AMI. Ctwm. 1003, 65, 1390-1398
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Chemometric Evaluation of the Multiple Mode Response of an Ion-Diffused Planar Optical Waveguide to Liquid-Phase Analytes Kevin J. Kuhn and Lloyd W. Burgess* Center for Process Analytical Chemistry, Department of Chemistry, BG-10,University of Washington, Seattle, Washington 98195
An ion-diffused planar optical waveguide was fabricated by exposing a soda-lime silica glass substrate with etched surface gratings to a AgN03 melt solution. The ion-diffused waveguide exhibited multiple mode output which was imaged with a photodiode array detector. The waveguide response to modulations in cover solution composition was tested for a series of model chemical systems emphasizing the distinction between surface-active and surface-inactive species. Model systems were defined to test the waveguide response to changes in solution refractive index and absorbance and the important case of simultaneous changes in refractive index and absorbance. Chemometrics were applied to the analysis of the resulting multiple mode response data to characterize waveguide response components and to provide for multivariate calibration. The iondiffused waveguide response to refractive index and absorbance changes is sufficiently different as to allow a PLS calibration to predict both components in mixture solutions. The waveguide response to surface-active analytes exhibited a large sensitivity enhancement relative to analytes which did not interact with the waveguide surface. INTRODUCTION Interest in the development of integrated optic components for computing and communication applications has driven research into the theory and fabrication of thin-film planar waveguides. The use of planar waveguides for the analysis of chemical systems was first proposed by Theoretical considerations led Midwinter to conclude that thin-film planar waveguides could be used as internal reflection elements (IRES)with sufficient evanescent path length to make attenuated total reflection (ATR) measurements at visible wavelengths. Subsequent experimental work by Polky and Harris3 confirmed that planar waveguides provided the necessary sensitivity to perform visible wavelength evanescent absorbance measurements. Further research into the absorbance sensitivity of thin-film planar waveguides4-7 has been conducted. Reports of planar waveguide chemical sensors include immunosensorsa10 and the fabrication of electrochemical devices by deposition of a (1) Midwinter, J. E. IEEE J . Quantum Electron. 1971, QE-7 (7), 339344. (2) Midwinter, J. E. I E E E J . QuantumElectron. 1971, QE-7(7), 345350. (3) Polky, J. N.; Harris, J. H. J.Opt. SOC.Am. 1972,62 (9), 1081-1087. (4) Mitchell, G. L. IEEE J. Quantum Electron. 1977, QE-13(4), 173176. (5) Choquette, S. J. Ph.D. Thesis, Virginia Polytechnic Institute and State University, 1988. 0003-2700/93/0365-1390$04.00/0
thin film of conductive SnOz over the waveguide layer to serve as an electrode.”J2 A number of recent publications describing chemicalsensor development have employed an ion-diffused waveguide element.8JlJ2 In general, the development of sensors involves chemical modification of the ion-diffused surface to impart chemical specificity toward a target analyte. In the rush to develop novel sensors based on a planar waveguide technology, an evaluation of the basic ion-diffused waveguide element has received little emphasis. A characterization of the inherent waveguide stability, selectivity, and sensitivity in response to changing physical and chemical properties in the solution phase is crucial to the functioning of sensor devices in the “real” world. Such an evaluation also provides the necessary feedback to design and fabricate improved iondiffused waveguide elements for specific applications. The doped glass of an ion-diffused waveguide retains the characteristic surface chemistry of silica glass. In particular, the hydroxyl groups at the silica glass surface are slightly acidic so that in aqueous electrolyte solutions the glass is anionic and can act as a cation-exchange surface.13J4 The effectiveness of silicate glasses in binding cations is highly dependent on solution pH and glass composition. Researchers working with SiOz-Ti02 thin-film waveguides have reported differential waveguide responses to various cationic and pHactive components in electrolyte s01utions.l~In addition, an enhancement in the glass waveguide response to polar molecules and cationic absorbance dyes has been observed and attributed to interfacial interactions.4Jb The majority of research in planar waveguide evaluation and sensor development is conducted using prisms to couple light to and from the optical waveguide. In general, stable coupling conditions are difficult to maintain in that coupled intensity is sensitive to input beam alignment and prism contact with the waveguide surface. The integration of grating couplers within the planar waveguide structure provides a more stable coupling structure for use in the development of waveguide sensors.l6 Surface gratings are fabricated in glass substrates by a combined photolithographic patterning and (6) DeGrandpre, M. D.; Burgess, L. W.; White, P. L.; Goldman, D. S. Anal. Chem. 1990, 62 (la), 2012-2017. (7) Saavedra, S. S.; Reichert, W. M. Anal. Chem. 1990,62 (20), 22512256. (8) Choquette, S. J.; Locascio-Brown, L.; Durst, R. A. Anal. Chem. 1992, 64 (l),55-60. (9) Seifert, M.; Tiefenthaler, K.; Heuberger, K.; Mosbach, K. Anal. Lett. 1986, 205-216. (10) Nellen, P. M.; Tiefenthaler, K.; Lukosz, W. Sens. Actuators 1988, 15, 285-295. (11) Fujishima, A.; Itoh, K. J. Phys. Chem. 1988,92 (25), 7043-7045. (12) Piraud, C.; Mwarania, E.; Wylangowski, G.; Wilkinson, J.; O’Dwyer, K.; Schiffrin, D. J. Anal. Chem. 1992, 64 (6), 651-655. (13) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (14) Smith, R. L.; Pietrzyk, D. J. Anal. Chem. 1984,56 (4), 610-614. (15) Spohn, P. K.; Seifert, M. Sens. Actuators 1988, 15, 309-324. (16) Moshrezadeh, R.; Mai, X.; Seaton, C. T.; Stegeman, G. I. Appl. Opt. 1987, 26 (13), 2501.
