Stimuli-Responsive Polymer Film that Autonomously Translates a Molecular Detection Event into a Macroscopic Change in Its Optical Properties via a Continuous, Thiol-Mediated Self-Propagating Reaction

Article abstract of DOI:10.1021/jacs.5b08582

This Communication describes a chemically responsive polymer film that is capable of detecting low levels of a specific applied molecular signal (thiol) and subsequently initiating a self-propagating reaction within the material that converts the nonfluorescent film into a globally fluorescent material. We illustrate that the intensity of the resulting fluorescent material is independent of the quantity of the applied thiol, whereas the rate to reach the maximum level of signal is directly proportional to the quantity of the signal. In contrast, a control film, which lacks functionality for mediating the self-propagating reaction, provides a maximum change in fluorescence that is directly proportional to the quantity of the applied thiol. This level of nonamplified signal is 78% lower in intensity (when initiated with 100 μM of applied thiol) than is achieved when the material contains functionality that supports the self-powered, self-propagating amplification reaction.

Full text of DOI:10.1021/jacs.5b08582

Communication
pubs.acs.org/JACS
Stimuli-Responsive Polymer Film that Autonomously Translates a
Molecular Detection Event into a Macroscopic Change in Its Optical
Properties via a Continuous, Thiol-Mediated Self-Propagating
Reaction
Hemakesh Mohapatra,† Hyungwoo Kim,† and Scott T. Phillips*
Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States
S
* Supporting Information
ABSTRACT: This Communication describes a chemi-
cally responsive polymer film that is capable of detecting
low levels of a specific applied molecular signal (thiol) and
subsequently initiating a self-propagating reaction within
the material that converts the nonfluorescent film into a
globally fluorescent material. We illustrate that the
intensity of the resulting fluorescent material is independ-
ent of the quantity of the applied thiol, whereas the rate to
reach the maximum level of signal is directly proportional
to the quantity of the signal. In contrast, a control film,
which lacks functionality for mediating the self-propagating
reaction, provides a maximum change in fluorescence that
is directly proportional to the quantity of the applied thiol.
This level of nonamplified signal is 78% lower in intensity
(when initiated with 100 μM of applied thiol) than is
Figure 1. Illustration of a polymeric film that transforms completely
from nonfluorescent to fluorescent when exposed to trace levels of
achieved when the material contains functionality that
supports the self-powered, self-propagating amplification
added thiol. The added thiol initiates a self-propagating reaction within
reaction.
the film that amplifies the fluorescent output. The bottom half of the
figure depicts the specific polymer (1) and self-propagating reaction
that provides the global change from nonfluorescent to fluorescent
across the entire material.
he ability of a self-propagating reaction to impart an
Tautonomous, amplified, macroscopic response to a
material (without assistance from reagents supplied in solution)
is exceedingly rare in synthetic materials, but is common in
living systems (e.g., Venus flytraps, touch-me-nots, or sea
cucumbers).1 Few reagents, reactions, and polymers are able to
achieve biological-like amplified responses in materials;2 thus,
the vast majority of stimuli-responsive materials are designed to
change macroscopically when large quantities of signal is
applied.3 As a step toward overcoming this limitation, herein we
describe a self-propagating reaction that occurs in a polymeric
material (Figure 1), thus offering a means for amplifying a
molecular detection event into a macroscopic change in a
material. The response arising from the self-propagating
reaction is analogous to living systems in that the material
changes equally regardless of whether the material is exposed to
trace levels of the specific applied signal or stoichiometric
quantities.
demonstrate the use of the reagent to create a polymeric
material that changes its macroscopic properties (optical
properties in this case) autonomously in response to local
and even fleeting stimuli. Such materials are highly sought after,
yet are largely unrealized. Possible applications of such
materials could include new types of reconfigurable camouflage,
self-healing materials, reconfigurable biomedical materials,
debondable adhesives, and materials that change porosity
and/or wetting properties to prevent exposure of underlying
objects, among a variety of other possible applications. This
Communication offers one path forward for realizing this long-
term potential, with a focus on changing the optical properties
of the material.
Polymer 1 is key to our design: each repeating unit in 1
contains the functionality needed to support the self-
propagating reaction. This functionality includes a 2,4-
dinitrobenzenesulfonyl (Nosyl) protected phenol, which
cleaves selectively in response to added thiol.4 Removal of
the Nosyl group allows the liberated phenol to cyclize into the
In brief, this Communication provides two key advances: (i)
A novel design for a small molecule signal amplification reagent
(which we ultimately convert into a monomer). This single
reagent enables a self-propagating amplification reaction while
simultaneously minimizing background reaction, thus enabling
substantial signal amplification in aqueous media. (ii) We then
Received: August 13, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b08582
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Journal of the American Chemical Society
Communication
thioester, releasing one equivalent of ethanethiol while
simultaneously forming an equivalent of fluorescent, polymer-
bound 7-alkoxycoumarin (2) (Figure 1). The released
ethanethiol is free to react with another Nosyl group on a
neighboring repeating unit in the polymeric material, thus
continuing the self-propagating reaction and, consequently,
amplifying the quantity of fluorescent 2. Since each reaction
between ethanethiol and the Nosyl group in 1 simultaneously
consumes and produces one equivalent of thiol, the net
concentration of thiol remains constant throughout the self-
propagating reaction. In other words, a single detection event
between a material made from 1 and an applied thiol should
lead to conversion of the entire material into 2.
