Synthesis and characterization of chromium aluminum carbide MAX phases (CrxAlCx-1) for potential biomedical applications

MAX phases, characterized as nanolaminates of ternary carbides/nitrides structure, possess a unique combination of ceramic and metallic properties, rendering them pivotal in materials research. In this study, chromium aluminum carbide ternary compounds, Cr₂AlC (211), Cr₃AlC₂ (312), and Cr₄AlC₃ (413) were successfully synthesized with high purity using a facile and cost-effective sol-gel method. Structural, morphological, and chemical characterization of the synthesized phases was conducted to understand the effects of composition changes and explore potential applications. Comprehensive characterization techniques including XRD for crystalline structure elucidations, SEM for morphological analysis, EDX for chemical composition, Raman spectroscopy for elucidation of vibrational modes, XPS to analyze elemental composition and surface chemistry, and FTIR spectroscopy to ensure the functional groups analysis, were performed. X-ray diffraction analysis indicated the high purity of the synthesized Cr₂AlC phase as well as other ternary compounds Cr₃AlC₂ and Cr₄AlC₃, suggesting its suitability as a precursor for MXenes production. Additionally, the antimicrobial activity against Candida albicans and biocompatibility assessments against Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and HepG2 cell line were investigated. The results demonstrated significant antifungal activity of the synthesized phases against Candida albicans and negligible impact on the viability of E. coli and S. aureus. Interestingly, lower concentrations of Cr₂AlC MAX phase induced cytotoxicity in HepG2 cells by triggering intercellular oxidative stress, while Cr₃AlC₂ and Cr₄AlC₃ exhibited lower cytotoxicity compared to Cr₂AlC, highlighting their potential in biomedical applications.

GRAPHICAL ABSTRACT

1 Introduction

MAX phases are synthesized by various methods at high temperatures. Different methods also prepared the Cr₂AlC MAX phase. Here, the question is what are the lowest temperature and cost-effective achievable methods for synthesizing MAX Phases? Firstly, the Cr₂GaC MAX phase was synthesized by the sol-gel method at low temperatures (Siebert et al., 2019). Classifications of MAX phases can be delineated according to distinct ‘n’ values, encompassing M₂AX (211), M₃AX₂ (312), and M₄AX₃ (413) phases, respectively (Sun, 2011). The distinctive combination of weakened M-A bonds and robust M-X bonds, coupled with a nano-layered structure, imparts to these solids a unique amalgamation of metallic properties and biological applications. Other MAX phases include 312 and 413 of Ti₂AlC (Rampai and Tokoloho, 2011; Galvin et al., 2018; Gao et al., 2020; Poulou et al., 2021). The synthesis of the Cr₂AlC MAX phase involves various methods such as the molten salt method (Abdelkader, 2016), ball milling (Ta et al., 2021; Mansouri et al., 2023), chemical vapor deposition (CVD) (Gorshkov et al., 2017), Physical vapor deposition (PVD) (Rueß et al., 2021; Li et al., 2024) and, notably, the sol-gel method. In the previous literature, the sol-gel method was utilized to fabricate the Cr₂GaC MAX phase (Siebert et al., 2019). This research investigated two materials, Cr₂AlC MAX phase, and Cr₂CT_x MXene-Cr, for their potential biomedical applications. Both materials showed free radical scavenging activity, with MXene-Cr being more effective. MXene-Cr also inhibited the enzyme alpha-amylase and displayed strong DNA nuclease activity. Furthermore, both materials exhibited significant antimicrobial activity against various bacteria, with better effects on Gram-positive bacteria. Notably, they inhibited microbial growth at low concentrations and MXene-Cr showed promising antibiofilm activity. Additionally, MXene-Cr demonstrated impressive antibiofilm activity against S. aureus (89.86%) and P. aeruginosa (87.01%), while the MAX phase displayed an antibiofilm activity exceeding 90%. These findings suggest promising potential for both materials in various biomedical applications (Kaya et al., 2024).

Originating from the mid-1800s, early investigations into silica gels form the foundational basis for the widely employed sol-gel chemistry, a method extensively applied in the synthesis of inorganic solids (Hench and West, 1990). Within the sol-gel methodology, the initial step entails the creation of a colloidal suspension termed “sol,” which then undergoes a structural transformation into a network referred to as a “gel.”

