Elsevier

Electrochimica Acta

Volume 197, 10 April 2016, Pages 152-158
Electrochimica Acta

Fluorinated Nanocarbon Film Electrode Capable of Signal Amplification for Lipopolysaccharide Detection

https://doi.org/10.1016/j.electacta.2015.12.100Get rights and content

Abstract

We describe a current amplification system that employs a fluorinated nanocarbon (F-nanocarbon) film electrode formed by unbalanced magnetron (UBM) sputtering with a short CF4 plasma treatment. The F-nanocarbon film exhibited the typical electrochemical reaction of a ferrocene-based mediator while strongly suppressing the electrochemical oxidation of Fe2+ ions. This selectivity provided the current amplification of ferrocene mediators with Fe2+ ions solely by using the F-nanocarbon film electrode without interference from the direct oxidation current of Fe2+ ions. The current amplification system was used to realize an electrochemical biosensor with superior performance for detecting lipopolysaccharides. A detection limit of 2 ng mL−1 with good reproducibility (RSD of 4.2%) was achieved thanks to the very low noise made by possible by the ultraflat and hydrophobic surface.

Introduction

The electrochemical performance of carbon film electrodes can be widely controlled by controlling the surface termination/doping/modification of other atoms [1]. Various surface terminations on carbon-based electrodes have been reported including hydrogen [2], [3], oxygen [4], nitrogen [5], fluorine [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] and metals [16], [17] for modulating electron transfer rates, which are dependent on analytes. Fluorine is a fascinating atom because it has high hydrophobicity and the highest electro-negativity in the periodic table. Fluorination has been reported for various carbon electrodes including graphite, glassy carbon (GC), carbon nanotubes (CNT), carbon nanofiber, graphene and boron-doped diamond (BDD) [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Although these fluorinated carbon electrodes provide unique characteristics such as improved hydrophobicity and a different electron transfer rate from those of original carbon electrodes, the property of a fluorinated GC electrode is frequently lost because fluorine atoms are often removed from the GC electrode during electrochemical measurement [7], [15]. Furthermore, the fluorinated GC electrode is unstable as regards high potential polarization or continuous measurement because its surface is easily oxidized [15], [18]. In contrast, fluorinated BDD electrodes exhibit better long-term stability [12], [14], suggesting that a fluorinated surface containing sp3 carbon exhibits less oxidization and damage under anodic polarization than GC.

We previously reported electrochemically stable fluorinated nanocarbon (F-nanocarbon) film electrode formed by electron cyclotron resonance (ECR) sputtering with a short CF4 plasma treatment [15]. The nanocarbon film electrode has a nanocrystalline sp2 and sp3 mixed-bond structure with an atomically flat surface. The fluorinated surface is easily prepared without losing the surface conductivity and surface flatness of the nanocarbon film electrode. The F-nanocarbon film electrode also exhibits high electrochemical selectivity for some species. For example, the F-nanocarbon film electrode suppresses the electrochemical oxidation of hydrophilic and inner-sphere species such as Fe2+/3+ and Fe(CN)63−/4− [15], thanks to its hydrophobic surface. In contrast, the responses of hydrophilic and outer-sphere Ru(NH3)63+/2+ are almost unchanged. The F-nanocarbon film has a very stable surface compared with fluorinated GC in terms of continuous electrochemical measurements [15]. In fact, the slow electron transfer rates for Fe2+/3+ and Fe(CN)63−/4− at the F-nanocarbon film electrode still remain after 20–50 potential cycles, whereas these slow electron transfer rates are easily recovered for fluorinated GC under the same conditions owing to the desorption of fluorine containing groups from the surface. We also employed the F-nanocarbon film electrode to selectively detect hydrophobic antioxidants in foods and drinks [19]. The F-nanocarbon film electrode exhibited fast electron transfer for hydrophobic α-tocopherol (vitamin E). In contrast, the electrochemical responses for hydrophilic antioxidants such as ascorbic acid (vitamin C) were effectively suppressed at the F-nanocarbon film electrode [19]. These properties allowed us to achieve selective and quantitative measurements of hydrophobic antioxidants while suppressing the responses of hydrophilic antioxidants in the analyte solution. We expect this selectivity to enable us to construct a current amplification system in combination with a relatively hydrophobic and outer-sphere ferrocene mediator and hydrophilic and inner-sphere Fe2+/3+ as a reductant. Current amplification systems for electron transfer mediators have been widely studied by using an enzyme-modified electrode to improve the sensitivity and detection limit of various biomolecules [20], [21]. If we achieve a current amplification system by redox cycling with an F-nanocarbon film electrode as shown Fig. 1(a), we can expect to use this system for bioelectroanalysis with a lower concentration and a high S/N ratio because the electrochemical inactivity of the F-nanocarbon film suppresses the direct oxidation of Fe2+ ions at a fluorinated surface.

Here we describe a current amplification system that uses an F-nanocarbon film electrode, which is unlike enzymatic amplification. We employed F-nanocarbon film to obtain the selective electrochemical reaction of ferrocene-based mediator against an Fe2+/Fe3+ redox couple. Our aim is to apply this approach to an electrochemical biosensor for detecting lipopolysaccharides (LPS) thus achieving superior performance to that reported in our previous studies [22], [23], [24].

Section snippets

Carbon film preparation and CF4 plasma treatment

In this study, nanocarbon film electrodes were deposited with the unbalanced magnetron (UBM) sputtering method [24], [25]. The film has relatively good electrochemical properties similar to those of our previously reported nanocarbon film formed using ECR sputtering [26], [27], [28], [29], [30], [31], [32], [33]. Briefly, the nanocarbon films were deposited on highly doped silicon (100) substrates with UBM sputtering equipment (Universal Systems, Japan) at room temperature (without substrate

Results and discussion

We first characterized the surface properties of the F-nanocarbon film (Table 1). The sp2/sp3 content estimated by XPS measurement was 50/50 and this was in good agreement with a previous report that used ECR sputtered nanocarbon films [25]. After fluorination, the sp2 content decreased from 50.1 to 34.6%. At the same time, the F/C ratio was 0.15, which is similar to the result of a previous study [15]. These results clearly indicate that the sp2 bond is selectively fluorinated with CF4 plasma.

Conclusion

We successfully used an F-nanocarbon film electrode to construct a current amplification system in combination with ferrocene-based mediators and Fe2+ ions as a reductant. The F-nanocarbon film exhibited a typical electrochemical reaction of ferrocene-based mediators while strongly suppressing the electrochemical oxidation of Fe2+ ions. This selectivity, which was realized solely by using the F-nanocarbon film electrode, provided fine current amplification without interference from the direct

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research (D.K. No. 25288071) from the Ministry of Education, Culture, Science, Sports and Technology of Japan. This work was conducted in part at the Nano-Processing Facility, AIST, Japan. We thank Masumi Hirashima for help with the LAL experiments.

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