Activated release of bioactive aldehydes from their precursors embedded in electrospun poly(lactic acid) nonwovens

Article abstract of DOI:10.1039/c8ra03137a

Hexanal and benzaldehyde are naturally-occurring aroma compounds from plants with enzyme-inhibition and antimicrobial properties. Although useful for food preservation applications, the end-use of these compounds can be challenging due to their volatility

Full text of DOI:10.1039/c8ra03137a

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PAPER
Activated release of bioactive aldehydes from their
precursors embedded in electrospun poly(lactic
Cite this: RSC Adv., 2018, 8, 19930
acid) nonwovens
a
Apratim Jash,a Gopinadhan Paliyathb and Loong-Tak Lim
*
Hexanal and benzaldehyde are naturally-occurring aroma compounds from plants with enzyme-inhibition
and antimicrobial properties. Although useful for food preservation applications, the end-use of these
compounds can be challenging due to their volatility and susceptibility to oxidative degradation. In this
study, stable precursors for benzaldehyde and hexanal were synthetized via reversible condensation
reactions with N,N0-dibenzylethane-1,2-diamine. The molecular structures of the resulting 1,3-
dibenzylethane-2-phenyl and 1,3-dibenzylethane-2-pentyl imidazolidines were confirmed by NMR
analyses. The precursors were encapsulated in poly(lactic acid) fibers via electrospinning, using a 90 : 10
ethyl formate : dimethyl sulfoxide blend as a solvent. Triggered release of benzaldehyde and/or hexanal
from the resulting active nonwovens was achieved by the addition of 1 N citric acid, which can be
described using a pseudo first order kinetic equation involving rapid and slow release steps.
Received 12th April 2018
Accepted 17th May 2018
DOI: 10.1039/c8ra03137a
rsc.li/rsc-advances
preserving cell membranes to increase the shelf-life of fruits
and vegetables.26,27
1. Introduction
Considering the different modes of action of benzaldehyde
and hexanal, simultaneous exposure of fresh fruits and vege-
tables to these compounds can synergistically extend their
shelf-life. However, these aldehydes are susceptible to oxida-
tive degradation.28,29 Although encapsulation may confer some
degree of protection to these aldehydes, the encapsulation
process is challenging due to the evaporative loss of these
volatile compounds. To address these issues, it is desirable to
convert benzaldehyde and hexanal into precursor compounds
that are relatively stable, and yet labile enough to allow for
rapid release under mild activating conditions. To this end,
reversible condensation of these aldehydes with an N,N0-
disubstituted-1,2-diamine to form the respective imidazoli-
dine precursors may be promising.30–33
This study investigated the feasibility of using N,N0-
dibenzylethane-1,2-diamine as a substrate for the formation of
hexanal and benzaldehyde imidazolidine precursors. The
precursors were further encapsulated within electrospun pol-
y(lactic acid) (PLA) bers to facilitate the release of the alde-
hydes, by leveraging the large surface area of the PLA
nonwoven carrier.34,35 Besides being a biodegradable polymer
derived from renewable plant sources, PLA can be solubilized
readily in organic solvents to form spin dopes optimal for
electrospinning.36–40 The release behaviors of hexanal and
benzaldehyde from the precursor-loaded nonwovens, when
activated with a mild acid, were evaluated.
