A novel fluorescent distinguished probe for Cr (VI) in aqueous solution

Article abstract of DOI:10.1016/j.saa.2009.06.031

Salicylaldehyde rhodamine B hydrazone (SRBH) was developed as a new spectrofluorimetric probe for the selective and sensitive detection of CrO₄^2- in acidic conditions. The proposed method was based on the special oxidation reaction b

Full text of DOI:10.1016/j.saa.2009.06.031

Spectrochimica Acta Part A 74 (2009) 265–270
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A novel fluorescent distinguished probe for Cr (VI) in aqueous solution
a
b,∗
b
a
b
AiFang Zheng , JinLong Chen , Genhua Wu , Ganlin Wu , Yuan Guang Zhang ,
HePing Weia
a
Department of Life Sciences, Anqing Normal College, Anqing 246003, PR China
School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Functional Molecule, Anqing Normal College,
b
Anqing 246003, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Salicylaldehyde rhodamine B hydrazone (SRBH) was developed as a new spectrofluorimetric probe for
the selective and sensitive detection of CrO4 in acidic conditions. The proposed method was based
on the special oxidation reaction between non-fluorescent SRBH by potassium dichromate to produce a
highly fluorescent rhodamine B, as a product. Under the optimum conditions described, the fluorescence
Received 15 December 2008
Received in revised form 6 June 2009
Accepted 7 June 2009
2−
2
−
−8
enhancement at 591 nm was good linearly related to the concentration of CrO4
from 1.0 × 10 to
Keywords:
Chromium (VI)
Salicylaldehyde rhodamine B
Spectrofluorimetric
Oxidation reaction
−7
−1
2
3
.0 × 10 M (0.42–12.6 ng mL ) with a correlation coefficient of R = 0.9989 (n = 10) and a detection limit
−9
−1
of 1.5 × 10 M (0.063 ng mL ). The relative standard deviation (R.S.D.) was 2.0% (n = 6). The proposed
method was also successfully applied to the determination of chromium (VI) in drinking water, river
water and synthetic samples.
Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
2
−
2−
1. Introduction
tral and alkaline environments, while Cr O
in acidic conditions.
2
7
Alternately, the CrO42 can convert into Cr O7 under acidic envi-
−
2
Chromium (Cr) exists in different oxidation states in the ground-
water, seawater, and soil of our environment [1]. Trivalent Cr is an
essential trace element in the human body as a part of the glucose
tolerance factor and also plays an important role in lipid and protein
metabolism. However, hexavalent Cr form is well known to be high
toxic and carcinogenic. Chromium (VI) enters natural waters mainly
through effluents from the electroplating and tanning industries,
from sanitary landfill leaching, and from water-cooling towers. It
can also enter the drinking water distribution system from the cor-
rosion inhibitors used in the water pipes [2]. So, the concentration
of Cr (VI) in drinking water is strictly regulated to a lower micro-
molar level by the governments of many nations [3]. To monitor the
quality of drinking water and the risks of industrial wastes, many
methods with high sensitivity is explored to ensure the availabil-
ity of Cr (VI) determination at low but harmful concentration levels.
Furthermore, good selectivity is also necessary to tolerate the inter-
ference of some commonly coexistent foreign ions, such as Na (I),
ronment, and vice versa. In the past 20 years, concern about the
presence of hexavalent Cr in the environment resulted in the devel-
opment of numerous analytical techniques for the determination
of Cr (VI) in different sample matrices. Besides the flow injection
analysis [7,8], on-line selective determination of both species is
by chromatography [9–11], mass spectrometry [12,13] or atomic
absorption spectrometry [14–17]. Solvent extraction [18,19] and
ion exchange [20,21] are still one of the most extensively used
methods for separation and preconcentration of Cr (VI). They tend
to be complicated and expensive to apply even though endowing
with high selectivity and sensitivity. Recent, two types of micro-
cantilever sensors for highly sensitive detection of CrO42 have
been reported [22,23]. Among them, the spectrofluorimetry meth-
ods utilizing Cr (VI)-selective fluorescent probes are of particular
interests because of its rapid, simple, highly sensitive and selec-
tive advantages. However, many reported fluorescent probes for Cr
(VI) generally undergo a fluorescence quenching upon the oxida-
tion by Cr (VI), which is generally not as sensitive as a fluorescence
enhancement response. A series of fluorescent nanoparticles (NPs)
were synthesized and applied in selective detection of Cr (VI) ions
in water samples [24–28]. Up to date, a fluorescence enhancement
method for detection of Cr (VI) has been proposed based on the
oxidation of non-fluorescent rhodamine B hydrazide by Cr (VI) in
acidic aqueous solutions [29].
