Luminescent oxygen sensors with highly improved sensitivity based on a porous sensing film with increased oxygen accessibility and photoluminescence

Accepted Manuscript Title: Luminescent Oxygen Sensors with Highly Improved Sensitivity based on a Porous Sensing Film with Increased Oxygen Accessibility and Photoluminescence Authors: Soyeon Lee, Jin-Woo Park PII: DOI: Reference:

S0925-4005(17)30721-9 http://dx.doi.org/doi:10.1016/j.snb.2017.04.112 SNB 22200

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

23-2-2017 15-4-2017 18-4-2017

Please cite this article as: Soyeon Lee, Jin-Woo Park, Luminescent Oxygen Sensors with Highly Improved Sensitivity based on a Porous Sensing Film with Increased Oxygen Accessibility and Photoluminescence, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.04.112 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Luminescent Oxygen Sensors with Highly Improved Sensitivity based on a Porous Sensing Film with Increased Oxygen Accessibility and Photoluminescence Soyeon Lee and Jin-Woo Park* Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea *Corresponding author’s contact: E-mail: [email protected], Phone: +82-221235834, Fax: +82221235834

Graphical abstarct

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HIGHLIGHTS: - Porous oxygen sensing films showed higher photoluminescence (PL) intensity and oxygen accessibility than solid films. - We quantitatively derive the oxygen accessibility from sensitivity saturation based on Langmuir-Hill adsorption. - Oxygen accessibility is more influential factor to sensitivity than PL. - Increasing the external surface area of the sensing films effectively improves its sensitivity.

ABSTRACT In this study, we present the effects of the morphological modification of an oxygen-sensing film on improving the sensitivity of luminescent oxygen sensors. Pores were made inside the volume and on the external surface of the oxygen-sensing film consisting of platinum(II) octaethylporphyrin (PtOEP) oxygen-sensitive dye embedded in a polystyrene (PS) polymer matrix. The size of the pores with diameters from 300 nm to 1 μm was controlled through the phase separation of the ternary system of PS, polyethylene glycol (PEG), and chloroform. Photoluminescence (PL) intensity and oxygen accessibility of the oxygen-sensing film were considered the main factors affecting the sensitivity of the sensors. PL intensity was analyzed through the diffused reflectance and absorbance of the oxygen-sensing film. Oxygen accessibility was analyzed based on the Langmuir-Hill absorption theory by considering the sensitivity saturation behaviors of the oxygen-sensing film above the excitation light source intensity of 1,000 cd/m2. The optimized porous-structured oxygen-sensing film showed 61% higher sensitivity than the solid oxygen-sensing film. According to the measurement results, the sensitivity enhancement in the porous sensing film was significantly more driven by the increase in oxygen accessible sites than the increase in PL intensity. Furthermore, the sensing

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film with pores only on its external surface and not inside its volume showed 72% enhanced sensitivity relative to the solid sensing film. Therefore, the external surface area of the sensing film affects the sensitivity of the oxygen-sensing film significantly more than the pores inside the volume of the sensing film because the external surface acts as an oxygen diffusion barrier that limits the amount of oxygen that can access the oxygen-sensitive dye embedded in the polymer matrix. Keywords: luminescent oxygen sensor, porous film, PtOEP, sensitivity, oxygen accessibility

1. Introduction Oxygen concentration is a key element in aerobic metabolism, which extends from tissue oxygenation [1], cell activity [2-4] and diseases [5-7] to the metabolic activity of various plants [8] and of seagrass biomass [9]. With the rapid growth of the human population, the metabolic activities of crops, seeds or other parts of plants have been a fundamental research area with regard to agriculture productivity [8, 10]. Thus, oxygen sensors for the adequate monitoring of plant tissues are required to have detectable range of 5% to 80% oxygen concentration and high oxygen sensitivity due to the high respiration rate of plants [8, 9]. The Clark electrode oxygen sensor, which is one type of electrochemical oxygen sensor, was the most extensively used measurement system in early works [8, 11]. Clark electrodes operate based on electrical current change in response to the oxygen reduction reaction [8, 11]. However, oxygen detection using the Clark electrode consumes oxygen during measurements, and is limited to a point analysis of the sample area and being unable to map out the oxygen distribution [12, 13]. In contrast, optical oxygen sensors [12-15] do not consume oxygen and can detect the oxygen concentration over a surface area, enabling accurate and real-time oxygen detection [12, 13]. Optical oxygen sensors determine oxygen concentration based on the variation of the light absorption or luminescence properties of their sensing probes 3

[12, 13]. Thus, optical oxygen sensors have been increasingly used [8, 9] as promising alternatives to electrochemical oxygen sensors for monitoring oxygen concentrations and understanding physiological processes in the development of plants [8, 13]. Among optical oxygen sensors, luminescent oxygen sensors based on the luminescence quenching mechanism of oxygen-sensitive probes (OSPs) can accurately detect oxygen and have been applied to various research fields [1, 13]. In addition to the real-time visualization of oxygen concentration [1, 2, 16], other physical quantities that are related to the metabolic activities of plant tissues [6, 8, 10, 16], such as pH, carbon dioxide and temperature, can be monitored using different kinds of OSPs [17]. The quenching of the triplet states for OSPs occurs by collisional quenching with the triplet oxygen species. As consequence, the transfer of energy induces the formation of singlet oxygen species, and reduction of the luminescence intensity and lifetime of the OSPs [12]. This luminescence quenching mechanism is described by the following Stern-Volmer (SV) equation:

