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Sensors and Actuators B 223 (2016) 1–8
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
A visible colorimetric sensor based on nanoporous polypropylene
fiber membranes for the determination of trihalomethanes in
treated drinking water
Evan K. Wujcik a,∗ , Stephen E. Duirk b , George G. Chase c , Chelsea N. Monty c,∗
a
Materials Engineering And Nanosensor (MEAN) Laboratory, Dan F. Smith Department of Chemical Engineering, Lamar University,
Beaumont, TX 77710, USA
b
Department of Civil Engineering, The University of Akron, Akron, OH 44325, USA
c
Department of Chemical & Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA
a r t i c l e
i n f o
Article history:
Received 6 May 2015
Received in revised form 31 August 2015
Accepted 1 September 2015
Available online 10 September 2015
Keywords:
Syndiotactic polypropylene
Nanoporous fiber membrane
Electrospinning
Halogentaed organic compounds
Trihalomethane
Visible colorimetric sensor
a b s t r a c t
Water is an integral part of our society, and it is of the utmost importance that this water is clean and
safe for potable use. Drinking water sources can become contaminated with halogenated organic compounds (HOCs) via improper disposal of refrigeration or air conditioning units, industrial usage, and even
through the disinfection of our drinking water (chlorination). One class of these HOCs regulated by the
U.S. Environmental Protection Agency (USEPA) is trihalomethanes (THMs). THMs have been found to
cause adverse health effects and can even be carcinogenic, they are limited to 80 ppb in treated water
by the USEPA. Currently THM concentrations are detected in treated water using expensive analytical
equipment, which have high sensitivity, but lack any portability or ease-of-use. Here we show a syndiotactic polypropylene (sPP) nanoporous electrospun fiber membrane capable of visible colorimetric
detection of trihalomethanes (THMs) at environmentally relevant levels (ppb-scale), without a separate
preconcentration step. The developed detection method includes a two-fold preconcentration technique
coupled with a colorimetric detection reaction, integrated into a single sensor device. Here, the utilization
of the colorimetric Fujiwara reaction serves as the detection reaction. This long-studied reaction has a
response that is normally limited to a beaker scale visible detection range on the order of 80 ppm THMs,
when a separate preconcentration step is not present. The two-fold integrated preconcentration technique consists of a thermodynamic method based on the utilization of the THM: water equilibrium in the
vapor phase of the system as well as a physical method using a sPP nanoporous membrane capable of concentrating THMs in a contaminated aqueous sample. The developed device has successfully lowered the
portable single-step visible THM detection concentration by an order of magnitude (80 ppm → 80 ppb).
This sensor was also successfully applied for the determination of the total THM (t-THM) concentration
in spiked and actual treated water samples, and these results correlated well with the GC/MS results of
the corresponding sample, showing its real world applicability.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Contamination of drinking water is a serious concern throughout the third and developed world, and can lead to a number of
long term problems. Drinking water sources can easily become
contaminated with halogenated organic compounds (HOCs) via
improper disposal of refrigeration or air conditioning units, industrial usage, and even through the disinfection of our drinking water
∗ Corresponding authors. Tel.: +1 4098808428; fax: +1 4098802197.
E-mail addresses: evan.wujcik@lamar.edu (E.K. Wujcik), cm78@uakron.edu
(C.N. Monty).
http://dx.doi.org/10.1016/j.snb.2015.09.004
0925-4005/© 2015 Elsevier B.V. All rights reserved.
(chlorination), resulting in the formation of disinfection byproducts (DBPs). DBPs are formed when chlorinated disinfectants
applied at drinking water treatment plants react with dissolved organic matter resulting in the formation of known
and suspected carcinogens [1]. Gem.-polyhalogens such as trihalomethanes (THMs), a common class of DBPs, are regulated and
monitored by the U.S. Environmental Protection Agency (USEPA),
which has set the maximum contaminant limit (MCL) of four THMs
(i.e.—chloroform, bromoform, bromodichloromethane, and dibromochloromethane) to a sum total concentration of the four species
to be 80 ppb in treated water [1]. HOCs, such as THMs and other
contaminants found in water, are known to cause adverse health
effects, damaging the liver, kidneys, and central nervous system
2
E.K. Wujcik et al. / Sensors and Actuators B 223 (2016) 1–8
(CNS), as well as being suspected carcinogens [2] and being environmentally harmful [3]. The environmental flux of chloroform
alone can total over 660 ± 220 Gg/year (±1) [4]. A novel method
capable of rapidly monitoring the potential t-THM concentration
of source and finished drinking water could help mitigate potential
health concerns as well as improve the quality of water being delivered to the homes of consumers. A number of detection methods to
determine the toxicity of water using a portable device have been
developed [5–7]; however, none of these portable devices are able
to detect THMs.
