WO2010130775A1

WO2010130775A1 - Combination of electrodeless quartz crystal microbalance and optical measurements

Info

Publication number: WO2010130775A1
Application number: PCT/EP2010/056531
Authority: WO
Inventors: Bengt Kasemo; Cristoph Langhammer; Igor Zoric; Malin Edvardsson; Elin Larsson
Original assignee: Insplorion Ab
Priority date: 2009-05-12
Filing date: 2010-05-12
Publication date: 2010-11-18

Abstract

The invention concerns an arrangement for measuring parameters characterizing surfaces in particular mass. Specifically, the invention concerns an arrangement for measuring parameters characterizing surfaces which may be used in combination with an optical measurement technique. The arrangement comprises at least two electrodes (2a, 2b), at least one piezoelectric sensor (3) located at a distance from the electrodes (2a, 2b), wherein each electrode is arranged with at least one optically transparent region (4). In an aspect of the invention, the arrangement combines a quartz crystal microbalance with dissipation monitoring (QCM-D) with a localized surface plasmon resonance sensor (LSPR sensor).

Description

COMBINATION OF ELECTRODELESS QUARTZ CRYSTAL MICROBALANCE AND OPTICAL MEASUREMENTS

TECHNICAL FIELD The present invention concerns an arrangement for measuring parameters characterizing surfaces. Specifically, the invention concerns an arrangement for measuring parameters characterizing surfaces which may be used in combination with an optical measurement technique.

BACKGROUND OF THE INVENTION

Surfaces are omnipresent in daily life and in areas of industrially important technology. They are central in energy and environmental technologies. Surface science and surface technology constitute a vast scientific field that explores properties and processes on surfaces e.g. in materials science, catalysis, corrosion science, biomaterials and biointerfaces. The development in the field has to a large extent been driven by the development of new or better experimental probes to study surfaces.

The surface systems under study are becoming increasingly more complex. Therefore, there is an ongoing development of better analytical tools that can give a comprehensive picture of the system under study by providing measurements of extensive sets of data.

One way of gaining information about complex surface systems is to perform separate measurements with complementary techniques to provide different aspects of information. Another possibility is to perform measurements using a multi-technique setup wherein several measurement techniques are combined. The multi-technique setup allows for measurements of several parameters simultaneously with different complementary techniques under identical conditions, preferably on one and the same surface, resulting in an extensive description of the surface under study.

The multi-technique setup offers advantages such as increased time efficiency due to a need for fewer experiments and less time spent on data treatment, and improved quality of data. Further, the risk for misinterpretation of data and erroneous conclusions is reduced compared to experiments where separate measurements with complementary techniques are used. This is because a multi-technique setup will always take place under identical conditions for all employed techniques, in terms of both sample preparation and handling as well as measurement conditions, whereas separate measurements may take place under presumably identical conditions which later turn out to be non-identical. Consequently, a direct comparison of the data is possible.

Surface properties and phenomena of interest include chemical composition, electron structure, crystalline structure, optical properties but also various processes on surfaces like adsorption or desorption of matter (e.g. molecules) on a specific surface. For the latter purpose, a number of techniques have been developed that can measure changes in mass on surfaces due to addition of mass from, or loss of mass to, the environment. Examples of mass surfaces include adsorbed layers, overlayers and surface films. The mass can be either a gas or a liquid. Such mass determining techniques are e.g. based on optical, mechanical, electro-acoustic or even pure electrical principles. A specific problem arises when the studied mass, which may be added or lost, is in a wet/hydrated state because the different techniques are more or less sensitive to the liquid content, e.g. water or solvent, of the mass. In other words the relative sensitivities to the water content and the non-water content are usually different, often very different. Generally speaking, there is "dry" and "wet" mass, where the wet mass includes both the dry mass and the liquid content associated with the dry mass.

Rev. Sci. Instr., 2008, 79, 075107, discloses the measurement of dry and wet masses on surfaces using a multi-technique setup in which Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) is combined with an optical technique. The QCM-D allows for measuring of the wet mass while the optical technique provides information about the dry mass.

