Fluorescence Probe Based on Hybrid Mesoporous Silica/Quantum Dot/Molecularly Imprinted Polymer for Detection of Tetracycline

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

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

Ultrasensitive Detection of Tetracycline Using Boron and Nitrogen Co-Doped Graphene Quantum Dots from Natural Carbon Source as the Paper-Based Nanosensing Probe in Difference Matrices

Herein, the boron and nitrogen co-doped 0-dimensional graphene quantum dots (B,N-GQDs) with high quantum yield (QY) were synthesized via microwave-assisted hydrothermal method at 170 °C for 20 min using fresh passion fruit juice and boric acid as the starting materials. The 3–6 layers of B,N-GQDs with mean particle size of 9 ± 1 nm were then used for ultra-sensitive and selective detection of tetracycline in aqueous and biological media. The hybridization of boron and nitrogen atoms into the GQD structures increases the intensity of electronegative, resulting in the enhancement of QY to 50 ± 1%. The B,N-GQDs show their excellent analytical performance on tetracycline determination after 2 min of reaction under an optimal condition at pH 5. The linear range of 0.04–70 µM and with limits of detection (LOD) of 1 nM in phosphate buffer saline (PBS), 1.9 nM in urine and 2.2 nM in human serum are obtained. Moreover, the high selectivity of tetracycline by B,N-GQDs over the other 23 interferences is observed. The π-π interaction and electron donor-acceptor principle play pivotal roles in enhancing the ultra-sensitivity and selectivity of B,N-GQDs toward TC detection. Moreover, the B, N-GQD based paper nanosensor exhibits an excellent analytical performance on visual detection of 0.1–30 µM TC in human serum. Results of this study clearly indicate the feasibility of synthesis of B,N-GQDs derived from passion fruit juice for ultrasensitive tetracycline detection, which can open an avenue to use natural products for the preparation of environmentally benign and biocompatible carbon nanomaterials for highly sensitive detection of drugs, antibiotics, organic compounds and biomarkers.

Metal-polydopamine framework based lateral flow assay for high sensitive detection of tetracycline in food samples

Gold nanoparticles (AuNPs)-based lateral flow assay (LFA) enables a rapid detection of tetracycline (TET) in food samples but suffers from low sensitivity. Herein, metal-polydopamine framework (MPF), as a label, was employed to load monoclonal antibodies (mAbs) directly as the probe in LFA for highly sensitive sensing of TET. Combining zeolitic imidazolate framework (ZIF-67) and polydopamine (PDA), a stable MPF with large size, well water-dispersible, excellent affinity and optical properties was prepared through a versatile layer-by-layer assembly (LLA) strategy. Under optimized conditions, the biosensor (MPF-LFA) exhibited a great linear relationship in the range of 0.09-6 ng/mL and a detection limit of 0.045 ng/mL for TET detection, which was over 66-fold more sensitive than traditional AuNPs based LFA. Furthermore, the MPF-LFA was successfully applied to the screening of TET in fish, chicken, milk and shrimp samples with satisfied recoveries from 91% to 114%.

The Use of Aptamers and Molecularly Imprinted Polymers in Biosensors for Environmental Monitoring: A Tale of Two Receptors

Effective molecular recognition remains a major challenge in the development of robust receptors for biosensing applications. Over the last three decades, aptamers and molecularly imprinted polymers (MIPs) have emerged as the receptors of choice for use in biosensors as viable alternatives to natural antibodies, due to their superior stability, comparable binding performance, and lower costs. Although both of these technologies have been developed in parallel, they both suffer from their own unique problems. In this review, we will compare and contrast both types of receptor, with a focus on the area of environmental monitoring. Firstly, we will discuss the strategies and challenges involved in their development. We will also discuss the challenges that are involved in interfacing them with the biosensors. We will then compare and contrast their performance with a focus on their use in the detection of environmental contaminants, namely, antibiotics, pesticides, heavy metals, and pathogens detection. Finally, we will discuss the future direction of these two technologies.

