Development and Evaluation of Gold 3D Cylindrical Nanoelectrode Ensembles

Gold 3D cylindrical nanoelectrode ensembles （NEEs）, 100 nm in diameter and 500 nm in length were prepared by electroless template synthesis in polycarbonate filter membranes, followed by selective controlled chemical etching. The morphology of the nanowires and cylindrical NEEs was imaged by scanning electron microscopy. The protruding nanoelectrodes were in good parallel order. EDX study showed that the nanoelectrode elements consisted of pure gold. The electrochemical evaluation of the 3D electrodes was conducted using the well known [Fe（CN）6]^3-/[Fe（CN）6]^4- couple. Cyclic voltammgrams （CV） show a very low double layer charging current and a higher ratio of signal to background current than 2D disc NEEs. Electrochemical impedance spectroscopy （EIS） indicates that the 3D cylindrical NEEs effectively accelerate the charge transfer process, which is in consistent with the results of CV. The linear relationship with a slope of 0.5 between lg Ipc and lg v shows that linear diffusion is dominant on the 3D cylindrical NEEs at conventional scan rates.     

Voltammetry of redox analytes at trace concentrations with nanoelectrode ensembles

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.     

Direct determination of pesticides in vegetable samples using gold nanoelectrode ensembles

Gold nanowires were produced by electrodeposition in polycarbonate membrane, with an average diameter of 200 nm and a height of about 2 μm. The nanowire array prepared by the proposed method can be considered as nanoelectrode ensembles (NEEs). An amperometric pesticides sensor based on gold NEEs has been developed and used for determination of phoxim and dimethoate in vegetable samples. The electrochemical performance of the gold NEEs has also been studied by the amperometric method. The electrode provided a linear response over a concentration range of 5.9 × 10^?5 to 1.2 × 10^?2 M for phoxim with a detection limit of 4.8 × 10^?6 M and 6.3 × 10^?5 to 1.1 × 10^?2 M for dimethoate. This sensor displayed high sensitivity and selectivity, long-term stability and wide linear range. In addition, the ellipsis of enzyme and the reactivation of enzyme make the operation simple. This sensor has been used to determine pesticides in a real vegetable sample.     

双极纳米电极阵列实现单个铂纳米颗粒上氢气析出反应的电致化学发光成像

本文制备了嵌于多孔阳极氧化铝（AAO）膜中直径为200 nm,间距为450 nm的高密度（5.7 × 10⁸ cm^-2）的金纳米电极阵列,纳米电极分布规则,尺寸高度均一。我们将该金纳米电极阵列作为双极电极阵列,可将电极一侧的电化学法拉第信号在另一侧电极上转化成电致化学发光（ECL）信号,从而实现对单个铂纳米颗粒上氢气析出反应（HER）进行亚微米空间分辨率的电化学成像。本文介绍的方法为高空间分辨率成像电催化材料、能源材料以及细胞过程的局部电化学活性提供了一个良好的平台。     

Recent advances in sensing and biosensing with arrays of nanoelectrodes

This review deals with recent advances in the field of electrochemical sensing and biosensing with nanoelectrode ensembles (NEEs) and nanoelectrode arrays (NEAs), focusing mainly on articles published since 2015. At first, a brief introduction on the properties and possible advantages which characterize electroanalytical signals at the NEE/NEA is presented, followed by an overview on the most recent theoretical advances concerning the modeling of relevant electrochemical signals. Novel nanofabrication methods and nanoelectrode materials are discussed together with original (bio)funtionalization procedures, suitable to obtain more sensitive and reliable sensors. Advanced applications of NEE/NEA-based sensors in the biological and biomedical field are presented, including their integration with living cells and application for neurochemical studies. Advances, present limits, and prospects for research in the area are finally discussed. As far as future research trends are concerned, on the one hand, there is a need for development of theoretical models which take into account specific effects that can rule electrochemistry with arrays of nanosized electrodes, such as double layer and quantum mechanical effects. On the other hand, frontier studies concerning the application of the NEE/NEA to the biomedical and neurochemical fields can open new tracks both to fundamental knowledge and application.     