0 1993 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 65, NO. 10. MAY 15, 1993
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waveguiderover interface at the original diffraction angle, WO). Totalinternalreflectionwillconfinelight incidentupon the interface a t angles greater than the critical angle,
e, = sin-'(n,/n,) . . . ISlSCT
modes
Figure 1. IondMusedwaveguide WlthagradedrehacHveindexproRle, Nx), and etched surface gratings.
plasmaetchpmedure. Amajoradvantageofgratingcouplers is that source and detector can be located behind the waveguide substrate, thus isolating optical components from the sensing interface. This is consistent with the design of integrated planar waveguide sensor prohes or detection windows into chemical processes (liquids, slurries, or solids). In evaluating the optical properties of the waveguides, we have observed that coupling a collimated laser beam to any one of the resonant modes of a multimode ion-diffused waveguide results in all modes being populated a t the output. The various waveguide modes are angularly resolved upon outcoupling from the second surface grating (or prism) and are conveniently imaged using a photodiode array detector. The research presented here considers the interaction between an ion-diffused waveguide and a number of model chemical systems. The modelsystemsarechosen totestthe waveguide response to surface-inactive(bulk) and surface-active species in the solution phase. The important case in which simultaneous changes in refractive index and absorbance occur is modeled by mixtures of bulk species. A considerable set of multivariate statistical techniques (chemometrics) are available to help in the analysis of chemical response data in the case where multiple measurements are ohtained for each sample." The common example is spectroscopic data where the absorbance a t each wavelength serves as a sample measurement. The photodiode array image of the iondiffused waveguide output is multivariate in that each mode measures the sample differentially. In this publication, we have applied chemometrics to characterize Components of the waveguide multiple mode response to modulations in solution composition and to calibrate the multivariate waveguide response against analyte concentration.
THEORETICAL SECTION An illustration of the ion-diffused waveguide device used in the described work is given in Figure 1. The planar structure consists of a Ag+-doped layer bounded by the sodalime silica substrate and a cover solution. The diffusion process yields a gradient distribution of Agf in the glasswhich results in a graded refractive index profile. The waveguide refractive index is maximum a t the surface, n, and monotonically decreases with depth, n(x), into the substrate. The mechanism of light confinement in the ion-diffused layer is total internal reflection. A light ray incident on the surface grating from the substrate side will be diffracted into thewaveguideand encounteranopticallayerofmonotonically decreasing refractive index. In accordance with Snell's law,
n,, sin B(r=O) = n(x) sin B ( x )
(1) the ray will be continuously refracted with depth until it is either totally reflected a t the turning point, xt, or exits through the substrate. After total reflection, transit through the waveguide layer in the opposite direction will reverse the angular refraction, W x ) , and the light ray will strike the (17)Beebe,K.R.;Kowdski,B. R.Anal. Chem. 1987,59,lW?A-l017A.
(2)
where nl is the cover solution refractive index. The confined light is subject to interference effects due to thesimilardimensionsofradiationwavelengthandwaveguide thickness. Constructive interference is associated with discrete angular paths of light propagation in the waveguide layer giving rise to the existence of discrete waveguide modes. The allowed angular paths of mode propagation are defined according to a resonance condition,
Ne,= n(x) sin e(x)
(4)
where ko is the free space wave vector (ko = 2r/X),xt is the turning point depth of the mode, Ne,is the mode effective refractive index, and rn is the integer mode order. The resonance condition requires that the phase shifts associated with reflections from the respective interfaces (01,2 and Q3.2) and path length through the ion-exchange layer combine to give a total phase shift which is an integer multiple of u. As waveguidemodeorderincr~es, theresonancecondition predicts that the grating diffraction angle, WO), for mode coupling will approach the critical angle for the waveguide cover interface. This means that light coupled into higher order modes must penetrate farther into the ion-diffused layer before sufficient refraction occurs for light confmement. Thus, there is an increase in effective waveguide thickness with increasing mode order. For monochromatic light coupled to the waveguide, the refraction process has a focusing effect such that all modes have approximately the same period. The important result is that waveguide modes propagating in an ion-diffused waveguide have approximately the same number of internal reflections. At each point of total internal reflection, the superposition of incident and reflected wave fronts creates a standing electromagnetic wave within the waveguide layer. The standing wave is continuous across the interface, giving rise to an evanescent wave which penetrates into the houndary layer. The electric field amplitude associated with the evanescent wave decays exponentiallywith distance from the interface. In general, evanescent path length into the cover solution increases as the mode order increases. For gradedindex waveguides, differences in the absorbance sensitivity among the various modes are expected to follow from the slight variation in evanescent path length.
