Titanium Carbide (Ti₃C₂T_x) MXene as Efficient Electron/Hole Transport Material for Perovskite Solar Cells and Electrode Material for Electrochemical Biosensors/Non-Biosensors Applications

Abstract

Recently, two-dimensional (2D) MXenes materials have received enormous attention because of their excellent physiochemical properties such as high carrier mobility, metallic electrical conductivity, mechanical properties, transparency, and tunable work function. MXenes play a significant role as additives, charge transfer layers, and conductive electrodes for optoelectronic applications. Particularly, titanium carbide (Ti₃C₂T_x) MXene demonstrates excellent optoelectronic features, tunable work function, good electron affinity, and high conductivity. The Ti₃C₂T_x has been widely used as electron transport (ETL) or hole transport layers (HTL) in the development of perovskite solar cells (PSCs). Additionally, Ti₃C₂T_x has excellent electrochemical properties and has been widely explored as sensing material for the development of electrochemical biosensors. In this review article, we have summarized the recent advances in the development of the PSCs using Ti₃C₂T_x MXene as ETL and HTL. We have also compiled the recent progress in the fabrication of biosensors using Ti₃C₂T_x-based electrode materials. We believed that the present mini review article would be useful to provide a deep understanding, and comprehensive insight into the research status.

1. Introduction

In the present scenario, the design and fabrication of bi-functional or multi-functional materials received extensive attention for various optoelectronic and electrochemical applications [1,2,3]. It is of great significance to summarize the bi-functional properties of such materials towards the development of perovskite solar cells (PSCs) and electrochemical biosensors [4,5,6]. The PSCs have been proven as the most efficient thin film-based photovoltaic technology compared to the conventional solar cells [7,8,9,10]. The poor stability and low charge transport properties of the PSCs are the major concerns for the development of highly stable and high-performance PSCs [11]. Therefore, it has been considered that the stability and charge transport properties of the PSCs can be further enhanced by employing efficient electron/hole transport layers (ETL or HTL) [12,13]. Ding et al. [14] demonstrated the fabrication of tandem solar cells which exhibited a PCE of 21.4%. Chen et al. [15] also reported the enhanced photovoltaic performance of PSCs. Ma et al. [16] optimized the surface morphology and reported the improved photovoltaic performance. Chen et al. [17] reported the passivation and buried interface study of 2D perovskite on ETL and achieved good performance.

The two-dimensional (2D) layered materials such as carbon nitride, hexagonal boron nitride, black phosphorus, transition metal dichalcogenides, and hybrid materials have received enormous attention because of their excellent optoelectronic properties [18,19,20]. The layered materials have been widely explored in various optoelectronic applications such as solar cells, supercapacitors, batteries, light emitting diodes, and photo-detectors [21,22,23,24,25,26,27,28,29,30,31,32,33]. Graphene has been proven as a highly efficient electrode material for various optoelectronic applications [34,35,36,37,38,39,40,41,42,43,44,45]. The 2D layered materials are promising material for their implantation in optoelectronic devices [17]. According to the reported literature, it was observed that the utilization of 2D materials enhanced the charge transport properties and reduced the recombination reactions [43]. The 2D layered materials can offer a perfect uniform surface due to the presence of the inherent confinement in the out-of-plane direction. Thus, 2D layered materials can be suitable as ETL or HTL for the fabrication of high-performance PSCs. Previously, various layered materials such as graphene, MoS₂, WS₂, SnS₂, and TiS₂, etc., have been reported in the development of PSCs. Tang et al. [45] reported the benign synthetic procedure for the preparation of MoS₂ and introduced it as ETL for the construction of PSCs. The introduction of MoS₂ significantly enhanced the photovoltaic performance of the fabricated PSCs and an interesting power conversion efficiency (PCE) of 20.55% was achieved. Other work also demonstrated the use of tungsten disulfide (WS₂) as ETL and reported a decent PCE of 12.44% [46].

Recently, Yin et al. [47] have proposed an exfoliation method for the preparation of titanium disulfide (TiS₂) films as ETL for the development of PSCs which demonstrated a PCE of 17.37%. Tin disulfide (SnS₂) has a CdI₂-like layered structure (where Sn atoms are sandwiched between two S atoms) and has been explored as ETL by Zhao et al. [48]. The aforementioned points show that layered materials can be used as efficient ETL or HTL materials to improve the PCE of the PSCs.

In 2011, Gogotsi and coworkers have discovered a new class of 2D materials which is known as MXenes [49]. In general, MXenes are a large family of 2D transition metal nitrides, carbonitrides, and carbides with the general chemical formula of M_n+1X_nT_x (where n = 1, 2 or 3; M = transition metal such as Mo, Ti, Cr, Hf, Zr, Nb, Ta; X = C or N and T_x = surface functional group such as –OH, –O, –F, and –Cl) [50]. MXenes possess excellent physiochemical features such as tunable optical, mechanical, biological, and electrical properties, which makes them a suitable candidate for optoelectronic devices [51,52,53]. Among different MXenes, Ti₃C₂T_x has attracted material scientists and electrochemists because of its high transparency, excellent electron mobility, thermal stability, and high electrical conductivity [54]. The work function of the Ti₃C₂T_x MXene can be tuned from 1.6 to 6.25 eV by employing different synthetic processes and post-treatments [55]. These features of Ti₃C₂T_x MXene open new doors for its application as highly efficient ETL/HTL for PSCs application. The Ti₃C₂T_x MXene can be prepared at low temperature, which makes it compatible with PSCs manufacturing technology.

