Editorial Feature

What are Optically Responsive Materials?

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At a very fundamental scale, most living organisms are composed of highly complex macromolecules. These macromolecules have highly complex structures defined by the rules of organic and inorganic chemistry. These molecules thus constitute more complex entities such as tissues which again constitute organs.

A variety of biological functions rely upon a feedback controlled communication mechanism among these molecules that include nucleic acids, proteins, and polypeptides. On a perceivable scale, our senses are direct results of such feedback based communication processes. The most intriguing of these is the sense of sight. Our eyes receive light from the surroundings which a lens focusses on the retina.

The retina is a thin layer of tissues that line at the back of the eye and is located near the optic nerve. It consists of a layer of light-sensitive photoreceptor cells which detects color and light intensity. This information is sent to the brain via the optic nerve for further processing. Researchers all over the world are trying to mimic this capability of optical responsiveness for other applications.

This has led to a variety of new developments, especially in the field of optically active polymers. There has been an overwhelming amount of research reported in past couple of decades for development of these polymers.

These photoresponsive polymers change their properties when irradiated with light of a particular wavelength. These changes occur by structural transformations of specific functional groups in a polymer matrix by the application of an incoming electromagnetic field.

There are various applications for such photoresponsive materials that include polymer viscosity control, stitching of biomolecules, targeted drug delivery, photomechanical transduction, and actuation. One of the most important aspects which makes these photoresponsive materials more attractive for real-life applications is their relatively straight-forward non-invasive mechanism to induce response behavior.

The science for these materials has matured and there has been a recent expansion to create much more complex macromolecular architectures. The most well-studied family of polymers are those which have Azobenzene group functionality. On application of radiation, a cis-to-trans isomerization is induced that is accompanied by a very fast change in electronic configuration, geometry, and polarity of the molecule.

To understand it better let's discuss the dipole moment of azobenzene. A comparatively stable trans-azobenzene has no dipole moment. On the other hand, the cis configuration is quite polar, having a dipole moment of 3D. Thus, by incorporating azobenzene derivatives into a polymer structure, we can achieve new materials with variable shape, polarity, and self-behavior.

For example, Wang and his team described the photo-induced deformation of epoxy based azobenzene-containing polymer colloids which changed their structural morphology from spheres to spindles and finally to rods.

The most useful application of photoresponsive materials come from their capability to have a non-invasive nature of light. One such demonstration is done by Hoffman and his team of researchers.

They employed polymers which contain azobenzene groups as switches to reversibly activate enzymes in response to distinct wavelengths of light and also used the same mechanism for reversibly controlling biotin-binding by site-specific conjugation to streptavidin. This shows the promising applications of photoresponsive materials for medical applications.  

Other than azobenzene functionalized polymers, some other materials have been used to impart optically responsive behavior. Spiropyran derivatives can be incorporated terminally or pendently to achieve light sensitivity.

This is because spiropyran groups are non-polar but  in presence of electromagnetic fields of appropriate wavelength, it leads to a zwitterionic merocyanine isomer which has a higher dipole moment. This is a reversible process which is also initiated by another wavelength of light.

Most optically responsive polymers rely on light-induced isomerization which causes a change in the molecular structure of the molecule. Another approach is to cash in on polarity changes that result from the cleavage of photo-liable esters that yield the corresponding alcohol and acid components.

Most of the examples of irradiation-induced isomerism based materials work in UV wavelengths. This limits its applications for some biomedical applications. However, IR radiation can penetrate skin with less risk of damaging the tissues and thus are more applicable for photoactivation of drug carriers within a living system.

One of the major use of optically responsive materials is its use in the semiconductor industry. Photoresists are fundamental materials related to photolithography which is the method of imprinting micro-scaled structures on top of a silicon wafer, which ultimately results in functional chips that are the core of electronics and information technology.

Photoresists consist of a polymer, a sensitizer, and a solvent. The polymer changes its structure on application of electromagnetic fields usually in UV wavelengths. Sensitizer controls the photochemical reactions.

The photoresist is spun on the surface of siilicon by a spin coater since it is in liquid form. Depending on the type of photoresists, they can be classified as either positive photoresist which weakens under light or negative photoresists which crosslink under the light.

In conclusion, optically responsive materials have various applications in STEM fields. Their applications, especially in the field of medicine, has enormous potential. Rigorous efforts are being made to bring the new theoretical ideas for optically active materials from drawing boards to the real life.

Sources:

  1. https://www.sciencedirect.com/science/article/pii/S0079670009001063
  2. https://www.ncbi.nlm.nih.gov/pubmed/12486222
  3. https://pubs.acs.org/doi/abs/10.1021/ma0710481
  4. https://pubs.acs.org/doi/full/10.1021/ma052002f?src=recsys

 

 

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