EXPERIMENTAL SECTION Waveguide Characterization. Gratings were etched into the surface of soda-lime silica glass substrates to serve as waveguide couplers.# A Lloyd's mirror configuration was used torecord interferencefringesinapositivephotoresist. Following photoresist development, the gratingswere etched into the glass surfaceby reactivesputteringin arf-diodesystem. Theresulting surface gratings exhibited a period of 0.49 pm (2040lineslmm), and the grating profile was sinusoidal. The two surfacegratings were separated by a distance of 1.4 em. An ion-exchanged waveguide was fabricated by immersing a soda-lime silica glass slide with etched surfacegratings in silver nitrate melt at 280 OC for 5 min. Themodestructureofthe waveguidewascharacterized by prism coupling. Two SF, prisms were fixed to the waveguide surface withmcderatepresaure. Theprism werelocatedbetween the gratings so that prism coupling and mode propagation was independent of grating effects. The collimated beam of a H e Ne laser (Uniphase, Model 1104P)was aligned to illuminatethe input prism such that transverse magnetic (TM) mode coupling would occur.
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ANALYTICAL CHEMISTRY, VOL. 85, NO. IO. MAY 15. 1993
Table I. Effective Refractive Index, New, for Each Mode' mode Nd xt (wm) 1.61
0.00
1.56 1.53 1.50
1.46 2.67 4.54
~
~~
Calculated from measured prism couplingangles and estimated mode tumine nointa. x,. 0
Table 11. Sample Set Solutions for Glycerol-Bromocresol Green (BCGI Exwriment soln glycerol BCG soln glycerol BCG conc (M) no, conc (dL) conc (M) no. conc (K/L) . 11 10.~60 1 . 5 10-5 ~ 1 OB52 12 5.0 x 10-5 2 2.820 3 5.780 13 0.468 5.0 X 4 8.628 14 2.168 5.0 X 5 5.0X 10" 15 4.368 5.0 X lV5 6 0.668 5.0 X 1W 16 1.0 x lo-" I 1.5X 17 0.864 1.0 X 8 0.892 1 . 5 10-5 ~ is 2.860 1.0 x 10-4 9 2.888 1 . 5 10-5 ~ 19 5.816 1.0 X 10-' 20 8.600 1.0 X lV4 10 6.420 1.5X
w.,"rg"lde 4' 1'0 I l e - N e ILLWI Flgure 2. Optical evaluatlon system components: FC. waveguide flow cell. BF, bandpass flner: Po, polarizer; NO, neutral densny niter; Ap, aperature; Ln. lens.
dual p ~ s m
1 wavcrjuidc
pump
The ion-diffused waveguideused in the followingexperiments was found to support three TM modes. Prism coupling angles for individual modes were well separated p2.59 and each resonance was sharp. Prism coupling to any given waveguide mode gave multiple modes at the output prism. Mode effective refractive indexes calculated from prism coupling angles were used as input into an inverse WKB analysis.18 Despite the small number of waveguide modes, the analysis of the mode structure gave reasonahle estimates for mode turning points, I,,and the surface index (Table I). Reagent Solutions. The hulk refractive index response of the ion-diffused waveguide was tested with 12 aqueous glycerol solutions spanning the concentration range from 0.70 to 34.4 g/L. This concentration range represents a change in refractive index with respect to deionized water carrier solution from 8.1 X to 4.0 X lo-%.To test the ion-diffused waveguideresponse to hulk absorbance, 10 bromocresol green (Aldrich, Milwaukee, WI) solutions in the range from 1.6 X to 1.0 X M were prepared in pH 5.9 phosphate buffer. To determine the waveguide response to simultaneous changes in hulk refractive index and absorbance, a seriesof mixture solutionsWBB prepared. The sample set consisted of 4 pure glycerol samples, 4 pure hromocresolgreensamples,and12 mixture solutionswithvarying concentrations of each component (Table 11). All mixture solutions in this sample set were prepared in pH 5.9 phosphate buffer. The surface-activerefractiveindex responseof the ion-diffused waveguidewas testedusinga lO-'M Ph(N03hsoIution in slightly basiccarrier. Thecarrier was prepared hy addingasmallvolume of dilute NaOH to deionized water to give a pH 7.6 solutiou. A 1.6 x lo-? M nitric acid solution was used to rinse and regenerate the waveguide surface. The waveguide response to a surface active absorbance dye was tested with five methylene blue (Aldrich) solutions covering the 1.0 X lk'-l.O X 10.' M concentration range. Phosphate buffer at a pH of 5.9 was used to prepare all methylene blue solutions. Optical System. The optical system for evaluating the solution-phase response of the ion-diffused waveguide is given in Figure 2. The entire optical system is conveniently isolated from the sensing interface hy illuminating the grating and collecting the mode output from the substrate side. The H e N e laser (632.8 nm) was aligned to grating couple directly to the TM9mode of the ion-diffusedwaveguide. The resulting modal output of the waveguidewas angularly resolved upon outcoupling from the second grating. There was some spreading of the beam in the horizontal plane associated with incomplete outcoupling at the leading edge of the grating. A lens was used to counter (18)
White,J. M.; Heidrich, P. F.Appl. Opt. 1976, 15 (l),151-155.
Flgure 3. So(utio~?+haseevaluatlon system components: Sa, sample loop; Va, injection valve: Wa. to waste. the spreading and obtain sharp, intense mode lines on the detector. The photodiode array detector (EG&G, S-series) has 512 diode elements on 25-pm centers. Flow System. A sandwich-type flow cell was designed and constructed to allow for the introduction of a solution phase across the waveguide surface. A polymeric gasket was dimensioned to include both waveguide surface gratings and the beam propagation region in the solution path. Figure 3 showsthe flow system in which a carrier (baseline) solution was continuously circulated over the waveguide hy a HPLC dual-piston pump (Dionex, Model GPM-2). The selector valve (Dionex, microinjection valve) was fitted with either a 100-or 200-pL sample loop for reproducible sample introduction. Experimental Procedure. At the beginning of each experiment, the coupled TM2 intensity was maximized with the waveguideexposedto flowingcarrier solution. Direct TM~mode couplingwas chosen as the highest order TM mode was expected togive thelargest effectiveevanescent path length for measuring hulk changes in solution composition. This optical alignment was maintained for the duration of the experiment. Computer software automated collection and storage of a full photodiode array image of the waveguide mode output at a selectable rate between 0.07 and 0.2 Hz. Data collectionwas initiated with the waveguideinflowingcarriertoestablisha baseline response prior to each sample injection. After a well-defined delay time, the sample loop wasswitchedin-line withthe flowingcarriersolution. In general, the sampling rate was chosen to oversample the waveguideresponseasatramientofsamplesolutionwas propelled through the flow cell. The waveguide surface was rinsed with carrier for several minutes after the sample passed through the flow cell before a subsequent injection was made. Data Analysis. The full 512elementarrayscan wastruncated to a 90- or 95-diode image which retained complete mode information. Theaveragebaseline response prior to eachsample injection was subtracted from the sample response to correct for long-term baseline drift. Rather than analyzethe variousTM modeoutputaindividually, a singular value decomposition'^ (SVD) was performed to aid in interpreting the multiple mode waveguide response. A SVD analysis attempts to describe the total variance in n-dimensional response space with a minimum of orthogonal eigenvectors.The variance described by each principal component eigenvector can he further decomposed into a loading vector and a score vector. (19) Goluh, G . H.; Van Loan,C. F.Matrix Computations, 2nd ed.; Johns Hopkins University Press: Baltimore, 1989.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993 --""I
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i
0.4
Position
Position (diode number)
Figure 4. Iondiffused waveguide multiple mode Image for TM2mode
0.02
coupling. 0
For the purposes of this study, a loading vector describes the distribution of variance among the waveguide TM modes in photodiode array image space. A score plot describes the temporal variance associated with changing the sample composition. Baseline-corrected response data were calibrated against analyte concentration using the partial least squares (PLS) calibration method. The eigenvectors defined in a PLS calibration may differ from those obtained from the SVD analysis because the PLS method considers concentration information when defining eigenvectors to describe response variance. The PLS method uses a leave-one-out prediction process for all samples in the calibration set to generate a root-mean-square error of cross-validation (RMSECV) statistic. A minimum in the value of the RMSECV statistic identifies the number of eigenvectors to include in the calibration model. The PLS calibration procedure was applied to glycerol, bromocresol green, and glycerol-bromocresol green mixture response data.
RESULTS AND DISCUSSION A typical photodiode array mode image for TM2 coupling to the ion-diffused waveguide in contact with a water cover solution is shown in Figure 4. The three modes were of similar intensity despite launch conditions which were optimum for TM2mode coupling. Vertical position on the input grating had some effect on the relative intensities of the modes, but multiple mode output was always observed. Substrate modes contribute a peak to the image which was well separated from the mode information. Substrate modes travel the entire thickness of the substrate and propagate via multiple total internal reflections from the waveguide-cover and substrateair interfaces. Glycerol. Aqueous glycerol solutions were defined as a model system for evaluating the bulk refractive index response of the ion-diffused waveguide. The glycerol molecule was chosen as it does not interact appreciably with soda-lime silica glass, has no inherent absorbance a t the He-Ne laser wavelength (632.8nm), and has a well documented refractive index behavior20 in aqueous solution. The initial launch conditions were set such that the collimated laser source matched the discrete TM2 mode coupling angle. The modulation in cover refractive index associated with a glycerol injection will change the discrete angular condition necessary for matching mode resonance. Since the laser input angle remained fixed, coupling to the waveguide becomes less efficient. Therefore, the primary component of the ion-diffused waveguide response to glycerol concentration was expected to be a decrease in TM2 mode inten sity
.
(20) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 65th ed.; CRC Press, Inc.: Boca Raton, FL, 1984; p D-235.
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8
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100
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300
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7M)
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PDA Scan Number (12 plslmin)
Figure 5. First principal component of the iondiffused wavegulde
response to glycerol in)ectlon: (a, top) loedinglmage space varlance;
(b, bottom) score-sample space varlance.
The waveguide response to duplicate injections of 12 glycerolsamples was subtractioncorrected, and the duplicate responses were averaged. After mean centering," a SVD was performed on the multiple mode waveguide response data. The first principal component describes 94.1 5% of the total experimental variance observed in the waveguide response to glycerol. For a data set consisting of a matrix of waveguide TM mode images, variance in a loading plot identifies positions on the array which measure correlated changes in intensity. The intensity variations described in a loadings plot require human interpretation in order to identify the underlying physical process responsible for mode intensity modulations. The first principal component loading plot (Figure 5a) shows that the waveguide response to glycerol includes variations in the intensity of all three TM modes. The relative magnitudes of the modes indicate that the variance described increases in the following order, TM1 < T& < TM2. The most striking feature in the loading plot is the inverse relationship between the TM2 and T& modes. That is, the primary waveguide response to glycerol injections is characterized by a decrease in the intensity of the TM2and TMI modes and an increase in the T& mode intensity. The position of each TM mode maximum taken froma mode image with the waveguide exposed to carrier is marked (+) to serve as a reference for possible position shifts. The position of the TM2 mode maximum in the loading plot indicates that the identified decrease in mode intensity is not due to a shift in mode position. Rather, the decrease in TM2 intensity reflecta a reduction in coupling efficiencyassociated with the increase in cover refractive index. The increase in the Th&intensity may indicate that an efficient mechanism of mode-mode
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993
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0.6, o.5t 0.4}
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Figure 6. Second prlnclpal component of the londlffused waveguide
response to glycerol InJectlons: (a,top) loadlnglmagespace variance; (b, bottom) score-sample space variance. cross-talk is involved in the waveguide glycerol response. The intensity modulations observed in the TM1 and TMo modes appear to have a peak shift contribution, as is evident in the asymmetry of these mode variance distributions. The score plot (Figure 5b) variance indicates that the waveguide response to injections of glycerol gives a series of smooth, transients offsets from a well-defined waveguide baseline response. The waveguide response to 0.7 g/L glycerol which corresponds to a 8.1 X 10-5 change in refractive index is well resolved above baseline noise. The score response amplitudes for test solutions above 21.6 g/L in concentration (ARI = 2.5 X are nearly identical but exhibit an increasing degree of asymmetry with increasing concentration. The second principal component accounts for 3.3% of the remaining variance in the glycerolexperiment. In the loading plot (Figure 6a), each TM mode exhibits an offset in the position of maximum loading response relative to the original mode positions (+) in the baseline image. The observed position shifts locate the mode maxima on the adjacent diode, and the direction of the position shift is the same for all three waveguide modes. The offsets in position are indicative of a change in the angle at which each TM mode propagates within the waveguide layer. This change in mode propagation angle reflects the increase in effective mode indexes resulting from an increase in the cover solution refractive index. In the score plot (Figure 6b),the low-concentrationsamples are again resolved above baseline response. The score response amplitudes for glycerol injections in the middle concentration range (6.4-12.9 g/L) are very similar until at 17.2 g/L the smooth rising edge of the response profile is broken by a sharp negative-going peak. For the more
Table 111. Root-Mean-SquareError of Cross Validation (RMSECV) Associated with a PLS Calibration of the Ion-Diffused Waveguide Response to Glycerol Injections principal component
RMSECV (g/L)
1 2 3 4 5 6
3.43 0.55 0.41 0.26 0.14 0.10
concentrated glycerol samples, the leading edge reaches a similar score response amplitude before the peak inversion occurs. The depth of the inversion increases with increasing concentration, thus differentiating among samples in the high concentration range. Additional principal component loading plots describe variance associated with different combinations of TM mode intensity correlations and mode position shift information. The response profiies in the corresponding score plots become increasingly complicated. The multiple mode waveguide response was nearly steady state (zero dispersion) across six sampled points on the transient glycerol concentration profile. Therefore, the corresponding TM mode images were averaged to define the ion-exchanged waveguide response to each glycerol sample. The waveguide response information and known glycerol concentrations were used to perform a PLS calibration. The RMSECV statistic for each of the first six calibration factors (principal components) is given in Table 111. A