Sensors and Actuators.2012.862.Fluorescent probe of Cu.

Sensors and Actuators B 173 (2012) 862–867Contents lists available at SciVerse ScienceDirectSensors and Actuators B:Chemicalj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s nbA highly selective and sensitive fluorescent sensor for Cu 2+and its complex for successive sensing of cyanide via Cu 2+displacement approachLijun Tang ∗,Mingjun CaiDepartment of Chemistry,Liaoning Key Laboratory for the Synthesis and Application of Functional Compounds,Bohai University,Jinzhou 121013,Chinaa r t i c l ei n f oArticle history:Received 7June 2012Received in revised form 29July 2012Accepted 31July 2012Available online 9 August 2012Keywords:Copper detection Cyanide recognition FluorescenceIndicator displacement approacha b s t r a c tA new benzimidazole-based imine linked sensor (1)was synthesized.Sensor 1exhibits highly selective and sensitive recognition properties to Cu 2+in CH 3OH/H 2O (1/1,v/v,HEPES 10mM,pH =7.0)solution with a 1:1binding stoichiometry.The in situ prepared 1–Cu 2+complex solution displays high selectiv-ity to cyanide through Cu 2+displacement approach and possesses excellent tolerance to other common interference anions.The detection limits of sensor 1to Cu 2+and 1–Cu 2+complex to cyanide were esti-mated to be 1.82×10−8M and 1.62×10−6M,respectively.This Cu 2+and cyanide successive recognition feature of sensor 1makes it a potential utility for Cu 2+and cyanide detection in aqueous media.© 2012 Elsevier B.V. All rights reserved.1.IntroductionThe development of artificial receptors for biologically impor-tant metal ions [1,2]and anions [3,4]has received considerable attention due to their fundamental roles in a wide range of chem-ical,environmental and biological processes.Copper (II)ion,being the third adequate element following ion and zinc,plays crucial roles in many fundamental physiological processes in organisms.It is well known that it serves as a catalytic cofactor for a variety of metalloenzymes including superoxide dismutase,cytochrome c oxidase and tyrosinase [5].Whereas,unregulated overloading of copper can lead to severe neurodegenerative diseases such as Alzheimer’s [6]or Parkinson’s diseases [7].Cu 2+is also suspected to cause infant liver damage in recent years [8,9].Thus,the highly selective,sensitive and rapid recognition of Cu 2+by chemosensors in aqueous media is still imperative.As a consequence,consider-able efforts have been devoted to the development of probes for the recognition of Cu 2+[10–24].On the other hand,among the significant anions,cyanide detec-tion has attracted increasingly more attention due to its high toxicity.For instance,cyanide ions can bind heme cofactors to inhibit the process of cellular respiration in mammals [25].Most environmental cyanides are released by industries involved in nylon and acrylic polymers synthesis,electroplating,and gold min-ing [26].Hence,great efforts have been devoted to the detection of∗Corresponding author.Tel.:+864163400302;fax:+864163400160.E-mail address:lijuntang@ (L.Tang).cyanide by using highly selective,sensitive and convenient chemi-cal probes [27].To date,a variety of detecting principles have been applied to cyanide recognition,including hydrogen bonding inter-actions [28],formation of cyanide complexes [29–31],utilizing of nucleophilic attack of CN −on activated carbonyl groups [32–41],Michael acceptor type of activated C C double bond [42–46]or a boron center [47,48],demetalation of preassembled complexed sensor [49–52].Taking the advantage of high copper-cyanide affin-ity has been proved to be a valuable strategy to built highly selective and sensitive chemosensors for cyanide in aqueous media [49,53–57],this methodology also can effectively avoid the inter-ference by other anions such as fluoride and acetate.Herein we designed and synthesized