Gold decorated polystyrene particles for lateral flow immunodetection of Escherichia coli O157:H7

Gold Nanoparticle-Decorated Polymer Particles for High-Optical-Density Immunoassay Probes

To realize a highly sensitive immunoassay, high-optical-density probes conjugated with antibodies for target antigens are needed to increase the detectability of antigen-antibody complex formation. In this work, gold nanoparticle (NP)-decorated polymer (GNDP) particles were successfully prepared by mixing positively charged polymer particles and negatively charged Au NPs. GNDP particles decorated with NPs of 20 nm in size had higher optical density than the original Au NPs and GNDPs decorated with smaller Au NPs. Using GNDP particles as a probe, a highly sensitive immunoassay for influenza H1N1 hemagglutinin was realized with a minimum detectable concentration of 32.5 pg/mL. These results indicate that GNDP particles have high potential as an immunoassay probe that can be used in practical immunoassay systems for detecting a wide variety of antigens.

One-pot rapid visual detection of E. coli O157:H7 by label-free AuNP-based plasmonic-aptasensor in water sample

Access to clean water for irrigation and drinking has long been a global concern. The need for fast, precise, and cost-effective methods to detect harmful bacteria like Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is high due to the potential for severe infectious diseases. Fortunately, recent research has led to developing and utilizing rapid bacterial detection methods. The creation of an aptamer-based biosensor (aptasensor) for the detection of E. coli O157:H7 using label-free aptamers and gold nanoparticles (AuNPs) is described in this study. The specific aptamers that can detect target bacteria are adsorbed on the surface of unmodified AuNPs to form the aptasensor. The detection is performed by target bacterium-induced aptasensor aggregation, which is associated with a red-to-purple color change under high-salt circumstances. We devised a quick and easy method for detecting bacteria using an anti-E. coli O157:H7 aptamer without the need for specialized equipment or pretreatment processes like cell lysis. The aptasensor could identify target bacteria with only as few as 250 colony-forming units (CFU)/ml in 15 min or less, and its specificity based on our test was 100%. This method not only provides a fast direct preparation process but also exhibits remarkable proficiency in promptly identifying the intended target with a heightened level of sensitivity and specificity. Therefore, it can serve as an intelligent tool for monitoring water reservoirs and preventing the transmission of infectious diseases associated with EHEC.

Smartphone-based colorimetric detection of cardiac troponin T via label-free aptasensing

We report an aptasensing platform for the detection of cardiac troponin T (cTnT) in the immediate and early phases of acute myocardial infarction (AMI). High-flow filter paper was used to fabricate a microfluidic paper-based analytical device (μ-PAD), which was further modified with gold-decorated polystyrene microparticles functionalized with a highly specific cTnT aptamer. Herein, cTnT detection is presented in two linear ranges (0.01-0.8 μg/ml and 6.25-50 μg/ml) with an LoD of 3.9X10^-4 μg/ml, which is in agreement with reference values determined by the American Heart Association. The proposed platform showed remarkable selectivity against AMI-associated cardiac biomarkers such as TNF-alpha, interleukin-6, cardiac troponin I, and reactive protein-C. This aptasensor is a label-free assay that relies only on smartphone-based image analysis and takes less processing time in comparison with traditional methods like ELISA. Furthermore, it exhibits outstanding stability over 23 days when devices are stored at 4 °C. The reported platform is a stable and cost-effective method for the on-site and user-friendly detection of cTnT in normal saline buffer and diluted human serum.

A ratiometric electrochemical strategy based on Fe (III) and Pt (IV) for immobilization-free detection of Escherichia coli

A new ratiometric electrochemical strategy for immobilization-free detection of Escherichia coli (E. coli) was constructed by using a capture DNA-polyaniline/copper ferrite nanoparticles/graphene oxide (cDNA-PANI/CuFe₂O₄/GO) composite as capture probes, which has a high specific surface area and good magnetic properties. Then trigger DNA/Au nanoparticles (tDNA/Au NPs) were used as signal amplification labels, and Pt (IV) and Fe (III) were chosen as the signal probes. In the presence of targets, the sandwich format among cDNA-PANI/CuFe₂O₄/GO, E. coli and auxiliary DNA (aDNA) was realized by using the aptamer recognition system. Then, the tDNA/Au binding could be anchored on the sandwich format due to the principle of base complementation between unpaired aDNA and tDNA. And the unbounded tDNA of tDNA/Au NPs could bind an amount of Pt (IV). After separation using a magnet, a handful of unbound Pt (IV) which remained in the supernatant reacted with a large number of Fe (III) ions, leading to a markedly increased I_Fe(III)/I_Pt(IV) value. Oppositely, the sandwich format could not appear in the absence of targets, and even the tDNA/Au could not be immobilized on it. So, the redox reaction between a large amount of Pt (IV) residue in the supernatant and Fe (III) was significantly successful, causing a low I_Fe(III)/I_Pt(IV) value. Under optimal conditions, we found that I_Fe(III)/I_Pt(IV) was linearly related to the logarithmic E. coli concentration with a low limit of detection (1.862 × 10³ cfu mL^-1). This devised ratiometric electrochemical method may develop into a powerful and effective means for the detection of E. coli in real samples, which may also be developed as a universal tool for another microorganism.

