Noncovalent functionalization of Ti₃C₂T_X using cationic porphyrins with enhanced stability against oxidation

Abstract

Ti₃C₂T_X, as the most explored MXenes, are a rising star among 2D materials due to their astonishing physicochemical properties. However, their practical applications remain extremely challenging because of chemical degradation into TiO₂ nanoparticles due to oxidation. Chemical functionalization is an effective way to improve their stability against oxidation and tune the physicochemical properties of 2D materials. In this paper, Ti₃C₂T_X is noncovalently functionalized using two different cationic porphyrins and the two hybrids show good stabilities in water against oxidation. The electrostatic interactions between the cationic porphyrins and the Ti₃C₂T_X nanosheets are confirmed by the changes in the zetapotential and the photophysical measurements. The hybrids show a red shifted Soret band of the porphyrins with a complete quenching of the fluorescence emission, which confirms the effective interactions and an energy/electron transfer between the porphyrins and the Ti₃C₂T_X nanosheets. The exfoliated and functionalized Ti₃C₂T_X are characterized using various microscopic and spectroscopic techniques. The two hybrids exhibit pH dependent release of cationic porphyrins particularly under acidic conditions. This study proposes a potentially useful strategy for the preparation of highly stable and functional MXenes towards promising applications in biomedicines, optoelectronics and sensors.

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Introduction

MXenes are a series of two-dimensional transition metal carbides and nitrides with a general formula of M_n+1X_nT_X, where M is an early transition metal (Ti, V, Nb, etc.), X denotes for C and/or N, T_X represents surface termination (–OH, –O, –F, etc.) and n = 1, 2, or 3.^1,2 These materials are synthesized by selective etching of an ‘A’ element (group 13 or 14 element) layer from their corresponding MAX phase.^3,4 Owing to their high electrical conductivity, hydrophilic surfaces, flexibility and transparency, large electrochemically active surfaces, tunable properties via compositional and structural modification, MXenes have attracted extensive attention in the research community and have been widely used in various applications including supercapacitors,^5–9 batteries,^10–14 electrocatalysis,^15–17 electrochromic devices,^18,19 field-effect transistors (FETs),²⁰ electromagnetic interference shielding²¹ and biomedicines.²²

Since the first discovery of MXenes in 2011, Ti₃C₂T_X has been one of the most widely investigated MXenes.²³ The poor stability against oxidation of Ti₃C₂T_X is a critical issue which hinders their practical applications.^24,25 The Ti₃C₂T_X nanosheets gradually degrade into TiO₂ nanoparticles in humid air or in water and lose their intrinsic physicochemical properties.^26,27 The ambient stability of Ti₃C₂T_X nanosheets against oxidation can be improved by storing the water dispersions protected with argon at low temperatures.²⁴ Chemical functionalization has been recognized as an effective strategy to improve the ambient stability and the physicochemical properties of 2D materials.^28–37 Particularly, noncovalent functionalization can provide a higher degree of functionalization and largely preserve the intrinsic properties of 2D nanosheets to a great extent.³⁸ Chemically functionalized Ti₃C₂T_X is expected to possess a higher stability against oxidation compared with the bare Ti₃C₂T_X dispersions in water because of the presence of organic molecules or polymers at the surfaces, which can protect MXene nanosheets against oxidation by isolating the Ti atoms from oxygen. It has been reported that Ti₃C₂T_X showed an improved stability in nonpolar organic media after surface modification with alkylphosphonic acids.³⁹ Kim and coworkers reported that the oxidation stability of MXene nanosheets can be enhanced by synthesizing Ti₃C₂T_X/polydopamine composites.⁴⁰ Different from these, it is anticipated that functional organic macrocyclic molecules with multiple positive charges can effectively interact with the functional groups of Ti₃C₂T_X and noncovalently stack on the surface of MXenes to prevent it from oxidation.

