Antonino Foti

SERS detection of Biomolecules at Physiological pH via aggregation of Gold Nanorods mediated by Optical Forces and Plasmonic Heating

Strategies for in-liquid molecular detection via Surface Enhanced Raman Scattering (SERS) are currently based on chemically-driven aggregation or optical trapping of metal nanoparticles in presence of the target molecules. Such strategies allow the formation of SERS-active clusters that efficiently embed the molecule at the “hot spots” of the nanoparticles and enhance its Raman scattering by orders of magnitude. Here we report on a novel scheme that exploits the radiation pressure to locally push gold nanorods and induce their aggregation in buffered solutions of biomolecules, achieving biomolecular SERS detection at almost neutral pH. The sensor is applied to detect non-resonant amino acids and proteins, namely Phenylalanine (Phe), Bovine Serum Albumin (BSA) and Lysozyme (Lys), reaching detection limits in the μg/mL range. Being a chemical free and contactless technique, our methodology is easy to implement, fast to operate, needs small sample volumes and has potential for integration in microfluidic circuits for biomarkers detection.

A Shape-Engineered Surface-Enhanced Raman Scattering Optical Fiber Sensor Working from the Visible to the Near-Infrared

Surface-enhanced Raman scattering (SERS) takes advantage of the giant electromagnetic field enhancement provided by localized surface plasmons in metal nanoparticles to amplify the weak Raman scattering of the molecules. Optical fibers coated with noble metal nanoparticles can therefore be used as SERS-based sensors for remote detection of molecular species. In this article, we report on the development of an optical fiber SERS sensor capable to operate on a range of excitation wavelengths from the visible to the near-infrared. We introduce a quasistatic chemical etching protocol to engineer the tip shape and investigate the effects of the tip shape on the sensor performances.

Double-Wall Nanotubes and Graphene Nanoplatelets for Hybrid Conductive Adhesives with Enhanced Thermal and Electrical Conductivity

Improving the electrical and thermal properties of conductive adhesives is essential for the fabrication of compact microelectronic and optoelectronic power devices. Here we report on the addition of a commercially available conductive resin with double-wall carbon nanotubes and graphene nanoplatelets that yields simultaneously improved thermal and electrical conductivity. Using isopropanol as a common solvent for the debundling of nanotubes, exfoliation of graphene, and dispersion of the carbon nanostructures in the epoxy resin, we obtain a nanostructured conducting adhesive with thermal conductivity of ∼12 W/mK and resistivity down to 30 μΩ cm at very small loadings (1% w/w for nanotubes and 0.01% w/w for graphene). The low filler content allows one to keep almost unchanged the glass-transition temperature, the viscosity, and the curing parameters. Die shear measurements show that the nanostructured resins fulfill the MIL-STD-883 requirements when bonding gold-metalized SMD components, even after repeated thermal cycling. The same procedure has been validated on a high-conductivity resin characterized by a higher viscosity, on which we have doubled the thermal conductivity and quadrupled the electrical conductivity. Graphene yields better performances with respect to nanotubes in terms of conductivity and filler quantity needed to improve the resin. We have finally applied the nanostructured resins to bond GaN-based high-electron-mobility transistors in power-amplifier circuits. We observe a decrease of the GaN peak and average temperatures of, respectively, ∼30 °C and ∼10 °C, with respect to the pristine resin. The obtained results are important for the fabrication of advanced packaging materials in power electronic and microwave applications and fit the technological roadmap for CNTs, graphene, and hybrid systems.

Optical Aggregation of Gold Nanoparticles for SERS Detection of Proteins and Toxins in Liquid Environment: Towards Ultrasensitive and Selective Detection

Optical forces are used to aggregate plasmonic nanoparticles and create SERS–active hot spots in liquid. When biomolecules are added to the nanoparticles, high sensitivity SERS detection can be accomplished. Here, we pursue studies on Bovine Serum Albumin (BSA) detection, investigating the BSA–nanorod aggregations in a range from 100 µM to 50 nM by combining light scattering, plasmon resonance and SERS, and correlating the SERS signal with the concentration. Experimental data are fitted with a simple model describing the optical aggregation process. We show that BSA–nanorod complexes can be optically printed on non-functionalized glass surfaces, designing custom patterns stable with time. Furthermore, we demonstrate that this methodology can be used to detect catalase and hemoglobin, two Raman resonant biomolecules, at concentrations of 10 nM and 1 pM, respectively, i.e., well beyond the limit of detection of BSA. Finally, we show that nanorods functionalized with specific aptamers can be used to capture and detect Ochratoxin A, a fungal toxin found in food commodities and wine. This experiment represents the first step towards the addition of molecular specificity to this novel biosensor strategy.

On the SERS depolarization ratio

Abstract The Raman depolarization ratio is a quantity that can be easily measured experimentally and offers unique information on the Raman polarizability tensor of molecular vibrations. In Surface Enhanced Raman Scattering (SERS), molecules are near-field coupled with optical nanoantennas and their scattering properties are strongly affected by the radiation patterns of the nanoantenna. The polarization of the SERS photons is consequently modified, affecting, in a non trivial way, the measured value of the SERS depolarization ratio. In this article we elaborate a model that describes how the SERS depolarization ratio is influenced by the nanoantenna re-radiation properties, suggesting how to retrieve information on the Raman polarizability from SERS experiments.

Nanoscale Discrimination between Toxic and Nontoxic Protein Misfolded Oligomers with Tip-Enhanced Raman Spectroscopy

Highly toxic protein misfolded oligomers associated with neurological disorders such as Alzheimers and Parkinsons diseases are nowadays considered primarily responsible for promoting synaptic failure and neuronal death. Unraveling the relationship between structure and neurotoxicity of protein oligomers appears pivotal in understanding the causes of the pathological process, as well as in designing novel diagnostic and therapeutic strategies tuned toward the earliest and presymptomatic stages of the disease. Here, it is benefited from tip-enhanced Raman spectroscopy (TERS) as a surface-sensitive tool with spatial resolution on the nanoscale, to inspect the spatial organization and surface character of individual protein oligomers from two samples formed by the same polypeptide sequence and different toxicity levels. TERS provides direct assignment of specific amino acid residues that are exposed to a large extent on the surface of toxic species and buried in nontoxic oligomers. These residues, thanks to their outward disposition, might represent structural factors driving the pathogenic behavior exhibited by protein misfolded oligomers, including affecting cell membrane integrity and specific signaling pathways in neurodegenerative conditions.

Low cost tips for tip-enhanced Raman spectroscopy fabricated by two-step electrochemical etching of 125 µm diameter gold wires

Tip-enhanced Raman spectroscopy (TERS) has become a well-applied technique for nanospectroscopy, allowing for single molecule sensitivity with sub-nanometer spatial resolution. The demand for efficient, reproducible and cost-effective probes for TERS is increasing. Here we report on a new electrochemical etching protocol to fabricate TERS tips starting from 125 µm diameter gold wires in a reproducible way. The process is reliable (50% of the tips have radius of curvature <35 nm, 66% <80 nm), fast (less than 2 min) and 2.5 times cheaper than the etching of standard 250 µm diameter wires. The TERS performance of the tips is tested on dyes, pigments and biomolecules and enhancement factors higher than 105 are observed. TERS mapping with a spatial resolution of 5 nm is demonstrated.