Publications by authors named "Ralph A Sperling"

21 Publications

  • Page 1 of 1

Automated Quantum Dots Purification via Solid Phase Extraction.

Nanomaterials (Basel) 2022 Jun 9;12(12). Epub 2022 Jun 9.

Fraunhofer Institute for Microengineering and Microsystems IMM, 55129 Mainz, Germany.

The separation of colloidal nanocrystals from their original synthesis medium is an essential process step towards their application, however, the costs on a preparative scale are still a constraint. A new combination of approaches for the purification of hydrophobic Quantum Dots is presented, resulting in an efficient scalable process in regard to time and solvent consumption, using common laboratory equipment and low-cost materials. The procedure is based on a combination of solvent-induced adhesion and solid phase extraction. The platform allows the transition from manual handling towards automation, yielding an overall purification performance similar to one conventional batch precipitation/centrifugation step, which was investigated by thermogravimetry and gas chromatography. The distinct miscibility gaps between surfactants used as nanoparticle capping agents, original and extraction medium are clarified by their phase diagrams, which confirmed the outcome of the flow chemistry process. Furthermore, the solubility behavior of the Quantum Dots is put into context with the Hansen solubility parameters framework to reasonably decide upon appropriate solvent types.
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http://dx.doi.org/10.3390/nano12121983DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC9230973PMC
June 2022

Single-cell ChIP-seq reveals cell subpopulations defined by chromatin state.

Nat Biotechnol 2015 Nov 12;33(11):1165-72. Epub 2015 Oct 12.

Epigenomics Program, The Broad Institute of MIT and Harvard, Cambridge, Massachusetts, USA.

Chromatin profiling provides a versatile means to investigate functional genomic elements and their regulation. However, current methods yield ensemble profiles that are insensitive to cell-to-cell variation. Here we combine microfluidics, DNA barcoding and sequencing to collect chromatin data at single-cell resolution. We demonstrate the utility of the technology by assaying thousands of individual cells and using the data to deconvolute a mixture of ES cells, fibroblasts and hematopoietic progenitors into high-quality chromatin state maps for each cell type. The data from each single cell are sparse, comprising on the order of 1,000 unique reads. However, by assaying thousands of ES cells, we identify a spectrum of subpopulations defined by differences in chromatin signatures of pluripotency and differentiation priming. We corroborate these findings by comparison to orthogonal single-cell gene expression data. Our method for single-cell analysis reveals aspects of epigenetic heterogeneity not captured by transcriptional analysis alone.
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http://dx.doi.org/10.1038/nbt.3383DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4636926PMC
November 2015

High-Throughput Single-Cell Labeling (Hi-SCL) for RNA-Seq Using Drop-Based Microfluidics.

PLoS One 2015 22;10(5):e0116328. Epub 2015 May 22.

Department of Physics and School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States of America.

The importance of single-cell level data is increasingly appreciated, and significant advances in this direction have been made in recent years. Common to these technologies is the need to physically segregate individual cells into containers, such as wells or chambers of a micro-fluidics chip. High-throughput Single-Cell Labeling (Hi-SCL) in drops is a novel method that uses drop-based libraries of oligonucleotide barcodes to index individual cells in a population. The use of drops as containers, and a microfluidics platform to manipulate them en-masse, yields a highly scalable methodological framework. Once tagged, labeled molecules from different cells may be mixed without losing the cell-of-origin information. Here we demonstrate an application of the method for generating RNA-sequencing data for multiple individual cells within a population. Barcoded oligonucleotides are used to prime cDNA synthesis within drops. Barcoded cDNAs are then combined and subjected to second generation sequencing. The data are deconvoluted based on the barcodes, yielding single-cell mRNA expression data. In a proof-of-concept set of experiments we show that this method yields data comparable to other existing methods, but with unique potential for assaying very large numbers of cells.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0116328PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4441486PMC
March 2016

Quantifying cell-generated mechanical forces within living embryonic tissues.

Nat Methods 2014 Feb 8;11(2):183-9. Epub 2013 Dec 8.

