WO2009095710A1

WO2009095710A1 - Nanosensor based on resonance and surface plasmon effects

Info

Publication number: WO2009095710A1
Application number: PCT/GB2009/050071
Authority: WO
Inventors: Shyqyri Haxha
Original assignee: University Of Kent
Priority date: 2008-01-29
Filing date: 2009-01-26
Publication date: 2009-08-06
Also published as: GB0801606D0

Abstract

An optical biosensor inclues a photonic crystal structure defining a photonic crystal optical waveguide (18) using a plurality of air cylinders (16). The waveguide operates a s a resonance type biosensor. The waveguide may have a spiral form. At least one (5) metallic layer (30) may be deposited along the waveguide for cooperating with the waveguide as a surface plasmon sensor.

Description

Nanosensor based on resonance and surface plasmon effects

The invention relates to a nanosensor, and particulalry to a nanosensor in the form of a spiral photonic crystal.

Background

Nanosensors are emerging as very attractive devices to be employed in a great number of application fields such as microbiology, medicine, environment, automotive, particle physics and defense. In last few years, one of the fastest growing areas in biosensor research has involved developing label- free affinity-based optical biosensors. Label- free biosensors are of particular interest - they allow one to study biomolecular complexes without using a fluorescence label or a radiolabel. This flexibility not only improves the assay simplicity but also allows time-resolved kinetic measurement of biomolecular interactions. In other words such optical label-free biosensors overcome the drawbacks of commercialized microarrays, which relay on the detection of labeled molecules.

Integrated optical devices present important advantage for biomolecular sensing such as high sensitivity, selectivity, compact size, and high-scale integration. In particular high sensitive and selective integrated sensor devices are becoming enormously popular as the demand for compact optical devices to detect label-free complex analytes would have tremendous potential applications in various environments. However, the inventor is not aware of an existing biosensor that fulfills the above criteria simultaneously. For example there are some of the sensors that fulfill only few of the said criteria. These integrated photonic sensors include Mach-Zehnder interferometers [1], directional couplers [2], microring [3-5] and disk [6] resonators, and photonic crystals [7].

There are various sensor devices that are based in different properties or so called effects. In other words the sensing is based on the following properties:

Mechanical properties: Changes in a microlayer's mass after binding may be quantified by measuring the shift of the resonant frequency of a quartz microbalance on which the microlayer changes in a microlayer's stress after absorption may be quantified by measuring the deflection of a microcantilever on which the microlayer is.

Electrical properties: Changes in electrical currents flowing in microlayers or changes in electrical potentials of microlayers arise from electrochemical or bioelectronic reactions taking place at the microlayers and may be directly measured using electronic circuits without the need of additional microtransducers.

Thermal properties: Any chemical and biological reactions or processes are either exothermic or endothermic and the enthalpy changes produced at a microlayer may be measured by using a thermal microsensor.

Optical properties: Chemiluminescence at a microlayer may be detected by photodetection or binding to a microlayer may be detected by measuring the change in the effective refractive index of the waveguide on which the microlayer is placed.

When it is possible to omit sorptive microlayers, for example when there is no need of any sorptive materials for a transduction process to occur or when the concentration of an analyte is high enough to make acceptable measures, then microsensors following another paradigm which relies on the direct measurement of the surroundings' properties and/or activities and/or reaction to stimulation may be used, such as in [9].

Although such microsensors could be preferred because they would permit direct measurement results, their implementation is often impracticable or inefficient for chemical or biological applications because of current microtransducer requirements, working principles, and characteristics and read-out device sensitivity. For microsensors following these paradigms, sensitivity and discriminating ability are of great importance. However, in microscale, chemical or biological properties and off- chip spontaneous signals and reactions rarely enable differentiation of one substance from another precisely. Even when using microlayers, which serve to collect substances for enabling some transduction processes to occur and/or for concentrating the substances to increase transduction processes effects (i.e. for overcoming sensitivity problems), the capability of knowing which substance corresponds to which measurement is needed since microlayers may indifferently absorb a few to several different types of entities.

It is already known that nanostructures such as nanowires, carbon nanotubes and quantum dots behave as sensors for various biological phenomena. Nanooptic sensors are devices which consist of biologically or biophysically-derived sensing elements (bioreceptors) integrated with a physical transducer that transforms a measurand into a reading entity or an output signal.

To detect fluoresence-based DNA, in general, optical sensors are widely used because of their immunity to electromagnetic interference. In these sensors, the optical signal provided by a laser propagates in an optical fibre until reaching the fibre section on which antibodies are immobilized. These sensors are used to detect the carcinogen benzo[a]pyrene (BaP) or to distinguish BaP and benzopyrene tetrol (BPT) [8, 9].

