Flexible SERS strip based on HKUST-1(cu)/biomimetic antibodies composite multilayer for trace determination of ethephon

Highlights

• A three-dimensional SERS sensor was developed for the detection of plant growth regulator residues.

• Innovative preparation of three-dimensional polyhedral metal-organic frame materials.

• A reusable SERS paper scrip which with multiple signal amplification strategies.

Abstract

A surface-enhanced Raman scattering (SERS) sensor based on the folding and assembly characteristics of the three-dimensional structure of paper fibers, the skeleton controllability of metal-organic framework materials (MOFs), and the morphology designability of plasmonic noble metal materials has been established for rapid on-site determination of ethephon in food. HKUST-1(Cu) was assembled onto a carbon-treated chromatographic paper matrix by electrodeposition, and its skeleton respiration and sponge effect were used to overcome the bottleneck problem of poor affinity of SERS substrate for target molecules. Further coupled with the targeted recognition specificity of biomimetic antibodies, a paper-based interface with high specificity of molecular sensitivity was constructed. A sandwich multi-stage progressive enhancement structure was designed to couple plasma pine branch-shaped silver material in situ at the interface to realize superposition and collaborative amplification of SERS signals. When the paper-based strip sensor was used to monitor ethephon, it demonstrated a linear range of 10⁻³ to 10 mg kg⁻¹ and a detection limit of around 1.39 × 10⁻⁴ mg kg⁻¹. The construction and application of the paper-based HKUST-1(Cu)/biomimetic antibodies/pine branch-shaped silver material sensor will provide technical means and theoretical support for the rapid and efficient identification of biological ripening agent residues in food with multi-level signal enhancement.

Introduction

With the development of modern science and technology, a large number of plant growth regulators, such as ethephon (ETH), gibberellin, abscisic acid and cytokines, have been widely used to improve crop yield [1]. Research shows that the intake of ETH-ripened vegetables and fruits may stimulate skin and gastrointestinal functions, as well as lead to cardiac dysfunction, and related diseases such as the central nervous system, liver, and kidney [2,3]. It can be seen that the excessive application of ETH will cause serious harm to this generation and even the next generation of human beings, and at the same time, it will bring irreparable damage to the environment and organisms. Hence, monitoring ETH content on fruits and vegetables can effectively predict any risk for human exposure. Therefore, due to the intense global interest, rapid and immediate detection method is required to analyze ETH residues.

For monitoring ETH content, some analytical methods including liquid chromatography-mass spectrometry (LC-MS) [4], gas chromatography-mass spectrometry (GC-MS) [[5], [6], [7]] and high performance liquid chromatography-mass spectrometry (HPLC-MS/MS) [8], etc. These methods have some detection advantages; however, they still have some limitations in responding to the actual needs of on-site and rapid detection [9,10]. Therefore, it is urgent to develop a simple, rapid, cheap, and real-time detection method. Benefiting from the rapid development of optical technology and nanotechnology, Surface-enhanced Raman spectroscopy (SERS) [11], as an emerging detection method, has appeared in the public eye, which can detect a single molecule and has ultra-high sensitivity [12,13]. Similar to the traditional Raman spectroscopy technique [14], SERS can directly provide the molecular information of the tested object, with short detection time and less required sample quantity [15,16]. Nowadays, SERS base is no longer a plasma material with single function, and the introduction of multi-component composites has made SERS test span multiple orders of magnitude [[17], [18], [19]].

In practical application, the integration of inorganic nanomaterials onto the surface or interior of metal-organic framework (MOFs) to construct high-performance SERS substrate opens a new road for the creation of innovative composites [20,21]. MOFs are inorganic-organic polymers with topological structures [22,23]. The regular porous structure of MOFs increases the specific surface area and provides the basis for the enrichment of the measured molecule. The good chemical modification created an opportunity for its combination with plasma metal nanoparticles, thus achieving efficient SERS enhancement.

However, it is noted that the applicability of MOFs can be expanded further if they can be endowed with predetermined selectivity toward target molecules. Biomimetic antibodies (BAs) as one distinctively identifiable material have received great attention. On account of their fascinating properties such as facile synthesis, reusability, and high selectivity/sensitivity for target molecules, BAs have been widespread applicated. Recently, hybridized MOF-BAs have been constructed to synergistically combine the excellent merits of both materials: specific identification sites for selective adsorption and abundant pores with large specific surface areas for effective adsorption. In the study by Zhang et al., BAs have also been coupled to SERS for detection purposes [24]. Therefore, combining MOFs and BAs for SERS sensing is expected to obtain better synergistic effect [25,26].

Current SERS chips have their respective limitations for Point of Care Testing (POCT) application, such as rigid nature and hence poor sample collection efficiency and costly fabrication process by the nanolithography [27] or oblique angle deposition [28] method. Flexible cellulose paper based on multiple functions has been widely used in the field of SERS sensing instruments because of its excellent chemical modification properties [29,30]. Paper-based material has abundant three-dimensional fiber structure and easily modified hydroxyl groups, which laid a foundation for enhancing the adsorption capacity of the sensor and is a potential candidate for SERS substrate [[31], [32], [33], [34]]. Besides, the integration of BAs as recognition elements with paper has opened new opportunities for SERS analytical devices with elevated selectivity and sensitivity, as well as a shorter assay time and a lower cost [35,36].

Hence, in this work, a SERS sensor with high signal sensitivity that could realize the rapid detection of ETH in crops has been proposed. The microfluidic paper strip with low cost, easy assembly was selected as the sensor base. HKUST-1(Cu) with polyhedral structure was prepared upon the paper base by electrodeposition. The polyhedral and porous structure endows the HKUST-1(Cu) a large specific surface area and high porosity, which could increase the adsorption capacity of the target molecule. In order to enhance the selective adsorption capacity of the SERS sensor, BAs technology was introduced in this experiment, and the BAs have been fabricated by UV polymerization. Finally, a dense layer of pine branch-shaped silver material (PBSM) was in situ grown on the surface of BAs through reduction reaction. When the target molecule was adsorbed on the surface, SRES “hot spot” will be generated at the irregular tip, which derived from the electromagnetic field produced by plasma resonance. Thereupon, the Raman signal intensity of the target molecule was then enhanced. Based on the HKUST-1(Cu)/BAs/PBSM composite multilayer proposed in this work, the efficient specific adsorption of ETH and electromagnetic-chemical compound enhancement effect of SERS signal have been realized (Scheme 1). The proposed SERS sensor provides a very dexterous, lightweight, fast, and accurate detection method for the on-site rapid detection of ETH in agricultural products and the environment.