Ensembles of nanoelectrodes modified with gold nanoparticles: characterization and application to DNA-hybridization detection

A new method to increase the active area (A _act) of nanoelectrode ensembles (NEEs) is described. To this aim, gold nanoparticles (AuNPs) are immobilized onto the surface of NEEs using cysteamine as a cross-linker able to bind the AuNPs to the heads of the nanoelectrodes to obtain the so-called AuNPs-NEEs. The analysis of the cyclic voltammograms recorded in pure supporting electrolyte showed that the presence of the nanoparticles reflects in an, approximately, ten-times increase in the electrochemically active area of the ensemble. The measurement of the amount of electroactive polyoxometalates, which can be adsorbed on the gold surface of NEEs vs. AuNPs-NEEs, confirmed a significant increase of active area for the latter. These evidences indicate that there is a good electronic connection between the AuNPs and the underlying nanoelectrodes. The possibility to exploit AuNPs-NEEs for biosensing application was tested for the case of DNA-hybridization detection. After immobilization on the gold surface of AuNPs-NEEs of a thiolated single-stranded DNA, the hybridization with complementary sequences labeled with glucose oxidase (GOx) was performed. The detection of the hybridization was achieved by adding to the electrolyte solution the GOx substrate (i.e., glucose) and a suitable redox mediator, namely the (ferrocenylmethyl) trimethylammonium (FA⁺) cation; when the hybridization occurs, an electrocatalytic increase of the oxidation current of FA⁺ is recorded. Comparison of electrocatalytic current recorded at DNA modified NEEs and AuNPs-NEEs indicate, for the latter, a significant increase in sensitivity in the detection of the DNA-hybridization event.     

Electrocatalytic H2O2 amperometric detection using gold nanotube electrode ensembles

Arrays of nanoscopic gold tubes were prepared by electroless plating of the metal within the pores of nanoporous polycarbonate track-etched membranes. A procedure for fabricating an ensemble of enzyme-modified nanoelectrodes has been developed based on the efficient immobilization of horseradish peroxidase (HRP) to the gold nanotubes array using self-assembled monolayers (mercaptoethylamine or mercaptopropionic acid) as anchoring layers. Hydrogen peroxide (H₂O₂) was determined electrochemically by using gold nanoelectrode ensembles (NEE) functionalized or not in phosphate buffer solution (PB) with or without a mediator (hydroquinone, H₂Q). Bare NEE displays a remarkable sensitivity (14 μA mM⁻¹ in H₂Q at −0.1 V versus Ag/AgCl) compared to a classical gold macroelectrode (0.41 μA mM⁻¹). The gold nanoparticles that form the tubular structure act as excellent catalytic surfaces towards the oxidation and the reduction of H₂O₂. The HRP modified NEE presents a slightly lower sensitivity (9.5 μA mM⁻¹) than bare NEE. However, this system presents an enhanced limit of detection (up to 4 × 10⁻⁶ M) and a higher selectivity towards the detection of H₂O₂ over a wide range of potentials. The lifetime, fabrication reproducibility and measurement repeatability of the HRP enzyme electrode were evaluated with satisfactory results.     

Gold nanoelectrode ensembles for direct trace electroanalysis of iodide

A procedure for the standardization of ensembles of gold nanodisk electrodes (NEE) of 30 nm diameter is presented, which is based on the analytical comparison between experimental cyclic voltammograms (CV) obtained at the NEEs in diluted solutions of redox probes and CV patterns obtained by digital simulation. Possible origins of defects sometimes found in NEEs are discussed. Selected NEEs are then employed for the study of the electrochemical oxidation of iodide in acidic solutions. CV patterns display typical quasi-reversible behavior which involves associated chemical reactions between adsorbed and solution species. The main CV characteristics at the NEE compare with those observed at millimeter sized gold disk electrodes (Au-macro), apart a slight shift in E_1/2 values and slightly higher peak to peak separation at the NEE. The detection limit (DL) at NEEs is 0.3 μM, which is more than one order of magnitude lower than DL at the Au-macro (4 μM). The mechanism of the electrochemical oxidation of iodide at NEEs is discussed. Finally, NEEs are applied to the direct determination of iodide at micromolar concentration levels in real samples, namely in some ophthalmic drugs and iodized table salt.     

