Raman Tweezers for Small Microplastics and Nanoplastics Identification in Seawater

Our understanding of the fate and distribution of micro- and nano- plastics in the marine environment is limited by the intrinsic difficulties of the techniques currently used for the detection, quantification, and chemical identification of small particles in liquid (light scattering, vibrational spectroscopies, and optical and electron microscopies). Here we introduce Raman Tweezers (RTs), namely optical tweezers combined with Raman spectroscopy, as an analytical tool for the study of micro- and nanoplastics in seawater. We show optical trapping and chemical identification of sub-20 μm plastics, down to the 50 nm range. Analysis at the single particle level allows us to unambiguously discriminate plastics from organic matter and mineral sediments, overcoming the capacities of standard Raman spectroscopy in liquid, intrinsically limited to ensemble measurements. Being a microscopy technique, RTs also permits one to assess the size and shapes of particles (beads, fragments, and fibers), with spatial resolution only limited by diffraction. Applications are shown on both model particles and naturally aged environmental samples, made of common plastic pollutants, including polyethylene, polypropylene, nylon, and polystyrene, also in the presence of a thin eco-corona. Coupled to suitable extraction and concentration protocols, RTs have the potential to strongly impact future research on micro and nanoplastics environmental pollution, and enable the understanding of the fragmentation processes on a multiscale level of aged polymers.

Polyphosphonate ligands: From synthesis to design of hybrid PEGylated nanoparticles toward phototherapy studies

The use of phosphonate ligands to modify the nanoparticle (NPs) surface has attracted a strong interest in the last years for the design of highly functional hybrid materials. Here, we applied a methodology to synthesize bisphosphonates having functionalized PEG side chains with a specific length in order to design a novel class of hybrid nanomaterials composed by tetraphosphonate-complex-gold COOH-terminated PEG-coated NPs (Bis-PO-PEG-AuNPs). The synthetic approach consist in three steps: (1) Complexation between new phosphonate ligands (Bis PO) and tetrachloroauric acid (HAuCl₄) to form gold clusters; (2) adsorption of COOH-terminated PEG molecules (PEG) onto Bis PO-Au complex; (3) reduction of metal ions in that vicinity, growth of gold particles and colloidal stabilization. The obtained snow-shape-like hybrid nanoparticles, have been characterized by ultra-violet/visible, Raman spectroscopies, and electron microscopy imaging, involving their optical properties and photothermal activity in pancreatic adenocarcinoma cancer cells (PDAC).

Fast and reliable fabrication of gold tips with sub-50 nm radius of curvature for tip-enhanced Raman spectroscopy

We have developed a new electrochemical etching procedure to fabricate gold tips with sub-50 nm apical radius of curvature with a production yield of 80% and production time lower than 5 min. The technique is based on a two-step self-terminating process in which a gold wire is first quickly (<1 min) pre-etched in an hydrochloric acid (HCl)∕ethanol solution at high voltage (10 VDC), and then slowly (2-4 min) etched at lower voltages (<2.5 VDC). The first step occurs under intense bubbling conditions and allows us to thin rapidly the wire. This reduces the time required by subsequent low-voltage process during which the tips are formed at the liquid∕air interface. A statistical analysis of the surface morphology has been carried out on a set of 60 tips by scanning electron microscopy. The results show that the surface roughness and the sharpness of the final tip are critically influenced by the intrinsic granularity of the gold wires. Moreover, there is a correlation between the tip quality and the time elapse required to complete the low-voltage etching step. Tips featuring smooth surfaces and radii of curvature <50 nm are produced whenever the etching times are lower than 250 s, while etching times larger than 300 s typically yield rough, blunt tips. Such a correlation can be used as a screening criterion to select sharp tips during production with an 80% yield. The high quality of the gold tips produced with such method is confirmed by the electromagnetic field enhancement measured both in tip-enhanced Raman scattering and surface-enhanced Raman scattering on the tip apex experiments.

Optical nanoantennas for multiband surface-enhanced infrared and Raman spectroscopy

In this article we show that linear nanoantennas can be used as shared substrates for surface-enhanced Raman and infrared spectroscopy (SERS and SEIRS, respectively). This is done by engineering the plasmonic properties of the nanoantennas, so to make them resonant in both the visible (transversal resonance) and the infrared (longitudinal resonance), and by rotating the excitation field polarization to selectively take advantage of each resonance and achieve SERS and SEIRS on the same nanoantennas. As a proof of concept, we have fabricated gold nanoantennas by electron beam lithography on calcium difluoride (1-2 μm long, 60 nm wide, 60 nm high) that exhibit a transverse plasmonic resonance in the visible (640 nm) and a particularly strong longitudinal dipolar resonance in the infrared (tunable in the 1280-3100 cm(-1) energy range as a function of the length). SERS and SEIRS detection of methylene blue molecules adsorbed on the nanoantenna's surface is accomplished, with signal enhancement factors of 5×10(2) for SERS (electromagnetic enhancement) and up to 10(5) for SEIRS. Notably, we find that the field enhancement provided by the transverse resonance is sufficient to achieve SERS from single nanoantennas. Furthermore, we show that by properly tuning the nanoantenna length the signals of a multitude of vibrational modes can be enhanced with SEIRS. This simple concept of plasmonic nanosensor is highly suitable for integration on lab-on-a-chip schemes for label-free chemical and biomolecular identification with optimized performances.