Metals have been used since a long time ago in human history. During the metal age, 3rd. 2nd and 1st millennia before common era (BCE), the bronze (an alloy composed mainly of copper) or the iron was started to be used in materials for practical objects, changing the population’s life completely. In this sense, the use of metals in the nanoscale, due to the different properties that exhibit in comparison with the bulk material, can also lead to essential changes.

Regarding the inorganic nanomaterials, exists a vast variety of examples using different metals, but we will give a brief introduction in Platinum (Pt), Silver (Ag) and Gold (Au).

Platinum

The platinum was first discovered in South America in 1735 by the Spanish navy officer and astronomer Antonio de Ulloa (but it had been in use by pre-Columbian Indians.). It was named as platinum due to the visual similitude with the silver (“plata” in Spanish).

Pt has an atomic number of 78 and an electronic configuration of [Xe] 4f14 5d9 6s1.

The dominant oxidation states for Platinum are Pt(II) and Pt(IV). Pt(II) forms almost exclusively low-spin, square planar complexes, while Pt(IV) forms a wide range of kinetically and thermodynamically inert octahedral complexes.

Bulk platinum has several applications, as catalysers for vehicles, electrical applications or in petroleum production. Also, it is used in medical applications, e.g. as the cis-platinum, used worldwide for the cancer treatment.

We can find in the literature several examples of Platinum nanoparticles (Pt NPs). Different shapes can be obtained, and we can see that is possible do Pt NPs in a spherical, cubic, tetrahedral, octahedral, or dendrimer type, among others shapes. The most extended use to Pt NPs is in catalysis. Electrocatalytic applications, as well as hydrogenations, or electro-oxidation, are reported applications. Sensing is also a use of Pt NPs, and the use in biomedicine is related.

Silver

Silver has one of the most ancient names and has an uncertain origin. Berzelius accepted the Latin name Argentum and used the symbol Ag consequently. The name that is used in Galician, Portuguese or Spanish (“prata”/”plata”) probably is a derivation from the Greek name “platys” (wide and plane). The name in English (“silver”) has an origin in the ancient German and Slavic languages (“silbar” in ancient German or “sirebro” in old Slavic).

Silver (Ag) has an atomic number of 47 and an electron configuration of [Kr] 4d10 5s1.

The only stable oxidation state of Silver is Ag(I). Their salts (except for AgF, AgNO₃ and AgClO₄) are sparingly soluble in water. Linear and tetrahedral complexes are common for Ag(I).

Silver is the best reflector of visible light, and was used in the fabrication of mirrors. The use in photography also was extended. Batteries, electrical contacts, or dental alloys are other uses of the silver.

The synthesis and applications of silver nanoparticles is a field in constant growth. The exciting properties of the silver in the nanometric range gives to these materials a vast range of applications. Different shapes can be obtained, such as spheres, platelets, cubes, or rods.

Silver nanoparticles have been used extensively for sensing, being used for detecting toxic ions like Hg(II), Pb(II) or As(V). Also, detection of Cu(II), Ni(II), Zn(II) or Co(III) is related. They are also used as small molecule detector (e.g. cysteine or dopamine). The use of silver nanoparticles for Surface Enhancement Raman Spectroscopy (SERS) is an active field with brilliant results. The studies of the antimicrobial property of the silver nanoparticles are also related in the literature, with a significant number of works reporting it.

Gold

The Latin name, “aurum”, probably comes from the ancient word “ausom”, which means yellow. In Galician, Portuguese or Spanish, “ouro”/“oro”, the name comes from the Latin “aurum”. In English, “gold”, probably is a derivation from the Anglo-Saxon word “geolo”, that can have the origin in the Sanskrit word “jual” (that shines).

Gold (Au) has an atomic number of 79 and an electron configuration of [Xe] 4f14 5d10 6s1.

For gold, Au(I) and Au(III) are the most usual oxidation states, but Au(III) is the dominant due to be the most stable. Au(I) disproportionate in water to yield Au(III) and Au(0). For Au(III), square planar coordination is the predominant, while for Au(I) complexes linear coordination is usual.

Jewellery is one of the primary uses of gold. Is also used in computer and electric materials due to the excellent conductivity or in medicine to treat rheumatoid arthritis.

The synthesis and applications of gold nanoparticles is a relevant area of research in the nanomaterials field. The applications of gold nanoparticles are broad, and the properties are different depending on the size and shape, so that we can find different synthetic methodologies for spheres, rods, triangles, stars, nanocubes or nanocages, among others.

The applications vary from sensing, plasmonics, photodynamic and photothermal therapy or drug delivery, citing just a few of them.