Probing phosphate ion via the europium(III)-modulated fluorescence of gold nanoclusters

Modulating the Photoluminescence of Europium through Crystalline Assembly Formation with Gold Nanoclusters and Then Phosphate Ions

We report delayed fluorescence enhancement of europium (Eu3+) ions through complexation with ligand-stabilized gold nanoclusters (Au NCs). The different Eu3+-centric emissions following complexation with Au NCs exhibited selective augmentation in the spectral lines attributed to the 5D0 → 7FJ transitions. The photoluminescence (PL) properties, including delayed Eu emission, from each component could be modulated through further functionalization of phosphate ions (Pi), leading to crystallization. The assembled crystalline structure of europium-containing Au NCs (Eu Au NCs) was corroborated by selected area electron diffraction analyses and high-resolution transmission electron microscopy analyses. On the basis of PL measurements and other experimental evidence, the two different lifetimes arising from the components, prompt emission of Au NCs and delayed emission of Eu3+, were affected in the assembled nanostructure. Such a design offers the possibility of developing an optical system by conjugating molecular NCs and atomic luminescent probes that has potential uses.

A Ratiometric Probe Based on Carbon Dots and Calcein & Eu3+ for the Fluorescent Detection of Sodium Tripolyphosphate

Sodium tripolyphosphate, a food additive, is applied broadly in food industry. However, excessive accumulation of sodium tripolyphosphate can result in electrolyte abnormality of the body. Therefore, it is of great importance to investigate an effective method for the detection of sodium tripolyphosphate. In this work, nitrogen-doped carbon dots (NCDs) with constant fluorescence were fabricated using a domestic microwave oven. A ratiometric fluorescent probe was constructed in which NCDs were as internal standard, calcein & Eu³⁺ were as the detection signal. The fluorescence of calcein at 515 nm was quenched by Eu³⁺, whereas the emission peak of NCDs at 446 nm was almost unchanged. Additionally, the fluorescence of calcein was recovered because of the strong interaction of sodium tripolyphosphate and Eu³⁺. The linear range for sodium tripolyphosphate was 0.5-6 µmol/L with detection limit of 0.12 µmol/L. Furthermore, the ratiometric fluorescent probe was applied for sodium tripolyphosphate detection in real milk samples.

Imine-Linked Covalent Organic Framework with a Naphthalene Moiety as a Sensitive Phosphate Ion Sensing

Due to the excellent ion-sensing potential of covalent organic frameworks (COFs), the new imine-linked conjugated COF (IC-COF) is synthesized through a water-based synthesis reaction between 1,5-diaminonaphthalene and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine to create a luminescence sensor. It is noteworthy that the green synthesized IC-COF shows excellent selectivity to phosphate ions (PO₄^3-) with a detection limit of 0.61 μM. The recyclability performance of IC-COF is high, indicating that it can be reused without a significant reduction in performance (5.2% decline after 5 cycles). Theoretical calculations using the density functional theory are performed on the IC-COF-PO₄^3- and IC-COF-Cu⁺ complexes to explore the sensing mechanism. The fluorescence quenching in the presence of PO₄^3- ions is attributed to the difference between PO₄^3- binding sites to the IC-COF compared to Cu⁺, which leads to the considerable change in the IC-COF absorption spectrum from 400 to 600 nm.

Ratiometric fluorescence sensor for sensitive detection of inorganic phosphate in environmental samples

Fast, simple, and low-cost on-site visualized detection of inorganic phosphate (Pi) is in great demand since phosphate is the major reason of eutrophication. In this work, a ratiometric fluorescent probe composed by green carbon dots (GCDs) and red carbon dots (RCDs) has been established for high-sensitivity and selective sensing of Pi. A trend of color change from red to green is observed for the detection of Pi under ultraviolet light and the detection limit is 0.09 μM in the range of 0 to 55 μM. Fluorescent test paper prepared from the probe solution was successfully applied to semi-quantitative visual detection of Pi in real-world water and soil samples, which shows great real-world application potentials.

