Poseidon 200

Milli-Q water (resistivity 18.2 MΩ cm at 25 °C) was used in all experiments. PTA (99.995%) and potassium permanganate (KMnO₄, >99.0%), sodium citrate, sodium chloride, bis(p-sulfonatophenyl)-phenylphosphine (BSPP, 97%), agarose and tetrachloroauric(III) acid (99%) were purchases from Sigma-Aldrich and used without any further purification. Gasket and O-rings for sealing of the liquid cell, PTFE tubings as well as small and large Poseidon E-chips were obtained directly from Protochips Inc²¹ . All used small E-chips had e-beam-transparent SiN membranes of nominal 550 μm × 20 μm expansion and flow spacers with 150 nm thickness (serial number: EPB-52DNF). As large E-chips, standard EPT-55W were used. RIE etched chips were obtained from Protochips company in the frame of a collaborative project. These modified chips are not catalog products but can be commercially requested from the company as of now. A possible fabrication routine is described in the physical prototyping of diffusion cells section. Two different LP-TEM sample holder setups, Poseidon Select™ and Poseidon 200™ (both Protochips Inc²¹ ), were used for experiments. All glassware was cleaned with aqua regia and rinsed thoroughly with Milli-Q water.

LP-TEM experiments were prepared by spin coating 1% w/v agarose gel with nominal thickness of 150 nm on a big chip (70 rev s⁻¹, 15 s; compare above) matching the selected spacer height (see below for details). Together with modified small chips, a diffusion cell was assembled in the tip of a bathtub with on-site mixing-type sample holder (Poseidon 200, Protochips Inc). The relevant geometric parameters of the flow setup comprised the height of the on-chip bypass (10 µm) and the extension of the central nanochannel (120 × 650 µm²) rendering diffusion the dominant mass transport mechanism in the nanochannel.
TEM imaging at a magnification of 27000× was applied to image particle movement in changing liquid environment with a temporal resolution of 2 s (acquisition time of 1 s per image and dose rate of 5 e⁻ nm⁻² s⁻¹). During the 8 min of experiment, the flow of aqueous NaCl (c = 100 mM) solution was interrupted by pure water for 5 min starting 1 min after the image acquisition was initiated. Supplementary Movie 1 depicts the obtained image sequence at a rate of 30 frames per second. The sequence was analyzed after the experiment by tracking the motion of particles using ImageJ software (TrackMate plugin^{45 (link)}).