Nanostructural properties of highly luminescent porous silicon

Tema istraživanja ovog doktorskog rada su strukturna i optička svojstva visoko luminiscentnog poroznog silicija (pSi) različitih poroznosti te njegova primjena kao predloška za izradu novih efikasnih SERS podloga. Uzorci nanostrukturnog pSi su izrađeni anodizacijom slabo dopiranih silicijevih pločica p-tipa mijenjanjem parametara jetkanja. Površina pSi se ponaša kao reducens srebrnih iona te se može uporabiti kao predložak za izradu SERS podloga. Nove SERS podloge su dobivene uranjanjem predložaka pSi u vodenu otopinu AgNO3. Uzorci su analizirani Ramanovom spektroskopijom s dvije laserske pobude (rezonantnom na 514,5 nm i nerezonantnom na 1064 nm), infracrvenom spektroskopijom s Fourierovom transformacijom (FTIR), pretražnom elektronskom mikroskopijom (SEM) te s energijski razlučujućom rendgenskom spektroskopijom (EDS). Snimljeni Ramanovi spektri pSi s nerezonantnom pobudom pokazali su crveni pomak i širenje O(Γ) fononske vrpce što upućuje na jako fononsko zatočenje. Pored toga opažen je i efekt kvantnog zatočenja elektrona u vidu plavog pomaka fotoluminiscentne vrpce. Kako energija fotoluminiscencije u malim nanokristalima ovisi i o kisiku na površini nanokristala, analizom FTIR i EDS spektara nađena je djelomična prekrivenost površine uslijed čega se valna duljina fotoluminiscentne vrpce ne mijenja na način kako bi se to očekivalo prema modelu kvantnog zatočenja. Ti rezultati dodatno ukazuju da pasivizacija površine pSi kisikom uz kvantno zatočenje određuje elektronska stanja silicijevih nanokristala poroznog silicija. SEM mikrografije priređenih SERS podloga pokazale su različite morfologije sa značajnom razlikom u srednjoj veličini čestica i distribuciji veličina srebrnih nanočestica. Aktivnost SERS podloga je testirana koristeći molekule rodamina 6G (R6G) s rezonantnom pobudom. SERS podloga napravljena od predloška makro/mezoporoznog silicija je pokazala veliku SERS aktivnost koja omogućava mjerenje ekstremno niske koncentracije molekula R6G sve do 10-15 M koja može ukazivati na detekciju jedne molekule.Light emitting porous silicon is nanostructured semiconductor material with distinguished optical properties. It has sponge-like structure containing silicon nanocrystals and a fairly large surface that can easily be accessed and chemically modified due to its porous characteristics. Unlike crystalline silicon, which is a typical example of the semiconductor with indirect energy bandgap, highly luminescent porous silicon shows the characteristics of a semiconductor with a direct bandgap whose photoluminescence quantum efficiency reaches up to 23%. Therefore, it has great potential to become main material in optoelectronic devices that generate light such as light emitting diodes and lasers. Due to the quantum confinement of charge within a nanocrystal, the reduction of size causes the gradual widening of its energy gap. This means that by changing the size of nanocrystals within the porous silicon the wavelength of the emitted light can be controlled. The aim of this study was to determine the size of porous silicon nanocrystals with different porosity using FT-Raman spectroscopy with non-resonant laser excitation and correlate it with the photoluminescence. Also, the aim was to analyse the molecular groups present on the surface of the nanocrystals and to determine their impact on photoluminescence energy

Structural and Oxidation Properties of Plasma Lipoproteins from Different Phenotypes: FT-IR and ESR Study

