Characterization of MXene, MXene-based aqueous black hair dye, and MXene-coated Hair
Transition metal carbide MXene, Ti3C2Tx (Tx = -O, -OH, -F, etc.), is highly desirable in bioengineering owing to its outstanding electrical and thermal conductivity and biocompatibility13,17,18. Scanning electron microscope (SEM) images reveal the two-dimensional planar and layered structures of MXene flakes (Fig. 1A and Supplementary Fig. 1A). Furthermore, the distinctive peaks of MXene and energy-dispersive spectrometer (EDS) analysis show the distribution of carbon (C), titanium (Ti), chlorine (Cl), fluorine (F), and oxygen (O) elements (Fig. 1B). The XRD pattern of Ti3C2Tx MXene exclusively comprises characteristic (002) peaks and rules out the presence of residual MAX phase or other impurities in the as-synthesized MXene (Fig. 1C). The Raman spectrum was measured under 785 nm laser excitation (Supplementary Fig. 1B). Raman modes observed at ~ 118 cm− 1 (Eg(Ti, C, O)) and ~ 196 cm− 1 (A1g(T, C, O)) can be attributed to the in-plane and out-of-plane vibrations of the entire MXene atomic structure (Ti, C, and surface terminations), respectively. Furthermore, Raman modes at ~ 289 cm− 1 (Eg(Tx = OH)) and ~ 368 cm− 1 (Eg(Tx = O)) are exclusively associated with the surface functional groups of MXene. Meanwhile, Raman modes at ~ 606 cm− 1 (Eg(C)) and ~ 718 cm− 1 (A1g(C)) were assigned to the in-plane and out-of-plane vibrations of carbon atoms within the MXene lattice, respectively.
The MXene-based dye used in this study comprised various concentrations, incorporating additives such as L-ascorbic acid and chitosan in a 1:1 ratio. L-ascorbic acid (vitamin C) nourishes hair and can be dissolved in solution. The chitosan exhibits excellent biocompatibility and enhances hair surface adhesion, ensuring a uniform coating. MXene dye was obtained in a consistently dispersed state through sonication for 30 minutes (Fig. 1D). Transmission electron microscopy (TEM) images reveal the smooth and transparent surfaces of the MXene nanosheets. The corresponding selected area electron diffraction (SAED) patterns confirm the hexagonal symmetry of these nanosheets (Fig. 1E and Supplementary Fig. 1C). The MXene nanosheets to exhibit weak acidic characteristics (pH 4.0–5.0), causing the hair cuticle layer to shrink and resulting in visibly tighter and firmer (Fig. 1F). Figure 1G shows schematic diagrams of blonde and MXene-coated hair. The MXene surface functional groups (F, O, OH-) contained in the MXene dye facilitated the coating of the keratin hair surface with an amine (NH2) and a carboxyl group (COOH-). Consequently, hair coated with MXene dye exhibited a black shade depending on the MXene concentration (Fig. 1H). The brightness was quantified by calculating the average RGB value of the uncoated (0 wt%) and hair coated with various concentration of MXene dye (0.1 wt%, 0.5 wt%, 1.0wt%), resulting in average RGB values of 175.89% and 41.2%, respectively, at a concentration of 1.0 wt%. The brightness of MXene-coated hair group decreased by 77.0% compared with that of the 0 wt% group at this concentration (Fig. 1I). SEM and microscopic images of hair samples show that the MXene dye was uniformly coated on the hair surface without aggregation (Fig. 1J and Supplementary Fig. 1D, E). Furthermore, EDS mapping analysis of the MXene-coated hair (1.0 wt%) indicates an even distribution of MXene (Supplementary Fig. 1F). The thickness of the hair coating layer after applying MXene dye coating was approximately 2.9 µm (Fig. 1K) compared to the control group. Moreover, no significant difference (Yield strength: 191.22 MPa for the control and 212.77 MPa for the coated group) in tensile strength was observed between 0 wt% and the MXene-coated hair group (Fig. 1L and Supplementary Fig. 1G).
The multifunctional performance of MXene-coated hair
Hair is highly susceptible to static electricity owing to its large friction area, particularly on dry days, which leads to discomfort and a flyaway effect on hair19,20. MXene-based coatings, with exceptional electrical conductivity, effectively and immediately dissipate static electricity in hair. We induced static electricity in hair samples to assess their antistatic properties by rubbing them with a rubber balloon ten times (Fig. 2A). The surface potential of each hair sample was measured using a static meter. Hair not coated with MXene dye exhibited an average surface potential of 6.6 kV against the induced static electricity from friction.
In contrast, hair coated with MXene dye exhibited surface potentials of approximately 3.96, 1.13, and 0.7 kV, respectively. The surface potential decreased as the MXene concentration increased (Fig. 2B). Furthermore, it exhibited antistatic properties even on hair coated with low concentrations of the MXene dye (Supplementary Figs. 2A, B). The high electrical conductivity of MXenes imparts antistatic properties to hair.
