Chemiluminescence Character of ZnS Quantum Dots with Bisulphite

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Chemiluminescence Character of ZnS Quantum Dots with Bisulphite-Hydrogen Peroxide System in Acidic Medium Syed Niaz Ali Shah, Yongzan Zheng , Haifang Li, and Jin-Ming Lin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01925 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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Chemiluminescence Character of ZnS Quantum Dots with BisulphiteHydrogen Peroxide System in Acidic Medium Syed Niaz Ali Shah,†, ‡ Yongzan Zheng,† Haifang Li, † Jin-Ming Lin*,†,‡ †

Department of Chemistry, Beijing Key Laboratory of Microanalytical Methods and

Instrumentation, The Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua University, Beijing 100084, China ‡

Collaborative Innovation Center of Functionalized Probes for Chemical Imaging at University

of Shandong, Shandong Normal University, Jinan 250014, China *Corresponding author: [email protected]

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ABSTRACT: In this work, ZnS quantum dots (QDs) capped with mercaptopropanoic acid were applied to hydrogen peroxide-hydrogen sulfite chemiluminescence (CL) system. The CL intensity of the system was significantly enhanced by ZnS QDs in acidic medium. The reactive oxygen species like superoxide ion (•O2−), sulfite (•SO3−), sulfate (•SO4−) and hydroxide (•OH) radicals were generated in the CL reaction. It is of worth to know that, the order of addition of reagents sturdily influences CL intensity, indicating different free radical generation in response to the different orders. Interestingly, the addition of water to an optimum concentration ensued further increase in CL signal, may be due to hydrolysis reaction at higher concentration of NaHSO3. The enhanced CL was induced by excited ZnS QDs, which could be produced from the combination of hole (QDs (h+)) and electron (QDs (e−)) injected QDs as well as by chemical resonance energy transfer (CRET) from 1O2 and SO2* to ZnS QDs. Four emitters such as 1O2, (O2)2*, SO2* and ZnS QDs* were detected in the CL system. Mechanistic investigation indicated that QDs acted as a catalyst, firstly decomposing H2O2 to generate free radicals and secondly prompting CL by energy transfer and electron-transfer annihilation effects. Distinct from most QDs CL reactions which classically attained in basic conditions, this system operates in acidic medium. This may intrigue an avenue for investigating the CL property of QDs in an acidic medium and promote its application in various fields.

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1. INTRODUCTION Oxidation of sulfite (SO32−) has been researched more than fifty years. In 1975, Hoffman proposed the oxidation of sulfite via a rapid nucleophilic displacement of HSO3− by hydrogen peroxide (H2O2). During this process, peroxomonosulfurous acid was formed as an intermediate, which generated SO42− ion through a rate-determining rearrangement.1-3 On the other hand, several groups suggested the oxidation of SO32− by H2O2 through radical pathway,4,5 as they have detected •SO3− and •OH radicals by electronic spin resonance (ESR) spectroscopy.6,7 The general features of this reaction are rationally well described and implicit, important aspects are yet to be explored. In addition, stoichiometry and stereochemistry of each reagent, and their exact kinetic role, as well as the pH-dependence, needs to be clarified for its comprehensive study.8 The most significant factors manipulating the S(IV) oxidation are relative humidity and •

OH radical formation, thus liquid water content (LWC) and pH have assured influence on its

oxidation rate. With increasing LWC the oxidation rate increases due to decomposition of disulfate ion (S2O72−), upon hydrolysis {during autoxidation of S(IV)}. Furthermore, in aqueous solution HSO3− can exist in two tautomeric forms, HOSO2− and HSO3−. HOSO2− is present in very low concentration compared to HSO3− and reacts quite differently due to availability of a pair of electrons on HOSO2− that would provide a stereochemically favorable attack by a nucleophile than the tetrahedral tautomer HSO3−. The latter would be more sterically hindered and consequently require higher activation energy. Moreover, at higher concentration HSO3− dimerizes, called Golding-dimer (HSO3−)2, which is in equilibrium with disulfite ion (S2O52−). Thereafter, another significant parameter is pH, ever since S(IV) species exhibit pH-dependent speciation in aqueous solution with different reactivity, and the chemistry of which is the basis for understanding complex reaction mechanisms.9 Consistent with pH, S(IV) species exist in

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different forms, i.e., SO2.H2O (pH 6.5) in aqueous solution. SO32− is more reactive than HSO3−, HSO3− in turn is more reactive than SO2.H2O.10 In contrast, H2O2 reacts hasty with HSO3− rather than with SO32−.11 Liquid-phase oxidation of HSO3− by H2O2 (reaction 1) subsidizes the formation of acid rain and follows third-order rate law at pH 3-6 (reaction 2).12 HSO3 − + H 2 O2 → SO42 − + H + + H 2O

(1)

− d [ HSO3− ] / dt → k h [ H + ] + [ HSO3− ] + [ H 2 O2 ]

(2)

Moreover, sulfur dioxide (SO2) and sulfite envisioned to have greater role in the industry and the environment because of their versatility, cheapness and efficiency. Additionally, sulfite is used as an antioxidant in pharmaceutical and food industries and as an inhibitor of yeasts and bacteria in the wine industry. Likely, SO2 is a major air pollutant, whispered to be responsible for the destruction of forests and acidifying soil and water, owed to acid rain.13 Thus the oxidation of S(IV) species have attracted substantial attention due to its role in acid rain and industrial desulfurization of plume gases,9 and were also positively used for hydroxylation, epoxidation and oxidative cleavage of DNA recently.8 In atmosphere the oxidation of S(IV) revenues in gaseous and liquid phases (clouds, fog, rain drops) by natural oxidants, among which H2O2 is the most potent one in the liquid phase.12 Chemiluminescence is an excellent analytical technique with many advantages1 and wide interests in various fields of analytical chemistry.2 To extend its applications, CL studies have been protracted from traditional molecular reactions to nanomaterials (NMs) based systems recently.14,15 Since, NMs have intensified the CL intensities, among which colloidal semiconductor nanocrystals or quantum dots (QDs), have attracted much attention due to their excellent and distinctive optical and electronic properties, and have been widely used as probes

