66
3 Confinement Effects on the Reactivity in Ultrathin …
Concentrated HCl (37%), cyclohexane, ammonium hydroxide (28–30%) were
purchased from Aldrich and used as received. Toluene (99.7%) and N,Ndimethylformamide (DMF) (99.8%) were purchased from Biosolve (the Netherlands) and used as received.
Preparation of Thin Films. All glassware used to prepare monolayers were
immersed in piranha solution (solution of 1:3 (vol.:vol.) 30% H 2 O 2 and concentrated H 2 SO 4 ) for 15 min, then rinsed with copious amounts of high purity water
(Millipore Milli-Q water). Caution: Piranha solution should be handled with
extreme caution; it has been reported to detonate unexpectedly. Gold substrates
(200 nm gold on 2 nm Ti primer on glass) were acquired from SSENS BV (Hengelo,
The Netherlands). The gold substrates were cleaned by oxygen plasma, subsequently
immersed in ethanol for 10 min to remove any possible oxide layer [34] and immersed
with minimal delay into 1.0 mM solutions of NHS-C10 in ethanol. The substrates
were removed from solutions after >16 h assembly time and rinsed extensively with
ethanol and water to remove any physisorbed material. Finally, the samples were
dried in a stream of nitrogen.
Polymer thin films were prepared by spin-coating polymer solutions in DMF (for
PNHSMA) and toluene (for PS n -b-PtBA m ) (typical concentration between 4 and
30 mg/ml) onto silicon wafers, which were cleaned by an oxygen plasma treatment
(Elektrotech PF 340 apparatus). The samples were spun at 3000 rpm for 30 s using
a P6700 spin-coater (Specialty Coating Systems Inc). All spin-cast samples were
annealed at 150 °C (for PNHSMA) or 135 °C (for PS n -b-PtBA m ) for 24 h in vacuum
before analysis. Mean film thicknesses were determined using a custom-built spectroscopic ellipsometer using a He–Ne laser (λ = 632.8 nm). The refractive index
used for PNHSMA film was 1.50 and for the block polymer films 1.513, while for
PtBA and PS we used 1.464 and 1.590, respectively.
Hydrolysis. The hydrolysis of NHS ester NHS-C10 monolayers and polymer films
was carried out by incubation of the corresponding sample for different reaction
times in 1.00 × 10
−2 M NaOH and in 25 × 10
−2 M aqueous NH 3 (PNHSMA),
respectively. The concentration of hydroxide ions in the aqueous NH 3 solution was
calculated based on tabulated values of the corresponding equilibrium constant and
its dependence on temperature. The films of PS n -b-PtBA m were immersed in 3 M
aqueous HCl acid solutions in closed beakers for different times or at different temperatures. The temperature was controlled between 25 and 70 °C using an oil bath. After
the reaction, the samples were thoroughly rinsed three times using Mill-Q water and
dried in vacuum.
Solvent Treatment of Thin Films. Cyclohexane was used to enrich the surface in PS
by placing a cross-linked poly(dimethylsiloxane) (PDMS) slab previously soaked in
cyclohexane onto the polymer surface [35]. After 3 h contact treatment, the PDMS
was removed and the samples were stored overnight in vacuum.
Atomic Force Microscopy (AFM). The tapping mode AFM measurements were
carried out with a NanoScope X multimode AFM (Digital Instruments/ Veeco, Santa
Barbara, CA) using a 10 μm scanner and microfabricated silicon tips/ cantilevers
3 Confinement Effects on the Reactivity in Ultrathin …
Concentrated HCl (37%), cyclohexane, ammonium hydroxide (28–30%) were
purchased from Aldrich and used as received. Toluene (99.7%) and N,Ndimethylformamide (DMF) (99.8%) were purchased from Biosolve (the Netherlands) and used as received.
Preparation of Thin Films. All glassware used to prepare monolayers were
immersed in piranha solution (solution of 1:3 (vol.:vol.) 30% H 2 O 2 and concentrated H 2 SO 4 ) for 15 min, then rinsed with copious amounts of high purity water
(Millipore Milli-Q water). Caution: Piranha solution should be handled with
extreme caution; it has been reported to detonate unexpectedly. Gold substrates
(200 nm gold on 2 nm Ti primer on glass) were acquired from SSENS BV (Hengelo,
The Netherlands). The gold substrates were cleaned by oxygen plasma, subsequently
immersed in ethanol for 10 min to remove any possible oxide layer [34] and immersed
with minimal delay into 1.0 mM solutions of NHS-C10 in ethanol. The substrates
were removed from solutions after >16 h assembly time and rinsed extensively with
ethanol and water to remove any physisorbed material. Finally, the samples were
dried in a stream of nitrogen.
Polymer thin films were prepared by spin-coating polymer solutions in DMF (for
PNHSMA) and toluene (for PS n -b-PtBA m ) (typical concentration between 4 and
30 mg/ml) onto silicon wafers, which were cleaned by an oxygen plasma treatment
(Elektrotech PF 340 apparatus). The samples were spun at 3000 rpm for 30 s using
a P6700 spin-coater (Specialty Coating Systems Inc). All spin-cast samples were
annealed at 150 °C (for PNHSMA) or 135 °C (for PS n -b-PtBA m ) for 24 h in vacuum
before analysis. Mean film thicknesses were determined using a custom-built spectroscopic ellipsometer using a He–Ne laser (λ = 632.8 nm). The refractive index
used for PNHSMA film was 1.50 and for the block polymer films 1.513, while for
PtBA and PS we used 1.464 and 1.590, respectively.
Hydrolysis. The hydrolysis of NHS ester NHS-C10 monolayers and polymer films
was carried out by incubation of the corresponding sample for different reaction
times in 1.00 × 10
−2 M NaOH and in 25 × 10
−2 M aqueous NH 3 (PNHSMA),
respectively. The concentration of hydroxide ions in the aqueous NH 3 solution was
calculated based on tabulated values of the corresponding equilibrium constant and
its dependence on temperature. The films of PS n -b-PtBA m were immersed in 3 M
aqueous HCl acid solutions in closed beakers for different times or at different temperatures. The temperature was controlled between 25 and 70 °C using an oil bath. After
the reaction, the samples were thoroughly rinsed three times using Mill-Q water and
dried in vacuum.
Solvent Treatment of Thin Films. Cyclohexane was used to enrich the surface in PS
by placing a cross-linked poly(dimethylsiloxane) (PDMS) slab previously soaked in
cyclohexane onto the polymer surface [35]. After 3 h contact treatment, the PDMS
was removed and the samples were stored overnight in vacuum.
Atomic Force Microscopy (AFM). The tapping mode AFM measurements were
carried out with a NanoScope X multimode AFM (Digital Instruments/ Veeco, Santa
Barbara, CA) using a 10 μm scanner and microfabricated silicon tips/ cantilevers