Enhancement of infrared absorption through a patterned thin film of magnetic field and spin-coating directed self-assembly of gold nanoparticle stabilised ferrofluid emulsion

Abstract

Molecular vibration signals were amplified by the gold strip gratings as a result of grating resonances and nearby electric field hotspots. Colloidal gold island films exhibit similar enhancement; however, the uneven geometrical characteristics of these films restrict the tunability of the vibrational enhancement. Infrared absorption is enhanced by regular metallic patterns such as arrays of strips fabricated using a top-down approach such as nanolithography, although this technology is expensive and difficult. The significant infrared absorption may serve as tuneable antenna sensitization to improve the sensor performance. In this article, we present a simple one-step process for fabricating optically sensitive ordered arrays of a gold nanoparticle ferrofluid emulsion in polyvinyl alcohol (PVA) using a magnetic field-directed and spin-coating self-assembly (MDSCSA) process. Techniques such as UV-visible absorption, scanning electron microscopy, and grazing-angle infrared spectroscopy were used to evaluate various parameters associated with the nanostructures. Unlike the gold strips, the chain-like features in the iron oxide nanoparticle arrays were discontinuous. The fabricated chain-like ordered arrays have been shown to increase the local field to enhance the infrared absorption corresponding to the symmetric vibration of the -CH₂ (2918 cm^-1) group present in PVA by ∼667% at a 45° grazing angle, as the chain thickness (CT) increased by 178%. This scalable and simple method can potentially generate low-cost patterns for antenna sensitisation.

Fig. 1. Schematic showing three stages of PEG-C-GM-pi-FF nano-emulsion manufacturing process and thin film preparation.

Fig. 2. (a) Schematic representation of the magnetized droplet of gold nanoparticle-stabilized pickering ferrofluid emulsion (PEG-C-GM-pi-FF) in an aqueous PVA solution. (b) Top view of the substrate with dimensions. (c) Measured spatial distribution of the magnetic field on the glass substrate surface. (d) Cross-sectional view of the experimental setup used for measuring magnetic fields and preparing magnetic field-directed self-assembly of PEG-C-GM-pi-FF via spin coating. The glass slide supporting magnet is represented by the blue feature, and the magnet itself is depicted as a red square. The distance between the magnet and the glass substrate is 5 mm.

Fig. 3. Characterization of colloids, nanoparticles, suspensions, and emulsions prepared using Transmission Electron Microscopy (TEM), contact angle measurements, UV-Vis absorption, and Thermogravimetric Analysis (TGA): (a) TEM image of prepared Fe₃O₄ nanoparticles, with an inset showing the size distribution (mean size of ∼15 nm and a standard deviation of 2.99 nm). (b) Size distribution of gold methacrylate (GM) nanoparticles with a mean size of ∼17 nm and a standard deviation of 2.57 nm, with an inset showing an image of the wine-coloured GM solution and methacrylate chains illustration. (c) PEG-coated gold methacrylate nanoparticles, with an inset showing the size distribution (mean size of ∼22 nm and a standard deviation of 3.8 nm). (d) Dried PEG-C-GM-pi-FF droplet. (e) UV-Vis spectra of PEG-C-GM, ferrofluid and PEG-C-GM-pi-FF. (f) TGA of GM and PEG-C-GM, with an inset showing the colour of dried droplets. (g) The contact angle (174.88°) for ferrofluid droplets in PEG-C-GM solutions, obtained using Low Bond Axisymmetric Drop Shape Analysis (LBADSA) plug-in of Image J®.

Fig. 4. (a) Shows a PEG-C-GM-pi-FF-PVA thin film on a glass slide prepared using MSCDS. The film is divided into three regions, labelled region 1 (r = 0 mm), region 2 (r = 4 mm), and region 3 (r = 7 mm), where “r” represents the radial location in millimetres from the centre of the film where the magnet was located. (b) Is an example of a dark field image of arrays of PEG-C-GM-pi-FF obtained using an Olympus BX41 microscope with a 20× 0.4 NA M-plan objective lens. (c) Illustrates the 1D pattern morphology, showing chain length (CL), chain thickness (CT), and chain gap (CG). (d and g) Are scanning electron microscope (SEM) images of the thin film at the centre of the substrate (r = 0 mm) at various resolutions (resolution 17 pixels and 1820 per μm, respectively), showing dense clusters of PEG-C-GM-pi-FF. (e and h) Are SEM images of the thin film at r = 4 mm, displaying longer chain structures with an average of 4 droplets thickness, (resolution 17 pixels and 1820 per μm, respectively). Lastly, (f and i) Are SEM images of the thin film at r = 7 mm, showing shorter chains with a maximum of 2 droplets thickness (resolution 17 pixels and 1820 per μm, respectively).

Fig. 5. The figure presents the results of specular reflectance FTIR spectroscopy of PEG-C-GM-pi-FF chains on a glass slide at different radial distances from the centre of the thin film, where a magnet was placed. The CT/CG ratio is highest at the centre of the film (r = 0 mm) and lowest at the outermost location on the film (r = 7.2 mm): using 2600 cm⁻¹ as the reference reflectance intensity point (R%), (a) displays the variation in the signal intensity of the –CH bond vibrations (2918 cm⁻¹ and 2845 cm⁻¹) at grazing angle of 20°, while (b) is for 45° grazing angle. (c) Depicts the change in vibrational intensity of the CO bond (1729 cm⁻¹ and 1707 cm⁻¹) at grazing angles of 20° and (d) for 45° grazing angle. Plots (e) illustrate the relationship between the intensity of vibrational peaks and the CT/CG values at grazing angle of 20° and (f) 45° grazing angle.