WO2020167224A1

WO2020167224A1 - Measurement system for magnetic samples

Info

Publication number: WO2020167224A1
Application number: PCT/SE2020/050143
Authority: WO
Inventors: Sobhan SEPEHRI; Alexei KALABOUHKOV; Dag Winkler
Original assignee: Sepehri Sobhan; Kalabouhkov Alexei; Dag Winkler
Priority date: 2019-02-11
Filing date: 2020-02-11
Publication date: 2020-08-20
Also published as: SE1950159A1

Abstract

The present invention relates to a measurement system(100) for analyzing a magnetic sample, the system comprising:a sensor (102) configured to detect a magnetic signal defined by a magnetic flux difference between magnetic samples located at spatially separated measurement locations (104, 106) of the sensor; wherein, when in use, the sensor is configured to simultaneously sense the magnetic flux of the magnetic samples and provide a detection signal (112) indicative of magnetic flux differences between the magnetic samples.

Description

MEASUREMENT SYSTEM FOR MAGNETIC SAMPLES

Technical Field

The present invention relates to a measurement system for analyzing a magnetic sample and to a method for analyzing a magnetic sample.

Background

Detection of magnetic flux has a vast variety of applications ranging from biomedical applications to magnetic particle inspection for detecting irregularities such as cracks in ferromagnetic materials.

In some applications, a weak magnetic signal is desirable to detect against a relatively high level background signal. For example, it may be desirable to detect a change in the content of magnetic material in a large sample of magnetic material. The change may be difficult to accurately detect due to the large background signal from the sample. Another example relates to detecting an alteration in a detected magnetic signal due to other characteristics of the sample that may have changed, such as the dynamic behavior of magnetic particles suspended in a liquid.

With regards to the above, it is desirable to improve the performance of systems for detecting magnetic signals.

Summary

In view of the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide a measurement system for analyzing a magnetic sample.

According to a first aspect of the present invention, it is therefore provided a measurement system for analyzing a magnetic sample, the system comprising: a sensor configured to detect a magnetic signal defined by a magnetic flux difference between magnetic samples located at spatially separated measurement locations of the sensor; wherein, when in use, the sensor is configured to simultaneously sense the magnetic flux of the magnetic samples and provide a detection signal indicative of magnetic flux differences between the magnetic samples.

The present invention is based on the realization to configure the sensor and the measurement locations for the magnetic samples such that the magnetic fluxes from the magnetic samples are simultaneously detectable and also subtracted at the sensor. In this way, the detection signal provided by the sensor is directly related to the flux difference between the magnetic samples.

Accordingly, a reference may be represented by one magnetic sample which is to be compared to another magnetic sample. Since the responses are subtracted by the sensor, the background, i.e. the reference is already considered when the sensor outputs its detection signal to a user. In other words, the suggested measurement system provides for directly measuring a magnetic differential measurement between two magnetic samples.

The sensor is designed such that the geometry of the sensor causes the subtraction of the responses. The positions of the measurement locations with respect to the sensor also influence the subtraction. In other words, the sensor geometry, the measurement locations, and an optional excitation field are adapted such that the responses of the sample couple to the sensor in such a way that they are subtracted by the sensor geometry.

Accordingly, it was realized to, when using the measurement system, arrange the magnetic samples at measurement locations so that the responses of the magnetic samples sensed by the sensor are subtracted by the sensor before the detection signal is determined. As mentioned above, the subtraction of the responses is advantageously performed by the sensor before the sensor provides the detection signal. This provides for improved sensitivity of the measurement system. It may further provide for improved dynamic range of the measurement system. According to some embodiments, the sensor may comprise: at least one sensing member configured to simultaneously sense the magnetic flux of the magnetic samples and to subtract a sensing signal of a respective magnetic sample from a sensing signal of another respective magnetic sample to provide a differential sensing signal; and at least one detection element coupled to the at least one sensing member and configured to provide the detection signal as a read out signal based on the differential sensing signal.

Accordingly, the sensing members which are part of the sensor geometry simultaneously sense the responses from the magnetic samples and provide a respective sensing signal therefrom. The sensing signals partly cancel each other so that a difference between the sensing signals is detected by the detection element. The sensing signal may be e.g. an induced current or voltage in the sensing member, or a measurable resistance.

According to some embodiments, the magnetic samples may be magnetic fluid samples, the system comprising: at least one channel, tube, or vial for providing at least two magnetic fluid samples to a respective measurement location of the sensor.

