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

Abstract

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 (Ti₃C₂T_x 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 Ti₃C₂T_x QDs, showing that a type Ⅱ heterojunction was built between TiO₂ and Ti₃C₂T_x 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).

1. 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.

TiO₂ 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 TiO₂ suffers from a high carrier recombination rate, which significantly hinders the signal generation and collection of PEC sensors [9]. Coupling TiO₂ 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 TiO₂. 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, Ti₃C₂T_x QDs possess more abundant surface hydrophilic groups (–O and –OH), making them connect tightly with photoactive supporters. Hence, the Ti₃C₂T_x QDs could be a good co-catalyst for boosting the performance of the PEC biosensor. Song et al. employed Ti₃C₂T_x QDs as a photoactive material to promote the performance of TiO₂-based PEC sensing.

Herein, a PEC biosensing platform was fabricated for microRNA-155 detection. Two-dimensional TiO₂ NS arrays were selected as the sensing active substrate. The exposed (001) crystal face of TiO₂ enables the material to have higher catalytic performance. The Ti₃C₂T_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 TiO₂ 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.

2. Experimental Section

2.1. Synthesis of Ti₃C₂T_x MXene QDs

An amount of 1 g Ti₃AlC₂ 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 Ti₃AlC₂. 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 Ti₃C₂ powder was added to 10 mL of tetramethylammonium hydroxide (TMAOH, 1 wt%) and stirred for 12 h. The TMAOH-intercalated Ti₃C₂ 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 (2.5wt%). The suspension was refluxed at 110 °C for a whole day, centrifuged at 12,000 rpm, and vacuum dried at 200 °C.

2.2. Synthesis of Ti₃C₂T_x QDs/(001) TiO₂/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 TiO₂ NSs arrays were prepared after annealing in an air atmosphere at 450 °C for 3 h. Subsequently, Ti₃C₂T_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 Ti₃C₂T_x QDs slowly self–Organized onto the surface of TiO₂ NSs with the volatilization of water. Finally, the Ti₃C₂T_x QDs/(001) TiO₂/FTO electrodes were washed with ultra-pure water to remove the unconnected Ti₃C₂T_x QDs.

2.3. PEC Detection of microRNA-155

To immobilize DNA, 20 μL of Au NPs (0.05 mg/mL) was added dropwise onto the Ti₃C₂T_x QDs/(001) TiO₂/FTO surface. An amount of 20 μL of 0.5 μM probe DNA immobilization buffer was cast onto the Ti₃C₂T_x QDs/(001) TiO₂/FTO electrode and incubated under humid condition for 12 h at 25 ºC. The electrode was denoted as DNA/Ti₃C₂T_x QDs/(001) TiO₂/FTO electrode. The electrode was washed with a washing buffer and incubated with 20 μL of mercaptohexanol (MCH, 0.1 mM) for 1 h. Then, the DNA/Ti₃C₂T_x QDs/(001) TiO₂/FTO electrode was incubated with 20 μL of different concentrations of microRNA-155 for 2 h. The RNA-DNA/Ti₃C₂T_x QDs/(001) TiO₂/FTO electrode was washed with 0.1×SSC hybridization buffer to eliminate the unhybridized microRNA-155. Subsequently, 20 μL of the S9.6 antibody (20 μg/mL) was further incubated with the electrode for 1 h at 25 ºC in a humid cell. The S9.6-RNA-DNA/Ti₃C₂T_x QDs/(001) TiO₂/FTO electrode was then washed with a buffer. Then, the electrode was incubated with 20 μL of IgG-ALP (25 μg/mL) at 37 °C for 1 h and keeping the surface moist. Finally, the PEC response of the ALP-IgG/antibody/RNA-DNA/Ti₃C₂T_x QDs/(001) TiO₂/FTO electrode was recorded in the detection buffer at 0V.

