Elsevier

Talanta

Volume 62, Issue 5, 19 April 2004, Pages 1055-1060
Talanta

Short communication
Voltammetry of redox analytes at trace concentrations with nanoelectrode ensembles

https://doi.org/10.1016/j.talanta.2003.10.024Get rights and content

Abstract

Gold nanoelectrodes ensembles (NEEs) have been prepared by electroless plating of Au nanoelectrode elements within the pores of a microporous polycarbonate template membrane. Cyclic voltammograms recorded in (ferrocenylmethyl) trimethylammonium hexafluorophosphate (FA+ PF6) solutions showed that these NEEs operate in the “total-overlap” response regime, giving well resolved peak shaped voltammograms. Experimental results show that the faradaic/background currents ratios at the NEE are independent on the total geometric area of the ensemble, so that NEE can be enlarged or miniaturized at pleasure without influencing the very favorable signal/noise ratio. Differential pulse voltammetry (DPV) at the NEE is optimized for direct determinations at trace levels. DPV at NEE allowed the determination (with no preconcentration) of trace amounts of FA+, with a detection limit of 0.02 μM. The use of NEE and DPV in cytochrome c (cyt c) solutions showed the possibility to observe the direct electrochemistry of submicromolar concentration of the protein, even without the need of adding any promoter or mediator.

Introduction

Nanoelectrode ensembles (NEEs) are nanotech-based electroanalytical tools which can find application in a variety of fields [1] including electrochemical sensors [2].

Nanoelectrodes ensembles can exhibit distinct voltammetric response regimes depending on the scan rate or on the reciprocal distance between the electrode elements [1], [2]. When radial diffusion boundary layers overlap totally (radius of diffusion hemisphere larger then average hemi-distance between electrodes, slow scan rates) NEEs behave as planar macroelectrodes with respect to faradaic currents [3]. The current response is dominated by radial diffusion at each single element only when diffusion hemispheres do not overlap, i.e. at high scan rates or when distances between the nanoelectrodes are large [4], [5], [6].

Martin and Menon [3] showed that NEEs behave like electrodes with partially blocked surface for which the nanodiscs area (active area) corresponds to the unblocked surface of the latter case. The theory developed by Amatore et al. [7] for electrodes with partially blocked surface has been applied successfully for measuring heterogeneous electron transfer rate constants at NEEs [3], [8]. Cyclic voltammetric responses at regular microdisc electrode arrays were also simulated recently [9], showing that there is a minimum number of elements in the array above which the normalized current response become independent on the size of the array.

As far as signal to background current ratios is concerned it was reported [2] that the cyclic voltammetric responses for a reversible redox couple at a NEE which operates in total overlap conditions, are characterized by faradaic peak currents (IF) and capacitive background currents (IC) given as follows:IF(NEE)=2.69×105n3/2Ageomv1/2D1/2cbIC(NEE)=AactvCdwhere Aact is the active area (nanodiscs surface), Ageom the total geometric area of the ensemble (nanodiscs plus insulator), v is the scan rate, D the analyte diffusion coefficient, cb its bulk solution concentration and Cd is the double layer capacitance.

At a conventional electrode of surface equal to Ageom, in the same experimental conditions, the same parameters obey , :IF(conv.)=2.69×105n3/2Ageomv1/2D1/2cbIC(conv.)=AgeomvCdBy combining , , , , one gets:IFICNEE=IFICconvAgeomAact

Eq. (5) puts in evidence that the signal/background current ratio at the NEE is higher than the signal/background current ratio at a conventional electrode of the same geometric area for a proportionality factor that is the reciprocal of the fractional electrode area f, defined asf=AactAgeom

In theory, this ratio should be independent on the overall geometric dimension of the ensemble.

Experiments showed that for f values between 10−3 and 10−2 detection limits at NEEs were 2–3 orders of magnitude lower than with conventional electrodes [3], [8], [10]. However, NEEs of rather large geometric area (typically 0.07 cm2) were used in these experiments.

For a fixed Ageom value, the voltammetric signal at a NEE is maximum when a total overlap regime is operative, being lower in the case of a pure radial regime. In the latter case, in fact, only a certain percentage of the geometric area of the ensemble contributes to produce a faradaic current while, in the total overlap regime this percentage is 100%. Note that the Faradaic currents at NEEs in total overlap regime are identical to those at conventional electrodes of the same geometric area as the ensemble and that the improvement produced by the use of the NEE is all in the dramatic lowering of capacitive current.

The ability of NEEs to furnish well resolved cyclic voltammograms for trace redox species is particularly attractive for analytical purposes since, in principle, it allows the electrochemical detection of low analyte concentrations avoiding the use of tedious and time consuming preconcentration steps (both faradic and non-faradic). This seems particularly interesting for direct “in field” analysis for in-real-time environmental monitoring and for “in situ” and “in vivo” electroanalysis in biological samples. Also the possibility to lower the overall NEE dimension while keeping unaltered the signal/background ratio is attractive for NEEs miniaturization and use in small volume electrochemical cells.

The present paper reports the results of a study aimed to examine the dependence of typical analytical parameters such as faradaic current/background current ratios, sensitivities and detection capabilities of NEEs as a function of geometric factors involved in the fabrication of these electrode systems as well as the improvements eventually achievable by combining the use of NEEs with differential pulse voltammetry (DPV). Target analytes used to show these possibilities are reversible “simple” redox probes such as the (ferrocenylmethyl)trimethylammonium cation as well as more complex redox systems like the redox protein cytochrome c (cyt c).

Section snippets

Chemicals and reagents

Polycarbonate filtration membranes (SPI-Pore, 47 mm filter diameter, 6 μm filter thickness) having a nominal pore diameter of 30 nm, a nominal pore density of 6×108 pores cm−2 and coated with the wetting agent polyvinylpyrrolidone were used as the templates to prepare the NEEs. Commercial gold electroless plating solution (Oromerse Part B, Technic Inc.) was diluted (40 times with water) prior to use. (Ferrocenylmethyl)dimethylamine (Aldrich) was reacted with methyl iodide to form the quaternary

Cyclic voltammetric characterization

Cyclic voltammograms (CVs) recorded at the NEEs used in this work in 5 μM FA+ (10−3 M NaNO3 as the supporting electrolyte) showed peak shaped diffusion controlled patterns since Ip depends linearly on v1/2 [17] (where Ip is the faradaic peak current). Other characteristics of the CVs were comparable with previous reports [3], [8]; in particular, the capacitive currents were significantly lower than those observed at macro-electrodes of the same geometric area.

A feature which was not yet examined

Conclusions

It is shown that improvements in signal/background current ratios at NEEs are independent on the total geometric area of the ensemble; this is true if the fractional area is kept constant and if the dimensions of the ensemble are lowered to a size still large enough to contain a large number of nanoelements (e.g. our NEE with Ageom of 0.005 cm2 contains 4.8×106 nanoelectrodes). Note that NEEs warranty such an independence on the ensemble size for overall geometric areas much lower than those

Acknowledgements

Financial support from MIUR (Rome) is acknowledged.

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