WO2014048671A1

WO2014048671A1 - A system and a method comprising an array of bending elements for determining a condition

Info

Publication number: WO2014048671A1
Application number: PCT/EP2013/067922
Authority: WO
Inventors: Klaus Bo MOGENSEN; Jörg Peter KUTTER
Original assignee: Danmarks Tekniske Universitet
Priority date: 2012-09-28
Filing date: 2013-08-29
Publication date: 2014-04-03

Abstract

A system comprising a sensor element and a sensing system, a method of operating it, a sensor element and a method of providing it, where the sensor element has a substrate from which a plurality of elongate, bendable elements extend. The elongated elements are configured to bend, in the same direction, when exposed to a condition, which may be a temperature, a pressure, a pH, a humidity or a presence of a predetermined molecule. The elongated elements may have a first surface and a second surface having different degrees of contraction/extension when exposed to the condition, where the first surfaces all point in the same direction. The sensing system may relate on a large number of elongate elements positioned within a given area on the sensor element.

Description

A SYSTEM AND A METHOD COMPRISING AN ARRAY OF BENDING ELEMENTS FOR DETERMINING A CONDITION

The present invention relates to a system and a method for determining a condition, such as the presence of a molecule or type of molecule, a temperature, or the like.

In particular, the invention relates to the providing of a plurality of bendable elements with a surface which will extend/contract upon presence of or exposure to the condition. In that situation, the bendable elements, when this surface is present on only one side thereof, will bend, and the bending of a plurality of elements is easier to detect.

In general, the bending of a bendable element upon presence of a molecule may be seen in e.g. US7612424, where a single lever will be bending when contacted with the molecule. Thus, the sensing system must be extremely precisely positioned and focused in order to be able to sense the single, bending lever without the signal disappearing in noise from the non- bending surroundings.

The present invention relates to a different set-up using a large number of bendable elements so that a much simpler sensing system and a much more relaxed positioning system may be used, as a plurality of bendable elements are used.

A system of this type which senses on the basis of direction changes of multiple elongate elements positioned in a periodical pattern may be seen in "Photonic Crystal Based All-Optical Pressure Sensor", Lu et al., MEMS 2011, Cancun, Mexico, January 23-27, 2011, pp. 621-624 and "Femtomolar Sensitivity DNA Photonic Crystal Nanowire Array Ultrasonic Mass Sensor", Lu et al., MEMS 2012, Paris, France, 29 January-2 February, 2012, pp. 88-91. However, in neither of these is the effect used of having the elongate elements bend due to the presence of a condition.

In a first aspect, the invention relates to a system according to claim 1.

In the present context, a system may be an apparatus where the sensor element and the sensing system are attached to each other, or the apparatus may have a holder, support or the like for holding the sensor element during operation of the sensing system.

The system may be divided up into separate parts, which do not engage, such as if the sensing system is based on a non-contact sensing method, such as an optical sensing method. In addition, the system may comprise a processor, computer, DSP, ASIC, FPGA or the like for quantifying the bending and/or the condition. This processor or the like may be positioned close to a remainder of the system or remotely therefrom, such as accessible via a network, such as the internet, if desired.

The condition may be one of a wide range of conditions or parameters, to which the surfaces are exposed, such as the presence of a particular molecule, a type of molecule or the like, a temperature, a degree of humidity, or the like.

Naturally, the condition/parameter may be the mere presence of the molecule, humidity or e.g. a temperature above a predetermined threshold, but preferably, the condition/parameter may be quantified, so that a concentration of the molecule (such as in a fluid provided to or on the bendable elements), an actual temperature or degree of humidity may be determined. In that situation, the degree or angle of bending of the bendable elements preferably depends on this concentration/temperature/degree of humidity, so that the

concentration/temperature/degree of humidity may be determined from the degree or angle of bending.

It is noted that the degree of bending may be determined in a number of manners, such as from a distance, in the plane of the substrate, between the positions of the two ends of a bendable element when projected on to the plane. In another manner, the bending may be determined from a curvature of the bent element or the curvature of a bent portion of the bent element, disregarding a straight or at least substantially straight portion, if such a portion exists. Yet another manner may be the determination or quantification of the bending from an angle between the outer, distal portion of the bent element, such as a straight line defined by the outer-most part of the bent element, and the plane.

Preferably, the substrate is at least substantially stiff so as to not itself bend when exposed to the condition. In this manner, the determined bending of the bending elements has no contribution from any bending of the substrate. Naturally, the substrate preferably is a monolithic element, but it may be divided into any number of elements each having one or more of the elongate elements. Especially when taking into account the below-mentioned preferred number of and dimensions of the bendable elements, a monolithic substrate is preferred.

Preferably, the substrate is made of a material which is not influenced or altered by the condition. Also, the elongate elements may be made of a such material, such as if subsequently added a material or layer which is sensitive to the condition. The sensor element may be monolithic, such as manufactured in a number of steps generating both the substrate and the elongate elements, or the sensor element may be manufactured by providing the substrate and the elongate elements in different steps followed by a step of fixing the elongate elements to the substrate. As mentioned above, the sensor element may also be manufactured from a number of elements so that the substrate may be assembled from a number of substrate elements either each having one or more elongate elements, or the elongate elements being fixed thereto subsequently.

