Tetracycline Generated Red Luminescence Based on a Novel Lanthanide Functionalized Layered Double Hydroxide Nanoplatform

Recent progress in lanthanide-based fluorescent nanomaterials for tetracycline detection and removal

Tetracycline (TC) has been widely used in clinical medicine and animal growth promotion due to its broad-spectrum antibacterial properties and affordable prices. Unfortunately, the high toxicity and difficult degradation rate of TC molecules make them easy to accumulate in the environment, which breaks the ecological balance and seriously threatens human health. Rapid and accurate detection of TC residue levels is important for ensuring water quality and food safety. Recently, fluorescence detection technology of TC residues has developed rapidly. Lanthanide nanomaterials, based on the high luminescence properties of lanthanide ions and the high matching with TC energy levels, are favored in the real-time trace detection of TC due to their advantages of high sensitivity, rapidity, and high selectivity. Therefore, they are considered potential substitutes for traditional detection methods. This review summarizes the synthesis strategy, TC response mechanism, removal mechanism, and applications in intelligent sensing. Finally, the development of lanthanide nanomaterials for TC fluorescence detection and removal is reasonably summarized and prospected. This review provides a reference for the establishment of a method for the accurate determination of TC content in complex food matrices.

Oxolinic Acid Generated Green Fluorescence Based on a Terbium-Functionalized Covalent Organic Framework

Oxolinic acid (OXO), a classic environmental contaminant, has a terrible detrimental effect on human health. The exploration of efficient strategies to detect and detecting OXO has remarkable significance. Herein, we reported a novel terbium(III)-functionalized covalent organic framework (Bpy-DhBt-COF@Tb3+) by fixing Tb3+ on the bipyridine-connecting COF (Bpy-DhBt-COF) as a turn-on fluorescent switch toward OXO for the first time. In this platform, Tb3+ acts as the specific recognition units for OXO and the response signal, while Bpy-DhBt-COF acts as the safehaven for Tb3+. Once introducing OXO to Bpy-DhBt-COF@Tb3+, OXO can instead water molecules coordinate with Tb3+ and sensitize Tb3+ instantly, thereby producing a significant fluorescence signal. Profiting from the excellent porosity of Bpy-DhBt-COF@Tb3+, it can obtain optimal response toward OXO only within 10 s with an ultrasensitive detection limit of 12.5 nM. Furthermore, Bpy-DhBt-COF@Tb3+ displayed outstanding selectivity toward OXO than other general quinolones. Based on these, a Tb3+-based COF was explored for the first time for the turn-on fluorescence detection of an OXO with rapid response, high sensitivity, and outstanding selectivity. In this work, we not only exhibit the attractive performance of Tb3+-functionalized COF to detect OXO but also propose a prospect strategy for creating other fluorescent sensors for multiple targets.

Development of europium(III) complex functionalized silica nanoprobe for luminescence detection of tetracycline

Increasing awareness of the health and environment impacts of the antibiotics misuse or overuse, such as tetracycline (TC) in treatment or prevention of infections and diseases, has driven the development of robust methods for their detection in biological, environmental and food systems. In this work, we report the development of a new europium(III) complex functionalized silica nanoprobe (SiNPs-Eu³⁺) for highly sensitive and selective detection of TC residue in aqueous solution and food samples (milk and meat). The nanoprobe is developed by immobilization of Eu³⁺ ion onto the surface of silica nanoparticles (SiNPs) as the emitter and TC recognition unit. The β-diketone configuration of TC can further coordinate with Eu³⁺ steadily on the surface of nanoprobe, facilitating the absorption of light excitation for Eu³⁺ emitter activation and luminescence "off-on" response. The dose-dependent luminescence enhancement of SiNPs-Eu³⁺ nanoprobe exhibits good linearities, allowing the quantitative detection of TC. The SiNPs-Eu³⁺ nanoprobe shows high sensitivity and selectivity for TC detection in buffer solution. Time resolved luminescence analysis enables the elimination of autofluorescence and light scattering for highly sensitive detection of TC in milk and pork mince with high accuracy and precision. The successful development of SiNPs-Eu³⁺ nanoprobe is anticipated to provide a rapid, economic, and robust approach for TC detection in real world samples.