a new benzimidazole based fluorescent sensor 1(Scheme 1)for Cu 2+recognition.The in situ formed 1–Cu 2+complex was further used for fluorescence “off–on”recognition of cyanide via Cu 2+displacement approach.2.Experimental2.1.Instruments and reagentsUnless otherwise stated,solvents and reagents were of ana-lytical grade from commercial suppliers and were used without further purification.1H NMR and 13C NMR spectra were recorded on Bruker Advance-600MHz spectrometer.Chemical shifts (ı)were expressed in ppm and coupling constants (J )in Hertz.High resolution mass spectrum was measured on an Agilent 1200time-of-flight mass spectrometer (Bruker,micrOTOF-Q).UV–vis spectra were measured on a SP-1900spectrophotometer (Shanghai,China).0925-4005/$–see front matter © 2012 Elsevier B.V. All rights reserved./10.1016/j.snb.2012.07.112L.Tang,M.Cai /Sensors and Actuators B 173 (2012) 862–867863NH NH 2NOHOH 3OHCN H NNHOHOCH 3EtOH reflux231Scheme 1.Synthesis of sensor 1.Fluorescence measurements were performed on a Sanco 970-CRT spectrofluorometer (Shanghai,China).The IR spectrum was mea-sured in KBr disks on a Nicolet Magna FT-IR 560Spectrometer.The pH measurements were made with a Model PHS-25B meter (Shanghai,China).2.2.Synthesis of 12-(Aminophenyl)-1H -benzimidazole (2)(200mg,0.96mmol)and 2-hydroxy-3-(hydroxymethyl)-5-methyl-benzaldehyde (3)[58](160mg,0.96mmol)were dissolved in 50mL of absolute ethanol,the resulting solution was heated to reflux for 3h.Upon cooling to r.t.,the precipitates formed were collected by filtration,which were washed by ice cold ethanol to give 246mg of 1.Yield:72%.Mp:219–219.7◦C.1H NMR (600MHz,DMSO-d 6)ı9.02(s,1H),7.98(dd,1H,J 1=7.8Hz,J 2=1.2Hz),7.65(d,1H,J =7.8Hz),7.26(d,1H,J =1.8Hz),7.25–7.22(m,2H),7.18–7.15(m,1H),7.07–7.04(m,2H),6.96(d,1H,J =7.8Hz),6.87(d,1H,J =7.8Hz),6.83–6.81(m,1H),6.54(d,1H,J =1.8Hz),5.50(t,1H,J =4.8Hz),4.63(m,2H),2.01(s,3H).13C NMR (150MHz,DMSO-d 6)ı149.8,147.9,144.3,143.9,133.3,131.9,128.9,128.6,126.9,125.7,125.0,122.5,122.4,119.0,118.3,115.2,112.1,110.7,65.4,60.1,65.89,20.8.HRMS (ESI+)calcd for C 22H 20N 3O 2[M+H]+358.1556,found 358.1549.2.3.General spectroscopic proceduresCompound 1was dissolved in MeOH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0)to afford the test solution (1.0␮M).Titration experiments were carried out in 10-mm quartz cuvettes at 25◦C.Metal ions (as chloride or nitrate salts,10mM)and anions (as sodium salts,10mM and 100mM)in MeOH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0)were added to the host solution and used for the titration experiment.1–Cu 2+solution for cyanide detection was prepared by addition of 1.0equiv.of Cu 2+to compound 1solution in MeOH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0).3.Results and discussionTo explore the photophysical properties,the fluorescence spec-trum of 1in MeOH/H 2O (1:1,v/v)solution under different pH conditions was examined first (Fig.S1).The results reveal that sen-sor 1exhibits strong fluorescence emission intensity at pH 7–8.The fluorescence intensity decrease of sensor 1at low pH values could be attributed to protonation of benzimidazole moiety that lead to proton-induced fluorescence quenching [59].Under high pH conditions,the phenol OH underwent deprotonation and might result in photoinduced electron transfer (PET)from the deproto-nated phenol to benzimidazole moiety,this PET effect quenched the fluorescence emission of sensor 1[60].Thus,pH of 7was selected as working moiety for further studies.Subsequently,thefluorescence spectrum changes of solution 1(1.0␮M)in MeOH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0)before and after addi-tion of different metal ions were examined.As depicted in Fig.1,metal free solution 1shows a strong fluorescence emission at 429nm which can be ascribed to the benzimidazole Schiff base skeleton.Interestingly,addition