Research Article www.acsami.org
Phosphorescent Oxygen and Mechanosensitive Nanostructured Materials Based on Hard Elastic Polypropylene Films Irina A. Okkelman,† Alla A. Dolgova,‡ Swagata Banerjee,† Joseph P. Kerry,† Aleksandr Volynskii,‡ Olga V. Arzhakova,‡ and Dmitri B. Papkovsky*,† †
School of Biochemistry and Cell Biology, University College Cork, Western Road, Cork T12 YN60, Ireland Faculty of Chemistry, Lomonosov Moscow State University, Leninskiye Gory, Moscow 119991, Russia
‡
ABSTRACT: It is well known that sensitivity of quenchedphosphorescence O2 sensors can be tuned by changing the nature of indicator dye and host polymer acting as encapsulation and quenching mediums. Here, we describe a new type of sensor materials based on nanostructured hard elastic polymeric substrates. With the example of hard elastic polypropylene films impregnated with Pt-benzoporphyrin dye, we show that such substrates enable simple one-step fabrication of O2 sensors by standard and scalable polymer processing technologies. In addition, the resulting sensor materials show prominent response to tensile drawing via changes in phosphorescence intensity and lifetime and O2 quenching constant, Kq. The mechanosensitive response shows reversibility and hysteresis, which are related to macroscopic changes in the nanoporous structure of the polymer. Such multifunctional materials can find use as mechanically tunable O2 sensors, as well as strain/deformation sensors operating in a phosphorescence-lifetimebased detection mode. KEYWORDS: Mechanosensitive materials, hard elastic polymers, phosphorescence-based sensors, mechanically tunable oxygen sensors, strain and deformation sensors
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INTRODUCTION Oxygen-sensitive materials comprising thin polymeric coatings or films impregnated with photoluminescent dyes are actively used in various applications, including process control, packaging, food, environmental, and biomedical applications.1−3 It is well-established that sensitivity of such materials can be tuned by changing the nature of the host polymer (O2 solubility and diffusion coefficient) and the reporter dye (unquenched lifetime and orientation factor, fluorescence/ phosphorescence).1,4 Here, we report a novel type of phosphorescent materials based on hard elastic polymers, the photophysical characteristics and O2-sensitivity of which are strongly influenced by tensile drawing. This allows preparation of mechanosensitive and strain-responsive phosphorescencebased sensors. Since their discovery in the 1960s,5 the so-called hard elastic or springy polymers have been under active research.6 Their basic and unique properties are relatively well described.7−11 One characteristic feature of hard elastic polymers based on semicrystalline polymers (e.g., polyethylene, polypropylene (PP), and polyamide with high level of crystallinity) is the development of high macroscopic internal porosity upon their tensile drawing in air, without any marked side contraction even at relatively high tensile strains. These materials also have a remarkably high strain recovery (up to 80−90%), similar to that of elastomers.7,10 Such features are mainly provided by their well-organized initial row-nucleated crystalline morphology, in which all stacked crystalline lamellas are oriented parallel to each other and perpendicular to the direction of tensile © 2017 American Chemical Society
drawing. The neighboring lamellas are bridged by microfibrils oriented parallel to the direction of drawing.8,10 Under the loading, the crystalline lamellas tend to separate, and this leads to the formation of interconnected nanoscale voids between the fibrils.9,10 As a result, these materials represent a special type of microporous polymeric matrices, which are suitable for simple and direct incorporation of various chemical cargo and fabrication of new functional materials. Also, the springy nature of hard elastic polymers can provide previously unknown properties to such composite materials. Polyolefines, such as PP and HDPE, are attractive polymeric host matrices for optochemical sensors and particularly quenched-phosphorescence O2 sensors. Bulk PP has moderate gas permeability and useful physicochemical and mechanical properties for such applications; however, it is not very compatible with many O2-sensitive indicator dyes and conventional processes of sensor fabrication.4,12 The latter issues have been addressed using advanced polymer processing technologies such as solvent crazing, which have enabled preparation of nanoporous polymeric substrates and their loading with O2sensitive dyes12,13 and other chemical additives.14−16 On the other hand, hard elastic polymers have not been studied or explored in optochemical sensing to a reasonable degree. However, hard elastic polymers such as PP can give rise to a family of advanced materials with useful functional attributes, Received: January 9, 2017 Accepted: April 3, 2017 Published: April 3, 2017 13587
