A molecularly imprinted fluorescence nanosensor based on upconversion metal–organic frameworks for alpha-cypermethrin specific recognition

A fluorescent nanocomposite probe of quantum dots and zinc oxide embedded in polymer for smartphone-assisted on-site determination of diflunisal

A fluorescent sensor based on nitrogen-doped graphene quantum dots (N-GQDs) was developed for the smartphone-assisted colorimetric determination of diflunisal. The fluorescence source was embedded with zinc oxide (ZnO) in a molecularly imprinted polymer (ZnO@N-GQDs@MIP). The quantitative analysis was based on the fluorescence quenching caused by electron transfer from the nanoprobe to diflunisal. The sensor demonstrated linearity in the range of 0.10-50.0 μg L-1 with a limit of detection of 0.03 μg L-1. Smartphone-assisted on-site determination produced linearity in the range of 1.0-50.0 µg/L with a limit of detection of 0.30 μg L-1. The developed sensor was applied to determine diflunisal in milk, egg and yogurt samples. Recoveries ranging from 94.8 to 103.7 % were achieved with a RSD below 2.0 % measured by fluorescence spectroscopy, and from 94.9 to 106.9 % with a RSD of <6 % smatphone-assisted measurement. Comparison of the detection outcomes of both methods with those of high-performance liquid chromatography revealed consistent results, demonstrating the accuracy of the developed method, which was also sensitive, selective, and fast. Notably, the portable and easy-to-read smartphone-assisted method is suitable for on-site application.

A Smartphone-Integrated Ratiometric Fluorescence Sensor for the Ultrasensitive and Selective Detection of 5-Heneicosylresorcinol in Whole Wheat Foods

Precise on-site monitoring of alkylresorcinols, a vital biomarker, is crucial for verifying whole wheat foods and accurately quantifying the whole wheat content in various consumer and industrial products. Herein, for the first time, we introduce a novel ratiometric fluorescence sensor (CDs@ZIF-8/CdTe@MIP) for ultrasensitive and selective detection of alkylresorcinols. 5-Heneicosylresorcinol (C21:0 AR), the primary alkylresorcinol homologue in whole wheat grains, was selected as the target analyte. This analyte was specifically and selectively recognized by the incorporation of a molecularly imprinted polymer (MIP) layer. Within this nanoreactor, blue-emitting carbon dots embedded in zeolitic imidazolate framework-8 (CDs@ZIF-8) and orange-emitting CdTe quantum dots served as the self-calibration signal and response signal, respectively. Exploiting a photoinduced electron transfer effect between CdTe and C21:0 AR, the established fluorescence sensor exhibited remarkable sensing performance, offering wide linear responses in 0.005-1 μg·mL-1 and 1-80 μg·mL-1 concentration ranges, and achieving a low detection limit of 1.14 ng·mL-1. The proposed assay effectively detected C21:0 AR in real samples, including 8 whole wheat foods and 19 whole wheat grains, demonstrating good recoveries and relative standard deviation. Furthermore, an intelligent sensing platform was established by integrating CDs@ZIF-8/CdTe@MIP with a smartphone-assisted device, thus validating the feasibility of visual and on-site monitoring of C21:0 AR. Because of its rapid response, portability, cost-effectiveness, superior sensitivity, and high selectivity, the proposed sensor serves as a reliable method for the analysis of C21:0 AR, thus having substantial potential for on-site monitoring of whole wheat foods.

A naphthalimide-based portable fluorescent sensor integrated with a photoelectric converter for rapid and on-site detection of type II pyrethroids in celery

