- Email: [email protected]
A sum parameter sensor for water quality
PII: S0043-1354(98)00317-0
Wat. Res. Vol. 33, No. 5, pp. 1147±1150, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
A SUM PARAMETER SENSOR FOR WATER QUALITY J. J. RAMSDEN* Biozentrum, 4056 Basel, Switzerland (First received March 1998; accepted in revised form July 1998) AbstractÐAn integrated optical grating coupler incorporated in an optical waveguide is used to monitor water quality. The primary measured parameter is the phase velocity of a selected guided mode. Upon exposure to a water sample, the device responds extremely rapidly to contaminants. The response is a sum parameter comprising contributions from the refractive index of the bulk sample, dissolved and suspended species which can adsorb at the surface of the waveguide, and small ions which can penetrate into the waveguiding layer. The waveguide surface can be readily modi®ed in order to confer a particular sensitivity pro®le to contaminants of interest. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐadsorption, biosensors, chemosensors, dissolved impurities, evanescent wave, integrated optics, sum parameter, suspended impurities, waveguides
INTRODUCTION
Clear natural water contains many substances apart from H2O, ranging from dissolved monatomic ions to suspended virus particles. The identi®cation and quantitative analysis of all the individual components is a lengthy and costly process. For many purposes, however, such detailed knowledge is not necessary and, if it can be measured rapidly and cheaply, a sum parameter to which many of the components contribute can be very useful, for example as an immediate warning of a change in water quality, which can be followed up by more detailed analysis, for example to determine whether the change is potentially hazardous. Here I describe how an integrated optics device, a grating coupler sensor, can be used to access such a sum parameter. The sensing pad is an optical waveguide, specially treated such that all impurities of interest interact with it. This interaction is transduced into an optical signal (the phase velocity of the guided light), which can be monitored with high precision.
PRINCIPLE OF OPERATION
An optical waveguide consists of a high refractive index slab F surrounded by lower refractive index material S (Tien, 1971, 1977). Light is con®ned within F. The solution of Maxwell's equations for an electromagnetic wave propagating in such a strati®ed medium corresponds to a standing wave in F and an exponentially decaying evanescent wave in S (Tien, 1971, 1977; Ramsden, 1993b). The standing *[Tel.: +41-61-2672193; Fax: +41-61-2672189; E-mail: [email protected]].
waves exist as discrete modes characterized by eigenvalues N, called the eective refractive index, which is the ratio of light velocity in vacuo to the phase velocity of the mode in the waveguide. To make such a structure into an environmental sensor, the material S is partly removed to expose the F slab to the environment C (Fig. 1). Any change of the composition of C within the evanescent ®eld will change the N of all the guided modes which can exist in the waveguide. The N can also be changed by the adsorption of material at the F,C interface to form an adlayer A and by penetration of material into F, changing its refractive index nF, if its structure allows such a process to take place. The relationship between the N and nF, nC, nA and adlayer thickness dA is given by the mode equations (Tien, 1977; Tiefenthaler and Lukosz, 1989; Ramsden, 1993b). An extremely accurate and convenient way to measure the N is via in- or out-coupling at a diffraction grating incorporated into the waveguide (Tien, 1977; Tiefenthaler and Lukosz, 1989; Ramsden, 1993b). A major advance in the practical application of the grating coupler to sensing problems was the development of inexpensive processes with which precision gratings could be produced a thousand times more cheaply than by ruling (Lukosz and Tiefenthaler, 1983; Heuberger and Lukosz, 1986). EXPERIMENTAL
Planar optical waveguides incorporating a grating coupler (grating constant L = 416.5 nm) were obtained from Arti®cial Sensing Instruments, ZuÈrich (type 2400). These are thin, so-called monomode waveguides capable of supporting only the zeroth transverse electric (TE) and trans-
1147
1148
J. J. Ramsden
Fig. 2. Schematic diagram of the ¯ow arrangement. The waveguide is mounted on a rotating goniometer with which the angle of incidence a of the external light beam L onto the grating G is measured. Photodiodes P detect the incoupled light. M is the membrane used to change the selectivity pro®le of the waveguide surface. RESULTS
Fig. 1. Electromagnetic ®eld distribution within the waveguide. The guided lightmodes are standing waves within F and evanescent waves in S and C. verse magnetic (TM) modes. The F layer was made from a pyrolyzed silica/titania precursor and had a surface composition of Si0.62Ti0.38O2 (measured by X-ray photoelectron spectroscopy), a surface roughness of 0.09 nm (measured using an atomic force microscope with a standard silicon nitride tip of radius 20±30 nm) and a porosity of 0.16 (Ramsden, 1994). The waveguides were equilibrated in triply distilled water before use. The mode spectrum was determined using an IOS-1 integrated optics scanner (Arti®cial Sensing Instruments, ZuÈrich). The waveguide formed one wall of a small ¯owthrough cuvette through which water samples were pumped at a wall shear rate g of 18 sÿ1 (Fig. 2). After ¯ushing with distilled water, the waveguides were regenerated for the next sample by brie¯y ¯ushing with decinormal HCl. The original baseline was regained a few minutes after switching back to distilled water. A complete measuring cycle thus comprised: (1) distilled water, (2) sample, (3) distilled water and (4) 0.1 M HCl. The same wall shear rate was used throughout. All measurements were carried out at a constant temperature of 25.38C. The absolute value of NTE,0 was determined to a precision of 21 10ÿ6 every 2.5 s. A second set of experiments was carried out with waveguides coated with a lipid bilayer membrane. Synthetic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl choline (POPC) was used as the lipid. The coating was carried out using a combination of the Langmuir±Blodgett and Langmuir±Schaefer techniques as described previously (Ramsden, 1993a).
