Anal. Chem. 1990,62.2012-2017
2012
(19) Mugnier, J.; Valeur. B.; Gratton, E. Chem. Phys. Lett. 1985, 179, 217-222. (20) Lakowicz, J. R.; Cherek, H. Chem. Phys. Lett. 1988, 722, 380-384. (21) Parasassi, T.; Contl, F.; Gratton, E. CeN. Mol. Siol. 1988, 3 2 , 103-108. (22) Lakowicz. J. R.; Cherek, H.;Laczko, G.; Gratton, E. Biochim. Biophys. Acta 1984. 777, 183-193. (23) Lakowicz. J. R.; Gryczynski, I.; Cherek, H. J . Bioi. Chem. 1986,261, 2240-2245. (24) Lakowicz. J. R.; Laczko. G.; Gryczynski, I. Biochemistry 1987,2 6 , 82-90. (25) Wczynski, I.; Cherek. H.; Lakowicz, J. R. Biophys. Chem. 1988,3 0 , 27 1-277. (26) Lakowicz, J. R.; Gryczynski, I.; Wiczk. W. Chem. Phys. Lett. 1988, 149. 134-139 _. (27) Lakowicz, J. R.; Cherek, H.; Gryczynski, I.; Joshi, N.; Johnson, M. L Biophys. Chem. 1987,28, 35-50. 128) ~. Alcala. R . R.: Gratton. E.: Prenderaast. - . F. G. Bjo&vs. J . 1987. 51. 587-596 and 597-604. (29) Alcala, R . R.; Gratton, E.; Prendergast, F. G. Siophys. J. 1987, 5 1 , 925-936. (30) Lakowlcz, J. R.; Johnson, M. L.; Wiczk, W.; Bhat, A.; Steiner, R. F. Chem. Phys. Lett. 1987, 738, 587-593. (31) Lakowicz, J. R.; Gryczynski, I.; Cheung, H.;Wonk, C.; Johnson, M. C.; Joshi, N. Biochemistv 1988,27, 9149-9160. (32) Lakowicz, J. R.; Joshi, N. B.; Johnson, M. C.; Szmacinski, H.; Gryczynski, I . J . Biol. Chem. 1987,262, 10907-10910. (33) Lakowicz. J. R.; Johnson, M. C.; Gryczynski, I.; Joshi, N.; Laczko. G. J . Pbys. Chem. 1987, 9 1 , 3277-3285. (34) Lakowicz, J. R.; Laczko, G. R e v . Sci. Instrum. 1986, 5 7 , 2499-2506. (35) Weber, G. J. Phys. Chem. 1981. 85. 949-953. (36) Jameson, D. M.; Weber, G. J . Phys. Chem. 1981, 85, 953-958. (37) Jameson, D. M.; Gratton, E. In New Directions in Molecukr Luminescence; Eastwood, D., Ed.; ASTM Special Technical Publication 882; American Standards for Testing and Materials: Philadelphia, PA, 1983: pp 67-81. (38) Barrow, D.; Lentz. 8. J. Biochem. Biophys. Methods 1983, 7 , 217-234. (39) Lakowicz, J. R.; Cherek. H.; Baker. A. J. Biochem. BioDhys. . . Methods 1981,5 , 131-146. (40) Dalbey, R. E.; Weiel, J.; Perkins, W. J.; Yount, R . G. J. Biochem. Biophys. Methods 1984,9 , 251-266. (41) Lakowicz, J. R.; Cherek. H. J . Biochem, Biophys. Methods 1981,5 , 19-35. (42) Lakowicz, J. R.; Cherek, H. J. Bioi. Chem. 1981,256, 6348-6353. (43) Vesolova, T. V.; Cherkasov. A. S.; Shirokov, V. I . Opt. Spectrosc. 1970,24, 617-616. (44) Mousa, J. J.; Winefordner, J. D. Anal. Chem. 1974,46, 1195-1206. (45) b m a s , J. N.; Keller, R. A. Anal. Chem. 1985, 57, 538-545. (46) van Hoek, A.; Vlsser, A. J. W. G. Anal. Instrum. 1985, 14, 143-154. (47) Nithipitikon, K.; McGown, L. B. Anal. Chem. 1987,59, 423-427. (48) Lakowicz, J. R.; Keating, S . J . Biol. Cbem. 1983,258, 5519-5524. (49) Nwipkekon, K.: McGown, L. B. Appl. Specirosc. 1987,41, 395-399.
