Functionalization Of Planar Surface Si (111)-H With Covalently Bound Molecular Monolayers And Polymer Brushes And Their Chemical Conversions To Immobilize Biomolecules

Various modifications have been carried out on silicon surface to satisfy the specific demands of different areas such as biochips, molecular devices and microelectronics in recent years. Chemical modifications on the silicon crystalline surface bring new chemical properties to prepare biosensors and biochips for protein and DNA assays.Chemical modifications can be realized including reactions between silane coupling reagents and the surface Si-OH as well as reactions between Grignard reagents and surface Si-Cl or Si-Br. However, the resulted Si-O-Si via the former reactions is apt to degrade under alkaline conditions. Meanwhile, the later reactions take place under stringent conditions and fewer sorts of Grignard reagents do not meet various practical demands.It demands robust covalent bonds between the interface of soft materials and silicon, which can suffer from diverse chemical environments and can be converted into different functional groups in practical use. The surface Si-C bond satisfies the requirement and silicon wafer is capable of enduring different organic solvents, acids and weak alkalis. Bond Si-C can be obtained by means of Si(111)-H, generated from etching with 40% NH4F solutions, coupling with terminal vinyl C=C of organic compounds with a functional group at the other end to fabricate monolayers on silicon surface. Further chemical modifications can be implemented until proper functionalized surface is yielded while the Si(111) surface remains atomic flat. All these properties of Si(111) forecast its extensive applications in biochemistry, molecular device and other areas. The research work described in this dissertation is as follows. First, a Si(111)-H surface was modified via a direct reaction between Si-H and 1-undecylenic acid (UA) under microwave irradiation to form molecular monolayers with terminal carboxyl groups. After esterifying carboxylic acid with N-hydroxysuccinimide (NHS), aminobutyl nitrilotriacetic acid (ANTA) was bound to the silicon surface through amidation (pH=8.0) between its primary amino group and NHS-ester, producing nitrilotriacetic acid (NTA) anions. Then hexa-histidine tagged thioredoxin-urodilatin (his-tagged protein) and FITC labeled hexa-histidine tagged thioredoxin-urodilatin (FITC-his-tagged protein) can be anchored after NTA was coordinated with Ni2+. Furthermore, the NTA-terminated chip was acidified with 0.1M HCl and subsequently esterified with NHS and then amidated with ANTA again to produce a second generation NTA. Thus the surface density of nitrilotriacetic acid anions was improved and resultantly that of anchored proteins was also enhanced through the iterative reactions. Both multiple transmission-reflection infrared spectroscopy (MTR-IR) and fluorescence scanning measurements demonstrated a proximate 1.63 times of anchored proteins on the second generation NTA/Ni2+ as that on the first generation NTA/Ni2+ monolayer.Apart from this, we developed a facile way of preparing a bioactive silicon surface through constructing multiple generation nitriloacetates from poly(poly(ethylene glycol)monomethacrylate) brushes on a planar silicon surface (Si-g-P(PEGMA)) to improve the capacity of binding proteins and biofouling repellence. Firstly the freshly etched Si(111)-H reacted with 10-undecen-l-ol (UO) under an optimized microwave irradiation condition to produce hydroxyl-terminated substrates. Secondly surface initiators for atom transfer radical polymerization (ATRP) were introduced by means of esterification between 2-bromo-2-methylpropionyl bromide (BMPB) and surface hydroxyl groups. Thirdly polymer brushes of Si-g-P(PEGMA) grown from surface initiators were carboxylated with succinic anhydride onto their side chains (Si-g-P(PEGMA-COOH)). Finally multiple generation nitriloacetates were obtained by iterative esterification of carboxylic acids with N-hydroxysuccinimide (NHS)/N,N'-dicyclohexylcarbodiimide (DCC) yielding amino-reactive NHS esters and successive amidation with aminobutyl nitrilotriacetic acid (ANTA) yielding nitrilotriacetic acid ligands for capturing histidine-tagged proteins. After coordinated with Ni(II), the first to third generation nitriloacetate surfaces were applied to bind histidine-tagged thioredoxin-urodilatin. All stepwise conversions were monitored with MTR-IR. By means of comparison of integrated peak areas of v(NH) from amide, we estimated the enhancement of protein loading by nitriloacetates as first generation/second generation/third generation= 1: 1.7:2.3.In addition, we reported a facile approach to building dendrons of polyamidoamine (PAMAM) derived from grafted polymer brushes of poly(poly(ethylene glycol) monomethacrylate) on planar silicon surface (Si-g-P(PEGMA-OH)). In the first place, Si-g-P(PEGMA-OH) brushes were built via surface-initiated atom transfer radical polymerization (ATRP) technique on silicon surface. The peripheral hydroxyl groups of Si-g-P(PEGMA-OH) were chlorinated with thionyl chloride to convert-OH into-Cl. Then the end chlorines were substituted with amino groups of ethylenediamine giving terminal primary amine. Each amino group reacted with methyl acrylate via Michael addition to produce two end esters of-(C=O)-OCH3 theoretically. The two resulted esters reacted with ethylenediamine (EDA) subsequently via amidation generating two terminal amino groups. We iterated such Michael addition to obtain terminal esters and subsequent amidation to achieve dendrons of PAMAM on such a biofouling-resistant surface of Si-g-P(PEGMA-OH). The dense amino groups could be activated via a cross-link reaction between peripheral amino groups of dendrons and N-succinimidyl-6-maleimidylhexanoate. Thus biomolecules such as oxytocins could be anchored on such functionalized surface, In addition, the dendrons of PAMAM on silicon surface could be used as a platform to synthesize a three amino acid peptide of H-Arg-Gly-Asp-OH (RGD). The modifications had been monitored and demonstrated by MTR-IR spectra, X-ray photoelectron spectroscopes (XPS), UV-Vis spectra and matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS).At last, two surface approaches were realized to complete click chemistry reactions at covalently grafted polymer brushes of poly(poly(ethylene glycol) monomethacrylate) on a planar silicon surface (Si-g-P(PEGMA-OH)). On one hand, the hydroxyls from Si-g-P(PEGMA-OH) brushes can be replaced by chlorines of thionyl chloride and then chlorines can be substituted with azides of sodium azide to achieve azide-terminated (Si-g-P(PEGMA-N3)) brushes. On the other hand, the terminal acetylene (Si-g-P(PEGMA-CH2C=CH)) brushes can be prepared easily by reaction between Si-g-P(PEGMA-OH) and propargyl bromide. Model compounds of propargylamine, propiolic acid and 10-undecynoic acid with terminal acetylene and of benzyl azide with terminal azide were chosen to investigate the surface click reactions catalyzed with Cu(Ⅱ)/sodium L-ascorbate by microwave irradiation under very mild conditions at 30℃for 1 h. The stepwise modifications were characterized by two surface-sensitive techniques, Multiple Transmission-Reflection Infrared Spectroscopy (MTR-IR) and XPS, and their spectra were analyzed in detail. The infrared band at 3139 cm-1 attributed to the triazole ring v(H-C=) and the XPS high-resolution scan of N 1s give the direct evidences to confirm the click reactions. By quantifying their infrared spectra before and after click reactions, we conclude that the click reactions on silicon surfaces by microwave irradiation possess high yield and efficiency. Hence, the microwave irradiated click reaction approaches on silicon might open convenient avenues to fabricate functional and hybrid organic/silicon devices.