Gold is the metal humanity first recognized and has continuously cherished: golden-colored, highly corrosion-resistant, with excellent ductility and conductivity, it simultaneously plays monetary, jewelry and industrial roles. Jewelry remains the largest outlet for global gold consumption; from wedding rings and necklaces to gold bars and coins, gold's ornamental value and wealth-storage function run through thousands of years of civilization, and central banks worldwide have long used gold as the ballast of foreign-exchange reserves. In modern industry, gold's electrical contacts and coatings are irreplaceable - its contact resistance is extremely low and it never tarnishes, used in mobile-phone connectors, motherboard keypads, spacecraft switches and high-reliability relays; although a smartphone contains only milligrams of gold, almost every critical connector is gold-plated. In dentistry, gold alloys, with good biocompatibility, corrosion resistance and wear resistance, have long been used in porcelain-fused-to-metal crowns, inlays and orthodontic wires; although partly displaced by ceramics and new alloys, they remain in high-end restorations. Gold is also used in infrared-reflective coatings (architectural energy-saving glass, aircraft windshields), satellite thermal-control coatings, chemical catalysis (gold nanoparticles catalyzing carbon-monoxide oxidation) and high-precision wire bonding. Taking 'never rusts' to the extreme is the underlying logic by which gold has moved from currency into industry.
Elemental gold (Au) is applied directly in forms such as gold leaf, gold powder, gold wire, gold plating and gold nanoparticles: gold leaf is used in traditional gilding and the decoration of temples and cultural relics; gold wire is used in high-end electronic wire bonding; and gold nanoparticles are widely used in biomedical diagnostics, immunochromatographic test strips and catalytic research. Gold trichloride (AuCl3) is the most basic gold compound, a red-brown crystal that is the starting material for all gold catalysts, gold-plating baths and gold nanoparticles, and also has a few traditional uses in red glass coloring and photographic toning. Gold(I) sulfide (Au2S) is the black sulfide of gold, used mainly historically in certain gilding processes and as a mineralogical specimen, with negligible modern industrial use. Beyond these, potassium gold cyanide is the main salt for industrial gold plating, chloroauric acid is used in electroless gold plating and gold-nanoparticle preparation, and gold complexes also carry academic value in certain anticancer-drug research and gold-catalyzed organic synthesis.
