Biomedical subjects

Kadir Aslan

Publications and source records attributed to Kadir Aslan.

At least 19 recordsLinked to original sources

PubMed2006

Fast and sensitive DNA hybridization assays using microwave-accelerated metal-enhanced fluorescence.

A new, fast, and sensitive DNA hybridization assay platform based on microwave-accelerated metal-enhanced fluorescence (MAMEF) is presented. Thiolated oligonucleotide anchors were immobilized onto silver nanoparticles on a glass substrate. The hybridization of the complementary fluorescein-labeled DNA target with the surface-bound oligonucleotides was completed within 20 s upon heating with low-power microwaves. In addition, the signal is optically amplified, a consequence of close proximity of the fluorophore to the silvered substrate. In this proof-of-principle methodology, as low as 50 nM of a target DNA was detected, although we envisage far-lower detection limits. Control experiments, where the surface-bound oligonucleotide was omitted, were also performed to determine the extent of non-specific binding. In these studies a significantly reduced non-specific adsorption was found when using microwave heating near to silvered structures as compared to room temperature incubation. These findings suggest that MAMEF could be a most useful alternative to the DNA hybridization assays used today, especially with regard to substantially increasing both the assay rapidity and sensitivity.

Aminoacridines↗

PubMed2006

Metal-enhanced chemiluminescence.

In this short paper we report the interactions of silver island films with chemiluminescing species. Our findings show that silver island films can increase the detectability of chemiluminescent reactions/species, with an approximately 5-fold increase in signal intensity. This finding not only suggests the use of silver nanostructures to amplify chemiluminscent signatures in assay platforms, and therefore increase the detectability of analytes or biospecies, but more importantly, suggests that surface plasmons can be directly excited by chemically induced electronically excited molecules. This finding is of significance towards our understanding of fluorophore-metal interactions, a relatively new near-field fluorescence concept, recently named metal-enhanced fluorescence and also radiative decay engineering.

Energy Transfer↗

PubMed2006

Microwave-Accelerated Metal-Enhanced Fluorescence (MAMEF) with silver colloids in 96-well plates: Application to ultra fast and sensitive immunoassays, High Throughput Screening and drug discovery.

Fluorescence detection is the basis of most assays used in drug discovery and High Throughput Screening (HTS) today. In all of these assays, assay rapidity and sensitivity is a primary concern, the sensitivity determined by both the quantum yield of the fluorophores and efficiency of the detection system, while rapidity is determined by the physical and biophysical parameters of temperature, concentration, assay bioaffinity, etc. In this paper we describe a platform technology that promises to fundamentally address these two physical constraints of sensitivity and rapidity. By combining the use of Metal-Enhanced Fluorescence (MEF), a near-field effect that can significantly enhance fluorescence signatures, with low power microwave heating, we can significantly increase the sensitivity of surface assays as well as >95% kinetically complete the assay within a few seconds. In addition, the metallic nanostructures used to facilitate MEF appear to be preferentially heated as compared to the surface assay fluid, advantageously localizing the MEF and heating around the nanostructures. To demonstrate proof of principle, a 96-well plate has been functionalized with silver nanostructures, and a model protein avidin-biotin assay studied. In our findings, a greater than 5-fold fluorescence enhancement coupled with a approximately 90-fold increase in assay kinetics was observed, but with no assay washing steps needed due to the silver-enhanced evanescent field mode of excitation. These findings promise to strongly facilitate high throughput fluorescence-based processes, such as in biology, drug discovery and general compound screening.

Colloids↗

PubMed2006

Metal-enhanced fluorescence-based RNA sensing.

In this communication, we present the proof-of-principle of a new RNA sensing technique, based on metal-enhanced fluorescence, which may improve the potential of solid substrate-based RNA capture assays. In this regard, the detection of RNA is accomplished by annealing a target RNA tagged with a fluorophore to a DNA anchor probe in a single step on a solid surface, where the fluorescence signal is enhanced by silver nanoparticles. Furthermore, the proposed method is highly amenable to high-throughput screening applications and is capable of detecting less than 25 fmol of RNA at a signal-to-noise ratio greater than 20, in several hours compared to that of reverse transcription-PCR at >24 h.

