first demonstrated two decades ago24 by in situ polymerization of
acrylamide for separation of proteins. Since then, several methods
have been used for many different polymer coatings on capillary
and microchannel surfaces. Researchers have modified micro-
fluidic channel surfaces through in situ graft-polymerization of
hydrogels or adsorption of polyelectrolyte layers within the
channels.25 However, these methods can have problems due to
poor surface adhesion and desorption of the coating from the
channel surface in addition to problems related to reproducibility
of the polymer coating.18 In contrast, covalent attachment of
preformed polymers as channel wall coatings is a promising path
toward the goal of achieving a high degree of control on channel
wall properties.14,16,18,19,26 Many different coatings have been
reported in the literature for myriad applications. These include
some covalently attached surface layers and many physisorbed
polymer coatings. Examples include mono-, di-, and trichloro-
silanes, ethoxy- and methoxysilanes, polymer coatings such as
acrylamide-derived polymers, cellulose-derived polymers, poly-
(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) ,26 poly-
(vinyl alcohol), oligourethanes, and poly(ethyleneimine) (PEI).27
These coatings have been used to modify the surface ú potential
and provide control and tunability of EOF in addition to minimizing
or eliminating analyte-wall interactions. Wetting characteristics
of the channels, capillaries, or both are also changed.17-19 PEO
or PEG is believed to coat surfaces by hydrogen bonding with
the surface silanol groups. It was modestly effective at suppressing
electroosmotic mobility at pH 7. Hydrophilicity of the coating is
important for bioseparations as increased hydrophilicity reduces
the analyte-wall interactions thereby increasing separation ef-
ficiency and improving resolution. However, the increased hy-
drophilicity also reduces the stability of the coating.19 This paper
reports on the goal of chemically modifying glass microchannel
surfaces by covalently attaching preformed polymers using well-
developed silane chemistries28 and “click”29 reactions. This paper
demonstrates a modular approach for surface attachment of
terminally functionalized linear and dendritic polymers to glass
microfluidic channels.
Previous work includes the demonstrations of this chemistry to
modify silica surfaces and silica gel,35 gold electrodes,31,38 and
single-wall carbon nanotubes.39
The purpose of this paper is to develop and demonstrate a
methodology for covalent attachment of preformed terminally
functionalized linear and dendritic polymers to silica surfaces and
glass microfluidic channels (Scheme 1a) to modify the surface
and thus change the ú potential by using well-known methods of
“click” chemistry. As a consequence of the altered ú potential,
the EOF in channels can be changed in a systematic manner.
EXPERIMENTAL SECTION
Materials and Chemicals. Glass slides (Corning 2947) and
cover glass (Premium No. 1, Fisher Scientific) were cleaned with
acetone, isopropyl alcohol, and deionized (DI) water and then
dried under a nitrogen stream. 11-Bromoundecyltrichorosilane
(BUTS; Gelest, Inc., Morrisville, PA) was used as received and
was vapor deposited using a MVD100 system (Applied Micro-
structures, Inc., San Jose, CA) in a chamber held at 35 °C. Sodium
azide, copper sulfate, sodium ascorbate, and N,N-dimethylforma-
mide (DMF) were purchased from Sigma-Aldrich (Saint Louis,
MO) and used without further purification. The terminal alkynes
used for “click” modification were synthesized in-house, and
information on synthesis methodology is available in the Support-
ing Information.
Silane Treatment. Surface modification was first tested on Si
wafers with 1 µm of thermal oxide. BUTS was vapor deposited in
a chamber held at 35 °C using a MVD100 system. BUTS was
heated to 130 °C to generate sufficient vapor, the vapor injection
pressure being maintained at 175 mTorr.
Nucleophilic Substitution. The bromo-terminated surface
was used as the starting point for “click” modification by conver-
sion to an azido-terminated surface through SN2 nucleophilic
substitution (Scheme 1a). The substitution reaction is carried out
overnight by exposing the bromo-terminated substrates to a
saturated solution of NaN3 in DMF in a covered container. The
sample was then rinsed with DMF followed by methanol and DI
water before drying in a stream of N2.
“Click” Modification. For surface treatments, solutions of the
alkyne-terminated polymers were prepared (5 or 10 mM) in either
ethanol or methanol. The azide-terminated substrates were
exposed to the polymer solutions for a minimum of 2 h in the
presence of CuSO4‚5H2O (0.1 mM) and sodium ascorbate (0.25-2
mol %). The samples were rinsed with copious amounts of water
followed by a rinse in 1% v/v NH4OH and DI water to remove
excess copper.
“Click” chemistry is defined as a class of robust and selective
chemical reactions with high yields, tolerant to a variety of solvents
(including water), functional groups, and air.29 Among these, the
previously known copper-catalyzed Huisgen cycloaddition of
alkynes and azides is considered to be particularly useful for rapid
and facile construction of combinatorial arrays29 for drug discov-
ery.30 The reaction proceeds in aqueous solvents and in the
presence of air to yield stable 1,2,3-triazoles. This reaction has
been utilized for rapid and facile modification of surfaces.31-37
Surface Characterization. All samples were characterized by
water contact angle measurements, X-ray photoelectron spectros-
copy (XPS),40,41 and Fourier transform infrared-attenuated total
reflection spectroscopy (FT-IR-ATR).42,43 In addition, the glass
microfluidic channels were characterized by measuring EOF44 in
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(34) Lin, P.-C.; Ueng, S.-H.; Tseng, M.-C.; Ko, J.-L.; Huang, K.-T.; Adak, A. K.;
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1662 Analytical Chemistry, Vol. 79, No. 4, February 15, 2007