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

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.

Luminescent Lifetime Regulation of Lanthanide-Doped Nanoparticles for Biosensing

Lanthanide-doped nanoparticles possess numerous advantages including tunable luminescence emission, narrow peak width and excellent optical and thermal stability, especially concerning the long lifetime from microseconds to milliseconds. Differing from other shorter-lifetime fluorescent nanomaterials, the long lifetime of lanthanide-doped nanomaterials is independent with background fluorescence interference and biological tissue depth. This review presents the recent advances in approaches to regulating the lifetime and applications of bioimaging and biodetection. We begin with the introduction of the strategies for regulating the lifetime by modulating the core-shell structure, adjusting the concentration of sensitizer and emitter, changing energy transfer channel, establishing a fluorescence resonance energy transfer pathway and changing temperature. We then summarize the applications of these nanoparticles in biosensing, including ion and molecule detecting, DNA and protease detection, cell labeling, organ imaging and thermal and pH sensing. Finally, the prospects and challenges of the lanthanide lifetime regulation for fundamental research and practical applications are also discussed.

A proposed insight into the anti-viral potential of metallic nanoparticles against novel coronavirus disease-19 (COVID-19)

BACKGROUND

Over the last ten months since December 2019, the world has faced infectious emerging novel coronavirus disease-2019 (COVID-19) outbreaks that had a massive global impact affecting over 185 countries.

MAIN BODY

Emerging novel COVID-19 is a global health emergency on a pandemic scale that represents a terror to human health through its ability to escape anti-viral measures. Such viral infections impose a great socioeconomic burden, besides global health challenges. This imposes a pressing need for the development of anti-viral therapeutic agents and diagnostic tools that demonstrate multifunctional, target-specific, and non-toxic properties. Nanotheranostics is regarded as a promising approach for the management of different viral infections. Nanotheranostics facilitates targeted drug-delivery of anti-viral therapeutics as well as contributing to the development of diagnostic systems. Multifunctional metallic nanoparticles (NPs) have emerged as innovative theranostic agents that enable sustainable treatment and effective diagnosis. Here we have reviewed current advances in the use of theranostic metallic NPs to fight against COVID-19, and discussed the application as well as limitations associated with nanotechnology-based theranostic approaches.

CONCLUSION

This review verified the potential use of some metal-based NPs as anti-viral nanotheranostic agents. Metal-based NPs could act as carriers that enable the sustainable and targeted delivery of active anti-viral molecules, or as diagnostic agents that allow rapid and sensitive diagnosis of viral infections.

Highly Luminescent and Anti-Photobleaching Core-Shell Structure of Mesoporous Silica and Phosphatidylcholine Modified Superparamagnetic Iron Oxide Nanoparticles

Highly fluorescent magnetic nanoparticles (Eu(TTA)₃(P(Oct)₃)₃@mSiO₂@SPION) [europium (III) chloride hexahydrate = Eu; 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione = TTA; trioctylphosphine = (P(Oct)₃); mesoporous silica = mSiO₂; superparamagnetic iron oxide nanoparticle = SPION] were developed as a dual-functional imaging agent. The hierarchical structure was composed of a magnetic core and mesoporous silica shell was constructed using a cationic surfactant template after coating with phosphatidylcholine of oleic acid coated SPION. Afterward, the surface and cavities of mSiO₂@SPION were modified with 3-(trimethoxysilyl) propyl methacrylate (TMSPMA) as a silane coupling agent to introduce methacrylate groups. Eu(TTA)₃(P(Oct)₃)₃ molecules are penetrated, located and bonded covalently inside of the cavities/mesopores of mSiO₂, it shows extremely stable anti-photobleaching properties. The emission spectra of Eu(TTA)₃(P(Oct)₃)₃@mSiO₂@SPION indicated typical hypersensitivity transition ⁵D₀→⁷F₂ at 621 nm. The concentration of Eu(TTA)₃(P(Oct)₃)₃@mSiO₂@SPION was varied between 10 and 500 μL/mL to evaluate the cytotoxicity with NCI-H460 (H460) cells using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. In addition, the presence of a strong red-emitting Eu(TTA)₃(P(Oct)₃)₃@mSiO₂@SPION in the cytoplasm was observed by fluorescence microscopy. Those results that it can be a potential candidate for dual-functional contrast agent and PL nanomaterials for fabricating the diagnostic kits to amplify the low signal.

Research Progress of Near-Infrared Fluorescence Immunoassay

Near-infrared fluorescence probes (NIFPs) have been widely used in immunoassay, bio-imaging and medical diagnosis. We review the basic principles of near-infrared fluorescence and near-infrared detection technology, and summarize structures, properties and characteristics of NIFPs (i.e., cyanines, xanthenes fluorescent dyes, phthalocyanines, porphyrin derivates, single-walled carbon nanotubes (SWCNTs), quantum dots and rare earth compounds). We next analyze applications of NIFPs in immunoassays, and prospect the application potential of lateral flow assay (LFA) in rapid detection of pathogens. At present, our team intends to establish a new platform that has highly sensitive NIFPs combined with portable and simple immunochromatographic test strips (ICTSs) for rapid detection of food-borne viruses. This will provide technical support for rapid detection on the port.

Development of a novel immunoassay for the simple and fast quantitation of neutrophil gelatinase-associated lipocalin using europium(III) chelate microparticles and magnetic beads

Neutrophil gelatinase-associated lipocalin (NGAL) is a promising biomarker for diagnosing acute kidney injury (AKI). Currently, there are few assays for determining NGAL and they are complex, time-consuming or expensive. We aimed to establish an efficient immunoassay to measure NGAL in human urine simply and rapidly. A novel immunoassay for NGAL determination was established by combining a dissociation-enhanced-free time-resolved fluoroimmunoassay (TRFIA) and immunomagnetic separation. Based on a "sandwich"-type immunoassay format, analytes in samples were captured by a pair of monoclonal antibodies (mAb) in which one mAb was coated in magnetic beads and the other mAb was labeled with europium(III) chelate microparticles (CM-EUs) as "fluorescent reporters". NGAL concentrations were determined in a linear range (10-1500 ng mL^-1) with a limit of detection of 0.32 ng mL^-1. The reproducibility, recovery, and specificity of our TRFIA were acceptable. Our method was compared with that of a chemiluminescence immunoassay (CMIA) using 115 urine samples, and the results showed good correlation (R² = 0.8677). We expect our novel method to be useful for the early diagnosis of AKI.

Inorganic Complexes and Metal-Based Nanomaterials for Infectious Disease Diagnostics

Infectious diseases claim millions of lives each year. Robust and accurate diagnostics are essential tools for identifying those who are at risk and in need of treatment in low-resource settings. Inorganic complexes and metal-based nanomaterials continue to drive the development of diagnostic platforms and strategies that enable infectious disease detection in low-resource settings. In this review, we highlight works from the past 20 years in which inorganic chemistry and nanotechnology were implemented in each of the core components that make up a diagnostic test. First, we present how inorganic biomarkers and their properties are leveraged for infectious disease detection. In the following section, we detail metal-based technologies that have been employed for sample preparation and biomarker isolation from sample matrices. We then describe how inorganic- and nanomaterial-based probes have been utilized in point-of-care diagnostics for signal generation. The following section discusses instrumentation for signal readout in resource-limited settings. Next, we highlight the detection of nucleic acids at the point of care as an emerging application of inorganic chemistry. Lastly, we consider the challenges that remain for translation of the aforementioned diagnostic platforms to low-resource settings.

Gold nanoparticle core–europium(iii) chelate fluorophore-doped silica shell hybrid nanocomposites for the lateral flow immunoassay of human thyroid stimulating hormone with a dual signal readout

Simultaneous colorimetric and fluorometric signal detection with hybrid nanocomposite labels.

Assessing inter lanthanide photophysical interactions in co-doped titanium dioxide nanoparticles for multiplex assays

This work assesses inter lanthanide photophysical interactions in titanium dioxide nanoparticles towards the development of multiplex assays.

