A dual-emission nano-rod MOF equipped with carbon dots for visual detection of doxycycline and sensitive sensing of MnO4−

Bimetallic Ag/Au nanoclusters encapsulated in ZIF-8 framework: A novel strategy for ratiometric fluorescence detection of doxycycline in food

This study successfully encapsulated the Ag+-doped Au nanoclusters (Ag/AuNCs) within the ZIF-8 framework to construct a novel Ag/AuNCs@ZIF-8 ratiometric fluorescent probe for the antibiotic doxycycline (DOX) detection. The incorporation of Ag+ contributed to the fluorescence enhancement of the nanoclusters through the "silver effect", consequently improving the stability of the developed bimetallic Ag/AuNCs. Furthermore, the encapsulation of bimetallic Ag/AuNCs within the ZIF-8 framework restricted their intramolecular vibrations, resulting in further amplification of fluorescence intensity at 595 nm. The ZIF-8 also sensitized the restoration of DOX green fluorescence at 515 nm. Within the concentration range of 0.001-20 μg mL-1, the ratio of fluorescence intensity (F515/F595) exhibited a favorable linearity for DOX concentration, with a detection limit of 36.8 ng mL-1. This ratiometric fluorescence approach had the promising potential for accurate and efficient quantitative detection of DOX residue in food and served as a valuable reference for rapid monitoring of food contaminants.

Metal-organic framework (MOF)/C-dots and covalent organic framework (COF)/C-dots hybrid nanocomposites: Fabrications and applications in sensing, medical, environmental, and energy sectors

Developing new hybrid materials is critical for addressing the current needs of the world in various fields, such as energy, sensing, health, hygiene, and others. C-dots are a member of the carbon nanomaterial family with numerous applications. Aggregation is one of the barriers to the performance of C-dots, which causes luminescence quenching, surface area decreases, etc. To improve the performance of C-dots, numerous matrices including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and polymers have been composited with C-dots. The porous crystalline structures, which are constituents of metal nodes and organic linkers (MOFs) or covalently attached organic units (COFs) provide privileged features such as high specific surface area, tunable structures, and pore diameters, modifiable surface, high thermal, mechanical, and chemical stabilities. Also, the MOFs and COFs protect the C-dots from the environment. Therefore, MOF/C-dots and COF/C-dots composites combine their features while retaining topological properties and improving performances. In this review, we first compare MOFs with COFs as matrices for C-dots. Then, the recent progress in developing hybrid MOFs/C-dots and COFs/C-dots composites has been discussed and their applications in various fields have been explained briefly.

Detection of Doxycycline Using Carbon Quantum dots as Probe Based on Internal Filtering Effect

The establishment of a convenient and effective detection method for doxycycline (DC) holds significant importance in drug monitoring and drug residue assessment. In this work, carbon quantum dots (CQDs) with excellent and stable luminescence performance (the quantum yield of CQDs was 21.8%) were synthesized by the melting method and employed as probes to monitor the fluorescence intensity variations before and after the introduction of DC. A fluorescence analytical method based on the internal filtration effect (IFE) was developed for DC determination. The mechanism of DC quenching CQDs was verified using fluorescence lifetime tests, absorption spectroscopy, and evaluation of internal filtration parameters. After optimizing experimental conditions, it was found that the DC concentration (CDC) exhibited a good linear relationship with the fluorescence quenching efficiency ((F0-F)/F0) of CQDs in the range of 5-30 µM. The fitted linear equation was Y = 0.01249*CDC+0.03625, R2 = 0.9987, and the detection limit was 2.343 µM (n = 8). This developed method has been successfully applied to accurately determine DC concentrations in both doxycycline hydrochloride tablets and human serum samples. It stands out for its simplicity, rapidity, and acceptable detection performance. Due to its advantages, this method holds great promise for application in the biomedical field for monitoring DC drug concentrations and ensuring quality control.

Smartphone-assisted colorimetric dual-mode sensing system based on europium-doped metal-organic frameworks for rapid on-site visual detection of Fe3+ and doxycycline

Exploring a rapid, sensitive, low-cost, in-situ intelligent monitoring multi-target fluorescence detection platform is important for food safety and environmental monitoring. A dual-mode ratiometric fluorescence sensing system integrated with a smartphone based on a luminescent metal-organic framework (NH2-MIL-53) and CdTe/Eu was developed for visual, in-situ analysis of Fe3+ and doxycycline (DOX) in this paper. Interestingly, with increasing Fe3+ concentration, the fluorescence sensing system exhibits dual-emission with CdTe QDs at 540 nM as the response signal and NH2-MIL-53 at 438 nm as the reference signal, resulting in a significant color shift of fluorescence color from blue-green to blue, with a linear range of 5--1550 nM and a detection limit of 1.08 nM. In the presence of DOX, the blue fluorescence of NH2-MIL-53 and the green fluorescence of CdTe QDs were quenched respectively by the internal filtering effect and the photoelectron transfer effect. While DOX enhances the red fluorescence of Eu3+ by the antenna effect, forming a triple-emission fluorescence sensor. The visual color of this fluorescent sensor shifted from blue green to grey to pink-white to pink to fuchsia to red as the DOX concentration increased with a detection limit of 0.11 nM. Furthermore, the developed intelligent sensing platform achieved real-time in-situ detection of Fe3+ and DOX with detection limit of 1.47 nM and 6.43 nM, respectively. The platform was applied to detection actual samples with satisfactory results, which proved a promising application for real-time on-site food safety monitoring and human health monitoring.

