Development of colorimetric cotton swab using molecular switching hydrazone probe in calcium alginate

Article abstract of DOI:10.1016/j.molstruc.2020.128301

Smart cotton fabric was developed to function as a swab for sweat pH monitoring in real-time toward a potential application for drug testing and monitoring human healthcare. A wearable, flexible and disposable microencapsulated chemical barcode was incorporated onto the highly absorptive cotton fabric to monitor sweat biochemical variations. A pH-responsive tricyanofuran hydrazone molecular switch encapsulated inside calcium alginate (Ca-alginate) microcapsule was loaded onto a cotton fabric matrix to keep an eye on sweat pH variations. Calcium alginate sensor microcapsules were prepared using tricyanofuran-hydrazone receptor sites as the core material and cross-linked calcium alginate biopolymer as the wall-former. The cotton fabric was dyed in-situ with a microencapsulated tricyanofuran-hydrazone in calcium alginate using the pad-dry-cure technique at room temperature. The chromogenic activity of the cotton detection tool depended mainly on the halochromic performance of the tricyanofuran hydrazone spectroscopic probe, which demonstrated color shift from light yellow to darker yellow, orange and purple due to protonation/deprotonation reversible mechanism of the tricyanofuran hydrazone chromophore. The visible color shift was monitored by CIELAB coordinates. The surface morphology, distribution and composition of fabrics were explored employing scanning electron microscopic (SEM) images, energy dispersive X-ray (EDX) spectra, and mapping analytical techniques. Microencapsulated tricyanofuran-hydrazone sensor probe on fabrics was characterized in terms of their air-permeability, stiffness and colorfastness.

Full text of DOI:10.1016/j.molstruc.2020.128301

Journal Pre-proof
Development of colorimetric cotton swab using molecular switching hydrazone probe
in calcium alginate
Meram S. Abdelrahman, Moustafa M.G. Fouda, Jamaan S. Ajarem, Saleh N.
Maodaa, Ahmed A. Allam, Tawfik A. Khattab
PII:
S0022-2860(20)30626-8
DOI:
https://doi.org/10.1016/j.molstruc.2020.128301
MOLSTR 128301
Reference:
To appear in: Journal of Molecular Structure
Received Date: 25 February 2020
Revised Date: 8 April 2020
Accepted Date: 18 April 2020
Please cite this article as: M.S. Abdelrahman, M.M.G. Fouda, J.S. Ajarem, S.N. Maodaa, A.A.
Allam, T.A. Khattab, Development of colorimetric cotton swab using molecular switching hydrazone
probe in calcium alginate, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2020.128301.
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Credit Author Statement
Meram S. Abdelrahman: Preparation of tricyanofuran and measuring melting
points. Moustafa M. G. Fouda: Preparation of tricyanofuran-hydrazone and study
NMR spectra. Jamaan S. Ajarem: Studying elemental analysis, mass spectra,
colorimetry and colorfastness measurements. Saleh N. Maodaa: Incorporation of
microcapsules onto cotton substrates. Ahmed A. Allam: Cotton sensor testing and
studying IR spectra. Tawfik A. Khattab: Studying SEM, EDX, writing, reviewing
and editing.

Graphical abstract
1

Development of colorimetric cotton swab using molecular switching
hydrazone probe in calcium alginate
Meram S. Abdelrahman1, Moustafa M. G. Fouda2, Jamaan S. Ajarem3, Saleh N.
Maodaa3, Ahmed A. Allam4, Tawfik A. Khattab1,*
1
Dyeing, Printing and Auxiliaries Department, National Research Centre, Cairo 12622,
Egypt
2
Pretreatment and Finishing of Cellulosic-based Fibers Department, National Research
Centre, 33 El-Buhouth Street, Dokki, Cairo 12622, Egypt
3 Department of Zoology, College of Science, King Saud University, P.O. Box 2455, Riyadh,
11451, Saudi Arabia
4 Department of Zoology, Faculty of Science, Beni-suef University, Beni-suef, 65211 Egypt.
(*) Corresponding author: Dr. Tawfik A. Khattab; Email address: ta.khattab@nrc.sci.eg
Abstract
Smart cotton fabric was developed to function as a swab for sweat pH monitoring in real-time
toward a potential application for drug testing and monitoring human healthcare. A wearable,
flexible and disposable microencapsulated chemical barcode was incorporated onto the
highly absorptive cotton fabric to monitor sweat biochemical variations. A pH-responsive
tricyanofuran hydrazone molecular switch encapsulated inside calcium alginate (Ca-alginate)
microcapsule was loaded onto a cotton fabric matrix to keep an eye on sweat pH variations.
Calcium alginate sensor microcapsules were prepared using tricyanofuran-hydrazone receptor
sites as the core material and cross-linked calcium alginate biopolymer as the wall-former.
The cotton fabric was dyed in-situ with a microencapsulated tricyanofuran-hydrazone in
calcium alginate using the pad-dry-cure technique at room temperature. The chromogenic
activity of the cotton detection tool depended mainly on the halochromic performance of the
tricyanofuran hydrazone spectroscopic probe, which demonstrated color shift from light
yellow to darker yellow, orange and purple due to protonation/deprotonation reversible
mechanism of the tricyanofuran hydrazone chromophore. The visible color shift was
monitored by CIELAB coordinates. The surface morphology, distribution and composition of
fabrics were explored employing scanning electron microscopic (SEM) images, energy
dispersive X-ray (EDX) spectra, and mapping analytical techniques. Microencapsulated
1

