WO2009115847A1

WO2009115847A1 - Monolithically integrated physical chemical and biological sensor arrays based on broad-band mach-zhender interferometry

Info

Publication number: WO2009115847A1
Application number: PCT/GR2009/000013
Authority: WO
Inventors: Ioannis Raptis; Konstantinos Misiakos; Sotiris Kakabakos; Panagiota Petrou; Eleni Makarona; Maria Kitsara
Original assignee: Ncsr "Demokritos"
Priority date: 2008-03-17
Filing date: 2009-03-09
Publication date: 2009-09-24
Also published as: GR1006491B

Abstract

Monolithically integrated optical interferometric sensor arrays for multi-analyte real-time label-free chemical and biological sensing are disclosed. The principle of operation of the said sensors is based on broad-band Mach-Zehnder Interferometry. Such an interferometry scheme exploits the full spectrum and modes of any given broad-band light source lowering the threshold limits and increasing the sensitivity of the disclosed arrays. Two embodiments of the sensor arrays are disclosed: in one embodiment/ the individual sensors consists of a light-emitting element, a photodetector in the form of a photodiode and planar waveguides self aligned and optically coupled to the emitter and the photodetector, monolithically fabricated on the same silicon chip using standard microfabrication technologies. Each sensor of the array uses a different light-emitting-element while the photodetector is common for all sensors. On the second embodiment, the sensor array includes the monolithically fabricated light-emitting elements and the coupled waveguides, and the array is coupled to a spectrophotometer. In both embodiments the sensing arm of each of the individual interferometric sensors of the array is properly functionalized in order to detect a specific analyte allowing thus real-time multi-analyte monitoring.

Description

MONOLITfflCALLY INTEGRATED PHYSICAL CHEMICAL AND BIOLOGICAL SENSOR ARRAYS BASED ON BROAD-BAND MACH-ZHENDER INTERFEROMETRY FIELD OF THE INVENTION

The invention relates to the field of optical multi-analyte sensors with multiplexed signals for label- free physical chemical and biological detection. The principle of operation of the invented optical sensor which is based on the broad-band Mach-Zehnder Interferometry is disclosed. A sensor design that allows the fabrication of a fully-integrated optical monolithic silicon-based sensor array without the need of any external optic elements is also disclosed. The said array may be produced in two embodiments, one allowing for the ultimate degree of integration, and the other for the exploitation of spectral features for substance identification. BACKGROUND OF THE INVENTION

A chemical or biological sensor (hereafter referred simply as "sensor") is an analytical device that consists of a chemical or biological recognition element and a signal transducer, which together relate the presence and/or the concentration of an analyte to a measurable response. The analyte of interest could be a chemical in a gaseous or liquid state, a new drug under investigation, a protein, an enzyme, an antibody, a receptor or nucleic acid and even cells or tissue slices. The transducer of the sensor is employed to convert the chemical/biological-recognition step into a signal for further recording and analysis. Such devices are usually divided into four broad categories depending on the transduction mechanism: optical, piezoelectric, calorimetric and electrochemical.

The detection principles invented and applied in the optical biosensors and biosensor arrays, are based either on labeling (enzymes, fluorescent or chemoluminescent molecules) or on optical signal changes (label-free sensors). The majority of the optical label-free approaches is based on the monitoring of changes of the optical properties of the areas where the chemical or biomolecular interactions are taking place. However, the quality of sensors does not depend on the transduction principles only, but on the total sensor system defined by this transduction: the sensitive layer, data acquisition electronics, and evaluation software.

The major label-free optical methodologies developed and applied for chemical or biological sensors are based on i) Reflectometric Interference Spectroscopy (RIfS) ii) Optical Porous Silicon Biosensors iii) Microcantilevers iv) Surface Plasmon Resonance (SPR) and v) Single-Wavelength Mach-Zehnder Interferometry (MZI).