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

A sensitive multicolor fluorescence sensing strategy for chlorotetracycline based on bovine serum albumin-stabilized copper nanocluster

Fluorescent probes with on-site visual detection function have received extensive attention in the detection of chlortetracycline (CTC), which was widely used in aquaculture and animal husbandry. Copper nanoclusters (Cu NCs) with excellent optical properties were prepared using bovine serum albumin (BSA) as a template, and a multicolor fluorescence strategy based on BSA-stabilized Cu NCs (BSA-Cu NCs) for detecting CTC was proposed. BSA-Cu NCs had a red emission at 640 nm. After the addition of CTC, the red emission of BSA-Cu NCs gradually decreased for internal filtering effect, while the green emission of CTC was significantly enhanced under the sensitization of BSA. This simple sensing process can be achieved in real time by directly mixing the target sample with BSA-Cu NCs, and the detection limit (LOD) of the system for CTC was 12.01 nM. Based on this sensing strategy, a fluorescence film sensing detection platform was constructed to achieve ultra-fast detection of CTC within 30 s. This work provided a fluorescent film sensor with the advantages of portability, ultra-fast and low cost, which provided a feasible alternative for on-site ultra-fast screening of CTC.

Nanoscale carbon dot-embedded metal-organic framework for turn-on fluorescence detection of water in organic solvents

An easy-to-use, highly selective, and real-time organic solvent quality assessment is desirable to detect water contamination in organic solvents. Herein, a one-step procedure using ultrasound irradiation was used for encapsulating nanoscale carbon dots (CDs) into metal-organic framework-199 (HKUST-1) to form CDs@HKUST-1 composite. The CDs@HKUST-1 exhibited very weak fluorescence due to photo-induced electron transfer (PET) from the CDs to the Cu²⁺ centers, acting as a fluorescent sensor in its off-state. The designed material can detect and discriminate water from other organic solvents, driven by turn-on fluorescence. This highly sensitive sensing platform could be applied for the detection of water in ethanol, acetonitrile, and acetone with wide linear detection ranges of 0-70% v/v, 2-12% v/v, and 10-50% v/v and limits of detection of 0.70% v/v, 0.59% v/v, and 1.08% v/v, respectively. The detection mechanism is attributed to the interruption of the PET process due to the release of fluorescent CDs after treatment with water. A smartphone-based quantitative test was successfully developed to monitor the water content in organic solvents utilizing CDs@HKUST-1 and a phone color processing application, thus making it possible to develop an on-site, real time and easy-to-use sensor for water detection.

Engineered organotin(IV) and vanadium(V) derivatives with distinct coordination modes and luminescent properties for the efficient detection and quantification of permanganate ions

The distinction in coordination modes of metal complexes leads to their versatile structural features and unique properties. Here, we report two tetradentate Schiff base ligands (H₂L¹ and H₂L²) bearing N₂O₂ donor sets, tactically selected to provide distinct coordination modes with different metal ions. The ligands were utilized to synthesize their organotin(IV) (1-4) and vanadium(V) (5) derivatives. The synthesized compounds were characterized using elemental analysis, FT-IR spectroscopy, multi-nuclei NMR (¹H, ¹³C, and ¹¹⁹Sn) spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The organotin(IV) derivatives (1-4) displayed hepta-coordination around both the Sn centres as they were achieved in their dimeric form. Contrariwise, the vanadium(V) compound (5) was isolated as a mononuclear entity exhibiting penta-coordinated geometry around the vanadium centre. The variation in the coordination modes was evident in their UV-vis and fluorescence spectra. The organotin(IV) compounds (1-4) exhibited a strong emission band centred at 468 nm when excited at a wavelength of 360 nm whereas the vanadium(V) (5) derivative displayed poor fluorogenic response. Compound 1 was further explored for the fluorogenic chemo-sensing of permanganate ions (MnO₄^-) amongst various anions by quenching response. A detailed investigation of the recognition of permanganate ions was accomplished by spectrofluorometric, spectroscopic (¹¹⁹Sn NMR), mass spectrometric, and computational studies.

Luminescent Guests Encapsulated in Metal-Organic Frameworks for Portable Fluorescence Sensor and Visual Detection Applications: A Review

Metal-organic frameworks (MOFs) have excellent applicability in several fields and have significant structural advantages, due to their open pore structure, high porosity, large specific surface area, and easily modifiable and functionalized porous surface. In addition, a variety of luminescent guest (LG) species can be encapsulated in the pores of MOFs, giving MOFs a broader luminescent capability. The applications of a variety of LG@MOF sensors, constructed by doping MOFs with LGs such as lanthanide ions, carbon quantum dots, luminescent complexes, organic dyes, and metal nanoclusters, for fluorescence detection of various target analyses such as ions, biomarkers, pesticides, and preservatives are systematically introduced in this review. The development of these sensors for portable visual fluorescence sensing applications is then covered. Finally, the challenges that these sectors currently face, as well as the potential for future growth, are briefly discussed.

A dual-function [Ru(bpy)3]2+ encapsulated metal organic framework for ratiometric Al3+ detection and anticounterfeiting application

In this work, a novel composite material (HPU-24@Ru) has been prepared by combining a blue-emission Cd-based metal-organic framework (MOF, [Cd₂(TCPE)(DMF)(H₂O)₃]_n, HPU-24) with a red-emission tris (2,2'-bipyridine) dichlororuthenium(II) hexahydrate ([Ru(bpy)₃]²⁺) molecule for ratiometric fluorescence sensing of Al³⁺ ions in aqueous medium and high-level dynamic anticounterfeiting application. The luminescence measurement results indicated that the fluorescence intensity of HPU-24 at 446 nm showed a red shift in the presence of Al³⁺ ions, and the new peak appeared at 480 nm and continued to increase with an increase in Al³⁺ ion concentration. Meanwhile, the fluorescence intensity of [Ru(bpy)₃]²⁺ almost showed no change. The detection limit was calculated as 11.63 μM, which was better than that for the MOF-based Al³⁺ ions in some reported examples in aqueous media and achieved through strong electrostatic interactions between HPU-24@Ru and Al³⁺ ions. Moreover, owing to the particularity of the tetrastyryl structure in HPU-24, HPU-24@Ru showed intriguing temperature-dependent emission behavior. This unique structure provides the composite material HPU-24@Ru with attributes for high-level information encryption that make it difficult for counterfeiters to identify all of the right decryption measures.

