Effects of Selenium Species on Belousov-Zhabotinsky Reaction | The

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Effects of Selenium Species on Belousov-Zhabotinsky Reaction Cuifang Xu, Xueqi Ye, Zuandi Luo, Yayun Shi, Chuang Gao, and Yan Bai J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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The Journal of Physical Chemistry

Effects of Selenium Species on Belousov-Zhabotinsky Reaction Cuifang Xu, Xueqi Ye, Zuandi Luo, Yayun Shi, Chuang Gao, Yan Bai



(Chemistry Department, Jinan University, Guangzhou 510632, P.R. China)

Abstract: The effects of selenium species on Belousov-Zhabotinsky (B-Z) reaction were investigated by adding them to the system before and during the oscillation. When selenium species were added into the system before the oscillation, sodium selenite prolonged the induction period, whose effect was strong as sodium selenite could consume malonic acid to prohibit the accumulation of bromomalonic acid. For selenomethionine and selenocystine, their effects were derived from their reaction with ·CH2COOH and ·Br2- producing radical cation of selenoamino acids, which prohibited the accumulation of bromomalonic acid. Here, the selenium atoms in selenoamino acids, as reactive centers, took part in the redox reaction. As a result, the induction period was prolonged. However, as a diselenide, selenocystine can reduce bromate in acidic medium, which led to shortening the induction period. Therefore, the effect of selenocystine on the induction period was the result of two opposite effects. Nanoselenium shortened the induction period in a certain concentration range, because bromate was directly reduced by nanoselenium and the accumulation of bromomalonic acid was promoted. Furthermore, the dose perturbation effect was investigated by the injection of nanoselenium during the oscillation. It was found that the amplitude was increased or decreased in a dose-dependent fashion, when nanoselenium was injected at peak or trough of the time-dependent redox potential curve.

1. INTRODUCTION Chemical oscillatory reaction refers to the fact that the concentration of some species, such as intermediates and catalyst, in the chemical reaction varies 

Corresponding author. E-mail address: [email protected]. 1

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periodically with the reaction time under certain conditions.1 The chemical oscillation is a branch of chemical kinetics and also a part of nonlinear dynamics and synergetics, which demonstrates all possible types of dynamical behavior in time and space.2 In many different families of chemical oscillators, the Belousov-Zhabotinsky (B-Z) reaction has been paid close attention by many researchers because of a rich dynamical behavior in chemical systems. The B-Z reaction involves simultaneous reactions of bromination and oxidation of organic substrate by transition metal ions as the catalyst in the strong sulfuric acid medium. In recent years, a lot of effort has been spent on exploring a qualitative understanding of complex behavior in B-Z reaction and some progresses have also been achieved. In the aspects of the chemical composition and kinetics, on the basis of classical FKN theory (elucidated by Field, Kotos and Noyes)3, detailed reaction mechanisms including reactions of organic radicals, the role of intermediates and kinetic models are proposed. Here are some examples. Zoltan Noszticzius’s group investigated the reaction of organic radicals with the inorganic bromine in the course of the autocatalytic periods and suggested new Marburg-Budapest-Missoula (MBM) model.4 Horst-Dieter Forsterling’s group revealed that the bromination reactions was considered to occur via the enol forms of the organic acids and in all cases the bromination by HBrO was slower than the bromination by Br2.5 Maria Wittmann’s group proposed that various radicals (·COOH, ·BrO2, ·BrO, ·Br and ·Br2-) appeared in the presence of oxidation state catalyst, causing a fundamental change in the mechanism.6 Besides, biochemical oscillations have been a hot area of research for decades, since the glycolytic oscillations in yeast were reported by Chance et al.7 Other biochemical oscillations were also found, such as rapid oscillations of membrane potential in nerve cells, slow cycles of ovulation in mammals and periodic growth and division of well-nourished cells.8 In recently, the effects of some small biological molecules, such as glucose,9 troxerutin,10 cysteine,11 and procyanidins12 and so on, are also one of the research hotspots, which also involves the determination based on B-Z oscillating reaction. Selenium has abundant species including inorganic and organic selenium compounds as well as nanoselenium. More importantly, these species have 2