Here, a newly devised detection method utilizes an electrospun syndiotactic polypropylene (sPP) fiber membrane. Through
the use of a functionalized, nanoporous membrane, THMs simultaneously undergo a two-fold preconcentration step simultaneously
coupled with visible colorimetric detection, integrated into a single
sensor device. Hydrophobic polymer membranes have previously
been used for organic–inorganic separations [8,9]. Electrospun sPP,
in particular, has shown superior membrane functionality and
hydrophobicity [10,11]. This electrospun membrane is hypothesized to enhance the selectivity of the organic analytes allowed
through the membrane, while reducing the water molecules which
are detrimental to the Fujiwara Reaction—entering the Fujiwara
reactants. Electrospinning is a popular fiber fabrication technique
that has gained much interest due to great control over the fibers
produced [12–17]. This technique can be used to produce continuous fibers of polymers and ceramics from nanometer to micrometer
scale widths. In this process, a solution or melt being extruded
out of a syringe is charged by a high voltage electric field, causing the formation of a Taylor cone – due to Coulombic repulsion
and the liquid surface tension being overcome – from which a
continuous stream of charged polymer is ejected [18]. Due to the
electrically-driven polymer jet instability, solidification and elongation occur, leading to the formation of micro- or nanofibers on
a grounded collector. The morphology of these fibers can further
be controlled via adjusting the electrospinning parameters, such as
solution concentration, solvent, flowrate, applied voltage, syringecollector distance and collector type [19,20]. The accumulation
of fibers will occur after electrospinning for an extended time,
leading to robust, easily fabricated, reproducible, and extremely
high-surface area nanoporous fiber membranes.
In this work, a nanoporous, electrospun membrane is functionalized for the preconcentration and detection of THMs at or
below the MCL set by the USEPA. Using differences in thermodynamic properties between the HOCs and water, one may show that
the concentration of THMs in the vapor phase (head-space) of a
vapor–water system is much higher than in the aqueous phase,
assuming the temperature of the system is kept below the boiling
point of water. In addition, manipulating the temperature of the
system will reduce the number of water molecules and increase
the ratio of THMs to water molecules in the headspace.
Once in the vapor phase, the THMs are further preconcentrated
using a functionalized sPP membrane capable of simultaneously
concentrating and detecting halogenated species in a contaminated
aqueous sample. The membrane reduces the amount of water vapor
diffusing through the membrane, as a function of temperature,
allowing the THMs to flow through and react with the colorimetric
detection chemistry—functionalized on top of the membrane.
The detection chemistry utilizes the Fujiwara reaction [21], first
discovered in 1916, has been employed in a number of schemes
to detect halogenated compounds in such media as blood [22],
bodily tissues [23], air [24], and water [25]. Through the use of this
colorimetric method, HOCs can be detected at concentrations of
approximately 100 ppm. However, the beaker-scale Fujiwara reaction cannot detect at the parts per billion (ppb) range – as needed
to meet USEPA regulations – without first being preconcentrated
[6,21,26]. HOCs can be detected colorimetrically using the Fujiwara reaction through conjugation with pyridine in the presence
of a base. The detection can be seen when the heated reactants
form an intermediate that changes the solution from clear to a
purple, red, pink, or yellow color, upon addition of HOCs. This is
the only reaction known to detect THMs spectrophotometrically
in the visible spectrum [26]. This is accomplished by the formation of an intermediate chromophore, at a high pH, that emits a
red/pink color when visible light is absorbed [25,27]. The original
pyridine/aqueous sodium hydroxide/THM reaction has been modified throughout the years in a number of ways to tailor it to specific
chemistries and increase the sensitivity, including experimenting
with one [26,28] and two [28] phase systems, a variety of basic
compounds [26], pyridine derivatives [26], solvents [24], and various compound ratios [29]. This has led to the Fujiwara reaction
being a highly customizable reaction, suiting the needs of many
various applications. One drawback of the Fujiwara reaction is its
accelerated intermediate decomposition in the presence of water
[24,30], causing no color change to be seen [26], and has proven
problematic in testing actual potable water samples. One solution
to this problem would be a simultaneous two-fold preconcentration system to isolate/concentrate THMs from the water, thereby
increasing the sensitivity of the detection method—as described.
The overall schematic of the sensor can be seen in Fig. 1a.
Initially, the water sample contains THMs in the solution (represented as black dots). After heating the system to a desired
temperature (between 60 ◦ C and 90 ◦ C), the THMs evaporate and
become highly concentrated in the headspace above the test solution (represented as more concentrated black dots in headspace).
The THM molecules then diffuse through the functionalized membrane and react to form a red/pink complex via the Fujiwara
reaction (Fig. 1b). In this work, the electrospun membrane and colorimetric sensor are investigated and optimized. The colorimetric
sensor is found to exhibit high analytical performance with excellent selectivity. This would be the first colorimetric sensor of its
Fig. 1. (a) Conceptual illustration of a colorimetric membrane in a proof-of-concept device used for sensing THMs in a contaminated water sample. Initially, the water
sample contains THMs in the solution (represented as black dots). After heating the system to a desired temperature, the THMs evaporate and become highly concentrated
in the headspace above the test solution. The THM molecules then diffuse through the sPP membrane and react with the Fujiwara reactants to form a red/pink complex.