1] School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA. [2] Wyss Institute for Biologically Inspired Engineering at Harvard University, Boston, Massachusetts, USA. [3] Vascular Biology Program, Children's Hospital, Boston, Massachusetts, USA. [4] Department of Pathology, Harvard Medical School, Boston, Massachusetts, USA.

Cell-generated mechanical forces play a critical role during tissue morphogenesis and organ formation in the embryo. Little is known about how these forces shape embryonic organs, mainly because it has not been possible to measure cellular forces within developing three-dimensional (3D) tissues in vivo. We present a method to quantify cell-generated mechanical stresses exerted locally within living embryonic tissues, using fluorescent, cell-sized oil microdroplets with defined mechanical properties and coated with adhesion receptor ligands. After a droplet is introduced between cells in a tissue, local stresses are determined from droplet shape deformations, measured using fluorescence microscopy and computerized image analysis. Using this method, we quantified the anisotropic stresses generated by mammary epithelial cells cultured within 3D aggregates, and we confirmed that these stresses (3.4 nN μm(-2)) are dependent on myosin II activity and are more than twofold larger than stresses generated by cells of embryonic tooth mesenchyme, either within cultured aggregates or in developing whole mouse mandibles.
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http://dx.doi.org/10.1038/nmeth.2761DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3939080PMC
February 2014

Synchronized reinjection and coalescence of droplets in microfluidics.

Lab Chip 2014 Feb;14(3):509-13

Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.

Coalescence of two kinds of pre-processed droplets is necessary to perform chemical and biological assays in droplet-based microfluidics. However, a robust technique to accomplish this does not exist. Here we present a microfluidic device to synchronize the reinjection of two different kinds of droplets and coalesce them, using hydrostatic pressure in conjunction with a conventional syringe pump. We use a device consisting of two opposing T-junctions for reinjecting two kinds of droplets and control the flows of the droplets by applying gravity-driven hydrostatic pressure. The hydrostatic-pressure operation facilitates balancing the droplet reinjection rates and allows us to synchronize the reinjection. Furthermore, we present a simple but robust module to coalesce two droplets that sequentially come into the module, regardless of their arrival times. These re-injection and coalescence techniques might be used in lab-on-chip applications requiring droplets with controlled numbers of solid materials, which can be made by coalescing two pre-processed droplets that are formed and sorted in devices.
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http://dx.doi.org/10.1039/c3lc51214bDOI Listing
February 2014

DNA sequence analysis with droplet-based microfluidics.

Lab Chip 2013 Dec;13(24):4864-9

University of California, San Francisco - Bioengineering and Therapeutic Sciences, San Francisco, California, USA.

Droplet-based microfluidic techniques can form and process micrometer scale droplets at thousands per second. Each droplet can house an individual biochemical reaction, allowing millions of reactions to be performed in minutes with small amounts of total reagent. This versatile approach has been used for engineering enzymes, quantifying concentrations of DNA in solution, and screening protein crystallization conditions. Here, we use it to read the sequences of DNA molecules with a FRET-based assay. Using probes of different sequences, we interrogate a target DNA molecule for polymorphisms. With a larger probe set, additional polymorphisms can be interrogated as well as targets of arbitrary sequence.
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http://dx.doi.org/10.1039/c3lc50905bDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4090915PMC
December 2013

A novel implantable glaucoma valve using ferrofluid.

PLoS One 2013 28;8(6):e67404. Epub 2013 Jun 28.

Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts, United States of America.

Purpose: To present a novel design of an implantable glaucoma valve based on ferrofluidic nanoparticles and to compare it with a well-established FDA approved valve.

Setting: Massachusetts Eye & Ear Infirmary, Boston, USA.

Methods: A glaucoma valve was designed using soft lithography techniques utilizing a water-immiscible magnetic fluid (ferrofluid) as a pressure-sensitive barrier to aqueous flow. Two rare earth micro magnets were used to calibrate the opening and closing pressure. In-vitro flow measurements were performed to characterize the valve and to compare it to Ahmed™ glaucoma valve. The reliability and predictability of the new valve was verified by pressure/flow measurements over a period of three months and X-ray diffraction (XRD) analysis over a period of eight weeks. In vivo assessment was performed in three rabbits.