Biosensing Techniques↗

PubMed2005

Microwave-accelerated metal-enhanced fluorescence (MAMEF): application to ultra fast and sensitive clinical assays.

In this rapid communication we describe an exciting platform technology that promises to fundamentally address two underlying constraints of modern assays and immunoassays, namely sensitivity and rapidity. By combining the use of Metal-enhanced Fluorescence (MEF) with low power microwave heating (Mw), we can significantly increase the sensitivity of surface assays as well as >95% kinetically complete the assay within a few seconds. This technology is subsequently likely to find significant importance in certain clinical assays, such as in the clinical assessment of myoglobin, where both the assay rapidity and sensitivity are paramount for the assessment and treatment of acute myocardial infarction.

Fluorescence↗

PubMed2005

Microwave-accelerated metal-enhanced fluorescence: platform technology for ultrafast and ultrabright assays.

We describe an exciting assay platform technology that promises to fundamentally address two underlying physical constraints of modern assays and immunoassays, namely, assay sensitivity and rapidity. By combining the use of metal-enhanced fluorescence with low-power microwave heating, we can indeed significantly increase the sensitivity of surface assays as well as >95 % kinetically complete the assay within a few seconds. Subsequently, this new technology promises to fundamentally change the way we currently employ immunoassays in clinical medicine. This new model platform system can be potentially applied to many other important assays, such as to the clinical assessment of myoglobin, where both assay speed and sensitivity is paramount for the assessment and treatment of acute myocardial infarction. To demonstrate the utility of microwave-accelerated metal-enhanced fluorescence (MAMEF), we show that a simple protein-based assay system can be optically amplified approximately 10-fold by using silver nanostructures, while being kinetically complete in less than 20 s. This new platform approach is subsequently over 10-fold more sensitive and approximately 90 times faster than a control assay that operates both at room temperature and without the use of metal-enhanced fluorescence. Finally, we show that low-power heating by microwaves in our model system does not denature proteins, as evidenced by no protein structural changes, probed by fluorescence resonance energy transfer.

Avidin↗

PubMed2005

Angular-ratiometric plasmon-resonance based light scattering for bioaffinity sensing.

We describe an exciting opportunity for affinity biosensing using a ratiometric approach to the angular-dependent light scattering from bioactivated and subsequently aggregated noble metal colloids. This new model sensing platform utilizes the changes in particle scattering from very small colloids, which scatter light according to traditional Rayleigh theory, as compared to the changes in scattering observed by much larger colloidal aggregates, formed due to a bioaffinity reaction. These larger aggregates no longer scatter incident light in a Cos(2) theta dependence, as is the case for Rayleigh scattering, but instead scatter light in an increased forward direction as compared to the incident geometry. By subsequently taking the ratio of the scattered intensity at two angles, namely 90 degrees and 140 degrees , relative to the incident light, we can follow the association of biotinylated bovine serum albumin-coated 20 nm gold colloids, cross-linked by additions of streptavidin. This new model system can be potentially applied to many other nanoparticle assays and has many advantages over traditional fluorescence sensing and indeed light-scattering approaches. For example, a single nanoparticle can have the equivalent scattered intensity as 10(5) fluorescing fluorescein molecules substantially increasing detection; the angular distribution of scattered light from noble metal colloids is substantially easier to predict as compared to fluorescence; the scattered light is not quenched by biospecies; the ratiometric measurements described here are not dependent on colloid concentration as are other scattering techniques; and finally, the noble metal colloids are not prone to photodestruction, as is the case with organic fluorophores.

Affinity Labels↗

PubMed2005

Metal-enhanced fluorescence using anisotropic silver nanostructures: critical progress to date.

