Nicolò Bontempi

Colloidal lenses as universal Raman scattering enhancers

SiO2 microspheres were tested as micro-lenses in a series of Raman experiments, in order to evaluate their potential application in analysis of thin films and molecular species. We demonstrate that colloidal lenses can act as versatile, universal Raman scattering enhancers, that can be easily implemented into conventional microspectroscopy experiments. Our results indicate that colloidal lenses can strongly enhance Raman scattering of all the analytes under investigation, extending their detection limits by several orders of magnitude. Colloidal lenses can be exploited as non-destructive, disposable tools for Raman detection of ultra-thin films. They can also be coupled to either metal- or all-oxide-based SERS active substrates to further boost Raman sensitivity, offering exciting perspectives for ultrasensitive detection and in situ monitoring of chemical and biochemical reactions under real-working conditions.

Probing the spatial extension of light trapping-induced enhanced Raman scattering in high-density Si nanowire arrays

This paper reports an experimental investigation of surface-enhanced Raman scattering in high-density Si nanowire arrays obtained by electroless etching. A direct relationship between light trapping capabilities of Si nanowires and enhanced Raman scattering was demonstrated. Optimized arrays allowed for a remarkable increase of Raman sensitivity in comparison to reference planar samples. As a result, the detection limit of molecular probes under resonant excitation (e.g. methylene blue) can be extended by three orders of magnitude. In addition, continuous ultrathin films, that cannot be analyzed in conventional Raman experiments, are made detectable. In the case of anatase thin films, the detection limit of 5 nm was reached. Raman spectra of Si/TiO₂ core/shell heterostructures demonstrate that the enhanced field resulting from surface multiple scattering is characterized by a large spatial extension (about fifty nanometers), making these materials a potential alternative to plasmonic metals for SERS experiments.

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.

Non-Plasmonic SERS with Silicon: Is It Really Safe? New Insights into the Optothermal Properties of Core/Shell Microbeads

Silicon is one of the most interesting candidates for plasmon-free surface-enhaced Raman scattering (SERS), because of its high-refractive index and thermal stability. However, here we demonstrate that the alleged thermal stability of silicon nanoshells irradiated by conventional Raman laser cannot be taken for granted. We investigated the opto-thermal behavior of SiO2/Si core/shell microbeads (Si-rex) irradiated with three common Raman laser sources (λ = 532, 633, 785 nm) under real working conditions. We obtained an experimental proof of the critical role played by bead size and aggregation in heat and light management, demonstrating that, in the case of strong opto-thermal coupling, the temperature can exceed that of the melting points of both core and shell components. In addition, we also show that weakly coupled beads can be utilized as stable substrates for plasmon-free SERS experiments.

Photo-induced heat generation in non-plasmonic nanoantennas

Light-to-heat conversion in non-plasmonic, high refractive index nanoantennas is a key topic for many applications, including Raman sensing, laser writing, nanofabrication and photo-thermal therapy. However, heat generation and propagation in non-plasmonic antennas is increasingly debated and contradictory results have been reported so far. Here we report a finite element analysis of the steady-state temperature distribution and heat flow in SiO2/Si core/shell systems (silicon nanoshells) irradiated with different continuous wave lasers (λ = 532, 633 and 785 nm), under real working conditions. The complex interplay among the optical properties, morphology, degree of crystallinity of the nanoshells, thickness dependence of thermal conductivity and interactions with the substrate has been elucidated. This study reveals that all of these parameters can be appropriately combined for obtaining either stable nanoshells for Raman sensing or highly efficient sources of local heating. The optimal balance between thermal stability and field enhancement was found for crystalline Si shell layers with thicknesses ranging from 40 to 60 nm, irradiated by a NIR laser source. On the other hand, non-conformal amorphous or crystalline shell layers with a thickness >50 nm can reach a very high local temperature (above 1000 K) when irradiated with a low power density (less than 1 mW μm-2) laser sources. This work provides a general approach for an extensive investigation of the opto-thermal properties of high-index nanoantennas.

Ultra-sensitive biosensing with dielectric nanoantennas

We demonstrate new direction in biosensing based on bio-compatible, non-toxic, robust and low-loss dielectric nanoantennas. Using biotin-coated silicon nanodisks with optically-induced magnetic resonances we detect streptavidin with concentration as low as 10−10 mol/L.

Design of All-Dielectric Photonic Crystals for Surface-Enhanced Raman Scattering

We present the design and optimization of all-dielectric photonic crystal devices for surface-enhanced Raman scattering based applications.