Thermochromic textile sensors for temperature measurements

Thermochromic textile sensors for temperature measurements Malwina Jaszczak-Kuligowska a,* , El˙ zbieta Sąsiadek-Andrzejczak a , Marta Safandowska b , Marek Kozicki a , Laura Florentino Madiedo c,d , Marcin Barburski e , David Ranz f , Reyes Mallada c,d a Department of Mechanical Engineering, Informatics and Chemistry of Polymer Materials, Faculty of Materials Technologies and Textile Design, Lodz University of Technology, ˙ Zeromskiego 116, 90-543 Lodz, Poland b Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland c Instituto de Nanociencia y Materiales de Arag´ on (INMA), CSIC-Universidad de Zaragoza, Zaragoza 50009, Spain d Chemical and Environmental Engineering Department, Univerisdad de Zaragoza 50018, Spain e Institute of Architecture of Textiles, Faculty of Materials Technologies and Textile Design, Lodz University of Technology, ˙ Zeromskiego 116, 90-543 Lodz, Poland f Department of Design and Manufacture Engineering, University of Zaragoza, María Luna 3, 50018 Zaragoza, Spain ARTICLE INFO Keywords: Thermochromic sensor Thermochromic pigment Temperature changes indicator Textile sensor Wool textile Screen printing ABSTRACT This study presents woollen textiles printed with thermochromic pigments by the screen-printing method for use as new sensors for temperature measurements. The sensors demonstrate reversible colour changes due to temperature variations, which were measured using reflectance spectrophotometry. The beginning of thermal activation of the pigment is registered below 23 ◦C, the main action takes place in the range of 30–45 ◦C, and its complete discolouration occurs at 50 ◦C. The uniformity of the print and pigment distribution on the fabric surface was confirmed using scanning electron microscopy (SEM). Furthermore, a comprehensive chemical analysis of the commercially available thermochromic pigment was performed using elemental analysis, energydispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) techniques. Based on this research, it was determined that the pigment has a spirobenzoxadiazine core structure with amide and imine functional groups. The paper also discusses potential applications of the developed sensors, highlighting their promise as temperature sensors, as well as security, marking, and decorative elements, usable independently or as a part of a composite. Additionally, their potential for two-dimensional temperature distribution measurements was indicated. 1. Introduction Thermochromic materials exhibit a colour change in response to temperature variations. This alteration may involve a transition between two distinct colours, from a colourless to a coloured form, or from a coloured to a colourless form. The colour change may be reversible or irreversible and can occur gradually or violently at a specific temperature threshold [1–4]. In thermochromic materials, thermochromic pigments or dyes are responsible for the colour change. The terms dye and pigment are often used interchangeably, but they differ in many features, mainly in (i) their chemical composition, (ii) behaviour in medium, (iii) the mechanism of colour change, (iv) stability, and (v) their possible applications. These differences influence the choice between using thermochromic pigments or dyes depending on the desired properties and applications. Thermochromic pigments are usually organic or inorganic compounds from the groups of inorganic salts, inorganic oxides or spiro compounds. They are insoluble in the medium in which they are used, and they are dispersed in it as particles. They do not undergo chemical reactions with the surrounding medium or substrate. The mechanism of their colour change involves selective absorption and/or scattering of light. Their colour properties depend on their chemical structure and physical characteristics of their particles, and their colour change may be reversible or irreversible. Furthermore, thermochromic pigments * Corresponding author. E-mail addresses: [email protected] (M. Jaszczak-Kuligowska), [email protected] (E. Sąsiadek-Andrzejczak), marta.safandowska@cbmm. lodz.pl (M. Safandowska), [email protected] (M. Kozicki), [email protected] (L.F. Madiedo), [email protected] (M. Barburski), dranz@ unizar.es (D. Ranz), [email protected] (R. Mallada). Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement https://doi.org/10.1016/j.measurement.2025.118698 Received 14 May 2025; Received in revised form 10 July 2025; Accepted 11 August 2025 Measurement 257 (2026) 118698 Available online 13 August 2025 0263-2241/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). generally exhibit high stability and resistance to factors such as light, heat, and chemical reagents, making them suitable for long-term use. For their application, pigments require attachment to the substrate by additional compounds, e.g., by the polymer in the paint in which they are dispersed. On the other hand, thermochromic dyes are predominantly organic compounds featuring a chromophore group (mainly leuco dyes). Unlike pigments, thermochromic dyes are soluble in the medium in which they are used and thus disperse at a molecular level. A key distinction is their capacity to undergo chemical reactions with the surrounding medium or substrate. Their colour change mechanism primarily relies on the selective absorption of light, and their colour properties are mainly determined by their chemical structure. The colour changes observed with dyes are typically irreversible. Regarding stability, thermochromic dyes are generally less permanent and light stable than pigments, and their exposure to UV radiation commonly leads to fading and degradation. For effective application, dyes must possess a specific affinity for the substrate onto which they are applied [2,5–10]. Thermochromic materials have been known since the 1970s. Their development significantly accelerated in the 21st century, with initial applications primarily focused on decorative elements and indicators that inform about temperature changes in textiles, packaging, and everyday products. Subsequently, thermochromic materials were developed for more advanced technological applications, such as anticounterfeiting technology, drug delivery systems, the pharmaceutical industry, safety equipment, food and agriculture, military technology, energy-efficient construction, and aerospace [1,4,11–13]. For these applications, various types of temperature-sensitive sensors are used. To produce thermochromic materials, thermochromic dyes and pigments are often added to polymer solutions, which can then be used to produce thermochromic coatings [14–17], microcapsules [14,18,19], or fibres [14,20]. They are also incorporated into inks, paints, and printing pastes, which can be used to cover plastics, paper, or textile surfaces [14,21–24]. The existing literature does not fully exhaust the topic of developing markers and sensors for measuring temperature changes, hence, the idea of developing a new thermochromic sensor appeared. A part of this study was additionally dedicated to the chemical analysis of the thermochromic pigment used, which is an aspect often neglected in other published works. Commercially available thermochromic pigments typically lack detailed chemical composition information in both distributor datasheets and literature. This study aimed to develop a thermochromic sensor on a textile substrate. For this purpose, a wool fabric was printed with a printing paste containing a thermochromic pigment using the screen-printing method. The selection of textile as the substrate ensures the flexibility of the sensor and the possibility of its use in numerous applications. To combine ecological aspects in line with the concept of sustainable development with innovative solutions, wool was chosen as the textile substrate due to its environmentally friendly and biodegradable nature. A reversible pigment was used as the thermochromic colourant, which Fig. 1. EDX spectrum of the thermochromic pigment (above) with SEM images showing where the EDX analyses were performed (below). Table 1 EDX analysis weight % results of thermochromic pigment. C N O Spectrum 1 (wt. %) 61.41 23.64 14.95 Spectrum 2 (wt. %) 61.54 23.99 14.47 Spectrum 3 (wt. %) 64.31 18.58 17.11 Spectrum 4 (wt. %) 64.25 19.56 16.19 Spectrum 5 (wt. %) 62.01 19.66 18.33 Mean (wt. %) 63 21 16 Std. Dev. 1 3 2 Atomic ratio 11 3 2 Atomic % 68 19 13 M. Jaszczak-Kuligowska et al. Measurement 257 (2026) 118698 2 ensures high stability and resistance to light and chemical reagents, as well as enables multiple uses of the sensor. The screen-printing method was used for the production of the sensors, which is a simple, economical, and environmentally friendly approach due to the minimal consumption of printing paste. This research introduces a novel and original approach by creating a new temperature-sensitive sensor that is simultaneously (i) flexible, (ii) reversible, (iii) reusable, (iv) easy to produce, use, and read, (v) developed on an ecological, natural, and