Measurement of dry and wet masses is also described in Anal. Chem., 2008, 80, pages 7988-7995, where a multi-technique setup combines QCM-D with localized surface plasmon resonance (LSPR) in transmission mode. The transmission measurement was enabled via an elaborate electrode arrangement on the QCM-D sensor, where a thin front electrode of about 30 nanometer (nm) thickness was made LSPR active by making nanoholes having a diameter of about 140 nm in the electrode, and a hole in the back electrode, which allowed for light transmission. Colloids Surf. B, 1996, 8, pages 39-50, and Langmuir, 2003, 19, pages 6837-6844, disclose a combination of Quartz Crystal Microbalance (QCM) measurements with the optical techniques ellipsometry and surface plasmon resonance (SPR). These combinations require expensive surface functionalization and advanced optical equipment. For example, the mentioned techniques require several of the following: mirrors, prisms, polarizers, precision goniometers which are not necessary for the present invention.

An aspect to consider when choosing what techniques to combine in the multi-technique setup is the compatibility in terms of construction; each technique's requirements regarding design of sensor area, electrode configuration, electrically conductive/insulating materials, transparency/reflectivity, access of light to the surface vs. closed chamber, etc.

In the case of QCM and its special version QCM-D, there is traditionally a requirement on having electrodes on the sensor crystal surface, which makes it complicated to combine with optical measurements in transmission mode. Transmission mode is often desirable since it is the simplest optical arrangement. Combining QCM with optical transmission measurements requires e.g. that an optically transparent region (e.g. a hole) is made in each of the electrodes. Making electrodes with an optically transparent region, however, requires the use of photolithography or other costly techniques. A second option could be to use optically transparent but electrically conducting electrode materials. Unfortunately the latter are few and put severe restrictions on other aspects of the measurements.

Analytica Chimica Acta 1991 , 243, pages 273-278, disclose so-called electrodeless versions of QCM in which one or both electrodes driving the QCM sensor crystal have been placed a short distance away from the crystal surface, so that there is a gap filled with liquid between the sensor surface and the electrodes. It was shown that the electrodeless piezoelectric quartz crystal oscillates in the same way as a piezoelectric crystal having two electrodes attached to its surface. Faraday Discussions 1997, 107, pages 91 -104, disclose so-called electrodeless versions of QCM in which both electrodes driving the QCM sensor crystal have been placed a short distance away from the crystal surface, so that there is a gap filled with air between the sensor surface and the electrodes. There is no disclosure of electrodeless QCM combined with an optical measurement technique. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved arrangement for mass measurement.

In accordance with the present invention there is provided an arrangement which is primarily characterized by at least two electrodes, at least one piezoelectric sensor located at a distance from the electrodes, wherein at least one electrode is arranged with at least one optically transparent region. The optically transparent region of the electrodes may be located at any position on the surface of the electrodes. The electrodes may be made of any electrically conducting material that is compatible with the environment in which the measurement is performed, e.g. stainless steel, platinum, brass, gold, aluminium or alloys. The piezoelectric sensor may be at any location with respect to the electrodes.

The piezoelectric sensor may be arranged with at least one optically transparent region.

The piezoelectric sensor may have a diameter or cross section of (measured in inches) e.g. 0.170"-1.00O¹¹, 0.318"-1.000", 0.340"-1.000", 0.170^0.538", or 0.318"-0.538". The fundamental mode resonance frequency of the piezoelectric sensor may be e.g. 1-30 MHz, 1-20 MHz, 4-20 MHz, 4-15 MHz, M O MHz, 4-10 MHz. The electrodes may be rectangular, square shaped, polygonal, elliptic or circular. The outer/transverse diameter of circular/elliptic electrodes or the length/vertex of the rectangle/square may be larger than, the same as or smaller than the outer diameter of the piezoelectric sensor. The electrodes may also constitute the proximate part of the measurement cell, i.e. the upper electrode may be constituted by the cell top, and the cell bottom may constitute the lower electrode. The two electrodes may have the same or different shapes. The distance between the electrodes and the piezoelectric sensor may e.g. be 0.001-5 (millimeter) mm, 1 -5 mm, 2-5 mm, 0.5 - 3 mm or 0.001-2 mm. The distance between the electrodes and the piezoelectric sensor may be the same or different.