of 1.0equiv.of Cu 2+resulted in a remarkably quench of fluorescence intensity.Whereas,upon addi-tion of other metal ions such as Hg 2+,Fe 3+,Na +,K +,Fe 2+,Ag +,Pb 2+,Sr 2+,Ba 2+,Cd 2+,Ni 2+,Co 2+,Mn 2+,Zn 2+,Al 3+,Cr 3+and Mg 2+(1.0equiv.for each),no significant variations in the fluorescence intensity of 1occurred.Time-dependent fluorescence change stud-ies revealed that the Cu 2+induced fluorescence quenching can complete within 1min (Fig.S2).These observations demonstrate that sensor 1is highly selective to Cu 2+over other meal ions.Subsequently,fluorescence titration of 1(1.0␮M)by different amounts of Cu 2+(0–1.0␮M)was then investigated.With increasing the Cu 2+concentration,the original strong fluorescence emission was gradually quenched and reached saturation when 1.0equiv.of Cu 2+was added.Further increasing of Cu 2+amount did not cause significant fluorescence emission changes (Fig.2).When assum-ing a 1:1binding mode,the association constant (K a )of 1and Cu 2+was determined by using the Benesi–Hildebrand equation.Plotting of 1/(F 0−F )versus 1/[Cu 2+]showed a good linear rela-tionship (R =0.994),which indicates the 1:1interaction between 1and Cu 2+(Fig.S3),and the association constant K a was calcu-lated to be 6.59×105M −1.The 1:1binding stoichiometry of 1and Cu 2+was further verified by Job’s plot.The fluorescence intensity at 429nm exhibited aturning point when the molar fraction of Cu 2+isFig.1.Fluorescence spectra changes of solution 1(1.0␮M)upon addition of a spe-cific metal ion (1.0␮M)in CH 3OH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0)with an excitation at 357nm.Inset:photograph of 1and 1+Cu 2+taken under illumination of a UV lamp at 365nm.864L.Tang,M.Cai /Sensors and Actuators B 173 (2012) 862–867F l u o r e s c e n c e I n t e n s i t y (a .u .)Wavelength (nm)Fig.2.Changes in fluorescence spectra of solution 1(1.0␮M)upon incrementaladdition Cu 2+ion (0–1.0␮M)in CH 3OH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0)with an excitation at 357nm.Inset:Benesi–Hildebrand plot (emission at 429nm)of 1by plotting 1/(F 0−F )as a function 1/[Cu 2+].50%,which further proved the 1:1binding stoichiometry between 1and Cu 2+(Fig.S4).These results reveal that sensor 1exhibits an excellent sensing property to Cu 2+ion and binds Cu 2+via a 1:1stoichiometry.UV–vis absorption changes of solution 1(20␮M)upon addition of miscellaneous metal ions were also explored.Upon addition of 1.0equiv.of Cu 2+to solution 1,the original absorption at 356nm greatly decreased,concomitantly,a new absorption band centered at 426nm appeared.Other tested metal ions did not induce such absorption changes (Fig.S5).The results of UV–vis absorption titra-tion of solution 1by Cu 2+showed two clear isosbestic points at 341and 378nm (Fig.S6),indicating the formation of a stable Cu 2+complex.Thus,the observed Cu 2+induced fluorescence quenching of solution 1could be attributed to the formation of 1–Cu 2+com-plex,in which the paramagnetic Cu 2+quenched the fluorescence emission of 1through electron or energy transfer [61,62].As a highly selective probe for the target analyte,the good capacity of resisting disturbance is required.Hence,the competi-tion experiments in the presence of potentially competitive metal ions were examined by fluorescence spectra.Except for Cu 2+,other metal ions (1.0equiv.to 1)do not produce significant fluorescence intensity changes (Fig.3).Upon addition of Cu 2+(1.0equiv.to 1)to the solution containing 1and other metal ion,the fluorescence emission of the solutions was almost completely quenched.These results demonstrate that the Cu 2+recognition by 1is barely inter-fered by other coexisting metal ions.According to the fluorescence titration data,the limit of detec-tion of sensor 1for Cu 2+is evaluated to be 1.82×10−8M (Fig.S7,see supplementary data for detailed