A highly sensitive and fast-responding oxygen sensor based on POSS-containing hybrid copolymer films

Yongyun Mao ab, Qian Zhao b, Jianchang Wu b, Tingting Pan b, Bingpu Zhou *a and Yanqing Tian *b
aInstitute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau, China. E-mail: bpzhou@umac.mo
bDepartment of Materials Science and Engineering, Southern University of Science and Technology, No. 1088, Xueyuan Rd, Xili, Nanshan District, Shenzhen, Guangdong 518055, China. E-mail: tianyq@sustc.edu.cn

Received 10th August 2017 , Accepted 12th October 2017

First published on 19th October 2017


Organic–inorganic hybrid 3-(trimethoxysily)propylmethacrylate-co-platinum porphyrin-co-methacrylolsobutyl-polyhedral oligomeric silsesquioxane (TPMA-PtTPP-POSS) copolymer films were synthesized and applied as high-performance oxygen sensors. High sensitivity and fast response characteristics originate from the POSS-containing copolymer composition and wormlike structures. The wormlike structures composed of the POSS-containing copolymers self-assembled on the film surface can ensure the homogeneous dispersion of PtTPP and can be perturbed by a trace oxygen environment. Compared with the state-of-the-art PtTPP-TPMA film, the TPMA-PtTPP-POSS copolymer sensors exhibit a swift response (approx. 0.6 s), long-term stability, higher sensitivity (increased by 4-fold), and a relatively large dynamic range, etc. We believe that the POSS-containing hybrid films should provide a new strategy for designing sensitive and fast responding optical oxygen sensors.


Introduction

Oxygen is one of the most popular and important analytes on the earth and almost all living organisms utilize oxygen for energy generation and respiration.1,2 Therefore, highly sensitive and ultrafast response oxygen sensors are required to follow rapid changes in the oxygen partial pressure (pO2) and oxygen concentration in both gas and liquid phases. Subsequently, optical oxygen sensors based on the quenching of the fluorescence of oxygen-sensitive probes (OSPs) have been proved to be versatile and convenient tools for oxygen determination.3,4 Up to now, many oxygen sensors have been developed with significantly different levels of sensitivity or response times based on various polymeric matrix materials, such as silicone rubbers, polydimethysiloxane (PDMS), polystyrene, co-polymers, hydrogels, ormosil and sol–gels, etc.5–10 As we all know, the polymer matrix plays an important role in defining the sensitivity of sensors as well as their response characteristics.11 Among most polymer materials, the larger pendant groups can efficiently prevent the polymer chains from packing together, resulting in higher oxygen solubility and higher diffusion coefficients.12 Therefore, a polymer matrix with larger pendant groups has been confirmed to be a critical criterion to regulate the sensor sensitivity. For example, Chen et al. reported three acrylate polymers: poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and poly(propyl methacrylate) (PPMA) as matrix materials to immobilise a platinum(II) complex.12 The results revealed that oxygen sensors constructed with a PPMA based luminophore matrix provided higher sensitivity and faster response times than those made with either PMMA or PEMA matrices. Additionally, some ormosils are also better than silica as a matrix for oxygen sensing, as the presence of organic side groups can effectively increase the oxygen solubility of the matrix as well as its permeability.13,14

Polyhedral oligomeric silsesquioxane (POSS) is the smallest silica nanoparticle with the general formula ((RSi)1.5)8, which contains a polyhedron silicon–oxygen cube skeleton with intermittent siloxane linkages and tuneable organic groups at the silicon atoms.15,16 More recently, POSS has attracted a great deal of attention because fine control of the polymer morphology on the nanoscale gives great opportunities for designing new materials, including films, with improved performance.17 It has been particularly reported that the presence of bulky POSS cages should not only enhance the fluorescence quantum yields, but also endow novel applications in optoelectronic devices, biological fluorescence labelling and imaging.18,19 Therefore, many POSS derivatives have been incorporated into light-emitting polymers and amphiphilic fluorescent polymers.18,20,21 Subsequently, this has led to the preparation of devices with improved performances in terms of brightness, life times and quantum efficiencies compared with the devices prepared without the introduction of the POSS cage.22 However, the fabrication of POSS-containing organic–inorganic hybrid copolymer films with ultra-sensitivity and fast response times for the determination of oxygen concentration and pO2 has been seldom reported until now. Additionally, most optical oxygen sensors generally consist of an oxygen sensitive luminescent molecule embedded in a solid matrix, such as a PDMS matrix.11,23–25 However, poor solubility of luminescent probes in the polymer matrix can lead to inhomogeneous dispersion of the molecules and self-quenching effects because of its hydrophobic character.26 Only a few materials based on OSPs covalently grafted into a copolymer matrix have been reported.27–29 Therefore, a novel method is necessary to permanently immobilize the luminescent probes into the copolymer matrix with successful prevention of aggregation and photo-bleaching of the luminescent probes.