The on-site detection of pyrethroids, particularly type II pyrethroids, remains a challenging task in complex vegetable samples. Herein, a novel method based on naphthalimide was developed to realize the specific detection of type II pyrethroids by hydrolyzing and utilizing the compound m-phenoxybenzaldehyde (3-PBD). Hydrazine group, used as the appropriate moiety, was introduced into the fluorescent dye 1,8-naphthalimide to construct the fluoroprobe NAP. In the presence of 3-PBD, NAP displayed the prominently enhanced fluorescence and also exhibited high selectivity. This proposed method exhibited high anti-inference effects in complex media, realizing sensitive detection of 3-PBD with linear range of 2.15-800 μM and a low detection limit (LOD) of 0.64 μM. The underlying fluorescence-responsive mechanisms were in-depth elucidated by combining spectral analyses with TD-DFT theoretical calculations. Additionally, a direct and rapid hydrolysis method for deltamethrin in celery was established, achieving a high hydrolysis efficiency of >90% within 15 min. Furthermore, a portable fluorescence sensor (PFS) was developed based on high-power LEDs and photodetectors. PFS supplied a LOD of 2.23 μM for 3-PBD and exhibited comparable stability by a fluorescence spectrometer when detecting celery hydrolysate. Moreover, external power source is not required for PFS operations, thereby enabling rapid and on-site detection by transmitting data to a smartphone via bluetooth. These findings extend the academic knowledge in the field of specific pyrethroids detection and contribute to the development of on-site methods for pesticide residual analyses in food matrices.

Inner filter effect-based upconversion nanosensor for rapid detection of thiram pesticides using upconversion nanoparticles and dithizone-cadmium complexes

Herein, we propose a method for detecting thiram based on the fluorescence inner filter effect using upconversion nanoparticles and dithizone-cadmium complexes (DZ-Cd2+). The ultraviolet absorption of DZ-Cd2+ was in the range of 480-600 nm under alkaline conditions, resulting in fluorescence quenching of the nanoparticles at 540 nm. Thiram had a stronger coordination effect with Cd2+ than dithizone; thus, more thiram-cadmium complex (T-Cd2+) formed when thiram was added, leading to fluorescence recovery at 540 nm. The standard thiram curve was found to have a detection limit of 6.75 ng/mL in the linear range of 0.01-1000 µg/mL. In addition, high-performance liquid chromatography results for detecting thiram in apple samples revealed good application performance. The results demonstrate that the developed method has great potential to detect thiram residues in food.

Novel dual-emission fluorescence imprinted sensor based on Mg, N-CDs and metal-organic frameworks for rapid and smart detection of 2, 4, 6-trinitrophenol

Rapid and real-time detection of 2, 4, 6-trinitrophenol (TNP) is of great importance for the living environment and human health. Herein, we constructed an innovative ratiometric fluorescence imprinted sensor with fast response and high selectivity based on magnesium and nitrogen co-doped carbon dots (Mg, N-CDs) and chromium telluride quantum dots (r-CdTe) self-assembled in zirconium-based metal organic frameworks (UiO-66) combined with imprinted polymers for the detection of TNP. In the protocol, the introduction of UiO-66 with large specific surface area and porosity using as carrier material significantly enhanced the mass transfer rate, which improved the sensitivity of the Mg, N-CDs/r-CdTe@UiO-66@MIP (LHU@MIP). And the Mg, N-CDs with high quantum yields and r-CdTe were selected as fluorescence emitting elements to yield fluorescence signal, achieving signal amplification. The dual-channel strategy enabled the sensor to not only display a fast fluorescence response, but also generate a dual-response signal under the action of internal filtering effect (IFE). Combining these advantages, the LHU@MIP had a wide linear range (1-100 μM), good detection sensitivity (0.56 μM), and a distinct color change (from blue to pink). Meanwhile, for accurate on-site analysis, we designed a portable smart sensing platform with a color recognizer application. The smartphone enabled visual sensing of TNP by capturing fluorescent images and converting them into digital values. More importantly, the platform was successfully utilized for the analysis of TNP in the simulated actual samples with considerable results. Therefore, the developed platform was characterized by low cost, portability, ideal specificity, and provided a strategy for on-site monitoring of TNP.

