Nanoscale & Nanoscale Advances Blog

From stochastic single atomic switch to nanoscale resistive memory device

Attila Geresdi, András Halbritter, András Gyenis, Péter Makk and György Mihály

Nanoscale, 2011, DOI:10.1039/C0NR00951B

Scientists from Hungary have published some important work in Nanoscale regarding the use of solid state ionic conductors as atomic-sized junctions in non-volatile computer memory devices. By varying the size of the junctions from single-atom-sized to 10 nm, the group concluded that there is a lower size limit of 3 nm for reliable ionic nano-switches, a size which is well below the resolution of recent lithographic techniques.

In this work, Garesdi et al. created the junctions by gently touching a silver thin film with an electrochemically sharpened tungsten tip. Exposure of the silver film to air established the ionic conductor surface layer, and the nanoscale ‘point-contact’ geometry was sufficient to form a reliable switching device above the 3 nm threshold. Below this value, the switching process was much less reliable. The storage density here, even with the 3 nm limit, would be higher than current NAND flash devices and similar to the proposed bit size threshold of magnetic media which arises due to the superparamagnetic limit.

The authors provide a detailed analysis of the physical properties of the nano-junctions, as well as an explanation of the underlying mechanisms. They conclude that their ionic conductor-based devices are good candidates for non-volatile memory cells.

To read this article, click here.

Scientists in China have made a synthetic bone material using a new technique for creating polymer nanocomposites incorporated with inorganic nanoparticles.

Yuandong Dou, Kaili Lin and Jiang Chang, Nanoscale, 2011, DOI: 10.1039/C1NR10028A

Fluorescent images of the composite films

Jiang Chang and colleagues created this material after coming up with a new approach to making the  nanocomposites,  allowing them to control both the spatial distribution and orientational organisation of the nanocomponents, a known limitation of current methods of fabrication.

Their method involves using electrospinning and hot pressing techniques. They firstly homogeneously dispersed the nanoparticles within a polymer matrix solution, which was then electrospun into a patterned “nanofibrous mat” using a specifically designed “collector”. This mat was then placed between two sheets of non-woven polymer nanofibre and hot pressed to create the nanocomposite.

Because bone tissue is, generally speaking, structurally similar to these composites (they involve mineral particles preferentially oriented in a collagen matrix), the researchers tested their new fabrication method by creating an synthetic bone material by incorporating calcium silicate hydrate nanowires into a polyvinyl butyral matrix.  Their artificial material showed remarkable mechanical properties, particularly when compared with the pure polymer (for instance, the bending strength of the researchers’ material reached 188 MPa, as compared to the 86 MPa of the polymer), which also matched those of real cortical bone tissue.

You can find out more about this work by reading the article here.

Encapsulated enhanced green fluorescence protein in silica nanoparticle for cellular imaging

Zhengwei Cai, Zhangmei Ye, Xiaowei Yang, Yanli Chang, Haifang Wang, Yuanfang Liu and Aoneng Cao
Nanoscale DOI:10.1039/C0NR00956C

Scientists in China have developed a simple method of capping green fluorescent protein (GFP) in silica, which is a vital step in improving the versatility of fluorescent proteins for use as imaging probes.

Cai et al. at Shanghai University developed a covalent attachment route, which means the capping precursors are chemically bonded to the protein, rather than just providing passive encapsulation. The silica shell is then grown from the precursor layer to provide a solid and stable shell. A simple reverse emulsion method was used, and the group achieved a very high encapsulation efficiency and high protein loading. Their characterisation results suggest that encapsulating GFP in silica significantly increases its fluorescence and stability as the capping provides an effective barrier from external interference, such as protease attack, denaturants, and excessive heating.

Fluorescent probes are widely used to image biological structures and processes, both in vivo and in vitro. The main concerns in the design of these probes are their optical properties and the way they interact with their environment. For example, you may have a probe which exhibits excellent optical properties, but is toxic and therefore adversely affects the things you are trying to image. Conversely, you could have a probe which is non-toxic, but is unstable and loses its fluorescence too quickly under excitation. Traditional organic dyes suffered from various problems, including a lack of stability, broad emission profiles and toxicity issues. These have gradually been replaced with modern ‘nanoprobes’ which consist of either fluorescent nanoparticles or nanoparticulate coatings for fluorescent molecules. Of all the nanoprobes developed, fluorescent quantum dots exhibit the best optical properties, however, as they generally contain heavy metals such as cadmium, there are toxicity concerns in many applications.

The silica encapsulated fluorescent protein nanoparticles developed in this work may prove to be an exciting new probe which exhibits excellent optical and stability properties whilst avoiding problems such as toxicity and instability,

To read this article, click here.

Nanoscale DOI:10.1039/C0NR00956C