"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

All-dielectric core/shell resonators: From plasmon-free SERS to multimodal analysis

All-dielectric materials are emerging as a new class of substrates for enhanced Raman scattering. As ohmic losses are reduced in the absence of plasmonic metals, Raman data obtained with dielectrics are very reproducible and reliable. This mini-review summarizes our recent work in the field of core/shell dielectric resonators designed for Raman purposes, with a special focus on SiO2/TiO2 (T-rex) core/shell beads. These systems are able to exploit the evanescent field generated by total internal reflection and multiple scattering of light at the sphere-to-sphere interface to multiply the number of Raman photons, improving the sensitivity of Raman detection and extending the application of surface enhanced Raman scattering for investigating surface chemical reactions. Examples of the application of T-Rex beads in detecting and monitoring environmental pollutants, greenhouse gases, biochemical species, and biochemical reactions are presented. The use of core/shell resonators for multimodal analysis based on the combination of surface enhanced Raman scattering with either mass spectrometry or refractive index optical sensing is also discussed, suggesting different possible future development

Plasmon-free SERS detection of environmental CO2on TiO2surfaces

SiO2/TiO2 core/shell beads (T-rex) were designed, fabricated and tested for Raman detection of environmental CO2 under real-working conditions, as those encountered, for example, in solar-to-fuel conversion reactions. The exploitation of light trapping and morphology dependent resonances was crucial for extending the limit of detection of CO2 adsorbed on TiO2 surfaces. T-rex beads allowed for achieving surface enhanced Raman scattering (SERS) without using plasmonic metals showing high-efficiency, fast response and reproducibility in CO2 detection in both air and solvents. The dependence of SERS activity on Mie-type resonances was investigated through a systematic comparison of experimental data and numerical simulations, demonstrating that T-rex beads can be tailored for the detection of gaseous environmental pollutants on the basis of simple, Mie-scattering based calculations. Three-dimensional T-rex colloidal crystals were also successfully tested in precise, in situ, real time detection of CO2 as a function of different temperature-sweep cycles

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

Here we investigate for the first time the opto-thermal behavior of SiO2/Si core/shell microbeads (Si-rex) irradiated with three common Raman laser sources (lambda=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.</p

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

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

<p>Here we investigate for the first time the opto-thermal behavior of SiO₂/Si core/shell microbeads (Si-rex) irradiated with three common Raman laser sources (lambda=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.</p

Abstract Book: ISEV2017

Introduction: Surface Enhanced Raman spectroscopy (SERS) promises to be a powerful resource to provide information about the biochemical content of Extracellular Vesicles (EVs) in a fast and reproducible way. We firstly explored the ability of plasmonic and non-plasmonic SERS to probe nanosized EV populations separated from human serum of patients affected by Multiple Myeloma (MM) or Parkinsos’s disease (PD) and from healthy (H) donors. Typically, metal nanoparticle (NP) with a plasmonic resonance (e.g. Au) are utilized to enhance the Raman response (plasmonic SERS). However excited plasmonic NPs generate local heating and energy release, thereby inducing instability and low reproducibility, especially with organic or biological analytes. For this reason we also considered to probe EVs with innovative T-rex beads made of SiO2/TiO2 core/shell colloids that enhance the Raman fingerprint of the analyte by non-plasmonic SERS, thus expected to show a lower ability impact on the stability of the adsorbed EVs. Methods: EVs from serum of H patients, with MM or PD were purified using sequential centrifugation steps and discontinuous sucrose gradients. Samples were biochemically characterized by Western Blot analysis. Positive fractions to typical exosomal markers were pooled and further characterized for biophysical characteristics by atomic force microscopy (AFM), colloidal nanoplasmonic assays, and an agarose gel. EVs were then targeted with 15 nm Au NPs and analyzed by conventional SERS. In alternative EVs were coupled with T-rex beads for non-plasmonic SERS . Results: The colloidal nanoplasmonic assay allowed us to assess purity and determine the molar concentration of the EV formulations. AFM imaging confirmed the formulation to be composed of nanosized EV populations (50-100 nm). Both plasmonic and non-plasmonic SERS experiments gave promising results in terms of the possibility to use SERS profiling to identify each of the H, MM and PD EV populations. Our contribution will focus on presenting and discussing the last updates of these results (further experiments are ongoing). The institutional review board of Azienda Ospedaliera Spedali Civili of Brescia approved the study in adherence with the Declaration of Helsinki. This project was financed by the BIOMANE grant from the University of Brescia 2015