Anal. Chem. 1992, 6 4 , 930-936
990
Determination of pH with Surface-Enhanced Raman Fiber Optic Probes Ken I. Mullen, DaoXin Wang, L. Gayle Crane, and Keith T. Carron*
Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071-3838
Methyl red, cresol red, and 4-pyrldlnethlol were examined for thelr aMHy to detetmlne the pH at a surface. Methyl red and cresol red were coupled to cystamlne. Thls produced a dlsulflde derlvatlzed Indicator whlch formed robust monolayers on sllver substrates. The spectroscoplc technlque used to examlne these compounds was surfaceanhanced Raman spectroscopy. We determlned that all of these compounds possessed surface-enhanced Raman bands which were characterktlc of the molecular structure assoclated wtth the kwllcator and tts conjugate ackl. Methyl red represents a near resonance Indicator. It showed a hear log ( IlU,,/f ,,,15) vs pH relatlonshlp trom pH 2 to 4.5. Some Interference due to buffer anlons was observed. Cresol red was examlned as a resonance Raman Indkator. Its llnear range was pH 2-8 for the ratlo of Zlm/I 1990. An Inflection In the callbratlon curve was observed near pH 5. Cresol red was tested on an optkal flber for Its usefulness as a remote pH probe. It functloned very well and was used to follow a flowing, pH gradlent pr+ duced by adjustlng a reservolr pH wlth acetlc acld. 4-Pyrldlnethlol lo a normal Raman Indlcator. It possessed a hear log (IlOw/Zllm) vs pH relationship between pH 5 and pH Q. The callbratlon curves, lncludlng the lnflectlon In the case of cresol red, are explalned wIth Gouy-Chapman-Stem theory.
INTRODUCTION In situ sensing with fiber optic probes offers many advantages over conventional analytical methods. In situ sensing can avoid all the problems associated with sample contamination due to collection and transportation. If the in situ technique is carried out with fiber optics, the sample size and perturbations to the system can be minimized by the small size of fiber optics. Many sensors have been developed around fiber optics.' Of the many prototype and commercial fiber optic sensors available, those based on Raman spectroscopy offer excellent selectivity through the fingerprint nature of vibrational spectroscopy. Raman spectroscopy uses visible light to produce a vibrational spectrum. This makes it well suited for use with optical fiber sensors which transmit visible light with high effi~iency.2~ Raman spectroscopy is also practical for the design of probes in water analysis since water does not interfere in Raman spectroscopy.4 However, the normal Raman signal is too weak to construct a sensitive probe. Surface-enhanced Raman scattering (SERS) can be used to increase the Raman signal by as much as 6 orders of m a g n i t ~ d e . ~However, .~ SERS requires that the analyte be brought into close proximity, within a few monolayers, of the surface. SERS has been shown to be an ultrasensitive technique for detecting numerous compounds which naturally adsorb to silver surface^.^-'^ Improved sensitivity can be achieved with the combination of S E W with resonance excitation. Surface-enhancedresonance *To whom correspondence should be sent.
Raman scattering (SERRS) leads to larger enhancements." The signal intensities observed with SERRS rival those of fluorescence spectroscopy.12 There have been many studies of pH with fiber optic probes.'"16 This is the first using SERS as the spectroscopic technique. As well we believe it is the first report of a pH sensors based on a single monolayer of indicator. We intend to show that a linear calibration curve is obtained over a large region due to the broadening of the transition range by charge build up in the monolayer. This is not the case with pH sensors which use encapsulated beads or polymer matrices to contain the indicator. The use of a single monolayer also alleviates any problems with dynamic response since the whole indicator interface is exposed directly to the solution. In our case, dynamic response becomes a simple one-dimensional diffusion problem. In a previous publication we demonstrated the use of an indicator molecule to detect the presence of an analyte using SERRS.17 This was accomplished by examining the changes in the Raman spectrum of Eriochrome Black T (EBT) when it was both uncomplexed and complexed with metal ions. In that study we applied a solution of EBT, mixed with varying concentrationsof analyte,to a SERS substrate. After allowing the solvent to evaporate we collected a SERRS spectrum of the physisorbed indicator. We were able to use shifta in peak positions to identify metal ions in solution and changes in relative peak intensities to determine the concentration of the metal ions. A methodology for abrasively roughening an optical fiber to fabricate a SERS substrate directly on the tip of the fiber has been demonstrated.18 The enhancement of surfaces produced by abrasive roughening were shown to be equivalent to the enhancement of other forms of vapor deposited substrates, i.e. island films and CaF2-roughenedsilver substrates. The work represented in this paper is a combination of the use of SERRS with pH indicators and the remote detection of pH changes with abrasively roughened fibers. Fabrication of a durable fiber optic sensor requires the indicators to be anchored to the silver substrate. It has been shown that organic disulfides react with silver surfaces to form anchored silver t h i o l a t e ~ . 