Optical modulators with 2D layered materials

Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solutions are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fibre integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.

Engineering metallic nanoparticles for enhancing and probing catalytic reactions

Recent developments in tailoring the structural and chemical properties of colloidal metal nanoparticles (NPs) have led to significant enhancements in catalyst performance. Controllable colloidal synthesis has also allowed tailor-made NPs to serve as mechanistic probes for catalytic processes. The innovative use of colloidal NPs to gain fundamental insights into catalytic function will be highlighted across a variety of catalytic and electrocatalytic applications. The engineering of future heterogenous catalysts is also moving beyond size, shape and composition considerations. Advancements in understanding structure-property relationships have enabled incorporation of complex features such as tuning surface strain to influence the behavior of catalytic NPs. Exploiting plasmonic properties and altering colloidal surface chemistry through functionalization are also emerging as important areas for rational design of catalytic NPs. This news article will highlight the key developments and challenges to the future design of catalytic NPs.

BLOCK COPOLYMERS CONTAINING A LOW-SURFACE-ENERGY BLOCK AND THEIR APPLICATIONS

This thesis reports the synthesis and/or applications of three types of block copolymers that each bear a low-surface-energy block. First, poly(dimethylsiloxane)-block-poly(2-cinnamoyloxyethyl acrylate) (PDMS-b-PCEA) was synthesized and characterized. Cotton coating using a micellar solution of this block copolymer yielded superhydrophobic cotton fabrics. X-ray photoelectron spectroscopy (XPS) and surface property analyses indicated that the PDMS block topped the polymer coating. Photocuring the cotton swatches crosslinked the underlying PCEA layer and yielded permanent coatings. More interestingly, hydrophilically patterned superhydrophobic cotton fabrics were produced using photolithography that allowed the crosslinking of the coating around irradiated fibers but the removal, by solvent extraction, of the coating on fibers that were not irradiated. Since water-based ink only permeated the uncoated regions, such patterned fabric was further used to print ink patterns onto substrates such as fabrics, cardboard, paper, wood, and aluminum foil. Then, another PDMS-based diblock copolymer poly(dimethylsiloxane)-block-poly(glycidyl methacrylate) (PDMS-b-PGMA) was prepared. Different from PCEA that photocrosslinked around cotton fibers, PGMA reacted with hydroxyl groups on cotton fiber surfaces to get covalently attached. Further, different PGMA chains crosslinked with each other. PDMS-b-PGMA-coated cotton fabrics have been used for oil-water separations. In addition, polymeric nanoparticles were grafted onto cotton fiber surface before PDMS-b-PGMA was used to cover the surfaces of the grafted spheres and the residual surfaces of the cotton fibers. These two types of fabrics, coated by the block copolymer alone or by the polymer nanospheres and then the copolymer, were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), and water repellency analyses. A comprehensive comparative study was made of their performances in oil-water separation. Finally, a fluorinated ABC triblock copolymer poly(acrylic acid)-block-poly(2-cinnamoyloxyethyl methacrylate)-block-poly(2-perfluorooctylethyl methacrylate) (PAA-b-PCEMA-b-PFOEMA) was used to iii encapsulate air nanobubbles. The produced air nanobubbles were thermodynamically stable in water and were some 100 times more stable than commercially available perfluorocarbon-filled microbubbles under ultrasound. These nanobubbles, due to their small sizes and thus ability to permeate the capillary networks of organs and to reach tumors, may expand the applications of microbubbles in diagnostic ultrasonography and find new applications in ultrasound-regulated drug delivery.

Surface-Enhanced Raman Spectroscopy as a Probe of the Surface Chemistry of Nanostructured Materials

Surface-enhanced Raman spectroscopy (SERS) is now widely used as a rapid and inexpensive tool for chemical/biochemical analysis. The method can give enormous increases in the intensities of the Raman signals of low-concentration molecular targets if they are adsorbed on suitable enhancing substrates, which are typically composed of nanostructured Ag or Au. However, the features of SERS that allow it to be used as a chemical sensor also mean that it can be used as a powerful probe of the surface chemistry of any nanostructured material that can provide SERS enhancement. This is important because it is the surface chemistry that controls how these materials interact with their local environment and, in real applications, this interaction can be more important than more commonly measured properties such as morphology or plasmonic absorption. Here, the opportunity that this approach to SERS provides is illustrated with examples where the surface chemistry is both characterized and controlled in order to create functional nanomaterials.