In this contribution, we report a simple synthesis of new organic–inorganic hybrid 3-(trimethoxysily)propylmethacrylate-co-platinum porphyrin-co-methacrylolsobutyl-polyhedral oligomeric silsesquioxane (TPMA-PtTPP-POSS) copolymer films by varying the monomer ratios and POSS loadings to investigate the performance of oxygen sensitivity and response time. One of the major advantages of the TPMA-PtTPP-POSS film is that the functional PtTPP, TPMA and POSS can be covalently bonded with each other through the polymerization reaction and efficiently prevent aggregation and photo-bleaching of the luminescent probes. To the best of our knowledge, for the first time, POSS-containing organic–inorganic hybrid films are adopted as optical oxygen sensors to enhance the sensitivity and response time. Consequently, the TPMA-PtTPP-POSS sensing films give rise to high fluorescence quenching efficiencies and ultrafast response/recovery times towards oxygen, which outperform current copolymer based sensing films. Thanks to the dramatically optimized response time, the prepared novel oxygen sensor film was employed for the real-time monitoring of DO in the glucose oxidase (GOx) catalytic process. The organic–inorganic hybrid sensor films exhibited fast response times, and are highly durable and reusable, and might be implemented as a promising candidate for sensitive oxygen sensors.

Experimental section

Materials

Tetrahydrofuran (THF) and 2,2-azobisisobutyronitrile (AIBN) were purchased from Sinopharm Chemical Reagent Co. Ltd. 3-(Trimethoxysily)propylmethacrylate (TPMA), 5-(4-methoxycarbonylphenyl)-10,15,20-triphenylporphyrin and K2[PtCl4] were purchased from Sigma-Aldrich (St. Louis, MO). Methacrylolsobutyl-polyhedral oligomeric silsesquioxane (POSS) was purchased from the Hybrid Plastics Company. β-D-Glucose was supplied by the Tianjin FuChen chemical reagents factory and glucose oxidase (GOx 180 U mg−1) was purchased from Aladdin. All chemical solvents were purchased from Sinopharm Chemical Reagent Co. Ltd. PtTPP-pendant monomers were synthesized and purified as described in the literature.30

