Making sense of sensors - finding a needle in a haystack

Ronen Fogel

For just about every field of science there is an enduring need to measure or even just detect something that can give valuable information. In the environmental sciences, scientists might be interested in the concentration of pesticides in water; in food, whether there are pathogens present; in medicine, simply just your glucose levels; for forensic scientists or the police, perhaps a bit of DNA that can link a criminal to a crime scene. Many people will be familiar with the glucose sensors used by diabetics to detect glucose in their blood, a life-saving tool for many.  When biological systems fail, a host of molecules in the body can serve as early warning indicators of the failure. Can scientists accurately detect these?

Once you know what you are hoping to detect, how do scientists go about developing these critical tools? This is where the field of biosensors comes in. To do it justice, there are three parts to explaining this story.

The "lock and key"

Trying to find a molecule in your blood that may be an early indicator of disease or a specific pollutant in water is a little like searching for a needle in a haystack. Unless you have the right key, it could take a very long time to find this molecule. But scientists have this covered: they look to see how nature does it…

Let’s start off with a very important, but often-ignored truth in biological science: when functioning properly, biological molecules (or biomolecules for short), such as antibodies  and enzymes, use very specific and selective interactions with each other to achieve their function. If they didn’t, things would get rather chaotic in living organisms. Glucose oxidase is an enzyme which has the molecule glucose as a substrate, catalysing the oxidation of this molecule. To develop a sensor, scientists can utilise this knowledge to design a sensor that can just detect glucose, and the key to this is the glucose oxidase enzyme.

Enzymes that catalyse reactions have a very narrow range of substrates that they interact with and this is very much dependent on the shape and environment of the binding site. Think of it then simply as a lock and a key, where the key has to have a specific shape to fit the lock. 

Much of life and its associated mechanisms relies on these specific events to occur at specific times and speeds. These are often called “biorecognition events”: where one molecule recognises another and binds to it or interacts with it in some way bringing about some change.

Scientists take advantage of these very specific events to design sensors able to detect bacteria in food or pesticides in water and even for detecting cancer. These are often termed “the targets” that you wish to detect.

For this, one requires at least two things: a biomolecule (e.g. glucose oxidase) capable of recognising and interacting with the target (e.g. glucose) that you wish to detect, and a method of measuring this interaction so that at the end of the day you get a rapid response that tells you whether the toxin is present or exactly what your blood glucose levels are.  This is where we get a bit more detailed. 

Once you know what you wish to detect and which biomolecule or biorecognition agent will be the key, the first step is layering this onto a surface to form a biorecognition layer packed with these molecules. The surface is really important as it is attached to or part of a device which translates any “recognition event” into a signal which can be detected. This device or the surface is known as a transducer. The image gives a brief schematic on the various components of these devices, and how they function.

Elements and components of biosensor during operation. From left to right: The biorecognition agent is attached to the transducer surface to form a layer.  The next step is bringing this surface into contact with the sample of blood or water that might contain the target you are interested in. If the target is present in the sample of water, for example, it travels to the biorecognition agent to interact with it. This interaction produces at least one biorecognition event, which results in a positive signal.

By simply changing the biorecognition agent, scientists are able to detect specifically a host of different targets in fluids, soil or air.

The biorecogniton agent selected is dependent on what it is you hope to detect (the target). And here is where scientists take advantage of nature again by seeing which molecules naturally bind to or recognise a specific target.

Thanks to decades of research by biological scientists, we can select biorecognition agents from a rather vast toolkit. These include enzymes (for example glucose oxidase to detect glucose) or antibodies that can recognise foreign bodies such as pathogens entering our bodies. We can also use DNA to detect specific DNA sequences (for example for forensic science) or even whole cells.
Probably the simplest example of this would be the use of enzymes. Due to the specificity of the reaction(s) of the biorecognition layer that ultimately generates the signal, these devices are considered to be much more specific than other analytical techniques.

But nature does not always provide a handy biorecognition agent and this is where the fun really begins – designing a biorecognition agent from scratch.  The term “aptamer” might be new to most scientists but, for those searching for needles in haystacks, it opens up a whole new dimension for designing specific biorecognition agents that can perhaps perform better than antibodies in recognising targets. See here for more on aptamers.

What about the transducer

A key question is how the signal is generated. What is the basis? Perhaps the two most well-known sensor transducers used are optical and electrochemical methods.
Optical or light-based methods are usually used due to their low manufacture costs and intuitive use. HIV/AIDs detection strip tests (or “lateral flow sensors”) form a classic example.  These use an indirect method of detecting HIV, by detecting some of the common antibodies formed against the viral particles. When this antibody is detected and bound by a secondary antibody (an antibody raised to bind to the original antibodies), it initiates a colour change in a localised area that is viewed as a stripe on these sensors.

This also explains why there is a window period where HIV infections are not detectable – it takes time for the immune system to manufacture enough antibodies to be detected by this method. Adding a stripe that always gives a positive result when the sensor is functioning (which scientists call a positive control) provides a method of quality assurance that ensures defective biosensors are disregarded.

A flagship example of electrochemical biosensors is, and remains, the blood glucose detection strip and this example is often used. It’s portable, sensitive, very cost-effective (per analysis), stable for a long time (as long as one has access to a fridge) and meant to be routinely used by people who aren’t necessarily technically proficient. Since it is routinely used by Type I diabetics to monitor their blood glucose levels, so it operates in a very complex sample filled with many potential interfering molecules and related compounds. And the operation is simple - immobilise the enzymes onto the electrode surface in such a manner as to keep them effectively confined while still retaining their catalytic activity, build a small electronic device that can monitor a product of the reaction between glucose and glucose oxidase (easily done, in today’s technology) and a functional sensor is produced.

Simple? Absolutely. Complex? Definitely. As legislation increases and as our knowledge of the human body expands, the demand for accurate, real-time sensors continues to grow and this is where a new wave of research is currently growing. Now would be an excellent time to invest in this technology!
 

* Dr Ronen Fogel has a PhD in Biotechnology and is based at Rhodes University