Author: SD

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Electrochromic device, Vanadium Oxide, Flexible screen

Chennai, India: Whenever we hear the word photochromic device, what comes to our mind is photochromic glasses. The glasses change color to black/brown in the presence of sunlight and return to normal colorlessness in its absence. This active switching protects the users/wearers from the harsh sunlight and soothes their sight. A stimulus of light is given which in response a color change is observed, this response can be activated by other stimuli one such is potential, called as electrochromic device. Electrochromic devices are far more appreciable than conventional photochromic devices due to their vast applications. Especially for smart displays, flexible screens, military camouflage and etc. Conventionally, tungsten oxide particles have been used for electrochromic displays since their discovery back in 1969. “They possess high transmittance contrast, cyclic stability against high temperature and strong light.”

Color tunability of these particles is possible by doping, however, the entire process is time-consuming, could be expensive due to the need of additional material and also affect the overall transmittance of the system.

In this regard, Wang et al., scientists from the University of Science and Technology, China, proposed a simple yet, and scalable method for the fabrication of highly tunable nanowire assembly-based electrochromic devices. Initially, they have synthesized tungsten oxide (W₁₈O₄₉) nanowires and vanadium oxide (V₂O₅) nanowires. These nanowires were then mixed and allowed to self-assemble on a fluorine-doped indium tin oxide coated glass. This process of self-assembly was accomplished by the Langmuir-Blodgett method. Upon drying, the coated glass was tested under various external potentials varying from – 0.5 to 2 V.

Upon application of potential, drastic changes in the chromic response were observed based on the ratio of W₁₈O₄₉ and V₂O₅ mixture. Switchability of the colors were visualized as orange, green and gray, which could be possible by varying the potential. Furthermore, transmittance of the film was tuned by changing the concentration of active material. Having such precise control over switchability and transmittance, these assembled nanowires potentially could be used in the display screen and by varying the substrates, they could be potentially be used as flexible screens also.

References:
[1] J. L. Wang, J. W. Liu, S. Z. Sheng, Z. He, J. Gao, and S. H. Yu, Nano Lett. 2021, 21, 21, 9203–9209

https://pubs.acs.org/doi/full/10.1021/acs.nanolett.1c03061

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Biodegradable plastic, Cellulose nanofibers, Mechanical and Thermal stability

Chennai, India: Plastics have become somewhat inseparable and part and parcel of our day-to-day life. The US flag hoisted on the moon by astronaut Neil Armstrong was made of nylon. The global plastic production has totaled 356 million tons at the end of 2018. From disposable spoons, plates all the way to IV tubes, plastics have become truly insuperable.

However, due to their non-recyclable and nonbiodegradable nature, they end up in landfills, rivers, and oceans. These then pose a grave threat to the survivability of the local ecology, which breakdowns the sustainability of the environment. To address this, there have been few laws imposed that did impact the usage of single usage plastics. However, to meet the growing demands alternatives to plastic needs to be ventured.

Before diving into the substitutes of plastics, let’s think why we are fascinated to use plastics? Plastics are of less weight and offer protection from water and rain. These are also flexible, affordable, possess good mechanical and thermal stability. Moreover cost of plastic is low as compared to many. So, the substitute that we have to find must offer these properties of plastics. Therefore it can be easily replaceable of plastics. However, mass awareness which is very important and highly appreciable to implement this.

We have plastics of various origins, natural, petrochemical and aliphatic polyesters. Petrochemical-derived plastics are albeit most durable, offer very high tensile strength and offer high thermal resistance, are the leading agents for damaging the environment due to their least biodegradability. In this regard, aliphatic polyesters offer some biodegradability but suffer from poor mechanical and thermal properties.

Cellulose is one of the most abundant natural polymers on earth which is biodegradable. Studies have shown that more than 10¹¹ tons of cellulose can be produced every year through photosynthesis. From this cellulose, cellulose nanofibers can be extracted, that can be used as a structural material due to its unique properties of having high tensile strength comparable to that of steel and high modulus. Owing to exploit these wonderful properties of cellulose nanofibers, Guan et al., of the University of Science and Technology of China from China realized a substitute for the petrochemical-based structural material. [1]

Cellulose nanofibers have a typical diameter in the range of 5-10 nm and these fibers were crosslinked by simply spraying aqueous calcium chloride solution. The resulting hydrogel was pressed under a pressure of ~ 1 MPa for 12 h and finally dried under ~ 50 MPa at 80 ^oC for 1 h. The resulting product was tested for its mechanical and biodegradability studies. They observed that due to crosslinking, the modulus rose to ~ 16 GPa and flexural strength to ~ 300 MPa. The system was so versatile to process, that they could drill holes through the product without damaging the overall structure and even process it into desired complex shapes. The crosslinked product degraded within 90 days when it is placed under soil. These observations are truly inspiring, one day in the near future substitute for plastics is truly possible by modifying the naturally available polymers.

