Design of oxygen sensing nanomaterial: Synthesis, encapsulation of phenylacetylide substituted Pd(II) and Pt(II) meso-tetraphenylporphyrins into poly(1-trimethylsilyl-1-propyne) nanofibers and influence of silver nanoparticles

Article abstract of DOI:10.1039/c5ra24817e

Room temperature phosphorescent oxygen sensors have been designed by embedding symmetric palladium(ii) or platinum(ii) meso-tetraphenylporphyrins in poly(1-trimethylsilyl-1-propyne) in the form of nanofibers along with/without silver nanoparticles. These materials combine the advantages of the high oxygen sensitivity of porphyrins and the enhanced surface area of porous nanofibers. Phenylacetylide bearing palladium(ii) or platinum(ii) meso-tetraphenylporphyrins (Pd-TPA, Pt-TPA, Pd-TPP and Pt-TPP) were designed to enhance phosphorescence quantum yields as well as sensitivity towards oxygen. Their syntheses were achieved by 2 alternative methods including successive metallation reactions and Sonogashira coupling via optimization of the synthetic strategies and conditions. The respective effect of the metal (Pd or Pt) and substituent on phosphorescence quantum yield as well as other photophysical properties was considered. The sensing performances of the corresponding silver-free and silver doped nanofibers were tested in the oxygen concentration range of 0.0-100.0%. The offered composites provided the advantages of fast response, enhanced sensitivity, reversible and long lasting response, higher Stern-Volmer constants (K_SV) and low limit of detection values extending to 7.5 ppm for oxygen.

Full text of DOI:10.1039/c5ra24817e

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Page 1 of 32
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DOI: 10.1039/C5RA24817E
Design of oxygen sensing nanomaterial: Synthesis, encapsulation
of phenylacetylide substituted Pd(II) and Pt(II) meso-
tetraphenylporphyrins into poly(1-trimethylsilyl-1-propyne)
nanofibers and influence of silver nanoparticles
Emel Önala, Zeynep Ayb, Zübeyde Yela, Kadriye Ertekinb, Ayꢀe Gül Güreka, Sevinc Zehra Topal*a
and Catherine Hirel*a
a Gebze Technical University, Faculty of Science, Department of Chemistry, P. O. Box 141, Gebze, 41400 Kocaeli,
Turkey
b Dokuz Eylul University, Faculty of Science, Department of Chemistry, 35160, Buca, Izmir, Turkey
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Abstract
Room temperature phosphorescent oxygen sensors have been designed by embedding symmetric
palladium(II) or platinum(II) meso-tetraphenylporphyrins in poly(1-trimethylsilyl-1-propyne) in
form of nanofibers along with silver nanoparticles. These materials combine the advantages of the
high oxygen sensitivity of porphyrins and enhanced surface area of porous nanofibers.
Phenylacetylide bearing Palladium (II) or platinum(II) meso-tetraphenylporphyrins (Pd-TPA, Pt-
TPA, Pd-TPP and Pt-TPP) were designed to enhance phosphorescence quantum yields as well as
sensitivity towards oxygen. Their syntheses were achieved by 2 alternative methods including
successive metallation reaction and Sonogashira coupling via optimization of the synthetic strategies
and conditions. The respective effect of the metal (Pd or Pt) and substituent on phosphorescence
quantum yield as well as other photophysical properties was considered. The sensing performances
of the corresponding silver–free and silver doped nanofibers were tested in the oxygen concentration
range of 0.0 - 100.0 %. The offered composites provided the advantages of fast response, enhanced
sensitivity, reversible and long lasting response, higher Stern-Volmer constants (KSV) and low limit
of detection values extending to 7.5 ppm for the oxygen.
Keywods: Oxygen sensor, Phosphorescence, Electrospun nanofiber, Silver nanoparticle,
Palladium(II), Platinum(II), meso-Porphyrin, Phenylacetylide, Poly(1-trimethylsilyl-1-propyne).
*Corresponding authors: chirel@gtu.edu.tr; sztopal@gtu.edu.tr
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Introduction
Development of nano-scale chemical sensors has been emerged as one of the most important
research areas over the past decades1, 2 as the physicochemical properties of bulk materials depend
essentially of their size. Reducing materials from macro-to nano-scale increases the surface-to-
volume ratio, influencing the photophysical and photochemical properties, which are important
parameters on the development of chemical sensors. Nanomaterials show unique surface chemistry,
thermal and electrochemical properties which can help to improve the sensor characteristics such as
sensitivity, response time and limit of detection. Recently, fabrication of nanofibers and nanoscale
beads has been studied to improve optical sensing properties1-5. Their porous structure with
enhanced surface area, feasibility of modification and reliable entrapment of fluorescent dyes within
the cavities made them useful especially for gas sensing studies6-10
.
On the other hand, accurate measurement of oxygen levels is of great importance in many process-
control systems, strict inert gas atmospheres in semiconductor production, nuclear or life science
studies, environmental and biomedical analysis as well as in industrial processes. The demanded
oxygen concentration measurement range may vary from trace levels to large amounts depending on
the application. Both, optical and electrochemical sensing approaches can measure the oxygen
levels. Actually, scientists and instrument designers privileged optical sensing technology to
overcome the electrochemically sensing devices problems such as slow response, frequent
calibration required, suffering from electrical interference and large sample sizes required.
