Eriochrome Black T–Eu3+ Complex as a Ratiometric Colorimetric and Fluorescent Probe for the Detection of Dipicolinic Acid, a Biomarker of Bacterial Spores

A novel colorimetric and fluorometric dual-signal identification of crude baijiu based on La-CDs

The correct classification of strong-flavored crude baijiu affects its quality and overall standard and is crucial for the intelligent development of the baijiu industry. In this work, we developed a novel optical sensing array using lanthanum-doped carbon dots (La-CDs). Using La-CDs with three metal complex dyes-chromium black T, alizarin red, and dimethylphenol orange-we were able to detect organic acids and tannic acid (TA) in crude baijiu in a way that was both colorimetric and fluorescent for the first time. Based on the indicator displacement (IDA) principle, organic acids competitively replace the dyes' binding sites on La3+, causing the dye colors to change to varying degrees. TA quenches the fluorescence of quantum dots through an internal filtering effect. We analyzed the data using pattern recognition algorithms such as HCA, PCA, and LDA, successfully classifying and identifying 16 types of strong-flavored crude baijiu, which included 10 types of carboxylic acids and various grades. In blind tests of 32 crude baijiu samples, the colorimetric method achieved a 94 % accuracy rate, while the fluorescence method achieved 100 %. The sensor demonstrates significant advantages in response speed.

Smartphone-assisted mobile fluorescence sensor for self-calibrated detection of anthrax biomarker, Cu2+, and cysteine in food analysis

Dipicolinic acid (DPA), as a biomarker for Bacillus anthracis, is highly toxic at trace levels. Rapid and on-site quantitative detection of DPA is essential for maintaining food safety and public health. This work develops a dual-channel self-calibrated fluorescence sensor constructed by the YVO4:Eu and Tb-β-diketone complex for rapid visual detection of DPA. This sensor exhibits high selectivity, fast response time, excellent detection sensitivity, and the detection limit is as low as 4.5 nM in the linear range of 0-16 μM. A smartphone APP and portable ultraviolet lamp can assemble a mobile fluorescence sensor for on-site analysis. Interestingly, adding Cu2+ ions can quench the fluorescence intensity of Tb3+. In contrast, the addition of cysteine can restore the fluorescence, allowing the accurate detection of Cu2+ ions and cysteine in environmental water and food samples. This work provides a portable sensor that facilitates real-time analysis of multiple targets in food and the environment.

Zn-CD@Eu Ratiometric Fluorescent Probe for the Detection of Dipicolinic Acid, Uric Acid, and Ex Vivo Uric Acid Imaging

Development of reliable methods for the detection of potential biomarkers is of the utmost importance for an early diagnosis of critical diseases and disorders. In this study, a novel lanthanide-functionalized carbon dot-based fluorescent probe Zn-CD@Eu is reported for the ratiometric detection of dipicolinic acid (DPA) and uric acid (UA). The Zn-CD@Eu nanoprobe was obtained from a simple room-temperature reaction of zinc-doped carbon dots (Zn-CD) and the EDTA-Eu lanthanide complex. Under optimal conditions, a good linear response was obtained for DPA in two concentration ranges of 0-55 and 55-100 μM with a limit of detection of 0.53 and 2.2 μM respectively, which is significantly below the infectious dosage of anthrax (∼55 μM). Furthermore, the Zn-CD@Eu/DPA system was employed for the detection of UA with a detection limit of 0.36 μM in the linear range of 0-100 μM. The fluorescent probe was successfully implemented for determining DPA and UA in human blood serum, sweat, and natural water bodies with considerable recovery rates. In addition, the potential of the nanoprobe for ex vivo visualization of UA was demonstrated in fruit fly (Drosophila melanogaster) as a model organism.

Dual-mode detection of 2,6-pyridinedicarboxylic acid based on the enhanced peroxidase-like activity and fluorescence property of novel Eu-MOFs

2,6-Pyridinedicarboxylic acid (DPA) is a significant biomarker of anthrax, which is a deadly infectious disease for human beings. However, the development of a convenient anthrax detection method is still a challenge. Herein, we report a novel europium metal-organic framework (Eu-MOF) with an enhanced peroxidase-like activity and fluorescence property for DPA detection. The Eu-MOF was one-step synthesized using Eu3+ ions and 2-methylimidazole. In the presence of DPA, the intrinsic fluorescence of Eu3+ ions is sensitized, the fluorescence intensity linearly increases with an increase in DPA concentration, and the fluorescence color changes from blue to purple. Simultaneously, the peroxide-like activity of the Eu-MOF is enhanced by DPA, which can promote the oxidation of TMB to oxTMB. The absorbance values increase linearly with DPA concentrations, and the colorimetric images change from colorless to blue. The dual-mode detection of DPA has good sensitivity with a colorimetric detection limit of 0.67 μM and a fluorescent detection limit of 16.67 nM. Moreover, a simple detection method for DPA was developed using a smartphone with the RGB analysis system. A portable kit with standard color cards was developed using paper test strips. The proposed methods have good practicability for DPA detection in real samples. In conclusion, the developed Eu-MOF biosensor offers a valuable and general platform for anthrax diagnosis.

