White-light emissive upconversion nanoparticles for visual and colorimetric determination of the pesticide thiram

A two-dimensional coordination polymer with high laccase-like activity for sensitive colorimetric detection of thiram

In contrast to natural enzymes, nanozymes show promising advantages of low cost and high stability for analytical applications. The simple mix of L-phenylalanine (F) and Cu2+ produces two-dimensional nanosheets of a coordination polymer with a high surface area ratio and rich exposed active sites as a novel catalyst. As the mimetic of natural laccase, this nanozyme (F-Cu) can catalyze the oxidative coupling reaction of 2,4-dichlorophenol (2,4-DP) and 4-aminoantipyrine (4-AP) to produce a distinct red product, thus establishing an intuitive and simple method for the detection of thiram. In the range of 0-7.5 μM, the absorbance intensity was linearly related to the concentration of thiram, and the detection limit was 0.0845 μM. The F-Cu nanozyme was successfully applied to the colorimetric detection of thiram in real samples.

Recent Advances in Developing Optical and Electrochemical Sensors for Monitoring Thiram and Future Perspectives

Thiram, as one widely used dithiocarbamate pesticide, has been considered seriously detrimental to food safety and human health because of poor efficiency, nonstandard/superfluous usage, and lack of a targeting effect. Developing high-performance sensors for thirams is strongly needed. With the rapid development of chemistry, biology, and materials science, many sensors have been constructed for thiram with high sensitivity and selectivity. Regarding the energy form of the signal, recognition mode, and detection principle, recent advances in the design and construction of optical and electrochemical sensors for thiram are summarized in this review, including colorimetric, luminescent, chemiluminescent, and electrochemical sensors. The advantages and disadvantages of the sensors for thiram including sensitivity, ability to avoid interference, recognition mechanism, signal output mode, and practicability are clarified in detail. Furthermore, the challenges faced, effective restrictions, and next direction of development are proposed for achieving more sensitive and selective analysis of thiram with less interference. We desire that this review will supply a solid theoretical basis and inspiration to generate innovative thinking for achieving new progress on thiram assays and the commercialization of the developed sensors in the future.

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.

A solid-phase capture probe based on upconvertion nanoparticles and inner filter effect for the determination of ampicillin in food

Ampicillin (AMP) is commonly used to treat diseases caused by bacterial infections as a veterinary drug. However, the abuse of AMP can lead to residues in food and ultimately cause harm to humans. Thus, it is significant to construct a reliable system for AMP detection. Here, we developed an inner filter effect system based on a solid-phase capture probe and the catalysis of platinum nanoparticles (PtNPs) for AMP determination in food. In the presence of AMP, PDMS captured AMP then combined with aptamer-functionalized PtNPs, which catalyzed the oxidation of 3,3',5,5'-tetramethylbenzidine, resulting in upconversion fluorescence quenching. The results showed the fluorescence intensity of upconversion nanoparticles was related to AMP concentration (0.5-100 ng·mL^-1) with an LOD of 0.32 ng·mL^-1, which made quantification of AMP possible. The method also achieved a satisfactory recovery rate (96.89-112.92%) and can be used for AMP detection in food samples with selectivity and sensitivity.

Full shell coating or cation exchange enhances luminescence

Core-shell structure is routinely used for enhancing luminescence of optical nanoparticles, where the luminescent core is passivated by an inert shell. It has been intuitively accepted that the luminescence would gradually enhance with the coverage of inert shell. Here we report an “off-on” effect at the interface of core-shell upconversion nanoparticles, i.e., regardless of the shell coverage, the luminescence is not much enhanced unless the core is completely encapsulated. This effect indicates that full shell coating on the luminescent core is critical to significantly enhance luminescence, which is usually neglected. Inspired by this observation, a cation exchange approach is used to block the energy transfer between core nanoparticle and surface quenchers. We find that the luminescent core exhibits enhanced luminescence after cation exchange creates an effective shell region. These findings are believed to provide a better understanding of the interfacial energy dynamics and subsequent luminescence changes.