−
−
2−
−
,
K (I), Ca (II), Cd (II), Ni (II), Cu (II), Zn (II), Hg (II), Cl , SO4 , NO3
relatively non-toxic Cr (III) [4] and another pollutant As (V) [5,6].
2
−
2−
and CrO4
But Cr (VI) was found in two forms such as Cr O7
2
−
in the water matrix. As our well known, CrO42 is stable in neu-
∗ Corresponding author at: School of Chemistry and Chemical Engineering, Anhui
Key Laboratory of Functional Molecule, Anqing Normal College, Anqing 246003, PR
China. Tel.: +86 556 5500690; fax: +86 556 5500090.
In this paper, salicylaldehyde rhodamine B hydrazine (SRBH) as
a novel fluorescent probe was facilely synthesized and explored
to indicate Cr (VI) in aqueous solution. Based on a special oxida-
E-mail address: chenjl 4@hotmail.com (J. Chen).
1386-1425/$ – see front matter Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.06.031

2
66
A. Zheng et al. / Spectrochimica Acta Part A 74 (2009) 265–270
Scheme 1. Procedures for the synthesis of SRBH.
tion effect of Cr (VI) on the non-fluorescent SRBH to produce highly
fluorescent rhodamine B in acidic conditions as a product, a new
highly sensitive fluorescent and colorimetric method for Cr (VI)
was proposed. The advantages of this method over other reported
fluorescent procedures are: long emission wavelength (>550 nm)
avoiding the influence of background fluorescence (<400 nm); even
higher sensitivity due to the significant fluorescence enhancement
signals; and facile analysis at room temperature without heat-
ing and preconcentration. Furthermore, a significant pink color
occurred in the presence of Cr (VI), which could be developed a new
IR light, a high product yield of RBH as a pink solid was obtained for
use of followings. Rhodamine hydrazide (1, 0.46 g, 1.0 mmol) was
dissolved in 20 mL absolute ethanol. An excessive salicylaldehyde
(4.0 mmol) was added then the mixture was refluxed in an air bath
for 6 h. After that, the solution was cooled (concentrated to 10 mL)
and allowed to stand at room temperature overnight. The precipi-
tate that appeared next day was filtered and washed 3 times with
10 mL cold ethanol. After being dried under reduced pressure, and
the reaction afforded 0.30 g, 2 (55%) as a white solid (as shown in
Scheme 1).
+
“naked-eye” method for indication of Cr (VI) in aqueous solutions.
Compound 2. ESI mass spectrometry: m/z 545.3 (95% [M+H] ),
567.1 (5% [M+Na] ); M calculated 544.3. 1H-NMR (CDCl ), ␦
+
+
In addition, SRBH could be served as an effective probe for illumina-
3
2−
2−
tion of the stoichiometric ratio as 1:2 of Cr O7 and CrO4 based
(ppm): 1.15 (t, 12H, NCH CH , J = 7.1 Hz), 3.31 (q, 8H, NCH CH ,
2
2
3
2
3
on their different response to probe.
J = 7.1 Hz), 6.24 (dd, 2H, Xanthene-H, J = 8.8 Hz, J = 2.3 Hz), 6.43 (d,
1 2
2
H, Xanthene-H, J = 2.3 Hz), 6.52 (dd, 2H, Xanthene-H, J = 8.8 Hz,
1
J2 = 2.3 Hz), 7.11 (d, 1H, Ar-H), 7.26 (m, 3H, Ben-H), 7.48 (m, 2H, Ben-
H), 7.54 (m, 2H, Ar-H), 7.98 (d, 1H, Ar-H), 8.65 (bs, 1H, N C–H).
