Turn-on Sensor Research Articles

Poly-l-lysine-Functionalized Green-Light-Emitting Carbon Dots as a Fluorescence Turn-on Sensor for Ultrasensitive Detection of Endotoxin.

Herein, we report a facile, ultrasensitive, and selective fluorescence turn-on sensing strategy based on green-light-emitting functional nanodots for the detection of bacterial lipopolysaccharide (LPS) endotoxin. In this protocol, first, the pure carbon dots (CDs) with a fairly high quantum yield were prepared by microwave-assisted pyrolysis of citric acid in the presence of urea. Subsequently, the carboxyl-group-rich surfaces of the CDs were allowed to conjugate with the poly-l-lysine (PLL) using an EDC-NHS amidization method to obtain the PLL-modified CDs (PLL-CDs). The LPS could specifically bind to the PLL at the PLL-CD surfaces, and this binding enabled an electron transfer from the phosphate groups of LPS to the carbon core through the PLL bridge, thus resulting in a fluorescence enhancement. Interestingly, this fluorescent turn-on sensor provided a detection limit of 68.3 fM in PBS (pH 7.4), which is the lowest ever reported among all of the synthetic assays for LPS detection. Furthermore, our fluorescent probe was able to show a remarkable selectivity toward LPS over a range of commonly known interfering substances. Thus, this study demonstrated the feasibility of using specific LPS binding to PLL to drive molecular recognition in aqueous medium and offered an effective fluorescence turn-on sensing strategy to detect bacterial endotoxin in diverse clinical and biological applications.

Ultrafast Size Expansion and Turn-On Luminescence of Atomically Precise Silver Clusters by Hydrogen Sulfide.

The formation of high-nuclearity silver(I) clusters remains elusive and their potential applications are still underdeveloped. Herein, we firstly prepared a chain-like thiolated AgI complex {[Ag18 (St Bu)10 (NO3 )8 (CH3 CN)2 (H2 O)2 ] ⋅ [Ag18 (St Bu)10 (NO3 )8 (CH3 CN)6 ]}n (abbreviated as Ag18 ) in which two similar Ag18 clusters are assembled by NO3 - anions. The solution containing Ag18 reacted with hydrogen sulfide with controlled concentration, promptly producing another identifiable and bright red-emitting high-nuclearity silver(I) cluster, Ag62 (S)13 (St Bu)32 (NO3 )4 (abbreviated as Ag62 ). We tracked the transformation using time-dependent electrospray ionization mass spectrometry (ESI-MS), UV/Vis absorption and photoluminescence spectra. Based on this cluster transformation, we further developed an ultra-sensitive turn-on sensor detecting H2 S gas with an ultrafast response time (30 s) at a low detection limit (0.13 ppm). This work opens a new way of understanding the growth of metal clusters and developing their luminescent sensing applications.

Blue/yellow emissive carbon dots coupled with curcumin: a hybrid sensor toward fluorescence turn-on detection of fluoride ion

Trace detection of fluoride ion has gained increasing attention due to fluoride’s close association with biological and environmental processes. Herein, we construct a novel hybrid nanosystem consisting of carbon dots and curcumin for sensitive and selective sensing of F–. Carbon dots are synthesized by hydrothermal treatment of 2,3-diaminopyridine and selenourea in hydrochloric acid. This material is employed as the fluorescent indicator that exhibits intense blue and yellow emission with quantum yields of 12% and 33%, respectively. Curcumin, possessing an absorption peak at 532 nm, can significantly quench the yellow fluorescence of carbon dots through inner filter effect. Curcumin is also used to specifically recognize F–. When F– is added, the curcumin-F– complex generates, which leads to the hypochromatic shift of the absorption band from 532 to 430 nm. In such a case, the inner filter effect reduces, and yellow fluorescence of carbon dots recovers. Thus, a fluorescence turn-on sensor of F– is built based on the carbon dots/curcumin system. The limits of detection and quantitation are measured to be 0.39 and 1.30 μM, respectively. For real usage, the proposed method is applied to determinate F– in tap water and milk samples with relative standard deviations below 7.9%.

