Quantification of grafted poly(ethylene glycol)-silanes on silicon by time-of-flight secondary ion mass spectrometry

Article abstract of DOI:10.1002/jms.330

Silicon grafted monodisperse poly(ethylene glycol) (PEG) silanes with various PEG chain lengths and mixtures of these were systematically analyzed with static time-of-flight secondary ion mass spectrometry (TOF-SIMS). The mass spectra show differences in the various relative signal intensities, an observation that was used to elucidate important aspects of the grafting process. The relationship between PEG-silane fragment ion abundances and Si⁺ ion abundances were used to (i) qualitatively describe layer thicknesses of grafted mixtures of PEG-silanes on silicon, (ii) construct a calibration curve from which PEG chain length (or molecular mass) can be determined and (iii) quantitatively determine surface mixture compositions of grafted monodisperse PEG-silanes of different chain lengths (3, 7 and 11 PEG units). The results suggest that discrimination does take place in the adsorption process. The PEG-silane with the shorter PEG chain is discriminated for mixtures containing PEG3-silane, whereas the PEG-silane with the longer PEG chain is discriminated in PEG7/PEG11-silane mixtures. The origin of this difference in adsorption behavior is not well understood. Aspects of the grafting process and the TOF-SIMS analyses are discussed. Copyrigh

Full text of DOI:10.1002/jms.330

JOURNAL OF MASS SPECTROMETRY
J. Mass Spectrom. 2002; 37: 699–708
Published online 17 June 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jms.330
Quantification of grafted poly(ethylene glycol)-silanes
on silicon by time-of-flight secondary ion mass
spectrometry
K. Norrman,1∗ A. Papra,2 F. S. Kamounah,3 N. Gadegaard1 and N. B. Larsen1
1
Danish Polymer Centre, Risø National Laboratory, DK-4000 Roskilde, Denmark
Eppendorf Instrumente GmbH, R&D, 22339 Hamburg, Germany
CISMI, Department of Chemistry, University of Copenhagen, DK-2100 Copenhagen, Denmark
2
3
Received 6 November 2001; Accepted 9 April 2002
Silicon grafted monodisperse poly(ethylene glycol) (PEG) silanes with various PEG chain lengths and
mixtures of these were systematically analyzed with static time-of-flight secondary ion mass spectrometry
(TOF-SIMS). The mass spectra show differences in the various relative signal intensities, an observation
that was used to elucidate important aspects of the grafting process. The relationship between PEG-silane
fragment ion abundances and Si+ ion abundances were used to (i) qualitatively describe layer thicknesses
of grafted mixtures of PEG-silanes on silicon, (ii) construct a calibration curve from which PEG chain length
(or molecular mass) can be determined and (iii) quantitatively determine surface mixture compositions of
grafted monodisperse PEG-silanes of different chain lengths (3, 7 and 11 PEG units). The results suggest
that discrimination does take place in the adsorption process. The PEG-silane with the shorter PEG chain
is discriminated for mixtures containing PEG3-silane, whereas the PEG-silane with the longer PEG chain
is discriminated in PEG7/PEG11-silane mixtures. The origin of this difference in adsorption behavior is
not well understood. Aspects of the grafting process and the TOF-SIMS analyses are discussed. Copyright
 2002 John Wiley & Sons, Ltd.
KEYWORDS: time-of-flight secondary ion mass spectrometry; quantification; poly(ethylene glycol); silanes; protein
adsorption
constituents result in such smooth adsorbed films. This
behavior could result from at least two different mechanisms.
The adsorption process might selectively favor one of the
components owing to kinetic or equilibrium differences in
reactivity. Another, less plausible, mechanism could be the
formation of an umbrella-like layer of longer chain PEG-
silanes below which the shorter chain PEG-silanes adsorb
without causing an increase in the surface roughness.
In order to understand better why the commercial PEG-
silanes exhibit such favorable properties, it is relevant
to study the monodisperse PEG-silanes systematically.
The properties of grafted monodisperse PEG-silanes and
mixtures of these have been examined comprehensively
by a multitude of techniques (N. Gadegaard, A. Papra,
N. B. Larsen, K. Norrman and F. S. Kamounah, unpublished
work). The current work was focused only on the problems
associated with quantifying mixture compositions of grafted
monodisperse PEG-silanes on silicon surfaces.