A single channel may transfer both a test and a control samples in separate volume packages to the respective measurement locations of the sensor, wherein, when in use, the measurement system is adapted for a differential measurement between the samples.

The channels provide for a simplified sample handling, in particular for small sample volumes.

The channels may be microfluidic channels. Furthermore, the microfluidic channels may be manufactured in a polymer material or glass, although other materials are conceivable. In some embodiments, the measurement system may comprise at least two channels for providing each of at least two magnetic fluid samples to a respective measurement location of the sensor.

Preferably, each of the channels is individually configured to transfer a respective sample to the measurement locations of the sensor, wherein, when in use, the measurement system is adapted for a differential

measurement between the samples. The comparative measurement advantageously provides for a direct measurement of a magnetic difference between the samples. The magnetic difference may relate to magnetic AC (alternating current) susceptibility difference.

According to some embodiments, when placed at the measurement locations, the magnetic samples are simultaneously excitable by an excitation magnetic field (H), wherein the sensor is configured to simultaneously sense the responses of the magnetic samples caused by the excitation magnetic field. The magnetic flux detectable may be enhanced by using a magnetic excitation field. Further, some applications benefit from using a magnetic excitation field.

According to some embodiments, the sensor may be a planar gradiometer. Preferably, the excitation magnetic field is applied substantially parallel to the plane of the planar gradiometer. A gradiometer advantageously cancels out homogenous background signals. Furthermore, with a

gradiometer, the measurement locations may be at a respective pick-up loop of the gradiometer. Since the gradiometer is sensitive to flux differences between the pick-up loops, the pick-up loops are beneficial positions for the measurement locations in order to subtract the responses at the sensor position before a read-out signal is provided.

In order to reduce the coupling between the excitation field and the gradiometer the excitation magnetic field is preferably applied in the plane of the gradiometer. In one embodiment, the excitation magnetic field is applied perpendicular to the base line of the planar gradiometer. The base line is along a longitudinal center axis through both pick-up loops of the planar gradiometer.

According to some embodiments, the channels may be configured to be aligned substantially parallel with the base line. The channels are adapted to be arranged along a conductive path of a respective pick-up loop of the planar gradiometer. In this way, the coupling between the samples responses and the sensor is increased.

According to some embodiments, the channels are provided in a comparator device which is separable from the sensor. Thus, the channels are advantageously removable which allows for a new configuration of channels to be tested with the sensor, or more convenient cleaning or maintenance of the sensor or channels. The comparator device may be provided in the form of a chip.

One of the samples may be a reference sample such as a negative control sample. In other words, the measurement system is configured to simultaneously measure the response of the negative control sample and a test sample.

The measurement system may advantageously be configured for a single shot analysis of a test sample, wherein, when in use, the test sample may be placed at one of the measurement locations, and a reference sample is placed at another measurement location. A single shot analysis provides for a fast analysis. Accordingly, it was realized to, when using the

measurement system, arrange the measurement locations so that the responses of the reference sample and the test samples sensed by the sensor are subtracted by the sensor before the detection signal is

determined. In this way, a direct differential measurement between the reference sample, such as a negative control, and the test sample is provided in a single shot measurement with a single sensor.

When the measurement system is in use, the test sample may be injected in a first channel or tube to one of the measurement locations, and the reference sample may be injected by a second channel or tube to another one of the measurement locations.

According to some embodiments, the sensor may be a

superconducting sensor comprising a superconducting quantum interference device coupled to at least one pick-up loop adapted for coupling of magnetic flux to the sensor. A superconducting quantum interference device provides for a sensor with high sensitivity to magnetic flux. The superconducting sensor may comprise two pick-up loops adapted for coupling of magnetic flux to the sensor.

The measurement system is advantageously configured to detect biomolecules using magnetic nanoparticles as detection labels, wherein the magnetic sample comprises magnetic nanoparticles suspended in a liquid.

For detection of biomolecules, magnetic nanoparticles may be functionalized with capturing molecules specific for the target analyte. Using magnetic nanoparticles for use in detecting biomolecules is often called magnetic assay and is perse known to the skilled addressee.

