Water-dispersible and (bio)functionalizable nanoclays have a considerable potential as inexpensive carriers for organic molecules like drugs and fluorophores. Aiming at simple design strategies for red-emissive optical probes for the life sciences from commercial precursors with minimum synthetic effort, we systematically studied the
dye loading behavior and stability of differently functionalized laponites. Here, we present a comprehensive study of the absorption and emission properties of the red emissive hydrophobic and neutral
dye Nile Red, a well-known polarity probe, which is almost insoluble and nonemissive in water. Adsorption of this probe onto disk-shaped nanoclays was studied in aqueous dispersion as function of
dye concentration, in the absence and presence of the cationic
surfactant cetyltrimethylammonium bromide (
CTAB) assisting
dye loading, and as a function of pH. This
laponite loading strategy yields strongly fluorescent nanoclay
suspensions with a fluorescence quantum yield of 0.34 at low
dye loading concentration. The
dye concentration-,
CTAB-, and pH-dependent absorption, fluorescence emission, and fluorescence excitation spectra of the
Nile-Red-nanoclay
suspensions suggest the formation of several
Nile Red species including emissive
Nile Red monomers facing a polar environment, nonemissive H-type dimers, and protonated
Nile Red molecules that are also nonfluorescent. Formation of all nonemissive
Nile Red species could be suppressed by modification of the
laponite with
CTAB. This underlines the great potential of properly modified and functionalized
laponite nanodisks as platform for optical probes with
drug delivery capacities, for example, for
tumor and
therapy imaging. Moreover, comparison of the
Nile Red dimer absorption spectra with absorption spectra of previously studied
Nile Red aggregates in
dendrimer systems and
micelles and other literature systems reveals a considerable dependence of the dimer absorption band on microenvironment polarity which has not yet been reported so far for H-type
dye aggregates.