Simultaneous detection of Escherichia coli O157∶H7 and Salmonella Typhimurium using quantum dots as fluorescence labels

Functionalized quantum dots for biosensing and bioimaging and concerns on toxicity

Considerable efforts have been devoted to the development of novel functionalized nanomaterials for bio-oriented applications. With unique optical properties and molar scale production, colloidal photoluminescent quantum dots (QDs) have been properly functionalized with controlled interfaces as new class of optical probes with extensive use in biomedical research. In this review, we present a brief summary on the current research interests of using fine engineered QDs as a nanoplatform for biomedical sensing and imaging applications. In addition, recent concerns on the potential toxic effects of QDs are described as a general guidance for the development on QD formulations in future studies.

Double core Infrared (CdSeTe) / ZnS quantum dots conjugated to Papiloma virus antibodies

Papers in the Appendix were published in electronic format as Volume 1534

Quantum dots: synthesis, bioapplications, and toxicity

This review introduces quantum dots (QDs) and explores their properties, synthesis, applications, delivery systems in biology, and their toxicity. QDs are one of the first nanotechnologies to be integrated with the biological sciences and are widely anticipated to eventually find application in a number of commercial consumer and clinical products. They exhibit unique luminescence characteristics and electronic properties such as wide and continuous absorption spectra, narrow emission spectra, and high light stability. The application of QDs, as a new technology for biosystems, has been typically studied on mammalian cells. Due to the small structures of QDs, some physical properties such as optical and electron transport characteristics are quite different from those of the bulk materials.

Lab-on-a-chip pathogen sensors for food safety

There have been a number of cases of foodborne illness among humans that are caused by pathogens such as Escherichia coli O157:H7, Salmonella typhimurium, etc. The current practices to detect such pathogenic agents are cell culturing, immunoassays, or polymerase chain reactions (PCRs). These methods are essentially laboratory-based methods that are not at all real-time and thus unavailable for early-monitoring of such pathogens. They are also very difficult to implement in the field. Lab-on-a-chip biosensors, however, have a strong potential to be used in the field since they can be miniaturized and automated; they are also potentially fast and very sensitive. These lab-on-a-chip biosensors can detect pathogens in farms, packaging/processing facilities, delivery/distribution systems, and at the consumer level. There are still several issues to be resolved before applying these lab-on-a-chip sensors to field applications, including the pre-treatment of a sample, proper storage of reagents, full integration into a battery-powered system, and demonstration of very high sensitivity, which are addressed in this review article. Several different types of lab-on-a-chip biosensors, including immunoassay- and PCR-based, have been developed and tested for detecting foodborne pathogens. Their assay performance, including detection limit and assay time, are also summarized. Finally, the use of optical fibers or optical waveguide is discussed as a means to improve the portability and sensitivity of lab-on-a-chip pathogen sensors.

Quantum dot enabled detection of Escherichia coli using a cell-phone

We report a cell-phone based Escherichia coli (E. coli) detection platform for screening of liquid samples. In this compact and cost-effective design attached to a cell-phone, we utilize anti-E. coli O157:H7 antibody functionalized glass capillaries as solid substrates to perform a quantum dot based sandwich assay for specific detection of E. coli O157:H7 in liquid samples. Using battery-powered inexpensive light-emitting-diodes (LEDs) we excite/pump these labelled E. coli particles captured on the capillary surface, where the emission from the quantum dots is then imaged using the cell-phone camera unit through an additional lens that is inserted between the capillary and the cell-phone. By quantifying the fluorescent light emission from each capillary tube, the concentration of E. coli in the sample is determined. We experimentally confirmed the detection limit of this cell-phone based fluorescent imaging and sensing platform as ∼5 to 10 cfu mL(-1) in buffer solution. We also tested the specificity of this E. coli detection platform by spiking samples with different species (e.g., Salmonella) to confirm that non-specific binding/detection is negligible. We further demonstrated the proof-of-concept of our approach in a complex food matrix, e.g., fat-free milk, where a similar detection limit of ∼5 to 10 cfu mL(-1) was achieved despite challenges associated with the density of proteins that exist in milk. Our results reveal the promising potential of this cell-phone enabled field-portable and cost-effective E. coli detection platform for e.g., screening of water and food samples even in resource limited environments. The presented platform can also be applicable to other pathogens of interest through the use of different antibodies.

