Metal-CdSe-metal (metal ) Au, Ni) nanowires were grown by electrochemical replication of porous aluminum oxide and polycarbonate track etch membranes with pore diameters of 350 and 70 nm, respectively. The lengths of the individual segments of the nanowires were controlled by varying the amount of charge that was passed. The composition of the CdSe segments was characterized by energy-dispersive X-ray spectroscopy. A 1:1 ratio could be obtained, and Cd-and Se-rich stoichiometries were also made by adjusting the concentrations of Cd 2+ and SeO 2 in the aqueous plating solutions. X-ray powder diffraction showed the presence of both zinc blende and wurzite phases, and grain sizes on the order of 10 nm were observed by TEM. The nanowires were resistive in the dark but showed pronounced visible light photoconductivity.

Metallic "barcodes" have been reported recently in which the size and location of distinguishable metal segments (e.g., Au and Ag) are used to encode information [Nicewarner-Pen ˜a et al., Science 2001, 294, 137-141]. Barcode readout is accomplished by conventional brightfield reflectance optical microscopy. Herein we report the wavelength-dependent optical reflectivity of individual stripes in metallic barcodes, and how this wavelength-dependence impacts the intensity of fluorescence from sandwich immuno-and hybridization assays performed on the particle surface. The encoded particles used in this study were striped nanowires on the order of 4-8 µm in overall length, with individual stripes typically on the order of 1-2 µm, and diameters ∼320 nm. Reflectivity measurements were made for several metals (Ag, Cu, Co, Ni, Pd, and Pt) relative to Au, which was used as an internal standard. Despite the subwavelength diameters of these nanowires, good agreement was found between experimentally determined reflectivities and bulk metal values. Under some conditions, fluorescence intensity patterns corresponding to the underlying metal segments could be observed. We find that the ratio of fluorescence intensities on different metal segments correlate with the metal reflectivity ratios at the excitation and emission wavelengths for the dye. Surface roughness and chemical effects may also play a role for some metals. We have shown that by choice of the underlying metal, particle striping patterns can be accentuated or hidden in the fluorescence image. This is demonstrated in a triplexed DNA hybridization assay.

A hybrid quadrupole orthogonal time-of-flight mass spectrometer optimized for MALDI and electrospray ionization has been equipped with a C 60 cluster ion source. This configuration is shown to exhibit a number of characteristics that improve the performance of traditional time-of-flight secondary ion mass spectrometry (SIMS) experiments for the analysis of complex organic materials, and potentially, for chemical imaging. Specifically, the primary ion beam is operated as a continuous rather than a pulsed beam, resulting in up to 4 orders of magnitude greater ion fluence on the target. The secondary ions are extracted at very low voltage into 8 millitorr of N 2 gas introduced for collisional focusing and cooling purposes. This extraction configuration is shown to yield secondary ions that rapidly lose memory of the mechanism of their birth, yielding tandem mass spectra that are identical for SIMS and MALDI. With implementation of ion trapping, the extraction efficiency is shown to be equivalent to that found in traditional TOF-SIMS machines. Examples are given, for a variety of substrates that illustrate mass resolution of 12,000-15,600 with mass range for inorganic compounds to m/z 40,000. Preliminary chemical mapping experiments show that with added sensitivity, imaging in the MS/MS mode of operation is straightforward. In general, the combination of MALDI and SIMS is shown to add capabilities to each technique, providing a robust platform for TOF-SIMS experiments that already exists in a large number of laboratories.

A new, low power ionization source for elemental MS analysis of aqueous solutions is described. The liquid sampling-atmospheric pressure glow discharge (LS-APGD) operates by a process wherein the surface of the liquid emanating from a 75 μm i.d. glass capillary acts as the cathode of the direct current glow discharge. Analyte-containing solutions at a flow rate of 100 μL min(-1) are vaporized by the passage of current, yielding gas phase solutes that are subsequently ionized in the <5 W (maximum of 60 mA and 500 V), ~1 mm(3) volume, plasma. The LS-APGD is mounted in place of the normal electrospray ionization source of a Thermo Scientific Exactive Orbitrap mass spectrometer system without any other modifications. Basic operating characteristics are described, including the role of discharge power on mass spectral composition, the ability to obtain ultrahigh resolution elemental isotopic patterns, and demonstration of potential limits of detection based on the injection of aliquots of multielement standards (S/N > 1000 for 5 ng mL(-1) Cs). While much optimization remains, it is believed that the LS-APGD ion source may present a practical alternative to high-powered (>1 kW) plasma sources typically employed in elemental mass spectrometry, particularly for those cases where costs, operational overhead, simplicity, or integrated elemental/molecular analysis considerations are important.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) can be utilized to map the distribution of various molecules on a surface with sub-micrometer resolution. Much of its biological application has been in the study of membrane lipids, such as phospholipids and cholesterol. Cholesterol is a particularly interesting molecule due to its involvement in numerous biological processes. For many studies, the effectiveness of chemical mapping is limited by low signal intensity from various biomolecules. Due to the high energy nature of the SIMS ionization process, many molecules are identified by detection of characteristic fragments. Commonly, fragments of a molecule are identified using standard samples, and those fragments are used to map the location of the molecule. In this work, MS/MS data obtained from a prototype C 60 + / quadrupole time-of-flight mass spectrometer was used in conjunction with indium LMIG imaging to map previously unrecognized cholesterol fragments in single cells. A model system of J774 macrophages doped with cholesterol was used to show that these fragments are derived from cholesterol in cell imaging experiments. Examination of relative quantification experiments reveals that m/z 147 is the most specific diagnostic fragment and offers a 3-fold signal enhancement. These findings greatly increase the prospects for cholesterol mapping experiments in biological samples, particularly with single cell experiments. In addition, these findings demonstrate the wealth of information that is hidden in the traditional ToF-SIMS spectrum.