Nanoporous gold-based dopamine sensor with sensitivity boosted by interferant ascorbic acid
Graphical abstract
Introduction
Nanoporous materials have attracted numerous interests in a myriad of electrochemistry research areas in recent years, which has been demonstrated by their extensive applications in energy devices (battery, supercapacitors, fuel cells) [[1], [2], [3]] and electroanalysis (electrochemical sensing and biosensing) [4,5]. The high electrochemical activity of such materials was previously attributed mainly to large surface area [6], specific crystalline facets [7] and associated structural defects [8]. Recently, the nanoporous morphology has been also found to significantly influence the electrochemical process through the nanoconfinement phenomenon [9]. Nanoporous gold (NPG) has metallic foam like structure characterized by interconnected network of nanometric grains with multiple pore sizes that have been prepared by electrochemical methods including dealloying [10], template assisted electrodeposition [11] and anodization of a gold surface [12], but the basic nanoporous architecture is strongly dependent on the preparation method and conditions. NPGs have been extensively employed as electrocatalyst for oxygen reduction [13], evolution of hydrogen [14] and oxidation of carbon monoxide [15] and methanol [16], as well as in the development of electrochemical sensors for nitrite [17], dopamine [18] and ascorbic acid [19].
Dopamine (DA) is a type of catecholamine and a neurotransmitter present in the human central nervous system which plays an important role in many brain functions and behavior response such as feelings, motivation, learning and addictions [20]. Its optimum amount in the body is very important to regulate the physiological processes taking place in the brain thus maintaining the neurological, psychiatric, endocrine and cardiovascular health of a person [21]. An excess or low secretion of DA in the brain may cause neurodegenerative disorders, including Alzheimer [22] and Parkinson disease [23], demonstrating the importance of dopamine sensors for monitoring the neurological health of patients.
In this context, electrochemical sensors [[24], [25], [26]] are interesting given the good sensitivity and selectivity, but their application is hampered by the interference of ascorbic acid (AA), which coexists with DA in human body fluids in more than 1000-fold larger concentrations, implying that only highly selective sensors can actually be used in real sample measurements. Such a challenge has been overcome conventionally either by applying a selective ion exchange membrane like Nafion [27] or by exploiting an adsorption preconcentration process using a platinum electrode [28]. However, the sensitivity of such platforms is often limited by higher noise level in the measurements. Electroanalytical platforms based on highly electrochemically active porous materials like zeolites [29] and NPG [[30], [31], [32]] have also been used to achieve sensitive and selective detection of DA. At such platforms, AA can be oxidized at different potentials, mainly due to the enhanced heterogeneous electron transfer kinetics. Besides the increased selectively and sensitivity obtained by using porous nanomaterials like NPG electrodes, no reports can be found in the literature suggesting that the presence of AA changes dramatically the voltammetric response of DA depending on the NPG morphology, increasing the relative intensity of its anodic signal.
Accordingly, hereon a simple strategy to boost the sensitivity of electrochemical sensors for quantification of DA is reported, which is an advancement from a previous work of our group [18] where an NPG microelectrode was synthesized by anodization of a gold microdisk and applied for DA detection. In the present work, a different type of NPG electrode prepared according to a bottom-up Dynamic Hydrogen Bubble Template (DHBT) electrodeposition approach [33] has been used and the structure and morphology of the film have been judiciously controlled in order to increase as much as possible the exposed electrocatalytically active sites. At the optimized platform the DA sensitivity is promoted by AA, which otherwise behaves as a potential interferant. Taking further our previous understanding of similar electrochemical processes, a mechanistic study on electrochemical measurements of AA and DA and their mutual interference has been performed with an emphasis to interlink structure, kinetics and electrochemical response of the materials. This was achieved by preparing the material at different electrodeposition conditions for further characterization of the structure, morphology and purity by electron microscopy and spectroscopy techniques, as well as X-ray diffraction. Systematic voltammetric studies were performed to correlate the electrodeposition parameters during NPG fabrication and its electroanalytical performance for detection of DA in the presence of AA. The electrochemical reactivity of different NPG surfaces was investigated by Scanning Electrochemical Microscopy (SECM) and the kinetics information extracted from such studies was correlated with the observed electroanalytical response towards AA and DA. Finally, the electrochemical sensing performance and analytical parameters of such platform for DA detection in large excess of AA were evaluated using DPV technique.
Section snippets
Materials and reagents
Analytical grade chemicals were used in this work without any further purification. All aqueous solutions were prepared using Milli-Q ultrapure water (resistivity ∼18 MΩ cm). Dopamine hydrochloride (C8H11NO2·HCl, Molecular weight: 189.64 g mol−1), l-ascorbic acid (C6H8O6, Molecular weight: 176.12 g mol−1, ≥99%), Gold(III) chloride trihydrate (Molecular weight: 393.83 g mol−1, HAuCl4·3H2O, ≥99.9%) and Phosphate Buffer Saline (PBS) tablets were purchased from Sigma-Aldrich. One tablet of PBS was
Electrochemical, morphological and structural characterization of NPG
The most relevant features of NPG electrodes are the interconnected nanoporous framework allied to exposed low index planes, which are absent in conventional gold electrodes that expose only the (111) plane. One of the simplest methods to confirm those features is by recording a CV in H2SO4 solution, typically in the 0.2–1.6 V range (Fig. 1). Accordingly, the NPG electrodes (NPG1, NPG2 and NPG3) present the characteristic anodic waves associated with the oxidation of gold (220), (200) and (111)
Conclusions
A simple strategy to overcome the problem of AA interference for detection of DA in real biological samples is presented. This was accomplished by exploiting the strong adsorption of DA on nanoporous gold electrodes, as compared with the much weaker adsorption of AA. A strong correlation of NPG film thickness, electrocatalytic activity and amount of electrocatalytic sites associated with higher surface energy of (220), (200) and (311) planes was demonstrated. AA electrooxidation was catalyzed
Notes
The authors declare no competing financial interest.
Acknowledgments
Authors are grateful to Sao Paulo State Research Foundation (FAPESP 2018/08782-1, FAPESP (2016-07461-1) and National Council for Scientific and Technological Development (CNPq 401581/2016-0, 303137/2016-9 and CNPq 402281/2013-6) for providing generous funding throughout the research project. Authors are also thankful to Fabiano Montoro at LNNano, CNPEM, Campinas for performing electron microscopy characterizations. Josué M. Gonçalves and Jéssica S. G. Selva (CNPq 141866/2016-0) thank CNPq for
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