A sensitive OFF–ON–OFF fluorescent probe for the cascade sensing of Al3+ and F− ions in aqueous media and living cells

A simple Schiff-base ligand 2-hydroxy-1-naphthaldehyde semicarbazone (HNS) was synthesized and characterized. Based on the combined effect of inhibition of CH N isomerization and chelation-enhanced fluorescence (CHEF), HNS functions as a fluorescence “turn on” sensor for Al³⁺ in buffered aqueous media. Based on the strong affinity of Al³⁺ to F⁻ ions, the in situ generated Al³⁺–HNS complex can also be utilized as an effective chemosensor for F⁻ sensing by metal displacement approach, ensuing quenching of fluorescence by the reversible return of HNS from Al³⁺–HNS complex. Thus a method using a single probe for the detection of both Al³⁺ and F⁻ ions is developed. The system exhibits high selectivity and sensitivity for Al³⁺ and F⁻ ions and the detection limits were found to be as low as 6.75 × 10⁻⁸ M and 7.89 × 10⁻⁷ M, respectively. Furthermore, the practical applicability of this probe has been examined in living cells.

A simple Schiff-base ligand 2-hydroxy-1-naphthaldehyde semicarbazone (HNS) was synthesized and applied to the sequential sensing of Al³⁺ and F⁻ ions in aqueous media and live cells.     

A new fluorescent and colorimetric chemosensor for Al3+ and F−/CN− based on a julolidine unit and its bioimaging in living cells

A novel multifunctional chemosensor HL bearing a julolidine unit and a Schiff base unit has been synthesized. As a fluorescent sensor, HL exhibited excellent selectivity and high sensitivity to Al³⁺ and F⁻/CN⁻ with a low detection limit in acetonitrile. Moreover, HL also showed good colorimetric selectivity to F⁻/CN⁻; a solution color change from colorless to light yellow in acetonitrile was observed by the ‘naked-eye’. The properties of HL with Al³⁺ and F⁻/CN⁻ were studied by UV-vis absorption spectroscopy, fluorescence spectroscopy, high-resolution mass spectrometry and ¹H NMR titration. Furthermore, the cell imaging experimental results indicated that the chemosensor HL could be applied for the detection of Al³⁺ in living cells.

A novel multifunctional chemosensor HL bearing a julolidine unit and a Schiff base unit has been synthesized.     

A highly sensitive and selective fluorescent probe for quantitative detection of Al3+ in food,water, and living cells

Three novel β-pinene-based fluorescent probes 2a–2c were designed and synthesized for the selective detection of Al³⁺. Probe 2a showed higher fluorescence intensity toward Al³⁺ than the other two compounds. Probe 2a determined the concentration of Al³⁺ with a rapid response time (45 s), wide pH range (pH = 1–9), excellent sensitivity (LOD = 8.1 × 10⁻⁸ M) and good selectivity. The recognition mechanism of probe 2a toward Al³⁺ was confirmed by ¹H NMR, HRMS and DFT analysis. Probe 2a was successfully used as a signal tool to quantitatively detect Al³⁺ in food samples and environmental water samples. Furthermore, probe 2a was successfully utilized to label intracellular Al³⁺, indicating its promising applications in living cells.

Probe 2a exhibiting high sensitivity, good selectivity, wide pH range, lower detection limit, and rapid detection for Al³⁺, probe 2a was applied for the successful detection of Al³⁺ in water samples, food samples and HeLa cells.     

Highly selective fluorescent turn-on–off sensing of OH−, Al3+ and Fe3+ ions by tuning ESIPT in metal organic frameworks and mitochondria targeted bio-imaging

Herein we report a multifunctional high performance metal organic framework (Zn-DHNDC MOF) based chemosensor that displays an exceptional excited state intramolecular proton transfer (ESIPT) tuned fluorescence turn-on–off response for OH⁻, Al³⁺ and Fe³⁺ ions along with mitochondria targeted bio-imaging. Properly tuning ESIPT as well as the hydroxyl group (–OH) allows Zn-DHNDC MOF to optimize and establish chelation enhanced fluorescence (CHEF) and chelation enhanced quenching (CHEQ) based sensing mechanisms. The MOF benefits from acid-base interactions with the ions which generate a turn-on bluish green fluorescence (λ_Em 492 nm) for OH⁻, an intense turn-on green fluorescence (λ_Em 528 nm) for Al³⁺ and a turn-off fluorescence quenching response for Fe³⁺ ions. The aromatic –OH group indeed plays its part in triggering CHEF and CHEQ processes responsible for the turn-on-off events. Low limits of detection (48 nM of OH⁻, 95 nM for Al³⁺, 33 nM for Fe³⁺ ions), high recyclability and fast response time (8 seconds) further assist the MOF to implement an accurate quantitative sensing strategy for OH⁻, Al³⁺ and Fe³⁺ ions. The study further demonstrates the MOF''s behaviour in cellular medium by subjecting it to live cell confocal microscopy. Along with a bio-compatible nature the MOF exhibited successful accumulation inside the mitochondria of MCF7 cancer cells, which defines it as a significant bio-marker. Therefore the present work successfully represents the multidisciplinary nature of Zn-DHNDC MOFs, primarily in sensing and biomedical studies.

