8/13/2019 Martin Narayanaswamy Chlorine

1/4

ELSEVIER Sensors and Actuators B 38-39 (1997) 330-333

SEflSORsACT@Ons

Studies on quenching of fluorescence of reagents in aqueous solutionleading to an optical chloride-ion sensorA. Martin, R. Narayanaswamy *

Department of Instrumentation and Analytical Science, UMJST, PO Box 88, Manchester M60 IQD, UK

AbstractThe sensitiv ity of quinoline- and acridine-type indicators towards the detection of low levels of chloride ions in aqueous and amine-dopedmedia, by fluorescence quenching, is reported in this paper. Solution studies are based on the fluorescence of quinine sulphate, acridine and3- (6-methoxyquinolino) propanesulphonate (SPQ) and the fluorescence response of these ndicators has been examined at different pH valuesand at various chloride ion concentrations in aqueous media.

Keywords: Chloride ions; Fluorescence quenching; SPQ

1. IntroductionThe importance of chloride-ion detection involves a widearea including pure, environmental, bio- and industrial chem-istry. Industrial monitoring of chloride ions in feedwater isnecessary since chloride ions are a major cause of corrosion

in the metal components of steam-generating systems. Cer-tain industrial specifications require the limit of detection forchloride ions in feedwater to be 1.7-57 FM and the feedwatertypically contains amines, dissolved oxygen, carbon dioxideand trace levels of copper and iron with a pfl range of 5-10.Many industries also requires the detection systems o be n-line for real-time process-control applications.Traditional methods of chloride-ion detection nvolve titra-tions using silver nitrate, silver fluoresceinate and silver chro-mate [ 1,2]. Titrations can be time consuming with theend-point often difficult to detect. Spectrophotometricmeth-ods involving reagents such as mercury thiocyanateiron( III) ,mercury chloranilate and mercury diphenylcarbazide can behazardous due to their toxicity [ 3-51.Fluorescence ntensity sensing s an establishedanalyticalmethod due to its high sensitivity and selectivity for a widevariety of analytes. Fluorescence quenching is a commontechnique used n the detection of gasesand metal ions [ 6,7].Collisional quenching by anions, such as alkyl halides, ofquinine- and acridine-type fluorophores has been previouslyreported [ 81. The fluorescence quenching has been related

* Corresponding author. Phone: f44 161200 4891. Fax: +44 1612004911.0925-4005/97/$17.00 0 1997 Elsevier Science S.A. All rights reservedPIISO925-4005(97)00044-O

to the halide concentration by Ihe Stern-Volmer equation[9]. This reaction has been adapted or fibre-optic chemicalsensing of chloride ions for analytical and clinical purposes[ lo]. A mechanism nvolving the fluorescencequenching ofN-( 6-methoxyquinolyl) acetoethyl ester (MQAE) by chlo-ride ions has beenutilized in the development of a fibre-opticprobe for measuring chloride in aqueoussolution [ 111.In this paper we describe a comprehensivesolution studybased on the collisional fluorescence quenching of quininesulphate, acridine and 3 (6methoxyquinolino) propane-sulfonate (SPQ) by low levels of chloride ions in aqueousmedia. The fluorescencestudies n aqueoussolution are pre-liminary and indispensable steps or a later sensordevelop-ment basedon this principle.

2. ExperimentalQuinine sulphate (BDH), acridine (BDH) and SPQ

(Sigma) were used as purchased. All quinine sulphate andacridine solutions were prepared. n 0.05 M sulphuric acid,while SPQ solutions were prepared in deionized water(Elgastat). The concentration of all the Eluorophore stocksolutions was 10 PM. The stock amine (2-amino-2-methyl-l-propanol, AMP, (97% Aldrich) ) solution (200 mg 1-I)was prepared n deionized water j.Yornwhich a working solu-tion of AMP (20 mg 1-l) was made. Potassium chloride(Aldrich) dissolved in either AMP or deionized waterwas used to prepare standards of various chloride-ionconcentrations.

8/13/2019 Martin Narayanaswamy Chlorine

2/4

A. Martin, R. Narayanaswamy/Sensors andActuators B 38-39 (1997) 330-333 331The solution studies were carried out on the Perk&ElmerLS-5 Luminescence Spectrometer using 90 excitation andemission alignment. In the solution studies a fixed volume o f3.1 ml was maintained and all measurementswere performedat room temperature. pH measurements were made using ahand-held meter (Mettler Toledo-Checkmate 90).

