Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope (AFM-SECM)

Atomic force microscopy (AFM) combined with scanning electrochemical microscopy (SECM), namely, AFM-SECM, can be used to simultaneously acquire high-resolution topographical and electrochemical information on material surfaces at nanoscale. Such information is critical to understanding heterogeneous properties (e.g., reactivity, defects, and reaction sites) on local surfaces of nanomaterials, electrodes and biomaterials.

This protocol employees a powerful and innovative technology so called Atomic Force Microscope coupled with Electro-chemicals Scanning Microscope which is AFM-SECM to scan the morphological and the electrochemical information on faceted nanomaterials and nanobubbles in water. AFM-SECM is capable of mapping electrochemically reactive surface based on tip current images and also enable simultaneous acquisition of nanoscale surface structures and electrochemistry information on sample materials. The sample preparation for this method require that the solid particles are immobilized on the substrate completely and the bindings between samples and substrates ensures electrical conductivity.

The choice of the Redox Mediator is critical as well. More so, demonstration is necessary to really show that it held under a dedicated, separate operation in this protocol such as sample preparation systems that happen under the imaging process. Deposit 10 microliters of epoxy on a cleaned silicon wafer using a pipette tip and tile it with a clean glass slide.

After about five minutes, drop 10 microliters of the copper oxide nanoparticle suspension on the epoxy coated silicon wafers substrates. Then vacuum dry the substrate at 40 degrees Celsius or six hours. To prepare oxygen nanobubbles directly inject compressed oxygen through a tubular ceramic membrane into deionized water.

Deposit 1.8 milliliters of the nanobubble suspension on a gold substrate in the electrochemical sample cell and stabilize it for 10 minutes. Replace the existing sample chunk with the SECM chunk. Then screw it in place using two M3, 6 mm Socket Head Cap Screws and a 2.5 millimeter hex wrench.

Install the strain release module onto the AFM scanner and connect it to the working electrode connector on the spring connector block with an extension cable. Double-click the two software icons to initialize the AFM system and the by potentiostat control interface. Prepare the electrostatic discharge field, surface package including an anti-static pad, electrostatic discharge protective probe stand, wearable anti-static gloves and wrist strap.

To prevent the AFM scanner from exposure to liquid, use a protective boot during AFM-SECM testing. Put the probe holder onto the electrostatic discharge protective probe stand and use a pair of plastic tweezers to attach the protective boot to the tip holder. Then align the small cut in the protective boot to the notch in the probe holder.

Open the box of AFM-SECM probes with a tip tweezer and grab the probe from both sides of the grooves. Use the disc gripper to hold the probe holder on the stand and put the probe wire into the hole of the stand. Then slide the probe into the slot of the probe holder.

After the probe is inside the slot, use the flat end of the tweezer to push it in. Attach the whole probe holder to the scanner and use the PTFE tip tweezer to grab the wire right below the copper ring and connect it to the module. Then put the scanner back to the dovetail.

Place the previously assembled electrochemical sample cell with the test sample on the central point of the SECM chunk. Then connect the pseudo reference electrode and the counter electrode to the spring connector block. In the AFM-SECM software, select SECM PeakForce QNM to load the workspace.

In setup, load the SECM probe and then align a laser to the tip using an alignment station. Go to Navigation and move the scanner downwards slowly to focus on the sample surface. Adjust the position of the electrochemical sample cells slightly to make sure the scanner doesn't touch the glass cover of the sample cell while moving.

After focusing on the sample, click Update Blind Engage Position. Click, Move to Add Fluid Position and add approximately 1.8 milliliters of the buffer solution into the sample cell. Making sure that the level of the solution is lower than the glass cover and that the wires are immersed in the solution, use a pipette to agitate the solution to remove any bubbles and wait for five minutes.

Click, Move to Blind Engage Position which will make the tip move back into the buffer solution. Adjust the laser slightly to make sure the laser is aligned on the tip. Open the electrochemical workstation software and click on the Technique Command on the toolbar to open the tech selector.

Select Open Circuit Potential Time and use the default setting to run the OCP measurement which should be almost zero and stable. Click the Technique Command again and select Cyclic Voltammetry then enter the cyclic voltammetry parameters and proceed with SECM imaging. Go back to the AFM-SECM software and click, Engage.

Next select chronoamperometry and set the chronoamperometry parameters with the initially as minus 0.4 volts the pulse width as 1000 seconds and the same sensitivity as the CV scan. With the program running, go back to the AFM-SECM software check the real time reading on the strip chart and click on start. Save the images in the AFM-SECM software.

Using the electric chemical sample cell as a clean water container, move the tip in and out of the liquid with the Blind Engage Functions in the navigation panel, changing the water three times. Then use clean wipes to carefully remove residual water from the probe holder and put the probe back in the probe box. This protocol was used to characterize individual oxygen nanobubbles, revealing both morphological and electrochemical information.

The comparison of the topography and the current image demonstrates the correlation between the locations of the nanobubbles and the current spots. The topography and current images of copper oxide nanoparticles are shown here. The tip current image indicates that the nanoparticle visible in the topography image is associated with an evident electric current spot.

Whereas the background current corresponds to the flat silicon substrate. Here are five representative cyclic voltammetry curves of the AFM-SECM tip at approximately one millimeter from the substrate. The diffusion limited tip current did not decrease with time.

The changes of the tip current as the tip approaches the sample surface are plotted here. The AFM-SECM tip, approached the substrate surface in the Z direction until it reached a set point indicating the physical tip substrate contact or bending. When this protocol make sure that the solid particle are immobilized as the substrate they're completely with electrical conductivity and there are no bubbles in the solution in the sample cell.

The sample preparation method are relevant to a wider range of applications that involves nanomaterials, especially for nano material characterization. The AFM-SECM technique can be used to acquire simultaneous topography and the electrochemistry image at the nanoscale which is important in the development and application of nanomaterials in research fields such as material science, chemistry and life science.