Selection of the highest performance functional dye
In order to select the best functional dye when applied to a substrate, 19 kinds of conventional dyes (Fig. 2) whose structure was known were dyed to cotton fabrics. All dyed fabrics were exposed to DMMP vapor for 24 h, and then the color changes were measured (Fig. 3 (a) and (b)). As a result, the cotton fabrics dyed with Direct Orange 26, Direct Red 23, and Direct Violet 7 exhibited superior color change with color differences of 10 or more, especially, in the case of Direct Violet 7, a color difference of over 30 was recorded.
Although the color differences were measured as 16.2 and 13.2, respectively, when the cotton fabrics dyed with Direct Orange 26 and Direct Red 23 were exposed to DMMP vapor, they showed mainly in color intensity change due to a hyperchromic shift. On the contrary, in the case of Direct Violet 7, the color difference was very large as 32.1, and the color pattern also changed both color intensity and color hue due to hyperchromic and hypsochromic shifts. It means that it might be much more recognizable when seen with the naked eye. The color spectra of the fabrics dyed with those three functional dyes before and after exposure to DMMP vapor were presented in Fig. 4. Therefore, Direct Violet 7 was selected to realize a high performance wearable chemical gas sensor.
Application to cotton fabrics
To fabricate the wearable chemical gas sensor exhibiting color change upon exposure to vapor phase of organophosphorus nerve agents, Direct Violet 7 was applied to cotton fabrics and the sensing properties were measured. To optimize concentration of the dye showing the maximum color change on exposure to DMMP, the cotton fabrics were applied with various dye concentrations (0.5–30% owf) and then the dyed fabrics were exposed to saturated vapor of DMMP at room temperature. Color strengths and color difference values according to dye concentrations were presented in Fig. 5. The color strength increased as the dye concentration increased and reached equilibrium at about 10% owf. However, the color differences before and after exposure showed the maximum value at 3% owf, and then decreased as the dye concentration increased more. The color of the sample was too light at lower concentration and too dark at higher concentration. Therefore, the optimal concentration of the dye was determined to be 3% owf.
Sensing performance of the dyed cotton fabrics
To perform the quantitative experiments, the cotton fabrics (15 mm × 20 mm) dyed with 3% owf of Direct Violet 7 were hung in a sealed vials and exposed to various concentrations (3-300 ppm) of DMMP and at different time (10–200 min). Even though detection is possible at room temperature, each test vial was heated to 70°C for complete evaporation for quantitative experiments. Although the test temperature was much lower than boiling points, it was enough to vaporize the 300 ppm of DMMP. The color change increased with the increase of the concentration of DMMP and exposure time, and then reached equilibrium. Since the chemical warfare agents, as nerve agents, have fatal toxicity to the human body when inhaled, it is important to quickly detect even under trace concentrations. As shown in Fig. 6 (a), the color differences were obtained about 5 within 10 min at 3 ppm of DMMP. Generally speaking, it is recognizable with naked eyes at the color difference value is 1.0 or more. It means that the color changed highly visibly even at a small concentration of 3 ppm. In this study, it was measured from 10 min, but it can be assumed that the color difference has changed by more than 1.0 within a few minutes, implying that detection is possible even at a time faster than 10 min. The CWAs are extremely toxic, therefore, they must be detected immediately even when present at ultra-trace of concentrations (e.g. ppb level). Although the textile-based sensor fabricated in this study could not achieve this level of sensitivity, it was confirmed that the chemical gases could be detected using this textile-based organic sensor. Figure 6 (b) depicts the change of L*, a*, and b* values when the wearable chemical gas sensors were exposed to 300 ppm of DMMP vapor. The L*, a*, and b* values for the sensor were from 37.03, 28.17, and − 25.29 to 39.30, 46.66, -6.80. It can be seen that the wearable chemical gas sensor changed from bluish purple to reddish violet upon exposure to DMMP vapor.
To visualize the performance of the wearable military chemical sensor, we dyed cotton yarn with the selected functional dye (Direct Violet 7) and then embroidered it as the lettering "CWAs" on a sheet of polyester fabric dyed with a conventional non-functional disperse dye of the similar color to Direct Violet 7, as depicted in Fig. 7. The wearable sensor was located in a closed box containing 200 ppm of DMMP vapor, and the sensor displayed the lettering (CWAs) by changing the color. The CWAs stands for chemical warfare agents.
