Surfaces designed for charge reversal

Article abstract of DOI:10.1021/ja028338b

We have created surfaces which switch from cationic at pH < 3 to anionic at pH > 5, by attaching aminodicarboxylic acid units to silica and gold substrates. Charge reversal was demonstrated by monitoring the adsorption of cationic dyes (methylene blue and

Full text of DOI:10.1021/ja028338b

Published on Web 05/03/2003
Surfaces Designed for Charge Reversal
James R. Matthews, Do¨nu¨s Tuncel, Robert M. J. Jacobs, Colin D. Bain, and
Harry L. Anderson*
Contribution from the Department of Chemistry, UniVersity of Oxford,
South Parks Road, Oxford, OX1 3QY, U.K.
Received August 29, 2002; E-mail: harry.anderson@chemistry.ox.ac.uk
Abstract: We have created surfaces which switch from cationic at pH < 3 to anionic at pH > 5, by attaching
aminodicarboxylic acid units to silica and gold substrates. Charge reversal was demonstrated by monitoring
the adsorption of cationic dyes (methylene blue and a tetracationic porphyrin) and an anionic sulfonated
porphyrin, at a range of pH using UV-vis absorption and reflection spectroscopy. The cationic dyes bind
under neutral conditions (pH 5-7) and are released at pH 1-4, whereas the anionic dye binds under
acidic conditions (pH 1-4) and is released at pH 5-7. Gold surfaces were functionalized with two different
amphoteric disulfides with short (CH2)2 and long (CH2)10CONH(CH2)6 linkers; the longer disulfide gave
surfaces exhibiting charge reversal in a narrower pH range. Adsorption is much faster on the functionalized
gold (t1/2 ) 62 s) than on functionalized silica (t1/2 ) 6900 s), but the final extents of coverage on both
surface are similar, for a given dye at a given pH, with maximal coverages of around 2 molecules nm-2
.
These charge-reversal processes are reversible and can be repeatedly cycled by changing the pH. We
have also created surfaces which undergo irreversible proton-triggered charge switching, using a carbamate-
linked thiol carboxylic acid which cleaves in acid. These surfaces are versatile new tools for controlling
electrostatic self-assembly at surfaces.
Introduction
The electrostatic assembly of molecules, nanoparticles, and
even viruses on charged surfaces is becoming a widely used
technique.1 The ability to switch this interaction on and off,
between attraction and repulsion, will have important applica-
tions in diverse fields ranging from material science to micro-
biology. Here we present the first examples of surfaces designed
for controlled charge reversal.2 We have created surfaces which
switch reversibly from cationic to anionic in a narrow pH range
and other surfaces which undergo irreversible proton-triggered
charge switching. These functional surfaces were created using
both silane-silica and thiol-gold chemistry.3,4 Charge reversal
was demonstrated by monitoring the adsorption of anionic and
cationic chromophores (1-3). Charge-reversible surfaces are
versatile control elements for the binding, manipulation, and
release of charged guests.
(1) Mrksich, M. Chem. Soc. ReV. 2000, 29, 267-273. Shipway, A. N.; Katz,
E.; Willner, I. ChemPhysChem 2000, 1, 18-52. Britt, D. W.; Buijs, J.;
Hlady, V. Thin Solid Films 1998, 327-329, 824-828.
(2) Many naturally occurring surfaces, such as proteins and metal oxides, are
amphoteric and undergo charge reversal over a wide pH range, due to
protonation in acid and deprotonation or coordination of hydroxide.
However these surfaces are not suitable for use as control elements in self-
assembly because very large changes in pH are required to achieve charge
reversal and because they are difficult to fabricate. The design of surfaces
capable of switching hydrophobicity in response to changes in electrical
potential was reported recently: Lahann, J.; Mitragotri, S.; Tran, T.-N.;
Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer,
R. Science 2003, 299, 371-374.
Results and Discussion
(3) Ulman, A. An introduction to ultrathin organic films: from Langmuir-
Blodgett to self-assembly; Academic Press: Boston, 1991. Ulman, A. Chem.