Reflectometric Interference Spectroscopy (RIfS): RIfS [1] is based on the interference of light beams reflected at interfaces of different refractive indices. At its typical configuration, a thin glass slide is typically coated with an interference Siθ₂ layer and illuminated with white light. At each interface the light beam is partly reflected, thus the reflected beams travel different optical paths (n*d), and a phase difference is introduced causing changes in the interference spectrum. RIfS systems provide measurement of real time kinetics of bioanalyte binding to a surface immobilized sensor molecule. Sensitivity in the order of 10 pg/mm² for protein-protein (avidin - anti-avidin) interaction [2] and detection of femtomole quantities of untagged DNA oligonucleotides in an array format have been shown. However, in RIfS an external light source, a spectrometer and optical fibers are required and thus it is not possible to fabricate a miniaturized integrated device. Optical Porous Silicon Biosensors: The photonic band gap (PBG) structures are highly sensitive to changes in the refractive index of the environment and thus are ideal candidates for ultra small and efficient "lab-on-a-chip" devices. Porous silicon (PSi)-based PBG structures [3] due to their high specific surface have been investigated thoroughly as optical label-free (bio)chemical sensors. The PSi advantages include ease of fabrication and compatibility with silicon microelectronics technology. However, external light sources and detectors are required making the development of an integrated device impossible.

Microcantilevers: Microfabricated cantilever array sensors have been applied for the monitoring of (bio-) chemical interactions through the detection of the beam deflection either optically or electrically. Adsorption of molecules on a receptor layer on the cantilever surface produces formation of surface stress and therewith bending of the cantilever [4, 5]. The major advantages of such sensors are their small size, fast response times, high sensitivity and direct signal transduction without the need of using any labels. However, an external light source and photodetector is required with very strict alignments requirements, while the functionalization of each cantilever for different analyte detection is very difficult. Therefore, this approach is equally not suitable for integrated biosensors arrays.

Surface Plasmon Resonance (SPR): In SPR the detection of molecular interactions is realized through excitation of surface plasmons using parallel-polarised light. The SPR signal, expressed in resonance units, is a measure of mass concentration at the sensor chip surface thus the biomolecular interactions can be observed [6]. This methodology even though it provides very high sensitivity, is not suitable for real integration due to the need of inclined illumination of the sample under investigation.

Mach-Zehnder Interferometer sensor (MZI): In the MZI [7, 8] sensor a single wavelength (hereafter referred to as SW-MZI) coherent beam is split into two arms, one as a reference and one as sensing, using a beam splitter (Y-junction). The difference in refractive indices of the evanescent fields between the sensing and reference arms caused by the analyte absorption/adsorption/binding to the sensing arm is recorded as intensity change at the output waveguide after a second Y-junction - recombining the two arms. The MZI device offers very high sensitivities in terms of refractive index changes [9], limited mainly by the sensing arm length, and can be fabricated on Si wafers with standard microelectronic processes [10]. However, the light used, should be monochromatic causing the requirement for external light sources (such light sources can not integrated on Si) and detectors. Mathematically, the output signal of the MZI (I_τ) is given by:

where I₅ and I_R are the light intensities in the sensor and reference arms, respectively. The term Δφ(t) is the phase difference between the light beams traveling in both arms, and is:

where Ns and N_R are the effective refractive indices of the sensing and reference arms, respectively, λ is the wavelength of the light and L is the length of the sensing area. The phase difference (Δφ) depends on the product of the changes of the effective refractive index and the length of the sensing area. However, in addition to the advantages of MZI device there are certain drawbacks when monochromatic light is used and in particular:

(a) Optical coupling: Monochromatic light sources (i.e. lasers) and detectors can be coupled to MZIs only through external optical fibers. Misalignments, even minor ones, of the optical components or changes in the environment (e.g. humidity change) will be detected by the SW-MZI because of its high sensitivity and will be misinterpreted as phase changes of the interferometer. In addition, the use of external light sources increases the packaging requirements and cost, renders the assembly of the system difficult and increases the volume of the final system.

(b) Ambiguity: it is not possible to deduce the direction of the phase changes when the phase difference between the two arms of the SW-MZI is an integer multiple of π (eq. 2). (c) Signal fading: the sensitivity depends on the initial phase difference between the interferometric arms. If the interferometer is tuned close to one of the extreme values of the transmission curve, small phase changes will generate low intensity variations. Even though there are several sensor designs offering very high sensitivities, their integration in an all-silicon device has been improved impossible so far and in all cases the analytical microsystems are assembled by hybrid integration or coupling with external optical parts (such as light sources and photodetectors). The only exception is a monolithic silicon optoelectronic transducer (WO 03046527, WO 2007/074348, [H]) where standard silicon micro-technology is applied for the fabrication of integrated biosensors incorporating arrays of light sources, and photodetectors coupled with waveguides all on the same silicon die. However, the sensing in these fully integrated devices is realized through the evanescent field changes along planar waveguides without applying any interferometric methods, while in some cases labeling through fluorescent dyes or nanoparticles is required.

