Optical sensor for detecting chemical substances dissolved or dispersed in water

A highly sensitive optical sensor in a simple structure for detecting a chemical substance dissolved or dispersed in water is provided. The optical sensor (1) comprises a detecting element (2) having a polymer thin film, a light source unit (3) for emitting light for irradiating the polymer thin film, and a photodetector (4) for detecting the intensity of light reflected from the polymer thin film. The detecting element (2), the light source unit (3), and the photo-detector (4) are integrally mounted in a housing. The polymer thin film is formed on a highly reflective substrate or an optically transparent substrate so as to interact with a chemical substance dissolved or dispersed in water flowing through a water channel (8).

The present invention relates generally to an optical sensor and more
 particularly to an optical sensor which utilizes a polymer thin film for
 directly detecting a chemical substance dissolved or dispersed in water,
 particularly dissolved organic carbon (hereinafter abbreviated as "DOC")
 in accordance with an optical detecting method such as a waveguide mode
 method (WG method), surface plasma resonance method (SPR method),
 interference enhanced reflection method (IER method), and so on. The
 polymer thin film interacts with a chemical substance such as hydrocarbon
 and so on which may be absorbed into or adsorbed on the polymer thin film.
 As a result, the polymer thin film exhibits a change in thickness and/or
 refractive index depending on the concentration of the chemical substance,
 so that such a physical change may be measured by an optical method to
 determine the concentration of the chemical substance dissolved or
 dispersed in water, particularly, DOC.
 A variety of reports have been made on the use of polymer thin films in
 optical sensors for detecting chemical species in gas phase and chemical
 substances dissolved in water. Many of these reports are related to fiber
 optic sensors or optical waveguide sensors based on evanescent waves or
 guided waves.
 As is well known in the art, when a light beam is incident on an interface
 between two dielectric materials having different refractive indices
 n.sub.1 and n.sub.2 (&gt;n.sub.1), respectively, total internal reflection
 occurs when the light beam is incident from the dielectric material of the
 refractive index n.sub.1 to the dielectric material of the refractive
 index n.sub.2 and when the angle of incidence is larger than a critical
 angle .theta.c. The critical angle .theta.c of total internal reflection
 is given by:
EQU .theta.c=sin.sup.-1 (n.sub.1 /n.sub.2) (1)
 In this case, the incident light is fully reflected back into the
 dielectric material of the refractive index n.sub.2 so that no light will
 enter the dielectric material of refractive index n.sub.1. However, there
 exists a wave function called an evanescent wave which propagates in
 parallel with the interface between the dielectric material of refractive
 index n.sub.1 and the dielectric material of the refractive index n.sub.2.
 The electric field E of the evanescent wave decays exponentially with the
 distance z from the interface, and can be expressed by an exponential
 function:
EQU E=E.sub.0 exp(-z/d.sub.p) (2)
 where E.sub.0 is the electric field on the interface, and d.sub.p is the
 depth of penetration defined as the distance where the electric field of
 the evanescent wave, produced when light is incident on the interface at
 an angle .theta., is reduced from the value at the interface to 1/e, and
 is expressed by:
EQU d.sub.p =.lambda.[2.pi.(n.sub.2.sup.2 sin.sup.2
 .theta.-n.sub.1.sup.2).sup.1/2 ] (3)
 As is well known in the art, optical waveguides operate based on the
 principle of total internal reflection. A planar waveguide, which is one
 type of the optical waveguides, simply consists of a first medium of
 refractive index n.sub.2 sandwiched between a second medium of refractive
 index n.sub.1 and a third medium of refractive index n.sub.3, where the
 refractive indices of the media are selected such that n.sub.2
 &gt;n.sub.3.gtoreq.n.sub.1 is satisfied. A light beam is confined in the
 first medium by successive total reflections when the light beam is
 traveling in the medium n.sub.2 at an angle .theta. larger than the
 critical angle of total internal reflection on interfaces of the first
 medium and the two other media (in this event, sin .theta.&gt;n.sub.3
 /n.sub.2.gtoreq.n.sub.1 /n.sub.2 is satisfied). In this case, waveguiding
 occurs, and the light waves existing in the first medium are called guided
 waves. Optical fibers are another type of waveguides consisting of a
 cylindrical core of refractive index n.sub.2 surrounded by a cladding
 layer of refractive index n.sub.1 (&lt;n.sub.2).
 In either evanescent wave sensors or guided wave sensors, light must travel
 at an angle larger than the critical angle of total internal reflection.
 Typical examples of optical chemical sensors based on evanescent waves can
 be found in many prior art documents. Carter et al. disclose in USP No.
 Re.33064 a method of identifying a chemical species in a solvent using an
 optical waveguide covered with a response film having a refractive index
 smaller than that of the waveguide layer. Light propagates through the
 optical waveguide by the action of total internal reflection.
 Within the propagating light, evanescent waves generated by the total
 reflection only are involved in interaction of the response film with a
 chemical species under detection. Thus, the method proposed by Carter et
 al. is limited only on interaction which is accompanied with absorption or
 scattering of light, or generation of fluorescence.
 Hinrich et al. have reported the use of polymer for detecting organic
 compounds in water on an internal reflection element in "Determination of
 organic compounds by IR/ATR spectroscopy with polymer-coated internal
 reflection elements" (Applied Spectroscopy, Vol. 44, No. 10, 1990, pp
 1641-1646). However, the detecting method of Hinrich et al. relies on the
 absorption of evanescent waves of infrared rays penetrating in the polymer
 film by organic compounds, wherein the polymer film is used to eliminate
 water and extract the organic compounds on the surface of the internal
 reflection element to thereby enhance an absorption signal.
 Burck et al. have reported a similar method except for the use of an
 optical fiber in "A fiber optic evanescent field absorption sensor for
 monitoring organic contamination in water" (Fresenius J. Anal. Chem.,
 (1994), 342, pp 394-400) and "Fiber-optic evanescent wave sensor for in
 situ determination of non-polar organic compounds in water" (Sensors and
 Actuators, B 18-19 (1994), pp 291-295).
 Japanese Laid-open Patent Application No. 7-85122 (1995) discloses a method
 for detecting an organic solvent in water with an optical fiber having a
 cladding layer made of a chitosan compound. Since the intensity of
 evanescent waves penetrating into the chitosan cladding layer depends on
 the degree of swelling, and the concentration of the chitosan cladding
 layer varies in accordance with the ratio of water to solvent, the
 intensity of light propagating the optical fiber is consequently a
 function of the concentration of organic solvent dissolved in water.
 A main disadvantage of a sensor utilizing evanescent waves, however, is
 that the sensitivity of the sensor is limited since only a portion of
 incident light is used for detection. Thus, a long interaction distance is
 required to realize a sensor having a high sensitivity. This imposes a
 limit on reduction in size of such sensors.
 A larger portion of incident light may be utilized for detection to provide
 sensors having higher sensitivities. WO95/20151 discloses a chemical
 sensor having a multi-layered optical fiber. Specifically, a sensing
 polymer layer is sandwiched between a core of the optical fiber and a
 cladding layer, and the refractive index of the polymer layer is larger
 than that of the cladding layer so that the polymer layer serves as an
 optical waveguide layer. With this structure, light incident to the
 optical fiber is refracted toward the polymer waveguide layer and
 propagates therethrough toward the end terminal of the sensor. However,
 since this structure requires an output light detector to be located near
 an output terminal, the chemical sensor disclosed in WO95/20151 is
 inconvenient for measuring a substance to be detected in water.
