Welcome to one of our guest columns, where active researchers can share their views on topics relevant to materials science. Professor Geoffrey Ozin from the University of Toronto shares his thoughts on who should be credited for the field of nanochemistry.
What isn’t chemistry? Nearly everything in the world around us is made from chemicals and chemistry pervades the physical, life and applied sciences and is often termed “the central science”. Synthetic chemistry is certainly playing a central role in modern nanochemistry, the hallmark of which is a bottom-up synthetic approach to nanoscale building blocks made from inorganic, organic and polymeric materials and composites thereof. And nanochemistry is playing a central role in nanoscience and nanotechnology as evidenced by the disruptive effect it is having on basic and directed research emerging across the disciplines of physics, materials science, engineering, biology and medicine.
But is there anything “really” new about nanochemistry? Is it just the next chapter in the long and illustrious history of colloid chemistry, the centerpieces of which are the same tiny pieces of matter and the forces between them that occupy much of nanochemistry research today?
So who owns nanochemistry? Should “all” the credit be given to chemistry pioneers of the past 20-30 years or were the foundations of nanochemistry already laid in the field of colloid chemistry, the origin of which can be traced to a century earlier?
The term colloid (Greek word kola meaning glue-like) was coined by Thomas Graham in 1861 and used to describe the distinctive behaviour of any form of matter, soft to hard, with a physical size in the 1-1000 nm range and with properties intermediate between that of a solution and a suspension (e.g., slow diffusion, difficult crystallization, scattering light, sol-gel formation). While Graham (1805-1869) is often credited with founding colloid chemistry, Wolfgang Ostwald (1883-1943 seen in the portrait above, adapted with permission from J. Chem. Ed. 1955, 32, 2, Copyright 1955 American Chemical Society – not to be confused with his Nobel Laureate father Wilhelm Oswald (1853 – 1932) renowned for catalysis, chemical equilibria and reaction kinetics) is given credit for propagating the field of colloid science, the physicochemical principles of which are expounded in his classic book The World of Neglected Dimensions, 1914.
Colloid science owes much to the pioneering contributions of early researchers like Aldar Buzagh (1895-??) and Ernst Hauser (1896-??) who came to the realization that all matter with at least one of its physical dimensions in the colloidal domain will display colloidal properties and it was also noted how colloid science impacts many fields of science and technology crisscrossing the boundaries of science from chemistry and physics to biology and medicine to geology and mineralogy. This is very similar to the nanosheets, wires, and dots which occupy much of modern nanochemistry research and is echoed by the multidisciplinary nature of the field of nanochemistry today. Herbert Freundlich (1880-1941) noted how the colloidal state of matter can be accessed through what he called “two doors” either from a molecularly dispersed system whereby the size of a dissolved species is increased continuously until the colloidal chemistry regime is reached or by constantly reducing the size of matter to the colloidal state until it can be continuously dispersed in another one. Today we refer to these as the bottom-up and top-down approaches to nanomaterials. In 1903 Richard Zsigmondy (1865-1938 – Nobel in chemistry) was the first to observe the colloidal state of matter in an optical microscope, which becomes visible when illuminated from one side despite the gold particles he was studying being smaller than the resolving power of the microscope. The work of Gustav Mie (1869-1957) on the scattering of light by metal colloids complements the optical studies of Zsigmondy and provided a theoretical foundation for the plasmon resonance of colorful gold colloids, originally observed by Michael Faraday (1791-1867). Nowadays, we routinely use absorption and scattering of light for studying the structure, stability and dynamics of gold nanomaterials and how they interact with each other and their environment which underpins exciting breakthroughs in nanoplasmonics and photonic metamaterials, as well as diagnostics, therapeutics and imaging in nanomedicine.
It is worth noting that in the early days of colloid science the unit name “nanometre” did not exist. In optical spectroscopy it was referred to as a milli-micron. Pieter Harting (1812-1885) a Dutch biologist and geologist and early pioneer of optical microscopy, invented in 1845 a new measure of length, the micron as the millimetre millimetre (mmm), later denoted µm, to study microscopic objects. In order to handle colloidal length scales Zsigmondy introduced a system with three size regimes, denoted in German, “Mikron, Amikron und Ultramikron”. The Ultramikron, translated as submicron, can be regarded as today’s nanometric ruler for defining the size of nanomaterials. The 1926 chemistry Nobel awarded to Theodore Svedberg (1884-1971) for his work on the analytical ultracentrifuge also cited colloid science research. This was followed soon after by the 1932 Nobel to Irving Langmuir, largely for the development (with Katharine Blodgett) of monolayer surface chemistry. So this was the high point in terms of recognizing colloid and affiliated surface-science research as core areas of chemistry, as opposed to industrial chemistry.
By 1959, when Richard Feynman delivered his after dinner speech on the future of miniaturization, “Plenty of Room at the Bottom”, colloid science was pretty well grounded as a field and colloid chemistry was the premier method to synthesize colloids, made for example of metals, metal alloys, metal oxides or metal chalcogenides, dispersed on the surface or within the spatial confines of supports, such as alumina, silica, clays, zeolites and polymers. These colloids were often targeted at the time for applications in the burgeoning field of heterogeneous catalysis. Precursors from inorganic, organometallic, cluster and metal vapor chemistry were employed for synthesizing and controlling the nucleation and growth of unimetallic and multimetallic catalytic colloids and it was well recognized that the catalytic activity and selectivity of the colloidal particles depended on their size and shape, accessible crystal surfaces and crystallographic sites, as well as electronic and steric effects associated with interaction of the colloidal particles with the support.
The “perfect colloid” was always a holy grail in the field of heterogeneous catalysis as this was seen as one of the best ways to control catalyst performance and many creative ways were devised to make single size and shape colloids, on and in a range of supports, expressly for this purpose. These days, we are still pursuing the perfect colloid, although now we call them monodispersed nanoparticles, no matter how they are made and what they are made of.
And simultaneous with the emergence of heterogeneous catalysis, a multi-analytical approach, including X-ray diffraction, electron microscopy, extended X-ray absorption fine structure, photoelectron, laser Raman and Fourier transform infrared spectroscopy was being developed and used to establish the structure and chemical reactivity of catalytic colloids. These analytical methods are amongst the major tools used for characterizing nanomaterials produced by nanochemistry today.
Yet in all of this early research employing colloidal particles particularly catalytic colloids in heterogeneous catalysis, to the best of my knowledge, the nano word was never used. The prefix “nano-“ was of course used in “nanosecond”, from the 1960s onward, because pulsed laser flashes had durations that were conveniently expressed in that unit.
Norio Taniguchi (1912-1999) is credited with the first use of the term “nanotechnology” in 1974 in reference to “production technology to get the extra high accuracy and ultrafine dimensions, i.e., the preciseness and fineness on the order of 1 nm (nanometre), 10-9 meter in length”. Whilst he was initially concerned with top down semiconductor fabrication techniques, his term and its descendents have come to be used across all branches of modern science.
So: who gets the credit for nanochemistry ?