Porphyrins are an important class of macrocyclic functional dyes with extraordinary physicochemical properties and hold potential applications in photomedicines,^41,42 and photovoltaic⁴³ and optoelectronic devices.⁴⁴ Covalent or noncovalent functionalization of 2D materials such as graphene,^45–47 transition metal dichalcogenides,⁴⁸ and black phosphorus nanosheets⁴⁹ using various porphyrins has already been reported to achieve novel hybrid materials with multifunctional properties such as nonlinear optical properties, photoenergy conversion, photo/electrochemical sensing and photocatalytic/electrocatalytic properties.^50–52 In the light of these studies, we anticipate that the electron rich functional groups on MXene surfaces can effectively interact with positively charged porphyrins, which can decorate MXene with new functional properties and enhance its stability against oxidation. Herein, we have synthesized two porphyrin–Ti₃C₂T_X hybrids based on two different cationic porphyrins and studied their stability against oxidation and the effect of pH on their interactions. The present study will further boost up the blossoming of MXene research towards chemically functionalized MXenes with functional moieties such as relevant molecules/polymers for biological and electronic applications.

Experimental

Materials

Ti₃AlC₂ was purchased from Forsman (purity 98%). LiF (99.995%), HCl (≥37%), 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (dye content 97%) and 5,10,15,20-tetrakis(4-trimethylammoniophenyl)porphyrin tetra(p-toluenesulfonate) (dye content 90%) were purchased from Sigma-Aldrich and were used as received.

Instruments

Zeta potential of the dispersions was analysed using Zetasizer Nano Zs (Malvern instruments UK). Thermogravimetric analysis (TGA) was carried out on a TGA 3+ star system (Mettler Toledo, Columbus, OH, USA) under N₂ flow (50 mL min⁻¹) at a ramp rate of 10 °C min⁻¹. Raman analysis was performed on a WITec alpha300 R confocal Raman microscopy system (excitation at 532 nm) and mean Raman spectra were obtained by averaging 400 point Raman spectra. X-Ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI 5000 VersaProbe III Scanning XPS Microprobe and the spectra were processed using the PHI multiPak software. Scanning electron microscopy (SEM) was performed on a Quanta 200 FEG ESEM. Transmission electron microscopy (TEM) and HR-TEM were performed on a FEI Tecnai T20 instrument at an acceleration voltage of 200 kV. Attenuated total reflectance infrared (ATR-IR) spectra were recorded using a PerkinElmer Frontier Infrared Spectrometer with a GladiATR, diamond crystal design. UV-visible absorption spectra were recorded on a Varian Cary 50 Bio UV-visible spectrophotometer and fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer.

Preparation of Ti₃C₂T_X

The etchant was prepared by adding 12 M LiF to 2 mL of 9 M HCl and the mixture was stirred with a Teflon magnetic bar for 5 minutes in a polypropylene centrifuging tube. To this etchant, 100 mg of Ti₃AlC₂ was slowly added and the mixture was stirred at 50 °C for 24 h. After stirring for 24 h, the etched solution was washed repeatedly with water by centrifuging at 4000 rpm for 10 minutes. At a pH of ∼5, delamination started. The dark green supernatant was collected by centrifuging at 4000 rpm for 10 minutes and the solid residues were stored as delaminated MXenes.

Preparation of Ti₃C₂T_X–porphyrin hybrids

The noncovalent functionalization of Ti₃C₂T_X was carried out by gradually adding 3.6 mL of Ti₃C₂T_X dispersion (0.6 mg mL⁻¹) into 75.56 mL of a TMPyP or a TTMAPP solution (3 μmol) in water and followed by shaking the mixture overnight. After shaking, almost all the solids were precipitated out with clear supernatants. The solids were separated by filtration using PTFE filter paper and the excess porphyrins were removed by gently washing the solids on the filter papers using water. The solid residues were dried under vacuum at 55 °C for 24 h and were directly used for further characterization studies.

