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Title: Semiconductor nanocrystal probes for biological applications and process for making and using such probes

Patent ·
OSTI ID:1079439

A semiconductor nanocrystal compound and probe are described. The compound is capable of linking to one or more affinity molecules. The compound comprises (1) one or more semiconductor nanocrystals capable of, in response to exposure to a first energy, providing a second energy, and (2) one or more linking agents, having a first portion linked to the one or more semiconductor nanocrystals and a second portion capable of linking to one or more affinity molecules. One or more semiconductor nanocrystal compounds are linked to one or more affinity molecules to form a semiconductor nanocrystal probe capable of bonding with one or more detectable substances in a material being analyzed, and capable of, in response to exposure to a first energy, providing a second energy. Also described are processes for respectively: making the semiconductor nanocrystal compound; making the semiconductor nanocrystal probe; and treating materials with the probe.

Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Sponsoring Organization:
USDOE
DOE Contract Number:
AC03-5F00098
Assignee:
The Regents of the University of California (Oakland, CA)
Patent Number(s):
8,288,153
Application Number:
12/408,462
OSTI ID:
1079439
Country of Publication:
United States
Language:
English

References (224)

Subpicosecond dynamics of confined excitons in CuCl nanocrystals journal October 1996
Rigid-Fluid Transition in Specific-Size Argon Clusters journal September 1988
Organometallic synthesis of gallium-arsenide crystallites, exhibiting quantum confinement journal December 1990
Study of the Core−Shell Nanoparticle Formed through the “Coil-to-Globule” Transition of Poly( N -isopropylacrylamide) Grafted with Poly(ethylene oxide) journal December 1997
Design of a Monomeric Arsinogallane and Chemical Conversion to Gallium Arsenide journal July 1988
Structural and spectroscopic investigations of CdS/HgS/CdS quantum-dot quantum wells journal May 1996
Packing and molecular orientation of alkanethiol monolayers on gold surfaces journal September 1989
Self-assembly of organic multilayers with polar order using zirconium phosphate bonding between layers journal October 1990
An EXAFS study on thiolcapped CdTe nanocrystals journal November 1998
Encoding methods for combinatorial chemistry journal June 1997
Colloidal synthesis of nanocrystals and nanocrystal superlattices journal January 2001
Electron–vibration coupling in semiconductor clusters studied by resonance Raman spectroscopy journal April 1989
Semiconductor nanocrystals covalently bound to metal surfaces with self-assembled monolayers journal June 1992
Penetration-controlled reactions in organized monolayer assemblies. 1. Aqueous permanganate interaction with monolayer and multilayer films of long-chain surfactants journal November 1987
Scanning tunneling microscopic imaging of cadmium sulfide Q particles prepared in acetonitrile solution journal November 1989
Randomly Ordered Addressable High-Density Optical Sensor Arrays journal April 1998
Synthesis and characterization of MSe (M=Zn, Cd) nanorods by a new solvothermal method journal March 1999
Radio Frequency Tag Encoded Combinatorial Library Method for the Discovery of Tripeptide-Substituted Cinnamic Acid Inhibitors of the Protein Tyrosine Phosphatase PTP1B journal November 1995
Direct Allelic Variation Scanning of the Yeast Genome journal August 1998
Selective Colorimetric Detection of Polynucleotides Based on the Distance-Dependent Optical Properties of Gold Nanoparticles journal August 1997
Single Molecule Rotational and Translational Diffusion Observed by Near-Field Scanning Optical Microscopy journal October 1997
Electroluminescence from CdSe quantum‐dot/polymer composites journal March 1995
Quantum‐dot size‐distribution analysis and precipitation stages in semiconductor doped glasses journal July 1990
Glycine immunoreactivity localized in the cochlear nucleus and superior olivary complex journal September 1987
Coadsorption of ferrocene-terminated and unsubstituted alkanethiols on gold: electroactive self-assembled monolayers
  • Chidsey, Christopher E. D.; Bertozzi, Carolyn R.; Putvinski, T. M.
