Microchim Acta (2012) 179:337–343 DOI 10.1007/s00604-012-0860-z
SHORT COMMUNICATION
Comparison of three different configurations of dual ultramicroelectrodes for the decomposition of S-Nitroso-L-glutathione and the direct detection of nitric oxide Achille Nassi & Loan To Thi Kim & Aurélie Girard & Laurent Griscom & Florence Razan & Sophie Griveau & Laurent Thouin & Fethi Bedioui
Received: 23 March 2012 / Accepted: 1 June 2012 / Published online: 11 July 2012 # Springer-Verlag 2012
Abstract We report on the characterization and use of three configurations of dual gold ultramicroelectrodes (UMEs), namely a ring-disc electrode, a disc-disc electrode, and band-band electrode. These can be used for the direct determination of nitric oxide that is released during the coppercatalyzed decomposition of S-nitroso-L-glutathione (GSNO). The dual UMEs were electrochemically characterized by
A. Nassi Laboratoire de Chimie Analytique, Département de Chimie, Faculté des Sciences, Université de Douala, B.P. 24157, 24167, Douala, Cameroun A. Nassi : L. To Thi Kim : S. Griveau : F. Bedioui (*) Chimie ParisTech, Ecole Nationale Supérieure de Chimie de Paris, Unité de Pharmacologie Chimique et Génétique et Imagerie, 75005, Paris, France e-mail:
[email protected]
using ferrocenemethanol as a soluble redox mediator during chronoamperometric measurements. The data are similar to those reported in the literature for various configurations and can be used to characterize the ability of copper ions to reach the vicinity of the NO sensor. One UME was electrochemically modified with layers of a poly(eugenol)/poly(phenol) composite to act as an NO sensor. The second UME was electrochemically coated with a copper layer that serves as a source for Cu(II) that is needed for the in-situ decomposition of GSNO. The mediated decomposition of GSNO is accomplished in presence of ascorbate that acts as reducing agent for Cu(II). The NO released from GSNO is detected at the same potential as applied to form the Cu(II)-based catalyst (+0.8 V vs Ag/AgCl). Keywords Nitrosothiols . Nitric oxide . Electrochemical sensor . Copper catalyst . Ultramicroelectrode
L. To Thi Kim : S. Griveau : F. Bedioui CNRS, Unité de Pharmacologie Chimique et Génétique et Imagerie UMR 8151, 75005, Paris, France L. To Thi Kim : S. Griveau : F. Bedioui Université Paris Descartes, Unité de Pharmacologie Chimique et Génétique et Imagerie, 75006, Paris, France L. To Thi Kim : S. Griveau : F. Bedioui INSERM, Unité de Pharmacologie Chimique et Génétique et Imagerie (N° 1022), 75006, Paris, France A. Girard : L. Griscom : F. Razan CNRS 8029 SATIE, Ecole Normale Supérieure de Cachan-Bretagne, 35170, Bruz, France L. Thouin CNRS 8640, Ecole Normale Supérieure, 75005, Paris, France
Introduction In living animal organisms, a part of the continuous production of nitric oxide (NO) by the vascular system is converted into S-nitrosothiols (RSNOs) via nitrosylation of thiols. Therefore, it is believed that RSNOs are carriers and reservoirs of NO in blood plasma and play important physiological roles [1, 2]. Although various techniques have been reported for RSNO detection and understanding their vital roles [3, 4], electrochemical methods are the only presently available means for their direct and real time monitoring. The principle of the electrochemical detection of RSNOs is based on the monitoring of NO released upon their catalytic
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decomposition. Indeed, RSNOs undergo homolytic cleavage of the S-N bond leading to the release of NO, with several factors affecting this reaction [5, 6]. In general, it is suggested that Cu2+ has a powerful effect on decomposing RSNOs in the presence of a reducing agent [2, 7–9] such as cysteine or glutathione (GSH), or the natural antioxidant ascorbate. In culture media, these molecules are endogenously present as Cu2+ species, which bind to amino acids, peptides and proteins accessible for thiolate ions and are reduced to Cu+, the catalytically active species, leading to NO production according to global scheme: GSNO
Cuþ ! GS
þ NO
