Crystallization in confined spaces is a widespread process
in nature
that also has important implications for the stability and durability
of many man-made materials. It has been reported that confinement
can alter essential crystallization events, such as nucleation and
growth and, thus, have an impact on crystal size, polymorphism, morphology,
and stability. Therefore, the study of nucleation in confined spaces
can help us understand similar events that occur in nature, such as
biomineralization, design new methods to control crystallization,
and expand our knowledge in the field of crystallography. Although
the fundamental interest is clear, basic models at the laboratory
scale are scarce mainly due to the difficulty in obtaining well-defined
confined spaces allowing a simultaneous study of the mineralization
process outside and inside the cavities. Herein, we have studied magnetite
precipitation in the channels of cross-linked protein crystals (CLPCs)
with different channel pore sizes, as a model of crystallization in
confined spaces. Our results show that nucleation of an Fe-rich phase
occurs inside the protein channels in all cases, but, by a combination
of chemical and physical effects, the channel diameter of CLPCs exerted
a precise control on the size and stability of those Fe-rich nanoparticles.
The small diameters of protein channels restrain the growth of metastable
intermediates to around 2 nm and stabilize them over time. At larger
pore diameters, recrystallization of the Fe-rich precursors into more
stable phases was observed. This study highlights the impact that
crystallization in confined spaces can have on the physicochemical
properties of the resulting crystals and shows that CLPCs can be interesting
substrates to study this process.