The plane of the substrate surface preferably is that seen when the elongate elements are not present. Usually, the substrate has a plane, upper surface, from which the elongate elements extend. Naturally, other shapes may be used, such as bent or twisted planes, if desired. In one example, the plane may have parts which are plane but which are at an angle to each other, where the elongate elements of the different parts have different properties, such as bending when exposed to different conditions, which may be different quantities or the like of a molecule, different molecules, different temperatures or the like. It is especially preferred that the elongate elements are at least substantially parallel prior to the exposure to the condition, where the elements preferably are at least substantially straight, so that a bending thereof may more easily be determined, quantified or, for example, summed, or a mean value may be determined, taking into account e.g. any variation in the exposure of the individual elongate element to the condition. Also, it is preferred that the elongate elements extend from the surface at least substantially perpendicularly to the surface at least at the position where the individual elongate element is attached to or connected to the substrate.

In this context, an elongate element is an element which has a length (between the two ends, one of which is attached to the substrate) is longer than a diameter across the length. Preferably, the length is at least 5 times, such as at least 10 times, preferably at least 20 times the largest diameter of the elongate element.

An elongate element may have a length of lOOnm-ΙΟΟμιη, depending on e.g. the detection method and the thickness thereof. Thicknesses may be e.g. lnm-ΐμιη and may be adapted to the surfaces, as the elongate element should be able to bend. A spacing, a mean spacing or minimum spacing between neighbouring elongate elements may be from a few nm to ΙΟΟμιη, where a spacing may be a spacing between neighbouring elongate element in a row or column of a regular array. If the pattern is not regular, a mean spacing or minimum spacing may be defined as a spacing from an elongate element and the closest neighbouring elongate element.

In particular, when the detection method, see below, is optical, such as e.g. in a photonic crystal, where the light beam probes a multitude of elements simultaneously. The first plurality of elongate elements may comprise 100 or more elements, such as 500 or more elements, preferably 1000 or more elements.

As mentioned above, the elongate elements may be monolithic with the substrate or may be fixed or attached thereto subsequent to manufacture thereof. This attachment may be a gluing, soldering, of the like. In a further alternative, the elongate elements may be grown on the substrate. The elongate elements extend away from the substrate.

The elongate elements are configured to bend when exposed to the condition. This exposure may be the exposing to a condition such as providing of a type of material, such as a particular molecule, a type of molecule, a material having predetermined properties, such as a temperature, a pH, or the like. The material "carrying" the condition/parameter may not be relevant. If the condition is a temperature, it may not matter whether the temperature is provided via a gas, a liquid or via radiation. If the condition is a humidity, the material may be a water containing liquid or fluid, but this material may comprise other materials without altering the read out of the system.

In many instances, the condition is the presence of a particular molecule or a type of molecule such as for use in a biochemical sensor. In this situation, the condition may be the presence of particular DNA fragments or an antibody / antigen, such as enzyme, protein, aptamer, hapten, lectin or a selective polymer layer, such as a self assembled layer, molecular imprinted polymer layer, block copolymer, porous polymer layer or the like.

Alternatively, the condition may be the presence of a combination of these where e.g. the polymer layer is functionalized with the antibody / antigen.

The dimensioning and choice of surfaces is well known to the skilled person, as is seen from e.g. the above-mentioned US reference. The type of elongate element and the types of materials forming the surfaces thereof need not vary from these prior art references.

All elongate elements are configured to bend in the same, predetermined direction. In this situation, the sensor element may be positioned so that all elongate elements bend or will bend to the right to a spectator or North, for example. This has the advantage that one elongate element will bend in the direction of another which will, consequently, be bending away from the first elongate element. Then, the risk of elongate elements bending toward each other and interacting with each other is reduced drastically. This bending in the same direction has even larger advantages in relation to the detection of the bending. This will be described further below. The system has a sensing system configured to determine the condition from a bending of a second plurality, comprised in the first plurality of the elements, of the elongate elements being within a predetermined area of the substrate when projected on to the plane.

Preferably, the second plurality of the elongate elements comprise at least 10 elongate elements, such as at least 50 elongate elements, preferably at least 100 elongate elements. A first advantage is seen in that the area of detection may be made larger than that seen in the prior art. Another advantage is that the area in which the second plurality of elongate elements are positioned, may be made smaller than a second area in which all of the first plurality of elongate elements are positioned, so that the actual positioning of the second plurality and the area may not need to be specific, as the bending may be seen over all of the second area, so that the area need merely be chosen within the second area.

Thus, the first plurality of elongate elements may be positioned within a second area, when projected on to the plane. The second area may be the smallest area, such as a square, triangle, oval, star or circle, for example, within which the first plurality is positioned. Then, the area of the area may be no more than 90%, such as no more than 50%, preferably no more than 40%, such a no more than 25% or no more than 10% of the second area. In this manner, when the position of the area is determined by sensing system and the second are by the sensor element, relative positioning there between puts less demands on e.g. a holding element or support of the sensing system supporting the sensor element during sensing.

In addition to this, the number of elongate elements in the second plurality may still be selected so that a sufficient number of elongate elements are used to obtain a desired determination quality.

The elements each has a first and a second, opposite sides, the first sides pointing at least substantially in the same direction, and each of the first sides having a surface configured to extend, to a first degree, upon existence of the condition, and the second side having a surface extending to a second degree, being different from the first degree upon existence of the condition. In this respect, a surface may just as well contract upon experiencing the condition. A contraction may be seen as a negative expanding.