Design of turn-on luminescent sensor based on nanostructured molecularly imprinted polymer-coated zirconium metal–organic framework for selective detection of chloramphenicol residues in milk and honey

Herein, an optical sensor based on nanostructured molecularly imprinted polymer (MIP) coated on a luminescent zirconium metal–organic framework (MIP/Zr-LMOF) is introduced, and its performance is investigated for the fluorescent determination of chloramphenicol (CAP) antibiotic residues in milk and honey. To fabricate the sensor, the surface of Zr-LMOF is modified with MIP in the presence of CAP template, resulting in the introduction of recognition sites for antibiotic molecules. The porous structure of Zr-LMOF with specific binding sites for CAP recognition benefiting from coated MIP leads to selective and sensitive detection of antibiotic. The probe yields a linear range for detection of CAP in trace concentrations (0.16–161.56 µg.L−1) and provides a detection limit of 0.013 µg.L−1. Acceptable recoveries are achieved for antibiotic in real samples, which are consistent with that obtained from liquid chromatography–tandem mass spectrometry (LC-MS/MS), confirm the favorable performance of sensor for accurate determination of CAP in practical applications.

A hemicyanine based fluorescence turn-on sensor for amyloid fibril detection in the far-red region

The designing or identification of extrinsic fluorescence probes for the sensitive and selective detection of amyloid fibrils, which can emit in the red to near infrared region, is an extremely important area of research. Long wavelength emission offers various crucial advantages such as preventing auto-fluorescence and photo-damage of specimen, reduced scattering, large penetration depth, etc. Herein, we have identified a hemicyanine based organic dye molecule, LDS722, which displays an impressive fluorescence turn-on emission (~200 fold increase), in the near infrared region of the spectrum, in presence of insulin fibrils. LDS722 also displays a very high selectivity to bind to the fibrillar form of insulin as opposed to its native conformation. Remarkably, the probe also registers a very large Stokes' shift of ~170 nm and a huge red shift of ~120 nm in the excitation spectra. The significant shift in the absorbance maximum of the fibril-bound probe, from its initial free form, results in the visual colour change facilitating naked eye detection of the fibril positive solution. LDS722 could also efficiently monitor the kinetic progression of the fibrillation process without any interference. Importantly, LDS722 also shows a good performance in the human serum matrix with a LOD of 55 nM.

Porphyrin-Based Metal-Organic Framework Probe: Highly Selective and Sensitive Fluorescent Turn-On Sensor for M3+ (Al3+, Cr3+, and Fe3+) Ions.

The development of porphyrin-based metal-organic frameworks (MOFs) has attracted significant interest in the scientific community in recent years because of their versatile applications particularly in optical and electronic fields. In this study, a highly selective and sensitive fluorescent turn-on sensor using a porphyrinic MOF, Tb-TCPP, is presented, which displays a 10-fold fluorescence enhancement in the presence of Al3+, Cr3+, and Fe3+ ions. The detection limit is in the nM region. For the Al3+ ion, it could be visually detected at concentrations as low as 5 mM within 15 min. Tb-TCPP could also be used as an indicator for acidic or alkaline solutions at pH values of >9 and <3. The studies on the detection mechanism illustrate that cation exchange proceed between Tb-TCPP and these M3+ ions, and consequently, energy transfer from TCPP to Tb3+ is suppressed and π*-π energy transfer of the porphyrin ligand is significantly enhanced.

A fluorescence red-shift and turn-on sensor for acetylacetone derived from ZnII-based metal–organic framework with new topology

JXUST-4 with a rare (3,3,7)-c topology net has been prepared, acting as a fluorescent sensor for acac through obvious fluorescence red-shift and enhancement with excellent properties.

Ultrasensitive and turn-on homogeneous Hg2+ sensing based on a target-triggered isothermal cycling reaction and dsDNA-templated copper nanoparticles.

In this work, an ultrasensitive and turn-on sensor for homogeneous Hg2+ detection has been constructed based on a target-triggered isothermal cycling reaction and rapid label-free signal output with dsDNA-templated copper nanoparticles (CuNPs). As the key component of the sensor, a hairpin DNA without any labels was designed to contain different functional sequence segments and to resist digestion by exonuclease due to the protruding 3'-terminus. In the presence of Hg2+, the formation of a T-Hg2+-T structure turned the protruding 3'-terminus of the hairpin DNA to a blunt end that could be efficiently digested by Exo III, accompanied by Hg2+ release, followed by another digestion cycle. Hence, the Hg2+-triggered isothermal cycling reaction accumulated numerous dsDNA templates that facilitated fluorescent CuNP generation and finally output an amplified signal used to identify the target. This protocol is capable of Hg2+ sensing in a concentration range of 5 orders of magnitude with a detection limit down to 3.9 pM. The as-constructed sensor also revealed high selectivity, as well as satisfactory results in recovery experiments of Hg2+ detection in real water samples.