calculation)[63],which is much lower than the limit of copper in drinking water (∼20␮M)and the typical concentration of blood copper (15.7–23.6␮M)in normal individuals defined by the U.S.Environmental Protection Agency [11,64].Such a low detection limit makes sensor 1is sensitive enough to monitor Cu 2+concentration in water.To get some insight into the binding mode between 1and Cu 2+,the IR spectra of 1and 1–Cu 2+were measured and compared (Fig.S8).IR spectrum of 1exhibited a strong band at 1618cm −1characteristic of CH N stretching vibration [65],which split and shifted to 1621and 1633cm −1after 1complexed with Cu 2+.The shifting of CH N band to the higher frequency side provided evi-dence in support of the involvement of CH N nitrogen atom in coordination with Cu 2+[66].Additionally,the in-plane bending vibration band of phenolic O H of 1at 1401cm −1disappearedinH B F PNC Z F CF l u o r e s c e n c e I n t e n s i t y (a .u .Fig.3.The fluorescence of solution 1(1.0␮M)at 429nm to various metal ions.The gray bars represent the fluorescence of 1in the presence of 1.0␮M of miscellaneous metal ions,the red bars represent the fluorescence of the above solution upon sub-sequent addition of 1.0␮M of Cu 2+.(For interpretation of the references to color in this figure legend,the reader is referred to the web version of the article.)the IR spectrum of 1–Cu 2+,which indicating the coordination of deprotonated phenolic oxygen atom to Cu 2+[67].In view of the extremely high stability constants of cyanide and copper ions [54],1–Cu 2+complex was expected to act as a poten-tial indirect sensor for cyanide recognition via Cu 2+displacement approach.Thus,the sensor 1–Cu 2+was prepared in situ by addi-tion of 1.0equiv.of Cu 2+to solution 1.The responses of sensor 1–Cu 2+(1.0␮M)to a varietyof anions including F −,Cl −,Br −,I −,AcO −,HSO 4−,H 2PO 4−,HPO 42−,PO 43−,SCN −,NO 3−,CO 32−,HCO 3−,ClO 4−and CN −(all used as sodium salt,100equiv.for each)were subsequently investigated (Fig.4).Only CN −could induce the fluo-rescence intensity of 1–Cu 2+solution greatly enhanced and almost restored to the original emission state of solution 1.The emission changes can be readily distinguished by naked eye (Fig.4,in set).Time-dependent fluorescence change evaluation revealed that the CN −induced fluorescence revival can complete within 1h (Fig.S9).Fig.4.Changes in fluorescence spectra of 1–Cu 2+(1.0␮M)to a specific anion (100␮M)in CH 3OH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0)with an excitation at 357nm.Inset:photograph of 1+Cu 2+and 1+Cu 2++CN −taken under illumination of a UV lamp at 365nm.L.Tang,M.Cai /Sensors and Actuators B 173 (2012) 862–867865Fig.5.Proposed mechanism of cyanide recognition by 1–Cu 2+.These results indicate that the added CN −can snatch Cu 2+from 1–Cu 2+complex and lead to the fluorescence revival of 1.To further understand the Cu 2+displacement approach,UV–vis absorption changes of 1–Cu 2+(20␮M)in the absence and pres-ence of different anions were also investigated.The absorption band at 426nm greatly decreased upon addition of 100equiv.of CN −,however,addition of other anions did not induce obvious absorption changes (Fig.S10).Upon incremental addition of CN −(0–100equiv.)into solution 1–Cu 2+,the absorption band at 426nm decreased gradually and finally restored to the original absorption state of 1when 100equiv.of CN −was added (Fig.S11).These results demonstrate that the CN −induced Cu 2+displacement indeed hap-pen.Based on the aforementioned IR spectra comparison results as well as the binding mode of Cu 2+to ligands that structural similar to 1[68–70],the binding of 1with Cu 2+and the mechanism of cyanide recognition by 1–Cu 2+were proposed and illustrated in Fig.5.The high selectivity and fluorescence “off–on”change make sensor 1–Cu 2+a good candidate for highly selective recognition