20 June 2018
Nanoparticles Sense Ionic Mercury
Fluorescence “On-Off” tools
Mercury, in its water-soluble form, is detrimental to several proteins and enzymes in the body. Though poisonous, mercury is used as salts and organomercury compounds in batteries, thermometers, barometers, skin lightening cosmetics, and as amalgams in dental applications. Effluents from these industries contain mercury compounds which are detrimental to life. So, sensing mercury salts in an aqueous medium is an important first step to design a removal strategy and to avoid its hazardous effects.
There are some techniques to sense mercuric ions in trace amounts. Fluorescent dyes, gold and silver nanoparticles, as well as polymers can be used as sensors. However, the functioning of these sensors in aqueous media is a challenge. Navneet Kaur and team from the Panjab University teamed up with Ashok Ganguli from the IIT Delhi to develop an efficient and reliable sensor for ionic mercury in aqueous medium.
Mercury is a soft acid. Sulphur and oxygen act as soft bases. The researchers leveraged on this principle. They synthesised four organic molecules of the urea/thio-urea framework bearing naphthyl and phenol groups. Two of these were dipodal and and two were tetrapodal frameworks with pseudocavity.
Then they fabricated nanoparticles from these molecules using a reprecipitation method. When tested, one of the organic nanoparticle showed enhanced fluorescence intensity when exposed to light of wavelength 390 nm in the presence of mercuric ions. This effect was specific to mercuric ions and not to other metal ions tested.
“Before binding with mercuric ions, when exposed to light, electrons are transferred from the highest occupied molecular orbital of the free receptor to the highest occupied molecular orbital of the excited fluorophore in the organic nanoparticle. But when the mercuric ion binds to the receptor, this photoinduced electron transfer diminishes”, explains Navneet Kaur, Panjab University.
“Organic nanoparticles, with urea or thiourea anchors, are used to efficiently recognize mercuric ions. The emission signatures of organic nanoparticles in the presence and absence of mercuric ions are different and, therefore, a clear “on-off” modus operandi works here”, adds Ashok Ganguli, IIT Delhi.
The scientists are hoping to integrate this concept into a prototype that can be used for field investigations into mercuric ion pollution.
J Mol Struct., 1161: 34-43 (2018)
6 November 2017
E-nose Sniffs Shelf Life
The shelf life – the duration for which a product can be stored and remains fit for use – is an important criterion especially for food items and medicines. To determine shelf life, there are various instrumental and manual methods. However, it is challenging to determine the shelf life of an aromatic product such as flavored cookies because flavour is a perception based assessment. Hence, reproducibility of results is poor.
Paramita Bhattacharjee from the Jadavpur University and collaborators from the Centre for Development of Advanced Computing, Kolkata took up the challenge. Bakery products degrade fast due to oxidation of rich fat content resulting in problem of rancidity. The team felt that adding black pepper and cardamom extracts, known for their antioxidant properties, may be useful to increase the shelf life of the cookies, besides making them flavoursome.
To test this, they have made five groups of designer cookies containing nutraceutical rich black pepper and cardamom extracts. The first two groups contained extract of black paper and small cardamom individually, while the other two groups contained extracted matrix of black pepper and cardamom individually. The fifth was the control group.
The scientists then compared the five groups of cookies. Rancidity was used as parameter to assess the shelf life of the cookies. The researchers used conventional biochemical tests and a state-of-art e-nose technology to evaluate the cookies.
The artificial olfactory system, the e-nose, developed by C-DAC, has eight metal oxide semiconductor gas sensors, capable of responding to a wide range of volatile organic compounds. A minicompressor pushes the volatiles through solenoid valves over the sensors and a small blower removes the volatiles.
With this system, the scientists could quantify the difference in the aroma of the cookies due to rancidity. The team observed that the shelf life of the cookies with black pepper and small cardamom extracts increased to 180 and 120 days respectively compared to 80 days for the control cookies. With statistical analysis, they found that the results of conventional perceptual tests and e-nose rancidity assessment were equivalent.
Based on their findings, the scientists claim that e-sniffing is a better, faster and more quantifiable way to assess the shelf life of cookies. This will also help consumers ask for an e-nose based quick look at the shelf life of sensitive consumer products. These findings provide technocrats an opportunity to develop a portable e-nose for general use.
Food and Bioprocess Technol, 10 (11): 2023-2033 (2018)
25 October 2017
Liquefied Petroleum Gas (LPG) fuels the majority of Indian kitchens and industries. It is a mixture of butane and propane. The mixture is compressed at low temperature, liquefied and distributed in cylinders. When connected to the regulator, it sometimes leaks. Since the gas is self-ignitable, leakage leads to explosions. So there is a need for early detection of gas leakage.
Usually, devices for early detection of LPG leakage use a polymer – polypyrrole – mixed with other nanocomposite metallic oxides such as titanium and zinc oxides. Such devices need significant amounts of LPG before they respond.
Recently, Choudhary from the Suresh Deshmukh College of Engineering and Waghuley from the Sant Gadge Baba Amravati University, Maharashtra reported a method to detect LPG leakage in the early stage.
