This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 6329–6331 6329

A highly sensitive, selective, colorimetric and near-infrared fluorescent

turn-on chemosensor for Cu2+

based on BODIPYw

Shouchun Yin,ab Volker Leen,a Sven Van Snick,a Noel Boensa and Wim Dehaen*a

Received 7th June 2010, Accepted 28th June 2010

DOI: 10.1039/c0cc01772h

A new colorimetric and NIR fluorescent chemosensor (1) for

Cu2+ based on BODIPY is reported, displaying a highly

sensitive and selective fluorescent enhancement with Cu2+

among various metal ions, upon excitation at 620 nm in CH3CN.

Fluorescent chemosensors for the recognition and measure-

ment of transition metal ions are indispensable tools in life

science, medicine, chemistry and biotechnology.1 In the past

few years, much effort has gone into the design and develop-

ment of fluorescent chemosensors for the detection of Cu2+

because of its importance in environmental and bio-

logical systems.2 Cu2+ is the third most abundant (after Fe3+

and Zn2+) among the essential transition metal ions in the

human body. Cu2+ plays a central role in a variety of funda-

mental physiological processes in organisms, ranging from

bacteria to mammals.2 Cu2+ can cause neurodegenerative

diseases, such asMenkes disease, Wilson disease, and Alzheimer’s

disease.3 Most of the reported Cu2+ ion fluorescent chemo-

sensors work in a ‘‘turn-off’’ (i.e., quenching) mode due to the

paramagnetic nature of Cu2+.4 For sensitivity reasons, fluoro-

ionophores showing fluorescence enhancement (‘‘turn-on’’) as

a result of metal–ion binding are to be favored over those

exhibiting fluorescence quenching (‘‘turn-off’’). Recently,

some ‘‘turn-on’’ examples with a selective response to Cu2+

have been reported.5 However, some of these have short-

comings for practical applications such as cross-sensitivities

toward other metal cations and low fluorescence quantum

yields. Therefore, development of new Cu2+-selective turn-on

fluorescent probes is of the utmost importance and necessity.

BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) is a

popular fluorescent dye with widespread applications as a

fluorescent chemosensor due to its valuable characteristics,

such as sharp and intense absorption and fluorescence peaks in

the visible spectral region, high molar absorption coefficients

and fluorescence quantum yields, and robustness against light

and chemicals. Furthermore, BODIPY dyes are amenable to

structural modification so that the absorption and emission

bands can be shifted to the red or near infra-red (NIR) spectral

range.6 Probes with spectra in this wavelength region are

preferable for applications in biological systems because they

reduce autofluorescence and limit photodamage to living

cells.7

With this in mind, herein we report a novel colorimetric

and NIR fluorescent turn-on sensor, 5-N-(2-picolyl)amine-4,4-

difluoro-3-(4-(ethynyl)phenylethynyl)-8-(4-tolyl)-4-bora-3a,4a-

diaza-s-indacene (1), based on BODIPY with di(2-picolyl)amine

(DPA) as the ion recognition subunit. The attachment of a

p-diacetylene benzene residue to BODIPY at the 3-position

extends the conjugation of the system, resulting in bathochromic

spectral shifts.8 1 demonstrates a high selectivity toward Cu2+

over a wide range of metal ions tested and a remarkable

fluorescent enhancement in the presence of Cu2+ upon excita-

tion at 620 nm in CH3CN. To the best of our knowledge, this

is the first NIR turn-on fluorescent sensor for Cu2+ based on

BODIPY.

As outlined in Scheme 1, 3,5-dichloro-8-(4-tolyl)BODIPY9

was coupled with one equivalent of 1-ethynyl-4-(trimethyl-

silylethynyl)benzene under Sonagashira conditions to afford

the monosubstituted product 2 in 31% yield.10 Subsequent

silyl-deprotection of 2 in the presence of Bu4NF at �78 1C

provided 3. The remaining chlorine atom of 3 was then

substituted by DPA in CH3CN yielding 1 (ESIw).The photophysical properties of 1 upon addition of several

metal cations (Na+, K+, Mg2+, Ca2+, Ba2+, Cu2+, Zn2+,

Cd2+, Fe2+, Hg2+, Pb2+, Ni2+, Co2+, Ag+) in CH3CN were

investigated by UV–vis absorption and fluorescence

Scheme 1 Synthetic route to sensor 1.

aDepartment of Chemistry, Katholieke Universiteit Leuven,Celestijnenlaan 200f—bus 02404, 3001 Heverlee (Leuven), Belgium.E-mail: Wim.Dehaen@chem.kuleuven.be; Fax: +32 16 327990;Tel: +32 16 327439

bCollege of Material Chemistry and Chemical Engineering,Hangzhou Normal University, Hangzhou 310036, China

w Electronic supplementary information (ESI) available: Experimentalprocedures and full characterization of 1, 2 and 3; UV–vis absorption,fluorescence emission (excitation at 520 nm) spectra and photographsof solutions of 1 in the presence of metal ions; determination of Kd;ESI-MS of 1 and 1–Cu2+ complex; fluorescent emission spectra of 1 inthe presence of Ag+ upon addition of Cu2+. See DOI: 10.1039/c0cc01772h

