Elsevier

Analytica Chimica Acta

Volume 1147, 22 February 2021, Pages 187-198
Analytica Chimica Acta

Simple paper-based colorimetric and fluorescent glucose sensor using N-doped carbon dots and metal oxide hybrid structures

https://doi.org/10.1016/j.aca.2020.11.023Get rights and content

Highlights

  • Metal oxide hybrid with N-doped carbon dots (MFNCDs) as a peroxidase-like catalyst was fabricated by a hydrothermal method.

  • A colorimetric and fluorescent sensor for H2O2 and glucose was developed.

  • The MFNCDs exhibited excellent selectivity towards glucose.

  • MFNCDs showed low cytotoxicity and excellent biocompatibility.

Abstract

A new strategy for the fluorescent and colorimetric sensing of hydrogen peroxide (H2O2) and glucose based on the metal oxide – carbon-dot hybrid structure was investigated. The sensing system is related to the catalytic oxidation reaction of glucose-by-glucose oxidase (GOx) to H2O2. In this study, a metal oxide hybrid with nitrogen-doped carbon dots (MFNCDs) that showed intrinsic peroxidase-like activity was synthesized and used as a catalyst instead of GOx to oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to blue-emitting oxidized TMB (oxTMB) in the presence of hydrogen peroxide (H2O2). The fluorescence of MFNCDs/TMB at 405 nm was quenched in the presence of H2O2 through the inner filter effect (IFE) and electron transfer within MFNCDs, oxTMB, and glucose system. Therefore, the fluorescence and absorbance intensity can be applied to the quantitative determination of the concentration of H2O2 and glucose with a wide linear range. The detection limit for H2O2 and glucose based on the colorimetric method were as low as 84 nM and 0.41 μM, respectively. In contrast, the detection limit for H2O2 and glucose based on the fluorescent method were as low as 97 nM and 0.85 μM, respectively. Furthermore, the colorimetric readout on the paper device based on the changing color of the solution could also be integrated with a smartphone platform to conduct the on-site analysis of glucose without the use of the spectrometer. In addition, this dual sensor can be applied to detect glucose in real serum with highly accurate results, making it a good candidate for biosensor applications.

Introduction

Diabetes mellitus is one of a group of metabolic disorders related to glucose, which can cause multi-system damage, such as eyes, kidneys, heart, and other human organs [1]. Hence, the early detection and regular monitoring of the glucose levels in the blood are key factors in preventing the complications of diabetes. In recent years, enormous efforts have been made to determine the glucose concentration with high accuracy, such as surface-enhanced Raman scattering, electrochemical, chemiluminescence, fluorescence, and colorimetric approaches [[2], [3], [4], [5], [6], [7]]. Among these approaches, colorimetric and fluorescent methods have attracted tremendous attention owing to their simplicity, low-experimental cost, high sensitivity, portability, and recognition of color discrimination. On the other hand, many studies have been based on single signal determination (colorimetric or fluorescence). On the other hand, the single signal output could be affected by external conditions, such as operation and instrument efficiency [8,9]. Moreover, there are few colorimetric and fluorescent dual-signal sensor for glucose with an expected rapid response and feasible measurement available. Therefore, a method with dual signals to detect glucose is needed.

In a typical colorimetric method, glucose would be oxidized by glucose oxidase (GOx) to generate gluconolactone and H2O2. The concentration of glucose is then measured by assessing the change from a colorless to a colored product of the peroxidase substrate in the presence of a peroxidase catalyst [10]. Nevertheless, the use of natural enzymes as a peroxidase catalyst still has some drawbacks, including adversely affected by environmental factors and the relatively high cost. To solve this problem, it is essential to investigate proper peroxidase mimetic nanomaterials to substitute for natural enzymes. Several studies have reported that various transition metal oxides nanoparticles (NPs) [11], metal particles [10], perovskite NPs [12], and metal oxides nanocomposite with carbon materials [13,14] can catalyze the reaction through the peroxidase-catalytic activity. Recently, ferric oxide and manganese oxide have received a great interest across technological applications, including energy storage [15,16], catalyst [17], magnetism [18,19], environmental [20], sensor [21,22], drug delivery [23], owing to their outstanding structural combined with chemical and physical properties. Apart from unique properties, ferric oxide and manganese oxide have been reported to show excellent peroxidase-like activity. The use of composites of both as the enzyme peroxidase material has not been reported widely. Accordingly, a well-designed morphology and enhanced active site of the catalytic material are desired.