Core-shell designs enhance the luminescence of lanthanide-doped upconversion nanoparticles (UCNPs), but the effect of shell coverage was insufficiently characterized. Here the authors demonstrate, on a series of core-shell UCNPs with various shell coverage ratios, an on-off effect by which luminescence is enhanced only when a full coverage is achieved.

Mesoporous silica-loaded gold nanocluster with enhanced fluorescence and ratiometric fluorescent detection of thiram in foods

A core-shell QDs@mSiO2@y-AuNCs nanoprobe was prepared, and a new ratiometric fluorescent sensor for thiram detection was developed. The mechanism of thiram sensing was investigated using FTIR, surface-enhanced Raman, XPS spectra, etc. The sensing of thiram was mainly ascribed to the formation of Au-S bonds between thiram and Au atoms on y-AuNCs surface, resulting in the dissociation of 11-MUA ligand from the y-AuNCs surface and the charge transfer between thiram and y-AuNCs. In the ratiometric fluorescence detection of thiram based on QDs@mSiO2@y-AuNCs, a linear range of 0.5-60 ng/mL was obtained with a LOD of 0.19 ng/mL. Compared with the fluorescence detection based on y-AuNCs, the ratiometric fluorescence detection of thiram demonstrated 3-fold enhanced sensitivity. The improvement was ascribed to two aspects: the fluorescence emission of y-AuNCs was enhanced after they were loaded onto the QDs@mSiO2 nanoparticles; the ratiometric detection mode provided more precise sensing. The detection of thiram can be completed immediately after mixing the nanoprobe with thiram. Good recoveries of thiram in apple and pear samples were achieved. All the above results demonstrated the high potential of this method in practical applications.

Lanthanide ion (Ln3+ )-based upconversion sensor for quantification of food contaminants: A review

The food safety issue has gradually become the focus of attention in modern society. The presence of food contaminants poses a threat to human health and there are a number of interesting researches on the detection of food contaminants. Upconversion nanoparticles (UCNPs) are superior to other fluorescence materials, considering the benefits of large anti-Stokes shifts, high chemical stability, non-autofluorescence, good light penetration ability, and low toxicity. These properties render UCNPs promising candidates as luminescent labels in biodetection, which provides opportunities as a sensitive, accurate, and rapid detection method. This paper intended to review the research progress of food contaminants detection by UCNPs-based sensors. We have proposed the key criteria for UCNPs in the detection of food contaminants. Additionally, it highlighted the construction process of the UCNPs-based sensors, which includes the synthesis and modification of UCNPs, selection of the recognition elements, and consideration of the detection principle. Moreover, six kinds of food contaminants detected by UCNPs technology in the past 5 years have been summarized and discussed fairly. Last but not least, it is outlined that UCNPs have great potential to be applied in food safety detection and threw new insight into the challenges ahead.

In situ reduction triggers the highly sensitive detection of pesticide by classic gold nanoparticle and quantum dots nanocomposite

Fluorescence "turn on" method is always preferable for target detection under the urgent demand to develop point-of-care portable sensors in practical applications due to its higher selectivity and less false positives. However, there is only few reports of pesticide monitoring based on this strategy so far most probably ascribed to its poor hydrophilicity and reactivity. In this work, triggered by reductant tris (2-carboxyethyl) phosphine hydrochloride (TCEP), initially fluorescence-quenched gold nanoparticles (Au NPs)-decorated quantum dots (QDs)-embedded nanobead shows obvious fluorescence "turn on" signal response to thiram with concentration response range of 0.01-20 μM and limit of detection (LOD) of 7 nM due to the target-induced dissociation of Au NPs from the surface of probe nanobead. Moreover, paper sensor has been successfully developed by immersing commercial drainage membrane in probe solution for visual detection of thiram with the ultrahigh LOD (50 nM) by the naked eye. More importantly, this work, for the first time, reported an in situ reduction strategy to improve the interaction between target and nanoprobe and thus bring obvious signal output for pesticide detection with high sensitivity, demonstrating the potential to expand the detection scope of nanomaterials.

