Boosting the Photocatalytic Ability of TiO2 Nanosheet Arrays for MicroRNA-155 Photoelectrochemical Biosensing by Titanium Carbide MXene Quantum Dots

The electrodes of two-dimensional (2D) titanium dioxide (TiO2) nanosheet arrays were successfully fabricated for microRNA-155 detection. The (001) highly active crystal face was exposed to catalyze signaling molecules ascorbic acid (AA). Zero-dimensional (0D) titanium carbide quantum dots (Ti3C2Tx QDs) were modified to the electrode as co-catalysts and reduced the recombination rate of the charge carriers. Spectroscopic methods were used to determine the band structure of TiO2 and Ti3C2Tx QDs, showing that a type Ⅱ heterojunction was built between TiO2 and Ti3C2Tx QDs. Benefiting the advantages of materials, the sensing platform achieved excellent detection performance with a wide liner range, from 0.1 pM to 10 nM, and a low limit of detection of 25 fM (S/N = 3).


Introduction
The ultrasensitive, rapid, and accurate detection of microRNA is very meaningful for the early diagnosis and prevention of disease [1]. Research has shown that the aberrant expression of microRNA-155 in the human body can be regarded as a critical detection index for some diseases, such as B-cell lymphoma [2] and breast cancer [3]. However, microRNA-155 is expressed only at the DNA level and not at the protein level; therefore, detecting microRNA-155 by traditional methods for early warning is very difficult [4]. Photoelectrochemical (PEC) biosensing is now attracting extensive attention for sensing nucleic acid and other diagnostic markers because of its inherently low limit of detection and high sensitivity. Generally speaking, there are two important parts in PEC biosensing [5]: (i) the PEC biosensing active species (catalytic signaling molecule to generate the detection signal) and (ii) the biological recognition elements (which are in contact with the active species). Therefore, active materials are very important for photoelectrochemical biosensing.
TiO 2 is one of the most charming candidates for PEC biosensing due to its outstanding chemical stability, biocompatibility, and accessibility [6]. Titanium dioxide nanomaterials have been widely used in biological monitoring [7,8]. Sadly, pristine TiO 2 suffers from a high carrier recombination rate, which significantly hinders the signal generation and collection of PEC sensors [9]. Coupling TiO 2 with other semiconductors can achieve spatial separation of the photogenerated charges [10]. Proper band alignment and electron trapping would increase the concentration and lifetime of the photogenerated charges, thereby improving the catalytic ability of the material [11][12][13]. For this purpose, the interface between the two materials needs to be rationally designed. In principle, the morphology and contacting pattern of active species have to be rationally considered to maximize the contact area while reducing the interfacial defects caused by lattice mismatch between the two phases. On the one hand, the optoelectronic properties of composite materials are closely related to the configuration between the materials. For instance, compared with other forms of allotropes (such as graphene and carbon nanotubes), 0D carbon materials (such as carbon quantum dots) exhibit unique optoelectronic properties when combined with TiO 2 . On the other hand, the charge behavior of the materials is different when the heterojunction is built on different exposed crystal planes because (i) the generation rates of the photogenerated carriers on different crystal planes are different, and (ii) different work functions of different crystal planes can change the direction of electron flow between the heterojunctions [14].
Since their discovery in 2011, MXene materials have come into the spotlight due to their chemical stability, rapid charge-transfer kinetics, and tight interfacial coupling. Quantum dots derived from 2D materials exhibit excellent properties as compared to their 2D counterparts, such as more abundant active edge sites, bandgap widening, and tunable physicochemical properties [15]. In addition, compared with the other QDs, Ti 3 C 2 T x QDs possess more abundant surface hydrophilic groups (-O and -OH), making them connect tightly with photoactive supporters. Hence, the Ti 3 C 2 T x QDs could be a good co-catalyst for boosting the performance of the PEC biosensor. Song et al. employed Ti 3 C 2 T x QDs as a photoactive material to promote the performance of TiO 2 -based PEC sensing.
Herein, a PEC biosensing platform was fabricated for microRNA-155 detection. Twodimensional TiO 2 NS arrays were selected as the sensing active substrate. The exposed (001) crystal face of TiO 2 enables the material to have higher catalytic performance. The Ti 3 C 2 T x QDs were used as a co-catalyst for the photocatalysis of ascorbic acid (AA) and to suppress the recombination of the charge carriers inside the electrode. Their appropriate energy-band structure enables them to form a type II heterojunction with TiO 2 to achieve efficient separation of electrons and holes. The S9.6 antibody was used as the microRNA recognition unit to identify DNA-RNA hybrid duplexes, and alkaline phosphatase (ALP) served as the catalytic signal generation unit. With reasonable material selection and interface design, excellent sensing performance can be expected.