When the capturing molecules, e.g. antibodies, complementary DNA or RNA strands, etc., capture the target analyte, the dynamics of the magnetic nanoparticles in the suspension liquid are altered. This alteration is an indication of the detection of the target analyte. A drawback of prior art detection systems for magnetic assays is that the detection is based on a detection of a small change of magnetic signal in the presence of high magnetic background. Reducing the concentration of magnetic nanoparticles may narrow the dynamic range of the detection scheme and makes it susceptible to disturbance. However, with the inventive concept one of the samples may be a negative control sample and the other a test sample. In this way, the background signal is directly subtracted by the sensor itself before the read-out signal is provided. This may for example provide for a homogeneous assay with improved performance. According to a second aspect of the present invention, there is provided a method for analyzing a magnetic sample, the method comprising: providing a measurement system comprising a sensor configured to detect a magnetic signal defined by a flux difference between magnetic samples located at spatially separated measurement locations of the sensor; placing a test sample at one of the measurement locations and a reference sample at another one of measurement locations; simultaneously sensing the magnetic flux of the test sample and the reference sample; providing a detection signal indicative of magnetic flux differences between the samples based on the magnetic fluxes.

According to embodiments, the method may comprise: simultaneously applying an excitation magnetic field to the test sample and to the reference sample; simultaneously sensing the response of the magnetic samples caused by the excitation magnetic field, wherein the detection signal is based on the sensed responses.

According to embodiments, the samples may be magnetic fluid samples comprising magnetic nanoparticles, wherein a first sample is a test sample and a second sample is a reference sample, the method may comprise: determining the presence of a target analyte in the test sample based on the detection signal.

According to embodiments, the method may comprise quantifying the concentration of the target analyte based on the magnitude of the magnetic flux differences. The reference sample may be a negative control sample.

Further embodiments of, and effects obtained through this second aspect of the present invention are largely analogous to those described above for the first aspect of the invention.

According to a third aspect of the invention, there is provided use of a measurement system according to embodiments of the first aspect, comprising: placing a test sample at one of the measurement locations and a reference sample at another one of the measurement locations; simultaneously sensing the magnetic flux of the test sample and the reference sample; and providing a detection signal indicative of magnetic flux

differences between the samples based on the sensed magnetic fluxes.

Further embodiments of, and effects obtained through this third aspect of the present invention are largely analogous to those described above for the first aspect and the second aspect of the invention.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled addressee realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

Brief Description of the Drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

Fig. 1 conceptually illustrates an example measurement system in accordance with embodiments of the present disclosure;

Fig. 2 conceptually illustrates an example measurement system in accordance with embodiments of the present disclosure;

Fig. 3A-B conceptually illustrates example measurement systems in accordance with embodiments of the present disclosure;

Fig. 4A-B conceptually illustrates a perspective views of an example measurement system in accordance with embodiments of the present disclosure;

Fig. 5 conceptually illustrates an application of an example

measurement system of the present disclosure; Fig. 6 illustrates example measurement data of a magnetic bioassay using the measurement system according to embodiments of the present disclosure;

Fig. 7 is a flow-chart of method steps according to embodiments of the invention;

Fig. 8 is a flow-chart of method steps according to embodiments of the invention; and

Fig. 9 conceptually illustrates an example measurement system in accordance with embodiments of the present disclosure.

Detailed Description of Example Embodiments

In the present detailed description, various embodiments of a measurement system according to the present disclosure are described. Flowever, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. Like reference characters refer to like elements throughout.

Fig. 1 conceptually illustrates a measurement system according to the present disclosure for analyzing a magnetic sample. The system comprises a sensor 102 configured to detect a magnetic signal defined by a magnetic flux difference between magnetic samples located at spatially separated measurement locations of the sensor. Here, a first measurement location 104 and a second measurement location 106 are shown. When placed at the measurement locations 104, 106, the magnetic fluxes 108, 110 of the magnetic samples are simultaneously detectable by the sensor 102. The sensor is configured to provide a detection signal 112 indicative of magnetic flux differences between the magnetic samples.

Optionally, the magnetic samples are simultaneously excitable by an excitation magnetic field H, when placed at the measurement locations 104, 106. The excitation magnetic field is adapted to magnetize or generally transfer energy to the magnetic samples to cause them to provide a response signal once the excitation magnetic field H is removed or altered.

The sensor 102 is in this case configured to simultaneously sense the responses 108, 110 of the magnetic samples caused by the excitation magnetic field and subtract the responses from each other to provide a detection signal 112 indicative of magnetic flux differences between the magnetic samples.

With the proposed measurement system a differential measurement between two magnetic samples is possible in a single shot measurement.

