New Approach for Measuring Lateral Diffusion in Langmuir Monolayers by Scanning Electrochemical Microscopy (SECM):  Theory and Application

Abstract

A new SECM approach for studying the lateral diffusion of redox-active amphiphiles in Langmuir monolayers at an air/water (A/W) interface is described. To apply this technique practically, a triple potential step transient measurement is utilized at a submarine ultramicroelectrode (UME) placed in the water phase close (1−2 μm) to the monolayer. In the first potential step, an electroactive species is generated at the UME by diffusion-controlled electrolysis of a precursor. This species diffuses to, and reacts with, the redox-active amphiphile at the A/W interface resulting in the formation of the initial solution precursor, which undergoes diffusional feedback to the UME. In this first step, the rate constant for electron transfer between the solution mediator and the surface-confined species can be measured from the UME current−time transient. In the second period, the potential step is reversed to convert the electrogenerated species to its initial form. Lateral diffusion of electroactive amphiphile into the interfacial zone probed by the UME occurs simultaneously in this recovery period. In the third step, the potential is jumped in the same direction as for the first step and the corresponding UME current−time transient can be used to determine either the distance between the UME tip and the monolayer at the water surface, or the lateral diffusion coefficient of the amphiphile. A theoretical treatment for this technique is developed and discussed in detail. The method is demonstrated experimentally with measurements of the lateral diffusion of N-octadecylferrocenecarboxamide in a 1:1 Langmuir monolayer with 1-octadecanol.

Evaluation of the Chemical Reactions from Two Electrogenerated Species in Picoliter Volumes by Scanning Electrochemical Microscopy

“…Measurable effects on this electrochemical signal are only possible when the probe and the substrate are close to each other, for example, to a distance shorter than about five times the UME radius where their diffusion layers can interact; a typical UME has a radius close to 10 mm or smaller. The electrogenerated species may be reactive towards chemical species at the substrate; thus, the feedback mode has been applied to study the heterogeneous reactivity, for instance, from mediators that "micro titrate" chemisorbed species at electrodes, [5,6] that chemically etch metals [7] involved in the study of the effects of mediator surface diffusion, [8][9][10] lateral charge propagation in polymer films, [11,12] and diffusion in monolayers, [13] as well as used in the detection of reactive species in living cells. [14] In these examples, the SECM probe acts both as a generator of the reactive species and as a transducing element as the feedback current indicates the existence and extent of chemical reaction.…”

“…Equations (13) and (14) are used to describe the flux of each species to the tip; in the case of the substrate, E R is replaced by E O (see Equations S2 and S3 of the Supporting Information). The following parameters were chosen for the potentials: E R = À0.4 V, E O = 0.4 V and formal potentials E 0' 1 = À0.2 V for the O1/R1 pair and E 0' 2 = 0.2 V for the O2/R2 pair; this ensured the recreation of the predominant regions and the I-E characteristics shown in Figure 2 A, panel A4.…”

“…Under these conditions, the electrodes exhibit the typical response for a system with positive feedback and collection [Eqs. (13) and (14)]:…”

Laser Scanning Confocal Microscopy Coupled with Hydraulic Permeability Measurements for Elucidating Fluid Flow across Porous Materials: Application to Human Dentine

“…These include: medically-related processes, such as the transport of water and solutes through cartilage [1][2][3] and transdermal drug delivery 4,5 basic processes pertinent to cellular processes, such as transport across lipid monolayers [6][7][8] and bilayers; [9][10][11][12][13] and applications in fuel cell technology. [14][15][16] In this paper, we describe the use of laser scanning confocal microscopy (LSCM), coupled with constant volume flow ratepressure measurements, as a new approach for visualising and quantifying the nature of fluid flow across porous materials.…”

Visualizing Chemistry: The Progess and Promise of Advanced Chemical Imaging

“…SECM provides a means to study interfacial charge-transfer dynamics, while minimizing some of the difficulties associated with other methods. The rates of electron transfer can be measured by both steady-state 253 and time-resolved chronoamperometric 254 SECM methods. Mass transport through films, 255 porous membranes, 256 and biological membranes (even in living cells) 257 can also be probed.…”