Endonuclease Controlled Aggregation of Gold Nanoparticles for the Ultrasensitive Detection of Pathogenic Bacterial DNA

The development of an ultrasensitive biosensor for the low-cost and on-site detection of pathogenic DNA could transform detection capabilities within food safety, environmental monitoring and clinical diagnosis. Herein, we present an innovative approach exploiting endonuclease-controlled aggregation of plasmonic gold nanoparticles (AuNPs) for label-free and ultrasensitive detection of bacterial DNA. The method utilizes RNA-functionalized AuNPs which form DNA-RNA heteroduplex structures through specific hybridization with target DNA. Once formed, the DNA-RNA heteroduplex is susceptible to RNAse H enzymatic cleavage of the RNA probe, allowing the target DNA to liberate and hybridize with another RNA probe. This continuously happens until all of the RNA probes are cleaved, leaving the nanoparticles unprotected and thus aggregated upon exposure to a high electrolytic medium. The assay is ultrasensitive, allowing the detection of target DNA at femtomolar level by simple spectroscopic analysis (40.7 fM and 2.45 fM as measured by UV-vis and dynamic light scattering (DLS), respectively). The target DNA spiked food matrix (chicken meat) is also successfully detected at a concentration of 1.2 pM (by UV-vis) or 18.0 fM (by DLS). In addition to the ultra-high sensitivity, the total analysis time of the assay is less than 3 hours, thus demonstrating its practicality for food analysis.

Mid-infrared photonic sensors based on metamaterial structures

In this work three different metallic metamaterials (MMs) structures such as asymmetric split ring resonators (A-SRRs), dipole and split H-shaped (ASHs) structures that support plasmonic resonances have been developed. The aim of the work involves the optimization of photonic sensor based on plasmonic resonances and surface enhanced infrared absorption (SEIRA) from the MM structures. The MMs structures were designed to tune their plasmonic resonance peaks in the mid-infrared region. The plasmonic resonance peaks produced are highly dependent on the structural dimension and polarisation of the electromagnetic (EM) source. The ASH structure particularly has the ability to produce the plasmonic resonance peak with dual polarisation of the EM source. The double resonance peaks produced due to the asymmetric nature of the structures were optimized by varying the fundamental parameters of the design. These peaks occur due to hybridization of the individual elements of the MMs structure. The presence of a dip known as a trapped mode in between the double plasmonic peaks helps to narrow the resonances. A periodicity greater than twice the length and diameter of the metallic structure was applied to produce narrow resonances for the designed MMs. A nanoscale gap in each structure that broadens the trapped mode to narrow the plasmonic resonances was also used. A thickness of 100 nm gold was used to experimentally produce a high quality factor of 18 in the mid-infrared region. The optimised plasmonic resonance peaks was used for detection of an analyte, 17β-estradiol. 17β-estradiol is mostly responsible for the development of human sex organs and can be found naturally in the environment through human excreta. SEIRA was the method applied to the analysis of the analyte. The work is important in the monitoring of human biology and in water treatment. Applying this method to the developed nano-engineered structures, enhancement factors of 10^5 and a sensitivity of 2791 nm/RIU was obtained. With this high sensitivity a figure of merit (FOM) of 9 was also achieved from the sensors. The experiments were verified using numerical simulations where the vibrational resonances of the C-H stretch from 17β-estradiol were modelled. Lastly, A-SRRs and ASH on waveguides were also designed and evaluated. These patterns are to be use as basis for future work.

