Rapid aggregation of gold nanoparticles induced by non-cross-linking DNA hybridization

Ordered hexagonal array of Au nanodots on Si substrate based on colloidal crystal templating

We report two types of site-selective metal deposition methods based on colloidal crystal templating. We discuss in particular the controllability of the morphology and crystallinity of Au nanodots depending of the choice of method.

Photothermal optical coherence tomography of epidermal growth factor receptor in live cells using immunotargeted gold nanospheres

Molecular imaging is a powerful tool for investigating disease processes and potential therapies in both in vivo and in vitro systems. However, high resolution molecular imaging has been limited to relatively shallow penetration depths that can be accessed with microscopy. Optical coherence tomography (OCT) is an optical analogue to ultrasound with relatively good penetration depth (1-2 mm) and resolution (approximately 1-10 microm). We have developed and characterized photothermal OCT as a molecular contrast mechanism that allows for high resolution molecular imaging at deeper penetration depths than microscopy. Our photothermal system consists of an amplitude-modulated heating beam that spatially overlaps with the focused spot of the sample arm of a spectral-domain OCT microscope. Validation experiments in tissuelike phantoms containing gold nanospheres that absorb at 532 nm revealed a sensitivity of 14 ppm nanospheres (weight/weight) in a tissuelike environment. The nanospheres were then conjugated to anti-EGFR, and molecular targeting was confirmed in cells that overexpress EGFR (MDA-MB-468) and cells that express low levels of EGFR (MDA-MB-435). Molecular imaging in three-dimensional tissue constructs was confirmed with a significantly lower photothermal signal (p<0.0001) from the constructs composed of cells that express low levels of EGFR compared to the overexpressing cell constructs (300% signal increase). This technique could potentially augment confocal and multiphoton microscopy as a method for deep-tissue, depth-resolved molecular imaging with relatively high resolution and target sensitivity, without photobleaching or cytotoxicity.

Ultrasensitive flow injection chemiluminescence detection of DNA hybridization using signal DNA probe modified with Au and CuS nanoparticles

A novel and sensitive flow injection chemiluminescence assay for sequence-specific DNA detection based on signal amplification with nanoparticles (NPs) is reported in the present work. The "sandwich-type" DNA biosensor was fabricated with the thiol-functionalized capture DNA first immobilized on an Au electrode and hybridized with one end of target DNA, the other end of which was recognized with a signal DNA probe labeled with CuS NPs and Au NPs on the 3'- and 5'-terminus, respectively. The hybridization events were monitored by the CL intensity of luminol-H2O2-Cu(2+) after the cupric ions were dissolved from the hybrids. We demonstrated that the incorporation of Au NPs in this sensor design significantly enhanced the sensitivity and the selectivity because a single Au NP can be loaded with hundreds of signal DNA probe strands, which were modified with CuS NPs. The ratios of Au NPs, signal DNA probes, and CuS NPs modified on the gold electrode were approximately 1/101/103. A preconcentration process of cupric ions performed by anodic stripping voltammetry technology further increased the sensor performance. As a result of these two combined effects, this DNA sensor could detect as low as femtomolar target DNA and exhibited excellent selectivity against two-base mismatched DNA. Under the optimum conditions, the CL intensity was increased with the increase of the concentration of target DNA in the range of 2.0 x 10(-14)-2.0 x 10(-12) M. A detection limit of 4.8 x 10(-15) M target DNA was achieved.

Colorimetric response to mercury-induced abstraction of triethylene glycol ligands from a gold nanoparticle surface

A simple method for specific colorimetric sensing of aqueous mercury(II) has been developed by using a gold nanoparticle supported by triethylene glycol ligand (Au:S-EG(3)). The unusual kinetic behavior of the sensing process of Hg(2+) and the TEM images of the Au core, as well as the electrospray ionization mass spectroscopic analysis of the analyte solution, have led us to propose a new color changing mechanism that involves ligand abstraction from a gold surface by Hg(2+).