Scano-magneto immunoassay based on carbon nanotubes/gold nanoparticles nanocomposite for Salmonella enterica serovar Typhimurium detection

To improve sensitivity of S. enterica serovar Typhimurium detection, multiwalled carbon nanotubes (MWCNTs) and gold nanoparticles (AuNPs) were combined and used as a label to amplify signal in a scanometric based assay. In this study, the MWCNTs/AuNPs nanocomposite was fabricated by directly assemble of Au(3+) to MWCNTs and allowed growing of AuNPs along the MWCNTs surface. This MWCNTs/AuNPs nanocomposite was then attached to anti-S. typhimurium antibody (MWCNTs/AuNPs/Ab(1)) and used as a detecting molecule. Upon binding to Salmonella, they were pre-concentrated by magenetic beads/antibody (MBs/Ab(2)) forming a sandwich immuno-complex which is later spotted on a nitrocellulose membrane coated slide. Silver reduction was applied to amplify signal. The detection limit of 42CFU/ml was achieved when 2% BSA was used as a blocking agent. Given different types of real samples testing, chicken broth was found to give lowest detection limit, followed by orange juice low fat and whole milk. Selectivity testing was performed by using Escherichia coli as interference and found slightly cross-reactivity which could be due to specificity of the Ab used. By virtue of using a slide for multi-samples spotting and a flatbed scanner for signal-read out acquisition, this scano-magneto immunoassay could enable low-cost detection as well as high throughput screening.

Portable microfluidic chip for detection of Escherichia coli in produce and blood

Pathogenic agents can lead to severe clinical outcomes such as food poisoning, infection of open wounds, particularly in burn injuries and sepsis. Rapid detection of these pathogens can monitor these infections in a timely manner improving clinical outcomes. Conventional bacterial detection methods, such as agar plate culture or polymerase chain reaction, are time-consuming and dependent on complex and expensive instruments, which are not suitable for point-of-care (POC) settings. Therefore, there is an unmet need to develop a simple, rapid method for detection of pathogens such as Escherichia coli. Here, we present an immunobased microchip technology that can rapidly detect and quantify bacterial presence in various sources including physiologically relevant buffer solution (phosphate buffered saline [PBS]), blood, milk, and spinach. The microchip showed reliable capture of E. coli in PBS with an efficiency of 71.8% ± 5% at concentrations ranging from 50 to 4,000 CFUs/mL via lipopolysaccharide binding protein. The limits of detection of the microchip for PBS, blood, milk, and spinach samples were 50, 50, 50, and 500 CFUs/mL, respectively. The presented technology can be broadly applied to other pathogens at the POC, enabling various applications including surveillance of food supply and monitoring of bacteriology in patients with burn wounds.

A facile and sensitive detection of pathogenic bacteria using magnetic nanoparticles and optical nanocrystal probes

We report a facile and sensitive analytical method for the detection of pathogenic bacteria. Salmonella bacteria in milk were captured by antibody-conjugated magnetic nanoparticles (MNPs) and separated from analyte samples by applying an external magnetic field. The MNP-Salmonella complexes were re-dispersed in a buffer solution then exposed to antibody-immobilized TiO(2) nanocrystals (TNs), which absorb UV light. After magnetically separating the MNP-Salmonella-TN complexes from solution, the UV-Vis absorption spectrum of the unbound TN solution was obtained. Because the light absorption intensity was reversely proportional to the Salmonella concentration, the assay exhibited high sensitivity toward low concentrations of Salmonella bacteria. The detection limit of Salmonella in milk was found to be more than 100 cfu mL(-1).

Quick and simple estimation of bacteria using a fluorescent paracetamol dimer-Au nanoparticle composite

Rapid, simple and sensitive detection of bacterial contamination is critical for safeguarding public health and the environment. Herein, we report an easy method of detection as well as enumeration of the bacterial cell number on the basis of fluorescence quenching of a non-antibacterial fluorescent nanocomposite, consisting of paracetamol dimer (PD) and Au nanoparticles (NPs), in the presence of bacteria. The composite was synthesized by reaction of paracetamol (p-hydroxyacetanilide) with HAuCl(4). The Au NPs of the composite were characterized using UV-Vis spectroscopy, transmission electron microscopy (TEM), X-ray diffraction and selected area electron diffraction analysis. The paracetamol dimer in the composite showed emission peak at 435 nm when excited at 320 nm. The method successfully detected six bacterial strains with a sensitivity of 100 CFU mL(-1). The gram-positive and gram-negative bacteria quenched the fluorescence of the composite differently, making it possible to distinguish between the two. The TEM analysis showed interaction of the composite with bacteria without any apparent damage to the bacteria. The chi-square test established the accuracy of the method. Quick, non-specific and highly sensitive detection of bacteria over a broad range of logarithmic dilutions within a short span of time demonstrates the potential of this method as an alternative to conventional methods.

