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

Detection of doxycycline via a smartphone-assisted ratiometric fluorescence platform with cerium‑nickel bimetallic nanoclusters

The accumulation of tetracycline antibiotics, such as doxycycline (DOX), in animal-derived foods poses considerable health risks to humans when found in excessive concentration levels. This research introduces a ratiometric fluorescence sensor for the detection of DOX utilizing ovalbumin-stabilized cerium‑nickel bimetallic nanoclusters (OVA-Ce/Ni NCs). The sensor exploits the sensitizing effect of ovalbumin on DOX, resulting in a fluorescence peak at 510 nm, while its inherent blue fluorescence at 420 nm gradually quenches. Under optimal conditions, the OVA-Ce/Ni NCs sensor exhibited a linear response range of 0.2-80 μM for DOX, with an exceptionally low detection limit of 51 nM. The sensor exhibited excellent selectivity for DOX and was successfully applied for the detection of DOX in milk and fish samples. Furthermore, smartphone-assisted detection facilitated visual identification of DOX. This study demonstrates a highly sensitive and selective approach for detecting DOX in animal-derived foods, with potential implications in pollutant identification and control.

Water-stable Eu(III) coordination polymer-based ratiometric fluorescence sensor integrated with smartphone for onsite monitoring of doxycycline hydrochloride in milk

The widespread misuse of doxycycline hydrochloride (Dox) in livestock farming has necessitated the development of rapid and reliable methods for monitoring its residues in food products. Herein, a water-stable europium coordination polymer-Eu(C2O4)1.5(H2O)ₙ (Eu-CP) with a layered structure was synthesized via a one-step hydrothermal approach. Leveraging its dual-emission properties (455 nm ligand-centered blue emission and 615 nm Eu(III)-based red emission), we engineered a ratiometric fluorescence sensor (I₆₁₅/I₄₅₅) for Dox detection. The sensing mechanism involves synergistic effects of the antenna effect and Dox@Eu-CP complexation, enabling selective Dox recognition with a wide linear range (10-100 μM) and a low detection limit (0.46 μM, S/N = 3). To facilitate onsite analysis, a smartphone-integrated platform was developed, translating the Dox concentration-dependent color transition (blue → red) into quantifiable R/G values via a custom Android application. Practical applicability was demonstrated in milk samples, achieving recoveries of 82.4-119.4% (fluorescence) and 87.8-113.3% (smartphone) with RSD < 5%. This work pioneers the integration of lanthanide coordination polymers with portable digital detection, offering a green and visual strategy for antibiotic residue monitoring in food safety.

Multienzyme-Activity Sulfur Quantum Dot Nanozyme-Mediated Cascade Reactions in Whole-Stage Symptomatic Therapy of Infected Bone Defects

Integrating the therapeutic efficacy of early bacterial clearance, midstage inflammatory remission, and late-stage effective tissue healing is considered a pivotal challenge in symptomatic treatment of infected bone defects (IBDs). Herein, a microenvironment-adaptive nanoplatform based on a sulfur quantum dot (SQD) nanozyme was proposed for whole-stage symptomatic therapy of IBDs by mediating the sequence of enzyme cascade reactions. The SQD nanozyme prepared by a size-engineering modification strategy exhibits enhanced multienzyme activity compared to conventional micrometer- and nanometer-sized sulfur particles. In the early stages of bacterial infection, the SQD nanozyme self-activates superoxide dismutase-peroxidase activity, resulting in the production of reactive oxygen species (ROS) that effectively eliminate bacteria. After disinfection, the SQD nanozyme self-switched to superoxide dismutase-catalase mimetic behavior and eliminated excess ROS, efficiently promoting macrophage polarization to an anti-inflammatory phenotype in the midinflammatory microenvironment. Importantly, SQD nanozyme-mediated M2 macrophage polarization significantly improved the damaged bone immune microenvironment, accelerating bone repair at late-stage tissue healing. Therefore, this strategy offers a promising and viable approach for the treatment of infectious tissue healing by developing multienzyme-activity nanozymes that respond intelligently to the microenvironment at different stages, effectively fighting bacteria, reducing inflammation, and promoting tissue regeneration for whole-stage symptomatic therapy.

Advances in the synthesis and properties of sulfur quantum dots for food safety detection and antibacterial applications

Food safety is closely related to human health and has become a worldwide, pressing concern. Food safety analysis is essential for ensuring food safety. Sulfur quantum dots (SQDs), a new type of zero-dimensional metal-free nanomaterials, have recently become the focus of scientific research due to their good luminescence properties, dispersibility, biocompatibility, and inherent antibacterial properties. This review focuses on recent advances in SQDs, with emphasis on their practical applications in the food field. First, commonly used methods for the synthesis of SQDs are presented, including traditional and emerging strategies. The properties of SQDs are then analyzed in detail, particularly their luminescence properties, catalytic activities, and reducing properties. Next, the use of SQDs in food safety detection and antibacterial fields are elaborated. Finally, this review discusses the challenges associated with the use of SQDs in food safety detection and antimicrobial applications.

