Eye Spy

Every day we awaken to the world around us, opening our eyes to see what the day may bring. But have you ever thought about how we’re able to see all that the world around us has to offer. How is it that the light reflecting off of objects is translated in our brains to produce the images we see in our head?

Although it may seem rather complex, vision can be broken down into several major steps. First, light enter the pupil, is focused by the lens, and then hits the retina, a surface covered in light-sensitive detectors. These detectors, known as rods and cones, convert the light into electrical impulses which are then transmitted to the brain via brain nerves. It is from this array of impulses that the brain then creates a picture, enabling us to see.

Isomerization of Retinal

The chemistry of vision lies in the photoreceptor cells lining the retina. Within the retina, there are 7 million cones cells, which provide color information and sharpness, while there are 120 million rods cells which are responsible for detecting white light and providing most of our night vision. Just under the surface of rod cells lie pigment discs which contain proteins bound to the molecule 11-cis-retinal. Upon absorption of light, 11-cis-retinal isomerizes into all-trans-retinal. 

Courtesy of chemistry.wustl.edu

When retinal changes shape, it causes a change in the shape of the protein it sits within, called opsin. Together, 11-cis-retinal and opsin are known as rhodopsin.  Upon isomerization to all-trans-retinal, the complex is known as bathorhodopsin. In order to fit properly in the protein, the retinal molecule has to twist into an unfavorable shape. Due to this instability, bathorhodopsin rapidly changes its shape and expels the retinal molecule from it. 

Just before the molecule leaves the protein (now called metarhodopsin II), the protein complexes with another protein, transducing, which then activates the enzyme photodiesterase. Photodiesterase leads to the hydrolysis of cyclic GMP, a molecule required to open Na⁺ channels in the cells membrane. With closed Na⁺ channels, the cell develops a difference in charge across it’s cell membrane, producing an electrical signal, which can then be sent to the brain and create vision!

Courtesy of RSC Publishing

In cone cells, the process is mostly the same. There are three different cone cells within the body, responsible for perceiving red, blue, and green. Each of these different cells contains a different protein bound to 11-cis-retinal. Given the number and type of cone cells activated by incoming light, the brain is able to decipher what images we are looking at. 

Just think, all of this is happening millions of times every second! The body is certainly an amazing thing!

Cool Videos about the Chemistry of Vision:

1. http://www.youtube.com/watch?v=r6v21W8zRIw
2. http://www.youtube.com/watch?v=Fm45A4yjmvo

References:

1. http://chemwiki.ucdavis.edu/Biological_Chemistry/Photoreceptors/Chemistry_of_Vision
2. http://www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/Vision.html

OLEDs: Brightening the World of Tomorrow

In today’s technological world, electronic devices continue to get smaller and thinner, including televisions. From the older days of rear projection TVs to today’s plethora of plasma, LCD (liquid crystal display), LED (light-emitting diodes), who knew it could be so difficult to buy a TV! One of the more recent developments in this area is the use of devices known as OLEDs, which stands for Organic Light Emitting Diodes. These newer devices can provide brighter, crisper displays than the more conventional systems while consuming less power. In other words, you can have your cake and eat it too!

Courtesy of howstuffworks.com

Internally, although somewhat complicated, OLED’s are similar to batteries (so hopefully you had a chance to read last week’s post).  The entire device is made up of a substrate, an anode, a conductive layer, an emissive layer, and a cathode. The substrate is simply the backing which supports the device and is typically made out of glass, plastic or foil. The anode, which is transparent, is the site where reduction occurs, a process in which electrons are removed from the conductive layer, creating “positive holes”. The conductive layer is made up of organic molecules which help transfer the formed positive holes from the anode to the emissive layer. Similar to the conductive layer, the emissive layer is also made up of organic molecules which, in contrast to the conductive layer, transport electrons from the cathode into the conductive layer. The cathode completes the system and is responsible for places electrons back into the system. But how does this process create the light we see?

When an OLED is attached to a power supply, electrons begin to flow from the cathode towards the anode, passing through the emissive layer. Simultaneously, electrons are removed from the conductive layer at the anode, sending positive holes back towards the cathode. When the electrons and positive holes meet in the middle at the interface of the emissive and conductive layers, energy is released in the form of light.

The particular color of light emitted depends on the organic molecule used in the emissive layer (some examples are shown below). The intensity of the light emitted can also easily be increased or decreased by changing the amount of current applied to the system. The more current, the brighter the light!

Courtesy of howstuffworks.com

Today there are a few different types of OLEDs which vary slightly in their construction, each of which is suited for a particular use. These categories include passive-matrix OLEDs (phones and MP3 players), Active-matrix OLEDs (computer monitors and TVs), transparent OLEDs (heads-up displays), top-emitting OLEDs (smart cards), foldable OLEDs (smart clothing), white OLEDs (commercial lighting). Some of constructions of these different OLED types are shown in adjacent image.

While OLED’s are thinner, lighter, brighter, more flexible, and consume less power, the manufacturing process of them remains expensive, limiting their use. As the technology of OLED’s continues to advance, this technology will become increasingly prevalent, from phones, to TVs, even to car lighting!

References:

• http://www.konicaminolta.com/about/research/oled/about/index.html
• http://electronics.howstuffworks.com/oled.htm
• http://en.wikipedia.org/wiki/OLED