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

Assembly of Glycochips with Mammalian GSLs Mimetics toward the On-site Detection of Biological Toxins

According to our previously proposed scheme, each of three kinds of glycosphingolipid (GSL) derivatives, that is, lactosyl ceramide [Lac-Cer (1)] and gangliosides [GM1-Cer (2) and GT1b-Cer (3)], was installed onto the glass surface modified with Au nanoparticles. In the present study, we tried to apply microwave irradiation to promote their installing reactions. Otherwise, this procedure takes a lot of time as long as a conventional self-assembled monolayer (SAM) technique is applied. Using an advanced microwave reactor capable of adjusting ambient temperatures within a desired range, various GSL glycochips were prepared from the derivatives (1)-(3) under different microwave irradiation conditions. The overall assembling process was programed with an IC controller to finish in 1 h, and the derived GSL glycochips were evaluated in the analysis of three kinds of biological toxins [a Ricinus agglutinin (RCA₁₂₀), botulinum toxin (BTX), and cholera toxin (CTX)] using a localized surface plasmon resonance (LSPR) biosensor. In the LSPR analysis, most of the irradiated GSL chips showed an enhanced response to the targeting toxin when they were irradiated under optimal temperature conditions. Lac-Cer chips showed the highest response to RCA₁₂₀ (an agglutinin with β-_D-Gal specificity) when the microwave irradiation was conducted at 30-35 °C. Compared to our former Lac-Cer glycochips with the conventional SAM condition, their response was enhanced by 3.6 times. Analogously, GT1b chips gained an approximately 4.1 times enhancement in their response to botulinum type C toxin (BTX/C) when the irradiation was conducted around at 45-60 °C. In the LSPR evaluation of the GM1-Cer glycochips using CTX, an optimal condition also appeared at around 30-35 °C. On the other hand, the microwave irradiation did not lead to a notable increase compared to the former GM1-Cer chips derived with the SAM technique. Judging from these experimental results, the microwave irradiation effectively promotes the installing process for all the three kinds of the GSL derivatives, while the optimal thermal condition becomes different from each other. Many bacterial and botanic proteinous toxins are composed of such carbohydrate binding domains or subunits that can discriminate both the key epitope structure and the dimension of glycoconjugates on the host cell surface. It is assumed that the optimal irradiation and thermal conditions are required to array these semi-synthetic GSL derivatives on the Au nanoparticles in a proper density and geometry for tight adhesion with each of the biological toxins.

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.

Circular Bioassay Platforms for Applications in Microwave-Accelerated Techniques

In this paper, we present the design of four different circular bioassay platforms, which are suitable for homogeneous microwave heating, using theoretical calculations (i.e., COMSOL™ multiphysics software). Circular bioassay platforms are constructed from poly(methyl methacrylate) (PMMA) for optical transparency between 400-800 nm, has multiple sample capacity (12, 16, 19 and 21 wells) and modified with silver nanoparticle films (SNFs) to be used in microwave-accelerated bioassays (MABs). In addition, a small monomode microwave cavity, which can be operated with an external microwave generator (100 W), for use with the bioassay platforms in MABs is also developed. Our design parameters for the circular bioassay platforms and monomode microwave cavity during microwave heating were: (i) temperature profiles, (ii) electric field distributions, (iii) location of the circular bioassay platforms inside the microwave cavity, and (iv) design and number of wells on the circular bioassay platforms. We have also carried out additional simulations to assess the use of circular bioassay platforms in a conventional kitchen microwave oven (e.g., 900 W). Our results show that the location of the circular bioassay platforms in the microwave cavity was predicted to have a significant effect on the homogeneous heating of these platforms. The 21-well circular bioassay platform design in our monomode microwave cavity was predicted to offer a homogeneous heating pattern, where inter-well temperature was observed to be in between 23.72-24.13°C and intra-well temperature difference was less than 0.21°C for 60 seconds of microwave heating, which was also verified experimentally.

