Supporting Information
Polyelectrolyte Negative Resist Patterns as Templates for the Electrostatic Assembly
of Nanoparticles and Electroless Deposition of Metallic Films**
Yuval Ofir,1 Bappaditya Samanta,1 Qijun Xiao,2 Brian J. Jordan,1 Hao Xu,1 Palaniappan
Arumugam,1 Rochelle Arvizo,1 Mark T. Tuominen2 and Vincent M. Rotello1*
1. Experimental Section:
1.1 Polymers.
Polyvinyl pyridine (PVP) (MW 60,000), sulfonated polystyrene (SPS) (MW 70,000),
methyl iodide (MeI) and solvents are purchased from Aldrich or Fisher Scientific and
used without further purification. PVP (1 g, 9.6 mmol monomer) was dissolved in
ethanol, and then 0.15 ml MeI (0.34g, 2.4 mmol) was injected. The solution was heated
up to 60 ° C and stirred overnight. Solvent was then evaporated under reduced
pressure. From NMR spectrum taken in DMSO-d6, the integration shows 25%
quaternization. XPS measurements on the film were reported before1 and show around
25% methylated pyridine groups, which was in accordance to NMR results.
1.2 Synthesis of NPs.
1.2.1 CdSe-ZnS NPs
Materials
Cadmium oxide (99.99), selenium powder (99.5%), trioctylphosphine oxide (TOPO,
90%), trioctylphosphine (TOP, 90%), 11-mercaptoundecanoic acid (MUA, 95%), and
diethylzinc (1.0 M solution in hexane) were purchased from Aldrich.
Bis(trimethylsilyl)sulfide (99.9%) was purchased from Fluka. Tetradecylphosphonic acid
(TDPA) was purchased from PCI synthesis. Tetramethylammonium hydroxide
pentahydrate (99%) was purchased from Acros while hexadecylamine (HDA, 90%) was
purchased from Alfa Aesar. The chemicals were used as received unless specified.
Synthesis of CdSe NPs
The cadmium selenide NPs were synthesized according to the literature method. In brief,
a Schlenk flask containing 0.0514 g (0.4 mmol) of cadmium oxide, 0.1116 g (0.4 mmol)
of TDPA and 3.8744 g (10 mmol) of TOPO was heated to 340-350 °C under argon flow
for 3 hrs resulting in a colorless solution. The temperature of the solution was then
reduced to 260 °C and a solution of selenium containing 0.0263 g (0.33 mmol) of
selenium powder in 2 mL of TOP was injected. The resulting mixture was cool down to
180 °C by lifting the reaction flask from the heating mantle. Then the temperature was
raised slowly to 250 °C and the solution was digested at that temperature for 1-2 hours,
depending on required particle size. The NPs were purified by dispersing in hexane
followed by precipitation with methanol and this cycle was repeated twice.
Synthesis of core-shell CdSe-ZnS
The CdSe-ZnS, core-shell, was obtained by the modified literature procedure. The
purified CdSe NPs from above were dissolved in 5 mL of hexane and transferred to a
Schlenk flask containing 5.0 g (13 mmol) TOPO and 1.5 g (6 mmol) HDA. The mixture
was heated to 150 °C for 2 hrs under constant Ar flow. After the complete evaporation of
hexane stock solution containing 1.6 mL of dimethylzinc and 0.28 mL of
bis(trimethylsilyl)sulfide in 6 mL of TOP was added very slowly (over 15 min.) at 150
°C. The reaction temperature was lowered down to 100 °C and the mixture was digested
for 1 hr at that temperature. Core-shell CdSe-ZnS material was purified by dispersing the
product in distilled dicholoromethane followed by precipitation with n-butanol/methanol
mixture and the cycle was repeated twice.