Trace detection of methcathinone in sewage using targeted extraction based on magnetic molecularly imprinted polymers coupled with liquid chromatography-tandem mass spectrometry

Methcathinone, a new psychoactive substance (NPS), poses a serious threat to public health. Therefore, there is an urgent need to develop a reliable, selective, sensitive and simple analytical technique for monitoring trace amounts of this target NPS in complex matrices. For this purpose, magnetic molecularly imprinted polymers (MMIPs) based on MIPs combined with nano-sized magnetic Fe3O4 were developed for the specific enrichment of methcathinone in wastewater. The binding properties and selectivity of MMIPs toward methcathinone were evaluated and compared with non-imprinted polymer (MNIPs). For sensitive and selective extraction and determination of the target methcathinone, magnetic solid-phase extraction (MSPE) based on MMIPs was combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Under optimized conditions, the proposed method was successfully used for the detection of methcathinone in wastewater, which provided a low limit of detection of 0.3 ng L-1 and a limit of quantification of 1.0 ng L-1 with relative standard deviations of less than 6.89% for intra- and inter-day analyses. Good linearity in the range of 1-2000 ng L-1 with a coefficient of determination (R2) greater than 0.98 was observed. Moreover, a certified reference material of water sample was successfully analyzed with satisfactory results and the recoveries of spike experiments ranged from 96.35-116.7%.

A Turn-Off Fluorescent Biomimetic Sensor Based on a Molecularly Imprinted Polymer-Coated Amino-Functionalized Zirconium (IV) Metal-Organic Framework for the Ultrasensitive and Selective Detection of Trace Oxytetracycline in Milk

Developing sensitive and effective methods to monitor oxytetracycline residues in food is of great significance for maintaining public health. Herein, a fluorescent sensor (NH₂-UIO-66 (Zr)@MIP) based on a molecularly imprinted polymer-coated amino-functionalized zirconium (IV) metal-organic framework was successfully constructed and first used for the ultrasensitive determination of oxytetracycline. NH₂-UIO-66 (Zr), with a maximum emission wavelength of 455 nm under 350 nm excitation, was prepared using a microwave-assisted heating method. The NH₂-UIO-66 (Zr)@MIP sensor with specific recognition sites for oxytetracycline was then acquired by modifying a molecularly imprinted polymer on the surface of NH₂-UIO-66 (Zr). The introduction of NH₂-UIO-66 (Zr) as both a signal tag and supporter can strengthen the sensitivity of the fluorescence sensor. Thanks to the combination of the unique characteristics of the molecularly imprinted polymer and NH₂-UIO-66 (Zr), the prepared sensor not only exhibited a sensitive fluorescence response, specific identification capabilities and a high selectivity for oxytetracycline, but also showed good fluorescence stability, satisfactory precision and reproducibility. The fabricated sensor displayed a fluorescent linear quenching in the OTC concentration range of 0.05-40 μg mL^-1, with a detection limit of 0.012 μg mL^-1. More importantly, the fluorescence sensor was finally applied for the detection of oxytetracycline in milk, and the results were comparable to those obtained using the HPLC approach. Hence, the NH₂-UIO-66 (Zr)@MIP sensor possesses great application potential for the accurate evaluation of trace oxytetracycline in dairy products.

Dual-template molecularly surface imprinted polymer on fluorescent metal-organic frameworks functionalized with carbon dots for ascorbic acid and uric acid detection

In this work, dual-template molecularly imprinted polymer surfaces imprinted on blue fluorescent Cr-based MOF (Cr-MOF) functionalized with yellow emissive carbon dots (Y-CDs) were prepared using l-ascorbic acid (AA) and uric acid (UA) as templates for simultaneous selective recognition of AA and UA. The as-prepared nanocomposite probe (Y-CDs/Cr-MOF@MIP) contains two recognition site cavities and emits a dual well-resolved fluorescence spectra when excited at 390 nm; blue emission (λ_em 450 nm) is due to Cr-MOF, and yellow emission (λ_em 560 nm) is due to Y-CDs. The yellow fluorescence emission of Y-CDs was quenched upon the addition of ascorbic acid, while Cr-MOF's emission remained unaffected. In the same way, the blue fluorescence emission of the Cr-MOFs was quenched in the presence of uric acid, while the yellow emission remained constant. Both emissions were quenched in a sample containing both AA and UA. This can be exploited to design a dual-template biosensor to detect UA and AA simultaneously. The Y-CDs/Cr-MOF@MIP sensor displayed a dynamic linear response for AA in the range 25.0 µM - 425.0 µM with a detection limit of 1.30 µM, and for UA in the range 25.0 µM - 425.0 µM with a detection limit of 1.10 µM. The dual-target probe Y-CDs/Cr-MOF@MIP was highly selective and sensitive for the detection of UA and AA in human urine samples due to the selectivity of the two recognition sites.

