Microwave-Accelerated Metal-Enhanced Fluorescence (MAMEF): Application to Ultra Fast and Sensitive Clinical Assays

Low-concentration trypsin detection from a metal-enhanced fluorescence (MEF) platform: Towards the development of ultra-sensitive and rapid detection of proteolytic enzymes

Proteolytic enzymes, which serve to degrade proteins to their amino acid building blocks, provide a distinct challenge for both diagnostics and biological research fields. Due to their ubiquitous presence in a wide variety of organisms and their involvement in disease, proteases have been identified as biomarkers for various conditions. Additionally, low-levels of proteases may interfere with biological investigation, as contamination with these enzymes can physically alter the protein of interest to researchers, resulting in protein concentration loss or subtler polypeptide clipping that leads to a loss of functionality. Low levels of proteolytic degradation also reduce the shelf-life of commercially important proteins. Many detection platforms have been developed to achieve low-concentration or low-activity detection of proteases, yet many suffer from limitations in analysis time, label stability, and ultimately sensitivity. Herein we demonstrate the potential utility of fluorescein derivatives as fluorescent labels in a new, turn-off enzymatic assay based on the principles of metal-enhanced fluorescence (MEF). For fluorescein sodium salt alone on nano-slivered 96-well plates, or Quanta Plates™, we report up to 11,000x enhancement for fluorophores within the effective coupling or enhancement volume region, defined as ~100 nm from the silver surface. We also report a 9% coefficient of variation, and detection on the picomolar concentration scale. Further, we demonstrate the use of fluorescein isothiocyanate-labeled YebF protein as a coating layer for a MEF-based, Quanta Plate™ enzymatic activity assay using trypsin as the model enzyme. From this MEF assay we achieve a detection limit of ~1.89 ng of enzyme (2.8 mBAEE activity units) which corresponds to a minimum fluorescence signal decrease of 10%. The relative success of this MEF assay sets the foundation for further development and the tuning of MEF platforms for proteolytic enzyme sensing not just for trypsin, but other proteases as well. In addition, we discuss the future development of ultra-fast detection of proteases via microwave-accelerated MEF (MAMEF) detection technologies.

Rapid sample preparation with Lyse-It® for Listeria monocytogenes and Vibrio cholerae

Sample preparation is a leading bottleneck in rapid detection of pathogenic bacteria. Here, we use Lyse-It^® for bacterial cellular lysis, genomic DNA fragmentation, and protein release and degradation for both Listeria monocytogenes and Vibrio cholerae. The concept of Lyse-It^® employs a conventional microwave and Lyse-It^® slides for intensely focused microwave irradiation onto the sample. High microwave power and a <60 second irradiation time allow for rapid cellular lysis and subsequent intracellular component release. The pathogenic bacteria are identified by quantitative polymerase chain reaction (qPCR), which subsequently demonstrates the viability of DNA for amplification post microwave-induced lysis. Intracellular component release, degradation, and detection of L. monocytogenes and V. cholerae has been performed and shown in this paper. These results demonstrate a rapid, low-cost, and efficient way for bacterial sample preparation on both food and water-borne Gram-positive and -negative organisms alike.

Microwave-accelerated method for ultra-rapid extraction of Neisseria gonorrhoeae DNA for downstream detection

Nucleic acid-based detection of gonorrhea infections typically require a two-step process involving isolation of the nucleic acid, followed by detection of the genomic target often involving polymerase chain reaction (PCR)-based approaches. In an effort to improve on current detection approaches, we have developed a unique two-step microwave-accelerated approach for rapid extraction and detection of Neisseria gonorrhoeae (gonorrhea, GC) DNA. Our approach is based on the use of highly focused microwave radiation to rapidly lyse bacterial cells, release, and subsequently fragment microbial DNA. The DNA target is then detected by a process known as microwave-accelerated metal-enhanced fluorescence (MAMEF), an ultra-sensitive direct DNA detection analytical technique. In the current study, we show that highly focused microwaves at 2.45 GHz, using 12.3-mm gold film equilateral triangles, are able to rapidly lyse both bacteria cells and fragment DNA in a time- and microwave power-dependent manner. Detection of the extracted DNA can be performed by MAMEF, without the need for DNA amplification, in less than 10 min total time or by other PCR-based approaches. Collectively, the use of a microwave-accelerated method for the release and detection of DNA represents a significant step forward toward the development of a point-of-care (POC) platform for detection of gonorrhea infections.

