Selenium

Some forms of selenium exhibit photoconductivity, a property by which its electrical conductivity increases upon receiving light. The light energy facilitates the generation of charge carriers capable of transporting current. This phenomenon aroused interest in the development of photocells, light sensors and image reproduction devices. Although many of these applications have been replaced by more modern technologies, the photoconductivity of selenium remains a notable example of the relationship between light and electricity.

Selenium presents several structural forms, known as allotropes, whose optical properties are different. Some amorphous varieties show reddish tones, while crystalline grey selenium has a dark appearance and semiconductor behaviour. These differences are due to the organisation of their atoms and the way their electronic structure interacts with light. That is why selenium's colour is not unique: it depends on the specific form of the material and the conditions in which it is found.

Certain selenium compounds are incorporated into glass to modify its colour and optical properties. Depending on the composition and manufacturing conditions, they can contribute to red tones or neutralise greenish tints caused by other impurities. The effect depends not exclusively on the amount of selenium, but also on its chemical form and the rest of the components present. That is why manufacturers carefully adjust the formulation to achieve a uniform appearance and the required visual characteristics.

Selenium exhibits photoconductive properties that allow transforming changes in illumination into variations of an electric signal. This feature was exploited in historic image reproduction technologies and continues to be relevant in certain specialised detection systems. In some digital radiography devices, for example, an amorphous selenium layer can directly convert X-rays into electric charges, without the need for first converting to visible light. This mechanism is known as direct conversion and allows recording images with detectors designed for this purpose.

Selenium is part of selenoproteins, a group of proteins that incorporate the amino acid selenocysteine. Some of them participate in the regulation of the cell's oxidation state and in protection against certain oxidative damage. Others intervene in the metabolism of thyroid hormones. The presence of selenium modifies the chemical properties of the active centre of these proteins, allowing certain biological reactions. Its function depends on incorporation into specific chemical forms, not simply on being present as an element.

Selenium and sulfur belong to the same group of the periodic table and share some chemical characteristics, but they do not behave identically. Selenium is more polarisable and its bonds exhibit different properties, which influences the stability and reactivity of its compounds. In addition, selenium can incorporate into proteins in the form of selenocysteine, while sulfur appears in amino acids such as cysteine and methionine. These differences affect both inorganic chemistry and biological processes.

Selenocysteine is an amino acid that contains selenium instead of the sulfur present in cysteine. It is incorporated into certain proteins through a specialised cellular mechanism, different from the one used for most amino acids. Its selenium atom can facilitate important chemical reactions, especially those related to electron transfer. Although it is structurally similar to cysteine, its chemical properties are different. This difference allows certain enzymes to perform functions that depend on the characteristic reactivity of selenium.