tricyanofuran-hydrazone sensor probe on fabrics was characterized in terms of their air-
permeability, stiffness and colorfastness.
Keywords: Smart Swab; Hydrazone; Alginate; Microcapsules; Colorimetric; Sweat
Monitoring.
1. Introduction
Smart technical textile materials are generally defined as clothing materials that are able to
detect or response to external stimuli, such as temperature, chemicals or gases, light,
magnetic or electric field, pH, pressure, and solvent polarity [1-6]. For instance, smart
garments can release medication onto human skin to help regulating muscular vibrations
during physical or sport activities. Smart cloths that produce sufficient responsiveness due to
an external stimulus, such as heat, high voltage electric field, hazardous chemicals and
irradiation, are prone to improve their protective function [7-9]. Sensing materials can be
incorporated into clothing to determine the biochemical variations in fluids of human body,
such as sweat, tears, saliva and urine [10-12]. Sweat is mainly composed of water as well as
small quantities of other biological substances, such as amino acids, potassium and sodium
ions, urea, lactate, and pyruvate [13, 14]. Sweat is naturally excreted by skin with a pH value
from 4 to 6.8. Increasing pH value of sweat fluid higher than 6.8 is usually an indication of an
abnormal performance of the human body. Therefore, its contents can be monitored to
introduce important diagnosis about the individual physiological case. A sweat sample can be
analyzed to monitor pathological disorders, dehydration, hyponatremia, which is significant
to optimize the reasons of rehydration and remineralization [15, 16]. In case of drug abuse,
sweat fluid might hold drug opiates, amphetamines, cocaine buprenorphine, and
cannabinoids, which usually correlate sweat pH to person health [17-19]. Thus, monitoring
human sweat fluid is beneficial for personal healthcare; however, the sweat fluid sample
collection is a difficult issue as the useful examination must be made in real-time [20]. The
use of illegal drugs leads to a huge number of related threats to public health particularly in
road traffic [21, 22]. The action of law against drunken drivers led to a decrease in alcohol
associated accidents. The breathing alcohol test can be simply performed on-site; however,
the case is different for drug-messed drivers as particular signs are less noticeable and can be
complicated to detect without the support of examining biological samples [23, 24]. Recently,
police authorities took a high concern in the recognition of an impaired driver by using
roadside screen examination and called upon the forensic employee to present non-invasive
2

sampling methods and appropriate screen approaches for on-site drug exams. However, such
techniques require complicated instrumentations, difficult operation, hard processing and
trained personnel, in addition to their failure to introduce real-time detection results [23-27].
Thus, monitoring sweat changes on-site in real-time can afford a preliminary beneficial
physiological evaluation. Colorimetric sensors are characterized by their simple fabrication
and operation, as well as their ability for naked-eye recognition making them particularly
attractive. Thus, colorimetric probes have been acknowledged as one of the most easy-to-use,
low-cost and effective analytical methods to monitor primary hazards or changes in the
surrounding environmental pollution [28-31]. Using colorimetric approach for sweat
examination is a helpful diagnostic tool due to the benefit of being non-invasive and simply
reachable. Additionally, colorimetric detection tools are characterized by their ability to
introduce real-time detection results, no need for complicated instrumentations or trained
personnel, simple operation and easy processing [32-37]. Herein, we developed a novel,
simple, robust and flexible halochromic cotton-based sensor with the ability to real-time
follow up the performance of biochemical changes in human body sweat fluid. Such cotton
sensor matrix is characterized by their being flexible, comfortable, high porosity and potential
high surface area that can be treated [38]. The current microencapsulated cotton swab sensor
device proved to function as a real-time detection tool for the variations in sweat without the
requirement of any electronic components, trained personnel, power source or complicated
instrumentation as it can be directly function by direct touch with human skin to guarantee
instant response to sweat contents.
2. Experimental
2.1. Materials and reagents
Plain weave cotton (100%) substrate, micronaire 3.88 μg/inch, thickness 0.4 mm, weft 30
yarn/cm, warp 36 yarn/cm, 150 g m-2 were used. The cotton substrates, supplied from
Spinning and Weaving Misr Mehalla of Egypt, were desized, scoured, and bleached
employing previous literature procedures [7]. Calcium chloride (CaCl2) and sodium alginate
(Na-alginate) were obtained from Sigma-Aldrich. Tricyanofuran and tricyanofuran-
hydrazone were prepared according to previous synthetic procedures [39, 40], in which
tricyanofuran and tricyanofuran-hydrazone were recrystallized and obtained in high purity
using flash column chromatography and reactions were visually monitored by TLC (PF254)
under ultraviolet lamp (365nm). Organic solvents were purchased from Aldrich, while
3

