Team:UC Davis/Electrochemistry Electrode Choice

Selecting the electrode was the first step in building our electrochemical system. In the selection process, many features were considered including price, portability, accuracy, solution volume, and compatibility with our hardware and software signal processing device. Our initial idea was to build our own electrode. However, after reviewing many literatures, we have concluded that purchasing one would be the smarter choice for our short time constraints and enhancing accuracy of chemical detection. We narrowed down our choice to two of the commercial electrodes which already have been proved to detect small concentration of NADH. One was DropSens, and the other was Pine Instrument. These two electrode have features which may or may not be suitable for our system.

DropSens Vs. Pine Instrument:

DropSens	Pine Instrument
Pros: Size and Portability: Electrode has dimensions of only 3.4 x 1.0 x 0.05cm Working and counter electrodes are 4mm diameter, made in carbon surface Price: Requires 50-60µL volume of solution Inexpensive than build our own Disposability Easily disposable after run experiment Accuracy proved to detect NADH in a smaller concentration (see figure(1) - excerpt from DropSens website)	Pros: Flexibility: gives up an option to self-polymerize electrode with chemicals of our choice add more adjustment for working/counter/reference electrode which may affect the sensitivity of detection Re-usable solution cap closed solution cap prevents contamination of solution helps portability with its closed-solution cap
Cons: Less flexibility: electrode comes with pre-polymerized surface	Cons: Price: Consumes 200 times more volume of solution than Dropsens (10mL vs. 50uL) closed-solution cap was pricy Complex electrode pre-treatment necessity of separate working electrode and counter electrode by gluing surface and wait until it dries up need to polymerize electrode each time of experiment Accuracy: due to complex steps for pre-treatment electrode, more possibility of errors could be caused(i.e: non-complete separation of working and counter electrode results in different signals)

Considering both pros and cons of each electrode, DropSens has more benefits than disadvantage to our electrochemical system in many ways. Therefore, we have been decided to select DropSens as our electrode.

The electrode system consists of a screen-printed chip embedded with three electrodes: the counter, working, and reference electrodes. Though the potential of the reference electrode is kept constant, a voltage bias is applied across the working and counter electrodes to facilitates a buildup of excess positive charge on the working electrode. This buildup induces the directional diffusion of NADH toward the working electrode. The working electrode consists of a carbon ink infused with polymer. Since some polymers have selective oxidative affinity for NADH, we decided to test two polymers that would work the best for our electrode.

Azure A	Meldola's Blue
Advantage: Showed selective oxidative affinity for NADH Dissolved and activated in hydrochloric acid and sodium nitrate Electropolymerized onto surface	Advantage: Showed selective oxidative affinity for NADH Dissolved and activated in Phosphate buffer Electropolymerized onto surface Displayed greater electron deposition and flow Demonstrated more reliable signals (NO indication of major chemical interference at the electrode surface)
Concerns: Displays second order responses (indicates a possibility of non-trivial chemical interactions on electrode surface) There is no pre-polymerized Azure A DropSens electrode.	Concerns: There's no significant concern compare to Azure A

Comparison experiment result:

Polymerization with both compounds, Azure A and Meldola’s Blue both gives some selective oxidative affinity for NADH by lowering the potential required to oxidize NADH at the electrode; therefore, expanding the voltage range favorable for us to conduct experiments at. Both AA and MB has a selective oxidative affinity for NADH, reducing the over potential necessary for NADH oxidation at the working electrode. Although both are advantageous in terms of selectivity for detecting NADH, we have decided to use a polycyclic aromatic monomer dye, Meldola’s Blue (MB) for the following reason:

1. MB facilitates greater electron deposition and flow.
   
2. Azure A demonstrated second order responses, which can be possible indicative of non-trivial chemical interactions at the electrode surface.
   
3. MB gave us more reliable readings, especially showed no second order responses.
   
4. We can specifically ordered MB-infused electrodes from Dropsens.