Fabrication of Copper Nanowires Through Multi-step Anodizing, Electrochemical Barrier Layer Thinning and Electrodeposition

Nanoporous anodic aluminum oxide (AAO) is one of the most commonly employed templates for nanofabrication since the pore diameter, interpore distance, thickness of the oxide, barrier layer and walls can be precisely controlled by a proper choice of anodizing conditions. For the fabrication of metallic nanowires (NW), it is necessary to decrease the barrier layer thickness. In addition, to achieve sufficient electrical contacts at the bottom of the pores, a thin layer of Au has to be deposited either by sputtering or by electrodeposition (ED) using a cyanide bath. Researchers at Military University of Technology, Poland and Delft University of Technology, The Netherlands have proposed a methodology that combines multi-step anodizing, electrochemical barrier layer thinning (BLT) and ED for the fabrication of Cu NW.

Commercial purity aluminum alloy (AA 1050 alloy) was degreased and electropolished (EtOH:HClO₄ 4:1, 0 °C, 20 V, 120 s, Pt grid cathode). A multi-step anodizing protocol was employed to obtain nanoporous anodic aluminium oxide (AAO) templates with a desired nanoporous structure. Mild anodization (MA) in 0.5 M H₂SO₄ with 20 vol.% ethylene glycol (EG) at 0 °C, 20 V and 60 min (Fig. 1, reaction I) was carried out as the first step. The voltage was increased up to 45 V with 0.5 V steps for each 5 s and hard anodizing was performed at 45 V for 1 h (Fig. 1, reaction II). To thin down the bottom of the barrier layer, mild anodizing was carried out in 0.3 M oxalic acid, at 30 °C, 45 V and 30 min (Fig. 1, reaction III). Electrochemical barrier layer thinning (BLT) of multi-step anodized Al alloy was performed in 0.3 M oxalic acid (Fig. 1, reaction IV). A step-wise decrease in voltage and the duration of each voltage step was varied and the suitable conditions for BLT were optimized. For effective opening of the pores at the bottom, the Al alloy at the base was chemically etched using 0.1 M CuCl₂ in HCl. The applicability of the membranes formed using a combination of MA, HA and BLT was ascertained through electrodeposition (ED) of Cu using 0.3 M CuSO₄ and 0.1 M H₃BO₃ at -0.3 V vs. Ag/AgCl for 30 min (Fig. 1, reaction V). The Cu nanowires (NW) were liberated from the AAO template by chemical etching in 5% H₃PO₄ at 30 °C for 45 min (Fig. 1, reaction VI).

Fig. 1 Schematic representation of the various stages involved in the fabrication of Cu nanowires

MA in 0.5 M H₂SO₄ with 20 vol.% EG at 0 °C, 20 V and 60 min enables the formation of a protective oxide layer. The presence of this oxide layer as well as a steady step wise increase in voltage (0.5 V steps for each 5 s) up to 45 V prevents destruction of the Al alloy anode by the high current density avalanche generated during HA at 45 V for 1 h. The MA/HA combination though improved ordering of nanoporous structure in the resultant AAO, the thickness of the barrier layer at the bottom is a critical issue. MA in 0.3 M oxalic acid,    at 30 °C, 45 V and 30 min decreased the thickness of the barrier layer by a reasonable extent, which is suitable for subsequent BLT process. Since the conditions of anodization are mild, the interpore distance and ordering of the pores are maintained. A step-wise decrease in voltage enables BLT of the anodized Al alloy in 0.3 M oxalic acid. Thinning of the barrier layer is effective at selective voltage step and time (Un+1 = 0.75.Un; Δt = 60 s). Under such conditions of BLT, the hexagonal honeycomb-like morphology is maintained at the bottom of the pores (Fig. 2(a)). The applicability of the AAO formed using a combination of MA, HA, MA and BLT is confirmed by ED of Cu NW with a high aspect ratio (Fig. 2(b)). The successful ED of Cu NW confirms efficient BLT and sufficient electrical contact at the electrolyte–aluminum interface that could have facilitated the reduction of Cu²⁺ ions at bottom of the pores.

