Most optical sensors for molecular oxygen were developed based on the quenching effect of the luminescence of oxygen-sensitive probes; however, the signal turn-off mode of these probes is undesirable to quantify and visualize molecular oxygen. Herein, we report a novel luminescence turn-on detection strategy for molecular oxygen via the specific oxygen-triggered bonding-induced emission of thiol-functionalized gold nanoclusters. Thiol-functionalized gold nanoclusters were prepared by a facile one-step synthesis, and as-prepared gold nanoclusters possess significant aggregation-induced emission (AIE) property. It is the first time to discover the oxygen-triggered bonding-induced emission (BIE) behavior of gold nanoclusters, which results in disulfide-linked covalent bonding assemblies with intensely red luminescence. This specific redox-triggered BIE is capable of quantitatively detecting dissolved oxygen in aqueous solution in a light-up manner, and trace amount of dissolved oxygen at ppb level is achieved based on this detection method. A facile and convenient test strip for oxygen detection was also developed to monitor molecular oxygen in a gas matrix. Covalent bonding-induced emission is proven to be a more efficient way to attain high brightness of AIEgens than a physical aggregation-induced emission process, and provides a more convenient and desirable detection method for molecular oxygen than the previous sensors.Keywords: bonding-induced emission; disulfide; gold nanoclusters; oxygen detection; test strip;

Unique aggregation-induced emission (AIE) property has been found and widely applied in chemo/biosensors for thiolated gold nanoclusters and copper nanoclusters; however, little is known about this property of thiolate-protected silver nanoclusters. In this work, specific aggregation-induced emission enhancement (AIEE) of glutathione-capped silver nanoclusters (AgNCs) was verified via its solid-state luminescence and enhanced emission in poor solvent, three stimuli responsive nanoswitches were constructed based on its AIEE property, and a reliable and sensitive PPase assay was developed via ion-triggered luminescence switch. Glutathione-capped AgNCs from a facile one-pot synthesis were found to possess bright red luminescence and aggregation-induced emission enhancement property. This AIEE feature enables AgNCs in sensitive response to pH and temperature in a reversible way, allowing the two nanoswitches to precisely monitor the change of environmental pH and temperature. Complexation reactions among AgNCs, aluminum cation and PPi were also designed for an ion-triggered luminescence nanoswitch, which allows selective response to aluminum cation or PPi in luminescence. This ion-driven luminescence switch is further utilized to design a novel detection strategy for PPase activity through competitive coordination reactions. Our method illustrates a novel detection strategy mediated by complexation reaction between Al3+ and AgNCs avoiding the involvement of copper cations in the detection, and this developed assay performed well in detection of PPase level in fresh rat serum. This work confirms unique aggregation-induced emission enhancement property of glutathione-capped AgNCs, constructs multiple luminescence switches based on its multistimuli responsive behaviors, and demonstrates an example of Al3+-mediated detection strategy for PPase assay.

A reversible luminescence nanoswitch through competitive hydrophobic interaction among copper nanoclusters, p-nitrophenol and α-cyclodextrin is established, and a reliable real-time luminescent assay for acid phosphatase (ACP) activity is developed on the basis of this luminescence nanoswitch. Stable and intensely luminescent copper nanoclusters (CuNCs) were synthesized via a green one-pot approach. The hydrophobic nature of CuNCs aggregate surface is identified, and further used to drive the adsorption of p-nitrophenol on the surface of CuNCs aggregate due to their hydrophobic interaction. This close contact switches off the luminescence of CuNCs aggregate through static quenching mechanism. However, the introduction of α-cyclodextrin switches on the luminescence since stronger host–guest interaction between α-cyclodextrin and p-nitrophenol causes the removal of p-nitrophenol from the surface of CuNCs. This nanoswitch in response to external stimulus p-nitrophenol or α-cyclodextrin can be run in a reversible way. Luminescence quenching by p-nitrophenol is further utilized to develop ACP assay using p-nitrophenyl phosphate ester as the substrate. Quantitative measurement of ACP level with a low detection limit of 1.3 U/L was achieved based on this specific detection strategy. This work reports a luminescence nanoswitch mediated by hydrophobic interaction, and provides a sensitive detection method for ACP level which is capable for practical detection in human serum and seminal plasma.

