Multifunctional Textiles from New Zealand Wool Coloured with Silver or Silver Halide Nanoparticles
Authors:   , Kelly Fern M. 
Published: November 2021

<p>Significant opportunities exist for the development of innovative multifunctional textiles for high value market applications. Composites that combine the inherent properties of their all precursor components in a synergistic manner in particular are sought after. Thus the unique chemical and physical properties of silver or silver halide nanoparticles are combined with the traditional properties of wool, thereby producing an innovative multifunctional composite. The prepared wool - silver or - silver halide nanoparticle composites retain the elasticity, thermal insulation and softness of the wool, whilst the colour, conductivity and antimicrobial properties owing to the nanoparticles are also incorporated. Due to the multi functions of silver the resulting high quality, high value product has numerous applications within the fashion and interior furnishings industries. The wools employed for the preparation of wool - silver or - silver chloride nanoparticle composites are merino wool and crossbred wool. Merino wool provides the main focus of the research.  The experimental approach for the colouring of merino by silver or silver halide nanoparticles follows a novel and proprietary approach. The preparation of wool - silver nanoparticle composites includes two different procedures: 1) the synthesis of nanoparticles in the presence of wool fibres, using an external reducing agent/stabilising agent (trisodium citrate (TSC)), with the in situ binding of nanoparticles to the surface of the fibre; and 2) the synthesis of nanoparticles in the presence of the merino wool substrate, using the reducing nature of wool, with the in situ binding of nanoparticles within the fibre. Merino wool - silver nanoparticle composites range in colour from very pale yellow, through gold to tan and brown. The successful preparation of wool - silver halide nanoparticle composites includes the in situ precipitation of nanoparticles within the wool fibre. This is accomplished by doping the wool, with one of the halides, Cl⁻, Br⁻ or I⁻, prior to treatment with a silver containing solution. The colour of merino wool - silver halide nanoparticle composites can be tuned from pink to peach to purple.  The colour of the wool - silver or - silver halide nanoparticle composites is due to surface plasmon resonances, i.e. the interaction of electromagnetic radiation of visible light with the nanoparticles. The reflected colour is dependent upon the size and shape of the nanoparticle, in addition to the refractive index of the stabilising agent surround the particle. The refractive index of silver chloride or silver bromide differs to that of the reducing/stabilising agent implemented, TSC, or merino, and thus the reflected colour is altered. The colour of silver iodide nanoparticles appears to be due to the interaction of light with the formed nanoparticles themselves and not due to the formation of silver nanoparticles within the silver iodide nanoparticles. In addition to the colour being measured by UV-vis in reflectance mode, the characterisation of the hues of the prepared composites were monitored by obtaining CIE L*, a*, b* values via the HunterLab Colourquest.  The morphological characterisation of merino wool coloured by silver or silver chloride nanoparticles was undertaken using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). When merino wool - silver nanoparticle composites are prepared using an external reducing agent, the formed nanoparticles predominantly bind to the wool fibres surfaces only. When the reducing nature of wool is used for composite preparation, nanoparticles are formed within the fibre and are dispersed throughout the fibres core, with few being present on the surface. Comparable studies of merino wool - silver halide nanoparticle composites showed that silver halide nanoparticles are formed and stabilised just below the fibres surface. The confirmation of silver or silver halide species within the prepared composites was undertaken using energy dispersive spectroscopy (EDS), scanning transmission spectroscopy (STEM), x-ray diffraction (XRD) and x-ray absorption near edge spectroscopy (XANES).  Colourfastness tests to washing, rubbing and exposure to chlorinated swimming pool water were undertaken to assess the robustness of the prepared composites in terms of their colour. These tests indicate that the colours of both merino wool - silver and - silver chloride nanoparticle composites are very stable. The leaching of silver during the washing process was noted to be insignificant, suggesting a strong and stable bond between the fibre substrate and the nanoparticles. X-ray photoelectron spectroscopy (XPS) was used to elucidate the chemical bonding between the wool fibre substrate and the silver or silver halide nanoparticles.  The colourfastness of merino wool - silver or - silver halide nanoparticle composites to light however, was not observed. When exposed to UV light for extended periods, a distinct change in colour occurs. Silver nanoparticle composites lighten considerably, whereas their silver chloride nanoparticle counterparts are noted to become grey in their colour. XPS was used in an attempt to determine what leads to the discolouration of the composites. Further research is required however, in order to reduce or halt the colour degradation of merino wool - silver or - silver chloride nanoparticle composites. Silver iodide nanoparticles, on the other hand, show great potential as colourants for wool, exhibiting stable colours over a long time period to light.  A range of desirable colours are obtained through the colouring of wool by silver or silver halide nanoparticles. These nanoparticles are strongly bound to the fibres and thus the colours are stable to washing and rubbing, exhibiting insignificant leaching of silver during such processes. Additionally, the prepared silver and silver halide nanoparticle composites tested positive for their antistatic properties, and their antimicrobial activity, providing a high value multifunctional material. Numerous applications within fashion and interior furnishing industries are therefore apparent. However, the evident setback for applications in these fields is the colour instability to light of silver, silver chloride and silver bromide nanoparticles, and thus further studies are required to eliminate this problem. Alternative options exist for the exploitation of the photosensitivity of silver halide nanoparticles within the merino wool composites, or the possibility of using silver or silver halide nanoparticles in combination with other strong dyes for the production of coloured woollen fabrics.</p>