malononitrile, 3-hydroxy-3-methylbutan-2-one and 4-nitroaniline were purchased from
Sigma Aldrich.
2.2. Methods and instrumentation
Melting points were obtained uncorrected (oC) on Stuart/SMP30. FTIR spectrum of the
prepared tricyanofuran-hydrazone was recorded on Nexus-670 (Nicolet, United States). NMR
spectra were explored by BRUKER/AVANCE/400 spectrometer at 400 MHz. Elemental
analysis was carried out on PerkinElmer/2400 (Norwalk, USA). Mass spectrum was
measured on Shimadzu GCMS-QP-1000-EX spectrometer. Scanning electron microscope
(SEM) was applied to investigate both morphological properties and elemental content using
Quanta FEG250 (Czech Republic) coupled to energy-dispersive X-ray (TEAM-EDXA). The
calcium alginate microparticle diameter was measured by Image J software.
2.3. Preparation of tricyanofuran [39]
Sodium metal (6.5 mmol) was dissolved in 15 mL of absolute ethyl alcohol in a water bath at
room temperature to introduce sodium ethoxide solution, followed by adding 3-hydroxy-3-
methylbutan-2-one (45 mmol) and malononitrile (90 mmol) with stirring. After stirring for
one hour at room temperature, an additional 20 mL of absolute ethyl alcohol was added. The
mixture was refluxed for another two hours. After cooling in a refrigerator, the resultant
precipitate was filtered under vacuum, subjected to washing with a minimal quantity of cold
ethyl alcohol, and air-dried to give an off-white crystalline product. The filtrate was then
concentrated, cooled and filtered off to give a second crop (total yield 59%). mp 202ºC–
204ºC. 1H NMR (400 MHz, CDCl3): ppm 2.36 (s, 3H), 1.64 (s, 6H).
2.4. Preparation of tricyanofuran-hydrazone [40]
Tricyanofuran-hydrazone was prepared by stirring 4-nitroaniline (2 mmol) and hydrochloric
acid (15 mL) on a magnetic stirrer at 0-5ºC. An aqueous solution of NaNO2 (2 mmol) in was
then added slowly to afford the corresponding diazonium salt, which was added slowly to a
solution of tricyanofuran (2 mmol) and sodium acetate (3 g) in acetone (15 mL) at 0-5ºC with
vigorous stirring. The precipitate was filtered, subjected to washing with distilled water,
crystallized from a mixture of ethyl alcohol/chloroform (2:1) to give a dark orange powder
(yield 81%); mp 225-226°C; 1H NMR (400 MHz, DMSO-d6): ppm 12.79 (singlet broad, 1 H,
NH), 8.26 (doublet, 2 H, J= 9.2 Hz), 7.99 (singlet, 1 H, =C-H), 7.38 (doublet, 2 H, J= 8.8
Hz), 1.81 (singlet, 6 H); 13C NMR (400 MHz, DMSO-d6): ppm 177.38, 171.91, 147.82,
142.86, 129.19, 126.53, 115.19, 113.14, 112.38, 110.87, 99.44, 99.30, 26.36; IR (neat, ν/cm-1)
3275 (for secondary amine group), 2224 (for cyanide group), 1577 (for C=N group), 1511
and 1309 (for nitro group); MS m/z (%): 347 [M-H]+; Elemental analysis for tricyanofuran-
4