3. Results and discussionBoth quinine sulphate and acridine exhibit fluorescence nethanol, but to be selectively quenched by haiide ions (chlo-ride) they must be in their protonated form. Both indicatorswere protonated in 0.05 M sulphuric acid as describedpre-viously [8,12]. Quinine sulphate will not fluoresce inhydrochloric acid due to its fluorescence quenching effect[ 131. SPQ is a neutral fluorophore and is therefore not pHdependent. It can fluoresce in acid, alkali, alcohol or water.Thus SPQ solutions were prepared n deionized water.To determine the effect o f amine-doped feedwater on thefluorescenceofthe indicators, excitation andemissionspectrawere recorded in its presence. These spectra comparedfavourably with previous work [9,14]. Table 1 shows theexcitation and emission wavelengths for the indicators.The AMP working solution (20 ppm) is alkaline, whereasdeionized water is almost neutral (pH 6.30). The pH of 0.05M sulphuric acid is 1.3. Table 2 shows the change in pH o fthe indicators in their appropriate solvents when added toAMP and deionized water. The sulphuric acid in which qui-nine sulphate and acridine are prepared protonates the weakAMP, The pH of AMP was not affectedwhen SPQwas added.No spectral shift or change n peak shapewas observed n the

spectra, indicating that the presence of AMP in the solventhad no effect on the fluorophores. A shift in wavelength ofthe emission spectrum observed in different solvents is acommon phenomenon when solvent polarity is changed.Water is more polar than alcohol, but in this case AMP ispresent at very low concentrations (20 ppm) and hence thesolvent properties are more characteristic of water.Table 1Excitation and emission wavelengths of quinine sulphate, acridine and SPQ

The fluorescence ntensities of fluorophore solutions havebeen experimentally observed o stabilize and then decreaseover a period of time. For fluorescence quenching the fluo-rophore solution should not be degrading, hence t is impor-tant to use the fluorophore solution only when thefluorescence ntensity is stable. The aim of this study was todetermine experimentally the time it takes for the fluores-cence of the solution to stabilize and therefore the optimumtime for use.All solutions were stored n the dark to minimizeany photobleaching effects due to ambient light. The fluores-cence ntensities of quinine sulphate, acridine and SPQ pre-pared in their appropriate solvents were monitored at 24 hintervals over a 96 h period. After 24 h the intensities of allthe fluorophores were noted to increase.For acridine, aperiodof 24 h appeared o be the optimum age. The excitation andemission intensities of SPQ and quinine sulphate remainedrelatively constant after 24 h. While SPQ appeared o be themost stable fluorophore, acridine was the least stable. It wastherefore decided o use only fluorophore solutions that havebeen stored for a period o f 24 h for further studies.To determine the most sensitive fluorophore towards lowlevels of chloride ions, a fluorescence quenching study wascarried out. By using the Stern-Volmer equationF,/F= Ksv[Q] + 1where F,, is the fluorescence ntensity in the absenceof thequencher, F is the fluorescence ntensity in the presenceofthe quencher, KS, is the Stern-Volmer quenching constantand Q is the concentration o f the quencher, calibration graphsfor quinine sulphate, acridine and SPQ were obtained(Figs. l-3). From thesecalibration graphs he detectionlimitof chloride ions, defined as the concentration equivalent toF,,/ (F. - 3sd) where sd s the standard deviation of the blank(Fo) , could be calculated. The detection limits, although the-oretical values [ 15,161, are valuable as a comparison of thedifferent detection processes mployed. Therefore, the detec-tion limits for the fluorescencequenching processof quininesulphate, acridine and SPQ with the experimental values ofthe lowest measurable concentration of chloride ions and the

Indicator Quinine sulphate(0.05 M H,SO,) A&dine(0.05 M H,SO,) SPQ(water)Aqueous medium waterExcitation wavelength (nm) 360Emission wavelength (nm) 465Table 2pH measurements for quinine sulphate, acridine and SPQ

AMP water362 367465 49.5

AMP water AMP367 330 330492 463 465

IndicatorSolventAqueous medium AMPPH 9.79 H2O6.30

QSH&bHz1.33

AC SPQ QSH,SO, H,O H804H,SO, H,O AMP1.31 5.37 2.47

HzWJ3202.32

ACWO4AMP2.46

SPQH,SO, Hz0 Hz0r-l,0 AMP Hz02.38 9.30 5.22

8/13/2019 Martin Narayanaswamy Chlorine

3/4

332 A. Martin, R. Narayanaswamy/Sensors and Actuators 3 38-39 (1997) 330-333

Fig. 1. Collisional quenching of quinine suiphate by chloride ions. (Solidand dashed ines are the regression plots.)

b 2'5 3'0 3'5 4'0 45 60cmc chloride on (ppm)0.5-l I I * 2 * I0 5 10 15 20 25 30 35 40 45Mnc chloride on?3ppm)

Fig. 3. Collisional quenching of SPQ by chloride ions. (Solid and dashedlines are the regression plots.)

