Flow-Injection Chemiluminescence Determinationof Lisinopril Using Luminol–KMnO4 Reaction Catalyzedby Silver Nanoparticles

YINHUAN LI,* YANKUN LI, and YUN YANGSchool of Science, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China

A simple, sensitive, and rapid chemiluminescence method combined with

flow-injection analysis was developed for the determination of lisinopril. It

was found that the chemiluminescence (CL) signal arising from the

reaction of alkaline luminol with KMnO4 could be significantly increased

by addition of lisinopril in the presence of silver nanoparticles. The

experimental parameters affecting the chemiluminescence reaction were

carefully studied and the chemiluminescence reaction mechanism was

briefly discussed. Under the selected conditions, the chemiluminescence

intensity was proportional to the concentration of lisinopril ranging from

0.1 mg/L to 20.0 mg/L. The detection limit was 0.027 mg/L lisinopril and

the relative standard deviation for 4.0 mg/L lisinopril solution was 1.9%

over eleven repeated measurements. The proposed method was applied to

the determination of lisinopril in tablets and spiked human urine samples.

Index Headings: Lisinopril; Chemiluminescence; Flow injection; Silver

nanoparticles; Ag nanoparticles.

INTRODUCTION

Lisinopril, chemically named as [N2-[(1S)-1-carboxy-3-phenylpropyl]-L-lysyl-L-proline dehydrate (Fig. 1), is an orallyactive angiotensin-converting enzyme inhibitor primarily usedin the treatment of hypertension, congestive heart failure, heartattack, and in preventing renal and retinal complications ofdiabetes.1 Its action is based on reducing the concentration ofagents that tighten the blood vessels so that blood flows moresmoothly and the heart can pump blood more efficiently.1

Adverse effects of lisinopril include infection, oliguria,anaphylaxis, jaundice, and syncope. Lisinopril is not signifi-cantly metabolized in humans; the absorbed drug is primarilyexcreted unchanged in urine. Therefore, taking into account itspracticability and possible health effects, the development ofsimple, sensitive, and rapid detection methods for thedetermination of lisinopril is of importance for industrialquality control and clinical monitoring. Various analyticaltechniques have been reported for the determination oflisinopril in pure form, dosage forms, and biological fluids,including spectrophotometry,2,3 fluorimetry,4 polarography,5

liquid chromatography with mass spectrometry detection (LC-MS),6 liquid chromatography with fluorescence detection (LC-F),7 and capillary electrophoresis (CE).8 The analytical meritsof the reported methods for the determination of lisinopril aresummarized in Table I.

‘‘Chemiluminescence’’ (CL), the emission of radiation as aresult of a chemical reaction, has been intensively investigatedfor its promising advantages of sensitivity, rapidity, extensive

dynamic range, and generally inexpensive instrumentation.9

Traditionally, studies of CL have been limited to somemolecular systems. Recently, more and more attention hasbeen paid to nanomaterials participating in CL systems.10 Inthese systems, nanomaterials participate in CL reactions as acatalyst,11,12 reductant,13 CL emitter,14 and energy acceptor.15

Low redox potential and high chemical activity promise silvernanoparticles (AgNPs) excellent catalytic activities. AgNPshave been used as a CL catalyst in the luminol–H2O2

reaction,16,17 luminol–K3Fe(CN)3–terbutaline sulfate reac-tion,18 and luminol–isoniazid reaction.19 However, the catalyticbehavior of AgNPs in CL reactions other than those mentionedabove still remains unknown.

We here find that AgNPs can enhance the CL signal from thereaction between alkaline luminol and KMnO4. The enhancingeffect is further enlarged in the presence of lisinopril. However,it is interesting to find that lisinopril alone doesn’t display anyenhancement of the luminol–KMnO4 reaction in the absence ofAgNPs. Based on these observations, the experimental condi-tions for the reaction were optimized and a flow-injection CLmethod for the determination of lisinopril was developed. Theapplication of the proposed method in the analysis of lisinoprilin tablets and spiked human urine samples was performed. Thepossible CL reaction mechanism was also discussed.