References:
[1] Q. F. Guan, H. B. Yang, Z. M. Han, Z. C. Ling, K. P. Yang, C. H. Yin, and S. H. Yu, Nano Lett. 2021, 21, 21, 8999–9004.
https://pubs.acs.org/doi/10.1021/acs.nanolett.1c02315

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Wearable electronics, Non-invasive, E-textile, hydrophobic, Health monitoring

Chennai, India: Wearable, non-invasive health monitoring sensors have an edge over conventional medical diagnostic tools. Lack of mobility, cost-ineffectiveness, a requirement of physical space and the requirement for trained personnel are some of the preliminary drawbacks of the medical diagnostic tools. With the ad of the internet of things, it has become much conducive for monitoring the necessary biomarkers on a daily basis. However, due to prolonged usage of such wearable sensors, especially on the skin, could lead to discomfort and wet-thermal management is crucial or it could lead to further health complications. Sweat consists of a vast variety of components apart from water, by non-invasively monitoring these components, could help in the overall health assessment of the health of the wearer. Fabrics are by far the most favorable for absorbing sweat, keeping the body cool and protecting one from extreme environment.

Traditional fabrics are ideal for adsorbing the sweat on the skin which helps in keeping the skin dry. However, they can transfer the moisture from the cloth to the skin once, the fabric is completely wet. Fabrics, however, can be customized based on the requirement. When I meant customization, it’s not merely changing the shape and adding colors. Fabrics’ chemical properties can be altered completely transforming the very nature of cloth. For example, we all know that natural fabrics are hydrophilic in nature, i.e., they absorb water. A single droplet of water immediately spreads on it. However, by chemical functionalization this property can be completely altered, transforming the cloth to a hydrophobic one similar to that of a lotus leaf.

He et al., of Shenzen University, China, chose natural fabric silk, then chemically treated it and transformed it into a hydrophobic cloth. [1] Subsequent exposure to oxygen plasma reverted the hydrophobic nature to the hydrophilic one. The hydrophilic part of the cloth comes in contact with the skin and the hydrophobic cloth faces the environment. By doing this, they observed that most of the sweat was absorbed by the cloth and the region where the cloth was in contact was far more uniform when compared to the untreated silk. Now, that one issue of wet-thermal management was addressed, let us think about what else we can use this hydrophobic cloth. Two silk yarns were then chosen, one was coated with conducting carbon paint and the other with Ag/AgCl paint. These two were then stitched on the hydrophilic side of the silk cloth transforming the fabric into E-fabric. Electrochemical measurement was performed using this modified cloth and presence of K⁺ ions, pH, uric acid, as well as glucose in sweat was measured. This E-fabric was connected to a microcontroller worn on a t-shirt and sweat analysis was performed and the concentration of these species was monitored through a mobile application.

Such E-fabrics with higher selectivity and sensitivity towards the various components of sweat could further modernize the field of wearable electronics for health monitoring, owing to its simple fabrication method.

Reference:
[1] X. He et al. Nano Lett. 2021, 21, 20, 8880–8887.
https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03426)

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Diabetes mellitus, Glucose sensor

Chennai, India: Diabetes mellitus (DM), also known as diabetes is a metabolic disorder caused by the presence of high sugar in blood for prolonged duration. Globally ~ 8.8% i.e., 463 million people suffer from this disorder, classified into two major types, type – I and type – II. Type-I diabetes, also called as “juvenile diabetes” is a rare condition, where the pancreas cannot sufficiently secrete insulin and Type-II is the increased resistance to insulin generally caused due to poor lifestyle. Can lead to ketoacidosis, chronic kidney failure, cognitive impairment, nerve damage and stroke. Therefore regular monitoring of blood glucose level of a diabetic patient is advisable. However, pathological examination of blood sugar level by any patient is hectic. In recent days, glucose is detected by using glucometer, where a small drop of blood is placed on a strip and upon analysis would give the concentration of glucose (mg/dL). Even though, this is a widely used commercialized technology, it has its limitations as the patient needs to constantly monitor his/her blood glucose levels and frequent pricking of blood is uncomfortable. Thus there is a need for noninvasive technique for detection of blood glucose level.