They took polypyrrole, an organic conducting polymer that has many biomedical applications, and made different nanocomposites with zirconium dioxide, titanium oxide and silver oxides.
They found that polypyrrole with silver oxide has less response time and showed more sensitivity and selectivity towards LPG than available technologies. The recovery time, too, was less than that taken by the composites with the other two oxides. Moreover, silver oxide with polypyrrole is sensitive even at a lower temperature.
Entrepreneurs can now come forward to develop and commercialize this technology. An affordable, cost-effective device that detects LPG leakages can reduce accidents in homes and restaurants.
Mater. Lett. 205: 36-39 (2017)
Nitin K S
4 November 2017
Sensing Ethanol Gas
Using ZnO thin films
A breath analyser is the tool used for measuring the amount of alcohol concentration in blood. This is based on thin films. The interaction between the thin film and ethanol vapour produces a signal which is detected. Thin films are used as chemical sensor for detecting gas leakages. Existing gas sensors are difficult to operate. They have low sensitivity and delayed response. Moreover, there are times when they may give false alarm. These sensors also take longer to become functional again after use.
Most ethanol gas sensors use ZnO thin films. Several factors affect ZnO thin film response: surface oxidation, electron exchange and desorption of the reaction products. Scientists claim that these problems can be controlled by doping with different elements. Cadmium is an emerging doping element that can improve the opto-electronic properties and sensitivity of ZnO.
Recently, a team of researchers from the NIT and the NMAMIT, Karnataka reported their investigations on enhancing the performance of CdxZn1-xO thin films for ethanol vapour detection. They prepared three concentrations of cadmium – 5, 10 and 20% – doped on ZnO thin films. The scientists then studied the composition, structure and surface morphology as well as the optical transmittance and electrical conductivity of the films.
Next, the team optimized the gas sensing properties of the CdxZn1-xO films. They investigated the activity of the sensor in the presence of acetone, ethanol and methanol to establish fixed resistance at 350ºC for 4-6 h. And reported that a 10% concentration of cadmium doped thin films was best for gas sensing.
The team also analysed the response and recovery time of the thin films for different gases. They found that the response time for all the gases increases with increase in concentration. At higher concentrations, more gas molecules interact with the sensor surface to bring down response time. However, increase in gas vapour concentration increases the recovery time of the sensor.
The researchers concluded that this highly sensitive, responsive and selective detection technology can be effectively used for monitoring ethanol gas. Moreover, the sensor developed is a miniature, inexpensive and compact.
J. Alloys Compd. 720: 39-46 (2017)
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Future Gas Sensors
Gas leaks are among the worst of industrial disasters. Every year thousands die due to these poisonous gases. Such tragedies could be avoided if we could detect leaks in time. Scientists from the N. C. College, Jaipur, recently developed a thin film gas sensor which can differentiate a gas on the basis of molecular weight.
They grew Zinc Oxide thin films on quartz surfaces by sol-gel process. They took a concentrated solution and matured it to form a gel and then heated the film in calcium. The x-ray diffraction pattern of the product showed that the ZnO film was pure and had highly textured planes. Surface studies showed that films, though crack-free and homogeneous, were porous.
The spectroscopy studies showed these films to be transparent in visible light. The transmitted light intensity decreased with increasing thickness of the film. The incorporation of defects in the film made the excitation energy rise up to 3.81 eV which made the operating temperature to rise providing sufficient energy to gas molecules to react with the adsorbed oxygen species.
They found gases with lower molecular weight have an increased response because they could go to the bottom of the film and get adsorbed. Thus hydrogen being highly flammable and explosive can be easily detected by these gas sensors.
It is expected that the standardisation of the technique and calibrating the product can help in detecting a wide variety of toxic or combustible gases with reasonable accuracy.
J. Alloys and Compounds, 695: 3552-3558 (2017)
Functional forms by doping
Use of dyes in textile industry poses a threat to the environment. Photocatalysis is one of the methods for the degradation of hazardous pollutants in wastewater from the industry. Transition metal oxides, including zinc oxide can be used as catalysts. Research has shown that the catalytic properties depend on morphologies and crystallographic forms of the catalysts. Moreover, doping with other metals also impact the photocatalytic activity.
Recently, scientists from the Shivaji University, Kolhapur, in collaboration with the Hanyang University, Korea, prepared a better photocatalyst by doping the zinc oxide with copper metal. They used a low cost, easy to prepare, template-free and morphology-controllable reflex method to prepare the catalyst in three different forms: cubes, maize corn seeds and rods.
The scientists characterised the samples for copper substitution in zinc sites. Among all, the corn seed-shaped form had better ability to degrade methyl orange dye in wastewater under UV light. The scientists found that increase in photogenerated electron-hole pairs and excess formation of superoxide, hydroxyl radicals due to doping is responsible for increasing the photocatalytic activity. Based on a scavenger test, the researchers found that both superoxide and hydroxyl radicals are main active species involved in methyl orange degradation.
The results show that a better catalyst is at hand. Who will develop this further to commercially available technologies remains to be seen.
J. Alloys and Compounds, 710: 102-113 (2017)