A Versatile and Highly Selective Colorimetric Sensor for the Detection of Amines

Article abstract of DOI:10.1002/chem.201700368

The utility of Meldrum's activated furan (MAF) for the colorimetric detection of sub ppm levels of amines in solution, on solid supports, and as vapors is reported. MAF is synthesized in one step from inexpensive and commercially available starting materials and exhibits high selectivity for primary and secondary amines in the presence of competing nucleophiles. The reaction between activated furans and amines results in a distinct color change, discernable by the naked eye. UV/Vis absorption spectroscopy was utilized to monitor reactions in solution and determine detection limits. Additionally, solutions of MAF were useful as stains for thin layer chromatography and for monitoring solid-phase synthesis of peptides and peptidomimetics. Finally, MAF was used to detect volatile amines released from fish samples, demonstrating potential for food spoilage applications.

Full text of DOI:10.1002/chem.201700368

A Journal of
Accepted Article
Title: A Versatile and Highly Selective Colorimetric Sensor for
Detection of Amines
Authors: Yvonne J Diaz, Zachariah A Page, Abigail S. Knight, Nicolas
J. Treat, James R Hemmer, Craig J Hawker, and Javier Read
de Alaniz
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201700368
Link to VoR: http://dx.doi.org/10.1002/chem.201700368
Supported by

Chemistry - A European Journal
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COMMUNICATION
A Versatile and Highly Selective Colorimetric Sensor for
Detection of Amines
Yvonne J. Diaz, Zachariah A. Page, Abigail S. Knight, Nicolas J. Treat, James R. Hemmer, Craig J.
Hawker,* and Javier Read de Alaniz*
Abstract: The utility of Meldrum’s activated furan (MAF) for the
colorimetric detection of sub ppm levels of amines in solution, on
solid supports, and as vapors is described herein. MAF is
synthesized in one step from inexpensive and commercially
available starting materials and exhibits high selectivity for primary
and secondary amines in the presence of competing nucleophiles.
The reaction between activated furans and amines results in a
distinct color change, discernable by the naked eye. UV-vis
absorption spectroscopy was utilized to monitor reactions in solution
and determine detection limits. Additionally, solutions of MAF were
useful as stains for thin layer chromatography and for monitoring
solid phase synthesis of peptides and peptidomimetics. Finally, MAF
was used to detect volatile amines released from fish samples,
demonstrating potential for food spoilage applications.
restricted to the detection of volatile amines, limiting their use as
a general amine colorimetric sensor. Recent investigation into
new negative photoswitches, termed donor acceptor Stenhouse
adducts (DASAs), revealed an avenue to increasing the
[
27,28]
versatility and selectivity of colorimetric detection of amines.
The ring opening of activated furans with secondary amines is a
rapid and efficient reaction that produces DASAs, which possess
5
−1
−1 [29–31]
high molar absorptivity values (ελmax ≈ 10 M cm ).
The
facile synthesis of activated furans from commercially available
and inexpensive starting materials coupled with the high ε of the
DASA product indicated the potential of activated furans as
[
30]
colorimetric sensors.
Herein, we present a simple methodology to monitor the
presence of amines in solution, on solid supports, and as vapors
that is available to non-experts (Scheme 1). These activated
furan-based sensors combine the sensitivity of state-of-the-art
detectors with the ability to distinguish various amines by the
naked eye. In illustrating the utility of this novel system across
different fields, applications including thin layer chromatography
The ability to selectively sense amine-containing
molecules is pertinent to a variety of research and commercial
[
1]
[2,3]
applications, including chemical synthesis, drug detection,
[
4–12]
and environmental monitoring.
Emerging methods that track
(
TLC) staining, solid phase peptide/peptoid synthesis, and food
[
13–15]
[2,7,16–18]
changes in resistivity
and luminescence
sensitivity and
and fluoresence-sensors
have
with
spoilage detection were demonstrated.
allowed
increased
selectivity
[7,16,20–23]
[
13,15,19]
chemiresistors
providing
sub-ppm limits of detection. However these systems often
require complex synthetic methods and the use of secondary
equipment for amine identification. Additionally, the majority of
these sensors are limited to detection of volatile and primary
amines.