Various synthesis methods are utilized for Cr₂AlC MAX phase fabrication, including the molten salt method (Liu et al., 2020; Shamsipoor et al., 2021; Liu et al., 2023), ball milling (Mansouri et al., 2023), chemical vapor deposition (CVD) (Lei and Lin, 2022), physical vapor deposition (PVD) (Gonzalez-Julian, 2021), and spark plasma sintering (Duan et al., 2015). Recent research has highlighted the potential of MAX phases as effective high-temperature coatings, with applications spanning turbojets, aircraft, automobiles, and the petrochemical industry. J Liu et al. Successfully produced Cr₂AlC thin films, a type of MAX coating, using magnetron sputtering techniques (Li et al., 2018; Liu et al., 2018). Both elemental and compound targets are employed for the deposition process, with substrate heating or post-annealing necessary to achieve crystallization. However, despite these efforts, challenges persist in achieving the desired stoichiometric ratio and homogeneity due to the presence of binary carbides or intermetallic impurities in the resulting thin films. Manoun et al. synthesized Cr₂AlC compounds by employing a Hot Isostatic Pressing (HIP) method. The synthesis involved mixing Cr, Al, and C, followed by subjecting the mixture to conditions of 1,200°C and approximately 100 MPa pressure (Manoun et al., 2006). Yukhvid et al. employed a self-propagating high-temperature synthesis (SHS) technique to prepare Cr₂AlC. The synthesis involved utilizing mixtures of Cr₂O₃, CrO₃, Al, and C, conducted under an argon gas atmosphere at 5 MPa pressure (Gorshkov et al., 2018). Additionally, the reaction kinetics and mechanical properties of the Cr₂AlC compound was investigated, which was synthesized via the hot pressing (HP) technique at 1,300 °C and 30 MPa (Yan et al., 2019). Despite their merits, these methods typically necessitate elevated temperatures (>1,000°C), high pressures (up to 100 MPa), and sophisticated equipment, thereby limiting their widespread applicability (Lin et al., 2022). Previous studies have predominantly utilized various compounds such as Al₄C₃, AlCr₂, Cr₂O₃, and CrCx (where x = 0.5) as raw materials to mitigate the risk of secondary compound formation during processing (Rajkumar et al., 2017). Generally, synthesizing the Cr₂AlC MAX phase directly from a mixture of elemental C, Al, and Cr powders is challenging due to the likely formation of intermediate phases such as CrxCy and Al-C compounds, as well as the oxidation tendency of Al and Cr (Tian et al., 2008). Consequently, there have been limited reports on the fundamental synthesis of the Cr₂AlC MAX phase from elemental powders without the need for high-pressure or complex tools (Yan et al., 2017; TAN et al., 2019).

