there has been growing interest during the last decade in the development of fluorescent molecular sensors for cations and anions in solution [ 1 - 8 ] . especially , fabricating fluorescent materials for the detection of zinc cation has drawn much more attention [ 9 - 13 ] , as zinc not only plays important roles in human bodies , but also closely relates to severe pathological diseases such as alzheimer s and parkinson s diseases . so far , much study has been done for the detection and real - time localization of zn(ii ) . yasuhiro shiraishi s group has synthesized a quinoline  polyamine conjugate as a fluorescent chemosensor for quantitative detection of zn(ii ) in water . maarten merkx et al . used chelating fluorescent protein chimeras for ratiometric detection of zn(ii ) in living cells and the detection range was from 10 nm to 1 mm . jinshi ma and coworkers have synthesized several bis(pyrrol-2-yl - methyleneamine ) ligands as fluorescent sensor for zn(ii ) , and their results revealed that the ligands exhibit excellent fluorescent properties . however , much of the work was just based on organic molecules as fluorescent chemosensors . for a few practical applications the attachment of the fluorescent units to a solid support has advantages like the possibility of recovering the materials for their repetitive use . for this point , scientists chose silica nanoparticles , nanosized boehmite particles   and silicon nanowires   to support fluoresence ligands as fluorescence sensors . and these materials exhibit excellent selectivity and sensitivity to sense metal ions . in this study , we chose multi - walled carbon nanotubes ( mwnts ) as fluorescent support . since its discovery , the fluorophore in this study is glycine - n-8-quinolylamide ( gnq ) molecule , in light of the fact that the 8-aminoquinoline derivatives could effectively coordinate with specific metal ions . the quinoline group has been covalently grafted to the surface of the mwnts that can behave as recognition center for metal ions depending on its actual protonation state . we find that this new material ( mwnts - gnq ) has high selectivity and sensitivity to detect zn(ii ) , and the sensitivity is down to 0.2 m . , which is about the same as the silica nanoparticles - supported fluorescence sensors . for other sensing materials , the fluorescence enhancement selectivity is not only for zn(ii ) but also for cd(ii ) , which may reduce selectivity when they are used just for zn(ii ) detection . on the other hand , the fluorescence enhancement of mwnts - gnq is only for zn(ii ) . as the use of organic  inorganic hybrid materials for bio - application has become a hot subject in the research field currently [ 28 - 31 ] , mwnts - gnq may be used to build nanosensor devices to sense directly in intracellular environment , because carbon nanotubes ( cnts ) can penetrate into cells and almost have no toxicity to organism . the multi - walled carbon nanotubes ( mwnts , diameters : 2040 nm , purity : 9598% ) prepared by the catalytic decomposition of ch4 were provided by shengzhen nanotech port ltd . glycine - n-8-quinolylamide ( gnq ) was synthesized according to the known method . in a typical experiment , 300 mg pristine - mwnts were added to a 180 ml 3:1 mixture of concentrated h2so4and hno3 . the mixture was treated in an ultrasonic bath ( 40 khz ) for 20 min and stirred at 60 c for 4 h under reflux . then , the mixture was vacuum - filtered through a 0.22 m millipore polytetrafluoroethylene membrane and washed with distilled water until ph of the filtrate was 7 . the filter cake was dried under vacuum at 40 c for 24 h to obtain mwnts - cooh . the acid - treated mwnts - cooh of 200 mg was reacted with 20 ml socl2for 24 h under reflux , and then the residual socl2was removed by the reduced pressure distillation , the solid was washed with anhydrous thf several times until the brown - colored supernatant became to colorless . the remaining solid acyl chloride - functionalized mwnts ( mwnts - cocl ) was dried under vacuum at 20 c for 24 h. functionalization of mwnts mwnts - cocl of 100 mg was dispersed in 10 ml anhydrous chloroform and the mixture was sonicated for 20 min to create a homogeneous suspension . the mixture was added with 50 mg gnq under a nitrogen atmosphere , and then immersed in an oil bath at 70 c accompained by mechanical stirring for 24 h. the resulting reaction medium was vacuum - filtered through a 0.22 m polycarbonate membrane three times to yield mwnts - gnq . fourier transform infrared ( ftir ) spectrometer ( bruker ifs66/s ) , dupont-1090 thermal gravimetric analysis ( tga ) instrument and gmbh varioel elementar analysensyteme were used to characterize the materials . perkin elmer ls 55 spectrofluorimeter was used to obtain the fluorescence spectra of the fluorescence material . the multi - walled carbon