The development of highly sensitive assays for glycosidases is of critical significance to understand their functions, facilely detect associated diseases and screen potential new drugs. In this work, we develop a universal assay strategy for glycosidase enzymes and inhibitor screening based on functional carbon quantum dots through a combined host–guest recognition and specific static quenching-induced signal transduction mechanism. This detection strategy is established in terms of the following facts: (1) β-cyclodextrin as a perfect host can selectively associate with p-nitrophenol due to its hydrophobic character and right size match of the cavity, which renders specific binding between β-cyclodextrin and p-nitrophenol via a host–guest recognition. (2) The formation of an inclusion complex between β-cyclodextrin modified carbon quantum dots (β-CD–CQDs) and p-nitrophenol results in fluorescence quenching with a high quenching efficiency due to the static quenching mechanism. Glycoconjugates of p-nitrophenol as the substrates could be rapidly hydrolyzed to corresponding glycose and p-nitrophenol in the presence of specific glycosidase, and the resulting p-nitrophenol induces the following host–guest interaction and static quenching leading to a change in the fluorescence signal. The activity of different glycosidase enzymes could be evaluated in the same way as long as the glycosyl unit of glycosylated substrates was changed. Here we take β-galactosidase as an example to demonstrate the applicability of the proposed detection strategy because it can act as a molecular target for primary ovarian cancers. A highly sensitive assay for β-galactosidase activity in terms of linear correlation of the fluorescence change with the β-galactosidase level was established with a low detection limit of 0.6 U L−1. Its function of inhibitor screening was also assessed by using D-galactal as the inhibitor for β-galactosidase, and the positive results indicated its feasibility to screen potential inhibitors. It is also illustrated that the nanoprobe possesses excellent biocompatibility, and can sensitively monitor the intracellular β-galactosidase level in ovarian cancer cells. This work provides a general detection method for glycosidase activity, demonstrates its applicability of monitoring the enzyme level in living cells, and broadens fluorogenic probes in fluorescence-guided diagnostics.

•A sensitive assay for ALP activity based on β-CD-CQDs nanoprobe was developed for the first time.•A novel detection strategy based on specific host-guest recognition and PET of CQDs was identified.•This detection approach for ALP level is ultra-sensitive with the detection limit of 0.9 U/L.A convenient, reliable and highly sensitive assay for alkaline phosphatase (ALP) activity in the real-time manner is developed based on β-cyclodextrin-modified carbon quantum dots (β-CD-CQDs) nanoprobe through specific host-guest recognition. Carbon quantum dots were first functionalized with 3-aminophenyl boronic acid to produce boronic acid-functionalized CQDs, and then further modified with hydropropyl β-cyclodextrins (β-CD) through B-O bonds to form β-CD-CQDs nanoprobe. p-Nitrophenol phosphate disodium salt is used as the substrate of ALP, and can hydrolyze to p-nitrophenol under the catalysis of ALP. The resulting p-nitrophenol can enter the cavity of β-CD moiety in the nanoprobe due to their specific host-guest recognition, where photoinduced electron transfer process between p-nitrophenol and CQDs takes place to efficiently quench the fluorescence of the probe. The correlation between quenched fluorescence and ALP level can be used to establish quantitative evaluation of ALP activity in a broad range from 3.4 to 100.0 U/L with the detection limit of 0.9 U/L. This assay shows a high sensitivity to ALP even in the presence of a very high concentration of glucose. This study demonstrates a good electron donor/acceptor pair, which can be used to design general detection strategy through PET process, and also broadens the application of host-guest recognition for enzymes detection in clinical practice.