Hair plays a pivotal role in the complex biological system regulating body temperature21. Improving the thermal conductivity of hair through MXene coating enhances the rapid heat dissipation from the head, thereby increasing comfort. We conducted heating and cooling tests at various rates to compare the thermal dispersion properties of hair coated with 1.0 wt% and 0 wt% groups. Initially, we brought hair samples into contact with a hot plate set to the human body temperature of 36.5 ℃. Images were captured using an infrared (IR) camera during heating (Fig. 2C). Hair coated with MXene dye exhibited a temperature increase that was 2 ℃ faster than the control group within 5 s of heating.
Next, we preheated hair samples to 36.5 ℃ and removed them from the hot plate to measure the cooling rate (Fig. 2D). The IR camera captured images revealing that hair coated with MXene dye dissipated heat 2 ℃ faster than the control group within 5 s (Fig. 2E). This temperature change is sufficiently significant to be perceived by the skin, and the rapid heat dispersion prevents local heat accumulation on the skin, thereby increasing comfort and decreasing hair damage. Moreover, in the MXene dye-coated hair (1.0 wt%), the electromagnetic interference (EMI) shielding was 2.5 times higher than that of uncoated hair (0 wt%) (0 wt%: 1.82 dB and 1.0 wt%: 5.1 dB) because of the excellent electrical conductivity of MXene (Fig. 2F and Supplementary Table 1).
The durability of MXene-coated hair dye
The abundant –O, –F, and –OH groups on the MXene surface interacted with the –NH2 and –OH groups of chitosan via hydrogen bonding22,23. MXene hair dye incorporates chitosan, which possesses high surface adhesion and adheres to the hair surface during the coating process, making it resistant to washing. Additionally, the amino group within the chitosan molecule becomes a cation under acidic conditions, whereas the hair surface becomes an anion under the same conditions. This results in ion interactions that lead to the adsorption of chitosan onto the hair surfaces. Cross-sectional SEM images and EDS mapping analysis of hair show that dyeing with MXene nanosheets did not damage the cuticle layer, did not penetrate the hair, and homogeneously coated the hair surface (Fig. 3A and Supplementary Fig. 3A). The MXene dye-coated hair samples were cleaned using shampoo to evaluate the sustainability of the MXene dye coating. The coated hair retained its color even after 30 washes (Fig. 3B). The brightness of hair was quantified by measuring the average RGB values before and after washing (Fig. 3C). Additionally, accelerated life test (85 ℃/85% test) results demonstrate that the color of coated hair was consistently maintained for 5 d (Supplementary Fig. 3B). These results indicate that the MXene dye effectively covered the hair surface and retained its color for an extended period.
Owing to its biological characteristics, such as its ability to promote cell proliferation and antimicrobial effects, MXenes have garnered considerable interest in biology24,25. However, several studies have reported the potential cytotoxicity of MXene26. To confirm the biocompatibility of hair coated with MXene dye, we evaluated the cell viability of HaCaT (human, adult, low calcium, high temperature) cells using a live/dead kit (Fig. 3D). The survival rate of HaCaT cells remained above 98% until D7 (Fig. 3E).
Comparison of MXene dye and commercial products
Traditional hair dyes incorporate chemical ingredients to permeate the hair and alter its pigmentation, which may lead to various health problems, including allergic reactions, skin irritation, and an increased risk of certain types of cancer27,28. To determine whether MXene-based dyes without chemical additives can replace dyes containing various chemical components for hair dyeing, blonde hair samples were dyed with MXene dye (1.0 wt%), commercial dye product (C1), and colored shampoo (C2) (Fig. 4A). Hair coated with MXene dye (1.0 wt%) and commercial dye (C1) exhibited brightness values of 17.3% and 10.6%, respectively. The brightness difference between the colored shampoo (C2) and the MXene dye (1.0 wt%) was 49.6% (Fig. 4B). To compare the multifunctional characteristics of MXene dyes with those of various commercial dye products, their antistatic properties were confirmed by inducing static electricity on hair samples coated with different dyes (Fig. 4C). The surface potential of the electrostatically induced hair was measured. The surface potential of the hair covered with the MXene dye was 0.73 kV, whereas that of the commercial products C1 and C2 was 4.8 and 5.9 kV, respectively. Hair coated with the MXene dye (1.0 wt%) exhibited the lowest surface potential (Fig. 4D). To compare the thermal properties of hair coated with MXene dye, the heat diseprsion characteristics of hair coated with commercial dye and colored shampoo were examined (Fig. 4E). The difference in temperature change between the MXene dye (1.0 wt%) and commercial dye (C1) was 2°C, indicating a higher rate of temperature change in hair coated with MXene dye (Fig. 4F). In addition, hair with MXene dye had approximately six times higher EMI shielding performance (Coated: 5.1 dB, C1: 0.8 dB) than the commercial dye product C1 (Fig. 4G).
