Apparatus for detecting particles

Abstract

Apparatus for detecting particles in a liquid, comprises an evanescent optical sensor having an electrically conducting layer, and means associated therewith for providing an electric field at or adjacent the sensor surface. Preferably, the optical sensor is a surface bound sensor based on a metal-clad leaky waveguide structure. A DC potential applied to the metal layer and a parallel counter electrode provides a uniform electric field, which drives deposition of the particles on the sensor surface. Alternatively, an alternating potential applied to a electrically conducting layer configured as an electrode and counter electrode induces an electro-osmotic bulk flow in the liquid which drives deposition of the particles on the sensor surface.

Description

The present invention is generally concerned with apparatus for enhanced detection of particles using optical sensors. The invention is particularly, but not exclusively, directed towards enhanced deposition of particles, such as bacteria and viruses, on surface bound optical sensors.

As used herein the term “surface bound sensor” describes an optical sensor including a sensing medium comprising a biological material capable of binding the particles to be detected—such as an antibody or lectin.

Optical sensors often comprise waveguide structures in which an evanescent wave, associated with an optical mode existing in the structure, extends into the sensing layer. A change in the refractive index of the sample by interaction with a particle leads to a change in an optical property of the mode, which can be readily detected.

One such optical sensor uses the phenomenon of surface plasmon resonance (SPR). Light incident a dielectric prism having an upper surface coated with a thin metal layer of gold or silver and a sensing layer comprising the biological sample, is coupled at a certain “resonant” angle or angles to oscillations of the electron cloud in the metal layer. A surface optical mode is propagated at the interface of the prism and metal layer and a drop in the amount of reflected light is recorded at a detector. The surface optical mode generates an evanescent field that extends into the sensing medium. A particle binding to the sample leads to a change in the refractive index of the medium affecting the surface mode and the angle at which resonance is excited.

The basic structure of “leaky” waveguide sensors, described in International Patent Application WO 99/44042, is similar but offers improved sensitivity in that an optical mode is supported in the bulk of the sensing layer.

International patent application WO 01/42768A describes flow cell apparatus in which detection of surface bound particles on an SPR sensor is improved by monitoring light scattered or emitted there from.

Improved detection is also obtained using metal-clad or dye-clad leaky waveguide sensors described in our co-pending international patent application PCT/GB02/045045 incorporated by reference herein. Here, an evanescent wave extends above the sensing surface and can be “tailored”, by choice of refractive indices in the stucture, to optimise overlap with the particles to be detected.

However, the sensitivity of flow cell apparatus incorporating these optical sensors is inherently limited by slow diffusion of the particles in the liquid to the sensor surface, even with stirring or agitation. Consequently, there is still a need to improve the detection of particles at surface bound sensors.

The present invention generally aims to improve detection at optical sensors, and in particular, surface bound optical sensors by providing an electric field acting on the particles so as to direct them to the sensor surface.

Under suitable fluid flow conditions, the electric field may act on charged particles or on an induced dipole on the particles to enhance the deposition of the particles to the sensor surface.

Accordingly, in one aspect, the present invention provides apparatus for detecting particles in a liquid, comprising an evanescent optical sensor, including an electrically conducting layer, and means associated therewith for providing an electric field at or adjacent the sensor surface.

It will be apparent, that the electrically conducting layer can in itself comprise an electrode. In a preferred embodiment of the present invention, therefore, the means providing an electric field comprise a plane surface counter electrode arranged in parallel with the conducting layer, a DC potential source and means for applying the potential across the electrode and the conducting layer. In this embodiment, the conducting layer and the plane surface electrode together define a gap electrode configuration in which the sensing layer partially extends.

It will be understood that the DC potential provides a uniform electric field capable of charging the particles and directing them to the sensor surface (electrophoresis). The magnitude of the DC potential, the extent of the gap between the plane surface electrode and the conducting layer, the flow rate of the liquid containing the particles across the sensor surface will all be determined having regard to each other and to the particle size. It will be appreciated that the force acting on the particles should be sufficient to overcome flow and drag effects.

In a preferred embodiment of the present invention, the optical sensor comprises a surface bound sensor such as the MCLW sensor mentioned above and described in our co-pending international patent application PCT/GB02/045045. Suitable electrically conducting layers comprise metals, in particular, aluminium, tantalum, zirconium, titanium or chromium, or crystalline dye materials.

Suitable applied potentials for the detection of bacillus subtilis var. Niger (bacillus globbiggi, BG) spores according to this embodiment range from 10 to 100 V, typically about 30 V for a gap size ranging from 20-50 μm and flow rates ranging from 50 to 300 μl min⁻¹.

The plane surface counter electrode may comprise a conducting, metal or metal oxide layer arranged on an inert substrate. Preferably, the counter electrode comprises a layer of indium tin oxide (ITO) arranged on a glass substrate since it is clear and known to resist optical degradation on prolonged polarisation by application of an alternating potential.

