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The application of pH-sensitive dye molecules onto textile materials is a promising method for the development of sensor materials. Ten commonly used pH-indicators, namely sulfonphthaleine dyes, are applied onto polyamide 6 using two distinct methods: conventional dyeing of fabrics and dye-doping of nanofibres. The influence of the substituents of each dye on their interaction with polyamide, as well as the difference between both application methods is investigated. For the conventionally dyed fabrics, halogen substituents are needed to result in a pH-sensitive fabric. This can be traced back to halogen bonding and is supported by theoretical simulations. Dye-doped nanofibrous non-wovens show significant dye leaching, which can be understood based on the very acidic electrospinning solution. The use of a complexing agent improves the leaching properties, especially for dyes containing four bromine substituents. These findings indicate the importance of halogen substituents on sulfonphthaleines for further research in the development of pH-sensitive sensors.

Here, we introduce a novel concept for the fabrication of colored materials with significantly reduced dye leaching through covalent immobilization of the desired dye using plasma-generated surface radicals. This plasma dye coating (PDC) procedure immobilizes a pre-adsorbed layer of a dye functionalized with a radical sensitive group on the surface through radical addition caused by a short plasma treatment. The non-specific nature of the plasma-generated surface radicals allows for a wide variety of dyes including azobenzenes and sulfonphthaleins, functionalized with radical sensitive groups to avoid significant dye degradation, to be combined with various materials including PP, PE, PA6, cellulose, and PTFE. The wide applicability, low consumption of dye, relatively short procedure time, and the possibility of continuous PDC using an atmospheric plasma reactor make this procedure economically interesting for various applications ranging from simple coloring of a material to the fabrication of chromic sensor fabrics as demonstrated by preparing a range of halochromic materials., Dye coating techniques for colored materials are often cost intensive or cause degradation of the material during processing. Here the authors demonstrate a fast, scalable and cost efficient plasma dye coating procedure, which allows for covalent immobilization of dye molecules on different polymer surfaces.

Nanofibrous membranes based on polycaprolactone (PCL) have a large potential for use in biomedical applications but are limited by the hydrophobicity of PCL. Blend electrospinning of PCL with other biomedical suited materials, such as gelatin (Gt) allows for the design of better and new materials. This study investigates the possibility of blend electrospinning PCL/Gt nanofibrous membranes which can be used to design a range of novel materials better suited for biomedical applications. The electrospinnability and stability of PCL/Gt blend nanofibers from a non-toxic acid solvent system are investigated. The solvent system developed in this work allows good electrospinnable emulsions for the whole PCL/Gt composition range. Uniform bead-free nanofibers can easily be produced, and the resulting fiber diameter can be tuned by altering the total polymer concentration. Addition of small amounts of water stabilizes the electrospinning emulsions, allowing the electrospinning of large and homogeneous nanofibrous structures over a prolonged period. The resulting blend nanofibrous membranes are analyzed for their composition, morphology, and homogeneity. Cold-gelling experiments on these novel membranes show the possibility of obtaining water-stable PCL/Gt nanofibrous membranes, as well as nanostructured hydrogels reinforced with nanofibers. Both material classes provide a high potential for designing new material applications.

‘Smart’ materials can be defined as materials that respond to a certain stimulus with a change in their properties. A specific class herein is halochromic textiles, i.e. fibrous materials that change color with pH. Such halochromic textiles play an important role in the continuous monitoring and visual reporting of the pH with applications in various fields, such as wound treatment and protective clothing. pH-sensitive nanofibrous nonwovens have high sensitivity and a fast response time, and are mostly fabricated by introducing a pH-responsive dye via dye-doping of the feed mixture before fabrication. However, this method suffers from leaching of the dye, which is an undesirable effect that not only reduces the output signal strength but can also be detrimental to the environment by causing, for instance, toxicological responses. In this paper, a new strategy is demonstrated for the reduction of dye leaching in electrospun, nanofibrous materials. Through blend electrospinning of polyamide-6 (PA6) with a dye-functionalized copolymer, large sheets of uniform, halochromic nanofibrous material can be fabricated showing a fast pH-sensitive color change. Polymeric entanglements within the nanofiber are proposed to immobilize the dye-functionalized copolymer in the PA6 matrix, resulting in drastically reduced dye leaching. Such stable nanofibrous, PA6-based, halochromic materials are particularly interesting in the design of new colorimetric sensors applicable in several sectors, including the biomedical field, agriculture, safety and technical textiles.

Gelatin nanofibrous structures, characterized by high specific surface area and high porosity, have been widely researched for biomedical and food applications. The present paper researches the potential of electrospinning to produce a nanofibrous cold-gelling (or instant) gelatin product. Our results show that gelatin nanofibers are cold-water-soluble due to their high surface-to-volume ratio, facilitating easy water penetration and dissolution, and this for several gelatin types. Additionally, fast gelation after dissolution in cold water indicates that the electrospinning process does not significantly reduce the gelatin molecular weight, nor compromise triple helix formation. These conclusions were supported by thorough investigation of the internal gelatin structure, using a new approach based on modulated temperature scanning calorimetry. Oscillation rheology revealed that the nanofiber-based gels have moduli comparable to powder-based gels. Gelatin nanofibers can thus be used as instant gelatin product, without the drawbacks of traditional amorphous instant gelatins such as sensitivity to moisture, low wettability and low modulus of the cold gel. Using the approach reported here, every electrospinnable, but non-cold-water-soluble gelatin can be transformed into a cold-water-soluble variant, regardless of the type or modification. Electrospinning can thus offer enormous flexibility in materials selection, enabling the production of cold gels loaded with temperature-sensitive components, UV-cross-linkable cold gels, etc. (C) 2016 Elsevier Ltd. All rights reserved.