Figure 3 shows the response of the waveguides to the dierent water samples. Since dierent waveguides have slightly dierent thicknesses dF and refractive indices nF, the results are normalized as NÄ, de®ned as: N~ NTE ÿ N0 =N0 ,
1
where N0 is the value of NTE in the presence of distilled water prior to exposure to the sample. After an initially rapid increase of NÄ, the rate of increase diminished and the response appeared to approach a plateau. In the case of tap water ¯owing over a POPC bilayer-coated waveguide, the signal returned to zero upon ¯ushing with distilled water. In the other cases investigated, a small residue remained, which was removed by the HCl ¯ush.
DISCUSSION
The initial state, N0, is determined by C = pure (distilled) water. When a water sample is introduced into C, three processes are possible, in order of decreasing rapidity: 1. The refractive index of C, nC, may change;
Water samples Rhine water was collected in February from the St. Johann bathing pontoon in Basel. Basel tap water was collected from the laboratory supply in the Biozentrum after allowing it to run for 5 min. Basel tap water predominantly comes from the Rhine, whose water is used to enrich the groundwater sources from which the tap water is pumped (Aschwanden, 1993).
Fig. 3. Plot of NÄ vs. time: (ÐÐÐ) Rhine water/Si(Ti)O2, (- - -) Basel tap water/Si(Ti)O2, (Ð Ð Ð) Rhine water/ lipid bilayer, ( ) Basel tap water/lipid bilayer. Samples began ¯owing at t = 0 and the arrows mark the start of ¯ushing with distilled water.
A sum parameter sensor for water quality
1149
Table 1. Sensitivity coecients Coecient @N/@nC @N/@G @N/@nF
Typical value
Unit
D
t1/2 (s)
0.1 3.2 10ÿ3 0.7
ÿ cm2/mg ÿ
10ÿ5 1 ng protein/cm2 10ÿ6
0.1 100 5000
D is an estimate of the minimum change in the value of the denominator required to produce a detectable signal, assuming that N can be measured to a precision of one part per million. t1/2 is an estimate of the time needed to reach half the level of response saturation.
2. Large molecules and aggregates, viruses, etc., may adsorb at the F,C interface, forming an adlayer A characterized by a surface excess G, de®ned as: G dA nA ÿ nC
dn , dc
2
where dn/dc is the mean refractive index increment of the adsorbed material. 3. Small molecules and ions may diuse into the ®lm F, changing nF and possibly even dF. Typical values of the sensitivity coecients and characteristic response times for these three processes are given in Table 1. Since the bulk refractive indices of the water samples diered by less than 10ÿ5 from that of pure water (measured with an LI3 (Carl Zeiss, Jena) Rayleigh interferometer), the measured response must result from either the formation of an adsorbed adlayer on the waveguide, or penetration of material into the F-layer. From the observed kinetics, it appears that the change in NÄ is predominantly due to the adsorption of macromolecules and particles (it is known, for example, that drinking water contains many viruses (Bergh et al., 1989)), since the kinetics characterizing penetration of material into the F-layer (Ramsden et al., 1993) are much slower than the rate of change of NÄ observed here. A sample of natural water can be characterized by hundreds or even thousands of parameters corresponding to dierent impurities, some of which may mutually interact, making their individual determination problematical (matrix eects). Here we have a response whose magnitude and kinetics (of both the interaction and its reversal upon ¯ushing with pure water) are globally characteristic of the sample under scrutiny. Two possible approaches to evaluating the sum parameter data are: (i) to identify a small number of key response parameters, such as the initial rate of change dN/dt and the fractional drop in NÄ when the sample is ¯ushed with distilled water, which can be correlated with the particular impurities present in the sample, and (ii) to record the values of N at predetermined intervals of time following the start of exposure to the sample, possibly using two or more dierent kinds of surface and including the change in NÄ upon ¯ushing with pure water, and feed them into a neural network trained to recognize a large variety
of dierent water samples (e.g. Goodacre et al., 1992; Cherquaoui and Villemin, 1994).