In 2009, Kojima et al. proposed the novel PSCs using methyl ammonium lead iodide (MAPbI₃) as absorber layers [56]. The PCE of the proposed PSCs was less than 4%. Thus, various efforts have been made by various research groups to further improve the performance of the PSCs. The PSCs consist of various components such as an absorber layer, ETL, and HTL [44,45,46]. Titanium dioxide (TiO₂) has been widely used as ETL for the fabrication of PSCs, but suffers from the presence of trapping states [45]. Many attempts were also made to overcome such issues using novel electron transport materials. The Ti₃C₂T_x MXene has excellent charge-carriers mobility and optoelectronic features, and can be utilized as ETL and HTL for the construction of PSCs. In addition, Ti₃C₂T_x MXene also possesses excellent electrochemical properties, conductivity, and high surface area [57]. Thus, Ti₃C₂T_x MXene has been used in the fabrication of various sensors and biosensors. In case of sensing-related applications, various enzymatic biosensors and non-biosensors have been reported using Ti₃C₂T_x MXene-based electrode materials as electro-catalysts [57,58,59,60].

Herein, we have compiled and discussed the recent progress of Ti₃C₂T_x MXene for the development of PSCs and electrochemical biosensors/non-biosensors applications. This review article would be beneficial for the scientific community working in the field of PSCs and electro-analytical sensing applications.

2. Synthetic Procedures for Ti₃C₂T_x MXene

The Ti₃C₂T_x MXene was discovered by Gogotsi and coworkers in 2011 [49]. The Ti₃C₂T_x MXene was obtained by HF treatment followed by the sonication process, as depicted in Figure 1a–d. The SEM image of the synthesized Ti₃C₂T_x MXene has been presented in Figure 1d, which demonstrates the presence of the layered structure of the prepared Ti₃C₂T_x MXene. Yu et al. [61] also reported the synthesis of Ti₃C₂T_x MXene. In brief, an appropriate amount of LiF was slowly added to the 9 molar HCl using stirring which yielded a homogeneous solution. In further steps, titanium aluminium carbide (Ti₃AlC₂) was milled by mortar and pestle and gradually mixed to the above-prepared homogeneous solution. This etching process was continued to 24 h at 25 °C using continuous stirring. Finally, the etched product was washed with DI water using centrifuge. This process was repeated for several times until the pH of the mixture reached above 6. Subsequently, handshaking was used to exfoliate the Ti₃C₂T_x into the few-layer flakes. The dispersion of Ti₃C₂T_x was degassed with Ar and stored in the fridge at 4 °C [61,62]. In 2019, Yang et al. [63] also reported the preparation of Ti₃C₂T_x. In the first step, Ti powder, aluminum powder, and graphite powder were mixed uniformly. This mixture was sintered at high temperature (1650 °C) for 2 h under Ar atmosphere to obtain the Ti₃AlC₂ (MAX phase). Then, Ti₃AlC₂ powder was grinded and added to the LiF/HCl mixture solution, and kept for the etching process for 24 h. After 24 h, the acidic mixture was centrifuged and washed with DI water until pH of the solution reached 6 and finally Ti₃C₂T_x MXene was obtained [63]. The schematic illustration for the preparation of Ti₃C₂T_x MXene is presented in Figure 1e. In the past few years, many synthetic procedures have been developed for the preparation of Ti₃C₂ MXene. In this section, we have briefly discussed two widely used and efficient etching and exfoliation synthetic procedures as given below.

2.1. Etching Method

A previous report showed that the MAX phase of the MXene can be chemically etched by using HF aqueous solution and the etching process can be described as below [64],

2Ti₃AlC₂ + 6HF    ⟶    2AlF₃ + 3H₂ + 2Ti₃C₂

(1)

Ti₃C₂ + 2H₂O    ⟶    Ti₃C₂ + Ti₃C₂(OH)₂ + H₂

(2)

Ti₃C₂ + 2HF    ⟶    Ti₃C₂F₂ + H₂

(3)

The Ti₃C₂T_x MXene can be prepared by the etching of Al from the Ti₃AlC₂ phase using HF as an etching agent. Studies have shown that H and F radicals broke down after being adsorbed onto Ti atoms, leading to the weakening of Al-Ti bonds. This resulted in the creation of surface terminals and, eventually, the formation of Ti₃C₂T_x [64,65,66]. Because chemical etching operates under kinetic control, it is necessary to take into account various significant reaction factors such as time, temperature, and the concentration of HF. It was documented that achieving proper etching of a significant quantity in M_n+1AX_n demands extended etching duration, a relatively elevated etching temperature, and a low pH level. The primary resulting compound during the etching process is AlF₃ (as shown in Equation (1)), which is insoluble in water. Hence, the careful selection of suitable temperature and duration is crucial to prevent the formation of AlF₃ precipitates. Moreover, the direct use of HF is very dangerous. Thus, to avoid the direct utilization of toxic HF, Halim and colleagues opted for a safer and less intense approach by using NH₄HF₂ instead of the hazardous HF for their etching process [67]. Similarly, Ghidiu et al. [68] took an alternative route by introducing Ti₃AlC₂ powder into a mixture of LiF and HCl (6 M) to produce Ti₃C₂T_x. They conducted the reaction at 40 °C for 45 h, and their findings highlighted that the presence of both fluorine ions and protons played a crucial role in the successful etching. Expanding on this method, Lipatov et al. [69] adjusted the ratios of MAX and LiF (MAX: LiF = 1:5 or 1:7.5), revealing that an excess of LiF facilitated the etching of Al, along with the insertion of Li⁺. As a result, Ti₃C₂T_x MXene nanosheets were achieved, boasting larger dimensions, uniform thickness, and fewer imperfections. The etching procedure essentially converted the compact MAX structure into a loosely arranged, accordion-like configuration, recognized as multilayer MXenes (ML-MXenes). Consequently, a separate exfoliation step became essential to isolate the ML-MXenes into individual monolayers. It is important to mention that wet-chemical techniques are unsuitable for etching nitride-based MAXs because they have a higher energy barrier for formation. Urbankowski and colleagues [70] came up with an alternative approach involving molten salt (like potassium fluoride, lithium fluoride, and sodium fluoride) to create Ti₄N₃-based MXene. They achieved this by selectively removing aluminum from Ti₄AlN₃ at 550 °C in an argon environment. The leftover fluoride in the produced powder was subsequently eliminated using a solution of sulfuric acid (H₂SO₄).