Results and discussion

The Ti₃C₂T_X was prepared by selective etching of Al from Ti₃AlC₂ using LiF and HCl at 50 °C and then by delamination using a hand shaking process (Scheme 1).³ The as-prepared Ti₃C₂T_X nanosheets appeared as a stable dispersion in water with a zetapotential value of −27.5 mV (Fig. S1, ESI†). This value indicates that the MXene surfaces are negatively charged possibly due to the presence of abundant electron rich hydroxyl, oxides and fluoride functional groups.⁵³ We have selected two different cationic porphyrins (with four positive charges on the peripheral groups) namely, 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) and 5,10,15,20-tetrakis(4-trimethylammoniophenyl)porphyrin tetra(p-toluenesulfonate) (TTMAPP) for the functionalization of the Ti₃C₂T_X (Scheme 1). The functionalization was carried out by titrating porphyrins with Ti₃C₂T_X in deoxygenated water and followed by an overnight shaking using a shaker. After functionalization, the zetapotential was increased to +2.68 for the TMPyP–Ti₃C₂T_X and −1.45 for the TTMAPP–Ti₃C₂T_X and the two hybrids were precipitated out from water (Fig. S1, ESI†). The changes in the zetapotential confirm the effective electrostatic interactions between the positively charged porphyrin molecules and the Ti₃C₂T_X surface. The TMPyP–Ti₃C₂T_X and TTMAPP–Ti₃C₂T_X hybrids were collected by centrifugation and dried under vacuum at 60 °C for 24 h.

Scheme 1 Schematic illustration of the preparation of Ti₃C₂T_X and its noncovalent functionalization using the cationic porphyrins, TMPyP and TTMAPP.

The interactions between Ti₃C₂T_X and the two cationic porphyrins were further investigated by analyzing the changes in the UV-visible absorption and fluorescence emission properties. The as-prepared Ti₃C₂T_X dispersion in water showed the characteristic absorption peaks at ∼800 and 320 nm for the Ti₃C₂T_X.⁵⁴ The absorption spectra of the TMPyP showed a Soret band at 422 nm and Q bands at 518, 555, 584 and 640 nm, respectively, in water. The TTMAPP showed a Soret band at 412 nm and Q bands at 514, 550, 580 and 635 nm in water (Fig. S2 and S3, ESI†). A titration was performed by gradually adding the Ti₃C₂T_X dispersion (0.2 mg mL⁻¹) into 1 μM solutions of the cationic porphyrins in water. For both the cationic porphyrins, the intensity of the Soret band was gradually decreased upon the addition of the Ti₃C₂T_X dispersion. After adding 18 μg of the Ti₃C₂T_X, the Soret band of the free porphyrins was completely vanished with the formation of a new peak at 455 nm with a red shift of 33 nm for the TMPyP–Ti₃C₂T_X and at 430 nm with a red shift of 18 nm for the TTMAPP–Ti₃C₂T_X. The red shift in the Soret band can be attributed to the flattening of porphyrins as the positively charged meso-groups can interact with the surface groups of the Ti₃C₂T_X in the hybrids. In addition, the number and the intensities of the Q bands were found to be decreased after the formation of the hybrids, suggesting an increased symmetry of the cationic porphyrins due to their proximity on the Ti₃C₂T_X surface (Fig. 1A and B).⁵⁵

Fig. 1 Changes in the UV-visible absorption and fluorescence emission of 1 μM of TMPyP (A and C) and 1 μM of TTMAPP (B and D), upon titration with 0.2 mg mL⁻¹ of Ti₃C₂T_X in water, respectively. (E) The graph shows the extent of emission quenching with respect to the amount of the Ti₃C₂T_X (I₀ and I are the intensity at fluorescence maxima of porphyrins before and after adding Ti₃C₂T_X, respectively).

The fluorescence spectra of the TMPyP showed a relatively lower intensity emission peak at 680 nm and a higher intensity peak at 708 nm (Fig. S2, ESI†). Similarly, the TTMAPP exhibited a higher intensity emission peak at 643 nm and a lower intensity emission peak at 702 nm (Fig. S3, ESI†). In both cases, the addition of Ti₃C₂T_X resulted in a complete quenching of the fluorescence, suggesting an effective electron and/or energy transfer between the cationic porphyrins and the Ti₃C₂T_X (Fig. 1C–E). Similar observations were reported for other 2D and 1D nanomaterials including graphene,⁴⁵ MoS₂,⁴⁸ black phosphorus nanosheets⁴⁹ and carbon nanotubes.⁵⁶