 A large number of highly sensitive polymers for detecting chemical
 substances in gas have also been reported. Gliliani et al. have reported a
 strip-shaped polymer waveguide having a thickness of 1 .mu.m for detecting
 the existence of several kinds of organic vapors in "Fabrication of an
 integrated optical waveguide chemical vapor microsensor by
 photopolymerization of a bifunctional oligomer" (Appl. Phys. Lett., 48
 (1986), pp 1311-1313) and "Integrated optical chemical vapor microsensor"
 (Sensors and Actuators, 15 (1988), pp 25-31). A method proposed by them
 involves introducing non-polarized light into a waveguide channel from one
 end of an optical fiber by fiber coupling, and extracting the light
 propagating through the waveguide from the other end to the outside. In
 this way, interaction between the polymer and an organic vapor is sensed
 as a change in intensity of the transmitting (propagating) light. The
 method of Gluliani et al., however, implies the following two
 difficulties: (1) a photopolymerized polymer is required to fabricate the
 strip-shaped waveguide; and (2) the strip-shaped waveguide must be coupled
 by end-fire coupling to a thin film having a thickness on order of
 micrometers.
 A planar polymer thin film optical waveguide as a sensor for detecting
 organic vapors has been reported by Bowman and Burgess in "Evaluation of
 polymer thin film waveguides as chemical sensors" (SPIE Proceedings, Vol.
 1368: Chemical, biochemical, and environmental II, 1990). The polymer film
 exhibits a change in waveguide characteristic as a result of absorbing
 chemical vapors. Bowman et al. use two gratings (diffraction gratings)
 embedded in a substrate for coupling incident and decoupling light. Such a
 grating coupler, however, is difficult to fabricate and expensive. A
 similar method using a simpler prism coupler has been reported by
 Osterfeld et al. in "Optical gas detection using metal film enhanced leaky
 mode spectroscopy" (Appl. Phys. Lett. 62 (19), 1993, pp 2310-2312). A
 metal reflective layer is sandwiched between a polymer film waveguide and
 an optical coupling prism such that light incident to the interface
 between the metal reflective layer and the prism is totally reflected at
 an optimal incident angle. Evanescent waves produced by total reflection
 excite a waveguide mode in the polymer film.
 Reference has not been made as to whether or not the polymer waveguides
 proposed by Bowman et al. and Osterfeld et al. can be used for detecting
 organic carbon in water. Further, since the polymer film (teflon AF) used
 by Osterfeld et al. nas a refractive index (=1.3034) smaller than the
 refractive index (=1.33) of water, the film made of teflon AF does not
 function as a waveguide in water.
 Optical sensors without using evanescent waves or guided waves have also
 been reported. For example, Gauglitz et al. have reported a method of
 reflection spectroscopy for detecting organic vapors using the swelling of
 polymer films (GIT Fachz. Lab., 889, 7/1990). In this method, a sensitive
 polymer thin film coated on a transparent substrate is irradiated with
 white light at a normal incidence from the substrate side, and reflected
 light from the polymer thin film is collected and analyzed by a
 spectrometer. Here, wavelength shift in the reflection spectra caused by
 polymer-vapor interaction is measured as an indication of organic vapor
 concentration. As will be later described, the normal incident arrangement
 is less sensitive and must therefore rely on spectral interferometry. In
 other words, the method of Gauglitz et al. is complicated and requires
 expensive and large equipment for implementation.
 The present invention has been made to solve the foregoing problems of the
 known techniques, and its object is to provide an optical sensor for
 detecting a chemical substance dissolved or dispersed in water,
 particularly DOC, which is simple in structure, highly sensitive, and easy
 to fabricate.
 To achieve the above object, the present invention provides an optical
 sensor for directly detecting a chemical substance dissolved or dispersed
 in water comprising:
 at least one detecting element having a polymer thin film capable of
 interacting with the chemical substance;
 at least one light source unit for emitting light for irradiating the
 polymer thin film; and
 a first photo-detector for detecting the intensity of light reflected from
 the polymer thin film.
 In the present invention, while the polymer thin film is capable of
 detecting any chemical substance which is absorbed or adsorbed thereby,
 the polymer thin film is preferably used to detect organic carbon in terms
 of the sensitivity and so on.
 The detecting element, the light source unit, and the photo-detector are
 integrally supported by a housing. The polymer thin film provided in the
 detecting element is preferably formed on a planar substrate. In one
 embodiment of the present invention, the polymer thin film is formed on a
 highly reflective substrate such as that made of silicon, metal or the
 like, and interaction between the polymer thin film and the organic carbon
 in water is detected in accordance with an IER method (so called the
 front-side IER, abbreviated as FS-IER). In another embodiment of the
 present invention, the polymer thin film is formed on an optically
 transparent substrate, with a light source and a photodetector being
 located on the substrate side, i.e., facing the side of the substrate on
 which the polymer thin film is not formed, and interaction between the
 polymer thin film and the organic carbon in water is detected in
 accordance with the IER method (so called the back-side IER, abbreviated
 as BS-IER). In another embodiment of the present invention, the polymer
 thin film is formed on a highly reflective metal layer deposited on a
 transparent substrate. The highly reflective metal layer has a thickness
 equal to or less than a wavelength of light from the light source unit,
 and is made of a material selected from a group including silver, gold,
 chrome, silicon, and germanium. In this embodiment, interaction between
 the polymer thin film and organic carbon in water is detected in
 accordance with one of a SPR method and a WG method.
 The polymer thin film preferably has a thickness of 10 .mu.m or less, more
 preferably 5 .mu.m or less, and further preferably 3 .mu.m or less.
 Desirably, the light source unit comprises a laser diode (LD) or a light
 emitting diode (LED), and the photo-detector is a photodiode or a
 phototransistor. An output of the photo-detector is applied to an electric
 circuit which generates a signal indicative of the concentration of a
 chemical substance in water.
 In the present invention, the polymer thin film absorbs or adsorbs a
 chemical substance in water to directly respond to the chemical substance.
 As a result of such interaction, the polymer thin film exhibits a change
 in thickness and/or refractive index. Since such a physical change is
 related to the concentration of the chemical substance, the physical
 change can be measured in accordance with an optical approach such as an
 IER method, SPR method, waveguide mode method, or the like to derive the
 concentration of the chemical substance in water.
 In the four optical methods, the concentration of the chemical substance in
 water may be measured as a function of the intensity of reflected light at
 a fixed detection angle. With the waveguide mode method, the concentration
 of the chemical substance is measured as a function of the reflectivity of
 the polymer thin film or an angular position of a waveguide mode.
 In the FS-IER method, a light source and a photodetector are located above
 the polymer thin film such that probe light from the light source and
 reflected light from the polymer thin film pass through water in which
 organic substances are dissolved. This method is not always desirable
 because bubbles and particles in water may scatter or block light beams,
 causing large fluctuations or attenuation of output signal.
 The BS-IER method takes an approach similar to that reported by Gauglitz et
 al., which employs a sensing element having a polymer thin film formed on
 an optically transparent substrate, with a light source and a
 photodetector being located on the substrate side, i.e., facing the side
 of the substrate on which the polymer thin film is not formed. The
 inventors of this invention, however, have found that when probe light is
 incident at an angle less than but close to the critical angle of total
 internal reflection, the reflectivity of the polymer thin film largely
 varies as compared with light incident normal to the polymer thin film, as
 reported by Gauglitz et al. This invention has been made based on this
 discovery and provides an optical sensor different from conventional
 evanescent wave sensors and guided wave sensors.
 In the BS-IER method, the substrate couples light from the light source
 unit to the polymer thin film at a predetermined angle, and functions as
 light coupling means for coupling light reflected by the sensing element
 to the first light detector, and the predetermined incident angle is set
 at a value smaller than a critical angle of total internal reflection on
 the interface between the polymer thin film and the water and close to the
 critical angle.
 In one embodiment of this invention, the optical sensor further comprises a
 second light detector for directly receiving light from the light source
 unit, and an electronic circuit to receive outputs of the first light
 detector and the second light detector for calculating the ratio of these
 outputs to generate a signal indicative of the concentration of the
 organic substance. The light source unit, the first light detector, the
 second light detector, and the light coupling means are mounted in a
 housing in a predetermined positional relationship with respect to the
 sensing element.