The piezoelectric sensor is made of a piezoelectric material, which may occur naturally. Examples of piezoelectric materials include naturally occurring materials such as quartz, non-natural materials such as gallium orthophosphate, ceramics and polymers. For instance, the piezoelectric material may be chosen among the family of ceramics with perskovite or tungsten-bronze structures or among lead-free piezoceramics. Examples of piezoelectric materials include:

- berlinite (AIPO₄)

- cane sugar - quartz

- Rochelle salt

- topaz

- tourmaline-group minerals

- gallium orthophosphate (GaPO₄), - Langasite (La₃Ga₅SiO₁₄),

- Langanite (La₃Ga_{5 5}Nb_{0 S}Oi₄)

- Langatate (La₃Ga_{5 5}Ta_{0 5}O₁₄)

- barium titanate (BaTiO₃)

- lead titanate (PbTiO₃) - lead zirconate titanate (Pb[Zr_xTi₁-JG₃ 0<x<1 )-— more commonly known as

PZT,

- potassium niobate (KNbO₃)

- lithium niobate (LiNbO₃)

- lithium tantalate (LiTaO₃) - sodium tungstate (Na₂WO₃)

- Ba₂NaNb₅O₅

- Pb₂KNb₅O₁₅

- BiFeO₃

- Na_xWO₃ - polyvinylidene fluoride (-CH₂-CF₂-)_n

- sodium potassium niobate (KNN).

- bismuth ferrite (BiFeO₃).

The arrangement according to the invention may be combined with an optical measurement technique to afford a multi-technique setup allowing for simultaneous measurements of various parameters. For example, parameters characterizing surfaces may be measured including measurement of dry and wet masses.

A particular advantage of the present invention is that the combination of the arrangement with various optical measurement techniques is facilitated considerably due to the fact that the electrodes are not physically attached to the piezoelectric sensor surface, but located at a distance from the piezoelectric sensor. For example, the piezoelectric sensor itself has high optical quality and transmission and allows for optical measurements in transmission mode which cannot normally take place when electrodes are present on the surface of the piezoelectric sensor. Additionally, the surface of the piezoelectric sensor may be modified, for instance in order to prepare it for sample preparation. Further, the absence of electrodes on the surface of the piezoelectric sensor eliminates any contact induced interaction between the electrodes and the material being studied.

Still another advantage of the present invention is that the piezoelectric sensor may be easily replaced without disposing of the electrodes. Flexibility is thus increased and costs can be kept at a minimum compared to techniques where both the electrodes and the piezoelectric sensor have to be disposed of, e.g. when the electrodes are attached to a piezoelectric sensor.

In one aspect of the invention the arrangement comprises a Quartz Crystal Microbalance (QCM) or a Quartz Crystal Microbalance with dissipation monitoring (QCM-D). The Quartz Crystal Microbalance is generally a mass sensor where a decrease in the resonance frequency of a piezoelectricaliy excited quartz crystal disk, f, can be related to the accumulation of mass, m_acoustιc, caused by the deposition of small amounts of material at the crystal surface via the well known equation formulated by Sauerbrey in 1959:

Δ/_z m acoustic C ^• z

where the mass sensitivity constant is C = -17.7 nanograms ng/(cm²Ηz) for a 5 MHz fundamental mode quartz crystal, and z (=1 ,3... ) is the overtone number. The crystal is excited to resonance via the application of an alternating voltage across the faces of the crystal, which generally has one vacuum deposited electrode on each face. The frequency shift and the mass sensed by QCM, m_acoustιc, includes the mass of all material that is coupled to the shear oscillating motion of the crystal surface, including both adsorbed molecules and molecules from the solution associated with them (typically water). The Sauerbrey equation will only hold when the material adsorbed on the surface follows the shear oscillatory motion as a "dead mass" and does not induce large energy losses (dissipation) during the forced shear oscillatory motion of the crystal. Strongly damped systems e.g., viscoelastic layers composed of highly hydrated polymers or biomolecular structures (e.g. gels or gel like structures), which do not follow the shear motion rigidly but are deformed due to the shear motion require more extensive modelling which requires information about the energy dissipation, D, (or equivalently, the damping) induced by the system. Strictly D is defined as the ratio between the energy dissipated during one oscillation cycle and the total energy stored in the oscillator. In the QCM-D technique the energy dissipation is captured by measuring the decay time of the induced (forced) oscillation, when the driving power is interrupted. The modelling of m_acoustιc can then be obtained by the use of an acoustic multilayer model, using data collected at multiple harmonics as input. D may also be measured by conventional techniques such as e.g. via impedance/admittance analysis and parameter fitting or bandwidth monitoring, via monitoring of the motional resistance in the oscillator circuit, or via monitoring of the quality factor (Q-factor) of the resonator.

In a further aspect of the invention the arrangement is combined with a localized surface plasmon resonance sensor (LSPR sensor). The LSPR sensor may be localized on the surface of the piezoelectric sensor.

LSPR is an inherent feature of metallic nanoparticles and means a collective coherent oscillation of the conduction electrons which can be excited by external photons/electromagnetic field. In other words, the LSPR is a possible excited state of the metal nanoparticle electron system, which can be excited by photons or, equivalently, by the electromagnetic field of light incident on the particle. The LSPR excitation is a consequence of the inter-electronic (i.e. the collective) interactions of the electrons combined with spatial confinement of the conduction band electron system within the nanoparticle volume. The spectral sensitivity of the LSPR to changes in the surrounding dielectric environment makes metallic nanoparticles attractive as sensors. In a further embodiment there is provided an arrangement according to any previous embodiment which is further arranged with a light source and a light detector. It is to be understood that the light source and light detector are suitable for transmission and reflection measurements, and also for measurements involving fluorescence. The light source and detector may operate in wavelength regions including 175 nm - 100 micrometers, 180 nm - 100 micrometers, 750 nm - 100 micrometers, 175-3500 nm, 350 nm - 1100 nm, 350 nm - 800 nm.

In a further embodiment there is provided an arrangement according to any previous embodiment, wherein the light detector is arranged to receive signals indicative of localized surface plasmon resonances, geometrical resonances, localized surface plasmon resonance enhanced fluorescence, surface enhanced Raman spectroscopy, transmission through a film, surface plasmon resonance enhanced infrared (IR) spectroscopy, diffuse reflection, surface plasmon resonance (SPR) spectroscopy, ellipsornetry, reflectometry, fluorescence spectroscopy, fluorescence microscopy, infrared spectroscopy, infrared absorption reflection spectroscopy (IRAS), extinction spectroscopy, sum frequency generation (SFG), second harmonic generation (SHG), circular dichroism or optical scanning probe microscopy.

In a further embodiment there is provided an arrangement according to any previous embodiment, wherein said piezoelectric sensor is arranged to receive a sample susceptible to detection by localized surface plasmon resonances, geometrical resonances, localized surface plasmon resonances enhanced fluorescence, surface enhanced Raman spectroscopy, transmission through a film, surface plasmon resonance enhanced infrared (IR) spectroscopy, diffuse reflection, surface plasmon resonance (SPR) spectroscopy, ellipsometry, reflectometry, fluorescence spectroscopy, fluorescence microscopy, infrared spectroscopy, infrared absorption reflection spectroscopy (IRAS), extinction spectroscopy, sum frequency generation (SFG), second harmonic generation (SHG), circular dichroism or optical scanning probe microscopy.