Multifunctional conjugated microporous polymers with pyridine unit for efficient iodine sequestration, exceptional tetracycline sensing and removal

In this work, two multifunctional conjugated microporous polymers (CMP-LS7-8) were obtained via the Pd-catalyzed Suzuki coupling reactions of 2,4,6-tris(4-bromophenyl)pyridine with two aromatic borates. The Brunauer-Emmett-Teller (BET) surface areas of CMP-LS7-8 were calculated to be 507 and 2028 m² g^-1. CMP-LS7-8 exhibit excellent volatile iodine adsorption about 2.77 and 5.29 g g^-1, respectively, and outstanding reversible adsorption. High adsorption capacity should be attributed to an integrated effect by excellent porous characteristics, effective sorption sites, and expanded π-conjugated network. In addition, this platform integrated two functions of sensing and adsorption of tetracycline (TC) into one material. The excellent luminescence of CMP-LS7-8 can be effectively quenched by TC, which demonstrates they can be acted as new sensitive and selective fluorescence probes toward TC. Simultaneously, CMP-LS7-8 also display high adsorption ability of TC. The adsorption kinetics of TC suggested that the process of adsorption followed a pseudo-second-order model, and the adsorption behaviour of these polymers fitted with the Langmuir model. These results suggest that CMP-LS7-8 posess high volatile iodine capture and exceptional TC detection and removal performance, which can be promising candidates for environmental remediation.

A long lifetime ratiometrically luminescent tetracycline nanoprobe based on Ir(III) complex-doped and Eu3+-functionalized silicon nanoparticles

Different from fluorescent dyes-doped or carbon materials-based ratiometric tetracycline nanoprobes, herein, a new Ir(III) complex-doped and europium(III) ion (Eu³⁺)-functionalized silicon nanoparticles (Ir(III)@SiNPs-Eu³⁺) with long luminescent lifetimes was firstly fabricated for selective detection of tetracycline (TC) in complex systems through time-resolved emission spectra (TRES) measurement. In the presence of TC, the red phosphorescence of Eu³⁺ is greatly enhanced by adsorption energy transfer emission (AETE) of TC, while the strong green luminescence of Ir(III)@SiNPs is quenched by the inner filtration effect (IFE) of TC. Based on these striking emission changes, Ir(III)@SiNPs-Eu³⁺ can sensitively detect TC in the linear range of 0.01-20 μM with a detection limit of 4.9 × 10^-3 μM. Benefitting from the long lifetime of Ir(III)@SiNPs-Eu³⁺, the nanoprobe demonstrates excellent TC detection performance through TRES in high background system of 5 % human serum. Furthermore, the formed Ir(III)@SiNPs-Eu³⁺/TC complex can be used to sensitively recognize Hg²⁺ via a ratiometric luminescence mode. Notably, the cytotoxicity of Ir(III)@SiNPs-Eu³⁺ is very low and thus the sensitive monitoring the detection of Ir(III)@SiNPs-Eu³⁺ to TC and Hg²⁺ also works well in porcine renal cells, demonstrating high application potential in real samples.

A novel fluorescent "turn-on" aptasensor based on nitrogen-doped graphene quantum dots and hexagonal cobalt oxyhydroxide nanoflakes to detect tetracycline

In this study, a novel fluorescent "turn-on" aptasensor was developed for sensitive and rapid detection of tetracycline (TC) in animal-derived food. It is based on aptamer-functionalized nitrogen-doped graphene quantum dots (N-GQDs-aptamer) coupled with cobalt oxyhydroxide (CoOOH) nanoflakes. The CoOOH nanoflakes are efficient fluorescence quenchers in homogeneous solutions, and this is due to their advantages of excellent optical properties, superior flexibility, and water dispersibility. The proposed method's mechanism is driven by quenching based on the fluorescence resonance energy transfer (FRET) between the donor (N-GQDs) and the acceptor (CoOOH nanoflakes). On the other hand, fluorescence recovery is caused by the structure switching behavior of the aptamer. Compared with previous methods, our developed method exhibits better behavior in terms of being easy to fabricate and being simple in detection procedure and maintains the detection limit low enough in TC determination: a linear range from 1 to 100 ng mL^-1 and a detection limit of 0.95 ng mL^-1 (S/N = 3). Furthermore, the proposed method was applied to five animal-derived food samples (milk, honey, fish, eggs, and chicken muscle) and demonstrated practical applicability. As well, the method has the advantages of simplicity in pre-treatment and convenience in instruments, saves times, and is cost-effective. Finally, the proposed method demonstrates significant potential for sensitive and rapid detection of specific components in real samples. Graphical abstract.

