In Situ Raman Spectroscopy of COOH-Functionalized SWCNTs Trapped with Optoelectronic Tweezers

Optoelectronic tweezers (OETs) were used to trap and deposit aqueous dispersions of carboxylic-acid-functionalized single-walled carbon nanotube bundles. Dark-field video microscopy was used to visualize the dynamics of the bundles both with and without virtual electrodes, showing rapid accumulation of carbon nanotubes when optical virtual electrodes are actuated. Raman microscopy was used to probe SWCNT materials following deposition onto metallic fiducial markers as well as during trapping. The local carbon nanotube concentration was observed to increase rapidly during trapping by more than an order of magnitude in less than one second due to localized optical dielectrophoresis forces. This combination of enrichment and spectroscopy with a single laser spot suggests a broad range of applications in physical, chemical, and biological sciences. 1. Introduction One persistent challenge in molecular sensing is the enriching of candidate analytes to concentrations high enough for detection. Optoelectronic tweezers (OET) recently have been used as a versatile platform for trapping objects such as polystyrene spheres, living cells [1], and solid-state nanowires [2], and both [3] single- and [4] multi-walled samples of carbon nanotubes using 100,000x less optical power than single-beam laser traps. Based on a combination of dielectrophoresis and optical image patterning, OET has the unique capability of massively parallel localization of organic and inorganic nanoscale structures for both direct visualization and spectroscopic characterization. In this paper, we use carboxylic-acid- (COOH-) functionalized single-walled carbon nanotubes as a model system to demonstrate analyte enrichment by over an order of magnitude with a low-power OET trapping laser that serves simultaneously as a Raman spectroscopic probe. 2. Materials and Methods 2.1. Carbon Nanotube Sample Preparation COOH-functionalized carbon nanotubes have been used as a surfactant-free alternative for aqueous SWCNT suspensions, with the hydrophilic COOH-surface functional groups serving as a means to suspend the nanotubes on polar solvents such as water [5]. In this work, COOH functionalized SWCNTs (P3, Carbon Solutions, Inc., ~4 atomic % COOH-functionalization) were dispersed as made in Milli-Q deionized water, bath-sonicated for 30？min, and centrifuged for 30？min at 16,000？g to remove large bundles and other metallic catalyst particles yielding a semitransparent solution. 2.2. Carbon Nanotube Sample Characterization Transmission electron microscopy (JEOL CM-300) and atomic force microscopy (Veeco)