 Also, the substrate may be a prism or a planar plate. When the substrate is
 a planar plate, a grating may be formed on a predetermined position of the
 substrate, or a grating layer formed with a grating may be disposed
 between the substrate and the polymer thin film or on a surface of the
 substrate on which the polymer thin film is not formed.

A certain kind of polymer thin film (later described) exhibits a change in
 thickness and/or refractive index when a chemical substance such as
 organic carbon or the like is absorbed into or adsorbed on the polymer
 thin film. The present invention measures such a physical change of the
 polymer thin film to sense a chemical substance in water. Several
 embodiments of an optical sensor according to the present invention will
 hereinafter be described with reference to the accompanying drawings. It
 should be noted that in the drawings, the same or similar components are
 designated by the same reference numerals, and repetitive explanation
 thereof will be omitted.
 FIG. 1A generally illustrates the configuration of a first embodiment of an
 optical sensor according to the present invention, and FIG. 1B illustrates
 in an enlarged view a detector element used in the optical sensor. The
 sensor of the first embodiment relies on the FS-IER method for detecting a
 chemical substance in water. Referring specifically to FIG. 1A, the
 optical sensor 1 comprises the detecting element 2; a light source unit 3
 for emitting light such that the light is incident to the detector element
 2 at an incident angle .theta.; and a first photo-detector 4 for detecting
 the intensity of light emitted from the light source unit 3 and reflected
 by a polymer thin film 2.sub.2 of the detecting element 2.
 The detecting element 2 is positioned on one surface of a base 5, and has a
 planar reflective substrate 2.sub.1 and the polymer thin film 2.sub.2
 formed on the substrate 2.sub.1 in contact with water, as illustrated in
 FIG. 1B. The reflective substrate 2.sub.1 is preferably a substrate having
 a high reflectivity and may be, for example, mirror, semiconductor, metal,
 or a thin film of any metal material or any semiconductor material
 deposited on a low reflective substrate.
 The light source unit 3 has a light source 31, a beam splitter 32, and a
 polarizing plate 33. The light source 31 may be a laser diode (LD) or a
 light emitting diode (LED) for emitting visual light or infrared rays.
 Light emitted from the light source 31 is split by the beam splitter 32
 into two portions, one of which is polarized by the polarizing plate 33
 and emits on the polymer thin film 2.sub.2 of the detecting element 2.
 Light reflected from the polymer thin film 2.sub.2 enters, through a
 window 41, the first photo-detector 4 which detects the intensity of
 received light. The other light portion split by the beam splitter 32 is
 directed to a second photo-detector 6 and transduced thereby into a signal
 representative of a reference light intensity.
 The first photo-detector 4 and the second photo-detector 6 may be
 photodiodes or phototransistors. Outputs of these photo-detectors 4, 6 are
 transferred to an appropriate electronic circuit to calculate the ratio of
 the output of the first photo-detector 4 to the output of the second
 photo-detector 6. This ratio is used to generate a signal indicative of
 the concentration of a chemical substance to be detected.
 The light source unit 3, the first photo-detector 4, and the second
 photo-detector 6 are mounted at appropriate locations in a housing 7. The
 housing 7 is mounted on the base 5 such that the housing 7 forms a water
 channel 8 with the base 5 and the polymer thin film 2.sub.2 of the
 detecting element 2 is in contact with water in the water channel 8. The
 light passing through the polarizing plate 33 is preferably s-polarized
 light which has an electric field vector of the light perpendicular to an
 incident plane of the polymer thin film 2.sub.2.
 The IER method is utilized to detect a change in thickness and/or
 refractive index of the polymer thin film 2.sub.2 in contact with water
 based on the fact that the intensity of light reflected from a thin
 dielectric film depends on the thickness of the dielectric film. The
 polymer thin film 2.sub.2 exhibits a change in thickness and/or refractive
 index when it absorbs or adsorbs a chemical substance in water. Thus, when
 the polymer thin film 2.sub.2 is irradiated with light from the light
 source unit 3, a change in thickness and/or refractive index of the
 polymer thin film 2.sub.2 appears as a change in intensity of light
 reflected from the polymer thin film 2.sub.2. It is therefore possible to
 measure the concentration of a chemical substance in water by measuring
 the intensity of the reflected light.
 FIG. 2 illustrates a graph which represents the relationship between the
 thickness of the polymer thin film 2.sub.2 of the detecting element 2 in
 the optical sensor 1 shown in FIG. 1A and the reflectivity of the polymer
 thin film 2.sub.2 to s-polarized light incident thereto, when the polymer
 thin film 2.sub.2 is formed on a silicon substrate and placed in water. In
 other words, the graph shows reflectivity curves derived in accordance
 with the IER method. A solid line indicates the reflectivity when the
 incident angle .theta. of the light is 80, and a broken line indicates the
 reflectivity when the incident angle .theta. is 70. In this event, the
 refractive index of the polymer thin film is 1.50.
 Although the thickness of the polymer thin film 2.sub.2 may be arbitrarily
 selected in a range of several nanometers (nm)-10 micrometers (.mu.m), the
 thickness is desirably set at a value away from minimum values of the
 reflectivity curves in FIG. 2 in order to appropriately detect the
 thickness of the polymer thin film 2.sub.2 in accordance with the IER
 method. Also, it can be seen from the reflectivity curves of FIG. 2 that
 the reflectivity more largely modulates as the incident angle .theta. is
 larger (the reflectivity exhibits a larger change in response to a change
 in thickness). Thus, the incident angle .theta. is preferably 70 or more.
 FIG. 3 generally illustrates a basic configuration of the BS-IER of a
 sensing element for use in the optical sensor according to this invention.
 Referring specifically to FIG. 3, a sensing element 51 comprises a
 transparent substrate 52 and a polymer thin film 53 formed on one surface
 of the substrate 52 by spin coating or the like. The polymer thin film 53
 has a thickness d and a refractive index n.sub.2. Other than the spin
 coating, the polymer thin film 53 may be formed by any of generally known
 methods such as vapor deposition, dip coating, roller coating, sputtering,
 chemical vapor deposition (CVD), and so on. Assume that the surface of the
 polymer thin film 53, opposite to the substrate 52, is in contact with
 water having a refractive index n.sub.3. A light source 54 for emitting
 linearly polarized monochromic light of wavelength .lambda. is located
 opposing the substrate 52. Monochromic light 55 emitted from the light
 source 54 is incident on the substrate 52 at an angle .theta. and
 reflected by an interface 56 between the polymer thin film 53 and the
 water and by an interface 57 between the polymer thin film 53 and the
 substrate 52, respectively. Thus, the intensity of reflected light from
 the sensing element 1 is a combination of the intensity of light 58
 reflected by the interface 56 and the intensity of light 59 reflected by
 the interface 57, and therefore is the sum or difference of the
 intensities of the light 58, 59 depending on optical path lengths of the
 respective light 58, 59.
 The reflectivity of the sensing element 51 illustrated in FIG. 3 can be
 calculated using the well-Known Fresnel formula. As to further details on
 the Fresnel formula, see "Principles of Optics", by M Born and E. Wolf,
 Pergmon Press, 1959. Assume herein that a sensing element is formed of an
 SF11 glass substrate having a refractive index n.sub.1 equal to 1.7786 and
 a poly(octadecyl methacrylate-co-glycidyl methacrylate) thin film
 (hereinafter referred to as "poly(ODMA-co-GLMA) thin film), having a
 thickness d equal to 1.8 .mu.m and a refractive index n.sub.2 equal to
 1.493, coated on one surface of the glass substrate. The sensing element
 is located such that the poly(ODMA-co-GLMA) thin film is in contact with
 water having a refractive index n.sub.3 equal to 1.332, with the exposed
 surface of the SF11 glass substrate irradiated with p-polarized light and
 s-polarized light of wavelength .lambda. equal to 632.8 nm. Then, the
 reflectivity of the poly(ODMA-co-GLMA) thin film is calculated in the
 measuring conditions mentioned above as a function of the incident angle
 of the light on the substrate. FIG. 4 illustrates the results of the
 calculations. As illustrated in the graph of FIG. 4, the critical angle of
 total internal reflection .theta..sub.c23 is equal to 48.495.degree. for
 the interface between the poly(ODMA-co-GLMA) thin film and the water. For
 reference, the critical angle of total internal reflection
 .lambda..sub.c12 (not shown) is equal to 57.079.degree. for the interface
 between the SF11 glass substrate and the poly(ODMA-co-GLMA) thin film.
