Light-regulated electrostatic interactions in colloidal suspensions

Article abstract of DOI:10.1021/ja054666a

The net charge of a colloidal particle was controlled using light and a new photocleavable self-assembled monolayer (SAM). The SAM contained a terminal ammonium group and a centrally located carboxylic acid group that was masked with an ortho-nitrobenzyl functionality. Once exposed to UV light, the 2-nitrobenzyl group was cleaved, therefore transforming the colloidal particle from a net positive (silica-SAM-NH₃⁺) to a net negative (silica-SAM-COO^-) charge. By varying the UV exposure time, their zeta potential could be tailored between +26 and -60 mV at neutral pH. To demonstrate a photoinduced gel-to-fluid phase transition, a binary colloidal suspension composed of silica-SAM-NH₃⁺ and negatively charged, rhodamine-labeled silica particles was mixed to form a gel. Exposure to UV light rendered all of the particles negative and therefore converted the system into a colloidal fluid that settles to form a dense sediment. Copyright

Full text of DOI:10.1021/ja054666a

Published on Web 09/30/2005
Light-Regulated Electrostatic Interactions in Colloidal Suspensions
Kyle N. Plunkett,§ Ali Mohraz,# Richard T. Haasch,† Jennifer A. Lewis,*,# and Jeffrey S. Moore*,§
Departments of Chemistry and Materials Science & Engineering and the Materials Research Laboratory,
UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Received July 13, 2005; E-mail: jsmoore@uiuc.edu; jalewis@uiuc.edu
Colloidal suspensions enjoy widespread use in applications
ranging from advanced materials to drug delivery. By tailoring the
interactions between particles, one can design colloidal fluids, gels,
or crystals needed for ceramic,1 coating,2 ink,3 photonic,4,5 and
pharmaceutical materials.6 One common strategy is to alter their
electrostatic interactions by varying either the pH or ionic strength
of the solution in which they are suspended.7 However, problems
such as nonuniform mixing and disruption of suspension micro-
structure arise when acid, base, or electrolyte species are added.
The ability to tailor electrostatic interactions between colloids with
spatial and temporal control in the absence of chemical additions
would not only alleviate these problems but also open new
possibilities in colloidal assembly. Here we demonstrate an approach
to modulate the surface charge of colloidal particles and, hence,
their electrostatic interactions by irradiation with UV light.
To generate light-sensitive colloidal particles, we designed a new
self-assembled monolayer (SAM) that undergoes charge inversion
upon photocleavage (Figure 1). Each molecule of the SAM is
+
initially terminated with a positively charged ammonium (-NH3
)
group. Photocleavage of the ortho-nitrobenzyl linker8 then exposes
an underlying negatively charged carboxylate (-COO-) group.
Accordingly, silica particles functionalized with this SAM are
expected to possess an effective charge (or zeta potential) that can
be tuned through a broad range of positive, neutral, and negative
values at fixed pH simply by exposure to UV light for varying
times. Because their zeta potential (ú) can be regulated by irradiation
alone, it should be possible to induce phase changes within colloidal
suspensions without externally altering their solution composition.
As described in detail in the Supporting Information, chlorosilane
1 is synthesized in three steps from the known 4-(hydroxymethyl)-
3-nitrobenzoic acid.9 Immobilization of 1 on dried silica micro-
spheres (∼1 µm diameter) is accomplished in toluene with
triethylamine as an acid scavenger (Figure 1). Brief exposure to
trifluoroacetic acid (TFA) in methylene chloride converts the BOC
group to an ammonium cation. This stable colloidal suspension
undergoes rapid flocculation upon exposure to TFA. After washing
with methylene chloride, the ammonium-terminated microspheres
(silica-SAM-NH3+) are washed with benzene and lyophilized.
To confirm the expected photochemical transformation, the
particles’ surface composition was examined via X-ray photoelec-
tron spectroscopy (XPS) before and after irradiation. As shown in
Figure 2, the nitrogen 1s photoelectron peak decreases substantially
upon exposure of colloidal suspensions to UV light. This observa-
tion is consistent with photoinduced cleavage of the bifunctional
SAM, which results in a loss of amino and nitro groups from the
microsphere surface. From the XPS integrated intensities, it is
estimated that 85-95% of the nitrogen has been removed from
the silica-SAM after 40 min of UV exposure.10
Figure 1. Immobilization of a bifunctional, photocleavable SAM on the
silica particle surface.
+
Figure 2. XPS spectra of the nitrogen 1s photoelectron. Silica-SAM-NH3
microspheres irradiated by UV light for 40 min (red) contained 85-95%
less nitrogen than their as-synthesized counterparts (blue).
To quantify the photosensitivity of particle charge, colloidal
suspensions of silica-SAM-NH3+ microspheres were systematically
exposed to UV light for varying times. Particles collected from
the various UV exposure times were then resuspended in an aqueous
solution (ca. pH 7)11 and subsequently analyzed by microelectro-
