Sensors for biological discovery

Project Summary

With the major goal of the CBNS being to understand the bio-nano interface and how to rationally design materials for bio-nanotechnology, a major shift in the Sensors and Diagnostics theme has been to develop sensing tools to understand the bio-nano interface direction inside cells.

This work pertains to the development of the first diagnostics for the detection of biomarkers from a single cell. There have been two major developments in this regard aimed at understanding the journey of nanoparticles into a cell and through the cell to the nucleus.

These studies are important because they provide tools for understanding how to develop nanoparticle based sensor and nanoparticle based drug delivery vehicles and how to get the nanoparticles to go to a given organelle of interest.

The first of these two studies is the development of a sensor for determining whether nanoparticles are internalised or not. This sensor is a DNA based sensor that is anchored to a nanoparticle.

This is important as classical fluorescence imaging of a fluorophore on a membrane cannot show whether the fluorescent species is on the inside or the outside of the membrane. This is naturally important as a drug delivery vehicle that is stuck to the outside of the cell will not have the same therapeutic impact as one inside the cell.

The way the sensor works is that attached to the nanoparticles for internalisation are single strands of DNA that possess a fluorophore on them. The cell is then exposed to a complementary sequence of DNA which can bind to the single strand of DNA if it is outside the cell. If it is inside the cell, then no binding occurs.

The presence of binding is determined from the complementary sequence possessing a quencher. Hence nanoparticles outside the cell are not fluorescent and nanoparticles inside the cell are. In this way the impact of particle design to promote internalisation by cells can be monitored.

In a second study a new microscope method was exploited to understand how the shape of nanoparticles influenced their ability to transit through cells. The new microscope method is called pair-correlation microscopy and also relies on the design of nanoparticles that have fluorescent tags.

The main advance here is that pair-correlation microscopy is a form of fluctuation analysis that permits quantification of how many nanoparticles are in the cell, and how many are moving and their rate of movement. The method has the sensitivity to detect a change of a single nanoparticle within a volume yet tracks many millions of nanoparticle so gives statistically relevant data.

The method uses standard confocal microscopy and looks at the fluctuations in fluorescence intensity at one pixel in the cell relative to another pixel. The correlation in fluctuations between this pair of pixels allows calculation of the speed of nanoparticle movement.

If the two pixels are on either side of a barrier, such as the nuclear envelope, then the speed different particles go across the barrier can be determined. CBNS researchers found nanoparticles of different shapes but the same surface chemistry delivered different results; rod shaped nanoparticles were more effective at killing cancer cells than spherical shaped ones because they managed to travel into the nucleus before releasing their drug payload. Such information is vital to understand how to develop more effective drug delivery vehicles.