 It can be seen from the graph of FIG. 4 that when the incident angle
 .theta. of light is larger than the critical angle .theta..sub.c23
 (=48.495.degree.), the reflectivity of the poly(ODMA-co-GLMA) thin film to
 the s-polarized light (TE wave) and the p-polarized light (TM wave) is
 unity, and does not at all depend on the thickness of the
 poly(ODMA-co-GLMA) thin film, and the reflectivity of the
 poly(ODMA-co-GLMA) thin film to the s-polarized light and the p-polarized
 light strongly depends on the thickness of the poly(ODMA-co-GLMA) thin
 film when the incident angle .theta. is smaller than the critical angle
 .theta..sub.c23.
 FIG. 5 illustrates the relationship between the thickness of the
 poly(ODMA-co-GLMA) thin film and the reflectivity of the same to
 s-polarized light when the sensing element used for deriving the graph of
 FIG. 4 is irradiated with light of the same wavelength (.lambda.=632.8 nm)
 for a plurality of incident angles smaller than the critical angle
 .theta..sub.c23 (=48.495.degree.). Comparison between reflectivity curves
 calculated with different incident angles illustrated in the graph of FIG.
 5 reveals that the reflectivity of the poly(ODMA-co-GLMA) thin film
 strongly depends on its thickness as the incident angle .theta. approaches
 the critical angle .theta..sub.c23 when the incident angle .theta. is
 smaller than the critical angle .theta..sub.c23, and that the dependency
 of the reflectivity of the poly(ODMA-co-GLMA) thin film on its thickness
 is largest when the incident angle .theta. is around 48. The depth of
 modulation is abruptly reduced as the incident angle .theta. is smaller
 than the critical angle .theta..sub.c23. At the incident angle .theta.
 equal to 10, the depth of modulation becomes extremely small.
 It will be apparent from the above discussion that the normal incident
 arrangement used by Gauglitz et al., as described previously, has an
 extremely small change by the thickness or refractive index of a polymer
 thin film, and is therefore not sensitive. One embodiment of this
 invention is intended to provide an optical sensor which has a high
 sensitivity in a simple structure.
 FIG. 6A is a cross-sectional view generally illustrating the BS-IER
 configuration of an optical sensor according to the present invention. The
 optical sensor 10 comprises a sensing element 11, a light source unit 12,
 a first light detector 13, and a second light detector 14. As can be best
 seen in an enlarged view of FIG. 6B, the sensing element 11 comprises a
 polymer thin film 16 formed on one surface of a prism 15, acting as the
 substrate 2 of FIG. 3, in a predetermined thickness. The prism 15 is
 mounted on a flow cell 17 for passing water therethrough such that the
 polymer thin film 16 is in contact with water flowing through the flow
 cell 17. The flow cell 17 has a flow inlet 18 and a flow outlet 19 for
 water.
 The light source unit 12 comprises a light source 20, a beam splitter 21,
 and a polarizing plate 22. The light source 20 may be, for example, a
 laser diode (LD) or a light emitting diode (LED) which emits visible light
 or infrared rays. Light emitted from the light source 20 is split by the
 beam splitter 21 into a probe beam and a reference beam. The probe beam
 passes through the polarizing plate 22 and becomes a linearly polarized
 beam. The polarization of this linearly polarized beam is preferably an
 S-polarization (i.e., the electric field of the light beam is oriented
 perpendicular to the plane of incident). The probe beam passes through the
 prism 15, is incident on the polymer thin film 16 at an incident angle
 .theta. smaller than a critical angle of total internal reflection
 .theta.on the interface between the polymer thin film 16 and water, and is
 reflected by the polymer thin film 16. The reflected probe beam is
 received by the first light detector 13 which transduces the probe beam
 into an electric signal indicative of the intensity of the reflected
 light. The reference beam, which is the other light beam split by the beam
 splitter 21, is received by the second light detector 14 which transduces
 the received light beam into an electric signal indicative of a light
 intensity for reference. The signals outputted from the first light
 detector 13 and the second light detector 14 are supplied to an
 appropriate electronic circuit having, for example, a sampling hold
 circuit, a comparator, and so on, for calculating the ratio between these
 signals. The concentration of an organic substance in water can be
 determined using this ratio.
 For implementing the emission, reflection, and detection of light beams,
 the light source unit 12, the first light detector 13, and the second
 light detector 14 are mounted in a housing 23 in a predetermined
 positional relationship with respect to the sensing element 11 as
 illustrated in FIG. 6A. Alternatively, optical fibers may be used to
 couple between the light source 12 and the prism 15 and between the prism
 15 and the first light detector 13. Photodiodes and phototransistors may
 be used as the first light detector 13 and the second light detector 14,
 by way of example.
 In the optical sensor according to the BS-IER method, the incident angle
 .theta. of light emitted from the light source unit is desirably smaller
 than the critical angle of total internal reflection .theta.on the
 interface between the polymer thin film and water and close to the
 critical angle .theta.c. By selecting the incident angle .theta. such that
 the reflectivity of the polymer thin film of zero thickness, is preferably
 0.1 or more, particularly preferably 0.2 or more, and further preferably
 0.3 or more, more highly sensitive sensors can be provided. For example,
 as illustrated in the graph of FIG. 5, s-polarized light having the
 wavelength at 632.8 nm is preferably incident at an incident angle .theta.
 ranging from 40.degree. and 48.degree. on a poly(ODMA-co-GLMA) thin film
 spin-coated on an SF11 glass substrate. On the other hand, while it is
 desirable that the thickness of a polymer thin film is generally in a
 range of several nanometers to 10 .mu.m, the thickness of a polymer thin
 film optimal to the detection in accordance with the IER method is not
 limited in particular. However, as will be understood from the graph of
 FIG. 5, since a change in reflectivity is small near a maximum value or a
 minimum value of the reflectivity, the thickness of the polymer thin film
 is desirably selected in the middle of a thickness at which the
 reflectivity is maximum and a thickness at which the reflectivity is
 minimum.
 The substrate for use in the sensing element of the optical sensor
 according to the BS-IER method is preferably transparent and may be made
 of materials including, for example, glass, plastic, polymer and
 semiconductor. In addition, an extremely thin metal layer, inorganic
 dielectric film or semiconductor film (50 nm or less) may be vapor
 deposited on such a transparent substrate. It should be noted, however,
 that the reflectivity of a polymer thin film varies depending on the
 refractive index of a material used for the substrate. FIG. 7 is a graph
 showing how the refractive index of a substrate influences the
 reflectivity of a polymer thin film. For the purpose of measurements,
 three sensing elements are prepared. Specifically, a poly(ODMA-co-GLMA)
 thin film having a thickness d equal to 1.8 .mu.m and a refractive index
 n.sub.2 equal to 1.493 is formed by spin-coating on one surface of each of
 three transparent substrates having a refractive index n1equal to 1,5143,
 1.7786 and 2.3513, reflectively. Each of the sensing elements is
 positioned such that the poly(ODMA-co-GLMA) thin film is in contact with
 water having a refractive index n.sub.3 equal to 1.332. The graph shows
 the relationship between the thickness of the polymer thin film and its
 reflectivity when each substrate is irradiated with a light beam having a
 wavelength equal to 632.8 nm. It can be seen from the graph that a
 substrate having a larger refractive index allows the reflectivity of
 polymer thin film to be more sensitive to a change in its thickness, and
 accordingly is desirable for use in the optical sensor.