Apart from blood, glucose is also present in tears, sweat and saliva. Monitoring these fluids is noninvasive and could provide some relief to the patients. In this regard, there has been substantial research [1] and few commercial products [2] have also been released which noninvasively monitors glucose. Need of constant sensor replacement and excessive cost are some of the major causes for these commercial products to reach the masses. Even though the amount of sweat or saliva one generates is sufficiently high, due to other components present, it could hamper the detection of glucose. Optical measurements of tear drops can detect the concentration of glucose, usually by fluorescence resonance energy transfer and also by surface plasmon resonance. [1] However, these two techniques require continuous tear generation and also laser, making them difficult for daily monitoring.

To address some of the major limitations and successfully monitor glucose concentrations noninvasively, Jeon et al., of Gwangju Institute of Science and Technology, Republic of Korea fabricated a smart contact lens. [3] This lens is comprised of nanoparticles of cerium oxide nanoparticles and glucose oxidase and polyethylene glycol complex embedded on the lens. As soon as glucose upon contact with the glucose oxidase, it oxidizes and produces hydrogen peroxide. This peroxide converts the Ce³⁺ to Ce⁴⁺ changing the colour of the lens in that process. These lenses are highly biocompatible and were tested for cytotoxicity using the human umbilical vein endothelial and human corneal epithelial cells. The testing rig is extremely simple, all it contains is a CCD camera, with a zoom lens of 0.45 X magnification. A photograph is initially captured of the lens, then by using simple image processing methodologies, the center of lens is tracked, the intensity of the image in red, blue and green channels were calculated and finally an average of all the three intensities estimates the glucose concentration. A schematic explaining the process is shown in the figure below.

Figure: Schematic illustration of colorimetric NECL and optical monitoring system. (a) NECL color was changed to yellow (colorless Ce³⁺ to yellow Ce⁴⁺) (b) A color CCD camera with a zoom lens for the demagnification (0.45×) record NECL surface color change (RGB). Reprinted with permission Copyright © 2021 American Chemical Society.

The system was tested both for rat and human tear, continuously monitored the glucose levels for diabetic rats when they were awake, sleeping due to inhalation and injected anesthesia. Every rats’ blood glucose levels was monitored prior to the experiment using the smart lens. They have observed almost 82 % and 71 % linear correlation between the blood and tear glucose concentration in case of humans and rats, respectively. Thus, this methodology is simple, could alleviate the discomfort and by developing a mobile phone application, it could potentially reach the masses.

References
[1] M. Chung, G. Fortunato, N. Radacsi, J. R. Soc. Interface, 2019, 16, 20190217.
[2] https://www.freestylelibre.co.uk/libre/
[3] H. J. Jeon et al., Nano Lett. 2021, 21, 8933–8940.

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Wearable electronics, Flexible solar cells, TiO₂ nanotube

Chennai, India: Whenever we hear about wearable electronics, the most successful and widely accepted technology that reached the masses is in the form of a smartwatch. It is not only showing time, but it is also capable of counting steps, total distance moved, measuring heart rate, body temperature and even conducting ECG of human body.  This invention has completely transformed our way of life. The major limitation for such kind of wearable technologies is the power supply, the battery used is limited and needs charging for continued usage. This limits the overall usability of the system. Imagine if we have a smartwatch that doesn’t need any charging!

In this context, extensive research has been conducting and researchers from Fudan University and EMPA were successful in fabricating solar concentrators for fiber solar cells.

So, before directly going to the fabrication procedure and the detail technology inside it, let’s first look into the basics of what is a solar cell and solar concentrator means. A solar cell is a device that converts light energy into electricity and a solar concentrator helps further in improving the solar cell’s efficiency. Sun is our abundant source of energy and thus we need to effectively and efficiently use that energy for powering our daily use electronic gadgets. Solar panels are one of the prime examples for efficient power production, however, the cost of the panels and carry such kind panels during mobility restrict their uses in such purpose.  Since, it lacks flexibility, solar panels, can’t be worn on clothes and is limits its further uses. Let’s limit the focus of this article towards mobility alone. In the past, a series of reports which have been addressed for this purpose and researchers have come up with flexible solar cells in this context. The seminal work by Prof. Graetzel of EPFL, Switzerland, who invented the dye-sensitized solar cell totally revolutionized the field of photovoltaics. A dye-sensitized solar cell consists of a photoanode which is made of semiconducting metal oxides typically of tin, zinc and titania. This photoanode is then bleached in a dye, which could be of natural or synthetic origin, is then immersed in an electrolyte solution and finally platinum or carbon is used as the photocathode.