  • Journal of the American Chemical Society, Vol. 112, Issue 11, p. 4301-4306 https://doi.org/10.1021/ja00167a028
journal May 1990
Quantum-size stark effect in semiconductor microcrystals journal March 1990
Photoemission investigation of compound semiconductor monodisperse clusters
  • Tobin, J. G.; Colvin, V. L.; Alivisatos, A. P.
  • Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, Vol. 9, Issue 3 https://doi.org/10.1116/1.577328
journal May 1991
Zero-dimensional "excitons" in semiconductor clusters journal September 1986
Melting of clusters and melting journal August 1984
Time-resolved luminescence of CdSe microcrystals journal May 1990
Nanoparticle precursor route to low‐temperature spray deposition of CdTe thin films journal October 1995
Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state journal May 1984
Electrochemical Fabrication of CdS Nanowire Arrays in Porous Anodic Aluminum Oxide Templates journal January 1996
Relation between electroabsorption in bulk semiconductors and in quantum wells: The quantum-confined Franz-Keldysh effect journal May 1986
Immunofluorescent labeling of cancer marker Her2 and other cellular targets with semiconductor quantum dots journal December 2002
Quantum size effects of in situ generated colloidal cadmium sulfide particles in dioctadecyldimethylammonium chloride surfactant vesicles journal February 1987
Radiofrequency Encoded Combinatorial Chemistry journal November 1995
Tagging in combinatorial chemistry: the use of coloured and fluorescent beads journal January 1997
Fluorescence intermittency in single cadmium selenide nanocrystals journal October 1996
Spectra and decay kinetics of radiative recombination in CdS microcrystals journal March 1990
Nucleation and Growth of CdSe on ZnS Quantum Crystallite Seeds, and Vice Versa, in Inverse Micelle Media journal February 1990
In vivo cancer targeting and imaging with semiconductor quantum dots journal July 2004
Three-dimensional orientation measurements of symmetric single chromophores using polarization microscopy journal May 1999
Prediction of Critical Micelle Concentration Using a Quantitative Structure−Property Relationship Approach. 1. Nonionic Surfactants journal January 1996
Rotational Analysis of the 2600 angstrom Absorption System of Benzene
  • Callomon, J. H.; Dunn, T. M.; Mills, I. M.
  • Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 259, Issue 1104 https://doi.org/10.1098/rsta.1966.0023
journal May 1966
Synthesis of Gallium Nitride Nanorods Through a Carbon Nanotube-Confined Reaction journal August 1997
Chemical functionality in self-assembled monolayers: structural and electrochemical properties journal March 1990
Structures of self-assembled monolayer films of organosulfur compounds adsorbed on gold single crystals: electron diffraction studies journal May 1988
Effects of resonance on low-frequency raman scattering from semiconductor nanocrystals journal December 1995
Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites journal September 1993
Kinetics of II-VI and III-V Colloidal Semiconductor Nanocrystal Growth:  “Focusing” of Size Distributions journal June 1998
Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility
  • Peng, Xiaogang; Schlamp, Michael C.; Kadavanich, Andreas V.