INTRODUCTION
Many applications require specific surface properties.1 Non-
specific protein adsorption is a prerequisite for many
biotechnical applications, e.g. optimized cell substrates.1
or specific recognition in biosensor applications.2 The
prevention of non-specific adsorption or any non-specific
binding is a prerequisite to ensure specific recognition for
biosensor applications.2 Poly(ethylene glycol)s (PEGs), also
known as poly(ethylene oxide) (PEO), are powerful reagents
towards providing protein repellent surfaces.3 A lot of effort
has been put into modifying surfaces with PEGs in order to
make them protein repellent.4,5 The surface properties of a
PEG layer are dependent on its molecular mass,6 interfacial
chain density.7,8 structural properties9 and whether or not
the PEG layer is adsorbed or covalently grafted.
It has been shown that grafting of low molecular mass
commercially available PEG-silanes on silicon (Fig. 1) yields
ultra-thin hydrophilic and stable PEG monolayers with a
In order to determine if the adsorption process selectively
favors one of the components owing to kinetic or equilib-
rium differences in reactivity, it is necessary to quantify the
mixture composition on the surface. This is a substantial
problem from a characterization point of view, because no
obvious characterization techniques exist that can measure
the composition of grafted PEG-silane mixtures on silicon
10
˚
roughness of <3 A. Commercially available PEG-silanes
are mixtures of PEG oligomers of various chain lengths. It
is surprising that large variations in the size of the solute
ŁCorrespondence to: K. Norrman, Danish Polymer Centre, Risø
National Laboratory, DK-4000 Roskilde, Denmark.
E-mail: kion.norrman@risoe.dk
Copyright  2002 John Wiley & Sons, Ltd.

700
K. Norrman et al.
OCH3
Si
OCH3
Toluene, H+, 18 h
O
OCH3
CH3O
OH
+
n
OCH3
Si
O
OCH3
n
OCH3
Figure 1. Reaction scheme for the grafting process. Grafting conditions: 3 mM solution of PEG-silane in toluene (with 0.8 ml lꢀ1
concentrated hydrochloric acid) at room temperature for 18 h.
surfaces. If the PEG-silanes were to be chemically cleaved
from the substrate, in order to analyze the composition
of the desorbed PEGs or PEG-silanes, there would be no
way of knowing whether or not a potential discrimination
was caused by the grafting process or the cleavage process.
We have employed time-of-flight secondary ion mass spec-
trometry (TOF-SIMS),11 which is not directly a quantitative
technique, to quantify the compositions of various grafted
monodisperse PEG-silane mixtures on silicon surfaces.
TOF-SIMS is a powerful tool for obtaining chemical
information on surfaces.11 A lot of valuable information can
be extracted from a surface mass spectrum, e.g. molecular
and structural information, and, as is going to be evident,
also quantitative information. TOF-SIMS is a technique based
on mass spectrometry and is therefore said not to be a
quantitative technique. However, in spite of TOF-SIMS not
being a quantitative technique in principle, many workers
have made extensive efforts to find ways to use TOF-SIMS
quantitatively.12–62 Whether or not TOF-SIMS can be said to
be quantitative is still debated.
The fundamental reason for mass spectrometry not being
directly quantitative in principle is the different response
factors associated with different species, i.e. different species
produce different signal intensities per unit amount of
material. For surface mass spectrometry it makes more sense
to use the related property termed the secondary ion yield,
which is the number of secondary ions detected relative to
the number of primary ions used. The secondary ion yield is
influenced by several factors, which are more or less linked,
e.g. ionization probability, element type, chemical structure
and surface topography and, to complicate things even more,
the matrix may influence the secondary ion yield. Matrix
effects prevent the signal intensities from the same type of
compound on different substrates or different compounds on
the same substrate from being compared, which is one of the
major problems when trying to use TOF-SIMS quantitatively.
However, the PEG-silanes in question differ from each other
only in the number of PEG units, i.e. they are structurally and
chemically very similar, so it is likely that the matrix effects
are similar for all adsorbed molecules in these systems.
There are many examples in the literature reporting
quantitative TOF-SIMS.12–62 However, they all have one
thing in common, namely that they employ calibration
curves to make TOF-SIMS quantitative. One could argue
that a quantification based on a calibration curve should
be termed semi-quantitative. To avoid confusion we shall
subsequently not distinguish between the terms quantitative
and semi-quantitative.
