Highly sensitive immunoassay of free prostate-specific antigen in serum using europium(III) nanoparticle label technology

Dye-sensitized lanthanide containing nanoparticles for luminescence based applications

Due to their exceptional luminescent properties, lanthanide (Ln) complexes represent a unique palette of probes in the spectroscopic toolkit. Their extremely weak brightness due to forbidden Ln electronic transitions can be overcome by indirect dye-sensitization from the antenna effect brought by organic ligands. Despite the improvement brought by the antenna effect, (bio)analytical applications with discrete Ln complexes as luminescent markers still suffers from low sensitivity as they are limited by the complex brightness. Thus, there is a need to develop nano-objects that cumulate the spectroscopic properties of multiple Ln ions. This review firstly gives a brief introduction of the spectral properties of lanthanides both in complexes and in nanoparticles (NPs). Then, the research progress of the design of Ln-doped inorganic NPs with capping antennas, Ln-complex encapsulated NPs and Ln-complex surface functionalized NPs is presented along with a summary of the various photosensitizing ligands and of the spectroscopic properties (excited-state lifetime, brightness, quantum yield). The review also emphasizes the problems and limitations encountered over the years and the solutions provided to address them. Finally, a comparison of the advantages and drawbacks of the three types of NP is provided as well as a conclusion about the remaining challenges both in the design of brighter NPs and in the luminescence based applications.

Highly sensitive chromatographic time-resolved fluoroimmunoassay for rapid onsite detection of streptomycin in milk

Antibiotic residues are major contaminants in milk because of their use in agriculture and animal husbandry. In particular, streptomycin, an aminoglycoside antibiotic, is a potential risk to consumers because of its ototoxicity, anaphylaxis, and growth inhibition. Herein, monoclonal antibodies for streptomycin were conjugated with europium microspheres to serve as detection probes for the development of a chromatographic time-resolved fluoroimmunoassay to detect streptomycin residues in milk. The method had a low detection limit of 0.58 µg/kg, a linear range of 0.8 to 6.25 μg/kg, and substantial recovery, from 85.6 to 108.3%. It showed slight cross-reactivity with another aminoglycoside analog. Strong correlations between the results of established chromatographic time-resolved fluoroimmunoassay and ultra-performance liquid chromatography-tandem mass spectrometry indicated that the established fluoroimmunoassay is a reliable method for rapid onsite detection of streptomycin in milk and it has great potential in food safety monitoring.

A novel bioassay for the monitoring of carcinoembryonic antigen in human biofluid using polymeric interface and immunosensing method

Carcinoembryonic antigen (CEA) is a member of a family of cell surface glycoproteins. Recognition of CEA is needed to monitor the physiological status of the patient for treatment and also it is important to assess the severity of the disease. In this work, we reported a novel sandwich-type electrochemical immunosensor based on gold nanoparticles functionalized cysteamine-glutaraldehyde (AuNPs-CysA-GA) and it successfully designed to detection of the CEA biomarker in a human plasma sample. The AuNPs-CysA-GA provides a large surface area for the effective immobilization of CEA antibody, as well as it ascertains the bioactivity and stability of immobilized CEA antigens. Biotinylated-anti-CEA antibody (Ab1) was immobilized on the surface of glassy carbon electrode (GCE) modified AuNPs-CysA-GA. Also, secondary antibody (HRP-Ab2) was costed immobilized to complete the sandwich part of immunosensor. Field emission scanning electron microscope (FE-SEM and EDS), was employed to monitor the sensor fabrication procedure. The immunosensor was used for the detection of CEA using differential pulse voltammetry (DPVs) technique. The proposed interface led to enhancement of accessible surface area for immobilizing high amount of anti-CEA antibody, increasing electrical conductivity, boosting stability, and biocompatibility. Finally, the low limit of quantitation (LLOQ) of the proposed immunosensor was obtained as 7 ng/mL with the linear range of 0.001-5 μg/L. The proposed immunoassay was successfully applied for the monitoring of the CEA in unprocessed human plasma samples. Obtained results paved that the proposed bioassay can be used as a novel bioassay for the clinical diagnosis of cancer based on CEA monitoring.

A novel label-free colorimetric aptasensor for sensitive determination of PSA biomarker using gold nanoparticles and a cationic polymer in human serum

In this colorimetric assay for sensitive detection of prostate specific antigen (PSA) tumor marker, adsorbed non-thiolated poly-Adenine aptamer (polyA Apt) on the gold nanoparticles (AuNPs) surface was used. By incubating the AuNPs and the PSA specific aptamer prior to target addition, polyA Apt adsorbed on the gold nanoparticles and could bind the target while preventing non-specific interactions. Adsorbed polyA Apt on the AuNPs prevents aggregation of them by poly(diallyldimethylammoniumchloride) (PDDA). Upon the addition of PSA, it bind to the polyA Apt and induce the formation of a secondary structure. Therefore, interaction between polyA Apt and PDDA is repressed and PDDA induce the aggregation of the AuNPs. This analytical platform produces a remarkable optical signal in the absence and presence of PSA that accompanied by a color change from red to blue. This effect as a sensing strategy can be observed with naked eyes and quantified by colorimetry via measurement of the ratio of absorbances at 680 nm and 520 nm. Fabricated aptasensor for detection of PSA is linear in the concentration range of 0.1-100 ng/ml with 20 pg/ml as the limit of detection (S/N = 3). Because of the selectively recognized for PSA in the presence of other interfering substances, this proposed assay applied to real samples for the rapid screening of PSA.