Effective assay of bacterial transglycosylation by molecular turn-on sensing and a second-order scattering effect

Instead of using the lipid II substrate that requires prior labelling with a radioactive isotope or fluorophore to probe the formation of peptidoglycan in bacterial transglycosylation, the released undecaprenyl pyrophosphate (UPP) product is quantitatively measured either using a terpyridine-zinc fluorescence turn-on sensor or simply by the second-order scattering effect of the in situ formed UPP-calcium complex. Both the assay methods are utilized to identify moenomycin A as a potent transglycosylase inhibitor with a consistent IC50 value.

Rational design of an HClO-specific triggered self-immolative fluorescent turn-on sensor and its bioimaging applications

The development of a rapid and intuitive method for the detection of a specific small molecule biomarker is important for understanding the pathogenesis of relevant diseases. Described here is the design and evaluation of an HClO-specific triggered self-immolative fluorescent sensor (RESClO) based on the structure of an N-protected Resorufin dye. Due to the interrupted π-conjugated structure of the Resorufin dye, the free sensor showed very weak absorption and fluorescence. It can quickly complete the response to HClO (within 10 s) with high selectivity and sensitivity (LOD = 16.8 nM) in aqueous solution. The sensor can be made into test strips to quickly detect HClO in the environment by obvious changes in color and fluorescence. It was successfully used for bioimaging of exogenous and endogenous HClO in cells and zebra fish. More importantly, it can also be used for visual imaging of mouse arthritis models. Thus, sensor RESClO can provide a simple and promising visual analytical tool for the detection of HClO in the environment and the early diagnosis of HClO-mediated related diseases.

MOF-on-MOF Membrane with Cascading Functionality for Capturing Dichromate Ions and p-Arsanilic Acid Turn-On Sensing.

It is very significant that functional porous metal-organic frameworks are used to manufacture hierarchical components to achieve cascading functions that cannot be achieved by a single-layer metal-organic framework (MOF). Here, we report two cases of novel MOFs constructed by the same ligand, Cu(I)-tpt and Cu(II)-tpt (Htpt = 5-[4(1H-1,2,4-triazol-1-yl)]phenyl-2H-tetrazole), and prepared a Cu(II)-tpt-on-Cu(I)-tpt membrane by a layer-by-layer approach ignoring the lattice mismatch problem. The first Cu(I)-tpt layer is grown on an oriented Cu2O nanostructured array by a "one-pot" approach. The aligned second Cu(II)-tpt layer can be deposited using liquid-phase epitaxy. Notably, the prepared Cu(II)-tpt-on-Cu(I)-tpt membrane combines adsorption and fluorescence sensing, which exhibited significant adsorption for Cr2O72- (203.25 mg g-1) as typical highly poisonous ions with a fluorescence quenching response. Hence, based on the oxidation-reduction between Cr2O72- and p-arsanilic acid (p-ASA), the Cu(II)-tpt-on-Cu(I)-tpt membrane's ability to adsorb Cr2O72- could be used to design "on-off-on" mode fluorescence probes to detect p-ASA with high sensitivity (limit of detection (LOD) = 0.0556 μg L-1). p-ASA can be degraded into highly toxic inorganic arsenic compounds in the natural environment and has received widespread attention. Therefore, the integration of adsorption and fluorescence properties makes the Cu(II)-tpt-on-Cu(I)-tpt membrane a feasible multifunctional material for pollution control and detection.

Theoretical Investigations on the Excited-State Dynamics of an Al3+ Fluorescence Sensor.

Twisted internal charge transfer (TICT) states are of fundamental importance during the photo-physical processes of dyes and sensors. In this contribution, excited-state dynamics of an Al3+ fluorescence sensor 1-{[(2-hydroxyphenyl)-imino]methyl}naphthalen-2-ol based on the turn-on signal is clarified. Two different dark TICT states are observed by exploring the excited-state potential energy surface. With the twist of the C2-N bond, the two dark states can be reached facilely, which induce the experimentally observed weak fluorescence of the sensor. The sensing mechanism is then uncovered by investigating the electronic coupling between the sensor and analyte. Al3+ is proved to form strong coordination bonds with the sensor, which restricts the motion of the C2-N bond. Consequently, the TICT states are eliminated, which generate the turn-on signal. This sensing mechanism is trustworthy and intrinsically different from the previously proposed one, which would shed some light on the design of turn-on sensors.

Dual-Emission Zr-MOF-Based Composite Material as a Fluorescence Turn-On Sensor for the Ultrasensitive Detection of Al3.

