Gold Nanopost-Shell Arrays Fabricated by Nanoimprint Lithography as a Flexible Plasmonic Sensing Platform

Plasmonic noble metal nanostructured films have a huge potential for the development of efficient, tunable, miniaturized optical sensors. Herein, we report on the fabrication and characterization of gold-coated nanopost arrays, their use as refractometric sensors, and their optimization through photonics simulations. Monolithic square nanopost arrays having different period and nanopost size are fabricated by nanoimprint lithography on polymer foils, and sputter-coated by gold films. The reflectivity of these gold nanopost-shell arrays present dips in the visible range, which are efficient for refractometric sensing. By finite-difference time-domain (FDTD) simulations we reproduce the experimental spectra, describe the electric fields distribution around the nanopost-shells, and then explain their good sensitivity, around 450 nm/RIU. Furthermore, we determine by simulations the influence of several geometrical parameters, such as array period, nanopost width, gold film thickness, and nanopost side coverage on both reflectivity spectra and sensing capabilities. Fully coated nanoposts provide an extremely deep reflectivity minimum, approaching zero, which makes the relative reflectivity change extremely high, more than two orders of magnitude higher than for partially coated nanoposts. These results contribute to the understanding of the plasmonic properties of metal coated nanopost arrays, and to the development of efficient platforms for sensing and other surface plasmon based applications

Fabrication and Characterization of Hybrid and Tunable ZnO@Ag Flexible Thin Films Used as SERS Substrates

Flexible substrates have known increased popularity over rigid ones due to their use in surface-enhanced Raman scattering (SERS). They provide irregular surfaces, ideal for in situ sensing. In this context, we report the SERS performance of hybrid ZnO@Ag thin films deposited by magnetron sputtering (MS) on flexible, thermoplastic substrates. This physical deposition method is acknowledged for obtaining high-quality and reproducible ZnO films that can be embedded in (bio)sensing devices with various applications. Three types of thermoplastic-based, commercially available substrates with different glass transition temperatures (Tg) were chosen for the variation in flexibility, transparency, and thickness. Zeonor® (Tg = 136 °C, thickness of 188 μm) and two types of Topas (Topas®: Tg = 142 °C, thickness of 176 μm; Topas2: Tg = 78 °C, thickness of 140 μm) thermoplastic sheets are nonpolar and amorphous cyclo-olefin polymer (COP) and cyclo-olefin copolymers (COC), respectively. Their thicknesses and different values of Tg can greatly affect the topographical and roughness properties of films with small thicknesses and, thus, can greatly influence the enhancement of the Raman signal. The ZnO films deposited on top of Zeonor® or Topas® have identical morphological properties, as shown by the scanning electron microscopy (SEM) characterization. Subsequently, by using the MS technique, we tuned the thickness of the deposited silver (Ag) films in the range of 7–30 nm to assess the growth influence on the morphology and the SERS signal amplification of the substrates with and without the ZnO intermediate layer. The SEM analysis showed that the Ag atoms migrated both into the interstitial areas, filling the voids between the ZnO granular structures, and over the latter, forming, in this case, isolated Ag clusters. SERS analysis performed on the ZnO-Ag hybrid films using crystal violet (CV) molecule revealed a limit of detection (LOD) of 10−7 M in the case of 15 nm thick Ag/Zeonor® interlayer films ZnO and relative standard deviation (RSD) below 10%

The Influence of the Preparation Method on the Physico-Chemical Properties and Catalytic Activities of Ce-Modified LDH Structures Used as Catalysts in Condensation Reactions

Mechanical activation and mechanochemical reactions are the subjects of mechanochemistry, a special branch of chemistry studied intensively since the 19th century. Herein, we comparably describe two synthesis methods used to obtain the following layered double hydroxide doped with cerium, Mg3Al0.75Ce0.25(OH)8(CO3)0.5·2H2O: the mechanochemical route and the co-precipitation method, respectively. The influence of the preparation method on the physico-chemical properties as determined by multiple techniques such as XRD, SEM, EDS, XPS, DRIFT, RAMAN, DR-UV-VIS, basicity, acidity, real/bulk densities, and BET measurements was also analyzed. The obtained samples, abbreviated HTCe-PP (prepared by co-precipitation) and HTCe-MC (prepared by mechanochemical method), and their corresponding mixed oxides, Ce-PP (resulting from HTCe-PP) and Ce-MC (resulting from HTCe-MC), were used as base catalysts in the self-condensation reaction of cyclohexanone and two Claisen–Schmidt condensations, which involve the reaction between an aromatic aldehyde and a ketone, at different molar ratios to synthesize compounds with significant biologic activity from the flavonoid family, namely chalcone (1,3-diphenyl-2-propen-1-one) and flavone (2-phenyl-4H-1benzoxiran-4-one). The mechanochemical route was shown to have indisputable advantages over the co-precipitation method for both the catalytic activity of the solids and the costs