Identification of bacteria in juice/lettuce using magnetic nanoparticles and selected reaction monitoring mass spectrometry

Ensuring food safety requires a rapid and reliable method for detecting food-borne pathogens. Mass spectrometry has been demonstrated as a powerful tool to classify pure bacterial species. However, matrix interference from food backgrounds may lead to false results because of the suppression of microbial signals. It is useful to develop a method for bacterial enrichment and marker identification in food samples. Magnetic zirconia nanoparticles were used to concentrate spiked microorganisms from apple juice/lettuce under specific conditions (pH 4.5). Bacterial identification was achieved using nanoLC-MS. Selected reaction monitoring of bacteria-related peptides was applied for the first time to identify bacteria including Staphylococcus aureus and Escherichia coli. This study presents an accurate means for bacterial identification in food matrixes using MS. The analysis time is less than 90 min and the minimum concentration of E. coli detected was 5 × 103 CFU/mL. The interaction between bacteria and the magnetic nanoparticles was electrostatic and nonspecific, in contrast to immunoassays which require specific antibodies. The targeted peptide analysis focuses on the bacterial markers, thus significantly simplifying the analysis and leading to an accurate identification of bacteria.

A novel classification of prostate specific antigen (PSA) biosensors based on transducing elements

During the last few decades, there has been a tremendous rise in the number of research studies dedicated towards the development of diagnostic tools based on bio-sensing technology for the early detection of various diseases like cardiovascular diseases (CVD), many types of cancer, diabetes mellitus (DM) and many infectious diseases. Many breakthroughs have been developed in the areas of improving specificity, selectivity and repeatability of the biosensor devices. Innovations in the interdisciplinary areas like biotechnology, genetics, organic electronics and nanotechnology also had a great positive impact on the growth of bio-sensing technology. As a product of these improvements, fast and consistent sensing policies have been productively created for precise and ultrasensitive biomarker-based disease diagnostics. Prostate-specific antigen (PSA) is widely considered as an important biomarker used for diagnosing prostate cancer. There have been many publications based on various biosensors used for PSA detection, but a limited review was available for the classification of these biosensors used for the detection of PSA. This review highlights the various biosensors used for PSA detection and proposes a novel classification for PSA biosensors based on the transducer type used. We also highlight the advantages, disadvantages and limitations of each technique used for PSA biosensing which will make this article a complete reference tool for the future researches in PSA biosensing.

An integrated lab-on-a-chip-based electrochemical biosensor for rapid and sensitive detection of cancer biomarkers

Recent advances in the area of biosensor technology and microfluidic applications have enabled the miniaturisation of the sensing platforms. Here we describe a new integrated and fully automated lab-on-a-chip-based biosensor device prototype (MiSens) that has potential to be used for point-of-care cancer biomarker testing. The key features of the device include a new biochip, a device integrated microfluidic system and real-time amperometric measurements during the flow of enzyme substrate. For ease of use, a new plug and play type sensor chip docking station has been designed. This system allows the formation of an ∼7 μL capacity flow cell on the electrode array with the necessary microfluidic and electronic connections with one move of a handle. As a case study, the developed prototype has been utilised for the detection of prostate-specific antigen (PSA) level in serum that is routinely used as a biomarker for the diagnosis of prostate cancer. The patient samples from a nearby hospital have been collected and tested using the MiSens device, and the results have been compared to the hospital results. The obtained results indicate the potential of the MiSens device as a useful tool for point-of-care testing. Graphical abstract Microfluidics integrated and automated electrochemical biosensor device "MiSens" has been designed and fabricated by a multidisciplinary team and utilised to detect PSA from clinical samples.

Lanthanide-doped luminescent nano-bioprobes for the detection of tumor markers

Sensitive and specific biodetection of tumor markers is essential for early-stage cancer diagnosis and therapy, and will ultimately increase the patient survival rate. As a new generation of luminescent bioprobes, lanthanide (Ln(3+))-doped inorganic luminescent nanoparticles have attracted considerable interest for a variety of biomedical applications due to their superior physicochemical properties. In this feature article, we provide a brief overview of the most recent advances in the development of Ln(3+)-doped luminescent nano-bioprobes and their promising applications for in vitro detection of tumor markers with an emphasis on the establishment of state-of-the-art assay techniques, such as heterogeneous time-resolved (TR) luminescent bioassay, dissolution-enhanced luminescent bioassay, upconversion (UC) luminescent bioassay, homogeneous TR Förster resonance energy transfer (TR-FRET) and UC-FRET bioassays. Some future prospects and efforts towards this emerging field are also envisioned.

