How optical microscopes allow detailed investigations of nanoparticles for biosensing
Monday 08 May 2023
It sounds like trying to scan a record with a hammer: Light is actually too "coarse" to image small particles on the nanometer scale. However, in their project 'Supercol'- funded by the European Union - scientists want to achieve just that: The investigation of nanoparticles with light. To make this possible, they are combining Nobel Prize-winning methods with modern computer algorithms. The goal: the development of novel nanoparticles for biomedical applications such as biosensing, drug delivery and cancer therapy. The results of the SuperCol project will be presented at Spring Meeting of the European Materials Research Society on June 1st in Strasbourg.
Using a combination of super-resolution microscopy and electron microscopy, scientists can now determine the position of molecules on the surface of nanoparticles much more precisely. In the future, this could enable new biomedical applications.
Nanoparticles - i.e. small particles with a size in the range of a few tens to hundreds of billionths of a meter - are a wide-ranging field of research. For example, they make the latest biomedical applications possible by acting as a kind of container to transport active ingredients to their target. Ideally, their surfaces are 'functionalized' - i.e. provided with a molecular puzzle piece, which allows them to dock only to desired target cells in the body.
However, studying such particles and the molecules on their surface is difficult: light is basically too 'coarse' to image such particles in a normal light microscope. Visible light in the range from UV to infrared can at most resolve particles with a size of 200 nanometers. Too large to determine where, for example, a molecular puzzle piece sits on its surface or to determine their number.
On to higher resolution
Therefore, the researchers use a method that won the 2014 Nobel Prize in Chemistry: In what is known as 'super-resolution microscopy', small fluorescent molecules - called fluorophores - are used and, in the case of nanoparticles, attached to molecules on its surface. These fluorophores have the property of blinking statistically in a microscope. The position of this blinking signal can be detected much more accurately than would be possible with conventional optical microscopy.
*So a lot of what we do in the lab is imaging fluorophores blinking on our nanoparticles", Rodolphe Marie, Associate Professor and Group Leader at DTU Health Tech, explains. "Because each time a fluorophore blinks on its own, we can in principle find its position with a high precision. Which would be impossible when all molecules are emitting light at the same time."
Using computer power to get to the truth
However, the image of the nanoparticle obtained in this way is only half the truth: Nanoparticles have properties that can distort this image - for example resonance phenomena that bring also part of the nanoparticle to glow, and not only the fluorophore. The scientists therefore imaged nanoparticles using both electron microscopy and super-resolution optical microscopy. While electron microscopy provides the 'true' position of the docked molecule, physical effects in the light microscope lead to a shift. Software now correlates both images - and can thus predict the true position based on the light microscope image.
"In a way the nanoparticle acts as a lens. Even when the nanoparticle is not transparent, the lensing effect allow us to see a blinking fluorophore even when it is behind the particle," Rodolphe Marie says. "Since we know the nanoparticle distorts our image, we might as well predict how it distorts it and use this to our advantage in finding the exact location of the blinking molecule on the nanoparticle."
The researchers now hope to use their method to study nanoparticles in the light microscope, which delivers faster results and does not destroy the particles. This will allow nanoparticles to be studied more precisely and comprehensively in the future, leading to new biomedical applications. One of these applications is biosensing, a technology that has become vitally important in the past years during the pandemic. New biosensors are urgently needed for a wide range of diseases, as is explained in the video below.
"We are so lucky that the team at DTU has physicists and chemists that can connect theoretical modeling to real applications of the health technology industry. The PhD students affiliated with DTU Health Tech are Teun A.P.M. Huijben, Masih Fahim and Shanil D. Gandhi", Rodolphe Marie finishes.