Carbon dots synthesized at room temperature for detection of tetracycline hydrochloride

Highly efficient and sensitive detection of tetracycline in environmental water: Insights into the synergistic mechanism of biomass-derived carbon dots and N-methyl pyrrolidone solvent

The tetracycline (TC) residue in water environment has caused serious public safety issue. Thus, efficient sensing of TC is highly desirable for environmental protection. Herein, biomass-derived nitrogen-doped carbon dots (N-CDs) synthesized from natural Ophiopogon japonicus f. nanus (O. japonicus) were used for TC detection. The unique solvent synergism efficiently enhanced detection sensitivity, and the detailed sensing mechanism was deeply investigated. The blue fluorescence of N-CDs was quenched by TC via static quenching and inner filter effect. Moreover, the enhancement of green fluorescence from deprotonated TC was firstly proposed and sufficiently verified. The solvent effect of N-methyl pyrrolidone (NMP) and the fluorescence resonance energy transfer (FRET) with N-CDs achieved an instantaneous enhancement of the green emission by 64-fold. Accordingly, a ratiometric fluorescence method was constructed for rapid and sensitive sensing of TC with a low detection limit of 6.3 nM within 60 s. The synergistic effect of N-CDs and solvent assistance significantly improved the sensitivity by 7-fold compared to that in water. Remarkably, the biomass-derived N-CDs displayed low cost, good solubility, and desired stability. The deep insights into the synergism with solvent can provide prospects for the utilization of biomass-based materials and broaden the development of advanced sensors with promising applications.

Self-ratiometric fluorescence approach based on room-temperature instantaneously synthesized carbon dots from Folin's reagent and ethanolamine for determination of nitroxinil in water, milk, and food samples

BACKGROUND

Nitroxinil has been used extensively to treat parasitic worms, mainly Fasciola, in food-producing cattle and sheep. The reported methods for nitroxinil analysis included expensive instrumentation, the need for skilled operators, and tedious procedures. Fluorimetry is one of the fastest and simplest methods widely used; hence, we aimed to develop a simple, cost-effective, and convenient fluorometric approach for the estimation of nitroxinil in various matrices. Compared with other detection methods, self-ratiometric fluorescent probes are considered a promising approach for the detection of analytes as their detection accuracy overcomes traditional fluorescence sensing probe in that it is not affected by the probe concentration, solution polarity, instrument parameters, and other factors. In this research, room temperature instantaneously synthesized carbon dots were used as a sensitive and selective self-ratiometric probe for the determination of the veterinary medicine nitroxinil in various matrices.

RESULTS

A room-temperature synthesized quinone-ethanolamine carbon dots (RTQECDs) was fabricated using the instantaneous reaction of sodium 1,2-naphthoquinone-4-sulfonate (Folin's) with ethanolamine, without any energy/catalyzing reagents, for the first time. The prepared carbon dots show green-blue fluorescence at 450 nm upon exposure to UV light at 365 nm with a quantum yield of 26.6 %. Upon interaction with nitroxinil, the fluorescence intensity of RTQECDs at 450 nm is quenched and shifted to a longer wavelength at 475 nm. Meanwhile, the fluorescence of RTQECDs at 400 nm (absorbance maxima of nitroxinil) was more extremely quenched under the same conditions. Taking this in hand, a new RTQECDs self-ratiometric probe was developed for the determination of nitroxinil using the decrease in peaks at 450 nm and 400 nm and the shift of the fluorescence maxima to 475 nm as built-in reference peaks. The probe showed a quantitative increase in signal output of F475/F400 in the range of 0.10-30.0 μg/mL nitroxinil with a limit of detection of 30.0 ppb. The nitroxinil-sensing mechanism using RTQECDs is mainly ascribed to the partial secondary blue-type inner filter effect (IFE). The designed study was applied for the estimation of nitroxinil in veterinary dosage forms (recoveries; 99.78 %-100.35 %), river water (recoveries; 98.55 %-101.53 %), and food products, including meat, liver, kidney, and milk (recoveries; 97.60 %-104.25 %).

SIGNIFICANCE

The novelty of our work includes the immediate synthesis of the sensing probe at room temperature, as well as its use as a self-ratiometric fluorescence probe for the determination of nitroxinil in veterinary samples, river water, and food products with excellent sensitivity down to 30.0 ppb. RTQECDs own the highest response and selectivity to nitroxinil compared with cations, anions, as well as other co-administered drugs, including cefotaxime and ivermectin.

Sensitive and selective detection of tetracycline using fluorescence-enhanced Eu(III)-functionalized silver nanoparticles with homocysteine

Utilizing metal luminescence enhancement to design fluorescent probes is a very sensible strategy. Herein, a fluorescent probe based on europium (III)-functionalized silver nanoparticles-conjugated homocysteine (AgNPs-Hcy-Eu3+) was proposed for the selective and sensitive detection of tetracycline (TC). In this probe, Eu(III) was employed as the detection signal unit for TC, while AgNPs-Hcy was used as the ligand of fluorescence enhancement. When TC exists, it can bind to Eu3+ immobilized in AgNPs-Hcy, leading to an enhanced fluorescence signal from Eu3+ through energy transfer. Under optimal conditions, the fluorescence intensity of AgNPs-Hcy-Eu3+ increased linearly with increasing TC concentration in the range of 0.1-30 μM (R2 = 0.9964). The fluorescent probe own fluorescence enhancement, paving the way for sensitive detection with a low detection limit of 0.083 μM. It also has good selectivity for common antibiotics and anions. This work can be applied to the determination of TC in tap water and milk with recoveries of 94-98.5%. We expect AgNPs-Hcy-Eu3+ to have potential applications in environmental testing and food safety.

Synthesis of Carbon Dots from Peltophorum Pterocarpum Flowers for Selective Fluorescence Detection of Carbendazim

In this study, carbon dots (CDs) were synthesized from Peltophorum pterocarpum flowers as the precursor material using the hydrothermal method. The fluorescence emission spectra of the resulting Peltophorum pterocarpum CDs (PP-CDs) exhibited excitation-independent behavior, showing the fluorescence emission peak at 410 nm when excited at 330 nm. This method is simple, rapid and well consistent with the green chemistry and sustainable analytical method development. The as-synthesized PP-CDs acted as a promising fluorescent probe for detecting carbendazim (CBZ) via aggregation-induced emission mechanism, showing a linear response to CBZ concentrations ranging from 1 to 30 μM, with a detection limit of 5.41 nM. This method was successfully applied to quantify CBZ in food samples, achieving excellent recoveries of 99% with a relative standard deviation (RSD) of less than 2%.

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.

