Ultrasensitive densitometry detection of cytokines with nanoparticle-modified aptamers

Tumor-Induced Inflammatory Cytokines and the Emerging Diagnostic Devices for Cancer Detection and Prognosis

Chronic inflammation generated by the tumor microenvironment is known to drive cancer initiation, proliferation, progression, metastasis, and therapeutic resistance. The tumor microenvironment promotes the secretion of diverse cytokines, in different types and stages of cancers. These cytokines may inhibit tumor development but alternatively may contribute to chronic inflammation that supports tumor growth in both autocrine and paracrine manners and have been linked to poor cancer outcomes. Such distinct sets of cytokines from the tumor microenvironment can be detected in the circulation and are thus potentially useful as biomarkers to detect cancers, predict disease outcomes and manage therapeutic choices. Indeed, analyses of circulating cytokines in combination with cancer-specific biomarkers have been proposed to simplify and improve cancer detection and prognosis, especially from minimally-invasive liquid biopsies, such as blood. Additionally, the cytokine signaling signatures of the peripheral immune cells, even from patients with localized tumors, are recently found altered in cancer, and may also prove applicable as cancer biomarkers. Here we review cytokines induced by the tumor microenvironment, their roles in various stages of cancer development, and their potential use in diagnostics and prognostics. We further discuss the established and emerging diagnostic approaches that can be used to detect cancers from liquid biopsies, and additionally the technological advancement required for their use in clinical settings.

A protein triggering exponential amplification reaction enables label- and wash-free one-pot protein assay with high sensitivity

Methods capable of sensitive and facile quantification of low-abundant proteins play critical roles in disease diagnosis and treatment. Herein, on a rationally designed aptamer-based hairpin structure-switching template, we developed a protein triggering exponential amplification reaction (PTEXPAR) method. The platelet-derived growth factor BB (PDGF-BB) is used as model analyte in the current proof-of-concept experiments. This method can detect PDGF-BB specifically with a detection limit as low as 4.9 fM. Additionally, the proposed PTEXPAR strategy allows label- and wash-free one-pot quantification of protein within ~35 min. Moreover, it is potentially universal because hairpin template can be easily designed for other proteins by changing the corresponding aptamer sequence.

A signal amplification probe enhances sensitivity of antibodies and aptamers based Immuno-diagnostic assays

One major unmet need is improving the sensitivity of immune-diagnostic assays. This is particularly important in the field of biomarker discoveries and monitoring. We have established a novel signal amplification probe system enabling a highly sensitive target detection platform to be used in immuno-assays. The probe consists of a double stranded DNA that can carry a large number of signaling elements such as biotin or fluorescent molecules. The DNA probe anchors to the recognition unit, whether an antibody or an aptamer, by covalent conjugation or by a simple and rapid molecular association process. Binding curves obtained by using the DNA amplification probe are dose dependent and linear over a wide range of antigen concentration. The optimal slopes are characterized by high signals and low background increasing the assay sensitivity and reducing the limit of detection by up to 10-fold compared to biotinylated antibodies commonly used in ELISA systems. When using aptamers in combination with the amplification probe for antigen recognition, the limit of detection is comparable to that obtained by biotinylated antibodies. Biotin labeled aptamers practically cannot be used for detection of low target levels. The DNA amplification probe system enables to expand the range of diagnostic assays including clinical samples and meet research needs.

Aptamer-aptamer linkage based aptasensor for highly enhanced detection of small molecules

The multi-target colorimetric aptasensors can be easily fabricated by using two different aptamer sequences. However, there have been no research reports about improvement or enhancing of colorimetric signals based on the aggregation properties of AuNPs. Herein, we report a simple and efficient method to control and enhance the function of the multi-target aptasensor using an aptamer-aptamer linkage method. The aptasensor was developed for highly sensitive multiple-target detection of small molecules. The extension of aptamer DNA sequences using this method resulted in the enhanced analytical sensitivity of this aptasensor in sensing applications for two small molecule targets. Furthermore, the mechanism of the interaction between DNA aptamer and AuNPs was studied by measuring the zeta potential to explain the enhancement of the sensitivity of this multi-target aptasensor. The limit of detection of this multi-target aptasensor was found to be 1 nM and 37 nM for kanamycin (KAN) and chlortetracycline (CHLOR), respectively. It is 25-fold lower than in the previous report using an AuNP-based sensor for defining the limit of detection (LOD) of KAN and five times lower than the LOD for CHLOR. This aptasensor has great potential in the simultaneous detection of a wide range of KAN and CHLOR concentrations.