EXPERIMENTAL

Apparatus. Chemiluminescence measurements were per-formed using a model IFFL-D flow injection CL analyzer(Xi’an Remex Eletronic Science-Tech Co. Ltd, China) and theschematic diagram of the CL flow system is shown in Fig. 2.Polytetrafluoroethylene (PTFE) tubing (0.8 mm i.d.) was usedto connect the components in the flow system. All solutionswere propelled with peristaltic pumps, each channel at a flowrate of 2.1 mL/min. Injection was carried out with the aid of asix-way valve equipped up with a 50 lL sample loop. The flowcell was made by coiling a 20 cm length of colorless glasstubing (1 mm i.d.) into a spiral disk shape (3.5 turns) andplacing it close to the photomultiplier tube. The CL signalproduced in the flow cell was detected with a CR105photomultiplier tube (Beijing Hamamatsu Photo Techniques

FIG. 1. Molecular structure of lisinopril.

Received 2 September 2010; accepted 20 December 2010.

* Author to whom correspondence should be sent. E-mail: liyh@mail.xjtu.edu.cn.

DOI: 10.1366/10-06115

376 Volume 65, Number 4, 2011 APPLIED SPECTROSCOPY0003-7028/11/6504-0376$2.00/0

� 2011 Society for Applied Spectroscopy

Inc.). CL data acquisition and treatment was performed usingthe IFFL-D data processing system.

The CL spectra were obtained with a refitted RF 5301fluorescence spectrophotometer (Shimadzu Corporation, Ja-pan) with the excitation source shutter closed. Absorptionspectra were taken on a TU-1901 ultraviolet–visible (UV-Vis)spectrophotometer (Beijing Currency Instrumental Ltd, China).

Chemicals. All solutions were prepared from analytical-grade chemicals with doubly distilled water. Lisinoprilstandard was obtained from the National Institute for theControl of Pharmaceutical and Biological Products (Beijing,China). Luminol was kindly offered by the Institute ofAnalytical Science of Shaanxi Normal University (purity.95%, Xi’an, China). Sodium borohydride was purchasedfrom Shanghai Shanpu Chemical (Shanghai, China). Silvernitrate, KMnO4, and sodium citrate were purchased from Xi’anChemical (Xi’an, China).

Lisinopril standard solution (500.0 mg/L) was prepared indoubly distilled water. This solution was stored at 4 8C in arefrigerator and protected from light. More dilute solutionswere freshly prepared from the stock solution by appropriatedilutions with water. Luminol solution (5.0 3 10�5 mol/L) wasprepared by diluting 1.0 3 10�2 mol/L luminol stock solution(prepared in 1.0 3 10�2 mol/L NaOH solution) with the desiredconcentration of NaOH solution. KMnO4 solution (2.0 3 10�2

mol/L), AgNO3 (0.1 mol/L), sodium citrate solution (1%,w/w), and sodium borohydride (0.1 mol/L) were prepared withdoubly distilled water before use.

Synthesis of Silver Nanoparticles. Silver nanoparticleswere prepared according to the method reported by Creightonet al.20 Briefly, a 25 mL 1 3 10�3 mol/L AgNO3 solution wasadded dropwise into 75 mL 2 3 10�3 mol/L sodiumborohydride solution with simultaneous vigorous stirring.Ten minutes later, a 5 mL 1% sodium citrate solution wasadded to stabilize the colloid. The colloid was stirred foranother 20 min and aged for two days at 4 8C before use. Thesize and dispersity of as-prepared AgNPs were determined

using a JEM-2100 transmission electron microscope (JEOL,Japan). The TEM image of the as-prepared AgNPs is shown inFig. 3; the diameter of the particles was 10–20 nm. Theconcentration of the as-prepared AgNPs was 2.38 3 10�4

mol/L, based on the concentration of AgNO3 solution used forthe preparation of AgNPs.

Chemiluminescence Measurements. As shown in Fig. 2,flow lines were inserted into the luminol solution, KMnO4

solution, AgNP solution, and lisinopril solution, respectively.Peristaltic pumps were started to deliver all solutions in the flowsystem. After a stable baseline was obtained, 50 lL of the mixtureof alkaline luminol with KMnO4 was injected into the mergedstream of AgNPs and lisinopril by a six-way valve to produce CL.The determination of lisinopril was based on the increment in theCL intensity, calculated as DI¼ I� Ib, where I is the CL signal inthe presence of lisinopril and Ib is the blank signal.

