Bias Modulated Scanning Ion Conductance Microscopy (BM-SICM)

Title slide

David PerryThe University of WarwickElectrochemistry and Interfaces GroupBias Modulated Scanning Ion Conductance Microscopy (BM-SICM)1

Novak et al., Nature Methods 6 , 279 (2009) SICM2Non contact, solution phase imaging technique used to map topography of samples2SubstrateAg/AgCl ElectrodesTip-Substrate SeparationIonic CurrentTipDiameterTipDiameter

100 mMKClV

SICM3So as the tip is brought towards the surface, in our case using piezo positioners, the resistance starts to increase within a tip diameter and the ionic current is seen to drop3SubstrateAg/AgCl ElectrodesTip-Substrate SeparationIonic CurrentTipDiameterTipDiameter

100 mMKClV

SICM4SubstrateAg/AgCl ElectrodesTip-Substrate SeparationIonic CurrentTipDiameterTipDiameter

100 mMKClV

SICM5DC CurrentTip DiameterTip-Substrate SeparationACAmplitudeTip DiameterVSubstrateDistance Modulated SICM6In order to map topography of substrates, robust feedback is needed so that when the tip is detecting the surface it can be held at a fixed distance. Typically this is done through oscilalting the tip up and down. As we can see when in bulk and there is no drop in ionic current, no AC signal would be seen whereas when the ionic current starts to drop, for the same sized oscillation we would now see a change in the dc current and hence an amplitude signal is recorded. This can be seen to increase the closer we get to the surface6No physical oscillation of tip

Can perform about 0V

Potential to scan at wide range of frequencies

Easier to model

A Simple Alternative-Bias Modulation7But why do we want to use this, well the physical oscillation of the tip has the limitation of not allowing you as close to the surface which could limit the investigation of surface properties. As well as this any convective effects of moving the tip are reduced. It is much easier to implement this set up in finite element models without the oscillting boundary of the tip. The piezos that control the pipette have a limit on oscillation frequency and hence the response time is limited whereas in the bias modulated mode we have the potential to scan at higher frequencies allowing a faster response and scanning. And finally, this will potentially allow us to peform scans about 0V meaning there is no net ion flow and so there will be no electroosmotic effects and the electrodes wont become polarised7

Bias Modulated SICM+10 mV-10 mVBiasDC Current 0 mVSubstrateTip DiameterMcKelvey et al., Anal. Chem. 86, 36393646 (2014) 8Here we propose a new mode of SICM feedback where instead we oscillate the bias we apply between the electrodes about 0V . As with previously mentioned SICM we would expect the DC current either side of 0 to drop off within a tip diameter so here is a brief schematic of what we would expect to see.8

Bias Modulated SICM+10 mV-10 mVBiasDC Current 0 mVSubstrateTip Diameter99



Bias Modulated SICM+10 mV-10 mVBiasDC Current

0 mVSubstrateTip Diameter1212











McKelvey et al., Anal. Chem. 86, 36393646 (2014) Experimental Approach Curves

18And a very similar pattern was seen. The ac ampliude and phase can be seen to decrease and increase respectively as the surface is detected. It can be seen that in the phase especialy a change is noted even at 30 kHz.18

McKelvey et al., Anal. Chem. 86, 36393646 (2014) Equivalent Circuit Model19Using the impedance measurements we could fit a simple randles circuit to the system modelling our set up in this way and using the randles circuit equations could generate a theoretical approach curves to a glass surface 19

McKelvey et al., Anal. Chem. 86, 36393646 (2014) Theoretical Approach Curves

20Which we see here. This is a simple model just combining the randles equations for impedance with the expected change in tip resistance as the tip reaches the surface. So we then tested what we saw experimentally20

TheoreticalExperimentalExperimental Vs. Theoretical

21Just to show them next to each other to show the strong agreement.21

GOLDGLASSMcKelvey et al., Anal. Chem. 86, 36393646 (2014) Distance Vs. Bias Modulated22Now that we know the response we get when approaching glass we wanted to do some topographical mapping to prove it has this capability and we can see here that we can geenrate some images of gold bands deposited on glass that strongly agree with scans carried out with distance modulated SICM22

AC Amplitude Set Point

AC Phase Set PointTopographical Mapping of Calcite23We also imaged a more interesting sample where we etched a calcite surface in maleic acid to produce etch pits which we imaged using both the AC amplitude and phase as set points with an oscillation frequency of 1 kHz.23SICM For Functional Imaging24

Finally to give a taste of what we are using this technique for now, you may have heard my colleague talk earliet today about using distance modulated SICM to look at surface charge effects in low electrolyte concentration. Well here we can do the same thing, this time oscillating our bias about a fixed potential and looking at the DC and phase responses shows we can map areas of different surface charge, here using a polystyrene film with glass pinholes as an example surface. 24ConclusionsSICM is a powerful tool for non-contact imaging

Improved feedback in the form of bias modulation

Large feedback signal over a range of frequencies

Good theoretical model

Can scan topography using either AC amplitude or AC phase

Extend SICM as a tool for functional imaging25AcknowledgementsDr. Kim McKelveySophie KinnearDr. Dmitry MomotenkoProf. Patrick Unwin

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Bias Modulated Scanning Ion Conductance Microscopy (BM-SICM)

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