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and biological luminescent markers. The application is grounded on the emission of the excited state QDs, which can be produced by electron injection, electron impact, or chemical reaction of QDs.3 QDs are also practiced as the emitting species or as catalysts of redox CL reactions.14 In fact, NMs acts as catalysts, reductants and luminophores or energy accepters in CL systems and results in improved sensitivity and stability. As, Zhou et al. undertook an investigation on ZnS QDs enhanced CL of NaClO-H2O2 in basic medium.16 In which ZnS QDs catalyzed the decomposition of H2O2, producing reactive intermediates to form 1O2 and (1O2)2*, which returned to its ground state by passing its energy to QDs through an energy-transfer process and resulted in enhanced CL emission. In addition, it was also demonstrated that H2O2 directly oxidized ZnS QDs to produce weak CL emission in basic medium.17 Unlike to the above reports, our enhanced CL system operated in acidic medium. From the outset, the enhancement, mechanism and potential applications of ultra-weak CL systems have received greater attention, among which peroxide induced ultra-weak CL systems are the most important ones.18 However, low quantum efficiency of ultraweak CL from the oxidation of inorganic molecules19 restricted the development of these CL systems from detecting demands.20 So, it is demanding to explore novel CL-reaction strategies providing new approaches to enhance the inherent sensitivity of ultraweak CL systems, to widen and deepen its applications. Herein, we choose a NaHSO3-H2O2 ultraweak CL system, with fairly green and cheap reagents but low luminescence yields. Thus, it is momentous to enhance the CL intensity of the proposed system for the purpose to accelerate its applications. This will not only broaden the application of this CL system, but will also provide a new route to study the properties of ZnS QDs. It was found that ZnS QDs could enhance the CL of NaHSO3-H2O2 system. Furthermore, for the first time it was observed that with the addition of water the CL intensity of

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the system increases. The enhanced mechanism was investigated based on photoluminescence (PL) spectra, CL emission spectrum and ESR spectra, as well as the effects of radical scavengers on CL intensity. The results revealed that ZnS QDs enhanced the CL, firstly decomposed H2O2 to generate radicals due to surface effect and secondly due to energy transfer and electrons and hole injection and annihilation processes. The investigation of the CL mechanism of ZnS QDs will be valuable both for understanding these QDs and for the use of this enhanced CL system in analytical fields. As reported in the literature, CL properties of NMs not only will be helpful to study its physical properties, but have applications in many fields such as luminescence devices, bioanalysis and multicolor labeling probes.21

2. EXPERIMENTAL 2.1. Chemical and Materials. All the chemicals used were of analytical grade. ZnSO4 was purchased from Heng Ye Jingxi Chemical Co. Beijing (Beijing, China) and Na2S was from Fuchen Chemical Reagents (Tianjin, China). H2SO4 and HCl were obtained from Tianjin Kaitong Chemicals Co. (Tianjin, China). In addition, 2,2,6,6-Tetramethyl-4-piperidine (TEMP) was bought from Sigma Aldrich (St. Louis, MO, USA), 2-mercaptopropanoic acid, H2O2, NaHSO3, NaN3, thiourea, ascorbic acid and NaOH were obtained from the Beijing Chemical Reagent Co. (Beijing, China). Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai Tesque Inc. (Tokyo, Japan) and 5,5-Dimethyl-1-pyrrolineN-oxide (DMPO) was purchased from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). The solutions were prepared in distilled water and stored at 4oC. 2.2. Apparatus. The CL aptitudes were done with a BPCL Luminescence analyzer (Institute of Biophysics, Chineses Academy of Sciences, Beijing, China). On the other hand, F-7000

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fluorescence spectrophotometer (Hitachi, Japan) was deployed for PL study and absorption spectra were fared by UV-3900s spectrophotometer (Hitachi). While the characterization of QDs was done by transmission electron microscope (TEM, Tecnai G2 20S-Twin, FEI Company, USA) at 200 kV and ESR analysis was performed on Bruker spectrometer (ESP-300E, Bruker, Germany). 2.3. Synthesis and Characterization of ZnS-QDs. ZnS QDs were disposed in one-pot at room temperature from an alkaline solution of ZnSO4 and Na2S as reported earlier22 with slight modification. 2-Mercaptopropanoic acid was used as capping agent. Briefly, 2 mL of 0.2 M ZnSO4 was diluted with 24 mL distilled water. Then, 2-mercaptopropanoic acid (75 µL, 1 mmol) was added under constant stirring and the mixture was alkalized with 2 mL of 1 M NaOH solution (2 mmol). Finally, 2 mL of 0.2 M Na2S solution (0.4 mmol) was drop wise added and stirred the mixture for 2h. The 2-mercaptopropanoic acid has a thiolate substituent. The thiol group often participates in enzymatic reactions, serving as a nucleophile. Here, the thiolate substituent of 2-mercaptopropanoic acid served as a nucleophile bound to ZnS QDs; the other part of 2-mercaptopropanoic acid are hydrophilic, which facilitates the modified QDs to become soluble in water.16 The sample for TEM was dialyzed before analysis to remove soluble electrolytes. Dialysis was performed by using MW cut off 3.5 kDa Biotech regenerated cellulose (RC) membrane for 12 h against distilled water under gentle stirring, and replaced the distilled water after each 2 h. The sample was prepared by molding a drop of ZnS QDs on to a 300-mesh holey carbon-coated copper grid. The ZnS QDs have an average diameter of about 5 nm (Figure S1 in Supporting Information). Furthermore, the experimental details are given in the supplementary information.

