A nanofluidic device for parallel single nanoparticle catalysis in solution

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Imagine you are a singer performing on the scene in a big arena with tens of thousands of spectators. During a ballade, they all wave lighters in sync with your music. So, at a first glance when looking into the stadium, they all do the same. However, focusing on the front rows where you can identify the individuals, reveals differences. Some have no lighter at all, some have a small flame, some a large flame, and some have flickering flames. In other words, at the individual level they all behave differently. The same goes for nanoparticles in a catalyst, where some particles may be very active and some not. The reasons are their structure since they are slightly different at the atomic level, their interaction with the support, and their relation with neighboring particles due to, for example, local reactant conversion effects. 

Therefore, researchers in the field of single particle catalysis are developing experimental methods to investigate catalyst nanoparticles “in action” one-by-one. In this quest, one of the main challenges is to measure single catalyst nanoparticle activities at conditions relevant for applications in terms of for example reactant concentration, and to ensure that other nanoparticles do not influence the one(s) under investigation. This is where our idea emerged to use nanofluidic structures decorated with single nanoparticles as a new paradigm for single particle catalysis. When localizing a single nanoparticle inside a fluidic structure, we can both isolate it, and thus prevent interaction with others, and confine the formed product in a volume so tiny that we drastically increase the possibility to detect it because it cannot get diluted or diffuse out of our field of view.

Figure 1. To the left: Schematic depiction of our nanofluidic platform, with inlets connected to microchannels that lead to an array of 50 nanochannels in the center, containing single nanoparticles of increasing size from 64 to 129 nm in diameter. To the right: Fluorescence image of the nanochannels during the catalytic reaction. Note the decrease in fluorescence intensity downstream of the particles due to the reduction of fluorescein, following the reaction scheme portrayed on the far right.

In this pilot study, we verified this hypotesis by simultaneously flowing reagents through parallel nanochannels decorated with single Au catalyst particles and monitoring them during reaction (Figure 1). We used the reaction of fluorescein with borohydride in which the fluorescence emission is “turned off”, enabling the use of fluorescence microscopy to measure catalyst activity. Using this platform, we measured the turnover frequency for 32 single Au nanoparticles simultaneously over a wide range of reaction conditions, and monitor in operando how it varied from the mass transport limited to the surface reaction limited regime, by changing the fluorescein concentration in the reactant mixture flushed through the nanochannels (Figure 2). We observed that the turnover frequency for single particles of different size is almost identical in the mass transport limited regime, while it varied widely and became particle-specific in the surface reaction limited regime even for particles of identical size. This is in line with the single-particle specific structure dictating the reaction rate and an example of the particle-to-particle heterogeneity often seen in nanoparticle catalysis. Consequently, generating rigorous and conclusive understanding of the main causes of this heterogeneity, and its impact on activity, remains an important question in the field. Our nanofluidic device is thus a contribution to the experimental toolbox for single particle catalysis, where it enables investigations of single catalyst particle heterogeneity at relevant reaction conditions.

Figure 2. Turnover frequencies for individual particles of varying sizes spanning from the mass transport limited regime (local fluorescein concentration of 0-2 µM) to the surface reaction limited regime (conc. > 2 µM).

For further details, the full article can be found at: https://www.nature.com/articles/s41467-019-12458-1

Sune Levin

Phd student, Chalmers