Topic 3:Micro/Nano electrodes Flashcards
1
Q
- How big are micro and nano electrodes?
A
- Micro: ≤ 50 µm
- Nano: ≤ 100 nm
2
Q
Nanoelectrodes refer to … electrodes with at least one dimension below …. They can be considered a special type of … (UMEs) with … … dimension
A
Nanoelectrodes refer to voltammetetric electrodes with at least one dimension below 100nm. They can be considered a special type of ultramicroelectrode (UMEs) with smaller critical dimension
3
Q
- What are three general applications of using micro/nano-electrodes?
A
- Measurement of ultra-fast electron transfer kinetics (ko)
- Sensor capable of species detection in unusual environments e.g. in-vitro/vivo brain chemistry
- Probe in scanned probe techniques scanning electrochemical microscopy.
4
Q
- What shape do macroelectrodes have?
A
- Also a disc like shape, similar to microscale
5
Q
In micro/nano scale electrodes, the current flowing through the circuit is … , so can now use a …-…setup, removing … and flow current through WE/…
A
- In micro/nano scale electrodes, the current flowing through the circuit is smaller, so can now use a 2-electrode setup, removing CE and flow current through WE/REF
6
Q
- Describe the shape of a microelectrode as well as its properties and values associated with it
A
- Glass sealed microwire: An inlaid circular disk (Pt/Au) surrounded in an insulator material (glass) with a polished flat tip
- RG relates to the dimensions of the probe
- RG = dprobe/delectrode
- Chemistry only occurs at exposed part of probe now

7
Q
- How are nanoelectrodes made differently to microelectrodes
A
Fine wires etched and sealed in insulator using nanoscale templates
8
Q
- Describe the diffusion profile of a Microelectrode and the effect a microelectode has on diffusional flux
A
- Hemispherical diffusion profile, divided in to a linear (top) and a significant radial part (sides)
- Diffusional flux to electrode increases at the micro level (increases kt)
- Profile maximises # molecules being turned over per unit area/time

9
Q
- What is the current equation for a ME and what type of current is it?
A
- ilim = 4nFDac*
- this is a limiting current
- a is radius of electrode (cm)
10
Q
- The limiting current of a microelectrode has no dependence on scan rate, v. Is this always the case? What would be the result of the CSV is it was
A
- Yes, however it assumes that the scan rate is not so fast that the system has not been allowed to achieve hemispherical flux (linear only).
- The resulting CSV would begin to from peaks (at corresponding peak current), similar to a macroscale electrode
11
Q
- What is the time difference equation in a ME?
A
- tdiff = a2/2D
12
Q
- Derive the equation for kt at an ME and state the relationship it has with electrode radius
A
- i = nAFktc*
- i =4naFDc*
- A = πa2
- kt =4D/πa
- kt ∝ 1/a (small electrode, large kt)
13
Q
- Describe the differences in resulting CSV’s of macro and microscale electrodes, and use it to suggest why a limiting current forms at a micro but not macro electrode
A
- As E increases, so does the rate of electron transfer
- Diffusion at large electrode is slow due to removal of species at the electrode diffusion can’t keep supplying electrode to maintain current
- Current diminishes due to drop in flux
- Diffusion at microelectrode is high so current maintained as supply to surface rate fives steady state
- Limiting current forms

14
Q
- How can a limiting current be achieved at a macro-sized electrode? What would the resulting CSV look like?
A
- Used forced convection to increase mass transport
- Means kt is now diffucion AND convection controlled

15
Q
- What is the limiting current at a rotating macro electrode?
A
- ilim = 1.554nAFD2/3v-1/6w1/2c*
- A – Area cm2
- V – viscosity
- W – rotation frequency (Hz)
16
Q
- What are some other types of microelectrodes that can be used? How do their limiting currents and mass transfer constants change?
A
- Need to know how they are made (Zhang)
- Spherical difficult as blocks diffusion path in attachment
- Hemisphere forms droplets via mercury in solution

17
Q
- What sort of current and diffusion profile does an intermediately sized electrode form?
A
- A microband electrode setup gives a hemispherical diffusion profile but over a macroscale length of longer exposed electrode
- Current generated is a non-steady state response

