**New Insights on Optical Biosensors: Techniques, Construction and Application**

Tatiana Duque Martins, Antonio Carlos Chaves Ribeiro, Henrique Santiago de Camargo, Paulo Alves da Costa Filho, Hannah Paula Mesquita Cavalcante and Diogo Lopes Dias

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/52330

**1. Introduction**

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Since Clark´s enzymatic electrode in 1962s [1], biosensors have been proposed for a range of application and, aiming clinical analysis application, the amperometrical, potentiometrical and optical are the biosensors which have achieved most significant development. In con‐ cept, optical biosensors are those based on the detection of changes on absorption of UV/ visible/Infrared light when chemical reactions occur or on the quantity of light emitted by some luminescent process. Regarding to supramolecular nanostructures and their ability of enhancing the sensing activity when applied to biosensors construction, a very instigating work, presented by Jin Shi et al. [2] showed a way of turning carbon nanotubes into more water-soluble compounds and, consequently, more biocompatible by modifying their sur‐ face with a synthetic DNA sequence. In this way, the carbon nanotubes can overlay the bio‐ sensor electrode more efficiently, enhancing the biosensing activity. Lieden et al. [3] also took advantage of the properties of nanotubes in biosensing. In their work, they showed that a biosensor can present a rate of detection tree times faster when carbon nanotubes are used in the nanobiosensor construction, preventing the attachment of protein to the device components. The change in electric resistance of carbon nanotubes when proteins touch them is immediate, which confer to the device a fast recognition ability, and leads to an in‐ creased efficiency of the biosensor. Yet, other fascinating works are the one presented by

© 2013 Martins et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Martins et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Chen et al. [4] and the work presented by Park et al.,[5] in which a piezoelectric nanogenera‐ tor was developed to feed several devices, including implantable biosensors, which make of this research field very promising, since nowadays, implantable biosensors present the dis‐ advantage of the need to be recharged or replaced when discharged.

In a common sense, biosensor is a device constructed to inform about a system, requiring as less human action as possible. It is formed of sample holder, a biological recognition element which must be selective, a physical transducer to generate a measurable signal proportional to the concentration of the analytes and the signal processing unit, which gives to the ana‐ lysts graphical, numerical or comparative information that they shall interpret (fig.1). The recognition element can be of almost any type of biological system, from antibodies, pro‐ teins and peptides to viruses, microbes, cells and tissues.The selection of the appropriate recognition element considers not only what is the information to be obtained, but also the ease of construction of the devices employing such element and, of course, their durability. As an example, some microbes have been employed instead of antibodies and proteins mostly due to their facile production via cell culture and in vitro stability. Nevertheless, it is evident that microbes might lead to a lack of selectiveness, due to their non-specific metabo‐ lisms. Recent research had proposed that highly selective microbial biosensors can be con‐ structed by inducing a desired microbe metabolic pathway and by adapting them to the substrate of interest, using selected conditions of cell-culturing. Also, as alternative to ma‐ nipulate the selectivity and sensitivity of microbial biosensors at the DNA level, the geneti‐ cally engineered microorganisms (GEMs) had been proposed. [6]

**Figure 1.** General scheme of a biosensor.

Although it is of extreme importance to accurately choose a recognition element to propose a suitable biosensor, it is of great importance to define a way to turn the recognition event into a signal that can be detected and interpreted. The transducer figures now as the element which is able to transform the biochemical response into a recognizable physical signal. For example, techniques of immobilizing microorganisms on transducers had played important roles in the fabrication of microbial biosensors. [7] Some traditional methods of immobiliza‐ tion include adsorption, encapsulation, entrapment, covalent binding, and cross-linking, mainly via sol-gel processes and, in special, immobilization of microorganisms in conduct‐ ing polymers are of great interest due to their unique electrochemical properties, which can be exploited on the transduction function. [8, 9]

