitive detection/identification of single or various analytes under easily manipulated conditions. A facile passive pumping method is used to deliver samples into the channels under flowing conditions that are highly conducive for SERS measurments. Also an unconventional nanofabrication approach is modified to produce efficient SERS substrates. Metallic nanopatterns of silver discs are transferred from a stamp onto PDMS to create nanocomposite substrates with regular periodic morphologies. The stamp with periodic arrays of square, triangular, and elliptical pillars is created via Electron Beam Lithography of ma-N 2403 resist. A modified cyclodextrin is thermally evaporated on the stamp to overcome the adhesive nature of the ebeam resist and to function as a releasing layer. Subsequently, the stamp is over coated with Ag by physical vapor deposition at a controlled rate and thickness and used directly for nanotransfer printing (nTP). Stamps, substrates, and the efficiency of the nTP process were explored by SEM. Ag nano-disc-PDMS substrates are studied by SERS using Rhodamine 6G as the probe analyte. The SERS response of metallic nano-discs of various shapes/sizes on the original stamp is compared to the corresponding nTP substrates. We demonstrate that physical manipulation of the PDMS post nTP can be used to alter morphology. Additionally, stamps are shown to be reusable after the nTP process

Enhancing Raman and Fluorescence Spectroscopies with Nanosphere Lithography Platforms

Localized surface plasmon resonance (LSPR) is of particular interest to enhance the limit of detection for spectroscopic techniques such as Raman and fluorescence via a surface enhancement from metallic nanostructures. In this study, using nanosphere lithography (NSL) technique, a series of gold nanostructures over glass surfaces are prepared. These nanostructures are used to record the surface enhanced Raman scattering (SERS) spectrum of benzenethiol and azobenzene thiol and the vibrational modes are compared to literature. Once protected with an ultrathin layer of SiO2, the gold nanostructures are investigated using scanning confocal fluorescence microscopy to detect the fluorescence from a dye solution. Herein, we show that the NSL-fabricated nanotriangle arrays made with particle sizes with dimensions closer to the excitation wavelength can be used to study the SERS spectrum of the molecule and, in the case of surface enhanced fluorescence (SEF), display the most intense hot-spots for each bow-tie assembly oriented along the polarization direction of the impinging light

Probing Trapped Extracellular Vesicles by Surface-Enhanced Raman Spectroscopy

Extracellular vesicles (EVs) are released by nearly all cell types within the human body and have been found to play important biological roles including cell-to-cell communication, apoptosis and tissue repair. Lacking cellular machinery, these nano-sized vesicles carry functional proteins and nucleic acids from their parent cells, providing insight into biomarkers present in healthy, cancerous and diseased cells. EVs may be isolated from biofluids such as from blood or urine. Their detection and characterization holds extreme potential in developing less invasive disease detection and treatment methods. In this work, we propose use of lithographic techniques to fabricate platforms to allow for molecular-level characterization by surface-enhanced Raman spectra (SERS). Two methods of lithography are proposed to probe spectral signatures of individual EVs without use of labelling agents. SERS spectra are acquired for EVs released from two cell lines, allowing for determination of the diversity existent within a cell line, and amongst different cell lines

Superfocusing, Biosensing and Modulation in Plasmonics

Plasmonics could bridge the gap between photonics and electronics at the nanoscale, by allowing the realization of surface-plasmon-based circuits and plasmonic chips in the future. To build up such devices, elementary components are required, such as a passive plasmonic lens to focus free-space light to nanometre area and an active plasmonic modulator or switch to control an optical response with an external signal (optical, thermal or electrical). This thesis partially focuses on designing novel passive and active plasmonic devices, with a specific emphasis on the understanding of the physical principles lying behind these nanoscale optical phenomena. Three passive plasmonic devices, designed by conformal transformation optics, are numerically studied, including nanocrescents, kissing and overlapping nanowire dimers. Contrary to conventional metal nanoparticles with just a few resonances, these devices with structural singularities are able to harvest light over a broadband spectrum and focus it into well-defined positions, with potential applications in high efficiency solar cells and nanowire-based photodetectors and nanolasers. Moreover, thermo-optical and electrooptical modulation of plasmon resonances are realized in metallic nanostructures integrated with either a temperature-controlled phase transition material (vanadium dioxide, VO2), or ferroelectric thin films. Taking advantage of the high sensitivity of particle plasmon resonances to the change of its surrounding environment, we develop a plasmon resonance nanospectroscopy technique to study the effects of sizes and defects in the metal-insulator phase transition of VO2 at the single-particle level, and even single-domain level. Finally, we propose and examine the use of two-dimensional metallic nanohole arrays as a refractive index sensing platform for future label-free biosensors with good surface sensitivity and high-throughput detection ability. The designed plasmonic devices have great potential implications for constructing nextgeneration optical computers and chip-scale biosensors. The developed plasmon resonance nanospectroscopy has the potential to probe the interfacial or domain boundary scattering in polycrystalline and epitaxial thin films