Syntheses of TPMA-PtTPP, TPMA-PtTPP-POSS and PtTPP-POSS hybrid copolymers

The detailed synthetic procedures and characterization methods are listed in the ESI. The synthetic procedures of TPMA-PtTPP, TPMA-PtTPP-POSS, and PtTPP-POSS and the chemical structures of the PtTPP-pendant and POSS are presented in Scheme 1. The polymers were characterized by 1H NMR and the representative 1H NMR spectra of the POSS, TPMA, TPMA-PtTPP, TPMA-PtTPP-POSS and PtTPP-POSS copolymers are collected in Fig. S2 (ESI). In Fig. S2A and B (ESI), the characteristic resonance peaks of TPMA were clearly exhibited at 2.25 ppm (CH2 of TPMA), 3.55 ppm (OCH3 of TPMA), and 4.09 ppm (OCO–CH2– of TPMA). The protons from the C[double bond, length as m-dash]CH2 group at δ 6.1 and 5.55 ppm and the protons from the CH2[double bond, length as m-dash]C–(CH3) group at δ 1.9 ppm (no. 1 and no. 2 in Fig. S2A, ESI) completely disappeared in the TPMA-PtTPP copolymer (Fig. S2B, ESI), revealing that a complete process of copolymerization has occurred between PtTPP and TPMA.31,32 Similarly, the characteristic resonance peaks of POSS were clearly observed at 0.97 ppm (CH2–CH–(CH3)2), 1.88 ppm (CH2–CH–(CH3)2), 0.61 ppm (CH2–CH–(CH3)2), and 0.69, 1.62, 4.11, 1.97, 6.11 and 5.55 ppm (the protons in order from Si–CH2–CH2–CH2–OCO–CH(CH3)CH2 of POSS) (Fig. S2C, ESI). Similarly, the protons from the C[double bond, length as m-dash]CH2 group at δ 6.1 and 5.55 ppm and the protons from the CH2[double bond, length as m-dash]C(CH3) group at δ of 1.9 ppm (no. 12 and no. 13 in Fig. S2C, ESI) completely disappeared in the PtTPP-POSS copolymer (Fig. S2D, ESI), revealing that a complete process of copolymerization has occurred between PtTPP and POSS. As shown in Fig. S2E (ESI), the protons from the C[double bond, length as m-dash]CH2 group occuring at δ 6.1 and 5.55 ppm completely disappeared in the TPMA-PtTPP-POSS copolymer, revealing that a complete process of copolymerization has occurred between PtTPP, POSS and TPMA. Finally, the molar ratio between TPMA (m) and POSS (h) in the copolymer could be calculated from the peak intensity ratios of the protons of the OCH3 group at δ 3.55 ppm of TPMA and δ 0.97 ppm (CH2–CH–(CH3)2) for POSS (at a molar ratio of TPMA[thin space (1/6-em)]:[thin space (1/6-em)]POSS = 33[thin space (1/6-em)]:[thin space (1/6-em)]1 for the TPMA-PtTPP-POSS-11% copolymer). The results of the 1H NMR test and experimental data (mTPMA[thin space (1/6-em)]:[thin space (1/6-em)]mPOSS = 32[thin space (1/6-em)]:[thin space (1/6-em)]1) are in good agreement.
image file: c7tc03606j-s1.tif
Scheme 1 Syntheses of TPMA-PtTPP (A), TPMA-PtTPP-POSS (B) and PtTPP-POSS (C) copolymers.

Preparation of sensing films

The hybrid copolymers were dissolved in acetone at a concentration of 4 mg mL−1. A glass plate (size of 10 × 10 × 2 mm) with active hydroxyl groups generated by oxygen plasma treatment was used as the substrate to prepare a polymer membrane for optical characterization. After setting the glass plate on the disk of a spin-coater, the sample solution (approx. 200 μL) was dropped and spin-coated at 1000 rpm for 10 s in air. This process made a precursor film on the glass substrate and then the films were dried in air at 60 °C for 10 min, and 25 μm thick copolymer films (observed using SEM) were obtained for oxygen measurements (Fig. S3, ESI).

Results and discussion

The surface characterization of the copolymer films was carried out by atomic force microscopy (AFM). Both the average roughness (Ra) of the copolymer film layers and the surface topography of the POSS-containing copolymer films were studied. Different dosages of added POSS were defined as the weight ratios of POSS to TPMA-PtTPP-POSS. The neat copolymer TPMA-PtTPP without any additional POSS corresponding to a dosage of 0% was also studied as a control. Significant differences in the surface topography between POSS-absent and POSS-containing copolymer films were observed. When 0%, 30% and 100% of POSS were added to the matrix of the sensing films, the morphologies were significantly different. As shown in Fig. 1A, the surface of the TPMA-PtTPP copolymer film seems to be relatively smooth and the roughness was 0.42 nm. Fig. 1B and C shows that the surface of the POSS-containing copolymer films became rougher with randomly existing nano-sized aggregations on the surface. It can be observed that with an increase in POSS loading, the surface roughness of the films increased correspondingly. With an increase in POSS loading from 30 wt% to 100 wt%, the Ra gradually increased from 30 nm to 119 nm. The average heights are 2 nm, 125 nm and 979 nm, respectively. As we all know, hybrid copolymers with an inorganic block (POSS) and organic (TPMA) block are promising for achieving domains with smaller feature sizes. When the POSS loading exceeds the limit, the POSS preferred to aggregate together to reduce the interfacial area. However, the strongly repulsive interactions between the organic (TPMA) and inorganic (POSS) blocks of the copolymers allow the formation of self-assembled nanostructures with smaller feature sizes compared with those of purely organic copolymer systems.33–36 Therefore, wormlike surface structures were formed on the copolymer sensing film of TPMA-PtTPP-POSS (Fig. 1B). From the above results, we can conclude that the introduction of POSS could lead to higher surface roughness of the sensing films and form self-assembled nanostructures with smaller feature sizes. More importantly, the wormlike surface structure could be considered as porous structures which provided an interesting platform for probing the interactions between the frameworks and gaseous molecules. Therefore, the wormlike surface structures on the sensing film are extremely effective at enhancing the sensitivity and response time as will be discussed later.
image file: c7tc03606j-f1.tif
Fig. 1 Topography and 3D images of the copolymer sensor films on glass substrates: (A, A-1) TPMA-PtTPP; (B, B-1) TPMA-PtTPP-POSS-30%; (C, C-1) PtTPP-POSS.

The accuracy of AFM scanning for the worm-like structures on the POSS-containing copolymer film surface is critically important for understanding the sensing films' sensitivity and swift responses in this study because the worm-like structures are different at different POSS content ratios. Therefore, different visual examinations of all the AFM images (four POSS-containing copolymer films) taken from the films' surfaces are shown in Fig. S4 (ESI). The phase images shown in Fig. S4A–D (ESI) have presented the typical surface morphologies of the samples containing 11, 30, 62 and 79 wt% POSS, respectively. In the samples containing 11 and 30 wt% POSS, a good dispersion of the worm-like structures could not be obviously observed with limited worm-like structured clusters randomly dispersed on the film surface (marked as red boxes in Fig. S4A and B, ESI). However, the samples containing 62 and 79 wt% POSS possessed a large area of worm-like structures on the film surface, as shown in Fig. S4C and D (ESI). Additionally, all the AFM phase images from the different POSS-containing copolymer films exhibited similar worm-like structural features and the amount of worm-like-structure areas gradually increased with increasing POSS content. This goes some way towards explaining the POSS-containing copolymer films’ superior sensitivity and swift time responses as the POSS content increases. As mentioned above, more worm-like structures on the sensor surface, which provided large specific surface area and high accessibility of gaseous molecules to the surface of the sensing film, can act positively for the sensitivity and response recovery of the sensors. Moreover, the cross-sectional morphologies of the POSS-containing sensing films (TPMA-PtTPP-POSS-11%, TPMA-PtTPP-POSS-30%, TPMA-PtTPP-POSS-62% and TPMA-PtTPP-POSS-79%) were also analyzed by FESEM, as shown in Fig S3 (ESI). All four POSS-containing sensing films showed solid inner structures and there were no pores or significant differences in the cross-sectional morphologies. Above all, the POSS-containing polymer sensing films’ solid inner structures have no noticeable impact on the sensor sensitivity. Therefore, higher sensitivity and swift response times can be mainly attributed to the worm-like structures on the film surface.

The sensitivity and response time of an optical oxygen sensor are partially determined by the polymer matrix and can be tuned by changing the sensor matrix in terms of manipulating oxygen permeability. Several different POSS-containing copolymer films were used to systematically fine-tune the sensitivity of the oxygen sensors to investigate the effect of POSS loadings on sensitivity and responses. Fig. 2A shows that the typical relative fluorescence intensities of the sensor film decreased upon changing the atmosphere from pure N2 to pure O2, which mainly contributed to the distinctly increased quenching effect of the sensor film when exposed to higher oxygen concentrations. The calibration plots of I0/I vs. pO2 are shown in Fig. 2B and Fig. S5 (ESI). The sensitivity of the hybrid copolymer sensing films increased with increasing POSS loading. Introducing pendant substituents in the copolymer could prevent the closing and packing of the polymer chains together. Consequently, the polymers with larger pendant groups will tend to have higher oxygen solubility and higher diffusion coefficients.12 Therefore, the higher sensitivity and faster response times of the POSS-containing sensor films can be attributed to the synergetic effect of the diffusion and solubility coefficient of gases in the hybrid copolymer matrix. Additionally, the worm-like surface structure might facilitate the presence of porous topologies, which could benefit a higher accessibility of gas molecules and free motion of oxygen molecules within the porous structures. The oxygen sensor constructed with 79% POSS loading exhibited the highest sensitivity and faster response time than those constructed with other POSS contents. Therefore, the external surface area of the sensing films covered with worm-like structures affects the sensitivity and response time significantly. However, the oxygen sensing film constructed with pure POSS conversely showed a certain reduction in sensitivity compared with the TPMA-TTPP-POSS sensing film. As we all know, the portions of the POSS-rich domains increase with increasing POSS contents of the hybrid sensing films.37 Additionally, as the mass fraction ratio of POSS increases, the POSS macromonomers reduce the reactivity in free-radical copolymerization due to the steric hindrance of the bulk and rigid moieties. Therefore, the decrease in sensitivity of the TTPP-POSS sensor is most likely due to the influence of POSS-rich domains which might facilitate the dye aggregation as shown in Fig. 1C. Finally, the oxygen sensing properties were fitted by the Stern–Volmer equation and the KSV values for the oxygen sensing films are summarized in Table 1. The POSS-containing copolymer based oxygen sensors typically exhibited a high linearity (R2 > 0.99) in the range from 0 to 1 kPa oxygen. It is clear that the TPMA-PtTPP-POSS-79% sensor exhibits the highest sensitivity to O2 at a KSV value up to 1.833 kPa−1, which is about 4 times higher than that of the TPMA-PtTPP sensor (0.462 kPa−1). Additionally, the KSV value for the POSS-containing copolymer oxygen sensing films was much higher than those reported for PtTFPP or other dyes in various polymer matrices and was quite close to those using silica nanoparticles as matrices, yielding a sensor with greater sensitivity and faster response time than most previously reported formulations (Table 2). From the above results, we can conclude that the POSS indeed exhibits a positive influence on the overall performance of the oxygen sensor including sensitivity, response, etc. The change in the sensitivity of the sensor corresponds to the predictions in the literature.12,13,38,39


