FYP viva

how does potassium lead to cardiac arrest?

1. potassium build up reduces potassium gradient across cell membranes

2. The cell membrane resting potential increases

3. resulting depolarization inactivates fast sodium channels

4. sodium channels can't generate cardiac upstroke, causing failure

how is blood usually taken for potassium blood tests?

1. extraction of blood sample

2. blood sample clots, unless if using anticoagulent tubes for plasma

3. serum extracted or plasma tested

How do ISEs work for potassium testing?

measures the different in electrochemical potential generated between the working and reference, creating a voltage

the working potassium electrode electrochem potential comes from...

ion-partitining between the ISM and serum, creating a phase bundary potential proportional to the potassium activity

the reference potassium electrode electrochem potential comes from

the stable Ag/AgCl redox system due to the internal fixed Cl concentration

a reference electrode is needed because...

it completes the circuit and allows ions to flow between electrodes, generating a baseline chem potential used to create a voltage

ion activity (define)

effective concentration of an ion in solution

why do we use activity instead of concentration for ISEs?

ions interact with eachother in solution, making the effective concentration lower than the measured concentration, the activity adjusts for this non-ideal behavior

why would a counter electrode be used in a printed strip?

to measure the current by balancing the electron flow at the working electrode

for voltammetric measurements, the counter electrode...

enables current flow and supports redox reactions

for electrolysis measurements, the counter electrode...

completes teh curcuit for bulk electrochemical exchange

carbon is often used for printed working or counter electrodes because...

chemically inert, wide potential window, low background current, easy to modify, and surface compatibility

Ag/AgCl ink is used for reference electrodes becuase...

has a well-defined electrochemical potential due to its stable redox reaction

haemolysis is an issue for small-volume measurements because...

devices are too small to contain enough sample for relable centrifugation, and it is difficult to account only for lysed cells in a whole sample

field effect transistor

transistor that uses an e-field to control the flow of current via a semiconducting channel

FETs are good because...

they have a high input impedence

high input impedence

doesn't need a lot of current to work so gives little load on the source, so won't disturb the signal and give better accuracy

FET parts

source, drain, gate

source

where charge carriers enter the channel

drain

where charge carriers exit the channel

gate

control terminal that applies voltage and creates an e-field that modulates the channel's conductivity

How do FETs work?

1. voltage is applied to the gate, between the source and drain

2. charges accumulate at the semiconductor's interface

3. accumulated charges form a conductive channel between the source and drain

IonGO PCB base consists of

flexible polyaniline base with coppre, nickel, gold plated contacts

why is it good to have a source/drain interdigitated array on the IonGO?

increases channel width without requiring large device footprint and gives for lower resistance

what is the role of the reference electrode in the IonGO?

to provide a stable and well-defined reference potential for the gate voltage applied to the graphene channel

what is graphene?

2D, sp2 single carbon atom material, ambipolar and highly conductive

in the IonGOs, the graphene serves as...

the semiconducting channel material between the source and drain

what is the graphene's CNP or Dirac point?

the point where the valence and conduction band energies overlap, leading to no band gap

because graphene's fermi level and Dirac point are aligned...

the sensing can be tuned via doping through the applied gate voltage

if a negative gate voltage is applied to a FET...

holes are attracted to the graphene channel, moving the fermi level below the Dirac point and making it p-doped

if a negative voltage is applied to a FET, the downward shift of the Fermi level is due to...

the injection of holes into the graphene

the potassium ISM consists of...

potassium ionophore, plasticizer, polymer matrix, and lipophilic additive

potassium ionophore role

selectively binds to potassium and through a gradient diffuses across the membrane to graphene

the potassium ionophore only fits potassium because...

it's internal cavity matches potassium's ionic radius and the carbonyl oxygens for iron-dipole bonds causing conformational changes to create a cage

plasticizer role

creates a hydrophobic environment and solvates the ionophore

plasticizers must be hydrophobic to...

prevent interferance from water or competing ions and stabilize the hydrophobic ionophore, allow for it to remain mobile

polymer matrix role

provides structural support

lipophilic additive role

maintains electroneutrality

it's important that the lipophilic additive maintains electroneutrality because...

it ensures stable ion transport and prevents charge imbalance or polarization effects taht can damage the membrane

electrostatic gating mechanism

1. negative bias applied, holes accumulate, graphene becomes p-doped

2. potassium added, membrane gradient established

3. electrons attracted to grpahene surface as more potassium reaches surface, reducing hole density

4. fermi level shifts upwards, transitioning to n-doped graphene

5. too much K+ is added, and the transduction efficiency is decreased causing less graphene modulation

membrane stencil addresses...

non-uniform membrane deposition and disparities in measurement

the membrane stencil ensures that...

no membrane can spread between the IonGO and stencil or accumualte on the sides

Ag/AgCl reference membrane addresses...

need for standalone IonGO reference

Ag/AgCl reference membrane creates...

a stable redox system with constant potential and a stable baseline

cuvette containment stencil addresses...

need for single-point measurements and hemolysis accountancy

monochromator addresses...

need for hemolysis accountancy

why wouldn't a light be integrated through the bottom to make a thinner strip?

would need to re-configer the IonGO and the orange polyaniline base

why was an 805 nm LED used?

oxy/deoxy isosbestic point with teh least carboxy/methemo interference and smallest change in coefficient between neighboring wavelengths

why use a fully lysed sample?

