Rapid, quantitative and ultra-sensitive detection of cancer biomarker by a SERRS-based lateral flow immunoassay using bovine serum albumin coated Au nanorods

Establishment of a Raman microsphere-based immunochromatographic method for the combined detection of influenza A and B viruses and SARS-CoV-2 antigen on a single T-line

A simple and rapid method based on Raman microsphere immunochromatography was developed in this study for the simultaneous detection of influenza A and B viruses and SARS-CoV-2 on a single test T-line. Three types of Raman microspheres with different Raman characteristics were used as the signal sources and were labelled with monoclonal antibodies against FluA, FluB and SARS-CoV-2, respectively. A mixture of antibodies containing anti-FluA monoclonal antibody, anti-FluB monoclonal antibody and anti-SARS-CoV-2 was sprayed on the detection line (T), and goat polyclonal antibody to chicken (IgY) encapsulated on the quality control line (C), for qualitative detection of these three viruses by the double antibody sandwich method. The results demonstrated that the LOD values were 0.5 ng mL-1 for FluA, 0.25 ng mL-1 for FluB, and 0.5 ng mL-1 for SARS-CoV-2. The method showed good repeatability for the respiratory viral antigens, with CV values below 15%. Oxymetazoline and commonly used oral medications did not interfere with the test results; the strips did not cross-react with common respiratory virus antigens, demonstrating good specificity. This method does not require any complicated pre-treatment, and all three viruses can be detected simultaneously by titrating one sample, which improves the detection efficiency. The Respiratory Pathogen Multiplex provides a scientific basis for preventing and controlling the spread of respiratory diseases by analyzing data to understand epidemiological trends and the spread of pathogens.

Highly sensitive multiplexed colorimetric lateral flow immunoassay by plasmon-controlled metal-silica isoform nanocomposites: PINs

Lateral flow assay (LFA) systems use metal nanoparticles for rapid and convenient target detection and are extensively studied for the diagnostics of various diseases. Gold nanoparticles (AuNPs) are often used as probes in LFAs, displaying a single red color. However, there is a high demand for colorimetric LFAs to detect multiple biomarkers, requiring the use of multicolored NPs. Here, we present a highly sensitive multiplexed colorimetric lateral flow immunoassay by multicolored Plasmon-controlled metal-silica Isoform Nanocomposites (PINs). We utilized the localized surface plasmon resonance effect to create multi-colored PINs by precisely adjusting the distance between the NPs on the surface of PINs through the controlled addition of reduced gold and silver precursors. Through simulations, we also confirmed that the distance between nanoparticles on the surface of PINs significantly affects the color and colorimetric signal intensity of the PINs. We achieved multicolored PINs that exhibit stronger colorimetric signals, offering a new solution for LFA detection with high sensitivity and a 33 times reduced limit of detection (LOD) while maintaining consistent size deviations within 5%. We expect that our PINs-based colorimetric LFA will facilitate the sensitive and simultaneous detection of multiple biomarkers in point-of-care testing.

Multiplexed immunolabelling of cancer using bioconjugated plasmonic gold-silver alloy nanoparticles

Reliable protein detection methods are vital for advancing biological research and medical diagnostics. While immunohistochemistry and immunofluorescence are commonly employed, their limitations underscore the necessity for alternative approaches. This study introduces immunoplasmonic labelling, utilizing plasmonic nanoparticles (NPs), specifically designed gold and gold-silver alloy NPs (Au:Ag NPs), for multiplexed and quantitative protein detection. These NPs, when coupled with antibodies targeting proteins of interest, enable accurate counting and evaluation of protein expression levels while overcoming issues such as autofluorescence. In this study, we compare two nanoparticle functionalization strategies-one coating based on thiolated PEG and one coating based on calix[4]arenes-on gold and gold-silver alloy nanoparticles of varying sizes. Overall results tend to demonstrate a greater versatility for the calix[4]arene-based coating. With this coating and using the classical EDC/sulfo-NHS cross-linking procedure, we also demonstrate the successful multiplexed immunolabelling of Her2, CD44, and EpCAM in breast cancer cell lines (SK-BR-3 and MDA-MB-231). Furthermore, we introduce a user-friendly software for automatic NP detection and classification by colour, providing a promising proof-of-concept for the practical application of immunoplasmonic techniques in the quantitative analysis of biopsies in the clinical setting.

