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Formation of Mono-layer-based Electrooptic Devices

Final Activity Report Summary - MEOD (Formation of Monolayer-Based Electrooptic Devices)

Several monolayer-based sensors were designed for the detection of water in organic solvents and fuels, carbon monoxide (CO), NOX, NO+, metal ions such as Fe(III) and chromium(VI). In particular, the detection of chromium(VI) was very important. Chromium is widely found in the environment and exists primarily in the oxidation states number +3, i.e. trivalent chromium, and +6, or hexavalent chromium. The perhaps most remarkable occurrence of trivalent chromium in nature is in the ruby gemstones, where it results in the characteristic red colour. Moreover, it is also an essential trace element in our dietary uptake and important for many biological processes. However, this belies the toxicity of its hexavalent counterpart, which is extremely dangerous to the environment. Hexavalent chromium causes kidney and liver damage, skin ulcers and, most importantly, is highly carcinogenic. This was best illustrated by the blockbuster movie ‘Erin Brockovich’, which was based on the true story of a legal assistant who helped in a lawsuit against a power company for hexavalent chromium contamination in the United States.

This dangerous hexavalent chromium enters the environment through various modern chemical and industrial processes, including oil and coal combustion, manufacturing of textile dyes, fabrication of nuclear weapons, chrome plating, metal finishing and leather and wood preservation. For example, in the United States alone 700 metric tons of hexavalent chromium is emitted annually as atmospheric dust, and an estimated 175 000 workers are exposed to hexavalent chromium on a daily basis. Therefore, the detection, quantification and removal of this dangerous pollutant, especially in natural water sources, are of utmost importance.

The ‘problem’ in the detection of hexavalent chromium in water systems, is that it is costly, time consuming and often requires trained laboratory personnel and special sample treatment. In the work presented, a new procedure, developed during my internship at the Weizmann Institute of Science, was established for the rapid detection and quantification of hexavalent chromium under environmental conditions. This procedure was based on the fabrication of a molecular-based thin film on a glass substrate, of 1.7 nm thick, which was able to quantify the amount of hexavalent chromium in water, even when present in parts per million levels. The metal complexes that were present in these films gave away their electrons to the hexavalent chromium, thereby changing their colour. This colour change could be optically readout, which was beneficiary since it did not need to be wired directly to larger-scale electronics.

This newly developed test could be performed in as little as one minute time and did not require any sample preparation. It could even be used in natural systems and contaminated soil samples. The molecular-based film could be returned to its original state by simply washing with water, it thus had significant reversibility. It was also highly selective towards hexavalent chromium and was stable under various conditions. In addition, the test did not require any qualified laboratory personnel and was very cost effective. The film could be deposited on inexpensive materials including, glass, silicon, optical fibres and plastics. The ease and low cost fabrication made it ideal for disposable sensors.

By using this method, not only hexavalent chromium could be detected and quantified accurately; it was also converted to the much less ‘toxic’ trivalent chromium. This conversion was important in waste water treatment as most processes achieved this conversion by using excessive amounts of iron (sulfur) metal salts. Those processes were undesirable, as they produced a lot of hazardous waste. In solution, we were able to demonstrate the catalytic conversion of toxic hexavalent chromium to the much less ‘toxic’ trivalent chromium, without the production of hazardous waste. Therefore, this recently discovered method was an important improvement towards the catalytic removal of hexavalent chromium from waste waters.