ReV. 1996, 96, 1533-1554.
Design. Several ways of switching the charge on a surface
can be envisaged, such as (a) electrochemical oxidation and
reduction, (b) cation or anion coordination, (c) protonation, and
(4) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109,
2358-2368. Bain, C. D.; Whitesides, G. M. AdV. Mater. 1989, 1, 506-
512.
9
6428
J. AM. CHEM. SOC. 2003, 125, 6428-6433
10.1021/ja028338b CCC: $25.00 © 2003 American Chemical Society

Surfaces Designed for Charge Reversal
A R T I C L E S
Scheme 1. Synthesis of a Charge-Reversible Silica Surfacea
Scheme 2. Synthesis of an Irreversible Proton-Triggered
Charge-Switching Surfacea
a Reagents: (a) freshly evaporated gold, DMSO; (b) TFA or HCl (10 M
aqueous).
disulfide is more readily accessible, but the longer disulfide was
expected to form more stable monolayers due to van der Waals
interaction between the long aliphatic chains and hydrogen-
bonding between the amides.6
a Reagents: (a) fused silica, toluene, 80 °C; (b) aqueous imidazole. This
representation of the surface is idealized; some silane polymerization occurs
giving CH2-Si-O-Si-CH2 links, as discussed in the text.
(d) covalent modification. We chose to focus on the last two of
these options. Proton transfer is fast, reversible, easy to monitor,
and easy to drive by pH. We set out to design surfaces exhibiting
charge reversal in response to small pH changes. Many
molecules undergo charge reversal over a wide pH range, for
example, natural R-amino acids are cationic below pH 3 (when
the amine and carboxylic acid are both protonated) but require
pH > 10 to fully deprotonate the nitrogen, leaving a wide pH
gap in which the molecule is weakly charged. This pH gap is
minimized by using two equivalent protonation sites, as for
example in N-propyl-3,3′-iminodipropionic acid, 4. At low pH
both carboxylic acids and the amine are protonated giving a
single positive charge. The pH need only be raised high enough
to deprotonate the carboxylic acids (pKa ≈ 3.5) for the net
charge to reverse from +1 to -1.
Irreversible proton-triggered charge-switching surfaces were
designed using a carbamate-linked disulfide carboxylic acid 8,
which can be cleaved at low pH to unmask ammonium
functionality as shown in Scheme 2. This type of acid-labile
linker is widely used in solid-phase synthesis.7
Synthesis. Detailed schemes and experimental procedures for
the synthesis of compounds 5-8 are provided in Support-
ing Information. Trimethoxysilane 5 was prepared in 92% yield
by treatment of (aminopropyl)trimethoxysilane with methyl
acrylate. Disulfide 6 was prepared by reacting cystamine with
tert-butyl acrylate, followed by cleavage of the tert-butyl esters
with TFA. Similar chemistry was used to prepare 7 in four steps
from 1,6-diaminohexane. The key step in the synthesis of
carbamate 8 was coupling of a disulfide isocyanate (generated
in situ by Curtis rearrangement) with a benzyl alcohol.
Fused silica microscope slides were functionalized by im-
mersion in a solution of trimethoxysilyl derivative 5, followed
by hydrolysis in aqueous imidazole8,9 (Scheme 1). Monitoring
by attenuated total reflection infrared (ATR-IR) confirmed that
these conditions hydrolyze the surface-bound methyl esters,
We decided to make charge-reversible surfaces by grafting
amino acid 4 to silica and gold surfaces.5 In both cases this
amphoteric unit was coupled to the surface via the central
nitrogen, by replacing the n-propyl group with a suitable linker.
