Fabrication of a nanoparticle gradient substrate by thermochemical manipulation of an ester functionalized SAM

Article abstract of DOI:10.1039/b712687e

The hydrolysis of methyl ester (-CO₂Me) and tert-butyl ester (-CO₂^tBu) functionalized SAMs as a function of subphase temperature and pH is described. Contact angle measurements show that the methyl ester functionalized monolayer does not hydrolyse in pH 1-13 aqueous solutions heated up to 80 °C. In contrast, the -CO₂^tBu functionalized monolayer hydrolysed below pH 5. The rate and the extent of the hydrolysis were dependent on the temperature and pH of the aqueous solution. Using the Cassie equation, the activation energy for the hydrolysis of CO ₂^tBu-phenyl functionalized SAM was determined as 75 ± 7 kJ mol^-1 from the contact angle measurements. Furthermore, the adhesion properties of -CO₂^tBu and -COOH functionalized SAMs were investigated by depositing -NR₂ and -COOH functionalized polystyrene nanoparticles onto the surfaces at pH 3 and 9. By AFM, it was observed that the particles bind preferentially to the -COOH functionalized SAM and the adhesion was pH dependent, with the largest coverage being observed at pH 3. Using the acquired understanding of the hydrolysis of -CO₂^tBu functionalized SAM and the particle adhesion properties, a simple and facile approach towards fabricating a particle density gradient on this surface is demonstrated. An acid gradient SAM (20 mm long) was prepared by mounting one end of a -CO₂^tBu functionalized SAM onto the hot side of a Peltier element (80 °C) in pH 1 aqueous solution. The substrate was subsequently immersed into a colloidal solution of -NR ₂ functionalized polystyrene nanoparticles, removed and rinsed. By AFM, the particle density was shown to be dependent on the surface coverage of -COOH moieties of the underlying SAM. The density started at 104 particles μm^-2 on the hydrolysed end down to 0 particles μm^-2 on the non-hydrolysed end. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712687e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Fabrication of a nanoparticle gradient substrate by thermochemical
manipulation of an ester functionalized SAM{
Parvez Iqbal,a Kevin Critchley,b James Bowen,a David Attwood,c David Tunnicliffe,c Stephen D. Evansd and
Jon A. Preece*a
Received 29th August 2007, Accepted 22nd October 2007
First published as an Advance Article on the web 14th November 2007
DOI: 10.1039/b712687e
t
The hydrolysis of methyl ester (–CO2Me) and tert-butyl ester (–CO2 Bu) functionalized SAMs as
a function of subphase temperature and pH is described. Contact angle measurements show that
the methyl ester functionalized monolayer does not hydrolyse in pH 1–13 aqueous solutions
t
heated up to 80 uC. In contrast, the –CO2 Bu functionalized monolayer hydrolysed below pH 5.
The rate and the extent of the hydrolysis were dependent on the temperature and pH of the
t
aqueous solution. Using the Cassie equation, the activation energy for the hydrolysis of CO2 Bu-
phenyl functionalized SAM was determined as 75 ¡ 7 kJ mol21 from the contact angle
t
measurements. Furthermore, the adhesion properties of –CO2 Bu and –COOH functionalized
SAMs were investigated by depositing –NR2 and –COOH functionalized polystyrene
nanoparticles onto the surfaces at pH 3 and 9. By AFM, it was observed that the particles bind
preferentially to the –COOH functionalized SAM and the adhesion was pH dependent, with the
largest coverage being observed at pH 3. Using the acquired understanding of the hydrolysis of
t
–CO2 Bu functionalized SAM and the particle adhesion properties, a simple and facile approach
towards fabricating a particle density gradient on this surface is demonstrated. An acid gradient
t
SAM (20 mm long) was prepared by mounting one end of a –CO2 Bu functionalized SAM onto
the hot side of a Peltier element (80 uC) in pH 1 aqueous solution. The substrate was subsequently
immersed into a colloidal solution of –NR2 functionalized polystyrene nanoparticles, removed
and rinsed. By AFM, the particle density was shown to be dependent on the surface coverage of
–COOH moieties of the underlying SAM. The density started at 104 particles mm22 on the
hydrolysed end down to 0 particles mm22 on the non-hydrolysed end.
lithographic techniques such as electron beam (e-beam),3–5
X-rays,6,7 UV/Vis8 irradiation and finally self-assembling
1 Introduction
The ability to immobilise single molecules, molecular aggre-
nanomaterials on to the modified surfaces.
gates or nanoparticles onto selective sites on surfaces of metals
Self-assembled monolayers (SAMs) provide a simple route
towards functionalising surfaces of metals,9 insulators10 and
and semiconductors is of major interest in nanotechnology
as it is seen as a possible route towards nanofabrication.1
The combination of top-down lithography processes with
bottom-up self-assembly processes termed ‘‘precision chemical
engineering’’2 is seen as a possible route towards nanofabrica-
tion. The approach involves the fabrication of ultrathin
films followed by chemical manipulation of the surfaces by
semiconductors.11 The terminal groups can be tailored to
provide control over the surface properties, and thus provide
the necessary control required over the reactions which take
place on the surface, hence allowing controlled fabrication of
three-dimensional structures. For instance, there have been
several groups who have used nitro functionalized SAMs as
building blocks towards fabricating three-dimensional struc-
tures, via selectively inducing chemical modification of the
nitro moieties to amine moieties by irradiating regions with an
e-beam,4,5 or X-rays7 and then performing coupling reactions,4
or by absorbing Au nanoparticles5 on the modified surfaces to
obtain three-dimensional structures.
aSchool of Chemistry, University of Birmingham, Edgbaston,
Birmingham, UK, B15 2TT E-mail: p.iqbal@bham.ac.uk;
j.bowen.1@bham.ac.uk; j.a.preece@bham.ac.uk;
Fax: +44 (0) 121 414 4403; Tel: +44 (0) 121 414 4426
Tel: +44 (0) 121 414 3528
bDepartment of Chemical Engineering, University of Michigan, Ann
Arbor, MI 48109, USA. E-mail: k.critchley@leeds.ac.uk;
Fax: +1 734 764 7453; Tel: +1 734 764 7487
Recently, chemical gradient SAMs, which exhibit a con-
tinuous spectrum of surface properties,12–16 have attracted
enormous interest, as such surfaces reduce the number of
substrates required to perform particle adhesion studies,
catalytic studies,12 combinatorial studies,13 protein attach-
ment,14,15 cell adhesion and cell mobility.16 Since Elwing et al.14
first introduced the concept, there have been a number of
approaches reported for fabricating such SAMs. These
methods either involve the controlled diffusion of surfactants
cMaterials Sciences Department, BAE SYSTEMS, Filton, Bristol, UK,
BS34 7QW
dDepartment of Physics and Astronomy, The University of Leeds, Leeds,
UK, LS2 9JT E-mail: s.d.evans@leeds.ac.uk; Fax: +44(0) 113 343 3900;
Tel: +44(0) 113 343 3850
{ Electronic supplementary information (ESI) available: Survey XPS
spectra; equations (1)–(5); second-order rate constants of the acid-
catalysed hydrolysis of MHBTE (I) SAM determined using the Cassie
equation. See DOI: 10.1039/b712687e
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onto a substrate,14,17,18 controlled immersion of a substrate
into a surfactant solution,19 applying an electrochemical
potential to a substrate during adsorption,12,20 using micro-
fluidic devices,21 chemical manipulation13,15,16,22–26 of ultra-
thin films by radio frequency plasma discharge,22 corona
discharge treatment,23 UV light25 and grafting.26 Recently,
Shosky and Scho¨nherr13 demonstrated a simple approach for
obtaining gradient SAMs by heating an aliphatic tert-butyl
ester functionalized SAM with the aid of a Peltier element,
which was used as a heat pump. One end of the substrate
(2.5 cm long) was mounted onto a Peltier element and heated
to the required temperature. A temperature gradient was
established along the substrate from the set temperature on the
hot end to room temperature on the cool end. When placed
in an acidic liquid cell, the extent of hydrolysis along the
substrate progressed from 100% hydrolysis at the hot end to
0% hydrolysis at the cool end. The approach was developed
to determine the kinetic studies for chemical reactions on
ultrathin films.