(b) Photograph of the laboratory testing setup, where the colorimetric response can be seen with the naked eye. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
E.K. Wujcik et al. / Sensors and Actuators B 223 (2016) 1–8
3
kind capable of the simultaneous preconcentration and detection
of THMs in treated water at environmentally relevant concentrations. The membrane developed in this work is advantageous to
currently available detection techniques, as it does not require a
separate preconcentration step, such as the commonly used purge
& trap, liquid–liquid–extraction (LLE), or bubbler steps followed by
gas chromatography/mass spectrometry (GC/MS).
2. Materials and methods
2.1. Reagents and materials
Cyclohexane [Burdick & Jackson Brand, high purity–0.001%
water], N,N-dimethylformamide (DMF) [Sigma-Aldrich, ≥99.9%],
acetone [Sigma-Aldrich, ≥99.9%], pyridine [Sigam-Aldrich,
99.9+%], sodium hydroxide (NaOH) [Fisher Scientific, certified
ACS pellets], 1,1,2 trichloroethane [Sigma-Aldrich, 96%], bromodichloromethane [Sigma-Aldrich], bromoform [Sigma-Aldrich],
carbon tetrachloride [Sigma-Aldrich, 99%], chloroform [SigmaAldrich], dibromochloromethane [Sigma-Aldrich], dichloroacetic
acid [Sigma-Aldrich, 99%], pyridine [Sigma-Aldrich, >99%],
tetrachloroethylene [Sigma-Aldrich, 99%], trichloroacetic acid
[Sigma-Aldrich, 100%], trichloroethylene [Sigma-Aldrich], 1,1,1trichloroethane [Fisher Scientific, purified grade], acetonitrile
[Fisher Scientific, 99%], and sodium hydroxide [Fisher Scientific,
99%], 1,1,2,2-tetrabromoethane [Eastman Kodak Company], bromoethane [Alfa Aesar, 98%], dichloromethane [Acros Organics,
HPLC grade] were used as purchased. The polymer used for
electrospinning was purchased from Aldrich Chemistry Co. and
used as received: polypropylene (sPP) [syndiotactic, average
MW ∼ 174,000, average Mn ∼ 75,000].
Fig. 2. Theoretical concentration of bromoform in the vapor () and liquid (䊉)
phases as a function of time at 70 ◦ C. This shows that the concentration of bromoform in the headspace of the sensor is three orders of magnitude higher than in
the bulk liquid.
Relative humidity measurements were conducted using an
Omega Engineering HX15 (USA) humidity probe with OM-CPQUADPROCESS-25MA data logger. The humidity was measured
using two different setups: one setup with the probe centered
0.7 cm above the top of the testing vial with no membrane in place,
and one setup with the probe centered 0.7 cm above the top of the
testing vial with a membrane in place. The percent reduction in
relative humidity due to the presence of the membrane was calculated with respect to the control (no membrane) as a function of
temperature. Physical setup of experiment was identical to setup
used during testing.
2.2. Membrane fabrication and electrospinning
2.4. Colorimetric sensor evaluation
The electrospinning procedure was initiated with a 1–4 wt%
polymer mixed thoroughly with a solution of 8:1:1 (wt/wt/wt)
cyclohexane/acetone/DMF, and then left in a heating block at 70 ◦ C
overnight. The solution was then electrospun onto an aluminum
foil at a voltage of 25 kV, a needle/collector distance of 25 cm, and a
syringe pump rate of 25 mL/h. These electrospun membranes were
left to dry overnight, and cut to the required geometry to perform
the experiments. The electrospinning procedure was adapted from
previous work [31]. This yielded membranes of a 0.02 ± 0.006 cm
thickness that were cut to a cross sectional area of 0.25 cm2 .
For the construction of the calibration curve and the selectivity study, contaminated water samples were made with a
standard amount of HOC. To test the environmental water samples (see Supplementary material), water was collected using
an existing USEPA collection method [32]. For confirmation of
HOC/THM content of the collected water, a second sample was
taken concurrently for GC/MS analysis, with 4 mL of hydrochloric acid (per 40 mL of sample) to prevent volatilization of the
HOCs. Gas chromatography/mass spectrometry (GC/MS) was performed at TestAmerica Laboratories, Inc. (North Canton, OH) using
USEPA testing protocol SW-846 Method 8260B for the four regulated THMs (i.e., chloroform, bromoform, bromodichloromethane,
dibromochloromethane).
Testing of the colorimetric membrane was conducted using a
scintillation vial, fabricated membrane holder (Fig. 2b), and the
functionalized membrane. A spiked (known amount of HOC) or
environmental (unknown amount of HOC, prior to GC/MS analysis)
water sample, kept to 70 ◦ C, was then put into the scintillation vial
and the membrane holder was assembled above. To functionalize
the membrane, pyridine and a saturated ethanol (EtOH) solution of
sodium hydroxide (NaOH) were added to the top of the membrane
in a ratio of 9:1 (v/v) [6].
