Topic 3:Micro/Nano electrodes

Question 1

• How big are micro and nano electrodes?

Answer 1

• Micro: ≤ 50 µm
• Nano: ≤ 100 nm

Question 2

Nanoelectrodes refer to … electrodes with at least one dimension below …. They can be considered a special type of … (UMEs) with … … dimension

Answer 2

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

Question 3

• What are three general applications of using micro/nano-electrodes?

Answer 3

• Measurement of ultra-fast electron transfer kinetics (k^o)
• Sensor capable of species detection in unusual environments e.g. in-vitro/vivo brain chemistry
• Probe in scanned probe techniques scanning electrochemical microscopy.

Question 4

• What shape do macroelectrodes have?

Answer 4

• Also a disc like shape, similar to microscale

Question 5

In micro/nano scale electrodes, the current flowing through the circuit is … , so can now use a …-…setup, removing … and flow current through WE/…

Answer 5

• 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

Question 6

• Describe the shape of a microelectrode as well as its properties and values associated with it

Answer 6

• 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 = d_probe/d_electrode
• Chemistry only occurs at exposed part of probe now

Question 7

• How are nanoelectrodes made differently to microelectrodes

Answer 7

Fine wires etched and sealed in insulator using nanoscale templates

Question 8

• Describe the diffusion profile of a Microelectrode and the effect a microelectode has on diffusional flux

Answer 8

• 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 k_t)
• Profile maximises # molecules being turned over per unit area/time

Question 9

• What is the current equation for a ME and what type of current is it?

Answer 9

• i_lim = 4nFDac*
• this is a limiting current
• a is radius of electrode (cm)

Question 10

• 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

Answer 10

• 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

Question 11

• What is the time difference equation in a ME?

Answer 11

• t_diff = a²/2D

Question 12

• Derive the equation for k_t at an ME and state the relationship it has with electrode radius

Answer 12

• i = nAFk_tc*
• i =4naFDc*
• A = πa²
• k_t =4D/πa
• k_t ∝ 1/a (small electrode, large k_t)

Question 13

• 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

Answer 13

• 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

Question 14

• How can a limiting current be achieved at a macro-sized electrode? What would the resulting CSV look like?

Answer 14

• Used forced convection to increase mass transport
• Means k_t is now diffucion AND convection controlled

Question 15

• What is the limiting current at a rotating macro electrode?

Answer 15

• i_lim = 1.554nAFD^2/3v^-1/6w^1/2c*
• A – Area cm²
• V – viscosity
• W – rotation frequency (Hz)

Question 16

• What are some other types of microelectrodes that can be used? How do their limiting currents and mass transfer constants change?

Answer 16

• Need to know how they are made (Zhang)
• Spherical difficult as blocks diffusion path in attachment
• Hemisphere forms droplets via mercury in solution

Question 17

• What sort of current and diffusion profile does an intermediately sized electrode form?

Answer 17

• 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

Question 18

• What is an advantage of intermediate sized electrodes?

Answer 18

Can measure larger currents than at ME via its current response that is in between Micro/macro electrodes.

Question 19

• How is a limiting current generated at an intermediate sized electrode? What is the equation associated with this?

Answer 19

• k_t increased by adding convection to system via a microfluid flowing over long electrode to achieve steady state current
• i_lim = 0.925nFcD^2/3(h/2)^-2/3l^2/32r^2/3V_f^1/3
• h – height of flow cell
• V – volume flow rate

Question 20

• … 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

Answer 20

• 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

Question 21

• What is the critical dimension of a nanoband electrode?

Answer 21

• Its width; length can be mm or even cm, leading to larger currents than for nanodisks, while maintaining properties of radial distribution

Question 22

• How are nanobands fabricated?

Answer 22

• 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

Question 23

• Describe two methods of froming a ring shaped nanoband electrode

Answer 23

• 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)

Question 24

• Give an advantage and disadvantage of disk nanoelectrodes

Answer 24

• 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

Question 25

• Describe the commonly used laser assisted pulling process of producing disk nanoelectrodes

Answer 25

• 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

Question 26

• What is a disadvantage of laser assisted pulling process to make disk nanoelectrodes?

Answer 26

• For lower MP metals (e.g. Au ~1060^OC) fabrication using high heat laser-based technique can be difficult in maintaining structure

Question 27

• Describe the formation of hemispherical nanoelectrodes

Answer 27

• 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

Question 28

• What are nanopore electrodes and what systems are they useful for (dntk)

Answer 28

• 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.

Question 29

• Briefly, how are nanopore electrodes fabricated?(dntk)

Answer 29

• Glass sealed disk electrodes.

Question 30

• What is an advantage of using carbon as an electrode material and what are two common forms used in electrochemistry?

Answer 30

• Low cost
• Inert electrochemistry
• Wide potential window
• Graphene
• SWCNTs

Question 31

• Give a method of fabricating SWCNT-based electrode

Answer 31

• Electrical attachment of SWCNT to Pt wire, insulated in phenol
• Tip cut by applying a voltage, exposing fresh tip

Question 32

• How does the voltammogram of a SWCNT electrode compare to a spherical nanoelectrode?

Answer 32

• Sigmoidal voltammogram characteristic of radial diffusion, like a spherical electrode (but with no blocking problems)

Question 33

• What is an advantage of using SWCNTs in electrodes?

Answer 33

• 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

Question 34

• What is a method of producing many SWCNTs in nano electrode?

Answer 34

• 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