Microwave-accelerated bioassay technique for rapid and quantitative detection of biological and environmental samples

Quantitative detection of molecules of interest from biological and environmental samples in a rapid manner, particularly with a relevant concentration range, is imperative to the timely assessment of human diseases and environmental issues. In this work, we employed the microwave-accelerated bioassay (MAB) technique, which is based on the combined use of circular bioassay platforms and microwave heating, for rapid and quantitative detection of Glial Fibrillary Acidic Protein (GFAP) and Shiga like toxin (STX 1). The proof-of-principle use of the MAB technique with the circular bioassay platforms for the rapid detection of GFAP in buffer based on colorimetric and fluorescence readouts was demonstrated with a 900W kitchen microwave. We also employed the MAB technique with a new microwave system (called the iCrystal system) for the detection of GFAP from mice with brain injuries and STX 1 from a city water stream. Control bioassays included the commercially available gold standard bioassay kits run at room temperature. Our results show that the lower limit of detection (LLOD) of the colorimetric and fluorescence based bioassays for GFAP was decreased by ~1000 times using the MAB technique and our circular bioassay platforms as compared to the commercially available bioassay kits. The overall bioassay time for GFAP and STX 1 was reduced from 4h using commercially available bioassay kits to 10min using the MAB technique.

Optical control of fluorescence through plasmonic eigenmode extinction

We introduce the concept of optical control of the fluorescence yield of CdSe quantum dots through plasmon-induced structural changes in random semicontinuous nanostructured gold films. We demonstrate that the wavelength- and polarization dependent coupling between quantum dots and the semicontinuous films, and thus the fluorescent emission spectrum, can be controlled and significantly increased through the optical extinction of a selective band of eigenmodes in the films. This optical method of effecting controlled changes in the metal nanostructure allows for versatile functionality in a single sample and opens a pathway to in situ control over the fluorescence spectrum.

5-color multiplexed microwave-accelerated metal-enhanced fluorescence: detection and analysis of multiple DNA sequences from within one sample well within a few seconds

We present a potentially highly sensitive and selective bio-assay for the potential detection of any five different DNA sequences from one sample in one well. The assay is based on a DNA "rapid catch and signal" (DNA-RCS) technology developed for the detection of different DNA sequences from a sample well area. Our signal amplification utilizes the metal-enhanced fluorescence (MEF) of dyes attached to the probe-DNAs, which hybridizes with the pre-formed mixture of anchor-DNA scaffolds on silver island films (SiFs). Low-power microwave irradiation accelerates both the formation of the anchor-DNA scaffold on the SiF-surface and anchor/probe DNA hybridization, i.e. "rapid catch" of target DNAs from a bulk solution, decreasing the assay run time from hours to only a few seconds. Localization of signaling dye-labels close to the SiFs make them extremely photostable, which allows for collecting/integrating the signal over a long time period. To demonstrate a 5 color DNA assay (5-plex) we have used a range of readily available Alexa™ dyes. Advantages and perspectives of the RCS-technologies ability to detect 5 different DNA sequences from within one plate-well are discussed.

