Design of a Capillary Flow Immunosensor with Increased Sensitivity 
 

Katrina L Schweiker, Penn State College of Medicine

Dr. Greg Zimmerli, Biosciences and Technology Branch, NASA Glenn Research Center 
 

In the increasing uncertainty in our world, biosensors have proven useful for everything from detecting bioagents to helping clean land mines by looking for chemical signatures of various explosives (1-5).  NASA is interested in them for a couple of reasons.  First, biosensors can easily be integrated into a robotic exploration system, which would allow us to look for traces of life on other planets.  Second, they could be used during manned missions to make sure the habitat is not getting contaminated with microbes against which we might not have an adequate immune response.  For these reasons, I was interested in starting the process of developing a biosensor that was both small enough to fit onto a robotic mission or into a habitat and sensitive enough to detect extremely small amounts of life. 
 

Perhaps the most straight forward method of detection for biosensors is based on the enzyme linked immunosorbent assay (ELISA), which is already common in biochemistry and immunology labs around the world.  So far, only femtomolar quantities of antigen have been detectable by these methods, which may not be sensitive enough if we are looking for signatures of life on other planets.  We chose the capillary design because the process can already be automated using off-the-shelf syringe pumps.  In addition, it seemed like it would be the easiest to miniaturize for spaceflight.  There are many methods that can be used to increase the sensitivity of the capillary sensor, which would also allow us to decrease its size.  Patterning controlled porous glass (CPG) with capture antibodies gives a higher surface to volume ratio, which should increase the signal of the ELISA.  Labeling the tracer antibody with quantum dots or colloidal gold particles instead of fluorophores could also significantly increase the sensitivity of the sensor, possibly into the attomolar concentration range.   
 

We chose to pack the capillary with CPG to increase the sensitivity.  This would also allow us to have the control for the experiment in the same tube.  A wire stopper was inserted into the tube to keep the CPG from being injected out when the experimental apparatus was hooked up to the syringe.  The CPG patterned with capture antibodies were placed in the tube first.  A mark was made where that region ended, and then the control CPG (treated to keep the antigen from sticking, but no capture antibodies) was placed on top.  The assay was run according to previously published protocol (4). 
 

Unfortunately, the fluorescently labeled tracer antibody got stuck in the CPG packed capillary tubes, and I was unable to figure out how to get it to wash out without a special pump.  When I tried running the same assay with one CPG bead in a glass dish, it seemed to work relatively well, so the future of this research will be figuring out how to make this happen in the capillary tube.  It is possible that this isn’t the best design for what we were trying to do.  If there had been more time, I would like to have tried using colloidal gold or quantum dot labeled antibodies, and then do a normal ELISA on a microscope slide.  This procedure has already been automated (4), so it is possible that using a either of these detectors would have increased the sensitivity relative to the fluorescently labeled antibody. 
 

References: 
 

1. Rowe, CA et al. (1999) Anal Chem 71:433-439.

2. Narang, U et al. (1997) Anal Chem 69:1961-1964.

3. Narang, U et al. (1997) Anal Chem 69:2779-2785.

4. Taitt, CR et al. (2004) Microb Ecol 47:175-185.

5. Crass, JJ et al. (1999) Biosens Bioelectron 14:683-689.