XPS can be used to analyze bio-surfaces like amino acid multilayers and contact lenses. XPS depth profiling with cluster ions allows intact profiling of amino acid multilayers, observing the expected alternating layers. Batch analysis of contact lenses uses fluorine peaks to measure coating thickness variations across a batch. Angle-resolved XPS can also characterize ultra-thin coatings on curved surfaces like catheter coatings without changing analysis conditions.

1. XPS Simplified
4. Analysis of Bio-surfaces using XPS
The world leader in serving science
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2. Webinar overview
• Introduction
• Why are we interested in surfaces?
• How XPS can assist with surface
problems?
• What is XPS?
• Theory
• Instrumentation
• What can we learn about
biosurfaces with XPS?
• Application examples
• Summary
2

3. Why are we interested in the surface of bio-materials?
• The surface of a solid is the point where it interacts with it’s environment.
• Physical, electronic and chemical properties can all depend on the first few
atomic layers of a material.
3

4. XPS of biosurfaces
4

5. What is XPS?
• Through the photoelectric effect, core
electrons are ejected from the surface
irradiated with the X-ray beam.
• These have a characteristic kinetic
energy depending on the
element, orbital and chemical state of
the atom
EBE = hn - EKE
• Layers up to ~10 nm thick can be
probed directly.
• Thicker layers can be analysed by ion
beam depth profiling
5

6. XPS instrumentation
• UHV System
• Ultra-high vacuum keeps surfaces clean Hemispherical
• Allows longer photoelectron path length analyser
• Electron analyser
• Lens system to collect photoelectrons
• Analyser to filter electron energies Detector
• Detector to count electrons
• X-ray source Ion gun
• Typically Al Ka radiation Electron transfer
• Monochromated using quartz crystal lens
• Low-energy electron flood gun
• Analysis of insulating samples
• Ion gun Mono
• Sample cleaning Flood gun crystal
• Depth profiling
• For polymers, cluster ion sources may be required
X-ray source
6

7. Application examples
• What can we learn about biosurfaces
with XPS?
• Depth profiling sensitive layers
• Amino acid biosensor
• Contact lens analysis
• Ultra-thin film analysis
• Using angle resolved XPS
• Catheter polymer coating
• Self-assembled monolayer
characterisation
7

8. XPS depth profiling
 XPS depth profiling
 XPS is extremely surface sensitive
 Signals are observed from <10nm into the
sample
 Many features of interest lie deeper into
sample
 Layers of up to a few microns thickness are
common
 There may be buried layers
 The interfaces between these layers are often
of interest
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9. XPS depth profiling
 XPS depth profiling
 How can we access the deeper layers for
analysis?
 By progressively removing the material from
the surface and performing XPS analysis at
each step
 Data collected after each etch period of milling
 Monatomic argon ion (Ar+) beam milling is the
most common method, but can damage
chemistry of the remaining surface, especially
polymers
 New Ar gas cluster ion sources minimise
chemical damage after sputtering – very useful
for biosurfaces
9

10. Cluster ions v monatomic ions
Monatomic ion beam Cluster ion beam
10

11. Monatomic v cluster profiling Cleaning polyimide
• Many polymers cannot be sputtered with monatomic argon
• Chemical information is destroyed & composition is modified
• C1s spectra shown for ion beam etched Kapton
Kapton4 keV clusters Kaptonmonatomic Ar+
C-O
C-C
N-C=O
C-N
Shake-up
296 292 288 284 280 296 292 288 284 280
Binding Energy (eV) Binding Energy (eV)
11

12. MAGCIS – Monatomic and Gas Cluster Ion Source
Cluster
Electrical Gas inlet
connections Skimmers
Nozzle
Ionization region
Monatomic
gas inlet
Focus & Mass selection
scanning
electrodes
12

13. 1. Amino acid multilayers for biosensor development
 Biosensor applications of amino acid
multilayer films
 Amino acid multilayer studied in this work
 Multilayer of phenylalanine (Phe) and tyrosine (Tyr)
 Films deposited by thermal evaporation
Schematic of expected structure of
amino acid multilayer
Phenylalanine (Phe)
Tyrosine (Tyr)
13

14. Amino acid multilayers Phe and Tyr references
Measured Expected Measured Expected
At% At% At% At%
Measured as received surface composition
Element Tyr Tyr Phe Phe is as expected for Tyr and Phe
C 69.67 69.23 74.05 75.00
O 21.62 23.08 15.85 16.67
N 8.71 7.69 10.11 8.33
Elemental quantification table
C1s
Tyr
O1s
N1s
CAuger OAuger
NAuger
1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0
Phe
Binding Energy (eV)
14

15. Amino acid multilayers Phe and Tyr references
 Chemical analysis of amino acid films Phenylalanineas received
 XPS is chemically sensitive
 Spectrum of phenylalanine shows components due Aromatic
to aromatic ring, C-C-NH2 and OH-C=O groups
 Quantitative chemical & elemental analysis
Observed At% Expected At%
Caromatic 53.34 50.00
CCCNH2 13.18 16.67
CCO2H 7.47 8.33
N 10.13 8.33
O 15.88 16.67 C-CNH2
Elemental quantification table
CO2H
p-p* shake-ups
298 296 294 292 290 288 286 284 282 280
Binding Energy (eV)
15

16. Amino acid multilayers Phe and Tyr references
 Chemical analysis of amino acid films Tyrosineas received
 XPS is chemically sensitive
 Addition of a single OH group to phenyl ring shows Aromatic
clearly in hi-resolution C1s spectrum
 XPS can easily chemically resolve carbon bonding
environments in Phe and Tyr
Aromatic-OH
Observed At% Expected At% and C-CNH2
Caromatic 40.50 38.46
CCCNH2 22.47 23.08
CCO2H 6.70 7.69
N 8.71 7.69
O 21.62 23.08
Elemental quantification table
CO2H
p-p* shake-up
296 292 288 284 280
Binding Energy (eV)
16

17. Amino acid multilayers Phe and Tyr references
 Chemical analysis of amino acid films Pheas received and Tyras received
 Oxygen chemical analysis
 High energy resolution O1s spectra allow extra OH
group in Tyr to be tracked and quantified
 Ratio of “red:blue” components in Tyr is Tyr
measured at 2:1, as expected
 Small amount of “contaminant” oxygen in Phe O1s
spectrum
Phe
542 540 538 536 534 532 530 528 526
Binding Energy (eV)
17

18. Amino acid multilayers Profiling of amino acid films
 Profiling of amino acid films
 Amino acid films cannot be sputtered with
monatomic argon
 Chemical information is destroyed & composition is
strongly modified
 Cannot observe expected layer structure
 Elemental composition strongly modified
 Chemical information is destroyed
p-p* shake-up
disappears
Elemental profile of amino acid layers with 200eV
monatomic Ar+ beam
C1s spectra from monatomic Ar+ profile of amino acid layers
18

19. Amino acid multilayers Tyrosine reference
 Profiling of Tyr films MAGCIS cluster profile of Tyr on Si
 Chemical stability of Tyr during argon cluster 70
C
profiling
 Chemistry of Tyr film NOT destroyed by cluster 60
profiling
50
Atomic percent (%)
40
30
p-p* peak O
retained
20
Depth Si
25nm 10 N
15nm
0 nm
0
0 10 20 30 40 50
Etch Depth (nm)
C1s spectra during profile
19

20. Amino acid multilayers Intact multilayer
 Profiling of amino acid multilayer
 Expected structure of multilayer 80
 Alternating Phe/Tyr layers, with layer of Phe on top
surface and 3 Tyr layers 70
 All three Tyr layers observed C
60 OPhe&Tyr
 Quantification change between Phe and Tyr as
expected OTyr
Atomic percent (%)
50
N
 Slight increase in carbon signal over 300nm depth
Si
(1.2 At%)
40
 Chemical resolution of Phe and Tyr oxygen
throughout profile
30
 Reasonable stability on OTyr quantification
 Depth resolution on last Tyr layer slightly degraded 20
10
0
0 100 200 300 400
Etch Depth (nm)
MAGCIS cluster profile of intact amino acid multilayer
20

21. Amino acid multilayers Damaged multilayer
 Profiling of amino acid multilayer
 Expected structure of multilayer
 Alternating Phe/Tyr layers, with layer of Phe on top 70
surface and 3 Tyr layers
 Top Phe layer not observed 60
C
 Damaged BEFORE analysis OPhe&Tyr
OTyr
 All three Tyr layers observed 50
Atomic percent (%)
N
 Quantification change between Phe and Tyr as Si
expected 40
 Slight increase in carbon signal over 300nm depth
(1.2 At%) 30
 Chemical resolution of Phe and Tyr oxygen
throughout profile 20
 Excellent stability on OTyr quantification
10
0
0 500 150 250 350
Etch Depth (nm)
MAGCIS cluster profile of damaged amino acid multilayer
21

22. 2. Batch analysis – contact lens coating thickness
• Disposable contact lenses are commonly
manufactured from a composite of
silicone rubber and hydrogel monomers.
• Silicone is hydrophobic, which results in
poor performance and wear comfort.
• Lenses can be plasma-coated to give
good hydrophilic properties
• The coating thickness is known to vary
depending upon the position of the lens
during the coating process
• XPS depth profiling can be used to
investigate the coating thickness
throughout a batch of lenses
22

23. Batch analysis – contact lens coating thickness
• Fluorine is in different chemical states in the
coating and the substrate, making it an excellent
marker for the coating thickness.
• The experiment is configured to use a pre-defined
peak table to process the data after
acquisition, calibrate to a thickness scale, and
export to excel
F1s Snap
500
450
400
Counts / s
350
300
250
696 694 692 690 688 686 684 682 680 678
Binding Energy (eV)
23

24. Batch analysis – contact lens coating thickness
• The final result of the experiment is a simple chart which
enables a non-expert analyst to determine trends from the data
Thickness (nm)
Lens 14
Lens 15
Lens 16
Lens 10
Lens 11
Lens 12
Lens 13
Lens 5
Lens 6
Lens 7
Lens 8
Lens 9
Lens 1
Lens 2
Lens 3
Lens 4
24

25. ARXPS - Varying the collection angle
• Information depth varies with • Spectra from thin films on
collection angle substrates are affected by the
• I = Iexp(-d/lcosq) collection angle
Varying the angle between the surface normal and the electron
analyser changes the surface sensitivity – leads to identifying the
structure and thickness of ultra-thin layers
25

26. The Parallel ARXPS Solution
• Theta Probe
• Measures Energy and Angle simultaneously
• ARXPS without tilting the sample
• Allows mapping of ultra thin film structures
26

27. 3. Catheter surface coating analysis
Live optical view from Theta Probe camera
 Fluoropolymer catheter
• ARXPS from a curved, insulating surface
• Live optical view for easy alignment of sample
• Analysis area DOES NOT change as a function of photoemission angle
• Charge neutralisation conditions DO NOT change as a function of
photoemission angle
• Depth distribution of carbon bonding states
27

28. Catheter surface coating analysis
Live optical view from Theta Probe camera
 Fluoropolymer catheter
• ARXPS from a curved, insulating surface
• Live optical view for easy alignment of sample
• Analysis area DOES NOT change as a function of photoemission angle
• Charge neutralisation conditions DO NOT change as a function of
photoemission angle
• Depth distribution of carbon bonding states
CF2 C1s spectrum
 Depth distribution of carbon bonding states C-C
C-O
• Depth integrated carbon chemistry
• High energy resolution spectrum of C1s region shows carbon O-*C=O
C-*C=O
bonding states within total XPS sampling depth (~10 nm)
• Fluorocarbon states easily observed CF3
• Excellent resolution due to high performance charge
neutralisation system
28

29. Catheter surface coating analysis
Live optical view from Theta Probe camera
 Fluoropolymer catheter
• ARXPS from a curved, insulating surface
• Live optical view for easy alignment of sample
• Analysis area DOES NOT change as a function of photoemission angle
• Charge neutralisation conditions DO NOT change as a function of
photoemission angle
• Depth distribution of carbon bonding states
ARXPS C1s spectra
 Depth distribution of carbon bonding states
• Depth distribution of carbon chemistry
• ARXPS C1s spectra acquired simultaneously at all angles
• Constant charge neutralisation conditions at all angles Bulk
• Constant analysis area at all angles
• ARXPS data was peak fit with the components shown on the
previous slide to generate a Relative Depth Plot
Surface
29

30. Catheter surface coating analysis
Live optical view from Theta Probe camera
 Fluoropolymer catheter
• ARXPS from a curved, insulating surface
• Live optical view for easy alignment of sample
• Analysis area DOES NOT change as a function of photoemission angle
• Charge neutralisation conditions DO NOT change as a function of
photoemission angle
• Depth distribution of carbon bonding states
Layer ordering of carbon bonding states
CF3
C-*C=O
 Depth distribution of carbon bonding states
CF2
• Depth distribution of carbon chemistry
• Relative depth plot shows the layer ordering of elements and
chemical states
• Method is model independent C-C
• Instant conversion of ARXPS data into depth information
O-*C=O
C-O
30