A ceramic microreactor for the synthesis of water soluble CdS and CdS/ZnS nanocrystals with on-line optical characterization

In this paper, a computer controlled microreactor to synthesize water soluble CdS and CdS/ZnS nanocrystals with in situ monitoring of the reaction progress is developed. It is based on ceramic tapes and the Low-Temperature Co-fired Ceramics technology (LTCC). As well the microsystem set-up, the microreactor fluidic design has also been thoroughly optimized. The final device is based on a hydrodynamic focusing of the reagents followed by a three-dimensional micromixer. This generates monodispersed and stable CdS and core-shell CdS/ZnS nanocrystals of 4.5 and 4.2 nm, respectively, with reproducible optical properties in terms of fluorescence emission wavelengths, bandwidth, and quantum yields, which is a key requirement for their future analytical applications. The synthetic process is also controlled in real time with the integration of an optical detection system for absorbance and fluorescence measurements based on commercial miniaturized optical components. This makes possible the efficient managing of the hydrodynamic variables to obtain the desired colloidal suspension. As a result, a simple, economic, robust and portable microsystem for the well controlled synthesis of CdS and CdS/ZnS nanocrystals is presented. Moreover, the reaction takes place in aqueous medium, thus allowing the direct modular integration of this microreactor in specific analytical microsystems, which require the use of such quantum dots as labels.

Silica-on-silicon waveguide integrated polydimethylsiloxane lab-on-a-chip for quantum dot fluorescence bio-detection

Integration of microfluidics and optical components is an essential requirement for the realization of optical detection in lab-on-a-chip (LOC). In this work, a novel hybrid integration of silica-on-silicon (SOS) waveguide and polydymethylsiloxane (PDMS) microfluidics for realizing optical detection based biochip is demonstrated. SOS is a commonly used platform for integrated photonic circuits due to its lower absorption coefficient of silica and the availability of advanced microfabrication technologies for fabricating complicated optical components. However, the fabrication of complex microfluidics circuits on SOS is an expensive process. On the other hand, any complex 3D and high-aspect-ratio microstructures for the microfluidic applications can be easily patterned on PDMS using soft lithography. By exploring the advantages of these two materials, the proposed hybrid integration method greatly simplifies the fabrication of optical LOC. Two simple technologies--namely, diamond machining and soft lithography--were employed for the integration of an optical microfluidic system. Use of PDMS for the fabrication of any complex 3D microfluidics structures, together with the integration of low loss silica-on-silicon photonic waveguides with a straight microfluidic channel, opens up new possibilities to produce low-cost biochips. The performance of SOS-PDMS-integrated hybrid biochip is demonstrated with the detection of laser induced fluorescence of quantum dots. As quantum dots have immense application potential for biodetection, they are used for the demonstration of biodetection.

Rapid, sensitive, and simultaneous detection of three foodborne pathogens using magnetic nanobead-based immunoseparation and quantum dot-based multiplex immunoassay

Losses caused by foodborne diseases are enormous in terms of human life, illness, medical costs, and food product recalls. Rapid detection of multiple bacterial pathogens in foods is extremely important to ensure food safety. The objective of this research was to develop a multiplex immunoassay by integrating magnetic nanobeads (MNBs) for immunoseparation with quantum dots (QDs) as fluorescent labels for rapid, sensitive, and simultaneous detection of three major pathogenic bacteria, Salmonella Typhimurium, Escherichia coli O157:H7, and Listeria monocytogenes, in food products. In this research, both streptavidin-conjugated MNBs (30- and 150-nm diameter) and QDs (530-, 580-, and 620-nm emission wavelength) were separately coated with biotinylated anti-Salmonella, anti-E. coli, and anti-Listeria antibodies. The immuno-MNBs were mixed with a food sample to capture the three target bacteria. After being magnetically separated from the sample, the MNB-cell conjugates were mixed with the immuno-QDs to form the MNB-cell-QD complexes, and unattached QDs were removed. The fluorescence intensity of the MNB-cell-QD complexes was measured at wavelengths of 530, 580, and 620 nm to determine the populations of Salmonella Typhimurium, E. coli O157:H7, and L. monocytogenes, respectively. This multiplex immunoassay simultaneously detected Salmonella Typhimurium, E. coli O157:H7, and L. monocytogenes at levels as low as 20 to 50 CFU/ml in food samples in less than 2 h without enrichment. The change in fluorescence intensity was linearly correlated (R(2) > 0.96) with the logarithmic value of bacterial level in the range of 10 to 10(3) CFU/ml. More than 85% of the three target pathogens could be simultaneously separated from food samples. The multiplex immunoassay could be expanded to detect more target pathogens, depending on the availability of specific antibodies and QDs with different emission wavelengths.

