12-Bromo-3,6,9,12-tetraoxadodecan-1-ol (b). 17 g LiBr
(0.2 mol) were dissolved in p.a. acetone (200 ml). 8 g of (a)
(0.023 mol) were added and the mixture was stirred for 4–5 h
at 80 °C. The mixture was then cooled to room temperature
and stirred overnight. The solvent was evaporated and 100 ml
chloroform was added to the residue. A white precipitate was
filtered and the solution was washed two times with water.
The organic layer was dried with Na SO and the solvent was
the surfaces of the glass slides and were mounted on Scotch-
Tape. Measurements were taken from 20°∏2h∏40° with a
step size of 0.02° and a step time of 60 s. SPS measurements
were performed in the Kretschmann configuration.16 Optical
coupling was achieved with a LASFN9 prism (n=1.85 at l=
632.8 nm) and index matching fluid (n=1.70) between the
prism and the BK270 glass slide. The surface plasmon was
excited with p-polarized electromagnetic light using a He–Ne
laser (632.8 nm, 5 mW). Kinetic experiments of the assembly
of the thiol on gold-coated glass slides were monitored by
following the plasmon reflectivity corresponding to an initial
reflectivity of 40%. The initial time was taken to correspond
to the injection of the solution into the cuvette. Tapping-mode
atomic force microscopy (AFM) images using silicon cantilev-
ers were acquired on colloidal samples that were spin-cast
onto mica substrates from aqueous solution. A Nanoscope
IIIa was used for this purpose. FTIR spectra were recorded
2
4
evaporated. Yield: 3.9 g (65%). (1H) NMR: 3.72 (t, 2 H,
CH OH), 3.63–3.49 (m, 12 H, OCH ), 3.36 (t, 2 H, CH Br).
2
2
2
EI-MS m/z: 256.41 (54.4%), 258.39 (21.2%).
12-Mercapto-3,6,9,12-tetraoxadodecan-1-ol (c). 20 g of (b)
(7.8×10−2 mol) were dissolved in ethanol (200 ml) and stirred
under reflux. A solution of 2.5 g of Na S O (0.11 mol) in
2 2 3
water (200 ml) was added dropwise. The mixture was stirred
at 20 °C for 48 h. The solvent was evaporated and 1 M HCl
(60 ml) was added to the residue. The mixture was heated
under reflux for 2 h. The solvent was evaporated and the
residue was dissolved in dichloromethane (50 ml) and dried
with Na SO . The solvent was evaporated and the residue was
in transmission mode (KBr pellets) using
a Mattson
Instrument Galaxy 2030 IR-spectrometer. FTIR spectra in
reflection mode were obtained by using a Nicolet (5DXC) GI-
FTIR spectrometer equipped with a Spectra-Tech specular
reflectance cell and an MCTA-detector with an angle of
incidence of 85°.
2
4
dissolved in acetone (5 ml). A yellow precipitate was filtered
off. The solution was evaporated and the product obtained as
a yellow oil. Yield: 6.7 g (40%). (1H) NMR: 3.72 (t, 2H,
CH OH), 3.57 (m, 12H, OCH ), 2.8 (m, 2 H, CH S). IR 3410
2
2
2
Results and discussion
(-OH), 2925–2850 (-CH ), 1454 (CH ), 1300–1020 cm−1
2
2
(C-O-C).
Self-assembly of the thiol on gold-coated glass substrates
Fig. 1 shows angle dispersive surface plasmon spectra of a
clean gold surface in water, and the plasmon spectra of a gold
surface exposed to the thiol 1. The shift of the plasmon curve
is corresponding to an angular change of 0.4° and could be
fitted using the Fresnel formula. Assuming the refractive index
of the thiol 1 to be no different from that of bulk tetraethylene
Study of the self-assembly of 1 on Au surfaces
After resistive evaporation (using a Balzers Baltec instrument
at a pressure of 10−5 hPa) of a 2 nm adhesion layer of Cr on
clean glass substrates, a further 50 nm of Au was deposited.
The rate of deposition was monitored using a quartz crystal
microbalance. The freshly prepared Au–glass substrates were
placed in solutions of the thiol 1 in ethanol (1 mM) overnight
for studies by surface plasmon spectroscopy.
˚
glycol (n=1.4598) a thickness of 18 A for the SAM of 1 on
gold was determined. Modeling the structure of the thiol using
molecular mechanics at the MM2 level (as implemented in
Chem3D@ version 3.5) of the thiol 1 suggests that chain
˚
Gold colloids protected by 1
expansions of as much as 15 A are reasonable from the point
of view of energetics. Considering additional gold–sulfur bind-
ing for the adsorbed thiol, we obtain the result that nearly
complete monolayer coverage of the gold surface by the thiol
has been achieved.
A 5 ml solution of HAuCl (2 g per 100 ml water) was diluted
with water to 40 ml. 50 ml of the thiol 1 were added then. A
4
solution of NaBH (0.01 g) in water (10 ml) was added slowly
4
with vigorous shaking until the mixture exhibited a dark red
The kinetics of the thiol binding could be followed from
changes in the surface plasmon. Fig. 2 displays the adsorption
of the thiol on flat gold surfaces followed from changes in the
plasmon reflectivity at a fixed scattering angle. The initial time
for this experiment was taken as the time of exposure of the
gold surface to the thiol (1 mM in water). It is seen that near-
saturation in the reflectivity takes as long as 100 min. After
150 min no further changes in the reflectivity are seen and the
complete spectrum recorded after this time is consistent with
a monolayer of the thiol being present. In comparison, long
chain alkylthiols adsorb much more rapidly from ethanolic
colour. In the absence of the thiol, there is immediate precipi-
tation of fine, black gold particles under these conditions. It
was also verified that the simple tetraethylene glycol (without
the thiol functionality) provides no protection to the gold
colloids. Excess thiol and salts left over from the reduction
could be removed by containing the solutions of the thiol-
protected colloids in cellophane dialysis bags and washing
with copious quantities of water.
Crystallization of CaCO in the presence of gold colloids
3
protected by the thiol 1
A 10 mM solution of CaCl was prepared in water (250 ml)
2
and a freshly prepared solution of the colloid (50 ml, 0.2 g per
100 ml) was added. Glass slides were placed at the bottom of
the vessel containing the solution. The vessel was placed in a
closed desiccator with solid (NH ) CO at the bottom for 2
4 2
3
days at 22 °C. In the CO -rich atmosphere, crystallites precipi-
2
tate from solution. The precipitated material was collected on
the glass slides and dried at 50 °C in air before being examined
by scanning electron microscopy and powder X-ray diffraction.
Instrumental techniques
X-Ray diffraction patterns were obtained using a Siemens
D5000 powder diffractometer equipped with a Ge(111) mon-
˚
ochromatized CuKa radiation (l=1.54056 A) in h/2h trans-
Fig. 1 Surface plasmon resonance spectra of (a) the bare gold-coated
glass surface and (b) after the coverage by the thiol 1.
1
mission geometry. The crystals were collected by scratching
1122
J. Mater. Chem., 1999, 9, 1121–1125
Bartz, Marcus
