All-dielectric optical metasurfaces for sensing of substances with identical real parts of refractive index

Metasurfaces are one of the most attractive research fields in recent years, in no small measure due to their use as refractometric sensors with unparalleled sensitivity, enabling sensing of even single atoms or molecules. This sensitivity is rooted in their ability to localize electromagnetic fields in volumes orders of magnitude below the diffraction limit. This is achieved by special geometry and refractive index contrast of metasurface nanocomposites. A plethora of both conductive (metals, TCO, graphene, MXenes...) and dielectric (oxides, semiconductors) materials can be used, all of them bringing different functionalities which further enhance the freedom of design when tailoring metasurfaces. However, the fundamental sensing mechanism is practically identical across all platforms and is based on the spectral shift of transmission or reflection due to a difference in the values of the real parts of refractive index between analyte and the environment. Here we propose an alternative approach, the use of exceptional capabilities of optical metasurfaces in transforming optical space to sense analytes with identical real parts of refractive index but different imaginary parts (losses). This is becoming increasingly important with the need to detect airborne viruses. We propose a metasurface formed by cruciform openings in a thin silicon layer on a SiO2 substrate, as shown in Fig. 1a. The structure is suspended in the air. For our FEM simulation we used measured values for Si and SiO2 refractive index taken from literature. The electric field distribution and its circular power flow, both at a wavelength of 630 nm, are shown in Fig. 1b. We gradually increase the imaginary part of the refractive index in the cruciform openings, starting with the lossless case, while maintaining the real part of refractive index equal to unity (air). The dispersive properties that depend on the value of the imaginary part are shown in Fig. 1c. The circular power flow that increases the optical path, the field localization and intrinsically low losses of the structure in the visible range all cause that adding even the smallest volumes of analyte with slightly increased optical absorption in comparison to the metasurface significantly reduces transmission through the structure, despite the exceptionally low structure thickness

Surface Characteristics of polyurethane/organoclay nanocomposites

Polyurethanes (PUs) are widely used polymers, with specific production able to be aimed at their notable industrial and biomedical applications by carefully changing their ingredients, their ratios and their preparation procedures. The popularity of PU nanocomposites is caused mainly by the simplicity of tuning their functional properties. The choice of nanofillers, for example, clay, graphene oxide, carbon nanotubes, silicon dioxide, nanosilver or nanoferrite is miscellaneous. In this work, PU/clay nanocomposites were prepared by in situ polymerization in the presence of organically modified clay (Cloisite 30B) with clay loading of 0.5 wt.%. We used hyperbranched polyester and 4,4'-methylenediphenyldiisocyanate as hard segment components, while poly(dimethylsiloxane) macrodiol as soft segment. The influence of the soft segment content on the properties of nanocomposites was investigated by swelling behavior, crosslinking density, degree of phase separation, water absorption and contact angle measurements as well as surface free energy determination. FTIR results showed the higher degree of phase separation in nanocomposites as soft segment content decreased. Moreover, the results showed that equilibrium swelling degree of PU nanocomposites decreases, while crosslinking density increases with decreasing soft segment content. Hydrophobicity of the PU nanocomposites increases with increasing soft segment content, due to the hydrophobic character of PDMS. Namely, the surface free energy of nanocomposites films decreases in the range of 39.8 to 28.0 mJ/m2 with increasing soft segment content, confirming good surface hydrophobicity. Therefore, PU nanocomposites could be considered as promising materials suitable for coating applications

Structural and thermal properties of PDMS/Triton/laser-induced graphene composites