2.2. Exfoliation Method

In 2013, the process of exfoliating MXene nanosheets into individual layers involved the introduction of large organic molecules into the spaces between the layers of the accordion-shaped structure [71]. This was followed by either mechanical vibration or the use of ultrasonic energy [72]. Common substances used to create these spaces, known as intercalants, included tetrabutylammonium hydroxide (TBAOH), dimethyl sulfoxide, hydrazine, urea, and NH₄⁺ [73,74,75,76]. For instance, researchers led by Chia employed TBAOH as the intercalant to exfoliate Ti₃C₂T_x MXene from products obtained through HF etching [59]. The process involved expanding the MAX powder containing larger flakes after removing Al. This resulted in Ti₃C₂-HF, which was then separated into individual or a small number of MXene layers by reducing the interaction between layers using TBAOH. Alternatively, when NH₄HF₂ was used as the etchant, the accordion-like multilayer Ti₃C₂T_x MXene already contained cations like NH₄⁺ within the aqueous solution.

2.3. Structural and Physiochemical Properties of Ti₃C₂T_x MXene

The Ti₃C₂T_x MXene consists of an interlayer region, intra-layer skeleton region, and surface terminating groups. The Ti and C atoms are stacked in the intra-molecular skeleton region to form the ionic bonds. In case of the interlayer region, the interaction between the layers was connected through the hydrogen bonding between either F/O atoms or van der Waals forces between O and F atoms. It was also observed that the strength of the hydrogen bonding depends not only on the number or distribution of –OH groups but also on the orientation of –OH. A large number of termination groups are also distributed on the Ti₃C₂T_x MXene surface. The properties of the Ti₃C₂T_x MXene can be influenced by the presence of functional groups. The Ti₃C₂T_x MXene has good charge carrier mobility, metallic conductivity, and work function, which suggest its potential applications in photovoltaic devices. The transformation of the MAX phase to MXene phase has been illustrated in Figure 1a–c, whereas the scanning electron microscopic (SEM) picture of the MXene is presented in Figure 1d.

2.4. Charge Transport Properties of Ti₃C₂T_x MXene

The Ti₃C₂T_x MXene possesses excellent charge transport properties, which makes it suitable ETL and/or HTL materials for the fabrication of high-performance PSCs. Here are several essential facets related to its properties of charge transport. The Ti₃C₂T_x MXene demonstrates excellent electrical conductivity, which is desirable and the most crucial feature for their applications in electronic devices. The presence of the metallic nature in Ti₃C₂T_x MXene arises from its unique layered structure, which includes transition metal carbide layers that facilitate the effective charge transport [49].

In addition, delocalized electronic transition states in the Ti₃C₂T_x MXene structure contribute to its presence of metallic properties. The presence of metallic Ti-C bonds in the Ti₃C₂T_x MXene allows the movement of electrons and is responsible for the high conductivity of Ti₃C₂T_x MXene. The charge carriers, such as electrons, experience the high mobility in the Ti₃C₂T_x MXene and contribute to its excellent charge transport properties [77]. Thus, it is clear that high carrier mobility is crucial for electronic applications, as it can allow for efficient and fast movements of charge carriers. Due to the favorable charge transport properties, Ti₃C₂T_x MXene can be used as efficient ETL or HTL for the development of PSCs [63].

It can be noted that research in the field of Ti₃C₂T_x MXene is ongoing, and new findings may further uncover additional details regarding the charge transport properties.

Figure 1. Schematic diagram (a–c) for the preparation of Ti₃C₂T_x MXene and SEM image (d) [49]. Schematic illustration for the preparation of Ti₃C₂T_x MXene (e) [63]. PXRD (f) of Ti₃C₂T_x MXene. UV-vis spectra (g) and J-V curves (h) of the Ti₃C₂T_x/MAPbI₃ [77]. Reprinted with permissions from Refs. [49,63,77].

Figure 1. Schematic diagram (a–c) for the preparation of Ti₃C₂T_x MXene and SEM image (d) [49]. Schematic illustration for the preparation of Ti₃C₂T_x MXene (e) [63]. PXRD (f) of Ti₃C₂T_x MXene. UV-vis spectra (g) and J-V curves (h) of the Ti₃C₂T_x/MAPbI₃ [77]. Reprinted with permissions from Refs. [49,63,77].

3. Ti₃C₂T_x MXene in PSCs

3.1. Ti₃C₂T_x MXene as Additive for PSCs

The Ti₃C₂T_x MXene has excellent stability under ambient conditions and moisture. Perovskite materials are sensitive to the moisture and ambient conditions. Therefore, the use of Ti₃C₂T_x MXene as additive to the perovskite material may significantly improve the stability of the perovskite materials. Hence, Ti₃C₂T_x MXene can be used as additive for the development of highly stable PSCs. Therefore, Ma and his research team investigated the role of the synthesized Ti₃C₂T_x MXene as an additive to control the crystallization of the absorber layer (MAPbI₃; where M = CH₃NH₃⁺) for PSCs [77]. Authors used the acidic etching method for the preparation of Ti₃C₂T_x MXene. The powder X-ray diffraction pattern (PXRD) of the Ti₃C₂T_x MXene showed the well-defined diffraction peaks (Figure 1f) which confirmed the formation of the Ti₃C₂T_x MXene phase. Further, authors developed PSCs using tin oxide (SnO₂) as an electron transport layer and introduced Ti₃C₂T_x MXene as an additive to improve the charge transportation process. Authors introduced different weight percentages of Ti₃C₂T_x to the SnO₂ layer and observed that the introduction of Ti₃C₂T_x enhanced the optical properties of the SnO₂ layer (Figure 1g). The developed PSCs with Ti₃C₂T_x as an additive exhibited the highest PCE of 16.80% (Figure 1h). This enhanced PCE may be due to the better electron transfer and lower charge transfer resistance.