2. Experimental
1
1
3C-NMR (DMSO-d6) ␦ (ppm): 12.9, 44.1, 66.0, 97.8, 106.0, 108.5,
23.5, 124.4, 127.3 (2C, 127.3, 127.4), 128.1, 129.4 (2C, 129.3, 129.4
respectively), 130.8, 134.4, 135.1, 148.4, 149.0, 151.6, 153.3 and
64.3.
2
.1. Apparatus and reagents
The absorption spectra were acquired on a Hitachi U-3010
1
spectrophotometer (Tokyo, Japan). All fluorescence measurements
were made with a Hitachi F-4500 Fluorescence Spectrophotome-
ter (Tokyo, Japan) equipped with a plotter unit and a 1 cm quartz
cell. NMR spectra were recorded using a JOEL JNMECA300 spec-
trometer operated at 300 MHz. All pH measurements were made
with a Model pHs-3c meter (Shanghai, China). Deionized water
2.3. Analytical procedure
The fluorescence “turn-on” reaction of SRBH with Cr (VI) was
conducted in 10 mM H2SO4 buffer. Typically, to a reaction solution
−
7
(
distilled) was used throughout the experiment as the solvent. All
containing 10 mM H2SO4 buffer and 1 × 10 M SRBH, an appropri-
the reagents were purchased from commercial suppliers (Beijing
Chemical Reagent Co., China; Acros; Fluka) and used without fur-
ther purification. The solutions of Cr (VI), As(V), S O
NO2 , ClO3 , BrO3 , IO3 and H O2 were prepared from K CrO ,
NaAsO , K S O , NaNO , KMnO , KClO , KBrO , KIO and 30% H O ,
respectively. The solutions of other metal ions were prepared from
their nitrate or chloride salts. Stock solutions of chromium (VI),
H SO , and rhodamine B hydrazide were prepared as follows: first,
ate volume of sample solution with a final Cr (VI) concentration
−
7
of not more than 1 × 10 M was added, and the final volume was
adjusted to 10 mL with deionized water. After 15 min, a 3 mL por-
tion of the solution was transferred to a 1-cm quartz cell, and the
fluorescence intensity/spectrum was measured at room tempera-
ture with ꢀex/ꢀem = 520/591 nm and both excitation and emission
slit widths of 5 nm. In the meantime, a blank solution containing no
Cr (VI) was prepared and measured under the same conditions for
comparison. Because of the determination of Cr (VI) at low concen-
2−
−
,
, MnO4
2
8
−
−
−
−
2
2
4
3
2
2
8
2
4
3
3
3
2
2
2
4
−3
Cr (VI) stock solution (1.00 × 10 M): in a 100 mL standard flask,
−3
−4
−5
Cr (VI) stock solution (1.00 × 10 M): in a 100 mL standard flask,
4.7 mg potassium dichromate (K Cr O7) and 19.4 mg potassium
trations, Cr (VI) stock solutions of 1.0 × 10 and 1.0 × 10 M were
prepared by diluting the original stock solution, and proper amount
of these solutions was used in the analytical procedure.
1
2
2
chromate (K CrO ) was dissolved in 5 mL water, and then diluted
2
4
to the mark respectively. Second, H SO4 stock solution (0.20 M): in
2
a 10 mL standard flask, 1.60 mL sulfuric acid solution was dissolved
in 5 mL water, and then diluted to the mark. Finally, salicylalde-
hyde rhodamine B hydrazide stock solution in DMF (1.00 × 10 M):
in a 10 mL standard flask, 5.45 mg salicylaldehyde rhodamine B
hydrazide was dissolved in 5 mL DMF, and then diluted to the mark
with DMF.