Laser-induced graphene (LIG) has recently been proposed as a viable option for fabricating various types of flexible electronic devices due to its excellent mechanical stability and electrical properties. During laser induction of graphene on polymers, the high temperature generated with the laser breaks C-O, C=O, and N-C bonds in polymers, leading to the recombination of C and N atoms. Additionally, the rapid release of carbonaceous and nitric gases results in the formation of 3D porous structures. This approach offers a one-step, chemical-free synthesis method for producing porous graphene on polymer surfaces. Moreover, it is a fast and cost-effective technique that is ideal for flexible electronics and energy storage devices. In this study, graphene was formed on a poly(dimethylsiloxane) (PDMS)/Triton substrate with varying concentrations of Triton (1-30 wt.%) using CO2 laser irradiation. The effects of Triton content on the structural, thermal, and surface characteristics of PDMS/Triton and PDMS/Triton/graphene materials were investigated. The prepared PDMS/Triton/graphene materials were thoroughly examined using X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and water contact angle analysis. XRD analysis confirmed the presence of graphene in the material. The thermal and surface properties of the proposed materials can be easily adjusted by manipulating the Triton concentration. The hydrophilicity of the prepared PDMS/Triton materials increased compared to pure PDMS, which is hydrophobic. It was found that the success of LIG formation depends on Triton content, increasing with higher concentration of Triton in the PDMS matrix. The presented results aim to address the existing challenges associated with stretchable polymers suitable for flexible electronic device applications

Laser-Induced Graphene for Heartbeat Monitoring with HeartPy Analysis

The HeartPy Python toolkit for analysis of noisy signals from heart rate measurements is an excellent tool to use in conjunction with novel wearable sensors. Nevertheless, most of the work to date has focused on applying the toolkit to data measured with commercially available sensors. We demonstrate the application of the HeartPy functions to data obtained with a novel graphene-based heartbeat sensor. We produce the sensor by laser-inducing graphene on a flexible polyimide substrate. Both graphene on the polyimide substrate and graphene transferred onto a PDMS substrate show piezoresistive behavior that can be utilized to measure human heartbeat by registering median cubital vein motion during blood pumping. We process electrical resistance data from the graphene sensor using HeartPy and demonstrate extraction of several heartbeat parameters, in agreement with measurements taken with independent reference sensors. We compare the quality of the heartbeat signal from graphene on different substrates, demonstrating that in all cases the device yields results consistent with reference sensors. Our work is a first demonstration of successful application of HeartPy to analysis of data from a sensor in development

Preparation of poly(dimethylsiloxane)-based materials for laser-induced graphenization

Laser induced graphenization (LIG) of polymer materials has recognized as the most promising method for fabrication of flexible electronic devices. Poly(dimethylsiloxane) (PDMS) is suitable elastomeric materials for flexible electronics devices fabrication due to outstanding mechanical and optical properties. Namely, the low carbon content and the lacking aromatic structures in PDMS material limit the graphenization process resulting in limited conduction properties. The aim of this study was the graphenization of PDMS and PDMS-based materials by CO2 laser radiation. We prepared pure PDMS elastomer, PDMS/ethylene glycol and PDMS/Triton composite materials by using 20 wt. % of ethylene glycol or Triton in PDMS matrix. Indeed, up to now the evidence of graphenization of these PDMS-based materials has never been observed. PDMS elastomer was prepared by hydrosilylation reaction, while composite materials by blending method. The prepared PDMS-based materials were characterized by Fourier-transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and Raman spectroscopy. The obtained results showed that surfaces of pure PDMS elastomer and PDMS/ethylene glycol composite cannot be graphenized by direct laser writing. However, by adding Triton as aromatic and carbon sources into the PDMS matrix it is possible to improve the graphenization of PDMS based materials and this material is good candidate for fabrication of flexible electrodes

Nanoparticle synthesis in microreactors, Preparation of poly(dimethylsiloxane)-based materials for laser-induced graphenization

Microreactors are microfluidic devices with a network of channels where chemical reactions can take place. The diameter of those channels is less than 1 millimeter. Advances of such devices are reduction of the time required for the synthesis, better control of the reaction and the size of nanoparticles. There are different materials from which microreactors can be fabricated. The most common are silicon (Si), polymers, glass, ceramics and metals. In this study, we used Si/Pyrex glass and poly(dimethylsiloxane) (PDMS) materials to fabricate two types of microreactors. Both types of microreactors had integrated heaters but different length and width of microchannels. Reaction was performed on the same temperature (80⁰C) and with the same reaction time in both microreactors. Synthesis of titanium (IV)-oxide nanoparticles was performed in these microreactors, in order to show how dimension of microchannels can affect the size of nanoparticles. Size distribution of nanoparticles was determined with dynamic light scattering (DLS). It was concluded that dimensions of microchannels had great influence on the size of the nanoparticles