The use of gold nanoparticles in diagnostics and detection

The widespread use of gold nanoparticles (GNPs) as labels in diagnostics and detection is due to a unique combination of chemical and physical properties that allow biological molecules to be detected at low concentrations. In this critical review detection methods based on GNPs are divided up and discussed based on the way in which signals are generated in response to specific target molecules. Particular attention is devoted to methods that allow target molecules to be detected with the unaided eye because these, more than any other, harness the full range of properties that make GNPs unique. Methods that are discussed include those in which specific target molecules induce a visible colour change, chromatographic methods that allow non-specialized users to perform sophisticated tests without additional equipment and methods in which trace amounts of GNPs are rendered visible to the unaided eye by catalytic deposition of a metal such as silver. The use of metal deposition as a means of enhancing the signal for optical and electrical detection is also reviewed. The other detection methods included in this review are based on interactions between GNPs and molecules located in close proximity to their surface. These include methods in which light emission from such molecules is enhanced (surface enhanced Raman scattering) or quenched (fluorescence), and methods in which the accumulation of specific target molecules induce subtle changes in the extinction spectra of GNPs that can be followed in real time with inexpensive equipment (166 references).

Enzymatic cleavage of nucleic acids on gold nanoparticles: a generic platform for facile colorimetric biosensors

The enzymatic cleavage of nucleic acids (DNA or DNA with a single RNA linkage) on well-dispersed gold nanoparticles (AuNPs) is exploited in the design of facile colorimetric biosensors. The assays are performed at salt concentrations such that DNA-modified AuNPs are barely stabilized by the electrostatic and steric stabilization. Enzymatic cleavage of DNA chains on the AuNP surface destabilizes the AuNPs, resulting in a rapid aggregation driven by van der Waals attraction, and a red-to-purple color change. Two different systems are chosen, DNase I (a DNA endonuclease) and 8-17 (a Pb(2+)-depedent RNA-cleaving DNAzyme), to demonstrate the utility of our assay for the detection of metal ions and sensing enzyme activities. Compared with previous studies in which AuNP aggregates are converted into dispersed AuNPs by enzymatic cleavage of DNA crosslinkers, the present assay is technically simpler. Moreover, the accessibility of DNA to biomolecular recognition elements (e.g. enzymes) on well-dispersed AuNPs in our assay appears to be higher than that embedded inside aggregates. This biosensing system should be readily adaptable to other enzymes or substrates for detection of analytes such as small molecules, proteases and their inhibitors.

Ultrasensitive optical biodiagnostic methods using metallic nanoparticles

Dramatic progress has been made over the recent decade in the applications of metallic nanoparticles in the field of biomolecule detection. The useful physical and chemical properties (e.g., availability of various synthetic methods of size- and shape-controlled nanoparticles, size- and shape-dependent optical properties, availability of various surface chemistries and biocompatibility) of metallic nanoparticles have brought development to the ultrasensitive detection of biomolecules at the attomolar level and this sensitivity enables the diagnosis of otherwise undetectable biomarkers of many fatal diseases, including Alzheimer’s disease. Furthermore, coupled with the strong physical properties and biocompatible nature of gold nanoparticles in in vivo conditions, the scope of applications for these particles have been broadened into the field of in vivo imaging, such as X-ray contrasting agents, and also cellular tracking. Here, we review synthetic methods and optical properties of metallic nanoparticles and their use in ultrasensitive, in vitro and in vivo biodiagnostic methods.