Also, in this respect, the direction may define an axis parallel thereto which, in a plane perpendicular to a longitudinal axis of an elongate element, and when going through a centre of the elongate element in the plane, at the surface of the elongate element intersects the first side of the elongate element and the second side of the elongate element. Preferably, an axis of this type may be defined for each elongate elements, and these axes are preferably parallel.

Preferably, the surface of the first side extends symmetrically from the point of intersection, in the plane, of the axis and the first side, to either side of the point of intersection. Then, the surface of the second side may also extend symmetrically along the circumference of the elongate element, in the plane, from the point of intersection of the axis and the second side.

The first and second degrees may be quantified so that the condition will make one of the first and second surfaces extend more than the other. When these surfaces are positioned on opposite sides of each elongate element, the extending of one surface in relation to the opposite surface will make the elongate element bend either toward the direction of the first surface or the direction of the second surface.

Naturally, one of the first and second degrees may be no or at least substantially no extending of the surface. In that situation, the bending will be caused by the surface having the higher degree of extending.

Of course, one surface may be expanding when exposed to the condition and the other may be contracting. This will also cause the desired bending.

Preferably, the two different surfaces are generated by providing a cover or layer on at least one of the surfaces. This cover or layer may be either of the first and second surfaces. The cover or layer may be more or less expanding, than the other surface, when subject to the condition.

Naturally, both the first and second surfaces may be provided by separate covers or layers if desired.

A layer or cover may be provided by deposition, sputtering, evaporation, chemisorption and/or physisorption. As an alternative to providing a cover or layer, a surface may be altered, such as by sputtering, ion implantation or the like.

The expansion may be along the longitudinal direction alone of the elongate elements but will typically be in all directions. However, the expansion along the longitudinal direction will cause the bending. Expansion in other directions will not cause a bending but e.g. a slight thickening of the elongate element, which may be insignificant.

The first and second surfaces are provided on opposite sides of the elongate elements. Then, in a cross section of the elongate element in the plane and/or across a longitudinal axis of the elongate element, the first surface preferably covers no more than 75% of the circumference of the elongate element.

It is noted that if the same expansion is seen on both sides of the elongate element, which would be the situation at the sides perpendicular (in the plane perpendicular to the longitudinal axis) to the first and second sides, this will not cause a bending to one of these perpendicular sides. This may be seen by pointing the first side North and allowing the first surface to extend from North to West and toward South as well as from North to East and toward South so that only e.g. 25% of the circumference around the South direction is not covered. The covering from the West will counter-act the covering from the East so that the bending will still be toward the North or the South.

Preferably, the first surface covers no more than 60%, such as no more than 50% of the circumference.

Naturally, the largest bending may be seen if all of the length of the elongate elements has the first and second surfaces. However, this may be difficult to achieve, and it may not be required. In many embodiments, at least 25% of the length of the elongate elements have the first and second surfaces, such as at least 50% of the length thereof. This may depend on the manner of manufacturing the elongate elements and/or the surface(s).

In one embodiment, the condition is the presence of a predetermined molecule. In this situation, one or both of the first and second surfaces may be so-called bio recognition layer(s) which are layers or surfaces extending/contracting upon contact with the molecule. A large number of different layers and surfaces are known which have this property. Typical bio recognition layers comprise one or more substances, such as antibodies, which engage with the molecules and cause the layer/surface to expand/extend. Contracting layers or surfaces will function equally well. Bio recognition layers may be antibody or antigens, such as enzymes, proteins, aptamers, haptens, lectins or a selective polymer layer, such as a self assembled layer, molecular imprinted polymer layer, block copolymer layer, porous polymer layer or the like.

Other types of conditions may be temperature, pH, humidity, or the like. Materials exist which expand/contract more than others at a given temperature or temperature change, pH or pH change, humidity or humidity change, so that the above bending may be obtained and the condition determined or quantified in the same manner. If the condition is humidity, one surface may be hydrophilic and the other hydrophobic.

Typical hydrophilic materials are glass/oxides, such as silicon dioxide, aluminium oxide, or nitrides, such as silicon nitride. Typical hydrophobic materials are carbon, such as carbon nanotubes, and polymers.

In relation to temperature, different materials have different expansion coefficients, so it would be simple to select two materials for the first and second surfaces to obtain a desired bending at e.g. a predetermined temperature change.

In relation to pH, pH sensitive layers that change stress upon exchange of protons (H+ ions), e.g. altering their charge during protonation/deprotonation, which can result in a change from a neutral to a charged state or reverse. In one situation, the elongate elements are positioned, in the plane, in a periodic manner, so that the two-dimensional pattern is a periodic pattern. In another situation, the positioning is less relevant and may be a stochastic positioning or pattern. In this manner, the

predetermined pattern is the determination that the pattern is not to be periodic.

In the latter situation the sensing system may determine a change in polarization of radiation transmitted to or through the second plurality before and after exposure to the condition. Alternatively, the elongate elements may be used as electrical field emitters, the angle or bending of which may be determined when emitting electrons toward a field emission display.