I0 τ0 = = K SV [O2 ] +1 I τ

(1)

where I0 and I are the luminescence intensity of the OSPs in the presence and in the absence of oxygen, respectively. Similarly, τ0 and τ are the luminescence lifetimes of OSPs in the presence and in the absence of oxygen, respectively. KSV is the SV constant, and [O2] is the oxygen concentration [12]. Among the most substantially studied OSPs, platinum(II) octaethylporphyrin (PtOEP) has a relatively long phosphorescent lifetime and a high quantum yield. Furthermore, PtOEP is effectively quenched in the dynamic [O2] range when using the visible wavelength range as its excitation light source. PtOEP has its light absorption peak at 535 nm and emission peak at 645 nm [12]. In a portable photoluminescence (PL)-based oxygen sensor platform, sensing

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films with PtOEP immobilized within a polystyrene (PS) polymer matrix are effectively excited with a green light source, which can be various forms of light emitting diodes (LEDs) [18, 19]. The PL intensity of the sensing film is then measured using a photodiode (PD) [18, 19]. However, the oxygen sensors composed of PtOEP dye in PS polymer matrix do not show a linear behavior in their sensitivity over the entire [O2] range of 0 to 100%. Their deviation from the SV linearity is ascribed to the low light quenching efficiency of dye due to the heterogeneous microenvironment of the polymer matrices and instrumental detection limit. The effects of heterogeneous microenvironment surrounding the dye molecules can be properly described by the two-site model [20] where the two KSV are derived and each representing the OSP being quenchable and being not quenchable or quenched at very different rates. Furthermore, the PL intensity of the sensing film decreases in the high [O2] region and thus, disabling the accurate detection of the sensing signals [18, 19, 21]. Therefore, to realize the SV linearity over the entire [O2] range, both the light quenching efficiency of the OSPs and signalto-noise ratio (SNR) of the oxygen sensors should be improved. Many researchers have made efforts to overcome the low SNR [18, 19, 21-24] and quenching efficiency of oxygen sensing films [25-29]. The modification of the polymer matrix encapsulating the OSPs has been extensively studied in order to solve these two major challenges. Some researchers were embedding nanoparticles into the polymer matrix [21] or incorporating the dyes into porous films or membranes [24, 26] in order to scatter the incident light through the voids and large surface that resulted in the enhancement of the PL intensity and sensitivity of the sensing films. On the other hand, other researchers controlled the spacing between polymer chains [28] or the distance between fluorophore films [29] thereby increasing the permeability of polymer matrices and reducing the nonradiative deactivation of excited states of the dyes [28, 29].

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However, for simple and effective preparation of the sensing films in solution-based processing, the morphological modification of oxygen-sensing films without any addition of extra components or chemical modification is desirable for improving their performance. Using phase separation, Cui et al. [27] developed microporous oxygen-sensing films and enhanced their PL intensity and KSV values. The enhanced PL intensity was attributed to the lengthened optical pathway due to light scattering by voids within the microporous sensing films [27]. However, the detailed mechanisms of KSV enhancement by the porous structure are not yet understood. In this study, we analyzed in detail the mechanism of the KSV increase in porous oxygensensing films. The porous PtOEP/PS oxygen-sensing film was prepared and the pore size and density were finely tuned through the phase separation of the PS, polyethylene glycol (PEG), and chloroform ternary system [30-32]. The oxygen-sensing performance of the porous PtOEP/PS films was compared to that of solid PtOEP/PS films. We applied the new concept of sensitivity saturation to determine the oxygen accessibility of the sensing films. Sensitivity saturation is characterized by I0/I100 convergence (where I100 corresponds to PL intensity of the sensing film at 100% [O2]) over a certain range of the luminescence intensity of LED. Thus, the oxygen-sensing performances of the films were examined through the KSV value and sensitivity saturation value (Vmax) based on Langmuir-Hill saturation curve. The diffused reflectance and absorbance of the sensing films were measured to investigate the PL intensity variation. In addition, we investigated whether the external surface of the sensing film and the pore surfaces inside the volume of the sensing film have a significant effect on the KSV value by fabricating sensing films with varying thicknesses. Finally, we described an approach for enhancing the KSV value of luminescence oxygen sensors in the high [O2] region through various surface modifications of the sensing films.

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2. Experimental procedures 2.1. Materials and fabrication of sensing films The luminescent oxygen-sensing films were prepared using PtOEP (98%) as the OSP, PS (Mw~192,000) as the polymer matrix and chloroform (≥ 99.5%) as the solvent. The solid sensing film listed as PS in Table 1 was prepared by mixing PtOEP and PS with a PtOEP-PS weight ratio of 1:40. Two types of porous sensing films were prepared by adding PEG (Mw~1,000 from Alfa Aesar, USA) was added into the two different mixtures of PtOEP and PS to obtain 1:32:8 and 1:36:4 PtOEP-PS-PEG weight ratios, which yielded the P4 and P9 sensing films in Table 1, respectively. Prior to casting the sensing films onto a glass mold, the polymer blends were dissolved in chloroform to obtain solutions with a concentration of 2.68 weight percent (wt%). The sample descriptions of the PS, P4 and P9 oxygen-sensing films are summarized in Table 1. The thickness of the PS and P9 sensing films was varied by controlling the amount of 1.36 wt% polymer blend solutions dropped onto a glass mold (Table 1). Films prepared with a higher PEG weight ratio within the polymer blend than that of P4 were unstable, forming cracks as the pores coalesced. In contrast, films prepared with a lower PEG weight ratio than that of P9 showed relatively small pores with diameters of less than 100 nm. Therefore, the P4 and P9 porous sensing films were the only stable sensing films, which is consistent with previous reports [18, 33].