In this critical and timely review, the effects of anisotropic silver nanostructures on the emission intensity and photostability of a key fluorophore that is frequently used in many biological assays is examined. The silver nanostructures consist of triangular, rod-like, and fractal-like nanoparticles of silver deposited on conventional glass substrates. The close proximity to silver nanostructures results in greater intensity and photostability of the fluorophore than for fluorophores solely deposited on glass substrates. These new anisotropic silver nanostructure-coated surfaces show much more favorable effects than silver island films or silver colloid-coated substrates. Subsequently, the use of metal-enhanced fluorescence (MEF) for biosensing applications is discussed.

Anisotropy↗

PubMed2005

Rapid deposition of triangular silver nanoplates on planar surfaces: application to metal-enhanced fluorescence.

A simple and rapid wet-chemical technique for the deposition of silver triangles on conventional glass substrates, which alleviates the need for lithography, has been developed. The technique is based on the seed-mediated cetyltrimethylammonium-bromide-directed growth of silver triangles on glass surfaces, where smaller spherical silver seeds that were attached to the surface were subsequently converted and grown into silver triangles in the presence of a cationic surfactant and silver ions. The size of the silver triangles was controlled by sequential immersion of silver seed-coated glass substrates into a growth solution and by the duration time of immersion. Atomic force microscopy studies revealed that the size of the silver triangles ranged between 100 and 500 nm. Interestingly, these new surfaces are a significant improvement over traditional silver island films for applications in metal-enhanced fluorescence. A routine 16-fold enhancement in emission intensity was typically observed, for protein-immobilized indocyanine green, with a relatively very low loading density of silver triangles on the glass surface.

Adsorption↗

PubMed2005

Nanogold plasmon resonance-based glucose sensing. 2. Wavelength-ratiometric resonance light scattering.

Gold colloids are well known to display strong plasmon absorption bands due to electron oscillations induced by the incident light. When the colloids are in proximity, the plasmon absorption bands are often perturbed. This has enabled us recently to successfully develop a glucose sensing platform based on the disassociation of dextran-coated gold colloids, cross-linked with Con A, by glucose. However, a much more useful and simpler property of gold colloids, which has been ill explored with regard to sensing, is their ability to efficiently scatter excitation light. We have found that our nanogold sensing aggregates are indeed efficient light scatters around the nanogold plasmon absorption band. By measuring the ratio of scattered light intensities at two different arbitrary wavelengths, 560 and 680 nm, glucose concentrations can be readily determined from a few millimolar up to approximately 60 mM, using a simple white light LED and detection system. Further, by measuring the ratio of the scattered intensities, this sensing approach is independent of the total sensing aggregate concentration and the excitation and detection instrumentation fluctuations or drifts. This simplistic and low-cost approach to glucose sensing, coupled with the sensing aggreagates' ability to scatter red light, suggests the potential use of these aggregates for use in physiological transdermal glucose monitoring, either for implantable skin sensors or glucose sensing tattoos (discussed later).

Biosensing Techniques↗

PubMed2005

Fast and slow deposition of silver nanorods on planar surfaces: application to metal-enhanced fluorescence.

Two methods have been considered for the deposition of silver nanorods onto conventional glass substrates. In the first method, silver nanorods were deposited onto 3-(aminopropyl)triethoxysilane-coated glass substrates simply by immersing the substrates into the silver nanorod solution. In the second method, spherical silver seeds that were chemically attached to the surface were subsequently converted and grown into silver nanorods in the presence of a cationic surfactant and silver ions. The size of the silver nanorods was controlled by sequential immersion of silver seed-coated glass substrates into a growth solution and by the duration of immersion, ranging from tens of nanometers to a few micrometers. Atomic force microscopy and optical density measurements were used to characterize the silver nanorods deposited onto the surface of the glass substrates. The application of these new surfaces is for metal-enhanced fluorescence (MEF), whereby the close proximity of silver nanostructures can alter the radiative decay rate of fluorophores, producing enhanced signal intensities and an increased fluorophore photostability. In this paper, it is indeed shown that irregularly shaped silver nanorod-coated surfaces are much better MEF surfaces as compared to traditional silver island or colloid films. Subsequently, these new silver nanorod preparation procedures are likely to find a common place in MEF, as they are a quicker and much cheaper alternative as compared to surfaces fabricated by traditional nanolithographic techniques.

