A new method to extend the stress response of triboluminescent crystals by using hydrogels

Polyacrylamide hydrogel entrapment of EuD 4 TEA or Cu(NCS)(py) 2 (PPh 3 ) radically extends the emission time of the triboluminescent (TL) crystalline particles by a factor of 10 3 , optimized when matching the hydrophilic/hydrophobic characteristics of the TL/gel components. Triboluminescence intensity improves with hydration of the TL/hydrogel composite. The composites may be used in impact-related sensor applications.


INTRODUCTION
Triboluminescence (TL) is defined as light emission resulting from the piezoelectric response of crystalline materials upon application of mechanical force [1,2]. TL research is an emerging field for the potential development of self-responsive materials in mechano-sensing technology. However, TL materials currently have limitations to be applied as a sensing platform because of powder nature of the bulk crystal form. Additionally, the time of TL emission is on the order of microseconds [3,4]. The integration of TL materials into solid matrices is critically important to enhance the applicability of these materials in a variety of force-induced sensor platform. The control over emission time (i.e. until any emission could be measured upon mechanical action) of TL signals can potentially increase the application of TL materials for example, in the chemical sensing of impact related damage to fragile and metastable components such as electronics and packaged foods. The synthesis of TL materials under different solvents can directly affect the size of crystals, and accordingly TL emission time of compounds [5][6][7][8]. Moreover, some materials like piperine [5], uranium [6], dimethyl methylphosphate (DMMP) [7], and caffeine [8] can enhance the emission when they are associated with TL systems. In terms of polymer-based TL composites, there are some 2 studies such as ZnS:Mn/polymer composite [9], ZnO tetrapod filled elastomers [10], EuD 4 TEA incorporated within polymer films [11], diblock copolymers integrated inorganic TL materials [12] that have been carried out to understand the concept of TL materials within solid matrices. Main advantage of polymer/TL composites is to transfer TL response to any polymeric systems such as films, fibers or any complex-shaped materials for different end-use. In this work, two organometallic crystals (europium tetrakis (dibezonlymethide) triethylammonium, EuD 4 TEA and copper (I) thiocyanate bipyridine triphenly phosphine Cu(NCS)(py) 2 (PPh) 3 ) with different emission wavelengths were obtained and integrated within hydrogel systems resulting in (previously unreported) extended TL emission. The emission of the TL materials of the two crystals was examined particularly in Nhydroxymethylacrylamide (NHMA), N-isopropylacrylamide (NIPAM) and acrylamide (AA) hydrogel matrices.