The morphology changes of Ti₃C₂T_X during the preparation and functionalization were analyzed using SEM and TEM analysis. The Ti₃AlC₂ phase showed a densely stacked layer structure while the Ti₃C₂T_X possessed a stacked layered morphology in the powder state (Fig. 2A and B). TEM images show the mono- to few-layer nanosheets of the Ti₃C₂T_X by drop-casting its water dispersion onto a TEM grid (Fig. 2C). In addition, the high resolution TEM image confirms the crystalline nature of the synthesized Ti₃C₂T_X (Fig. 2D). After the functionalization with porphyrin molecules, the Ti₃C₂T_X nanosheets were found to be more aggregated which is in accordance with the zetapotential changes and the aggregation in water (Fig. 2E and F).

Fig. 2 SEM images of Ti₃AlC₂ (A) and Ti₃C₂T_X (B) in the powder form. TEM image (C) and high-resolution TEM image (D) of Ti₃C₂T_X. TEM images of the TMPyP–Ti₃C₂T_X (E) and the TTMAPP–Ti₃C₂T_X (F).

XPS was used to study the chemical compositions and surface electronic states in the Ti₃C₂T_X before and after functionalization. The survey spectra showed the presence of Ti, C, F, and O in all samples, while the presence of N was only found in the functionalized samples (Fig. S4, ESI†). The high resolution Ti2p, C1s, O1s and F1s XPS spectra of the bare Ti₃C₂T_X are shown in the Fig. 3A–D. In the Ti2p XPS spectrum, the 2p_3/2 peaks were located at 455.26, 456.19, 457.20, 458.45 and 459.61 eV, corresponding to the Ti–C, Ti²⁺, Ti³⁺, TiO₂ and Ti–F, respectively.⁵⁷ The C1s XPS spectrum can be fitted to five peaks at 281.94, 282.55, 284.84, 286.41 and 289.34 eV, assigned to the two C–Ti–T_X peaks, and C–C, C–O and C=O/C–F, respectively.^58,59 The O1s spectrum has been fitted to four peaks at 529.81, 530.68, 532.10 and 533.39 eV, attributed to the O–Ti (TiO₂), O–Ti (O/OH), O–C and adsorbed water molecules via hydrogen bonding to the terminal functional groups of the MXene.⁵⁹ The F1s spectrum showed two peaks at 685.24 and 686.77 eV, assigned to the F–Ti and F–C, respectively.⁶⁰Fig. 3E and F show the high resolution Ti2p XPS spectra of the TMPyP–Ti₃C₂T_X and the TTMAPP–Ti₃C₂T_X, respectively. Similar to the fitting for the bare Ti₃C₂T_X, the Ti2p spectra of both the hybrids were fitted with a higher degree of oxidation (higher intensity of the TiO₂ peak), probably due to the exposure to ambient atmosphere during the preparation and washing processes. The N1s spectra of the hybrids show three peaks at 397.85, 399.67 and 401.76 eV for the TMPyP–Ti₃C₂T_X (Fig. 3G) and at 397.45, 399.58 and 402.73 eV for the TTMAPP–Ti₃C₂T_X (Fig. 3H), corresponding to the aza (–NH–), pyrrol (–C=N–) and peripheral pyridinium/quaternary ammonium nitrogen, respectively.⁶¹

Fig. 3 High resolution Ti2p (A), C1s (B), O1s (C) and F1s (D) XPS spectra of Ti₃C₂T_X. High resolution Ti2p XPS spectra (E and F) and N1s spectra (G and H) of the TMPyP–Ti₃C₂T_X and the TTMAPP–Ti₃C₂T_X, respectively.