The above-mentioned methods are more expensive and are high-temperature and high-pressure based. Based on the aforementioned studies, ensuring phase purity is of paramount importance for MAX phase materials. This study aimed to create a facile method for producing the Cr₂AlC MAX phase, utilizing elemental powders as initial ingredients without applying pressure. In the initial stages, the sol-gel method was employed to prepare the Cr₂AlC MAX phase. Additionally, the ternary compounds Cr₃AlC₂ and Cr₄AlC₃, which exhibit similarities to Cr₂AlC MAX phase, were also synthesized using the sol-gel method by adjusting the concentration ratios of chromium nitrate for the 312 and 413 phases. Sol-gel method is cost cost-effective method and easy to maintain. It is low low-temperature-based technique for synthesizing the Cr₂AlC MAX phase. Other phases Cr₃AlC₂ and Cr₄AlC₃, (312 and 413) are not mentioned in previous studies, but we prepared those phases successfully by using the sol-gel method. The prepared phases are more stable. we called it ternary compounds. Cr₃AlC₂ and Cr₄AlC₃, are probably MAX phases. Other novel phases of Ti₂AlC are mentioned in the literature (Tzenov and Barsoum, 2000; Sarkar et al., 2024). The MAX phases are synthesized by the novel sol-gel route and applied for the potential biomedical applications. This work’s novelty lies in utilizing a cost-effective the sol-gel technique for synthesizing MAX phases with three different ratio. The development of novel MAX phases such as Cr₃AlC₂ and Cr₄AlC₃ was realized using the sol-gel method. The developed materials were successfully confirmed by several analytical techniques with excellent biomedical application. Herein, the facile sol-gel method has been opted for the synthesis of the Cr₂AlC MAX phase and related ternary compounds. In the sol-gel method, nitrate precursors and citric acid are used for the synthesizing of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC_3. Comprehensive characterization through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, confirmed the Cr₂AlC MAX phase and other prepared Cr₃AlC₂ and Cr₄AlC₃ were found to have comparable structural morphology with Cr₂AlC MAX phase with the prepared Cr₂AlC showing promising results across all analyses, and the ternary compounds exhibiting significant characteristics comparable to the Cr₂AlC MAX phase, further validated through a comparative analysis with the 211 MAX phase. The prepared materials were assessed for biomedical applications including Anti-bacterial, Anti-fungal, and Anti cancerous. The anti-bacterial activity was done against E coli and S. aureus,anti-fungal activity against Candida albacians, and anti-cancerous activity against the HepG2 cell line (liver cancer cell). The thorough characterization and biomedical assessments underscore the potential of these materials for various applications, offering insights into their structural, chemical, and functional properties, and positioning them as valuable candidates for future research and technological advancements. The realm of material science has been perennially enriched by the discovery and synthesis of novel materials that offer a blend of desirable properties for a wide range of applications.

2 Materials

The chemicals used in this study are Chromium (III) Nitrate Nonahydrate [Cr(NO₃)₃·9H₂O, 99.9%, Honeywell], Aluminum Nitrate Nonahydrate [Al(NO₃)₃·9H₂O, 99.9%, ChemPUR], and Citric Acid [99.5%, Alfa Aesar]. The deionized water was used throughout the research. All the chemicals used were of analytical grade and used without further purification (Siebert et al., 2019).

2.1 Synthesis of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ MAX phases

In this experimental procedure, For the 2:1 ratio, 16.0 g of Cr(NO₃)₃·9H₂O and 7.5 g of Al(NO₃)₃·9H₂O were used as precursors, along with 8.4 g (9 equivalents) of citric acid. This same amount of Al(NO₃)₃·9H₂O and citric acid was used for the 3:1 and 4:1 ratios, while the amount of Cr(NO₃)₃·9H₂O was increased to 24.0 g and 32.0 g, respectively. The precursor chemicals were dissolved in water using a magnetic stirrer bar in a beaker. The resulting mixture was then heated within a temperature range of 70°C–80 °C, forming a viscous liquid. Once it gelled, the material was moved to an Al₂O₃ crucible for subsequent heat treatment. This annealing process occurred in a horizontal tube furnace (Carbolite) at 900 °C in a Nitrogen atmosphere (N₂,99.99% purity), with 2 °C/min heating rate. The temperature was maintained for 5 hours before allowing the sample to cool gradually to room temperature. Various materials were heated at a temperature of 900 °C for 5 hours to gain a deeper understanding of the reaction dynamics. Following thermal treatment, the specimens underwent a 30-min grinding process. After this initial grinding, further grinding was carried out before subjecting the samples to comprehensive characterization using various analytical techniques (Siebert et al., 2019). The schematic diagram of synthesis is shown in Figure 1.

Figure 1

Figure 1. Schematic Diagram for the synthesis of Chromium Aluminum Carbide MAX Phases (Cr_xAlC_x-1).