Fresh fruits and vegetables are important parts of a healthy
diet to prevent chronic diseases.1–5 However, worldwide
wastage of fruits and vegetables remains an issue due to
inadequate post-harvest preservation.6,7 Active packaging
systems based on advanced controlled release materials,
coupled with modied atmosphere packaging, are instru-
mental in delaying the spoilage of fresh produce during
distribution, as well as extending the shelf-life at the retail
level.8–11 In response to increasing consumer preference for
natural over synthetic preservatives, researchers and technol-
ogists have been exploiting bioactive compounds (e.g., alde-
hydes, organic acids, esters, ketones, and terpenoids), derived
from edible plants and herbs, as potential antimicrobial
agents in active packaging and food preservation.12–18 For
example, hexanal and benzaldehyde, which are food avor
additives with a GRAS (generally recognized as safe) status, are
potent antimicrobial compounds due to their ability to
interact with the amino and sulydryl groups of protein
moieties in the microbial cytoplasmic membrane, disrupting
the membrane transport function and thereby causing cell
death.19–25 Hexanal is also a potent inhibitor of phospholipase
D
– an enzyme responsible for the hydrolysis of cell
membranes in plant tissues. By inactivating this enzyme and
its gene expression, hexanal has been shown to be effective in
aDepartment of Food Science, University of Guelph, ON, N1G2W1, Canada. E-mail:
llim@uoguelph.ca; Tel: +1 519 824 4120 extn 56586
bDepartment of Plant Agriculture, University of Guelph, ON, N1G2W1, Canada
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inuenced by the distance between the spinneret and the
grounded collector plate. Reducing the distance resulted in
jetting at lower applied voltages. However, thicker and fused
bers were observed due to reduced bending instability essential
for stretching the polymer jet, as well as inadequate ight time
for solvent evaporation. On the other hand, increasing the spin-
neret–collector distance produced thinner bers, although
a higher applied voltage was needed, which tended to cause
charge leakage. As a compromise, a constant operating voltage of
18 kV and a working distance of 20 cm were maintained between
the spinneret tip and the collector – a condition that resulted in
a stable electrospinning process with consistent ber morphol-
ogies. Electrospinning was conducted in an environmental
chamber (Model MLR-350H, Sanyo Corporation, Japan) main-
2. Experimental
2.1. Materials
Benzaldehyde, hexanal, ethyl formate (EF), citric acid mono-
hydrate, and dimethyl sulfoxide (DMSO) were purchased from
Sigma-Aldrich (Oakville, ON, Canada). PLA (6201 D) was
provided by NatureWorks LLC (Minnetonka, MN, U.S.A.).
Anhydrous ethanol and N,N0-dibenzylethane-1,2-diamine were
purchased from Commercial Alcohol (Brampton, ON, Canada)
and Alfa Aesar (Haverhill, MA, U.S.A.), respectively. Chloroform-
d was obtained from Cambridge Isotope Labs (Tewksbury, MA,
U.S.A.).
2.2. Precursor formation
ꢁ
tained at 10 ꢀ 1% relative humidity and 22 ꢀ 0.5 C. Residual
The hexanal precursor (HP) was synthesized following the
procedure reported by Jash and Lim.41 To prepare the benzal-
dehyde precursor (BP), benzaldehyde (170 mg) was dissolved in
2 mL of anhydrous ethanol at 22 ꢀ 2 ꢁC. An equimolar amount
of N,N0-dibenzylethane-1,2-diamine was added and stirred for
2 h. The resulting white precipitates were ltered and dried
under vacuum at 40 ꢁC to obtain the BP. The precursor sample
was analyzed using an NMR spectrometer (AVANCE 600 MHz
(14.1 T), Bruker Corporation, Billerica, MA, U.S.A.). The sample
was prepared in a 5 mm NMR tube by dissolving 5 mg of
vacuum-dried sample in 400 mL of deuterated chloroform.
TopSpin™ soware (Version TS3.5pl6, Bruker Corporation,
Billerica, MA, U.S.A.) was used for spectral analysis.
solvents from electrospun nonwovens were removed by drying
under vacuum at 40 ꢁC for 12 h.
2.5. Fourier transformed infrared (FTIR) analysis
Infrared spectra of benzaldehyde, N,N0-dibenzylethane-1,2-
diamine, and BP were acquired using a FTIR spectrometer
(Model IRPrestige21, Shimadzu Corporation, Kyoto, Japan)
tted with an attenuated total reectance (ATR) cell (Pike
Technologies, Madison, WI, U.S.A.). For the pristine and
precursor-loaded nonwovens, the samples were compressed on
top of the ATR cell and scanned. Each spectrum was taken by
averaging 40 scans at 4 cmꢂ1 resolution in the mid-IR region
(600 to 4000 cmꢂ1). Spectrum analysis was done by using IR
Solution soware (Shimadzu Corporation, Kyoto, Japan).