Efficient separation and sensitive detection of Listeria monocytogenes using an impedance immunosensor based on magnetic nanoparticles, a microfluidic chip, and an interdigitated microelectrode

Listeria monocytogenes continues to be a major foodborne pathogen that causes food poisoning, and sometimes death, among immunosuppressed people and abortion among pregnant women. In this study, magnetic nanoparticles with a diameter of 30 nm were functionalized with anti-L. monocytogenes antibodies via biotin-streptavidin bonds to become immunomagnetic nanoparticles (IMNPs) to capture L. monocytogenes in a sample during a 2-h immunoreaction. A magnetic separator was used to collect and hold the IMNPs-L. monocytogenes complex while the supernatants were removed. After the washing step, the nanoparticle-L. monocytogenes complex was separated from the sample and injected into a microfluidic chip. The impedance change caused by L. monocytogenes was measured by an impedance analyzer through the interdigitated microelectrode in the microfluidic chip. For L. monocytogenes in phosphate-buffered saline solution, up to 75% of the cells in the sample could be separated, and as few as three to five cells in the microfluidic chip could be detected, which is equivalent to 10(3) CFU/ml of cells in the original sample. The detection of L. monocytogenes was not interfered with by other major foodborne bacteria, including E. coli O157:H7, E. coli K-12, L. innocua, Salmonella Typhimurium, and Staphylococcus aureus. A linear correlation (R(2) = 0.86) was found between the impedance change and the number of L. monocytogenes in a range of 10(3) to 10(7) CFU/ml. Equivalent circuit analysis indicated that the impedance change was mainly due to the decrease in medium resistance when the IMNPs-L. monocytogenes complexes existed in mannitol solution. Finally, the immunosensor was evaluated with food sample tests; the results showed that, without preenrichment and labeling, 10(4) and 10(5) CFU/ml L. monocytogenes in lettuce, milk, and ground beef samples could be detected in 3 h.

Prostate specific antigen in cord blood

Recent studies have demonstrated the presence of prostate specific antigen (PSA) in cord blood of male as well as female babies. The placental progesterone and estradiol up-regulate the synthesis and secretion of PSA in Placenta. This PSA is presumed to play a role in intrauterine growth of fetus by virtue of its proteolytic action on several substrates including insulin-like-growth-factor-binding-protein-3, insulin chains and Interleukin-2. This study was planned with the objective of correlating the levels of PSA in cord blood to gestation at delivery, the type of delivery and gender of the fetus. Fifty-seven cord blood samples were collected from the umbilical cord during delivery or mid-trimester abortion and analyzed for PSA using 'Active PSA DSL-9700 ultra sensitive' kit employing two-site immuno-radiometric assay principle and having a detection limit of 0.001 ng/ml. Mean PSA levels in cord blood were found to be 0.112 ± 0.027 ng/ml. The concentration of PSA in cord blood was found to be higher in case of higher gestational age, male baby and operative delivery. 50 % of cord bloods for female babies had PSA below detection limit (range <0.001-0.460 ng/ml), while all the male samples had detectable PSA (range 0.11-0.973 ng/ml). Higher Progesterone levels found in prenatal maternal blood in case of male babies may be responsible for the higher cord blood PSA. Mean cord blood PSA was 0.150 ± 0.150 ng/ml in forceps delivery and 0.078 ± 0.012 ng/ml in normal vaginal delivery. Forceps delivery causes much more stress and strain as compared to a normal vaginal delivery, resulting in increased levels of adrenal glucocorticoids, and therefore, higher cord blood PSA.

Nanobiotechnologies for the detection and reduction of pathogens

Advances in the manipulation of nanomaterials has permitted the development of nanobiotechnology with enhanced sensitivities and improved response times. Low levels of infection of the major pathogens require the need for sensitive detection platforms and the properties of nanomaterials make them suitable for the development of assays with enhanced sensitivity, improved response time and increased portability. Nanobiotechnologies focusing on the key requirements of signal amplification and pre-concentration for the development of sensitive assays for food-borne pathogen detection in food matrices will be described and evaluated. The potential that exists for the use of nanomaterials as antimicrobial agents will also be examined.

Lanthanide-based time-resolved luminescence immunoassays

The sensitive and specific detection of analytes such as proteins in biological samples is critical for a variety of applications, for example disease diagnosis. In immunoassays a signal in response to the concentration of analyte present is generated by use of antibodies labeled with radioisotopes, luminophores, or enzymes. All immunoassays suffer to some extent from the problem of the background signal observed in the absence of analyte, which limits the sensitivity and dynamic range that can be achieved. This is especially the case for homogeneous immunoassays and surface measurements on tissue sections and membranes, which typically have a high background because of sample autofluorescence. One way of minimizing background in immunoassays involves the use of lanthanide chelate labels. Luminescent lanthanide complexes have exceedingly long-lived luminescence in comparison with conventional fluorophores, enabling the short-lived background interferences to be removed via time-gated acquisition and delivering greater assay sensitivity and a broader dynamic range. This review highlights the potential of using lanthanide luminescence to design sensitive and specific immunoassays. Techniques for labeling biomolecules with lanthanide chelate tags are discussed, with aspects of chelate design. Microtitre plate-based heterogeneous and homogeneous assays are reviewed and compared in terms of sensitivity, dynamic range, and convenience. The great potential of surface-based time-resolved imaging techniques for biomolecules on gels, membranes, and tissue sections using lanthanide tracers in proteomics applications is also emphasized.