Results: In the in vitro experiments, the opening and closing pressures of the valve were 10 and 7 mmHg, respectively. The measured flow/pressure response was linearly proportional and reproducible over a period of three months (1.8 µl/min at 12 mmHg; 4.3 µl/min at 16 mmHg; 7.6 µl/min at 21 mmHg). X-ray diffraction analysis did not show oxidization of the ferrofluid when exposed to water or air. Preliminary in vivo results suggest that the valve is biocompatible and can control the intraocular pressure in rabbits.

Conclusions: The proposed valve utilizes ferrofluid as passive, tunable constriction element to provide highly predictable opening and closing pressures while maintaining ocular tone. The ferrofluid maintained its magnetic properties in the aqueous environment and provided linear flow to pressure response. Our in-vitro tests showed reliable and reproducible results over a study period of three months. Preliminary in-vivo results were very promising and currently more thorough investigation of this device is underway.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0067404PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3696055PMC
January 2014

Microwave dielectric heating of non-aqueous droplets in a microfluidic device for nanoparticle synthesis.

Nanoscale 2013 Jun;5(12):5468-75

School of Engineering and Applied Sciences, Department of Physics, Harvard University, Cambridge, MA 02138, USA.

We describe a microfluidic device with an integrated microwave heater specifically designed to dielectrically heat non-aqueous droplets using time-varying electrical fields with the frequency range between 700 and 900 MHz. The precise control of frequency, power, temperature and duration of the applied field opens up new vistas for experiments not attainable by conventional microwave heating. We use a non-contact temperature measurement system based on fluorescence to directly determine the temperature inside a single droplet. The maximum temperature achieved of the droplets is 50 °C in 15 ms which represents an increase of about 25 °C above the base temperature of the continuous phase. In addition we use an infrared camera to monitor the thermal characteristics of the device allowing us to ensure that heating is exclusively due to the dielectric heating and not due to other effects like non-dielectric losses due to electrode or contact imperfection. This is crucial for illustrating the potential of dielectric heating of benzyl alcohol droplets for the synthesis of metal oxides. We demonstrate the utility of this technology for metal oxide nanoparticle synthesis, achieving crystallization of tungsten oxide nanoparticles and remarkable microstructure, with a reaction time of 64 ms, a substantial improvement over conventional heating methods.
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http://dx.doi.org/10.1039/c3nr00500cDOI Listing
June 2013

Observation of spatial propagation of amyloid assembly from single nuclei.

Proc Natl Acad Sci U S A 2011 Sep 26;108(36):14746-51. Epub 2011 Aug 26.

Department of Physics and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.

The crucial early stages of amyloid growth, in which normally soluble proteins are converted into fibrillar nanostructures, are challenging to study using conventional techniques yet are critical to the protein aggregation phenomena implicated in many common pathologies. As with all nucleation and growth phenomena, it is difficult to track individual nuclei in traditional macroscopic experiments, which probe the overall temporal evolution of the sample, but do not yield detailed information on the primary nucleation step as they mix independent stochastic events into an ensemble measurement. To overcome this limitation, we have developed microdroplet assays enabling us to detect single primary nucleation events and to monitor their subsequent spatial as well as temporal evolution, both of which we find to be determined by secondary nucleation phenomena. By deforming the droplets to high aspect ratio, we visualize in real-time propagating waves of protein assembly emanating from discrete primary nucleation sites. We show that, in contrast to classical gelation phenomena, the primary nucleation step is characterized by a striking dependence on system size, and the filamentous protein self-assembly process involves a highly nonuniform spatial distribution of aggregates. These findings deviate markedly from the current picture of amyloid growth and uncover a general driving force, originating from confinement, which, together with biological quality control mechanisms, helps proteins remain soluble and therefore functional in nature.
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http://dx.doi.org/10.1073/pnas.1105555108DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3169119PMC
September 2011

Dielectrophoretic trapping of DNA-coated gold nanoparticles on silicon based vertical nanogap devices.

Phys Chem Chem Phys 2011 Jun 8;13(21):9973-7. Epub 2011 Mar 8.

Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching, Germany.