Heterometallic Eu/Zn-MOF-based ratiometric sensing platform: Highly sensitive fluorescence / second-order scattering identification of tetracycline analogs and its molecular informatization applications

The traditional preparation method of ratiometric probes faces challenges such as cumbersome preparation and low sensitivity. Thus, there is an urgent need to provide a simple method of preparing a highly sensitive ratiometric probe. Here, Eu3+-doped zinc-based organic framework (Eu/Zn-MOF) was prepared through hydrothermal method for the detection of tetracycline analogs (TCs). Under the same excitation conditions, the probe can simultaneously display valuable fluorescence and second-order scattering signals. The developed probe enabled specific identification and fast detection (1 min) of TCs, including tetracycline, oxytetracycline, doxycycline, and chlortetracycline. The linear detection ranges of tetracycline, oxytetracycline, doxycycline and chlortetracycline were respectively 100 nM - 200 μM, 100 nM - 200 μM, 98 nM - 195 μM, and 97 nM - 291 μM, and the corresponding detection limits were respectively 15.79 nM, 20.83 nM, 15.31 nM, and 28.30 nM. The developed sensor was successfully applied to detect TCs in real samples, and the recovery rate was from 92.54 % to 109.69 % and the relative standard deviation was from 0.04 % to 2.97 %. Moreover, the heterometallic Eu/Zn-MOF was designed as a ratiometric neuron for Boolean logic computing and information encryption based on the specific identification of TCs. As a proof of concept, molecular steganography was successfully employed to encode, store, and conceal information by transforming the specific identification patterns of Eu/Zn-MOF into binary strings. This study is anticipated to advance the application of metal-organic frameworks in logic detection and information security, and bridging the gap between molecular sensors and the realm of information.

Construction of colorimetric sensor arrays using steel slag-based composites for highly sensitive detection of tetracycline antibiotics

Sensor array methods have received much attention in recent years. In this study, a colorimetric sensor array consisting of three kinds of steel slag-based composites (including porphyrin-functionalized non-magnetic steel slag (NMSS-Por), alkali-excited steel slag (A-SS), and platinum modified steel slag (ALANH-Pt)) was developed for the detection and recognition of tetracycline antibiotics (TCs) such as tetracycline (TC), oxytetracycline (OTC) and doxycycline (DOX). Linear discriminant analysis (LDA) and hierarchical cluster analysis (HCA) showed that the colorimetric sensor array has excellent recognition ability for TCs. The detection limits of this sensor array for TC, OTC, and DOX were 0.059 μM, 0.111 μM and 0.118 μM, respectively, which provided higher sensitivity compared to the colorimetric sensors composed of a single steel slag-based composite material. At the same time, the array sensor has anti-interference ability, and this study provides a new application route for steel slag.

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.

A tailored ratiometric fluorescent sensor based on CdTe and MgF2 quantum dots for trace N-ethylpentylone detection

The development of low-cost and highly sensitive ratiometric fluorescence sensor, CdTe@MIPs/MgF2, for N-Ethylpentylone (NEP) detection in wastewater samples is described. In this system, CdTe@MIPs (λex = 370, λem = 570) are employed as the receptor and response unit for NEP, with MgF2 (λex = 370, λem = 470) as the reference signal to enhance stability. Under optimal conditions, the sensor shows fluorescent quenching response at 570 nm to NEP in linear range of 2-200 nM, with LOD of 0.6 nM. The sensor also demonstrates significant selectivity for NEP over other analogues and interferents, making it ideal for practical applications in wastewater analysis. This approach is potentially more cost-effective and sensitive than conventional mass spectrometry in detecting abused substances in sewage. Additionally, the MgF2 fluorescent nano-material was first-ever developed and investigated, which may be significant in future research.