Sensing mechanism: Solvatochromism
The sensing mechanism reflected the interplay of three factors, namely solvatochromism, aggregative properties of the dye molecule, and amount of DMMP adsorbed on the cotton fabrics.
In order to investigate the solvatochromism of the functional dye, the absorbance was measured by dissolving the dye in each solvent such as water, DMSO, ethylene glycol, DMF, acetonitrile, MeOH, acetone, pyridine, THF, and DMMP (the nerve agent simulant) with different polarity. The absorption spectrum shift by dielectric constants of the solvents was presented in Fig. 8 (a) and the relationship between the dielectric constant of the solvents and the maximum absorption wavelengths was in Fig. 8 (b). The functional dye exhibits positive solvatochromism showing bathochromic shift with increasing solvent polarity. The first sensing mechanism of the wearable gas sensor was the solvatochromic effect inside the fiber. The cotton fabrics as a substrate are hydrophilic material usually containing 7–10% water at standard condition. This implies that the dye molecules are placed at water-rich environment inside cellulose structure and the dye molecules are affected by water which is the extremely polar solvent. Upon exposure to less polar DMMP (dielectric constant; 22.3000) than water (dielectric constant; 78.3553), the internal polarity of cellulose may be lowered, and it brings about the hypsochromic shift of absorption spectrum of the dye as presented in Fig. 4 (c). Herein, the effect of relative humidity does not have to be taken into account. Even when the fabricated textile-based gas sensor was exposed to saturated water vapor for 24 h, there was no color change at all. It means that the interaction between the functional dye and water is already in equilibrium with the amount of moisture in the air. However, there is definitely an effect on moisture. When moisture is completely removed through vacuum drying, it exhibits a hypsochromic shift and returns to its original color as soon as it is exposed to the atmosphere.
To observe the solvatochromism of the functional dye from the molecular orbital energy point of view, density functional theory (DFT) calculation was performed. After geometry optimization, solvation effect was measured for the 9 solvents used in the solvatochromism experiment. In Fig. 9, HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy were calculated and compared to the experimental results of maximum absorption wavelength of the dye in each solvent. The energy gap between HOMO and LUMO of Direct Violet 7 became narrower as the dielectric constants increases. This was consistent with the Fig. 8 that the maximum absorption wavelength shifted to a longer wavelength as it was dissolved in a more polar solvents.
Sensing mechanism: Aggregative characteristics
The functional dye has highly aggregative properties because it is high planar and linear structural dye. When exposed to DMMP vapor, the DMMP molecules permeate into the dye molecule aggregates and disassemble dye crystals, causing color change. To verify the change of the distance between the dye molecular planes due to exposure to DMMP, XRD analysis was performed. Figure 10 depicted the XRD patterns before and after exposure to DMMP. The dye showed diffraction peaks at 27.3°, 31.7°, and 45.5°, etc. before exposure and it can be calculated the molecular interplanar distance using the Bragg Equation as 3.26 Å, 2.82 Å, and 1.99 Å, respectively. After exposure to DMMP, the intensity of the peaks significantly reduced and new peaks were created in the low theta region. It indicates that the DMMP molecules partially broke the dye crystals or aggregates.
Organic dyes are dissolved as monomeric states in good solvents but aggregates in poor solvents. The aggregates can be theoretically divided into two forms: J- and H-aggregates. According to the exciton theory, the excitonic state of the dye aggregate splits into two levels through the interaction of their transition dipoles. The molecules may form head-to-tail arrangement (end-to-end stacking) in J-aggregates. It can be simplified two levels of the excitonic state in the molecule arrangements through the interaction of their transition dipoles, such as unstable aggregation exhibiting higher transition energy by charge repulsion and stable aggregation with lower transition energy by charge attraction compared to the monomeric state. In H-aggregate, the molecules may aggregate in a parallel way (side-by-side stacking) and it also can be simplified as two arrangements. However, the unstable aggregative form in J-aggregates and the stable aggregative form in H-aggregates are forbidden transitions according to the exciton theory. Therefore, J-aggregates show the red shifted absorption and H-aggregates show the blue shifted absorption based on free molecules (monomeric state) absorbance spectrum (Klymchenko 2013; Kasha et al. 1965; Wuthner et al. 2011). When the dyed fabric was exposed to DMMP vapor, the molecules adsorbed to the dye molecules aggregated inside substrate, and then the dye molecules become to the more monomeric state. It caused the hyperchromic shift of the wearable gas sensors under DMMP exposure (Fig. 4 (c)). From this, the dye formed J-aggregates in the fabrics, and DMMP vapor decomposed the aggregates and finally they changed to monomeric states. Therefore, the sensing mechanism from the dye point of view was attributed to the solvatochromism and the deformation of dye aggregates.
