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Free-Standing Networks of Core-Shell Metal and Metal Oxide Nanotubes for Glucose Sensing Falk Muench, Luwan Sun, Tintula Kottakkat, Markus Antoni, Sandra Schaefer, Ulrike Kunz, Leopoldo MolinaLuna, Michael Duerrschnabel, Hans-Joachim Kleebe, Sevda Ayata, Christina Roth, and Wolfgang Ensinger ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13979 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016

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ACS Applied Materials & Interfaces

Free-Standing Networks of Core-Shell Metal and Metal Oxide Nanotubes for Glucose Sensing Falk Muench,*,†,ǁ Luwan Sun,† Tintula Kottakkat,‡ Markus Antoni,† Sandra Schaefer,† Ulrike Kunz,† Leopoldo Molina-Luna,† Michael Duerrschnabel,† Hans-Joachim Kleebe,† Sevda Ayata,§ Christina Roth‡ and Wolfgang Ensinger† * corresponding author: [email protected] † Technische Universität Darmstadt, Department of Materials and Earth Sciences, AlarichWeiss-Straße 2, 64287 Darmstadt, Germany ‡ Freie Universität Berlin, Department of Physical and Theoretical Chemistry, Takustraße 3, 14195 Berlin, Germany § Dokuz Eylul University, Science Faculty, Department of Chemistry, Tinaztepe Kampusu, Buca, 35160 Izmir, Turkey

KEYWORDS Electroless plating, metal nanotubes, core-shell nanostructures, ion-track technology, Kirkendall effect, enzyme-free glucose sensing.

ACS Paragon Plus Environment

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ABSTRACT

Nanotube assemblies represent an emerging class of advanced functional materials, whose utility is however hampered by intricate production processes. In this work, three classes of nanotube networks (monometallic, bimetallic, metal oxide) are synthesized solely using facile redox reactions and commercially available ion track membranes. First, the disordered pores of an ion track membrane are widened by chemical etching, resulting in the formation of a strongly interconnected pore network. Replicating this template structure with electroless copper plating yields a monolithic film composed of crossing metal nanotubes. We show that the parent material can be easily transformed into bimetallic or oxidic derivatives by applying a second electroless plating or thermal oxidation step. These treatments retain the monolithic network structure, but result in the formation of core-shell nanotubes of altered composition (thermal oxidation: Cu2O-CuO; electroless nickel coating: Cu-Ni). The obtained nanomaterials are applied in the enzyme-free electrochemical detection of glucose, showing very high sensitivities between 2.27 and 2.83 A M-1 cm-2. Depending on the material composition, varying reactivities were observed: While copper oxidation reduces the response to glucose, it is increased in the case of nickel modification, albeit at the cost of decreased selectivity. The performance of the materials is explained by the network architecture, which combines the advantages of onedimensional nano-objects (continuous conduction pathways, high surface area) with those of a self-supporting, open-porous superstructure (binder-free catalyst layer, efficient diffusion). In summary, this novel synthetic approach provides a fast, scalable and flexible route toward freestanding nanotube arrays of high compositional complexity.


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1. INTRODUCTION Electroless plating (EP) utilizes the surface-selective chemical reduction of metal ions to achieve conformal metallization with simple wet-chemical means.1-3 The plating solutions contain metastable redox pairs formed by metal complexes and reducing agents, whose conversion is restricted to catalytic surfaces which lower the activation barrier of the process. Due to the autocatalytic nature of EP reactions – the deposited metal is able to oxidize the reducing agent and thus provides electrons for further metal reduction – a continuously growing metal film is obtained. Compared to metallization techniques such as electrodeposition,1 atomic layer deposition,4 chemical vapor deposition5 or sputtering,6 EP stands out for its facility and versatility. It does not involve complex instrumentation, can be applied to sophisticated substrates (e.g. 3D printed objects,7 nanochannel network membranes,8 diatom frustules),9 is operated at ambient temperatures and does not require electrically conducting or heat-resistant samples.1-3 Making use of these favorable characteristics to produce nanomaterials with unusual and enhanced functional properties displays a major thrust in modern EP research.8-16 Due to the flexibility of the technique, a substantial range of application fields can be addressed, including sensing,8,11 plasmonics,9,11 flexible and transparent electrodes,10 heterogeneous catalysis,12,14 self-propelled micro-objects13 and self-cleaning coatings.15,16 In conjunction with specifically designed templates, the conformal growth mode of EP allows to create materials of tailored morphology,9,17,18 including 1D nanostructures and their assemblies.8,19-23 1D nano-objects such as nanotubes, nanowires or nanorods feature unique properties, represent important model systems and are promising for implementation in key


