Design and synthesis of a vanadate-based ratiometric fluorescent probe for sequential recognition of Cu2+ ions and biothiols

Dual-mode sensing strategy based on carbon dots for sensitive and selective detection of molybdate ions

Two kinds of carbon dots with the maximum fluorescence peak of 492 nm (named as G-CDs) and 607 nm (named as R-CDs) were synthesized. In the presence of MoO42- ions, the fluorescence of R-CDs at 607 nm can be quenched, which can probably be assigned to their aggregation caused by MoO42-, while that of G-CDs at 492 nm remained unchanged. For the first time, a ratiometric fluorescence probe was developed for MoO42- ions detection. In the range 0.25 ~ 100 μM, the fluorescence ratio (F492/F607) of the probe was linearly related to MoO42- concentration, and the detection limit was 61.5 nM, which fully meets the minimum detection requirements of MoO42- ions in drinking water. On the other hand, when MoO42- was introduced, a significant fading phenomenon of R-CDs can be observed with the naked eye; thereby, the colorimetric method can also be proposed. Based on above, the ratiometric fluorometric/colorimetric dual-mode sensing method was established for MoO42- anion quantification. Compared with the traditional analysis methods, the results obtained by multimodal sensing can be mutually verified, which effectively improves the accuracy and reliability. The dual-mode assay proposed in this work provides an alternative scheme to meet the need of sensing target compounds in complex matrices.

Low Cu(II) Concentration Detection Based on Fluorescent Detector Made from Citric Acid and Urea

Carbon quantum dots (CQD) are an advanced fluorescent material, which has attracted more and more attention in theoretical research and practical applications. To obtain stable CQDs with high fluorescence characteristics for detecting trace metal ions in water, nitrogen-doped carbon quantum dots (N-CQDs) based fluorescent sensors were synthesized by the hydrothermal method, using citric acid and urea as source. Transmission electron microscopy (TEM) images showed that the synthesized N-CQDs maintained a narrow particle size distribution bellow 10 nm, and its average size was 3.07 nm. Fourier transform infrared spectroscopy (FT-IR) indicated that abundant hydroxyl and carboxyl functional groups existed on N-CQDs surface, which helped N-CQDs highly disperse in water. In addition, UV-vis spectroscopy and photoluminescence demonstrated that the N-CQDs obtained a 10.27% of quantum yield (QY) with relatively high and stable fluorescence performance. As a fluorescent sensor, the N-CQDs showed a fluorescence "ON-OFF" mechanism during the Cu2+ detection, which was induced from the electrons transition in surface functional groups. The final N-CQDs exhibited a wide linear relationship between fluorescence response and concentration of Cu2+ in range of 0.3-0.7 μM with a detection limit of 0.071 μM. Furthermore, the detection of Cu2+ in the simulating surface water (by adding interfering metal ions in purified water) and the tap water (from municipal water in Beijing) were used to verify N-CQDs practical application.

Two near-infrared phosphorescent iridium(III) complexes for the detection of GSH and photodynamic therapy

GSH is one of the most important reducing agents in biological systems. The depletion of GSH in the human body is linked to many diseases. Therefore, it is necessary to develop suitable and efficient probes for detecting GSH concentrations in real samples. In this work, we designed and synthesized two near-infrared emitting iridium(III) complex probes containing a novel ligand functionalized with an α,β-unsaturated ketone for the rapid and sensitive detection of GSH. The molecular structure of Ir2 was determined by X-ray crystallography. Due to their large Stokes shift, long luminescence lifetime and NIR emission, these probes were successfully applied in the imaging of GSH in living cells. In addition, two iridium(III) complexes have strong singlet oxygen generation ability which can be used for photodynamic therapy (PDT) upon visible light irradiation. On the basis of these findings, our iridium(III) complexes may serve as GSH probes for HeLa cell imaging and as photosensitizers for PDT.

