Microwave-Accelerated and Metal-Enhanced Fluorescence Myoglobin Detection on Silvered Surfaces: Potential Application to Myocardial Infarction Diagnosis

Plasmonic Fluorescence Enhancement in Diagnostics for Clinical Tests at Point-of-Care: A Review of Recent Technologies

Fluorescence-based biosensors have widely been used in the life-sciences and biomedical applications due to their low limit of detection and a diverse selection of fluorophores that enable simultaneous measurements of multiple biomarkers. Recent research effort has been made to implement fluorescent biosensors into the exploding field of point-of-care testing (POCT), which uses cost-effective strategies for rapid and affordable diagnostic testing. However, fluorescence-based assays often suffer from their feeble signal at low analyte concentrations, which often requires sophisticated, costly, and bulky instrumentation to maintain high detection sensitivity. Metal- and metal oxide-based nanostructures offer a simple solution to increase the output signal from fluorescent biosensors due to the generation of high field enhancements close to a metal or metal oxide surface, which has been shown to improve the excitation rate, quantum yield, photostability, and radiation pattern of fluorophores. This article provides an overview of existing biosensors that employ various strategies for fluorescence enhancement via nanostructures and have demonstrated the potential for use as POCT. Biosensors using nanostructures such as planar substrates, freestanding nanoparticles, and metal-dielectric-metal nanocavities are discussed with an emphasis placed on technologies that have shown promise towards POCT applications without the need for centralized laboratories.

Applications of Microwave Energy in Medicine

Microwaves are a highly utilized electromagnetic wave, used across a range of industries including food processing, communications, in the development of novel medical treatments and biosensor diagnostics. Microwaves have known thermal interactions and theorized non-thermal interactions with living matter; however, there is significant debate as to the mechanisms of action behind these interactions and the potential benefits and limitations of their use. This review summarizes the current knowledge surrounding the implementation of microwave technologies within the medical industry.

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.

Elucidation of a non-thermal mechanism for DNA/RNA fragmentation and protein degradation when using Lyse-It

Rapid sample preparation is one of the leading bottlenecks to low-cost and efficient sample component detection. To overcome this setback, a technology known as Lyse-It has been developed to rapidly (less than 60 seconds) lyse Gram-positive and-negative bacteria alike, while simultaneously fragmenting DNA/RNA and proteins into tunable sizes. This technology has been used with a variety of organisms, but the underlying mechanism behind how the technology actually works to fragment DNA/RNA and proteins has hitherto been studied. It is generally understood how temperature affects cellular lysing, but for DNA/RNA and protein degradation, the temperature and amount of energy introduced by microwave irradiation of the sample, cannot explain the degradation of the biomolecules to the extent that was being observed. Thus, an investigation into the microwave generation of reactive oxygen species, in particular singlet oxygen, hydroxyl radicals, and superoxide anion radicals, was undertaken. Herein, we probe one aspect, the generation of reactive oxygen species (ROS), which is thought to contribute to a non-thermal mechanism behind biomolecule fragmentation with the Lyse-It technology. By utilizing off/on (Photoinduced electron transfer) PET fluorescent-based probes highly specific for reactive oxygen species, it was found that as oxygen concentration in the sample and/or microwave irradiation power increases, more reactive oxygen species are generated and ultimately, more oxidation and biomolecule fragmentation occurs within the microwave cavity.

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.

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.

Lab-on-a-chip based immunosensor principles and technologies for the detection of cardiac biomarkers: a review

This review examines the current state of the art lab-on-a-chip and microfluidic based biosensor technologies used in the detection of cardiac biomarkers. The determination and quantification of blood based, cardiac biomarkers are crucial in the triage and management of a range of cardiac related conditions, where time delay has a major impact on short and longer-term outcomes of a patient. The design and manufacturing of biomarker detection systems are multi-disciplinary in nature and require researchers to have knowledge of both life sciences and engineering for the full potential of this field to be realised. This review will therefore provide a comprehensive overview of chip based immunosensing technology as applied to cardiac biomarker detection, while discussing the potential suitability and limitations of each configuration for incorporation within a clinical diagnostics device suitable for point-of-care applications.

