In situ fabrication of aloe-like Au–ZnO micro/nanoarrays for ultrasensitive biosensing of catechol

Highlights

• A three-dimensional aloe-like Au–ZnO nanocomposite was in-situ synthesized via a seed-assisted hydrothermal approach.

• The Au–ZnO micro/nanoarrays immobilized by laccase were applied for the real-time detection of catechol.

• This biosensor shows an ultrasensitive detection of catechol with ultralow limit in real water samples.

Abstract

Currently, the large-scale and controllable fabrication of nanostructures on substrates remains a great challenge for further practical applications. In this work, a novel 3D aloe-like Au–ZnO nanocomposite was designed for in situ synthesis on an ITO substrate, achieving real-time detection of trace catechol (CC) in water. A seed-assisted hydrothermal approach was proposed to control the crystal distribution and growth direction to build a ZnO aloe-like architecture. To eliminate the natural weak conductivity of ZnO, Au nanoparticles were further deposited on all ZnO arrays to construct Au–ZnO micro/nanostructures. The synergetic effects derived from the aloe-like ZnO with a large specific area and Au nanoparticles with high conductivity resulted in both high electrocatalysis and fast electron transfer in enzymatic reactions. After laccase immobilization, the as-prepared biosensor exhibited specific recognition of catechol among other dihydroxybenzenes and phenol with an ultrahigh sensitivity of 131 μA mM⁻¹, as well as an extremely wide linear range from 75 nM to 1100 μM and an ultralow detection limit of 25 nM. In addition, in the detection of real lake samples, this biosensor showed satisfactory anti-interference ability and provided reliable assay results.

Introduction

Phenolic compounds, which are hypertoxic and non-degradable pollutants, have caused severe damage to the environment and human health in the last decade (Abbas et al., 2019). With the rapid societal and economic development, phenolic substances are being widely discharged into water and soil due to various human industrial activities (Bensalah et al., 2005; Liu et al., 2019). According to the integrated wastewater discharge standard of China, the maximum permitted concentration of phenolic substances is 0.4 mg/L. Catechol (CC) is a typical phenolic compound that is listed as a potential human carcinogen even at a low concentration (Ameer and Adeloju, 2009; Wang et al., 2007). Therefore, its fast and in situ monitoring in industrial regions is essential to providing early warning of sudden leaks to prevent further pollutant diffusion. However, traditional CC analysis techniques, such as high-performance liquid chromatography (HPLC) (Jiao et al., 2018) and gas chromatography/mass spectrometry (GC/MS) (Moldoveanu and Kiser, 2007), are rarely used for immediate and on-site reports of pollutant concentrations due to their long detection period, complicated pre-treatment operation and large instrument requirements (Cardoso et al., 2019).

Compared with other analysis methods, electrochemical biosensors are advanced in terms of their fast response, portable size and real-time detection mode, arousing wide attention in the fields of environmental monitoring, the food industry, fermentation processes and clinical medicine (Bansod et al., 2017; Jiang et al., 2016; Samphao et al., 2018; Sani et al., 2018). For CC detection, laccase often serves as the bio-recognizer; its redox ability, derived from the copper in the flavin adenine dinucleotide (FAD) (Palanisamy et al., 2017; Jiang et al., 2019), enables the following reaction:

$$\text{Catechol}\stackrel{\mathit{laccase}}{\to }\text{o}-\text{benzoquinone}+{2\text{H}}^{+}+{2\text{e}}^{-}$$

Normally, in polluted lakes or seas, CC coexists with many other phenolic compounds with low oxidation potentials. Hence, accurate determination of CC is subject to great interference from the extra electro-oxidation of other compounds. However, biosensing sensitivity is strongly dependent on the potential. A higher overpotential often produces higher performance but low selectivity (Chu et al., 2017; Hu et al., 2020). In this case, enhancing the electrocatalysis of the electrode material under a low potential is desired to satisfy the needs of practical applications.

Nanostructured zinc oxide (ZnO), a well-known semiconductor material, plays a critical role in optics, optoelectronics and sensors due to its excellent catalysis, high specific surface area, good biocompatibility and non-toxicity (Shetti et al., 2019; Yue et al., 2019). In addition, enzymes can easily adsorb on the ZnO surface owing to its high isoelectric point (IEP) of approximately 9.5 (Shetti et al., 2019), showing the potential to promote enzymatic reactions. Among various nanostructures, the aloe-like shape of ZnO nanocrystals shows an extremely large 3D surface that is able to provide more catalytic sites than other materials for electrochemical oxidation. Nevertheless, in the electrochemical biosensing process, its deficiency of poor conductivity will strongly block electron transfer, reducing performance (Wang et al., 2019; Usha and Gupta, 2018). Meanwhile, the high activation potential of ZnO nanocrystals for the redox of zinc can easily cause interference, affecting the detection accuracy (Zhao et al., 2013).

To overcome the above obstacles, we here proposed an ultrasensitive and accurate catechol biosensor constructed from Au nanoparticle-decorated aloe-like ZnO arrays. As shown in Fig. 1, anisotropic ZnO nanorods derived from ZnO seeds were simultaneously grown to assemble an aloe-like shape. Abundant AuNPs with a size of 90 nm were then electrodeposited to decorate the ZnO surface to strongly enhance the conductivity. The synergistic effects from the high catalytic area of the aloe-like architecture and the fast electron transfer ability of the AuNPs obviously magnify the current response signal during the electrochemical process. This nanocomposite promotes the adsorption of laccase for the fabrication of a catechol biosensor and shows specific recognition of catechol at a very low working potential with outstanding sensitivity and accuracy in real lake samples.