Collagen-modified MXene dye (MXene@Col) characteristics
Type 1 Collagen is the most abundant protein in the human body and exhibits excellent biocompatibility and bioactivity29. Collagen can also improve the radiance of nails, hair, and skin. In particular, collagen structurally resembles keratin, a hair component that strengthens hair texture. To modify the MXene surface with collagen, the positively charged collagen was ionically bonded to the negatively charged MXene surface in a slightly acidic environment30. Figure 5A shows a schematic diagram of the MXene surface and collagen interactions. Successful modification of the MXene surface with collagen was demonstrated by X-ray photoelectron spectroscopy (XPS) measurements (Fig. 5B). The survey scan spectra of the MXene and MXene@Col dyes exhibited Ti 2p, C 1s, O 1s, F 1s, and Cl 2p peaks, which are typically observed for MXene (Supplementary Fig. 4A, B). However, the MXene@Col dye exclusively exhibited an N 1s peak originating from collagen. Furthermore, the chemical bonds of collagen resulted in significant changes in the high-resolution C 1s spectra of the MXene@Col dye compared to those of the MXene dye (1.0 wt%). The C 1s spectrum of the MXene dye can be deconvoluted into five peaks corresponding to C–Ti, C–O–Ti, C–C, C–O, and O–C–O bonds. In addition to the enhanced C–O peak, the C 1s spectrum of the MXene@Col dye contains new peaks corresponding to the C–N and N–C = O bonds of collagen (Fig. 5C). Moreover, to confirm the surface modification of collagen, the zeta potential of each group (0 wt%, 1.0 wt%, MXene@Col) was measured (Fig. 5D and Supplementary Table 2). MXenes exhibit a negative charge because of their abundant –OH groups. Consequently, the hair surface coated with MXene dye exhibited a negative zeta potential of -21.97 mV. Meanwhile, collagen has a positive charge; therefore, MXene@Col comprises MXene surface-modified with collagen. The zeta potential of hair coated with MXene@Col dye was − 7.81 mV, higher than that of hair coated with MXene dye. As previously described, an evenly dispersed MXene@Col dye was produced using MXene modified with collagen (Supplementary Fig. 5A). The TEM image of the fabricated MXene@Col dye reveals the clean surface of the MXene@Col nanosheet. The SAED pattern shows the hexagonal symmetry of the nanosheet (Supplementary Fig. 5B). Hair samples were coated with the dye prepared using collagen surface-modified MXene (Fig. 5E). The brightness of hair coated with MXene@Col dye was nearly identical to that of hair coated with MXene dye (1.0 wt%), with a difference of approximately 4.8% in darkness (Fig. 5F). Additionally, the color properties of 0 wt% and 1.0 wt% or MXene@Col groups were examined based on changes in the Commission Internationale de l’Elcairage (CIE) L*a*b* values. The 0 wt% group was blonde, with L*, a*, and b* values of 76.45, 3.02, and 25.36, respectively. However, after MXene dye (1.0 wt%) and MXene@Col dye coating, the hair color changed to black (Supplementary Fig. 5C and Supplementary Table 3). The SEM images and EDS mapping analysis show that the hair surface was smoothly coated with the MXene@Col dye (Fig. 5G and Supplementary Fig. 5D). To determine whether collagen affects hair roughness, we measured the surface roughness of hair coated with the MXene@Col dye using a 3D Optical Profiler. Hair coated with MXene@Col dye exhibited lower surface roughness than hair coated with MXene dye with no surface modification (Figs. 5H and 5I).
Biological properties of MXene@Col dye
To confirm the biocompatibility of MXene@Col dye, Live/dead assay on keratinocytes was performed (Fig. 6A). Cell viability showed a high survival rate of over 99.12% up to 7 days (Fig. 6B). Additionally, we confirmed the expression of KRT5 and KRT14 by immunofluorescence staining analysis (Fig. 6C). As a result, overexpression of KRT5 and KRT14 was observed in the group coated with MXene dye and MXene@Col dye (Fig. 6D). We further examined KRT5 and KRT 14 gene expression levels in cultured HaCaT in three groups (0 wt%, 1.0 wt%, MXene@Col). The expression of KRT4 was upregulated by 3.02-fold and 5.21-fold in the 1.0 wt% and MXene@Col groups, respectively, compared to 0 wt%. The expression of KRT14 was also upregulated the most in the MXene@Col group, 5.81-fold compared to 0 wt% (Fig. 5E). To confirm whether the MXene@Col dye coating can block ultraviolet (UV), we evenly applied each group (0 wt%, 1.0 wt%, MXene@Col) to BALB/c-nu mice and irradiated UV-A with a wavelength of 395 nm to penetrate the skin. The extent of skin damage was confirmed (Fig. 6F). UV-induced skin damage was assessed through histological analysis (Fig. 6G). In hematoxylin-eosin staining, except for the 0 wt% group, almost no damage to the stratum corneum, which exists in the outermost layer of the epidermis layer, was observed. Still, in the UV-treated 0 wt% group, the stratum corneum thickness was significantly thinner (9.28 µm) (Fig. 6H). Additionally, TUNEL immunofluorescence staining results were used to confirm cell damage caused by UV. The cell apoptosis rate by UV treatment showed the highest rate of apoptosis (7.85%) in the 0 wt% group (Fig. 6I).