Over the last decades, dielectric Whispering-Gallery Mode (WGM) micro-resonators, such as spherical or toroidal structures, have demonstrated their effectiveness for highly sensitive biochemical sensing [1]. When a WGM resonance is excited, light is tightly concentrated within a small modal volume, greatly enhancing the interaction with analytes. Conversely, Surface-Enhanced Raman Spectroscopy (SERS) is a powerful technique that merges the molecular fingerprinting capabilities of Raman spectroscopy with the optical near-field enhancement provided by localized surface-plasmon resonances (LSPRs) on metallic nanostructures. This combination enables the highly sensitive and selective detection of various substances, even in liquid environments [2].

In this work, we present a sensor for the detection of melamine, a nitrogen-rich compound that has been illegally used to enhance the apparent protein content in dairy products. Due to its toxicity, strict regulatory limits have been established: 1 ppm in infant formula and 2.5 ppm in other food products. Hence, the development of reliable, rapid, and highly sensitive detection methods is essential. Conventional analytical techniques used for melamine detection are High-performance liquid chromatography (HPLC), Gas chromatography (GS), and Enzyme-linked immunosorbent assay (ELISA) [ 1-3]. Despite their good reliability, these methods suffer from high costs and long turnaround time. Surface-Enhanced Raman Scattering (SERS) has overcome these disadvantages, being a simpler, quicker, and cheaper tool for detecting trace amounts of toxic molecules across multiple fields, namely in food safety. In our work, we used a highly efficient and easily fabricated SERS substrate based on a cold-plasma-assisted silver film. This approach allows us to achieve signal amplification factors of the order of 10(7), while maintaining excellent reproducibility. To evaluate the performance of our biosensor, we calibrated the system using melamine dissolved in water. Then, we extended our study to detect melamine in a milk matrix. Unlike many previously reported methods that require chemical treatments or extensive sample processing, we adopted a straightforward approach undergoing the milk samples to a simple centrifugation step. This effectively removed macromolecules such as fats and proteins, which could otherwise interfere with detection. Our results are particularly promising, as we achieved a detection limit of 200 ppb, well below the regulatory limits for melamine in food. This demonstrates the strong potential of our SERS-based sensor as a rapid, cost-effective, and highly sensitive tool for food safety monitoring.

The development of easy, robust and cheap approaches for nanopatterning metallic surfaces is of great importance for many plasmon-based techniques, including surface-enhanced Raman spectroscopy (SERS). Herein, we report on the fabrication of bimetallic plasmonic nanopatterns for SERS applications. The fabrication protocol includes two steps: in the first step, a porous silver-based nanopattern was achieved by applying an inductively coupled plasma (ICP) on a flat Ag-film, which gives rise to a coral-like structure. In the second step, a gold layer was deposited on this pattern, acting as a protective layer against silver oxidation. Our analysis reveals that the developed bimetallic nanopatterns exhibit a relevant amplification and broad spectral response. Moreover, the fabrication protocol is easy and cost effective, therefore potentially usable also for mass production. Globally, it holds promise for its use in SERS-based sensing platforms for sensitive detection of targets molecules and in-vitro analysis of cells.

We present a novel and versatile strategy for fabricating high-performance surface-enhanced Raman scattering (SERS) substrates by depositing thin gold films onto coral-like silver (Ag) or silver oxide (AgO) nanostructured templates. These porous architectures, produced via a cold plasma-assisted process, exhibit broad plasmonic responses and a large fractal surface area conducive to molecular adsorption. By varying the gold coating thickness between 2 and 55 nm, we identify an optimal enhancement factor (EF) of similar to 5.7 x 10(7)at a thickness of similar to 40 nm under 785 nm excitation. Finite element method simulations-parametrized by SEM and AFM-derived geometries-reveal that this enhancement originates from a synergistic combination of lightning rod effects and enhanced optical backscattering. The fabrication protocol enables both Ag-and Au-coated substrates within a unified process. Notably, gold coatings impart superior chemical stability and biocompatibility, with durability tested over more than two weeks of continuous exposure to air and immersion in water. These properties, along with structural robustness, make the proposed substrates ideally suited for integration into microfluidic platforms for real-time biosensing of proteins, DNA, and living cells. Furthermore, the proposed fabrication process is well-suited for sustainable, high-throughput, and cost-effective industrial-scale production.