SERS biosensors for liquid biopsy towards cancer diagnosis by detection of various circulating biomarkers: current progress and perspectives

Liquid biopsy has emerged as a promising non-invasive strategy for cancer diagnosis, enabling the detection of various circulating biomarkers, including circulating tumor cells (CTCs), circulating tumor nucleic acids (ctNAs), circulating tumor-derived small extracellular vesicles (sEVs), and circulating proteins. Surface-enhanced Raman scattering (SERS) biosensors have revolutionized liquid biopsy by offering sensitive and specific detection methodologies for these biomarkers. This review comprehensively examines the application of SERS-based biosensors for identification and analysis of various circulating biomarkers including CTCs, ctNAs, sEVs and proteins in liquid biopsy for cancer diagnosis. The discussion encompasses a diverse range of SERS biosensor platforms, including label-free SERS assay, magnetic bead-based SERS assay, microfluidic device-based SERS system, and paper-based SERS assay, each demonstrating unique capabilities in enhancing the sensitivity and specificity for detection of liquid biopsy cancer biomarkers. This review critically assesses the strengths, limitations, and future directions of SERS biosensors in liquid biopsy for cancer diagnosis.

Immunoassays: Analytical and Clinical Performance, Challenges, and Perspectives of SERS Detection in Comparison with Fluorescent Spectroscopic Detection

Immunoassays (IAs) with fluorescence-based detection are already well-established commercialized biosensing methods, such as enzyme-linked immunosorbent assay (ELISA) and lateral flow immunoassay (LFIA). Immunoassays with surface-enhanced Raman spectroscopy (SERS) detection have received significant attention from the research community for at least two decades, but so far they still lack a wide clinical commercial application. This review, unlike any other review that we have seen, performs a three-dimensional performance comparison of SERS IAs vs. fluorescence IAs. First, we compared the limit of detection (LOD) as a key performance parameter for 30 fluorescence and 30 SERS-based immunoassays reported in the literature. We also compared the clinical performances of a smaller number of available reports for SERS vs. fluorescence immunoassays (FIAs). We found that the median and geometric average LODs are about 1.5-2 orders of magnitude lower for SERS-based immunoassays in comparison to fluorescence-based immunoassays. For instance, the median LOD for SERS IA is 4.3 × 10-13 M, whereas for FIA, it is 1.5 × 10-11 M. However, there is no significant difference in average relative standard deviation (RSD)-both are about 5-6%. The analysis of sensitivity, selectivity, and accuracy reported for a limited number of the published clinical studies with SERS IA and FIA demonstrates an advantage of SERS IA over FIA, at least in terms of the median value for all three of those parameters. We discussed common and specific challenges to the performances of both SERS IA and FIA, while proposing some solutions to mitigate those challenges for both techniques. These challenges include non-specific protein binding, non-specific interactions in the immunoassays, sometimes insufficient reproducibility, relatively long assay times, photobleaching, etc. Overall, this review may be useful for a large number of researchers who would like to use immunoassays, but particularly for those who would like to make improvements and move forward in both SERS-based IAs and fluorescence-based IAs.

Recent advances in SERS-based immunochromatographic assay for pathogenic microorganism diagnosis: A review

Infectious diseases caused by bacteria, viruses, fungi, and other pathogenic microorganisms are among the most harmful public health problems in the world, causing tens of millions of deaths and incalculable economic losses every year. The establishment of rapid, simple, and highly sensitive diagnostic methods for pathogenic microorganisms is important for the prevention and control of infectious diseases, guidance of timely treatment, and the reduction of public safety risks. Lateral flow immunoassay (LFA) based on the colorimetric signal of colloidal gold is the most popular point-of-care testing technology at present, but it is limited by poor sensitivity and low throughput and hardly meets the needs of the highly sensitive screening of pathogenic microorganisms. In recent years, the combination of surface-enhanced Raman scattering (SERS) and LFA technology has developed into a novel analytical platform with high sensitivity and multiple detection capabilities and has shown great advantages in the detection of pathogenic microorganisms and infectious diseases. This review summarizes the working principle, design ideas, and application of the existing SERS-based LFA methods in pathogenic microorganism detection and further introduces the effect of new technologies such as Raman signal encoding, magnetic enrichment, novel membrane nanotags, and integrated Raman reading equipment on the performance of SERS-LFA. Finally, the main challenges and the future direction of development in this field of SERS-LFA are discussed.