In another report published in 2019, Agresti et al. [78] proposed novel strategies to tune the work function of the Ti₃C₂T_x MXene. The PCE of Ti₃C₂T_x MXene incorporated MAPbI₃-based PSCs, which can be further improved. In this work, authors investigated the critical role of Ti₃C₂T_x MXene for work function tuning and interfacial engineering of PSCs. The Ti₃C₂T_x MXene with different termination groups (Tx) was employed to engineer the perovskite/electron transport layer interface and tune the work function of the perovskite light absorber and TiO₂ electron transport layer. Authors found that MXenes can efficiently tune the work function of perovskite light absorber and electron/hole transport layers without affecting other properties of the perovskite or electron transport layers. This showed the potential applications of MXene in the development of PSCs. However, the critical role of MXenes in the work function or interfacial engineering needs to be further investigated for various optoelectronic applications [79]. In other work, Zhang et al. [80] decorated MAPbBr₃ by few-layered Ti₃C₂T_x sheets by employing the in situ solution growth method.

3.2. Ti₃C₂T_x MXene as ETL for PSCs

Since efficient electron transportation is crucial for achieving high efficiency of the PSCs, ETL plays a vital role in the development of high-performance PSCs. The Ti₃C₂T_x MXene has excellent properties such as high electrical conductivity, which is a desirable feature for an ETL. The ETL with high electrical conductivity provides the better electron transportation and reduces the electron recombination process. Ti₃C₂T_x MXene has suitable electron affinity, which may be useful to collect and transport the electrons efficiently. The physiochemical properties of the Ti₃C₂T_x MXene can be tuned or modified through surface functionalization or by incorporating it into composite materials. These features suggested that Ti₃C₂T_x MXene may be used as ETL in the development of PSCs. The performance of the PSCs largely depends on the electron transport layer, perovskite light absorber layer, and hole transport layer. Hence, Chen et al. [81] developed Ti₃C₂T_x quantum dots to engineer the perovskite/electron transport layer interface and an improved PCE of 21.64% was reported. Yang et al. [82] prepared a novel electron transport layer (Ti₃C₂T_x/SnO₂) to improve the photovoltaic performance of the PSCs. Ti₃C₂ MXene was prepared by etching of Ti₃AlC₂ as shown in Figure 2a. The developed PSCs using Ti₃C₂T_x/SnO₂ as an electron transport layer exhibited the enhanced PCE of 18.34%. The lowest PCE of 5.28% was achieved for the Ti₃C₂ MXene-based PSCs device, while a relatively high PCE of 17.23% was achieved for the SnO₂-based PSCs device. This showed that the presence of Ti₃C₂T_x MXene enhanced the electron transportation and an improved PCE of 18.34% was achieved [82]. The energy level diagram of the Ti₃C₂T_x/SnO₂-based PSCs is presented in Figure 3a. The incorporation of Ti₃C₂T_x/SnO₂ as an electron transport layer not only improves the charge extraction but also enhances the photovoltaic performance of the developed PSCs. Huang et al. [83] also developed a multi-dimensional conductive network (MDCN) electron transport layer using Ti₃C₂T_x MXene for the construction of high-performance PSCs (Figure 2b). The enhanced PCE of 18.44% was reported for the MDCN-based PSCs device. Moreover, the developed PSCs device showed good stability in air for more than 45 days. Wang et al. [84] also developed a PSCs device using a Ti₃C₂T_x MXene-modulated electrode/SnO₂ interface. The Ti₃C₂T_x MXene was prepared by the chemical exfoliation method and employed as charge transport material. The energy level values of the Ti₃C₂T_x MXene were found to be well suited with the energy level values of the SnO₂ (Figure 3b).

The developed PSCs device exhibited a high PCE of 20.6% with good stability up to 3 months. The introduction of Ti₃C₂T_x MXene enhanced the device stability as well as photovoltaic performance. This suggested the potential application of Ti₃C₂T_x MXene in the development of PSCs. In 2021, Yang et al. [85] prepared Ti₃C₂T_x MXene and investigated its properties for photovoltaic applications. The schematic picture of the developed PSCs is presented in Figure 3d. The energy level values of the Ti₃C₂T_x MXene and other components have been summarized in Figure 3d. Authors found that energy level values of the Ti₃C₂T_x MXene are well-matched with the energy values of the ITO and TiO₂. The developed PSCs using Ti₃C₂T_x MXene showed the highest PCE of 18.29%. In other work, an interesting PCE of 15.71% was also obtained using Ti₃C₂T_x MXene as dopant [86]. Yang et al. [63] developed the planar structured PSCs using Ti₃C₂T_x MXene as an electron transport layer. The schematic diagram of the fabricated PSCs is depicted in Figure 4a. The cross-sectional SEM image of the developed PSCs is presented in Figure 4b, which is clearly showing the presence of interlayers. Yang et al. [63] investigated the effect of UV-ozone treatment. The observations showed that UV-ozone treatment slightly influenced the work function of the Ti₃C₂T_x MXene (Figure 4c). The UV-ozone treatment enhanced the surface Ti-O bonds without affecting electron mobility, which suggested its potential use as an electron transport layer. The developed PSCs using Ti₃C₂T_x MXene as an electron transport layer exhibited the excellent PCE of 17.17% [63]. In some other recent reports, Ge et al. [87] investigated the role of Ti₃C₂ quantum dots in PSCs and obtained the highest PCE of 16%.

3.3. Ti₃C₂T_x MXene as HTL for PSCs

The high conductivity of Ti₃C₂T_x MXene may also be useful for its potential applications as HTL for the development of PSCs. The high conductivity Ti₃C₂T_x MXene may facilitate the hole extraction and transportation, which alternatively improve the efficiency of the PSCs. Thus, Ti₃C₂T_x MXene can be used as HTL for the development of PSCs. Cao et al. [79] also developed electrodes using 2D MXenes for the construction of HTL-free PSCs applications. The MXene-incorporated electrode-based PSCs exhibited the good PCE of 13.8% [79]. Saranin et al. [88] developed inverted p-i-n PSCs using Ti₃C₂T_x MXene decorated with NiO and obtained a PCE of 19.2%. The recent progress in the enhancement of the PCE is depicted in Figure 4d. The above results showed that Ti₃C₂T_x MXene has the potential for photovoltaic applications. The photovoltaic performances of the previously reported PSCs with Ti₃C₂T_x MXene are summarized in

Table 1. Photovoltaic performance of the recently reported PSCs.