Zinc Ion-Based Switch-on Fluorescence-Sensing Probes for the Detection of Tetracycline

Tetracycline (TC) is an antibiotic that has been widely used in the animal husbandry. Thus, TC residues may be found in animal products. Developing simple and sensitive methods for rapid screening of TC in complex samples is of great importance. Herein, we demonstrate a fluorescence-sensing method using Zn²⁺ as sensing probes for the detection of TC. Although TC can emit fluorescence under the excitation of ultraviolet light, its fluorescence is weak because of dynamic intramolecular rotations, leading to the dissipation of excitation energy. With the addition of Zn²⁺ prepared in tris(hydroxymethyl)amino-methane (Tris), TC can coordinate with Zn²⁺ in the Zn²⁺-Tris conjugates to form Tris-Zn²⁺-TC complexes. Therefore, the intramolecular motions of TC are restricted to reduce nonradiative decay, resulting in the enhancement of TC fluorescence. Aggregation-induced emission effects also play a role in the enhancement of TC fluorescence. Our results show that the linear dynamic range for the detection of TC is 15-300 nM. Moreover, the limit of detection was ~7 nM. The feasibility of using the developed method for determination of the concentration of TC in a complex chicken broth sample is also demonstrated in this work.

Eu doped Ti3C2 quantum dots to form a ratiometric fluorescence platform for visual and quantitative point-of-care testing of tetracycline derivatives

Antibiotic residues have become a public health issues, the fast detection of tetracycline (Tc) in the environment is urgently required. In this work, Ti₃C₂ quantum dots (Ti₃C₂ QDs) and Europium ions jointly constructed a ratiometric fluorescence (FL) platform for the detection of Tc, based on synergistic impact of the Foster Resonance Energy Transfer (FRET) from Ti₃C₂ QDs to Eu³⁺ ions and the Antenna Effect (AE) between Tc and Eu³⁺ ions. And we proposed a ratiometric FL platform for detecting Tc with good linear response range (100-1000 uM) and low detection limit (48.79 nM). Meanwhile, we applied this platform to detect a serious of β-diketone ligands of Eu³⁺ ions, demonstrating the platform's versatility for this category of chemical. Furthermore, based on the color changes of QDs@Eu³⁺ from blue to red at 365 nm ultraviolet light, an intelligent detection smart device was built for the visual semi-quantitative detection of Tc in actual samples. We proved the applicability of the device in complicated samples and the potential for rapid, sensitive, intuitive and point-of-care detection in the field of environment, food, pharmaceutical and agriculture.

Design of terbium (III)-functionalized covalent organic framework as a selective and sensitive turn-on fluorescent switch for ochratoxin A monitoring

For the first time, we develop a terbium (III)-functionalized covalent organic framework named Dpy-NhBt-COF@Tb³⁺, through anchoring Tb³⁺ onto a two-dimensional imine COF (Dpy-NhBt-COF), as the selective and sensitive turn-on fluorescent switch for ochratoxin A (OTA) monitoring. Of particular significance, Tb³⁺ actually plays two roles during sensing process: the specific response signal, and exclusive recognition sites for OTA, while Dpy-NhBt-COF acts as the protector for Tb³⁺. The sensing process involves the replacement of coordinated water molecules from Tb³⁺ by OTA and the energy transfer from OTA to Tb³⁺ centers, resulting in remarkable fluorescence emergence of Tb³⁺. The stabilization of Tb³⁺ via coordination with bipyridine of Dpy-NhBt-COF not only reduces the nonselective binding of naturally occurring ligands, but also avoids the non-radiative quenching caused by solvents molecules. As a sensing platform, Dpy-NhBt-COF@Tb³⁺ possesses noticeable selectivity and high sensitivity toward OTA with an ultralow detection limit of 13.5 nM and rapid response of 10 s. Taken together, our work not only demonstrates great prospect of Tb³⁺-functionalized COF for OTA detection, but also provides a potential way to explore other functionalized materials as promising sensors for other targets.

