Highly Selective and Sensitive Detection of Cu(2+) Ions Using Ce(III)/Tb(III)-Doped SrF2 Nanocrystals as Fluorescent Probe

Surface Amide-Mediated Synthesis of Bright Blue Fluorescent Carbon Dots for High-Sensitivity Detection of Hg2+ Ions

Mercury ions (Hg2+) seriously harm the central nervous system of humans, leading to brain damage and even heart failure and death. Therefore, effective detection of Hg2+ in water quality has become an urgent research field. It is very important to develop economically efficient fluorescent sensors to achieve rapid and sensitive detection of Hg2+. Therefore, the high fluorescence quantum yield fluorescent carbon dots (CDs) with amide group were prepared. The process of preparing CDs was regulated by multiple key factors (carbon source, proportion, time), and the CDs with the best fluorescence performance were selected. It was comprehensively characterized, including fluorescence performance, surface structure, phase, and morphological characteristics. The amide group endows CDs with the ability to act as both donors and acceptors for hydrogen bonding, forming complexes with metal ions, thus making them suitable for the detection of Hg2+. It is worth noting that CDs can quickly detect Hg2+ within 1 min, and there is a good linear relationship within the ranges of 0.001-200 μM and 200-500 μM. The detection limit of UC-CDs is 8.2 nM. This study provides a fluorescent sensor with fast reaction, excellent sensitivity, and selectivity for the efficient detection of Hg2+ in water.

Orotic acid-capped Tb(III)-doped calcium sulphate nanorods for the selective detection of tryptophan

Lanthanide-based luminescent materials have gained huge attention due to their applications in optoelectronic devices, sensing, bio-imaging, anti-counterfeiting, and more. In this work, we report a luminescence-based sensor for the detection of tryptophan using orotic acid-capped Tb3+-doped CaSO4 nanorods (NRs). Orotic acid (OA) was found to play a dual role as a capping agent to control the growth of the nanorods and as a sensitizer for Tb3+ ions. The resulting nanorods exhibited excellent dispersibility and strong photoluminescence signals characteristic of Tb3+ ions in the visible region. Nearly 10-fold enhancement in the emission intensity was noted through OA sensitization compared to direct excitation of Tb3+ ions (acceptors). Interestingly, the strong emission intensity of the NRs reduced significantly with the addition of tryptophan. In contrast, hardly any change was noted with the addition of other amino acids and metal ions, suggesting greater selectivity for tryptophan. Moreover, there is barely any notable interference from other amino acids toward the detection of tryptophan. The limit of detection is found to be ∼0.61 μM. Finally, the sensing study was extended to biological samples to detect tryptophan present in blood plasma, urine, and saliva samples. The nanorods demonstrated high detection abilities, indicating the potential of the developed materials for biomedical applications.

Ratio-fluorescent and naked-eye visualized dual-channel sensing strategy for Cu2+ and alkaline phosphatase activity assay

The levels of Cu2+ and alkaline phosphatase (ALP) are the important indicators of the developed stage of the relative diseases. Herein, a binary ratio-fluorescent and smartphone-assisted visual strategy basing on 4'-aminomethyl-4, 5', 8-trimethylpsoralen (AMT) and the oxidation of o-phenylenediamine was developed. Under the action of Cu2+, the fluorescent molecule, 3-diaminophenazine (DAP) formed which can act as a fluorescent acceptor of the ratio-fluorescent sensor. The emission spectrum of AMT overlapped with the excitation spectrum of DAP and, thus, it can act as the fluorescent donor of the ratio-fluorescent sensor. With the increasing concentration of Cu2+ and ALP, the fluorescent intensity of AMT decreased and the fluorescent intensity of DAP increased. The dual-emission reverse change ratio-fluorescent sensor realized the sensitive detection Cu2+ and ALP with the detection limits of 2 nM and 0.03 U/mL, respectively. In addition, the acceptable recoveries were obtained when the Cu2+ and ALP in spiked samples were detected. Furthermore, the relative activity of ALP was assessed by increasing the concentrations of the inhibitor Na3VO4 and IC50 of 25 μM was obtained. Importantly, the target concentration-dependent color change of DAP allowed us to utilize R/B ratio values to design the smartphone-assisting visual detection model of Cu2+ and ALP activity with the detection limits of 0.1 μM and 0.18 U/mL. This simple, flexible, dual-mode sensor strategy has a potential for disease diagnosis and drug screening.