Galuska21,22,24 described extensive work regarding the
quantification of copolymers and polymer blends, mainly
by the use of TOF-SIMS. The systems investigated included
copolymers of ethylene and vinyl acetate and copolymers of
ethylene and methyl acrylate. He also managed to quantify
the fractions of ethylene and propylene in their copolymers,
which is noteworthy since these copolymers are entirely
hydrogen and carbon based and structurally and chemically
very similar. He used a variety of pure C2 –C3 copolymers
as standards with random sequence distributions, which
were verified by 13C NMR. The mass spectral differences
between polyethylene and polypropylene are, not surpris-
ingly, minor, which means that to make a calibration curve
one needs to look at small differences in selected relative
signal intensities. Furthermore, there are no unique signals;
both polymers produce the same set of signals, but with
minor differences in the relative signal intensities, so the sig-
nals from each component will be mixed. This will typically
result in a non-linear calibration curve. The smaller the ions
monitored, the less specific they are, i.e. they can be formed
via various reaction channels and originate from different
parts of the polymer structure.
In another study, Galuska23 used TOF-SIMS to determine
molecular masses from polymeric surfaces and microscopic
phases. He analyzed a variety of elastomers and thermo-
plastics of known molecular mass and polydispersity index.
Once again he studied relative signal intensities and corre-
lated these with the known molecular masses. He obtained
non-linear calibration curves that leveled off at ¾20 000 mass
units, which means that under the given conditions the
mass spectra did not change for molecular masses >20 000
mass units. The appearance of TOF-SIMS mass spectra is
very sensitive to changes in the chemical and physical sur-
face properties, a fact that Galuska21–24 used effectively to
quantify mixture compositions and molecular masses.
Copyright  2002 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2002; 37: 699–708

TOF-SIMS of Si-grafted monodisperse PEG-silanes
701
The approach that Galuska21–24 used has general appli-
cability; thus any property for which a property change
will result in a change in relative signal intensities can
be quantified by constructing a calibration curve. In this
work, the same approach was used to illustrate how a
potential unknown chain length (or molecular mass) of a
monodisperse PEG-silane adsorbed on a silicon surface can
be determined from the construction of a calibration curve.
Furthermore, it is demonstrated how TOF-SIMS can be used
quantitatively to determine binary mixture compositions of
grafted monodisperse PEG-silanes on silicon by the use of a
calibration.
monotosylated using p-toluenesulfonyl chloride and KOH in
°
dichloromethane (0 C) to give allyloxytetra(ethylene glycol)
tosyl ether. The allyloxytetra(ethylene glycol) tosyl ether was
then reacted with hepta(ethylene glycol) monomethyl ether
°
(from the PEG7-silane synthesis) and NaH in THF (0–25 C)
to give allyloxyundeca(ethylene glycol) methyl ether. The
allyloxyundeca(ethylene glycol) methyl ether was then sily-
°
lated using trimethoxysilane and H2PtCl6 in toluene (90 C,
96 h) to give the PEG11-silane.
PEG40-silane
Commercially available polydisperse tetracosa(ethylene gly-
col) monomethyl ether was allylated using allyl chloride
°
and tert-C4H9OK in THF (60 C, 24 h) to give allyloxyte-
EXPERIMENTAL
tracosa(ethylene glycol) monomethyl ether. Allyloxytetra-
cosa(ethylene glycol) monomethyl ether was then silylated
using trimethoxysilane and H2PtCl6 in toluene (90 C, 96 h)
Synthesis
°
Three monodisperse PEG-silanes were synthesized for this
study: (i) a PEG3-silane (three monomers), (ii) a PEG7-silane
and (iii) a PEG11-silane. A polydisperse PEG40-silane was
also synthesized. Gas chromatographic/mass spectrometric
(GC/MS) analysis showed the purities for the PEG3-silane
and the PEG7-silane to be ½97%. The PEG11-silane partly
decomposed during GC/MS and the PEG40-silane was too
large to be analyzed using GC/MS.
All chemicals used were products of Fluka (Buchs,
Switzerland). The tetrahydrofuran (THF) used in the syn-
theses was freshly distilled from sodium benzophenone.
Toluene was dried using metallic sodium, filtered and
stored with activated molecular sieves. Tri(ethylene glycol),
tetra(ethylene glycol) and tri(ethylene glycol) monomethyl
ether were dried with activated molecular sieves. All reac-
tions were performed under a nitrogen atmosphere.
to give the PEG40-silane.