Role of metal and metal oxide nanoparticles as diagnostic and therapeutic tools for highly prevalent viral infections

Nanotechnology is increasingly playing important roles in various fields including virology. The emerging use of metal or metal oxide nanoparticles in virus targeting formulations shows the promise of improved diagnostic or therapeutic ability of the agents while uniquely enhancing the prospects of targeted drug delivery. Although a number of nanoparticles varying in composition, size, shape, and surface properties have been approved for human use, the candidates being tested or approved for clinical diagnosis and treatment of viral infections are relatively less in number. Challenges remain in this domain due to a lack of essential knowledge regarding the in vivo comportment of nanoparticles during viral infections. This review provides a broad overview of recent advances in diagnostic, prophylactic and therapeutic applications of metal and metal oxide nanoparticles in human immunodeficiency virus, hepatitis virus, influenza virus and herpes virus infections. Types of nanoparticles commonly used and their broad applications have been explained in this review.

Multivalent linkers for improved covalent binding of oligonucleotides to dye-doped silica nanoparticles

This paper describes the fabrication of oligonucleotide-coated Cy5-doped silica nanoparticles using a combination of multivalent linkers and their use in surface-based DNA sandwich hybridization assays. Dipodal silane is introduced as a means to fabricate amine-coated silica nanoparticles and its advantages compared to monopodal silanes are discussed. The use of dipodal silane in conjunction with three different polymer linkers (oxidized dextran, linear and 8-arm polyethylene glycol (PEG)) to immobilize single-stranded DNA to Cy5-doped nanoparticles is investigated and dynamic light scattering measurements and Fourier transform infrared spectroscopy are used to follow the progression of the functionalization of the nanoparticles. We observe a significant improvement in the binding stability of the single-stranded DNA when the dipodal silane and 8-arm PEG are used in combination, when compared to alternative conjugation strategies. Both 8mer and 22mer oligonucleotides are securely conjugated to the high-brightness nanoparticles and their availability to hybridize with a complementary strand is confirmed using solution-based DNA hybridization experiments. In addition, a full surface-based sandwich assay demonstrates the potential these nanoparticles have in the detection of less than 500 femtomolar of a DNA analogue of micro RNA, miR-451.

A Novel Europium Chelate Coated Nanosphere for Time-Resolved Fluorescence Immunoassay

A novel europium ligand 2,2',2'',2'''-(4,7-diphenyl-1,10-phenanthroline-2,9-diyl) bis (methylene) bis (azanetriyl) tetra acetic acid (BC-EDTA) was synthesized and characterized. It shows an emission spectrum peak at 610 nm when it is excited at 360 nm, with a large Stock shift (250 nm). It is covalently coated on the surface of a bare silica nanosphere containi free amino groups, using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-Hydroxysuccinimide. We also observed an interesting phenomenon that when BC-EDTA is labeled with a silica nanosphere, the chelate shows different excitation spectrum peaks of about 295 nm. We speculate that the carboxyl has a significant influence on its excitation spectrum. The BC-EDTA/Eu3+coated nanosphere could be used as a fluorescent probe for time-resolved fluorescence immunoassay. We labeled the antibody with the fluorescent nanosphere to develop a nanosphere based hepatitis B surface antigen as a time-resolved fluorescence immunoassay reagent, which is very easy to operate and eliminates potential contamination of Eu3+ contained in the environment. The analytical and functional sensitivities are 0.0037 μg/L and 0.08 μg/L (S/N≥2.0) respectively. The detection range is 0.08-166.67 μg/L, which is much wider than that of ELISA (0.2-5 μg/L). It is comparable to the commercial dissociation-enhanced lanthanide fluoro-immunoassay system (DELFIA) reagents (0.2-145 μg/L). We propose that it can fulfill clinical applications.

