A competitive self-powered sensing platform based on a visible light assisted zinc–air battery system

Junlun Zhu a, Wei Nie a, Qin Wang a, Wei Wen *a, Xiuhua Zhang a, Fujun Li b and Shengfu Wang *a
aHubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, People's Republic of China. E-mail: wangsf@hubu.edu.cn; wenwei@hubu.edu.cn
bKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin 300071, P. R. China

Received 13th February 2020 , Accepted 25th April 2020

First published on 27th April 2020

We developed a novel competitive self-powered sensing platform based on the discharge process of a visible light assisted zinc–air battery system for the detection of targets with a high open circuit potential signal (Eocpt = 1.5 V), which is a splendid signal among state-of-the-art self-powered sensing platforms.

Self-powered sensing systems (SPSs) are emerging detection platforms that have attracted wide attention due to their detection capabilities without the need for external power supplies or voltage biasing compared to conventional electrochemical sensors.1,2 According to the definition of an SPS, the system has a galvanic cell structure, requires no energy input, and provides a signal in the presence of the analyte.3,4 SPSs dominate most fields and have been widely applied in the detection of small molecules,5 biomarkers,6 tumor cells,7 enzyme activity8 and environmental pollutants.9 Among them, most SPSs have a number of defects, including low output voltage, low output power, and microamp current, which require more sophisticated and expensive instruments to detect, thus limiting the development of SPSs. In particular, there are still some bottlenecks that need to be overcome, including the narrow range of linear detection due to too small open circuit potential signals (Eocpt < 0.6 V), insufficient signal output stability, etc. Thus, the design of an SPS with high output power signal and stability for the detection of targets is of great significance. Encouragingly, metal–air batteries, promising energy storage systems, generate electricity through a redox reaction between the metal anode and oxygen in the air, which has the potential to address the bottlenecks mentioned above. These battery systems are characterized by an open-cell structure that allows continuous and almost unlimited supply of the cathode active substance (oxygen) from an external source (air).10,11 Zinc–air batteries (ZABs), a special kind of fuel cell with the characteristics of a primary battery, use zinc metal as the anode and oxygen as the cathode reactant. Compared with other metal–air batteries (Table S1, ESI), ZABs have some advantages, such as high energy density (theoretical value: 1086 W h kg−1), tailorability, safety, simple structure, stability, low cost, greenness, appropriate theoretical voltage and high efficiency.12,13 At present, ZABs are the most mature technology among the different types of metal–air batteries, and have been used as power supplies in devices such as hearing aids, radar and navigation lights.12 The actual output voltage of a ZAB can be up to 1.38 V, but in photoassisted ZABs, the actual output voltage and energy density can be increased to 1.78 V and 129.0%, respectively.14 Therefore, the introduction of high energy density ZAB systems into SPSs will establish a new generation of promising electrochemical detection devices.

However, SPSs based on simple and stable ZABs have not been reported. The oxygen reduction reaction (ORR) is a key factor in the construction of ZABs. Photoassisted ORR which combines the merits of both electrocatalysis and photocatalysis is one of the most promising yet challenging strategies.15,16 Polytriphenylthiophene (pTTh), a p-type photoresponsive conducting polymer, has been widely studied in the solar cell field due to its visible light response and oxygen reduction performance.17 These promising features would make pTTh films appropriate photocathodes for photoassisted ZABs to achieve high and stable energy output.

Herein, we firstly proposed a competitive SPS based on the discharge process of a visible light assisted ZAB system. As shown in Scheme 1A, the photocathode is formed by the electropolymerization of a pTTh film on an ITO electrode. The photocathode adsorbs gold nanoparticles (Au NPs) and glucose oxidase (Gox) on the pTTh film surface through the formation of Au–S bonds. During the discharge, the pTTh modified air photocathode undergoes ORR to obtain electrons under light conditions, and is finally reduced to hydrogen peroxide (H2O2). The zinc anode is oxidized to zinc ions and combines with the hydroxide in the solution to form an intermediate product, which eventually forms zinc oxide and releases electrons to the external circuit. As depicted in Scheme 1B, the competitive mechanism of the SPS involves the Gox on the photocathode catalyzing the decomposition of glucose to reduce the oxygen content in the solution and reduce the progress of the photoassisted ORR, thereby indirectly detecting glucose by using changes in the oxygen concentration. The obtained detection range was 0.1 μM to 200 μM with a credible detection limit of 73.7 nM. This strategy achieved an outstanding analytical performance for the SPS and provided a promising idea for point-of-care devices, which exhibits good prospects for rapid diagnosis of disease with stable and high energy output in the enzyme-label biosensing field.