Synthesis of Ti 3 C 2 T x MXene QDs
An amount of 1 g Ti 3 AlC 2 was slowly added into 10 mL concentrated hydrofluoric acid solution (40 wt%). The mixture was stirred for 12 h to fully etch the aluminum atomic layer in Ti 3 AlC 2 . Afterward, the mixture was centrifuged until the pH was near neutral. After vacuum filtration, the sample was vacuum dried at 200 • C overnight. Then, 0.1 g of Ti 3 C 2 powder was added to 10 mL of tetramethylammonium hydroxide (TMAOH, 1 wt%) and stirred for 12 h. The TMAOH-intercalated Ti 3 C 2 powder was centrifuged at 8000 rpm, vacuum filtered, and vacuum dried at 200 • C. Finally, 50 mg of the sample was added to 10 mL solution of TMAOH (2.5 wt%). The suspension was refluxed at 110 • C for a whole day, centrifuged at 12,000 rpm, and vacuum dried at 200 • C.

Synthesis of Ti 3 C 2 T x QDs/(001) TiO 2 /FTO Electrode
Initially, 10 mL of concentrated hydrochloric acid was mixed with equal amounts of deionized water to configure a dilute solution. Then, 385 µL of tetrabutyl titanate and 0.158 g of ammonium fluorotitanate were added into the mixture with constant stirring until a transparent color solution formed. The fluorine-doped tin oxide (FTO) substrates were ultrasonically cleaned with a glass cleaner and poured into a Teflon reaction kettle with a perforated Teflon base. After heating at 170 • C for 12 h, the FTO substrates were rinsed with water. The (001) exposed TiO 2 NSs arrays were prepared after annealing in an air atmosphere at 450 • C for 3 h. Subsequently, Ti 3 C 2 T x QDs (20 mg) were dispersed in 20 mL of water. In order to carry out the self-assembly process, the substrates were dropped vertically into the solution. The solution was placed in an oven at 50 ºC overnight. The Ti 3 C 2 T x QDs slowly self-Organized onto the surface of TiO 2 NSs with the volatilization of water. Finally, the Ti 3 C 2 T x QDs/(001) TiO 2 /FTO electrodes were washed with ultra-pure water to remove the unconnected Ti 3 C 2 T x QDs.

Electrode Construction and Sensing Mechanism of PEC Sensor
As shown in Figure 1A, layered Ti 3 C 2 T x MXene were fabricated by a top-down method by etching the Al atomic layer in Ti 3 AlC 2 with HF. The Ti 3 C 2 T x QDs were prepared by the reflux hydrothermal method with TMAOH as the intercalating agent. Using ammonium fluorotitanate as a seed, (001) TiO 2 NSs were grown on FTO glass through the hydrolysis of titanate in an acidic solution ( Figure 1B). Ti 3 C 2 T x QDs and (001) TiO 2 NSs were joined together by a self-assembly process. The microRNA-155 detection process is shown in Figure 1C. Au NPs served as the reagent of the immobilization matrix for the thiol modified probe DNA. MCH was used for end capping of the electrode surface. After probe DNA hybridization with target RNA, rigid DNA:RNA double helix hybrids were combined with the S9.6 antibody. Afterward, the immunoreaction between the IgG and S9.6 antibody would lead to alkaline phosphatase immobilization. The alkaline phosphatase on the electrode surface could catalyze phosphorylated ascorbic acid in the detection solution to generate electron donor ascorbic acid, thereby increasing the electrode photocurrent and realizing the quantitative analysis of target microRNA-155.

