Physics of Biosensors
Nabendu Sekhar Mishra
BS-MS 4th year, Physical Sciences
With the world pacing towards automation and reduction of manual labour, sensing applications have witnessed a tremendous uprise. In particular, the advent of biosensors in the 1960s has revolutionized the fields of biomedicine and biotechnology. For biosensors, the science is a tantalizing blend of disciplines and the scientific challenge is formidable. The multidisciplinary expertise that underpins successful research and development in biosensors include life sciences such as pharmacology, protein chemistry, immunology, genetic engineering and biochemistry; and from the physical sciences side, materials science, nanotechnology, electronics, surface chemistry or, more succinctly, physics.
Biosensor: Definition and Components
As per IUPAC, 1996, “A biosensor is a self-contained integrated device that is capable of providing a specific quantitative or semi-quantitative analytical information using a biological recognition element which is in direct spatial contact with a transduction element.” In Layman terms, this can be rephrased as: “a device that senses some changes in a biological environment and produces a proportionate response in terms of a parameter that can be measured conveniently.” The sensor is composed of three essential components: the detector, which recognizes the physical stimulus; the transducer, which converts the stimulus to a measurable output; and the output system itself, which involves amplification, display, etc. in an appropriate format. In case of a biosensor, the detector is a bio-recognition element or a bioreceptor that detects changes in a biochemical environment. This component is predominantly the domain of a biologist or a biochemist and shall not be a topic of discussion here. The third essential component, the output system deals with hi-tech electronics and information technology. The other component, the transducer system very much falls in the domain of a physicist and would be the pivotal subject of further discussion. In a simple choice of words, a transducer converts the form of signal which is received from a bioreceptor, to a more readable and convenient form of signal. Based on the types of signals to which the bioreceptor signals can be converted, and on the mechanism of manifestation of these signals, biosensors can be of numerous types. The studies and development pertaining to these fall in the domain of physics and engineering.
Basic principle
The device is placed in the required working environment where the bioreceptor is exposed to the ‘analyte medium’. Analyte medium refers to the medium containing the biomolecules that are to be sensed and measured. Upon exposure, the analyte poses a response so as to trigger a change in a physical parameter. The transducer recognizes this trigger and the amount of change in the parameter, and further converts this change into a change in a more readable parameter. The change in parameter that the transducer receives and the one that it produces, together decide the category to which the biosensors belong. The output from the transducer is sent to an output system which contains an amplifying unit and a display unit. The amplifying unit amplifies the obtained signals to an amount/scale that can be read conveniently on the display unit. Figure 1 shows a schematic diagram of the components of a typical biosensor.
This is the basic construction of a biosensor:
Types of biosensors
Depending on the trigger reception from the analyte and the process of transduction, biosensors can be broadly of the following types:
I. Electrochemical Biosensor
II. Piezoelectric Biosensor
III. Thermometric Biosensor
IV. Optical Biosensor
I. Generally, an electrochemical biosensor is based on the reaction of enzymatic catalysis that consumes or generates electrons. Such types of enzymes are named as Redox Enzymes. The substrate of this biosensor generally includes electrodes which are connected to the transducer system. The analyte’s response triggers the electrodes and is translated into a more convenient parameter by the transducer, depending on the principle involved in transduction. Based on the principle of converting the transduced signal to a readable output, electrochemical biosensors are sub-divided into Amperometric Biosensors, Potentiometric Biosensors, Impedimetric Biosensors, Voltammetric Biosensors.
II. A piezoelectric biosensor uses a piezoelectric crystal (Aluminium Phosphate, Barium Titanate, etc.) which senses mechanical stress when the analyte medium changes (depending on the mass of the analyte), and this causes a change in the oscillation frequency of the AC circuit in the transducer system.
III. Thermometric biosensors can be thought of as a fancy and precise version of a simple thermometer. These biosensors are used to sense heat flow to the substrate from the analyte and project the response in a linear scale of temperature. These are most commonly used for sensing lipids and cholesterol which exchange heat during oxidation.
IV. Optical Biosensors work on the principle of change in an optical parameter due to change in the analyte medium. The most widely used ones exploit the principles of total internal reflection and evanescent wave coupling to detect a change in reflectance curves with respect to a change in refractive index. The details of the processes involved and an insight on the design of these biosensors have been entailed below.
Optical Biosensors: A Closer Outlook
An optical biosensor may use a multitude of processes for signal transduction as well as numerous optical structures as the transducer. The figure above explains it in a schematic form. However, all the optical structures work on a common principle: coupling of evanescent waves to the analyte medium and detecting the change in an optical parameter (most commonly refractive index) due to any changes in analyte. The structures are usually multi-layered, with the optical constants - refractive index and absorption - correctly matched between the layers so as to achieve effective coupling of the light waves with the carriers (usually electrons) on the surface of the bioreceptor. If the layer is dielectric, the evanescent waves are called Bloch Surface Waves; if the layer is metallic, the waves are called Surface Plasmons. There are two basic approaches for optical sensing: (i) Angular Interrogation, and (ii) Spectral Interrogation. In angular interrogation, light of a particular wavelength is incident at various angles in a certain angle range, and the net reflectance is plotted as a function of angle of incidence. Correspondingly, spectral interrogation requires the angle of incidence to be fixed and reflectance to be plotted as a function of wavelength of incident light. Here comes the real physics! When light is incident upon an optical structure, the carriers inside the material interact with the incident photons and at a certain wavelength or angle of incidence when their momenta match, the coupling is maximum, a condition called resonance. This causes a dip (called resonance dip) in the reflectance plot at that wavelength / angle. In particular, when metal layers are involved, the resonance occurs as a consequence of interaction between photons and plasmons and the phenomenon is called Surface Plasmon Resonance (SPR). Now, a change in analyte medium causes a change in refractive index and this in turn, causes a shift in the resonance dip. The shift in resonance dip (in terms of wavelength/angle) per unit refractive index change is termed as sensitivity of the sensor. In all sensing techniques including biosensing, having sensitivity on the higher side is always an advantage, and optical biosensors offer excellent sensitivity. This is one of the reasons why optical biosensors have received such dedicated focus in recent times. The layers of materials in the structure are chosen and arranged keeping in mind the efficiency of coupling and the strength of resonance that they can manifest. For example, a hybrid structure of dielectric and metal (e.g. SF7/SiO 2 /TiO 2 /Ag) can excite both Bloch surface waves and surface plasmons in the analyte medium, thus enhancing the sensitivity as well as electromagnetic fields exponentially over what would have been obtained from an ordinary (say, purely dielectric) structure.
The methods involving other processes (fluorescence, absorption, Raman scattering, etc.) are not as widely used owing to the heavy costs they incur. Optical biosensors offer several pros in addition to sensitivity: high resolution, immunity to external disturbances, low noise, low signal processing requirements, and most importantly, simple design and moderate cost. Nevertheless, the scope of improved biosensing in future, be it in optical or other arenas, is immense. Clearly, optical sensors shine bright in the future of sensor technology; the exploration is still in its dawn!