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Simultaneous Opto- and Spectro-Electrochemistry – Reactions of Individual Nanoparticles Uncovered by Dark-Field Microscopy Kevin Wonner, Mathies V. Evers, and Kristina Tschulik J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02367 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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Journal of the American Chemical Society

Simultaneous Opto- and Spectro-Electrochemistry – Reactions of Individual Nanoparticles Uncovered by Dark-Field Microscopy Kevin Wonner, Mathies V. Evers, Kristina Tschulik* Chair of Analytical Chemistry II and Center for Electrochemical Sciences (CES), ZEMOS 1.45, Ruhr-University Bochum, Universitätsstr. 150, D-44801 Bochum, Germany

Supporting Information Placeholder ABSTRACT: Despite the frequent use of silver nanoparticles in consumer products and medical treatments, their reactivity and degradation in aqueous suspensions is still under debate. Here we elucidate this reactivity by an in situ opto- and spectro-electrochemical approach. Using darkfield microscopy coupled to a spectrophotometer and to an electrochemical cell, redox reactions of individual silver nanoparticles are studied in the presence of chloride. The intensity and spectral position of the plasmon resonance of an individual particle are tracked simultaneously in real-time during cyclic voltammetry. They both change almost instantaneously with the detected current in a chemically reversible way. Thus, it is evidenced that the intensity decrease of the optical signal at the silver peak position is caused by the reversible formation of silver chloride and not by dissolution of silver. Moreover, at large positive potentials, further transformation to silver oxide or chlorite is revealed spectroscopically, although the electrochemical current is hidden by water and chloride oxidation. Thus, the combination of electrochemistry with dark-field microscopy and hyperspectral imaging is introduced as a new tool for real-time analysis of (electro-)chemical reactions of nanoparticles on a single entity level. Despite their frequent application in medicine based on their antibacterial and anti-inflammatory effects, the reactivity and metabolism of silver nanoparticles (AgNPs) is still 1,2 poorly understood today. Even in well-defined electrochemical conditions and for most commonly used AgNPs several, partly in part contradictory, results have been reported. Some authors described a single-step full oxidation of suspended AgNPs when impacting a potentiostated elec3–6 trode. Others reported a stepwise and incomplete oxida7–9 tion albeit working at similar ionic strengths, pH and potentials. Probing myriads of particles at the same time will also not provide the understanding of the process and its variation to resolve this debate. Thus, single particle studies have to be performed and linked to additional analysis techniques. In situ opto- and spectro-electrochemical studies like fluorescence microscopy, surface enhanced Raman spectroscopy (SERS) or dark-field microscopy (DFM) are suitable op10–17 tions. Kanoufi et al. used 3D-DFM holography and Zhang

et al. used fluorescence microscopy combined with electrochemistry to visualize the motion of AgNPs along an electrode during their oxidation, explaining the electrochemical18–20 An inherent limitation of ly observed step-wise oxidation. these optical approaches is their inability to reveal changes in the size or aggregation state of the reacting single entity, due to being aberration-limited to micrometric scales. Also, it is difficult to distinguish if a loss in signal intensity is caused by dissolution, chemical conversion or loss of the particle from the field of view. These drawbacks can be circumvented by probing the localized surface plasmon resonance (LSPR). It is a sensitive feature of nanoparticles, based on their confined size, and hence electronic structure that can be tracked by scattering in the near-infrared region. The excitation energy and intensity of this plasmon depend on the particle shape, composition, size and dielectric environment, which also strongly 21–24 affect their reactivity in a liquid environment. Thus, probing the spectral position and intensity of the LSPR during (electro-)chemical reaction will provide insights into reactivities at the nanoscale not readily accessible by established in situ measurements. Here we employ this concept of measuring the change in near-infrared scattering intensity and energy level to study the electrochemical oxidation of individual AgNPs in the presence of Cl . This allows us to follow compositional changes of a nanoparticle even if its size remains almost unaltered. The combination of in situ spectroelectrochemical DFM used herein, needs a fraction of the measurement time of, e.g. SERS or fluorescence microscopy. Hence, real-time video streaming of individual nanoparticle reactions is performed. To mimic the Ag oxidation in biological or environmental 25 systems, we control the reaction by applying a potential and measuring the current of the reacting AgNPs. The species formed in this oxidation has strong implications on the + fate and release of Ag into the surrounding liquid. AgCl may 26,27 be formed during the oxidation(): ‫ ݃ܣ‬+ ‫ ↓ ݈ܥ݃ܣ ⟶ ି ݈ܥ‬+݁ ି

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The low solubility of AgCl causes slow release of Ag into the solution. At higher potentials Ag2O3 is formed in presence of an oxygen source or potentially AgClO2 in a chloride 28 solution. Alternatively, the oxidation may directly yield

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Ag (aq.) in high local concentrations, with potentially toxic 29 effects on cells. Herein, the change in chemical composition of individual AgNPs is monitored in situ by the change in their plasmon resonance frequency and intensity, employing hyperspectral imaging (HSI) and charge coupled device (CCD) video recording. Thus, the electrochemical signal at a microelectrode during cyclic voltammetry (CV) and the optical and spectroscopic response of these individual AgNPs are tracked in real-time. This reveals the formation of AgCl, and Ag2O3 or AgClO2 upon AgNP oxidation and re-formation of Ag upon reduction of AgCl.