 Continuing the explanation on the substrate, the prism 15 of the sensing
 element 11 in the optical sensor illustrated in FIG. 6A acts as a light
 coupling means for coupling the probe beam from the light source unit 12
 to the polymer thin film 16 and for coupling a reflected beam therefrom to
 the first light detector 13 as well as constitutes the substrate 2 for
 forming the polymer thin film 16 thereon. However, such a light coupling
 means is not limited to the prism but may be realized by a variety of
 other means. Typical examples of such coupling means may be those
 utilizing grating coupling and side-coupling.
 In the following, sensing elements utilizing the grating coupling will be
 described with reference to FIGS. 8A-8D, and a sensing element utilizing
 the side-coupling with reference to FIG. 8E. FIG. 8A illustrates a sensing
 element which has a grating 25 on a portion of a surface of a transparent
 substrate 24 and a polymer thin film 26 coated on the surface having the
 grating 25 formed thereon. A sensing element illustrated in FIG. 8B has a
 grating layer 28 formed with a grating 27 positioned between a substrate
 24 and a polymer thin film 26. A sensing element illustrated in FIG. 8C is
 formed with a grating 25 on a surface of a substrate 24 in a portion on
 which light is incident and in a portion from which light reflected by a
 polymer thin film 26 exits. FIG. 8D illustrates a sensing element which
 employs a grating layer 28 formed with a grating 27 on a surface, on which
 light is incident, and mounted on a surface of a substrate 24 opposite to
 a polymer thin film 26. A sensing element illustrated in FIG. 8E, in turn,
 utilizes the side-coupling such that light is detected to be incident on
 one side surface 29 perpendicular to a polymer thin film 26 on a substrate
 24 and reflected light from the polymer thin film 26 is led out through
 the other side surface 30.
 FIG. 9A generally illustrates the configuration of a third embodiment of
 the optical sensor according to the present invention, and FIG. 9B is an
 enlarged cross-sectional view illustrating the structure of a detecting
 element shown in FIG. 9A. The third embodiment differs from the second
 embodiment of FIG. 6A in that the second embodiment uses a polymer
 waveguide formed on a metal cladding layer.
 Referring specifically to FIGS. 9A and 9B, a metal layer 2.sub.3 is
 deposited on the bottom 91 of a prism 9, and a polymer thin film 2.sub.2
 serving as a polymer waveguide is formed on the metal layer 2.sub.3 to
 complete a detecting element 2. The prism 9 is mounted on a flow cell 17
 having a water flow inlet 18 and a water flow outlet 19 such that the
 polymer thin film 2.sub.2 faces water passing through the flow cell 17.
 Light emitted from a light source unit 3 and polarized by a polarizing
 plate 33 is incident to the bottom 91 of the prism 9 at an angle larger
 than an internal total reflection angle of the prism 9, and the intensity
 of light reflected by the bottom 91 is measured by a first photo-detector
 4. The metal layer 2.sub.3 has a thickness equal to or less than the
 wavelength of the light emitted from a light source 31, and preferably
 made of silver, gold, chrome, silicon, or germanium.
 When the light emitted from the light source 31 is totally reflected by the
 bottom 91 of the prism 9, evanescent waves are produced and light waves in
 a waveguide mode are excited by the evanescent waves. Such excitation of
 the waveguide mode in the polymer thin film 2.sub.2 i.e., optical coupling
 is the strongest at the incident angle at which a tangential component of
 an evanescent wave vector on the bottom 91 of the prism 9 is equal to a
 wave vector of the light waves in the waveguide mode. Thus, under such a
 condition, the energy of incident light from the light source 31
 transitions to light waves in the waveguide mode internal to the polymer
 thin film 2.sub.2, whereby the intensity of light reflected from the metal
 layer 2.sub.3 is abruptly decreased.
 Thus, when the reflectivity of the polymer thin film 2.sub.2 is measured
 while varying the incident angle .theta. of the light from the light
 source 31, excitation of light waves in the waveguide mode can be
 recognized as abrupt attenuation of a curve representing the reflectivity
 at a certain resonance coupling angle. FIG. 10 is a graph of values
 measured in an experiment for showing changes in reflectivity of the
 polymer thin film 2.sub.2 with respect to an incident angle .theta. of
 light emitting on the detecting element 2 comprising a poly(ODMA-co-GLMA)
 layer of 2 .mu.m in thickness. In the curve illustrated in FIG. 10, four
 waveguide modes TM.sub.1, TM.sub.2, TM.sub.3, TM.sub.4 can be recognized.
 The poly(ODMA-co-GLMA) layer may be spin coated on the surface of a gold
 layer having a thickness of approximately 50 nm vapor-deposited on the
 bottom of a rectangular prism made of SF11 glass (the refractive index of
 which is 1.7780 at wavelength of 632.8 nm). Since the poly(ODMA-co-GLMA)
 layer has a refractive index of approximately 1.46 in water, which is
 larger than those of water and gold, the poly(ODMA-co-GLMA) layer
 functions as a waveguide.
 When the polymer thin film 2.sub.2 responds to a chemical substance in
 water, i.e., absorbs or adsorbs the chemical substance, the polymer thin
 film 2.sub.2 exhibits a change in thickness and/or refractive index to
 cause a shift of a resonance coupling angle at which a certain waveguide
 mode is excited. Such a shift of angle is a function of the concentration
 of the chemical substance. Thus, the concentration of a chemical substance
 in water can be sensed by measuring a shift of the resonance coupling
 angle associated with a certain waveguide mode. FIG. 11 is a graph
 representing a shift of the resonance coupling angle associated with the
 waveguide mode TM.sub.4 (FIG. 10) when the polymer thin film 2.sub.2
 responds to toluene in concentration of 2 ppm, resulted in a change
 .delta. of reflectivity. Specifically, a solid line represents the
 relationship between an incident angle of light emitting on the polymer
 thin film 2.sub.2 and the reflectivity of the polymer thin film 2.sub.2
 when the concentration of toluene is 0 ppm, while a broken line represents
 the relationship between the incident angle and the reflectivity after the
 polymer thin film 2.sub.2 has responded to toluene in concentration of 2
 ppm.
 Instead of measuring a shift of the resonance coupling angle, the
 concentration of a chemical substance in water can be sensed by fixing an
 incident angle .theta. of light from the light source 31 at one side of
 the waveguide mode resonance and measuring a change in intensity of light
 reflected from the detecting element 2. Since the resonance is quite
 sharp, even a very small shift of the resonance coupling angle appears as
 a large change in reflectivity.
 For supporting at least one waveguide mode, the polymer thin film 2.sub.2
 illustrated in
 FIG. 9A must have a sufficient thickness. For example, a cutoff thickness
 of the polymer thin film 2.sub.2 having a refractive index of 1.45 in
 water is approximately 284 nm in order for the polymer thin film 2.sub.2
 to have a TEO mode. When the thickness of the polymer thin film 2.sub.2 is
 equal to or less than the cutoff thickness, any waveguide mode cannot
 exist in the polymer thin film 2.sub.2. It is however possible to observe
 a different phenomenon referred to as "surface plasmon resonance"
 (hereinafter abbreviated as "SPR").