Blind evaluation of the microwave-accelerated metal-enhanced fluorescence ultrarapid and sensitive Chlamydia trachomatis test by use of clinical samples

Accurate point-of-care (POC) diagnostic tests for Chlamydia trachomatis infection are urgently needed for the rapid treatment of patients. In a blind comparative study, we evaluated microwave-accelerated metal-enhanced fluorescence (MAMEF) assays for ultrafast and sensitive detection of C. trachomatis DNA from vaginal swabs. The results of two distinct MAMEF assays were compared to those of nucleic acid amplification tests (NAATs). The first assay targeted the C. trachomatis 16S rRNA gene, and the second assay targeted the C. trachomatis cryptic plasmid. Using pure C. trachomatis, the MAMEF assays detected as few as 10 inclusion-forming units/ml of C. trachomatis in less than 9 min, including DNA extraction and detection. A total of 257 dry vaginal swabs from 245 female adolescents aged 14 to 22 years were analyzed. Swabs were eluted with water, the solutions were lysed to release and to fragment genomic DNA, and MAMEF-based DNA detection was performed. The prevalence of C. trachomatis by NAATs was 17.5%. Of the 45 samples that were C. trachomatis positive and the 212 samples that were C. trachomatis negative by NAATs, 33/45 and 197/212 were correctly identified by the MAMEF assays if both assays were required to be positive (sensitivity, 73.3%; specificity, 92.9%). Using the plasmid-based assay alone, 37/45 C. trachomatis-positive and 197/212 C. trachomatis-negative samples were detected (sensitivity, 82.2%; specificity, 92.9%). Using the 16S rRNA assay alone, 34/45 C. trachomatis-positive and 197/212 C. trachomatis-negative samples were detected (sensitivity, 75.5%; specificity, 92.9%). The overall rates of agreement with NAAT results for the individual 16S rRNA and cryptic plasmid assays were 89.5% and 91.0%, respectively. Given the sensitivity, specificity, and rapid detection of the plasmid-based assay, the plasmid-based MAMEF assay appears to be suited for clinical POC testing.

Design and Proof-of-Concept Use of a Circular PMMA Platform with 16-Well Sample Capacity for Microwave-Accelerated Bioassays

We demonstrate the design and the proof-of-concept use of a new, circular poly(methyl methacrylate)-based bioassay platform (PMMA platform), which affords for the rapid processing of 16 samples at once. The circular PMMA platform (5 cm in diameter) was coated with a silver nanoparticle film to accelerate the bioassay steps by microwave heating. A model colorimetric bioassay for biotinylated albumin (using streptavidin-labeled horse radish peroxidase) was performed on the PMMA platform coated with and without silver nanoparticles (a control experiment), and at room temperature and using microwave heating. It was shown that the simulated temperature profile of the PMMA platform during microwave heating were comparable to the real-time temperature profile during actual microwave heating of the constructed PMMA platform in a commercial microwave oven. The model colorimetric bioassay for biotinylated albumin was successfully completed in ~2 min (total assay time) using microwave heating, as compared to 90 min at room temperature (total assay time), which indicates a ~45-fold decrease in assay time. Our PMMA platform design afforded for significant reduction in non-specific interactions and low background signal as compared to non-silvered PMMA surfaces when employed in a microwave-accelerated bioassay carried out in a conventional microwave cavity.

Rapid and sensitive detection of troponin I in human whole blood samples by using silver nanoparticle films and microwave heating

BACKGROUND

Cardiovascular diseases are among the leading causes of mortality in developed countries. It is widely recognized that troponin I (TnI) can be used for the assessment of a myocardial infarction.

METHODS

We investigated the use of the microwave-accelerated and metal-enhanced fluorescence (MA-MEF), a technique based on the combined use of low-power microwave heating, silver nanoparticle films (SNFs), and fluorescence spectroscopy for the detection of TnI from human whole blood samples. SNFs were deposited onto amine-modified glass microscope slides by use of Tollen's reaction scheme and characterized by optical absorption spectroscopy and scanning electron microscopy. The detection of TnI from buffer solutions and human whole blood samples on SNFs was carried out by using fluorescence-based immunoassays at room temperature (control immunoassay, 2 h total assay time) or microwave heating (MA-MEF-based immunoassay, 1 min total assay time).

