A Ratiometric Fluorescent Nano-Probe for Rapid and Specific Detection of Tetracycline Residues Based on a Dye-Doped Functionalized Nanoscaled Metal-Organic Framework

Design and application of dual-emission metal-organic framework-based ratiometric fluorescence sensors

Metal-organic frameworks (MOFs) are novel inorganic-organic hybridized crystals with a wide range of applications. In the last twenty years, fluorescence sensing based on MOFs has attracted much attention. MOFs can exhibit luminescence from metal nodes, ligands or introduced guests, which provides an excellent fluorescence response in sensing. However, single-signal emitting MOFs are susceptible to interference from concentration, environment, and excitation intensity, resulting in poor accuracy. To overcome the shortcomings, dual-emission MOF-based ratiometric fluorescence sensors have been proposed and rapidly developed. In this review, we first introduce the luminescence mechanisms, synthetic methods, and detection mechanisms of dual-emission MOFs, highlight the strategies for constructing ratiometric fluorescence sensors based on dual-emission MOFs, and classify them into three categories: intrinsic dual-emission and single-emission MOFs with luminescent guests, and non-emission MOFs with other luminescent materials. Then, we summarize the recent advances in dual-emission MOF-based ratiometric fluorescence sensors in various analytical industries. Finally, we discuss the current challenges and prospects for the future development of these sensors.

Turn-off/turn-on biosensing of tetracycline and ciprofloxacin antibiotics using fluorescent iron oxide quantum dots

A fluorescence probe based on iron oxide quantum dots (IO-QDs) was synthesized using the hydrothermal method for the determination of tetracycline (TCy) and ciprofloxacin (CPx) in aqueous solution. The IO-QDs were characterized using high-resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (P-XRD), vibrating sample magnetometry (VSM), and Fourier-transform infrared spectroscopy (FTIR). The as-prepared IO-QDs are fluorescent, stable, and with a fluorescence quantum yield (QY) of 9.8 ± 0.12%. The fluorescence of IO-QDs was observed to be quenched and enhanced in the presence of TCy and CPx, respectively. The fluorescence intensity ratio shows linearity at concentrations from 1-100 μM and 5-100 μM for TCy and CPx, respectively; the detection limit for TCy and CPx was estimated to be 0.71 μM and 1.56 μM, respectively. The proposed method was also successfully utilized in the spiked samples of drinking water and honey with good recoveries. The method offered convenience, rapid detection, high sensitivity, selectivity, and cost-efficient alternative options for the determination of TCy and CPx in real samples.

Eu MOF-enhanced FeNCD nanozymes for fluorescence and highly sensitive colorimetric detection of tetracycline

The constrained enzymatic activity and aggregation challenges encountered by small-sized nanozymes pose obstacles to their practical utility, necessitating a strategy to mitigate aggregation and boost enzymatic catalytic efficiency. In this work, a negatively charged Eu MOF was utilized as the encapsulation matrix, encapsulating the small-sized nanozymes FeNCDs into the Eu MOF to synthesize an FeNCDs@Eu MOF. The dispersibility of the encapsulated FeNCDs was increased, and owing to the negative charge of the FeNCDs@Eu MOF, electrostatic pre-concentration of the positively charged target molecule tetracycline (TC) was facilitated, thereby amplifying the enzymatic catalytic efficiency of the FeNCDs. The response of the FeNCDs to TC increased by nearly 6 times upon encapsulation. The TC detection limit (LOD) of the FeNCDs@Eu MOF-based sensor is as low as 11.63 nM. The incorporation of fluorescence detection expanded the linear range of the sensor, rendering it more suitable for practical sample detection.

Dual-state dual emission from precise chemically engineered bi-ligand MOF free from encapsulation and functionalization with self-calibration model for visual detection

Synthesis of dual-state dual emitting metal-organic frameworks (DSDE-MOFs) is uncommon and challenging. Additionally, DSDE-MOFs can fulfil the expanding need for on-site detection due to their stability and self-reference for a variety of non-analyte variables. In the present work, a novel intrinsic DSDE of chemically engineered bi-ligand Eu-based MOF (UoZ-1) was designed. The prepared UoZ-1 spherical particles were small-sized around 10-12 nm and displayed blue (425 nm) and red fluorescence (620 nm) at both states, dispersed in liquid and in solid state, when excited at 250 nm. A ratiometry platform was developed since the red emission was quenched by the addition of folic acid and the blue emission was almost remained unaffected. In the fluorometric ratiometric-mode, a dynamic linear range was recorded from 10 to 200 µM with LOD about 0.4 µM. Visual-based detection with assistance of smartphone was developed for quantification based on RGB analysis using Color Grab App. In the visual-mode, LOD as small as 2.3 µM was recorded. By utilizing the intrinsic dual-emitting UoZ-1, highly stable, recyclable, sensitive, and selective on-site visual detection of folic acid can be achieved. UoZ-1, a DSDE-MOF with no encapsulation or functionalization requirements, exhibits great potential for diverse applications.