A simple paper-based ratiometric luminescent sensor for tetracyclines using copper nanocluster-europium hybrid nanoprobes

Tetracyclines (TCs), as one of the broad-spectrum antibiotics, are widely used to treat bacterial infections. The residues of TCs in animal-origin foods and drinking water have raised safety concerns and affected the public health. Thus, there is a high demand to develop a simple and rapid method for the detection of TCs. In this work, we developed a ratiometric luminescence probe for the sensitive and visualized detection of TCs. Specifically, tannic acid-stabilized copper nanoclusters (TA-CuNCs) with blue emission at 433 nm were synthesized. The luminescence of TA-CuNCs attenuated partially by the europium ions (Eu³⁺) due to the aggregation-induced quenching. When TCs were added to the TA-CuNCs-Eu³⁺ system, the luminescence of TA-CuNCs at 433 nm can be further quenched by the inner-filter effect, and the characteristic luminescence of Eu³⁺ at 617 nm emerged due to the formation of Eu³⁺-TCs complex. The ratio of the luminescence at 617 nm-433 nm increased linearly to the concentration of TCs. Additionally, we demonstrated the detection of oxytetracycline in real samples such as tap and lake water, milk, pharmaceutical industry wastewater, honey and soil extract with high recovery rate (97.25%-103.44%). Furthermore, a portable paper device is fabricated by the luminescent probe to conduct the on-site analysis of TCs.

Polydopamine coated copper nanoclusters with aggregation-induced emission for fluorometric determination of phosphate ion and acid phosphatase activity

The preparation of aggregation-induced emission-type copper nanoclusters (CuNCs) capped with polydopamine (PDA) is described. PDA was formed via in situ polymerization of dopamine in the presence of alkaline polyethylenimine. The PDA-capped CuNCs (PDA-CuNCs) exhibit orange fluorescence with maximal emission at 580 nm upon excitation at 340 nm, a storage stability of at least 2 weeks, and a quantum yield (QY) of 2.54% in aqueous solution. The QY is 28-fold higher than that of sole CuNCs. The fluorescence of the PDA-CuNCs is quenched by Fe³⁺ ion while it is recovered by PO₄^3- due to its stronger affinity for Fe³⁺. On this basis, a fluorometric phosphate assay was developed that has a 1.5 nM detection limit and a linear range over 0.003-70 μM. The method was satisfactorily applied to the determination of phosphate in local tap water and human sera, and the results agreed well with those obtained by a colorimetric method. In the presence of acid phosphatase (ACP), PO₄^3- is produced by the catalytic hydrolysis of adenosine triphosphate (ACP substrate). Thus, a fluorogenic assay for screening ACP activity was established. Response is linear over the activity range 0.0012-25 U L^-1, with a detection limit of 0.001 U L^-1 (at S/N = 3). Graphic abstract We proposed an effective polydopamine-templating strategy for the in situ synthesis of highly emissive and stable CuNCs and demonstrated its use as an ion-driven fluorescence switch for the determination of phosphate and acid phosphatase activity.

Europium-Pyridinedicarboxylate-Adenine Light-Up Fluorescence Nanoprobes for Selective Detection of Phosphate in Biological Fluids

Phosphate (Pi) plays important roles in various physiological processes. Its quantification in biological fluids is highly crucial for timely warning of Pi accumulation. Herein, an europium (Eu)-based coordination polymer nanoprobe (Eu/DPA/Ade) is prepared by coordinating 2,6-pyridinedicarboxylic acid (2,6-DPA) and adenine (Ade) with Eu³⁺. Eu/DPA/Ade exhibits light-up fluorescence response to Pi. The strong coordinating interaction between Eu³⁺ and O atoms in the Pi group not only shortens the Eu³⁺-ligand distance to improve the energy transfer from 2,6-DPA to Eu³⁺ but also attenuates the fluorescence quenching from water molecules in the coordinating sphere of Eu³⁺. Eu/DPA/Ade produces red emission at λ_em 618 nm via the "antenna effect". The coligand Ade further promotes the fluorescent emission. The selective recognition of Pi within 10-60 μM is achieved with a detection limit of 4.65 μM. In addition, a certain level of Pi (100-170 μM) causes an exponential increment on the fluorescence of Eu/DPA/Ade and makes it feasible for visual estimation of Pi under irradiation by an ultraviolet lamp at 254 nm. The quantitative detection and visual estimation of Pi in human urine and saliva have been demonstrated.