2.2 Characterizations

The Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ samples generated in the synthesis were subjected to a series of analytical techniques for comprehensive characterization. A Bruker D2 Phaser X-ray Diffractometer equipped with Cu-Kα radiation (λ = 1.5406 Å). The scanning range extended from 20° to 70° in terms of 2θ. Particle size and morphology were examined through SEM Cube II, Emcraft South Korea with EDX. Vibrational spectra were obtained using the Confocal Micro Raman MNSTEX PRI 100, DONGWOO South Korea system. The diverse stretching and bending vibrations of functional groups within the synthesized materials were identified using Fourier Transform Infrared Spectroscopy (FTIR) with an Agilent Technology Cary 360 FTIR spectrophotometer. Additionally, X-ray Photoelectron Spectroscopy (XPS) analysis, performed with ESCALAB-250 (Thermo Scientific, United Kingdom), delved into the electronic states present in the material. 1mL Dimethyl sulfoxide (DMSO) was used to prepare the 40 mg/mL dose to perform biomedical activity. Strains of E. Coli, B. cereus, and strains of Fungi were assembled from the Department of Microbiology lab in Govt. College University Faisalabad, Pakistan.

2.3 Antifungal activity

The procedure involves creating Candida albicans inoculum in Sabouraud Dextrose Broth, adjusting turbidity, and applying MAX Phase materials to agar plates (Bakht et al., 2011). Plates were incubated, and inhibition zones around the MAX Phase material were measured to assess antifungal effectiveness.

2.4 Anti-bacterial activity

This involves preparing E. coli inoculum in LB broth, followed by turbidity adjustment, and applying MAX Phase materials on agar plates. Followed by incubation, zones of inhibition are observed to assess antibacterial effectiveness the same procedure applied for Cr₃AlC₂ and Cr₄AlC₃.

2.5 Anti-bacterial activity against S. aureus

A standardized procedure was opted by preparing S. aureus inoculum, applying MAX Phase materials on Mueller-Hinton agar plates, and observing inhibition zones after incubation to assess antibacterial effectiveness (Sharmin et al., 2021). The same method was applied for Cr₃AlC₂ and Cr₄AlC₃.

2.6 Anti-cancerous activity

Protocol for assessing cytotoxicity on HepG2 ([HEPG2]-HB-8065-ATCC) cells includes culturing cells, treating with Cr₂AlC MAX phase materials, performing MTT assay, and analyzing data to determine IC50 value (Younas et al., 2021). The systematic approach ensures accurate evaluation of cytotoxic effects.

3 Results and discussion

3.1 Material’s characterization

The analysis using Fourier transform infrared (FTIR) spectroscopy was performed to ascertain the chemical properties of the synthesized material. FTIR analysis was accomplished to illustrate the surface functional groups present in the Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ (Figure 2). The peaks observed between 430 and 820 cm⁻¹ corresponded to the stretching vibration modes of Al-O and Cr-O bonds, indicating the presence of Cr-C and Al-C bonds (Reghunath et al., 2021) for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃. The FTIR characteristics of prepared Cr₃AlC₂ and Cr₄AlC₃ were aligned with the FTIR spectrum of the Cr₂AlC MAX phase. Peaks appeared between 550 and 800 cm^-1 are ascribed to the Al-C bonding. In all the FTIR spectra the presence of peaks can be realized confirming the Al-C stretching vibration mode (Shalini Reghunath et al., 2021).

Figure 2

Figure 2. FTIR Analysis of prepared samples of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃.