Plasmon-Enhanced Fluorescence Biosensors: a Review

Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

Microwave-accelerated surface modification of plasmonic gold thin films with self-assembled monolayers of alkanethiols

A rapid surface modification technique for the formation of self-assembled monolayers (SAMs) of alkanethiols on gold thin films using microwave heating in <10 min is reported. In this regard, SAMs of two model alkanethiols, 11-mercaptoundecanoic acid (11-MUDA, to generate a hydrophilic surface) and undecanethiol (UDET, a hydrophobic surface), were successfully formed on gold thin films using selective microwave heating in (1) a semicontinuous fashion and (2) a continuous fashion at room temperature (24 h, control experiment, no microwave heating). The formation of SAMs of 11-MUDA and UDET was confirmed by contact angle measurements, Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The contact angles for water on SAMs formed by the selective microwave heating and conventional room temperature incubation technique (24 h) were measured to be similar for 11-MUDA and UDET. FT-IR spectroscopy results confirmed that the internal structures of SAMs prepared using both microwave heating and room temperature were similar. XPS results revealed that the organic and sulfate contaminants found on bare gold thin films were replaced by SAMs after the surface modification process had been conducted using both microwave heating and room temperature.

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.

Surface plasmon-coupled emission: what can directional fluorescence bring to the analytical sciences?

Surface plasmon-coupled emission (SPCE) arose from the integration of fluorescence and plasmonics, two rapidly expanding research fields. SPCE is revealing novel phenomena and has potential applications in bioanalysis, medical diagnostics, drug discovery, and genomics. In SPCE, excited fluorophores couple with surface plasmons on a continuous thin metal film; plasmophores radiate into a higher-refractive index medium with a narrow angular distribution. Because of the directional emission, the sensitivity of this technique can be greatly improved with high collection efficiency. This review describes the unique features of SPCE. In particular, we focus on recent advances in SPCE-based analytical platforms and their applications in DNA sensing and the detection of other biomolecules and chemicals.

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.

Metal-enhanced chemiluminescence: advanced chemiluminescence concepts for the 21st century

Chemiluminescent-based detection is entrenched throughout the biosciences today, such as in blotting, analyte and protein quantification and detection. While the biological applications of chemiluminescence are forever growing, the underlying principles of using a probe, an oxidizer and a catalyst (biological, organic or inorganic) have remained mostly unchanged for decades. Subsequently, chemiluminescence-based detection is fundamentally limited by the classical photochemical properties of reaction yield, quantum yield, etc. However, over the last 5 years, a new technology has emerged which looks set to fundamentally change the way we both think about and use chemiluminescence today. Metal surface plasmons can amplify chemiluminescence signatures, while low-power microwaves can complete reactions within seconds. In addition, thin metal films can convert spatially isotopic chemiluminescence into directional emission. In this forward looking tutorial review, we survey what could well be the next-generation chemiluminescent-based technologies.

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.

Microwave-accelerated surface plasmon-coupled directional luminescence 2: a platform technology for ultra fast and sensitive target DNA detection in whole blood

The application of Microwave-Accelerated Surface Plasmon-Coupled Luminescence (MA-SPCL) to fast and sensitive DNA hybridization assays in buffer and whole blood is presented. In this regard, a model DNA hybridization assay whereby a fluorophore-labeled target ssDNA specific to human immunodeficiency, Hepatitis C (Hep C), is probed by an anchor probe immobilized on thin gold films, is driven to completion within 1 min with microwave heating, as compared to an identical assay completed in approximately 4 h at room temperature. Finite-Difference Time-Domain calculations show that gold disks are preferentially heated around the edges creating a temperature gradient along the disks, which in turn results in the larger influx of complementary DNA towards anchor probe-modified surface. Thermal images of the assay platform during microwave heating also provide additional information on the microwave heating pattern in the microwave cavity. Finally, the effects of low power microwave heating on the ability of DNA to re-hybridize with the complimentary target on the surface gold films, which allows the multiple re-use of the gold films, is demonstrated. The MA-SPCL technique offers an alternative approach to current DNA based detection technologies, especially when speed and sensitivity are required, such as in the identification of DNA or even RNA-based diseases using whole blood samples that affect human health.

Fluorescence microscopy in a microwave cavity

Optical microscopy is a well-established technique that has wide ranging applications for imaging molecular dynamics of biological systems. Typically, these applications rely on external temperature controllers to maintain or change reactions rates of these biological systems. With increasing interest in applying low power microwaves to drive biological and chemical reactions, we have combined optical and microwave based technologies and developed a fluorescence microscope in a microwave cavity. With this instrument, we have found a means to optically image biological systems inside microwave cavities during the application of microwave pulses.