Luminescence-based oxygen sensors work on the principle of fluorescence or phosphorescence
quenching of different types of organic dyes in oxygen permeable solid matrices11. Especially,
palladium(II) and platinum(II)tetraphenylporphyrins have been widely used in phosphorescence-
based oxygen sensors12, 13 owing to their easy synthesis, strong and long-lived room temperature
near infrared (NIR) phosphorescence and also high sensitivity even at low oxygen concentrations.14
However, there are a few porphyrin-based optical O2 sensor studies have been performed in
nanoscale12-17 where decrease of material size magnifies the sensitivity, photostability and working
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range. Especially, meso-tetraphenylporphyrins are candidates of choice as their molecular structure
can be efficiently and simply tuned by modifying the number, position, and nature of the functional
groups introduced onto the (meso-)phenyl substituent. Therefore synthetic modification of such
important dyes seems worthwhile to ameliorate the oxygen sensing properties. Among the available
library of O2 sensing molecules, such as palladium, platinum or iridium complexes, the one
containing phenylacetylide substituent18 has shown interesting phosphorescent and lifetime
enhancement which led us to design symmetric palladium(II)
and platinum(II) meso-
tetraphenylporphyrins substituted by phenyl acetylide moieties as oxygen probe molecules (Pd-
TPA; Pt-TPA).
Poly(1-trimethylsilyl-1-propyne) [poly(TMSP)] polymer matrix along with perfluorochemical
additive (PFC) has previously attracted our attention for its high permeability versus oxygen O2(g)19.
All these developments prompted us to investigate the preparation of pioneering poly(TMSP)-based
nanofibers including Pd-TPA or Pt-TPA by electrospinning method to obtain (Pd-TPA)NF and
(Pt-TPA)NF. On the other hand, presence of the conducting metallic surfaces like gold and silver,
affect emissive spectral properties of nearby fluorophores as demonstrated previously for
[Ru(bpy)3]2+ dyes20 and influence the sensitivity, photostability and calibration characteristics of
oxygen sensors21. These studies encouraged us to investigate the effect of AgNPs on the nanofibers.
Herein, a detailed study about porphyrins including nanofiber preparation and their oxygen sensor
properties in a large concentration range were reported. Poly(TMSP) matrix and targeted porphyrins
(See Scheme 1) were suitable for electrospinning in presence of ionic liquid, silver nanoparticles
(AgNPs) and perfluorochemical additive (PFC). As far as we know, this is the first study on
phosphorescence-based oxygen sensing properties of porphyrins improved by silver nanoparticles.
Moreover, the influence of phenylacetylide substituent on the Stern-Volmer quenching constant
(Ksv), I0/I100 and limit of detection (LOD) that are representative of the interactions between O2 and
the electrospun nanofibers were compared by studying the oxygen sensing properties obtained with
palladium(II) and platinum(II)meso-tetraphenylporphyrin-based nanofibers ((Pd-TPP)NF; (Pt-
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TPP)NF) as reference probes.
Scheme 1. Oxygen sensing probes (M=Pd, Pt) investigated in this study
Results and Discussion
This study was devoted to prepare nanoscale optical oxygen sensors. Our strategy was based on two
main axes: i) incorporation of Pd(II) and Pt(II) meso-tetraphenylporphyrin complexes in
poly(TMSP) matrix to obtain innovative oxygen sensing nanofibers combining room temperature
phosphorescence and oxygen gas permeability, ii) tune of oxygen sensing properties of nanofibers
by addition of silver nanoparticles.
Design and synthesis
In order to increase the oxygen sensing sensitivity and phosphorescence quantum yield of the dyes,
novel meso-tetraphenylporphyrins (TPP) were designed by introducing phenylacetylide moieties at
the para-position of the meso-phenyl substituent (Pd-TPA; Pt-TPA), a non-trivial feature, which
provide a more rigid structure, enhance the π-conjugation and increase the luminescence properties
at room temperature. The phosphorescence properties of Pd(II) and Pt(II) complexes at room
temperature are of particular interest, but for such applications a large quantities of complexes have
to be produced in an easy and efficient route. Two synthetic pathways (Scheme 2) were tested which
both involve two sequential reactions, a metal insertion and a Sonogashira coupling in the reverse
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order resulting to the formation of Pd-TPA and Pt-TPA. Both synthesis were startedDfOroI:m10.1t0h3e9/wC5eRlAl2-4817E
known meso-tetrakis(4-bromophenyl)porphyrin (H2-TPPBr) as starting synthon, which can be
easily prepared on a large scale following the Adler method22. Method 1 involved first the
metallation of H2-TPPBr to obtain Pd-TPPBr and Pt-TPPBr followed by a Sonogashira coupling
unlike Method 2 was started by a Sonogashira coupling to obtain H2-TPA and finished by metal
insertion.
Among the range of Sonogashira coupling conditions, Cu-free cross coupling was preferred to avoid
polymerization sub-products and metal insertion was improved by microwave irradiation heating.
Scheme 2. Two-Steps synthetic strategies for Pd-TPA and Pt-TPA
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Metal insertion. The limit to synthesize Pd(II) and Pt(II) metalloporphyrins lies in the slow rate
incorporation of these transition metal ions into the porphyrin core, lowering drastically the reaction
yield. To insert Pd(II), the classical approach is the use of Pd(II)chloride or acetate in refluxing DMF
or benzonitrile with reaction times from 45 min. to days.23, 24 Generally, Pt(II) insertion is more
difficult and gives lower yield25, 26 requiring refluxing for days in a degassed atmosphere. Reaction
conditions were optimized against molar ratio of reagent, metal salt, organic solvent, heating
(microwave assisted metallation or classical heating) to develop an efficient Pd(II) and Pt(II)
insertion method.
Method 1 involved the metallation of H2-TPPBr as substrate. Generally excess of metal salts is
used; however this process spends a large amount of metal salts that are very expensive in bulk
scale. When the molar ratio of metal salt:porphyrin is 2:1 or 2.5:1, yield decreases. The optimal ratio
of metal salt:porphyrin is 3:1 (as observations in reports)27, 28. The choice of metal salt is crucial.
Commercial availability, stability and ease of use of the metal salts were preferred. The metal salt
should be enough soluble, but should not coordinate with the solvent of reaction thus competing with
the porphyrin ring. PtCl2 is a good example of unproper metal salt source that coordinates very fast
in hot benzonitrile and can be easily isolated by precipitation with hexane29, 30 Other disadvantage of
halogen salts like PdCl2 and PtCl2, they are causing arcing in microwave field since they can be
easily converted to metallic elements such as Pd0 and Pt0.27 Among metal salt, M(acac)2 salts are
interesting because they are commercially available and highly soluble in weakly coordinating
solvents. Our investigation compared M(acac)2, MCl2, M(OAc), and K2PtCl4 salt and the most
efficient metal insertion was observed for M(acac)2. Solvent should also be chosen very carefully as
porphyrins and metal salts must be soluble in it. Moreover, for microwave heating, solvent must
possess a high dielectric constant to drive the reaction more efficiently.31 In this optic, N-methyl-2-
pyrrolidinone (NMP) is an excellent choice due to its high polarity (6.7) and dielectric constant (32.2
o
at 25 C). The results were summarized in Table S1 and S2. The best conditions were 3 equiv.
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Pd(acac)2/ NMP/ 180 oC/ MW 15 min. and 81% yield for Pd-TPPBr and 3 equiv. Pt(acac)2/ NMP/
110 oC/ MW 45 min. and 25% yield for Pt-TPPBr.
For method 2, the optimized metallation method was transposed to H2-TPA substrate. Thanks to
microwave power and adequate NMP solvent, Pd-TPA and Pt-TPA were obtained despite the
weakness and reactivity of the triple bond (see Table 1). Especially, during the purification process,
aggregation can easily occur and the use of usual technique such as precipitation in water or organic
solvent has to be avoided.
Table 1: Pd(II) and Pt(II) metal insertion to H2-TPA.
Equiv.
Metal salt
3 equiv. Pd(acac)2
3 equiv. Pt(acac)2
Organic solvent
NMP
NMP
Conditions
% yield
86
3
3
180 oC, MW, 30 min.
200 oC, MW, 45min.
52
Sonogashira coupling. Cu-free Sonogashira reaction conditions32 were modified to obtained a
controlled 4-fold coupling of phenylacethylene to H2-TPPBr under the conditions outlined in
Scheme 2. In method 2, The coupling reaction occurred with 84 % yield using Pd(OAc)2, PPh3, dry
Et3N and dry THF without purification problem. Unfortunately, when the same conditions adapted
to method 1, it did not allow Sonogashira coupling of Pd-TPPBr and Pt-TPPBr. Regarding these
results method 2 was privileged.
Preparation and Characterization of Nanofibers
Although sensing compounds are of great importance in sensor design, the choice of the matrix is to
be considered carefully in order to achieve desired sensor’s characteristics. For example, matrix
affects the gas diffusion rate which result in different relative signal change and response time. In
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our preliminary work, according to the relative intensity change criteria (I0/I100), poDlyO(IT: 1M0.10S3P9/)C5wRAa2s4817E
found to be the ideal polymer matrix for porphyrin macrocycle in accordance with its moderate
diffusion and partition coefficients.19 On the other hand, the chemically and biochemically inert
perfluorochemical (PFC); nonadecafluorodecanoic acid (CF3(CF2)8CO2H) was added to the
preparation, called thereafter as cocktail, as oxygen solubility enhancing agent to increase the
oxygen response of the sensing materials. Ionic liquids have been also utilized as additives to gain
superior characteristics for the sensor materials such as high stability and sensitivity.33 Presence of
ionic liquid positively affects the nanofiber preparation process by increasing the electrical
conductivity. In recent works, effect of the ionic liquid on porous nature of the nanofibers as well as
oxygen sensing capabilities of the oxygen sensing molecule was studied. When 1-butyl-3-
methylimidazolium tetrafluoroborate was added into the sensing composite, larger pores were
handled compared to ionic liquid free ones.34 Therefore, porous texture of nanofibers affects the
response and recovery times as well as the sensitivity and selectivity of sensors. In this work, the
fabrication of oxygen sensing nanofibers were achieved using poly(TMSP) matrix, optimum amount
of PFC; 1.9 × 10-4 M (compared to cocktail weight) and 1-butyl-3-methylimidazolium
tetrafluoroborate considering polymer/ionic liquid ratio in the literature.34 On the other hand, to
investigate the influence of AgNPs on phosphoresce-based oxygen sensitivity, AgNPs were also
added to cocktails.
Various techniques can be used such as electrospinning, melt blowing, bicomponent spinning, force
spinning and flash spinning. Electrospinning is a broadly used technique for electrostatic fiber
formation which relies on repulsive electrostatic forces to produce nanofibers with diameter ranging
from 2 nm to several micrometers.35 Electrospinning has many advantages like ease of material
combination, relatively low cost, ease of fiber deposition and handle of long and continuous fibers.
To our knowledge, it is the first time that nanofibers of poly(TMSP) was prepared by electrospinning
technique.
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SEM and TEM imaging: The scanning electron microscope (SEM) images of (Pd-TPA)NF and
Ag-(Pd-TPA)NF at various magnifications were shown in Fig. 1. The empty spaces of the holes
within the network structures allow higher diffusion of O2. It was observed that both electrospun
membranes have a 3D network like structure with a random fiber orientation that was evenly
distributed on the substrate. The diameters of the fibers varied between 283 and 727 nm for (Pd-
TPA)NF and between 141 and 467 nm for Ag-(Pd-TPA)NF, respectively. The morphology of
nanofibers was also investigated by Transmission electron microscopy (TEM). TEM images of (Pd-
TPA)NF and Ag-(Pd-TPA)NF were shown in Fig. 1B and 1D. We can hypothesized from TEM
pictures that the black spots localized on the surface of Ag-(Pd-TPA)NF correspond to AgNPs.
This type of fibrous-structure provides higher surface area than thin films. Further increase of the
surface area may be achieved by changing the conditions of the electrospinning process such as
solvent composition, viscosity, concentration, temperature, humidity and working distance, which
results in either smaller diameter fibers or increased porosity at the fiber surface.
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Figure 1. (A) SEM and (B) TEM images of (Pd-TPA)NF, (C) SEM and (D) TEM images of Ag-
(Pd-TPA)NF.
Spectral Characterization
Absorption spectra of Pd-TPP, Pt-TPP, Pd-TPA and Pt-TPA were recorded in toluene solution
and poly(TMSP) matrix. As expected, the absorption spectra exhibited a strong Soret band around
410 nm and two weak Q bands around 520 and 550 nm (see Fig. S27 and Table S3), with a 5-10 nm
red shifts when porphyrin derivatives are incorporated in poly(TMSP) compared to toluene solution.
The molar absorptivity coefficients of the Soret Bands which were in accordance with literature were
286 286± 596, 269 714± 5095, 104 881± 3024 and 130 962± 712 (n=3) for Pd-TPP, Pt-TPP, Pd-
TPA and Pt-TPA, respectively (See Fig. S28, S29, S30 and S31).36 All of the compounds obeyed
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Beer’s law in the concentration range of 2.0 x 10-6 - 8.0 x 10-6 M. It was obsDeOrvI:e1d0.10t3h9a/Ct5tRhAe24817E
phenylacetylide moieties decrease the ε values.
At room temperature, upon excitation around 420 nm, the molecules in deoxygenated toluene
solutions exhibited strong NIR phosphorescence. Pd-TPA, Pt-TPA, Pd-TPP and Pt-TPP showed
typical double phosphorescence bands with large Stoke’s shifts (extending to 180 nm) that almost
completely eliminates self-absorption of the emitted light (See Fig. S32, S33, S34 and S35).
Luminescence-based data in solution phase and nanofibers were summarized in Table 2. The
maximum phosphorescence excitation and emission wavelengths were similar for both the solution
phase and the nanofibers.
Table 2. Phosphorescence spectra related data of the molecules in toluene and poly(TMSP)
ex
em
λ
λ
λ
ex :excitation wavelength, max : maximum emission wavelength, max : maximum excitation
(
wavelength and ΦP: phosphorescence quantum yield).
em1
λmax
em2
λmax
ex
λmax
Compound
Medium
ΦP
λex
(nm)
(nm)
(nm)
(nm)
Toluene
Poly(TMSP)
Toluene
415
415
415
415
420
698
698
664
664
704
771
770
731
728
770
421
419
398
400
428
Pd-TPP
Pt-TPP
Pd-TPA
0.054±0.002
0.037±0.002
0.185±0.01
0.103±0.002
0.025±0.01
Poly(TMSP)
Toluene
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Poly(TMSP)
Toluene
420
420
420
704
770
734
732
426
406
407
0.020±0.001
0.092±0.005
0.061±0.003
670
668
Pt-TPA
Poly(TMSP)
Phosphorescence quantum yields (φp) were calculated in toluene. H2-TPP was used as a reference
standard whose φF value is 0.12.37 The ΦP values of Pd-TPP, Pt-TPP, Pd-TPA, and Pt-TPA were
found to be 0.054±0.002, 0.185±0.01, 0.025±0.01 and 0.092±0.005 in toluene, respectively. In
parallel to molar absorptivity, phenylacetylide substitution resulted in nearly two fold decreased
quantum yields. In contrast to the solution phase, in embedded forms, quantum yields did not exhibit
any dramatic change. For both TPP and TPA derivatives, the platinum complexes exhibited higher
quantum yield values.
Oxygen Sensing Properties
The phosphorescence emissions of metallo-porphyrin complexes decreased proportionally to the
concentration of oxygen by dynamic quenching based on the energy transfer between the triplet state
of the metal complex and the molecular (triplet) oxygen.11 Quenching often is accompanied by the
formation of singlet oxygen. However, herein we did not attempt to detect singlet oxygen. In the
oxygen sensor, the relationship between intensity and the concentration of O2 is reflected by the
Stern-Volmer equation. The Stern-Volmer constant (KSV) quantifies the quenching efficiency and
therefore the sensitivity of the sensor. The O2 sensitivity is also represented by the ratio (I0/I100),
where higher value means higher sensitivity. For an ideal homogeneous environment like in
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solution, the Stern-Volmer plots are linear. But, the Stern-Volmer plots for (Pd-TPP)NF, (Pd-
TPA)NF, (Pt-TPP)NF and (Pt-TPA)NF exhibit downward curvature for the concentration range of
0.0-100.0 pO2 typical of heterogeneous material. It means that porphyrin derivatives embedded into
poly(TMSP)-based nanofibers are present in two different micro environments which possess
different gas permeabilities, one being quenchable very efficiently, other either not being quenched
at all or being quenched at a very different rate. The two-site model equation (Eq. (1)) is used to
describe this kind of quenching behavior, and this equation can be used to calculate the two Stern-
Volmer constants (KSV1 and KSV2).38
em1
max
λ
The first and more intense phosphorescence emission maxima (
quenching of the excited state phosphorescence intensity by O2.
) were used to monitor the
Eq. 1
In Equation 1, KSV1 and KSV2 were obtained for the same chromophores: where P accounts for
fraction of nanofibers which can be interact with oxygen effectively, (1-P) presents for fraction of
nanofibers which can’t be interact with oxygen. In the equation, KSV2 was fixed as zero (KSV2=0) to
reduce the fit parameters, thus fitting excellently the nonlinear Stern-Volmer plots.38 Calculated
I0/I100 parameters, KSV and LOD values of all derivatives were summarized in Table 3. All of the
Ksv parameters were given for the concentration range of 0.0 –100.0 % pO2. Linear working ranges
were also mentioned in Table 3. All of the nanofibers exhibited good Ksv values and high I0/I100
parameters in the chosen matrix material. Varying the metal affected the oxygen sensitivity of the
nanofibers. In all formulations, the Pd-based composites exhibited higher Stern-Volmer constants
with respect to the Pt-based ones. However characteristics of the response were different in terms of
the working range. The Pd derivatives exhibited dramatic drops and linear response when exposed to
low concentrations of oxygen between 0.0-5.0% pO2. The high Ksv values and I0/I parameters were
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the evidences of the extreme sensitivity of the Pd complexes to oxygen (See inset of DFOigI:.102.1A03,9T/Ca5RbAle24817E
3). This kind of response make the Pd derivatives appropriate for the detection of trace amounts of
oxygen. The Pt derivatives also exhibited good sensitivity to the oxygen in terms of KSV and I0/I
values, but for larger concentration ranges (0.0–100.0% pO2; See inset of Fig. 2B). Additionally the
response was more linear than that of the response observed for the palladium derivatives.
When phenylacetylide substituents were introduced, small variations on the oxygen sensitivity were
observed. As can be seen from comparing Fig. 3 and Fig. 2B, (Pt-TPA)NF yielded slightly higher
KSV (0.6374) with respect to (Pt-TPP)NF (0.4763). In case of the Pd derivatives, the Ksv value of
the (Pd-TPP)NF was higher than that of the (Pd-TPA)NF.
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Figure 2. Oxygen induced emission spectra of A) (Pd-TPP)NF, B) (Pt-TPP)NF under small steps
of O2 (pO2 %): 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 20.0, 40.0, 60.0, 80.0, 100.0 %. Inset A: Linear
working range of the sensing agent; [pO2 ]%: 0.0-5.0 Inset B: [pO2 ]%: 0.0-10.0.
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Figure 3. Oxygen induced emission spectra of (Pt-TPA)NF under small steps of O2 partial
pressures (pO2 %): 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 20.0, 40.0, 60.0, 80.0, 100.0 %.
Table 3. Phosphorescence emission wavelengths and oxygen sensitivities of the embedded
molecules with/without AgNPs (I0/I: parameter exhibit sensitivity; KSV: Stern-Volmer coefficients,
LOD: limit of detection).
em1
em2
Compounds in
nanofiber form
I0/I100
KSV
LOD
(%-1)
λmax
λmax
(%-1)
(nm) (nm)
698
704
664
770
770
728
33.2 3.0584 0.00098
27.9 2.4854 0.0120
23.7 0.4763 0.0062
(Pd-TPP)NF
(Pd-TPA)NF
(Pt-TPP)NF
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668
694
702
662
666
728
768
770
728
734
20.2 0.6374 0.0047
(Pt-TPA)NF
Ag-(Pd-TPP)NF
Ag-(Pd-TPA)NF
Ag-(Pt-TPP)NF
Ag-(Pt-TPA)NF
49.4 3.9823 0.00075
20.5 3.5073 0.0016
23.1 0.4339 0.0069
17.3 0.4165 0.0072
3.5. Effect of AgNPs additive
Effect of AgNPs on oxygen sensitivity was different for Pd and Pt derivatives. Fig. 4 reveals the
response of (Pd-TPA)NF and Ag-(Pd-TPA)NF. Presence of the AgNPs enhanced the Ksv values of
the (Pd-TPP)NF from 3.0584 to 3.9823 and of (Pd-TPA)NF from 2.4854 to 3.5073. Whereas the
Ksv values of the platinum derivatives slightly dropped. Among them Ag-(Pd-TPP)NF exhibited
the highest Stern-Volmer constant and I0/I100 parameter (See Fig. S36-S38). The enhanced oxygen
sensitivity of the silver doped composites can be attributed to the efficient interaction of surface
plasmon polaritons of AgNPs with Pd complexes during oxygen induced collisional quenching. The
competing surface plasmon absorption of the AgNPs with the excitation light (and following
intrinsic emission of the silver) tunes the emission based response of the Pd derivatives. However,
identification of the mechanism taking place in the solid state requires calculation of the Förster
distances and/or transition dipoles and will be subject of further studies. The effect was not as
significant for platinum porphyrin probes probably due to the lack of sensitivity induce by the metal
effect (See Fig. 2).
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Figure 4. Oxygen induced emission spectra of A) (Pd-TPA)NF, B) Ag-(Pd-TPA)NF under O2
partial pressures (pO2 %): 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 20.0, 40.0, 60.0, 80.0, 100.0%.
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3.6. Limit of detection
LOD is governed by both the initial slope (KSV) of the quenching plot and by the resolution of the
instrument. Assuming a 0.1% uncertainty in light intensity measurement, the detection limit is
0.003/KSV (KSV expressed in %-1 units and at a signal-to-noise ratio of 3). The limit of detection
(LODs) of the corresponding porphyrin-based nanofibers in the absence and presence of silver
nanoparticles were determined by using the Stern–Volmer constants (See Table 3). Pd containing
nanofibers exhibited better LOD values with respect to the Pt- based ones. In all cases presence of
the silver nano particles enhanced the detection limit. Among the corresponding 8 nanofibers, the
best LOD was found to be for Ag-(Pd-TPP)NF with the value of 0.00075% pO2 (7.5 ppm).
3.7. Response time, regeneration time and repeatability
In all cases oxygen sensing responses of the porphyrin-based nanofibers were reversible. The
variation of the phosphorescence emission changes of (Pd-TPA)NF and Ag-(Pd-TPA)NF under
deoxygenated ([O2] =0.0%) and oxygenated conditions ([O2]=100.0 % pO2) were shown in Fig. 5.
Similarly, reversible response characteristics of the (Pt-TPA)NF and Ag-(Pt-TPA)NF were shown
in Fig. 6. (Pd-TPA)NF and (Pt-TPA)NF exhibited 13 and 18 % of signal drifts after 10th cycle (See
Fig. 5A and 6A). The drifts for (Pd-TPP)NF and (Pt-TPP)NF were 20 % and 18 %, respectively
(See Fig. S39A and S40A). Addition of AgNPs negatively affected the regeneration cycles. The Ag-
(Pd-TPA)NF and Ag-(Pt-TPA)NF lost 39 % and 32 % of their original signal intensity after second
cycle (See Fig. 5B and 6B).
Response and regeneration times are defined as the time reach to 95 % of signal change when the
gas is switched between O2 partial pressures of 0.0 to 100.0 % pO2. When the pO2 sharply increased
from 0.0 to 100.0%, the response times of (Pd-TPA)NF and (Pt-TPA)NF were found to be 6 and 5
seconds, respectively. The regeneration times of the same species were 117 and 46 seconds. On the
other hand, (Pd-TPP)NF and (Pt-TPP)NF exhibited slightly longer response times of 13 and 6
seconds, respectively. Response and regeneration performances of (Pd-TPP)NF, Ag-(Pd-TPP)NF,
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(Pt-TPP)NF, Ag-(Pt-TPP)NF were presented in Fig. S39 and S40. These values aDreOI:l1i0k.e10l3y9/tCo5RbA2e4817E
limited by the time needed for the gas to flow-through tubing, in fact response and regeneration
performances should be faster. In summary, response times of phenylacethylide substituted
porphyrins were slightly faster than tetraphenylporphyrins. Besides, the addition of AgNPs into
nanofibers did not affect the response and regeneration time. However, these response times were
faster than that of PtTFPP incorporated into poly(2-hydroxyethylmethacrylate) (50 seconds),39 but
slower compared to that of PdTFPP and PtTFPP-based core shells (0.24 and 0.36 seconds,
respectively).15
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Figure 5. Time-based kinetic response of A) (Pd-TPA)NF, B) Ag-(Pd-TPA)NF in an alternating
atmosphere of 100.0 % N2 and 100.0 % O2 .
Figure 6. Time-based kinetic response of A) (Pt-TPA)NF, B) Ag-(Pt-TPA)NF in an alternating
atmosphere of 100.0 % N2 and 100.0 % O2 .
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3.8. Storage and stability
Concentration dependent oxygen responses of the sensing nanofibers were investigated for the 1st
and 240th-days. The gathered concentration dependent oxygen response curves of (Pd-TPA)NF, (Pt-
TPA)NF, Ag-(Pd-TPA)NF and Ag-(Pt-TPA)NF for 1st and 240th -days were presented in Fig. 7.
Responses of (Pd-TPP)NF, (Pt-TPP)NF, Ag-(Pd-TPP)NF and Ag-(Pt-TPP)NF were presented in
Fig. S41 and Fig. S42. After 240 days of period, (Pd-TPP)NF and (Pt-TPP)NF lost their original
signal intensities by ~21% and 20%, whereas (Pd-TPA)NF and (Pt-TPA)NF were almost stable
over 8 months stored under ambient conditions and in the absence of light. However, presence of
AgNPs affected the reproducibility negatively. After 8 months, oxygen sensing signal changes (in
terms of I0/I100) of the Ag-(Pd-TPA)NF, Ag-(Pt-TPA)NF, Ag-(Pd-TPP)NF and Ag-(Pt-TPP)NF
were lost 4%, 28%, 59% and 56%, respectively.
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Figure 7: Storage stability test results of A) (Pd-TPA)NF, B) (Pt-TPA)NF C) Ag-(PDdO-IT: 1P0.A103)N9/CF5RDA2)4817E
Ag-(Pt-TPA)NF.
4. Conclusion
In this study, Pd-TPA and Pt-TPA were designed to improve the phosphorescence oxygen sensing
properties of porphyrin dyes. The synthesis, characterization and oxygen sensitivities of four
different symmetric palladium(II) and platinum(II) meso-tetraphenylporphyrins were successfully
achieved in absence and presence of silver nanoparticles. Palladium(II) and Platinum(II) metallation
of meso-tetraphenylporphyrins were obtained in good and reproducible yield using the combination
of microwave irradiation, acetate salt and NMP as solvent. Palladium(II) and platinum(II) meso-
tetraphenylporphyrins were studied both in solution phase and in polymeric nanofibers obtained
using electrospun technique. As far as we know, it is the first time that nanofibers of poly(TMSP)
have been prepared by this technique.
The influences of metals (Pd or Pt) as well as variation of the substituent moieties were
investigated. All nanofibers exhibited sensitive, reversible and long lasting steady-state response to
oxygen. Among them, (Pd-TPP)NF exhibited the highest oxygen sensitivity and its oxygen sensing
response was greatly improved by adding AgNPs. Therefore, we envision that more O2 sensing (Pd-
TPP)NF can be obtained by tuning the size of the nanofiber and AgNPs. Also varying the amount of
AgNPs should influence the oxygen sensing properties. Our further studies will be on this direction.
Experimental
The following abbreviations were used in this article and in the figures: N-methyl-2-pyrrolidone
(NMP), dichloromethane (DCM), tetrahydrofurane (THF), meso-tetraphenylporphyrin (H2-TPP),
meso-tetrakis(4-bromophenyl)porphyrin
(H2-TPPBr),
palladium(II)
meso-tetrakis(4-
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bromophenyl)porphyrin (Pd-TPPBr), platinum(II) meso-tetrakis(4-bromophenyl)porphyrin (Pt-
TPPBr), poly(trimethylsilylpropyne) [poly(TMSP)], silver nanoparticles (AgNPs), palladium(II)
meso-tetraphenylporphyrin (Pd-TPP), platinum(II) meso-tetraphenylporphyrin (Pt-TPP), meso-
tetrakis(4-phenylethynyl)phenylporphyrin
(H2-TPA),
(Pd-TPA),
palladium(II)
platinum(II)
meso-tetrakis(4-
meso-tetrakis(4-
phenylethynyl)phenylporphyrin
phenylethynyl)phenylporphyrin (Pt-TPA), Pd-TPP-based nanofiber [(Pd-TPP)NF], Pd-TPP-based
nanofiber in presence of silver nanoparticles [Ag-(Pd-TPP)NF], Pt-TPP-based nanofiber [(Pt-
TPP)NF], Pt-TPP-based nanofiber in presence of silver nanoparticles [Ag-(Pt-TPP)NF], Pd-TPA-
based nanofiber [(Pd-TPA)NF], Pd-TPA-based nanofiber in presence of silver nanoparticles [Ag-
(Pd-TPA)NF], Pt-TPA-based nanofiber [(Pt-TPA)NF], Pt-TPA-based nanofiber in presence of
silver nanoparticles [Ag-(Pt-TPA)NF], Stern-Volmer constant (Ksv), Acetylacetonate (acac),
Acetate (OAc).
General information
Starting materials were of reagent grade quality, purchased from Aldrich for Pyrrole (reagent grade
%98), from Merck for propionic acid, from Across Organics for N-methyl-2-pyrrolidone (NMP).
Tetrahydrofurane (THF) was dried over Na, preparative separations were performed by biobeads
SM-2 Resin (300–1180 µm mesh size) and silica gel column chromatography Merck-60 (43–63
mesh). TLC was done on aluminum sheets precoated with silica gel 60F254 (Merck). The
microwave device (Milestone Inc.) operates at fixe frequency (2450 MHz). The equipment can
control the temperature (up to 250oC) in a range of power values from 0 to 1000 watt. The
perfluorochemical; nonadecafluorodecanoic acid; (CF3(CF2)8CO2H) was purchased from Alfa
Aesar. Poly(1-trimethylsilyl-1-propyne) was supplied from ABCR company. NaBH4 and AgNO3
were purchased from Merck. 1-butyl-3-methylimidazolium tetrafluoroborate was supplied from
Fluka.
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Physical Measurements
UV-Vis, Steady-state luminescence, IR, NMR, MASS, SEM and TEM measurements. UV-Vis
spectra were recorded by using a Shimadzu 2101 UV-Visible spectrophotometer. Steady-state
luminescence emission spectra were recorded using a FLS920 from Edinburg Instruments, using 1
cm fluorescence cuvette (Hellma, 117100F-10-40). IR spectral data between 4000 and 600 cm−1
were recorded using a Perkin Elmer 100 FT-IR spectrometer with an attenuated total reflection
1
(ATR) accessory featuring a zinc selenide (ZnSe) crystal. H NMR spectra were recorded on a
Varian 500 MHz spectrometer (with the deuterated solvents as the lock and tetramethylsilane as the
internal reference). MALDI-MS spectra were obtained using 2,5-dihydroxybenzoic acid as the
MALDI matrix, acquired in linear modes with an average of 50 shots on a Bruker Daltonics
Microflex mass spectrometer equipped with a nitrogen UV-Laser operating at 337 nm and ESI-MS
spectra were recorded by Bruker Daltonics MicrOTOF mass spectrometer equipped with an
orthogonal electrospray ionization (ESI) source. SEM images were obtained with JEOL 6510-LV
JSM scanning electron microscope. TEM observations were performed on a JEOL 2000FX electron
microscope at an acceleration voltage of 160 kV. TEM samples were prepared on mesh copper grids.
Luminescence quantum yield determinations. Absorbance and luminescence spectra of the
sample (for example Pd-TPA) and a reference standard were measured three times under identical
conditions (absorbance at both the excitation wavelengths were 0.05). In our study, meso-
tetraphenylporphyrin (H2-TPP) (ΦF= 0.12 in toluene40) was used as reference standard. All solutions
were thoroughly deoxygenated by pump and freeze follows by nitrogen bubbling through for 10 min
for three times. The ratio of the luminescence of the sample to reference standard was compared to
find out quantum yield.
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Oxygen sensing measurements. Oxygen and nitrogen gases were mixed in the concentration range
0-100% in a gas diluter (Sonimix 7000A gas blending system). The total pressure was maintained at
760 Torr. The output flow rate of the gas mixture was maintained at 550 mL min−1. Gas mixtures
were introduced into the sensing agent containing cuvette via a diffuser needle under ambient
conditions.
Synthetic Procedures
Synthesis
of
meso-tetraphenylporphyrin
(H2-TPP)
and
meso-tetrakis(4-
bromophenyl)porphyrin (H2-TPPBr)
meso-tetraphenylporphyrin (H2-TPP) and meso-tetrakis(4-bromophenyl)porphyrin (H2-TPPBr)
were synthesized following Adler Method as described in literature22 with 30% yield for H2-TPP
and 25% for H2-TPPBr. [H2-TPPBr] FT-IR ν (cm-1): 3320 (NH), 3022-3058 (CHAr), 1472 (C=N),
1
1348-1390 (=C-N), 1011 (C-Hpyrrole), 964(C-Hporp. ring) , 782(C-Hpyr. ring), 724 (NH bending). H
NMR (CDCl3), δ (ppm)= -2.94 (s, 2H, NH), 7.82 (d, 8H, CHAr-phenyl), 7.98 (d, 8H, CHAr-phenyl), 8.76
(s, 8H, CHpyrrole). UV-visible (CHCl3), λmax(nm) = 418; 512; 547; 588; 644. Mass (ESI-MS, m/z):
C44H26N4Br4 [calcd]: 930.3200, [found]: 930.6624 [M]+ .
meso-tetrakis(4-phenylethynyl)phenylporphyrin (H2-TPA): In tri-necked reaction flask, 2,5 g of
H2-TPPBr (2,7 mmol), 11 mg(0,05 mmol) of palladium(II) acetate and 40 mg of triphenlphosphine
were dissolved in the mixture of degazed and dry triethylamine/THF solution (35 mL/25 mL) under
o
argon. 2,5 mL (21,6 mmol) of phenylacethylene was added and the solution was refluxed at 91 C
for one night. 10 mL of water was added after cooling of the solution at room temperature. The
resulting purple precipitate was filtered on büchner, and washed with hot water. Purification was
achieved by column chromatography on silica gel with a gradient of eluent from acetone/hexane
(1:2; 1:1), acetone, toluene/hexane (1:1) to toluene. Yield 84 %. FT-IR ν (cm-1): 3306 (NH), 3032
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(CHAr), 2216(C≡C), 1473 (C=N), 1347-1398 (=C-N), 1020 (C-Hpyrrole), 968(C-Hporp. ring) , 795(C-
H
pyr. ring), 753 (NHbending). 1H NMR: δ (ppm), CDCl3 = -2.75 (s, 2H, NH), 7.45 (m, 12H, CHAr-phenyl),
7.70 (d, 8H, CHAr-phenyl), 7.96 (d, 8H, CHAr-phenyl), 8.23 (d, 8H, CHAr-phenyl) 8.91 (s, 8H, CHpyrrole) UV-
visible (CHCl3), λmax(nm) = 418; 512; 547; 588; 644. Mass (MALDI-MS, m/z) : C76H46N4 [calcd]:
1014.37, [found]: 1015.403 [M+H]+.
Insertion of Pd(II) and Pt(II)
General procedure: In a schlenk tube 1 eqv. of free base porphyrin was dissolved in NMP (0.05
mM) and 3 eqv. of Pd(acac)2 or Pt(acac)2 was added at once (or two times for TPA derivatives). The
reactions were achieved under microwave irradiation (see Table 1, S1 and S2). The reaction was
cooled to room temperature and NMP was removed under reduced pressure. The orange residue was
purified on silica gel with different equivalent of hexane/DCM eluent system for Pd-TPP, Pt-TPP,
Pd-TPPBr and Pt-TPPBr; and purified on bio-beads for Pd-TPA and Pt-TPA eluted by DCM,
ethanol and ethyl acetate.
Pd-TPP: Eluent hexane/DCM 4:1. Yield 100%. IR ν (cm-1): 3052 (CHAr), 1440 (C=N), 1363-1351
1
(=C-N), 1071 (C-Hpyrrole), 963(C-H por. ring), 788(C-H pyr. ring). H NMR (CDCl3), δ (ppm)= 7.71 (m,
12H, CHAr-phenyl), 8.17 (d, 8H, CHAr-phenyl), 8.80 (s, 8H, CHpyrrole). UV-visible (Toluene), λmax (nm) =
417; 524; 555. Mass (MALDI-MS, m/z): C44H28N4Pd, [calcd]: 718.130, [found]: 719.947 [M+H]+.
Pd-TPPBr: Eluent hexane/DCM 9:1, 3:1, 2:1 successively. Yield 81%. IR ν (cm-1): 2922 (CHAr ),
1449 (C=N), 1350 (=C-N), 1070 (C-Hpyrrole), 1007(C-H por. ring) , 792(C-Hpyr. ring). 1H NMR (CDCl3):
δ (ppm), 7.82 (d, 8H, CHAr-phenyl), 7.96 (d, 8H, CHAr-phenyl), 8.74 (s, 8H, CHpyrrol) UV-visible
(CHCl3), λmax(nm)= 418; 523, 644. Mass (MALDI-MS, m/z) : C44H24N4Br4Pd, [calcd]:1034.74 ,
[found]:1034.571[M]+
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Pd-TPA: Eluent DCM, ethanol and ethyl acetate. Yield 86%. IR ν (cm-1): 2924 (CHAr
DOI: 10.10)3,9/C154R4A224817E
1
(C=N), 1352 (=C-N), 1075 (C-Hpyrrole), 1012(C-H por. ring) , 794(C-H pyr. ring). H NMR (CDCl3): δ
(ppm), 7.36 (m, 12H, CHAr-phenyl), 7.61 (d, 8H, CHAr-phenyl), 7.88 (d, 8H, CHAr-phenyl), 8.09 (d, 8H,
CHAr-phenyl) 8.79 (s, 8H, CHpyrrol) UV-visible(Toluene), λmax(nm)= 426; 526, 559. Mass (MALDI-
MS, m/z) : C76H44N4Pd, [calcd]: 1118.26, [found]: 1119.548 [M+H]+.
Pt-TPP: Eluent hexane/DCM 4:1. Yield 73%. IR ν (cm-1): 3053 (CHAr), 1440 (C=N), 1358 (=C-N),
1
1074 (C-Hpyrrole), 1016(C-Hpor. ring), 794(C-Hpyr. ring). H NMR (CDCl3), δ (ppm)= 7.66 (m, 12H,
CHAr-phenyl), 8.07 (d, 8H, CHAr-phenyl), 8.67 (s, 8H, CHpyrrole). UV-visible (CHCl3), λmax(nm)= 402;
510; 540. Mass (MALDI-MS, m/z): C44H28N4Pt, [calcd]: 807.80, [found] : 808.497[M+H]+.
Pt-TPPBr: Eluent hexane/DCM 9:1, 3:1, 2:1, 1:1. Yield 25%. IR ν (cm-1): 2919 (CH Ar), 1455
1
(C=N), 1356 (=C-N), 1069 (C-Hpyrrole), 1009(C-Hpor. ring) , 792(C-Hpyr. ring). H NMR (CDCl3), δ
(ppm): 7.82 (d, 8H, CHAr-phenyl), 7.93 (d, 8H, CHAr-phenyl), 8.67 (s, 8H, CHpyrrol) UV-visible (CHCl3),
λ
max(nm)= 402; 514, 543. Mass (MALDI-MS, m/z) : C44H24N4Br4Pt, [calcd]:1118.84 , [found]:
1119.857 [M+H]+.
Pt-TPA: Eluent DCM, ethanol and ethyl acetate. Yield 52%. IR ν (cm-1): 2919 (CHAr ), 1462
(C=N), 1364 (=C-N), 1056 (C-Hpyrrole), 968(C-H por. ring) , 796(C-H pyr. ring). 1H NMR (CDCl3), δ
(ppm): 7.05 (m, 12H, CHAr-phenyl), 7.46 (d, 8H, CHAr-phenyl), 7.83 (d, 8H, CHAr-phenyl), 7.73 (d, 8H,
CHAr-phenyl) 8.68 (s, 8H, CHpyrrol). UV-visible (Toluene), λmax(nm)= 409, 515, 574. Mass (MALDI-
MS, m/z): C76H44N4Pt, [calcd]: 1207.32, [found]: 1208.710. [M+H]+.
Synthesis of silver nanoparticles
29