Among the ideal nanomaterials, carbon dots (CDs) are potential candidates for chemical sensing and bioimaging applications owing to their low cytotoxicity, good biocompatibility, superior photostability, eco-friendly, tunable fluorescence, and distinct optical properties. The introduction of an N atom to the graphitic carbon structures can enhance their electrical property and chemical stability, which leads to improved working performance towards specific target materials. Recently, CDs as well as N-doped carbon dots (NCDs) have been utilized for a range of applications, such as oxygen reduction reaction [24,25], water splitting [26,27], biosensors [28,29], batteries [30], bioimaging [31], and photocatalysts [32]. A versatile method to fabricate novel NCDs fluorescent materials used the hydrothermal process of different saccharides [33,34]. After the hydrothermal process, saccharides form carbonaceous nanomaterials, which can be functionalized further to improve the optical and fluorescent properties of the obtained carbon dots. Moreover, to enrich the potential applications, decorating other metal oxides with NCDs is an efficient way of tuning the unique electronic, catalytic, and optical properties. Thus, the introduction of NCDs to metal oxides will enhance their versatile properties further.

Along with the development of information technology, the smartphone has become more easily accessible. Recently, some proposed methods using smartphones as detectors for advanced detection applications such as colorimetric, fluorescence and electrochemiluminescence sensing were developed [[35], [36], [37], [38], [39], [40]]. This approach not only provides portable, rapid response, cost-effective diagnostic equipment but also enables on-demand analysis with reliable results. Therefore, the development of an integrated visual testing platform based on a combination of smartphone and composite material has become a great potential alternative to traditional diagnostic testing devices in analytical applications.

In this research, uric acid was used as a direct monomer source for the fabricated of NCDs along with glucose using a bottom-up method. Owing to the interaction between the aldehyde groups of glucose and the amine groups of uric acid without the assistance of a catalyst [41], it was expected that the product of the reaction would lead to N-containing carbon nanostructures. The composite with NCDs decorated on manganese oxide/ferric oxide (MFNCDs), as a new peroxidase mimetic catalyst, was fabricated using a simple one-pot hydrothermal process. The results showed that the NCDs were well dispersed over the metal oxide surface, which provided more beneficial interface active sites and enhanced the intrinsic peroxidase-like catalytic activity towards TMB in the presence of H2O2 to generate the blue color product (oxTMB). Moreover, the fluorescence of the MFNCDs/TMB system can be quenched with the presence of H2O2 owing to the inner filter effect (IFE) and electron transfer between the components in the system. Hence, with the aforementioned properties, a dual sensor based on colorimetric and fluorescent method for the quantitative determination of the concentration of H2O2 and glucose was developed. The additional advantages, including color readout on the paper-sensing device via an application (APP) integrated with a smartphone, low cytotoxicity, good biocompatibility as well as feasibility in the real samples, were outstanding characteristics for practical applications.

Section snippets

Experimental details

Detailed information on the materials and analysis apparatus is described in the supporting materials.

Synthesis and characterization of MFCNDs

Scheme 1 presents the approach for fabricating the MFCNDs. The crystallographic phase of the as-obtained samples was characterized using X-ray diffraction (XRD). As shown in Fig. S2, the XRD pattern of NCDs showed a broad peak at approximately 25.3° 2θ, which corresponded to graphitic-like carbon systems (002) (JCPDS 08–0415) [34,42]. The interlayer spacing of d002 = 0.38 nm was higher than that of the ideal graphite due to the incorporation of heteroatoms on the basal plane, which was

Conclusions

The dual signal sensor for H2O2 and glucose were developed successfully using fluorescent and colorimetric methods based on the oxidation of TMB in the presence of MFNCDs as a peroxidase mimetic catalyst. The sensing system delivered a wide detection range and a low detection limit for the detection of both H2O2 and glucose. The obtained MFNCDs-based sensor exhibited high selectivity against various common interfering species, long-term stability, excellent bio-imaging properties, and low

CRediT authorship contribution statement

Yen-Linh Thi Ngo: Methodology, Investigation, Visualization, Data curation, Writing - original draft. Phi Luan Nguyen: Methodology, Investigation, Visualization, Data curation. Jayasmita Jana: Software, Validation. Won Mook Choi: Resources, Investigation. Jin Suk Chung: Project administration. Seung Hyun Hur: Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by The Ministry of Science, ICT and Future Planning (2019R1A2B5B02069683).

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