Chemical Engineering Journal 416 (2021) 129143
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Chemical Engineering Journal
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Review
Current analytical methods for the determination of persulfate in aqueous
solutions: A historical review
Yunjiao Liu a, Lingli Wang a, Yongxia Dong a, Wenya Peng a, Yu Fu a, Qingchao Li a, Qingya Fan a,
Yifan Wang a, Zhaohui Wang a, b, c, *
a
School of Ecological and Environmental Sciences, Shanghai Key Laboratory for Urban Ecological Process and Eco-Restoration, East China Normal University, Shanghai
200241, China
Shanghai Engineering Research Center of Biotransformation of Organic Solid Waste, Shanghai 200241, China
c
Technology Innovation Center for Land Spatial Eco-restoration in Metropolitan Area, Ministry of Natural Resources, 3663 N. Zhongshan Road, Shanghai 200062,
China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Persulfate
Sulfate radical based-AOPs
Traditional titration
Electrochemical methods
Spectrometry
Persulfate (peroxymonosulfate or peroxydisulfate) ions, as strong oxidants, have a broad application in the field
of advanced oxidation processes (AOPs) for oxidative degradation of organic pollutants in soil and contaminated
water. Persulfate concentration is an important operational parameter in persulfate degradation of organic
pollutants based on SO•−
4 -AOPs. At present, the analytical methods used for quantification of persulfate mainly
include traditional titration, polarography, electrochemiluminescence, electrochemical methods, spectrometry
and chromatography. These methods vary in their limit of detection (LOD), execution time, operational diffi­
culty, accuracy and sensitivity. The traditional methods are cheap and easy to operate, but the poor sensitivity
and the high LOD limit its application. Electrochemiluminescence methods are highly sensitive with low
detection limit, but requires rather specialized instruments. Electrochemical methods are repaid and have high
sensitivity, low LOD and wide detection range, but relatively large reagent volume is needed in the determi­
nation process. Spectrophotometry has high accuracy with low consumption rate of sample and reagents, but it is
susceptible to colored substances. Ion chromatography has the advantages of high sensitivity, less reagent
consumption and simultaneous detection of multiple ions, but no advantage in detection time and LOD. This
review presents the principles, merits and demerits of various existing analytical methods for persulfate deter­
mination, and discusses the limitations and applicability of various methods. This review is expected to provide
some references for the establishment of novel detection methods for persulfate in the future.
1. Introduction
Advanced oxidation processes (AOPs) have gained increasing inter­
est in both science and practice of wastewater and drinking water
treatment because some refractory organic pollutants in water are
difficult to be removed by traditional physicochemical and biological
treatment methods. In comparison, AOPs have the advantages of rapid
reaction rates, ability to completely mineralize most organic pollutants
and environmental friendliness in the treatment of toxic and refractory
organic pollutants [1–11]. The traditional AOPs mainly refer to the
technology based on •OH, which is generated by activating stable pre­
cursors, such as H2O2 [12,13]. Recently, alternative sulfate radical
based-AOPs (SO•−
utilizing persulfate (PS) including
4 -AOPs)
peroxymonosulfate (PMS, HSO-5) and peroxydisulfate (PDS, S2O28)
instead of H2O2 gradually emerge.
2−
PS are strong oxidants with higher redox potentials (E0(S2O28 /SO4 )
0
2−
= 2.01 V vs NHE; E (HSO5/SO4 ) = 1.85 V vs NHE) than hydrogen
peroxide (E0(H2O2/OH− ) = 1.78 V). PDS typically decays in a radical
pathway usually producing SO•−
4 , while PMS may also decompose into
•
SO•−
4 and OH [14]. Due to the higher redox potential of free radicals
0
•
0
(SO•−
4 : E = 2.5–3.1 V, OH: E = 2.7–2.8 V) produced by cleaving the
peroxide bond (O-O) in the PS molecule, they are widely used in the
degradation of organic contaminants [4,6,9,10,15–21]. In addition to
the application of AOPs, PS has also been used in other fields including
but not limited to polymerization, metal surface oxidation, adhesives
preparation, circuit board fabrication, determination of total organic
* Corresponding author.
E-mail address: zhwang@des.ecnu.edu.cn (Z. Wang).
https://doi.org/10.1016/j.cej.2021.129143
Received 16 November 2020; Received in revised form 11 February 2021; Accepted 20 February 2021
Available online 28 February 2021
1385-8947/© 2021 Elsevier B.V. All rights reserved.
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
carbon (TOC), bleaching of cosmetics and textiles and in site chemical
•−
oxidation (ISCO) [22–28]. In SO•−
4 -AOPs, SO4 is usually formed in situ
by cleaving the O-O in the PS molecule in an isomeric or hetero­
polymeric manner through energy and/or electron transfer reactions
[14,29–31]. There are various methods for PS activation, including heat,
piezoelectricity, transition metal, ultraviolet, microwave and ultrasonic
[17,18,32–39]. In addition, compared to H2O2 with •OH as the main
oxidant, unactivated PS can also oxidize organic matter without free
radicals, such as singlet oxygen and other nonradical oxidation path­
ways, as illustrated in Fig. 1.
It is well known that the degradation of organic pollutants is affected
by PS concentration [40–42]. As the application of PS-based AOPs for
remediation of contaminated groundwater, wastewater and soil, it is
necessary to monitor the concentration of PS at designated time in­
tervals to evaluate its efficiency. The importance of monitoring PS is
mainly reflected in: (1) it can guide the further optimization of condi­
tions and improve the utilization rate of PS. For example, PS persistence
(lifetime and diffusion distance) in aquifer materials, which is critical to
the design of field-scale application of ICSO can be predicted in terms of
kinetic modeling of PS depletion in laboratory tests [43]. In addition,
information on the amount of residual PS after water treatment is
necessary to avoid the waste of oxidant and release of excess PS into
treated water; (2) the reaction mechanism can be proposed according to
its consumption. For instance, Sheng et al. measured about 0.35 mM
residual PMS in Fe(II)/PMS system when pollutant degradation ceased,
indicating Fe(II) was not effectively recycled [44]. Accelerated PMS
decomposition in the presence of MoS2 demonstrates the pivotal role of
MoS2 in facilitating redox cycling of Fe(III)/Fe(II).
Several methods for the determination of S2O28 have been reported in
the literature. The traditional methods include iodometry [45–47],
ferrometry [48–51] and polarographic method [52,53]. Some new
methods have been developed, such as, electrochemiluminescence
(ECL) [54–60], electrochemical method [61–64], spectral analysis
method, including the UV–vis spectrophotometry [32,37,40,42,65–67],
fluorescence method [68,69] and resonant Rayleigh scattering (RRS)
method [70], and liquid chromatography (LC) [71–75]. These methods
vary in their limit of detection, execution time, accuracy and sensitivity.
Although Wacławek et al. [14] mentioned the determination methods
for PDS and PMS while reviewing the chemistry of PS in wastewater
treatment, they only briefly compared the measurement time and limit
of detection of several methods, and did not analyze the limitations and
applicable scope of various methods. Despite these burgeoning de­
velopments of PS-related applications, there is no exclusive review
summarizing the determination of PS in various application scenarios.
The aim of this review is to describe and discuss the applicability of these
methods and to conduct a comprehensive comparison of the various
detection methods.
2. Methods
2.1. Traditional methods
2.1.1. Iodometric method
In early years, iodometry (or iodometric method) and ferrometry (or
ferrometric method) are effective methods in quantification of PS via
oxidation–reduction reactions. According to the production of iodine
(I2) and the consumption of thiosulfate, iodometry is used to calculate
the concentration of oxidizing agents, such as PDS and PMS. Iodide ions
(I-) can be oxidized to I2 by PDS in neutral or more acidic solution (Eq. 1)
[45,76].
2S2O28 + 2I = I2 + 2SO4 (1)
In this context, Kolthoff and Carr [46] and Wahba et al. [47] pro­
posed iodometric technique which depends on the color disappearance
of solution and the dosage of reductant-sodium thiosulfate (NaS2O3). In
neutral or acidified solution, the reaction can be accelerated by catalysts
(such as ferric chloride (FeCl3) or sulfuric acid (H2SO4)) and completed
in 15 min. The experimental procedure is described in scheme 1. Firstly,
carbon dioxide removes dissolved oxygen in solution to avoid the in­
fluence of oxygen. Then the sample, iodide solution and glacial acetic
acid containing catalysts (FeCl3 or H2SO4) are mixed well and react in a
dark environment for 15 min. This operation requires the addition of an
excess of iodide so that the PDS can be consumed completely. Finally
Fig. 1. Possible oxidation pathways induced by PMS and PDS activation. .
Adapted from [30]
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Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
Scheme 1. Schematic diagram of iodometric method.
using starch as an indicator, the mixed solution is titrated with thio­
sulfate until the color disappears (Eq. 2) [46]. It may take 15–45 min to
complete one measurement. The concentration of sodium perox­
ydisulfate (Na2S2O8) is estimated via Eq. (3) [46].
22S2O23 + I2 = S4O6 + 2I (2)
Na2S2O8 (mg/mL) = [C×(X1 ± X2)/2] × M × DF/V (3)
Where C is the concentration of Na2S2O3 (mol/mL); X1 and X2 is the
volume of Na2S2O3 used in sample and blank sample (mL), respectively;
DF is the dilution factor; V is the sample volume (mL); M is the molar
mass of Na2S2O8: 238.104 × 103 mg/mol;
When testing PDS concentration in samples (30 mL solution con­
taining 30–400 mg PDS), the relative error of iodometry is 3%–5%.
