Metal oxide microrings with femtoliter capacity for Raman microspectroscopy

"RaMassays": Synergistic Enhancement of Plasmon-Free Raman Scattering and Mass Spectrometry for Multimodal Analysis of Small Molecules

SiO₂/TiO₂ core/shell (T-rex) beads were exploited as "all-in-one" building-block materials to create analytical assays that combine plasmon-free surface enhanced Raman scattering (SERS) and surface assisted laser desorption/ionization (SALDI) mass spectrometry (RaMassays). Such a multi-modal approach relies on the unique optical properties of T-rex beads, which are able to harvest and manage light in both UV and Vis range, making ionization and Raman scattering more efficient. RaMassays were successfully applied to the detection of small (molecular weight, M.W. <400 Da) molecules with a key relevance in biochemistry and pharmaceutical analysis. Caffeine and cocaine were utilized as molecular probes to test the combined SERS/SALDI response of RaMassays, showing excellent sensitivity and reproducibility. The differentiation between amphetamine/ephedrine and theophylline/theobromine couples demonstrated the synergistic reciprocal reinforcement of SERS and SALDI. Finally, the conversion of L-tyrosine in L-DOPA was utilized to probe RaMassays as analytical tools for characterizing reaction intermediates without introducing any spurious effects. RaMassays exhibit important advantages over plasmonic nanoparticles in terms of reproducibility, absence of interference and potential integration in multiplexed devices.

Cavitands Endow All-Dielectric Beads With Selectivity for Plasmon-Free Enhanced Raman Detection of Nε-Methylated Lysine

SiO2/TiO2 microbeads (T-rex) are promising materials for plasmon-free surface-enhanced Raman scattering (SERS), offering several key advantages in biodiagnostics. In this paper we report the combination of T-rex beads with tetraphosphonate cavitands (Tiiii), which imparts selectivity toward Nε-methylated lysine. SERS experiments demonstrated the efficiency and selectivity of the T-rex-Tiiii assays in detecting methylated lysine hydrochloride (Nε-Me-Lys-Fmoc) from aqueous solutions, even in the presence of the parent Lys-Fmoc hydrochloride as interferent. The negative results obtained in control experiments using TSiiii ruled out any other form of surface recognition or preferential physisorption. MALDI-TOF analyses on the beads exposed to Nε-Me-Lys-Fmoc revealed the presence of the Tiiii•Nε-Me-Lys-Fmoc complex. Raman analyses based on the intensity ratio of Nε-Me-Lys-Fmoc and cavitand-specific modes resulted in a dose-response plot, which allowed for estimating the concentration of Nε-methylated lysine from initial solutions in the 1 × 10(-3) to 1 × 10(-5) M range. These results can set the basis for the development of new Raman assays for epigenetic diagnostics.

"Stainless" Gold Nanorods: Preserving Shape, Optical Properties, and SERS Activity in Oxidative Environment

One of the main limitations to the application of gold nanorods (Au NRs) as surface-enhanced Raman scattering (SERS) probes for in situ monitoring of chemical processes is their instability in oxidative environments. Oxidation induces progressive anisotropic shortening of the NRs, which are eventually dissolved once this process has been completed. This paper compares two types of Au NRs, obtained through different routes and characterized by similar aspect ratios but different sizes. The key factors influencing the resistance of Au NRs to oxidation were systematically investigated, showing that the reduction of free bromide species and the increase of the particle size allowed the NRs to maintain their stability under harsh environments for several weeks. The most stable Au NRs were also demonstrated to be highly efficient SERS substrates in a series of Raman experiments involving molecular probes, treated under either oxidizing or nonoxidizing conditions, which simulate the oxidation of organic pollutants in water. These hallmarks make these "stainless" Au NRs attractive tools for ultrasensitive diagnostic under real working conditions.

All-oxide Raman-active traps for light and matter: probing redox homeostasis model reactions in aqueous environment

Core-shell colloidal crystals can act as very efficient traps for light and analytes. Here it is shown that Raman-active probes can be achieved using SiO2-TiO2 core-shell beads. These systems are successfully tested in monitoring of glutathione redox cycle at physiological concentration in aqueous environment, without need of any interfering enhancers. These materials represent a promising alternative to conventional, metal-based SERS probes for investigating chemical and biochemical reactions under real working conditions.

Triggering and monitoring plasmon-enhanced reactions by optical nanoantennas coupled to photocatalytic beads

Plasmonic metal/semiconductor nanocomposites promise to be a breakthrough for boosting and investigating photon-assisted processes at the nanoscale, with exciting perspectives for energy conversion and catalysis. However, the efficiency and selectivity of these surface processes are still far from being controlled. Here, shown for the first time, is a new class of photocatalyst which is based on the synergistic combination of bowtie-like gold nanoantennas and SiO2 /TiO2 core/shell oxide beads. These systems are exploited as efficient near-field optical light concentrators, stimulating photon-driven processes at the metal-semiconductor interface. Extraordinary enhancements of photodegradation rates (minutes instead of hours) result from matching the nanoantenna surface plasmon resonance with the optical absorption of organic dyes and the excitation source wavelength. Moreover, strong Raman enhancements are observed allowing for direct in-situ monitoring of reaction progress of different analytes on the same site.

Simple, large-scale patterning of hydrophobic ZnO nanorod arrays

Here we describe a simple, versatile technique to produce large-scale arrays of highly ordered ZnO nanorods. Patterning of three distinct ZnO crystal morphologies is demonstrated through use of different ZnO seed layers. Array formation is accomplished through a simple variation on nanosphere lithography that imprints a thickness variation across a PMMA mask layer. The area of exposed seed layer is controlled through etching time in an oxygen plasma. Subsequent hydrothermal growth from the patterned seed layer produces high-quality ZnO crystals in uniform arrays. The high uniformity of the patterned array is shown to induce a high contact angle hydrophobic state even without the need for chemical modification of the ZnO surface. This technique provides a straightforward way to integrate the optical and electrical properties of high-quality ZnO nanorods with the tunable fluidic properties at the surface of well-ordered arrays.

Recyclable SERS substrates based on Au-coated ZnO nanorods

Vertically aligned Au-coated ZnO nanorods (Au-ZnO NRs) were investigated as cheap, efficient and recyclable SERS-active substrates. The ZnO NRs were prepared through a simple, low-temperature hydrothermal route and made SERS-active through deposition of gold nanoislands by sputtering at room temperature. Optimized samples were able to detect methylene blue over a wide range of low concentrations (from 1 × 10(-4) to 1 × 10(-12) M), with good reproducibility. The photocatalytic properties of Au-ZnO NRs were exploited to recycle these substrates through UV-assisted cleaning. The experimental results showed that these substrates are characterized by high reproducibility and long shelf life, which make them promising as SERS platforms for multiple detection of different molecular species.