3
. Results and discussion
−
3
3.1. Spectral characteristics of salicylaldehyde rhodamine B
hydrazone (SRBH)
It has been reported that the rhodamine derivative SRBH is a col-
orless and non-fluorescent substance due to its stable “spirolactam
form” [33]. Inthis work, itwasalsoobservedthatthe fluorescence of
2.2. Synthesis of salicylaldehyde rhodamine B hydrazide (SRBH)
−5
MSRBHevenin10 mMH SO aqueousmediumwasveryweak
2 4
1
0
−
5
The SRBH as a fluorescent probe for Cr (VI) was synthesized
and only about 1/300 of that of 10 M rhodamine B (RB) under the
same condition. This suggested the possibility of fluorogenic deter-
mination of substances, which could successfully convert SRBH to
RB under this acidic condition. Recently, salicylaldehyde fluorescein
through two steps according to literatures [30,31]. First, rhodamine
hydrazide (RBH) was synthesized by a modified procedure accord-
ing to Yang’s [32] and Tong’s methods [29]. After drying under an

A. Zheng et al. / Spectrochimica Acta Part A 74 (2009) 265–270
267
Fig. 1. Fluorescence excitation (left) and emission (right) spectra of SRBH in the
Fig. 2. Effect of sulfuric acid concentration on the fluorescence intensity. Condi-
tions: 1.0 × 10−7 M SRBH, 1.0 × 10−7 M Cr (VI) and 15 min reaction time at room
−
7
absence and presence of Cr (VI): (from bottom to top (×10 M): 0.0, 0.1, 0.2, 0.3, 0.5,
−7
temperature. Excitation/emission was performed at ꢀ /ꢀ = 520/591 nm.
0
.75, 1.0 1.5, 2.0 and 3.0 Cr (VI). Conditions: 1.0 × 10 M SRBH, 10 mM H2SO4 and
ex
em
15 min reaction time at room temperature.
schematic of interaction between SRBH and Cr (VI) was shown in
Scheme 2. In addition, a more direct evidence was obtained by
comparing the pure RB solution fluorescence spectra to SRBH-Cr
hydrazone were applied in a colorimetric logic chemosensor for pH
and Cu (II) [31]. Furthermore, salicylaldehyde rhodamine B hydra-
zone displayed a selective and sensitive for sensing of Cu (II) based
on metal–ligand complex and opening-spirolactam ring effect [30].
With careful investigations, a significant enhancement of fluores-
cence and continuous red shift of emission peak from 573 to 585 nm
was observed upon the addition of Cu (II) compared to that of only
SRBH under the same conditions. The optical changes including flu-
orescence enhancement and wavelength red shift were attributed
to the opening ring and the increase of molecule’s conjugation due
to Cu (II)-binding to SRBH, respectively. In addition, the utilization
of the conversion of RBH to RB had already been carried out to
detect Cu (II) [33] and peroxynitrite [32] in neutral aqueous media.
In this work, interestingly, it was found that in highly acidic condi-
tions, the addition of Cr (VI) to SRBH aqueous solution could cause a
significant enhancement of fluorescence, which was similar to that
of RB molecule. The fluorescence spectra of SRBH in the absence
and presence of different amounts of Cr (VI) are shown in Fig. 1. It
can be found that SRBH displays no obvious spectral characteristics
in either its excitation or emission spectra. However, a significant
restore of fluorescence with an excitation maximum at 564 nm and
an emission maximum at 591 nm was observed when Cr (VI) was
added.
(VI) under the same environments (these results are not shown).
The characteristics exaction around 358, 413, 522 and 564 nm, and
emission centered 591 nm of SRBH-Cr (VI) system were completely
same as the pure RB solution.
3
.2. Optimization of the analytical procedures
.2.1. Effect of H SO concentration
3
2
4
−
1
As our well known, 0.1 mol L of H SO4 was generally chosen
2
to control the acidity of system. The effect of H SO4 concentration
2
−
7
on the fluorescence intensity at 591 nm for 1.0 × 10 M Cr (VI) and
SRBH was illustrated in Fig. 2. No fluorescence enhancement could
be observed when the reaction system was lack of H SO , suggest-
2
4
ing the important role of acidic condition in the oxidation reaction
of Cr (VI) and SRBH. The fluorescence intensity at 591 nm increased
with the concentration of H SO4 first, and then slightly decreased.
2
The maximum and stable fluorescence enhancement occurred in
the range of 7.5–30 mM H SO . High concentration of H SO could
2
4
2
4
cause a significant enhancement of the blank fluorescence of RBH
in water due to the formation of open ring form. Therefore, a con-
centration of 10 mM H SO4 was then chosen.