Biosensing based on upconversion nanoparticles for food quality and safety applications

Food safety and quality regulations inevitably call for sensitive and accurate analytical methods to detect harmful contaminants in food and to ensure safe food for the consumer. Both novel and well-established biorecognition elements, together with different transduction schemes, enable the simple and rapid analysis of various food contaminants. Upconversion nanoparticles (UCNPs) are inorganic nanocrystals that convert near-infrared light into shorter wavelength emission. This unique photophysical feature, along with narrow emission bandwidths and large anti-Stokes shift, render UCNPs excellent optical labels for biosensing because they can be detected without optical background interferences from the sample matrix. In this review, we show how this exciting technique has evolved into biosensing platforms for food quality and safety monitoring and highlight recent applications in the field.

Engineering of a Dual-Recognition Ratiometric Fluorescent Nanosensor with a Remarkably Large Stokes Shift for Accurate Tracking of Pathogenic Bacteria at the Single-Cell Level

Rapid, accurate, reliable, and risk-free tracking of pathogenic microorganisms at the single-cell level is critical to achieve efficient source control and prevent outbreaks of microbial infectious diseases. For the first time, we report a promising approach for integrating the concepts of a remarkably large Stokes shift and dual-recognition into a single matrix to develop a pathogenic microorganism stimuli-responsive ratiometric fluorescent nanoprobe with speed, cost efficiency, stability, ultrahigh specificity, and sensitivity. As a proof-of-concept, we selected the Gram-positive bacterium Staphylococcus aureus (S. aureus) as the target analyte model, which easily bound to its recognition aptamer and the broad-spectrum glycopeptide antibiotic vancomycin (Van). To improve the specificity and short sample-to-answer time, we employed classic noncovalent π-π stacking interactions as a driving force to trigger the binding of Van and aptamer dual-functionalized near-infrared (NIR) fluorescent Apt-Van-QDs to the surface of an unreported blue fluorescent π-rich electronic carbon nanoparticles (CNPs), achieving S. aureus stimuli-responsive ratiometric nanoprobe Apt-Van-QDs@CNPs. In the assembly of Apt-Van-QDs@CNPs, the blue CNPs (energy donor) and NIR Apt-Van-QDs (energy acceptor) became close to allow the fluorescence resonance energy transfer (FRET) process, leading to a remarkable blue fluorescence quenching for the CNPs at ∼465 nm and a clear NIR fluorescence enhancement for Apt-Van-QDs at ∼725 nm. In the presence of S. aureus, the FRET process from CNPs to Apt-Van-QDs was disrupted, causing the nanoprobe Apt-Van-QDs@CNPs to display a ratiometric fluorescent response to S. aureus, which exhibited a large Stokes shift of ∼260 nm and rapid sample-to-answer detection time (∼30.0 min). As expected, the nanoprobe Apt-Van-QDs@CNPs showed an ultrahigh specificity for ratiometric fluorescence detection of S. aureus with a good detection limit of 1.0 CFU/mL, allowing the assay at single-cell level. Moreover, we also carried out the precise analysis of S. aureus in actual samples with acceptable results. We believe that this work offers new insight into the rational design of efficient ratiometric nanoprobes for rapid on-site accurate screening of pathogenic microorganisms at the single-cell level in the early stages, especially during the worldwide spread of COVID-19 today.