phoresis. Figure 3 shows that ú varied from an initial positive value
of +26 mV and reached an asymptotic limit in the negative range
(-60 mV) after ca. 40 min. It is worthwhile to note that these
changes are accomplished at constant pH.
To demonstrate a photoinduced phase transition in silica-SAM-
NH3+ microsphere suspensions, a binary colloidal suspension was
prepared by slowly adding a suspension of the photosensitive silica-
§ Department of Chemistry.
+
SAM-NH3 microspheres to a suspension of negatively charged,
# Department of Materials Science & Engineering.
† Materials Research Laboratory.
rhodamine-labeled silica microspheres (this mixture will be referred
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J. AM. CHEM. SOC. 2005, 127, 14574-14575
10.1021/ja054666a CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
tions via irradiation with UV light. The approach opens up new
avenues for investigating the phase transitions and dynamic structure
evolution of both unary and binary colloidal suspensions. As one
example, we demonstrated the gel-to-fluid transition in binary
mixtures that were initially oppositely charged and, ultimately, like-
charged after exposure to UV light for an appropriate time.
However, a myriad of phase changes are possible using this
approach, including fluid-to-gel, gel-to-fluid, or gel-to-crystal
+
transitions for pure silica-SAM-NH3 suspensions of increasing
colloid volume fraction as well as a fluid-to-gel transition in binary
mixtures, depending on whether the colloids begin as like-charged
or neutral species that are converted into neutral-, like-, or oppositely
charged species, respectively, upon irradiation. These light-
responsive systems will further enable novel assembly routes for
creating colloidal structures via photoinduced patterning without
the use of photopolymerizable resins.
+
Figure 3. Zeta potential of silica-SAM-NH3 microspheres as a function
of UV irradiation time. Solid line was added to show the trend.
to as the control suspension). Confocal microscopy of this mixture
reveals the presence of clusters of oppositely charged particles that
form immediately upon adding the suspensions together (Figure
4). More specifically, chainlike aggregates comprised of alternating
silica-SAM-NH3+ and rhodamine-labeled microspheres can be seen
in the control suspension, as expected due to the heterofloccula-
tion.12-15 A second colloidal suspension was prepared in the same
way as the control, except that it was irradiated with UV light for
40 min after mixing. A dramatic change in the colloidal packing
efficiency accompanies UV irradiation, which is visually apparent
by comparing the light-exposed suspension to the control. The
sediment height for the control suspension is substantially higher
than that of the UV exposed sample, consistent with a gel-to-fluid
phase transition in response to light-based stimuli (Supporting
Information). Confocal images of the sediment assembled from the
light-exposed suspension reveals the dense structure formed under
gravity-driven microsphere settling. Time-resolved microscopy
carried out on this sample after gentle shaking confirmed the
particles to be fully dispersed, undergoing uncorrelated Brownian
displacements (data not shown) that suggest all of the silica
microspheres now possess a net negative charge.
Acknowledgment. This material is based upon work supported
in part by the U.S. Department of Energy, Division of Materials
Sciences under Award No. DEFG02-91ER45439, through the
Frederick Seitz Materials Research Laboratory at the University
of Illinois at UrbanasChampaign and by the Nanoscale Science
and Engineering Initiative of the National Science Foundation under
NSF Award No. DMR-0117792. XPS measurements were per-
formed in the Center for Microanalysis and Materials, University
of Illinois, which is partially supported by the U.S. Department of
Energy Award No. DEFG02-91ER45439.
Supporting Information Available: Experimental protocols, image
of colloidal assembly in sedimentation tube. This material is available
free of charge via the Internet at http://pubs.acs.org.
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(10) The C, N, and O content of these samples before and after cleavage is
+
quantified as: 52.6% C, 2.8% N, and 44.6% O for silica-SAM-NH3
before UV exposure and 45.7% C, 0.2% N, and 54.1% O after 40 min
exposure. Because the signal-to-noise is low, an exact quantification of
the cleavage yield was not obtainable.
(11) Not surprisingly, added salt attenuates the effects described here. Thus,
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Figure 4. Confocal images (x-y scans) of a binary colloidal suspension
of rhodamine-labeled silica microspheres (red, imaged in fluorescent mode)
and silica-SAM-NH3+ (green, imaged in reflection mode) before (left) and
after (right) irradiation by UV light for 40 min. Scale bar ) 4 µm.
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In conclusion, a novel silica-based photocleavable SAM has been
developed that provides exquisite control over interparticle interac-
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