Fabrication of a ratiometric fluorescence nanoprobe for detecting tryptophan enantiomers

A novel ratiometric fluorescence nanoprobe was developed to detect tryptophan enantiomers with high selectivity and sensitivity. Fluorescein (FL) was doped into SiO2 nanoparticles with little dye leakage, acting as an internal reference signal. The chiral center of L-histidine (L-His) was introduced by partially replacing the carboxyl on the surface of SiO2. The response signal of Eu3+ was coordinated with carboxyl to obtain the FL@SiO2-Eu/His nanoprobe. The nanoprobe shows enantioselective luminescence responses for tryptophan enantiomers. The red emission of Eu3+ can be effectively quenched by the tryptophan enantiomers, but the quenching efficiency is different due to the different interactions with the chiral recognition sites. The fluorescence intensity ratio (I613/I515) shows excellent linearity with tryptophan enantiomers in the range of 0-100 μM (R2 > 0.99). The limit of detection (LOD) is 1.0 μM and 1.3 μM for L-tryptophan (L-Trp) and D-tryptophan (D-Trp), respectively. The FL@SiO2-Eu/His nanoprobe shows good selectivity and specific recognition in serum actual samples. In addition, the nanoprobe can realize visual detection under UV light due to the obvious color change. This work provides an innovative idea for the development of new probes in the detection field.

Ratiometric fluorescent and electrochemiluminescent dual modal assay for detection of 2,6-pyridinedicarboxylic acid as an anthrax biomarker

2,6-pyridinedicarboxylic acid (DPA) is an excellent biomarker of Bacillus anthracis (B. anthracis). The sensitive detection of DPA, especially through visual point-of-care testing, was significant for accurate and rapid diagnosis of anthrax to timely prevent anthrax disease or biological terrorist attack. Herein, a ratiometric fluorescent (R-FL) and electrochemiluminescent (ECL) dual-mode detection platform with a lanthanide ion-based metal-organic framework (Ln-MOF, i.e., M/Y-X: M = Eu, Y = Tb, and X = 4,4',4″-s-triazine-1,3,5-triyltri-m-aminobenzoic acid) was developed. Eu/Tb-TATAB nanoparticles were constructed to identify DPA. The R-FL detection platform quantitatively detected DPA by monitoring the I545/I617 ratio of the characteristic fluorescence peak intensities of Tb3+ ions and Eu3+ ions. The ECL sensing platform successfully quantified DPA by exploiting the burst effect of DPA on the ECL signal. The above methods had highly sensitive and rapid detection of DPA in water and serum samples. The results showed that this dual-mode detection platform may be projected to be a powerful instrument for preventing related biological warfare and bio-terrorism.

Amorphous amEu-NH2BDC and amTb-NH2BDC as ratio fluorescence probes for smartphone-integrated naked eye detection of bacillus anthracis biomarker

The abnormal concentration of anthrax spore biomarker 2,6-pyridinedicarboxylic acid (2,6-DPA) will seriously affect public health. Therefore, a sensitive and rapid assay for 2,6-DPA monitoring is of vital importance. In this work, novel nano-sized amorphous Eu-NH2BDC (amEu-NH2BDC) and amorphous Tb-NH2BDC (amTb-NH2BDC) metal organic frameworks are prepared by adjusting the ratio of metal and ligand, respectively. Both of them exhibit highly sensitive and selective ratiometric fluorescence detection for 2,6-DPA with wider linear range and lower detection limit in aqueous solutions and human serum. Attributed to the coordination effect of 2,6-DPA in triggering the characteristic fluorescence emissions of Eu3+or Tb3+ by replacing coordinated solvent molecules, as evidenced by ultraviolet-visible spectroscopy, the fluorescence lifetimes analysis, thermal gravimetric analysis, Fourier-transform infrared spectroscopy, density functional theory (DFT) simulations and X-ray photoelectron spectroscopy. In addition, the amEu-NH2BDC or amTb-NH2BDC loaded paper-based microsensors are constructed for real-time and sensitive detection of 2,6-DPA and coupled with a smartphone-assisted visual portable device.

Self-calibrated HAp:Tb-EDTA paper-based probe with dual emission ratio fluorescence for binary visual and fluorescent detection of anthrax biomarker

Development of the portable device is significant for sensitive and rapid detection of an anthrax biomarker dipicolinic acid (DPA), existing in the B. anthracis. In this work, a novel HAp:Tb-EDTA paper-based ratiometric fluorescent sensor was obtained by a simple one-pot method for rapid and sensitive DPA detection. With the increased DPA concentration, the luminescence intensity of HAp (hydroxyapatite) remained constant, and thus applied as the stable reference signal, while the luminescence signal of Tb3+-EDTA was significantly enhanced due to the antenna effect. Therefore, the HAp:Tb-EDTA paper-based sensor was endowed with self-calibrated and ratiometric fluorescent detection performance for DPA. The proposed sensor showed excellent detection performance with a detection limit as low as 10.8 nM in the linear range of 0.5-30 μM. After combination with a smartphone, rapid visual and fluorescent detection of DPA was achieved. The proposed sensor was successfully applied to detect DPA from B. subtilis spore real samples, showing the application prospects of the paper-based sensors and opening a new horizon to develop novel paper-based point-of-care testing (POCT) devices.