Detection of single-nucleotide polymorphisms using gold nanoparticles and single-strand-specific nucleases

The current study reports an assay approach that can detect single-nucleotide polymorphisms (SNPs) and identify the position of the point mutation through a single-strand-specific nuclease reaction and a gold nanoparticle assembly. The assay can be implemented via three steps: a single-strand-specific nuclease reaction that allows the enzyme to truncate the mutant DNA; a purification step that uses capture probe-gold nanoparticles and centrifugation; and a hybridization reaction that induces detector probe-gold nanoparticles, capture probe-gold nanoparticles, and the target DNA to form large DNA-linked three-dimensional aggregates of gold nanoparticles. At high temperature (63 degrees C in the current case), the purple color of the perfect match solution would not change to red, whereas a mismatched solution becomes red as the assembled gold nanoparticles separate. Using melting analysis, the position of the point mutation could be identified. This assay provides a convenient colorimetric detection that enables point mutation identification without the need for expensive mass spectrometry. To our knowledge, this is the first report concerning SNP detection based on a single-strand-specific nuclease reaction and a gold nanoparticle assembly.

Molecular imaging of epidermal growth factor receptor in live cells with refractive index sensitivity using dark-field microspectroscopy and immunotargeted nanoparticles

We present a study using plasmonic nanoparticles (NPs) to image epidermal growth factor receptor (EGFR) in live cells. Through detailed analysis of the NP scattering spectra, we determine the intracellular refractive index (RI) within attoliter volumes inside of the living cells. Molecular imaging is demonstrated using anti-EGFR labeled gold nanospheres delivered to cancer cells that overexpress EGFR, with targeted binding confirmed by appropriate control experiments. RI determination is achieved by measurement of the bound NPs' scattering spectra, acquired using a precision dark-field microspectroscopy system and through careful characterization of the NP properties throughout the immuno-labeling procedure. To demonstrate the effect of receptor-mediated uptake, the data are compared to similar spectral measurements using antibody-free NPs, taken up by the cells through nonspecific mechanisms. In these experiments, NP aggregation introduces interparticle effects in the scattering spectra, suggesting that EGFR-mediated internalization of NPs may provide an advantage for maintaining NP isolation upon uptake. The results of this study show the potential utility of dark-field microspectroscopy and labeled NPs for live cell imaging. By demonstrating RI sensitivity over nanometer length scales, this study also presents a potential new avenue for assessing the structure and dynamics of live cells.

Optical sensing of biomedically important polyionic drugs using nano-sized gold particles

A simple optical method for the sensing of biomedically important polyionic drugs, protamine and heparin based on the reversible aggregation and de-aggregation of gold nanoparticles (AuNPs) is described. The polycationic protamine induces the aggregation of negatively charged citrate-stabilized AuNPs, resulting in a shift in the surface plasmon (SP) band and a consequent color change of the AuNPs from red to blue. Addition of polyanionic heparin dissipates the aggregated AuNPs due to its strong affinity to protamine and the blue color changes to the native color. The color change was monitored using UV-vis spectrophotometry. The aggregation and de-aggregation was confirmed by transmission electron microscopic (TEM) measurements. The degree of aggregation and de-aggregation is proportional to the concentration of added protamine and heparin, allowing their quantitative detection. The change in the absorbance and SP band position has been used to monitor the concentration of protamine and heparin. This optical method can quantify protamine and heparin as low as 0.1 microg/ml and 0.6 microg/ml, respectively and the calibration is linear for a wide range of concentration.

Peptide nanowires for coordination and signal transduction of peroxidase biosensors to carbon nanotube electrode arrays

A strategy of metallizing peptides to serve as conduits of electronic signals that bridge between a redox enzyme and a carbon-nanotube electrode has been developed with enhanced results. In conjunction, a protocol to link the biological elements to the tips of carbon nanotubes has been developed to optimize contact and geometry between the redox enzyme and the carbon nanotube electrode array. A peptide nanowire of 33 amino acids, comprised of a leucine zipper motif, was mutated to bind divalent metals, conferring conductivity into the peptide. Reaction between a thiolate of the peptide with the sulfenic acid of the NADH peroxidase enzyme formed a peptide-enzyme assembly that are fully primed to transduce electrons out of the enzyme active site to an electrode. Scanning electron microscopy shows immobilization and linking of the assembly specifically to the tips of carbon nanotube electrodes, as designed. Isothermal titration calorimetry and mass spectrometry indicate a binding stoichiometry of at least three metals bound per peptide strand. Overall, these results highlight the gain that can be achieved when the signal tranducing units of a biosensor are aligned through directed peptide chemistry.

