A fast-response, fluorescent 'turn-on' chemosensor for selective detection of Cr3+

Article abstract of DOI:10.1039/c5ra11460h

A fast-response, highly selective and sensitive chemosensor, 3, for Cr³⁺ detection with turn-on fluorescence behavior in the physiological pH range was designed and synthesized. The chemosensor contained a combined push-pull system in which the

Full text of DOI:10.1039/c5ra11460h

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A fast-response, fluorescent ‘turn-on’ chemosensor for selective
3+
detection of Cr
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
c
a
c
Chunhua Fan , Ximing Huang , Cory A. Black , Xingxing Shen , Junjie Qi , Yuanping Yi , Zhengliang
a,
a,
Lu *, Yong Nie, Guoxin Sun *
DOI: 10.1039/x0xx00000x
3
+
www.rsc.org/
A fast-response, highly selective and sensitive chemosensor, 3, for Cr detection with turn-on fluorescence behavior in the
physiological pH range was designed and synthesized. The chemosensor contained a combined push-pull system in which
the fluorescent phenanthro[9,10-d]oxazole moiety acts as both the electron donor and a potential binding site. An
electron deficient nitrile group served as the electron acceptor. A significant enhancement of fluorescene emission
3+
intensities was observed with increasing Cr concentration upon excition at 300 nm. The emission intensity reached its
3
+
3+
maximum on adding 8 equiv. of Cr where the quantum yield of 3-Cr was found to be 0.917, ca. 7-fold larger than the
3
+
chemosensor 3.The selectivity mechanism of 3 for Cr was found to be based on the combined effects of the inhibition of
ICT and CHEF. Remarkably the entire process was virtually complete in only 10 seconds, with a minimum detection limit
3
+
–8
–1
M .
for Cr of 1.72 × 10
mechanism which involves paramagnetic luminescence quenching of the
3
+
13-15
Introduction
Cr fluorophore.
Great efforts have also been made on designing turn-
on sensors because turn-off sensors tend to produce a low signal output
Fast-response sensors for detection of heavy and transition metal ions have
been attracting considerable attention due to their potential application in
upon binding and are therefore prone to interfere with the temporal
1
6-19
1
-4
separation of similar complexes with time-resolved fluorometry.
biological and environmental systems.
micronutrients in the human body, the trivalent form of chromium, Cr
plays crucial role in effectively maintaining the metabolism of
As one of the essential
3
+
3
+
,
Unfortunately, most turn-on chemosensors for detection of Cr have
3+
focused on rhodamine derivatives, which can probe Cr by conversion of
a
5
3+
the non-fluorescent rhodamine spirolactam to the highly fluorescent ring-
carbohydrates, adipose cells and proteins. However, Cr deficiency within
20-25
open amide form upon binding.
Furthermore, the long equilibrium time
biological systems adversely increases the risk of maturity-onset diabetes,
3
+
and high limits of detection of rhodamine derivatives do not satisfy the
requirement of fast-response and high sensitivity under pharmacological
conditions. The limitations of rhodamine-based chemosensors have
therefore inspired us to develop novel fast-response, highly selective and
cardiovascular diseases and nervous system disorders. High levels of Cr can
negatively affect cellular structures and damage cellular components by
6
-8
forming reactive oxygen species. Furthermore, the use of chromium in
common industrial processes such as, dyes and paints manufacturing, alloy
production and metallurgy also engenders serious environmental pollution
3
+
sensitive chemosensors for Cr detection.
3
+
6+
9, 10
as Cr could be converted to the more toxic Cr by redox cycling.
Thus
A number of different mechanisms, including intramolecular charge transfer
from a health and environmental point of view, it is urgent to develop highly
(ICT), chelation enhanced fluorescence (CHEF) and fluorescence resonance
3
+
selective chemosensors for Cr detection. At present, the fluorescence
energy transfer (FRET), have been extensively used to design fluorescent
turn-on chemosensors. We envisioned that combination of ICT and CHEF
mechanisms could be simultaneously applied to construct fluorenscence
3
+
spectroscopy for trace Cr detection is an effective method due to its high
sensitivity and selectivity, operational simplicity for real-time imaging. In
contrast,
traditional
analytical
techniques
such
as
atomic
3+
turn-on sensors of Cr . Usually ICT will take place upon excitation of a
absorption/emission spectroscopy of inductively coupled plasma mass
molecule containing both an electron rich group and an electron deficient
group. CHEF could turn on or off intramolecular charge transfer from the
donor to the acceptor, which thereby affects the fluorescence emission
intensity of the fluorophore. Based on our speculation above and
continuation of our work, we herein describe the design and synthesis of a
novel fluorescent probe 3'-(1H-phenanthro[9,10-d]imidazol-2-yl)-4'-(pyridin-
1
1, 12
spectrometry are less effective.