2
A gradual blue-shift of fluorescence spectra of SRBH were found
in the presence of Cr (VI) compared to original SRBH solution
resulting from the oxidation of SRBH to RB by Cr (VI) in the acidic
conditions. A significant enhancement of fluorescence intensity of
solution was attributed to opening-spirolactam ring, as a prod-
uct of RB. These results were similar to Tong groups’ findings. The
3.2.2. Effect of SRBH concentration
Effect of SRBH concentration on the sensitivity of the proposed
method for detection of Cr (VI) was investigated. The results shown
in Fig. 3 indicated that a maximum fluorescence enhancement was
observed when 0.05–2.0 ␮M SRBH reacted with 0.1 ␮M Cr (VI) in
Scheme 2. Possible reaction mechanism of SRBH and Cr (VI).

2
68
A. Zheng et al. / Spectrochimica Acta Part A 74 (2009) 265–270
Fig. 4. Kinetic behavior of the fluorescence of the SRBH–Cr (VI)–H2SO4 reaction
Fig. 3. Effect of RBH concentration on the fluorescence intensity Conditions: 10 mM
H2SO4, 0.1 ␮M Cr (VI) and 15 min reaction time at room temperature. Excita-
tion/emission was performed at ꢀex/ꢀem = 520/591 nm.
−7
−7
system. Squares: 1.0 × 10 M SRBH only; dots: 1.0 × 10 M SRBH and 0.5 ␮M Cr
−7
(
1
VI); triangles: 1.0 × 10 M SRBH and 1.0 ␮M Cr (VI); conditions: 10 mM H2SO4 and
5 min reaction time at room temperature. Excitation/emission was performed at
520/591 nm.
1
0 mM H SO for 15 min. Excess SRBH was found to give only mild
2 4
Table 1
Tolerance of foreign ions in the determination of 1.0 × 10 M Cr (VI).
interference to the fluorescence blank signal. In this work, 0.10 ␮M
SRBH was chosen. This concentration of SRBH was also applied to
the determination of Cr (VI) at the lower micromolar level to ensure
the acceptable reaction rate with desirable analysis characteristics.
−
7
Ions added Tolerance ratio (mol/mol)
Na (Cl ), K (Cl ), Ca (NO3 ), Mg2+(NO3 ),
+
−
+
−
2+
−
2+
−
2+
>500
Ba +(NO3 ), Al (Cl ), Mn (NO3 ), Co (NO3 ),
2
−
3+
−
−
−
Ni (SO4 ), Cu2+(SO4 ), Zn2+(NO3 ), Pb (NO3 ),
2
+
2−
−
2−
2−
−
−
−
2+
−
2−
3.2.3. Effect of reaction time
2
+
2+
3+
+
Hg (NO3 ), Cd (NO3 ), Cr (NO3 ), (Na )SO4 ,
The kinetic characteristics of the reaction system of SRBH and
+
+
−
(
K )CO3 , (K )NO3
Cr (VI) were first investigated and shown in Fig. 4. Upon the addi-
Fe (Cl−), Fe3+ (SO4 ), (K )S2O8
2+
2−
+
2−
20
10
5
−
7
tion of 0.5 and 1.0 ␮M Cr (VI) to the SRBH (1.0 × 10 M) solution in
H2O2
+
−
+
−
+
−
(
(
K )ClO3 , (K )BrO3 , (K )IO3
K )MnO4 , (Na )NO2
the presence of 10 mM H SO , a stable fluorescent signal at 591 nm
2
4
+
−
+
−
0.5
could be obtained within a reaction time of 10 min and remained
constant for at least 1 h. It was also found that higher concentra-
tion of the reagents was of benefit to the rate of the reaction. An
incubation time of 15 min was chosen for this work as a practical
matter. An accurate and reproducible result could be obtained at
this relatively short reaction time.
−
7
Conditions: 10 mM H2SO4, 1.0 × 10 M SRBH and 15 min reaction time at room
temperature. Excitation/emission was performed at 520/591 nm.
more than 10-fold of that of Cr (VI), while H O2 might be oxidized
2
by Cr (VI) and its concentration should be lower than 5-fold. The
−
major interferences come from these strong oxidants such as ClO
,
3
−
−
−
−
3.3. Interference of foreign ions
BrO3 , IO3 , MnO4 and NO2 which might oxidize SRBH or bleach
RB. However, the proposed method is still of good selectivity.