Improved detectability and signal strength for rotating phase fluorescence immunoassays through image processing

Fluorescence immunoassays based on rotating solid phase have shown promise of lowered detection limits, among other advantages. However, intrinsic background distortion effects have limited their utility. Here, novel image processing strategies are used to minimize these effects and improve the estimate of concentration and lower the detection limit. This initial demonstration of a new processing capability is performed on data for a protein, myoglobin, which is a biomarker for acute myocardial infarction. For these data, compared with published results, the detection limit is improved by a factor of approximately one hundred (to 700 fM), which is competitive with or better than other immunoassay strategies (ELISA, for example) that are fully developed. This work suggests that image and video processing technologies can provide a valuable alternative approach to biochemical detection and concentration estimation.

Quantitative Comparison of Protein Surface Coverage on Glass Slides and Silver Island Films in Metal-Enhanced Fluorescence-based Biosensing Applications

The use of Metal-Enhanced Fluorescence (MEF) phenomenon in fluorescence-based bioassays affords for increased sensitivity to be realized by incorporating metal nanoparticles onto planar surfaces. The close-range interactions of metal-fluorophores result in increased fluorescence emission from the bioassays, which in turn affords for the detection of target biomolecules at lower concentrations. Moreover, the use of silver nanoparticles increases the photostability of fluorophores improving the detectability of fluorescence emission under prolonged use of excitation light. Although numerous reports on MEF-based biosensing applications exist, the contribution of protein coverage on Silver Island Films (SIFs) on the increased fluorescence emission was never investigated. This work presents our findings on the quantitative comparison of protein surface coverage on SIFs and blank glass slides. In this regard, identical protein bioassay for a model protein (biotinylated bovine serum albumin, b-BSA) on these surfaces is constructed and the relative extent of protein surface coverage on SIFs and blank glass slides was determined using radio-labeled biomolecules. It was found that the total scintillation counts on SIFs and blank glass slides were similar for BSA concentrations ranging from 1 μM to 1 pM, which implies that increased fluorescence in MEF-based biosensing applications is only due to metal-fluorophore interactions.

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 and Sensitive Colorimetric ELISA using Silver Nanoparticles, Microwaves and Split Ring Resonator Structures

We report a new approach to colorimetric Enzyme-Linked Immunosorbent Assay (ELISA) that reduces the total assay time to < 2 min and the lower-detection-limit by 100-fold based on absorbance readout. The new approach combines the use of silver nanoparticles, microwaves and split ring resonators (SRR). The SRR structure is comprised of a square frame of copper thin film (30 µm thick, 1 mm wide, overall length of ~9.4 mm on each side) with a single split on one side, which was deposited onto a circuit board (2×2 cm(2)). A single micro-cuvette (10 µl volume capacity) was placed in the split of the SRR structures. Theoretical simulations predict that electric fields are focused in and above the micro-cuvette without the accumulation of electrical charge that breaks down the copper film. Subsequently, the walls and the bottom of the micro-cuvette were coated with silver nanoparticles using a modified Tollen's reaction scheme. The silver nanoparticles served as a mediator for the creation of thermal gradient between the bioassay medium and the silver surface, where the bioassay is constructed. Upon exposure to low power microwave heating, the bioassay medium in the micro-cuvette was rapidly and uniformly heated by the focused electric fields. In addition, the creation of thermal gradient resulted in the rapid assembly of the proteins on the surface of silver nanoparticles without denaturing the proteins. The proof-of-principle of the new approach to ELISA was demonstrated for the detection of a model protein (biotinylated-bovine serum albumin, b-BSA). In this regard, the detection of b-BSA with bulk concentrations (1 µM to 1 pM) was carried out on commercially available 96-well high throughput screening (HTS) plates and silver nanoparticle-deposited SRR structures at room temperature and with microwave heating, respectively. While the room temperature bioassay (without microwave heating) took 70 min to complete, the identical bioassay took < 2 min to complete using the SRR structures (with microwave heating). A lower detection limit of 0.01 nM for b-BSA (100-fold lower than room temperature ELISA) was observed using the SRR structures.