There are many literature reports of
chemosensors that have been designed to make use of the turn-off
2
-ylmethoxy)-[1,1'-biphenyl]-4-carbonitrile (3) which displays
a
fast
3
+
fluorescent turn-on response to Cr . Sensor 3 contains a combined push-
pull system in which the fluorescent phenanthro[9,10-d]oxazole moiety not
only acts as the electron donor but also provides a potential binding site for
3
+
Cr . 3'-formyl-4'-(pyridin-2-ylmethoxy)biphenyl-4-carbonitrile was chosen
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as the electron acceptor and the potential chelating unit, in which the an (0.063 g, 0.2 mmol) and a drop of acetic acid were added to the mixture,
electron deficient CN group is able to promote ICT. As anticipated, sensor 3 which was again heated to reflux for 2 h, cooled, and the crude product was
3
+
gives a fast turn-on response for Cr detection after only 10 seconds.
filtered and washed with ethanol to afford 0.045 g of compound 3 (0.045 g,
1
4
9
8
=
7
5%) as a light yellow solid. H NMR (400 MHz, CDCl
3
) δ ppm: 12.98 (s, 1H),
.00 (t, J = 5.69 Hz, 2H), 8.84 (d, J = 7.59 Hz, 1H), 8.79 (d, J = 8.26 Hz, 1H),
.74 (d, J = 8.26 Hz, 1H), 8.40 (d, J = 7.78 Hz, 1H), 7.75-7.85 (m, 6H), 7.73 (d, J
4.93 Hz, 1H), 7.67 (d, J = 7.78 Hz, 2H), 7.60 (dd, J = 8.70, 2.42 Hz, 1H), 7.42-
Experimental
Materials and instruments
1
3
3
.48 (m, 2H), 7.24 (d, J = 8.46 Hz, 1H), 5.53 (s, 2H). C NMR (100 MHz, CDCl )
All solvents were purified using standard methods. All starting materials
δ ppm: 155.4558, 155.3922, 149.9561, 146.5190, 144.5912, 137.1551,
1
13
were used as received. H and C NMR were performed on a 400 MHz/100
MHz Bruker Advance DRX 400 spectrometer. High resolution mass
measurements were carried out on a Waters-Q-TOF-Premier (ESI) or a
Shimadzu LCMS-IT-TOF (ESI). Elemental analysis (C, H and N) was carried out
using a Perkin-Elmer 4100 elemental analyzer. UV-Vis absorption spectra
were measured on a Shimadzu UV-2100 spectrophotometer. Fluorescence
spectra were obtained on an F-380 spectrofluorophotometer.
1
1
5
32.5776, 127.4694, 123.5267, 121.5182, 121.3903, 119.0652, 113.3245,
+
+
10.6370, 70.5648. HRMS (ESI , Fig. S8) calc. for C34
H
22
N
4
O (M + H )
03.1866, found 503.1894.
Synthesis of 2-(pyridin-2-ylmethoxy)benzaldehyde (4)
Compound 4 was synthesized using the same procedure as compound 2
1
(0.671 g, 31.5%). H NMR (400 MHz, CDCl
3
) δ ppm : 10.61 (s, 1H), 8.60 (d, J =
4
7
.74 Hz, 1H), 7.85(d, J = 7.76 Hz, 1H), 7.74(td, J = 7.78, 1.81 Hz, 1H), 7.49-
Synthesis of 3'-formyl-4'-hydroxybiphenyl-4-carbonitrile (1)
1
3
.56 (m, 2H), 7.25(t, J = 5.04 Hz, 1H), 7.05(t, J = 7.82 Hz, 2H), 5.31 (s, 2H).
) δ ppm: 189.5491, 160.5508, 156.2593, 149.3338,
37.0619, 136.0331, 128.7990, 125.0443, 122.9736, 121.2581, 112.9863,
0.9946. Anal. calc. for C13 : C, 73.23; H, 5.20; N, 6.57; found: C, 73.28;
C
26, 27
Compound 1 was synthesized according to a modified procedure.
