Localised polymerisation of acrylamide using single-barrel scanning electrochemical cell microscopy

Single-barrel scanning electrochemical cell microscopy has been adapted for polymerisation of acrylamide in droplet cells formed at gold electrode surfaces. Localised electrochemical atom transfer radical polymerisation enables controlled synthesis and deposition of polyacrylamide or synthesis of polyacrylamide brushes from initiator-functionalised electrode surfaces.

and a layer of Araldite glue was applied. They were then left to dry overnight before use the next day. Au and Au/SAM substrates within the SECCM setup were connected to a custom electrometer head (100 pA -100 fA sensitivity) to measure surface currents. (SEM) and using these conditions was found to consistently be 1000 nm ± 10%. Ag/AgCl QRCEs were used and placed in the nanopipette barrels as required, fabricated by electrolysing silver wire in saturated potassium chloride solution.

-SECCM -Single Barrel Setup
The basic instrumentation setup is similar to that used by the Unwin group and was built following specifications provided by them. The nanopipette probes were mounted on a mechanical micro-positioner (Newport, M-461-XYZ-M) for coarse probe positioning over a surface. The fine horizontal movement of the probe was controlled using a two-axis   (4) substrate stage, where the gold/AFM disc substrates are sticked down; (5) Pixel-Link camera used to find the nanopipette opening once the pipette is mounted on the pipette holder, used to help deliver the pipette to the substrate; (6) crocodile clip which attaches QRCE 1 (one QRCE for single barrel mode, two for dual-barrel mode); (7) crocodile clip which attaches QRCE 2; (8) x-y piezoelectric positioner; (9) electrometer head for surface current measurements; (10) crocodile clip which attaches the electrometer head to the copper wire that had been glued to the gold substrate.

-Carrying out a Single-Barrel SECCM Measurement
Ag/AgCl quasi reference counter electrode (QRCEs) was inserted into the barrel of the pipette, with a potential difference applied to the gold substrate, V surf , depending on the experiment, and as stated in the manuscript (some value between -0.05V and -0.5V). The probe was brought close to the surface with the micro-positioner, and with the help of a camera (Pixel link with Edmund Optics lens, 6.0x magnification, 65 mm focal length) and a light box. The SECCM probe was moved vertically towards the biased gold WE surface using the z-piezo via the 'Approach and CV' (cyclic voltammetry) or 'Approach and IT' (constant potential) program, using a surface current threshold of -10 pA to detect when the meniscus had made contact with the surface and halt further approach when this threshold had been met. Slow approach speeds of 1 µm per second were used to prevent tip crashing. Note that the probe itself never makes contact with the WE surface. Electrochemical measurements were performed in the localized area defined by the meniscus cell formed between the SECCM probe tip and WE surface. Cyclic voltammetry was carried out using a fixed potential window (-0.6 V to 0.4 V) and fixed scan rate (0.1 vps), whilst constant potential experiments used a particular V surf value depending on the experiment. If we assume the probe was 10 µm above the surface before making contact, the probe would then move automatically back to that position in the z-axis (or whatever height it was before the approach and CV/IT program was executed), guided by the piezoelectric positioners. In the LabVIEW program used, the probe would then be programmed by manual user input to move (for example) 40 µm in the y-axis to the next position (see diagram), and the probe would then execute another approach and CV/IT procedure as outlined. The surface is biased (V surf ) and the current (I surf ) is measured at the Ag/AgCl QRCE as schematically shown below (Fig. S3). The components of the solution used in our nanopipette are as described in section 1.

-Scanning Electron Microscopy
Nanopipette images were taken using a Zeiss Supra 55VP SEM instrument, with a working distance of 6.1 mm, and an EHT of 5 kV. A Zeiss Gemini 500 Field Emission instrument was used to take images of the polymeric gels, using a weak EHT of 0.5 kV (lower voltages are enabled by the Gemini device) to prevent damage to the soft polymer gels, and a working distance of 3.3 mm.

-Atomic Force Microscopy
Atomic force microscopy images were taken using a Bruker-Nano Enviroscope AFM using the SCANASYST regime which monitors image quality and makes automatic parameter adjustments, using a scan rate of 0.1 microns per second and a SCANASYST-Air Bruker AFM tip.   The mean width of each spot, before rinsing, from figures S16A, S16C, S16E and S17

Supporting Figures
versus the time of deposition.