Liquid metal based junctions

Fig. 5 Schematic representation of LAJs based on liquid metal electrodes, (a) The two Hg drops junction. The drops are extruded from two microsyringes and covered singularly by similar or different SAMs before being brought in contact, (b) An Hg-drop electrode covered by SAM(l) (usually formed by hexadecane thiol) is brought in electrical contact with a SAM(2) formed on a solid metal surface, (c) A drop of In/Ga eutectic alloy (E-Gain) contacts a SAM formed on a solid electrode surface...

The two metal surfaces covered by SAMs are brought into contact by the use of a micro-manipulator in the presence of a liquid medium, such as hexadecane the presence of hexadecane transforms the defects of the SAMs into insulating sites. The use of a semitransparent solid surface (Au or Ag) allows (1) evaluation of the contact area by collecting the image of the contact area by a mirror and (2) electrical measurements under irradiation of the SAMs through the Au surface. The disadvantage of Hg-based junctions is related to the environmental unfriendly characteristics of Hg, which prevent any application. For this main reason, these junctions are valuable only as versatile test-beds for organic electronics. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

Tao et al. [32] pioneered a technique based on the formation of single molecular junctions between the tip of an STM and a metal substrate. The method was adapted by other groups, modified and applied to a large number of molecular conductance studies at (electrified) solid/liquid interfaces [33, 113-119]. For details we refer to Sect. 2.3. 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

In many other cases, however, there is a direct influence of the reducing agent on the metal deposition reaction. This is observed for NiP autocatalytic deposition, de Minjer22 showed that it is possible to deposit nickel cathodically when the electrode is connected through a liquid junction to another nickel electrode, at which the reducing agent oxidation proceeds independently. However, the rate of deposition under these conditions, based on de Minjer s observations, is much lower when the two reactions are separated than when they occur simultaneously at the same surface. 

Albert A, Avidity of terramycin and aureomycin for metallic cations. Nature, 172,201 (1953). Cited in Perrin Bases 3329 ref. A17. NB The study used pH measurements wiBi a glass electrode and liquid junction potentials. 

A large variety of quasi-reference electrodes have been used based on metal wires such as silver [40], platinum [41], aluminium [42], tungsten [43], magnesium [44], etc. Typically electrodes are placed directly into the IL solution or separated from the main solution inside a fritted glass tube/compart-ment. In order to minimise liquid junction potentials in the latter case, the same ionic liquids as per the measurements is used as electrolyte. It is the authors experience that separation of the quasi-reference electrode allows for a reduction in potential drift when compared to direct insertion into the electrochemical cell. 

---