Lab-on-a-Chip - Flow Systems
 

Flow Effects on Amperometric determination using Microelectrodes

Experimental Scheme

1: Fabrication of the Micro channel
2: Bonding of the channel to a substrate (Glass Slide)
3: Preparation of Microelectrodes (Gold and Screen Printed Carbon)
4: Hydrodynamic Voltammetry for Characterisation of Electrode Behaviour
5: Testing of Amperometric Response with Gold Electrodes
6: Testing of Amperometric Response with SP Carbon Electrodes


1: Fabrication of Micro channel

The micro channel was fabricated using injection moulding onto a brass mould, prepared using a Datron CAT 3DNC-milling machine. The channel dimensions were 200micron by 20omicron by 30mm. There was a raised ridge around the channel, 100mm wide, which was used eliminate problems seen with previous systems where any bonding method resulted in a blockage of the channel. The channels were fabricated from polystyrene, with 4 channels per device. These were then cut into two sections, two channels on each and once channel was used over the electrodes, the second channel was not utilised.


2: Bonding of the channel to a substrate.

Bonding the channels required the use of an adhesive, which could be used between he plastic and glass surfaces of the components.  The major difficulty was using an adhesive, which could be easily applied and resulted in a rapid seal around the channel.

In the initial stages, a UV cure epoxy was used. This was painted onto the polymer device, around the channel and as the device was exposed to UV radiation. However due to glass being opaque to UV and the transmission co-efficient of the polymer being low, the bond was of poor quality.

This method was abandoned, and a low viscosity cyanoacrylate glue  (Hot Stuff –FM570, Craft Supplies Ltd, UK) was obtained and used to seal the polystyrene chips to the electrode slides. Due to the nature of the glue, it was possible to simply flow the glue around the ridged structure, ensuring even coverage, and simply bond the two components. This gave a success rate of 99%, with problems only arising if the components were knocked before the glue had time to set. Once bonded, the seal was shown to be capable of withstanding flow rates in excess of 4 ml minute, giving a linear flow velocity of  1.112 m S-1 for the channel dimensions (using a Kontron HPLC pump). A 25cm ODS reverse phase HPLC column (Spherisorb, UK) is placed between the pump and the flow chip, to minimise the large back-pressures required for HLPC. The weakest part of the device being the connection wells required for interface with the outside world. This problem was solved using araldite adhesive to secure the tubing in place.


3: Preparation of Microelectrodes (Gold and Screen Printed Carbon)

The slides containing the gold electrodes were obtained from Glasgow University as part of the LOAC project, using a photo-mask design prepared by Dr Sara Baldock. All measurements are given in microns for the electrode widths and spacing.
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The screen-printed carbon electrodes were prepared on polycarbonate sheet, and screen-printed by University of Luton, using a template designed by Dr Sara Baldock. These sheets were cut into the appropriate sub-units and the channels bonded in the usual manor.
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Access to the electrodes was achieved using a connection frame, constructed from Perspex, with spring-loaded contacts obtained from RS. This allowed easy connection to the coated electrodes and caused no damage to the electrode surface.

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The need for a reference electrode was tackled but the addition of a “pot”, connected to the exit well of the chip, with a standard reference electrode located below the liquid level. Concerns were voiced regarding the moving level of liquid in this pot, however with slight modifications, a waste pot allowed for a semi-constant liquid level.
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4: Hydrodynamic Voltammetry for Characterisation of Electrode Behaviour


All work carried out used a ferrocyanide solution of approximately 0.01M, with a supporting electrolyte of 0.1M KCl.

All flow rates were adjusted to take into account the channel dimensions and are therefore stated as linear flow velocities in m s-1.


A new electrode device was taken from the batch obtained from Glasgow University, and prepared in the manner described before.

Cyclic Voltammetry was used to obtain the maximum amount of data during the reduction and oxidation steps in the presence of variable solution flow, thus giving hydrodynamic voltammetric data. The ferrocyanide solution was pumped through the micro channel, across the electrodes, and voltammogrames were obtained.

The reaction used to characterise the electrode behaviour was: -
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The operating parameters used were: -
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Flow rates used were: -

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Where all flow rates are given in ml min-1, as measured from the HPLC pump system and all linear velocities are given in m s-1, calculated from the dimensions of the channel and the flow rate.

The standard electrode arrangement of Counter or Auxiliary electrode being up stream from the working electrode was used in the initial stages or the work, however as a comparison the electrode positions were reversed to determine what, if any, effect the production of redox species at the counter electrode had on the working electrode and the sensitivity of the response.


5: Testing of Amperometric Response with Gold Electrodes
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Current range and response - C/W

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Current range and response - W/C
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6: Testing of Amperometric Response with Screen Printed Electrodes


All work was carried out useing a ferrocyanide solution of approximately 0.01M, with a supporting electrolyte of 0.1M KCl.

All flow rates were adjusted to take into account the channel dimensions and are therefore stated as linear flow velocities in m s-1.
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Current range and response - C/W
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Discussion and Conclusions

From the data obtained, it is apparent that the gold electrodes perform significantly better then the screen printed electrodes with respect to current response, with the typical current response being of the order of 10-6 A. The screen-printed carbon electrodes typically operate in the 10-8 A range, which indicates a lower response to current for a set concentration of analyte (in this case 0.1M ferrocyanide solution.

This is to be expected since metal electrodes are significantly more conductive than screen-printed carbon. However the response of the screen-printed electrodes offers significant advantages, especially where low cost electrodes are required. The costs and time scales involved in metal deposition and etching compared to the simplicity of screen-printing makes the latter a viable alternative, particularly for one shot disposable systems, where the reduction is size must be accompanied by the reduction in cost.

Linearity for the electrodes is an area of contention. The linearity of the screen-printed carbon systems was more consistent over the different electrode arrangements, however it should be noted that often only one of the two sets of electrodes in the channel were usable.

The linearity of the metal electrodes was significantly more affected by electrode dimensions than the carbon electrodes, with no real trends being observed. It should be noted that a single device was used throughout the metal electrodes procedures, since all electrodes were present in the micro channel, where as for the screen-printed systems a new system was used for each different electrode pair. This may have an effect, since electrode fouling would not be observed in these latter systems.

It is quite feasible that electrode fouling contributed to the reduced linearity of the electrode systems for the metal devices. When the effects of reversing the working and counter electrodes are taken into account, this becomes more obvious.  The initial experimental system for the metal electrodes consisted of the counter electrode being placed before the working electrode, subsequently the experiments were repeated with the working electrode placed before the counter electrode, and on the whole the linearity of response dropped. The carbon electrodes were treated differently, with both C/W and W/C runs being conducted on back to back. As such there is not the same degree of reduction in linearity for the electrode systems, which leads to the conclusion that electrode fouling may well be an influencing factor.

The arrangement of the metal electrodes in the micro channel might give some indication of the effects of electrode fouling, however since this was not considered at the time, the electrode response was measured in a random order for the varying electrode pairs. This was not a problem with the screen-printed systems since as stated before, different devices were used for each run. With the metal electrodes, some of the electrodes pairs would have suffered more from fouling due to their extended exposure to both analyte and reagents.

This is once again supported by the consistently high linearity of the C/W experiments for metal electrodes, vs. the consistently low response for the W/C experiments, an effect resulting from the delay between the experiments rather than due to the change in electrode positions.

Again the data also indicates that the current response is similar for both C/W and   W/C electrodes systems, a further pointer to electrode fouling as a contributory factor in poor linearity rather than electrode arrangement.