A Polymer Dots-Based Photoelectrochemical pH ... - ACS Publications

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A Polymer Dots-Based Photoelectrochemical pH Sensor: Simplicity, High Sensitivity, and Broad-Range pH Measurement Xiao-Mei Shi, Li-Ping Mei, Nan Zhang, Wei-Wei Zhao, Jing-Juan Xu, and Hong-Yuan Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02291 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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Analytical Chemistry

A Polymer Dots-Based Photoelectrochemical pH Sensor: Simplicity, High Sensitivity, and Broad-Range pH Measurement Xiao-Mei Shi,†,1 Li-Ping Mei,†,1 Nan Zhang,† Wei-Wei Zhao,*,†,‡ Jing-Juan Xu† and Hong-Yuan Chen*,† †

State Key Laboratory of Analytical Chemistry for Life Science and Collaborative Innovation

Center of Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China. ‡

Department of Materials Science and Engineering, Stanford University, Stanford, California

94305, United States 1These authors contributed equally to this work. * To whom correspondence should be addressed. * E-mail: [email protected]; [email protected] * E-mail: [email protected]

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Abstract: This work reported the photoelectrochemical (PEC) pH sensor for sensitive and broadrange pH measurement on the basis of semiconducting polymer dots (Pdots). The sensor was fabricated by immobilizing Pdots onto the surface of indium tin oxide (ITO). Experimental results revealed that the carboxylic acid groups of Pdots were sensitive to pH variation, which could result in conformational changes and further diffusion of carriers. Besides, different pH value could change the redox properties of the Pdots and the photocurrent response was hence altered by the carriers produced on the Pdots. Further results demonstrated that the developed sensor exhibited variable photocurrent sensitively as responding to different pH value. This pH sensor is of high sensitivity, stability and reversibility, which provides a bright prospect for future pH measurements in bioanalytical field. Keywords: Photoelectrochemical; pH Sensor; Polymer Dots; Sensitivity; Broad Range


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The pH value acts as an important qualitative, diagnostic or surrogate parameter in industrial, physiological or clinical practices.1 The pH sensor thus has wide range of applications, such as food and beverage, environmental monitoring, chemical processing, biomedical applications (e.g. body fluid and blood analysis), as well as laboratory pH measurements.2-5 These applications generally require highly reliable and accurate pH sensors with the reduced maintenance and long lifetime. Various electrochemical and non-electrochemical techniques have been explored for pH measurements. Currently, due to its high selectivity toward protons, the most widespread one has been the glass-bulb-based electrode, which also has been well-established as commercial products of high stability and accuracy (except in too high/low conditions). Nevertheless, they still have some disadvantages, such as high fragility of the glass membrane electrodes, spaceconsuming of the total devices, difficulty to miniaturize and frequently requirement of calibration. These drawbacks generate a strong motivation to develop new pH sensors,6,7 and many advanced materials (e.g. micro Ir/IrOX wires,8 bovine serum albumin (BSA) nanoparticles,9 boron-dipyrromethene (BODIPY) and other pH-sensitive dyes.10,11) have been explored to this end. Table S1 lists the comparison of various materials for pH detection. Photoelectrochemical (PEC) sensor has attracted increasing interest12-15 and recent have witnessed its tremendous advances towards various purposes.16-20 However, it is surprising that the PEC pH sensor has not been reported to date. Inspired by this, we expect the possibility of PEC pH sensor since it might be a good choice of economical and simple way for pH monitoring. Semiconducting polymer dots (Pdots) have attracted great attention for biology and medicine applications.21 As fluorescent probes, their application for cell tracking, tumor imaging, ultrafast hemodynamic imaging, and chemiluminescence imaging of drug-induced injury have been reported.22 Besides, they also exhibit good photothermal property and was applied for photothermal therapy (PTT).23 On the basis of their ability to convert light energy into acoustics,


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photoacoustic imaging (PAI) of tumors and enzyme activity was also realized.24 Recently, our group also exploited their light-harvesting property and demonstrated their feasibility for PEC bioanalysis.25-27 Scheme 1. Schematic Mechanism of the Operating PEC pH Sensor System