In this work, a novel zirconium-based metal-organic framework (MOF) composite material, UiO-(OH)2@RhB, has been solvothermally prepared with zirconyl chloride octahydrate, 2,5-dihydroxyterephthalic acid, and rhodamine B (RhB) for ratiometric fluorescence sensing of Al3+ ions in an aqueous medium. The luminescence measurement results showed that, at the single excitation wavelength of 420 nm, the fluorescence intensity of the ligand at 500 nm increased significantly in the case of Al3+, while that of RhB at 583 nm changed slightly, together with an apparent color change. Under optimal conditions, UiO-(OH)2@RhB exhibited an extraordinary sensitivity (10 nM), good selectivity, and a fast response (2 min) for Al3+. As far as we know, the limit of detection is superior to that of the current reported MOF-based Al3+ fluorescence sensors. The response mechanism suggested that -OH could capture Al3+ in water through coordination and high electrostatic affinity and achieved turn-on ratiometric fluorescence through the excited-state intramolecular proton transfer process and stable fluorescence of RhB. In addition, this sensor was also applied to actual food samples (grain beans), with the recoveries ranging from 89.08% to 113.61%. Such a turn-on ratiometric fluorescence sensor will provide a constructive strategy for the ultrasensitive detection of Al3+ in practical applications.

Novel paper- and fiber optic-based fluorescent sensor for glucose detection using aniline-functionalized graphene quantum dots

Aniline functionalized graphene quantum dots (a-GQDs) is synthesized by microwave-assisted pyrolysis of fructose. After a-GQDs are modulated using phenyl boric acid (PBA), resulting in fluorescence quenching, the a-GQDs/PBA system are employed as a fluorescent turn-on sensor for the glucose detection by the specific reaction of PBA with glucose. The prepared fluorescent sensor using a-GQDs/PBA exhibits high sensitivity and selectivity toward glucose. In addition, the paper-based fluorescent sensor is prepared for portable sensing devices by printing the a-GQDs ink with a commercial inkjet printer, demonstrating a superior visual glucose detection and a good sensitive performance for the detection of glucose in a blood serum and tear samples. Furthermore, the a-GQDs/hydrogel film is evaluated for a wearable sensor as well as fiber optic-based sensor for highly sensitive and fast on-site glucose detection. The prepared fiber optic sensor exhibits remarkable glucose sensing performance with a fast response, a low detection limit of 2.1 μM and excellent recyclability. Finally, the a-GQD/PBA system exhibits excellent biocompatibility and was applied to detect glucose in HeLa cells with satisfactory results. These results provide a novel approach for developing low-cost and flexible sensors for the portable and visual detection of glucose with high sensitivity and selectivity.

Host-guest co-assembly triggered turn-on and ratiometric sensing of berberine and its detoxicating

Fluorescent sensing for specific detection of berberine is an important issue in view of its potential jeopardization to food safety and human health, but remains less investigated. To the best of our knowledge, there is no fluorescence turn-on and ratiometric sensors available for specific detection of berberine. In this study, calix[4]carbazole (3) has been synthesized and its property of recognizing berberine has been evaluated by UV–vis, fluorescence, NMR, DLS and TEM techniques. The results show that 3 selectively recognizes berberine among the tested drugs and detects it with turn-on and ratiometric fluorescence due to their co-assembly nature. Moreover, 3 is not only low toxic and can reduce toxicity of berberine to human normal liver L02 cell, but also can release berberine to tumor HepG2 cells at acid micro-environment. It therefore holds a great potential for further exploration

A novel supramolecule-based fluorescence turn-on and ratiometric sensor for highly selective detection of glutathione over cystein and homocystein.

Acyclodextrin-based fluorescence light-up and ratiometric sensor is reportedfor highly selective and sensitive recognition of glutathione over cystein and homocystein. The sensing scheme developed builds up on a supramolecular assembly formed between a molecular rotor dye (ThT) and a polyanionic supramolecular host (sulfated-β-cyclodextrin, SCD). The detection scheme is accomplished as follows: firstly, the bivalent Cu2+ quenches the emission from ThT-SCD assembly by causing the dissociation of ThT molecules from SCD surface. Secondly, when GSH is added to the copper-quenched system, owing to specific interaction between Cu2+ and GSH, Cu2+ is removed from the SCD which again allows the formation of ThT-SCD assembly. Indeed, this scheme of disassembly and reassembly successivelycaused by Cu2+ and GSH in the aqueous solution empowers our sensor framework to work as a good ratiometric sensor for the detection of GSH. The sensor scheme shows a linear response in the range 0-250μM with a LOD of 2.4 ± 0.2μM in aqueous solution and 13.6 ± 0.5μM in diluted human serum sample. The sensor system is excited at 410nm and the emission signal is plotted as a ratio of intensity at 545nm (aggregate band) and 490nm (monomer band). This ratiometric sensor system is highly selective to glutathione over cystein, homocystein, and other amino acids. Additionally, response of the sensor system towards GSH in complex biological media of serum samples demonstrates its potential for practical utility. Graphical abstract.