Morphology Characterization of Ti 3 C 2 T x QDs/(001) TiO 2 /FTO Electrode
Atomic force microscopy was used to observe the topography and size of the quantum dots. From Figure S1, the average thickness of Ti 3 C 2 T x QDs was about~1.0 nm, indicating that they were mostly single layer. FESEM was used to study the morphology of the (001) TiO 2 NSs. The TiO 2 NSs with a side length of about 2 µm and a thickness of about 150 nm uniformly grew on the surface of FTO glass ( Figure S2). The FESEM images of the Ti 3 C 2 T x QDs/(001) TiO 2 composite are provided in Figure 2a,b. Compared with pure TiO 2 NSs, there were no significant changes in the morphology of the composite electrodes after loading with Ti 3 C 2 T x QDs. Transmission electron microscopy (TEM) images were provided to characterize the crystal information of TiO 2 NSs. The HRTEM image (Figure 2c insert, middle part) revealed (200) and (020) atomic planes with a lattice spacing of 0.19 nm and an interfacial angle of 90 • . The bright, periodically arranged diffraction spots in selected-area electron diffraction (SAED, Figure 2c insert, top right-hand corner) patterns indicated that the TiO 2 NSs prepared were a single crystal with excellent crystallinity [16]. Proofread with standard PDF cards, the main exposed crystal plane of TiO 2 nanosheets was (001) [17]. The introduction of Ti 3 C 2 T x QDs was further identified by TEM images. Compared with pure TiO 2 NSs, many small scales (~10 nm) appeared on the Ti 3 C 2 T x QDs/(001) TiO 2 composite Nanomaterials 2022, 12, 3557 4 of 12 ( Figure 2d). The HRTEM image of the Ti 3 C 2 T x QDs/(001) TiO 2 composite is presented in Figure 2e. The lattice fringes with widths of 0.19 and 0.21 nm can be assigned to the (200) plane of TiO 2 and the (100) plane of Ti 3 C 2 T x QDs. The elemental mapping dots of the Ti 3 C 2 T x QDs/(001) TiO 2 composite for Ti and O were dense and apparent (Figure 2f-i) because TiO 2 was dominant in this composite, whereas those for C were relatively scarce and primarily found around the sheet edges, indicating that Ti 3 C 2 T x QDs successfully combined with the (001) crystal plane of TiO 2 NSs.

Composition Characterization of Ti 3 C 2 T x QDs/(001) TiO 2 /FTO Electrode
XRD pattern, Fourier transform infrared (FTIR) spectroscopy, and XPS analyses were performed for electrode composition characterization. The XRD results in Figure 3a indicated that FTO had peaks at 26  . No distinct characteristic diffraction peak of Ti 3 C 2 T x QDs was found in the Ti 3 C 2 T x QDs/(001) TiO 2 sample, which was due to the low crystallinity and low content of the Ti 3 C 2 T x QDs in the composites [19]. To further determine the functional group information of Ti 3 C 2 T x QDs and TiO 2 , the Fourier transform infrared spectroscopy (FTIR) spectra of TiO 2 NSs, Ti 3 C 2 T x QDs, and Ti 3 C 2 T x QDs/(001) TiO 2 were presented in Figure S3. The (001) TiO 2 composite film had some characteristic peaks at 3439, 1633, 1380, and 1110 cm −1 , which were assigned to surface hydroxyl groups and adsorbed oxygen. Compared with pure TiO 2 , two new peaks emerged at 561 and 613 cm −1 after self-assembly, and they can be assigned to Ti-C and Ti-O, respectively [20].

Morphology Characterization of Ti3C2Tx QDs/(001) TiO2/FTO Electrode
Atomic force microscopy was used to observe the topography and size of the quantum dots. From Figure S1, the average thickness of Ti3C2Tx QDs was about ~1.0 nm, indicating that they were mostly single layer. FESEM was used to study the morphology of the (001) TiO2 NSs. The TiO2 NSs with a side length of about 2 μm and a thickness of about 150 nm uniformly grew on the surface of FTO glass ( Figure S2). The FESEM images of the Ti3C2Tx QDs/(001) TiO2 composite are provided in Figure 2a,b. Compared with pure TiO2