In another embodiment of the present invention, the electrically conducting layer of the optical sensor is itself configured, as described in our co-pending GB patent application No. 0303305.7—incorporated by reference herein, so as to provide a planar electrode and planar counter electrode.

In this embodiment, the means providing an electric field at or adjacent the sensor surface comprise an alternating potential. The alternating potential when applied at a predetermined frequency and magnitude to the layer induces a non-uniform electric field that can induce an electro-osmotic flow in the bulk of the liquid so as to focus particles onto the sensor surface (abnormal dielectrophoresis).

It will be understood that, in some embodiments of the present invention, the fluid layer in contact with the upper surface of the sensor comprises a sensing layer. The fluid layer, which is semi-infinite, will contain the particles to be detected. However, a surface bound sensor is preferred, particularly where it comprises a sensing layer of an antibody or lectin.

In a second aspect, the present invention provides a method for detecting particles in a liquid comprising i) introducing the liquid to an evanescent optical sensor, including an electrically conducting layer, having means associated therewith for providing an electric field at or adjacent the sensor surface and ii) generating the electric field.

In a preferred embodiment, the means providing an electric field comprise a plane surface counter electrode arranged in parallel with the conducting layer, a DC potential source and means for applying the potential across the electrode and the conducting layer. In this embodiment, the conducting layer and the plane surface electrode together define a gap electrode configuration in which the sensing layer partially extends.

The magnitude of the DC potential, the extent of the gap and the flow rate of the liquid containing the particles across the sensor surface will all be determined having regard to each other and to the force necessary for the particles to overcome flow and drag effects.

In a particularly preferred embodiment, the optical sensor comprises a surface bound optical sensor such as the MCLW sensor described in our co-pending international patent application PCT/GB02/045045.

The DC voltage may be applied to one or both of the electrodes. Preferably, however, the counter electrode is earthed. Suitable applied potentials for the detection of bacillus subtilis var. Niger (bacillus globbiggi BG) spores according to this embodiment from 10 to 100 V, typically about 30 V for a gap size ranging from 20-50 μm and flow rates ranging from 50 to 300 μl min⁻¹.

Other embodiments in this aspect of the present invention will be apparent from the foregoing description. In addition, it will be apparent that a combination of uniform and non-uniform fields (electrophoresis and dielectrophoresis) maybe used where it is desired to manipulate different particles.

The present invention will now be described by reference to several embodiments and the following examples and drawings in which

FIG. 1 is a schematic illustration of one embodiment of the present invention;

FIGS. 2a) to c) are photographs showing electrophoretic enhancement for the collection of bacillus globiggi BG spores on the embodiment of FIG. 1;

FIG. 3 is a graph showing the enhancement at various concentrations of spores;

FIG. 4 is a graph showing the capture of spores on a preferred embodiment of the present invention; and

FIGS. 5a) and b) show the scattering image and fluorescence image of the spores captured on the preferred embodiment.

Having regard now to FIG. 1, a basic MCLW chip 11 comprises an upper surface of a 300 nm silica sol layer 12 (n=1.43) provided on a thin layer 13 (8.5 nm) of titanium coating a 1 mm glass substrate layer 14 (n=1.5). The thickness and refractive index of the silica sol layer 12 is chosen to support a single sharp-guided optical mode at a wavelength of incident light of 685 nm or 488 nm and to optimise the extent of the evanescent field above the surface to about 1.5 to 2.0 μm.

As mentioned previously, a final sensing layer (not shown) may comprise a liquid layer containing the particles to be analysed. Alternatively or additionally the sensing layer can comprise an antibody layer deposited on the silica sol layer.

A counter electrode comprises a glass substrate 15 coated with a conducting layer 16 of ITO. The counter electrode is joined to the chip 11 by a 30 μm (gap size) double-sided adhesive tape 17 through which a 12 mm by 2 mm section, defining a flow cell 18, has been cut. Two 1.5 mm holes drilled in the counter electrode provide inlet and outlet means for the flow of liquid through a delivery and collection tube 19 to the cell 18.

The titanium layer 13 of the sensor chip is provided with a silver loaded epoxy contact (not shown) at one end of the chip 11. The ITO layer 16 of the counter electrode is provided with similar contacts (not shown) at positions along its length so as to reduce voltage gradient due to the resistance of the metal and ITO layers. The contacts connect the sensor chip and counter electrode to a DC potential source.

The assembly is used in conjunction with an equilateral, coupling prism (not shown 30 mm) of BK 7 glass and refractive index 1.510. The interrogation of particle deposition at the upper surface of the chip may be conducted using the basic arrangement including an optical source and detection means described in our co-pending international patent application PCT/GB02/045045.

EXAMPLE 1

The sensor surface was blocked by exposure to 0.1% w/v BSA in PBS/Tween® 20 and stored overnight at 4° C. BG spores (4.7×10⁷ spores/ml) in 50 mM histidine buffer were introduced to the flow cell using a MINIPULS-3, MP4 peristaltic pump (Gilson, Canada) at 50 μl min⁻¹. A positive potential of 30 V was applied to the metal sensor layer with respect to the counter electrode for a total of 2 min. After a further period, a negative potential of 1.5 V was applied to the metal sensor layer with respect to the counter electrode.