In addressing the challenges with these techniques,
colorimetric sensors are attractive alternatives due to their
simple and rapid detection of amines in solution, as vapors, and
Scheme 1. General reaction for activated furan-based amine sensor.
Activated furan (1) reacts with ammonia, or a primary or secondary amine to
form a colored DASA (2) . Inset: Photographs of Meldrum’s activated furan in
THF (20 mM, left) and 5 min after the addtion of diethylamine (200 ppm, right).
[
24]
on solid supports.
competitive sensitivities (<1 ppm).
polyacrylonitrile nanofibers have been used for the visible
Recent improvements have also provided
[
11,25,26]
For example,
[
11,25]
detection of ammonia vapors of <1 ppm.
Suslick and
Activated furan-carbon acids (1) are rapidly synthesized by
reacting furfural (≈$2/kg), a derivative of non-edible biomass,
coworkers have achieved <1 ppm sensitivities by utilizing a
combination of metalloporphyrins and pH acid/base indicators in
colorimetric arrays with the additional benefit of merging modern
[
29,30]
with cyclic 1,3-dicarbonyl compounds on water.
[
26]
Conveniently, both Meldrum’s acid activated furan (MAF,
digital imaging and pattern recognition techniques.
Although
compound 3) and 1,3-dimethyl barbituric acid activated furan
advances to colorimetric sensors have allowed for the
identification and quantification of specific analytes, these
methods often involve the use of specialized equipment and are
(
BAF, compound 5) (Figure 1) precipitate out of the reaction
mixture and can be isolated in near quantitative yields by
[
30]
filtration.
Additionally, MAF and BAF have excellent stability
under ambient conditions as well as when dissolved in non-
nucleophilic organic solvents (e.g., tetrahydrofuran and
dichloromethane), as evidenced by monitoring the solution over
[*]
Y. J. Diaz, J. R. Hemmer, Prof. J. Read de Alaniz
Department of Chemistry and Biochemistry
University of California, Santa Barbara
Santa Barbara, CA 93106 (USA)
1
time using H NMR spectroscopy (Figure S1). These activated
furans then react rapidly with amines to produce highly colored
E-mail: javier@chem.ucsb.edu
[29–31]
and thermodynamically stable triene forms of DASAs (2).
Although DASAs are light-responsive dyes, basic “donors”, such
as aliphatic amines, require a highly nonpolar matrix (e.g.,
toluene, hexanes) for photoswitching (color bleaching) to be
observed, making them excellent candidates for amine sensing
Dr. Z. A. Page, Dr. A. S. Knight, Dr. N. J. Treat, Prof. C. J. Hawker
Materials Research Laboratory, University of California
Santa Barbara, CA 93106 (USA)
E-mail:hawker@mrl.ucsb.edu
[
31–33]
(
Figure S2).
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
Figure 1. General reaction with amines for a) MAF, 3, and b) BAF, 4
UV-Vis absorption spectroscopy was utilized to compare
the optical properties and amine reactivities of MAF and BAF.
For equimolar solutions (10 µM in THF) of the two activated
furans, MAF solution was found to be colorless to the naked eye,
while BAF solution was slightly yellow (Figure 1a, 1b). The near
colorless initial MAF solutions are favorable for sensing
applications, given the more distinguishable “turn-on” of color
that occurs upon amine exposure (Figure S3-S5). Introducing
(
photographs represent respective 20 mM solutions of MAF and BAF in THF).
c) Reaction of MAF (red triangles and BAF (blue squares) with DEA (0.6 ppm).
To showcase MAF as an amine-selective stain for thin
layer chromatography (TLC),
a tryptamine-based synthetic
sequence was monitored (Figure 2). TLC staining is one of the
most common methods to track the progress of a chemical
reaction and determine chromatographic purification conditions.