Silica surfaces were functionalized with the trimethoxysilane 5
(Scheme 1). We chose to use the methyl ester 5, which requires
hydrolysis on the surfaces, rather than the free carboxylic acid,
because this acid was found to be unstable. Gold was function-
alized with two different amphoteric disulfides, 6 and 7, with
short (CH2)2 and long (CH2)10CONH(CH2)6 linkers. The short
(6) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey,
C. L. Langmuir 1998, 14, 124-136.
(5) The stability of the silane-silica linkage under the conditions of our dye
adsorption experiments (pH 1-7 water) was verified by testing fused silica
functionalization with methyl orange. 4-Dimethylaminoazobenzene-4′-
sulfonyl chloride was reacted with (aminopropyl)trimethoxysilane in toluene
and then used directly to functionalize fused silica. These surfaces were
immersed in water over the pH range 1-7 over a period of 14 days. UV-
vis spectra were acquired to confirm that the dye, with an approximate
coverage of 2.0 molecules nm-2, remained attached to the surface.
(7) Eggenweiler, H.-M. Drug DiscoVery Today 1998, 3, 552-560. Wang, S.-
S. J. Am. Chem. Soc. 1973, 95, 1328-1333.
(8) Menegheli, P.; Farah, J. P. S.; El Seoud, O. A. Ber. Bunsen-Ges. Phys.
Chem. 1991, 95, 1610-1615.
(9) As a control experiment, we investigated the hydrolysis of 3,3′-imino-
dipropionic acid dimethyl ester, in 2.6 M imidazole in D2O by 1H NMR.;
this reaction reaches completion in 7 days at 20 °C.
9
J. AM. CHEM. SOC. VOL. 125, NO. 21, 2003 6429

A R T I C L E S
Matthews et al.
without removing the silane, as discussed below. Harsher
hydrolysis conditions were avoided so as not cleave the silane
from the surface.
Monolayers of 6-8 were assembled on gold from solutions
in water or DMSO.
on functionalized fused silica, and on gold functionalized with
7 are shown in Figure 1. All adsorption and desorption
experiments were reproducible within experimental error. The
values for surface coverage obtained in this way were corrected
to account for the optical properties of the interfaces, by
comparison of experimental and theoretically produced spec-
tra.12,15,16 The dye absorption spectra were modeled as Lorentz
oscillators (two each for dyes 1 and 3 and one for dye 2). The
solvent and silica real refractive indices were modeled with
Cauchy functions with zero for their imaginary parts. For the
solution spectra, this was combined with the dye indices using
a Bruggeman effective medium approximation to produce
effective optical constants for the system.12 The surface absorp-
tion data were fitted assuming a thin film of dye (film thickness
<10 nm).
Surface Analysis. ATR-IR spectroscopy of an oxidized
silicon prism functionalized with the ester, 5, on one face only,
shows a coverage of 50 ( 10 molecules nm-2, which corre-
sponds to a thickness of 26 nm and is about 20 times as thick
as expected for a monolayer. The coverage was determined by
comparison of the intensities of the ATR-IR signal of the ester
group at 1738 cm-1 from the functionalized silicon surface with
that of a clean silicon surface immersed in 0.1 M aqueous
N-propyl-3,3′-iminodipropionic acid dimethyl ester (the methyl
ester of 4). The penetration depth of the evanescent wave into
the aqueous solution was determined from the known optical
properties of water. While the coverage on oxidized silicon and
fused silica may not be identical, this result implies that the
adsorbed film on fused silica is a multilayer, due to some degree
of polymerization. Horizontal and vertical polymerization
frequently occur when trimethoxysilanes adsorb onto silica, due
to traces of moisture.3,10 Vertical polymerization of 5, via
(CH2)SiOSi(CH2) links, increases the layer thickness but should
not affect the surface functionality. AFM images of oxidized
silicon functionalized with 5 (Supporting Information) show an
undulating surface with an root-mean-square roughness of 2 nm,
together with a significant number of randomly distributed
approximately hemispherical particles with a typical size of 100
nm. This is in contrast to the same substrates prior to
functionalization which show a root-mean-square roughness of
0.4 nm. Comparison with the mean film thickness of 26 nm
determined by ATR-IR suggests that the uniform undulating
part of the film has a thickness of about 20 nm, with roughly
one-fifth of the absorbed material being present as larger
aggregates. The increased roughness of the surface by func-
tionalization does not significantly increase the surface area
(<3%).