Composition Characterization of Ti3C2Tx QDs/(001) TiO2/FTO Electrode
XRD pattern, Fourier transform infrared (FTIR) spectroscopy, and XPS analyses were performed for electrode composition characterization. The XRD results in Figure 3a indicated that FTO had peaks at 26.58°, 33.77°, 37.77°, 51.76°, and 65.19°, consistent with SnO2 (JCPDS No. 46-1088) [18]. Meanwhile, the TiO2 NS arrays had diffraction peaks at 25.28°, 37.80°, 48.05°, and 55.06°, assigned to the anatase TiO2 diffraction peaks (JCPDS No. . No distinct characteristic diffraction peak of Ti3C2Tx QDs was found in the Ti3C2Tx QDs/(001) TiO2 sample, which was due to the low crystallinity and low content of the Ti3C2Tx QDs in the composites [19]. To further determine the functional group information of Ti3C2Tx QDs and TiO2, the Fourier transform infrared spectroscopy (FTIR) spectra of TiO2 NSs, Ti3C2Tx QDs, and Ti3C2Tx QDs/(001) TiO2 were presented in Figure S3. The (001) TiO2 composite film had some characteristic peaks at 3439, 1633, 1380, and 1110 cm −1 , which were assigned to surface hydroxyl groups and adsorbed oxygen. Compared with pure TiO2, two new peaks emerged at 561 and 613 cm −1 after self-assembly, and they can be assigned to Ti-C and Ti-O, respectively [20]. The chemical bonding and functional groups of (001) TiO 2 and Ti 3 C 2 T x QDs/(001) TiO 2 composite were also investigated by XPS spectrum. In Figure 3b, the high-resolution spectrum of Ti 2p of (001) TiO 2 revealed two peak components at 458.8 eV (2p 3/2 ) and 464.4 eV (2p 1/2 ). After loading Ti 3 C 2 T x QDs, the peak components of Ti 2p 3/2 and 2p 1/2 centered from low binding energy to high binding energy were attributed to the Ti-C, Ti-X from substoichiometric TiC x (x < 1) or Ti 3 AlC 2 , Ti 2+ ions and Ti 4+ ions, respectively [21]. The spectrum of O 1s had two peaks located at 530.98 and 529.83 eV (Figure 3c), which were assigned to the Ti-OH species and the lattice oxygen [Ti-O 6 ] species. As for the O 1s XPS spectra after Ti 3 C 2 T x QDs were loaded, two new peaks were found at 531.78 and 533.58 eV, ascribed to the Ti-C-OH and Ti-C-O species, demonstrating the surface groups of Ti 3 C 2 T x QDs were O and −OH [22]. The C 1s of (001) TiO 2 can be divided into three characteristic peak components located at 288.4 eV, 286.5 eV, and 284.7 eV, which can be assigned to O-C=O, C=O, and C-C. Compared with pure TiO 2 , the introduction of Ti 3 C 2 T x QDs led to the appearance of two new characteristic peaks. The characteristic peak at 282.3 can be assigned to the Ti-C inside the Ti 3 C 2 T x QDs. Interestingly, compared with (001) TiO 2 composite, a new component appeared at 283.03 eV after the self-assembly process, which could be assigned to the C−Ti−O x bonding at the interfaces between Ti 3 C 2 T x QDs and (001) TiO 2 (Figure 3d) [23]. We believe that the O and -OH on the surface of Ti 3 C 2 T x may act as rivet sites to connect to the five coordinated titanium atoms in (001) of TiO 2 and form an atomic-scale interfacial heterojunction between 0D Ti 3 C 2 T x QDs and 2D TiO 2 NSs.