Although the ability to rapidly distinguish different amine
moieties by eye is beneficial for synthetic chemists, traditional
0
.6 ppm diethylamine (DEA) to MAF and BAF solutions (20 mM
in THF), and monitoring the reactions in situ, revealed that MAF
reacts ~2 times faster (Figure 1c). Given the beneficial optical
properties and faster response time of MAF based sensor, it was
selected for in-depth investigation. Initially, the limit of detection
by eye for DEA in a 20 mM solution of MAF (in THF) was found
to be ~0.4 ppm after 1 h of exposure (Figure S6). Significantly,
other secondary amines, primary amines, and ammonia were all
found to form the characteristic pink color (λmax ≈ 532 nm),
however, with varying reaction rates (Figure S7). After 5 min,
the absorbance of 4 using DEA is 11-fold higher than the
reaction with butylamine (1° amine), and 33-fold higher than the
reaction with ammonia (Figure S7). The same trend was
observed for other 2° and 1° amines; 10 ppm of dimethylamine
and piperidine (2° amines) resulted in pink color after 5 min,
[35]
stains, such as ninhydrin, often do not provide this distinction.
Tryptamine derivatives are prevalent in the pharmaceutical
industry and natural product synthesis, making them exemplary
candidates for TLC staining with MAF. Four derivatives,
possessing various aliphatic and/or conjugated amines, were
synthesized from tryptamine, spotted onto silica TLC plates, and
stained with MAF (20 mM in THF). After staining for <1 min at
room temperature, a wide range of vibrant colors became
apparent, making it possible to clearly distinguish the different
reaction stages by eye (Figure 2). To quantify the color
difference between stained tryptamine derivatives, ΔE* values
were determined following International Commission on
[
34]
while 100 ppm of cadaverine (a common biogenic 1° amine
)
[36–38]
Illumination (CIE) guidelines (Figure S10-S14).
All pairs
was required to turn the solution pink within 5 mins (Figure S8).
This difference in reaction rates and absorbance provides a
pathway to distinguish between primary and secondary amines
in solution. To establish selectivity for amines, the MAF solution
was reacted with other nucleophiles, such as water, alcohols,
thiols, and organophosphorus compounds (Figure S9). As
expected only amines were found to be reactive with the MAF
solution, highlighting the amine-selectivity.
had ΔE* values > 7, which is generally considered to be
[38–40]
distinguishable by eye.
This supported that MAF stains are
an effective method to distinguish between amine-containing
compounds with subtle chemical differences (see SI for more
details on color quantification).
Figure 2. Tryptamine-based synthesis and TLC staining with MAF (20 mM in
THF). Reagents and conditions: i) secondary amine. ii) methyl chloroformate,
3 3
NaOH(aq):DCM (1:1), rt, >95%. iii) Et SiH, CF COOH, 60°C, 48%. iv) LAH,
THF, 60°C, >95%.
MAF solutions were further used to stain amine-functional
resins found in solid phase synthesis of peptides and
peptidomimetics. For modular solid phase synthesis, the
quantitative conversion at each synthetic step is critical for
[
41]
obtaining the desired product.
As such, stains have been
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Chemistry - A European Journal
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COMMUNICATION
[
42–46]
developed to identify the presence of 1° and 2° amines.
filter membrane was captured. Calculating ΔE* for each image
[
42]
However, commonly used stains such as ninhydrin
(a.k.a.,
revealed that filter membrane exposed to 0.5 ppm of DMA and
[
43]
[38–40]
Kaiser test) and chloranil,
used for detection of 1° and 2°
ammonia vapors is distinguishable (ΔE* > 5)
from the MAF
amines respectively, are unstable and require the use of toxic
reagents such as acetaldehyde and potassium cyanide. To
highlight the ease of reaction monitoring with MAF, peptide and
peptoid-functionalized resins were tested. A stable stock solution
of MAF in THF (200 mM) was prepared and diluted tenfold into
methanol prior to adding 2-3 drops to functionalized resins. For
peptides (1° amines), exposure to MAF resulted in beads with a
light pink color in 5 min with heat (Figure 3a). Unlike the Kaiser
test, the MAF solution is capable of sensing 1° and 2° amines,
allowing for the detection of all amino acids and many
peptidomimetics (Figure S15). The addition of 2-3 drops of our
MAF solution to resin-bound peptoids, immediately turned the
beads bright pink under ambient conditions (Figure 3b). These
results confirmed that MAF stains are a simple and effective
alternative to classic methods used to determine reaction
completion in solid phase peptide and peptoid synthesis.
coated reference (no amine exposure) (Figure 4). Although
MAF has a similar limit of detection for both vapors, the relative
change in color for the 2° amine provides ΔE* values of 68 at 2
ppm DMA, as compared to 31 at 50 ppm ammonia. The more
pronounced color change for the secondary amine is consistent
with prior observations made for amines in solution and on solid
supports. This demonstrated the ability to use MAF coated nylon
filters for quick and sensitive detection of volatile amines.