Some organic substances such as methanol, ethyl alcohol or acrylonitrile
have little influence because they do not react with I− or I2 [46].
However, the titration process is easily interfered by the color and
turbidity of water samples, and nitrite, free chlorine, iron ions in water.
Several measures may be applied to reduce the relative error including
overdose of KI, rapid titration, pH and temperature controls, preparation
of fresh KI and starch solutions. Indeed, high limit of detection (LOD)
and narrow detection range make it more suitable to quantify 6–30 mg
PDS in 30 mL solution. Iodometry was ever considered as an accurate
and rapid method, although its reaction operation revolves backtitration, which makes the whole operation tedious and time-
strictly requires the pH to be close to neutrality, although H+ does not
appear in the rate expression. At the near-neutral pH, the reaction at
high concentrations is almost instantaneous, whereas at a pH lower than
6.0 or higher than 8.0, autoxidation of I− becomes appreciable while I−3
is easily destructed via Eq. (5) [50]. Amin and Hareez [48] also proposed
a method for titrimetric micro-determination of PDS by amplification
reactions, which combined titrimetric method with amplification pro­
cedure. According to Leipert reaction for determining iodide, iodide (I− )
can be oxidized to iodate (IO−3 ) by PDS in alkaline solution when boiling
for 1 min (Eq. 6) [51]. Then sulphuric acid is added, which prompts the
reaction between excessive I− and IO−3 (Eq. 7) [48]. The formed I2 is
extracted with chloroform or benzene and then is reduced to I− by so­
dium hydrogen sulphite solution (NaHSO3) (Eq. 8) [48]. Through the
series of operations, the iodide has been given a 6-fold amplification
[49]. The formed I− is oxidized to iodate (IO−3 ) by bromine (Br2). IO−3 is
oxidized to I2 by I− and then is titrated by Na2S2O3.
−
2−
−
S2O2−
8 + 3I → 2SO4 + I3 (4)
3I−3 + 6OH− → IO−3 + 8I− + 3H2O (5)
−
−
2−
I− + 3S2O2−
8 + 6OH → IO3 + 6SO4 + 3H2O (6)
−
+
−
IO3 + 5I + 6H → 3I2 + 3H2O (7)
I2 + HSO−3 + H2O → 2I− + HSO−4 (8)
The whole reaction process is briefly demonstrated in Eq. (9)
(Adapted from Amin and Hareez [48]).
consuming.
(9)
2.1.2. Improved iodometric method
Based on the iodometric method, Frigerio [50] developed a method
for macro and micro determination of PDS with the detection range 1–2
mM and 0.01–0.15 mM, respectively. Without heat, catalysts or dis­
solved oxygen removal, iodide can be directly oxidized by PDS to form
triiodide (I−3 ) with high molar ratios in neutral solution (Eq. 4) [50].
Then the I−3 is titrated with S2O2−
3 or determined spectrophotometrically
at 355 nm. In this way it is possible to obtain replicate runs with a
precision of ± 0.02% or better. This reaction provides the basis for a
rapid and precise method which can be applied to not only macro
titration but also microspectrophotometry. However, this method
In addition, a 24-fold amplification of the original iodide was also
tested. This method uses chloroform to extract I2 on the basis of 6-fold
amplification, and then reduce I2 to I− with NaHSO3 solution. The
formed I− is oxidized by periodate to form iodate (Eq. 10) [48]. Then
iodate reacts with iodide to give a 24-fold amplification in the presence
of sulfuric acid (H2SO4).
I- + 3IO-4 → 4IO-3 (10)
The reaction process can be expressed as Eq. (11) (Adapted from
Amin and Hareez [48]).
(11)
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Chemical Engineering Journal 416 (2021) 129143
•
SO•−
4 + CH3CH2OH → CH3CHO + HSO4 (16)
•
•−
2SO4 + Br → SO4 + Br (17)
Br• + Fe2+ → Br- + Fe3+ (18)
However, possible problems of ferrometry in the practical applica­
tion may include: (1) in the PDS test, pollutants exist in the solution
together with PS. Adding Fe2+ promotes PDS to produce SO•−
4 , which
will react with organic substances and affect the determination result;
(2) even if Br- is added, the formation of Br• will react with organic
matter to make the measurement inaccurate. Like iodometry, ferrometry
is also widely applied in PDS analysis in early times. However, use of
catalysts and removal of dissolved oxygen make the process difficult to
control, reducing the accuracy of test. In addition, the operation process
involves the back-titration, making the whole reaction complex and
tedious.
Compared with traditional titrimetric method, which is suitable for
the determination of 7.3 × 10-4–3.69 × 10-3 M PDS, the combination of
titrimetric method and amplification reactions allows evaluation of a
lower concentration (2.4 × 10-4–1.23 × 10-5 M) with a higher sensi­
tivity. The relative error of the 6-fold and the 24-fold amplification re­
actions are 0.5%-2% and 0.3%-1.9%, respectively, both less than the
3%–5% of iodometry. However, the process requires heating and a large
excess of iodide.
2.1.3. Ferrometric method
Some organic substances (such as ethyl alcohol) can react with SO•−
4
yielded from PDS, affecting PDS measurement. As a good substitute for
iodometry, ferrometry is able to eliminate the interference from or­
ganics. Similar to iodometry, ferrometry also applies the color change of
solution and the amount of titration solutions to evaluate PDS concen­
tration. Specific operational process can be described as: excesive
ferrous ions (Fe2+) is added to the sample and reacts with PDS and SO•−
4
(Eqs. 12–13) [46]. Then using ferrous phenanthroline as an indicator,
potassium permanganate or ceric sulfate solution is added to oxidize the
remaining Fe2+ (Eq. 14 or Eq. 15) until the color changes [46,47]. Ethyl
alcohol, as a representative of organic compounds, can react with SO•−
4
and affect PDS measurement (Eq. 16) [46]. To avoid this influence,
halides especially bromide (Br− ) are added because it can compete with
•
organic compounds and Fe2+ for SO•−
4 and transformed into Br , then
2+
react with Fe (Eqs. 17–18) [46]. Suitable level of bromide (1 M) can
eliminate the influence of organic compounds and the relative error of
this method is − 2.1%–1.5%. Thus, ferrometric method is preferred in
the presence of co-existing organic substances.
2+
3+
•−
S2O2+ SO28 + Fe → Fe
4 + SO4 (12)
2+
3+
•−
2Fe + SO4 → Fe + SO4 (13)
MnO-4 + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O (14)
Ce4+ + Fe2+ → Ce3+ + Fe3+ (15)
2.1.4. Polarography
Polarography is a kind of electrochemical analysis method in terms
of the current–potential (or potential-time) curve of polarized electrode.
Polarographic determination of PDS has been reported by Hakolia [52],
Kolthoff and Woods [53] and Amin [77]. The experimental procedure is
described in scheme 2. When the mercury drop of the mercury electrode
(DME) is covered by an insoluble film of the adsorbed halide, a response
peak due to peroxydisulfate appears in the alternating-current (AC)
polarographic at a potential more negative than that of the halide po­
tential. The corresponding PDS concentration is determined according
to the wave height owing to the reduction of iodate on a mercury drop
electrode. Polarography is suitable for measuring PDS with 10-3 M
concentration and its relative error ranges from − 10% to 2%. Then Amin
[77] proposed the combination of AC polarography and amplification
method, which gives a 36-fold or 144-fold amplification. The main re­
actions can be described as Eq. (19) (Adapted from Amin [77]).
Scheme 2. Schematic diagram of polarography.
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Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
(19)
Compared with polarographic method, this combination has a higher
sensitivity and accuracy. It can be used to analyze PDS with concen­
tration of 2.6 × 10-6 M and its relative error rate does not exceed 2% in
solution containing 10–2000 μg of PDS. However, this method needs
heating and pH control to neutral. Because the height of the AC
polarogram is affected by the PDS ion concentration, the halide ion
concentration and the electrolyte, the concentration of halide ions must
be controlled in the range of 1 × 10-2–2 M and the appropriate elec­
trolyte should be selected in the determination of PDS. Organic sub­
stances, ammonium salts, hydrogen peroxide and metal ions such as
copper (I) and sliver (I) are expected to interfere the analysis. In addi­
tion, substances that yield half-wave potentials close to that of iodate
from NaOH or H2SO4 supporting electrolyte also cause interference
[53].
Table 1 summarizes the differences among traditional methods.
Accompanied by amplification reaction, titration and polarography
have higher sensitivity and lower detection limit. Especially, amplifi­
cation/polarographic method can be used for the determination of as
low as 2.6 μM of PDS. In general, the titration methods and polarogra­
phy are often used in the determination of PDS in the early stage,
however, these methods are susceptible to pH, temperature, organic
substance and might involve back titration, which is rather tedious and
time consuming. Therefore, special attention should be paid to con­
trolling parameters such as temperature, pH, organic substance content
and reaction time when determining persulfate by traditional methods.
In addition, the conventional methods are suitable to measure high
concentration of PDS. Simply judging from reaction mechanism, these
traditional methods are also theoretically suitable for the detection of
PMS.
