Single-Molecule Charge Transport through Positively Charged Electrostatic Anchors.

The charge transport in single-molecule junctions depends critically on the chemical identity of the anchor groups that are used to connect the molecular wires to the electrodes. In this research, we report a new anchoring strategy, called the electrostatic anchor, formed through the efficient Coulombic interaction between the gold electrodes and the positively charged pyridinium terminal groups. Our results show that these pyridinium groups serve as efficient electrostatic anchors forming robust gold-molecule-gold junctions. We have also observed binary switching in dicationic viologen molecular junctions, demonstrating an electron injection-induced redox switching in single-molecule junctions. We attribute the difference in low- and high-conductance states to a dicationic ground state and a radical cationic metastable state, respectively. Overall, this anchoring strategy and redox-switching mechanism could constitute the basis for a new class of redox-activated single-molecule switches.

Plasmon-Driven Chemistry in Ferri-/Ferrocyanide Gold Nanoparticle Oligomers: A SERS Study.

The plasmon-driven chemistry of ferri-/ferrocyanide ions inside surface-enhanced Raman spectroscopy (SERS) active hot spots associated with gold nanoparticle oligomers is studied with continuous wave (CW) pump-probe SERS. By comparing with solution-phase normal Raman spectra, the characteristic spectral variations observed upon 532 nm optical pumping can be attributed to an oxidation process that occurs on the surface species followed by desorption of the oxidized surface species from the gold nanoparticles. Interrogating the plasmon-driven processes over a wide range of temperatures reveals that neither process is purely driven by the thermal effects associated with the optical pumping, and the apparent activation energies of both steps are estimated based on semiquantitative SERS analysis. Our observation identifies a more detailed reaction pathway for this classic model system under considerably simplified reaction conditions, adding to the current mechanistic background for future plasmon-driven chemistry studies and applications.

Monitoring Cobalt-Oxide Single Particle Electrochemistry with Subdiffraction Accuracy.

By partially overcoming the diffraction limit, superlocalization techniques have extended the applicability of optical techniques down to the nanometer size-range. Herein, cobalt oxide-based nanoparticles are electrochemically grown onto carbon nanoelectrodes and their individual catalytic properties are evaluated through a combined electrochemical-optical approach. Using dark-field white light illumination, edges superlocalization techniques are applied to quantify changes in particle size during electrochemical activation with down to 20 nm precision. It allows the monitoring of (i) the anodic electrodeposition of cobalt hydroxide material and (ii) the large and reversible volume expansion experienced by the cobalt hydroxide particle during its oxidation. Meanwhile, the particle light scattering provides chemical information such as the Co redox state transformation, which complements both the particle size and the recorded electrochemical current and provides in operando mechanistic information on particle electrocatalytic properties.

Electrochemistry of Single Nanodomains Revealed by Three-Dimensional Holographic Microscopy.

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.

Platinum Nanoparticle Impacts at a Liquid|Liquid Interface.

Single nanoparticle (NP) electrochemistry detection at a micro liquid|liquid interface (LLI) is exploited using the catalyzed oxygen reduction reaction (ORR). In this way, current spikes reminiscent of nanoimpacts were recorded, which corresponded to electrocatalytic enhancement of the ORR by Pt NPs. The nature of the LLI allows exploration of new phenomena in single NP electrochemistry. The recorded impacts result from a bipolar reaction occurring at the Pt NP straddling the LLI. O2 reduction takes place in the aqueous phase, while ferrocene hydride (Fc-H+ ; a complex generated upon facilitated interfacial proton transfer by Fc) is oxidized in the organic phase. Ultimately, the role of reactant partitioning, NP bouncing, or the ability of NPs to induce Marangoni effects, is demonstrated.

Opto-electrochemical In Situ Monitoring of the Cathodic Formation of Single Cobalt Nanoparticles.

Single-particle electrochemistry at a nanoelectrode is explored by dark-field optical microscopy. The analysis of the scattered light allows in situ dynamic monitoring of the electrodeposition of single cobalt nanoparticles down to a radius of 65 nm. Larger sub-micrometer particles are directly sized optically by super-localization of the edges and the scattered light contains complementary information concerning the particle redox chemistry. This opto-electrochemical approach is used to derive mechanistic insights about electrocatalysis that are not accessible from single-particle electrochemistry.

Correlated Electrochemical and Optical Detection Reveals the Chemical Reactivity of Individual Silver Nanoparticles.

Electrochemical (EC) impacts of single nanoparticles (NPs) on an ultramicroelectrode are coupled with optics to identify chemical processes at the level of individual NPs. While the EC signals characterize the charge transfer process, the optical monitoring gives a complementary picture of the transport and chemical transformation of the NPs. This is illustrated in the case of electrodissolution of Ag NPs. In the simplest case, the optically monitored dissolution of individual NPs is synchronized with individual EC spikes. Optics then validates in situ the concept of EC nanoimpacts for sizing and counting of NPs. Chemical complexity is introduced by using 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.