Regenerable and Highly Stable Two-Dimensional Imine-Based Covalent Organic Framework for Simultaneous Rapid Detection and Adsorption of Cu2+ Ions

The development of efficient fluorescent probes and adsorbents for detecting and removing Cu2+, which pose potential environmental and health risks, is a highly active area of research. However, achieving simultaneously improved fluorescence detection efficiency and enhanced adsorption capacity in a single porous probe remains a significant challenge. In this study, we successfully synthesized a two-dimensional imine-based TAP-COF using 2,4,6-triformylphloroglucinol and tri(4-aminophenyl)amine as raw materials. TAP-COF exhibited excellent properties, including a large specific surface area of 685.65 m2·g-1, exceptional thermal stability (>440 °C), chemical stability, temporal stability, and recyclability. Fluorescence testing revealed that TAP-COF exhibited remarkable specificity and high sensitivity for detecting Cu2+. The fluorescence mechanism, in which the excited state intramolecular proton transfer was impeded by the interaction of Cu2+ with C═O and C-N bonds on TAP-COF upon the addition of Cu2+, was further elucidated through experimental and theoretical methods. Furthermore, the adsorption capacity of TAP-COF toward Cu2+ was investigated, confirming the excellence of TAP-COF as a fluorescent probe and adsorbent for the specific detection and removal of Cu2+. This work holds significant implications for improving environmental and human health concerns associated with Cu2+ contamination.

Ecotoxicity of non- and PEG-modified lanthanide-doped nanoparticles in aquatic organisms

Various types of nanoparticles (NPs) have been widely investigated recently and applied in areas such as industry, the energy sector, and medicine, presenting the risk of their release into the environment. The ecotoxicity of NPs depends on several factors such as their shape and surface chemistry. Polyethylene glycol (PEG) is one of the most often used compounds for functionalisation of NP surfaces, and its presence on the surfaces of NPs may affect their ecotoxicity. Therefore, the present study aimed to assess the influence of PEG modification on the toxicity of NPs. As biological model, we chose freshwater microalgae, a macrophyte and invertebrates, which to a considerable extent enable the assessment of the harmfulness of NPs to freshwater biota. SrF₂:Yb³⁺,Er³⁺ NPs were used to represent the broad group of up-converting NPs, which have been intensively investigated for medical applications. We quantified the effects of the NPs on five freshwater species representing three trophic levels: the green microalgae Raphidocelis subcapitata and Chlorella vulgaris, the macrophyte Lemna minor, the cladoceran Daphnia magna and the cnidarian Hydra viridissima. Overall, H. viridissima was the most sensitive species to NPs, which affected its survival and feeding rate. In this case, PEG-modified NPs were slightly more toxic than bare ones (non-significant results). No effects were observed on the other species exposed to the two NPs at the tested concentrations. The tested NPs were successfully imaged in the body of D. magna using confocal microscopy; both NPs were detected in the D. magna gut. The results obtained reveal that SrF₂:Yb³⁺,Er³⁺ NPs can be toxic to some aquatic species; however, the structures have low toxicity effects for most of the tested species.

Simultaneous Observation of Visible Upconversion and Near-Infrared Downconversion in SrF2:Nd3+/Yb3+/Er3+ Nanocrystals and Their Application for Detecting Metal Ions under Dual-Wavelength Excitation

In this work, a sequence of Nd3+, Yb3+, and Er3+ tridoped SrF2 nanocrystals (NCs) is synthesized by a hydrothermal method. Both the efficient near-infrared downconversion luminescence (DCL) and visible upconversion luminescence (UCL) of the Er3+ and Nd3+ ions are simultaneously observed and systematically demonstrated under dual-wavelength excitation (808 and 980 nm continuous-wave lasers). Subsequently, the SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs with the strongest luminescence were utilized for detecting the metal ion concentrations under 808 nm excitation. The results reveal that both the UCL and DCL gradually decrease as the metal ion concentrations increase, and high sensitivity is obtained for Cu2+ ions with a detection limit of 0.22 nM (∼650 nm) and 0.63 nM (∼976 nm). In addition, these SrF2:Nd3+/Yb3+/Er3+ NCs are further demonstrated to achieve a solid-state display under 980 nm excitation, exhibiting obvious "red" and "green" patterns by varying the doping rare earth ion concentrations.