Simultaneous colorimetric and electrochemical detection of trace mercury (Hg2+) using a portable and miniaturized aptasensor

We report a novel aptasensor for the simultaneous colorimetric and electrochemical detection of mercury (Hg²⁺). This device consists of a paper-based microfluidic component (μ-PAD) incorporated into a miniaturized three-electrode system fabricated through printed circuit board (PCB) technology. This biosensor is portable, rapid, versatile, and can detect Hg²⁺ down to 0.01 ppm based on 3σ of the blank/slope criteria. Moreover, it is highly selective against As²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, Mg²⁺, and Fe²⁺, reaching up to 13 times more of the input signal than the other heavy metals. The colorimetric detection mechanism uses aptamer functionalized polystyrene (PS)-AgNPs and Ps-AuNPs microparticles' specific aggregation. The Ps-AuNPs-based system allows qualitative detection (LOD 5 ppm) and stability over seven days (up to 97.59% signal retention). For the Ps-AgNPs-based system, the detection limit is 0.5 ppm with a linear range from 0.5 to 20 ppm (adjusted R²= 0.986) and stability over 30 days (up to 94.95% signal retention). The electrochemical component measures changes in charge transfer resistance upon target-aptamer hybridization using a [Ru (NH₃)₆]³⁺Cl₃] redox probe. The latest component presents a linear range from 0.01 to 1 ppm (adjusted R²= 0.935) with a LOD of 0.01 ppm and performance stability over seven days (up to 102.52 ± 11.7 signal retention). This device offers a universal dual detection platform with multiplexing, multi-replication, quantitative color analysis, and minimization of false results. Furthermore, detection results in river samples showed recoveries up to 91.12% (RSD 0.85) and 105.61% (RSD 1.62) for the electrochemical and colorimetric components, respectively. The proposed system is highly selective with no false-positive or false-negative results in an overall wide linear range and can safeguard the accuracy of detection results in aptasensing platforms in general.

Nanomaterial application in bio/sensors for the detection of infectious diseases

Infectious diseases are a potential risk for public health and the global economy. Fast and accurate detection of the pathogens that cause these infections is important to avoid the transmission of the diseases. Conventional methods for the detection of these microorganisms are time-consuming, costly, and not applicable for on-site monitoring. Biosensors can provide a fast, reliable, and point of care diagnostic. Nanomaterials, due to their outstanding electrical, chemical, and optical features, have become key players in the area of biosensors. This review will cover different nanomaterials that employed in electrochemical, optical, and instrumental biosensors for infectious disease diagnosis and how these contributed to enhancing the sensitivity and rapidity of the various sensing platforms. Examples of nanomaterial synthesis methods as well as a comprehensive description of their properties are explained. Moreover, when available, comparative data, in the presence and absence of the nanomaterials, have been reported to further highlight how the usage of nanomaterials enhances the performances of the sensor.

Polymethylolacrylamide/AuNPs Nanocomposites: Electrochemical Synthesis and Functional Characteristics