A novel micron europium cluster coordination polymer as a strong electrochemiluminescent emitter for accurate and sensitive detection of tetracycline

In this work, a self-luminescent micron europium cluster coordination polymer (Eu-CCP) cathode electrochemiluminescence (ECL) emitter is first reported. The mass percentage of Eu in Eu-CCP is 50.1%, indicating that Eu-CCP has a high-nucleation luminescence center. In addition, our Eu-CCP possesses a stable and efficient ECL red emission performance, and the intensity is approximately 6.5-fold higher than that of the traditional tris(2,2'-bipyridyl)ruthenium(II) dichloride. The enhancement of Eu-CCP luminescence in our system is due to the following reasons: (1) the mixed ligand and high nuclear europium luminescent center can cooperate to improve the quenching effect induced by water or hydroxyl groups; and (2) external coreaction accelerator and coreactant enhancement. We also investigate the application of Eu-CCP in ECL sensors by sensitive detection of tetracycline (TC). The low detection limit (73.5 fmol·L^-1), high selectivity, good stability and satisfactory recoveries indicate that our ECL strategy can be used to detect TC accurately and sensitively.

Nanomaterial-based fluorescent biosensors for the detection of antibiotics in foodstuffs: A review

Antibiotics are widely used as bacteriostatic or bactericidal agents against various microbial infections in humans and animals. The excessive use of antibiotics has led to an accumulation of their residues in food products, which ultimately poses a threat to human health. In light of the shortcomings of conventional methods for antibiotic detection (primarily cost, proficiency, and time-consuming procedures), the development of robust, accurate, on-site, and sensitive technologies for antibiotic detection in foodstuffs is important. Nanomaterials with amazing optical properties are promising materials for developing the next generation of fluorescent sensors. In this article, advances in detecting antibiotics in food products are discussed with respect to their sensing applications, with a focus on fluorescent nanomaterials such as metallic nanoparticles, upconversion nanoparticles, quantum dots, carbon-based nanomaterials, and metal-organic frameworks. Furthermore, their performance is evaluated to promote the continuation of technical advances.

A wearable gloved sensor based on fluorescent Ag nanoparticles and europium complexes for visualized assessment of tetracycline in food samples

The abuse of tetracycline antibiotics leads to accumulating residues in the human body, seriously affecting human health. Establishing a sensitive, efficient, and reliable method for qualitative and quantitative detection of tetracycline (TC) is necessary. This study integrated silver nanoclusters and europium-based materials into the same nano-detection system to construct a visual and rapid TC sensor with rich fluorescence color changes. The nanosensor has the advantages of a low detection limit (10.5 nM), high detection sensitivity, fast response, and wide linear range (0-30 μM), which can meet the analysis requirements of different types of food samples. In addition, portable devices based on paper and gloves were designed. Through the smartphone's chromaticity acquisition and calculation analysis application (APP), the real-time rapid visual intelligent analysis of TC in the sample can be realized, which guides the intelligent application of multicolor fluorescent nanosensors.

Intelligent visual detection of OTC enabled by a multicolor fluorescence nanoprobe: Europium complex functionalized carbon dots

It is of great significance to realize ultra-sensitive and visual detection of oxytetracycline (OTC) residues, especially for public health and environmental safety. In this study, a multicolor fluorescence sensing platform (CDs-Cit-Eu) for OTC detection was constructed by using rare earth europium complex functionalized carbon dots (CDs). The blue-emitting CDs (λ_em = 450 nm) prepared by one-step hydrothermal method using nannochloropsis were not only the scaffold of Eu³⁺ ion coordination, but also the recognition unit of OTC. After adding OTC to the multicolor fluorescent sensor, the emission intensity of CDs decreased slowly, and the emission intensity of Eu³⁺ ions (λ_em = 617 nm) enhanced significantly, accompanying by a significant color change of the nanoprobe from blue to red. The detection limit of the probe for OTC was calculated to be 3.5 nM, manifesting ultra-high sensitivity towards OTC detection. In addition, OTC detection in real samples (honey, lake water, tap water) was successfully achieved. Moreover, a semi-hydrophobic luminescent film SA/PVA/CDs-Cit-Eu was also prepared for OTC detection. With the help of smartphone color recognition APP, real-time intelligent detection of OTC was realized.

