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Correlated Electrochemical and Optical Detection Reveals the Chemical Reactivity of Individual Silver Nanoparticles Vitor Brasiliense, Anisha N. Patel, Ariadna Martinez-Marrades, Jian Shi, Yong Chen, Catherine Combellas, Gilles Tessier, and Frédéric Kanoufi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b13217 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016
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Journal of the American Chemical Society
Correlated Electrochemical and Optical Detection Reveals the Chemical Reactivity of Individual Silver Nanoparticles Vitor Brasiliense,1 Anisha N. Patel,1 Ariadna Martinez-Marrades, 2 Jian Shi,3 Yong Chen,3 Catherine Combellas,1 Gilles Tessier,2,* Frédéric Kanoufi1,* 1
Sorbonne Paris Cité, Université Paris Diderot, ITODYS CNRS-UMR 7086, 15 rue J. A. Baif, F-75013 Paris Sorbonne Paris Cité, Université Paris Descartes, Neurophotonics CNRS-UMR 8250, 45 rue des Saints-Pères, F-75006 Paris 3 Ecole Normale Supérieure-PSL Research University, Chemistry Dpt. CNRS-UMR 8640, 24 Rue Lhomond, F-75005 Paris Supporting Information Placeholder 2
ABSTRACT: Individual electrochemical (EC) impacts of nanoparticles (NPs) on an ultramicroelectrode (UME) and optics are coupled to identify chemical processes at individual NPs. While the EC signals characterize the charge transfer, the optical monitoring gives a complementary picture of the transport and chemical transformation of the NPs. It is illustrated for Ag NPs electrodissolution. In the simplest case, the optically monitored individual NP dissolution is synchronized with an individual EC spike. Then optics validates in situ the concept of EC nanoimpacts for sizing and counting NPs. Chemical complexity is introduced from a precipitating agent, SCN-, which tunes the overall electrodissolution kinetics. Particularly, the charge transfer and dissolution steps occur sequentially as the synchronicity between the EC and optical signals is lost. This demonstrates the level of complexity that can be revealed from such electrochemistry/optics coupling.
If nanoparticles (NPs) have been developed in all branches of science, most studies are achieved over large ensembles of NPs, while their special properties often rely on individual behaviors. It is therefore imperative to understand the chemistry of individual NPs. In particular, the popular antimicrobial properties of silver NPs essentially rely on their ability to controllably deliver Ag+ ions.1 This dissolution mechanism is driven by their oxidation and also by the chemical environment of the Ag NPs. A challenge in this field is then to observe the action of a single antimicrobial agent, e.g. a single Ag NP, directed towards a single bacterial agent. Crucially, one needs to address the chemical transformation of individual Ag NPs in situ. The highest spatial resolutions are provided by environmental transmission electron microscopy (TEM). However it is far from routine use,2 and uses ionizing beams invasive in terms of NPs chemistry. A powerful and simple alternative is provided by nanoelectrochemistry, through the timeresolved analysis of stochastic collisions of individual NPs on UMEs,3,4 or from nanoconfined EC cells.5 If such EC nanoimpact experiments provide, in situ, much valuable information about the electron transfer during the NP impact, it fails to fully characterize the associated chemistry. Examples of phenomena invisible to EC alone include partial oxidation, slow dissolution, chemical or phase transformation. To resolve these situations, correlating EC signals to in situ visualization of the process is required.2,6-12 This study demonstrates for the first time the simultaneous EC and optical visualization of individual nanoimpact events of Ag NPs on an UME. The approach delivers a complete picture of the electrodissolution process, i.e. electron transfer detection through individual EC nanoimpact signatures, whilst the associated chemical process is monitored at individual NPs by 3D superlocalization optical microscopy. Indeed, optical microscopies such as surface plasmon resonance (SPR),9 dark field imaging,10 holography,11 and fluorescence imaging-7,8,12 have emerged to image electrodes under operation and consequently study EC processes at individual domains of µm7 or nm (NPs, vesicles8)
sizes. Most studies focused on reactive processes occurring within the optical field of view from the electrode: < 2 µm7,8,10,12 or within the evanescent wave (< 200 nm) for SPR.9 However, 3D is also essential: holography11 allows measuring NP-surface distance and analyze NP movements and size while chemical processes unfold over a large volume (100x100x30 µm3). With a 50 mW dark field laser illumination, NPs down to 10 nm are detected,13 which compares to SPR sensitivities.9 For NP electrochemistry, 3D holography allowed in situ NP sizing, evidencing an electrochemically-triggered diffusiophoretic transport mode of NPs, or distinguishing between NP dissolution and desorption from the electrode. We also set quantitative grounds for optical estimates of individual NP dissolution rates from the evolution of the intensity of the light each NP scatter.11b However, while optical methods allow visualizing