  • Journal of the American Chemical Society, Vol. 119, Issue 30, p. 7019-7029 https://doi.org/10.1021/ja970754m
journal July 1997
Synthesis, morphology and optical properties of thiol-stabilized CdTe nanoclusters in aqueous solution journal November 1997
Scanned probe investigations of chemically derived nanostructures journal December 1996
Protein-Sized Quantum Dot Luminescence Can Distinguish between "Straight", "Bent", and "Kinked" Oligonucleotides journal September 1995
Molecular dynamics study of melting and freezing of small Lennard-Jones clusters journal September 1987
Optical properties of quantized semiconductor particles
  • Weller, Horst
  • Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences, Vol. 354, Issue 1708, p. 757-766 https://doi.org/10.1098/rsta.1996.0029
journal March 1996
Resonance Raman spectroscopy of electron-hole pairs - polar phonon coupling in semiconductor quantum microcrystals journal June 1991
Formation of monolayer films by the spontaneous assembly of organic thiols from solution onto gold journal January 1989
Luminescence properties of CdSe quantum crystallites: Resonance between interior and surface localized states journal January 1992
Size dependence of acoustic and optical vibrational modes of CdSe nanocrystals in glasses journal May 1996
Accessing Genetic Information with High-Density DNA Arrays journal October 1996
Formation of multilayers by self-assembly journal January 1989
Preparation of Monodisperse CdS Nanocrystals by Size Selective Photocorrosion journal January 1996
Enhancement of electron-hole exchange interaction in CdSe nanocrystals: A quantum confinement effect journal January 1996
MICROSCOPY:Semiconductor Beacons Light Up Cell Structures journal September 1998
Electroluminescent Display Materials journal August 1989
Layer-by-Layer Assembled Films of HgTe Nanocrystals with Strong Infrared Emission journal June 2000
Fluorescence recovery spectroscopy as a probe of slow rotational motions journal December 1984
DNA SEQUENCING: Massively Parallel Genomics journal July 1997
Spectroscopic Analysis of Semiconductor Colloids: An Experiment in Materials Science for the Advanced Inorganic or Physical Chemistry Laboratory journal January 1993
Combinatorial Chemistry and New Drugs journal April 1997
Perspectives on the Physical Chemistry of Semiconductor Nanocrystals journal January 1996
Bifunctional Capping of CdS Nanoparticles and Bridging to TiO2 journal June 1995
Characterization of peptide-coated cadmium-sulfide crystallites journal April 1990
Size-dependent electron-hole exchange interaction in CdSe quantum dots journal November 1995
Preferential Adsorption of a “Kinked” DNA to a Neutral Curved Surface:  Comparisons to and Implications for Nonspecific DNA−Protein Interactions journal January 1996
Synthesis of Highly Luminescent GaSe Nanoparticles journal February 2002
Structure of BRIJ-35 Nonionic Surfactant in Water:  A Reverse Monte Carlo Study journal January 2006
Directionally Aligned Helical Peptides on Surfaces journal July 1993
Electroluminescence and electrical transport in poly(3‐octylthiophene) diodes journal July 1992
Biomolecular Self-Assembly of Quantum-Dot Composites journal January 1993
Self-Assembly of CdSe−ZnS Quantum Dot Bioconjugates Using an Engineered Recombinant Protein
  • Mattoussi, Hedi; Mauro, J. Matthew; Goldman, Ellen R.
  • Journal of the American Chemical Society, Vol. 122, Issue 49, p. 12142-12150 https://doi.org/10.1021/ja002535y
journal December 2000
Synthesis and antihypertensive activity of 4-(substituted-carbonylamino)-2H-1-benzopyrans journal September 1990
Core−Shell Quantum Dots of Lattice-Matched ZnCdSe 2 Shells on InP Cores:  Experiment and Theory journal December 2000
Optical studies of ZnSe-ZnSGaAs(100) single quantum wells grown by photo-assisted vapor phase epitaxy journal December 1995
Synthesis of Nanosized Gold−Silica Core−Shell Particles journal January 1996
Electronic wave functions in semiconductor clusters: experiment and theory journal June 1986
Surface recombination of nonequilibrium electron-hole plasma in laser-modified semiconductor-doped glasses journal August 1993
Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection journal September 1998
Semiconductor Nanocrystals as Fluorescent Biological Labels patent September 1998
A DNA-based method for rationally assembling nanoparticles into macroscopic materials journal August 1996
Scintillation proximity assay: a versatile high-throughput screening technology journal July 1996
