A turn-on Schiff ba fluorescence nsor for zinc ion
Wei Hsun Hsieh a ,Chin-Feng Wan b ,De-Jhong Liao a ,An-Tai Wu a ,⇑
a Department of Chemistry,National Changhua University of Education,Changhua 50058,Taiwan b
School of Applied Chemistry,Chung Shan Medical University,Taichung City 40201,Taiwan
a r t i c l e i n f o Article history:
Received 23June 2012Revid 24July 2012
Accepted 10August 2012
Available online 26August 2012Keywords:Chemonsor Schiff ba Fluorescence Turn-on
翁的组词组a b s t r a c t
如何培养创新意识A simple Schiff ba type fluorescent receptor 1was prepared and evaluated for its fluorescence respon to heavy metal ions.Receptor 1exhibits an ‘off–on-type’mode with high lectivity in the prence of Zn 2+ion.The lectivity of 1for Zn 2+is the conquence of combined effects of chelation-enhanced fluo-rescence (CHEF),C @N isomerization,and inhibition of photoinduced electron transfer (PET).
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The design and synthesis of new chemonsors for the efficient detection of trace metal ions are among the most important re-arch topics in environmental chemistry and biology.1The zinc ion (Zn 2+)is the cond most abundant and esntial trace element in the human body behind iron it is indispensable for mediating many enzyme-catalyzed reactions and therefore plays very important roles in a wide variety of physiological and pathological process.2–5Although Zn 2+is considered to be relatively nontoxic,particularly if taken orally,manifestations of overt toxicity symp-toms (naua,vomiting,lethargy,and fatigue)will occur with ex-tremely high Zn 2+intakes.Zn 2+is also known to have a role in neurological disorders,such as Parkinson’s dia,Alzheimer’s dia,amyotrophic lateral sclerosis,and epileptic izures.Therefore,the detection of Zn 2+in biological samples is of signifi-cant interest and importance.Plenty of fluorescent chemonsors for Zn 2+have been developed in the past decades.Most of them have been developed bad on quinoline,anthracene,coumarin,BODIPY,and fluorescein fluorophores;6–14however,some of them require complicated synthesis and are insoluble in polar solutions.Thus,for practical applications,it is necessary to develop polar Zn 2+nsors that are easily prepared,and posss lective and nsi-tive signaling mechanisms.
Schiff bas (imines)are known to be good ligands for metal ions.15–19In addition,Schiff ba deriva
tives incorporating a fluorescent moiety are appealing tools for optical nsing of metal ions.20–24Herein,we reported a highly nsitive fluorescence turn-on receptor 1to Zn 2+.More importantly,receptor 1can be readily prepared by a simple and low-cost Schiff ba reaction of 2-hydroxynaphthalene-1-carboxaldehyde with tris(2-amino-ethyl)amine,as shown in Scheme 1.
The fluorescence respon behaviors of receptor 1on various metal ions were investigated in EtOH–H 2O (95:5v/v).As shown in Figure 1,receptor 1alone displayed a very weak,single fluores-cence emission band at 450nm when it was excited at 308nm.In addition,only weak changes in fluorescence intensity were obrved upon addition of the other cations (Li +,Na +,K +,Ca 2+,Mn 2+,Hg 2+,Co 2+,Ni 2+,Cu 2+,Pb 2+,and Cd 2+)except for Zn 2+.Upon addition of Zn 2+,receptor 1exhibited a prominent fluorescence enhancement,indicating that a Zn 2+-lective off–on fluorescent signaling behavior occurred.Bad on the u of a UV lamp,only in the prence of Zn 2+,the solution of receptor 1showed a dra-matic color change from colorless to fluorescent blue,which could easily be detected by the naked-eye,while the addition of other metal ions did not show any detectable color change (Fig.2).
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Corresponding author.Fax:+88647211190.
E-mail address:ue.edu.tw (A.-T.Wu).
The fluorescence enhancement efficiency obrved at 450nm was 92-fold greater than the control in the abnce of Zn 2+(Fig.3).The obrved fluorescence enhancement may be attributed to the formation of a rigid system after binding with Zn 2+,causing the chelation-enhanced fluorescence (CHEF)effect.Alternatively,
this could also be explained by blocking the photoinduced electron transfer (PET)process and causing an enhancement in the emis-sion 25,26(Scheme 2).