of CN −.The CN −sensing property of 1–Cu 2+was then evaluated by flu-orescence titration experiments and the results were depicted in Fig.6.Upon incremental addition of CN −into 1–Cu 2+solution,the fluorescence intensity increased gradually and reached saturation when 100equiv.of CN −was added.Fluorescence titration results show that the original strong emission of 1was gradually restored200400600800F l u o r e s c e n c e I n t e n s i t y (a .u .)Wavelength (nm)Fig.6.Changes in fluorescence spectra of 1–Cu 2+(1.0␮M)upon incremental addi-tion CN −ion (0–100␮M)in CH 3OH/H 2O (1:1,v/v,HEPES 20mM,pH =7.0)with an excitation at 357nm.with the increasing of CN −amount,which indicating the Cu 2+abstraction of 1–Cu 2+by cyanide ions.The fluorescence titration results also demonstrate the good sensitivity of 1–Cu 2+to CN −.Subsequently,fluorescence competition experiments were con-ducted to further evaluate the anti-jamming ability of sensor 1–Cu 2+.The results reveal that coexistence of equal amount of other anion does not induce any significant interference on CN −recognition (Fig.7).The fluorescence intensities of the competi-tive anion containing tested solutions were almost restored to the level that caused by CN −only.Results of these competition exper-iments reveal that the CN −recognition by sensor 1–Cu 2+is barely interfered by other coexisting anions.In addition,the detection limit of 1–Cu 2+for CN −was also eval-uated and was calculated to be 1.62×10−6M (Fig.S12),which is lower than the upper limit 1.9␮M for cyanide in drinking water set by the World Health Organization [71].This result indicates that sensor 1–Cu 2+hasa good sensitivity toward cyanide and has the potential utility to detect cyanide concentration in water.Different from the previously developed monofunctional Cu 2+sensing probes [10–24],sensor 1has the ability of highly selective and sensitive sequential recognition to Cu 2+and CN −.The in situ prepared cyanide sensor 1–Cu 2+can overcome the drawbacks such as moderate selectivity and only work in organic solvent of someFig.7.The fluorescence of 1–Cu 2+(1.0␮M)at 429nm to various anions.The graybars represent the fluorescence of 1in the presence of 100␮M of miscellaneous anions,the red bars represent the fluorescence of the above solution upon subse-quent addition of 100␮M of CN −.(For interpretation of the references to color in this figure legend,the reader is referred to the web version of the article.)866L.Tang,M.Cai/Sensors and Actuators B173 (2012) 862–867cyanide probes[28,30].Moreover,the limitations such as lack of reversibility or the use of elevated reaction temperature of the chemodosimetric cyanide probes[32–48]can also be nullified. 4.ConclusionsIn summary,a new benzimidazole based sensor(1)for highly selectivefluorescent recognition of Cu2+has been developed.The in situ prepared sensor1–Cu2+for successive recognition of cyanide was also investigated.Sensor1behaves highly selectivity and sen-sitivity toward Cu2+over other heavy and transition metal ions.The in situ prepared1–Cu2+complex exhibits highly selective cyanide recognition property in the presence of other common interfer-ence anions.The Cu2+and cyanide successive recognition feature and the low detection limits make1has potential utilities for Cu2+ and cyanide detection in environmental monitoring work. AcknowledgementsWe are grateful to the NSFC(21176029),the Natural Science Foundation of Liaoning Province(20102004)and the Program for Liaoning Excellent Talents in University(2012)forfinancial sup-port.This work was also sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars,State Edu-cation Ministry.Appendix A.Supplementary dataSupplementary data associated with this article can be found,in the online version,at /10.1016/j.snb.2012.07.112. 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