[I]. J. Piehler, A. Brecht, G. Gauglitz Anal. Chem. 68 139(1996) "Affinity Detection of Low

Molecular Weight Analytes" [2]. J. Lu, CM. Strohsahl, B.L. Miller, LJ. Romberg Anal. Chem. 76 4416(2004) "Reflective

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"Towards a label-free optical porous silicon DNA sensor" [4]. M. Calleja, M. Nordstrom, M. Alvarez, J. Tamayo, L.M. Lechuga, A. Boisen Ultramicroscopy

105 215(2005) "Highly sensitive polymer-based cantilever-sensors for DNA detection" [5]. T. A. Betts, CA. Tipple, MJ. Sepaniak, P.G. Datskos Anal. Chim. Acta 422 89(2000) "Selectivity of chemical sensors based on micro-cantilevers coated with thin polymer films"

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Silicon Optoelectronic Transducer as a Real-Time Affinity Biosensor"

BRIEF DESCRIPTION OF THE DRAWINGS

Figure Ia: Top view of the fully-integrated embodiment of a single BB-MZI bio/chemical sensor (not drawn to scale). Figure Ib: Top view of the semi-integrated embodiment of a single BB-MZI bio/chemical sensor (not drawn to scale).

Figure 2a: Cross section of the fully-integrated BB-MZI bio/chemical sensor based on silicon technology (not drawn to scale)

Figure 2b: Cross section of the fully-integrated BB-MZI bio/chemical sensor based on compound semiconductor technology with conductive substrates (not drawn to scale) Figure 2c: Cross section of the fully-integrated BB-MZI bio/chemical sensor based on compound semiconductor technology with non-conductive substrates where mesa structures are required (not drawn to scale)

Figure 3a: Top view of an array of BB-MZI bio/chemical sensors in the fully-integrated embodiment where all sensors converge and optically couple to a common integrated photodiode (not drawn to scale). Each sensor is functionalized for a different analyte/measurand, while one of them is used as a reference.

Figure 3b: Top view of an array of BB-MZI bio/chemical sensors in the semi-integrated embodiment where all sensors converge and optically couple to a single planar waveguide. The planar waveguide is optically coupled to an external optical fiber and an external photodetector (photodiode or spectrophotometer). Each sensor is functionalized for a different analyte/measurand, while one of them is used as a reference (not drawn to scale).

DETAILED DESCRIPTION OF THE INVENTION

The present innovation relates to the development, method of operation and applications of a fully integrated monolithic sensor array for ultra-sensitive, real-time, label-free, multi-analyte detection of biological and chemical entities. It is an object of the present invention to overcome the aforementioned drawbacks and limitations associated with the state-of-the-art SW-MZI.

Said sensor array comprises (i) a plurality of integrated light emitting elements,(ii) a plurality of integrated planar optical interferometric waveguides each one self-aligned and optically coupled to one of the said plurality light-emitting elements, (iv) one single optical detector coupled to each of the said plurality of optical waveguides via (v) a single planar waveguide where all the optical waveguides of the said plurality converge. Each light-emitting element of the said array is a broadband emitting light source emitting in the visible or near-infrared part of the spectrum. Each planar waveguide of the said plurality is an integrated Mach-Zehnder Interferometer, the operation of which is based on Broad-band Mach-Zehnder Interferometry. All elements that comprise the said sensor array are fabricated with standard microelectronic - micromachining techniques. The sensing arm of each of the said plurality of Mach-Zehnder waveguides after appropriate functionalization is selectively absorb or adsorb or bind a specific analyte which will alter the effective refractive index atop the sensing arm. The effective refractive index change along the sensing arm is in turn translated to a phase change between the two propagating modes which when recombined at the second Y- divisor will interfere in a way dictated by the induced phase change. The interference will occur for every wavelength of the spectrum and the final result will be a change in the output spectrum and the integrated intensity of the output signal recorded by the said common optical detector. Each said element of the plurality of emitters along with its said corresponding appropriately functionalized Mach-Zehnder interferometer consist an individual sensor of the array (hereinafter referred simply as BB-MZI sensor) tailored for the detection of a specific analyte. Multiplexed operation of the said plurality of the light-emitting elements and the interferometric basis of operation of each of the said individual BB-MZI sensors allows for real-time multi-analyte detection of bio/chemical reactions not requiring labeling (label-free detection).