Bioelectroanalysis with nanoelectrode ensembles and arrays

This review deals with recent advances in bioelectroanalytical applications of nanostructured electrodes, in particular nanoelectrode ensembles (NEEs) and arrays (NEAs). First, nanofabrication techniques, principles of function, and specific advantages and limits of NEEs and NEAs are critically discussed. In the second part, some recent examples of bioelectroanalytical applications are presented. These include use of nanoelectrode arrays and/or ensembles for direct electrochemical analysis of pharmacologically active organic compounds or redox proteins, and the development of functionalized nanoelectrode systems and their use as catalytic or affinity electrochemical biosensors.     

Nanoelectrode ensembles based on conductive polyaniline/poly(acrylic acid) using porous sol–gel films as template

Porous sol–gel (PSG) film has been utilized as a template for the electrochemical polymerization of aniline in presence of poly(acrylic acid) (PAA). The presence of electroactive polyaniline (PAn)/PAA within the porous skeleton of the sol–gel films has been confirmed using cyclic voltammetry, UV–vis spectrometry and atomic force microscopic measurements. The densities and the sizes of the nanoelectrodes can be controlled easily using electrochemical methods. The conductive polymer “wires” of PAn/PAA formation in PSG matrix can behave as an ensemble of closely-spaced but isolated nanoelectrodes. Moreover, the nanoelectrode ensembles based on conductive PAn/PAA for glucose biosensing are fabricated by immobilization of glucose oxidase (GOx) and Nafion onto the surface of conductive polymer. Owing to the biocompatibility of PSG and electro-activity of PAn/PAA at neutral pH regions, the glucose biosensor shows excellent characteristics and performance, such as low detection limit and fast response time.     

纳米阵列电极研究*

纳米阵列电极作为一种人工组装的纳米结构体系,具有高传质速率、低双电层充电电流、小时间常数、小IR降及高信噪比、可操作性强和测量灵敏度高等优势,因而在电化学理论研究、生物传感器、电催化材料和高能化学电源电极材料等方面等具有广阔的应用前景。迄今为止,人们采用多种材料设计制备出包括圆盘状、圆柱形、球形、圆锥形、叉指状和井状等各种形状的纳米阵列电极。其制作方法主要包括模板法、刻蚀法和自组装法等,电极的表征主要采用电子显微镜技术和电化学方法。本文结合我们的工作和国内外文献,就纳米阵列电极制作方法、表征和应用等方面进行了评述,并对目前纳米阵列电极研究中存在的问题及发展前景进行了探讨和展望。     

Nanoelectrode ensembles for the direct voltammetric determination of trace iodide in water

Procedures for the preparation and characterisation of ensembles of gold nanodisk electrodes (NEE) of 30 nm diameter are presented, in particular focusing on improvements in the signal/background current ratios and detection limits with respect to the electrochemical oxidation of iodide and its analytical determination in water samples. At NEEs iodide undergoes a quasi-reversible diffusion controlled oxidation with a slight shift in E _1/2 values and slightly higher peak to peak separation with respect to conventional gold disk electrodes. The double layer charging current at the NEE is significantly lower than at conventional electrodes so that the detection limit (DL) by cyclic voltammetry with NEEs in tap water is significantly lower than DL at the Au-disk millimetre-sized electrode (DL 0.3 µM at NEE vs. 4 µM for Au-disk). Finally, it is shown that NEEs in combination with square wave voltammetry can be applied for the direct determination of iodide in water samples from the lagoon of Venice, with a detection limit of 0.10 µM.     