Understanding Antibiotic Detection with Fluorescence Quantum Dots: A Review

The utilization of fluorescent quantum dots (FL QDs) has gained significant traction in the realm of antibiotic detection, owing to their exceptional FL properties and versatility. Various types of QDs have been tailored to exhibit superior FL characteristics, employing diverse capping agents such as metals, surfactants, polymers, and biomass to protect and stabilize their surfaces. In their evolution, FL QDs have demonstrated both "turn-off" and "turn-on" mechanisms in response to the presence of analytes, offering promising avenues for biosensing applications. This review article provides a comprehensive overview of the recent advancements in antibiotic detection utilizing FL QDs as biosensors. It encompasses an extensive examination of different types of FL QDs, including carbon, metal, and core-shell QDs, deployed for the detection of antibiotics. Furthermore, the synthesis methods employed for the fabrication of various FL QDs are elucidated, shedding light on the diverse approaches adopted in their preparation. Moreover, this review delves into the intricate sensing mechanisms underlying FL QDs-based antibiotic detection. Various mechanisms, such as photoinduced electron transfer, electron transfer, charge transfer, Forster resonance energy transfer, static quenching, dynamic quenching, inner filter effect, hydrogen bonding, and aggregation-induced emission, are discussed in detail. These mechanisms provide a robust scientific rationale for the detection of antibiotics using FL QDs, showcasing their potential for sensitive and selective sensing applications. Finally, the review addresses current challenges and offers perspectives on the future improvement of FL QDs in sensing applications. Insights into overcoming existing limitations and harnessing emerging technologies are provided, charting a course for the continued advancement of FL QDs-based biosensing platforms in the field of antibiotic detection.

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.

Ratiometric fluorescence immunoassay based on silver nanoclusters and calcein-Ce3+ for detecting ochratoxin A

Ochratoxin A (OTA), a dangerous mycotoxin, is found in many crops. It is essential to create sensitive OTA detection techniques to ensure food safety. Based on the principle of p-nitrophenol (PNP) quenched the fluorescence of bovine serum albumin silver nanocluster (BSA-AgNCs) through an internal filtering effect, and phosphate activated fluorescence of calcein-Ce3+ system, a ratiometric fluorescence immunoassay for OTA detection was developed. In this strategy, the value of F518/F640 was used as a signal for response of OTA concentration. The detection range of this strategy was 0.625-25 ng/mL, the limit of detection (LOD) was 0.04 ng/mL. This new immunoassay offered a brand-new platform for detecting OTA.

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.

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.

Synthesis of oil-soluble carbon dots via pyrolysis and their diverse applications in doxycycline detection, fluorescent ink and film

The overuse of doxycycline poses a risk for ecological environment. Advanced materials such as anti-counterfeiting and photovoltaic materials are urgently needed to develop innovative strategies for exploiting solar cells and protecting valuable products. Herein, oil-soluble CDs (o-CDs) were successfully fabricated from citric acid, tris-base and oleylamine as precursors via pyrolysis method. The o-CDs with uniform size distribution exhibited a high quantum yield of 0.48 and excellent photostability. The fluorescence of o-CDs was rapidly quenched by doxycycline at room temperature without further modification. Optimal conditions were selected to construct a fluorescence probe with high selectivity and good sensitivity to detect doxycycline. Interestingly, the probe achieved two linear ranges of 0.85--16.7 µM and 16.7--33.4 µM with a low detection limit of 0.26 µM. Furthermore, inner filter effect (IFE) was dominated in the process in which doxycycline interact with the oxygen-containing groups of o-CDs. This sensing platform has been further successfully applied to the detection of doxycycline in milk with recovery rates of 96.8%- 102.7% and relative standard deviations of 0.98%- 1.02%, suggesting that the novel probe has the potential to be applied in real samples. Moreover, o-CDs directly serve as fluorescence ink and work as fluorescence film using PVA as matrix because of strong fluorescence in the solid state, indicating that they have potential applications in anti-counterfeiting and photovoltaic materials. This is the first report that oil-soluble CDs via pyrolysis is applied in the detection of doxycycline in milk. Importantly, this work provides efficient strategies for the construction of anti-counterfeiting and photovoltaic materials.

Histidine-capped copper nanoclusters for in situ amplified fluorescence monitoring of doxycycline through inner filter effect

There is a significant need to accurately measure doxycycline concentrations in view of the adverse effects of an overdose on human health. A fluorescence (FL) detection method was adopted and copper nanoclusters (CuNCs) were synthesized using chemical reduction technology. Based on FL quenching with doxycycline, the prepared CuNCs were used to explore a fluorescent nanoprobe for doxycycline detection. In an optimal sensing environment, this FL nanosensor was sensitive and selective in doxycycline sensing and displayed a linear relationship in the range 0.5-200 μM with a detection limit of 0.092 μΜ. A characterization test demonstrated that CuNCs offered active functional groups for identifying doxycycline using electrostatic interaction and hydrogen bonds. Static quenching and the inner filter effect (IFE) resulted in weakness in the FL of His@CuNCs with doxycycline with great efficiency. This suggested nanosensor was revealed to be a functional model for simple and rapid detection of doxycycline in real samples with very pleasing accuracy.