image file: c7tc03606j-f2.tif
Fig. 2 (A) Oxygen induced emission spectra of the copolymer sensor film under different O2 partial pressures (pO2); (B) Stern–Volmer plots for different copolymer sensing films at 25 °C with a linear fit (pO2: 0–1 kPa).
Table 1 Comparison of the key parameters of the oxygen sensors
Sensor films K SV (kPa−1) R 2[thin space (1/6-em)]a K SV (kPa−1) R 2[thin space (1/6-em)]b Response timeb (s) Recovery timeb (s)
a p O2: 0–1 kPa. b p O2: 0–21 kPa.
TPMA-PtTPP 0.462 0.996 0.416 0.994 4.8 21.54
TPMA-PtTPP-POSS-11% 0.862 0.999 0.767 0.997 3 18.6
PtTPP-POSS 1.212 0.997 0.97 0.997 3 6.48
TPMA-PtTPP-POSS-30% 1.394 0.999 1.183 0.997 1.2 6.36
TPMA-PtTPP-POSS-62% 1.512 0.999 1.287 0.997 0.8 6.06
TPMA-PtTPP-POSS-79% 1.833 0.997 1.501 0.999 0.6 2.58


Table 2 Comparison of optical sensors for oxygen measurements
Indicator probe Polymer matrix Sensitivity Response time/s Recovery time/s Ref.
PtTFPP TEOS ormosil I 0/I100 = 22 0.6 5 8
PtTFPP TEOS ormosil and silica nanoparticles I 0/I100 = 166 1.3 18.6 9
PtTFPP TriMOS/TEOS/octyl-triEOS I 0/I100 = 101 2 68 10
PtTFPP n-Propyl-triMOS/TEOS/octyl-triEOS I 0/I100 = 155 3 23 10
PtTFPP PDMS-2 I 0/I21 = 133 2 11
PtTFPP PS I 0/I100 = 3.0 18 60 40
PtTFPP Poly-TFEM I 0/I100 = 15.4 5.6 32 41
PtTFPP n-Propyl-triMOS and TFP-triMOS ormosil I 0/I100 = 68.7 3.7 5.3 42
PtOEP Poly-IBM-co-TFPM I 0/I100 = 86.4 6.1 45.3 43
PtOEP Poly-Styn-co-PFSm I 0/I100 = 24 5.6 30 44
PtOEP Poly-Styn-co-TFEMm I 0/I100 = 20.6 5.5 90 45
PtOEP Poly(styrene-co-PFS) I 0/I100 = 18 5.66 30 46
PtTFPP TPMA-PtTPP-POSS-79% I 0/I100 = 147 1.8 6 This work