•Au/Ag nanoclusters exhibit good stability and excellent fluorescence performance.•Detection strategy based on a photoinduced election transfer pathway was designed.•A sensitive assay for tyrosinase activity based on Au/Ag nanoclusters was developed.•This work broadens the application of bimetallic nanoclusters in the bioanalysis.Due to the vital role of tyrosinase in melanin biosynthesis and its function as an important biomarker for melanoma cancer, highly sensitive detection of its activity using biocompatible materials is in urgent demand. Herein we report a convenient and highly sensitive fluorometric biosensor for tyrosinase activity detection based on biocompatible dopamine-functionalized Au/Ag nanoclusters (Dopa-Au/Ag NCs). Dopamine with redox property was covalently linked to Au/Ag NCs surface and formed a Dopa-Au/Ag NCs bioconjugate with strong blue fluorescence. Dopamine is readily oxidized by molecular oxygen under the catalysis of tyrosinase. After dopamine is transformed to o-dopaquinone, an intraparticle photoinduced election transfer (PET) process occurs between Au/Ag NCs and o-dopaquinone moiety, leading to the fluorescence quenching of the Dopa-Au/Ag NCs bioconjugate. Thus, this biosensor was utilized for sensitive and selective detection of tyrosinase in terms of the relationship between fluorescence quenching efficiency and tyrosinase activity. This study discovers that Au/Ag NCs and dopaquinone can serve as a good electron acceptor and donor pair which results in an efficient intraparticle photoinduced electron transfer process, and also provides another alternative way for tyrosinase activity monitoring.

A convenient, reliable, and highly sensitive real-time assay for alkaline phosphatase (ALP) activity in the continuous and recyclable way is established on the basis of aggregation and disaggregation of carbon quantum dots (CQDs) through the competitive assay approach. CQDs and adenosine triphosphate (ATP) were used as the fluorescent indicator and substrate for ALP activity assessment, respectively. Richness of carboxyl groups on the surface of CQDs enables their severe aggregation triggered by cerium ions, which results in effective fluorescence quenching. Under the catalytic hydrolysis of ALP, ATP can be rapidly transformed to phosphate ions. Stronger affinity of phosphate ions to cerium ions than carboxyl groups is taken advantage of to achieve fluorescence recovery induced by redispersion of CQDs in the presence of ALP and ATP. Quantitative evaluation of ALP activity in a broad range from 4.6 to 383.3 U/L with the detection limit of 1.4 U/L can be realized in this way, which endows the assay with high enough sensitivity for practical detection in human serum. The assay can be used in a recyclable way for more than three times since the generated product CePO4 as a precipitate can be easily removed from the standard assay system. This strategy broadens the sensing application of fluorescent CQDs with excellent biocompatibility and provides an example based on disaggregation in optical probe development.

A reversible fluorescence nanoswitch by integrating carbon quantum dots nanoassembly and pyrophosphate ion is developed, and a reliable real-time fluorescent assay for acid phosphatase (ACP) activity is established on the basis of the fluorescence nanoswitch. Carbon quantum dots (CQDs) abundant in carboxyl groups on the surface, nickel(II) ion and pyrophosphate ion comprise the fluorescent nanoswitch, which operates in the following way: the nanoassembly consisting of CQDs and nickel ions can be triggered by pyrophosphate ion serving as an external stimulus. At the same time, the fluorescence nanoswitch switches between two fluorescence states (OFF and ON) accompanying shifts in their physical states aggregation and disaggregation. Based on the nanoswitch, the introduction of ACP leads to breakdown of pyrophosphate ions into phosphate ions and resultant fluorescence quenching due to catalytic hydrolysis of ACP toward pyrophosphate ions (PPi). Quantitative evaluation of ACP activity in a broad range from 18.2 U/L to 1300 U/L, with a detection limit of 5.5 U/L, can be achieved in this way, which endows the assay with sufficiently high sensitivity for practical detection in human serum and seminal plasma.

•The detection of lead (II) was achieved based on PET between GQDs and GO.•Controllable fluorescence turn-on process between GQDs-probes and GO is achieved.•The nanosensor shows high sensitivity and selectivity for Pb2+ detection.•The nanosensor shows a broad linear scope and ultralow detection limit for Pb2+ detection.The sensitive detection of heavy metal ions in the organism and aquatic ecosystem using nanosensors based on environment friendly and biocompatible materials still remains a challenge. A fluorescent turn-on nanosensor for lead (II) detection based on biocompatible graphene quantum dots and graphene oxide by employment of Pb2+-induced G-quadruplex formation was reported. Graphene quantum dots with high quantum yield, good biocompatibility were prepared and served as the fluorophore of Pb2+ probe. Fluorescence turn-off of graphene quantum dots is easily achieved through efficient photoinduced electron transfer between graphene quantum dots and graphene oxide, and subsequent fluorescence turn-on process is due to the formation of G-quadraplex aptamer–Pb2+ complex triggered by the addition of Pb2+. This nanosensor can distinguish Pb2+ ion from other ions with high sensitivity and good reproducibility. The detection method based on this nanosensor possesses a fast response time of one minute, a broad linear span of up to 400.0 nM and ultralow detection limit of 0.6 nM.