Herein we have designed, fabricated and characterized the Pdots-based PEC pH sensor within a broad range of pH 1.0-13.0 (see Supporting Information for Experimental Section). As deomonstrated in Scheme 1, the poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}thiadazole)] (PFBT) Pdots contained tetraphenylporphyrin (TPP) dopants were immobilized onto the indium tin oxide (ITO) electrode and applied as the pH sensor. As illustrated, carboxylic acids (-COOH) functionalized TPP-doped PFBT Pdots would be protonated or deprotonated at different pH buffer.28 With different pH values, the configuration of Pdots will convert between coiled (I state) and stretched one (II state) due to the repulsive electrostatic.29 At low pH value, small-sized one could decrease the impedance and enhance the diffusion of protons.30 Besides, the potential of hydrogen reduction is higher because of the high content of the protons.31 The photocurrent thus exhibited high response due to the increase of carriers (I and III states). At high pH value, the photogenerated electrons were transferred to dissolved oxygen.25 With the increase of the pH value, fewer electrons were transferred from Pdots to dissolved oxygen upon light


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excitation. Therefore, the photocurrent exhibited low response as the decrease of the carriers (II and IV states). To our knowledge, such a Pdots-based PEC pH sensor has not been reported. RESULTS AND DISCUSSION

Figure 1 (a) TEM image of the Pdots. Inset above: The photograph and fluorescence image of the Pdots after a 365 nm UV-lamp excitation. Inset below: the emission (red) and the UV-vis absorption (blue) spectra of the Pdots. (b) IR spectrum of the Pdots. (c) Photocurrent responses under pH 6.0, 8.0 and 10.0. (d) Effects of 1 µM different ions on the signal of the sensor in Tris-HCl buffer (pH 7.4). Inset: Reversible variation of the signal upon switching the pH between 2.0 and 12.0 (start and end at pH 7.0).

The preparation of the Pdots was displayed in Scheme S1. As shown in Figure 1a, the quasispherical particles of Pdots were observed in transmission electron microscope (TEM) and their average diameter was ca. 3 nm. Its digital and fluorescence images were embedded in the upper part of Figure 1a, it could be seen that its color turned from luminous yellow to orange-red after the excitation of 365 nm UV-lamp. The fluorescence emission spectra and ultraviolet-visible


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(UV-vis) absorption of the Pdots were embedded in the lower part of Figure 1a. The absorption spectrum (blue curve) possessed a primary absorption range between 400 and 500 nm, and the relatively weak absorption bands belonged to the TPP dopants. The fluorescence spectrum (red curve) was measured from 500 to 750 nm at the 450 nm excitation.22 Especially, as shown in Figure 1b, the functionalization of the Pdots with -COOH groups32 were confirmed by infrared (IR) spectroscopy. The peaks at 1660 cm-1, 1566 cm-1 and 1450 cm-1 peaks were the C=C from benzene ring. The broadband of 3300_2400 cm-1 represented the characteristic absorption peak of -COOH, which was the obvious indication of -COOH in the Pdots. The peaks at 1700 cm-1,1358 cm-1 and 1028 cm-1 were ascribed to the stretching vibration of C=O and C-O, which were also originated from -COOH groups. Figure 1c shows the response of the developed sensor against the buffers of pH 6.0, 8.0 and 10.0, respectively, where the signal was acquired by every 36 h. The scarcely changed signal indicated the good stability of the pH sensor. As shown in Figure 1d, the sensor was tested in the presence of various ions. Compared with other ions, the response to H+ was obviously higher, confirming that only H+ can protonate the -COOH groups and further influence its photocurrent response. Figure 1d inset demonstrates that little change was happen when the pH varied from 2.0 to 12.0 and vice versa. When the pH changed from 7.0 to 2.0, the signal decreased quickly, while it quickly increased when the pH changed from 2.0 to 12.0. After repeating for five times, the photocurrent intensity kept almost the same level, indicating the good reversibility of the sensor. The photocurrent responses of the sensor were then investigated at different pH values. As shown in Figure 2a, the corresponding signal gradually decreased with the increased pH value, which was due to the enhanced deprotonation process. Figure 2b shows that the dependence of signals on the pH from 2.0 to 12.0 exhibited two linear ranges, which could be ascribed to two kinds of redox reaction occurred in the Pdots. The photocurrent varied rapidly in pH from 2.0 to


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6.0 because of the reduction reaction of the proximal hydrogen.34 The high content of the protons caused the increase of the potential of hydrogen reduction, resulting in the large amount carriers and further enhancement of photocurrent. On the other side, the low content of the protons resulted in the decline of photocurrent. The photocurrent varied more gently in pH from 6.0 to 12.0 because of the reduction reaction of the oxygen.25 These results manifested the high sensitivity of the developed PEC pH sensor.