S. No.	Device Structure	Voc (V)	Jsc (mA/cm2)	FF	PCE (%)	References
1.	ITO/Ti3C2Tx/CH3NH3PbI3/ Spiro-OMeTAD/Ag	1.08	22.63	0.70	17.17	[63]
2.	ITO/SnO2/perovskite: Ti3C2Tx/Spiro-MeOTAD/Au	1.03	22.26	0.76	17.41	[77]
3.	FTO/c-TiO2 +MXene/ m-TiO2 + MXene/MXene/perovskite + MXene/ spiro-OMeTAD/Au	1.09	23.82	0.77	20.14	[78]
4.	FTO/TiO2/CH3NH3PbI3/ MXene	0.95	22.96	0.63	13.83	[79]
5.	c-TiO2/m-TiO2-TQD/perovskite/ Spiro-OMeTAD-Cu1.8S	1.13	23.64	0.77	21.72	[81]
6	ITO/SnO2-Ti3C2 MXene/MAPbI3/Spiro-OMeTAD/ Ag	1.06	23.14	0.75	18.34	[82]
7.	FTO/SnO2-MXene/ (FAPbI3)0.97(MAPbBr3)0.03/spiro-OMeTAD	1.07	24.52	0.77	19.14	[83]
8.	FTO/MXene-SnO2/Perovskite/Spiro- OMeTAD/Au	1.11	24.34	-	20.65	[84]
9.	ITO/HO-Ti3C2Tx@Ti3C2Tx/CH3NH3PbI3/ Spiro-OMeTAD/Ag	1.07	23.11	0.74	18.29	[85]
10.	ITO/SnO2/(BA)2(MA)4Pb5I16-Ti3C2 MXene/Spiro-OMeTAD/Ag	1.11	20.87	0.67	15.71	[86]
11.	FTO/c-TiO2/ m-TiO2-2D MXene/ perovskite-0D Ti3C2 QDs/Spiro-OMeTAD/Au	0.92	19.6	0.66	17.1	[87]
12.	FTO/SnO2/perovskite: Ti3C2Tx/Spiro-MeOTAD/Au	1.12	23.48	0.73	19.27	[89]
13.	ITO/SnO2-MQDs/perovskite/ Spiro/MoO3/Au	1.17	24.96	0.79	23.34	[90]
14.	FTO/Ti3C2Tx@TiO2 (0.2 wt%)/Cs2AgBiBr6/Spiro/MoO3/Ag	0.96	4.14	0.70	2.81	[91]
15.	FTO/TiO2/CsPbBr3/ Ti3C2-MXene/C	1.44	8.54	0.73	9.01	[92]
16.	FTO/c-TiO2/CsPbBr3/C+ CNTs+MXene	1.35	7.16	0.72	7.09	[93]

.

4. Electrochemical Sensing Applications

4.1. Electrochemical Properties of Ti₃C₂ MXene

The distinctive attributes of MXene (with Ti₃C₂ being the predominant variant) in contrast to other two-dimensional substances have led to its growing application in fabricating electrochemical sensors. These distinctive features can be outlined as follows.

MXene boasts a distinct benefit in terms of its high electrical conductivity, a crucial factor for enhancing the speed of electron transfer in heterogeneous reactions when compared to other 2D materials [94]. This exceptional conductivity characteristic forms a fundamental basis for its application in electrochemical sensors. The synthesis of MXene with favorable solution dispersibility and stability is straightforward. This is of paramount importance for creating electrochemical sensors, given that the predominant technique for preparing modified electrodes involves pre-preparation of a well-dispersed coating solution for drop-casting. MXene serves as a robust material for substrate applications in printing scenarios. The methods of printing and pre/post-patterned coating offer an array of uncomplicated, economically viable, adaptable, and environment-friendly manufacturing methods for devices. Printing facilitates intricate 3D structures and multi-functional qualities, highly sought after in diverse applications. Hence, the introduction of MXene could propel printing/coating towards a more potent tool for fabricating devices and advancing industrial processes [95]. Leveraging its high stretch capacity and compatibility with living organisms, MXene stands as an excellent substrate choice for producing flexible conductive platforms. These platforms hold significant potential in crafting wearable electrochemical sensors, a pressing need for health monitoring and clinical analysis. With its two-dimensional layered structure and distinctive surface featuring numerous chemical groups, MXene displays considerable promise for integration with various functional materials or biomolecules for diverse analytical objectives. Various MXene-based nanostructures showcase a range of distinct and vibrant properties, offering opportunities for designing electrochemical sensors or devices with assorted functions, particularly in the realm of novel ECL or PEC sensors [96]. MXene’s compatibility with biomolecules such as enzymes, proteins, and nucleic acids, coupled with its non-toxic nature, renders it as an excellent carrier in biosensors or biomedical applications. The exceptional photothermal conversion capability of MXene enables the realization of a dual-mode detection strategy in electrochemical sensor design. This expands the array of signal strategies available for electrochemical sensors, as evidenced by recent advancements in this field [97]. Herein, we have compiled recent works on the development of Ti₃C₂-based biosensors.