A novel and efficient fluorescence sensing platform based on biocompatible graphene quantum dots and graphene oxide was established. It showed high selectivity and sensitivity for DNA detection.

Heteroatom doping of carbon quantum dots not only enables great improvement of fluorescence efficiency and tunability of fluorescence emission, but also provides active sites in carbon dots to broaden their application in sensor. Silicon as a biocompatible element offers a promising direction for doping of carbon quantum dots. Si-doped carbon quantum dots (SiCQDs) were synthesized through a facile and effective approach. The as-prepared Si-doped carbon quantum dots possess visible fluorescence with high quantum yield up to 19.2%, owing to fluorescence enhancement effect of introduced silicon atoms into carbon dots. The toxicity test on human Hela cells showed that SiCQDs have lower cellular toxicity than common CQDs, and bioimaging experiments clearly demonstrated their excellent biolabelling ability and outstanding performance in resistance to photobleaching. Strong fluorescence quenching effect of Fe(III) on SiCQDs can be used for its selective detection among general metal ions. Specific electron transfer between SiCQDs and hydrogen peroxide enables SiCQDs as a sensitive fluorescence sensing platform for hydrogen peroxide. The subsequent fluorescence recovery induced by removal of hydrogen peroxide from SiCQDs due to formation of the stable adducts between hydrogen peroxide and melamine was taken advantage of to construct effective sensor for melamine.Keywords: bioimaging; hydrogen peroxide; melamine; sensor; Si-doped carbon quantum dots;

Fluorescent B-doped carbon quantum dots (BCQDs) were prepared by a facile one-pot solvothermal route. The BCQDs can be used as a novel fluorescence sensing system for hydrogen peroxide and glucose detection.

P-doped carbon quantum dots (PCQDs) were synthesized by a solvent-thermal method using phosphorous tribromide and hydroquinone as precursors. The as-prepared PCQDs present strong visible fluorescence with quantum yield up to 25%. The toxicity and bioimaging experiments showed that PCQDs have low cell toxicity and excellent biolabeling ability.

Dominant components of oxidized products of multi-walled carbon nanotubes were separated by column chromatography, the origin of highly visible fluorescence from carbon quantum dots was revealed, and the nature of weak near-UV-Vis fluorescence of oxidized carbon nanotubes from isolated sp2 carbon clusters was supported among the four proposed explanations through experimental and theoretical approaches. It was found that three dominant components including carbon quantum dots, short and long oxidized carbon nanotubes were produced during the oxidation of carbon nanotubes. The highly visible fluorescence was mainly originated from carbon quantum dots, while short and long oxidized carbon nanotubes only exhibited weak near-UV-Vis fluorescence. For the nature of fluorescence of oxidized carbon nanotubes, two proposed explanations including defects mechanism and an isolated carbon cluster mechanism were compared and discussed through theoretical analysis of corresponding model compounds. It was supported that the fluorescence is dominantly originated from sp2 carbon clusters isolated by sp3 carbons due to oxidation depending on the comparison between experimental data and calculated values. The results also indicated that carbon nanotubes can be transformed to large graphene oxide during oxidation. This work not only clearly demonstrated the origin of highly visible fluorescence in an oxidized carbon nanotube mixture and a reasonable explanation for fluorescence of oxidized carbon nanotubes, but also provided an example to understand visible fluorescent graphene oxide and carbon quantum dots.