A schematic description of the casting of the oxygen-sensing films is shown in Fig. 1. The prepared polymer blend solutions described in Table 1 were dispensed onto a 15 mm × 15 mm square glass mold under a pure nitrogen gas atmosphere inside a glove box. The sensing

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films were then dried and stabilized inside the nitrogen gas-filled glove box for 10 min and 1 h, respectively. The sensing films were then placed inside a fume hood under ambient air conditions for an additional 12 h. After the previous step, the sensing films were immersed in a deionized (DI) water bath to rinse the remaining PEG from the sensing films. The DI water immersion did not affect any elements other than PEG, as determined in previous reports [27, 30-32].

2.2. Instruments Atomic force microscopy (AFM, performed on a Nanowizard I from JPK Instrument, Germany) was used in intermittent height measurement mode to observe the pore structure and calculate the top surface area of the sensing films. Field-emission scanning electron microscopy (FE-SEM, performed on a JSM-7001F from JEOL Ltd., Japan) was used to analyze the thickness and morphology of the sensing films. The PL intensity was measured using a digital storage oscilloscope (DSO7054B with 500 MHz from Agilent Technologies, USA) with a femtosecond regenerative amplifier laser (Libra, peak wavelength (λpeak) at 400 nm from Coherent, USA) as the excitation light source. The total reflectance and absorbance spectra of each sensing film were measured with a UV-Vis-NIR spectrophotometer (Cary 5000, Agilent, USA) equipped with an integrating sphere. 2.3. Gaseous-oxygen-sensing configuration The optical sensor system used in this study contained three major groups of components: the gas injection components, sensing elements and data acquisition tools, as shown in Fig. 2 and Fig. 3. All sensing elements, which included the PD (DET100A/M from

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Thorlabs, USA), long-pass filter (FBH650-40, λpeak at 650 nm and full width at half maximum (FWHM) of 40 nm from Thorlabs, USA), LED (assembled in-house, λpeak at 520 nm and FWHM of 55 nm) and the sensing film, were set inside an inner chamber enclosed by a larger outer chamber (a black acrylic box assembled in-house). The outer chamber protected the inner chamber from stray light and other ambient atmospheric factors (Fig. 2 and Fig. 3a). The oxygen concentration inside the inner chamber was controlled by oxygen and nitrogen mass flow controllers (MFCs, MPR 3000S from Korea Instrument T&S, Korea) that directly connected to the inner chamber. The inner chamber shown in Fig. 3b physically supported the sensing elements in place and helped maintain the desired gas environment conditions. The long-pass filter mounted on the PD was used to block all light other than the PL coming from the sensing films (Fig. 3c). The PD equipped with the long-pass filter was mounted on the cover of the inner chamber (Fig. 3b). As shown in Fig. 3d, the PL from the sensing film was detected through the hole on the inner chamber case that was set at the center between the four LEDs. The 15 mm × 15 mm square sensing film was glued at its edges to a 25 mm × 25 mm square glass substrate and was attached to the LEDs using a spacer (Fig. 3e). The detected PL was amplified by a PD amplifier (PDA200C from Thorlabs, USA) for its precise detection and conversion to a photocurrent signal. An electrical source-meter (Keithley 2400, USA) was used to apply the necessary voltage to the LED.

3. Results and discussion 3.1. Morphology of porous sensing films The surfaces and cross-sections of the films (PS, P4 and P9 in Table 1) analyzed using AFM and FE-SEM, respectively, are shown in Fig. 4. As shown in Fig. 4 and Table 2, the P4

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and P9 films both showed a porous structure, with P4 having the larger average pore diameter of 1,082.0 μm compared to 536.7 μm for P9. Similarly, the average pore height in P4 of 259.3 nm was higher than the pore height of 142.9 nm in P9. These measurements were obtained from the cross-sectional height profile using AFM (Fig. S1 and S2 in the supplementary information). In contrast, P9 had the higher average pore density of 1.2/μm2 compared to 0.30/μm2 for P4. The distribution of the pores on the top surface is shown in Fig. S3 in the supplementary information. No pores were detected in the PS film. In solvent-induced phase separations, miscible polymer blends with a smaller fraction of PEG have more delayed onsets of phase separation relative to those of blends with a larger fraction of PEG [31, 32]. A delayed onset of phase separation in miscible polymer blends results in an increase in the viscosity of the solution and a reduction in the pore size in the resulting polymer blend films [31, 32]. As a consequence, polymer blend films with smaller pores and higher pore density can be obtained by changing the mass balance of PEG [32]. Therefore, the trends in pore size and distribution of P4 and P9 were in good agreement with previous studies [31, 32]. However, due to the fast evaporation rate of chloroform from the thinner films (with film thicknesses in the range of 4 to 20 μm) prepared in our study, the P4 and P9 films had smaller pores and a higher density of pores relative to thicker films (40 to 80 μm in film thickness) prepared by other groups [30, 32].