RESULTS

We found that the lower limits of detection for TnI from buffer solutions in the control immunoassay and MA-MEF-based immunoassay were 0.1 μg/L and 0.005 μg/L, respectively. However, we were unable to detect TnI in whole blood samples in the control immunoassays owing to the coagulation of whole blood within 5 min of the incubation step. The use of the MA-MEF technique allowed detection of TnI from whole blood samples in 1 min with a lower detection limit of 0.05 μg/L.

CONCLUSIONS

The MA-MEF-based immunoassay is one of the fastest reported quantitative detection methodos for detection of TnI in human whole blood and has low detection limits similar to those obtained with commercially available immunoassays.

Two-color, 30 second microwave-accelerated Metal-Enhanced Fluorescence DNA assays: a new Rapid Catch and Signal (RCS) technology

For analyses of DNA fragment sequences in solution we introduce a 2-color DNA assay, utilizing a combination of the Metal-Enhanced Fluorescence (MEF) effect and microwave-accelerated DNA hybridization. The assay is based on a new "Catch and Signal" technology, i.e. the simultaneous specific recognition of two target DNA sequences in one well by complementary anchor-ssDNAs, attached to silver island films (SiFs). It is shown that fluorescent labels (Alexa 488 and Alexa 594), covalently attached to ssDNA fragments, play the role of biosensor recognition probes, demonstrating strong response upon DNA hybridization, locating fluorophores in close proximity to silver NPs, which is ideal for MEF. Subsequently the emission dramatically increases, while the excited state lifetime decreases. It is also shown that 30s microwave irradiation of wells, containing DNA molecules, considerably (~1000-fold) speeds up the highly selective hybridization of DNA fragments at ambient temperature. The 2-color "Catch and Signal" DNA assay platform can radically expedite quantitative analysis of genome DNA sequences, creating a simple and fast bio-medical platform for nucleic acid analysis.

Development of a microwave-accelerated metal-enhanced fluorescence 40 second, <100 cfu/ml point of care assay for the detection of Chlamydia trachomatis

An inexpensive technology to both lyse Chlamydia trachomatis (CT) and detect DNA released from CT within 40 s is demonstrated. In a microwave cavity, energy is highly focused using 100-nm gold films with "bow-tie" structures to lyse CT within 10 s. The ultrafast detection of the released DNA from less than 100 cfu/mL CT is accomplished in an additional 30 s by employing the microwave-accelerated metal-enhanced fluorescence technique. This new "release and detect" platform technology is a highly attractive alternative method for the lysing of bacteria, DNA extraction, and the fast quantification of bacteria and potentially other pathogenic species and cells as well. Our approach is a significant step forward for the development of a point of care test for CT.

Rapid Whole Blood Bioassays using Microwave-Accelerated Metal-Enhanced Fluorescence

The proof-of-principle demonstration of rapid whole blood bioassays based on microwave-accelerated metal-enhanced fluorescence (MAMEF) method using silver nanoparticle-deposited surfaces is presented. In this regard, spherical silver nanoparticles were deposited onto glass slides (silver nanoparticle films, SNFs) in a highly reproducible manner, which was assessed by optical absorption spectroscopy. Atomic force microscopy was employed to determine the size of the deposited silver nanoparticles. A model bioassay, based on the well-known interactions of biotinylated bovine serum albumin (b-BSA) and streptavidin was constructed on SNFs. The model bioassay was run at room temperature (metal-enhanced fluorescence (MEF)-based bioassay without microwave heating) for 60 minutes and with microwave heating (MAMEF-based bioassay) for 1 minute. In contrast to MEF-based bioassays that only allowed the use of samples in buffer solution, MAMEF-based bioassays afforded the use of whole blood samples. A lower detection limit of 1 nM and 0.01 nM for b-BSA was determined in MEF-based and MAMEF-based bioassays, respectively.