Fluorescent silica nanoparticles as nano-chemosensors for the sequential detection of Pb2+ ions and bacterial-spore biomarker dipicolinic acid (DPA) in aqueous solution

Herein, we report fluorescein-labelled silica nanoparticles (FSNP) which serve as fluorescent nano-chemosensors for sequential detection of Pb2+ (which is a toxic heavy metal) and dipicolinic acid (DPA) (which is a distinctive indicator biomarker of bacterial spores) with high sensitivity and selectivity. The fluorescence of FSNP is quenched because of the complex formation between Pb2+ ions and surface amide groups, however, the fluorescence is recovered in contact with DPA, resulting from the association of DPA with surface bound Pb2+ ions. FSNP-Pb2+ complexes show high sensitivity towards DPA with a low detection limit of 850 nM which is approximately seventy times lower than the infectious dosage of bacterial spores (60 μM). Lateral flow test platform was further developed to show the applicability and practicability of our system.

Smartphone-integrated ratiometric fluorescence sensing platform based on bimetallic metal-organic framework nanowires for anthrax biomarker detection

Developing an intelligent, sensitive, and visual strategy for quickly identifying anthrax biomarkers is crucial for ensuring food safety and preventing disease outbreaks. Herein, a smartphone-integrated ratiometric fluorescent sensing platform based on bimetallic metal-organic framework (Eux/Tb1-x-MOF) nanowires was designed for specific recognition of pyridine-2,6-dicarboxylic acid (DPA, anthrax biomarker). The Eux/Tb1-x-MOF was prepared by coordinating Eu3+ and Tb3+ with BBDC ligands, which exhibited a uniform fibrous morphology and dual-emission fluorescence at 543 and 614 nm. After the introduction of DPA, the red emission at 614 nm displayed obvious fluorescence quenching, while the green emission at 543 nm was gradually enhanced. The ratiometric sensing offered a wide linear equation in the range of 0.06-15 µg/mL and a low detection limit (LOD) of 20.69 ng/mL. Furthermore, a portable smartphone installing the color recognition application can achieve sensitive, real-time, and visual detection of DPA. As a simple and effective smartphone-assisted sensing platform, this work holds admirable promise to broaden the applications in biomarker real-time determinations and other fields.

Designed Metal-Containing Peptoid Membranes as Enzyme Mimetics for Catalytic Organophosphate Degradation

The detoxification of lethal organophosphate (OP) residues in the environment is crucial to prevent human exposure and protect modern society. Despite serving as excellent catalysts for OP degradation, natural enzymes require costly preparation and readily deactivate upon exposure to environmental conditions. Herein, we designed and prepared a series of phosphotriesterase mimics based on stable, self-assembled peptoid membranes to overcome these limitations of the enzymes and effectively catalyze the hydrolysis of dimethyl p-nitrophenyl phosphate (DMNP)─a nerve agent simulant. By covalently attaching metal-binding ligands to peptoid N-termini, we attained enzyme mimetics in the form of surface-functionalized crystalline nanomembranes. These nanomembranes display a precisely controlled arrangement of coordinated metal ions, which resemble the active sites found in phosphotriesterases to promote DMNP hydrolysis. Moreover, using these highly programmable peptoid nanomembranes allows for tuning the local chemical environment of the coordinated metal ion to achieve enhanced hydrolysis activity. Among the crystalline membranes that are active for DMNP degradation, those assembled from peptoids containing bis-quinoline ligands with an adjacent phenyl side chain showed the highest hydrolytic activity with a 219-fold rate acceleration over the background, demonstrating the important role of the hydrophobic environment in proximity to the active sites. Furthermore, these membranes exhibited remarkable stability and were able to retain their catalytic activity after heating to 60 °C and after multiple uses. This work provides insights into the principal features to construct a new class of biomimetic materials with high catalytic efficiency, cost-effectiveness, and reusability applied in nerve agent detoxification.

Competitive substitution in europium metal-organic gel for signal-on electrochemiluminescence detection of dipicolinic acid

Metal-organic gels (MOGs) emerged as an attractive luminescent soft material for electrochemiluminescence (ECL). In this work, a cathodic ECL-activated europium metal-organic gel (Eu-MOG) has been synthesized by a facile mixing of Eu3+ with 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine (Hcptpy) under mild conditions. The prepared Eu-MOG is highly mesoporous for co-reactant permeation to produce an ultra-stable and high-efficient ECL, based on the antenna effect of Eu3+ coordinating with Hcptpy. Moreover, dipicolinic acid (DPA) can competitively coordinate with Eu3+ instead of water molecules, producing an enhanced ECL signal. Therefore, an ECL enhancement assay was developed for DPA detection. There was a linear relationship between the ECL intensity and the logarithmic concentration of DPA in the 0.01-1 μM range, and the detection limit is 7.35 nM. This work displays the promising application of Eu-MOG in the ECL field, opening a broad inspection for seeking a new generation of ECL luminophores.

A highly stable chain-based EuIII metal-organic framework as a turn-on and blue-shift luminescent sensor for dipicolinic acid

The development of a rapid and selective method for the identification of dipicolinic acid (DPA), a specific biomarker in Bacillus anthracis spores, is of great importance for the avoidance of anthrax infection. Herein, a chain-based Eu^III metal-organic framework with the formula {[Eu₃(BTDB)₃(μ₃-OH)₃(H₂O)]·solvents}_n (JXUST-38, H₂BTDB = (benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic acid) was obtained using 2-fluorobenzoic acid as the pH regulator. JXUST-38 exhibits good chemical and thermal stability and can specifically recognize DPA in N,N-dimethylformamide solution through luminescence enhancement and blue-shift effects with a detection limit of 0.05 μM. Furthermore, the significant luminescence enhancement and blue shift under UV lamps are obviously observable by the naked eye. The luminescence sensing mechanism is attributed to absorbance-induced enhancement between JXUST-38 and DPA. Test paper and mixed-matrix membrane based on JXUST-38 are designed for DPA detection. In addition, the feasibility of using JXUST-38 in biosensing is discussed in detail.