The mass flux equations for the evaporation of low-solubility
contaminants from water developed by Mackay and Leinonen were
used to estimate the evaporation of THMs from a water solution
over time [33]. The mass flux of THMs (N
i ) across the phase boundary is described as: Ni = KiL Ci − Pi /Hi mol/m3 h, where KiL is the
overall liquid mass transfer coefficient (m/h), Ci is the bulk liquid concentration (mol/m3 ), Pi is the partial pressure (atm), and
Hi is the Henry’s law constant (atm m3 /mol). Combining the mass
flux equation with an unsteady state mass balance leads to a
2.3. Physical characterization
Scanning electron microscopy (SEM) was performed on a JOEL
JSM-6510LV operating at an accelerated voltage of 5 kV and highvacuum mode. All samples were sputter-coated in a gold/palladium
(Au/Pd) alloy for 1.5 min.
FTIR measurements were conducted using Nicolet Nexus 670
FTIR ESP with a Thermo electron corporation smart orbit Diamond
ATR using 64 scans. The data was analyzed using OMNIC software
(version 5.2a). Measurements were done (1) on a dry 3 × 3 cm membrane, (2) the same membrane with 189 L of pyridine and 27 L
of EtOH/NaOH solution added, and (3) the same membrane after
running a full experiment at 70 ◦ C with 500 ppb chloroform solution.
Contact angle measurements were done using the sessile
drop technique, conducted on a drop shape analyzer goniometer (DSA20E, KrüssUSA, Mathews, NC). The deionized water drops
were applied using a micro-syringe pointed vertically downward
onto the membrane surfaces and the instrument-provided image
analysis software, which was used for contact angle determination.
4
E.K. Wujcik et al. / Sensors and Actuators B 223 (2016) 1–8
Fig. 3. SEM micrographs of electrospun polypropylene membranes ranging from 1 to 4 wt% sPP, showing the drastic changes in morphology.
differential equation
expressing
the change in concentration over
time: dCi /t = −KiL Ci − Pi /Hi /L. Integrating this equation using
Ci0 being
at time zero,
the boundary condition
the
concentration
yields Ci = Pi /Hi + Ci0 − Pi /Hi exp −KiL t/L for concentration of
THM as a function of time. The aforementioned equations describe
the first stage of preconcentration, based on manipulation of the
THM: water equilibrium in the vapor phase of the system. Using
these equations, the theoretical concentration of bromoform in the
vapor and liquid phases was determined to be a function of time
at 70 ◦ C (Fig. 2). Notice that the concentration of bromoform in the
headspace is three orders of magnitude higher than in the bulk
liquid.
2.5. Image intensity analysis
Image intensity analysis was adapted from previous works
dealing with the quantification of colorimetric sensor responses
[34–36]. As an overview, through the use of Image J open-source
software (available on the NIH website: http://rsbweb.nih.gov/ij/),
the color intensity was determined to quantify the color change of
the colorimetric reaction. Here, the area of interest was selected,
inverted, and then the mean color intensity was taken from the
histogram. This intensity was used for further image intensity
analysis.
3. Results and discussion
3.1. Optimization and characterization of the nanoporous
colorimetric sensor membrane
The SEM images in Fig. 3 show that the sPP wt% greatly affects
the morphology of the membrane, hence controlling its surface
area, surface energy, and wettability. At 1 wt% sPP, the less viscous solution simply sprays out of the tip during electrospinning
resulting in a porous layered membrane. It can also be seen that
using a 1 wt% sPP solution, at these electrospinning conditions, can
Fig. 4. SEM micrograph of desired nanoporous sPP fiber membrane morphology at
2.6 wt% polypropylene.
result in the production of microspheres—similar to electrospraying methods [37]. At 2 wt% sPP, however, the morphology is a hybrid
between a porous layered membrane (1 wt% sPP) and a fiber structure (3 wt% sPP). The image of the 3 wt% sPP shows predominately
the formation of fiber-bound beads. At 4 wt% sPP, the fiber diameters are much larger, on the order of 10 m, and beads are not
observed at all. The fiber mats with the larger diameter behave
more similarly to bulk polymer films.