Dry
NMR (100 MHz, CDCl
3
paraformaldehyde (6.6 g) was added to a mixture of 4'-hydroxybiphenyl-4-
carbonitrile (3.1 g, 16 mmol), triethylamine (8.4 mL, 61 mmol) and
1
7
H11NO
2
2
anhydrous MgCl (2.3 g, 24 mmol) in dry acetonitrile (50 mL). The mixture
H, 5.15; N, 6.61.
was heated to reflux for 6 h, and then cooled to room temperature, acidified
with 1M HCl, and extracted with ethyl acetate (3 × 20 mL). The combined
Synthesis
of
2-(2-(pyridin-2-ylmethoxy)phenyl)-1H-
4
organic layer was washed with water and dried over MgSO . The crude
phenanthro[9,10-d]imidazole (5)
material was purified by column chromatography to give 1.95 g of the title
1
A mixture of 9,10-phenanthrenequinone (0.42 g, 2 mmol), ammonium
acetate (1.5 g, 2 mmol) was suspended in a solution of ethanol (30 mL) and
dichloromethane (3 mL). The suspension was heated to reflux until all solids
were dissolved and then cooled to room temperature. 2-(Pyridin-2-
ylmethoxy)benzaldehyde (0.426 g, 2 mmol) and a drop of acetic acid were
added to the mixture which was again heated to reflux for 2 h, cooled, and
compound as a white solid in a 55% yield. H NMR (400 MHz, CDCl
3
) δ ppm:
1
7
1
7
2
1.11 (s, 1H), 9.99 (s, 1H), 7.73-7.78 (m, 4H), 7.66 (dt, J = 8.59, 2.12 Hz, 2H),
1
3
.12 (d, J = 8.37 Hz, 1H). C NMR (100 MHz, CDCl
3
) δ ppm: 196.4555,
61.9312, 135.5569, 132.8403, 132.2187, 127.1455, 120.8202, 111.0172,
+
+
7.4160, 77.0977, 76.7798. HRMS (ESI , Fig. S3) calc. for C19
H19NO
2
(M + H )
23.0633, found 223.0734.
the crude product was filtered and washed with ethanol to afford 0.62 g of
1
compound 5 (0.62 g, 78%) as a yellow solid). H NMR (400 MHz, CDCl
3
) δ
Synthesis
carbonitrile (2)
(1.1 g, 5 mmol), 2-(chloromethyl)pyridine hydrochloride (0.82 g, 5 mmol),
anhydrous potassium carbonate (3.5 g, 25 mmol) and potassium iodide
0.42 g, 2.5 mmol) were dissolved in dry acetonitrile (35 mL). The mixture
was refluxed for 6 h, cooled to room temperature and filtered. The filtrate
was washed with ethyl acetate (3 × 20 mL), dried over MgSO and the
of
3'-formyl-4'-(pyridin-2-ylmethoxy)biphenyl-4-
ppm : 12.93 (s, 1H), 9.00 (dt, J = 4.35, 1.54 Hz, 1H), 8.82 (dd, J = 7.96, 1.12
Hz, 1H), 8.76 (dt, J = 7.79, 1.75 Hz, 2H), 8.71 (d, J = 8.33 Hz, 1H), 8.37 (dd, J =
2
7
2
.95, 1.13 Hz, 1H), 7.80 (td, J = 7.70, 1.77 Hz, 1H), 7.66-7.76 (m, 2H), 7.62 (m,
H), 7.41 (d, J = 7.59 Hz, 3H), 7.23 (td, J = 7.82, 1.07 Hz, 1H), 7.18 (d, J = 8.24
(
13
3
Hz, 1H), 5.49 (s, 2H). C NMR (100 MHz, CDCl ) δ ppm: 155.7294, 155.1141,
1
1
49.8573, 147.1222, 137.0730, 130.1922, 129.8430, 128.2731, 126.7600,
24.9979, 123.3831, 122.3351, 121.5181, 119.3915, 112.6859, 70.4111.