In the current article written by Huang et al., authors initially fabricated a flexible fiber-based dye-sensitized solar cell. [1] Consisting of a titanium wire which was anodized to form flexible titanium dioxide nanotubes (TN). Further, carbon nanotube (CN) fiber was initially fabricated by a technique known as floating catalyst chemical vapor deposition. As a result of this process, the obtained CN fibers intertwined with the TN fibers were achieved and this structure was used as photoanode of the device. This composite fiber-wire is then immersed in N719 dye and the electrolyte and cell has been fabricated by making a sandwich structure with the photoanode and photocathode. Finally, a methacryloxypropyl- terminated polydimethylsiloxane with a UV-initiator along with a fluorescent dye known Comarin 6 was used as a solar concentrator. Multiple flexible solar cells were taken and the above solution was poured on these fibers and a film was made. Upon drying, the film was tested under a solar simulator and the researcher observed an enhancement of 84 % in the conversion efficiency and taking the device efficiency to ~7.89 %. With multiple flexible solar cell fibers with a 3 cm² area of the solar concentrator, they have obtained a power output of 0.89 mW, which is substantial to energize the smart watch.

References:
[1] Chieh-Szu Huang et al. J. Mater. Chem. A, 2021, DOI: https://doi.org/10.1039/D1TA04984D  (Just accepted)

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Chlorofluorocarbons (CFC), Montreal treaty, Global warming, Greenhouse

Chennai, India: Before directly going into the Montreal protocol, let’s travel back in time where we have read that the earth’s surface is protected from the harmful UV radiations coming from the sun. It’s a thin layer of ozone (O3) that is formed when a nascent oxygen species react with oxygen forming a particularly dense molecule that is highly efficient in absorbing the UV radiations. Typically present at 25 km from the earth’s surface and protecting us from skin cancer. Thinning of this protective layer was observed by Farman, Gardiner and Shanklin in 1985 over the Halley and Faraday stations in Antarctica [1]. There was about ~16 % decrease in the overall density of the O3 layer and it has been raised a serious concern. Upon further investigation into what led to this decrease, they understood the chlorofluorocarbons (CFCs) are the main culprits. These CFCs were commonly present in propellants, as coolants in refrigerators and also for aerosolizing paint and scent. The global body met in 1987 in Montreal and signed a treaty to phase out the manufacture and sale of CFC-based gadgets by 2010. This has been a success story, where a collective work led to an overall decrease in the CFC consumption and there has been a significant O3-hole coverage. However, due to the high lifetime (~ 50 years) of the CFCs, the atmosphere can fully recover only by 2050.

Now, let us consider a scenario wherein if there was no Montreal treaty and we continued the rampant usage of CFCs coupled with the increased consumption of fossil fuels, which caused an increase in the overall concentration of greenhouse gases in the atmosphere. Recently, a paper was published where the scientists have simulated a scenario that deals with the above question, the outcome is rather warming (pun aside). In their simulation, they have considered 3 scenarios; the first one, where the simulation follows the present time(with no control over the production of CO₂ and the other greenhouse gases, but there is a ban on CFCs), the second one where the ozone layer was fixed in 1960 but no control over the production of CO₂, and other greenhouse gases and the third is where there was no Montreal treaty and there is rampant usage of CFCs. The simulation was based on past historical data and predictions till 2100. Let’s look into their extreme scenario where there was no control over the production and consumption of the CFCs.

In figure 1a, we see the depletion in the ozone layer from the year of 2000 in both scenarios 1 and 3.However, there was a significant decrease after 2040 in the case of third scenario, where there was no Montreal treaty. Overall ~70 % decrease in the ozone layer was projected at the end of this century.

