# What is the @ symbol used for in material science?

I came across the phrase "Silicone Rubber/Paraffin@Silicon Dioxide" also written as $$\ce{SR/pa@SiO2}$$, utilising the @ symbol.

How can I understand what this means?

The at symbol @ universally means "at the site" or "at the rate of". It is routinely used in supramolecular chemistry to denote guest@host relationship because

1. it follows the original designation of denoting the binding site;
2. its shape resembles a guest (letter "a") entrapped within a host shell (circle sign "○") providing a visual cue for an intuitive image (e.g. see endohedral fullerene).

The symbol has been introduced by Chai et al. [1] as a part of nomenclature for complex fullerenes:

Ordinarily, a fullerene composed of $$n$$ carbon atoms with a metal atom, $$\ce{M},$$ will simply be represented as $$\ce{MC},$$ regardless of whether the metal atom is inside or outside the cage. However, to facilitate discussion of these and more complicated fullerenes with one or more atoms inside, some attached outside and possibly one or more heteroatoms substituting for carbons in the cage network itself, a more explicit symbolism is essential. We will use a set of parentheses around the symbol $$@$$ to indicate that the atoms listed within the parentheses are grouped to form a fullerene. Within this group all atoms listed to the right of the $$@$$ symbol are assumed to be part of the cage network, and all to the left are situated somehow inside the cage. Buckminsterfullerene, under this notation is then $$\ce{(@C60)},$$ and a $$\ce{C60}$$-caged metal species is written $$\ce{(M@C60)}.$$ A more complex example that will be encountered below is $$\ce{K2(K@C59B)},$$ which denotes a 60-atom fullerene cage with one boron atom substituted for a carbon in the geodesic network, a single potassium trapped inside, and two potassium atoms adhering to the outside.

We have adopted this symbolism since it has the virtue of being concise, while still containing the critical information. It is readily printed and transmitted electronically, and it is compatible with ordinary chemical formula conventions. It also has the virtue of being visually suggestive, and it emphasizes the “superatom” aspects of the fullerenes as new chemical entities.

In the context of the paper by Guo et al. [2] you are referring to, paraffin is encapsulated inside $$\ce{SiO2}$$ shell (obtained in situ), and these silicon dioxide particles were then embedded in silicone rubber matrix. Here, in the notation $$\ce{SR/Pa@SiO2}$$ the forward slash “/” implies composite material (phase separator), and the at sign “@” refers to host-guest interactions as mentioned before. A brief infographics to illustrate the concept:

The symbol has also been listed in style guides [3, p. 203]:

3.6.17 At Symbol (@)

The at symbol $$(@)$$ is most commonly used in email addresses. Its modern use is primarily in accounting (where it means “at the rate of”) and should not be used in scientific writing as a substitute for “at.”

Two legitimate uses of the at symbol are

• In chemical formulae, the $$@$$ is used to denote trapped atoms or molecules. For instance, $$\ce{La@C60}$$ means lanthanum inside a fullerene cage.
• In genetics, an at symbol after a gene symbol indicates that it is part of a gene cluster.

### References

1. Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. Fullerenes with Metals Inside. J. Phys. Chem. 1991, 95 (20), 7564–7568. DOI: 10.1021/j100173a002.
2. Guo, Y.; Yang, W.; Jiang, Z.; He, F.; Zhang, K.; He, R.; Wu, J.; Fan, J. Silicone Rubber/Paraffin@silicon Dioxide Form-Stable Phase Change Materials with Thermal Energy Storage and Enhanced Mechanical Property. Solar Energy Materials and Solar Cells 2019, 196, 16–24. DOI: 10.1016/j.solmat.2019.03.034.
3. The Manual of Scientific Style: A Guide for Authors, Editors, and Researchers, 1st ed.; Rabinowitz, H., Vogel, S., Eds.; Elsevier/Academic Press: Amsterdam; Burlington, MA, 2009. ISBN 978-0-12-373980-3.