"Click" Patterning of Self-Assembled Monolayers on Hydrogen-Terminated Silicon Surfaces and Their Characterization Using Light-Addressable Potentiometric Sensors

China Scholarship Council for funding (J.W., F.W., and J.Z.

USING CROSS-SECTIONED MULTILAYER POLYMER FILM AND SURFACE MODIFICATION TO FORM CHEMICALLY PATTERNED SUBSTRATES

Highly layered structures are important to micro- and nanofabrication technologies for understanding and controlling surface structures through manipulation of chemical and physical interactions. The objective of this work was to develop a new approach to create micro- and nanopatterned surfaces using multilayer polymer films of commercially available and inexpensive polymers instead of inorganic substrates. As an example, linear low density polyethylene (LLDPE) and ethylene-co-acrylic acid copolymer (EAA) were used as alternating inert and reactive polymers, respectively. Thin cross-sections of the multilayer molded sheets were prepared by ultra-microtoming. As a precursor to the multilayer work, surface modification of EAA was conducted to carefully control the chemical functionality on the surface by a variety of methods. Dansyl cadaverine and polyethylene glycol (PEG) derivatives were grafted on the surface of EAA film and in its subsurface region through formation of amides and esters, respectively. First, EAA film was activated with PCl5 and then the acid chloride was reacted with dansyl cadaverine or a PEG derivative. Moreover, two other reaction schemes were developed to covalently graft PEG chains on EAA surfaces. The schemes involved surface grafting of linker molecules l-lysine or polypropyleneamine dendrimer (AM64), with subsequent covalent bonding of PEG chains to the linker molecules. Combining the data from ATR-FTIR, XPS, and contact angle goniometry, it was found that the PEG chains were grafted on the surface of the EAA film and larger surface coverage was achieved when the dendrimer was used as intermediate layer. Research was then conducted on the EAA-LLDPE multilayer cross-sectioned templates. Regionally confined chemical functionality was confirmed by grafting an amine-terminated biotin to the alternating layers of EAA. Subsequently, fluorescently labeled streptavidin selectively adsorbed on the biotin-modified EAA layers. As a further development, polyelectrolyte multilayers (PEM) were adsorbed on the nanopatterned surfaces to significant increase the areal density of reactive groups. Using PAH and PAA as the polyelectrolytes, the EAA nano-stripes were successfully modified by PEM films, forming a nanopatterned template with alternating hydrophilic and hydrophobic regions. This kind of nano-striped surface could serve as a template for many applications, including biomedical, separation, and electronics

Silicon dioxide microstructures based on macroporous silicon for biomedical applications.

