A supramolecular microfluidic optical chemosensor

Article abstract of DOI:10.1021/ja010176g

A supramolecular microfluidic optical chemosensor (μFOC) has been fabricated. A serpentine channel has been patterned with a sol-gel film that incorporates a cyclodextrin supramolecule modified with a Tb³⁺ macrocycle. Bright emission from the T

Full text of DOI:10.1021/ja010176g

Published on Web 02/01/2002
A Supramolecular Microfluidic Optical Chemosensor
Christina M. Rudzinski, Albert M. Young, and Daniel G. Nocera*,†
†
‡,§
Contribution from the Department of Chemistry, 6-335, Massachusetts Institute of Technology,
7
7 Massachusetts AVenue, Cambridge, Massachusetts 02139-4307, and Lincoln Laboratory,
Massachusetts Institute of Technology, 244 Wood Street, Lexington, Massachusetts 02420-9108
Received June 1, 2001. Revised Manuscript Received November 6, 2001
Abstract: A supramolecular microfluidic optical chemosensor (µFOC) has been fabricated. A serpentine
channel has been patterned with a sol-gel film that incorporates a cyclodextrin supramolecule modified
3+
3+
with a Tb macrocycle. Bright emission from the Tb ion is observed upon exposure of the µFOC to
biphenyl in aqueous solution. The signal transduction mechanism was elucidated by undertaking steady-
state and time-resolved spectroscopic measurements directly on the optical chemosensor patterned within
the microfluidic network. The presence of biphenyl in the cyclodextrin receptor site triggers Tb3 emission
by an absorption-energy transfer-emission process. These results demonstrate that the intricate signal
transduction mechanisms of supramolecular optical chemosensors are successfully preserved in microfluidic
environments.
+
specially tailored supramolecular chemosensors1
1-15
in which
Introduction
a noncovalent molecular recognition event at a receptor site is
communicated by physical or chemical means to a reporter site,
which produces a measurable optical signal. This “3R” sensing
scheme of recognize, relay, and report typically relies on the
chemical and physical compatibility between the analyte and
the crafted supramolecular binding site, relay mechanisms
involving intricate energy or electron transfer and specially
tailored photophysical properties of the reporter site.
Microfluidic (µF) devices are emerging as an important
technology for chemo- and bioanalytical applications. The
miniaturization offered by µF devices allows for the analysis
of fluid samples to be performed on a chip, leading to
advantages such as reduced sample volume, increased reaction
speed, and the possibility of massive parallelism.1-8 One method
1
for rapidly prototyping µF networks involves conformally
contacting a micromolded elastomeric polymer such as poly-
Following the 3R approach, we have created a cyclodextrin
(
dimethylsiloxane) to a glass or quartz substrate, thus forming
3
+
9,10
(CD) supramolecule appended with a Tb macrocycle (1 in
enclosed microchannels.
To date, optical analysis on µF
Figure 1) for the optical detection of polyaromatic hydrocarbons
platforms has been performed by direct spectroscopic detection
methods. The advantage of this approach is that signal trans-
duction is easy to implement within the constraints imposed by
the µF environment. The simplicity of the detection method,
however, has inherent limitations; these include difficulties
associated with selectively distinguishing analyte (usually
because an optical signature must be identified from a multi-
component spectral envelope) and the measurement of an optical
signal against a bright background, to name a few. Such
limitations may be overcome when analytes are detected by
1
6,17
in aqueous solutions.
hydrocarbon in the CD bucket is signaled by the appearance of
Molecular recognition of the aromatic
3
+
green luminescence from the encrypted Tb ion. Detailed
mechanistic investigations show signal transduction to proceed
via an absorption-energy transfer-emission (AETE) mechanism
in which excitation energy absorbed by the bound analyte is
3+
transferred from its long-lived triplet excited state to the Tb
1
8
reporter site. The appearance of green luminescence against
a dark background enables polyaromatics to be detected at
submicromolar concentrations with no need for amplification.
In view of the benefits gained in selectivity and sensitivity, the
†
Department of Chemistry, MIT.