A fluorescent sensor based on sulfur nanodots encapsulated into zeolitic imidazolate framework-8 for ultrasensitive detection of tartrazine

A new composite material (SDs@ZIF-8) was synthesized by integrating sulfur nanodots (SDs) into metal-organic frameworks (ZIF-8) through a facile one-step self-assembly strategy. The obtained SDs@ZIF-8 has not only the high adsorption performance of ZIF-8 but also the superior fluorescence characteristics of SDs. The composite featured good dispersibility, stable structure as well as excellent fluorescence in water solution, and can be used as an ideal fluorescent sensor for tartrazine detection. Due to the high specific surface area and adsorption performance of ZIF-8, the prepared composite material can significantly enrich tartrazine, further enhancing the sensitivity of analysis. The fluorescence of SDs @ZIF-8 composite can be effectively quenched by tartrazine through the inner filter effect. The sensing technique exhibited exceptional sensitivity, as evidenced by its impressive detection limit of 6.5 nM across a broad linear range spanning from 0.02 to 90 μM. In addition to its high sensitivity, the technique displayed rapid response times and excellent selectivity. Moreover, the fluorescent sensing technology we developed has been employed successfully for the detection of tartrazine in real samples, which is expected to promote the development of the food safety industry.

A Smartphone-Integrated Molecularly Imprinted Fluorescence Sensor for Visual Detection of Chlortetracycline Based on N,P-Codoped Carbon Dots Decorated Iron-Based Metal-Organic Frameworks

The residue of chlortetracycline is potentially hazardous to human health; it is meaningful to exploit a portable, rapid, sensitive, and selective method for detection of chlortetracycline (CTC). In this study, a novel fluorescence bionic sensing probe (NH2-MIL-53&N,P-CDs@MIP) was successfully prepared based on the nitrogen and phosphorus codoped carbon dots decorated iron-based metal-organic frameworks combining with molecular imprinted polymer for the detection of CTC. A fluorescence intensity-responsive "on-off" detection of CTC on account of the inner-filter effect (IFE) was achieved by NH2-MIL-53&N,P-CDs@MIP. Under the optimal conditions, the fluorescence quenching degree of NH2-MIL-53&N,P-CDs@MIP presented a good linear relationship with the CTC concentration in the range 0.06-30 μg mL-1 and the limit of detection (LOD) was 0.019 μg mL-1. The fluorescent probe was applied to detect CTC in milk samples, and experimental results showed a good recovery rate (88.73%-96.28%). Additionally, a smartphone-integrated fluorescence sensing device based on NH2-MIL-53&N,P-CDs@MIP was exploited to replace the expensive and bulky fluorescence spectrophotometer for quantitative determination of CTC with the LOD of 0.033 μg mL-1. The sensing system showed high selectivity, strong stability, high specificity, and portability, which provide a great strategy for the quantitative detection of antibiotic residue.

Highly selective and sensitive determination of doxycycline integrating enrichment with thermosensitive magnetic molecular imprinting nanomaterial and carbon dots based fluorescence probe

Doxycycline (DOX), a typical tetracycline antibiotic, is widely used because of its excellent antibacterial activity. To develop effective method for DOX has attracted much more attention. Herein, a new detection technology integrating magnetic solid phase extraction (MSPE) based on thermosensitive magnetic molecularly imprinted polymers (T-MMIPs) and fluorescence spectrometry based on carbon dots (CDs) was established. Thermosensitive magnetic molecularly imprinted polymers (T-MMIPs) was designed for selective enrichment of trace DOX. The synthesized T-MMIPs showed excellent selectivity for DOX. The adsorption performance of T-MMIPs varied with temperature in different solvents, which could achieve the enrichment and rapid desorption of DOX. In addition, the synthesized CDs had stable fluorescent property and better water-solubility, and the fluorescence of CDs was significantly quenched by DOX due to the internal filtration effect (IFE). Under the optimized conditions, the method resulted in good linearity over the range from 0.5 to 30 μg L-1, and the limit of detection was 0.2 μg L-1. The constructed detection technology was validated with real water samples, and excellent spiked recoveries from 92.5 % to 105.2 % were achieved. These data clearly indicated that the proposed technology was rapid, highly selective, environmentally friendly, and possessed significant potential application and development prospects.

Y3+@CdTe quantum dot nanoprobe as a fluorescence signal enhancement sensing platform for the visualization of norfloxacin

Quinolone antibiotics (norfloxacin) pose a serious threat to animal and human health due to their misuse and difficulty in being broken down in surface water and food. Rapid and effective detection of norfloxacin (NOR) is essential for environmental testing and ecosystems. In this study, yttrium was coordinated with mercaptopropionic acid (MPA)-modified CdTe quantum dots (QDs) to obtain a novel fluorescence sensor Y³⁺@CdTe QDs for the sensitive detection of NOR. NOR can bind to Y³⁺ to form a complex (NOR-Y³⁺). This complex enhances the luminescence of NOR and blue-shifts to 423 nm. The fluorescence intensity of NOR-Y³⁺ at 423 nm (I₄₂₃) gradually increased with increasing NOR concentration; meanwhile, the fluorescence intensity of CdTe QDs at 634 nm (I₆₃₄) gradually decreased due to aggregation induction. The ratio of I₄₂₃ to I₆₃₄ was used for the quantitative determination of NOR. The linear range of the constructed fluorescent probes was from 1.0 to 150.0 μM, with a detection limit of 31.8 nM. CdTe QDs act as a red fluorescent background, and with the addition of NOR, the color of the system transitions from red to purple and finally blue. This method was rapid (immediate) and visual, providing a simple analysis of various actual samples (tap water, lake water, honey, milk and human serum) for NOR.