In this paper, a novel, simple and facile approach for
fabricating nanoparticle gradient SAMs is described. Using
the approach described by Shosky and Scho¨nherr13 an acid
gradient SAM is produced by hydrolysing a tert-butyl ester
functionalized SAM with the aid of a Peltier element followed
by deposition of the particles (Fig. 1). The –COOH moieties
provide the active sites for the binding of the particles, as it has
previously been demonstrated that –COOH functionalized
SAMs are amenable to particle adhesion,27 as well as protein
attachment28 and cell adhesion.29 Such particle gradient
surfaces provide the opportunity to study arrays of surface
Fig. 1 Schematic representation of the approach used for the fabrication of the nanoparticle gradient substrate: (a) hydrolysis of ester
functionalized monolayer (–CO2R) using a Peltier element, (b) acid gradient SAM prepared, (c) substrate immersed in colloidal solution, and (d)
nanoparticle gradient.
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bulky ester functionalized SAMs such as –CO2 Bu13,30 and
t
hydroxysuccinimide ester,31 to our knowledge the hydrolysis
behaviour of the smaller esters such as methyl ester (–CO2Me)
has not been reported. As the esters are less bulky the
packing in the monolayer will be different compared to
the bulkier ester moieties, and also the mechanisms for the
hydrolysis of the ester moieties are different, which may
influence the hydrolysis behaviour in the SAM, where the
molecules are confined in a quasi-crystalline structure.32,33
Thus, in this paper the hydrolysis behaviour of a –CO2Me
functionalized SAM prepared by 4-(6-mercapto-hexyloxy)
benzoic acid methyl ester (MHBME, II) (Fig. 2) is studied
Fig. 2 Molecular structures of the molecules used in the investigation.
t
and compared to the –CO2 Bu functionalized (MHBTE, I)
SAM as a function of pH and temperature. Finally, to
optimise the adhesion of particles on the acid gradient SAM,
the attachment of tertiary amine functionalized and acid
functionalized polystyrene nanoparticles on both MHBTE (I)
and acid functionalized SAM 4-(6-mercaptohexyloxy) benzoic
acid (MHBA, III) SAMs (Fig. 2) in acidic and basic colloidal
solutions is investigated.
properties such as optoelectronics and catalysis as a function
of particle density on a single substrate, which would provide
valuable information for preparing future nanodevices that
contain nanoparticles. Previously, there have been only a few
examples reported where gradient nanoparticle substrates
have been prepared. Such substrates were prepared either by
immersion of a Si/SiO2 substrate into a colloidal solution
of nanoparticles as a function of immersion time19 or by
adsorbing citrate Au nanoparticles on a gradient (3-amino-
propyl)triethoxysilane (APTES) SAM on a silica substrate,
where the gradient SAM was prepared by vapour deposition of
APTES on a Si/SiO2 substrate as a function of exposure time
along the substrate.17 There are two major differences in our
approach compared to previously reported approaches. Firstly
the particle gradient is fabricated on a Au substrate, and
secondly it is fabricated on a complete monolayer containing
two functional moieties at each terminus of the SAM, hence
providing the possibility to further chemically manipulate the
ester moieties for constructing additional three-dimensional
structures. Furthermore, to optimise the conditions required
2 Results and discussion
2.1 Synthesis of the arylthiols (I–III)
The thiols (I–III) were synthesised from commercially avail-
able 4-hydroxybenzoic acid (Scheme 1). The first step was the
esterification of 4-hydroxybenzoic acid (1) to the correspond-
ing esters (2a and 2b). 2a was obtained by DCC coupling with
tBuOH in THF,34 whereas 4-hydroxy-1-methyl benzoate (2b)
was obtained under methanolic acidic (H2SO4) conditions.
The phenol derivatives (2a and 2b) were alkylated with
1,6-dibromohexane in the presence of a base (K2CO3) to
obtain 3a and 3b.35 Finally, the thiol moiety was introduced
through the reaction of 3a and 3b with thiourea to form the
intermediate isothiouronium salt, which was cleaved under
basic conditions (NaOH) to yield the thiols (I–II).36 The acid
(III) was formed by addition of two more equivalents of
aqueous NaOH to II.
t
for the hydrolysis of –CO2 Bu functionalized SAMs prepared
from 4-(6-mercapto-hexyloxy) benzoic acid tert-butyl ester
t
(MHBTE, I) (Fig. 2), the hydrolysis of –CO2 Bu as a function
of pH and temperature is investigated. Although there
have been a number of papers reporting the hydrolysis of
Scheme 1 Synthesis of the aryl based thiols.
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Table 1 Wetting properties (water contact angles) and monolayer thickness
Contact angle measurements
Observed
Literature
SAM
ha/u
hr/u
ha/u
SAM thicknessa/nm
Molecular lengthb/nm
MHBTE (I)
MHBME (II)
MHBA (III)
a
89 ¡ 1
64 ¡ 1
10 ¡ 3
78 ¡ 1
48 ¡ 1
0
8930
6537
y10u
1.31 ¡ 0.04
1.60 ¡ 0.12
1.40 ¡ 0.10
1.79
1.64
1.45
37–39
b
The SAM thickness obtained from ellipsometry The molecular length obtained from Chem 3D software
2.2 Preparation of the SAMs
XPS survey spectra for MHBTE (I), MHBME (II) and MHBA
(III) SAMs show the presence of peaks at binding energies 284,
400, 581 and 162 eV, which are indicative of the presence of C,
N, O and S, respectively (the XPS survey spectra are shown
in ESI,{ Fig. S1).
The SAMs were formed by immersion of the Au substrates in
1 mM solutions of I–III in EtOH (HPLC grade) for 24 h. After
24 h the Au substrates were thoroughly rinsed with EtOH
(HPLC grade) and dried with a stream of N2.