As shown in Fig. 4 and Table 2, both P4 and P9 were thinner than the PS. However, the 3 μm difference in thickness between the PS and the two porous films (P4 and P9) was negligible, allowing valid comparisons of their sensitivity. The size and density of the pores inside the volume of P4 were similar to those of P9, while the pores on the top surface of P4 and P9 were clearly different, as shown in Fig. 4. The similar pore structures inside the volume

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of P4 and P9 were attributed to the mold casting process during which the solvent starts to evaporate from the top surface. Furthermore, as the chloroform rapidly evaporates, the porous structure of the films are mostly determined during the initiation of the solvent evaporation process [30]. Consequently, the distinct porous structures on the top surfaces of the films (P4 and P9) were formed in a manner determined by the polymer composition. The number of pores was multiplied by the average surface area of the individual pore to calculate the top surface area of the films. The individual pore cross-sectional height profile was obtained using AFM and was fitted with the parabolic equation presented in Fig. S2 in the supplementary information. Consequently, the average top surface areas of the P4 and P9 films were 3.96% (233.92 μm2) and 6.69% (240.06 μm2) larger, respectively, than that of the PS film (225 μm2). Therefore, we successfully prepared three different types of sensing films: one without pores and two with pores but having different external porous structures and top surface areas.

3.2. Sensitivity to gaseous oxygen The gaseous oxygen-sensing performance of each sensing film sample was examined in relation to its photocurrent and SV plots. As shown in Fig. 5, the photocurrent was measured under various levels of [O2]. For the three sensing films in Table 1, the photocurrent increased with a decrease in [O2], but P4 and P9 showed significantly higher photocurrents relative to that of PS. This result indicates that the additional surface area of the pores in both P4 and P9 enhanced their photocurrent signal relative to that of PS. The photocurrents of PS, P4 and P9 at 10% [O2] exposure were 1.6 μA, 3.16 μA and 3.85 μA, respectively; at 70% [O2] exposure, they were 0.61 μA, 1.09 μA and 1.13 μA, respectively. The photocurrent of P9 was slightly higher than that of P4, which is attributed to the different optical properties between the two porous sensing films, including the PL intensity, in relation to their porous structures.

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Furthermore, the response time (insets in Fig. 5) of PS, P4 and P9 was 113, 97 and 93s, respectively, when the [O2] was varied continuously from 100% down to 10% and back up again to 100%. The faster response time of P4 and P9 in comparison with PS might be due to the porous structures both on their external surfaces and inside their volume. However, there was no significant difference between the response time of P4 and P9.

In Fig. 6, the linear sensing properties of each sample are surveyed using the photocurrent under various [O2] of 0, 2.5, 5, 8, 10, 15, and 21% as well as 10% intervals from 30 to 100%. For all three sensing films, a larger KSV value in the [O2] range of 0 to 15% (denoted as Range I) and a smaller KSV value in the [O2] range of 15 to 100% (denoted as Range II) were observed. The linear sensing properties of each sample were fitted separately in the two ranges below (Range I) and above 15% [O2] (Range II). As shown in Table S1 in the supplementary information, no significant difference in KSV was observed between PS, P4 and P9 in Range I. In contrast, both P4 and P9 had larger KSV values of 0.129 and 0.153, respectively, than the 0.095 value for PS in Range II. The SV plots of the sensing films were linearly fitted in Range II with R2 values of 0.9808, 0.9920 and 0.9921 for PS, P4 and P9, respectively.

As shown in Table 3, the KSV values in Range II increased in order of PS, P4 and P9, and the KSV improvements in P4 and P9 were represented as the relative change of KSV, i.e., (KSV - KSVPS) / KSVPS  100 = Δ KSV / KSVPS (%), where KSV is the SV constant of either P4 or P9 and KSVPS is the SV constant of PS. The ΔKSV / KSVPS values of P4 and P9 were 35.89% and 60.75%, respectively. Some of the previous reports [24, 34, 35] on the porous sensing films obtained two to five times enhanced sensitivity with the variation of the pore size ranging from

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several hundred nanometers up to few micrometers, which had similar optimized pore size obtained in this study. Their higher sensing improvements were mainly due to the three dimensionally micro-structured membrane support [24, 34] or nanoparticle with some additional chemical treatment [35, 36] in order to enlarge the surface effect on sensitivity. However, in the case of sensing films formed using phase separation processes, we achieved a larger KSV improvement relative to the results published in earlier reports [19, 27] when the sensing performance of porous and solid films were compared. In the earlier reports, they had sensing films with pore size of 1 to 4 μm, while we obtained that of 300 nm to 1 μm, which resulted in the increase in pore density and total surface area that further enhanced the KSV values. The difference in pore distribution was attributed primarily to the different solvents used for each study, since the chloroform solvent used in this study evaporates eight times faster than the toluene solvent[30] used by the other group[19]. Interestingly, the KSV improvement was achieved more effectively with P9 than with P4 despite the similar pore morphology inside their film volume. Therefore, the increase in the external surface area from the pores was considered the main driving force inducing the KSV improvement through increased PL and oxygen accessibility. Furthermore, the KSV difference between Range I and Range II was represented as the relative change of KSV, i.e., (KSV2 - KSV1) / KSV1  100 = Δ KSV / KSV1(%), where KSV1 is the SV constant in Range I and KSV2 is the SV constant in Range II (Table S1 in the supplementary information). Because P9 exhibited the highest KSV improvement in Range II among the three samples, this sample’s preparation method and its sensing properties suggest a new approach towards achieving linear detection in sensing films over the entire [O2] range.