RESULTS AND DISCUSSION

Chemiluminescence of Lisinopril in Luminol–KMnO4

Reaction Catalyzed by Silver Nanoparticles. The CL

TABLE I. Comparison of various methods for the determination of lisinopril.

Methods Reagents used Linear range (mg/L) Detection limits (mg/L) Ref.

Spectrophotometry Hydroxylamine hydrochloride, dicyclohexylcarbodiimide;derivatization at 40 8C for 10 min

0.568 2

1-Fluoro-2,4-dinitrobenzene derivatization at 60 8C for 45 min 20.0–110.0 3Fluorimetry o-Phthalaldehyde 0.3–10.0 0.1 4Polarography Nitrous acid 0.1–20.0 0.02 5HPLC-MS 0.0025–0.32 0.0025 6LC-F Fluorescamine 0.005–0.2 for plasma and

0.025–1.0 for urine0.002 for plasma and

0.01 for urine7

CE 26–520 8CL Luminol, KMnO4, AgNPs 0.1–20.0 0.027 This method

FIG. 2. Schematic diagram of the CL flow system. FIG. 3. TEM image of the as-prepared Ag nanoparticles.

APPLIED SPECTROSCOPY 377

behaviors of lisinopril in the luminol–KMnO4 reaction withand without AgNPs were thoroughly investigated using a staticmethod. As shown in Fig. 4, no obvious difference in CL signalwas observed upon the injection of 0.5 mL alkaline solution of5.0 3 10�5 mol/L luminol into 0.5 mL 7.5 3 10�5 mol/LKMnO4 solution (peak a) and into 0.5 mL of the mixture of 7.53 10�5 mol/L KMnO4 with 1.0 mg/L lisinopril (peak b), whichsuggested that lisinopril alone couldn’t enhance the CL signalfrom the luminol–KMnO4 reaction in the absence of AgNPs.The CL signal taken from the mixture of 7.5 3 10�5 mol/LKMnO4 with 5.0 3 10�5 mol/L AgNPs (peak c) wasremarkable higher than that of 7.5 3 10�5 mol/L KMnO4

solution alone (peak a). Obviously AgNPs displayed acatalyzing effect on the CL reaction of luminol with KMnO4.It was interesting to find that this catalyzing effect could befurther enhanced in the presence of lisinopril (peak d).However, only very weakly enhanced CL was observed whenAgNPs were replaced by the supernatant of AgNPs (peak e),the precipitation of AgNPs by 0.1 mol/L KNO3, indicating thatthe CL caused by the concomitants in the solution could beignored. Accordingly, the enhancement of Ag colloid on theCL did not arise from the concomitants in the colloid but fromAgNPs. It was found that the rate of the CL reactions withAgNPs was fast; from reagent mixing to the peak maximumrequired only 2 s and it took about another 60 s for the signal todecline to baseline.

Optimization of Experimental Conditions. Flow rate is acritical parameter in the flow-injection-based CL detectionsystem. It influences the degree of diffusing and mixing of thereactants, determines the time that is taken from the finalmixing point of the reagents (six-way valve) to the point thatCL signal is detected (flow cell), and consequently influencesthe CL signal. The effect of flow rate in the range of 1.4 to 2.8mL/min was examined. The signal-to-blank ratio increasedwith increasing flow rate from 1.4 mL/min to 2.1 mL/min,above which the signal-to-blank ratio decreased. Therefore, aflow rate of 2.1 mL/min was employed.

The luminol CL reaction occurs well in an alkaline medium.

In this work the alkalinity of the reaction medium is controlledby adjusting the NaOH concentration in the luminol solution.The effect of NaOH concentration was tested in the range of0.01 to 0.4 mol/L (Fig. 5a). The experiments showed that 0.025mol/L NaOH provided the maximum signal-to-blank ratio.

Luminol concentration plays a critical role in the CLreaction. Both the CL signal and the blank signal increased asthe luminol concentration increased from 1.0 3 10�5 mol/L to2.0 3 10�4 mol/L (Fig. 5b). The reaction has the maximumsignal-to-blank ratio at 2.5 3 10�5 mol/L luminol. Finally, 5.03 10�5 mol/L luminol was employed as a compromise betweenthe signal-to-blank ratio and the magnitude of the CL signal.

