Nile-Red–Nanoclay Hybrids: Red Emissive Optical Probes for Use in Aqueous Dispersion

Abstract

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.

Multifold Fluorescence Enhancement in Nanoscopic Fluorophore–Clay Hybrids in Transparent Aqueous Media

“…Hdimers absorb at about5 50 nm, monomers at 600 nm and the protonated species at 620 and 640 nm, as reported previously. [9] It is of interest to note that the protonated form of Nile Red on laponite occurs despite ap Ho fa bout1 0o ft he dispersion, whichi sa scribed to the peculiar charged istribution in this colloidal system,t hat is, interlayerB r ø nsted acid sites characteristic of laponite clays. [13] The various absorption bands are dependent on several parameters like the mpd ratio, solventc ontent and polarity,i onic strength,p Hc onditions and further modifications, revealing am ultifaceted colloidal system.…”

“…Intuitivelya nticipated fluorescencee nhancements of dye-clay hybridsbyc o-adsorption of long-chain quaternary alkyl ammonium cations (e.g.,C TAB, cetylammonium bromide), that is, CTAB cation exchange, have previously been shown to remain only moderate at best, as demonstrated for Nile Red [9] and rhodamin 6G-laponite hybrids; [11] the latter work on Rhodamine solid films even includes exchange rates in excess of 100 %. While such high CTAB concentrations on Nile Red laponite hybrids inevitably lead to non-dispersible nanoclays due to the transition to an organoclay, [12] the absorption spectra reveal ac omplete absence of protonated specieso fN ile Red even at degrees of cation exchange (x ce ) as low as 3-5 %o ft he cation exchange capacity (cec).…”

“…In the presence of nanoclays, Nile Red dispersions ("NRLD") still retained some emission even when solubilized in aqueous dispersions, at least at low dye loading concentrations. [9] Unfortunately,t he temptingly simple deposition of dyes on clay surfaces has substantiald rawbacks. At higher dye loads of the nanoclay,Hdimers, and additionally,p rotonated species [10] appear,b oth of which are non-fluorescent even on the nanoclay surface.…”

Synthesis and optical properties of covalently bound Nile Red in mesoporous silica hybrids – comparison of dye distribution of materials prepared by facile grafting and by co-condensation routes

“…28,29 In addition, these hybrids could be ligated to biomolecules by decorative functionalization of the outer surface of silica particles. Finally, these NR-silica hybrids should be still luminescent in the solid sol state.…”

Phthalocyanine blue in aqueous solutions

“…3a). The spectra were taken from dispersions at a constant 1 wt% with respect to laponite, which, using data of the supplier (r ¼ 2.53 g cm À3 , disk diameter of 25 nm) 14 and a value of 1.5 nm for the disk thickness accounting for the inclusion of interlayer water, 13 can be recalculated to correspond to 5. aggregates could be re-dispersed completely in pure H 2 O to obtain 1 wt% -laponite dispersions (solutions (2)). Absorption spectra of the rst ltrates (solutions (1), Fig.…”