CONCLUSIONS
Fully hydrated silica±titania waveguides are inde®nitely stable in pure (distilled) water. In the presence of high-quality potable water, however, the phase velocities of guided lightmodes in the waveguides rapidly and markedly slow down, apparently due to the adsorption of dissolved and suspended matter at the surface of the waveguide. The phase velocities can be conveniently measured via a grating coupler. The kinetics of the interaction and its reversal upon ¯ushing with pure (distilled) water can be followed with good time resolution and these kinetics, as well as the absolute magnitudes of the reponses, are characteristic of the water impurities. By modifying the surface coating the waveguide, dierent sensitivity pro®les to a range of impurities can be obtained and, presumably, even a highly selective response to a particular impurity. In order to cause a response, the impurities must be optically detectable (i.e. have a refractive index dierent from that of pure water) and must be enriched at or in the waveguide, either by the formation of an adsorbed adlayer (larger molecules, viruses, bacteria) or by diusing into the waveguide (small molecules). The response saturates upon prolonged exposure to impurities. Hence the waveguide should be exposed intermittently to the sample, followed by ¯ushing with pure water and regeneration (e.g. with HCl). An important advantage of integrated optics for water monitoring is that the whole device can be miniaturized and interfaced to a data-processing electronic chip and in the not too distant future chips incorporating both sensing and optical dataprocessing functions should become available (Kunz, 1992). Therefore, low-cost devices could easily be made widely available to domestic and recreational users for the maintenance of public health, supplied to members of expeditions and dispersed over large tracts of terrain for environmental monitoring.
1150
J. J. Ramsden REFERENCES
Aschwanden W. (1993) Geschichte und Gegenwart der Basler Wasserversorgung (Water supply in Basel-past and present). Energie Wasser 2, 6±9. Bergh é., Bùrsheim K. Y., Bratbak G. and Heldal M. (1989) Nature (Lond.) 340, 467±468. Cherquaoui D. and Villemin D. (1994) Use of a neural network to determine the boiling point of alkanes. J. Chem. Soc. Faraday Trans. 90, 97±102. Goodacre R., Kell D. B. and Bianchi G. (1992) Neural networks and olive oil. Nature (Lond.) 359, 594. Heuberger K. and Lukosz W. (1986) Embossing technique for fabricating surface relief gratings on hard oxide waveguides. Appl. Opt. 25, 1499±1504. Kunz R. E. (1992) Totally integrated optical measuring sensors. Proc. SPIE 1587, 98±113. Lukosz W. and Tiefenthaler K. (1983) Embossing technique for fabricating integrated optical components in hard inorganic waveguiding materials. Opt. Lett. 8, 537± 539.
Ramsden J. J. (1993a) Partial molar volume of solutes in bilayer lipid membranes. J. Phys. Chem. 97, 4479±4483. Ramsden J. J. (1993b) Review of new experimental methods for investigating random sequential adsorption. J. Statist. Phys. 73, 853±877. Ramsden J. J. (1994) Porosity of pyrolyzed sol-gel waveguides. J. Mater. Chem. 4, 1263±1265. Ramsden J. J., Roemer D. U. and Prenosil J. E. (1993) Microporous monomode sensors as waveguides. In Proc. 6th European Conference on Integrated Optics, Neuchatel, pp. 12-38±12-39. Tiefenthaler K. and Lukosz W. (1989) Sensitivity of grating couplers as integrated-optical chemical sensors. J. Opt. Soc. Am. B 6, 209±220. Tien P. K. (1971) Light waves in thin ®lms and integrated optics. Appl. Opt. 10, 2395±2413. Tien P. K. (1977) Integrated optics and new wave phenomena in optical waveguides. Rev. Mod. Phys. 49, 361±420.