Aptamers in analytics

Nucleic acid aptamers are promising alternatives to antibodies in analytics. They are generally obtained through an iterative SELEX protocol that enriches a population of synthetic oligonucleotides to a subset that can recognize the chosen target molecule specifically and avidly. A wide range of targets is recognized by aptamers. Once identified and optimized for performance, aptamers can be reproducibly synthesized and offer other key features, like small size, low cost, sensitivity, specificity, rapid response, stability, and reusability. This makes them excellent options for sensory units in a variety of analytical platforms including those with electrochemical, optical, and mass sensitive transduction detection. Many novel sensing strategies have been developed by rational design to take advantage of the tendency of aptamers to undergo conformational changes upon target/analyte binding and employing the principles of base complementarity that can drive the nucleic acid structure. Despite their many advantages over antibodies, surprisingly few aptamers have yet been integrated into commercially available analytical devices. In this review, we discuss how to select and engineer aptamers for their identified application(s), some of the challenges faced in developing aptamers for analytics and many examples of their reported successful performance as sensors in a variety of analytical platforms.

Colorimetric detection based on localised surface plasmon resonance of gold nanoparticles: Merits, inherent shortcomings and future prospects

Localised surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) has been exploited for two decades in analytical science and has proven to be a powerful tool for the detection of various kinds of substances including small molecules, ions, macro biomolecules and microbes. Detection can be performed by visual colour change observations, photometry or resonance light scattering. A wide range of applications have been studied in the areas of environmental, pharmaceutical and biological analysis and clinical diagnosis. In this article, some fundamental aspects and important applications involving LSPR of AuNPs are reviewed. Several inherent shortcomings of these techniques and possible strategies to circumvent them are discussed.

Enhanced Sensitivity for Detection of HIV-1 p24 Antigen by a Novel Nuclease-Linked Fluorescence Oligonucleotide Assay

The relatively high detection limit of the Enzyme-linked immunosorbent assay (ELISA) prevents its application for detection of low concentrations of antigens. To increase the sensitivity for detection of HIV-1 p24 antigen, we developed a highly sensitive nuclease-linked fluorescence oligonucleotide assay (NLFOA). Two major improvements were incorporated in NLFOA to amplify antibody-antigen interaction signals and reduce the signal/noise ratio; a large number of nuclease molecules coupled to the gold nanoparticle/streptavidin complex and fluorescent signals generated from fluorescent-labeled oligonucleotides by the nuclease. The detection limit of p24 by NLFOA was 1 pg/mL, which was 10-fold more sensitive than the conventional ELISA (10 pg/mL). The specificity was 100% and the coefficient of variation (CV) was 7.8% at low p24 concentration (1.5 pg/mL) with various concentrations of spiked p24 in HIV-1 negative sera. Thus, NLFOA is highly sensitive, specific, reproducible and user-friendly. The more sensitive detection of low p24 concentrations in HIV-1-infected individuals by NLFOA could allow detection of HIV-1 infections that are missed by the conventional ELISA at the window period during acute infection to further reduce the risk for HIV-1 infection due to the undetected HIV-1 in the blood products. Moreover, NLFOA can be easily applied to more sensitive detection of other antigens.

Bioanalytical chemistry of cytokines--a review

Cytokines are bioactive proteins produced by many different cells of the immune system. Due to their role in different inflammatory disease states and maintaining homeostasis, there is enormous clinical interest in the quantitation of cytokines. The typical standard methods for quantitation of cytokines are immunoassay-based techniques including enzyme-linked immusorbent assays (ELISA) and bead-based immunoassays read by either standard or modified flow cytometers. A review of recent developments in analytical methods for measurements of cytokine proteins is provided. This review briefly covers cytokine biology and the analysis challenges associated with measurement of these biomarker proteins for understanding both health and disease. New techniques applied to immunoassay-based assays are presented along with the uses of aptamers, electrochemistry, mass spectrometry, optical resonator-based methods. Methods used for elucidating the release of cytokines from single cells as well as in vivo collection methods are described.

Aptamer degradation inhibition combined with DNAzyme cascade-based signal amplification for colorimetric detection of proteins

Combining the inhibition of aptamer degradation with DNA Enzyme (DNAzyme) cascade-based signal amplification, a label-free and sensitive colorimetric protein assay strategy was proposed.

