mauzeroll-et-al-2016-scanning-electrochemical-microscopy-a-comprehensive-review-of-experimental-parameters-from-1989-to

Introduction

• Scanning Electrochemical Microscopy (SECM) is an electroanalytical scanning probe technique introduced in 1989.

• Capable of imaging substrate topography and local reactivity with high resolution.

• Applications have expanded into various fields including biology, corrosion, energy, and surface modification.

• Over 1800 peer-reviewed publications related to SECM have been produced.

• The review aims to summarize experimental parameters used in SECM research from 1989 to 2015, making it a practical guide.

Principles of SECM

2.1 Instrumentation

• A typical SECM setup consists of:

  • Bipotentiostat: Measures and controls current and potential.

  • 3D Positioning System: Allows precise movement of the probe and substrate.

  • Probe (SECM tip): Defines the resolution of measurements, generally in micrometer to nanometer size.

  • Data Acquisition System: Coordinates the functions of the SECM setup.

• Additional components may include inverted optical microscopes, fluorescence detection systems, and constant distance units.

• Commercially available systems include setups from BioLogic and CH Instruments.

2.2 Modes of Operation

• SECM can operate in various modes including:

  1. Feedback Mode: Most common mode, involves a biased probe close to a substrate, and depends on substrate topography and reactivity.

  2. Generation/Collection Modes: Involves either substrate generation/tip collection (SG/TC) or tip generation/substrate collection (TG/SC) for measuring concentration profiles.

  3. Redox Competition Mode: Measures the competitive reaction rates of the same redox species between the tip and substrate.

  4. Direct Mode: Utilizes the microelectrode tip as a counter electrode, primarily for surface modification applications.

  5. Potentiometric Mode: Measures potential rather than current, suitable for examining electroactive species.

2.3 Key Parameters

• Current Normalization: Measured current is normalized by the steady-state current to allow comparisons across different probes.

• Redox Mediators: Essential for the operation of SECM, selected based on stability and compatibility with operational mode.

Experimental Design

3.1 Mediators

• Direct Redox Mediators: Present in solution before experimentation (e.g., O2).

• Indirect Redox Mediators: Added to the solution (e.g., FcMeOH).

• Ideal mediator properties include electrochemical reversibility, fast heterogeneous kinetics, and suitable diffusion properties.

• Table 1 provides an exhaustive list of redox mediators categorized by usage and application in feedback mode SECM.

3.2 Solvents

• Selection of solvent systems is critical in SECM experiments for influencing conductivity and chemical stability.

• Over 99% of experiments conducted in solutions containing electrolytes (e.g., KCl) in the concentration range greater than 0.1 mM.

3.3 Probes

3.3.1 Amperometric Probes

• Most commonly used type in SECM, responds to faradaic processes and consists of an electroactive core surrounded by an insulating sheath.

• Advantages include robustness and quick response times, but may experience convolution of signals from multiple faradaic processes.

3.3.2 Potentiometric Probes

• Used mainly in biological and localized pH measurements.

• Provides high selectivity but has slower response times.

Applications

4.1 Instrumental Development

• Ongoing efforts in SECM focus on the development of operational modes and smaller, more sensitive probes.

• Recent advancements include temperature-controlled measurement systems and new methodologies for dynamic imaging.

4.2 Biological Research

• SECM is effectively applied to biological substrates, including measuring enzymatic activity, cellular respiration, and protein expression.

4.3 Enzymatic Activity

• Feedback mode used to correlate electrochemical activity with enzyme activity; SG/TC mode used for mapping concentration profiles.

4.4 Living Cells Studies

• SECM's non-invasive approach allows for measurements of metabolic processes and enzyme interactions in single cells.

4.5 Corrosion Studies

• SECM effectively investigates corrosion mechanisms in metallic substrates, utilizing feedback and various generation/collection modes to understand local reactivity.

4.6 Energy Applications

• Characterization of materials for batteries, energy harvesting devices, and fuel cells using SECM to evaluate electrochemical activity.

4.7 Surface Modification

• SECM's resolution allows for precise surface modifications, including deposition of metals and polymers.

4.8 Kinetics

• SECM is a powerful tool in measuring reaction kinetics across various interfaces, including solid/liquid and liquid/liquid systems.

Summary and Future Perspectives

• This review highlights the significant advancements in SECM in over 25 years.

• Future focus areas include improving imaging speed, enhancing probe performance, and expanding applications in complex biological and material systems.

Introduction

Scanning Electrochemical Microscopy (SECM) is an advanced electroanalytical scanning probe technique that was introduced in 1989. This method is notable for its ability to image both the topography of substrates and their local reactivity with high spatial resolution, opening pathways for in-depth analysis in various scientific fields. As of today, there are over 1800 peer-reviewed publications detailing numerous applications of SECM across domains such as biology, corrosion science, energy devices, and surface modification techniques. This review is designed to summarize important experimental parameters employed in SECM research from its inception to the present, serving as a practical guide for researchers.