In a further embodiment there is provided an arrangement according to any previous embodiment, wherein the sample is placed on the surface of said piezoelectric sensor.

in a further embodiment there is provided an arrangement according to any previous embodiment, wherein said piezoelectric sensor is provided with at least one LSPR sensor. In this document, LSPR sensors are understood to comprise nanoparticles susceptible to LSPR. Examples of nanoparticles susceptible to LSPR include, but are not limited to, disks, triangles, spheres, cubes, stars, holes in thin metal films, nanoshells and core-shell particles, nanorice and nanorings. In this process, the nanoparticle(s) is(are) directly exposed to and interacting with the environment to be detected. The process may be, but is not limited to, absorption of atoms/molecules into the LSPR sensor nanoparticle and/or adsorption of layers onto the LSPR sensor surface. Alternatively, the LSPR sensor may be used in a process in which a thin material layer separates the optically active sensor nanoparticle(s) from the environment to be sensed and from the sensing material, i.e. the sensing process takes place in an indirect way. The nanoparticles of the LSPR may be the same or different. When the LSPR sensor comprises nanodisks there are typically 10⁷~10⁸ nanodisks/cm².

In a further embodiment there is provided an arrangement according to any previous embodiment, wherein the LSPR sensor is arranged to receive a sample.

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that said piezoelectric sensor is a quartz crystal.

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that said piezoelectric sensor is made from a piezoelectric material.

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that said piezoelectric sensor is made from a piezoelectric material selected from one or more of the following: berlinite (AIPO₄), cane sugar, quartz, Rochelle salt, topaz, tourmaline-group minerals, gallium orthophosphate (GaPO₄), langasite (La₃Ga₅SiO₁₄), langanite (La₃Ga_{5 5}Nb₀ SOi₄), langatate (La₃Ga_{5 5}Ta_{0 5}O₁₄), barium titanate (BaTiO₃), lead titanate (PbTiO₃), lead zirconate titanate (Pb[Zr_xTi₁-JO₃ 0<x<1 )-— more commonly known as PZT, potassium niobate (KNbO₃), lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃), sodium tungstate (Na₂WO₃), Ba₂NaNb₅O₅, Pb₂KNb₅O₁₅, BiFeO₃, Na_xWO₃, polyvinylidene fluoride (-CH₂-CF₂-)_n, sodium potassium niobate (KNN), bismuth ferrite (BiFeO₃). In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that said piezoelectric sensor is made from a piezoelectric material selected from one or more of the following: berlinite (AIPO₄), cane sugar, quartz, Rochelle salt, topaz, tourmaline-group minerals, gallium orthophosphate (GaPO₄), langasite (La₃Ga₅SiO₁₄), langanite (La₃Ga₅₅Nb_{0 5}O₁₄) and langatate (La₃Ga_{5 5}Ta_{0 5}O₁₄).

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that said piezoelectric sensor is made from a piezoelectric material selected from one or more of the following; barium titanate (BaTiO₃), lead titanate (PbTiO₃), lead zirconate titanate (Pb[Zr_xTi₁-JO₃ 0<x<1 ) — more commonly known as PZT, potassium niobate (KNbO₃), lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃), sodium tungstate (Na₂WO₃), Ba₂NaNb₅O₅, Pb₂KNb₅O₁₅, and BiFeO₃, Na_xWO₃.

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that said piezoelectric sensor is made from a piezoelectric material selected from a polymer such as polyvinylidene fluoride (-CH₂-CF₂-),-,.

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that said piezoelectric sensor is made from a piezoelectric material selected from one or more of the following: sodium potassium niobate (KNN) and bismuth ferrite (BiFeO₃).

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that the distance between the electrodes and the piezoelectric sensor may be the same or different and is selected from 0.001 -5 millimeters (mm), 1 -5 mm, 2-5 mm 0.5 - 3 mm or 0.001-2 mm.