Turbidimetric detection of ATP using polymeric micelles and DNA aptamers

Two types of turbidimetric detection of adenosine 5'-triphosphate (ATP) by the naked eye were achieved through a combination of non-cross-linking aggregation of DNA-linked polymeric micelles and molecular recognition of ATP by a DNA aptamer.

In Situ Observation of Spherical DNA Assembly in Water and the Controlled Release of Bound Dyes

Three strands of 30-mer oligodeoxyribonucleotides (ODNs) were designed to form three-way junctions that possess self-complementary sticky ends. The morphology of self-assembled ODNs in water was observed in situ by confocal laser scanning fluorescence microscopy. The three-way junctions self-assembled into spherical assemblies, in accordance with transmission and scanning electron microscopy. The size of nucleospheres was in the range of several tens of nanometers to micrometers, which varied depending on the concentration of ODNs and added salts. Fluorescence images of spherical ODN assemblies suggested that the nucleospheres possess multiwalled structures. The fluorescence of sodium 1-anilinonaphthalene-8-sulfonate in the presence of nucleospheres revealed that the interior of nucleospheres possesses polarity corresponding to that between methanol and ethanol. A dye-inclusion experiment showed that cationic and even anionic dyes were adsorbed to the interior of the nucleospheres. The dye-included nucleospheres released dyes by thermal dissociation or digestion of the constituent ODNs.

Microarray detection of duplex and triplex DNA binders with DNA-modified gold nanoparticles

We have designed a chip-based assay, using microarray technology, for determining the relative binding affinities of duplex and triplex DNA binders. This assay combines the high discrimination capabilities afforded by DNA-modified Au nanoparticles with the high-throughput capabilities of DNA microarrays. The detection and screening of duplex DNA binders are important because these molecules, in many cases, are potential anticancer agents as well as toxins. Triplex DNA binders are also promising drug candidates. These molecules, in conjunction with triplex-forming oligonucleotides, could potentially be used to achieve control of gene expression by interfering with transcription factors that bind to DNA. Therefore, the ability to screen for these molecules in a high-throughput fashion could dramatically improve the drug screening process. The assay reported here provides excellent discrimination between strong, intermediate, and weak duplex and triplex DNA binders in a high-throughput fashion.

Complexes of DNA bases and Watson-Crick base pairs with small neutral gold clusters

The nature of the DNA-gold interaction determines and differentiates the affinity of the nucleobases (adenine, thymine, guanine, and cytosine) to gold. Our preliminary computational study [Kryachko, E. S.; Remacle, F. Nano Lett. 2005, 5, 735] demonstrates that two major bonding factors govern this interaction: the anchoring, either of the Au-N or Au-O type, and the nonconventional N-H...Au hydrogen bonding. In this paper, we offer insight into the nature of nucleobase-gold interactions and provide a detailed characterization of their different facets, i.e., geometrical, energetic, and spectroscopic aspects; the gold cluster size and gold coordination effects; proton affinity; and deprotonation energy. We then investigate how the Watson-Crick DNA pairing patterns are modulated by the nucleobase-gold interaction. We do so in terms of the proton affinities and deprotonation energies of those proton acceptors and proton donors which are involved in the interbase hydrogen bondings. A variety of properties of the most stable Watson-Crick [A x T]-Au3 and [G x C]-Au3 hybridized complexes are described and compared with the isolated Watson-Crick A x T and G x C ones. It is shown that enlarging the gold cluster size to Au6 results in a rather short gold-gold bond in the Watson-Crick interbase region of the [G x C]-Au6 complex that bridges the G x C pair and thus leads to a significant strengthening of G x C pairing.