•−
pathways [58]. For instance, the reaction between Ru(bpy)+
3 and SO4 to
2+*
+
generate Ru(bpy)3 (an excited state of Ru(bpy)3 ) (Eqs. 20–22). Or
•−
strong oxidants (Ru(bpy)3+
3 and SO4 ) yielded by the quenching reaction
2+*
2between Ru(bpy)3 and S2O8 can be recycled to produce another Ru
(Eqs. 23–25). The relative intensity of ECL based on Ru(bpy)2+
(bpy)2+*
3
3
has close relationship with S2O28 concentration. The maximum intensity
2+
is obtained at S2O28 concentration of 15–20 mM when the Ru(bpy)3
concentration is 1 mM. However, at high S2O2concentration
(>30
8
mM), the excited state is not only quenched, and electrogenerated Ru
22+
(bpy)+
3 may be removed by reaction with S2O8 to obtain Ru(bpy)3 ,
which reduces the steady-state concentration of Ru(bpy)+
3 near the
electrode surface and inhibits the luminescence intensity. Thus, ECL
2based on Ru(bpy)2+
3 is not suitable for testing high content of S2O8 .
2+
+
[Ru(bpy)3] + e → [Ru(bpy)3] (20)
2+
•−
[Ru(bpy)3]+ + S2O2+ SO28 → [Ru(bpy)3]
4 + SO4 (21)
2+*
2[Ru(bpy)3]+ + SO•−
→
[Ru(bpy)
]
+
SO
(22)
4
3
4
3+
•−
+ SO2[Ru(bpy)3]2+* + S2O28 → [Ru(bpy)3]
4 + SO4 (23)
3+
2[Ru(bpy)3]2++ SO•−
→
[Ru(bpy)
]
+
SO
(24)
4
3
4
[Ru(bpy)3]3++[Ru(bpy)3]+ → [Ru(bpy)3]2+* + [Ru(bpy)3]2+ (25)
The potentials of [Ru(bpz)3]2+ (bpz = 2,2′ -bipyrazine) are shifted by
around 0.5 V toward more positive potentials compared with Ru(bpy)2+
3
[79], which can avoid the interference of proton reduction at negative
potential. Based on the electrolytic reduction of [Ru(bpz)3]2+, a direct
current electrogenerated chemiluminescent method was developed to
determine PDS concentration within 10-9–103 M [60]. Similar to the
2+
2transformation of Ru(bpy)2+
3 , Ru(bpz)3 is oxidized by S2O8 to produce
bright orange luminescence [58]. Detailed mechanism is expressed as
Eqs. (26–29) [54]. It is worth noting that light appears in the trans­
formation of [Ru(bpz)3]2+* to [Ru(bpz)3]2+. In this system, ECL in­
tensity is easily affected by NO−2 , Br− , I− and certain metal ions or
complexes, as these ions react with SO•−
4 (Eqs. 30–31). Also, its intensity
is affected by pH, and the maximum intensity appears in the range of pH
−
3.0–5.0, which may be caused by the rapid consumption SO•−
4 by OH at
higher pH or fast decomposition of the electrogenerated [Ru(bpz)3]2+ at
lower pH [80–82]. In general, ECL based on [Ru(bpz)3]2+ has a wide
detection range and is suitable for the microdetermination of S2O28.
[Ru(bpz)3]2+ + e- → [Ru(bpz)3]+ (26)
2+
•−
+ SO2[Ru(bpz)3]+ + S2O28 → [Ru(bpz)3]
4 + SO4 (27)
2+*
2[Ru(bpz)3]+ + SO•−
→
[Ru(bpz)
]
+
SO
(28)
4
3
4
[Ru(bpz)3]2+* → [Ru(bpz)3]2+ + hv (29)
−
1
−
−
2−
SO•−
4 + X → SO4 + /2 X2 (X = Br , I ) (30)
+
−
2−
−
2SO•−
+
NO
+
3H
O
→
2SO
+
NO
4
2
2
4
3 + 2H3O (31)
Also ECL is vulnerable to the proton reduction at low negative po­
tential [60]. The redox potential of Cr(bpy)3+
3 (-0.5 V) is about 1.0 V
2+
higher than that of Ru(bpy)2+
3 and 0.5 V higher than that of Ru(bpz)3 ,
3+
2indicating that ECL based on Cr(bpy)3 /S2O8 system is a reliable and
sensitive method for the determination of PDS [59]. When the concen­
tration of PDS is 10-6–7 × 10-4 M, ECL intensity is linearly correlated
with PDS concentration. The detection limit (signal/noise: S/N = 3) and
relative standard deviation (RSD) is 1 μM and 2%, respectively. The
analysis result of sample in organic solvent-saturated aqueous solutions
shows that the recovery is 98 ± 3% (mean ± standard deviation).
However, the pH has an important effect on its intensity. At higher pH,
−
3+
SO•−
4 reacts with OH faster and Cr(bpy)3 decomposes faster, which
may cause the decrease in ECL intensity [80–82]. In addition, similar to
−
−
[Ru(bpz)3]2+/S2O28 system, ECL intensity is easily affected by NO2 , Br
or I− , but it is not susceptible to the presence of organic compounds and
2.2. Electrochemiluminescence
Electrochemiluminescence (also called as electrogenerated chem­
iluminscence and abbreviated ECL) involves the generation of species at
electrode surfaces that then undergo electron-transfer reaction to form
excited states that emit light. Based on this phenomenon, PDS concen­
tration can be measured by ECL intensity through addition of poly­
pyridine transition-metal complexes (such as [Cr(bpy)3]2+, [Ru
(bpy)3]2+ or [Ru(bpz)3]2+) as luminescent materials [54–60,78], which
can produce bright light through electrochemical and chemical re­
actions on the electrode surface. The experimental procedure is
described in scheme 3. ECL has the advantages of fast analysis (within 1
min), good selectivity and wide selection range.
2′
The reaction of Ru(bpy)2+
3 (bpy = 2,2 -bipyridine) with S2O8 at -1.5
V potential can lead to intense ECL, which involves a variety of reaction
Table 1
Comparison of traditional methods for S2O28 detection.
Method
LOD
(μM)
Linear concentration range
(LCR) (mM)
Reference
Titration
Macro-Micro/Titration
Amplification/
Titration
Polarographic
Amplification/
Polarographic
——
——
——
0.73–3.69
1.00–2.00,0.01–0.15
0.52–5.15
[46]
[50]
[48]
103
2.6
1.00–5.00
——
[53]
[77]
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Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
Scheme 3. Schematic diagram of ECL sensor.
NO−3 .
2+
3+
Besides Ru(bpz)2+
3 , Ru(bpy)3 and Cr(bpy)3 , luminol (5-Amino-2,3dihydro-1,4-phthalazinedione) is also a typical ECL reagent [83,84]. It
can react with PMS in alkaline solutions to produce intense chem­
iluminescence [85]. The maximum intensity is linearly correlated with
PMS concentration in its range of 4.0 × 10-7–2.0 × 10-6 M. This method
has a good selectivity for common ions or organic substance, with
detection limit of 5 × 10-8 M and RSD of 1.5%–3%.
In addition to the electrochemiluminescence, Tang et al. [86] also
proposed a fast method for the determination of PMS by flow injection
chemiluminescence (FIA-CL) using the Tb (III) ligand in micelle me­
dium. Under the optimized conditions, the maximum intensity is
Table 2
Comparison of different ECL systems for S2O28 detection.
System
Working point (V)
LOD(μM)
LCR(μM)
Reference
2Ru(bpy)2+
3 -S2O8
2-S
O
Ru(bpz)2+
3
2 8
2Cr(bpy)3+
3 -S2O8
Luminol-HSO-5
Tb (III)- HSO-5
− 1.5
− 1.0
− 0.5
——
——
——
——
1.00
0.05
0.5
15000–20000
1 × 10-3–1000
7–100
4 × 10-1–2
4.0–200
[58]
[60]
[59]
[85]
[86]
Scheme 4. Schematic diagram of modified electrode. Adapted from Guo et al. [96].
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Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
linearly correlated with PMS concentration ranging from 4.0 × 10-6 to
2.0 × 10-4 M. The LOD is 5.0 × 10-7 M (S/N = 3) and the RSD is 2.4%.
This is comparable to the luminol-HSO-5 system. The variation of ECL
based on different luminescent materials has been summarized in
Table 2. ECL method can complete the analysis of five samples per
minute and can be applied to study the PMS decomposition profiles.
However, this method is susceptible to interference of Fe2+, Co2+, Fe3+,
which may be related to their participation in PMS decomposition.
Compared with the traditional methods, chemiluminescent method
has many advantages, especially high sensitivity, high recovery, low
cost, wide LCR and low LOD. However, ECL methods also have many
deficiencies. For example, ECL intensity is easily affected by pH, the
electrode materials and some ions (such as Br− , I− , NO−2 and Fe3+, Cu2+,
Ni2+). This is because in the presence of some ions or high pH, SO•−
4 will
have a scavenging reactions with some ions or OH− . The accuracy can be
improved by controlling pH or adding appropriate masking agents. In
addition, ECL requires specialized instruments, which are not common
in ordinary laboratories.
hour), high precision (RSD no more than 2.2%), less reagent consump­
tion (only tens or even hundreds of microliters each time), wide appli­
cation and high repeatability [105]. In view of these advantages, De
Oliveira et al. [61] used a prussian blue film modified glassy carbon disk
electrode (GCE) in combination with wall jet configuration electro­
chemical cell for automated determination of PDS by a flow injection
amperometry. The methodology allows the efficient determination of
PDS concentration (160 samples per hour) with good sensibility. By
optimizing the reaction parameters, 20 mL injection volume, 4 mL/min
flow rate and 0.025 V detection potential were selected. The peak cur­
rent of modified electrode showed good linear correlation with PDS
content in the concentration range of 1 × 10-4–1 × 10-3 M. In this
method, the correlation coefficient is 0.9998, while the detection limit is
9.0 × 10-5 M, sensitivity and RSD (n = 12) are 3.6 × 103 μA L mol− 1 and
2.2%, respectively. In the determination of PDS in commercial hairbleaching sample, it was consistent with the titrimetry with good re­
coveries of 95–112%. Compared with PB/CPE alone, PB/GCE combined
with FIA has lower optimal working point (0.025 V) and is more suitable
for analysis of large amounts of samples.
