Nanoscale & Nanoscale Advances Blog

Future energy storage solutions require a combination of high energy density, high reliability and low manufacturing cost. Conducting polymer hydrogels (CPHs) have emerged in recent years as a viable alternative for energy storage applications, as a Feature article by Shi et al. reports.

CPHs exhibit highly advantageous properties such as a large surface area, tunable mechanical properties and high conductivity compared to other polymers. These materials combine a conductive π–conjugated backbone with a porous structure.

Two synthesis routes are presented: template-guided synthesis (e.g. polymerization of monomers in a non-conductive hydrogel matrix) and direct formation using phytic acid as a gelator and dopant of the polymer.

Independent of synthesis route, CPHs were successfully demonstrated as bulk materials for electrochemical capacitors (also termed “supercaps”), as well as functional binders within Li-ion batteries. By careful modification of the polymer properties, a stable material able to withstand over 10000 charge-discharge cycles was demonstrated. Finally, the current hurdles for mass-market adoption, such as limited mechanical strength, lower conductivity than currently utilized material combinations and a lower capacity are explained, and paths to overcome these are discussed.

Nanostructured conducting polymer hydrogels for energy storage applications
Ye Shi, Lele Peng and Guihua Yu
Nanoscale, 2015, 7, 12796-12806. DOI: 10.1039/C5NR03403E

Sebastian Axmann is a guest web-writer for the Nanoscale blog. His interests comprise manufacturing and metrology of nanostructures as well as their usage in current semiconductor devices. He also posts links to interesting research articles on Twitter: @SebastianAxmann.

Besides the wide field of photovoltaics research, additional technologies, such as the direct conversion of H₂ via solar water splitting, are currently being researched. A recent article by Zhang et al. presents their findings on improved manufacturing routes for these cells.

For the time being, Ta₃N₅ is the material of choice as the band gap and structure are both well suited for light absorption. To form a large interfacial area for efficient light conversion within the cells, arrays of hollow nanorods are employed. As the authors describe in their article, earlier attempts of a one-step synthesis route, also evaluated by other researchers, led to weak adhesion of the brittle nanorod film on the substrate. Their new approach utilizes a two-step synthesis route: first, a nanorod layer of Ta₂O₅ is formed via anodization in a solution with a lower HF concentration compared to that employed by other groups. Next, this weakly adhering layer is removed by sonication and a second layer is formed. The formation of the second layer also employs a low reaction temperature to limit the reaction rate. Finally, this second nanorod layer is nitridated, forming Ta₃N₅ from the Ta₂O₅ layer.

(a) Schematic illustration of the synthetic process, (b) top-view SEM image and (c) cross-sectional SEM image of Ta3N5 NTAs.

The resulting layer was found to adhere well on the substrate surface and to exhibit only a few cracks. By further optimization of processing times and the additive material used during nitridation, a maximum current density of 11 mA/cm² at 1.6 V was demonstrated by the authors.

Bridging the transport pathway of charge carriers in a Ta₃N₅ nanotube array photoanode for solar water splitting
Peng Zhang, Tuo Wang, Jijie Zhang, Xiaoxia Chang and Jinlong Gong
Nanoscale, 2015, Advance Article. DOI: 10.1039/C5NR03013G

Sebastian Axmann is a guest web-writer for the Nanoscale blog. His interests comprise manufacturing and metrology of nanostructures as well as their usage in current semiconductor devices. He also posts links to interesting research articles on Twitter: @SebastianAxmann.

Reversible engineering of the pore size and philicity of mesoporous SBA via dynamic covalent chemistry triggered by changes in pH.

Control of surface chemistry is important for utilisation of mesoporous silicas in many applications such as catalysis, drug delivery and separation sciences. Due to the mostly irreversible nature of covalent functionalisations and lack of rigidity in supramolecular approaches, in this study, dynamic covalent chemistry was used to reversibly alter the chemical coating on the surface of amine functional mesoporous silica SBA-15. This was achieved through the condensation reaction of the primary amine on SBA-15 with 4-decyloxybenzaldehyde (4-DB) to form an imine containing a hydrophobic decyl chain. This step both reduced the pore size and hydrophilicity of the pore surface, however, could be reversed by cleaving the imine at low pH in an ethanol/water mixture.

Dynamic control of the pore properties was demonstrated using the catalytic reduction of p-nitrophenol in aqueous conditions by gold nanoparticles, which were imbedded into both amine and imine/decyl SBA-15 materials. The more porous/hydrophilic amine-based material completely reduced p-nitrophenol to p-aminophenol after 30 minutes, whereas the more hydrophobic imine-based surface exhibited no catalytic activity. The authors suggest that this methodology could be used to generate a host of new covalently functionalised materials with tuneable surface properties.