In situ ATR-IR was also used to monitor the hydrolysis of
the surface ester groups, by observing the intensity of the CdO
stretch; after 10 days the hydrolysis is >95% complete, and
after 14 days no residual ester groups are detectable. Several
other peaks, including those in the CH region (2900-3100
cm-1) are unchanged during hydrolysis, which confirms that
aqueous imidazole does not cleave the silane from the surface.
Films of disulfide 7 on gold were characterized by FTIR
spectroscopy. From the intensity of the CH stretch band, we
estimate11,12 that the surface coverage is 2.3 ( 0.4 molecules
nm-2, which is consistent with a monolayer of a molecule with
two tail groups.13 FTIR spectra of gold treated with carbamate
disulfide 8 show similar levels of surface coverage.
Adsorption Characteristics. Preliminary experiments were
carried out to test the rate of dye adsorption. A series of
functionalized fused silica samples were immersed in a solution
of methylene blue 1 at pH 7 for varying lengths of time before
analysis. Under these conditions the cationic dye adsorbs onto
the anionic surface. The results displayed in Figure 2 show that
the adsorption process reaches equilibrium with a half-life (t1/2
)
of 6900 s. Similar results were obtained with the porphyrins 2
and 3. Adsorption of dyes onto functionalized gold surfaces
reachs equilibrium much more rapidly, as illustrated by the
adsorption of porphyrin 2 onto gold functionalized with 7
(Figure 3, t1/2 ) 62 s).
(11) The CH stretch region of the FTIR spectrum of 7 on gold was analyzed12
using the isotropic imaginary part for the refractive index (kiso), obtained
by averaging published values of kx, ky, and kz for the CH2 groups of behenic
acid methyl ester (BAME, CH3(CH2)20CO2CH3; Pelletier, I.; Bourque, H.;
Buffeteau, T.; Blaudez, D.; Desbat, B.; Pe´zolet, M. J. Phys. Chem. B 2002,
106, 1968-1976). The real part of the refractive index, n, was generated
from the values of k by numerical Kramers-Kronig transformation with
n∞ as 1.41. These parameters were used to determine the thickness of an
isotropic BAME film that would have an integrated absorbance in the CH
stretch region equal to that in the reflectance spectrum of 7, on gold at the
same angle of incidence. Assuming that the BAME monolayer from which
the optical constants were derived (Flach, C. R.; Gericke, A.; Mendelsohn,
R. J. Phys. Chem. B 1997, 101, 58-65) had a typical coverage of 5.0
molecules nm-2 (Small, D. M. The physical chemistry of lipids: from
alkanes to phospholipids; Plenum Press: New York, 1986), we estimate
an equivalent coverage of BAME of 2.3 ( 0.5 molecules nm-2. If we
assume that all the CH2 groups in 7 have the same oscillator strength as
the CH2 groups in BAME (there are an equal number of CH2 groups in
BAME and 7) and that they are isotropically distributed, the coverage of
7 will also be 2.3 ( 0.4 molecules nm-2
.
(12) Film Wizard, 6.5.1 Scientific Computing International, 1999.
(13) Surprisingly, the CH2 frequencies observed in films of 7 on gold (2849
and 2919 cm-1) are typical of densely packed all-anti hydrocarbon chains.
(Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437-
463.) If the layer is densely packed, it must be highly tilted to give the
estimated area per molecule.
(14) McCallien, D. W. J.; Burn, P. L.; Anderson, H. L. J. Chem. Soc., Perkin
Trans. 1 1997, 2581-2586.