The present invention of BB-MZI optical sensors solves the SW-MZI limitations as follows: (a) Optical coupling: This major issue is radically dealt with through the monolithic integration of the light-emitting elements which are optically coupled to the self-aligned waveguides and common detector in a way similar to the approach described in WO03046527 and WO 2007/074348 for the Si- based array, (b) Ambiguity: the problem of ambiguity is circumvented since the phase change for every wavelength is different, and therefore it is possible to deduce from the full spectrum the exact changes, (c) Signal Fading: For a monochromatic source very careful design of the SW-MZI has to be performed prior to any application in order to avoid operation near the quadrature points. This renders each SW-MZI applicable only to a limited number of sensing schemes. When a broad-band source is employed, there are wavelengths in the spectrum that will be far off from the extreme values of their corresponding transmission curves enhancing the output signal and allowing thus the BB-MZI sensor arrays to be applied to a large number of detection schemes. Moreover, the use of a broad-band spectrum increases the sensitivity in the detection. Every wavelength is "affected" in the sensing arm because of any binding event in a larger or lesser degree producing a phase change of its own. This in turn is translated into the output spectrum both as changes the intensity and as peak shifts. The changes procured by every binding event would have the potential to be used for substance fingerprinting, while integration of the spectrum intensity will also yield different results for various applications.

The BB-MZI sensor can be operated in two different embodiments as is depicted in Figures Ia and Ib. The first embodiment (referred to hereafter as the "fully-integrated embodiment") depicted in Figure Ia consists of a photodetector 140 integrated on the same die 100 as the light emitting device 110 and its fabrication process is part of the fabrication of the said BB-MZI sensor. The light-emitting device is self-aligned to the planar waveguide 120 fabricated on top of the bottom cladding layer 130 over the substrate 100. The said waveguide is microfabricated to a BB-MZI with a reference arm 122 and a sensing arm 121 atop of which the cladding layer 131 may be removed with microfabrication techniques leaving an opening 361. After the second Y-divisor the two arms converge to the output waveguide 123. The output planar waveguide 123 is then optically coupled to an integrated photodiode 140 fabricated on the same substrate 100 at the same time as the light-emitting device 110. Detection of each of the analyte(s) for which the said BB-MZI is customized, is achieved through changes in the output intensity of the light-emitting device 110 due to the changes of the refractive index atop the sensing arm 121 that are in turn transduced into changes in the photocurrent detected at the photodetector 140. Metallic contact pads 141 and 142 are employed for wire bonding of the light emitting diode and the photodiode.

In the second embodiment (referred to hereafter as the "semi-integrated embodiment") of the said BB- MZI sensor, depicted in Figure Ib, the integrated photodiode 140 is replaced by an external spectrometer or photodiode 153 coupled to the said output waveguide 123 through a butt-coupled external fiber 152. The detection on this said embodiment is based on the changes of the spectrum of the said light-emitting element 110. Depending on the experimental setup an objective lens 150 and a polarizer 151 may be employed and inserted between the output waveguide 123 and the external optical fiber 152. The semi-integrated embodiment, even though it does not allow for the ultimate degree of integration, allows for fingerprinting of chemical or biological reactions through the development of an appropriate algorithm.