Tourmaline/pyrite dual mineral photocatalysis with a powerful surface electric field for efficient antibiotic removal

Pyrite (FeS2) has garnered attention due to its narrow bandgap, high light absorption, and low cost. However, the rapid recombination of charge carriers hinders its practical application. Surface electric field is a unique characteristic of tourmaline, which can induce effective separation of photo generated electrons and holes. This study successfully combined two directly mined natural minerals, tourmaline and pyrite, to form TFS. Characterization and experiments show that the surface electric field of tourmaline can significantly enhance the photocatalytic activity of TFS. Tetracycline (TC, 50 ppm) was degraded by 95% with 60 min, and the TFS reaction rate constant reached 0.0439 min-1, which is 6.1 times and 17.3 times higher than that of tourmaline and FeS2. Additionally, it significantly improved light absorption and charge carrier separation capabilities. After simulating various natural environmental factors, TFS demonstrated practicality. Considered analysis of active substances and detection revealed that h+ and 1O2 radicals are significant contributors, and the photocatalytic mechanism was proposed. Furthermore, the transformation pathways and toxicity of metabolites were studied. This research offers further inspiration and insights for improving photocatalytic material performance and the green governance environment of natural resources.

Development of a "Signal-On" Fluorescent Aptasensor for Highly Selective and Sensitive Detection of ZEN in Cereal Products Using Nitrogen-Doped Carbon Dots Based on the Inner Filter Effect

This study aimed to develop a novel fluorescent aptasensor for the quantitative detection of zearalenone (ZEN), addressing the limitations of conventional detection techniques in terms of speed, sensitivity, and ease of use. Nitrogen-doped carbon dots (N-CDs) were synthesized via the hydrothermal method, resulting in spherical particles with a diameter of 3.25 nm. These N-CDs demonstrated high water solubility and emitted a bright blue light at 440 nm when excited at 355 nm. The fluorescence of N-CDs was quenched by dispersed gold nanoparticles (AuNPs) through the inner filter effect, while aggregated AuNPs induced by NaCl did not affect the fluorescence of N-CDs. The aptamer could protect AuNPs from NaCl-induced aggregation, but the presence of ZEN weakened this protective effect. Based on this principle, optimal conditions for ZEN detection included 57 mM NaCl, 12.5 nM aptamer concentration, incubation of AuNPs with NaCl for 15 min in Tris-EDTA(TE) buffer, and incubation of aptamer with ZEN and NaCl for 30 min. Under these optimized conditions, the "signal-on" fluorescent aptasensor for ZEN detection showed a linear range of 0.25 to 200 ng/mL with a low detection limit of 0.0875 ng/mL. Furthermore, the developed aptasensor exhibited excellent specificity and could rapidly detect ZEN in corn flour samples or corn oil, achieving satisfactory recovery rates ranging from 84.7% to 108.6%. Therefore, this study presents an economical, convenient, sensitive, and rapid method for accurately quantifying ZEN in cereal products.

Microwave-assisted preparation of yellow fluorescent graphitic carbon nitride quantum dots for trace tetracycline-specific detection

Tetracycline (TC) is extensively utilized in livestock breeding, aquaculture, and medical industry. TC residues seriously harm food security, the environment, and human health. There is an urgent need to exploit a highly efficient and sensitive testing method to monitor TC residue levels in aquatic environments. In this study, graphitic carbon nitride quantum dots (g-CNQDs) were successfully synthesized by a one-step microwave-assisted method using citric acid and urea as precursors. The as-prepared g-CNQDs with size of 1.25-3.75 nm exhibited bright yellow fluorescence at 523 nm when excited at 397 nm. Interestingly, this characteristic fluorescence emission of g-CNQDs could be selectively and efficiently quenched by TC. Based on this phenomenon, for TC detection was successfully explored and applied in real water samples. Wide linear scope of 7-100 μM, low detection limit (LOD) of 0.48 μM, satisfactory recovery of 97.77%-103.4%, and good relative standard deviation (RSD) of 1.05-5.87% were obtained. Mechanism investigations revealed that the static quenching and the inner filter effect (IFE) were responsible for this fluorescence quenching between g-CNQDs and TC. This work not only provided a facile approach for g-CNQDs synthesis but also constructed a g-CNQDs-based fluorescent sensor platform for the highly sensitive and selective detection of TC in aquatic environments.

An asynchronous response fluorescence sensor combines machine learning theory to qualitatively and quantitatively detect tetracyclines

Excess use of tetracyclines poses significant health risks arising from animal-derived foods, meaning simple and sensitive methods to detect tetracyclines would be beneficial given current laboratory methods are complex and expensive. Herein, we describe an asynchronous response fluorescence sensor constructed based on Zn-based metal-organic framework and Ru(bpy)32+ (denoted as Ru@Zn-BTEC) for the qualitative and quantitative detection of tetracyclines in foods. Under excitation at 365 nm, the sensor emitted red fluorescence at 609 nm. When tetracyclines were present, these molecules aggregated in the Ru@Zn-BTEC framework, causing green fluorescence emission at 528 nm. The developed sensing system accurately distinguished the different categories of tetracyclines with a classifier accuracy of 94 %. The Ru@Zn-BTEC sensor demonstrated a detection limit of 0.012 μM and satisfactory recovery (87.81 %-113.84 %) for tetracyclines in food samples. This work provides a pathway for constructing asynchronous response fluorescence sensors for food analysis.

A novel fluorescence platform for specific detection of tetracycline antibiotics based on [MQDA-Eu3+] system

Tetracycline antibiotics (TCs) are extensively used in clinical medicine, animal husbandry, and aquaculture because of their cost-effectiveness and high antibacterial efficacy. However, the presence of TCs residues in the environment poses risks to humans. In this study, an inner filter effect (IFE) fluorescent probe, 2,2'-(ethane-1,2-diylbis((2-((2-methylquinolin-8-yl)amino)-2-oxoethyl)azanediyl))diacetic acid (MQDA), was developed for the rapid detection of Eu3+ within 30 s. And its complex [MQDA-Eu3+] was successfully used for the detection of TCs. Upon coordination of a carboxyl of MQDA with Eu3+ to form a [MQDA-Eu3+] complex, the carboxyl served as an antenna ligand for the effective detection of Eu3+ to intensify the emission intensity of MQDA via "antenna effect", the process was the energy absorbed by TCs via UV excitation was effectively transferred to Eu3+. Fluorescence quenching of the [MQDA-Eu3+] complex was caused by the IFE in multicolor fluorescence systems. The limits of detection of [MQDA-Eu3+] for oxytetracycline, chlorotetracycline hydrochloride, and tetracycline were 0.80, 0.93, and 1.7 μM in DMSO/HEPES (7:3, v/v, pH = 7.0), respectively. [MQDA-Eu3+] demonstrated sensitive detection of TCs in environmental and food samples with satisfactory recoveries and exhibited excellent imaging capabilities for TCs in living cells and zebrafish with low cytotoxicity. The proposed approach demonstrated considerable potential for the quantitative detection of TCs.

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.