Thiolate-protected copper nanoclusters (CuNCs) with aggregation-induced emission (AIE) have emerged as a novel kind of luminescent material, but their applications in neutral solution are greatly limited by their ultra-low emission efficiency under such conditions. Herein, we report a facile synthesis of glutathione-protected CuNCs with AIE properties, and their self-assembly was driven by aluminum cations. The bright red luminescence of solid CuNCs illustrates their significant AIE nature, but CuNCs dispersed in neutral water exhibit almost no luminescence. It was found that aluminum cations were capable of driving the self-assembly of the CuNCs, and the resulting CuNC dots with controllable sizes retain their bright luminescence under neutral conditions. The strong affinity of GSH ligands to the CuNCs contributes to this good stability in neutral and even in weakly alkaline solutions. The stable existence of the luminescent CuNC dots enables them to function as a luminogen under physiological conditions. This ability was employed to detect β-galactosidase activity using 4-nitrophenyl-β-D-galactopyranoside as the substrate. The strong quenching effect of the CuNC dots by p-nitrophenol was used to achieve a sensitive measurement of the β-Gal level. This work proposes the preparation of CuNC dots with bright luminescence in neutral solution via the self-assembly of GSH-capped CuNCs by aluminum ions, and demonstrates their sensing application in the detection of β-galactosidase activity under physiological conditions.

A novel bonding-induced emission (BIE) phenomenon of silyl-protected copper nanoclusters was observed and identified, and a new detection method for trace water in organic solvents was established based on a water-triggered BIE process. This assay employs simple and commercially available reagents and is capable of determining trace water at the ppm level.

•Multi-stimuli responsive copper nanoclusters were prepared by a facile one-pot synthesis.•Two excellently reversible nanoswitches driven by pH and temperature were designed.•A luminescent assay for acid phosphatase was developed based on redox-controlled switch.Thiolate-protected copper nanoclusers (CuNCs) are emerging as a promising class of luminescent materials since its unique optical properties such as aggregation-induced emission (AIE) and intriguing molecular-like behavior have been explored for sensing application. In this work, multi-stimuli responsive property of CuNCs was first investigated in depth and further adopted to develop a reliable and sensitive ACP assay. Penicilamine-capped CuNCs from a facile one-pot synthesis possess bright red luminescence and distinctive multi-stimuli responsive behaviors. Its sensitive and reversible response in luminescence to pH and temperature is originated from its inherent AIE property, and can be constructed as luminescent nanoswitches controlled by these external stimuli for precisely monitoring the change of environmental pH or temperature. The specific redox-responsive behavior of CuNC aggregates is found from severe luminescence quenching in the presence of a small amount of ferric or silver ions, and this sensitive response in luminescence to the preceding species is proved to be due to the conversion of Cu(II) from copper atoms with lower valence inside CuNCs. The luminescence switch of CuNC aggregates controlled by specific external potentials is further utilized to design a novel detection strategy for ACP activity. The great difference in luminescence quenching of CuNCs induced by iron(III) pyrophosphate (FePPi2) complex and free ferric ions enables us to quantitatively monitor ACP level by the luminescence change as variation of ACP activity in the assay solution. This assay is able to detect ACP level as lower as 0.8 U/L, and covers a broad linear scope of 100.0 U/L. This work reports redox-responsive property of CuNCs and its underlying nature due to the oxidation of its interior copper atoms, and provides a sensitive assay method for ACP activity which is sufficiently sensitive for practical measurement in real samples.Download high-res image (417KB)Download full-size image

•Glutathione-stabilized fluorescent Cu NCs are prepared by a green one-pot method.•The Cu NCs show high quantum yield, ultrafine size and excellent photostability.•The Cu NCs are utilized as sensitive and selective fluorescent probe for sensing Fe3+ in real water samples.Herein, we reported a simple one-pot green method for preparation of fluorescent copper nanoclusters (Cu NCs) with a high quantum yield up to 8.6%, using glutathione as the stabilizing agent. The as-prepared Cu NCs had good water solubility, excellent stability and ultrafine size through the characterization of UV–vis absorption spectroscopy, fluorescence spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. Experimental results showed that the fluorescence of Cu NCs was linearly quenched by the Fe3+ concentrations in the range of 1–100 μM, with the detection limit of 0.3 μM. The as-formed Cu NCs were explored for the assay of Fe3+ in real water samples.