They observed an increase in the production of greenhouse gases till 2075, however, thisrise was decelerated in all of their simulations. What does this lead to? There was a steady rise in the temperature of the earth due to global warming. With the absence of the ozone layer by the end of the century, they observed an increased temperature by 6 k compared to that of 3.3 k. This increased temperature surely impacts everyone on the planet. Plants are the major source for carbon fixation, i.e., they inhale CO₂, convert them into energy and release O₂ into the atmosphere. What happens to this conversion when there is abundant UV radiation in the absence of the protective ozone layer? For every 10% increase in UV radiation, they observed a 3% reduction in the fixation. We might feel 3% is so minuscule and should it matter? Because of the poor carbon fixation, by the end of the century, the CO₂ concentration doubled from 400 ppm to ~900 ppm. This news not only predicts the grim future if we don’t take care of our environment, but also the importance of the Montreal treaty which will save our planet.

The Montreal treaty is a success story, we need to remind ourselves that we all must towards the collective good over individual greed and make this world a better place for the future generation. All the governing bodies must meet regularly and implement the Paris Accords at the individual level, so that we can reduce global warming to less than 1.5 ^oC.

Source:
[1] PaulJ.Young et al.Nature,2021, 596, 384–388.
https://www.nature.com/articles/s41586-021-03737-3

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: SARS-CoV-2, RT-PCR, COVID, Biosensor, Liquid-gated FET

Chennai, India: Presently, the world is dealing with the outbreak of a respiratory illness caused by the severe acute respiratory syndrome coronavirus – 2 also known as (SARS-CoV-2). Being a successor to SARS-CoV-1 which was responsible for the 2002-2004 SARS outbreak, is highly contagious as the transmission primarily occurs via aerosols. [1] The virion typically ranges between 50-200 nm in diameter having four structural proteins known as the spike, envelope, membrane and nucleocapsid. [2] Testing-tracking and treating is the current strategy followed globally for controlling the pandemic. Nucleic acid testing using quantitative reverse transcription-polymerase chain reaction (qRT-PCR) is the golden standard for diagnosing COVID-19. However, this methodology requires sample purification, amplification and trained professionals which reduces the overall efficiency of the testing system.

To mitigate some of these limitations, there have been strides made using colloidal gold particles-based lateral flow assay, enzyme-linked immunosorbent assay and luminescence-based biosensors. However, for most of these sensors, the detection limit reaches a maximum of 2.8 fM. (where 1fM is 10^-15 M). This limitation could become a factor while managing COVID when the antibody concentration is below this value. Therefore, while studying the vaccine efficacy, a sensor that can detect even the smallest concentration of antibodies is needed. In this regard, Hang et al., from Fudan University recently developed a graphene-based field-effect transistor for detecting SARS-CoV2 antibodies. Monolayered graphene films were initially prepared using chemical vapor deposition on copper foils. Graphene from these copper foils was then transferred onto SiO2/Si substrate using the wetting method. In this method, graphene is first transferred to phenyl methyl ether solution containing polymethyl methacrylate. Graphene was later transferred onto a SiO2/Si substrate from PMMA. The graphene is then functionalized with 1-pyrenebutyric acid N-hydroxysuccinimide ester (PASE) molecules through the pi-pi coupling. The spike S1 protein of SARS-CoV2 is then anchored to PACE via the amine group present on the protein with the hydroxyl-free succinimide esters present on PASE. Finally, an open well was created on the SiO2/Si substrate using PDMS, this helps in holding the sample solution.

This sensor comprises a liquid-gated FET with Ag/AgCl reference electrode which is inserted in the electrolyte as gate electrodes. The electrolyte/graphene interface serves as a dielectric layer when a liquid-gate bias is applied externally. Current response to external voltage was measured. At a particular voltage, a drop in current was observed. As soon as the antibody was dropped on the sensor within 15 min there was a shift in the voltage. A calibration curve was obtained by varying the concentration of antibodies and the voltage shifts were noted. Using this sensor, a concentration of 2.6 aM was achieved. How is this possible? Is a simple question that everyone gets, graphene is wonder material having an atomic layer thickness due to this the surface is covered completely with the receptor which helps in higher sensitivity. The detection result is read out directly from the electric response without needing any complex further processing and data analysis. Such chips can be manufactured large-scale at a relatively low cost. It does hold a great promise for on-site point-of-care detection of SARS-CoV-2 thus mitigating cross-infection and accessibility for testing at home. These results have been published in the journal NANO Letters. [3]

Sources:
[1] https://www.nature.com/articles/d41586-020-02058-1
[2]Philip V’kovski et al. Nat Rev Microbiol.  2020, 1–16.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7592455/
[3] Hua Kang et al. Nano Lett.2021, 21, 19, 7897–7904.
https://doi.org/10.1021/acs.nanolett.1c00837