To investigate the surface composition and valence states of the Cr₂AlC MAX phase, XPS analysis of its structural elements was conducted. The results are illustrated in Figure 3. The survey scan of the Cr₂AlC Max phase, Cr₃AlC₂, and Cr₄AlC₃ are presented in Figure 3A. The presence of Cr, Al, O, and C elements was confirmed, aligning with findings from prior research (Wei et al., 2015; Soundiraraju et al., 2020). In the high-resolution spectra of the Cr₂AlC Max phase, the Cr spectrum (Figure 4A) exhibits discernible peaks at 575.0 eV and 586.4 eV for Cr 2p (i.e., 2p3/2 and 2p1/2 respectively. These peaks are indicative of the presence of the Cr–C bond characteristic of chromium carbide, consistent with findings reported by Zamulaeva and co-workers (Zamulaeva et al., 2016). Peaks corresponding to Al 2p were detected at 73.9 eV (Figure 3B) (Hauert et al., 1993), attributed to the Al-C bond. The strong reactivity between aluminum and carbon facilitates the formation of a layered Cr₂AlC structure, characterized by alternating layers of chromium and aluminum. In the C1s spectrum (Figure 4B), a peak at 282.4 eV is attributed to the Cr-C bond (Zamulaeva et al., 2013). Therefore, the high-resolution XPS results suggest the formation of a high-purity Cr₂AlC MAX phase (Monireh et al., 2023a). In Cr₃AlC₂, the Cr spectrum (Figure 4C) exhibits discernible peaks at 574.8 eV and 584.6 eV for Cr 2p (i.e., 2p3/2 and 2p1/2 respectively. These peaks are indicative of the presence of the Cr–C bond characteristic of chromium carbide. Peaks corresponding to Al 2p were detected at 73.1 eV (Figure 3B), attributed to the Al-C bond. In the C1s spectrum (Figure 4D), a peak at 282.6 eV is attributed to the Cr-C bond. In Cr₄AlC₃, the Cr spectrum (Figure 4E) exhibits discernible peaks at 574.8 eV and 584.6 eV for Cr 2p (i.e., 2p3/2 and 2p1/2 respectively. These peaks are indicative of the presence of the Cr–C bond characteristic of chromium carbide. Peaks corresponding to Al 2p were detected at 73.0 eV (Figure 3B), attributed to the Al-C bond. In the C1s spectrum (Figure 4F), a peak at 281.1 eV is attributed to the Cr-C bond. All the results of XPS for Cr₃AlC₂ and Cr₄AlC₃ aligned with the Cr₂AlC MAX phase. There are some shifts in peaks, which may be due to material and may be due to changes in composition.

Figure 3

Figure 3. (A) XPS analysis of prepared Cr₂AlC, Cr₃AlC₂, and Cr₄AlC_3, (B) High resolution XPS spectra of Al 2p.

Figure 4

Figure 4. High resolution XPS spectra of (A, B) Cr 2p&C 1s for Cr₂AlC, (C,D) Cr 2p&C 1s for Cr₃AlC₂, and (E,F) Cr 2p&C 1s for Cr₄AlC₃.

The morphologies of prepared Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ were examined by Scanning electron microscope (SEM). The lamellar sheet structure, characteristic of the prepared materials is evident in Figures 5A, C, E. The layers exhibit delamination, with noticeable kink bands present. The uneven structural morphology with varying surface energies reflects versatility in structural composition thereby effective physicochemical characteristics of the materials. All the results of Cr₃AlC₂, and Cr₄AlC₃ agreed with Cr₂AlC. The lamellar sheets were nonuniform in thickness (Sharma and Pandey, 2019; Sharma et al., 2023).

Figure 5

Figure 5. SEM-EDX analysis (A,B) Cr₂AlC, (C,D) Cr₃AlC₂, and (E,F) Cr₄AlC₃.

The elemental analysis of the Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ powder (Figures 5B, D, F) verified the presence of carbon (C), chromium (Cr), aluminum (Al), and oxygen (O), with no detection of additional elements. Atomic percentages within the EDX spectrum are reported, and the observed values closely correspond to the formula of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ (Monireh et al., 2023b). The expected stoichiometric ratios for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ are 2.5:1:1.0, 2.8:1:2.3 and 3.9:1:2.7 for Cr:Al: C respectively. The obtained ratios closely match this, with a slight excess of Chromium (Cr). These ratios indicate a composition of approximately Cr_2.5Al₁C_1.0, Cr_2.8Al₁C_2.3, and Cr_3.9Al₁C_2.7 suggesting a nearly stoichiometric Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ with a slight deviation. This deviation could be due to experimental error, sample inhomogeneity, or actual variations in stoichiometry. The purity of the prepared material in EDX is evident as it only contains Cr, Al, C, and O. There are no other elements present.