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technologies. For instance, metal nanotubes can act as unsupported electrocatalysts of excellent aging stability, metal utilization, conductivity and activity.5,8,24,25 The functionality of 1D nano-objects can be further augmented by assembling them to superstructures.26 Free-standing nano-architectures do not require binders,8,20,27,28 which cause diffusion barriers and adversely affect material performance.28,29 Likewise, nanostructure interconnection reduces deformation, fragmentation or detachment of individual nano-objects, which contributes to the activity loss during operation.30 A fully interconnected, open-porous superstructure simultaneously ensures continuous conduction pathways, high surface area and efficient mass transfer.8,20,27,28 Also, free-standing morphologies facilitate handling and device integration of 1D nanomaterials.26,30,31 Despite the relevance of metal and metal oxide nanotube networks (NTNWs), present synthetic approaches rely on intricate deposition techniques and / or template materials.8,20,32 For nanocasting purposes in research, dedicated ion track etched template membranes are available which are prepared using highly parallelized ion beams.12,20 The high degree of pore alignment translates to a precise orientation of the nanostructure replica, which is vital to realize welldefined material anisotropy.33,34 To produce crosslinked nanostructures, multiple irradiation steps are applied, which complicates membrane production.8,20,34 Regarding industrial use, ion track etched membranes are mainly employed in biomedical and filtration applications, which do not require strict pore alignment.35 For this target market, high irradiation yields and thus reduced production costs are beneficial. Accordingly, less efforts are devoted to a precisely aligned, parallelized irradiation, which results in a somewhat random orientation of the pores in commercial membranes.36 Moreover, with angular pore distributions, high membrane porosities can be realized without creating many pores overlapping alongside


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their full length, which in filtration or sterilization applications allows to increase flow rate and lifetime without losing size selectivity.37 While in certain electronic, magnetic or plasmonic applications exact 1D nanostructure arrangement plays a crucial role,33,34,38,39 it is not required to devise e.g. efficient catalyst architectures.40 In this study, we present a flexible and scalable approach to produce metal and metal oxide NTNWs, which only relies on commercially available membranes, EP and thermal oxidation. Intriguingly, the key factor enabling the fabrication of free-standing nano-networks is the disordered pore structure of the ion track membranes. Like the pore inhomogeneities in anodized alumina membranes enable the deposition of more complex nanowire structures with enhanced performance,30,41 we exploit the presence of random pore intersections in the commercial membranes to deposit monolithic metal nanotube films. In addition, we show that the Cu NTNW can be easily transformed into bimetallic or oxidic derivatives, which are still free-standing, but possess a unique core-shell structure. The materials chosen for our NTNWs (Cu, Cu oxides, Ni) are frequently employed in the electrochemical oxidation of glucose,42-48 in which they share mechanistic similarities: The named compounds probably oxidize carbohydrates by trivalent metal species acting as mediators.49,50 Employing the disordered NTNWs in this reaction thus allows us to demonstrate their catalytic efficiency, to investigate composition-performance relationships and to derive design strategies for nanostructured glucose sensors.

2. EXPERIMENTAL 2.1 CHEMICALS Purified water (Milli Q, > 18 MΩ cm) was employed in all procedures. Following chemicals were used as received: Ascorbic acid (Merck, 99.7%); borane dimethylamine complex (Sigma-


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Aldrich, 97%), CuSO4 · 5H2O (Sigma-Aldrich, ACS reagent), dichloromethane (Sigma-Aldrich, puriss. p.a.), ethanol (Brenntag, 99.5 %), ethylenediamine (Sigma-Aldrich, ReagentPlus), formaldehyde solution 36.5 % (Fluka, puriss. p.a.), fructose (Merck, puriss.); glucose (AppliChem, puriss. p.a.); HCl 37 % (AppliChem, p.a.), L-glutamine (Sigma, 99.5%); NaCl (Merck, suprapur); lactose (Merck, puriss.); NaOH solution 32% (Sigma-Aldrich, purum), NiSO4 · 7H2O (Sigma-Aldrich, purum p.a. cryst.), PdCl2 (Alfa Aesar, 99.9% metal basis), potassium sodium tartrate tetrahydrate (Fluka, puriss. p.a.), trisodium citrate dihydrate (SigmaAldrich, puriss. p.a.); urea (Sigma, BioReagent); uric acid (Sigma, 99%).

2.2 NANOSTRUCTURE SYNTHESIS Ion track etched polycarbonate membranes (ipPORE by it4ip, thickness = 25 µm) with a pore density of 1.5 × 108 cm-2 and a nominal pore size of 0.4 µm were additionally etched in stirred NaOH solution (6 M, 50 °C) for 10 min. Subsequently, the membranes were washed with water three times and dried in air. The as-obtained templates were activated by coating with nanoparticle seeds, which was performed according to a previously reported procedure.23 Compared to the named study, the seed metal Ag was replaced by Pd. Briefly, the polycarbonate membranes were sensitized in a Sn(II) solution, washed and transferred to an aqueous solution containing the nanoparticle precursor (0.01 M PdCl2, dissolved with 0.06 M HCl). After 3 min immersion, the templates were washed with ethanol. This process was repeated three times to increase the nanoparticle density.23 Cu deposition was performed at 8 °C by storing the activated template in the EP bath for 45 min, which contained 0.1 M CuSO4 (metal source), 0.22 M tartrate (ligand), 0.24 M ethylenediamine (ligand), 0.26 M NaOH (pH adjustment) and 1 M formaldehyde (reducing