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3. RESULTS AND DISCUSSION 3.1. UV-Visible and PL Spectroscopy. The prepared ZnS QDs displayed UV absorption at 290 nm,16,17,22 and the maximum PL emission located at 428 nm (Figure 1). The PL intensity increased as the excitation wavelength increased up to 300 nm beyond that decreased. In order to ascertain the radiant species, PL spectra of ZnS QDs were recorded before and after the CL reaction, in which the altered peaks, pointed different emitting species. Whereas, the decrease in PL intensity of ZnS QDs may be due to dilution. On the other hand, the appearance of new peaks may be attributed to the presence of other reagents and vicissitudes in pH (from 12 to 0) after the reaction (Figure S2 in Supporting Information).20 Thus the decrease in PL intensity and appearance of new bands indicated the reaction among ZnS QDs, H2O2 and NaHSO3 in acidic medium. (Figure 1) 3.2. Batch CL Analysis. A batch system was used for NaHSO3−H2O2 CL reaction. There was weak but fast luminescence when 100 µL of 0.1 M NaHSO3 was injected into 100 µL of 0.1 M H2O2, which was roughly increased 200 times by ZnS QDs in acidic medium (Figure 2). A series of experiments were examined on CL properties, by varying one variable at a time while keeping other conditions fixed. Mixing order of reagents played an important role in CL emission,1 and was evaluated by changing orders of injections and different mixing orders of reagents. The highest CL intensity was achieved by the injection of NaHSO3 compared to the injection of H2O2 and ZnS QDs. In addition, the injection of NaHSO3 to ZnS QDs-H2O2-H2SO4, H2SO4-ZnS QDs-H2O2 and H2O2-H2SO4-ZnS QDs resulted in different CL intensity. The highest intensity was achieved in the first case. This can be described as a decomposition of H2O2 due to the surface effect of QDs,2 and generation of •OH radicals in basic medium. While in the last two 8 ACS Paragon Plus Environment

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cases the ZnS QDs were added to acidic medium which may be the reason for the lower generation of radicals from H2O2. Thus the order of addition of reagents has a prominent effect upon the CL intensity (Figure S3 in Supporting Information). (Figure2) Similarly, the role of concentration (Figure 3) and volume (Figure 4) of each reagent on CL signals were scrutinized. The concentration and volume of each reagent was optimized by changing one parameter at a time while keeping others constant. The CL intensity of the system increased with increase in concentration of reagents to a certain limit beyond that decreases. Similarly, the volume of HCl, H2O2 and NaHSO3 have a direct relation with the CL intensity. On the other hand, the volume of ZnS QDs have very little effect on CL signals. The relatively no volume effect of ZnS QDs and very poor CL signals in its absence pointing the catalytic behavior of ZnS QDs in the system. The concentration of H2O2, H2SO4 and NaHSO3 played an important role, and the CL intensity increased to an optimal concentration of 1, 1 and 1.5 M, respectively. Interestingly, the addition of water to an optimum reagents concentrations boosted the CL intensity further (Table 1). Furthermore, the effect of volume of water was also studied (Figure S4 in Supporting Information). (Figure 3,4) 3.3. Free Radicals Studies. In order to monitor the spawned intermediates, different radical scavengers on the CL intensity of H2O2-NaHSO3 system in the presence and absence of ZnS QDs were scrutinized (Figure S5 in Supporting Information) and the results are listed in Table 1. Ascorbic acid being a classical reducing agent is generally used as a free radical scavenger. The CL intensity was significantly inhibited by the addition of ascorbic acid due to its dehydrogenation by ROS (• OH, •O2−). This supported the radical pathway of the NaHSO3-H2O2

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system in the presence of ZnS QDs in acidic medium. Since, •OH radical was deliberated to be one of the most potent oxidizer and apparently play an important role in CL of NaHSO3-H2O2ZnS QDs-H2SO4 system. The generation of •OH radical was indicated by the restraint effect of thiourea, which is commonly used as •OH radical scavenger. The short lived •OH, •SO3− (reaction 3) and SO4•− radicals were deliberated in the system by DMPO scavenger. Similarly, the existence of •OH and •SO3− radicals were confirmed by ESR studies, while SO4•− anionic radical was supposed to be one of the products of chain reactions of O2 and •SO3−.20,23 DMPO + • SO3− / •OH → DMPO / • SO3− , DMPO / •OH

(3)

NBT was frequently used for •O2− radicals detection,5 which specifically scavenges •O2− radicals (reaction 4).24 When 0.05 M NBT was added to ZnS QDs-H2O2-H2SO4-NaHSO3 or H2O2-H2SO4-NaHSO3 reaction systems, a color change from yellow to blue was observed due to the reduction of NBT to its deep-blue diformazan form by •O2−. The decrease in CL intensity by the addition of NBT, indicated the existence of •O2− in the system.20,23 The smaller decrease in CL intensity of NBT relative to other scavengers (Table 1), led to the smaller generation of •O2− radicals, which may be due to strong acidic medium or its protonation. NBT 2 + + •O2− → • NBT + + O2

(4)

Sodium azide (NaN3) was used in this system, which physically quenches 1O2 (reaction 5).24 The addition of 0.1 M NaN3 significantly quenched the CL intensity (Table 1), indicating 1O2 generation.3,20 So consistent with these results, we cannot exclude the 1O2 generation although there were no signals in ESR spectra for 1O2 with TEMP. N 3− + 1O2 → N 3− + 3O2

