Published in Journal of Materials Chemistry C
According to the reviewers
“In this manuscript, the authors provided a new design of MEMS based liquid flow system with a unique on-chip microfluidic channel and a microheater, which enables the quick replenishment of fresh solution and uniform heating of the liquid solution.”
Connecting in situ to ex situ
One of the most fundamental challenges that any microscopist experiences is the question whether the phenomenon you observe inside the microscope is representative of what happens outside. You can see interesting things happening inside the microscope, but if there is no link to the outside world, the knowledge is not so useful.
To solve this challenge, we design our products so that the user has full control of all the relevant parameters during the in situ experiment. In the Stream system, this relates to controlling the solution conditions. Especially temperature and concentration. The sample should experience the same conditions inside and outside the TEM. To achieve this, the Stream system has a flow channel that enables rapid replenishment of the solution to ensure continuous supply of fresh reactant species. Meanwhile, the microheater accurately controls the temperature.
Temperature is a highly important variable to control. For this reason, all our product lines include the possibility to manipulate temperature. In liquid, the speed of chemical reactions is often dictated by the temperature. Moreover, completely different reaction pathways can be found at different temperatures. During an in situ experiment, the increase in temperature can be used to trigger a phenomenon. Many people rely on the electron beam to induce the dynamics. However, it’s normally desirable to decouple the stimulus from the imaging. In other words, the beam is used for imaging, while the MEMS device supplies the heat to start a reaction.
We chose to design the MEMS device to generate a uniform temperature throughout the Nano-Cell. In other words, no temperature gradients are present that could lead to complications. This also allows to accurately measure and control the temperature of the liquid and the sample.
Silica nanoparticle experiment
To validate the effect of the combined flow channel and microheater, we looked at the etching process of silica nanoparticles in NaOH. This process is quite sensitive to temperature; increasing the temperature substantially accelerates the reaction kinetics. In-flask, the etching time in NaOH with pH 13.8 is reduced from ~500 to ~10 minutes when increasing the temperature from 20 to 60 °C. This was found by measuring the transmittance of the solution. The TEM allows us to observe this process in real time, at the nanoscale. In the Stream, we aimed to reproduce the reaction conditions from the in-flask experiment. In the flask, the bulk liquid acts as a large reservoir of available reactant species, while in the Nano-Cell, the space is much more confined. A constant flow was used to refresh the solution to make sure that the silica particles are etched by fresh reactants continuously.
We found very good similarity between the results obtained in-flask and in situ. In the Nano-Cell, the etching time reduced from 360 to 4 minutes for the same temperature increase from 20 to 60 °C. So in both cases, the same order of magnitude increase in etching rate is observed, indicating that the Nano-Cell meticulously mimics the situation outside the microscope. This was the most important finding from the paper. The e-beam seems to slightly accelerate the etching process, but the low dose imaging procedure ensured that the effect of the e-beam was reduced to a minimum.
Department of Chemistry and the department of Materials Science and Engineering,
University of California, Irvine, USA
Collaboration with customers
DENSsolutions actively participates in the scientific community. We work closely together with our customers to make sure that our products help them to generate impact. This study is a good example where our expertise in the design and engineering of the in situ system was combined with the expertise at TU Eindhoven and UC Irvine.
In Eindhoven they were already very experienced working with the silica nanoparticle samples and with the ex situ etching behavior at different temperatures. So when the MEMS devices for Stream Liquid Heating were launched, they proposed to run this experiment inside the microscope. We anticipated that one of the key parameters to control during the experiment would be the e-beam, as it could interfere with the etching process. Fortunately the groups at Eindhoven and Irvine have a thorough background in imaging soft matter, so we managed to adhere to a low dose imaging protocol to successfully minimize the beam effect.