PEC Performance Characterization of Ti 3 C 2 T x QDs/(001) TiO 2 /FTO Electrode
To evaluate the catalytic ability of the materials to catalyze AA, time-resolved current response curves were obtained in an aqueous O 2 -saturated PBS solution containing AA (0.1 M) under light irradiation (365 nm). In Figure 4a, the photoelectric response of TiO 2 NSs significantly improved after loading Ti 3 C 2 T x QDs. To explain the enhanced catalytic ability, the photoelectric properties of the catalysts were evaluated. Photoluminescence (PL) spectra were also obtained to reveal the recombination efficiency of the carriers. In general, fluorescence emission at 420 nm represents the recombination of free excitons inside a material, whereas fluorescence emission at 480 nm represents surface state-trapping recombination [18]. Compared with TiO 2 NSs, the emission intensity of Ti 3 C 2 T x QDs/(001) TiO 2 electrodes significantly decreased in both ranges (Figure 4b). The reduced recombination rate of photogenerated carriers could supply sufficient holes to activate -OH on Ti 3 C 2 T x QDs, thereby significantly promoting the formation of reactive species (·OH) during the photocatalytic redox reaction. Time-resolved photoluminescence (TRPL) spectroscopy was performed to survey the lifetime of the electrons in (001) TiO 2 and Ti 3 C 2 T x QDs/(001) TiO 2 electrodes. The average lifetime (τ ave ) for the (001) TiO 2 and Ti 3 C 2 T x QDs/(001) TiO 2 electrodes was 2.14 ns and 3.73 ns (Figure 4c). The carrier density of the electrodes was also investigated by the Mott-Schottky Equation (1) [23].
The carrier density N D can be obtained from the slope of the linear region of the Mott-Schottky plots (Figure 4d) on the basis of Equation (2).
where N D is the electron density, e is the element charge value, ε is the dielectric constant (48 for anatase), ε 0 is the vacuum permittivity, C is the space charge capacitance, and U S is the applied potential. The calculated N D for the (001) TiO 2 and Ti 3 C 2 T x QDs/(001) TiO 2 electrodes were 4.04 × 10 18 and 8.18 × 10 18 , respectively. The photoelectric property tests implied that the introduction of Ti 3 C 2 T x QDs could reduce the recombination rate, prolong the lifetime, and increase the density of the carriers in the electrode, thereby improving the catalytic ability of the electrode. The chemical bonding and functional groups of (001) TiO2 and Ti3C2Tx QDs/(001) TiO2 composite were also investigated by XPS spectrum. In Figure 3b, the high-resolution spectrum of Ti 2p of (001) TiO2 revealed two peak components at 458.8 eV (2p3/2) and 464.4 eV (2p1/2). After loading Ti3C2Tx QDs, the peak components of Ti 2p3/2 and 2p1/2 centered OH on Ti3C2Tx QDs, thereby significantly promoting the formation of reactive (·OH) during the photocatalytic redox reaction. Time-resolved photoluminescence spectroscopy was performed to survey the lifetime of the electrons in (001) Ti Ti3C2Tx QDs/(001) TiO2 electrodes. The average lifetime (τave) for the (001) TiO2 and QDs/(001) TiO2 electrodes was 2.14 ns and 3.73 ns (Figure 4c). The carrier density electrodes was also investigated by the Mott-Schottky Equation (1) [23].