Bacillus spore germination: mechanisms, identification, and antibacterial strategies

Bacterial spores are metabolically inactive and highly resistant to harsh environmental conditions in nature and during decontamination processes in food and related industries. However, inducing germination using specific germinants in dormant spores can convert them into vegetative cells which are metabolically active and fragile. The potential utility of a "germinate to eradicate" strategy, also known as germination-inactivation, has been validated in foods. Meanwhile, the strategy has sparked much interest in triggering and maximizing spore germination. Although many details of the spore germination process have been identified over the past decades, there remain many uncertainties, including some signal transduction mechanisms involved in germination. In addition, the successful implementation of the germination-inactivation strategy relies on the sensitive detection of germinative biomarkers within minutes of germination initiation and the optimal timing for the subsequent inactivation step. Meanwhile, the emergence of biomarkers has renewed attention to the practical application of the spore germination process. Here, this review presents the current knowledge of the germination mechanisms of Bacillus spore, influencing factors, and germination biomarkers. It also covers a detailed discussion on the development of germination-inactivation as a spore eradication strategy.

Zirconium-Directed Supramolecular Self-Assembly of Coenzyme A@GNCs with Enhanced Phosphorescence for Developing Ultrasensitive Tracer Probe of Dipicolinic Acid, a Biomarker of Bacterial Spores

Luminescent gold nanoclusters (GNCs) are a class of attractive quantum-sized nanomaterials bridging the gap between organogold complexes and gold nanocrystals. They typically have a core-shell structure consisting of a Au(I)-organoligand shell-encapsulated few-atom Au(0) core. Their luminescent properties are greatly affected by their Au(I)-organoligand shell, which also supports the aggregation-induced emission (AIE) effect. However, so far, the luminescent Au nanoclusters encapsulated with the organoligands containing phosphoryl moiety have rarely been reported, not to mention their AIE. In this study, coenzyme A (CoA), an adenosine diphosphate (ADP) analogue that is composed of a bulky 5-phosphoribonucleotide adenosine moiety connected to a long branch of vitamin B5 (pantetheine) via a diphosphate ester linkage and ubiquitous in all living organisms, has been used to synthesize phosphorescent GNCs for the first time. Interestingly, the synthesized phosphorescent CoA@GNCs could be further induced to generate AIE via the PO32- and Zr4+ interactions, and the observed AIE was found to be highly specific to Zr4+ ions. In addition, the enhanced phosphorescent emission could be quickly turned down by dipicolinic acid (DPA), a universal and specific component and also a biomarker of bacterial spores. Therefore, a Zr4+-CoA@GNCs-based DPA biosensor for quick, facile, and highly sensitive detection of possible spore contamination has been developed, showing a linear concentration range from 0.5 to 20 μM with a limit of detection of 10 nM. This study has demonstrated a promising future for various organic molecules containing phosphoryl moiety for the preparation of AIE-active metal nanoclusters.

Fluorescence on and off sensing platform based on europium nanosheets for the detection of DPA and Cu2+ ions

With the development of society, the modern environment has put forward higher requirements for analysis and detection. This work proposes a new strategy for the construction of fluorescent sensors based on rare-earth nanosheets. Organic/inorganic composites were obtained by the intercalation of 4,4'-stilbene dicarboxylic acid (SDC) into layered europium hydroxide, and then the composites were exfoliated to form nanosheets. Taking advantage of the fluorescence emission characteristics of SDC and Eu³⁺, a ratiometric fluorescent nanoprobe was constructed, which realized the detection of dipicolinic acid (DPA) and Cu²⁺ in the same system. With the addition of DPA, the blue emission of SDC gradually decreased and the red emission of Eu³⁺ gradually increased, when Cu²⁺ was added, the emission of SDC and Eu³⁺ were gradually weakened. The experimental results showed that the ratio of fluorescence emission intensity (I₆₁₉/I₃₉₄) of the probe had a positive linear relationship with the concentration of DPA, and a negative linear relationship with the concentration of Cu²⁺, thus realizing the high sensitivity detection of DPA and a wide detection range of Cu²⁺. In addition, this sensor also exhibits potential visual detection possibilities. This is a multifunctional fluorescent probe that provides a novel and efficient method for the detection of DPA and Cu²⁺, which broadens the application field of rare-earth nanosheets.