Simple and rapid colorimetric enzyme sensing assays using non-crosslinking gold nanoparticle aggregation

Non-crosslinking gold nanoparticle (AuNP) aggregation induced by the loss (or screen) of surface charges is applied for enzymatic activity sensing and potentially inhibitor screening.

Unusual fluorescence enhancement of a novel carbazolyldiacetylene bound to gold nanoparticles

A novel carbazolyldiacetylene derivative of 5-[1,2]dithiolan-3-ylpentanoic acid 1,6-bis((N-carbazol-3-yl)methyl)-2,4-hexadiyne ester (DCHD-HS) was synthesized. DCHD-HS showed unusual fluorescence enhancement when it was bound to gold nanoparticles. The studies also showed that varied solvents and ratios of ligand/gold particles that served as the driving forces to control aggregation resulted in emission enhancement. A rational explanation for this phenomenon was discussed and elucidated.

Gold nanoparticle aggregation-based highly sensitive DNA detection using atomic force microscopy

The potential ability of atomic force microscopy (AFM) as a quantitative bioanalysis tool is demonstrated by using gold nanoparticles as a size enhancer in a DNA hybridization reaction. Two sets of probe DNA were functionalized on gold nanoparticles and sandwich hybridization occurred between two probe DNAs and target DNA, resulting in aggregation of the nanoparticles. At high concentrations of target DNA in the range from 100 nM to 10 microM, the aggregation of gold nanoparticles was determined by monitoring the color change with UV-vis spectroscopy. The absorption spectra broadened after the exposure of DNA-gold nanoparticles to target DNA and a new absorption band at wavelengths >600 nm was observed. However, no differences were observed in the absorption spectra of the gold nanoparticles at low concentrations of target DNA (10 pM to 10 nM) due to insufficient aggregation. AFM was used as a biosensing tool over this range of target DNA concentrations in order to monitor the aggregation of gold nanoparticles and to quantify the concentration of target DNA. Based on the AFM images, we successfully evaluated particle number and size at low concentrations of target DNA. The calibration curve obtained when mean particle aggregate diameter was plotted against concentration of target DNA showed good linearity over the range 10 pM to 10 nM, the working range for quantitative target DNA analysis. This AFM-based DNA detection technique was three orders of magnitude more sensitive than a DNA detection method based on UV-vis spectroscopy.

Ultra-sensitive colorimetric method to quantitate hundreds of polynucleotide molecules by gold nanoparticles with silver enhancement

An ultra-sensitive colorimetric method to quantitate hundreds of polynucleotide molecules by gold nanoparticles with silver enhancement has been developed. The hybridization products from the target polynucleotides with biotin-labeled probes and gold nanoparticle-functioned oligonucleotides were immobilized to microplates via avidin-biotin system, and the absorbance signals of gold nanoparticles were amplified by silver enhance solution. This sandwich colorimetric assay can detect as few as 600 molecules for single-strand oligonucleotides and as few as 6000 molecules for double-strand polynucleotides in a 50 microL reaction system.

Enhanced oligonucleotide-nanoparticle conjugate stability using thioctic acid modified oligonucleotides

Metallic nanoparticles of gold functionalized with oligonucleotides conventionally use a terminal thiol modification and have been used in a wide range of applications. Although readily available, the oligonucleotide–nanoparticle conjugates prepared in this way suffer from a lack of stability when exposed to a variety of small molecules or elevated temperatures. If silver is used in place of gold then this lack of stability is even more pronounced. In this study we report the synthesis of highly stabilized oligonucleotide–nanoparticle conjugates using a simple oligonucleotide modification. A modified solid support was used to generate 3′-thioctic acid modified oligonucleotides by treatment with an N-hydroxysuccimidyl ester of thioctic acid. Unusually, both gold and silver nanoparticles have been investigated in this study and show that these disulphide-modified oligonucleotide probes offer significant improvements in nanoparticle stability when treated with dithiothreitol (DTT) compared with monothiol analogues. This is a significant advance in oligonucleotide–nanoparticle conjugate stability and for the first time allows silver nanoparticles to be prepared that are more stable than standard gold-thiol functionalized nanoparticles. This opens up the possibility of using silver nanoparticles functionalized with oligonucleotides as an alternative to gold.