Whispering-gallery mode (WGM) microresonators are amongst the most promising optical sensors for detection of biochemical targets. A number of laser interrogation methods have been proposed and demonstrated over the last decade, based on scattering and absorption losses or resonance splitting and shift, harnessing the high quality factor and light confinement in the ultra-small volume of WGMs. Regardless of the sensitivity gain, in chemical sensing applications where the chemical targets are present only in rare traces, WGM sensors operation may be augmented by plasmonic interaction, e.g. metallic nanostructures or substrates that provide impressive local field amplification via the so-called Surface-Enhanced Raman Scattering (SERS). Here, we use a silica microsphere and efficiently excite WGMs collecting the relevant information from the transmitted and back-scattered light, which is then analyzed to extract the Raman spectral signature a chemical analyte. We show that a near-infrared diode laser locked on a WGM resonance provides real-time, fast sensing of dielectric nanoparticles approaching the surface with no need for signal post-processing. Also, the Raman spectrum of light scattered from the microsphere surface clearly shows the enhancement of Purcell effect due to the oscillating WGMs. The implementation of a SERS-modified spectroscopic scheme is underway.

AbstractTwo-dimensional transition metal dichalcogenides, particularly MoS2, are interesting materials for many applications in aerospace research, radiation therapy and bioscience more in general. Since in many of these applications MoS2-based nanomaterials can be placed in an aqueous environment while exposed to ionizing radiation, both experimental and theoretical studies of their behaviour under these conditions is particularly interesting. Here, we study the effects of tiny imparted doses of 511 keV photons to MoS2 nanoflakes in water solution. To the best of our knowledge, this is the first study in which ionizing radiation on 2D-MoS2 occurs in water. Interestingly, we find that, in addition to the direct interaction between high-energy photons and nanoflakes, reactive chemical species, generated by γ-photons induced radiolysis of water, come into play a relevant role. A radiation transport Monte Carlo simulation allowed determining the elements driving the morphological and spectroscopical changes of 2D-MoS2, experimentally monitored by SEM microscopy, DLS, Raman and UV–vis spectroscopy, AFM, and X-ray photoelectron techniques. Our study demonstrates that radiolysis products affect the Molybdenum oxidation state, which is massively changed from the stable + 4 and + 6 states into the rarer and more unstable + 5. These findings will be relevant for radiation-based therapies and diagnostics in patients that are assuming drugs or contrast agents containing 2D-MoS2 and for aerospace biomedical applications of 2DMs investigating their actions into living organisms on space station or satellites.

Optical tweezers exploit light-matter interactions to trap particles ranging from single atoms to micrometer-sized eukaryotic cells. For this reason, optical tweezers are a ubiquitous tool in physics, biology, and nanotechnology. Recently, the use of deep learning has started to enhance optical tweezers by improving their design, calibration, and real-time control as well as the tracking and analysis of the trapped objects, often outperforming classical methods thanks to the higher computational speed and versatility of deep learning. In this perspective, we show how cutting-edge deep learning approaches can remarkably improve optical tweezers, and explore the exciting, new future possibilities enabled by this dynamic synergy. Furthermore, we offer guidelines on integrating deep learning with optical trapping and optical manipulation in a reliable and trustworthy way.

Surface-enhanced Raman scattering (SERS) is of growing interest for a wide range of applications, especially for biomedical analysis, thanks to its sensitivity, specificity, and multiplexing capabilities. A crucial role for successful applications of SERS is played by the development of reproducible, efficient, and facile procedures for the fabrication of metal nanostructures (SERS substrates). Even more challenging is to extend the fabrication techniques of plasmonic nano-textures to atomic force microscope (AFM) probes to carry out tip-enhanced Raman spectroscopy (TERS) experiments, in which spatial resolution below the diffraction limit is added to the peculiarities of SERS. In this short review, we describe recent studies performed by our group during the last ten years in which novel nanofabrication techniques have been successfully applied to SERS and TERS experiments for studying bio-systems and molecular species of environmental interest.