3.3. Optical properties of sensing films

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The PL intensity of the sensing films was investigated as one of the main factors that contributed to KSV improvement of the samples. PL intensity is generally influenced by the optical absorption efficiency and quantum yield of the dyes. The optical absorption efficiency is related to the amount of incident light that is absorbed by the sensing film and excites the dyes embedded inside the sensing film, while the quantum yield is related to the intrinsic photodynamic of a dye. The diffused reflectance and absorbance of the samples were measured to examine their optical absorption efficiency. The total amount of incident light penetrating the sensing films was determined from the diffused reflectance, which represents the total amount of reflected light at all angles from the film [37]. As shown in Fig. 7, the normalized diffused reflectance decreased in the order of PS, P9 and P4, while the normalized absorbance increased in order of PS, P9 and P4 within the 300 to 750 nm wavelength range of the light spectrum. The porous P4 and P9 films had much less reflectance and much more absorbance than the solid PS film. The light was scattered or trapped inside the porous films. Consequently, the scattered or trapped light effectively induces the excitation of the dyes embedded in the sensing films, which results in the enhanced PL intensity of the sensing films.

Between the two porous films, P4 had a slightly lower reflectance and a slightly higher absorbance than P9 between the wavelengths of 400 and 500 nm in the light spectrum. These differences in reflectance and absorbance seem to be caused by the higher pore density of P9 relative to that of P4. The edges of the pores on the top surface of the sensing films reflected portions of the incident light, thereby reducing the amount of light that penetrated the volume of the sensing films. However, this higher reflectance of P9 from 400 to 500 nm wavelength region in the spectrum may not significantly influence the sensing performance of the films

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since PtOEP effectively absorbs light in the 500 to 550 nm wavelength range than the any other wavelength region in the spectrum. As shown in Fig. 8, P4 and P9 showed a 4.3- and 2.9-fold higher PL intensity, respectively, than PS under ambient air conditions, as expected from the previous analysis of reflectance and absorbance (shown in Fig. 7a and 7b, respectively). Clearly, the porous P4 and P9 films both showed a higher PL intensity than PS since they absorbed the incident light more effectively than PS. On the other hand, the P9 has higher density of smaller pores while P4 had lower density of bigger pores, therefore, the P9 absorbed less number of photons than P4 from the incident light due to the reflection from densely distributed pore edges on its external surface. As consequence, the PL intensity of P4 was 1.5-fold higher than that of P9 under ambient air conditions. Consequently, the higher light absorption efficiency of P4 and P9 relative to that of PS gave rise to more dye excitation and an improvement in the SNR. However, as shown in Table 3, the KSV improvement of P9 was higher than that of P4 when they were compared to PS. Thus, as another contributing factor to the KSV improvement, the effects of the oxygen accessibility on the sensing performance of P4 and P9 were investigated to compare the extent of their influence on the KSV improvement to that of the PL intensity.

3.4. Oxygen-quenching efficiency of sensing films based on sensitivity saturation behaviors The varying KSV values originating from the different PL capacities among the sensing films must be reduced to compare the oxygen accessibility between samples. Thus, the luminescence intensity of the LED was gradually increased to raise the amount of dyes in their excited states. In this manner, the sensing films would have the appropriate amount of dyes in their excited states to participate in effective quenching and exceed the minimum SNR for

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accurate signal detection. Therefore, the ideal sensing performances of the sensing films were expected to be obtained when using a certain luminescence intensity of the light source. As shown in Fig. 9, the photocurrent increased at regular intervals as the luminescent intensity of the LED was increased incrementally from 40 to 3500 cd/m2 by increasing the DC voltage applied to the LED at 0.05-V intervals. The photocurrent ratio between 0% [O2] and 100% [O2] (I0/I100, sensitivity) in relation to the luminescent intensity of LED was then obtained and is presented in Fig. 10. As shown in Fig. 10, all the sensitivity saturation curves agreed well with the LangmuirHill equation. The Langmuir-Hill equation couples the results of the Langmuir gas adsorption theory and the Hill equation [38, 39]. The Hill equation was originally derived to explain the binding interaction between a hemoglobin molecule and an oxygen molecule as the quantity of the bound complex of the two molecules increases [38]. The Langmuir-Hill equation has the form of [38] y = Vmax x n/ (k n+ x n)

(2)

where y is the concentration of the ligand-receptor complex, Vmax is the total saturated receptor concentration, x is the concentration of the free ligands, k is the concentration of the free ligands when the receptor concentration is half its saturation value, and n is the Hill coefficient, which indicates the degree of interaction between ligand binding sites [38, 39]. The interaction between the oxygen and dye within the sensing film can be considered similar to that between the ligand and receptor. The photophysical process [40] between the oxygen and dye occurs when the oxygen molecule is adsorbed on the surface of excited dye and can be described by the following equation [26]: 3

hv A* + 3 O2 ( 3Σ)   3 A + 1 O2 ( 1Δ )

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(3)