Dynamic covalent B–O bonds are introduced to the design of a reversible fluorescence nanoswitch in response to the external stimulus of pH. This nanoswitch is based on a phenylboronic acid functionalized carbon quantum dot (PBA–CQD) nanoprobe, and constructed with reference to the two facts that the PBA–CQD probe can bind to p-nitrophenol to form a non-fluorescent conjugate via B–O bonds, and that the making and breaking of the B–O bonds between them can be controlled by pH changes. Excellent reversibility of this nanoswitch is illustrated by switching of the pH from 4 to 8. The reaction between the PBA–CQD nanoprobe and the p-nitrophenol resulting in switching off the fluorescence is further utilized to design a general detection strategy for enzyme activity when substrates that can generate p-nitrophenol through enzymatic reactions are chosen. The feasibility of the detection strategy is qualitatively assessed using α-glucosidase and β-galactosidase, and its practicability to quantitatively monitor enzyme activity is also demonstrated by taking α-glucosidase as an example. The detection limit of this method can be as low as 0.33 U L−1, which is much lower than those reported previously and is sufficiently low to be capable of α-glucosidase level detection in practical human samples. This study demonstrates excellent usability of dynamic covalent B–O bonds in the design of reversible switches and in the general detection of enzyme activity, and provides a sensitive, real-time assay for α-glucosidase based on carbon quantum dots.

Brightly luminescent copper nanoclusters (CuNCs) were prepared via a facile one-step synthesis in organic phase, and a novel luminescent nanoswitch on the basis of CuNCs through alternation of the physical states between aggregation and dispersion in response to specific external stimuli was designed. Two states including aggregation state and disaggregation state corresponding to fluorescence on and off signaling can be readily switched in a reversible way based on the aggregation-induced emission and disaggregation-induced quenching mechanism, respectively. This reversible nanoswitch can be controlled by the external stimulus water or N,N′-dicyclohexylcarbodiimide (DCC). The bright luminescence due to aggregation of CuNCs in organic solvents can be effectively quenched by the introduction of a small amount of water, where a disaggregation-induced quenching takes place. This specific behavior is capable to quantify an ultralow level (ppm) of water in aprotic solvents. The excellent reversibility of the nanoswitch enables one to monitor water content in a continuous and recyclable way.

•A fluorescent dual-channel chemosensor with BPMA-CQDs nanoprobe was constructed.•It can separately assay copper and sliver ions with high sensitivity.•It can discriminatively detect GSH from its analogues with high selectivity.•It was utilized to monitor GSH level in live cells with good biocompatibility.A convenient, fluorescent dual-channel chemosensor on the basis of bis(3-pyridylmethyl)amine-functionalized carbon quantum dots (BPMA-CQDs) nanoprobe was constructed, and it can discriminatively detect glutathione from its analogues cysteine and homocysteine based on two distinctive strategies. Two distinct fluorescence responses of BPMA-CQDs probe to Cu(II) and Ag(I) were identified and further employed to achieve selective detection of Cu(II) and Ag(I) respectively. Based on the BPMA-CQDs/Cu(II) conjugate, discriminative detection of GSH was achieved in terms of correlation between the amounts of GSH and fluorescence recovery. The addition of GSH into BPMA-CQDs/Cu(II) system induces the reduction of Cu(II) to Cu(I), which could efficiently block PET process resulting in the following fluorescence recovery. Based on the BPMA-CQDs/Ag(I) conjugate, GSH assay could also be established on the basis of fluorescence response to GSH. The introduction of GSH into the preceding system triggers the competitive coordination to Ag(I) between BPMA and GSH, and silver ions are finally taken away by GSH from the probe, where the fluorescence is restored to its original weak state. Both of the detection strategies can achieve discriminative detection of GSH from Cys and Hcy. The assays showed good stability and repeatability, and covered a broad linear range of up to 13.3 μM with a lowest detection limit of 42.0 nM. Moreover, both of them were utilized to monitor GSH level in live cells.