hydrazone (C17H12N6O3; 348.10): calculated C 58.62, H 3.47, N 24.13; established C 58.51,
H 3.62, N 24.03.
2.5. In-situ preparation of microcapsules on cotton substrates
Sodium alginate (0.5wt % to water volume) and hydrazone probe (0.1 (H1), 0.5 (H2), 1 (H3),
1.5 (H4) and 2 (H5) wt% to the weight of sodium alginate) dissolved in 1-3mL acetone were
added to distilled water (100 mL). The mixture was stirred on a mechanical stirrer for 1 hour
to ensure a homogenous mixture. Cotton fabrics were subjected to padding in the prepared
solutions for 15 minutes, and then padded in a calcium chloride aqueous solution (500 mM)
for 15 minutes. The treated cotton fabrics were then washed with distilled water to get rid of
any remained depositions of NaCl or CaCl2, and air-dried. The In-situ preparation of
microcapsules onto cotton fabrics is displayed in Figure 1.
Figure 1: In-situ preparation of microcapsules immobilized onto cotton fibers.
2.6. Colorimetric and colorfastness measurements
The color changes were reported on a Texflash ACS/Datacolor 600 spectrophotometer. The
color data were explored by CIE (L*, a*, and b*) coordinates and color strength. The
colorfastness properties against rubbing, light, washing and perspiration were examined
according to ISO standards; ISO105-X12(1987), ISO/105-B02 (1988), ISO/105-C02 (1989)
and ISO/105-E04 (1989), respectively.
2.7. Cotton sensor test
Sweat mimic solution was assembled depending on ISO/105/E04:1989 standard test. L-α-
Amino-β-(4-imidazolyl)propionic acid monohydrochloride (0.25 g·L-1), sodium chloride
(NaCl; 10 g/L), and sodium dihydrogen phosphate (NaH2PO4; 1 g L-1) were added to distilled
water (1 L). The pH was tuned between 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5 and 8 using aqueous
solutions of acetic acid (0.1 N) and NaOH (0.1 N) to reduce and to raise the pH value,
5

respectively. The designed nine sweat mimic solutions with different pH values was sprayed
on the cotton fabrics to show an instant color shifting from light yellow to darker yellow,
orange and purple relying on the pH of the medium. The pH values of sweat mimic solutions
were monitored by BECKMAN-COULTER pHI340.
The reversibility of dyed cotton sensor (1 wt %) was tested by exposing the cotton fabric to
sweat fluid (pH 7.5) to demonstrate purple color. The fabric was then washed with an
aqueous solution of acetic acid (pH 6) in distilled water to remove the effect of the sweat
fluid and finally air-dried. The absorption wavelength was recorded on Texflash ACS
Datacolor. The same procedure was repeated several cycles.
3. Results and discussion
3.1. Preparation of microcapsules immobilized onto cotton fabric
There are a diversity of materials, such as cellulose acetate [41], mesoporous silica [42] and
silicate sol-gel [43] that have been employed as supporting carrier substrates to fabricate
sensor tools. However, most of those materials comprise several drawbacks, such as ageing,
poor sensitivity and reversibility, expensive, difficult processing, as well as long-time
sensing. Alginate is a polysaccharide, naturally occurred in brown algae, that has been
applied for several purposes, such as nontoxic wound dressing, foodstuff, thickener, stabilizer
and gelator, and other medical substrates. Alginate-based microencapsulated devices can be
assembled by the crosslinking process of Na-alginate polymer chains. This is carried out by
the replacement of monovalent Na(I) with divalent Ca (II) to function in an aqueous medium
at mild conditions [44, 45]. Immobilization of one or more active detection sites into calcium
alginate (Ca-alginate) microcapsules can be accomplished by co-precipitating those active
sites within Ca-alginate capsules.
Tricyanofuran hydrazones are pH-stimuli responsive dyes characterized by excellent ability
to create pH-responsive colorimetric sensor devices due to their excellent color changes at
different values, reversibility, and thermal and photochemical stability. Tricyanofuran-
hydrazone sensor dyes have been known as multi-stimuli responsive dyes to pH and
temperature. They have been used as solution probes and as solid state sensors in polymer
matrices for the production halochromic tools, and determination of ammonia and amines.
The tricyanofuran (1) starting material bearing an active-methyl was simply prepared
depending previous literature procedure [40, 41]. The tricyanofuran-hydrazone sensor probe
(2) was prepared via azo-coupling of heterocyclic compound 1 and diazonium salt of 4-
nitroaniline (Figure 2).
6

