Category Archives: Science Communication

Plastic replacements: some new conSQUIDerations

The amount of plastic that has been produced to date now exceeds 8300 million metric tonnes (Mt). To put this into perspective, the average blue whale weighs approximately 180 Mt, thus 46 million blue whale’s worth of plastic has been produced since humans started commercializing plastics in 1950. Our society had become extremely dependent on plastic products and synthetic (petroleum- based) textiles which cause serious consequences such as microplastic and microfiber pollution as I’ve discussed in previous blog posts.

Figure 1. Size reference for blue whales. Wikipedia Commons

Bioplastics have more recently taken the stage as a potential avenue for replacing petroleum-based plastics. Biologically based polymers have structural elements such as helices, β-turns, β-sheets and coils which provide structural integrity and resilience and can replicate the desirable polymeric interactions in plastics. Additionally, a lot of new material is being developed based on the protein polymers that are naturally occurring in biological systems (including ourselves).

Figure 2. Fibrous protein polymers have molecular architecture that can include (i) helices and coils, (ii) β-turns and β-spirals, and (iii) β-sheets. Source

Video 1. SRT- coated fabrics that self-heal in water.

Squid ring teeth (SRT) are an especially promising candidate for making functional fibres and films due to its strength, conductivity and self-healing properties. The SRT are located inside the suction cups of the tentacles of squids and are composed of a naturally occurring protein complex. Fortunately, it is not necessary to harvest squid to obtain the SRT proteins as they can be biosynthetically produced since having their genome sequenced. The SRT-inspired monomer is repeated to create a polymeric chain and the resulting protein that is produced is named accordingly as tandem repeat (TR) proteins.

Figure 3. The Squid ring teeth of a giant squid. Wikipedia Commons

Figure 4. Optical images of squid ring teeth (SRT) and the six common squid species they originate from. Source

Films produced with SRT proteins consist of disordered domains that provide elasticity and flexibility to the materials in addition to ordered domains such as β-sheets that provide mechanical strength. One study designed four TR proteins denoted as TR-nX where X was the number of repeat units within the molecule. They measured the mechanical force of these four TR proteins and found that the ultimate strength of the protein’s scale linearly, with TR-n25 reaching an ultimate strength of 40 MPa. As seen in Figure 4 below, there is a limitation of the study due to sample size. The error bars of the TR-n12 and TR-n25 overlap, so it is not possible to say there is a statistically significant difference from each other.

Figure 4. Mechanical testing of fully hydrated TR proteins (inset shows 1/n dependence) Source

Additionally, SRT protein films have been suggested as a potential solution to the issues related to the release of microfibers into the water from washing. A cloth made from polyester was coated with an SRT protein film and was found to dramatically increase the cloths resistance to abrasion (and microfiber release) compared to cloths that were not coated with SRT protein films.

While there is still a lot of further research to be done, SRT-based proteins are a promising avenue for making our world a little bit less plastic.

~Isla

Photoactivated Self-healing Copolymers- It’s Lit

A scratch on your car may no longer need a trip to the auto-shop. Simply applying heat or light could remedy this issue. This idea could soon be a reality using vitrimers, a new class of plastics that have thermal and chemical stability, but can also be self-healing on a small scale and fully recyclable on a larger scale.

Taylor Wright from Dr. Wolf’s group at UBC. Source

In 2018, at the University of British Columbia, Taylor Wright under the supervision of Dr. Michael Wolf investigated the photoactive self-healing properties of vitrimeric copolymers.

Photoactive materials undergo physical and chemical changes in response to illumination. The development of responsive materials to both heat and light were explored for the first time through the incorporation of functional molecular groups into the polymeric backbone of these systems.

Wright and Wolf’s focus on the molecule’s response to light also offered a new aspect into vitrimeric research compared to the previous studies, that exclusively focused on the vitrimers’ response to heat.

Figure 1. Comparison of thermoplastics and thermosets upon heating. Source

So what exactly are vitrimers? Vitrimers are a new class of polymeric material that was first created in 2011 by a Polish physicist, Dr. Ludwik Leibler. By combining characteristics of thermosets and thermoplastics, Leibler was able to develop a material that is strong and durable, yet moldable and recyclable.

Thermoplastics are made of plastics linked by intermolecular forces. They can be easily molded and shaped under heat, then cooled down to produce the final structure. This allows for ease when it comes to processing. Additionally, this property allows them to be recycled to produce new products.

Conversely, thermosets involve irreversible cross-linking, connecting the backbones of the polymer chains with molecular bridges. This results in enhanced chemical and heat resistance, making the material less susceptible to stress-cracking. However, due to their cross-linked bonds, these materials do not melt upon exposure to heat,  unable to remold and recycle.