Sulfur quantum dots as a fluorescent sensor for N-acetyl-beta-D-glucosaminidase detection

N-acetyl-beta-D-glucosaminidase (NAG) is an important biomarker for early clinical diagnosis of renal disease, suggesting the necessity to develop a fast and sensitive method for its detection. In this paper, we developed a fluorescent sensor based on polyethylene glycol (400) (PEG-400)-modified and H2O2-assisted etched sulfur quantum dots (SQDs). According to the fluorescence inner filter effect (IFE), the fluorescence of SQDs can be quenched by the p-nitrophenol (PNP) generated by NAG-catalyzed hydrolysis of p-Nitrophenyl-N-acetyl-β-D-glucosaminide (PNP-NAG). We successfully used the SQDs as a nano-fluorescent probe to detect the NAG activity from 0.4 to 7.5 U·L-1, with a detection limit of 0.1 U·L-1. Furthermore, the method is highly selective and was successfully used in the detection of NAG activity in bovine serum samples, suggesting its great application prospect in clinical detection.

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.

Synthesis and application of quantum dots in detection of environmental contaminants in food: A comprehensive review

Environmental pollutants can accumulate in the human body through the food chain, which may seriously impact human health. Therefore, it is of vital importance to develop quick, simple, accurate and sensitive (respond quickly) technologies to evaluate the concentration of environmental pollutants in food. Quantum dots (QDs)-based fluorescence detection methods have great potential to overcome the shortcomings of traditional detection methods, such as long detection time, cumbersome detection procedures, and low sensitivity. This paper reviews the types and synthesis methods of QDs with a focus on green synthesis and the research progress on rapid detection of environmental pollutants (e.g., heavy metals, pesticides, and antibiotics) in food. Metal-based QDs, carbon-based QDs, and "top-down" and "bottom-up" synthesis methods are discussed in detail. In addition, research progress of QDs in detecting different environmental pollutants in food is discussed, especially, the practical application of these methods is analyzed. Finally, current challenges and future research directions of QDs-based detection technologies are critically discussed. Hydrothermal synthesis of carbon-based QDs with low toxicity from natural materials has a promising future. Research is needed on green synthesis of QDs, direct detection without pre-processing, and simultaneous detection of multiple contaminants. Finally, how to keep the mobile sensor stable, sensitive and easy to store is a hot topic in the future.

Shaping Sulfur Precursors to Low Dimensional (0D, 1D and 2D) Sulfur Nanomaterials: Synthesis, Characterization, Mechanism, Functionalization, and Applications

Low-dimensional sulfur nanomaterials featuring with 0D sulfur nanoparticles (SNPs), sulfur nanodots (SNDs) and sulfur quantum dots (SQDs), 1D sulfur nanorods (SNRs), and 2D sulfur nanosheets (SNSs) have emerged as an environmentally friendly, biocompatible class of metal-free nanomaterials, sparking extensive interest in a wide range application. In this review, various synthetic methods, precise characterization, creative formation mechanism, delicate functionalization, and versatile applications of low dimensional sulfur nanomaterials over the last decades are systematically summarized. Initially, it is striven to summarize the progress of low dimensional sulfur nanomaterials from versatile precursors by using different synthetic approaches and various characterization. Then, a multi-faceted proposed formation mechanism with emphasis on how these different precursors produce corresponding SNPs, SNDs, SQDs, SNRs, and SNSs is highlighted. Besides, it is essential to fine-tune the surface functional groups of low dimensional sulfur nanomaterials to form new complex nanomaterials. Finally, these sulfur nanomaterials are being investigated in bio-sensing, bio-imaging, lithium-sulfur batteries, antibacterial activities, plant growth along with future perspective and challenges in emerging fields. The purpose of this review is to tailor low dimensional nanomaterials through accurately selecting precursors or synthetic approach and provide a foundation for the formation of versatile sulfur nanostructure.