Colorimetric Differentiation of Multiple Oxidizing Anions Based on Two Core-Shell Au@Ag Nanoparticles with Different Morphologies as Array Recognition Elements

The efficient discrimination of oxidizing anions is of considerable importance in environmental monitoring. Here, for the first time, we have developed a simple and fast colorimetric sensor array for detection and identification of oxidizing anions, which takes advantage of the etching of the Ag shell of two core-shell Au@Ag nanoparticles (Au@Ag nanospheres (Au@Ag NPs) and Au@Ag nanocubes (Au@Ag NCs)) by oxidizing anions. The differential etching ability of various oxidizing anions to the Ag shell of the two Au@Ag nanoparticles resulted in different absorbance and color change of the nanoparticles. Thus, employing Au@Ag NPs and Au@Ag NCs as the array's receptors and the indicators, six oxidizing anions (i.e., BrO₃^-, Cr₂O₇^2-, ClO₄^-, IO₃^-, IO₄^-, and MnO₄^-) down to 10 nM could be identified from each other by their own colorimetric response patterns. Moreover, the complex mixtures of oxidizing anions could be well discriminated. Most importantly, the sensor array was successfully applied to the discrimination of oxidizing anions in river water and tap water samples.

Porphin-Based Carbon Dots for "Turn Off-On" Phosphate Sensing and Cell Imaging

Porphin-based carbon dots (denoted as PCDs) are prepared through a one-step hydrothermal method by using meso-tetra (4-carboxyphenyl) porphin (TCPP), citric acid, and ethanediamine as precursor. PCDs give rise to the optimal photoluminescence at λex/λem = 375/645 nm, exhibit an excitation-independent property, excellent water solubility, and good biocompatibility, which provide red emission and avoid the autofluorescence as an efficient fluorescent imaging probe. On the other hand, when Eu3+ is added into PCDs, the carboxylate groups located on the surface of PCDs exhibit high affinity to Eu3+, resulting in the fluorescence of PCDs turning off via static quenching. In the presence of phosphate, owing to the strong coordination with Eu3+, the fluorescence of PCDs turns on. Based on this performance, a novel "turn off-on" phosphate sensing system is developed. The detection limit of this sensing system can attain 3.59 × 10-3 μmol L-1. This system has been utilized for the detection of phosphate in real samples successfully, which further demonstrates potential applications in biological diagnostic and environmental analysis.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

An overview of nanoparticles commonly used in fluorescent bioimaging

This article gives an overview of the various kinds of nanoparticles (NPs) that are widely used for purposes of fluorescent imaging, mainly of cells and tissues. Following an introduction and a discussion of merits of fluorescent NPs compared to molecular fluorophores, labels and probes, the article assesses the kinds and specific features of nanomaterials often used in bioimaging. These include fluorescently doped silicas and sol-gels, hydrophilic polymers (hydrogels), hydrophobic organic polymers, semiconducting polymer dots, quantum dots, carbon dots, other carbonaceous nanomaterials, upconversion NPs, noble metal NPs (mainly gold and silver), various other nanomaterials, and dendrimers. Another section covers coatings and methods for surface modification of NPs. Specific examples on the use of nanoparticles in (a) plain fluorescence imaging of cells, (b) targeted imaging, (c) imaging of chemical species, and (d) imaging of temperature are given next. A final section covers aspects of multimodal imaging (such as fluorescence/nmr), imaging combined with drug and gene delivery, or imaging combined with therapy or diagnosis. The electronic supplementary information (ESI) gives specific examples for materials and methods used in imaging, sensing, multimodal imaging and theranostics such as imaging combined with drug delivery or photodynamic therapy. The article contains 273 references in the main part, and 157 references in the ESI.