Pesticide detection with covalent-organic-framework nanofilms at terahertz band

Chromatic covalent organic frameworks enabling in-vivo chemical tomography

Covalent organic frameworks designed as chromatic sensors offer opportunities to probe biological interfaces, particularly when combined with biocompatible matrices. Particularly compelling is the prospect of chemical tomography - or the 3D spatial mapping of chemical detail within the complex environment of living systems. Herein, we demonstrate a chromic Covalent Organic Framework (COF) integrated within silk fibroin (SF) microneedles that probe plant vasculature, sense the alkalization of vascular fluid as a biomarker for drought stress, and provide a 3D in-vivo mapping of chemical gradients using smartphone technology. A series of Schiff base COFs with tunable pKa ranging from 5.6 to 7.6 enable conical, optically transparent SF microneedles with COF coatings of 120 to 950 nm to probe vascular fluid and the surrounding tissues of tobacco and tomato plants. The conical design allows for 3D mapping of the chemical environment (such as pH) at standoff distances from the plant, enabling in-vivo chemical tomography. Chromatic COF sensors of this type will enable multidimensional chemical mapping of previously inaccessible and complex environments.

In Situ Growth of COF/PVA-Carrageenan Hydrogel Using the Impregnation Method for the Purpose of Highly Sensitive Ammonia Detection

Flexible ammonia (NH3) gas sensors have gained increasing attention for their potential in medical diagnostics and health monitoring, as they serve as a biomarker for kidney disease. Utilizing the pre-designable and porous properties of covalent organic frameworks (COFs) is an innovative way to address the demand for high-performance NH3 sensing. However, COF particles frequently encounter aggregation, low conductivity, and mechanical rigidity, reducing the effectiveness of portable NH3 detection. To overcome these challenges, we propose a practical approach using polyvinyl alcohol-carrageenan (κPVA) as a template for in the situ growth of two-dimensional COF film and particles to produce a flexible hydrogel gas sensor (COF/κPVA). The synergistic effect of COF and κPVA enhances the gas sensing, water retention, and mechanical properties. The COF/κPVA hydrogel shows a 54.4% response to 1 ppm NH3 with a root mean square error of less than 5% and full recovery compared to the low response and no recovery of bare κPVA. Owing to the dual effects of the COF film and the particles anchoring the water molecules, the COF/κPVA hydrogel remained stable after 70 h in atmospheric conditions, in contrast, the bare κPVA hydrogel was completely dehydrated. Our work might pave the way for highly sensitive hydrogel gas sensors, which have intriguing applications in flexible electronic devices for gas sensing.

Research progress of the combination of COFs materials with food safety detection

Covalent organic frameworks (COFs) have received lots of attention due to their multiple advantages such as high specific surface area, controlled pore size, and excellent stability. When detecting food contaminants, the matrix effect brought by complex food samples can significantly affect the accuracy of the results. How to attenuate matrix effect has always been a major challenge. Utilizing the advantages of COFs and applying them to detect food contaminants is currently a key research direction. The aim of this work is to provide a systematic summary of sample pretreatment techniques and detection techniques combined with COFs, which include almost all current techniques combined with COFs. In addition, the principles of combining COFs with different techniques are explained. Finally, the research foci and development direction of COFs in food contaminant detection are discussed. This is an important reference for the future development of food safety and the design of COFs.

Single-Crystal 2D Covalent Organic Frameworks for Plant Biotechnology

Molecules chemically synthesized as periodic two-dimensional (2D) frameworks via covalent bonds can form some of the highest-surface area and -charge density particles possible. There is significant potential for applications such as nanocarriers in life sciences if biocompatibility can be achieved; however, significant synthetic challenges remain in avoiding kinetic traps from disordered linking during 2D polymerization of compatible monomers, resulting in isotropic polycrystals without a long-range order. Here, we establish thermodynamic control over dynamic control on the 2D polymerization process of biocompatible imine monomers by minimizing the surface energy of nuclei. As a result, polycrystal, mesocrystal, and single-crystal 2D covalent organic frameworks (COFs) are obtained. We achieve COF single crystals by exfoliation and minification methods, forming high-surface area nanoflakes that can be dispersed in aqueous medium with biocompatible cationic polymers. We find that these 2D COF nanoflakes with high surface area are excellent plant cell nanocarriers that can load bioactive cargos, such as the plant hormone abscisic acid (ABA) via electrostatic attraction, and deliver them into the cytoplasm of intact living plants, traversing through the cell wall and cell membrane due to their 2D geometry. This synthetic route to high-surface area COF nanoflakes has promise for life science applications including plant biotechnology.

A terahertz metasurface sensor with fingerprint enhancement in a wide spectrum band for thin film detection

Terahertz spectroscopy is a powerful tool to resolve molecular fingerprints by detecting their vibrational and rotational modes, and has great application potential in chemistry and biomedicine. However, the limited sensitivity and poor specificity restrict its applications in these areas, where trace amounts of analytes need to be identified effectively and accurately. Here, we propose a sensing scheme for enhancing molecular fingerprints based on angle-scanning of terahertz waves on an all-silicon metasurface. The metasurface consists of a periodic array of silicon cylinder dimers arranged in a square lattice. An ultrasharp guided mode resonance governed by bound states in the continuum can be excited by elaborately arranging the silicon cylinder dimers. By utilizing the angle-scanning strategy, two kinds of saccharides are successfully identified with extremely high sensitivity. Specifically, the detection limits for lactose and glucose are 1.53 μg cm^-2 and 1.54 μg cm^-2, respectively. Our study will provide a new idea for the detection of trace amounts of analytes, and promote the application of terahertz technology in chemistry and biomedicine.

Defect-rich graphene-coated metamaterial device for pesticide sensing in rice

Performing sensitive and selective detection in a mixture is challenging for terahertz (THz) sensors. In light of this, many methods have been developed to detect molecules in complex samples using THz technology. Here we demonstrate a defect-rich monolayer graphene-coated metamaterial operating in the THz regime for pesticide sensing in a mixture through strong local interactions between graphene and external molecules. The monolayer graphene induces a 50% change in the resonant peak excited by the metamaterial absorber that could be easily distinguished by THz imaging. We experimentally show that the Fermi level of the graphene can be tuned by the addition of molecules, which agrees well with our simulation results. Taking chlorpyrifos methyl in the lixivium of rice as a sample, we further show the molecular sensing potential of this device, regardless of whether the target is in a mixture or not.

A Novel Method for Carbendazim High-Sensitivity Detection Based on the Combination of Metamaterial Sensor and Machine Learning

Benzimidazole fungicide residue in food products poses a risk to consumer health. Due to its localized electric-field enhancement and high-quality factor value, the metamaterial sensor is appropriate for applications regarding food safety detection. However, the previous detection method based on the metamaterial sensor only considered the resonance dip shift. It neglected other information contained in the spectrum. In this study, we proposed a method for highly sensitive detection of benzimidazole fungicide using a combination of a metamaterial sensor and mean shift machine learning method. The unit cell of the metamaterial sensor contained a cut wire and two split-ring resonances. Mean shift, an unsupervised machine learning method, was employed to analyze the THz spectrum. The experiment results show that our proposed method could detect carbendazim concentrations as low as 0.5 mg/L. The detection sensitivity was enhanced 200 times compared to that achieved using the metamaterial sensor only. Our present work demonstrates a potential application of combining a metamaterial sensor and mean shift in benzimidazole fungicide residue detection.