To test this design, we prepared polymer 1 in six linear steps
using the route depicted in Scheme 1. We also used a similar
a
Scheme 1. Synthesis of Polymer 1 and Control Polymer 10
a
Reagents and conditions: (a) BrCH2CO2tBu, NaHCO3, 70 °C, DMF
(42%); (b) TFA, CH2Cl2; (c) HBTU, DIEA, DMAP, MeCN (42%
over 2 steps); (d) DNBS-Cl, DIEA, CH2Cl2, −78 to 20 °C (87%); (e)
K2CO3, 18-crown-6, THF, −20 °C (66% for 8 and 43% for 9); (f) (i)
Grubb’s second gen. cat., CH2Cl2, 20 °C; (ii) ethyl vinyl ether (81%
for 1 and 82% for 10).
sequence to prepare control polymer 10, which differs from 1
only by replacement of the ethyl thioester (1) with an ethyl
ester (10) (Scheme 1). Control polymer 10 is capable of
consuming one equivalent of added thiol and producing one
equivalent of fluorescent product 2, but not of releasing thiol to
continue propagating the reaction. Thus, 10 provides a
benchmark for a stoichiometric response between the plastic
and a specific quantity of an applied thiol.
Before evaluating whether 1 is capable of supporting a self-
propagating reaction in the context of a macroscopic plastic, we
first tested monomer 8 since it is readily soluble and the
products of its reaction with added thiol are easily analyzed
using liquid chromatography coupled to a mass spectrometer
(LCMS). Treatment of 8 (2 mM) in 1:1 MeCN−buffered
water (pH 7.4) with 0.1 equiv of L-cysteine (a model thiol)
yielded the expected 7-alkoxycoumarin product (11, Figures
2a,b), ethanethioether 12, and L-cysteine adduct of the cleaved
Nosyl group (13) (Figures 2a,b). Production of ethane-
thioether 12, in particular, is consistent with a self-propagating
reaction, as is the observation that nearly all of 8 is consumed
during the experiment despite the 10-fold excess of 8 relative to
the added thiol.
Figure 2. Demonstration that monomer 8 is capable of supporting the
self-propagating reaction. (a) The reaction conditions and products
when 8 is exposed to thiol (cysteine in this example). (b) LCMS
chromatograms that reveal the temporal change in concentration of
reactants and products when 8 (2 mM) in 1:1 MeCN−buffered water
(pH 7.4) is exposed to 0.1 equiv of L-cysteine at 23 °C. (c) Kinetics
(obtained from fluorescence measurements) that support the self-
propagating reaction in (a). These results demonstrate that even trace
levels of added thiol elicit a maximum fluorescence response over time.
The graph includes all data corresponding to three replicates per
equivalent of added cysteine. The inset clarifies the response for 1
equiv of cysteine. The λex for coumarin 11 is 320 nm, while the λem is
380 nm. The lines are simulated data for the reaction A + B → C + A
(see the text for details) superimposed on the experimental data
points.
and hence the relative quantity of 11 produced during the self-
propagating reaction (Figure 2c). These experiments revealed
that the same level of fluorescent signal is obtained regardless of
whether 8 is treated with 1 equiv or 0.2 equiv of added thiol
(Tables S1 and S3), and that only the time required to reach
this maximum level of signal varies between reactions. In the
absence of added thiol, minimal background fluorescence was
Similar experiments using 8 and different initial concen-
trations of added L-cysteine were analyzed using fluorescence
spectroscopy to quantify the intensity of fluorescence from 11
B
DOI: 10.1021/jacs.5b08582
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Journal of the American Chemical Society
Communication
observed over the duration of the experiment, which is a highly
desirable feature for an amplification reaction.5
Moreover, the experimental fluorescence data is consistent
with the self-propagating reaction A + B → C + A, where A =
thiol, B = reagent 8, and C = coumarin 11. Simulations of this
reaction (eq 1) superimpose with the experimental data, as
illustrated in Figure 2c.
([B]0 −[A]0)kt
[A]0[B]0(e
− 1)
Imeasured = y =
([B]0 −[A]0)kt
[B]0e
− [A0]
(1)
In this equation, Imeasured is the normalized fluorescence
intensity, [A]0 and [B]0 represent the concentrations of A
and B (respectively) at the outset of the reaction, k is the rate
constant for the reaction, and t is exposure time.6
We next tested whether this self-propagating reaction would
be effective in the context of a solid-state material made from
polymer 1. Since we planned to use the same 1:1 MeCN−
buffered water solvent system as used for 8, we first evaluated
whether solid plastics made from 1 would dissolve in the
solvent. The goal of this experiment was to use a solvent that
did not dissolve solid 1 to ensure that subsequent experiments
involved measurements of the self-propagating reaction at the
interface of solid 1 and an applied signal in an aqueous solution.