We report on the successful dielectrophoretic trapping and electrical characterization of DNA-coated gold nanoparticles on vertical nanogap devices (VNDs). The nanogap devices with an electrode distance of 13 nm were fabricated from Silicon-on-Insulator (SOI) material using a combination of anisotropic reactive ion etching (RIE), selective wet chemical etching and metal thin-film deposition. Au nanoparticles (diameter 40 nm) coated with a monolayer of dithiolated 8 base pairs double stranded DNA were dielectrophoretically trapped into the nanogap from electrolyte buffer solution at MHz frequencies as verified by scanning and transmission electron microscopy (SEM/TEM) analysis. First electrical transport measurements through the formed DNA-Au-DNA junctions partially revealed an approximately linear current-voltage characteristic with resistance in the range of 2-4 GΩ when measured in solution. Our findings point to the importance of strong covalent bonding to the electrodes in order to observe DNA conductance, both in solution and in the dry state. We propose our setup for novel applications in biosensing, addressing the direct interaction of biomolecular species with DNA in aqueous electrolyte media.
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http://dx.doi.org/10.1039/c0cp02718aDOI Listing
June 2011

The effect of PEG-coated gold nanoparticles on the anti-proliferative potential of Specific Nutrient Synergy.

Nanotoxicology 2010 Jun;4(2):177-85

Dr. Rath Research Institute, Santa Clara, CA, USA.

The role of PEG-coated gold nanoparticles (Au NPs) on the anti-proliferative effect of Specific Nutrient Synergy (SNS) on HTLV-1 infected (C91-PL and HuT-102) and non-infected (CEM and Jurkat) malignant T-lymphocytes cells, was investigated. When PEG-coated Au NPs (of different molecular weights) were added alone, there was no effect on either viability or proliferation of the leukemic cell lines studied. Treatment of cells with SNS and PEG (5 or 10 kDa) coated Au NP reduced significantly the proliferation in all cell lines tested; this reached more than 50% reduction as compared to the control for cells treated for 96 h. Data showed that the best anti-proliferative effect was obtained using SNS and Au NP coated with PEG of molecular weights of 5 and 10 kDa with almost no effect of PEG of lower molecular weights (0.75 and 2 kDa) or higher ones (20 kDa). This was true as well for HTLV-1 infected as for non-infected malignant T-lymphocytes. Electron microscopy results showed uptake of the gold particles to Jurkat cells. All described effects are specific to leukemia cell lines, and no effects were observed with freshly activated human mononuclear lymphocytes as control.
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http://dx.doi.org/10.3109/17435390903569621DOI Listing
June 2010

Biodistribution of PEG-modified gold nanoparticles following intratracheal instillation and intravenous injection.

Biomaterials 2010 Sep 9;31(25):6574-81. Epub 2010 Jun 9.

Comprehensive Pneumology Center, Institute of Lung Biology and Disease and Focus Network Nanoparticles and Health, Ingolstaedter Landstrasse 1, 85764 Neuherberg, Germany.

Besides toxicity tests, biokinetic studies are a fundamental part of investigations to evaluate a safe and sustainable use of nanoparticles. Today, gold nanoparticles (Au NPs) are known to be a versatile tool in different areas such as science, engineering or medicine. In this study, we investigated the biokinetics after intravenous and intratracheal applications of poly(ethylene glycol) (PEG) modified Au NPs compared to plain Au NPs. Radioactive-labeled Au NPs of 5 nm inorganic core diameter were applied to rats and the NP content in tissues, organs and excretion were quantified after 1-hour and 24-hours. After intravenous injection, a prolonged blood circulation time was determined for Au NPs with 10 kDa PEG chains. Non-PEGylated Au NPs and 750 Da PEG Au NPs accumulated mostly in liver and spleen. After intratracheal application the majority of all three types of applied NPs stayed in the lungs: the total translocation towards the circulation did not differ considerably after PEGylation of the Au NPs. However, a prolonged retention time in the circulation was detected for the small fraction of translocated 10 kDa PEG Au NPs, too.
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http://dx.doi.org/10.1016/j.biomaterials.2010.05.009DOI Listing
September 2010

Inorganic engineered nanoparticles and their impact on the immune response.

Curr Drug Metab 2009 Oct;10(8):895-904

Institut Català de Nanotecnologia, Barcelona, Spain.