The XRD pattern of the Cr₂AlC MAX phase as prepared is displayed in Figure 6A. The distinct sharp peaks observed confirm the formation of the Cr₂AlC MAX phase. Furthermore, the discernible peaks observed at the 2θ = 22.0^°, 24.7^°, 26.5^°, 36.6^°, 42.1^°, 50.9^°, 55.6^° and 65.9^° correspond to the (0 0 2), (2 1 0), (0 0 4), (1 0 0), (1 0 3), (1 0 4), (1 1 6) and (1 1 0) planes of the synthesized sample, respectively. This observation affirms the hexagonal structure of the prepared MAX phase (Crisan and Crisan, 2018; TUNES et al., 2021). For Cr₃AlC₂ and Cr₄AlC₃ (Figures 6B, C, respecctively) all the 2θ values corresponding to the planes were matched with the Cr₂AlC MAX phase. This finding validates the hexagonal structure of the prepared Cr₃AlC₂ and Cr₄AlC₃. Furthermore, the crystallite size of the Cr₂AlC was determined to be 55.4 nm using the Debye–Scherrer equation (Zhang et al., 2019) based on the intense XRD peak observed at 2θ = 42.1 (Guan, 2016). Furthermore, the Crystallite size of the Cr₃AlC₂ and Cr₄AlC₃ were determined to be 23.1 nm and 21.3 nm using the Debye–Scherrer equation, based on the intense XRD peak observed at 2θ = 33.70^°and 33.50^° respectively and the Crystallite size Cr₃AlC₂ and Cr₄AlC₃ were also determined to be 21.8 nm and 23.6 nm using the Debye–Scherrer equation, based on the intense XRD peak observed at 2θ = 36.4^° and 36.39^°respectively. The values of the lattice constant (a and c) for the hexagonal pattern were designed employing Eq. 1. The equation represents the inter-planar spacing (dhkl) for the plane (hkl) and is expressed as:

$\frac{1}{d^2}=\left(\frac{h^2}{a^2}+\frac{hk}{a^2}+\frac{k^2}{a^2}+\frac{l^2}{c^2}\right)(1)$

Figure 6

Figure 6. XRD analysis of prepared samples of (A) Cr₂AlC, (B) Cr₃AlC₂, and (C) Cr₄AlC₃.

The determined unit cell parameters are a = 3.03 Å and c = 12.3 Å for the Cr₂AlC MAX phase (Siebert et al., 2019; Reghunath et al., 2021). The unit cell parameters have been determined as follows: for the Cr₃AlC₂, a = 2.46 Å and c = 10.28 Å, and for the Cr₄AlC₃ plane, a = 2.47 Å and c = 10.31 Å. The X-ray diffraction (XRD) analysis confirmed the hexagonal structure of the synthesized material. This is further supported by the match between the observed peaks and the reference pattern in JCPDS card no. 29–0017 (Shalini Reghunath et al., 2021). The a and c parameters for Cr₃AlC₂, and Cr₄AlC₃ aligned with the Cr₂AlC. Cr₃AlC₂ and Cr₄AlC₃ can be probably called MAX phases.

Raman spectroscopy was performed on the samples across a spectral range from 400 cm⁻¹–2000 cm⁻¹, as illustrated in Figure 7. The observed peaks at 327 cm⁻¹ and 553.8 cm⁻¹,361.4 cm⁻¹ and 553.3 cm⁻¹ correspond to the signature peaks of CAC(Cr₂AlC) and Cr₂O₃, confirming the formation of Cr₂O₃ due to surface oxidation. The obtained results have been confirmed form several relevant publications (Spanier et al., 2005; Shtansky et al., 2009; Bortolozo et al., 2010; Presser et al., 2012; Vishnyakov et al., 2014; Bentzel et al., 2016; Zhang et al., 2021). Additionally, the peak observed at 809.0 cm⁻¹, 806.7 cm⁻¹ and 872.0 cm⁻¹ in the Raman spectra of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ respectively is attributed to the vibrational mode of the carbide (Bortolozo et al., 2007). The Raman spectrum showed the D band at a Raman shift of 1,356.1 cm⁻¹,1359.2 cm⁻¹ and 1,360.4 cm⁻¹ and the G band at a Raman shift of 1,563.3 cm⁻¹,1568.4 cm⁻¹, and 1,565.3 cm⁻¹ for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ respectively. Carbon’s D peaks are observable at 1,356.1 cm⁻¹,1359.2 cm⁻¹ and 1,360.4 cm⁻¹ and G peaks at 1,563.3 cm⁻¹,1568.4 cm⁻¹, and 1,565.3 cm⁻¹ for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ respectively. The G band is referred to as the graphitic band. Its presence indicated the presence of carbon atoms. The D band is also recognized as the disorder-induced band, being linked to structural defects or disorders present in the carbon material. The I_D/I_G intensity ratio for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ 0.86 suggests the presence of disorder and graphitization in the carbon structure (Patel et al., 2023).

