New technique to study ultrasmall particles in cells

The technique exploits the quantum emission properties of erbium-doped nanoparticles

Researchers from IIT Madras and IISER Kolkata have deve- loped a method to detect mi- nute quantities of chemicals in solution. They use a varia- tion of absorption spectros- copy that surpasses the sys- temic limits imposed by conventional absorption spectroscopy. With this tech- nique, they can, illuminate the insides of cells and detect minuscule quan- tities of substances present there. The work was pu- blished in Nanoscale.

Spectroscopy, the tool Absorption spectroscopy is a tool to detect the presence of elements in a medium. Light is shone on the sample, and after it passes through the sample is examined using a spectroscope. Dark lines are seen in the observed spec- trum of the light passed through the substance, which correspond to the wa- velengths of light absorbed by the intervening substance and are characteristic of the elements present in it. In usual methods, about a cubic ic centimetre of the sample is needed to do this experi- ment.In the method deve- loped here, minute amounts of dissolved substances can be detected easily.

Usually in absorption spectroscopy, the principle used is that light because of its wavelike nature, shows diffraction patterns, that is, dark and light fringes, when it scatters off any object. A related concept called the Abbe criterion sets a natural limit on the size of the object being studied.

According to this criterion, the size of the observed object has to be at least of the order of the wa- velength of the light being shone on it. “If you want to perform absorption spec- troscopy using visible light, namely, blue, green and red, the wavelengths [of these co- lours] are about 400 nm, 500 nm and 600 nm, res- pectively.

the diffraction li- mit is typically half of that, about 200 nm for the blue light,” explains Basudev Roy, from the Department of Physics of IIT Madras and one of the corresponding authors of the study along with Ayan Banerjee of IISER Kolkata.

In the method used by the researchers here, tiny, nano- sized particles that can ab- sorb light being shone on them and re-emit red, blue and green light were em- ployed. “We use a nanoparti- cle of sodium yttrium fluo- ride (a kind of glass) with some dopants, which has the special property that when you excite this with infra-red light at 975 nm, it emits blue, green and red light from the particle itself,” says Dr Roy. These particles were made by M. Gunasaeelan at department of physics, university of madras.

ultra small particle

Like a bar magnet The particles emit electric fields that are analogous to how a tiny magnet would give off magnetic lines of force – this is called a dipole, and the particle is like a tiny mobile phone’s antenna.

“Our dipole… generates an electromagnetic field de- pending upon the quantum properties of the erbium do- pants in the glass. Our emission pattern is typically limit ed to a cone of 45 degrees, starting from a diameter of the size of the particle,” he adds.

The absorption leaves a gap in the reflected light, which is what is observed and used to analyse the na- ture of the absorbing mate- rial. Since this works at the level of photons, this sur- passes the limit on the size of the substance or sample be- ing studied.

Inside living cells There are many potential ap- plications. “We are ourselves going to put these particles inside living cells, and the emission can be used as a ti- ny flash lamp to look for ab- sorption from individual molecules in close proximity to the particle,” he says. “This is way in which small molecules almost ten- millionth of a mm in diame ter can be detected while these pass the emission re- gion of the glass particle.. The future is to use it to mea- sure individual molecules, see an absorption spectros- copy of a single DNA or pro- tein molecule.”