A highly selective and sensitive ICT-based Cu2+ fluorescent probe and its application in bioimaging

Cu²⁺ is an essential trace element for the organism, but its excess can also cause irreversible damage to the organism. As such, a "Turn-Off" fluorescent probe DPAP for the specific detection of Cu²⁺ was successfully constructed. DPAP exhibits large Stokes shift (120 nm), fast reaction speed (1 min), low detection limit (15.2 nM), low toxicity, and good cell permeability. Cu²⁺ quenches the fluorescence of DPAP by blocking its intramolecular charge transfer process to achieve the detection of Cu²⁺ and has been confirmed by HRMS, ¹H NMR and DFT calculations. Excitingly, the five-cycle detection of Cu²⁺ and the successful recovery of trace Cu²⁺ in environmental water samples fully demonstrate the potential of DPAP for practical applications. In particular, DPAP can observe the distribution and translocation patterns of exogenous Cu²⁺ in HeLa cells and zebrafish in real-time. This research concept has offered important theoretical support for the study of the environmental behavior of heavy metal ions.

A naphthalimide functionalized chitosan-based fluorescent probe for specific detection and efficient adsorption of Cu2

As one of the most abundant metal ions, Cu²⁺ has turned into a great threat to human health and the natural environment due to its widely utilized in various industries. In this paper, a chitosan-based fluorescent probe CTS-NA-HY for detection and adsorption of Cu²⁺ was rationally prepared. CTS-NA-HY exhibited a specific "turn off" fluorescence response to Cu²⁺ and the fluorescence color changed from bright yellow to colorless. It possessed satisfactory detection performance to Cu²⁺ including good selectivity and anti-interference, low detection limit (29 nM) and wide pH range (4-9). The detection mechanism was confirmed by Job's plot, X-ray photoelectron spectroscopy, FT-IR and ¹H NMR analysis. Additionally, the probe CTS-NA-HY was capacity of determining Cu²⁺ in environmental water and soil samples. Besides, CTS-NA-HY-based hydrogel could also remove Cu²⁺ in aqueous solution effectively, which the ability of adsorption was greatly improved compared with original chitosan hydrogel.

A nanotheranostic agent based on Nd3+-doped YVO4 with blood-brain-barrier permeability for NIR-II fluorescence imaging/magnetic resonance imaging and boosted sonodynamic therapy of orthotopic glioma

The specific diagnosis and treatment of gliomas is a primary challenge in clinic due to their high invasiveness and blood-brain barrier (BBB) obstruction. It is highly desirable to find a multifunctional agent with good BBB penetration for precise theranostics. Herein, we design and construct a core-shell structured nanotheranostic agent (YVO₄:Nd³⁺-HMME@MnO₂-LF, marked as YHM) with YVO₄:Nd³⁺ particles as the core and MnO₂ nanosheets as the shell. Sonosensitizer hematoporphyrinmonomethyl ether (HMME) and lactoferrin (LF) were further loaded and modified on the surface, giving it a good ability to cross the BBB, near-infrared fluorescence imaging in the second window (NIR-II)/magnetic resonance imaging (MRI) bimodality, and highly efficient sonodynamic therapy (SDT) of orthotopic gliomas. The YVO₄:Nd³⁺ (25%) core exhibited good NIR-II fluorescence properties, enabling YHM to act as promising probes for NIR-II fluorescence imaging of vessels and orthotopic gliomas. MnO₂ shell can not only provide O₂ in the tumor microenvironments (TME) to significantly improve the healing efficacy of SDT, but also release Mn²⁺ ions to achieve T₁-weight MRI in situ. Non-invasive SDT can effectively restrain tumor growth. This work not only demonstrates that multifunctional YHM is promising for diagnosis and treatment of orthotopic glioma, but also provides insights into exploring the theranostic agents based on rare earth-doped yttrium vanadate nanoparticles.

Ratiometric fluorescent nanoprobes based on carbon dots and multicolor CdTe quantum dots for multiplexed determination of heavy metal ions