A dual-signal fluorescent colorimetric tetracyclines sensor based on multicolor carbon dots as probes and smartphone-assisted visual assay

The widespread presence of tetracyclines in the environment has raised concerns about the potential risks to ecosystems and human health. The ratiometric fluorescence sensor for detecting tetracyclines was developed using europium-doped carbon dots (Eu-CDs) as probes under alkaline conditions. The sensing mechanism of sensor for tetracyclines was considered as inner filter effect (IFE), antenna effect (AE), and self-quenching effect (SQE). The sensor had a wide linear detection range than the reported europium ions-based tetracyclines sensors. The linear detection ranges of oxytetracycline (OTC), tetracycline (TC), doxycycline (DC) and chlorotetracycline (CTC) were respectively 0.00-603.75 μM, 0.00-623.82 μM, 0.00-594.61 μM and 0.00-601.54 μM, and the corresponding detection limits were respectively 9.50 nM, 15.80 nM, 10.40 nM and 90.30 nM. The smartphone with RGB Color Picker was further employed to analyze the concentration of tetracyclines, which provided a new method for visual tetracyclines detection. The application of Eu-CDs test paper was also explored, and the results showed that the Eu-CDs test paper has great potential application in the visual detection of tetracyclines. In addition, the accuracy of the established tetracyclines sensor was compared with that of the China national standard method by high-performance liquid chromatography (HPLC), and the results showed that the established method in this work has similar accuracy to the China national standard method. The sensor has been employed to detect tetracyclines in the actual samples with satisfactory results, which indicated that this method has promising applications in the real-time monitoring tetracyclines of food and environment.

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.

An ion-coordination hydrogel based sensor array for point-of-care identification and removal of multiple tetracyclines

Misuse and overuse of tetracycline antibiotics (TCs) brings serious issues to ecological environment, food safety and human health. It is urgent to develop unique platform for high efficient identification and removal of TCs. In the present investigation, an effective and simple fluorescence sensor array was constructed based on the interaction between metal ions (Eu³⁺ and Al³⁺) and antibiotics. Benefiting from the different affinities between the ions and TCs, the sensor array can identify TCs from other antibiotics, which also can further differentiating four kinds of TCs (OTC, CTC, TC and DOX) from each other via linear discriminant analysis (LDA) technique. Meanwhile, the sensor array performed well in quantitative analysis of single TC antibiotic and differentiation of TCs mixtures. More interestingly, Eu³⁺ and Al³⁺-doped sodium alginate/polyvinyl alcohol hydrogel beads (SA/Eu/PVA and SA/Al/PVA) were further constructed, which can not only identify the TCs but simultaneously remove the antibiotics with high efficiency. The investigation provided an instructive way for rapid detection and environment protection.

Inner filter effect-based red-shift and fluorescence dual-sensor platforms with sulfur quantum dots for detection and bioimaging of alkaline phosphatase

Alkaline phosphatase (ALP), one of the vital biomarkers in several diseases, plays a role in indicating disease presence or severity in early diagnosis. Here, a simple H2O2 assisted top-down method was used to synthesize sulfur quantum dots (SQDs) with excitation and emission at 355 nm and 440 nm. Adding ALP into p-nitrophenyl phosphate (p-NPP) and SQDs was found to exhibit a red shift in the emission wavelength and fluorescence intensity quenching of SQDs, respectively, allowing us to propose dual-sensor platforms of red shift of emission wavelength (RSEW) and fluorescence quenching of SQDs. These dual-sensor platforms were highly sensitive and selective in ALP detection, with a linear response to ALP in the concentration range of 0.25 to 100 U L-1 and detection limits of 0.08 and 0.10 U L-1, respectively. The absorption of p-NP at 400 nm showed a good overlap with the excitation and emission of SQDs, leading to inner filter effect-based RSEW and fluorescence quenching of SQDs. This sensor platform was successfully applied in ALP sensing of serum samples as well as monitoring of ALP in cells. More importantly, this platform can serve as an example of using RSEW to detect ALP.

Facile Synthesis of Molecularly Imprinted Ratiometric Fluorescence Sensor for Ciguatoxin P-CTX-3C Detection in Fish

Ciguatoxin (CTX) detection methods are essential due to the serious hazard that bioaccumulation in fish and transmission along the food chain poses to human health. We report the rapid and simple development of a dual-emitting, molecularly imprinted, ratiometric fluorescence sensor (MIPs@BCDs/RCDs@SiO₂) to detect ciguatoxin P-CTX-3C with high sensitivity and selectivity. The sensor was fabricated via sol–gel polymerization using monensin as the fragmentary dummy template molecule, blue carbon dots (BCDs) as the response signal, and red carbon dots (RCDs) as the reference signal. The fluorescence emission of BCDs was selectively quenched in the presence of P-CTX-3C, leading to a favorable linear correlation between the fluorescence intensity ratio (I₄₄₀/I₆₇₅) and the P-CTX-3C concentration in the range of 0.001–1 ng/mL with a lower detection limit of 3.3 × 10⁻⁴ ng/mL. According to LC-MS measurement results, the proposed sensor can rapidly detect ciguatoxin P-CTX-3C in coral reef fish samples with satisfactory recoveries and standard deviations. This study provides a promising strategy for rapid trace analysis of marine toxins and other macromolecular pollutants in complex matrices.