At 2.6 wt% sPP (Fig. 4), electrospinning produces a highly porous
membrane of thin (∼1–5 m) fibers featuring spherical beads
strung along them. These beads may contribute to enhanced surface
area for the colorimetric reaction. By implementing electrospun
fibers with the observed beads, the membrane’s surface roughness is increased in comparison to that of a smooth flat continuous
layer of sPP. Previous work has shown that an increase in surface roughness leads to an increase in porosity [38]. An increase
in porosity will lead to a decrease in diffusion time for THMs
across the membrane, as there are more pathways for diffusion. The
2.6 wt% membrane porosity, calculated as 1 − membrane /bulk , is
E.K. Wujcik et al. / Sensors and Actuators B 223 (2016) 1–8
5
Fig. 5. FTIR of the neat sPP nanoporous membrane, the same membrane after exposure to 9 washes with the Fujiwara reactants (pyridine and NaOH in a 9:1 ratio), and the
same membrane then used in the developed colorimetric sensor in the presence of 500 ppm of the analyte—in this case, chloroform. The lessening of the intensity of the
Fujiwara reactant peaks indicates the progression of the Fujiwara reaction.
0.89 ± 0.04. The density of the membrane was determined by dividing the weight of the membrane by the volume of the membrane,
for five membranes fabricated using the standard electrospinning
conditions discussed in the experimental procedure.
The contact angles of the 1–4 wt% sPP nanoporous membranes
were all between 90 and 150◦ (as compared to 102◦ for a smooth
flat layer of sPP), characterizing the material as hydrophobic.
This contact angle reaches an apparent maximum for concentrations greater than 2 wt% sPP, where the enhanced morphological
hydrophobicity effects of the nanoporous membrane seem to peak,
seen in Fig. S1 of the supplementary material. Due to its overall
morphology, its ability to be thick and robust enough to easily use
in the sensor device (ease of handling), and peak hydrophobicity
(138.2◦ ), the 2.6 wt% sPP fiber nanoporous membrane was selected
for use in the visible colorimetric sensor. Characterization of these
membranes yielded an average thickness of 0.02 ± 0.006 cm, with
a pore volume of 4.5 ± 1.5 L.
FTIR was implemented to confirm the achievement of the Fujiwara reaction. The spectra of the neat sPP mat, the same mat with
a 9:1 (v/v) ratio of pyridine to NaOH, and the same mat with
the entire reaction chemistry – including analyte – are shown
in Fig. 5. Characteristic sPP transmittance peaks are shown in
each of the spectra, indicating the presence of the alkane strong
C H stretch (2948–2837 cm−1 ), C H bend/scissoring (1454 cm−1 ),
and C H methyl rock (1373 cm−1 ). The spectrum of the membrane with the Fujiwara reactants presented shows the pyridine
molecule aromatic in-ring C C stretch (1612–1340 cm−1 ), inplain C H bending (1078 1051 cm−1 ), in-ring C N C bending
(880 cm−1 ), and the C H out-of-plane bands (825 cm−1 ). The broad
peak at 3310 indicates the O H stretch of the NaOH. Once the
membrane is used in a sensor experiment with 500 ppm of the
analyte – chloroform in this case – all of the peaks associated
with the Fujiwara reactants, specifically those associated with pyridine, diminish in intensity, suggesting that the reaction is taking
place.
3.2. Visible colorimetric sensor evaluation for detection of THMs
in treated drinking water
The electrospun sPP nanoporous membrane was then tested in
a device for the preconcentration and detection of analytes (in this
case, THMs) from contaminated water samples using image intensity analysis. As the Fujiwara reaction is hindered by the presence of
water, both the vapor–liquid equilibrium (THM/water ratio in the
head space) of the system and the nanoporous membrane properties must be optimized [25]. Experimentation was conducted
to determine the optimum functionalization and operating temperature based on the highest response as determined by image
intensity analysis (Fig. S2 of the Supplementary material). Here,
the sensors were tested for optimum Fujiwara reactant ratios and
optimum temperature, across concentrations. The obtained results
agree with previous studies examining ratios of Fujiwara reactants
[29]. Additionally, testing shows that the sPP nanoporous membrane decreases %RH by 28% at 70 ◦ C and 5% at 90 ◦ C, indicating
that the sPP membrane inhibits the transport of water across the
membrane until the membrane is saturated due to the increased
presence of water at higher temperatures. This result may explain
the observed decrease in intensity at increased temperatures, as the
Fujiwara reaction is inhibited by the presence of water. This sensor scheme and chemistry has been used to show the colorimetric
response of the preconcentration system to varying concentrations
of a number of THMs. Again, it should be noted that the beaker-scale
detection limit for the Fujiwara reaction is on the ppm scale, without preconcentration [26,39] (i.e., 80 ppb total THM concentration).
A calibration curve for t-THM concentration can be seen in Fig. 6,
which shows the working colorimetric response range is an order
of magnitude lower than previously developed detection methods
and is able to detect t-THMs in the regulatory relevant concentration range [39]. The insets (top-bromoform, bottom-chloroform)
of this figure show the average colorimetric intensity response
at a concentration of 80 ppb. At concentrations of 8 ppb, 80 ppb,
6
E.K. Wujcik et al. / Sensors and Actuators B 223 (2016) 1–8
Table 1
Visible colorimetric nanoporous fiber membrane sensor response to potential
interferents.