(5)

(Table 1)

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3.4. Spin Trapping Studies. In order to detect radical intermediates, ESR spectroscopy was carried out at room-temperature (Figure 5). ESR studies show that TEMP preferentially react with

1

O2 to form 2,2,6,6-tetramethyl-4-piperidine-N-oxide (TEMPO)21 which is a stable

nitroxide radical with a characteristic spectrum.1,20 In Figure 5B the signal of TEMP, favored the generation of 1O2, only in ZnS QDs-H2O2 system. However, no ESR signals were observed in other possible combinations, indicating that at lower pH the production of 1O2 diminishes, or it was possible that 1O2 did not bind with TEMP due to its protonation. DMPO was used to detect •

OH, •SO3−,24 and HO2• radicals in the reaction system.25 The ESR signals of DMPO−•OH/•SO3−

adducts at different combinations of reactants, showed the generation of these radicals in the studied system (Figure 5A). The peak splitting in last 3 spectra revealed the coexistence of •OH and •SO3− radicals, while no such splitting was observed in the first spectrum, due to absence of •

SO3− radical.23 Moreover, in spectrum (iii) a new peak appeared due to absence of acid, which

was further boosted in the absence of ZnS QDs (iv). These phenomena proved that in different pH systems different radicals were generated and the ZnS QDs along with catalytic properties also resulted in a pH change of the system. (Figure 5) 3.5. Emitting Species. To ascertain the emitting species, CL spectrum (Figure 6) and PL spectra before and after the reaction (Figure S2 in Supporting Information) were recorded. The CL spectrum was performed by Hitachi F-7000 spectrophotometer with a flow cell, when the lamp was turned off and the width of the emission slit was 20 nm. On the basis of research results of Shi,6 Ozawa,7 and Xue,20 it was assumed that SO2* and 1O2 as CL emitters might be formed by simultaneous processes of decomposition and radical recombination. The SO2* radiated in the range of 450-600 nm,26,27 while excited 1O2 and singlet dimole specie (O2)2*

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emitted in the range of 490-550 nm.28 The peak at 420-470 nm was attributed to ZnS QDs, which was consistent with its PL spectra (Figure 1).24 Therefore, we hypothesized from the CL spectrum that ZnS QDs along with 1O2 and SO2* acted as CL emitters in ZnS QDs-H2O2-H2SO4NaHSO3 system. (Figure 6) 3.6. Mechanism of CL reaction. The main objective of the present work was to explore the role of acid in the oxidation of S(IV) by H2O2 in the presence of ZnS QDs and to develop a detailed mechanism for interpretation of experimental observations. The H2O2 oxidizes S(IV) and results in the formation of emitters such as 1O2, SO2* and ZnS QDs*. Three different types of mechanisms for the oxidation of HSO3− by H2O2, radical, nonradical and combined one are reported in the literature.9 Many researchers advocated the major contribution possibly from a nucleophilic substitution (Nonradical mechanism)1-3 when the pH ranged between 0-3, while some argued in favor of a radical pathway at 4-7 pH value.4,5 In their sentiments, free-radical pathway in complete reaction seemed improbable, but certainly could not be excluded as radical intermediates have been detected in the reaction. For example, •SO3− and •OH radicals were detected by Shi6 in the reaction of acidic H2O2 with sulfite. Ozawa and Hanaki7 also detected •

SO3− in the nonenzymatic reaction of sulfite with H2O2 by ESR.20,27 Based on the mechanistic study of free radical scavengers, ESR, PL and CL spectrum of ZnS

QDs-H2O2-H2SO4-NaHSO3 system, a proposed mechanism is depicted in Figure 7. CL emission from NaHSO3-ZnSQDs-H2O2-H2SO4 was measured to identify the emitting species in the system. The dimeric form of HSO3• radical produced SO2* (low quantum yield), which on deexcitation resulted in very weak CL emission in the range of 400-600 nm. Similarly, the generation of 1O2 also resulted in CL emission. The contribution from 1O2 cannot be excluded

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due to constrain effect of NaN3 on the CL intensity of the system (table 1) as well as the appearance of the peak nearly at 490 nm in the CL spectrum (Figure 6), though no evidence from ESR studies. Thus the simultaneous decomposition and radical recombination resulted in 1O2 and SO2*,26 along with ZnS QDs* as emitting species, which released the excess energy immediately and produced CL emission. The hypothesized mechanism for the CL system showed that the QDs acted as a catalyst, firstly decompose H2O2 to generate free radicals and secondly, CL was prompted by energy transfer and electron-transfer annihilation effects. The contribution of each step is explained in detail below. 3.6.1.

Catalysis (Decomposition of H2O2). The enhancement was firstly attributed to

the catalytic behavior of ZnS QDs, which was assumed to facilitate radical generation and accelerate the electron transfer process. ZnS QDs acting as a catalyst, generated active oxygencontaining reactant intermediates, like •O2− and •OH.15 HO2− was firstly produced from H2O2 (reaction 6), which resulted in •O2−, •OH and HO2• radicals (reaction 7,8).17,28 H 2O2 + −OH → HO2− + H 2O

(6)

H 2 O2 + HO2− → •OH + •O2− + H 2 O

(7)

•

(8)

OH + H 2 O2 → HO2• + H 2O

So it can be concluded that the enhancement of CL by ZnS QDs was attributed to the formation of •OH. On the other hand, a lot of •OH might be stabilized on the surface of ZnS QDs, and the probability of collision of the excited state with water molecules was small.15 Moreover, •

O2− being unstable initiating the reactive oxygen chain reaction, leading to the production of

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reactive oxygen radicals and 1O2.25 The •OH and •O2− radical anion then reacted to form 1O2 and (O2)2* (reaction 9-11).18,29,30 O2− + •OH → 1O2 + −OH