In preferred embodiments of the invention the said BB-MZI sensors are fabricated monolithically on silicon wafers according to the approach described in WO03046527 and WO 2007/074348 where the said light-emitting elements 110 are fully integrated silicon avalanche photodiodes biased beyond their breakdown voltage and emitting in a broad range in the visible and near-infrared. The basic layout of the said sensor in cross-section is depicted in Figure 2a. The said planar waveguides 120, 123 fabricated into BB-MZIs with sensing arm 121 and reference arm 122 are made out of silicon nitride (Si₃N₄) or another suitable inorganic dielectric material lithographically defined and etched with conventional etching techniques such as reactive ion etching (RIE), plasma-enhanced reactive ion etching (PE-RIE), electron cyclotron reactive ion etching (ECRIE), wet etching or a combination thereof. The said integrated photodetector 140 is also a silicon photodiode. The bottom cladding layer 130 made out of silicon dioxide (SiO₂) or another suitable dielectric may be used also as a spacer for the smooth bending of the waveguides 120 and 123 and their effective optical coupling to the light emitting diode 110 and the photodiode 140, respectively. Silicon dioxide (SiO₂) or another suitable inorganic dielectric 210 may be used for the electrical isolation of the integrated devices within the same chip. SiO₂ or another suitable dielectric material is used as the top cladding layer 131 which is removed atop the sensing arm by employing appropriate lithographic and etching techniques such as RIE, PE-RIE, ECRDE, wet etching or a combination thereof. The sensing arm depending on the sensing application may be totally exposed to the analytes or be covered with a suitable layer 361 where bio/chemical reactions may take place. A metallic film that can form p-type ohmic contact with Si, is used as metallic contacts and pads 141 and 142, deposited with any conventional deposition techniques (such as e-beam evaporation, sputtering, electroplating, metal fuse etc) and patterned lithographically. Contact pads 141 and 142 serve as the contacts for wire bonding or direct probing of the light emitting diode 110 and the photodiode 140 respectively. A metallic thin layer or multi-layers of metals 230 that can form n-type ohmic contacts, are deposited with any conventional deposition techniques (such as e-beam evaporation, sputtering, electroplating, metal fuse etc) at the bottom of the silicon wafer and serve as the lead for the common ground 231 for both devices 110 and 140. Programmable bias voltages 232 and 233 are applied via the metallic pads 141 and 142 with respect to the common ground 231 to drive electrically the light emitting diode 110 and the photodiode 140. Another realization of the embodiments may be selected among different classes of semiconducting materials, such as IL-VI, DI-V and DI-Nitride compound semiconductors covering a very wide spectrum from the UV to the DR.. There are two possible realization schemes for compound semiconductors depending on whether the substrate is conducting or not, depicted in cross sections in Figures 2b and 2c respectively.

With reference to Figure 2b depicting the case of a BB-MZI sensor fabricated from compound semiconductors with a conductive substrate, a conducting substrate (referred to hereafter as "epi- wafer") 300 is used atop of which all subsequent layers required for the diodes are grown using any one or any combination of various growth techniques, such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), metallorganic chemical vapor deposition (MOCVD) etc. A buffer layer 371 is directly grown atop the epi-wafer followed by an extra layer 372 that may be a conducting n-type superlattice (n-SL) for improved n-type conductivity, or an in-situ Distributed Bragg Reflector (DBR) for enhanced light confinement and directionality of the emitted light, or a combination of a n-SL and an in-situ DBR. Then, the pin-structure required for the formation of the diodes (both the light emitting diode 310 and the photodiode 320) is grown comprising an n-type layer 301, an active region 302 that may be a thin layer of intrinsic semiconductor or a single quantum well (SQW) or a multiple quantum well (MQW) region, and a p-type layer 303. Depending on the nature of the compound semiconductor used, an electron-blocking layer 373 may be inserted atop the active region 302 and a p-type SL (p-SL) 374 may be added on top of or beneath the p-type layer 303 for enhanced p-type conductivity. In addition, an ex-situ or in-situ DBR 375 grown by sputtering or MBE or CVD or MOCVD may be added for increased directionality of the emitted light atop the p- type layer 303. A metallic thin layer or multi-layers of metals that can form p-type ohmic contacts with the p-type material are used as metallic contacts 311 and 321, and are lithographically patterned and deposited with any conventional deposition techniques (such as e-beam evaporation, sputtering, electroplating, metal fuse etc). The contacts 311 and 321 may be ultra-thin semi-transparent films in case there are current spreading issues related to the chosen compound semiconductor system, or might be of a semi-circular or half-square or any other shape that exposes the p-type material. Metallic multilayers 312 and 322 sitting atop the passivation layers 340 made out of a suitable inorganic dielectric material, serve as the contact pads for wire bonding or direct probing of the light emitting diode 310 and the photodiode 320. A metallic thin layer or multi-layers of metals 380 that can from n-type ohmic contacts are deposited with any conventional deposition techniques (such as e- beam evaporation, sputtering, electroplating, metal fuse etc) at the bottom of the epi-wafer serves as the lead for the common ground 381 for both the light emitting diode 310 and the photodiode 320Programmable bias voltages 313 and 323 are applied via the metallic pads 312 and 322 with respect to the common ground 381 to drive electrically the light emitting diode 310 and the photodiode 32O.The BB-MZI waveguide 350 is made out of a dielectric material and defined lithographically and etched with conventional etching techniques such as reactive ion etching (REE), plasma-enhanced reactive ion etching (PE-RIE), electron cyclotron reactive ion etching (ECRIE), wet etching or a combination thereof. The BB-MZI might be fabricated on top of a spacer layer 341 that also serves as the bottom cladding layer and can be made out of the same material and during the same process steps as the passivation layer 340. A top cladding layer 360 may be employed and may be fabricated out of a dielectric material with suitable index of refraction that ensures adequate waveguiding. The sensing arm depending on the sensing application may be totally exposed to the analytes or be covered with a suitable layer 361 where bio/chemical reactions may take place.