Lincoln Laboratory, MIT.
Current address: IBM Emerging Products, Microelectronics Division,
‡
§
Hopewell Junction, New York 12533.
(11) Rudzinski, C. M.; Nocera, D. G. In Optical Sensors and Switches; Schanze,
K., Ramamurthy, V., Eds.; Marcel Dekker: New York, 2001; Vol. 7, p 1.
(12) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.;
McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515.
(13) Chemosensors of Ion and Molecular Recognition; Desvergne, J.-P., Czarnik,
A. W., Eds.; NATO Advanced Study Institute Series C; Kluwer Aca-
demic: Dordrecht, The Netherlands, 1997; Vol. 492.
(
1) Manz, A., Becker, H., Eds. Microsystem Technology in Chemistry and
Life Sciences. Topics in Current Chemistry; Springer-Verlag: Berlin, 1998;
Vol. 194.
(
2) Jakeway, S. C.; de Mello, A. J.; Russel, E. L. Fresenius’ J. Anal. Chem.
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000, 366, 525.
(
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(
3) Sanders, G. H. W.; Manz, A. TrAC, Trends Anal. Chem. 2000, 19, 364.
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Rooij, N. F. Chimia 1999, 53, 87.
(14) Fluorescent Chemosensors for Ion and Molecule Recognition; Czarnik, A.
W., Ed.; ACS Symposium Series 538; American Chemical Society:
Washington, DC, 1993.
(6) Delamarche, E.; Bernard, A.; Schmid, H.; Bietsch, A.; Michel, B.; Biebuyck,
(15) Czarnik, A. W. Acc. Chem. Res. 1994, 27, 302.
H. J. Am. Chem. Soc. 1998, 120, 500.
7) Weigl, B. H.; Yager, P. Science 1999, 283, 346.
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9) Kim, E.; Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1996, 118, 5722.
(16) Mortellaro, M. A.; Nocera, D. G. J. Am. Chem. Soc. 1996, 118, 7414.
(17) Rudzinski, C. M.; Hartmann, W. H.; Nocera, D. G. Coord. Chem. ReV.
1998, 171, 115.
(
(
(
(18) Rudzinski, C. M.; Engebretson, D. S.; Hartmann, W. K.; Nocera, D. G. J.
Phys. Chem. A 1998, 102, 7442.
(
10) Kim, E.; Xia, Y.; Whitesides, G. M. Nature 1995, 376, 581.
10.1021/ja010176g CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC.
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VOL. 124, NO. 8, 2002 1723

A R T I C L E S
Rudzinski et al.
relative to monodispersed polystyrene standards purchased from
Polymer Laboratories. Spin casting was carried out on a Lavall
Technologies WS-400-6NPP\Lite spin coater.
Materials. Lanthanide-modified CD chemosensor 1 was prepared
according to published procedures.16 Starting reagents and solvents for
synthesis were reagent grade or better and were used as received from
Aldrich Chemical Co. Samples of the chemosensor gave satisfactory
analyses, which were performed at H. Kolbe Mikroanalytisches
Laboratorium. The starting material, m-nitrophenyl acrylate, was
prepared by the dropwise addition of 5.0 g (55.0 mmol, 1.0 equiv) of
acryloyl chloride to a stirring solution of 2.5 g of KOH and 3.5 g (25.0
mmol, 0.5 equiv) of m-nitrophenol in 250 mL of water at 0 °C. The
precipitate formed after 10 min of continued stirring and was filtered
and recrystallized from ether to afford long off-white needles of the
product. 13C NMR (d
1
-DMSO) δ/ppm: 165.5, 151.8, 149.6, 136.1,
31.6, 129.7, 127.9, 121.7, 118.0.
Quartz substrates were modified by deposition from aqueous alcohol
6
20
solutions onto substrates cleaned according to the RCA SC-1 method.