Sonication-assisted metal-enhanced fluorescence-based bioassays

A new bioassay technique, sonication-assisted metal-enhanced fluorescence, which is based on the combined use of ultrasound waves and metal-enhanced fluorescence (MEF), is reported. In this technique, low-intensity ultrasound waves significantly reduce the bioassay time by creating a temperature gradient between the bulk and the surface, which is thought to result in a mass transport of biomolecules from the bulk to the surface. After the assay is completed in 1 min, fluorescence emission is enhanced due to the MEF phenomenon. For proof-of-concept, a model bioassay based on the interactions of biotin and fluorophore-labeled avidin was constructed on SIFs and was subsequently completed in <1 min using low-intensity ultrasound at 40 kHz. The end-point values for fluorescence emission from sonicated assays were compared to those measured from assays carried out at room temperature without sonication to confirm to accuracy of the new technique. The effect of sonication on the assay components were studied using optical absorption spectroscopy, atomic force microscopy, and fluorescence spectroscopy techniques. Real-time thermal imaging was used to measure the changes in temperature of the bioassay components during the sonication process. Fluorescence resonance energy transfer (FRET) was also employed to investigate the effect of sonication on potential surface protein denaturation and conformational changes.

New tools for rapid clinical and bioagent diagnostics: microwaves and plasmonic nanostructures

In this timely review, we summarize recent work on ultra-fast and sensitive bioassays based on microwave heating, and provide our current interpretation of the role of the combined use of microwave energy and plasmonic nanostructures for applications in rapid clinical and bioagent diagnostics. The incorporation of microwave heating into plasmonic nanostructure-based bioassays brings new advancements to diagnostic tests. A temperature gradient, created by the selective heating of water in the presence of plasmonic nanostructures, results in an increased mass transfer of target biomolecules towards the biorecognition partners placed on the plasmonic nanostructures, enabling diagnostic tests to be completed in less than a minute, and in some cases only a few seconds, by further microwave heating. The diagnostic tests can also be run in complex biological samples, such as human serum and whole blood.

MALDI MS analysis of oligonucleotides: desalting by functional magnetite beads using microwave-assisted extraction

The presence of alkali cation adductions of oligonucleotides commonly deteriorates matrix-assisted laser desorption/ionization (MALDI) mass spectra. Thus, desalting is required for oligonucleotide samples prior to MALDI MS analysis in order to prevent the mass spectra from developing poor quality. In this paper, we demonstrate a new approach to extract traces of oligonucleotides from aqueous solutions containing high concentrations of salts using microwave-assisted extraction. The C18-presenting magnetite beads, capable of absorbing microwave irradiation, are used as affinity probes for oligonucleotides with the addition of triethylammonium acetate as the counterions. This new microwave-assisted extraction approach using magnetite beads as the trapping agents and as microwave-absorbers has been demonstrated to be very effective in the selective binding of oligonucleotides from aqueous solutions. The extraction of oligonucleotides from solutions onto the C18-presenting magnetite beads takes only 30 s to enrich oligonucleotides in sufficient quantities for MALDI MS analysis. After using this desalting approach, alkali cation adductions of oligonucleotides are dramatically reduced in the MALDI mass spectra. The presence of saturated NaCl (approximately 6 M) in the oligonucleotide sample is tolerated without degrading the mass spectra. The detection limit for d(A)6 is approximately 2.8 fmol.

Real-time Thermal Imaging of Microwave Accelerated Metal-Enhanced Fluorescence (MAMEF) Based Assays on Sapphire Plates

In this paper, we describe an optical geometry that facilitates our further characterization of the temperature changes above silver island films (SiFs) on sapphire plates, when exposed to microwave radiation. Since sapphire transmits IR, we designed an optical scheme to capture real-time temperature images of a thin water film on sapphire plates with and without SiFs during the application of a short microwave pulse. Using this optical scheme, we can accurately determine the temperature profile of solvents in proximity to metal structures when exposed to microwave irradiation. We believe that this optical scheme will provide us with a basis for further studies in designing metal structures to further improve plasmonic-fluorescence clinical sensing applications, such as those used in microwave accelerated metal-enhanced fluorescence (MAMEF).

