Development of novel nano-structured materials for Enhanced Raman Spectroscopies: an insight in SERS and TERS applications

Owing to their ultrahigh selectivity and sensitivity, the plasmon-enhanced Raman spectroscopies (PERS) methods have emerged as diverse and exciting cutting-edge techniques for the investigation of biosystems at nanometric scales in air or water environments. By exploiting the plasmonic properties of noble metal nanoparticles, the PERS methods enable to remove the main obstacle of the Raman spectroscopy represented by the small Raman cross- section. Among them, the Surface Enhanced Raman Spectroscopy (SERS) is certainly the most important in terms of the number of applications in many fields of science (physics, chemistry, and biomedicine). One of the interesting features of the SERS is that the huge amplification of inelastic Raman photons can reach up to 12 orders of magnitude allowing even the detection at single-molecule level. In addition, the strong distance dependence of the plasmonic near-field effect (∼ 10-20 nm) makes effective the SERS only for molecules in proximity to metal-nanostructured surfaces and, thus, suitable not only for the bio-analysis of membranes but also for the surface characterization in science materials. Anyway, beyond the high sensitivity, the limitation of the SERS is represented by the diffracted-limited spatial resolution. A significant improvement is given by the modern tip-enhanced Raman spectroscopy (TERS) technique. By combining the high resolution of scanning-probe microscope (SPM) technology and the sensitivity of SERS, TERS is capable to correlate topographical and chemical information of a sample at nanoscale level. In fact, the Raman signal coming from the probed molecules is strongly enhanced via SERS effect when they are in proximity of the apex of a metalized or metallic SPM tips. Moreover, the scattering efficiency of TERS signal is greatly increased when the metal surface of the probe is nano-structured. The spatial resolution of TERS signals is mainly ruled by the tip-radius, which is typically of few tens of nanometers, therefore allowing to reach a lateral resolution in the range of 10-50 nm, far beyond the diffraction limit. Anyway, the development of reliable and effective plasmonic devices for SERS and TERS applications represents the major obstacle towards a wider diffusion of TERS/SERS as powerful analytical tools in material science and life science. In the case of TERS, the main technological challenge is based on the fabrication of metal nano-structures on the tip. Compared to SERS substrates that are produced on large-area surfaces, the sub-micron dimensions of the tip apex make the nano-structuring task more tricky. In this frame, the current thesis work aims to present a novel and versatile method for the preparation of appropriate AFM-TERS tips and SERS substrates. The innovative approach is based on the application of a radio-frequency discharge produced by an inductively coupled plasma (ICP) on commercial Ag-covered AFM probes. The plasma treatment produces an intriguing metallic porous nanotexture resembling a coral-like structure. The so-produced probes have been characterized by showing an amplification up to six orders of magnitude and a spatial resolution down to 10 nm, which render these devices particularly attractive for nanometer chemical characterization. In addition, this method has been successfully implemented for the fabrication of broad-band SERS-active platforms. This protocol has shown to be effective to produce substrates that can amplify the Raman signal up to seven orders of magnitude. Finally, another method for the fabrication of SERS substrates, based on the self-assembly of block copolymer (BCP) loaded with Ag-NPs, is proposed. The sensitivity of the so-prepared substrates has been tested by revealing the over-expression of target proteins in membranes of cancer cells

Feasibility of SERS-Active Porous Ag Substrates for the Effective Detection of Pyrene in Water

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants that are typically released into the environment during the incomplete combustion of fossil fuels. Due to their relevant carcinogenicity, mutagenicity, and teratogenicity, it is urgent to develop sensitive and cost-effective strategies for monitoring them, especially in aqueous environments. Surface-enhanced Raman spectroscopy (SERS) can potentially be used as a reliable approach for this purpose, as it constitutes a valid alternative to traditional techniques, such as liquid and gas chromatography. Nevertheless, the development of an SERS-based platform for detection PAHs has so far been hindered by the poor adsorption of PAHs onto silver-and gold-based SERS-active substrates. To overcome this limitation, several research efforts have been directed towards the development of functionalized SERS substrates for the improvement of PAH adsorption. However, these strategies suffer from the interference that functionalizing molecules can produce in SERS detection. Herein, we demonstrate the feasibility of label-free detection of pyrene by using a highly porous 3D-SERS substrate produced by an inductively coupled plasma (ICP). Thanks to the coral-like nanopattern exhibited by our substrate, clear signals ascribable to pyrene molecules can be observed with a limit of detection of 23 nM. The observed performance can be attributed to the nanoporous character of our substrate, which combines a high density of hotspots and a certain capability of trapping molecules and favoring their adhesion to the Ag nanopattern. The obtained results demonstrate the potential of our substrates as a large-area, label-free SERS-based platform for chemical sensing and environmental control applications

A simple and reliable approach for the fabrication of nanoporous silver patterns for surface-enhanced Raman spectroscopy applications

The fabrication of plasmonic nanostructures with a reliable, low cost and easy approach has become a crucial and urgent challenge in many fields, including surface-enhanced Raman spectroscopy (SERS) based applications. In this frame, nanoporous metal films are quite attractive, due to their intrinsic large surface area and high density of metal nanogaps, acting as hot-spots for Raman signal enhancement. In this paper, we report a detailed study on the fabrication of nanoporous silver-based SERS substrates, obtained by the application of two successive treatments with an Inductively Coupled Plasma (ICP) system, using synthetic air and Ar as feeding gases. The obtained substrates exhibit a quite broad plasmonic response, covering the Vis–NIR range, and an enhancement factor reaching 6.5 ×107, estimated by using 4-mercaptobenzoic acid (4-MBA) as probe molecule at 532 nm. Moreover, the substrates exhibit a quite good spatial reproducibility on a centimeter scale, which assures a good signal stability for analytical measurements. Globally, the developed protocol is easy and cost effective, potentially usable also for mass production thanks to the remarkable inter-batches reproducibility. As such, it holds promise for its use in SERS-based sensing platforms for sensitive detection of targets molecules