The redox dyes are widely used as electrode modification since their
formal potential is closed to the redox potential of most biomolecules
[106]. However, most of these modified electrodes exhibit quasireversible electrochemical behavior and even show ambiguous cyclic
voltammograms at large background currents. Nanocomposite as
modified electrode attracted great interest due to its high electrical
conductivity, surface area and mechanical stability [93,94,107–109].
Nanoparticle can promote electron transfer in electrochemical device.
Dyes immobilized by nanostructures on the electrode surfaces are more
stable than the dye-modified electrode alone. For example, glassy car­
bon modified electrode formed by Poly brilliant cresyl blue (PBCB)
electrodeposited on multi-walled carbon nanotubes (MWCNT) by cyclic
voltammetry is used as an amperometric (Amp) sensor due to its good
stability at different pH and various scan rates, low over-potential and
high current response to PDS [62]. Based on the interesting chemical
and physical properties of carbon nanotubes (CNTs) and PBCB, the
sensor has good chemical stability, conductivity, ductility, electro­
catalytic activities and electron transfer capability [110–113]. The
studies on pH dependence at different scanning rates showed that the
electrode modified by PBCB-MWCNT had stable electrochemical activ­
ity at different scanning rates and within pH range of 1.0–13.0. The peak
currents have linear correlation with the concentration of S2O28 as Eqs:
(34–35).
Ip(μA) = 0.035 Ci (μM) + 5.8 (R2 = 0.997) (34)
Ip(μA) = 0.006 Ci (μM) + 24.8 (R2 = 0.995) (35)
It is calculated with the sensitivity of 21.2 and 124.5 μA mM− 1 cm− 2
for the linear concentration range of 3.1 × 10-3–1.01 × 10-1 M and 105
–10-4 M, and detection limit of 1 μM (S/N = 3).
In view of the excellent film-forming ability and high water perme­
ability of multi-wall carbon nanotubes-chitosan (MWCNT-Chit) nano­
composite, Roushani et al. [114] fabricated a novel PDS detection sensor
based on the modification of GCE by MWCNT-Chit and azo dye metail
yellow (MY). The proposed sensor showed a linear amperometric
respond to the PDS concentration from 1 × 10-7 M− 1 × 10-3 M with
detection limit of 0.03 μM (S/N = 3) and sensitivity of 860.1nA nM− 1.
In addition to MWCNT, graphene quantum dots (GQDs) are also an
ideal candidate for nanoscale optical device construction due to its
quantum confinement and edge effects. Riboflavin (6,7-dimethyl-9-(d-1ribityl)-isoalloxazine, RF), known as vitamin B2, is a water-soluble
vitamin. Roushani and Abdi [93] proposed an amperometric sensor
for S2O28 detection with RF as a modifier and GQDs as a fixator. The
catalytic reactions of RF are shown as Eqs: (36–37).
Riboflavin (oxidized form) + 2H+ + 2e− → Riboflavin (reduced
form) (36)
Riboflavin (reduced form) + S2O2−
8 → kcat Riboflavin (oxidized
form) + 2SO2−
4 (37)
The glassy carbon electrode modified by RF/GQDs has strong
2.3. Electrochemical methods
Electrochemical method is a general term for a class of analysis
methods established on electrochemical principles. The forementioned
polarography is one of electrochemical methods and is establish­
ed early. Then voltammetry, electrolysis analysis and potentiometric
titration were developed. In electrochemical catalytic reaction, the
chemically modified electrodes not only have a significantly lower
activation overpotential, but are less prone to form surface contamina­
tion and oxide compared to inert substrate electrodes [87]. And the
modified electrode provides a new electrochemical sensor with good
stability and selectivity [88–91]. Thus, the application of chemically
modified electrodes for the determination of PDS has aroused great in­
terest due to its simplicity, good stability, and sensitivity in past two
decades [61–64,92–95]. The schematic diagram of modified electrode is
described in scheme 4.
Several electrode modification methods have been developed, such
as adding clays, metal oxides, redox polymers, metal cyanometallates
and ion-exchange polymers [97–101]. Among these additives, prussian
blue [PB, hexacyanoferrate(II)] and analogous compounds such as
ruthenium purple (RP) or osmium purple (OP) modified electrodes are
greatly attractive because of its simple preparation and wide application
range [61,62,102]. In 1992, Weißenbacher et al. [64] took the lead in
reporting the electrocatalytic effect in the reduction of PDS by direct
current voltammetry (DCV) using PB, RP or OP modified carbon paste
electrodes (CPE), which reveals that PDS generates a linear catalytic
current response at the modified carbon paste electrode within the
concentration of 5 × 10− 5–3 × 10− 3 M. The determination limit and
coefficient of variation of PB, RP, OP modified electrode are about 1 μg
O2−
2 /mL (as peroxydisulfate) and 4%, respectively. The best working
points obtained in the direct current voltammetry are − 0.6 V, − 0.5 V
and − 0.5 V for PB, RP and OP, respectively. The concentration of PDS is
evaluated according to the liner relationship between current and con­
centration. The electron-transfer mechanism may be formulated as Eqs.
II
II
(32–33) [103,104]. The soluble form of FeIII
4 [Me(CN)6]3 (Me = Fe , Ru
or OsII) can be reduced electrochemically on CPE in the presence of
excess potassium ion. Then S2O2−
8 is reduced by direct current voltam­
+
III
metric and a mixture of SO2−
4 , K and Fe4 [Me(CN)6]3 may be yielded.
The modified electrode, obtained by electrochemical deposition of this
film in carbon paste electrode, exhibits a stronger current and a better
mechanical stability than inert substrate electrodes. However, this
method is time-consuming and susceptible to interference by other ox­
ides unless the analyte is preconcentrated.
+
−
II
FeIII
4 [Me(CN)6]3 + 4 K + 4e → K4Fe4 [[Me(CN)6]3 (32)
+
II
2−
2−
K4Fe4 [Me(CN)6]3 + S2O8 → SO4 + FeIII
4 [Me(CN)6]3 + 4 K (33)
As a new continuous flow analysis technique, flow injection
amperometry (FIA) has fast analysis speed (around 160 samples per
7
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
conductivity, high stability, direct electron transfer ability, excellent
electrocatalytic activity and reproductivity. Like PBCB-MWCNT modi­
fied electrode, the RF/GQDs modified electrode also shows an excellent
reproducibility, wide applicability of pH (1.0–10.0) and ion selectivity
2+
2+
2+
22(IO-3, ClO-4, NO–2, SO23 , S2O3 , S2O6 , Ni , CO , Zn ), which is largely
attributed to the low operating potential used in the determination. The
fabricated sensor has excellent reproducibility with RSD about 2.5%.
The catalytic reduction allows an amperometric detection of S2O28 at a
potential of -0.1 V with concentration calibration range of 1 × 10-6–1 ×
10-3 M, detection limit of 0.2 μM and sensitivity of 4.7 nA M− 1. Conse­
quently, the RB/GQDs modified electrode can be used as an ideal sensor
for the S2O28.
Compared with MWCNT and GQDs, metallic nanowires (MNWs)
possess two advantages: one is the precise control of material properties
through a controlled synthesis process, and the other is the inherent
oxide layer on the surface of nanowires (NWs), which provides oppor­
tunities for various functionalization and blocking chemical reaction
[91]. Natural red (NR: N8, N8, 3-trimethylphenazine-2, 8-diamine
chloride) has been considered as redox mediator in the application of
electrochemical biosensors, in view of their low redox potential and
efficient electron transfer [115,116]. By immobilizing dyes on metallic
nanostructure carbon paste electrode, a novel nanoparticles-based
electrochemical sensor was developed and has been widely applied
[94,117–119]. Savari et al. [63] systematically studied the voltammetric
and amperometric behavior of electrode modified by neutral red/nickel
oxide nanowires (NR/NiOx-NWs) and employed them for the determi­
nation of PDS (E0 = -0.2 V in a buffer solution at pH 2.0). There is a
linear relationship between the current and S2O28 concentration with a
correlation coefficient of 0.998. Using NR/NiOx-NWs to determine PDS
in real samples, the recovery can reach 96%, demonstrating the ability
of the sensor for the determination of PDS in real sample. The modified
electrode demonstrates a good stability and reproducibility at wide
range of pH (1.0–12.0). In the study of catalytic activity and selectivity,
due to the low operation potential, the electrode modified by NR/NiOxNWs has excellent electrocatalytic activity toward S2O28 and selectivity
in the presence of common ions of Ni2+, Co2+, Fe3+, Al3+, Pb2+, Zn2+,
222Ca2+, Bi3+, SO23 , ClO4, S , S2O3 , IO4 . The sensor modified by NR/NiOxNWs was evaluated by amperometry under the stirred condition. The
linear concentration range is 1.0 × 10-7–1.2 × 10-2 M, whereas the
detection limit is 30 nM and the sensitivity is 647.33 μA mM− 1 cm− 2.