Fig. 2 FE-SEM micrographs of (a) bottom side of the AAO (after removal of Al alloy using 0.1 M CuCl₂ in HCl) indicating the effectiveness of BLT performed at U_n+1 = 0.75 Un; Δt = 60 s; and (b) ED Cu NW

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: W.J. Stepniowski et al., Journal of Electroanalytical Chemistry, 809 (2018) 59–66.

Detecting cancer from urine sample – Will the development of a nanowire based device open the gates for early diagnoses and timely medical checkups for cancer?

MicroRNAs (miRNAs) encapsulated by extracellular vesicles (EVs) are found in body fluids of patients with malignant diseases as well as with those having a better health. The difference in the EV-encapsulated miRNAs between these two groups of people could be used as a signature to identify various diseases. The miRNAs present in urine could serve as biomarkers for detecting cancer. Unfortunately, the concentration of EVs in urine is extremely low (<0.01 volume %) and hence the most commonly used method of extraction – ultracentrifugation is not capable of extracting nearly 90% of the miRNA species, due to their low abundance. Recently, a nanowire-based device anchored to a microfluidic substrate is fabricated for the efficient collection of EVs and in situ extraction of various miRNAs of different sequences, which is believed to open the gates for urine-based early diagnoses and medical checkups for cancer (Yasui et al., Sci. Adv. 2017;3: e1701133).

Si (100) served as the base substrate (Fig. 1(a)), which was coated with a positive photoresist followed by channel patterning using photo lithography (Fig. 1(b)). A 140 nm thick Cr layer was sputter deposited (Fig. 1(c)), followed by removal of the photoresist layer and thermal oxidation of the Cr layer at 400 °C for 2 h (Fig. 1(d)). The thermally oxidized Cr layer served as the seed layer for subsequent growth of ZnO nanowires using a solution mixture of 15 mM hexamethylenetetramine (HMTA) and 15 mM zinc nitrate hexahydrate at 95 °C for 3 h (Fig. 1(e)). PDMS was poured over the ZnO nanowire grown substrate and cured (Fig. 1(f)). Subsequently, the PDMS was removed from the Si substrate (Fig. 1(g)) and the ZnO nanowires in the PDMS were transferred to another PDMS substrate. The transferred nanowires were uniformly and deeply buried into PDMS while their slightly emerged heads served as growth points for the second nanowire growth (Fig. 1(h)), which was carried out by immersing the PDMS in a solution mixture of 15 mM HMTA and 15 mM zinc nitrate hexahydrate at 95 °C for 3 h (Fig. 1(i)). To enhance contact events between the ZnO nanowires and the EVs as well as to avoid any pressure drop, the ZnO nanowire embedded PDMS substrate was anchored to a herringbone-structured PDMS substrate (Fig. 1(j)).

Fig. 1 (a-j) Various stages involved in the fabrication of nanowire-based device; and (k) schematic of the collection and extraction of EV–encapsulated miRNAs.

The nanowire-based device is capable of detecting around 1000 types of species of miRNAs when compared to the conventional ultracentrifugation method. Using this device, it is possible to extract EV–encapsulated miRNAs within 40 min (collection, 20 min; extraction, 20 min) by introducing just 1 ml of urine sample followed by 1 ml of lysis buffer into the device (Fig. 1(k)). In contrast, the ultracentrifugation method requires 20 ml of urine sample and more than 5 h for collection and extraction. The device enables a four-fold increase in the miRNA expression level with a larger variety of extracted species of miRNAs. The ZnO nanowire-based device is found to be superior to the commonly used ultracentrifugation method in terms of treatment time and RNA extraction efficiency. This attribute is due to its large surface area of ZnO nanowires and their positively charged surface (isoelectric point of 9.50 at pH 6 to 8), which electrostatically attracts the negatively charged EVs in urine sample at pH 6-8. In addition, the mechanical stability of ZnO nanowires, which are firmly anchored to the PDMS substrate helps to retain their strength during buffer flow and enhances the extraction efficiency. The positively charged surface of ZnO nanowires offers benefit in collecting negatively charged objects in urine samples, including exosomes, microvesicles, and EV-free miRNAs.

The ZnO nanowire-based device is believed to help in the early diagnoses and timely medical checkups based on urine miRNA analysis. The method is capable of identifying urinary miRNAs that could potentially serve as biomarkers for detecting bladder, prostate lung, pancreas, and liver cancer.