4
solvent was removed under reduced pressure. The crude product was
+
+
19 3
HRMS (ESI , Fig. S11) calc. for C27H N O (M + H ) 402.1601, found 402.1599.
purified by column chromatography to afford 0.8 g of the pure product as a
1
yellow solid in 51% yield. H NMR (400 MHz, CDCl
3
) δ ppm: 10.65 (s, 1H),
8
=
7
.64 (d, J = 5.57 Hz, 1H), 8.11 (d, J = 2.51 Hz, 1H), 7.76–7.78 (m, 2H), 7.72(d, J
8.34 Hz, 2H), 7.67 (d, J = 8.60 Hz, 2H), 7.55 (d, J = 7.82 Hz, 1H), 7.30 (dd, J =
1
3
.34, 5.10 Hz, 1H), 7.18 (d, J = 8.75 Hz, 1H), 5.40 (s, 2H). C NMR (100 MHz,
ppm : 189.1033, 160.8735, 155.8735, 149.5142, 143.7822,
37.1566, 134.3457, 125.3517, 123.1937, 121.4113, 113.9511, 111.0266,
3
CDCl ) δ
1
7
7
1.4160. Anal. calc. for C20
6.38; H, 4.52; N, 8.95.
14 2 2
H N O : C, 76.42; H, 4.49; N, 8.91; found: C,
Synthesis of 3'-(1H-phenanthro[9,10-d]imidazol-2-yl)-4'-(pyridin-2-
ylmethoxy)-[1,1'-biphenyl]-4-carbonitrile (3)
A
mixture of 9,10-phenanthrenequinone (0.042 g, 0.2 mmol) and
ammonium acetate (0.15 g, 2 mmol) was suspended in a solution of ethanol
15 mL) and dichloromethane (1.5 mL). The suspension was heated to reflux
until all solids were dissolved and then cooled to room temperature. 3
Scheme 1. Syntheses of chemosensor 3 and reference compound 5.
(
2
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2
9
charge transfer (ICT) as found previously. The emission intensity reached
Results and discussion
3
+
its maximum on adding 8 equiv. of Cr , which increased by 15 fold as
The synthesis of compound 3 is shown in Scheme 1. Briefly, 2 was prepared
by an elimination reaction between 2-(chloromethyl)pyridine hydrochloride
and 1, which was synthesized according to a previously reported procedure.
Compound 3 was obtained in fair yield by heating a mixture of 9,10-
phenanthrenequinone and 2 to reflux in acetonitrile for 6 h. All compounds
3+
compared with that of free 3. The quantum yield of 3-Cr was found to be
0
.917, a ca. 7-fold enhancement on the free sensor.
Furthermore, sensor 3 exhibited a good linear relationship between the
3+
2
intensity at 412 nm and Cr concentration with a R value of 0.996 (shown
1
13
were characterized by H NMR, C NMR, and MS (SI, Fig. S1-S13). A similar
−8
in Fig. 2). Based on this, the detection limit was evaluated to be 1.72 × 10
compound 2-(2-(pyridin-2-ylmethoxy)phenyl)-1H-phenanthro[9,10-
−1
M
(Table S1), which is 180-fold lower than the maximum level (0.96 μM) of
d]imidazole 5, without the cyanophenyl group found in 3, was synthesized in
30
total chromium in drinking water permitted by the WHO. Indeed, this
good yield with the same procedure, but using the commercially available 2-
3+
detection limit was found to be superior to all but two of the reported Cr
(pyridin-2-ylmethoxy)benzaldehyde as a starting material. Comparison of
21, 31
sensors in the literatures (Table S2).
A Job plot indicated a 1:1 binding
the fluorescence emission spectra of 3 and 5 showed a very weak
fluorescence emission peak at 393 nm for 3 which can be attributed to the
ICT from the phenanthro[9,10-d]imidazole moiety to the CN group.
stoichiometry with a maximum emission change observed at a mole ratio of
3+
3
1
a
:1 for sensor 3 and Cr (Fig. 3). The association constant (K ) of 4.44 × 10
–1
M
was calculated from a Benesi-Hildebrand plot (Fig. 4) using data
obtained from a UV-Vis titration. This value was found to be comparable to
most turn-on sensors reported in the literature (Table S2).
UV-Vis absorption studies
With compound 3 in hand, first absorption and fluorescence properties
were investigated in DMF-water (1:1, v/v) with HEPES buffer solution (pH =
2
8
7
.0) under excitation at 300 nm. The solution of 3 was colourless and
exhibited two mild absorption bands at 329 nm and 364 nm (Fig. S14). The
bands increased upon gradual addition of an aqueous solution of CrCl (0–8
5
000
000
7
Y = 444.84 + 6.77*10 X
3
4
2
equiv.), which could be attributed to an electron density increase of the
R = 0.996
imidazole moiety due to inhibition of ICT arising from metal chelation. After
3
+
3000
2000
binding Cr , the solution turned from colourless to light-blue immediately
upon exposure to UV light (λem = 365 nm). Such a dramatic colour change
under UV light could make compound 3 a sensitive turn-on chemosensor for
3
+
Cr detection.
