Zinc vacancy mediated electron–hole separation in ZnO nanorod arrays for high-sensitivity organic photoelectrochemical transistor aptasensor

Abstract

A novel strong solvent coordination leaching method was developed to prepare surface zinc vacancies in ZnO nanorod arrays. Remarkably, the surface-zinc-vacancy-rich ZnO nanorod arrays exhibit high electron–hole separation efficiency and excellent photoelectrochemical performance for use as a promising candidate for the next generation of organic photoelectrochemical transistor aptasensors.

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As an ideal combination of organic electrochemical transistor (OECT) and photoelectrochemical (PEC) technologies, the organic photoelectrochemical transistor (OPECT) sensor, which is increasing in popularity, shows great potential in the sensitive detection of chemical and biological species.^1–3 Typically, an OPECT sensor incorporates a photo-electric conversion interface in gate electrode and intrinsic signal amplification channel that is ultrasensitive to small variations of the photo-induced surface potential, exhibiting the advantages of high sensitivity, easy miniaturization, and power saving.^4–6 Serving on photo-electric conversion interface in gate electrode, the PEC process involves the conversion of light to an electrical signal resulting from electron excitation and the subsequent charge transfer generated by illumination of the photoelectric material.^7–9 Consequently, photoactive materials play a key role in this process. One-dimension-based ZnO nanorod arrays have been widely researched as significant and promising photoactive materials due to their high surface area for efficient collection and conversion of photocarriers, good structural stability, and environmental benignity.¹⁰ However, their favourable PEC performance is usually accompanied by a limited light-absorption wavelength range and inefficient separation and transfer for photoexcited electron–hole pairs.¹¹ The rational design and superficial regulation and control of ZnO nanorod arrays from a system-level angle remain worth studying.

Surface-vacancy engineering has attracted much attention in the fields of PEC and photocatalysis, in which the vacancies could act as centres for capturing photoexcited electrons and hence increase the carrier separation efficiency.^12–14 Nonetheless, studies of the surface-vacancies of metals are rare, while vacancy engineering is an emerging field. Metal-vacancy dispensing on a surface facilitates the generation of new energy levels, hence lowering the reaction energy barrier, increasing the photo responsive range, promoting the carrier separation and transfer efficiency, and ultimately enhancing the photoconversion efficiency.^15–18 Therefore, the development of a material model with the major defects for metal distribution on the surface is significant.

Herein, we developed a novel strong solvent coordination leaching method and an ideal photoactive material model of ZnO nanorod arrays and deliberately created lots of zinc vacancies (Zn_vac) on the surface for the first time. Specifically, Zn_vac are generated by using a deep eutectic solvent (DES) which leaches the Zn atoms from the surface of the nanorod arrays. The large surface area of the nanorod array structure results in numerous defect sites, thus providing new opportunities to explore the correlation between the defect sites and PEC performance. Of note, the introduction of Zn_vac notably accelerated the electron–hole separation efficiency, enhanced the interface charge transport process, and promoted the efficiency of the photoelectric conversion, which can be demonstrated by PL spectra, EIS Nyquist plots and I–t curves. Taking the Zn_1−xO nanorod array electrode as the photo-gate electrode, the as-constructed OPECT aptasensor exhibited a superior gating effect and a corresponding three orders of magnitude increase in the channel response, and hence showed high sensitivity in the detection of ochratoxin (OTA).

ZnO nanorod arrays were attached to a fluorine-doped tin oxide (FTO) substrate by a two-step method.¹⁹ As shown in Fig. 1A, surface zinc vacancies in the ZnO nanorod arrays were prepared through a novel strong-coordination leaching method with DES, which is a new class of green and sustainable solvent that can act as an effective leaching agent in the metal extraction process, even with a solid metallic compound.^20,21 Specifically, the ZnO electrode was directly immersed in a DES solution to leach Zn atoms from ZnO lattices after being heated at different temperatures and for different amounts of time. After being optimized in DES, the ZnO electrodes exhibited the best PEC performance at 80 °C for 10 h (Fig. S1, ESI†).