Figure 4. a) Images of nylon filters that correspond to data points from graphs
b and c. Visual responses of MAF coated nylon filters after 5 minute amine
exposures to b) 0.3, 0.4, 0.5, 1, and 2 ppm DMA (red squares, detection limit
of ~0.4 ppm) and c) 0.5, 1, 10, 50, and 100 ppm ammonia (blue circles,
detection limit of ~0.5 ppm)
Figure 3. a) General reaction between MAF and resin bound
peptides/peptoids. b) Images of resin beads 5 min after adding 2-3 drops of a
MAF solution (20 mM methanol) where 1° amine containing beads (peptides)
turn pale pink after heating and 2° amine containing beads (peptoids) turn
bright pink under ambient conditions. Control image represents resin beads
To expand the utility of this sensor to applications in food
spoilage, the release of volatile amines from two commercially
important fish samples, cod and tilapia, was studied. Freshly
thawed fish samples (~15 g) were sealed into separate glass
jars containing five nylon filter membranes coated with MAF
solutions in THF at concentrations ranging from 56 to 450 mM
(see Figure S16 for experimental setup). As a control, an
analogous setup was constructed with no added fish samples.
The sensors were monitored over the course of 48 h with time
lapse imaging (Figure 5). Interestingly, the cod sample resulted
in a noticeable change in color after ~8 h, while the tilapia had a
significantly delayed response, changing color after ~24 h
(Figure 5). Ultimately, ΔE* values of 60 and 38 were obtained
for the cod and tilapia samples (respectively) after 48 h relative
to the control sample, highlighting the very distinct color change
for MAF samples upon exposure to volatile amines. These
(
no amine) after heating for 5 min.
Given the facile detection of amines in solution and on
solid supports with MAF, we investigated its utility to sense
amine vapors. Detection of amine vapors is important for various
[
4–7,19,47,48]
applications, including food spoilage
amines such as dimethylamine (DMA) and ammonia are
, where volatile
[
34]
released from aging meat.
To evaluate the sensitivity of our
sensor toward volatile amines, MAF was exposed to known
concentrations of amine vapors. First, nylon filter membranes
were dipped into a 450 mM solution of MAF in THF, dried, and
sealed in septa-capped scintillation vials. The MAF coated filters
were then exposed to vapors of DMA and ammonia. After 5 min,
the filters were removed from the vials and a digital image of the
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results show the potential applicability for this type of sensor in
real-time monitoring of food spoilage.
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identified as a versatile and simple to use sensor for the
selective detection of amines in solution, on solid supports, and
in the vapor phase. With an amine detection limit <1 ppm MAFs
are among the most sensitive, all organic, colorimetric tests for
amines, highlighting the capacity of activated furans to be used
as simpler alternatives to state-of-the-art sensors. Further utility
of activated furans for sensing in organic synthesis, drug
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Keywords: Activated furan • colorimetric sensor • stain • peptide
•
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Chemistry - A European Journal
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COMMUNICATION
COMMUNICATION
The utility of Meldrum’s activated
furan (MAF) for the colorimetric
detection of sub ppm levels of amines
in solution, on solid supports, and as
vapors is described herein. To
illustrate the utility of this novel system
across different fields, applications
including thin layer chromatography,
solid phase peptide/peptoid synthesis,
and food spoilage detection are
demonstrated.
Yvonne J. Diaz, Zachariah A. Page,
Abigail S. Knight, Nicolas J. Treat,
James R. Hemmer, Craig J. Hawker*,
and Javier Read de Alaniz*
Page No. – Page No.
A Versatile and Highly Selective
Colorimetric Sensor for Detection of
Amines
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