Multicolor microwave-triggered metal-enhanced chemiluminescence

We describe a novel platform technology for both significantly enhancing and obtaining chemiluminescence signatures "on-demand", subsequently named Microwave-Triggered Metal-Enhanced Chemiluminescence. By combining the use of silver nanoparticles to plasmon enhance chemiluminescence with the use of low power microwaves to localize heating around the nanostructures, we can both optically amplify and trigger chemiluminescence reactions. This approach is a significant advantage over traditional chemiluminescence techniques and methodologies, such as those used for Western Blots, which typically require long periods of time to collect chemiluminescence and offer few possibilities of optically amplifying the signatures.

Microwave-accelerated metal-enhanced fluorescence: an ultra-fast and sensitive DNA sensing platform

In this paper, we investigated the effects of low-power microwave heating on the components of the recently described new approach to surface DNA hybridization assays, based on the Microwave-Accelerated Metal-Enhanced Fluorescence (MAMEF) platform technology. Thiolated oligonucleotides have been linked to surface-bound silver nanostructures which partially coat a glass slide. The addition of a complementary fluorescein-labeled oligonucleotide results in metal-enhanced fluorescein emission as the probe is brought into close proximity to the silver upon hybridization. In addition, the combined use with low-power microwave heating, which is thought to locally heat around the silvered surface, affords for both the assay kinetics and optical amplification to also be localized to the surface. In our model DNA target assay reported here, we can detect 23-mer targets in less than 20 s, up to a 600-fold decrease in the assay run time as compared to control samples hybridized to completion at room temperature. Importantly, the use of MAMEF also reduces the extent of unwanted non-specific DNA absorption, further increasing specific DNA target detection limits. It was also found that low-power microwave heating did not denature DNA and the bulk temperature increase near to silver nanoparticles was only ca. 1 degrees C.

Spatial and temporal control of microwave triggered chemiluminescence: a protein detection platform

We have combined the principles of microwave circuitry and antenna design and our recent work in microwave-triggered metal-enhanced chemiluminescence to now "trigger" chemically and enzyme-catalyzed chemiluminescent reactions with spatial and temporal control. With this technology platform, we achieve spatial and temporal control of enzyme and chemically catalyzed chemiluminescence reactions to achieve more than 500-fold increases in "on-demand" photon flux from chemically catalyzed chemiluminescent reactions. We also report a 6-fold increase in photon flux from HRP-catalyzed assays on disposable coverslips functionalized with HRP and placed proximal to the substrates modified with thin-film aluminum triangle disjointed "bow-tie" structures. In addition, we demonstrate the applicability of this technology to develop multiplexed or high-throughput chemiluminescent assays. We also demonstrate the clinical and biological relevance of this technology platform by affixing aluminum structures in proximity to HRP protein immobilized on nitrocellulose to improve the sensitivity for this model Western blot scheme by 50-fold. We believe analytical applications that rely on enzyme-catalyzed chemiluminescence, such as immunoassays, may greatly benefit from this new platform technology.

Microwave-Accelerated Surface Plasmon-Coupled Directional Luminescence: application to fast and sensitive assays in buffer, human serum and whole blood