Keating-Nakamoto, S. M.; Cherek, H.; Lakowicz, J. R. Anal. Chem. 1987,5 9 , 271-278. Gratton, E.; Jameson, D. M. Anal. Chem. 1985,57, 1694-1697. KeatingNakamoto, S. M.; Cherek, ti.;Lakowicz, J. R . Anal. Sbchem. 1985, 148, 349-356. Keating-Nakamoto, S . M.; Cherek, H.; Lakowicz, J. R. Sbphys. Chem. 1988,24, 79-95. Bevington, P. R . Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York. 1969. Matayoshi. E. D. Biochemistry 1980, 19, 3414-3422. Spencer, R. D.; Weber, G. Ann. N . Y . Acad. Sci. 1969. 158, 361-376. Lakowicz, J. R.; Baker, A. Biophys. Chem. 1982. 16, 99-115. McGown, L. B.; Bright, F. V. CRC Crit. Rev. Anal. Chem. 1987, 18. 245-295. McGown, L. B. Anal. Chem. Acta 1984, 157, 327-332. Bright, F. V.; McGown, L. B. Anal. Chem. 1985,57, 2877-2880. Eftink. M. R.; Wasyiewski, 2.; Ghiron, C. A. Biochemlstry 1987, 26. 8338-8346. Peilegrino, F.; Aifono. R. R. Biobgical Events Robed by Ultrafast Laser Spectroscopy; Academic Press: New York, 1982; Chapter 2, pp 27-53. Bittermann, E.; Holzwarth, A. R.; Agei, G.; Nuksch. W. Photochem. Photobiol. 1988, 47, 101-105. Sebban, P.; Moya. I. Biochim. Siophys. Acta 1983, 727, 436-447. Sebban. P.; Jolchine, 0.; Moya, I . Photochem. Photobiol. 1984,39, 247-253. Sebban, P.; Bruno, R.; Jolchine. G. Photochem. Photobioi. 1985,42, 573-578. Laczko, G.; Gryczynski, I.; Gryczynski. 2 . ; Wiczk. W.; Malak, H.; Lakowicz, J. R. Rev. Sci. Instrum., in press. Lakowicz, J. R.; Baker, A. Siophys. Chem. 1982, 15, 353-360. Lakowicz. J. R.; Thompson, R. B.;Cherek, H. Siochim. Siophys. Acte 1983. 734. ..., . , 294-308 -- - -. Gkowicz, J. R.; Cherek, H.; Maliwal, B.; Laczko, C.; Gratton, E. J . BIOI. Chem. 1984,259, 10967-10972. Lakowicz, J. R.; Cherek, H.; Laczko, G.; Gratton, E. Biocbh. Biopbys. Acta 1977. 777. 183-193. Deciemy, A.;Rulliere, C. Chem. Phys. Lett. l98& 146, 1-6. Castner, E. W.; Bagchi, B.; Maroncelli, M.; Webb, S. P.; Ruggiero, A. J.; Fleming, G. R. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 363-372. Nagarajan, V.; Brearley, A. M.; Kank, T. J.; Barbara, P. F. J. Chem. PhyS. 1987,86. 3183-3196. Anthon. D. W.; Clark, J. H. J. Phys. Chem. 1988, 91, 3530-3536.
-.
.
I
RECEIVED for review December 28, 1989. Accepted June 8, 1990. This work was supported by grants from the National Science Foundation (DMB-8804931 and DMB-8502835). Joseph R. Lakowicz acknowledges support from the Medical Biotechnology Center of the University of Maryland.
Thin Film Planar Waveguide Sensor for Liquid Phase Absorbance Measurements Michael D. DeGrandpre a n d Lloyd W. Burgess* Center for Process Analytical Chemistry, Department of Chemistry, BG- 10, University of Washington, Seattle, Washington 98195
Patricia L. White a n d Don S. Goldman Battelle Pacific Northwest Laboratory, Richland, Washington 99352
A thin fllm planar waveaulde Is studled for appllcatlon as a chemical &r for Ilquid-phase absorbance measurements. The wavegulde Is comprised of a thin fllm of tantalum pentoxide deposited on a glass substrate with a pair of dmractlon gatlngs etched Into the substrate surface. The burled g r a m couplers aHow the launch and collection optics to be Isolated fromthe Hqukl sample. The response to an absorMng dye and dlfferent refractive Index ( R I ) solutions Is studied and compared to theoretical predictions. The sensor has an absorbance sensitivity equlvalent to a 1 mm path length in a conventlonal transmlsslon measurement. A method to reduce the Intensity changes due to solution R I is demonstrated.