Place exchange reactions
(NP1) Negatively charged core-shell nanoparticles were obtained by exchanging the
surface TOPO with 11-MUA. In a typical experiment, 0.03 g of 11-MUA was dissolved
in 20 mL of methanol and the pH of the solution was adjusted to ~10-11 using
tetramethylammonium hydroxide pentahdrate. The resulting solution, after purged with
Ar gas for 10 min., was transferred to the Schlenk flask containing ~0.01 g of CdSe-ZnS
precipitate and stirred at 40 °C for 6 hrs. The product was precipitated with diethyl ether
and re-dissolved in methanol and this cycle was repeated twice. The methanol dispersion
of purified 11-MUA capped core-shell nanoparticles was stored in dark.
(NP2) Positively charged CdSe-ZnS nanocrystals were obtained by treating the pyridine
coated CdSe-ZnS material with thiol-TEG-ammonium ligand (TTMA). The TTMA was
synthesized by following our earlier work. In a typical experiment, the TOPO capped
CdSe-ZnS was first place exchanged with pyridine following the literature procedure.
Next, the TTMA ligand was dissolved in methanol and the pH of the solution was
adjusted to ~10-11, as before, by addition tetramethylammonium hydroxide pentahydrate.
The basic TTMA solution was transferred to a flask containing pyridine coated CdSeZnS precipitate and stirred at 40 °C for 24 hrs under Ar atmosphere. The product was
purified by treating with diethyl either and methanol solution.
1.2.2 Fe3O4 NPs
Synthesis of citric acid functionalized Fe3O4 NPs
(NP3) Fe3O4 NPs were synthesized according to a literature procedure.2 In a typical
reaction, 0.43 g FeCl2_4H2O (2.16 mmol) and 1.17 g FeCl3_6H2O (4.32 mmol) were
mixed in 20 mL DI water and heated to 80 ºC under Argon atmosphere. While
vigorously stirring the reaction mixture, 5 mL of NH4OH was introduced by a syringe
and heating was continued for thirty minutes. The reaction mixture was cooled down to
room temperature and all the water was decanted and fresh 20 mL DI water was added.
Another 1 mL of NH4OH was introduced to make the solution alkaline. Following, 1g of
citric acid in 2mL water was introduced, and the temperature raised to 60 ºC and the
stirring continued for an additional two hours to get water soluble NPs. Purification was
achieved by ultra-centrifugation at 50K rpm for 30 minutes following addition of fresh
DI water. The resulting NPs are negatively charged and have a mean size of 12 nm.
Synthesis of Trimethylammonium carboxylic acid Fe3O4 NPs
Synthesis of Fe3O4 NPs followed a literature procedure.3Fe(acac)3 (acac =
acetylacetonate) (2 mmol), 1,2-hexadecanediol (10 mmol), lauric acid (6 mmol),
dodecylamine (6 mmol), was dissolved in benzyl ether (20 mL) and heated to 200 °C in
nitrogen atmosphere. Nitrogen flow was maintained throughout the reaction. The
temperature of the resulting solution was maintained for 2 hrs at 200 °C and was heated
further to reflux (300 °C) for 1 h. It was later allowed to cool down to room temperature
and the nitrogen flow was disconnected at this point. The black colored product was
precipitated by adding 20 mL ethanol and separated via centrifugation. The precipitate
was then dispersed in hexane (10 mL) in the presence of lauric acid (0.05 mL) and
dodecylamine (0.05 mL). These NPs of ~5 nm were re-precipitated by using ethanol (15
mL) and collected using centrifugation. Finally the purified product was re-dispersed in
hexane (10 mL).
Fe(acac)3 (2 mmol), 1,2-hexadecanediol (10 mmol), benzyl ether (20 mL), lauric acid (2
mmol), and dodecylamine (2 mmol) were mixed and stirred under nitrogen. 90 mg of ~5
nm Fe3O4 nanoparticles dispersed in hexane (6 mL) was added. Nitrogen flow was
maintained throughout the experiment. The mixture was first heated to 100 °C and
maintained this temperature for 45 min to remove hexane. Then mixture was heated to
200 °C for 1 h and then, it was refluxed (300 °C) for 30 min. The black colored mixture
was cooled to room temperature. Following the workup procedures described above, a
brown dispersion of 7 nm NPs in hexane was produced.