Vitrimers combine the best properties of both materials; structural integrity is improved through cross-linking, as well as self-healing and fully recyclable properties.

Figure 2. The molecular structure of the photoactivated vitrimeric copolymer created by Wright and Wolf. Source

As seen above in Figure 2, the vertical wiggly line splits the system into the two unique parts that make this a copolymer. The left side shows the aromatic anthracene molecule that crosslinks into a dimer in response to UV radiation.  The amine on the right side behaves like a more traditional vitrimer and responds to heat to form reversible exchanges.

Originally, their aim was to create a single polymeric system that responds to both heat and light simultaneously. However, during their research, they found that amines directly bonded to the anthracene molecules simply do not engage in the bond exchange process. They believed the electronics of the ring alters the behaviour of the molecule in comparison with non-aromatic amines.

Studying the photodimerization and thermally exchangeable functionalities of the copolymer based on the vinylogous urethane vitrimer, the self-healable properties can be seen in the video above. Self- healing polymers are a class of materials that enable the repair of micro-scale damage in the coating, ultimately restoring the passive state of the metal substance.  This enables reprocessability or longer lifetimes in cross-linked polymeric materials. The systems containing anthracene undergo self-healing through reversible reactions, allowing monomers and polymer chains to link and unlink.

Figure 3. Polymer sample, P2, mounted on a glass slide. A scratch from a razor blade can be observed. Source

Wright and Wolf tested the modification of surface properties by using a razor blade to scratch a polymer sample (that Wright denoted as P2), that was mounted on a glass slide. By using optical microscopy, the scratches were observed as dark lines crossing the sample, as seen below in Figure 4. The scratches were seen to decrease in width and ultimately close during heating through a series of expansions and contractions of the material, which can be seen in the video above.

Figure 4. Optical microscope image of (a) sample P2a initial scratch, (b) P2a after heating (c) sample P2b initial scratch, (d) and (e) P2b after heating. Black scale bar is 300 μm. Source

These specific Wright and Wolf vitrimeric copolymers will not be scaled up for commercial use, due to the difficulties of incorporating the two components of the copolymer together. However, the general idea of vitrimeric materials has “almost limitless applications”. For example, they can be incorporated into products that have a long lifetime, such as shipping materials and plastic stadium seats which can be recycled into new products once they start to deteriorate.

Additionally, Wright is currently working on vitrimers that start as a viscous liquid, much like thermoplastics, that can be easily molded and processed. This possible advancement will provide more flexibility with processing the starting material and ease in the synthesis process.

~Brina, Isla and Taiki (Group 4)

Using uncertainty-based strategies for modelling atmospheric pollutants

After World War II, thousands of synthetic chemicals became commercially available for use in agriculture, manufacturing, or disease control. Some of these chemicals were classified as “persistent organic pollutants”, or POPs, because of how they resist degradation, persist in the environment, and are toxic to plants or animals.

One well-known example is DDT, an insecticide that became infamous for its health and environmental impacts after Rachel Carson’s Silent Spring was published in 1962, and was ultimately banned for American agricultural use in 1972. Like many POPs, DDT magnifies along the food chain and accumulates in fish. In 2018, University of Maine researchers found that children who eat fish from rivers fed by the Eastern Alaska Mountain Range have a cancer risk above the Environmental Protection Agency’s threshold limit.

A biogeochemical cycle of PAHs in the environment. Source: Microbial Biodegradation and Bioremediation (Das et al. 2014)

In order to better understand the atmospheric chemistry of POPs, Colin Pike-Thackray—a graduate student in Dr. Noelle Selin’s group at the Massachusetts Institute of Technology—used quantitative models based on uncertainty. One class of pollutants that Pike-Thackray focused on in his thesis were polycyclic aromatic hydrocarbons (PAHs), which result from fuel or biomass consumption. Similar work in the field had revealed that uncertainty in simulations of DDT concentrations result from estimated emission and degradation constants, while uncertainty in simulations of mercury concentrations in the air and ocean surface were due to partition coefficients and reaction rate constants.

Uncertainty distributions for the atmospheric concentrations of different PAHs (colour-coded) in the Northern Hemisphere and the Arctic. Anuual (solid), winter averaged (dot-dashed), and summer (dotted) averages are shown. Source: Environmental Science and Technology (Pike-Thackray et al. 2015)

Using the mathematical method of “polynomial chaos”, in which each parameter of a dynamic system is a source of uncertainty, Pike-Thackray et al. (2015) found that a variety of factors increased the uncertainty of estimated PAH concentrations. One leading contributor was the black carbon-air partition coefficient, which describes the relative concentrations of PAHs trapped in in air or black carbon at equilibrium. The oxidation rate constants of PAHs were also significant sources of uncertainty. In addition to uncertainty arising from parameters specific to PAHs, the researchers also considered the uncertainty associated with precipitation or advection (the horizontal mass motion of the atmosphere).