The differential measurement is a comparison between magnetic properties of two samples placed at the selected measurement locations. The geometry of the sensor and optionally the direction of the excitation field provides for a subtraction of the responses from the samples by the sensor itself. In other words the magnetic fluxes, either directly or in the form responses, of the magnetic samples sensed by the sensor may be subtracted by the sensor before the detection signal is determined. Accordingly, for determining an alteration in magnetic properties of a test sample compared to a reference sample, both the test sample and the reference sample are measurable simultaneously and the detected signal reflects the alteration of the test sample compared to the reference sample which may be a negative control sample.

The excitation magnetic field may be an alternating (AC) magnetic field swept through a frequency range. The responses of the magnetic samples are detected at several frequencies in the range. Generally, with this type of excitation the response is considered a magnetic AC susceptibility. However, other types of excitation magnetic fields are also conceivable.

Fig. 2 conceptually illustrates an example measurement system 100 of the present disclosure. The sensor 201 here comprises a first sensing member 202 and a second sensing member 204, each associated with a respective measurement location 104, 106. The sensing members 202 and 204 may be pick-up coils or loops or elements of the sensor 201. The sensing signal 208 from the first sensing member 202 is subtracted from the sensing signal 210 from the second sensing member 204, or vice versa, by the coupling between the sensing members to provide a differential sensing signal 205 that reaches a detection element 206 coupled to the sensing members 202 and 204. The detection element 206 is configured to provide the detection signal as a read out signal 112 based on the sensing signals. The detection element 206 is thus configured to provide a readable signal for a user based on the differential sensing signal. As conceptually illustrated in fig. 2, the sensor 201 is configured to subtract one sensing signal from the other one before a read-out signal is provided, by the geometry of the sensor.

Fig. 3A illustrates a conceptual configuration of a measurement system 300 according to the present disclosure. The sensor 301 in fig. 3A is provided in the form of a planar gradiometer 301 comprising sensing members in the form of pickup loops 202 and 204.

The measurement system 300 further comprises two channels, a first channel 304 and a second channel 305 for transporting magnetic fluid samples to a respective measurement location 306, 307. The measurement locations 306 and 307 are located along a conductive path of the pickup loops, i.e. the channels 304 and 305 are aligned with a respective conductive path of the pickup loops 202 and 204. Furthermore, the channels 304 and 305 are arranged on the same side of the base line 308 which defines a longitudinal centerline of the planar gradiometer 301. The channels 304 and 305 are arranged parallel with the base line 308, and the excitation magnetic field H is applied perpendicular to the base line 308 of the planar gradiometer 301 , in the plane of the gradiometer. With this configuration, the response magnetic flux from the magnetic fluid samples is locally coupled to the pickup loop they are aligned with. A detection element 302 is coupled to the pick-up loops 202 and 204. The detection element 302 may be provided in the form of a superconducting quantum interference device (SQUID). Preferably, the pick-up loops 202 and 204 may also be made from a superconducting material.

Superconducting materials are perse known to the skilled addressee.

Generally, a superconducting material has no electrical resistance when cooled to below a transition temperature. One example superconducting material is YBaCuO (YBCO) which may advantageously be cooled using liquid nitrogen instead of liquid helium as is the case for so-called low- temperature superconductors. Superconducting materials other than YBCO are also conceivable.

Accordingly, the SQUID is kept at a temperature below its operational temperature by means known to the skilled person and the magnetic samples are preferable kept at room temperature. This type of configuration is well known in the art and will not be described further herein.

Furthermore, SQUIDs are perse known to the skilled person.

Generally, a SQUID is comprised of a superconducting ring interrupted by at least one“weak link”, a so-called Josephson junction.

A superconducting planar gradiometer comprising a SQUID may be manufactured on a substrate 319 here only conceptually shown, using thin- film technology including pulsed laser deposition, sputtering,

chemical/physical vapor deposition, photolithography, e-beam lithography, etc.

The magnetic fluid samples provided to the measurement locations 306 and 307 by the channels may preferably comprise magnetic

nanoparticles suspended in a liquid. Using an alternating excitation magnetic field H, the response of the magnetic nanoparticles is determined. In this example the magnetic nanoparticles are excited by the excitation magnetic field H applied in the plane of the gradiometer 301 , whereby they align their magnetic moments along that the direction of the excitation field. Removal of alteration of the excitation magnetic field H causes a response from the magnetic nanoparticles. The responses are coupled into the pick-up loops 202 and 204 of the planar gradiometer 301.