CN
CN
H2
C
H2C
CH3COCH3
CH3COONa
CN
CN
CN
CN
H
Cl N N
NO2
+
O
O
diazonium salt
carbanion form
tricyanofuran 1
0-5ºC
H
N
H
N
CN
CN
CN
N
N
CN
CN
O2N
O2N
O
O
hydrazone form 2
azo form
CN
Figure 2: Preparation of tricyanofuran-hydrazone probe
Illegal drugs and other health problems can be easily reflected spectroscopically by apparent
shifts in the pH value of human sweat, which diffuses through the porous film of alginate
microcapsules on cotton fabric into the tricyanofuran-hydrazone detection active sites to
change their color between light yellow and purple. Mechanism of perspiration solution
sensing depended mainly on the protonation-deprotonation reversible process of
tricyanofuran-hydrazone which was integrated within Ca-alginate microcapsules on cotton
substrates. The strong electron-withdrawing nitro group at para position of the
phenylhydrazone moiety led to deprotonating NH hydrzone introducing the corresponding
hydrazone anion bearing negative charges as a result of the increased acidity of this NH
hydrazone proton. Consequently, the hydrazone anion form functions as an electron-donor
fragment conjugated with the electron-withdrawing tricyanofuran heterocycle in a push-pull
extended conjugation molecular structure (Figure 3).
CN
O
H
N
CN
CN
CN
N
CN
CN
sweat mimic
fluid
N
N
:
O
O
washing with
CH3COOH
in distilled water
O
N
O
N
O
:
:
Extended conjugation
tricyanofuran-hydrazone anion
(purple)
tricyanofuran-hydrazone
(yellow)
Figure 3: Mechanism of tricyanofuran-hydrazone protonation-deprotonation reversible
process
7

The sensor microcapsules were assembled by concurrent co-precipitation of Na-alginate and
tricyanofuran-hydrazone into cross-linked Ca-alginate enclosing tricyanofuran-hydrazone
dispersed active sites through the alginate polymer matrix. Cotton samples were immersed in
an aqueous Na-alginate and tricyanofuran-hydrazone, and air-dried followed by immersion in
an aqueous CaCl2 to commence the cross-linking operation. In the alginate polymer strains,
Upon adding Na-alginate into an aqueous solution of CaCl2, the divalent Ca++ replaced the
monovalent Na+. Each Ca++ can be cross-linked to two polymer strains. Alginate is a
polysaccharide that has been applied in foodstuffs, pharmaceutical and dental purposes,
thickeners, stabilizing and gelling agents, and wound dressings. Five different solutions were
prepared depending on tricyanofuran-hydrazone concentration in solution (0.1 (H1), 0.5 (H2),
1 (H3), 1.5 (H4) and 2 (H5) wt %).
3.2. Characterization and morphological properties
Cotton fabric conventionally dyed with tricyanofuran-hydrazone probe did not demonstrate
color change to the sweat mimic solution, which could be attributed to the steric hindrance
that tricyanofuran-hydrazone molecules experience in the fibrous matrix of cotton fabric,
where both the active tricyanofuran-hydrazone probe and cotton fibers are not in the
nano/microscale and accordingly cannot be stimulated easily by the targeted molecules to be
monitored. In comparison to large diameter particles, microcapsules are distinguished by
their high surface area, outstanding mechanical features and high surface functionality [46-
50]. For instance, the solid powder of tricyanofuran-hydrazone probe did not demonstrate any
sensing performance as was already demonstrated by the same material probe in solution
phase. This is because the guest molecules cannot diffuse easily through the solid powder of
the tricyanofuran-hydrazone probe. On the other hand, microcapsules possess higher surface
area compared to the corresponding solid powder of the same material, which enabled
diffusion of the targeted molecules to be detected through the sensing device matrix. The
current pH-responsive tricyanofuran hydrazone molecular switch can be classified as a
disperse dye because it is water-insoluble and characterized with small molecular structure.
Thus, this tricyanofuran hydrazone disperse dye has absolutely no affinity to dyeing cotton
fibers and it can only be attached to cotton fibers via encapsulation within the calcium
alginate cross-linked mediator [46, 51-54]. Compared to previously report blank cotton fibers
[7], scanning electron microscope images of treated samples proved the formation of calcium
alginate microcapsules incorporated on the surface of cotton fabric, which resulted in a higher
surface area with porous structure to allow better diffusion of sweat fluid into the
tricyanofuran-hydrazone active sites. The morphological properties and elemental
8

composition of the treated cotton substrate surface dip-coated with Ca-alginate microcapsules
molecularly imprinted by tricyanofuran-hydrazone were explored (Figure 4). The scanning
electron microscopic images of pad-dry-cured cotton demonstrated successful dip-coating of
cotton surface with clusters of Ca-alginate microcapsules displaying nano/microstructures of
irregular shapes. The alginate microcapsules diameter distribution was measured on software
program employing SEM images. The size distribution of the obtained nano/microstructured
Ca-alginate microcapsules on cotton fabric surface was in the range from ~550nm to ~5µm.
The main size average of the Ca-alginate microcapsules was about ~2µm. Such
nano/microstructural Ca-alginate microcapsules tended to agglomerate, and accordingly
dispersed slightly heterogeneous onto the pad-dry-cured cotton fabric, which could be
attributed to the sort of chemical and physical interactions of Ca-alginate macromolecules
with the cotton fabric. Moreover, SEM images demonstrated no physical variations happened
to the surface of pad-dry-cured cotton fabric.
Figure 4: Scanning electron microscope images of dip-coated (pad-dry-cured at room
temperature) cotton fabric (Sample H3)
9