'We ~ ~ have ~ coupled our indicator compounds with cystamine to form an indicator disulfide compound. Most indicators possess a sulfonic acid or carboxylic acid group for enhanced aqueous solubility. We have used these groups to form a sulfonamide or amide linkage to the cystamine. Our approach should be quite general for most indicators. When the SERS substrate is exposed to a solution of the modified indicator it is chemisorbed and is held in close proximity to the SERS surface for maximum enhancement. The compact layers formed when disulfides assemble on a silver surface also act to protect the silver surface against degradation. We found that when immersed in aqueous solution the SERRS spectrum of the indicator changed with pH. The relative intensity of peaks associated with the indicator compared to peaks associated with its conjugate acid correlated well with pH. The we of relative intensity changes, eliminated
0003-2700/92/0364-0930$03.00/00 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1892
problems associated with variations in substrates, laser power, and collection geometry. We chose three indicators as test cases for our sensors. Methyl red (MR) and cresol red (CR) were used for our SERRS studies. MR provided data about monoprotic indicators. CR was used to show the potential for using a diprotic indicator. We tested Cpyridinethiol(4PT)88 a normal Raman pH indicator. The need for high laser powers with 4PT and MFt illustrated the advantages of SERRS indicators. A demonstration of our probes under dynamic flow conditions was performed with CR.
THEORY In a solution the relative concentration of the two forms of an indicator should follow the Henderson-Hasselbalch equation
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pH bulk
7
Flgure 1. A plot of the ratio of (a) In/InH+ vs pH, and (b) ,p H , for a monoprotic indicator, pK, = 6.5. The effects of surface charge are clearly seen at low pH. At low pH the pH ,,, is much larger than pH., The pHWa, ranges from 5.40 to 8.82 as pH, changes from 3 to 7.
vs pH,
where [In] and [InH+]are the concentrations of indicator and ita conjugate acid. Normal Raman varies linearly with concentration, thus the log of the ratio of the intensities of Raman bands associated with the indicator and ita conjugate acid should be a linear function of pH. On a surface the problem becomes more complex. First, it has been established that the surfaceenhanced signals from normal Raman are not linear with surface coverage.21i22This is due to the coverage-dependent depolarization of the plasmon resonances which are responsible for the electromagnetic contribution to SEW. SERRS exaggerates this situation even more due to the presence of much stronger electric dipoles which produce fields that are out of phase with the plasmon In this preliminary study we will assume that the enhanced Raman signal is linear with concentration of In and InH+ on the surface. There is some justification for this since we are not actually changing the coverage, but rather the relative concentration of two species on the surface. It there is not a large polarizability difference between the two species, we would not expect the relative concentrations to affect the surface enhancement. Equation 1 is not valid when the indicator is bound to a The charges which are present on the conjugate acid of the indicator will produce a potential at the surface, and this local potential will create a gradient in the concentration of charged species around the surface. This effect is well-known and is described by the Gouy-Chapman-Stern theory. The local pH, pHsurf,,, is given by
where e is the electronic charge, k is Boltzmann's constant, T i s temperature, and rC, is the potential at the surface. For a specifically adsorbed layer, rC, is given by
where c is the bulk concentration of ions (in this case H+), n is the area density of InH+ species on the surface/cm2,D is the dielectric constant of the bulk solution, R is the gas constant, and t is the permittivity of free space. n is given by
P"
7
kdk Flgure 2. A plot of the ratio of (a) In/InH+ vs pH, and (b) ,p H , vs p H , for a dlprotlc indicator, pK,, = 2 and pK, = 8. The effects of surface charge are clearly seen at b w pH and high pH. At low pH the p b Is iargerthan &. At high pH the pHIs smaller than pH., The p b , ranges from 3.30 to 5.57 as pH, changes from 3 to 7.