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.

Ultra-small highly fluorescent zinc-based metal organic framework nanodots for ratiometric visual sensing of tetracycline based on aggregation induced emission

Color tonality by intrinsic fluorescent metal-organic frameworks (MOFs) is highly desirable in bioanalytical applications due to its stability, low-cost and robustness with no need for functionalization and/or encapsulation of fluorophores. In the present work, ultra-small and higly fluorescent zinc-based MOFs (FMOF-5) were synthesized. The prepared FMOFs were around 5 nm in size, and gave strong blue emission at 440 nm when excited at 350 nm. Interestingly, tetracycline (TC) selectively tuned the blue emission of FMOF-5 to greenish-yellow emission (520 nm) with dramatic enhancement through aggregation induced emission (AIE). The fluorimetric analysis of TC was carried out through the ratiometric peak intensities of F520/F440, with detection limit (LOD) of 5 nM. To realize quantitative point-of-care based on color tonality, a smartphone integrated with the ratiometric visual platform was thereby design. Hence, TC was visually detected with LOD of 10 nM. The prepared FMOF-5-based probe showed high stability (3 months) and reusability (∼10 times). The developed visual-based platform presents great promise for practical point of care testing due to its low-cost, robustness, ruggedness, simple operation, and excellent selectivity and repeatability.

Highly selective and multicolor ultrasensitive assay of dipicolinic acid: The integration of terbium(III) and gold nanocluster

A novel multicolor fluorescent nano-probe based on the hybridization of Tb³⁺ ion with gold nanoclusters (Au NCs) was synthesized to monitor and on-site visual assay of 2,6-pyridinedicarboxylic acid (DPA), a biomarker of bacterial spores. DPA can replace the water molecule in the center of Tb³⁺ and strongly coordinate with Tb³⁺ based on the analyte-triggered antenna effect. Simultaneously, the red fluorescence of Au NCs is not influenced after addition of DPA and can be used as steady inside fluorescence reference channel to measure background noise. On this basis, the multicolor fluorescence nano-probe based on Tb³⁺-doped Au NCs for fast analysis of DPA was fabricated. The linear range of this method is 0 to 12.5 μM and the limit of detection is 3.4 nM, which is well below the quantity of DPA concentration of 60 μM released by the spore transmission dose of anthrax infection. The proposed multicolor fluorescence nano-probe was successfully detecting DPA in actual sample with good sensitivity and specificity. In addition, the visual paper-based nano-probe is designed to detect DPA by using the color scanning application of smart phone. This developed platform possesses abroad application prospects with advantages of effective, convenient carrying, simple operation, good selectivity and repeatability.

Ratiometric fluorescence sensor based on europium-grafted ZnO quantum dots for visual and colorimetric detection of tetracycline

An europium functionalized ZnO quantum dots (QDs) ratiometric fluorescent nanoprobe is designed to establish a real time, on-site visual, and highly sensitive probe method for tetracycline (TC). The yellow-emitting ZnO QDs serves as the internal reference, while the Eu³⁺ chelated on the surface of ZnO QDs is used as the signal reporting unit. This nanoprobe exhibits rapid response, excellent selectivity, and high sensitivity with a detection limit of 4 nM in detecting the levels of TC. In addition, fluorescence of the nanoprobe can change from yellow to red as the concentration of TC increases. Thus, naked eye detection of TC was realized using the test paper processed by nanoprobe, followed by RGB value analysis function on the mobile phone APP.

Current Progress on Antibiotic Sensing Based on Ratiometric Fluorescent Sensors

Antibiotics, which can be used as veterinary drugs, are widely used in the prevention and treatment of infectious diseases for animals. However, overuse of antibiotics had caused serious problems on food contamination and human harm. For control such public issues, several of techniques have been in recent years. Ratiometric fluorescent (RF) technique, as one of the most promising strategies for quantitatively evaluated analytes, had been extensively developed for the readily measurements on the two different fluorescent emission intensities. In this review, the construction strategies for recent RF sensors will be mainly focused on. Meanwhile, the recent advances and new tendencies for detection of antibiotics based on RF technique shall be introduced. Finally, outlooks on the opportunities and challenges for quantitative fluorescence sensing on antibiotics will be summarized.