In the situation where the bendable elements are positioned in a periodic pattern on the surface, this pattern may be a number of columns and rows (a so-called matrix pattern), or a pattern of a number of rows of equidistant elongate elements but where the elongate elements of one row are offset compared to the neighbouring rows. This periodicity makes a large class of interesting sensing set-ups possible, in addition to the above-mentioned methods, such as when the sensing system comprises elements for providing radiation to at least the second plurality of the elongate elements and determining the condition from a diffraction or an interferometric determination. The elongate elements can be arranged uniformly as in a photonic crystal array. Here, the distance between the individual elements is in the same range as the wavelength of light/radiation used for detection, such as from λ=200 nm to 2000 nm. This means that the light does not probe the individual elements, but measures the consorted action of a number of elements. The property of light/radiation detected, can e.g. be a change in polarization, phase or amplitude. In another situation, the sensing system comprises an electrode, means for applying an electric field between the electrode and at least one of the elongate elements, as well as a sensor for determining an impact position there-on of charged particles, such as electrons, emitted by the at least one elongate element.

In this context, an electrode may be any type of conducting element configured to receive a voltage. The electrode may form a part of the sensor or may be configured to be positioned beside, behind or in front of the sensor to provide the field in a direction from the substrate to the sensor.

When the elongate element bends, the position of impact of charged particles emitted thereby will differ, so that the degree of bending may be determined. Usually, this type sensing is performed under vacuum or in a gas phase in order to not interfere too much with the travelling charged particles.

The sensor may be any type of element configured to sense a position of impact of a charged particle. The sensor may be a sensor array of individual sensors, such as a CCD.

The sensor may be a field emission display (FED), such as a phosphorescent screen imaged by a photo detector array.

Naturally, the position or position change for a single or for each individual elongated element may be determined, or an average may be used in order to lower background noise and fluctuations, as an average is employed.

Another aspect of the invention relates to a method according to claim 7. As mentioned above, the substrate may be manufactured separately from the elongate elements or at the same time, and the sensor element may be assembled from a number of elements together forming the substrate and elongate elements. The exposure of the elongate elements to the condition may be performed as in the prior art, where the sensing element is exposed to a fluid, a solid, or the like, which is allowed to contact the first and second surfaces. The fluid may comprise a substance, such as a particular type of molecule, the presence or quantity of which forms the actual condition, or the fluid may itself have a condition, such as a temperature, a pH value, a water content, which forms the condition.

It may be desired that the exposure step is followed by a purging step where irrelevant or surplus material is removed. This may be so as to facilitate the sensing step and/or allow the elongate elements to bend without having to struggle with surplus material and the like.

This purging step can comprise cleansing with a solvent or buffer solution, or evaporation of the solvent before performing the subsequent measurement.

The determination of the condition may be from the mere fact that the elongate elements bend, or the bending may be quantified. This quantity may be compared to a threshold value to determine whether the condition is present or not, or the quantity or value may be used to quantify the condition, such as a temperature, temperature shift, quantity, concentration or volume of a material or the like.

As mentioned above, the determination is made on the basis of the bending of a second plurality of the elements, the second plurality being comprised in the first plurality, and the second plurality being positioned within the area.

As mentioned above, the area may form only a part of the overall area covered by the first plurality of elongate element, so that e.g. the positioning of the sensing element in relation to the sensor system is less critical, and the area may be selected to obtain a suitable determination quality.

As mentioned above, one of the surfaces or both surfaces may be provided as a layer or coating. Additionally, a surface may contract, which in this context is merely seen as a negative extending and will function in exactly the same manner and cause the same bending. In one embodiment, the exposing step comprises exposing the elongate, bendable elements to a predetermined molecule. In this situation, one or both sides may be a so-called bio recognition layer extending upon contact with e.g. a predetermined molecule or type of molecule. This molecule may be provided, in the exposure step, carried by a fluid, for example, provided to the sensor element.

In one situation, the elongate elements are not provided in any particular pattern or the like over the surface of the substrate. In that situation, the determining step may comprise launching radiation to or through the sensor element before and after the exposing step and determining the condition from at least a change in polarization of the radiation before and after the exposing step. Alternatively, the determining step may comprise emitting electrons from the second plurality toward a field emission screen (such as an old fashioned TV screen with fluorescent/phosphorescent material) or an electron detector, such as a scintillator or an electron multiplier and determining the bending from a change in position of interaction of the electrons before and after bending. In another situation, the bendable elements are positioned in a periodic pattern on the surface. This pattern may be any pattern, such as a matrix, where the elongate elements are positioned equidistantly in rows and columns. Another pattern is one wherein the elongate elements are positioned equidistantly in rows which, however, are offset in relation to each other. In such situations, the determining step preferably comprises determining the condition based on a diffraction or an interferometric determination.

In one embodiment, the sensing step comprises emitting at least one charged particle from an elongated element, guiding the charged particle toward a sensor and determining a position of impact of the charged particle on the sensor.

As described above, an average of positions of particles emitted from a plurality of the elongate elements may be used.

Often, a position may be determined both before and after bending so as to be able to determine a difference caused by the bending.

A third aspect of the invention relates to a sensor element according to claim 12.

As mentioned above, the substrate and elongate elements may be provided in a number of manners. In one embodiment, the substrate and elongate elements is monolithic. In other embodiments, the elongate elements and the substrate are generated in different steps and in different materials. In many embodiments, the elongate elements further have a coating or layer on at least one side.

The different aspects and embodiments of the sensor elements, such as the layers/coatings, dimensions, densities etc. are equally applicable in this aspect of the invention.