solved for [H+],&- iteratively until a self-consistency of l/loo is met. The solution of eq 4 requires the knowledge of the pKa of the indicator and the coverage on the surface. Figure l a shows a plot of R vs pHba for a monoprotic indicator with pKa of 6.5 and a coverage of 2 X 1013molecules/cm2. R, the ratio of [In]/[InH+], can be seen to increase nearly exponentially with pH. Figure l b illustrates the effect of surface charge on pHSurfawAt low pH the indicator is mostly in ita conjugate acid form. The surface is positively charged, electrostatically repels H+, and therefore, pH,da, is higher than pHb& As the pH increases toward the pK, the surface charge drops and PH8urface approaches PHbulk. Figure 2 shows the same plots as Figure 1 except in this case we have modelled a diprotic indicator with a pK,, of 2 and pKa2of 8. The inflection represents the point at which the positive potential at the surface drops off and the concentration gradient of H+ equilibrates with the bulk. Further leveling off of the ratio with increasing pH is due to the build up of In- at the surface. Figure 2b illustrates the change in pHsurface due to the surface charge going from positive at low pH to negative at high pH.
EXPERIMENTAL SECTION where [Inlo is the coverage of indicator on the surface, [H+],,,, is the concentration of H+ at the surface. We have
In order to optimize the conditions for the pH sensor we initially did not use the somewhat tedious procedures associated with
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SERS on optical fibers. Two types of SERS substrates were used: island films and silver foil roughened with nitric acid. Island film SERS substrates were fabricated from 1-in.2glass slides. The slides were rinsed with acetone, scrubbed with soap and water, and then cleaned with concentrated ammonium hydroxide and finally by rf sputtering in a Harrick PDC-3XG plasma cleaner. Careful pretreatment of the slides was necessary to achieve reproducibility. Island films were produced by coating the cleaned glass slides with 5 nm of Ag (99.9%, Aldrich) at 4 X 10" mbar using an Edwards E306A vacuum coating system. The silver foil SERS surfaces were prepared by etching 0.1 mm silver foil (99.9%,Aldrich) in rapidly stirred 30% nitric acid for 60 to 90 seconds and rinsed with DI water. This has been shown to produce strongly enhancing SERS substrate^.^' These substrates were used in the studies involving 4PT. CR and MR were bound to cystamine using 1,34icyclohexycarbodiimide (DCC) (Aldrich) as shown in Figure 3. DCC is commonly used as a coupling reagent in the synthesis of pepThe synthesis of the MR derivative was accomplished by combining 0.321 g (2.11 mmol) of cystamine (Aldrich), 1.310 g (4.86 mmol) of methyl red (Aldrich),and 0.882 g (4.27 mmol), of DCC (Aldrich)in 25 mL of dimethylformamide (DMF) (Aldrich). The solution was stirred for 24 h at room temperature. The DMF was removed under reduced pressure. The desired product was isolated by flash column chromatography. The initial eluent was 5:l methylene chloride (Fischer) to methanol. Separation was achieved by increasing the methanol proportion to a final ratio of 1:l. The CR derivative was synthesized similarly. The following were added to 20 mL of DMF: 0.761 g (0.5 mmol) of cystamine, 0.404 g (1.0 mmol) of cresol red (Aldrich),and 0.205 g (1.0 "01) of DCC. The mixture was stirred for 24 h at room temperature. The DMF was removed under reduced pressure and the desired product isolated by flash column chromatography with a mixture of acetonitrile (Fischer)and methanol (Fischer). We began with 4:l ratio of acetonitrile to methanol and gradually increased the methanol portion to a final ratio of 1:2. In both cases the structure of the products was verified by proton and carbon-13 NMR. Absorption spectra of the indicators at various pH were obtained with a Perkin-Elmer Lamda 9 spectrometer. The modified surfaces were prepared by soaking the SERS substratesin a solution of the derivatized indicator for a minimum of 24 h. Island films were treated with M cresol red with crystamine (CRC) in methanol (spectral grade, Baker) and M methyl red with cystamine (MRC) in acetone (spectral grade, M Baker). Nitric acid etched silver films were exposed to a solution of 4PT (90% Aldrich) in acetone. The substrates were thoroughly rinsed in the appropriate solvent to remove any