TGA was performed under an N₂ atmosphere to study the thermal stability of the Ti₃C₂T_X and the two hybrids (Fig. 4A). The Ti₃C₂T_X showed a 2.6% weight loss from room temperature to 200 °C attributed to the adsorbed water molecules.⁶² Another weight loss from 200–400 °C could be attributed to the decomposition of the OH and F functional groups. Apart from these, the hybrids showed a higher weight loss (around 5.5%) between 310 and 530 °C ascribed to the degradation of the porphyrins. Fig. 4B shows the mean Raman spectra of the Ti₃C₂T_X and the hybrids, measured using a laser at an excitation wavelength of 532 nm. The Ti₃C₂T_X showed several Raman bands: the one at 148 cm⁻¹ is assigned to the Eg vibrational mode of anatase TiO₂,⁶³ the two bands at 260 and 403 cm⁻¹ correspond to the Eg modes of the surface groups attached to Ti, and the band at 609 cm⁻¹ is associated with the Eg vibrational modes of C.^64,65 The two peaks centered at 1341 and 1569 cm⁻¹ are attributed to the D and G band of graphitic carbon, respectively.⁶⁶ After functionalization, the peak at 403 cm⁻¹ was shifted to 426 cm⁻¹ indicating the effective interactions between the cationic porphyrins and the surface functional groups of the Ti₃C₂T_X. Furthermore, ATR-IR spectra were recorded and the hybrids showed the C–H and C–N vibrational frequencies corresponding to the cationic porphyrins (Fig. S5, ESI†).

Fig. 4 TGA curves (A) and Raman spectra (B) of the Ti₃C₂T_X, TMPyP–Ti₃C₂T_X and TTMAPP–Ti₃C₂T_X.

The stability of the Ti₃C₂T_X and the hybrids against oxidation were studied and compared by investigating the changes in the absorption after storing the dispersions in undeoxygenated water at 5 °C for one week. Since the two hybrids were in the aggregated state in water, we have chosen the absorption peak at 260 nm corresponding to TiO₂ to track the oxidative degradation process, instead of using the Ti₃C₂T_X characteristic peaks at 320 and 800 nm. After one week, the peak at 260 nm was found to be considerably increased in the case of the bare Ti₃C₂T_X compared with those of the two hybrids, suggesting an improved stability against oxidation for the two hybrids (Fig. 5A–C). Moreover, the binding interactions between the Ti₃C₂T_X and the cationic porphyrins were found to be highly stable because free porphyrins are not released in water even after one week. Furthermore, high resolution XPS was employed to confirm the improved stability against oxidation for the two hybrids. The Ti2p XPS spectra for the bare Ti₃C₂T_X and the two hybrids were recorded after storing the samples for one week in undeoxygenated water (Fig. 5D–F) and the peak areas under Ti–O (TiO₂) region was compared with that of the spectra of the first day. After one week, the Ti₃C₂T_X exhibited a vast increase of ∼104% in the intensity of the TiO₂ peak because of the fast degradation (Fig. S6, ESI†). In contrast, the TMPyP–Ti₃C₂T_X and the TTMAPP–Ti₃C₂T_X showed an increase of ∼21 and ∼6%, respectively, confirming that the noncovalent functionalization of the Ti₃C₂T_X using the two cationic porphyrins resulted in a dramatic improvement of stability against oxidation in water.

Fig. 5 Absorption changes (A–C) in water and Ti2p XPS spectra (D–F) of the Ti₃C₂T_X, TMPyP–Ti₃C₂T_X and TTMAPP–Ti₃C₂T_X after storing in water for one week.

Studying the effect of external stimuli like pH on the interactions between 2D materials and functional moieties in hybrid systems is highly important by considering both fundamental research in term of interactions and also various applications aspects of such systems particularly biological applications in drug loading and releases at different pH values.^67,68 Since porphyrins are sensitive to the changes in pH, it is anticipated that the pH of the medium can tune the noncovalent interactions between the cationic porphyrins and the Ti₃C₂T_X in the two hybrids. The free porphyrins exhibited different absorption and emission behavior with respect to the changes in pH due to the protonation or deprotonation of the porphyrins (Fig. S2 and S3, ESI†).⁵⁵ To study the effect of pH on the two hybrids, we have investigated the changes in the absorption and emission properties of the hybrids by varying the pH from neutral to both acidic and basic conditions. In the case of the TMPyP–Ti₃C₂T_X hybrid, a peak at 422 nm corresponding to the free TMPyP was observed at pH 4. From pH 4 to pH 2, the intensity of the peak at 422 nm was found to be increased, suggesting an increased release of the free TMPyP from the hybrid (Fig. 6A). Correspondingly, we have observed an increase in the intensity of emission spectra as the pH decreases from 6 to 2 (Fig. 6B). The free TMPyP showed no obvious change in the absorption and emission until pH 2 (Fig. S2A, ESI†). At pH 4 and pH 2, the percentage of the TMPyP released from the TMPyP–Ti₃C₂T_X hybrid was calculated to be 2.7% and 11.2%, respectively, based on the absorption intensity of the TMPyP at 422 nm. On the other hand, increasing pH to 12 did not have too much influence on the interaction as there was no remarkable absorption or emission peaks appeared corresponding to the free TMPyP (Fig. S7A and B, ESI†).