Paper-based device for the selective determination of doxycycline antibiotic based on the turn-on fluorescence of bovine serum albumin-coated copper nanoclusters

An enhanced ratiometric fluorescence sensor was built for on-site visual detection of doxycycline (DOX) through the interaction with bovine serum albumin on the surface of red emissive copper nanoclusters. Upon the addition of weakly fluorescent DOX, the red fluorescence from copper nanoclusters gradually decreased through the inner-filter effect (IFE), while a green fluorescence appears and significantly increases, forming an interesting fluorescent isosbestic point, which was assigned to DOX due to sensitization effect of bovine serum albumin. On the basis of this ratiometric fluorescence, the system possessed good limit of detection (LOD) of 45 nM and excellent selectivity for DOX over other tetracyclines. Based on these findings, a paper-based sensor has been fabricated for distinct visual detection of trace DOX and combined with smartphone color recognizer for quantitative detection of DOX (LOD = 83 nM). This method shows broad application prospects in environmental monitoring and food safety.

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.

Ratiometric fluorescence and visual determination of tetracycline antibiotics based on Y3+ and copper nanoclusters-induced cascade signal amplification

A ratiometric fluorescence probe is proposed for sensitive and visual detection of tetracyclinee (TC) based on cascade fluorescence signal amplification induced by bovine serum albumin-stabilized copper nanoclusters (BSA-CuNCs) and yttrium ions (Y³⁺). TC can combine with Y³⁺ to form the complex (TC-Y³⁺) to enhance the fluorescence of TC at 515 nm. Then, positively charged TC-Y³⁺ and negatively charged BSA-CuNCs was bonded together by electrostatic interactions to achieve the fluorescence resonance energy transfer (FRET) process. With the increase of TC concentration, the fluorescence intensity of TC-Y³⁺ at 515 nm (F₅₁₅) gradually increased; meanwhile, the fluorescence intensity of BSA-CuNCs at 405 nm (F₄₀₅) decreased gradually. The ratio of F₅₁₅ and F₄₀₅ was used for the quantitative determination of TC. The linear range of the constructed fluorescent probe is 1.0 to 60.0 μM, and the limit of detection is 0.22 μM. The method was successfully applied to the determination of TC in spiked milk with recoveries ranging from 94.3 to 112%. Furthermore, the color of this platform can be observed from dark violet to bright green under the UV lamp. Since the response time of the reaction is less than 10 s, an intelligent sensing platform based on the use of the smartphone as image acquisition equipment was also established to realize rapid on-site and portable detection of TC through the colorimetric recognition application.

Full Green Detection of Antibiotic Tetracyclines Using Fluorescent Poly(ethylene glycol) as the Sensor and the Mechanism Study

Tetracyclines are well-known antibiotics and widely used against a variety of bacterial infections. Their monitoring and detection have been an important issue. To this end, a vast number of methods have been developed; fluorescence sensing is one of the most reported. However, most of the reported sensors are made from transition metals with sophisticated multiprocesses; polymers are hardly seen for this purpose, particularly biocompatible ones. Herein, an aqueous solution of poly(ethylene glycol) (PEG), well known for being biocompatible, is shown to emit under excitation of 280 nm, while the solutions of selected tetracyclines, namely, doxycycline (DOX) and tetracycline (TC), are non-emissive under the same conditions. In the binary solutions of PEG-DOX or PEG-TC, PEG emission is sharply quenched with high sensitivity and selectivity. PEG was then used as a sensor for DOX and TC detections in water with high performance compared to reported studies. The same tests were also done by DOX spiking in milk and tap water, demonstrating that DOX was practically fully recovered. The quenching mechanism was ascribed to the interaction between the O atoms of PEG in clusters and specific heteroatom groups on tetracycline molecules through hydrogen bonding, elucidated from FTIR and NMR analyses. Therefore, this work provides a novel, fully green, easy to operate, low cost, and reliable protocol for tetracycline monitoring and detection and opens new potential application for PEG.

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.

Research Update of Emergent Sulfur Quantum Dots in Synthesis and Sensing/Bioimaging Applications

Due to their unique optical property, low toxicity, high hydrophilicity, and low cost, sulfur quantum dots (SQDs), an emerging luminescent nanomaterial, have shown great potential in various application fields, such as sensing, bioimaging, light emitting diode, catalysis, and anti-bacteria. This minireview updates the synthetic methods and sensing/bioimaging applications of SQDs in the last few years, followed by discussion of the potential challenges and prospects in their synthesis and sensing/bioimaging applications, with the purpose to provide some useful information for researchers in this field.