DOI: 10.1021/acsami.7b00405 ACS Appl. Mater. Interfaces 2017, 9, 13587−13592
Research Article
ACS Applied Materials & Interfaces
and in a hypoxia chamber (Coy Laboratory Products), respectively. Sensor temperature was monitored with a built-in infrared temperature sensor of the Optech reader. Optical microscopy measurements were performed on a custommade system (Becker&Hickl, Germany) based on an upright AxioExaminer Z1 microscope (Carl Zeiss) with 5×/0.25 FLUAR air objective (Carl Zeiss). The microscope was connected to a DCS-120 confocal scanner (Becker&Hickl) with two excitation and two emission channels and picosecond supercontinuum 400−650 nm laser SC400-4 (Fianium, U.K.). Emission was detected with an R10467U-40 photon counting detector (Hamamatsu Photonics K.K.) connected to the scanner and system hardware. The microscope was controlled using MicroToolBox software (Carl Zeiss) and image acquisition and data processing were performed with SPCImage software (Becker&Hickl). Phosphorescence lifetime imaging microscopy (PLIM) measurements were performed with film samples fixed in the mechanical clamp, both in relaxed and stretched (60%) states. The film was excited at 614 nm and emission (maximum at 760 nm) was collected with a 750−810 nm bandpass filter. Atomic force microscopy (AFM) images of sensor films were obtained on a PRO-M atomic force microscope with a NSG11 cantilever (NMDT, Russia), in a semicontact scanning mode in air. For transmission electron microscopy (TEM), ultrathin sections were cut from the samples with a diamond knife (ultramicrotome, Reichert Jung) at room temperature and then placed onto copper grids for electron microscopy, coated with formvar. The samples were examined on a Leo_912, AB Omega transmission electron microscope (Carl Zeiss).
particularly for quenched-phosphorescence O2 sensing and related applications. Furthermore, mechanoresponsive and conjugated polymers are gaining much attention in recent years,17−20 but so far their phosphorescent versions are significantly underdeveloped and should be studied more systematically.21 To explore these opportunities, we have produced a panel of phosphorescent materials based on hard elastic polypropylene (HEPP) films impregnated with platinum(II)-benzoporphyrin (PtBP) reporter dye22 and studied their structural and spectroscopic properties and oxygen- and mechano-sensitive behavior.
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EXPERIMENTAL SECTION
Sensor Fabrication. HEPP precursors/substrates in the form of a 105 μm thick tape were produced from a conventional blown PP film (Tver Khimvolokno, Russia; thickness 200 μm; Mw = 3 × 105) as follows: 2 cm wide pieces of PP film tape were uniaxially stretched in air at room temperature by 500% with respect to their original length. The necked part of the resulting tape was then cut out, placed in a dry oven, and allowed to relax in a free state at 160 °C for 30 min (annealing23). During such thermal treatment, the stretched tape shrunk by ∼52−55% and converted into HEPP. The HEPP substrates thus produced were impregnated with PtBP dye as follows: 10 mm × 50 mm strips of the HEPP precursor were fixed in a hand-operating clamp (Figure 1), stretched in air by 90%,
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RESULTS AND DISCUSSION Let us first consider the structure of the HEPP precursors as substrates for the impregnation and sensor fabrication. These precursors were produced by stretching in air, which produces necking, followed by annealing in the free-standing state at 160 °C for 30 min.23 Upon such annealing, the samples shrunk, reducing their dimensions along the length by about 52−55%. Figure 1 illustrates that subsequent stretching in air of such PP substrates is accompanied by development of marked porosity, evidenced by the formation of light-scattering irregularities and “milky” color of the stretched PP film. Such a behavior is characteristic for HEPP, which has been extensively studied and characterized.10,24 We also studied the structure of the HEPP substrates by AFM and TEM techniques. Although high-resolution scanning probe imaging of all nanostructured polymer-based materials is challenging, both the AFM and TEM images obtained by us clearly reveal the fine structure of the HEPP matrices, which contains nanoscale fibrils and pores between them (Figure 2). Direct proof of the nanoporosity developed in HEPP samples upon their stretching in air was also obtained by measuring sample geometrical dimensions at different elongations. Figure 3 shows the plot of measured porosity versus elongation, from which we can determine that stretching of the relaxed HEPP film by 100% increases its porosity from zero to ∼26%. The parameters of the porous structure of the HEPP substrate at the time of its loading with the dye were also determined, using the pressure-driven liquid permeability method according to the hydrodynamic Hagen−Poiseuille model.25 The porosity of the air-stretched HEPP samples is open, and liquids can pass through them. The overall flux of isopropanol through the sample of HEPP stretched by 90% was equal to 9 L/(m2 h bar), and the corresponding pore size was found to be 25 nm. All this allowed us to conclude that the airstretched HEPP samples can be used as nanoporous substrates