Development of novel series and parallel sensing system based on nanostructured surface enhanced Raman scattering substrate for biomedical application

With the advance of nanofabrication, the capability of nanoscale metallic structure fabrication opens a whole new study in nanoplasmonics, which is defined as the investigation of photon-electron interaction in the vicinity of nanoscale metallic structures. The strong oscillation of free electrons at the interface between metal and surrounding dielectric material caused by propagating surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) enables a variety of new applications in different areas, especially biological sensing techniques. One of the promising biological sensing applications by surface resonance polariton is surface enhanced Raman spectroscopy (SERS), which significantly reinforces the feeble signal of traditional Raman scattering by at least 104 times. It enables highly sensitive and precise molecule identification with the assistance of a SERS substrate. Until now, the design of new SERS substrate fabrication process is still thriving since no dominant design has emerged yet. The ideal process should be able to achieve both a high sensitivity and low cost device in a simple and reliable way. In this thesis two promising approaches for fabricating nanostructured SERS substrate are proposed: thermal dewetting technique and nanoimprint replica technique. These two techniques are demonstrated to show the capability of fabricating high performance SERS substrate in a reliable and cost efficient fashion. In addition, these two techniques have their own unique characteristics and can be integrated with other sensing techniques to build a serial or parallel sensing system. The breakthrough of a combination system with different sensing techniques overcomes the inherent limitations of SERS detection and leverages it to a whole new level of systematic sensing. The development of a sensing platform based on thermal dewetting technique is covered as the first half of this thesis. The process optimization, selection of substrate material, and improved deposition technique are discussed in detail. Interesting phenomena have been found including the influence of Raman enhancement on substrate material selection and hot-spot rich bimetallic nanostructures by physical vapor deposition on metallic seed array, which are barely discussed in past literature but significantly affect the performance of SERS substrate. The optimized bimetallic backplane assisted resonating nanoantenna (BARNA) SERS substrate is demonstrated with the enhancement factor (EF) of 5.8 × 108 with 4.7 % relative standard deviation. By serial combination with optical focusing from nanojet effect, the nanojet and surface enhanced Raman scattering (NASERS) are proved to provide more than three orders of enhancement and enable us to perform stable, nearly single molecule detection. The second part of this thesis includes the development of a parallel dual functional nano Lycurgus cup array (nanoLCA) plasmonic device fabricated by nanoimprint replica technique. The unique configuration of the periodic nanoscale cup-shaped substrate enables a novel hybrid resonance coupling between SPR from extraordinary (EOT) and LSPR from dense sidewall metal nanoparticles with only single deposition process. The sub-50nm dense sidewall metal nanoparticles lead to high SERS performance in solution based detection, by which most biological and chemical analyses are typically performed. The SERS EF was calculated as 2.8 × 107 in a solution based environment with 10.2 % RSD, which is so far the highest reported SERS enhancement achieved with similar periodic EOT devices. In addition, plasmonic colorimetric sensing can be achieved in the very same device and the sensitivity was calculated as 796 nm/RIU with the FOM of 12.7. It creates a unique complementary sensing platform with both rapid on-site colorimetric screening and follow-up precise Raman analysis for point of care and resource limited environment applications. The implementations of bifunctional sensing on opto-microfluidic and smartphone platforms are proposed and examined here as well