Detection of anthrax toxin by an ultrasensitive immunoassay using europium nanoparticles

We developed a europium nanoparticle-based immunoassay (ENIA) for the sensitive detection of anthrax protective antigen (PA). The ENIA exhibited a linear dose-dependent pattern within the detection range of 0.01 to 100 ng/ml and was approximately 100-fold more sensitive than enzyme-linked immunosorbent assay (ELISA). False-positive results were not observed with serum samples from healthy adults, mouse plasma without PA, or plasma samples collected from mice injected with anthrax lethal factor or edema factor alone. For the detection of plasma samples spiked with PA, the detection sensitivities for ENIA and ELISA were 100% (11/11 samples) and 36.4% (4/11 samples), respectively. The assay exhibited a linear but qualitative correlation between the PA injected and the PA detected in murine blood (r=0.97731; P<0.0001). Anthrax PA was also detected in the circulation of mice infected with spores from a toxigenic Sterne-like strain of Bacillus anthracis, but only in the later stages of infection. These results indicate that the universal labeling technology based on europium nanoparticles and its application may provide a rapid and sensitive testing platform for clinical diagnosis and laboratory research.

Nanostructures as analytical tools in bioassays

We critically evaluate the usefulness of different nanostructures described as labels, nanoscaffolds or separation media in immunoassays and nucleic-acid hybridization assays. Many of the great number of publications describe only theoretical aspects of using these nanostructures or nanoparticles, but do not verify their applicability in the presence of potential interferents that can be present in the sample matrix. We attempt a systematic study of the advantages and the limitations of using these new reagents in bioassays, the different assay formats for individual and multiplexed detection, and the capability of these assays in analyzing real samples.

Multiple Fluorescent Labeling of Silica Nanoparticles with Lanthanide Chelates for Highly Sensitive Time-Resolved Immunofluorometric Assays

Background: Time-resolved immunofluorometric assays (TrIFA) using lanthanide-labeled nanoparticles have greatly increased the sensitivity of immunoassays. Current labeling strategies, however, use either physical doping of lanthanide chelates into preformed nanoparticles or covalent linking of lanthanide chelates to precursors used for making nanoparticles; both these strategies have drawbacks.

Methods: Luminescent Eu(III) and Tb(III) chelates were covalently coated on the surface of preformed silica nanoparticles to which detection antibodies or bridging proteins for antibody binding were conjugated. We used the resulting conjugates in TrIFA for detection of hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg), both individually and simultaneously. We compared the results of the newly established method with results of an ELISA for serum samples. Positive samples identified by TrIFA but not by ELISA were confirmed by additional assays, including real-time PCR detection of viral DNA.

Results: The prepared nanoparticle conjugates were homogeneous in size, at ∼55 (5) nm in diameter [mean (SD)], were stable for long-time storage (&gt;2 years), and contained more chelates [6.86 × 105 for Eu(III), 4.73 × 104 for Tb(III)] per nanoparticle than particles made as previously reported. The TrIFA established for HBsAg had a comparable or lower detection limit (0.0092 μg/L) than existing nanoparticle-based TrIFA or ELISA. The TrIFA for HBeAg had a much lower detection limit [10.0 National Centre Unit (NCU)/L] than ELISA and detected HBeAg in 5 samples missed by the ELISA method. Simultaneous TrIFA for both HBsAg and HBeAg was achieved with detection limits (0.033 μg/L for HBsAg and 27.0 NCU/L for HBeAg) close to those of the individual assays.

Conclusions: Covalent surface labeling of silica nanoparticles with lanthanide chelates provides good fluorescent labels that can be used in TrIFA for highly sensitive and robust detection of clinical targets.

Enhancing antimicrobial activity of lysozyme against Listeria monocytogenes using immunonanoparticles