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physiology and pharmacology activities and exhibit various chemical activities. Some simple species are as follows. Firstly, sodium selenite is viewed as the reagent for cancer prevention and treatment, which help to repair the damage of DNA fragments and stimulate the cellular immune system.13 Secondly, selenoamino acids such as selenomethionine and selenocystine play an important role in living organisms. Selenomethionine has a strong antioxidant capacity and improves immunity.14 Selenocystine is present in a few enzymes, such as glutathione peroxidase and thioredoxin reductase, and plays an important role in keeping body’s oxidative balance.15 Additionally, nanoSe0 has some biological activities in antioxidant and anti-tumor in vitro.16 Complex feedback loops are common characteristics of biochemical networks, whereby the rates of some reactions are affected by the products of one reaction. The B-Z reaction is very similar to some complex behaviors in nature, especially the behavior of the biological system, whose positive-feedback loop is created by a pair of antagonistic species.8 If the composition, especially antagonistic species, were influenced by different species of selenium, the B-Z reaction will be perturbed. Herein, the effects of sodium selenite, selenomethionine, selenocystine and nanoselenium on the B-Z reaction were investigated to obtain some valuable information about the non-linear phenomena.

2 EXPERIMENTAL SECTION 2.1 Materials Malonic acid (CH2(COOH)2), potassium bromate, manganese sulfate, sodium selenite, sulfuric acid, selenium powder (gray), graphite powder and sodium sulfate were analytical grade chemicals. Selenocystine (SeCys, purity > 99%) and selenomethionine

(SeMet,

purity

> 99%)

were

purchased

from

Dingguo

Biotechnology Co. Ltd. (Beijing, China). Nanoselenium colloidal solution (nanoSe0) was made in our laboratory. According to our previous work,17 nanoSe0 was prepared by constant-potential electrolysis using a selenium doped carbon paste as working electrode, a silver 3



chloride electrode as reference electrode and a platinum electrode as counter electrode. The electrolysis was performed at -1.2 V for 20 minutes under magnetic stirring in 10 mL 0.01 mol/L K2SO4. The Zeta potential of the colloidal solution was -3.48±0.9 mV and particles were spherical shape with average particle size of 106±2 nm. NanoSe0 could be stable for several hours, which could ensure that the B-Z reaction carries out in homogenous solution. All the reagents were prepared using ultrapure water except that SeCys stock solution was prepared with 0.01 mmol/L H2SO4. 2.2 Methods The chemical reaction was carried out according to the classical B-Z reaction:3 Thoroughly mixed the solutions of KBrO3, CH2(COOH)2, H2SO4 for 80 s, then added the solution of MnSO4. The final concentrations of KBrO3, CH2(COOH)2, H2SO4 and MnSO4 were 0.083 mol/L, 0.167 mol/L, 1 mol/L and 0.0186 mol/L, respectively. The reaction was performed under magnetic stirring at medium speed. The perturbation effect of selenium species on the B-Z reaction by the additives added before the oscillation: In the same experimental conditions of the B-Z reaction, equal volume (0.50 mL) of Na2SeO3, nanoSe0, SeCys and SeMet solutions were added to the KBrO3-CH2(COOH)2-H2SO4 system, respectively and the mixture was stirred homogeneously for 80 s, then added the solution of MnSO4. The dose perturbation effect of nanoSe0 on the B-Z reaction by the additive injected during the oscillation: After the fourth oscillation cycle, its oscillation was perturbed by injecting 0.5 mL of nanoSe0 with five different doses at peak or trough of the time-dependent redox potential curve. The experiment of the B-Z reaction was carried out at 28 ± 2 °C in a water bath and all solutions including additives were kept under 28 °C for 10 minutes before the B-Z reaction. The B-Z reaction was monitored using electrochemical workstation (DY2312 Bipotentiostat) with a three-compartment electrolytic cell. A glass carbon electrode was used as working electrode, a silver chloride electrode was the reference electrode and a platinum electrode was the counter electrode. The time-dependent redox 4


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potential curve of the reaction was recorded with sampling every 1 s. 2.3 Statistical analysis All the experiments were carried out at least in quintuplicate to ensure the reproducibility of the results. Average values and standard deviations from these independent experiments were calculated. Further, an ordinary one-way ANOVA was utilized to detect statistically no significant difference. In addition, the t test was performed to assess statistical significance in the average values of the induction period, period and amplitude of the B-Z reaction between with and without selenium species. The difference was considered significant if the confidence level of was more than 95 %.