Molecularly Imprinted Sites Translate into Macroscopic Shape-Memory Properties of Hydrogels

The polymerization of acrylamide, dopamine methacrylamide, and bis-acrylamide in the presence of one of the electron acceptors, N,N'-dimethyl-4,4'-bipyridinium, (1), N,N'-dimethylbipyridinium-4,4'-ethylene, (2), or bipyridinium dithienylethene, (3), yields hydrogel matrices of high stiffness that are cooperatively cross-linked by bis-acrylamide and electron donor (dopamine)-acceptor complexes. Washing off the diffusional electron acceptor units yields molecularly imprinted matrices of lower stiffness, stabilized only by the bis-acrylamide bridges that include specific binding sites for the selective association of the electron acceptor (1), (2), or (3). These imprinted hydrogel matrices show selective recovery of the stiff properties upon binding the respective electron acceptor units to the imprinted sites. The control over the stiffness properties enables the development of shape-memory, molecularly imprinted hydrogels and stiffness-based sensors. The results show how molecularly imprinted sites translate into macroscopic shape-memory properties of hydrogels.

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

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

Molecularly imprinted mesoporous silica incorporating C3N4 dots and CdTe quantum dots as ratiometric fluorescent probe for determination of Malachite Green

A new dual-emission ratiometric fluorescent probe was synthesized and successfully used for the determination of Malachite Green (MG) in fish farming water. The ratiometric fluorescent probe was successfully composited by sol-gel method using C₃N₄ and CdTe quantum dots as fluorescent materials combined with mesoporous molecularly imprinted polymers. MG quenches the red fluorescence of the CdTe QDs (with excitation/emission wavelengths at 350/680 nm) while the blue fluorescence of C₃N₄ (with excitation/emission wavelengths at 350/458 nm) remains unchanged. The change of fluorescence color and fluorescence intensity ratio can be successfully used for quantification of malachite green. In addition, the mesoporous structure has a large surface and good adsorption capacity for malachite green. The normalized intensity of fluorescence increases linearly in the 50-1000 ng·mL^-1 MG concentration range, and the detection limit is 10 ng·mL^-1. The imprinting factor is 3.2. The nanoprobe was applied to the determination of MG in fish farming water samples. Recoveries and relative standard deviations were 92.5-97.8% and 2.5-6.2%, respectively. Graphical abstract Schematic representation of synthesis of molecularly imprinted mesoporous silica ratiometric fluorescent probes incorporating C₃N₄ dots and CdTe quantum dots for determination of malachite green.

Semiconductor nanocrystal-polymer hybrid nanomaterials and their application in molecular imprinting

Quantum dots (QDs) are attractive semiconductor fluorescent nanomaterials with remarkable optical and electrical properties. The broad absorption spectra and high stability of QD transducers are advantageous for sensing and bioimaging. Molecular imprinting is a technique for manufacturing synthetic polymeric materials with a high recognition ability towards a target analyte. The high selectivity of the molecularly imprinted polymers (MIPs) is a result of the fabrication process based on the template-tailored polymerization of functional monomers. The three-dimensional cavities formed in the polymer network can serve as the recognition elements of sensors because of their specificity and stability. Appending specific molecularly imprinted layers to QDs is a promising strategy to enhance the stability, sensitivity, and selective fluorescence response of the resulting sensors. By merging the benefits of MIPs and QDs, inventive optical sensors are constructed. In this review, the recent synthetic strategies used for the fabrication of QD nanocrystals emphasizing various approaches to effective functionalization in aqueous environments are discussed followed by a detailed presentation of current advances in QD conjugated MIPs (MIP-QDs). Frontiers in manufacturing of specific imprinted layers of these nanomaterials are presented and factors affecting the specific behaviour of an MIP shell are identified. Finally, current limitations of MIP-QDs are defined and prospects are outlined to amplify the capability of MIP-QDs in future sensing.

Fluorescent probe based on carbon dots/silica/molecularly imprinted polymer for lysozyme detection and cell imaging

The abnormal concentration of lysozyme in body fluids and tissues is a potential indicator for diseases such as leukemia and meningitis. In this work, by combining the excellent optical properties of carbon dots (CDs) with the favorable selectivity of molecularly imprinted polymer (MIP), a novel fluorescent probe for lysozyme detection and cell imaging was constructed, where silanized CDs with low cytotoxicity (CDs/SiO₂) were used as the fluorescence signal reporter and N-isopropylacrylamide (NIPAM) was used as the temperature-sensitive monomer. The as-prepared CDs/SiO₂/MIP showed a thermo-sensitive property for the response to lysozyme. Moreover, this probe could be used for quantitative detection of lysozyme, with a wider detection range (0.001~0.01 mg/mL), a low detection limit (0.55 μg/mL), and a high selectivity. Importantly, the MTT assay testified that the fluorescent CDs/SiO₂/MIP probe had low cytotoxicity. In addition, human hepatoma carcinoma cells (HepG-2 cells) could be stained by the CDs/SiO₂/MIP, and showed a bright intracellular green fluorescence, indicating that the imaging of live cells was possible. This study provides a new way to detect lysozyme in vitro and an attractive perspective to probe intracellular lysozyme in vivo.

