Bright and persistent green and red light-emitting fine fibers: A potential candidate for smart textiles

Highlights

• Forcespun nanoparticle encapsulated PVA fiber.

• High thermal stability.

• Exceptionally fine and monodisperse in nature.

• Bright green and red photoluminescence depicted by fiber.

• Fiber also displayed persistent light emission.

Abstract

The properties of smart light-emitting textile/fabric with a persistent afterglow can be beneficial for use in anti-counterfeiting, night surveillance, and security trooping among other potential applications. Keeping this in mind, this work is a step forward in designing bright green and red emitting poly vinyl alcohol (PVA) fine fibers. We first synthesized green emitting Zn₂GeO₄:Mn²⁺ (ZGOM) and red emitting ZnGa₂O₄:Cr³⁺ (ZGOC) nanoparticles using a hydrothermal process. Field Emission Scanning Electron Microscope (FESEM) images depicted the formation of nanorods and sub-10 nm nanoparticles for ZGOM and ZGOC, respectively. These nanoparticles were mixed in a PVA solution and spun into fiber using the Forcespinning® technology. FESEM images of the fiber samples show the presence of long, bead-free, defect-free, smooth surfaces with the diameters ranging from 200 nm to 1.2 μm depending on selected processing parameters; a fiber system was selected considering fiber output and fiber diameter, the selected system had fibers with average diameter of 900 nm for both ZGOM and ZGOC encapsulated PVA fine fibers (MG-PF and CR-PF). Another favorable property was that the thermal stability of the PVA fine fibers was not affected by the nanoparticles. Both MF-PF and CR-PF fine fibers depicted a bright green and red luminescence, respectively, under UV excitation. The fiber can easily accommodate loading up to 1.0% weight without doing any kind of fluorescence quenching. MG-PF fiber displayed a bright and substantial green persistence luminescence up to 500 s. In the case of CR-PF there is a quenching of trap states and therefore intrinsic red persistence luminescence is not retained beyond 100 s. We believe such green fiber if smartly woven in any kind of textile materials/fabric can be an efficient solution to deal with cloth counterfeiting and night surveillance.

Introduction

In the photonic application, the one-dimensional (1D) nanoarchitectures such as nanorods, nanowires, and nanofibers have emerged as the superior building blocks over their bulk counterpart because they exhibit unique properties such as high aspect-ratio, super elongated shape, well-defined polarization, enhanced spontaneous emission, and efficient energy transfer, which make them suitable candidates for short-distance networking, light-emitting diodes, nano-lasers, solar cells, optical sensors, and drug delivery applications among others [[1], [2], [3], [4]]. Among these various nanostructures, polymer-based1D nanofibers have gained recent attention due to their hosting ability towards easily tunable embedding emissive systems such as incorporation of functional luminescent materials(i.e. quantum dots [5,6], rare-earth ions [7], dye molecules [8,9], noble metal nanoparticles, etc [10]). The integration of tunable luminescent materials over non-emissive 1D polymeric host nanostructures opens up substantial opportunities for nanoscale electronic and optoelectronic devices [1,8,[11], [12], [13]]. Moreover, 1D polymer nanofibers have mechanical flexibility, good biocompatibility, and versatile functionality, which make them very appropriate for high-performance optical devices [[12], [13], [14], [15], [16]]. We believe such nanofibers can be integrated into a type of smart clothing that will glow after being exposed to UV irradiation [17,18].

The polymer nanofibers can be functionalized by introducing luminescent materials into the polymer solution and then be drawn into fibers [19]. Among several fiber drawing methods, it is worth mentioning that the electrospinning technique is the most versatile and effective method to produce these fibers [4,15,20]. However, this technology presents a challenge for bulk production and involves use of toxic organic solvents given its dielectric requirements, therefore its wide application at the industrial level is limited [21,22]. Recently, Forcespinning® (FS) has been introduced as the potential alternative, which can eliminate the previously mentioned issues [23]. FS® has a unique operating system based on centrifugal forces instead of electric fields, and fibers can be drawn from a polymeric solution. Moreover, there is no specific requirement for the dielectricity of the solvent since an electric field is not used.

In recent years, only limited efforts [24] have been dealt to build luminescent multifunctional materials, particularly with ternary metal oxide nanoparticles doped with transition metal ions. Although, it is expected that these solution-processable, bright persistent luminescent nanoparticles will be readily accommodated within the polymer matrix. There are few reports on the synthesis of photoluminescent fibers. Some of the recent ones are on PDMS elastomers with aggregation induced emission luminogens (AIEgens), silk fiber modified with an organic fluorophore, cellulose fiber modified with core-shell nanoparticles, electrospun PVP/PVDF/PMMA nanofibers, Ag nanoparticles, modified PVP nanofibers, etc. [17,18,[25], [26], [27]] However, none of them explored the fascinating properties observed in encapsulated nanostructures within PVA fibers; water dispersibility, persistent photoluminescence, biocompatibility and high photoluminescence (PL) quantum yield. In this study we designed fine fibers doped with transition metal ternary mixed metal oxide nanoparticles and explored their tunable and bright, persistent optical luminescence. Herein, we have selected two such emissive nanoparticles viz. (1) green luminescent Mn²⁺-doped Zn₂GeO₄ (ZGOM) nanorods (NRs) [28] and (2) red luminescent Cr³⁺-doped ZnGa₂O₄ (ZGOC) nanoparticles (NPs) having a size of sub-10 nm which demonstrate stable persistent luminescence emission more than 40 min after excitation [[29], [30], [31]]. These nanomaterials possess well-defined morphology with high quantum yields and water dispersibility, which make them suitable for use in luminescent fine fiber fabrication. Moreover, their persistent luminescent nature makes them advantageous for applications in medical diagnostics as well as light-emitting devices. As a solid powder particle, however, they suffer from aggregation-induced quenching, and as a result they have to be dispersed in very low concentrations, which limits their application in display devices and other photonic applications [19,27]. These issues can certainly be resolved if such nanostructures are encapsulated inside the flexible, and solution-processable polymer.

Polyvinyl alcohol was chosen as the polymer base material for ForceSpinning® due to its non-toxicity and biocompatibility, along with several other advantageous properties such as high aspect ratio, high tensile strength, relatively high modulus of elasticity, etc. [4,32] Therefore, in this present investigation we demonstrate not only a facile strategy to produce bright and persistent photoluminescent polymeric fine fibers, but it also leads to an improved distribution method of the two mixed metal oxide nanoparticles, which prevent any kind of non-radiative emission, thus, maintaining the original fluorescent properties of the nanoparticles. The ultimate goal of this work is to produce a material that can be integrated into clothing as security against counterfeiting as well as aiding during nighttime by increasing visibility.