Researchers use new self-assembly techniques to create microstructures that can be used in early detection of Alzheimer’s disease.
For years, computer chips and other electronics have been produced on a mass scale using lithographic techniques, printing the required patterns on palm-sized chips. As chips get smaller to be used in hand-held diagnostic tools, these printing techniques prove insufficient. These devices have micrometre-sized tubes carrying liquids for some pathological tests, which were once done in laboratories, to be now performed on a small chip. The ‘lab-on-chip’ concept is revolutionising diagnostics and needs a better technique for its precise manufacture.
As an alternative to lithographic techniques, scientists are exploring self-assembly methods, where materials are engineered to self-organize into required structures on the chip. Diphenylalanine, a protein polymer, is one such self-assembling material known for its stability at high temperatures. It is also stable in the presence of chemicals and is touted as the future for bio-compatible nanodevices.
In a recent study, researchers at the Indian Institute of Science Education and Research Kolkata (IISER Kolkata) have devised new techniques to create micrometre-sized rods and rings wit diphenylalanine. They have demonstrated that these rods and rings can transmit some specific frequencies of light with minimum energy loss. As a result, they can be used in light-based circuits and devices. Their work was published in the Journal of Materials Chemistry C and funded by the Science and Engineering Research Board (SERB) under its IMPRINT funding scheme.
Currently, self-assembly processes with diphenylalanine do not use electric or magnetic fields. Instead, the polymers are left to themselves to arrange into required patterns. This process is not only slow but also hard to control. To address this shortcoming, the researchers of the current study had developed a faster 'laser-assisted self-assembly' technique. In the present study, they have used this technique to create micro-rings and micro-rods.
The proposed method involves dispersing diphenylalanine solution in water in a small glass cell. On top of this cell is a heat-absorbent coating that heats up when a laser beam is focused, so as to create a water vapour bubble in the solution. While still in cold water, this bubble is stuck to the hot spot on the coating. The difference in the temperature around the bubble creates unequal surface tension, causing a flow of diphenylalanine from the solution to the base of the bubble. The diphenylalanine then crystallizes into beautiful micro-rings at the coating.
"The intensity of the laser beam controls the size of the bubble. Greater the intensity is, the bigger the bubble and bigger is the size of the ring," explains Prof. Ayan Banerjee, a professor at IISER Kolkata and one of the corresponding authors of the study.
Thus, by adjusting the laser beam, one can control the size and the location at which the micro-ring is formed. When the polymer dispersion in water was agitated with sound waves, and allowed to dry on a thin glass slide, it formed micro-rods.
The researchers then examined if the micro-rods and micro-rings can transmit specific frequencies of light with minimum energy loss. They sent a beam of light from one end and observed it coming out at the other end. They found that thinner micro-rods showed lesser loss of energy than thicker ones, irrespective of their length. Besides, the micro-rings did better than micro-rods when both were of the same thickness.
"These materials slightly absorb the light of the wavelength we use. Thus, the thicker we make the rods, the more is the absorption," explains Roshan Tiwari, a research scholar at IISER Kolkata, who carried out the experiments.
The energy loss also depends on an optical phenomenon called total internal reflection—where all of the incident light is reflected off a surface beyond a certain incident angle. "The total internal reflection is probably better in rings, so there are lower losses than in rods of the same thickness. We are currently performing simulations to determine this," he adds.
What makes these micro-rods and micro-rings more exciting is a phenomenon called Fano resonance. It is described as asymmetric peaks formed at specific frequencies in the scattered white light due to the interaction of the total internal reflection in the rings and rods with the overall scattering by these structures.
“This property of these microstructures could lead to them being used in sensing and switching applications,” says Prof. Nirmalya Ghosh, another corresponding author of the study.
In fact, they are currently building a device to detect Alzheimer's disease using these rods and rings.
"Congo red dye is specifically used for the detection of Alzheimer's disease. We are trying to make Congo red dye-doped diphenylalanine micro-rings. When these rings bind to the Aβ peptide, a protein present in the serum of an Alzheimer's patient, there is a change in its optical properties," elaborates Prof. Banerjee.
The researchers have observed that the energy loss of the polymer micro-rings increases when dyed with Congo-red. Hence, it has to be factored in when manufacturing the rings. The study is a step towards incorporating self-assembly techniques to fabricate nano and micro-scale devices. It could also help to develop a diagnostic tool for early detection of Alzheimer's disease.
"The final aim is to use some variety of the polymer that would bind strongly with the actual Aβ peptide," concludes Prof. Debasish Haldar, who is also a corresponding author of the study.
This article has been run past the researchers, whose work is covered, to ensure accuracy.