Figure 5c depicts the effect of 5.0 3 10�6 to 7.5 3 10�5

mol/L AgNPs on the signal-to-blank ratio. The signal-to-blankratio continued to increase by increasing the AgNP concentra-tion up to 5.0 3 10�5 mol/L. Concentrations greater than 5.0 310�5 mol/L AgNPs decreased the signal-to-blank ratio.Therefore, 5.0 3 10�5 mol/L AgNPs was selected as theoptimum.

The effect of KMnO4 concentration on the signal-to-blankratio is shown in Fig. 5d. As KMnO4 concentration increasedfrom 5.0 3 10�5 mol/L to 7.5 3 10�5 mol/L, the signal-to-blankratio increased. The maximum signal-to-blank ratio wasobserved when 7.5 3 10�5 mol/L KMnO4 was used.

Analytical Figures of Merit. Under the optimum condi-tions, the calibration graph of the increment in CL intensity(DI) versus lisinopril concentration (c, mg/L) was linear in therange of 0.1–20.0 mg/L. The regression equation was DI ¼235.49cþ 174.08, with a correlation coefficient of 0.9977. Therelative standard deviation was 1.9% for 4.0 mg/L lisinoprilsolution over eleven repeated measurements. The detectionlimit, calculated as the concentration of lisinopril yielding a CLsignal greater than three times the standard deviation of theblank signal, was 0.027 mg/L.

Interference. The effect of foreign species was studied byanalyzing 1.0 mg/L lisinopril standard solution to whichincreasing amounts of foreign species were added. Thetolerance level was established as the maximum amount offoreign species that produced a relative error in the measure-ment not exceeding 65%. No interference could be foundwhen including up to 100-fold glucose, sucrose, glycin,dextrine, starch, SO4

2�, CO32�; 10-fold hydrochlorothiazide,

PO43�, Al3þ, 1-fold Ca2þ, Mg2þ, Cu2þ, Zn2þ, Ni2þ Fe2þ; or 0.2-

fold ascorbic acid and uric acid. Concentrations higher than thetolerance limit of Ca2þ, Zn2þ, and Ni2þ produced a positiveinterference on the signal, whereas ascorbic acid, uric acid,Mg2þ, Cu2þ, and Fe2þ produced a negative effect.

Determination of Lisinopril in Tablets. The average tabletweight was obtained from the weight of ten tablets. They werefinely powdered and homogenized, and a portion of thepowder, equivalent to about 20 mg of lisinopril, was accuratelyweighed and dissolved in 30 mL of water. The resultingmixture was filtered and the filtrate was diluted with water to50 mL for further sample analysis. The results are shown inTable II. Statistical analysis of the results by the t-test showedthat there were no significant differences between the proposedmethod and the Chinese pharmacopoeia method based on high-performance liquid chromatography (HPLC)21 at a confidencelevel of 95%.

Determination of Lisinopril in Spiked Urine Samples.Urine samples were offered by three healthy volunteers. Into0.50 mL urine sample, known amounts of lisinopril standard

FIG. 4. CL curves of the injection of 0.5 mL alkaline solution of 5.0 3 10�5

mol/L luminol into 0.5 mL of (a) 7.5 3 10�5 mol/L KMnO4 solution, (b) themixture of 7.5 3 10�5 mol/L KMnO4 with 1.0 mg/L lisinopril, (c) the mixtureof 7.5 3 10�5 mol/L KMnO4 with 5.0 3 10�5 mol/L AgNPs, (d) the mixture of7.5 3 10�5 mol/L KMnO4 with 5.0 3 10�5 mol/L AgNPs and 1.0 mg/Llisinopril, and (e) the mixture of 7.5 3 10�5 mol/L KMnO4 with the supernatantof AgNPs and 1.0 mg/L lisinopril.

378 Volume 65, Number 4, 2011

solution and 0.05 g Na2CO3 (to remove reducing substances)22

were added. The resulting mixture was diluted to 5.0 mL withwater, heated in a 60 8C water bath for 10 min, and centrifugedat 4000 rpm for 30 min. One milliliter (1.0 mL) supernatantwas then diluted to 50 mL with water for analysis. Blankexperiments were performed according to the same procedurewithout adding lisinopril. The results showed that with theabove pretreatment procedure, the CL signal from the urinesample without adding lisinopril showed no significant

difference from that of water, which indicated that the

interfering substances existing in the urine sample were

effectively eliminated by the present pretreatment procedure.

Table III shows the results and the recoveries are satisfactory.

