IIT Madras team produces white light using pomegranate, turmeric extract

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Dr. Vikram Singh produced white light by irradiating the pomegranate extract and carbon nanoparticles made from pomegranate extract with UV light.

Dr. Vikram Singh, former research scholar in the Department of Chemistry, IIT Madras won the BIRAC Gandhian Young Technological Innovation (GYTI) Award 2017 for his work on producing white light emission using natural extracts.

Dr. Singh and Prof. Ashok Mishra from the Department of Chemistry, IIT Madras used a mixture of two natural extracts — red pomegranate and turmeric — to produce white light emission. The researchers used a simple and environment-friendly procedure to extract dyes from pomegranate and turmeric.

While polyphenols and anthocyanins present in red pomegranate emit at blue and orange-red regions of the wavelength respectively, curcumin from turmeric emits at the green region of the wavelength. White light emission is produced when red, blue and green mix together. This is probably the first time white light emission has been generated using low-cost, edible natural dyes. The results were published in the journal Scientific Reports.

“We had to mix the two extracts in a particular ratio to get white light,” says Dr. Singh, the first author of the paper; he is currently at Lucknow’s CSIR-Central Drug Research Institute (CDRI). By changing the concentration of the two extracts the researchers were able to get different colour temperatures (tunability).

WLE in Gelatin Gel-Optimized-1

White light produced by irradiating UV at 380 nm.

“When we mix the two extracts and irradiate it with UV radiation at 380 nm, we observed energy transfer (FRET mechanism) taking place from polyphenols to curcumin to anthocyanins, which helps to get perfect white light emission,” says Dr. Singh. For FRET mechanism to take place there must be spectral overlap between a donor and an acceptor.

In this case, there is a perfect overlap of emission of polyphenols with absorption by curcumin so the energy from polyphenols is transferred to curcumin. Since there is also a perfect overlap of emission of curcumin with absorption by anthocyanin, the energy of curcumin is transferred to anthocyanin.

As a result of this energy transfer from one dye to the other, when the extract is irradiated with UV light at 380 nm (blue region of the wavelength), the polyphenols emit in the blue region of the wavelength and transfers its energy to curcumin. The excited curcumin emits in the green region of the wavelength and transfers its energy to anthocyanin, which emits light in the red region of the wavelength.

“Because of the energy transfer, even if you excite in the blue wavelength we were able to get appropriate intensity distribution across the visual wavelength,” says Prof. Mishra, who is the corresponding author of the paper.

White light sans turmeric  

Taking the work further, the duo produced carbon nanoparticles using pomegranate and to their surprise it was producing fairly green emission. So instead of using turmeric to get green wavelength, the researchers used carbon nanoparticles made from pomegranate extract. “We could get white emission, though it is not as white as when we used turmeric. It’s slightly bluish but well within the white zone,” says Prof. Mishra. “It is attractive to use a single source to create white light emission.”

The principle by which the pomegranate extract and carbon nanoparticles made from the extract is the same as in the case when pomegranate and turmeric extracts were used. The results were published in the Journal of Materials Chemistry C.

Though this natural mixture of dye can be used in a wide variety of applications such as tunable laser, LEDs, white light display, much work needs to be done in terms of photostability and chemical stability before it becomes ready for translation. Biosystems have an inherent tendency to breakdown and so this has to be addressed.

Published in The Hindu on May 6, 2017

IISER Pune researchers turn insulating MOFs into semiconductors

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(From left) Dr. Nirmalya Ballav, Vikash Kumar and Barun Dhara of the Indian Institute of Science Education and Research team that achieved the feat.

After four years of intensive screening, researchers at Pune’s Indian Institute of Science Education and Research (IISER) have transformed insulating metal-organic frameworks (MOFs), which are generally used for gas storage and solvent separation, into semiconductor MOFs by incorporating polymers.

A team led by Dr. Nirmalya Ballav from the Department of Chemistry at IISER Pune has converted a cadmium-based MOF insulator into a semiconductor at room temperature through nanochemistry. The electrical conductivity increased nine-fold (a billion-fold increase) when chains of conducting polymers were introduced into the nanochannels of MOFs. The results were published recently in The Journal of Physical Chemistry Letters.

Initially, the pores of metal-organic frameworks are loaded with pyrrole monomers, which are not electrical conductors. The addition of iodine brings about an oxidation reaction and converts the monomers into polymers. Unlike monomers, polymers are electrically conducting in nature and this helps turn the metal-organic framework into a semiconductor.

The weak interaction between the MOF and the conducting polymer is the key behind the unusual increase in conductivity.“The size of the pyrrole monomer nearly matches the dimension of nanochannels of the metal-organic framework. So no branch polymer was formed but only a single-chain (linear) polymerisation took place,” says Dr. Ballav. “Branch polymers are generally less electrically conducting in nature than single-chain polymers.”

The amount of polymer loaded inside the one-dimensional MOF pores was only about 10 per cent. Though electrical conductivity may increase if more polymer is packed inside the pores, the restricted diffusion in the pore nanospace does not allow more polymers to be loaded.

The MOF continued to retain its fluorescence even after becoming electrically conducting. “If you bring about electrical conductivity in a fluorescent MOF the fluorescence is expected to vanish. But it was not so in our case,” he says. “It indicates the weak, non-covalent interaction between the conducting polymer and the MOF. The weak interaction was sufficient enough for the electrons to flow across the material and is the key behind the unusual increase in conductivity.”

“The unusual enhancement of electrical conductivity of the MOF was due to the presence of conducting polymer and the electronic interaction between the MOF and the polymer,” says Barun Dhara of IISER Pune and the first author of the paper.

“Conductivity and fluorescence is a rare combination that could provide a route towards multifunctonal MOFs suitable for optoelectronics, including solar cells and imaging devices,” says a news item in Chemistry World.

Though the researchers have been able to produce a nine-fold increase in the electrical conductivity of the MOF-nanocomposite, it is still far less than silicon. “But our work shows promise that organic materials can be used in electronic industry where silicon is primarily used. It will be an economic approach for the development of future electronic applications,” says Dr. Ballav.

By using a right combination of MOFs and conducting polymer, Dr. Ballav is confident of designing nanocomposites for specific purposes. The team is working on other monomers such as aniline and thiophene.

The hybrid nanocomposite can be used for fabricating electronic devices for gas sensing applications and for making electrochemical devices such as super capacitors, he says.

Published in The Hindu on November 1, 2016.