Amino-Functionalized Al-MOF for Fluorescent Detection of Tetracyclines in Milk

A fluorescent method for detection of tetracyclines (TCs) in milk was developed by using the NH₂-MIL-53(Al) nanosensor synthesized via a one-pot hydrothermal method. The nanosensor had a crystalline nanoplates structure with rich groups of -NH₂ and -COOH. The -NH₂/-COOH of NH₂-MIL-53(Al) reacted with the -CO-/-OH of TCs to form a complex. The electron of -NH₂/-COOH from the NH₂-BDC ligand transferred to the -CO-/-OH of TCs. -NH₂ of the NH₂-MIL-53(Al) interacted with the -CO-/-OH of TCs by hydrogen bonding. The quenching efficiency of the inner filter effect (IFE) was calculated to contribute 57-89%. The synergistic effect of photoinduced electron transfer (PET) and IFE account for fluorescence quenching. TCs were quantitatively detected in milk samples with recoveries of 85.15-112.13%; the results were in great accordance with high-performance liquid chromatography (HPLC) ( P > 0.05), confirming the NH₂-MIL-53(Al) nanosensor has potential applicability for the detection of TCs in food matrix.

An eco-friendly imprinted polymer based on graphene quantum dots for fluorescent detection of p-nitroaniline

An eco-friendly fluorescent molecularly imprinted polymer anchored on the surface of graphene quantum dots (GQDs@MIP) was developed with an efficient sol–gel polymerization for highly sensitive and selective determination of p-nitroaniline (p-NA).

Selective electrochemical detection of bisphenol A using a molecularly imprinted polymer nanocomposite

A molecularly imprinted nanocomposite with covalently connected polyacrylate, β-cyclodextrin and graphene is synthesized for selective capture and electrochemical detection of bisphenol A.

Post-imprinting modification based on multilevel mesoporous silica for highly sensitive molecularly imprinted fluorescent sensors

p-MIFPs based on multilevel mesoporous silica exhibited improved sensitivity as compared to those prepared by the doping method due to their well-maintained fluorescence intensity and low background.

New insights into the structure-performance relationships of mesoporous materials in analytical science

Mesoporous materials are ideal carriers for guest molecules and they have been widely used in analytical science. The unique mesoporous structure provides special properties including large specific surface area, tunable pore size, and excellent pore connectivity. The structural properties of mesoporous materials have been largely made use of to improve the performance of analytical methods. For instance, the large specific surface area of mesoporous materials can provide abundant active sites and increase the probability of contact between analytes and active sites to produce stronger signals, thus leading to the improvement of detection sensitivity. The connections between analytical performances and the structural properties of mesoporous materials have not been discussed previously. Understanding the "structure-performance relationship" is highly important for the development of analytical methods with excellent performance based on mesoporous materials. In this review, we discuss the structural properties of mesoporous materials that can be optimized to improve the analytical performance. The discussion is divided into five sections according to the analytical performances: (i) selectivity-related structural properties, (ii) sensitivity-related structural properties, (iii) response time-related structural properties, (iv) stability-related structural properties, and (v) recovery time-related structural properties.

A nanocomposite optosensor containing carboxylic functionalized multiwall carbon nanotubes and quantum dots incorporated into a molecularly imprinted polymer for highly selective and sensitive detection of ciprofloxacin