Figure 1. Experimental setup of the dark-field microscope and the used electrochemical cell. An electrochemical dark-field cell (Figure 1) was designed to study the electrochemical reaction of 50 nm diameter AgNPs in situ. The experiments were carried out in a threeelectrode system using a Ø=15µm Pt90Ir10 alloy wire working electrode (PtWE) in a suspension of AgNPs in 0.05M KCl(aq). Hence, in situ modification of the PtWE results from sticking of AgNPs upon their Brownian motion-based collisions with the electrode (See SI for details). The successful immobilization of AgNPs on the electrode is visible in the dark-field microscopy CCD (DFM-CCD) color camera image (Figure 2A). Afterwards, DFM-CCD videos are recorded during CV, sweeping the potential through +0.05V, +1.5V, -1.0V, to 1.5V with a sweep rate of 0.05V/s (Figure 3, see Movie 1 in the SI).. Snapshots at selected potentials are summarized in Figure 2A-F and linked to the recorded current trace (Figure 2).

Figure 2. Top. (A-F) DFM CCD images recorded during CV at selected potentials (indicated by letters A-F) between 1.0V and 1.5V; red rectangles: zoom in of 3 reacting AgNPs; exposure time: 70ms. Bottom. CV of AgNPs reaction in KCl(aq). Initially, a distinct color change is observed of the individual nanoparticles during their oxidation at ca. 0.1V (Figure 2B). In the backward scan, at 0.0V (Figure 2D), this process is reversed, reducing AgCl back to Ag. This is concluded from the observed drastic shift in color of the individual particles back to the original values with simultaneous detection of a cathodic current in the CV (Figure 2, bottom). Repetitive cycling of the potential shows the reversible formation of AgCl at 0.1V (Figure 2F) and of Ag at 0.0V, linked to a shift in color and brightness, indicating a change in spectral position and intensity of the LSPR. Note that AgNPs not electrically connected to the PtWE remain chemically unaltered and allow to ensure minimal drift of the system during the measurements. Blank measurements conducted in the absence of AgNPs did neither show any specific color changes of scattering features in the DFM analysis, nor any electrochemical signals, as shown in Movie 2 and Figure SI.7. Although the current at potentials above 1.1V is dominated by water and potentially chloride oxidation (see blank measurement in SI7), an additional change of particle color towards blue is clearly visible at potentials above 1.15V (Figure 2C). This indicates a further blue shift of the nanoparticle LSPR. We suggest that this shift is caused by the formation of 30 either a silver oxide species (Ag2O3) , in line with the Pour-

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baix diagram in aqueous solution, or silver chlorite (AgClO2), 28,31 in line with the Pourbaix diagram in a Cl solutions. Since these chemical species might be unstable ex situ, and since their electrochemical signal is hidden by side reactions, in situ detection and analysis is necessary, e.g. by DFM. The associated changes of the dielectric properties of the NP enable the spectroscopic detection of this hidden electrochemical transformation to a single species. In contrast, previous in situ Raman studies conducted in alkaline solutions and at particles ensembles, reported the formation of 17,32–34 Ag2O and AgO. The discussed changes are more clearly visible in the overlaid current and spectral intensity plot provided in Figure 3. A simultaneous increase of scattering intensity and current is detected during oxidation of Ag to AgCl (at time=1s and time=102s) and further oxidation of AgCl to Ag2O3/AgClO2 (at time=20s and time=120s). During the reverse scan this oxide is (meta-)stable under the experimental conditions, until the reduction to Ag is detected at about 0V (Figure 3, time=58s). A cathodic current and a sudden decrease in signal intensity are detected. Since the same intensity is seen as prior to the particle oxidation, chemical reversibly of the process is further substantiated.

intensity. For better comparison, selected difference-spectra (referring to the initial spectrum at 0.05V) are plotted in Figure 4B. By conversion to AgCl two maxima are observed in the extinction difference spectrum at about 500nm and 640nm (red curve). Overlaying the simultaneously measured LSPR intensities (Figure 3, red curve) and current traces (black curve), it is clearly visible that the spectral changes are linked to the formation of AgCl. When the potential is further increased to ca. 1.15V, another steep change in the spectrum of the particle is detected (Figure 4). The blue line in Figure 4B, shows a single broad feature at about 520nm. In addition to the previously discussed increase in scattering intensity, this shift in the spectral position indicates another change in particle composition. This indicates the reaction to Ag2O3 or AgClO2. The formation of silver oxides during electrochemical oxidation of Ag has previously been evidenced by SERS. However, relying on bulk samples or nanoparticle ensembles, these studies provide integral information in contrast to the analysis of individual reacting nanoparticles, 34,40,41 presented here.