Owing to the high risk to human and environmental health, heavy metal pollution has become a global problem. Rapid, accurate and multiplexed determination of heavy metal ions is critical. In this work, we reported a promising approach to designing ratiometric fluorescent nanoprobes for multiplexed determination of Hg²⁺, Cu²⁺, and Ag⁺ ions. The nanoprobes (CDs-QDx) were designed by mixing the CDs and multicolor CdTe QDs without the involvement of recognition elements. The CDs were insensitive to heavy metal ions while CdTe QDs showed the size-dependent fluorescence response to different heavy metal ions, thereby establishing a ratiometric detection scheme by measuring the fluorescence intensity ratios of CDs-QDx systems. By evaluating the detection performance, the CDs-QDx (x = 570, 650, and 702) were successfully used for differentiation and quantification of Hg²⁺, Cu²⁺, and Ag⁺ ions. In addition, we also carried out the detection of heavy metal ions in actual samples with acceptable results. We believed that this work offers new insight into the design of ratiometric fluorescent nanoprobe for multiplexed determination of not only heavy metals but also some other analytes by combining the CDs with CdTe QDs with fine-tuned sizes.

Ratiometric fluorescence probe of Cu2+ and biothiols by using carbon dots and copper nanoclusters

A novel dual-emission ratiometric fluorescent probe based on N-doped yellow fluorescent carbon dots (y-CDs) and blue fluorescent copper nanoclusters (CuNCs) was established for quantitative determination of Cu²⁺ and biothiols. In this work, the Cu²⁺-(y-CDs) complexes formed by the chelation of y-CDs with Cu²⁺, showed an absorption peak at 430 nm that not only enhanced the fluorescence of y-CDs through inhibiting photoinduced electron transfer (PET) but also effectively quenched the fluorescence of CuNCs due to Förster resonance energy transfer (FRET). In addition, the chelation of y-CDs with Cu²⁺ could be inhibited by biothiols that prevented the fluorescence of y-CDs from being enhanced and the fluorescence of CuNCs from being quenched. On account of the changes of ratiometric signal, a dual-emission fluorescence probe for Cu²⁺ and biothiols determination was achieved. The proposed method exhibited high sensitivity for Cu²⁺ and biothiols in the ranges of 0.5-100 μM and 0.8-50 μM and the limits of detection (LODs) of Cu²⁺, glutathione (GSH), cysteine (Cys) and homocysteine (Hcy) were 0.21 μM, 0.33 μM, 0.39 μM and 0.46 μM, respectively. Subsequently, the established strategy presented an application prospect for the detection of Cu²⁺ and biothiols in real samples.

Praseodymium selective fluorescence recognition based on GdPO4: Tb3+ probe via energy transfer from Tb3+ to Pr3+ ions

A novel strategy is proposed based on the efficient energy transfer from Tb³⁺ to Pr³⁺ for the sensitive and selective discrimination of praseodymium ions due to the matched energy levels of ⁵D₄ (Tb³⁺) and ³P₀ (Pr³⁺). The electron of Tb³⁺ transfers from the ground state to the excited state under the excitation of ultraviolet light and relaxes to the ⁵D₄ level. In the presence of Pr³⁺ the electron has no time to return to the ground state, thus it transfers to the ³P₀ level of Pr³⁺ resulting in the quenching of Tb³⁺ luminescence. In the case of GdPO₄: Tb³⁺ nanowire, its fluorescence intensity at 545 nm linearly decreased when Pr³⁺ concentration ranged from 1 × 10^-7 to 1 × 10^-5 M, and the detection limit was 75 nM. To further investigate the sensing mechanism, CePO₄: Tb³⁺, YPO₄: Tb³⁺, and YBO₃: Tb³⁺ nanoparticles were also synthesized for Pr³⁺ ion detection. For all materials, similar fluorescence quenching by Pr³⁺ ions occurred, which confirmed the efficient energy transfer from Tb³⁺ to Pr³⁺ ions. Utilizing the matched energy levels of ⁵D₄ (Tb³⁺) and ³P₀ (Pr³⁺), for the first time, we proposed a novel strategy (taking GdPO₄: Tb³⁺ probe as the example) based on the efficient energy transfer from Tb³⁺ to Pr³⁺ for the sensitive and selective discrimination of praseodymium ions.