In Situ Synthesis of Highly Fluorescent, Phosphorus-Doping Carbon-Dot-Functionalized, Dendritic Silica Nanoparticles Applied for Multi-Component Lateral Flow Immunoassay

The sensitivity of fluorescent lateral flow immunoassay (LFIA) test strips is compromised by the low fluorescence intensity of the signaling molecules. In this study, we synthesized novel phosphorus-doped carbon-dot-based dendritic mesoporous silica nanoparticles (DMSNs-BCDs) with a quantum yield as high as 93.7% to break this bottleneck. Meanwhile, the in situ growth method increased the loading capacity of carbon dots on dendritic mesoporous silica, effectively enhancing the fluorescence intensity of the composite nanospheres. Applied DMSNs-BCDs in LFIA can not only semi-quantitatively detect a single component in a short time frame (procalcitonin (PCT), within 15 min) but also detect the dual components with a low limit of detection (LOD) (carbohydrate antigen 199 (CA199) LOD: 1 U/mL; alpha-fetoprotein (AFP) LOD: 0.01 ng/mL). And the LOD of PCT detection (0.01 ng/mL) is lower by 1.7 orders of magnitude compared to conventional colloidal gold strips. For CA199, the LOD is reduced by a factor of four compared to LFIA using gold nanoparticles as substrates, and for AFP, the LOD is lowered by two orders of magnitude compared to colloidal gold LFIA. Furthermore, the coefficients of variation (CV) for intra-assay and inter-assay measurements are both less than 11%.

Rapid sub-nanomolar protein determination in serum using electropolymerized molecularly imprinted polymers (E-MIPs)

Rapid detection of biologicals is important for a range of applications such as medical screening and diagnostics. Antibodies are typically employed for biosensing with high sensitivity and selectivity but can take months to prepare. Here, we investigate electropolymerized molecularly imprinted polymers (E-MIPs), which are produced in minutes as alternative-antibody rapid biosensors for the selective recognition of model proteins bovine haemoglobin (BHb) and bovine serum albumin (BSA). We evaluated two disposable screen-printed electrodes (SPE) designated AT-Au and BT-Au based on their different annealing temperatures. E-MIPs for BHb demonstrated an imprinting factor of 146 : 1 at 1 nM and 12 : 1 at 0.1 nM, showing high effectiveness of E-MIPs compared to their control non-imprinted polymers. The BHb imprinted E-MIP, when tested against BSA as a non-target protein, gave a selectivity factor of 6 : 1 for BHb. Sensor sensitivity directly depended on the nature of the SPE, with AT-Au SPE demonstrating limits of detection in the sub-micromolar range typically achieved for MIPs, while BT-Au SPE exhibited sensitivity in the sub-nanomolar range for target protein. We attribute this to differences in electrode surface area between AT-Au and BT-Au SPEs. The E-MIPs were also tested in calf serum as a model biological medium. The BT-Au SPE MIPs detected the presence of target protein in <10 min with an LOD of 50 pM and LOQ of 100 pM, suggesting their suitability for protein determination in serum with minimal sample preparation. Using electrochemical impedance spectroscopy, we determine equilibrium dissociation constants (KD) for E-MIPs using the Hill-Langmuir adsorption model. KD of BHb E-MIP was determined to be 0.86 ± 0.11 nM.

SERS immuno- and apta-assays in biosensing/bio-detection: Performance comparison, clinical applications, challenges