Comparison of adhesion layers of gold on silicate glasses for SERS detection

Gold is one of the most widely used metals for building up plasmonic devices. Although slightly less efficient than silver for producing sharp resonance, its chemical properties make it one of the best choices for designing sensors. Sticking gold on a silicate glass substrate requires an adhesion layer, whose effect has to be taken into account. Traditionally, metals (Cr or Ti) or dielectric materials (TiO2 or Cr2O3 ) are deposited between the glass and the nanoparticle. Recently, indium tin oxide and (3-mercaptopropyl)trimethoxysilane (MPTMS) were used as a new adhesion layer. The aim of this work is to compare these six adhesion layers for surface- enhanced Raman scattering sensors by numerical modeling. The near-field and the far-field optical responses of gold nanocylinders on the different adhesion layers are then calculated. It is shown that MPTMS leads to the highest field enhancement, slightly larger than other dielectric materials. We attributed this effect to the lower refractive index of MPTMS compared with the others.

Compact discs as versatile cost-effective substrates for releasable nanopatterned aluminium films

We demonstrate that standard polycarbonate compact disk surfaces can provide unique adhesion to Al films that is both strong enough to permit Al film nanopatterning and weak enough to allow easy nanopatterned Al film detachment using Scotch tape. Transferred Al nanohole arrays on Scotch tape exhibit excellent optical and plasmonic performance.

Study of nanostructured targets for plasma production via laser ablation

Metal nanowires (NWs) - nanostructures 20-100 nm in diameter and up to tens of micrometers long - behave as waveguides when irradiated with light with wavelength much greater than their diameter. This is due to collective excitations of free electrons (plasmons) in the metal which couple to light and travel on the surface of the nanowire. This effect can be used to efficiently absorb laser pulses to produce dense and hot plasma on special nanostructured targets with metal nanowires vertically aligned on the surface. In this thesis work, nanostructured targets with different parameters (length, diameter, metal and fabrication process) have been irradiated with infrared laser light. X-ray flux emitted by the cooling plasma is measured during irradiation, and the depth of craters formed on the target is measured later. This data is used to choose which target parameters are best for plasma production. Different targets are compared with each other and against a control, non-nanostructured (bulk) target. As will be shown, highly significant (> 5 sigma) differences are found between targets with different nanostructures, and between nanostructured and bulk target. This differences are very difficult to explain whithout accounting for the nanostructures in the targets. Therefore, data collected and analized in this thesis work supports the hypotesys that nanostructured targets perform better than bulk targets for laser plasma production purposes, and provides useful indications for optimization of NWS' parameters.

Role of Shiga toxins in the pathogenesis of hemolytic uremic syndrome

During the pathogenesis of hemolytic uremic syndrome (HUS), a severe sequela of Shiga toxin (Stx)-producing Escherichia coli (STEC) gastrointestinal infections, before the toxin acts on the target endothelial cells of the kidney and brain, several Stx forms are transported in the bloodstream: free Stx; Stx bound to circulating cells through Gb3Cer and TLR4 receptors; and Stx associated to blood cell-derived microvesicles. The latter form is mainly responsible for the development of life-threatening HUS in 15% of STEC-infected patients. Stx consist of five B subunits non-covalently bound to a single A subunit (uncleaved Stx) which can be cleaved in two fragments (A1 and A2) held by a disulfide bond (cleaved Stx). After reduction, the enzymatically active A1 fragment responsible for toxicity is released. Cleaved and uncleaved Stx are biologically active but functionally different, thus their presence in patients’ blood could affect the onset of HUS. Currently, there are no effective therapies for the treatment of STEC-infected patients and the gold standard strategies available for the diagnosis are very expensive and time-consuming. In this thesis, by exploiting the resolving power of SERS technology (Amplified Raman Spectroscopy on Surfaces), a plasmonic biosensor was developed as effective diagnostic tool for early detection of Stx in patients’ sera. An acellular protein synthesis system for detecting cleaved Stx2a in human serum based on its greater translation inhibition after treatment with reducing agents was developed and used to identify cleaved Stx in STEC-infected patients’ sera. Pathogenic microvesicles from Stx2a-challenged blood from healthy donors were isolated and characterized. The antibiotic NAB815, acting as inhibitor of toxin binding to TLR4 expressed by circulating cells, was found to be effective in impairing the formation of blood cell-derived microvesicles containing Stx2a, also having a protective effect in cellular models. This approach could be proposed as an innovative treatment for HUS prevention.