Biosensors have revolutionized the fields of medicine and biotechnology through the detection and characterization of target molecules in complex biological fluids. A biosensor is a device that uses a biological receptor molecule, such as an enzyme, to detect a specific compound. Biosensors use different methods, such as electrochemistry, mechanical properties, or optics, to detect the binding of the specific molecule to the receptor. This video will introduce biosensors and the biosensing field by discussing some basic techniques and types of biosensors as well as their applications.

First, let's discuss the basics of how a typical biosensor works. A biosensor consists of a biological recognition system, often called a bioreceptor or the probe molecule. This is usually an immobilized biomolecule, such as an enzyme, antibody, or nucleic acid, which is used to selectively capture a target molecule. The binding between the probe and target molecules causes a measurable event, such as a pH change, optical change, or redox event. This change is measured by the transducer, which converts the binding information into a quantifiable signal. Some biosensors quantify the amount of bound target, providing the user with a concentration. However, some are used to simply confirm the presence of a molecule, like in a pregnancy test. The first biosensor was the oxygen sensor developed by Leland C. Clark. The sensor utilized glucose oxidase, which is reduced when glucose binds. In the presence of oxygen, glucose oxidase is then oxidized, producing hydrogen peroxide in a side reaction. When hydrogen peroxide is oxidized, an electron is lost and measured by the electrode. The more oxygen present, the more hydrogen peroxide produced and the more electrons measured. This discovery paved the way for modern biosensors. Now that we've introduced biosensors and a bit of their history, let's take a look at some common types of biosensors.

Optical sensors use light to detect the binding of a target molecule to the probe molecule. A very simple example is a fluorescence-based sensor. Here, the surface of the sensor is coated with a polymer, which has a low basal fluorescence. When the polymer surface is functionalized with the probe molecule, in this case, single-stranded DNA, the fluorescence increases. When the target molecule, the complementary DNA strand, binds to the target strand, the increased fluorescence goes away, enabling the user to determine the quantity and location of binding based on fluorescence intensity. Another common type of biosensor is the electrochemical sensor, which uses electrodes to sense redox reactions between the probe and target molecules. This is commonly done using an enzyme bound to the electrode surface. When the target molecule binds to the enzyme under a specific applied potential, the reduction or oxidation of the complex occurs. This creates either a surplus or deficit of electrons, which is directly proportional to the amount of bound target molecule.

The field of biosensing is not without its challenges. First, one key challenge is the contamination of the biosensor surface by other molecules in the sample, called biofouling, which can occur in complex mixtures. This contamination can block the probe molecules from sensing the target molecule in the sample, thereby greatly diminishing the sensor's detection ability. As a result, some biosensors have a limited lifetime and are either regenerated or disposed of. Alternatively, anti-biofouling coatings may be developed to mitigate this effect. The detection limit of a sensor refers to the lowest quantity of substance that can be reliably distinguished from the absence of that substance. A low detection limit is advantageous in detecting trace amounts of a substance with certainty, with single molecule detection being the ideal scenario. However, this can be difficult, as low concentrations often result in weak signals that are below the noise and are difficult to quantify. Much of current research is aimed at improving detection limits by improving binding efficiency and reducing noise.

Now that we've discussed some common types of biosensors along with their challenges, let's take a look at some applications of these basic concepts. A commonly used sensor is the quartz crystal microbalance, or QCM. QCM consists of two gold electrodes separated by a quartz crystal, which has piezoelectric properties. When an alternating current is applied, oscillations with a specific resonant frequency are induced. This resonant frequency changes when molecules bind to the surface. This change is used to detect the binding of the target molecules and the amount. Specialized cantilevers use mechanical properties to detect the binding of target molecules. Here, the cantilevers are functionalized with probe molecules and then exposed to the target molecule. Upon binding of the target molecule, the cantilever deflects due to the changes in surface stress. This deflection is then measured using a laser.

You've just watched JoVE's Overview of Biosensing. You should now be familiar with the basics of biosensors, some key types of sensors and their challenges, as well as some applications in the field. Thanks for watching.