20 mg of Fe3O4 NPs from above were taken in 5 mL dichloromethane. 60 mg of 10carboxydecyltrimethylammonium bromide in 0.5 mL of ethanol was added into the NPs
dispersion and stirred for three days. The resulting brown precipitate was isolated using
centrifugation and purified using repeated washing (three times) with mixture of ethanol
and dichloromethane (1:10 volume). Finally, it was dispersed in MilliQ water.
Trimethylammonium carboxylic acid synthesis:
11-bromoundecanoic acid (7.5 mmol) was dissolved in ethanolic solution (20 mL) of
trimethylamine (4.2 M) and stirred for three days. Solvent was removed under reduced
pressure to isolate white compound. NMR (1HNMR, 400 MHz, D2O): 3.16-3.26 (m, 2H),
3.0(s, 9H), 2.22-2.32(t, 2H), 1.63-1.75(m, 2H), 1.45-1.56(m, 2H), 1.15-1.30 (m, 12H).
1.2.3 Citric acid functionalized gold NPs
(NP4) Gold Citrate stabilized NPs were synthesized following the reported methods.4 A 1
L round bottom used in the preparation was scratch free and thoroughly cleaned with
aqua regia then rinsed well DI water and further with MilliQ water. Once cleaned, the
round bottom was dried in an oven for several hours. 0.175 g hydrogen tetrachloroaurate
(Strem Chemicals) was added to 500 mL of filtered MilliQ water (1mM). This solution
was added to the clean and dried round bottom flask and brought to a rolling boil while
stirring. After refluxing for 15 minutes, a filtered solution of sodium citrate (38.8 mM,
50 mL filtered MilliQ water) that was preheated to 60 °C was rapidly added to the gold
mixture. This solution changed color from a pale yellow to burgundy. Reflux was
continued for 25 more minutes. The solution was removed from the heat and cooled
down to room temperature. Once cooled, it was filtered with 0.45 µM PVDF filters and
stored at 4 °C.
1.2.4 FePt NPs
FePt NPs were synthesized according to a literature procedure5 using a combination of
oleic acid and oleyl amine while reducing platinum acetylacetonate by a diol and
decomposition of Fe(CO)5 at high temperature solution. In a typical reaction: under N2
conditions, platinum acetylacetonate (197 mg, 0.5 mmol), 1,2-hexadecanediol (390 mg,
1.5 mmol) and dioctylether (20 mL) were mixed and heated to 100 °C. Oleic acid (0.16
mL, 0.5 mmol), oleyl amine (0.17 ml, 0.5 mmol), and Fe(CO)5 (0.13 mL, 1 mmol) were
added. The mixture was refluxed for 30 min, cooled down to room temperature,
precipitated with ethanol and centrifuged. Purification of the NPs was achieved through
re-dispersion in hexane and precipitation in ethanol.
10-carboxydecyltrimethylammonium (TMA) coated FePt NPs
20 mg of FePt NPs from above were taken in 5 mL of dichloromethane. To this 60 mg of
10-carboxydecyltrimethylammonium bromide and 60 mg of 11mercaptoundecyltrimethylammonium chloride, which was synthesized following the
reported procedure,6 in 0.5 ml of ethanol were added. The dispersion was stirred for 3
days and the resulting black precipitate was isolated using centrifugation. The product
was purified by repeated washing (3 times) with mixture of ethanol and dichloromethane
(1:10 volume). Finally, it was dissolved in MilliQ water.
Mercaptoundecanoic acid (MUA) coated FePt NPs
20 mg of FePt NPs from above were taken in 5 mL of dichloromethane. To this 60 mg of
MUA were added. The dispersion was stirred for 3 days and the resulting black
precipitate was isolated using centrifugation. The product was purified by repeated
washing (3 times) with dichloromethane. Finally, it was dissolved in MilliQ water with a
small amount of NaOH.
Examples of NPs used in the assembly step:
1.3 Test pattern in SPS.
Dosages were scanned from 25 µC/cm2 to 10000 µC/cm2.
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