Surface Enhanced Raman Spectroscopy is increasingly used as a sensitive bioanalytical tool for detection of variety of analytes ranging from viruses and bacteria to cancer biomarkers and toxins, etc. This comprehensive review describes principles of operation and compares the performance of immunoassays and aptamer assays with Surface Enhanced Raman scattering (SERS) detection to each other and to some other bioassay methods, including ELISA and fluorescence assays. Both immuno- and aptamer-based assays are categorized into assay on solid substrates, assays with magnetic nanoparticles and assays in laminar flow or/and strip assays. The best performing and recent examples of assays in each category are described in the text and illustrated in the figures. The average performance, particularly, limit of detection (LOD) for each of those methods reflected in 9 tables of the manuscript and average LODs are calculated and compared. We found out that, on average, there is some advantage in terms of LOD for SERS immunoassays (0.5 pM median LOD of 88 papers) vs SERS aptamer-based assays (1.7 pM median LOD of 51 papers). We also tabulated and analyzed the clinical performance of SERS immune and aptamer assays, where selectivity, specificity, and accuracy are reported, we summarized the best examples. We also reviewed challenges to SERS bioassay performance and real-life application, including non-specific protein binding, nanoparticle aggregation, limited nanotag stability, sometimes, relatively long time to results, etc. The proposed solutions to those challenges are also discussed in the review. Overall, this review may be interesting not only to bioanalytical chemist, but to medical and life science researchers who are interested in improvement of bioanalyte detection and diagnostics.

Lateral Flow Immunoassay for Proteins

Protein biomarkers are useful for disease diagnosis. Identification thereof using in vitro diagnostics such as lateral flow immunoassays (LFIAs) has attracted considerable attention due to their low cost and ease of use especially in the point of care setting. Current challenges, however, do remain with respect to material selection for each component in the device and the synergistic integration of these components to display detectable signals. This review explores the principle of LFIA for protein biomarkers, device components including biomaterials and labeling methods. Medical applications and commercial status are examined as well. This review highlights critical methodologies in the development of new LFIAs and their role in advancing healthcare worldwide.

Lateral flow assays for detection of disease biomarkers

Early diagnosis saves lives in many diseases. In this sense, monitoring of biomarkers is crucial for the diagnosis of diseases. Lateral flow assays (LFAs) have attracted great attention among paper-based point-of-care testing (POCT) due to their low cost, user-friendliness, and time-saving advantages. Developments in the field of health have led to an increase of interest in these rapid tests. LFAs are used in the diagnosis and monitoring of many diseases, thanks to biomarkers that can be observed in body fluids. This review covers the recent advances dealing with the design and strategies for the development of LFA for the detection of biomarkers used in clinical applications in the last 5 years. We focus on various strategies such as choosing the nanoparticle type, single or multiple test approaches, and equipment for signal transducing for the detection of the most common biomarkers in different diseases such as cancer, cardiovascular, infectious, and others including Parkinson's and Alzheimer's diseases. We expect that this study will contribute to the different approaches in LFA and pave the way for other clinical applications.

Simultaneously ultrasensitive and quantitative detection of influenza A virus, SARS-CoV-2, and respiratory syncytial virus via multichannel magnetic SERS-based lateral flow immunoassay

Respiratory viruses usually induced similar clinical symptoms at early infection. Herein, we presented a multichannel surface-enhanced Raman scattering-based lateral flow immunoassay (SERS-based LFA) using high-performance magnetic SERS tags for the simultaneous ultrasensitive detection of respiratory viruses, namely influenza A virus (H1N1), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and respiratory syncytial virus (RSV) in biological samples. As-prepared magnetic SERS tags can directly enrich and capture target viruses without pretreatment of samples, avoiding the interference of impurities in the samples as well as improving the sensitivity. With the capture-detection method, the detection limits of the proposed assay reached 85 copies mL^-1, 8 pg mL^-1, and 8 pg mL^-1 for H1N1, SARS-CoV-2 and RSV, respectively. Moreover, the detection properties of the proposed method for target viruses in throat swab samples were verified, suggesting its remarkable potential for the early and rapid differential diagnosis of respiratory viruses.

Expanded detection range of lateral flow immunoassay endowed with a third-stage amplifier indirect probe

Here, a third-stage amplifier indirect probe (TsAIP) based lateral flow immunoassay (LFIA) was proposed to detect furazolidone (FZD) with Prussian blue nanoparticles (PBNPs) as carrier to label the goat anti-mouse antibody-horseradish peroxidase conjugation [GAMA(HRP)]. In this strategy, owing to the fact that one monoclonal antibody (mAb) can combine several GAMA molecules simultaneously, the indirect probe can generate primary signal amplification, then realize second-stage amplification attributing to PBNPs, and finally achieve third-stage amplification because of the conjugated HRP. The TsAIP-based LFIA shows improved performance for FZD metabolite derivative with a detection limit of 1 ng mL^-1. The detection range is expanded about 2-fold compared with the original outcome. Besides, the proposed sensor could be successfully applied in food samples. This method provides a platform for broadening the detection range and application of PBNPs based LFIAs.