With reference to Figure 2c depicting the case of a BB-MZI sensor fabricated from compound semiconductors with a non-conductive epi-wafer, a non-conducting substrate 300 is used atop of which all subsequent layers required for the diodes are grown using any one or any combination of various growth techniques, such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), metallorganic chemical vapor deposition (MOCVD) etc. A buffer-layer 371 is directly grown atop the epi-wafer followed by an extra layer 372 that may be a conducting n-type superlattice (n-SL) for improved n-type conductivity, or an in-situ Distributed Bragg Reflector (DBR) for enhanced light confinement and directionality of the emitted light, or a combination of a n-SL and an in-situ DBR. Then, the pin-structure required for the formation of the diodes (both the light emitting diode 310 and the photodiode 320) is grown comprising an n-type layer 301, an active region 302 that may be a thin layer of intrinsic semiconductor or a single quantum well (SQW) or a multiple quantum well (MQW) region, and a p-type layer 303. Depending on the nature of the compound semiconductor used, an electron-blocking layer 373 may be inserted atop the active region 302 and a p-type SL (p-SL) 374 may be added on top of or beneath the p-type layer 303 for enhanced p-type conductivity. In addition, an ex-situ or in-situ DBR 375 grown by sputtering or MBE or CVD or MOCVD may be added for increased directionality of the emitted light atop the p-type layer 303. All layers grown atop the epi- wafer are referred to hereafter as the "epi-structure". In order to access the n-type layer, the epi- structure is selectively etched down to the n-type layer into a mesa structure using any or a combination of etching techniques such as RIE, PE-RIE, EC-REE, or wet etching. A passivation layer 340 of a suitable dielectric is deposited on the mesa structure with any conventional deposition technique such as CVD, PE-CVD or sputtering. A metallic thin layer or multi-layers of metals that can form p-type ohmic contacts with the p-type material are used as metallic contacts 311 and 321, and are lithographically patterned and deposited with any conventional deposition techniques (such as e-beam evaporation, sputtering, electroplating, metal fuse etc). The contacts 311 and 321 may be ultra-thin semi-transparent films in case there are current spreading issues related to the chosen compound semiconductor system, or might be of a semi-circular or half-square or any other shape that exposes the p-type material. A metallic thin layer or metallic multilayers 312 and 322 sitting atop the passivation layer(s) 340, serve as the contact pads for wire bonding or direct probing of the light emitting diode 310 and the photodiode 320. A metallic thin layer or multi-layers of metals 330 and 331 that can from n-type ohmic contacts are deposited with any conventional deposition techniques (such as e-beam evaporation, sputtering, electroplating, metal fuse etc) at the exposed n-type layer of the mesa structures as the n-contact of the light emitting diode 310 and the photodiode 320 respectively. Programmable bias voltages 313 and 333 are applied via the metallic pads 312 and 332 to drive electrically the light emitting diode 310, while programmable bias voltages 323 and 335 are applied via the metallic pads 322 and 334 to drive electrically and the photodiode 320. The BB-MZI waveguide 350 is made out of a dielectric material and defined lithographically and etched with conventional etching techniques such as RIE, PE-RIE, EC-RIE, wet etching or a combination thereof. The BB-MZI might be fabricated on top of a spacer layer 341 that also serves as the bottom cladding layer and can be made out of the same material and during the same process steps as the passivation layer 340. A top cladding layer 360 may be employed and may be fabricated out of a dielectric material with suitable index of refraction that ensures adequate waveguiding. The sensing arm depending on the sensing application may be totally exposed to the analytes or be covered with a suitable layer 361 where bio/chemical reactions may take place. In other preferred aspects of the embodiments of the present invention, the said BB-MZI sensors may be fabricated by organic materials provided the doped organic layers can produce p-i-n junctions operating as organic light-emitting diodes 110 (OLEDs) and photodiodes 140 in order to achieve the full integration of both the said broad-band light sources and said photodetectors. The planar waveguides 120 123 fabricated into BB-MZIs 121 and 122 can be chosen either among organic or inorganic dielectric materials with the only restriction that their refractive index is higher than the underlying organic semiconductor in order to achieve waveguiding. This embodiment may be realized in any of the aspects depicted in Figures 2a-2c depending on the choice of materials and substrates. It is a further objective of the invention to use a plurality of the described BB-MZI sensors -each one of them functionalized with a different material- integrated on the same substrate to form a sensor array to be applied for chemical and biological sensing. The plurality of sensors 421 forming the array uses a plurality of light emitting devices 410 and input waveguides 420. All BB-MZI sensors 421 of the said plurality are connected via waveguides 422 to a common output waveguide 423 and share thus the same integrated photodiode 440 (figure 3a), or the external photodiode or spectrometer 153 (figure 3b) through a butt-coupled external fiber 152 and the possible use of an objective lens 150 and a polarizer 151. In conjunction with figures Ia, Ib, 2a, 2b and 2c, the plurality of the said BB-MZI sensors is fabricated on a common substrate 400 with a bottom cladding layer 430 and a top cladding layer 431 opened lithographically atop the sensing arm of each BB-MZI 421. Metallic contacts and pads 441 as described in figures 2a-2c, are used for the wiring and electrical driving of the light emitting diodes 410, while a metallic contact and pad 442 as described in figures 2a-2c, is used for the wiring and electrical driving of the common integrated photodiode 440. For both embodiments, software controlled sequential turning-on and off of the emitters (multiplexed) operation of the plurality of the light-emitting elements allows for the real-time detection of different analytes within the same die and from the same detector.

