Amperometric biosensors display changes in current corresponding to changes in the concentration of an analyte. For in vitro biosensing where analytes will be provided to a batch of PBS for example, you can see a step function of currents as you inject increasing concentrations of an analyte. My question is for a glucose biosensor. Amperometric glucose sensor is based on enzymatic activity of glucose oxidase oxidizing glucose molecules to produce hydrogen peroxide, which can again be oxidized into 2H+, O2, and 2e-. Once those 2 electrons are released from H2O2, do they stay in the solution to keep the current at a constant #? What if the current at a certain concentration of glucose drops? what does this mean?
2 electrons are taken by electrode which is charged by applied potential. Otherwise 2 electrons are not free in solution because H2O2 is in solution, it is transformed by current.
well the syeady state is necessary to be established in amperometric biosensors. ! Current will be stable and dependent by the concentration of the analyte only if a steady state is established !
This can be obtained also by protecting the enzyme with a suitable membrane in order to establish the steady state.
Your amperometric sensor consists on absorbing the electrons of the oxidation, which provide the increase in the current value. That's the way you measure. Electrons are rarely free, maybe in the space as beta radiation they are, but certainly not in solution. Regarding current drop: if it's too high, let's say much more than 1 molar, the conductivity of your solution drops. If it's too low, or if GOx has been too long consuming its substrates, one of them (glucose or oxygen) becomes short and the reaction does not proceed as fast as the initial time.
I agree with both Marco Mascini and Gareth Keeley. This is observed very often and you can explain it by a release of the enzyme from the electrode or the fact that you didn't reach the steady state current of your enzyme from a michaelis menten point of vue...
But electrons are never in solution...
The electrons go to the electrode and are measured as the current, which is your sensor readout. If the current goes down it (probably) means you are generating less H2O2, which is because you are oxidizing less glucose, which is because the mass flux of glucose to the sensors surface is lower. This, in turn, is normally because you have a lower concentration in the sample IF you continuously supply new solution to the sensor. Otherwise I guess it would means that you have oxidized all the glucose you had close to the surface and reach some kind of diffusion limitation in the flux.
Thanks everyone! Summing up all comments, I suspect that my enzyme was not sufficiently attached to the electrode. The current drop is in the magnitude of uA and it does go back to almost to the baseline again, which didn't quite make sense to me. Understanding the enzymes were in hydrogel matrix, I'm not sure how good of adhesion I had between hydrogel matrix and hydrophobic platinum electrode. Does anyone have an idea on how to make a good adhesion between hydrogel and platinum surface?
Coate titanium with a thin pyrrole or something else layer and via hydrogel residues bind them each other
What you suggest is only for a stirred solution or solution pumped through a flow cell. Normally a mediator is used such as ferricyanide or oxidized dye molecule to replace the oxygen as the source of oxidizing power. There are thousands of papers on glucose sensors over 40+ years, but most are very similar. Peroxide (as you mention it) is typically measured at a platinum electrode, but more modern glucose sensors use carbon electrodes and some use palladium or gold films. You reach a steady state if the volume of solution is huge vs the surface area of the electrode. Often this is not the case, as in glucose meters purchased at a pharmacy. There the volume is tiny and the process is not a steady-state process at all.
you can see more informations here:
http://web.uconn.edu/rusling/Wang_G-sensorREv_2008.pdf
The peak current increase with the increase of glucose, so electrons will change the current in the same proportions. You have a reference electrode with a constant potential and the measures are made in a relation (versus) this electrode. good luck!
- Badges
- Science topic
- Similar topics
- Chemistry
- Electrochemistry