Fig. 3A shows the response capabilities of the TPMA-PtTPP-POSS-79% sensor with the applied pO2 step changing between 0 kPa and 21 kPa. Quenching and recovery cycles were fully reversible. The curves exhibited excellently stable optical signals in 15 cycles when pO2 was continuously switched within 600 s. Additionally, it takes about 0.6 s and 2.58 s, respectively, to accomplish 90% of total fluorescence intensity when pO2 varied from 0 kPa to 21 kPa and vice versa (Fig. 3A). The kinetic fluorescence spectra of the sensor film suspension by alternating cycles of different pO2 values indicated that the TPMA-PtTPP-POSS-79% sensor film possessed better reversible oxygen quenching. The excellent performance of the sensor film can be attributed to the optimized diffusion and solubility coefficient of the gas in the POSS-containing copolymers. The stable optical signals, fast responses and recovery times can well satisfy the requirement for the real-time continuous monitoring of oxygen concentrations and pO2. Moreover, the dynamic responses of the TPMA-PtTPP-POSS-79% oxygen sensor were tested against smaller steps of pO2 (Fig. 3B). The oxygen sensing films with low pO2 exhibited intense emission and were sensitive to pO2 with their luminescence changing stepwise and reversibly to different pO2. Such a detailed study revealed that the emission intensity changed obviously with different pO2, which was used to evaluate the oxygen sensitivity and response time. Photostability is always of particular concern for the online continuous monitoring of oxygen under strengthened light density situations. In the case of sensing films, photo-bleaching can be overcome by increasing the thickness of the sensing layer. Thanks to the organic–inorganic hybrid copolymer structure, a moderate increase in the film thickness does not lead to deteriorated response times of the sensors. The stability results of the fluorescence intensity were acquired under ambient atmosphere, as shown in Fig. S6 (ESI). It should be noted that the sensor film exhibited nearly constant fluorescence intensities without an obvious decrease/increase. Additionally, thermal analysis was also performed to investigate the thermal stability of the POSS-containing copolymer sensing films, as provided in Fig S7 (ESI). The results exhibited that the copolymer sensing films possessed excellent thermal stability and thermal degradation occurred only above 230 °C. The experimental result proves the possibility of the as-prepared sensing films for potential applications under high temperature surroundings.


image file: c7tc03606j-f3.tif
Fig. 3 (A) Response of the oxygen sensing film to alternating atmospheres (0 and 210 hPa O2); (B) the dynamic emissive intensity response of the oxygen sensing film under different pO2.

The response and recovery properties of the POSS-containing organic–inorganic copolymer sensing films are critical for the evaluation of oxygen sensors. Generally, the response time needed for I/I0 to change from 20% to 90% of the total variation range was obtained and vice versa. To obtain these parameters, each sensor film was exposed three times to pO2 from 0 kPa to 21 kPa, and the value of I/I0 was maintained until the response reached a stationary stage after every pO2 variation. The responses of the POSS-containing sensing films to the dynamic variations of pO2 from 0 kPa to 21 kPa repeatedly are shown in Fig. 4A, and the detailed recovery profiles are displayed in Fig. 4B. The values of the response time obtained from the experimental results are shown in Table 1. The response time of the hybrid copolymer sensing films increased with increasing POSS loading. The POSS-containing copolymer sensing films exhibited faster responses, and the response times from the POSS-containing sensors were found to be similar in the range of 3 s to 0.6 s. The TPMA-PtTPP-POSS-79% sensor has the fastest target response time (0.6 s), which exhibited over an ∼8-fold increment when compared with contemporary oxygen sensors (TPMA-PtTPP). More importantly, the duration for the recovery to the baseline has been dramatically decreased from 21.54 s of the PMA-PtTPP sensor to 2.58 s of the TPMA-PtTPP-POSS-79% sensor as shown in Fig. 4B. From this perspective, the introduction of POSS into the copolymer is extremely effective at enhancing the sensitivity and response capability of the oxygen sensors. However, the recovery time and response time of the TTPP-POSS sensor are delayed and almost the same as that of the TPMA-PtTPP-POSS-30% sensor, which might originate from the enhanced dye aggregation influenced by the POSS-rich domains. This has also been examined by AFM, as shown in Fig. 1C.


image file: c7tc03606j-f4.tif
Fig. 4 Response of the POSS-containing sensing films to the dynamic variations of the partial oxygen pressure from 0 kPa to 21 kPa periodically (A); detailed curve profiles for the demonstration of the recovery time for each of the sensing films (B).