(15) A slight modification of the data was required before fitting to take into
account the dye adsorbed onto both sides of the functionalized fused silica
slides, because it is not possible to model a macroscopic layer (the 1 mm
thick silica) with the software. Hence, the absorbance at each wavelength
point in the spectra was halved, making the assumption that absorption
and reflection at each air/silica interface contributes equally. The resulting
transmission spectra were compared with those simulated for a single air/
dye/functionalized silica interface. The data for absorption of dyes onto
gold functionalized with 6 and 7 were normalized by reference to a protected
aluminium mirror. The reflectivity data on gold were then fitted using the
software material file for gold12 and only varying the parameters of the
dye thin film Lorentz functions. The simulation allows the absorbance of
a free-standing film of dye to be calculated directly. Several sets of data at
varying dye coverage for each dye were compared with simulated spectra,
enabling correction factors to be calculated for the coverage obtained by
direct integration of the spectra.
Dye Adsorption: Determining Surface Coverage. Dyes
1-3 were adsorbed onto the surfaces from aqueous solutions,
to probe the charge-reversal properties of the surfaces. The
copper(II) sulfonated porphyrin, 3, was used, rather than the
free base, to prevent protonation at the center of the macrocycle.
The amount of dye adsorbed on the surface was determined by
comparing the integral of the UV-vis. absorption band of the
surface-bound dye with that of the same dye in solution at
known concentration.14 Raw spectra for porphyrin 2 in solution,
(16) The calculations assume a random orientation of the chromophores on the
surfaces. If the dye molecules were aligned face-on (0° to the surface),
then their absorptions relative to that of a randomly oriented configuration
would be 150%. If they were aligned edge-on (90° to the surface), then
their absorptions would be 75% for a dye of D4h symmetry with two
oscillators and 150% for methylene blue, 1, with only one oscillator.
(10) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274.
9
6430 J. AM. CHEM. SOC. VOL. 125, NO. 21, 2003

Surfaces Designed for Charge Reversal
A R T I C L E S
Figure 3. Time dependence of porphyrin 2 adsorption onto gold func-
tionalized with disulfide 7, at pH 1, fitted to a first-order curve with t1/2
62 s.
)
elucidated and may be associated with a slow release of
countercations (e.g., Na+) from the functionalized silica.
The surface coverages of each dye at pH 1 and 7 are listed
in Table 1, together with calculated maximum coverages for
close-packed monolayers.17 Methylene blue molecules have an
approximate face-on area of 1.1 ( 0.1 nm2. Adsorption of
methylene blue to mica gives a coverage of ca. 1.5 molecules
nm-2, with the molecules packing at an angle of 65-70° to the
surface.18 This is similar to the maximum coverage of 1.6 (
0.1 molecules nm-2 for adsorption to our functionalized silica.
The porphyrins, 2 and 3, give similar maximum coverages to
methylene blue, yet their calculated footprint areas are more
than twice as large, implying that they must form multilayers
to achieve these high surface densities.
The fixed incident angle of 6° to normal in our reflection
equipment prevents us from obtaining good reflectance spectra
of surface-bound methylene blue on gold, because the imaginary
part of the refractive index is too high in this wavelength region
(540-750 nm), resulting in a very high reflectivity and a node
in the electric field at the interface. Fortunately, this is not a
problem in the region of the porphyrin Soret band (370-460
nm). Comparison between adsorption of the porphyrins onto
the functionalized silica and gold (Table 1) shows larger
maximum adsorbances on the functionalized gold surfaces. The
maximum charge density of 7 adsorbed on gold is 2.3 ( 0.5
charges nm-2 (assuming complete ionization of the carbox-
ylates). Thus the observation that tetracationic porphyrin 2
adsorbs to a coverage of 2.3 ( 0.1 molecules nm-2 implies
that some other interaction must augment electrostatic adsorption
of the porphyrins onto this surface. Gold surfaces functionalized
with the two disulfides, 6 and 7, have similar levels of dye
adsorption, as is to be expected if the adsorption is determined
mainly by the amphoteric tail groups.