An Excitation Navigating Energy Migration of Lanthanide Ions in Upconversion Nanoparticles

Upconversion nanoparticles (UCNPs) doped with lanthanide ions that possess ladder-like energy levels can give out multiple emissions at specific ultra-violet or visible wavelengths irrespective of excitation light. However, precisely controlling energy migration processes between different energy levels of the same lanthanide ion to generate switchable emissions remains elusive. Herein, a novel dumbbell-shaped UCNP is reported with upconverted red emission switched to green emission when excitation wavelength changed from 980 to 808 nm. The sensitizer Yb ions are doped with activator Er ions and energy modulator Mn ions in NaYF₄ core nanocrystal coated with an inner NaYF₄ :Yb shell to generate red emission after harvesting 980 nm excitation light, while an outer NaNdF₄ :Yb shell is coated to form a dumbbell shape to generate green emission upon 808 nm excitation. Such specially designed UCNPs with switchable green and red emissions are further explored for imaging of latent fingerprint and detection of explosive residues in the fingerprint simultaneously. This work suggests a novel research interest in fine-tuning of upconversion emissions through precisely controlling energy migration processes of the same lanthanide activator ion. Furthermore, use of these nanoparticles in other applications such as simultaneous dual-color imaging or orthogonal bidirectional photoactivation can be explored.

Fluorescent assay based on phenyl-modified g-C3N4 nanosheets for determination of thiram

Phenyl-modified graphitic carbon nitride nanosheets (Ph-g-C₃N₄ NSs) were synthesized by a thermal copolymerization and ultrasonic exfoliation method. The Ph-g-C₃N₄ NSs are used as a fluorescent assay for determination of thiram. The results of X-ray photoelectron spectroscopy, ¹³C solid-state nuclear magnetic resonance and Fourier transform infrared spectra confirm that phenyl group is integrated into the heptazine network of g-C₃N₄. Compared to the g-C₃N₄ NSs, the Ph-g-C₃N₄ NSs show bigger stokes shift about 185 nm and higher fluorescence intensity. The fluorescence of Ph-g-C₃N₄ NSs is quenched by Cu²⁺ via the photo-induced electron transfer mechanism, which then recovers in the presence of thiram. The fluorescence restoring of Ph-g-C₃N₄ NSs is correlated with the concentration of thiram. Under the optimized conditions, the fluorescent intensity of g-C₃N₄ NSs at excitation/emission wavelengths of 310/455 nm give a linear range of 33.0-670 nM with detection limit of 9.90 nM. While fluorescent assay based on the Ph-g-C₃N₄ NSs show the linear range of 6.70-1300 nM at excitation/emission wavelengths of 310/495 nm with detection limit of 2.01 nM. Graphical abstract Schematic representation of fluorescent "on-off-on" assay based on phenyl-modified graphitic carbon nitride nanosheets (Ph-g-C₃N₄ NSs) for determination of thiram.

Surface lanthanide activator doping for constructing highly efficient energy transfer-based nanoprobes for the on-site monitoring of atmospheric sulfur dioxide

The sensitive and on-site detection of sulfur dioxide (SO₂) is in great demand in the fields of food safety and environmental protection. Here, we developed a novel upconversion nanoprobe based on the luminescence energy transfer mechanism for monitoring the atmospheric SO₂ concentrations. The lanthanide emitters, Tm³⁺ ions, were optimized to be doped on the surface layer of the upconversion nanoparticles to improve their energy transfer efficiency by minimizing the distance between the emitters and the surface quencher, a cyanine dye. As a proof-of-concept, the optimal nanoprobe was utilized to detect SO₂ water derivatives, bisulfite ions, exhibiting a linear luminescence increase in the range of 1 nM to 10 nM. Furthermore, we assembled the cyanine-modified upconversion nanoparticles onto a test paper, and used a smartphone-based detection platform to achieve portable and visual detection of SO₂. The test paper showed a strong luminescence stability, homogeneity and good anti-interference. The limit of detection for SO₂ gas was found to be 1 ng L^-1. This novel upconversion test paper was also demonstrated to directly monitor the concentration of SO₂ gas in atmosphere.