Benzoindole-based bifunctional ratiometric turn-on sensor with an ICT effect for trapping of H+ and Al3+ in dual-channel cell imaging and samples.

Abnormal changes in H+ and Al3+ concentrations in living cells can alter neurological diseases. A small-molecule sensor combined with a fluorescence imaging technique holds great promise for monitoring changes in proton and metal-ion concentration. In this work, a bifunctional ratiometric naked-eye fluorescence sensor (BIBC) was developed for turn-on detection of H+ and Al3+ in H2O/EtOH (v/v=1:1) mixtures. BIBC exhibits a pKa value of 4.58 within a linear pH variation from 4.1 to 4.7 (R2=0.9939). Moreover, the fluorescence intensity ratio (I566 nm/I524 nm) shows a good linear relationship (R2=0.9965) within an Al3+ concentration range of 7.0-10.0μM. The detection limit (DL) for the sensor was calculated to be 1.58μM. The practical application of BIBC for Al3+ detection in real samples was further discussed, and satisfactory results were obtained. Furthermore, the sensor was applied to real-time visualization of changes in H+ and Al3+ concentration in living cells, with great photostability and low cytotoxicity observed. Fluorescence images of H+ and Al3+ were collected by using a fluorescence microscope in a dual-channel configuration, wherein they were labeled green and yellow, respectively.

Synthesis, application and AIE properties of novel fluorescent tetraoxocalix[2]arene[2]triazine: The detection of a hazardous anion, cyanate

A highly effective, new heterocalixarene fluorescent receptor comprised of 2-(2-aminophenyl)benzothiazole and tetraoxacalix[2]arene[2]triazine was designed and synthesized by one-step reaction. The sensor candidate exhibiting aggregation induced emission (AIE) was tested for its photophysical behaviour towards detection of various anions. The results showed that our receptor undergo AIE in >40% H2O-DMSO along with large pseudo Stokes shift (Δλ = 219 nm) and exhibit selective and sensitive detection towards hazardous cyanide’s oxidation product, cyanate (CNO-) ion over other tested anions. The blue-shifted fluorescence emission (λem = 492 nm) enhancement with large Stokes shift (Δλ = 144 nm) was observed with the increase in cyanate concentration. The synthesized turn-on sensor towards cyanate detection could be applied in real sample analyses as an improvement to the method currently carried out by international standards and hereby a different approach has been made for the detection of cyanide through its oxidation form, cyanate.

A chromogenic and fluorescence turn-on sensor for the selective and sensitive recognition of Al3+ ions – A new approach by Schiff base derivative as probe

In this present study, A newly designed and synthesized probe specifically (E)-1-(((2-(phenylthio)phenyl)imino)methyl)naphthalen-2-ol (NAPTA) as efficient chemosensor was portrayed through the utilization of various spectroscopic techniques like 1H, 13C NMR and ESI-MS. The probe exhibits fantastically specific and sensitive towards Al3+ ions than the other contending cationic metal ions. This designed probe indicates chromogenic and turn-on fluorescence behavior which may be noticeable through naked-eye and UV light individually. The 1:1 binding interaction of NAPTA with Al3+ ions was revealed from the Job’s plot investigation. The limit of detection was determined as 13x 10-7 M which is very lower level than the permitted level. The DFT figuring and calculations have furthermore upheld to the proposed mechanism as CHEF and ICT from the acquired spectroscopic outcomes.

Highly selective and sensitive chemosensor for Al(III) based on isoquinoline Schiff base.

As a colorimetric and fluorescent turn-on sensor to Al3+, N'-(2-hydroxybenzylidene)isoquinoline-3-carbohydrazide (HL) has been easily synthesized. The fluorescence intensity increases by 273 times in the presence of Al3+ at 458nm. Meanwhile, the experiment data indicate that the limit of detection for Al3+ is 1.11×10-9M. Remarkably, the blue fluorescence signal of HL-Al3+ could be specially observed by the naked eye under UV light and is significantly different from those of other metal ions. Fluorescence switch based on the control of Al3+ and EDTA proved HL could act as a reversible chemosensor. According to ESI-MS result and the Job's plots, the 2:1 coordination complex formed by HL and Al3+ could be produced. Density functional theory calculations were performed to illustrate the structures of HL and complex. The cell imaging experiment indicates that HL can be applied for monitoring intracellular Al3+ levels in cells.