Colorimetric biosensors based on DNA-nanoparticle conjugates

In this review, we present an overview of the technologies in colorimetric biosensors based on DNA-nanoparticle conjugates. Two types of DNA-nanoparticles aggregation assays are summarized. One of the methods relies on cross-linking of the gold nanoparticle (GNP) by hybridization. The crosslinking system was used not only to detect target DNA sequences, but also to detect metal ions or small molecules which were recognized by DNAzymes. The other method is the GNP non-crosslinking system. This approach shows high performance in the detection of single nucleotide polymorphisms. These methods do not need special equipment and open up a new possibility of point-of-care diagnoses.

Quantum dot-mediated biosensing assays for specific nucleic acid detection

Two new classes of quantum dot (QD)-mediated biosensing methods have been developed to detect specific DNA sequences in a separation-free format. Both methods use 2 target-specific oligonucleotide probes to recognize a specific target. The first method is based on cross-linking of 2 QDs with distinct emission wavelengths caused by probe-target hybridization. The second method uses QDs as both fluorescent tags and nanoscaffolds that capture multiple fluorescently labeled hybridization products, resulting in amplified target signals. The presence of targets is determined according to spatiotemporal coincidence of 2 different wavelength fluorescent signals emitted from the QD/DNA/probe complexes. With a single wavelength-excitation, dual wavelength-emission confocal spectroscopic system, the fluorescent signals can be measured with single-dot/molecule sensitivity. Compared with other nanoparticle-based, separation-free assays, our method shows advantages in simplicity, testing speed, and multiplexed applications.

Homogeneous, unmodified gold nanoparticle-based colorimetric assay of hydrogen peroxide

An unmodified gold nanoparticle-based colorimetric assay system in homogeneous format has been developed using hydrogen peroxide (H(2)O(2)) as a model analyte. H(2)O(2) is added to o-phenylenediamine/horseradish peroxidase solution, and allowed to react for 10 min. Then, unmodified gold nanoparticles that serve as "reaction indicators" are added to the reaction solution. The resulting mixture color changes dramatically from red to blue. The reason is that azoaniline, a horseradish peroxidase-catalyzed oxidation product, induces the nanoparticle aggregation. Using this approach, H(2)O(2) can be semiquantitatively determined over the concentration range of approximately 4 orders of magnitude by the naked eye. If the observed peak intensity at 420 nm is used for the construction of the calibration plot, hydrogen peroxide can be accurately determined down to concentration levels of 1.3 x 10(-6) M. Compared with the conventional electrochemical protocol, this sensing system offers several important advantages: (1) ability to be monitored by the naked eye, (2) avoiding the need of surface modification of electrodes or gold nanoparticles and (3) detection in homogeneous solution. It is worthy of note that this efficient and convenient strategy is also suitable for the detection of other species, such as glucose and cholesterol.

Nanodiagnostics: fast colorimetric method for single nucleotide polymorphism/mutation detection

Advances in nanosciences are having a significant impact in many areas of research. The impact of new nanotechnologies has been particularly large in biodiagnostics, where a number of nanoparticle-based assays have been introduced for biomolecules detection. To date, applications of nanoparticles have largely focused on DNA-functionalised gold nanoparticles used as the target-specific probes. These gold nanoparticle-based systems can be used for the detection of specific sequences of DNA (pathogen detection, characterisation of mutation and/or single nucleotide polymorphisms) or RNA (without prior retro-transcription and amplification). Here a rapid and inexpensive nanoparticle-based method for single-base mismatch detection (single nucleotide polymorphism/mutation) in DNA samples is reported. Gold nanoparticles derivatised with thiol modified oligonucleotides complementary to DNA targets -- Au-nanoprobes -- are used to distinguish fully complementary from mismatched sequences, with a single-base mismatch. The authors have successfully applied this strategy to detect common mutations within the beta-globin gene.