A novel core-shell coordination assembled hybrid via postsynthetic metal exchange for simultaneous detection and removal of tetracycline

Metal-organic frameworks (MOFs) can perform later transformation without compromising the integrity of the overall framework, and a variety of chemical reactions can be used to modify framework components. Postsynthetic modification (PSM) of MOFs has been developed as an alternative strategy that can expand the range of MOF functional groups. Considering the p-type semiconducting visible light active performance of CuBi₂O₄ (CBO) and the unique porous nanostructure and stability of zeolitic imidazolate framework-8 (ZIF-8), in this work, a novel core-shell coordination assembled hybrid based on p-type semiconductor@MOFs (Eu-CBO@ZIF-8) is prepared for the first time via in-situ growth and postsynthetic metal exchange. A series of detailed characterizations were conducted to confirm the successful synthesis of the material. Moreover, we are focusing on using this material as a new dual-functional sensing material for simultaneous detection and removal of tetracycline (TC), which shows an outstanding analytical performance with a low detection limit of 17 nM, relatively broad linear range (0-70 μM), fast response of less than 120 s, and excellent adsorption performance (377.07 mg g^-1) toward TC. In addition, the sensitive luminescence response caused by TC makes Eu-CBO@ZIF-8 undergo a significant color transition from dark to red under UV-lamps, which is beneficial for visual analysis with the naked eye. The possible detection and adsorption mechanisms, including coordination between Eu³⁺ and the detected substance, the hydrogen bond between ligand and the detected substance, were further discussed. In addition, the practical feasibility of Eu-CBO@ZIF-8 for TC sensing was also studied, with a satisfactory recovery rate of 96.9%-104.6% and RSD ≤3.32%. These results indicate that this material can be used for the detection and adsorption of TC in actual samples.

Tandem Förster resonance energy transfer induced visual ratiometric fluorescence sensing of tetracyclines based on zeolitic imidazolate framework-8 incorporated with carbon dots and safranine T

With increasing TC concentration, tandem FRET1 from CDs to TC, then FRET2 from TC to safranine T were occurred. TC could be easily recognized by naked eye. Besides, we could perform on-site detection of TC with the help of a mobile phone.

Recent Advances in Layered Double Hydroxide-Based Electrochemical and Optical Sensors

Layered double hydroxides (LDHs) have attracted considerable attention as promising materials for electrochemical and optical sensors owing to their excellent catalytic properties, facile synthesis strategies, highly tunable morphology, and versatile hosting ability. LDH-based electrochemical sensors are affordable alternatives to traditional precious-metal-based sensors, as LDHs can be synthesized from abundant inorganic precursors. LDH-modified probes can directly catalyze or host catalytic compounds that facilitate analyte redox reactions, detected as changes in the probe's current, voltage, or resistance. The porous and lamellar structure of LDHs allows rapid analyte diffusion and abundant active sites for enhanced sensor sensitivity. LDHs can be composed of conductive materials such as reduced graphene oxide (rGO) or metal nanoparticles for improved catalytic activity and analyte selectivity. As optical sensors, LDHs provide a spacious, stable structure for synergistic guest-host interactions. LDHs can immobilize fluorophores, chemiluminescence reactants, and other spectroscopically active materials to reduce the aggregation and dissolution of the embedded sensor molecules, yielding enhanced optical responses and increased probe reusability. This review discusses standard LDH synthesis methods and overviews the different electrochemical and optical analysis techniques. Furthermore, the designs and modifications of exemplary LDHs and LDH composite materials are analyzed, focusing on the analytical performance of LDH-based sensors for key biomarkers and pollutants, including glucose, dopamine (DA), H2O2, metal ions, nitrogen-based toxins, and other organic compounds.

A Review of Off-On Fluorescent Nanoprobes: Mechanisms, Properties, and Applications

With the development of nanomaterials, fluorescent nanoprobes have attracted enormous attention in the fields of chemical sensing, optical materials, and biological detection. In this paper, the advantages of "off-on" fluorescent nanoprobes in disease detection, such as high sensitivity and short response time, are attentively highlighted. The characteristics, sensing mechanisms, and classifications of disease-related target substances, along with applications of these nanoprobes in cancer diagnosis and therapy are summarized systematically. In addition, the prospects of "off-on" fluorescent nanoprobe in disease detection are predicted. In this review, we presented information from all the papers published in the last 5 years discussing "off-on" fluorescent nanoprobes. This review was written in the hopes of being useful to researchers who are interested in further developing fluorescent nanoprobes. The characteristics of these nanoprobes are explained systematically, and data references and supports for biological analysis, clinical drug improvement, and disease detection have been provided appropriately.