It is yet a further objective of the invention to develop sensor arrays -which after appropriate functionalization or use of specific adsorbing/absorbing thin films or appropriate calibration for exposed sensing arms- will be encapsulated in a single-use chip equipped with an appropriately designed microfluidic system which will allow for the easy delivery of the samples to be analyzed and will also ensure the facile contact with the external low-noise electronic components. The whole chip will be encapsulated and hence immune to any environmental changes.

It is a further objective of the present invention, to develop sensors tailored to specific diagnostic applications. This involves appropriate functionalization of the sensing arm surface with specific recognition biomolecules (e.g. binding protein, antibody, ss-DNA) that can specifically bind to their counterpart molecules from a sample (e.g. human serum sample, PCR product). An example is the use of an antibody designed to bind a specific antigen in human serum sample or an oligonucleotide designed to bind a specific DNA sequence. Biomolecules can be immobilized onto the sensing arm surface either by covalent bonding or by physical adsorption. In the first case, the sensing arm surface has to be modified in order to introduce reactive groups which will be used for the covalent bonding of recognition biomolecules through standard coupling chemistries known to the art. In the latter case, a film or monolayer with protein or DNA binding ability can be created onto top of the sensing arm and used for immobilization of recognition biomolecules through passive adsorption. In addition, the sensing arm of each said BB-MZI sensor in an array of sensors may be covered with a film or monolayer to which a biomolecule (protein or ss-DNA), different for each sensor might be immobilized by spotting or other known to the art techniques. Each sensor array will be comprised of individually functionalized BB-MZI sensors coupled to a single detector for multiplexing operation allowing for the real-time synchronous detection of the specific counterpart molecules of the immobilized recognition molecules (analytes). In addition, another objective of the present invention is the use of specially selected thin organic or inorganic films (suck as photoresists, PDMS etc) on the exposed sensing arms of the sensors whereas absorption or adsorption of chemicals on the said films will alter their thickness and refractive index allowing thus for detection of compounds either in gaseous or liquid solutions. One example could be the use of thin polymer films for the detection of humidity or volatile organic compounds. The thickness of the proposed films should be in the range of a few nm to 500nm, preferably in the range of 100 to 300nm in order to ensure adequate penetration of the evanescent field within the films.