A fluorescence turn-on probe for hydrogen sulfide and biothiols based on PET & TICT and its imaging in HeLa cells

In this paper, a photoinduced electron transfer (PET)& twisted intramolecular charge transfer (TICT)-based fluorescent probe (1) for detecting biothiols (GSH/Cys/Hcy) and hydrogen sulfide with fluorescence turn on was developed. The probe could recognize hydrogen sulfide over primary ions and selectively detect GSH/Cys/Hcy over other amino acids with fluorescence turn-on (an ESIPT process). H₂S can be distinguished from GSH/Cys/Hcy with wavelength shift by UV-Vis spectra. In addition, detection limits for H₂S/GSH/Cys/Hcy of probe 1 were 1.42 μM (0-100 μM), 0.13 μM (0-40 μM), 0.27 (0-30 μM), 0.22 μM (0-40 μM), respectively. The proposed thiolysis of the 2,4-dinitrochlorophenyl ether reaction in identification process was verified by the characteristic peak in ¹H NMR and HRMS spectra. Finally, the biological imaging experiments and low cytotoxicity investigations in HeLa cells demonstrated that probe 1 could provide a promising method for the determination of H₂S and biothiols in aqueous solution and living cells.

A phenanthrene[9,10-d]imidazole-phenol-based fluorescent probe combining ESIPT and AIE for the “turn-on” detection of Cu2+ with green-emission and improved Stokes’ shift, and its application

The “turn-on” probe PIA(OH)-Py responds to Cu2+ in living cells and can determine the concentration of Cu2+ in blood samples.

A highly selective fluorescent probe for the sensing of Cu2+ based on the hydrolysis of a quinoline-2-carboxylate and its application in cell imaging

A highly selective OFF–ON fluorescent probe is developed for the sensing of Cu2+ based on the hydrolysis of a quinoline-2-carboxylate moiety. The probe is weakly fluorescent due to esterification of the phenolic group. Upon treatment with 1 equiv. of Cu2+, the probe exhibits strong fluorescence at 570 nm. The probe also exhibits high selectivity for Cu2+ over other cations with a low detection limit of 0.2 μM, which is sensitive enough to meet the standard of the World Health Organization for Cu2+ in drinking water (30 μM). Moreover, the probe shows a very low cell cytotoxicity, and imaging experiments demonstrate that the probe can be used for the sensing of Cu2+ in living cells.

A Coumarin-Based Fluorescent Probe for Ratiometric Detection of Cu2+ and Its Application in Bioimaging

The fluorescent probe L, based on naphthalimide-modified coumarin, was designed, synthesized, and characterized, which could recognize Cu2+ from other cations selectively and sensitively in HEPES buffer (10 mM, Ph = 7. 4)/CH3CN (1:4, V/V). When the probe L interacted with Cu2+, the color and the fluorescent intensity changed obviously and it provided the naked-eye detection for Cu2+. The recognition mode between them was achieved by Job's plot, IR, MS, SEM, and 1HNMR. In addition, test strips made from L could still interact with Cu2+ in tap water effectively. The limit of detection (LOD) of L was 3.5 × 10-6 M. Additionally, the density functional theory (DFT) calculation method was used to analyze the action mechanism of L toward Cu2+. Importantly, the fluorescent probe L could demonstrate favorable selectivity toward Cu2+ in Caenorhabditis elegans. Thus, L was considered to have some potential for application in bioimaging.

A novel fluorescent sensor for specific recognition of GSH based on the copper complex and its bioimaging in living cells

Given the important role of biothiols in various physiological processes, there is a need to develop novel fluorescent sensors for detecting them. Herein, a novel "on-off-on" fluorescent sensor (E)-N'-((7-(diethylamino)-2-oxo-2H-chromen-3-yl)methylene)-6-((quinolin-8-yloxy)methyl)picolinohydrazide (PQC) was synthesized and its absorbance and fluorescence properties were characterized. The sensor PQC could form a stable complex and showed a significant fluorescence quenching response to Cu²⁺ with a quenching efficiency of approximately 100%, and the PQC-Cu²⁺ complex showed a fluorescence enhancement response to GSH with a higher recovery rate of above 80% in a CH₃OH/HEPES (9:1 v/v, pH = 7.23) buffer system. Its detection limits were determined to be 0.17 μM for Cu²⁺ and 0.20 μM for GSH, and the binding stoichiometry of PQC-Cu²⁺ was determined to be 1: 1 by Job's plot method. Importantly, the sensor PQC can be used for filter paper strip tests and bioimaging in living cells.