Electrical currents induced in the pick-up loops by the responses are here indicated by arrows 312 and 313. Since the measurement locations are on the same side of the base line 308 and the excitation magnetic field H is applied to both samples simultaneously, the magnetized samples will both couple to the respective pick-up loop in the same direction. Thereby, the induced currents, i.e. the sensing signals, from the pick-up loops will circulate in the same directions in the pick-up loop 202 and 204. Accordingly, the combined electrical current that reaches the detection element 302, coupled to both pick-up loops, will partly cancel each other if the samples are different. Thus, the detection element 302 measures the difference between magnetic flux in the pickup loops 202 and 204.

In other words, when the channels 304 and 305 are filled with a respective magnetic fluid sample, the measured detection signal reflects the difference in magnetic signal caused by the two samples.

If the magnetic content is the same in both samples, but one sample has been affected by a test substance such as a target analyte as will be described below, then the subtraction between the sensing signals causes a subtraction of the background magnetic signal caused by the magnetic material and only the difference is detected.

Fig. 3B illustrates another possible implementation of a measurement system 3000 in which a single channel 3004 is used for transporting two samples to their respective measurement location 306 and 307. In other words, a single channel having inlet 3018 and outlet 3020 may transfer both a test and a control samples in separate volume packages 3100, 3101 to the respective measurement locations 306, 307 of the sensor 301.

Fig. 4A illustrates the measurement system 300 with the channels 304 and 305 provided in a comparator device 316 in the form of a comparator chip which is separable from the sensor 301. Fig. 4A shows the comparator chip 316 separated from the sensor 301 and fig. 4B illustrates the comparator chip 316 arranged near the sensor 301 for a differential measurement.

The channel 304 comprises an inlet 318 and an outlet 320 for a first magnetic fluid sample to enter and exit the channel 304. Similarly, the channel 305 comprises an inlet 322 and an outlet 324 for a second magnetic fluid sample to enter and exit the channel 305.

The channels 304 and 305 are connectable to suitable pumps for circulating the magnetic fluid samples though the channels 304, 305 for a differential measurement.

The channels 304 and 305 may be microfluidic channels manufactured from e.g. a polymer such as PDMS, or glass.

Each of the channels 304 and 305 is individually configured to transfer a respective sample to the measurement locations 306, 307 of the sensor, wherein, when in use, the measurement system is adapted for a differential measurement in which the samples are different.

It should be understood that the measurement locations 306 and 307 may cover the entire conductive path of the pickup loop, in other words, anywhere along the pickup loop where magnetic sample may couple to the respective pickup loop.

It should be understood that the channel configuration shown in the herein depicted embodiments serves merely for exemplifying the

measurement system and that many different shapes and paths of the channels are possible and within the scope of the enclosed claims. For example, the channels may have curved portions, or may be provided by a respective single straight portion.

An example application for a measurement system according to the present disclosure will be described next with reference to fig. 5. This example application will be described with reference to the measurement system 300 although other configurations of the specific measurement system may be tailored for a specific application or implementation.

Furthermore, the application that is described next relates to detecting biomolecules using magnetic nanoparticles as detection labels, i.e. a magnetic assay. For this, the magnetic samples comprise magnetic

nanoparticles suspended in liquid.

Fig. 5 illustrates application of the measurement system 300. A first magnetic fluid sample 332 comprising magnetic nanoparticles 331 suspended in a fluid is injected in the first channel 304. A second magnetic fluid sample 333 comprising magnetic nanoparticles 331 suspended in a fluid is injected also in the second channel 305. The concentration of magnetic nanoparticles is the same or at least similar in both magnetic fluid samples 332, 333.

The detection of a target analyte is based on detecting a change in the magnetic response caused by the presence of the target analyte. Generally, bio-assays are bioanalytical methods for qualitative and quantitative determination of the pathogens that relies on specific biomolecular reactions between target (analyte) and labeled markers, in this case magnetic nanoparticles. The reactions involve e.g. antigens-antibodies

(immunoassays), DNA/RNA molecules, bacterial culture, proteins, enzymes, etc.

In fig. 5, the first sample 332 is a reference sample without target analyte and the second sample 333 is a test sample which, in this case comprises a target analyte, whereby clusters 334 of magnetic nanoparticles may be formed, as a result of specific biomolecular reactions between target (analyte) and labeled magnetic nanoparticles. The difference between the reference sample 332 and the test sample 333 is the presence of target analyte in the test sample 333. The reference sample 332 may therefore represent a negative control sample. Thus, the inventive concept

advantageously enables a simultaneous measurement of the negative control sample and the test sample to provide a direct comparison of the test sample with the negative control sample. The readout signal from the sensor 301 has intrinsically subtracted the negative control sample background signal.