ESIPT tuned fluorescence sensing of OH⁻, Al³⁺ and Fe³⁺ ions and mitochondria targeted bio-imaging by a Zn-DHNDC MOF.     

Correction: A highly selective ratiometric fluorescent probe for the cascade detection of Zn2+ and H2PO4− and its application in living cell imaging

Correction for ‘A highly selective ratiometric fluorescent probe for the cascade detection of Zn²⁺ and H₂PO₄⁻ and its application in living cell imaging’ by Kui Du et al., RSC Adv., 2017, 7, 40615–40620.

Affiliation c was incomplete in the original publication; the corrected version is shown below.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

A novel fluorescent off–on probe for the sensitive and selective detection of fluoride ions

A highly sensitive and selective fluorescent probe for fluoride ions has been developed by incorporating the dimethylphosphinothionyl group as a recognition moiety into the fluorophore of coumarin. The detection mechanism is based on the fluoride ion-triggered cleavage of the dimethylphosphinothionyl group, followed by the release of coumarin, which leads to a large fluorescence enhancement at 455 nm (λ_ex = 385 nm). Under the optimized conditions, the fluorescence enhancement of the probe is directly proportional to the concentration of fluoride ions in the range of 0–30 μM with a detection limit of 0.29 μM, which is much lower than the maximum content of fluoride ions guided by WHO. Notably, satisfying results have been obtained by utilizing the probe to determine fluoride ions in real-water samples and commercially available toothpaste samples. The proposed probe is rather simple and may be useful in the detection of fluoride ions in more real samples.

A sensitive and selective fluorescent off–on probe is developed for fluoride ion detection, and its applicability has been demonstrated by determining fluoride ions in real-water samples and toothpaste samples.     

A mitochondrion-targeted dual-site fluorescent probe for the discriminative detection of SO32− and HSO3− in living HepG-2 cells

Sulfur dioxide, known as an environmental pollutant, produced during industrial productions is also a common food additive that is permitted worldwide. In living organisms, sulfur dioxide forms hydrates of sulfite (SO₂·H₂O), bisulfite (HSO₃⁻) and sulfite (SO₃²⁻) under physiological pH conditions; these three exist in a dynamic balance and play a role in maintaining redox balance, further participating in a wide range of physiological and pathological processes. On the basis of the differences in nucleophilicity between SO₃²⁻ and HSO₃⁻, for the first time, we built a mitochondrion-targeted dual-site fluorescent probe (Mito-CDTH-CHO) based on benzopyran for the highly specific detection of SO₃²⁻ and HSO₃⁻ with two diverse emission channels. Mito-CDTH-CHO can discriminatively respond to the levels of HSO₃⁻ and SO₃²⁻. Besides, its advantages of low cytotoxicity, superior biocompatibility and excellent mitochondrial enrichment ability contribute to the detection and observation of the distribution of sulfur dioxide derivatives in living organisms as well as allowing further studies on the physiological functions of sulfur dioxide.

Rational design and sensing mechanism of a dual-site fluorescence probe for HSO₃⁻ and SO₃²⁻.     

A straightforward and sensitive “ON–OFF” fluorescence immunoassay based on silicon-assisted surface enhanced fluorescence

A straightforward immunoassay based on silicon-assisted surface enhanced fluorescence (SEF) has been demonstrated using a silicon-based fluorescent immune substrate and silver-antibody nanoconjugate (SANC). The P-doped, (100) oriented silicon wafers are used for both fluorophore attachment and antigen immobilization. The silicon substrate offers a very low blank signal in the “OFF” state, due to its fluorescence quenching effect. In the detection process, the capture of the SANCs by the surface-immobilized antigen leads to an effectively enhanced fluorescence to produce an “ON” state. The analytical performance of the presented scheme has been investigated and a limit of detection of 31.4 pg mL⁻¹ has been obtained. Besides the broadened application range compared with the conventional immunoassays, the presented scheme is straightforward, cost effective and sensitive, and is hence expected to find widespread applications in immunoassays as well as other fluorescence-based assays.