1000
3
+
6000
6
5
4
3
2
1
000 0-8 equiv of Cr
0
4500
3000
1500
0
0
.00000 0.00002 0.00004 0.00006
000
000
000
000
000
0
3+
[Cr ]/M
Fig. 2. Fluorescence intensity at 412 nm of 3 versus increasing
3
+
3+
0
2
4
63 8
+
concentrations of Cr (Cr concentration: 0, 0.5, 1, 1.5, 2, 2.5, 3
equiv. of Cr
equiv., λex = 300 nm). Each spectrum was acquired 5 minutes after
3
+
Cr addition at room temperature.
8
6
4
2
00
00
00
00
0
300
400
500
λ/nm
600
700
Fig. 1. Change of emission spectra of sensor 3 (20 μM) upon the
3
+
gradual addition of Cr (0 to 8 equiv.). Inset: the plot of fluorescent
3
+
emission intensity at 412 nm as a function of Cr concentration
em = 300 nm).
(
λ
Fluorescence emission studies
3+
A weak fluorescence emission band for 3 at 393 nm in the absence of Cr
was observed in a DMF-water solution (1/1, v/v, 20 mM HEPES buffer at pH
7.0) upon excitation at 300 nm (Fig. 1). This band could be assigned to a
0.2
0.4
0.6
0.8
=
[
M]/[R]+[M]
strong intra-molecular charge transfer (ICT) band transition. The
fluorescence quantum yield was evaluated as being 0.127 with anthracene
3+
Fig. 3. Job plot for the complexation of Cr ion with 3 determined
by UV-vis method (at 412 nm, λex = 300 nm). Total concentration of
3
+
as a reference. Addition of 0.5 equiv. of Cr led to a strong emission at 412
3+
3
and Cr ions is 20 μM.
nm. This red shift from 393 nm to 412 nm can be attributed to the internal
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0
0
0
0
.0012 Y = A + B * X
A = 5.38268E-5
B = 1.21221E-8
.0008
.0004
.0000
0
20000 40000 60000 80000 100000
3
+
1
/[Cr ]
3
+
Fig. 4. Benesi-Hilderbrand plot of 3 with Cr (F is the fluorescence
3
+
Fig. 6. Fluorescence intensity of 3 (20 μM) in DMF-H
2
O (1:1, v/v) at
intensity of 3 in the presence of Cr , F
of free 3).
0
is the fluorescence intensity
4
12 nm after 8 equiv. of various caions (blue bars) and those after
3+
3+
further addition of 8 equiv. of Cr (red bars). 1, Blank; 2, Cr ; 3,
Ag ; 4, Al ; 5, Ba ; 6, Cd ; 7, Cu ; 8, Fe ; 9, Hg ; 10, K ; 11, Mn ;
+
3+
2+
2+
2+
3+
2+
+
2+
+
2+
2+
2+
1
2, Na ; 13, Ni ; 14, Pb ; 15, Zn . λex = 300 nm.
6000
5000
4000
3000
2000
1000
0
Fig. 7. Colour change of 3 upon interaction with tested cations (top:
under natural light; bottom: under UV light). From left to right:
3
+
+
3+
2+
2+
2+
3+
2+
+
2+
+
2+
2
4
6
8
10
12
14
Blank, Cr , Ag , Al , Ba , Cd , Cu , Fe , Hg , K , Mn , Na , Ni ,
pH
2+
2+
.
Pb , Zn
Fig. 5. Fluorescence response at 393 nm for chemosensor 3 (black
3
+
line, 20 μM, λex = 300 nm) and at 412 nm for complex 3-Cr (red
6
4
3
1
000
line) as a function of pH in DMF-H O (1:1, v/v); the pH was adjusted
Cr(NO3)3
2
CrCl3
using 1 M aqueous solutions of HCl or NaOH.