Like NiOx, Ruthenium oxide (RuOx) is also a popular metallic oxide
nanoparticle with good electrochemical or photochemical catalysis and
excellent charge storage. Therefore Roushani and Karami [94] proposed
a new nanoparticle amperometric sensors by fixing redox dyes thionine
(TH) or celestin blue (CB) onto the glassy carbon electrode surface with
RuOx. The sensor was successfully applied to the determination of S2O28
and showed excellent electrocatalytic activity at a potential of + 0.1 V.
The sensitivities of the electrode modified by GC/RuOx/TH or GC/
RuOx/CB were 3nA μM− 1 or 1nA μM− 1 at the concentration range of 105
M− 11 × 10-3 M or 10-5 M− 6 × 10-3 M, and the detection limit was
1.46 μM or 2.64 μM, respectively. The sensor had high stability in 0.1 M
phosphate solution with pH = 7 by continuous potential cycling be­
tween − 0.5 and − 0.05 V. The electrode not only has good stability and
reproducibility in a wide range of pH 2.0–12.0, but also is less affected
22by interfering ions such as Ni2+,Bi3+, Co2+, SO23 , SO4 , S2O3 . Thus, the
electrochemical method stands out owing to its wide detection range,
excellent reproducibility, sensitivity and selectivity.
However, some of the electrode materials used are not environ­
mentally friendly and easy to cause pollution especially using dyes as
composites. Guo et al. [120] reported a facile fabrication of
H4SiMo12O40 decorated the poly (3,4-ethylenedioxythiophene)-elec­
trochemically reduced graphene oxide (pEDOT-ERGO) on the Indium
tin oxide (ITO) electrode, designated as SiMo12/pEDOT-ERGO/ITO, by
electrodepositing the out-standing electrocatalytic capacity of SiMo12
into the excellent conductivity of the pEDOT-ERGO/ITO nanocomposite
film by cyclic voltammetry. In this composite electrode, H4SiMo12O40,
an inorganic metal oxide cluster compound, was used as a redox mate­
rial, and pEDOT-ERGO was used as connector and modifier because of
their excellent conductivity, good solubility and large surface area
[121–123]. The current–time response of SiMo12/pEDOT-ERGO/ITO
with successive addition of S2O28 was measured by amperometry at a
fixed potential of − 0.1 V. The novel sensor has a good linear response in
the range of 1.5 × 10-6–1.32 × 10-4 M with regression coefficient value
of 0.99987, a low detection limit (0.48 μM), a better sensitivity (0.22 μA
μM-1cm− 2, S/N = 3) compared to other electrochemical electrodes (see
Table 3). The modified electrode shows significant stability in neutral
solution, which could be attributed to the promoting effect of pEDOT on
the stability of SiMo12. In addition, SiMo12/pEDOT-ERGO composite
modified ITO has an excellent selectivity for electrochemical detection
of S2O28 from various inorganic ions of Na2SO3, KCl, NiCl2, MnCl2 and
CoCl2. For example, the recoveries of the tap and lake water were
100.2% and 93.8%, respectively, indicating that the modified electrode
can be employed for the determination of S2O28 in real sample.
To sum up, the analytical performance of SiMo12/pEDOT-ERGO/ITO
modified electrode is comparable or even superior than other electro­
chemical modified electrodes. And, the proposed modified electrode is
eco-friendlier, indicating that it is more suitable for the determination of
S2O28 . While the current literature on electrochemical method mostly
focus on PDS, its application for PMS quantification deserves further
investigations.
In order to facilitate the comparison of various modified electrode
methods, a summary has been made in Table 3. In general, the analytical
performance of electrode modified by nanocomposite is comparable or
even superior than simple electrode modified by dye or SiMo12-pEDOTERGO/ITO, but SiMo12-pEDOT-ERGO/ITO electrode is the most ecofriendliest. Compared with the traditional methods and the ECL, the
electrochemical method has a wider pH range and stronger ions selec­
2+
tivity in presence of common ions (Ni2+, Fe3+, Pb2+, Zn2+, S2O23 , Co ,
23+
2,
ClO
,
S
,
Al
and
IO
).
In
addition,
the
electro­
Ca2+, Bi3+, SO23
4
4
chemical method has other merits, such as low redox potential, efficient
electron transfer, simple function, low cost and quick response.
2.4. Spectral analysis
2.4.1. UV–vis spectrophotometry
Spectrophotometric analysis is a common method for quantitative
Table 3
Comparison of different electrochemical measurements for S2O28 detection.
Modified electrode
Methodology
Working point (V)
LOD (μM)
LCR(mM)
Sensitivity
Reference
PB/RP/OP/CPE
PB/GCE
PBCB-MWCNT/GCE
MY-MWCNT-Chit/GCE
RF-GQDs/GCE
NR-NiOx NWs/CPE
TH-RuOx/ GCE
CB-RuOx/ GCE
SiMo12-pEDOT-ERGO/ITO
DCV
FIA
Amp
Amp
Amp
Amp
Amp
Amp
Amp
− 0.6
0.025
− 0.03
+0.2
− 0.1
− 0.2
+ 0.1
+ 0.1
− 0.1
7.81
0.9
1.0
0.03
0.2
0.03
1.46
2.64
0.48
0.05–3
0.1–1
3.1–1000.01–0.1
0.0001–1
0.001–1
0.0001–12
0.01–11
0.01–6
0.0015–0.132
——
3.6 × 103 μA L mol− 1
21.2124.5 μA mM− 1 cm− 2
860.1 nAnM− 1
4.7 nA M− 1
647.3 μA mM− 1 cm− 2
3.0 nA μM− 1
1.0 nA μM− 1
0.22 μA μM− 1 cm− 2
[64]
[61]
[62]
[114]
[93]
[63]
[94]
[94]
[120]
8
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
Scheme 5. Schematic diagram of spectrophotometric analysis.
and qualitative detection of substance due to its advantages of simple
operation, high accuracy, good reproducibility, and low consumption
rates of samples and reagents. It is based on the principle that the
absorbance of substances at the maximum absorption wavelength is
proportional to the concentration. PS can be activated by transition
metal [18,124], microwave (MW) [42], UV [125] or heat [32] to pro­
duce sulfate radical and hydroxyl radical with redox potential E0 =
2.5–3.1 V and E0 = 2.7–2.8 V, respectively (PDS: Eqs. 38–40, PMS: Eqs.
•
41–47). SO•−
4 and OH can oxidize dyes and other colored substances,
resulting in decolorization of the dyes and other colored substances (Eq.
48–49). It is found that the decolorization degree at the maximum ab­
sorption wavelength is proportional to the concentration of PS. The
spectrophotometric analysis is demonstrated in scheme 5.
•−
S2O28 + heat → SO4 (38)
2•−
S2O8 + MW → SO4 (39)
2+
3+
2/ Co2+ → SO•−
/ Co3+ (40)
S2O28 + Fe
4 + SO4 + Fe
Co2+ + H2O ↔ CoOH– + H+ (41)
HSO-5 + CoOH– → CoO+ + SO•−
4 + H2O (42)
CoO+ + 2H+ ↔ Co3+ + H2O (43)
+
Co3+ + HSO-5 → Co2+ + SO•−
5 + H (44)
−
•−
−
•−
2SO5 ↔ O3SOOOOSO3 ↔ [SO4 O2SO•−
4 ] (45)
•−
•−
[SO•−
4 O2SO4 ] ↔ O2 + 2SO4 (46)
•
+
2SO•−
4 + H2O → OH + H + SO4 (47)
+
dye
→
decolorized
dye
+
SO2SO•−
4
4 (48)
•
OH + dye → decolorized dye + OH– (49)
The spectrophotometric method for quantitative determination of PS
by correlating dyes coloration/decolorization with PS concentration has
been widely used [32,37,40,42,65–67]. As early as 1963, Villegas et al.
[126] proposed a colorimetric determination of persulfate with alcian
blue. The optical density at 615 nm is linearly related to the persulfate
concentration in the range of 10–80 μg/mL. And this system is not
affected by bromate and iodate. Based on the improved iodometric
titration method, Liang et al. [37] proposed a rapid UV–Vis spectro­
photometric method for the determination of S2O28 in ISCO, which
reduced the reaction time to 15 min by iron activation. This method is on
the basis of the absorbance change of yellow iodine color formed from
the reaction between S2O28 and I , rather than the back-titration with
thiosulfate. The absorption spectra of iodine yellow color in the presence
of NaCO3 showed that the maximum absorption in the UV–Vis range is
288 nm and 352 nm. Since the spectrum is easily affected by the reagent
matrix at 288 nm, 352 nm is selected as the analysis wavelength. Iron as
an activator has no significant interference to the absorption measure­
ment at pH close to neutral. The LCR is 0–70 mM at 352 nm. The
sensitivity is 1.179 × 102 Abs M− 1. At room temperature, the reaction of
PDS and iodide can be accelerated and completed in 15 min in the
presence of excess iodine. Based on modification of traditional iodo­
metric titration, this method is simple and time-saving, but the process
needs pre-adjustment of pH to neutral and the coefficient of variation is
8%, as high as 7% for the traditional iodometric titration.
In addition to S2O28 detection, spectrophotometric determination of
HSO-5 based on modification of the iodometric titration method has also
been reported [127]. At 395 nm, the absorbance is linearly correlated
with PMS concentration ranging from 1.35 to 13.01 ppm (2.2 × 10-6 M
to 2.1 × 10-5 M), without any interference by pH in the range of
2.5–10.0. The limit of detection and the limit of quantification are 0.41
ppm (6.7 × 10-7 M) and 1.35 ppm (2.2 × 10-6 M), respectively. The RSD
is 1.1%, lower than the traditional iodometric titration. In addition, this
2–
method is not affected by Cl-, SO24 and CO3 . More importantly, iron and
cobalt commonly used as free radical activators have no effect on
absorbance.