3. Results and Discussion

3.1. Electrode Construction and Sensing Mechanism of PEC Sensor

As shown in Figure 1A, layered Ti₃C₂T_x MXene were fabricated by a top-down method by etching the Al atomic layer in Ti₃AlC₂ with HF. The Ti₃C₂T_x QDs were prepared by the reflux hydrothermal method with TMAOH as the intercalating agent. Using ammonium fluorotitanate as a seed, (001) TiO₂ NSs were grown on FTO glass through the hydrolysis of titanate in an acidic solution (Figure 1B). Ti₃C₂T_x QDs and (001) TiO₂ 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.

3.2. Morphology Characterization of Ti₃C₂T_x 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 Ti₃C₂T_x 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 Ti₃C₂T_x QDs/(001) TiO₂ composite are provided in Figure 2a,b. Compared with pure TiO₂ NSs, there were no significant changes in the morphology of the composite electrodes after loading with Ti₃C₂T_x QDs. Transmission electron microscopy (TEM) images were provided to characterize the crystal information of TiO₂ 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 TiO₂ NSs prepared were a single crystal with excellent crystallinity [16]. Proofread with standard PDF cards, the main exposed crystal plane of TiO₂ nanosheets was (001) [17]. The introduction of Ti₃C₂T_x QDs was further identified by TEM images. Compared with pure TiO₂ NSs, many small scales (~10 nm) appeared on the Ti₃C₂T_x QDs/(001) TiO₂ composite (Figure 2d). The HRTEM image of the Ti₃C₂T_x QDs/(001) TiO₂ 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 TiO₂ and the (100) plane of Ti₃C₂T_x QDs. The elemental mapping dots of the Ti₃C₂T_x QDs/(001) TiO₂ composite for Ti and O were dense and apparent (Figure 2f–i) because TiO₂ was dominant in this composite, whereas those for C were relatively scarce and primarily found around the sheet edges, indicating that Ti₃C₂T_x QDs successfully combined with the (001) crystal plane of TiO₂ NSs.

3.3. Composition Characterization of Ti₃C₂T_x 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. 21-1276). No distinct characteristic diffraction peak of Ti₃C₂T_x QDs was found in the Ti₃C₂T_x QDs/(001) TiO₂ sample, which was due to the low crystallinity and low content of the Ti₃C₂T_x QDs in the composites [19]. To further determine the functional group information of Ti₃C₂T_x QDs and TiO₂, the Fourier transform infrared spectroscopy (FTIR) spectra of TiO₂ NSs, Ti₃C₂T_x QDs, and Ti₃C₂T_x 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) TiO₂ and Ti₃C₂T_x 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 (2p_3/2) and 464.4 eV (2p_1/2). After loading Ti₃C₂T_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 Ti₃AlC₂, Ti²⁺ ions and Ti⁴⁺ 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–O₆] species. As for the O 1s XPS spectra after Ti₃C₂T_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 Ti₃C₂T_x QDs were O and −OH [22]. The C 1s of (001) TiO₂ 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 TiO₂, the introduction of Ti₃C₂T_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 Ti₃C₂T_x QDs. Interestingly, compared with (001) TiO₂ 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 Ti₃C₂T_x QDs and (001) TiO₂ (Figure 3d) [23]. We believe that the O and –OH on the surface of Ti₃C₂T_x may act as rivet sites to connect to the five coordinated titanium atoms in (001) of TiO₂ and form an atomic-scale interfacial heterojunction between 0D Ti₃C₂T_x QDs and 2D TiO₂ NSs.

3.4. PEC Performance Characterization of Ti₃C₂T_x QDs/(001) TiO₂/FTO Electrode

To evaluate the catalytic ability of the materials to catalyze AA, time-resolved current response curves were obtained in an aqueous O₂-saturated PBS solution containing AA (0.1 M) under light irradiation (365 nm). In Figure 4a, the photoelectric response of TiO₂ NSs significantly improved after loading Ti₃C₂T_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 TiO₂ NSs, the emission intensity of Ti₃C₂T_x QDs/(001) TiO₂ electrodes significantly decreased in both ranges (Figure 4b). The reduced recombination rate of photogenerated carriers could supply sufficient holes to activate –OH on Ti₃C₂T_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) TiO₂ and Ti₃C₂T_x QDs/(001) TiO₂ electrodes. The average lifetime (τ_ave) for the (001) TiO₂ and Ti₃C₂T_x QDs/(001) TiO₂ 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].

