Integrated gold superparticles into lateral flow immunoassays for the rapid and sensitive detection of Escherichia coli O157:H7 in milk

A nanoparticle-assisted signal-enhancement technique for lateral flow immunoassays

Lateral flow immunoassay (LFIA), an affordable and rapid paper-based detection technology, is employed extensively in clinical diagnosis, environmental monitoring, and food safety analysis. The COVID-19 pandemic underscored the validity and adoption of LFIA in performing large-scale clinical and public health testing. The unprecedented demand for prompt diagnostic responses and advances in nanotechnology have fueled the rise of next-generation LFIA technologies. The utilization of nanoparticles to amplify signals represents an innovative approach aimed at augmenting LFIA sensitivity. This review probes the nanoparticle-assisted amplification strategies in LFIA applications to secure low detection limits and expedited response rates. Emphasis is placed on comprehending the correlation between the physicochemical properties of nanoparticles and LFIA performance. Lastly, we shed light on the challenges and opportunities in this prolific field.

A Review on Self-Assembly of Colloidal Nanoparticles into Clusters, Patterns, and Films: Emerging Synthesis Techniques and Applications

The colloidal synthesis of functional nanoparticles has gained tremendous scientific attention in the last decades. In parallel to these advancements, another rapidly growing area is the self-assembly or self-organization of these colloidal nanoparticles. First, the organization of nanoparticles into ordered structures is important for obtaining functional interfaces that extend or even amplify the intrinsic properties of the constituting nanoparticles at a larger scale. The synthesis of large-scale interfaces using complex or intricately designed nanostructures as building blocks, requires highly controllable self-assembly techniques down to the nanoscale. In certain cases, for example, when dealing with plasmonic nanoparticles, the assembly of the nanoparticles further enhances their properties by coupling phenomena. In other cases, the process of self-assembly itself is useful in the final application such as in sensing and drug delivery, amongst others. In view of the growing importance of this field, this review provides a comprehensive overview of the recent developments in the field of nanoparticle self-assembly and their applications. For clarity, the self-assembled nanostructures are classified into two broad categories: finite clusters/patterns, and infinite films. Different state-of-the-art techniques to obtain these nanostructures are discussed in detail, before discussing the applications where the self-assembly significantly enhances the performance of the process.

The influence of hapten spacer arm length on antibody response and immunoassay development

Antibodies against small molecules with high titer and high affinity are always pursued in the field of vaccines for drugs of abuse, antidotes to toxins and immunoassays in medical, environmental, and food safety. The exposure degree of the target molecule to the immune system is critical to induce a strongly specific antibody response, thus, the spacer arm length between the target molecule and carrier protein plays an important role. However, the influence of spacer arm length on antibody titer, affinity, and assay performance is not yet clear and highly demanded to be addressed. In the present study, we proposed a model study to answer the question by using two typical small molecules, melamine and p-nitroaniline, which were introduced by varied spacer arms with increasing alkane linear length from 2 to 12 carbon atoms brick by brick. The spacer arm lengths of the haptens were obtained by computational chemistry. The titer and affinity of mouse antisera were analyzed and compared, showing that all haptens with spacer arms of 6-8 carbon atoms, i.e. 6.3-8.8 Å in length, induced strong antibodies represented by the highest titer and affinity without exception, while the haptens with spacer arms of 2-4 carbon atoms and 10-12 carbon atoms, i.e. 1.5-3.9 Å and 11.3-13.9 Å in length, failed to induce high-quality antibody response. Moreover, the titer and sensitivity of the subsequently developed immunoassays were significantly affected by using coating haptens with different spacer arm lengths, and coating haptens with a spacer arm of 6.3-8.8 Å in length delivered the optimum detection performance. The antibody recognition mechanism study further confirmed that the hapten spacer arm length had a critical effect on the recognition properties of the induced antibody, which should be interactive with the spacer arm each other. This study showed that the hapten with appropriate spacer arm length is important to antibody response and immunoassay development, providing a valuable and general clue for the rational design of hapten.

