Determination of trace uranium by resonance fluorescence method coupled with photo-catalytic technology and dual cloud point extraction

Article abstract of DOI:10.1016/j.saa.2016.06.040

In this paper, two kinds of salophens (Sal) with different solubilities, Sal1 and Sal2, have been respectively synthesized, and they all can combine with uranyl to form stable complexes: [UO₂ ^2?+-Sal1] and [UO₂ ^2?+

Full text of DOI:10.1016/j.saa.2016.06.040

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 208–215
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Determination of trace uranium by resonance fluorescence method
coupled with photo-catalytic technology and dual cloud point extraction
⁎
Jiekang Li, Guirong Li , Qian Han
College of Public Health, University of South China, Hengyang, Hunan 421001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, two kinds of salophens (Sal) with different solubilities, Sal1 and Sal2, have been respectively syn-
thesized, and they all can combine with uranyl to form stable complexes: [UO22+-Sal1] and [UO22+-Sal2]. Among
them, [UO22+-Sal1] was used as ligand to extract uranium in complex samples by dual cloud point extraction
(dCPE), and [UO22+-Sal2] was used as catalyst for the determination of uranium by photocatalytic resonance fluo-
rescence (RF) method. The photocatalytic characteristic of [UO22+-Sal2] on the oxidized pyronine Y (PRY) by
potassium bromate which leads to the decrease of RF intensity of PRY were studied. The reduced value of RF in-
tensity of reaction system (ΔF) is in proportional to the concentration of uranium (c), and a novel photo-catalytic
RF method was developed for the determination of trace uranium (VI) after dCPE. The combination of photo-
catalytic RF techniques and dCPE procedure endows the presented methods with enhanced sensitivity and
selectivity. Under optimal conditions, the linear calibration curves range for 0.067 to 6.57 ng mL−1, the linear re-
gression equation was ΔF = 438.0 c (ng mL−1) + 175.6 with the correlation coefficient r = 0.9981. The limit of
detection was 0.066 ng mL−1. The proposed method was successfully applied for the separation and determina-
tion of uranium in real samples with the recoveries of 95.0–103.5%. The mechanisms of the indicator reaction and
dCPE are discussed.
Received 29 January 2016
Received in revised form 26 June 2016
Accepted 27 June 2016
Available online 29 June 2016
Keywords:
Uranyl
[UO22+-salophen] complex
Photo-catalytic resonance fluorescence method
Dual cloud point extraction
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
A novel double cloud point extraction (dCPE) was developed recent-
ly which has played important roles in the field of environment science
Uranium is a ubiquitous trace radioactive element that has received
much attention, owing to its high toxicity and radio-activity [1,2].
Recently, low concentration of uranium has been widely detected in
most of environmental water, tap water, and ground water [3]. The
U.S. Environmental Protection Agency defined the maximum contami-
nation level of UO22+ in drinking water to be 130 nM. Due to the devel-
opment of current nuclear industry, there is an increasing amount of
waste water containing radioactive uranium, which is harmful to natu-
ral environment and human being [4]. Therefore, highly selective and
sensitive determination of trace uranium is of great significance in envi-
ronmental protection and public security.
To date, many methods for the determination of uranium have been
developed, such as spectrophotometry [5], inductively coupled plasma-
mass spectrometry [6], atomic emission spectrometry [7], X-ray fluores-
cence [8], time-resolved laser fluorescence spectroscopy [9], resonance
light scattering assay [10] and capillary zone electrophoresis [11] and
surface enhanced Raman spectroscopy [12]. However, it is generally
considered those most of them either require sophisticated equipment
and skilled operators, or involve tedious and painstaking procedures.
and is becoming increasingly popular owing to the simplicity of opera-
tion, rapidity of extraction, low cost, environmental friendliness, and
high enrichment factors [13,14]. To our knowledge, no work has so far
been reported on the determination of uranyl based on dCPE technique.
In this investigation, we integrate the advantages of photo-catalytic res-
onance fluorescence (RF) and dCPE technology for detecting trace
amount of uranyl. In consideration of the cationic nature of uranyl,
salophen (Sal), a tetradentate ligand, was used to combine with the cat-
ion to form complex, which is based on the fact that the increased inter-
actions between the metal cations and Sal via not only electrostatic but
also spatial coordination forces could enhance the extraction efficiency
[15].