Measured (black) and simulated (blue) monthly concentrations for different PAHs, where the shaded regions mark one and two standard deviations for the uncertainty distribution. Source: Environmental Science and Technology (Pike-Thackray et al. 2015)

Notably, Pike-Thackray et al. modified their models to be consistent with experimental observations. The researchers also compared the different strategies and amount of computational power needed for different modelling approaches, and claimed that their methods offer “a significant advantage over traditional model parameter sensitivity tests” because of how they “quantify the relative importance of each parameter, as well as account for their interactions in the model system”.

Best estimates, uncertainties, and literature values for various physical and chemical parameters associated with a specific PAH. Source: Environmental Science and Technology (Pike-Thackray et al. 2015)

Overall, this research reveals which parameters cause the greatest uncertainty in modelling the concentration and transport of PAHs in the atmosphere. My opinion is that this research is extremely interesting and worthwhile because targeting these parameters could allow the development of better environmental models and predictions, which could in turn influence both government regulation and commercial use of POPs. Furthermore, the work presented in Pike-Thackray’s thesis is an interesting example of how chemistry, environmental science, statistics and mathematics can all intersect and be applied towards a real-world issue.

— Jessica Li

References

  1. United States Environmental Protection Agency. Persistent Organic Pollutants: A Global Issue, A Global Response. https://www.epa.gov/international-cooperation/persistent-organic-pollutants-global-issue-global-response (accessed Mar 22, 2019).
  2. United States Environmental Protection Agency. DDT – A Brief History and Status. https://www.epa.gov/ingredients-used-pesticide-products/ddt-brief-history-and-status(accessed Mar 22, 2019).
  3. The University of Maine. DDT in Alaska meltwater poses cancer risk for people who eat lots of fish.  https://umaine.edu/news/blog/2018/12/06/ddt-in-alaska-meltwater-poses-cancer-risk-for-people-who-eat-lots-of-fish/(accessed Mar 22, 2019).
  4. Schenker, U.; Scheringer, M.; Sohn, M. D.; Maddalena, R. L.; McKone, T. E.; Hungerbühler, K. Using information on uncertainty to improve environmental fate modeling: A case study on DDT. Env. Sci Technol 2009, 43, 128–134.
  5. Qureshi, A.; MacLeod, M.; Hungerbuhler, K. Quantifying uncertainties in the global mass balance of mercury. Glob. Biogeochem Cycles 2011, 25.
  6. Kim K.; Shen, D.E.; Nagy, Z.K.; Braatz, R.D. Wiener’s Polynomial Chaos for the Analysis and Control of Nonlinear Dynamical Systems with Probabilistic Uncertainties. IEEE Control Systems Magazine, 2013, 33, 5.
  7. Thackray, C.P.; Friedman, C.L.; Zhang, Y.; Selin N.E. Quantitative Assessment of Parametric Uncertainty in Northern Hemisphere PAH Concentrations. Env. Sci Technol 2015, 49, 15, 9185–9193.
  8. Pike-Thackray, C.M. An uncertainty-focused approach to modeling the atmospheric chemistry of persistent organic pollutants. Ph.D. Dissertation, Massachusetts Institute of Technology, Cambridge, MA, 2016.

Steps in Improving Transistors for a New Era of Computing

Transistors are a piece of technology universally used in modern devices today, and a smartphone in your pocket will contain about two billion. Transistors are used to amplify electronic signals and switch them on and off, and are the reason for the sophisticated technology we have access to. They are semiconductors, meaning they conduct electricity worse than a metal but better than a resistor, and commonly made of silicon or germanium.

A diagram showing the energy difference an electron needs to overcome for movement in a metal, semiconductor, and resistor.

A basic transistor. Source: Wikimedia Commons

However, there is a limit to the size of a transistor, one that we grow ever closer to. This has prompted research into quantum technology, designing new devices with new components that have exponentially faster computing speeds than our current technology. Rather than semiconductors, quantum devices use superconductors, materials with zero electrical resistance when cooled below a certain temperature.