Where 3 A* and 3 A are the excited and ground triplet states of PtOEP dye, respectively, 3O2 (3 Σ) and 1O2 (1Δ) are the triplet and low-lying singlet states of the oxygen molecules, respectively, h is the Planck constant, and v is the light frequency. The population of 3 A* within the sensing films is linearly related to the luminescence intensity of the LED, while I0/I100 represents the quenching efficiency. 3 A* and I0/I100 correspond to the x and y components of the LangmuirHill equation, respectively. Therefore, because the sensitivity saturation curve is in good agreement with the Langmuir-Hill equation, the oxygen accessibility can be derived from the relationship between 3 A* and I0/I100 in the photophysical process between the oxygen and dyes in the sensing films. As shown in Fig. 10, the sensitivity of PS, P4 and P9 steeply increased when the luminescence intensity of LED was less than 1,000 cd/m2. This strong dependency of sensitivity on the luminescent intensity of the LED was due to the small SNR and insufficient amount of dyes in their excited states. In contrast, when the value of the light source luminescence was greater than 1,000 cd/m2, the sensitivity of the sensing films showed no dependence on the light source intensity since the PL of the films exceeded the minimum SNR and there was a sufficient amount of excited dyes that could interact with the oxygen molecules. A possible determining factor of the sensitivity saturation value (Vmax) is the oxygen molecule accessibility limit, which in turn can depend strongly on the surface area of the sensing films. Consequently, Vmax can represent the quenching efficiency of the sensing films. In Table 3, P9 showed the highest Vmax among all the samples, indicating that the highest oxygen accessibility was achieved in P9, followed by P4. Thus, the dyes were less accessible to the oxygen molecules in PS than in they were in the porous P9 and P4 films. The relatively high oxygen accessibility of P9 was attributed to its larger total surface area, which arose from the pores on its external surface and inside its volume. Furthermore, as shown in Table 3, the Vmax improvements in P4 and P9 were smaller than the KSV improvements in Range II. However,

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there were similar trends in both the KSV and Vmax improvements when P4 and P9 were compared with PS. Therefore, a large portion of the KSV improvements is attributed to the Vmax improvements rather than to the improvements of the PL since the values of both KSV and Vmax indicated that the sensing performance of P9 was superior to that of P4 despite P9 having a lower PL than P4. In other words, the contribution of oxygen accessibility outweighs the contribution of PL to the KSV improvements, indicating the importance of oxygen accessibility from the top surface of the sensing films. 3.5. External surface effects on KSV 3.5.1. Morphology of the sensing films A comparison of the Vmax and KSV values of P4 and P9 (presented in Table 3 and discussed in Section 3.4) indicates that the top surface of the porous-structured sensing films played a significant role in the KSV improvement. The growth of the pores inside the volume of the sensing films was suppressed by reducing the thickness of the sensing films to verify the significant role played by the external porous structure on the sensing film surface in enhancing KSV. To achieve this objective, two thickness variants of PS and P9 were prepared (denoted as PSUT, PST, P9UT and P9T in Table 1). As shown in Table 1, the thinner sensing films were fabricated by changing the solution concentration and reducing the volume of the drop-casted solution.

As shown in Fig. 11, the surfaces and cross-sections of the thinner versions of P9 (P9UT and P9T) were analyzed using AFM and FE-SEM, respectively. As presented in Table 4, P9UT had a smaller average pore diameter of 352.17 nm compared to 470.1 nm for P9T. However, P9UT had a higher average pore density of 1.32 /μm2 compared to 0.27 /μm2 for P9T. The

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distribution of the pores is shown in Fig. S4 in the supplementary information. As shown in Fig. S5 in the supplementary information, no pores were observed in either PSUT or PST. As shown in Fig. 11, P9UT and P9T had different porous structures on their top surfaces despite having the same polymer composition. These results may be due to the different volumes of the solutions used in preparing each sample, which thereby induced different solvent evaporation rates and convection [41] and different polymer-solvent interactions [42]. The smaller solution volume used in P9UT than in P9T resulted in a more rapid evaporation of the solvent, thereby generating smaller pores with a higher density on the surface of P9UT than on the surface of P9T. The variation in the formation of the pores on the surface of P9UT and P9T was in good agreement with PS-PEG phase separation results reported by other groups [30-32].

3.5.2. Sensitivity and saturation behavior The effects of the pores on the external surface of the sensing films on the sensitivity were compared to that of films with similar thicknesses. The sensing performance of P9UT was compared to that of PSUT; similarly, P9T was compared to PST. Fig. 12 and Fig. 13 show the SV plots and sensitivity saturation curves, respectively, of the P9UT, P9T, PSUT and PST sensing films. As shown in Fig. 12 and Table S3, the Δ KSV / KSV1 (%) value of each sample was compared by the same processes that was used in Table S1, and the P9UT and P9T (P9type samples) had a smaller difference between KSV1 and KSV2 than PSUT and PST (PS-type samples), respectively. As consequences, the P9-type samples achieved higher KSV improvement in Range II than the PS-type samples, and thus the P9-type samples could potentially achieve linear detection over the entire [O2] range.

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As shown in Table 5, the ΔKSV / KSVPS value in Range II was 73.3% when the sensing properties of P9UT were compared to those of PSUT. However, the ΔKSV / KSVPS value in Range II was smaller at 28.0% for the P9T and PST sensing properties comparison. To analyze the KSV improvement of P9UT and P9T with respect to PSUT and PST, respectively, the Δ KSV / KSVPS value should be compared to the previous enhancement in Table S1, where Δ KSV / KSVPS was 60.75% for P9 and 35.89% for P4 (discussed in Section 3.2). The ΔKSV / KSVPS value of 72.3% for P9UT (with respect to PSUT) was larger than the 60.75% value for P9 (with respect to PS). This variation in ΔKSV / KSVPS is attributed to the slightly smaller pores and higher pore density on the top surface of P9UT than on the top surface of P9, resulting in a slightly larger surface area. Similarly, the lower ΔKSV / KSVPS value of 28.0% for P9T (with respect to PST) compared to the 35.89% value of P4 (with respect to PS) is attributed to the smaller pore size and lower pore density on the top surface of P9T than on the top surface of P4, corresponding to a slightly smaller area of the top surface of P9T than on the top surface of P4. Therefore, the larger ΔKSV / KSVPS value for P9UT than that for P9T is due to its larger external surface area.