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that the optically transparent region is a hole, an optically transparent material, a grid or a combination thereof.

In a further embodiment there is provided an arrangement according to any previous embodiment, characterized in that the size of the optically transparent regi mm², 10-500 mm², 1 -100 mm² or 0.1-25 mm². In a further embodiment there is provided an arrangement according to any previous embodiment, which is arranged within a measurement cell.

In a further embodiment there is provided an arrangement according to any previous embodiment, wherein the measurement cell is under vacuum or filled with gas or liquid. The measurement in the cell may thus take place in liquid, gas or vacuum.

In a further embodiment there is provided the use of an arrangement according to any previous embodiment in biosensing applications such as bilayer formation and protein adsorption.

In a further embodiment there is provided the use of an arrangement according to any previous embodiment for hydrogen storage.

In a further embodiment there is provided the use of an arrangement according to any previous embodiment for applications involving corrosion, oxidation, catalysis (e.g. NOx storage and reduction), electrodeposition, electropolishing, electroplating, anodic oxidation, humidity measurements, drying, phase transitions, water uptake and swelling.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described more closely with reference to the Figures.

Figure 1 illustrates an arrangement according to the invention.

Figure 2 illustrates an arrangement according to the invention further comprising a light source and a detector.

Figure 3a shows an assembly in which nanodisks are placed on a piezoelectric sensor. The nanostructure is covered by a layer.

Figures 3b and 3c are graphs illustrating the formation of a biotin labelled supported lipid bilayer followed by subsequent binding of streptavidin and thereafter binding of biotin labelled vesicles. Figure 4a shows an assembly in which a nanodisk is placed on a piezoelectric sensor.

Figure 4b is a graph showing kinetic curves of hydrogen absorption and desorption as obtained from QCM and LSPR.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows an embodiment of an arrangement 1a which has two electrodes 2a and 2b, a piezoelectric sensor 3 located between and at a distance from the electrodes, wherein each electrode is arranged with an optically transparent region 4 in the form of a hole.

Figure 2 shows an embodiment of an arrangement 1 b which has two electrodes 2a and 2b, a piezoelectric sensor 3 located between and at a distance from the electrodes, wherein each electrode is arranged with an optically transparent region 4 in the form of a hole, in combination with a light source 5 and a detector 6.

Figure 3a shows an assembly 7 in which circular nanodisks 8 are placed on a piezoelectric sensor 3. The nanodisks 8 may be made of gold (Au) and fabricated by Hole-Mask Colloidal Lithography. The nanodisks 8 have a diameter D and a height h. The diameter D may be 76 nanometer (nm) and the height h may be 25 nm. A layer 9 of for example chromium (Cr) is deposited onto the nanodisks 8. The thickness of the layer 9 may be equal to or less than 3 nm. A layer 10 of for example SiO₂ covers the nanostructure. The layer 10 may be Radio Frequency plasma (RF) sputtered and the thickness may be equal to or less than 20 nm.

Figure 3b is a graph illustrating the formation of a biotin labeled supported lipid bilayer followed by subsequent binding of streptavidin and thereafter binding of biotin labelled vesicles. The QCM response, Δf and ΔD are shown. F5/5 means the fifth overtone of the crystal resonance frequency divided by 5. D5 is the dissipation factor signal. More information about Figure 3b is provided in the Examples.

Figure 3c is a graph illustrating the formation of a biotin labelled supported lipid bilayer followed by subsequent binding of streptavidin and thereafter binding of biotin labelled vesicles. The QCM parameter Δf is plotted together with the optical response. F5/5 means the fifth overtone of the crystal resonance frequency divided by 5. Centroid means the centre of mass of the LSPR peak. More information about Figure 3c is provided in the Examples.

Figure 4a shows an assembly 1 1 wherein a circular nanodisk 8 is fabricated on a piezoelectric sensor 3 in the form of a QCM crystal. The nanodisk may be made of palladium and fabricated by Hole-Mask Colloidal Lithography. The diameter D may be 260 nm when the height h is 100 nm.