Fig. 6 Changes in the UV-visible absorption and fluorescence emission of the TMPyP–Ti₃C₂T_X (A and B) and the TTMAPP–Ti₃C₂T_X (C and D), respectively, under acidic conditions.

In the case of the TTMAPP–Ti₃C₂T_X hybrid, no obvious peak for the free TTMAPP was observed until pH 5. At pH 4, an absorption peak at 412 nm corresponding to the free TTMAPP appeared (Fig. 6C). A slight increase in the emission intensity was observed from neutral to pH 5 and then a considerable increase at pH 4, suggesting a gradual release of the free TTMAPP in water (Fig. 6D). Since the free TTMAPP shows a change in its absorption and emission structure at pH 3 due to protonation (Fig. S3A and B, ESI†), the effect of pH on the TTMAPP–Ti₃C₂T_X was studied until pH 4. The percentage of the free TTMAPP released from the TTMAPP–Ti₃C₂T_X hybrid at pH 4 is ∼1.7%, which is lower than that of the TMPyP–Ti₃C₂T_X hybrid released at the same pH, suggesting a stronger interaction in the case of the TTMAPP with the Ti₃C₂T_X. This observation further supports a higher stability against oxidation in the TTMAPP–Ti₃C₂T_X hybrid compared with that of the TMPyP–Ti₃C₂T_X hybrid in water. Furthermore, the pH of the hybrid dispersion was gradually increased to pH 12 and no peaks corresponding to the free TTMAPP were observed. However, a small increase in the emission intensity was seen from pH 9 to 12, indicating a slight release of the TTMAPP (Fig. S7C and D, ESI†). For both the hybrids, acidic conditions favor considerable release of the porphyrins compared with basic conditions. This can be attributed to the protonation of the functional groups present on the Ti₃C₂T_X surface and consequently a lower interaction with the positively charged peripheral groups in the porphyrins.

Conclusions

In summary, Ti₃C₂T_X was synthesized and noncovalently functionalized using two different cationic porphyrins. The electrostatic interactions between the Ti₃C₂T_X and the porphyrins were studied by investigating the changes in the zetapotential and the photophysical properties. The exfoliated Ti₃C₂T_X and the functionalized Ti₃C₂T_X were further characterized using SEM, TEM, XPS, TGA and Raman analyses. The functionalized Ti₃C₂T_X showed a largely improved stability against oxidation in water which was confirmed by UV-visible absorption measurement and high resolution XPS analyses. Finally, the effect of pH on the interactions between the Ti₃C₂T_X and the cationic porphyrins was investigated and it was found that pH dependent release of the porphyrins is possible especially under acidic conditions. This study will further fuel the growing research in MXenes and will serve as an example for chemical functionalization of MXenes using attractive functional molecules for applications towards drug loading and release,⁶⁹ nonlinear optical or optical limiting devices,⁷⁰ and photo- or electro-catalysis.^71,72

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Stiftelsen Chalmers Tekniska Högskola (215213), Adlerbertska Forskningsstiftelsen (C 2020-1230 and C2021-1258), Swedish Foundation for International Cooperation in Research and Higher Education (IB 2020-8789), Göteborg Energi (Tänk:Om Stipendiet) and Swedish Research Council Starting Grant (2020-04903).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm01326b

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