Abstract
In Chap. 1, we have detailed the potentialities of the CNT-FET configuration for label-free transduction of an analyte/receptor interaction. As mentioned, such interaction can be transduced into a readable electrical signal by following up the conductance changes of a semiconducting CNT.
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Notes
- 1.
As said above in the liquid-gated CNT-FET a voltage is applied between the CNT and a reference electrode immersed in the solution. Owing to electrostatics interactions, a layer of counter ions is formed close to the CNT and acts as a dielectric layer (double layer).
- 2.
The differences in the thickness of the double layer (in the liquid-gated CNT-FET) and the oxide thickness (in the back-gated CNT-FET) have effects in their capacitance values. The lower the thickness of the dielectric the higher the capacitance (the capacitance, C, is inversely proportional to the separation between conducting sheets). That parameter affects in the accumulation of charge (Q) at the capacitor and on the voltage (V) that is needed for accumulating that charge (Q = CV). If one wants to accumulate a certain amount of charge it would need lower voltages if the capacitance is large. In the case of a FET operation, higher capacitances can induce more effective charge accumulation and hence more modulation in the CNT conductance. All these effects make that the voltage that is needed to accumulate certain charge and modulate the CNT conductance will be smaller in the case that higher capacitances (smaller dielectric thickness) are involved. In that case it is said that the gate coupling is more efficient and large Fermi level shifts can be achieved.
- 3.
The total capacitance (Ct) in a CNT-FET system is composed of the gate capacitance (Cd, capacitance of the dielectric, the double layer capacitance in wet conditions) and the quantum capacitance (Cq) of the tube. The quantum capacitance comes from the Pauli Exclusion Principle, which forces electrons to occupy higher energy levels once the lower ones are occupied, leading to an extra increase in the electrical potential. These two capacitances are in series, 1/Ct = 1/Cd + 1/Cq. It may happen that under certain circumstances in the liquid-gated CNT-FET, Cd can be large enough that Cq can become the dominant term in Ct and the conductance of the tube does not depend significantly on Cd and thus on the liquid composition.
- 4.
At room temperature and in aqueous solutions the Debye length (λD) can be expressed as \( \lambda_{D} (\text{nm}) = \frac{0.304}{{\sqrt {I(\text{M})}}} \) where λD is in nanometers and the ionic strength (I) in mol/L.
- 5.
We have shown in the experimental section that pyrene adsorbs on the CNT walls, a process that has been checked with confocal fluorescence microscopy.
- 6.
Just for recalling some experimental tests, the carbodiimide coupling was successfully followed by XPS measurements.
- 7.
When the concentration of a buffer varies, the ratio of the concentration of buffer components remains constant. However it is the variation in the ratio of the activity coefficient of the solution components that leads to variations in the pH value of the buffer solution. The effect is an increase of the pH depending on the dilution factor.
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Pacios Pujadó, M. (2012). Results and Discussion: Electronic Response of Carbon Nanotube Field-Effect Transistors to Biorecognition Processes. In: Carbon Nanotubes as Platforms for Biosensors with Electrochemical and Electronic Transduction. Springer Theses. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-31421-6_7
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