Luminescent lanthanide nanomaterials: an emerging tool for theranostic applications

Lanthanide materials have been gaining popularity for use in various theranostic applications, primarily due to their unique optical properties such as narrow emission bands, multiple emission wavelengths, emission tunability, long fluorescence lifetime and large Stokes shift. Apart from these, some lanthanide materials also exhibit magnetic and light-up conversion properties. Such nanomaterials have been used for a wide range of applications ranging from detection of biomarkers, in vitro and in vivo imaging to therapeutic applications. Recently, combined modalities of lanthanide nanomaterials for simultaneous detection/imaging and delivery of therapeutic agents (termed ‘theranostics’) have been explored. The various advantages and disadvantages of using lanthanide nanomaterials as theranostic agents and potential areas for future development have been discussed in this review.

A simple method for the quantification of molecular decorations on silica particles

A simple, rapid quantitative approach to determining attachment density on silica nanoparticles has been demonstrated using attenuated total reflectance Fourier transform infrared spectroscopy and verified by thermogravimetric analysis. A very high attachment of approximately 5 attachments per nm² has been achieved through photoinduced thiol-ene click reaction of 11-bromo-1-undecene with a thiol functionalized silica nanoparticle formed from mercaptopropyltrimethoxysilane as the sole precursor. Attachment density with concentration of alkene and reaction time is shown to be highly nonlinear and appears to be limited by accessability of thiols on the surface of the particle. This method opens the opportunity to form nanoparticles with controlled functionality including multifunctional particles, which have been produced in this work.

Silica-coated dimers of silver nanospheres as surface-enhanced Raman scattering tags for imaging cancer cells

Surface-enhanced Raman scattering (SERS) tags have been actively explored as a multiplexing platform for sensitive detection of biomolecules. Here, we report a new type of SERS tags that was fabricated by sequentially functionalizing dimers made of 50 nm Ag nanospheres with 4-mercaptobenzoic acid as the Raman reporter molecule, silica coating as a protective shell and antibody as a targeting ligand. These dimer-based tags give highly enhanced and reproducible Raman signals owing to the presence of a well-defined SERS hot spot at the junction between two Ag nanospheres in the dimer. The SERS enhancement factor (EF) of an individual dimer tag supported on a glass slide can reach a level as high as 4.3 × 10(6). In comparison, the EFs dropped to 2.8 × 10(5) and 8.7 × 10(5), respectively, when Ag nanospheres and nanocubes with sizes similar to the spheres in the dimer were used to fabricate the tags using similar procedures. The SERS signals from aqueous suspensions of the dimer-based tags also showed high intensity and good stability. Potential use of the dimer-based tags was demonstrated by imaging cancer cells overexpressing HER2 receptors with good specificity and high sensitivity.

Influences of surface charge, size, and concentration of colloidal nanoparticles on fabrication of self-organized porous silica in film and particle forms

Studies on preparation of porous material have attracted tremendous attention because existence of pores can provide material with excellent performances. However, current preparation reports described successful production of porous material with only partial information on charges, interactions, sizes, and compositions of the template and host materials. In this report, influences of self-assembly parameters (i.e., surface charge, size, and concentration of colloidal nanoparticles) on self-organized porous material fabrication were investigated. Silica nanoparticles (as a host material) and polystyrene (PS) spheres (as a template) were combined to produce self-assembly porous materials in film and particle forms. The experimental results showed that the porous structure and pore size were controllable and strongly depended on the self-assembly parameters. Materials containing highly ordered pores were effectively created only when process parameters fall within appropriate conditions (i.e., PS surface charge ≤ -30 mV; silica-to-PS size ratio ≤0.078; and silica-to-PS mass ratio of about 0.50). The investigation of the self-assembly parameter landscape was also completed using geometric considerations. Because optimization of these parameters provides significant information in regard to practical uses, results of this report could be relevant to other functional properties.