Aptamer--nanoparticle-based chemiluminescence for p53 protein

A simple colorimetric biosensing technique based on the interaction of gold nanoparticles (AuNPs) with the aptamer was developed for detection of p53, a tumor suppressor protein, in the current study. Aggregation of AuNPs was induced by desorption of the p53 binding RNA aptamer from the surface of AuNPs as a result of the aptamer target interaction leading to the color change of AuNPs from red to purple. The detection limit of p53 protein by the colorimetric approach was 0.1 ng/ml after successful optimization of the amount of aptamer, AuNPs, salts, and incubation time. Furthermore, the catalytic activity of the aggregated AuNPs was greatly enhanced by chemiluminescence (CL) reaction, where the detection limit was enhanced to 10 pg/ml with a regression coefficient of R2 = 0.9907. Here the sensitivity was increased by 10-fold compared with the AuNP-based colorimetric method. Hence, the sensitivity of detection was increased by employing CL, by using the catalytic activity of aggregated AuNPs, on the luminol-hydrogen peroxide reaction. Thus, the combination of colorimetric and CL-based aptasensor can be of great advantage in increasing the sensitivity of detection for any target analyte.

Colorimetric and ultrasensitive bioassay based on a dual-amplification system using aptamer and DNAzyme

Rapid detection of ultralow amount of biomarkers in a biologically complex mixture remains a major challenge. Herein, we report a novel aptamer-based protein detection assay that integrates two signal amplification processes, namely, polymerase-mediated rolling-circle amplification (RCA) and DNA enzyme-catalyzed colorimetric reaction. The target biomarker is captured in a sandwich assay by primary aptamer-functionalized microbeads (MBs) and a secondary aptamer that is connected to a RCA primer/circular template complex. RCA reaction, which amplifies the single biomarker binding events by a factor of hundreds to thousands (the first amplification) produces a long DNA molecule containing multiple DNAzyme units. The peroxidase-like DNAzyme catalyzes the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (the second amplification), which generates a blue-green colorimetric signal. This new biosensing platform permits the ultrasensitive, label-free, colorimetric detection of biomarker in real time. Using platelet-derived growth factor B-chain (PDGF-BB) as a model system, we demonstrated that our assay can detect a protein marker specifically in a serum-containing medium, at a concentration as low as 0.2 pg/mL in ∼2 h, which rivals traditional assays such as ELISA. We anticipate this simple methodology for biomarker detection can find utility in point-of-care applications.

Highly sensitive detection of protein with aptamer-based target-triggering two-stage amplification

Highly sensitive detection of proteins is essential to biomedical research as well as clinical diagnosis. However, so far most detection methods rely on antibody-based assays and are usually laborious and time-consuming with poor sensitivity. Here, we develop a simple and sensitive method for the detection of a biomarker protein, platelet-derived growth factor BB (PDGF-BB), based on aptamer-based target-triggering two-stage amplification. With the involvement of an aptamer-based probe and an exponential amplification reaction (EXPAR) template, our method combines strand displacement amplification (SDA) and EXPAR, transforming the probe conformational change induced by target binding into two-stage amplification and distinct fluorescence signal. This detection method exhibits excellent specificity and high sensitivity with a detection limit of 9.04 × 10(-13) M and a detection range of more than 5 orders of magnitude, which is comparable with or even superior to most currently used approaches for PDGF-BB detection. Moreover, this detection method has significant advantages of isothermal conditions required, simple and rapid without multiple separation and washing steps, low-cost without the need of any labeled DNA probes. Furthermore, this method might be extended to sensitive detection of a variety of biomolecules whose aptamers undergo similar conformational changes.

Sensitive bioassay for detection of PPARα potentially hazardous ligands with gold nanoparticle probe

There are so many kinds of peroxisome proliferator-activated receptor α (PPARα) ligands with hazardous effect for human health in the environment, such as certain herbicides, plasticizers and drugs. Among these agonists, perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), and mono-(2-ethylhexyl) phthalate (MEHP) are mostly investigated due to their persistence and accumulation in environment and their potential toxicity via PPARα. This investigation aims at developing a bioassay method to detect PPARα ligands based on the ligand-receptor interaction on microplate. PPARα, which formed heterodimers with retinoid X receptor-α (RXRα), were activated by PPARα ligands to form ligands-PPARα-RXRα complexes. Then the complexes were transferred into a microplate and captured via monoclonal anti-PPARα antibody. The PPARα responsive elements (PPRE) modified-gold nanoparticle probes were captured by the ligand-PPARα-RXRα complexes immobilized on the microplate, and then could be quantified through measuring the optical density after silver enhancement. The results showed that PFOS was quantified with a linear range from 100 pM to 1 μM and the detection limit was 10 pM. In addition to PFOS, PFOA and MEHP were also quantified within a proper range through the proposed bioassay. This bioassay was compared with that of liquid chromatography tandem-mass spectrometry (LC-MS) for water spiked samples with a significant correlation (r = 0.9893). This study provides a high-throughput detection method for PPARα ligands in microplate with high sensitivity and wide linear range. It may serve as an assistant of LC-MS for prescreening of PPARα ligands like PFOS.

