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Unformatted text preview: Acoustically Levitated Droplets A Contactless Sampling Method for Fluorescence Studies J ORK L EITERER , a M ARKUS G RABOLLE , a K NUT R URACK , a U TE R ESCH-G ENGER , a J AN Z IEGLER , b T HOMAS N ANN , b AND U LRICH P ANNE a a Federal Institute for Materials Research and Testing (BAM), Berlin, Germany b University of East Anglia, School of Chemical Sciences, Norwich, United Kingdom Acoustic levitation is used as a new tool to study concentration-dependent processes in fluorescence spectroscopy. With this technique, small amounts of liquid and solid samples can be measured without the need for sample supports or containers, which often limits signal acquisition and can even alter sample properties due to interactions with the support material. We demonstrate that, because of the small sample volume, fluorescence measurements at high concentrations of an organic dye are possible without the limitation of inner-filter effects, which hamper such experiments in conventional, cuvette-based measurements. Furthermore, we show that acoustic levitation of liquid samples provides an experimentally simple way to study distance-dependent fluorescence modulations in semiconductor nanocrystals. The evaporation of the solvent during levitation leads to a continuous increase of solute concentration and can easily be monitored by laser-induced fluorescence. Key words: acoustic levitation; dyes; energy transfer; fluorescence; quantum dots; nanocrystals; ultrasonic trap Introduction Many of today’s analytical problems are charac- terized by small sample volumes and can be solved only through a corresponding miniaturization of the analytical instrumentation. 1 , 2 In principle, analytical methods that are based on spectroscopic techniques are sensitive enough for the analysis of small sample amounts, but handling of small volumes is inherently difficult because of desorption of contaminants from and adsorption of analytes at the walls of containers. Furthermore, miniaturized systems that rely, for in- stance, on analytical chips often suffer from optical in- terferences at chip surfaces, and for micro total analysis systems, analyte adsorption at interfaces can hamper the reliability of the detected signal. 3 Acoustic levitation is a powerful tool to circumvent such drawbacks. 4–6 This method allows the contact- free handling of solid and liquid samples in a gaseous environment as well as the trapping of solid objects in Address for correspondence: Jork Leiterer, Dipl.-Phys., Div. I.3, Federal Institute for Materials Research and Testing (BAM), Richard-Willst¨atter- Str. 11, 12489 Berlin-Adlershof, Germany. Fax: + 49(30)-8104-1137. [email protected] condensed (flowing) phases by means of a stationary ultrasonic field. For that purpose, a concave reflector is adjusted concentrically at a distance of several multiple half-wavelengths of the so-called sonotrode that emits an ultrasonic wave. As a result of multiple reflections between sonotrode and reflector, a standing wave is...
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This note was uploaded on 07/11/2010 for the course SPECTOGRAP 545 taught by Professor Gdf during the Spring '10 term at AIB College of Business.
- Spring '10