Sensing mechanism: Adsorption amounts on cellulosic fabrics.
To induce the solvatochromism and the deformation of dye aggregates, it is essential for the textile substrates to adsorb the stimulant molecules. Therefore, the adsorption amounts of stimuli on cotton substrates and sensing performance may be closely related. The correlation between the amounts of solvents adsorbed on cotton fabrics and color difference values before and after exposure to solvents in vapor state were in Fig. 11. The adsorption amounts to the substrates were linearly proportional to the color differences. It means that the more the amounts of solvents adsorbed to the substrates, the stronger is the color change.
The relationship between the rate of adsorption of DMMP on cotton fabrics and color change under 300 ppm was depicted in Fig. 12. The behavior and speed of adsorption and color change were very similar, therefore, the adsorption amount DMMP and the color difference of the wearable chemical gas sensor were related closely.
To sum up, the sensing performance of the cotton-based VOC sensors in this study is affected by the three factors together: solvatochromism, aggregative properties of the dyes, and adsorption amount of DMMP on to substrates. The major factor of color change of the sensors is thought to be the solvatochromism first rather than the change in aggregative characteristics and the adsorption amounts of solvents.
The solvatochromism comes into view when dye molecules are surrounded by solvent molecules. Before exposure to DMMP, the dye molecules are surrounded by cotton-containing water, and when exposed to DMMP, the solvent surrounding the dye molecules change from pure water to a mixed solvent of water and DMMP, changing the HOMO and LUMO energy gap of the dye molecule and inducing a color change. The aggregative properties changed upon exposure to DMMP. The intermolecular distance of the dye was increased as the surrounding solvent (DMMP) molecules increased. The adsorption amount of DMMP on to the substrates was also significant. This is because that the dye and DMMP molecules must establish contact for the above two phenomena to occur (Fig. 13).
Reversibility
One of the advantages of the fabricated wearable chemical gas sensor is that it can be reused continuously. The color change property of the sensor can be maintained after repeated exposure to vapor phase of DMMP and ventilation cycles. It indicates the color changed upon exposure to DMMP and returned to original colors by desorption. It was because that the solvatochromism disappeared and the aggregates of the dye molecules were recovered by desorption of DMMP molecules. As displayed in Fig. 14, the sensitivity was maintained at almost the same level as the initial sensing performance during 10 cycles.
Aftertreatment
Water-soluble dyes with high molecular weight have many advantages on the cellulosic fabrics, however, they have poor washing fastness. Therefore, the wearable gas sensor was employed aftertreatment with a commercial polycationic fixing agent to improve the fastness. The durability of the dyed samples against washing, rubbing, and light was evaluated by the procedure of textile standard test methods and summarized in Table 1. The fastness ratings were improved overall. Even after treatment, the color difference was maintained at 22.2 over 80% when exposed to 300 ppm of DMMP for 200 min.
Table 1
Color fastness of untreated and treated wearable chemical gas sensor with a polycationic fixing agent
Color fastness | Untreated | Treated |
Washing | Change in color | 3–4 | 4 |
Staining | Acetate | 3–4 | 4 |
Cotton | 1 | 2–3 |
Nylon | 2–3 | 3–4 |
PET | 4 | 4 |
Acrylic | 4 | 4 |
Wool | 3–4 | 4 |
Rubbing | Staining | Dry | 4 | 4 |
Wet | 1–2 | 3–4 |
Light | Change in color | 3–4 | 4–5 |