A novel nanostructured organic framework sensor for selective and sensitive detection of doxycycline based on fluorescence enhancement

It is critical for human health to develop sensitive and rapid analytical methods for detecting doxycycline (DOX) residues in food. This paper presents a novel metal-organic framework nanomaterial (Zn-MOF) based on dithiodiglycolic acid and its application in DOX detection by fluorescent probe method. Zn-MOF itself does not fluoresce. When DOX is added, the system exhibits strong fluorescence (100-fold) at 530 nm. The fluorescence intensity displayed an excellent linear relationship with DOX concentration with a detection limit of 2.7 nM. The reaction solution's fluorescence displayed a visible color shift from colorless to yellow that was concentration-dependent. A smartphone was used to detect DOX by recognizing the red, green, and blue values of the reaction solution and the corresponding test paper. The use of smartphones can speed up the detection process and streamline operations, offering a sensitive and visible method for the quantitative detection of DOX residues in actual samples. Interestingly, Zn-MOF can discriminate DOX from other tetracyclines with high selectivity. This material has been used successfully as a fluorescent probe to determine DOX in fish samples with an average spiked recovery of 94.6 % ∼ 95.1 %. The DOX levels in the measured perch samples were 1.25 ∼ 157 μg/kg. There are 2 batches of DOX exceeding the standard in 14 batches.

Recent research progress in CDs@MOFs composites: fabrication, property modulation, and application

Carbon dots (CDs) have exhibited a promising application prospect in many fields because of their good fluorescence properties, biocompatibility, low toxicity, and easy functionalization. In order to improve their photoelectricity and stability, metal-organic frameworks (MOFs) can be used as host materials to provide ideal carriers for CDs to realize the multifunctional composites of CDs and MOFs (CDs@MOFs). At present, CDs@MOFs composites have shown tremendous application potential because they have various advantages of both CDs and MOFs. In this review, the synthesis methods of CDs@MOFs composites are firstly introduced. Then, the influence of the synergy between CDs and MOFs on the regulation of their structures and optical properties is highlighted. Furthermore, the recent application researches of CDs@MOFs composites in fluorescent probes, solid-state lighting, and photoelectrocatalysis are generalized. Finally, the critical issues, challenges, and solutions on their structure and property regulation and application are put forward, and their commercialization direction is also prospected.

A Novel Dual-Emission Fluorescence Probe Based on CDs and Eu3+ Functionalized UiO-66-(COOH)2 Hybrid for Visual Monitoring of Cu2

In this work, CDs@Eu-UiO-66(COOH)₂ (denoted as CDs-F2), a fluorescent material made up of carbon dots (CDs) and a Eu³⁺ functionalized metal-organic framework, has been designed and prepared via a post-synthetic modification method. The synthesized CDs-F2 presents dual emissions at 410 nm and 615 nm, which can effectively avoid environmental interference. CDs-F2 exhibits outstanding selectivity, great sensitivity, and good anti-interference for ratiometric sensing Cu²⁺ in water. The linear range is 0-200 µM and the limit of detection is 0.409 µM. Interestingly, the CDs-F2's silicon plate achieves rapid and selective detection of Cu²⁺. The change in fluorescence color can be observed by the naked eye. These results reveal that the CDs-F2 hybrid can be employed as a simple, rapid, and sensitive fluorescent probe to detect Cu²⁺. Moreover, the possible sensing mechanism of this dual-emission fluorescent probe is discussed in detail.

Design of "turn-off" luminescent Ln-MOFs for sensitive detection of cyanide anions

Two novel 2D lanthanide metal-organic frameworks (Ln-MOFs), namely {[Eu₂(DBTA)₃(DMF)₂]·DMF}_n (1) and {[Tb₂(DBTA)₃(DMF)₂]·DMF}_n (2) (H₂DBTA = 2,5-dibromoterephthalic acid), have been successfully synthesized by the solvothermal method. Single-crystal X-ray diffraction results proved that the complexes possess the same topological structure of a (4²·6)₂(4²·8⁴)(4⁷·6³)₂-connected net. The recognition of CN^- from interfering anions with a low detection limit by "turn-off" luminescence makes them promising candidates for the highly selective and sensitive detection of the cyanide ion. The Ln-MOFs 1 and 2 exhibit excellent chemical sensing properties for CN^- with efficiency, selectivity, and excellent performance in various mixed anions. The evaluation parameters, including the quenching constant and detection limit, have been investigated to obtain the detection performance for CN^-.

Promising energy transfer system between fuorine and nitrogen Co-doped graphene quantum dots and Rhodamine B for ratiometric and visual detection of doxycycline in food

A novel sensor based on dual emissive fluorescent graphene quantum dots is developed for a rapid, selective, sensitive and visual detection of doxycycline (DOX). The ratiometric fluorescent probe is designed by grafting fluorescent group (Rhodamine B, RhB) on F, N-doped graphene quantum dots (FNGQDs). In the presence of DOX, the fluorescence at 466 nm is remarkably quenched due to inner filter effect and fluorescence resonance energy transfer, whereas the peak at 592 nm is attenuated slightly due to the energy transfer in the emission peaks of FNGQDs and RhB functional group. The sensor shows good linear relationship from 0.04 to 100 µM with a low detection limit of 40 nM. Furthermore, the flexible solid-state fluorescent sensing platform is used for detecting DOX in milk, pork and water samples. Therefore, this dual-emission FGQD-RhB can be used as a high-performance fluorescent and visual sensor for food safety and environmental monitoring.