A quantum computer developed by IBM. Source: Flickr

Some researchers have considered a new design to develop more advanced computing: a hybrid semi/superconducting transistor. Yan et al. published a study describing a new method of producing these in March of 2018. They took advantage of epitaxial semiconductor crystal growth, meaning highly ordered growth, on top of a crystalline superconductor to achieve this. This allowed for the production of a semiconducting transistor on top of a superconductor. Here is a video simulating some different forms of epitaxial growth that show its very consistent order:

This new transistor design has the potential to be used as a superconductor or semiconductor at will, only requiring a temperature change to activate or deactivate superconductivity. As well, Yan et al. achieved this using nitride based semiconductors and superconductors, which are non-toxic and very stable, presenting some interesting applications. This differs from many common arsenic-containing semiconductors, like indium arsenide, that are quite toxic. They also found that the produced transistor had a property called negative differential resistance (NDR) when superconductive, which allows for amplification of electric signals. Telephone lines use this property, and the ability of the transistor to activate or deactivate NDR with temperature has potential use.

A Gallium Nitride crystal, the semiconductor Yan et al. used. Source: Wikimedia Commons

While Yan et al.’s method shows promise for the future, there were some significant issues in their specific method. The most relevant was that they found fairly low electron mobility in their material, something that another semiconductor like indium arsenide excels in. This presents one of the biggest limitations of this current design, as matching the mobility of indium arsenide will be difficult.

Indium Arsenide, a highly effective yet toxic semiconductor. Source: Wikimedia Commons

While Yan et al.’s specific design may not be the solution to transistor limitations, it does present a promising method for designing a different hybrid transistor. Hopefully, other researchers can use this result to produce an even more effective transistor so that widespread quantum computing can become a reality. If you would like a more scientific summary of Yan et al.’s paper, you can find it here, published by the Nature journal.

– Nicholas Patterson

Replacing rare, costly metals in electronic and pharmaceutical applications

Some metals such as nickel, aluminum and steel are ubiquitous in our daily lives, and can be found in coinage, cookware, bridges, and more. Other metals, known as “precious metals”, are rarer and more expensive—but if you’ve ever owned a smartphone or taken medication, then you’ve likely benefitted from them as well.

An average iPhone contains approximately 0.034 grams of gold and 0.34 grams of silver, as well as smaller amounts of rare Earth elements such as yttrium, terbium, and neodymium. Precious metals are also used in the large-scale syntheses of commercial drugs. A common example is palladium, which catalyzes “cross-coupling” reactions—in which two molecules are coupled together—used to prepare Losartan (to treat high blood pressure) and Diflunisal (to treat fever and pain).

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Developments in the Future of Cancer Treatment with Photodynamic Therapy

Cancer has long been a devastating condition and one that is difficult to treat, thanks to its ability to quickly propagate throughout the body. As well, the fact that cancer is contained within the body poses the issue of how to kill it off, as toxins will kill both cancer cells and normal cells. This is a problem that researchers are trying to solve by investigating a treatment named Photodynamic Therapy.

Photodynamic Therapy involves the injection of a compound called a “photosensitizer” into the bloodstream. A photosensitizer is a compound that is activated by exposure to light, and in this case produces toxic chemicals once activated. The photosensitizer is allowed to cycle throughout the bloodstream for 1-3 days, at which point it will only remain in cancer cells and not normal cells. A fiber optic cable can then be inserted into the body in order to reach the area with the tumor so that the photosensitizer can be activated.

An example of fiber optic cables used in surgery. Source: Max Pixel

However, new research from Columbia University may present an easier way of approaching Photodynamic Therapy, one that does not require invasive procedures. The team of B. D. Ravetz et al. have discovered a method for activating photosensitizers from outside the body. They do this by using two compounds, one to absorb near-infrared (NIR) light and transfer that energy to the other, which then emits higher energy light.

A diagram showing IR compared to visible light. Source: Wikimedia Commons

The fact that NIR light can be used for this presents some interesting applications. Unlike visible light, NIR is able to penetrate human skin and flesh quite far, meaning photosensitizers can be activated from outside the body, no incisions required! Another advantage of this is that NIR light is lower in energy than visible, meaning it has a very low risk of damaging surrounding cells.

Now, how does this low energy NIR light become high energy visible light? This is done by utilizing a process called Triplet Fusion Upconversion. This same process is actually used in modern solar cells! The “sensitizer” is first excited by the NIR light, and soon loses energy from being excited and transfers it to the “annihilator.” If two energized annihilators interact, it generates a single more energized form through “triplet fusion.” This final high energy form is what then emits the visible light. Here is a brief animation that shows a similar upconversion process.

A diagram showing upconversion processes. “Emitter” is the same as Annihilator. Source: Wikimedia Commons

While this development is very promising, more testing has to be done before this can be used on humans. The toxic chemicals produced by the photosensitizer still hold a risk of killing normal cells, so tests will have to be able to control the production of these toxins. As well, the toxicity of the sensitizer and annihilator molecules will need to be evaluated too. Hopefully this procedure can be perfected in the near future, so that a safe and effective method for killing cancer can become widespread.

– Nicholas Patterson