As shown in Table 1, the interference of some foreign ion species
to 0.1 ␮M Cr (VI) under the optimum condition was examined. To
maintain the relative error within 5.0%, most of the ions could be
allowed at high concentrations (>100-fold) of that of Cr (VI)). Fe
3.4. Cr (VI) ion analysis properties of SRBH probe
According to the above procedure, the calibration curve for
the determination of Cr (VI) was constructed under the opti-
2−
(
III) and S O
had mild interference and should be kept at no
2
8
−
6
Fig. 5. (A) Photographs of 1.0 ␮M SRBH as a selective ‘naked-eye’ probe for Cr (VI) in 10 mM H2SO4 medium, the concentration of Cr (VI) from left to right are (×10 M):
−
6
0
0
.00, 0.05, 0.10, 0.30, 0.50, 1.00 and 5.00; (B) UV–vis absorption spectra of SRBH in 10 mM H2SO4. Concentrations of Cr (VI) are (×10 M): 0.00, 0.01, 0.03, 0.05, 0.10, 0.30,
.50, 0.75, 1.50 and 3.00.

A. Zheng et al. / Spectrochimica Acta Part A 74 (2009) 265–270
269
Table 2
Comparison the analytical performances of familiar method for detection Cr (VI) ion.
−
6
−6
−6
Reagents
Regression equation (10 M)
Linear range (10 M)
LOD (10 M)
Method
Microcantilever sensor
AN/PAM NPs
MagneticFe3O4/Py/PAM nanocomposites
Poly-4-vinylaninline NPs
Rhodamine B hydrazide
–
0.001–1000
0.769–39.5
1.92–269.8
1.92–250.6
0.05–2.00
0.01–0.30
0.001
0.384
0.384
0.384
0.0055
0.0015
[23]
[24]
[27]
[28]
[29]
This work
F0/F = 0.94667 + 2.1606C (␮g mL−1)
−
1
F0/F = 0.9573 + 3.6572C (g mL
)
F0/F = 1.055 + 0.067C (␮g mL−1)
–
2
−
Salicylaldehyde rhodamine B hydrazone
ꢁIF = −11.97 + 2429C CrO4
−
7
mum condition, which was: 10 mM H SO , 1.00 × 10 M SRBH,
2
4
1
5 min reaction time, excitation/emission at 520/591 nm at room
−
7
temperature. The linear range was at least 0.10–3.00 × 10
M
−1
2
(
(
(
0.42–12.6 ng mL ) with a correlation coefficient of R = 0.9989
n = 10). The detection limit, based on the definition by IUPAC
−1
−9
−1
LOD = 3S m ), was found to be 1.5 × 10 M (0.063 ng mL ) from
b
1
0 blank solutions. The relative standard deviation (R.S.D.) for six
−1
repeated measurements of 4.2 ng mL
Cr (VI) was 2.0%. When
SRBH was incubated with various amounts of Cr (VI) ion, a dramatic
color change from colorless to purple was observed (Fig. 5A) and
the characteristic absorption of initial RB molecule at about 585 nm
gradually returned (Fig. 5B). Under the acidic condition of 10 mM
H SO , there was a linear relationship between the absorbance at
2
4
585 nm and the concentration of Cr (VI). These results suggested
that SRBH could severe a simple “naked-eye” probe for accurate
indication of Cr (VI) ion in the water samples.
In view of the linear range and detection limit of methods, some
familiar methods for Cr (VI) ions with organic fluorescent dyes
and functionalized organic and inorganic nanoparticles are sum-
marized in Table 2. Evaluation from analytical performances, we
can find very clearly that our proposed method exhibited lower
detection blank and detection limit than most of current used and
recently described fluorescent methods for detection of Cr (VI) ion
2
−
Fig. 6. Two plots of fluorescence intensity of SRBH via the concentrations of Cr2O7
2
−
2−
and CrO4 . Black squares represented Cr2O7 anions and red triangles represented
CrO4 anions. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
2
−
(see the results in Table 2).
