Materials and Characteristics of Biosensors

Sunday, November 26th, 2017 - Chemical, Resistive Transducers

Materials and Characteristics of Biosensors

The most important characteristics of biosensors include selectivity (or specificity), sensitivity, response time, regenerability, and simplicity. Selectivity, meaning detection selectivity, depends entirely on the inherent binding capability of the bioreceptor molecule, whereas sensitivity depends on both the nature of the biological element and the sensing material. The biomaterials that can be used as biological sensing elements include organisms, tissues, cells, organelles, membranes, enzymes, receptors, antibodies, and nucleic acids. The bioagents that can be recognized by the biorecognition elements are metabolic chemicals (oxygen, methane, ethanol), enzyme substrates (glucose, penicillin, urea), ligands (hormones, pheromones, neurotransmitters), antigens and antibodies (human immunoglobulin, antihuman immunoglobulin), and nucleic acids (DNA, RNA).

A biosensor should be :

  1. highly specific for the analyte of interest;
  2. responsive in the appropriate concentration range and have a moderately fast response time;
  3. reliable, reproducible, accurate, and sensitive; and
  4. miniaturizable.

Electrodes used for bioelectrical recording and stimulation can be classified as noninvasive or invasive types. Noninvasive (mostly surface type) electrodes are used outside the body (e.g., on the skin), while invasive electrodes use a needle to punch through the skin and are inserted beneath the tissue. An invasive electrode is only used for specialized cases (e.g., anesthetized patients and skin infections) and in veterinary applications.

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Figure 1. Structure of an Ag–AgCl electrode.

Another type of invasive electrodes is the indwelling electrode. An indwelling electrode, inserted into the body rather than into the layers beneath the skin, is typically a tiny exposed metallic contact at the end of a long insulated catheter (e.g., the catheter for veins or arteries). There are two categories of electrode materials: perfectly polarized (or called perfectly nonreversible) and perfectly nonpolarized (or called perfectly reversible). In the first type, there is no net transfer of charge across the metal–electrolyte interface, while in the second type, there is a free transfer of charge between the metal and the  electrolyte. Silver–silver chloride (Ag–AgCl) is an ideal polarized (Ag) and nonpolarized (AgCl) pair, which becomes excellent standard, and is by far the most common reference electrode used today due to its simplicity, low cost, stability, reproducibility, and toxin-free character. Figure 1 shows an Ag–AgCl electrode consisting of a silver (Ag) substrate with a thin layer of silver chloride (AgCl) deposited onto its surface. The AgCl provides a free two-way exchange of Ag+  and Cl ions between the metal Ag and the electrolytic solution.

Besides silver–silver chloride, other noble metals and alloys, for example, gold and platinum, tungsten alloys, and platinum–platinum black, are also used to make practical bioelectrodes, not only because of their ideal features but also their resistance to corrosive biofluids inside the human body. A conductive gel or paste is also applied when using bioelectrodes to reduce the impedance between skin and electrodes. One of the commonly used bioelectrode gels is a 0.5% saline-based electrode gel. It has high conductivity and is hypoallergenic, bacteriostatic, water soluble, and nongritty. Note that not all electrodes are acceptable for bioapplications. For example, zinc–zinc sulfate, although it has ideal features, is toxic to living tissues.

I hope this information about “Materials and Characteristics of Biosensors” is useful.