biodegradable textile substrate in the idea of sustainable development, (vi) manufactured by printing on fabric with only one active side to prevent potential skin allergies upon contact, and (vii) contains a thermochromic pigment that has been chemically characterised within this study. As part of this work, a comprehensive characterisation of the sensor was performed, including (i) its temperature response and stability measurements using reflectance spectrophotometry, (ii) evaluation of print uniformity using RGB channel and scanning electron microscopy (SEM) analysis, (iii) chemical composition characterisation of the commercially available thermochromic pigment using the elemental analysis, as well as energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), and Fourier transform infrared spectroscopy (FTIR) techniques, and (iv) thermal analysis of the pigment using differential scanning calorimetry (DSC). Furthermore, the sensors were incorporated into composites, and their effect on temperature changes and the possibility of their use in such structures were assessed. Additionally, potential applications of the developed thermochromic sensors were presented, including their use as decorative elements, protective elements for textile products, temperature sensors on everyday accessories, sensors measuring temperature distribution in 2D, as well as parts of the composites for various applications, for example, drone casings. 2. Materials and methods 2.1. Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopic (EDX) analysis The morphology of the samples was analysed using a field-emission scanning electron microscope (SEM, FEI Inspect F50; Thermo Fisher Scientific, formerly FEI, Waltham, MA, USA) with a typical acceleration voltage of 10 kV. The samples were deposited on carbon conductive tape and metallised with palladium. The particle size was measured from these images using ImageJ software (National Institutes of Health and the Laboratory for Optical and Computational Instrumentation, LOCI, University of Wisconsin). The compositional analysis was performed using Energy-Dispersive X-ray Spectroscopy (EDX) detector, at an accelerating voltage of 20 kV, and a working distance of approximately 10 mm. For the EDX analysis, the samples were coated with a carbon layer to ensure optimal signal detection. Fig. 2. XPS spectra of thermochromic pigments: broad scan (A), deconvoluted carbon 1s spectra (B), deconvoluted nitrogen 1s spectra (C), deconvoluted oxygen 1s spectrum (D). Fig. 3. Fourier transform infrared (FTIR) spectra of thermochromic pigment. M. Jaszczak-Kuligowska et al. Measurement 257 (2026) 118698 3 2.2. Elemental analysis The elemental analysis of the thermochromic pigment was performed with a CHNS analyser model 3018 from EuroVector S.p.A. (Italy). A sample, either analysed or standard, with a weight ranging from 1 to 5 mg and measured with an accuracy of 0.001 mg, was sealed in a tin capsule and loaded into the elemental analyser’s autosampler. The sample was then burnt in an automated process, and the resulting gaseous products were analysed chromatographically. 2.3. X-ray photoelectron spectroscopy (XPS) analysis The surface atomic composition and chemical states of the thermochromic pigment were measured by X-ray photoelectron spectroscopy (XPS), using a KRATOS AXIS ULTRA spectrometer equipped with a DLD analyser (Kratos-Shimadzu, Kyoto, Japan). The X-ray radiation source was a monochromatic Al K⍺ (1486.74 eV) with a 120 W X-ray power and an anode voltage of 15.00 kV. The photoexcited electrons were analysed Fig. 4. 13 C CP MAS NMR spectrum of thermochromic pigment. Fig. 5. XRD patterns of photochromic pigment, without baseline correction, scanned at 1.7◦/min, from 2θ 3 to 50◦, no peaks were detected from 45 to 50◦. Fig. 6. Differential scanning calorimetry (DSC) thermogram of thermochromic pigment. Table 2 The colour analysis of the wool sample before and after printing with the paste containing the blue thermochromic pigment. The photographs were taken with an iPhone 13 Pro Max (12 MP sensor, 1.9 µm pixels, 26 mm equivalent f/1.5aperture lens, sensor-shift OIS, Dual Pixel AF, Apple, Cupertino, CA, USA) at 23 ◦C in standard D65 light. The measurement error of Lab values is 0.1 %. Wool sample before printing Wool sample after printing with the paste containing the blue thermochromic pigment CIELab analysis L* a* b* L* a* b* 87.73 −0.10 7.53 57.84 2.92 −47.62 M. Jaszczak-Kuligowska et al. Measurement 257 (2026) 118698 4 in constant pass energy mode, using a pass energy of 160 eV for the survey spectra and 20 eV for the high-resolution core level spectra. CasaXPS software was used for data processing. 