Plasmon color-preserved gold nanoparticle clusters for high sensitivity detection of SARS-CoV-2 based on lateral flow immunoassay

Lateral flow immunoassays (LFI) have shown great promise for point-of-care (POC) sensing applications, however, its clinical translation is often hindered by insufficient sensitivity for early detection of diseases, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This is mainly due to weak absorption signals of single gold nanoparticles (AuNPs). Here, we developed AuNP clusters that maintain the red color of isolated individual AuNPs, but increase the colorimetric readout to improve the detection sensitivity. The plasmon color-preserved (PLASCOP) AuNP clusters is simply made by mixing streptavidin-coated AuNP core with satellite AuNPs coated with biotinylated antibodies. The biotinylated antibody-streptavidin linker forms a gap size over 15 nm to avoid plasmon coupling between AuNPs, thus maintaining the plasmonic color while increasing the overall light absorption. LFI sensing using PLASCOP AuNP clusters composed of 40 nm AuNPs showed a high detection sensitivity for SARS-CoV-2 nucleocapsid proteins with a limit of detection (LOD) of 0.038 ng mL⁻¹, which was 23.8- and 5.9-times lower value than that of single 15 nm and 40 nm AuNP conjugates, respectively. The PLASCOP AuNP clusters-based LFI sensing also shows good specificity for SARS-CoV-2 nucleocapsid proteins from other influenza and coronaviruses. In a clinical feasibility test, we demonstrated that SARS-CoV-2 particles spiked in human saliva could be detected with an LOD of 54 TCID₅₀ mL⁻¹. The developed PLASCOP AuNP clusters are promising colorimetric sensing reporters that present improved sensitivity in LFI sensing for broad POC sensing applications beyond SARS-CoV-2 detection.

'Three-To-One' multi-functional nanocomposite-based lateral flow immunoassay for label-free and dual-readout detection of pathogenic bacteria

Sandwich lateral flow immunoassays (LFIAs) based on paired antibodies are the most frequently used platform for food-borne pathogen detection. Although label-free strategies are used in LFIAs to avoid the utilization of paired antibodies, challenges of probe design and detection reliability still remain. Here, we report a new label-free and dual-readout LFIA (LD-LFIA) mediated by a 'Three-To-One' multi-functional nanocomposite with a unique combination of magnetic-adhesion-color-nanozyme properties. The strengths of the new designed nanocomposite are: (i) the Fe₃O₄ magnetic core simplifies the separation processes; (ii) surface adherent polydopamine (PDA) films exhibit a strong adhesion to pathogenic bacteria and provide colorimetric detection signal; and (iii) the deposited platinum nanoparticles (Pt NPs) can function as nanozymes to generate an extra catalytic signal for constructing a dual-readout mode to improve the detection accuracy. The resulting Fe₃O₄@PDA@Pt nanocomposite-based LD-LFIA can detect highly pathogenic Escherichia coli O157:H7 with limits of detection of 10² and 10 CFU mL^-1 for colorimetric and catalytic quantitative analyses, respectively. Systematic results also reveal that the proposed method exhibited high specificity and applicability for drinking water and chicken samples, serving as a promising tool for real bacterial sample testing. The multi-functional Fe₃O₄@PDA@Pt nanocomposite-based LD-LFIA can provide new ideas for designing new multi-functional probes for improving detection performance of conventional label-free LFIA and constructing more accurate and sensitive detection systems.

An Overview for the Nanoparticles-Based Quantitative Lateral Flow Assay

The development of the lateral flow assay (LFA) has received much attention in both academia and industry because of their broad applications to food safety, environmental monitoring, clinical diagnosis, and so forth. The user friendliness, low cost, and easy operation are the most attractive advantages of the LFA. In recent years, quantitative detection has become another focus of LFA development. Here, the most recent studies of quantitative LFAs are reviewed. First, the principles and corresponding formats of quantitative LFAs are introduced. In the biomaterial and nanomaterial sections, the detection, capture, and signal amplification biomolecules and the optical, fluorescent, luminescent, and magnetic labels used in LFAs are described. The invention of dedicated strip readers has drawn further interest in exploiting the better performance of LFAs. Therefore, next, the development of dedicated reader devices is described and the usefulness and specifications of these devices for LFAs are discussed. Finally, the applications of LFAs in the detection of metal ions, biotoxins, pathogenic microorganisms, veterinary drugs, and pesticides in the fields of food safety and environmental health and the detection of nucleic acids, biomarkers, and viruses in clinical analyses are summarized.

A gold nanoparticle based colorimetric sensor for the rapid detection of Yersinia enterocolitica serotype O:8 in food samples

Foodborne diseases from Yersinia enterocolitica serotype O:8 represent global public health problems.