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agent). After plating, the product was thoroughly washed with water, dried, and the template matrix was removed with dichloromethane. The core-shell NTNWs were fabricated by applying a second reaction step to the Cu parent structure: (i) Oxide NTs were obtained by annealing the Cu NTNW in air (225 °C or 400 °C, 2 h reaction time, followed by quenching to room temperature). (ii) Bimetallic nanostructures were obtained by immersing the Cu NWNT in an electroless Ni deposition bath for 3 min, which was composed of 0.1 M NiSO4, 0.1 M trisodium citrate and 0.1 M borane dimethylamine complex.

2.3 CHARACTERIZATION Scanning electron microscopy (SEM; JEOL JSM–7401F, 5-10 kV acceleration voltage) and Xray diffraction (XRD; STOE STADI P, operating with Cu Kα radiation in transmission geometry) was used to characterize the template-freed samples. For transmission electron microscopy (TEM) experiments, microtome cuts of the samples embedded in Araldite resin were prepared. Morphology analysis by TEM was performed with a FEI CM20 (200 kV acceleration voltage, LaB6 cathode). For scanning transmission electron microscopy (STEM) imaging in combination with energy-dispersive X-ray microanalysis (EDX) and electron energy-loss spectroscopy (EELS) experiments an aberration-corrected JEOL JEM ARM-200F (scanning) transmission electron microscope was used. The microscope was operated at 120 kV to reduce beam damage and to enhance the relevant EELS scattering cross-sections. It is equipped with a “Schottky” field-emission gun (FEG) and a Gatan Enfina EELS spectrometer. The energyresolution in EELS, as measured by the full-width half-maximum of the zero-loss peak, was about 0.8 eV. For STEM imaging a camera length of 6 cm, a 30 µm condenser aperture, a 3 mm bright-field aperture and a 4C spot size were used. This yields a convergence angle α = 24.5


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mrad, a maximum bright-field detector angle of 23 mrad and a high-angle angular dark-field (HAADF) angular detection range of 90-370 mrad. EELS and EDX experiments were carried out with a 50 µm condenser aperture corresponding to a convergence angle α = 42.2 mrad increasing the EELS signal. The camera length was reduced to 4 cm yielding a collection semi-angle β = 10.5 mrad. For low-loss EELS a 1 mm spectrometer entrance aperture was used whereas for core-loss it was a 3 mm one. Wide range spatially resolved STEM-EELS maps were acquired for measuring the O and Cu edges simultaneously using the following settings: 3 mm spectrometer entrance aperture, 8.8 nm pixel size, 10 s acquisition time per pixel, and 0.5 eV energy dispersion. A power law function and the Hartree-Slater model were used to remove the background and to quantify the composition. The integration widths for the quantification were 528.0 – 558.0 eV for the O–K and 930.0 – 970.0 eV for the Cu-L2,3 edge. In order to identify the phase, i.e. CuO or Cu2O, via the energy-loss near-edge structure (ELNES), two additional spatially resolved STEM-EELS maps were acquired at the same sample area with the same acquisition settings as before except the energy dispersion of the spectrometer, which was decreased to 0.1 eV per channel. All acquired EELS data was filtered using principal component analysis51 with 5 components to enhance the EELS signal to background ratio.52

2.4 ELECTROCHEMISTRY The electrochemical measurements were performed in a three electrode setup using a Gamry Reference 600 potentiostat. A Hg|HgO electrode (filled with 1 M NaOH, resulting in a potential of 140 mV versus the standard hydrogen electrode) and a Pt grid were used as reference and counter electrodes, respectively. The working electrode was prepared by mounting rectangular pieces of the template-freed catalyst film (approx. 4 × 4 mm size) with carbon paste8 on the tip of


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a glassy carbon electrode (3 mm diameter, polymer isolation). For all experiments, the electrolyte temperature was adjusted to 25 °C with a thermostat. Cyclic voltammetry (CV) was performed using a scan rate of 10 mV s-1 and 10 mL of 0.1 M NaOH as electrolyte, which has been optionally modified with 5 mM or 10 mM glucose. The shown CVs were recorded after cycling the electrodes 10 times in pure NaOH electrolyte, in order to achieve a stable response. The amperometric measurement of the catalyst sensitivity and selectivity was conducted at a potential of 0.7 V using 10 mL of 0.1 M soda lye as base electrolyte. Before the measurement, the working electrodes were held at the potential for 10 min to achieve a steady-state current. During amperometry, 20 µL aliquots of glucose stock solutions were injected into the stirred electrolyte with a pipette (5 additions per stock solution, four solutions containing 5, 10, 100 and 400 mM glucose, prepared freshly using 0.1 M NaOH as solvent). Depending on the concentration of the employed stock solution, each addition corresponds to an increase of the glucose concentration in the electrolyte by approximately 10 µM, 20 µM, 200 µM and 800 µM. The selectivity measurements were performed accordingly, but various analyte types were employed. Analyte injection was targeted not directly at the working electrode to avoid concentration and current spikes. Data points were accumulated each 0.3 s, which is a suitably short value to resolve the response times.