Chemical Phenomena↗

PubMed2005

Enhanced ratiometric pH sensing using SNAFL-2 on silver island films: metal-enhanced fluorescence sensing.

We have explored the opportunities for enhanced ratiometric pH sensing using the well-known carboxy seminaphthofluorescein (SNAFL-2) and silver island films (SiFs). Our results show that the metallic surfaces can provide up to a 40-fold increase in probe fluorescence intensity as compared to nonmetallic surfaces with the same probe coverage. However, while the S/N is significantly better for pH sensing, the emission wavelength ratiometric values are similar to that obtained in solution, due to the fact that the emission of both the acidic and basic forms of the probe are enhanced to similar extents. To the best of our knowledge this is the first report of enhanced ratiometric fluorescence sensing on metallic surfaces.

Fluoresceins↗

PubMed2005

Metal-enhanced fluorescence from plastic substrates.

We report the first findings of Metal-Enhanced Fluorescence (MEF) from modified plastic substrates. In the past several years our laboratories have reported the favorable effects of fluorophores in close proximity to silver nanoparticles. These effects include, enhanced fluorescence intensities, (increased detectability), and reduced lifetimes, (enhanced fluorophore photostability). All of these reports have featured silver nanostructures and fluorophores which have been immobilized onto clean glass or quartz surfaces. In this report we show how plastic surfaces can be modified to obtain surface functionality, which in turn allows for silver deposition and therefore metal-enhanced fluorescence of fluorophores positioned above the silver using a protein spacer. Our findings show that plastic substrates are ideal surfaces for metal-enhanced phenomena, producing similar enhancements as compared to clean glass surfaces. Subsequently, we speculate that plastic substrates for MEF will find common place, as compared to the more expensive and less versatile traditional silica based supports.

Fluorescence↗

PubMed2005

Annealed silver-island films for applications in metal-enhanced fluorescence: interpretation in terms of radiating plasmons.

The effects of thermally annealed silver island films have been studied with regard to their potential applicability in applications of metal-enhanced fluorescence, an emerging tool in nano-biotechnology. Silver island films were thermally annealed between 75 and 250 degrees C for several hours. As a function of both time and annealing temperature, the surface plasmon band at approximately 420 nm both diminished and was blue shifted. These changes in plasmon resonance have been characterized using both absorption measurements, as well as topographically using Atomic Force Microscopy. Subsequently, the net changes in plasmon absorption are interpreted as the silver island films becoming spherical and growing in height, as well as an increased spacing between the particles. Interestingly, when the annealed surfaces are coated with a fluorescein-labeled protein, significant enhancements in fluorescence are observed, scaling with annealing temperature and time. These observations strongly support our recent hypothesis that the extent of metal-enhanced fluorescence is due to the ability of surface plasmons to radiate coupled fluorophore fluorescence. Given that the extinction spectrum of the silvered films is comprised of both an absorption and scattering component, and that these components are proportional to the diameter cubed and to the sixth power, respectively, then larger structures are expected to have a greater scattering contribution to their extinction spectrum and, therefore, more efficiently radiate coupled fluorophore emission. Subsequently, we have been able to correlate our increases in fluorescence emission with an increased particle size, providing strong experiment evidence for our recently reported metal-enhanced fluorescence, facilitated by radiating plasmons hypothesis.

Journal Article↗

PubMed2005

Plasmon light scattering in biology and medicine: new sensing approaches, visions and perspectives.

