Electrode “horses for courses”, no one electrode material is suitable for all applications
Key to the components used in electrochemical-based biosensors is the underlying electrode material and fabrication process. Choosing the right material for the right analyte and chosen biochemical architecture is key. Below I’ve summarised some pros and cons of a range of different electrode materials based on my first-hand experience. While I could wax lyrically about this topic all day, I’ll stick to a list for now!
After identifying your target molecule to detect and what sample type it will be accessed in, then determine the most likely biochemical pathways I which to render is specifically and sensitively detectable in an E-chem device. At this point, determine what type of electrode material you’ll need. Make a smart and informed choice of the right material, and don’t try to force the assay to work on a single material type – assay development is hard enough!
Screen printed Carbon/graphite
- Pro: Low cost, stable, inert, well characterized, good mediator options. Can embed enzymes/mediators.
- Con: Limited sensitivity, limited surface chemistries. High resistance (RCT), too many options. Not solvent resistant.
Screen printed Graphene paste
- Pro: Slightly more sensitive than carbon, not quite as well characterized.
- Con: Expensive paste yielding marginal benefit over carbon. More fragile than carbon print, not solvent resistant.
Screen printed gold
- Pro: Very sensitive due to surface area, good functionalization options.
- Con: Very expensive, very rough and variable surface, limited scalability due to cost, not solvent resistant.
Laser ablated Thin film gold (CVD)
- Pro: Very sensitive, high fidelity, versatile, good functionalization options. Very scalable.
- Con: Expensive equipment outlay, surface needs to be pacified prior to sample testing, thin delicate surface.
Laser induced graphene (LIG)
- Pro: Sensitive compared to carbon, versatile process.
- Con: Fragile, limited surface options, surface needs to be pacified prior to testing (3D micro-structure makes it challenging), compatibility with downstream processes, unproven commercially.
Laser ablated Carbon nano tubes (CNT)
- Pro: Very sensitive compared to carbon, functionalization options, versatile process.
- Con: Expensive equipment outlay, Material scalability challenges, very fragile.
Laser ablated stainless steel
- Pro: Antifouling characteristics, robust material, versatile process.
- Con: Expensive equipment outlay, limited functionalization options, may contain electrochemically active species in metal.
Laser ablated platinum
- Pro: Sensitive, versatile process, catalytic activity.
- Con: Expensive, fragile, functionalization limitations.
Supplementary layers to consider and be aware of:
Introducing carbon nanotubes to the surface of carbon is a great way to enhance the sensitivity of carbon, but adds extra cost. Nanotubes also enable some solution phase functionalization prior to coupling to the surface of the electrode, giving better control and reduce cost to the functionalization process. Similarly, coupling conductive polymers (such as PEDOT:PSS) is another great way to enhance sensitivity of carbon (and to a lesser expect gold surfaces). This can be introduce in various ways depending on the functionality required.
When developing biosensors for 3rd parties, it’s important to start the journey with an electrode type that will bring the sensor to life as quickly as possible (and which has a clear line of sight to a cost effective and manufacturable design and process). Below are what I see as the strongest materials from which to start the development journey. Each biosensor and client needs are different, so as we encounter performance challenges that are potentially linked to the underlying electrode, we evaluate alternatives and pivot if necessary (All with the limits of Design Control of course!).
At FMS, we don’t really care what electrode material we use, we just want to make sure it’s right for our clients specific application. In fact, if we can’t fabricate the chosen electrode in-house, we will source and procure – with past and current examples including carbon nanotubes, pure graphene and silicon transistors.
Homogeneous enzymatic assays (eg: glucose)
- What: Screen printed carbon
- Why: low cost, very versatile, an entire industry is based on it!
Homogenous Immunoassay (eg: CRP)
- What: Screen printed Gold
- Why: Sensitive, versatile, predictable conjugation strategies.
Heterogeneous Immunoassay (eg: TnI)
- What: Laser ablated gold, SAM functionalization
- Why: Excellent reproducibility (precision!), predictable conjugation strategies
Continuous monitoring enzymatic assays (eg: Lactate)
- What: Screen printed mediated carbon
- Why: enzyme conjugation methods established, low protein binding
Ion-selective Electrode (eg: Potassium)
- What: Electrochemically activated Laser ablated gold
- Why: Excellent precision (needed for ISE), good for membrane formation
And Remember – not all assays are suited to electrochemical detection
We do a lot of electrochemical product development and manufacturing, because its one of our super powers as an organization: these devices are robust, low cost and allow for very low cost instrumentation (glucometers!). The reality is we don’t always recommend electrochemical methods for our clients projects. We evaluate our clients vision and product concept, and working within their specifications, we routinely recommend alternative assay architectures such as optical, fluorescence, chemiluminescence and electroluminescence….again, the client is king, so where necessary we are happy to advise against electrochemistry methods if it is not suitable.
Donogh FitzGerald
Chief Scientific Officer