Nitrogen and Chlorine Co-doped Carbon Dots as a Highly Selective and Sensitive Fluorescent Probe for Sensing of PH, Tetracycline Detection and Cell Imaging

Carbon dots have been widely focused on the field of sensing and detection due to their excellent optical property. Herein, novel orange fluorescent nitrogen and chlorine co-doped carbon dots (N,Cl-CDs) are obtained by one-pot hydrothermal method using o-phenylenediamine and neutral red. Based on the inner filter effect, the prepared N,Cl-CDs can be innovatively developed as an effective "signal-off" multifunctional sensing platform for sensitive determination of tetracycline. The proposed sensor was utilized to realize the determination of tetracycline in Rirver water samples/milk samples (λex = 390 nm, λem = 606 nm) with satisfactory recoveries and relative standard deviations. The linear range of are 0.05 to 45 μM and 45 to135 μM, and detection limit is 3.9 nM (3σ/m). Meanwhile, the luminescent intensity of N,Cl-CDs was reduced gradually when pH changed continuously from 12 to 2, showing a pH-responsive fluorescence property with two linear ranges of pH 3-7 and pH 7-10. In addition, due to the characteristics of low toxicity and excellent biocompatibility, the N, Cl-CDs were also used in the imaging of oocystis cells, which is hopeful to realize the detection of tetracycline in living cells.

Stability indicating high performance thin layer chromatography method development and validation for quantitative determination of tetracycline hydrochloride in tetracycline hydrochloride active pharmaceutical ingredient (API) and its dosage forms

Simple, quick, cost-effective, and environmentally friendly analytical methods for quality assurance and control roles for different medicines, including Tetrcyclines, are most significantly needed. Also, different thin layer chromatography (TLC)-based methods for tetracycline identification exist, but high performance thin layer chromatography methods based on modern state- of- the art equipment are still nonexistent. Thus, in this study, analytical method development and verification were done by high performance thin layer chromatography (HPTLC) (using an automated equipment model) using glass plates coated with silica gel 60 F254 after treating with 10% Na2EDTA. Validation was carried out according to International Council for Harmonization (ICH) guidelines. A mobile phase formed from ethyl acetate, acetonitrile, methanol, and 1% aqueous ammonia in the composition of 4.4:19.6:10:6 volume to volume ratio (V/V) was used. Rf value, percentage recoveries, linearity ranges, limit of detection (LOD), and limit of quantitation (LOQ) for the developed HPTLC method were 0.28, 100.83-106.25%, 160-560 ng/band (r2 values of 0.9999), 31.9 ng/band, and 96.7 ng/band, respectively. The results of the sample assays conducted using the new method and the United States Pharmacopoeia (USP) high performance liquid chromatography (HPLC) method were 91.59% to 108.3% and 90.83% to 102.85%, respectively. The F test for the aforementioned methods was 2.01, which is smaller than the tabulated F value of 5.05 with 5 degrees of freedom at a 95% confidence range, proving that the newly developed HPTLC and HPLC pharmacopoeial methods can be used interchangeably.The newly developed HPTLC method is easy, economical, specific, accurate, and roboust, thus it can be employed in a range of settings that require quality control and assurance activities of tetracycline hydrochloride (TC-HCl) in bulk and ointment dosage forms.

CARBON DOTS: Bioimaging and Anticancer Drug Delivery

Cancer, responsible for approximately 10 million lives annually, urgently requires innovative treatments, as well as solutions to mitigate the limitations of traditional chemotherapy, such as long-term adverse side effects and multidrug resistance. This review focuses on Carbon Dots (CDs), an emergent class of nanoparticles (NPs) with remarkable physicochemical and biological properties, and their burgeoning applications in bioimaging and as nanocarriers in drug delivery systems for cancer treatment. The review initiates with an overview of NPs as nanocarriers, followed by an in-depth look into the biological barriers that could affect their distribution, from barriers to administration, to intracellular trafficking. It further explores CDs' synthesis, including both bottom-up and top-down approaches, and their notable biocompatibility, supported by a selection of in vitro, in vivo, and ex vivo studies. Special attention is given to CDs' role in bioimaging, highlighting their optical properties. The discussion extends to their emerging significance as drug carriers, particularly in the delivery of doxorubicin and other anticancer agents, underscoring recent advancements and challenges in this field. Finally, we showcase examples of other promising bioapplications of CDs, emergent owing to the NPs flexible design. As research on CDs evolves, we envisage key challenges, as well as the potential of CD-based systems in bioimaging and cancer therapy.

Multi-component determination based on high quantum yield "on-off-on" carbon quantum dots sensor

The nitrogen(N)-sulfur(S)-sodium(Na(I)) co-doped carbon quantum dots (CQDs) were synthesized via a one-step hydrothermal method, which exhibited a remarkably high fluorescence quantum yield (24.58%) and exceptional optical properties. The fluorescence "on-off-on" sensor was constructed. The fluorescence of CQDs was rapidly quenched with Fe(III) and the fluorescence recovered by ascorbic acid (Asc) partially and arginine (Arg)/histidine (His) completely. The CQDs fluorescence sensor demonstrated rapid response, exceptional sensitivity, excellent stability, remarkable selectivity, and robust anti-interference performance, which was feasible to simultaneously determine the concentrations of multiple analytes in the sample with satisfactory recovery rates. The "on-off-on" fluorescence mechanism of CQDs was investigated, revealing the significant potential of carbon nano-functionalized materials in the field of drug detection through fluorescence sensing.

A dual-mode sensing system based on carbon quantum dots and Fe nanozymes for the detection of α-glucosidase and its inhibitors

In this research, a sensitive fluorometric and colorimetric dual-mode sensing platform based on nitrogen-doped carbon quantum dots (NCDs) and magnetic Fe nanoparticles with peroxidase-like activity (Fe nanozymes, Fe NZs) was established, and was further applied for the detection of α-glucosidase (α-glu) and its inhibitors. The ⋅OH that produced by H2O2 catalyzed by Fe NZs can oxidize the colorless diammonium 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) to green oxABTS, and a noticeable absorption peak at 417 nm appeared. Simultaneously, oxABTS can quench the fluorescence of NCDs at 402 nm via fluorescence resonance energy transfer (FRET). 2-O-α-D-glucopyranosyl-L-ascorbic acid (AAG) can be decomposed by α-glu to glucose and ascorbic acid (AA), AA can prevent the oxidation of ABTS, resulting in the absorption at 417 nm decreased. Moreover, the quenching effect of oxABTS on NCDs is weakened, and the fluorescence at 402 nm is restored. Therefore, based on the change of absorption at 417 nm and fluorescence at 402 nm, the fluorometric and colorimetric dual-mode sensing method can be used for the determination of acarbose and voglibose that are the inhibitors of α-glu.