Sal can combine with UO22+ to form stable [UO22+-Sal] complexes,
so it is a good reagent used for recognition of uranium(VI) [16,17]. In
this paper, two kinds of Sals (Sal1 and Sal2) were synthesized and
used to the dCPE and the photo-catalytic reaction process, respec-
tively (as shown in Scheme 1). First, in consideration of the cationic
nature of UO22+, Sal1, a hydrophobic ligand, could be used to combine
with the positive ion to form complexes without charge that could
enhance the extraction efficiency in dCPE. Second, as the UO22+ cation
typically adopts a pentacoordinate environment, a tetradentate Sal2 li-
gand with hydrophilicity in [UO22+-Sal2] complexes leaves one site
⁎
Corresponding author.
E-mail address: liguirong525@126.com (G. Li).
http://dx.doi.org/10.1016/j.saa.2016.06.040
1386-1425/© 2016 Elsevier B.V. All rights reserved.

J. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 208–215
209
Scheme 1. Procedure of synthesizing Sal and [UO22+-Sal] synthetic process diagram.
open for coordination to another molecule [18,19]. Published papers
demonstrated that [UO22+-Sal] complex can bind phosphate derivatives
or amino compounds with strong affinity and high selectivity [20].
The pyronine Y (PRY) is a reducing fluorescent reagent with two
amino groups in the molecule, and can bind with UO22+ of [uranyl-
Sal2] complex at fifth coordination site of uranyl to form complex
[PRY-uranyl-Sal2] which will result in the enhancing of reducing prop-
erty of PRY.
with a 1 cm quartz cell. The slit widths of excitation and emission light
were kept at 2.5 and 5 nm, respectively. The excitation spectrum of PRY
was obtained by scanning the excitation monochromator from 450 to
650 nm with λem = 564 nm, and the emission spectrum can be obtain-
ed by scanning the emission monochromator from 450 to 650 nm with
λex = 547 nm. The RF spectra of PRY can be obtained by scanning both
monochromators simultaneously from 450 to 650 nm at constant-
wavelength differences of Δλ = 0. A Shimadzu UV-2450 spectropho-
tometer (Kyoto, Japan) was used to measure the absorption spectra. A
thermostatic water bath was used to control the reaction temperature.
A stopwatch was used for recording the reaction time. PB-21 standard
pH meter (Beijing) was used for pH adjustment.
All reagents were of analytical grade, and doubly distilled water
was used throughout. Uranium nitrate hexahydrate were purchased
from Shanghai First Reagent Plan (Shanghai, China). Pyronine Y and
potassium bromate and Tris(trihydroxymethyl aminomethane) were
purchased from Qingzhou Zhenhua Chemical Reagent Company
(Qingzhou, China). Uranium nitrate hexahydrate was dissolved in dou-
bly distilled water to make a stock solution (UO22+ 1.00 mg L−1). The
KBrO3 (0.010 mol L−1), PRY (0.10 mmol L−1) and Triton X-114 (TX-
114) (2.0% w/v) working solutions were prepared by dissolving an
appropriate amount of them in doubly distilled water, then diluting to
100 mL with doubly distilled water. A Tris-HCl buffer solution
(0.100 mol L−1, pH 5.5) was prepared by dissolving 7.880 g
Tris(hydroxymethyl)aminomethane hydrochloride in 500 mL doubly
distilled water and then adjusting pH to 5.5 with 1:1 HCl.
Salophen-1 was synthesized according to the following procedures.
2.46 g of o-hydroxybenzaldehyde and 1.14 g of o-phenylenediamine
were mixed and dissolved in 80 mL of anhydrous methanol. The mixture
was stirred for 1 h for the synthesis of N,N′-bis (salicylidene)-o-
phenylenediamine (i.e. Sal1). The brown solid Sal1 products can be ob-
tained by distilling the mixture solution for removal of methanol solvent.
Take 2.46 g of o-hydroxybenzaldehyde and 1.68 g of 3,4-bis(amino)-
benzoic acid, salophen-2 was synthesized in according with the same
procedure as the synthesis of Sal1. The brown solid products obtained
was N,N′-bis(salicylidene)-3-carboxyl-o-phenylenediamine (i.e.
Sal2). A Sal1 or Sal2 ethanol solution (0.10 mmol L−1) were prepared
by dissolving an appropriate amount of them in 100 mL anhydrous
ethanol.