In one embodiment, the elements each has a first and a second, opposite sides, the first sides pointing at least substantially in the same direction, and each of the first sides having a surface configured to extend, to a first degree, upon existence of the condition, and the second side having a surface extending to a second degree, being different from the first degree upon existence of the condition.

In one embodiment, as is described further above, the condition is the presence of a predetermined molecule. This presence may be the presence of a molecule of a

predetermined type or family of molecules and/or a quantification of the amount of, concentration of or number of such molecule(s).

As mentioned above, a larger amount of sensing techniques is available, when the bendable elements positioned in a periodic pattern on the surface. Also, the number of the first plurality of elongate elements may be desired to be at least 100 elongate elements. Even more may be desired, as may the dimensions, stiffness etc. thereof, depending on the actual use. It is noted that the skilled person will know how to dimension an elongate element in order for it to be bendable by the operation of e.g. one or two surfaces or layers sensitive to e.g. the presence of a molecule. A final aspect of the invention relates to a method according to claim 16.

As mentioned above, the basic element may be provided as a single element or from multiple elements together with or separately from the elongate elements. Also, the elongate elements may be attached to the substrate subsequent to the manufacture thereof, or they may be made in a monolithic element. In an interesting embodiment, the elongate elements are grown from the substrate by e.g. catalysis or through a template with holes that is subsequently removed/dissolved/etched.

At least one of the surfaces may be a surface of a basic material of the elongate elements, whereas the other of the surfaces may be provided subsequent to the manufacture of the elongate shape or structure of the elongate elements. This may be in the form of a coating or layer and may be provided in one of a plurality of manners, such as by glancing angle deposition.

In the following, preferred embodiments of the invention will be described with reference to the drawing, wherein: Figure 1 illustrates a sensor element, seen from the side,

Figure 2 illustrates a manner of manufacturing the sensor element of figure 1,

Figure 3 illustrates bending of an elongated nanotube with selective sidewall functionalization comprising bio recognition molecules,

Figure 4 illustrates an example of a sensor element and a sensing system, Figure 5 illustrates nanostructure dimensions, and

Figure 6 illustrates an area inside which the determination could be made.

The preferred embodiment relates to a nano-mechanical biochemical sensor 1 based on an array 3 of vertically aligned nanostructures 20 or elongated, bendable elements 20. An array 3 of vertically aligned nanostructures 20 is provided on a planar substrate 5, which may be positioned inside or form a part of a microfluidic channel network indicated by 100. The array 3 of nanostructures 20 can be organized uniformly, or the nanostructures may be positioned randomly on the substrate 5 and/or in relation to each other.

The sensing principle relies on measurement of the bending of the nanostructures 20, such as when target analytes are adsorbed selectively onto one side of the nanostructure surface. Selectivity, in terms of adsorption, is achieved by e.g. providing a recognition layer on one side of the vertical nanostructures.

This type of sensor is an improvement over traditional mechanical sensors, such as cantilever sensors, because alignment to the individual nanostructure is not necessary, as was hitherto required, as only a single cantilever was used, only alignment to the whole array is needed, thereby relaxing the alignment tolerances more than e.g. a factor of 100, which makes the system much more robust and simple. The manufacture also is significantly simplified, as etching and release of cantilevers is not necessary, when the nanostructures 20 can be fabricated perpendicular to the substrate 5 instead of in the plane of the substrate. The sensor can be used both in the gas phase and in the liquid phase.

In figure la, the bending of the nanostructures 20 is seen over the full length thereof. In figure lb, the bending is only seen in the upper part thereof. Example of an implementation of the invention

In fig. 2, fabrication of a sensor element is seen. An array of Ni nanodots is provided by means of holographic photolithography and metal lift off (step a + b). Carbon nanotubes are grown (step c) and gold is deposited on the sidewall of the carbon nanotube array by means of glancing angle deposition (step d). The gold layer 30 is subsequently selectively functionalized with biorecognition molecules 10 on one sidewall of the carbon nanotubes 20 by traditional biochemical methods (Fig. 3.).

The sensing principle is depicted in Fig 3a. Binding between the recognition molecules 10 and the target analytes results in a change in the surface stress of one side of the nanotubes 20, which leads to bending of the individual tubes 20. In figure 3b, a cross section of a nanotube 20 perpendicular to a longitudinal axis thereof is seen in which a centre, c, is illustrated as well as an axis, a, intersecting the centre, c, and circumference of the nanotube 20 at two positions, pi and p2. A layer or deposition, such as the gold layer and/or the bio recognition molecules 30, is illustrated. The layer 10/30 extends symmetrically around the circumference on either side of the point pi. Thus, the absence of the layer 10/30 also extends symmetrically around the circumference on either side of the point p2.

This symmetry will make the nanotube 20 bend in a plane defined by the axis and the longitudinal axis. When all nanotubes have the same structure or symmetry axis, they will bend in the same direction - the direction of the axis. Naturally, the bending direction will be given by not only the extent of the layer 10/30 but also, in some situations, the thickness thereof or a concentration of e.g. bio recognition molecules on the surface thereof. The symmetry thus may in some situations be corrected based on a concentration value on the surface.

Detection of the collective bending of the nanotubes 20 is shown in Fig. 4. In figure 4a, the readout of the array is spectrophotometric, where radiation is launched (b) toward the array 3 and a change in the backscattered (a) or forward scattered (c) light, such as peak position, peak height or peak width, is related to the bending of the nanotubes 20 and hence the concentration of the target analytes.