In another example of the embodiments of the present invention, the sensing arms of the said BB- MZI sensors can be fully exposed to the measurands without the need of any surface functionalization or the use of any adsorbing/absorbing thin film. Such a scheme may allow the determination of the concentration of given analytes (such as hazardous substances or proteins) in gaseous or liquid solutions as compared to calibration curves obtained right after the fabrication process.

Claims

1. An optical interferometric device for label-free physical, chemical and biological sensing consisting of (a) a broad-band light source emitting in the UV, visible or IR spectrum,

(b) a planar waveguide patterned into a Mach-Zehnder Interferometer,

(c) top and bottom cladding layers,

(d) a sensing arm of the said Mach-Zehnder Interferometer the cladding layer atop of which is removed and the sensing area of which is either exposed to the analytes, or chemically and/or physically modified, or covered with an appropriate thin absorbing/adsorbing/physisorbing layer for sensing,

(e) a photodiode, and

(f) spacers that help the optical coupling of the said waveguide to the said light source and photodetector, all integrated on the same chip and detecting analytes through changes of the refractive index atop the sensing arm of the said Mach-Zehnder that are translated into photocurrent changes of the said photodetector

2. A device according to claim 1 that is fabricated with standard silicon technology on silicon substrates where the (a) light source and the photodiode are pn-junctions, and (b) the planar waveguide and top and bottom cladding layers are made of dielectric materials with suitable refractive index that ensure waveguiding

3. A device according to claim 1 that is made of compound semiconductors with conductive substrates where the (a) light source and the photodiode are pn-junctions, and (b) the planar waveguide and top and bottom cladding layers are made of dielectric materials with suitable refractive index that ensure waveguiding

4. A device according to claim 1 that is made of compound semiconductors with non-conductive substrates where the (a) light source and the photodiode are mesa structure pn-junctions, and (b) the planar waveguide and top and bottom cladding layers are made of dielectric materials with suitable refractive index that ensure waveguiding

5. A device according to claim 1 that is made of organic semiconducting materials where the (a) light source and the photodiode are either planar or mesa structure pn-junctions, and (b) the planar waveguide and top and bottom cladding layers are made of dielectric materials with suitable refractive index that ensure waveguiding

6. The use of any of the devices according to claims 1 to 5 for measuring refractive index changes of gaseous or liquid solution

7. The use of any of the devices according to claims 1 to 5 the sensing arm of which is modified with a recognition molecule for specific binding of analytes of biological interest from a sample, and where the recognition molecule can be (a) an antibody, (b) an olgonucleotide, or (c) a binding protein

8. The use of any of the devices according to claims 1 to 5 where the sensing arm is coated with an appropriate organic thin film the refractive index and thickness of which change due to the absorption/adsorption/physisorption of the analytes

9. An optical interferometric device for label-free physical, chemical and biological sensing consisting of (a) a broad-band light source emitting in the UV, visible or IR spectrum, (b) a planar waveguide patterned into a Mach-Zehnder Interferometer ,

(c) top and bottom cladding layers,

(e) spacers that help the optical coupling of the said waveguide to the said light source

(f) a planar waveguide butt-coupled to

(g) an external optical fiber connected to (h) a spectrophotometer

10. A device according to claim 9 and fabricated according to any of the claims 2 to 5

11. The use of any of the devices according to claims 9 and 10 with any of the detection principles according to claims 6 to 8

12. An array of any of the devices according to claims 1 to 8 sharing the same light source and having different photodiodes, where each of the said devices of the array is modified for sensing different analytes, and where the output signal is acquired through a suitable multiplexing electronics circuit

13. An array of any of the devices according to claims 1 to 10 sharing the same photodetector and having different light sources, where each of the said devices of the array is modified for sensing different analytes, and where the output signal is acquired through a suitable multiplexing electronics circuit