individual particles, they have rarely7 been associated to complementary individual EC events detection. An UME working as a low-current detector and enabling optical monitoring is mandatory to provide both optical and electrochemical individual signatures. By using microfabricated semitransparent gold UMEs (see Figure 1), we are able to record the EC impact for the oxidation of individual Ag NPs at the same time as optically monitor, by holography,13b the NP properties. In addition to the correlation between two signatures, the complementary methods allow visualizing two different steps: the charge transfer, from EC, and the chemical step (dissolution) from the optics. It is exemplified by Ag NPs electrodissolution in the absence or presence of a precipitating agent, SCN-, which tunes the dissolution dynamics. Indeed, precipitating anions may slow down (SCN-)11b or retard (Cl-)11a NPs dissolution by tens of seconds after their ensemble oxidation. Here, the overall electrodissolution mechanism is recorded by coupled EC and optical signals at single NPs. In this respect, this work also extends to the in situ monitoring of the transformation of individual Ag NPs into AgSCN NPs, or more generally electrocrystallization processes.2 A 50 x 50 µm2 UME was defined from SU-8 photoresist lithography on an Au-coated microscope glass slide (Figure 1, details in Supporting Information, SI, §I). It is the size of the optical field of view, so that the whole UME in contact with the solution is monitored optically by a 100x objective. The UME is then built into a microfluidic cell, of ~10x5x0.1 mm3 volume, provided with micropipettes and an Ag wire (used as Quasi-Reference, AgQRE, and counter electrode). We first address the correlation between individual optical and EC collisions when the Ag NPs dissolution is fast (in KNO3 or in > 0.1 M SCN-). A 0.18 pM colloidal solution of 50 nm Ag NPs in 0.05 M KNO3 was injected into the microfluidic EC cell. NPs subjected to Brownian motion eventually reach the UME surface that is polarized positively enough to ensure diffusion-controlled NP oxidation and dissolution (typically 0.6 – 0.9 V vs AgQRE). As described previously,3 every time the UME oxidizes a NP, a current spike is recorded associated to its electrodissolution: Ag → Ag+ + e-
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from which the charge, Q, and therefore the NP size (here an EC radius, denoted rEC) is inferred from Faraday’s law: ସ
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drodynamic radius, rH, inferred from mean-square displacement (MSD) analysis, corroborates these measurements of rEC (SI §IV). Figures 2 (left) and SI3 exemplify the other type of situation encountered: a NP that arrived on the UME before its polarization, disappears ~8.7 s after the polarization began. Here again, an EC signal is recorded concomitantly with the optical disappearance of the NP (evidenced from the successively recorded 2x2.7 µm2 reconstructed images of the UME plane shown as the inset of Figure 2 left). As the exchanged EC charge and the optical intensity are two independent measures of the NP volume, both signals (onset and kinetics) are expected to overlap, as in Figure 2. This study in KNO3 (absence of precipitating agent), shows that the dissolution of Ag NPs is concomitant with their oxidation that may not take place immediately upon landing: NPs remain for a certain period of time in the vicinity of the surface and stochastically oxidize. This can be due to different reasons. For instance, the UME or NP might be partially active and the NP-UME contact upon landing is inefficient. In this case, the NP residence time would relate to a target-search strategy14 in which the diffusion time required until a reactive site of the NP/UME is found may be quite long. Indeed, the diffusion of NPs in solution is hindered near walls (the UME surface) and a one order of magnitude slower diffusion is expected.15 It is also likely that this near-wall hindered diffusion freezes the NP within 0.7 µm, they interact during a few frames before merging together into a NP-dimer. The agglomerate formation is corroborated by the tracking of its trajectory whose corresponding MSD analysis (Fig. SI9) reveals a diffusion coefficient significantly smaller than that of a single 50 nm NP (3.1 µm2/s vs 4.8 µm2/s); it corresponds to an agglomerate of apparent rH = 70 nm (SI §IV), compatible with a NP-dimer agglomerate. Later on, the same agglomerate hits the UME. The optical signature recorded at the pixel where the agglomerate collides with the UME is presented in Figures 2 (right, blue) and SI4b along with the EC impact (brown); the agglomerate is not fully oxidized as the charge involved corresponds to rEC = 50 nm, i.e. that of a single NP. Such partial dissolution of the agglomerate is corrobo-
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the level of individual NPs. Different examples of Ag NPs electrodissolution profiles in 50 mM KSCN solution are given in Figures 4 (left) and SI10. The individual dissolution times, ∆t, were obtained from optical data by considering only the sharp decreasing slopes, such as those shown in Fig. 4 left. Their values (N=18) agree with previous results.11b norm. scattered light (a. u.)