Xylenol orange-modified CdTe quantum dots as a fluorescent/colorimetric dual-modal probe for anthrax biomarker based on competitive coordination

Bacillus anthracis spores can make humans infected with vicious anthrax, so it is significant to detect their biomarker 2,6-pyridinedicarboxylic acid (DPA). The development of dual-modal methods for DPA detection that are more flexible in practical applications remains a challenge. Herein, colorimetric xylenol orange (XO) was modified on fluorescent CdTe quantum dots (QDs) for dual-modal detection of DPA through competitive coordination. After the binding of XO on CdTe QDs via coordination with Cd²⁺, CdTe QDs displayed quenched red fluorescence and the bound XO was presented as red color. The competitive coordination of DPA with Cd²⁺ made XO released from CdTe QDs, causing the enhanced red fluorescence of CdTe QDs and the yellow color of free XO. On this basis, DPA was rapidly (1 min) quantified through fluorescent and colorimetric modes within the ranges of 0.1-5 μM and 0.5-40 μM, respectively. The detection limits for DPA were calculated as low as 42 nM and 240 nM, respectively assigned to fluorescent and colorimetric modes. The level of urinary DPA was further measured. Satisfactory relative standard deviations (fluorescent mode: 0.1%-10.2%, colorimetric mode: 0.8%-1.8%) and spiked recoveries (fluorescent mode: 100.0%-115.0%, colorimetric mode: 86.0%-96.6%) were obtained.

Tb3+-Based Off-On Fluorescent Platform for Multicolor and Dosage-Sensitive Visualization of Bacterial Spore Marker

Developing a novel strategy for the sensitive and rapid detection of pathogenic bacterial spores in field or on-site settings will be helpful in minimizing their potential threats to human health, environmental safety, and food safety. In this study, Tb³⁺ was combined with glutathione (GSH)-modified copper nanoclusters (CuNCs), and an aggregation-induced emission (AIE) fluorescent probe based on Tb-GSH-CuNCs was fabricated for dipicolinic acid (DPA, a pathogenic bacterial spore marker) sensing. Making use of the competitive binding of Tb³⁺ between GSH-CuNCs and DPA, a multicolor sensing of DPA was facilely realized without introducing fluorescent materials as the reference. Due to an "off-on" response mechanism of the AIE fluorescent probe, this multicolor response to DPA exhibited a feature of rich color gradients and highly discriminative color change, allowing a dosage-sensitive visual quantification of DPA. The DPA with a concentration even as low as 0.5 μM can still be identified by the naked eye. Moreover, together with a smartphone app, which can extract the R (red), G (green), and B (blue) values from the probe system, a portable platform can be established for sensitive DPA quantification in the range of 0.5-70 μM, showing great potential for the practical monitoring of DPA in field or on-site settings.

Dual-mode strategy for 2,6-dipicolinic acid detection based on the fluorescence property and peroxidase-like activity inhibition of Fe-MIL-88NH2

This work designed a fluorometric/colorimetric dual-mode sensor for detecting 2,6-dipicolinic acid (DPA) based on the blue emission property and peroxidase-like activity of Fe-MIL-88NH₂. The fluorescence of Fe-MIL-88NH₂ was obviously turned off by Cu²⁺, but DPA was able to bring it back because it has a strong chelate bond with Cu²⁺. Fe-MIL-88NH₂ also displayed high peroxidase-like activity, which accelerated the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to the blue oxidation product (oxTMB) when H₂O₂ was present. When DPA was added, it efficiently inhibited the peroxidase-like activity of Fe-MIL-88NH₂, causing less oxTMB and less absorbance at 652 nm. The fluorescence recovery of Fe-MIL-88NH₂ and the change in absorbance at 652 nm were used as analytical signals for dual-mode detection of DPA. The linear responses in the range of 10-60 μM and 60-160 μM were achieved for the fluorometric mode, and the limit of detection (LOD) was 1.46 μM. The respective values of linear range and LOD for the colorimetric mode were 5-25 μM and 3.00 μM, respectively. In summary, the dual-mode testing strategy successfully detected DPA in aqueous environmental samples, suggesting great potential in disease prevention and environmental analysis.

A self-designed device integrated with a Fermat spiral microfluidic chip for ratiometric and automated point-of-care testing of anthrax biomarker in real samples

A desirable lanthanide-based ratiometric fluorescent probe was designed and integrated into a self-designed Fermat spiral microfluidic chip (FS-MC) for the automated determination of a unique bacterial endospore biomarker, dipicolinic acid (DPA), with high selectivity and sensitivity. Here, a blue emission wavelength at 425 nm was generated in the Fermat spiral structure by mixing the europium (Eu³⁺) and luminol to form the Eu³⁺/Luminol sensing probe. DPA in the reservoir can be used to specifically bind to Eu³⁺ under the negative pressure and transfer energy from DPA to Eu³⁺ sequentially via an antenna effect, thus resulting in a significant increase in the red fluorescence emission peak at 615 nm. According to the fluorescence intensity ratio (F615/F425), a good linearity can be obtained with increasing the concentration of DPA from 0 to 200 μM with a limit of detection as low as 10.11 nM. Interestingly, the designed FS-MC can achieve rapid detection of DPA in only 1 min, reducing detection time and improving sensitivity. Furthermore, a self-designed device integrated with the FS-MC and a smartphone color picker APP was employed for the rapid automatic point-of-care testing (POCT) of DPA in the field, simplifying complex processes and reducing testing times, thus confirming the great promise of this ready-to-use measurement platform for in situ inspection.