 The surface plasmon is plasma oscillation of free electrons existing on the
 boundary of a metal. This plasma oscillation is affected by the refractive
 index of a substance proximate to a surface of a metal. For example, when
 p-polarized light is incident to the bottom 91 of the prism 9 in the
 optical sensor constructed as illustrated in FIG. 9A and evanescent waves
 are produced by internal total reflection, surface plasma oscillation can
 be excited. The plasma oscillation is excited at an incident angle .theta.
 at which a tangential component of an evanescent wave vector on the bottom
 91 of the prism 9 matches a wave vector of plasma waves on an interface
 opposing to the polymer thin film 2.sub.2 (i.e., on the interface with the
 substrate) with respect to the metal layer 2.sub.3. In this event, the
 energy of the incident light is transferred to plasma waves to cause the
 intensity of reflected light to abruptly attenuate. This phenomenon is the
 surface plasmon resonance (SPR). The position of the resonance coupling
 angle of SPR largely depends on the refraction of the polymer thin film on
 the surface of the metal layer, so that the SPR method may also be
 unitized to sense a chemical substance in water.
 When the reflectivity to total internal reflected light is measured while
 varying an incident angle .theta. of incident light, the SPR can be
 experimentally observed as abrupt attenuation of the reflectivity at a
 certain resonance coupling angle. As an example of observed SPR, FIG. 12
 illustrates a reflectivity curve (SPR curve) showing how the reflectivity
 of the polymer thin film used in the optical sensor illustrated in FIG. 9A
 varies with respect to an incident angle 6 when the optical sensor
 comprises a poly(ODMA-co-GLMA) layer having a thickness of 107 nm which is
 spin coated on the surface of a gold layer having a thickness of 50 nm
 vapor-deposited on the bottom of a rectangular prism made of SF11 glass
 (the refractive index of which is 1.7780 at wavelength of 632.8 nm).
 Another optical sensor having the structure illustrated in FIG. 9A and
 relying on the SPR method can also be realized. While this optical sensor
 is also irradiated with p-polarized light, the polymer thin film should be
 as thin as possible, and preferably has a thickness in a range between
 several nanometers and several hundreds of nanometers. This is because the
 surface plasmon is a surface phenomenon and is sensitive to a change which
 may occur at a position several nanometers to several hundreds of
 nanometers away from the surface of the metal layer. When the polymer thin
 film absorbs or adsorbs a chemical substance in water and swells to cause
 a change in thickness and/or refractive index, the resonance coupling
 angle of SPR is shifted. It is therefore possible to sense the
 concentration of the chemical substance in water by measuring a shift of
 the resonance coupling angle of SPR or by measuring a change in intensity
 of light reflected from the polymer thin film with an incident angle of
 light from the light source being fixed at the near resonance coupling
 angle of SPR.
 Materials for the polymer thin film used in the optical sensor according to
 the present invention preferably include a homopolymer or copolymer having
 a recurring unit represented by the following chemical formula (I):
 ##STR1##
 where X represents --H, --F, --Cl, --Br, --CH.sub.3, --CF.sub.3, --CN, or
 --CH.sub.2 --CH.sub.3 ;
 R.sup.1 represents --R.sup.2 or --Z --R.sup.2 ;
 Z represents ----, --S--, --NH--, --NR.sup.2 --, --(C.dbd.Y)--,
 --(C.dbd.Y)--Y--, --Y--(C.dbd.Y)--, --(SO.sub.2)--, --Y'--(SO.sub.2)--,
 --(SO.sub.2)--Y'--, --Y'--(SO.sub.2)--Y'--, --NH--(C.dbd.O)--,
 --(C.dbd.O)--, --(C.dbd.O)--NH--, --(C.dbd.O)--NR.sup.2 '--,
 --Y'--(C.dbd.Y)--Y'--, or --O--(C.dbd.O)--(CH.sub.2)n--(C.dbd.O)--O--;
 Y represents the same or different O or S;
 Y' represents the same or different O or NH;
 n represents an integer ranging from 0 to 20; and
 R.sup.2 and R.sup.2 ' represent the same or different hydrogen, a linear
 alkyl group, a branched alkyl group, a cycloalkyl group, an un-saturated
 hydrocarbon group, an aryl group, a saturated or un-saturated hetero ring,
 or substitutes thereof. It should be noted that R.sup.1 does not represent
 hydrogen, a linear alkyl group, or a branched alkyl group.
 In the formula, X is preferably H or CH.sub.3 ; R.sup.1 is preferably a
 substituted or non-substituted aryl group or --Z--R.sup.2 ; Z is
 preferably --O--, --(C.dbd.O)--O--, or --O--(C.dbd.O)--; R.sup.2 is
 preferably a linear alkyl group, a branched alkyl group, a cycloalkyl
 group, an un-saturated hydrocarbon group, an aryl group, a saturated or
 un-saturated hetero ring, or substitutes thereof.
 A polymer used as the polymer thin film 2.sub.2 for the present invention
 may be a polymer consisting of a single recurring unit (I), a copolymer
 consisting of another recurring unit and the above-mentioned recurring
 unit (I), or a copolymer consisting of two or more species of the
 recurring unit (I). The recurring units in the copolymer may be arranged
 in any order, and a random copolymer, an alternate copolymer, a block
 copolymer or a graft copolymer may be used by way of example.
 Particularly, the polymer thin film is preferably made from
 polymethacrylic acid esters or polyacrylic acid esters. The side-chain
 group of the ester is preferably a linear or branched alkyl group, or a
 cycloalkyl group with the number of carbon molecules ranging preferably
 from 4 to 22.
 Polymers particularly preferred for the polymer thin film are listed as
 follows:
 poly(dodecyl methacrylate);
 poly(isodecyl methacrylate);
 poly(2-ethylhexyl methacrylate);
 poly(2-ethylhexyl methacrylate-co-methyl methacrylate);
 poly(2-ethylhexyl methacrylate-co-styrene);
 poly(methyl methacrylate-co-2-ethylhexyl acrylate);
 poly(methyl methacrylate-co-2-ethylhexyl methacrylate);
 poly(isobutyl methacrylate-co-glycidyl methacrylate);
 poly(cyclohexyl methacrylate);
 poly(octadecyl methacrylate);
 poly(octadecyl methacrylate-co-styrene);
 poly(vinyl propionate);
 poly(dodecyl methacrylate-co-styrene);
 poly(dodecyl methacrylate-co-glycidyl methacrylate);
 poly(butyl methacrylate);
 poly(butyl methacrylate-co-methyl methacrylate);
 poly(butyl methacrylate-co-glycidyl methacrylate);
 poly(2-ethylhexyl methacrylate-co-glycidyl methacrylate);
 poly(cyclohexyl methacrylate-co-glycidyl methacrylate);
 poly(cyclohexyl methacrylate-co-methyl methacrylate);
 poly(benzyl methacrylate-co-2-ethylhexyl methacrylate);
 poly(2-ethylhexyl methacrylate-co-diacetoneacrylamide);
 poly(2-ethylhexyl methacrylate-co-benzyl methacrylate-co-glycidyl
 methacrylate);
 poly(2-ethylhexyl methacrylate-co-methyl methacrylate-co-glycidyl
 methacrylate);
 poly(vinyl cinnamate) poly(butyl methacrylate-co-methacrylate);
 poly(vinyl cinnamate-co-dodecyl methacrylate);
 poly(tetrahydrofurfuryl methacrylate);
 poly(hexadecyl methacrylate);
 poly(2-ethylbutyl methacrylate);
 poly(2-hydroxyethyl methacrylate);
 poly(cyclohexyl methacrylate-co-isobutyl methacrylate);
 poly(cyclohexyl methacrylate-co-2-ethylhexyl methacrylate);
 poly(butyl methacrylate-co-2-ethylhexyl methacrylate);
 poly(butyl methacrylate-co-isobutyl methacrylate);
 poly(cyclohexyl methacrylate-co-butyl methacrylate);
 poly(cyclohexyl methacrylate-co-dodecyl methacrylate);
 poly(butyl methacrylate-co-ethyl methacrylate);
 poly(butyl methacrylate-co-octadecyl methacrylate);
 poly(butyl methacrylate-co-styrene);
 poly(4-methyl styrene);
 poly(cyclohexyl methacrylate-co-benzyl methacrylate);
 poly(dodecyl methacrylate-co-benzyl methacrylate);
 poly(octadecyl methacrylate-co-benzyl methacrylate);
 poly(benzyl methacrylate-co-benzyl methacrylate);
 poly(benzyl methacrylate-co-tetrahydrofurfuryl methacrylate);
 poly(benzyl methacrylate-co-hexadecyl methacrylate);
 poly(dodecyl methacrylate-co-methyl methacrylate);
 poly(dodecyl methacrylate-co-ethyl methacrylate);
 poly(2-ethylhexyl methacrylate-co-dodecyl methacrylate);
 poly(2-ethylhexyl methacrylate-co-octadecyl methacrylate);
 poly(2-ethylbutyl methacrylate-co-benzyl methacrylate);
 poly(tetrahydrofurfuryl methacrylate-co-glycidyl methacrylate);
 poly(styrene-co-octadecyl acrylate);
 poly(octadecyl methacrylate-co-glycidyl methacrylate);
 poly(4-methoxystyrene);
 poly(2-ethylbutyl methacrylate-co-glycidyl methacrylate);
 poly(styrene-co-tetrahydrofurfuryl methacrylate);
 poly(2-ethylhexyl methacrylate-co-propyl methacrylate);
 poly(octadecyl methacrylate-co-isopropyl methacrylate);
 poly(3-methyl-4-hydroxystyrene-co-4-hydroxystyrene);
 poly(styrene-co-2-ethylhexyl methacrylate-co-glycidyl methacrylate);
 It should be noted that in the methacrylate ester polymers or copolymers
 listed above, acrylate may be substituted for methacrylate. The polymers
 may be crosslinked on their own, or they may be crosslinked by introducing
 a compound that has crosslinking reactive groups. Suitable crosslinking
 reactive groups include, for example, an amino group, a hydroxyl group, a
 carboxyl group, an epoxy group, a carbonyl group, an urethane group, and
 derivatives thereof. Other examples include maleic acid, fumaric acid,
 sorbic acid, itaconic acid, cinnamic acid, and derivatives thereof.
