RSC Advances
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spectroscopy, contact angle measurements, X-ray photoelectron was at about 520 nm and the peak absorbance was about 2.6.
spectroscopy (XPS), atomic force microscopy (AFM), and Ell- For preparation of gold colloid monolayers, the stock AuNP
man's reagent to determine surface mercaptan concentration, solution was diluted by 1% sodium citrate solution until the
and examined the ambient stability of the lms. Furthermore, absorbance was 2.0. As such, the concentration of the diluted
we have explored the use of MPS as an adhesive lm for AuNP solution is about 10 nM.20 Histograms of the images
anchoring gold nanoparticles on silica surfaces, and compare derived from transmission electron microscopy (TEM) revealed
its performance to that of MPTMS.
that the mean diameter of the AuNPs was 13 ꢂ 1 nm.
Experimental
Materials
Preparation of mercaptan-functionalized substrates
Glass slides with dimensions 5.5 cm ꢃ 0.8 cm were cleaned by
soap and then sonicated sequentially in methanol, acetone,
puried water, methanol, acetone, and puried water, with
sonication time of 10 min in each step. The glass slides were
dried in an oven at 70 ꢀC and then stored at room temperature.
Before use, the glass slides were treated by oxygen plasma (0.6
torr, Harrick Scientic Products, Inc.) for 20 min.
The MPTMS-modied substrates were prepared as follows. A
solution of 5% MPTMS in toluene (v/v, about 0.25 M) was
allowed to prehydrolysis for 12 h. Then the glass slides were
immersed into a 5% MPTMS solution for 12 h. Subsequently,
the modied glass slides were sonicated sequentially in meth-
anol–toluene (1 : 1, v/v), methanol, and puried water for 15
min in each step, and then dried under a nitrogen stream.
The MPS-modied substrates were prepared as follows. The
stock MPS solution was diluted to 1 : 1000 (v/v, about 0.46 mM)
in methanol and the glass slides were immersed into the solu-
tion for 30 min. Subsequently, the modied glass slides were
rinsed by methanol and puried water and then dried by
nitrogen stream.
The following chemicals, (3-mercaptopropyl)trimethoxysilane
(MPTMS, Sigma Aldrich), tetrachloroauric acid (Sigma Aldrich),
¨
triethanolamine (Riedel-de Haen), toluene (Macron), dichloro-
methane (Macron), sodium citrate (J.T. Baker), methanol (J.T.
Baker), pentane (J.T. Baker), potassium carbonate (Shimakyu),
sodium sulfate (Shimakyu), acetonitrile (Tedia), dimethyl sulf-
oxide (DMSO, Acros), and 5,50-dithio-bis(2-nitrobenzoic acid)
(DTNB, Aldrich) were used as received. All aqueous solutions
were prepared with water that had been puried by using a
Millipore Milli-Q water purication system (Millipore) with a
specic resistance of 18.2 MU cm.
Synthesis of MPS
MPS was prepared using a synthetic method reported earlier19
but with slight modications. A solution of MPTMS (4.693 g,
20.3 mmol) and triethanolamine (3.2 g, 21.4 mmol) in 30 mL
toluene was heated at 110 ꢀC in a round-bottomed ask and
then reuxed for about 30 h. The progress of the reaction was
followed by thin layer chromatography. Aer cooling to room
temperature, the resulting product was washed by cold n-
pentane and then dried by a rotary-vacuum evaporator. The
dried powder was re-dissolved in 1 : 1 dichloromethane/n-
pentane. Aer re-crystallization twice, a white solid 2.30 g MPS
(9.2 mmol) with a yield of 45.5% was obtained. The deposit was
re-dissolved in DMSO (20 mL) as a stock solution for storage.
Fourier transform infrared spectroscopy (FTIR) and
hydrogen nuclear magnetic resonance (1H NMR) were used to
Quantication of mercaptan surface density
Samples of MPS and MPTMS lms on glass slides were
immersed in a cuvette containing 1 mL of Ellman's reagent, (4
mM DTNB in 0.1 M pH 8.0 phosphate buffer). The reaction was
allowed to proceed for 12 h at room temperature and the
absorbance was measured at the wavelength of 412 nm.
Controlled samples were elaborated with non-modied glass
slides. The amount of mercaptan moieties was calculated from
the corresponding standard curve elaborated between 2 ꢃ 10ꢁ8
M and 5 ꢃ 10ꢁ6 M of mercaptoethanol.
1
examine the purity and structure. H 200 MHz NMR (CDCl3):
0.285 (SiCH2, 2H, t, J ¼ 16 Hz), 1.22 (HS, 1H, t, J ¼ 12 Hz), 1.55
(CH2, 2H, m, J ¼ 42.6 Hz), 2.64 (SCH2, 2H, t, J ¼ 16 Hz), 2.788
(OCH2, 6H, t, J ¼ 11.2 Hz), 3.647 (NCH2, 6H, t, J ¼ 11.2 Hz). FTIR
(KBr, cmꢁ1): 2928 and 2876 (vs., nCH2), 2545 (w, nSH), 1454 and
1277 (m, dCH2), 1124 and 1100 (vs., nCO), 939 (s, nCC), 909 (s, nCN),
776 (s, n(as)SiO), 616 (m, n(s)SiC), and 582 (m, nN/Si).14,19
Ambient stability test
The modication of the substrates by MPTMS (toluene 16 h) or
MPS (MeOH 30 min) was under dark and room temperature
Preparation of AuNPs
AuNPs were synthesized according to the following procedure. conditions according to the procedures as described above.
HAuCl4$3H2O (0.88 mM, 20 mL) was heated to boiling with Once a substrate had nished the modication procedures, the
vigorous stirring. A fresh 1% sodium citrate solution (2.4 mL) stability of the modied substrate is tested immediately under
was added in the boiling solution rapidly. The color of the room light and room temperature conditions for standing time
solution turned from yellow to colorless, then very dark purple, periods of 0, 1, 3, 24, and 48 h. Then the modied substrate was
then ruby-red. The resulting solution was kept boiling for 20 immersed in a diluted AuNP solution to anchor AnNPs on the
min, then cooled down to room temperature and stored at 4 ꢀC modied surface. The result of the stability test was observed by
as a stock solution. The stock AuNP solution was characterized measuring the UV-visible spectra of the AuNPs anchored on the
by a UV-visible spectrometer. Typically, the peak wavelength modied substrate.
46528 | RSC Adv., 2014, 4, 46527–46535
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