In this study the advantages of the electrochemical approach to the formation of polymer/metal nanoparticle composites are demonstrated. The method enables one to simplify the multistage processes of traditional technologies for the production of such materials through combining all intermediate processes in one stage and reducing the total formation time to 3–10 min. The possibility of a single-stage formation of a polymethylolacrylamide/AuNPs composite through including AuNPs into an electrically non-conducting polymethylolacrylamide film (carrier) formed by electropolymerization through potentiostatic electrolysis is also demonstrated for the first time. It is established that the addition of tetrachloroauric acid (HAuCl₄·4H₂O) into a monomeric composition containing acrylamide, formaldehyde, N,N′-methylene-bis-acrylamide, zinc chloride, and H₂O results in simultaneous electrochemical initiation of polymerization with the formation of a polymer film on the cathode, electrolytic reduction of gold ions to Au⁰, and immobilization of AuNPs particles into the growing polymer matrix. It was found that the formation of the PMAA / AuNPs composite is energetically more favorable than the synthesis of the main PMAA film, since it proceeds at a lower cathodic potential. The inclusion of AuNPs into the polymethylolacrylamide film was confirmed visually, as well as by X-ray phase analysis, small-angle X-ray scattering, microscopy, and element analysis. The gold content in the composite increases along with the increase of the concentration of HAuCl₄ in the electrolyte. The radius of the AuNPs particles was found to range between 3 and 7 nm. The AuNPs particles are spherical in shape and can combine into larger clusters containing up to 10 or more particles. The dynamics of formation, structure, and morphology of the polymethylolacrylamide/AuNPs composite were investigated. It was revealed that gold nanoparticles are mainly concentrated in the near-electrode and near-solution layers of the composite. We found that the composite has electrocatalytic activity. The possibility of its use as a sensor for hydrogen peroxide is demonstrated.

A molecule capturer analysis system for visual determination of avian pathogenic Escherichia coli serotype O78 using a lateral flow assay

A method for rapid and accurate determination of avian pathogenic Escherichia coli serotype O78 (APEC O78) by the gold nanoparticle-labeled lateral flow strip method, entitled molecule capturer analysis system (MCAS), is described. Target virulence-associated gene of APEC O78 is adopted as the analyte. After pre-amplification with the designed functional primer set, numerous new-formed amplicons are simultaneously labeled with fluorescein isothiocyanate (FITC) and digoxin. AuNPs with a diameter of 18 nm and the characteristic plasmonic peak at 526 nm are utilized for labeling. These two labels of FITC and digoxin are further captured and measured with the AuNP-labeled lateral flow strip, and the AuNPs are retained on the test line through the immunoreaction for signal output. Under optimized conditions, this MCAS protocol can determine the target APEC O78 with excellent determination limit of 4.3 cfu mL^-1 based on the optical density of AuNPs on the test line of lateral flow strips. The working range is 2.52 × 10¹ to 1.63 × 10⁷ cfu mL^-1. Spiked serum samples are rapid and accurately measured, and the results are highly correlated with those of the real-time PCR. With this MCAS protocol, rapid and on-site determination of APEC O78 can be realized without expensive instruments or professional personnel. This MCAS protocol can be easily applied to other analytes by just replacing the traditional primer set with functionalization primer set. Graphical abstract Schematic illustration of molecule capturer analysis system for rapid and accurate determination of avian pathogenic Escherichia coli serotype O78.

Development and optimization of thermal contrast amplification lateral flow immunoassays for ultrasensitive HIV p24 protein detection

Detection of human immunodeficiency virus (HIV) p24 protein at a single pg/ml concentration in point-of-care (POC) settings is important because it can facilitate acute HIV infection diagnosis with a detection sensitivity approaching that of laboratory-based assays. However, the limit of detection (LOD) of lateral flow immunoassays (LFAs), the most prominent POC diagnostic platform, falls short of that of laboratory protein detection methods such as enzyme-linked immunosorbent assay (ELISA). Here, we report the development and optimization of a thermal contrast amplification (TCA) LFA that will allow ultrasensitive detection of 8 pg/ml p24 protein spiked into human serum at POC, approaching the LOD of a laboratory test. To achieve this aim, we pursued several innovations as follows: (a) defining a new quantitative figure of merit for LFA design based on the specific to nonspecific binding ratio (BR); (b) using different sizes and shapes of gold nanoparticles (GNPs) in the systematic optimization of TCA LFA designs; and (c) exploring new laser wavelengths and power regimes for TCA LFA designs. First, we optimized the blocking buffer for the membrane and running buffer by quantitatively measuring the BR using a TCA reader. The TCA reader interprets the thermal signal (i.e., temperature) of GNPs within the membrane when irradiated by a laser at the plasmon resonance wavelength of the particle. This process results in higher detection and quantitation of GNPs than in traditional visual detection (i.e., color intensity). Further, we investigated the effect of laser power (30, 100, 200 mW), GNP size and shape (30 and 100 nm gold spheres, 150 nm gold-silica shells), and laser wavelength (532, 800 nm). Applying these innovations to a new TCA LFA design, we demonstrated that 100 nm spheres with a 100 mW 532 nm laser provided the best performance (i.e., LOD = 8 pg/ml). This LOD is significantly better than that of the current colorimetric LFA and is in the range of the laboratory-based p24 ELISA. In summary, this TCA LFA for p24 protein shows promise for detecting acute HIV infection in POC settings.