2.4 pH and temperature dependence of the hydrolysis
2.3 SAM characterisation
2.4.1 pH dependence of the hydrolysis. The hydrolysis of
MHBTE (I) and MHBME (II) SAMs was studied over a pH
range of 1–13. The SAMs were immersed in pH 1–13 aqueous
solutions for 5 h at 20 uC and 80 uC. At 20 uC, neither ester
showed any significant change in ha and hr over the pH range
(Fig. 3a and b), suggesting little or no hydrolysis had taken
The SAMs were characterised by water contact angle
measurements, X-ray photoelectron spectroscopy (XPS) and
ellipsometry. As expected the MHBME (II) SAM is more
hydrophilic than MHBTE (I), which is presumably due to the
bulkier tert-butyl moiety screening the influence of the polar
ester from the monolayer surface. The monolayer formed
from MHBA (III) is extremely hydrophilic (advancing angle ha
10 ¡ 2u and receding angle hr y 0u) and exhibits similar
contact angles to literature values.37–39 The monolayer thick-
nesses obtained for the SAMs are smaller than the molecular
length of the molecules as shown in Table 1. The discrepancy
between the SAM thicknesses and the molecular lengths
suggests that the monolayers are tilted, which is typically
found for alkanethiol molecules adsorbed onto gold.40 The
t
place. At the elevated temperature of 80 uC, for the –CO2 Bu
functionalized SAM (Fig. 3c) it can be seen from pH 13 down
to pH 5 there is no change in the contact angle, but below pH 5
the contact angle drops as a function of pH down to ha 15u and
hr y 0u at pH 1. These lower contact angles are characteristic
for –COOH functionalized monolayers, and are similar to the
contact angle observed for MHBA (III). Thus, the hydrolysis
of the ester is near to complete at pH 1. It should be noted that
Fig. 3 Contact angle measurements taken after immersing (a) MHBTE (I) SAM at room temperature, (b) MHBME (II) SAM at room
temperature, (c) MHBTE (I) SAM at 80 uC and (d) MHBME (II) SAM at 80 uC in aqueous solution with pH ranging from 1–13 for 5 h.
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alkanethiol-Au SAMs are known to desorb at elevated
temperatures,41 and therefore in order to ensure that the
change in wetting behaviour was due to a chemical change and
not to desorption, the thickness of the modified SAM was
determined by ellipsometry and the chemical composition was
determined by XPS. The monolayer thickness was determined
as 1.58 ¡ 0.10 nm, which is comparable to the MHBA (III)
SAM thickness (1.40 ¡ 0.10 nm). Furthermore, the survey
spectra for the hydrolysed MHBTE (I) SAM reveal the
presence of C (1s), S (2p) and O (1s) (see ESI,{ Fig. S2). Hence,
the change in the wetting properties of the surface is due
to the hydrolysis of the ester. In the case of the –CO2Me
functionalized SAM there is no change in the contact angle
(Fig. 3d), suggesting no hydrolysis had taken place in either
acidic or basic medium.
demonstrated that the extent of the hydrolysis on MHBTE (I)
SAM can be controlled by pH and temperature.
2.5 Partial hydrolysis of MHBTE (I) SAM using the Peltier
element
2.5.1 Determination of the temperature gradient. The experi-
mental set-up for the preparation of the gradient SAM is
shown in Fig. 5. One end of the Au substrate was mounted
onto the Peltier element (5 mm) with the setpoint (0 mm) being
2 mm from the edge which was mounted on the Peltier
element. The temperatures along the substrate were taken at
increments of 2 mm beginning from the setpoint using a
thermocouple that was attached to a thermocouple module.
The hot end of the substrate was heated to 80 uC, because
preliminary studies showed complete hydrolysis of the
To summarise, at 80 uC, hydrolysis only takes place on the
MHBTE (I) SAM in acidic conditions (pH , 5), whereas no
hydrolysis was observed on the MHBME (II) SAM under
either acidic or basic conditions. One can postulate that the
lack of reactivity of the –CO2Me functionalized SAM relative
t
–CO2 Bu functionalized SAM was obtained in pH 1 solution
heated at 80 uC for 5 h. The temperature gradient along the
substrate is shown in Fig. 6, which also illustrates that the
gradient is stable over 5 h.
t
to the –CO2 Bu functionalized SAM is due to the different
2.5.2 Determining the extent of the hydrolysis on the MHBTE
(I) SAM. All the contact angles reported from now are static
contact angles. The contact angles were determined as static
contact angles rather than dynamic contact angles to eliminate
the risk of the water droplet expanding onto the next
measuring point. The static contact angle for MHBTE (I)
SAM was determined as 88 ¡ 1u and MHBA (III) SAM it was
determined as 10 ¡ 2u.
types of mechanism that are in operation. The hydrolysis of
CO2Me esters requires carbonyl oxygen protonation, followed
by attack of water on the electrophilic carbonyl carbon atom
(AAC1 mechanism).32,33 However, the hydrolysis of CO2 Bu
t
esters does not require the attack of a nucleophile water, as a
stable tert-butyl cation can be lost (AAL1 mechanism) which
subsequently loses H+ and forms gaseous isopropene.32,33
Thus, one can envisage that in the quasi-crystalline SAM it is
difficult for nucleophilic water to penetrate the SAM, and
thus, the CO2Me ester SAM resists hydrolysis.
The change in the contact angles along the substrate after
one end of the MHBTE (I) has been heated to 80 uC by a
Peltier element in a cell containing pH 1 aqueous solution for
2.4.2 Temperature dependence of the ester hydrolysis. It has
already been demonstrated that the hydrolysis of MHBTE (I)
in pH
1 aqueous solution is temperature sensitive, as
hydrolysis was achieved at 80 uC and no hydrolysis was
noticeable at 20 uC. However, for the proposed preparation
of an acid gradient SAM, we require the hydrolysis to be
progressive between temperatures 20–80 uC, therefore, the
hydrolysis of MHBTE (I) SAM over four temperatures (35, 44,
50 and 65 uC) in pH 1 aqueous solution was investigated
(Fig. 4). It was observed that the hydrolysis is progressive over
the temperature range 35–80 uC and contact angle measure-
ments show no evidence of hydrolysis taking place below
35 uC. From the pH and temperature studies it has been
Fig. 4 The hydrolysis of MHBTE (I) SAM as
a
function of
Fig. 5 Schematic representation of the experimental setup used for
temperature.
preparing the acid gradient SAM.
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In addition, at the setpoint (0 mm) the static contact angle is
32 ¡ 2u, which suggests that the hydrolysis is not complete.
2.5.3 Determination of the activation energy (Ea).
Bimolecular reactions are defined as second order reactions
with the rate of reaction dependent on the concentration of the
two reactants and the second order rate constant. In the case of
the acid-catalysed hydrolysis reaction, the rate of reaction is
dependent on the concentration of H+ ([H+]) and MHBTE (I)
([MHBTE]) as shown in ESI{ equation (1). The second order
rate constants (k) obey the Arrhenius equation (see ESI{
equation (2)).11,42 If k is known for a number of temperatures,
Ea for the reaction can be determined. According to previously
reported studies on the hydrolysis of ester terminated SAMs,
the hydrolysis is shown to follow a pseudo-first-order reaction.
If [H+] is much larger than [MHBTE], [H+] can be regarded
as a constant and hence k[H+] y k9 (pseudo-first-order rate
constant), hence leaving the rate only dependent on [MHBTE]
(see ESI{ equation (3)). k9 can be determined by integrating
equation (3) (see ESI{ equation (4)) from which k can be
obtained.
Fig. 6 The temperature gradient along the MHBTE (I) SAM between
the hot end and cold end of the substrate over 300 min.
30, 60, 180 and 300 min is shown in Fig. 7a. The hydrolysis is
progressive from 0 to 10 mm. After 10 mm the contact angles
plateau with the static contact angles measured as 88–89u.
These data are in good agreement with the contact angle
observed from the temperature study where no change in ha
was seen when hydrolysing the MHBTE (I) SAM at 35 uC.