In this study, sensing films with different thicknesses were also prepared and the thickness effects on the time response performance were investigated. The thickness of each film was controlled by the speed of spin-coating. The sensing films with different thicknesses were fabricated by dropping and spinning at 2000, 1000 and 500 rpm, respectively. Subsequently, porous films with thicknesses of 5, 25 and 50 μm were obtained. The thicknesses of the thin films obtained were observed by FESEM, as shown in Fig. S8A–C (ESI). As the spinning speed increased from 500 to 2000 rpm, the thickness of the film decreased from 50 μm to 5 μm. Fig. S8D (ESI) shows the time responses of the sensor films with different thicknesses. It is apparent that the sensors’ response time became slower with the increase of film thickness, because for the thicker films the oxygen molecules need more time to diffuse out from the films. Therefore, the time response of the sensing film becomes slower with the increase of film thickness (Fig. S8D, ESI). All the above research results are in agreement with previous observations.11

To illustrate the potential biological applications of the POSS-containing copolymer sensor films, the kinetics of oxygen consumption during the enzymatic oxidation of β-D-glucose was used as a model reaction to evaluate the as-prepared sensor. In experiments, the β-D-glucose solution was added to a quartz cuvette and the concentration of β-D-glucose was set as 0.2 M. Then, the TPMA-PtTPP-POSS sensor film was plugged in the cuvette. To conduct the reaction, 0.1 mL of GOx solutions with concentrations of 50, 100, 200, 400 and 600 U mL−1 was added to the test solutions, respectively. Emission measurements were taken every 0.01 s at a maximum emission peak of 660 nm with an excitation wavelength of 405 nm. All the reactions were conducted at room temperature. From the fluorescence kinetics data illustrated in Fig. 5A, the fluorescence kinetics intensities of the sensor film rapidly increased with increasing GOx concentration in the aqueous solution, corresponding to a higher catalytic reaction efficiency and faster consumption of DO in the β-D-glucose solution. The more rapid fluorescence enhancement was observed when a higher concentration of GOx was introduced into the β-D-glucose solutions, matching well with the fact that DO is more rapidly consumed with increased GOx concentration. Additionally, information on DO concentration can be obtained from the fluorescence intensities through the luminescence quantification method as provided in Fig. 5B. From the above perspectives, the POSS-containing copolymer sensor film, which exhibited highly durable and reusable superiorities, can be suitable for integration into biological systems (cell culture, etc.) for the real-time monitoring of oxygen concentrations. It is worthy of being generalized and applied.


image file: c7tc03606j-f5.tif
Fig. 5 Time-dependent fluorescence emission from the TPMA-PtTPP-POSS sensor film in β-D-glucose solution for the monitoring of oxygen consumption in the process of the enzymatic catalytic oxidation of β-D-glucose (A); the corresponding DO concentrations at different times in the process of enzymatic catalytic oxidation of β-D-glucose (B).

Conclusions

In summary, for the first time, the authors demonstrated that the introduction of organic–inorganic POSS cages as oxygen sensor films can obviously improve the sensitivity and response time properties. A series of novel POSS-containing copolymer sensor films with self-assembled wormlike surface structures of the POSS domains on the glass surface were fabricated and studied. The produced wormlike surface structure was proved to function over a wide operational relative pO2 range with fast response and reversible behaviour. The sensitivity characteristics are dependent on both the POSS-containing copolymer composition and the geometrical characteristics of the wormlike structures. It was experimentally confirmed that with the precise control of the POSS contents, the oxygen sensor film can achieve a high sensitivity with a KSV of 1.833 kPa−1, swift response capability (0.6 s) and excellent stability against photo-bleaching. In the future, the POSS-containing hybrid films should provide a new strategy for designing self-assembled sensitive films as high performance optical oxygen sensors for applications in biological systems, environmental monitoring, etc.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The author would like to acknowledge the support from the Start-up Research Grant (SRG2016-00067-FST) from the Research & Development Administration Office at the University of Macau, the Science and Technology Development Fund from Macau SAR (FDCT-073/2016/A2), the National Science Foundation of China (21574061), the Shenzhen Fundamental Research Program (JCYJ20150630145302243), and the start-up fund of SUSTC (Y01256009).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc03606j

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