Spherical Assemblies of Semiconductor Nanoparticles in Water-Soluble Block Copolymer Aggregates journal April 1998
Electroluminescence of doped organic thin films journal May 1989
Strong molecular alignment in anisotropic fluid media journal September 1996
Characterization of CdS thin film in high efficient CdS/CdTe solar cells journal June 2000
Quantum-Confined Stark Effect in Single CdSe Nanocrystallite Quantum Dots journal December 1997
Formation of self-assembled monolayers by chemisorption of derivatives of oligo(ethylene glycol) of structure HS(CH2)11(OCH2CH2)mOH on gold journal January 1991
Epitaxial films of the 3D semiconductor CdS on the 2D layered substrate MX2 prepared by Van der Waals epitaxy journal January 1995
Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids journal September 1996
Structural and optical characteristics of ZnSe nanocrystals synthesized in the presence of a polymer capping agent journal March 1998
Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules journal July 2001
Subpicosecond dynamics of confined excitons and optical nonlinearities of CuCl quantum dots journal December 1995
Synthesis of cadmium sulfide in situ in reverse micelles: influence of the preparation modes on size, polydispersity, and photochemical reactions journal March 1992
Melting in Semiconductor Nanocrystals journal June 1992
Multicolor Spectral Karyotyping of Human Chromosomes journal July 1996
Structure and reactivity of alkylsiloxane monolayers formed by reaction of alkyltrichlorosilanes on silicon substrates journal July 1989
Resonance Raman scattering and optical absorption studies of CdSe microclusters at high pressure journal November 1988
Formation of monolayers by the coadsorption of thiols on gold: variation in the head group, tail group, and solvent journal August 1989
Nanocrystal targeting in vivo journal September 2002
(CdSe)ZnS Core−Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites journal November 1997
Lattice reorganization in electronically excited semiconductor clusters journal March 1990
Characterization of quantum-confined CdS nanocrystallites stabilized by deoxyribonucleic acid (DNA) journal April 1992
Immunogold labeling in scanning electron microscopy journal July 1996
Optical properties of Mn-doped CdS nanocrystals journal February 1996
Surface Derivatization of Nanocrystalline CdSe Semiconductors journal January 1996
Synthesis of CdS Nanoparticles in Solution and in a Polyphosphazene Matrix journal June 1997
Inorganic phosphors as new luminescent labels for immunocytochemistry and time-resolved microscopy journal January 1990
Formation of monolayers by the coadsorption of thiols on gold: variation in the length of the alkyl chain journal August 1989
Growth of CdSe nanocrystals in ion-implanted SiO2 films journal May 1995
Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS) journal March 1997
Immunosensors: technology and opportunities in laboratory medicine journal February 1996
Electronic Energy Transfer in CdSe Quantum Dot Solids journal February 1996
One-dimensional nanostructures: Chemistry, physics & applications journal August 1998
Preparation of II–VI quantum dot composites by electrospray organometallic chemical vapor deposition journal December 1994
Strongly Photoluminescent CdTe Nanocrystals by Proper Surface Modification journal October 1998
Micro photoluminescence spectra of CdTe and CdMnTe self-organized quantum dots journal November 1999
Size quantization in semiconductor particulate films journal May 1991
Quantum Dots as Inorganic DNA-Binding Proteins journal January 1996
Polaron and Exciton-Phonon Complexes in CuCl Nanocrystals journal February 1995
Adsorption of ordered zirconium phosphonate multilayer films on silicon and gold surfaces journal May 1988
Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum journal October 1988
Electroabsorption of highly confined systems: Theory of the quantum‐confined Franz–Keldysh effect in semiconductor quantum wires and dots journal June 1988
A simple model for the ionization potential, electron affinity, and aqueous redox potentials of small semiconductor crystallites journal December 1983
The Enzymatic Synthesis of Thiol-Containing Polymers to Prepare Polymer−CdS Nanocomposites journal June 1997
Single molecule polarization spectroscopy: pentacene in p-terphenyl journal November 1996
Quantum confinement effects on the optical phonons of CdTe quantum dots journal May 1998