image file: d0cc01163k-s1.tif
Scheme 1 (A) Schematic illustration of the SPS based on the discharge process of a ZAB system. (B) Competitive mechanism: in the presence of glucose and oxygen, glucose oxidase catalyzes glucose decomposition to produce gluconic acid and hydrogen peroxide (left hand). Visible light excites the pTTh film to form electron–hole pairs, and the electrons in the conduction band reduce oxygen to generate hydrogen peroxide (right hand).

The air photocathode played a significant role in the construction of the photoassisted ZAB. Gox, Au NPs, and the pTTh film formed an all-in-one structure on the ITO electrode with Au NPs acting as bridges via Au–S bonds. Transmission electron microscopy (TEM) images indicated that our methods coated the pTTh film with Au NPs and Gox. The freshly prepared ITO electrodes were further characterized by scanning electron microscopy (SEM) images. Fig. 1 shows TEM and SEM images of pTTh, pTTh + Au NPs, and pTTh + Au NPs + Gox, which revealed that Gox and Au NPs can successfully modify the surface of pTTh films. The TEM image of the pTTh film showed a graphene-like thin layer structure (Fig. 1A), which contributed a large number of sites for loading of Au NPs. Fig. 1B shows that the pTTh + Au NPs film is thicker relative to pTTh. As shown in the inset of Fig. 1B, a large number of Au NPs were uniformly dispersed on the film, which confirmed the successful loading of Au NPs onto the pTTh film. Further thickening of the film can be observed in the edge portion of Fig. 1C, which proves the successful loading of Gox, that is, the formation of an all-in-one film. It can be observed from the inset of Fig. 1C that the loading of Gox does not cause aggregation of the Au NPs. SEM images provided convincing evidence to further confirm the formation of the all-in-one film. Obviously, the pTTh film displayed a highly overlapping thin sheet structure (Fig. 1D). Furthermore, due to the adhesion of Au NPs, the film structure of pTTh + Au NPs is significantly thicker than that of the pTTh film (Fig. 1E), which implies that the Au NPs were subsequently successfully loaded on the surface of the film. As shown in Fig. 1F, the pTTh + Au NPs + Gox film manifested thicker and more adherent nanosheets compared to the former, which further illustrates the formation of an all-in-one film on the electrode. The analysis of the TEM and SEM images revealed the formation process and highly consistent results were obtained. The UV-Vis absorption spectrum, TEM image and particle size distribution chart of Au NPs are shown in Fig. S1–S3 (ESI).

image file: d0cc01163k-f1.tif
Fig. 1 TEM images of (A) pTTh, (B) pTTh + Au NPs, (C) pTTh + Au NPs + Gox. The insets of A–C show high magnification images. SEM images of (D) pTTh, (E) pTTh + Au NPs, (F) pTTh + Au NPs + Gox.