3 RESULTS AND DISCUSSION 3.1 Addition of selenium species before the oscillation The essence of B-Z reaction is a kind of chemical oscillatory reaction of KBrO3 with CH2(COOH)2 catalyzed by transition metal ions such as Mn(II) ions in acidic medium. From the standard electrode potential of each component, KBrO3 can oxidize CH2(COOH)2 under acidic condition. But if there was no Mn(II) ions, reaction of KBrO3 with CH2(COOH)2 occurred slowly and oscillatory reaction of KBrO3 with CH2(COOH)2 even couldn’t occur. Na2SeO3 has strong oxidizability and its effect on B-Z reaction is shown in Figure 1A and Table 1. The induction period was prolonged by 21−173 s in the presence of 0.16−16 mmol/L Na2SeO3, especially at high concentrations. And the amplitude was decreased by 21−24 mV (Table 1). Further, when Na2SeO3 (0.1 mol/L) was added to 0.17 mol/L CH2(COOH)2-1 mol/L H2SO4 system, the red element selenium was generated, confirming that Na2SeO3 was reduced by CH2(COOH)2 in acidic conditions. While the same amount of Na2SeO3 was added to the 0.08 mol/L KBrO3-0.17 mol/L CH2(COOH)2-1 mol/L H2SO4 system, no red element selenium was observed, but the solution turned into yellow with a pungent smell, because KBrO3 could immediately oxidize element selenium and produce Br2. In short, Na2SeO3 directly or indirectly interacted with the components in the B-Z reaction, 5



resulting in changes in chemical oscillatory rhythms and parameters. Amino acids with methylene group could be regarded as an organic substrate 18,19 or an additive for the B-Z reaction. In addition, selenoamino acids contain selenium atoms which have high reactivity in acidic medium. Thus, selenoamino acids should affect oscillating reaction. In the presence of low concentrations of SeMet (0.016−0.16 mmol/L), the B-Z reaction wasn’t changed obviously. And in the presence of high concentrations of SeMet (0.8−1.6 mmol/L), the induction period was prolonged by 67−92 s and the amplitude was decreased by 14−17 mV (Figure 1B and Table 1). However, in the presence of SeCys, the B-Z reaction wasn’t changed obviously (Figure 1C and Table 1). Additionally, when SeMet and SeCys was added to the KBrO3-CH2(COOH)2-H2SO4 system, respectively, no change was observed with the naked eye, indicating that their interaction with KBrO3 or CH2(COOH)2 was weaker than that of Na2SeO3 or their paths of perturbation might be different from that of Na2SeO3. The elemental selenium with an intermediate oxidation state has a strong reactivity, especially nanoSe 0 . It should also interact with the components of the B-Z reaction. Herein, we used nanoSe 0 without surfactant to obtain the information about the effects of nanoSe 0 on the B-Z system, rather than that of surfactant. Unlike Na2SeO3 and selenoamino acids, the induction period was shortened by 20−55 s and the amplitude wasn’t changed in the presence of nanoSe 0 (Figure 1D and Table 1).

6


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A

B

60 mV 16 mmol/L Na2SeO3

0.8 mmol / L SeMet

1.6 mmol/L Na2SeO3

0.16 mmol / L SeMet

0.16 mmol/L Na2SeO3

0.016 mmol / L SeMet

no additive

no additive

850

0

60 mV 1.6 mmol / L SeMet

300

Time/s

900

600

C

850

900 0

300

D

60 mV

900

600

Time/s

60 mV

0

3.02 umol/L nanoSe

1.6 mmol / L SeCys

900

0

0.925 umol/L nanoSe

0.16 mmol / L SeCys

0

0.310 umol/L nanoSe

0.016 mmol / L SeCys

no additive

no additive

850

0

300

Time/s

900

600

900 0

850

300

Time/s

900

600

900

Figure 1. Time-dependent redox potential curves for the B-Z reaction in the absence and the presence of Na2SeO3 (A), SeMet (B), SeCys (C) and nanoSe0 (D). Scale bar: 60 mV

7



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Table 1 Effects of the selenium species on induction period and amplitude of the B-Z reaction Induction period

Amplitude

Final concentration of Averagea Differenceb Averagea Differenceb additive (s) (s) (mV) (mV) no additive 0.16 mmol/L Na2SeO3

