Surface-enhanced Raman scattering: from the colloid to the stabile substrate

Supstrati za površinski pojaĉano Ramanovo raspršenje (SERS) pripremljeni su na tri razliĉita naĉina: (i) sintezom koloidnih suspenzija srebra, (ii) depozicijom srebra na prethodno elektrokemijski jetkan silicij i (iii) naparavanjem srebra na monosloj polistirenskih mikrosfera. Koloidne suspenzije sintetizirane su redukcijom srebrove soli (AgNO3) s razliĉitim reducirajućim sredstvima. Prvi tip ĉvrstih podloga dobiven je depozicijom metala na porozni silicij metodom uranjanja u otopinu AgNO3, nanošenjem koloidne suspenzije srebra ili postupkom pulsne laserske ablacije. Drugi tip ĉvrstih SERS supstrata dobiven je depozicijom monodisperznih polistirenskih sfera (350 i 1000 nm) u monosloju na hidrofilnu staklenu podlogu te naparavanjem razliĉitih debljina metalnih filmova pomoću evaporatora. Sve pripremljene podloge testirane su na SERS aktivnost korištenjem testnih molekula (piridin, rodamin 6G, metilensko modrilo). Za koloidnu suspenziju gdje je veliĉina Ag ĉestica iznosila oko 40 nm i uz korištenje natrijevog borhidrida kao agregirajućeg sredstva postignut je faktor pojaĉanja za piridin reda veliĉine 106. Za podloge bazirane na poroznom Si najniţe dobivene granice detekcije su 10-10 mol L-1 za metilensko modrilo (uz pobudu 633 nm) i 10-9 mol L-1 za rodamin 6G (uz pobudu 514,5 nm). Za supstrate bazirane na polistirenskim sferama najbolje postignute granice detekcije su 10-9 mol L-1 za rodamin 6G i 1,2 ×10-3 mol L-1 za piridin. Sintetizirane koloidne SERS podloge primijenjene su za određivanje niskih koncentracija histamina i aflatoksina B1.The substrates for surface enhanced Raman scattering (SERS) were prepared in three different ways: (i) the synthesis of silver colloidal suspension, (ii) the deposition of silver on the previously electrochemically etched silicon and (iii) vapor deposition of silver on a monolayer of polystyrene microspheres. Colloidal suspensions were synthesized by reduction of silver salts (AgNO3) with various reducing agents. The first type of solid substrates was obtained by the deposition of metal on the porous silicon using immersion plating method, by applying Ag colloidal solution or by pulsed laser ablation. Another type of solid SERS substrates was prepared by deposition of monodisperse polystyrene spheres (350 and 1000 nm) in a monolayer on a hydrophilic glass surface. Different thicknesses of metal films were evaporated on the substrates using e-beam deposition. All prepared substrates were tested for their SERS activity using the test molecules (pyridine, rhodamine 6G, methylene blue). For the colloidal suspension where the size of Ag particles was about 40 nm and with the use of sodium borohydride as aggregating agent the enhancement factor for pyridine of the order of 106 was reached. For the substrates based on porous Si the lowest limits of detection obtained were 10-10 mol L-1 for methylene blue (with 633 nm excitation) and 10-9 mol L-1 for rhodamine 6G (with 514.5 nm excitation). For substrates based on polystyrene spheres best achieved detection limits were 10-9 mol L-1 for rhodamine 6G and 1.2 × 10-3 mol L-1 for pyridine. Synthesized colloidal SERS substrates have been applied for the detection of low concentrations of histamine and aflatoxin B1

Surface Enhanced Raman Spectroscopy for Molecular Identification- a Review on Surface Plasmon Resonance (SPR) and Localised Surface Plasmon Resonance (LSPR) in Optical Nanobiosensing

Surface plasmon resonance (SPR) allows for real-time, label-free optical detection of many chemical and biological substances. Having emerged in the last two decades, it is a widely used technique due to its non-invasive nature, allowing for the ultra-sensitive detection of a number of analytes. This review article discusses the principles, providing examples and illustrating the utility of SPR within the frame of plasmonic nanobiosensing, while making comparisons with its successor, namely localized surface plasmon resonance (LSPR). In particular LSPR utilizes both metal nanoparticle arrays and single nanoparticles, as compared to a continuous film of gold as used in traditional SPR. LSPR, utilizes metal nanoparticle arrays or single nanoparticles that have smaller sizes than the wavelength of the incident light, measuring small changes in the wavelength of the absorbance position, rather than the angle as in SPR. We introduce LSPR nanobiosensing by describing the initial experiments performed, shift-enhancement methods, exploitation of the short electromagnetic field decay length, and single nanoparticle sensors are as pathways to further exploit the strengths of LSPR nanobiosensing. Coupling molecular identification to LSPR spectroscopy is also explored and thus examples from surface-enhanced Raman spectroscopy are provided. The unique characteristics of LSPR nanobiosensing are emphasized and the challenges using LSPR nanobiosensors for detection of biomolecules as a biomarker are discussed. This work is licensed under a Creative Commons Attribution 4.0 International License