Fluorescent and smartphone imaging detection of tetracycline residues based on luminescent europium ion-functionalized the regular octahedral UiO-66-NH2

Antibiotic residues cause extensive damage to food security, thus arousing serious concerns. Hence, rapid and sensitive detection of antibiotic residues is crucial to food safety. This study aimed to propose a portable, visual, intelligent and rapid method for tetracycline detection. We developed a ratiometric fluorescent sensor based on the Eu3+-functionalized regular octahedral UiO-66-NH2 material. The developed sensor could quantify tetracycline in the concentration range of 0.5-200 μM with a detection limit as low as 0.2 μM under the optimum conditions. Furthermore, the analytical results obtained using the designed sensor in the actual samples were basically consistent with those obtained using high-performance liquid chromatography. Based on these achievements, a smartphone application-integrated fluorescent testing paper was designed for facile, intelligent, and visual detection of tetracycline. The integrated portable sensor not only saved cost and time for testing but also provided a forward-looking approach to fast, sensitive detection of antibiotic residues.

Room temperature cost-effective synthesis of carbon quantum dots for fluorescence pattern recognition of metal ions

Herein, a type of low-consuming carbon quantum dot (CD) has been synthesized at room temperature in just 45 minutes via Schiff base reaction between o-phthalaldehyde (OPA) and polyethyleneimine (PEI). These CDs are pH-dependent, so a novel label-free florescent sensor array can be constructed by utilizing buffers with various pH levels, which leads to distinctive fluorescence response patterns upon being challenged with metal ions for their pattern recognition. The results demonstrate that large-scale detection of metal ions can be achieved with as little as 3 types of sensors. Additionally, the sensors are able to discriminate between various metal ion concentrations and mixtures of different metal ions. The technique demonstrates potential uses in water quality monitoring by promising straightforward, quick, sensitive, and potent discrimination of metal ions.

Synthesis of water-soluble CdS quantum dots for the fluorescence detection of tetracycline

An effective strategy for combating antibiotic-resistant bacteria (ARB) entails the early detection of antibiotics during the initial stages of water treatment facilities. In this context, cadmium sulfide quantum dots (CdS QDs) were employed for the precise detection of tetracycline (TET), an emerging contaminant, in water. CdS QDs with fluorescence properties were synthesized by culturing Citrobacter freundii bacteria. The CdS QDs were characterized by spectroscopy techniques, and the quantum efficiency was estimated to be 55.8% which is ∼2-fold high compared to the standard rhodamine-B solution. The fluorescence of CdS QDs was quenched at 440 nm in the presence of TET. The linear range of TET was varied from 10 to 100 μM with a lower limit of detection of ∼23 nM. The CdS QDs were used to detect TET in river water, tap water, and milk which showed an excellent recovery rate. Therefore, the novel biosynthesis CdS QDs can be a significant fluorescence probe for the detection of TET that shows exceptional sensitivity and selectivity.

Dual-functional molecularly imprinted doped carbon dot based on metal-organic frameworks for tetracycline adsorption and determination

A carbon dot (CD) was prepared by using tryptophan as a single carbon source and demonstrated its good selective fluorescence quenching effect on tetracycline (TC). The modified metal-organic frameworks (MOF) NH2-MIL-101 was chosen as matrix, doped with CD, molecularly imprinted polymer (MIP) prepared with TC as the template, and finally CD-MOF-MIP complexes (CD@MIP) was synthesized. For comparison, MIP were also prepared without CD as well as non-imprinted polymers and their ability was tested, respectively. CD@MIP is a nanomaterial with bright fluorescence under the irradiation of ordinary UV equipment (λ = 360 nm), which has a fast and stable fluorescence quenching for TC and a good linear relationship for TC in the concentration range 0-400 μmol L-1. The quantum yield of CD@MIP was 12.75% and the 3σ limit of detection (LOD) for CD@MIP was 0.59 μmol L-1. The maximum adsorption capacity of CD@MIP reached 304.6 mg g-1 and the adsorption equilibrium was reached after about 75 min. The adsorption of CD@MIP to tetracycline spiked in milk samples reached 90.0 mg g-1 within 2 h, which was much higher than that of NIP (48.4 mg g-1) under the same conditions, as demonstrated by high performance liquid chromatography (HPLC). The results obtained showed that CD@MIP combined the high adsorption capacity of MOF, the specific adsorption of molecular imprinting and the fluorescence properties of CD, can determine and rapidly removeTC in the environment.

Paper test strip for fluorescence detection of iron ion based on nitrogen, zinc and copper codoped carbon dots

In this study, a test strip for fluorometric analysis of iron ion (Fe3+) was constructed based on nitrogen, zinc and copper codoped carbon dots (NZC-CDs) as fluorescence probes. NZC-CDs were synthesized by hydrothermal method. The morphology, size, components, crystal state and optical properties of NZC-CDs were characterized by transmission electron microscope (TEM), Fourier-transform infrared (FT-IR), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), UV-vis absorption and fluorescence spectroscopy techniques, respectively. NZC-CDs exhibited bright blue fluorescence under UV lamp with a quantum yield at 17.76%. The fluorescence of NZC-CDs was quenched by Fe3+possibly due to the static quenching. The possible fluorescence quenching mechanism was also discussed. The quenching fluorescence was linear with the concentration of Fe3+in the range of 2.5-400μM with a low detection limit of 0.5μM. For the convenient detection, the test strips based on filter paper were employed for Fe3+assay. Moreover, the present approach was successfully applied in the determination of Fe3+in real samples including black fungus, duck blood and pork liver. The sensing method had the potential application in more food analysis.

A smartphone-integrated portable platform based on polychromatic ratiometric fluorescent paper sensors for visual quantitative determination of norfloxacin

The emergence of "superbugs" due to antibiotics overuse poses a significant threat to human health and security. The development of sensitive and effective antibiotics detection is undoubtedly a prerequisite for addressing antibiotics overuse-associated issues. However, current techniques for monitoring antibiotics typically require costly equipment and well-trained professionals. Hence, we developed herein a rapid, instrument-free, and on-site detection method for antibiotic residues such as norfloxacin (NOR) based on a ratiometric sensing platform utilizing "on-off-on" response properties of polychromatic fluorescence for direct visual quantitative NOR analysis. Specifically, this platform integrated iron ions (Fe3+)-chelated blue carbon dots (BCDs) for signal sensing and red carbon dots (RCDs) as an internal reference. The sensor mechanism is selective quenching of BCDs' blue fluorescence by Fe3+ via an inner filter effect with unaffected RCDs' red fluorescence. Further NOR addition led to competitive binding with BCDs due to Fe3+ shedding from the BCDs' surface for a recovered blue fluorescence signal. Notably, the ratiometric fluorescence sensor demonstrated rapid and highly sensitive NOR detection in a concentration range of 1-70 μM with an impressive detection limit of 6.84 nM. The ratiometric fluorescence sensing platform was constructed by integrating smartphone and paper-based strategies, which exhibited exceptional sensitivity, selectivity, and rapid response for portable, instrument-free, visual quantification of NOR in real samples.