A straightforward immunoassay based on silicon-assisted surface enhanced fluorescence (SEF) has been demonstrated using a silicon-based fluorescent immune substrate and silver-antibody nanoconjugate (SANC).     

A sensitive BODIPY-based fluorescent probe for detecting endogenous hydroxyl radicals in living cells

The hydroxyl radical (˙OH) has been suggested to play very vital roles in many physiological and pathological processes. However, selective detection of ˙OH is highly challenging owing to its extremely high reactivity and short lifetime. Herein, we designed and synthesized a sensitive “turn on” fluorescent probe for detecting endogenous ˙OH based on a BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) platform. The probe had shown good properties including high sensitively, ideal selectively and low cytotoxicity, and was successfully employed to image endogenous hydroxyl radical in living cells.

A novel “turn-on” NIR fluorescent probe was designed and used for monitoring endogenous hydroxyl radical in living cells, which also showed higher selectivity toward hydroxyl radical over other reactive oxygen/nitrogen species.     

A label-free fluorescent peptide probe for sensitive and selective determination of copper and sulfide ions in aqueous systems

A label free fluorescent peptide probe (HDSGWEVHH) was used for Cu²⁺ and S²⁻ determination in aqueous solution. Our results demonstrated that HDSGWEVHH is highly selective and sensitive for monitoring free Cu²⁺ concentration via quenching of the probe fluorescence upon Cu²⁺ binding. The mechanism of the complexation is investigated with Cyclic Voltammetry (CV), ¹H nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) spectroscopy and computational techniques. Theoretical calculation results indicated the binding ratio of the probe to Cu²⁺ is 2 : 1 and the binding constant was obtained as 1.72 × 10 ⁸ M⁻¹. Cu²⁺ concentration can be detected with the detection limit of 16 nM. Free Cu²⁺ concentration released from the metallothionein–Cu complex at different pH values was detected. Cu²⁺ concentration in real water and tea samples was also detected, and the results were consistent with the ones monitored by atomic absorption spectrometer. Because of the exceedingly small K_sp value of CuS (1.27 × 10⁻³⁶), S²⁻ can sequester Cu²⁺ from HDSGWEVHH to restore the tryptophan (W) fluorescence. Thus the HDSGWEVHH–Cu²⁺ complex can also be used for S²⁻ detection. The S²⁻ concentrations can be monitored with a detection limit of 19 nM. The assay is also amenable to measurement of S²⁻ concentration in pure water samples. Thus the probe designed herein is sensitive, label free, low cost, and environmentally friendly for Cu²⁺ and S²⁻ determination in aqueous solutions.

A label-free fluorescence “on–off–on” peptide probe for selective determination of Cu²⁺ and S²⁻ in a pure water system.     

Development of a fused imidazo[1,2-a]pyridine based fluorescent probe for Fe3+ and Hg2+ in aqueous media and HeLa cells

A new fluorescent sensor 5 based on a fused imidazopyridine scaffold has been designed and synthesized via cascade cyclization. The reaction features the formation of three different C–N bonds in sequence. Imidazopyridine based fluorescent probe 5 exhibits highly sensitive and selective fluorescent sensing for Fe³⁺(‘turn-on’) and Hg²⁺(‘turn-off’). The excellent selectivity of imidazopyridine for Fe³⁺/Hg²⁺ was not hampered in the presence of any of the competing cations. The limit of detection (LOD) of 5 toward Fe³⁺ and Hg²⁺ has been estimated to be 4.0 ppb and 1.0 ppb, respectively, with a good linear relationship (R² = 0.99). Notably, 5 selectively detects Fe³⁺/Hg²⁺ through fluorescence enhancement signalling both in vitro and in HeLa cells.

A new fluorescent sensor 5 having fused imidazopyridine scaffold has been synthesized via cascade cyclization. It exhibits highly sensitive and selective detection of Fe³⁺ (‘turn-on’) and Hg²⁺ (‘turn-off’) in vitro and in HeLa cells.     

