Photoinduced carbene generation from diazirine modified task specific phosphonium salts to prepare robust hydrophobic coatings

Article abstract of DOI:10.1021/la301975u

3-Aryl-3-(trifluormethyl)diazirine functionalized highly fluorinated phosphonium salts (HFPS) were synthesized, characterized, and utilized as photoinduced carbene precursors for covalent attachment of the HFPS onto cotton/paper to impart hydrophobicity t

Full text of DOI:10.1021/la301975u

Article
pubs.acs.org/Langmuir
Photoinduced Carbene Generation from Diazirine Modified Task
Specific Phosphonium Salts To Prepare Robust Hydrophobic
Coatings
Sara Ghiassian, Hossein Ismaili, Brett D. W. Lubbock, Jonathan W. Dube, Paul J. Ragogna,*
and Mark S. Workentin*
Department of Chemistry and the Centre of Materials and Biomaterials Research (CAMBR), Western University, London, Ontario,
Canada N6A 5B7
S
* Supporting Information
ABSTRACT: 3-Aryl-3-(trifluormethyl)diazirine functional-
ized highly fluorinated phosphonium salts (HFPS) were
synthesized, characterized, and utilized as photoinduced
carbene precursors for covalent attachment of the HFPS
onto cotton/paper to impart hydrophobicity to these surfaces.
Irradiation of cotton and paper, as proof of concept substrates,
treated with the diazirine-HFPS leads to robust hydrophobic cotton and paper surfaces with antiwetting properties, whereas the
corresponding control samples absorb water readily. The contact angles of water were determined to be 139° and 137° for cotton
and paper, respectively. In contrast, water placed on the untreated or the control samples (those treated with the diazirine-HFPS
but not irradiated) is simply absorbed into the surface. Additionaly, the chemically grafted hydrophobic coating showed high
durability toward wash cycles and sonication in organic solvents. Because of the mode of activation to covalently tether the
hydrophobic coating, it is amenable to photopatterning, which was demonstrated macroscopically.
on the phosphorus center, which is much more difficult to
obtain in related C- or Si-centered and even polymer systems.
In this way, using a small number of simple quantitative
transformations one can impart specific functionality onto the
cation. A drawback of the HFPS we described earlier and
indeed other systems mentioned above is that they lack a
universal tether or they need to have functionality tailored for a
specific surface. So, despite the many successes, there remains a
need for the development of materials that allow for facile,
efficient, and mild approaches to impart hydrophobic character
more universally on any surface.
Photolinkers are a versatile method for covalent attachment
of molecules onto surfaces. Common highly efficient photo-
linkers used generally include diaryldiazo,16−19 azide,20−22 and
diazirine23−26 molecules that generate highly reactive carbene
or nitrene intermediates that then react with functionality
native to or deliberately functionalized onto the desired surface.
To the best of our knowledge, only diaryldiazo and azide
photoactivation have been used to tether hydrophobic
substrates onto surfaces. For example, very recently Ling et
al. reported the use of azide functionalized silica nanoparticles
as a way to impart hydrophobicity to cotton fiber.27 To date,
the diazirine moiety has not been used in this sort of
technology. However, it has been suggested that diazirine as
a photolinker can be more beneficial because aryl azides can
INTRODUCTION
■
There is a host of potential applications of hydrophobic
coatings in such things as microfluidic1 or biomedical devices,2
self-cleaning and impermeable textiles,3,4 and in the printing
and packaging industries.5 Recent successful efforts to prepare
superhydrophobic surfaces have involved the incorporation of
nanoparticles (to provide uniform surface roughness) or using a
mechanically roughened rigid surface prior to deposition of the
hydrophobic layer.6,7 The chemical modification of surfaces
with low-surface-energy functionalities, especially fluorine-
containing hydrocarbons or perfluorosilanes, is an efficient
and practical method to fabricate a hydrophobic surface. In
addition to the more common use of small fluorinated
molecules for preparing coatings, fluorinated silica nano-
particles have also been used to produce rough super-
hydrophobic surfaces.8−10 However, in many cases the
deposited layer on the surface should have a thickness equal
to or higher than the particle diameter, and having such a thick
layer of coating can change the intrinsic properties of the
substrate and damage its transparency.
Highly fluorinated phosphonium salts (HFPS) have
demonstrated promise in a wide variety of fields over the
past several years11 in areas such as surfactants12 and solvents
for phase transfer catalysis,13 but only more recently have we
demonstrated it as an effective medium for imparting
hydrophobic properties to various metal surfaces such as Au
and Ag.14,15 The advantage the HFPS systems possess,
compared to other strategies, is that the phosphonium salts
can be readily modified using selective substitution chemistry
Received: May 14, 2012
Revised: July 26, 2012
Published: July 27, 2012
© 2012 American Chemical Society
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Synthesis of [1-HFPS][Br]. Under an inert atmosphere (N2) a
solution of the synthesized diazirine derivative [1.8] (200 mg, 0.55
mmol) in DMF (1 mL) and CF3(C6H5) (1 mL), was added to
(CH3)3CCH2CH(CH3)CH2P((CH2)2(CF2)2CF3)2 (380 mg, 0.60
mmol). The reaction mixture was stirred for 4 weeks at 45 °C and
continually monitored by 31P {1H} NMR. The precursor fluorinated
phosphine (δP = −30.4) was converted to a singlet in the region
consistent with the target product phosphonium species (δP = 35.2),
an oxide was also present in the reaction mixture (δP = 44.0) but was
removed by purification.
Synthesis of [1-HFPS][I]. First [1.8] was converted to [1.9] using
sodium iodide. To prepare [1-HFPS][I], under an inert atmosphere
(N2) a solution of [1.9] (225 mg, 0.55 mmol) in DMF (1 mL) and
CF3(C6H5) (1 mL) was added to (CH3)3CCH2CH(CH3)CH2P-
((CH2)2(CF2)2CF3)2 (380 mg, 0.60 mmol). The reaction mixture was
stirred for 2 weeks at 45 °C. The reaction was continually monitored
by 31P {1H} NMR. The fluorinated phosphine (δP = −30.4) was
converted to a singlet in the region consistent with the target product
phosphonium species (δP = 35.9), an oxide was also present in the
reaction mixture (δP = 44.1) but was removed by purification.
Synthesis of [1-PMe3S]. Under an inert atmosphere (N2), PMe3
(19 mg, 0.25 mmol, 26 mL) was added to a solution of the diazirine
[1.8] (109 mg, 0.238 mmol) in DMF (2 mL), and the reaction
mixture was stirred for 3 h at 45 °C. The reaction was monitored by
31P {1H} NMR and was purified upon completion.
Surface Modification of Cotton Fabric and Paper. [1-
HFPS][X] (X = Br or I) 1, 2, 5, 10, and 20 mg/mL solutions of
[1-HFPS] in THF were prepared. Prior to photocoating, cotton
substrate was pretreated by washing with deionized water and
detergent three times to remove any surfactants or chemical coatings
and subsequently washed with distilled water and then dried in a 60 °C
oven. The paper samples were used as obtained with no pretreatment.
Both the cotton and paper substrates were treated by submersion into
solution of [1-HFPS][X] and were allowed to air-dry until the solvent
evaporated. Next, the 1-HFPS-coated substrates were placed in a
sealed vessel that was purged with argon. All of the substrates were
irradiated with ultraviolet light on both sides, using a medium-pressure
mercury lamp (Hanovia S9 PC 451050/805221), which was contained
in quartz water jacket, ∼10 cm from the samples (ca. 1800 mJ/cm2).
After irradiation, all substrates were thoroughly rinsed with THF and
DCM to remove any unbound HFPS and yield the light treated
samples.
Patterning Paper. Simple plain white printer paper, 92 Bright, was
soaked in a solution of [1-HFPS][I] in THF for 15 min and allowed to
air-dry until no solution remained. A photomask made out of
cardboard with a circular gap (1 cm in diameter) was placed on top of
the HFPS-coated paper substrate, and the backside was covered with a
full mask to avoid any light exposure from the backside. Next, it was
placed in a sealed vessel that was purged with argon and irradiated
with ultraviolet light, using a medium-pressure mercury lamp (Hanovia
S9 PC 451050/805221). After irradiation, the treated paper was
thoroughly rinsed with THF and DCM to afford the patterned surface.
Simulated Wash Cycles. Two washing methods were used for
coated samples. In a typical experiment, a piece of 2 × 2 cm treated
cotton fabric was washed in a 50 mL flask that contained 30 mL of
aqueous detergent solution and a stir bar. The fabric was stirred at 500
rpm for 15 min. It was then rinsed with distilled water and stirred for
another 15 min in a flask containing 30 mL of distilled water to
remove any adsorbed detergent. Fabric was then dried for 10 min in a
60 °C oven before it was used for contact angle measurement. In a
separate experiment a piece of 2 × 2 cm treated cotton was immersed
in three different solvents and sonicated for 5 min periods: EtOH,
DCM, THF subsequently. Water contact angles were measured before
and after sonication.
generate a singlet nitrene that can undergo a ring expansion and
to a much less reactive species, and many diazo compounds
suffer from low thermal stability. The use of diazirine, however,
combines the high reactivity of a carbene with its inherent high
stability. Compounds containing 3-aryl-3-(trifluormethyl)-
diazirine moieties are accessible synthetically useful carbene
precursors. Their most common application is in photoaffinity
labeling, prolific in biological applications.28−30 The ease of
synthesis, excellent thermal and chemical stability of the
diazirine, and its nearly quantitative photochemical generation
of a reactive carbene, which reacts with a wide range of
functionalities (e.g., CC; X−H bonds),23,30,31 are the basis of
its widespread use in these applications.32 However, utilizing
the diazirene functionality for the modification of substrates in
polymer and materials science remains relatively unexplored.
Hayes and co-workers in particular have utilized this chemistry
creatively, mainly for the modification of polymers.23,30,33−35
We have recently established the utility of this diazirine moiety
as the carbene precursor for the modification of other materials,
including CNT, diamond, graphene, and glass surfaces with
gold nanoparticles.24−26 In these studies the photochemistry of
3-aryl-3-(trifluormethyl)diazirine is utilized to generate the
carbene to tether the attached gold nanoparticles (AuNPs) via
insertion into the native CC, C−H, or O−H functionality
inherent on these substrates. The result is a suite of covalent
hybrid materials containing the properties of both the host
material and the AuNP. Lawrence et al. have used this protocol
recently to append acylferrocene as a redox-active functional
group onto CNT.36,37
In the present report we extend the applications of the
photochemistry of diazirines to new functional materials by
incorporating the photoactive diazirine moiety onto the
structure of a novel hydrophobic highly fluorinated phospho-
nium salt (HFPS). The high hydrophobicity imparted by this
HFPS especially given the relatively small fluorinated chain
length makes this material commercially viable from an
environmental perspective, compared to many that use longer
fluorinated hydrocarbons. We utilize the high reactivity of the
photogenerated carbene to attach the HFPS to form robust
hydrophobic coatings on model cotton and paper surfaces
simply and efficiently. Because the carbene reacts with virtually
any functional group, the approach provides a way to marry the
high reactivity of the carbene with the newly discovered use of
HFPS to introduce hydrophobicity efficiently and generally to
any substrate/surface. The process is photochemically activated
and allows for the coverage of macroscale substrates and surface
patterning, which has implications for the application of this
technology at the micro- and nanoscale. Because the highly
fluorinated phosphine used in the synthesis of our novel
diazirine highly fluorinated phosphonium salt (HFPS) is
available in commercial quantities and can be made in the
100 g to 1 kg scale in our lab and the required diazirine can be
prepared in multigram quantities through a series of simple
organic transformations, the approach allows for the explora-
tion of potential applications of the proof of concept results
described herein.
EXPERIMENTAL SECTION
Contact Angle Measurement. Contact angles (CAs) were
measured with deionized water using a Kruss DSA 100 goniometer
with DSA (drop shape analysis) software, at room temperature (21
°C). All the static water contact angles were determined by averaging
values measured for 5 μL droplets at five different spots on each
substrate. The Laplace−Young fitting method was used to calculate all
■
Synthesis of 3-(3-(6-Bromohexyloxy)phenyl)-3-(trifluoro-
methyl)-3H-diazirine [1.8]. All steps in preparation and synthesis
were performed in accordance with the literature procedure,38 with the
exception that 1,6-dibromohexane was used in place of 1,12-
dibromododecane. Details are provided in the Supporting Information.
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the static contact angles. Contact angle hysteresis was determined by
placing a 5 μL droplet on the surface, rotating the stage 30°, and
measuring the difference between the advancing and receding contact
angles.
Other Techniques. The decomposition temperature for com-
pounds [1-HFPS][I] and [1-PMe3S] were determined using thermal
gravimetric analysis (Q600 SDT, TA Instruments). A 0.005−0.010 g
sample was heated at a rate of 10 °C/min over a temperature range of
30− 600 °C under a flow of N2(g) (100 mL/min). X-ray
photoelectron spectroscopy (XPS) was carried out using a Kratos
Axis Ultra spectrometer using a monochromatic Al Kα source (15 mA,
14 kV). Scanning electron microscopy (SEM) images were recorded
on a TM3000 Hitachi scanning electron microscope.
model compound 1-PMe3S was also prepared in a similar
manner using trimethylphosphine; in this case the reaction
proceeds much more readily (3 h) because of the more
nucleophilic and less sterically encumbered phosphine.
Compound 1-PMe3S was used as a model to demonstrate
that any modification observed in the surface properties of the
substrates is in fact due to grafting the HFPS on the surfaces
and not simply due to the diazirine CF3 or phosphonium ion
moieties.
As a proof of principle of using the diazirine moiety as the
carbene precursor to insert into the native functionality of the
surfaces to act as a molecular tether, the reactivity of the salt 1-
HFPS was studied upon exposure to UV light in the presence
of methanol as a model O−H containing molecule. The m-
methoxyphenyl(trifluoromethyl)carbene is known to be a
ground state triplet; however, its reactivity generally corre-
sponds to that of the singlet carbene (namely insertion
reactions).40 A solution of [1-HFPS][I] in methanol was
purged with argon to remove any oxygen and then irradiated (λ
>300 nm) with a medium-pressure Hg lamp. The course of the
reaction was monitored with 19F NMR spectroscopy. The
starting material peak at δF = −65 (CF3 of diazirine)
disappeared, and a new peak at δF = −77 known to be that
due to the methanol insertion product appeared quantitatively,
verifying the reactivity of carbene for insertion into O−H
bonds.38 The details of these model reactions are provided in
the Supporting Information.
RESULTS AND DISCUSSION
■
The 3-aryl-3-(trifluormethyl)diazirine functionalized perfluori-
nated phosphonium salts 1-HFPS were prepared by reacting
the 3-(3-(6-halohexyloxy)phenyl)3-(trifluoromethyl)-3H-dia-
zirine derivatives (halo = Br or I) with a highly fluorinated
alkyl phosphine (RP[(CH2)2Rf4]2 (R = 2,4,4-trimethylpentyl,
Rf4 = (CF2)4CF3) via a quaternization reaction, with bromide
or iodine as the counterion. The reaction involved mixing a
solution of the diazirine in dimethylformamide with a solution
of C8H17P(CH2Rf4)2 in trifluorotoluene under an inert
atmosphere (N2) (Scheme 1).
Scheme 1. Reaction Pathway toward the Synthesis 1-HFPS
and the Model Compound 1-PMe3S
To demonstrate the utility of 1-HFPS as a photoprecursor to
a reactive carbene that can be used to graft the hydrophobic
properties of the HFPS, we chose cotton fabric and paper as
proof of concept materials. Both of these substrates are known
to be rich in O−H and C−H bonds on the surface, which can
react with the photogenerated carbene via insertion reactions.
The 100% cotton fabric was purchased and cleaned by washing
with detergent and deionized water three times and then dried
in a 60 °C oven. The paper substrate (simple plain white
printer paper, 92 Bright) was used as obtained with no
pretreatment. For photocoating, cotton or paper substrate was
immersed into a solution of the prepared 1-HFPS in THF for
10 min and allowed to dry in the dark. This process was
repeated up to three times to ensure a film of the 1-HFPS was
on the surface of each substrate. Next, the coated sample was
placed in a sealed Pyrex vessel, which was purged with argon
gas for 30 min and then irradiated with UV light (λ > 300 nm)
on both sides using a medium-pressure Hg lamp. Upon
irradiation, the diazirine loses N2 and generates a carbene
reactive intermediate that inserts into X−H bonds.30,38 The
general photochemical reaction that was utilized for grafting the
salts 1-HFPS and 1-PMe3S onto the surfaces of cotton and
paper is illustrated in Scheme 2. After irradiation, to remove any
unbound, physisorbed 1-HFPS from the surface, the HFPS-
photocoated sample was washed repeatedly with THF and
dichloromethane and then allowed to air-dry. In parallel, dark
control samples consisting of cotton/paper samples that were
similarly prepared with the 1-HFPS were left in the dark for 24
h (not exposed to UV irradiation) and then washed using the
same protocol as used for the light-treated samples.
The reaction mixture was stirred at 45 °C and monitored by
31P {1H} NMR spectroscopy over this period, by following the
disappearance of the phosphine (δp = −30) and the appearance
of the signal from the phosphonium ion. Upon disappearance
of the characteristic resonance of the precursor phosphine, the
reaction was exposed to air to convert any trace unreacted
phosphine to the oxide and then worked up with water. The
product was characterized using multinuclear NMR and IR
spectroscopy, ESI MS, and TGA. Although both the bromide
and iodide salts of 1-HFPS behaved similarly in further
chemical tests and exhibited all the specifications that were
sought for surface modification of various substrates, the I− salt
is the preferred target: the quaternization reaction is generally
slow due to the attenuated nucleophilicity of the fluorinated
phosphine and because the reaction is performed at temper-
atures not exceeding 45 °C to avoid thermal degradation of the
diazirine. To achieve a faster and more efficient method for
synthesizing the target, the 3-(3-(6-iodohexyloxy)phenyl)3-
(trifluoromethyl)-3H-diazirine derivative formed by halide
exchange from the corresponding bromo derivative is preferred,
and using it results in a pure product.39 The 1-HFPS were
viscous liquids, exhibiting single signals in the 31P NMR
spectrum (δp = 35 ppm), and the expected signals in the 19F
NMR spectrum (δF = −65 (CF3 of diazirine) and −81, −114,
−124, and −126 (fluorinated alkyl chains). The glass transition
temperature (Tg = −27.2 °C) and decomposition temperature
(Td = 304.2 °C) were also determined for 1-HFPS[I]. The
To demonstrate the influence of the 1-HFPS solution
concentration on the resulting wetting behavior, we prepared
[1-HFPS]-coated surfaces on cotton and paper using the same
protocol explained above, with c = 1, 2, 5, 7, 10, and 20 mg/mL
of [1-HFPS][I] in THF and irradiated them for 1 h. Figure 1
shows the dependency of the water contact angle on the
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samples show a high hydrophobicity (antiwetting). The second
is that while there is an increase in contact angle with the
increase in UV exposure no significant change was observed
over 60 min of irradiation under our conditions. Using more
lamps (increased intensity) will lead to shorter irradiation
times, but for consistency, especially when treating the small
analytical samples in this study, we used 60 min of irradiation
under our conditions to prepare the hydrophobic substrates for
further testing.
Scheme 2. Illustration of Photochemical Reaction Used for
Surface Modification of Cotton and Paper: 1-HFPS and 1-
PMe3S
We investigated the surface characteristics of treated samples
and compared with those of the untreated samples visually and
analytically using SEM, XPS, and contact angle measurements.
The surface morphology of treated and untreated cotton/paper
was examined using SEM (see Supporting Information Figure
S1). In this study we used the optimized concentrations of the
HFPS, which leads to a very thin layer on the surface. SEM
micrographs of the modified samples and their unmodified
counterparts did not show any obvious change in diameter or
structure of the fibers. There were likewise no other visible
changes in color or bulk morphology to the irradiated samples
under the conditions of the irradiation. The fact that no
obvious change was observed after treating/irradiating
substrates supports the thinness of the coatings. As a result,
we did not damage the transparency or color of the cotton or
paper. The surface of modified samples was also smooth, and
no destruction of cotton/cellulose fibers was indicated. More
dramatic was the change in the surface wetting properties of the
samples, which were evaluated visually and by measuring water
contact angles (Figure 2).
The stability of the water droplets on treated cotton/paper
was in complete contrast with the untreated ones. It is well-
known that both cotton and paper are rich in hydroxyl groups
and are therefore very hydrophilic. After chemically grafting the
surface with HFPS all the light-treated samples were found to
be highly hydrophobic as illustrated by the photographs in
Figure 2, while the water droplet soaked into the untreated and
dark surfaces immediately after deposition (Figures 2A,E and
2B,F, respectively). In Figure 2, the water droplets were colored
for more easily visualization. Also included in Figure 2 are
images showing the hydrophobic nature from contact angle
measurements using water placed on the irradiated cotton and
paper substrates (Figure 2D,H). The water contact angles were
measured 30 s after a 5 μL water droplet was placed on the
surface. It is worth noting that those fibers protruding out from
the cotton surface made the contact angle measurement
somewhat difficult. Because of the inherent roughness of the
cotton fabric, determining the baseline is less straightforward,
and this can lead to possible underestimation of the contact
angle data. For each specimen, the contact angle reported is the
average of at least five different readings on different spots on
the surface. The irradiated cotton and paper surfaces treated
with 1-HFPS exhibited average water contact angles (θ) as high
as 139.5° and 137.4°, respectively. The lack of hydrophobicity
of the dark samples demonstrates that the phototreatment is
necessary and suggests that it is the formation of the carbene
and subsequent covalent bonding to the substrate via carbene
insertion reactions that leads to the robust hydrophobic
coating. The magnitudes of the observed contact angles are
approaching those of surfaces that have a micro/nanoscale
roughness and are ultimately superhydrophobic (θ ≥150°).8−10
The effect is dramatic especially when one considers that the
surfaces that did not undergo irradiation prior to washing
Figure 1. Variation in water contact angles as a function of irradiation
time of 5 mg/mL samples (square, top scale) and as a function of the
[1-HFPS][I] concentration on both cotton (inverted triangle, bottom
scale) and paper (circle, bottom scale).
concentration of the solution used for coating. Even surfaces
coated with very dilute solution (1 mg/mL) exhibit high
contact angles after the irradiation and washing protocol.
Although there is an increase in the water repellency when
using more concentrated solutions, we observed that with
concentrations higher than 10 mg/mL we not only damage the
transparency of the substrate but also decrease the contact
angle. We believe that when too concentrated a solution is
used, the density of the diazirine molecules on the surface
increases. When exposed to UV light, this high density can lead
to dimerization of the reactive carbene species at the expense of
reactions of the carbene reacting with the OH groups present
on the surface. These dimers would simply be rinsed away in
the washing protocol.
Using the optimized coating concentration (5 mg/mL
solution of [1-HFPS][I] in THF), we then performed
experiments to monitor the effect of irradiation time on the
resulting hydrophobicity of the light-treated substrates while all
the other factors were kept constant. Figure 1 also shows the
change in the water contact angles on cotton with time under
our irradiation conditions. There are two important observa-
tions. The first is that even after very short irradiation times the
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Figure 2. Pictures of colored water droplets (colored to aid
visualization) placed on standard printer paper samples (left column)
and cotton (right column): (A, E) untreated, (B, F) 1-HFPS treated
dark control (no irradiation), and (C, G) 1-HFPS treated and
irradiated. Panels D (paper) and H (cotton) are representative
pictures of water contact angle measurements.
Figure 3. XPS spectra of treated, treated with no light (dark treated),
and untreated cotton (top) and paper (bottom) with [1-HFPS].
the light-treated cotton spectrum that is not observed in XPS of
the native sample. This proves that they are in fact coated with
a film of HFPS. Although the fluorine peak is also present in
XPS of the dark sample, the relative intensity is significantly
smaller (1.9%) by comparison to the C and O signals. With the
paper sample the results are 7.4% and 1.4%, respectively.
Contact angle measurements confirm that the much lower
fluorine incorporation in the dark control sample (likely due to
some physical absorption or thermal carbene activation) is not
enough for the sample to exhibit hydrophobicity.
Control over interfacial adhesion is particularly important
when integrating the material into a device design. In our case,
the photogenerated carbene can be used to impart well-defined
surface functionality with water-repellent properties. The
chemical stability and overall robustness of this coating are
dictated by the molecular interaction between the coating and
the substrate. As a result, in order to remove the chemically
grafted HFPS coating from the substrate, covalent bonds must
be broken. To find out how strongly the hydrophobic layer is
attached to the surface, we subjected the treated samples to two
washing protocols. Hydrophobic substrates were immersed in
sonication baths of three different solvents that wet the surface
completely (EtOH, THF, and DCM), for 5 min periods each
time. Water contact angle measurements reveal that even after
repeated sonication there is no significant change in the wetting
properties of these surfaces. For a representative piece of
treated cotton, water contact angles were measured to be 138.7
2.4° and 136.8 3.7° before and after the sonication process,
respectively. We also subjected the 1-HFPS treated and
irradiated cotton samples to simulated wash cycles where the
samples were washed in water containing detergent for 15 min
simply absorb the water due to the abundance of the hydroxyl
groups in the structure.
On the light-treated surfaces (Figure 2C,G) the water droplet
remained on the surface indefinitely while maintaining their
shape and high contact angles, only disappearing through
evaporation. These droplets could be simply removed by
picking up the substrate and shaking them off or wicking using
a Kimwipe or pipet (see Figure S3). Interestingly, even though
the surface was coated with a low surface energy compound
and exhibited large water contact angles, while measuring
contact angle hysteresis we observed the “petal effect” on all the
treated surfaces.41,42 For all the paper/cotton fabric surfaces
contact angle hysteresis was relatively high (>10°). The small
water droplets stuck to the surface when the sample was tilted
vertically (90°) or even turned upside down (180°) (see Figure
S4). This indicates that there is a strong adhesion between the
water droplet and the modified surfaces. Such surfaces are
expected to have many potential applications, such as sticky
tapes,43 for liquid transportation in microfluidic devices44 or
two-dimensional lab on paper devices.45 Larger water droplets,
like those from a Pasteur pipet bead, roll off of the substrates.
This is shown for cotton in a movie supplied in the Supporting
Information.
To obtain further evidence of coating of the HFPS on the
surface of cotton and paper, XPS analysis was performed on
substrate treated light and dark control samples and also on
untreated samples. Analysis of the spectra (Figure 3) indicates
that there is a relatively large peak related to fluorine (10.3%) in
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and then rinsed in water for 15 min. After each wash cycle the
substrates were dried before measuring the contact angle.
Illustrated in Figure S4 is a measure of the WCA as a function
of wash cycle; no significant change was observed in the water
contact angles even after 10 of these wash cycles. These results
support the claim of robustness of our coating that originates
from covalent bond between the HFPS and the surface.
Having demonstrated that the 1-HFPS can modify the
wetting behavior of the cellulose fiber material surfaces and
produce hydrophobic surfaces, we also carried out additional
sets of control experiments. Samples of cotton and paper were
treated with the nonfluorinated salt 1-PMe3S, according to the
previously explained procedure. Hydrophobicity tests per-
formed on these treated surfaces after irradiation and washing
cycles showed no hydrophobic properties, with the water
absorbing promptly after deposition. This suggests that it is the
highly fluorinated phosphonium moiety that imparts the
(majority of) hydrophobicity and that the CF3 of the diazirine
is not sufficient. Of course, the alkylphosphonium salt (no
fluorinated chains) would be expected to make the sample
hydrophilic and mask any potential hydrophobicity imparted by
the CF3 of the diazirine or the appended alkyl chain. Cotton/
paper samples treated with either 3-(3-methoxyphenyl)3-
trifluoromethyl)-3H-diazirine or 3-(3-(6-iodohexyloxy)phenyl)-
3-(trifluoromethyl)-3H-diazirine at the same concentrations
used for 1-HFPS and subjected to identical irradiation
conditions were similarly found to be nonhydrophobic.
By taking advantage of the photosensivity of the diazirine in
1-HFPS, patterning can be achieved on the surfaces using a
crude photomask. This is a simple and inexpensive method that
enables us to develop any hydrophobic pattern on various
surfaces even in the microscale that can be utilized in
biochemical assay devices.46−48 As a proof of concept, to
produce a simple patterned hydrophobic surface, a paper
substrate coated with 1-HFPS was covered with a shadow mask
with only a circular gap allowing the substrate to be exposed to
the light in that area. After irradiation followed by rinsing the
paper with THF, the surface wetting properties were tested.
The circular area exposed to light (indicate by a dark circle
drawn with pencil to aid in visualization) exhibited the high
hydrophobicity while water soaked into the other masked areas
(see Figure 4). With such a simple method for patterning one
can create well-defined hydrophobic and hydrophilic channels.
This method is compatible with small pieces of paper as well as
large. By using the patterning method, we will have control over
the hydrophobicity of the paper surface. Therefore, these
surfaces can serve as paper-based lab-on-a-chip devices (lab-on-
paper) that allow the transport, mixing, or sampling of the
liquid droplets all on the paper surfaces.45
Figure 4. Cartoon of the macropatterning and a photograph of the
paper surface after patterning. The spot on the left, indicated by the
penciled circle, is where the mask let light through. The spot on the
right is where the mask was dark.
diazo and azide substrates used in materials chemistry, this
work further demonstrates that the diazirine as an emerging
new platform for material modification. The diazirine-HFPS
was prepared in a convergent synthesis, bringing the two task
specific moieties together in the final step. The materials are
easy to prepare and can be made in large quantities. The
phosphine precursor can be prepared in our laboratory in the
100 g−1 kg scale but is made commercially in tanker car
quantities. The synthesis of the diazirine, while requiring several
steps to prepare, is routine and can be made in multigram
quantities.50 While the applications of robust hydrophobic
coatings on cotton and other fabrics (water-proofing) and
paper (document protection) are fairly obvious, because of the
high reactivity of the carbene intermediate, this protocol can be
used to form robust hydrophobic coatings on a host of
materials including glass, CNT, graphene, diamond, hair, and
other fabrics, to name a few. These studies along with a
systematic examination of altering the ligands of the HFPS and
demonstration of the photopatterning allowed by this method
are currently being examined.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details and characterization of compounds 1-
HFPS and 1-PMe3S, control reaction products, SEM of treated
and untreated paper and cotton samples, WCA measures on
cotton and paper samples, also as a function of washing cycle,
and a movie showing the water beading effect. This material is
available free of charge via the Internet at http://pubs.acs.org.
CONCLUSIONS
■
We have demonstrated for the first time a powerful, new
efficient and easy photochemical approach utilizing the
photochemistry of diazirine-modified HFPS to imposing highly
effective water barrier coatings on two model substrates: cotton
and paper. Evidence presented here suggests that the
photosensitive salt was covalently grafted onto the cotton and
paper surfaces via the generated carbene intermediate by
insertion into one of the surface-active groups (likely O−H).
This work represents a new class of materials for forming
hydrophobic coatings that marry the recently demonstrated
hydrophobic character of the HFPS to a diazirine moiety as a
photoactivated tethering agent.49 Like the more ubiquitous
AUTHOR INFORMATION
Corresponding Author
*E-mail: mworkent@uwo.ca (M.S.W.); pragogna@uwo.ca
(P.J.R.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC) and The
University of Western Ontario, WorldDiscoveries@Western.
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