Inorganic Recognizer-Assisted Homogeneous Electrochemiluminescence Determination of Organophosphorus Pesticides via Target-Controlled Emitter Release

Given the relevance of organophosphorus pesticides (OPs) with food safety, it is highly urgent to develop sensitive and reliable sensors for OPs. However, most of the OP sensors are developed based on colorimetric and fluorescent techniques, which are limited to severe interference of color and fluorescence from pigments and organic acids in agricultural crops. Herein, we develop an inorganic recognizer-based homogeneous electrochemiluminescence (ECL) sensor for the highly sensitive and credible determination of OPs based on manganese dioxide and tris(2,2'-bipyridine)ruthenium [Ru(bpy)₃]²⁺. Through electrostatic interaction, manganese dioxide nanoflakes-[Ru(bpy)₃]²⁺ nanocomposites (MnNFs-Ru) are formed and exhibit a weak ECL signal due to the confinement of [Ru(bpy)₃]²⁺ in MnNFs-Ru. Interestingly, MnNFs-Ru are capable of recognizing thiols due to the analyte-initiated reduction of MnNFs into Mn²⁺ and release of [Ru(bpy)₃]²⁺ from MnNFs-Ru into solution. Further, MnNFs-Ru are employed for the homogeneous ECL determination of OPs, where acetylcholinesterase (AChE) catalyzes the hydrolysis of acetylthiocholine (ATCh) into thiocholine, which in turn decomposes MnNFs of MnNFs-Ru into Mn²⁺, and OPs inhibit AChE activity. This study widens the application of inorganic recognizers from colorimetry/fluorescence to homogeneous ECL and effectively avoids the interference of color and fluorescence, opening up a new path to the development of high-performance OP sensors and supplying a promising tool for guaranteed OP-related food safety.

pH-Response Quantum Dots with Orange-Red Emission for Monitoring the Residue, Distribution, and Variation of an Organophosphorus Pesticide in an Agricultural Crop

Development of simple, sensitive, and reliable fluorescence sensors for monitoring the residue, distribution, and variation of organophosphorus pesticides (OPs) in agricultural crops is highly urgent but remains challenging, which is ascribed to deprivation of an ideal fluorophore and ingenious detection strategy. Herein, we report the fabrication of cadmium telluride quantum dots (CdTe QDs) with bright emission, good water dispersion, and long emission wavelength for OP screening based on the unique response of CdTe QDs to pH and the inhibition of OPs on acetylcholinesterase (AChE) activity. AChE catalyzed hydrolysis of acetylcholine (ACh) into CH₃COOH, which protonated CdTe QDs to decline the fluorescence, whereas target OP impeded AChE from catalyzing hydrolysis of ACh into CH₃COOH, making little influence in fluorescence of CdTe QDs. On the basis of the change in fluorescence, sensitive detection of OP was acquired, with the limit of detection at 0.027 ng/mL, which was comparable or lower than that of most known OP sensors. Furthermore, the CdTe-QD-based sensor was successfully applied for precisely monitoring the residue, distribution, and variation of methidathion in Chinese cabbage and cultivated soil. Therefore, the proposed sensor was anticipated to supply a promising alternative for food safety guarantee and was an valuable application for OP screening.