No noticeable change in size of a cup-shaped object made from
1 (Figure S1) was observed when the object was immersed in
1:1 MeCN−buffered water (pH 7.4) for 45 h, thus confirming
the suitability of this solvent for the solid-state experiments.7
Since the plastic made from 1 was not affected dramatically
by the aqueous solution, we solvent-cast films of the polymer as
well as control polymer 10 on glass slides. Separate films were
exposed for 3 h to 1:1 MeCN−buffered water containing L-
cysteine. After rinsing and drying the films, we used infrared
spectroscopy (IR) to evaluate whether the esters (in films of
10) and thioesters (in films of 1) had transformed into
lactones. Indeed, when solutions containing L-cysteine were
exposed to the films, the single carbonyl IR signature of the
starting polymer films (e.g., ∼ 1750 cm−1) converted into two
peaks (∼1750 and ∼1720 cm−1) (Figures 3a,b). In the absence
of L-cysteine, no change in IR signature was observed for either
polymer. These combined results confirm that cleavage of the
Nosyl group, and subsequent cyclization to the lactone of 7-
alkoxycoumarin indeed occurs when the solid films are wet with
solutions containing thiols.
To test the effects of self-propagation on global changes in
the optical properties of a material made from 1, we used the
technique of spin coating to create 0.5 cm-wide × 1 cm-long ×
7.2 1.9 nm-thick films of polymer 1 and 10 (6.7 0.9 nm-
thick) on strips of polypropylene. Individual films were
immersed at 23 °C in 0.2 mL of 1:1 MeCN−buffered water
(pH 7.4) containing different initial concentrations of added
thiol (L-cysteine). At specified time points, samples were
removed from solution and dried. Time-dependent measure-
ments of fluorescence (λex = 320 nm; λem = 430 nm) for these
dry films revealed the expected relationship (for a self-
propagating reaction) between the quantity of added thiol,
and the reaction time required for the film to reach a maximum
(and equal) level of fluorescence signal (Figure 3c). We also
observed minimal background reaction in the absence of L-
cysteine (the background signal increased only by 10% after 72
h of exposing a film of 1 to the MeCN−buffered water; Table
S8).
Figure 3. Self-propagating reactions in films. (a,b) Change in IR
signature in the carbonyl region when a film of 1 (a) or 10 (b) is
exposed to 10 mM thiol (L-cysteine) in 1:1 MeCN−buffered water
(pH 7.4) for 3 h. (c) Kinetics (obtained from fluorescence
measurements) when 0.1 mM (pink data), 1 mM (blue data), and
10 mM (black data) solutions of L-cysteine were exposed to films of
polymers 1 and 10 at 23 °C. The circular data points correspond to
films made from 1; the triangular data in the inset graph correspond to
films made from 10 (the axis labels are the same as the larger graph).
The data represent the averages of three measurements, and the error
bars reveal the standard deviations from these averages. In the context
of solid state films, the λex = 320 nm; λem = 430 nm. The lines are
provided to visually differentiate the data sets.
In contrast, but also expected, the kinetics for solid-state
experiments using films of 10 were consistent with 1:1
responses between signal (added thiol) and the readout
(fluorescence) (Figure 3c, inset). Films made from 10 reached
a maximum fluorescence signal faster than films made from 1,
which is a result that is consistent with self-propagating
reactions continuing at the solid−liquid interface between films
of 1 (but not 10) and the surrounding solution until all
accessible Nosyl groups react and convert 1 into 2. Thus, these
solid-state studies confirm the ability of thiols to propagate a
macroscopic change in the optical properties of a plastic when
the plastic encounters a low dose of an applied signal at the
interface of the plastic and an aqueous solution.
These findings support the concept of using self-propagating
amplification reactions to create materials with the ability to
autonomously and completely transform themselves in
response to trace levels of specific applied signals. When
viewed together with two recent demonstrations of auto-
inductive reactions in materials (one in a solid-state film8 and
the other dispersed in a hydrogel9), it is becoming increasingly
apparent that self-propagating reactions can be used to impart a
dramatic and preplanned change to the global properties of
materials. These advances, the most critical of which is the
signal-induced self-propagating reaction, are key for enabling a
new type of biomimetic, amplified response in synthetic
materials.
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Journal of the American Chemical Society
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Future expansions of this type of stimuli-responsive material
may benefit from (i) use of polymer backbones other than
poly(norbornene) and (ii) recent discoveries of autoinductive10
(including dendritic chain reaction11), autocatalytic,12 and
catalytic signal amplification reactions13 in small molecules.
Inclusion of polymer-bound detection reagents (that release
thiols in response to another analyte, such as an enzyme10,11
)
into a material along with polymer 1 will substantially expand
the scope of the materials as well.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b08582.
■
S
Materials, procedures, tables of data, and figures (PDF)
AUTHOR INFORMATION
Corresponding Author
*sphillips@psu.edu
■
Author Contributions
†These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health:
R01GM105686. We thank Gregory G. Lewis for preparing the
sample holder for fluorescence measurements.
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(5) Over the course of the 50 h exposure time in Figure 2c, the level
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DOI: 10.1021/jacs.5b08582
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX