  Incorporation of guest additives within inorganic single crystals offers a
unique strategy for creating nanocomposites with tailored properties. While
anionic additives have been widely used to control the properties of crystals,
their effective incorporation remains a key challenge. Here, we show that
cationic additives are an excellent alterative for the synthesis of
nanocomposites, where they are shown to deliver exceptional levels of
incorporation of up to 70 wt% of positively charged amino acids, polymer
particles, gold nanoparticles, and silver nanoclusters within inorganic single
crystals. This high additive loading endows the nanocomposites with new
functional properties including plasmon coupling, bright fluorescence, and
surface-enhanced Raman scattering (SERS). Cationic additives are also shown to
outperform their acidic counterparts, where they are highly active in a wider
range of crystal systems, owing to their outstanding colloidal stability in the
crystallization media and strong affinity for the crystal surfaces. This work
demonstrates that although often overlooked, cationic additives can make
valuable crystallization additives to create composite materials with tailored
composition-structure-property relationships. This versatile and
straightforward approach advances the field of single-crystal composites and
provides exciting prospects for the design and fabrication of new hybrid
materials with tunable functional properties.