A 95%-ethanol/5%-water solution was adjusted to pH 5.0 with acetic
acid, and 3-acryloxypropyltrimethoxysilane (United Chemical Tech-
nologies, Inc.; Bristol, PA) was added with stirring to yield a 2% final
concentration. Five-minute reaction times were allowed for hydrolysis
and silanol formation. The quartz slides were coated with the resulting
solution for a period of 2 min, and then rinsed by dipping in ethanol;
the treated slides were cured for 24 h at room temperature before use.
CD chemosensor 1 was immobilized in an acryloyl polymer film
by adapting previously reported procedures21 to achieve the site-directed
functionalization of the secondary rim of 1 with a vinyl acrylate moiety.
Figure 1. A microfluidic optical chemosensor fabricated in a serpentine
channel configuration. The white, sol-gel squares contain the supra-
molecular chemosensor (1), a cyclodextrin strapped by a DTPA macrocycle
in which a Tb ion resides. The conical bucket schematically represents
the cyclodextrin receptor site.
-2
Chromatographically purified samples of 1 (0.10 g, 6.7 × 10 mmol,
3
+
1.0 equiv) in 10 mL of pH 11 carbonate buffer were combined with
-
2
0
.012 g (6.7 × 10 mmol, 1.0 equiv) of m-nitrophenyl acrylate in
acetonitrile in air. The clear solution immediately turned bright yellow
upon shaking, indicating the presence of m-nitrophenolate in solution.
Shaking was continued for 5 min after which enough 0.1 M HCl was
added to neutralize the solution, causing the solution to turn colorless.
Water was removed by rotary evaporation to yield a pale yellow solid.
The acryloyl-modified product of 1 was purified on a G-15 Sephadex
incorporation of supramolecular optical chemosensors onto µF
platforms presents the opportunity for the discovery of new
sensing applications. The design of such devices, however,
demands that the functional requirements of the supramolecular
optical chemosensor are retained within a µF environment. The
host matrix of the µF chemosensor must permit analyte to access
and recognize the supramolecule binding site without hindering
selectivity or specificity and without perturbing the stereoelec-
tronic factors that control the relay and reporting steps of the
signal transduction mechanism. Accordingly, we sought to
ascertain whether optical supramolecular chemosensor function
could be preserved in a µF platform. Here we report methods
to immobilize chemosensor 1 within inorganic and organic
polymer host matrixes and show by steady-state and transient
laser spectroscopy that the 3R sensing strategy may be
implemented on a patterned µF device, thus allowing us to
construct the first supramolecular microfluidic optical chemo-
sensor (µFOC).
(
Aldrich Chemical Co.) size exclusion column to give purified product
1
in 24% yield. H NMR (D O) δ/ppm: 2.5-4.0 (m, 84H), 4.9 (m, 7H),
2
6.0 (m, 3H). 13C NMR (D O) δ/ppm: 176.8, 172.3, 168.0, 132.4, 127.3,
2
102.0, 84.5, 81.5, 73.2, 72.3, 60.5, 57.7, 57.3, 56.5, 53.2, 52.3, 49.4,
41.8, 39.1. Anal. Calcd for C59H N O42: C, 45.88; H, 6.07; N, 4.53;
93 5
O, 43.51. Found: C, 45.73; H, 6.12; N, 4.23; O, 43.80.
Acryloyl-modified 1 was polymerized by dissolving 80 mg of the
compound in 10 mL of H
,2′-azobis(isobutyronitrile) was added. The resulting solution was
freeze-pump-thawed four times and filled with N prior to radical
2
O to which a methanol solution of 0.1%
2
2
initiation, which was induced by heating to 50 °C. Solvent was removed
by rotary evaporation after 24 h to give the water-soluble solid product.
13
(M
n
) 44 000, M
w
) 83 000). C NMR (D
2
O) δ/ppm: 176.8, 172.3,
1
5
2
68.0, 102.0, 84.5, 81.4, 73.2, 72.2, 69.7, 65.5, 64.0, 60.5, 57.9, 55.4,
1
3.6, 51.3, 49.1, 47.5, 46.8, 40.9, 29.6. H NMR (D
2
O) δ/ppm: 2.5 (t,
H), 2.9-3.8 (m, 109H), 4.9 (m, 7H). Acryloyl polymers of 1 were
Experimental Section
introduced onto a quartz substrate by performing the identical polym-
erization reaction on the surface of a quartz platform surface modified
with vinyl acrylate.