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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6330 Chem. Commun., 2010, 46, 6329–6331 This journal is c The Royal Society of Chemistry 2010

spectroscopic measurements and titration studies. The UV–vis

absorption spectrum of 1 in CH3CN exhibits a main peak with

a maximum at 526 nm assigned to the S0 - S1 transition of

the BODIPY chromophore (Fig. 1). By using the above

mentioned set of metal cations (50 equiv.), it is clear that only

Cu2+ induces a new, significant, red-edge absorption band

(Fig. S1, ESIw). Upon titration of 1 with Cu2+, the main

absorption band at 526 nm progressively decreases in intensity

and shifts to 536 nm, while a new band at 621 nm increases

continuously in intensity. Similar changes are found in the

fluorescence excitation spectra. The new peak at 621 nm upon

addition of Cu2+ results in a naked eye color change from

pink to blue, indicating that 1 can be used as a colorimetric

probe for Cu2+ (Fig. S2, ESIw). The presence of three well-

defined isosbestic points at l = 357, 455 and 558 nm is con-

sistent with the presence of an equilibrium process corresponding

to apo 1 and copper complex 1–Cu2+.

Fig. 2 displays the fluorescence emission spectra of 1 (2 mM)

with the above mentioned metal cations (50 equiv.) measured

in CH3CN upon excitation at lex = 620 nm, where apo 1 has a

vanishingly small absorbance while 1–Cu2+ shows an

absorption maximum. As a consequence, apo 1 emits almost

no fluorescence upon excitation at this wavelength. When

Cu2+ ions are added to 1 in CH3CN, a new strong fluores-

cence emission with maximum at 651 nm appears. Under

the same conditions, no obvious fluorescence changes are

observed for the other metal ions tested. Using shorter excita-

tion wavelengths (e.g., 520 nm (Fig. S3, ESIw)) does not

produce significant shifts in the emission spectrum of 1 in

the presence (50 equiv.) of all metal ions tested except Cu2+.

Excitation at lex = 520 nm (as used in the inset of Fig. 2)

shows that the emission maximum of 1 located at 599 nm is

shifted to B640 nm upon addition of Cu2+. Those observa-

tions indicate that 1 has a very high sensitivity and selectivity

for Cu2+.

The fluorescence titration of 1 in the presence of different

Cu2+ concentrations was then performed. As shown in Fig. 3,

the fluorescence intensity with excitation at 620 nm increases

significantly by addition of Cu2+. Upon increasing the con-

centration of Cu2+ to 100 mM, there is an impressive fluores-

cence enhancement from the background level for 1 as a

result of the increase of the fluorescence quantum yield from

0.09 � 0.01 (for apo 1) to 0.72 � 0.03 (for 1–Cu2+) and the

parallel rise of absorbance at lex = 620 nm. The non-

linear fitting of the fluorometric titration data reveals a 1 : 1

stoichiometry for the 1–Cu2+ complex (Fig. S4, ESIw) with

Kd = 8.7 � 0.5 mM. Direct evidence for the formation of a

1 : 1 complex was obtained by comparing the ESI mass

spectra of 1 and 1–Cu2+ (Fig. S5, ESIw).11 The unique peak

at m/z = 666.3 (calcd = 666.2) corresponding to [1 + Cu]2+

was clearly observed when 10 equiv. of Cu2+ was added to 1 in

CH3CN.

To explore the use of 1 as an ion-selective fluorescent probe

for Cu2+, competition experiments were carried out. 1 (2 mM)

was treated with 5 equiv. Cu2+ in the presence of other metal

ions (25 equiv.). As shown in Fig. 4, the competing metal ions

exhibit only a small or no interference with the detection of

Cu2+ ions except for Ag+, which strongly quenches the

fluorescence of 1–Cu2+. The fluorometric titration of 1

(lex = 620 nm) in the presence of 25 equiv. Ag+ (Fig. S6, ESIw)

Fig. 1 UV–vis absorption spectra of 1 (2 mM) in the presence of

different concentrations of Cu2+ (0, 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 2.5,

3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 15.0, 20.0 mM) in CH3CN.

Fig. 2 Fluorescence emission spectra of 1 (2 mM) upon addition of

various metal ions (100 mM) in CH3CN (lex = 620 nm). Inset:

fluorescence emission spectra of 1 and 1–Cu2+ in CH3CN with

lex = 520 nm.

