Researchers from IIT Madras and IISER Kolkata have developed a method to detect minute quantities of chemicals in solution. They use a variation of absorption spectroscopy that surpasses the systemic limits imposed by conventional absorption spectroscopy. With this technique, they can, in principle, illuminate the insides of cells and detect minuscule quantities of substances present there. The work was published 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 spectrum of the light passed through the substance, which correspond to the wavelengths of light absorbed by the intervening substance and are characteristic of the elements present in it. In usual methods, about a cubic centimetre of the sample is needed to do this experiment.In the method developed 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 wavelength of the light being shone on it. “If you want to perform absorption spectroscopy using visible light, namely, blue, green and red, the wavelengths [of these colours] are about 400 nm, 500 nm and 600 nm, respectively. the diffraction limit 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 absorb light being shone on them and re-emit red, blue and green light were employed. “We use a nanoparticle of sodium yttrium fluoride (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. Gunaseelan at Department of Physics, University of Madras.
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 depending upon the quantum properties of the erbium dopants in the glass. Our emission pattern is typically limited 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 nature of the absorbing material. Since this works at the level of photons, this surpasses the limit on the size of the substance or sample being studied.
Inside living cells
There are many potential applications. “We are ourselves going to put these particles inside living cells, and the emission can be used as a tiny flash lamp to look for absorption from individual molecules in the close proximity to the particle,” he says. “This is way in which small molecules almost ten-millionth of a mm in diameter can be detected while these pass the emission region of the glass particle… The future is to use it to measure individual molecules, see an absorption spectroscopy of a single DNA or protein molecule.”
