dso darpa integrated nanoscale ion-channel sensor
TRANSCRIPT
DSO DARPA
Integrated
Nanoscale Ion-
Channel Sensor
DSO DARPA
Project Goals
Goal 1: Embed channels in an integrated device that maintains stable potential across them and allows recording of stable, artifact free current through them.
Goal 2: Find simulants that bind and transiently block conduction of ions through OmpF.*
* we shall work with DARPA and other groups within the MOLDICE network to incorporate ion channels that show desired properties
AgCl Electrode
OxideSU-8 Resist
Si
Lipid Bilayer with
Ion Channels
Important building blocks of a fully integrated biosensor with on-chip sensing and signal processing
DSO DARPA
Technical Approach
• silicon substrates are used
• layers are structured by conventional optical lithography
• the aperture that supports the bilayers is constructed using deep silicon dry etching
• relation between the size of the lipid bilayer and its stability and the signal-to-noise ratio of the ion channel response
• ultimate limit for the size scaling of the sensor
• optimal surface treatment for bilayer attachment
• stability of the integrated reversible Ag/AgCl electrodes
• manufacturability of the sensor
• usability issues (reusability, cleaning, automation)
Challenges we are facing For the fabrication …
• impedance analysis of bilayers
•current-voltage measurements of bilayers and porin channels
•studying the influence of surface modification layers on bilayer Gigaseal formation
Experiments involve …
DSO DARPA
Summary sheet
• maintain stable potential (± 1 mV for 1 hour) across a single channel of OmpF porin
• recording of stable, artifact- free current voltage curves (± 100 pA for 1 hour) from a single channel of OmpF porin using external electrodes
• recording stable current voltage curves using inte- grated Ag/AgCl electrodes
Milestones Accomplishments
• design and process flow-chart for a silicon bilayer support chip
• working proof-of-concept in form of a silicon chip as a direct Teflon membrane replacement
• Gigaseal formation proven
• channel insertion succeeded
• PTFE layers deposited by plasma CVD facilitate bilayer formation
• planar AgCl electrodes exhibit desired properties
DSO DARPA
Summary sheet
• measure sealing resistance on samples with different geometries and surface properties
• measure Nernst potential of Ag/AgCl electrodes
• measure DC potential across porin
• measure current through porin
Demonstration of Results Technology Transition
• construct a silicon-based sensor template (reusable if possible) along with a fixture to allow easy bilayer formation and protein insertion
• development of a procedure to reproducibly create bilayers with Gigaseals
• work with DARPA and other groups within the MOLDICE net- work to incorporate ion channels that show desired properties
DSO DARPA
Microfabrication
Details
(ASU)
DSO DARPA
Small Hole Etching
825 Resist, 1m thickness
AZ 4330 Resist, 2.6m thickness
Si Substrate
50m
300m
SU-8 Resist
Si
1 mm250m
Si
150m
150mSi
Thermally Grown Oxide, d = 500 nm
Si
150m
Si
Photoresist
SU-8 Resist
Si
AgCl
Hydrophobic Layer
SU-8 Resist
Si
AgCl
Bilayer
Resist for Initial Hole Etching
Thermal Oxidation
Resist for Small Hole Etching
Large Hole Etching
SU-8 Resist (Epoxy)
Surface Modification Layer
AgCl Electrode
AgCl Electrode, up to 1m thickness
SU-8 Resist
Si
Lipid Bilayer Attachment
Process Flow
DSO DARPA
250 m
• deep silicon etch process that is optimized on high etch rate (4.7 m/min), good selectivity (220:1) and a concave bottom profile
• etch process that exhibits vertical sidewalls and a low aspect ratio dependent etch rate of 3.7 m/min with planar bottom profiles below 100 m ridge width
Process optimization
DSO DARPA
250 m
• switch to double-side polished 100 mm (4”) wafer with 380 m thickness allows the fabrication of multiple samples per run with identical geometry
• front and backside have a smooth surface and the etching does not roughen the lower surface
• optimized backside alignment re- sults in good centering of the hole
Process optimization
DSO DARPA
250 m
• conventional hole preparation using electrical discharge to create an aperture in a PTFE sheet of 25 m thickness
• using deep silicon dry etching and back side alignment photo- lithography a small hole (150 m) was created inside a recess
Sample comparison
DSO DARPA
PTFE Surface Modification• the stability of the lipid bilayer is related to the contact angle between the bilayer and the supporting substrate
• water contact angle measure- ments can be used to determine the substrate’s surface energy
• coating the oxide surface with a Teflon film changes its properties from hydrophilic to hydrophobic (small to large contact angle)
• using Plasma CVD is a novel method that provides an easy way to deposit thick PTFE layers
Bilayer
Torus
Substrate
DSO DARPA
Lipid Bilayer
Experiments
(Rush)
DSO DARPA
• Experiment showing the opening of a single OmpF porin channel. The vertical lines through the red current trace are an artifact from stirring of the bath to facilitate the insertion of porin into the bilayer membrane.
• Plot showing the different levels of OmpF porin (Trimer). Level 1 is not shown. All the traces in the above plot are from the same OmpF porin bilayer experiment using the silicon wafer coated with PTFE (Teflon).
Lipid Bilayer Experiments
Hole diameter = 150 m
PTFE coated surface
DSO DARPA
Lipid Bilayer Experiments
• physiological behavior of OmpF
• response is indistinguishable from channels in Teflon supported membranes
• reproducibility of measurements and voltage dependence indicates that switching is not an artifact but real channel activity
DSO DARPA
Ag/AgCl
Electrodes
(ASU)
DSO DARPA
Integrated AgCl ElectrodesAgCl ring on SU-8
(chloridized)
• chloridization in 5% NaOCl for 30 sec
• measurements are performed using 0.1M or 0.5M KCl reference solutions
AgCl ring on oxide
3 mm
Schematic view of the electrode layout
• silver is evaporated on both sides of the wafer (> 500 nm)
• layer patterning by photo- lithography and etching
DSO DARPA
Integrated AgCl ElectrodesAgCl Electrode Potential, Single substrate
Simulation Measurement
0 1 2 3 4 5-100
-80
-60
-40
-20
0
Pot
entia
l diff
eren
ce (
mV
)
KCl Molarity difference (M)
0.1M KClReference
solution
• no notable difference between electrodes on oxide and epoxy
• good potential stability of the microstructured electrodes
• minimal difference between the expected and measured Nernst potential variation with KCl concentration
0.5M (trans) and 0.6M (cis) KCl Test solutions
AgCl layer, chloridized in 5% NaOCl
Pot
entia
l diff
eren
ce (
mV
)
0 1 2 3 4 5-10
-8
-6
-4
-2
0
2
4
6
8
10
Time (h)
DSO DARPA
AgCl Electrode
• difference between expected and measured potential due to partially chloridized surface
• longterm failure mechanism: AgCl gets dissolved in the KCl electrolyte
AgCl layer before measurement AgCl layer after 5 h measurement
DSO DARPA
Making a Calcium
Channel
(Rush)
DSO DARPA
Make a Calcium Channelby Site-directed Mutagenesis
Theory, Simulation, Experiment show
Crowded Charge Selectivity
George Robillard, Henk Mediema, Wim Meijberg
BioMaDe Corporation, Groningen, Netherlands
DSO DARPA
Strategy
Use site-directed mutagenesis to put in extra glutamates
and create an EEEE locus in the selectivity filter of OmpF
Site-directed
mutagenesis
R132
R82E42
E132
R42 A82
Wild type WT EAE mutant
E117 E117
D113D113
George Robillard, Henk Mediema, Wim MeijbergBioMaDe Corporation, Groningen, Netherlands
DSO DARPA
-100 -50 50 100
-150
-50
50
150
ECa
WT
EAE
Current (pA)
Voltage (mV)
Cis Trans
1 M CaCl2 0.1 M CaCl2
Ca2+
Ca2+
IV-PLOT
Cis Trans Cis Trans
IV-plot EAE: current reverses at equilibrium potential of Ca2+ (ECa),
indicating the channel can discriminate between Ca2+ and Cl-
Zero-current potentialor reversal potential = measure of ion selectivity
Henk MediemaWim Meijberg
Ca2+ over Cl- selectivity (PCa/PCl)recorded in 1 : 0.1 M CaCl2
IV-Plot
DSO DARPA
Selectivity arises from Electrostatics and Crowding of Charge
Precise Arrangement of Atoms is not involved
Make a Calcium Channelby constructing the right
Charge, Volume, Dielectric
DSO DARPA
Conclusions
• measure single channels in an integrated device
• study the relation between the size of the lipid bilayer and the signal-to-noise ratio
• find optimal surface treatment for bilayer attachment
• find simulants that bind and transiently block conduction of ions through ompF
• work with DARPA and other groups MOLDICE groups to incorporate ion channels that show desired properties
Future work under Phase IAccomplishments
• a silicon bilayer support chip has been constructed and successful Gigaseal formation has been demonstrated
• channel insertion succeeded
• first milestones have been achieved
• integration of the reversible electrodes demonstrated
• PTFE layers deposited by plasma CVD exhibit excellent properties
DSO DARPA
1) Project DetailsTitle: Integrated Nanoscale Ion Channel SensorStart Date: December 15th 2003End Date: December 31st 2004 (Phase I)Partners: Marco Saraniti (IIT)
Bob Eisenberg (Rush)Steve Goodnick (ASU)Trevor Thornton (ASU)
Plus: Dr. J. Tang (Rush), Dr. M. Goryll (ASU), Dr. G. Laws (ASU), Mr. S. Wilk (ASU) and Mr. D. Marreiro (IIT)
2) Project Goals
resist
silicon
sealingring
electrode 1
membrane channel
electrode 2 electrode 2
• embed channels in a membrane device that maintains stable potential across them and allows recording of stable, artifact free current through them.
• Simulants will be found that bind and transiently block conduction of ions through ompF.
3) “Phase I’ Deliverables
▪ demonstrate ‘Gigaseal’ properties ▪ demonstrate reversible electrodes ▪ measure single channels with integrated device ▪ characterize stability of integrated device
Si
Bilayer
4) Future Plans - issues to be addressed
• membrane stabilization
• simulants detection
• identifying stochastic signatures
• ………..