Electrochemical transformation of individual nanoparticles revealed by coupling microscopy and spectroscopy.

Although extremely sensitive, electrical measurements are essentially unable to discriminate complex chemical events involving individual nanoparticles. The coupling of electrochemistry to dark field imaging and spectroscopy allows the triggering of the electrodissolution of an ensemble of Ag nanoparticles (by electrochemistry) and the inference of both oxidation and dissolution processes (by spectroscopy) at the level of a single nanoparticle. Besides the inspection of the dissolution process from optical scattering intensity, adding optical spectroscopy reveals chemical changes through drastic spectral changes. The behaviours of single NPs and NP agglomerates are differentiated: in the presence of thiocyanate ions, the transformation of Ag single nanoparticles to AgSCN is investigated in the context of plasmonic coupling with the electrode; tentative interpretations for optically unresolved groups of nanoparticles are proposed.

Electron Transfer to a Phosphomolybdate Monolayer on Glassy Carbon: Ambivalent Effect of Protonation.

The polyoxomolybdate hybrid TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] K(Mo)Sn[N2(+)] was prepared through Sonogashira-type coupling between TBA4[PMo11O39{Sn(C6H4)I}] K(Mo)Sn[I] and an excess of 3,3-diethyl-1-(4-ethynylphenyl)triaz-1-ene bearing a protected diazonium function, followed by its deprotection by the addition of trifluoroacetic acid (TFA). This enlarges the family of organic-inorganic polyoxomolybdate-based hybrids, which has been far less investigated than their related polyoxotungstates. The diazonium function allows for the electrochemical grafting on glassy carbon, and the K(Mo)Sn-modified electrode was further probed by cyclic voltammetry. The PMo11Sn core was found to be highly sensitive to protonation, and five bielectronic proton-coupled electron transfer processes were detected in the presence of an excess of TFA, thus corresponding to the injection of up to 10 electrons in the potential range between 0.15 and -0.45 V/SCE. The gain observed in the thermodynamic potentials is however detrimental to the apparent kinetics of the electron transfer, which drops from 500 s(-1) in the absence of acid to 12 s(-1) in the presence of an excess of TFA.

Deciphering the Elementary Steps of Transport-Reaction Processes at Individual Ag Nanoparticles by 3D Superlocalization Microscopy.

Transport-reaction processes at individual Ag nanoparticles (NPs) are studied using electrochemistry coupled with in situ 3D light scattering microscopy. Electrochemistry is used to trigger a (i) diffusiophoretic transport mode capable of accelerating and preconcentrating NPs toward an electrode and (ii) subsequent diffusion-controlled oxidation of NPs. Individual NP dissolution rate, analyzed using optical modeling, suggests the intervention of insoluble products. New insights into diverse NPs behaviors highlight the strength of coupled optical-electrochemical 3D microscopies for single-NP studies.

Nanometrology based control: taming radical grafting reactions with attoliter precision.

Precisely controlled micropatterning with organic moieties is a promising route for designing smart surfaces, enabling the development of microsensors and actuators with optimal usage of reactants. Such applications require fine control over the surface modification process, which in turn demands detailed knowledge about the surface modification process. As complex surface kinetics often emerge as a result of even slight modifications of the grafting entity, non-invasive, sensitive and precise closed loop control strategies are highly desirable. In this paper we demonstrate that a nanometrology approach based on quantitative phase imaging (QPI) fulfill all these requirements. We first use the technique to monitor surface photografting kinetics of aryl radicals, comprehensively analyzing the effect of substituents on surface addition reactions. We demonstrate that several aspects of the grafting process are affected in complex ways, rendering open-loop strategies impossible to implement precisely. Then, we show that the operando optical phase signal can be used as a direct feedback, guiding the grafting reaction process. Using relatively simple instrumentation, we demonstrate that general and precise control strategies can be designed and used to control the volume of the grafting material with attoliter precision, in spite of radically different surface modification kinetics spanning several orders of magnitude.

Operando surface optical nanometrology reveals diazonium salts' visible photografting mechanism.

High resolution and quantitative surface modification through photografting is a highly desirable strategy towards the preparation of smart surfaces, enabling chemical functions to be precisely located onto specific regions of inert surfaces. Although promising, the mechanisms leading to direct (without the use of any additive) photoactivation of diazonium salts using visible wavelengths are poorly understood, precluding the generalization of popular diazonium-based electrografting strategies into high resolution photografting ones. In this paper, we employ quantitative phase imaging as a nanometrology tool for evaluating the local grafting rate with diffraction-limited resolution and nanometric precision. By carefully measuring the surface modification kinetics under a range of different conditions, we reveal the reaction mechanism while evaluating the influence of key parameters, such as the power density, the radical precursor concentration and the presence of side reactions.

