ELECTROCHEMICAL BIOSENSORS
Why is the topic relevant
Biosensors are very relevant because there is an increasing need in our society for inexpensive point of care diagnostics, portable field microanalyzers and highly sensitive substance-recognition systems for a wide variety of security applications, due to the importance that healthcare is achieving with the aim of improving the quality of life of the patient.
Biosensors were first reported in 1960s. They are a class of devices that have found a widespread use, ranging from the detection of gas molecules to the real time tracking of chemical signals in biological cells.
In general a sensor comprises an active sensing element and a signal transducer and produces an electrical, optical, thermal or magnetic output signal. The sensing element is responsible for the selective detection of the analyte and the transducer converts a chemical event into an appropriate signal that can be used with or without amplification.
In biosensors, the sensing element consists of a biological material (protein, cell receptor, antibody, enzyme). The sensor is used to monitor biological processes or for the recognition of biomolecules. The sensing of the biomaterial can be made in vitro or in vivo:
-In vitro biosensing
The sample solution (blood, urine) is dropped atop the biosensor and the output signal gives information on the composition of the solution. For example: measurements of blood glucose concentrations.
-In vivo biosensing
Dynamic systems aiming for instance to measure the rate of uptake of relevant species or to estimate the spatial distribution of the concentration of an analyte in a living organism. For example: portablesensingdevicesforcontinuousmonitoringofparametersofbloodglucose, carbondioxide, PH, neurochemicalanalysis.
Lab on chip devices have to be mentioned too. LOCs integrate one or several laboratory functions on a single chip of only millimeters to few square centimeters in size. Apart from the small size, this devices have the advantages of low fluid volume consumption, faster analysis and response time, lower fabrication cost, etc. For example: October 31, 2007 Researchers in Singaporehavesuccessfullydeveloped a miniaturizeddevicethat can be usedtodetectthehighlypathogenicavianflu (H5N1) virus.
What is added value of being nano
The special properties that low dimensional systems have, are the key point for the advances made in the field of biosensors. This field is increasing rapidly due to advances made in new materials (nanomaterials) and new fabrication processes.
Previously low sensor to sensor reproducibility and rather poor signal stability have negatively affected the mass production of biosensors and their commercial use. Today however, nanotechnology plays a major role in the development of biosensors.Nanoscience and nanotechnology involve the synthesis, characterization, exploration, manipulation and utilization of nanostructured materials which are characterized of being at least one dimension smaller than 100nm.
Nowadays, biosensors are chips that integrate arrays of sensors, microfluidic actuators and processing in the same chip. The objective is to create more selective biosensors that permit the realization of various tests simultaneously in the same chip, using small sample quantities, reduce the response time and the size/cost of the dispositive.
For example, the construction of DNA biosensors,is based on the immobilization of the ssDNA or dsDNA onto the surface or in the bulk of the working electrode. The working principle is based on the detection of specific interactions, such as DNA hybridization and association interactions with low molecular weigth compounds (drugs, risk chemicals) in addition to the structural damage of DNA. During recent years nanoparticles and carbon materials, like carbon nanotubes, nanofibers, fullerenes and diamonds have begun to be used widely in the preparation of DNA biosensors.
Nanoparticles have now found applications in novel electronic devices, drug delivery systems, biomaterials and biomedicine. The main aim of applying nanomaterials to DNA electrochemical biosensors is to improve the immobilization of DNA molecules, as well as to enhance molecular recognition and signal transduction events. Although the primary advantage of nanostructured materials is their high surface area, the chemical modification of these materials by enzymes and electroactive molecules has led to significant improvements in electrochemical sensing.
What are the relevant questions that need to be answered?
The field of electrochemical biosensors has still many aspects that need to be improved and better understood and there are also many important biomoleculesor diseases that can´t be detected with biosensors yet, so there is a lot of work to do in this field. Here I mention some research lines:
-Biosensors for cancer related testing.
-Single molecule detection using intracellular nanosensors.
Implantable nanosensors can provide unique insights into biological or cellular metabolism in the very near future.
-Innovative signal transducers fabricated using silicon nanowires.
Selectively detect individual virus particles in solution.
-New methods of layer-by-layer fabrication.
Improved designs of biosensor platforms. These simple methods provide a mechanism for fine tuning the sensing layer and providing a greater degree of flexibility in biosensor design.
-Interfacing sensors with tissue, skin, and other biological structures.
-The types and classes of materials used as the primary building blocks of biological based sensors are being expanded.
A number of biosensor designs exploit neuronal tissue networks as the basic sensing element.
For new advances in biosensors, sciencist are also trying to understand and mimick biology: In what ways biosensors can become smarter than they have already been? Perhaps a “smart sensor” should have some capability of “comprehension” of those important features that distinguish the human brain from the most advanced digital computers. One way to achieve this is to turn to biology for inspiration.Biomimetic science (imitating nature in an intelligent way).
Is quantum confinement relevant?
It was answered in the second question, that nano plays a major role in the advances made in biosensors therefore, quantum confinement will be relevant.
Nanostructured materials, materials whose structural elements have dimensions in the 1 to 100nm range, are used in biosensors because of their high surface area, high reactivity, easy dispersability and rapid fabrication. Dendrimers, polymers, nanoparticles, nanotubes, oxides, enzymes and their hybrids are used as catalyst for various sensors such as glucose sensors, DNA sensors, etc. We will see the application of CNTs in electrochemical biosensors in more detail:
CNT in electrochemical biosensors.
Unique behavior of carbon nanotubes including their remarkable electrical, chemical, mechanical and structural properties. CNT can display metallic, semiconducting and superconducting electron transport, posseses a hollow core suitable for storing guest molecules and have the largest elastic modulus of any known material. These properties make CNTs extremely attractive for a wide range of electrochemical biosensors ranging from amperometric enzyme electrodes to DNA hybridization biosensors. To take advantages of this materials in electrochemical biosensing, the CNT need to be properly functionalized and immobilized.
A promising research direction is to directly ‘wire’ redox enzymes to carbon nanotubes. This way a measurement of the electrochemical current gives direct information on the activity of the enzymes. We aim to study the turnover of enzymes at the single-molecule level by using the small dimensions of carbon nanotubes to relay electrons from a single enzyme.