Figure 1. Hand-operating clamp with the initial HEPP precursor (after necking and thermal annealing): before (left) and after (right) stretching in air by 90%. dip-coated under isometric conditions with a PtBP solution in nheptane (9 mg/mL), and then allowed to relax and dry at room temperature. The porosity of the stretched HEPP samples was calculated from measured changes in their geometrical dimensions: W = (Vc − V0)/Vc × 100%, where V0 and Vc are the volumes of unstretched and stretched samples, respectively. Mechanical tests of the samples were conducted on an Instron 4301 tensile machine at a strain rate of 5 mm/min. Phosphorescence Intensity and Lifetime Measurements. Phosphorescence intensity and lifetime measurements with HEPPPtBP-based sensors were performed on handheld reader Optech (Mocon), which operates in time domain with pulsed excitation.3 Sensor calibrations were performed with standard O2/N2 gas mixtures (0−21 kPa O2 range) produced by a precision gas mixer (LN Industries SA, Switzerland), in a flow cell with a glass window.12 In stretching/relaxation experiments, films were gradually elongated from 0 to ∼30% of their initial length in a mechanical clamp and after each step phosphorescent lifetimes were monitored for 10 min. After reaching the maximal elongation rate, the film was relaxed (once-off or gradually) and its parameters were measured in a similar manner. HEPP film sensors were tested under three environmental conditions: 27 °C/21 kPa O2, 8 °C/21 kPa O2, and 32 °C/5 kPa O2. Lowtemperature and low-O2 experiments were performed in a cold room 13588
DOI: 10.1021/acsami.7b00405 ACS Appl. Mater. Interfaces 2017, 9, 13587−13592
Research Article
ACS Applied Materials & Interfaces
0.3 wt %. This is close to the theoretical value of 0.28 wt %, calculated with the assumption that all pores were filled with the dye solution. Overall, this simple procedure allows fabrication of densely, uniformly, and permanently stained, highly phosphorescent HEPP films. The method is readily applicable to other indicator dyes and chemistries, including those which are poorly compatible or incompatible with the bulk polymer. Several modifications of this procedure were also tested with PtBP but were seen to produce the HEPP sensor materials with less favorable characteristics in terms of uniformity, stability, phosphorescent signals, and photobleaching (results not shown). The initial testing of the as-produced HEPP sensors for sensitivity to O2 revealed moderate quenching of their phosphorescence, with approximately 4-fold reduction in lifetime in air (20.86 kPa O2) at 20 °C. The corresponding Stern−Volmer plots show excellent linearity (R = 0.997, Figure 4A,B), which reflects high homogeneity of the polymeric
Figure 2. TEM (top panel) and AFM (bottom panel) images of HEPP substrates that underwent 90% stretching in air.
Figure 4. Calibration of the phosphorescent oxygen-sensitive HEPP film in relaxed state at 20 °C. (A) Lifetime (τ) vs [O2] dependence. (B) Linearization in Stern−Volmer plots.
material and uniform loading of the PtBP dye in it.1,2 Note that linear Stern−Volmer plots are common for nanostructured polymers, such as solvent-crazed sensors,13,26 whereas O2 sensors produced by casting of polymeric “cocktails” are often heterogeneous.1,4 The nanostructured O2 sensors showed no cross-sensitivity to humidity and no drift of lifetime signals over time, which is also advantageous.27 Therefore, their onceoff lifetime calibration is valid for both gaseous and aqueous samples. As the HEPP sensors were produced by the stretching/ relaxation protocol, we decided to test their sensitivity to changes in tensile strain during stretching−shrinking cycles. We found that upon minor to moderate elongation (3−30% from the relaxed state) they show marked optical response, reducing their phosphorescence lifetime signals (Figure 5). The initial experiment at 21 kPa O2 and 27 °C produced rather small lifetime changes (blue curves), but we attributed this to the flat part of the lifetime versus O2 dependence (Figure 4A). Indeed,
Figure 3. Porosity of the HEPP substrate upon its stretching in air.