Ultrasensitive electrocatalytic DNA detection at two- and three-dimensional nanoelectrodes

Electrochemical DNA detection systems are an attractive approach to the development of multiplexed, high-throughput DNA analysis systems for clinical and research applications. We have engineered a new class of nanoelectrode ensembles (NEEs) that constitute a useful platform for biomolecular electrochemical sensing. High-sensitivity DNA detection was achieved at oligonucleotide-functionalized NEEs using a label-free electrocatalytic assay. Attomole levels of DNA were detected using the NEEs, validating the promise of nanoarchitectures for ultrasensitive biosensing.     

One step fabrication of nanoelectrode ensembles formed via amphiphilic block copolymers self-assembly and selective voltammetric detection of uric acid in the presence of high ascorbic acid content

A novel one-step approach to glassy carbon nanoelectrode ensembles (NEEs) with the pores of 20-120 nm in radii has been developed using an amphiphilic block copolymer [polystyrene-block-poly (acrylic acid)] self-assembly. This procedure is simple and fast, and requires only conventional, inexpensive electrochemical instrumentation. Electrochemical methods were used to characterize the NEEs prepared using this new procedure. The NEEs drastically suppressed the response of ascorbic acid (AA) and resolved the overlapping voltammetric response of uric acid (UA) and AA into two well-defined peaks with a large anodic peak difference (ΔE_pa) of about 310 mV. The peak current obtained from differential pulse voltammetry (DPV) was linearly dependent on the UA concentration in the range of 0.25-50 μM at neutral pH (PBS, pH 6.86) with a correlation coefficient of 0.999, and the detection limit was 0.04 μM (S/N = 3). The NEEs has also been demonstrated to be applicable in the detection of UA in serum and urine samples with excellent sensitivity and selectivity. The NEEs will hopefully be of good application for further sensor development.     

Nanofabrication of the gold scanning probe for the STM-SECM coupling system with nanoscale spatial resolution

Scanning probe is the key issue for the electrochemical scanning probe techniques (EC-SPM) such as EC-scanning tunnel microscopy (STM), EC-atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM), especially the insulative encapsulation of the nanoelectrode probe for both positioning and electrochemical feedbacks. To solve this problem, we develop a novel fabrication method of the gold nanoelectrodes: firstly, a micropipette with nanomter-sized orifice was prepared as the template by a laser puller; secondly, the inside wall of micropipette apex was blocked by compact and conic Au nano-piece through electroless plating; thirdly, the Au nano-piece was grown by bipolar electroplating and connected with a silver wire as a current collector. The fabricated Au nanoelectrode has very good voltammetric responses for the electrodic processes of both mass transfer and adsorption. The advantage lies in that it is well encapsulated by a thin glass sealing layer with a RG value lowered to 1.3, which makes it qualified in the SECM-STM coupling mode. On one hand, it can serve as STM tip for positioning which ensures the high spatial resolution; on the other hand, it is a high-quality nanoelectrode to explore the local chemical activity of the substrate. The nanofabrication method may promote the SPM techniques to obtain simultaneously the physical and chemical images with nanoscale spatial resolution, which opens a new approach to tip chemistry in electrochemical nanocatalysis and tip-enhanced spectroscopy.     

Amplified Electrochemical DNA Sensor Based on Polyaniline Film and Gold Nanoparticles

In this work, an electrochemical DNA biosensor, based on a dual signal amplified strategy by employing a polyaniline film and gold nanoparticles as a sensor platform and enzyme‐linked as a label, for sensitive detection is presented. Firstly, polyaniline film and gold nanoparticles were progressively grown on graphite screen‐printed electrode surface via electropolymerization and electrochemical deposition, respectively. The sensor was characterized by scanning electron microscopy (SEM), cyclic voltammetry and impedance measurements. The polyaniline‐gold nanocomposite modified electrodes were firstly modified with a mixed monolayer of a 17‐mer thiol‐tethered DNA probe and a spacer thiol, 6‐mercapto‐1‐hexanol (MCH). An enzyme‐amplified detection scheme, based on the coupling of a streptavidin‐alkaline phosphatase conjugate and biotinylated target sequences was then applied. The enzyme catalyzed the hydrolysis of the electroinactive α‐naphthyl phosphate to α‐naphthol; this product is electroactive and has been detected by means of differential pulse voltammetry. In this way, the sensor coupled the unique electrical properties of polyaniline and gold nanoparticles (high surface area, fast heterogeneous electron transfer, chemical stability, and ease of miniaturisation) and enzymatic amplification. A linear response was obtained over a concentration range (0.2–10 nM). A detection limit of 0.1 nM was achieved.     