Optical tweezers (OT) have become an essential technique in several fields of physics, chemistry, and biology as precise micromanipulation tools and microscopic force transducers. Quantitative measurements require the accurate calibration of the trap stiffness of the optical trap and the diffusion constant of the optically trapped particle. This is typically done by statistical estimators constructed from the position signal of the particle, which is recorded by a digital camera or a quadrant photodiode. The finite integration time and sampling frequency of the detector need to be properly taken into account. Here, we present a general approach based on the joint probability density function of the sampled trajectory that corrects exactly the biases due to the detector's finite integration time and limited sampling frequency, providing theoretical formulas for the most widely employed calibration methods: equipartition, mean squared displacement, autocorrelation, power spectral density, and force reconstruction via maximum-likelihood-estimator analysis (FORMA). Our results, tested with experiments and Monte Carlo simulations, will permit users of OT to confidently estimate the trap stiffness and diffusion constant, extending their use to a broader set of experimental conditions.

The development of sensitive methods for the detection of endotoxin molecules, such as lipopolysaccharides (LPS), is essential for food safety and health control. Conventional analytical methods used for LPS detection are based on the pyrogen test, plating and culture-based methods, and the limulus amoebocyte lysate method (LAL). Alternatively, the development of reliable biosensors for LPS detection would be highly desirable to solve some critical issues, such as high cost and a long turnaround time. In this work, we present a label-free Surface-Enhanced Raman Spectroscopy (SERS)-based method for LPS detection in its free form. The proposed method combines the benefits of plasmonic enhancement with the selectivity provided by a specific anti-lipid A antibody (Ab). A high-enhancing nanostructured silver substrate was coated with Ab. The presence of LPS was quantitatively monitored by analyzing the changes in the Ab spectra obtained in the absence and presence of LPS. A limit of detection (LOD) and quantification (LOQ) of 12 ng/mL and 41 ng/mL were estimated, respectively. Importantly, the proposed technology could be easily expanded for the determination of other biological macromolecules.

We investigate a quasi-2D suspension of Brownian particles in an optical speckle field produced by holographic manipulation of a laser wavefront. This system was developed to study, in a systematic and controllable way, a distinctive instance of diffusion, called Fickian yet Non Gaussian diffusion (FnGD), observed, during the last decade, for colloidal particles in a variety of complex and biological fluids. Our setup generates an optical speckle field that behaves like a disordered set of optical traps. First, we describe the experimental setup and the dynamics of the particles, focusing on mean square displacements, displacement distributions and kurtosis. Then, we present Brownian Dynamics simulations of point-like particles in a complex energy landscape, mimicking that generated by the optical speckle field. We show that our simulations can capture the salient features of the experimental results, including the emergence of FnGD, also covering times longer than the ones so far achieved in experiments. Some deviations are observed at long time only, with the Gaussian restoring being slower in simulations than in experiments. Overall, the introduced numerical model might be exploited to guide the design of upcoming experiments targeted, for example, to fully monitor the recovery of Gaussianity.

Benzodiazepines, psychotropic drugs, are among the most frequently found pharmaceuticals in aquatic matrices. An increasing number of studies are reporting their harmful effects on adults' behaviour and physiology, while little information is available regarding developing organisms exposed since early stages. Improper activation of GABA receptors during embryonic development is likely to induce relevant consequences on the morphogenesis and, at later stages, on behaviour. This study investigated the negative effects of three increasing concentrations of delorazepam on Xenopus laevis retinal and skeletal muscle morphogenesis. Morphological and ultrastructural investigations were correlated with gene expression, while Raman spectroscopy highlighted the main biochemical components affected. Conventional phototactic response and orientation in the magnetic field were assessed as indicators of proper interaction between sensory organs and the nervous system. Results confirm the profound impact of delorazepam on development and return an alarming picture of the amphibians' survival potentialities in a benzodiazepine-contaminated environment.