Scanning Electron Microscopy and Raman Spectroscopy Characterization of Structural Changes Induced by Thermal Treatment in Innovative Bio-Based Polyamide Nanocomposites

A comprehensive Raman scattering-based characterization of a full bio-based polyamide loaded with graphene nanoplatelets or layered double hydroxides (LDH) was assessed. The potential of the Raman spectroscopy was used to reveal several particularities of the nanocomposite structures induced by thermal treatment. Thus, a complete morpho-structural picture was obtained in combination with scanning electron microscopy (SEM) analysis of the neat polyamide and polyamide nanocomposites exposed at different thermal conditions (room temperature, 80 °C, and 145 °C). The analysis of G, D and 2D Raman peaks and their relative intensity ratio ID/IG, revealed the fact that the presence of graphene in polyamide is suitable for improving the essential physical properties and is also responsible for the decrease in the defects’ occurrence in the graphene layers. The surface of nanocomposites based on full bio-based polyamide, with different 2D fillers (graphenic and non-graphenic structures), was carefully evaluated before and after the thermal treatment by employing SEM and Raman analyses. The two thermal treatments allowed different chain mobility of the polymer (first temperature being over the polymer Tg and second one close to the melting phase in the viscoelastic stage). The spectroscopic and microscopic investigation was used to determine the conformational changes in filler aggregates and polymer surface, respectively

Detection and Characterization of Nodularin by Using Label-Free Surface-Enhanced Spectroscopic Techniques

Nodularin (NOD) is a potent toxin produced by Nodularia spumigena cyanobacteria. Usually, NOD co-exists with other microcystins in environmental waters, a class of cyanotoxins secreted by certain cyanobacteria species, which makes identification difficult in the case of mixed toxins. Herein we report a complete theoretical DFT-vibrational Raman characterization of NOD along with the experimental drop-coating deposition Raman (DCDR) technique. In addition, we used the vibrational characterization to probe SERS analysis of NOD using colloidal silver nanoparticles (AgNPs), commercial nanopatterned substrates with periodic inverted pyramids (KlariteTM substrate), hydrophobic Tienta® SpecTrimTM slides, and in-house fabricated periodic nanotrenches by nanoimprint lithography (NIL). The 532 nm excitation source provided more well-defined bands even at LOD levels, as well as the best performance in terms of SERS intensity. This was reflected by the results obtained with the KlariteTM substrate and the silver-based colloidal system, which were the most promising detection approaches, providing the lowest limits of detection. A detection limit of 8.4 × 10−8 M was achieved for NOD in solution by using AgNPs. Theoretical computation of the complex vibrational modes of NOD was used for the first time to unambiguously assign all the specific vibrational Raman bands

Mo-LDH-GO Hybrid Catalysts for Indigo Carmine Advanced Oxidation

This paper is focused on the utilization of hybrid catalysts obtained from layered double hydroxides containing molybdate as the compensation anion (Mo-LDH) and graphene oxide (GO) in advanced oxidation using environmentally friendly H2O2 as the oxidation agent for the removal of indigo carmine dye (IC) from wastewaters at 25 °C using 1 wt.% catalyst in the reaction mixture. Five samples of Mo-LDH-GO composites containing 5, 10, 15, 20, and 25 wt% GO labeled as HTMo-xGO (where HT is the abbreviation used for Mg/Al in the brucite type layer of the LDH and x stands for the concentration of GO) have been synthesized by coprecipitation at pH 10 and characterized by XRD, SEM, Raman, and ATR-FTIR spectroscopy, determination of the acid and base sites, and textural analysis by nitrogen adsorption/desorption. The XRD analysis confirmed the layered structure of the HTMo-xGO composites and GO incorporation in all samples has been proved by Raman spectroscopy. The most efficient catalyst was found to be the catalyst that contained 20%wt. GO, which allowed the removal of IC to reach 96.6%. The results of the catalytic tests indicated a strong correlation between catalytic activity and textural properties as well as the basicity of the catalysts