Figure 4b is a graph showing hydrogen absorption and desorption kinetic curves measured simultaneously by monitoring the frequency shift of the QCM and the optical extinction shift of the LSPR signal. A negative frequency and extinction shift corresponds to hydrogen absorption and hydride formation. Opposite shifts correspond to desorption of hydrogen. F1 is the fundamental mode of the crystal resonance frequency. By extinction is meant the height of the LSPR peak. More information about Figure 4b is provided in the Examples.

The invention is illustrated, but not limited, by the following examples.

EXAMPLES

Biosensing

The performance of the combined LSPR/electrodeless QCM setup in the case of biosensing applications is demonstrated via the formation of a biotin labelled lipid bilayer via the vesicle adsorption-rupture-fusion process (where lipid vesicles adsorb rupture and fuse to a lipid bilayer), followed by the binding of streptavidin to biotin molecules in the bilayer, and thereafter binding of biotin labelled vesicles to the adsorbed streptavidin. The lipid bilayer was formed on a nanofabricated surface, comprising Au nanodisks fabricated by Hole-Mask Colloidal Lithography with diameter D=76nm and height h=25nm, a 3nm thick Cr layer deposited through the hole-mask onto the Au disks and finally a 20nm thick RF-sputtered SiO₂ layer covering the nanostructures (see Figure 3a). The latter is necessary to facilitate the formation of lipid bilayers.

Initially, biotin labelled vesicles were inserted into the chamber and started to adsorb intact to the surface (t = 31.5 min, figures 3b and 3c). This was reflected as an increase in coupled mass (decrease in Δf and increase in the centroid position) as well as increase in energy losses (increase in AD) induced by the viscoelastic vesicles. As a critical surface coverage is reached, the vesicles start to rupture and thereby release enclosed water. When the net amount of rupturing vesicles becomes larger than the amount of those adsorbing, the energy losses and coupled mass have reached their extreme values, and will start to decrease. At this point the optical signal, which measures the non-hydrated adsorbed mass, will continue to increase, as lipid material is still adsorbing to the surface. When the bilayer formation process was complete Af = -30.8 Hz and ΔD = 0.3- 10^"6, which was slightly higher than for a perfect bilayer and hence indicated that there were still a small amount of intact vesicles remaining at the sensor surface. At t = 81 min the protein was exposed to the bilayer doped surface, and started to bind, as revealed by the decrease in Δf and the increase in ΔD as well as in the optical signal. The equilibrium responses for the protein binding, Δf ~38 Hz and ΔD -1.3-10^"6, was slightly higher than what was previously reported for a monolayer of streptavidin, for example in Anal. Chem.,2003, 75, pages 5080-5087. In the final step, the streptavidin monolayer was exposed to biotin-doped vesicles which (t - 146 min) resulted in monotonously increasing signals up to saturation. The high final value in ΔD (~12-10^"6), indicated binding of intact vesicles to the surface.

Hydrogen Storage in Pd Nanodisks

The performance of the combined LSPR/electrodeless QCM setup for application in the area of characterizing nanoscopic hydrogen storage systems is demonstrated by simultaneously measuring the LSPR and QCM response for hydrogen absorption and desorption in Pd nanodisks. As a proof-of-concept experiment for the operation of the combined LSPR/electrodeless QCM in gas phase, i.e. in the absence of a conducting liquid, we studied the hydrogen storage kinetics in Pd nanodisks. The latter were fabricated by the Hole-Mask Colloidal Lithography method, yielding a mean nanodisk diameter D = 260 nm at a height of 100 nm (figure 4a). This nanofabrication method yields a random distribution of nanodisks on a substrate which, combined with a large particle-particle separation, eliminates both far and near field coupling between the particles, i.e. the measured optical properties reflect a single particle optical response.