Using the Cassie equation (see ESI{ equation (5)) the extent
of the hydrolysis can be determined from the contact angle.43
Fig. 7 (a) Static contact angle measurements along the substrate after hydrolysis of the MHBTE (I) SAM in pH 1 aqueous solution at 80 uC for
30, 60, 180 and 300 min, (b) xtbu along the substrate determined from the static contact angle measurements using the Cassie equation, (c) the linear
relationship between ln(xtbu) and the reaction time (the lines are least-square fits) and (d) the linear relationship between lnk9 versus the inverse of
the temperature.
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The static contact angles along the substrate after reaction
times 30, 60, 180 and 300 min are shown in Fig. 7a. It can be
observed that the static contact angle measurements show that
there are only significant changes in the contact angle along
the first 8 mm of the SAM, after which point there is no sign of
hydrolysis taking place after 30, 60, 180 and 300 min. At the
higher temperatures the contact angle decreases more rapidly,
which indicates a faster rate of reaction. The surface coverage
t
for the –CO2 Bu functionalized monolayer (xtbu) along the
substrate during the hydrolysis determined from the Cassie
equation is shown in Fig. 7b.
Fig. 7c shows the natural logarithm of xtbu plotted against
reaction time; the linear relationship between the variables
reaffirms that the hydrolysis reaction of MHBTE (I) in the
conditions investigated is pseudo-first order. The slopes are
least square fits that have R2 values in the range of 0.98–1.
From the determined k9 for the five temperatures investigated,
the k values were determined by multiplying the values with
[H+], which was 0.1 M. The k values are shown in ESI{ Table
S1. As expected, k increases with increasing temperature.
The second-order rate constants obtained from Fig. 7c are
linearised according to the Arrhenius equation as shown in
Fig. 7d. From the gradient of the plot in Fig. 7d, Ea for the
hydrolysis of MHBTE (I) in aqueous pH 1 solution was
determined as 75 ¡ 7 kJ mol21. Previously, Ea for the acid
Fig. 8 Tapping mode AFM topography images of SAMs, after
immersion in colloidal solution of COOH-PL nanoparticles: (a) on the
MHBTE (I) SAM at pH 3, (b) on the MHBTE (I) SAM at pH 9, (c) on
the MHBA (III) SAM at pH 3 and (d) on the MHBA (III) SAM at
pH 9.
for 2 h, followed by rinsing with UHQ H2O and drying
with N2. The surfaces were then analysed by AFM (an area of
5 6 5 mm2 was scanned).
t
catalysed hydrolysis of –CO2 Bu functionalized SAM deter-
mined by the use of the Cassie equation was reported as
28 kJ mol21 13
However, this was for an aliphatic tert-butyl
2.6.1 Attachment of COOH-PL nanoparticles onto the
SAMs. The particles were shown to selectively bind to the
MHBA (III) SAM at pH 3 solution relative to MHBA (III)
SAM at pH 9 and to MHBTE (I) SAM at both pHs as shown
in Fig. 8a–d. The particle density was determined by visually
counting the number of particles on the AFM images.45 The
particle density on MHBA (III) SAM at pH 3 was observed as
29 particles mm22 (Table 2), whereas at pH 9 (COO2 func-
tionalized SAM) the particle density was 0 particles mm22. The
particle density on MHBTE (I) SAM at pH 3 is much lower at
1.9 particles mm22 and at pH 9 is 0 particles mm22. The
discrepancy in the particle density on both substrates and at
the different pHs can be rationalised by considering the
intermolecular interactions between the functionalized surface
and the nanoparticles. The –COOH functionalized SAM and
the COOH-PL have complementary groups which can bind
.
ester and not an aromatic tert-butyl ester. As the initial step in
t
the RCO2 Bu ester hydrolysis is the protonation of the
carbonyl moiety, we postulate that the difference between
the Ea values may be due to the difference in the pKa values of
t
the protonation of the respective esters. The pKa of CO2 Bu-
aliphatic is reported as 26.5,44 whereas the pKa of the
t
protonation of CO2 Bu-phenyl is reported as 27.4.44 Hence,
t
the protonation of CO2 Bu-phenyl is more difficult. Thus, it
t
would be expected that the Ea value of CO2 Bu-phenyl would
t
be larger than that for the CO2 Bu-aliphatic functionalized
SAM, as indeed is found to be the case.
2.6 Adhesion of functionalized nanoparticles on MHBTE (I) and
MHBA (III) SAMs
t
To optimise the particle adhesion on the acid gradient SAM,
two types of nanoparticles were deposited on the MHBTE (I)
and MHBA (III) SAMs at pH 3 and 9 using unbuffered
aqueous solutions containing:
through hydrogen bonding at low pH, whereas the –CO2 Bu
moiety is a relatively hydrophobic group, and thus has less
affinity for the –COOH/COO2 moiety on the nanoparticles.
Therefore, it would be expected that the COOH-PL nanopar-
ticles would preferentially bind to the –COOH functionalized
(i) –COOH functionalized polystyrene latex (COOH-PL)
nanoparticles (diameter: 40 nm);
(ii) –NR2 functionalized polystyrene latex (NR2-PL) nano-
particles (diameter: 60 nm).
Table 2 Particle density on the MHBTE (I) and MHBA (III) SAMs
after being immersed in colloidal solution of NR2-PL nanoparticles at
pH 3 and 9
These nanoparticles were chosen to utilise electrostatic
interactions between the –COOH/COO2 functionalized sur-
face and the nanoparticles. Particle attachment was studied at
two different pH values (3 and 9), so that the protonated as
well as the deprotonated surfaces (i.e. –COOH and –COO2,
respectively) could be investigated.
Particle density/particles mm22
SAM
pH
COOH-PL
NR2-PL
–COOH
3
9
3
9
29.0
0
1.9
0
104.0
1.1
0.3
t
–CO2 Bu
The particles were deposited onto the SAMs by immersing
the SAMs in an aqueous solution of the particles at pH 3 and 9
0.1
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t
surface over the –CO2 Bu functionalized surface at low pH.
pKa of tertiary amine is reported to be between 10.6–10.7 when
in free solution.49,50 However, the pKa of primary amines at
the periphery of SAMs has been reported as 6,51 which is lower
than the pKa of primary amines in free solution. Thus, the pKa
of the NR2-PL nanoparticles could be lower than expected.
Presuming the pKa for the tertiary amine on the nanoparticles
is between 6–10, most of the particles are not protonated, and
therefore there is no complementary interaction between the
SAM and the NR2-PL nanoparticles.
The selective binding of the COOH-PL to the –COOH func-
tionalized surface from pH 3 solution compared to pH 9 solu-
tion can be explained by referring to the pKa of the –COOH
moiety. In free solution the pKa of a carboxylic acid is 4–5,46,47
however, numerous studies on determining the pKa of –COOH
moieties at the terminus of SAMs have reported it to be higher
than in free solution (pKa 5–8).48 Hence, the pKa of the MHBA
(III) SAM lies most probably between 4–8. Thus, at pH 3 both
the monolayer and the COOH-PL particles will be near to
fully protonated (COOH), and thus, the particles can bind to
the surface through hydrogen bonding, whereas at pH 9 the
COOH moieties in the monolayer and the particles will be near
to fully deprotonated. The anionic charges will repel each
other, and thus the particles will not attach at pH 9.
2.6.3 Summary of the attachment of nanoparticles on the
SAMs. Both COOH-PL and NR2-PL nanoparticles exhibit
stronger affinities for the –COOH functionalized monolayer
t
compared to the –CO2 Bu functionalized monolayer at pH 3.