nanotubes ( mwnts , diameters : 2040 nm , purity : 9598% ) prepared by the catalytic decomposition of ch4 were provided by shengzhen nanotech port ltd . in a typical experiment , 300 mg pristine - mwnts were added to a 180 ml 3:1 mixture of concentrated h2so4and hno3 . the mixture was treated in an ultrasonic bath ( 40 khz ) for 20 min and stirred at 60 c for 4 h under reflux . then , the mixture was vacuum - filtered through a 0.22 m millipore polytetrafluoroethylene membrane and washed with distilled water until ph of the filtrate was 7 . the filter cake was dried under vacuum at 40 c for 24 h to obtain mwnts - cooh . the acid - treated mwnts - cooh of 200 mg was reacted with 20 ml socl2for 24 h under reflux , and then the residual socl2was removed by the reduced pressure distillation , the solid was washed with anhydrous thf several times until the brown - colored supernatant became to colorless . the remaining solid acyl chloride - functionalized mwnts ( mwnts - cocl ) was dried under vacuum at 20 c for 24 h. functionalization of mwnts mwnts - cocl of 100 mg was dispersed in 10 ml anhydrous chloroform and the mixture was sonicated for 20 min to create a homogeneous suspension . the mixture was added with 50 mg gnq under a nitrogen atmosphere , and then immersed in an oil bath at 70 c accompained by mechanical stirring for 24 h. the resulting reaction medium was vacuum - filtered through a 0.22 m polycarbonate membrane three times to yield mwnts - gnq . fourier transform infrared ( ftir ) spectrometer ( bruker ifs66/s ) , dupont-1090 thermal gravimetric analysis ( tga ) instrument and gmbh varioel elementar analysensyteme were used to characterize the materials . perkin elmer ls 55 spectrofluorimeter was used to obtain the fluorescence spectra of the fluorescence material . figure 1shows the infrared spectra of a gnq and b mwnts - gnq . in the ftir spectrum of gnq , , the characteristic peaks of amino groups in the spectrum of mwnts - gnq disappeared , demonstrating that the amino groups on gnq have reacted with acyl chloride groups on the surface of mwnts . a new peak that appeared around 1686 cmis attributed to the amide carbonyl ( c = o ) stretch . another new peak at 1629.87 cmis attributed to secondary amide band which accompanies the absorption at 1686 cm . another peak at 1082.47 cmattributed to ( c  n ) has also been found , and it has obviously been enhanced . ftir spectra ofagnq andbmwnts - gnq the tga curves of mwnts - cooh and mwnts - gnq were recorded on a dupont-1090 thermogravimeter in ar atmosphere at the heating rate 10 c / min from 20 c to 500 c ( fig . 2a , there is a continuous weight lose of the mwnts - cooh , and the amount of the weight loss is about 10% typical for acid - functionalized mwnts . 2b , the major weight loss happened in the temperature range from 200 c to 450 c due to the degradation of the gnq grafted to the mwnts . the content of the gnq grafted to the mwnts is about 12 wt% , which is similar to the calculation result of the elemental microanalysis ( table 1 ) . tga ofamwnts - cooh andbmwnts - gnq elemental microanalysis for mwnts - gnq the fluorescence spectra of gnq and mwnts - gnq are shown in fig . fluorescence spectrum of gnq ( 1  10 m ) and mwnts - gnq ( 1  10 m ) . ex = 324 nm gnq is expected to be a stronger coordinating agent because it is a tridentate ligand . when gnq is selectively coordinated with metal ions , the fluorescence from gnq this phenomenon can be utilized to construct a material for the detection of metal ions based on mwnts ( scheme 2 ) . accordingly , titration of various metal ions in the presence of mwnts - gnq in methanol solution was performed , and the results are summarized in fig . , it is observed that the intensity of fluorescence from mwnts - gnq containing zn(ii ) is much higher than that of other metal ions . this is very nice because under many conditions ( e.g. , physiological conditions ) various metal ions may exist at certain concentrations compared to zn(ii ) . fluorescent chemosensor ( mwnts - gnq ) for detection of zn(ii ) relative fluorescence intensity of mwnts - gnq ( 1  10 m ) in the presence of variouse metal ions alone ( 1  10 m ) . methanol solution . ex = 324 nm it was reported that most of the previous zn(ii ) sensors do not exhibit good selectivity to these metal cations   ( for instance , the selectivity in many cases is close to 1:1 for zn(ii):cd(ii ) ) . this may bring trouble to certain applications where co(ii ) , ni(ii ) , cu(ii ) , or cd(ii ) may interfere ( e.g. , in environmental science ) . herein , mwnts - gnq shows 2.8-fold fluorescence enhancement for zn(ii ) versus just minimal fluorescence enhancement for cd(ii ) and ni(ii ) . from these results , it is evident that mwnts - gnq have a high selectivity to zn(ii ) . as for selectivity of mwnts - gnq to metal ions , when zn(ii ) forms a complex