A stick-like intelligent multicolor nano-sensor for the detection of tetracycline: The integration of nano-clay and carbon dots

In recent years, the overuse of antibiotics has caused more and more serious environmental pollution, the uncontrolled abuse of antibiotics makes bacteria produce resistance to antibiotics faster than the replacement rate of antibiotics themselves, leading to the emergence of super drug-resistant bacteria. Therefore, it is of great practical significance to establish a simple, rapid and sensitive method for the detection of antibiotics. By integrating natural nano-clay (Atta) and carbon dots (CDs), the real-time and rapid visual detection of tetracycline (TC) in the sample can be realized by chromaticity pick-up APP on smartphone. The nano-sensor can detect tetracycline in the concentration between 25 nM and 20 μM with the detection limit of 8.7 nM. The low detection limit coupled with good accuracy, sensitivity and specificity meets the requirements for the detection of tetracycline in food. More importantly, the test paper and fluorescent stick-like nano-sensor are designed to detect tetracycline by polychromatic fluorescence changes. In addition, a logic gate for semi-quantitative identification of the concentration of tetracycline is designed, which makes it possible for the application of the nano-sensor in the field of smart devices.

Anodic near-infrared electrochemiluminescence from Cu-doped CdTe quantum dots for tetracycline detection

A sensitive anodic near-infrared electrochemiluminescence (ECL) immunosensor for the detection of tetracycline, based on Cu-doped CdTe quantum dots, was fabricated for the first time in this work. We have synthesized Cu-doped CdTe quantum dots by co-precipitation. The emission spectrum of the Cu-doped CdTe quantum dots could reach the near-infrared region at 730 nm in a short reflux time. More importantly, the ECL intensity of the CdTe quantum dots was enhanced by 2 fold after Cu element doping, which was attributed to the Cu d-orbital mixed with the conduction band and valence band of the host CdTe quantum dots. Inspired by the strong anodic ECL intensity of Cu-doped CdTe quantum dots, the anodic near infrared ECL sensor was constructed to detect tetracycline by competitive immunoassay. The detection range of the developed biosensor was 0.01-10 ng mL-1 and the detection limit was 0.0030 ng mL-1. In addition, the biosensor showed outstanding selectivity, long-term stability and high reproducibility, which has great potential in the field of analysis and detection.

Ultrasensitive and visual detection of tetracycline based on dual-recognition units constructed multicolor fluorescent nano-probe

Ultrasensitive and visual detection of tetracycline antibiotic (TC) residues is of great significance to public health and environmental safety. A novel dual-response ratiometric fluorescent nano-probe (SiQDs-Cit-Eu) has been elaborately tailored for the determination and on-site visual assay of tetracycline, by grafting citric acid and europium (Eu³⁺) ions onto the surface of silicon quantum dots (SiQDs). The blue-emissive SiQDs (λ_em = 455 nm) fabricated by a one-step facile method act as both scaffold for coordination with Eu³⁺ ions and recognition unit for TC owing to the inner filter effect (IFE). The coordinate unsaturated red-fluorescent Eu³⁺ ions (λ_em = 617 nm) bond to the surface of SiQDs, serving as the specific recognition element for TC due to the antenna effect. In the presence of TC, the as-synthesized nano-probe exhibits double (λ_em = 455 and 617 nm) and reverse response signals which are accompanied by a marked color change from blue to purple, and then red, thus achieving ultra-high sensitivity with a detection limit of 7.1 nM and instant visual detection of TC in real samples (milk, honey, lake and river water). Furthermore, smartphone-assisted point-of-care testing platform is also constructed based on nano-probe-immobilized test paper by using the color scanning APP.

Nanoscale Metal-Organic Frameworks as Fluorescence Sensors for Food Safety

Food safety has attracted attention worldwide, and how to detect various kinds of hazardous substances in an efficient way has always been a focus. Metal-Organic Frameworks (MOFs) are a class of hybrid porous materials formed by organic ligand and metal ions. Nanoscale MOFs (NMOFs) exhibit great potential in serving as fluorescence sensors for food safety due to their superior properties including high accuracy, great stability, fast response, etc. In this review, we focus on the recent development of NMOFs sensing for food safety. Several typical methods of NMOFs synthesis are presented. NMOFs-based fluorescence sensors for contaminants and adulterants, such as antibiotics, food additives, ions and mycotoxin etc. are summarized, and the sensing mechanisms are also presented. We explore these challenges in detail and provide suggestions about how they may be surmounted. This review could help the exploration of NMOFs sensors in food related work.