FIG. 5. Effect of the concentrations of reagents on the CL reaction. (a) 1.0 3 10�4 mol/L luminol, 1.0 3 10�4 mol/L KMnO4, 5.0 3 10�5 mol/L AgNPs, 2.1 mL/minflow rate, 1.0 mg/L lisinopril; (b) 0.025 mol/LNaOH, 1.0 3 10�4 mol/L KMnO4, 5.0 3 10�5 mol/L AgNPs, 2.1 mL/min flow rate, 1.0 mg/L lisinopril; (c) 5.0 3 10�5

mol/L luminol, 0.025 mol/L NaOH, 1.0 3 10�4 mol/L KMnO4, 2.1 mL/min flow rate, 1.0 mg/L lisinopril; (d) 5.0 3 10�5 mol/L luminol, 0.025 mol/L NaOH, 5.0 310�5 mol/L AgNPs, 2.1 mL/min flow rate, 1.0 mg/L lisinopril.

TABLE II. Determination of lisinopril in tablets.

Samples Labeled (mg)Proposed

method (mg) RSD (n ¼ 3)Pharmacopoeiamethod (mg)21

Tablet 1 10 10.06 1.6% 9.71Tablet 2 10 9.94 1.7% 10.12Tablet 3 10 10.63 1.4% 10.76

TABLE III. Determination of lisinopril in spiked urine samples.

Samples Added (mg/L) Found (mg/L) RSD (n ¼ 3) Recovery

No. 1 0.0 0.02.0 2.05 0.9% 102.5%4.0 3.83 1.1% 95.8%

No. 2 0.0 0.02.0 1.98 1.4% 99.0%4.0 3.84 2.5% 96.0%

No. 3 0.0 0.02.0 1.91 2.4% 95.5%4.0 4.40 1.8% 110.0%

APPLIED SPECTROSCOPY 379

Mechanism Discussion. Chemiluminescence spectra of thereactions were obtained by continuously pumping solutions ofluminol, KMnO4, AgNPs, and lisinopril into a Y-piecepositioned immediately prior to the flow cell of a model RF5301 spectrofluorimeter with the excitation source shutterclosed and in scanning made from 300 nm to 650 nm (Fig. 6).All CL spectra were identical with a maximum emissionwavelength at about 425 nm, which indicated that they had thesame CL emitter, the excited state of the 3-aminophthalateanion.23 Moreover, the CL intensity of the luminol–KMnO4–AgNP reaction (peak b) is obviously higher than that of theluminol–KMnO4 reaction (peak a), indicating the catalyzingeffect of AgNPs on the luminol–KMnO4 reaction. Whenlisinopril was present in the luminol–KMnO4 system catalyzedby AgNPs (peak c), the CL intensity increased dramatically.Based on these facts, we propose that the enhanced CL signalsare thus ascribed to the possible catalysis from AgNPs.

The purple color of the KMnO4 solution changed graduallyinto green when alkaline luminol solution was added into theKMnO4 solution. This result indicated that during the reactionof luminol with KMnO4, KMnO4 was firstly reduced toK2MnO4,24 and luminol was oxidized to 3-aminophthalateanions.25 When lisinopril was added into the above reactionmixture, the color of the solution changed from green to brown,which suggested that lisinopril could further react withK2MnO4 and K2MnO4 was further reduced to MnO2.24 Figure7 depicts the UV-Vis absorption spectra of (a) 5.0 3 10�5

mol/L luminol solution, (b) 5.0 3 10�5 mol/L AgNPs solution,(c) 7.5 3 10�5 mol/L KMnO4 solution, (d) the mixture of 7.5 3

10�5 mol/L KMnO4 with 5.0 3 10�5 mol/L luminol, and (e) thereaction mixture of 5.0 3 10�5 mol/L luminol, 7.5 3 10�5

mol/L KMnO4, 5.0 3 10�5 mol/L AgNPs, and 10.0 mg/Llisinopril. When KMnO4 was mixed with luminol, theabsorbance peaks of KMnO4 at 525 nm and 545 nm decreasedobviously and one new absorbance peak at about 600 nmappeared in the spectrum. The new absorbance peak at about600 nm was assigned to K2MnO4.24 For the absorbancespectrum of the reaction mixture (curve e), the absorbance peakof AgNPs at 392.0 nm decreased significantly, which revealed

that the structure of AgNPs was destroyed and it participated inthe reaction.

