Abstract
Understanding polarity gradients inside nanomaterials is essential to capture their potential as nanoreactors, catalysts or in drug delivery applications. We propose here a method to obtain detailed, quantitative information on heterogeneous polarity in multicompartment nanostructures. The method is based on a 2-steps procedure, (i) deconvolution of complex emission spectra of two solvatochromic probes followed by (ii) spectrally resolved analysis of FRET between the same solvatochromic dyes. While the first step yields a list of polarities probed in the nanomaterial suspension, the second step correlates the polarities in space. Colocalization of polarities falling within few nanometer radius is obtained via FRET, a process called here nanopolarity mapping. Here, Prodan and Nile Red are tested to map the polarity of a water-dispersable, multicompartment nanostructure, named PluS nanoparticle (NPs). PluS NPs are uniform core-shell nanoparticles with silica cores (diameter ~10 nm) and Pluronic F127 shell (thickness ~7 nm). The probes report on a wide range of nanopolarities among which the dyes efficiently exchange energy via FRET, demonstrating the coexistence of a rich variety of environments within nanometer distance. Their use as a FRET couple highlights the proximity of strongly hydrophobic sites and hydrated layers, and quantitatively accounts for the emission component related to external water, which remains unaffected by FRET processes. This method is general and applicable to map nanopolarity in a large variety of nanomaterials.
Highlights
Polarity and proticity of confined nanoenvironments play a fundamental role in many chemical and biochemical processes, since they govern the strength of noncovalent interactions such as H-bonds, electrostatic and van der Waals interactions or π-π stacking
We performed spectral deconvolution of the emission spectra of solvatochromic fluorescent probes: the complex spectrum of each solvatochromic probe in a multicompartment nanostructure is fitted using a set of spectra of the same probe in pure solvents
Previous data on morphology and on chemical properties of the components suggest a possible model for the nanoarchitecture that consists of a dense silica shell, in which the hydrophobic polypropylene oxide (PPO) segments are deeply buried, and of a permeable soft polymeric shell[31,34,36]
Summary
Polarity and proticity of confined nanoenvironments play a fundamental role in many chemical and biochemical processes, since they govern the strength of noncovalent interactions such as H-bonds, electrostatic and van der Waals interactions or π-π stacking. Nanomaterials are expressing a rich potential inasmuch as they can implement such heterogeneity in polarity at the nanoscale, featuring several environments with different physicochemical properties in a single structure This richness is comparable to the complexity of biological species such as proteins and DNA, and it is at the basis of their peculiar versatility. A correct description of complex multicompartment nanostructures, on the contrary, requires to find unambiguous interpretation of multi-component data In this contribution, we describe a method based on fluorescence spectra deconvolution and FRET analysis that yields polarity mapping of complex nanoarchitectures. The second step of our proposed method allows to estimate the approximate distance between probed compartments This is possible carefully selecting at least two solvatochromic fluorescent dyes with suitable photophysical features to give efficient FRET processes in all combinations of environments. The estimation of the energy transfer efficiency for each individual spectral component of the probes yields a fine mapping of the nanoenvironment polarities within the nanostructure
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