2.4. Fourier transform infrared spectroscopy (FTIR) analysis FTIR spectra of the thermochromic pigment were collected at room temperature on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with a deuterated triglycine sulphate (DGTS) detector. The technique of attenuated total refraction (ATR) was used for measurements. The spectra were obtained by adding 128 scans at a resolution of 2 cm −1 . Table 3 The colour changes of the wool sample with blue thermochromic pigment at various temperatures (15, 23, 30, and 50 ◦C). The photographs were taken with an iPhone 13 Pro Max (12 MP sensor, 1.9 µm pixels, 26 mm equivalent f/1.5-aperture lens, sensor-shift OIS, Dual Pixel AF, Apple, Cupertino, CA, USA) in standard D65 light. The measurement error of Lab values is 0.1 %. 15 ◦C 23 ◦C 30 ◦C 50 ◦C CIELab analysis L* a* b* L* a* b* L* a* b* L* a* b* 50.18 11.35 −55.98 57.84 2.92 −47.62 88.18 −4.71 −3.26 91.68 −1.83 2.69 Fig. 7. The reflectance spectra of wool fabric printed with a paste containing thermochromic pigment measured at different temperatures in the range of 15–50 ◦C (A) with the corresponding temperature responses (B, C) and a fitted curve for the linear temperature range (C). Measurements were performed immediately after the sample reached the tested temperature. The results are the average of three measurements. M. Jaszczak-Kuligowska et al. Measurement 257 (2026) 118698 5 2.5. Nuclear magnetic resonance spectroscopy (NMR) analysis The solid-state 13 C CP MAS NMR spectra were recorded on a Bruker Avance III 400 spectrometer (Bruker BioSpin, Germany) operating at resonance frequencies of 100.627 MHz. In CP MAS experiments, powder samples were packed into a 4 mm ZrO 2 rotor and spun at a spinning rate of 8 kHz. 2.6. X-ray diffraction (XRD) analysis The crystal structure of the thermochromic pigment was characterised by X-ray diffraction (XRD) on a PANalytical Empyrean diffractometer with Cu K⍺ radiation (Malvern Panalytical, Malvern, United Kingdom). A quick wide 2θ range scan was performed (5-110◦, 9.9 ◦C/min) as well as a slower scan of the area of interest (3-50◦, 1.7 ◦C/ min) to ensure that the structure did not change during the slow scan. 2.7. Differential scanning calorimetry (DSC) Thermal analysis of the thermochromic pigment was conducted by using a DSC TA Q20 analyser (TA Instruments, New Castle, DE, USA). The instrument was calibrated with temperature and heat flow using indium as a standard. The samples (5–6 mg) were weighed accurately into hermetic aluminium pans and pressed slightly to ensure good contact with the DSC cell surface. The data were recorded during heating at a constant rate of 10 ◦C/min under nitrogen flow with an empty pan as the reference probe. 2.8. Preparation of printing paste In this study, a bleached wool fabric with a 1/1 plain weave (warp: 270/dm, weft: 260/dm, thickness: 0.38 ±0.01 mm, and a surface mass of 1.45 ±0.02 g per 10 cm 2 , Tomtex S.A., Tomasz´ ow Mazowiecki, Poland) was used. This fabric was covered with a printing paste using the screen-printing method. The printing paste contained 15 % w/w acrylic binder Helizarin Binder ET 95 (BASF, Ludwigshafen, Germany), 5 % w/w acrylate-based thickener Lutexal HIT Plus (BASF, Ludwigshafen, Germany), 10 % w/w blue thermochromic pigment TO-NI30 (Chaos Trade, Warsaw, Poland), and 70 % w/w water. The paste was prepared by mixing the binder with the thickener and water and then adding the pigment. All components were weighed using a laboratory balance with an accuracy of ±0.1 mg (model: AS220.X2 PLUS, RADWAG, Radom, Poland). The paste was left for 24 h before being used for Fig. 8. Reflectance values at a selected wavelength (600 nm) of a wool sample printed with a paste with thermochromic pigment, heated to 50 ◦C, and cooled over time at room temperature (23 ◦C). Sample photo Blue Channel map Green Channel map Red Channel map Wool sample before printing Wool sample after printing Printed wool sample after heating (50°C) Fig. 9. Non-printed wool and wool printed with printing paste containing thermochromic pigment at 23 ◦C and 50 ◦C after RGBreader analysis. Colour scale utilised by an algorithm for the analysis of tinge distribution on the surface of textile samples (0: ideal black; 255: ideal white). M. Jaszczak-Kuligowska et al. Measurement 257 (2026) 118698 6 printing. The composition of the printing paste was selected based on recommendations of the manufacturers of individual ingredients. 