Surface plasmons are collective oscillations of free electrons at metallic surfaces. These oscillations can give rise to the intense colors of solutions of plasmon resonance nanoparticles and/or very intense scattering. While the use of plasmonic particle absorption based bioaffinity sensing is now widespread throughout biological research, the use of their scattering properties is relatively ill explored. We refer to the use, utility and control of surface plasmons as plasmonics. In this review and forward-looking article, we discuss the current opinions and uses of plasmonics, as well as speculate on areas of future research. These include the use of plasmon scatter for long-range immunosensing and macromolecular conformation studies, as well as the ability to Stokes shift plasmon scatter, a plasmonics phenomenon recently referred to as metal-enhanced fluorescence.

Fluorescence↗

PubMed2005

Metal-enhanced fluorescence: an emerging tool in biotechnology.

Over the past 15 years, fluorescence has become the dominant detection/sensing technology in medical diagnostics and biotechnology. Although fluorescence is a highly sensitive technique, where single molecules can readily be detected, there is still a drive for reduced detection limits. The detection of a fluorophore is usually limited by its quantum yield, autofluorescence of the samples and/or the photostability of the fluorophores; however, there has been a recent explosion in the use of metallic nanostructures to favorably modify the spectral properties of fluorophores and to alleviate some of these fluorophore photophysical constraints. The use of fluorophore-metal interactions has been termed radiative decay engineering, metal-enhanced fluorescence or surface-enhanced fluorescence.

Biotechnology↗

PubMed2004

Nanogold-plasmon-resonance-based glucose sensing.

Noble metal nanoparticles are well known for their strong interactions with light through the resonant excitations of the collective oscillations of the conduction electrons on the particles, the so-called surface plasmon resonances. The close proximity of two nanoparticles is known to result in a red-shifted resonance wavelength peak, due to near-field coupling. We have subsequently employed this phenomenon and developed a new approach to glucose sensing, which is based on the aggregation and disassociation of 20-nm gold particles and the changes in plasmon absorption induced by the presence of glucose. High-molecular-weight dextran-coated nanoparticles are aggregated with concanavalin A (Con A), which results in a significant shift and broadening of the gold plasmon absorption. The addition of glucose competitively binds to Con A, reducing gold nanoparticle aggregation and therefore the plasmon absorption when monitored at a near-red arbitrary wavelength. We have optimized our plasmonic-type glucose nanosensors with regard to particle stability, pH effects, the dynamic range for glucose sensing, and the observation wavelength to be compatible with clinical glucose requirements and measurements. In addition, by modifying the amount of dextran or Con A used in nanoparticle fabrication, we can to some extent tune the glucose response range, which means that a single sensing platform could potentially be used to monitor microM --> mM glucose levels in many physiological fluids, such as tears, blood, and urine, where the glucose concentrations are significantly different.

Biosensing Techniques↗

PubMed2004

Saccharide sensing using gold and silver nanoparticles--a review.

We review new methodologies for glucose sensing from our laboratories based on the specific biological interactions between Con A, dextran-coated gold nanoparticles and glucose, and the interactions between dextran, glucose, and boronic-acid capped silver nanoparticles in solution. Our new approaches promise new tunable glucose sensing platforms. Dextran-coated gold nanoparticles were aggregated with the addition of Con A resulting in increase an in absorbance of nanoparticles at 650 nm, where the post-addition of glucose caused the dissociation of the aggregates and thus a decrease in the absorbance at 650 nm. The interaction of glucose and dextran with boronic acid-capped silver nanoparticles in solution resulted in enhanced luminescence intensity cumulatively due to surface-enhanced fluorescence and the decrease in absorbance at 400 nm, with an increase in absorbance at 640 nm. Lifetime measurements were used to distinguish the contribution from the surface-enhanced fluorescence. TEM was employed to assess the aggregation of nanoparticles.

Algorithms↗