(9)

HO2• + HO2• → 1O2 + H 2 O2

(10)

ZnSQDs H 2O2  → •OH + •O2− → 1O2 + (O2 )*2 Decomposition

(11)

•

It is known that singlet oxygen had a higher energy than the ground-state triplet oxygen and was one of the CL emitter (reaction 12).3 1

O2 → 3O2 + hv

3.6.2.

hʋ = 381, 580, 634 nm

(12)

Substation reaction. The nucleophilic reaction of HSO3− with H2O2 resulted in

the formation of peroxomonosulfurous acid intermediate. The acidified bisulfite ion (HSO3−), was oxidized by H2O2 generating HSO3•.29 The proton-catalyzed pathway is generally described by the sequence of steps presented in (reaction 13-15), within the intermediate peroxysulfurous acid, HOOSO2H.12 Whereas, the decomposition of HSO4− formed HSO3• and HO• radicals (reaction 14),23,27 which were the main radicals in NaHSO3−H2O2 CL system.26 +

H HOOH + HSO3− → HOOSO2− + H 2O → HOOSO2 H

(13)

HOOSO2 H → •OH + HSO3•

(14)

2 HSO3• → S 2O62 − + 2 H +

(15)

The above reaction only takes place in acidic solution,9 because of the absence of oxygen.11 The recombination of HSO3• radicals produces S2O62− ion immediately (reaction 15), which upon decomposition resulted in an important excited intermediate SO2* (reaction 16), generated CL

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emission band around 500 nm (reaction 17),14 having a wide emission range of 450–600 nm.23,27,29 Besides 1O2, SO2* was also found to account partially as CL emitter.18,21 S 2 O62 − → SO42 − + SO2* SO2* → SO2 + hv

3.6.3.

(16) hʋ ≈ 500 nm

(17)

Radical pathway contribution. •O2− and •OH were generated in H2O2–NaHSO3–

ZnS QDs system and were the key intermediates in CL system. The HSO3− may either trigger a radical chain reaction by generating •SO3− radical or may directly oxidize S(IV) to S(VI).8 As •

OH being a strong oxidant reacted with HSO3− to give •SO3− radical promptly (reaction 18).23

Similarly, •SO3− reacted with excess H2O2 to produce HO2• (reaction 19), which decompose to yield •O2− (reaction 20). •

OH + HSO3− → • SO3− + H 2 O

(18)

H 2O2 + • SO3− → HSO3− + HO2•

(19)

HO2• → H + + •O2−

(20)

In NaHSO3−H2O2 system, the existence of •OH radical was proved by the quenching effect of thiourea, ascorbic acid and was detected by the ESR spin-trapping.26 As •O2− reacted with •OH to form 1O2 (reaction 9),24 or HO2• reacted among themselves to produce 1O2 (reaction 10).19,30 Thus, the simultaneous processes of decomposition and radical recombination,1 produced 1O2 and SO2*, which released the excess energy immediately, resulted in CL emission. 3.6.4.

Electron Transfer and Annihilation Effect. In HSO3−–H2O2–ZnS QDs system,

reactions of radicals with ZnS QDs play an important role. The oxidant radicals, such as HO• and •

SO4−, injected holes in the 1Sh quantum-confined orbital of ZnS QDs (reaction 21).17,18 on the

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other hand, •SO3− and •O2− radicals served as electron donors, which can easily inject an electron into the 1Se quantum-confined orbital of ZnS QDs (reaction 22).14,17,18 •

S O4− / •OH + ZnSQDs → S O42 − / −OH + ZnSQDs ( h + 1sh )

(21)

•

SO3− / •O2− + ZnSQDs → SO3• / •O2 + ZnSQDs (e − 1se)

(22)

The resulted QDs upon electron annihilation give rise to QDs in the excited state (reaction 23), with one electron in 1Se (ZnS (e−1Se)) and one hole in 1Sh (ZnS (h+1Sh)) quantum-confined orbital, respectively, which upon deexcitation illuminates (reaction 24).

ZnSQDs(h +1sh) + ZnSQDs(e− 1se) → ZnSQDs*

(23)

ZnSQDs* → ZnSQDs + hv

(24)

The CL intensity significantly depends on the rates of generation and annihilation of ZnS QDs*. Therefore, the formation of 1O2 and SO2* was competed with the generation of ZnS QDs*.1 (Figure 7) 3.6.5.

CRET. Chemical Resonance Energy Transfer (CRET) may be attributed to the

different energy matching between the chemical energy engendered during CL reaction and the required excitation energy for the formation of different excited state of QDs. The chemical energy generated during the CL emission from 1O2 and SO2* matches the smallest energy band gap of ZnS QDs. The more chemical energy matches the required excitation energy, the stronger the CL intensity will be, and the higher efficiency could be achieved.3 In fact, the carboxyl and thiol group of the 2-mercaptopropanoic acid on the outer surface of ZnS QDs behaved as the receptor sites through the acid–base pairing interaction, and was very important in the CL energy transfer process.16 The excited SO2* may transfer its energy to ZnS QDs through CRET (reaction 25). In addition, ZnS QDs served as the energy acceptor from 1O2 and (O2)2* to become excited 16 ACS Paragon Plus Environment

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ZnS QDs* (reaction 26), which returned to its ground state with the enhanced CL emission (reaction 24).18,29 SO2* + ZnSQDs → SO2 + ZnSQDs * 1

O2 / (O2 )*2 + ZnSQDs → 3O2 + ZnSQDs *

(25) (26)