won't have any discrepencies on which cells burst and which ones didn't before or during measurement

original IonGO configuration purpose

generates initial calibration curve and serves as a control

built-in IonGO configuration purpose

serves to compare the built-inreference membrane with the commercial membrane

cuvette IonGO configuration purpose

serves to compare novel single-point measurements with control continuous ones, relying on the built-in reference

blood IonGO configuration purpose

pulls everything together and tests in new media

why was Na PBS chosen as the buffer?

has the isotonicity and pH stability similar to blood

what is isotonicity?

same osmotic pressure and ion concentration

why can't K PBS be used?

because potassium is being tested, so need to have a 0MM background that's not elevated

sweeps purpose

verify the gate/drain are working as intended, charactterize electronic properties, and assess the presence of hysterisis

hysteresis

lag effect in device response attributed to charge trapping or interfacial effects that can reduce accuracy

why doesn't the presence of hysteresis matter here?

because a single, constant applied gate voltage is being used

why were the sweeps done between -0.4V and 0.4V?

it is the GFET operational window and demonstrates graphene's ambipolarity

the drain sweeps gave a...

U-shaped graph where the minima was teh Dirac point and a hysteresis of 0.14

the gate sweeps gave a...

oval, duck shape rgraph representing the electric current that flows in and out of the gate as voltage changes

for the gate sweeps, the current should be minimal because...

the insulating barrier that separates the gate from the channel

the gate current comes from...

capacitive current and leakage current

capacitive current

charging/discharging of gate capacitance during the sweep

leakage current

charge transfer at the dielectric and gate interface

why wasn't the total potassium concentration used to measure the solution values?

total potassium depends on the hematocrit and intracellular potassium concentrations, which may vary between donors and samples and cannot be quantified as a single constant

current resulting graph

step-wise graph with errors ranging from 0.02 uA to 0.05 uA, depending on the configuration and sample

IonGO accuracy issue - because there is no set current value for a single concentration...

a calibration curve must be used instead

IonGO calibration curve issue

curve relies on deposited graphene surface potential and the inherent IonGO resistance, which varies between IonGOs

a one-to-one current-concentration graph is difficult to establish with the IonGOs because...

a calibration curve must be used, which relies on graphene deposition and inherent resistance, which varies between chips

why does it take longer for the curves to stabilize in the cuvettego measurements?

it is a non-dynamic environment where potassium transprot to the membrane relies solely on diffusion as there is no convection or bulk flow

why do the current drops decrease?

1. valinomycin available to transport decreases, and teh membrane gradient flattens

2. the electric double layer becomes more compact, and potassium ions screen each other, decreasing the debye length

3. additional ions can't easily traverse teh membrane and effectively modulate the graphene due to the screening

potassium ions in the innner helmholtz layer have reduced relative permittivity so...

they contribute to stronger screening

Jaworska is more linear and lower spread due to...

1. more valinomycin increases transport sites, delaying saturation effects

2. DOS has a lower dielectric constant and higher membrane resistance, slowing transport and delaying ion saturation

Reference membrane shows dips in measurement and less linear raw data because...

1. suddent increase in chem potential creates a rapid influx of potassium acros steh membrane

2. ions redistribute across the interface and the curent stabilizes, dynamic equilibration

the initial dips in the reference membrane data show...

the diffusion to teh membrane and re-establishment of equilibrium and gradual stabilization

the reference membraen shows a smaller spread of data because...

the membrane creates a physical barrier that prevents rapid mixing of sample with the environment, creating a potential that isn't really interacting with the solution

the cuvettego membrane reset helped with...

encouraging diffusion away from teh graphene and towards teh water, pulling potassium out of the membrane

resetting the membrane gave more linear values because...

the membrane can establish a new equilibrium without the bottleneck, saturated exchange after long-term exposure, giving no ion build-up or screening effects, allowing for a higher resolution

for absorption values, the nanodrop showed a range of ...

32-48 mmol/L

the monochromator showed lower values because...

light leakage from external plug ins and uncollimated light source

unlike previous measurements, the blood current was positive. this could've been due to...

1. contamination of source/drain

2. dirac shift

3. switch to charge transfer

contamination of source/drain means that...

blood components got through the ISM and contaminated the source/drain

contamination of source/drain didn't happen because...

ISM is extremely selective and smaller blood components wouldn't form the ion-dipole interactions necessary to create the ionophore cages, and amino acids would be repelled by the hydrophobicity

Dirac shift means that...

the total potassium was so positive that there was an inversion and the Dirac point shifted, so we are operating on the positive portion of the curve

Dirac shift didn't happen because...

solution potassium tests with the blood concentration showed a downwards trend, so the concentration was not positive enough to induce this, and sweeps showed no discernable shift

how was the solution potassium concentration determined?

possible serum potassium x (1 - hematocrit) + intracellular potasssium x hematocrit

the solution potassium is actually lower than just summing the serum and intracellular because...

we need to account for the volume that the cells and potassium take up in the blood, represented by the hematocrit and plasma volume fractions

if there was a positive shift in dirac point, the current increase would come from...

the high density of positive potassium ions shifting the fermi level to the conduction band and flipping the channel to an n-type as electron density increases

the dirac point shift would only be possible if...

there is a constant reference potential and a constant gate potential

switch to charge transfer means that...

the blood components permeate the gate electrode's porous gold contact layer and reacted with the underlying nickel

switch to charge transfer occured because...

there was a proportional inverse decrease in gate current, where electrons were transfered between the contacts and the blood medium

how could there be a switch in mechanism at the gate and nothing at the source/drain?

the drain current is highly sensitive to the gate potential, so ionic or faradaic processes can alter the gate current and potential, altering teh channel conductivity

blood components that can react with a nickel PCB

albumin, fibrinogen, amino acids