The immune system is the responsible for body integrity and the prevention of external invasion. In principle, the immune system has not been evolutionarily trained to respond against inorganic engineered nanoparticles (NPs). However, how it will react against them will determine developments on the use of NPs as medical devices and their toxicological impact on human and environmental health. Initial observations show a broad range of results as a function of size, shape, concentration and surface state of NPs, and a variety of immune responses from absent to acute inflammation. In particular for the case of NP, the composition of the material, which strongly influences its physical properties, appears not to be the main determining factor for their behavior in biological environments as compared to surface state or size.
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http://dx.doi.org/10.2174/138920009790274577DOI Listing
October 2009

Tracking of cellular uptake of hydrophilic CdSe/ZnS quantum dots/hydroxyapatite composites nanoparticles in MC3T3-E1 osteoblast cells.

J Nanosci Nanotechnol 2009 Apr;9(4):2758-62

Department of Biomedical Engineering, R&D Center for Membrane Technology and Center for Nano Bioengineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan, Republic of China.

We report the fluorescent labeling of osteoblast cells using the biocompatible hydroxyapatite (HA) grown with nucleating seed of hydrophilic CdSe/ZnS quantum dots (QDs) allowing the real-time observation of cell under confocal microscope. We found that the MC3T3-E1 osteoblast cells can engulf HA with surface-tailored QDs showing fluorescent spots in the cytoplasm, while HA and QDs nanoparticles were not engulfed. It is interesting to see that the fluorescence was only displayed in the cytoplasm of MC3T3-E1 osteoblast cells. It can be envisioned that the nano-sized hydroxyapatite bearing fluorescent QD can only be internalized in the cytoplasm. Therefore, it is worth utilizing these composite particles to observe cellular physiology with minimal toxicity to the osteoblast cells.
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http://dx.doi.org/10.1166/jnn.2009.463DOI Listing
April 2009

Chloroform- and water-soluble sol-gel derived Eu+++/Y2O3 (red) and Tb+++/Y2O3 (green) nanophosphors: synthesis, characterization, and surface modification.

IEEE Trans Nanobioscience 2009 Mar 16;8(1):43-50. Epub 2009 Mar 16.

Department of Chemistry, Motilal Nehru National Institute of Technology (MNNIT), Allahabad 211004, India.

Eu+++ and Tb+++ ions have been incorporated into nanodimensional yttrium oxide host matrices via a sol-gel process using Y5O(OPr(i))13 as precursor (OPr(i) = isopropoxy). The as-synthesized white powders have been annealed at different temperatures. Photoluminescence (PL) spectroscopy and X-ray diffraction (XRD) have been used as tools for documenting the characteristics of these powders. For Eu+++-doped powders, a comparison of the Eu+++, 5D0-->7F1, and 5D0-->7F2 peak intensities in the emission spectra reveals that the dopant ions are occupying unsymmetrical sites in the host yttrium oxide in all the samples. For Tb+++-doped powders, the characteristic terbium 5D3-->7Fn and 5D-->7Fn (n = 2-6) transitions were visible only in the samples that had been annealed above 500 degrees C. Samples of the doped particle powders were suspended in chloroform by fragmenting the powder with and without sonification under the presence of trioctylphosphine oxide, or a mixture of oleic acid and dioctyl ether. The resulting clear colorless (for Eu+++) and light green translucent (for Tb+++) solutions of the suspended particles showed red and green luminescence upon UV excitation, respectively. In addition, suspension in water has been achieved by fragmenting the powder in the presence of dichloroacetic acid. Transmission electron micrograph investigation of the soluble particles shows single dispersed particles along with agglomerates. The changes in the luminescence due to fragmentation of the particle powder and due the influence of the surfactant of the suspended colloidal particles are discussed.
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http://dx.doi.org/10.1109/TNB.2009.2017316DOI Listing
March 2009

Synthesis, characterization, and bioconjugation of fluorescent gold nanoclusters toward biological labeling applications.

ACS Nano 2009 Feb;3(2):395-401

Department of Biomedical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan, ROC.