INTRODUCTION Research on the theory and fabrication of thin film planar waveguides began about 20 years ago, primarily driven by the desire to make integrated optical circuits. The potential applications of thin film waveguides in chemical analysis were quickly recognized (1-3). Midwinter ( I ) first suggested that thin film waveguides could be used as internal reflection elements (IRES) in attenuated total reflection (ATR)spectroscopy but most research following Midwinter’s theoretical treatise has addressed the spectroscopic properties of the films themselves. Bohn recently reviewed the analytical spectroscopic applications of thin film waveguides ( 4 ) . Specific
0003-2700/90/0362-2012$02.50/062 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
studies cited include Raman and absorption of thin films (5-7) and fluorescence excitation of molecules adsorbed on the waveguide surface (8). In contrast to the thin film and adsorption studies, this study addresses application of a thin film planar waveguide for quantitative measurements of absorbing liquids, analogous to conventional ATR spectroscopy. ATR operates through absorption of the evanescent wave that is present at each point of total internal reflection. The evanescent wave penetrates only a fraction of the wavelength into the sample, allowing measurements to be made in highly scattering liquids, a definite advantage over most transmission or reflectance techniques. Furthermore, only a very small portion of the total guided energy is in the evanescent wave so ATR has effective path lengths 3-4 orders of magnitude less than a 1cm transmission measurement. These sampling advantages have been exploited to measure IR spectra with minimal sample preparation and have made ATR a popular spectroscopic technique among process chemists (9). Quantitative ATR of liquids is possible because the liquid makes intimate and reproducible contact with the IRE, although the short effective path length limits quantitation to major constituents of the liquid. The sensitivity is primarily limited by the total number of reflections possible in conventional IREs, from 10 to 100 for an IRE of practical physical dimensions. New waveguide technology can be used to provide a much wider range of reflections. In a previous study, we used fiber optics as IREs (10). The long length and small radius of multimode fiber optics increase the total number of reflections by a t least 1 or 2 orders of magnitude over conventional IREs. Thin film planar waveguide technology can also be used to increase the effective pathlength of ATR measurements. A thin film, on the order of 1 pm thick, can have thousands of reflections per centimeter. The planar structure is very durable and it readily lends itself to miniaturization. Unlike fiber optics, there are numerous materials which may be used as thin film waveguides, many of which may be easily spun or dip-coated onto the substrate (11). Several other researches have investigated thin film waveguides for ATR-type measurements (3, 12, 13). In these studies, prisms were used to couple light into the waveguide. Prism coupling presents several practical disadvantages for integration into a sensor. Since the prisms must be placed on the surface of the thin film, it is difficult to isolate the prisms from the liquid sample. Coupling efficiency is very sensitive to the input beam alignment and prism contact with the surface. Prism coupling is, therefore, only practical for benchtop applications. An alternative is to use grating couplers that are integrated directly into the waveguide structure. Surface gratings have been used by other researchers for immunosensing on the waveguide surface (14). In our study, gratings are fabricated in the substrate and are buried beneath the thin film as shown in Figure 1. The buried grating couplers are much more amenable to a sensor format because the beam input and collection optics are located behind the waveguide and are completely isolated from the liquid sample. This report describes the fabrication and response characteristics of a thin film waveguide sensor with buried grating couplers. EXPERIMENTAL S E C T I O N Grating Fabrication. Gratings were fabricated by the method described in ref 15. A Kr+ laser (Model 171, Spectra Physics) at 406 nm and Lloyd's mirror were used to record interference fringes in a positive photoresist (AZ1370, Hoescht Celanese). After the photoresist was developed, the substrate was etched either by reactive sputtering in an rf-diode system (SEM-8620,Materials Research Corp.) or by chemical etch with dilute buffered hydrofluoric acid (Transene Co., Inc.). The gratings used in this study were made by the plasma etch although the chemical etch
2013
sample
n3
n2
glass substrate
i/
Figure 1. Thin film planar waveguide with buried grating coupler (no = 1.0003, n , = 1.517, n 2 = 2.07, n 3 = sample RI).
is presently used because of its simplicity and reproducibility. The reactive sputtering gases argon and CHF, were chosen, based on their comparative etch rates (16). An argon plasma pre-etch was used to remove any residual photoresist from the grating wells followed by CHFBplasma, which etched the exposed pattern into the glass substrate. A 0.4-pm groove spacing was chosen to make coupling angles reasonably close to the waveguide normal, based on the expected film thickness and refractive index (RI) of tantalum pentoxide, the thin film chosen for this study. The input and output gratings were recorded 1.0 cm apart. Field emission scanning electron micrographs (SEM's) (Model S800, Hitachi, Inc.), shown in Figure 2 were taken of a cross section of the waveguide. The gratings can be seen at the interface between the substrate and the lighter colored thin film in Figure 2a. From these photographs the grating wells were measured to be approximately 700 A deep, sinusoidal in shape, with a groove spacing of 0.375 0.006 km. The grating period was slightly lower than expected due to the uncertainty in the angle of the incident beam when generating the interference pattern. Thin Film Fabrication. Metal oxide thin films such as Ti02, Vz05,and Ta205typically have very good mechanical and chemical durability and are very dense (low porosity). Tantalum pentoxide was chosen as the primary candidate because films have been fabricated with losses less than 1 dB/cm (17,18). Postthermal oxidation of sputtered @-tantalummetal and reactive rf sputtering techniques have been used to produce low loss Ta205waveguides. Reactive rf sputtering was used to fabricate the films used in this study. An rf-diode (Materials Research Corp.) system using a power density of 1 W/cm2, with a 5:l ratio of argon to oxygen at 14 mTorr, deposited Ta205at a rate of approximately 40A/min. The SEM's in Figure 2 show a corrugated pattern on the surface of the thin film indicating that the grating was replicated on the surface of the film. Surface gratings have also been observed with A1203films sputtered on grating substrates under similar conditions (19). Thin Film Characterization. Initial estimates of the Ta206 film RI and thickness were obtained by measuring the interference fringes from a UV-vis transmission spectrum of the coated substrate (20). The estimated RI and thickness (RI = 0.5 rm) Kern Internawere then used in a BASIC program (MODCHART, tional Inc.) that solves eq l, the eigenvalue equation, to obtain the film mode angles for a specific wavelength (6) K2t2- tan-' (K3/K2) - tan-' ( K 1 / K 2 )= mr (1) Ki = Vi =
- k02n,211/2 TE modes
(2)
K i = V i / $ TM modes (3) @ = k,n2 sin B2 (4) where t 2 is the film thickness, m is the mode order (0, f l , f2, ...), ko is the free space wave vector which is equal to %/A, ni is the RI of region 1,2,or 3 in Figure 1, p is the propagation constant, and B2 is the angle of the mode in the film (Figure 1). The estimated coupling angles were calculated by the guided-wave grating equation @ = kono sin 8,
+ P2* A
(5)
where no is the RI of air, Bo is the incident angle in air, p is the
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 18. SEPTEMBER 15. 1990
now in
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now out
Instrumentation f w thin film waveguide sensw measure-
ments. Table I. Modes and Angles (deg) for the Tantalum Pentoxide Waveguide mode
coupling angle ($)
TEo
film angle (8,)
reflections/cm 1780 3620 2230 4420
19.333
76.555
TE.
9.333
63.027 .~
TW
18.417 5.583
74.864 59.312
TG
account the Goos-Hanchen shift that occurs for each reflection (21).
Scanning electron micrograph of (a1 grating and thin film section and (b) surface of Ta,OS film.
Flgure 2. Cross
diffraction order (*I, +2, ...), A is the grating period, and 0 is the propagation constant determined by eqs 1-3. The waveguide was set up as shown in Figure 3 to determine the actual coupling angles. The thin film waveguide sensor was mounted on a rotation stage (Model 471, Newport Corp.). Coupling angles were measured by approximating zero, or normal incidence, as the coincidence of the incident and reflected beams and then rotating until coupling was observed. The approximation of normal incidence limited the angular resolution to approximately i1 arc min. A translation stage (Model 420, Newport Corp.) was used to adjust the input heam to the leading edge of the grating and a lab jack (Model 270, Newport Corp.) provided vertical movement to select different portions of the waveguide for measurement. Output from a 4-mW 5OO:l polarized HeNe laser (Model 1104P, Uniphase, Inc.) was coupled into the waveguide. The collimated beam was used unless noted otherwise. Grating coupling was found to he greatly superior to prism coupling in the ease with which mode angles can be found. Finding a mode involves simply rotating the waveguide and keeping the input heam on the leading edge of the grating. The modes and angles for the waveguide used in this study are shown in Table I along with the number of reflections per centimeter for each mode. The number of reflections was calculated by taking into
After the coupling angles were determined, a more accurate estimate of the waveguide RI and thickness wa8 obtained by iteratively solving eq 1 using two different modes (22). A Fortran program was written for this purpose. The two TM modes yield a thin film RI of 2.0754 and thickness of 0.530 pm while the two T E modes yield an RI of 2.0734 and thickness of 0.539 pm. The differencein RI between the two modes is greater than the *O.oOOS error in the film RI introduced by the *1 arc rnin angle precision. A slight birefringence, which has previously been observed in Ta,O, films (18), may account for the difference. With these parameters a second order mode (TE, or TM,) is predicted to be allowed but very ne= cutoff. Became the TE, and TM, modes were never observed, we assume that the calculated RI and/or thickness of the film are slightly in error. Thin film attenuation due to ahsorption and scattering was measured by moving a fiber optic (High numerical aperture (NA) 400 pm diameter HCS fiber, Ensign Bickford) along the scattered streak of the guided wave. Decibels (dB) were calculated from dB = 10 log (P,/P,) (6) with P,the intensity at some point along the waveguide and P, the intensity at the starting point (approximately 2 mm from the input grating). The slope of the linear attenuation curves ranged from 12 to 18 dB/cm depending upon the mode and portion of the film examined. The losses from the first-order modes were greater than the zero-order mode losses in all cases. Higher order modes often have increased losses due to surface scattering becaw the electric field (E-fold) is larger for these modes at the film intmface (23). It has been shown that the primary loss mechanism in Ta,O, films is scattering at the dielectric interfaces (18)and judging hy the high degree of surface roughness shown in Figure Zh, this is probably an important factor. The SEMs show a large variability in film thickness which can also lead to high losses.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
. 2 w
TM,
A
TE,
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C
p '
0
B
d
Table 11. Experimental and Theoretical Absorbance Response of the Tantalum Pentoxide Thin Film Waveguide Sensor Reported as the Path Length Equivalent to a Transmission Measurement
TM,,
0
g
0
0.20-
".A
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-
0.0
-0.4
2015
-
0.8
I 1.2
1.6
2.0
2.4
Beer's Law Response (Cm' ) Figure 4. Absorbance response of thin film waveguide sensor for different modes and polarizations versus the Beer's law response obtained with a UV-vis spectrophotometer. Film quality was shown to be quite sensitive to small changes in oxygen content, sputtering pressure, and gas flow rate (17). Since the film used in this study was fabricated, we have improved the control of these parameters and reduced losses to approximately 5 dB/cm. Additional studies included ESCA (electron spectroscopy for chemical analysis) scans of the waveguide surface which indicated a chemical shift characteristic of Ta205. Surface contact angles with H 2 0 were larger than expected for a metal oxide surface, although surface roughness can cause a large increase in contact angle compared to a smooth surface (24). A Ta205thin film deposited on a substrate without gratings was tested for resistance to various concentrated acids. Using prism couplers, no changes in waveguide propagation angles were observed after immersing the waveguide for 10 min in 10 M HC1, "OB, and H2S0,. Sample Measurements. In order to evaluate the thin film waveguide response to solution absorbance and refrative index, a flow cell was fixed over the waveguide surface as shown in Figure 3. The flow channel was approximately 1.2 cm long and 0.3 cm wide to ensure that the gasket did not contact the film in the waveguiding region. The flow cell gasket was made from silicone rubber (PS2061, Petrarch, Inc.). Samples were introduced over the sensor using a peristaltic pump (Mino-S 860, Isco, Inc.). Light from the outcoupling grating was focused onto a photodiode detector (DF-633, EG&G Photon Devices). The input beam was mechanically chopped at 200 Hz and the detector output was processed by a lock-in amplifier (SR530, Stanford Research Systems, Inc.). The time constant was set at 1 s. Laser signalto-noise (S/N) and intensity drift measured with this equipment were approximately 700 and *1.5% per hour, respectively. Total system throughput was 0.17% of the incident beam intensity for the TE&mode measured with a power meter (22XLC, Photodyne, Inc.). The low throughput is a combination of film losses and poor grating incoupling efficient (estimated to be less than 10%). An order of magnitude improvement in throughput could be achieved by reducing film loss to 1 dB/cm. Grating efficiency may be improved by empirically optimizing groove depth and profile (25). All sample RI's were measured with an Abbe refractometer (Bausch and Lomb) calibrated with RI standards (f0.0002 RI units) (R.P. Cargille Laboratories).
RESULTS AND DISCUSSION The absorbance responses of the thin film waveguide sensor using the two different modes and polarizations are shown in Figure 4. The data represent the sensor's full response at 632.8 nm with an absorbing dye flowing over the surface. Concentrations of bromothymol blue (BTB) (Aldrich Chemical) ranging from 4 X lo4 to 1 X lo4 M in pH 8.02 phosphate buffer were used as the test solutions. A blank correction was applied by measuring Io with only buffer (no dye present) flowing over the waveguide surface. The stream was then switched to buffer containing various concentrations of the dye to obtain I. The sensor response is plotted versus Beer's
experimental values (Figure 4) y int (X10-2) slope, cm
mode
corr coef
TEO TEl TMO TMi
0.9799
0.9841 0.9815 0.9860
theory, cm
0.243
0.0110
0.759 0.537
0.0357
0.0095 0.0484
0.0273
0.0198
0.0969
0.1080
1.90
Table 111. Analytical Performance of the Thin Film Waveguide Sensor (LOD Calculated from 3 X Root Mean Square Noise) mode TEO TE1 TMO TM,
LOD (AU X 1.7 1.7
1.9 2.6
LOD (M X lo*)
6.9 2.1 3.1 1.2
law response obtained from a conventional UV-vis spectrophotometer (Model 3840, Perkin-Elmer Corp.). The ratio of the sensor response to the Beer's law response (the slope of the lines) gives the effective path length that can be compared to theoretical values. The slopes and other curve statistics are shown in Table 11. The TM1 mode exhibits nearly 10% of the absorbance sensitivity of a 1 cm transmission measurement; thus, the sensor has an effective path length of approximately 1mm. The TM1 mode is more sensitive than the other modes due to the larger number of reflections and the higher Eopresent a t the Ta205/solution interface (normal component of the incident polarized beam) (26). The expected response, based on integration of the square of the E-field (evanescent field) in the outer solution (27) is also reported in Table 11. The theoretical values were normalized to 1 W/unit width of waveguide. Theory and experiment match very well considering the many parameters that are required to make the theoretical calculations (Le., film RI and thickness, substrate RI, sample RI and grating period). Other researchers found that theory underestimated their waveguide response because surface adsorption concentrated the dye at the waveguide surface (3). The rapid response and reproducible baseline of the Ta205waveguide sensor indicated that the BTB was not adhering to the surface, and the theoretical response supports these observations. Table I11 summarizes the analytical performance of the thin film waveguide sensor. Limit of detection (LOD) was calculated by using 3 times root mean square noise over a set time period. Laser intensity fluctuations were the primary source of noise although the TM, mode proved to be sensitive to flow variations and had a slightly higher LOD. Concentration LOD's calculated from Figure 4 and using the BTB molar absorptivity ( t = 2.24 X lo4 M-' cm-' at pH 8.02 and X = 632.8 nm) are also shown in Table 111. Stray light levels were approximately 0.5% which limits the dynamic range to 2 AU, although this was not tested. Each measurement was repeated a t least once for each concentration and these data are also plotted in Figure 4. It is believed the flow cell gasket allowed diffusion of the absorbing solution around the edges, which limited the precision of these measurements, particularly at the higher dye concentrations. Five earlier replicate measurements at a single concentration (7 x lo" M BTB) using the TM1 mode indicated that a reproducibility of *2.7% (f0.004 AU) could be achieved. The sensor full response was achieved in less than 30 s and is limited by the flow cell dead volume, not the dynamics of the sensor itself.
2016
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990 0.58
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.0.25
-0.75
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1.450 1,450
0.25
0.75
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Figure 7. Intensity versus launch angle with collimated and focused input beams (TE, mode).
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Refractive Index (n,) Figure 6, Experimental and theoretical (dashed line) changes in coupling angle measured for different R I solutions (glycerol in water) for the TE modes.