Organosilylated complex [Eu(TTA)3(Bpy-Si)]: a bifunctional moiety for the engeneering of luminescent silica-based nanoparticles for bioimaging

A new highly luminescent europium complex with the formula [Eu(TTA)3(Bpy-Si)], where TTA stands for the thenoyltrifluoroacetone, (C4H3S)COCH2COCF3, chelating ligand and Bpy-Si, Bpy-CH2NH(CH2)3Si(OEt)3, is an organosilyldipyridine ligand displaying a triethoxysilyl group as a grafting function has been synthesized and fully characterized. This bifunctional complex has been grafted onto the surface of dense silica nanoparticles (NPs) and on mesoporous silica microparticles as well. The covalent bonding of [Eu(TTA)3(Bpy-Si)] inside uniform Stöber silica nanoparticles was also achieved. The general methodology proposed could be applied to any silica matrix, allowed high grafting ratios that overcome chelate release and the tendency to agglomerate. Luminescent silica-based nanoparticles SiO2-[Eu(TTA)3(Bpy-Si)], with a diameter of 28 ± 2 nm, were successfully tested as a luminescent labels for the imaging of Pseudomonas aeruginosa biofilms. They were also functionalized by a specific monoclonal antibody and subsequently employed for the selective imaging of Escherichia coli bacteria.

Fluorescent Ru(phen)3(2+)-doped silica nanoparticles-based ICTS sensor for quantitative detection of enrofloxacin residues in chicken meat

A Ru(phen)3(2+)-doped silica fluorescent nanoparticle (FN)-based immunochromatographic test strip (ICTS) sensor was developed for rapid, high sensitivity, easy to use, and low cost quantitative detection of enrofloxacin (ENR) residues in chicken meat. The fluorescence signal intensity of the FNs at the test line (FI(T)) and control line (FI(C)) was determined with a prototype of a portable fluorescent strip reader. Unique properties of Ru(phen)3(2+) doped silica nanoparticles (e.g., large Stokes shift, high emission quantum yield, and long fluorescence lifetime) were combined with the advantages of ICTS and an easy to make portable fluorescent strip reader. The signal was based on FI(T)/FI(C) ratio to effectively eliminate strip to strip variation and matrix effects. Various parameters that influenced the strip were investigated and optimized. Quantitative ENR detection with the FNs ICTS sensor using 80 μL sample took only 20 min, which is faster than the commercial ELISA kit (that took 90 min). The linear range of detection in chicken extract was established at 0.025-3.500 ng/mL with a half maximal inhibitory concentration at 0.22 ± 0.02 ng/mL. Using the optimized parameters, the limit of detection (LOD) for ENR using the FNs ICTS sensor was recorded at 0.02 ng/mL in chicken extract. This corresponds to 0.12 μg/kg chicken meat which is two (2) orders of magnitude better that the maximum residue limits (MRLs) imposed in Japan (10 μg/kg) and three (3) orders of magnitude better than those imposed in China. The intra- and inter-assay coefficient of variations (CVs) were 6.04% and 12.96% at 0.5 ng/mL, 6.92% and 12.61% at 1.0 ng/mL, and 6.66% and 11.88% at 2.0 ng/mL in chicken extract, respectively. The recoveries using the new FNs ICTS sensor from fifty (50) ENR-spiked chicken samples showed a highly significant correlation (R(2) = 0.9693) with the commercial enzyme-linked immunosorbent assay (ELISA) kit. The new FNs ICTS sensor is a simple, rapid, sensitive, accurate, and inexpensive quantitative detection of ENR residues in chicken meat and extracts.

Preparation and time-gated luminescence bioimaging applications of long wavelength-excited silica-encapsulated europium nanoparticles

