Asymmetric interfacial oxygen sites of porous CeO₂-SnO₂ nanosheets enabling highly sensitive and selective detection of 3-hydroxy-2-butanone biomarkers

Highlights

• Porous CeO₂-SnO₂ nanosheets with finely modulation of active sites is constructed.

• Gas sensor is fabricated to detect 3-hydroxy-2-butanone, a biomarker of microorganisms.

• The sensor exhibits high sensitivity, selectivity, fast response and recovery rate.

• Asymmetric Ce-O-Sn site is recognized as active site to improve sensing performance.

Abstract

The local environment of active sites and the number of oxygen vacancies can greatly affect the reactivity of semiconductor metal oxides (SMOs). However, rare work has been reported to investigate the relationship between the state of oxygen vacancies and sensing performance of SMOs. Herein, a series of porous CeO₂-SnO₂ hetero-structure nanosheets with fine modulation of the local environment active sites and oxygen defect activity have been successfully constructed. We have investigated their sensing performance towards 3-hydroxy-2-butanone, which is a biomarker of food pathogenic microbe Listeria monocytogenes. We found that the amount of asymmetric Ce-O-Sn sites rather than total oxygen vacancies amount of CeO₂/SnO₂ materials can greatly affect their sensing performance. Impressively, the porous CeO₂/SnO₂-400 nanosheets, which possessed abundant active O^-(ad) species originated from the asymmetric Ce-O-Sn sites, exhibited high sensitivity (R_a/R_g=637.94 to 50 ppm), fast response (29 s) and recovery (172 s), excellent selectivity and high stability toward 3-hydroxy-2-butanone at a working temperature of 160 °C. Both the surface O^-(ad) species associated with the asymmetric oxygen sites and porous nanosheet hetero-structure contribute to the enhanced gas adsorption and diffusion process, further boosting their sensing performance. Our work provides new sights for identification of active sites in sensing materials as well as paves the way for detection of pathogenic microbes in food.

Introduction

Listeria monocytogenes (LMs) is a contagious food pathogen that can cause a variety of diseases including localized enteritis and systemic infections in immunocompromised and immunocompetent populations with a lethality rate of 20–30 %[1], [2], [3], [4], [5]. LMs is widely distributed in fruits, vegetables, or other foods and exhibit high ability to reproduce at low temperatures, making prevention of its spread extremely difficult. Therefore, the accurate and quantitative detection of LMs is of great importance for food industry. Nowadays, various methods including nucleic acid test[6], immunoassay[7] and microbiological studies[8] have been explored to detect LMs. Although significant progresses have been made, these techniques still suffer the problems of being time-consuming and costly; hence, it is highly desirable to develop an effective, low cost and accurate method to monitor the pathogenic microbes in food. In general, microorganisms including LMs can produce species-specific microbial volatile organic compounds (MVOCs), which can be characterized as biomarkers. When LMs multiplies in food media, it produces large amounts of exhaled gases, which are considered as the MVOCs. Among them, 3-hydroxy-2-butanone (3H-2B), also known as acetoin, was the main gas species in the volatile metabolites of LMs with an abundance of 32.2 %, and the concentration of this metabolite is directly related to the growth of LMs in foods[9]. Therefore, the 3H-2B is a useful biomarker for detection of LMs.

Recently, semiconductor metal oxides (SMOs)-based gas sensors have been regarded as a promising candidate for monitoring LMs through detection of the biomarker (3H-2B) due to the merits of low cost, high stability, easy operation, and rapid detection[10]. Although remarkable achievements have been made to detect the 3H-2B biomarker by using various SMOs (including WO₃[11], Cr-WO₃[4], Pt-SnO₂[12], Au-WO₃[13], PtCu-WO₃[2], ZnO-Co₃O₄[14], SnO₂@Al₂O₃[3], NiO[5], Pd-BiVO₄[1] and ZnO@Al₂O₃[15]), the in-depth understanding of heterogeneous interfaces are still urgent to guide the rational design of the active site local environment of SMOs to achieve the high performance gas sensors[16].

Tin oxide (SnO₂) is a typical n-type semiconductor with a wide band gap of 3.6 eV and variable conductivity sensitive to the amount of oxygen vacancy, which has been widely used in detection of several target gases [17], [18], [19], [20], [21], [22], [23], [24]. In order to improve the sensing performance of the gas sensors, various nano-structures including hollow structure, core-shell structure and mesoporous structure have been developed[25], [26], [27], [28]. Among them, the construction of porous nanosheet structure (NS) with abundant available active sites, highly open porous structure and large surface area-to-volume ratio has been proved to be an effective strategy to enhance the performance. Additionally, the construction of hetero-junction structures is an another effective way to enhance the gas sensing performance of metal oxides sensors[29], [30]. Particularly, cerium oxide (CeO₂) is a cracking catalyst in the petrochemical industry due to its excellent properties such as superior oxygen storage capacity, abundant oxygen vacancies, and high thermal stability[31], [32], which have been regarded as an emerging candidate to construct the hetero-structure to enhance their sensing performance. Recently, many efforts have been devoted to fabricating CeO₂-SnO₂ hetero-structures to detect various target gases[33], [34]. Usually, the surface oxygen species are recognized as the active sites to display a vital role in the sensing process[35]. The oxygen species absorbed on the surface of metal oxides can react with the target molecules and release electrons to perform the interfacial redox reaction, and the sensing performance is enhanced in pace with the increased oxygen vacancies amount[36]. However, it is found that the reactivity of these SMOs not only depends on the number of oxygen vacancies but also highly relative to their local environment of active sites, in particular, the symmetry of the oxygen vacancies[37], [38]. In the CeO₂-SnO₂ hetero-structures, eight-coordinated Ce-O will bond to six-coordinated Sn-O at the interface, and the asymmetrical oxygen vacancies (Ce-O-Sn) will be created neighboring the Ce to stabilize the coordination number. Typically, the oxygen vacancies generated around asymmetrical sites prefer to be filled by adsorbed oxygen species, and compared with the M-O-M site the asymmetrical M₁-O-M₂ structure is more reactive in redox reactions[39]. Although significant works have been performed to confirm this conception in catalytic reaction[40], it is still quite rare in the sensing process; therefore, to get more insights into the relationship between the state of oxygen vacancies and sensing performance is of great importance.

Herein, we have successfully synthesized CeO₂/SnO₂-x (x denote the treatment temperature, x = 330, 400, 500) nanosheets, systematically modulated the state of local environment active sites and further investigated the sensing performance towards .3H-2B. We found the sample of CeO₂/SnO₂-500 NSs after treatment at 500 °C possessed higher oxygen defect concentration but exhibited poor 3H-2B sensing performance, indicating that local environment of active sites plays a vital role in the sensing process. Notably, the CeO₂/SnO₂-400 NSs exhibited excellent gas sensing performance, showing the highest sensing response (R_a/R_g=637.94), fast response and recovery rate, and excellent selectivity for 50 ppm 3H-2B at a low operating temperature (160 °C). Detailed structural characterization results illustrated the active O^-(ad) species originated from the asymmetric Ce-O-Sn sites are more reactive than the symmetric Sn-O-Sn sites. Typically, CeO₂/SnO₂-400 NSs with porous nanosheets hetero-structure possessed more active O^-(ad) species, hence exhibiting a much higher sensing performance and fast response and recovery rate.