3. RESULTS AND DISCUSSION 3.1 SYNTHESIS AND DERIVATIZATION OF THE CU NANO-NETWORK The NTNW fabrication is summarized in Fig. 1. Depositing robust nano-networks in porous membranes necessitates an adequate density of intersections. In the commercial ion track etched polycarbonate membrane we use in this synthesis, these features correspond to inclined,


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overlapping pores, and their number increases with both pore density and size.37 To ensure a sufficient pore interconnection, we therefore employed a template membrane with a relatively high density of 1.5 × 108 pores per cm2, and additionally widened the pore diameter from initially about 430 nm to 770 nm by etching with soda lye (Fig. 1, step 1). A control experiment showed that without the increased pore diameter (and thus linkage), the mechanical stability of the produced NTNW is reduced, and fragmentation occurs during template removal (see Supporting Information, Fig. S1 a, b). In section 2 of the Supporting Information, a detailed analysis of the pore size distribution is given, and augmented by porosity calculations. While the porosity of the pristine membrane (≈ 19%) is close to the typically employed maximum value of about 20% (due to the membrane stability loss at high porosities caused by pore overlap),53 it is increased to ≈ 50% in the case of the etched membrane, demonstrating the pronounced effect of this step.

Figure 1. Scheme of the NTNW fabrication. After pore widening by chemical etching (step 1), Pd nanoparticles are deposited on the template membrane (step 2), which act as nucleation sites for Cu deposition (step 3). The Cu NTNW is isolated by dissolution of the template matrix (step 4), followed by optional derivatization reactions to create core-shell NTNWs (step 5).


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Pore widening is followed by attaching nanoparticles to the template surface (Fig. 1, step 2), which initiate the subsequent metal deposition. Here, we employed a standard seeding technique based on the reaction of superficially bound Sn2+ with a metal salt solution to form nanoparticles.23 In the cited study, Ag nanoparticles were produced by immersing the sensitized template in an ammoniac Ag+ solution. To obtain mechanically stable nanotubes with closed walls, a dense nucleation of the metal film on the whole inner template surface is vital, which strongly depends on the employed seed type.54 The Ag nanoparticles obtained with the named approach only showed moderate activity in our EP reaction. Replacing them with Pd seeds ensured homogeneous metal nanofilm deposition (Supporting Information, Fig. S1 c, d). For EP (Fig. 1, step 3), a Cu bath based on the reducing agent formaldehyde was chosen, which contained ethylenediamine in addition to the standard ligand tartrate55,56 to stabilize the Cu2+ source and to moderate the deposition speed, which contributes to avoid reagent depletion and inhomogeneous deposition in the template pores.57 In the course of the Cu-catalyzed reaction, hydrogen gas is formed according to (eq. 1):58

Cu2+ + 2HCHO + 4OH- Cu↓ + 2HCOO- + 2H2O + H2↑

(eq. 1)

The formation of gas bubbles accelerates the replenishment of the plating bath in the template pores.58 Due to the gas evolution, the Cu bath allows the fabrication of high aspect ratio nanotubes in comparably short time. Within 45 min deposition time, free-standing NTNWs are obtained. Other plating reactions which do not involve gas evolution usually require multiple hours to days of deposition to produce well-defined nanotubes of high aspect ratio.57,60


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After EP, the template matrix is removed by washing with dichloromethane (Fig. 1, step 4), resulting in the isolation of the Cu NTNW. Using additional reaction steps such as a second EP reaction or thermal oxidation, core-shell NTNWs can be created (Fig. 1, step 5). Electron micrographs and a photograph of the template-freed Cu NTNW are shown in Fig. 2. Due to the frequent intersections, the nanotubes form a free-standing film (Fig. 2 a). In accordance with the very low probability of isolated tubes in this system (Supporting Information, section 2), we could not find a clear example of a non-interconnected tube at the breaking edge of the NTNW film. On higher magnification, the disordered alignment and the pronounced linkage of the nanotubes is clearly visible (Fig. 2 b). While most of them are oriented perpendicular to the surface, some adopt inclinations of up to approximately 50° as compared to the perpendicular arrangement (Fig. 2 b). Viewed from top, the nanotube openings can be seen (Fig. 2 c), showing that during EP, the pore entrances were not blocked by the growing metal deposit, which would obstruct further metal deposition within the template.57 TEM reveals the fine-grained internal structure of the deposited Cu film (Fig. 2 d, e), which adopts a thickness of 125 ± 16 nm (see Supporting Information, Fig. S3 a). The relatively low roughness and homogeneous thickness of the nanotubes indicates that the employed seeding procedure ensures uniform and dense metal nucleation during EP.54,61 Due to the pronounced interconnection of the individual nanostructures, the collective material is robust enough to allow straightforward manipulation on the macroscale (Fig. 2 f). The network shown in the photograph has a size of 1.3 cm × 1.5 cm and is composed of approximately 300 million nanotubes.