As shown in Fig. 13, P9UT had a larger Vmax value of 16.31 than the 12.22 value for PSUT; similarly, the Vmax value of 19.30 of P9T was larger than the value of 16.10 of PST. Table 5 indicates that P9-type samples had higher Vmax values than PS-type samples, and the Vmax improvements in both P9UT and P9T had similar trends as those observed with the KSV improvements in Range II. However, the comparison of the thickness dependence of the

20

sensitivity between the P9-type samples and PS-type samples may provide a clearer picture of the effects of the external surface area on KSV improvements. As the thickness of the films increases, the amount of dye inside the volume of the films was also expected to increase, resulting in improvements in the sensitivity of the films. As shown in Table 6, the thickness dependence of the sensing performance of the films was examined through the KSV and Vmax enhancements within the PS-type samples and P9-type samples. The improvement of sensing performance was represented as the relative change of KSV and Vmax, i.e., ΔKSV / KSVUT = (KSVT- KSVUT) / KSVUT and ΔVmax/VmaxUT = (VmaxT- VmaxUT) / VmaxUT. The KSVT and VmaxT correspond to the KSV and Vmax values of the P9T or PST (thin films), while KSVUT and VmaxUT correspond to the KSV and Vmax of the P9UT or PSUT (ultrathin films). The variations in KSV and Vmax in relation to the thickness of the films are shown in Fig. 13. ΔKSV / KSVUT and ΔVmax/VmaxUT were much larger for the PSUT and PST comparison than for the P9UT and P9T comparison. The higher thickness dependence of the PS-type samples than that of the P9-type samples indicates that the P9-type samples were less influenced by the amount of dye within its volume due to the presence of the external porous structure on their surfaces.

4. Conclusions We presented a method on how to analyze the sensitivity enhancement caused by morphological modifications of sensing films in terms of PL intensity and oxygen accessibility. Varying the polymer compositions of a PtOEP, PS and PEG ternary blend resulted in two different types of porous-structured sensing films (P4 and P9 films). The surface areas of P4 and P9 were 3.96% and 6.7% larger, respectively, than that of the solid PS film. The degree of light trapping was improved in the porous films, which resulted in 2.9- and 4.3-fold increases in the PL intensity from P9 and P4, respectively, relative to the PL

21

intensity from PS. The lower PL intensity of P9 than that of P4 was attributed to the higher reflection of incident light on its surface, thereby reducing the amount of light penetrating the film. The oxygen accessibility determined through the Vmax value was 5.2% and 20% higher in P4 and P9, respectively, than in the solid PS films. This increase in oxygen accessibility was proportional to the external surface area variation in the samples. Consequently, the porous sensing films of P4 and P9 achieved a 35% and 61.71% higher KSV, respectively, relative to that of PS. Therefore, the higher KSV improvement in P9 despite its lower PL intensity than P4 indicates that the superior oxygen accessibility of P9 to P4 outweighs the effects of PL intensity on the KSV improvement. Furthermore, the sensing films with pores present only on their surface and not within their volume still showed a 72% higher sensitivity than the solid films. Thus, the porous structures on the external surface of the sensing films had a more significant impact on the sensing performance than the porous structures inside the volume of the sensing films, possibly because the exterior surface acts as a diffusion barrier for oxygen molecules entering the volume of the sensing films.

Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (grant number 2015R1D1A1A01061340) and by the Joint Program for Samsung Electronics-Yonsei University.

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Author Biographies

Ms. Soyeon Lee 2016.3 ~ present

Graduate student, M.S degree candidate Dep. of Materials Science and Engineering Yonsei University, Korea

2012.3 ~ 2016.2

YONSEI UNIVERSITY B.S. in Materials Science and Engineering

Prof. Jin-Woo Park Experience 2007.3 ~ present

Assistant/Associate Professor Dep. of Materials Science and Engineering 27

Yonsei University, Korea 2003.10 – 2007.2

Principal Engineer Fundamental Technology Center CORPORATE R&D INSTITUTE, SAMSUNG ELECTROMECHANICS, Korea

2002.3 – 2003.8

Research Assistant Professor Dep. of Materials Science and Center for Materials Processing UNIVERSITY OF TENNESSEE (UT), USA

2002.3 – 2003.8

Postdoctoral Research Associate Joining Group, Metals and Ceramics Division OAK RIDGE NATIONAL LABORATORY (ORNL), USA

Education 1997.2 – 2002.3

1992.3 – 1996.2

MASSACHUSETTS INSTITUTE OF TECHNOLOGY (MIT) Ph.D. in Materials Engineering Department of Materials Science and Engineering Advisor: Professor Thomas W. Eagar YONSEI UNIVERSITY B.S. in Metallurgical Engineering

28

Fig. 1. (a) Schematic of the procedure of the fabrication of the sensing film from the PtOEP, PS, and PEG polymer blend-dye system. (b) Schematic of the phase separation process.

Fig. 2. Schematic diagram of the sensor system.