Sensing performance and mechanism of carbon dots encapsulated into metal-organic frameworks

Metal-organic frameworks (MOFs) can be combined with nanomaterials and the combined composites have excellent optical properties. Carbon dots (CDs) with tiny particle size, non-toxic and rich surface functional groups are novel fluorescent materials. Carbon dots@metal-organic frameworks (CDs@MOFs) are synthesized by encapsulating CDs into MOFs. CDs@MOFs are promising composites for the preparation of a new generation of fluorescence sensors, which combine the hybrid properties of MOFs and the special optical properties of CDs. Urged as such, we are encouraged to categorize according to the sensing mechanisms. These include fluorescence resonance energy transfer (FRET), aggregation-caused quenching (ACQ), static quenching, dynamic quenching, photo-induced electron transfer (PET), inner filter effect (IFE) and so on. Based on the above mechanisms, CDs@MOFs can specifically interact with target analytes to generate fluorescence quenching. This review covers the research progress of CDs@MOFs in recent five years (with 103 refs), synthetic design of CDs@MOFs and introduces the sensing mechanism. The current challenges and future research directions are discussed briefly. The sensing mechanism and applications of CDs@MOFs.

A luminescent Eu3+-functionalized MOF for sensitive and rapid detection of tetracycline antibiotics in swine wastewater and pig kidney

Tetracyclines (TCs), a type of antibiotics, are widely used in human therapy and animal husbandry. Public concerns about tetracyclines residues have been raised due to their negative impact on the environment and food, causing bacterias drug resistance and human health concerns. In this work, a luminescent europium MOF (EuUCBA) is constructed via post-synthetic attachment of Eu³⁺ into a UiO-66 type MOF. The luminescent of EuUCBA exhibits high stability in aqueous media in the pH range of 4-11. Among 36 common veterinary drugs, the synthesized probe is highly selective and sensitive to six tetracyclines with low detection limits of 0.118 μM, 0.228 μM, 0.102 μM, 0.138 μM, 0.206 μM, and 0.078 μM for oxytetracycline (OTC), chlortetracycline (CTC), methylenetetracycline (MTC), minocycline (MOC), tetracycline (TC), and doxycycline (DOXY), respectively. Furthermore, the probe shows good anti-interference ability and fast response. Finally, EuUCBA was successfully to detect DOXY in swine wastewater and pig kidney with good recoveries. This work provides an excellent fluorescent sensor for highly selective and rapid detection of TCs residues in wastewater and complex biological samples.

Recent Advances in the Application and Mechanism of Carbon Dots/Metal-Organic Frameworks Hybrids in Photocatalysis and the Detection of Environmental Pollutants

Metal-organic frameworks (MOFs) are a class of crystalline porous materials with simple synthesis conditions, large specific surface area, structural diversity, and a wide range of interesting properties. The integration of MOFs with other materials can provide new multifunctional composites that exhibit both component properties and new characteristics. In recent years, the integration of carbon dots (CDs) into MOFs to form composites has shown improved optical properties and fascinating new characteristics. This review focuses on the design and synthesis strategies of CDs@MOFs composites (including pore-confined synthesis, in situ encapsulation, post-synthesis modification and impregnation method) and their recent research progress in photocatalysis and detection of environmental pollutants. Both the achievements and problems are evaluated and proposed, and the opportunities and challenges of CDs@MOF composite are discussed.

Confinement of Luminescent Guests in Metal–Organic Frameworks: Understanding Pathways from Synthesis and Multimodal Characterization to Potential Applications of LG@MOF Systems

This review gives an authoritative, critical, and accessible overview of an emergent class of fluorescent materials termed “LG@MOF”, engineered from the nanoscale confinement of luminescent guests (LG) in a metal–organic framework (MOF) host, realizing a myriad of unconventional materials with fascinating photophysical and photochemical properties. We begin by summarizing the synthetic methodologies and design guidelines for representative LG@MOF systems, where the major types of fluorescent guest encompass organic dyes, metal ions, metal complexes, metal nanoclusters, quantum dots, and hybrid perovskites. Subsequently, we discuss the methods for characterizing the resultant guest–host structures, guest loading, photophysical properties, and review local-scale techniques recently employed to elucidate guest positions. A special emphasis is paid to the pros and cons of the various methods in the context of LG@MOF. In the following section, we provide a brief tutorial on the basic guest–host phenomena, focusing on the excited state events and nanoscale confinement effects underpinning the exceptional behavior of LG@MOF systems. The review finally culminates in the most striking applications of LG@MOF materials, particularly the “turn-on” type fluorochromic chemo- and mechano-sensors, noninvasive thermometry and optical pH sensors, electroluminescence, and innovative security devices. This review offers a comprehensive coverage of general interest to the multidisciplinary materials community to stimulate frontier research in the vibrant sector of light-emitting MOF composite systems.

A ratiometric fluorescence-scattering sensor for rapid, sensitive and selective detection of doxycycline in animal foodstuffs

The residue problem of tetracycline antibiotics, especially doxycycline (DC), in animal foodstuffs has attracted much attention. This paper reported ZIF-8 and bovine serum albumin (BSA) as a ratiometric fluorescence-scattering sensor for DC. The mechanism relied on the disassembly of ZIF-8 caused by DC, bringing weakened second-order scattering, and the double fluorescence amplification of DC under ZIF-8 with BSA, inducing enhanced fluorescence. The response of the sensor was completed within 1 min, and the detection limit for DC (3.4 nM) was 1-2 orders of magnitude lower than the reported ones. The distinguishment of DC from other tetracycline antibiotics was also achieved by the sensor. The sensor was applied to detecting DC in animal foodstuffs with satisfactory recoveries (80.0-104.0%). Hence, this work develops a rapid, sensitive and selective ratiometric sensor to monitor the DC residue in animal foodstuffs, also opens the window to construct ratiometric DC sensors with the fluorescence-scattering strategy.

