# 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).

In the context of the paper by Guo et al. [1] 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 SR/Pa@SiO2 the forward slash "/" implies composite material (phase separator), and the at sign "@" refers to host-guest interactions as mentioned before. Here is a brief infographics I roughly created with Inkscape to illustrate the concept:

An obligatory reference quotation [2, 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. 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.
2. 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.