540±2.6 561±3.8

/ 21

59±5.8 36±1.9

/ -23

1.6 mmol/L Na2SeO3 16 mmol/L Na2SeO3

610±6.1 713±5.8

70 173

38±2.0 35±1.2

-21 -24

no additive 0.016mmol/L SeMet 0.16 mmol/L SeMet 0.8 mmol/L SeMet 1.6 mmol/L SeMet

539±6.2 537±12 536±12 606±3.1 631±5.6

/ no no 67 92

51±1.6 45±2.0 44±1.6 37±1.7 34±1.2

/ -6 -7 -14 -17

no additive 0.016 mmol/L SeCys 0.16 mmol/L SeCys 1.6 mmol/L SeCys

543±12 517±14 526±7.1 535±12

/ no no no

59±3.6 53±3.3 55±2.6 52±1.7

/ no no -7

no additive 533±8.9 / 59±1.7 / 0.310 μmol/L nanoSe0 480±14 -53 56±3.0 no 0 0.925 μmol/L nanoSe 478±19 -55 59±3.5 no 0 3.02 μmol/L nanoSe 513±6.0 -20 57±1.4 no a The average ±standard deviation of five experiments. b “Difference” is the difference between the two results with and without selenium species and the difference with confidence level 95% (significance level 5%) was considered statistically significant. no: there is no significant difference.

3.2 Mechanism on the effects of selenium species on the B-Z reaction The manganese-catalyzed the B-Z reaction is inherently connected to the chemistry of bromine.20 The reaction consists of two main parts: Positive feedback process is that bromate oxidizes Mn (II) ions to produce autocatalytic bromic acid and bromine dioxide under acidic conditions and negative feedback process is that Mn (III) ions react with bromomalonic acid to release Br- ions. Then Br- ions react with bromic acid rapidly, which is an autocatalytic intermediate. In this way, Br- ions control the switch between positive and negative feedback process. According to references,1,6 8


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this B-Z reaction might involve the following reaction: 2BrO3  2H   2HBrO 2  O 2

k forword  6.0  10 10

BrO3  HBrO 2  H   Br2 O 4  H 2 O

k forword  48

Br2 O 4  BrO 2

k forword  7.5  10 4



k reverse  1.3  10 4

(R1)

k reverse  3.2  103

(R2)

k reverse  1.4  109

(R3)

Br   BrO3  2H   HBrO 2  HOBr

(R4)

Br   HBrO 2  H   2HOBr

(R5)

Br   HOBr  H   Br2  H 2 O

(R6)

CH 2 (COOH ) 2  CH 2 (COOH ) 2 (enol)

k forword  3.0  10

Br2  CH 2 (COOH ) 2 (enol)  BrCH(COOH ) 2  Br  H 

3



k reverse  200

(R7)

k  1.5  10

(R8)

8

HBrO 2  BrO3  H   BrO 2  H 2 O

(R9)





BrO 2  Mn(II)  H   Mn(III)  HBrO 2 k forword  6.4  10 4

k reverse  1.3  10 4

(R10)

HBrO 2  HBrO 2  HOBr  BrO 3

(R11)

Mn(III)  BrCH(COOH) 2  HOBr  3H 2 O  Mn(II)  2Br   4HOCH(COOH ) 2  6H 

(R12)

Mn(III)  CH 2 (COOH) 2  CO 2  Mn(II)  H   CH 2 COOH

(R13)



k  0.1



CH 2 COOH  Br2  CO 2  Br  BrCH 3

(R14)



Br  CH 2 (COOH) 2  CO 2  H   Br   CH 2 COOH

(R15)



Br  Br   Br2

(R16)



CH 2 COOH  Br2  CO 2  Br2  CH 2 Br2  H 



CH 2 COOH  Br2  BrH 2 CCOOH  Br 







(R17)



k  7  109



(R18)

The induction period is considered as the time required for the accumulation of sufficient bromomalonic acids and the induction period gets shorter as bromomalonic acids increases. Bromomalonic acid is mainly formed by the reaction of Br2 with an enolized CH2(COOH)2 (enol) (R7, R8). When

