You're right to be suspicious of the reusability of the gold chloride solution being a factor in the current decrease you're observing. Here's a breakdown of the reusability and potential reasons for the current decrease:

Reusability of 3 mM Gold Chloride Solution:

• Fresh solution is recommended: While gold chloride solutions (HAuCl₄) can technically be reused a few times, it's generally recommended to prepare a fresh solution for each experiment, especially when dealing with sensitive procedures like electrochemical deposition.
• Reasons for degradation:Decomposition: Gold chloride can slowly decompose over time, especially upon exposure to light or heat. This can lead to the formation of inactive gold species that won't effectively deposit nanoparticles. Contamination: Even minor contamination from previous uses or storage containers can interfere with the deposition process.

Possible reasons for current decrease:

• Degraded gold chloride: As mentioned above, a degraded gold chloride solution might not provide enough active gold ions for efficient nanoparticle deposition. This can lead to insufficient coverage on the electrode and a subsequent decrease in current during further experiments (like square-wave voltammetry with the probe).
• Electrolyte Issues (less likely): While less likely, there could be an interaction between the thiol-modified probe and the sodium nitrate (NaNO3) electrolyte used. This could potentially affect its adsorption or electron transfer properties on the electrode. However, NaNO3 is a common and generally non-interfering electrolyte in such experiments.

Recommendations:

By following these steps, you can isolate the cause of the current decrease and ensure successful immobilization of your thiol-modified probe on the gold nanoparticle-modified glassy carbon electrode.

Thanks!