Figure 1. Representative transmittance (T) and reflectance (R) spectra of
porphyrin 2 Soret band, (a) in water, 3.0 µM, (b) adsorbed on functionalized
silica, and (c) adsorbed on gold functionalized with 7. Dashed lines show
background spectra for blank substrates.
Variation in adsorption with pH: Demonstration of
Charge Reversal. Surfaces were placed in dye solutions of
varying pH; after 24 h each sample was removed, its UV-vis
spectrum was recorded, and then the surface was reimmersed
in a fresh dye solution of different pH. As the pH drops, the
surface protonates and the charge is reversed. The affinity of
Figure 2. Time dependence of methylene blue, 1, adsorption onto
amphoteric functionalized fused silica, at pH 7, fitted to a first-order curve
with t1/2 ) 6900 s.
(17) Van der Waals footprint areas of molecules of dyes 1-3 were calculated
using molecular mechanics (CaChe, 4.1, Oxford Molecular Ltd., 1999) and
used to estimate monolayer coverages, assuming either flat or vertical
orientations of the chromophores on the surface.
(18) Ha¨hner, G.; Marti, A.; Spencer, N. D.; Caseri, W. R. J. Chem. Phys. 1996,
104, 7749-7757.
It is remarkable that equilibration on the gold surfaces is 2
orders of magnitude faster than that on functionalized silica.
The structural origins of this kinetic difference have yet to be
9
J. AM. CHEM. SOC. VOL. 125, NO. 21, 2003 6431

A R T I C L E S
Matthews et al.
Table 1. Calculated and Observed Dye Coverage (molecules nm-2) at pH 1 and pH 716
calcd monolayer
coverage
coverage on silica
functionalized with 5
coverage on gold
functionalized with 6
coverage on gold
functionalized with 7
dye
flat
vertical
pH 1
pH 7
pH 1
pH 7
pH 1
pH 7
1
2
3
0.9 ( 0.05
0.30 ( 0.02
0.20 ( 0.01
1.8 ( 0.09
0.84 ( 0.04
0.57 ( 0.03
0.02 ( 0.03
0.07 ( 0.03
1.2 ( 0.09
1.6 ( 0.1
1.8 ( 0.1
0.19 ( 0.03
0.37 ( 0.04
1.8 ( 0.1
2.2 ( 0.1
0.20 ( 0.04
0.04 ( 0.03
2.5 ( 0.2
2.3 ( 0.1
0.14 ( 0.03
Figure 5. Sequential adsorption and desorption of porphyrin 2 to gold
functionalized with disulfide 7, by alternating (O) pH 1 and (0) pH 7.
the same as that of the dyes, revealing a low level of nonspecific
binding. Control experiments were performed to test the
adsorption of charged dyes to clean unfunctionalized substrates.
The anionic dye 3 does not show any significant adsorption to
silica at pH 1-7 (coverage <0.25 molecules nm-2), whereas
the cationic dyes 1 and 2 adsorb at pH 4-7, with coverages of
up to 1.5 molecules nm-2, as expected because clean silica has
a negative surface charge in water at neutral pH (pKa ≈ 4.3).19
Blank gold surfaces do not adsorb detectable levels of dyes 1-3.
The similar maximum coverages of the anionic and cationic
porphyrins, which are of comparable molecular sizes and charge
densities, on all the amphoteric surfaces indicates that there is
charge reversal on these surfaces across the pH range studied.
The surface charge can be reversed repeatedly by switching the
pH of the solution. Figure 5 shows a typical adsorption response
to repeated switching of pH. The surface remains charge
reversible through multiple cycles and thus has potential for
reuse.