five-factor calibration model results in an RMSECV value of 0.14 g/L. For a change in glycerol concentration of 0.14 g/L, the change in bulk refractive index relative to the carrier solution is calculated to be 1.6 X 10-5. BromocresolGreen. The indicator dye bromocresol green serves as a convenient model system to test the ion-diffused waveguide response to bulk absorbance for a number of reasons. First, bromocresolgreen is a large nonpolar molecule which carries a negative charge in the basic form such that any interaction with the waveguide glass should be repulsive. Second, complete conversion to the anionic form of the dye occurs a t relatively low pH (pK, = 4.7) so that basic conditions which tend to degrade soda-lime silica glass are avoided. Finally, the basic dye has a large extinction coefficient (38 000 L mol-' cm-I) at 632.8 nm. The waveguide response to bromocresol green samples was expected to be primarily due to evanescent absorbance resulting in a decrease in mode intensity propagating within the waveguide. As discussed earlier, modes propagating in a graded index waveguide have approximately the same number of internal reflections. Therefore, differences in sensitivity among the waveguide modes should be due to differencesin evanescent path length. In general,higher order modes have slightly greater evanescent path length into the cover solution. The waveguide response to 10 bromocresol green sample solutions was tested in triplicate. Following subtraction correction, the responses were averaged and mean centered, and a SVD was performed. The first principal component of the waveguide response to bromocresol green injections describes 98.2% of the total experimental variance. The loading plot (Figure 7a) preserves the shape and position of each TM mode in describing relative variance contributions. Thus, the loading plot confirms that the primary waveguide response to bromocresol green is a loss of propagating intensity in all three waveguide modes. The variance described by each TM mode is in proportion to the intensities in the original baseline mode image. Thus, the evanescent sensitivities of
ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993
1391 I
0, -0.05 -
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.-F
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Flgure 7. First prlnclpal component of the bndiffused waveguide
responseto bromocresol green injectlons: (a,top) loadingimagespace variance: (b, bottom) score-sample space variance. Table IV. Root-Mean-Square Error of Cross Validation (RMSECV) Associated with a PLS Calibration of the Ion-Diffused Waveguide Response to Bromocresol Green Injections principal component RMSECV (10-6 M) 3.64 2.48 2.32 2.24 2.24
the various TM modes are observed to be approximately equivalent. The score plot (Figure 7b) reveals a series of transient profiles which indicate a monotonic waveguide response with increasing dye concentration. A qualitative interpretation of the score plot indicates that the entire concentration range down to 7.58 X l t 7M bromocresol green gave an observable waveguide response. The steady-state waveguide responseto bromocresol green was obtained by averaging three mode images from the transient response profile. The average mode image for each dye concentration was used in a PLS calibration. The results of a PLS calibration of the waveguide multiple mode response against bromocresol green concentration are given in Table IV. A four-factor PLS calibration for the waveguide response to bromocresol green gives a RMSECV of 2.24 X 10-6 M. Glycerol-Bromocresol Green Mixtures. The waveguide response to mixtures of glycerol and bromocresol green is expected to result in attenuation of the guided beam due to two different mechanisms. The incoupling efficiency for the TM:! mode is expected to change with glycerol concentration
Frguro 8. First plnclpal component of the bndlffused wavegulde response to glycerol-bromocresol green mixture Injections: (a, top) badln@lmage space variance: (b, bottom) score-sample space
variance.
since the boundary conditions for mode excitation are modulated with refractive index. Also, the incoupled TM modes are expected to undergo attenuation due to evanescent absorbance as the propagating beam interacts with bromocresol green. The waveguide response to triplicate injections of 20 sample solutions was subtraction corrected and averaged. The average waveguide response was mean centered, and a SVD was performed. The first principal component accounts for 77.9% of the total experimental variance. The loading plot (Figure 8a) describes variance associated with all three TM modes. The variance distribution maintains the shape and position of the various TM modes relative to the baseline image. Sirnilareto the case of a pure bromocresol green response, the variance contribution of each mode indicates a loss in mode intensity propagating within the waveguide. The primary waveguide response to a mixture of glycerol and bromocresol green is much like an absorbance response. However, the relative absorbance sensitivities of the waveguide modes differ significantly in the mixture matrix compared to in the pure dye solutions. The score plot (Figure 8b) description of sample space variance is unable to distinguish the waveguide response to pure glycerol solutions (solutions 1-4) from the baseline response. In contrast, all pure bromocresol green Solutions (solutions 5,7,12, and 16) give a response which is resolved from the baseline. The remaining mixture solutions exhibit responses which are perturbations of the pure bromocresol green responses. Thus, the first principal component of the waveguide response to the mixture sample set is due to absorbance of waveguide mode intensity.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993