2.9. Screen printing The printing paste was applied onto the wool fabric by screen printing using a printing screen (EX 63–063/160 PW screen: 63 mesh/ cm, thread diameter 63 µm, colour of mesh: white, tension: 18 N/cm, NBC, Tokyo, Japan) and a squeegee. The screens were prepared by covering them with photopolymerisation emulsion and drying at 30 ◦C for 24 h. Subsequently, foils with the printed designed patterns were placed on the screens and exposed to light (halogen lamp, Halogenfluter 500 W 930037; Düwi GmbH, Breckerfeld, Germany). The light exposure initiated the polymerisation process of the photosensitive emulsion. In areas where the pattern was applied, the polymerisation process did not occur, and the emulsion remained unpolymerised, allowing it to be easily washed away with water. After drying, the screens were used for printing with the printing paste prepared as described in Section 2.8. The paste was spread across the screens using a medium-soft squeegee. Printed wool textiles were dried in a laboratory dryer (model: FD 23, BINDER, Tuttlingen, Germany) at 100 ◦C for 5 min and then heated at 150 ◦C for 5 min. For printing, 0.21 ±0.01 g of the printing paste was used per 10 cm 2 of the wool fabric surface. The A4-sized wool sheets were printed, and samples for each study were cut out from them. For application tests, wool fabric was printed using screens with designed patterns. The patterns were printed with a printing paste containing thermochromic pigment or standard blue pigment (Light blue, BASF, Ludwigshafen, Germany). 2.10. Reflectance of light measurements The reflectance spectra of the printed wool samples were measured with a Spectraflash light reflectance instrument (Spectraflash 300, DataColor, Rotkreuz, Switzerland). The spectrophotometer was equipped with an illuminant D65, and the measurements were registered with a step of 10 nm and a measurement error of 0.1 % at an angle of 10◦. The device was calibrated before measurements in accordance with the procedure recommended by the manufacturer. The measurements were performed in the wavelength range of 400–700 nm, with UV light automatically cut off by the software (microMATCH v. 3.6; DataColor, Rotkreuz, Switzerland). To determine the colour response of the sample as a function of temperature change, the samples were cooled in a refrigerator and heated in a hydraulic press (Blue Press, Schulze, Ravensburg, Germany). After the sample reached the desired temperature, which was verified with a temperature indicator, it was immediately measured with the spectrophotometer. Based on the spectrophotometric measurements, the colour coordinates were also determined with the CIELab colour system, which describes the perceived colour according to the ISO/CIE 11664–4 standard [25]. Chromatic colours are expressed by the notations L*,a*, b*, where “L*” describes the lightness of the colour ranging from 0 (black) to 100 (white), the “a*” determines the colour components on the green–red axis, and the “b*” determines the colour components on the blue-yellow axis. The accuracy of the measurement of colour coordinates in the CIELab system has been investigated and described in a previous paper [26]. 2.11. Analysis of the surface uniformity of printed textiles The uniformity of the printed textile surface was analysed. For this purpose, the textile samples measuring 3 ×3 cm 2 were scanned at a resolution of 300 dpi with an Epson Perfection V750 Pro scanner (Nagano, Japan; cold cathode fluorescent lamp; optical resolution Main 6400 DPI ×Sub 9600 dpi; 48 bit/colour; reflection mode; colour depth 24-bit RGB). Based on the scans, each sample was depicted using a threecolour RGB scale (red, green, and blue), which was conducted with the aid of a script for reading RGB channels (RGBreader; Python Script with Python Imaging Library; DosLab [27]). After preliminary analysis of the test samples, the green channel was selected to create sample profiles because it exhibited the largest changes in values. 2.12. Stability tests The sensors were exposed to UVA radiation ageing (lamp 8 W, type F8T5 Blacklight (range: 315–400 nm; a peak at 369 nm, Hitachi, Japan) in a UVA cabinet (UVP, Upland, Canada). The specified UVA radiation dose was emitted automatically, as the device has a built-in detector with a control system (for example, the emission time of 0.1 J/cm 2 UVA was 57 s). Additionally, changes in reflectance spectra for samples stored for 4 months after their production were examined using a reflectance spectrophotometer (Section 2.10) and compared with the spectra of samples measured immediately after manufacturing. 