Analysis and interpretation of bimetallic plasmonic metamaterial properties for forensic applications

Plasmonic biochemical sensors, as a subgroup of optical refractometric sensors, are a topic of rapid research and development due to their ability to detect trace amounts of biochemical agents, even allowing detection of a single molecule. As such, plasmonic sensors find wide applications in forensic engineering, medicine and clinical diagnostics, food processing, environmental protection, and many more. Additionally, the use of plasmonic metametrials is not limited to sensing, and they can be used in various fields paramount to forensic sciences, such as enhanced microscopy and spectroscopy, photodetector enhancement (entire optical range – UV, VIS, and IR light), signal processing, etc

Characterization of poly(dimethylsiloxane)/laser-induced graphene composites

Laser-induced graphene (LIG) is one of the most promising graphene-based materials for the fabrication of flexible electronic devices. However, despite huge efforts to develop LIG on new substrates there is a lack of stretchable polymers convenient for laser graphenization. Poly(dimethylsiloxane) is elastomer suitable for flexible electronic fabrication due to excellent flexibility, optical transparency, hydrophobicity, UV-resistance and good thermal and oxidative stability. Unfortunately, PDMS cannot be easily graphenized by direct laser writing because of the low amount of carbon linked to the siloxane chains, mainly consisting of methyl groups. In this study, a series of PDMS/Triton materials with different concentrations of Triton (1-30 wt.%) was prepared by casted-based approach starting from divinyl-terminated-PDMS and poly(methylhydrogensiloxane). Furthermore, direct laser graphenization of PDMS/Triton materials is proposed for the first time. Different laser parameters and multiple writing steps have been examined in order to induce a graphene-like structure on PDMS/Triton and to increase as much as possible its conductivity. The prepared PDMS/Triton/graphene composites were characterized by Raman spectroscopy, scanning electron microscopy (SEM) and sheet resistance measurements. The results revealed that by adding Triton it is possible to enhance the graphenization degree. The obtained PDMS/Triton/graphene composites are suitable candidates for flexible microsupercapacitor fabrication

Photolithography-based Fabrication of Interdigitated Electrodes with Integrated Gold Microheater: Temperature Distribution Study

Interdigitated electrodes with integrated heaters fabricated on a Si platform were investigated in this study. The electrodes and heaters were made of gold, using standard photolithography processes. The primary objective was to analyze the maximum temperature achievable on the microheater and characterize its temperature distribution. The integrated heater achieved a maximum temperature of 420 °C with an applied voltage of 16 V. The temperature distribution was uniform across the entire surface of the heater, which was located on the underside of the chip beneath the interdigitated electrodes. At higher temperatures, the silver paste, utilized as a bonding agent between the copper wires and heater, underwent melting

Laser-induced graphene on PEO/PDMS composites

Laser-induced graphene (LIG) has emerged as one of the most promising materials for flexible functional devices. One-step fabrication of LIG offers advantages such as low cost, patterning of desired geometries, and high sensitivity. However, previous attempts to obtain LIG on elastomeric substrates have been unsuccessful, limiting its potential for use in stretchable electronics. In this study, we propose using a substrate composed of polydimethylsiloxane (PDMS) and poly(ethylene oxide) (PEO) with a low molecular weight as a platform for manufacturing LIG. A series of PDMS/PEO materials with varying concentrations of PEO (1, 5, 10, 20, 30, 40 and 50 wt.%) were prepared using a cast-based approach, starting from divinyl-terminated-PDMS and poly(methyl-hydrogensiloxane). The prepared PDMS/PEO/graphene composites were characterized using Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) analysis. FTIR analysis confirmed the structure of the prepared PDMS/PEO and PDMS/PEO/graphene materials. The results demonstrated that the prepared PDMS/PEO composites exhibited a higher degree of graphenization compared to pure PDMS networks. SEM analysis revealed the formation of a porous graphene structure. Based on these findings, the PDMS/PEO/graphene composites show promise for further investigation as electronic device applications