Multicolor AIE polymeric nanoparticles prepared via miniemulsion polymerization for inkjet printing

Highlights

• Multicolor AIE polymeric NPs for inkjet printing are prepared via miniemulsion polymerization.

• The AIE polymeric NP inks are made by simple addition of a nonionic surfactant to the initial emulsions.

• The printed images on the cotton textiles display a bright and high-quality fluorescence under UV light.

Abstract

Owing to the strong solid-state luminescence of aggregation-induced emission luminogen (AIEgen), polymer/AIEgen nanoparticles (NPs) have great potential in application as high-quality luminescent inks. In this work, polymer/AIEgen NPs with a particle size below 70 nm were efficiently prepared through encapsulation of AIEgens with various colors within polymeric matrix via miniemulsion polymerization. The prepared polymer/AIEgen NPs emitted bright and stable blue, green, or orange-red fluorescence. The polymer/AIEgen NP inks suitable for inkjet printing were conveniently made through addition of a non-ionic surfactant to the emulsions with corresponding polymer/AIEgen NPs. The bright, high-resolution, and UV-responsive images were printed on cotton textiles with polymer/AIEgen NP inks. The microscopic observation indicated that the polymer/AIEgen NPs could form homogenous films onto the cotton fibers. More promisingly, the prepared polymer/AIEgen NPs with various colors had good compatibility, and they could be directly mixed to prepare multicolor polymer/AIEgen NP inks for handwriting.

Introduction

Inkjet printing technique is one of the most widely used deposition techniques for preparation of versatile functional materials, taking advantage of its high printing resolution, material-effective utility, excellent compatibility with various ink materials, and low pollution [[1], [2], [3], [4], [5]]. Fluorescent nanoparticles (NPs), as a kind of emerging star luminescent nanomaterials, have also been used as nanocolorants for inkjet printing to prepare diverse functional materials that can be applied in many advanced fields like anti-counterfeiting labels, signage and displays, and light-emitting diodes [[4], [5], [6], [7], [8], [9], [10], [11]]. Versatile fluorescent NPs for inkjet printing have been reported, including quantum dots (QDs) [4,5,11], semiconducting polymer dots [7,9], upconversion NPs [10], or NPs with small organic chromophores [12]. However, the reported fluorescent NPs may still have some defects, for example the difficult synthesis, limited variety, and possible cytotoxicity of QDs, and the low solid-state luminescence efficiency due to aggregation-caused quenching (ACQ) effect [4,10,12].

In 2001, Tang et al. first coined a unique aggregation-induced emission (AIE) phenomenon, which is exactly opposite to the ACQ effect [13,14]. AIE luminogens (AIEgens) are nonemissive or weak emissive in benign solvents, but become highly emissive in the aggregate or solid state due to the restriction of intramolecular motion [15,16]. In virtue of the AIE properties, AIEgens are ideal candidates as emitting components to fabricate fluorescent NPs for wide applications, including sensing, biological imaging, disease diagnosis and treatment, and photoelectric devices [[17], [18], [19], [20], [21], [22]]. Among various AIE NPs, AIE polymeric NPs hold high potential in application as nanocolorants for inkjet printing because of the good water dispersibility, well-controlled particle and surface properties, high brightness, and good photostability. Many methods have been devised to prepare AIE polymeric NPs, including encapsulation of AIEgens through self-assembly of block copolymers [23,24], supramolecular interaction [25], ultrasound-assisted multicomponent reaction [26,27], click chemistry [28], emulsion polymerization [29,30], and miniemulsion polymerization [[31], [32], [33], [34], [35]]. The self-assembly based techniques are limited by the preparation efficiency, because they are often carried out under a relatively low concentration for controlling the particle properties and colloidal stability of the systems [24,30]. Limited by the intrinsic polymerization mechanism [36], the AIEgen should diffuse into the newly-nucleated latex particles during the emulsion polymerization process, but the AIEgen diffusion is not a spontaneous process. Therefore, the AIEgen loading capacity of emulsion polymerization technique is relatively low [29,30].

In typical miniemulsion polymerization systems, many monomer droplets are homogenously dispersed in the aqueous continuous phase, and latex particles are formed through droplet nucleation [[36], [37], [38]]. This technique has been used to efficiently prepare AIE polymeric NPs with a particle size below 100 nm through copolymerization of AIE-active monomers or in situ encapsulation of AIEgens that were pre-dissolved in the monomer droplets [[31], [32], [33]]. The miniemulsion polymerization technique displayed a high AIEgen loading capacity, and the photoluminescence (PL) intensity of AIE polymeric NPs could be accurately tuned by the loaded amount of AIEgens [31]. Moreover, the surface of AIE polymeric NPs could be flexibly functionalized through copolymerization with functional comonomers or utilisation of the chemical reaction of the surface reactive groups [33,35]. Therefore, the miniemulsion polymerization technique holds great potential in preparation of AIE polymeric NPs for inkjet printing.

For printable inks with nanocolorants, they should fulfil the strict requirements of the particle size of nanocolorants, and the surface tension, density, and viscosity of inks [1,39]. Normally, the particle size of the nanocolorants should be small enough to avoid clogging the inkjet nozzle and acquire high quality printing images [40]. Furthermore, the Fromm number (Z), a nondimensional number embodying the rheological property of inks, should be in the range of 4 < Z < 14, as proposed in the literature [1,41].

In this work, blue, green, and orange-red polymer/AIEgen NPs with a particle size below 70 nm were efficiently prepared through encapsulation of various AIEgens within polymeric matrix in miniemulsions (Scheme 1). The prepared polymer/AIEgen NPs emitted bright fluorescence, and displayed excellent photostability and colloidal stability. The printable inks with appropriate Z numbers were conveniently obtained through addition of a suitable amount of non-ionic surfactant. The printed images on cotton textiles were invisible under the daylight, but the high-quality images could be clearly seen under UV light irradiation. Furthermore, the prepared polymer/AIEgen NPs had good compatibility, and the inks with various colors could be conveniently prepared by delicately mixing three primary polymer/AIEgen NP emulsions prepared by miniemulsion polymerization.