2.3. Preparation of spin dope solutions for electrospinning
2.6. Microstructural analysis
EF and DMSO were used as the solvents for the spin dope due to
their relatively low toxicity.42 PLA resin pellets were dissolved in
EF : DMSO (90 : 10, w/w) binary solvent at 64 ꢁC for 2 h to obtain
a 10% PLA (w/w) solution. HP and BP were separately dispersed
in the PLA solution at a ratio of 10 : 90 (w/w) to form the HP-PLA
and BP-PLA spin dopes, respectively. The third spin dope
(HPBP-PLA) was prepared by dissolving HP and BP together in
PLA solution at a ratio of 5 : 5 : 90 (w/w/w). The three nal spin
dopes were stirred for 2 h to obtain homogenous solutions
before electrospinning.
A scanning electron microscope (SEM) (Model S-570, Hitachi
High-Technologies Corporation, Tokyo, Japan) was used to
examine the morphology of the nonwovens. Three samples were
randomly chosen from the electrospun bers and mounted
onto metal stubs by using double-adhesive carbon tapes. A
sputter coater (Model K550, Emitech, Ashford, Kent, U.K.) was
used to provide a thin layer (20 nm) of gold coating on the
surfaces of the samples. Microscopic analysis was carried out
under an accelerating voltage of 10 kV. The mean ber diameter
was measured by using image processing soware (Image Pro-
Plus 6.0, Media Cybernetics Inc., Bethesda, MD, U.S.A.).
2.4. Electrospinning
The spin dopes prepared in Section 2.3 were electrospun using
a vertical setup. The solution was drawn into a 3 mL plastic
2.7. Headspace analysis
syringe (KD Scientic, Holliston, U.S.A.) and pumped at 5 mL hꢂ1 An automatic headspace analysis system was used to determine
ow rate using an infusion pump (Model 780100; KD Scientic the amounts of released aldehydes from the precursor-
Inc., Holliston, MA, U.S.A.). A 20-gauge blunt tip stainless steel containing nonwovens. This system is made up of a gas chro-
needle was used as a spinneret. The spinneret was connected to matograph (GC) equipped with a ame ionization detector (FID)
the positive electrode of a direct current power supply (Model (GC 6890, Agilent Technologies Inc., Santa Clara, CA, U.S.A.)
ES50P-50W/DAM, Gamma High Voltage, Ormond Beach, FL, interfaced with a steam selection valve and a gas sampling valve
U.S.A.). A circular stainless-steel plate, covered with aluminum (EMTCA-CE, VICI Valco Instruments, Houston, TX, U.S.A.)
foil, was used as the collector of the electrospun bers. The connected by stainless steel tubing (OD 1/1600). The GC was
collector was attached to a ground electrode to develop a elec- tted with a capillary column (Agilent J&W DB-624, Agilent
trical potential difference between the spinneret and the Technologies Inc., Santa Clara, CA, U.S.A.) with dimensions of
collector. From a preliminary experiment, it was observed that 30 m ꢃ 0.53 mm ꢃ 3.00 mm. Other GC operational parameters
the morphology of the electrospun bers was substantially are: H2 ow rate: 50 mL minꢂ1; air ow rate: 200 mL minꢂ1
;
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Fig. 1 Formation of the benzaldehyde precursor (BP), 1,3-dibenzylethane-2-phenyl imidazolidine, via condensation of benzaldehyde with N,N0-
dibenzylethane-1,2-diamine. Acid catalyzed hydrolysis of the BP resulted in the rapid release of the aldehyde.