13
1
General Methods. C and H NMR and 2-D total correlation spectra
1
9
(
TOCSY) were obtained at the MIT Department of Chemistry
Instrumentation Facility using VARIAN Inova-500 MHz NMR spec-
trometers with dual broadband RF hosting an Oxford Instruments Ltd.
superconducting magnet. TOCSY was run at the set temperature of 25
Chemosensor was immobilized in sol-gel films by following the
methods of Bright and Prasad et al.22 A partially hydrolyzed sol was
prepared by stirring tetraethyl orthosilicate TEOS (Aldrich Chemical
Co.), ethanol, and 0.1 N HCl (1.5:2:0.5) in a closed container in air
for 5 h. To 100 µL of this sol, 100 µL of a 0.025-0.032 M aqueous
°
C with a mixing time of 60 ms. Data were processed using a Sun
Ultra 5 workstation. Polymer molecular weights were determined with
a Hewlett-Packard 1100 series HPLC equipped with a PL gel mixed-C
column (5 µm) using THF as the mobile phase at a rate of 1.0 mL/
min. Gel permeation chromatography (GPC) measurements were made
(20) Elam, J. H.; Nygren, H.; Stenberg, M. J. Biomed. Mater. Res. 1984, 18,
9
53.
(
(
21) Harada, A.; Furue, M.; Nozakura, S. Macromolecules 1976, 9, 9.
22) Narang, U.; Dunbar, R. A.; Bright, F. V.; Prasad, P. N. Appl. Spectrosc.
1993, 47, 1700.
(19) Ikeda, H. Chem. ReV. 1998, 98, 1755.
1
724 J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002
9

Supramolecular Microfluidic Optical Chemosensor
A R T I C L E S
solution of 1 was added while stirring; mixing was continued in a closed
container for an additional 45 min. Aliquots of the CD-doped sol (∼20
Scheme 1
2
µL for a 1 cm quartz substrate) were introduced onto the surface of a
clean quartz substrate (RCA SC-1), which was then sealed in an airtight
container for 72 h to allow the sol to gel and age. Unbound CD was
removed from monoliths by washing with spectroscopic grade water.
Profilometry measurements (Tencor Alpha-Step 200; KLA-Tencor
Corp., San Jose, CA) showed that film thicknesses of 2-6 µm were
obtained from this procedure. Thinner films (1 µm thickness) were
achieved by spin casting sol-gel films at 2000 rpm for 30 s.
Chemosensor-coated quartz substrates (ESCO Products, Oak Ridge,
NJ) were patterned using deep UV exposure in an AZ327 developer
after application of Hoechst Celanese (Somerville, NJ) AZ1512 positive
photoresist. Dip-etching was accomplished by employing a buffered
HF solution, and photoresist was subsequently removed in an 80 °C
Act 1 photoresist stripper (ACT, Inc., Allentown, PA). The thicknesses
of the coated surfaces were measured by profilometry.
Physical Methods. Steady-state emission spectra were recorded on
a high-resolution instrument that has been previously described.18
Terbium ion luminescence, produced upon excitation with 273 nm
wavelength light from a Hg-Xe lamp, was captured with a R943-02
Hamamatsu (Bridgewater, NJ) photomultiplier tube interfaced to a
Stanford Research Instruments (Palo Alto, CA) SR400 gated photon
counter. Spectra were corrected for detector response. All measurements
were performed at 25(2) °C. Quartz substrates, modified with sol-gel
films of 1 (3 µm films unless otherwise specified), were exposed to
solutions of varying concentrations of biphenyl in water; the maximum
group is introduced under mild conditions onto the secondary
rim of the CD bucket. The reaction is conveniently monitored
by following the vinyl resonances, which appear in a spectrally
-
5
concentration of analyte was limited by its solubility (5 × 10 M).
