A nanobeam probe (handle showing) inserted into a cell……credit: Gary Shambat, Stanford School of Engineering
Scientists, like journalists, like to get the inside story. In the case of biologists, it amounts to an unending push to get inside the workings of living things and see what ‘really’ goes on. For example, how great it would be to monitor the changes happening inside a living cell. Unfortunately, it’s been much easier to dream than do. For one thing, most cells are very small – a matter of microns (roughly from 8 microns to about 50 microns, that is, millionths of a meter). Whatever goes into the cell must itself be even smaller, or thinner. Yet how could anything that small still do anything? If it’s too big or in any way destructive – the cell dies. Plus, there’s the added problem of guiding something very small into a cell and finding specific targets.
Historically, observation of cells – living or dead – is a matter of external observation. Essentially that means some kind of device that observes visually (optical microscope), scans with electromagnetism (electron microscope), or prepares the cell in some way to make it ‘visible’ (such as staining). Most of the techniques work, if at all, with dead cells. Living cells, especially in a dynamic (growing) state are much more difficult.
The problem is not insurmountable. With the aid of powerful microscopes and ultrathin needles, cells are penetrated as a matter of routine, for example, during an in-vitro fertilization procedure. However, that is just poking something into the cell, temporarily. It’s another thing altogether to leave something in the cell, which can monitor the inner workings.
To make this work some demanding conditions need to be met:
1. Whatever goes into the cell must be smaller, often much smaller, than the cell itself
2. Whatever goes into the cell must not be toxic or destructive to the cell
3. Once inside the cell, the device must have some way of targeting, attaching, or attracting whatever is under study
4. Once inside the cell, the device must be largely self-contained and protected from the cell environment
5. The device must have its own source of energy – battery, chemical interaction with the cell fluid, fluorescence etc.
6. The device must have some way of communicating to something outside the cell
Until about twenty years ago, this list was all but impossible. Nanotechnology changed the picture. Scientists realized almost immediately that devices built on the nanoscale were more than small enough to operate within a cell, satisfying #1 on the list above. Choosing the right nanomaterial, for example some forms of carbon (such as carbon nanotubes), could be biologically neutral. This was successfully demonstrated in many research studies. So #2 in the list was possibly tricky but possible. Then #3 on the list was a matter of coating the nanomaterial with molecules that either were drawn to or attracted the right molecules within the cell – the targeting capability.
This much of the list formed the core of a great deal of nanomedicine, especially the delivery of drugs into targeted cells. There are literally thousands of studies and ongoing research efforts in this area. Still, this is not penetrating a cell and leaving a functional monitor in place. It’s the last three items on the list, which present a formidable challenge. Only a composite nanodevice, something made of several functional elements could do the job inside a cell. Manufacturing such a device is not easy.
Enter the nanobeam, a nanoscale resonating device to embed in a living cell. Developed by a research team at Stanford University (Berkeley, CA USA) and published in the journal Nano Letters [06 February 2013, paywalled, Single-Cell Photonic Nanocavity Probes] the nanobeam resembles a steel I-beam with a series of holes in the center – only it’s a few nanometers in width and thickness and a micron or two in length.
The holes in the nanobeam are the hangout for the most important components – nanocavities containing illuminating quantum dots. This needs some explaining.
Quantum dots are one of the darlings of the most up-to-date research in many fields. They’re not easy to describe in the length of, say, a tweet. They are very small (nanoscale) crystals of various materials that have electronic properties somewhere between traditional semiconductors (such as in your computer) and individual molecules. Depending on the type and size of crystal, the quantum dot absorbs a certain amount of energy and then slowly releases it until reaching a resting state. Loading a quantum dot with energy is called ‘excitation.’ One of the things it does in releasing energy is fluoresce, glow. That’s why one of the first applications of quantum dots was in fluorescent dye.
There are plenty of other fluorescent materials, for example made with naturally fluorescing material such as phosphorus, which are routinely used in biological work. However, fluorescing quantum dots have a big advantage, controlling the size and properties of the crystal also controls the frequency and other properties of the emitted energy. This is where the Stanford nanobeam picks up the quantum dot.
Fluorescing quantum dots are placed inside the nanocavities of the nanobeam. When the nanobeam is inserted into a cell, penetrating cell walls without damaging the cell, the beam emits light, which can be detected and interpreted to get a ‘reading’ of things going on within the cell.
To test this approach, the Stanford researchers studied cells from a prostate tumor. By coating the nanobeam with the appropriate protein molecules, for example antibodies, it attracts the desired target protein and collects it on the nanobeam surface. Depending on how much of the target protein is present, determines the thickness of the coating, which in turn determines how much light energy escapes from the nanobeam. Technically, it causes a shift in the wavelength of emitted light, which a very sensitive photonic reader device can detect. The results produce an interpretation of the level of the target protein, which in turn indicates something about the progress of the tumor growth.
The variety of applications for this approach could be enormous. Conceptually, the ability to monitor chemical changes within individual cells is sort of the bedrock for biological analysis. Detecting cancer or the development of other cell-originated diseases becomes a matter of embedding nanobeams with the appropriate attractor coatings and monitoring the phase shifts of the emitted light. This is medical diagnostics at a fundamental level.
Constructing nanobeams is similar other semiconductor processes that begin with growing extremely thin layers of gallium arsenide, alternating with layers of quantum dots (light-emitting crystal). The trick discovered by the Stanford team was how to peel off the photonic nanobeams from the stiff underlying substrate on which the material is built. The nanobeams are then glued onto ultrathin fiberoptic cable for steering the probe into a cell.
The potential power of this technique is the ability of the nanobeam to reside inside the cell while the cell grows, migrates and eventually splits for reproduction. Nanobeams can even transfer from parent to child cells during mitosis (cell reproduction), allowing researchers to track the chemical composition of the cell during the process. In their experiment, the Stanford researchers were able to follow the targeted protein history of a cell for over eight days, a current record (soon to be eclipsed, no doubt).
It is hard to imagine, because the scale is so tiny, but the ability to ‘watch’ or monitor the interior workings of a living cell presents research biologists with a new and previously unattainable perspective for their work. Whether it unlocks secrets of the cell, of DNA, RNA and all the other components of cell biology remains to be seen, but this technique along with others that are in the works, may give science something it has long desired – the inside story.