The antimicrobial efficacy of lysozyme may be reduced by undesirable interactions with food components and nontarget bacteria. Immunonanoparticles, i.e., nanoparticles functionalized with pathogen-specific antibodies, may serve as an antimicrobial carrier for improving the stability and activity of antimicrobials in foods. The objective of this research was to study the antimicrobial activity of lysozyme-carrying immunonanoparticles against Listeria monocytogenes. Polystyrene nanoparticles with active carboxyl groups were conjugated with anti-L. monocytogenes antibody through covalent bonding. Enhanced antimicrobial activity of lysozyme-carrying immunonanoparticles was achieved when 0.04 microg/ml anti-L. monocytogenes antibody was used for coating nanoparticles and the resulting immunonanoparticles were then coated with lysozyme for 6 h. Lysozyme-carrying immunonanoparticles with a final concentration of 35 microg/ml reduced L. monocytogenes Scott A populations from ca. 5 log CFU/ml to below the detection limit (< 1 log CFU/ml) within 3 h. However, when 500 microg/ml lysozyme was used, ca. 2 log CFU/ml concentration of L. monocytogenes cells remained culturable after 5 h of treatment. The addition of lysozyme-carrying immunonanoparticles (37 microg/ml) to an L. monocytogenes solution of ca. 7 log CFU/ml for 6 h resulted in 0.9-, 1.0-, and 2.3-log greater reductions of L. monocytogenes cells than that achieved with lysozyme-carrying nanoparticles and lysozyme solutions of 500 and 50 microg/ml, respectively. Overall, lysozyme-carrying immunonanoparticles had significantly more anti-L. monocytogenes activity (P < 0.05) than did lysozyme-carrying nanoparticles and lysozyme solutions at higher concentrations (500 and 50 microg/ml). Our study revealed that the use of lysozyme-carrying immunonanoparticles is more effective than direct addition of lysozyme for inactivating L. monocytogenes in nutrient broth.

Comparison of infrared-excited up-converting phosphors and europium nanoparticles as labels in a two-site immunoassay

Research in the field of immunoassays and labels used in the detection has been recently focused on particulate reporters, which possess very high specific activity that excludes the label as a sensitivity limiting factor. However, the large size and shape of the particulate labels may produce additional problems to immunoassay performance. The aim of this work was to study with two identical non-competitive two-site immunoassays whether up-converting phosphor (UCP) particles are comparable in performance with europium(III) chelate-dyed nanoparticles as particulate labels. In addition we strived to verify the common assumption of the photostability of up-converting phosphor particles supporting their potential applicability in imaging. Detection limits in two-site immunoassay for free prostate-specific antigen (free-PSA) were 0.53 ng L(-1) and 1.3 ng L(-1) using two different up-converting phosphors and 0.16 ng L(-1) using europium(III) nanoparticle. Large size distribution and non-specific binding of up-converting phosphor particles caused assay variation in low analyte concentrations and limited the analytical detection limit. The non-specific binding was the major factor limiting the analytical sensitivity of the immunoassay. The results suggests the need for nanoscaled and uniformly sized UCP-particles to increase the sensitivity and applicability of up-converting phosphor particles. Anti-Stokes photoluminescence of up-converting phosphor particles did not photobleach when measured repeatedly, on the contrary, the time-resolved fluorescence of europium nanoparticles photobleached relatively rapidly.

Biosensor developments: application to prostate-specific antigen detection

Prostate-specific antigen (PSA) is the best serum marker currently available for the detection of prostate cancer and is the forensic marker of choice for determining the presence of azoospermic semen in some sexual assault cases. Most current assays for PSA detection are processed on large analyzers at dedicated testing sites, which require that samples be sent away for testing. This leads to delays in patient management and increased administration costs. The recent emphasis placed on the need for point-of-care patient management has led to the development of novel biosensor detection strategies that are suitable for the miniaturization of assays for various targets including PSA. This review highlights the current and novel analytical technologies used for PSA detection, which will benefit clinicians, patients and forensic workers in the future.

Practical time-gated luminescence flow cytometry. I: Concepts

The method of time-gated detection of long-lifetime (1-2,000 micros) luminescence-labeled microorganisms following rapid excitation pulses has proved highly efficient in suppressing nontarget autofluorescence (<0.1 micros), scatterings, and other prompt stray light (Hemmila and Mukkala, Crit Rev Clin Lab Sci 2001;38:441-519). The application of such techniques to flow cytometry is highly attractive but there are significant challenges in implementing pulsed operation mode to rapid continuous flowing sample to achieve high cell analysis rates (Leif R, Vallarino L, Rare-earth chelates as fluorescent markers in cell separation and analysis, In: Cell Separation Science and Technology, ACS Symposium Series 464, American Chemical Society, 1991, pp 41-58; Condrau et al., Cytometry 1994;16:187-194; Condrau et al., Cytometry 1994;16:195-205; Shapiro HM, Improving signals from labels: Amplification and other techniques, In: Practical Flow Cytometry, 4th ed., Wiley, New York, 2002, p 345). We present here practical approaches for achieving high cell analysis rates at 100% detection efficiency, using time-gated luminescence (TGL) flow cytometry. In particular, we report that new-generation UV LEDs are practical sources in TGL flow cytometry. Spatial effects of long-lived luminescence from the target fluorophore in a fast-flowing sample stream have been investigated; excitation and detection requirements in TGL flow cytometry were theoretically analyzed; two practical approaches, a triggered model and a continuous flow-section model, were considered as a function of flow speed, sizes and relative positions of the excitation/detection spots, label lifetime, excitation pulse duration/intensity, and detection duration. A particular configuration using LED excitation to detect europium dye-labeled targets in such a system has been modeled in detail. In the triggered model, TGL mode is confined to a low repetition rate (<1 kHz) and engaged only while a target particle is present in the excitation zone. In the flow-section model, TGL mode is engaged continuously at high repetition rates to permit much higher cell arrival rates. The detection of 5.7-microm europium calibration beads in a UV LED-excited TGL flow cytometer has been shown to be feasible with a calculated signal-to-background ratio up to 11:1.

