Electrochemical sensor for the discrimination of bilirubin in real human blood based on Au nanoparticles/ tetrathiafulvalene –carboxylate functionalized reduced graphene oxide 0D-2D heterojunction
Graphical abstract
In this work, the reduced graphene oxide (RGO) was functionalized with tetrathiafulvalene-carboxylate (TTF-Ce the interface resistance, and also enhance the dispersity of TTF-COOH/RGO nanosheets in water. What's more, the S atoms of TTF-COOH can adsorb OOH) to fabricate a 0D-2D heterojunction for sensitive detection of bilirubin. The selected TTF-COOH molecules could repair electron conductivity of RGO nanosheets, decreasgold nano-particles (AuNPs) to fabricate a 0D-2D heterojunction with excellent biocompatibility of enhanced specific surface area. And then, the bilirubin oxidases were self-assembled on the surface of AuNPs, and constructed a specific recognition interface as a sensor for detection. The heterojunction showed an enhanced interface electron transfer rate, excellent biocompatibility, and also prominent electrocatalytic activity for the high efficiency catalysis of bilirubin. The detector showed a linear range from 2.66 μmol L−1 to 83 μmol L−1 and a low detection limit of 0.74 μmol L−1 at 3σ, which fully covers the content of bilirubin in normal man body (1.71–21 μmol L−1). Specially, this detection process could finish in 1 min with the volume of blood at microliter level. This work provides a novel approach for the detection of bilirubin by functional RGO nanosheets, and broadens the application of RGO nanosheets in selective catalysis and detection of molecules in real human blood.
Introduction
Reduced graphene oxide (RGO) is an alternative material of graphene with some functionalized groups on its surface [[1], [2], [3]], which was always obtained by reduction of graphene oxide nanosheets. RGO shown great potential application in sensor [[4], [5], [6], [7]], supercapacitor [2,3,8,9], and electromechanical resonators [10,11] because of it could be produce in industry more easily, and it show more high dispersity in several kinds of solvent, and the properties could be tune by chemical groups on the surface [12]. In the course of its application, how to design synthetic methods of RGO to avoid more or less conglomeration and re-graphitized of graphene and give RGO good chemical stability, high electro-conductivity and excellent biocompatible interface is still necessary. Among many reported preparation methods [[13], [14], [15]], noncovalent functionalizations of RGO nanosheets through π-π interactions were supposed to produce high quality RGO nanosheets with high dispersity, excellent biocompatibility and high electroconductivity. In the process of functionalizing graphene, the organic molecules with conjugated electronic structure were regarded as suitable approach to repair the electronic structure of RGO and realize surface functionalization in the same time. Therefore, what organic molecule is selected as functional monomers is the key part in the process of functionalizing graphene. Among the multitudinous organic molecules, tetrathiafulvalene (TTF) is an excellent electron donor with a sulfur-rich structure [[16], [17], [18], [19]], showing great potentials application in organic conducting materials [16,21], superconductors [20], fluorescence switches [4,22], and nonlinear optics [23]. For the enhanced S⋯S interaction and the extension of the π-conjugation of TTF derivatives [24], it has been widely used for fabricating charge-transfer complexes and enhancing charge transfer process [19,25,26]. Given the interest of TTF in π-electron-donating, the π-extended TTF derivatives can act as a bridge that allow electron transfer and form an electroactive architectures [18,25,[27], [28], [29]], when linked to some 2D materials, such as graphene and carbon nanotubes.
In this work, the multifunctional tetrathiafulvalene-carboxylate (TTF-COOH) was selected to fabricate the 0D-2D heterojunction with biocompatible interface. The S atoms of TTF-COOH play a key role in this fabrication process. Firstly, the four S atoms can form a large delocalization π bond with the C=C bond of TTF molecule, and then the large π bond of TTF-COOH could stabilize the π bond of RGO to form TTF-COOH/RGO nanosheets through interlayer π-π bond interaction. The electron conductivity and electronic transfer rate of RGO could be both enhanced with this method. Secondly, the -COOH group of TTF-COOH could reverse the natural hydrophobic surface of RGO nanosheets to hydrophilic, and greatly enhanced the dispersity of RGO. Thirdly, the four S atoms of TTF-COOH could also act as interaction site to chemical adsorb AuNPs to form a biocompatible surface via Au-S bond, and fabricate a 0D-2D heterojunction with enhanced specific surface area. The HRTEM was used to confirm the AuNPs were adsorbed uniformly on the surface of TTF-COOH/RGO nanosheets. The Raman spectra and UV–vis spectra were also used to confirm the interaction between AuNPs and TTF-COOH/RGO nanosheets. And the high resolution XPS spectra was used to confirm the Au-S has been formed. Finally, the bilirubin oxidase (BBO) was self-assembled on the surface of AuNPs also via Au-S bond, constructing a specific recognition interface as the selectivity biomimetic catalyst. The fabricated 0D-2D heterojunction was used to investigate the catalytic degradation process of bilirubin. This layered heterojunction could be used to detect bilirubin in real human blood.
Section snippets
Reagents and apparatus
Graphite was purchased from Alfa Aesar. Sodium borohydride, Bilirubin, Bilirubin Oxidase, and tetrathiafulvalene (TTF) were purchased form J&K SCIENTIFIC LTD. Phosphates were purchased from TianJinguangfu Chemical Co. All of the other chemical reagents were of analysis grade and were used as received. Ultrapure water was used throughout this study.
Characteristics were achieved via high resolution transmission electron microscopy (HRTEM, JEM 2100), field-emission scanning electron microscopy
Characterization of as-synthesized materials
The functionalization process of RGO is shown in Fig. 1a: the naturally hydrophilic TTF-COOH could self-assembly on RGO nanosheets due to interlayer π-π bond interaction. The layer structure of fabricated 0D-2D heterojunctions is shown in Fig. 1b: the AuNPs were adsorbed on TTF-COOH/RGO nanosheets to form a uniform biocompatible interface [33].
The interface binding energy between RGO and TTF-COOH was calculated by ab initio density functional theory (DFT) in Fig. 1c. The result shows that the
Conclusion
In summary, we have developed a TTF-COOH mediated stabilize strategy to produce stable few-layer RGO nanosheets in water-phase at a large scale. Because of the naturally conjugate electron structure of TTF-COOH, it can't only repair electron conductivity of RGO, and also enhance the transfer rate of interface electron, similar with one smaller RGO nanosheets stacking on n layer RGO. Due the strong multi Au-S bond, the AuNPs were spontaneously adsorbed on the surface of TTF-COOH/RGO to form a
Conflicts of interest
The authors declare no competing financial interest.
Declaration of interests
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.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (51862029, 51802266, 11634010,11572251), the Shaanxi's Key Project of Research and Development Plan (2018KA110052) the Fundamental Research Funds for the Central Universities in Northwestern Polytechnical University (3102016QD071, 3102017jc1003and 3102017jc11001),A special Fund of Gansu Province Guiding Scientific and Technological Innovation and Development (2018ZX-03),Major scientific projects of GanSu Institute of
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