Based on the reaction of PDS with aromatic amine, o-dianisidine
(ODA), a flow injection system for the determination of PDS by colori­
metric or electrochemical detection was developed [18]. Simple opti­
mization and automated response surface mapping were implemented
on the flow injection system. ODA can be oxidized by PDS to produce a
stable substance with a maximum absorbance at 450 nm. Cu(II) is the
recommended catalyst owing to its excellent catalytic activity and sta­
bility. The percentage by volume of acetone in water is recommended
between 20% and 40%, and the maximum response and sensitivity are
obtained at pH 6.5–7.5. As for organic substances interference, meth­
anol and ethanol show the least effect. The sensitivity is 2.4 × 103 Abs
M− 1, in the linear dynamic range from 2.5 × 10-5 M to 7.5 × 10-4 M. LOD
in this system and RSD of 10 repetitions are 5.0 × 10-7 M and 1.5%
respectively. In conclusion, the spectrophotometric method with o-dia­
nisidine as flow injection has many advantages, such as high precision,
low sample consumption rates.
9
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
Fig. 2. The chemical structures of five dyes.
Table 4
Experimental optimization condition for each dye-based system.
System
λmax
(nm)
Reaction
time
(min)
Initial pH
Fe2+ or Co2+
concentration
(μM)
Dye
concentration
(μM)
RhB/
Fe2+/
S2O28
MB/
Fe2+/
S2O28
MV/
2+
Fe /
S2O28
Orange
II/
Fe2+/
S2O28
MO/
2+
Fe /
S2O28
RhB/
Co2+/
HSO-5
MB/
Co2+/
HSO-5
MV/
Co2+/
HSO5AO7/
Co2+/
HSO-5
MO/
Co2+/
HSO-5
MB/
MW/
S2O28
556
2.0
2.5
1 × 102
20.0
663
2.0
2.5
1 × 102
20.0
590
7.0
2.5
1 × 102
24.0
485
7.0
2.5
1 × 102
40.0
507
2.5
2.5
2 × 102
25.0
556
2.0
5.0
1 × 103
19.2
663
2.0
5.0
1 × 103
29.6
590
2.0
5.0
1 × 103
62.4
475
2.0
5.0
1 × 103
113.6
464
1.0
4.0 ~ 8.0
2 × 103
77.0
644
1.0
Little
influence
——
31.3
Table 5
Comparison of LOD, LCR and sensitivity of different dyes system for PS
detection.
System
LOD
(μM)
LCR(μM)
Sensitivity (Abs
M− 1)
Reference
RhB/Fe2+/S2O28
MB/Fe2+/S2O28
MV/Fe2+/S2O28
Orange II/Fe2+/
2S2O8
MO/Fe2+/S2O28
RhB/Co2+/HSO-5
2+
MB/Co /HSO-5
MV/Co2+/HSO-5
AO7/Co2+/HSO-5
MO/Co2+/HSO-5
MB/MW/S2O28
0.62
0.42
0.50
8.10
2.0–150.0
2.0–100.0
4.0–150.0
20.0–150.0
7.57 × 10-3
9.24 × 10-3
6.72 × 10-3
4.45 × 10-3
[32]
[32]
[32]
[32]
0.17
0.04
0.10
0.08
0.08
0.09
2.80
0.5–100.0
0–40.0
0–100.0
0–80.0
0–80.0
0.2–100.0
0–1500.0
8.41 × 10-3
4.75 × 10-2
1.59 × 10-2
1.92 × 10-2
1.82 × 10-2
1.63 × 10-2
——
[40]
[41]
[41]
[41]
[41]
[67]
[66]
reaction time, initial pH and initial Fe2+ or Co2+ concentration for the
five dyes to approach the maximum absorbance (ΔA) are summarized in
Table 4. Under optimum conditions, the liner range, detection limit and
sensitivity are shown in Table 5. Compared with the traditional methods
and the ECL, the dyes/Fe2+ or Co2+ system is well effective to tolerate
the interference of typical transition metal ion and background con­
stituents. The dyes/Fe2+ or Co2+ system-based spectrophotometric
method was used to monitor the remediation process of practical
wastewater samples. The results showed that the deviation between the
proposed method and the iodometric method was<5% [46], demon­
strating that the dyes/Fe2+ or Co2+ system for the determination of PS is
feasible.
Although dyes/Fe2+ or Co2+ system achieved satisfactory results
when it was applied to determine PS concentration in practical waste­
water samples, the system requires addition of Fe2+ or Co2+ as an
activator and the pre-adjustment of pH to neutral, which makes the
system tedious and complicated. Instead of Fe2+ or Co2+ activation
[32,67], domestic microwave activation (MW) [66] can effectively
avoid this defect, and requires neither any additives nor any pretreat­
ment. The accuracy of the proposed method was better than the classic
iodometric spectrophotometry. In this system, RSD is 3.71% and the
recovery is 89.75%–103.04%. Under the optimized conditions (Table 4),
the PS concentration in the range 0–1.5 mmol/L showed a good liner
correlation with the decolorization extent, and the detection limit was
0.0028 mmol/L.
Here, the results of these dye-based systems have been compared
There have been many reports on the PS determination by Fe2+ or
Co or MW induced oxidative decolorization of dyes [32,40,67]. In
view of the decolorization of N = N bearing dye probes, a spectropho­
tometric method has been developed for the determination of PDS and
PMS by measuring the decolorization extent of dyes such as methyl vi­
olet (MV), methylene blue (MB), rhodamine B (RhB), orange II, methyl
orange (MO) via Fe2+ or Co2+ activation [32,40,41,67].
The chemical structures of dyes are shown in Fig. 2. The optimized
2+
10
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
(Table 5). The cobalt-ion-activated system has higher sensitivity and
lower detection limit than the iron-activated one, although the LCR and
activator dosage have no advantages. The MW-activated system has the
widest LCR and highest LOD. The azo dyes/Fe2+ or Co2+ or MW system
based on dyes decolorization extent not only has as good accuracy as the
classical iodometric, electrochemical and ECL methods, but also has the
advantages of saving time (only 1 min), simpleness (no back-titration),
convenience (using conventional analytical instruments) and sensitivity.
Nevertheless, these reactions require addition of high concentration
of KI, toxic transition metal ions, and persistent dye contaminants,
which may result in harmful wastewater contaminants after PS analysis.
Also, this method is susceptible to interference from other colored
substance. Thus, Gokulakrishnan et al. [65] and Zou et al. [42] devel­
oped a safe spectrophotometric method based on the direct oxidation of
the colorless intermediates such as N,N-diethyl-pphenylenediamine
(DPD) and 2,2′ -azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS)
by PDS/PMS to generate pink-colored DPD•+ or green-colored ABTS•+
(Eqs. 50–51). PS could be quantitatively determined at 510 nm and 551
nm in DPD system and at 415 nm, 650 nm, 732 nm and 820 nm in ABTS
system. This oxidative coloration method does not require addition of
catalyst peroxidase enzyme or transition metal ions and toxic dye indi­
cator. In addition, DPD, ABTS and their oxidized waste solution are nontoxic towards Escherichia coli (E. coli). At room temperature and neutral
pH, the absorbance of the generated DPD•+ and ABTS•+ increased lin­
early with increasing concentration of PS in the range of 10–100 μM and
3–500 μM, respectively. The detection limit is 10 μM in DPD system and
1 μM in ABTS system, respectively. Compared with other dyes systems,
the DPD and ABTS systems have multiple characteristic peaks in the
wide wavelength coverage, which can greatly reduce the interference of
other dye contaminants. This method can theoretically be used for PMS
and PDS detection, when a suitable dye is selected.
and has the advantages of high sensitivity, high selecitivity and high
specificity. In recent years, the determination of PS with a chemo­
dosimeter has been reported [68,69].
Based on the change of fluorescence from blue to green caused by the
reaction of anthracenethiosemicarbazone (ATSC) with PS, Badekar and
Kumbhar [68] developed an ATSC fluorescence “turn on” chemo­
dosimeter for the detection of PDS with a good linear correlation be­
tween fluorescence intensity and PS concentration at 485 nm. Through
the 1H NMR and single crystal X-ray structure experiments, it is eluci­
dated that ATSC as a molecular probe reacts with S2O28 in 90% DMSO
solution to produce anthraquinone with fluorescence characteristics
(Eq. 52). Selectivity studies have shown that ATSC have remarkable
selectivity for S2O28 in the presence of other competitive anions and
cations (i.e., Na+, K+, Mg2+, Ca2+, Ba2+, Al3+, Fe3+, Co2+, Ni2+, Cu2+,
- 2Zn2+, Ag+, Hg2+, Cd2+, Pb2+ and HSO-4, SO24 , SO3 , H2PO4, F , Cl , Br , I ,
–
–
2–
NO3, CO3 , ClO4, and CH3COO ). However, the co-existence of other
peroxides like hydrogen peroxide and tert-butyl peroxide, will drasti­
cally reduce the response of ACTS. This is possibly because the selfannihilation of radicals inhibits the oxidation of ATSC [128]. SO•−
4 is
−
formed by the reaction of S2O28 ions with OH in the 10% water, which
makes the reaction rate very low and 110 min are needed before
measuring the maximum response.
(52)
In addition to ACTS, other substances that can be oxidized to produce
fluorescence can also serve as indicators. Given that aromatic com­
pounds can be hydroxylated in sulfate radical system and the hydrox­
ylated products may possess a strong fluorescent property, Huang et al.