Potential interferents
Response
Trichloroacetic acid
Dichloroacetic acid
Trichloroethylene
Chloroamphenicol
1,1,2-Trichloroethane
Carbon tetrachloride
Tetrachloroethylene
1,1,1-Trichloroethane
1,1,2,2-Tetrabromoethane
Bromoethane
Dichloromethane
Slight interference*
Slight interference*
Slight interference*
Slight interference (yellow/orange)†
No response
No response
No response
No response
No response
No response
No response
*
Interference was at concentrations multiple orders of magnitude higher than
environmentally relevant levels to show the desired pink/red response
†
Interference was at 5 ppb, but with a yellow/orange response—not the desired
red/pink response. Hence, the slight interferents do not affect the THM selectivity
of the sensor in an environmentally relevant range.
Fig. 6. Developed calibration curve for t-THM concentration, using the 2.6 wt% sPP
fiber nanoporous membrane visible colorimetric sensor. Data shown for bromoform
(䊐), chloroform (), and the averaged data (䊉) at the corresponding concentrations include error bars showing the standard deviation obtained from three runs
(n = 3). The insets show the colorimetric response of the sensors to the analytes
(bromoform-top and chloroform-bottom) at the US EPA MCL of 80 ppb.
and 250 ppb bromoform the sensor yielded a color intensity of
114.5 ± 6.4, 127.6 ± 8.7, and 136.3 ± 4.9, respectively. Error bars
show the standard deviation of 3 runs – each run with a different
membrane – for each concentration, where the average standard
deviation of the color intensity was 6.7. At concentrations of 8 ppb,
80 ppb, and 250 ppb chloroform the sensor yielded a color intensity of 90.7 ± 3.2, 116.0 ± 3.6, and 137.0 ± 1.7, respectively. Error
bars show the standard deviation of 3 runs for each concentration, where the average standard deviation of the color intensity
was 2.9. These training set data were then averaged at concentrations of 8 ppb, 80 ppb, and 250 ppb t-THM – an order of magnitude
both above and below the EPA detection limit for THMs – for color
intensities of 102.6 ± 5.8, 121.6 ± 6.2, and 136.7 ± 3.3, respectively.
A logarithmic fit of the average data (C.I. = 9.7 × log10 (Ct-THM ) + 81.7,
where C.I. is the color intensity of the colorimetric response
and Ct-THM is the total THM concentration) showed a correlation coefficient (R2 ) of 98.6% based on the regression line. The
working calibration curve for t-THMs was thus calculated to be
Ct-THM = 2.14285(10−4 ) × EXP(0.103391 × C.I.). The data for the calibration curve developed is statistically significant, with very little
percent deviation (±4.4%), which can be credited to the high sensitivity of the developed visible colorimetric membrane sensor. The
deviation at low THM concentrations can be attributed the increasingly small amount of analyte (ppb-scale), which is lower than
many recent water analysis sensors [40,41], but still maintains a
high predictive ability.
It is known that the Fujiwara reaction has been used to sense
a number of halogenated species [24], therefore other HOCs were
tested to evaluate the selectivity of the functionalized nanoporous
membrane as a visible colorimetric sensor. The selectivity of a
sensor – the ability of a sensor to respond to a particular analyte without interference from others – is particularly important
for environmental samples as the possible interferents are often
unpredictable. The sensor was able to detect only a few other
interferents HOCs (shown in Table 1) at concentrations orders of
magnitude higher than environmentally relevant levels and should
not affect the THM selectivity of the sensor in the environmentally
relevant range. This small response in the trichloroacetic acid and
dichloroacetic acid at extremely high concentrations is most likely
caused by the THM-like interactions between the ␣-carbon – with
Fig. 7. Plot showing the bromoform (+) and chloroform (×) sensor-predicted THM
concentrations for spiked samples (10, 50, 100, 200 ppb). The dotted lines indicate
one average standard deviation, based on the calibration curve. It can be seen that
the sensor very closely predicts the THM concentrations in treated water.
three chlorine and two chlorine atoms attached, respectively – and
the Fujiwara reactants. A similarly small response to trichloroethylene is theorized to be due to the conversion of trichloroethylene to
trichloroacetic acid in aqueous media [42]. These haloacetic acids
(HAAs) are known to interact with the Fujiwara reactants, but in
the current design they do not interfere at relevant concentrations. The reactions with no response – even at concentrations
orders of magnitude higher than environmentally relevant levels
– further show the high selectivity of the developed colorimetric
membrane system to THMs at environmentally relevant concentrations (ppb-scale), without the use of a separate preconcentration
step.
To test the predictive ability of the optimized sPP nanoporous
membrane sensor, the calibration curve was evaluated using a test
set of spiked samples to determine accuracy of the sensor. The
concentration of the spiked sample was fit using the calibration
curve of the training set for t-THM. Seen in Fig. 7, the predicted
concentrations versus the actual concentrations of bromoform and
chloroform are plotted very close to the 45◦ line—indicating a
highly accurate prediction. The slight over prediction seen can be
advantageous for an environmental sensor, as an under prediction
E.K. Wujcik et al. / Sensors and Actuators B 223 (2016) 1–8
can cause unnecessary exposure. Notice that the sPP nanoporous
membrane sensor predicts t-THM concentration very closely or
within the average percent deviation (dotted lines) of the developed calibration curve (±4.4%). Using the developed calibration
curve, this highly sensitive developed sensor could accurately be
used to predict either a concentration of a single THM or t-THM in
treated water.