The measurement system for detecting biomolecules is particularly effective for so-called homogeneous assays. A homogenous assay is characterized by its simplicity and lack of washing and separation steps for the magnetic fluid samples. Typically, in a homogenous assay the test substance including the target analyte is mixed with the labeled magnetic nanoparticles, mixed, and subsequently measured without the need for washing and separation steps. The magnetic nanoparticles produce a large background signal in which the alteration of the magnetic flux from the magnetic fluid samples caused by the presence of the target analyte may be difficult to detect. This is alleviated with the inventive concept in which the negative control may be intrinsically subtracted by the sensor configuration.

Fig. 6 illustrates example measurement data of a magnetic bioassay using the measurement system 300 as described with reference to fig. 5.

Many different protocols for enabling a detectable specific binding reaction that causes an alteration in the magnetic signal from each sample are conceivable. For the measurement data shown in fig. 6, differential measurements of test samples containing V. Cholera DNA targets and a negative control sample are performed. For the measurements, the test samples are placed at a first one of the measurement locations, and the negative control sample is placed at a second one of the measurement locations.

The V. Cholera target DNA molecule are detected by padlock probe ligation and then volume amplified into DNA coils for 20 minutes using the rolling circle amplification (RCA) technique. In this example case, positive samples contain different concentrations of RCA coils ranging from 200 fM to 200 pM.. 250 pg/mL functionalized magnetic nanoparticles are added to the dilutions of RCAs as magnetic markers and the mix is incubated at 55°C for 20 minutes. All the RCA dilutions contain same amount of functionalized magnetic nanoparticles for binding. The negative control sample is prepared by incubating the functionalized magnetic nanoparticles with only the hybridization buffer and therefore, contains no RCA coils. The negative control is the control sample filled in channel 304 and the RCA coil dilutions are filled in channel 305 for differential measurement.. The zero-response is defined by filling both channels 304 and 305 with the negative control.

One of the challenges in homogenous magnetic bioassays is the magnetic background which undermines the resolution of small signals from the background. The concentration of the magnetic nanoparticles may be reduced in order to reduce this background signal and to make the bioassay more sensitive to lower concentration of target.. However, lowering the concentrations increases the time for reaching equilibrium during incubation and also makes the bioassay more susceptible to systems disturbances.

Using the differential ac susceptibility technique enabled by the measurement system 300, it is possible to increase the MNP label concentration 5 times to 250 pg/mL compared to prior art systems (see Sepehri et al. ALP Bioeng. 2,

016102), and still measure 200 fM of RCA coils in a 3.6pL sample volume. Having large number of MNP labels available is advantageous for reasonably fast binding kinetics during hybridization specially at low concentration of RCA coil.

Fig. 6 shows the imaginary part (c”) of differential ac susceptibility as a function of frequency for the negative control and positive test samples with the RCA coil concentrations ranging from 200 fM to 200 pM. The imaginary component shows relaxation peaks that correspond to the Brownian relaxation of magnetic nanoparticles consumed due to conjugation with the RCA coils. The magnitude of the imaginary part (c”) of differential ac susceptibility at its peak frequency increases as the concentration of RCA coils is increased. In other words, using a calibration curve, it is possible to quantify the amount of RCA coils in a test sample. Fig. 7 is a flow-chart of method steps according to embodiments of the invention. In step S102, providing a measurement system comprising a sensor configured to detect a magnetic signal defined by a flux difference between magnetic samples located at spatially separated measurement locations of the sensor. Such measurement system may be any of the described systems herein. Next, a test sample is placed S104 at one of the measurement locations and a reference sample at another one of

measurement locations. The magnetic flux of the test sample and the reference sample are sensed in step S105. A detection signal is provided in step S110 indicative of magnetic flux differences between the samples based on the sensed magnetic fluxes.

Fig. 8 is a flow chart of method steps according to embodiments of the present invention. In step S102, providing a measurement system comprising a sensor configured to detect a magnetic signal defined by a flux difference between magnetic samples located at spatially separated measurement locations of the sensor. Such measurement system may be any of the described systems herein. Next, a test sample is placed S104 at one of the measurement locations and a reference sample at another one of

measurement locations. Subsequently, an excitation magnetic field is simultaneously applied to the test sample and to the reference sample in step S106. The response of the magnetic samples caused by the excitation magnetic field is simultaneously sensed in step S108. The response signal is here the magnetic flux response caused by applying the excitation field to the samples. A detection signal is provided in step S110 indicative of magnetic flux differences between the samples based on the sensed response.