Recent Advances in Nanoparticle-Based Optical Sensors for Detection of Pesticide Residues in Soil

The excessive and unreasonable use of pesticides has adversely affected the environment and human health. The soil, one of the most critical natural resources supporting human survival and development, accumulates large amounts of pesticide residues. Compared to traditional spectrophotometry analytical methods, nanoparticle-based sensors stand out for their simplicity of operation as well as their high sensitivity and low detection limits. In this review, we focus primarily on the functions that various nanoparticles have and how they can be used to detect various pesticide residues in soil. A detailed discussion was conducted on the properties of nanoparticles, including their color changeability, Raman enhancement, fluorescence enhancement and quenching, and catalysis. We have also systematically reviewed the methodology for detecting insecticides, herbicides, and fungicides in soil by using nanoparticles.

Simultaneous and Sensitive Detection of Three Pesticides Using a Functional Poly(Sulfobetaine Methacrylate)-Coated Paper-Based Colorimetric Sensor

Chlorpyrifos (CHL), profenofos (PRO) and cypermethrin (CYP) are widely used in combination to increase crop yields. However, these three pesticides can cause serious harm to human health and do not easily degrade. In this study, a novel visible paper sensor has been prepared successfully and different colorimetric reactions were utilized to detect the three pesticides simultaneously. The sensor was constructed by grafting a zwitterionic polymer onto a cellulose filter (CF) and placing it on a glass surface modified with PDMS. The branch shape was designed to form multiple detection areas, which were modified with specific pesticides and corresponding chromogenic reagents. The as-prepared colorimetric platform exhibited high sensitivity, a short detection time, a good linear response and a low detection limit (LOD) for the three pesticides (chlorpyrifos: y = 46.801 - 1.939x, R² = 0.983, LOD = 0.235 mg/L; profenofos: y = 40.068 + 42.5x, R² = 0.988, LOD = 4.891 mg/L; cypermethrin: y = 51.993 + 1.474x, R² = 0.993, LOD = 4.053 mg/L). The comparison of the results obtained by the proposed paper sensor and those obtained by spectrophotometry further revealed the stability and reliability of the paper sensor. In particular, the color intensity of the interaction between the pesticides and coloring agents could be directly observed by the human eye. The consistency of the colorimetric/optical assay was proven in real target pesticide samples. Thus, this sensing strategy provides a portable, cost-effective, accurate and visualized paper platform, which could be suitable for application in the fruit and vegetable industry for monitoring CHL, PRO and CYP in parallel.

Electrochemical sensor for ultrasensitive detection of paraquat based on metal-organic frameworks and para-sulfonatocalix[4]arene-AuNPs composite

The widespread occurrence of pesticides in surface water, groundwater, soil, and food has received increasing attention towards environmental safety. Paraquat (PQ) is world widely used as a rapid sterilant herbicide and is highly toxic to humans. A simple, rapid, sensitive, and on-site detection method for the water environment to detection of PQ is urgently required. Here, we prepared a zeolite imidazole skeleton-8 (ZIF-8) and para-sulfonylcalix[4]arene (pSC₄) coated gold nanoparticles composite (pSC₄-AuNPs@ZIF-8) by one-step method. An electrochemical biosensor assay for PQ was established based on pSC₄-AuNPs@ZIF-8 modified glassy carbon electrode through host-guest recognition of PQ and pSC₄. Under the optimal conditions, recoveries of targets determination results were 92.7%-103% (n = 3), respectively. The quantity PQ detection limit was found to be 0.49 pM. Therefore, the signal amplification strategy based on pSC₄-AuNPs@ZIF-8 has potential value in detecting trace pollutants in the water environment.