The catalytic kinetic methods exhibit a number of advantages in-
cluding low-cost, simplicity of application and high sensitivity for the
determination of trace levels of environmental pollutants, but the
coexisting substances in complex samples seriously interfere with the
catalytic action of the analytes resulting in poor reproducibility and se-
lectivity [21,22]. The photo-catalytic kinetic analysis methods have
good reproducibility and selectivity because most of coexistent sub-
stances are not the photo-catalytic activity. Uranium is a kind of actinide
elements with a special outer electronic structure 5f3 6d1 7s2 and UO22+
can act as the photo-catalytic agent. Some uranyl-containing com-
pounds are considered as a real photocatalyst and have considerable po-
tential for use due to it can result in the high efficient photo-catalytic
degradation of organic molecules at visible-light irradiation [23–26],
but the work on the determination of uranyl based on photo-catalytic
technology has hardly been reported. In the present study, the strong
RF of PRY is employed for the first time to determine trace uranyl in en-
vironmental water samples after dCPE based on the sensitive photo-cat-
alytic effect of [UO22+-Sal] on the redox reaction of PRY with KBrO3. The
proposed method has the additional advantages of speed, high selectiv-
ity and sensitivity, and simple instrumentation. Furthermore, the limit
of detection is below the maximum contamination level of 130 nM for
uranium in drinking water defined by the U.S. Environmental Protection
Agency. In addition, the mechanism of the indicator reaction and dCPE is
discussed.
2. Experimental
2.1. Apparatus and chemicals
The fluorescence spectra and the intensities of fluorescence were
measured with a Hitachi RF–4500 Spectrofluorometer (Japan) equipped

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2.2. Experimental method
scanning with Δλ = 0 would include RF signal and resonance light scat-
tering (RLS) signal. Fig. 1(b) showed that the two signals overlap with
each other in the range of 516–587 nm, which indicates that the emis-
sion signal contains RF and resonance light scattering. The experimental
results indicated that the RF signal of PRY was very strong while the RLS
signal was weak in 516–587 nm. The intensity values of the emission
light signal were measured at 558 and 516 nm, respectively, and were
showed in Fig. 1(b). Because the intensity of RLS becomes weak with
the increase of the wavelength in accordance with the Rayleigh scatter-
ing law, namely I∝1/λ4, λ = 558 nm I ≈ IRF = 5260, λ = 516 nm
I ≈ IRLS = 36 and IRS,558 ﹤ IRS,516. The intensity ratio of the two signals
at 558 nm was about 1000:7. So, when the RF is measured at 558 nm,
the effect of RLS was negligible. In addition, the RF spectra (Fig. 1(a))
of PRY displayed some characteristics of spiky, narrow, symmetrical
and slippery, which could eliminate the effect of other concomitant
fluorescence substances, including Sal2 and [UO22+-Sal2] complex, on
the RF signal of PRY. Therefore, the accuracy and selectivity of this ana-
lytical method would be effectively enhanced.
2.2.1. Double cloud point extraction
An aliquot (6.0 mL) of standard or sample solution containing uranyl
was pipetted into 10 mL scaled-glass test tubes with conical bottom.
Then, 2.0 mL of pH 5.5 Tris-HCl buffer solution was added to the tube
and mix well. A mixture of 0.40 mL of Sal1 ethanol solution (as coordi-
nation agent) and 1.0 mL of TX-114 aqueous solution (2.0% w/v) (as ex-
traction solvent) was injected rapidly into the solution. After gently
shaking and heating in a thermostatic bath at 68 °C for 15 min, a cloudy
solution which consisted of very fine droplets of TX-114 dispersed into
the aqueous sample was formed. The mixture was then centrifuged for
3 min at 3500 rpm after cooling in an ice bath for 5 min, causing the dis-
persed fine droplets of the extraction phase to settle down to the bot-
tom of the tube. The supernatant aqueous phase was discarded. The
sticky micellar phase was diluted to 0.40 mL with 0.0050 mol L−1
H2SO4 solution and heated once again at 68 °C for 15 min. The cloudy so-
lution is divided into two layers by centrifuging after cooling, and the
supernatant aqueous phase containing uranyl was detected by photo-
catalytic RF method. Because the amount of uranyl in 6 mL sample solu-
tion is measured after preconcentration by dCPE in a final volume of
0.4 mL, the solution is concentrated by a factor of 15.