Wat. Res. Vol. 33, No. 5, pp. 1147±1150, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
A SUM PARAMETER SENSOR FOR WATER QUALITY J. J. RAMSDEN* Biozentrum, 4056 Basel, Switzerland (First received March 1998; accepted in revised form July 1998) AbstractÐAn integrated optical grating coupler incorporated in an optical waveguide is used to monitor water quality. The primary measured parameter is the phase velocity of a selected guided mode. Upon exposure to a water sample, the device responds extremely rapidly to contaminants. The response is a sum parameter comprising contributions from the refractive index of the bulk sample, dissolved and suspended species which can adsorb at the surface of the waveguide, and small ions which can penetrate into the waveguiding layer. The waveguide surface can be readily modi®ed in order to confer a particular sensitivity pro®le to contaminants of interest. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐadsorption, biosensors, chemosensors, dissolved impurities, evanescent wave, integrated optics, sum parameter, suspended impurities, waveguides
INTRODUCTION
Clear natural water contains many substances apart from H2O, ranging from dissolved monatomic ions to suspended virus particles. The identi®cation and quantitative analysis of all the individual components is a lengthy and costly process. For many purposes, however, such detailed knowledge is not necessary and, if it can be measured rapidly and cheaply, a sum parameter to which many of the components contribute can be very useful, for example as an immediate warning of a change in water quality, which can be followed up by more detailed analysis, for example to determine whether the change is potentially hazardous. Here I describe how an integrated optics device, a grating coupler sensor, can be used to access such a sum parameter. The sensing pad is an optical waveguide, specially treated such that all impurities of interest interact with it. This interaction is transduced into an optical signal (the phase velocity of the guided light), which can be monitored with high precision.
PRINCIPLE OF OPERATION
An optical waveguide consists of a high refractive index slab F surrounded by lower refractive index material S (Tien, 1971, 1977). Light is con®ned within F. The solution of Maxwell's equations for an electromagnetic wave propagating in such a strati®ed medium corresponds to a standing wave in F and an exponentially decaying evanescent wave in S (Tien, 1971, 1977; Ramsden, 1993b). The standing *[Tel.: +41-61-2672193; Fax: +41-61-2672189; E-mail: [email protected]].
waves exist as discrete modes characterized by eigenvalues N, called the eective refractive index, which is the ratio of light velocity in vacuo to the phase velocity of the mode in the waveguide. To make such a structure into an environmental sensor, the material S is partly removed to expose the F slab to the environment C (Fig. 1). Any change of the composition of C within the evanescent ®eld will change the N of all the guided modes which can exist in the waveguide. The N can also be changed by the adsorption of material at the F,C interface to form an adlayer A and by penetration of material into F, changing its refractive index nF, if its structure allows such a process to take place. The relationship between the N and nF, nC, nA and adlayer thickness dA is given by the mode equations (Tien, 1977; Tiefenthaler and Lukosz, 1989; Ramsden, 1993b). An extremely accurate and convenient way to measure the N is via in- or out-coupling at a diffraction grating incorporated into the waveguide (Tien, 1977; Tiefenthaler and Lukosz, 1989; Ramsden, 1993b). A major advance in the practical application of the grating coupler to sensing problems was the development of inexpensive processes with which precision gratings could be produced a thousand times more cheaply than by ruling (Lukosz and Tiefenthaler, 1983; Heuberger and Lukosz, 1986). EXPERIMENTAL
Planar optical waveguides incorporating a grating coupler (grating constant L = 416.5 nm) were obtained from Arti®cial Sensing Instruments, ZuÈrich (type 2400). These are thin, so-called monomode waveguides capable of supporting only the zeroth transverse electric (TE) and trans-
1147
1148
J. J. Ramsden
Fig. 2. Schematic diagram of the ¯ow arrangement. The waveguide is mounted on a rotating goniometer with which the angle of incidence a of the external light beam L onto the grating G is measured. Photodiodes P detect the incoupled light. M is the membrane used to change the selectivity pro®le of the waveguide surface. RESULTS
Fig. 1. Electromagnetic ®eld distribution within the waveguide. The guided lightmodes are standing waves within F and evanescent waves in S and C. verse magnetic (TM) modes. The F layer was made from a pyrolyzed silica/titania precursor and had a surface composition of Si0.62Ti0.38O2 (measured by X-ray photoelectron spectroscopy), a surface roughness of 0.09 nm (measured using an atomic force microscope with a standard silicon nitride tip of radius 20±30 nm) and a porosity of 0.16 (Ramsden, 1994). The waveguides were equilibrated in triply distilled water before use. The mode spectrum was determined using an IOS-1 integrated optics scanner (Arti®cial Sensing Instruments, ZuÈrich). The waveguide formed one wall of a small ¯owthrough cuvette through which water samples were pumped at a wall shear rate g of 18 sÿ1 (Fig. 2). After ¯ushing with distilled water, the waveguides were regenerated for the next sample by brie¯y ¯ushing with decinormal HCl. The original baseline was regained a few minutes after switching back to distilled water. A complete measuring cycle thus comprised: (1) distilled water, (2) sample, (3) distilled water and (4) 0.1 M HCl. The same wall shear rate was used throughout. All measurements were carried out at a constant temperature of 25.38C. The absolute value of NTE,0 was determined to a precision of 21 10ÿ6 every 2.5 s. A second set of experiments was carried out with waveguides coated with a lipid bilayer membrane. Synthetic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl choline (POPC) was used as the lipid. The coating was carried out using a combination of the Langmuir±Blodgett and Langmuir±Schaefer techniques as described previously (Ramsden, 1993a).