Visual Luminescent Probes Constructed by Eu3+ Complex-Functionalized Silica Nanocomposites and Their Langmuir-Blodgett Films at Interfaces

The trivalent europium ion (Eu³⁺) has garnered a great deal of interest for the design of luminescent materials possessing compound-independent emission bands, strong luminescent intensity, and long emission lifetimes. We herein introduce a synthetic methodology capable of constructing visual luminescent probes from Eu³⁺ complex-functionalized silica nanocomposites and their Langmuir-Blodgett (LB) films at interfaces. In order to facilitate the coordinative stabilization of Eu³⁺ over carrier surfaces, silica nanoparticles (nanoSiO₂) were pregrafted with terpyridyl (TPy) to make nanoSiO₂TPy linkers. Then, a well-designed coordination reaction of nanoSiO₂TPy with EuCl₃ and 2,6-pyridinedicarboxylic acid (DPA) was carried out at solid-liquid and air-water interfaces, where our desired material (denoted as nanoSiO₂TPy@EuDPA) and its corresponding LB film are obtained. The presence of TPy and DPA interacting with Eu³⁺ plays a key role in regulating the chemical nature of the particle surface, hence giving rise to closely packed nanocomposite arrays in the film. As a result, the improvement in uniformity and stability is achieved alongside the enhancement in emission intensity and lifetime. With such advantageous optical properties, we find them workable as facile, green, and affordable luminescent sensors, by which a range of common toxic anions (Cr₂O₇^2-, MnO₄^-, and PO₄^3-) can be visually and quantitatively recognized. Notably, the LB film-based material could afford a higher K_sv value (1.53 × 10⁵ M^-1), a lower detection limit (0.157 μM), and better recyclability than its original powder analogue, showcasing its utility as a more promising candidate for practical use.

Smartphones and Test Paper-Assisted Ratiometric Fluorescent Sensors for Semi-Quantitative and Visual Assay of Tetracycline Based on the Target-Induced Synergistic Effect of Antenna Effect and Inner Filter Effect

Development of selective and sensitive methods for on-site assay of tetracycline (TC) is of great significance for public health and food safety. Herein, a valid ratiometric fluorescence strategy using g-C₃N₄ nanosheets coupled with Eu³⁺ is designed for the assay of TC. In this strategy, both Eu³⁺ and g-C₃N₄ nanosheets serve as the recognition units of TC. The blue fluorescence of g-C₃N₄ nanosheets can be quenched by TC via the inner filter effect (IFE); meanwhile, the red fluorescence of Eu³⁺ can be enhanced by TC through the antenna effect (AE). The synergistic effect of AE and IFE caused by TC makes the developed ratiometric fluorescent sensor display a wide linear range for TC from 0.25 to 80 μM with a detection limit of 6.5 nM and a significant fluorescence color evolution from blue to red. Given its simplicity, free-label, excellent selectivity, high sensitivity, and recognizable color change, point-of-care testing systems, including smartphones and test paper-based assays, are developed for the visual sensing of TC. The integration of smartphones and test paper on a ratiometric fluorescent sensor greatly reduces the detection cost and time, providing a promising method for the qualitative discernment and semi-quantitative assay of TC on-site. Moreover, the potential application of the approach is also verified by detecting TC in milk.

Rapid conversion from common precursors to carbon dots in large scale: Spectral controls, optical sensing, cellular imaging and LEDs application

The commercial production of carbon dots will be concerned with the simplicity and energy consumption. Herein, maleic acid and m-phenylenediamine form elegantly simple sources for carbon dots. The two precursors are dissolved in formamid (abbreviated as FA) or N,N-dimethylformamide (abbreviated as DMF) and the dehydration-condensation processes have been performed at 30 min or 120 min under room temperature. No external energy/irradiations, reactants or high temperature will be required and the afforded carbon dots (abbreviated as CDs) are collected by extraction, centrifugation, dialysis and column chromatography. It has been found for the first time the choice of organic solvents has been correlated with emission color. The blue-emitting CDs (abbreviated as B-CDs) and green-emitting CDs (abbreviated as G-CDs) are yielded in FA and DMF respectively. Facts support that the increase of -CONH- units causes red-shift in emissions. The optical sensing of tetracycline has been explored and the detection limit of blue-emitting CDs is as low as 25 nM. Live cells exposed to B-CDs and G-CDs (0.5 mg/ml) show no apparent changes via both Cell Counting Kit-8 and Annexin V/7-AAD analysis. The blue and green fluorescent signals can be easily tracked in cells. It has been demonstrated that the two carbon dots can be fabricated as multiple-color light-emitting diodes (abbreviated as LEDs).