3.2. The characteristics of PRY fluorescence resonance
The RF spectra of the reaction system were shown in Fig. 2. PRY itself
has very strong RF in the range of 516 to 587 nm with the maximum
wavelength located at 558 nm (curve 1). Sal2 or [UO22+-Sal2] had no
RF (curves 2 or 3), which shows that concomitant Sal2 or [UO22+-Sal2]
did not interfere with the RF signal of PRY. The RF of PRY was almost un-
changed after the addition of uranyl (curve 4). The RF of PRY was slight-
ly decreased (curve 5) after addition of the oxidizing agent KBrO3 to the
solution of PRY. Adding of Sal2 did not influence the reaction speed of
PRY and KBrO3 (curve 6). The RF of the system observably decreased
after adding the UO22+ to the solution of PRY-KBrO3 (curve 7) due to
the photo-catalytic action of UO22+. When Sal2 was added to the solu-
tion of PRY-KBrO3-UO22+, the more obvious decrease of the RF intensity
was observed (curve 8), which indicated that Sal2 could effectively en-
hance the photo-catalytic activity of UO22+. When the concentration of
Sal2 remains constant, compared with the reagent blank solution with-
out UO22+, the decrease value of RF intensity at 558 nm was proportional
to the concentration of UO22+ (curves 9–15). Based on this, a new photo-
catalytic RF method could be established for the determination of ultra-
trace uranium.
2.2.2. Procedure
Into a 25 mL colorimeter glass tube several solutions were added in
the following order: 2.0 mL of Tris-HCl buffer solution, 2.0 mL of KBrO3
solution, 2.0 mL of PRY solution, 0.40 mL of Sal2 ethanol solution and the
extraction from the uranyl calibration solution or samples solution by
dPCE. The mixture was diluted to 25 mL with redistilled water and be
shaken well. The mixture was placed in the circulating tap water to
keep in room temperature, and at strong visible-light irradiation for
15 min. The RF spectrum was obtained by synchronously scanning
with Δλ = 0 from 500 to 650 nm in a 1-cm quartz cell with slit widths
of 2.5 nm for excitation and 5 nm for emission. The intensity of RF was
measured at 558 nm. The decreased RF intensity of the reaction system
was represented as ΔF = F0 − F, here F and F0 were the RF intensities of
the reaction system with and without uranyl, respectively.
3. Results and discussion
3.1. The fluorescence spectral characteristics of PRY
3.3. The comparison of the absorption and RF spectra of the system
The fluorescence excitation, emission and RF spectra of PRY were
shown in Fig. 1(a). The maximum excitation and emission wavelengths
are located at 547 and 564 nm, respectively, and the excitation spectra is
overlapped with the emission spectra over a wide wavelength range be-
cause the Stokes shift is very small. The maximum overlap of the spectra
is located at 558 nm, which is equal to the RF peak.
A comparison of the absorption and RF spectra of the system are
shown in Fig. 3. Fig. 3 shows that both Sal2 and [UO22+-Sal2] have a
strong absorption band in the wavelength range of 305–355 nm. The
maximum absorption of Sal2 was located at 323 nm (curve 1). After
adding the UO22+, the absorption decreased, accompanying the blue
shift of the maximum absorption from 323 to 311 nm (curve 2). This
showed that UO22+ might react with Sal2 to form a complex compound
The three dimensional fluorescence spectrum of PRY was obtained
and showed in Fig. 1(b). The light signal obtained by synchronously
Fig. 1. (a) The RF fluorescence (1), emission (2), excitation (3) spectra of PRY. (b) The three dimensional fluorescence spectra.

J. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 208–215
211
KBrO3 can oxidize PRYred to nonfluorescence product (PRYox) [27,28]
at strong light irradiation, which resulted in slightly quenching of RF.
−
BrO3 þ PRYred þ 6Hþ→Br− þ 3H2O þ PRYox
ð1Þ
This reaction speed was very slow because the reaction activation
energy was too high. The RF of system showed a larger decrease with
adding of uranyl, which showed that the RF quenching of PRY-KBrO3
system could be photo-catalyzed by free uranyl. The reason might be
that UO22+ could be reduced by PRYred to UO+2 , and the reaction mecha-
nism changed:
UO2 þ þ PRYred→UO2þ þ PRYox
ð2Þ
ð3Þ
2
BrO3 þ UO2þ þ 6Hþ→Br− þ UO2 þ þ 3H2O:
−
2
However, when Sal2 was added to PRY-KBrO3-UO22+ system, a more
significant decrease of RF had been observed, which indicated that Sal2
could enhance the photocatalytic activity of uranyl, and accelerate the
fluorescence quenching reaction of the system. According to the infer-
ence from Fig. 2, this reason might be that Sal2 could combine uranyl
to form the complex [UO22+-Sal2]. This showed that the reaction mech-
anism might again be changed and the reaction activation energy might
be further reduced. Thereby, a possible photo-catalyzed reaction mech-
anism could be shown as following:
Fig. 2. The RF spectra of the reaction system. 1. PRY, 2. Sal2; 3. UO22+ + Sal2; 4.