Sensitive assay of tyrosinase (TYR) activity is in urgent demand for both fundamental research and practical application, but the exploration of functional materials with good biocompatibility for its activity evaluation at the intracellular level is still challenging until now. In this work, we develop a convenient and real-time assay with high sensitivity for TYR activity/level monitoring and its inhibitor screening based on biocompatible dopamine functionalized carbon quantum dots (Dopa-CQDs). Dopamine with redox property was functionalized on the surface of carbon quantum dots to construct a Dopa-CQDs conjugate with strong bluish green fluorescence. When the dopamine moiety in Dopa-CQDs conjugate was oxidized to a dopaquinone derivative under specific catalysis of TYR, an intraparticle photoinduced electron transfer (PET) process between CQDs and dopaquinone moiety took place, and then the fluorescence of the conjugate could be quenched simultaneously. Quantitative evaluation of TYR activity was established in terms of the relationship between fluorescence quenching efficiency and TYR activity. The assay covered a broad linear range of up to 800 U/L with a low detection limit of 7.0 U/L. Arbutin, a typical inhibitor of TYR, was chosen as an example to assess its function of inhibitor screening, and positive results were observed that fluorescence quenching extent of the probe was reduced in the presence of arbutin. It is also demonstrated that Dopa-CQD conjugate possesses excellent biocompatibility, and can sensitively monitor intracellular tyrosinase level in melanoma cells and intracellular pH changes in living cells, which provides great potential in application of TYR/pH-associated disease monitoring and medical diagnostics.Keywords: carbon quantum dots; fluorometric assay; inhibitor screening; intracellular imaging; tyrosinase

A Cu-catalyzed divergent hydroboration of thioacetylenes has been achieved, providing (Z)-1-thio- or (Z)-2-thio-1-alkenyl boronates in moderate to high yields with excellent regio- and stereoselectivity, by using pinacolborane or bis(pinacolato)diboron as the hydroborating reagents, respectively. DFT calculations indicate that the sulfur atom plays a key role in determining the regioselectivity through polarizing the C–C triple bonds and participating in the HOMO orbitals. Moreover, the SR group can serve as a good leaving group, resulting in the concise synthesis of six regio- and stereoisomers of trisubstituted alkenes 5 via the iterative cross-coupling of C–B and C–S bonds. Clearly, it will be valuable for assembling stereochemically diverse trisubstituted olefins in organic synthesis.

A simple and efficient method to prepare halogenated carbon quantum dots (CQDs) through solvent-thermal reaction was reported. The halogenated CQDs were synthesized and further surface functionalization with diamines was achieved based on the halogenated CQDs as intermediates. The halogenated CQDs provide an alternative approach for surface modification of CQDs.

A simple and effective method to prepare highly efficient fluorescent multi-walled carbon nanotubes (MWCNTs) is reported. The quantum yield of the as-prepared MWCNTs is up to 0.25, and increases with the shortening of the aliphatic chain of the diamines functionalized on the tubes. The as-prepared MWCNTs have high selectivity for several transition metal ions based on fluorescence quenching which is due to the complexes of tubes and metal ions.

The facile synthesis of halogenated multi-walled carbon nanotubes with Cl, Br and I under high pressure and high temperature was reported, and the unique photoluminescence of the halogenated carbon nanotubes was investigated by means of experimental and theoretical approaches. The one-step and facile method to incorporate these halogens onto nanotubes’ surface involves high temperature and high pressure using a sealed vessel. This method is easily handled using SOCl2, Br2 and I2 as halogen sources and is accessible for the large-scale production of halogenated carbon nanotubes. The concentration of halogen in the samples by this method reaches up to 3–8% in weight. The UV-Vis photoluminescence of these halogenated carbon nanotubes was observed, and attributed to fluorescence, depending on their lifetimes. The fluorescence of the halogenated carbon nanotubes can be remarkably enhanced by the increase of introduced halogen atoms resulting from prolonging the reaction time. Different species of halogen atoms have little influence on the fluorescence emission behaviour, but affect their fluorescence lifetimes. The fluorescence is more likely to originate from sp2 carbon clusters isolated from sp3 carbons formed by the addition of halogen atoms, but a defect mechanism cannot be excluded from the present results. A series of compounds with different aromatic rings were modelled to predict the size of the isolated sp2 carbon clusters by comparing the calculated excitation and emission maximum wavelengths with their experimental data. Basing on these theoretical results, it is thought that the UV-Vis fluorescence of the halogenated carbon nanotubes may originate from the isolated sp2 carbon clusters containing 4–8 aromatic rings.