Surface chemistry provides an alternative approach to modulate the emission colour and efficiency of graphene quantum dots. We systematically investigated the surface chemistry of graphene quantum dots functionalized with a series of small organic molecules combining experimental and theoretical approaches. Experimental results indicated that surface functionalization with functional groups such as alcohol, amine and thiol can effectively tune the fluorescence of graphene quantum dots, and proved that amino groups can highly elevate the quantum yields of modified graphene quantum dots. The emission efficiency of 1,2-ethylenediamine functionalized graphene quantum dots reached up to 17.6% due to specific proton transfer to the conjugated fluorophore-like structure from ammonium formed by protonation. The polyaromatic structure within the graphene quantum dots was proposed to explain the fluorescence enhancement mechanism of graphene quantum dots functionalized by diamines. The computational results suggested that not only the size of the polyaromatic structures within graphene quantum dots can change their emissions, but surface functionalization can also tune their photoluminescence through modulating their band gaps. Toxicity experiments indicated that diamine-functionalized graphene quantum dots showed low cell toxicity similar to that of pristine graphene quantum dots. Moreover, the bioimaging experiments suggested that functionalized graphene quantum dots had identical abilities to label cells at a lower concentration than pristine graphene quantum dots owing to their higher quantum yields.

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.

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.

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.

The development of highly sensitive assays for glycosidases is of critical significance to understand their functions, facilely detect associated diseases and screen potential new drugs. In this work, we develop a universal assay strategy for glycosidase enzymes and inhibitor screening based on functional carbon quantum dots through a combined host–guest recognition and specific static quenching-induced signal transduction mechanism. This detection strategy is established in terms of the following facts: (1) β-cyclodextrin as a perfect host can selectively associate with p-nitrophenol due to its hydrophobic character and right size match of the cavity, which renders specific binding between β-cyclodextrin and p-nitrophenol via a host–guest recognition. (2) The formation of an inclusion complex between β-cyclodextrin modified carbon quantum dots (β-CD–CQDs) and p-nitrophenol results in fluorescence quenching with a high quenching efficiency due to the static quenching mechanism. Glycoconjugates of p-nitrophenol as the substrates could be rapidly hydrolyzed to corresponding glycose and p-nitrophenol in the presence of specific glycosidase, and the resulting p-nitrophenol induces the following host–guest interaction and static quenching leading to a change in the fluorescence signal. The activity of different glycosidase enzymes could be evaluated in the same way as long as the glycosyl unit of glycosylated substrates was changed. Here we take β-galactosidase as an example to demonstrate the applicability of the proposed detection strategy because it can act as a molecular target for primary ovarian cancers. A highly sensitive assay for β-galactosidase activity in terms of linear correlation of the fluorescence change with the β-galactosidase level was established with a low detection limit of 0.6 U L−1. Its function of inhibitor screening was also assessed by using D-galactal as the inhibitor for β-galactosidase, and the positive results indicated its feasibility to screen potential inhibitors. It is also illustrated that the nanoprobe possesses excellent biocompatibility, and can sensitively monitor the intracellular β-galactosidase level in ovarian cancer cells. This work provides a general detection method for glycosidase activity, demonstrates its applicability of monitoring the enzyme level in living cells, and broadens fluorogenic probes in fluorescence-guided diagnostics.

Dominant components of oxidized products of multi-walled carbon nanotubes were separated by column chromatography, the origin of highly visible fluorescence from carbon quantum dots was revealed, and the nature of weak near-UV-Vis fluorescence of oxidized carbon nanotubes from isolated sp2 carbon clusters was supported among the four proposed explanations through experimental and theoretical approaches. It was found that three dominant components including carbon quantum dots, short and long oxidized carbon nanotubes were produced during the oxidation of carbon nanotubes. The highly visible fluorescence was mainly originated from carbon quantum dots, while short and long oxidized carbon nanotubes only exhibited weak near-UV-Vis fluorescence. For the nature of fluorescence of oxidized carbon nanotubes, two proposed explanations including defects mechanism and an isolated carbon cluster mechanism were compared and discussed through theoretical analysis of corresponding model compounds. It was supported that the fluorescence is dominantly originated from sp2 carbon clusters isolated by sp3 carbons due to oxidation depending on the comparison between experimental data and calculated values. The results also indicated that carbon nanotubes can be transformed to large graphene oxide during oxidation. This work not only clearly demonstrated the origin of highly visible fluorescence in an oxidized carbon nanotube mixture and a reasonable explanation for fluorescence of oxidized carbon nanotubes, but also provided an example to understand visible fluorescent graphene oxide and carbon quantum dots.

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 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.

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.