A nanocomposite optosensor consisting of carboxylic acid functionalized multiwall carbon nanotubes and CdTe quantum dots embedded inside a molecularly imprinted polymer (COOH@MWCNT-MIP-QDs) was developed for trace ciprofloxacin detection. The COOH@MWCNT-MIP-QDs were synthesized through a facile sol-gel process using ciprofloxacin as a template molecule, 3-aminopropylethoxysilane as a functional monomer and tetraethoxysilane as a cross-linker at a molar ratio of 1:8:20. The synthesized nanocomposite optosensor had high sensitivity, excellent specificity and high binding affinity to ciprofloxacin. Under optimal conditions, the fluorescence intensity of the optosensor decreased in a linear fashion with the concentration of ciprofloxacin and two linear dynamic ranges were obtained, 0.10-1.0 μg L^-1 and 1.0-100.0 μg L^-1 with a very low limit of detection of 0.066 μg L^-1. The imprinting factors of the two linear range were 17.67 and 4.28, respectively. The developed nanocomposite fluorescence probe was applied towards the determination of ciprofloxacin levels in chicken muscle and milk samples with satisfactory recoveries being obtained in the range of 82.6 to 98.4%. The results were also in good agreement with a HPLC method which indicates that the optosensor can be used as a sensitive, selective and rapid method to detect ciprofloxacin in chicken and milk samples.

Magnetic carbon dots based molecularly imprinted polymers for fluorescent detection of bovine hemoglobin

A simple and effective method was proposed to prepare an imprinted polymer layer at the surface of magnetic carbon dots with dopamine as the functional monomer for selectively and sensitively fluorescent recognizing bovine hemoglobin. The magnetic fluorescence imprinted polymers were investigated by Fourier-transform infrared spectra, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, and vibrating sample magnetometer. Under optimum conditions, the fluorescent intensity decreased linearly coincided with the concentration of bovine hemoglobin in the range of 0.05-16.0 μM with a detection limit of 17.3 nM. The magnetic fluorescence imprinted polymers were used for detecting bovine hemoglobin in real samples with the recoveries of 99.0-104.0%. These results indicated that a convenient method was proposed to develop fluorescent probes for rapid recognition and sensitive detection of trace proteins in real samples.

Modulation of the carboxamidine redox potential through photoinduced spiropyran or fulgimide isomerisation

Carboxamidines functionalized with either a spiropyran or fulgimide photoswitch were prepared on multigram scales. The thermal, electrochemical, and photochemical ring isomerizations of these compounds were studied and the results compared with related systems. The photochemical isomerisations were found to be reversible and could be followed by 1H NMR and UV-vis spectroscopy. The spiropyran/merocyanine couple was thermally active and an activation enthalpy of 116 kJ mol-1 was measured for ring-opening. These measurements yielded an enthalpy difference of 25 kJ mol-1 between the open and closed states which is consistent with DFT calculations. DFT calculations predicted a charge transfer to the carboxamidine group upon ring closure in the fulgimide and a charge transfer from the carboxamidine group upon switching the spiropyran to the merocyanine form. This was confirmed experimentally by monitoring the change in the oxidation potential assigned to the carboxamidine group. The potential of these molecules to therefore act as a new class of photoresponsive ligands that can modulate the ligand field of a complex is discussed.

Switch-on fluorescent strategy based on crystal violet-functionalized CdTe quantum dots for detecting L-cysteine and glutathione in water and urine

The concentration of L-cysteine (Cys) and glutathione (GSH) is closely related to the critical risk of various diseases. In our study, a new rapid method for the determination of Cys and GSH in water and urine samples has been developed using a fluorescent probe technique, which was based on crystal violet (CV)-functionalized CdTe quantum dots (QDs). The original QDs emitted fluorescence light, which was turned off upon adding CV. This conjugation of CV and QDs could be attributed to electrostatic interaction between COO^- of mercaptopropionic acid (MPA) on the surface of QDs and N⁺ of CV in aqueous solution. In addition, Förster resonance energy transfer (FRET) also occurred between CdTe QDs and CV. After adding Cys or GSH to the solution, Cys or GSH exhibited a stronger binding preference toward Cd²⁺ than Cd²⁺-MPA, which disturbed the interaction between MPA and QDs. Thus, most MPA was able to be separated from the surface of QDs because of the participation of Cys or GSH. Then, the fluorescence intensity of the CdTe QDs was enhanced. Good linear relationships were obtained in the range of 0.02-40 μg mL^-1 and 0.02-50 μg mL^-1, and the detection limits were calculated as 10.5 ng mL^-1 and 8.2 ng mL^-1, for Cys and GSH, respectively. In addition, the concentrations of biological thiols in water and urine samples were determined by the standard addition method using Cys as the standard; the quantitative recoveries were in the range of 97.3-105.8%, and relative standard deviations (RSDs) ranged from 2.5 to 3.7%. The method had several unique properties, such as simplicity, lower cost, high sensitivity, and environmental acceptability. Graphical abstract Crystal violet-functionalized CdTe quantum dots for detecting L-cysteine and glutathione with switch-on fluorescent strategy.