Figure 5: Elemental mapping image (top) and energy dispersive X-ray analysis spectrum
(bottom) of dip-coated (pad-dry-cured at room temperature) cotton fabric (Sample H3)
The elemental composition of the dip-coated (pad-dry-cured) cotton was also explored by
EDAX spectroscopic analysis via measuring the elemental weight percent at three different
spots on the surface of the pad-dry-cured fabric as shown in Figure 5. The elemental
composition selected at these three spots was closely similar. This proved the homogenous
incorporation of Ca-alginate microcapsules on cotton surface. The mapping diagram of the
major elements confirmed the consistent distribution of Ca-alginate microcapsules on the
surface of the pad-dry-cured cotton. The Ca-alginate microcapsules were immobilized onto
cotton fabric to establish porous three-dimensional architecture bearing highly porous surface
area. Thus, solid state cotton sensor showed high sensitivity toward sweat fluid due to the
high surface to volume ratio and large porous structural design, which facilitated the diffusion
of the guest sweat fluid within the Ca-alginate microcapsules integrated within cotton matrix.
Fourier transform infrared (FTIR) spectra were employed to investigate the key characteristic
peaks of the dyed cotton including both hydroxyl and CH aliphatic groups. In general, there
are no considerable shifts were detected. Slight reduction was monitored in the absorbance
10

band at 3305 cm-1 for OH groups, which could be attributed to H-bond formation among the
hydroxyl groups on the cellulose polymer strains of the cotton fabric and the carboxylate
anion comprised from the alginate biopolymer. Also, slight increments in the absorbance
peaks were monitored at 2965 and 1013 cm-1 for both aliphatic C-H stretching and bending,
respectively. This could be assigned to the extra aliphatic C-H from the alginate layer.
3.3. Colorimetric measurements toward sweat monitoring
Solid-state detection tools are more practical than liquid probes because they are
characterized by portability, reversibility and simplicity leading to fast online detection at low
cost. Thus, it is very interesting and also challenging, to assemble solid state sensing devices
for real time detection of a trace material. Solid state devices could potentially be applied as
detection devices for general laboratory assays and household purposes as commercial
sensors [55-59]. Hence, the research for an environmentally friendly simple solid sensor is of
high importance. Cotton is originally natural solid cellulosic material with a fibrous-like
porous structure characterized by low density, high porosity and high surface area. The
calcium alginate biopolymer-based microcapsules were immobilized into cotton fabric to
present porous three-dimensional architecture characterized by highly porous scaffolds with
large surface area. The colorimetric cotton sensor has the ability to function directly as a
swab on human skin. To inspect the solid state cotton fabric detector, we designed a sweat
mimic solution to be sprayed, which is similar to swabbing human skin, onto the surface of
the dip-coated cotton to display an immediate color shift from light yellow to darker yellow,
orange and purple relying on the medium pH of the sweat mimic system as demonstrated in
Figure 6 and Table 1. Changes in K/S and CIE color space values were explored to evaluate
the cotton fabric sensing ability of sweat biochemical changes. The coloration data of the dip-
coated cotton fabrics at different concentration values of tricyanofuran-hydrazone probe
before and after exposing to sweat solution are demonstrated in Table 2. For both situations,
before and after exposing to sweat solution, a significant increase in K/S was detected due to
variation to better K/S upon increasing the concentration values of tricyanofuran hydrazone
within the range of 0.1-1.0 wt %. A slight change was observed in K/S upon increasing the
concentration values of tricyanofuran hydrazone within the range of 1.0-2.0 wt %. This is
because the uptake of Ca-alginate/tricyanofuran hydrazone from the solution bath into the
cotton fabric under treatment was increased between 0.1-1.0 wt%, while the uptake of Ca-
alginate/tricyanofuran hydrazone reached equilibrium within the range of 1.0-2.0 wt%.
Therefore, Ca-alginate/tricyanofuran hydrazone content on the fabric was at highest
saturation at 1.0 wt%. The fabric at this equilibrium stage paused absorption of Ca-
11

alginate/tricyanofuran hydrazone during the padding process. Thus, the best coloration values
of the cotton fabric were monitored at 1 wt%. The exposure of cotton fabric to sweat
solutions resulted in an increase in the absorption maxima of the fabric from 455 (pH 4) to
515 nm (pH 8). The cotton fabric sensor was integrated with tricyanofuran-hydrazone
immobilized within Ca-alginate microcapsules. The instantaneous color change was visually
recognized from light yellow to darker yellow, orange and purple relying on the biochemical
variations of the sweat fluid. The three dimensional color parameters (L*, a*, b*) of the pad-
dry-cured fabrics treated at different concentration values of tricyanofuran hydrazone before
and after exposure to sweat fluid were displayed in Table 2. The lightness/darkness range
was denoted by the symbol L*, green/red range was denoted by the symbol a*, and
blue/yellow range was denoted by the symbol b*. The dip-coated cotton fabrics possessed
yellow color, while the blank untreated cotton possessed white color (L*= 95.83, a*= 0.02,
b*= -1.05). The color coordinates of the Ca-alginate microcapsules incorporated on cotton
fabrics were measured before and after exposing to the sweat solution. Depending on the
concentration of tricyanofuran-hydrazone, all treated cotton fabrics, before and after exposing
to perspiration solution, showed significantly dissimilar L*, a* and b* magnitudes comparative
to blank cotton. For both conditions, before and after exposing to perspiration solution, L*
was comparatively decreased with increasing tricyanofuran-hydrazone total content within
the range of 0.1-2.0 %. However, a significant increase was monitored in L*, mostly for the
tricyanofuran-hydrazone concentration at 1.0 wt %, after exposing to perspiration solution to
signify darker color. Before exposing to perspiration solution, the decreased b* and increased
a* positive values indicated yellow color of all pad-dry-cured cotton fabrics at all
tricyanofuran-hydrazone concentrations. After subjecting to perspiration solution, the
increased positive a* and negative b* values indicated purple color of all pad-dry-cured cotton
fabrics. For all samples, after exposure to perspiration solution, a* positive magnitudes were
increased to designate more red, while positive b* magnitudes were shifted to negative to
designate more blue.
12

520
510
500
490
480
470
460
450
3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5
pH value
Figure 6: Relationship between maximum absorption wavelength and different pH values of
sweat fluid
Table 1: Changes in color and absorption wavelength according to changes in pH values of
sweat fluid
pH value λmax Color of fabric
4
4.5
5
455
455
455
460
475
495
510
515
515
Light yellow
Light yellow
Light yellow
Light yellow
Yellow
5.5
6
6.5
7
Orange
Orange
7.5
8
Purple
Purple
13

Table 2: Color coordinates of Ca-alginate microcapsules incorporated on cotton fabrics with
different contents of tricyanofuran-hydrazone, and before (B) and after (A) exposure to the
perspiration mimic mixture.
L*
a*
b*
K/S
Probe
wt%
B
A
B
A
B
A
B
A
H1
H2
H3
H4
H5
72.67 48.26 3.09
68.92 47.08 6.78
67.01 37.85 8.55
65.83 36.36 8.34
65.12 36.58 8.15
4.21
7.89
18.25 - 6.14
14.47 - 8.33
3.23
4.19
5.00
5.87
7.23
7.85
8.12
10.32 11.71 - 11.59 5.07
11.29
12.13
8.56 - 13.90 5.72
5.82 - 17.05 6.10
3.4. Durability and comfort measurements
The main reason of applying the pad-dry-curing technique is to establish a smooth fabric
surface with low film thickness and low roughness, while maintaining the cotton substrate’s
flexibility and air-permeability. Shirley Stiffness Testing Machine was employed to measure
the bending length of the pad-dry-cured cotton fabrics (Table 3). Mainly, the pad-dry-curing
procedure did not affect on air-permeability, but a slight decrease in flexibility of the pad-
dry-cured cotton fabrics was detected in warp/weft with raising the concentration of
tricyanofuran-hydrazone probe. The pad-dry-cured film possessed a surface free energy (35
mN/m) as demonstrated by a tensiometer (KRÜSS/DSA30S). This proved that the pad-dry-
cured cotton fabric surface had a high wettability. The durable performance of the pad-dry-
cured cotton to light, perspiration, washing and rubbing was explored. No changes were
detected for the pad-dry-cured cotton after wash. The pad-dry-cured cotton fabrics showed
softness to touch. The colorfastness and photostability were satisfactory for all concentrations
of tricyanofuran-hydrazone as shown in Table 4. However, a little decrease in the
colorfastness and photostability was monitored upon increasing tricyanofuran-hydrazone
concentration.
14