Highly sensitive chemiluminescence technology for protein detection using aptamer-based rolling circle amplification platform

A robust, selective and highly sensitive chemiluminescent (CL) platform for protein assay was presented in this paper. This novel CL approach utilized rolling circle amplification (RCA) as a signal enhancement technique and the 96-well plate as the immobilization and separation carrier. Typically, the antibody immobilized on the surface of 96-well plate was sandwiched with the protein target and the aptamer–primer sequence. This aptamer–primer sequence was then employed as the primer of RCA. Based on this design, a number of the biotinylated probes and streptavidin–horseradish peroxidase (SA–HRP) were captured on the plate, and the CL signal was amplified. In summary, our results demonstrated a robust biosensor with a detection limit of 10 fM that is easy to be established and utilized, and devoid of light source. Therefore, this new technique will broaden the perspective for future development of DNA-based biosensors for the detection of other protein biomarkers related to clinical diseases, by taking advantages of high sensitivity and selectivity.

Fluorescent ferritin nanoparticles and application to the aptamer sensor

We synthesized fluorescent ferritin nanoparticles (FFNPs) through bacterial expression of the hybrid gene consisting of human ferritin heavy chain (hFTN-H), spacer (glycine-rich peptide), and enhanced green (or red) fluorescent protein [eGFP (or DsRed)] genes. The self-assembly activity of hFTN-H that leads to the formation of nanoparticles (12 nm in diameter), the conformational flexibility of the C-terminus of hFTN-H, and the glycine-rich spacer enabled eGFPs (or DsReds) to be well displayed on the surface of each ferritin nanoparticle, resulting in the construction of green (or red) FFNPs [gFFNPs (or rFFNPs)]. As compared to eGFP (or DsRed) alone, it is notable that the developed FFNPs showed significantly amplified fluorescence intensity and also enhanced stability. DNA aptamers were chemically conjugated to gFFNP via each eGFP's cysteine residue that was newly introduced through site-directed mutagenesis (Ser175Cys). The DNA-aptamer-conjugated gFFNPs were used as a fluorescent reporter probe in the aptamer-based "sandwich" assay of a cancer marker [i.e., platelet-derived growth factor B-chain homodimer (PDGF-BB)] in phosphate-buffered saline buffer or diluted human serum. This is a simple two-step assay without any additional steps for signal amplification, showing that compared to the same aptamer-based assays using eGFP alone or Cy3, the detection signals, affinity of the reporter probe to the cancer marker, and assay sensitivity were significantly enhanced; i.e., the limit of detection was lowered to the 100 fM level. Although the PDGF-BB assay is reported here as a proof-of-concept, the developed FFNPs can be applied in general to any aptamer-based sandwich assays.

Optical detection systems using immobilized aptamers

Advances in the development and the applications of optical biosensing systems based on immobilized aptamers are presented. These nucleic acid sequences have been used as new molecular recognition elements to develop heterogeneous assays, biosensors and microarrays. Among different detection modes that have been employed, optical ones which are described here are among the most used. Since their first report in 1996, numerous optical detection systems using aptamers and mainly based on fluorescence have been developed. Two main approaches have been used: label-based (using fluorophore, luminophore, enzyme, nanoparticles) or aptamer label-free detection systems (e.g. surface plasmon resonance, optical resonance). Most methods are based on a labeling approach. Some targets can be optically detected using not only colorimetry, chemiluminescence or the most developed fluorescence mode but also more recent non conventional optical methods such as surface plasmon-coupled directional emission (SPCDE). The first SPCDE-based aptasensor for thrombin detection has recently been reported in 2009. Aptasensors based on surface-enhanced Raman scattering spectroscopy (SERS) which presents advantages compared to fluorescence have also been described. Different label-free techniques have recently been shown to be suitable for developing performant aptasensors or aptamer-based microarrays, such as surface plasmon resonance (SPR), diffraction grating, evanescent-field-coupled (EFC) waveguide-mode, optical resonance or Brewster angle straddle interferometry (BASI). Important advances have been realized on optical aptamer-based detection systems that appear as highly efficient devices with enormous potential.