carrier N2 ow rate: 30 mL minꢂ1; oven temperature: 150 ꢁC; the sealed glass jar. The lid was equipped with a septum, so that
FID temperature: 200 ꢁC. The sampling of the headspace gas the needle of the sampling port could pierce through the lid to
and acquisition of the FID signals were carried out automati- extract the headspace gas at predetermined time intervals. To
cally using a controller (SRI Instruments Inc., Las Vegas, NV, trigger the release of volatiles from the nonwoven membrane,
U.S.A.). Chromatographs were analyzed using PeakSimple 1 mL of 1 N citric acid was added to the precursor containing
soware (4.44–64 bit, SRI Instruments, CA, U.S.A.). Calibration nonwoven by using a disposable syringe (KD Scientic, Hollis-
of the FID was achieved by injecting known amounts of benz- ton, U.S.A.) tted with an 18-gauge needle (BD Precision Glide
aldehyde and hexanal ranging from 0.2–1.0 mL Lꢂ1 into 1000 mL Needle, NJ, U.S.A.). The release of volatiles was monitored for
hermetically sealed glass jars.
5 h at 25 ꢁC. At a given sampling point, the amounts of hexanal
The release kinetics of benzaldehyde and hexanal were and benzaldehyde released were calculated by adding the
studied by enclosing the precursor-loaded nonwovens inside recorded amount (Mr) to the accumulated amount lost (Ml) up
to that point:
Mr ¼ CrVr
(1)
(2)
Ml ¼ rꢂ1 ðCrꢂiVeÞ
X
i¼1
Mt ¼ Mr + Ml
(3)
where Cr (mL Lꢂ1) is the concentration of hexanal or benzalde-
hyde released at that time point; Vr (L) is the volume of the jar
and Ve (L) represents the volume of the gas extracted from the
headspace of the jar during sampling; Mt (mL) is the nal
amount of hexanal or benzaldehyde released at any given
sampling point.
2.8. Data analysis
Statistical soware, R (Version 3.3.2., R Foundation for Statis-
tical Computing, Vienna, Austria), was used for statistical
analysis. One-way ANOVA was conducted to detect the differ-
ences between various factors and treatments at a 95% con-
dence interval level with Tukey's honest signicant difference
test. All treatments were performed in triplicate.
3. Results and discussion
3.1. Formation and characterization of the benzaldehyde
precursor (BP)
The BP, 1,3-dibenzylethane-2-phenyl imidazolidine, was
synthesized through a partial Schiff base reaction, summarized
in Fig. 1. According to Kallen, the imidazolidine formation
involves an intramolecular aminoalkylation between the
secondary amine and the carbonyl group, followed by
1
Fig. 2 (a) H NMR and (b) 13C NMR spectra of 1,3-dibenzylethane-2-
phenyl imidazolidine.
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Fig. 3 FTIR spectra of 1,3-dibenzylethane-2-phenyl imidazolidine, N,N0-dibenzylethane-1,2-diamine, and benzaldehyde.
dehydration to produce a cationic Schiff base intermediate that (CH–Ph–C(20) or CH–Ph–C(30)); 89.1 (N–CH–N); 57.0 (Ph–CH2–
undergoes rapid cyclization to form the nal imidazolidine N); 50.7 (N–CH2–CH2–N). These results are consistent with
product.43 In another study, Jurcik and Wilhelm synthesized those reported in the literature.32,33
a series of imidazolidines, including 1,3-dibenzylethane-2-
phenyl imidazolidine, by emulsifying N,N0-dibenzylethane-1,2- 2-phenyl imidazolidine was compared against the spectra of
diamine in water, followed by the addition of benzaldehyde to the starting substrates, benzaldehyde and
N,N0-
The FTIR spectrum of the synthesized 1,3-dibenzylethane-
the emulsion.44 In the present study, the reaction of benzalde- dibenzylethane-1,2-diamine (Fig. 3). The absorbance signals
hyde with N,N0-dibenzylethane-1,2-diamine in ethanol resulted between 3000 and 2700 cmꢂ1 are related to symmetric and
in the formation of white precipitates, which could be ltered asymmetric C–H stretching vibrations and the absorbance
and dried to yield the BP. In accordance with the literature,30 the band at 1500–1400 cmꢂ1 is attributable to aromatic C]C
BP could be hydrolyzed readily in the presence of a mild acid, stretching. Out of plane aromatic C–H bending is responsible
for the strong peaks present between 800 and 650 cmꢂ1
.
45
liberating the benzaldehyde.
Spectral data from 1H and 13C NMR analyses conrmed the Vibrations due to the carbonyl group of benzaldehydes, which
formation of the heterocyclic ring structure of the BP (Fig. 2). include C]O stretching at 1700 cmꢂ1 and in-plane C–CHO
1H NMR (CDCl3, 600 MHz, d in ppm): d ¼ 7.70–7.21 (15H, Ph– bending at 825 and 1200 cmꢂ1, were not observed in the
H); 3.84 (d, J ¼ 13.08 Hz, 2H, –Ph–CH2–N–); 3.25 (d, J ¼ spectrum of the BP, implying the depletion of these IR-active
13.08 Hz, 2H, –Ph–CH2–N–); 3.89 (s, 1H, –N–CH–N–); 3.24–3.21 groups during the condensation reaction, due to the forma-
(m, 2H, (–N–CH2–CH2–N–) or (–N–CH2–CH2–N–)); 2.56–2.53 tion of the heterocyclic imidazolidine precursor.46,47
(m, 2H, (–N–CH2–CH2–N–) or (–N–CH2–CH2–N–)). 13C NMR
(CDCl3, 600 MHz, d in ppm): d ¼ 139.7 (CH2–Ph–C(10)); 128.7
(CH2–Ph–C(20) or CH2–Ph–C(30)); 128.2 (CH2–Ph–C(20) or CH2–
3.2. Electrospinning solvent selection
Ph–C(30)); 126.9 (CH2–Ph–C(40)); 140.4 (CH–Ph–C(10)); 129.6
(CH–Ph–C(20) or CH–Ph–C(30)); 128.7 (CH2–Ph–C(40)); 128.3
The solvents reported in the literature for the electrospinning of
PLA are generally toxic (e.g., chloroform, dimethylformamide
(DMF), hexauoro-2-propanol, dichloromethane). For food
applications, the use of non-toxic solvents is essential. For this
purpose, Hansen solubility parameters (HSPs) were used to
Table 1 Typical solvents used for the electrospinning of PLA and their
distance (R) from PLA
R, MPa1/2
Reference
formulate a blend of EF and DMSO for the electrospinning of
PLA. EF and DMSO were selected because of their low toxicity
(Class 3 ICH list). In the HSP concept, the Hildebrand solubility
parameter (d) is substituted by a system of multicomponent
solubility parameters, on the basis that a liquid's total cohesive
energy of vaporization is related to polar and nonpolar inter-
active forces between the liquid molecules:48,49
Solvent
Dichloromethane : DMF (70 : 30) (w/w)
Chloroform : acetone (70 : 30) (w/w)
Chloroform : DMF (90 : 10) (w/w)
Chloroform
1,1,1,3,3,3-Hexauoro-2-propanol
EF : DMSO (90 : 10) (w/w)
3.84
5.49
5.98
6.99
10.61
6.18
50
51
52
53
54
Present study
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Fig. 4 Scanning electron micrographs of the electrospun nonwovens: (a) pristine-PLA, (b) HP-PLA, (c) BP-PLA, and (d) HPBP-PLA. The diameter
distributions of the fibers are shown as histograms, where the Y-axis represents the count fraction, and the X-axis represents the diameter range.
The arrows indicate the locations of benzoic acid crystals.
Table 2 Mean diameter and standard deviation values of the pristine
where dD, dP, and dH represent the Hansen dispersion, polar
and precursor-loaded electrospun fibers (n ¼ 200). Means within
a column with different superscript letters (a, b, and c) indicate
a statistically significant difference (p < 0.05)
interaction and hydrogen bonding parameters, respectively.
Subscripts “A” and “B” depict the dD, dP, and dH values for
compounds A and B, respectively, and R is the distance between
the HSP components of compounds A and B in three-
dimensional Hansen space. A value of R close to zero indi-
cates great thermodynamic compatibility between the two
compounds and vice versa.49
Type
Fiber diameter, mm
Pristine-PLA
HP-PLA
BP-PLA
1.05 ꢀ 0.31a
0.88 ꢀ 0.27b
0.75 ꢀ 0.24c
0.74 ꢀ 0.15c
Based on the HSP concept, a blend of 90 : 10 EF : DMSO was
HPBP-PLA
chosen as the solvent for PLA with an R value of 6.18 MPa1/2
,
which is within the range of various solvents previously used by
researchers to dissolve PLA for electrospinning purposes
(Table 1). The PLA polymer dissolved readily in the binary
solvent to produce a transparent solution, which can be elec-
trospun into continuous bers (Section 3.3).
2
2
2
d2 ¼ dD + dP + dH
(4)
where d is the Hildebrand solubility parameter; dD, dP, and dH
are the Hansen partial solubility parameters, representing
dispersion, polar interaction and hydrogen bonding parame-
ters, respectively, with a unit of MPa1/2. The partial solubility
parameters of a binary solvent blend can be calculated as:
3.3. Morphology of the electrospun membrane
The pristine PLA solution was electrospun into bead-free bers
with smooth surfaces (Fig. 4a). The surface morphologies were
markedly different from those reported in the literature for the
PLA electrospun using 90 : 10 chloroform : DMF,41 70 : 30
chloroform : acetone,51 and dichloromethane,55 where the
bers had porous surface morphologies. The smooth bers
observed in the present study can be attributable to the lower
volatility of the 90 : 10 EF : DMSO solvent used in the present
di (blend) ¼ 41di,1 + 42di,2 (i ¼ D, P, H)
(5)
where 41 and 42 represent the volume fraction of the 1st and 2nd
solvent. The compatibility of the polymer with the used solvent
can be calculated using eqn (6):
R2 ¼ 4(dDA ꢂ dDB)2 + (dPA ꢂ dPB)2 + (dHA ꢂ dHB
)
(6)
2
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Fig. 5 FTIR spectra of the electrospun pristine and precursor-loaded PLA nonwovens.
binary solvent. By delaying the solidication of the polymer
jet, additional whipping and bending motions were imposed
on the ber, thereby stretching the polymer into thinner
bers. It is noteworthy that rod-shaped entities exogenous to
the electrospun bers were observed in the BP-PLA and HPBP-
PLA nonwovens (indicated with arrows in Fig. 4c and d), which
are likely benzoic acid crystals produced due to the oxidation
of benzaldehyde released during the sample preparation and
sputter coating/SEM analysis processes.
3.4. FTIR analysis of the pristine and precursor-loaded PLA
nonwovens
The pristine PLA bers exhibited one strong peak at 1750 cmꢂ1
due to carbonyl (–C]O) stretching of the polymer (Fig. 5).
Symmetric and asymmetric deformational vibrations of the C–H
bonds in the CH3 groups of the PLA appeared as peaks between
1400 and 1350 cmꢂ1, while the peaks at 1185 and 1085 cmꢂ1 are
due to C–O vibration in the ester (C–O–C) backbone of PLA. Two
peaks at 1130 and 1045 cmꢂ1 were observed due to vibrations
associated with CH3 rocking and C–CH3 stretching in PLA.56
The main distinction between the pristine and precursor-
containing PLA bers is the appearance of new peaks between
800 and 650 cmꢂ1, which are due to the vibrations of the phenyl
Fig. 6 (a) Release profiles of hexanal and benzaldehyde from HP-PLA
and BP-PLA, respectively; (b) simultaneous release of hexanal and groups of the precursors. The absorbance bands at 864 and
755 cmꢂ1 are due to the amorphous and crystalline phases of
PLA.36,57 These bands were not affected when the precursor
benzaldehyde from HPBP-PLA. Solid lines and symbols represent fitted
curves based on eqn (7) and the experimental data, respectively.
compounds were added. Moreover, no detectable wavenumber
shis were observed for the characteristic –C]O and C–O–C
bands of PLA. These observations imply that the precursors are
study. The mean diameters of the PLA bers decreased
signicantly (p < 0.05) as the hexanal and/or benzaldehyde
mainly physically entrapped within the PLA ber matrix, rather
precursors were incorporated into the electrospun bers
than involving site-specic interaction with the PLA
(Table 2), due to the suppression of the vapor pressure of the
macromolecules.
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Table 3 Hexanal and benzaldehyde release profiles from the precursor-loaded electrospun membrane at 25 ꢁC. The values are estimated using
eqn (7), and are presented in the format of mean ꢀ standard deviation (n ¼ 3). Means within a column with different superscript letters (a, b, and c)
indicate a statistically significant difference (p < 0.05). Statistical analysis was conducted using one-way ANOVA with Tukey's honest significant
difference test
Ce1 (mL
Ce2 (mL
Volatile
per L per mg ber per mg precursor) k1 (minꢂ1
)
per L per mg ber per mg precursor) k2 (minꢂ1
)
Hexanal from HP-PLA
0.062 ꢀ 0.002a
0.026 ꢀ 0.006b
0.049 ꢀ 0.004a
353.868 ꢀ 16.466a 0.193 ꢀ 0.050a
203.561 ꢀ 80.621b 0.054 ꢀ 0.010a
139.663 ꢀ 29.815c 0.192 ꢀ 0.113a
55.183 ꢀ 30.442c 0.106 ꢀ 0.070a
1.194 ꢀ 0.510a
4.729 ꢀ 2.155a
2.351 ꢀ 1.638a
1.907 ꢀ 0.878a
Benzaldehyde from BP-PLA
Hexanal from HPBP-PLA
Benzaldehyde from HPBP-PLA 0.024 ꢀ 0.011b
Table 4 The HSPs for EF, DMSO, and PLA were obtained from the
library of the HSPiP software package (5.0.04, Hansen Solubility
Parameters in Practice, https://www.hansen-solubility.com). The HSP
values of the hexanal precursor (HP) and benzaldehyde precursor (BP)
were predicted by using the Stefanis–Panayiotou method incorpo-
rated into HSPiP. The compatibility of HP and BP with a solvent or
polymer has been represented by the distance parameter R
3.5. Activated release of benzaldehyde and hexanal
The electrospun PLA bers acted as a physical carrier for the
precursors and facilitated end-use delivery of the aldehyde
vapors. The release of hexanal and benzaldehyde was activated
by the addition of 1 N citric acid to the precursor-loaded
nonwoven. From the GC headspace analysis, the release
proles of hexanal and benzaldehyde exhibited rapid initial
release followed by slow release (Fig. 6). This two-step
phenomenon can be described by an empirical model
comprising two pseudo-rst order kinetics:
Spin dope components
HSP values
MPa1/2
EF
DMSO PLA Hexanal Benzaldehyde HP BP
dD
15.5 18.4
7.2 16.4
7.6 10.2
14.2 14.7
19.2 17.4
11.2 11.6
18.6 15.8
9.9 8.4
19.4
7.4
5.3
—
—
3.0
22.3 24.9
5.8 6.6
dP
ꢂk2t
C ¼ Ce1(1 ꢂ eꢂk t) + Ce2(1 ꢂ e
)
(7)
dH
6
5.3
—
3.7
—
—
—
4
1
R(HP)
R(BP)
R(PLA)
8.8
13.2
—
—
—
—
—
where C is the concentration of the volatile at time t; Ce1 and Ce2
are the initial and nal volatile equilibrium concentrations,
respectively; k1 and k2 are the diffusion rate constants for the
fast and slow phases, respectively. The Ce1, Ce2, k1 and k2 values
were estimated using the Solver function in a Microso Excel
spreadsheet (Microso Office 365, Redmond, WA, U.S.A.), and
are summarized in Table 3.
5.8
precursor tended to distribute uniformly through the ber
thickness (Table 4). However, for BP-PLA, a greater amount of
BP will likely be partitioned within the ber, due to the lower R
value of BP to DMSO (17.4 MPa1/2) than BP to EF (19.2 MPa1/2
)
In general, the release of aldehydes from the precursor-
loaded PLA is governed by four main mechanisms: (1) diffu-
sion of water and acid into the PLA bers; (2) hydrolysis of the
precursors; (3) diffusion of the released aldehydes through the
ber matrix to the surface; and (4) desorption of the aldehydes
from the ber surface to the air. The two-step release
phenomenon is likely related to two populations of precursors
existed in the bers, i.e., one population located near/on the
surface, and the other fraction encapsulated within the ber.
The k1 values observed in Table 3 can be attributed to the
surface precursors, the aldehyde release from which was mainly
due to mechanisms (2) and (4). On the other hand, the slower
process, characterized by the k2 values, was mainly due to the
precursors embedded within the bers, involving all four
mechanisms.
During the electrospinning process, EF tends to migrate
towards the surface of the ber because of its higher volatility
than DMSO (the vapor pressures of EF and DMSO are 25.6 and
0.049 kPa at 20 ꢁC, respectively).58,59 The preferential evapo-
ration of EF will result in the formation of an EF-rich outer
layer and DMSO-rich inner layer. For HP-PLA, since HP has
a comparable compatibility in EF and DMSO (the R values of
HP to EF and DMSO are 14.2 and 14.7 MPa1/2, respectively), the
(Table 4). The less volatile DMSO will evaporate aer the
evaporation of EF. Because the precursors were thermody-
namically more compatible with PLA than DMSO (the R values
of HP from PLA and DMSO are 8.8 and 14.7 MPa1/2, respec-
tively; the R values of BP from PLA and DMSO are 13.2 and 17.4
MPa1/2, respectively; Table 4), the precursors will tend to be
trapped within the PLA ber, instead of migrating with DMSO
towards the surface.
Both hexanal and benzaldehyde are fairly compatible with
PLA (the R values from PLA are 5.8 and 3.0 MPa1/2, respec-
tively), implying that both aldehydes can diffuse through the
PLA matrices readily. However, the k1 values were signicantly
higher for hexanal than benzaldehyde, which can be attributed
to the higher vapor pressure of hexanal than benzaldehyde
(11.3 and 1.27 mm Hg at 25 ꢁC, respectively; https://
pubchem.ncbi.nlm.nih.gov; accessed on 21 Mar 2018). On
the other hand, the k2 values for the slow release step were two
orders of magnitude lower than k1. Moreover, the differences
in the k2 value were not statistically signicant (P > 0.05) (Table
3). The lack of difference suggests that the release of aldehydes
from the encapsulated precursors was limited by the diffusion
of water/acid into the ber (mechanism 1) and/or hydrolysis of
the precursors (mechanism 2). Simultaneous releases of
19936 | RSC Adv., 2018, 8, 19930–19938
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hexanal and benzaldehyde from HPBP-PLA followed similar
trends, i.e., the presence of one precursor did not substantially
affect the release kinetics of aldehyde from the other
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This work was carried out with the aid of a grant from Canada's
International Development Research Centre (IDRC) and with
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and Engineering Research Council of Canada (NSERC) and
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