There are a lot of fun things about my job. And I'd like to devote a couple of posts to one of my favorite aspects of developing optical sensors. One challenge that arises occasionally is when the optical or fluorescent analysis instruments that we have on hand in the lab are not sufficient for analysis of the optical sensor platforms that I've developed. This is often the case when dealing with flat, planar substrates onto which a sensor architecture or layer has been applied or when we apply a sensing mechanism onto the tip of an optical fiber. When this happens, I have to build a bench-top optics system using our inventory of detectors (PMTs and hand-held spectrometers), light sources (LEDs and lasers), and optomechanics components that we keep in the lab just for this type of situation. And let me just say this: building a bench-top optics platform is outrageously fun. It's like playing with LEGOs. But these LEGOs are bigger, studier, and able to achieve precise alignments. They also cost thousands of dollars.
My research project is nearing a point at which we will begin applying our fluorescent sensing material onto optical fibers (I'll discuss the application of a sensing mechanism onto optical fibers another time). This has left me with a couple of specific needs that must be addressed before the project can move forward: 1) the sensor material must be dip-coated onto the optical fiber tip and to do this, the rate of withdrawal of the fiber tip from the precursor solution must be kept constant at a particular rate, and 2) I have to have some way of accurately measuring the fluorescence emission from the layer coated onto the fiber tip. In this post, I'm going to focus on the dip coating apparatus. As it turns out, the optomechanics components that are used to build bench-top optics systems are also quite useful for constructing a home-built dip-coater. Here's how I did it:
First, I started with a plain breadboard.
Next, I bolted one large post into the center of the breadboard. The large post allowed me to align the entire system vertically.
Onto the post, I clamped a small system of rails. The top and bottom of the rails are secured using filter/lens holders. I used these because they have large holes cut out of their centers. In between the two filter/lens holders is a modified filter/lens holder that has a small metal cylinder attached with epoxy. The cylinder is just the right size to a hold a 4 ml vial - and this will hold the precursor fluid that is to be dip-coated onto the optical fibers. While the top and bottom holders are clamped tightly to the rails, the vial holder in the center was left loose, allowing it to slide up and down the rails freely.
Above the rail system I added a small v-clamp. The clamp will hold the optical fiber in place during the dip-coating process.
Last, I added a small actuator and the accompanying controller. The actuator moves the vial holder up and down, like an elevator.
When I clamp an optical fiber into place, you can hopefully see how this apparatus works. The fiber is submerged into the fluid to be dip-coated onto the exposed tip. Then I just flip a switch on the actuator and the vial holder slowly descends, withdrawing the fiber tip from the liquid. When the fiber tip has been completely removed from the fluid, it can be removed from the apparatus and the thin layer that has been deposited on the tip is allowed to crosslink and/or cure into a solid film.
Now this setup was pretty simple. And because no light sources or detectors were involved, alignment of the entire apparatus was not a big deal. I just needed the little elevator to withdraw the fiber from the fluid at a constant rate. My next project - which I'll be working on over the next couple of days - will be to build a system for measuring the fluorescence of the thin film dip-coated onto the fiber tip. This will be slightly more difficult, though using the fiber itself to gather the fluorescence emission does make things much easier. But more on that to come.
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