Different sizes of graphene oxide with tunable fluorescence colours were prepared by continuous chemical oxidation with multi-walled carbon nanotubes (MWCNTs) as precursor. The photoluminescence of as-prepared GO was attributed to fluorescence according to their luminescence lifetimes. The as-prepared GO exhibit colourful fluorescence, which can be tuned by the level of oxidation. It is found that the fluorescence emission can be bathochromically shifted with the increase of pH value, and the fluorescence can be strongly quenched by usual heavy metal ions except for Cd2+.

Nanosized N-doped graphene oxide (GO) with visible fluorescence in water was prepared by cutting and unzipping of N-doped carbon nanotubes (NCNTs) and used to distinguish between normal and transition metal ions. It is found that the fluorescence is bathochromically shifted as the level of oxidation is increased.

Identification of aluminium polyoxocations, MO4Al12(OH)24(H2O)127/8+ (M = Al, Ga and Ge) (K–MAl12) and Al30O8(OH)56(H2O)2618+ (Al30), by their luminescence is reported. The fluorescence behavior of K–Al13 has been found to differ with different metal ions and anions, implying a new discovery of a potential ion sensor.

A novel and simple method to prepare well dispersed single-walled carbon nanotubes with strong visible fluorescence in water is reported. The visible fluorescence was found to be responsive to pH value and metal ions, and tunable emission ability of oxidized SWCNTs depending on the excitation wavelength and a novel self-excitation and emission process were found.

The limiting dissociative (D) and interchange dissociative (Id) water exchange pathways on Al(III) inside and outside single-walled carbon nanotubes (SWCNTs) were modelled using ONIOM calculations with density functional theory, and the influence of SWCNTs on both D and Id pathways was examined. The interchange dissociative water exchange pathway was revealed, in which the zigzag SWCNTs (13,0), (14,0) and (15,0) with the same length were modelled for interaction. The results indicate that the confinement effect of SWCNTs on the energy barriers is strong when the reaction takes place inside SWCNTs with small diameter relative to reaction complexes, and varies heavily along with the change of diameters of SWCNTs. The results also indicate that SWCNTs act as trans-activating ligand to effectively lower the energy barriers of both D and Id pathways outside SWCNTs. The interaction between aluminium-water complexes and SWCNTs can effectively lower the energy barriers in general and may accelerate the reaction rates, which has great importance for the influence of carbon nanotubes on dissolution and transformation rates of minerals such as aluminium (hydr)oxide.

The formation mechanism of bipyridyl molecule catalyzed by nickel catalyst with pyridine precursor has been studied using density functional theory calculations. The formation of bipyridyl on Ni(111) surface from two pyridine molecules is considered as the initial process of N-doped graphene growth, and the minimum energy pathway for the formation has been investigated in detail. The whole formation processes mainly includes three steps, i.e., the dehydrogenation of the first pyridine, adsorption and dehydrogenation of the second pyridine, and formation of the bipyridyl molecule. It is found that the C–H bond of pyridine could be selectively dissociated while the C–C and C–N bond connections are retained during the catalytic processes. The N-doped graphene formed by pyridine only contains pyridine-like nitrogen atoms, suggesting a possible way to produce N-doped graphene with pure pyridine-like nitrogen atoms. The comparison of formation mechanisms between bipyridyl and biphenyl molecules was carried out, and the results imply a lower temperature process for synthesis of N-doped graphene from pyridine than that for graphene from benzene.