Sol-gel route to zirconia-Pt-nanoelectrode arrays 8 nm in radius: their geometrical impact in mass transport

The fabrication of advanced nanoelectrode arrays and their electrochemical characterization are presented. These nanoelectrode arrays are constituted of nanoperforations of 8 nm in radius leading to platinum and protected by an inorganic matrix made of crystalline zirconia. These nanoelectrodes arrays provide a ceramic support with a high thermal and chemical stability. These devices present a well characterized structure with a control of size, shape, and spacing of the nanoelectrodes, allowing studying in depth both the mass transport and the charge transfer properties in the nanometer range. The radial diffusion occurs when the experimental scan rate is superior to a theoretical scan rate estimated from the model proposed by Amatore and colleagues. The coupling between electrochemical analysis and nanoscale structural characterizations successfully demonstrates that the theory defined for microelectrode arrays can be directly transposed for well-defined metal-ceramic nanocomposite nanoelectrodes.     

Flow Injection Amperometric Detection at Enzyme‐Modified Gold Nanoelectrodes

Gold nanotubular electrode ensembles were prepared by using electroless deposition of the metal within the pores of polycarbonate particle track‐etched membranes. Glucose oxidase (Gox), used as a model enzyme, has been immobilized onto preformed self‐assembled monolayers (mercaptoethylamine or mercaptopropionic acid) on electroless gold via cross‐linking with glutaraldehyde or covalent attachment by carbodiimide coupling. Flow‐injection analysis systems in flow‐through or wall‐jet configurations using these Gox‐modified nanoelectrodes are described. The influence of different experimental parameters (i.e., applied potential, flow rate, interferents…?) on the analytical response of the sensor to glucose has been evaluated. Under optimized conditions, very reproducible results (standard deviations <4%, n=38) were obtained, linear calibration was achieved in the 2×10^?4 M to 3×10^?2 M concentration range and the detection limit was 2×10^?4 M. Moreover, no significant interferences from species like ascorbic and uric acids were observed at a potential of +0.9 V.     

Development and Characterization of Nickel‐NTA‐Polyaniline Modified Electrodes

The engineered addition of hexa‐histidine sequences to biomolecules such as antibody fragments has been found to be an excellent means of purifying these materials. This tagging methodology has also been extended to its use as a tool for immobilization and orientation of antibodies on transducer surfaces. Polyvinyl sulfonate‐doped polyanilne (PANI/PVS) can be used as a mediator in amperometric biosensors. This short communication looks at the effect of nickel chelate materials and nickel chelation on this conducting polymer and evaluates it as a potential surface for the immobilization of his‐tagged biomolecules. N‐nitrilotriacetic acid (NTA) was doped into the electropolymerized PANI/PVS at a screen‐printed carbon paste electrode. The resulting NTA‐PANI/PVS film was shown to have comparable electrochemical properties of polymer without the chelating agent. When Ni²⁺ was applied to the electrode, the incorporated NTA was found to efficiently chelate the metal ions at the electrode surface.     

普鲁士蓝纳米修饰电极的制备及表征

采用激光加热拉伸的方法制备铂纳米电极,并通过交流电刻蚀的方法制备纳米孔电极,在这两种电极上可通过电化学方法原位合成单颗普鲁士蓝微晶. 结果表明,普鲁士蓝微晶在纳米微孔电极上的机械附着强度增强. 这种方法可用于制备纳米修饰电极或研究功能微晶体材料的电化学性质.