Optionally, in the case of samples being magnetic fluid samples comprising magnetic nanoparticles, where a first sample is a test sample and a second sample is a reference sample, the method may comprise a subsequent step S112 comprising to determine the presence of a target analyte in the test sample based on the detection signal. Optionally, the concentration of the target analyte may be quantified based on the magnitude of the magnetic flux difference in step S114.

The detection signal output by the sensors of the present disclosure and the actuation circuits to provide various forms of excitation fields may be processed and analyzed by a control unit for providing readable data.

The control unit may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control unit may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a

programmable logic device, or a digital signal processor. Where the control unit includes a programmable device such as the microprocessor,

microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device.

The magnetic nanoparticles discussed herein may be provided in a variety of forms. One example is Streptavidin coated of magnetite (Fe304) multicore-shell nanoparticles with a median particle diameter of 100 nm suspended in PBS solution (Micromod Partikeltechnologie GmbH,

Rostock, Gennany). The magnetic cores in the multi-core particles may have a median size of 15 nm and the streptavidin surface allows binding of biotinylated molecules which may be used for targeting specific target analytes. Brownian relaxation is the dominating dynamics for these magnetic nanoparticles.

A superconducting gradiometer may be of first order although higher order gradiometers are conceivable. A superconducting gradiometer comprising a SQUID may be fabricated on from YBa2Cu307 film grown on a 10 by 10 mm SrTi03 bicrystal substrate with 24-degree misorientation for the forming of Josephson junctions. The SQUID is thus made of bicrystal grain boundary junctions, although other types of Josephson junctions are conceivable. The baseline of the superconducting gradiometer loops may be about 4 mm with a linewidth of about 400 pm. The SQUID is placed in the center of the gradiometer and is directly connected to the two pickup loops, sensing only the difference in the magnetic flux in the two superconducting pickup loops.

As is known in the art, the operation of a superconducting sensor requires sufficient cooling so that the sensor reaches and remains in the superconducting state. For this, The SQUID chip is placed on a sapphire rod in contact with liquid nitrogen in a cryostat preferably made from non- magnetic materials. The cold SQUID is advantageously placed close, e.g. less than 1 mm from the top surface of a 250 pm thick sapphire window which separates the cold parts in vacuum from the room temperature environment where the magnetic samples are located.

In order to measure magnetic ac susceptibility a Helmholtz coil is used as an excitation coil to generate alternating excitation magnetic field. The coil provides homogenous excitation fields of sufficient strength (e.g. up to 250 pT) in the frequency range of 1 Hz - 10 kHz.

The comparator chip comprising microfluidic channels may be manufactured using precision machining for fabricating the master mold and PDMS (polydimethylsiloxane) as the casting material. The channels are sealed by a thin, i.e. less than 100 pm thick PDMS membrane in order keep the distance between the sensor and the samples in the channels small. An example channel has a 1 mm by 1 mm sized cross-section. Teflon tubes may be used for connecting the channels with the magnetic fluid sample reservoir. A peristaltic pump may be arranged to transfer the magnetic fluid samples in and out of each channel.

Although the figures may show a sequence the order of the steps may differ from what is depicted. Also, two or more steps may be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations for operation of the system could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps. Additionally, even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, different types of sensors are conceivable although a superconducting sensor is specifically described herein. Other types or sensors include, induction coils, flux gates, Hall sensors, giant magnetoresistance (GMR) sensors, etc.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A measurement system (100;300;) for analyzing a magnetic sample, the system comprising:

- a sensor (102) configured to detect a magnetic signal defined by a magnetic flux difference between magnetic samples located at spatially separated measurement locations (104, 106) of the sensor;

wherein, when in use, the sensor is configured to simultaneously sense the magnetic flux of the magnetic samples and provide a detection signal (112) indicative of magnetic flux differences between the magnetic samples.

2. The measurement system according to claim 1 , wherein the measurement locations are located so that the magnetic fluxes of the magnetic samples sensed by the sensor are subtracted by the sensor before the detection signal is determined.

3. The measurement system according to any one of claims 1 and 2, wherein the sensor comprising:

- at least one sensing member (202, 204) configured to simultaneously sense the magnetic flux of the magnetic samples and to subtract a sensing signal of a respective magnetic sample from another sensing signal of another respective magnetic sample to provide a differential sensing signal;

- at least one detection element (206, 302) coupled to at least one sensing member and configured to provide the detection signal as a read out signal based on the differential sensing signal.