Temperature sensing using bulk and nanoparticles of Ca0.79Er0.01Yb0.2MoO4 phosphor

Optical temperature sensing is widely realized by using upconversion (UC) emission in lanthanide-doped phosphors. There are so many various parameters that are responsible for UC intensity of the phosphor like particle shape and size, type of symmetry that exist at the site position, distribution of lanthanide ions in the phosphor, and so on. However, a comparative study of the bulk and nanostructure on the temperature sensing ability of such phosphor is rare. In the present work, we have taken Ca0.79Er0.01Yb0.2MoO4 phosphors as a model system and synthesized its bulk (via solid-state reaction method, named SCEY) and nanostructures (via solution combustion route, named CCEY). We further studied their phase, crystal structure, phonon frequency, optical excitation, and emission (upconversion & downshifting) properties. Finally, the optical temperature sensing behavior of SCEY and CCEY, in the range 305 K - 573 K, have been compared. The maximum relative sensitivity of the phosphor SCEY and CCEY are 0.0061 K-1 at 305 K and 0.0094 K-1 at 299 K, respectively, while, the maximum absolute sensitivities are 0.0150 K-1 at 348 K, and 0.0170 K-1 at 398 K, respectively. We thus conclude that the temperature sensing ability of nanoparticle-based Ca0.79Er0.01Yb0.2MoO4 phosphor is better compared to its bulk phosphor.

A highly sensitive and selective ratiometric sensing platform based on 7-amino-4-methylcoumarin for naked-eye visual fluorescence sensing of Cu2

Cu²⁺ is a major environmental pollutant. An efficient measurement for Cu²⁺ is urgently needed. In this report, we have developed a new sensitive and selective ratiometric sensing platform using 7-amino-4-methylcoumarin (AMC) for detecting Cu²⁺ in real samples. In the presence of Cu²⁺, o-phenylenediamine (OPD) could be catalytically oxidized to 2,3-diaminophenazine (DAP), which could react with AMC, leading to quench the fluorescence intensity of AMC at 438 nm. Meanwhile, DAP provided a new emission peak at 557 nm. Based on the efficient overlapped spectrum of AMC and DAP, a ratiometric sensing platform through fluorescence resonance energy transfer (FRET) was carried out. Furthermore, the as-proposed strategy displayed the linear relationship in the wide range from 6 to 250 μM with a low detection limit of 0.059 μM, and the recoveries of the spiked samples in real samples ranged from 86.5% to 110.1%. Moreover, comparing the visual fluorescence colors of the real samples with the standard colorimetric card, we used the as-proposed strategy as a solid-based platform for realizing an efficient semi-quantitative detection of Cu²⁺ via naked-eye visual fluorescence mode without any complicated instrument and operation. The above results implied that the as-proposed strategy could be used in the practice determination of Cu²⁺.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Simultaneous Visual Detection and Removal of Cu2+ with Electrospun Self-Supporting Flexible Amidated Polyacrylonitrile/Branched Polyethyleneimine Nanofiber Membranes

Sensitive detection and effective removal of copper ions (Cu²⁺) from water are still arduous tasks required to protect public health and environmental safety because of the serious impacts of Cu²⁺ on humans and other organisms. Herein, we report the design and fabrication of self-supporting flexible amidated polyacrylonitrile/branched polyethyleneimine nanofiber membranes (abbreviated as aPAN/BPEI NMs) via facile electrospinning and a subsequent hydrothermal method, which are used not only as strips for the visual detection of Cu²⁺ but also as effective adsorbents for the removal of Cu²⁺ from water. Because aPAN/BPEI NMs are self-supporting, they can be easily removed from the solution to reduce secondary pollution to the environment. Based on the high Cu²⁺ binding capacity of BPEI, Cu²⁺ ions are adsorbed on the aPAN/BPEI NMs, which leads to the appearance of new absorbance bands at 280 and 636 nm and a color change from yellow to blue. aPAN/BPEI NMs are utilized for the visual detection of Cu²⁺ with a linear range of 50-700 μM and limits of detection of 11.5 and 4.8 μM (absorption peaks at 280 and 636 nm). More importantly, aPAN/BPEI NMs exhibit excellent selectivity and certain recovery with a simple treatment. Furthermore, by utilizing the adsorption characteristics of Cu²⁺ in aqueous media, it can be effectively removed by aPAN/BPEI NMs with a remarkable adsorption capacity of 209.53 mg·g^-1. Additionally, the removal of Cu²⁺ by aPAN/BPEI NMs does not exhibit interference by other foreign ions. The adsorption process conforms well to the pseudo-second order (PSO) kinetic model and Jovanovich model, proving that adsorption occurs via chemical and monolayer adsorption mechanisms. Accordingly, this work will provide theoretical and technical support for the design and fabrication of novel heavy metal ion detection-removal integrated materials exhibiting high sensitivity and strong adsorption.