[69] established a fast and sensitive fluorescence method to determine
PMS in aqueous solution. This method used benzoic acid (BA) as a
chemical probe and Co2+ as a PMS activator to initiate BA hydroxylation
via activating PMS to generate SO•−
4 . Compared with the unactivated
PMS/ACTS system, Co2+ as homogeneous catalyst can greatly improve
the reaction rate. A series of emission spectra and excitation-emission
matrix spectra (EEMS), electron paramagnetic resonance (EPR),
ultrahigh-performance liquid chromatography (UHPLC) and gas
chromatography-mass spectrometry (GC–MS) analyses showed that
salicylic acid (SA) was identified as the fluorescent molecule. Consid­
ering the previous studies on hydroxylation of aromatic compounds
[129–132], the reaction mechanism of BA/PMS/Co2+ system was pro­
posed, as shown in Fig. 3. Firstly, Co2+ activates PMS to generate SO•−
4
(R1), which then attacks the aromatic rings of BA to form carboncentered free radical cation HOOCC6H•þ
5 (R2). The subsequent reac­
tion between HOOCC6H•þ
5 with water produces an OH-adduct radical
(50)
(51)
2.4.2. Fluorescence method
Although spectrophotometry has many advantages such as simple
operation and rapid reaction, it is susceptible to interference from other
colored substances in dyeing/decolorization reactions. Fluorescence
spectroscopy is a type of photochemical analysis. Using a beam of light,
usually ultraviolet light, excites the electrons in molecules of certain
compounds and make them emit light. In the special case of the mon­
omolecule fluorescence spectroscopy, intensity fluctuations from the
emitted light are measured from either single fluorophores, or pairs of
fluorophores. Therefore, it is not disturbed by other colored substance,
11
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
Fig. 3. Proposed radical chain reactions for the generation of SA in the BA/PMS/Co2+ system. Quoted from Huang et al. [69].
obvious advantages over other methods in previous studies, as shown in
Table 6. BA fluorescence method has better anti-interference perfor­
mance against common organic matters in actual wastewater, and the
Co2+ concentration of 50 μM is only one-fortieth [67] and one-twentieth
[41] of those used the previously reported spectrophotometry. Based on
the above advantages, the BA fluorescence method seems promising to
quantify PMS.
However, the BA/PMS/Co2+ fluorescence system also has many
shortcomings in practical application. High-content Fe3+, reaching the
level of more than 1 mg/L, will reduce the fluorescence intensity of BA/
PMS/Co2+ system, which may be attributed to the reduction in the
electron cloud density of the p-π conjugated structure of the SA mole­
cules, due to the coordination with Fe3+ [133]. Therefore, it is still
necessary to improve the performance of the BA/PMS/Co2+ system in
the treatment of samples containing high concentration of Fe3+.
Although BA fluorometry possesses an excellent specificity for PMS in
complex aqueous solution (i.e. PDS, H2O2), due to the higher reduction
•−
potential of the Co3+/Co2+ redox pair (1.92 V) than those of S2O28 /SO4
•
(1.39 V) and H2O2/ OH (0.80 V), how to simultaneously determine
PMS, PDS and H2O2 in complex aqueous also warrants further
investigations.
Table 6
Comparison of LOD and LCR between the BA fluorometry and other PS deter­
mination methods.
Method
LOD (μM)
LCR(μM)
Reference
Iodometric method
Polarographic method
Chemiluminescence
PB/GCE electrochemical method
MO/Co2+ spectrophotometry
AO7/Co2+ spectrophotometry
MB/Fe2+ spectrophotometry
IC column-switching HPLC
BA/Co2+ fluorescence method
Not available
2.60
0.05
90.00
0.09
0.08
0.42
9.84
0.01
730.00–3690.00
0.05–10.42
0.01–1000.00
100.00–1000.00
0.20–100.00
0–80.00
2.00–100.00
1.00–50000.00
0–100.00
[46]
[60]
[85]
[93]
[67]
[116]
[32]
[71]
[69]
product (R3), which can also be obtained by secondary R4 and R5,
where the •OH generated by the reaction of SO•−
4 with water directly
attacks BA aromatic ring. Finally, OH-adduct radical product is oxidized
by Co3+ to generate a SA, accompanied by the cycling of Co2+/Co3+.
The BA fluorescence method possessed a good linearity (R2 =
0.9998), a high precision (the narrow intervals at 95% confidence), a
good reproducibility, an excellent selectivity (i.e., PDS, H2O2) and
−
resistibility (i.e., K+, Ca2+, Mg2+, Fe3+, Cl− , SO2−
4 , NO3 ), a rapid reac­
tion equilibrium (<1 min), a good recovery (95%–105%), a low RSDs
(<3%), a wide liner concentration range (LCR = 0–100 μM) and a high
sensitivity (LOD = 10 nM, LOQ: limit of quantification = 33 nM). It is
noteworthy that the LOD and LCR of this method have considerable and
2.4.3. Resonant Rayleigh scattering method
Resonant Rayleigh scattering (RRS) is considered as a fast, sensitive,
selective, simple and inexpensive spectral analysis using single probe,
which has been widely used in the determination of inorganic ions,
Fig. 4. Illustration on the interaction between CuO-NGs and PS anions. Adapted from Qasem et al. [70].
12
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
Scheme 6. Separation mechanism of liquid chromatography.
biological macromolecules, pharmaceutical analysis and other sub­
stances in nanometer and non-nanometer reaction systems [134–142].
Based on the large surface area structure, high stability and high thermal
conductivity of copper oxide nanograins (CuO-NGs), Qasem et al. [70]
designed a new method for the determination of PDS using cetyl tri­
methylammonium bromide (CTAB) capped copper oxide nanograins as
a sensing probe. This is the first application of RRS technique in the
determination of PS. The sensing mechanism is based on the non-crosslinking aggregation of CuO-NGs, which results from the electrostatic
interaction between S2O28 and CuO-NGs (Fig. 4) [70]. In this case, the
RRS signal increases with the CuO-NGs size [143]. Therefore, it is
feasible to quantify PDS with the RRS signal using CuO-NGs as a sensing
probe.
According to Qasem et al. [70], the RRS intensity of CuO-NGs
nanosensor was linearly correlated with PDS concentrations in the
range of 1 to 100 μM at λ = 376 nm. And the LOD was equal to 1.03 μM,
which was higher than other spectral methods. The synthesized CuONGs sensing probe was highly selective towards to S2O28 , when a vari­
3–
2–
ety of ions such SO24 , PO4 , NO3 and CO3 co-existed. In addition, the
recovery percentages of a real sample proved to be between 98.4% and
100.3%. The RRS method is as reliable, sensitive and selective as the
aforementioned spectrophotometric method for the determination of
PS. However, the RRS technique requires that the molecule should have
scattering properties. Therefore, the RRS method is suitable for the
detection of high concentration and scattering properties substances.
reagents in the reaction, which causes a problem of applicability. Then
Weidenauer et al. [75] built up a microbore system for separation of
sulphur-anions (specifically PDS) by optimizing the conventional liquid
chromatography unit. This is the pioneering work on the determination
of PS by IC. Later, IC for the determination of PS was formally proposed
by Khan and Adewuyi. [73]. This method can simultaneously determine
PDS and other common inorganic ions by gradient elution. Firstly,
elution with low concentration (<20 mM KOH-internal eluent genera­
tion) was used to remove common inorganic ions, then high concen­
tration of elution (180 mM NaOH-external eluent and 18 mM KOHinternal eluent) for PS. IC is simple and fast, because it can simulta­
neously determine several common ions in 25 min. Also it has higher
accuracy at low concentration of PS compared to spectrophotometry
[37]. The retention time and the area under the peak have little effect on
ions quantification (RSD of the retention time and area is<0.2% and 5%,
respectively). In addition, the range of analysis and the detection limit
are 50–1900 ppm and 0.2 ppm, respectively. However, high concen­
tration of eluent may cause high back conductivity, high limit of
detection and chromatographic separation column contamination.
In order to optimize the IC method proposed by Khan and Adewuyi
[73], Huang et al. [72] proposed a simple ion chromatography with
column-switching. The column-switching is an effective sample pre­
treatment technique and has been widely used in complex samples
analysis [149–151]. In this system, the sample is first injected into the
IonPac AG 11 guard column through the control of six-port value and
ten-port value. Under the solution rinsing, the weakly retention ions can
be quickly separated by passing through both IonPac AG 11 guard col­
umn and IonPac AS 11 separation columns, but PDS was retained as a
strong retention ion and washed down slowly. Then switch ten-port
value to the load position and increase the eluent concentration so
that PDS can be quickly eluted into the detector by only passing through
the Ion Pac AG 11 guard column. This process not only reduces the back
conductivity and protects the chromatographic column and detector,
but also shortens the analysis time to 18 min in a single run. The test
results are in good agreement with the spectrophotometric method
described by Liang et al. [37]. Therefore, this method has lower detec­
tion limits due to the low back conductivity from low concentration
elution. The stable baseline makes it more suitable for determination of
low concentration of PS. However, its analysis time is longer than
spectrophotometry.