The applicability of a new technique, Microwave-Accelerated Surface Plasmon-Coupled Luminescence (MA-SPCL) for fast and sensitive bioassays in buffer, serum and whole blood using quantum dots as luminescence reporters is demonstrated. In this regard, a model bioassay based on the well-known interactions of biotin and streptavidin is used. Using MA-SPCL, the bioassay was kinetically completed within 1 min with the use of low power microwave heating as compared to the identical bioassay which took in excess of 30 min to reach >95% completion at room temperature, a 30-fold increase in assay kinetics. The luminescence emission from the quantum dots was coupled to surface plasmons of the gold film, enabling the detection of the luminescence emission in a highly directional fashion as compared to the normal isotropic emission, for enhanced sensitivity and detection. The combined effect of microwaves for faster assay kinetics, with surface plasmon-coupled luminescence for sensitive luminescence measurements, has also made possible the demonstration of the use of the MA-SPCL technique for assays run in complex media, such as human serum and whole blood, where the same assay could not be performed at room temperature due to the coagulation of blood. In the MA-SPCL assay run in serum and whole blood, the luminescence intensity from 33 nM quantum dots was 75% and 20% that of the luminescence intensity from the assay run in buffer, with a signal to noise ratio of 12.5 and 3, respectively.

Microwave triggered metal enhanced chemiluminescence: Quantitative protein determination

We present a new technology that offers a faster alternative to the chemiluminescence-based detection that is used in protein assay platforms today. By combining the use of silver nanostructures with chemiluminescent species, a technique that our laboratories have recently shown can enhance the system photon flux over 50-fold, with the use of low-power microwave heating to additionally accelerate, in essence "trigger", chemiluminescence-based reactions, then both ultrafast and ultrabright chemiluminescence assays can be realized. In addition, the preferential heating of the nanostructures by microwaves affords for microwave triggered metal enhanced chemiluminescence (MT-MEC) to be localized in proximity to the silvered surfaces, alleviating unwanted emission from the distal solution. To demonstrate MT-MEC, we have constructed a model assay sensing platform on both silvered and glass surfaces, where comparison with the identical glass substrate-based assay serves to confirm the significant benefits of using silver nanostructures for metal-enhanced chemiluminescence. Our new model assay technology can detect femtomoles of biotinylated BSA in less than 2 min and can indeed be modified to both detect and quantify a great many other biomolecules as well. As compared to traditional western blot approaches, MT-MEC offers protein quantification, high-sensitivity detection combined with ultrafast assay times, i.e., <2 min.

Microwave-accelerated ultrafast nanoparticle aggregation assays using gold colloids

In this paper, the proof of principle of microwave-accelerated aggregation assay technology, which shortens the solution-based aggregation assays' run time to seconds (>100-fold increase in kinetics) with microwave heating, was demonstrated using a model aggregation assay based on the well-known interactions of biotin and avidin. Biotinylated gold colloids were aggregated in solution with the addition of streptavidin, which takes 20 min at room temperature to reach >90% completion and only 10 s with microwave heating. The initial velocity (after 1-s microwave heating) of the biotinylated gold colloids reaches up to 10.5 m/s, which gives rise to greater sampling of the total volume but not a large increase in bulk temperature. The room-temperature, steady-state velocity of the colloids was <0.5 microm/s. In control experiments, where streptavidin preincubated with d-biotin in solution is added to biotinylated gold colloids and microwave heated, gold colloids did not aggregate, demonstrating that nonspecific interactions between biotinylated gold colloids and streptavidin were negligible.

Microwave-Triggered Metal-Enhanced Chemiluminescence (MT-MEC): Application to Ultra-fast and Ultra-sensitive Clinical Assays

In this rapid communication we describe a new approach to protein detection with chemiluminescence. By combining common practices in protein detection with chemiluminescence, microwave technology, and metal-enhanced chemiluminescence, we show that we can use low power microwaves to substantially increase enzymatic chemiluminescent reaction rates on metal substrates. As a result, we have found that we can in essence trigger chemiluminescence with low power microwave (Mw) pulses and ultimately, perform on-demand protein detection assays. Using microwave triggered metal-enhanced chemiluminescence (MT-MEC), we not only improve the sensitivity of immunoassays with enhanced signal-to-noise ratios, but we also show that we can accurately quantify protein concentrations by integrating the photon flux for discrete time intervals.

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.

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.

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.