A new technique has been developed by researchers from IIT Madras and IISER Kolkata to detect minute quantities of chemicals in solution. They used a variation of absorption spectroscopy that surpasses the systemic limits, imposed by conventional absorption spectroscopy. By doing this, they can, in principle, illuminate the insides of cells and detect minuscule quantities of substances present there.
The work was published in the scientific journal, Nanoscale, in August 2021.
Absorption Spectroscopy Technique
Absorption spectroscopy is a tool to detect the presence of elements in a medium. In this technique, 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. Thus, minute amounts of dissolved substances can be detected easily.
Experiment
In this technique, the researchers used tiny, nano-sized particles that can absorb light being shone on them and re-emit red, blue, and green light. They used a nanoparticle of sodium yttrium fluoride (a kind of glass) with some dopants, which emits blue, green, and red light from the particle itself, when it is excited with infra-red light at 975 nm (nanometre).
Findings of the Experiment
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. The particle is like a tiny mobile phone’s antenna. Experiment dipole generates an electromagnetic field depending upon the quantum properties of the erbium dopants in the glass. Emission pattern is typically limited to a cone of 45 degrees, starting from a diameter of the size of the particle. 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.
Abbe Criterion
A related concept, 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 someone wants 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.
Potential Application of the Technology
To put these particles inside living cells, the emission can be used as a tiny flash lamp to look for absorption from individual molecules in the close proximity to the particle. This is the 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 particles.
In future, this technique can be used to measure individual molecules, to see an absorption spectroscopy of a single DNA or protein molecule. It will be a potential weapon in the application of remote sensing. Infrared gas analysers with the help of this method, can be used to identify pollutants present in the air. It can also make a distinction of different pollutants and their sources like nitrogen, oxygen, water, and others.
Conventional Absorption Spectroscopy
Absorption spectroscopy is a method of elemental analysis. It is particularly useful for determining trace metals in liquids and is mostly independent of molecular form of the metal in sample.
Conventionally, about a cubic centimetre of the sample is required for this experiment. Usually, the principle used is that light shows diffraction patterns because of its wavelike nature, i.e., dark and light fringes, when it scatters off any object.
It is used to study the structures of atoms and molecules. The large number of wavelengths emitted by these systems help in investigating their structures in detail, including the electron configurations of ground and various excited states.
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