A dissociative (D) and a solvent-assisted dissociative interchange (Id) water-exchange pathways for magnesium(II) in aqueous solution were simulated with density functional theory calculations. The D   mechanism of Mg(H2O)62+ includes a five-coordinated intermediate, while the Id   water-exchange pathway of Mg(H2O)62+ proceeds with the assistance of a solvent water molecule, which supports the experimental assignment of the reaction mechanism. The intrinsic activation volume was used to differentiate between Id and Ia mechanisms despite of the exclusion of the contribution of transmission coefficient. The calculated intrinsic activation volume for the Id mechanism is consistent with the experimental data, and is closer to the experimental data than that for D mechanism. The Id   mechanism is suggested as the dominate water-exchange pathway of Mg(H2O)62+ depending on the intrinsic activation volume with the assistance of the activation entropy. The calculations also showed that the influences of the explicit and bulk waters on energy barriers for D and Id mechanisms are obviously different.A dissociative (D) and a solvent-assisted dissociative interchange (Id) water-exchange pathways for magnesium(II) in aqueous solution were simulated with density functional calculations. The Id mechanism was identified depending on the calculated intrinsic activation volume, which supports the experimental assignment.

The formation mechanism of bipyridyl molecule catalyzed by nickel catalyst with pyridine precursor has been studied using density functional theory calculations. The formation of bipyridyl on Ni(111) surface from two pyridine molecules is considered as the initial process of N-doped graphene growth, and the minimum energy pathway for the formation has been investigated in detail. The whole formation processes mainly includes three steps, i.e., the dehydrogenation of the first pyridine, adsorption and dehydrogenation of the second pyridine, and formation of the bipyridyl molecule. It is found that the C–H bond of pyridine could be selectively dissociated while the C–C and C–N bond connections are retained during the catalytic processes. The N-doped graphene formed by pyridine only contains pyridine-like nitrogen atoms, suggesting a possible way to produce N-doped graphene with pure pyridine-like nitrogen atoms. The comparison of formation mechanisms between bipyridyl and biphenyl molecules was carried out, and the results imply a lower temperature process for synthesis of N-doped graphene from pyridine than that for graphene from benzene.

Thiolate-protected copper nanoclusters (CuNCs) with aggregation-induced emission (AIE) have emerged as a novel kind of luminescent material, but their applications in neutral solution are greatly limited by their ultra-low emission efficiency under such conditions. Herein, we report a facile synthesis of glutathione-protected CuNCs with AIE properties, and their self-assembly was driven by aluminum cations. The bright red luminescence of solid CuNCs illustrates their significant AIE nature, but CuNCs dispersed in neutral water exhibit almost no luminescence. It was found that aluminum cations were capable of driving the self-assembly of the CuNCs, and the resulting CuNC dots with controllable sizes retain their bright luminescence under neutral conditions. The strong affinity of GSH ligands to the CuNCs contributes to this good stability in neutral and even in weakly alkaline solutions. The stable existence of the luminescent CuNC dots enables them to function as a luminogen under physiological conditions. This ability was employed to detect β-galactosidase activity using 4-nitrophenyl-β-D-galactopyranoside as the substrate. The strong quenching effect of the CuNC dots by p-nitrophenol was used to achieve a sensitive measurement of the β-Gal level. This work proposes the preparation of CuNC dots with bright luminescence in neutral solution via the self-assembly of GSH-capped CuNCs by aluminum ions, and demonstrates their sensing application in the detection of β-galactosidase activity under physiological conditions.

Identification of aluminium polyoxocations, MO4Al12(OH)24(H2O)127/8+ (M = Al, Ga and Ge) (K–MAl12) and Al30O8(OH)56(H2O)2618+ (Al30), by their luminescence is reported. The fluorescence behavior of K–Al13 has been found to differ with different metal ions and anions, implying a new discovery of a potential ion sensor.

The limiting dissociative (D) and interchange dissociative (Id) water exchange pathways on Al(III) inside and outside single-walled carbon nanotubes (SWCNTs) were modelled using ONIOM calculations with density functional theory, and the influence of SWCNTs on both D and Id pathways was examined. The interchange dissociative water exchange pathway was revealed, in which the zigzag SWCNTs (13,0), (14,0) and (15,0) with the same length were modelled for interaction. The results indicate that the confinement effect of SWCNTs on the energy barriers is strong when the reaction takes place inside SWCNTs with small diameter relative to reaction complexes, and varies heavily along with the change of diameters of SWCNTs. The results also indicate that SWCNTs act as trans-activating ligand to effectively lower the energy barriers of both D and Id pathways outside SWCNTs. The interaction between aluminium-water complexes and SWCNTs can effectively lower the energy barriers in general and may accelerate the reaction rates, which has great importance for the influence of carbon nanotubes on dissolution and transformation rates of minerals such as aluminium (hydr)oxide.