In figure 4b, a detection method similar to a field emission display device is illustrated wherein the nanotube functions as a electron field emitter, where the emission is read out on a field emission display. When the nanotubes bend, the position of the electrons impinging on the display change, since they are emitted at an angle. This change is related to the concentration of the target analyte and therefore used for detection.

In the following, alternatives to the above preferred embodiment are given.

Nanostructures and their manufacture Depending on the type of readout, the vertically aligned nanostructures 20 can either be arranged in a uniform array, such as a photonic crystal array, or organized randomly on the surface of the substrate.

The nanostructures 20 can be fabricated by bottom up fabrication methods, such as catalytic growth of carbon nanotubes, silicon nanowires, zink oxide nanowires or polymeric nanowires, etc.

The nanostructures 20 can also be manufactured by top-down methods, such as etching of silicon nanowires or nano-imprinting/moulding of the structures.

When a mask is used, either for pattern definition of catalyst particles or an etching mask, processes such as: holographic lithography, deep UV lithography, step-and-flash lithography, e-beam lithography or nanoimprint lithography can be used.

Obtaining different selectivity on the sidewalls of the nanostructures

Selective sidewall functionalization can be obtained by several methods

• Glancing angle deposition (GLAD) of metal, semiconductor or polymer layers, as depicted in fig 2d. This layer can serve as a template for further binding of recognition molecules, for example. • Photopolymerization using oblique light. Light from a glancing angle is shone onto the array 3, and the biomolecules or selective polymer layer are photopolymerized selectively on one side of the nanostructures. · Transverse electrical field through the nanostructures 20. A transverse electric field is applied across the nanostructure array. This polarizes the nanostructures, thereby making one sidewall partially positive and the other partially negative. This is used for selective binding of recognition molecules on one sidewall.

• Altering of the material of the nanostructure, such as by ion implantation.

Types of recognition elements/molecules on the sensor array

Various types of metals can be used, such as, Ti, Au, Ag, Al, Ni, Fe, etc. as well as alloys/combinations thereof. The metal layer can be functionalized by e.g. antibodies, aptamers, haptens, lectins, enzymes or other recognition elements for selective

measurements of target analytes.

The sensor can also be used as a universal sensor by e.g. making one sidewall hydrophobic and the other hydrophilic. This can e.g. be done by depositing Al on hydrophobic carbon nanotubes followed by a brief oxidation. This will oxidize Al to Al oxide, which is hydrophilic, while leaving the other parts or sides of the carbon nanotube surface hydrophobic. Such a sensor can be used in e.g. a separation system (capillary electrophoresis, liquid

chromatography, HPLC or gas chromatography) for label-free detection of all the analyte bands that pass through the detector.

Therefore the detector can be used both in situations where the analytes bind permanently to the sensors (e.g. in a biosensor) or where they bind momentarily (e.g. in a separation system).

In addition, the sensor may be used also for simply detecting the presence of humidity.

Actually, the sensor may equally well be used for sensing or detecting pressure, pH, temperature, or the like.

Detection principles A simple example of a spectrophotometric detection scheme is seen in Figure 4a, where the array is illuminated (a) from one side by a radiation emitter 50, and the spectral response is detected either as the reflected light, by a detector 70, or the back scattered light, by a detector 60. If a transparent array is used, the transmitted/refracted light can also be measured by a detector 80.

Electronics 90 are provided for controlling the emitter/detectors as well as receiving signals from the detectors and determine and/or quantify the bending and/or the

condition/parameter.

Other optical detection principles can also be utilized, such as:

• Interferometry, where the difference between two beams is employed for detection The two beams can impinge on the substrate from different angles, so one beam is constant when the nanotubes bend (reference signal), while the other changes, e.g. a change in the phase, polarization and/or amplitude (detection signal). The interference pattern/signal from these two beams is correlated with the concentration of the target analyte and can therefore be used for detection.

• A change in the diffraction pattern of e.g. a laser beam reflected from the array 3 or a change in a holographic pattern generated from the array.

• Standard polarimetry can also be used. Here, the sensor array is placed between two polarizing filters, one having a polarizing axis rotated a given angle in relation to the other's axis. A change in the transmitted light is seen, when the nanostructures 20 bend, due to interaction with the polarized light. In this case, the array 3 does not need to be uniform, but the set-up also works in a random configuration. This is equivalent to measuring the polarization of e.g. liquid crystals or other optically active substances.

• Electrical readout (fig. 4b) can furthermore also be used, where the nanotubes are used as electrical field emitters. Here, bending of the nanotubes will result in a change in the direction of the emitted electrons from the nanotubes, which can be readout on a field emission display.

Both a single channel (as in Figure 4a) and dual channel detection can be used. By using two light sources or splitting the light into two beams, the array can be illuminated at e.g. a 90 degree angle at the same position or at two different positions. The light can furthermore be polarized using polarization optics. This configuration can be arranged so one beam is perpendicular to the bending direction (sensing beam), while the other is in the same direction as the bending (reference beam). The two beams can afterwards be combined, by using e.g. a interferometer in order to employ the difference of the two beams for detection. In relation to figure 4b, carbon nanotubes have long been known to be excellent electron emitters, due to their high conductivity and very sharp tip, which means that the bending may generally be detected as a change in position of impact of electrons emitted toward a sensing surface. Due to he high conductivity and sharp tip, a relatively low applied voltage is needed for efficient electron emission.