PRY 5. PRY KBrO3; 6. PRY KBrO3 Sal2; 7.
UO22+
PRY + KBrO3 + UO22+(1.0 ng mL−1); 8. PRY + KBrO3 + Sal2 + UO22+; Insets: 9–15.
+
;
+
+
+
PRY + KBrO3 + Sal2 + UO22+ (0, 0.5, 1.5, 2.5, 3.5, 4.8, 5.4) ng mL−1. cPRY
=
0.008 mmol L−1, cSal2 = 0.016 mmol L−1. All were in Tris-HCl buffer solution of pH 5.5.
[UO22+-Sal2]. The maximum absorption wavelength of PRY was 546 nm
(curve 3). The absorption of PRY was not changed after the addition of
uranyl (curve 4). When KBrO3 was added to the PRY solution, the ab-
sorption of the system was very slightly decreased (curve 5). The adding
of UO22+ resulted in the decrease of absorption intensity of the system at
546 nm (curve 6) because the oxidative reaction of PRY with KBrO3 was
photo-catalyzed by UO22+. With the addition of Sal2, the absorption in-
tensity of PRY decreased (curve 7) obviously compared to in the ab-
sence of Sal2 (curve 6), which indicated the photo-catalysis of UO22+
on the reaction was increased by Sal2. These conclusions were in agree-
ment with those obtained by RF spectra (Fig. 2). From the comparison of
the RF spectrum (curve 8) with absorption spectra, it can be seen that
the RF spectrum of the system was located in the absorption band
with a sharp, narrow, and symmetrical shape. There is a valid resonance
of the absorption and emission in this system.
h
i
UO2þ þ Sal2→ UO2 þ−Sal2
ð4Þ
ð5Þ
2
h
i
ꢀ
ꢁ
UO2 þ−Sal2 þ PRYred→ UO2þ−Sal2 þ PRYox
2
h
i
ꢀ
ꢁ
BrO3 þ UO2þ−Sal2 →Br2 þ UO2 þ−Sal2
ð6Þ
ð7Þ
−
2
Br2 þ PRYred→2Br− þ PRYox
:
Reactions (4)–(7) are all fast. Therefore, the oxidation of PRYred to
PRYox by KBrO3 was photo-catalyzed by [UO22+-Sal2] Total reaction
can was expressed as the following:
−
BrO3 þ PRYred þ 6Hþ½uranyl−Sal2ꢀphoto−catclyzeBr− þ 3H2O þ PRYox
:
ð8Þ
3.4. Mechanism of the photo-catalytic reaction
As previously mentioned, PRY itself has a strong RF in Tris-HCl buffer
solution (Fig. 2). The RF of PRY was slightly decreased after adding the
oxidizing agent KBrO3 to the PRY solution. The reason might be that
According to published research [16,17], the composition ratio of
UO22+ and Sal2 in [UO22+-Sal2] complexes was 1:1. There are two
amino and two hydroxyl groups in the Sal2 molecule. Sal2 coordinates
to UO22+ through amino nitrogen and hydroxyl oxygen atoms to form
a larger conjugation plane with two six-membered rings and a penta-
membered ring (as showed in Scheme 1) [16] . The sensitization of
[UO22+-Sal2] complex might be explained from two aspects: On the
one hand, the photo-catalysis activity of uranyl was enhanced due to
formation of a larger electron conjugated plane in [UO22+-Sal2] which
results in the reducing of stability and/or the increasing of reaction ac-
tivity of whole molecule.
On the other hand, both Sal2 and [UO22+-Sal2] complexes were good
hydrophilicity as there is a carboxyl groups in their molecular. When
PRY is added to the solution of [UO22+-Sal2] complex, the [UO22+-Sal2]
complex can bind PRY at fifth coordination site of UO22+ to form ternary
complex [PRY-UO22+-Sal2]. The chemical structures of the ternary com-
plexes are shown in the Scheme 2. This UO22+ in [PRY-UO22+-Sal2] can
bind the nitrogen of amino of PRY with strong affinity and high selectiv-
ity [29]. The formation of [PRY-UO22+-Sal2] complexes would results in
the offset of the electron cloud density of the conjugate system in PRY
molecule due to the strong attraction of the positive charge of the
UO22+, and the reducibility of PRY molecule would be enhanced. As
the results, the photo-catalytic activity of the [UO22+-Sal2] complexes
is greater than that of free UO22+ and the sensitivity of purpose method
Fig. 3. The absorption spectra (1–7) and the RF spectra (8) of the reaction system. 1. Sal2,
2. UO22+ + Sal2, 3,8. PRY, 4. PRY + UO22+, 5. PRY + KBrO3, 6. PRY + KBrO3 + UO22+
(1.0 ng mL−1), 7. PRY + KBrO3 + UO22++Sal2. cPRY = 0.008 mmol L−1, cSal2
=
0.016 mmol L−1, All were in Tris-HCl buffer solution of pH 5.5).