4.2. Ti₃C₂-Based Enzymatic Biosensors

The fundamental aspect of constructing electrochemical biosensors involves the essential direct transfer of electrons (DET) between enzymes and electrodes. MXene materials exhibit a range of unique characteristics, such as a high specific surface area and remarkable electrical conductivity [98]. Hence, the integration of MXene could potentially serve as an effective approach to promote the electron transfer process. The pioneering MXene in this context, namely Ti₃C₂, was employed in developing electrochemical sensors. Notably, the inaugural Ti₃C₂-based electrochemical sensor, crafted in 2014, was an enzyme-based biosensor, designed to detect H₂O₂ [99]. In this regard, Ti₃C₂ MXene was utilized for immobilizing the enzyme hemoglobin (Hb). This immobilization showcased not only proficient enzyme immobilization capabilities but also furnished an advantageous microenvironment for sustaining the activity and stability of the protein. Furthermore, it facilitated the direct transfer of electrons within Hb, underscoring the efficacy of anchoring enzymes onto the surface of Ti₃C₂ MXene as a means to produce mediator-free enzyme-centric biosensors. Endeavors were also undertaken to immobilize other enzymes, such as acetylcholinesterase (AChE) and tyrosinase, onto the surface of Ti₃C₂ MXene [100,101]. These endeavors highlighted that Ti₃C₂ MXene, with its expansive specific surface area, favorable biocompatibility, hydrophilic surface, and exceptional metallic conductivity, indeed stands as a promising choice for serving as an excellent platform for immobilizing enzymes in the construction of enzyme-based biosensors. To enhance the performance of enzyme-based biosensors, many researchers have employed a strategy involving the integration of Ti₃C₂ MXene with various functional materials, particularly diverse nanomaterials, to form composite structures. For instance, Wang et al. improved the Ti₃C₂ MXene by attaching TiO₂ nanoparticles (NPs), resulting in an increased surface area for protein adsorption and preservation of enzymatic stability and activity [102]. This modified Ti₃C₂ MXene was utilized to create a biosensor for Hb (hemoglobin), exhibiting superior detection capabilities for H₂O₂, with a limit of detection (LOD) of 14 nM, surpassing the performance of the TiO₂NP-free biosensor. Moreover, Rakhi et al. developed a glucose oxidase (GOx)-based biosensor by constructing a nanocomposite of Au/Ti₃C₂ MXene [103]. The integration of Au NPs endowed the composite with distinctive electrocatalytic properties through synergistic effects. The resulting GOx/AuNPs/Ti₃C₂/Nafion/GCE biosensor showcased a relatively high amperometric sensitivity of 4.2 μA mM⁻¹ cm⁻² and an LOD of 5.9 μM for glucose detection. Similarly, Jiang et al. [104] employed an Ag@Ti₃C₂ nanocomposite as a carrier for AChE (acetylcholinesterase), fabricating a biosensor for malathion detection. In a separate study, Song et al. [105] developed an AChE-based biosensor for the identification of organophosphorus pesticide (OP) methamidophos. This biosensor was founded on a three-dimensional (3D) composite structure of MnO₂@Mn₃O₄/MXene/AuNPs, achieving a remarkably low LOD of 0.134 pM for methamidophos. In the realm of glucose biosensing, the enzymatic process of glucose oxidation generates a potentially harmful byproduct, H₂O₂ [57]. This byproduct, however, frequently hinders the effectiveness of GOx (glucose oxidase) in practical applications. In order to address this concern, Wu, M. et al. devised a novel solution [57]. They engineered a hybrid nanoreactor utilizing a combination of Ti₃C₂, poly-L-lysine (PLL), and glucose oxidase (GOx) (Figure 5a,b). This nanoreactor exhibited the capability to drive both the sequential reactions of glucose oxidation and the subsequent breakdown of H₂O₂.

Interestingly, the Ti₃C₂ MXene component demonstrated proficiency in catalyzing the breakdown of H₂O₂. By incorporating GOx onto this platform, a cascading reaction for glucose oxidation was initiated. To realize this concept, the researchers fabricated Ti₃C₂/PLL/GOx nanoreactors, distinguished by their exceptional catalytic performance. These nanoreactors were then affixed to a glassy carbon electrode, creating a glucose biosensor with an impressive limit of detection (LOD) of 2.6 μM. Another notable advancement was accomplished by Wang et al. [106], who established a dual-enzyme biosensor for inosine monophosphate (IMP) detection. In this approach, Ti₃C₂ MXene was combined with Au@Pt nanoflowers to harness robust catalytic abilities. Subsequently, two enzymes (5′-nucleotidase and xanthine oxidase) were immobilized on this composite. The resultant biosensor displayed an LOD of 2.73 ng/mL for inosine monophosphate detection in meat samples. The Ti₃C₂ MXene has the potential to serve as a constituent in conductive foundations within electrode arrangements beyond the glassy carbon electrode (GCE). Researchers created a composite called Ti₃C₂/graphene oxide (Ti₃C₂-GO), which was employed in crafting an inkjet-printed biosensor for hydrogen peroxide detection. This study revealed that the printable Ti₃C₂-GO composite exhibited remarkable capabilities as a sensing platform for electrochemical analysis [107]. Additionally, they developed a hybrid composite, Pt/PANI/MXene, by combining platinum particles, polyaniline, and Ti₃C₂ MXene [108]. This composite was utilized to modify a screen-printed carbon electrode (SPCE) to formulate a biosensor capable of detecting both hydrogen peroxide and lactate. The resulting SPCE demonstrated a low detection limit of 1.0 μM for H₂O₂. Following the immobilization of lactate oxidase, the biosensor facilitated lactate detection through amperometric measurements, achieving a detection limit of 5.0 μM for lactate. This biosensor proved its applicability for lactate measurement in milk samples, showcasing strong durability and dependability. The reported biosensors using Ti₃C₂ are compiled in

Table 2. Sensing performance for enzymatic biosensors.

Materials	Sensing Analyte	Sensing Technique	Linear Range	LOD	References
Ti3C2	Glucose	CV	0.02–1.1; 4.0–20 mM	2.6 μM	[57]
Ti3C2	β-hydroxybutyrate	Amperometric	0.36–17.9 mM	45 μM	[58]
Ti3C2	Glucose	Amperometric	50–27,750 μM	23 μM	[59]
Ti3C2/Au-PdNPs	Paraoxon	Amperometric	0.1–1000 μg/L	1.75 ng/L	[60]

.

Zhao et al. [60] also fabricated a biosensor for the determination of pesticides. Figure 6 exhibits the fabrication of the biosensor. In this research, bimetallic nanoparticles consisting of a combination of gold and palladium (Au-Pd NPs) were synthesized through self-reduction occurring on the surface of ultrathin MXene nanosheets (Ti₃C₂T_x).