The electrochemical method consists then in detecting NO generated from decomposition of RSNOs. In the last 10 years, Meyerhoff et al. developed NO-sensor polymeric materials containing immobilized copper [10–13] or selenium species [14, 15] aimed at decomposing RSNOs to NO. The authors showed the possibility of electrochemically detecting RSNOs by utilizing these materials as coating layer at the distal end of the amperometric NO-sensor in phosphate buffer solution containing EDTA and ascorbate. Recently, we presented the design, microfabrication and amperometric performance of a sensing arrayed device for the direct detection of NO generated from decomposition of S-nitroso-L-gluthatione GSNO through gold ring-disc ultramicroelectrodes (RD-UMEs) [16] where each pair of RDUMEs consists in a central copper disc aimed at being the source of Cu+ catalyst for RSNO decomposition, surrounded by a ring gold electrode aimed at acting as NOsensor with an insulating gap in between. In this study we report on the comparative evaluation of the RD-UMEs configuration with two of the most conventional dual configurations, dual disc-disc (DD) and dual band-band (BB) UMEs [16–34]. Although dual electrode RD, DD and BB models were developed specifically for electroanalytical applications, their use in a combined configuration as source of catalytic moieties and as a sensor has almost never been developed. In the particular case of NO detection, where concentration is low, the dual UMEs dimensions were deliberately chosen large enough to enhance measurement performance and signal intensity. We also deliberately limited the choice of the dual UMEs configurations to those which are at our disposal in order to have preliminary indications on the tendency of the influence of their geometrical parameters and their size on the analysis of RSNO. Firstly, the characterization of each electrode configuration in generation-collection mode was performed with a soluble redox couple. Secondly, dual microelectrodes were used, after their chemical modification, for the detection of NO released from GSNO and their performances were evaluated in phosphate buffer solution (pH07.4).
Experimental Chemicals NO donor compound, DEA-NONOate (diethylammonium (Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate) was from Cayman Chemical (USA, www.caymanchem.com) and SNitroso-L-glutathione (GSNO) was from Alexis Biochemical, both were kept at −20 °C. Silver (99.9 %) wires were obtained from Goodfellow (UK, www.goodfellow.com). All other chemicals were reagent grade and were used without further purification. All aqueous solutions were made using ultra-pure water with a resistivity of 18.2 M Ω.cm. Phosphate buffer solution was prepared by mixing 78.2 mL of NaOH (0.1 mol L−1) and 100 mL of NaH2PO4 (0.1 mol L−1). GSNO solution was freshly prepared by dissolving GSNO in ultra-pure water. The obtained solutions were aliquoted and kept at −20 °C in the dark for maximum 2 months. Design and microfabrication of the dual UMEs Dual RD-UMEs were patterned to fit on a 50 mm diameter borosilicate substrate where the electrode area is contained within a 15 mm diameter defined by the holding cell [35]. Its fabrication method was the same as previously reported [35]. Briefly, two titanium 30 nm and gold 200 nm layers were evaporated on a glass substrate and patterned using photolithography to create an array of ultramicroelectrode pairs and passivated with a 1 μm thick parylene layer. The array consists of 7 RD-UMEs pairs and 4 integrated counter microelectrodes. The RD-UMEs pairs are configured concentrically with a 50 μm diameter central disc electrode surrounded by a ring electrode, which is in fact a 3/4th semi-circle with a 100 μm inner-diameter and 200 μm outer-diameter (Fig. 1a). This design allows a 25 μm separation between the disc and the ring, which have respective areas of 0.002 mm2 and 0.07 mm2. Dual DD-UMEs were prepared by sealing short pieces of gold wires (Goodfellow, UK) in θ-type borosilicate glass capillaries (Hilgenberg, Malsfeld, Germany, www.hilgenberg-gmbh.com). Gold wires (diameter050 μm) were inserted into each of the wide ends of the capillary and
a
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gap g
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width w
Fig. 1 Structure of the Ring-Disc, Disc-Disc and Band-Band UMEs
Three different configurations of dual ultramicroelectrodes
gently pushed to the pulled end. The tip was then sealed by heating under vacuum and polished carefully using emery paper (Presi, France, www.presi.com) and diamond suspension of particle sizes 1 and 0.25 μm on a polishing cloth (Lamplan, France, www.polishing-tehcnology.com). The polishing step was carefully controlled in such a way that the electrode surface was perpendicular to the polishing wheel. Electrical contact with the gold in the capillary was established by inserting a piece of Cu wire from the open end of the capillary, and the wire was kept in place with silver glue. This design allows a 150 μm separation between the two disc UMEs, which have 0.002 mm2 area (Fig. 1b). Dual