Nanoporous silver films produced by solid-state dewetting for SERS applications

Detection of analytes in aqueous solution with high specificity and sensitivity is of paramount importance in many fields of science, ranging from biomedicine, environmental control, and food quality assessment. Surface-enhanced Raman scattering (SERS) has proven to be a cutting-edge analytical technique for this purpose, by combining the high selectivity of Raman features with the high sensitivity deriving from the plasmonic amplification of Raman signals. Herein, we report a facile and quite effective approach to fabricate large-area Ag-based SERS substrates, exhibiting a porous, coral-like nanotexture. Due to their intrinsic large surface-area and high hot-spot density, the produced substrates appear quite promising for the detection of analytes at trace levels. The nanoporous substrates are produced by Solid-State Dewetting (SSD) of thin Ag-films. In particular, ~30 nm thickness Ag-films are first deposited on glass coverslips by magnetron sputtering. Then, marked roughening is induced by exposing the films to an Inductively Coupled Plasma (ICP) discharge, using synthetic air as feeding gas. The performances of our SERS substrates are characterized in terms of morphology and enhancement factor using CV as probe molecule

Isolation of a 25-kDa protein binding to a curved DNA upstream the origin of the L strand replication in the rat mitochondrial genome

The presence of a curved DNA sequence in the gene for the NADH-dehydrogenase subunit 2 of rat mitochondrial genome, upstream from the origin of the light strand replication have been demonstrated through theoretical analysis and experimental approaches. Gel retardation assays showed that this structure makes a complex with a protein component extracted from the mitochondrial matrix. The isolation and purification of this protein is reported. With a Sepharose CL-6B and magnetic DNA affinity chromatography a polypeptide was purified to homogeneity having 25-kDa mass as shown by gel electrophoresis. To functionally characterize this protein, its capability to bind to other sequences of the homologous or heterologous DNA and to specific riboprobes was also investigated. A role for this protein as a trans-acting agent required for the expression of the mammalian mitochondrial genome is suggested

Step-by-step iconographic description of a prolonged but still favourable course of orbital cellulitis in a child with acute rhinosinusitis: an iconographic case study

Orbital cellulitis is an infrequent complication of acute ethmoiditis possibly leading to life- or visual-threatening complications. Despite its natural history is well known, its clinical evolution may widely vary among patients, and even in the most favourable cases long-term sequelae may persist. We here provide a step-by-step iconographic description of a periorbital and orbital cellulitis occurring in a child with ipsilateral acute rhinosinusitis. Our report shows that an unusual long-term evolution of periorbital and orbital cellulitis is possible also in apparently favourable cases

Upper Respiratory Tract Microbiome and Otitis Media Intertalk : Lessons from the Literature

Otitis media (OM) is one of the most common diseases occurring during childhood. Microbiological investigations concerning this topic have been primarily focused on the four classical otopathogens (Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pyogenes) mainly because most of the studies have been conducted with culture-dependent methods. In recent years, the introduction of culture-independent techniques has allowed high-throughput investigation of entire bacterial communities, leading to a better comprehension of the role of resident flora in health and disease. The upper respiratory tract (URT) is a region of major interest in otitis media pathogenesis, as it could serve as a source of pathogens for the middle ear (ME). Studies conducted with culture-independent methods in the URT and ME have provided novel insights on the pathogenesis of middle ear diseases through the identification of both possible new causative agents and of potential protective bacteria, showing that imbalances in bacterial communities could influence the natural history of otitis media in children. The aim of this review is to examine available evidence in microbiome research and otitis media in the pediatric age, with a focus on its different phenotypes: acute otitis media, otitis media with effusion and chronic suppurative otitis media

Single-Cell Photothermal Analysis Induced by MoS2 Nanoparticles by Raman Spectroscopy

Two-dimensional nanomaterials, such as MoS2 nanosheets, have been attracting increasing attention in cancer diagnosis and treatment, thanks to their peculiar physical and chemical properties. Although the mechanisms which regulate the interaction between these nanomaterials and cells are not yet completely understood, many studies have proved their efficient use in the photothermal treatment of cancer, and the response to MoS2 nanosheets at the single-cell level is less investigated. Clearly, this information can help in shedding light on the subtle cellular mechanisms ruling the interaction of this 2D material with cells and, eventually, to its cytotoxicity. In this study, we use confocal micro-Raman spectroscopy to reconstruct the thermal map of single cells targeted with MoS2 under continuous laser irradiation. The experiment is performed by analyzing the water O-H stretching band around 3,400 cm−1 whose tetrahedral structure is sensitive to the molecular environment and temperature. Compared to fluorescence-based approaches, this Raman-based strategy for temperature measurement does not suffer fluorophore instability, which can be significant under continuous laser irradiation. We demonstrate that irradiation of human breast cancer MCF7 cells targeted with MoS2 nanosheets causes a relevant photothermal effect, which is particularly high in the presence of MoS2 nanosheet aggregates. Laser-induced heating is strongly localized near such particles which, in turn, tend to accumulate near the cytoplasmic membrane. Globally, our experimental outcomes are expected to be important for tuning the nanosheet fabrication process