Electron Transfer Mechanism of Ti 3 C 2 T x QDs/(001) TiO 2 /FTO Electrode
Ultraviolet-visible diffuse reflection spectrum (UV-vis DRS) and ultraviolet photoelectron spectroscopy (UPS) were combined to study the band structure and interface electron states of Ti 3 C 2 T x QDs and (001) TiO 2 (Figure 5a-d). Figure S4 depicts the optical bandgap (E g ) of the (001) TiO 2 and Ti 3 C 2 T x QDs, as derived from the Tauc Equation (3).
where α is the absorption coefficient, h is the Planck constant, ν is the photon frequency, n = 1/2 is the indirect bandgap semiconductors, A is a constant, and E g is the bandgap. The bandgaps of (001) TiO 2 , Ti 3 C 2 T x QDs were obtained as 3.16 and 2.91 eV, respectively. The cutoff energies (E cut off ) of (001) TiO 2 and Ti 3 C 2 T x QDs were obtained as 16.67 (Figure 5b) and 17.05 eV (Figure 5d) from the UPS spectra. Their work functions (W) were calculated to be 4.55 and 4.15 eV, respectively. The valence band maximum (VBM) of (001) TiO 2 and Ti 3 C 2 T x QDs were determined from the binding energy onset as 2.49 ( Figure 5a) and 1.93 eV (Figure 5c), which were −7.04 and −6.08 eV. The conduction band minimum (CBM) positions were -3.88 and −2.77 eV, which is the VBM plus the optical bandgap. The band structures and schematic of electrode electron transfer of (001) TiO 2 and Ti 3 C 2 T x QDs are shown in Figure 5e,f. A type II heterojunction was built between TiO 2 and Ti 3 C 2 T x QDs (Figure 5e). Because the CBM and VBM of Ti 3 C 2 T x QDs were more positive than those of (001) TiO 2 , the photogenerated electrons from the conduction band (CB) of Ti 3 C 2 T x QDs flowed to the CB of (001) TiO 2 due to the lower energy level (Figure 5f). Given that the single crystalline (001) TiO 2 were grown in situ on conductive substrate, the photogenerated electrons on the (001) plane were rapidly transferred to the FTO electrode. These electrons would flow to the counter electrode. As for the hole in the valance band (VB) Nanomaterials 2022, 12, 3557 8 of 12 of (001) TiO 2 , it will be injected to the VB of Ti 3 C 2 T x QDs to oxidize the (-OH) groups on the Ti 3 C 2 T x QD surface into ·OH free radicals (-OH+ h + (hv) →·OH). These surface hydroxyl radicals can be regarded as the active species, thereby greatly improving the photocatalytic ability of the material. When the recognition process is completed, the ALP on the electrode surface converts Ascorbic acid-2-phosphate (AAP) into electron donor AA, and these active species can catalyze the oxidation of AA to generate dehydroascorbic acid (DHA) and generate photocurrent at the same time. The band structures and schematic of electrode electron transfer of (001) TiO2 and Ti3C2Tx QDs are shown in Figure 5e and Figure 5f. A type Ⅱ heterojunction was built between TiO2 and Ti3C2Tx QDs (Figure 5e). Because the CBM and VBM of Ti3C2Tx QDs were more positive than those of (001) TiO2, the photogenerated electrons from the conduction band (CB) of Ti3C2Tx QDs flowed to the CB of (001) TiO2 due to the lower energy level (Figure 5f). Given that the single crystalline (001) TiO2 were grown in situ on conductive substrate, the photogenerated electrons on the (001) plane were rapidly transferred to the FTO electrode. These electrons would flow to the counter electrode. As for the hole in the valance band (VB) of (001) TiO2, it will be injected to the VB of Ti3C2Tx QDs to oxidize the (-OH) groups on the Ti3C2Tx QD surface into ·OH free radicals (-OH+ h + (hv) →⋅OH). These surface hydroxyl radicals can be regarded as the active species, thereby greatly improving the photocatalytic ability of the material. When the recognition process is completed, the ALP on the electrode surface converts Ascorbic acid-2-phosphate (AAP) into electron do- Figure 5. Ultraviolet photoelectron spectra: valence band spectra of (a) (001) TiO 2 and (b)Ti 3 C 2 T x QDs, cutoff energies spectra of (c) (001) TiO 2 and (d)Ti 3 C 2 T x QDs (e) band structure of (001) TiO 2 and Ti 3 C 2 T x QDs, and (f) schematic of electrode electron transfer.

MicroRNA-155 Analytical Performance
The PEC response current of stepwise modified electrodes was presented to corroborate the electrode modification process. In Figure 6a, a stable PEC response was obtained after Ti 3 C 2 T x QDs were coated on the (001) TiO 2 NSs substrate (curve a). Afterward, the PEC response was further raised after Au NPs were loaded, probably due to the good conductivity of Au NPs (curve b). The PEC response current of electrodes dropped gradually with the introduction of probe DNA, MCH, microRNA-155, and S9.6 antibody (curve c-f). This may be due to the poor electrical conductivity of nucleic acid and protein structures.
However, when IgG-ALP was introduced into the system, the photoelectric response current of the electrode greatly improved (curve g). This is because the alkaline phosphatase can catalyze AAP to generate electron donor AA to enhance the photoelectric response. Figure 6b illustrates the EIS spectra of stepwise modified electrodes. The Ti 3 C 2 T x QDs/(001) TiO 2 electrode shows a semicircle (curve a) in the high-frequency region relating to the electron transfer resistance. Then, the electron transfer resistance decreased significantly when AuNPs were loaded (curve b). However, the electron transfer resistance increased continuously after probe DNA immobilization (curve c), MCH blocking (curve d), and hybridization with microRNA-155 (curve e). This could be due to the electrostatic repulsion between the negative ions (phosphate and acetate) and the redox probe of Fe(CN) 6 3−/4− . Electron transfer resistance further successively increased after the electrodes were incubated with S9.6 (curve f) and IgG-ALP (curve g) because of the insulativity of the protein structure. To explore the impact of the concentration of S9.6 and ALP-IgG, a concentration parameter adjustment experiment was performed in Figure 6c,d. It can be seen that the change of the response current also increases with the increase in the concentration. When the concentration of S9.6 reaches 20 µg/mL, and the concentration of ALP-IgG reaches 25 µg/mL, the current change reaches the maximum.
The response currents of the PEC platform with various microRNA-155 concentrations were tested (Figure 7a). The response current (I) showed a logarithmic relationship with the microRNA-155 concentrations (c), and the regression equation was I = 1.25lgc + 8.05 (R 2 = 0.9964) (Figure 7b). Moreover, according to the literature [13], the LOD was calculated as 3.0×σ/S = 0.025 pM, where σ is the standard deviation of five times blank tests, and S is the sensitivity. The stability of the PEC platform was studied by continuous scanning under periodic light irradiation. Based on the relative standard deviation (RSD = 0.59%) of the response current in Figure 7c, the detection platform we built is very stable. Furthermore, the selectivity of the PEC platform was investigated by performing an anti-interference test with 1 nM microRNA-141, microRNA-121, and microRNA-21 as interferents. It can be seen that the response current of the detection platform to the interference is much smaller than that of the target, indicating that the detection platform has good anti-interference performance (Figure 7d). The performance of the detection platform is compared with the reported articles in Table 1.