At pH 9, there are virtually no particles attached to either
surface. In addition, it has been shown that the NR2-PL
nanoparticles have
a
stronger affinity for the –COOH
in comparison with
2.6.2 Attachment of NR2-PL nanoparticles onto the SAMs.
Similar behaviour is observed for the NR2-PL nanoparticles.
They show a preference for attachment on the MHBA (III)
SAM at pH 3 relative to MHBA (III) SAM at pH 9 and
MHBTE (I) SAM at both pHs as shown in Fig. 9a–d. The
particle density on the –COOH functionalized surface at pH 3
is observed as 104 particles mm22 (Table 2), whereas the
particle density at pH 9 is 1.1 particles mm22. The particle
functionalized monolayer at pH
3
COOH-PL nanoparticles, which is reflected in the particle
densities on the respective substrates which are observed as 104
and 29 particles mm22, respectively.
2.6.4 NR2-PL attachment onto the acid gradient SAM. After
the gradient hydrolysis of the MHBTE (I) SAM using the
Peltier element, the substrate was immediately immersed into
NR2-PL solution at pH 3 for 2 h, followed by rinsing with
UHQ H2O, dried with a stream of N2 and stored in sealed
sample holders until the samples were analysed. NR2-PL
nanoparticles were chosen as these particles had the largest
difference in particle attachment between the –COOH and
t
densities on the –CO2 Bu functionalized surface at pH 3 and 9
were 0.3 particles mm22 and 0.1 particles mm22, respectively.
We postulate that the selective binding to the –COOH over
t
–CO2 Bu functionalized SAMs at pH 3 is due to the NR2-PL
particle being in the protonated form and thus binding to the
–COOH functionalized SAM through hydrogen bonding,
t
whereas on the –CO2 Bu functionalized surface it cannot
t
–CO2 Bu functionalized surfaces. AFM images taken along the
substrate were taken within 24 h of being removed from the
particle solution to minimise contaminants adsorbing on
the surfaces. From the images (Fig. 10a) it is evident that the
particle density along the substrate gradually decreases to
zero. Fig. 10b depicts the average particle density along the
substrate determined from the AFM images. The particle
density decreases exponentially from 64 particles mm22 at
0 mm to 0 particles mm22 at 12 mm from the setpoint, and this
correlates with the fractional surface coverage of –COOH
moieties on the underlying SAM.
hydrogen bond. The preference for NR2-PL to bind at pH 3
over pH 9 could also be explained by referring to the pKa
values of the substrate and the particles. The monolayer as
described earlier will be nearly fully deprotonated at pH 9. The
3 Experimental
3.1 Chemicals
Commercially available chemicals were purchased from
Aldrich, Acros or Lancaster and used as received. The
functionalized polystyrene nanoparticles were purchased from
Bangs Labs, USA. The different pH solutions were prepared
either by the addition of HCl (0.5 M) to obtain the aqueous
solutions with pH in the range 1–5 or by the addition of NaOH
(0.5 M) to obtain aqueous solutions with pH in the range 6–13
to UHQ H2O. pH was measured using a thin stem stainless
pH reference probe (ISFET electrode), which was two point
calibrated with phosphate buffers. For measuring aqueous
solutions in the pH range 1–7, phosphate buffers pH 4.0 and
7.0 were used for the two point calibration of the pH probe
and for measuring the higher pHs (8–13), phosphate buffers
pH 7.0 and pH 9.18 were used. Thin-layer chromatography
Fig. 9 Tapping mode AFM topography images of the SAMs after
immersion in aqueous solution of NR2-PL nanoparticles: (a) on the
MHBTE (I) SAM at pH 3, (b) on the MHBTE (I) SAM at pH 9, (c) on
the MHBA (III) SAM at pH 3 and (d) on the MHBA (III) SAM at
pH 9.
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Fig. 10 (a) Tapping mode AFM topography images taken along the modified substrate after being immersed in colloidal solution of NR2-PL
solution at pH 3 and (b) the average particle density along the acid gradient substrate determined from the AFM images.
(TLC) was carried out on aluminium plates coated with silica
gel 60 F254 (Merck 5554). For the aryl-based compounds the
TLC plates were air-dried and analysed under a short wave
UV lamp (254 nm), whereas for the aliphatic compound the
TLC plates were air dried and developed in an I2 chamber.
Column chromatographic separations were performed on
118–120 uC; nmax/cm21 (Nujol): 3302 (OH), 2923 (CH), 2854
(CH), 1678 (CLO), 1608 (benzene ring), 1590 (benzene ring);
dH(300 MHz; CDCl3; Me4Si) 7.89 (2 H, d, J = 8.64 Hz, ArH),
6.85 (2 H, d, J = 8.64 Hz, ArH), 6.18 (1 H, br s, ArOH), 1.58
(9 H, s, C(CH3)3); dC(75 MHz; CDCl3; Me4Si) 169.0, 161.8,
133.4, 125.2, 116.9, 83.1, 30.0; m/z (ESMS): 217 ([M + Na]+,
100%); HRMS: found 217.0846; calc. mass for C11H14O3Na:
217.0841.
˚
silica gel 120 (ICN Chrom 32–63, 60 A).
3.2 Synthesis of the aryl-based thiols
4-Hydroxybenzoic acid methyl ester (2b). A solution of
4-hydroxybenzoic acid (1) (5.00 g, 36.22 mmol) and conc.
H2SO4 (0.5 ml) in MeOH (100 ml) was heated under reflux for
20 h. The reaction mixture was allowed to cool to room
temperature and concentrated in vacuo (20 ml). Water (100 ml)
was added and the aqueous layer was extracted with Et2O
(3 6 75 ml). The combined organic layers were washed with
brine (50 ml), dried (MgSO4) and filtered. The solvent was
removed in vacuo to afford a white solid (5.08 g, 92%). Mp:
118–120 uC; nmax/cm21 (Nujol): 3281 (OH), 2926 (CH), 2854
(CH), 1681 (CLO), 1608 (benzene ring), 1587 (benzene ring);
dH(300 MHz; CD3OD; Me4Si) 7.89 (2 H, d, J = 8.64 Hz,
ArH), 6.84 (2 H, d, J = 8.64 Hz, ArH), 3.84 (3 H, s, OCH3);
dC(75 MHz; CD3OD; Me4Si) 169.0, 163.1, 133.3, 123.1, 116.7,
52.8; m/z (ESMS): 175 ([M + Na]+, 100%).
4-Hydroxybenzoic acid tert-butyl ester (2a). To a solution of
4-hydroxybenzoic acid (1) (5.00 g, 36.22 mmol), 4-DMAP
(0.17 g, 1.40 mmol) and tert-butanol (100 ml) in dry THF
(150 ml) under N2 atmosphere, a solution of DCC in dry THF
(50 ml) was added dropwise at room temperature for 30 min.
The reaction mixture was stirred at room temperature under
N2 atmosphere for 20 h. The residue mixture was filtered and
the filtrate was concentrated in vacuo (y20 ml). The filtrate
was washed with 0.3 M Na2CO3 solution (3 6 40 ml), dried
(MgSO4), filtered and concentrated in vacuo. The pale yellow
crude product was purified by flash silica gel chromatography
(graded elution: 0 to 30% EtOAc in hexane, increase polarity
in increments of 5% per 100 ml of eluent used). The solvent
was removed in vacuo to yield a white solid (6.68 g, 95%). Mp:
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1-(6-Bromohexyloxy)benzoic acid tert-butyl ester (3a). A
suspension of K2CO3 (5.45 g, 39.49 mmol) in a solution of 2a
(3.83 g, 19.74 mmol), 1,6-dibromohexane (9.63 g, 39.47 mmol)
in MeCN (150 ml) was heated under reflux with a CaCl2 guard
for 20 h. The reaction mixture was allowed to cool to room
temperature and concentrated in vacuo (20 ml). Water (100 ml)
was added and the aqueous layer was extracted by washing
with EtOAc (3 6 50 ml). The combined organic layers were
washed with brine (20 ml), dried (MgSO4), filtered and solvent
removed in vacuo, yielding a white solid as the crude product.