BB-UMEs were composed of a set of parallel gold microband electrodes (width w012, 20 or 30 μm wide and 2 cm long) patterned on a glass substrate by optical lithography and liftoff techniques, as previously reported [36]. The microbands were separated by a well-defined gap g varying from 10 to 100 μm. The characteristic dimensions (w, g) of each double-band assembly were controlled by optical microscopy (Fig. 1c). The effective microband length (l≈0.4–0.6 cm) was delimited by an adhesive tape before immersion of the device into the solution. After the microfabrication stage, 500 nm of copper was electrodeposited onto the central gold disc UME of the RD configuration or onto one disc of the dual DD configuration or on one band of the BB configuration by applying −0.4 V vs. Ag/AgCl (faradic charge: 0.106 mC) in aerated solution of 0.1 mol L−1 CuSO4 to form the required Au/Cu UMEs device. Electrochemical technique and surface characterization Dual RD- and BB-UMEs platforms were inserted in a twopart holding cell that includes pogo for external electrical contacts. Conventional electrochemical cell was used for the dual DD-UMEs device. All experiments were achieved at room temperature (25±3 °C) in aerated conditions. An Ag wire electrochemically coated by AgCl was used as external pseudo-reference electrode. Electrochemical experiments were carried out using a Priceton Applied Research potentiostat (model 263A) or Picostat (eDAQ, Australia) for cyclic voltammetry experiments or QuadStat (eDAQ) for amperometric measurements. Simultaneous chronoamperometric measurements of NO detection from decomposition of GSNO at one of the dual UMEs and oxidation of copper from the second UME were achieved by using common reference and counter electrodes. Preparation of NO-sensor and detection of GSNO Gold ring, disc or band UMEs dedicated to the electrochemical NO detection were coated with poly(eugenol) and poly (phenol) layers to ensure adequate selectivity [34]. For NO
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measurements, a potential of 0.8 V vs. Ag/AgCl was applied. NO sensor calibration was performed with DEANONOate solutions [35] within a dynamic range from 0.2 to 500 μmol L−1. For example a sensitivity of 0.13 nA/ μ mol L−1 was determined for the RD-UMEs configuration and the calculated limit of detection (signal/noise ratio03) is 27 nM. NO-release from decomposition of GSNO by Cu2+ based catalyst was measured at modified UME during generation of copper species from paired UME at the same potential (0.8 V vs. Ag/AgCl) in presence of ascorbate. More precisely, the dissolution of copper from band or disc UME was achieved by oxidation at 0.8 V vs. Ag/AgCl in 0.1 mol L−1 phosphate buffer solution+10 μmol L−1 EDTA+100 μmol L−1 ascorbate +10 μmol L−1 GSNO few minutes after the baseline current of the NO-sensor poised at +0.8 V vs Ag/AgCl becomes constant.
Results Electrochemical characterization of the dual UMEs configurations As mentioned above, the devices are dedicated to the direct detection of NO generated from decomposition of GSNO by Cu2+ based-catalyst. To do so, the transport of copper ions generated from the copper UME (disc or band) to the adjacent ring, disc or band UME that constitutes the NOsensor should be relatively rapid and controlled by diffusion process to allow in situ local decomposition of GSNO. The collection efficiency (CE), defined as the ratio collector/ generator currents of a conventional soluble redox probe such as ferrocenemethanol Fc-MeOH, can be used as indicator of the efficiency of the three different electrode configurations. Although the electrochemical couple Cu/Cu2+ involved in the required process does not imply a simple electron transfer, the estimated CE from Fc-MeOH can be indicative of the effective transport of Cu2+ ion from the copper-coated UME to the vicinity of the second UME that serves as NO-sensor. When using Fc-MeOH as a soluble simple redox mediator, the collection efficiencies were determined from chronoamperometric measurements as illustrated in Fig. 2. The values are reported in Table 1 and gathered together with previously reported ones for various and different configurations of dual UMEs. In fact, the dimensions and the shape of the dual RD, DD and BB electrodes, as well as the inter-electrode distance have important implications on the observed electrochemical response due to the overlap of the diffusion profiles of adjacent electrodes. The best collection efficiencies (≈40 %) are obtained for RD and BB configurations with interelectrode distances of 25 μm and 21 μm, respectively.