CrK(SO4)
2
500
000
500
0
(
CH COO) Cr
pH range
3
3
K CrO4
2
The influence of pH on fluorescence intensity of chemosensor 3 in the
Blank
3
+
absence and presence of Cr was investigated at various pH values in the
DMF-H O (1:1, v/v) solution. The fluorescence response at varying pH is
shown in Fig. 5, with maximum emissions at 393 nm and 412 nm for the free
2
3
+
sensor and the complex 3-Cr , respectively. The emission at 390 nm was
observed when the solution of the free chemosensor 3 was in a more acidic
environment (pH < 4.0) (λex = 360 nm). This is most likely because the
imidazole and pyridine groups become protonated at low pH. In contrast,
400
500
600
700
λ
/nm
3
+
the fluorescence intensity of 3-Cr was sharply quenched at pH 8.5 with the
3
+
Fig. 8. Fluorescence response of 3 (20 µM) with 10 equiv. of various
chromium salts in DMF-H O (1:1, v/v, HEPES, 20 mM, pH 7.0, λex
00 nm).
addition of base. This could likely be attributed to a release of Cr from the
2
=
complex at high pH which gave the free chemosensor in which ICT is on. This
3
+
3
good fluorescence response of 3 toward Cr in the 6.5-8.5 pH range
demonstrates that it can be used as a sensitive chemosensor under
physiological conditions.
Competition and response time
Competition experiments to study the selectivity of chemosensor 3 with
various metal ions were performed and the respective fluorescence
4
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3
+
intensities are displayed in Fig. 6. When the titration was conducted in a Table 1 Determination of Cr with sensor 3 in samples (n = 5).
3
+
2
DMF-H O solution (1/1, v/v, HEPES 20 mM, pH 7.0), only Cr induced a
3
+
3+
significant fluorescence enhancement although Al and Fe induced a weak
fluorescence change. However, the colour change from colourless to blue
3+
3+
3+
was only observed for Cr and not for Al and Fe . Other competitive metal
+
+
+
2+
2+
2+
2+
+
2+
2+
2+
2+
3+
ions including Na , K , Ag , Hg , Cd , Cu , Ni , Li , Ni , Ba , Mn , Zn , Fe
2
+
and Pb did not show any obvious absorbance and fluorescence emission
change, even at a concentration of 50 equiv. of metal ions under
physiological conditions (Fig. 7). This remarkable colour change and
fluorescence response clearly demonstrated the good selectivity of sensor 3
3
+
7500
for Cr . Competition titration also revealed that sensor 3 demonstrated a
high affinity for trivalent cations with a strong positive charge (Fig. 6).
Similar results were observed by other research groups where rhodamine
0s
6
000
500
10s
3
2-34
derivatives were used as probe molecules.
on the fluorescent properties of sensor 3 was also explored (Fig. 8). In each
case the choice of anion did not have a significant effect although Cr(NO
The effect of choice of anion
30s
90s
4
120s
3 3
)
3
+
3000
gave a slightly higher intensity to the other Cr salts. This is likely due to the
difference between the binding ability and steric hindrance of the anions
1
500
used in this study. K
2
CrO
addition of Cr to a mixture of 3 and the other potentially competitive
metal ions (40 equiv.) mentioned above in a DMF-H O (1:1) solution, led to a
4
also demonstrated only a negligible response. The
3
+
0
2
400
500
600
significant fluorescence intensity enhancement (Fig. 9). These observations
λ/nm
demonstrated that sensor 3 could be used as an efficient fluorescence
3
+
chemosensor for Cr ion with high selectivity.
Fig. 10. Effect of time on the fluorescence intensity of 3 (20 μM) in
the presence of 8 equiv. of CrCl
(HEPES 20 mM).
3 2
in DMF-H O (1:1, v/v) at pH 7.0
7500
6000
4500
3000
1500
0
3
3+other cations
3
+
3+other cations+Cr
400
500
600
700
λ/nm
Scheme 2. Proposed mechanism for enhanced fluorescence
response of 3 upon addition of CrCl
3
.
Fig. 9. Fluorescence intensity of 3 (20 µM) in the absence of
metal ions (red curve), and presence of 8 equiv. of all kinds of Fluorescence recognition mechanism
competitive metal ions (blue curve) in DMF-water (1:1, v/v, pH 7.0,
It is well known that fluorescent turn-on chemosensors are preferable due
λ
ex = 300 nm). The black curve represents the addition of CrCl
3
to
35-37
to their highly selectivity, sensitivity and ease of observation . The
the above mixture.