Figure 7

Figure 7. Raman spectra for (A,B) Cr₂AlC, (C,D) Cr₃AlC₂, and (E,F) Cr₄AlC₃.

3.2 Biomedical applications of Cr₂AlC MAX phase, Cr₃AlC₂, and Cr₄AlC₃

The Cr₂AlC MAX phase and prepared ternary compounds demonstrate a suppressive effect on the growth of microorganisms. It enhances the production of reactive oxygen species (ROS), which in turn disrupts the integrity of cell walls and membrane structures. This disruption impairs vital cellular functions such as DNA and RNA synthesis and increases the susceptibility of microorganisms to ROS. The approach for evaluating biomedical activity (such as antibacterial, antifungal, and anticancer effects) remains consistent remains consistent and its schematic view is shown in Figure 8.

Figure 8

Figure 8. Schematic view of biomedical activity mechanism (antibacterial, antifungal, and anticancer) Activity.

The antimicrobial activity of the samples synthesized via the sol-gel method was examined. It was found that these particles exhibit potent antimicrobial activity at a concentration of 40 mg/mL. In the context of this research article, we can delve into the biomedical applications of the synthesized Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ particularly focusing on its differential efficacy against Candida albicans and bacteria like E. coli and S. aureus (Chacko et al., 2024). Emphasize its noteworthy antifungal activity, demonstrated by an inhibition zone of 16 mm, 17 mm, and 15 mm (positive control = 15 mm) against Candida albicans for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ respectively, underscoring its potential as an antifungal agent as shown in Figure 9 and Table 1. In contrast, it discussed its lack of effectiveness against both Gram-negative (E. coli) and Gram-positive (S.aureus) bacteria, exploring potential reasons for this selective antimicrobial activity. Consider factors such as the distinct interaction mechanisms of the Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ with fungal cells versus bacterial cells, or variations in cell wall structures and metabolic pathways between fungi and bacteria (Berardo et al., 2024).

Figure 9

Figure 9. (A) Antifungal activity of S1, S1*(Cr₂AlC MAX phase), S2 (Cr₃AlC₂), and S3 (Cr₄AlC₃) for Candida albicans (B) for S. aureus, and (C) for E. coli.

Table 1

Table 1. Antifungal activity of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ for Candida albicans.

To provide a comprehensive perspective on the biomedical potential of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ materials, it is imperative to acknowledge their limitations and advocate for further research to enhance understanding and improve antibacterial properties. The antifungal properties of the Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ against Candida albicans can be ascribed to various inherent characteristics of nanomaterials like Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃. Nanocomposites possess distinct physical properties, including a large specific surface area and unique interaction mechanisms with fungal cells, enabling effective targeting and inhibition of fungal growth. Conversely, the lack of efficacy against bacteria such as E. coli and S. aureus may be attributed to disparities in cell wall structures and metabolic pathways between fungi and bacteria (El-Zahed et al., 2024). Bacteria typically exhibit more complex and robust cell wall structures, potentially rendering them less susceptible to the mechanisms employed by Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ nanomaterials. Moreover, the interaction mechanisms of these nanomaterials may be more potent against fungi’s cell structures and reproductive mechanisms than bacteria. This specificity in antimicrobial activity underscores the significance of comprehending the intricate interactions between nanomaterials and various microorganisms for the advancement of targeted antimicrobial therapies (Warsi et al., 2022; Krasian et al., 2024).