Investigation of enhanced degradation of the antibiotic under visible in novel B/ZnO/TiO2nanocomposite and its electrical energy consumption

Both ZnO and TiO2are common semiconducting metal oxides with high mechanical and chemical durability. However, they only have good photocatalytic ability in the UV region, besides the rapid recombination between electrons and holes reduces the efficiency of the decomposition of organic substances. To improve their catalytic efficiency, in this study, ZnO and TiO2were doped with B to produce the novel B/ZnO/TiO2nanocomposites for degrading tetracycline hydrochloride (TCH) in the aqueous solution. The characteristics of samples were analyzed by the diffuse reflectance ultraviolet-visible (DR/UV-vis), scanning electron microscope (SEM), energy-dispersive (EDS), Fourier transform infrared spectroscopy (FT-IR), and x-ray diffraction (XRD) techniques. The 3B/ZnO/TiO2sample had a band gap energy (Eg) of 3.21 eV. Although the B/ZnO/TiO2sample had a tightly aggregated morphology composed of many nanoparticles in 33-137 nm, it still exhibited a higher uniformly and photocatalytic efficiency than ZnO and ZnO/TiO2. At the optimal doped B of 3 wt%, the degradation efficiency (DE) was achieved at 96.33% with a rate constant of 0.067 min-1. The factors that affect the photocatalytic process such as the initial TCH concentration, the catalyst content, and the pH solution were comprehensively investigated. In addition, the stability of 3B/ZnO/TiO2nanocomposite was evaluated via three consecutive cycles and the DE was 69.75% in 3rd cycle. The Z-scheme mechanism was proposed for the photocatalytic mechanism of TCH in the B/ZnO/TiO2catalyst. In addition, electrical energy consumption was estimated that the electrical energy per order only was 29.05 kW.h.l-1.

An electronic tongue based on conjugated polymers for the discrimination and quantitative detection of tetracyclines

A fluorescent sensor array has been developed based on conjugated polymers (CPs) having six different skeletons for the detection of tetracyclines (TCs), which are known as environmental pollutants. CPs were synthesized from confined nanoreactors in a controlled manner. The fluorescent response occurs through the fluorescence resonance energy transfer (FRET) effect. By utilizing linear discriminant analysis (LDA), effective differentiation of TCs was accomplished with a very low detection concentration (66 nM). Moreover, the sensor array exhibited a highly sensitive ability to quantitatively distinguish different concentrations of TCs. Finally, the sensor array's potential for detecting TCs in aqueous solutions has been successfully demonstrated, widening its applications in practical environments. With simple preparation process, a low cost of detection and high sensitivity, the experimental results indicate that the CP-based sensor array is a promising platform for the sensitive and quantitative detection of TCs, and provides a good reference for future scientific research.

Upcycling Waste: Citrus limon Peel-Derived Carbon Quantum Dots for Sensitive Detection of Tetracycline in the Nanomolar Range

In this work, a sustainable method was developed for the production of water-soluble carbon quantum dots employing a green approach. The synthetic protocol was employed using the microwave pyrolysis technique, while lemon peel served as a carbon precursor. Fabrication of highly fluorescent lemon-peel-derived CQDs (LP-CQDs) having inherent nitrogen functionality was validated by X-ray photoelectron spectroscopy, FTIR, X-ray diffraction, Raman spectroscopic analysis, and TEM techniques. The average particle size of fabricated LP-CQDs was 4.46 nm. LP-CQDs yielded a remarkable quantum yield of 49.5%, which displayed excellent salinity, photostability, storage time, conditions, and pH stability. LP-CQDs displayed encouraging results for tetracycline (TC) detection using a PL turn-off approach. The sensitivity of LP-CQDs toward TC was seen in a nanomolar range having a detection limit of 50.4 nM. Method validation was comprehensively studied to ensure the precision of the nanosensor. A complete analysis of different photophysical parameters of LP-CQDs was performed with TC to gain a deeper understanding of the sensing mechanism. Fabricated LP-CQDs showed fluorescence quenching toward TC, elucidated by the inner filter effect (IFE) mechanism. The synthesized nanoprobe demonstrated a lesser detection limit with a broad linear range, enabling facile, cheap, environmentally friendly, and fast detection of TC. Practicality of the detection method was assessed through analysis of real samples, resulting in satisfactory recovery percentage and relative standard deviation with respect to the developed probes. Furthermore, LP-CQDs were used as fluorescent inks and to fabricate paper-based fluorescent strips. This study lays the door for the sensing platform of LP-CQDs toward detection of TC, which may impact the potential role of environmental sustainability.

One-Pot Synthesis of Dual-Emissive Carbon Dots for Ratiometric Fluorescent Determination of Hg2

Mercury ion is a global toxic and hazardous environmental pollutant. In this work, a facile and selective ratiometric fluorescent probe was constructed for the detection of mercury ion. The dual-emissive carbon dots (BYCDs) were fabricated by a one-pot hydrothermal method utilizing o-phenylenediamine and glycine as raw materials, and the prepared BYCDs had two independent fluorescence emission peaks at 426 nm and 543 nm under a single excitation wavelength. Based on the change of the intensity ratio of the two fluorescence emission peaks after the addition of Hg2+, a sensitive and selective ratiometric fluorescent probe based on BYCDs was constructed for the detection of Hg2+ with good linearity ranging from 0.95-50 μM and a detection limit of 0.27 μM. In addition, the recovery of this probe was satisfactory in the standard addition experiments of real water samples, and it could be applied to the analysis of Hg2+ in real water samples.

Waste to value transformation: Converting Carica papaya seeds into green fluorescent carbon dots for simultaneous selective detection and degradation of tetracycline hydrochloride in water

Rampant use of antibiotics has resulted in their seepage into groundwater and ultimately ending up in the food chain, causing antimicrobial resistance. To address this issue, it is imperative to not only quantitatively detect but eliminate them from water. An eco-friendly, one-step microwave-induced pyrolysis of waste papaya seeds (PS) with ethylenediamine (EDA) for just 5min gave green fluorescent nitrogen-doped carbon dots (PS-CDs), which are capable of detecting and photocatalytically degrading TC. The fluorescence properties of PS-CDs displayed that it has high sensitivity and selectivity towards sensing of TC with a detection limit as low as 120 nM. Also, the method gave satisfactory recovery results when extrapolated to determine TC in spiked milk, orange juice, tap water, and honey samples. On the other hand, PS-CDs alone potentially function as an efficient photocatalyst for the degradation of TC. PS-CDs' dual functionality provides an effectual method for the simultaneous detection and degradation of TC by a single nanoprobe.