The oxygen reduction reaction includes two-electron and four-electron processes, and the two-electron process can be used to produce H2O2. Fortunately, recent work has reported the introduction of photocatalytic activity and oxygen reduction catalysts for enhancing the two-electron oxygen reduction process to generate H2O2.18–20 pTTh has attracted wide attention due to its excellent oxygen reduction ability and photocatalytic activity. As shown in Fig. S4 (ESI), pTTh has a certain oxygen reduction activity, which is consistent with previous reports.18–20 In addition, it was observed that pTTh produced a small amount of H2O2 during the photocatalytic oxygen reduction reaction (Fig. S5, ESI). Under light conditions, photoelectrons are generated in the conduction band of pTTh and are accepted by O2 molecules to produce H2O2, which drives the oxidation of Zn to divalent zinc at the anode. The pTTh films were fabricated by electropolymerization in an acetonitrile solution containing 2,2′:5′,2′′-terthiophene (TTh) and lithium perchlorate, which is a polymerization reaction that occurs at the Cα–H sites of two TTh molecules (Fig. S6 and S7, ESI). Fig. S8 (ESI) displays the electropolymerization mechanism of the conversion of TTh into pTTh.21 In order to confirm the mechanism of electropolymerization, we used Fourier transform infrared spectroscopy (Fig. S9, ESI) and Raman spectroscopy (Fig. S10, ESI), which were consistent with previous reports.14 These results further confirmed that TTh forms pTTh through electropolymerization at the Cα–H sites.

To resolve the band structure, ultraviolet photoelectron spectroscopy (UPS) and UV-Vis diffuse reflectance spectroscopy (DRS) were carried out. Fig. 2A and B exhibit the UPS spectra of the pTTh film. Fig. 2C and D display the UV-Vis DRS spectra of pTTh. From UPS and UV-Vis DRS spectrum analysis, the pTTh of EHOMO and ELUMO is equal to −5.04 eV and −3.09 eV, respectively. Combining the UPS and UV-Vis DRS results, it can be concluded that the pTTh energy levels corresponding to the electrochemical potentials of the CB and VB (vs. NHE) were −0.54 V and 1.41 V, respectively (Fig. 2E). It is exciting that these energy levels are favorable for the catalysis of the two-electron oxygen reduction process.

image file: d0cc01163k-f2.tif
Fig. 2 UPS spectra of pTTh. (A) The secondary edge region and (B) the HOMO region of pTTh. (C) UV/Vis DRS absorption spectrum of pTTh. (D) Energy band gap of pTTh. (E) Energy diagram displaying the calculated HOMO and LUMO levels of pTTh as absolute vacuum energies and as potentials relative to the NHE.

In order to investigate the SPS sensor assembly process, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed. The semicircle diameter in the EIS reveals the electron transfer resistance. The Fe(CN)63−/Fe(CN)64− redox probe was used to monitor the gradual modification of the electrode surface. As shown in Fig. 3A, the ITO, ITO/pTTh, ITO/pTTh/Au NPs, and ITO/pTTh/Au NPs/Gox electrodes have different electron transfer resistance. The bare ITO electrode displayed a low electron transfer resistance (black curve). After the TTh solution was electropolymerized into a pTTh film on the ITO electrode, the electron transfer resistance increased significantly (red curve), which indicated that the pTTh film was successfully deposited on the ITO electrode surface. When the Au NPs were adsorbed onto the pTTh film, the electron transfer resistance dropped sharply owing to the electrical conductivity and hydrophilicity of Au NPs (blue curve). After the immobilization of Gox, the increase in electron transfer resistance was attributed to steric hindrance and poor conductivity, preventing the redox probe from accessing the electrode surface (purple curve). As can be seen in Fig. S11 (ESI), the CV of the modified electrodes was measured. The obtained results were consistent with those of EIS. These results implied that the electron transfer resistances of all the electrodes were consistent with the changes in redox peak current in the CV curves, which confirmed the successful construction of our sensor.

image file: d0cc01163k-f3.tif
Fig. 3 (A) Nyquist diagram of bare ITO (black curve), pTTh (red curve), pTTh + Au NPs (blue curve) and pTTh + Au NPs + Gox (purple curve). (B) Eocpt response of the bare ITO (black curve), pTTh (red curve), pTTh + Au NPs (blue curve), pTTh + Au NPs + Gox + 10 μM glucose (purple curve) and pTTh + Au NPs + Gox without glucose (green curve).

