Green Nanomaterials. Siddharth Patwardhan

connected with ssDNA (red) and dsDNA (blue). (ii) Spectral position as a function of time after addition of complementary DNA. Discrete states indicated by horizontal dashed lines (reproduced from [36] with permission of Springer).

      When two gold nanoparticles are brought close together, the effect of proximity couples their plasmons, red shifting the SPR peak to larger wavelengths as two gold nanoparticles come closer together and blue shift to smaller wavelengths as they move apart. This phenomenon can be used to sense and measure tiny distances and form a time resolved molecular ruler (figure 2.10(B)) [36]! Here the DNA is attached to one nanoparticle, thus a sulphur linker, while the other end is functionalised with biotin which allows it to bind to the streptavidin coated nanoparticle. A shift from green to orange particles is immediately seen on this coupling. The molecular ruler could be used to measure the effect of hybridising the single-stranded DNA (ssDNA) to double-stranded DNA (dsDNA) (figure 2.10(B)) [36].

2.4.2 Nanodevice technologies

      2.4.2.1 Lab-on-a-chip diagnostics

      There is much overlap between the last section on nanosensors, and this section on nanodevices. Indeed, much of the research in the area of nanodevices is centred around medical sensing and the future goal of completely personalised and rapid multiple diagnostics healthcare built into one small chip, which can then be easily processed in an simple hand-held electronic reader (figure 2.11(A)). Here a drop of blood could be distributed via microfluidics to multiple nanosensors to detect a complete set of diseases and wellbeing factors with unparalleled sensitivity and speed. This idea of bringing multiple nanosensors and microfluidics together on one surface ‘chip’ has been termed lab-on-a-chip and has coalesced into a dedicated research field driven by this goal, with many excellent reviews on the subject [37, 40].

      Figure 2.11. (A) Diagram of a point-of-care test chip and reader to offer immediate diagnostics (reproduced from [37], copyright 2015 with permission from Elsevier). (B(i)) Image of a hard disc drive (HDD), (ii) diagram describing how a magnetic patter can be read as binary code for (a) transverse magnetic recording and (b) perpendicular magnetic recording (reproduced with permission from Scott Bird). (iii) Schematic of conventional granular magnetic recording media (left) compared to high-density patterned media (right) (reproduced with permission from [38], copyright 2008 IEEE). (C(i)) Schematic to show the central premise of spintronics. The electronic spin can be manipulated with a magnetic field. (ii) A 2D spin field-effect switch of MoS₂ on graphene (reproduced with permission from Macmillan Publishers Ltd [39], copyright 2016).

      2.4.2.2 Data storage technologies

      In this information age, we see the collection, processing and storage of data and information becoming more important in our daily lives, and also see that the remit for electronic data storage devices goes beyond utility only in a PC. Increasingly consumer products are becoming more user-friendly, intelligent or ‘smart’, and require the ability to access and store information. The invention of the magnetic hard disk drive (HDD) in 1956 by IBM was the birth of modern information storage as we know it, and magnetic HDDs are still found within most modern PCs (figure 2.11(Bi)). They remain the most commercially viable choice for high volume data storage, something that may become more important if the current shift towards cloud computing continues.

      A magnetic HDD uses the same principle of magnetic tape recording, where an electronic signal writes information in a magnetic pattern on the tape. For a magnetic HDD, this information is recorded in binary from ‘bits’. Simply, the two binary components synonymous to either of two magnetic spins align in the same direction (0) as the neighbour, or the opposing direction (1) in longitudinal magnetic recording (figure 2.11(Biia)), or align anti-parallel (0) or parallel (1) to their neighbour in perpendicular magnetic recording (figure 2.11(Biib)). The other key difference between magic tape recording and a magnetic HDD is that an HDD can be randomly accessed rather than being read and written serially. This is possible due to the nature of the magnetic medium being a granular thin film on a spinning disc which can be accessed anywhere by a flying read–write head (figure 2.11(Biii)). The data storage capacity and economy of HDDs have vastly increased over the years: 2011 saw storage capacities of 500 Gb in⁻² at a cost of $0.2 per GB compared to 2 kb in⁻² in 1956 for the first 305 Random Access Method of Accounting and Control (RAMAC), which weighed over a ton and cost $3200 per month (equivalent to approximately $160000 today) to lease. The revolutionary nature of computing and electronic data processing/storage was not imagined 60 years ago, when Thomas Watson (then chairman and CEO of IBM) famously said ‘I think there is a world market for maybe five computers’. While the development over time has been phenomenal, there is still ever-increasing demand for miniaturisation and higher density data storage capacity. However, reducing the size of each bit is reaching its physical limit. It is likely that conventional granular recording mediums will soon reach storage capacities of approximately 1 Tbit in⁻², with bit sizes of approximately 625 nm² formed by aligning the polarities of multiple grains. At very small sizes the thermal energy is greater than the magnetic stabilisation energy with the onset of superparamagnetism, meaning the magnetic orientation and thus the data is lost at room temperature.

      Two key new approaches to magnetic data storage are currently in development, with the aim of continuing the trend of increasing storage densities and capacities. These are: energy assisted recording and lithography patterned media (figure 2.11(Biii)). Energy assisted recording applies energy in the form of heat (heat-assisted magnetic recording) or microwaves (microwave-assisted magnetic recording) to the granular recording media, allowing the write head to orient the magnetic polarity of a higher anisotropy medium. Lithographically patterned media replaces the featureless granular recording medium used in current magnetic HDDs with a nanopatterned magnetic medium [38]. In nanopatterned media the size of the bit is defined by the nanopattern and not the write head, with the non-magnetic spaces reducing the risk of magnetic coupling and disorientation between neighbours, overall increasing the areal density. These patterns can take the form of discrete tracks (discreet track media) or each bit being retained on a discreet nano-island or nanoparticle (bit patterned media (BPM)) (figure 2.11(Biii)) [41]. With the use of current head materials thermally stable magnetic islands or nanoparticles suitable for BPM could be as small as 8 nm in diameter; with the addition of spaces between the islands’ bit sizes could be as small as 12 nm². As a result it has been predicted that storage densities could be pushed beyond 50 Tbit in⁻². However, the development of a cost effective and industrially scalable manufacturing technique for BPM mean that currently this technology is still very much in the development stage [41].

      Although common in computer systems, magnetic hard disk drives (HDDs) are not the only technologies available, with optical and solid-state semiconductor drives now found in many products. Semiconductor memories are electronic data storage devices which are compact and can operate at very high speeds due to a lack of moving parts. As such these devices have become the primary internal memories within computers. Semiconductor memories are either termed ‘volatile devices’ which lose stored information without a power source, or ‘non-volatile devices’ which will retain stored information in the absence of power. The two most common semiconductor memories, which fall into the volatile class, are dynamic random access memory (DRAM) and static random access memory (SRAM) [37]. DRAM stores bits of information inside capacitors, with each capacitor encoding a bit of information. A 1 or 0 can be formed by the capacitor being either charged or discharged, but