340 Fig. 2 Generation-collection experiments for the three dual UMEs configurations in 5 mmol L−1 Fc-MeOH in phosphate buffer solution: (a) RD (b) DD and (c) BB (g0 30 μm and w012 μm). The generating electrode is poised at +0.35 V while that collecting is poised at −0.2 V. For RD experiments (a), the disc acts as generator and the ring as collector
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those of the classical BB-UMEs configuration with equivalent dimensions (CE≈40 %, Table 1), thus indicating that our proposed RD-UME configuration has rather good performances and can be used confidently. Electrochemical detection of NO from decomposed GSNO NO-release from the decomposition of GSNO by Cu+ species were measured at the NO-sensor UME at +0.8 V. Micromolar GSNO concentrations were used, as expected for plasma RSNO carriers (1–100 μmol L−1) [42, 43], in presence of ascorbate and EDTA (used to chelate free copper residues initially present in the buffer medium). Figure 3 illustrates the typical amperometric responses of the NOsensor in the three configurations associated to NO release from GSNO upon the electrical connection of the Cu-UME at +0.8 V. As soon as the Cu-UME is connected, the dissolution of the copper coating starts, releasing Cu2+ ions, followed by the catalytic decomposition of GSNO in close proximity to the NO-sensor. The shape of the amperograms is indicative of a fast formation of NO through the decomposition of GSNO by Cu+ which lasts for 20 to 200 s, depending on the configuration of the dual UMEs. The shape of the amperograms are similar to those reported in the literature for the decomposition of GSNO in presence of
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Cu2+ in bulk solution and in presence of various types of reducing agents [7]. Although there is a significant difference in the chemical nature of the catalytic processes, the shape of the amperograms are also similar to those related to NO released from the proton-catalysed decomposition of NO donor moieties (DEA-NONOate, for example [35]) in similar conditions (data not shown). In the case of the DD-UMEs, with an inter-electrode distance of 150 μm, the amperogram resulted in a very sharp shape and a low intensity indicative of a very low efficiency of the catalytic decomposition of GSNO. This is probably due to large distance between the copper source electrode and the NO-sensor that affects the decomposition profile of GSNO. Although the decrease of the current of the NO sensor below the baseline is intriguing, we have previously observed such behaviour in different situations where GSNO was decomposed using various reducing agents and Cu amounts [7]. For instance, we have no explanation. Table 2 recapitulates the obtained data. The reported values show that dual BB and RD-UMEs of similar CE values (≈40 %) gave the highest performances towards the decomposition of GSNO. It is noticeable that BB and RD-UMEs configurations give similar results in terms of current intensities for GSNO decomposition (0.77 and 0.81 nA, respectively), which is consistent since both configurations have similar CE (43 %, Table 1). These results thus show the proof of concept for the use of RD-UMEs configuration for the detection of NO from GSNO decomposition. Concerning our RD configuration, seven RD-UMEs are available on the chip [16] so that the current intensities of RSNO decomposition and thus the sensitivity of the measurements could be largely improved by using the array of RD-UMEs. In addition, their performances could be certainly improved by decreasing the gap between adjacent electrodes. The spacing between each electrode pair is one of the key parameter in the detection system proposed. Work is now on progress to make comparison with electrodes of different geometries (DD, RD, BB) with similar dimensionless parameters.
Table 2 Amperometric current measured at the NO-sensor at 0.8 V vs Ag/AgCl upon the oxidation of the Cu UME at 0.8 V vs. Ag/AgCl in 0.1 mol L−1 phosphate buffer solution +10 μmol L−1 M EDTA +100 μmol L−1 ascorbate +10 μmol L−1 GSNO
Fig. 3 Amperograms of NO- sensor UME at +0.8 V vs Ag/AgCl in 0.1 M phosphate buffer solution +10 μmol L−1 EDTA +100 μmol L−1 ascorbate +10 μmol L−1 GSNO upon the dissolution of copper layer by oxidation at 0.8 V after a stable baseline is reached (a) RD-UMEs; (b) DD-UMEs and (c) BB-UMEs
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Conclusion Novel sensing devices made from RD, DD and BB-UMEs for direct detection of NO released from GSNO was developed in this work. This was achieved through electrochemically controlled release of Cu2+ used as catalyst within the NO-sensor in phosphate buffer solution containing ascorbate and EDTA. NO released from GSNO is rapidly detected with significant intensity of current. The obtained results demonstrate the possibility of applying a RD-UMEs sensing device for detection of endogenous RSNO in biological fluids. A systematic study of the parameters governing the design of the dual UMEs should be carried out in order to lay down a precise rule. Acknowledgements Financial for this research was supported by “Agence Nationale de la Recherche” (ANR France) in the framework of the project MECANO ANR-08-PCVI-0018. AN is thankful to Université de Douala (Cameroun) for offering him the opportunity to travel to Paris for this work.
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