To further verify that the SPS was successfully constructed, the open circuit potentials (Eocpt) of the different modified electrodes were measured. As shown in Fig. 3B, Eocpt of the ITO (black curve), ITO/pTTh (red curve), ITO/pTTh/Au NPs (blue curve), and ITO/pTTh/Au NPs/Gox (purple and green curves) electrodes were measured. Due to the electropolymerization of the pTTh film, the occurrence of the photoassisted oxygen reduction reaction resulted in a higher Eocpt response. The adsorption of Au NPs on the pTTh film surface caused a surface plasmon resonance (SPR) phenomenon (Fig. S12, ESI), in which the absorption of light increased, and the Eocpt further increased. When Gox was loaded on the surface of the Au NPs, it was found that the Eocpt was reduced, which was mainly because the covering effect of the Gox reduced the light absorption. When glucose was added, the oxygen concentration changed due to the glucose oxidase breaking down the glucose, which further decreased the Eocpt. These results further confirm the successful construction of the SPS and its response to the target.

The air photocathode is a significant part of the battery system and plays a vital role in the performance of the SPS. To investigate the effect of light illumination, H2O2, and oxygen content, the catalytic activity of the pTTh cathode under dark and light conditions was investigated in 1 mM H2O2 and O2 or N2 saturated electrolyte (Fig. S13, ESI). To further prove the effect of light illumination, H2O2, and oxygen content, the Eocpt responses were measured under the different conditions (Fig. S14, ESI). These results demonstrated that the proposed SPS is a suitable candidate for the development of monitoring devices with high and safe energy signal output.

Under the best experimental conditions, the Eocpt responses of the proposed SPS to different concentrations of glucose (0.1 μM to 200 μM) were measured. The intensity of the Eocpt response decreased gradually with increasing concentration of glucose. Fig. 4A illustrates the Eocpt responses of the constructed SPS to different concentrations of glucose. The calibration plot exhibited a good linear relationship between the Eocpt response and the logarithm of glucose concentration in the range from 0.1 μM to 200 μM with a squared correlation coefficient of 0.9966 (Fig. 4B). The linear equation was Eocpt = −0.0362[thin space (1/6-em)]log[thin space (1/6-em)]Cglucose + 1.42, where Eocpt was the open circuit potential and Cglucose was the concentration of glucose. The calculated detection limit was 73.7 nM at a signal-to-noise ratio of 3, which shows that this is a promising method for low glucose concentration detection. To highlight the advantages of the SPS for glucose detection, this method was compared with other glucose sensors, which are listed in Table S2 (ESI). Compared with other SPSs, this SPS based on a ZAB system has certain advantages, including high circuit potential and wide voltage range, providing new ideas for wearable devices and portable detection (Table S3, ESI). The selectivity (Fig. S15, ESI), reproducibility (Fig. S16, ESI) and stability (Fig. S17 and S18, ESI) of the proposed SPS were measured under the optimal conditions. These results reveal that the SPS can be used for sensitive detection of targets.

image file: d0cc01163k-f4.tif
Fig. 4 (A) Eocpt responses of the SPS to glucose concentrations of 0.1, 0.5, 1, 5, 10, 20, 50, 100 and 200 μM. (B) Linear calibration curve of the Eocpt response.

In summary, a competitive SPS based on the discharge process of a ZAB system was reported for the first time. The concentration of oxygen in the solution is affected by Gox breaking down glucose, and the visible light responsive pTTh is sensitive to the concentration of oxygen. Through this competitive mode, the change in oxygen concentration is used to detect the glucose target. This SPS for the detection of glucose had a good response in an acceptable linear range with good reproducibility, selectivity, and stability. The calibration plots exhibited a good linear relationship between the Eocpt response and the logarithm of glucose concentration in the range from 0.1 μM to 200 μM with a squared correlation coefficient of 0.9966. Notably, the SPS achieved a high Eocpt signal of 1.50 V. This work provided a novel method for the construction of an SPS with high and stable energy output and exhibited promising prospects for rapid diagnosis of disease by wearable, portable, and implantable electronic devices.

This work was partly financially supported by the National Natural Science Foundation of China (No. 21775033, 21405035).

Conflicts of interest

There are no conflicts to declare.

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Electronic supplementary information (ESI) available: Spectra and discussion. See DOI: 10.1039/d0cc01163k

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