In the optical waveguide system described above, signal attenuation may be caused not only by evanescent absorbance but also by RI changes in the sample. At least three effects can modulate the light intensity as a function of changing RI in the sample layer. The first is a change in the surface grating input and output contributions as a function of changing reflectivity at the surface grating/sample interface. A second effect is a change in the amount of surface scatter, which will vary as the RI difference between the waveguide and sample varies. And finally, the presence of the superstrate RI, n3, in eqs 2 and 3 indicates that the propagation angle will vary with changes in the sample RI. To test the magnitude of these effects, samples ranging in RI from 1.3355 to 1.4507 were prepared by mixing glycerol (RI = 1.4746) with deionized water (RI = 1.332). Each sample was pumped through the flow cell and the angle was manually returned to give the maximum intensity. The change in coupling angle (0,) is shown for the T M and T E modes in Figures 5 and 6. The TMI mode undergoes the largest angle change and the TEo mode the least, in the same order as the response to absorbing solutions. The coupling angle increased with increasing RI for all modes. The TM1 mode is sensitive to a 3 x lo4 change in RI, calculated from Figure 5, assuming a f 1 2 arc sec resolution of the intensity peak. Theoretical curves, calculated by changing n3 in eqs 2 and 3, are shown as dashed lines in Figures 5 and 6. The coupling angle sensitivity to R I is greater than that predicted by theory for all modes. This may be due to glycerol concentrating a t the interface, although large deviations in the theoretical slope
I
138
1 39
1 40
(n,)
Figure 8. Response to different R I solutions (glycerol in water) for 100 mm focal length lens and a collimated beam (TE, mode).
may occur solely because of the *60 A error in the measured grating period. In order to minimize the error introduced by the grating period in the calculations, the grating period must be known to within 10 A. The surface grating coupling efficiency is dependent upon n3 (28)so changes in n3 would modulate the output intensity if the surface grating was also coupling light into the waveguide. It was noted that after returning the waveguide to the new coupling angle, the total light throughput remained approximately the same. This is a good indication that the grating present on the top of the film did not greatly influence the response of the sensor. We discovered that by focusing the input beam at the incoupling grating, and thus providing a range of incident angles on the grating to accommodate small changes in resonance coupling conditions for a given allowed mode, intensity changes can be reduced over a wide range of sample RIs. Figure 7 shows how different focal length lenses change the outcoupled intensity distribution as one changes input angle or the TE, mode. The shoulder on the intensity profile using the collimated input beam is a result of nonuniform film thickness. The responses due to changes in sample RI are shown in Figure 8 for both collimated and focused input beams. The response with a focused beam initially shows a slight increase in intensity which may be due to decreased scattering at the interface as n3 increases (23). Nonetheless, sensitivity to RI is greatly diminished versus the collimated beam response. Although intensity changes due to variations in sample RI are reduced by focusing the input beam, the absorbance response is still dependent upon sample RI because the eva-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
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nescent field intensity changes and alters the effective path length of the sensor. We did not measure the magnitude of this effect, but theoretical calculations (27) indicate that variations in RI up to kO.1 do not significantly change the sensor response. The properties of the thin film, both in the bulk and on the surface, affect propagation; therefore, it was important to determine if the film had any chemical interactions with the sample. Ionic strength and pH effects have been reported previously for a SOz-TiOz thin film waveguide (29). We tested the TA205waveguide with different ionic strength solutions (1.1-7.2 M NaCl and 1.2-5.9 M NH,CL) and different pH solutions (5.8-8.0 phosphate buffer). In all cases, the response of the waveguide coincided with the experimental curves in Figures 5 and 6, showing strictly a RI response with no additional effects due to interactions on the surface or within the bulk of the film. In addition, the response times were the same as those for the glycerol samples, slower responses would be expected if chemical reactions or ion exchange was taking place. It should be noted that a dilute surfactant (Triton-X, Sigma Chemical) (0.1 vol %) in a pH 8 buffer caused angle changes (TEI mode) opposite of that expected for the bulk solution RI measured by an Abbe refractometer. In addition, the uncharacteristic slow return to baseline indicated that the surfactant had a strong affinity for the surface. Absorbing analytes with similar surface interactions would concentrate at the waveguide surface, resulting in an analytical response that is not a direct representation of the bulk concentration. Surface derivitization using silane reagents and/or polymers may be used to deactivate the structure in some cases.
wavelength encountered in ATR may be exacerbated with thin film wavegides because the angle of incidence is smaller for longer wavelengths. Both of these conditions result in greater effective path lengths. Determining the broad band response of the thin film waveguide sensor is presently an active area of research in our lab.