(1)$\frac{1}{C^2}=\frac{2}{{\epsilon }_0\epsilon e{N}_{\mathrm{D}}}\left[\left(U_{\mathrm{S}}-U_{\mathrm{FB}}\right)-\frac{k_{\mathrm{B}}T}{e}\right]$

The carrier density $N_{\mathrm{D}}$ can be obtained from the slope of the linear region of the Mott–Schottky plots (Figure 4d) on the basis of Equation (2).

(2)$N_{\mathrm{D}}=-\left(\frac{2}{e\epsilon {\epsilon }_0}\right){\left(\frac{\mathrm{d}\left(1/C^2\right)}{\mathrm{d}\left(U_{\mathrm{S}}\right)}\right)}^{-1}$

where $N_{\mathrm{D}}$ is the electron density, e is the element charge value, ε is the dielectric constant (48 for anatase), ε₀ is the vacuum permittivity, C is the space charge capacitance, and U_S is the applied potential. The calculated N_D for the (001) TiO₂ and Ti₃C₂T_x QDs/(001) TiO₂ electrodes were 4.04 × 10¹⁸ and 8.18 × 10¹⁸, respectively. The photoelectric property tests implied that the introduction of Ti₃C₂T_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.

3.5. Electron Transfer Mechanism of Ti₃C₂T_x QDs/(001) TiO₂/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 Ti₃C₂T_x QDs and (001) TiO₂ (Figure 5a–d). Figure S4 depicts the optical bandgap (E_g) of the (001) TiO₂ and Ti₃C₂T_x QDs, as derived from the Tauc Equation (3).

(3)${\left(\mathsf{\alpha }\mathrm{h}\mathsf{\nu }\right)}^n=\mathrm{A}\left(\mathrm{h}\mathsf{\nu }-\mathrm{Eg}\right)$

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) TiO₂, Ti₃C₂T_x QDs were obtained as 3.16 and 2.91 eV, respectively. The cutoff energies (E_{cut off}) of (001) TiO₂ and Ti₃C₂T_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) TiO₂ and Ti₃C₂T_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) TiO₂ and Ti₃C₂T_x QDs are shown in Figure 5e,f. A type Ⅱ heterojunction was built between TiO₂ and Ti₃C₂T_x QDs (Figure 5e). Because the CBM and VBM of Ti₃C₂T_x QDs were more positive than those of (001) TiO₂, the photogenerated electrons from the conduction band (CB) of Ti₃C₂T_x 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 Ti₃C₂T_x QDs to oxidize the (–OH) groups on the Ti₃C₂T_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.

3.6. 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 Ti₃C₂T_x 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 Ti₃C₂T_x 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.

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 (R² = 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.

4. 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 Ti₃C₂T_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 TiO₂ nanosheets. The better separation ability of the photogenerated carriers was due to the Ti₃C₂T_x QDs/TiO₂ 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 Ti₃C₂T_x and TiO₂ allowed the electrons generated by the detection to be collected more smoothly. The PEC sensor comprising the Ti₃C₂T_x QDs/(001) TiO₂ electrode exhibited high stability, sensitivity, and selectivity for microRNA-155 detection.

Author Contributions

Conceptualization: B.Y.; Methodology: B.Y.; Software: C.L.; Validation: H.P.; Formal analysis: R.Y.; Investigation: C.Z.; Resources: J.T.; Data curation: B.Q.; Writing—original draft preparation: B.Y.; Writing—review and editing: Q.W.; Visualization: Z.C.; Supervision: J.T.; Project administration: Q.W.; Funding acquisition: Q.W. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data available in a publicly accessible repository. The data presented in this study are openly available in [FigShare] at [10.6084/m9.figshare.21291828], reference number [10.1000/data.20120401].