Smartphone-assisted ratiometric fluorescence sensing platform for the detection of doxycycline based on BCNO QDs and calcium ion

A novel colorimetric and ratiometric fluorescence sensor has been established based on boron carbon oxynitride quantum dots (BCNO QDs) and Ca²⁺ for the detection of doxycycline (DOX). BCNO QDs were synthesized by microwave-assisted method with boric acid and ethylenediamine. The fluorescence of BCNO QDs at 425 nm was quenched due to the electrostatic interaction and inner filter effect with doxycycline. Meanwhile, doxycycline was combined with Ca²⁺ to form a fluorescence complex, which generated a new fluorescence peak at 520 nm. The fluorescence intensity ratio (F₅₂₀/F₄₂₅) has a good linear relationship with doxycycline concentration, and the detection limit is 25 nM. Moreover, the fluorescence of the reaction solution showed a concentration-dependent visual color change from blue to green. In order to facilitate further application, a portable fluorescent test paper which is easy to store was prepared. The RGB values of the reaction solution and corresponding test paper were identified by smartphone, and the visual detection of doxycycline was performed by digital image colorimetric analysis. The application of smartphone and fluorescent test paper can effectively shorten the detection time and simplified the operation, providing an effective scheme for quantitative detection of doxycycline in actual samples. Overall, this work provides a method for the detection of doxycycline and shows that the BCNO QDs have great potential application in food safety.

UiO-66-NH2 based fluorescent sensing for detection of tetracyclines in milk

In this work, a fluorescent sensor based on a zirconium-based metal organic framework was prepared for the detection of tetracyclines (TCs) in milk. The UiO-66-NH₂ fluorescent sensor was synthesized by a simple microwave-assisted method with 2-aminoterephthalic acid and zirconium chloride as precursors. In the presence of target TCs, the synergistic effect of the inner filter effect (IFE) and photo-induced electron transfer (PET) was responsible for the fluorescence quenching of UiO-66-NH₂, and the fluorescence sensor showed a rapid fluorescence quenching response (5 min) to target TCs. The proposed UiO-66-NH₂ sensor had good sensitivity and selectivity, and under the optimal conditions possessed detection limits of 0.449, 0.431, and 0.163 μM for tetracycline (TET), oxytetracycline (OTC), and doxycycline (DOX), respectively. Besides, the UiO-66-NH₂ sensor was successfully applied to the quantitative detection of TCs in milk samples with reasonable recoveries of 93.26–115.17%, and the detection results achieved from the as-fabricated fluorescence sensing assay were consistent with those of high-performance liquid chromatography (HPLC), indicating the potential applicability of the UiO-66-NH₂ sensor for detecting TCs in actual food samples.

We have developed a UiO-66-NH₂ fluorescent sensor by a simple microwave-assisted method. The synthetic UiO-66-NH₂ sensor has good sensitivity and selectivity for tetracyclines determination in milk samples.

Facile synthesis of dual-emission fluorescent carbon nanodots for a multifunctional probe

In this study, we developed a facile method for synthesizing dual-emission carbon nanodots (CDs) through trimesic acid and o-phenylenediamine through electrolysis for 2 h. The synthesized CDs were mainly 3-7 nm in size, with an average size of 5.17 nm. The dual-emission fluorescent property of these CDs could be observed under two different excitation wavelengths. The green emission of the CDs could be quenched after the addition of mercury ions or copper ions, and the blue emission of the CDs could be inhibited using hydroxychloroquine (HCQ). Furthermore, the quenched fluorescence of CDs/Cu2+ could be recovered through the addition of glyphosate. We developed a multifunctional chemical sensor by using these special fluorescence materials. Under optimal conditions, the detection limits of mercury ions, glyphosate, and HCQ were 0.42 μM, 1.1 mg L-1, and 0.14 μM, respectively. Moreover, this method can be used to detect mercury ions, glyphosate, and HCQ in environmental water, cereals, and urine samples, respectively.

Preparation of Eu0.075Tb0.925-Metal Organic Framework as a Fluorescent Probe and Application in the Detection of Fe3+ and Cr2O72

Luminescent Ln-MOFs (Eu0.075Tb0.925-MOF) were successfully synthesised through the solvothermal reaction of Tb(NO3)3·6H2O, Eu(NO3)3·6H2O, and the ligand pyromellitic acid. The product was characterised by X-ray diffraction (XRD), TG analysis, EM, X-ray photoelectron spectroscopy (XPS), and luminescence properties, and results show that the synthesised material Eu0.075Tb0.925-MOF has a selective ratio-based fluorescence response to Fe3+ or Cr2O72-. On the basis of the internal filtering effect, the fluorescence detection experiment shows that as the concentration of Fe3+ or Cr2O72- increases, the intensity of the characteristic emission peak at 544 nm of Tb3+ decreases, and the intensity of the characteristic emission peak at 653 nm of Eu3+ increases in Eu0.075Tb0.925-MOF. The fluorescence intensity ratio (I653/I544) has a good linear relationship with the target concentration. The detection linear range for Fe3+ or Cr2O72- is 10-100 μM/L, and the detection limits are 2.71 × 10-7 and 8.72 × 10-7 M, respectively. Compared with the sensor material with a single fluorescence emission, the synthesised material has a higher anti-interference ability. The synthesised Eu0.075Tb0.925-MOF can be used as a highly selective and recyclable sensing material for Fe3+ or Cr2O72-. This material should be an excellent candidate for multifunctional sensors.