for incorporation of O2-sensitive dyes and preparation of sensors. At a tensile strain of 90%, which corresponds to 25% porosity, the HEPP samples were impregnated with PtBP. For that, the substrate was soaked under isometric conditions in a solution of the dye (9 mg/mL in heptane) and then allowed to dry and relax in air at room temperature. Upon such relaxation in a free state, the HEPP substrate shrunk back to its original length (by 90−95%) so that its nanopores were almost fully healed and the polymer became dense again. The dye content in the relaxed HEPP sample was estimated gravimetrically to be 13589
DOI: 10.1021/acsami.7b00405 ACS Appl. Mater. Interfaces 2017, 9, 13587−13592
Research Article
ACS Applied Materials & Interfaces
this process is accompanied by the development of marked porosity in the sample. Curves 1 and 2 in Figure 6 show the mechanical response to stretching and relaxation of the initial HEPP substrate during its loading with the dye. Curves 3 and 4 show the response of the dye-loaded sample to stretching and relaxation. Importantly, the hysteresis shown by curves 3 and 4 is fully reproduced in subsequent cycles, thus reflecting the reversible character of loading and high strain recovery. The range of strains in Figure 6 corresponds to the range where a marked decrease in lifetime is observed (Figure 4). Upon unloading, stress relaxation takes place, the pores get healed, and porosity decreases. However, the different rates of onward and reverse processes produce hysteresis. Hence, one can conclude that hysteresis in the stress−strain curve of the sample is responsible for the hysteresis in the lifetime-elongation dependence, which follows a similar scenario. On the other hand, it is known from HE polymer chemistry28 that strain rate has no influence on the results of these experiments (in the studied range of such rates). We also performed the structural analysis of the relaxed and stretched (60% elongation, 23 °C, 21 kPa O2) HEPP film sensors by PLIM. The PLIM images and lifetime distribution histograms in Figure 7A,B show the relatively uniform distribution of the phosphorescence lifetime across the film (close to normal distribution) and prominent changes in the mean lifetime values from 14.6 ± 0.8 μs in the relaxed state to 10 ± 1 μs in the stretched state (23 °C).
Figure 5. Changes in the phosphorescence lifetime of the HEPP sensor film upon loading/unloading cycles. (A) Response under different environmental conditions: 21 kPa O2, 27 °C, blue curves; 21 kPa O2, 8 °C, red curves; 5 kPa O2, 32 °C, green curves. The arrows show the direction of film deformation. (B) Two repetitive elongation/stretching cycles under constant environmental conditions (25 °C, 21 kPa O2).
when similar experiments were performed at a lower environmental temperature (red curves) or O2 concentration (green curves), that is, under the conditions which corresponded to the higher basal lifetime values, the response to the mechanical treatment becomes markedly enhanced. A prominent hysteresis was observed in all of the cases (Figure 5A); however, the response was reversible and upon relaxation of the HEPP sensor film the lifetime signals returned to their original values (Figure 5B). To explain this rather uncommon optomechanical behavior, we analyzed the mechanical characteristics of the sample in response to the applied tensile stress (loading−unloading cycles). The stress−strain curve in Figure 6 shows that with increasing tensile strain, the stress in the sample increases, and
Figure 7. Phosphorescence lifetime microscopy images (256 × 256 pixels) of the relaxed and stretched (60%) HEPP film sensors at 21 kPa O2 and 23 °C (A) and corresponding distributions of phosphorescence lifetime signals (B). Scale bars on (A), 200 μm.
Figure 6. Stress−strain curves for the HEPP in air, 20 °C: loading (1 and 3) and unloading (2 and 4) curves for the original HEPP substrate and the derived sensor material impregnated with PtBP. 13590
DOI: 10.1021/acsami.7b00405 ACS Appl. Mater. Interfaces 2017, 9, 13587−13592
Research Article
ACS Applied Materials & Interfaces
based sensors overcome these limitations and possess new attractive features. With regard to practical applications, the results presented here illustrate that phosphorescent HEPP sensors provide a basis for a new type of “adaptive” materials for optical O2 sensing, sensitivity of which can be tuned by mechanical means. The current range of tuning corresponds to about 2−3-fold change in Kq, but we believe that this can be extended further, for example, with other polymeric substrates or sensor fabrication procedures. Such studies are outside the scope of this article. In addition, phosphorescent HEPP films can be used as simple strain or deformation sensors, which operate in the medium elongation/shrinking range (currently 1−60%). Their operation principle would be similar to that of the O2 sensors and it allows calibration-free, contactless sensing by phosphorescence lifetime measurements on commercial handheld instruments such as Optech. Even using a rather crude mechanical setup (manual stretching in the hand-operating clamp), HEPP films were able to reliably sense elongations of about 2% at 27 °C and