Figure 4b shows hydrogen absorption and desorption kinetic curves measured simultaneously by monitoring the frequency shift of the QCM and the optical extinction shift of the LSPR signal. The measurements were carried out at 22°C and in a constant gas flow rate of 60 rnl/rnin trough the measurement eel!. The gas concentration was alternated between 100% Ar and 4% H₂ in Ar (corresponding to a hydrogen pressure of 30.4 Torr). The hydrogen concentration was thus high enough to ensure total conversion of the Pd to hydride, as the corresponding hydrogen pressure was above the equilibrium plateau at 22°C. Surprisingly, the frequency signal from the QCM and the optical LSPR response are following each other very well, revealing the absorption and desorption kinetics of hydrogen in Pd. From the frequency shift Af, by using the Sauerbrey equation, the hydrogen concentration in the Pd disks can be calculated. This information can now be used to calibrate the optical LSPR signal in order to e.g. facilitate the development of quantitative LSPR-based hydrogen sensors and platforms for the characterization of novel hydrogen storage materials.

Claims

1. An arrangement comprising at least two electrodes, at least one piezoelectric sensor located at a distance from the electrodes, wherein at least one electrode is arranged with at least one optically transparent region.

2. An arrangement according to claim 1 , wherein the piezoelectric sensor is arranged with at least one optically transparent region.

3. An arrangement according to claim 1 or 2 further arranged with a light source and a light detector.

4. An arrangement according to any previous claim, wherein the light detector is arranged to receive signals indicative of localized surface plasmon resonances, geometrical resonances, localized surface plasmon resonances enhanced fluorescence, surface enhanced Raman spectroscopy, transmission through a film, surface plasmon resonance enhanced infrared spectroscopy, diffuse reflection, surface plasmon resonance spectroscopy, ellipsometry, reflectometry, fluorescence spectroscopy, fluorescence microscopy, infrared spectroscopy, infrared absorption reflection spectroscopy, extinction spectroscopy, sum frequency generation (SFG), second harmonic generation

(SHG), circular dichroism or optical scanning probe microscopy.

5. An arrangement according to any previous claim, wherein said piezoelectric sensor is arranged to receive a sample susceptible to detection by localized surface plasmon resonances, geometrical resonances, localized surface plasmon resonances enhanced fluorescence, surface enhanced Raman spectroscopy, transmission through a film, surface plasmon resonance enhanced infrared spectroscopy, diffuse reflection, surface plasmon resonance spectroscopy, ellipsometry, reflectometry, fluorescence spectroscopy. fluorescence microscopy, infrared spectroscopy, infrared absorption reflection spectroscopy, extinction spectroscopy, sum frequency generation (SFG), second harmonic generation (SHG), circular dichroism or optical scanning probe microscopy..

6. An arrangement according to claim 5, wherein the sample is placed on the surface of said piezoelectric sensor.

7. An arrangement according to any previous claim, wherein said piezoelectric sensor is provided with at least one localized surface plasmon resonance sensor.

8. An arrangement according to claim 7, wherein the localized surface plasmon resonance sensor is arranged to receive a sample.

9. An arrangement according to any previous claim, characterized in that said piezoelectric sensor is a quartz crystal.

10. An arrangement according to any previous claim, characterized in that the distance between the electrodes and the piezoelectric sensor may be the same or different and is selected from 0.001-5 mm, 1-5 mm, 2-5 mm 0.5 - 3 mm or 0.001-2 mm.

1 1 . An arrangement according to any previous claim, characterized in that the optically transparent region is a hole, an optically transparent material, a grid or a combination thereof.

12. An arrangement according to any previous claim, characterized in that the size of the optically transparent region is 0.1 -500 mm², 10-500 mm², 1 -100 mm² or 0.1-25 mm².

13. An arrangement according to any previous claim, which is arranged within a measurement cell.

14. An arrangement according to claim 13, wherein the cell is under vacuum or filled with gas or liquid.

15. Use of an arrangement according to any previous claim in biosensing applications or for hydrogen storage.