The solid was absorbed onto silica and purified via silica gel
column chromatography (graded elution: 0 to 30% EtOAc in
hexane, increase polarity in increments of 5% per 100 ml of
eluent used). The solvent was removed in vacuo to yield a white
solid (5.36 g, 76%). Mp: 45–46 uC; nmax/cm21 (film): 2924
(CH), 2854 (CH), 1702 (CLO), 1605 (benzene ring), 1511
(benzene ring); dH(300 MHz; CDCl3; Me4Si) 7.91 (2 H, d, J =
8.82 Hz, ArH), 6.86 (2 H, d, J = 8.82 Hz, ArH), 3.99 (2 H, t,
J = 6.25 Hz, OCH2C5H10Br), 3.41 (2 H, t, J = 6.44 Hz,
OC5H10CH2Br), 1.90–1.78 (4 H, m, OCH2CH2C2H4CH2-
CH2Br), 1.58 (9 H, s, C(CH3)3), 1.54–1.50 (4 H, m,
OC2H4C2H4C2H4Br); dC(75 MHz; CDCl3; Me4Si) 167.7,
164.4, 133.9, 126.5, 116.4, 83.0, 70.4, 36.3, 35.2, 31.5, 30.8,
30.4, 27.8; m/z (EIMS): 379 ([M]+, 100%), 381 ([M+], 80%);
HRMS: found 379.0892; calc. mass for C17H25O3BrNa:
379.0885.
in increments of 5% per 100 ml of eluent used). This afforded a
white solid (0.72 g, 78%). Mp: 42–43 uC; elemental analysis
found: C, 65.82%; H, 8.55%. Calc. for C17H26O3S: C, 65.77%;
H, 8.44%; nmax/cm21 (film): 2925 (CH), 2854 (CH), 1705
(CLO), 1606 (benzene ring), 1512 (benzene ring); dH(300 MHz;
CDCl3; Me4Si) 7.91 (2 H, d, J = 8.83 Hz, ArH), 6.86 (2 H, d,
J = 8.83 Hz, ArH), 3.99 (2 H, t, J = 6.44 Hz, OCH2C5H10SH),
2.54 (q, J = 7.72 Hz, 2H, OC5H10CH2SH), 1.81–1.77 (2 H, m,
OCH2CH2C4H8SH), 1.67–1.60 (2 H, m, OC4H8CH2CH2SH),
1.57 (9 H, s, C(CH3)3), 1.48–1.45 (4 H, m, OC2H4C2H4C2H4-
SH), 1.34 (1 H, t, J = 7.43 Hz, SH); dC(75 MHz, CDCl3;
Me4Si) 163.9, 160.0, 129.5, 122.5, 112.0, 78.6, 66.1, 32.0, 27.1,
26.4, 26.2, 23.6, 22.6; m/z (ESMS): 333 ([M + Na]+, 100%); m/z
(HRMS): found 310.1612; calc. mass for C17H26O3S: 310.1609.
4-(6-Mercaptohexyloxy)benzoic acid methyl ester (MHBME
(II)). The procedure described for the synthesis of compound
MHBTE (I) was followed, however using compound 3b (1.00 g,
3.18 mmol), thiourea (0.27 g, 3.55 mmol) and 5 M NaOH
(0.64 ml, 3.18 mmol) in EtOH (40 ml). This afforded a white
solid (0.61 g, 73%). Mp: 45–46 uC; elemental analysis found: C,
62.64%; H, 7.50%. Calc. for C14H20O3S: C, 62.66%; H, 7.51%;
n
max/cm21 (Nujol): 2922 (CH), 2854 (CH), 1724 (CLO), 1608
(benzene ring), 1510 (benzene ring); dH(300 MHz; CDCl3;
Me4Si) 8.19 (2 H, d, J = 9.20 Hz, ArH), 6.93 (2 H, d, J =
9.20 Hz, ArH), 4.04 (2 H, t, J = 6.44 Hz, OCH2C5H10SH),
3.87 (3 H, s, OCH3), 2.54 (2 H, q,
J = 7.72 Hz,
4-(6-Bromohexyloxy)benzoic acid methyl ester (3b). The
procedure described for the synthesis of compound 3a was
followed, however using compound 2b (3.00 g, 19.74 mmol),
1,6-dibromohexane (9.63 g, 39.47 mmol) and K2CO3 (5.45g,
39.49 mmol) in MeCN (150 ml). This afforded a white solid
(4.76 g, 77%). Mp: 50–51 uC; nmax/cm21 (Nujol): 2925 (CH),
2858 (CH), 1723 (CLO), 1608 (benzene ring), 1510 (benzene
ring); dH(300 MHz; CDCl3; Me4Si) 7.97 (2 H, d, J = 8.64 Hz,
ArH), 6.89 (2 H, d, J = 8.64 Hz, ArH), 4.00 (2 H, t, J = 6.44 Hz,
OCH2C5H10Br), 3.87 (3 H, s, OCH3), 3.42 (2 H, t, J = 6.78 Hz,
OC5H10CH2Br), 1.91–1.79 (4 H, m, OCH2CH2C2H4-
CH2CH2Br), 1.52–1.49 (4 H, m, OC2H4C2H4C2H4Br);
dC(75 MHz; CDCl3; Me4Si) 167.0, 162.6, 132.6, 123.2, 115.0,
68.9, 52.8, 34.7, 33.6, 29.9, 28.9, 26.2; m/z (ESMS): 339 ([M +
Na]+, 97%), 337 ([M + Na]+, 100%); m/z HRMS: found
377.0430; calc. mass for C14H19O3NaBr: 337.0415.
OC5H10CH2SH), 1.85–1.80 (2 H, m, OCH2CH2C4H8SH),
1.67–1.63 (2 H, m, OC4H8CH2CH2SH), 1.49–1.47 (4 H, m,
OC2H4C2H4C2H4SH), 1.34 (1 H, t, J = 7.72 Hz, SH);
dC(75 MHz; CDCl3; Me4Si) 166.0, 161.7, 130.4, 121.2, 112.9,
66.8, 50.6, 32.7, 27.8, 26.9, 24.3, 23.3; m/z (ESMS): 291 ([M +
Na]+, 100%); m/z (HRMS): found 291.1027; calc. mass for
C14H20O3SNa: 291.1031.