Surface plasmon resonance imaging on a microchip for detection of DNA-modified gold nanoparticles deposited onto the surface in a non-cross-linking configuration

Recently we reported that gold nanoparticles (GNPs) with fully matched duplexes on their surfaces are selectively deposited onto walls of poly(dimethylsiloxane) (PDMS) microchannels at high salt concentrations. In this study, the surface plasmon resonance (SPR) imaging technique was applied to monitor this phenomenon for improvement of detection sensitivity and elucidation of the phenomenon. The microchip was fabricated by bonding a surface-patterned PDMS plate and a gold thin film-deposited glass substrate. Probe oligonucleotide-modified GNPs were hybridized with target oligonucleotides to make fully matched or single-base-mismatched duplexes. The hybridized GNP solution was mixed with an NaCl solution in a Y-shaped microchannel. The deposition of the GNPs onto the gold sensor surface was detected by SPR imaging. Discrimination of the targets was possible with limit of detection of 32 nM (19 fmol) without temperature control in 5 min. Detailed analysis indicated that a seed layer of GNPs was initially adsorbed onto the sensor surface regardless of the target sequence. Therefore, in combination with a portable SPR device, the proposed method is promising for point-of-care testing of single-nucleotide polymorphsims.

DNA dissociation and degradation at gold nanoparticle surfaces

The instability of DNA-functionalized gold nanoparticles (Au-DNA conjugates) upon exposure to high temperatures is characterized using fluorescence spectroscopy, gel electrophoresis and ion-exchange chromatography. Above 70 degrees C, aqueous Au-DNA conjugates decompose within hours due to both desorption of thiol-terminated DNA from the gold nanoparticle surface and chemical degradation of DNA in the presence of colloidal gold. Although the chemical mechanism for DNA degradation was not identified in this study, the gold surface participates directly in the cleavage reaction. These results have important implications for the use of Au-DNA conjugates in biotechnological and clinical applications that require high temperatures, such as polymerase chain reaction (PCR).

Quantitative detection and fixation of single and multiple gold nanoparticles on a microfluidic chip by thermal lens microscope

A detection and fixation method of single and multiple gold nanoparticles on the wall of a microfluidic channel is demonstrated. A thermal lens microscope (TLM) with continuous-wave excitation (wavelength, 532 nm) and probe (wavelength, 670 nm) laser beams was used to realize the sensitive detection of heat generated by light absorption of individual gold nanoparticles (50 nm in diameter); fixation of the individual nanoparticles was realized simultaneously. The fixation mechanism was investigated and attributed to an absorption-based optical force. In addition to single nanoparticle detection, multiple-nanoparticle detection and fixation was demonstrated. An acceleration of fixation was observed when the number of fixed particles was increased. TLM is expected to be a powerful tool for both the quantitative detection and precise fixation of individual nanoparticles.

A gold nanoparticle based approach for screening triplex DNA binders

Nanoparticle assemblies interconnected with DNA triple helixes can be used to colorimetrically screen for triplex DNA binding molecules and simultaneously determine their relative binding affinities based on melting temperatures. Nanoparticles assemble only when DNA triple helixes form between DNA from two different particles and a third strand of free DNA. In addition, the triple helix structure is unstable at room temperature and only forms in the presence of triplex DNA binding molecules which stabilize the triple helix. The resulting melting transition of the nanoparticle assembly is much sharper and at a significantly higher Tm than the analogous triplex structure without nanoparticles. Upon nanoparticle assembly, a concomitant red-to-blue color change occurs. The assembly process and color change do not occur in the presence of duplex DNA binders and therefore provide a significantly better screening process for triplex DNA binding molecules compared to standard methods.