Grafting
We have developed a simple method to coat silicon surfaces,
which has been described previously.10 We used silicon
wafers (100, single side polished) purchased from Topsil
(Frederiksund, Denmark). All chemicals used were products
of Fluka (Buchs, Switzerland) and Aldrich (Milwaukee, WI,
USA) and used without further purification for the grafting
procedure. Ultrapure water was supplied by a Milli-Q system
from Millipore (Bedford, MA, USA). The wafers were cut
into suitable sizes (a few square centimeters) and cleaned
by sonication in ethanol–water (1 : 1, v/v) for 5 min. The
substrate was oxidized in a mixture of hydrogen peroxide
(30%, w/v) and sulfuric acid (96%) (20 : 80, v/v) for 10 min
°
at 120 C. Caution: the mixture should be used with extreme
care owing to its oxidizing power and risk of explosions.63
The wafers were washed with copious amounts of water,
sonicated in water for 10 min, blown dry and immersed
immediately in the respective solution. Grafting (Fig. 1),
was performed in a 3 mM solution of PEG-silane in toluene
(with 0.8 m lꢀ1 concentrated hydrochloric acid) at room
temperature for 18 h.10 The samples were rinsed in toluene
(once), ethanol (twice) and water (twice) and finally sonicated
inwaterfor2 mintoremovenon-graftedmaterial. Thewafers
were blown dry and stored dry under ambient conditions.
PEG3-silane
Commercially available tri(ethylene glycol) monomethyl
ether was allylated using allyl chloride and tert-C4H9OK
in THF (0–60 C) to give allyloxytri(ethylene glycol) mono-
methyl ether. This intermediate was silylated using trimetho-
xysilane and H2PtCl6 in toluene (90 C, 96 h) to give the
PEG3-silane.
°
°
PEG7-silane
Commercially available tri(ethylene glycol) monomethyl
ether was chlorinated using thionyl chloride in benzene
(pyridine, reflux) to give chloroethyloxydi(ethylene glycol)
methyl ether. This intermediate was reacted with com-
mercially available tetra(ethylene glycol) and Na in THF
TOF-SIMS
The TOF-SIMS analyses were performed using a TOF-SIMS
IV (ION-TOF, Mu¨nster, Germany) operated at a pressure of
5 ð 10ꢀ9 Torr (with sample, 1 Torr D 133.3 Pa); 15 ns pulses of
15 keV GaC (primary ions) were bunched to form ion packets
with a nominal temporal extent of <0.9 ns at a repetition rate
of 10 kHz, which produced a target current of 2.5 pA. These
primary ion conditions were used to scan a 500 ð 500 µm
area of the sample for 30 s. A flood gun was used to prevent
built-up charge in the surface. Desorbed secondary ions were
accelerated to 2 keV, mass analyzed in the flight tube and
post-accelerated to 10 keV before detection. The scan area
used (500 ð 500 µm) only allowed a mass resolution of ¾6000,
which was more than enough to resolve all relevant mass
spectral peaks. The ion abundances were normalized relative
°
(100 C) to give hepta(ethylene glycol) monomethyl ether.
The hepta(ethylene glycol) monomethyl ether was then ally-
lated with allyl chloride and Na in THF (0–25 C) to give
allyloxyhepta(ethylene glycol) methyl ether. The allyloxy-
hepta(ethylene glycol) methyl ether was then silylated using
trimethoxysilane and H2PtCl6 in toluene (90 C, 96 h) to give
°
°
the PEG7-silane.
PEG11-silane
Commercially available tetra(ethylene glycol) was monoally-
lated using allyl chloride and NaOH in water (85 C) to give
allyloxytetra(ethylene glycol). This intermediate was then
°
Copyright  2002 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2002; 37: 699–708

702
K. Norrman et al.
to the total ion abundance to compensate for differences
in the numbers of GaC shots that each sample received
together with other instrumental factors. Each sample was
analyzed five times at different locations and the average
value used. All measurements were repeated with a target
current of 200 fA as a test for possible detector saturation. The
same results were obtained, but with significantly increased
uncertainties caused by the reduced signal intensities.
as the PEG-silanes in question, there is a paucity of data. We
observed a sampling depth, that is significantly larger than
1 nm. Furthermore, the substrate ion, SiC, is so small that
its escape depth could easily be larger than the PEG-silane
fragments. Hence it is not surprising that the substrate signal
is detectable for all samples. However, the common surface
contaminant silicone oil is also a possible source for the SiC
signal. The lack or negligible amount of m/z 147 (Si2C5H15OC,
a very characteristic ion for silicone oil) in the mass spectra,
Fig. 3(a)–(d), supports the fact that the samples were not
contaminated prior to analysis, so it is safe to assume that
all SiC detected originates from the substrate or the silane
interface.