 Materials having chemical structures capable of forming carbene or nitrene
 by irradiation of visible light, ultraviolet light, or high energy
 radiation may also be used as crosslinking agents. Since a film formed
 from crosslinking polymer is insoluble, the polymer forming the polymer
 thin film of the sensor may be crosslinked to increase the stability of
 the sensor. The crosslinking method is not particularly limited, and
 methods utilizing irradiation of light or radioactive rays may be used in
 addition to known crosslinking methods, for example, a heating method.
 It should be noted that in the optical sensor according to this invention,
 the refractive index of the polymer thin film and swelling of the polymer
 thin film vary with temperature, so that the characteristics of the
 optical sensor are inevitably affected somewhat by ambient temperature.
 For this reason, temperature control techniques or temperature
 compensation techniques are required for highly accurate measurements.
 Specifically, such temperature control and temperature compensation may be
 implemented by appropriate techniques including: (1) arrangement of the
 optical sensor in a temperature controlled housing or control of the
 temperature of water flowing through the flow cell; (2) creation of a
 temperature compensating signal using a temperature sensitive element
 attached on the optical sensor; (3) utilization of a polymer more
 sensitive than the polymer thin film to temperature to create a reference
 signal for temperature correction; and so on.
 Further, the optical sensor according to this invention may be modified to
 allow for measurements of the concentration of a mixture of organic
 substances (for example, hydrocarbons) dissolved in water. Such
 modifications may be realized by one or a combination of the following
 options:
 (1) a method using a kind of polymer thin film which has a low or small
 response to a plurality of kinds of chemical substances in water;
 (2) a method using a plurality of polymer thin films, each of which is
 selectively responsive to a different kind of chemical substance in water,
 wherein responses derived from these polymer thin films are combined to
 output a signal indicative of the concentration of a mixture composed of a
 plurality of chemical substances in water;
 (3) a method using a plurality of polymer thin films, each of which
 exhibits a different response to the same or different chemical substance
 in water, wherein responses derived from these polymer thin films are
 combined to output a signal indicative of the concentration of a mixture
 composed of a plurality of chemical substances in water; and
 (4) a method using one or a plurality of the above-mentioned polymer thin
 films in combination of different optical approaches (for example, the IER
 method, surface plasmon resonance, guided wave mode spectroscopy), to
 generate a signal indicative of the concentration of a mixture composed of
 a plurality of chemical substances in water.
 It should be noted however that a plurality of sensing elements, light
 source units, and/or light sensing elements may be required depending on
 particular implementations using one or a combination of the foregoing
 approaches.
 Actually, the sensitivity and response time of the optical sensor according
 to this invention may vary depending on the kind of organic substance to
 be measured. Thus, in order to accurately measure the total amount of
 organic substances and the concentrations of individual organic substances
 or groups of organic substances in a variety of applications, a
 multi-channel configuration of optical sensors may be configured using the
 methods (1)-(4) mentioned above, in combination of known pattern
 recognition techniques (for example, matrix analysis, neural network
 analysis, and so on).
 Several examples of the optical sensor according to the present invention
 will be described below.
 EXAMPLE 1
 FIG. 13 is a graph showing how a response of an optical sensor having the
 structure illustrated in FIG. 9A varies over time when the concentration
 of toluene in water is 4 ppm. An employed detecting element comprised a
 poly(ODMA-co-GLMA) layer of 2 .mu.m in thickness which was spin coated on
 a gold layer of 50 nm in thickness deposited on the bottom of a
 rectangular prism made of SF11 glass. A laser diode emitting light at
 wavelength of 670 nm was used for a light source. Light from the laser
 diode was split into a reference beam and measuring beam. The reference
 beam was directed to a silicon photodiode for reference, while the
 measuring beam was passed through a polarizing plate and directed to the
 bottom of the rectangular prism as p-polarized light. An incident angle of
 the measuring beam was set at a smaller angle of resonance in the TM.sub.4
 waveguide mode illustrated in FIG. 11. When the intensity of the measuring
 beam reflected by the bottom of the prism was measured by a silicon
 photodiode for measurement, four waveguide modes were observed as
 illustrated in FIG. 10.
 Outputs of the two photodiodes for reference and for measurement were
 supplied to an electronic divider to calculate the ratio of the output of
 the photodiode for measurement to the output of the photodiode for
 reference. The calculated ratio was supplied to an electric circuit to
 generate an output signal indicative of a response of the optical sensor.
 The result of the measurement revealed that for detecting toluene in
 water, a detection limit was lower than the concentration at 1 ppm and a
 90% response time (a time required to reach a complete response) was
 within three minutes.
 EXAMPLE 2
 FIG. 14 shows how the response of the same optical sensor used in Example 1
 varies as the concentration of toluene in water changes. It should be
 noted that the response from the optical sensor was measured in the form
 of a change in reflectivity with respect to a change in concentration of
 toluene. The measurement revealed that the optical sensor linearly
 responded to toluene in water in concentration ranging from 0 to 20 ppm.
 EXAMPLE 3
 FIG. 15 is a graph showing how an output signal of the same optical sensor
 used in Example 1 varies over time when the concentration of toluene in
 water is 20 ppm. The optical sensor used in this measurement employed a
 polymer thin film formed of a poly(ODMA-co-GLMA) layer of 107 nm in
 thickness and a helium-neon laser at wavelength of 632.8 nm for the light
 source. Surface plasmon resonance was observed as illustrated in FIG. 12
 by the use of the polymer thin film of 107 nm in thickness.
