Mapping fluxes of radicals from the combination of electrochemical activation and optical microscopy

Abstract

The coating of gold (Au) electrode surfaces with nitrophenyl (NP) layers is studied by combination of electrochemical actuation and optical detection. The electrochemical actuation of the reduction of the nitrobenzenediazonium (NBD) precursor is used to generate NP radicals and therefore initiate the electrografting. The electrografting process is followed in situ and in real time by light reflectivity microscopy imaging, allowing for spatio-temporal imaging with sub-micrometer lateral resolution and sub-nanometer thickness sensitivity of the local growth of a transparent organic coating onto a reflecting Au electrode. The interest of the electrochemical actuation resides in its ability to finely control the grafting rate of the NP layer through the electrode potential. Coupling the electrochemical actuation with microscopic imaging of the electrode surface allows quantitative estimates of the local grafting rates and subsequently a real time and in situ mapping of the reacting fluxes of NP radicals on the surface. Over the 2 orders of magnitude range of grafting rates (from 0.04 to 4 nm s(-1)), it is demonstrated that the edge of Au electrodes are grafted -1.3 times more quickly than their centre, illustrating the manifestation of edge-effects on flux distribution at an electrode. A model is proposed to explain the observed edge-effect, it relies on the short lifetime of the intermediate NP radical species.

Optical Nanoimpacts of Dielectric and Metallic Nanoparticles on Gold Surface by Reflectance Microscopy: Adsorption or Bouncing?

“…Comparing the signal recorded at each pixel I(x,y,t) to that recorded on the first image I(x,y,t = 0) allows monitoring of the local relative variation in reflectivity, ∆R(x,y,t)/R, from, Under the λ = 490 nm blue light illumination used herein, an ideal reflecting Au substrate ( ñ M . = 1.1 + 1.83i) immersed in water (n A = 1.33) is expected to reflect toward the CCD camera 37% of the incident light, and our setup was shown to detect local variation of reflectivity down to ∆R/R = 0.002 [16][17][18], meaning variation in collected signal down to 0.2%.…”

“…The detection of individual NPs freely moving in solution or adsorbed on an Au-coated Si wafer (Aldrich) was investigated in real time by light reflectivity microscopy. It was performed, as previously described [16][17][18], with an in-house developed setup consisting of a standard microscope (Olympus) equipped with a water-immersion objective (Olympus LUMP-LFLN 40XW, 40 ×, NA 0.8), and a CCD camera (Photon-Focus MV-D1024E-160-CL). A light halogen source, filtered at 490 nm with an interference filter (spectral bandwidth of 20 nm), was used to illuminate, from the top solution and via the objective, the Au substrate.…”

“…Simpler and yet quantitative strategies consist in analyzing images collected by a standard microscope operating in the reflection mode. Indeed, our group showed that the analysis of the light reflected by a reflecting surface illuminated under normal incidence [16][17][18], allowed quantifying and mapping heterogeneous electrochemical processes with sub-monolayer formation or deposition sensitivity. Particularly, we demonstrated the versatility of the strategy to the in situ imaging, using water-immersion objectives, of various electrode systems such as microfabricated Au-coated Si wafer [17], the standard glass-shielded microelectrodes [16], or various electrode materials, such as iron [18], for operando corrosion studies.…”

Calibration procedures for quantitative multiple wavelengths reflectance microscopy

“…It is also seen to be non-zero and electrochemicallydriven on the substrate (x < 16 pix), thereby proving its non-mechanical origin. This field actually corresponds to the reflectivity field used to monitor surface chemical affinities 4 , and illustrates (wavelength-dependent) electroreflectance phenomena in this particular case 19 . It also shows a slight charge accumulation over the cycles.…”

“…Imaging techniques usually reveal the heterogeneity of surface affinities at the micrometer scale 4 , so that the assessment of the chemical homogeneity of the considered surface is required when focusing on chemo-mechanical coupling phenomena. As surface chemical composition modifications induce (complex) reflection coefficient changes, Jin et al 5 proposed an ellipsometric imaging set-up to measure the optical thickness of thin adsorbed films, when Li et al 6 use interferometry to measure locally the concentration profiles of reactants near an electrode.…”