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.

Conversion of just-continuous metallic films to large particulate substrates for metal-enhanced fluorescence

We report the effects of thermally annealing, non-, just-, and thick continuous silver films for their potential applications in metal-enhanced fluorescence, a near-field concept which can alter the free-space absorption and emissive properties of close-proximity fluorophores (excited states). We have chosen to anneal a noncontinuous particulate film 5 nm thick and two thicker continuous films, 15 and 25 nm thick, respectively. Our results show that the annealing of the 25 nm film has little effect on close-proximity fluorescence when coated with a monolayer of fluorophore-labeled protein. However, the 15 nm continuous film cracks upon annealing, producing large nanoparticles which are ideal for enhancing the fluorescence of close-proximity fluorophores that are indeed difficult to prepare by other wet-chemical deposition processes. The annealing of 5 nm noncontinuous particulate films (a control sample) has little influence on metal-enhanced fluorescence, as expected.

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.

Characterization of cap-shaped silver particles for surface-enhanced fluorescence effects

Surface-enhanced fluorescence has potentially many desirable properties as an analytical method for medical diagnostics, but the effect observed so far is rather modest and only in conjunction with fluorophores with low quantum yields. Coupled with the fact that preparation of suitable surfaces at low costs has been difficult, this has limited its utilities. Here we report a novel method for forming uniform and reproducible surfaces with respectable enhancement ratios even for high-quantum-yield fluorophores. Formation of dense surface-adsorbed latex spheres on a flat surface via partial aggregation, followed by evaporation of silver, results in a film consisting of cap-shaped silver particles at high densities. Binding of fluorescence biomolecules, either through physisorption or antigen-antibody reaction, was performed, and enhancements close to 50 have been observed with fluorophores such as R-phycoerythrin and Alexa 546-labeled, bovine serum albumin, both of which have quantum yields around 0.8. We attribute this to the unique shape of the silver particle and the presence of abundant gaps among adjacent particles at high densities. The effectiveness of the new surface is also demonstrated with IL-6 sandwich assays.

Metal enhanced fluorescence solution-based sensing platform 2: fluorescent core-shell Ag@SiO2 nanoballs

In this Rapid Communication, we present the development of monodisperse core-shell (silver core-silica shell) nanoparticles with various shell thicknesses featuring a fluorophore, subsequently named Metal-Enhanced Fluorescence (MEF) nanoballs. MEF nanoballs consist of a approximately 130 nm silver nanoparticle core, a silica shell with up to 35 nm thickness and fluorophores doped within the silica shell. Fluorescent nanobubbles where the silver core is removed by chemical etching are used as control samples to show the benefits of using silver nanoparticles, i.e, Metal-Enhanced Fluorescence. Finally, we demonstrate the broad potential biological applications of MEF nanoballs by employing near-infra red emitting probes (Rhodamine 800) within the silica shell, for potential applications in cellular imaging and solution-based sensing.

Metal-Enhanced Fluorescence from Gold Surfaces: Angular Dependent Emission

The first observation of Metal-Enhanced Fluorescence (MEF) from large gold colloids is presented. Gold colloids, 40 and 200 nm diameter, were deposited onto glass substrates in a homogeneous fashion. The angular-dependent fluorescence emission of FITC-HSA, adsorbed onto gold colloids, was measured on a rotating stage which was used to evaluate MEF at all spatial angles. The emission intensity of FITC-HSA was found to be up to 2.5-fold brighter than the emission on bare glass substrates at an angle of 270 degrees. This is explained by the Radiating Plasmon Model, whereby the combined system, composed of the fluorophore and the metal colloids, emits with the photophysical characteristics of the fluorophore, after the excitation and the partial radiationless energy transfer between the excited states of the fluorophore and the surface plasmons of the gold colloids. The fluorescence enhancement was found to be higher with 200 nm gold colloids as compared to 40 nm colloids due to the increased contribution of the scattering portion of the 200 nm gold colloid extinction spectrum. These observations suggest that gold colloids could be used in MEF applications, offering more stable surfaces than the commonly used silvered surfaces, for applications requiring longer term storage and use.