Protein detection using biobarcodes

Over the past 50 years the development of assays for the detection of protein analytes has been driven by continuing demands for higher levels of sensitivity and multiplexing. The result has been a progression of sandwich-type immunoassays, starting with simple radioisotopic, colorimetric, or fluorescent labeling systems to include various enzymatic or nanostructure-based signal amplification schemes, with a concomitant sensitivity increase of over 1 million fold. Multiplexing of samples and tests has been enabled by microplate and microarray platforms, respectively, or lately by various molecular barcoding systems. Two different platforms have emerged as the current front-runners by combining a nucleic acid amplification step with the standard two-sided immunoassay. In both, the captured protein analyte is replaced by a multiplicity of oligonucleotides that serve as surrogate targets. One of these platforms employs DNA or RNA polymerases for the amplification step, while detection is by fluorescence. The other is based on gold nanoparticles for both amplification as well as detection. The latter technology, now termed Biobarcode, is completely enzyme-free and offers potentially much higher multiplexing power.

Disposable noncompetitive immunosensor for free and total prostate-specific antigen based on capacitance measurement

This work reports on the successful integration of a one-step lateral flow immunoassay format and impedance detection of the specific affinity event using an electrochemical transducer coated with a pH-sensitive polymer layer. This approach was particularly applied to the development of a rapid single-use immunosensor for the sensitive detection of free and total prostate-specific antigen (f-PSA, t-PSA) tumor marker. Strips of nitrocellulose membrane were coated with appropriate antibodies to f-PSA and t-PSA and used as solid supports for the performance of noncompetitive immunoassays where PSA was allowed to react with both immobilized anti-PSA antibody and anti-PSA urease enzyme conjugate for less than 1 min. An additional piece within the device consisting of a storage blister filled with a urea solution allowed the rapid washing of unbound species from the membrane strips and simultaneous urea hydrolysis catalyzed by the bound urease conjugate in an automatic fashion. The hydrolysis of urea increased the pH of the reaction media, which in turn induced a breakdown of the polymer layer on the transducer and a consequent measurable change in capacitance of the system. This was easily recorded at a given frequency over a 30-min period. Overall, we describe a one-step immunosensor prototype that exhibits enough sensitivity to detect both forms of PSA at concentration levels down to 3 ng/mL. With the possibility of being portable and considering its ease of use, robustness, and simplicity, this device has great potential as a tool for the screening and early detection of prostate cancer.

A simple hybridoma screening method for high-affinity monoclonal antibodies using the signal ratio obtained from time-resolved fluorescence assay

In hybridoma screening, quantitative kinetic evaluation is difficult since the concentration of each antibody in the hybridoma supernatant is unknown. From modeling calculations, we hypothesized that the ratio of two different antigen-antibody concentrations might allow discrimination of high-affinity monoclonal antibodies irrespective of the antibody concentration. Using anti-alpha-fetoprotein monoclonal antibodies of known affinity, we set the signal ratio of a time-resolved assay at >0.1, in which the antigen concentrations were 10 and 100 ng/mL. From anti-alpha-fetoprotein hybridoma screening with this assay, it was possible to effectively select high-affinity monoclonal antibodies with KD values below 1x10(-8) M. High-sensitivity sandwich enzyme-linked immunosorbent assay which detects domain III of alpha-fetoprotein has been established using selected high-affinity monoclonal antibodies. This screening method is useful for selection of high-affinity monoclonal antibodies of potential diagnostic value.

Application of europium(III) chelate-dyed nanoparticle labels in a competitive atrazine fluoroimmunoassay on an ITO waveguide

We have demonstrated the use of an optical indium tin oxide (ITO) (quartz) waveguide as a new platform for immunosensors with fluorescent europium(III) chelate nanoparticle labels (Seradyn) in a competitive atrazine immunoassay. ITO as a solid surface facilitated the successful use of particulate labels in a competitive assay format. The limit of detection in the new nanoparticle assay was similar to a conventional ELISA. The effect of particle size on bioconjugate binding kinetics was studied using three sizes of bioconjugated particle labels (107, 304, and 396nm) and a rabbit IgG/anti-IgG system in a 96-well plate. A decrease in particle size resulted in faster binding but did not increase the assay sensitivity. Flux calculations based on the particle diffusivity prove that faster binding of the small particles in this study was primarily due to diffusion kinetics and not necessarily to a higher density of antibodies on the particle surface. The results suggest that ITO could make a good platform for an optical immunosensor using fluorescent nanoparticle labels in a competitive assay format for small molecule detection. However, when used in combination with fluorescent particulate labels, a highly sensitive excitation/detection system needs to be developed to fully utilize the kinetic advantage from small particle size. Different regeneration methods tested in this study showed that repeated washings with 0.1 M glycine-HCl facilitated the reuse of the ITO waveguide.