3D rhombohedral microcrystals metal-organic frameworks for electrochemical and fluorescence sensing of tetracycline

Zirconium-based metal-organic frameworks (MOF) exhibiting 3D rhombohedral microcrystals were synthesized by the solvothermal method. The structure, morphology, composition, and optical properties of the synthesized MOF were carried out using different spectroscopic, microscopic, and diffraction techniques. Synthesized MOF was rhombohedral in shape and the cage structure of these crystalline molecules was the active binding site of the analyte, tetracycline (TET). The electronic property and size of the cages are chosen such that a specific interaction with TET was observed. Sensing of the analyte was demonstrated by both the electrochemical and fluorescent techniques. The MOF had significant luminescent properties and exhibited excellent electro-catalytic activity due to embedded zirconium metal ions. An electrochemical and fluorescence sensor was fabricated towards TET where TET binds via hydrogen bond to MOF, and causes fluorescence quenching due to the transfer of electrons. Both approaches exhibited high selectivity and good stability in the presence of interfering molecules such as antibiotics, biomolecules, and ions; and showed excellent reliability in tap water and wastewater sample analysis.

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.

Alkali-Etched Imprinted Mn-Based Prussian Blue Analogues with Superior Oxidase-Mimetic Activity and Precise Recognition for Tetracycline Colorimetric Sensing

As a typical antibiotic pollutant, tetracycline (TC) is producing increasing threats to the ecosystem and human health, and exploring convenient means for monitoring of TC is needed. Here, we proposed alkali-etched imprinted Mn-based Prussian blue analogues featuring superior oxidase-mimetic activity and precise recognition for the colorimetric sensing of TC. Simply etching Mn-based Prussian blue analogues (Mn-PBAs) with NaOH could expose the sites and surfaces to significantly improve their catalytic activity. Density functional theory calculations were employed to screen the molecularly imprinted polymer (MIP) layer for target identification. Consequently, the designed Mn-PBA_NaOH@MIP possessed the rich channels for substrates to get in touch with the active Mn-PBA_NaOH core, showing an excellent catalytic capacity to trigger the chromogenic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) without the use of H₂O₂. If TC was introduced, it would be recognized selectively by the MIP shell and masked the channels for TMB access, resulting in the obstruction of the chromogenic reaction. According to this mechanism, selective optical detection of TC was achieved, and performance stability, reusability, and reliability as well as practicability were also verified, promising potential for TC monitoring in complex matrices. Our work not only presents an effective way to enhance the enzyme-like activity of Prussian blue analogues but also provides a facile approach for TC sensing. Additionally, the work will inspire the exploration of molecularly imprinted nanozymes for various applications.

Deep insights to explain the mechanism of carbon dot formation at various reaction times using the hydrothermal technique: FT-IR, 13C-NMR, 1H-NMR, and UV-visible spectroscopic approaches

A well-explained mechanism for synthesizing carbon dots (CDs) is not yet explored and is still a subject of great debate and challenge. This study used a one-step hydrothermal method to prepare highly efficient, gram-scale, excellent water solubility, and blue fluorescent nitrogen-doped carbon dots (NCDs) with the particle size average distribution of around 5 nm from 4-aminoantipyrine. The effects of varying synthesis reaction times on the structure and mechanism formation of NCDs were investigated using spectroscopic methods, namely FT-IR, 13C-NMR, 1H-NMR, and UV-visible spectroscopies. The spectroscopic results indicated that increasing the reaction time affects the structure of the NCDs. As the hydrothermal synthesis reaction time is extended, the intensity of the peaks in the aromatic region decreases, and new peaks in the aliphatic and carbonyl group regions are generated, which display enhanced intensity. In addition, the photoluminescent quantum yield increases as the reaction time increases. The presence of a benzene ring in 4-aminoantipyrine is thought to contribute to the observed structural changes in NCDs. This is due to the increased noncovalent π-π stacking interactions of the aromatic ring during the carbon dot core formation. Moreover, the hydrolysis of the pyrazole ring in 4-aminoantipyrine results in polar functional groups attached to aliphatic carbons. As the reaction time prolongs, these functional groups progressively cover a larger portion of the surface of the NCDs. After 21 h of the synthesis process, the XRD spectrum of the produced NCDs illustrates a broad peak at 21.1°, indicating an amorphous turbostratic carbon phase. The d-spacing measured from the HR-TEM image is about 0.26 nm, which agrees with the (100) plane lattice of graphite carbon and confirms the purity of the NCD product with a surface covered by polar functional groups. This investigation will lead to a greater understanding of the effect of hydrothermal reaction time on the mechanism and structure of carbon dot synthesis. Moreover, it offers a simple, low-cost, and gram-scale method for creating high-quality NCDs crucial for various applications.

A simple paper-based nickel nanocluster-europium mixed ratio fluorescent probe for rapid visual sensing of tetracyclines

In this work, a ratiometric fluorometric sensor based on nickel nanoclusters (NiNCs)-europium complex (NiNCs-Eu³⁺) was constructed for the highly selectivity detection of tetracyclines (TCs) in water samples. In the presence of TCs, the blue fluorescence of the sensor NiNCs-Eu³⁺ was quenched at 430 nm and the characteristic red fluorescence of Eu³⁺-TCs appeared at 620 nm because of the combined help of inner filter effect (IFE) and antenna effect. Under the optimized conditions (100 mM Eu³⁺ (100 µL); temperature (25℃); reaction time (10 min), HEPES buffer solution (pH = 7.0)), the sensor offered a wide detection range of tetracycline (TC) and oxytetracycline (OTC) from 0.1 to 50 μM with the detection limit (LOD) of 25 nM and 21 nM, respectively. Moreover, the sensor was able to detect of TC and OTC in tap and lake water with high recovery rate (89.10%-97.60%). In addition, the portable paper-based sensor was constructed using filter paper embedded with NiNCs-Eu³⁺. The distinct fluorescent color of the paper-based sensor varied from bright blue to red against different concentrations of TC and OTC. These above findings demonstrated the potential for wide application of as-prepared ratio metric fluorescence sensor for visual detection of TCs in water samples.

Nitrogen-doped carbon dots: Recent developments in its fluorescent sensor applications

Doped carbon dots have attracted great attention from researchers across disciplines because of their unique characteristics, such as their low toxicity, physiochemical stability, photostability, and outstanding biocompatibility. Nitrogen is one of the most commonly used elements for doping because of its sizeable atomic radius, strong electronegativity, abundance, and availability of electrons. This distinguishes them from other atoms and allows them to perform distinctive roles in various applications. Here, we have reviewed the most current breakthroughs in nitrogen-doped CDs (N-CDs) for fluorescent sensor applications in the last five years. The first section of the article addresses several synthetic and sustainable ways of making N-CDs. Next, we briefly reviewed the fluorescent features of N-CDs and their sensing mechanism. Furthermore, we have thoroughly reviewed their fluorescent sensor applications as sensors for cations, anions, small molecules, enzymes, antibiotics, pathogens, explosives, and pesticides. Finally, we have discussed the N-CDs' potential future as primary research and how that may be used. We hope that this study will contribute to a better understanding of the principles of N-CDs and the sensory applications that they can serve.