Highly Sensitive Nanomagnetic Quantification of Extracellular Vesicles by Immunochromatographic Strips: A Tool for Liquid Biopsy

Extracellular vesicles (EVs) are promising agents for liquid biopsy-a non-invasive approach for the diagnosis of cancer and evaluation of therapy response. However, EV potential is limited by the lack of sufficiently sensitive, time-, and cost-efficient methods for their registration. This research aimed at developing a highly sensitive and easy-to-use immunochromatographic tool based on magnetic nanoparticles for EV quantification. The tool is demonstrated by detection of EVs isolated from cell culture supernatants and various body fluids using characteristic biomarkers, CD9 and CD81, and a tumor-associated marker-epithelial cell adhesion molecules. The detection limit of 3.7 × 105 EV/µL is one to two orders better than the most sensitive traditional lateral flow system and commercial ELISA kits. The detection specificity is ensured by an isotype control line on the test strip. The tool's advantages are due to the spatial quantification of EV-bound magnetic nanolabels within the strip volume by an original electronic technique. The inexpensive tool, promising for liquid biopsy in daily clinical routines, can be extended to other relevant biomarkers.

Surface engineering strategies of gold nanomaterials and their applications in biomedicine and detection

Gold nanomaterials have potential applications in biosensors and biomedicine due to their controllable synthesis steps, high biocompatibility, low toxicity and easy surface modification. However, there are still various limitations including low water solubility and stability, which greatly affect their applications. In addition, some synthetic methods are very complicated and costly. Therefore, huge efforts have been made to improve their properties. This review mainly introduces the strategies for surface modification of gold nanomaterials, such as amines, biological small molecules and organic small molecules as well as the biological applications of these functionalized AuNPs. We aim to provide effective ideas for better functionalization of gold nanomaterials in the future.

Silver-Assembled Silica Nanoparticles in Lateral Flow Immunoassay for Visual Inspection of Prostate-Specific Antigen

Prostate-specific antigen (PSA) is the best-known biomarker for early diagnosis of prostate cancer. For prostate cancer in particular, the threshold level of PSA <4.0 ng/mL in clinical samples is an important indicator. Quick and easy visual detection of the PSA level greatly helps in early detection and treatment of prostate cancer and reducing mortality. In this study, we developed optimized silica-coated silver-assembled silica nanoparticles (SiO2@Ag@SiO2 NPs) that were applied to a visual lateral flow immunoassay (LFIA) platform for PSA detection. During synthesis, the ratio of silica NPs to silver nitrate changed, and as the synthesized NPs exhibited distinct UV spectra and colors, most optimized SiO2@Ag@SiO2 NPs showed the potential for early prostate cancer diagnosis. The PSA detection limit of our LFIA platform was 1.1 ng/mL. By applying each SiO2@Ag@SiO2 NP to the visual LFIA platform, optimized SiO2@Ag@SiO2 NPs were selected in the test strip, and clinical samples from prostate cancer patients were successfully detected as the boundaries of non-specific binding were clearly seen and the level of PSA was <4 ng/mL, thus providing an avenue for quick prostate cancer diagnosis and early treatment.

SERS-based test strips: Principles, designs and applications

Test strips represent a class of point-of-care testing (POCT) tools for analysis of a variety of biomarkers towards diagnostics. Conventional test strips offer benefits of simple operation, visualization, and short detection time, along with the drawbacks of relatively low sensitivity and unavailability of quantitative analysis. Recently, the combination of surface-enhanced Raman scattering (SERS) and test strips have evolved to provide a powerful platform capable of ultrasensitive and multiplex detection of extensive analytes of interest. In this review, we focus on the working principles, design strategies and POCT applications of SERS-based test strips. Initially, both lateral and vertical flow test strips are briefly introduced, followed by presentation of various strategies for reforming SERS-based test strips with better detection performance. Applications of SERS-based test strips in diagnosis of disease biomarkers, nucleic acids and toxins are reviewed, with an emphasis on SERS tag design, sensitivity and analytical applicability. Finally, conclusions are made and perspectives on futuristic research directions are given.