4-(6-Mercaptohexyloxy)benzoic acid (MHBA (III)). The
procedure described for the synthesis of compound MHBTE
(I) was followed, using compound 3b (1.00 g, 3.18 mmol),
thiourea (0.27 g, 3.55 mmol) and 5 M NaOH (1.54 ml,
9.54 mmol) in EtOH (40 ml). This afforded a white solid
(0.59 g, 73%). Mp: 112–114 uC; elemental analysis found: C,
61.51%; H, 6.93%. Calc. for C13H18O3S: C, 61.39%; H, 7.13%;
n
max/cm21 (Nujol): 2924 (CH), 2853 (CH), 1674 (CLO), 1607
(benzene ring), 1514 (benzene ring); dH(300 MHz; CDCl3:
Me4Si) 8.04 (2 H, d, J = 8.64 Hz, ArH), 6.93 (2 H, d, J =
8.64 Hz, ArH), 4.02 (2 H, t, J = 6.44 Hz, OCH2C5H10SH), 2.54
(2 H, q, J = 7.27 Hz, OC5H10CH2SH), 1.84–1.79 (2 H, m,
OCH2CH2C4H8SH), 1.67–1.63 (2 H, m, OC4H8CH2CH2SH),
1.48–1.45 (4 H, m, C2H4C2H4C2H4SH), 1.34 (1 H, t, SH);
dC(75 MHz; CDCl3; Me4Si) 166.8, 162.0, 131.1, 122.8, 114.0,
67.4, 33.1, 28.2, 27.2, 24.7, 23.5; m/z (EIMS): 254 ([M]+, 50%).
4-(6-Mercaptohexyloxy)benzoic acid tert-butyl ester
(MHBTE (I)). A solution of 3a (1.13 g, 3.18 mmol) and
thiourea (0.27 g, 3.50 mmol) in anhydrous EtOH (40 ml) was
heated under reflux and N2 atmosphere for 20 h. 5 M aqueous
NaOH (0.64 ml, 3.18 mmol) was added and the reaction
mixture further heated under reflux for 20 h. The mixture
was allowed to cool to room temperature and concentrated
in vacuo (20 ml). Water (50 ml) was added and acidified with
2 M HCl to yield a white precipitate. The white precipitate was
dissolved by CH2Cl2 (50 ml) and the aqueous layer extracted
with CH2Cl2 (3 6 50 ml). The combined organic layers were
dried (MgSO4), filtered and solvent removed in vacuo to yield a
white solid as the crude product. The solid was absorbed onto
silica and purified via silica gel column chromatography
(graded elution: 0 to 25% EtOAc in hexane, increase polarity
3.3 Compound characterisation
3.3.1 NMR. 1H Nuclear magnetic resonance (NMR) spectra
were recorded on a Bruker AC 300 (300.13 MHz) spectro-
meter. 13C NMR spectra were recorded on a Bruker AV 300
(75.5 MHz) using Pendent pulse sequences. All chemical shifts
are quoted in ppm to higher frequency from Me4Si using either
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deuterated chloroform (CDCl3) or methanol (CD3OD) as the
lock and the residual solvent as the internal standard. The
coupling constants are expressed in hertz (Hz) with multi-
plicities abbreviated as follows: s = singlet, d = doublet, dd =
double doublet, t = triplet, q = quartet and m = multiplet.
temperature for 10 min with occasional stirring, followed by
thorough rinsing with UHQ H2O, then with EtOH (HPLC
grade) and immediately immersed in the desired 1 mM
solution of MHBTE (I), MHBME (II) and MHBA (III) for
24 h. Finally, the SAMs were rinsed thoroughly with EtOH
(HPLC grade) and dried with a stream of N2.
3.3.2 Mass spectrometry (MS). Electron impact mass
spectroscopy (EIMS) was performed on a VG Prospec. Low
and high resolution electrospray mass spectrometry was
performed on a Micromass time of flight (TOF) instrument
using methanol as the mobile phase.
3.6 SAM characterisation
3.6.1 Water contact measurements. Contact angles were
determined using a home-built contact angle apparatus,
equipped with a charged coupled device (CCD) KP-M1E/K
camera (Hitachi) that was attached to a personal computer for
3.3.3 Infrared spectroscopy (IR). The IR spectra were
recorded as thin solid films on NaCl discs using a Perkin
Elmer 1600 FT-IR. The solids were mixed with Nujol to form
a paste which was spread between the NaCl discs to form a
thin film.
˚
video capture. FTA Video Analysis software v1.96 (First Ten
Angstroms) was used for the analysis of the contact angle of a
droplet of UHQ H2O at the three-phase intersection. The
dynamic contact angles were recorded as a micro-syringe was
used to quasi-statically add liquid to or remove liquid from the
drop. The drop was shown as a live video image on the PC
screen and digitally recorded for future analysis. The acquisi-
tion rate was 4 frames per second. The static contact angles
were measured by placing a H2O (UHQ) droplet (1 mL) on the
substrate and the images were recorded digitally in the same
way as the dynamic contact angles. The contact angles were
determined from an average of fifteen different measurements
made on each type of SAM. Three samples were prepared for
each SAM and five measurements were taken from different
areas on each sample, except for the contact angles measured
on the acid gradient SAMs where three contact angles were
taken for each position along each of the three acid gradient
SAMs prepared. The errors reported for the contact angle
measurements are standard errors.
3.3.4 Elemental analysis. Elemental analyses were carried out
on a Carlo Erba EA 1110 (C H N) instrument.
3.4 Preparation of Au substrates
The gold substrates were prepared using an Auto 306 vacuum
evaporation chamber (Edwards) in a two pump system, the
pressure was reduced to y1024 bar followed by a subsequent
reduction to y1027 bar. Gold was deposited onto glass
microscope slides (BDH). Prior to the evaporation of gold
onto the glass slides, a Cr layer (6 nm) was evaporated onto the
glass slides by heating Cr pieces (Agar Scientific, 99.99%
purity) of y5 mm3 volume by electrical resistance using a
voltage of 30 V and a current of 3 A, to promote adhesion of
the gold to the base material. Au was deposited in a similar
manner, with Au wire (Advent Research Materials, 99.99+%
purity) of 0.5 mm diameter, which was placed into a Mo boat
(Agar Scientific) and heated. The Au wire was heated by
electrical resistance using a voltage of 10 V and a current of
3 A until y100 nm of Au had been deposited onto the desired
surface within the auto 306 vacuum evaporation chamber.
Deposition and deposition rate were monitored using a quartz
crystal microbalance (QCM) thickness monitor. A deposition
rate between 0.05–0.1 nm s21 was used for both Cr and Au
layers. The Au substrates were cut up into smaller pieces (1 6
1 cm2) for the preparation of samples for characterisation and
(2.5 6 2.5 cm2) for the preparation of the acid gradient SAMs
prior to use, using a diamond tipped scriber.
3.6.2 Ellipsometry. The thickness of the deposited mono-
layers was determined by spectroscopic ellipsometry. A Jobin-
Yvon UVISEL ellipsometer with a xenon light source was used
for the measurements. The angle of incidence was fixed at 70u.
A wavelength range of 280–820 nm was used. The DeltaPsi
software was employed to determine the thickness values and
the calculations were based on a three-phase ambient/SAM/Au
model, in which the SAM was assumed to be isotropic and
assigned a refractive index of 1.50. The thickness reported is
the average of six measurements taken on each SAM.
3.6.3 X-Ray photoelectron spectroscopy (XPS). Elemental
composition of the SAMs were analysed using an Escalab 250
system (Thermo VG Scientific) operating with Avantage v1.85
software under a pressure of y5 6 1029 mbar. An Al Ka
X-ray source was used, which provided a monochromatic
X-ray beam with incident energy of 1486.68 eV and a circular
spot size of y0.2 mm2 was employed.