Principles of SECM

Instrumentation

A standard SECM setup includes several critical components:

• Bipotentiostat: This device measures and controls both current and potential within the system, ensuring accurate readings.

• 3D Positioning System: This allows for precise movement of the probe relative to the substrate, crucial for maintaining appropriate distances during measurements.

• Probe (SECM tip): The probe is responsible for defining the resolution of measurements, generally ranging from micrometers to nanometers in size.

• Data Acquisition System: This coordinates the various functions of the SECM setup and collects data for analysis.

• Additional Components: Systems often include inverted optical microscopes for enhanced visualization, fluorescence detection systems for specific imaging applications, and constant distance units for maintaining probe-substrate distance.Commercially available SECM systems are produced by companies such as BioLogic and CH Instruments.

Modes of Operation

SECM operates in several different modes tailored for various research needs:

• Feedback Mode: This is the most common mode, where a biased probe is placed close to a substrate, relying on the topography and reactivity of the substrate for data collection.

• Generation/Collection Modes: These modes involve substrates generating and tips collecting species (SG/TC) or tips generating and substrates collecting species (TG/SC) for profiling concentration differences.

• Redox Competition Mode: Used to investigate competitive reaction rates of identical redox species between the probe and substrate.

• Direct Mode: In this mode, the microelectrode tip functions as a counter electrode, making it particularly useful for surface modification studies.

• Potentiometric Mode: Focuses on measuring potential rather than current, ideal for analyzing the presence of electroactive species.

Key Experimental Parameters

• Current Normalization: Current measurements are normalized against a steady-state current to facilitate comparisons across different probes.

• Redox Mediators: The choice of redox mediators is critical for SECM operation, requiring selection based on stability and compatibility with the intended mode of operation.

Experimental Design

Mediators

• Direct Redox Mediators: These are present in the solution prior to experimentation, such as oxygen (O2), which directly facilitates redox reactions.

• Indirect Redox Mediators: These are added during the experimental process, like ferrocene methanol (FcMeOH), to enhance measurement accuracy.Ideal properties for mediators include electrochemical reversibility, rapid heterogeneous kinetics, and suitable diffusion characteristics. A comprehensive list of redox mediators clarified by usage and application in feedback mode SECM is provided in Table 1.

Solvents

The choice of solvent systems is paramount in SECM experiments, as they considerably influence both conductivity and chemical stability. Remarkably, over 99% of SECM studies utilize solutions that contain electrolytes, typically in concentrations greater than 0.1 mM solution, such as potassium chloride (KCl).

Probes

Amperometric Probes

These are the most frequently employed probes in SECM. They respond to faradaic processes and consist of an electroactive core insulated by a sheath. Their advantages include robustness and rapid response times; however, they may suffer from signal convolution due to multiple concurrent faradaic processes.

Potentiometric Probes

Potentiometric probes are primarily utilized for biological applications, particularly localized pH measurements. They offer high selectivity but typically exhibit slower response times compared to amperometric probes.

Applications

Instrumental Development

Current efforts in SECM focus on advancing operational modes and developing smaller, more sensitive probes. Recent innovations entail temperature-controlled measurement systems and novel methodologies aimed at improving dynamic imaging capabilities.

Biological Research

SECM has proven effective in studying biological substrates, facilitating the measurement of enzymatic activity, cellular respiration, and protein expression levels.

Enzymatic Activity

SECM utilizes feedback mode to correlate electrochemical responses with enzyme activity, while SG/TC mode is applied for mapping concentration profiles pertinent to their biological functions.

Living Cell Studies

The non-invasive nature of SECM permits it to measure metabolic processes and enzymatic interactions within living single cells, providing critical insights without disrupting their natural environment.

Corrosion Studies

SECM is adept at investigating corrosion mechanisms in metallic substrates, employing feedback and generation/collection modes to comprehend local reactivity phenomena.

Energy Applications

Through SECM, researchers can characterize materials employed in batteries, energy harvesting mechanisms, and fuel cells by assessing their electrochemical activities.

Surface Modification

The high resolution achievable via SECM allows for precise surface modifications, including the deposition of metals and polymers on various substrates.

Kinetics

SECM stands out as a formidable method for measuring reaction kinetics across diverse interfaces, ranging from solid/liquid to liquid/liquid systems.

Summary and Future Perspectives

This review encapsulates the substantial advancements in SECM over the past 25 years. Future trajectories for SECM research will likely concentrate on enhancing imaging speeds, improving probe performance, and extending the applications of SECM into complex biological systems and innovative material study projects.