Ivano Alessandri

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.

Enhanced Raman Scattering with Dielectrics

Dielectrics represent a new frontier for surface-enhanced Raman scattering. They can serve as either a complement or an alternative to conventional, metal-based SERS, offering key advantages in terms of low invasiveness, reproducibility, versatility, and recyclability. In comparison to metals, dielectric systems and, in particular, semiconductors are characterized by a much greater variety of parameters and properties that can be tailored to achieve enhanced Raman scattering or related effects. Light-trapping and subwavelength-focusing capabilities, morphology-dependent resonances, control of band gap and stoichiometry, size-dependent plasmons and excitons, and charge transfer from semiconductors to molecules and vice versa are a few examples of the manifold opportunities associated with the use of semiconductors as SERS-active materials. This review provides a broad analysis of SERS with dielectrics, encompassing different optical phenomena at the basis of the Raman scattering enhancement and introducing future challenges for light harvesting, vibrational spectroscopy, imaging, and sensing.

Tailoring the Pore Size and Architecture of CeO2/TiO2 Core/Shell Inverse Opals by Atomic Layer Deposition

Hierarchically ordered porous materials have been receiving attention because of their large number of applications in several fields of materials science, including catalysis, optoelectronics, molecular sieves, drug delivery, and electrodes for fuel, solar, and thermophotovoltaic cells. Among the variety of templating agents for preparing organized mesostructures, colloidal crystals are widely used to generate twoand threedimensional (2D and 3D) photonic-bandgap materials, which exhibit Bragg diffraction within frequency ranges for which light propagation through the material is forbidden. Stopband reflections have been recently proposed to be exploited in the field of photocatalysis. Indeed, it has been demonstrated that, at the frequency edges of these stop bands, photons propagatewith strongly reduced group velocity. If the energy of these slow photons overlaps with the absorbance of the material, the effective optical path length is significantly enhanced, thus improving the catalytic activity of the system. Recently, Chen et al. demonstrated that the photoactivity of anatase TiO2 inverse opals is remarkably enhanced by using slow photons with energies close to the electronic bandgap of the semiconductor. In a following paper, the same authors demonstrated that a certain degree of microstructural disorder, which is inherently associated with these materials, can be tolerated without preventing their use in some important applications, such as purification of water from environmental pollutants (dyes, viruses, etc.). In addition, Mallouk and co-workers and, more recently, Corma and coworkers investigated the use of the photonic crystal topology for photoelectrochemical solar cells. TiO2 inverse opals exhibited a conversion efficiency of light that is five times higher than that of nonphotonic crystal reference samples.

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.

In Situ Plasmon-Heating-Induced Generation of Au/TiO2 “Hot Spots” on Colloidal Crystals

SERS you right: The plasmon heating of gold nanoshells is exploited to yield the local conversion of amorphous TiO(2) into anatase on the surface of polymeric colloidal crystals (see scheme). The resulting Au/TiO(2) spots are active substrates for surface-enhanced Raman spectroscopy and allow surface reactions and processes to be followed directly on-site.

Plasmonic heating assisted deposition of bare Au nanoparticles on titania nanoshells

A very simple procedure for depositing bare Au nanoparticles onto the surface of oxide nanoshells is presented. This method is based on plasmonic heating assisted processes which are triggered by a continuous wave laser source that is focused through the objective of a Raman microscope. The Au nanoparticles are obtained upon laser irradiation of colloidal crystals consisting of Au semishells buried in titania overlayers. SERS activity and heating effects induced by laser irradiation are demonstrated in a series of microRaman experiments.

Using plasmonic heating of gold nanoparticles to generate local SER(R)S-active TiO2 spots

The plasmonic heating generated by laser irradiation of gold nanoparticles was exploited for fabricating SER(R)S-active anatase spots.

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.

Laser-induced modification of polymeric beads coated with gold nanoparticles

A direct laser writing method for modifying colloidal crystals and single colloids is presented. This method takes advantage of the highly efficient conversion of photons into heat exhibited by gold nanoparticles. The easy control of experimental parameters allowed control of the spatial resolution of the patterns. This may open the way to practical applications for the technology.

Writing, self-healing, and self-erasing on conductive pressure-sensitive adhesives.

A simple strategy for enabling conductive pressure sensitive adhesives (PSAs) to work as light-responsive materials is reported. Direct laser-writing of PSA substrates was achieved by means of a continuous-wave He-Ne laser focused through the objectives of an optical microscope. This approach takes advantage of cooperative interplay between viscoelastic properties of PSAs and enhanced thermal conductivity provided by an extra overlayer of gold. In particular, the thickness of the gold layer is a crucial parameter for tuning the substrate responsiveness. Self-healing and self-degradation processes can be exploited for controlling the lifetime of the written information, whereas additional protective coatings can be introduced to achieve permanent storage.