The samples were attached onto a stainless steel holder using
double-sided carbon sticky tape (Shintron tape). In order to
minimise charge retention on the sample, the samples were
clipped onto the holder using stainless steel or Cu clips. The
clips provided a link between the sample and the sample holder
for electrons to flow, which the glass substrate inhibits.
Low resolution survey spectra were obtained using a pass
energy of 150 eV over a binding energy range of 210 eV to
3.5 Preparation of SAMs
Prior to the preparation of the SAMs the glassware and the
gold substrates were cleaned thoroughly to remove contami-
nants. Initially, the glassware was washed thoroughly with
piranha solution (conc. H2SO4: 30% H2O2 = 7:3) followed by
rinsing with UHQ H2O. The subsequent steps were followed:
sonication in UHQ H2O for 30 min, drying in an oven at
120 uC for 30 min, allowed to cool to room temperature,
sonication in EtOH for 30 min, drying in an oven at 120 uC for
30 min and wrapping in aluminium foil before use to prevent
exposure to airborne contaminants and use within 24 h. The
Au substrates were immersed in piranha solution at room
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1200 eV obtained using 1 eV increments. The spectra recorded
were an average of 3 scans. The high resolution spectra
were obtained using a pass energy of 20 eV and 0.1 eV
increments over a binding energy range of 20–30 eV, centred
on the binding energy of the electron environment being
studied. A dwell time of 20 ms was employed between each
binding energy increment.
heating, the aqueous solution was removed followed by the Au
substrate being removed from the reaction cell and great care
was taken to avoid damage to the substrate. The substrate was
thoroughly rinsed with copious amount of UHQ H2O and
dried with a stream of N2.
3.10 Attachment of nanoparticles
3.10.1 MHBTE (I) and MHBA (III) SAMs. After their
preparation, the SAMs were immersed in the colloidal
solutions (5 mg ml21, 2 ml) at either pH 3 or pH 9 for 2 h,
followed by rinsing with copious amounts of UHQ H2O for
30 s and drying with a stream of N2. The average particle
density was determined by counting the number of particles on
three AFM images taken from each sample.
3.7 Hydrolysis of MHBTE (I) and MHBME (II) SAMs
To avoid any unnecessary contamination, the freshly prepared
SAMs were immersed immediately into the preheated aqueous
acidic/basic solution (20 ml) at the required temperature for
5 h. After this duration, the substrates were rinsed with
copious amounts of UHQ H2O and dried with a stream of N2.
Contact angle measurements were taken immediately to avoid
contamination of the surface, which may influence the data
obtained.
3.10.2 Acid gradient substrate. The modified surface was
immersed immediately after being modified by the Peltier
element in NR2-PL nanoparticle solution (5 mg ml21) at pH 3
(25 ml) for 2 h, followed by rinsing with copious amounts of
UHQ H2O for 1 min and drying with a stream of N2. The
average particle density at each point along the substrate was
determined from three AFM images taken across the substrate
for each point.
3.8 Temperature gradient on the MHBTE (I) SAM
The temperature gradient was obtained by mounting 5 mm of
the Au substrate with MHBTE (I) SAM (dimensions 25 mm 6
20 mm) onto the Peltier element with the setpoint as shown in
Fig. 5 being 3 mm from one of the edges. The temperature
along the substrate was measured in air and determined using
K-type thermocouples which were connected to a thermo-
couple module (iso-tech ITA11) and readings taken on a
voltmeter (Caltek instrument, CM1200T). The current to the
Peltier element was supplied by a Ranger power unit (0–13 V,
8 A). The Au substrate (dimensions 25 6 20 mm) was
mounted on the Peltier element (dimensions: 30 6 30 mm)
with Loctite glass bond glue. The Peltier element with the
substrate was placed upon a metal block to act as a heat sink,
so that the temperature gradient through the substrate
remained stable. Three temperature readings were taken at
each point along the Au substrate (0, 2, 4, 6, 8, 10, 12, 14, 16,
18 and 20 mm); the variation in the measurements across the
substrate is shown by the standard error.
4 Conclusion
The work described has demonstrated that the confinement of
the ester moieties at the terminus of quasi-crystalline SAMs
limits the hydrolysis of the esters. Thus, –CO2Me moieties do
t
not hydrolyse in either acidic or basic media and –CO2 Bu
moieties do not hydrolyse in basic media. It is concluded that
the lack of hydrolysis is due to the molecules being confined in
closely packed monolayers, which does not allow the attacking
nucleophilic water access to the electrophilic carbonyl carbon
atom, i.e. the attack trajectory is sterically hindered. This steric
argument is supported by considering the relative ease of the
t
acid-catalysed hydrolysis of –CO2 Bu moieties which proceeds
via the AAL1 mechanism, whereby there is no requirement for
attack by a nucleophile, only protonation of the carbonyl
oxygen that leads to the release of a tBu cation (which
collapses to isobutene and a proton). Furthermore, detailed
analysis of the activation energy of the SAM hydrolysis
revealed that the pKa of the ester functional group had a
profound effect on the activation energy, further supporting
3.9 Preparation of the acid gradient substrate on the Peltier
element
A Au substrate with dimensions 25 6 20 mm was mounted
onto a custom built rectangular shaped glass cell (25 6 20 mm)
with Loctite glass bond glue to form the reaction cell. 1 mM
MHBTE (I) ethanolic solution (5 ml) was poured into the
reaction cell and covered with aluminium foil for 24 h. After
24 h, the MHBTE (I) solution was removed and the reaction
cell was rinsed thoroughly with copious amounts of HPLC
EtOH followed by drying with N2. The reaction cell was
immediately glued onto the Peltier element with Loctite glass
bond glue and allowed to dry for 5 min, covered with
aluminium foil to avoid contaminating the surface. The hot
end of the cell (end mounted onto the Peltier element) was
heated to 80 uC, followed by the addition of the aqueous pH 1
solution (0.5 ml). The temperature was maintained at 80 uC for
the duration of the hydrolysis by monitoring the temperature
at the exposed corners of the Au substrate. Also the volume of
the aqueous solution was monitored. After the duration of the
the AAL1 mechanism in the case of the –CO2 Bu SAM.15 Thus,
t
when designing SAMs on which surface chemistry is to be
carried out it is important to pay attention to the mechanisms
associated with the chemical modification to ensure there is
adequate space around the target functional group for the
transformation to occur.
Moreover, a novel facile approach for the fabrication of a
nanoparticle gradient substrate has been successfully demon-
t
strated by the thermochemical manipulation of a –CO2 Bu
functionalized SAM. An acid gradient SAM moving from
t
–COOH to –CO2 Bu moieties provided the ideal underlying
SAM onto which NR2-PL nanoparticles were attached at
pH 3. The particles were shown to attach preferably onto the
t
–COOH moieties and not to the –CO2 Bu moieties, resulting in
5108 | J. Mater. Chem., 2007, 17, 5097–5110
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particle density starting from 104 particles mm22 on the
hydrolysed acid end reducing to 0 particles mm22 on the non-
hydrolysed end. The extent of the particle gradient could
potentially be tuned by varying the pH at which the acid
gradient SAM is immersed in the particle solution, or by the
extent of the acid gradient in the underlying SAM, which can be
controlled by the temperature and pH of the aqueous solution.
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Acknowledgements
This work was supported by BAE SYSTEMS and EPSRC
(DTA to PI), as well as the EU (Nano3D, contract number
NMP4-CT-2005-014000). We thank the research group of
Prof. Graham Leggett for use of their thermal evaporation
apparatus to fabricate the Au substrates.
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