Samples were mounted 45° to the incident beam to enable front-face
detection; a machined holder allowed the quartz substrate to be
reproducibly positioned. Excitation light was eliminated with 450 and
1
13
uncongested region of both H and C NMR spectra of 1. For
the former spectrum, the vinyl and CD protons integrate as 3H
13
and 90H, respectively. The C NMR spectrum is more
4
70 nm long pass filters. Spectroscopic measurements on the µF
distinguishing with the appearance of three doublets, which we
ascribe to the carbonyl (168.0 ppm) and the two vinylic carbons
prototype were performed by flowing aqueous solutions of analyte
through the capillary channels of the µF device.
Time-resolved data were collected on a nanosecond laser instrument
utilizing a Coherent (Auburn, CA) Infinity XPO tunable laser (fwhm
(132.4 and 127.3 ppm) of the acryloyl substituent. The crucial
step in the synthetic protocol is the inclusion of the polymer
precursor, m-nitrophenyl acrylate, within the CD to direct
functionalization to a single secondary hydroxyl group. The
limiting capacity of the CD cavity to incorporate only one
conformationally restricted m-nitrophenyl acrylate precursor is
responsible for functionalization at a single site. Mono func-
tionalization is crucial to the overall performance of the µFOC
since the entrance of the CD is the least sterically encumbered,
thus providing maximum access of analyte to the receptor site
of the immobilized supramolecule. Polymerization of acryloyl-
modified 1 by the free-radical initiator 2,2′-azobis(isobutyro-
nitrile) affords a polymer possessing the pendant supramolecular
)
7 ns) as the source. The Infinity Nd:YAG laser system consisted of
an internal diode pumped, Q-switched oscillator, which provided the
seed pulse for a dual rod, single lamp, amplified stage. Sequential
second and third harmonics of 532 and 355 nm radiation were generated
from the Nd:YAG 1064 nm fundamental via Type I polarization and
frequency mixing in tuned BBO crystals. The resultant 355 nm third
harmonic was subsequently passed thorough Type I tunable XPO and
second harmonic cavities capable of producing the required 273 nm
excitation wavelength. Emitted light from the sample passed through
f/4 collimating and f/4 focusing lenses onto the entrance slit of an
Instruments SA (Edison, NJ) Triax 320 monochromator. The signal
wavelengths were dispersed by a grating possessing a blaze wavelength
of 500 nm and 300 grooves/cm and detected with a Hamamatsu R928
PMT. The output from the PMT was fed into a LeCroy (Chestnut Ridge,
NY) 1 GHz 9384CM digital oscilloscope, which was triggered from
the Q-switch sync output of the laser. Monochromator operation, data
storage, and data manipulation were managed by National Instruments
driver software (Labview) incorporated into programs written at MIT.
Communication between a Dell Optiplex GX-1 computer and the
instrumentation was achieved through an IEEE-488 (GPIB) interface.
Excited-state kinetics data were obtained by signal averaging (1000
1
3
chemosensor. Disappearance of the vinyl C resonances at
27.3 and 132.4 ppm is indicative of complete polymerization.
1
A concomitant growth of resonances at 45-70 ppm for the
polymer chain carbons was observed over the course of the
2
3
1
reaction. The corresponding alkyl H resonances at 3.3 and
1
2.5 ppm were identified in the 2-D H NMR spectrum.
Whereas a well-characterized functionalized polymer was
obtained from the procedures of Scheme 1, attempts to im-
mobilize the polymer onto quartz substrates resulted in low and
inconsistent surface coverages. For this reason, we turned our
efforts to alternative supports. Specifically, we investigated sol-
gel films as a support of 1 owing to their superior properties in
3
+
pulses at 20 Hz) Tb emission at 546 nm.
Results and Discussion
24-28
The construction of a µFOC requires the immobilization of
the optical chemosensor in a support that is compatible with
lithographic patterning protocols. Scheme 1 presents a site-
directed functionalization strategy that leads to the successful
incorporation of 1 into organic host matrixes. A vinyl acrylate
regard to optical sensing applications.
Sol-gels are optically
transparent throughout the ultraviolet spectral region, and
(
23) Britton, D.; Heatley, F.; Lovell, P. A. Macromolecules 1998, 31, 2828.