Aptamer-based molecular recognition for biosensor development

Nucleic acid aptamers are an emerging class of synthetic ligands and have recently attracted significant attention in numerous fields. One is in biosensor development. In principle, nucleic acid aptamers can be discovered to recognize any molecule of interest with high affinity and specificity. In addition, unlike most ligands evolved in nature, synthetic nucleic acid aptamers are usually tolerant of harsh chemical, physical, and biological conditions. These distinguished characteristics make aptamers attractive molecular recognition ligands for biosensing applications. This review first concisely introduces methods for aptamer discovery including upstream selection and downstream truncation, then discusses aptamer-based biosensor development from the viewpoint of signal production.

Multianalyte electrochemical biosensor based on aptamer- and nanoparticle-integrated bio-barcode amplification

In the present work, a signal-on electrochemical sensing strategy for the simultaneous detection of adenosine and thrombin is developed based on switching structures of aptamers. An Au electrode as the sensing surface is modified with two kinds of thiolated capture probes complementary to the linker DNA that contains either an adenosine aptamer or thrombin aptamer. The capture probes hybridize with their corresponding linker DNA, which has prehybridized with the reporter DNA loaded onto the gold nanoparticles (AuNPs). The AuNP contained two kinds of bio-barcode DNA: one is complementary to the linker DNA (reporter), whereas the other is not (signal) and is tagged with different metal sulfide nanoparticles. Thus a "sandwich-type" sensing interface is fabricated for adenosine and thrombin. With the introduction of adenosine and thrombin, the aptamer parts bind with their targets and fold to form the complex structures. As a result, the bio-barcoded AuNPs are released into solution. The metal sulfide nanoparticles are measured by anodic stripping voltammetry (ASV), and the concentrations of adenosine and thrombin are proportional to the signal of either metal ion. With the dual amplification of the bio-barcoded AuNP and the preconcentration of metal ions through ASV technology, detection limits as low as 6.6 x 10(-12) M for adenosine and 1.0 x 10(-12) M for thrombin are achieved. The sensor exhibits excellent selectivity and detectability in biological samples.

Differentiation and detection of PDGF isomers and their receptors by tunable aptamer capillary electrophoresis

Tunable aptamer capillary electrophoresis (CE) techniques were developed to enable the separation and detection of platelet derived growth factor (PDGF) isomers and their receptors. Using an aptamer that formed a stable complex with the B chain but not with the A chain of PDGF, we were able to tweak the electrophoretic mobilities of the PDGF isomers for their separation. PDGF-AB bound to a single aptamer molecule was well resolved from PDGF-BB bound to two aptamer molecules. Simultaneous determination of 50 pM of two isomers was accomplished in a single analysis. Furthermore, PDGF-AB was used as a connector to bring receptor alpha and fluorescent aptamer into a single complex molecule. As a result, the formation of a (receptor alpha)-(PDGF-AB)-(fluorescent aptamer) ternary complex enabled the detection of the receptor alpha by tunable aptamer CE. A competitive assay was developed to determine receptor beta, making use of the competition between the receptor beta and fluorescent aptamer in binding to PDGF-BB. Detection limits were 0.5 nM for PDGF receptor alpha and 3 nM for receptor beta. Determination of PDGF isomers and their receptors in diluted serum samples showed no interference from the sample matrix.

Aptamer-linked assay for thrombin using gold nanoparticle amplification and inductively coupled plasma-mass spectrometry detection

We describe a sensitive and specific sandwich assay for human alpha-thrombin. The assay takes advantage of sandwich binding of two affinity aptamers for increased specificity, gold nanoparticles for signal amplification, magnetic beads for fast magnetic separation, and inductively coupled plasma mass spectrometry for ultrasensitive detection. Other proteins, such as immunoglobulin G, serum albumin, transferrin, fibrinogen, and lysozyme did not show interference with the assay for human alpha-thrombin. The detection limit of human alpha-thrombin was as low as 0.5 fmol, corresponding to 10 pM thrombin in 50 microL, and the dynamic range covered approximately 3 orders of magnitude.