Detection of anthrax biomarker and metallic ions in aqueous media using spherical-shaped lanthanide infinite coordination polymers

We report a lanthanide-based infinite coordination polymer (ICP) system synthesized using pyrazole-3,5-dicarboxylic acid as linker, malonic acid as coordination modulator and water as solvent. The precursors self-assembly into microspherical particles, which are water-stable and exhibit excellent dispersibility. Bimetallic samples based on Tb³⁺ doped with Eu³⁺ were investigated as ratiometric dipicolinic acid (DPA) sensors, which is a biomarker for Bacillus anthracis spores. Along with the calibration curves, a detection in a real sample extracted from Bacillus subtilis (model organism) was performed. The samples proved to be highly sensitive and selective for ratiometric DPA detection. In a secondary study, the monometallic sample containing only Tb³⁺ was also investigated as a sensor for ionic species in aqueous media. The Cr³⁺, Fe³⁺, Cu²⁺, and Cr₂O₇^2- ionic species could be detected in water by luminescence quenching mechanism. Therefore, we found that the reported ICP system can be judiciously constructed in order to act as a multimodal probe for several chemical species.

Bifunctional ratiometric fluorescent probe for sensing anthrax spore biomarker and tetracycline at different excitation channels

Multifunctional fluorescent probes have received increasing attention for the sake of atom economy and high-density integration. Herein, CdTe quantum dots (QDs) modified with Eu³⁺ were synthesized as the bifunctional ratiometric fluorescent probe for sensing two hazardous substances tetracycline (TC) and anthrax spore biomarker 2,6-dipicolinic acid (DPA) at different excitation channels, based on the discrepant excitation wavelengths of Eu³⁺ and the fluorescence quenching of CdTe QDs after interaction with them. Both DPA and TC enhanced the red emission of Eu³⁺ via antenna effect but caused the green emission of CdTe QDs to quench. Interestingly, the excitation wavelengths of Eu³⁺ after coordinating with DPA and TC were 275 and 386 nm, respectively. On this basis, CdTe QDs-Eu³⁺ achieved the bifunctional ratiometric detection of DPA (λ_ex = 275 nm) and TC (λ_ex = 386 nm) with different excitation channels. Both DPA and TC were selectively detected by CdTe QDs-Eu³⁺ with rapid response (DPA-1 min, TC-1 min) and high sensitivity (DPA-LOD 0.3 μM, TC-LOD 2.2 nM). CdTe QDs-Eu³⁺ were applied to analyzing DPA and TC in food, biological and environmental samples. Satisfactory spiked recoveries (80.0-119.0 %) and relative standard deviations (0.5-8.4 %) were obtained for measuring DPA and TC in these samples.

A Smartphone Integrated Platform for Ratiometric Fluorescent Sensitive and Selective Determination of Dipicolinic Acid

A desirable lanthanide-based ratiometric fluorescence probe was designed as a multifunctional nanoplatform for the determination of dipicolinic acid (DPA), a unique bacterial endospore biomarker, with high selectivity and sensitivity. The carbon dots (CDs) with blue emission wavelengths at 470 nm are developed with europium ion (Eu³⁺) to form Eu³⁺/CDs fluorescent probes. DPA can specifically combine with Eu³⁺ and then transfer energy from DPA to Eu³⁺ sequentially through the antenna effect, resulting in a distinct increase in the red fluorescence emission peak at 615 nm. The fluorescence intensity ratio of Eu³⁺/CDs (fluorescence intensity at 615 nm/fluorescence intensity at 470 nm) showed good linearity and low detection limit. The developed ratiometric nanoplatform possesses great potential for application in complex matrices owing to its specificity for DPA. In addition, the integration of a smartphone with the Color Picker APP installed enabled point-of-care testing (POCT) with quantitative measurement capabilities, confirming the great potential of the as-prepared measurement platform for on-site testing.

An Emerging Fluorescent Carbon Nanobead Label Probe for Lateral Flow Assays and Highly Sensitive Screening of Foodborne Toxins and Pathogenic Bacteria

By virtue of the fascinating merits of low cost, rapid screening, and on-site detection, fluorescence lateral flow assays (FLFAs) have attracted considerable attention. Their detection limits are closely associated with the label probes used. The development of high-performance and robust phosphors remains a great challenge. Herein, we presented a new label probe, i.e., fluorescent carbon nanobeads (FCNBs), for FLFAs. Monodispersive, water-soluble, and highly emissive FCNBs were facilely prepared via a hydrothermal carbonization manner. Their abundant amino groups were beneficial for versatile surface functionalization. After being modified by biomolecules, the fabricated FCNB reporter probes were adopted for the construction of lateral flow test strips toward representative foodborne toxins, i.e., aflatoxin B1 (AFB1), and pathogenic bacteria, i.e., Staphylococcus aureus (S. aureus), respectively. The detection limits (0.01 ng/mL for AFB1 and 10² cfu/mL for S. aureus) were about 1 or 2 orders of magnitude lower than most reported methods. Furthermore, the proposed test strips were successfully applied for the quantitative, accurate, and rapid screening of AFB1 and S. aureus in food samples. This work provided a promising label probe for FLFAs and would open the opportunity to exploit a sensing platform by modifying different ligands onto the FCNBs.