Silica-encapsulated luminescent lanthanide nanoparticles have shown great potential as biolabels for various time-gated luminescence bio-detections in recent years. The main problem of these nano-biolabels is their short excitation wavelengths within the UV region. In this work, a new type of silica-encapsulated luminescent europium nanoparticle, with a wide excitation range from UV to visible light in aqueous solutions, has been prepared using a conjugate of (3-isocyanatopropyl)triethoxysilane bound to a visible light-excited Eu(3+) complex, 2,6-bis(1',1',1',2',2',3',3'-heptafluoro-4',6'-hexanedion-6'-yl)-dibenzothiophene-Eu(3+)-2-(N,N-diethylanilin-4-yl)-4,6-bis(pyrazol-1-yl)-1,3,5-triazine (IPTES-BHHD-Eu(3+)-BPT conjugate), as a functionalized precursor. The nanoparticles, which are prepared by the copolymerization of the IPTES-BHHD-Eu(3+)-BPT conjugate, tetraethyl orthosilicate and (3-aminopropyl)triethoxysilane in a water-in-oil reverse microemulsion consisting of Triton X-100, n-octanol, cyclohexane and water in the presence of aqueous ammonia, are monodisperse, spherical and uniform in size. Their diameter is 42 ± 3 nm and they are strongly luminescent with a wide excitation range from UV to ∼475 nm and a long luminescence lifetime of 346 μs. The nanoparticles were successfully used for streptavidin labeling and the time-gated luminescence imaging detection of two environmental pathogens, cryptosporidium muris and cryptosporidium parvium, in water samples. The results demonstrated the practical utility of the new nanoparticles as visible light-excited biolabels for time-gated luminescence bioassay applications.

Dye doped Eosin yellowish silica nanoparticles as novel fluorophore for a peroxyoxalate chemiluminescence system

In this work, we report the preparation and characterization of novel dye doped fluorophore Eosin yellowish silica nanoparticles (ESNPs). We synthesized ESNPs by the Stöber method via encapsulation of Eosin Yellowish in silica particles by the condensation of tetraethyl orthosilicate under alkaline condition at room temperature. The resulted ESNPs were characterized by transmission electron microscopy, atomic force microscopy; UV-Visible, fluorescence and Fourier transform infrared spectroscopy. The sizes of the nanoparticles have been found to be 300.0 (±1.0), 400.0 (±1.1) and 500.0 (±5.2) nm depending the reaction conditions under which they were synthesized. Furthermore, because of intense light emission, the ESNPs were used as fluorophore in a peroxyoxalate chemiluminescence system. The effect of solvent and concentrations of necessary reagents, bis(2,4,6-trichlorophenyl)oxalate, sodium salicylate, hydrogen peroxide and the effects of size of the ESNP and temperature on the luminescence efficiency of the system were examined. The activation kinetic parameters of the system were also evaluated from the temperature investigation.

Biological applications of rare-earth based nanoparticles

Biomedicine and cell and molecular biology require powerful imaging techniques of the single molecule scale to the whole organism, either for fundamental science or diagnosis. These applications are however often limited by the optical properties of the available probes. Moreover, in cell biology, the measurement of the cell response with spatial and temporal resolution is a central instrumental problem. This has been one of the main motivations for the development of new probes and imaging techniques either for biomolecule labeling or detection of an intracellular signaling species. The weak photostability of genetically encoded probes or organic dyes has motivated the interest for different types of nanoparticles for imaging such as quantum dots, nanodiamonds, dye-doped silica particles, or metallic nanoparticles. One of the most active fields of research in the past decade has thus been the development of rare-earth based nanoparticles, whose optical properties and low cytotoxicity are promising for biological applications. Attractive properties of rare-earth based nanoparticles include high photostability, absence of blinking, extremely narrow emission lines, large Stokes shifts, long lifetimes that can be exploited for retarded detection schemes, and facile functionalization strategies. The use of specific ions in their compositions can be moreover exploited for oxidant detection or for implementing potent contrast agents for magnetic resonance imaging. In this review, we present these different applications of rare-earth nanoparticles for biomolecule detection and imaging in vitro, in living cells or in small animals. We highlight how chemical composition tuning and surface functionalization lead to specific properties, which can be used for different imaging modalities. We discuss their performances for imaging in comparison with other probes and to what extent they could constitute a central tool in the future of molecular and cell biology.