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Figure 2. (a, b) SEM images of the Cu NTNW film, viewed from the side. (c) SEM image of nanotube openings in the partly cracked Cu surface film, viewed from the top. (d) TEM image of a Cu nanotube cross-section. (e) TEM image of a nanotube opening cross-section. (f) Photograph of a template-freed Cu NTNW.

To demonstrate the flexibility of the approach toward multi-component materials, different core-shell NTNWs were prepared from the Cu parent structure. First, EP was used to create a bimetallic NTNW. By shortly immersing the Cu NTNW in a borane-based electroless Ni plating bath, the typical salmon color of the material turns greyish, indicating coverage with Ni. SEM and TEM characterization of the resulting Cu-Ni NTNW can be found in Fig. 3 a-c. In the TEM survey image of the network cross-section, it can be seen that the Ni deposit partially peels off the Cu walls due to the microtome cutting (see arrows in Fig. 3 b). The change of the nanotube surface structure also suggests Ni deposition (Fig. 3 c). As compared with the Cu parent material, the surface of the Ni-modified nanotubes exhibits less internal structuring, which can be explained by the amorphizing effect of boron-based reducing agents on electroless Ni deposits.62


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Figure 3. Electron micrographs of the core-shell NTNWs, depicting (a-c) the bimetallic Cu-Ni and (d-f) the oxidized Cu NTNWs.

Second, the oxidation state of the Cu parent material was adjusted with a simple thermal oxidation step. Oxidation in air at 225 °C for 2h transformed the single-walled Cu tubes into a multi-walled material, but maintained the overall network architecture (Fig. 3 d, e). TEM analysis of the cross-section of an oxidized wall reveals a trilayer structure, which is composed of two roughened, compact layers, which surround a porous central layer (Fig. 3 f). The evolution of this structural motif can be explained by the nanoscale Kirkendall effect:63-65 During oxidation of the Cu wall, oxide ions diffuse more slowly from the forming oxide shell towards the Cu core as Cu ions move outwards, resulting in a net material transport to the outer shell. Upon supersaturation, the vacancies in the core layer form voids. Due to the recrystallization during oxidation, the roughness of the outer layers increases. EDX mapping confirmed the


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presence of oxygen in all layers, indicating the complete conversion of metallic Cu to oxide (Supporting Information, Fig. S6). To determine the Cu oxidation state in the different layers,65 STEM-EELS was employed (Fig. 4). Figure 4 a-d are STEM bright-field and HAADF Z-contrast images at two different magnifications showing the structure of the nanotubes in longitudinal section. Two points labelled 1 and 2 in Fig. 4 c were selected for a detailed EELS analysis. Fig. 4 e shows the measured O-K edge exhibiting a detailed ELNES at point 1 (red curve) and point 2 (blue curve). By comparing the ELNES of both curves to reference spectra of CuO (black curve) and Cu2O (green curve),66,67 point 1 is identified as CuO and point 2 as Cu2O. The same type of analysis was applied to the ELNES of the Cu-L2,3 edge shown in Fig. 4 f. The ionization edges lie at 931 eV (Cu-L3) and 951 eV (Cu-L2). By comparing the red curve from point 1 to the reference spectra one finds that it neither pure CuO nor pure Cu2O, but rather a mixture of both. For point 2 (blue curve), the situation was more obvious, since it almost fits to the Cu2O reference spectrum. This is a slightly different observation compared to the O-K edge, since the acquisition was carried posterior to the O-K measurement due to instrumental limitations. Thus, either a slight shift by a few nanometers is probable or partial reduction by the intense convergent electron beam. In summary, also by considering the 2D STEM-EELS mapping in Figure S7, one can state that CuO preferentially occurs at the wall surface whereas Cu2O was more likely found in the volume.


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Figure 4. Low magnification STEM bright-field (a) and HAADF Z-contrast image (b). (c) and (d) are magnified views of the part marked by the dashed, white rectangle in (a) and (b), respectively. (e) ELNES of the O-K edge acquired from position 1 (red) and 2 (blue) indicated in (c). (f) ELNES of the Cu-L2,3 edge acquired from position 1 (red) and 2 (blue) indicated in (c). The ELNES data is compared to reference data.66,67

By increasing the oxidation temperature to 400 °C, the Cu NTNW could be fully converted to CuO (see Supporting Information, Fig. S8 and Fig. S9). However, the CuO NTNW was highly