To test the proof-of-concept real-world applicability and analytical reliability of the developed sensor, environmental samples
– obtained from sites local to The University of Akron – were tested.
Fig. S3 of the Supplementary material shows the concentration of
a positive sample (78 ppb) and the concentration of the same sample as determined by GC/MS (70 ppb). This environmental sample
did not cause any obvious interference in the performance of the
sPP nanoporous membrane sensor, and the close agreement shows
that the developed sensor is sensitive enough to accurately predict t-THM concentrations at the USEPA set MCL in environmental
samples.
4. Conclusions
Through the optimization of a sPP nanoporous membrane sensor capable of simultaneous preconcentration and detection, the
authors have developed a detection method for THMs that is up
to three orders of magnitude lower than the normal beaker-scale
sensing reaction—using no separate preconcentration step. The
optimized nanoporosity of the membrane and the reaction chemistry significantly improved the Fujiwara reaction detection limit
for aqueous samples. This sensor was also found to be able to
detect t-THM concentrations at environmentally relevant conditions, as defined by the USEPA (ppb-scale). The visible colorimetric
nanoporous membrane sensor was found to have a dynamic range
that extends both an order of magnitude above and below the
current USEPA t-THM MCL. Comparison of this sensor to other
developed THM detection techniques [6,43,44] can be found in
Table S1 of the supplementary material. The sensor was also able to
accurately predict the concentration of spiked and environmental
samples, showing that it may be used as a direct qualitative and/or
quantitative sensor for t-THM analysis. This sensor holds potential as a portable t-THM concentration determination tool for an
inexpensive, on-site environmental monitoring tool. Future work
must be done to characterize sensor response to a wide variety of
environmental samples as well as more robust mixtures of halogenated species—although no interference was observed in this
study. From this work, the authors plan to develop a colorimetric
water contaminant detection assay, utilizing a number of sensing
chemistries simultaneously for an accurate evaluation of water
samples.
Acknowledgments
The authors gratefully acknowledge The University of Akron and
Lamar University funds supporting this work. The authors would
also like to thank Associate Professor Christopher Miller (Department of Civil Engineering, The University of Akron) for his insight, as
well as Brittany Apone, Nathaniel Blasdel, Isaac Afreh, Max Duckworth, Brad Vielhaber, Benjamin C. Sauer, and Akshay Jagtap for
experimental contributions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.snb.2015.09.
004.
7
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Biographies
Dr. Evan K. Wujcik is currently an Assistant Professor of Chemical Engineering in the
Dan F. Smith Department of Chemical Engineering at Lamar University (Beaumont,
TX, USA). He obtained his Ph.D. in Chemical and Biomolecular Engineering from
The University of Akron (2013) and his M.B.A. (2011), M.S. in Chemical Engineering
(2009), B.S. in Applied Mathematics (2010), and B.S. in Chemical Engineering (2008)
from The University of Rhode Island. Prof. Wujcik directs the Materials Engineering And Nanosensor (MEAN) Laboratory at Lamar University, where his research
interests include bionanotechnology, nanocomposites, and nanosensor design &
development.
Dr. Chelsea N. Monty is currently an Assistant Professor of Chemical and Biomolecular Engineering at The University of Akron (Akron, OH, USA). She obtained her
Ph.D. in Chemical and Biomolecular Engineering (2009) and M.S. in Chemical and
Biomolecular Engineering (2005) from The University of Illinois Urbana-Champaign,
and her B.S. in Chemical Engineering (2004) from Carnegie Mellon University. Prof.
Monty’s research interests include the development of nanocomposite materials for
sensing applications methods and advanced electrochemical methods.
SUPPLEMENTARY MATERIAL FOR:
A visible colorimetric sensor based on nanoporous polypropyelene fiber membranes for the
determination of trihalomethanes in treated drinking water
Evan K. Wujcik,a,* Stephen E. Duirk,b George G. Chase,c and Chelsea N. Montyc,*
a
Materials Engineering And Nanosensor (MEAN) Laboratory, Dan F. Smith Department of Chemical
Engineering, Lamar University, Beaumont, TX, 77710 USA
b
c
Department of Civil Engineering, The University of Akron, Akron, OH, 44325, USA
Department of Chemical & Biomolecular Engineering, The University of Akron, Akron, OH, 44325,
USA
* To whom correspondence should be addressed:
(E.K.Wujcik) Evan.wujcik@lamar.edu;
(C.N. Monty) cm78@uakron.edu
S1
Contact angle measurements were conducted on varying sPP concentrations in a viable
electrospinnable range (1-4 wt%), as shown in Fig. S1. A plateau in the contact angle is observed at
values greater than 2 wt% sPP. This leveling shows indiscernible statistical variations. 2.6 wt% sPP was
used in the development of the visible colorimetric sensor, due to the overall morphology, ease of
handling, and peak hydrophobicity (138.2°).