A “weak acid and weak base” type fluorescent probe for sensing pH: mechanism and application in living cells

A simple pH fluorescent probe, N-(6-morpholino-1, 3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl) isonicotinamide (NDI), based on naphthalimide as the fluorophore and isonicotinic acid hydrazide as the reaction site was synthesized and characterized. It is useful for monitoring acidic and alkaline pH. The results of pH titration indicated that NDI exhibits obvious emission enhancement with a pK_a of 4.50 and linear response to small pH fluctuations within the acidic range of 3.00–6.50. Interestingly, NDI also displayed strong pH-dependent characteristics with pK_a 9.34 and linearly responded to an alkaline range of 8.30–10.50. The sensing response mechanism was confirmed by ¹H NMR and ESI-MS spectroscopy. The mechanism of the optical responses of NDI toward pH was also determined by density functional theory (DFT) calculations. In addition, NDI displayed a highly selective and sensitive response to hydrogen ions and hydroxyl ions. The probe was successfully applied to image acidic and alkaline pH value fluctuations in HeLa cells and has lysosomal targeting ability.

When the probe was in the protonation process, the fluorescence intensity gradually decreased, whereas when the probe was in the deprotonation process, the fluorescence intensity gradually increased.     

One pot fabrication of fluorescein functionalized manganese dioxide for fluorescence “Turn OFF–ON” sensing of hydrogen peroxide in water and cosmetic samples

In recent decades, H₂O₂ has been promoted as a health indicator because its moderate to high levels can cause some health problems. Herein, we developed a new fluorescent nanoprobe for rapid, selective and sensitive detection of H₂O₂. The fluorescent nanoprobe is composed of fluorescein dye (FLS) as a fluorescent probe and MnO₂ nanosheets (MnO₂ NS) as a quencher. In this study, H₂O₂ can reduce MnO₂ NS in the synthesized composite and release FLS, causing sufficient recovery of fluorescent signal related to the concentration of H₂O₂. The nanoprobe, with λ_ex/λ_em at 495/515 nm, has a linear range of 0.04–30 μM, with a limit of detection (LOD) of 7.5 nM and a limit of quantitation (LOQ) of 21 nM. The mean relative standard deviation (RSD) was 2.6% and the applicability of the method was demonstrated by the determination of H₂O₂ in water and cosmetic samples.

The fluorometric nanoprobe was fabricated via doping of fluorescein dye in MnO₂ nanosheets (FLS/MnO₂ NS) via facile co-precipitation method. It was used for analysis of H₂O₂ in different matrices through liberation of FLS after reduction of MnO₂ NS.     

A channel-structured Eu-based metal–organic framework with a zwitterionic ligand for selectively sensing Fe3+ ions

A novel Eu-based MOF [Eu(IMS1)₂]Cl·4H₂O (1) was successfully constructed based on a semi-rigid zwitterionic 1,3-bis(4-carboxylbenzyl)-imidazolium (IMS1) ligand, featuring a 3-fold interpenetrating dia net structure with a point symbol of 6⁶ and charged permanent micropores. Considering its excellent luminescent property as well as thermal and chemical stability, complex 1 was explored as a potential sensor for detecting Fe³⁺ ions. The results show that complex 1 has a high sensitivity and selectivity for Fe³⁺ based on a ‘turn-off’ effect, for which the electrostatic interaction between Fe³⁺ ions and the inner surface of the micropores may play a critical role. The fluorescence quenching mechanism reveals that dynamic quenching and competitive adsorption between Fe³⁺ and 1 lead to the quenching effect of 1.

A channel-structured Eu-based metal–organic framework with a zwitterionic ligand may serve as a sensor for selectively detecting Fe³⁺ ions.     

Correction: Poly(adenine)-mediated DNA-functionalized gold nanoparticles for sensitive detection of mercury ions in aqueous media

Correction for ‘Poly(adenine)-mediated DNA-functionalized gold nanoparticles for sensitive detection of mercury ions in aqueous media’ by Jinjin Yin et al., RSC Adv., 2019, 9, 18728–18733, DOI: 10.1039/C9RA03041G.