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.

Multiplex bioanalytical methods for food and environmental monitoring

Recent advances in miniaturization of analytical systems and newly emerging technologies offer platforms with greater automation and multiplexing capabilities than traditional biological binding assays. Multiplexed bioanalytical techniques provide control agencies and food industries with new possibilities for improved, more efficient monitoring of food and environmental contaminants. This review deals with recent developments in planar-array and suspension-array technologies, and their applications in detecting pathogens, food allergens and adulterants, toxins, antibiotics and environmental contaminants.

Nanoparticles: Emerging carriers for drug delivery

The core objective of nanoparticles is to control and manipulate biomacromolecular constructs and supramolecular assemblies that are critical to living cells in order to improve the quality of human health. By definition, these constructs and assemblies are nanoscale and include entities such as drugs, proteins, DNA/RNA, viruses, cellular lipid bilayers, cellular receptor sites and antibody variable regions critical for immunology and are involved in events of nanoscale proportions. The emergence of such nanotherapeutics/diagnostics will allow a deeper understanding of human longevity and human ills that include cancer, cardiovascular disease and genetic disorders. A technology platform that provides a wide range of synthetic nanostructures that may be controlled as a function of size, shape and surface chemistry and scale to these nanotechnical dimensions will be a critical first step in developing appropriate tools and a scientific basis for understanding nanoparticles.

Colistin-functionalised CdSe/ZnS quantum dots as fluorescent probe for the rapid detection of Escherichia coli

Intensely fluorescent, colistin-functionalised CdSe/ZnS QDs (Colis-QDs) nanoparticles, are synthesized and used as sensitive probes for the detection of Escherichia coli, a Gram-negative bacteria. Colistin molecules are attached to the terminal carboxyl of the mercaptoacetic acid-capped QDs in the presence of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) as amide bond promoters. The TEM analysis of bacteria treated with Colis-QDs conjugates showed the accumulation of Colis-QDs in the cell wall of E. coli. Under the recommended working conditions, the method provides a detection limit as few as 28 E. coli cells per mL, which is competitive which more elaborate detection systems. The simplicity of the method together with short analysis time (< 15 min, without including preparation and photoactivation of the Colis-QDs conjugate) make the proposed approach useful as quick bacteria screening system.

Assembly kinetics of nanocrystals via peptide hybridization

The assembly kinetics of colloidal semiconductor quantum dots (QDs) on solid inorganic surfaces is of fundamental importance for implementation of their solid-state devices. Herein an inorganic binding peptide, silica binding QBP1, was utilized for the self-assembly of nanocrystal quantum dots on silica surface as a smart molecular linker. The QD binding kinetics was studied comparatively in three different cases: first, QD adsorption with no functionalization of substrate or QD surface; second, QD adsorption on QBP1-modified surface; and, finally, adsorption of QBP1-functionalized QD on silica surface. The surface modification of QDs with QBP1 enabled 79.3-fold enhancement in QD binding affinity, while modification of a silica surface with QBP1 led to only 3.3-fold enhancement. The fluorescence microscopy images also supported a coherent assembly with correspondingly increased binding affinity. Decoration of QDs with inorganic peptides was shown to increase the amount of surface-bound QDs dramatically compared to the conventional methods. These results offer new opportunities for the assembly of QDs on solid surfaces for future device applications.