24) Avnir, D. Acc. Chem. Res. 1995, 28, 328.
(
(25) Dunn, B.; Zink, J. I. J. Mater. Chem. 1991, 1, 903.
J. AM. CHEM. SOC.
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Rudzinski et al.
Figure 3. The time evolution of Tb3 emission from a patterned thin film
(6 µm in thickness) of the microfluidic device shown in Figure 1 after (a)
exposure to a 50 µM aqueous solution of biphenyl followed by (b) a
methanol rinse to remove biphenyl from film. Signal reproducibility is
maintained over repeated cycles.
+
Figure 2. Emission from a patterned thin film of the microfluidic device
shown in Figure 1 in contact with (a) a 50 µM aqueous solution of biphenyl
and (b) an aqueous solution in the absence of analyte. The intensity of the
luminescence of spectrum a is ×45-fold that of spectrum b.
monoliths can be made with varying porosity to permit analyte
to diffuse to and from an irreversibly immobilized chemosensor
active site. In addition, numerous casting methodologies permit
films of various thicknesses to be readily synthesized.
One millimeter squares of sol-gel films of 1, photolitho-
graphically patterned within the serpentine channel of the µF
platform shown in Figure 1, exhibit excellent optical responses
to polyaromatics. Whereas weak Tb3 emission is observed from
the direct irradiation (λexc ) 273 nm) of thin films (patterned
squares of 3 µm thickness containing 4.8 nmoles of 1), the
luminescence intensity of Tb3 is significantly enhanced when
analyte is flowed through the µF network of Figure 1. As shown
+
+
3+
in Figure 2, a ×45-fold enhancement of the Tb luminescence
intensity is triggered when a 50 µM solution of biphenyl contacts
Figure 4. The growth and subsequent decay of Tb3 luminescence (λdet )
+
3+
the patterned microstructure. Sol-gel films containing only Tb
5
46 nm) from 1 upon the excitation of biphenyl with a Nd:YAG nanosecond
3
+
ion or the Tb macrocycle (not appended to CD) exhibit no
variation in luminescence intensity upon addition of biphenyl.
Equivalent concentrations of analyte added to aqueous solutions
of 1 generate significantly smaller enhancements (e.g., ×8-fold
enhancement for [biphenyl] ) 50 µM), suggesting that analyte
more strongly associates to the CD bucket of 1 when im-
mobilized in sol-gel films. The Tb3 luminescence enhance-
ment proves to be concentration-dependent, increasing mono-
tonically with the concentration of biphenyl; detection of
biphenyl at 5 µM is easily achieved. The detection response
varies approximately linearly with the concentration of 1 in sol-
gel monoliths. For example, the enhancement in luminescence
intensity is ×8, ×27, and ×45 upon exposing films containing
laser (λexc ) 273 nm). The time axis for the magnified rise time trace is
-
5
from 0 to 3 × 10 s. The solid lines display the exponential fits of the
data from which the rise time and decay rate constants were derived.
considerable structural and optical integrity; the signal response
of 1 is maintained over a period of greater than 3 months with
little degradation of the signal response (<10% change in signal
intensity).
The signal transduction mechanism for the µFOC was
investigated by undertaking steady-state and time-resolved
luminescence spectroscopic measurements directly on patterned
squares in contact with aqueous solutions of analyte. An AETE
process was confirmed by examining the emission characteristics
of the µFOC as the wavelength of the excitation light was
+
1
)
.2, 3.6, and 4.8 nmols of 1 to aqueous solutions of [biphenyl]
50 µM. We estimate from the extrapolation of these data to
3+
scanned. The intensity of the green luminescence from Tb
tracked the absorption profile of biphenyl and not the Tb ion
3+
the limits of our detector response that analyte can be detected
reliably with µFOC platforms patterned with 700 nm thick films.