29

Fig. 3. Photographs showing the (a) configuration of the sensor system instrument, (b) top view of the inner chamber, and (c) schematic of the arrangement of the sensing components. Detailed photographs of the (d) PD equipped with the long-pass filter and (e) bottom view of the inner chamber cover without and (f) with the sensing film on the spacer.

30

Fig. 4. AFM surface images (left column) and their corresponding FE-SEM cross-sectional images (right column) of (a) PS, (b) P4 and (c) P9.

31

Fig. 5. Photocurrent versus time graphs of (a) PS, (b) P4 and (c) P9 under various levels of [O2]. The insets in the graphs indicate the responses of the films when the [O2] was successively varied from 100% down to 10% and back up again to 100%.

32

Fig. 6. Linearly fitted SV plots of (a) PS, (b) P4 and (c) P9 under an [O2] of 0-15% (Range I) and 15-100% (Range II). The KSV1 and KSV2 values within the graphs represent the KSV in Range I and Range II, respectively.

Fig. 7. (a) The diffused reflectance and (b) diffused absorbance of PS, P4 and P9, as measured with an integrating sphere.

33

PL intensity (a.u.)

10 P4 P9 PS

8 6 4 2 0 0.0

20.0

40.0

60.0

Time (s)

80.0

Fig. 8. PL intensity decay curve (with n = 3) of PS, P4 and P9.

34

Fig. 9. Photocurrent versus response time of (a) PS, (b) P4 and (c) P9 at 0% [O2] and 100% [O2] exposure. The luminescence intensity of LED was gradually increased by raising the applied voltage at 0.05-V intervals.

Fig. 10. The sensitivity saturation responses of (a) PS, (b) P4 and (c) P9 upon increasing the luminescence intensity of the LED.

35

Fig. 11. AFM top surface images (left column) and their corresponding FE-SEM crosssectional images (right column) of (a) P9UT and (b) P9T.

36

Fig. 12. Linearly fitted SV plots of (a) PSUT, (b) P9UT, (c) PST and (d) P9T over an [O2] range from 0 to 100%; The KSV1 and KSV2 values within the graphs represent the KSV in Range I and Range II, respectively.

Fig. 13. Sensitivity saturation behavior of (a) PSUT and P9UT and (b) PST and P9T.

37

0.18

22

KSV P9-type KSV PS-type

0.16

KSV

0.14

18

0.12

16

Vmax

20

0.10

14 Vmax P9-type

0.08

Vmax PS-type

12

0.06 4

6

8

10

12

Thickness (m)

14

16

Fig. 14. The dependence of KSV and Vmax on the thickness of porous films (P9UT and P9T) and solid films (PSUT and PST).

Table 1 Sample descriptions of the sensing films. Sample

PS:PEG ratio

Polymer concentration (wt%)

Volume of solution (μL)

PS

-

2.68

200

P4

4:1

2.68

200

P9

9:1

2.68

200

PSUT

-

1.36

100

PST

-

1.36

200

P9UT

9:1

1.36

100

P9T

9:1

1.36

200

Table 2 Physical dimension and pore specifications of the solid and porous films.

Sample

Thickness (μm)

Pore diameter (nm)

Pore height Pore density Surface area (nm)

38

(/μm2)

(μm2)

PS

18.39± 0.065 -

-

-

P4

14.64 ± 0.4

1082.0±183.3 259.3± 23.47 0.30 ± 0.016

233.92 ± 7.01

P9

15.21 ± 0.25

536.7 ± 40.7

240.06±13.89

142.9 ± 47.3

225.0

1.2 ± 0.32

Table 3 Comparisons of KSV in Range II and Vmax of PS, P4 and P9, and Δ KSV / KSVPS (%) and Δ Vmax / VmaxPS (%) of P4 and P9 with respect to PS. Δ KSV / KSVPS

Δ Vmax / VmaxPS

Sample

KSV

PS

0.095

-

16.56

-

P4

0.129

35.89

17.42

5.19

P9

0.153

60.75

19.91

20.23

(%)

Vmax

(%)

Table 4 Physical dimensions and pore specifications on the top surface region of the solid and porous films.

Sample

Thickness (μm)

PSUT

Pore diameter Pore

height Pore

density

(nm)

(nm)

(/μm2)

6.41 ± 0.95

-

-

-

PST

11.30 ± 1.50

-

-

-

P9UT

4.54 ± 0.87

352.17 ± 28.44

138.58 ± 5.88

1.32 ± 0.06

P9T

12.6 ± 2.28

470.1 ± 14.88

159.77 ± 0.82

0.27 ± 0.0093

39

Table 5 Comparisons of the KSV values in Range II and Vmax of PSUT, P9UT, PST and P9T, and Δ KSV / KSVPS (%) and Δ Vmax / VmaxPS (%) of the P9-type samples with respect to the PStype samples. Δ KSV / KSVPS

Δ Vmax / VmaxPS

Sample

KSV

PSUT

0.074

-

12.22

-

P9UT

0.127

72.33

16.31

33.47

PST

0.127

-

16.10

-

P9T

0.164

29.36

19.3

19.88

(%)

Vmax

(%)

Table 6 Summary of the variations in the KSV and Vmax values of PSUT, PST, P9UT and P9T. Sample

KSV

Δ KSV / KSVUT (%)

Vmax

Δ Vmax / VmaxUT (%)

PSUT

0.074

-

12.22

-

PST

0.127

71.51

16.10

31.75

P9UT

0.127

-

16.31

-

P9T

0.164

28.74

19.30

18.33

40