3.3 Anti-cancerous activity

The established medical use of synthesized Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ extends to treating various types of cancer cells (Khaled et al., 2024). The prepared samples of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ were evaluated for their anti-cancer activity against liver cancer HepG2 cell line was conducted. The cells underwent treatment with a 200 μg/mL dosage for 48 h. In this investigation, the cytotoxic impact of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ was evaluated on HepG-2 (human hepatocellular cancer cells), uncovering pronounced cytotoxicity against the HepG-2 cells. Particularly, at a concentration of 200 μg/mL, that composite exhibited noteworthy anti-cancer activity by markedly inhibiting the growth of hepatocellular carcinoma and reducing cellular viability in HepG-2 cells, as illustrated in Figure 10 (Al-Thubaiti et al., 2022; Kumar et al., 2024). Three distinct plots illustrate data for absorbance, Viability%, and % inhibition in the graphical depiction. The minimum absorbance value was observed as 1.64 (mean value 1.71, S. D 0.05), 2.4(mean value 2.4, S. D 0.19), and 1.99 (mean 2.06, S. D 0.05) for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ respectively, as showed in Figure 10A and the maximum viability value was recorded as 50.4% (mean value 48.86, S. D 1.46), 62.4 (mean value 66.3, S. D 5.84) and 60.4 (mean value 58.8, S. D 1.18) as indicated in Figure 10B for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ respectively. Figure 10C represented the maximum inhibition value observed as 52.46% (mean value 51.13, S. D 1.19), 38 (mean value 33.6, S. D 5.84), and 42.4 (mean value 41.1, S. D 1.18)for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ respectively (Sedky et al., 2024).

Figure 10

Figure 10. Anti-cancerous activity of Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ for HepG2 cell line (A) Absorbance, (B) %Viability and (C) % Inhibition.

4 Conclusion

In the initial attempt, a cost-effective sol-gel approach was utilized to synthesize Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃ MAX phases. This novel synthesis method successfully produced these compounds through a sol-gel wet chemistry approach. Various characterization techniques were employed to confirm the formation of the MAX phases. The synthesized powders exhibited hexagonal structures, indicating high purity and stability. One notable advantage of this wet chemical technique was its enhanced handling of the precursor mixture, resulting in the formation of lamellar sheet structures for Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃. Analysis of X-ray diffraction data confirmed the presence of highly crystalline Cr₂AlC, Cr₃AlC₂, and Cr₄AlC₃, along with minor quantities of additional phases such as Cr₂O₃ and Cr₃C₂. Modifying the precursor mixture by increasing the excess of citric acid showed promise in reducing the level of oxide content in the final products. Furthermore, the synthesized materials exhibited notable antifungal efficacy against Candida albicans. Their anti-cancer activity was evaluated against the HepG2 cell line, with minimum absorbance values observed as follows: 1.64 (mean value 1.71, S.D 0.05) for Cr₂AlC, 2.4 (mean value 2.4, S.D 0.19) for Cr₃AlC₂, and 1.99 (mean value 2.06, S.D 0.05) for Cr₄AlC₃. Maximum viability values were recorded as 50.4% (mean value 48.86, S.D 1.46) for Cr₂AlC, 62.4% (mean value 66.3, S.D 5.84) for Cr₃AlC₂, and 60.4% (mean value 58.8, S.D 1.18) for Cr₄AlC₃. Additionally, maximum inhibition values were observed as 52.46% (mean value 51.13, S.D 1.19) for Cr₂AlC, 38% (mean value 33.6, S.D 5.84) for Cr₃AlC₂, and 42.4% (mean value 41.1, S.D 1.18) for Cr₄AlC₃.

Data availability statement

The raw data supporting the conclusion of this article will be available upon request.

Ethics statement

Ethical approval was not required for the studies on animals in accordance with the local legislation and institutional requirements because only commercially available established cell lines were used.

Author contributions

MS: Data curation, Formal Analysis, Methodology, Writing–original draft. NS: Conceptualization, Resources, Supervision, Writing–review and editing. NA: Software, Validation, Writing–review and editing. ZZ: Investigation, Validation, Writing–original draft. MZ: Methodology, Resources, Visualization, Writing–review and editing.

Funding

The author(s) declare that no financial support was received for the research, authorship, and/or publication of this article.

Acknowledgments

The authors extend thanks to Dr. Imran Shahid, Qatar University for characterization and to Dr. Azhar Rasul, Department of Zoology, GC University Faisalabad, Pakistan for biological and anticancer activities. This cell line was gifted by Prof. Dr. Xiaomeng Li, Northeast Normal University, China to Dr. Azhar Rasul, Department of Zoology, GC University Faisalabad, Pakistan.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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