Peptide-mediated constructs of quantum dot nanocomposites for enzymatic control of nonradiative energy transfer

A bottom-up approach for constructing colloidal semiconductor quantum dot (QDot) nanocomposites that facilitate nonradiative Förster-type resonance energy transfer (FRET) using polyelectrolyte peptides was proposed and realized. The electrostatic interaction of these polypeptides with altering chain lengths was probed for thermodynamic, structural, and morphological aspects. The resulting nanocomposite film was successfully cut with the protease by digesting the biomimetic peptide layer upon which the QDot assembly was constructed. The ability to control photoluminescence decay lifetime was demonstrated by proteolytic enzyme activity, opening up new possibilities for biosensor applications.

Quantum dot-based array for sensitive detection of Escherichia coli

A fluorescent quantum dot-based antibody array, used in sandwich format, has been developed to detect Escherichia coli O157:H7. Numerous parameters such as solid support, optimal concentration of immunoreagents, blocking reagents, and assay time were optimized for array construction. Quantum dot-conjugated anti-IgG was used as the detecting system. The array allows the detection of E. coli O157:H7 at concentrations below 10 CFU mL(-1) without sample enrichment, exhibiting an increase of three orders of magnitude in the limit of detection compared to ELISA. The interference caused by Gram (+) and Gram (-) bacteria was negligible at low concentrations of bacteria.

Nanoparticles as fluorescent labels for optical imaging and sensing in genomics and proteomics

Optical labelling reagents (dyes and fluorophores) are an essential component of probe-based biomolecule detection, an approach widely employed in a variety of areas including environmental analysis, disease diagnostics, pharmaceutical screening, and proteomic and genomic studies. Recently, functional nanomaterials, as a new generation of high-value optical labels, have been applied to molecular detection. The great potential of such recent optical labels has paved the way for the development of new biomolecule assays with unprecedented analytical performance characteristics, related to sensitivity, multiplexing capability, sample throughput, cost-effectiveness and ease of use. This review aims to provide an overview of recent advances using different nanoparticles (such as quantum dots, rare earth doped nanoparticles or gold nanoparticles) for analytical genomics and proteomics, with particular emphasis on the outlook for different strategies of using nanoparticles for bioimaging and quantitative bioanalytical applications, as well as possibilities and limitations of nanoparticles in such a growing field.

Rapid and quantitative detection of Legionella pneumophila applying immunomagnetic separation and flow cytometry

Legionella is a pathogenic bacterium that establishes and proliferates well in water storage and distribution systems. Worldwide it is responsible for numerous outbreaks of legionellosis, which can be fatal. Despite recent advances in molecular and immunological methods, the official, internationally accepted detection method for Legionella spp. in water samples (ISO 11371) is still based on cultivation. This method has major disadvantages such as a long assay time of 10 days and the detection of cultivable cells only. Therefore, we developed a cultivation-independent, quantitative, and fast detection method for Legionella pneumophila in water samples. It consists of four steps, starting with (1) a concentrating step, in which cells present in one litre of water are concentrated into 5 ml by filtration (pore size 0.45 microm), (2) then cells are resuspended with sterile filtered buffer and double-stained with FITC- and Alexa-conjugated Legionella-specific antibodies, (3) subsequently, the cells are immunomagnetically caught, and (4) finally, fluorescently labeled Legionella cells were flow cytometrically detected and quantified. The efficiency of each step was tested separately. The whole method allows detection of L. pneumophila in 180 min with a detection limit of around 500 cells/l and a recovery of Legionella cells of 52.1 % out of spiked tap water. Fluorescence microscopy and flow cytometric cell-counting correlated well.

Nanobioimaging and sensing of infectious diseases

New methods to identify trace amount of infectious pathogens rapidly, accurately and with high sensitivity are in constant demand to prevent epidemics and loss of lives. Early detection of these pathogens to prevent, treat and contain the spread of infections is crucial. Therefore, there is a need and urgency for sensitive, specific, accurate, easy-to-use diagnostic tests. Versatile biofunctionalized engineered nanomaterials are proving to be promising in meeting these needs in diagnosing the pathogens in food, blood and clinical samples. The unique optical and magnetic properties of the nanoscale materials have been put to use for the diagnostics. In this review, we focus on the developments of the fluorescent nanoparticles, metallic nanostructures and superparamagnetic nanoparticles for bioimaging and detection of infectious microorganisms. The various nanodiagnostic assays developed to image, detect and capture infectious virus and bacteria in solutions, food or biological samples in vitro and in vivo are presented and their relevance to developing countries is discussed.