1.170- +1.145- - N&r - y,+&?r A A$p ......' .Qfp

1.120-

0 5 10 15 20 25 30 35 40 45 50ME chloride ons (ppm)

bilized indicator in solution. This reduction was explained byimmobilization limiting the mobility of the fluorophore andtherefore lowering the probability of the quenching processoccurring. Krapf et al. [ 171 used SPQ to detect ntercellularchloride ion via fluorescencequenching, with a KS, value of118M - . Kar and Arnold have determined the KS, for chlo-ride-ion quenching of MQAE as 199M - [ 111. n our workwe have found that the sensitivity of SPQ quenching to chlo-ride ions (KS,) was greatly enhanced (717 M-l) withoutthe aid of preconcentration techniques,which could be agreatadvantage for sensing applications.

Fig. 2. Collisional quenching of acridine by chloride ions. (Solid and dashedlines are the regression plots.)I&v values are given in Table 3. The lowest measurablecon-centration values are higher than the detection limits. Thelowest measurable concentrations of chloride ions are realvalues and hence more useful in comparing the differentdetection processesemployed in the working system. TheKS, values indicate the order of sensitivity of the differentfluorophores for chloride ions. The larger the KS, value, themore sensitive the fluorophore is to chloride ions. The orderof sensitivity was found to be SPQ>quinine sulphateB acridine.The fluorescence quenching of quinoline and acridinederivatives, such as SPQ, by halide ions is not a novel phe-nomenon, and has beenpreviously used n solution and sensorapplications. Wolfbeis and coworkers [lo] applied thisquenching system to develop an optical sensor or continuousmonitoring of halide ions with a KS, value of 118 M- forchloride ions and a limit of detection of 10 r&I. This KS,value was around lO-20% lower than thoseof the non-immo-

It is well known that bromide and iodide quench the fluo-rescenceof the dyes investigated in this work much morestrongly than the chloride ion [ lo]. However, interferencestudies were not carried out in this investigation due to thefact that the feedwater contains only chloride ions.

4. ConclusionsThe presenceof AMP at low concentrations did not affectthe fluorescencequenching of quinine sulphate, acridine andSPQ. While SPQ s a neutral fluorophore, the fluorescenceofquinine sulphate and acridine were quenchedby ch1orid.eonsin their acidified forms. The dilute sulphuric acid in whichquinine sulphate and acridine were prepared protonated theAMP solution; hencequenching by chloride ions occurred.Quinine sulphate and SPQ were found to be photostable,whereas the fluorescence ntensity of acridine was found todecrease fter 24 h. Subsequently all solutions were stored nthe dark for 24 h prior to use, herefore etting thelhiorophores

diffuse and equilibrate in solution.Table 3Detection limits and Stem-Volmer quenching constants for quinine sulphate, acridine and SPQIndicator Quinine sulphate Acridine SPQ(0.05 M H2S04) (0.05 M H,SO,) (water)Aqueous medium water AMP water AMP water AMPDetection limit (mM) 0.5 0.6 0.9 1 0.1 0.1Lowest measureable concentration (mM) 0.7 0.8 1 1.1 0.3 0.3f&v Of-) 213 213 112 111 717 717

8/13/2019 Martin Narayanaswamy Chlorine

4/4

A. Martin, R. Narayanaswamy/Sensors and Actua tors 3 38-39 (1997) 330-333 333

SPQ was found to be the most sensitive fluorophore withthe lowest calculated detection limit and the lowest measur-able chloride-ion concentration. Immobilization of a lumi-phore onto solid supports can enhance the rigidity of themolecule and improve its luminescence characteristics, suchas ntensity and quantum yield [ 181, hus increasing the sen-sitivity of the lumiphore. This can lead to lower detectionlimits of the analyte to be determined. Solid surface analysisis most commonly applied to room-temperature phosphores-cence but can also be applied for fluorescence. Thereforestudies involving the immobilization of SPQ with a view tolowering the detection limit of chloride ions are being cur-rently performed. A detection of chloride ion in the range1.7-57 p,M in feedwater is required for the ntended ndustrialapplication. Investigations on the physical immobilization ofSPQ onto anion-exchange resins and chemical immobiliza-tion by covalent bonding to amine gels and entrapment intosol-gels are currently in progress with a view to developingan optical chloride-ion sensorwith the above detection imits.