Na2SeO3

was

added

into

KBrO3-CH2(COOH)2-H2SO4

system,

CH2(COOH)2 was consumed by Na2SeO3 rapidly, which inhibited the accumulation of bromomalonic acid (R8), resulting in an increase in the induction period. Herein, the elemental selenium reduced from Na2SeO3 didn't perturb further the induction period, because elemental selenium was produced at a later time and missed the key point of perturbing the induction period. The effect of the selenoamino acids might be related to the selenium atoms attacked by free radicals, such as ·Br2-, ·CH2COOH, ·BrO2, produced during the 9



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reactions R3, R9 and R13 to R17. For example, the SeCys or SeMet could react with ·Br2- radicals as follows 21:

HOOC H3C

H3C

Se

Br2-

Se

Se

H+ HOOC

NH3+

Se

Br22

H+

the reaction of SeMet with ·Br2-

CH3 HOOC

HOOC NH3+

NH3+

NH3+

HOOC NH3

COOH +

Se

Se

NH3+

the reaction of SeCys with ·Br2-

According to the above reactions, SeMet reacted with free radicals to produce an unstable selenium-centered radical cation which could coordinate with one selenium atom of another SeMet molecule to form a relatively stable dimer.21 Since the free radicals’ attack on SeMet prohibited the accumulation of bromomalonic acid (R18), the induction period was significantly prolonged. SeCys also reacted with free radicals to produce a stable selenium-centered radical cation which was the radical cation intra-molecularly through the p-orbitals of the two selenium atoms.21 The reaction tended to prolong the induction period, but, unlike SeMet, the addition of SeCys did not result in a prolongation of the induction. Because the Se-Se bond in SeCys was possibly converted to organic selenite (R-SeO2H) and organic selenate (R-SeO3H) in the presence of oxidant KBrO3, which tended to shorten the induction period. Although R-SeO2H and R-SeO3H could consume CH2(COOH)2 to prolong the induction period as Na2SeO3, this effect could be neglected because the reactivity of R-SeO2H and R-SeO3H was not as high as that of Na2SeO3 and Na2SeO4. From the discussion above, there were two opposite effects on the induction period in the presence of SeCys. When nanoSe0 was added before oscillation, KBrO3 was reduced into Br2 by nanoSe0, which promoted the accumulation of bromomalonic acid and shortened the induction period (R8). 10


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3.3 Dose perturbation effect of nanoSe0 injected during the oscillation We have investigated the perturbation effects of four kinds of selenium species on the B-Z reaction by adding them before the oscillation and results showed that significant changes have taken place in the induction period. In this section, taking nanoSe0 as an example, we study the perturbation effect of the additive on the amplitude of the B-Z reaction by injection of the additive during the oscillating. As shown in Figure 2 curve 4 and Table 2, when nanoSe0 was injected at the trough of the oscillating curve, the amplitude was decreased and when nanoSe0 was injected again at the next point, the amplitude was decreased continuely. Moreover, nanoSe0 was injected successively five times and the amplitude was decreased linearly (ΔE=1.34+1.6x, ΔE: amount of change in amplitude, x: cumulative concentration injected into nanoSe0, R2=0.999), indicating that the amplitude was decreased in a dose-dependent fashion by nanoSe0 (Inset I in Figure 2). However, when nanoSe0 was injected at the peak of the oscillating curve, the single spike amplitude was increased but returned back the initial states after a period (Figure 2 curve 5 and Table 2). And the amplitude was increased quadratically with increasing concentration of nanoSe0 injected (ΔE=36.1-46x+26x2, ΔE: amount of change in amplitude, x: cumulative concentration injected into nanoSe0, R2=0.997) (Inset II in Figure 2). Furthermore, the electrolyte solution (0.01 mol/L K2SO4) was injected at the trough and peak of the oscillating curve, separately and the amplitude didn’t show obvious change (Figure 2 curve 2 and 3), indicating the effects on the B-Z reaction from nanoSe0 not the electrolyte.

