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Scientists at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, are using surface-enhanced Raman spectroscopy (SERS) to test the properties of star-shaped gold nanoparticles. They have found that these particles have optical qualities that outshine the competition and could make them useful in chemical and biological sensing and imaging. Seeing stars is not usually a concern of NIST researchers, but they describe these gold nanoparticles of unique shape as having the potential to diagnose disease or identify contraband goods much more effectively than other SERS materials. SERS relies on metallic nanoparticles, most commonly gold and silver, to amplify signals from molecules present in a sample in only trace quantities. The conventional procedure is to shine laser light on an aqueous solution containing both the nanoparticles and the molecules of interest and to record the spectrum of the scattered light. The specific optical properties of the nanoparticles and of the sample molecules affect the way in which the laser light is scattered and so produce a characteristic fingerprint, the vibrational signature, for the molecule. NIST's Angela Hight Walker and her team point out that by amplifying this signature using specific nanoparticles, they can detect very low concentrations of sample molecules in the solution. Hight Walker and her team in NIST's Optical Technology Division, perfected the process of making gold nanostars using a bottom-up approach often favoured by chemists - the seed-mediated growth method - rather than sculpting the particles from particles of a larger feedstock material. They used surface alterations to manipulate their growth and control their shape. They add that researchers have observed that aggregates of metallic nanoparticles can generate very intense enhanced Raman signals at the junction between two nanoparticles - hot spots. With this in mind, the team suspended their gold nanostar particles in solution, they guide the nanoparticles to aggregate into multiple "hot spots". It is at these hot spots that the Raman enhancement is most dramatic, certainly being much stronger than for single nanostar particles. The NIST team has tested its nanostars with two target molecules, 2-mercaptopyridine and the dye molecule crystal violet. These two compounds are loosely similar to various biological molecules of interest. But, importantly, they have a large number of delocalized electrons and so lend themselves to SERS. "Molecules with delocalized electrons such as lone pairs or pi clouds (e.g. aromatic amines, phenols) often demonstrate strong Raman enhancement," the researchers explain, "The electromagnetic component of the enhancement results from an increased field at the metallic nanoparticle surface. The field enhancement is a consequence of the interaction of the incoming laser radiation with electrons in the metal surface, which activates surface plasmons, or collective oscillations of the metal electrons." Indeed, the team found that the Raman signal for 2-mercaptopyridine was five orders of magnitude - 100,000 - stronger with the nanostars suspended in the solution than without enhancing particles. The stars really shone when it came to testing crystal violet, boosting the Raman signature by ten times as much as the previous best effort - gold nanorods. The team emphasises that the nanostars and nanorods worked much more effectively than the conventional nanospheres used in most SERS experiments. They were able to obtain significant enhancement at crystal violet concentrations as low as 100 nanomolar. Hight Walker says that the ease with which gold nanostars can be mass-produced and their obvious prowess in SERS should prompt other researchers to look for other possible applications, perhaps one day, they might even become the stars of the nanoworld, she quips. Reference:
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Gold nanostars outshine the competition in SERS |