Nanoparticles in molecular diagnostics

The aim of this chapter is to provide an overview of the available and emerging molecular diagnostic methods that take advantage of the unique nanoscale properties of nanoparticles (NPs) to increase the sensitivity, detection capabilities, ease of operation, and portability of the biodetection assemblies. The focus will be on noble metal NPs, especially gold NPs, fluorescent NPs, especially quantum dots, and magnetic NPs, the three main players in the development of probes for biological sensing. The chapter is divided into four sections: a first section covering the unique physicochemical properties of NPs of relevance for their utilization in molecular diagnostics; the second section dedicated to applications of NPs in molecular diagnostics by nucleic acid detection; and the third section with major applications of NPs in the area of immunoassays. Finally, a concluding section highlights the most promising advances in the area and presents future perspectives.

Tandem conjugation of enzyme and antibody on silica nanoparticle for enzyme immunoassay

We present a new type of enzyme-antibody conjugate that simplifies the labeling procedure and increases the sensitivity of enzyme-linked immunosorbent assay (ELISA). The conjugates were prepared through layer-by-layer immobilization of enzyme and antibody on a silica nanoparticle scaffold. A maximal amount of enzyme was immobilized on the nanoparticle, followed by antibody linkage through Dextran 500. The conjugate could be easily purified from unreacted reagents by simple centrifugations. In comparison with the conventional antibody-enzyme conjugate used in ELISA, which often has one or two enzyme molecules per antibody, the new type of conjugate contained more enzyme molecules per antibody and provided a much higher signal and increased sensitivity. When used in an ELISA detection of the hepatitis B surface antigen (HBsAg), the detection limit was three times lower than that of the commercially available ELISA kit.

Lanthanide doped silica nanoparticles applied to multiplexed immunoassays

Luminescent samarium (Sm) and europium (Eu) doped silica nanoparticles (SiO(2) NPs) were investigated for use in multiple target immunoassays. Particles with a diameter of 40 +/- 10 nm were synthesized and applied to multiplexed immunoassays in a single well of an assay plate. The luminescent behavior of the lanthanide doped NPs was compared to conventional dyes in solution and in surface confined model immunoassays against two analytes. Luminescence from the dyes and NPs was detected with an imaging spectroscopic system and this could be processed with a simple least squares procedure. In solution, NPs exhibited linear mixing behaviour while conventional fluorescent dyes required careful control to solution conditions, particularly pH. A conventional dye system consisting of FITC and Cy3 applied to a two analyte model system resulted in up to 70% energy transfer from FITC to Cy3 under conditions of high surface coverage. This made quantitative analysis intractable for unknown samples. The lanthanide doped silica nanoparticles responded well to both analytes (<5% relative std. dev.). The results demonstrate that NPs provide alternatives to conventional dyes due to low susceptibility to pH and related solution conditions, absence of energy transfer, single excitation wavelength range, and high stokes shift. These properties allowed quantitative analysis of multiple analytes in the same well.

Lateral Flow Immunoassay Using Europium Chelate-Loaded Silica Nanoparticles as Labels

Background: Despite their ease of use, lateral flow immunoassays (LFIAs) often suffer from poor quantitative discrimination and low analytical sensitivity. We explored the use of a novel class of europium chelate-loaded silica nanoparticles as labels to overcome these limitations.

Methods: Antibodies were covalently conjugated onto europium chelate-loaded silica nanoparticles with dextran as a linker. The resulting conjugates were used as labels in LFIA for detection of hepatitis B surface antigen (HBsAg). We performed quantification with a digital camera and Adobe Photoshop software. We also used 286 clinical samples to compare the proposed method with a quantitative ELISA.

Results: A detection limit of 0.03 μg/L was achieved, which was 100 times lower than the colloidal gold-based LFIAs and lower than ELISA. A precise quantitative dose-response curve was obtained, and the linear measurement range was 0.05–3.13 μg/L, within which the CVs were 2.3%–10.4%. Regression analysis of LFIA on ELISA results gave: log (LFIA) = −0.14 log (ELISA) + 1.03 μg/L with r = 0.99 for the quantification of HBsAg in 35 positive serum samples. Complete agreement was observed for the qualitative comparison of 286 clinical samples assayed with LFIA and ELISA.

Conclusions: Europium chelate-loaded silica nanoparticle labels have great potential to improve LFIAs, making them useful not only for simple screening applications but also for more sensitive and quantitative immunoassays.