According to above-proposed procedure, the calibration curves
2−
2−
anions
for the determination of Cr (VI) for Cr O7
and CrO4
2
3
.5. Exploration to indication of Cr (VI) ion formations in the
were constructed under the optimal conditions. The linear range
for Cr2O72 and CrO4 all is 0.01–0.3 ␮M and the linear regression
−
2−
water
2
−
equation is as follows: ꢁIF = −11.97 + 2429C CrO4
correlation coefficient of 0.9989 (n = 10) and ꢁIF = −29.12 + 5044C
(␮M), with a
To our well known, Cr (VI) ion was found two anionic forms
2−
2−
2−
2−
(␮M), with a correlation coefficient of 0.9993 (n = 10),
such as Cr O7 and CrO4 in the water matrix. The CrO4 form
could be stable in neutral and alkaline environments, while Cr O7
in acidic condition. Furthermore, the form of CrO
into Cr O7
dependent dimerization equilibrium for chromate dianion could
be expressed as followings equation:
CrO7
respectively. We can cleanly find that the slope of the linear regres-
2
2
−
2
2−
2−
sion equation for Cr2O7 anions is approximately double than its’
can convert
4
2−
2−
for CrO4 . The result is consistent with the stoichiometric ratio
under acidic environment, and vice versa. The pH-
2
between Cr2O72 and CrO4
−
2−
as 1:2. These results suggested that
SRBH could serve an effective fluorescent probe for indication of Cr
2
− 2−
VI) formations: Cr O7 and CrO4 in water solution.
2
(
CrO42 + 2H+ ↔ Cr O7 +H O
−
2−
(1)
2
2
2
3.6. Applications of the method in real samples determination of
In the 10 mM H SO4 medium, the fluorescence “turn-on” of SRBH
Cr (VI)
2
2−
occurred in the presence of Cr (VI) as Cr O7 specie. As illustrated
2
2
−
2−
in Eq. (1), the stoichiometric ratio of Cr O7 and CrO4 was 1:2,
Under optimal experimental conditions, the present method
2
2
−
which displayed one molecule of Cr O7
corresponding to two
was applied to determine Cr (VI) in synthetic samples and wastew-
aters. Table 3 shows, the results obtained for the synthetic samples.
The results show that the present method can be applied to direct
determination of Cr (VI) without separation of Cr (III). Samples of
2
molecules of CrO42−. Under the same conditions, the two plots of
2−
the fluorescence intensity of SRBH via the concentration of Cr O7
2
2
−
and CrO4 were shown in Fig. 6.
Table 3
Determination of Cr (VI) in the synthetic samples.
−
7
−7
−7
Recoverya (%)
Cr (VI) in samples (10 M)
Cr (III) in samples (10 M)
Cr(VI) found (10 M)
R.S.D. (%)
0.5
1.0
1.5
2.0
0.5
2.0
3.0
5.0
0.504
1.02
1.52
2.06
101
102
101
103
1.8
1.9
2.0
2.5
−7
Conditions: 10 mM H2SO4, 1.0 × 10 M SRBH and 15 min reaction time at room temperature. Excitation/emission was performed at 520/591 nm.
a
Average of six determinations.

2
70
A. Zheng et al. / Spectrochimica Acta Part A 74 (2009) 265–270
Table 4
Determination of Cr (VI) in real water samples.
Found value (mean ± ꢂ, n = 5) (10− M)
7
Added standard solution (10 M)
−7
Found total value (10 M)
−7
Recovery (%)
Water sample
DPCI methoda
The presented methoda
Drinking-water
River-water
Waste-water sample 1
Waste-water sample 2
–
–
1.00
1.00
10.0
10.0
1.05 ± 0.02
3.65 ± 0.02
60.8 ± 0.3
86.5 ± 0.2
105
102
99.6
101
2.50 ± 0.05
50.0 ± 0.1
75.0 ± 0.2
2.60 ± 0.04
51.0 ± 0.2
75.5 ± 0.3
a
Average of six determinations.
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−
7
−1
−1
a linear range of 0.10–3.00 × 10 M (0.42–12.6 ng mL ) Cr (VI)
−
9
with a detection limit of 1.5 × 10 M (0.063 ng mL ). Further-
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2−
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[
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[
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Acknowledgments
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1
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The authors acknowledge the support from the Education
Commission Natural Science Foundation of Anhui Province (No.
[
2
006kj153B, 2006jql204, 2008jp1101).
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