# Scanning Electrochemical Impedance Microscopy-Based Assessment of Glucose Biosensors

## TL;DR

Researchers used a scanning electrochemical method to assess glucose biosensors, finding that even tiny amounts of an enzyme can reliably detect glucose levels.

## Contribution

The study demonstrates the use of SEIM for localized glucose detection with low enzyme concentrations on nonconductive surfaces.

## Key findings

- Glucose oxidase activity was reliably detected at surface concentrations as low as 50 fg/mm².
- Localized impedance measurements enabled glucose detection on nonconductive substrates.
- SEIM detected glucose concentrations between 2–20 mM using 10 Hz AC perturbation.

## Abstract

Scanning electrochemical impedance microscopy (SEIM)
was assessed
as an electrochemical method for developing glucose biosensors based
on glucose oxidase (GOx). To determine the lowest detectable GOx activity,
scanning electrochemical microscopy (SECM) in the feedback mode (FB-SECM)
was applied. During the measurement procedure, an ultramicroelectrode
(UME) was moved vertically over the surface modified by immobilized
GOx. A positive feedback response of the FB-SECM mode was determined
during the assessment of surfaces modified by 5 fg/mm2 to
20 μg/mm2 surface concentration of GOx. The lowest
surface concentration of GOx, which still provided reliable measurement
results, was 50 fg/mm2. The approach curves registered
using the FB-SECM mode were assessed using a mathematical model adapted
for the calculation of reaction kinetics by SECM. According to this
model, the reaction kinetics constant λ was calculated for differently
modified surfaces in the presence of the same glucose concentration.
For the surface not modified by GOx, the constant λ was determined
to be 0.14, while for the GOx-modified surface λ gradually increased
with increasing GOx surface concentrations, the λ value reached
0.34 when it was determined on the surfaces modified by 500 ng/mm2 of GOx. Any statistically significant changes in FB-SECM
were detected when the surface concentration of GOx exceeded 50 pg/mm2. Notably, localized electrochemical impedance measurements
using the SEIM mode enabled one to detect GOx activity even when GOx
was immobilized on a nonconductive substrate surface. The results
show that redox competition-based SEIM can be used to determine glucose
concentrations in the range of 2–20 mM, while using 10 Hz AC
perturbation. An FB-SECM configuration allows the reuse of the ultramicroelectrode
while providing the localized impedance-based glucose concentration
and enzyme activity measurements.