The detection response is reversible and reproducible (Figure
3
+
itself, unequivocally establishing that the Tb emission is
excited by light passing through biphenyl. The kinetics of the
AETE mechanism were ascertained by focusing the excitation
laser pulse of a nanosecond transient laser system onto the
µFOC. The singlet excited state of biphenyl, produced upon
absorption of the exciting photon, decays within the 7 ns laser
3
), though cycle times are slow (∼1 h when purging with
methanol/aqueous solutions). Increased response times (∼10
min) are observed for spin-cast films owing to the decreased
film thickness (1 µm thickness). The µFOC platform exhibits
3
+
pulse. As shown in Figure 4, the Tb luminescence appears at
significantly longer times; a monoexponential fit of the rise time
(
(
(
26) Wolfbeis, O. S.; Reisfeld, R.; Oehme, I. Optical and Electronic Phenomena
in Sol-Gel Glasses. Structure and Bonding; Springer-Verlag: New York,
3+
of the Tb emission yields a first-order rate constant of 4.0 ×
1
996; Vol. 85, p 51.
4
-1
27) Harrod, J. F., Laine, R. M., Eds. Applications of Organometallic Chemistry
in the Preparation and Processing of AdVanced Materials; Kluwer
Academic: Boston, 1995.
10 s . The incommensurate time scales between the decay of
3+
the singlet excited state of biphenyl and the appearance of Tb
luminescence signifies the participation of biphenyl’s triplet
28) Hench, L. L.; West, J. K. Chem. ReV. 1990, 90, 33.
1726 J. AM. CHEM. SOC.
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Supramolecular Microfluidic Optical Chemosensor
A R T I C L E S
excited state in mediating the overall AETE process. The
Figure 1. Signal amplification should allow even further
reduction of µFOC length scales (including a reduction in optical
chemosensor film thickness), enabling the development of
higher-performance µFOC devices.
1
initially prepared ππ* excited state of biphenyl decays to its
3
corresponding ππ* excited state, from which energy transfer
5
3+
to the D4 emitting state of the Tb occurs. A fit of the decay
curve of Figure 4 establishes that the green luminescence
emanating from the Tb3+ excited state decays with a lifetime
Acknowledgment. C.M.R. is a Corning Foundation Fellow.
This research was initiated by an ACC project grant (Project
No. 271) from MIT Lincoln Laboratory. Additional support was
provided by grants from the Center for Sensor Materials, a
Materials Research Science and Engineering Center sponsored
by the National Science Foundation (DMR-9400417), and from
the Air Force of Scientific Research, Air Force Material
Command, USAF, under grant number F49620-98-1-0203. The
U.S. Government is authorized to reproduce and distribute
reprints for governmental purposes notwithstanding any copy-
right notation thereon. Opinions, interpretations, conclusions,
and recommendations contained herein are those of the authors
and are not necessarily endorsed by the United States Air Force,
the Air Force Office of Scientific Research, or the U.S.
Government.
of 1.5 ms. Such long decay lifetimes are observed only when
3
+
the O-H oscillators of H2O are excluded from the Tb
coordination sphere by the DTPA strap of 1.
Chemosensor 1 operates by one of the most intricate optical
signal transduction pathways reported to date. That such a
mechanism is preserved in the µF network of Figure 1 bodes
well for the general development of supramolecular µFOC
devices. As the architectural elements in which optical chemosen-
2
9
sors function move toward the micro- and even nanoscale,
sensitivity will be compromised because fewer active sites will
be available to produce a signal. Accordingly, attendant to the
dimensional reduction inherent to µFOC design will be the need
to increase the intensity of the signal response. Current efforts
3
0,31
are focused on utilizing sensory amplification
and conjugated
fluorescence polymers32 as gain media for the µFOC shown in
Supporting Information Available: Experimental details
(
(
(
(
29) Mirkin, C. A. Inorg. Chem. 2000, 39, 2258.
(PDF). This material is available free of charge via the Internet
30) Swager, T. M. Acc. Chem. Res. 1998, 31, 201.
at http://pubs.acs.org.
31) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537.
32) Wang, J.; Wang, D. L.; Miller, E. K.; Moses, D.; Bazan, G. C.; Heeger, A.
J. Macromolecules 2000, 33, 5153.
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