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brittle, fragmented during attachment to the electrode or the electrochemical experiments, and thus is not discussed in the next section. The phase composition of the Cu, the Cu-Ni and the Cu2O-CuO NTNW was analyzed with XRD (Fig. 5). Reflex broadening indicates the nanocrystalline structure of all NTNW components. The Cu NTNW mostly consists of elemental Cu, with a small contribution of Cu2O, which probably is formed during storage of the material in ambient conditions (Fig. 5 a). After the thermal treatment at 225 °C, the elemental Cu is converted to similar proportions of Cu2O and CuO (Fig. 5 b). This result is in accordance with the chemical speciation provided by the TEM-EELS measurements, which identified a central Cu+ layer surrounded by Cu2+. The diffractogram of the Cu-Ni NTNW is dominated by metallic Cu (Fig. 5 c). While a faint intensity increase around 36.5° indicates the presence of a slight amount of Cu2O, no distinct Ni reflexes could be identified. This observation can be explained by the short Ni plating time, resulting in a large excess of Cu, and the low crystallinity of boron-doped Ni deposits obtained with reducing agents such as aminoboranes.62 The marked suppression of the Cu2O phase in the Cu-Ni NTNW corroborates our interpretation of the Cu2O phase in the Cu NTNW as a superficial oxidation product, whose formation is impeded in the presence of a protective Ni shell (compare Fig. 5 a with Fig. 5 c). Thus, EP of Ni displays a compelling synthetic option to improve the chemical stability of nanomaterials. It can achieve conformal deposition of a nanoscale protective layer on complex network architectures in short time, and can be performed more simply than competing methods (e.g. Ni electrodeposition, which has been used to prevent the degradation of twodimensional Ag nanowire networks).68


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Figure 5. XRD analysis of the network types employed in the glucose sensing, including reference reflex positions and intensities for elemental Cu, cuprite (Cu2O) and tenorite (CuO). (a) Cu NTNW. (b) Cu2O-CuO NTNW. (c) Cu-Ni NTNW.

3.2 APPLICATION IN ENZYME-FREE GLUCOSE SENSING Prior to glucose sensing, the electrochemical responses of the three NTNW types were evaluated with CV (Fig. 6). When determining the scanning range, the minimum potential was chosen high enough to avoid reduction of Cu2+ and Cu+,49 in order to not electrochemically


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revert the oxidation treatment of the Cu2O-CuO NTNW. The upper potential limit was set at the beginning of the oxygen evolution regime.

Figure 6. CV characterization of (a) the Cu NTNW, (b) the Cu2O-CuO NTNW and (c) the CuNi NTNW. As electrolyte, 0.1 M NaOH with varying glucose concentrations was used.

Both in the case of the Cu NTNW (Fig. 6 a) and the oxide NTNW (Fig. 6 b), oxidation currents are found which begin to more steeply increase in the anodic scan above circa 0.8 V, and turn to reduction currents in the cathodic scan. This feature can be explained by the redox


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conversion of Cu3+ and Cu2+, which is overlaid by the onset of oxygen evolution.49 The presence of glucose causes the oxidation current to increase, showing the electrocatalytic conversion of the analyte. The CV response of the metallic and oxidic Cu network is similar, which is not surprising considering that the Cu NTNW is subjected to anodic potentials in alkaline electrolyte, which results in a likewise oxidized surface. The CV of the Cu-Ni NTNW (Fig. 6 c) is differently shaped. It features a defined redox peak pair (maxima at 0.36 V and 0.73 V), which is typically attributed to the redox transition of divalent and trivalent Ni.50 Similar to the Cu materials, the oxidation region is followed by oxygen evolution,50 and the oxidation current rises upon glucose addition. In the case of all NTNW types, the glucose oxidation onset concurs with the material oxidation in the absence of glucose, corroborating the central role of oxidized surface species in the analyte conversion. Amperometry was performed at a potential of 0.7 V. At this value, glucose oxidation was proceeding efficiently in the case of all three catalysts (Fig. 6), while oxygen evolution – which contributes to the background current and can partially block the electrode surface by gas bubble formation – was still not dominant. The results are summarized in Fig. 7. Upon sequential addition of glucose aliquots, current increases due to analyte oxidation (Fig. 7 a). Already the first addition of 10 µM glucose could be well resolved with all NTNW types (Fig. 7a, inset). In the inset, it can also be seen that the catalysts adapt to changed glucose concentration with differing speed. The Cu NTNW network quickly responded to altered glucose concentrations and required approx. 3 s to reach 95% of the new steady state current (averaged over the first three measurements). Slower response times of approx. 10 s and 6 s were found for the Cu2O-CuO NTNW and the Cu-Ni NTNW, respectively. The fast response of the Cu NTNW can be explained by the glucose oxidation kinetics. Contrary to the Ni-coated and oxidized core-


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shell NTNWs, the reaction proceeds on the Cu NTNW under diffusion control (see Supporting Information, Fig. S10). An amperometric measurement covering an approximately doubled glucose range can be found in Fig. S11 of the Supporting Information.

Figure 7. (a) Measured curves of the catalysts’ response to increasing glucose concentrations. The inset shows the magnified first addition of glucose (10 µM) for each NTNW type. (b) Determination of the catalysts’ sensitivity, which is obtained as the slope of the current response over the analyte concentration in the linear range. All fits show R2 correlations of >0.996.