3+
enhanced fluorescence response of chemosensor 3 when binding Cr could
be attributed to an interference of both intra-molecular charge transfer
Response time is a fundamental parameter for most reaction-based
(ICT) and chelation energy transfer (CHEF). To investigate the mechanism of
chemosensors, and the kinetic profile of the reaction of chemosensor 3 and
ICT, compound 5 was designed and synthesized and the fluorescence
3
+
Cr at room temperature was examined (Fig. 10). The fluorescence emission
3+
intensity of 5 in the absence and presence of Cr was performed (Fig. S15).
3
+
reached equilibrium within 10 seconds of injection of Cr into a solution of
chemosensor 3 (DMF-H O, 1:1, v/v, HEPES 10 mM, pH 7.0). The emission
intensity hardly changed over the subsequent 120 s, which confirmed that
As expected, there was no ICT observed for reference 5 due to the lack of an
2
3+
electron withdrawing group. Upon binding to Cr , a small enhancement in
fluorescence intensity was found as well as a peak shift from 393 nm to 408
nm, likely attributable to CHEF. A proposed mechanism for the enhanced
fluorescence response of 3 is depicted in scheme 2. In contrast to 5, the free
3
+
3
-Cr was stable. These results implied that our proposed chemosensor
3
+
3+
would provide a rapid analytical method for the detection of Cr . Cr in
river water and tap water samples was determined by sensor 3 to examine
the applicability of the proposed method. The satisfactory recovery results
were summarized in Table 1.
2
chemosensor 3 in a DMF/H O (1/1, v/v) solution shows a very weak
fluorescence emission at 393 nm, likely because electrons can transition
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DOI: 10.1039/C5RA11460H
ARTICLE
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from the highest occupied molecular orbital (HOMO) to the lowest
Conclusions
unoccupied molecular orbital (LUMO), which make ICT on in sensor 3. When
3
+
In summary, we have successfully designed and synthesized a simple
fluorescent chemosensor based on the combination of ICT and CHEF
mechanisms. The experimental results clearly indicated that chemsensor 3
the imidazole and pyridyl groups of sensor 3 chelate to Cr , the redox
potential of the donor is raised so that the relevant HOMO lowers in energy
compared to that of the fluorophore. Electrons in the excited state cannot
therefore return to the ground state and the intra-molecular charge transfer
is switched off due to the lack of conjugation in the phenanthroimidazole
moiety. Furthermore coordination likely causes a locally excited band so
that the emission band at 393 nm shifts bathochromically and a very large
enhancement of fluorescence at 412 nm is observed as a consequence.
3
+
was a highly sensitive and selective chemosensor for Cr in a DMF-H
2
O (1:1,
v/v, pH = 7.0) solution. Remarkably, the probe exhibited a fast turn-on
3+
fluorescence response to Cr within 10 seconds. The fluorescence
enhancement with high selectivity and sensitivity was attributed to ICT and
–8
-1
CHEF. It was found that the probe had a detection limit of 1.72×10
M and
worked within a pH range of 6.5-8.5 demonstrating its value for practical
application in physiological systems. However, the short emission
Theoretical study
wavelength of 412 nm of
3 limits its application. These excellent
3
+
To get insight into the optical response of 3 to Cr , The geometries of 3, 5,
and 3-CrCl in DMF solution were optimized by density functional theory
DFT) with the B3LYP functional and the LANL2DZ basis set for Cr and 6-
fluorescence results allow us to design new chemosensors with a long
emission wavelength through removing the electron-withdrawing groups or
changing the CN group to one electron donating group and the related
investigation is fully under way.
3
(
3
1G** basis set for the other atoms (Fig. S16), respectively. The polarizable
continuum model (PCM) was implemented to consider the solvent effect.
The calculation results showed that the ground state of 3-CrCl is a quartet
3
state and the spin density is mainly distributed on the Cr atom (see Fig. S17).
Acknowledgements
It can be seen that the flexibility of 3 would be decreased due to the
3
+
We thank the national Natural Science Foundation of China (grant No.
coordination with Cr .
2
1101074), Shandong Provincial Natural Science Foundation of China
(
grant No. ZR2013BQ009) and the Doctor’s Foundation of University of
Based on the optimized geometries, the singlet excited states for
compounds 3 and 5 in water and DMF solutions were obtained by time-
dependent DFT (TDDFT) at the B3LYP/6-31G** level in combination with the
Jinan (Grant No. XBS1320) for funding.
PCM. For both 3 and 5, the S
1
state arises from the HOMO → LUMO
transition. As shown in Fig. S18, the HOMO and LUMO for 5 are almost
Notes and references
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1
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