Subclasses of lipoproteins VLDL, LDL and HDL were isolated from plasma of two different phenotypes, A and B. Phenotype B is present in people with diabetes, atherosclerosis and higher risk for cardiovascular disease. Lipoproteins were studied by FT-IR spectroscopy to investigate conformational and structural differences reflected in spectra. Differences were observed in spectra of LDL and VLDL. In protein domain differences were observed in amide I band profile and discussed in terms of conformation¬al change of apo B-100 in small LDL and large VLDL particles. The changes in lipid bands from pheno¬type B are associated with differences in core composition and lipids\u27 ordering in monolayer. The study of slow oxidation was done by measuring the oxygen consumption in lipoprotein solutions. Oxidation was faster in HDL and VLDL from phenotype B, while for LDL it was similar for A and B samples and signif¬icantly faster than in other two lipoproteins. (doi: 10.5562/cca2236

Interaction of High Density Lipoprotein with Nicotine – an IR and Raman Study

The direct interaction of high density lipoprotein (HDL) with nicotine, one of the major components of cigarette particulate matter, has been studied here at molecular level. Nicotine affects the vibration modes of HDL due to its embedment within the lipid monolayer. The changes in the positions and intensities of vibration bands in protein and lipid domain of the particle were studied by IR and Raman spectroscopy. Three types of samples were prepared: native HDL sample and two HDL samples with added nicotine. The molecular ratio of nicotine /phospholipids in two samples was 1: 12 and 1: 6. The same types of samples were prepared from liposomes containing phosphatidylcholine, and sphingomyelin with addition of cholesterol. Spectra of liposome samples were used for distinguishing and attribution of lipid bands in spectra of HDL samples, where the majority of changes were observed. The incorporation of nicotine into lipid monolayer induces changes in the lipid bands from the vibrations in acyl chains and head groups of phospholipids. The changes in vibration bands from particular amino acids\u27 residues confirm that nicotine molecule is located within lipid monolayer but close to lipid-protein interface

Interaction of High Density Lipoprotein with Nicotine – an IR and Raman Study

The direct interaction of high density lipoprotein (HDL) with nicotine, one of the major components of cigarette particulate matter, has been studied here at molecular level. Nicotine affects the vibration modes of HDL due to its embedment within the lipid monolayer. The changes in the positions and intensities of vibration bands in protein and lipid domain of the particle were studied by IR and Raman spectroscopy. Three types of samples were prepared: native HDL sample and two HDL samples with added nicotine. The molecular ratio of nicotine /phospholipids in two samples was 1: 12 and 1: 6. The same types of samples were prepared from liposomes containing phosphatidylcholine, and sphingomyelin with addition of cholesterol. Spectra of liposome samples were used for distinguishing and attribution of lipid bands in spectra of HDL samples, where the majority of changes were observed. The incorporation of nicotine into lipid monolayer induces changes in the lipid bands from the vibrations in acyl chains and head groups of phospholipids. The changes in vibration bands from particular amino acids\u27 residues confirm that nicotine molecule is located within lipid monolayer but close to lipid-protein interface

Micro and Nano Structure of Electrochemically Etched Silicon Epitaxial Wafers

Silicon epitaxial wafers, consisting of 280 μm thick n-type substrate layer and 4–5 μm thick epitaxial layer, were electrochemically etched in hydrofluoric acid ethanol solution, to produce porous silicon samples. The resistivity of epitaxial layer was 1 Ω cm, while the substrate was much better conductor with resistivity 0.015 Ω cm. By varying the etching time, the micro- and nano-pores of different sizes were obtained within the epitaxial layer, and on the substrate surface. Due to the lateral etching the epitaxial layer was partially detached from the substrate and could be peeled off. The influence of etching time duration on the optical and structural properties of porous samples was investigated by Raman, infrared and photoluminescence spectroscopy. The samples were analysed immediately after the etching and six months later, while being stored in ambient air. The Raman spectra showed the shift in positions of transversal optical (TO) phonon bands, between freshly etched samples and the one stored in ambient air. Infrared spectra indicated the presence of SiHx species in the freshly etched samples, and appearance of oxidation after prolonged storage. Photoluminescence spectra were very weak in freshly etched samples, but their intensity has increased substantially in six month period. (doi: 10.5562/cca1971