Figure 3 shows the response of the waveguides to the dierent water samples. Since dierent waveguides have slightly dierent thicknesses dF and refractive indices nF, the results are normalized as NÄ, de®ned as: N~ NTE ÿ N0 =N0 ,
1
where N0 is the value of NTE in the presence of distilled water prior to exposure to the sample. After an initially rapid increase of NÄ, the rate of increase diminished and the response appeared to approach a plateau. In the case of tap water ¯owing over a POPC bilayer-coated waveguide, the signal returned to zero upon ¯ushing with distilled water. In the other cases investigated, a small residue remained, which was removed by the HCl ¯ush.
DISCUSSION
The initial state, N0, is determined by C = pure (distilled) water. When a water sample is introduced into C, three processes are possible, in order of decreasing rapidity: 1. The refractive index of C, nC, may change;
Water samples Rhine water was collected in February from the St. Johann bathing pontoon in Basel. Basel tap water was collected from the laboratory supply in the Biozentrum after allowing it to run for 5 min. Basel tap water predominantly comes from the Rhine, whose water is used to enrich the groundwater sources from which the tap water is pumped (Aschwanden, 1993).
Fig. 3. Plot of NÄ vs. time: (ÐÐÐ) Rhine water/Si(Ti)O2, (- - -) Basel tap water/Si(Ti)O2, (Ð Ð Ð) Rhine water/ lipid bilayer, ( ) Basel tap water/lipid bilayer. Samples began ¯owing at t = 0 and the arrows mark the start of ¯ushing with distilled water.
A sum parameter sensor for water quality
1149
Table 1. Sensitivity coecients Coecient @N/@nC @N/@G @N/@nF
Typical value
Unit
D
t1/2 (s)
0.1 3.2 10ÿ3 0.7
ÿ cm2/mg ÿ
10ÿ5 1 ng protein/cm2 10ÿ6
0.1 100 5000
D is an estimate of the minimum change in the value of the denominator required to produce a detectable signal, assuming that N can be measured to a precision of one part per million. t1/2 is an estimate of the time needed to reach half the level of response saturation.
2. Large molecules and aggregates, viruses, etc., may adsorb at the F,C interface, forming an adlayer A characterized by a surface excess G, de®ned as: G dA nA ÿ nC
dn , dc
2
where dn/dc is the mean refractive index increment of the adsorbed material. 3. Small molecules and ions may diuse into the ®lm F, changing nF and possibly even dF. Typical values of the sensitivity coecients and characteristic response times for these three processes are given in Table 1. Since the bulk refractive indices of the water samples diered by less than 10ÿ5 from that of pure water (measured with an LI3 (Carl Zeiss, Jena) Rayleigh interferometer), the measured response must result from either the formation of an adsorbed adlayer on the waveguide, or penetration of material into the F-layer. From the observed kinetics, it appears that the change in NÄ is predominantly due to the adsorption of macromolecules and particles (it is known, for example, that drinking water contains many viruses (Bergh et al., 1989)), since the kinetics characterizing penetration of material into the F-layer (Ramsden et al., 1993) are much slower than the rate of change of NÄ observed here. A sample of natural water can be characterized by hundreds or even thousands of parameters corresponding to dierent impurities, some of which may mutually interact, making their individual determination problematical (matrix eects). Here we have a response whose magnitude and kinetics (of both the interaction and its reversal upon ¯ushing with pure water) are globally characteristic of the sample under scrutiny. Two possible approaches to evaluating the sum parameter data are: (i) to identify a small number of key response parameters, such as the initial rate of change dN/dt and the fractional drop in NÄ when the sample is ¯ushed with distilled water, which can be correlated with the particular impurities present in the sample, and (ii) to record the values of N at predetermined intervals of time following the start of exposure to the sample, possibly using two or more dierent kinds of surface and including the change in NÄ upon ¯ushing with pure water, and feed them into a neural network trained to recognize a large variety
of dierent water samples (e.g. Goodacre et al., 1992; Cherquaoui and Villemin, 1994).