rated by the optical trace, which shows that some optically scattering material is still left on the UME surface; Isc decreases by a factor of two, also consistent with the dissolution of a single 50 nm NP from the dimer. This indicates that NPs agglomerates do not behave electrochemically as larger particles, especially if the electrical contact between the NPs within the agglomerate is inefficient. Such coupled monitoring provides evidence for previous theories of incomplete agglomerate oxidation, related to either a loss of contact between NPs upon dissolution (and NP escape in bulk) or to NPs deactivation due to the capping agent.16 The optical monitoring leans in favor of the latter route. While a few groups have studied individual NPs via EC impacts or by optical methods, this is the first time that both are performed simultaneously, correlating the charge injection and the NP chemical dissolution. It provides insights into the dissolution mechanism at the individual NP level and/or into the chemical processes associated to the electron transfer step. To illustrate the potentialities of our setup for unraveling chemical processes, we have then introduced chemical complexity. Anions such as SCN-, acting as precipitating and/or complexing agents, may influence the Ag NP oxidation. As a result, at low [SCN-], a product of low solubility AgSCNs should be formed with Ag+. The oxidation of the Ag NP is then expected to produce a AgSCNs NP (3) before it is dissolved by (4) (with 0 ≤ x ≤ 3, see SI §VI). Therefore, [SCN-] allows tuning the dissolution of AgSCNs and, in turn, the chemistry associated to Ag NPs oxidation. The latter was then performed with different [SCN-], under diffusion-control (E ~ 0.7 V vs AgQRE). The experimental results are summarized in Figure 3.
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Figure 4: Oxidation of Ag NPs in 50 mM KSCN. (left) Optical intensity profiles of 4 individual dissolving NPs. (right) Correlation between optical scattered intensity (blue) and EC oxidation (brown) for a 0.18 pM Ag NP solution. (3) and (4) are sequential since the charge injection ends before the onset of dissolution. As above, the coupling with individual EC nanoimpacts allows inspection of the charge injection process (3) preceding this dissolution step (4). In a typical example (Fig. 4 right, Movie 2 and SI §III), a current spike is observed suggesting that an Ag NP has been oxidized. However, even though both optical and electrochemical acquisition are triggered simultaneously, the optical change (blue) of the NP, obtained from image analysis of Movie 2, indicates that the onset of a NP dissolution is delayed by several seconds from the EC spike associated to the individual NP oxidation (brown). This spike is also much broader and less intense than at higher [SCN-] (or in NO3-, see Fig. 1 right). The following scenario was reproduced for N=10 EC spikes, which were found associated to NPs optical disappearances: (i) the characteristic duration of the EC spikes is 1-2 s and, (ii) a time lag, of variable duration, is observed between the EC spike and the onset of the optical disappeareance. By integration, the charge of the spike (14 pC, rEC= 70 nm), as that of all the recorded spikes (N=10, Fig. SI6), is consistent with the size distribution of the NPs. The EC spikes suggest the Ag NPs are completely oxidized, but, contrary to observations in KNO3, the dissolution (4) is not synchronized to the oxidation (3). These results illustrate the advantages of coupled individual optical-EC monitorings as they report complementary events: the individual EC nanoimpacts characterize the charge transfer, while the individual optical monitoring characterizes the dissolution (chemical) process. To validate these results, EC nanoimpact experiments were performed at higher acquisition frequency and current resolution at a Pt UME (without optical coupling, under Faraday cage shielding) for [SCN-]=50 mM. They confirmed the opto-EC results, as 1-2 s long oxidation times (N=21, Fig. 3) are detected with an associated charge in agreement with both the opto-EC results and the NP size distribution (Fig. SI6 and SI11). The characteristic EC time is then a direct estimate of the slow electrocrystallization process depicted by (3). If the growth of a solid phase on another one is generally initiated from defects sites,2,17 the latter may be rare on a nm size surface, as e.g. during the growth of Ag NP on Pt nanoelectrodes.6b Moreover, the growth of these crystals is also an intricate succession of dissolution (Ag into Ag+) and precipitation (AgSCNs) steps from a supersaturated solution.18 It is expected that the supersaturation is higher in the region of the NP closer to the UME, suggesting the precipitation occurs preferentially from the UME surface. Both the low conductivity of AgSCNs and the low diffusivity (
