Optimizing nitrogen incorporation in nanodiamond film for bio-analyte sensing

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Abstract

Nitrogen incorporated nanodiamond electrodes were fabricated using microwave plasma enhanced chemical vapor deposition. The N2 incorporation was achieved by the introduction of N2 gas along with H2 and CH4 gases in the plasma. In the context of this work, the H2:CH4 ratio was held fixed at 9:1 and the N2 content was varied. Three different levels of N2 incorporation were examined: 30 sccm (S1), 60 sccm (S2) and 90 sccm (S3). The Fe(CN)63−/4− redox couple in 0.1 M KCl was used for initial evaluation of the electrochemical properties of the 3 electrodes. Cyclic voltammetry was then used to study the detection of dopamine (DA), which is an important neurotransmitter, in 0.1 M phosphate buffered saline (PBS) at the physiologic pH of 7.4. Electrode S1, which has a microstructure resembling that of ‘ridges’, showed excellent electrochemical response as compared to the other two electrodes, S2 and S3 which exhibited sluggish reaction kinetics. Cyclic voltammograms from S1 show well defined and closely spaced redox peaks, not only for dopamine, but also for its redox active metabolites which are produced due to reactions downstream of DA oxidation. The calibration curves for the three electrodes show linear behavior but S1 shows superior sensitivity towards DA detection. Also, a linear relationship between the DA concentration and √scan rate was observed which is consistent with semi-infinite linear diffusion limited mass transport mechanism for planar electrodes. These results have been achieved without the need of functionalization or modification of the electrode surface, using optimum N2 incorporated nanodiamond electrode.

Introduction

CVD diamond, such as boron-doped diamond and, more recently, nanodiamond, has shown interesting electrochemical behavior due to properties like chemical inertness, a wide working potential window, low background currents, resistance to fouling and mechanical stability [1], [2]. All these properties make diamond an ideal sensor for analysis of biological samples, like blood, urine and cerebral fluids.

Catecholamine neurotransmitters belong to a biologically important group of compounds which are mediators in transmitting messages between neurons. Dopamine (DA), widely distributed in the mammalian brain, is of particular importance among the class of neurotransmitters. It plays an essential role in functioning of the central nervous, renal, hormonal and cardiovascular systems. Abnormal dopamine level is related, among others, to the Parkinson's and Alzheimer's diseases, Tourette's syndrome, schizophrenia as well as pituitary tumors [3], [4]. Since dopamine is easily oxidized, electrochemical methods appear to be suitable for its quantitative determination, like cyclic voltammetry in our case. In this paper, we report the detection of DA in 0.1 M phosphate buffered saline (PBS) at physiologic pH 7.4 using nanodiamond electrodes having different levels of N2 incorporation.

It has been found that nanodiamond films can be made more conductive, by several orders of magnitude, by addition of nitrogen gas (up to 20%) in the PECVD gas mixture [5], [6]. Nitrogen containing nanocrystalline diamond films, where the N2 flow rate was varied from 0% to 5% in the feed gas consisting of Ar/CH4/N2 mixture, was used to study the electrochemical behavior by Chen [5]. With increasing N2 content, the electrochemical response of the nanodiamond electrodes showed wide working potential window, low background currents and fast reaction kinetics. Another study by Pleskov using nanodiamond films grown using a CH4/Ar/H2/N2 gas mixture revealed results consistent with previous observations [7]. They saw a transition in the nanodiamond film conductivity from poor to semi-metallic conductor as the N2 incorporation was varied from 0–25%. There was a significant enhancement in the electrochemical behavior in the electrode with 25% nitrogen.

In our previous experiments, N2 incorporated nanodiamond film with ‘ridge’ surface morphology was used for electroanalytical studies and exhibited excellent sensitivity towards detection of bio-analytes. The microstructure had distinctive ‘ridge’ like features, ~ 1 μm in length but only a few nanometers wide, with nanocrystallites on the walls of these so-called ‘ridges’ [8]. This nanodiamond film was grown at a lower N2 incorporation level, only ~ 9% of the input gas flow in the PECVD process. The current work involved growing nanodiamond films under similar conditions while increasing the N2 flow rates to higher levels, from ~ 16.7% to ~ 37.5% and studying the corresponding effect on the electrochemical behavior and bio-sensing response from the resulting nanodiamond electrodes.

Section snippets

Experimental

Three N2 incorporated nanodiamond electrodes were fabricated using microwave plasma enhanced chemical vapor deposition process on a highly doped n-type silicon substrate (ρ = 0.005 Ω cm). To aid in the nucleation and growth process, the substrate was first polished using diamond paste and then rinsed with methanol and deionized water. The flow rates of H2:CH4 gases were kept constant at 9:1 while increasing the N2 flow rate. This resulted in 3 electrodes having the following gas compositions: (H2

Results and discussions

Fig. 1 shows high-resolution scanning electron microscope (SEM) micrographs of the nanodiamond films. The images show a transition from a ‘ridge’ microstructure in case of electrode S1 to a more ‘cauliflower’ like microstructure of S3. The low magnification micrographs (not shown here) reveal a uniform and conformal growth of the nanodiamond films necessary for use as a planar electrode. The microstructure of S1 resembles to that seen previously, the only difference being in the size of those

Conclusions

Nitrogen incorporation during the PECVD process plays a very important role in the electrochemical properties of nanodiamond films. Not only the presence, but the relative content of N2 gas is very critical as well. Our studies reveal that nanodiamond electrodes, with excellent electrochemical and bio-sensing properties, can be fabricated using a contribution of N2 < 17% (S1) in the PECVD gas mix. Another aspect of the electrodes is the microstructure which can be correlated to the N2 content.

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