A Review on Graphene Quantum Dots for Electrochemical Detection of Emerging Pollutants

Graphene quantum dots which are known as zero-dimensional materials are gaining increasing attention from researchers all over the world. This is predicated upon their relatively unique chemiluminescent, fluorescent, electrochemiluminescent, and electronic properties. The precise mechanism of electrochemiluminescence continues to be a subject of debate in the research world, and this is important in identifying synthetic pathways for graphene quantum dots. Heavy metals and other emerging pollutants are global health and environmental concerns. Several studies have reported the sensitivity and limit of detection of graphene quantum dots up to the nano-, pico-, and femto- levels when used as sensors. This review seeks to bridge information gaps on the reported electrochemiluminescence chemosensors for emerging pollutants using graphene quantum dots under the sub-headings, synthesis, characterization, electrochemiluminescence chemosensor detection, and comparison with other detection methods.

Designing of robust and sensitive assay via encapsulation of highly emissive and stable blue copper nanocluster into zeolitic imidazole framework (ZIF-8) with quantitative detection of tetracycline

Metal–organic frameworks (MOFs) with high stability and porosity have gained great attention in bioanalysis due to their potential in improving sensitivity and robustness of assays. Herein, to improve both the stability and the emission intensity of Cu nanoclusters (CuNCs), in situ entrapment strategy of CuNCs into zeolitic imidazolate framework-8 (ZIF-8) is described. Blue emissive and stable CuNCs was prepared, for the first time, using thiamine hydrochloride as capping agents, and showed strong and stable emission at 440 nm when excited at 375 nm with fluorescence quantum yields 12%. Encapsulation of CuNC into ZIF-8 showed dramatic enhancement of the fluorescence intensity up to 53% fluorescence quantum yield. Furthermore, the CuNCs@ZIF-8 possesses better stability (more than three months) due to protective and confinement effect of MOFs. Upon the addition of tetracycline to CuNCs@ZIF-8 solution, the blue emission intensity was significantly decreased. The fluorescence ratio (Fo/F) against the concentration of tetracycline exhibited a satisfactory linear relationship from 1.0 to 10.0 µM with a detection limit (LOD) of 0.30 µM. The current probe was applied for quantification of tetracycline in drug sample with satisfactory accuracy and precision.

Graphical abstract

A High-Luminescence Biomimetic Nanosensor Based on N, S-GQDs-Embedded Zinc-Based Metal-Organic Framework@Molecularly Imprinted Polymer for Sensitive Detection of Octopamine in Fermented Foods

In this study, a novel fluorescent molecularly imprinted nanosensor (N, S-GQDs@ZIF-8@MIP) based on the nitrogen and sulfur co-doped graphene quantum dots decorated zeolitic imidazolate framework-8 was constructed for the detection of octopamine (OA). Herein, ZIF-8 with a large surface area was introduced as a supporter of the sensing system, which effectively shortened the response time of the sensor. Meanwhile, high green luminescent N, S-GQDs and a maximum emission wavelength of 520 nm under 460 nm excitation and a 12.5% quantum yield were modified on the surface of ZIF-8 as a signal tag that can convert the interactions between the sensor and OA into detectable fluorescent signals. Finally, N, S-GQDs@ZIF-8@MIP was acquired through the surface molecular imprinting method. Due to the synergy of N, S-GQDs, ZIF-8, and MIP, the obtained sensor not only demonstrated higher selectivity and sensitivity than N, S-GQDs@ZIF-8@NIP, but also displayed faster fluorescence response than N, S-GQDs@MIP. Under optimal conditions, the developed sensor presented a favorable linear relationship in the range of 0.1-10 mg L-1 with a detection limit of 0.062 mg L-1. Additionally, the proposed N, S-GQDs@ZIF-8@MIP strategy was effectively applied to the detection of OA in fermented samples, and the obtained results had a satisfactory correlation with those of HPLC.