Irreversible Proton-Triggered Charge Switching. Gold
functionalized with carbamate disulfide 8 was tested by adsorp-
tion of cationic and anionic porphyrins 2 and 3, and the surface
coverage was determined as described above. The kinetics of
adsorption to gold surfaces functionalized with the three thiols
6-8 are essentially the same, with half-lives of about 1 min. In
the pH range 2-7, no cleavage of the carbamate occurs (Figure
6a). The cationic porphyrin 2 adsorbs strongly at pH 7, when
the surface carboxylates are ionized, and the coverage decreases
reversibly with decreasing pH as the carboxylates protonate.
As expected, the anionic porphyrin 3 does not adsorb strongly
to this anionic surface. Solution NMR experiments show that
carbamate 8 is rapidly cleaved by TFA (Scheme 2). Thus when
gold surfaces functionalized with 8 are treated with hydrochloric
acid (10 M), or TFA, there is an abrupt reversal in the adsorption
characteristics as the surface functionality is transformed from
Figure 4. Adsorption of dyes 1 (0), 2 (O), and 3 (b) onto (a) fused silica
functionalized with 5, (b) gold functionalized with disulfide 6, and (c) gold
functionalized with disulfide 7, as a function of pH. Arrows indicate the
direction of pH increment.
the functionalized silica surface for positively charged dyes 1
and 2 increases with pH, while its affinity for negatively charged
dye 3 decreases (Figure 4a). Similar behavior is seen for
functionalized gold (Figure 4b,c). Gold functionalized with
disulfide 7 shows a sharp charge reversal between pH 2 and
pH 5, whereas the surface functionalized with disulfide 6
responds over a wider pH range. The level of adsorption does
not drop to zero even when the charge on the surface should be
(19) Chaiyasut, C.; Takatsu, Y.; Kitagawa, S.; Tsuda, T. Electrophoresis 2001,
22, 1267-1272.
9
6432 J. AM. CHEM. SOC. VOL. 125, NO. 21, 2003

Surfaces Designed for Charge Reversal
A R T I C L E S
by vapor deposition of 5 nm of chromium followed by 200 nm of gold
onto 16 × 16 mm2 glass cut from microscope slides 1.2 mm thick and
used fresh from the evaporator.
Substrate Functionalization. The fused silica slides were func-
tionalized on both sides by heating at 80 °C for 4 h in a toluene solution
of 10% (v/v) N-(trimethoxysilylpropyl)-3,3′-iminodipropionic acid
dimethyl ester, 5, with 0.1% (v/v) diisopropylethylamine.13 The slides
were rinsed in toluene, sonicated in fresh toluene for 30 s to remove
physisorbed material, further rinsed in methanol, and then dried in a
stream of nitrogen. These functionalized slides were then immersed in
aqueous imidazole (2.6 M) for 14 days to achieve hydrolysis of the
surface esters. The gold surfaces were functionalized by immersion
for 1 h in a 0.1 mg/mL aqueous solution of disulfide, 6 or 7, or 0.1
mg/mL 8 in DMSO, rinsed in ultrapure water to remove physisorbed
material and dried in a stream of nitrogen. Newly functionalized
substrates were used immediately to avoid accidental contamination.
Measurements of IR Spectra. Reflectance IR spectra of the
monolayers on gold were measured with a Bio-Rad FTS6000 FTIR
spectrometer, with a ceramic source, KBr beam splitter and linearized
liquid nitrogen cooled MCT detector. Spectra were recorded at 60°
with a Graseby Specac 19650 series variable angle accessory. ATR-
IR spectra were also measured on this instrument using a Spectra-Tech
Micro ATR prism holder, with a silicon prism, mounted in a Spectra-
Tech beam condenser. A liquid cell was constructed, from stainless
steel and Teflon, so that only one face of the silicon prism was exposed
to the liquid during coating and analysis, while in the sample
compartment of the spectrometer the other faces were open to the air,
which was scrubbed to remove water, carbon dioxide, and hydrocarbons.