The resulting multi-dimensional nanocomposites (MXene/Au-Pd) demonstrate excellent conductivity and stability that prove advantageous for facilitating electron transfer and enzyme immobilization. By incorporating these nanocomposites into a disposable screen-printed electrode (SPE), a high-performance enzymatic biosensor was developed for swiftly detecting organophosphates (OPs). The electrochemical platform relies on the use of MXene/Au-Pd nanocomposites, as illustrated in Figure 6. The specific model pesticide chosen for this study was paraoxon, due to its high toxicity and the potential for its conversion from other OPs.

Figure 7a,b depict the Differential Pulse Voltammetry (DPV) outcomes of the SPE/MXene/Au-Pd under varying conditions: (a) diverse durations of Au-Pd NP growth, and (b) distinct ratios of Au³⁺ to Pd²⁺ concentrations, all conducted in a 0.1 M KCl solution containing 5.0 mM K₃[Fe(CN)₆]. In Figure 7a, the electric current response progressively rises until the 5-min mark, beyond which it declines. This decrease is possibly attributable to the congestion of NPs, which affects the kinetics of electron transfer. Evaluating both the morphology of the NPs and the DPV response, the researchers selected a growth period of 5 min as the optimal timeframe for Au-Pd NP formation. Additionally, the researchers fine-tuned the ratio of Au³⁺ to Pd²⁺ precursor concentrations. The most substantial DPV response occurs when the ratio stands at 1:2, as demonstrated in Figure 7b. Considering these findings, the researchers opted for the 1:2 concentration ratio for the Au³⁺-Pd²⁺ precursor. The resultant sensor displayed commendable performance.

4.3. Electrochemical Non-Biosensors

MXene could be utilized in creating electrochemical non-biosensors which do not require the immobilization of enzymes. In 2018, the initial electrochemical non-biosensor utilizing pristine Ti₃C₂ MXene was manufactured. This sensor was designed to detect the contaminant BrO₃⁻ in drinking water [109]. In this application, Ti₃C₂ MXene served as both a signal-enhancing matrix and a reducing agent, displaying remarkable electrocatalytic characteristics that facilitated effective reduction of BrO₃⁻. Following this, two electrochemical sensors similar to the first one were developed using Ti₃C₂ MXene-modified glassy carbon electrodes (GCE). These sensors were used for the detection of the pesticide carbendazim [110] and the neurotransmitter dopamine [111]. In the case of carbendazim detection, the sensor achieved carbendazim redox at lower overpotentials compared to a graphene-based sensor [112]. The dopamine sensor demonstrated robust sensitivity, with a limit of detection (LOD) of around 3 nM for dopamine detection in actual samples. The incorporation of MXenes into other electrode systems, such as graphite composite paste electrodes (GCPE) [113] and screen-printed electrodes (SPE) [114], was also explored. An MXene/GCPE electrochemical sensor was devised for the detection of adrenaline. Notably, this was the first instance of introducing Ti₂C MXene into an electrochemical sensing system. The developed sensor achieved an LOD of 9.5 nM and could be further applied for detecting adrenaline in pharmaceutical samples, recovering between 99.2 and 100.8%. Another MXene-based electrochemical sensor was created using a screen-printed electrode (SPE) configuration for simultaneous voltammetric determination of acetaminophen (ACOP) and isoniazid (INZ). The Ti₃C₂ MXene displayed excellent electrocatalytic performance in the oxidation of ACOP and INZ compared to a bare SPE electrode in 0.1 M H₂SO₄. The distinct oxidation peak potentials allowed simultaneous detection of both targets. Consequently, this sensor attained LODs of 0.048 μM and 0.064 mM for ACOP and INZ, respectively. Additionally, the utilization of Ti₃C₂ MXene extended to the creation of various composites or combinations involving alternative substances or molecules, serving distinct analytical objectives. For instance, a hybrid of NiO and Ti₃C₂ was employed for detecting H₂O₂ without enzyme reliance [115]. Another example involves a three-dimensional porous composite of Ti₃C₂ and NiCo-LDH (Nickel-Cobalt layered double hydroxide) for glucose sensing without enzymes [116]. Furthermore, a composite of Au NPs and Ti₃C₂ was devised to sensitively detect nitrite [117]. A nanocomposite featuring Pd deposited on Ti₃C₂ was developed for swift, real-time detection of l-cysteine (l-Cys) [118]. A composite of Mn₃(PO₄)₂ and Ti₃C₂, synthesized using adenosine triphosphate (ATP) as a template, was applied for amperometric detection of superoxide anions O₂^•− released from HepG2 cells [119]. Another innovation involved a self-assembled nanocomposite of Ti₃C₂ and MWCNTs (multi-walled carbon nanotubes) for simultaneous electrochemical detection of hydroquinone (HQ) and catechol (CT) [120]. Moreover, a composite of methylene blue (MB), Cu NPs, and Ti₃C₂ was created for ratiometric electrochemical detection of piroxicam [121]. Throughout these studies, the performance of Ti₃C₂ MXene consistently proved highly effective in the design and operation of electrochemical sensors. The tendency of MXene to stack together is driven by hydrogen bonding and van der Waals interactions among its layers. This stacking can significantly reduce the effective surface area, thereby limiting its electrochemical performance. To counteract this issue, researchers like Tu et al. [122] introduced carbon nanohorns (CNHs) as spacers and created Ti₃C₂/CNHs nanocomposites. This layered MXene/CNHs structure displayed excellent conductivity, enhanced catalytic activity, and increased pathways for ion diffusion. Using this nanocomposite, they developed an electrochemical sensor for carbendazim with an impressive low detection limit of 1.0 nM. Similarly, Huang et al. [123] employed nitrogen-doped porous carbon derived from MOF-5-NH₂ (N-PC) as a spacer to prevent restacking of Ti₃C₂ MXene sheets, as shown in Figure 8.