Lanthanide complex-based fluorescence label for time-resolved fluorescence bioassay

Different from organic fluorescence dyes, fluorescent lanthanide complexes have the fluorescence properties of long fluorescence lifetime, large Stokes shift and sharp emission profile, which makes them favorable be used as the fluorescent labeling reagents for microsecond time-resolved fluorescence bioassay. Lanthanide complex-based fluorescence labels have been successfully used for highly sensitive time-resolved fluorescence immunoassay, DNA hybridization assay, cell activity assay, and bio-imaging microscopy assay. Since the technique allows easy distinction of the specific fluorescence signal of the long-lived label from short-lived background noises associated with biological samples, scattering lights (Tyndall, Rayleigh and Raman scatterings) and the optical components (cuvettes, filters and lenses), the sensitivity of fluorescence bioassay has been remarkably improved. This paper summarized the recent developments of lanthanide complex-based fluorescence labels and their applications in time-resolved fluorescence bioassays mainly based on the authors' researches and relative publications.

Nanoprobe-Based Affinity Mass Spectrometry for Selected Protein Profiling in Human Plasma

In recent decades, magnetic nanoparticles have emerged as a promising new platform in biomedical applications, particularly bioseparations. We have developed an immunoassay using antibody-conjugated magnetic nanoparticles as an efficient affinity probe to simultaneously preconcentrate and isolate targeted antigens from biological media. We combined this probe with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI MS) to profile proteins in diluted human plasma. The nanoparticles were designed to detect several disease-associated proteins and could be used directly in MALDI MS without an elution step, thereby facilitating multiple antigen screening and the characterization of antigen variants. Plasma antigens bound rapidly (approximately 10 min) to the antibody-conjugated nanoparticles, allowing the assay to be performed within 20 min. With sensitivity of detection in the femtomole range, the nanoscale immunoassay is superior to assays using microscale particles. We applied our method to comparative protein profiling of patients with gastric cancer and healthy individuals and found differential protein expression levels associated with the disease as well as individuals. Given the flexibility of manipulating functional groups on the nanoprobes, their low cost, robustness, and simplicity of the assay, our approach shows promise for targeted proteome profiling in clinical settings.

Design of europium(III) complexes with high quantum yield

This work describes a rational planning of a new light-conversion molecular device with high quantum yield. For this, we made modifications in the 3-amino-2-carboxypyridine and 3-amino-2-carboxypirazine acid ligands, generating eight different complexes. Theoretical methods have been used to calculate the quantum yield of each of the complexes. We first used the Sparkle model to calculate the ground-state geometries of the eight complexes. These data were used to perform theoretical predictions of the energy transitions using the INDO/S-CI method. After having obtained the geometry and the energy transitions, the energy transfer rates and quantum yield were calculated using a theoretical approach based on the application of the 4f-4f transition theory. The results show that the modifications in the 3-amino-2-carboxypyridine ligand had generated three complexes with high quantum yield (about 52.8, 51.6 and 52.8%). On the other hand, the modifications in the 3-amino-2-carboxypirazine led to only one complex with quantum yield larger than 50%, but it is the most efficient complex projected.

Progress in Lanthanides as Luminescent Probes

Lanthanides have recently found applications in different fields of biomolecular and medical research. Luminescent lanthanide chelates have created interest mainly due to their unique luminescent properties, such as their long Stokes' shift and exceptional decay times allowing efficient temporal discrimination of background interferences in the assays, such as immunoassays. Recently, new organometallic complexes have been developed giving opportunities to novel applications, in heterogeneous and homogeneous immunoassays, DNA hybridization assays, high-throughput screening as well as in imaging. In addition, encapsulating the chelates into suitable matrix in beads enables the use of new members of lanthanides extending the emission wavelength to micrometer range and decays from a few microseconds to milliseconds. As the luminescence is derived from complicated intra-chelate energy transfer, it also gives novel opportunities to exploit these levels in different types of energy transfer based applications. This review gives a short overview of recent development of lanthanide chelate-labels and discusses in more details of energy levels and their exploitation in new assay formats.

Synthesis, Characterization, and Application of Eu(III), Tb(III), Sm(III), and Dy(III) Lanthanide Chelate Nanoparticle Labels

Preparation and characterization of europium(III), terbium(III), samarium(III), and dysprosium(III) polystyrene nanoparticle labels with lanthanide-specific fluorescence properties has been presented. Emulsion copolymerization of styrene and acrylic acid was used to synthesize uniform-sized nanoparticles approximately 45 nm in diameter. Europium(III) and samarium(III) lanthanides were chelated with 2-naphthoyltrifluoroacetone and trioctylphosphine oxide to dye the spherical particles, whereas terbium(III) and dysprosium(III) chelate complexes contained a newly synthesized ligand, 4-(2,4,6-tridecyloxyphenyl)pyridine-2,6-dicarboxylic acid. The fluorescence properties of the four lanthanides-including a wide Stokes shift, a narrow emission peak, and long fluorescence lifetime-were retained despite the incorporation into the nanoparticles. Furthermore, the nanoparticles, containing more than 1000 lanthanide chelates, were detectable at label concentrations 3 orders of magnitude lower than the corresponding soluble lanthanide chelate labels. The applicability of the labels prepared was demonstrated by a heterogeneous sandwich-type immunoassay for human prostate-specific antigen, where the lowest limits of detection of 1.6, 2.4, 10.1, and 114.2 ng/L were achieved using europium(III), terbium(III), samarium(III), and dysprosium(III) nanoparticles, respectively. The spectral and functional properties of the lanthanide-embedded polystyrene nanoparticles developed here suggest that the technology is applicable for high-sensitivity multicolor assays.