Surface potential modulation as a tool for mitigating challenges in SERS-based microneedle sensors.

Raman spectroscopic-based biosensing strategies are often complicated by low signal and the presence of multiple chemical species. While surface-enhanced Raman spectroscopy (SERS) nanostructured platforms are able to deliver high quality signals by focusing the electromagnetic field into a tight plasmonic hot-spot, it is not a generally applicable strategy as it often depends on the specific adsorption of the analyte of interest onto the SERS platform. This paper describes a strategy to address this challenge by using surface potential as a physical binding agent in the context of microneedle sensors. We show that the potential-dependent adsorption of different chemical species allows scrutinization of the contributions of different chemical species to the final spectrum, and that the ability to cyclically adsorb and desorb molecules from the surface enables efficient application of multivariate analysis methods. We demonstrate how the strategy can be used to mitigate potentially confounding phenomena, such as surface reactions, competitive adsorption and the presence of molecules with similar structures. In addition, this decomposition helps evaluate criteria to maximize the signal of one molecule with respect to others, offering new opportunities to enhance the measurement of analytes in the presence of interferants.

Local Surface Chemistry Dynamically Monitored by Quantitative Phase Microscopy.

Surface modification by photo grafting constitutes an interesting strategy to prepare functional surfaces. Precision applications, however, demand quantitative methods able to monitor and control the amount and distribution of surface modifications, which is hard to achieve, particularly in operando conditions. In this paper, a label-free, cost-effective, all-optical method based on wavefront sensing which is able to quantitatively track the evolution of grafted layers in real-time, is presented. By positioning a simple thin diffuser in the close vicinity of a camera, the thickness of grafted patterns is directly evaluated with sub-nanometric sensitivity and diffraction-limited lateral resolution. By performing an in-depth kinetic analysis of the local modification of an inert substrate (glass cover slips) through photografting of arydiazonium salts, different growth regimes are characterized and several parameters are estimated, such as the grafting efficiency, density and the apparent refractive index distribution of the resulting grafted layers. Both focused and widefield-grafting can be quantitatively monitored in real time, providing valuable guidelines to maximize functionalization efficiency. The association of a well-characterized versatile photografting reaction with the proposed flexible and sensitive monitoring strategy enables functional surfaces to be prepared, and puts surface micro- to submicro-structuration within the reach of most laboratories.

Effect of the driving force on nanoparticles growth and shape: an opto-electrochemical study.

Most protocols developed to synthesize nanoparticles (NPs) and to control their shape are inspired from nucleation and growth theories. However, to rationalize the mechanisms of the shape-selective synthesis of NPs, experimental strategies allowing to probe in situ the growth of NPs are needed. Herein, metal Au or Ag nanoparticles (NPs) are produced by reaction of a metallic ion precursor with a reversible redox reducer. The process is explored by an oxidative electrosynthesis strategy using a sacrificial Au or Ag ultramicroelectrode to both trigger the metallic ion generation and control the local concentrations of the different reactants. The effect of the driving force for the metallic ion reduction over metal NP growth dynamics is inspected in situ and in real time at the single NP level by high-resolution optical microscopy from the tracking of the Brownian trajectories of the growing NPs in solution. The NP reductive growth/oxidative etching thermodynamics, and consequently the NP shape, are shown to be controlled electrochemically by the reversible redox couple, while the intervention of an Au(i) intermediate ion is suggested to account for the formation of gold nanocubes.

Light Driven Design of Dynamical Thermosensitive Plasmonic Superstructures: A Bottom-Up Approach Using Silver Supercrystals.

When narrowly distributed silver nanoparticles (NPs) are functionalized by dodecanethiol, they acquire the ability to self-organize in organic solvents into 3D supercrystals (SCs). The NP surface chemistry is shown to introduce a light-driven thermomigration effect, thermophoresis. Using a laser beam to heat the NPs and generate steep thermal gradients, the migration effect is triggered dynamically, leading to tailored structures with high density of plasmonic hot spots. This work describes how to manipulate the hot spots and monitor the effect by holography, thus providing a complete characterization of the migration process on a single object basis. Extensive single object tracking strategies are employed to measure the SCs trajectories, evaluate their size, drift velocity magnitude and direction, allowing the identification of the physical chemical origins of the migration. The phenomenon is shown to happen as a result of the combination of thermophoresis (at short length scales) and convection (long-range), and does not require a metallic substrate. This constitutes a fully optical method to dynamically generate plasmonic platforms in situ and on demand, without requiring substrate nanostructuration and with minimal interference on the chemistry of the system. The importance of the proof-of-concept herein described stems from the numerous potential applications, spanning over a variety of fields such as microfluidics and biosensing.