Photonic Crystal of Polystyrene Nanomembrane: Signal Amplification and Low Triggered Potential Electrochemiluminescence for Tetracycline Detection

In this work, a low triggered potential electrochemiluminescence strategy based on gold-filled photonic crystals (GPCs) electrodes composed of photonic crystals self-assembled with polystyrene spheres and gold nanoparticles embedded in the gaps of the photonic crystals was proposed. The GPCs electrodes served as the detection platform to bind antigen, and Ru(bpy)₃²⁺-COOH as a luminophore was labeled on the antibody (Ab). Then, Ru(bpy)₃²⁺-COOH/Ab was connected to the immobilized antigen on the surface of the photonic crystals by the immunoreaction to avoid direct contact with the gold nanoparticles surface. ECL emission can only be initiated by electrochemical oxidation of tripropylamine (TPrA) since Ru(bpy)₃²⁺-COOH cannot be oxidized directly on the electrode surface. The TPrA^·+ and TPrA^· radicals generated by the oxidation of TPrA can spread to the vicinity of Ru(bpy)₃²⁺-COOH over a short distance and react with the Ru(bpy)₃²⁺-COOH, eventually producing ECL emission. The potential of ECL emission caused by TPrA oxidation was about 300 mV lower than that caused by Ru(bpy)₃²⁺-COOH oxidation because the oxidation potential of TPrA (0.95 V vs SCE) was lower than Ru(bpy)₃²⁺-COOH (1.25 V vs SCE). Furthermore, the photonic crystals nanomembrane has the capability to enhance electrochemiluminescence. Thereafter, tetracycline antibiotic as a model compound was successfully detected via competitive immunoassay on GPCs electrodes with a detection limit of 0.075 pg/mL (S/N = 3), which has broad application prospects in the field of analysis and detection.

Antimonene Quantum Dots as an Emerging Fluorescent Nanoprobe for the pH-Mediated Dual-Channel Detection of Tetracyclines

Antimonene quantum dots (AMQDs) are attracting considerable attention due to their fascinating physicochemical properties. However, research on their semiconductor characteristics, especially the photoluminescence performance, is still in a preliminary stage and the experimental verification is scarcely reported, significantly restricting their further applications. Herein, the photoluminescence property of AMQDs is experimentally verified. The AMQDs are prepared by probe sonication-assisted liquid-phase exfoliation and show robust blue fluorescence, and the photoluminescence is hardly affected by pH. In view of the derivatization reaction of tetracyclines (TET) at different pHs, AMQDs are developed as a pH-mediated dual-channel ratiometric fluorescent probe for TET detection. Under acidic conditions, the AMQDs' probe exhibits unique recognition behavior due to the inherent fluorescence of TET and the solvent-enhancing effect, that is, the fluorescence changes from blue to red. Under alkaline conditions, this fluorescent probe realizes the transition from blue to yellow-green because of the decomposition of TET. The limits of detection are 27 × 10^-9 and 74 × 10^-9 m, respectively. The high sensitivity and remarkable fluorescence changes make AMQDs ideal probes for TET sensing. Additionally, this is the first report on the photoluminescence property of AMQDs. It is believed that this work will open a new avenue for AMQDs in optical sensing fields.

Smartphone-based optical assays in the food safety field

Smartphone based devices (SBDs) have the potential to revolutionize food safety control by empowering citizens to perform screening tests. To achieve this, it is of paramount importance to understand current research efforts and identify key technology gaps. Therefore, a systematic review of optical SBDs in the food safety sector was performed. An overview of reviewed SBDs is given focusing on performance characteristics as well as image analysis procedures. The state-of-the-art on commercially available SBDs is also provided. This analysis revealed several important technology gaps, the most prominent of which are: (i) the need to reach a consensus regarding optimal image analysis, (ii) the need to assess the effect of measurement variation caused by using different smartphones and (iii) the need to standardize validation procedures to obtain robust data. Addressing these issues will drive the development of SBDs and potentially unlock their massive potential for citizen-based food control.

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Optical smartphone based sensors in the food safety field are systematically reviewed.

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Recommendations on image analysis optimization are given.

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The analytical performance of smartphone based sensors is discussed.

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Available commercial devises are critically compared.

Recent Progress in Heavy Metal Ion Decontamination Based on Metal-Organic Frameworks

Heavy metals are inorganic pollutants which pose a serious threat to human and environmental safety, and their effective removal is becoming an increasingly urgent issue. Metal-organic frameworks (MOFs) are a novel group of crystalline porous materials, which have proven to be promising adsorbents because of their extremely high surface areas, optimizable pore volumes and pore size distributions. This study is a systematic review of the recent research on the removal of several major heavy metal ions by MOFs. Based on the different structures of MOFs, varying adsorption capacity can be achieved, ranging from tens to thousands of milligrams per gram. Many MOFs have shown a high selectivity for their target metal ions. The corresponding mechanisms involved in capturing metal ions are outlined and finally, the challenges and prospects for their practical application are discussed.