2.13. 2D temperature distribution measurements Thermochromic sensors were heated or cooled to a selected temperature. The sample temperature was then verified with a temperature indicator, and a photograph of the sample was immediately taken with an iPhone 13 Pro Max. Subsequently, the bitmaps obtained from the captured images were processed using the polyGeVero ® -CT software package (v.1.2, GeVero Co., Lodz, Poland) and polyGeVero ® software package (v.2.0, GeVero Co., Lodz, Poland) for further analysis [28]. The software performed image filtering, calibration, and conversion of the obtained results into temperature distribution, illustrated by 2D/3D maps. 2.14. Composites preparation Thermochromic sensors were used as composite elements with a bioepoxy resin matrix (SR GreenPoxy 33 epoxy resin and SD4775 hardener in a weight ratio of 100:27; Sicomin, Chˆ ateauneuf-les-Martigues, France) and reinforcement in the form of a fabric made of wool (weave structure: 1/1 plain, surface weight: 400 g/m 2 , Zakłady Przemysłu Wełnianego TOMTEX S.A., Tomasz´ ow Mazowiecki, Poland), flax (weave structure: 2/2 twill, surface weight: 500 g/m 2 , Libeco, Belgium), glass (weave structure: 1/1 plain, surface weight: 300 g/m 2 , SP-TEX Sp. z o.o., Czechowice-Dziedzice, Poland), or carbon (weave structure: 2/1 twill, surface weight: 280 g/m 2 , SP-TEX Sp. z o.o., CzechowiceDziedzice, Poland) fibres. The composites were produced by an infusion method. Fig. 10. The green RGB channel profiles (1 pix =0.1 mm) for non-printed, printed with the paste containing thermochromic pigment, as well as printed and heated to 50 ◦C wool samples (A), together with (B) the images of the RGBreader green channels of samples with red dashed lines to indicate the position of the profiles (from the top: non-printed, printed, printed and heated samples). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) M. Jaszczak-Kuligowska et al. Measurement 257 (2026) 118698 7 3. Results and discussion 3.1. Chemical analysis of thermochromic pigment A significant challenge lies in accurately determining the origin and chemical composition of commercially available thermochromic colourants. This lack of clarity stems from the frequent omission of complete chemical structure information in manufacturers’ product data sheets. Furthermore, literature often incorrectly uses the terms ’dye’ and ’pigment’ interchangeably when referring to thermochromic compounds, despite their significantly different properties [2,5–10]. However, knowledge of the chemical composition and colourant type is crucial for both basic and application-oriented research involving these compounds. In textile dyeing, the type of colourant directly influences the mechanism of reaction and binding with fibre, and the dyeing techniques used. When dyeing textiles with dyes, selecting a dye with appropriate fibre affinity is essential for establishing permanent fibredye chemical bonds. Dyes are typically applied to dye textiles in dye baths or by printing methods. Pigments, on the other hand, are only used in textile dyeing through printing due to their insolubility in water and common solvents. In this case, a dispersion of pigment in a printing paste consisting of water, thickener, and binder is used. This paste, as a result of the polymerisation process, permanently combines the pigment with the textile material. In this work, an attempt was made to assess the type of colourant used (dye or pigment) and identify its chemical composition using solubility assessment, elemental analysis, as well as EDX, XPS, XRD, FTIR-ATR, NMR, and DSC techniques. First, a colourant solubility test was carried out using commonly used solvents such as water, acetone, methanol, ethanol, toluene, pyridine, isopropyl alcohol, and dimethylformamide (DMF). For this purpose, 1 mg of the colourant was mixed with 10 ml of pure solvent using a magnetic stirrer (120 min, 250 rpm). In the case of acetone and DMF, the solution was additionally heated to 60 ◦C. After 7 days from preparation, the solubility of the colourant was assessed organoleptically, and in no A B Fig. 11. SEM images of non-printed wool at 160×magnification (A), wool printed with printing paste with thermochromic pigment at 160×magnification (B) and 2400×magnification (C), pigment particles at 10000×magnification (D), and the size distribution histogram of the