Nanoparticles for detection and diagnosis

Nanoparticle based platforms for identification of chemical and biological agents offer substantial benefits to biomedical and environmental science. These platforms benefit from the availability of a wide variety of core materials as well as the unique physical and chemical properties of these nanoscale materials. This review surveys some of the emerging approaches in the field of nanoparticle based detection systems, highlighting the nanoparticle based screening methods for metal ions, proteins, nucleic acids, and biologically relevant small molecules.

Multifunctional magnetic-optical nanoparticle probes for simultaneous detection, separation, and thermal ablation of multiple pathogens

Multifunctional nanoparticles possessing magnetization and near-infrared (NIR) absorption have warranted interest due to their significant applications in magnetic resonance imaging, diagnosis, bioseparation, target delivery, and NIR photothermal ablation. Herein, the site-selective assembly of magnetic nanoparticles onto the ends or ends and sides of gold nanorods with different aspect ratios (ARs) to create multifunctional nanorods decorated with varying numbers of magnetic particles is described for the first time. The resulting hybrid nanoparticles are designated as Fe(3)O(4)-Au(rod)-Fe(3)O(4) nanodumbbells and Fe(3)O(4)-Au(rod) necklacelike constructs with tunable optical and magnetic properties, respectively. These hybrid nanomaterials can be used for multiplex detection and separation because of their tunable magnetic and plasmonic functionality. More specifically, Fe(3)O(4)-Au(rod) necklacelike probes of different ARs are utilized for simultaneous optical detection based on their plasmon properties, magnetic separation, and photokilling of multiple pathogens from a single sample at one time. The combined functionalities of the synthesized probes will open up many exciting opportunities in dual imaging for targeted delivery and photothermal therapy.

Activation of nanoparticles by biosorption for E. coli detection in milk and apple juice

Two types of silver nanoparticles were activated by specific sorption of biomolecules for the detection of Escherichia coli. The capture of this bacterium was performed using polyclonal antibodies (anti-E. coli) biosorbed onto nanospheres or nanorice through a protein-A layer. The bacterial detection was achieved using surface enhancement Raman scattering in order to compare the performance of these two nanoparticles. The activated silver nanospheres showed a better performance mainly due to the dimension of these nanoparticles. The detection limit has been established using the automated Raman mapping system. The technique was capable of detecting 10(3) cells/mL in milk and apple juice without any pre-enrichment. With an overall assay time less than 1 h, the process could be easily adapted to detect other pathogens by selecting the pertinent antibody. Furthermore, PCR was used for the DNA verification to assess whether the selected bacterial strain was identical before and after detection.

Toward a multiplexed solid-phase nucleic acid hybridization assay using quantum dots as donors in fluorescence resonance energy transfer

Solid-phase assays using immobilized quantum dots (QDs) as donors in fluorescence resonance energy transfer (FRET) have been developed for the selective detection of nucleic acids. QDs were immobilized on optical fibers and conjugated with probe oligonucleotides. Hybridization with acceptor labeled target oligonucleotides generated FRET-sensitized acceptor fluorescence that was used as the analytical signal. A sandwich assay was also introduced and avoided the need for target labeling. Green and red emitting CdSe/ZnS QDs were used as donors with Cy3 and Alexa Fluor 647 acceptors, respectively. Quantitative measurements were made via spectrofluorimetry or fluorescence microscopy. Detection limits as low as 1 nM were obtained, and the discrimination of single nucleotide polymorphisms (SNPs) with contrast ratios as high as 31:1 was possible. The assays retained their selectivity and at least 50% of their signal when tested in bovine serum and against a large background of noncomplementary genomic DNA. Mixed films of the two colors of QD and two probe oligonucleotide sequences were prepared for multiplexed solid-phase hybridization assays. It was possible to simultaneously detect two target sequences with retention of selectivity, including SNP discrimination. This research provides an important precedent and framework for the future development of QD-based bioassays and biosensors.

An overview of recent strategies in pathogen sensing

Pathogenic bacteria are one of the major concerns in food industries and water treatment facilities because of their rapid growth and deleterious effects on human health. The development of fast and accurate detection and identification systems for bacterial strains has long been an important issue to researchers. Although confirmative for the identification of bacteria, conventional methods require time-consuming process involving either the test of characteristic metabolites or cellular reproductive cycles. In this paper, we review recent sensing strategies based on micro- and nano-fabrication technology. These technologies allow for a great improvement of detection limit, therefore, reduce the time required for sample preparation. The paper will be focused on newly developed nano- and micro-scaled biosensors, novel sensing modalities utilizing microfluidic lab-on-a-chip, and array technology for the detection of pathogenic bacteria.