The oxidation products of S(IV) oxides are SO42− and S2O62−. S2O62− is thermodynamically unstable, but kinetically stable with respect to disproportionation and oxidation reaction at room temperature. The disproportionation in acidic solution increases with increasing acidity.9 So, at lower pH S2O62− easily converts to SO42− and SO2*. •

SO3− + • SO3− → S 2 O62 −

(27)

The above reaction only takes place in acidic solution,9 due to lack of oxygen.11 Since, oxidation of sulfite to •SO3− radical has been proven only under acidic conditions. Which may participate in consequent reactions with oxygen (if present) and HSO3− to yield more potent sulfur oxide derived radicals (e.g. •SO4− and •SO5−). 3.7. Effect of ZnS QDs. Among the three probable mechanisms for QDs enhanced CL systems, the direct oxidation can be definitely presumed when QDs is the only luminescent specie in the system. While in case of more than one emitting species in a CL system, the mechanism can either be contributed to CRET, when QDs are final emitter or to catalytic process, when the final emitting specie is the luminophore.29 In a NaHSO3-H2O2-ZnS QDsH2SO4 system the direct oxidation, catalytic process and CRET take place simultaneously. The enhancing effect of ZnS QDs was attributed to the following possible mechanisms: (1) ZnS QDs facilitated the radical generation from H2O2 and formation of SO2* and 1O2 and thus acted as a catalyst. (2) SO2* and 1O2 transmitted the energies to fluorescent ZnS QDs, which operated as

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emitters in the boosted CL system. (3) The intermediate radicals spawned in H2O2-NaHSO3 reaction such as •OH, •O2−, SO4•− and SO3•− reacted with ZnS QDs to produce positively (ZnS QD•+) and negatively (ZnS QD•−) charged QDs, then excited-state (ZnS QDs*) were formed through the electron-transfer annihilation effect (reaction 21-24). Finally, the improved CL emission arose when ZnS QDs* returned to the ground-state. The inspiration of Seyed et al,17 on the CL mechanism of ZnS QDs led to the proposition of the third mechanism.20 3.8. The addition of water. The oxidation of S(IV) specie increases by addition of water may be due to the possible formation of disulfate ion (S2O72−) during the reaction which decomposes upon hydrolysis (reaction 28).9 This may be a reason for high CL intensity by the addition of water. S 2O72 − + H 2 O → 2 SO42 − + 2 H +

(28)

Secondly, the addition of water may convert the tautomeric form (S2O52−) to (HSO3−)2, because the HSO3− was in high concentration (Figure S6 in supplementary Information). •

O2− + H 2 O → HO2• + −OH

(29)

•

O2− + •O2− + 4 H 2O → [ 1O2 ]2 + 2 H 2 O2 + 4 −OH

(30)

The order of addition of water also had a prominent effect on the enhancement of CL intensity (Figure S7 in Supplementary Information). When water was added first, there was no enhancement, while its addition in last resulted in enhancement. Therefore, we concluded that the water may act as stabilizer of HSO3− species or ions produced in the reaction (reaction 29, 30). 3.9. Effect of pH. The main variable in the catalytic oxidation of S(IV) species was pH,31 which controls the speciation of S(IV),11 having different reactivity and the reaction mechanism 18 ACS Paragon Plus Environment

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was based on the chemistry of these species.9 In alkaline medium the dominant S(IV) specie is SO32−, 20-40 times more reactive than HSO3−, a principal specie in the pH range of 2-6. At lower pH (< 2), SO2.H2O is the main specie which is 50 times less reactive than HSO3−. As a result of this pH dependent speciation of S(IV) species, the pH dependent autoxidation of S(IV) species will depend on the catalytic activity and reactivity of each contributing oxidant.10 In literature, nonradical and radical mechanisms were suggested for oxidation of S(IV) in a pH range of 0-3 and 4-7 respectively. Additionally, at a pH range of 0-3 the rate of oxidation depends on H+ while in the range of 3-7 there is no such dependence. Moreover, the catalyzed process, in aqueous solution has been reported in different reaction order as a function of pH. The oxidation of S(IV) species by H2O2 were characterized by a strong pH dependence. The increase in ionic strength caused an increase in the rate of oxidation of S(IV) species. The oxidation of S(IV) species ensued via several reaction pathways, the prominence of each pathway mainly depends on pH. Furthermore, •SO4− radical can be protonated at lower pH (reaction 31), the protonated sulfate radical (HSO4), reacts faster with water (reaction 32).9,10

H + + • SO4− → HSO4

(31)

HSO4 + H 2O → O2 + otherproducts

(32)

Oxidation HSO3−  → SO42 − + H +

(33)

The oxidation of HSO3− to SO42− releases H+ ions (reaction 33) which are partially used in HSO3−/SO22− equilibrium, but no buffer effect is observed at strong acidic conditions, e.g. below pH 2.5.10 In strong acidic medium the S(IV) will be in HSO3− form which reacted rapidly with H2O2 than SO32−.11 It is noteworthy, that the pH of the solution was a key factor, controlling the form of S(IV) species, and consequently the reaction rate of S(IV) oxidation.31

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4. CONCLUSIONS In this study, ZnS QDs were used for the enhancement of NaHSO3-H2O2 CL system and the mechanism in acidic medium has been elucidated. The order of addition of reagents sturdily influences CL intensity, indicating different free radical generation in response to different addition orders. Interestingly, the addition of water enhanced the CL intensity by hydrolysis reaction or favoring the formation of Golding-dimer (HSO3−)2. The enhanced CL was induced by the excited ZnS QDs*, which could be produced from the combination of hole (QDs (h+)) and electron (QDs (e−)) injected QDs as well as by CRET from 1O2 and SO2* to ZnS QDs. Four emitters, such as 1O2, (O2)2*, SO2* and ZnS QDs* were detected in the CL system. The probable mechanism revealed that ZnS QDs acting as a catalyst, firstly decomposed H2O2 to generate free radicals and secondly prompted CL by energy transfer and electron-transfer annihilation. In contrary to most QDs CL reactions, this system operates in acidic medium, which may intrigue an avenue for researchers in investigating the CL property of QDs in an acidic medium and promoting step towards its application in various fields. This work is also helpful for comprehension of the CL mechanism correspondingly.