CdTe QDs@ZIF-8 composite-based recyclable ratiometric fluorescent sensor for rapid and sensitive detection of chlortetracycline

The residue problem in animal food products caused by the abuse of chlortetracycline (CTC) is one of the food safety issues that have attracted much attention. Herein, a composite was generated by embedding CdTe quantum dots (QDs) into ZIF-8 for ratiometric fluorescent analysis of CTC. With adding CTC, the green luminescence of CTC appeared under the sensitization effect of Zn²⁺ in ZIF-8, but the red luminescence of CdTe QDs was reduced by the inner filtration effect of CTC. On this basis, CTC was detected by the composite with a short response time of 1 min, and the limit of detection was calculated to be 37 nM that was 17 times lower than the maximum residue limit of CTC in animal food products (626 nM). Excellent recyclability of the composite was also observed, and CTC was consecutively measured at least six times. The composite was used to determine CTC in basa fish and pure milk with satisfactory recoveries (91.0-110.0%). Portable test strips were further manufactured and the visual determination of CTC was obtained. These results convictively demonstrate that CdTe QDs@ZIF-8 composite as a recyclable ratiometric fluorescent sensor achieves the rapid and sensitive measurement of CTC residue in animal food products.

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.

Ratiometric detection of doxycycline in pharmaceutical based on dual ligands-enhanced copper nanoclusters

A water-soluble, stable, simple and dual ligands (bovine serum albumin and L-histidine)-enhanced copper nanoclusters (BSA-CuNCs@L-His) was synthesized by one-step wet chemical method. Interestingly, the introduction of L-His ligand could improve evidently the quantum yields (QYs, 3.47%) and stability of BSA-CuNCs due to forming the stronger interaction of L-His and Cu and producing bigger diameter CuNCs by coordination-induced aggregation. Thus, a new ratiometric fluorescent probe (RF-probe) was successfully exploited for sensitively and selectively mensurating doxycycline (DOX) because DOX could simultaneously regulate the fluorescence (FL) intensities of BSA-CuNCs@L-His at 410 and 520 nm. The FL quenching of BSA-CuNCs@L-His at 410 nm by DOX was mainly originated from the static quenching process, while DOX could bind to Trp-212 in BSA from the skeleton of BSA-CuNCs@L-His by electrostatic interaction causing the appearance of new emission peak at 520 nm. The content of DOX was monitored by the RF-probe with a linear range of 0.05-14.0 μM and a LOD (limit of detection) and LOQ (limit of quantification) of 6.4 and 21.3 nM (at 3σ/slope and 10σ/slope). Moreover, compared to the standard HPLC method, the proposed RF-probe was extended to the detection of DOX in doxycycline hydrochloride (DOXH) tablets, DOXH injections and DOXH capsules with satisfactory results.

Enhanced peroxidase-like activity of bimetal (Fe/Co) MIL-101 for determination of tetracycline and hydrogen peroxide

The peroxidase-like activity of MIL-101(Fe/Co) is improved by adding tetracycline. On the basis of MIL-101(Fe/Co), fast colorimetric sensors of tetracycline and H2O2 have been successfully constructed.

Europium Doped Silicon Quantum Dot As a Novel FRET Based Dual Detection Probe: Sensitive Detection of Tetracycline, Zinc, and Cadmium

The imbalance of Zn²⁺ /Cd²⁺ in the human body can lead to many serious diseases due to the overuse of antibiotics and deposition in animal products. Developing a functional material for detecting is challenging and in demand. Herein, silicon quantum dots (SiQDs) are designed as a functional platform for the detection of tetracycline and Zn²⁺ /Cd²⁺ . The COOH functionalized SiQDs with the emission wavelength of 450 nm are chelated with Eu(NO₃ )₃ to form SiQDs-Eu³⁺ ratio fluorescent probes, which can be used to detect tetracycline (TCs) and Zn²⁺ /Cd²⁺ by fluorescence resonance energy transfer (FRET) principle sequentially. The fluorescent probe showed good linearity between ion concentration and fluorescence enhancement. The detection limit of TCs and Zn²⁺ /Cd²⁺ are 0.2 × 10^-6  m and 3 × 10^-6  m, respectively, when the pH of the solution is 7.4. In addition, the synthesized SiQDs-Eu³⁺ exhibited good stability (from 94.9% to 103.1%). The relative standard deviations (RSD, n = 10) of human serum and urine were both less than 3%. Therefore, the SiQDs-Eu³⁺ ratio fluorescence probe will provide a good application prospect in actual sample detection.