A highly selective ratiometric fluorescent probe for doxycycline based on the sensitization effect of bovine serum albumin

Fluorescent probes with in-situ visual feature have received numerous attentions for detecting doxycycline (DC), a semisynthetic tetracycline antibiotic widely used in animal husbandry. However, reported fluorescent probes commonly fail to selectively detect DC among tetracycline antibiotics due to their structural similarity. In this work, bovine serum albumin-capped gold nanoclusters (BSA-AuNCs) were ingeniously used as the ratiometric fluorescent probe for detecting DC over other tetracycline antibiotics through the selective sensitization effect of BSA on DC. After adding DC, the red fluorescence of BSA-AuNCs almost remained unchanged, while the green fluorescence of DC also emerged under the sensitization of BSA. BSA-AuNCs showed the highest response toward DC among tetracycline antibiotics ascribed to the strongest sensitization effect of BSA on DC. BSA-AuNCs also displayed the features of simple synthesis, short response time (1 min) and low detection limit (36 nM). BSA-AuNCs were finally applied to detecting DC in fish samples, and further fabricated into test strips for ease of carrying. Thus, this work proposes an efficient strategy to design fluorescent probe for selectively detecting DC among tetracycline antibiotics.

Inner-filter effect induced fluorescent sensor based on fusiform Al-MOF nanosheets for sensitive and visual detection of nitrofuran in milk

Developing highly sensitive and visual methods for rapid detection of antibiotics is significant to ensure food quality and safety. To meet the requirement of nitrofuran antibiotics detection, luminescent fusiform Al(III)-containing metal-organic frameworks (Al-MOF) nanosheets were successfully synthesized by one-step hydrothermal method. And then, the nanosheet served as a fluorescent probe to detect nitrofuran via the inner-filter effect mechanism. The developed sensor allowed sensitive and selective detection of nitrofuran with good linear relationships. And, the detection limit (LOD) values were estimated to be 0.53, 0.838 and 0.583 μM for nitrofurazone, nitrofurantoin and furazolidone detection, respectively. The practical application of the proposed system was verified by HPLC in spiked milk samples with satisfying recoveries ranging from 88.14 to 126.21% and low relative standard deviations of 2.85 ~ 8.13%. Moreover, we designed fluorescent test papers for semi-quantitative detection of nitrofuran via naked-eye colorimetric assay. The established method provides an alternative strategy for semiquantitative detection of nitrofuran.

A ratiometric fluorescent probe based on sulfur quantum dots and calcium ion for sensitive and visual detection of doxycycline in food

The residue of doxycycline in food can cause harm to human. Therefore, the detection of doxycycline residue is necessary. Herein, a ratiometric fluorescent probe was designed based on sulfur quantum dots (S dots) and Ca²⁺. Due to static quenching and inter filter effect between doxycycline and S dots, doxycycline quenched fluorescence of S dots at 450 nm. Meanwhile, doxycycline and Ca²⁺ formed fluorescent complex through coordination to produce new peak at 520 nm. The ratio of fluorescence intensity (F₅₂₀/F₄₅₀) and doxycycline concentration showed good linear relationship with detection limit of 0.19 μM. The fluorescence color of S dots/Ca²⁺ changed from blue to light green with increasing doxycycline concentration, which was applied for visual semi-quantitative detection of doxycycline. Moreover, the method was used for detecting doxycycline in milk and fish samples with recoveries in the range of 91%-110%. The method showed good application potential in detection of doxycycline in food samples.

Encapsulation of Luminescent Guests to Construct Luminescent Metal-Organic Frameworks for Chemical Sensing

Metal-organic frameworks (MOFs), which are a class of coordination polymers constructed by metal ions or clusters with organic ligands, have emerged as exciting inorganic-organic hybrid materials with the superiorities of inherent crystallinity, adjustable pore size, clear structure, and high degree of functionalization. The MOFs have attracted much attention to develop good luminescent functional materials due to their inherent luminescent centers of both inorganic and organic photonic units. Furthermore, the pores within MOFs can also be used to encapsulate a large number of luminescent guest species, which provides a broader luminescent property for MOF materials. MOFs possess the incomparable multifunctional advantages of inorganic and organic luminescent materials. A large number of luminescent MOFs (LMOFs) have been synthesized for applications in sensing, white-light-emitting diodes (LED), photocatalysis, biomedicine, etc. This paper reviews the encapsulation of various luminescent guests such as lanthanide ions, dyes, quantum dots, and luminescent complexes in metal-organic frameworks to construct luminous sensors with single- or double-emission centers, as well as the research progress of these sensors in chemical sensing. Finally, the challenges in these fields were outlined and the prospects for future development were put forward.

Carbon Dots: An Emerging Smart Material for Analytical Applications

Carbon dots (CDs) are optically active carbon-based nanomaterials. These nanomaterials can change their light emission properties in response to various external stimuli such as pH, temperature, pressure, and light. The CD's remarkable stimuli-responsive smart material properties have recently stimulated massive research interest for their exploitation to develop various sensor platforms. Herein, an effort has been made to review the major advances made on CDs, focusing mainly on its smart material attributes and linked applications. Since the CD's material properties are largely linked to their synthesis approaches, various synthesis methods, including surface passivation and functionalization of CDs and the mechanisms reported so far in their photophysical properties, are also delineated in this review. Finally, the challenges of using CDs and the scope for their further improvement as an optical signal transducer to expand their application horizon for developing analytical platforms have been discussed.