Fig. 1 (A) Schematic diagram of Zn atoms leaching from ZnO lattices. HRTEM images (B and C) of Zn_1−xO. (D) EPR spectra of ZnO (a) and Zn_1−xO (b).

The ZnO nanorod arrays had diameters of ≈150 nm and thicknesses of ≈1.5 μm (Fig. S1A and B, ESI†). Notably, the morphology of the ZnO electrode was unchanged after the defect etching, which maintained well the grain size and thickness of the pristine sample (Fig. S1C and D, ESI†). In addition, the XRD patterns could be readily indexed to the hexagonal structure of ZnO (JCPDS no. 74-0534) (Fig. S3, ESI†). No characteristic peaks of any other impurities were observed, indicating the high purity of the products.

To confirm the formation of Zn_vac, high-resolution transmission electron microscopy (HRTEM) of Zn_1−xO was used. In the HRTEM image (Fig. S4A, ESI†), the lattice fringes of ZnO could be observed. The corresponding selected area electron diffraction (SAED) pattern displayed a spot pattern, indicating the single-crystalline character of the fabricated nanorods (Fig. S4B, ESI†). Intriguingly, both distinct and blurry lattice fringes were observed in the red frame region of Fig. 1B, implying that an abundance of Zn_vac existed on the surface of the ZnO.^17,22 In addition, the magnified HRTEM image in Fig. 1C convincingly shows the presence of numerous vacancies. In this figure, the spacing values of the lattice fringes are 0.38 nm and 0.25 nm, corresponding to the (100) and (002) planes of wurtzite ZnO, which is in good agreement with the XRD results. Electron paramagnetic resonance (EPR) was also utilized to verify the existence of Zn_vac. As depicted in Fig. 1D, the samples had similar EPR signals (g = 2.004),^17,23 and the different EPR signal intensities showed that the Zn_1−xO possessed a higher concentration of Zn_vac compared with the intrinsic Zn_vac of ZnO. X-ray photoelectron spectroscopy (XPS) was also carried out (Fig. S5, ESI†). The O1s spectra of the DES-treated ZnO electrodes after 10 and 20 h exhibited a peak characteristic of absorbed oxygen, which could be considered as left behind after the formation of Zn_vac.²⁴

To investigate the light absorption and electronic structure of ZnO and Zn_1−xO, further characterizations were carried out. The UV-vis diffuse reflection spectra show that the absorption edges of ZnO are around 389 nm, and Zn_1−xO was slightly red-shifted relatively (Fig. 2A). The band gaps of ZnO and Zn_1−xO were estimated to be 3.07 and 3.01 eV, respectively (Fig. 2B). To analyze the energy band structures of ZnO and Zn_1−xO, electrochemical Mott–Schottky experiments were performed (Fig. S6, ESI†), and the calculated energy band positions of ZnO and Zn_1−xO are schematically drawn in Fig. 2C following conversion. Compared to ZnO, the E_VB and E_CB of Zn_1−xO are clearly increased and the band gap is reduced. The reduced band gap facilitates electron transfer from the VB to the CB.²⁵ For the PEC reaction process, the shift of the band edges is conducive to the oxidation of water, thus consuming the holes left by the electron transition from the VB to the CB and reducing the recombination of electrons and holes.²⁴ Room-temperature photoluminescence (PL) spectroscopy of ZnO and Zn_1−xO was used to further certify the results (Fig. 2D). As shown in Fig. 2D, the PL emission intensities of the ZnO-450 °C and Zn_1−xO are much lower than that of the ZnO, which suggests that the electron–hole pair recombination rate was suppressed effectively.²⁶

Fig. 2 (A) UV-vis absorption spectra of ZnO (a), ZnO-450 °C (b) and Zn_1−xO (c). (B) Plots of (αhν)^1/2versus energy (hν) for the band gap energy of ZnO (a), ZnO-450 °C (b) and Zn_1−xO (c). (C) Band alignments of ZnO and Zn_1−xO. (D) PL spectra of ZnO (a), ZnO-450 °C (b) and Zn_1−xO (c).