11



60 mV

a

b

160

c

140

d

e  E/ mV

5

II

120 100 80

2

E=36.1-46x+26x

R2=0.997

60 40

a

20

e

b

c

d

0

e

4 3

60

2

b 1.0

c

d

e

1.5

2.0 2.5 3.0 3.5 Concentration of nanoSe0 injected at peak /umol/L

I

40

E=1.34+16x R2=0.999

30 20

200 uL K2SO4 injected 0 200 uL nanoSe injected

1 0

a 0.5

50

 E/ mV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200

400

600

800

10

a 0.5

b 1.0

c

d

e

1.5

2.0 2.5 3.0 3.5 Concentration of nanoSe0 injected at trough /umol/L

1000

Time / s Figure 2. Time-dependent redox potential curves for the B-Z reaction under the condition of additives injected during the oscillating at the trough (Curve 2 and 4) and peak (Curve 3 and 5). Inset: The amplitude (E) of B-Z reaction in designated concentration of nanoSe0 injected during the oscillating at the trough (I) and peak (II) of the curve. Scale bar: 60 mV. No additive (Curve 1); Additives: K2SO4 (Curve 2 and 3) and nanoSe0 (Curve 4 and 5). Table 2 Effects of nanoSe0 injected during the oscillating on amplitude of the B-Z reaction Amplitude Final Averagea Differenceb Averagea Differenceb concentration of 0 (mV) (mV) (mV) (mV) nanoSe (μmol/L) nanoSe0 injected at trough nanoSe0 injected at peak No additive 0.62 (a) 1.22 (b) 1.83 (c) 2.43 (d) 3.01 (e)

62±1.0 51±1.7 39±3.1 30±1.9 21±1.2 15±1.0

-11 -23 -32 -41 -52

60±1.0 76±1.8 81±1.1 89±6.9 138±9.9 205±8.9

16 21 39 78 145

a

the average ±standard deviation of five experiments. “Difference” is the difference between the amplitude value /the period value with and without additives and the difference with confidence level 95% (significance level 5%) was considered statistically significant. b

NanoSe0 showed different effects on the B-Z reaction when nanoSe0 was injected at the trough and peak of oscillating wave. At the trough, the dominant reaction transfers to positive feedback process, which involves the reaction between 12


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KBrO3 and Mn (II) ions (R9, R10). Since nanoSe0 injected at trough could consume KBrO3 to prohibit positive feedback process (R9, R10), resulting in the amplitude decreasing. And the amplitude was decreased with increasing concentration of nanoSe0 due to consumption of initial reactant KBrO3. At the peak, the dominant reaction transfers to negative feedback process, which involves the reduction reaction between bromomalonic acid and Mn (III) ions (R12). Since nanoSe0 reduced KBrO3 into Br2 to promote the production of bromomalonic acid, which facilitated the reduction reaction of the B-Z oscillating, the single spike amplitude increased. Thus, the influence of nanoSe0 on negative feedback process may be through intermediate rather than original reactant.

CONCLUSION We investigated the effects of different species of selenium on the B-Z reaction by observing the curves of time-dependent redox potential. These four selenium species could affect the B-Z reaction through their interactions with antagonistic species of the B-Z oscillation with different degree. For nanoSe0, if nanoSe0 was added before the oscillation, it directly reduced BrO3- and promoted the accumulation of bromomalonic acid to shorten the induction period. And if nanoSe0 was injected during the oscillating, it showed strong effect on the amplitude. Especially, when nanoSe0 was injected at trough or peak, the amplitude was decreased or increased in a concentration fashion by nanoSe0. For Na2SeO3 added before the oscillation, the induction period was prolonged because Na2SeO3 consumed CH2(COOH)2 to inhibit the accumulation of bromomalonic acid. For SeMet and SeCys added before the oscillation, both of them could react with ·CH2COOH and ·Br2- to prohibit the accumulation bromomalonic acid, which resulted in prolonging of the induction period. But SeCys showed different effects on the B-Z reaction to that of SeMet, since SeCys could be oxidized by KBrO3 before reacting with the free radicals, which led to shortening of the induction period. It was also known that chemical oscillating behavior is very similar to some complex behaviors in nature, especially the biological system behavior. Therefore, the perturbation effects of selenium species 13



with high biological activity on the chemical oscillation reaction could provide the basic data and information for further research on the biological and physiological effects of selenium.

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ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (21075053). We are very grateful for Professor Tianfeng Chen (Chemistry Department of Jinan University), who gave a lot of helpful guidance on this work. We also would like to thank the anonymous reviewers for their constructive comments on the earlier version of this manuscript, which helped us to improve the presentation of the paper.

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Effects of Selenium Species on Belousov-Zhabotinsky Reaction | The

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