CONCLUSIONS Typical internal reflection elements, with light propagating a t angles similar to the angles in the Ta205thin film waveguide, may have a total number of reflections ranging from 10 to 100. Thin films have from 1 to 3 orders of magnitude more reflections as noted in Table I, and consequently, 1-3 orders of magnitude more sensitivity. The buried gratings permit integration of the couplers into the sensor and this has proven to be a significant practical improvement over prism coupling. The buried gratings make coupling light into the film as straightforward as coupling into conventional IRES. An important difference from ATR is that both the propagation angle and coupling angle depend upon wavelength. Polychromatic light must be input over a wide angular range and each wavelength will have different effective path lengths in the sample. The effective path length dependence on
Kogelnik, H. I n Integrated Optics; Tamir, T., Ed.; Springer-Verlag: New York, 1979;Vol. 7,pp 26-27. Ulrich, R.; Torge, R. Appl. Opt. 1973, 72(12),2901. Zernike, F. I n Integrated Optics; Tamir, T., Ed.; Springer-Verlag: New York, 1979;Vol. 7,p 217. Adam, N. K. I n Advances in Chemistry Series; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964;Vol. 43,p 55. Tamir, T. I n Integrated Optics; Tamir, T., Ed.; Springer-Verlag: New York, 1979;Vol. 7,p 113. Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corporation: New York, 1979. Heuberger, K.; Lukosz, W. Appl. Opt. 1986, 25 (9),1499. Taylor, H. F.; Yariv, A. Roc. I€€€ 1974, 62(8),1044. Spohn, P. K.; Seifert, M. Sens. Actuators 1988, 75,309.
ACKNOWLEDGMENT Sincere thanks to Dave Tracy of Perkin-Elmer Corporation for aiding with the theoretical calculations. LITERATURE CITED Midwinter, J. E. IEEE J. Qauntum Electron. 1971, E - 7 (7), 339. Midwinter, J. E. IEEE J . Quantum Electron. 1971, E - 7 (7), 345. Polky, J. N.; Harris, J. H. J. Opt. SOC.Am. 1972, 62(9),1081. Bohn, P. W. Prog. Anal. Spectrosc. 1989, 72,41. RaboR, J. F.i Santo, R.; Swalen, J. D. Appl. Spectrosc. 1979, 33,
549. Bohn, P. W. Anal. Chem. 1985, 57 (7),1203. Stephens, D. A.; Bohn, P. W. Anal. Chem. 1987, 59(21),2563. Ives, J. T.; Reichert, W. M. Appl. Spectrosc. 1988, 42(1),68. Hutchinson, R. J. Anal. Roc. 1987, 24,213. DeGrandpre, M. D.; Burgess, L. W. Anal. Chem. 1988, 60(23),2582. Swalen, J. D.; Santo, R.; Tacke, M.; Fischer, J. IBM J. Res. Dev.
1977, 27,168. Mitchell, G. L. I€€€ J. Quantum Electron. 1977, E - 7 3 (4),173. Choquette, S.J. Ph.D. Thesis, Virginia Polytechnic Institute and State University, 1988. Lukosz, W.; Tiefenthaler, K. Sens. Actuators 1988, 75,273. Moshrezadeh, R.; Mai, X.; Seaton, C. T.; Stegeman, G. I.Appl. Opt.
1987, 26 (13).2501.
Lehmann, H. W.; Wdmer, R. Appl. Phys. Left. 1978, 32 (3),163. Davis, R. L.; Hickernell, F. S. SPIE vol. 408, Integrated Optics I11
(1983),27. Hensler. D. H.; Cuthbert, J. D.; Martin, R . J.; Tien, P. K. Appl. Opt. 1971, 70 (5),1037. Chen, B. U.; Ghizoni, C. C.; Tang, C. L. Appl. Phys. Left. 1976, 28
(ll),651. Manifacier, J. C.; Gasiot, J.; Fellard, J. P. J. Phys. E : Sci. Instrum.
1978, 9,1002.
RECEIVEDfor review April 4, 1990. Accepted June 19, 1990. This work was supported by a grant from the NSF Center for Process Analytical Chemistry (Project Number 84-1). M.D. thanks the Department of Energy for a Science and Engineering Research Semester Fellowship.