Ellipsometry
The adsorbed mass per surface area (effective layer thickness)
was determined with a Picometer Ellipsometer (Beagle-
hole Instruments, Wellington, New Zealand) using phase-
modulated ellipsometry at the Brewster angle of silicon. The
resulting optical parameters were used in the accompany-
ing analysis package for modeling the surface structure in
a three-layer model consisting of silicon, silicon oxide and
PEG. The refractive index, n, of the adsorbed PEG layer
was assumed to be 1.46 in the model. This assumption is
not critical as modeling shows that variations of the over-
layer refractive index of š0.02 will affect the calculated layer
thickness by only 2%. We analyzed three 0.8 mm diameter
spots at different locations on each sample to capture spatial
variations and took the average of these measurements as
the effective layer thickness.
Quantification of chain length
For larger PEG-silane chains, the layer will be thicker, and
subsequently the likelihood of observing substrate signals
will decrease. In Fig. 4, the normalized intensities of various
positively charged fragment ions are shown for the PEG3-,
PEG7-, PEG11- and PEG40-silane.
The substrate signal (SiC) is seen, as expected, to decrease
for longer PEG-silane chains. The desorbed ions with the
elemental composition SiCH3OC most likely originate mainly
from the silane part of the PEG-silanes (Fig. 2). However,
SiCH3OC can also be formed from the silicon substrate
reacting with surrounding organic material. Since the silane
group, owing to the chemical bond, is in close vicinity of the
surface (Fig. 2), the SiCH3OC is expected, from wherever it
originates, to follow the same qualitative trend as SiC. It is
evident from Fig. 4 that this is indeed observed.
For larger PEG-silane chains there will be a greater
likelihood for larger fragment pieces to be formed [compare
Fig. 3(a) with Fig. 3(d)]. Furthermore, from the previous
discussion it was suggested that larger PEG-silane chains
result in less substrate signal. All this suggests that the
abundance of the fragment ions should increase relative to
the substrate signal for larger PEG-silane chains. Figure 4
shows the abundances of several fragment ions, and it is
evident that the expected trend is observed. The trend is
observed for all PEG fragment ions, i.e. the ions shown in
Fig. 4 are only a few representative examples. However, the
RESULTS AND DISCUSSION
The samples were all analyzed with TOF-SIMS. The ion
bombardment of the sample surfaces results in desorption of
fragmented pieces of PEG, of the silane and of the substrate
(Fig. 2). TOF-SIMS spectra of the pure monodispersive PEG-
silanes grafted on silicon are shown in Fig. 3(a)–(d). The
PEG ion series observed are chain cleavage products and
analogue ions with one or two hydrogens either added or
subtracted. As is evident, the mass spectra show differences
in the various relative signal intensities, a fact that will
be used to elucidate important aspects of the grafting
process.
The sampling depth for static TOF-SIMS is normally said
to be ¾1 nm.11 However, for polymers and oligomers such
Si+, SiCH3O+, Si(CH3O)2+, CH3O+
OCH3
Si
O
O
O
O
CH3O+
C2H5O+
C3H7O+
OCH3
C3H7O+
+
C3H7O2
+
C4H9O2
C5H11O2
+
Si+
+
C5H11O3
+
C6H13O3
+
C7H15O3
Figure 2. Possible fragmentation pathways during ionization forming various desorbed fragment ions, which can be detected by the
mass spectrometer. Hydrogen abstraction will result in the formation of analogue ion series containing plus or minus one or two
hydrogens.
Copyright  2002 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2002; 37: 699–708

TOF-SIMS of Si-grafted monodisperse PEG-silanes
703
(a)
100
90
80
70
60
50
40
30
20
10
0
3
PEG3-silane
+
1) CH3
11) C4H5O+
12) C4H7O+
+
2) C2H3
+
+
+
+
+
+
3) Si+ + C2H4
13) C3H5O2
14) C4H7O2
15) C4H9O2
16) C5H7O2
17) C5H9O2
1
4) CH3O+
+
5) C3H3
10
+
6) C3H5
7) C2H3O+
8) C2H5O+
9) C3H5O+
8
+
+
+
18) C5H11O2
19) C6H11O2
20) C6H13O3
7
10) C3H7O+
+
2
SiCH3O+
4
6
5
12
11
9
13
17
16
15
14
18
20
19
20
40
60
80
100
120
140
0
m/z
(b)
100
90
80
70
60
50
40
30
20
10
0
8
10
PEG7-silane
+
1) CH3
11) C4H5O+
12) C4H7O+
+
2) C2H3
+
+
3) Si+ + C2H4
13) C3H5O2
14) C4H7O2
15) C4H9O2
16) C5H7O2
17) C5H9O2
+
+
+
+
4) CH3O+
+
5) C3H3
+
7
6) C3H5
1
7) C2H3O+
8) C2H5O+
9) C3H5O+
3
+
18) C5H11O2
19) C6H11O2
20) C6H13O3
+
+
4
10) C3H7O+
+
SiCH3O+
2
13
12
6
18
14
15 17
16
9
5
11
20
19
20
40
60
80
100
120
140
0
m/z
Figure 3. TOF-SIMS of (a) PEG3-, (b) PEG7-, (c) PEG11- and (d) PEG40-silane grafted on silicon. Primary ion, 15 keV GaC; target
current, 2.5 pA; scan area, 500 ð 500 µm. scan time, 30 s; repetition rate, 10 kHz; secondary ion energy, 2 keV; post-acceleration,
10 kV; mass resolution, ¾6000. A flood gun was used for charge compensation.
C3H7OC ion is the only one not to follow the trend. The silane
group is connected to the PEG chain via a spacer consisting of
propylene. If the spacer, including the oxygen to which it is
bound, is cleaved, a C3H7OC ion can be formed if a hydrogen
is abstracted from somewhere (Fig. 2). This is one possible
explanation for C3H7OC not following the trend. Another
possible explanation could be that C3H7OC is a product of a
rearrangement. No attempts have been made to investigate
this further.
From an analysis of all abundant ions it was found that
the abundance of C4H7OC is most sensitive towards changes
in the PEG chain length. For the silane part the abundance
of SiCH3OC showed the largest sensitivity towards changes
in the PEG chain length. The logarithm of the abundances
for these two ions was plotted against the logarithm of the
number of PEG units (Fig. 5), which in turn corresponds to
the chain length or the molecular mass. Both of these plots
can be used as a calibration for a grafted monodisperse PEG-
silane sample with an unknown chain length. If the sample is
prepared and analyzed in parallel with the standard samples,
the signal intensity of C4H7OC or SiCH3OC will give the chain
length.
Quantification of mixture composition
Since the abundance of SiC is inversely proportional to the
abundances of PEG fragment ions with an increase in the PEG
chain length, the abundance ratio between PEG fragment
ions and SiC must be a sensitive measure of changes in
the PEG-silane chain length or of the PEG-silane mixture
composition. This relationship can be used to determine the
surface composition of binary PEG-silane mixtures. If a given
PEG fragment/substrate abundance ratio is considered, a
crude two-point calibration can be performed based on
the pure PEG-silanes, e.g. if the ratio in question is 2 for
PEG3-silane and 6 for PEG7-silane, then a ratio of 4 would
correspond to 50%. The result is observed to differ slightly
depending on which PEG-silane fragment is considered.
To compensate for this, the calculations were performed
on seven of the most abundant PEG fragment ions (except
C3H7OC), and average values with corresponding standard
Copyright  2002 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2002; 37: 699–708

704
K. Norrman et al.
(c)
100
90
80
70
60
50
40
30
20
10
0
8
PEG11-silane
+
1) CH3
11) C4H5O+
12) C4H7O+
+
2) C2H3
+
10
+
3) Si+ + C2H4
13) C3H5O2
14) C4H7O2
15) C4H9O2
16) C5H7O2
17) C5H9O2
+
+
+
+
4) CH3O+
+
5) C3H3
+
7
6) C3H5
7) C2H3O+
8) C2H5O+
9) C3H5O+
+
+
+
18) C5H11O2
19) C6H11O2
20) C6H13O3
1
3
4
10) C3H7O+
+
SiCH3O+
2
13
6
14
15
18
17
16
12
11
9
5
20
19
0
20
40
60
80
100
120
140
m/z
(d)
100
90
80
70
60
50
40
30
20
10
0
8
PEG40-silane
+
1) CH3
11) C4H5O+
12) C4H7O+
+
2) C2H3
+
+
+
+
+
+
3) Si+ + C2H4
13) C3H5O2
4) CH3O+
14) C4H7O2
15) C4H9O2
16) C5H7O2
17) C5H9O2
+
10
5) C3H3
+
6) C3H5
7) C2H3O+
8) C2H5O+
9) C3H5O+
+
+
+
18) C5H11O2
19) C6H11O2
20) C6H13O3
4
7
2
10) C3H7O+
+
SiCH3O+
1
13
3
6
12
11
15
18
17
16
9
5
20
19
0
20
40
60
80
100
120
140
m/z
Figure 3. (Continued).