 An incident angle of a measuring beam, i.e., p-polarized light to the
 bottom of a prism was 60.82 which was rather a small incident angle in
 FIG. 12 illustrating the resonance coupling angle in the SPR method. As
 the output signal of the optical sensor was recorded as a function of
 time, the output signal increased in response to toluene in concentration
 of 20 ppm, and a 90% response time was approximately 3 minutes.
 EXAMPLE 4
 The optical sensor having the structure illustrated in FIG. 1A was used to
 measure toluene in concentration ranging from 0 to 300 ppm in water in
 accordance with the FS-IER method. A detecting element had a
 poly(ODMA-co-GLMA) layer of 1.95 .mu.m in thickness spin-coated on a
 silicon substrate, and was mounted in a housing as illustrated in FIG. 1A.
 Water was introduced into the housing by suction. A laser diode for
 emitting light at wavelength of 670 nm was employed for a light source.
 Light from the laser diode was divided into a reference beam and a
 measuring beam. The measuring beam was passed through a polarizing plate
 to be linearly polarized s-polarized light which was incident to the
 polymer thin film at an incident angle of 80 through a glass window.
 The intensity of the measuring beam reflected by the polymer thin film was
 measured by a silicon photodiode for measurement, while the intensity of
 the reference beam was measured by a silicon photodiode for reference.
 Outputs of these two silicon photodiodes were supplied to an electronic
 divider to calculate the ratio of the output of the silicon photodiode for
 reference to the output of the silicon photodiode for measurement. The
 calculated ratio was used to generate an output signal by an appropriate
 electric circuit. FIG. 16 illustrates the reflectivity measured by the
 optical sensor in Example 4 which is plotted as a function of the
 concentration of toluene in water. It can be seen from FIG. 16 that the
 response of the optical sensor is not linear in the range of 0-300 ppm.
 EXAMPLE 5
 The optical sensor used to obtain the measurement results has the same
 structure as the optical sensor illustrated in FIG. 6A. A sensing element
 comprises a poly(ODMA-GLMA) thin film spin-coated on an SF11 glass
 substrate in a thickness of 1 .mu.m and a 90.degree. glass prism mounted
 on a surface of the glass substrate, on which the poly(ODMA-GLMA) thin
 film is not formed, by a refractive index matching oil. A He-Ne laser with
 a wavelength of 632.8 nm is used as a light source, while photodiodes are
 used as first and second light detectors. As previously explained, a laser
 beam emitted from the light source is split by a beam splitter into a
 probe beam and a reference beam. The probe beam is transformed into
 s-polarized light by a polarizing plate and directed to be incident on the
 prism at an incident angle of 45.degree., and the probe beam reflected by
 the poly(ODMA-GLMA) thin film is supplied to the first light detector. The
 reference beam, in turn, is supplied directly to the second light
 detector. Signals representative of the intensities of the probe beam and
 the reference beam are generated by the first and second light detectors,
 respectively, and supplied to an electronic circuit for producing
 measurement results.
 FIG. 17 is a graph representing the reflectivity of the optical sensor in
 response to 10-200 ppm toluene dissolved in water as a function of time.
 It can be seen from this graph that the optical sensor of this invention
 has a high sensitivity and a fast response. Detection limit could be
 around 1-2 ppm, and the 90% response time is less than 2 minutes.
 The relationship between the response of the sensing element and the
 concentration of toluene in water is as illustrated in FIG. 18 which
 reveals that the reflectivity of the sensing element linearly changes in
 proportion to the concentration of toluene in water.
 EXAMPLE 6
 Next, for examining how the response of the sensing element is related to
 the concentration of another organic substance in water, the reflectivity
 of the sensing element is measured while changing the concentrations of
 benzene and p-xylene in addition to toluene. The results are shown in FIG.
 19. The graph of FIG. 19 reveals that the response of the sensing element
 linearly changes in proportion to the concentrations of the three kinds of
 organic substances, i.e., toluene, benzene, and p-xylene, and the
 sensitivity of the sensing element to toluene and p-xylene are 3.5 and
 9.25, respectively, when the sensitivity of the sensing element to benzene
 is assumed to be one.
 EXAMPLE 7
 It is desirable that a polymer thin film has response to as many kinds of
 chemical substances under detection, i.e., DOC as possible for detecting
 DOC in water. The optical sensor used in Example 1 having a
 poly(ODMA-co-GLMA) layer of 2 .mu.m in thickness was used to measure a
 large number of different kinds of DOC in water. The results of the
 measurements are listed in the following table. In the table,
 "Sensitivity" is a value representing a change in reflectivity in percent,
 and "time" represents a 90% response time.
 TABLE 1
 Concentration Time
 Organic compound (ppm) Sensitivity (Minute)
 toluene C.sub.7 H.sub.8 2 41.72 6
 benzene C.sub.6 H.sub.6 2 10.34 4
 chlorobenzene C.sub.6 H.sub.5 Cl 2 87.07 9
 nitrobenzene C.sub.6 H.sub.5 NO.sub.2 2 23.28 1.5
 -xylene C.sub.8 H.sub.10 2 126.86 15
 chloroform CHCl.sub.3 2 8.71 3
 carbon tetrachloride CCl.sub.4 2 21.55 11.5
 1,2-dichloroethane 2 6.03 1.5
 ClCH.sub.2 CH.sub.2 Cl
 dichloromethane CH.sub.2 Cl.sub.2 2 1.38 1
 diethyl ether C.sub.4 H.sub.10 O 100 11.21 1.5
 tetrahydrofuran O(CH.sub.2).sub.4 100 9.48 1
 acetone C.sub.3 H.sub.6 O 500 8.62 1
 propanol C.sub.3 H.sub.8 O 500 6.03 1
 methanol CH.sub.4 O 500 0.00
 acetic acid C.sub.2 H.sub.4 O.sub.2 500 3.45 &lt;1
 hydrochloric acid HCl 2000 25 &lt;1
 sulfuric acid H.sub.2 SO.sub.4 2000 6.67 &lt;1
 It can be seen from Table 1 that the sensitivity and response time of the
 optical sensor vary depending on the kind of DOC to be detected. A
 multi-channel system comprising an array of detecting elements or an array
 of polymer thin films is therefore necessary to detect a large number of
 DOC in variety of industrial applications and environment protecting
 applications. In addition, known pattern recognition techniques such as
 matrix analysis, neural network analysis, or the like may be applied
 together with such a multi-channel system in order to accurately determine
 a total amount of DOC, the concentration of each DOC, or the
 concentrations of a DOC group.
 The pattern recognition techniques applicable to the detection of chemical
 substances in water using the optical sensor of the present invention are
 described in an article entitled "Detection of Hazardous Vapors Including
 Mixtures Using Pattern Recognition Analysis of Responses from Surface
 Acoustic Wave Device" by Susan L. Rosepehrsson et al. published in "Anal.
 Chem.", 1088, 60, pp 2801-2811; an article entitled "Development of
 Odorant Sensor Using SAW Response Oscillator
 Incorporating Odorant-Sensitive LB Films and Neural-Network Pattern
 Recognition Scheme" by Sang-Mok Chang et al, published in "Sensors and
 Materials", Vol. 1 (1995), pp 013-022; an article entitled "Polymer-based
 sensor arrays and multicomponent analysis for the detection of hazardous
 organic vapors in the environment" by Andreas Hierlemann et al. published
 in "Sensors and Actuators B" 26-27 (1995), pp 126-134; and so on.
 As will be apparent from several embodiments and examples of the present
 invention described above in detail, the optical sensor according to the
 present invention is advantageous over the optical fiber sensor in that it
 directly detects a change in thickness and/or refractive index of a
 polymer thin film resulting from interaction with a chemical substance in
 accordance with an optical approach such as the IER method, WG method, SPR
 method, or the like, so that even a trace of change in thickness and/or
 refractive index can be detected with a high sensitivity. In addition,
 since the polymer thin film can be readily formed by an ordinary method
 such as spin coating, the optical sensor itself can be readily
 manufactured.