Figure 2. (a) Photocurrent responses of the sensor at different pH, ranging from 1.0 to 13.0. The excitation wavelength was 450 nm and the working potential was 0.0 V. (b) The corresponding calibration curve.

Essentially, the TPP-doped PFBT Pdots are a kind of weak polyelectrolyte. The sensitivity of the pH sensor depends on its -COOH groups, which can be protonated or deprotonated as the follow equilibrium:33 Pdots-COOH + H2O ↔ Pdots-COO- + H3O+.28,33 As shown in Scheme 1, the conformation of the Pdots was regulated by the pH value. When at low pH, the Pdots owned a coiled conformation (I) with a hydrophobic surface, which was due to the interaction of intramolecular hydrogen bonds in the Pdots.34,35 With increased pH value, the coiled conformation converted to stretched state (II) because of the repulsive electrostatic of negative charges.29 In this situation, the hydrophilic surface was formed due to the interaction of the intermolecular hydrogen bonds between water and the Pdots.36 Upon the switched illumination at low pH (I and III states), because of the high proton content, the potential of hydrogen reduction


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was increased (E = E0 _ 0.059pH).28,31 Afterwards, the photogenerated electron-hole pairs were separated, with more electrons transferred from Pdots to the protons and thus generated high cathodic signal. Specifically, once the Pdots was irradiated by light at low pH, the electrons were excited from the highest occupied molecular orbit (HOMO) to the lowest unoccupied molecular orbit (LUMO). These electrons would transfer to the electrolyte and probably be trapped by protons, generating the hydrogen radical (H.), while the holes in the HOMO would be neutralized by electrons from ITO electrodes. In addition, the small size of the coiled conformation of Pdots in this state would contribute to the rapid diffusion of protons and the fast photocurrent responses.30,37 When the light was switched off, the hydrogen radical (H.) would combine with holes in the Pdots and resulted in reversible photocurrent responses. In the case of high pH, the photogenerated electrons were captured by dissolved oxygen.25 The potential of the oxygen reduction was decreased (E = E0 _ 0.059pH) with the increased pH value. The fewer carriers were produced at the surface of Pdots (right centre in scheme 1), the lower photocurrent responses were induced subsequently (II and VI states).31,38 CONCLUSIONS In summary, a simple Pdots-based PEC sensor capable of broad-range pH measurement was developed. Due to the presence of -COOH groups in these Pdots, the configuration of the Pdots and the production of carriers after photoexcitation can be easily regulated by pH value, leading to the sensitive response of photocurrents. Compared with other pH sensors, this Pdots-based pH sensor was very simple and exhibited good stability, reversibility and sensitivity. This work extended the applicability of PEC sensor. Besides, it offered a new prospect for the future development of advanced PEC pH sensor on the basis of various functional conjugated polymer nanomaterials.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.xxxxx. Materials and apparatus; Preparation of the TPP-doped PFBT Pdots; Preparation of TPP-doped PFBT Pdots/ITO electrode and Comparison of different pH sensors (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the Science and Technology Ministry of China (Grant No. 2016YFA0201200), National Natural Science Foundation of China (Grant nos. 21327902 and 21675080), and the Natural Science Foundation of Jiangsu Province (Grant BK20170073). REFERENCES (1) Wencel, D.; Abel, T.; McDonagh, C. Anal. Chem. 2014, 86, 15−29. (2) Nareoja, T.; Deguchi, T.; Christ, S.; Peltomaa, R.; Prabhakar, N.; Fazeli, E.; Perala, N.; Rosenholm, J. M.; Arppe, R.; Soukka, T.; Schaferling, M. Anal. Chem. 2017, 89, 1501−1508. (3) Yue, S.; Sun, X. T.; Wang, N.; Wang, Y. N.; Wang, Y.; Xu, Z. R.; Chen, M. L.; Wang, J. H. ACS Appl. Mater. Interfaces 2017, 9, 39699−39707. (4) Frankaer, C. G.; Hussain, K. J.; Rosenberg, M.; Jensen, A.; Laursen, B. W.; Sorensen, T. J.


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