A local look at sensors

Effective sensors allow for the quick and easy detection of a substance of interest, through a chemical reaction. In the relatively short history of sensors they have been applied to a wide range of industries as gas sensors, biosensors, and electrochemical sensors. Sensors have notably been applied in the field of healthcare with the highly successful blood glucose sensor. For a sensor to be useful it should allow for a fast and reversible chemical reaction. One method to study potential sensor systems is to use Scanning ElectroChemical Microscopy (SECM), a scanning probe microscopy technique measuring the local electrochemistry of a sample. Introduced in the late 1980s SECM uses an UltraMicroElectrode (UME) probe in close proximity to a sample surface to measure the Faradaic current of a redox mediator in solution. Because the interaction of the sample with the redox mediator affects the concentration of the mediator available to the probe the signal measured in SECM reflects the activity of the sample with respect to the mediator. This inherent chemical selectivity of SECM makes it ideal for use in the investigation of potential sensor systems.

Why use SECM in studies of sensors? SECM has been widely used in the study of sensor systems, particularly in the investigation of biosensors. The chemical selectivity of SECM is certainly of great benefit in the study of sensor systems, however it is not the only benefit offered by the use of SECM.

• The local specificity of the SECM measurement is beneficial in the study of sensor systems. In bulk electrochemical measurements, the result is a global, average signal of the sample. When sensor systems are measured local changes are sometimes expected, which cannot be detected through the use of a bulk technique. Furthermore, the local structure of the sensor can affect its performance, therefore the ability of SECM to investigate a sensor system locally provides useful insight.

• Unlike other techniques SECM allows high throughput screening of libraries of sensors. In these studies, a single sample comprising the sensor library is prepared and measured using an SECM area scan. The local specificity of the SECM techniques means that the local signal can be directly correlated to the local sensor composition. This is especially beneficial because the experimental parameters are exactly the same during the measurement of each sensor composition, allowing for direct comparison of the library.

• SECM can be performed further from the sample surface than other scanning probe microscopies, without the need to contact the sample. This opens the use of SECM measurement to particularly delicate sensor systems, for example those with enzymes adsorbed on the sensor surface, which would be damaged by the use of contact measurements.

• Unlike other methods of electrochemical analysis SECM can measure sensors without the need to make electrical contact to the sample. This expands the technique to sensor systems which would typically be difficult to prepare for measurement, including those where the conductive regions are isolated within an insulator.

• SECM is not just limited to area scans – approach curves can also be used to characterize a sample. In this case, the probe approaches the sample surface and the effect of the reduced probe to sample distance on the probe current is plotted. An analysis of the approach curve can provide valuable insight into the kinetics of the analyte interaction at the sensor.

The growing application areas of sensors means reliable methods to analyze the systems of interest are necessary. The local chemical specificity of SECM makes it well-suited to applications in sensor studies, providing researchers with information which is not easily measured by any other means.

Interested in using SECM to enhance your sensor studies?

Have a look to our SCAN-Lab AN#3: SECM height relief with OSP: An application in sensors introduces the use of SECM in the study of sensors.

This note describes the use of the Optical Surface Profiler (OSP) technique to initially measure the topography of a sensor sample. The OSP topography output is then applied to the subsequent SECM area scan. The resulting constant distance SECM area scan allows the activity of the sample to be measured without the influence of sample topography.

To learn more about the SECM systems available from Bio-Logic please visit the M470 and SECM150 pages.