4. The measurement system according to any one of the preceding claims, wherein the magnetic samples are magnetic fluid samples, the system comprising: - at least one channel (304, 305), tube, or vial for providing at least two magnetic fluid samples to a respective measurement location (306, 307) of the sensor.

5. The measurement system according to claim 4, comprising:

- at least two channels (304, 305) for providing each of at least two magnetic fluid samples to a respective measurement location (306, 307) of the sensor.

6. The measurement system according to any one of the preceding claims, wherein, when placed at the measurement locations, the magnetic samples are simultaneously excitable by an excitation magnetic field (H), wherein the sensor is configured to simultaneously sense the responses of the magnetic samples caused by the excitation magnetic field.

7. The measurement system according to any one of the preceding claims, wherein the sensor is a planar gradiometer (301 ).

8. The measurement system according to claim 6 and 7, wherein the excitation magnetic field is adapted to be applied substantially parallel to the plane of the planar gradiometer.

9. The measurement system according to claim 8, wherein an excitation magnetic field is applied perpendicular to a base line (308) of the planar gradiometer.

10. The measurement system according to any one of claims 4 to 9, wherein the channels are configured to be aligned substantially parallel with the base line of the planar gradiometer.

11. The measurement system according to any one of claims 4 to 10, wherein the channels are adapted to be arranged along a conductive path of a respective pick-up loop of the planar gradiometer.

12. The measurement system according to any one of claims 4 to 11 , wherein the channels are provided in a comparator device which is separable from the sensor.

13. The measurement system according to any one of claims 4 to 12, wherein the channels are microfluidic channels.

14. The measurement system according to claim 13, wherein the microfluidic channels are manufactured in a polymer material or glass.

15. The measurement system according to any one of claims 4 to 14, wherein, each of the channels is individually configured to transfer a respective sample to the measurement locations of the sensor, wherein, when in use, the measurement system is adapted for a differential measurement between the samples.

16. The measurement system according to any one of the preceding claims, wherein one of the samples is a negative control sample.

17. The measurement system according to any one of the preceding claims, wherein the measurement system is configured for a single shot analysis of a test sample, wherein, when in use, the test sample is placed at one of the measurement locations, and a reference sample is placed at another measurement location.

18. The measurement system according to any one of claims 4 to 17, wherein when in use, the test sample is injected in a first channel to one of the measurement locations, and the reference sample is injected by a second channel to another one of the measurement locations.

19. The measurement system according to any one of the preceding claims, wherein the sensor is a superconducting sensor comprising a superconducting quantum interference device coupled to at least one pick-up loops adapted for coupling of magnetic flux to the sensor.

20. The measurement system according to claim 19, wherein the superconducting sensor comprises two pick-up loops adapted for coupling of magnetic flux to the sensor.

21. The measurement system according to any one of the preceding claims, configured to detect biomolecules using magnetic nanoparticles as detection labels, wherein the magnetic sample comprises magnetic

nanoparticles suspended in a liquid.

22. A method for analyzing a magnetic sample, the method comprising:

- providing (S102) a measurement system comprising a sensor configured to detect a magnetic signal defined by a flux difference between magnetic samples located at spatially separated measurement locations of the sensor;

- placing (S104) a test sample at one of the measurement locations and a reference sample at another one of measurement locations;

- simultaneously (S105) sensing the magnetic flux of the test sample and the reference sample;

- providing (S110) a detection signal indicative of magnetic flux differences between the samples based on the sensed magnetic fluxes.

23. The method according to claim 22, comprising:

- simultaneously applying (S106) an excitation magnetic field to the test sample and to the reference sample;

- simultaneously sensing (S108) the response of the magnetic samples caused by the excitation magnetic field, wherein the detection signal is based on the sensed responses.

24. The method according to any one of claims 22 or 23, wherein the samples are magnetic fluid samples comprising magnetic nanoparticles, wherein a first sample is a test sample and a second sample is a reference sample, the method comprising:

- determining (S112) the presence of a target analyte in the test sample based on the detection signal.

25. The method according to claim 24, comprising:

- quantifying (S114) the concentration of the target analyte based on the magnitude of the magnetic flux differences.

26. The method according to any one of claims 22 to 25, wherein the reference sample is a negative control sample.

27. Use of a measurement system according to any one of claims 1 to 21 , comprising:

- placing a test sample at one of the measurement locations and a reference sample at another one of measurement locations;

- simultaneously sensing the magnetic flux of the test sample and the reference sample; and

- providing a detection signal indicative of magnetic flux differences between the samples based on the sensed magnetic fluxes.