100
90
80
70
60
50
40
30
20
10
0
PEG3
PEG7
PEG11
PEG40
+
Si+ SiCH3O+ C3H7O+ C2H3O+ C2H5O+ C3H5O+ C4H7O+ C4H9O2
Figure 4. Normalized abundances measured by TOF-SIMS for selected fragment ions for PEG3-, PEG7-, PEG11- and
PEG40-silane grafted on silicon. The error bars shown are standard deviations based on five measurements at different locations on
the sample surface.
deviations were calculated for binary mixtures of PEG3-,
PEG7- and PEG11-silane. The PEG40-silane was excluded to
limit the extensiveness of the work. The results are given
in Table 1 together with the mixture compositions for the
solutions used in the grafting procedure.
corresponding solution mixture values. The PEG3-silane
was clearly discriminated during the grafting procedure.
The PEG7/PEG11 systems behave differently. In contrast to
the PEG3 systems, discrimination in the grafting procedure
is observed here for the longer PEG-silane. The longer the
PEG chain, the less pronounced effect is expected for PEG-
silane mixtures of similar size. This is probably the reason
As is evident, the mixture compositions for the two
PEG3 systems are similar, with values greater than the
Copyright  2002 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2002; 37: 699–708

TOF-SIMS of Si-grafted monodisperse PEG-silanes
705
5
4
3
2
1
R = 0.997
C4H7O+
SiCH3O+
R = 0.9995
1.0
1.5
2.0
2.5
3.0
3.5
Ln number of PEG units
Figure 5. Calibration curves for determination of chain length (or molecular mass) of a grafted PEG-silane on silicon based on the
abundance of a PEG chain fragment ion (C4H7OC, logarithmic fit) or a silane interface fragment ion (SiCH3OC, linear fit). The error
bars shown are standard deviations based on five measurements at five different locations on the sample surface.
Table 1. PEG-silane mixture compositions in the grafting
solutions and the TOF-SIMS measured compositions of
grafted PEG-silane mixtures on silicon surfacesa
composition in the solution is plotted against the relative
abundances between C4H7OC and SiC for binary mixtures of
PEG3-, PEG7- and PEG11-silane. As is evident from Fig. 6,
the predicted trend is indeed observed.
One could argue that possible variations in the grafting
density could explain the phenomenon in Fig. 6. However,
performing the plot without including the substrate signal
produces the same qualitative plot, but with considerably
larger uncertainty. This rules out the possibility of variations
in grafting densities being the cause of the observed
phenomenon in Fig. 6.
Grafting
solution (% of
longer
Silicon
surface
Silicon
surface
Silicon
surface
PEG3/PEG7 PEG3/PEG11 PEG7/PEG11
PEG-silane)
(% PEG7)
(% PEG11)
(% PEG11)
20
50
80
52 š 2
80 š 2
81 š 4
47 š 4
72 š 4
93 š 2
3 š 2
19 š 14
60 š 15
It could also be argued that an unknown factor could
be the cause of the phenomenon with the PEG7/PEG11
mixtures and not the layer thickness. To verify that this is
not the case, the effective layer thicknesses were measured
with ellipsometry, which is based on a completely different
fundamental principle. The result is shown in Fig. 7, and
even though the data are somewhat scattered, it is reasonably
clear that the thickness values are located above the line for
the PEG3-systems and below the line for the PEG7/PEG11
system. This supports the qualitative TOF-SIMS effective
layer thickness results presented in Fig. 6.