Figure 6. Adsorption of dyes 2 (O) and 3 (b) onto gold functionalized
with carbamate-linked disulfide 8 as a function of pH (a) sweeping pH 7
f 2 f 7 (no irreversible charge switching) and (b) sweeping pH 7 f -1
f 7 (showing irreversible charge switching; see Scheme 2). Arrows indicate
the direction of pH increment.
Dye Adsorption. As a means of probing the amphoteric nature of
the surfaces, charged dyes were adsorbed from solutions with pH in
the range of 1-7. The surfaces were immersed in 0.1 mg/mL aqueous
solutions of both cationic and anionic dyes at an extreme of pH, either
1 or 7, for 24 h, removed and rinsed in ultrapure water for 10 s, dried
in a stream of nitrogen, and then placed in the UV-vis spectrometer
for analysis. The absorbance is measured, the surface is placed into a
solution one pH increment on, and the process is repeated. All
adsorption and desorption experiments were repeated more than once
and produced consistent results within experimental error.
carboxylate to ammonium. In Figure 6b, the surface coverages
were first monitored as the pH was scanned 7 f 0, and then
the surface was briefly exposed to 10 M hydrochloric acid (pH
≈ -1), before remonitoring the adsorption as the pH was
returned 0 f 7. Before acid treatment, the surface binds the
cationic dye but not the anionic one; after acid treatment this
selectivity is reversed, providing clear evidence for proton-
triggered charge reversal.
Measurements of Absorption and Reflection Spectra. Absorption
spectra of chromophore layers adsorbed onto silica surfaces were
measured with a Perkin-Elmer Lambda 20 UV-vis spectrometer. The
samples were secured vertically so that the beam passed perpendicularly
through the layers adsorbed on both sides of the slides. The absorption
of the layers adsorbed onto gold surfaces was measured using a Perkin-
Elmer specular reflectance accessory fitted into the beam path of the
Perkin-Elmer Lambda 20 UV-vis spectrometer; a protected aluminum
mirror was used to normalize the data. The beam was reflected off the
gold surface at 6° to the normal. Since all the samples, both fused silica
and gold, showed some background absorbance, spectra of the surfaces
prior to dye adsorption were obtained for each sample and subtracted
from subsequent spectra.
Conclusions
We have created a range of surfaces which exhibit reversible,
and irreversible, proton-induced charge reversal, as demonstrated
by their changing affinity for positively and negatively charged
dyes. The reversible surfaces are robust enough to allow their
charge to be switched many times, whereas the irreversible
surfaces undergo a single switch from anionic to cationic. These
surfaces could be used as versatile control elements for the
binding, manipulation and release of charged guests, such as
dyes, pharmaceuticals, nanoparticles, or viruses.1 Irreversible
proton-triggered charge switching may also enable surfaces to
be patterned by shadow-masked photoacid generation.20
Acknowledgment. We thank the EPSRC and the Leverhulme
Trust for support.
Supporting Information Available: Synthetic schemes and
experimental procedures for preparation of compounds 5-8,
examples of ATR-IR and FTIR spectra, adsorption of dyes 1
and 3 onto clean silica as a function of pH, details of AFM
measurements, and 13C and 1H NMR spectra of 10, 12, 14, and
17. This material is available free of charge via the Internet at
http://pubs.acs.org.
Experimental Procedures
Preparation of Substrates. Fused silica microscope slides (15 ×
15 × 1 mm from UQG Ltd.) were cleaned in 7:3 (v/v) H2SO4:30%
H2O2 at 70 °C for 2 h (Hazard: Corrosive! May react violently with
organic matter!) After cooling, the slides were rinsed with ultrapure
water and dried in a stream of nitrogen. Gold surfaces were obtained
(20) Wallraff, G. M.; Hinsberg, W. D. Chem. ReV. 1999, 99, 1801-1821.
JA028338B
9
J. AM. CHEM. SOC. VOL. 125, NO. 21, 2003 6433