Rapid and Reliable Excitation Wavelength-Dependent Detection of 2,6-Dipicolinic Acid Based on a Luminescent Cd(II)-Tb(III) Nanocluster

A Cd(II)-Tb(III) nanocluster {[Cd₁₀Tb₉L₈(OH)₁₆(OAc)₂₃(H₂O)₃][Cd₁₀Tb₉L₈(OH)₁₆(OAc)₂₃(H₂O)₄]}·3H₂O (1), which contains two crystallographically independent components, was constructed from a tridentate ligand (HL, 3-ethoxysalicylaldehyde). It exhibits rapid and reliable excitation wavelength-dependent luminescence response to 2,6-dipicolinic acid (DPA) [limit of detection = 0.23 nM], which is not influenced by aromatic carboxylates, amino acids, and ions. The test papers of 1 can be used to check DPA in solution. The equation I_Ex272nm/I_Ex329nm = 0.0109 × [DPA]2 + 0.106 × [DPA] + 2.39 of 1 for the luminescence response could be used to quantitatively measure the concentration of DPA in tap water. 1 displays rapid and stable luminescence response to DPA, with the sensing times shorter than 5 s and no changes for the lanthanide luminescence over 24 h.

A luminescent terbium(iii) probe as an efficient ‘Turn-ON’ sensor for dipicolinic acid, a Bacillus Anthracis biomarker

This work drives the potential of lanthanide luminescence in the quantification and detection of the B. Anthracis bacterial spore by targeting dipicolinic acid (DPA), a principal component of anthrax spores.

A time-resolved ratiometric luminescent anthrax biomarker nanosensor based on Ir(III) complex-doped coordination polymer network

Herein, an Ir(III) complex-doped coordination polymer networks (Ir(III)@GMP-Eu3+) is firstly fabricated for the ratiometric luminescent detection of anthrax biomarker 2,6-dipicolinic acid (DPA) through time-resolved emission spectra (TRES) measurement. The detection...

Construction of a Cd8Tb4 nanoring for luminescence response to 2,6-dipicolinic acid as an anthrax biomarker

One 12-metal Cd(ii)–Tb(iii) nanoring (1.2 × 2.8 × 2.8 nm) was constructed from a flexible Schiff base ligand, and it shows luminescent response to 2,6-dipicolinic acid with high sensitivity and selectivity.

Smart intercalation and coordination strategy to construct stable ratiometric fluorescence nanoprobes for the detection of anthrax biomarker

L@Mg-Al-Ln-LDHs (Ln = Tb, Eu) constructed by the intercalation coordination strategy exhibited a strong and stable fluorescence reference signal and achieved reliable ratiometric detection of DPA in complex environments and actual spores.

Three novel metal-organic frameworks with different coordination modes for trace detection of anthrax biomarkers

Dipicolinic acid (DPA) is an anthrax biomarker. Its serious consequences make its detection a great need. In this paper, three novel metal-organic frameworks (MOFs) with different coordination modes were synthesized by a simple solvothermal method, which can be used as highly efficient fluorescence sensors for the highly selective and sensitive trace detection of DPA. MOFs 1-3 showed rapid responses to DPA (<30 s), and the limits of detection (LODs) were calculated to be 1.01 × 10^-6 M^-1 (MOF 1), 1.17 × 10^-6 M^-1 (MOF 2) and 2.07 × 10^-6 M^-1 (MOF 3). DPA detection based on MOFs 1-3 in fetal bovine serum is highly reliable based on the high recovery rates (90% to 115%). Hence, the three MOF-based sensors can be used in the real-time detection of DPA.

Ratiometric fluorescent detection of dipicolinic acid as an anthrax biomarker based on a high-nuclearity Yb18 nanoring

An 18-metal lanthanide nanoring [Yb₁₈(L¹)₈(HL²)₂(OAc)₂₀(MeOH)₈(EtOH)₆(H₂O)₄] (1), which shows a ratiometric fluorescent response to DPA, was constructed through the strategy of using two types of polydentate organic ligands. The addition of DPA increases the visible ligand-centered emission, but decreases the NIR lanthanide luminescence of 1. The limit of luminescent detection of 1 for DPA is 1.5 μM. The high fluorescence sensitivity of 1 to DPA is not affected by the existence of interferents such as aromatic carboxylates and ions.

One high-nuclearity Eu18 nanoring with rapid ratiometric fluorescence response to dipicolinic acid (an anthrax biomarker)

One 18-metal Eu(iii) nanoring (size: 1.0 × 2.7 × 2.7 nm) was constructed as a rapid ratiometric fluorescent probe for the detection of dipicolinic acid with high sensitivity and selectivity, by using two types of polydentate organic ligands.

A structure-dependent ratiometric fluorescence sensor based on metal-organic framework for detection of 2,6-pyridinedicarboxylic acid

In the present work, a structure-dependent ratiometric fluorescence (RF) sensor constructed with boric acid-modified carbon quantum dots (B-CQDs) and Tb-MOF(MOF-76) was developed for sensing 2,6-pyridinedicarboxylic acid (DPA). Based on the distinct fluorescent responses of B-CQDs and MOF-76 to DPA, MOF-76/B-CQDs can be developed as a RF sensor for DPA detection. In this RF sensor, the reticulated cross-linked structure of MOF-76/B-CQDs can be destroyed by DPA due to a strong coordination effect between DPA and the Tb of MOF-76, resulting in the quenching of the fluorescence of B-CQD and the restoration of the fluorescence of MOF-76 after the addition of DPA. Benefiting from the confinement effect of the special structure change, the presented sensor showed high sensitivity toward DPA with a detection limit of 3.05 μM and excellent selectivity over the monochromatic fluorescence sensor.