The sensitivity of the catalysts was analyzed by plotting the glucose oxidation current over the analyte concentration (Fig. 7 b). In the linear range up to 1.1 mM glucose, the Cu parent structure exhibited a sensitivity of 2.62 mA mM-1 cm-2. While decoration of the Cu NTNW with Ni resulted in an increased sensitivity (2.83 mA mM-1 cm-2) and linearity (up to 1.9 mM


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glucose), thermal oxidation did not alter linearity, but reduced the sensitivity to 2.27 mA mM-1 cm-2. Based on these sensitivity values, the detection limits were calculated as 0.4 µM (Cu NTNW), 1 µM (Cu2O-CuO NTNW) and 1.2 µM (Cu-Ni NTNW) (for the detailed procedure, see Supporting Information, section 7). The comparably high detection limit of the Ni-modified network is related to its background fluctuation, which is more pronounced than in the case of the Cu and Cu oxide samples (see the comparably large starting current and baseline fluctuation for the Cu-Ni NTNW in Fig. 7 a and the inset therein). Also, the network architecture showed a good aging stability and reliability (see Supporting Information, section 8). Generally, the NTNW-based sensors possess very high sensitivities, which surpass a wide range of other nanostructured materials composed of Cu, Ni or their oxides (see Supporting Information, Table S1). The references were chosen with a focus on one-dimensional nanomaterials (or arrays created thereof), to match some morphological features of our network films. Most of the nanostructures require binders to form a coherent catalyst layer,43,46-49,69,70 while the NTNWs possess the advantage of a free-standing architecture8,44,45 and can be employed in pure form. The absence of conductive or cohesive additives helps to prevent catalyst occlusion.29 The open-porous structure of the NTNWs ensures a high surface area, a good accessibility of the catalyst sites and the presence of continuous conduction paths, which all contributes to the sensitivity of the material. Finally, the hollow structure of the tubes provides additional surface and thus activity as compared to nanowires or nanorods.45 The unique architecture, compositional flexibility and electroanalytical performance of the NTNWs suggests application in related fields. For instance, micro- and nanostructured Cu electrodes for CO2 reduction can greatly benefit from defined porosity and catalyst activation by surface oxide


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reduction,71 while the internal junctions in 1D core-shell nanostructures can provide additional functionality.72 Finally, the electrochemical response to the analyte glucose was compared with other sugars (fructose, lactose), ascorbic acid and several nitrogen-containing organic compounds (uric acid, urea, L-glutamine). To ensure a reasonably intense response and thus a dependable selectivity calculation, the interferents were added on a 1:1 stoichiometric basis. In addition, a higher concentration of NaCl was added, which can poison metal catalysts during glucose oxidation.73 The chosen interferents can be found in blood, which displays an important physiological sample for glucose determination. Fig. 8 depicts the selectivity measurements.

Figure 8. Amperometric selectivity study, showing the amperometric response of the three catalysts to the addition of 50 µM of glucose (a) and 50 µM of the following compounds: Fructose (b), lactose (c), ascorbic acid (d), uric acid (e), urea (f), and glutamine (g). Before the repeated injection of the main analyte, 10 mM NaCl were added (h).

A detailed list of the interferent-related currents is provided in the Supporting Information (Table S2). Glucose and fructose produced very similar responses on all catalysts, indicating that the carbohydrate oxidation is not limited to aldoses. Similarly, the disaccharide lactose was


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efficiently converted on all NTNWs. These observations are in agreement with the general reactivity of Cu and Ni electrodes in alkaline medium, which are able to efficiently oxidize a wide range of carbohydrates (and related compounds such as polyols or glucose derived acids).49,50 Ascorbic acid produced a response similar to or higher than glucose, which was lingering in the case of the core-shell NTNWs: While the Cu catalyst responded in ≈ 4 s, this value was considerably increased in the case of the oxidized and the Ni-coated network. Due to the especially sluggish response in the latter case, the conventional equilibration time of 60 s after interferent addition was increased to 120 s in this specific situation (Cu-Ni NTNW catalyst, ascorbic acid addition). The reaction of this reducing agent with the highly oxidized metal species present in the Cu and Ni electrocatalysts is not surprising, and frequently observed in similar studies.42,43,45,46 The catalyst films, especially the Cu NTNW, are more selective toward nitrogen-containing compounds and showed good stability against NaCl. Uric acid and glutamine only produced a weak response on the Cu NTNW (39% and 17% as compared to glucose), and the relatively stable diamide urea could neither be detected with the Cu NTNW nor with the Cu2O-CuO NTNW. However, in accordance with the literature,74 the Ni-modified network was able to oxidize urea (33% of the glucose current). While both the metallic and the oxidized Cu network did not significantly react to the addition of NaCl, a slight response was found in the case of the Ni-decorated catalyst (11% of the glucose current). The addition of all interferents did not hamper the detection of a second aliquot of glucose solution, demonstrating the capability of the sensors to operate in complex environments. However, when analyzing mixtures, the limited catalyst selectivity must be taken into account. Given blood as an example, the concentration levels of strong interferents such as other sugars or