The PEC performances of ZnO, ZnO-450 °C and Zn_1−xO were evaluated using light illumination. As shown in Fig. S7 (ESI†), the Zn_1−xO electrode possessed a photocurrent four times higher and a smaller resistance than ZnO, indicating that the presence of Zn_vac enhanced the interface charge transport process and promoted the photoelectric conversion. After treatment in DES the electrochemically active areas of the Zn_1−xO electrode increased compared with ZnO-450 °C (Fig. S7C, ESI†).

As shown in Scheme 1A, the Zn_1−xO gate electrode was then integrated with a PEDOT:PSS channel-coated screen-printed carbon electrode to structure the OPECT device, which was preliminarily studied by measuring the channel current (I_DS) and gate photocurrent (I_G) responses upon illumination by a xenon lamp. Under illumination, Zn_1−xO was excited to generate photoinduced electrons which moved from the valence band (VB) to the conduction band (CB), and then the electrons migrated to the FTO to form an anodic photocurrent, which gave a positive voltage to the channel. Under this positive voltage, the cation from the electrolyte will be inducted into the PEDOT:PSS channel, leading PEDOT to be de-doped, thus resulting in fewer carriers-holes in the channel. As a result, the I_DS decreased as the photovoltage (V_photo) and I_G increased. Under illumination, the photocurrent absolute value of the gate electrode (I_G = |I_G,on − I_G,off|) was ≈0.58 μA (Fig. 3A(a)) and the channel current absolute value of the channel electrode (I_DS = |I_DS,on − I_DS,off|) was up to ≈535 μA (Fig. 3A(b)). Note that a small variation in I_G of ≈0.58 μA gave rise to a considerable change in I_DS of ≈535 μA.

Scheme 1 Schematic diagram of the response mechanism (A) and the construction (B) of the OPECT aptasensor.

Fig. 3 (A) I_G (a) and I_DS (b) of the OPECT device with a Zn_1−xO gate electrode under illumination (I–time, V_G = 0 V, V_DS = −0.2 V). (B) Transfer characteristics (I_DS–V_G, V_DS = −0.2 V) of the OPECT device with ZnO (a) and Zn_1−xO (b) gate electrodes without illumination; and ZnO (c) and Zn_1−xO (d) gate electrodes with illumination. (C) I_DS of the OPECT aptasensor to detect different concentrations of the OTA toxin (a–h: 5 pg L⁻¹–50 μg L⁻¹). (D) The corresponding linear calibration curve for OTA toxin determination.

The Zn_vac gating effect on the OPECT aptasensor was further investigated. The typical transfer characteristics of the OPECT device were recorded to assess the capacity of the gate voltage (V_G) to adjust I_DS (Fig. 3B). The significant decrease of I_DS as V_G increased demonstrated a good modulation performance of the OPECT device. In addition, the ZnO gate electrode and Zn_1−xO gate electrode exhibited different transfer character (black curves a and b) without illumination, whereas the two curves shifted in different manners toward lower V_G upon illumination (red curves c and d). Specifically, with illumination, the transfer curve of the ZnO gate electrode shifted by a small amount to lower V_G, while the Zn_1−xO gate electrode shifted significantly, showing that the prepared OPECT aptasensor has an obvious gating effect because of Zn_vac. The same conclusion can be reached from Fig. S8A and B (ESI†). The Zn_1−xO gate electrode produced a larger I_DS than the ZnO-450 °C and ZnO electrodes (Fig. S8A, ESI†), corresponding to the largest I_G (Fig. S8B, ESI†). This indicates that Zn_1−xO can generate a large photovoltage, which can control the voltage range and achieve high sensitivity detection in the OPECT system.