ASSOCIATED CONTENT Supporting Information. The experimental detail for the synthesis of ZnS QDs, Batch CL system, CL spectrum measurements and ESR analysis are given in supplementary materials. Furthermore, TEM image of ZnS QDs (Figure S1), photoluminescent spectra after the CL reaction (Figure S2), CL graph showing the effect of order of reagents (Figure S3), effect of addition of water (Figure S4), effect of free radical scavengers on the CL intensity of the system (Figure S5), conversion of different form of NaHSO3 at different concentration and the role of

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water in its conversion (Figure S6), as well as the effect of order of addition of water (Figure S7) are given in supplementary materials. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone/Fax: +86-10-62792343. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENT This work was financially supported by National Natural Science Foundation of China (Nos. 21435002, 21227006, 81373373) and Mr. Shah acknowledges the Chinese Scholarship Council for the award of doctoral studies.

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Liu, J.-X.; Chen, H.; Lin, Z.; Lin, J.-M. Preparation of Surface Imprinting Polymer Capped Mn-Doped ZnS Quantum Dots and Their Application for Chemiluminescence Detection of 4-Nitrophenol in Tap Water. Anal. Chem. 2010, 82, 7380–7386.

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(11) Brandt, C.; Fabian, I.; Eldik, R. V. Kinetics and Mechanism of the Iron(III)-Catalyzed Autoxidation of Sulfur(IV) Oxides in Aqueous Solution. Evidence for the Redox Cycling of Iron in the Presence of Oxygen and Modeling of the Overall Reaction Mechanism. Inorg. Chem. 1994, 33, 687-701.

(12) Maa, F.; Elias, H.; Wannowius, K. J. Kinetics of the Oxidation of Hydrogen Sulfite by Hydrogen Peroxide in Aqueous Solution: Ionic Strength Effects and Temperature Dependence. Atmos. Environ. 1999, 33, 4413-4419. (13) He, Z.; Wu, F.; Meng, H.; Yuan, L.; Luo, Q.; Zeng, Y. Chemiluminescence Determination of Sulfur Dioxide in Air Using Tris(1,10-Phenanthroline)Ruthenium-KIO4 System. Anal. Lett. 1999, 32, 401-410.

(14) Khataeea, A.; Hasanzadeh, A.; Iranifam, M.; Joo, S.W. A Novel Flow-injection Chemiluminescence Method for Determination of Baclofen Using L-Cysteine Capped CdS Quantum Dots. Sensors and Actuators B 2015, 215, 272–282. (15) Li, S.; Li, X.; Zhang, Y.; Huang, F.; Wang, F.; Wei, X. Enhanced chemiluminescence of the Luminol–KIO4 System by ZnS Nanoparticles. Microchim. Acta 2009, 167, 103-108. (16) Zhou, Y.; Chen, H.; Ogawa, N.; Lin, J.-M. Chemiluminescence from NaClO–H2O2 and Enhanced by L-Cysteine capped Mn-doped ZnS Quantum-Dots. J. Lumin. 2011, 131, 1991-1997. (17) Azizi, S. N.; Chaichi, M. J.; Shakeri, P.; Bekhradnia, A.; Taghavi, M.; Ghaemy, M. Chemiluminescence of Mn-Doped ZnS Nanocrystals Induced by Direct Chemical Oxidation and Ionic Liquid-Sensitized Effect as an Efficient and Green Catalyst. J. Spectroscopy 2013, Article ID 803592, 1–8.

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(18) Lin, Z.; Chen, H.; Lin, J.-M. Peroxide Induced Ultra-Weak Chemiluminescence and its Application in Analytical Chemistry. Analyst 2013, 138, 5182–5193. (19) Chen, H.; Li, R.; Li, H.; Lin, J.-M. Plasmon-Assisted Enhancement of the Ultraweak Chemiluminescence Using Cu/Ni Metal Nanoparticles. J. Phys. Chem. C 2012, 116, 14796−14803. (20) Xue, W.; Lin, Z.; Chen, H.; Lu, C.; Lin, J.-M.

Enhancement of Ultraweak

Chemiluminescence from Reaction of Hydrogen Peroxide and Bisulfite by Water-Soluble Carbon Nanodots. J. Phys. Chem. C 2011, 115, 21707–21714. (21) Chen, H.; Xue, W.; Lu, C.; Li, H.; Zheng, Y.; Lin, J.-M. Plasmonic Luminescent Core– Shell Nanocomposites-Enhanced Chemiluminescence Arising from the Decomposition of Peroxomonosulfite. Spectrochim. Acta Part A 2013, 116, 355–360. (22) Bochev B.; Yordanov, G. Room Temperature Synthesis of Thioglycolate-Coated Zinc Sulfide

(ZnS)

Nanoparticles

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Physicochemical

Characterization. Colloid and Surf. A 2014, 441, 84–90. (23) Li, R.; Kameda, T.; Toriba, A.; Hayakawa, K.; Lin, J.-M.

Determination of

Benzo[α]pyrene-7,10-quinone in Airborne Particulates by Using a Chemiluminescence Reaction of Hydrogen Peroxide and Hydrosulfite. Anal. Chem. 2012, 84, 3215−3221. (24) Chen, H.; Li, H.; Lin, J.-M.