Inkjet Printed Nanopatterned Aptamer-Based Sensors for Improved Optical Detection of Foodborne Pathogens

The increasing incidence of infectious outbreaks from contaminated food and water supply continues imposing a global burden for food safety, creating a market demand for on-site, disposable, easy-to-use, and cost-efficient devices. Despite of the rapid growth of biosensors field and the generation of breakthrough technologies, more than 80% of the platforms developed at lab-scale never will get to meet the market. This work aims to provide a cost-efficient, reliable, and repeatable approach for the detection of foodborne pathogens in real samples. For the first time an optimized inkjet printing platform is proposed taking advantage of a carefully controlled nanopatterning of novel carboxyl-functionalized aptameric ink on a nitrocellulose substrate for the highly efficient detection of E. coli O157:H7 (25 colony forming units (CFU) mL^-1 in pure culture and 233 CFU mL^-1 in ground beef) demonstrating the ability to control the variation within ±1 SD for at least 75% of the data collected even at very low concentrations. From the best of the knowledge this work reports the lowest limit of detection of the state of the art for paper-based optical detection of E. coli O157:H7, with enough evidence (p > 0.05) to prove its high specificity at genus, species, strain, and serotype level.

Single cell immunodetection of Escherichia coli O157:H7 on an indium-tin-oxide electrode by using an electrochemical label with an organic-inorganic nanostructure

A rapid and highly sensitive method is described for the detection of enterohemorrhagic Escherichia coli O157:H7. An organic-inorganic nanostructure in which numerous gold nanoparticles (AuNPs) are enclosed with polyaniline (PANI) was utilized as an electrochemical label. The nanostructure showed (a) strong light scattering intensity due to the coupling effect of the surface plasmon resonance based on the presence of AuNPs, and (b) high electrochemical response due to the redox activity of PANI. To achieve selectivity, antibody against E. coli O157:H7 was immobilized on the surface of the nanostructure. The method exploits the combination of strong adsorption of bacterial cells onto the indium-tin-oxide (ITO) glass electrode without any special processing and specific binding of the nanostructured label to E. coli O157:H7. This enables the electrochemical detection of a single cell on the ITO electrode. The electrochemical response to E. coli O157:H7 was 30-fold higher than that to other types of bacteria. This procedure can be applied to the determination of E. coli O157:H7 even in the presence of other bacteria. Graphical abstract Schematic of a voltammetric immunoassay for Escherichia coli O157:H7 by using a nanocomposite consisting of gold nanoparticles and polyaniline on an ITO electrode.

Electrochemical sandwich immunoassay for Escherichia coli O157:H7 based on the use of magnetic nanoparticles and graphene functionalized with electrocatalytically active Au@Pt core/shell nanoparticles

A highly sensitive electrochemical sandwich immunoassay is described for determination of Escherichia coli O157:H7 (E. coli O157:H7). Silica coated magnetite nanoparticles (Fe₃O₄) were modified with primary antibody to capture E. coli O157:H7. Gold-platinum core/shell nanoparticles (Au@Pt NPs) with different Pt shell thicknesses were prepared via changing the molar ratio of H₂PtCl₆ to HAuCl₄ in the precursor solution. The optimized Au@Pt NPs exhibit enhanced activity in the electrocatalytic reduction of hydrogen peroxide (H₂O₂). The Au@Pt NPs were modified with graphene that was functionalized with Neutral Red, and then used as an electrochemical label for secondary antibodies and horseradish peroxidase (HRP). The sandwich immunocomplexes were magnetically absorbed on a 4-channel screen printed carbon electrode. Due to the catalysis of the Au@Pt NPs and HRP, the signal is strongly amplified in the presence of H₂O₂ when using thionine as the electron mediator. Under optimal conditions, the immunoassay has a linear response in the 4.0 × 10² to 4.0 × 10⁸ CFU·mL^-1 concentration range, with a limit of detection of 91 CFU·mL^-1 (at an S/N ratio of 3). Graphical abstract Preparation of Au@Pt core/shell nanoparticles with different Pt shell thickness (A), rGO-NR (B), rGO-NR-Au@Pt-Ab₂-HRP (C) and the preparation and the detection process of the immunoassay (D). rGO: reduced graphene oxide, GO: graphene oxide, NR: Neutral Red, HRP: horseradish peroxidase, AuNPs: gold nanoparticles, Fe₃O₄@SiO₂: Silica coated magnetite nanoparticles, 4-SPCE: 4-channel screen printed carbon electrode.