Raman spectroscopy has been employed for biosensing; this sensor is based on the Raman effect that results from energy exchange between incident photons and scattering molecules. This sensor has been used for human immunodeficiency virus (HIV) detection [10] and in DNA fragments sequencing [H]. The integrated optical biosensors adopt a waveguide to confine light within its guiding film. Part of the guided light travels through a region outside into the medium surrounding the waveguide, and can interact with the environment.

Prieto et al. [12] have proposed a Mach-Zehnder optical biosensor capable of monitoring antigen-antibody immunoreactions with a lower detection limit [13]. This kind of guiding structure supports optical modes having a weak evanescent field and exhibits a reduced sensitivity.

The integrated optical biosensors exploiting surface plasmon (SP) resonance [13, 14] are formed from a single mode waveguide. A small region of the waveguide is coated with a thin metal film, this metal-coated part of the sensor can support a SP waves and forms the interaction region with the solution containing the analyte. Also, Bragg gratings have been proposed as building blocks for fabrication of highly sensitive and compact integrated optical biosensors [15, 16].

The micro racetrack resonator biosensor with Si₃N₄ZSiO₂ microring based integrated optical sensors has been proposed [17]. This type of sensor has been adopted for biochemical measurements and exhibits a minimum detectable concentration change of a glucose solution at around 0.024% [18]. The detection limit for glucose can be lowered by increasing the quality factor. The physics of the SP sensor surface on which the biological receptors are immobilized is an important factor affecting the detection sensitivity and selectivity. It can adversely affect the detection by contributing to nonspecific adsorption. Various types of chemistries have been researched as platforms for antibody based SP detection. These include streptavidin-biotin, carboxymethyl dextran and SAMs [19] of thiols. The SAMs of different types can be formed on substrates such as gold, silver, copper, etc. for various applications. They can be tuned to suit various biosensor applications and are suitable for immobilization of receptors. SAMs of single and mixed thiol have been widely used for pathogen detection applications, including detection of Salmonella typhimurium [20], Legionella pneumophila, Salmonella paratyphi and Yersinia enterocolita [21].

Brief Description of Invention

According to the invention there is provided a biosensor according to claim 1.

When using prior microring resonators for a sensing application the complex or analyte mixture is placed on top of the waveguide. As a result of this, the light properties will change Δn, and a frequency shift response will be registered. Such change will provide sensing information which can be further processed. However, the inventors have realised that the downside of these sensors is that when using a mixture of a very small analytes, there is a real chance that some of these analytes will not fall on the region where the light is propagating. Therefore the light will not change its properties. As a result of this prior sensors will not be able to detect some of the analyte's samples.

In order to avoid such phenomenon, we have proposed a spiral waveguide based on photonic crystal structure. The light will follow the spiral waveguide and may cover almost the whole sensing area. In other words, there is a greatly reduced chance that the analyte mixture will fall outside the waveguide region. In this way the selectivity will increase significantly.

Furthermore, it is already known that nanosensors based on surface plasmons are very sensitive. Therefore, in order to increase the sensitivity and selectivity, we may deposit gold or silver metallic layers in the waveguide region. So, the proposed sensor becomes significantly highly sensitive and selective. This biosensor is based on a silicon on insulator (SOI) photonic crystal (PC) structure that simultaneously operates on SP effects and resonance frequency.

In many sensors demonstrated so far by many research groups, in order to increase only the selectivity, they integrated an array of bulky sensors. Our proposed biosensor would be very compact with significant advantages compared to current sensors in literature.

This device will accordingly combine high selectivity, sensitivity and a single compact device in a way not previously taught.

Detailed Description Figure 1 shows a biosensor according to a first embodiment. A silicon on insulator structure has a silicon wafer 10, a silicon oxide cladding layer 12 as insulator on wafer 10, and a thin silicon layer 14 on the insulator. The oxide cladding layer has a refractive index n=1.45, and the thin silicon has a refractive index n=3.45.

A plurality of air cylinders 16 are formed in the silicon layer 14 and the insulator oxide cladding layer 12 as set out in Bogaerts et al, "Fabrication of Photonic Crystals in Silicon-on-Insulator using 248-nm Deep UV lithography, IEEE Journal of Selected Topics in Quantum Electronics, vol 8, No 4, (2002). The air cylinders define an optical photonic crystal waveguide 18. The air cylinders in dielectric can be arranged in triangular or square lattice. The device size would be drastically reduced due to high refractive index contrast in both the horizontal and vertical directions. The optical photonic crystal waveguide 18 has two input ports 20 and two output ports 22. The optical photonic crystal waveguide 18 has a spiral structure extending across the plane of the substrate wafer 10.

The SOI structure consists of a thin silicon (n = 3.45) layer on top of an oxide cladding layer (n = 1.45) carried on a bare silicon wafer. Design of gently curved elements such as S bends is not necessary because in the waveguides with photonic band gap effect very abrupt bends and junctions can be constructed. By introducing and adjusting defect points or defect lines in the waveguide sections, we will be able to achieve almost 100% light transmission through the sharp bend for several frequencies. In this way we are able to design the spiral waveguide. Also, both the silicon and the SiO₂ layer are compatible with low cost CMOS fabrication processes. It is already known that membranes or high vertical index structure such as SOI structures offer much lower propagation loss than low vertical index structures such as GaAs/ AlGaAs or InP/InGaAsP [22]. To the best of our knowledge there is no such sensor based on PC spiral waveguide operating simultaneously on SP and resonance effects.

Figure 2 illustrates an alternative structure with two input ports and one output port.

Figure 3 illustrates an alternative structure with one input port and one output port.

Figure 4 illustrates an alternative structure with one input port and four output ports.

As illustrated schematically on Figures 1 to 4, where there are a number of input frequencies and multiple output ports, different frequencies may arrive at different ports.

Figure 5 illustrates schematically the sensor in use having detector area 24 and analytes 26.

The transmission properties are changed by the analyte on the surface and the resulting measured signals are used to sense the analyte. The spiral shape of waveguide 18 means that it covers substantially detector area 24, ensuring that wherever analyte is in contact with the surface in this detector area 24, it substantially increases the chance that the analyte is over the waveguide and hence changes the optical properties.

Figure 6 shows a further development having a plurality of metallic silver or gold sections 30 with gaps 32 therebetween deposited on top of the waveguide. In this way, the biosensor operates with both resonance and surface plasmon effect hence achieving both selectivity and sensing. The resonance effect is similar to that used in microring resonators. An alternative arrangement is shown in Figure 7.

Preferably, there is a balance along the length of the waveguide between the metallic sections 30 and the gap to ensure a balance of the resonance and surface plasmon effect. In embodiments, the metallic sections cover from 10% to 90%, 20% to 80% or even 30% to 70% of the length of the waveguide in the detector area 24.

Figure 8 shows the arrangement of Figure 6 in use with analyte 26 on detector area 24.

The proposed device can operate as a MUX/DEMUX as well. The structure can be based on square or triangle lattice. It can be designed on different materials, SOI... and other semiconductor materials, depending of fabrication methods and material performances.

The biosensor as shown can be combined with optical circuitry arranged to output information regarding an analyte present over the photonic crystal optical waveguide using both surface plasmon and resonance information. The circuitry may include a laser for providing an optical signal to the input port and detectors at each of the output port together with a computer programmed to interpret the information and output a result.

In particular, it would be very promising to increase nanosensor sensitivity and selectivity (as you can see from the various options shown here). Many other nanosensor design options can be designed based on this model. The size of this nanosensor would be very small compared to our previous sensors. Bend losses and other structure geometric parameters can be easily optimized. Although the above embodiment relates to a SOI structure, this is not essential and other substrate arrangements are possible.

The size of the sections of silver or gold can be varied. However, if larger gold or silver sections are used then the refractive index change due to mechanical effect will be smaller. A suitable length of waveguide and the number and length of the metallic sections should be chosen to select a suitable balance between surface plasmon and resonance effects.

An alternative approach could cover substantially all of the waveguide with gold or silver, to provide a sensor that substantially uses only the surface plasmon effect. SUch sensors are however less preferred than the sensor described above.

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Claims

1. An optical biosensor comprising: a semiconductor layer on a dielectric layer on a substrate; a photonic crystal structure defining a photonic crystal optical waveguide in the semiconductor layer and/or the dielectric layer, the photonic crystal optical wavguide having a plurality of input and/or output ports for coupling with optical components; and at least one metallic layer deposited along the waveguide for cooperating with the waveguide as a surface plasmon sensor; wherein there are a plurality of sections of the metallic layer arranged along the length of the photonic crystal optical waveguide with a plurality of gaps therebetween.

2. A biosensor according to claim 1 having a detector area, wherein the metallic sections cover from 10% to 90% of the length of the waveguide in the detector area.

3. A biosensor according to claim 1 or 2 wherein the metallic layer is gold or silver.

4. A biosensor according to any preceding claim wherein the photonic crystal optical waveguide is defined by a plurality of air cylinders in the dielectric layer arranged in a triangular or square lattice.

5. A biosensor according to any preceding claim wherein the photonic crystal optical waveguide has a spiral structure.

6. A biosensor according to any preceding claim, further comprising circuitry arranged to output information regarding an analyte present over the photonic crystal optical waveguide using both surface plasmon and resonance information.

7. An optical biosensor, comprising: a semiconductor layer on a dielectric layer on a substrate; a photonic crystal structure defining a photonic crystal optical waveguide in the semiconductor layer and/or the dielectric layer, the photonic crystal optical waveguide having a plurality of input and/or output ports for coupling with optical components; wherein the photonic crystal optical waveguide has a spiral shape.

8. A biosensor according to claim 7, further comprising: at least one metallic layer deposited along the waveguide for cooperating with the waveguide as a surface plasmon sensor.