Table V. Root-Mean-Square Error of Cross Validation (RMSECV) Associated with a PLS Calibration of the Ion-Diffused Waveguide Response to Glycerol-Bromocresol Green Mixture Injections
TM(2)
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1
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Flgurr 0. Second principal component of the iondiffused waveguide
response to glycerol-bromocresol green mixture Injactions: (a, top) loadlnglmage space variance; (b, bottom) score-sample space variance. The second principal component accounts for 14.8% of the remainingexperimentalvariancein the waveguide mixture responses. The loading plot (Figure 9a) exhibits the characteristic variance pattern observed in the pure glycerol experiment. The TM2 mode intensity decreases while the TMo mode intensity increases. The relative contributions of the various modes in describing system variance increase in the order TM1 < TMo < TM2. The score plot (Figure 9b) reveals that the pure glycerol sample responses are well resolved from the baseline. Pure bromocresol green solution responses are also apparent but are inversely related to the waveguide glycerol responses. Mixture solutions give negative or positive score responses, depending on the relative amounts of the two components. The second principal component of the waveguide mixture response is clearly related to changes in solution refractive index. The first two principal components account for 93.8% of the total variance in the mixture experiment. The remaining sample-relatedvariancerequires severalprincipal Components to describe and the physical meaning becomes increasingly difficult to interpret. However, a consideration of the first two principal components indicates an ability to differentiate between refractive index changes and absorbance changes in the mixture samples. The steady-state waveguide response to mixture solutions was defined by averagingfour mode images from the transient waveguide responseprofile. The average mode image for each mixture sample was combined with known concentration information to perform a PLS calibration. The PLS calibration results are given in Table V and indicate that a five-
principal component
RMSECV (g/L)
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0.61 0.47
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0.31
glycerol
0.25 0.24 0.25
bromocresol green RMSECV (lo$M) 6.16 5.52 4.14 4.11 3.71 3.67
factor model gives a RMSECV of 0.24 g/L (ARI = 2.8 X 10-5) for glycerol and 3.71 X 10-6 M for bromocresol green. Pb(NO&. Pb(N03)2waschosenasamodelsystemtotest the waveguide sensitivity to changes in surface-active refractive index. The divalent Pb2+ cation was chosen as it has no inherent absorbance at 632.8 nm, has a large molar refractivity, and interacts strongly with silica glass surfaces.2l To enhance the exchangecapacity of the s o d a - l i e silica glass surface, the waveguide was continuously exposed to a slightly basic carrier solution (pH 7.6). The binding of cations a t the waveguide surface constitutes the formation of an adlayer of increased refractive index relative to the solution phase. The primary response component was expected to be a decrease in coupled TM2 mode intensity as previously discussedfor changesin bulk refractive index. However, the waveguide sensitivity to an interfacial cation-exchangeinteraction should be significantly enhanced due to the location of the maximum in evanescent field strength at the waveguide surface. The waveguide response to Pb2+ was tested with five consecutive injections of 10-4 M Pb(N03)~. Each 100-pL injection corresponded to the introduction of 3.31 pg of Pb2+ into the flow system. Following multiple Pb2+ injections, a 1.6 X M nitric acid solution was injected to rinse and regenerate the glass cation-exchange surface by displacing Pb2+cations. The waveguide response to multiple Pb2+injections was mean centered, and a SVD was performed. The first principal component of the waveguide response to Pb2+ injections describes 98.5% of the total experimental variance. The loading plot (Figure loa) indicates that all three modes contribute variance to the waveguide Pb2+ response. No position shifting is observed, and all TM modes exhibit a decreasein intensity as Pb2+interacts with the glass waveguide surface. Rather than the transient response profiles observed in the glycerol experiment, the score plot (Figure lob) reveals that the waveguide response to each Pb2+ injection results in a step offset from the original baseline response. This is consistent with accumulation of bound Pb2+ cations at the waveguide surface. The first two injections gave response steps of approximately the same amplitude and reached a steady-state before the next injection was made. The waveguide response to the third injection was of similar amplitude but did not reach a steady state. The increased response amplitude for the fourth and fifth injections indicated an enhanced interaction between the glass surface and the Pb2+ cation. The observed variations in exchange sensitivity may follow from an increase in interfacial pH as surface hydroxyl groups are converted to Pb2+ binding sites. The original waveguide baseline response was recovered with a single injection of dilute nitric acid. The displacement of (21) Petersen,J. V.;Deasy,R.E.Proc. SPIE-Chem.,Biochem.,Enoiron. Fibers Sem. II 1990, 1368, 61-72.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993
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Flguro 10. First prlnclpal component of the londlffused wavegukle response to Pb2+ injections: (a, top) loadingimage space variance; (b, bottom) score-sample space variance.
First principal component of the iondiffused waveguide response to methylene blue injections: (a, top) loading-image space variance; (b, bottom) score-sample space variance.