Conjugated polyelectrolyte-sensitized fluorescent detection of thrombin in blood serum using aptamer-immobilized silica nanoparticles as the platform

The detection of disease-related proteins in complex biological media demands sensitive, reliable, and low-cost sensing methods. We present here an assay with aptamer-functionalized silica nanoparticles (NPs) as the sensory platform for thrombin detection in blood serum. As a result of specific protein/aptamer interaction, the presence of thrombin induces the primary aptamer-functionalized NPs to form a sandwich structure with fluorescein-labeled secondary aptamer and results in fluorescent NPs. A conjugated polyelectrolyte is further added to the NP solution to amplify the fluorescent signal. The assay shows high specificity for thrombin detection in serum. In addition, a detection limit of 1.06 nM is obtained, which is significantly improved as compared to that for many fluorescence-based thrombin detection assays. Using lysozyme as a second example, the NP-based detection strategy is shown to be generic and could be readily applicable to the detection of other proteins in practical samples.

Gold nanoparticle-based enhanced chemiluminescence immunosensor for detection of Staphylococcal Enterotoxin B (SEB) in food

Staphylococcal enterotoxins (SEs) are major cause of foodborne diseases, so sensitive detection (<1 ng/ml) methods are needed for SE detection in food. The surface area, geometric and physical properties of gold nanoparticles make them well-suited for enhancing interactions with biological molecules in assays. To take advantage of the properties of gold nanoparticles for immunodetection, we have developed a gold nanoparticle-based enhanced chemiluminescence (ECL) immunosensor for detection of Staphylococcal Enterotoxin B (SEB) in food. Anti-SEB primary antibodies were immobilized onto a gold nanoparticle surface through physical adsorption and then the antibody-gold nanoparticle mixture was immobilized onto a polycarbonate surface. SEB was detected by a "sandwich-type" ELISA assay on the polycarbonate surface with a secondary antibody and ECL detection. The signal from ECL was read using a point-of-care detector based on a cooled charge-coupled device (CCD) sensor or a plate reader. The system was used to test for SEB in buffer and various foods (mushrooms, tomatoes, and baby food meat). The limit of detection was found to be approximately 0.01 ng/mL, which is approximately 10 times more sensitive than traditional ELISA. The gold nanoparticles were relatively easy to use for antibody immobilization because of their physical adsorption mechanism; no other reagents were required for immobilization. The use of our simple and inexpensive detector combined with the gold nanoparticle-based ECL method described here is adaptable to simplify and increase sensitivity of any immunological assay and for point-of-care diagnostics.

Plastic-adherent DNA aptamer-magnetic bead and quantum dot sandwich assay for Campylobacter detection

DNA aptamers were developed against MgCl(2)-extracted surface proteins from Campylobacter jejuni. The two highest affinity aptamers were selected for use in a magnetic bead (MB) and red quantum dot (QD)-based sandwich assay scheme. The assay was evaluated using both heat-killed and live C. jejuni and exhibits detection limits as low as an average of 2.5 colony forming unit (cfu) equivalents in buffer and 10-250 cfu in various food matrices. The assay exhibits low cross-reactivity with bacterial species outside the Campylobacter genus, but exhibits substantial cross-reactivity with C. coli and C. lari. The assay was evaluated with a spectrofluorometer and a commercially available handheld fluorometer, which yielded comparable detection limits and ranges. Remarkably, the sandwich assay components adhere to the inside face of polystyrene cuvettes even in food matrices near neutral pH, thereby enabling a rapid homogeneous assay, because fluorescence is concentrated to a small, thin planar area and background fluorescence from the bulk solution is minimized. The plastic cuvette-adherent technology coupled to a sensitive handheld fluorometer may enable rapid (15-20 min), portable detection of foodborne pathogens from "farm-to-fork" by obviating the slow enrichment culture phase used by other food safety tests.

Development of an optical RNA-based aptasensor for C-reactive protein

The development of a RNA-aptamer-based optical biosensor (aptasensor) for C-reactive protein (CRP) is reported. CRP is an important clinical biomarker; it was the first acute-phase protein to be discovered (1930) and is a sensitive systemic marker of inflammation and tissue damage. It has also a prognostic value for patients with acute coronary syndrome. The average concentration of CRP in serum is 0.8 ppm and it increases in response to a variety of inflammatory stimuli, such as trauma, tissue necrosis, infection and myocardial infarction. The interaction between the 44-base RNA aptamer and the target analyte CRP is studied. In particular, the influence of the aptamer immobilization procedure (chemistry, length, concentration), as well as the binding conditions, i.e., the influence on the binding of different buffers, the presence of Ca2+ ion and the specificity (against human serum albumin) have been evaluated. Using the best working conditions, we achieved a detection limit of 0.005 ppm, with good selectivity towards human serum albumin. Some preliminary experiments in serum are reported.