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J. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 208–215
Scheme 2. The chemical structures of the [UO22+-Sal2] complexes.
is enhanced [30]. The experimental results show that the limit of detec-
tion was lowered by 10 times in comparison with that without Sal2.
could be achieved at the pH value where the electrostatically neutral
complexes formed between UO22+ and Sal1. Therefore, 0.10 mol L−1 of
Tris-HCl, Na2HPO4-citric acid, HAc-NaAc, Na2HPO4-NaH2PO4 buffer so-
lution were investigated, respectively, in pH values range of 3.0–8.0.
As can be seen from Fig. 4, the best extraction capabilities were obtained
when Tris-HCl was used at the optimum pH 5.5 for UO22+, because the
[UO22+-Sal1] complexes have greater solubility in surfactant phase,
leading to higher extraction efficiencies. The most likely explanation of
such experimental result might be that pH influences the uranyl ion
speciation in solution, and an uranyl species was produced at pH 5.0–
5.5, which is the most effective pH for carrying out the coordination re-
action. When pH values are higher than 5.5, hydrolyzed species such as
UO2(OH)+ start to dominate, and at pH N 7, the precipitation of
UO2(OH)2 can be observed [32]. Contrary, at pH b 4.5, Sal1 would hardly
be ionized and form the coordinated negative ion, which were all unfa-
vorable for the interaction of UO22+ with Sal1. Therefore, the Tris-HCl
buffer solution of pH 5.5 was selected for controlling the acidity of the
solution.
3.5. Mechanism of double cloud point extraction for uranyl
In the initial stage of dCPE, Sal1, a hydrophobic molecule, was used
as a complexing extraction agent of UO22+, which can selectively com-
bine with UO22+ by the strong electrostatic force and the rigorous steric
effects between Sal1 anion and UO22+ at pH≥ 5.5 to forming strong hy-
drophobic complex [UO22+-Sal1] enriched in surfactant phase. The ex-
traction is based on the ternary solvent system, a surfactant extracting
(TX-114) solvent, a coordination agent (Sal1) solvent and the aqueous
sample solution (UO22+). In this method, a mixture of the extractant
and coordination agent solvent is quickly injected into the aqueous
sample as very fine droplets, resulting in the formation of a cloudy solu-
tion after heated and cooled, into which the analytes (UO22+) are
enriched. Because of the considerably large contacting area between
the extractant solvent and the aqueous sample, the equilibrium state
is achieved rapidly. The extractant phase of a small volume can be sep-
arated rapidly by centrifugation, and used for the following back
extraction.
In the process of back extraction, the sulfuric acid aqueous solution is
used as a back-extraction solvent, which could both provide aqueous
phase and adjust the solution to strong acid environment (pH ≈ 1.5).
In strong acid condition, the [UO22+-Sal1] complexes would be
decomposed to form UO22+ (into aqueous phase) and neutral Sal1 (in
surfactant phase) (see Scheme 3). After the thermostatic bath and certi-
fication, the interested UO22+ was back-extracted into aqueous phase.
The obtained aqueous extract is used to the photo-catalytic RF method
as sample. Due to the introduction of the second cloud point procedure,
the effects of the surfactant on the photo-catalytic RF method are elim-
inated. The removal of interfering species through the dCPE procedure
improves the analysis method selectivity extensively, and besides the
aqueous sample solution is naturally compatible with the photo-cata-
lytic RF method condition.
3.6.2. Effects of TritonX-114, Sal1, sulfuric acid dosages
The influences of the adding volumes of TX-114 (2% w/v), Sal1
(0.10 mmol L−1) and sulfuric acid (0.005 mol L−1) on the ΔF of the sys-
tem were studied in the range of 0.4–1.5, 0.1–1.0 and 0.1–0.8 mL, re-
spectively. The study results showed that the maximums ΔF were
observed when the amounts of TX-114, Sal1 and sulfuric acid in turn
were in 1.0, 0.4 and 0.4 mL. So, their optimal dosages selected were re-
spectively in 1.0, 0.4 and 0.4 mL.
