Abstract
There are many types of biosensors such as: enzyme-based, tissue-based, cell-based, immunosensors, DNA sensors, RNA sensors, ion-selective electrodes. These sensors have many applications and the use of biosensors is expanding in line with movements such as the Internet of Things (IoT).
Some fields benefiting from biosensors are:
- Food Industry - Sensors and Biosensors are finding their way into the Food and Beverage Industry, where they are use for Food Quality and Food Safety. These applications include the measurement of the hotness of food to the E. Coli in food.
- Agriculture - Sensors and Biosensors are finding their way into the Agricultural industry, which like many industries is moving to a more precise applications of chemical inputs such as nitrate fertilizers.
- Medical - Sensors and Biosensors have always been synonymous with medical and human health. The largest medical market for biosensors is self monitoring blood glucose (SMBG), where diabetics are able to monitor their blood glucose by testing capillary blood. At the time of writing this article continuous glucose monitoring (CGM) is rapidly growing and though the market is dominated by three vendors, new players such as Apple and Glucose are also trying to enter the market.
Introduction
Biosensors are transducers that convert a biological event into an electrical signal. Biosensors were invented by Leyland Clarke in 1956. The first electrochemical sensors made by Clarke was an oxygen sensor, but the later addition of glucose oxidase turned the oxygen sensor into a glucose sensor.
The development and manufacture of biosensors is complex due to their inherent multi-disciplinary nature. A modern biosensor team will include engineers and scientists with backgrounds in: biology, chemistry, materials science, electrochemistry, electromechanical engineering, manufacturing engineering, regulatory affairs etc.
The materials used in biosensors are in four main categories:
- Enzymes - The commonly used enzymes are the oxidases, peroxidases, the dehydrogenases and the isomerase. These types of sensors are commonly interrogated by amperometry and/or voltammetry.
- Bioaffinity - The commonly used molecules here are: antibodies, aptamers, single stranded DNA/RNA nucleic acids, molecular imprinted polymers (MIPS). These types of sensors are commonly interrogated by amperometry and/or voltammetry.
- Ionophores - For the detection of ions such as sodium, potassium, chloride, calcium, ammonium etc, a corresponding ionophore exists. These types of sensors are commonly interrogated by potentiometry.
- Microbial - These are sensors where microbes are used to sense. These types of sensors are often based on electrochemical fuel cells, where the power out put of the microbial fuel cells is proportionate to the health of the microbial population, and so nutrients and/or toxins in the sample effect the power (signal) proportionally.
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Main discussion
Enzyme Biosensors - As discussed above Leyland Clark developed the first biosensor in 1956, this original sensors was for the measurement of dissolved oxygen (DO). Subsequently Professor Clark converted this initial sensor into a glucose sensor by the addition of glucose oxidase to the sensor. The Clark glucose sensor ran in a cathodic amperometric mode.
Bioaffinity Biosensors - Immunosensors were initially established on the principle that antibodies have high affinity towards their antigens. In more recent years aptamers have become more interesting. The proponents of aptamer based sensors like the fact that they can have a synthetic route to these recognition elements. For genetic diseases, such as cystic fibrosis, then commercial electrochemical biosensors are on the market from GenMark Dx (GenMark Dx was purchased Roche), these sensors use nucleic acid capture probes. One of the fastest routes to a recognition element can be molecular imprinted polymers (MIPS).
Ionophores - Ions such as sodium, potassium, calcium, chloride, pH can be detected by ion-selective electrode (ISE) using potentiometry. The larges ISE commercially used is the pH probe, click for an example.
The main application of biosensors has historically been the medical field, and as discussed above the biggest biosensor by manufacturing volume and value is SMBG, but as discussed above the fastest growing technology in the diabetes application space is CGM.