Since pH value affects the charge distribution of receptor 1and may change its indigenous fluorescence properties,we also evaluated the effect of pH on the emission bands of receptor 1in EtOH–H 2O (95:5v/v).The fluorescence intensity of free receptor 1in EtOH–H 2O (95:5v/v)remained unaffected and no emission band shifts occurred over a wide pH span of 2–10(Fig.S3).There-fore,the pH change causing from exogenous addition of metal ions
Figure 2.Fluorescence changes excited by UV lamp of 1(35l M)in EtOH–H 2O (95:5v/v)upon addition of 10equiv of Zn 2+.
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450 nm
W.H.Hsieh et al./Tetrahedron Letters 53(2012)5848–5851
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白蛇传说
was eliminated as a possible factor contributing to the obrved fluorescence intensity changes.
To further investigate the chemonsing properties of receptor1,fluorescence titration of receptor1with Zn2+was performed.From thefluorescence titration profile(Fig.4),the association const
ant for1-Zn2+in EtOH–H2O(95:5v/v)was determined as 1.23Â107MÀ1by a Hill plot(Fig.S4).The obrved high K a value clearly indicates the strong affinity of Zn2+to receptor1.A Job plot indicated a1:1complexation stoichiometry between receptor1 and Zn2+(Fig.5).In addition,the formation of1:1complex between receptor1and Zn2+was further confirmed by the appearance of a peak at m/z827,assignable to[1+Zn2++3EtOH+H2OÀH+]in the ESI/MS(Fig.S5).The formation of such a Zn2+complex can induce a p–p stacking interaction between three naphthalene rings leading to a rather rigid structure with quite strongfluorescence properties, as compared to free receptor1.By using the above-mentionedfluo-rescence titration results,the detection limit for Zn2+was deter-mined as4.89Â10À8M.The detection limit was sufficiently low to detect submicromolar concentration of Zn2+,which belongs to the range found in many chemical and biological systems.
The lectivity toward Zn2+was further ascertained by the com-petition experiment.As shown in Figure S6,in the prence of Li+, Na+,K+,Ca2+,Mn2+,Pb2+,and Cd2+,the emission spectra were al-most identical to that obtained in the prence of Zn2+alone.In the ca of Co2+,Cu2+,Hg2+,and Ni2+,the emission intensities diminished to a different extent to that obtained in the prence of Zn2+alone,but they still had a sufficient turn-on ratio for the detection of Zn2+except Cu2+.Therefore,receptor1was shown to be a promising lectivefluorescent nsor for Zn2+in the pr
es-ence of most competing metal ions.It is worth noting that the receptor1for Zn2+did not have any interference from Cd2+ion. Cd2+and Zn2+have similar properties and generally cau a strong interference when they are placed in a solution together,which makes it difficult to distinguish each other.10,27Thus,the receptor 1displayed a considerable ability to distinguish Zn2+from Cd2+in a common solution.
To better understand the complexation behavior of receptor1 with Zn2+,1H NMR experiments were carried out in CD3OD.The spectral differences are depicted in Figure6.In the prence of 1.0equiv of Zn2+,the protons of imine(H2)and naphthalene were shifted down-field by0.24and0.19ppm,‘respectively’.It possibly resulted from naphthalene–metal p–d orbital interactions28,29 indicating the indirect naphthalene–metal interactions occurred through space.The proton in ethylene(H3)linker was shifted up-field by0.15ppm(Fig.6).In contrast,the proton in ethylene(H4) linker was almost unaffected.Meanwhile,the phenolic proton (H1)disappeared.The results suggested that receptor1chelated Zn2+through interactions with imine nitrogen and oxygen by a deprotonation process of the phenolic proton.
In summary,a simple Schiff ba receptor1was synthesized for lective detection of Zn2+.Upon complexation with Zn2+,receptor1 exhibited a pronounced enhancement in thefluorescence emissio
n spectrum.The formation of a rigid structure derived from imine nitrogen,oxygen of the phenolic group,and Zn2+inhibits the C@N isomerization and PET,leading tofluorescence enhancement. Acknowledgement
We thank the National Science Council of Taiwan forfinancial support.
Supplementary data
Supplementary data associated with this article can be found, in the online version,at /10.let.2012.
08.058.
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