Reversible control of pore size and surface chemistry of mesoporous silica through dynamic covalent chemistry: philicity mediated catalysis
Dheeraj Kumar Singh, B. V. V. S. Pavan Kumar and M. Eswaramoorthy
Nanoscale, 2015, Advance Article. DOI: 10.1039/C5NR02959G

Dr Mike Barrow is a guest web writer for the Nanoscale blog. He currently works as a Postdoctoral Researcher at the University of Liverpool.

A gold nanohole array based surface-enhanced Raman scattering biosensor for detection of silver(I) and mercury(II) in human saliva

Researchers from West Virginia University have developed a method for detecting heavy metal ions in human saliva. Silver (I) (Ag) and mercury (II) (Hg) ions form part of dental fillings so it is important to have non-invasive analytical methods to monitor the toxicity of these metal ions should they be accidentally released into saliva.

Here, the authors exploited the strong electromagnetic coupling between gold (Au) nanostars and a Au nanohole array to detect Ag (I) and Hg (II) using surface enhanced Raman scattering (SERS). The Au nanostars and nanohole array were functionalised with mismatched pairs of single-stranded (ssDNA) probes that hybridise to form stable duplexes in the presence of the corresponding metal ions. Hybridisation allows the Au nanostars to come into close proximity with the Au nanohole array, which results in a large amplification of the SERS signal.

In this way, the authors were able to detect Ag (I) and Hg (II) ions in human saliva with limits of detection (LODs) of 0.17 nM and 2.3 pM for Ag (I) and Hg (II), respectively. This demonstrates the applicability of the SERS-based detection platform for on-site, non-invasive detection of analytes in body fluids.

A gold nanohole array based surface-enhanced Raman scattering biosensor for detection of silver(I) and mercury(II) in human saliva
Peng Zheng, Ming Li, Richard Jurevic, Scott K. Cushing, Yuxin Liu and Nianqiang Wu
Nanoscale, 2015, 7, 11005-11012. DOI: 10.1039/C5NR02142A

Dr Lee Barrett is a guest web writer for the Nanoscale blog. Lee is currently a postdoctoral researcher in the Centre for Molecular Nanometrology at the University of Strathclyde. His research is currently focused on the development of nanoparticle-based sensors and surface enhanced Raman scattering (SERS). Follow him on twitter @L_Bargie.

Single-layer nanosheets as layered double hydroxides (LDH) are of widespread interest both in research and application as they offer unique advantages for electronic devices. A recent communication in Nanoscale by Yu et al. reports on a new approach to simplify the overall manufacturing process.

Direct growth of single-layer nanosheets with the assistance of layer growth inhibitors

Common techniques yielding LDH nanosheets are based on exfoliation of synthesized multi-layer stacks. Due to the high binding forces in between the single layers, obtaining single layers requires aggressive chemicals and long processing times.

The approach presented by the authors comprises the utilization of formamide during the material growth process to suppress vertical growth for MgAl. Thus, only in-plane layer formation continues, leading to large single-layer LDH nanosheets. The authors demonstrate that only LDH single-layer nanosheets are obtained by means of TEM, XRD and AFM measurement techniques to compare this new method with the standard synthesis procedure.

The underlying mechanism is believed to be based on the weakened layer interaction by formamide. In addition, the same approach was also successfully applied to LDH nanosheets based on Co and Al.

One-step direct synthesis of layered double hydroxide single-layer nanosheets
Jingfang Yu, Benjamin R. Martin, Abraham Clearfield, Zhiping Luo and Luyi Sun
Nanoscale, 2015, 7, 9448-9451. DOI: 10.1039/C5NR01077B

Sebastian Axmann is a guest web-writer for the Nanoscale blog. His interests comprise manufacturing and metrology of nanostructures as well as their usage in current semiconductor devices. He also posts links to interesting research articles on Twitter: @SebastianAxmann.