A novel label-free dual-mode aptasensor based on the mutual regulation of silver nanoclusters and MoSe2 nanosheets for reliable detection of ampicillin

Current methods for the rapid detection of trace antibiotics in the environment remains problems of low accuracy and false negative or false positive, making the development of fast, and accurate, and reliable methods for antibiotic testing a major challenge that needs to be addressed. Herein, we developed a novel label-free colorimetric and fluorescent dual-mode aptasensor assembled by the strong interaction of layered MoSe₂ nanosheets (MoSe₂ NSs) with ampicillin (AMP) aptamer functionalized silver nanoclusters (Apt-AgNCs) that specifically bind AMP to allow the sensitive and selective detection of AMP. Apt-AgNCs could be adsorbed on the surface of MoSe₂ NSs via van der Waals force to form a nanocomposite, Apt-AgNCs/MoSe₂ NSs. Interestingly, Apt-AgNCs/MoSe₂ NSs act together to construct dual mode aptasensor through modulation of the intrinsic peroxidase activity of MoSe₂ NSs and the fluorescence of Apt-AgNCs. In the presence of AMP, Apt-AgNCs could specifically bind AMP, triggering desorption from the MoSe₂ NSs surface, leading to a decrease in the peroxidase activity of the system with the recovery in Apt-AgNCs fluorescence. The dual-signal aptasensor exhibited good linear colorimetric and fluorescence responses in the AMP concentration ranges of 0.115-2.00 μM and 6-100 nM, respectively. Furthermore, the aptasensor was successfully measured AMP levels in commercially-bought milk and lake water with satisfactory results. Unlike single-signal aptasensors, the constructed dual-signal aptasensor could not only improve the detection precision, but also reduce the false positive or false negative results. These promising results suggest that the dual-readout strategy as demonstrated is general mode for the detection of other antibiotics or compounds using various aptamers functionalized AgNCs in concert with MoSe₂ NSs.

A sensitized ratiometric fluorescence probe based on N/S doped carbon dots and mercaptoacetic acid capped CdTe quantum dots for the highly selective detection of multiple tetracycline antibiotics in food

A ratiometric fluorescent probe based on N/S doped carbon dots (N/S-CQDs) and mercaptoacetic acid capped CdTe quantum dots (TGA-CdTe QDs) with sensitized and self-calibration functions was constructed to sensitively detect multiple tetracycline antibiotics (TCs). N/S-CQDs could attach stably to TGA-CdTe QDs and form a new composite ratiometric fluorescent probe that had a more than tenfold increase in sensitivity to TCs compared with each single QD. The probe could detect four common TCs as the color of the probe changed from bright red to dark red, and the limit of detection (LOD) was 1.47 × 10^-2-1.78 × 10^-2 mg/L. Practical applications of the probe in food and urine were also verified with recovery rates of 95.21%-104.97%. Due to the abundant spectral fingerprints provided by both QDs, this novel probe could accurately recognize not only different single TCs but also mixed TC samples even in actual samples combined with chemometrics.

Molecularly imprinted ratiometric fluorescence detection of tetracycline based on its fluorescence enhancement effect caused by tungsten trioxide quantum dots

This paper reports a novel probe developed based on the tungsten trioxide quantum dots (WO_3-x QDs) and molecularly imprinted polymers for the detection of trace tetracycline (TC) in the complex food matrix. Tungsten ion (W⁶⁺) in WO_3-x QDs has a fluorescence enhancement effect on TC, and TC has a fluorescence quenching effect on WO_3-x QDs. The blue emission of the WO_3-x QDs (λ_em = 470 nm) as a reference and the yellow emission of the TC (λ_em = 550 nm) as a response were utilized for the ratiometric fluorescence detection. In order to improve its selectivity, the molecular imprinting technology was combined to construct molecularly imprinted ratiometric fluorescent probes (MIRFPs). Therefore, the MIRFPs can not only selectively detect TC, but also realize the visual detection from blue to yellow. Under the optimal conditions, the linear ranges of 0.01 ∼ 10.0 μmol/L and 20.0 ∼ 80.0 μmol/L were obtained with the limits of detection of 3.23 nmol/L and 6.37 μmol/L, respectively. Furthermore, the MIRFPs had been successfully applied to the detection of TC in milk and eggs. The satisfactory recoveries were in the range of 92.7 ∼ 102.9 % with relative standard deviations (RSD, n = 3) below 1.59 %. This work offers a good strategy for the detection of food hazards.

The synthesis of N-doped carbon dots for visual differentiating and detection of tetracyclines

N-doped carbon dots (N-CDs) were synthesized from L-glutamine and triethanolamine using a one-step hydrothermal method. The N-CDs emitting blue fluorescence had selective responses to tetracyclines (TCs) and could be used as a fluorescent probe to realize the quantitative detection and qualitative analysis of TCs. A method for the determination of TCs using the N-CDs in actual samples was successfully established. The recovery rate was maintained at 97.50-105.60%, and the relative standard deviation (RSD) was less than 3%. In addition, TCs can be visually distinguished using filter paper by the different fluorescence colours (light green, dark blue, and yellow-green) of the N-CDs/TCs system under ultraviolet light. This study provides a relatively simple method to detect and identify TCs.

Novel preparation of red fluorescent carbon dots for tetracycline sensing and its application in trace determination

The controllable design of red-emitting carbon dots and further exploration of their application in the trace determination of environmental pollutants remains a tremendous challenge. Herein, the novel strategy for red fluorescent carbon dots (R-CDs) with a higher quantum yield of 58.9% was proposed by doping small-molecule urea into the bio-dye of resazurin for the first time, which can retain the luminophore of precursors and exhibit exceptional optical, advantageous reversibility and outstanding photostability. Importantly, the R-CDs exhibit a remarkable fluorescence reduction towards tetracyclines (TCs) accompanied by a noticeable color change of R-CDs solution from red to yellow, which can realize the trace detection of TCs at strelatively low levels, including tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC). The linear range of TC, CTC, and OTC are 3-40 μM, 4-50 μM, and 2-50 μM, and the corresponding detection limits are 38.5 nM, 64.6 nM, and 45.4 nM, respectively (S/N = 3). Furthermore, the R-CDs demonstrate sensitivity to the physiological pH in the linear range of 4.0-5.0 and 5.0-6.2 with a pKa of 5.61. As a multifunctional fluorescent sensor, R-CDs can provide a new perspective for the preparation of long-wavelength CDs, and further realize the trace determination of environmental pollutants.