3.6.3. Effect of extraction temperature and time
The equilibration temperature and time play important roles in the
CPE. The surfactant-rich volume decreased with the equilibration tem-
perature increased, but the concentration of the uranyl in the surfactant
phase increased. Generally, the optimum equilibration temperature of
CPE is 15–20 °C higher than the cloud point temperature of surfactant
[14]. Considering the low cloud point temperature (22–30 °C) of TX-
114, we tested the effect of the equilibration temperature in the range
from 40 to 80 °C. The extraction efficiency increased with increase in
temperature in the range from 40 to 68 °C and then decreased above
68 °C (Fig. 5a). This might be attributed to the heat instability of pheno-
lic hydroxyl groups in Sal1. This could result in the Sal1 being changed
into a quinone or another compound at high temperature. Therefore,
the Sal1 might be transformed into other compounds which could not
be extracted into the micelles when the temperature was above 68 °C
3.6. Optimization of extraction conditions
3.6.1. Selection of buffer solution, acidity and dosage
The pH of solution is a significant factor which affects the extraction
performance of the analytes with two positive charges during dCPE pro-
cess [31]. Because UO22+ is a highly water-soluble cationic compounds
and Sal1 is a hydrophobic ligand, the maximum extraction efficiency
Scheme 3. The schematic diagram of back extraction.

J. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 208–215
213
[15]. So the equilibration temperature was set at 68 °C. Interestingly, for
the back-extraction, the trend of the equilibration temperature was
similar to that for extraction (Fig. 5b), and the equilibrium temperature
was also chosen as 68 °C. It is worth mentioning that the optimal equi-
librium temperature is about 38 °C higher than the critical temperature
of TX-114. The most likely explanation of such experimental result
might be that temperature influences the reaction speed of uranyl and
Sal1, and the fastest reaction speed was observed. At a lower tempera-
ture, the reaction is slower, because the two phenolic hydroxyl groups
in Sal1 molecule must be ionized in solution before the formation of
the complexes, which requires a greater energy.
The effect of the equilibration time was studied in the range from 10
to 60 min at 68 °C. Consequently, the optimum equilibration times all
were 15 min.
3.7. Selection of indication reaction conditions
3.7.1. Effects of the buffer solution and pH on ΔF
Fig. 5. The effects of the water-bath temperature and time on ΔF.
Because the indication reactions take place in weak acidic medium,
four buffer types such as Tris-HCl, Na2HPO4-citric acid, HAc-NaAc,
Na2HPO4-NaH2PO4 were tested; the test results showed that the opti-
mum buffer solution and pH for the indication reaction were in agree-
ment with that obtained in dCPE process. The experiments also
indicated that the maximum ΔF was obtained by adding 2.0 mL
pH 5.5 Tris-HCl buffer solutions. Therefore, a 2.0 mL pH 5.5 Tris-HCl
buffer solution was selected as optimum dosage.
complexes [UO22+-Sal2] which have the sufficient stability in aqueous
solution. The influences of Sal2 (0.40 mmol L−1) dosage on the ΔF
were studied in the range of 0.1–2.0 mL (as shown in Fig. 6). The result
shows that ΔF reached a maximum in 0.4 mL. So, 0.4 mL of Sal2 was se-
lected as optimal dosage.
3.7.3. Effect of reaction temperature and light irradiation time
3.7.2. Effects of pyronine Y, potassium bromate and Sal2 dosages on ΔF
The dosage of PRY directly affects the maximum fluorescence inten-
sity of the system and the sensitivity of analytical method. The effect of
PRY dosage on the ΔF values was studied in the range of 0.5–4.0 mL (as
shown in Fig. 6). The results showed that when the dosage of PRY is
2.0 mL ΔF reached a maximum. Therefore, 2.0 mL was selected as opti-
mal dosage.
The effect of temperature on ΔF was examined every 5 °C in the
range of 15–40 °C. The experimental results indicated that the ΔF
reached the maximum at 20 °C - 25 °C (at room temperature). The RF
intensity remained stable for at least 1 h after stopping light irradiation,
thus, this system exhibits good stability in the experiment. According to
the experimental results, the light irradiation time selected was 15 min.