physisorbed reagent. We found that the substrates were quite stable for long periods of time when stored in distilled water. They were regularly kept for a month or more with no apparent deterioration. Raman spectra were collected with the custom-built Raman instrument described previously." A Spectra Physics 2025 Kr+ ion laser operating at 647 nm provided the excitation for MRC and 4PT. Laser powers ranging from 15 to 30 mW were used. An Omnichrome Model 532 air cooled Ar+ ion laser provided 514.5-nm light for resonant excitation of the CRC. Spectra of CRC were collected with 3 mW of 514.5-nm light on the island films, and 5 mW on the fibers. All laser powers are reported at the sample, after the beam passed through an appropriate interference filter (Edwards Scientific Co.) to remove spurious plasma lines. The monochromator slit width was kept at 860 pm for all Raman measurements. This corresponds to 8-cm-' resolution with 514.5-nm excitation and 5-cm-' resolution with 647.1-nm excitation. Spectra were obtained by cutting the island film coated glass slides to fit in 10-mm glass cuvettes. The cuvettes were filled with solutions of appropriate pH and placed in the sample holder of the Raman instrument. The Raman spectra of CRC, MRC, and 4PT were obtained in Hydrion (Aldrich)buffer solutions. A study of counterion dependence was made with MRC. These spectra were obtained in HC104and HC1 solutions. A Markson Model 93 pH meter with a Hach 1 pH probe was used to measure the pH of the HCl and HC104solutions. Cuvettes and slides were thoroughly rinsed with distilled water between scans. Laboratory-grade deionized water was boiled and used to make all solutions. Fibers were prepared and coupled to the monochromator as described previously.'* Soft-clad 6OO-pm silica core fibers obtained from General Fiber Optics were used in all cases. The fibers were roughened with 15-pmpolishing paper. After careful rinsing with deionized water followed by rinsing in concentrated ammonium hydroxide, the fiber surface was prepared by vacuum deposition of 35 nm of Ag. CRC was coated on fibers by soaking in a M methanol solution. Since the Ar+ ion laser beam diverged significantlya 50-mm plano-convexachromat doublet lens (Melles Griot) was used to focus the beam onto the fiber tip. The SERS fiber optic spectra were obtained with a programmed data collection which slewed between two points in the spectrum. This avoided acquiring data between the relevant Raman peaks. A house built Kel-F flow cell held the fiber. The flow cell has a glass window in the front and is mounted on Daedal and Newport translators to align the fiber in the laser beam. Buffer solutions were pumped through the cell with a peristaltic pump to obtain the pH calibration curve. Between scans copious amounts of distilled water were pumped through the cell to rinse out the previous buffer solution. The flow cell was used to monitor the pH change of a solution under flow conditions as acetic acid was added to a reservoir.
RESULTS AND DISCUSSION We tested our indicators to determine their capacity as a remote pH sensor. MRC was tested as a near resonant Raman indicator, for ita pH response, the effect of solution composition, and the effect of ionic strength. CRC was tested as a SERRS pH indicator and to evaluate the influence of a diprotic indicator on the sensor characteristics. Finally, we examined 4PT as a nonresonant indicator. Methyl Red. The coupling of cystamine to MR was accomplished through the carboxylic acid group of MR and the amine of cystamine. The carboxylic acid group is responsible for the solubility of MR in water. This modification made MRC insoluble in water. The transition range of MRC was determined by dissolving the compound in acetone and adding this solution to water. A visual transition from red to yellow was observed between pH 1.5 and 2.5. The transition for MR is from red to yellow between pH 4.8 and 6.30 Figure 4 shows the molecular changes that occur when MR is p r ~ t o n a t e d . ~ ~ The protonated form is stabilized by resonance effects and hydrogen bonding between the added proton and the carboxylic acid. Replacing the carboxylic acid with an amide
ANALYTICAL CHEMISTRY, VOL. 64,NO. 8, APRIL 15, le92
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loo0 cm-1 Flgw 1. SERRS spectra of MRC. The integration tlme for these spedrawas1Os. LabekdareWpeabwedforthedeter“bn of pH: (a) pH 2 bufter, (b) pH 4 buffer, (c)pH 5 bufter. 1700
destabilizes the protonated form forcing the transition to OCCUT at a lower pH. MRC inthe h y d ” e f 0 r m poeeessed an absorption at& nm and a shoulder at 550 nm. As the pH increases the shoulder grows and we obeerved a single, broad ped centered at 460 nm. The tail of the high pH peak extended out to the laeer excitation frequency a t 647 nm. This should lead to a small near resonance enhancement. We attempted resonance excitation of .MRC with 514.5-nm light; however, the absorption caused it to degrade even at very low laser powers. The irreversible changes appeared to be due to photochemical degradation. Figure5 showsthree spectra of MRC on an island film slide immersed in pH 2,4, and 5 buffer. We found with the metal ion indicator Eriochrome Black T the strongest Raman bauds are associated with the stretching frequencies of the diazo nitrogens.82 With MRC we found that the intensity of the 1400-cm-l diazo stretching frequency increasea with pH. This cormponds well with the structural changes which occur in MR with pH, see Figure 4. At high pH MR is a diazo compound. As the pH decreases the structure becomes that of a hydrazone. It is desirable to have an intemal intensity standard in our probes. In this case we have used the hydrazone stretching frequency. In Figure 4 it is illustrated that
