Article pubs.acs.org/accounts
Electrochemistry of Single Nanodomains Revealed by ThreeDimensional Holographic Microscopy Published as part of the Accounts of Chemical Research special issue “Nanoelectrochemistry”. Vitor Brasiliense,† Pascal Berto,‡ Catherine Combellas,† Gilles Tessier,*,‡ and Frédéric Kanoufi*,† †
Université Sorbonne Paris Cité, Université Paris Diderot, ITODYS CNRS UMR 7086, 15 rue Jean de Baïf, F-75013 Paris, France Université Sorbonne Paris Cité, Université Paris Descartes, Neurophotonics Laboratory CNRS UMR 8250, 45 rue des Saints-Pères, F-75006 Paris, France
‡
CONSPECTUS: Interest in nanoparticles has vigorously increased over the last 20 years as more and more studies show how their use can potentially revolutionize science and technology. Their applications span many different academically and industrially relevant fields such as catalysis, materials science, health, etc. Until the past decade, however, nanoparticle studies mostly relied on ensemble studies, thus leaving aside their chemical heterogeneity at the single particle level. Over the past few years, powerful new tools appeared to probe nanoparticles individually and in situ. This Account describes how we drew inspiration from the emerging fields of nanoelectrochemistry and plasmonics-based high resolution holographic microscopy to develop a coupled approach capable of analyzing in operando (electro)chemical reaction over one single nanoparticle. A brief overview of selected optical strategies to image NPs in situ with emphasis on scattering based methods is presented. In an electrochemical context, it is necessary to track particle behavior both in solution and near a polarized electrode, which is why 3D optical observation is particularly appealing. These approaches are discussed together with strategies to track NPs beyond the diffraction limit, allowing a much finer description of their trajectories. Then, the holographic setup is used to study electrochemically triggered Ag NP oxidation reaction in the presence of different electrolytes. Holography is shown to be a powerful technique to track and analyze the trajectory of individual NPs in situ, which further sheds light on in operando behaviors such as electrogenerated NP transport, aggregation, or adsorption. We then show that spectroscopy and scattering-based optical methods are reliable and sensitive to the point of being used to investigate and quantify NP (electro)chemical reactions in model cases. However, since real chemical reactions usually take place in an inherently complex environment, approaches based exclusively on optical imaging only reach their limitations. The strategy is then taken one step further by merging together electrochemical nanoimpact experiments with 3D optical monitoring. Previous strategies are validated by showing that in simple cases, these two independent ways of probing NP size and reactivity yield the same results. For more complicated reactions (e.g., multistep reactions), one must go beyond either technique by showing that the two approaches are perfectly complementary and that the two signals contain information of different natures, thus providing a much better characterization of the reaction. This point is illustrated by studying Ag NP oxidation (single or agglomerates) in the presence of a precipitating agent, where the actual oxidation is uncoupled from the dissolution of the particle, thus proving the point of our symbiotic approach.
1. INTRODUCTION
observation of the activity of individual nanoobjects to convey and deliver drugs or to act as a local nanosurgical tool. “Electrochemical microscopes”1−3 can be built by combining optical microscopies and electrochemistry. Compared to the
The observation of chemical processes at individual nanoparticles (NPs) is crucial for various products and concepts such as the rational design of smart nanocatalysts for chemical syntheses and energy or key early diagnosis developments in health sciences. Similarly, nanocuring relies on the in situ © XXXX American Chemical Society
Received: June 30, 2016
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DOI: 10.1021/acs.accounts.6b00335 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
used,14,15 and one can track the apparent “hot-sphere” (indirect heating effect of the absorption),15−17 rendering single NP tracking18 difficult to transpose to multiple objects. Overall, absorption-based techniques, better performing than scattering ones on particles below 10−20 nm, are essentially limited to single-point measurements or long acquisition times. 2.1.2. Scattering-Based Methods. Therefore, most techniques aimed at tracking multiple NPs in solution rely on scattering. Associated with a DF illumination, they are adapted to full-field imaging without blinding the camera and allow realtime tracking of multiple particles in 2D or 3D. Classical DF illuminators19 use broadband halogen light sources, suitable for spectroscopic purposes. However, the fast detection of NPs often requires higher optical power provided by laser sources. DF illumination is obtained using commercial total internal reflection (TIR) objectives, TIR illumination through a prism,15 or illumination of the object laterally, at any angle avoiding direct illumination of the objective (e.g., in Nanosight products20). When NPs are directly illuminated, the investigated field can be large, but in TIR configurations, the illuminated depth is limited to the close vicinity (ca. 100 nm) of the surface, which is useful in densely populated volumes. A similar vertical selectivity is obtained under surface plasmon polariton illumination, with the added benefit of strong field enhancements, as is discussed elsewhere in this series.21
current scanning electrochemical probe microscopes,4,5 they offer promising performances (high throughput and submicrometer spatial resolution) for the in situ imaging of local or nano(electro)chemistry. This Account addresses their potential to image electrochemistry at individual NPs. Emerging electroanalytical strategies consist of catching NP signatures, such as discrete electrochemical current spikes, either indirectly when NPs obstruct6 or cross7 single nanopores or directly when individual particles collide on ultramicroelectrodes (UMEs). Originally developed to count NPs in a solution, discrete electrochemical signals also provide relevant information on NP size, chemistry, transport, or reactivity. This strategy, fostered by Bard8 in electrochemical nanoimpact experiments, allows detection of chemical phenomena, typically electrocatalysis or electroassisted transformation (e.g., electrodissolution in Compton’s group),9 at individual NPs. Optics has witnessed a similar reduction of its characteristic size, long limited by diffraction. Plasmonics now allows intense light concentration in nanometer-sized regions, leading to sensitivive detection and transduction methods such as exalted Raman spectroscopy (SERS) or surface plasmon-based detection (SPR). Imaging techniques have also broken the diffraction limit (2014 Chemistry Nobel Prize)10 and are being extended to 3D. This Account reviews the principles and performances of new optical microscopies combined with NP electrochemistry. An electrochemical reaction implies a succession of volume (transport, reaction) and interfacial processes (adsorption/ desorption, (electro)chemical transformation) occurring at an electrode. The importance of surface and volume effects is emerging in electrochemical nanoimpact experiments. They can be resolved through optical imaging. Strategies allowing the in situ real-time and high-spatial resolution imaging of electrochemical processes have been reviewed.11 The seminal works from Crooks (fluorescent sub-microbeads)12 or from Tao (SPR of single NPs)13 have revealed the wealth of information gathered by combined optoelectrochemical investigations. This Account particularly addresses those 3D strategies revealing surface and volume effects during NP electrochemistry.
2.2. Superlocalization of Fluorescent Objects
In conventional imaging, the image of an isolated NP has the size of the point spread function (PSF) of the microscope. The NP position, which corresponds to the center of the PSF, is then determined with subdiffraction-limited accuracy. Applied to single fluorescent molecules, this is the core of several superresolution imaging techniques (photoactivated localization microscopy (PALM), stochastic optical reconstruction microscopy (STORM)), some of which were awarded the 2014 Nobel Prize in Chemistry.10 The concept can be extended to metallic NPs, provided that they are separated by at least the width of the PSF. A wealth of algorithms and software have been developed to precisely localize (fluorescent) NPs in 2D.22 Various 3D superlocalization methods have been developed. They consist either in altering the PSF of the microscope using special lens23 or phase masks,24 extrapolating the z-position from the wavefront,25 or acquiring images in several planes.26,27 However, most of them have limited vertical extension (