Bright Mn-doped carbon dots for the determination of permanganate and L-ascorbic acid by a fluorescence on-off-on strategy

A one-pot hydrothermal synthesis of manganese-doped carbon dots (Mn-CDs) is reported for fluorescent "on-off-on" determination of Mn(VII) and L-ascorbic acid (L-AA) in aqueous solution and living cells. Mn-CDs were prepared by using sulfanilic acid, tetrakis(hydroxymethyl)phosphonium chloride, and Mn(II) chloride as precursors. Mn-CDs were characterized by several spectroscopic methods and microscopic techniques. Mn-CDs show distinctly long fluorescence lifetime (12.39 ± 0.07 ns) and high absolute fluorescence quantum yield (around 37%) with excitation and emission wavelengths of 362 and 500 nm, respectively. Mn-CDs exhibit no significant cytotoxicity to human cervical carcinoma HeLa cells and human embryonic kidney HEK-293T cells at 200 μg mL^-1 level after 48 h incubation. The fluorescence of Mn-CDs at 500 nm (excited at 362 nm) is quenched efficiently by Mn(VII) and can be further recovered after the addition of L-AA, resulting in a fluorescent "on-off-on" assay for the determination of Mn(VII) and L-AA. Under optimal experimental conditions, the linear response covers the 3 to 150 μM Mn(VII) concentration range and the 3 to 140 μM L-AA concentration range. This method offers relatively low detection limits of 0.66 μM for Mn(VII) and 0.90 μM for L-AA. This strategy was applied to visual determination of Mn(VII) and L-AA in living HeLa cells with satisfying results. Graphical abstract Schematic presentation of bright Mn-CD-based fluorescence "on-off-on" assay for both Mn(VII) and L-AA. This fluorescent assay possessed low detection limit of 0.66 μM for Mn(VII) and 0.90 μM for L-AA. This strategy was applied for visual determination of Mn(VII) and L-AA in living HeLa cells with satisfying results.

A stable LnMOF as a highly efficient and selective luminescent sensor for detecting malachite green in water and real samples

A series of isostructural 3D lanthanide metal–organic frameworks (LnMOFs), with the formula n(H₃O)[Ln(L)(H₂O)]_n·nH₂O (Ln = Gd 1, Eu 2 and Tb 3, H₄L = 3,5,3′,5′-oxytetrabenzoic acid), have been successfully synthesized by solvothermal reactions. Single-crystal X-ray diffraction analysis reveals that 1–3 are constructed from wave-like Ln–carboxylate chains which are further connected by the ligands to form 3D channel-type frameworks. Further experiments suggest that 3 is thermally stable up to 322 °C and exhibits outstanding chemical stability in aqueous solutions with the pH ranging from 3 to 11. Significantly, 3 can be utilized for the first time to detect malachite green (a synthetic antibiotic to cure saprolegniasis) in aqueous media even in the presence of other interfering antibiotics, with a high sensitivity (K_sv = 8.33 × 10⁴ M⁻¹), low detection limit (DL = 0.25 μM) and good recyclability. On a more practical note, we found that the luminescence intensity of 3 showed almost no response to pH changes (pH 3–11), allowing steady sensing in real samples such as river water, simulated human serum and urine with satisfactory recoveries and RSD.

A Tb-MOF with chemical stability was synthesized. It showed excellent luminescent sensing for MG in river water, simulated serum and urine.

Synthesis and sensitive detection of doxycycline with sodium bis 2-ethylhexylsulfosuccinate based silver nanoparticle

The monitoring of residual antibiotics in the environment has gained a significant importance for the effective control, because of the high risk to human health. A simple strategy was designed for the green synthesis and detection of doxycycline (Dox) by using anionic surfactant sodium bis 2-ethylhexylsulfosuccinate based silver nanoparticles (AOT-AgNPs). The chemical reduction and capping of Ag⁺¹ ions was achieved by sulfonyl and carbonyl functional groups of AOT molecule. The AOT-AgNPs were found to have excellent stability at variable environmental parameters (i.e. temperature, storage period, salt concentration and pH) possibly due to the strong emulsifying nature of the surfactant. Mechanism of interaction between the AOT-AgNPs and Dox was established with UV/visible, Fourier transform infrared (FTIR) spectroscopy, Atomic force microscopy (AFM) and Dynamic light scattering (DLS) techniques, which suggests the interaction via aggregates formation. The synthesize probe could detect the Dox within 15 min over a wide range of concentrations from 0.1 to 140μM with limit of detection (LOD) of 0.2 μM. As proof of strategy, we have illustrated that the AOT-AgNPs also detect Dox in biological and environmental samples with negligible interference and very significant recovery rates. Moreover, non-toxic nature against various tested cell lines (i.e. normal mouse fibroblast (NIH-3 T3) and cancerous non-small lung carcinoma (NCI-H460)) and significant antimicrobial, antibiofilm and biofilm eradicating potential of AOT-AgNPs were provide ideal nanomaterial for further applications.

Integration of fluorescent probes into metal–organic frameworks for improved performances

Recent years have witnessed a rapid development of fluorescent probes in both analytical sensing and optical imaging. Enormous efforts have been devoted to the regulation of fluorescent probes during their development, such as improving accuracy, sensitivity, selectivity, recyclability and overcoming the aggregation-caused quenching effect. Metal–organic frameworks (MOFs) as a new class of crystalline porous materials possess abundant host–guest chemistry, based on which they display a great application potential in regulating fluorescent probes. This review summarized the research works on the regulation of fluorescent probes using MOFs, with emphasis on the methods of integrating fluorescent probes into MOFs, the regulation effects of MOFs on fluorescent probes, the superiorities of MOFs in regulating fluorescent probes, and the outlook of this subject. It is desirably hoped that this review can provide a useful reference for the researchers interested in this field.