the intensity of this band should decrease as the pH increaseg. We have assigned the 16&cn-’ band to the C - N stretch of the hydrazone.@ The ratio of 1400/1615 cm-l scale well with the pH. We found the useful range in buffer solutions for MRC as a pH indicator to be from pH 2 to 4.5. Below pH 2 the molecule was irreversibly damaged, mcet likely by protonation of the second nitrogen in the diazo linkage. Above pH 4 the peak at 1615 cm-I abruptly diminishes as seen in Figure 5. We interpret this change as being due to the composition of the buffer solutions. The pH 2 and 3 buffers both contained potassium biphthalate in addition to sulfamic and tartaric acid, respectively. The pH 4 buffer consisted of pure potassium biphthalate. The pH 5 buffer consieted of a phosphate mixture. All of these buffer components appear to coordinate with the surface or ion pair with the conjugate acid of the indicator to varying degrees, and thereby, alter the ped intensitiea This behavior can be explained as a change in the surface charge due to coordination of the buffer components with the surface or ion pairing with the charged indicator. This phenomena has been O ~ S S N with ~ ~ reduction/oxidation of viologen in self amembled monolayers.M The coordination of anions with the surface or film will decrease the surface potential resulting in a higher concentration of protons near the surface and a decrease in the pHB-.. The net effect of changing the composition of the buffers is to create discontinuities in R vs pH plots. When spectra were obtained in distilled water adjusted with HC1 the dramatic change in the 1615-cm-l peak did not occur between pH 4 and 5. The 1615-cm-’ peak decreased in a more linear fashion. This is due to the presence of a single anion species throughout the calibration. Calibration curves for MRC are shown in Figure 6. The R2values for the u p p r and lower curves are 0.95 and 0.96, respectively. The upper line wae obtained in distilled water adjusted to the appropriate pH with HC10,. HCIOl was chosen since it doea not coordinate with metal surfaces. For the lower curve buffer solutions were used. The smaller Ilm/I1815 of the buffer plot is indicative of Mer coodination with the surface and a smaller P H , due ~ ~to a~ decreased surface potential. In a related test the effects of ionic strength were investigated. This teat was conducted in distilled water adjusted to pH 2 and 3 with HC1. Spectra were run in solutions of 1.0, 0.1, and 0.01 M and no NaCl. No effecta from the ionic strength of the solution were found. The ratio of peak ib tensities cooresponded to those of the buffer solutions. This is reasonable aince C1- does not ion pair with ~ I H + . ~ Cresol Red. The second compound which we have teatad as a viable pH indicatoris CRC. CR poeeeseee two transitions between pH 0.2 and 1.8 and 7.2 and 8.8.90 When coupled with cystamine through the sulfonic acid group, the transitions were
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Table I. Cresol Red Spectral Order pH
1600/1390
pH
1600/1390
7 2 4 6
1.85 1.37 1.44 1.76
8 5 3 7
1.91 1.71 1.42 1.82
found to shift to pH 3.0 to 4.0 and 4.8 to 6.0. There was also a noticeable change in the optical absorptions associated with the transitions. CRC at low pH possesses an optical absorption at 507 nm. As the pH increases to pH 4 this absorption shifta hypsochromically to 460 nm. A third absorption was found as the pH increased further. At pH 8 the optical absorption occurred at 561 nm. In all m e 8 we anticipate strong resonance enhancement of the Raman scattering with 514.5nm excitation. CRC appeared to be stabile under moderate to low laser powers. The calibration curve for CRC is shown in Figure 7. The upper curve waa obtained using island f h slides coated with CRC and immersed in buffer solutions. The lower curve was obtained on a fiber in buffer solutions and will be discussed in the next section. To evaluate the reversibility of the indicator we obtained spectra in a random order. The order in which the spectra were obtained and the ratios observed are listed in Table I. The system is clearly reversible. The spectra were obtained on the same substrate over the period of 1week, and no degradation of signal was observed. The sigmoidal shape of the calibration curve for CRC a k a from the presence of two tramitions in CRC. The calibration curve closely resembles Figure 2. At low pH CRC is mostly present as ita conjugate acid, and therefore, the surface is positively charged. This will inhibit the ratio of [In]/[InH+] from following eq 1and will tend to flatten out the changes in