US scientists are using a reaction between two sheets of chemically treated paper to create patterns that could deter counterfeiters. 

In traditional anti-forgery methods, the surfaces of official documents and currency are patterned using deterrents such as holograms and microprinting. But these methods are often easy to mimic or forge. Now George Whitesides and colleagues at Harvard University, US have devised a way to pattern paper from within its pores using a simple precipitation reaction. 

One sheet of paper is printed with a barrier of wax or toner in a predetermined two-dimensional design and acts as a stamp, explains Whitesides. The stamp is then 'inked' with a salt solution of copper nitrate and pressed into a second sheet which has been soaked in a solution of sodium hydroxide. As the sheets come together the reagents mix, forming a cyan-coloured solid (copper hydroxide) which is trapped within the pores of the second sheet in the same two-dimensional pattern. The paramagnetic nature of the solid means it can also be manipulated with a magnet. 

'The combination of patterned paper and local chemistry is very easy to implement, and can make structures that would be difficult to make otherwise,' comments Whitesides. The technique can also be used to store enzymes and other catalysts within the pores of paper for simple detection and diagnostics applications. 

Emanuel Carrilho an expert in forensic analysis from the University of Sao Paulo, Brazil, is interested in this unusual and creative method saying it is 'simple and uniquely selective' and a 'nice tool that opens up opportunities to add new function to paper and other porous substrates.' 

Whitesides and colleagues are now looking into other ways in which chemistry might be used to generate unique signatures in materials. 

Hilary Burch 

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