The authors regret that an incorrect grant number was shown in the Acknowledgements section of the published article. The corrected section should read:This work is supported by the Natural Science Foundation of Tianjin [No. 18JCQNJC06000] and the Youth Innovation Foundation of Tianjin University of Science and Technology [No. 2016LG16].The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

An effective biocompatible fluorescent probe for bisulfite detection in aqueous solution,living cells,and mice

Sulfur dioxide, an air pollutant, is easily hydrated to sulfites and bisulfites and extremely harmful to human health. On the other hand, endogenous sulfur dioxide is the fourth gasotransmitter. In view of the above, it is worth developing an effective method for the detection of these compounds. In this paper, a novel colorimetric fluorescent probe (Hcy-Mo), based on hemi-cyanine, for bisulfites is reported. Hcy-Mo shows excellent selectivity for bisulfites over various other species including cysteine, glutathione, CN⁻, and HS⁻, and undergoes 1,4-addition reactions at the C-4 atom of the ethylene group. The reaction can be completed in 30 s in a PBS buffer solution and displays high sensitivity (limit of detection is 80 nM) for bisulfites. Test paper experiments show that the probe can be used for bisulfite detection in aqueous solutions. In addition, Hcy-Mo exhibits excellent cell permeability and low cytotoxicity for the successful detection of bisulfites in living MDA-MB-231 cells and in living mice, implying that this probe would be of great benefit to biological researchers for investigating the detailed biological and pharmacological functions of bisulfites in biological systems.

Sulfur dioxide, an air pollutant, is easily hydrated to sulfites and bisulfites and extremely harmful to human health.     

A highly sensitive and selective fluorescent probe without quencher for detection of Pb2+ ions based on aggregation-caused quenching phenomenon

Lead is a highly toxic heavy metal, and various functional nucleic acid (FNA)-based biosensors have been developed for the detection of Pb²⁺ in environmental monitoring. However, most fluorescence biosensors that have been reported were designed on the basis of a double-labeled (fluorophore and quencher group) DNA sequence, which not only involved an inconvenient organic synthesis but also restricted their wider use in practical applications. Here, we utilized a G-rich DNA sequence as a recognition probe and conjugated fluorene (CF) to develop a fluorescence sensor without a quencher based on the aggregation-caused quenching (ACQ) effect. In the presence of Pb²⁺, the degree of aggregation of CF was reduced because Pb²⁺ induced the formation of a G-quadruplex structure of the CF-DNA probe, and the fluorescence signal increased with the concentration of Pb²⁺ (0–1 μM), with a limit of detection of 0.36 nM. This fluorescent probe without a quencher enables the sensitive and selective detection of Pb²⁺. On the basis of these advantages, the CF-DNA probe represents a promising analytical method for detecting Pb²⁺.

Fluorescent probe with only a fluorophore but no quencher for detecting Pb²⁺ on the basis of the aggregation-caused quenching (ACQ) phenomenon.     

Correction: A colorimetric and far-red fluorescent probe for the highly sensitive detection of silver(i)

Correction for ‘A colorimetric and far-red fluorescent probe for the highly sensitive detection of silver(i)’ by Yong-jun Wang et al., RSC Adv., 2017, 7, 55567–55570.

The published article incorrectly indicates four corresponding authors. The two corresponding authors should be Jin-wu Yan and Lei Zhang. In addition, a link to the equal contribution footnote, confirming that Yong-jun Wang and Jing-gong Liu contributed equally to the work, is missing in the published article. These errors are corrected herein.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

A simple fluorescent probe for detection of Ag+ and Cd2+ and its Cd2+ complex for sequential recognition of S2−

In this study, we designed and synthesized a simple probe 2-(8-((8-methoxyquinolin-2-yl)methoxy)quinolin-2-yl)benzo[d]thiazole (DQT) for detection of Ag⁺ and Cd²⁺ in a CH₃OH/HEPES (9 : 1 v/v, pH = 7.30) buffer system. Its structure was characterized by NMR, ESI-HR-MS and DFT calculations, and its fluorescence performance was also investigated. Probe DQT showed fluorescence quenching in response to Ag⁺ and Cd²⁺ with low detection limits of 0.42 μM and 0.26 μM, respectively. Importantly, the complexation of the probe with Cd²⁺ resulted in a red shift from blue to green, making it possible to detect Ag⁺ and Cd²⁺ by the naked eye under an ultraviolet lamp. The DQT-Cd²⁺ complex could be used for sequential recognition of S²⁻. The recovery response could be repeated 3 times by alternate addition of Cd²⁺ and S²⁻. A filter paper strip test further demonstrated the potential of probe DQT as a convenient and rapid assay.

A fluorescent probe for detection of Ag⁺ and Cd²⁺ and its Cd²⁺ complex for sequential recognition of S²⁻.