31. 4. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Molecular orientation information and depth profile of single molecules
ASEMBLON, INC
[1] www.asemblon.com
31

32. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Molecular orientation information and depth profile of single molecules
[1] www.asemblon.com
32

33. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Molecular orientation information and depth profile of single molecules
 Theta Probe ARXPS measurement
• Experimental advantages
• Data from all angles comes from same analysis point
• Imaging ARXPS is possible, allowing film uniformity
to be studied
• Rapid snapshot acquisition reduces X-ray spot dwell 3 mm
time
Imaging ARXPS of samples damaged in transit
[1] www.asemblon.com
33

34. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Molecular orientation information and depth profile of single molecules
Self-assembled
monolayer materials
Nonanethiol Hydroxy undecanethiol used in this work
Dodecanethiol 1-mercapto-11-undecyl-tri(ethylene glycol)
Hexadecanethiol Images from AsemblonTM, 15340 NE 92nd Street, Suite B, Redmond, WA 98052-3521,
USA. www.asemblon.com
34

35. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Molecular orientation information and depth profile of single molecules
2.5
 Non-destructive ARXPS thickness measurement 2
Layer Thickness
• Thickness as a function of organic chain length 1.5
• Film thickness measured on Theta Probe 1
• Thickness increases linearly with organic chain length 0.5
0
0 5 10 15 20
Number of Carbon Atoms
Theta Probe measured layer thickness
[1] www.asemblon.com
35

36. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Orientation information and depth profile of single molecules
Non-destructive ARXPS profile of alkanethiol on Au
100
100
 Alkanethiols non-destructive depth profiles C Au
Concentration/%
80
Concentration/%
• Thickness and molecular orientation information 80
• Confirms that organic bonds to gold at sulphur 60
60
• Relative layer thickness is observed in profiles
40
40
20 S
20
0 Dodecanenanethiol
0
0 1 2
0 Depth / nm 1 2
Depth/nm
[1] www.asemblon.com Depth/nm
36

37. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Orientation information and depth profile of single molecules
Non-destructive ARXPS profile of hydroxy
undecanethiol on Au
100
100
 Functionalised alkanethiols non-destructive depth Au
Concentration/%
80 CH2
Concentration/%
profiles 80
• Thickness and molecular orientation information 60
60
• Confirms that organic bonds to gold at sulphur
40
• Chemical state information is preserved 40
CH2OH
• Possible to observe CH2OH at top surface, then alkane 20
20
chain, then thiol group at Au interface S
0
0
0 1 2 3
0 Depth / nm
1 2 3
Depth/nm
[1] www.asemblon.com Depth/nm
37

38. Analysis of self-assembled monolayers
 Self-assembled monolayers
• Non-destructive depth profiling of single molecule
• Self-assembled monolayers allow controlled modification of surface
properties1
• Possible application in molecular electronics and biomaterials1
• Organosulfur chemistry often used to form layers on gold
• Layer thickness as a function of organic chain length
Schematic of self-assembled monolayer
• Orientation information and depth profile of single molecules
Non-destructive ARXPS profile of 1-mercapto-11-
undecyl-tri(ethylene glycol) on Au
100
100
 Functionalised alkanethiols non-destructive depth Au
Concentration/%
80 CH2
Concentration/%
profiles 80
• Thickness and molecular orientation information 60 C4H2O
60
• Confirms that organic bonds to gold at sulphur
40
• Chemical state information is preserved 40 CH2OH
• Possible to observe CH2OH at top surface, then alkane 20
chain, then thiol group at Au interface 20
0
S
0
0 1 2 3
0 Depth / nm
1 2 3
Depth/nm
[1] www.asemblon.com Depth/nm
38

39. Summary
Theta Probe
• XPS is great!
E250Xi
K-Alpha
39

40. Acknowledgements
Theta Probe
• J.J. Pireaux, P. Louette
• Laboratoire Interdisciplinaire de
Spectroscopie
Electronique, Facult´es
Universitaires Notre Dame de la
Paix, Namur, Belgium
• Dan Graham
• Assemblon Inc
• University of Washington
E250Xi
K-Alpha
40