AcknowledgementsThe authors acknowledge Rolls Royce and AssociatesLtd.for their financial support of this project.

References[l] AI. Vogel, A Text-book of Quantitative inorganic Analysis ,Longmans, London, 3rd edn., 1961.[2] F.D. Snell, PhotometricandFIuorometric Methods ofAnalysis-Non-metals, John Wiley, Chichester, 1981.[3] D.F. Boltz and J.A. Howell, Colourimetric Determination of Non

Metals, John Wiley, Chichester, 1978.[4] F.3. Krug, L.C.R. Pessenda, E.A.G. Zagatto, A.O. Jacintho and B.F.Reis, Spectrometric flow injection determination ofchlorldeinethanol,Anal. Chim. Acta, 130 (1981) 409413.1.51R.E. Humphrey and W.L. Hinze, Mercuric iodate as a analyticalreagent - determination of chloride by spectrophotometricmeasurement of mercuric chloride with phenolphthalein complexoneor xylenol orange, Anal. Chem, 45 (1973) 1747-1749.[6] M.G. Baron, R. Narayanaswamy and SC. Thorpe, A kineto-opticalmethod for the determination of chlorine gas, Sensors and ActuatorsB, 29 (1995) 3.58-362.

[7] M.H. Noire and B. Dureault, A ferrous ion optical sensor based onfluoresence quenching, Sensors and Actuators 3,29 (1995) 386-391.

[S] OS. Wolfbeis and E. Urbana, Fluorescence quenching method fordetermination of two or more components in solution, Anal. Chem.,55 (1983) 1904-1906.

[9] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, PlenumPress,London, 1983.[lo] E. Urbano, H. Offenbacher and O.S. Wolfbeis, Optical sensor forcontinuous determlnation of halides, Anal. Chem., 56 (1984) 427-429.[ll] S. Kar and M.A. Arnold, Fiber-optic chlorine probe based onfluorescence decay of N-(B-methoxyquinolyl)-acetoetbyl ester,

Talanta, 42 (1995) 663-670.[ 121 R.A. Velapoldi, Considerations on organic compounds in solution andinorganic ions in glasses as fluorescent standard reference materials,.J. Rex Nat. Bar. Stand. -A. Phys. Chem., 76A (1972) 641-654.[ 131 G.G. Stokes, Onacertain reaction ofquinine, J. Chem. Soc.,ZZ (1869)174-185.[ 141 LB. Berlman, Handbook of Fluorescence Spectra of AromaticMolecules, Academic Press,London, 2nd edn., 1971.[ 151 G.L, Long and J.D. Wineforder, Limit of detection - a closer look atthe IUPAC definition, Anal. Chem., 55 (1983) 713A-724A.[ 163 J.C. Miller and J.N. Miller, Statist ics for Analytical Chemistry, EllisHorwood, London, 3rd edn., 1993.[ 171 R. Krapf, C.A. Berry and AS. Verkman, Estimation of intercellularchloride activity in isolated perfused rabbit proximal convolutedtubules using a fluorescent indicator, Biophys. J., 53 (1988) 955-962.[IS] A. Sanz-Medel, Solid surface photoluminescence aud flow analysis: ahappy marriage, Anal. Chim. Acta , 283 (1993) 367-378.

BiographiesA. Martin obtained her I3 Sc. n chemistry with informationtechnology and instrumentation in 1993 at Glasgow Cale-donian University. She s currently studying towards a Ph.D.at the Department of Instrumentation and Analytical Science(DIAS) at UMIST.R. Narayanaswamy is currently a senior lecturer in DIASat UMIST. He obtained his Ph.D. in 1972 (Imperial College,

London, UK) in analytical chemistry and his D.Sc in 1995(University of London) in analytical science. He was a lec-turer in chemistry at the University of Sri Lanka, Peradeniya,Sri Lanka (1967-1978) and subsequently a postdoctoralresearch fellow at the University of Southampton, UK(1978-1981) and at the University of Warwick, UK ( 1982).He joined UMIST, UK, in 1983 as a senior postdoctoralresearch associate and became manager of the Optical Sen-sors ResearchUnit (1984-1987) and lecturer in instrumen-tation and analytical science (October 1984). He leads theresearch group in DIAS which deals with fundamental andapplied studies in molecular spectroscopy and chemicallysensitive optical-fibre sensorsand devices.