En aquesta tesi hem desenvolupat materials microestructurats basats en silici macroporós, centrant-nos en la producció de microestructures per la seva aplicació en biomedicina. El silici macroporós es forma per atac electroquímic de silici en electròlits basats en àcid fluorhídric. Es fabriquen mostres de silici macroporós ordenat i aleatori. Amb un procés litogràfic, es pot crear un patró predisenyat en el silici, i així definir els punts de nucleació i aconseguir porus amb un creixement ordenat i un diàmetre uniforme. L’oxidació tèrmica del silici macroporós permet la formació de noves estructures, com micropilars de SiO2. El SiO2 es normalment acceptat com un material biocompatible. Tot i això, utilitzem l’espectroscòpia infraroja per realitzar una caracterització exhaustiva i una modificació adequada de la química de superfície orientada cap a la conjugació de biomolècules. La peculiar arquitectura d’aquests substrats va permetre la creació de partícules multifuncionals amb una doble funcionalització selectiva en les cares interior i exterior. Aquestes microestructures van ser concebudes com a materials per al transport de fàrmacs. Així doncs, aquestes micropartícules de SiO2 van ser proposades com a sistemes d’alliberament de fàrmacs per control de pH quan es combinen amb polielectròlits sensibles al pH. Finalment, la doble funcionalització va ser explotada per crear micropartícules multifuncionals per l’alliberament de fàrmacs dirigida cap a cèl•lules diana. La viabilitat del sistema va ser provada amb cèl•lules cancerígenes in vitro.En esta tesis hemos desarrollado materiales microestructurados basados en silicio macroporoso, centrándonos en la producción de plataformas y partículas para su aplicación en biomedicina. El silicio macroporoso se forma por ataque electroquímico de silicio en electrolitos basados en ácido fluorhídrico. Se fabricaron muestras de silicio macroporoso ordenado y aleatorio. Con un proceso litográfico, se puede crear un patrón prediseñado en el silicio, y así definir los puntos de nucleación y conseguir poros con un crecimiento ordenado y un diámetro uniforme. La óxidación térmica del silicio macroporoso permite la formación de nuevas estructuras, como micropilares de SiO2. El SiO2 es normalmente aceptado como un material biocompatible. A pesar de esto, utilizamos la espectroscopía infraroja para realizar una caracterización exhaustiva y una modificación adecuada de la química de superficie orientada hacia la conjugación de biomoleculas. La peculiar arquitectura de estos sustratos permitió la creación de partículas multifuncionales con una doble functionalización selectiva en las caras interior y exterior. Estas microestructuras fueron concebidas como materiales para el transporte de fármacos. Así pues, estas micropartículas de SiO2 fueron propuestas como sistemas de liberación de fármacos por control de pH cuando se combinan con polielectrolitos sensibles al pH. Finalmente, la doble funcionalización fue explotada para crear micropartículas multifunctionales para la liberación de fármacos dirigida hacia células diana. La viabilidad del sistema fue probada con células cancerígenas in vitro.This thesis has explored the fabrication of silicon oxide (SiO2) microstructures based on macroporous silicon (macro-pSi), with a focus on producing suitable platforms and particles for application in biomedicine. Macroporous silicon was formed by the electrochemical etching of low doped p-type silicon in hydrofluoric acid based solutions. Both random and ordered structures were fabricated. A patterning lithography prior etching led to an ordered pore nucleation and consequently tubular structures of uniform size were produced. Thermal oxidation of macro-pSi allowed the formation of novel structures such as SiO2 micropillars, with identical arrangement and dimensions of those in the preceding macro-pSi. SiO2 is generally accepted as a biocompatible material; nevertheless, a methodical study of the surface chemistry and its modification was performed by infrared (IR) spectroscopy to generate surfaces capable of interfacing with living cells. The particular architecture of these substrates allowed creating multifunctional particles with a selective dual functionality in nanometrically separated internal and external sides. We also foresaw these microstuctured materials as drug carriers. Thus, SiO2 microparticles were proposed as pH-controlled drug delivery system when they are combined with pH-responsive polyelectrolytes. Finally, a dual-functionalization of the inner/outer sides was employed for creating multifunctional microparticles, which were demonstrated to be cancer-targeted in in vitro tests

Light-addressable potentiometric sensors based on self-assembled organic monolayer modified silicon on sapphire substrates