Colorimetric sensors for monitoring and visual reporting of acidic environments both in water and air are highly valuable in various fields, such as safety and technical textiles. Until now sol-gel-based colorimetric sensors are usually nonflexible bulk glass or thin-film sensors. Large-area, flexible sensors usable in strong acidic environments are not available. Therefore, in this study organically modified silicon oxide nanofibrous membranes are produced by combining electrospinning and sol-gel technology. Two pH-indicator dyes are immobilized in the nanofibrous membranes: methyl yellow via doping, methyl red via both doping, and covalent bonding. This resulted in sensor materials with a fast response time and high sensitivity for pH-change in water. The covalent bond between dye and the sol-gel network showed to be essential to obtain a reusable pH-sensor in aqueous environment. Also a high sensitivity is obtained for sensing of HCl and NH3 vapors, including a memory function allowing visual read-out up to 20 min after exposure. These fast and reversible, large-area flexible nanofibrous colorimetric sensors are highly interesting for use in multiple applications such as protective clothing and equipment. Moreover, the sensitivity to biogenic amines is demonstrated, offering potential for control and monitoring of food quality.

PCL/chitosan blend nanofibres could provide an excellent nanostructured material for biomedical use since their beneficial properties are combined into one material. This paper focuses on the solvent use in the production of PCL/chitosan nanofibres by solution electrospinning, since this is a crucial parameter in the electrospinning process. It was established that an acetic acid/formic acid shows great potential for the stable electrospinning of the blend. Additionally, the fibre morphology using this solvent system was analysed.

Sensors supplying a change in optical properties, easily detectable with the naked eye (visual) or inexpensive equipment such as compact spectrometers (optical), are a very powerful tool to visualise a wide range of parameters, including temperature, light, pH and concentration of chemical substances. Most of these sensors rely on indicator compounds showing a change in optical absorbance (colour) or fluorescence under the influence of a certain parameter. Halochromic dyes, for instance, change colour with pH. Since the use of nanofibres improves sensor sensitivity and response time due to their large surface area to volume ratio, the incorporation of indicator compounds into nanofibres is one of the current challenges in sensor design. This chapter discusses the production of colorimetric and fluorescent nanofibrous membranes for visual and optical monitoring (Sects. 7.3 and 7.4), supplemented by some fundamental information on those sensing systems (Sect. 7.2) and some interesting applications (Sect. 7.5).

This paper discusses the melting behaviour and crystal structure of electrospun (polymorphous) polyamide (PA) fibres and presents the importance of the heating rate used for their study by thermal analysis. A case study of PA6, PA46 and PA69 nanofibres is performed using a Rapid Heat-Cool DSC (RHC), a fast-scanning calorimeter capable of controlled heating rates above 1000 K min−1. Thermal lag becomes significant at scan rates above 500 K min−1 and the ideal scan rate for analysis of the crystal structure depends on the PA type. Measurements at the most suitable heating rate indicate that the thermal stability of the crystals is higher for nanofibres compared to bulk material. Additionally, more stable crystals are favoured in finer nanofibres. This is especially the case for PA6 nanofibres, having an increasing fraction of stable α-crystal phase with decreasing fibre diameter, as shown by both thermal analysis and X-ray diffraction.

Nanofibres functionalised with pH-sensitive dyes could greatly contribute to the development of stimuli-responsive materials. However, the application of biocompatible polymers is vital to allow for their use in (bio)medical applications. Therefore, this paper focuses on the development and characterisation of pH-sensitive polycaprolactone (PCL) nanofibrous structures and PCL/chitosan nanofibrous blends with 20% chitosan. Electrospinning with added Nitrazine Yellow molecules proved to be an excellent method resulting in pH-responsive non-wovens. Unlike the slow and broad response of PCL nanofibres (time lag of more than 3 h), the use of blends with chitosan led to an increased sensitivity and significantly reduced response time (time lag of 5 min). These important effects are attributed to the increased hydrophilic nature of the nanofibres containing chitosan. Computational calculations indicated stronger interactions, mainly based on electrostatic interactions, of the dye with chitosan (Δ G of −132.3 kJ/mol) compared to the long-range interactions with PCL (Δ G of −35.6 kJ/mol), thus underpinning our experimental observations. In conclusion, because of the unique characteristics of chitosan, the use of PCL/chitosan blends in pH-sensitive biocompatible nanofibrous sensors is crucial.

Blend nanofibres composed of chitosan and polycaprolactone (PCL) are highly valuable for biomedical applications since the benefits of natural and synthetic polymers are thus combined. Research towards the stable and reproducible electrospinning of these blends is, however, essential. Therefore, this paper focuses on the novel solvent system 3/7 acetic acid/formic acid for electrospinning PCL/chitosan nanofibres. Addition of chitosan to PCL solutions led to a significant increase in the solution's conductivity because of the polycationic character of chitosan. Moreover, since adding chitosan also considerably raised the viscosity, polymer solutions with a low total polymer concentration could be electrospun. Owing to both effects, the fibre diameter and distribution were significantly lowered with increasing chitosan content. Furthermore, chitosan led to the formation of an ultrafine nanofibrous web formed in between the main fibres. In conclusion, the solvent mixture acetic acid/formic acid is an excellent system for successfully electrospinning blend nanofibres containing PCL and chitosan.