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Designer gates for tiny, charged particles

Control over charge/ion transport in nanosized channels is essential in realising devices based on electromechanical energy conversion, in drug delivery and in controlled catalysis. This by no means trivial though larger dimensions are relatively easily accessible. 

Innovative materials are required to achieve charge reversal in tiny spaces. One strategy is to use surface charge to attract or repel charged particles called ions. This is called “ion transport”, a natural process used by human cells to transport different ions.

Publishing in the journal Angewandte Chemie, researchers from IISc and Jawaharlal Nehru Center for Advanced Scientific Research (JNCASR) have described how the magnitude and polarity of surface charge can be dynamically controlled. This provides a mechanism to enforce “ion gating”: controlling the charge and number of ions that can be transported across an interface.

The substrate designed is made of silica (major component found in sand) in the form  mesoporous silica film (MF), with extremely small, uniform pores in nanometre dimensions. Because of the size, the pores are called “nanopores” and the channels they form are called “nanochannels”. When MF is treated with a molecule called viologen, the nanopores get coated with a layer of viologen (MF-V).  Viologen makes the surface of nanopores positively charged. Thus, MF-V attracts negative ions. The positive charges on MF-V can be neutralised or even reversed by  dipping with  negatively charged pyranine molecules. This charge reversal (between positive and negative charges) is reversible and can be achieved at will.

The authors tested ion gating with a pH sensitive molecule, coronene tetracarboxylate (CS) in place of pyranine. CS exhibits different magnitudes of negative charge at different pH levels. MF-V is dipped in CS just as before. In an acidic environment, the substrate dipped in CS attracts more negatively charged molecules; in a mildly acidic environment, the substrate attracts both positively and negatively charged molecules; and in an alkaline environment, the substrate attracts positively charged molecules. Using CS, one can potentially control the polarity and magnitude of surface charge.

“Generally, molecules/polymers containing acidic as well as basic groups exhibit charge reversal with respect to pH. Surfaces linked with photo responsive molecules [those that respond to light] like spiropyran and malachite green could show light responsive charge reversal in nanochannels”, say the authors.

Earlier, some strategies based on covalent bonding have been used to achieve surface charge reversal. As compared to those, the non-covalent strategies that the authors have proposed are dynamic, flexible and modular. The modular nature allows the use of single material to achieve charge reversal in response to variety of stimuli (like pH, light etc.,) responsive molecules. The nature of ion transport can be switched from anion selective to ambipolar to cation selective.

Finally, the applications of this technique are enormous. The authors say, “We intend to use this facile control of ion transport for manipulating the rate of catalysis by restricting or encouraging the access of charged reactants to catalytic sites. This also can be used to fabricate abiotic analogues of protein channels for use in protocell designs and to electrostatically gate the release of charged drug molecules.”

Authors

Prof. S. Sampath is faculty in Department of Inorganic and Physical Chemistry, IISc. http://ipc.iisc.ernet.in/sampath.html

Dr. Muthusamy Eswaramoorthy is an Associate Professor at the Chemistry and Physics of Materials Unit, Jawaharlal Nehru Center for Advanced Scientific Research (JNCASR), Bangalore. B. V. V. S. Pavan Kumar is a PhD student in this lab. http://www.jncasr.ac.in/eswar

Dr. Subi J. George is an Associate Professor in Supramolecular Chemistry Laboratory, New Chemistry Unit, JNCASR.  K. Venkata Rao was a Ph.D student in the same lab. http://www.jncasr.ac.in/george

Contact

Prof Sampath. 080 22933315; sampath@ipc.iisc.ernet.in

Dr. Subi George 080-22082964; george@jncasr.ac.in

Dr. Muthusamy Eswaramoorthy. 080-22082870; eswar@jncasr.ac.in

The paper will be published in the prestigious international journal Angewandte Chemie. It was published as “hot paper” online on 26th September 2014. Online: http://onlinelibrary.wiley.com/doi/10.1002/ange.201406448/abstract