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Mercury reduction and oxidation on ultrathin gold electrodes was investigated by simultaneous voltammetry and surface plasmon resonance (SPR). The angle dependencies of SPR signal were analyzed quantitatively. The results indicate partially reversible formation of amalgam with optical properties in between of optical properties of mercury and gold. Up to 12% content of mercury in the gold layer is suggested.
This chapter presents a procedure to prepare and calibrate chemical sensor for mercury vapor and discusses application of the sensor for measurements of mercury vapor. The sensor is based on high affinity of gold to mercury and on chemoresistive properties of ultrathin gold layers: adsorption of mercury leads to increase in the surface resistance. However, this effect is not selective: similar changes are caused by adsorption of water vapor and sulfuric compounds. The use of monomolecular layer of alkylthiols as a filter excludes this interference completely. Sensor calibration is performed by thermoinjection of nanogram-amount of mercury quantitatively deposited by electrochemical reduction. It is found that the sensors display no sensitivity to such typical interferents of mercury chemoresistors based on uncoated gold as volatile thiols and water. Preliminary data demonstrate a possibility to measure Hg(0) even in aqueous solutions.
A number of analytical methods were developed for determination of elemental mercury. They include traditional analytical techniques, such as atomic adsorption spectroscopy (AAS), atomic fluorescence spectroscopy (AFS), and atomic emission spectroscopy (AES). Other analytical methods include neutron-activation analysis and laser photo-acoustic spectroscopy (detection limit about 5 μg/l). To increase sensitivity and selectivity of these methods, a preconcentration of mercury is widely performed. This can be realized by exploring high affinity of mercury to gold. Mercury and its volatile compounds can easily be adsorbed on gold at normal temperatures and can be released quantitatively upon heating. Hence, gold has been used as an adsorbent for the accumulation of mercury to a concentration sufficient for further spectroscopic analyses. The main disadvantage of mercury sensors based on bare gold layers is their poor selectivity and applications of such sensors for real probes and makes necessary a pre-treatment of probes. Mercury sensors based on thin gold layers require regular calibrations. In ideal cases, the calibration should be performed before each measurement. Therefore, a calibration technique should be compatible with design and concept of the mercury vapor sensor.
A simple technique for the calibration of mercury vapor chemosensors is described. It is based on a reductive deposition of a well-defined quantity of mercury (0) onto gold wire followed by thermal evaporation of the mercury into the sensor cell. The quantity of mercury on the gold wire was measured electrochemically by oxidation before and after heating, as well as after storage under different conditions. It is shown that the technique can be used for calibrated injections of nanogram quantities of mercury. The approach was applied to test the performance of ultrasensitive mercury chemoresistors.
The lateral electrical conductivity of gold layers with thickness in the nanometre range was found to decrease due to adsorption of mercury, water, iodine and sulphuric compounds onto the gold surface. A self-assembled monolayer of hexadecanethiol, deposited on the gold surface, blocked the effects of water and volatile sulphuric compounds on the lateral conductivity of the gold layers, but does not block the effects of iodine and mercury. The results indicate a selective permeability of hexadecanethiol monolayers for mercury vapour, thus providing a method for development of ultrathin filters for chemical sensors.