PhDLight-addressable potentiometric sensors (LAPS) have become attractive in many chemical and biological sensor applications. This thesis introduces the use of self-assembled organic monolayers (SAMs) as the insulator in LAPS and scanning photo-induced impedance microscopy (SPIM) for the first time. Two types of monolayer assemblies with alkenes (1-octadecene or undecylenic acid) and alkynes (1, 8-nonadiyne) were immobilised on hydrogenated silicon on sapphire (SOS) or silicon through thermal hydrosilylation. Further derivations were performed on the 1, 8-nonadiyne monolayers via “click” reactions. The monolayers were characterised by water contact angle, ellipsometry and X-ray photoelectron spectroscopy (XPS). LAPS/SPIM measurements with SAM-modified SOS showed the same good spatial resolution that was previously obtained with a conventional SiO2 insulator on SOS, but also a significant improvement in the accuracy of LAPS and the sensitivity of SPIM. Surface potential imaging using LAPS insulated by SAMs was validated by studying micropatterns of poly(allylamine hydrochloride) (PAH), poly(styrene sulfonate) (PSS) and DNA on a PAH template. Two potential strategies for chemically patterning SAMs on oxide-free SOS or Si substrates were investigated and compared. Microcontact printing (μCP) followed by “click” chemistry is a mild and efficient means of modifying the surface, whereas the combination of photolithography and “click” chemistry is not. LAPS was also shown to be extremely sensitive to surface contamination. LAPS/SPIM insulated by SAMs can also generate impedance images with high resolution and high sensitivity. Microcapsules labelled with gold nanoparticles (AuNPs) integrated with a femtosecond laser were used for the validation. In contrast, capsules without AuNPs showed no SPIM response at all, indicating that the impregnation with AuNPs can significantly increase the impedance of microcapsules. Finally, new instrumentation to integrate two-photon fluorescence microscopy with LAPS/SPIM was proposed. Preliminary results have shown that the new technique is promising to produce two-dimensional electrochemical images and two-photon fluorescent images of the cell-attachment area with subcellular resolutionChina Scholarship Council Queen Mary University of London

Fabrication of robust (bio)interfaces based on reactive polymer films : surface confinement, reactivity and pattern fabrication on multiple length scales

The aim of the work described in this Thesis was to investigate interfacial reactions in confinement on ultrathin homopolymer and diblock copolymer films, the immobilization of (bio)molecules and the fabrication of biomolecular patterns by reactive microcontact printing (µCP) on these reactive polymer films. Taking advantage of the microphase separation of diblock copolymer films, the fabrication of nanopatterns was investigated, which could contribute to the future development of a model system that enables one to area-selectively deposit (write) and address (read out) (bio)molecules

NANOSCALE PATTERNING AND 3D ASSEMBLY FOR BIOMEDICAL APPLICATIONS

Due to the inherent planarity of nanoscale patterning, there is a pressing need to develop novel approaches for parallel and cost-effective three dimensional (3D) patterning and assembly at the nanoscale. The 3D devices formed by such approaches are important for chem-bio sensing, nanoelectronics and photonics, nanorobotics, and nanobiotechnology. The body of work presented in this thesis is focused on developing scalable and manufacturable processes to create curved and foldable 3D nanostructures with precise surface patterns, in a highly parallel and cost-efficient method. Specifically, two new approaches were developed which include the spontaneous curving of nanostructures using grain reflow and creation of nanopatterned channels, wells, and semiconducting conical nanopores using metal assisted plasma etching process. During plasma etching of silicon with carbon tetraflouride and oxygen, it was discovered that certain metals present during the process undergo characteristic changes. In the grain reflow process, tin grains were found to undergo grain coalescence, resulting in the spontaneous curving of structures with tight radii of curvature of the order of a few nanometers. Another approach presented in the thesis for the large scale parallel patterning in the nanoscale involves using catalytic etching of silicon, assisted by lithographically patterned noble metal geometries. Using this method, three dimensional structures such as nanopore arrays and gold (Au) nanoparticles (NPs) coated micro or nano wells and channels can be fabricated in silicon in a highly parallel fashion. I also investigated the applications of the 3D nanostructures formed by the aforementioned processes. Conical nanopore arrays were used for voltage gated biomolecular sensing and separations. Ionic transport through these pores was investigated and it was found that the rectification ratios could be enhanced by a factor of 100 by voltage gating on the semiconducting substrate alone, and that these pores could function as ionic switches with high on-off ratios. Further, multifunctional 3D nanostructures were also combined with bacteria to create a nanoscale bionic system that can be remotely controlled using a laser. Overall, these results present important advancements in the development of nanoscale patterning and 3D assembly of curved and porous nanostructures with applications in biomedical sciences and microorganism robotics

Protein and Cell Micropatterning and its Integration With Micro/Nanoparticles Assembly

Ph.DDOCTOR OF PHILOSOPH