the ratio with pH. As the pH increases the indicator is transformed into ita conjugate base and the surface becomes negatively charged. This electrostatically repels OH-, and again the ratio [In]/[InH+] will be flattened out compared with that predictad by eq 1. The sharp inflection represents the pH at which the surface charge is neutralizing and ratio follows the eq 1. We found several combinations of peak ratioe correlate well with pH in the pH 5-8 range for CRC. The following peak pairs which correlated well with pH are 1630/460 cm-', 1600/1390 cm-', and 1490/1390 cm-'. The modes amociated with the indicator are the 1630-cm-' band which can be assigned to the c--O stretch, the 1490 cx-' may correspond to a C-H deformation associated with the B carbons of the carbonyl group, and 1600 cm-' most likely corresponds to a ring breathing mode. The modes associated with the conjugate
1800 cm
200
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Ftgure 8. Spectra of 4PT. Indicated are the peaks used for the pH determination. The integration time was 5 s: (a) In a pH 5 buffer, (b) obtained at pH 9.
acid are the 460 cm-' which corresponds to a bending mode of a CCO group and 1390 cm-' is most likely associated with the C 4 H + Comparing MRC with CRC demonstrates the importance of resonance enhancement. Signal to noise ratios ( S I N ) were calculatd for both compounds in terms of milliwatts of laser power and integration times. The appropriate SIN equation is
S/N =
IBid /
d
G
mwdintegration time The denominator was added to normalize the observed SIN from the spectra against laeer power and integration time. The signal intensity increases linearly with laser power, and the noise improves as the square root of the integration time. Comparing the corrected SIN ratios shows a 15-fold increase in SIN for CRC over MRC. If this is corrected for the 0 ' dependence of Raman scattering the remnant effect gives a Sfold enhancementof SIN. In terms of laser power it requires a laser with 15 times the power output to obtain the same signal for the near resonant MRC as the resonant CRC. An estimation of the precision of the CRC sensor was obtained from a regression analysis of the two linear segments of the CRC curve. We found that the precision was 0.115 pH units at low pH and 0.0095 pH units at high pH. 4-Pyridinethiol. It is of interest to test a nonresonantly enhanced compound for ita qualities as a SERS pH indicator. The methodology which we have developed is applicable to nonchromophoric compounds as well aa indicator dyes. We have choeen 4PT as a poesible nonmonantly e n h a n d Ra" pH indicator. 4PT has been shown to form compact amembled layers on s i l ~ e r . 3The ~ pK, of 4PT is assumed to be around 4.9 which corresponds to that of 4,4'-dithi0pyridine.~ Figure 8 shows the bands which correlated well with pH are the 1100 cm-' (4PT) and 1005 cm-' in (4PTH+). The 1100cm-' peak corresponds to an in-plane C-H deformation and the 1005-cm-' peak is the totally symmetric ring breathing
ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1992
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Flgure 11. A plot of the ratio of I,,,,lI,,,, monitored continuously as acetic acid was metered into the resewoh. The right Y-axis Is taken from the calibration curve for a fiber shown in Figure 7b. One can see that the initial pH is 7.9 and rapidly drops to pH 3 as acetlc acid is added to the reservoir.
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Flgure 9. Spectra of CRC on an optical fiber. The integration time for these spectra was 30 s. Indicated are the peaks used to form the calibration curve: (a) obtained in a pH 6 buffer, (b) obtained in a pH 3 buffer. Monochromator
Kcl-F cell
DI Water + Analyte
added glacial acetic acid at a rate of 0.22 mL/min. The water in the analyte beaker was initially adjusted to pH 7.9 with NaHC03. This allowed us to scan over the entire active range of CRC. After scanning the initial solution for approximately 10 min the second peristaltic pump was switched on to begin the flow of acetic acid into the analyte beaker. The ratio of the 1600/1390-~m-~ peak heights was monitored continuously. This ratio versus time is shown as the left Y-axis in Figure 11plotted with respect to the quantity of acetic acid added. The right Y-axis shows the pH corresponding to the ratio of peak intensities from the calibration curve in Figure 7b. The pH values correspond well to those calculated from the K, for acetic acid. For 2 mL of glacial acetic acid in 500 mL of distilled water the calculated pH is 2.9. The addition of 10 mL of glacial acetic acid should result in a pH of 2.6. We found that response times were quite rapid. The ratio of peak intensities began to fall immediately when the acetic acid pump was switched on and the peak intensity ratio corresponding to pH 2.9 was observed nearly simultaneously with the addition of 2 mL of acetic acid.