PEI functionalized NaCeF4:Tb3+/Eu3+ for photoluminescence sensing of heavy metal ions and explosive aromatic nitro compounds

This work reports an eco-friendly hydrothermal approach for the synthesis of hexagonal NaCeF4:Tb3+/Eu3+ nanophosphors. The phase, morphology and optical properties were characterized by Powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy and photoluminescence (PL) spectroscopy respectively. Herein, the as-synthesized nanophosphor was functionalized with amine rich polyethylenimine (PEI) resulting in development of a luminescent nanoprobe bearing dual sensing functions for hazardous nitroaromatics and heavy metal ions. The strong photoluminescence emission of Eu3+ ions was selectively quenched upon addition of toxic analytes at concentrations from 10 to 100 ppm due to complex formation between the analytes and PEI functionalized nanostructure. The synthesized nanomaterial shows sharp emission peaks at 493, 594, 624, 657 and 700 nm. Significantly, the peak at 594 nm shows a noticeable quenching effect on addition of toxic analytes to the aqueous solution of the nanocrystals. The nanophosphors are sensitive and efficient for the PA and Fe3+ ion detection with an LOD of 1.32 ppm and 1.39 ppm. The Stern-Volmer (SV) quenching constant (K SV) is found to be 2.25 × 105 M-1 for PA and 3.8 × 104 M-1 for Fe3+ ions. The high K SV value and low LOD suggest high selectivity and sensitivity of the nanosensor towards PA and Fe3+ ions over other analytes. Additionally, a reduced graphene oxide and nanophosphor based nanocomposite was also synthesized to investigate the role of energy transfer involving delocalized energy levels of reduced graphene oxide in regulating the luminescence properties of the nanophosphor. It was observed that PEI plays central role in inhibiting the quenching effect of reduced graphene oxide on the nanophosphor.

Ion-Imprinted Polymer Modified with Carbon Quantum Dots as a Highly Sensitive Copper(II) Ion Probe

Fluorescence analysis technology and ion imprinting technology are combined to prepare a copper ion fluorescence sensor. Carbon quantum dots (CQDs), with a quantum yield of 79%, were synthesized by a hydrothermal process using citric acid as the carbon source. The prepared CQDs, acting as the fluorophore, were grafted onto the surface of an SBA-15 mesoporous molecular sieve by an amidation reaction. Then, the fluorescent sensor CQDs@Cu-IIP was prepared using a surface imprinting technique with the modified SBA-15 as the substrate, copper ions as a template, tetraethoxysilane as the crosslinker, and 3-aminopropyl-3-ethoxysilane as the functional monomers. The sensor showed strong fluorescence from CQDs and high selectivity due to the presence of Cu(II)-IIP. After the detection conditions were optimized, the fluorescence intensity of the sensor had good linearity with Cu(II) concentration in a linear range of 0.25-2 mg/L and 3-10 mg/L. This CQDs@Cu-IIP was applied to the determination of traces Cu(II) in real water samples and good recoveries of 99.29-105.42% were obtained. The present study provides a general strategy for fabricating materials based on CQDs for selective fluorescence detection of heavy metals.

PAA Modified Upconversion Nanoparticles for Highly Selective and Sensitive Detection of Cu2+ Ions

Detection of the Cu²⁺ ions is crucial because of its environmental and biological implications. The fluorescent-based organic sensors are not suitable for Cu²⁺ detection due to their short penetration depth caused by the UV/visible excitation source. Therefore, we have demonstrated a highly sensitive and selective near-infrared (NIR) excitable poly(acrylic acid) (PAA) coated upconversion nanoparticles (UCNPs) based sensor for Cu²⁺ detection. We construct the PAA modified Na(Yb, Nd)F₄@Na(Yb, Gd)F₄:Tm@NaGdF₄ core-shell-shell structured UCNPs based sensor via a co-precipitation route. The upconversion emission intensity of the PAA-UCNPs decreases linearly with the increase in the Cu²⁺ concentration from 0.125 to 3.125 μM due to the copper carboxylate complex formation between Cu²⁺ and PAA-UCNPs. The calculated detection limit of the PAA-UCNPs based sensor is 0.1 μM. The PAA-UCNPs based sensor is very sensitive and selective toward detecting the Cu²⁺ ions, even when the Cu²⁺ co-exist with other metal ions. The EDTA addition has significantly reversed the upconversion emission quenching by forming the EDTA-Cu²⁺ complex based on their greater affinity toward the Cu²⁺. Therefore, the PAA-UCNPs based sensor can be a promising candidate for Cu²⁺ detection because of their higher sensitivity and selectivity under 980 nm NIR excitation.

Dual information encryption of carbon dots endowed with recoverable functions after interception

Hiding real information in deluded information can mislead others.