The small gradual shift of the intensity signal seen during the CV (Figure 3) is associated with the change of the dielectric environment of the particles, e.g. due to a voltage16,35–39 induced charging or specific adsorption.

Figure 3. Correlation spectra of applied potential (blue), current (black) and HSI intensity (red) in 50mM KCl(aq) vs. time. For a quantitative analysis of the LSPR frequency of one of these particles, a hyperspectral imaging (HIS) video with an exposure time of 250ms per image is recorded next for the same experimental conditions. The time-dependent extinction spectra of this single AgNP during the CV are shown in Figure 4A. An initial scattering spectrum of the LSPR of the (unreacted) AgNP is recorded at a potential of 0.05V. When sweeping the potential positively, a shift in the extinction spectrum and a steep increase in intensity is observed at 0.1V. This marks the oxidation of Ag to AgCl, as dissolution + of the particle under Ag (aq) formation would cause a loss in

Figure 4. A) Corrected spectra of a single AgNP during CV; B) Averaged difference spectra at the indicated potentials after subtraction of the initial spectrum. Sweeping the potential back to lower values, a current peak is obtained at 1.0V, indicating the reduction of a previ42 ously formed platinum oxide on the PtWE. The spectra and intensity of the LSPR remain unaffected, which agrees with the experimental results (Figure 4A). Further decreasing the potential to 0.05V, a reduction peak with a small shoulder is observed in Figure 3, indicating the reduction to be a twostep process. Accordingly, we suggest that AgCl is formed at first by the reduction of the previously formed silver species

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since the measured intensity and extinction spectra are related to the previous ones at 0.05V (which was assigned to AgCl, Figure 3 and SI4). Reaching a potential of 0.0V, the second reduction step is investigated, which can be assigned to the reduction back to silver, as discussed before (Figure 3 and 4A).

This work was financially supported by the Ministry of Innovation, Science and Research of North Rhine-Westphalia (NRW Rückkehrer Programm) and by the Cluster of Excellence RESOLV (EXC 1069) funded by the DFG. We also thank DFG and BMBF for support of ZEMOS facilities.

The cathodic current at -0.7V can be related to H2Oreduction.Notably, no change in the LSPR is seen by DFM during the formation of hydrogen. Hence, it is concluded that hydrogen (or adsorbed cations) has no significant effect on the dielectric environment of the AgNP, in line with 37 previous reports. When increasing the potential again, the oxidation of AgNP to AgCl is repeated. Accordingly, the chemical reversibility and the pathway of AgNP oxidation has successfully been evidenced by combining DFM-HSI with electrochemistry. Blank measurements conducted in the absence of AgNPs did not show any distinct change in the extinction spectra at the individual potentials (see Figs. SI8 and SI9). Hence, these spectral changes are linked to reactions of AgNPs.

References

In brief, we demonstrated in situ spectro-electrochemical monitoring of individual nanoparticle reactions by electrochemical DFM-HSI. It was possible to correlate the obtained current signals to the chemical conversion steps of single nanoparticles, based on the change in spectral position and intensity of their individual LSPR. Thus, spectroelectrochemical DFM-HSI was introduced as a general and fast tool for the real-time investigation of chemical reactivity at a single entity level. We applied this new method to study AgNP oxidation in the presence of chloride . Hence, it was revealed that initially formed AgCl is further oxidized at higher potentials. Resolving this process required in situ spectroscopy, as the simultaneous occurrence of water splitting and/or chloride oxidation dominated the electrochemical response. Since the used Cl solution may serve as a model for extracellular medium, this mechanistic insight helps understanding the observed long life-time of nanoparticles in 1 biosystems. Thanks to facilitating real-time video streaming of the involved processes, electrochemical DFM-HSI may also improve both, studies on cell uptake and toxicity of 29 nanomaterials, and electrocatalyst degradation under harsh 43 working conditions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Movie 1, Movie 2, Movie 3 Experimental details, movie caption CV’s, Nanoparticle immobilization, Hyperspectral imaging, data processing, extinction spectra, 3D-HSI-plot

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT

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Experimental setup of the dark-field microscope and the used electrochemical cell.________________________________________ 343x156mm (150 x 150 DPI)



Figure 2. Top. (A-F) Dark-field CCD images recorded during CV at selected potentials between -1.0V and 1.5V; red rectan-gles: zoom in of 3 reacting AgNPs; exposure time: 70ms. Bottom. CV of AgNPs reaction in KCl(aq). 57x92mm (300 x 300 DPI)


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Figure 3. Correlation spectra of applied potential (blue), current (black) and intensity obtained from hyperspectral video (red) in 50mM KCl(aq) vs. time. 272x208mm (300 x 300 DPI)



Figure 4. A) Corrected spectra of a single AgNP during CV; B) Averaged difference spectra at the indicated potentials after subtraction of the initial spectrum. 75x92mm (300 x 300 DPI)


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Graphical Abstract 340x121mm (150 x 150 DPI)


Simultaneous Opto- and Spectro-Electrochemistry ... - ACS Publications

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