METHOD AND APPARATUS FOR MEASURING CHARGE AND SIZE OF SINGLE OBJECTS IN A FLUID

• US 20150081228A1
• Filed: 03/28/2013
• Published: 03/19/2015
• Est. Priority Date: 04/03/2012
• Status: Active Grant

• Current Assignee: Madhavi Krishnan
• Original Assignee: Universität Zürich
• Inventors: Krishnan, Madhavi
• Granted Patent: US 9,958,374 B2
• Time in Patent Office: Days
• US Class Current: 702/29
• CPC Class Codes: B82Y 35/00 Methods or apparatus for measurement or analysis of nanostructures
• CPC Class Codes: G01N 15/0211 Investigating a scatter or diffraction pattern
• CPC Class Codes: G01N 15/0266 with electrical classification
• CPC Class Codes: G01N 15/10 Investigating individual particles
• CPC Class Codes: G01N 15/1031 by measuring electrical or magnetic effects
• CPC Class Codes: G01N 15/1433 using image recognition
• CPC Class Codes: G01N 2015/0038 Investigating nanoparticles
• CPC Class Codes: G01N 2015/1027 Determining speed or velocity of a particle
• CPC Class Codes: G01N 2015/1029 Particle size
• CPC Class Codes: G01N 25/00 Investigating or analyzing materials by the use of thermal means G01N3/00 - G01N23/00 take precedence

Abstract

In a method for determining charge and/or size of an object (15) suspended in a fluid, the object (15) is introduced, together with the fluid, into an electrostatic trap (1) defining an electrostatic confining potential. The thermal motion of the object (15) in the fluid is observed under influence of the confining potential, and charge and/or size are determined from the observed thermal motion. In particular, the viscous drag on the object yields a measure of its size, while the stiffness of its confinement can be compared with a potential model to reveal the total charge it carries. Also disclosed are an apparatus and software for carrying out the method.

Claims

1-18. -18. (canceled)

19. A method for determining at least one of charge and size of an object suspended in a fluid, the method comprising:

introducing the object, together with the fluid, into an electrostatic trap defining an electrostatic confining potential (Δ

U);

observing thermal motion of the object in the fluid under influence of the confining potential (Δ

U); and

determining at least one of charge and size from the observed thermal motion.

20. The method of claim 19, comprising:

determining at least one characteristic of the confining potential (Δ

U) from the observed thermal motion; and

comparing the determined characteristic with a potential model to determine at least one of charge and size of the object.

21. The method of claim 20, wherein the characteristic of the confining potential (Δ

U) is determined by measuring at least one characteristic of a probability density distribution (P(r)) of object displacement under influence of the confining potential and deriving the characteristic of the confining potential from said characteristic of the probability density distribution (P(r)).

22. The method of claim 20, wherein the characteristic of the confining potential is a stiffness parameter (k) of the confining potential (Δ

U).

23. The method of claim 22, wherein determining the stiffness parameter comprises:

imaging the thermal motion of the object with at least two different exposure times (σ

);

determining, for the at least two different exposure times (σ

), a width parameter of the apparent probability density distribution;

deriving a relaxation time (τ

) of the object from a comparison of the width parameters at the different exposure times (σ

); and

calculating the stiffness parameter (k) from the relaxation time (τ

).

24. The method of claim 23, further comprising:

determining a drag parameter (γ

) for the object,

wherein the stiffness parameter (k) is calculated from the relaxation time (τ

) using the drag parameter (γ

).

25. The method of claim 24, wherein the drag parameter (γ

) is determined from an observation of short-term diffusion behavior of the object.

26. The method of claim 23, wherein both the relaxation time (τ

) and a drag parameter (γ

) are derived from a comparison of the width parameters at the different exposure times (σ

).

27. The method of claim 20, further comprising:

obtaining the potential model by carrying out a calculation of system free energy.

28. The method of claim 19, comprising:

determining a drag parameter (γ

) of the object within the confining potential; and

calculating the size of the object from the drag parameter (γ

).

29. The method of claim 19, wherein the electrostatic trap comprises two substrates arranged in an opposing configuration so as to form a fluidic slit, the substrates having non-zero surface charge densities inducing an electrostatic potential between the substrates, at least one of the substrates having a surface nanostructure for modulating the electrostatic potential between the substrates so as to generate said electrostatic confining potential.

30. The method of claim 19, wherein a plurality of objects are observed in parallel in an array of traps, and wherein at least one of size and charge is determined for each one of said plurality of objects.

31. An apparatus for determining at least one of size and charge of an object suspended in a fluid, the apparatus comprising:

an electrostatic trap for receiving the object together with the fluid and for trapping the object in an electrostatic confining potential;

an imaging device for observing thermal motion of the object under influence of the confining potential; and

an analyzing device configured to receive information about the thermal motion from the imaging device and to determine at least one of size and charge of the object from the observed thermal motion.

32. The apparatus of claim 31, wherein the analyzing device is configured to determine at least one characteristic of the confining potential (Δ

U) from the observed thermal motion and to compare the determined characteristic with a potential model to determine the charge of the object.

33. The apparatus of claim 31, comprising an array of electrostatic traps for receiving a plurality of objects in parallel, wherein the analyzing device is configured to determine at least one of size and charge of each one of the plurality of objects.

34. The apparatus of claims 31, wherein the electrostatic trap comprises two substrates arranged in an opposing configuration so as to form a fluidic slit, the substrates having non-zero surface charge densities inducing an electrostatic potential between the substrates, at least one of the substrates having a surface nanostructure for modulating the electrostatic potential between the substrates so as to generate said electrostatic confining potential.

35. A computer program product comprising computer program code which is configured to carry out the following steps when executed on a computer:

receiving information from an imaging device about thermal motion of an object suspended in a fluid under influence of a confining potential in an electrostatic trap; and

determining at least one of charge and size of the object from the received information.

36. The computer program product of claim 35, wherein the computer program code is configured to carry out the following steps when executed on a computer:

determining at least one characteristic of the confining potential (Δ

U) from the received information; and

comparing the determined characteristic with a model potential to determine the charge of the object.