CONCLUSIONS
Fully hydrated silica±titania waveguides are inde®nitely stable in pure (distilled) water. In the presence of high-quality potable water, however, the phase velocities of guided lightmodes in the waveguides rapidly and markedly slow down, apparently due to the adsorption of dissolved and suspended matter at the surface of the waveguide. The phase velocities can be conveniently measured via a grating coupler. The kinetics of the interaction and its reversal upon ¯ushing with pure (distilled) water can be followed with good time resolution and these kinetics, as well as the absolute magnitudes of the reponses, are characteristic of the water impurities. By modifying the surface coating the waveguide, dierent sensitivity pro®les to a range of impurities can be obtained and, presumably, even a highly selective response to a particular impurity. In order to cause a response, the impurities must be optically detectable (i.e. have a refractive index dierent from that of pure water) and must be enriched at or in the waveguide, either by the formation of an adsorbed adlayer (larger molecules, viruses, bacteria) or by diusing into the waveguide (small molecules). The response saturates upon prolonged exposure to impurities. Hence the waveguide should be exposed intermittently to the sample, followed by ¯ushing with pure water and regeneration (e.g. with HCl). An important advantage of integrated optics for water monitoring is that the whole device can be miniaturized and interfaced to a data-processing electronic chip and in the not too distant future chips incorporating both sensing and optical dataprocessing functions should become available (Kunz, 1992). Therefore, low-cost devices could easily be made widely available to domestic and recreational users for the maintenance of public health, supplied to members of expeditions and dispersed over large tracts of terrain for environmental monitoring.
1150
J. J. Ramsden REFERENCES
Aschwanden W. (1993) Geschichte und Gegenwart der Basler Wasserversorgung (Water supply in Basel-past and present). Energie Wasser 2, 6±9. Bergh é., Bùrsheim K. Y., Bratbak G. and Heldal M. (1989) Nature (Lond.) 340, 467±468. Cherquaoui D. and Villemin D. (1994) Use of a neural network to determine the boiling point of alkanes. J. Chem. Soc. Faraday Trans. 90, 97±102. Goodacre R., Kell D. B. and Bianchi G. (1992) Neural networks and olive oil. Nature (Lond.) 359, 594. Heuberger K. and Lukosz W. (1986) Embossing technique for fabricating surface relief gratings on hard oxide waveguides. Appl. Opt. 25, 1499±1504. Kunz R. E. (1992) Totally integrated optical measuring sensors. Proc. SPIE 1587, 98±113. Lukosz W. and Tiefenthaler K. (1983) Embossing technique for fabricating integrated optical components in hard inorganic waveguiding materials. Opt. Lett. 8, 537± 539.
Ramsden J. J. (1993a) Partial molar volume of solutes in bilayer lipid membranes. J. Phys. Chem. 97, 4479±4483. Ramsden J. J. (1993b) Review of new experimental methods for investigating random sequential adsorption. J. Statist. Phys. 73, 853±877. Ramsden J. J. (1994) Porosity of pyrolyzed sol-gel waveguides. J. Mater. Chem. 4, 1263±1265. Ramsden J. J., Roemer D. U. and Prenosil J. E. (1993) Microporous monomode sensors as waveguides. In Proc. 6th European Conference on Integrated Optics, Neuchatel, pp. 12-38±12-39. Tiefenthaler K. and Lukosz W. (1989) Sensitivity of grating couplers as integrated-optical chemical sensors. J. Opt. Soc. Am. B 6, 209±220. Tien P. K. (1971) Light waves in thin ®lms and integrated optics. Appl. Opt. 10, 2395±2413. Tien P. K. (1977) Integrated optics and new wave phenomena in optical waveguides. Rev. Mod. Phys. 49, 361±420.