Chen et al. [4] and the work presented by Park et al.,[5] in which a piezoelectric nanogenera‐ tor was developed to feed several devices, including implantable biosensors, which make of this research field very promising, since nowadays, implantable biosensors present the dis‐

In a common sense, biosensor is a device constructed to inform about a system, requiring as less human action as possible. It is formed of sample holder, a biological recognition element which must be selective, a physical transducer to generate a measurable signal proportional to the concentration of the analytes and the signal processing unit, which gives to the ana‐ lysts graphical, numerical or comparative information that they shall interpret (fig.1). The recognition element can be of almost any type of biological system, from antibodies, pro‐ teins and peptides to viruses, microbes, cells and tissues.The selection of the appropriate recognition element considers not only what is the information to be obtained, but also the ease of construction of the devices employing such element and, of course, their durability. As an example, some microbes have been employed instead of antibodies and proteins mostly due to their facile production via cell culture and in vitro stability. Nevertheless, it is evident that microbes might lead to a lack of selectiveness, due to their non-specific metabo‐ lisms. Recent research had proposed that highly selective microbial biosensors can be con‐ structed by inducing a desired microbe metabolic pathway and by adapting them to the substrate of interest, using selected conditions of cell-culturing. Also, as alternative to ma‐ nipulate the selectivity and sensitivity of microbial biosensors at the DNA level, the geneti‐

Although it is of extreme importance to accurately choose a recognition element to propose a suitable biosensor, it is of great importance to define a way to turn the recognition event

advantage of the need to be recharged or replaced when discharged.

112 State of the Art in Biosensors - General Aspects

cally engineered microorganisms (GEMs) had been proposed. [6]

**Figure 1.** General scheme of a biosensor.

A number of possible transducers can be proposed, once the interaction between analyte and recognition element is defined. Available techniques are of a large number and to elect the proper one very often is not a trivial task. Among various sensing techniques, there are piezoelectrical, calorimetrical, enthalpimetrical, DNA microarray, Surface Plasmon Reso‐ nance (SPR), Impedance Spectroscopy, Scanning Probe Microscopy (SPM), Atomic Force Mi‐ croscopy (AFM), Quartz Crystal Microbalance (QCM), Surface Enhanced Raman Spectroscopy (SERS), but electrochemical and optical techniques are the widest used in the development of microbial biosensors, due to their numerous possibilities, which turn possi‐ ble the construction of a number of selective sensors.Electrochemical biosensors are classi‐ fied as amperometric, potentiometric, conductometric, voltammetric, depending on which detection principle is employed in the biosensor. In a quick overview on these important de‐ vices, an amperometric biosensor is the one that operates at a given applied potential be‐ tween the working and the reference electrodes. A current signal, related to analyte´s concentration in the sample, is then generated, due to the reduction or oxidation process suffered by an electroactive metabolic product. A conductometric biosensor is that in which a conductivity change is observed upon production or consumption of ionic species in‐ volved in the metabolic process. It became a very attractive device due to its enhanced sensi‐ tivity and fastness brought about through sophisticated modern analytical techniques. Additionally, they are suitable for miniaturization once it requires no reference electrode in the system. [10] Its disadvantage lies on that all charge carriers lead to a change of conduc‐ tivity, which directly affects the device selectivity and is known as relatively poor.

The potentiometric biosensor is based on the potential difference between working and ref‐ erence electrodes. In these biosensors, the measured species is not consumed, as it is in the amperometric biosensor. Its response is on the activity of the species in comparison to the reference electrode, with the output signal recorded in voltage units and, independently of the sensor size, the signal is proportional to natural the analyte concentration. Its great ad‐ vantage lies on sensitivity and selectivity, if the working electrode is species-selective. How‐ ever, a highly stable and accurate reference electrode is always a requirement.

The most versatile electrochemical technique applied to biosensors is voltammetry, since both current and potential difference combined, consist on a reasonable system response. Its major advantage its successfully application as a multi-component detector.