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figure and Tables

Figure 1. Schematic of PEC electrode construction and detection mechanism of microRNA-155 (A) synthesis process of Ti3C2Tx QDs (B) synthesis process of (001) TiO2/FTO electrode (C) detection process of microRNA-155 (a) dropping AuNPs (b) probe DNA loading (c) incubation with MCH (d) incubation with microRNA-155 (e) incubation with antibody (f) incubation with IgG-ALP.

Figure 2. (a,b) FESEM of Ti3C2Tx QDs/(001) TiO2; TEM of (c) (001) TiO2 inset (middle part, HRTEM image, top right-hand corner, SAED) and (d) Ti3C2Tx QDs/(001) TiO2; (e) HRTEM of Ti3C2Tx QDs/(001) TiO2; and (f–i) mapping of Ti3C2Tx QDs/(001) TiO2 ((g): Yellow, titanium; (h): blue, oxygen; (i): cyan, carbon).

Figure 3. (a) X-ray diffraction (XRD) pattern of FTO glass, (001) TiO2 and Ti3C2Tx QDs/(001) TiO2 and X-ray photoelectron spectroscopy (XPS) results of (b) Ti 2p, (c) O 1s, and (d) C 1s orbital of (001) TiO2 and Ti3C2Tx QDs/(001) TiO2.

Figure 4. (a) Time-resolved current response curves, (b) Photoluminescence (PL) spectroscopy, (c) Time-resolved photoluminescence (TRPL) emission spectroscopy spectra, and (d) Mott–Schottky plot of pristine (001) TiO2 and Ti3C2Tx QDs/(001) TiO2 electrode.

Figure 5. Ultraviolet photoelectron spectra: valence band spectra of (a) (001) TiO2 and (b)Ti3C2Tx QDs, cutoff energies spectra of (c) (001) TiO2 and (d)Ti3C2Tx QDs (e) band structure of (001) TiO2 and Ti3C2Tx QDs, and (f) schematic of electrode electron transfer.

Figure 6. (a) Photocurrent response in detection buffer and (b) EIS plot in 5.0 mM Fe(CN)63−/4− solution of different electrodes: curve a, Ti3C2Tx QDs/(001) TiO2/FTO; curve b, after dropping AuNPs; curve c, after probe DNA loading; curve d, after incubation with MCH; curve e, after incubation with 1 nM microRNA-155; curve f, after incubation with antibody; curve g, incubation with IgG-ALP, (c) the change of the response current with different concentrations of S9.6 and (d) the change of the response current with different concentrations of IgG-ALP.

Figure 7. (a) Photocurrent response in detection buffer of the biosensor with different microRNA-155 concentrations (b) calibration curve, (c) stability of the PEC biosensor with 1 nM microRNA-155, and (d) selectivity of the PEC microRNA-155 biosensor with 1 nM different microRNAs.

Table 1

Analytical performance of several microRNA-155 biosensors.

Biosensors	Dynamic Range (M)	Detection Limit (M)	References
CV	5.6 × 10−12–5.6 × 10−5	1.87 × 10−12	[24]
ECL	1 × 10−12–1 × 10−5	6.7 × 10−13	[25]
ECL	1 × 10−14–1 × 10−5	1.6 × 10−10	[26]
PEC	1 × 10−11–2 × 10−8	5 × 10−12	[27]
PEC	1 × 10−13–1 × 10−8	3.3 × 10−13	[28]
PEC	1 × 10−13–1 × 10−8	2.5 × 10−13	This work

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/nano12203557/s1, Table S1. Specific oligonucleotide sequence. Figure S1. Atomic force microscopy (AFM) image of Ti₃C₂T_x QDs. Figure S2. FESEM of (001) TiO₂ NSs. Figure S3. FTIR spectra of TiO₂ NSs, Ti₃C₂T_x QDs, and Ti₃C₂T_x QDs/(001) TiO₂ composite. Figure S4. UV−vis DRS of (001) TiO₂ and Ti₃C₂T_x QDs.

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