Authors fabricated an alk-Ti₃C₂/N-PC electrochemical sensor for detecting hydroquinone (HQ) and catechol (CT) in industrial wastewater. In addition to using N-PC to prevent restacking, they treated the Ti₃C₂ MXene sheets with an alkaline intercalation process. This treatment led to the presence of abundant -OH groups on the MXene surface, which facilitated hydrogen-bond interactions for sensing HQ and CT. The resulting alk-Ti₃C₂/N-PC electrochemical sensor displayed low detection limits of 4.8 nM for HQ and 3.1 nM for CT, covering a broad linear range from 0.5 to 150 μM. Selectivity poses a challenge in the practical use of electrochemical sensors for targeted measurements. Molecularly imprinted polymers (MIPs) offer a solution to enhance sensor selectivity through specific recognition properties, cost-effectiveness, and quick synthesis. Ma et al. [124] developed a sensitive and selective electrochemical sensor for detecting fisetin using a hierarchical porous Ti₃C₂ MXene/amino carbon nanotubes (MXene/NH₂-CNTs) composite combined with MIP (Figure 9).

The composite was formed by assembling negatively charged Ti₃C₂ flakes and positively charged NH₂-CNTs. The amino-functionalized CNTs not only provided good conductivity but also introduced positive charges on their surface, acting as spacers to prevent Ti₃C₂ MXene aggregation. The resultant MIP/Ti₃C₂ MXene/NH₂-CNTs/GCE sensor exhibited excellent analytical performance for fisetin detection, achieving a low LOD of 1.0 nM. Additionally, numerous advancements have been made in the field of electrochemical non-biosensors utilizing MXene-based materials. To facilitate the real-time and in situ detection of hydrogen peroxide (H₂O₂) released by living cells, Dang and colleagues [125] developed an electrochemical biosensor. This biosensor was constructed using titanium carbide (Ti₃C₂) nanosheets intercalated with Prussian blue nanoparticles (PB NPs/Ti₃C₂), achieving a notably low limit of detection (0.20 μM) for H₂O₂. Moreover, the PB NPs/Ti₃C₂ composite exhibited minimal harm to normal fibroblast cells across different time intervals and concentrations, underscoring its potential applicability in areas concerning living cells. In the realm of sulfadiazine detection, Kokulnathan et al. [126] synthesized nanocomposites of Ti₃C₂ and boron nitride (Ti₃C₂/BN) and designed an electrochemical catalytic sensor by modifying an electrode with Ti₃C₂/BN. This sensor exhibited a remarkably sensitive limit of detection (3.0 nM) for sulfadiazine. Kalambate and co-workers [127] developed an electrochemical sensor capable of detecting ifosfamide (IFO), acetaminophen (ACOP), domperidone (DOM), and sumatriptan (SUM). This sensor relied on a self-assembled nanocomposite thin film consisting of MXene, multi-walled carbon nanotubes (MWCNT), and chitosan. The limits of detection achieved for IFO, ACOP, DOM, and SUM were 0.00031, 0.00028, 0.00034, and 0.00042 μM, respectively. The reported sensors are summarized in

Table 3. Sensing performance for non-biosensors.

Materials	Sensing Analyte	Sensing Technique	Linear Range	LOD	References
Ti3C2/PBNPs	Hydroquinone (HQ)	DPV	-	4.8 nM	[123]
Ti3C2/PBNPs	Catechol (CT)	DPV	0.5–150 μM	3.1 nM	[123]
Ti3C2/NH2-CNTs	Fisetin	DPV	0.003–20.0 μM	1 nM	[124]
Ti3C2/BN	Sulfadiazine	DPV	0.01–44; 59–186 μM	3 nM	[126]
Ti3C2/MWCNT	Ifosfamide (IFO)	Adsorptive Stripping	0.0011–1.0 μM	0.00031 μM	[127]
Ti3C2/AuNPs	Folic acid (FA)	Amperometric	0.02–3580 μM	6.2 nM	[128]
Ti3C2/AuNPs	Uric acid (UA)	Amperometric	0.03–1520 μM	11.5 nM	[128]
Ti3C2/TiO2	NO2−	DPV	0.003–0.25 mM	850 nM	[129]
Ti3C2/BiNPs	Pb2+	SWV	0.06–0.6 μM	10.8 nM	[130]
Ti3C2/ZIF-8	Hydrazine	Amperometric	10 μm to 7.7 mM	5100	[131]

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5. Conclusions and Future Perspective

MXenes, especially Ti₃C₂T_x MXene, possess unique optoelectronic properties, and an adjustable composition/structure. The Ti₃C₂T_x MXene is one of the most important 2D materials which has been used in various applications. The work function of the Ti₃C₂T_x MXene can be easily tuned, which makes it the most suitable candidate as an electron and hole transport layer for perovskite solar cells. In the last 3–4 years, Ti₃C₂T_x MXene has been used as an additive, electron/hole transport layers, and electrode materials for the development of perovskite solar cells. The reported literature showed that the incorporation of Ti₃C₂T_x MXene not only enhances the photovoltaic performance of the perovskite solar cells but also improves long term stability. Therefore, it is clear that it would be beneficial to develop the novel device architectures of the perovskite solar cells with Ti₃C₂T_x MXene-assisted metal oxide as an electron transport layer. This may improve the photovoltaic performance as well as long term stability of the perovskite solar cells. In the realm of future applications of MXene in electrochemical sensors, there are several potential research avenues. Firstly, in the context of biosensors, MXene is often utilized as a carrier for biomolecules. The common methods for immobilizing biomolecules on MXene surfaces involve the assistance of Au nanoparticles or electrostatic adsorption. However, we believe that establishing a direct chemical linkage between biomolecules and MXene holds greater value due to the enhanced stability of resulting biocomposites and the rich surface chemistry of MXene. This underscores the significance of surface functionalization techniques like amination or carboxylation of MXene. Further exploration of diverse combinations in this regard is warranted. In summary, leveraging MXene as a powerful tool opens up the possibility of developing more advanced electrochemical sensors in the future.