Pb2+ with an excess of H+ occured quickly and without any apparent hysteresis. Estimates of the density of surface hydroxyl groups have been made by assuming various crystalline silica structures.22 Although the ion-diffusedwaveguide has an amorphous silica structure, the cited estimates serves as an upper limit for the density of binding sites on the soda-lime silica waveguide surface. Taking an estimated surfacedensity of 4.6 hydroxyls1 nm2results in the prediction of 1.2 X 1014cation binding sites in the surface area probed by the propagatingwaveguide beam. In general, only 10-2076 of the theoretical number of surface hydroxyls are available for ion exchange at favorable solution pH conditions.14 Therefore, saturation of all active cationexchange sites constitutes much less than a monolayer of cations. The occupation of all active binding sites with a single Pb2+ cation would result in a total bound weight of 13 ng of Pb2+. The waveguide response to each Pb2+injection indicates that the cation-exchange capacity of the glass waveguide surface was never reached. Thus, the ion-diffused waveguide responded with submonolayer sensitivity to Pb2+ cations. The second principal component is responsible for 1.3% of the remaining experimental variance. In the loading plot, the TM:, mode intensity decreases while the TMI and TMo mode intensities increase in response to Pb2+ injections. No mode position shifting is observed,and the variance described increases in the order TMo < TMI < TM2. The score plot indicates a series of transient response profiles which are
difficult to interpret due, in part, to an unstable baseline response. Methylene Blue. Methylene blue was chosen as a model system to test the waveguide response to a surface-active absorber. The interaction between the methylene blue cation and silica glass surfaceshas been described in the l i t e r a t ~ r e . ~ ~ ~ ~ The dye interaction is typically associated with an enhancement in waveguide responseover that predicted in considering simple evanescent absorbance theory. Significant tailing in the response profile to transient dye injections is also described. In addition, the choice of methylene blue is appropriate in that the blue cationic form of the dye is obtained at relatively low pH (i.e., 5.9) and has a large extinction coefficient (60 OOO L mol-' cm-l) a t the He-Ne wavelength. The primary waveguide response to methylene blue injections was expected to be a loss in the intensity of all TM modes due to evanescent absorbance. However, considerable enhancement in the waveguide sensitivity relative to the bulk absorbance case was expected due to the Coulombic interaction between the anionic glass and cationic dye. The attraction of dye to the waveguide surface would expose an increased concentration of methylene blue to a strong evanescent field. In general, the dye-glass interaction was not expected to be of sufficient strength to result in the formation of a stable adlayer on the glass surface. The waveguide response to five methylene blue sample solutions was tested in triplicate. For comparison, duplicate injections of 1 X 10-4 M bromocresol green were also made. The individual injections were subtraction corrected, and replicate responses were averaged. The resulting response
(22) Unger, K. K. Porous Silica; Elsevier Scientific Publishing Co.: New York, 1979; p 7.
Figure 11.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993
data was mean centered, and a SVD was performed. The first principal component accounts for 99.8% of the total experimental variance. The loading plot (Figure l l a ) indicates that the variance is distributed among the three TM modes with the variance contribution increasing as TMo C TM1C TM2. The actual mode sensitivities are approximately the same since the relative mode contributions to loading variance are consistent with the mode intensities in the baseline image. The modes maintain the same positions and peak shapes observed in the baseline image. The score plot (Figure I l b ) reveals that the waveguide response to dye injections results in smooth, transient profiles. The waveguide did not respond to 1 X M methylene blue. However, the response to 1 x lo4 M methylene blue completely extinguished the propagating intensity of all three waveguide modes. The waveguide response to 1 X lo4 M methylene blue is of approximately the same amplitude as the 1X lo4 M bromocresol green response. The factor of 1.6 in relative dye extinction coefficienb is insufficient explanation for a sensitivity enhancement of 2 orders of magnitude. The waveguide response profiles for methylene blue also show a large degree of tailing, further indicating an interaction between the dye and the glass waveguide surface. The conclusion is that the first principal component describes the absorbance of mode intensity by methylene blue with a large sensitivity enhancement relative to the bulk absorbance case which is associated with concentrating the cationic dye at the anionic waveguide surface. The second principal component accounts for 0.16% of remaining experimental variance in the waveguide response to methylene blue. The loading variance indicates that the TM2 mode intensity decreases while the TM1 and TMo mode intensities increase. The variance distribution is similar to that observed in the second loading plot in the Pb2+
experiment. The score plot response profiles corresponding to methylene blue injectionsexhibit some of the peakinversion features observed earlier in the waveguide response to refractive index changes. In contrast, the transient bromocresol green response has a smooth profile. The second principal component may be related to the refractive index perturbation associated with the formation of a transient adlayer at the waveguide surface.
CONCLUSIONS The mechanism of multiple mode population in ion-diffused waveguides is not well understood but is potentially of great importance in the development of planar waveguide sensors. We have observed mode-mode interactions as components of the ion-diffused waveguide response to solution-phase modulations. The analysis of these interactions provides more information about the chemical system that is in contact with the waveguide surface than is available by monitoring a single waveguide mode. The inclusion of multiple mode waveguide response information in a multivariate calibration provides a means to predict component concentrations in mixture solutions defining simultaneous changes in refractive index and absorbance.
ACKNOWLEDGMENT We thank Bruce Kowalski and the Chemometrics group at the University of Washington for numerous discussions on multivariate statistics.
RECEIVED for review October 19, 1992. Accepted January 29, 1993.