EXPERIMENTAL
Synthesis of EuD 4 TEA [13] and Cu(NCS)(py) 2 (PPh 3 ) [14] were carried out using modified methods described elsewhere. The fabrication of TL integrated hydrogels (NIPAM, NHMA, and AA) is described as follows. AA, 48% (w/v) NHMA, NIPAM, N,N'-methylenebisacrylamide (MBA), ammonium persulphate (APS), N,N,N',N'tetramethylethyldiamine (TEMED) were all purchased from Sigma-Aldrich, Poole, Dorset, UK. Hydrogels were synthesised using a family of acrylamide-based monomers, namely AA, NHMA and NIPAM to form individual respective polymer hydrogels along with MBA as a cross-linker as previously reported [15]. In the preparation of a 1 mL hydrogel each of functional monomer was used individually along with MBA, APS as an initiator, and TEMED as a cross-linker for each of the hydrogels. Each solution was then immediately added to 50 mg of either EuD 4 TEA or Cu(NCS)(py) 2 (PPh 3 ). Solutions were purged with nitrogen. Each solution was then separately poured into a glass Petri dish (5 cm inner diameter) and onto a pre-layered cutting of Parafilm® tape to fit the glass surface. The polymerising solution (containing TL material) was then covered with a second layer of Parafilm® tape to sandwich the solution. The sandwiched solution was left to polymerise overnight at room temperature. The dishes containing TL material-entrapped gels were then stored at 4° C prior to use. Specifically designed drop tower system (Fig. 1a) was used for the measurement of TL emission. The material was placed into sample holder, within a black box (to exclude ambient light interference). A 50 gram steel ball with a diameter of 1 cm was positioned on a pullable pin at a set distance of 27 inches (70 cm) above the material. When the pin is pulled, then the ball falls and hits the material. As a result, TL emission occurs. A fiber optic cable pre-inserted directly through a small hole inside the black box, can capture and transfer the impact radiation through to Ocean optics USB2000+ spectrophotometer. The 3 detector type is a high-sensitivity 2048-element CCD array and the integration time is from 3 to 30000 ms. By using LabVIEW program, the spectra was recorded by quick view fluorescence mode as a graph of TL emission with respect to wavelength. Figure 1 presents morphology and optical feature of the both TL crystals. Based on scanning electron microscopy (SEM) images, Eu(III)-based crystals (Fig. 1b) show rectangular, while the Cu(I)-based crystals (Fig. 1c) possess triangular prisms. Fluorescence spectrophotometry measurements were conducted in polar aprotic solvent, N,Ndimethyl formamide and photoluminescence (PL) emission of EuD 4 TEA and Cu(NCS)(py) 2 (PPh 3 ) are given in Fig.   1d and 1e, respectively. PL signal of the former crystal is centered on 614 nm, and the latter one is at 496 nm. TL spectra of EuD 4 TEA and Cu(NCS)(py) 2 (PPh 3 ) are given in Fig. 1f and 1g, respectively. In both cases, the higher height translates to a larger compression force onto crystals; therefore, higher intensity in TL emission. Figure 2 shows TL emission spectra of TL/gel composites. Fig. 2a and 2b depicts TL spectra of the composites consisting of EuD 4 TEA before and after water treatment, respectively. TL intensity increases after water soaking of the hydrogels. TL spectra of Cu(NCS)(py) 2 (PPh 3 ) consisting of gel composite before and after treatment are shown in Fig. 2c and 2d, respectively. Even though, the incorporation of EuD 4 TEA within an NHMA gel shows a better TL response, NIPAM shows the same tendency when integrated with Cu(NCS)(py) 2 (PPh 3 ). The percentage change of TL emissions can be ordered as 37%, 40%, and 2% for NHMA, NIPAM, and AA composites integrated with EuD 4 TEA, respectively. With Cu(I) based composites this order was found to be 5%, 22%, and 75% respectively. and NIPAM, respectively. In the latter case, the total emission time apparently increased with increasing hydrophobicity of the hydrogel. The Cu(NCS)(py) 2 (PPh 3 ) compound comprises of hydrophobic ligands. A similar effect has been reported using cyclotriveratrylenes as fluorescent markers, which have their emission enhanced with binding of similarly hydrophobic quaternary ammonium molecules but not with hydrophilic (or charged) species 4 [16]. Therefore, it is possible that preferential non-polar interactions between the ligands and hydrophobic NIPAM could be stabilizing the complex and enhancing the emission time. In both cases, the TL materials demonstrate optimum TL when the gels are fully hydrated.

RESULTS and DISCUSSION
The medium in which the light propagates plays a significant role on the photo-physical properties of the TL crystals [17]. The propagation of light through the gel volume depends on the optical density of the solvent in addition to polymeric crosslinking points, where the scattering is induced. Hydrogels provide a dense and viscous medium compared to air. Not surprisingly, TL emission time increases at high viscosity medium [18].
High intensity emission within hydrogels may originate from the better dispersion of TL crystalline particles throughout the hydrogel volume. It is well established that within a cross-linked polymer chain architecture the crystalline particles would be physically entrapped and have less chance to diffuse and/or undergo agglomeration [19]. Figure 4 presents SEM images of TL composites in dry gel (panel a) and in hydrated gel states (panel b), respectively. In the dry state, TL crystals are dispersed into particles as large domains in aggregated and agglomerated form. On the other hand, upon soaking with water, the particle domains are separated and become dispersed due to gel swelling. Thus, TL crystals may individually emit light, resulting in higher overall emission from the assembly of individual crystals.

CONCLUSION
In summary, two TL crystals (red: EuD 4 TEA and blue: Cu(NCS)(py) 2 (PPh 3 )) were incorporated within hydrogels via in situ radical polymerization. The hydrogel matrix was found to have no effect on spectral shape of the TL emission; however, it plays a significant role on the luminescence efficiency and emission time of the TL crystals.
The luminescence intensity is improved presumably due to the better particle dispersion. Moreover, sharp TL response of the crystalline particles can be extended to the order of minutes in an appropriate gel matrix. The control over TL response can be achieved by the adjustment of the hydrophilicity/hydrophobicity of the surrounding gel medium.