The effect of potassium bromate dosage on ΔF was studied in the
range of 1.0–4.0 mL (as shown in Fig. 6). The results showed that
when the dosage of potassium bromate is 2.0 mL ΔF reached a maxi-
mum. Therefore, 2.0 mL was selected as optimal dosage.
Compared with hydrophobic Sal1, Sal2 is a water-soluble Sal be-
cause of the hydrophilic carboxyl group in its molecule. Therefore, in
the process of dCPE, the Sal1 is used to combine with UO22+ for forming
the strong hydrophobic complexes [UO22+-Sal1] that enter easily the
surfactant phase. On the contrary, in the photo-catalytic reaction pro-
cess, the use of Sal2 is conducive to the formation of the hydrophilic
3.8. Analytical parameters
The analytical curve for the determination of UO22+ was described at
the optimum conditions. There was a linear relationship between the ΔF
at 558 nm and the concentration of UO22+ in the range of (0.067–
6.57 ng mL−1) (as shown in Fig. 2). According to the IUPAC definition
(IUPAC 1998), the limit of detection (LOD) was estimated as
0.066 ng mL−1 for UO22+ calculated by the equation LOD = 3Sb/S,
where Sb is the standard deviation of the blank measurements (n =
11) and S is the slope of the analytical curve. The linear regression equa-
tion was ΔF = 438.0 c (ng mL−1) + 175.6 r = 0.9981. The relative stan-
dard deviations for the determination of UO22+ concentrations of 0.10,
Fig. 4. The effects of the type of buffer solution and pH on RF.
Fig. 6. The effects of the reagent dosage on ΔF.

214
J. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 208–215
Table 1
Tolerance of foreign substances.
Content
Deviation
(%)
Content
Deviation
(%)
Foreign substances
(mol L−1
)
Foreign substances
(mol L−1
)
Cl−
,
5.0 × 10−3
5.0 × 10−3
5.0 × 10−3
5.0 × 10−4
5.0 × 10−3
5.0 × 10−5
2.0 × 10−4
4.0 × 10−5
5.0 × 10−4
5.0 × 10−4
3.6
−4.7
2.1
−3.5
3.1
NH+4
1.0 × 10−4
1.0 × 10−3
5.0 × 10−5
1.0 × 10−5
2.0 × 10−5
2.0 × 10−6
2.0 × 10−4
1.0 × 10−4
1.0 × 10−4
2.0 × 10−6
4.5
−4.1
3.8
4.5
3.3
4.8
4.5
3.7
−4.2
−4.6
NO−3
,
Na+
SO24−
Mg2+
K+
Zn2+
Glucose
Urea
Ascorbic acid
Phenol
Resorcinol
Pyrocatechol
2,4,6-Trinitrylphenol
Ca2+
Al3+
Hg2+
Pb2+
Cr3+
2.8
−3.7
−3.6
4.2
−4.6
2.0 and 5. 0 ng mL−1 were 1.78%, 2.93% and 3.25%, respectively. The pro-
posed method has some advantages over others, such as better sensitiv-
ity [33], lower limit of detection [10,33], higher selectivity [34] and
lower cost [6,8].
combination of dCPE and photo-catalytic RF detection, using the Sal1
as an extraction coordination reagent of uranyl and [UO22+-Sal2] as the
photo-catalysts, respectively. It has been applied successfully for the de-
termination of trace uranium in complex real samples. The results were
in good agreement with those obtained by ICP-MS. The proposed meth-
od integrates the advantages of photo-catalytic RF methods and dCP
technology including high sensitivity, good selectivity, high extraction
efficiencies, excellent preconcentration capability, wide practicality
and low cost. Although only applied to determine uranium as described
in this paper, the proposed method may open new strategy for the sep-
aration and determination of other targets.
3.9. Effect of foreign substances
The effects of foreign substances on the proposed procedure were
assessed by analyzing synthetic samples solutions containing
1.40 ng mL−1 of UO22+, where a series of metal cations, inorganic anions
and organics had been added according to the likely interference com-
position in actual samples. The tolerance limit is defined as the ratio of
foreign substance that gives a relative error of not more than
5%
Acknowledgements
and the results are listed in Table 1. The common metal cations, inorgan-
ic anions, and some organic compounds may be tolerated at high con-
centration. So the selectivity of the proposed method was better than
other methods such as catalytic beacon [34].
The authors gratefully acknowledge the support of the Hunan
Science and Technology Department Foundation in China (No.
2015SK20012) and the Hunan Education Department Foundation in
China (No. 14A123).
3.10. Analysis of water samples
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of this developed method in complex system. The samples were also an-
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