Assessment of Macrolide Transport Using PAMPA, Caco-2 and MDCKII-hMDR1 Assays

The transport of commercially available macrolide antibiotics erythromycin, clarithromycin, roxithromycin, azithromycin, and telithromycin was studied by PAMPA, a model of passive diffusion, and by two models of active transport, Caco-2 and MDCKII-hMDR1. An involvement of efflux pump P-glycoprotein (P-gp) in macrolide transport was also examined. Generally, in Caco-2 and MDCKII-hMDR1 assays without P-gp inhibition, a significantly lower apparent permeability (Papp) in apical-to-basolateral (AB) than basolateral-to-apical (BA) direction was observed [efflux ratio, Pappratio(BA/AB) > 4.2]. Upon P-gp inhibition, Papp(AB) was increased and Papp(BA) was decreased. The observed Papp values, their efflux ratios, as well as a significant decrease in efflux ratios caused by P-gp inhibition (> 50 %) qualify all tested macrolides as P-gp substrates. Results obtained with PAMPA suggest that this assay is not a method of choice for screening of macrolide permeability due to the lack of P-gp efflux pump and/or other transporters potentially involved in macrolide transport

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

Structural Properties of Iron/Titanium Oxide Nanoparticles Synthesized by Sol-gel Method in the Presence of Poly(ethylene glycol)

Iron/Titanium oxide nanoparticles with initial molar ratio Fe/(Fe+Ti) = 0.05 (low loading of Fe in TiO2) and Fe/(Fe+Ti) = 0.15 (high loading of Fe in TiO2) were synthesized by a modified sol-gel procedure using Ti(IV)-isopropoxide, FeSO4 ∙ 7H2O and poly(ethylene glycol) (PEG) as starting material. The amount of doped iron strongly affected structural properties as well as the degree of dispersion of iron in the Fe- Ti-O system. The highly-doped sample contained poorly crystallized hydrated iron(III)- sulphates or basic iron(III)-sulphates in a considerable amount. The lowly-doped Fe-TiO2 sample contained about 22 wt % of brookite that had taken up iron virtually in the same proportion as anatase. Contrary to quasi- homogeneous distributions of iron in the lowly- doped Fe-TiO2 sample, the uptake of iron in the highly-doped Fe-TiO2 sample heated at 500 °C is significant, but anatase is free from iron, thus suggesting a heterogeneous distribution of iron. The photocatalytic activity of the highly-doped Fe-TiO2 sample was better than that of the lowly-doped Fe-TiO2 sample, but still below the photoactivity of Degussa P25 reference photocatalyst

Metal Nanoparticles Deposited on Porous Silicon Templates as Novel Substrates for SERS

In this paper, results on preparation of stable and uniform SERS solid substrates using macroporous silicon (pSi) with deposited silver and gold are presented. Macroporous silicon is produced by anodisation of p-type silicon in hydrofluoric acid. The as prepared pSi is then used as a template for Ag and Au depositions. The noble metals were deposited in three different ways: by immersion in silver nitrate solution, by drop-casting silver colloidal solution and by pulsed laser ablation (PLA). Substrates obtained by different deposition processes were evaluated for SERS efficiency using methylene blue (MB) and rhodamine 6G (R6G) at 514.5, 633 and 785 nm. Using 514.5 nm excitation and R6G the limits of detection (LOD) for macroporous Si samples with noble metal nanostructures obtained by immersion of pSi sample in silver nitrate solution and by applying silver colloidal solution to pSi template were 10–9 M and 10–8 M respectively. Using 633 nm laser and MB the most noticeable SERS activity gave pSi samples ablated with 30000 and 45000 laser pulses where the LODs of 10–10 M were obtained. The detection limit of 10–10 M was also reached for 4 mA cm–2-15 min pSi sample, silver ablated with 30000 pulses. Macroporous silicon proved to be a good base for the preparation of SERS substrates

Silicon Nanowires Substrates Fabrication for Ultra-Sensitive Surface Enhanced Raman Spectroscopy Sensors

The silicon based substrates for surface enhanced Raman spectroscopy (SERS) have been synthesized and tested. The silver-assisted electroless wet chemical etching method has been utilized for silicon nanowires production which has been proved as the promising SERS substrate. The morphology of the silicon nanowires coated with silver nanoparticles has been examined by scanning electron microscopy. The SERS measurements tested on rhodamine 6G molecules indicated the optimal silicon nanowire substrate production obtained for 5 M hydrofluoric acid and 30 mM silver nitrate etching solution. The results show SERS detection limit of 10–8 M rhodamine in aqueous solution. This work is licensed under a Creative Commons Attribution 4.0 International License

Influence of Sample Matrix on Determination of Histamine in Fish by Surface Enhanced Raman Spectroscopy Coupled with Chemometric Modelling

Histamine fish poisoning is a foodborne illness caused by the consumption of fish products with high histamine content. Although intoxication mechanisms and control strategies are well known, it remains by far the most common cause of seafood-related health problems. Since conventional methods for histamine testing are difficult to implement in high-throughput quality control laboratories, simple and rapid methods for histamine testing are needed to ensure the safety of seafood products in global trade. In this work, the previously developed SERS method for the determination of histamine was tested to determine the influence of matrix effect on the performance of the method and to investigate the ability of different chemometric tools to overcome matrix effect issues. Experiments were performed on bluefin tuna (Thunnus thynnus) and bonito (Sarda sarda) samples exposed to varying levels of microbial activity. Spectral analysis confirmed the significant effect of sample matrix, related to different fish species, as well as the extent of microbial activity on the predictive ability of PLSR models with R2 of best model ranging from 0.722–0.945. Models obtained by ANN processing of factors derived by PCA from the raw spectra of the samples showed excellent prediction of histamine, regardless of fish species and extent of microbial activity (R2 of validation > 0.99)