Abstract
Under ambient conditions, the behavior of a solid surface is often dominated by a molecularly thin adsorbed layer (adlayer) of small molecules. Here we develop an optical approach to unveil the nanoscale structure and composition of small-molecule adlayers on glass surfaces through spectrally resolved super-resolution microscopy. By recording the images and emission spectra of millions of individual solvatochromic molecules that turn fluorescent in the adlayer phase, we obtain ~30 nm spatial resolution and achieve concurrent measurement of local polarity. This allows us to establish that the adlayer dimensionality gradually increases through a sequence of 0D (nanodroplets), 1D (nano-lines), and 2D (films) for liquids of increasing polarity. Moreover, we find that in adlayers, a solution of two miscible liquids spontaneously demixes into nanodroplets of different compositions that correlate strongly with droplet size and location. We thus reveal unexpectedly rich structural and compositional behaviors of surface adlayers at the nanoscale.
Highlights
Under ambient conditions, the behavior of a solid surface is often dominated by a molecularly thin adsorbed layer of small molecules
Recent advances in super-resolution fluorescence microscopy (SRM)[14,15], including those based on single-molecule imaging like stochastic optical reconstruction microscopy (STORM)[16] and points accumulation for imaging in nanoscale topography (PAINT)[17], provide noninvasive, optical means to achieve nanoscale spatial resolution
By illuminating the sample with a total internal reflection (TIR) configuration to excite a depth of ~100 nm from the glass surface, we found that the stochastic insertion of individual Nile red molecules from an aqueous imaging buffer into the adlayers led to bursts of single-molecule fluorescence with low background (Fig. 1a–c)
Summary
The behavior of a solid surface is often dominated by a molecularly thin adsorbed layer (adlayer) of small molecules. We develop an optical approach to unveil the nanoscale structure and composition of small-molecule adlayers on glass surfaces through spectrally resolved super-resolution microscopy. By recording the images and emission spectra of millions of individual solvatochromic molecules that turn fluorescent in the adlayer phase, we obtain ~30 nm spatial resolution and achieve concurrent measurement of local polarity. This allows us to establish that the adlayer dimensionality gradually increases through a sequence of 0D (nanodroplets), 1D (nano-lines), and 2D (films) for liquids of increasing polarity. We reveal how the nanoscale adlayer structure on a glass surface varies as a function of chemical polarity for 8 different solvents, and discover that a solvent mixture spontaneously decomposes into nanodroplets of varying compositions and sizes on the surface
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