The exposure of bacteria to CdTe-core quantum dots: the importance of surface chemistry on cytotoxicity

A series of water-soluble CdTe-core quantum dots (QDs) with diameters below 5.0 nm and functionalized at their surface with polar ligands such as thioglycolic acid (TGA) or the tripeptide glutathione (GSH) were synthesized and characterized by UV-vis absorption spectroscopy, their photoluminescence measurements, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Because cell elongations and growth inhibitions were observed during labeling experiments, the cytotoxicity of CdTe-core QDs was investigated. Using growth inhibition tests combining different bacterial strains with different CdTe-core QDs, it was possible to demonstrate that the cytotoxicity of QDs towards bacteria depends on exposure concentrations, surface chemistry and coating, and that it varied with the strain considered. Growth inhibition tests carried out with heavy-metal-resistant bacteria, as well as ICP-AES analyses of cadmium species released by CdTe@TGA QDs, demonstrated that the leakage of Cd2+ is not the main source of QD toxicity. Our study suggests that QD cytotoxicity is rather due to the formation of TeO2 and probably the existence of CdO formed by surface oxidation. In this respect, QDs possessing a CdO shell appeared very toxic.

Simultaneous detection of multifood-borne pathogenic bacteria based on functionalized quantum dots coupled with immunomagnetic separation in food samples

This paper reports a method that simultaneously detects three food-borne pathogenic bacteria, Salmonella typhimurium, Shigella flexneri, and Escherichia coli O157:H7, via an approach that combines magnetic microparticles for the enrichment and antibody-conjugated semiconductor quantum dots (QDs) as fluorescence markers. Using the water-in-oil reverse microemulsions method, the gamma-Fe(2)O(3) magnetic nanoparticles were coated with silica to empower the particles with high dispersibility and broad compatibility to biomacromolecules. The magnetic beads were then modified with amino silane, which could immobilize antibodies by glutaraldehyde treatment. The immunized magnetic beads and pathogenic bacteria formed "bead-cell" complexes in the enrichment procedure. QDs with different emission wavelengths (620, 560, and 520 nm) were immobilized with anti-S. typhimurium antibody, anti-S. flexneri antibody, and anti-E. coli O157:H7 antibody, respectively. Fluorescence microscope images and the fluorescence intensity of QDs labeled "sandwich" complexes (conjungated with antibodies against S. typhimurium, S. flexneri, and E. coli O157:H7, respectively) demonstrated that antibody-conjugated QDs could attach to the surface of bacterial cells selectively and specifically. In our method, we could detect food-borne pathogen bacteria in a food matrix at 10(-3) cfu/mL. We determined that a high concentration of proteins in food matrix would decrease the sensitivity of this method. This method, of which the detection procedures are completed within 2 h, can be applied to the rapid and cost-effective monitoring of bacterial contamination in food samples.

Backscattering particle immunoassays in wire-guide droplet manipulations

A simpler way for manipulating droplets on a flat surface was demonstrated, eliminating the complications in the existing methods of open-surface digital microfluidics. Programmed and motorized movements of 10 μL droplets were demonstrated using stepper motors and microcontrollers, including merging, complicated movement along the programmed path, and rapid mixing. Latex immunoagglutination assays for mouse immunoglobulin G, bovine viral diarrhea virus and Escherichia coli were demonstrated by merging two droplets on a superhydrophobic surface (contact angle = 155 ± 2°) and using subsequent back light scattering detection, with detection limits of 50 pg mL^-1, 2.5 TCID₅₀ mL^-1 and 85 CFU mL^-1, respectively, all significantly lower than the other immunoassay demonstrations in conventional microfluidics (~1 ng mL^-1 for proteins, ~100 TCID₅₀ mL^-1 for viruses and ~100 CFU mL^-1 for bacteria). Advantages of this system over conventional microfluidics or microwell plate assays include: (1) minimized biofouling and repeated use (>100 times) of a platform; (2) possibility of nanoliter droplet manipulation; (3) reprogrammability with a computer or a game pad interface.