An Enhanced Lateral Flow Assay Based on Aptamer-Magnetic Separation and Multifold AuNPs for Ultrasensitive Detection of Salmonella Typhimurium in Milk

In this paper, a novel and ultrasensitive lateral flow assay (LFA) based on aptamer-magnetic separation, and multifold Au nanoparticles (AuNPs) was developed for visual detecting Salmonella enterica ser. Typhimurium (S. Typhimurium). The method realized magnetic enrichment and signal transduction via magnetic separation and achieved signal amplification through hybridizing AuNPs-capture probes and AuNPs-amplification probes to form multifold AuNPs. Two different thiolated single-strand DNA (ssDNA) on the AuNPs-capture probe played different roles. One was combined with the AuNPs-amplification probe on the conjugate pad to achieve enhanced signals. The other was connected to transduction ssDNA1 released by aptamer-magnetic capture of S. Typhimurium, and captured by the T-line, forming a positive signal. This method had an excellent linear relationship ranging from 8.6 × 102 CFU/mL to 8.6 × 107 CFU/mL with the limit of detection (LOD) as low as 8.6 × 100 CFU/mL in pure culture. In actual samples, the visual LOD was 4.1 × 102 CFU/mL, which did not carry out nucleic acid amplification and pre-enrichment, increasing three orders of magnitudes than unenhanced assays with single-dose AuNPs and no magnetic separation. Furthermore, the system showed high specificity, having no reaction with other nontarget strains. This visual signal amplificated system would be a potential platform for ultrasensitive monitoring S. Typhimurium in milk samples.

A strip of lateral flow gene assay using gold nanoparticles for point-of-care diagnosis of African swine fever virus in limited environment

Recombinase polymerase amplification (RPA) was combined with lateral flow to develop a gold nanoparticles test strip for point-of-care diagnosis of African swine fever virus (ASFV), which is called lateral flow gene assay (LFGA). Common diagnostic techniques, including polymerase chain reaction (PCR) and immunochromatography, are time-consuming and labor-intensive, and generally require costly instruments. For improvement, this assay used tailed primers to produce DNA duplexes with a single-stranded tail at one end which can hybridize with a gold nanoparticle (AuNP)-labeled oligonucleotide detection probe. And then, biotin attached to the other end of the product bound to streptavidin, which previously fixed to the test line. Therefore, there would form a sandwich structure, and gold nanoparticles labeled on the detection probe would show a red band on the test line of strip. With the low reaction temperature (37~42 °C) and short reaction time (30 min), LFGA can specifically identify ASFV in blood samples infected with ASFV and classical swine fever virus (CSFV), and the LOD was 10² copies/μL, which was comparable to that of agarose gel electrophoresis. In addition, blood samples infected with ASFV and CSFV were tested, and it was found that the LFGA can specifically identify ASFV DNA. In conclusion, LFGA achieves visual observation of the product after rapid RPA amplification and does not require any expensive instruments during the entire process, which is very helpful for early diagnosis of ASFV. Combined recombinase polymerase amplification (RPA) with lateral flow, we developed a gold nanoparticles test strip for point-of-care diagnosis of African swine fever virus. The upstream primers of RPA were modified with biotin, and the downstream primers were modified with a C3 spacer and an oligonucleotide tail that can be hybridized to a gold nanoparticle-labeled oligonucleotide detection probe. On the strip, the test line and control line were sprayed with streptavidin and an oligonucleotide control probe. In the presence of positive products, RPA products can form a sandwich structure on the test line. Therefore, two red lines will be displayed both on the test line and control line. When there is no positive product, only the control line is shown in red. Its low reaction temperature (37~42 °C) and short time of amplification and detection (30 min) make ASFV realizing point-of-care diagnosis in limited environment.

Binding affinity-guided design of a highly sensitive noncompetitive immunoassay for small molecule detection

Small molecules are immunochemically classified as hapten that lacking of at least two epitopes, usually using competitive format for establishing immunoassays. However, theoretically, noncompetitive immunoassay format is more sensitive and has a wider analytical range. In the present study, a novel hapten of halofuginone was synthesized and used to produce a monoclonal antibody (mAb). By analyzing the binding kinetics, we found that the affinity of analyte-enzyme to mAb was much greater than that of analyte, which could result in a low sensitivity of competitive assay format. Based on this, we established a novel noncompetitive immunoassay by using a replacement approach. The noncompetitive format has obvious advantages in sensitivity and analytical range, which promoted approximately 3.5- and 5-fold, respectively, compared to the competitive immunoassay. Ultimately, the newly designed noncompetitive immunoassay in this work will provide insights as well as alternative method to traditional small molecule competitive assays.