Assembly and applications of the inorganic nanocrystals in polymer networks journal August 1998
Dynamics of excitons in CuBr nanocrystals: Spectral-hole burning and transient four-wave-mixing measurements journal January 1998
The Band Edge Luminescence of Surface Modified CdSe Nanocrystallites journal January 1996
Photoluminescence polarization of semiconductor nanocrystals journal October 1996
Probing of the quantum dot size distribution in CdTe‐doped glasses by photoluminescence excitation spectroscopy journal January 1995
Full Color Emission from II-VI Semiconductor Quantum Dot-Polymer Composites journal July 2000
Combining spontaneous molecular assembly with microfabrication to pattern surfaces: selective binding of isonitriles to platinum microwires and characterization by electrochemistry and surface spectroscopy journal August 1989
Characterization of solution-synthesized CdTe and HgTe journal April 1993
Organization of 'nanocrystal molecules' using DNA journal August 1996
Nanocrystals of II-VI Semiconductor Materials journal January 1994
Surface derivatization and isolation of semiconductor cluster molecules journal May 1988
Spontaneously organized molecular assemblies. 4. Structural characterization of n-alkyl thiol monolayers on gold by optical ellipsometry, infrared spectroscopy, and electrochemistry journal June 1987
Size-selective resonant Raman scattering in CdS doped glasses journal January 1998
Comparison of self-assembled monolayers on gold: coadsorption of thiols and disulfides journal May 1989
Optics of Zero Dimensional Semiconductor Systems journal January 1991
Enhanced Photoluminescence and Characterization of Mn-Doped ZnS Nanocrystallites Synthesized in Microemulsion journal November 1997
Melting transition of small molecular clusters journal July 1981
The influence of adenine content on the properties of Q-CdS clusters stabilized by polynucleotides journal February 1995
Epitaxial Growth of Size-Quantized Cadmium Sulfide Crystals Under Arachidic Acid Monolayers journal April 1995
Light-directed synthesis of high-density oligonucleotide arrays using semiconductor photoresists journal November 1996
Synthesis and Isolatin of a Homodimer of Cadmium Selenide Nanocrystals journal February 1997
Dynamics of nonlinear optical response of CuBr-doped glasses journal March 1993
Optical Properties of Semiconductor Quantum Dots in Glass Matrix journal January 1991
Dictation of the shape of mesoscale semiconductor nanoparticle assemblies by plasmid DNA journal December 1996
Semiconductor particulate films on solid supports journal January 1991
Spontaneously organized molecular assemblies. 3. Preparation and properties of solution adsorbed monolayers of organic disulfides on gold surfaces journal April 1987
Insights into US public biotech sector using patenting trends journal June 2006
Stabilization of CdS semiconductor nanoparticles against photodegradation by a silica coating procedure journal April 1998
Removal of Metal Pollutants (Cd(II) and Cr(III)) from Phosphoric Acid Solutions by Chelating Resins Containing Phosphonic or Diphosphonic Groups journal June 1998
Optical properties of oxide glasses doped by semiconductor nanocrystals journal December 1995
Hybrid organic–inorganic semiconductor-based light-emitting diodes journal October 1997
Photoluminescence Spectroscopy of Single CdSe Nanocrystallite Quantum Dots journal October 1996
Donor-like exciton in zero-dimension semiconductor structures journal February 1989
Luminescence properties of semiconductor quantum dots journal June 1997
Spontaneously organized molecular assemblies. 2. Quantitative infrared spectroscopic determination of equilibrium structures of solution-adsorbed n-alkanoic acids on an oxidized aluminum surface journal January 1985
Synthesis of a Small Molecule Combinatorial Library Encoded with Molecular Tags journal May 1995
Symmetry of Annealed Wurtzite CdSe Nanocrystals: Assignment to the C3v Point Group journal November 1995
Spontaneously organized molecular assemblies. 1. Formation, dynamics, and physical properties of n-alkanoic acids adsorbed from solution on an oxidized aluminum surface journal January 1985
PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors journal October 2005
Rapid processes of darkening and bleaching in Cds doped glasses journal January 1991
Photochemistry and radiation chemistry of colloidal semiconductors. 23. Electron storage on zinc oxide particles and size quantization journal January 1988