Functional Hybrid Micro/Nanoentities Promote Agro-Food Safety Inspection

The rapid development of nanomaterials has provided a good theoretical basis and technical support to solve the problems of food safety inspection. The combination of functionalized composite nanomaterials and well-known detection methods is gradually applied to detect hazardous substances, such as chemical residues and toxins, in agricultural food products. This review concentrates on the latest agro-food safety inspection techniques and methodologies constructed with the assistance of new hybrid micro/nanoentities, such as molecular imprinting polymers integrated with quantum dots (MIPs@QDs), molecular imprinting polymers integrated with upconversion luminescent nanoparticles (MIPs@UCNPs), upconversion luminescent nanoparticles combined with metal-organic frameworks (UCNPs@MOFs), magnetic metal-organic frameworks (MOFs@Fe₃O₄), magnetic covalent-organic frameworks (Fe₃O₄@COFs), covalent-organic frameworks doped with quantum dots (COFs@QDs), nanobody-involved immunoassay for fast inspection, etc. The presented summary and discussion favor a relevant outlook for further integrating various disciplines, like material science, nanotechnology, and analytical methodology, for addressing new challenges that emerge in agro-food research fields.

Upconversion Nanoparticles Encapsulated with Molecularly Imprinted Amphiphilic Copolymer as a Fluorescent Probe for Specific Biorecognition

A fluorescent probe for specific biorecognition was prepared by a facile method in which amphiphilic random copolymers were encapsulated with hydrophobic upconversion nanoparticles (UCNPs). This method quickly converted the hydrophobic UCNPs to hydrophilic UNCPs. Moreover, the self-folding ability of the amphiphilic copolymers allowed the formation of molecular imprinting polymers with template-shaped cavities. LiYF4:Yb3+/Tm3+@LiYF4:Yb3+ UCNP with up-conversion emission in the visible light region was prepared; this step was followed by the synthesis of an amphiphilic random copolymer, poly(methacrylate acid-co-octadecene) (poly(MAA-co-OD)). Combining the UCNPs and poly(MAA-co-OD) with the templates afforded a micelle-like structure. After removing the templates, UCNPs encapsulated with the molecularly imprinted polymer (MIP) (UCNPs@MIP) were obtained. The adsorption capacities of UCNPs@MIP bound with albumin and hemoglobin, respectively, were compared. The results showed that albumin was more easily bound to UCNPs@MIP than to hemoglobin because of the effect of protein conformation. The feasibility of using UCNPs@MIP as a fluorescent probe was also studied. The results showed that the fluorescence was quenched when hemoglobin was adsorbed on UCNPs@MIP; however, this was not observed for albumin. This fluorescence quenching is attributed to Förster resonance energy transfer (FRET) and overlap of the absorption spectrum of hemoglobin with the fluorescence spectrum of UCNPs@MIP. To our knowledge, the encapsulation approach for fabricating the UCNPs@MIP nanocomposite, which was further used as a fluorescent probe, might be the first report on specific biorecognition.

A highly sensitive upconversion nanoparticles@zeolitic imidazolate frameworks fluorescent nanoprobe for gallic acid analysis

In this work, a core-shell structured upconversion nanoparticles@zeolitic imidazolate frameworks (ZIF-8) fluorescent nanoprobe (NaErF₄:Tm@SiO₂@ZIF-8) has been designed for the detection of gallic acid (GA). The mechanism is according to the 3, 3', 5, 5'-tetramethylbenzidine (TMB) can be oxidized to oxidized TMB (oxTMB) by Ag⁺. Under 980 nm laser excitation, NaErF₄:Tm@SiO₂@ZIF-8 can emit red light at 652 nm, which have a good overlap with the absorption spectra of oxTMB, resulting in the fluorescence quenching at 652 nm. Continually adding GA into the above solution, oxTMB will restore to TMB, and the fluorescence intensity at 652 nm gradually recovers, which can realize the detection towards GA. The linear detection range of GA is from 0 to 30 μM, and the limit of detection (LOD) of GA is 0.35 μM. The ZIF-8 can largely enhance the sensitivity of the nanoprobe, due to the physical absorption and the electrostatic attraction between ZIF-8 and the oxTMB. More importantly, this is the first time to realize the detection of GA with high sensitivity by using upconversion fluorescence. Besides, we have realized the analysis of GA in real samples, which certify the feasible of the nanoprobe in potential applications.