Silver Eco-Solvent Ink for Reactive Printing of Polychromatic SERS and SPR Substrates

Numerical Simulation of Solvent Evaporation in a Reactive Silver Ink Droplet Deposited on a Heated Substrate

Understanding the movement of silver ions (Ag+) in the solvent of a thermally evaporated particle-free reactive silver ink droplet is essential for optimizing the electronic inkjet printing process. In this work, a numerical study based on the Navier-Stokes equations is used to examine the microflows inside the evaporating solvent of a reactive silver ink droplet and to predict the morphology of the resultant Ag particle aggregations that form during the heat-activated processes. The droplet evaporation of the water-ethylene glycol ink solvent (H2O-(CH2OH)2) is simulated using COMSOL Multiphysics software. The model assumes that the evaporating fluid is heterogeneous due to the mass transfer of ethylene glycol molecules throughout the droplet by capillary flow. A layer of concentrated ethylene glycol forms at the fluid-substrate interface during solvent evaporation if the substrate is heated. The concentrated ethylene glycol molecules are then transported inward by the capillary action, and the resultant Ag particles, arising from the thermally driven reactions, accumulate at the bottom center of the drying droplet. The numerical simulations demonstrate that the droplet evaporation process depends on the water concentration in the solvent, substrate temperature, surface tension, and natural convection. Furthermore, the capillary flow dominates the fluid flow inside the evaporating droplet, causing some Ag particles to accumulate at the contact line, the commonly observed "coffee-ring effect". The results provide new insights into the chemical reactions that produce experimentally observed silver particle aggregations during the reactive silver ink droplet evaporation process and help establish realistic process parameters for improving the quality of inkjet-printed conductive silver films and electronic circuit microtraces.

Dendrimer stabilized nanoalloys for inkjet printing of surface-enhanced Raman scattering substrates

Research on paper substrates prepared by inkjet deposition of metal nanoparticles for sensing applications has become a hot topic in recent years; however, the design of such substrates based on the deposition of alloy nanoparticles remains less explored. Herein, we report for the first time the inkjet printing of dendrimer-stabilized colloidal metal nanoalloys for the preparation of paper substrates for surface-enhanced Raman scattering (SERS) spectroscopy. To this end, nanoassemblies containing variable molar ratios of Au:Ag were prepared in the presence of poly(amidoamine) dendrimer (PAMAM), resulting in plasmonic properties that depend on the chemical composition of the final materials. The dendrimer-stabilized Au:Ag:PAMAM colloids exhibit high colloidal stability, making them suitable for the preparation of inks for long-term use in inkjet printing of paper substrates. Moreover, the pre-treatment of paper with a polystyrene (PS) aqueous emulsion resulted in hydrophobic substrates with improved SERS sensitivity, as illustrated in the analytical detection of tetramethylthiuram disulfide (thiram pesticide) dissolved in aqueous solutions. We suggest that the interactions established between the two polymers (PAMAM and PS) in an interface region over the cellulosic fibres, resulted in more exposed metallic surfaces for the adsorption of the analyte molecules. The resulting hydrophobic substrates show long-term plasmonic stability with high SERS signal retention for at least ninety days.

Dual modal plasmonic substrates based on a convective self-assembly technique for enhancement in SERS and LSPR detection

In this study, surface-enhanced Raman scattering (SERS) scheme is combined with localized surface plasmon resonance (LSPR) detection on a thin gold film with stripe patterns of gold nanoparticles (GNPs) via convective self-assembly (CSA) method. The potential of dual modal plasmonic substrates was evaluated by binding 4-ABT and IgG analytes, respectively. SERS experiments presented not only a high sensitivity with a detection limit of 4.7 nM and an enhancement factor of 1.34 × 10⁵, but an excellent reproducibility with relative standard deviation of 5.5%. It was found from plasmonic sensing experiments by immobilizing IgG onto GNP-mediated gold film that detection sensitivity was improved by more than 211%, compared with a conventional bare gold film. Our synergistic SERS-LSPR approach based on a simple and cost-effective CSA method could open a route for sensitive, reliable and reproducible dual modal detection to expand the application areas.

Immobilization of Cubic Silver Plasmonic Nanoparticles on TiO2 Nanotubes, Reducing the Coffee Ring Effect in Surface-Enhanced Raman Spectroscopy Applications

Surface-enhanced Raman spectroscopy (SERS) substrates prepared by immobilizing silver cubic nanoparticles (Ag CNPs) on titanium dioxide nanotubes (TiO₂ NTs) were used for investigations of the "coffee ring" (CR) effect and its impact on spatial reproducibility of measured Raman signals in comparison with flat surfaces (Ti and Si) where the CR effect is usually significant. The immobilization of nanoparticles from drops, which is a very simple technique, usually does not permit a homogeneous distribution of deposited NPs because there is significant accumulation of the material at the boundary of the drying area. Our proposed SERS substrates effectively reduced the CR effect through the use of well-ordered nanostructures where a smaller number of Ag CNPs were transferred to the boundary region. It was not only the surface morphology that was important but also the physicochemical properties of TiO₂ NTs, such as wettability. The wettability of the prepared samples was determined by measuring the static water contact angle (WCA), and the chemical composition near the boundary of the drying area was studied using Auger electron spectroscopy. The morphology of the substrates obtained was characterized using scanning electron microscopy. Our studies showed that reducing the coffee ring effect increased the spatial reproducibility of the measured SERS signal in the area of the deposited CNPs. Therefore, the platforms obtained may be very useful in commercial SERS applications.