thermochromic pigment particles (E). M. Jaszczak-Kuligowska et al. Measurement 257 (2026) 118698 8 case was its dissolution observed. Based on this, it can be concluded that the thermochromic colourant used is a pigment. The data obtained from EDX measurements allowed for the preliminary determination of the elemental composition on the sample surface. Five analyses were performed on the particles shown in Fig. 1. This analysis revealed the presence of carbon (63 ±1 wt%), nitrogen (21 ±3 wt%), and oxygen (16 ±2 wt%) (Table 1). The absence of inorganic elements, above the detection limit of the equipment (i.e., around 0.1–0.5 wt%) evidenced the organic composition of the pigment. Elemental analysis (CHNS), which provides information on precise percentages of mass of individual elements in the entire sample, confirmed the high content of carbon and also indicated the presence of hydrogen and nitrogen in the sample: carbon (C) – 67.81 wt%, hydrogen (H) – 9.69 wt%, nitrogen (N) – 11.12 wt%. Assuming that, in addition to the abovementioned chemical elements, oxygen is also present in the composition of the pigment and its content – determined by subtracting the total percentages of carbon, nitrogen and hydrogen – is 11.38 % wt. %. Based on the information obtained from the EDX and the CHNS elemental analysis, it can be concluded that the thermochromic pigment is an organic compound containing C, H, N, and O. The XPS results confirm the presence of carbon, nitrogen, and oxygen on the sample surface (Fig. 2A). Additionally, a small amount of sodium, 0.12 % atomic, was detected. The analysis of the C 1s XPS spectra indicates the presence of carbon in different hybridisations (sp 2 and sp 3 ), which is typical for organic compounds with aromatic rings and/or aliphatic chains (Fig. 2B). The peak at approximately 288 eV is assigned to carbon in the N-C=O bond (amide functional group) (Fig. 2B) [29]. Fig. 2C shows the N 1s spectra, the peak at 399.7 eV could be assigned also to the N-C=O bond (amide functional group), and the peak at 398.2 eV corresponds to the R=N-R bond, with R being aromatic (imine functional group) [29,30]. This is in agreement with the fact that in EDX the N/O ratio is higher than 1, indicating that a part of the amide group N is also present in another form. The O 1s spectrum indicates again the presence of an N-C=O bond (amide functional group) (Fig. 2D) [29]. Thus, the XPS analysis has confirmed the presence of aromatic carbons, as well as amide and imine functional groups. To determine the structural composition of the compound, the elemental composition, FTIR-ATR and NMR analyses were additionally performed. The FTIR spectrum, depicted in Fig. 3, provides insight into the chemical bonding structure of the thermochromic pigment. The absorption band at 3287 cm −1 corresponds to O–H stretching vibrations, indicating the presence of water in the sample. The peak at 2916 cm −1 is attributed to the asymmetric C–H stretching vibration of methylene (CH 2 ) groups, while the sharp band at approximately 2848 cm −1 is assigned to the symmetric C–H stretching vibration of methylene groups. An absorption peak around 1730 cm −1 in an infrared spectrum is attributed to the stretching vibration of the carbonyl (C=O) functional group [31]. Taking into account the conclusions obtained from the XPS analysis, the peak may correspond to the C=O stretching vibration of the amide functional group. The analysed spectrum also exhibits multiple Fig. 12. Reflectance spectra of wool samples not irradiated and irradiated with a dose of 10 J/cm 2 UVA (A), and samples of wool printed with a printing paste with a thermochromic pigment, not irradiated and irradiated with doses of 1, 3, 5, and 10 J/cm 2 UVA (B). Measurements were carried out at 23 ◦C. Table 4 The colour changes of the thermochromic sensor irradiated fractionally and nonfractionally with 100 J/cm 2 of UVA radiation at room temperature (23 ◦C). The photographs were taken with an iPhone 13 Pro Max (12 MP sensor, 1.9 µm pixels, 26 mm equivalent f/1.5-aperture lens, sensor-shift OIS, Dual Pixel AF, Apple, Cupertino, CA, USA) in standard D65 light. The measurement error of Lab values is 0.1 %. UVA 100 J/cm 2 given non-fractionally UVA 100 J/cm 2 given fractionally CIELab analysis L* a* b* L* a* b* 66.51 −6.55 −6.32 63.11 −4.39 –33.21 Fig. 13. Reflectance spectra of thermochromic sensors 1 day and 4 months after preparation at room temperature (23 ◦C) and heated to 50 ◦C. M. 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