Mie and bragg plasmons in subwavelength silver semi-shells

2D arrays of silver semi-shells of 100 and 200 nm diameter display complex reflection and transmission spectra in the visible and near-IR. Here these spectral features are deconstructed and it is demonstrated that they result from the coupling of incident light into a delocalized Bragg plasmon, and the latter's induction of localized Mie plasmons in the arrays. These phenomena permit the excitation of transverse dipolar plasmon resonances in the semi-shells despite an ostensibly unfavorable orientation with respect to normally incident light. The resulting spectral feature in the mid-visible is strong and tunable. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA

Optical properties of metal nanoparticles and their influence on silicon solar cells

The optical properties of metal nanoparticles have been investigated by simulation and experimental techniques. The aim of this investigation was to identify how to use metal nanoparticles to improve light-trapping in silicon solar cells. To do this we require nanoparticles that exhibit a high scattering efficiency and low absorption (i.e. high radiative efficiency) at near-infrared wavelengths. The simulation results identified Ag, Au, Cu and Al as potential candidates for use with silicon solar cells. The optical properties of Ag, Au and Cu nanoparticles are very similar above 700 nm. Below this wavelength Ag was found to be the preferred choice due to a decreased effect from interband transitions in comparison with Au and Cu. Al nanoparticles were found to exhibit markedly different optical properties to identical noble metal nanoparticles, with broader, weaker resonances that can be excited further into the UV. However, Al nanoparticles were found to exhibit higher absorption than noble metals in the NIR due to a weak interband region centred at around 825 nm.Tuning of the resonance position into the NIR was demonstrated by many methods, and extinction peaks exceeding 1200 nm can be achieved by all of the metals studied. However, it is important that the method used to red-shift the extinction peak does not also decrease the radiative efficiency. Core-shell nanoparticles, triangular nanoparticles and platelet-type nanoparticles were found to be unsuitable for silicon solar cells applications due their low radiative efficiencies. Instead, we propose the use of large (> 150 nm) Ag spheroids with moderate aspect ratios. A maximum radiative efficiency of 0.98 was found for noble metal nanospheres when the diameter exceeded 150 nm.The optical properties of Au and Al nanoparticles fabricated by electron-beam lithography were found to be in good agreement with simulations, provided that the substrate and local dielectric environment were accounted for by inclusion of an effective medium in the model. Cr adhesion layers were found to substantially weaken the extinction peaks of Au nanoparticles, and also result in a strong decrease of radiative efficiency. Adhesion layers were not required for Al nanoparticles. The morphological and optical properties of Ag island films were found to be highly dependent on the layer thickness, deposition speed and anneal temperature. Dense arrays containing average particle sizes ranging from 25 nm to 250 nm were achieved using anneal temperatures lower than 200oC. The largest nanoparticles were found to exhibit high extinction from 400 nm to 800 nm.Depositing Ag nanoparticles onto a-Si:H solar cells was found two have two effects on the spectral response. At short wavelengths the QE was decreased due to absorption by small particles or back-scattering by larger particles. At longer wavelengths large maxima and minima are present in the QE spectra. This latter effect is not due to excitation of surface plasmons, but is instead related to modification of interference effects in the thin-film layer stack

The use of Surface Enhanced Raman Spectroscopy (SERS) for biomedical applications

Recent advances in nanotechnology and the biotechnology revolution have created an immense opportunity for the use of noble metal nanoparticles as Surface Enhanced Raman Spectroscopy (SERS) substrates for biological sensing and diagnostics. This is because SERS enhances the intensity of the Raman scattered signal from an analyte by orders of 106 or more. This dissertation deals with the different aspects involved in the application of SERS for biosensing. It discusses initial studies performed using traditional chemically reduced silver colloidal nanoparticles for the SERS detection of a myriad of proteins and nucleic acids. It examines ways to circumvent the inherent aggregation problems associated with colloidal nanoparticles that frequently lead to poor data reproducibility. The different methods examined to create robust SERS substrates include the creation of thermally evaporated silver island films on microscope glass slides, using the technique of Nanosphere Lithography (NSL) to create hexagonally close packed periodic particle arrays of silver nanoparticles on glass substrates as well as the use of optically tunable gold nanoshell films on glass substrates. The three different types of SERS surfaces are characterized using UV-Vis absorption spectroscopy, Electron Microscopy (EM), Atomic Force Microscopy (AFM) as well as SERS using the model Raman active molecule trans-1,2-bis(4-pyridyl)ethylene (BPE). Also discussed is ongoing work in the initial stages of the development of a SERS based biosensor using gold nanoshell films for the direct detection of b-amyloid, the causative agent for Alzheimer's disease. Lastly, the use of gold nanoshells as SERS substrates for the intracellular detection of various biomolecules within mouse fibroblast cells in cell culture is discussed. The dissertation puts into perspective how this study can represent the first steps in the development of a robust gold nanoshell based SERS biosensor that can improve the ability to monitor biological processes in real time, thus providing new avenues for designing systems for the early diagnosis of diseases