Rare-Earth Eu3+/Gold Nanocluster Ensemble-Based Fluorescent Photoinduced Electron Transfer Sensor for Biomarker Dipicolinic Acid Detection

The use of metal ions to bridge the fluorescent materials to target analytes has been demonstrated to be a promising way to sensor design. Herein, the effect of rare-earth ions on the fluorescence of l-methionine-stabilized gold nanoclusters (Met-AuNCs) was investigated. It was found that europium (Eu³⁺) can significantly suppress the emission of Met-AuNCs, while other rare-earth ions showed a negligible impact. The mechanism on the observed fluorescence quenching of Met-AuNCs triggered by Eu³⁺ was systematically explored, with results revealing the dominant role of photoinduced electron transfer (PET). Eu³⁺ can bind to the surface of Met-AuNCs by the coordination effect and accepts the electron from the excited Met-AuNCs, which results in Met-AuNC fluorescence suppression. After introducing dipicolinic acid (DPA), an excellent biomarker for spore-forming pathogens, Eu³⁺ was removed from the surface of Met-AuNCs owing to the higher binding affinity between Eu³⁺ and DPA. Consequently, an immediate fluorescence recovery occurred when DPA was present in the system. Based on the Met-AuNC/Eu³⁺ ensemble, we then established a simple and sensitive fluorescence strategy for turn-on determination of biomarker DPA, with a linear range of 0.2-4 μM and a low limit of detection of 110 nM. The feasibility of the proposed method was further validated by the quantitative detection of DPA in the soil samples. We believe that this study would significantly facilitate the construction of metal-ion-mediated PET sensors for the measurement of various interested analytes by applying fluorescent AuNCs as detection probes.

A smartphone-integrated multicolor fluorescence probe of bacterial spore biomarker: The combination of natural clay material and metal-organic frameworks

The metal-organic frameworks (MOFs) functionalized palygorskite (Pal) hybrid as a novel multicolor fluorescence probe for the detection of bacterial spore biomarker-dipicolinic acid (DPA), had been prepared via in-situ growth. The MOFs can effectively encapsulate dye molecules on the surface of Pal, and the rich carboxyl groups on its surface can coordinate with europium ions (Eu³⁺), forming a highly sensitive recognition group. The results indicated that the limit of detection (LOD) of this multicolor fluorescence probe was as low as 9.3 nM and was obviously lower than the amount of anthrax spores infecting the human body (60 μM). Moreover, a wide linear range from 0 to 35 μM was obtained. The high specific surface area of Pal, as well as the permanent porosity and suitable binding sites of Eu³⁺-doped MOFs may play a major role in the sensitivity and linear detection range. The multicolor fluorescence strategy made full use of the diversity of fluorescence signals collected by dye molecules and lanthanide ions, which can realize the real-time and on-site detection through the smartphone with a color-scanning application (APP). The practicability of this probe was further verified by detecting DPA released by non-infectious Bacillus subtilis.

Graphene Oxide-Supported Lanthanide Metal-Organic Frameworks with Boosted Stabilities and Detection Sensitivities

The photoluminescent (PL) properties of lanthanide metal-organic frameworks (Ln-MOFs) are intrinsically subtle to water molecules, which remains the major challenge that severely limits their applications as fluorescent probes in aqueous samples. Herein novel composite fluorescent probes were prepared by growing Ln-MOFs (Tb-MOF, Eu-MOF, and Tb/Eu-MOF) on carboxylated porous graphene oxide (PGO-COOH). The 3D thorny composites presented significantly longer fluorescent lifetimes and higher quantum yields than that of the bare Ln-MOFs and exhibited long-term PL stabilities in aqueous samples up to 15 days. The stable and improved PL properties demonstrated that the highly hybrid composite structures protected the MOF components from the adverse effects of water. Furthermore, the unexpected antenna effect of the PGO-COOH substrate on Ln³⁺ was supposed to be another reason for the improved PL properties. The composites present ultralow detection limits as low as 5.6 nM for 2,4-dinitrotoluene and 2.3 nM for dipicolinic acid as turn-off and ratiometric fluorescent probes, respectively, which was attributed to the incoporation of PGO-COOH that dramatically enahnced inner filter effects and effectively protected the energy transfer process in the MOF components from the interference of the surrounding water. This work presents an effective strategy for creating ultrasensitive and stable fluorescent probes based on Ln-MOFs for applications in aqueous samples.

Tb(iii)-doped nanosheets as a fluorescent probe for the detection of dipicolinic acid

A new fluorescent probe based on terbium(iii)-doped nanosheets was designed for detecting low-levels of dipicolinic acid (DPA), a biomarker of bacterial spores. The ability to detect ultra-low concentrations of DPA is therefore of great significance. First, Tb(iii)-doped ytterbium hydroxide nanosheets were obtained by mechanical exfoliation from layered rare-earth hydroxide (LRH) materials. The morphology of the as-synthesized nanosheets was studied by transmission electron microscopy and atomic force microscopy. The Tb(iii)-doped nanosheets are demonstrated to be highly sensitive to DPA, which remarkably enhances Tb(iii) luminescence intensities at a wavelength of 544 nm. Furthermore, Tb(iii) emission increases linearly with DPA concentration. Selectivity studies were conducted by adding different competing aromatic ligands to the sensing solution; however, their fluorescence responses were observed to be negligibly small in comparison with that of DPA. Our findings provide a basis for the application of Tb(iii)-doped nanosheets for accurate, sensitive, and selective monitoring of DPA as a biomarker of anthrax.