A combination of dark-field microscopy and electrochemistry can make individual nanoparticles in a liquid medium visible. The technique is suited to determine the activity of catalysts during their use. Likewise, the decomposition mechanisms of nanomaterials in liquid systems can be investigated. The latter makes it possible to study the reaction of silver nanoparticles in a chloride-containing solution in order to better understand what happens to these frequently used particles when discarded into the environment.

Characterizing nanoscale materials – a tough challenge

Nanoparticles have served us in our daily lives for some time. Silver nanoparticles are used in sportswear to kill odor-causing bacteria. Gold nanoparticles are supposed to protect us against the free radicals in skin cream, straighten out the skin and prevent wrinkles. However, researchers are unsure about what exactly happens with these nanoparticles – both within our bodies and in the natural environment [1].

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Fig. 1 Immersion objective of a dark-field microscope. The leads are connected to microfiber electrodes under the coverslip.

Even laboratory experiments on the reactivity of nanoparticles have, in part, yielded contradictory results [2]. This is also due to the nanoparticles’ manufacturing process, which results in a wide range of particle sizes. Tiny size differences can have a significant effect on the properties and thus on the resulting measurement results.

Conventional methods need a large number of nanoparticles to be applied to a surface and the total value of all these nanoparticles measured, a value that can vary greatly in another sample with a slightly different size distribution and that often does not reveal much about the behavior of individual particles. Observing individual nanoparticles has so far been mostly limited to electron microscopy, which is performed dry or in a high vacuum – environments that are very unlike the human bloodstream or a pond with fish, for example. To increase the relevance for natural environments, we have combined electrochemistry with dark-field microscopy (Fig. 1) [3].

Let there be electrical current and light

A dark-field microscope detects only scattered light or light generated by the sample itself, delivering valuable information about the sample’s properties. Light that is scattered by a nanoparticle allows conclusions to be drawn about its composition and size. The interaction that occurs is called plasmon resonance, i.e. collective oscillation of the free electrons in the particle [4]. In silver nanoparticles, plasmons are excited by incident light and the light absorbed. This creates a starry sky of colored dots on the detector.

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Fig. 2 Dark-field image of silver nanoparticles on a platinum-iridium microwire. Nanoparticles appear as colored dots.

However, plasmon resonance is also sensitive to the shape and environment of the nanoparticles. When metals are oxidized, the wavelength of the sharp plasmon resonance signal changes. This change in color is detected across the observed area by a dark-field microscope (Fig. 2). The scattered light is recorded by a hyperspectral camera, resolved in space and time based on spectra between 400 and 1,000 nm wavelength. Unlike methods described earlier in the literature, this allows us to draw conclusions about the processes that cause the signal change. We combined this advantage with electrochemical methods for the first time. Our aim was to run reactions in a controlled manner to investigate them.

In an electrochemical measurement chamber under the dark-field microscope, we applied an electric potential to nanoparticles in an aqueous medium in order to drive a particular reaction. For this purpose, microelectrodes were brought together in a flat chamber of liquid between a microscopic slide and its coverslip (Fig. 3). On the working electrode we then observed individual nanoparticles optically and spectrally while the applied electric potential drove the oxidation or reduction. The spatially resolved operando recording of spectral information complemented the response of the current, as changes to nanoparticles can be clearly distinguished from side reactions at the electrode.

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Fig. 3 Schematic structure of a dark-field microscope with a hyperspectral camera (HSI camera) and an optical camera (CCD camera)

Although the electrically conductive support material is basically inert, side reactions such as solvent decomposition at the large surface area may lead to relatively high currents in the range of nanoamps or microamps. Individual nanoparticles, however, often display catalytic currents in the picoampere range. Nevertheless, the scattered light and the measured current can still be used to track which chemical processes take place on the individual nanoparticles.

Hence electrochemical methods allow us to control the driving forces of reactions and measure small current signals. However, monitoring individual nanoparticles requires spatially resolved dark-field microscopy, especially when background reactions conceal the electrochemical signals.

The fate of the silver nanoparticles

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Fig. 4 A result of electrochemical dark-field microscopy. Arrows indicate the oxidation of silver particles to AgCl. Other signals are very pronounced and conceal further signals. However, the scattered light allows the chemical changes on the nanoparticles to be detected, despite the pronounced current signals.

We applied the approach of combining electrochemistry and dark-field microscopy to the oxidation of silver nanoparticles (see Fig. 4). In potassium chloride solution, silver is oxidized to silver chloride (AgCl) at 0.10 V against an Ag/AgCl reference electrode. During the process, a small current signal can be measured and a significant difference in the scattered light spectrum observed. As the applied electric potential is further increased, the particle reacts a second time to yield silver (I, III) oxide (Ag₂O₃) or silver chlorite (AgClO₂) while electrochemically cleaving water. The reaction back to elemental silver occurs at 0.0V. The particles can be repeatedly oxidized and reduced, almost reversibly. However, they are difficult to dissolve in chloride-containing solution.

Chloride ions are abundant in most environments. This is why silver is easily oxidized but not very soluble. Our results lead us to conclude that silver particles may have a longer retention time in aqueous systems than previously assumed. What this means for their biocompatibility needs to be further investigated in the future.

Combining electrochemistry and dark-field microscopy opens up a new path to investigating nanoparticle reactivity. It can easily be used for many other nanoparticles and solutions. The resulting spatial resolution can reveal many of the secrets that nanoparticles keep.

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Category: Electrochemistry | Nanoparticle Characterization

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