Ultrasensitive surface enhanced Raman scattering nanomotors : for location predicable biochemical detection, single-cell bioanalysis, and controllable biochemical release and real-time detection

Localized surface plasmon resonance resulting from the concerted oscillations of conduction-band electrons in noble-metal (Au, Ag) nanostructures greatly induces enhanced electric ([italic E]) fields in confined nanoscale locations, such as on the tips of nanorods or in the junctions of nanodimers. These [italic E]-field enhanced locations are called hot spots. In the vicinity of hot spots, Raman scattering spectra of biochemicals can be substantially amplified with an [italic E]⁴ dependence due to the [italic E]-field enhancement of both the incident light and Raman spectra. This is called surface enhanced Raman scattering (SERS). SERS is known for its high sensitivity in providing fingerprint vibrational information of molecules. It has triggered intense interest because of its potential applications for label-free and multiplex biochemical detection relevant to medical, environmental and defense purposes. However, the tremendous potential of SERS for ultrasensitive detection has still not materialized because of four major obstacles: (1) it is extremely difficult to obtain a large number of hotspots for sensitive and reproducible detection due to the stringent requirement of hot spots of only a few nanometers; (2) it is arduous to achieve ultrasensitivity for the detection of a single/few molecules; (3) it is challenging to assemble the hot-spots at designated positions for location predicable sensing; and (4) it is even more difficult to change the state-of-the-art static/passive sensing schemes into dynamic/robotized schemes and also to incorporate multi-functionality into a single SERS nanostructure. In this research, we addressed the aforementioned problems by rational design, fabrication and robotization of ultrasensitive SERS nanomotor sensors. A nanomotor sensor consists of a tri-layer structure with a three-segment Ag/Ni/Ag nanorod as the core, a thin layer of silica in the center, and uniformly distributed Ag nanoparticles as the outer layer. The inner metallic nanorod core is the key structure in realizing the concept of the robotization of nanosensors, which can be electrically polarized and thus efficiently manipulated by electric tweezers. The presence of the Ni segment in the metallic nanowire core also allows manipulation and assembling by magnetic interactions. The central silica layer provides a supporting substrate for the synthesis of the Ag nanoparticles and separates the Ag nanoparticles from the metallic nanorod core to eliminate the effect of plasmonic quenching. Finally, the outermost layer made of Ag nanoparticles with optimized sizes and junctions provides a large number of hot spots (~1200/μm²) for ultrasensitive SERS detection with single molecule sensitivity and an enhancement factor (EF) of 1.1×10¹⁰. Moreover, two additional SERS enhancement mechanisms were investigated, i.e., the optical management with nanophotonic devices and the near field effect, which can readily increase the EF by 10 and 2 times, respectively, to at least 10¹¹. Finally, three applications of the SERS nanomotor sensors have been demonstrated: (1) the ultrasensitive SERS nanomotors were transported and assembled into a 3×3 array for location predicable sensing of multiplex molecules; (2) ultrasensitive SERS nanomotors were transported to individual living cells amidst many cells for single-cell bioanalysis; and (3) the SERS nanomotor sensors can be controlled to rotate by the electric tweezers for tunable biochemical release and detection. This research, exploring robotized nanosensors by integrating SERS and NEMS, is innovative in both material design and device concept, which could inspire a new device scheme for various bio-relevant applications.Materials Science and Engineerin