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Building nanostructures, economically

Imagine a scenario in the not-too-distant future when someone wants to have his blood tested. The process will not be as complicated or expensive as it is right now. The diagnostic device of the future would be so common that it would be easily available in our homes. These devices would conclusively show the results within seconds of placing a tiny drop of blood on it. This new diagnostic device of the future would be based on microfluidic channels on special paper. Microfluidic channels are networks of pipes whose diameters are much smaller than that of a human hair. Working towards such goal is Dr. Venugopal Santhanam, an Assistant Professor specializing in Nanoparticle engineering, at the Indian Institute of Science.

An alumnus of Purdue University and previously a post-doctoral researcher in Germany, Dr. Venugopal’s current focus is on developing a low-cost method for fabrication of nanostructures. Dr. Venugopal’s recent chapter on “Scalable Synthesis of Noble Metal Nanoparticles” was published in the book ‘Nanoscale and Microscale Phenomena’ earlier this year. In this chapter he describes how his research has led to the development of a new method for low-cost fabrication of silver nanostructures on paper using an inkjet printer. Yes, you read it right…an inkjet printer!

The first step in his work was to find how to synthesise gold and silver nanoparticles in the required size range of 2-20 nm at room temperature. For this, Dr. Venugopal turned to “Green Chemistry” which he defines as “the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances”. He used tannic acid (a compound derived from plants) and sodium citrate (a sodium salt of citric acid) of varying concentrations to prepare gold and silver nanoparticles by reducing their precursor compounds.

An interesting observation in his research is how changing the order of adding the reacting compounds affects the chemical process. He explains, “Mixing, in general, has a major effect on the reaction. This is well-known. But we weren't aware of any previous studies that looked at changing the order of addition. Adding a "metal salt" into the reducing solution (reverse addition---which is reminiscent of the old adage in cooking of always adding salt at the end) enables us to form smaller and more particles of uniform size, as well as speeds up the formation of nanoparticles.”

His primary objective was to synthesise metal nanoparticles below the size of 10 nm in a continuous fashion in water based processes. Hence, he built a “micro-reactor”, essentially a test-tube in the micro-meter scale, using a polymer called polydimethylsiloxane (PDMS). The micro-reactor contains an outer tube (in which the tannic acid was present) and an inner tube (through which the metal salt is introduced). The diameter of the outer tube is almost equal to the thickness of a standard toothpick, while that of the inner tube is equal to the thickness of a small needle. But due to some practical difficulties in using this technology, Dr. Venugopal had to design another reactor.

The new flow reactor has channels which are slots in a polymethylmethacrylate (PMMA) sheet. During nanoparticle synthesis, tannic acid flows through the channels, while metal salt is added dropwise at selected points using syringes. This resulted in gold nanoparticles of uniform size distribution.

The most recent technology involves tinkering with an average HP Deskjet 1000 desktop printer. Dr. Venugopal quips, “We just open up standard HP cartridges and empty the ink, clean it and fill it up with the desired salt solutions.” This helps deposit desired patterns accurately on paper using a standard computer interface within seconds. Silver nanoparticle patterns deposited on the paper can be then used as catalysts to generate conductive copper features on paper. Being ever ready to prove the concept, Dr. Venugopal fabricated a fully functional ultrawideband (UWB) antenna using his technology. He further adds, “Bio-diagnostic devices based on electrochemistry (such as glucose test strips) use platinum electrodes. Such "active" electrodes can also be obtained by our technique.”

Perhaps in the near future, we could print our very own diagnostic kits on our desktops and send the results electronically instead of waiting in long queues in medical institutions!

Dr Venugopal Santhanam is an Assistant Professor in the Department of Chemical Engineering, Indian Institute of Science, Bangalore. He can be reached at 080 - 2293 3113 or venu@chemeng.iisc.ernet.in