For the field emission detection, a high electric field, such as ranging from 100 V to 10 kV is applied between the carbon nanotube substrate 5 and an electrode 100 located above the substrate. In such a device, detection is preferably carried out in vacuum or in the gas phase, where the emitted electron impinges e.g. on a field emission display (FED) 110, such as a phosphorescent screen that is imaged by a photo-detector array.

For detection, the emitted electrons from each element can be measured. Either electrons from an individual element can be used for detection or the average of a number of elements can be used. The latter approach will lower the background noise and fluctuations of the system because the statistical average is employed.

In the present embodiment, the electrode 100 has an opening wherein the sensor 110 is positioned. Alternatively, the electrode 100 may be positioned behind the sensor 110 or be embedded therein.

Typically, position determinations are made both before and after bending, so that a difference is easily determined.

In general (see figure 6), the elongate elements are positioned over an area which may be quadratic, for example, where a part of the elongate elements are used for the determination of the bending and thus of the condition to be detected or quantified (the oval).

Any number of elongate elements may be provided within the quadratic area, such as at least 10, preferably at least 100, such as at least 1000, preferably at least 10,000, and any number of elongate elements may be used in the sensing or determination, such as at least 5, preferably at least 50, such as at least 500, preferably at least 1000.

The area of the substrate (when projected on to a plane of the substrate at a side from which the elongate elements extend) may have any size, and the proportion thereof covered by the elongate elements (the area of the oval) used in the sensing may be as desired, such as 5- 90%, preferably 10-80%, such as 25-75%. Detection schemes for uniformly spaced structures

For uniform structures 20 organized in an array 3, all the detection principles described above can be used, such as:

- Principles based on interferometry, - measurement of diffraction patterns (such as holography),

- Measurement of the polarization, using e.g. polarization optics as in commercial

polarimeters,

- Electrical readout in a field emission display device

Detection principles for stochastic structures: When stochastic positioned structures are used, the following measurement principles can be used :

- Measurement of a change in polarization of the probe light when the elongate elements are bending. Here, the detection can also be through the structure in the plane of the device. In this configuration, it is important that the light is not absorbed to a too high degree, which puts a limit on how deep the structure can be, depending on the wavelength of the light and on the absorptivity of the structure. Polarization can also be measured when the light impinges on the structure from the top. In this situation, there is not a limitation on size of the structure given by absorption of the probe light.

- Electrical readout with a field emission display. Each structure functions as an electron emitter where the emitted electrons are detected on a field emission display. In this case carbon nanotubes are very suitable, because it is well established that they are excellent electron emitters.

Implementation of the biosensor

• The use of the sensor influences the type of choice of the sensing layer. It can both be used for gases, liquids and as a physiochemical sensor (e.g temperature, pressure, humidity and pH) As a gas sensor it can be used for e.g. detection of

- explosives in the air for e.g. security control or demining purposes.

- pesticides

- poisonous/toxic/noxious gases in the environment, such as combustion products, carbon monoxide

- gases evaporated from solvents, e.g. petrol.

As a liquid sensor, it can be used in similar applications as traditional immunoassays, e.g. as a non-competitive assay, where an antigen in the unknown sample binds with the antibodies immobilized on the sensor surface, which results in a change in film stress and hence a signal.

The sensor can be used as a stand-alone sensor or integrated in a fluidic system 100, such as a flow injection system (FIA) or separation system, based on gas

chromatography, liquid chromatography (HPLC), capillary electrophoresis or capillary electro-chromatography. These systems can either be micro-fabricated or based on capillaries/tubes/cuvettes.

A single sensor can be used or an array of sensors can be used for detection of multiple analytes.

The sensor can, as an alternative to the molecule detection, be used as a

physiochemical sensors for measurement of:

- Temperature. Here, the sidewall is coated with a temperature sensitive layer that e.g. changes surface stress with a change in temperature.

- Pressure. Here, a pressure sensitive layer is coated on one sidewall. This could be a layer that compressed upon an increase and pressure, resulting in a change of surface stress.

- Humidity. Here, a layer sensitive to humidity is fabricated on the sidewall. This could e.g. be a layer capable of absorbing water, resulting in a change in the film stress.

- pH. Here, a layer sensitive to the pH of the environment is fabricated on the sidewall. This could be a layer that is sensitive to protonation/deprotonation due to acid-base reactions with the environment/solvent, resulting in a change in the film stress. In relation to the obtaining of the bending, it is naturally not required that the full length of the elongate elements 20 bend (fig. 1). A bending at any position along the length thereof will bring about a bending of the top part, which is the primary part detected in most of the above detection methods. The overall bending naturally will depend on the difference in extension/contraction of the layers/surfaces, and the larger a portion along the length of the elongate element is provided with the bending surfaces, the larger the overall bending.

It is preferred that at least 10%, such as at least 20%, preferably at least 30%, such as at least 40% of the overall length (along a longitudinal axis) of the elongate elements is provided with the bending surfaces.

In many situations, the bending surfaces are provided from the top down, i.e. at or from the other end of the elongate elements than that engaging or attached to the substrate.

In figure 5, the dimensions of the elongate elements 20 are illustrated where: w, the width of the elongate elements across the longitudinal axis, preferably ranges from 1 nm (diameter of single wall carbon nanotube) to 1 μιη, h, the length or height along the longitudinal axis, preferably ranges from 100 nm to 100 μιη, d, the spacing/distance between neighbouring elongate elements, preferably ranges from 10 nm to 100 μιη.

For the optical detection systems based on interference, the spacing (d) preferably is comparable to the wavelength of light/radiation used in the detection in order for interference effects to occur, while for embodiments in which the elongate elements are positioned stochastically, the spacing can be e.g. within the whole above range depending on the size of the elongate elements.

The thickness of a layer provided on an elongate element in order to facilite bending primarily must be sufficient to actually bring about the bending in the presence of the situation or condition to be sensed. The strength caused by the contraction/extension may depend on the thickness of the layer and should be sufficient to bring about a sufficient bending of the basic elongate structure on which the layer is provided. This layer thickness may, e.g. be in the range of 1 nm to 1 μιη depending on the width/thickness/stiffness of the supporting nanostructure.

Claims

1. A system for determining a condition, the system comprising: a sensor element comprising :

o a substrate having a surface defining a predetermined plane, and o a first plurality of elongate, bendable elements attached at one end to the substrate in a predetermined two-dimensional pattern over the surface and each element has a first and a second, opposite sides, the first sides pointing at least substantially in the same direction, and each of the first sides having a surface configured to extend, to a first degree, upon existence of a condition, and the second side having a surface extending to a second degree, being different from the first degree upon existence of the condition, and

a sensing system configured to determine the condition from a bending of a second plurality, comprised in the first plurality of the elements, of the elongate elements being within a predetermined area of the substrate when projected on to the plane.

2. A system according to claim 1, wherein the condition is the presence of a predetermined molecule.

3. A system according to any of claims 1 or 2, wherein the sensing system is configured to determine a change in polarization of radiation transmitted to or through the second plurality before and after exposure to the condition.

4. A system according to any of the preceding claims, wherein the bendable elements are positioned in a periodic pattern on the surface.

5. A system according to claim 4, wherein the sensing system comprises elements configured to provide radiation to at least the second plurality of the elongate elements and to determine the condition from a diffraction or an interferometric determination.

6. A system according to any of the preceding claims, wherein the sensing system comprises an electrode, means for applying an electric field between the electrode and at least one of the elongate elements, as well as a sensor for determining an impact position there-on of charged particles emitted by the at least one elongate element.

7. A method of determining a condition, the method comprising the steps of: providing a sensor element comprising a substrate having a surface defining a predetermined plane, and a first plurality of elongate, bendable elements attached at one end to the substrate in a predetermined two-dimensional pattern over the surface and each element has a first and a second, opposite sides, the first sides pointing at least substantially in the same direction, and each of the first sides having a surface configured to extend, to a first degree, upon existence of a condition, and the second side having a surface extending to a second degree, being different from the first degree, upon existence of the condition, exposing the elongate elements to the condition so as to have the bendable elements bend in a predetermined direction, and determining the condition from a bending of a second plurality, comprised in the first plurality of the elements, of the elongate elements being within a predetermined area of the substrate when projected on to the plane.

8. A method according to claim 7, wherein the exposing step comprises exposing the elongate, bendable elements to a predetermined molecule.

9. A method according to any of claims 7 and 8, wherein the determining step comprises launching radiation to or through the sensor element before and after the exposing step and determining the condition from at least a change in polarization of the radiation before and after the exposing step.

10. A method according to any of claims 7-9, wherein the bendable elements are positioned in a periodic pattern on the surface, and wherein the determining step comprises determining the condition based on a diffraction or an interferometric determination.

11. A method according to any of claims 7-10, wherein the sensing step comprises emitting at least one charged particle from an elongated element, guiding the charged particle toward a sensor and determining a position of impact of the charged particle on the sensor.

12. A sensor element for use in the system according to any of claims 1-6, the sensor element comprising : a substrate having a surface defining a predetermined plane, and a first plurality of elongate, bendable elements attached at one end to the substrate in a predetermined two-dimensional pattern over the surface and each element has a first and a second, opposite sides, the first sides pointing at least substantially in the same direction, and each of the first sides having a surface configured to extend, to a first degree, upon existence of a condition, and the second side having a surface extending to a second degree, being different from the first degree upon existence of the condition.

13. A sensor element according to claim 12, wherein the condition is the presence of a predetermined molecule.

14. A sensor element according to any of claims 12 or 13, wherein the bendable elements positioned in a periodic pattern on the surface.

15. A sensor element according to any of claims 12-14, wherein the first plurality of elongate elements comprises at least 100 elongate elements.

16. A method of preparing a sensor element, the method comprising : providing a basic element comprising: a substrate having a surface defining a predetermined plane, and - a first plurality of elongate, bendable elements attached at one end to the substrate at the surface and in a predetermined two-dimensional pattern over the surface, wherein the providing step comprises providing each of the elements with a first and a second, opposite sides, the first sides pointing at least substantially in the same direction, and each of the first sides having a surface configured to extend, to a first degree, upon existence of the condition, and the second side having a surface extending to a second degree, being different from the first degree upon existence of the condition.

17. A method according to claim 16, wherein the step of providing the elongate elements with the first and second surfaces comprises providing a coating or layer on one of the first and second surfaces.