Expression of secreted frizzled-related protein 1 and 3, T-cell factor 1 and lymphoid enhancer factor 1 in clear cell renal cell carcinoma

Frequency and mortality of renal cell carcinoma (RCC) are increasing for decades. However, the molecular background of RCC tumorigenesis is still poorly understood. In current study we investigated the expression of TCF/LEF and SFRP family members (SFRP1 and SFRP3) to gain a better understanding of biological signaling pathways responsible for epidemiology and clinical parameters of clear cell RCC (cRCC). Thirty-six pairs of paraffin-embedded clear cRCC and adjacent nontumoral tissues samples using immunohistochemistry (IHC) were analyzed and compared with corresponding clinicopathological parameters. Immunohistochemistry indicated statistically significant decreased SFRP3 expression in tumor tissues but no consistency in SFRP1 expression in analyzed normal and tumor tissue. The TCF1 expression level was significantly weaker in normal tissue compared to tumor samples while LEF1 protein levels were significantly weaker in tumor tissue. To our knowledge, this is the first report on analysis of the expression of transcription factors TCF1 and LEF1 in clear cell renal cell carcinoma and their comparison with Wnt signal pathway antagonists belonging to SFRP family

FT-IR spectroscopy of lipoproteins – a comparative study

FT-IR spectra, in the frequency region 4000 – 600 cm-1, of four major lipoprotein classes: very low density lipoprotein (VLDL), low density lipoprotein (LDL) and two subclasses of high density lipoproteins (HDL2 and HDL3) were analyzed to obtain their detailed spectral characterization. Information about the protein domain of particle was obtained from the analysis of amide I band. The procedure of decomposition and curve fitting of this band confirms the data already known about the secondary structure of two different apolipoproteins: apo A-I in HDL2 and HDL3 and apo B-100 in LDL and VLDL. For information about the lipid composition and packing of the particular lipoprotein the well expressed lipid bands in the spectra were analyzed. Characterization of spectral details in the FT-IR spectrum of natural lipoprotein is necessary to study the influence of external compounds on its structure

Structural and Oxidation Properties of Plasma Lipoproteins from Different Phenotypes: FT-IR and ESR Study

Subclasses of lipoproteins VLDL, LDL and HDL were isolated from plasma of two different phenotypes, A and B. Phenotype B is present in people with diabetes, atherosclerosis and higher risk for cardiovascular disease. Lipoproteins were studied by FT-IR spectroscopy to investigate conformational and structural differences reflected in spectra. Differences were observed in spectra of LDL and VLDL. In protein domain differences were observed in amide I band profile and discussed in terms of conformation¬al change of apo B-100 in small LDL and large VLDL particles. The changes in lipid bands from pheno¬type B are associated with differences in core composition and lipids\u27 ordering in monolayer. The study of slow oxidation was done by measuring the oxygen consumption in lipoprotein solutions. Oxidation was faster in HDL and VLDL from phenotype B, while for LDL it was similar for A and B samples and signif¬icantly faster than in other two lipoproteins. (doi: 10.5562/cca2236

Near-Infrared Surface-Enhanced Raman Scattering on Silver-Coated Porous Silicon Photonic Crystals

Surface-enhanced Raman scattering (SERS) with near-infrared (NIR) excitation offers a safe way for the detection and study of fragile biomolecules. In this work, we present the possibility of using silver-coated porous silicon photonic crystals as SERS substrates for near-infrared (1064 nm) excitation. Due to the deep penetration of NIR light inside silicon, the fabrication of photonic crystals was necessary to quench the band gap photoluminescence of silicon crystal, which acts as mechanical support for the porous layer. Optimal parameters of the immersion plating process that gave maximum enhancement were found and the activity of SERS substrates was tested using rhodamine 6G and crystal violet dye molecules, yielding significant SERS enhancement for off-resonant conditions. To our knowledge, this is the first time that the 1064 nm NIR laser excitation is used for obtaining the SERS effect on porous silicon as a substrate