Immunoassay of total prostate-specific antigen using europium(III) nanoparticle labels and streptavidin-biotin technology

Nanoparticle labels conjugated with biomolecules are used in a variety of different assay applications. We investigated the possibility of using europium(III)-labeled 68-nm nanoparticles coated with monoclonal antibodies or streptavidin (SA) to detect prostate-specific antigen (PSA) in serum. The selection of a suitable antibody pair and interference caused by the combination of nanoparticle label and structurally complex analyte were of special interest. A set of antibodies recognizing different epitope areas of PSA was mapped to find the optimal antibody pair for the immunometric nanoparticle-based assay. Different assay configurations were tested to obtain a good correlation with a conventional method based on biotinylated detection antibodies and europium(III) chelate-labeled streptavidin. Monoclonal capture antibody 5E4 was covalently coated on a microtitration well surface; biotinylated 5H6 monoclonal antibody (Mab) was used for detection, and europium(III)-labeled streptavidin-coated nanoparticles were utilized for signal generation. Total PSA concentrations were determined from a panel of male serum samples to test the developed assay. The correlation of the nanoparticle-based and reference assays was good; y=0.9844x-0.1252, R2=0.98, n=27; and the lowest limit of detection of the assay (LLD=0.83 ng/l) was 35-fold lower than for the reference method. The assay application presented here, where a structurally complex analyte is detected, combines the exceptionally high affinity of streptavidin-biotin technology and the high specific activity of long lifetime fluorescence nanoparticle labels. The general characteristics of this combination should permit the development of various immunoassay applications featuring high sensitivity, rapidity, and low consumption of reagents.

Fluorescent nanoparticles as labels for immunometric assay of C-reactive protein using two-photon excitation assay technology

We describe the use of fluorophore-doped nanoparticles as reporters in a recently developed ArcDia TPX bioaffinity assay technique. The ArcDia TPX technique is based on the use of polymer microspheres as solid-phase reaction carrier, fluorescent bioaffinity reagents, and detection of two-photon excited fluorescence. This new assay technique enables multiplexed, separation-free bioaffinity assays from microvolumes with high sensitivity. As a model analyte we chose C-reactive protein (CRP). The assay of CRP was optimized for assessment of CRP baseline levels using a nanoparticulate fluorescent reporter, 75 nm in diameter, and the assay performance was compared to that of CRP assay based on a molecular reporter of the same fluorophore core. The results show that using fluorescent nanoparticles as the reporter provides two orders of magnitude better sensitivity (87 fM) than using the molecular label, while no difference between precision profiles of the different assay types was found. The new assay method was applied for assessment of baseline levels of CRP in sera of apparently healthy individuals.

A sensitive adenovirus immunoassay as a model for using nanoparticle label technology in virus diagnostics

BACKGROUND

Currently, PCR-based hybridization assays are widely applied in adenovirus diagnostics. However, the technology requires tedious sample preparation, and the amplification phase is susceptible to various contaminants leading to inconvenient and time-consuming assay procedure. Methods relying on viral antigen detection, e.g. immunofluorometric assays (IFMAs) and enzyme immunoassays (EIAs), are less complicated to carry out, but they provide limited sensitivity.

OBJECTIVE

Our aim was to develop a simple and sensitive adenovirus assay based on direct antigen detection via sandwich-forming immunoreaction. The assay employed highly fluorescent europium(III)-chelate-doped nanoparticle labels and selection of high affinity monoclonal antibodies (anti-hexon) coated on label particles and microtitration wells.

RESULTS AND CONCLUSIONS

The extremely high specific activity of the nanoparticle labels enabled the detection limit over 5000 virus particles per millilitre with purified virus particles. The sensitivity was improved by three orders of magnitude (800-fold) compared to concurrent time-resolved IFMA. Furthermore, the nanoparticle assay showed reasonably low coefficients of variation (4.0-20%) and excellent linearity of more than four orders of magnitude (from below 10(5) to 10(9) virus particles per millilitre). Analyzed nasopharyngeal patient specimens revealed a minor disturbance of matrix components, which could be avoided by dilution. The average signal difference between negative and positive samples was nearly four orders of magnitude. The developed assay was sensitive and more convenient approach to adenovirus screening compared to available assays. In addition, the study demonstrates the potential of nanoparticles in sensitive screening of viral analytes.