Distinct Interfacial Fluorescence in Oil‐in‐Water Emulsions via Exciton Migration of Conjugated Polymers

Abstract

aforementioned types of dyes are the amphiphilic fluorescent probes that are structurally more challenging to synthesize owing to meeting the optimum balance between hydrophobicity and hydrophilicity. [5] Furthermore, the equilibrium of solubility results in the partial amount of dyes residing in water and/or oil phases in addition to interfaces, which could reduce the contrast of fluorescence images between interfaces and backgrounds.Poly(phenylene ethynylene)s (PPEs), fluorescent conjugated polymers, have been extensively investigated as optoelectronic and fluorescent sensory materials. [6][7][8] They possess both large extinction coefficients and high fluorescence quantum yields, resulting from an electronic structure with a direct bandgap [9] as well as rigid butatrienelike excited state structures. [10] The strong brightness makes them suitable materials as fluorescent dyes. One distinct feature of the PPE as a fluorescent probe, compared to other conventional small molecular probes, is that their fluorescence can be switched via exciton migration that can be modulated by their conformations [11] and interpolymer distances. [12,13] We reported [13] that surfactant-type PPEs having low-energy emissive endgroups can display dual fluorescence, which is cyan (backbone) in solution and red (endgroup) in thin films at a single excitation wavelength through energy transfer. In their Langmuir monolayers at the air-water interface, interpolymer distances are controlled, resulting in the ratiometric fluorescence changes. Herein we aim to apply this dual-fluorescence concept to more general amphiphilic colloidal systems. As a proof of concept, we choose oil-in-water emulsions that are stabilized by PPE surfactants. We found that PPEs show cyan fluorescence in water, which switches to red fluorescence in oil-in-water emulsions (Figure 1a) because of the closer interpolymer distance on the confined spherical oil surface, and therefore, the red emission is observed exclusively from the oil-water interface. We envision that this switchable fluorescence between bulk and interface can pave a novel way of high-contrast visualization techniques for many amphiphilic systems found in colloids and bioimaging. Results and DiscussionWe synthesized several perylene end-capped water-soluble PPEs to make oil droplets in water. The perylene is functionalized Fluorescence Commercial dyes are extensively utilized to stain specific phases for the visualization applications in emulsions and bioimaging. In general, dyes emit only one specific fluorescence signal and thus, in order to stain various phases and/or interfaces, one needs to incorporate multiple dyes and carefully consider their compatibility to avoid undesirable interactions with each other and with the components in the system. Herein, surfactant-type, perylene-endcapped fluorescent conjugated polymers that exhibit two different emissions are reported, which are cyan in water and red at oil-water interfaces. The interfacially distinct red emission results from enhanced exciton...

Encapsulated Nanodroplets for Enhanced Fluorescence Detection by Nano‐Extraction

“…The significant enhancement of fluorescence intensity from the encapsulated nanodroplets, in contrast to the dark host droplet and surrounding solution, may be mainly due to the lens effect of the curved surface of the encapsulated nanodroplets. The closer intermolecular distance of the fluorophore on the confined spherical nanodroplet surface improves the collection of light emitted from FR, [17] compared to the molecules freely distributed in the bulk solution. Meanwhile, any accumulation of FR or enhanced quantum yield from molecular alignment at the interface of the nanodroplets may also contribute to the strong fluorescence intensity.…”

“…Moreover, the highly curved surface of the encapsulated nanodroplets may act as microlenses to focus light locally and increase the sensitivity of optical detection. [ 16–19 ] Combining nano‐extraction with the lensing effect can potentially be applied to ultrasensitive detection in many spectrometry‐ and imaging‐based analyses.…”