An OPECT aptasensor was then constructed (Scheme 1B) using the synthetic Zn_1−xO as the gate electrode and an aptamer as the recognition element. First, aldehyde groups for Zn_1−xO were introduced by chitosan (CHI) and glutaraldehyde (GA) in a controlled fashion, after which the amidogen-containing aptamer was connected. The aptamer was anchored by an aldimine condensation reaction on the Zn_1−xO electrode. An evident decrease of I_DS could be observed after immobilizing the aptamer, causing a decrease in I_G (Fig. S9A, ESI†), which was put down to the large resistance of the aptamer.²⁷ The same result can be concluded from curve b of Fig. S9B (ESI†); the steric hindrance of the electrode interface was increased after modifying the aptamer.²⁸ Then, bovine serum albumin (BSA) was used to block the superfluous aldehyde group. Then fabrication of this aptasensor was accomplished. The aptasensor could specifically recognize and capture OTA toxin, which hindered the transfer of electrons. As a consequence, the I_DS decreased and the value of the electron transfer resistance (R_et) increased further (Fig. S9A and B, ESI†). Therefore, the signal changes arising from the specific binding of the aptamer and OTA can result in the quantitative detection of the OTA toxin.

The OPECT aptasensor has a good response to OTA under the optimal experimental conditions previously mentioned (Fig. S10, ESI†). As shown in Fig. 3C, the I_DS responses decreased gradually with increasing concentration of OTA, indicating that the I_G responses are suppressed by OTA on the biointerface. A good linear relationship with a regression equation of I_DS = 63.89 − 9.58[lg C_OTA (μg L⁻¹)] (R² = 0.994) can be derived for the detection of the OTA concentration ranging from 5 pg L⁻¹ to 50 μg L⁻¹ and the detection limit was experimentally found to be 1.66 pg L⁻¹ (S/N = 3), as shown in Fig. 3D.

To investigate the selectivity of the fabricated OPECT aptasensor, ochratoxin A (OTA), ochratoxin B (OTB), ochratoxin C (OTC), aflatoxin B1 (AFB₁), T-2 toxin (T-2), and their mixture were employed as interferents. 100 pg L⁻¹ of these interferents did not lead to significant alteration of I_DS compared with 10 pg L⁻¹ of OTA and their mixture, indicating excellent selectivity of the constructed OPECT aptasensor for OTA detection (Fig. S11A, ESI†). Furthermore, the signal decreased by nearly 4.5% after several on/off illumination cycles within 1000 s, indicating the photocurrent signal of Zn_1−xO was quite stable for the detection (Fig. S11B, ESI†). After the Zn_1−xO electrode had been stored at 4 °C for 20 days, the I_DS signal maintained 95% of the initial response, indicating considerable robustness and long-term storage stability (Fig. S11C, ESI†).

The potential for practical application of the OPECT aptasensor was explored by detecting different concentrations of OTA in corn flour samples using the standard addition method. The pretreatment of the corn flour was according to a previously reported method.²⁹ As shown in Table S1 (ESI†), the recoveries are in the range of 95.7% to 100.2% and the relative standard deviation (RSD) varied from 3.6% to 6.3%, clearly indicating that this platform has satisfactory applicability for the detection of OTA in real samples. Furthermore, compared with other published reports as shown in Table S2 (ESI†), this sensor platform has a wider detection range and lower detection limit.

In summary, we successfully introduced surface Zn_vac into ZnO nanorod arrays using a novel strong solvent coordination leaching method. HRTEM images, EPR spectra and XPS spectra demonstrate the introduction of Zn_vac. In addition, PL spectra, EIS Nyquist plots and photocurrent results revealed that the presence of Zn_vac on the surface notably accelerates the electron–hole separation, enhances the interface charge transport process and promotes the photoelectric conversion. The Zn_1−xO electrode with excellent PEC performance as a photo-gate electrode was used to design a highly efficient OPECT device which showed a three orders of magnitude increase in the channel response and a good performance for OTA determination. Given these results, engineering metal defects on a surface is a new avenue for photoactive material design with high PEC performance. Additionally, this work has successfully developed a promising pathway in the electrochemical field for high sensitivity sensing devices with a wider range of applications.

This work was supported by Key R&D Program of Jiangsu Province (No. BE2020677); National Natural Science Foundation of China (No. 22174055).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cc05735b

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