18
Q
- What is an advantage of intermediate sized electrodes?
A
Can measure larger currents than at ME via its current response that is in between Micro/macro electrodes.
19
Q
- How is a limiting current generated at an intermediate sized electrode? What is the equation associated with this?
A
- kt increased by adding convection to system via a microfluid flowing over long electrode to achieve steady state current
- ilim = 0.925nFcD2/3(h/2)-2/3l2/32r2/3Vf1/3
- h – height of flow cell
- V – volume flow rate
20
Q
- … microscopy is useful for obtaining quick/direct info about the … and … of a nanoelectrode
- Resulting info often limited by … resolution of instrumentation, skill of … and sample properties of nanoelectrode
- Size can be extracted from a … … CV but info on the … is more difficult
A
- Electron microscopy is useful for obtaining quick/direct info about the shape and size of a nanoelectrode
- Resulting info often limited by spatial resolution of instrumentation, skill of operator and sample properties of nanoelectrode
- Size can be extracted from a steady state CV but info on the shape is more difficult
21
Q
- What is the critical dimension of a nanoband electrode?
A
- Its width; length can be mm or even cm, leading to larger currents than for nanodisks, while maintaining properties of radial distribution
22
Q
- How are nanobands fabricated?
A
- Deposition of a thin metal on a substrate, with an inculating layer on top, isolating it and exposing the end of the electrode.
- E.g. Au/Pt on a glass microscope slide
23
Q
- Describe two methods of froming a ring shaped nanoband electrode
A
- Au/Pt deposition on a glass road or the inside of a glass capillary
- Au/Pt painted films around an outer capillary followed by an epoxy resin insulating sheath (Macpherson)
24
Q
- Give an advantage and disadvantage of disk nanoelectrodes
A
- Advantageous for examining faradaic reactions over a small area or probing single redox molecules/nanoparticles.
- Such small electrodes result in small current, the detection of which may be limited by the instrumentation
25
Q
- Describe the commonly used laser assisted pulling process of producing disk nanoelectrodes
A
- Pt/Au microwire inserted into a thick quartz capillary
- Microwire sealed inside capillary through laser heating/vacuum
- Pulling force applied, drawing encapsulated Pt/Au microwire into two ultrasharp tips
- End of Pt/Au wire enclosed in a sheath of glass and exposed by mechanical polishing
26
Q
- What is a disadvantage of laser assisted pulling process to make disk nanoelectrodes?
A
- For lower MP metals (e.g. Au ~1060OC) fabrication using high heat laser-based technique can be difficult in maintaining structure
27
Q
- Describe the formation of hemispherical nanoelectrodes
A
- STM style technique where electrochemically etched metal (Au/Pt)/carbon microwire coated in an insulting material (glass/electrophoretic paint) then exposing the very end of the tip
- Controlling the size and shape of the resulting nanoelectrode is difficult
28
Q
- What are nanopore electrodes and what systems are they useful for (dntk)
A
- Solid-state nanopore with an electrode created at the bottom used to monitor flux of redox species through orifice
- Useful for studying molecular transport through solid-state nanopores
- Extremely small volume inside pore can be utilized to study electrochemical properties of trapped single redox molecules.
29
Q
- Briefly, how are nanopore electrodes fabricated?(dntk)
A
- Glass sealed disk electrodes.
30
Q
- What is an advantage of using carbon as an electrode material and what are two common forms used in electrochemistry?
A
- Low cost
- Inert electrochemistry
- Wide potential window
- Graphene
- SWCNTs
31
Q
- Give a method of fabricating SWCNT-based electrode
A
- Electrical attachment of SWCNT to Pt wire, insulated in phenol
- Tip cut by applying a voltage, exposing fresh tip
32
Q
- How does the voltammogram of a SWCNT electrode compare to a spherical nanoelectrode?
A
- Sigmoidal voltammogram characteristic of radial diffusion, like a spherical electrode (but with no blocking problems)
33
Q
- What is an advantage of using SWCNTs in electrodes?
A
- Can be metallic or semiconducting in nature depending on the chirality of the molecule
- The side of the nanotube exposed therefore makes a difference and gives more variety in application
34
Q
- What is a method of producing many SWCNTs in nano electrode?
A
- Catalysed chemical vapor deposition of multiple SWCNT on an insulating surface
- Behaves similarly to a metallic film
- Can then be used as a substrate to deposit metal particles to from a network electrode