T.S.N. Sankara Narayanan

Template-assisted deposition of metal nanowires

Fabrication of metal nanowires have received considerable attention and among them template-assisted (ion tracked etched polymers or porous aluminium oxide templates) electrodeposition of metal nanowires assumed significance. Usually, a thin film (< 200 nm) is sputtered over the template and it is subsequently reinforced with a thick (up to 10 μm) metallic layer by plating. However, the difficulty in making electrical connection with the thin and fragile sputtered film as well as in removing the electrodeposited layer to facilitate the release of metal nanowires are the major limitations. Researchers at Centre for Manufacturing and Materials Engineering and Faculty of Health and Life Sciences, Coventry University, UK and Energy Technology Research Group, University of Southampton, UK for the first time have described a new procedure for fabrication of metal nanowires (Cu nanowires) by template-assisted electrodeposition using porous polycarbonate templates.

Polycarbonate templates (pore sizes: 60 nm, 100 nm and 200 nm; thickness: 25 μm) were washed using 1 v./v. % of Neutracon at 40 ºC for 5 min, rinsed, air-dried. They were sputter coated with silver for 3 min on one side of the template (Ar bombardment gas, 15 mA current) followed by electroless plating of Cu using an electroless copper bath at 46 ºC for 10 min to form the electrode layer, rinsed and air dried. Subsequently a layer of Cu was deposited by electrodeposition at -75 mV vs. saturated calomel electrode (SCE) for 120 min to grow the Cu nanowires. A titanium/mixed metal oxide mesh served as a counter electrode. After plating, the coated template was removed from the plating cell, rinsed and air dried. The Cu nanowires were freed from the template by etching away the bottom electrode layer using a 3 v./v.% solution of hydrogen peroxide/sulphuric acid and then by dissolving the polycarbonate template in dichloromethane. The various stages involved in the fabrication of Cu nanowire is schematically illustrated in Fig. 1.

Fig. 1 Schematic illustration of template-assisted deposition of Cu nanowires

The sputtered Ag acts as a seed layer and served as an effective catalyst for electroless deposition of Cu. After 3 min sputtering, a uniform but porous layer of Ag (thickness: ≈ 15 nm) is deposited (Fig. 2(a)). Electroless plating of Cu over the sputtered Ag film for 10 min completely covered and sealed the pores and provides an excellent coverage (Figs. 2(b) and 2(c)). Analysis performed at the reverse side of the electrode layer after dissolving the template indicates that the electroless deposited Cu starts to fill the bottom of the pores and forms the base of the nanowire. The sputtering process directs the Ag atoms into the pores wherein they adhere to the side of the walls and trigger deposition of Cu. Filling-up the bottom and side walls of the porous structure provides an ideal base for subsequent electroplating step to build uniform Cu nanowires (Fig. 2(d)). Plating of Cu into the pores offers an additional advantage of mechanically keying the electrode layer to the smooth surface of the template. A magnified acquired by SEM at the bottom of the Cu nanowires (Fig. 2(e)) clearly indicate the electroless Cu and sputter-coated Ag layers and the interconnection between the electroless Cu and electroplated Cu layer is good.

Fig. 2 SEM images (a) after sputter coating with Ag for 3 min (thickness: ≤ 15 nm); (b, c) after electroless plating with Cu for 10 min (thickness: 300–500 nm); (d) Cu nanowires formed after 120 min of electrodeposition of Cu at -75 V vs. SCE over the sputtered Ag seed layer/electroless Cu; and (e) bottom portion of the Cu nanowire showing a good interconnection between the electroless Cu and electroplated Cu layer

A simple protocol is suggested for the fabrication of template-assisted electrodeposition of metal nanowires. Sputter deposited Ag thin film (≤15 nm) on one side of the polycarbonate template acts as a seed layer and catalyze electroless deposition of a uniform and highly conductive Cu layer (300–500 nm) for subsequent electrolytic deposition of a thick Cu layer. Removal of the electrode layer at the bottom of the template by chemical etching as well as dissolution of the template in dichloromethane yields free standing Cu nanowires.

T.S.N. Sankara Narayanan

For more details, the reader may kindly refer: J.E. Graves et al., A new procedure for the template synthesis of metal nanowires, Electrochemistry Communications (2017) (article in press), doi:10.1016/j.elecom.2017.11.022