FIG. 2 shows the distribution of spores before (a) and on (b) applying the positive potential. As may be seen, the number of cells incident the sensor surface is greatly increased during the application of the potential. The application of the negative potential (c) highlights negligible non-specific adsorption of the spores to the chip surface.

These effects were repeated for BG spore concentrations ranging from 10³ to 10⁶ spores/ml. An increase in the electrical conductivity of the buffer (to 150 mM NaCl), however, greatly decreased the number of cells incident the sensor surface—presumably through ionic screening and restriction of the electric field to the double charge layer adjacent the electrodes.

EXAMPLE 2

A direct immunoassay utilised two DC potential application steps. A biotinylated-labelled anti BG capture antibody was deposited on the chip by repeated introduction of 50 μg/ml suspension in water to the flow cell (200 μl/min) and applying a positive potential of 20 V. Unbound antibody was removed by washing with 50 mM histidine buffer. A BG spore solution (4×10⁷ spores in 50 mM histidine buffer) was introduced to the flow cell at a flow rate of 50 μl/min. A positive potential of 30 V was applied to the metal sensor layer with respect to the counter electrode during 2 min. After a short period, a negative potential (1.5 V) was applied to the metal sensor layer with respect to the counter electrode. The assay was visualised in real time by detection of scattered light from captured BG spores using a CCD camera. These effects were repeated for BG spore concentrations ranging from 10³ to 10⁶ spores/ml (FIG. 3).

FIG. 4 shows the number of cells captured by subtraction of the number of spores removed from the sensor surface on application of the negative potential from the number of spores. The values are in good agreement with the number of captured spores remaining on the surface. Comparison of the number of captured spores with the number of spores captured using the MCLW chip alone revealed a 30 fold enhancement equal to an increase in local concentration from 10³ to 10⁸ spores.

EXAMPLE 3

A sandwich format immunoassay utilised three DC potential application steps. A biotinylated-labelled anti BG capture antibody was deposited on the chip and BG spores were captured on the antibody layer as previously described. Finally a suspension of CY5-labelled BG antibody (10 μg ml⁻¹ in 50 mM histidine buffer) was introduced to the cell at a flow rate of 50 μl min⁻¹ over 2 min. A positive potential of 30 V was applied to the metal sensor layer with respect to the counter electrode during 2 min. Unbound CY5-labelled antibody was washed from the flow cell by purging with 50 nM histidine buffer for 3 min at 200 μl min⁻¹.

FIG. 5 shows the scattering image of captured BG spores and the fluorescence image following exposure to CY5 labelled antibody. Although not shown it is found that the fluorescence emitted from the labelled antibody is greater when attached to the captured spores than when the labelled antibody alone is deposited at the sensor surface.

Repetition of the experiment with varying concentration of labelled antibody (3, 10, 20 μg ml⁻¹) showed that the optimum for detection of captured spores by fluorescence using this method was 10 μg ml⁻¹.

Claims

1. Apparatus for detecting particles in a liquid, comprising an evanescent optical sensor having an electrically conducting layer, and means associated therewith for providing an electric field at or adjacent the sensor surface.

2. Apparatus according to claim 1, in which the optical sensor comprises a surface plasmon resonance chip.

3. Apparatus according to claim 1, in which the optical sensor comprises a metal-clad leaky waveguide chip.

4. Apparatus according to claim 3, in which the means for providing an electric field comprise a plane surface counter electrode and a DC potential source.

5. Apparatus according to claim 4, in which the counter electrode is arranged parallel to the conducting layer.

6. Apparatus according to claim 4, in which the counter electrode and sensor define a gap there between ranging from 20 to 50 μm.

7. Apparatus according to claim 4, in which the counter electrode comprises a layer of indium tin oxide provided on a glass substrate.

8. Apparatus according to claim 3, in which the electrically conducting layer comprises titanium.

9. Apparatus according to claim 1, in which the optical sensor is a surface bound sensor.

10. A method for detecting particles in a liquid comprising i) introducing the liquid to an evanescent optical sensor, including an electrically conducting layer, having means associated therewith for providing an electric field at or adjacent the sensor surface and ii) generating the electric field.

11. A method according to claim 10, in which the optical sensor comprises a surface plasmon resonance chip.

12. A method according to claim 10, in which the optical sensor comprises a metal-clad leaky waveguide chip.

13. A method according to claim 12, in which the generated electric field is uniform.

14. A method according to claim 13, in which a DC potential is applied to a gap electrode configuration comprised by the sensor and a parallel planar surface counter electrode at magnitude ranging from 10 to 100 V for a gap size ranging from 20 to 50 μm and a flow rate ranging from 50 to 300 μl min−1.

15. A method according to claim 10, in which the optical sensor is a surface bound sensor.

16. (canceled)

17. (canceled)