Fig. 3 Fluorescent emission spectra of 1 (2 mM) upon excitation at

620 nm in the presence of different concentrations of Cu2+ (0, 0.25,

0.5, 0.75, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 15.0,

20.0, 50.0, 100.0 mM) in CH3CN.

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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 6329–6331 6331

shows that the fluorescence intensity attributable to the

1–Cu2+ complex increases considerably upon increasing

Cu2+ concentration, indicating that Cu2+ competes favorably

with Ag+ for binding by the chemosensor.

In conclusion, we have reported a new NIR fluorescent

chemosensor 1 based on BODIPY with DPA as ligand. Using

a convenient red excitation wavelength (lex = 620 nm),

chemosensor 1 displays a significant fluorescence enhancement

in the presence of Cu2+ and a high selectivity toward Cu2+

among various metal ions in CH3CN.

Notes and references

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Fluorescent Probes and Labeling Technologies, Invitrogen-Molecular Probes, 10th edn, 2005.

2 E. Gaggelli, H. Kozlowski, D. Valensin and G. Valensin, Chem.Rev., 2006, 106, 1995; E. L. Que, D. W. Domaille and C. J. Chang,Chem. Rev., 2008, 108, 1517.

3 K. J. Barnham, C. L. Masters and A. I. Bush, Nat. Rev. DrugDiscovery, 2004, 3, 205.

4 S. H. Kim, J. S. Kim, S. M. Park and S. K. Chang, Org. Lett.,2006, 8, 371; J. Xie, M. Menand, S. Maisonneuve and R. Metivier,J. Org. Chem., 2007, 72, 5980; H. S. Jung, P. S. Kwon, J. W. Lee,J. I. Kim, C. S. Hong, J. W. Kim, S. H. Yan, J. Y. Lee, J. H. Lee,T. H. Joo and J. S. Kim, J. Am. Chem. Soc., 2009, 131, 2008;W. B. Chen, X. J. Tu and X. Q. Guo, Chem. Commun., 2009, 1736.

5 X. R. He, H. B. Liu, Y. L. Li, S. Wang, Y. J. Li, N. Wang,J. C. Xiao, X. Xu and D. B. Zhu, Adv. Mater., 2005, 17, 2811;Z. C. Wen, R. Yang, H. He and Y. B. Jiang, Chem. Commun.,2006, 106; X. Qi, E. J. Jun, L. Xu, S. J. Kim, J. S. J. Hong,Y. J. Yoon and J. Y. Yoon, J. Org. Chem., 2006, 71, 2881;Y. Xiang, A. J. Tong and Y. Ju, Org. Lett., 2006, 8, 2863;G. K. Li, Z. X. Xu, C. F. Chen and Z. T. Huang, Chem. Commun.,2008, 1774; M. X. Yu, M. Shi, Z. G. Chen, F. Y. Li, X. X. Li,Y. H. Gao, J. Xu, H. Yang, Z. G. Zhou, T. Yi and C. H. Huang,Chem.–Eur. J., 2008, 14, 6892; Y. Zhao, X. B. Zhang, Z. X. Han,L. Qiao, C. Y. Li, L. X. Jian, G. L. Shen and R. Q. Yu, Anal.Chem., 2009, 81, 7022; S. Goswami, D. Sen and N. K. Das, Org.Lett., 2010, 12, 856.

6 A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891; G. Ulrich,R. Ziessel and A. Harriman, Angew. Chem., Int. Ed., 2008, 47,1184.

7 B. A. Smith, W. J. Akers, W. M. Leevy, A. J. Lampkins,S. Z. Xiao, W. Wolter, M. A. Suckow, S. Achilefu andB. D. Smith, J. Am. Chem. Soc., 2010, 132, 67.

8 W. W. Qin, T. Rohand, W. Dehaen, J. N. Clifford, K. Driesen,D. Beljonne, B. Van Averbeke, M. Van der Auweraer andN. Boens, J. Phys. Chem., 2007, 111, 8588.

9 M. Baruah, W. W. Qin, N. Basaric, W. M. De Borggraeve andN. Boens, J. Org. Chem., 2005, 70, 4152; M. Baruah, W. W. Qin,R. A. L. Vallee, D. Beljonne, T. Rohand, W. Dehaen andN. Boens, Org. Lett., 2005, 7, 4377; W. W. Qin, T. Rohand,M. Baruah, A. Stefan, M. Van der Auweraer, W. Dehaen andN. Boens, Chem. Phys. Lett., 2006, 420, 562.

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Fig. 4 Fluorescence response of 1 (2 mM) containing 10 mM Cu2+ to

the selected metal ions (50 mM). F1+Cu and F1+Cu+M denote the

fluorescence signals of 1 in the presence of Cu2+ only and in the

presence of Cu2+ as well as the competing ion, respectively. Excitation

was at 620 nm and emission was at 651 nm.

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