Synthesis of ultrasmall water-soluble fluorescent gold nanoclusters is reported. The clusters have a decent quantum yield, high colloidal stability, and can be readily conjugated with biological molecules. Specific staining of cells and nonspecific uptake by living cells is demonstrated.
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http://dx.doi.org/10.1021/nn800632jDOI Listing
February 2009

Biological applications of gold nanoparticles.

Chem Soc Rev 2008 Sep 17;37(9):1896-908. Epub 2008 Jul 17.

Fachbereich Physik, Philipps Universität Marburg, Renthof 7, 35037 Marburg, Germany.

This critical review gives a short overview of the widespread use of gold nanoparticles in biology. We have identified four classes of applications in which gold nanoparticles have been used so far: labelling, delivering, heating, and sensing. For each of these applications the underlying mechanisms and concepts, the specific features of the gold nanoparticles needed for this application, as well as several examples are described (142 references).
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http://dx.doi.org/10.1039/b712170aDOI Listing
September 2008

Design of an amphiphilic polymer for nanoparticle coating and functionalization.

Small 2008 Mar;4(3):334-41

Center for NanoScience, Ludwig-Maximilians-Universität München, Munich, Germany.

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http://dx.doi.org/10.1002/smll.200700654DOI Listing
March 2008

Gold nanostoves for microsecond DNA melting analysis.

Nano Lett 2008 Feb 26;8(2):619-23. Epub 2008 Jan 26.

Photonics and Optoelectronics Group, Physics Department, CeNS, Ludwig-Maximilians-Universität München, Amalienstrasse 54, 80799 Munich, Germany.

In traditional DNA melting assays, the temperature of the DNA-containing solution is slowly ramped up. In contrast, we use 300 ns laser pulses to rapidly heat DNA bound gold nanoparticle aggregates. We show that double-stranded DNA melts on a microsecond time scale that leads to a disintegration of the gold nanoparticle aggregates on a millisecond time scale. A perfectly matching and a point-mutated DNA sequence can be clearly distinguished in less than one millisecond even in a 1:1 mixture of both targets.
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http://dx.doi.org/10.1021/nl073028iDOI Listing
February 2008

Synthesis and characterization of polymer-coated quantum dots with integrated acceptor dyes as FRET-based nanoprobes.

Nano Lett 2007 Sep 11;7(9):2613-7. Epub 2007 Aug 11.

Center for Nanoscience, Ludwig-Maximilans Universität Munich, Amalienstrasse 54, Munich, D-80799 München, Germany.

A fluorescence resonance energy transfer pair consisting of a colloidal quantum dot donor and multiple organic fluorophores as acceptors is reported and the photophysics of the system is characterized. Most nanoparticle-based biosensors reported so far use the detection of specific changes of the donor/acceptor distance under the influence of analyte binding. Our nanoparticle design on the other hand leads to sensors that detect spectral changes of the acceptor (under the influence of analyte binding) at fixed donor/acceptor distance by the introduction of the acceptor into the polymer coating. This approach allows for short acceptor-donor separation and thus for high-energy transfer efficiencies. Advantageously, the binding properties of the hydrophilic polymer coating further allows for addition of poly(ethylene glycol) shells for improved colloidal stability.
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http://dx.doi.org/10.1021/nl070971dDOI Listing
September 2007

Size and surface effects on the MRI relaxivity of manganese ferrite nanoparticle contrast agents.

Nano Lett 2007 Aug 20;7(8):2422-7. Epub 2007 Jul 20.

Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany.

Superparamagnetic MnFe2O4 nanocrystals of different sizes were synthesized in high-boiling ether solvent and transferred into water using three different approaches. First, we applied a ligand exchange in order to form a water soluble polymer shell. Second, the particles were embedded into an amphiphilic polymer shell. Third, the nanoparticles were embedded into large micelles formed by lipids. Although all approaches lead to effective negative contrast enhancement, we observed significant differences concerning the magnitude of this effect. The transverse relaxivity, in particular r2*, is greatly higher for the micellar system compared to the polymer-coated particles using same-sized nanoparticles. We also observed an increase in transverse relaxivities with increasing particle size for the polymer-coated nanocrystals. The results are qualitatively compared with theoretical models describing the dependence of relaxivity on the size of magnetic spheres.
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http://dx.doi.org/10.1021/nl071099bDOI Listing
August 2007
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