Abstract
At the nanoscale measures can move from a mass-scale analogue calibration to counters of discrete units. The shift redefines the possible levels of control that can be achieved in a system if adequate selectivity can be imposed. As an example as ionic substances pass through nanoscale pores, the quantity of ions is low enough that the pore can contain either negative or positive ions. Yet precise control over this selectivity still raises difficulties. In this issue researchers address the challenge of how to regulate the ionic selectivity of negative and positive charges with the use of an external charge. The approach may be useful for controlling the behaviour, properties and chemical composition of liquids and has possible technical applications for nanofluidic field effect transistors [1].
Selectivity is a critical advantage in the administration of drugs. Nanoparticles functionalized with targeting moieties can allow delivery of anti-cancer drugs to tumour cells, whilst avoiding healthy cells and hence reducing some of the debilitating side effects of cancer treatments [2]. Researchers in Belarus and the US developed a new theranostic approach—combining therapy and diagnosis—to support the evident benefits of cellular selectivity that can be achieved when nanoparticles are applied in medicine [3]. Their process uses nanobubbles of photothermal vapour, referred to as plasmonic nanobubbles, generated by plasmonic excitations in gold nanoparticles conjugated to diagnosis-specific antibodies. The intracellular plasmonic nanobubbles are controlled by laser fluence so that the response can be tuned in individual living cells. Lower fluence allows non-invasive high-sensitive imaging for diagnosis and higher fluence can disrupt the cellular membrane for treatments.
The selective response of carbon nanotubes to different gases has leant them to be used within various different types of sensors, as summarized in a review by researchers at the University of California, Riverside [4]. Mangu et al in the US have developed highly sensitive and selective room temperature gas sensors made from composites of multiwalled carbon nanotubes and polymers [5]. They report sensitivities as high as 28% when exposed to 100 ppm of NH3 and 29.8% to 100 ppm of NO2.
Nanopore structures are also showing increasing promise for sensing and biophysical characterization applications, in particular DNA [6]. An applied potential drives negatively charged DNA molecules through nanopores in a membrane and gives rise to current blockage pulses that are characteristic of specific analytes. Solid-state nanopore structures hold advantages over biological pores, such as those in α-haemolysin protein, as they are more resilient to experimental conditions, and ideally should also allow control of the nanopore diameter, channel length and surface composition. Asghar and colleagues have now reported a method that enables just that, 'a rapid solid-state nanopore fabrication and controlled pore shrinking process which does provide simultaneous in situ control of surface properties' [7]. In addition, they demonstrate the viability of the approach for single molecule sensor applications using double-stranded DNA.
In this issue, researchers in China report on a different approach which allows control over the transport of ionic fluids through nanopore-type structures. They describe the rapid field effect control of electrical conductance in single nanotube nanofluidic transistors [1]. Rather than seeking to control the charge of thenanotube inner surface, Gong and colleagues control polarity switching based on negative and positive ion selectivity using an external charge. 'The polarity of the nanotube can be reversed and tuned by the external field, which could find interesting applications in the field of ion separation and energy conversion', they explain, adding that the system may also find a use as a voltage sensor through the detection of the type of ions across the channel.
The aim of achieving selectivity encompasses a huge range of fields in nanotechnology research, from sensing and medicine to nanoelectronics and self-assembly. As our understanding of how nanosystems behave deepens, so too does the hunger to improve our capabilities, allowing greater precision and control in manipulating these systems. Selectivity is far from trivial when shrinking to systems of nanoscale dimensions, but the range of opportunities it brings just keeps on growing.
References
[1] Gong X, Li J, Guo C, Xu K and Hui Y 2012 Molecular switch for tuning ions across nanopores by an external electric field Nanotechnology 24 025502
[2] Brannon-Peppas L and Blanchette J O 2004 Nanoparticle and targeted systems for cancer therapy Adv. Drug Deliv. Rev 56 1649–59
[3] Lukianova-Hleb E Y, Hanna E Y, Hafner J H and Lapotko D O 2010 Tunable plasmonic nanobubbles for cell theranostics Nanotechnology 21 085102
[4] Zhang T, Mubeen S, Myung N V and Deshusses M A 2008 Recent progress in carbon nanotube-based gas sensors Nanotechnology 19 332001
[5] Mangu R, Rajaputra S and Singh V P 2011 MWCNT–polymer composites as highly sensitive and selective room temperature gas sensors Nanotechnology 22 215502
[6]Meller A, Nivon L, Brandin E, Golovchenko J and Branton D 2000 Rapid nanopore discrimination between single polynucleotide molecules Proc. Natl Acad. Sci. 97 1079–84
[7] Asghar W, Ilyas A, Deshmukh R R, Sumitsawan S, Timmons R B and Iqbal S M 2011 Pulsed plasma polymerization for controlling shrinkage and surface composition of nanopores Nanotechnology 22 285304