Fluorescence and Redox Activity of Probes Anchored through an Aminotrithiol to Polycrystalline Gold journal February 1998
Improved efficiencies in light emitting diodes made with CdSe(CdS) core/shell type nanocrystals and a semiconducting polymer journal December 1997
Organic electroluminescent diodes journal September 1987
Growth and optical properties of semiconductor nanocrystals in a glass matrix journal October 1996
Size effect on the melting temperature of gold particles journal June 1976
Photochemistry of colloidal semiconductors. 20. Surface modification and stability of strong luminescing CdS particles journal September 1987
Adsorption of bifunctional organic disulfides on gold surfaces journal June 1983
All-Inorganic Field Effect Transistors Fabricated by Printing journal October 1999
Semiconductor Clusters, Nanocrystals, and Quantum Dots journal February 1996
The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state journal June 1997
Biosynthesis of cadmium sulphide quantum semiconductor crystallites journal April 1989
Self-assembling double layers on gold surfaces: the merging of two chemistries journal November 1989
Free Energy and Temperature Dependence of Electron Transfer at the Metal-Electrolyte Interface journal February 1991
Interface effects on the properties of confined excitons in CuCl microcrystals journal April 1994
Measurement of rotational motion in membranes using fluorescence recovery after photobleaching journal October 1981
Simultaneous Imaging of Individual Molecules Aligned Both Parallel and Perpendicular to the Optic Axis journal December 1998
GaAs Clusters in the Quantum Size Regime: Growth on High Surface Area Silica by Molecular Beam Epitaxy journal July 1989
Nonlinear Optics of Semiconductor-Doped Glasses journal December 1988
The “New” Science of “Complex Fluids” journal January 1996
Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals journal January 1996
Deactivation of Q-cadmium sulfide photoluminescence through polynucleotide surface binding journal December 1992
Synthesis and characterization of thiol-stabilized CdTe nanocrystals journal November 1996
Imaging and Time-Resolved Spectroscopy of Single Molecules at an Interface journal April 1996
Single Molecules Observed by Near-Field Scanning Optical Microscopy journal November 1993
Inorganic Clusters as Single-Source Precursors for Preparation of CdSe, ZnSe, and CdSe/ZnS Nanomaterials journal April 2002
X‐ray structural characterization of larger CdSe semiconductor clusters journal December 1989
Nucleotides as Structural Templates for The Self-Assembly of Quantum-Confined Cds Crystallites journal January 1990
Ionic recognition and selective response in self-assembling monolayer membranes on electrodes journal March 1988
Optically Induced Polarization Anisotropy in Porous Si journal September 1996
Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer journal August 1994
Photochemistry and radiation chemistry of colloidal semiconductors. 33. Chemical changes and fluorescence in CdTe and ZnTe journal July 1989
Oxidation of a sulfide group in a self-assembled monolayer journal July 1989
Magnetic circular dichroism study of CdSe quantum dots journal March 1998
Materials Science: Small Clusters Hit the Big Time journal February 1996
Synthesis of Luminescent Thin-Film CdSe/ZnSe Quantum Dot Composites Using CdSe Quantum Dots Passivated with an Overlayer of ZnSe journal January 1996
Spin-flip and acoustic-phonon Raman scattering in CdS nanocrystals journal July 1998
Single-molecule spectroscopy with 27 fs pulses: Time-resolved experiments and direct imaging of orientational distributions journal July 1998
CdS nanocrystal growth in thin silica films: evolution of size distribution function journal February 1998
Exciton fine structure in CdSe nanoclusters journal May 1998
Enhancement of Exciton Exchange Interaction by Quantum Confinement in CdSe Nanocrystals journal January 1995
The structure of self-assembled monolayers of alkylsiloxanes on silicon: a comparison of results from ellipsometry and low-angle x-ray reflectivity journal July 1989
Poly( p ‐phenylenevinylene) light‐emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement journal December 1992
Theory of the linear and nonlinear optical properties of semiconductor microcrystallites journal May 1987
Synthesis and characterization of surface-modified colloidal cadmium telluride quantum dots journal November 1993
QUANTUM DOTS MEET BIOMOLECULES: Method uses brightly fluorescing semiconductor nanocrystals as labels journal September 1998