Nanoparticles (NPs) coated with hyaluronic acid (HA) seem to be increasingly promising for targeted therapy due to HA chemical versatility, which allows them to bind drugs of different natures, and their affinity with the transmembrane receptor CD-44, overexpressed in tumor cells. However, an essential aspect for clinical use of NPs is formulation stability over time. For these reasons, analytical techniques capable of characterizing their physico-chemical properties are needed. In this work, poly(lactide-co-glycolide) (PLGA) NPs with an average diameter of 100-150 nm, coated with a few 10 s of nm of HA, were synthesized. For stability characterization, two complementary investigative techniques were used: Dynamic Light Scattering (DLS) and Surface-Enhanced Raman Scattering (SERS) spectroscopy. The first technique provided information on size, polidispersity index, and zeta-potential, and the second provided a deeper insight on the NP surface chemicals, allowing distinguishing of HA-coated NPs from uncoated ones. Furthermore, in order to estimate formulation stability over time, NPs were measured and monitored for two weeks. SERS results showed a progressive decrease in the signal associated with HA, which, however, is not detectable by the DLS measurements.

The pursuit of environmentally friendly solvents has become an essential research topic in sustainable chemistry and nanomaterial science. With the need to substitute toxic solvents in nanofabrication processes becoming more pressing, the search for alternative solvents has taken on a crucial role in this field. Additionally, the use of toxic, non-economical organic solvents, such as N-methyl-2 pyrrolidone and dimethylformamide, is not suitable for all biomedical applications, even though these solvents are often considered as the best exfoliating agents for nanomaterial fabrication. In this context, the success of producing two-dimensional transition metal dichalcogenides (2D TMDs), such as MoS2 and WS2, with excellent captivating properties is due to the ease of synthesis based on environment-friendly, benign methods with fewer toxic chemicals involved. Herein, we report for the first time on the use of cyrene as an exfoliating agent to fabricate monolayer and few-layered 2D TMDs with a versatile, less time-consuming liquid-phase exfoliation technique. This bio-derived, aprotic, green and eco-friendly solvent produced a stable, surfactant-free, concentrated 2D TMD dispersion with very interesting features, as characterized by UV–visible and Raman spectroscopies. The surface charge and morphology of the fabricated nanoflakes were analyzed using ς-potential and scanning electron microscopy. The study demonstrates that cyrene is a promising green solvent for the exfoliation of 2D TMD nanosheets with potential advantages over traditional organic solvents. The ability to produce smaller-sized—especially in the case of WS2 as compared to MoS2—and mono/few-layered nanostructures with higher negative surface charge values makes cyrene a promising candidate for various biomedical and electronic applications. Overall, the study contributes to the development of sustainable and environmentally friendly methods for the production of 2D nanomaterials for various applications.

Controlling light at the nanoscale, the fascinating goal of nanophononics, allows to mold the flow of light in previously unknown ways, providing access to the study of the interaction of single molecules with confined optical fields. One of the most powerful spectroscopic approach to nanoscale is Tip-Enhanced Raman Spectroscopy (TERS). TERS combines the fingerprint character of Raman spectroscopy with the high spatial resolution of scanning probe microscopies (SPM), by taking advantage of plasmonic nanostructures placed at the apex of a SPM probe. Thanks to the excitation of Localized Surface Plasmon Resonances (LSPR), such system behaves as a nano-antenna, able to explore surfaces with nanometric spatial resolution and sensitivity up to single molecule level. Herein, we report on the fabrication and characterization of newly conceived nano-antennas. In particular, we illustrate a top-down approach for TERS tip fabrication, based on the solid-state dewetting of metal-sputtered Atomic-Force Microscopy probes. Such approach produces AFM-TERS tips, exhibiting high reproducibility and efficiency.

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.

Intracavity optical tweezers are a powerful tool to trap microparticles in water using the nonlinear feedback effect produced by the particle motion when it is trapped inside the laser cavity. In such systems two configurations are possible: a single-beam configuration and counterpropagating one. A removable isolator allows to switch between these configurations by suppressing one of the beams. Trapping a particle in the counterpropagating configuration, the measure of the optical power shows a feedback effect for each beam, that is present also when the two beams are misaligned and the trapped particle periodically jumps between them.

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.