**5. Some footnotes about impedimetric biosensors**

mined, which is most important glucose. Several studies were carried out glucose biosens‐ ing. Using living cells as biorecognition receptors provides an opportunity for high sensitivity in a broad range of biologically active substances that affect the response of the cells. In this study, 3T3-L1 fibroblast cells are able to metabolise glucose through the activa‐ tion of specific glucose transporters (Glut 1 and Glut 4). Biological cells are very poor con‐ ductors at low frequencies, for this reason, force electrical currents to bypass them. Impedance spectrums were proportional to fibroblast cell growth on electrode; especially gaps between cells will affect impedance spectrums. An ITO (Indium Tin Oxide) electrode was used for this biosensor, fibroblast cells were attached on electrode, after 2-4 days a thin layer was formed by fibroblast cells. The effect of additional glucose concentrations, Nyquist diagram was chosen, which allows more sensitive measurement. Glucose and cell interac‐ tion depended on the glucose intake ability of cells through Glut 1 and Glut 4. To examine the specificity of the biosensor, D-mannitol was chosen instead of glucose and no variation was observed. No signal variation was obtained by inhibition of glucose transporters, there‐ fore glucose intake provides metabolic process in order to detect glucose molecules, not in‐ corporated by increased osmotic pressure. Addition of carbohydrate not metabolized by cells did not give any signal variation in impedance curves. The calibration curve was con‐ structed from 0 to 14mM glucose concentration [32]. On the other hand for this study, as a biorecognition receptor, cell surface transporters were used for biosensor indirectly. This

provided selective measurement.

190 State of the Art in Biosensors - General Aspects

**Figure 7.** Schematic representation of fibroblast cell based impedimetric biosensor.

One of the most important points of impedimetric biosensors is to find proper molecule that shows affinity to your analyte. Secondly your measurement process will be considered; es‐ pecially modification steps and oxidation/reduction ability of redox probe will define exper‐ imental parameters such as frequency, bias potential, electrical circuit model. In order to obtain sensitive detection, faradaic impedance is proposed to reach lower frequencies, be‐ cause of reduction/oxidation properties of redox probe, the electrons, which produced by this oxidation/reduction, can move easier. This transportation can be measured as electron transfer resistance. On the other hand non-faradaic impedance is only measure resistance and surface capacitance, that technique is for whole surface by electrical circuit model.

Electrical circuit model is constructed by characteristic of impedance curve, which depends on the electrical conductibility of electrochemical measurement solution, electrode surface and interaction between electrode-electrolyte. Impedance spectroscopy provides sensitive and label free detection by its sensitive electrode surface characterization ability. Especially for sensitive measurements faradaic experiments are considered that is better, by using re‐ dox probes, it is possible to reach lower frequencies, thus lower detection limits can be reached by examination of the modified electrode surface in lower frequencies. Electrode surface modification plays a crucial role for impedimetric measurements that inhomogenici‐ ty on electrode surface, pin holes or a direct interaction with bare electrode surface after a failed modification and electrolyte, the electrons, which through electrode surface without confront any electrical resistance, will give a false impedance spectrum. In this case of inho‐ mogenity, capacitance element of circuit model is redefined as constant phase element to solve inhomogenity problem. In case of lower or less altered signals, impedance spectrosco‐ py provides signal amplification by modifying electrode surface with a molecule or elec‐ trode surface can be charged same as redox probe's charge. Another challenging factor for impedimetric biosensor is the affinity, low affinity or affinity for wide range will reduce the importance or usefulness of the impedimetric biosensor.


**Table 1.** Comparison of Some Impedimetric Biosensor Systems
