A tunable resistive pulse sensing device is a method for single particle analysis. It provides quantitative size measurements that have a range of applications in the life science and nanotechnology fields.

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Tunable resistive pulse sensing is a fast and accurate alternative to previous sizing methods including electron microscopy, ultracentrifugation, chromatography, and gel electrophoresis. Resistive pulse sensors move particles through a pore, one at a time. The particle is detected as a transient blockade of current, and the magnitude of the blockade can be measured. The magnitude of the blockade is proportional to the size of the particle.

Quantitative sizing of nanoparticles

Current methods of sizing nanoparticles include electron microscopy, static and dynamic light scattering, ultracentrifugation, chromatography, and gel electrophoresis. Quantitative resistive pulse sensing has been shown to be a viable alternative to those methods.

In resistive pulse sensing, high-throughput single particle measurements can be made as molecules pass through the pore, one at a time. The particles are detected by a change in the current as they pass through. This is called a blockade event, and the magnitude of the blockade is proportionate to the size of the particle.

Detection and controlled gating of DNA molecules

Biological nanopores allow large molecules to pass into and out of cell membranes, compartments, and vesicles. It follows that a synthetic pore could be used in a similar way to detect and control the movements of molecules such as DNA.

In one study, a pore was created through precise penetration of a polyurethane membrane with a sharpened probe. The result was a self-sealing nanometer-scale aperture. The aperture could be adjusted by stretching and relaxing the material.

The researchers were able to demonstrate controlled gating of DNA using the aperture. This suggests that methods could be developed for single particle trapping and controlled translocation through a catch-and-release mechanism.

Diagnostics and genomics

A major practical use of TRPS is in diagnostics and genomics. TRPS has been used to distinguish 220 nm organosilica nanoparticles modified with DNA from unmodified particles. As well, researchers have studied non-specific aggregation of citrate capped gold nanoparticles in the presence of a mixed base peptide nucleic acid.

DNA interactions have been studied through the use of functionalized dextran-based magnetic beads with 23 bp DNA complementary to a target. TRPS sensing has also been used to detect target DNA through induced aggregation of gold nanoparticles.

TRPS can also be used to count molecules. In a study of synthetic strand of DNA from P. Aeruginosa, strands of DNA attached to beads were quantified based on increases of baseline pulse duration. The method was more sensitive than fluorescence-based methods, with a total assay and analysis time of less than one hour.

Aptamers

Aptamers are short pieces of single-stranded DNA or RNA with specific binding affinity for a target molecule. In one study, scientists looked at aptamers through aggregation of cylindrical nanorods in the presence of the target protein. Each rod was coded with a unique pattern of gold and nickel segments. This enabled detection in the femtomolar range by superparamagnetic properties of the nickel-containing rods.

Extracellular vesicles

Extracellular vesicles are compartments surrounded by lipid bilayers that exist outside of the cell membrane. They may play a role in intercellular communication and are thought to be involved in some disease processes. In a study of leukemia cells, TRPS was used to measure the concentration and size distribution of extracellular vesicles. In addition, TRPS confirmed that the cell medium was free of extracellular vesicles.

Further Reading

• All Single Molecule Experiments Content
• History of Single Molecule Experiments
• Single molecule detection of proteins in single cells
• Novel sensor selectively screens single molecules passing through nanopores
• Tunable Resistive Pulse Sensing

Written by

Dr. Catherine Shaffer

Catherine Shaffer is a freelance science and health writer from Michigan. She has written for a wide variety of trade and consumer publications on life sciences topics, particularly in the area of drug discovery and development. She holds a Ph.D. in Biological Chemistry and began her career as a laboratory researcher before transitioning to science writing. She also writes and publishes fiction, and in her free time enjoys yoga, biking, and taking care of her pets.