In general, IC and its improvement for the PS detection have supe­
riority in terms of material consumption, sample volume, accuracy,
specificity and simplicity, but the analysis time is longer (10–30 min for
a single run) and the LOD is higher compared with spectrophotometry
2.5. High performance liquid chromatography
Liquid chromatography (LC) is a method of analyzing samples with
liquid as a mobile phase. The separation mechanism is based on the
difference in the affinity of each component of the sample to the two
phases. The separation mechanism of LC is described in scheme 6. Ion
chromatography (IC), a kind of LC, measures the concentration of ionic
species through separating them based on their ionicity. This technique
can quickly detect 7 common anions and 6 common cations. It has been
often used for ions quantification since 1980 s [144–148] because of
high sensitivity, good selectivity and fast detection. According to the
separation ways of ion species, it can be divided into three types: ion
exchange chromatography, ion exclusion chromatography and ion pair
chromatography.
Early in 1988, Weidenauer et al. [74] established ion-pair chroma­
tography to quantify inorganic sulphur-anions (such as PS ions). How­
ever, when measuring PS and other common ions at the same time, this
method is not only inefficient, but also involves multiple ion pairs
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Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
Table 7
The comparison results between IC and IC-column switching method for S2O28 detection.
System
LCR (mM)
LOD (S/N = 3)(μM)
Recovery (%)
Analysis time(min)
IC
IC column-switching
Modified HPLC
0.078–2.920
0.010–50.000
0.075–300.000
0.31
9.84
6.60
95.0–120.0
97.6–103.4
——
25
18
6
RSD (n = 6) (%)
Retention
Area
<0.20
0.04
——
0.5%–5%
0.9
——
Table 8
Comparison of different PS detection methods.
Method
Sensitivity
LOD(μM)
LCR(mM)
Unaffected by
Reference
Traditional method
ECL
Electrochemical method
UV–vis spectrophotometry
Fluorometry
RRS
IC
Not available
Not available
0.22μA μM− 1 cm− 2
1.92 × 10-2(Abs M− 1)
Not available
Not available
Not available
Not available
0.5
0.48
0.08
0.01
1.03
0.31
0.73–3.69
4.00 × 10-3–2 × 10-1
1.5 × 10-3–1.32 × 10-1
0–8 × 10-2
0–1 × 10-1
1 × 10-3–1 × 10-1
7.8 × 10-2–2.92
Ethyl alcohol
2–
–
2–
Mg2+, Na+, Zn+, Ca2+, Ba2+, Cl-, NH+
4 , SO4 , NO3, HCO3, CO3 ,
+
2+
2+
2+
Na+, SO23 , K , Cl , Ni , Mn , Co
–
2+
3+
2+
+
+
Mn2+, SO24 , Fe , Fe , Cl , Cu , Na , K , NO3, BrO3
K+, Ca2+, Mg2+, Fe3+, Cl− , SO42− , NO3−
3–
2–
SO24 , PO4 , NO3, CO3
All ions
[46]
[86]
[120]
[41]
[69]
[70]
[73]
[32,66,67]. Based on multi-system combination [152], Baalbaki et al.
[71] developed flow injection/spectroscopy via high performance liquid
chromatography (HPLC) coupled with bypass capillary columns and a
diode array detector (DAD), which is an improved spectrophotometry
proposed by Liang et al. [37]. The special HPLC configuration was
equipped with a vacuum degasser, a quaternary pump, a thermo­
electrically cooled autosampler maintained at 4 ◦ C and three capillary
columns connected in series. Concentrated potassium iodide (KI) solu­
tion as mobile phase to reduce S2O28 . The reaction was carried out in a
capillary column, which promotes the production of I2 suspension under
moderate pressure, and finally forms I-3 complex anion in the presence of
excess I-. I-3 absorbs at 352 nm, which minimizes interference from other
organic contaminants. This method combines spectral and chromato­
graphic techniques. Consequently, it has a series of advantages: high
automation, high accuracy, fast spectral analysis and low reagent and
sample consumption. Under optimized conditions, the analysis time can
be controlled within 6 min, and LOD and LOQ were 6.6 × 10-6 M and 2.2
× 10-5 M, respectively. In addition, the method can effectively tolerate a
wide range of pH, salinity and organic pollutants.
A comparison of ion chromatography [73], ion chromatographic
with column-switching method [72] and the modified HPLC configu­
ration [71] is given in Table 7. Except for the LOD, the LCR, recovery
and analysis time of the IC-column switching technique are superior to
that of IC method. The analysis time of the modified HPLC is superior to
the forementioned IC method. On the whole, chromatographic method
including IC and HPLC, is one of the ideal methods for the determination
of PS, because of its accuracy, reliability, time saving, wide pH range,
wide LCR, low LOD, high sensitivity, simple operation, automatic
analysis, no interference from matrix organic contaminants and salt
ions.
the laboratory, the only method that can be used is titration, which is
cheap and easy to operate by non-professionals. However, this method
has poor reproducibility and high detection limit. Neither iodometric
titration nor its improvement method does fundamentally overcome the
shortcoming of the complex operation. Polarography is an electro­
chemical method based on the combination of AC polarography and
titration. Although this method is more sensitive than titration, and the
amplification reaction can detect 2.6 μM of PDS, it is still suitable for
macro-determination since the high LOD limits its application.
ECL is highly sensitive with low detection limit (0.05 μM) and short
detection time. However, this method requires rather specialized in­
struments, which are not common in ordinary laboratories. The elec­
trochemical methods have the advantages of fast analysis, high
sensitivity, wide detection range and pH window, However, relatively
large reagent volume of each tested sample is needed, and the dye
modified electrode is not environmentally friendly. The electrochemical
method is recommended for the sample with large reagent volume and
low pH requirement. The UV–vis spectrophotometry has the advantages
of simple operation, high accuracy, good reproducibility, low con­
sumption rate of samples and reagents, but it is susceptible to interfer­
ence from other colored substances in dyeing/decolorization reactions.
Compared with UV–vis spectrophotometry, fluorescence method has
higher sensitivity and is less susceptible to the interference of colored
substances. BA fluorometry possesses an excellent specificity for PMS in
complex aqueous solution (i.e., PDS and H2O2), but the equilibrium time
is long and the reaction rate is low. Therefore, fluorescence method is
suitable for PMS measurement where PMS, PDS and H2O2 coexist, as
well as the measurement of colored samples, and UV–vis spectropho­
tometry can be chosen for colorless samples.
The RRS method features with speed, simplicity, high sensitivity,
good selectivity and low cost, but the detection limit is equal to 1.03 μM,
higher than other spectral methods. The IC has the advantages of high
separation efficiency, less reagent consumption, high sensitivity, auto­
matic operation, wide application range and simultaneous detection of a
variety of ions. However, compared with other detection methods, IC
has no advantages in the LOD and detection time. In conclusion, RRS
and IC have advantages in the determination of PS when multiple ions
co-exist. For simultaneous determination of multiple ions in samples, IC
is more appropriate.
3. Conclusion and future perspectives
3.1. Conclusion
This review covers various analytical techniques most commonly
used for the determination of PS in aqueous solutions and evaluates the
merits and limitation of each technique. The PS detection methods are
summarized in Table 8. In general, except the traditional methods
(titration and polarography), the other methods can achieve relatively
accurate and rapid detection of persulfate.
The choice of a method for PS detection depends on the accuracy
needed, measurement frequency, the nature of the tested matrices, the
available laboratory equipments, as well as application scenarios (online, in situ, ex situ). In case while no specific equipments are available in
3.2. Future perspectives
In comparison to the burgeoning developments and radicals identi­
fication/quantification in SR-AOPs [153], establishing new detection
methods for PS lags considerably behind and is even often neglected.
14
Y. Liu et al.
Chemical Engineering Journal 416 (2021) 129143
The development direction for PS detection is clear, that is, to meet the
criteria of being ASSURED (i.e. affordable, sensitive, specific, userfriendly, rapid and robust, equipment-free, and deliverable to endusers). Firstly, new materials that can directly improve the sensor per­
formance (high sensitivity, low loss and long life) will continue to be
explored. For example, improving the diffusion coefficient of the
permeable film may considerably enhance measurement accuracy and
shorten response time in the electrochemical sensors. It is still one of the
key issues in ECL studies to develop innovative, efficient and costeffective ECL luminophores since to date, most nanoemitters have
inferior ECL efficiency to the classic Ru(bpy)2+
3 . In addition, new envi­
ronmentally friendly materials/reagents should be developed to avoid
the generation of toxic wastes after analysis
Secondly, much work is needed for successful application of these
detection methods to real-world samples. In reality, water samples may
contain multiple ions and organics at varied concentrations. The design
of novel strategies to sensitively quantify PS in a complex environment
remains a substantial challenge. LC and its modified system have the
advantages of fast analysis speed and simultaneous detection of multiple
ions but at present often have insufficiently low detection limits.
Selecting appropriate eluent ratio, improving sample purity and
reducing background signals may be important ways to optimize ion
chromatography. In addition, most works mainly focus on the deter­
mination of PDS, while the study of PMS detection is still in its infancy.
When PMS, PDS and H2O2 coexist in a complex matrix, how to simul­
taneously determine them remains unresolved. More importantly, cur­
rent methods are difficult to realize in situ continuous online
measurements.
In recognition of such challenges, it is expected that more accurate,
sensitive and reliable PS measurement sensors will be constructed,
miniaturized and commercialized to make the laboratory models
portable devices. Intelligent PS sensor technology will be the focus of
future research and next-generation development. The novel intelligent
sensor is expected to perform real-time dynamic compensation and
correction for salinity, temperature and other influencing factors,
thereby significantly reducing the staff workload and manual operation
errors.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by National Key Research Development
Program of China (2019YFC0408304), the National Natural Science
Foundation of China (Nos. 21677031, 41977313) and the Science &
Technology Innovation Action Plan of Shanghai under the Belt and Road
Initiative (No. 19230742800).
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