It was reported that the decomposition of MnO4� to MnO4

2�

in alkaline conditions is a free radical reaction. The possiblemechanism of the reaction can be expressed in reactions (1)through (5) given in Scheme 1.26 The produced reactiveoxygen species can oxidize luminol in alkaline conditions toproduce CL.27,28 When metal nanoparticles were used as thecatalysts in CL reactions, the formation of active oxygen-containing reactant intermediates such as �OH and O2

�� werefrequently reported (reaction 6).11,16,17,29,30 So, in this system itcan be deduced that the enhancement effect on CL by AgNPswas also attributed to the formation of �OH and O2

��. Thishypothesis was supported by the experiments showing thatabout 73% of the CL signal was inhibited by 1.0 3 10�5 g/mLascorbic acid and about 99% of CL signal was inhibited by 1.03 10�4 g/mL ascorbic acid, which is a common oxygen radicalscavenger.31 The generated �OH was stabilized on the surfaceof AgNPs via partial electron exchange interactions.32 �OHradicals reacted with luminol anion generating luminolradicals. It was followed by the reaction between O2

�� andluminol radicals and generated luminol intermediate, whichcould enhance the CL of luminol.27

The reaction of alkaline luminol with AgNPs produced veryweak CL signal in the absence of KMnO4. This weak CLsignal could be slightly enhanced in the presence of lisinopril.But the enhancement was observably lower than that in thepresence of KMnO4, about 1.3% in enhancement of the latterfor 1.0 mg/L lisinopril.

Cui et al. reported that some adsorbates such as I�, Br�,cysteine, mercaptoacetic acid, mercaptopropionic acid, andthiourea could decrease the oxidation potential of AgNPs andinduce an enhancement in the AgNP-involved CL reaction.33,34

We suppose that lisinopril may have a similar role sincelisinopril can adsorb on the surface of AgNPs through its –NH2,–NH–, and –COO– groups.35,36 The adsorption of lisinopril onAgNPs decreases the oxidation potential of AgNPs and results

FIG. 6. CL spectra of the reactions of (a) 5.0 3 10�5 mol/L luminolþ 7.5 310�5 mol/L KMnO4, (b) 5.0 3 10�5 mol/L luminolþ 7.5 3 10�5 mol/L KMnO4

þ 5.0 3 10�5 mol/L AgNPs , and (c) 5.0 3 10�5 mol/L luminolþ 7.5 3 10�5

mol/L KMnO4þ 5.0 3 10�5 mol/L AgNPsþ 1.0 mg/L lisinopril.

FIG. 7. Ultraviolet–visible absorption spectra of (a) 5.0 3 10�5 mol/L luminolsolution, (b) 5.0 3 10�5 mol/L AgNPs solution, (c) 7.5 3 10�5 mol/L KMnO4

solution, (d) the mixture of 7.5 3 10�5 mol/L KMnO4 with 5.0 3 10�5 mol/Lluminol, and (e) the reaction mixture of 5.0 3 10�5 mol/L luminol, 7.5 3 10�5

mol/L KMnO4, 5.0 3 10�5 mol/L AgNPs, and 10.0 mg/L lisinopril.

380 Volume 65, Number 4, 2011

in an enhancement in the CL signal of the luminol–AgNPs

reaction and luminol–AgNPs–KMnO4 reaction.

The CL mechanism of the reaction may be attributed,

therefore, to the reactions shown in Scheme 1.

CONCLUSION

In summary, silver nanoparticles were found to enhance the

CL reaction between luminol and KMnO4, and lisinopril

displays a significant enhancement in the luminol–KMnO4–

AgNPs CL reaction. Based on these findings, the experimental

conditions were carefully studied and a flow-injection CL

method was established for the determination of lisinopril for

the first time. The proposed method is simple, sensitive, and

rapid and has been applied to the analysis of lisinopril in tablets

and spiked human urine samples. At the same time, a possible

CL reaction mechanism was proposed by the study of CL

spectra, UV-Vis spectra, and related references.

ACKNOWLEDGMENT

The authors gratefully acknowledge financial support from Xi’an JiaotongUniversity (Grant No. 08140009).

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SCHEME 1. Suggested CL reaction mechanism.

APPLIED SPECTROSCOPY 381