MicroRNA-155 Analytical Performance
The PEC response current of stepwise modified electrodes was presented to corroborate the electrode modification process. In Figure 6a, a stable PEC response was obtained after Ti3C2Tx QDs were coated on the (001) TiO2 NSs substrate (curve a). Afterward, the PEC response was further raised after Au NPs were loaded, probably due to the good conductivity of Au NPs (curve b). The PEC response current of electrodes dropped gradually with the introduction of probe DNA, MCH, microRNA-155, and S9.6 antibody (curve c-f). This may be due to the poor electrical conductivity of nucleic acid and protein structures. However, when IgG-ALP was introduced into the system, the photoelectric response current of the electrode greatly improved (curve g). This is because the alkaline phosphatase can catalyze AAP to generate electron donor AA to enhance the photoelectric response. Figure 6b illustrates the EIS spectra of stepwise modified electrodes. The Ti3C2Tx QDs/(001) TiO2 electrode shows a semicircle (curve a) in the high-frequency region relating to the electron transfer resistance. Then, the electron transfer resistance decreased significantly when AuNPs were loaded (curve b). However, the electron transfer resistance increased continuously after probe DNA immobilization (curve c), MCH blocking (curve d), and hybridization with microRNA-155 (curve e). This could be due to the electrostatic repulsion between the negative ions (phosphate and acetate) and the redox probe of Fe(CN)6 3−/4− . Electron transfer resistance further successively increased after the electrodes were incubated with S9.6 (curve f) and IgG-ALP (curve g) because of the insulativity of the protein structure. To explore the impact of the concentration of S9.6 and ALP-IgG, a concentration parameter adjustment experiment was performed in Figure 6c,d. It can be seen that the change of the response current also increases with the increase in the concentration. When the concentration of S9.6 reaches 20 μg/mL, and the concentration of ALP-IgG reaches 25 μg/mL, the current change reaches the maximum.  0.59%) of the response current in Figure 7c, the detection platform we built is very stable. Furthermore, the selectivity of the PEC platform was investigated by performing an antiinterference test with 1 nM microRNA-141, microRNA-121, and microRNA-21 as interferents. It can be seen that the response current of the detection platform to the interference is much smaller than that of the target, indicating that the detection platform has good anti-interference performance (Figure 7d). The performance of the detection platform is compared with the reported articles in Table 1.

Conclusions
In this article, arrays of titanium dioxide nanosheets with a highly active (001) crystal plane were successfully prepared for microRNA-155 PEC detection. Zero-dimensional Ti3C2Tx QDs were successfully synthesized and used in titanium dioxide. The excellent

Conclusions
In this article, arrays of titanium dioxide nanosheets with a highly active (001) crystal plane were successfully prepared for microRNA-155 PEC detection. Zero-dimensional Ti 3 C 2 T x QDs were successfully synthesized and used in titanium dioxide. The excellent performance was related to the higher surface energy due to the exposed (001) facet on TiO 2 nanosheets. The better separation ability of the photogenerated carriers was due to the Ti 3 C 2 T x QDs/TiO 2 type II heterostructure being able to reduce the loss of electron transfer inside the electrode. The faster electron transport caused by the 0D/2D nanostructure and lattice connection at the interface between Ti 3 C 2 T x and TiO 2 allowed the electrons generated by the detection to be collected more smoothly. The PEC sensor comprising the Ti 3 C 2 T x QDs/(001) TiO 2 electrode exhibited high stability, sensitivity, and selectivity for microRNA-155 detection.