Nanosized N-doped graphene oxide (GO) with visible fluorescence in water was prepared by cutting and unzipping of N-doped carbon nanotubes (NCNTs) and used to distinguish between normal and transition metal ions. It is found that the fluorescence is bathochromically shifted as the level of oxidation is increased.

A novel and simple method to prepare well dispersed single-walled carbon nanotubes with strong visible fluorescence in water is reported. The visible fluorescence was found to be responsive to pH value and metal ions, and tunable emission ability of oxidized SWCNTs depending on the excitation wavelength and a novel self-excitation and emission process were found.

The facile synthesis of halogenated multi-walled carbon nanotubes with Cl, Br and I under high pressure and high temperature was reported, and the unique photoluminescence of the halogenated carbon nanotubes was investigated by means of experimental and theoretical approaches. The one-step and facile method to incorporate these halogens onto nanotubes’ surface involves high temperature and high pressure using a sealed vessel. This method is easily handled using SOCl2, Br2 and I2 as halogen sources and is accessible for the large-scale production of halogenated carbon nanotubes. The concentration of halogen in the samples by this method reaches up to 3–8% in weight. The UV-Vis photoluminescence of these halogenated carbon nanotubes was observed, and attributed to fluorescence, depending on their lifetimes. The fluorescence of the halogenated carbon nanotubes can be remarkably enhanced by the increase of introduced halogen atoms resulting from prolonging the reaction time. Different species of halogen atoms have little influence on the fluorescence emission behaviour, but affect their fluorescence lifetimes. The fluorescence is more likely to originate from sp2 carbon clusters isolated from sp3 carbons formed by the addition of halogen atoms, but a defect mechanism cannot be excluded from the present results. A series of compounds with different aromatic rings were modelled to predict the size of the isolated sp2 carbon clusters by comparing the calculated excitation and emission maximum wavelengths with their experimental data. Basing on these theoretical results, it is thought that the UV-Vis fluorescence of the halogenated carbon nanotubes may originate from the isolated sp2 carbon clusters containing 4–8 aromatic rings.

A simple and effective method to prepare highly efficient fluorescent multi-walled carbon nanotubes (MWCNTs) is reported. The quantum yield of the as-prepared MWCNTs is up to 0.25, and increases with the shortening of the aliphatic chain of the diamines functionalized on the tubes. The as-prepared MWCNTs have high selectivity for several transition metal ions based on fluorescence quenching which is due to the complexes of tubes and metal ions.

Dynamic covalent B–O bonds are introduced to the design of a reversible fluorescence nanoswitch in response to the external stimulus of pH. This nanoswitch is based on a phenylboronic acid functionalized carbon quantum dot (PBA–CQD) nanoprobe, and constructed with reference to the two facts that the PBA–CQD probe can bind to p-nitrophenol to form a non-fluorescent conjugate via B–O bonds, and that the making and breaking of the B–O bonds between them can be controlled by pH changes. Excellent reversibility of this nanoswitch is illustrated by switching of the pH from 4 to 8. The reaction between the PBA–CQD nanoprobe and the p-nitrophenol resulting in switching off the fluorescence is further utilized to design a general detection strategy for enzyme activity when substrates that can generate p-nitrophenol through enzymatic reactions are chosen. The feasibility of the detection strategy is qualitatively assessed using α-glucosidase and β-galactosidase, and its practicability to quantitatively monitor enzyme activity is also demonstrated by taking α-glucosidase as an example. The detection limit of this method can be as low as 0.33 U L−1, which is much lower than those reported previously and is sufficiently low to be capable of α-glucosidase level detection in practical human samples. This study demonstrates excellent usability of dynamic covalent B–O bonds in the design of reversible switches and in the general detection of enzyme activity, and provides a sensitive, real-time assay for α-glucosidase based on carbon quantum dots.