Biosensing based on nanophotonic structures has shown a great potential for cost-efficient, high-speed and compact personal medical diagnostics. While plasmonic nanosensors offer high sensitivity, their intrinsically restricted resonance quality factors and strong heating due to metal absorption impose severe limitations on real life applications. Here, we demonstrate an all-dielectric sensing platform based on silicon nanodisks with strong optically-induced magnetic resonances, which are able to detect a concentration of streptavidin of as low as 10 M (mol L) or 5 ng mL, thus pushing the current detection limit by at least two orders of magnitudes. Our study suggests a new direction in biosensing based on bio-compatible, non-toxic, robust and low-loss dielectric nanoresonators with potential applications in medicine, including disease diagnosis and drug detection.

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

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.

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.

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 developments.

Rohit Chikkaraddy opened the discussion of the Introductory Lecture: Regarding quantifying the chemical enhancement, you showed a systematic change in the SERS enhancement for halide substituted molecules due to charge transfer from the metal. Is the extra enhancement due to an inherent increase in the Raman cross-section of the molecule? How do you go about referencing, as the charge transfer changes the vibrational frequency? Richard Van Duyne answered: The extra enhancement is not due to an increase in the Raman cross section, as that is ratioed out in the calculation of the enhancement factor. The charge transfer (CT) process does not transfer a complete electron, it is a fractional degree of CT. Thus the change in vibrational frequency is small. DFT calculations that provide eigenvectors allow one to reference the vibrational modes of the free molecule with those of the adsorbed molecule. Sylwester Gawinkowski asked: You have shown that the enhancement factor curve is redshifted relative to the plasmon resonance band and has a maximum at about 800 nm. This means that the SERS signal should be strongest for excitations in the near infrared spectral region. Why do most SERS reports, particularly related to single molecule SERS, have the excitation in the green or red spectral range and not in the near infrared? Richard Van Duyne replied: The SERS excitation spectrum for isolated nanoparticles (e.g. the NSL nanotriangles that I showed in Fig. 1 of the introductory lecture 1 ) is redshifted with respect to the localized surface plasmon resonance (LSPR) by half the Stokes frequency of the vibrational mode. As the nanoparticle size is decreased the LSPR shifts to the blue so it is only for a specific size that one gets an LSPR maximum at 800 nm. Essentially all single molecule SERS experiments are done with dye molecules and the laser excitation wavelength is chosen to get maximum resonance Raman (RR) as well as SERS enhancement. For Rhodamine 6G (R6G) the laser excitation wavelength of 532 nm is close to the absorption maximum of R6G. SMSERS should be possible in the NIR for a wide range of dye molecules with absorption maxima in that spectra region. 1 A.-I. Henry, T. W. Ueltschi, M. O. McAnally and R. P. Van Duyne, Faraday Discuss., 2017, DOI: 10.1039/c7fd00181a. Marc Porter asked: Why is the oxidized form of nitrobenzene (I may not have the name of the reactant correct; my notes are a bit fuzzy, which I blame on jet lag) more sensitive to the local environment than its reduced from. Does the supporting electrolyte play a role here? Richard Van Duyne replied: The redox system you are referring to is the dye Nile Blue. The oxidized form is positively charged and the adsorption has electrostatic character. Hence it is more sensitive to the electrostatics of the local environment than the neutral reduced form. Sumeet Mahajan commented: In your work on surface-enhanced FSRS with a high rep rate laser why does the signal to noise not increase when there are 10× more pulses with the 1 MHz setup compared to the 100 kHz...

Binder and effector molecules that allow studying and manipulating epigenetic processes are of biological relevance and pose severe technical challenges. We report the first example of a synthetic receptor able to recognize mono-methylated lysines in a histone H3 tail peptide, which has relevant functions in epigenetic regulation. Recognition is robust and specific regardless of the position and the number of mono-methylated lysines along the polypeptide chain. The peptide is first captured in solution by a tetraphosphonate cavitand (Tiiii) that selectively binds its Lys-NMe moieties. Separation from solution and detection of the peptide-Tiiii complexes is then enabled in one single step by an all dielectric SiO-TiO core-shell resonator (T-rex), which captures the complex and operates fully reproducible signal transduction by non-plasmonic surface enhanced Raman scattering (SERS) without degrading the complex. The realized abiotic probe is able to distinguish multiple mono-methylated peptides from the single mono-methylated ones.

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.