This review surveyed the research works for the regulation of fluorescent probes with metal–organic frameworks based on host–guest chemistry.

Safranin O-functionalized cuboid mesoporous silica material for fluorescent sensing and adsorption of permanganate

A new safranin O-based chelating fluorophore coupled, dual-functionalized organic–inorganic hybrid material has been prepared for simultaneous MnO₄⁻ detection and adsorption in aqueous media and living organisms such as limnodrilus claparedianus and zebrafish.

A water-stable zinc(ii)-organic framework as a multiresponsive luminescent sensor for toxic heavy metal cations, oxyanions and organochlorine pesticides in aqueous solution

A novel metal-organic framework with the formula [Zn3(DDB)(DPE)]·H2O (1) (H5DDB = 3,5-di(2',4'-dicarboxylphenyl)benzoic acid and DPE = 1,2-di(4-pyridyl)ethylene) has been solvothermally synthesized by employing a rigid carboxylate ligand H5DDB to assemble with Zn(ii) ions in the presence of a flexible bis(pyridyl) linker DPE. The Zn-MOF is a 3D framework with six-nuclear clusters and possesses remarkable water stability and pH stability. Interestingly, complex 1 can sensitively and selectively sense Fe(iii), Cr(iii), Cr(vi), Mn(vii) and the pesticide 2,6-Dich-4-NA with low detection limits in aqueous solution. Moreover, complex 1 also exhibits selectivity for 2,6-Dich-4-NA detection in real samples including carrot, grape and nectarine extracts, and its detection ability is almost unchanged in the presence of the surfactant sodium dodecyl sulfate (SDS). The possible mechanisms of luminescence quenching have been explained by the weak affinity of nitrogen atoms, resonance energy transfer, and photoinduced electron transfer. To our knowledge, this is the first example of a MOF-based multiresponsive fluorescent probe for the simultaneous detection of Fe(iii), Cr(iii/vi), Mn(vii) and the pesticide 2,6-Dich-4-NA in aqueous solution.

A novel ratiometric fluorescence probe for highly sensitive and specific detection of chlorotetracycline among tetracycline antibiotics

It is of great importance to detect chlorotetracycline (CTC) in a highly sensitive and specific way because of its wide distribution in aquaculture and animal husbandry. Herein, we propose a novel ratiometric fluorescence strategy to assay CTC by using bovine serum albumin stabilized gold nanoclusters (BSA-AuNCs). The BSA-AuNCs consisting of 25 gold atoms (Au₂₅NCs) display a red emission at 640 nm (λ_ex = 370 nm). In the presence of CTC, a new blue emission at 425 nm is emerged and its intensity dramatically increases with the addition of more the analyte; meanwhile the red emission at 640 nm shows a linear decrease reversely. However, at identical conditions neither the analogues of CTC as tetracycline (TC), oxytetracycline (OTC) or doxycycline (DC) induces similar response of BSA-AuNCs. Such interesting phenomenon is proven related to the conversion from large Au₂₅NCs to smaller nanoclusters composing 8 gold atoms (Au₈NCs), which intrinsically originate from the interaction between CTC and the ligand BSA. Therefore, a ratiometric probe is established to sensitively detect CTC in the wide range (0.2-10 μM) with a low limit of detection (LOD) at 65 nM. In addition, this strategy can also be applied to assay CTC in human serum, showing great promise for practical applications in future.

Dual emission carbon dots from carotenoids: Converting a single emission to dual emission

Dual emission carbon dots have a high potential for use as fluorescence-based sensors with higher selectivity and sensitivity. This study demonstrated the possibility of conversion of a biological molecular system with a single emission peak to a double emission carbon dots system. This report is the first to describe the synthesis of dual emission carbon dots by tuning the electronic environment of a conjugated system. Here we prepared carbon dots from a natural extract, from which carotenoids were used as a new source for carbon dots. Formation of the carbon dots was confirmed by images obtained under a transmission electron microscope as well as from a dynamic light scattering study. The prepared carbon dots system was characterized and its optical property was monitored. The study showed that, after irradiation with microwaves, the fluorescence intensity of the whole system changed, without any change in the original peak position of the carotenoid but with the appearance of an additional peak. A Fourier transform infrared study confirmed breaking of the conjugated system. When using ethylene glycol as a surface passivating agent added to these carotenoid carbon dots, the dual emission spectra became more distinct.

Green synthesis of fluorescent PEG-ZnS QDs encapsulated into Co-MOFs as an effective sensor for ultrasensitive detection of copper ions in tap water

In this work, a novel and highly fluorescent (FL) metal-organic framework (MOF)-based host-guest hybrid system was developed through encapsulation of polyethylene glycol (PEG)-capped ZnS quantum dots (QDs) into zeolitic imidazolate framework (ZIF)-67 at ambient temperature. This new composite material was characterized by FT-IR, XRD, TEM, UV-Vis absorption and FL spectroscopy, and then exploited as fluorescence sensor for highly sensitive and selective detection of Cu (II) ions in water samples. The as-prepared PEG-ZnS QDs@ZIF-67 nanohybrids took advantages of both accumulation effect in ZIF-67 and FL sensitivity and selectivity in ZnS QDs toward analyte-Cu²⁺. In this regard, ZIF-67 was treated as absorbents to capture and enrich Cu (II) ions, and ZnS QDs were exploited as tentacle to selectively and sensitively sense the bonding interactions between ZIF-67 and Cu (II) ions, and further transduce these chemical events to the detectable fluorescence signals. By this approach, Cu²⁺ could be detected in a wide concentration range of 3 to 500 nM with a LOD as low as 0.96 nM. The proposed FL-sensor can be promising in the field of preparation of various QDs@MOFs platforms for application in high-performance optical sensing.