Determination of Ammonia in Water Based on

Chemiluminescence Resonance Energy Transfer between Peroxymonocarbonate and Branched NaYF4:Yb3+/Er3+ Nanoparticles. Anal. Chem. 2012, 84, 8871−8879. (25) Dou, X.; Lin, Z.; Chen, H.; Zheng, Y.; Lu, C.; Lin, J.-M. Production of Superoxide Anion Radicals as Evidence for Carbon Nanodots Acting as Electron Donors by the Chemiluminescence Method. Chem. Commun., 2013, 49, 5871-5873.

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(26) Li, R.; Chen, H.; Li, Y.; Lu, C.; Lin, J.-M. Enhancing Effect of Alcoholic Solvent on Hydrosulfite−Hydrogen Peroxide Chemiluminescence System. J. Phys. Chem. A 2012, 116, 2192−2197. (27) Zhou, Y.; Xing, G.; Chen, H.; Ogawa, N.; Lin, J.-M. Carbon Nanodots Sensitized Chemiluminescence on Peroxomonosulfate–Sulfite–Hydrochloric Acid System and its Analytical Applications. Talanta 2012, 99, 471–477. (28) Zhang, L.; Zhang, Z.; Lu, C.; Lin, J.-M. Improved Chemiluminescence in Fenton-Like Reaction via Dodecylbenzene-Sulfonate-Intercalated Layered Double Hydroxides. J. Phys. Chem. C 2012, 116, 14711−14716.

(29) Chen, H.; Lin, L.; Li, H.; Lin, J.-M. Quantum Dots-Enhanced Chemiluminescence: Mechanism and Application. Coord. Chem. Rev. 2014, 263–264, 86–100. (30) Shah, S. N. A.; Li, H.; Lin, J.-M. Enhancement of Periodate-Hydrogen Peroxide Chemiluminescence by Nitrogen Doped Carbon Dots and its Application for Determination of Pyrogallol and Gallic Acid. Talanta 2016, 153, 23-30. (31) Novic, M.; Grgic, I.; Poje, M.; Hudnik, V. Iron-Catalyzed Oxidation of S(IV) Species by Oxygen in Aqueous Solution: Influence of pH On the Redox Cycling of Iron Atmos. Environ. 1996, 30, 4191-4196.

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Table 1. Effect of free radical scavengers on the CL intensity of NaHSO3-H2O2 system in acidic medium with and without ZnS QDs Free radical scavengers

Radicals

Concentration (M)

H2O2 + H2SO4 + NaHSO3

ZnS QDs + H2O2 + H2SO4 + NaHSO3

-

-

-

-

31263

water

-

-

-

47425

O2

0.1

190

12686

OH, •O2−

0.1

2083

3077

O2 −

0.05

123

9618

OH,•SO3−

0.1

156

2151

0.1

59

872

1

NaN3 ascorbic acid

•

•

NBT DMPO T. urea

•

•

OH

Each result is an average of triplicate readings. The conc. of NaHSO3, H2O2 and H2SO4 were 1.5, 1 and 1M respectively. The volume of each reagent used was 100 µL.

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Figure Captions Figure 1. Photoluminescent spectra of ZnS QDs. The PL intensity increases as the excitation wavelength increased from 250 to 300 nm and beyond that decreases. There was no effect on the position of emission wavelength by changing the excitation wavelength. The inset is the absorption spectrum of ZnS QDs with a maximum absorption at 290 nm. Figure 2. CL intensity achieved by different concentration and different combinations of NaHSO3-H2O2 system (1) (0.1M)NaHSO3-(0.1M)H2O2, (2) (0.1M)NaHSO3-(0.1M)H2O2(1M)H2SO4, (3) (0.1M)NaHSO3-(0.1M)H2O2-ZnSQDs, (4) (0.1M)NaHSO3-(0.1M)H2O2ZnSQDs-(1M)H2SO4 and (5) (1.5M)NaHSO3-(1M)H2O2-ZnS QDs-(1M)H2SO4. The volume of each reagent used was 100 µL. Figure 3. Concentration optimization of each reagent in NaHSO3-H2O2-ZnSQDs-Acid system. The optimum concentration for HNO3, HCl, H2O2 and H2SO4 was 1 M while for NaHSO3 was 1.5 M. The volume of each reagent was 100 µL. Each point is a mean of triplicate readings. Figure 4. Optimization of volume of each reagent in NaHSO3-H2O2-ZnS QDs-Acid system. The optimum volume for HCl, H2O2 and NaHSO3 was 100 µL while for ZnS QDs the volume effect was negligible. The optimum concentration (as specified in Figure 3) for each reagent was used. Each point is a mean of triplicate readings. Figure 5. EPR spectra of NaHSO3-H2O2 system in the presence of ZnS QDs in acidic medium. (A)

DMPO/•OH/•SO3−

ZnSQDs−H2O2−H2SO4−NaHSO3

adducts (iii)

(i)

ZnSQDs−H2SO4−H2O2 ZnSQDs−H2O2−NaHSO3

(ii) (iv)

H2O2−H2SO4−NaHSO3 (B) ZnSQDs−H2O2. Conditions: DMPO and H2SO4 were 0.1M 0.1 M, NaHSO3 were 1.5 M and H2O2 was 1 M while TEMP was 10% by volume. The volume of each reagent was 50 µL and all the solutions were prepared in water. Figure 6. CL spectrum of NaHSO3-H2O2-ZnS QDs-H2SO4 system. The concentration of H2O2 and H2SO4 was 1 M while that of NaHSO3 was 1.5 M. The flow rate of each reagent was 3 mL/min. Figure 7. The proposed mechanism for the CL enhancement of NaHSO3-H2O2 system by ZnS QDs in acidic medium.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Table of contents (TOC)

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