Our results suggest that discrimination does take place
in the adsorption process. The discrimination is dependent
on what binary mixture system is considered, hence the
mechanism is not well understood. A possible explanation
for this observation could be that two or more competing
processes exist, e.g. discrimination in the adsorption process
and some other unknown process/mechanism that we have
not considered. It is highly unlikely that the unknown process
is the initially described formation of an umbrella-like layer
of longer chain PEG-silanes below which the shorter chain
PEG-silanes is situated. Since we are confident that the
grafting density is the same in all cases, the ‘umbrella’
effect would not have any influence on the layer thickness.
The ‘umbrella’ effect would probably have an influence on
the relative PEG fragment ion abundances in the TOF-SIMS
analysis, i.e. because of the limited probe depth of TOF-SIMS
a The standard deviations are the result of considering different
fragment ions for the quantification.
for the significantly greater standard deviations observed
for the PEG7/PEG11 systems compared with the PEG3
systems. We have found no rational explanation for why
the shorter PEG-silane is discriminated for two of the
systems and the longer PEG-silane is discriminated for one
system.
One could argue that our calculated mixture composi-
tions could be a result of discrimination in the desorption
process during the TOF-SIMS analysis. In order to confirm
that this is not the case, we made the following considera-
tions. A discrimination of the shorter PEG chain will increase
the mass per unit surface area (hereafter termed effective
layer thickness) and vice versa. Hence, for the PEG3-silane
mixtures the expected values for the effective layer thick-
ness versus the solution phase composition should lie above
a straight line between the layer thickness of pure PEG3-
silane and the pure PEG7-silane or PEG11-silane. For the
PEG7/PEG11 systems the expected values for the effective
layer thickness should, in contrast, lie below a straight line
between the layer thickness of pure PEG7-silane and the
pure PEG11-silane. The relative abundances between the
PEG fragment ions and the substrate ion, SiC, must be a
qualitative measure of the layer thickness. In Fig. 6, the
Copyright  2002 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2002; 37: 699–708

706
K. Norrman et al.
100
90
80
70
60
50
40
30
20
10
0
100%
PEG3
100%
PEG7
100%
PEG11
100%
PEG3
Composition in solution (%)
Figure 6. Normalized relative signal intensities for PEG–substrate measured by TOF-SIMS versus the composition (100, 80, 50, 20
and 0%) in the grafting solution. The dashed line represents the situation when there is no discrimination in the grafting process. The
error bars shown are standard deviations based on five measurements at five different locations on the sample surface.
10
9
8
7
6
5
4
3
2
1
100%
PEG3
100%
PEG7
100%
PEG11
100%
PEG3
Composition in solution (%)
Figure 7. Effective layer thicknesses measured by ellipsometry versus the composition (100, 80, 50, 20 and 0%) in the grafting
solution. The dashed line represents the situation when there is no discrimination in the grafting process. The error bars shown are
standard deviations based on three measurements at different locations on the sample surface.
with respect to larger molecular fragments. The PEG
fragment ions from the shorter PEG-chain would most likely
be discriminated. Since the qualitative layer thickness profile
in Fig. 6 (TOF-SIMS) is qualitatively consistent with the
quantitative layer thickness profile in Fig. 7 (ellipsometry),
it supports the fact that the ‘umbrella’ effect is either non-
existent or at most negligible. Furthermore, the ‘umbrella’
effect would produce inconsistent mixture compositions
when calculated from various PEG fragment ion abundances
relative to the corresponding SiC abundances. There is no
correlation between the size of the PEG fragment ions used
in the quantification and the small differences observed for
the calculated mixture compositions. This further supports
the contention that the ‘umbrella’ effect is either non-existent
or negligible.
CONCLUSIONS
Silicon-grafted monodisperse PEG-silanes with various PEG
chain lengths and mixtures of these were systematically
analyzed with TOF-SIMS. The mass spectra show differences
in the various relative signal intensities, an observation that
was used to elucidate important aspects of the grafting
process.
The signal intensity of the SiC ion decreases for longer
PEG-silane chains as a result of increased layer thickness.
The desorbed ions with the elemental composition SiCH3OC
follow the same qualitative trend as SiC. The signal intensities
of the PEG fragment ions increase relative to the SiC ion signal
intensities for larger PEG-silane chains. The C3H7OC ion is
seen to deviate from this trend. It is suggested that C3H7OC
Copyright  2002 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2002; 37: 699–708

TOF-SIMS of Si-grafted monodisperse PEG-silanes
707
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N.B.L., A.P and F.S.K. gratefully acknowledge financial support from
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