Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy

A common theme in our research is using light matter interactions and electrical energy to measure, drive, and control interfacial chemical transformations at the nanoscale. In particular, we seek to understand how local environments and reaction intermediates affects selectivity and electric catalysis. Catalytic transformations are traditionally evaluated with ensemble average measurements of the products and the catalyst attributes.

The frontier challenges to produce chemical products selectively include reducing this measurement averaging while maintaining the measurement sensitivity so that we can better understand the actions of individual molecules in the reactive sites of the catalyst. Electrochemical techniques alone do not provide any chemical information on species that form and transform at the electrode surface. Our protocol enables electrochemical measurements at a single nanoparticle using vibrational spectroscopy as a readout, enabling correlations between electrochemical processes and molecular changes.

Moving forward, our laboratory will continue to push the boundaries of measurement resolution in space and time to understand chemical and material processes at the single molecule level and at the timescale of chemical reactions. To begin, deposit copper and silver onto the cleaned cover slips using the electron beam thin film deposition system following standard procedures as recommended by the manufacturer. For copper deposition, increase the emission current gradually at 10 milliamperes per minute until the sensor reads a deposition rate close to 10 angstroms per second.

Once the desired deposition rate is achieved, close the shutter and set the platinum position to zero degrees. Open the shutter to start the deposition process and monitor the thickness on the display of the deposition sensor. Close the shutter when the desired thickness for copper is reached.

Then rotate the crucible holder using the knob to direct the beam towards the crucible containing silver pellets and perform deposition as demonstrated. Next, add 500 microliters of a 50 micromolar Nile Blue solution onto the surface of the silver thin film. After 15 minutes, rinse the silver thin film thoroughly with ultrapure water to remove weekly absorbed Nile Blue molecules.

Finally, dry the silver thin film with nitrogen gas. Drop cast 500 microliters of a 100 fold dilution of the silver nanoparticle colloid onto the same region of the silver thin film, drop cast with Nile Blue solution. After 20 minutes, rinse the gap-mode Surface-Enhanced Raman Scattering or SERS substrate with ultrapure water.

Then dry the substrate using nitrogen gas. The ultraviolet visible spectrum of the good silver thin film demonstrates that the film is partially transparent for the visible portion of the electromagnetic spectrum. A representative AFM image of the good substrate is shown here.

The variation in the height of the silver thin film substrate is represented by the line profile demonstrating the uniformity and smoothness of the film. The SEM image of silver nanoparticles drop cast and air dried on a silicon wafer showed an average diameter of approximately 79.2 nanometers. To obtain a five centimeter long glass well, using one hand, firmly grasp the glass tube cutter tool by its handle.

On the other hand, hold the glass tube and rotate it continuously so that the wheels in the chain start cutting the glass. Gently squeeze the tool by gradually increasing the pressure on the handles. Use 120-grit sandpaper to smoothen the broken end of the glass well, then polish with 220-grit sandpaper.

To attach the cut glass well to the surface of the substrate, dispense two part epoxy resin onto a small sheet of aluminum foil. Blend the product using a stir stick or a pipette tip. Then apply the mixture to the bottom rim of the glass well.

Glue the glass well to the surface of the gap mode substrate. Then apply the remaining mixed product on the outside of the well where it meets the substrate. To attach the electrical connection to the gap-mode SERS substrate, mixed two part conductive epoxy resin onto a small sheet of aluminum foil using a five centimeter long copper wire.

Attach the wire to the surface of the substrate. Add 10 milliliters of 0.5 millimolar Nile Blue and 0.1 molar phosphate buffer at pH5 to a 20 milliliter beaker. Insert a mechanically polished silver disc electrode, a platinum wire, and a silver/silver chloride electrode into the electrolyte solution.

Attach each electrode to its respective potentiostat clip as determined by the manufacturer. Ensure that the electrodes are not in contact with each other. Then perform cyclic voltammetry from 0 to 0.6 volts with a scan rate of 50 millivolts per second.

Place the electrochemical cell prepared with the gap-mode SERS substrate on the stage of an inverted optical microscope. Secure the substrate to the microscope stage by taping the edges to prevent movement during spectroelectrochemical measurements due to the tension of the wires connecting the cell to the potentiostat. Place the silver/silver chloride reference electrode into the home-built stand and fix its position by tightening the screw on the electrode holder stand.

Attach the reference electrode to the potentiostat's reference electrode alligator clip. Then attach the platinum wire counter electrode to potentiostat's counter electrode alligator clip. Finally, clip the copper wire attached to the silver film to the potentiostat's working electrode alligator clip.

Insert the platinum wire and alligator clip into the electrode holder and tighten the screw to secure its position. Place the electrode holder over the electrochemical cell to insert the electrodes. Then turn on the 642 nanometer laser and adjust the power to 500 microwatts.

Next, add a drop of immersion oil to the objective lens. Then move the focus knob to raise the objective until the oil contacts the bottom of the substrate. Focus the laser onto the surface of the gap-mode SERS substrate.

After removing one of the eye pieces from the microscope, insert the adapter in its place. Change the mode to video on the camera application, and zoom in as much as possible. Scan the gap-mode SERS substrate by moving the microscope stage to search for an isolated donut-shaped SERS emission pattern.

Once the donut-shaped emission pattern is located, move the light diverter lever of the microscope to direct the admitted light to the spectrometer. Set the grading position to 1000 wave numbers to detect Stokes shifted Raman scattering from 400 to 1600 wave number region. Keeping the laser light focused on the donut-shaped emission pattern, add three milliliters of a 0.1 molar phosphate buffer of pH5 into the electrochemical cell.

In the potentiostat's software, prepare a cyclic voltammogram experiment with at least three cycles from 0 to 0.6 volts versus silver/silver chloride, and a scan rate of 50 millivolts per second. Then run the simultaneous cyclic voltammetry and SERS experiments. Finally, move the light diverter lever so that the light is directed to the phone camera, and start recording a video while running the cyclic voltammetry experiment.

Individual silver nanoparticles on the silver thin film can be unambiguously identified by a donut-shaped emission pattern in contrast to a solid emission pattern produced by nanoparticle dimers, trimers, or multimers. The SERS cyclic voltammograms were measured for single nanoparticles and the Nile Blue molecules in and around the gap between the silver nanoparticle and the silver film were electrochemically reduced. Spectroelectrochemical measurements were done with the same applied potential range.

Electrochemical modulation of the Nile Blue SERS spectrum by stepping the potential causes the peak intensity at the 592 wave number region to decrease with time due to the reduction of the Nile Blue molecules. The magnitude of the electric bias altered the reduction kinetics as evidenced by the decay of the area under the 592 wave number region peak.