Table 3: Bending length and air-permeability properties of the pad-dry-cured fabrics
Probe
wt%
Bend length
(cm)
Air-permeability
(cm3/cm2/s1)
weft
2.50
2.87
3.10
3.38
3.53
3.61
wrap
2.91
3.25
3.43
3.47
3.68
3.75
Blank
H1
52.58
48.91
47.08
47.28
47.39
47.24
H2
H3
H4
H5
Table 4: Colorfastness properties of the pad-dry-cured fabrics
Wash
Perspiration
Acidic Basic
Alt. Alt.
Rubbing
Probe
wt%
Light
Dry Wet
Alt.
St.
St.
4-5
4-5
4
St.
4-5
4-5
4
H1
H2
H3
H4
H5
4-5
4-5
4
4-5
4-5
4
4
4-5
4
4-5
4-5
4
4
4
4
6
6
4
3-4
3-4
3-4
3-4
3-4
3
6
4
4
3-4
4
4
4
3-4
3-4
5-6
5-6
3-4
4
4
3-4
Alt. = color alteration; St. = cotton staining.
The reversibility of treated cotton (H3) was explored after perspiration monitoring by
exposing the cotton fabric to sweat fluid (pH 7.5) to demonstrate purple color, the maximum
absorbance wavelength were repeated back-and-forth as the fabric exhibited purple (515 nm)
color after exposure to perspiration solution and yellow color (475 nm) after washing with an
aqueous solution of acetic acid (pH 6) in distilled water and air-drying to remove the effect of
the perspiration solution. The absorbance wavelength was reported on Texflash ACS
15

Datacolor. This cycle of exposure to perspiration solution followed by washing was repeated
(Figure 7) to indicate high stability.
520
510
500
490
480
470
0
1
2
3
4
5
6
7
8
9
Number of Cycle
Figure 7: Changes in the absorbance wavelength (475nm for yellow and 515nm for purple)
of dip-coated (pad-dry-cured at room temperature) cotton fabric (Sample H3)
4. Conclusions
We prepared, characterized and assessed the activity of a simple solid state pH responsive
cotton fabric for potential monitoring sweat biochemical variations. The microcapsules were
prepared by cross-linkable coupling of Ca+2 with the aqueous gel of Na-alginate.
Microcapsules were assembled from a cross-linked Ca-alginate capsule shell integrated with
tricyanofuran-hydrazone capsule core. Those microcapsules were loaded on smart cotton
fabric to be applied for naked-eye sweat pH monitoring depending on the color change of the
deprotonated-protonated tricyanofuran-hydrazone from light yellow to darker yellow, orange
and purple associated with the medium pH from acidic to alkaline. The detector performance
was demonstrated by colorimetric measurements. The results were confirmed by exploring
surface morphological properties and composition of pad-dry-cured fabric employing
scanning electron microscopy, energy dispersive X-ray spectra, and mapping of elements.
The comfort properties of the pad-dry-cured cotton fabrics were evaluated to show acceptable
results of colorfastness, stability to wash and light, bend-length and air-permeability. This
method can be potentially employed for identification of sweat fluid status, such as on-site
16

real time drug test device and monitoring sporting activities. Compared to earlier
identification techniques which require sophisticated instrumentation, electric power, trained
personnel and electronic parts, the environmentally friendly cotton/alginate colorimetric
detector is an easy-to-fabricate and easy-to-apply with high response time technique for on-
site real-time sweat status identification.
Compliance with ethical standards: All procedures by this study were in accordance with
international ethical standards. The research involved no human participants.
Funding: This research was funded by King Saud University, Researchers Supporting
Project (RSP-2019/149), King Saud University, Riyadh, Saudi Arabia.
Acknowledgment: The authors acknowledge King Saud University, Riyadh, Saudi Arabia,
for funding this work through Researchers Supporting Project (RSP-2019/149).
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22

Highlights
•
•
•
•
Simple colorimetric cotton fabric for sweat monitoring was introduced.
Probe encapsulated within calcium alginate was immobilized in-situ onto cotton.
Potential chromogenic sensor to be applied as a drug test by direct swab of skin.
Cotton displayed color change from yellow to purple upon exposure to sweat mimic.
Page 1 of 1

Declaration of interests
√ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
Signature (on behalf of all co-authors)
Corresponding author
Name: Tawfik A. Khattab
Affiliation: National Research Centre, Dokki, Cairo 12622, Egypt
Tel.: +201011014356
Fax: +20233370931
E-mail address: ta.khattab@nrc.sci.eg / tkhattab@kent.edu