AceUc Acid
Flgm 10. Block diagam of the apparatus used to generate the acetic acid curve in Figure 1 1. A peristattlc pump was used to meter acetic acid into a reservoir. A second peristaltic pump flowed the reservoir contents through the flow cell.
mode.37 Below pH 5 the curve levels off. This is most likely due to saturation of the 4PT monolayer in the conjugate acid form. The increased pK, of 4PT on the surface, above 4.9 in the bulk, can be rationalized as the thiol group deprotonating and forming a thiolate on the silver surface. The negative charge on the thiolate will make it easier to protonate, and therefore, the pK, will become larger. The SIN ratio of 4PT is identical to that of MRC. This is indicative of the small off resonance excitation of MRC. Fiber Optic pH Probes. The purpose of this research is to use indicators with optical fibers. Figure 7b shows the calibration curve for CRC on an abrasively roughened optical fiber. There is a smaller change in the ratio of peak intensities and a less prominent inflection than with island T i s . These effects are due to the different substrates. We found that on fibers a sloping background existed with our abrasively roughened fibers. This caused the 1390-cm-' peak to appear larger than with the flat background found with island films. In both cases the calibration was formed with absolute intensities. It was felt that this best represented a realizable fiber pH sensor. If we had corrected for background the two calibrations would be nearly identical. Figure 9 shows spectra of CRC taken at the end of a fiber in pH 6 and pH 3 buffers. To illustrate the viability of measuring analytes with optical fibers we used a CRC coated fiber to monitor a dynamic system. We used the arrangement diagramed in Figure 10. One peristaltic pump continuously cycled water from the analyte beaker through the Kel-F flow cell. The other pump
CONCLUSION We have examined three compounds for their quantities as SERRS/SERS pH indicators. MRC was tested as a near resonance SERRS indicator. It showed good pH correlations a t the low end of the pH range. However, it suffered from deleterious effects due to the solution composition and laser induced damage when resonance Raman was attempted. CRC had much better pH determining characteristics. The useful range was between pH 2 and pH 8. No laser photcdegradation was observed a t powers less than 5 mW. This allowed us to use SERRS for our measurements and work in a laser power range easily attainable with low cost HeNe or diode lasers. 4PT was examined as a nonresonantly enhanced compound. It showed good stability under laser powers up to 25 mW. Above this value deviations from the calibration curve were observed. 4PT forms compact monolayers and did not show any affects due to varying anions in solution. We have demonstrated the stability and dynamic response of our fiber optic probe under flowing conditions. We were able to observe a pH gradient in a flowing system. No deleterious effects were observed due to the flow or desorption of the indicator from the surface. ACKNOWLEDGMENT We would like to acknowlege the generous support of the US.Geological Survey (USGS),Department of Interior, under USGS award number 14-08-0001-G2097;the donors of the Petroleum Research Fund, administered by the American Chemical Society; and the NSF EPSCoR grant number RII-8610680. The views and conclusions contained in this document are those of the authors and should not be interpreted necessarily as representing the official policies, either expressed or implied, of the US.government.
B30
Anal. Chem. 1092,64, 936-946
REFERENCES (1) For a revlew see: Angel. S. M. Spectroscopy 1987, 2 , 38-48. (2) McCreery, R. L.: Fkischmann, M.; Mndra, P. Anal. Chem. 1983, 55, 146-148. (3) Schwab, S.D.; McCreery, R. L. Anal. Chem. 1984, 56, 2199-2210. (4) Schwab, S.D.; McCreery, R. L. Appl. SpmYosc. 1987, 41, 126-130. (5) Chane, R. K.: Furtak, T. E. Swface EnhancedRamen Scattering; Pienum:-New York, 1982. (6) Wokaun, A. Sol& State Physics 1884, 46, 233-294. (7)Carrabba, M.; Edmonds, R.; Rauh, R. Anal. Chem. 1987, 5 9 , 2559-2583. (8) Aiak, A.; Vo-Dinh Anal. Chem. Acta 1988, 206, 333-337. (9) Bello, J. M.; Vo-Dlnh, T. Appl. Spectrosc. 1980, 4 4 , 63-69. (10) Enlow, P. D.; Buncick, M.; Warmack, R. J.; Vo-Dinh, T. Anal. Chem. 1988, 58, 1119-1123. (11) Zeman, E. J.; Canon, K. T.; Schatz, 0.C.; Van Duyne, R. P. J. Chem. PhyS. 1987, 7 . 4189-4200. (12)Canon, K. T. Ph.D. Thesis, Northwestern University, Evanston, Illinois, 1985;p 103. (13) Wangbai, M.; Zhujun, 2.; Seitz, W. R. Chemical Sensors and Micronshxnentetion; ACS Symposium Series 403;American Chemical Society: Washington, DC, 1989;Chapter 18. (14)Saarl, L. A,; Seitz, W. R. Anal. (2".1983, 5 4 , 821-823. (15) Jordan, D. M.; Walt, D. R.; Milanovich. F. P. Anal. Chem. 1987, 5 9 , 437-440. (16) Boisde, G.; Blanc, F.; Perr, J. Talenta 1988, 3 5 , 75-82. (17) Canon, K.; Mullen. K.; Lanoutte, M.; Angersbach, H. Appl. Specfrosc. 1991, 45,420-423. (18) Muiien, K. I.; Carron, K. T. Anal. Chem. 1991, 63, 2196-2199. (19) Bryant, M.; Pemberton, J. J . Am. Chem. SOC. 1991, 773, 3629-3637.
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RECEIVED for review October 16,1991. Accepted Janurary 31, 1992.
Heuristic Evolving Latent Projections: Resolving Two-way Multicomponent Data. 1. Selectivity, Latent-Projective Graph, Datascope, Local Rank, and Unique Resolution Olav M.Kvalheiml and Yi-zeng Liang' Department of Chemistry, University of Bergen, N-5007 Bergen, Norway
HeutWlc evolvtng latent proJ6ctlonr (HELP) are proposed as a new method to resolve two-way blllnear multlcomponent data Into spectra and chromatograms of the pure w"k. The method Is founded on four elements: (I) the use of the so-called zero-component reglons In order to establkh the detectlon llmlt for the chemlcal specles, (11) the use of latent-projective graphs (datascope) to reveal selective (onecomponent) chromatographlc and/or spectral reglonr, (Ill) local rank analysls In order to check the selectlvlty of the reglons found by vlwal detection, and (Iv) the use of the selectlve lnformatlon for unlque resolutlon Into spectra and chromatograms d the pure chwnkal conrtltuents. The HELP method works In an Inductlve, stepwb manner, Le. like a datascope penetrating Into the local structure of the data. I n the flrcrt part of thls paper, the bask concepts and the thee retlcal foundatlon of the method Is explalned. Results from slmulated analytlcal systems are used to Illustrate the olmpllclty and efflclency of the proposed method.
INTRODUCTION Two-way chemical methods, such as liquid chromatography with diode array detection (LC-DAD), are becoming in-
* Corresponding author.
On leave from Department of Chemistry and Chemical Engineering, Hunan University, Changsha, PRC.
creasingly important for the analysis of multicomponent The reason for this trend is obvious. No training set is needed as in one-way multicomponent anal~sis.~ From a single chemical analysis one can, in principle, determine the number of species present in a mixture and, by means of self-modeling curve resolution methods,&12resolve the mixture into spectra and concentration profiles of the pure constituents. In practice, the situation is not this simple. The determination of the number of species rests upon an estimation of the rank of the two-way data. Rank analysis is usually performed by dmmpoeing the data into principal Components and selecting the rank as the number of so-called significant components. This partition of the data into variance corresponding to chemical species and no& is not straightforward, however, and ambiguties are more the rule than the exception. The remark above applies also to the combination/rotation of the principal components into spectra and concentration profiles of the pure constituents of a mixture. However, the latter situation is relieved if selective variablea can be detected. Thus, methods developed by Lawton and S y l ~ e s t r eKnorr ,~ and Futrell? and Malinowski7provide unique resolution in the presence of at least one selective variable for each chemical species. The more recently developed methods of iterative target transformation factor analysis (I"FA)'O and evolving factor analysis (EFA)11J2do not explicitly use selective information. However, in practice both methods are unable to provide unique resolution in the absence of some selective
0003-2700/92/0384-0938$03.00/00 1992 American Chemical Society
