“[Our work] is the first preparation of nanoparticles that uses the principles of Green Chemistry,” said researcher Scott Wallen. “This approach provides a simple, robust means of producing nanoparticles that could be transferred to biological systems, used as water-soluble pharmaceuticals and, potentially, as in situ sensor materials.”

Wallen and colleagues employed water as an environmentally benign solvent. For the reducing agent they used β-D-glucose rather than potentially harmful chemicals such as hydrazine, sodium borohydride and dimethyl formamide (DMF). β-D-glucose acts as a mild, renewable, cheap and non-toxic reducing agent under gentle heating conditions.

Finally, for the capping material to protect the nanoparticle surfaces and prevent the particles from aggregating, the scientists chose a starch solution. They believe the great number of hydroxyl groups in starch can help the complexation of silver ions. Starch is also renewable, soluble in water and binds to the nanoparticles relatively weakly so that the protection should be easily reversible at higher temperatures.

Heating these reactants together with a solution of silver nitrate to 40 °C for 20 hours produced silver nanoparticles with sizes predominantly below 10 nm. The scientists reckon that the nanoparticles grew through reduction of silver ions inside the nanoscopic starch templates, with the starch’s hydroxyl groups acting as passivation contacts for the stabilization of the nanoparticles. Wallen says the particles have so far been stable for more than 10 months and show no signs of precipitation.

The nanoparticles were comparable in size and polydispersity to those produced using typical methods. The scientists believe it may be possible to control particle size and polydispersity better by choosing different reducing and protecting carbohydrates, as well as by changing the reaction conditions.

“The group’s current focus is to understand the mechanism of the starch stabilization using spectroscopy, and investigate control of the particle sizes and the particle size distribution, as well as the incorporation of the particles into real biological systems,” said Wallen.

The researchers reported their work in the Journal of the American Chemical Society.