A ratiometric fluorescence sensor for tetracycline detection based on two fluorophores derived from Partridge tea

Blue fluorescent carbon dots (PCDs) were prepared by hydrothermal method with Partridge tea. The ethanol extract of Partridge tea (PEE) was found to emit red fluorescence. Thus, a novel ratiometric sensor was constructed by simply mixing the two fluorophores derived from Partridge tea. The presence of tetracycline (TET) at lower concentrations enhanced the emission peak at 508 nm of PCDs and had a negligible effect on the emission peak at 680 nm of PEE. TET at higher concentrations led to  quenching  both the fluorescence of PCDs and PEE via inner filter effect and fluorescence resonance energy transfer, separately. Good linearities for the detection of TET were obtained in the ranges 0.67 to 15.00 μM and 33.33 to 266.67 μM, with limit of detection of 0.095 μM. The sensor was successfully applied to detect TET in lake water and milk samples with good recoveries ranging from 93.27 ± 4.04% to 107.30 ± 6.16%. This study provided a simple, selective, sensitive, rapid, and environmentally friendly method of monitoring TET residues in the environment and food.

Dual-mode detection sensor based on nitrogen-doped carbon dots from pine needles for the determination of Fe3+ and folic acid

In this study, nitrogen-doped carbon dots (N-CDs) from pine needles were obtained by one-step hydrothermal synthesis without any chemical reagents. The fluorescence quenching and absorbance enhancement of N-CDs occurred when Fe³⁺ and folic acid (FA) were added. Based on this, the dual-mode detection sensor by fluorescence and ultraviolet-visible (UV-Vis) spectrophotometry for the determination of Fe³⁺ and FA was established. Detected by the dual-mode detection sensor under the optimized condition, the linear range of Fe³⁺ was 0.1-540 μM and FA was 0.1-165 μM. At the same time, the two inputs "NOR" and "OR" logic gates are constructed successfully according to the dual-mode sensor signals. The proposed dual-mode detection sensor is simple, efficient and stable; it can be applied to determinate Fe³⁺ and FA in practical samples successfully and the results are satisfactory.

Smartphone-assisted miniature device based on nitrogen and sulfur co-doped carbon dots for point-of-care testing of tetracycline

A miniature device was design for the point-of-care testing (POCT) of tetracycline (TC) including a ratio fluorescence test strip, a sample slot, a UV lamp and a smartphone. The nitrogen and sulfur co-doped carbon dots (N, S-CDs) and Eu³⁺ were dropped onto the filter paper to construct the ratio fluorescence test strips for the specific detection of TC. Under the excitation at 390 nm, the fluorescence emission of N, S-CDs at 530 nm decreases through inner filter effect (IEF) after addition of Eu³⁺. When the further addition of TC, the emission of N, S-CDs at 530 nm kept unchanged while the emission of Eu³⁺ at 616 nm was obviously enhanced for the antenna effect (AE) between Eu³⁺ and TC. The ratio changes of the two-fluorescence emission realized the quantitative detection of TC. In addition, the test strips with different concentrations of TC showed different fluorescence color from green to red under a 365 nm UV lamp. The miniature device was designed as a fluorescence photo reader with the merits of the powerful functions of smartphones and the portability of test strips. The smartphone camera takes a fluorescent color image of the test strips and the photos are recognized by a color recognizer on the smartphone to obtain RGB (red-greenblue) values which reflect the concentrations of the analytes. Therefore, we established a fast, sensitive and efficient POCT of TC. In particular, the proposed nanomaterial-based POCT platform will open a new route towards the development of ratio fluorescence probe for TC analysis for environment samples.

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

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

Multifunctional Carbon Dots-Based Fluorescence Detection for Sudan I, Sudan IV and Tetracycline Hydrochloride in Foods

Sudan dyes are strictly prohibited from being added to edible products as carcinogens and tetracycline hydrochloride (TC) remaining in animal-derived food may cause harm to the human body. Therefore, it is necessary to establish a high-sensitivity, simple and convenient method for the detection of Sudan dyes and TC in foods for safety purposes. In this work, multifunctional blue fluorescent carbon dots (B-CDs) were prepared by a one-step hydrothermal synthesis using glucose as the carbon source. The results show that the fluorescence intensity of B-CDs was significantly affected by the acidity of the solution and can be quenched by Sudan I, IV and TC through selective studies. Interestingly, the fluorescence quenching intensities of B-CDs have a good linear relationship with the concentration of Sudan I and IV at pH = 3-7. The wide range of pH is beneficial to broaden the application of B-CDs in a practical samples analysis. The method has been successfully applied to real food samples of tomato paste, palm oil and honey, and the detection limits are 26.3 nM, 54.2 nM and 31.1 nM for Sudan I, Sudan IV and TC, respectively. This method integrates Sudan dyes and TC into the same multifunctional B-CDs, which shows that the sensor has a great potential in food safety detection.

Nitrogen-doped cyan-emissive carbon quantum dots for fluorescence tetracycline detection and lysosome imaging

Tetracyclines (TCs) prevent the growth of peptide chains and the synthesis of proteins, and they are widely used to inhibit Gram-positive and -negative bacteria. For the detection of tetracyclines in cell and in vitro, a convenient and simple detection system based on nitrogen-doped cyan carbon quantum dots (C-CQDs) was developed. C-CQDs have excellent excitation-independent properties, the best optimal excitation peak is 360 nm and the best emission peak is 480 nm. Based on the inner filter effect (IFE), the fluorescence intensity of C-CQDs in solution decreases with the increase of tetracyclines. In the range of 0-100 μM, C-CQDs present a good linear relationship with three tetracyclines (CTC, TET, OCT), with R ² all greater than 0.999. C-CQDs can detect tetracycline in milk samples with recovery in the range of 98.2-103.6%, which demonstrates their potential and broad application in real samples. Furthermore, C-CQDs exhibit excellent lysosomal targeting, as indicated by a Pearson's coefficient of 0.914 and an overlap of 0.985. The internalisation of C-CQDs was mainly affected by lipid raft-mediated endocytosis in endocytic pathway experiments. These experiments indicate that C-CQDs can be effectively used to detect TC content and target lysosomes as an alternative to commercial dyes.

A fluorescent and scattering dual-mode probe based on a carbon dot@cerium-guanosine monophosphate coordination polymer network for tetracycline detection

Dual-mode sensing with a two-signal read-out is conducive to the improvement of detection accuracy. Herein, a fluorescent and scattering dual-mode chemosensor for tetracycline (TC) is proposed based on a carbon dot@cerium-guanosine monophosphate (CD@GMP-Ce) coordination polymer network. The inexpensive CD@GMP-Ce was prepared by exploiting the adaptive inclusion capability of coordination polymers and possessed remarkable fluorescence and strong Rayleigh scattering. The functional CD@GMP-Ce demonstrated fluorescence and scattering, the two optical-signal responses to TC simultaneously. Based on TC-specific fluorescence and scattering decline, the dual-mode detection of TC was established and the probe's detection limits were 43 nM in the fluorescence mode and 77 nM in the scattering mode, respectively. Furthermore, the potential application of the dual-mode sensor was verified by measuring TC in milk and tap-water samples. The study not only provides a new perspective for the development of assay methods for TC but also expands the applications of cerium coordination polymers.