with mwnts - gnq with a suitable radius and an electronic structure 3d4 s , the electron - transfer process of mwnts - gnq is forbidden , and an extended electron conjugation system is formed synchronously . this conjugation system is involved in an internal charge transfer process from the ligand donor to the zn(ii ) acceptor , and simultaneously inhibits the excited - state proton transfer and photo - induced electron transfer that strongly suppress the fluorescence of mwnts - gnq . thus zn(ii ) considerably enhances the fluorescence of mwnts - gnq . to further characterize the performance of the sensing material for zn(ii ) , a series of comparison experiments were carried out with mwnts - gnq . because zn(ii ) always coexists with cu(ii ) or cd(ii ) , titration addition of 1  10 m cu(ii ) and cd(ii ) to mwnts - gnq solution containing zn(ii ) when cu(ii ) was added to the solution , it led to about 97.5% quenching of the total fluorescence intensity , probably because cu(ii ) formed some complex with gnq group , resulting in quenching of fluorescence as reported . but cd(ii ) almost had no influence on the fluorescence intensity of the mwnts - gnq solution containing zn(ii ) , because the electronic structure of cd(ii ) is fairly similar to that of zn(ii ) . to resolve the problem of fluorescence quenching by cu(ii ) , a masking agent of na2s2o3 was chosen . as is well known that s2o3 can form a very stable complex with cu(ii ) , the coordination number is three , and na(i ) almost has no influence to fluorescence intensity of mwnts - gnq . the results are shown in fig . 5 , from which we can see that the masking agent almost does not affect the experimental results except in the case when the system contains zn(ii ) and cu(ii ) . when three stoichiometry s2o3 were added to the zn(ii ) and cu(ii ) solution , the fluorescence intensity was greatly enhanced . although , compared to the solution only containing zn(ii ) , the intensity was a little weaker , na2s2o3 is still a good masking agent to mask cu(ii ) . we thus conclude that the presence of cd(ii ) does not affect the sensitivity of mwnts - gnq for zn(ii ) detection , and na2s2o3 could be used as a masking agent when cu(ii ) coexists in the system . relative fluorescence intensity of mwnts - gnq ( 1  10 m ) or mwnts - gnq ( 1  10 m ) containing s2o3(3  10 m ) in the presence of zn(ii ) ( 1  10 m ) and interfering ions with cu(ii ) ( 1  10 m ) or cd(ii ) ( 1  10 m ) , respectively . ex = 324 nm the sensitivity of fluorescence enhancing from mwnts - gnq by zn(ii ) was further investigated , and the results are shown in fig . 6b ) , which suggested the 1:1 stoichiometry of the ligand to the zinc ions . because zn(ii ) desires a square planar geometry when coordinated , while the three nitrogen on mwnts - gnq can only provide a tridentate ligand , the fourth coordination can come from the solvent methanol oxygen . it can be seen from fig . 6b that when the concentration of zn(ii ) is lower than 0.2 m , the relative fluorescence intensity is about 1 . but when the concentration of zn(ii ) is higher than 0.2 m , the relative fluorescence intensity is also increased . it can be expressed by the following formula:(1 ) wherein , ais the value of the fluorescence intensity , cis the concentration of zn(ii ) , and the range ofcis from 0.2 m to 10 m in this study . so , it is evident that the detection limit for zn(ii ) is established at 0.2 m under the experimental conditions in our study . afluoresence spectra of mwnts - gnq ( 1  10 m ) with zn(ii),brelative fluoresence intensity of mwnts - gnq at different concentration of zn(ii ) . we have prepared a new organic  inorganic hybrid sensing material based on cnts as support and glycine - n-8-quinolylamide as fluorescent center . the results of the fluorescence characterization show that the composite has a highly selective and sensitive ( 0.2 m ) detection for zn(ii ) , and reveal that ratiometric zn(ii ) sensing is possible with fluorophore chemically modified carbon nanotubes . this novel fluorescent material may be used as a fluorescent device in intracellular environment for the detection of zn(ii ) .
carbon nanotubes ( cnts ) were covalently modified by fluorescence ligand ( glycine - n-8-quinolylamide ) and formed a hybrid material which could be used as a selective probe for metal ions detection . 
 the anchoring to the surface of the cnts was carried out by the reaction between the precursor and the carboxyl groups available on the surface of the support . 
 fourier transform infrared spectroscopy ( ftir ) and thermogravimetric analysis ( tga ) unambiguously proved the existence of covalent bonds between cnts and functional ligands . 
 fluorescence characterization shows that the obtained organic  inorganic hybrid composite is highly selective and sensitive ( 0.2 m ) to zn(ii ) detection .