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ascorbic acid are much lower as compared to the analyte glucose, but in sum, a contribution to the oxidation current is to be expected. To improve the selectivity of the electrosensors, different optimization strategies can be pursued. Perhaps most importantly, the intrinsic selectivity of the electrocatalyst should be enhanced, e.g. by mixing the reactive metal with a second element. For instance, Ni2P showed a much higher glucose oxidation selectivity as expected from pure Ni.75 Other viable options include the tuning of the sensing conditions (e.g. the applied potential),76 the introduction of specific surface functionalities (e.g. by attaching biomolecules such as chitosan)77 or the exclusion of interferents (e.g. the electrostatic repulsion of ascorbate by a negatively charged barrier).78 Such measures usually require a trade-off between selectivity and sensitivity.78 In summary, the Cu NTNW allowed the enzyme-free electrochemical detection of glucose with high sensitivity, fast response and diffusion-controlled kinetics. Due to the superficial Cu oxidation during storage and the anodic measurement conditions, it is not required to purposefully apply oxidizing treatments to as-prepared Cu nanomaterials. Although an oxidative transformation of Cu can result in advantages such as a distinct surface increase due to nanostructuring,44 it is not required to devise efficient glucose oxidation catalysts, and can adversely affect properties such as the electrical conductivity. In our case, the Cu2O-CuO NTNW showed reduced performance as compared to the Cu parent material. Ni coating resulted in increased sensitivity and linearity, albeit at the cost of reduced selectivity, slower response and higher background current.

4. CONCLUSIONS


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We outlined a synthetic strategy allowing the production of metallic, bimetallic and oxidic NTNWs in any wet-chemical lab with basic equipment. The approach only requires commercial ion track membranes, chemical etching and the application of simple and fast redox reactions. Neither post-synthetic nanostructure assembly steps nor complex instrumentation is required. Interestingly, the key to obtain free-standing nanostructures is the disordered pore structure of the commercial membranes. The presence of pore intersections can be exploited to deposit nanostructures which form an interconnected, monolithic network. While the synthesis is only based on facile reactions, several aspects have to be considered in order to obtain well-defined and robust NTNWs. The template should contain a sufficient number of pore crossings, which can be adjusted by the etching step. Replication of the template structure with a conformal nanoscale metal film requires optimized EP reactions and a high surface density of active seeds. As demonstrated in the amperometric detection of glucose, disordered NTNWs display efficient electrocatalysts and allow to examine composition-property relationships. Our approach is not limited to the specific NTNW types shown in this study. Other base metals aside Cu (e.g. Ni, which can also be quickly deposited and conveniently oxidized)72 or derivatization reactions (e.g. sulfidation or galvanic replacement) are also applicable, and open up many synthetic opportunities to create tailored NTNWs with or without core-shell architecture. These materials are promising choices for application in e.g. heterogeneous catalysis, miniaturized batteries, supercapacitors, sensing or plasmonics.

ASSOCIATED CONTENT


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Supporting Information. Impact of important synthetic parameters (template pore diameter, seed metal) on the Cu NTNW structure, porosity analysis of the membranes, complete thermal oxidation of the Cu NTNW, additional electrochemical characterization (glucose oxidation CV using different scan rates, selectivity values for amperometry, amperometry covering higher glucose concentrations, LOD determination), additional TEM characterization (EDX mapping, STEM-EELS mapping), sensor performance comparison, aging and reliability studies. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. Present Address ǁ Department of Materials and Interfaces, Weizmann Institute of Science, Herzl St. 234, 7610001 Rehovot, Israel Author Contributions The manuscript was written by F. M., except for the discussion of the EELS experiments (added by L. M.-L. and M. D.), and refined through contributions of T. K., L. M.-L., M. D., S. A., H.-J. K., C. R. and W. E. The syntheses and electrochemical experiments were designed by F. M., and performed by L. S., M. A., S. S., S. A. and F. M. Nanostructure characterization was executed by


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F. M. and M. A. (SEM and EDX), T. K. (XRD), U. K., L. M.-L. and M. D. (TEM and related techniques). All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We sincerely thank it4ip (Louvain-la-Neuve, Belgium) for suppling us with the track etched polycarbonate membranes employed in this study. M. D. and L. M. acknowledge financial support from the Hessen State Ministry of Higher Education, Research and the Arts via LOEWE RESPONSE. The transmission electron microscope used in this work was partially funded by the German Research Foundation (DFG/INST163/2951). ABBREVIATIONS CV, cyclic voltammetry; EDX, energy-dispersive X-ray spectroscopy; EELS, electron energy loss spectroscopy; EP, electroless plating; HAADF, high angle annular dark field; NTNW, nanotube network. SEM, scanning electron microscopy; STEM, scanning transmission electron microscopy; TEM, transmission electron microscopy; XRD, X-ray diffraction.

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