Fig. S1 - The effect of electrospun polypropylene weight percent on the contact angle of the
nanoporous membrane using the Sessile Drop Technique. Inset shows an image of a drop of water on
the 3 wt% polypropylene membrane.
Experimentation was conducted to determine the optimum functionalization and operating
temperature based on the highest response as determined by image intensity analysis (Fig. S2).
Fujiwara reactant ratios (pyridine:NaOH) and temperatures (70-90 °C) were varied to determine
optimum sensor conditions. Fig. S1—with the best overall results outlined in red—shows that the sPP
nanoporous membrane had the higher colorimetric response, as determined by color intensity, at both
80 ppm and 1 ppm bromoform at 70°C and a 9:1 ratio of pyridine to saturated sodium hydroxide in
ethanol. The reactant ratios were chosen in reference to previous literature. The temperatures were
S2
chosen based on the thermodynamic properties of the THM:water system and the minimization of
water vapor.
Fig. S2 – Evaluation of the 2.6 wt% sPP nanoporous membrane for colorimetric response highlights
the optimized sensor operating conditions at each concentration. The sample temperature (70°C and
90°C, top and bottom row of images, respectively), exposed THM concentration (1 ppm or 80 ppm, left
six or right six, respectively), and pyridine to sodium hydroxide ratio (v/v, 9/1, 1/1, or 1/9, for their
respective columns) were varied as shown above. One may see from the image intensity values (within
the colorimetric results) that the higher (9/1) ratio of pyridine to sodium hydroxide at the lower (70°C)
temperature yielded the best results at both concentrations.
To illustrate the proof-of-concept real-world applicability of the developed sensor,
environmental samples–obtained from sites local to The University of Akron–were tested. Four of the
five collected samples showed no colorimetric response to THMs. The lack of THMs in these samples
was confirmed by GC/MS. However, in the fifth sample, a colorimetric response predicted a total
regulated trihalomethane (t-THM) concentration of 78 ppb from the chloroform calibration curve. The
environmental t-THM concentration of this sample was determined—via GC/MS—to be 70 ppb (Fig.
S3). The chloroform calibration curve was used as this THM is the most likely to be found in treated
water, due to disinfection by-products of water treatment (chlorination). The close agreement shows
that the developed sensor is sensitive enough to accurately predict t-THM concentrations at the USEPA
S3
set MCL. It was found that the calibration curve was in agreement and the environmental sample–with
t-THM concentrations confirmed via GC/MS–were within the error predicted by the calibration curve.
This shows that the developed sensor was able to detect environmentally-relevant THM concentrations
accurately without interference from any of the other chemicals found in a real environmental sample.
Fig. S3 - Map showing the environmental testing site. Insets show the concentration predicted from the
sensor, as well as the confirmed concentration—as determined by GC/MS.
Below (Table S1), a comparison amongst a variety of THM detection techniques and the current
work are presented. Nearly all of the techniques that involve detection of THMs using the Fujiwara
reaction found utilized gas chromatography with similar lower limits of detection. It can be seen that
the developed nanoporous sPP membrane visible colorimetric sensor has a lower limit of detection
(LLOD) an order of magnitude below that of the USEPA t-THM limit in treated water (80 ppb), and is
lower than some LLODs based on non-portable, expensive analytical laboratory equipment. The
working range of the developed sensor—extending a full order of magnitude above the USEPA t-THM
limit in treated water—is also one of the most dynamic.
S4
Table S1 – Comparison of halogenated organic compound sensors’ analytes, detection methods, working ranges, and LLODs.
Analyte(s)
Working Rangea
Detection Method
LLODa
Bromoform,
dibromochloromethane,
bromodichloromethane Electrochemical Sensor using the Fujiwara Reaction
80,000-300,000 ppb 80,000 ppb
Chloroform
80,000-800,000 ppb 80,000 ppb
Bromoform
43-199 ppb
43 ppb
Chloroform
37-245 ppb
37 ppb
Dibromochloromethane
49-221 ppb
49 ppb
Bromodichloromethane
48-238 ppb
48 ppb
0.9-35 ppb
0.3 ppb
[44]
8 ppb
This
work
[6]
Solvent Extraction/Chemical Quantification via UV-Vis
a
Ref.
[43]
t-THM
Capillary Membrane Sampling–Gas Chromatography
t-THM
Visible Colorimetric Nanoporous sPP Fiber Membrane
8 ppb-250 ppb
Sensor using the Fujiwara Reaction
Working range and LLOD were normalized to ppb, if reported values were of other units, for ease of comparison.
S5
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