Insight into the mechanism of intense NIR-to-red upconversion luminescence in Er3+ doped and Er3+–Yb3+ co-doped SrF2 nanoparticles

SrF₂:Yb³⁺,Er³⁺ NPs were synthesized by the hydrothermal method and their luminescence mechanism was discussed in detail, which provided a theoretical basis for further understanding the properties of the materials.

Efficient energy transfer from Ce3+ to Tb3+ in BaF2: green-emitting phosphors for potential applications in the detection of Cu2+ ions

A succession of Ce³⁺ and Tb³⁺ co-doped BaF₂ nanoparticles were synthesized by co-precipitation and hydrothermal methods.

Plasmonic gold nanostructures for biosensing and bioimaging

As one kind of noble metal nanostructures, the plasmonic gold nanostructures possess unique optical properties as well as good biocompatibility, satisfactory stability, and multiplex functionality. These distinctive advantages make the plasmonic gold nanostructures an ideal medium in developing methods for biosensing and bioimaging. In this review, the optical properties of the plasmonic gold nanostructures were firstly introduced, and then biosensing in vitro based on localized surface plasmon resonance, Rayleigh scattering, surface-enhanced fluorescence, and Raman scattering were summarized. Subsequently, application of the plasmonic gold nanostructures for in vivo bioimaging based on scattering, photothermal, and photoacoustic techniques  has been also briefly covered. At last, conclusions of the selected examples are presented and an outlook of this research topic is given.

Universal Platform for Ratiometric Sensing Based on Catalytically Induced Inner-Filter Effect by Cu2

Integrating two kinds of fluorescent probes in one system to develop a ratiometric sensing platform is of prime importance for achieving an accurate assay. Inspired by the efficient overlapped spectrum of 2-aminoterephthalic acid (PTA-NH₂) and 2,3-diaminophenazine (DAP), a new sensitive ratiometric fluorescent sensor has been developed for Cu²⁺ on the basis of in situ converting o-phenylenediamine (OPD) into DAP through the catalysis of Cu²⁺. Here, the presence of Cu²⁺ induced the emission of DAP, which acted as an energy acceptor to inhibit the emission of PTA-NH₂. This dual-emission reverse change ratiometric profile based on the inner-filter effect improved sensitivity and accuracy, and the highly sensitive determination of Cu²⁺ with a detection limit of 1.7 nmol·L^-1 was obtained. The proposed sensing platform displayed the wide range of detection of Cu²⁺ from 5 to 200 nmol·L^-1 by modulating the reaction time between Cu²⁺ and OPD. Moreover, based on the specific interaction between glutathione (GSH) and Cu²⁺, this fluorescent sensor showed high response toward GSH in a range of 0.5-80 μmol·L^-1 with a detection limit of 0.16 μmol·L^-1. The successful construction of this simple ratiometric sensing platform without the participation of enzymes provides a new route for the detection of small biological molecules that are closely related to human health.

High-Yield Production of Water-Soluble MoS2 Quantum Dots for Fe3+ Detection and Cell Imaging

Uniform water-soluble MoS2 quantum dots (WS-MSQDs) were synthesized via a sequential combination of sintering/etching/exfoliation method and solvothermal route. The obtained WS-MSQDs with average size of approximately 3.4 nm exhibited sufficient water solubility and remarkable fluorescence properties. The WS-MSQDs were utilized as a probe for detection of Fe3+ ions with high selectivity and specificity. Furthermore, the WS-MSQDs exhibited high fluorescence stability under different conditions. Finally, the WS-MSQDs were successfully applied for the fluorescence imaging of Fe3+ in living cells, which exhibited practical potential for biomedical applications.

A Fluorimetric Method Based on an Imidazole Compound for Cu2+ Determination in Tap Water

Ion sensor properties of 4-(1-(4-hydroxy-3-methoxybenzyl)-1H-benzo[d]imidazol-2-yl)-2-methoxyphenol (L) in acetonitrile-water (1 : 1) were evaluated by fluorescence spectrometry. Pronounced quenching in the fluorescence spectra of the ligand was only observed for the Cu2+ ion among many metal ions. Linear fluorescence responses of the ligand at 360 nm as the function of the Cu2+ concentration were used for the determination of the Cu2+ ion in spiked tap water samples. Recovery values (R%) were satisfactory, and relative standard deviation (RSD%) was below 5.00 in intraday and interday measurements. Detection and quantification limits were 0.28 and 0.84 μg/L, respectively. The assay based on external calibration only took a few minutes.

A novel “on–off–on” fluorescence assay for the discriminative detection of Cu(ii) and l-cysteine based on red-emissive Si-CDs and cellular imaging applications

Copper ions (Cu²⁺) and l-cysteine (l-Cys) in the human body always play critical roles in various physiological processes, while abnormal Cu²⁺ and l-Cys concentrations in the biological system lead to many diseases.

Colorimetric determination of copper(II) by using branched-polyethylenimine droplet evaporation on a superhydrophilic-superhydrophobic micropatterned surface

A colorimetric method is described for the determination of Cu(II). It is based on branched polyethylenimine (BPEI) droplet evaporation on a superhydrophilic-superhydrophobic polystyrene micropatterned surface. Exposure to Cu(II) leads to a color change from colorless to light blue and dark blue. The micropatterned surface was fabricated via combining electrospinning with oxygen plasma and served as a detection substrate. Analysis requires only a single drop of blood. The method has a linear response in the 5.0 μM to 2.5 mM Cu(II) concentration range which is within the physiological range (15.7 ∼ 23.6 μM). Compared to an assay in solution, the detection limit is decreased from 386 nM to 89 nM. Excellent selectivity over other metal ions and anions was achieved. Graphical abstract A rapid and sensitive colorimetric detection platform for Cu(II) was fabricated by using branched-polyethylenimine droplet evaporation on a superhydrophilic-superhydrophobic micropatterned surface. Only a single drop of blood was needed for the analysis. The sensitivity was improved about 4.3 times.

Highly selective and sensitive long fluorescence lifetime polyurethane foam sensor based on Tb-complex as chromophore for the detection of H2PO4- in water

Developing novel rare-earth complexes to rapidly and reliably sensing anions in pure water is highly challenging. Here, a series of long fluorescence lifetime polyurethane foam bonded Tb(BAA)₃ chromophore porous material (Tb-PUFs) have been designed and synthesized via a simple one-step co-polycondensation reaction as an efficient fluorescent sensor for H₂PO₄^- in pure water. All Tb-PUFs exhibited strong green emission and long fluorescence lifetime in water, which was ascribed to Tb-complex can be dispersed very well in polyurethane foam, effectively avoiding the emission quenching of Tb³⁺ ions caused by water molecules vibration. The titration experimental results showed that precursor Tb(BAA)₃ can effective recognize F^-, CH₃COO^-, and H₂PO₄^- in DMSO solution. Interestingly, Tb-PUFs can only selectively recognize H₂PO₄^- in pure water, this phenomenon can be explained that H₂PO₄^- is acidic, and can occur deprotonation with -NH group of ligand BAA, thus making the fluorescence quenching effect was more sensitive for H₂PO₄^- in pure water. In addition, the sensing ability of Tb-PUFs for H₂PO₄^- is highly reversible by washing them with deionized water for three times. In a word, all results implied that Tb-PUFs was an excellent candidate for H₂PO₄^- ions detection selectively in pure water.

Design of Lanthanide-Doped Colloidal Nanocrystals: Applications as Phosphors, Sensors, and Photocatalysts

The unique optical characteristics of lanthanides (Ln³⁺) such as high color purity, long excited-state lifetimes, less perturbation of excited states by the crystal field environment, and the easy spectral conversion of wavelengths through upconversion and downconversion processes have caught the attention of many scientists in the recent past. To broaden the scope of using these properties, it is important to make suitable Ln³⁺-doped materials, particularly in colloidal forms. In this feature article, we discuss the different synthesis strategies for making Ln³⁺-doped nanoparticles in colloidal forms, particularly ways of functionalizing hydrophobic surfaces to hydrophilic surfaces to enhance their dispersibility and luminescence in aqueous media. We have enumerated the various strategies and sensitizers utilized to increase the luminescence of the nanoparticles. Furthermore, the use of these colloidal nanoparticle systems in sensing application by the appropriate selection of capping ligands has been discussed. In addition, we have shown how the energy transfer efficiency from Ce³⁺ to Ln³⁺ ions can be utilized for the detection of toxic metal ions and small molecules. Finally, we discuss examples where the spectral conversion ability of these materials has been used in photocatalysis and solar cell applications.

A C3-substituted cyclotriveratrylene derivative with 8-quinolinyl groups as a fluorescence-enhanced probe for the sensing of Cu2+ ions

C₃-Substituted CTV derivatives with 8-quinolinyl groups exhibited a selective response of fluorescence enhancement toward Cu²⁺ ions even in the presence of other co-existing cations.

Self-Assembly of Europium-Containing Polyoxometalates/Tetra-n-alkyl Ammonium with Enhanced Emission for Cu2+ Detection

Lanthanide-containing polyoxometalates (POMs) can be used to detect various materials, but their luminescence in water has suffered enormous limitations due to the strong fluorescence quenching. Herein, to resolve this problem, three-dimensional nanoparticles built by mixed Weakley-type europium-containing POMs (Na9[EuW10O36]·32H2O, abbreviated to EuW10) and tetra-n-alkyl ammonium (TA) with enhanced fluorescent properties have been designed in aqueous solution using an ionic self-assembly (ISA) technique, which is mainly driven by the electrostatic interaction between EuW10 and TA. The morphology and fluorescent properties of the system as well as some influencing factors (alkyl chain length, amino group, and inorganic salt concentration) were systematically investigated. The results indicated that the fluorescent intensity of EuW10/tetramethylammonium bromide (TMAB) composite increased about 14 times, whereas the extent of increase of fluorescence for EuW10/tetraethylammonium bromide (TEAB) and EuW10/tetrabutylammonium bromide (TMAB) composites gradually decrease due to the bulkier steric hindrance of the longer alkyl chain. Besides, the luminescence of EuW10/TMAB nanoparticles is pH responsive, and the reversibility of their structures and luminescence can be realized upon the addition of NaOH/HCl. Moreover, the EuW10/TMAB system also shows great fluorescence-sensing behavior, which could detect Cu2+ with a detection limit of 0.15 μM. Our work provides a facile construction strategy for a functional fluorescent complex via POMs-based supramolecular self-assembly in aqueous solution, which will be further used in biomarkers and sensors.

Plasmon Coupling Effect-Enhanced Imaging of Metal Ions in Living Cells Using DNAzyme Assembled Core-Satellite Structures

We demonstrate a core-satellite plasmonic nanoprobe assembled via metal-ion-dependent DNA-cleaving DNAzyme linker for imaging intercellular metal ion based on plasmon coupling effect at a single-particle level. As metal ions are present in the system, the DNAzyme linker will be cleaved, and thus, disassembly of the core-satellite nanoprobes occurs, which results in distinct blue shift of the scattering spectra of Au core-satellite probes and naked color change of the scattering light. This change in scattering spectra has been supported by theoretical simulations. As a proof of concept, sensitive detection of Cu²⁺ with a limit of detection down to 67.2 pM has been demonstrated. The nanoprobes have been further utilized for intracellular Cu²⁺ imaging in living cells. The results demonstrate that the present strategy provides a promising platform for detection and imaging of metal ions in living cells and could be potentially applied to imaging other interesting target molecules simply by substituting the oligonucleotide sequence.

Polystyrenesulfonate-coated nanoparticles with low cytotoxicity for determination of copper(II) via the luminescence of Tb(III) complexes with new calix[4]arene derivatives

The authors describe new ligands with two 1,3-diketone groups and two heteroaromatic (pyridyl or quinolyl) moieties embedded to the upper and lower rims of dibromo-substituted calix[4]arene scaffold. The ligands bind Tb(III) ions in alkaline DMF solutions to form 1:1 complexes. The strong Tb(III)-centered luminescence (with excitation/emission peaks at 330/545 nm) of the complexes results from efficient ligand-to-metal energy transfer. The complexes were incorporated into polystyrenesulfonate (PSS) colloids by diluting a DMF solution of the complex with aqueous solution of PSS. The luminescence of the colloids is quenched by copper(II), and this was used to develop a method for its fluorometric determination in nanomolar concentrations. The lower limit of detection is 0.88 nM. Quenching is a result of (a) ion exchange which converts the terbium complexes into their copper counterparts, and (b) energy transfer from Tb(III) to Cu(II) complexes. The low cytotoxicity of the colloidal nanoprobe conceivably makes it a promising tool for use in cellular imaging. Graphical abstract New calix[4]arene derivative provide efficient binding sites for Tb(III) and Cu(II) ions. The Tb(III) complexes were embedded to core-shell nanoparticles by solvent-mediated aggregation followed by polystryrenesulfonate deposition. The nanoparticles exhibit luminescence response on copper ions in nanomolar concentration range.

An Ultrasensitive Fluorescence Sensor with Simple Operation for Cu2+ Specific Detection in Drinking Water

Whether short-term or long-term, overexposure to an abnormal amount of copper ion does significant harm to human health. Considering its nonbiodegradability, it is critical to sensitively detect copper ion. Herein, a novel fluorescent strategy with a "turn-on" signal was developed for highly sensitive and specific detection of copper ion (Cu2+). In the present investigation, we found that Cu2+ exhibits excellent peroxidase-like catalytic activity toward oxidizing the nonfluorescent substrate of Amplex Red into the product of resofurin with outstanding fluorescence emission under the aid of H2O2. Thus, an enzyme-free and label-free sensing system was constructed for copper ion detection with quite simple operation. To ensure the detection sensitivity and reproducibility, the amount of H2O2 and incubation time were optimized. The limit of detection can reach as low as 1.0 nM. In addition, the developed assay demonstrated excellent specificity and could be utilized to detect copper ion in water samples including tap water and bottled purified water without standing recovery.

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 "off-on" colorimetric and fluorometric assay for Cu(II) based on the use of NaYF4:Yb(III),Er(III) upconversion nanoparticles functionalized with branched polyethylenimine

The authors describe an "off-on" colorimetric and fluorometric assay for the determination of Cu(II). It is based on the use of upconversion nanoparticles (UCNPs) of type NaYF₄:Yb(III),Er(III) that were functionalized with branched polyethylenimine (BPEI). A color change from colorless to blue occurs within 2 s after addition of Cu(II) to a solution of the modified UCNPs. The color change can be visually detected at Cu(II) concentrations down to 80 μM. The upconversion fluorescence of the modified UCNPs, measured at excitation wavelength of 980 nm, is reduced due to the predominant inner filter effect caused by the formation of the BPEI-Cu(II) complex. Normalized fluorescence intensity drops linearly in the 50 nM to 10 μM Cu(II) concentration range, and the fluorometric detection limit is 45 nM. Both the color and the fluorescence are recovered on addition of EDTA. Excellent selectivity over other metal ions and anions is achieved. Graphical abstract Upconversion nanoparticles of type NaYF4:Yb,Er were functionalized with branched polyethylenimine (UCNP/BPEI) and used in an "off-on" colorimetric and fluorometric assay for Cu(II). The upconversion fluorescence is selectively quenched on addition of Cu(II), and this is accompanied by a rapid colorless-to-blue color switch. The colorimetric changes and quenched fluorescence can be reversed by adding EDTA.

Tuning the sensitivity of lanthanide-activated NIR nanothermometers in the biological windows

Lanthanide-activated SrF₂ nanoparticles with a multishell architecture were investigated as optical thermometers in the biological windows. A ratiometric approach based on the relative changes in the intensities of different lanthanide (Nd³⁺ and Yb³⁺) NIR emissions was applied to investigate the thermometric properties of the nanoparticles. It was found that an appropriate doping with Er³⁺ ions can increase the thermometric properties of the Nd³⁺-Yb³⁺ coupled systems. In addition, a core containing Yb³⁺ and Tm³⁺ can generate light in the visible and UV regions upon near-infrared (NIR) laser excitation at 980 nm. The multishell structure combined with the rational choice of dopants proves to be particularly important to control and enhance the performance of nanoparticles as NIR nanothermometers.

Nitrogen-doped carbon dots as a fluorescent probe for the highly sensitive detection of Ag+ and cell imaging

An easy hydrothermal synthesis strategy was applied to synthesize green-yellow emitting nitrogen-doped carbon dots (N-CDs) using 1,2-diaminobenzene as the carbon source, and dicyandiamide as the dopant. The nitrogen-doped CDs resulted in improvement in the electronic characteristics and surface chemical activities. N-CDs exhibited bright fluorescence emission and could response to Ag⁺ selectively and sensitively. Other ions produced nearly no interference. A N-CDs based fluorescent probe was then applied to sensitively determine Ag⁺ with a detection limit of 5 × 10^-8  mol/L. The method was applied to the determination of Ag⁺ dissolved in water. Finally, negligibly cytotoxic, excellently biocompatibile, and highly fluorescent carbon dots were applied for HepG2 cell imaging and the quenched fluorescence by adding Ag⁺ , which indicated its potential applications.

Bis(1-pyrenylmethyl)-2-benzyl-2-methyl-malonate as a Cu2+ Ion-Selective Fluoroionophore

A new malonate possessing two pyrene moieties was synthesized as a fluoroionophore, and its structure and fluorescence spectroscopic properties were investigated. When excited at 344 nm in acetonitrile/chloroform (9:1, v/v), the synthesized bispyrenyl malonate has the fluorescence of intramolecular excimer (λ_em = 467 nm) emissions and not a pyrene monomer emission (λ_em = 394 nm). A large absolute fluorescence quantum yield was obtained in the solid state (Φ_PL = 0.65) rather than in solution (Φ_PL = 0.13). X-ray crystallography analysis clarified the molecular structure and alignment of the bispyrenyl malonate in the crystal phase, elucidating its fluorescence spectroscopic properties. Such analysis also suggests there are intramolecular C-H···π interactions and intermolecular π···π interactions between the pyrenyl rings. Interestingly, the synthesized bispyrenyl malonate exhibits excellent fluorescence sensing for the Cu²⁺ ion. Remarkable fluorescence intensity enhancement was only observed with the addition of the Cu²⁺ ion.