Self-assembled supramolecular nanoprobes

The synthesis of nanoprobes for application in more than one imaging technology is becoming more popular. So-called “bi-modal” probes can reduce limitations of single imaging modalities such as sensitivity or penetration depth. In this Hot article, the synthesis of two amphiphilic, dual-modality, optical imaging/MRI nanoprobes is reported. Each probe, containing both a fluorophore and a gadolinium complex, was specifically engineered using hydrophobic and hydrophilic components so they would self-assemble above the critical micelle concentration (CMC), which in turn would improve the MRI performance. The materials could offer potential advantages compared to conventional, unimolecular probes. Flow cytometry was used to confirm that both negatively charged assemblies were efficient at labelling KB-3-1 (human cervical cancer) cells at different labelling concentrations and incubation periods, through measurement of cell fluorescence. Cells viability was not compromised for each condition. The authors are now looking to further improve performance of self-assembled cell tracking agents through synthetic manipulation of construct size and surface charge.

Design and assembly of supramolecular dual-modality nanoprobes
Shuang Liu, Pengcheng Zhang, Sangeeta Ray Banerjee, Jiadi Xu, Martin G. Pomper and Honggang Cui
Nanoscale, 2015, 7, 9462-9466. DOI: 10.1039/C5NR01518A

Dr Mike Barrow is a guest web writer for the Nanoscale blog. He currently works as a Postdoctoral Researcher at the University of Liverpool.

Graphene oxide (GO) is a versatile material with applications ranging from electronics to energy storage and biosensors. As a biosensing substrate, GO has many favorable attributes such as low cost, high signal-to-noise ratio, and the ability to efficiently quench fluorescence.

Scheme for the GO-aptamer based sensor for detection of thrombin.

This ability to quench fluorescence has inspired a range of biosensors using GO and Förster resonance energy transfer (FRET) for the sensitive detection of proteins using labelled probes, such as aptamers. However, the target proteins in such assays can non-specifically adsorb onto the surface of GO, thereby reducing the sensitivity.

To address this, Gao and co-workers implemented the use of polyethylene glycol (PEG) to prevent the non-specific adsorption of thrombin onto GO while implementing an aptamer-binding assay.  The authors report that the detection limit could be improved by optimizing the GO:PEG concentration. This manuscript helps to establish GO as promising tool in the biomedical and biotechnology fields.

Highly sensitive detection for proteins using graphene oxide-aptamer based sensors
Li Gao, Qin Li, Raoqi Li, Lirong Yan, Yang Zhou, Keping Chen and Haixia Shi
Nanoscale, 2015, Advance Article. DOI: 10.1039/C5NR01187F

Dr Lee Barrett is a guest web writer for the Nanoscale blog. Lee is currently a postdoctoral researcher in the Centre for Molecular Nanometrology at the University of Strathclyde. His research is currently focused on the development of nanoparticle-based sensors and surface enhanced Raman scattering (SERS). Follow him on Twitter @L_Bargie.

Recently, the 50th anniversary of Moore’s law has been celebrated by the semiconductor industry. Although the current ITRS (International Technology Roadmap for Semiconductors) continues to propose “traditional” transistor materials such as Si and GaAs, attractive, two-dimensional alternatives have become visible on the horizon.

Schwierz et al. have recently published a Feature article in Nanoscale compiling the current knowledge on 2D materials. Their article focuses on the application of these as the channel material in field effect transistors.

According to the authors, two large trends can currently be observed: More Moore and More than Moore. The former utilizes well known semiconductors and improves performance by sophisticated manufacturing and scaling techniques. In contrast, the latter one employs compound semiconductors and novel alloys as 2D materials with a wide range of new properties.

Two-dimensional materials and their prospects in transistor electronics

The main material classes researched to date are explained further, covering X-anes, Fluoro-X-enes, TMDs (transition metal–chalcogen combinations), SMCs (semimetal–chalcogen combinations), MX-enes and (currently theoretical) group IV-IV and III-V 2D materials.

To assess the viability of the materials for applications within electronic circuits, Schwierz et al. also describe a wish list of ideal properties of next generation channel materials: bandgap, carrier mobility, heat conductivity, contact resistance and scale length. Based on experiences from established semiconductors, they also derive practical values needed for high performance FET.

The final part of the review examines the current status of the material classes. Notable performance records of well researched materials (e.g. 100 GHz graphene FET) as well as first demonstrators (e.g. first X-ane based FET) are given here. Finally, some arguments for a revival of silicon as a promising material for future applications are also given.

Two-dimensional materials and their prospects in transistor electronics
F. Schwierz, J. Pezoldt and R. Granzner
Nanoscale, 2015, 7, 8261-8283. DOI: 10.1039/C5NR01052G

Sebastian Axmann is a guest web-writer for the Nanoscale blog. His interests comprise manufacturing and metrology of nanostructures as well as their usage in current semiconductor devices. He also posts links to interesting research articles on Twitter: @SebastianAxmann.