Metal Nanoparticles/MoS2 Surface-Enhanced Raman Scattering-Based Sandwich Immunoassay for α-Fetoprotein Detection

The detection of cancer biomarkers at an early stage of tumor development is vital for effective diagnosis and treatment of cancer. Current diagnostic tools can often detect cancer only when the biomarker levels are already too high, so that the tumors have spread and treatments are less effective. It is urgent therefore to develop highly sensitive assays for the detection of such biomarkers at the lowest possible concentration. In this context, we developed a sandwich immunoassay based on surface-enhanced Raman scattering (SERS) for the ultrasensitive detection of α-fetoprotein (AFP), which is typically present in human serum as a biomarker indicative of early stages of hepatocellular carcinoma. In the immunoassay design, molybdenum disulfide (MoS2) modified with a monoclonal antibody was used as a capture probe for AFP. A secondary antibody linked to an SERS-encoded nanoparticle was employed as the Raman signal reporter, that is, the transducer for AFP detection. The sandwich immunocomplex "capture probe/target/SERS tag" was deposited on a silicon wafer and decorated with silver-coated gold nanocubes to increase the density of "hot spots" on the surface of the immunosensor. The developed SERS immunosensor exhibits a wide linear detection range (1 pg mL-1 to 10 ng mL-1) with a limit of detection as low as 0.03 pg mL-1 toward AFP with good reproducibility (RSD < 6%) and stability. These parameters demonstrate that the proposed immunosensor has the potential to be used as an analytical platform for the detection of early-stage cancer biomarkers in clinical applications.

(Nano)tag-antibody conjugates in rapid tests

Antibodies (Abs) are naturally derived materials with favorable affinity, selectivity, and fast binding kinetics to the respective antigens, which enables their application as promising recognition elements in the development of various types of biosensors/bioassays, especially in rapid tests. These tests are low-cost and easy-to-use biosensing devices with broad applications including medical or veterinary diagnostics, environmental monitoring and industrial usages such as safety and quality analysis in food, providing on-site quick monitoring of various analytes, making it possible to save analysis costs and time. To reach such features, the conjugation of Abs with various nanomaterials (NMs) as tags is necessary, which range from conventional gold nanoparticles to other nanoparticles recently introduced, where magnetic, plasmonic, photoluminescent, or multi-modal properties play a critical role in the overall performance of the analytical device. In this context, to preserve the Ab affinity and provide a rapid response with long-term storage capability, the use of efficient bio-conjugation techniques is critical. Thanks to their prominent role in rapid tests, many studies have been devoted to the design and development of Abs-NMs conjugates with various chemistries including passive adsorption, covalent coupling, and affinity interactions. In this review, we present the state-of-the-art techniques allowing various Ab-NM conjugates with a special focus on the efficiency of the developed probes to be employed in in vitro rapid tests. Challenges and future perspectives on the development of Ab-conjugated nanotags in rapid diagnostic tests are highlighted along with a survey of the progress in commercially available Ab-NM conjugates.

Surface-Enhanced Raman Scattering-Based Lateral-Flow Immunoassay

Lateral flow immunoassays (LFIAs) have been developed and used in a wide range of applications, in point-of-care disease diagnoses, environmental safety, and food control. However, in its classical version, it has low sensitivity and can only perform semiquantitative detection, based on colorimetric signals. Over the past decade, surface-enhanced Raman scattering (SERS) tags have been developed in order to decrease the detection limit and enable the quantitative analysis of analytes. Of note, these tags needed new readout systems and signal processing algorithms, while the LFIA design remained unchanged. This review highlights SERS strategies of signal enhancement for LFIAs. The types of labels used, the possible gain in sensitivity from their use, methods of reading and processing the signal, and the prospects for use are discussed.

Rapid, Quantitative, High-Sensitive Detection of Escherichia coli O157:H7 by Gold-Shell Silica-Core Nanospheres-Based Surface-Enhanced Raman Scattering Lateral Flow Immunoassay

Escherichia coli O157:H7 is regarded as one of the most harmful pathogenic microorganisms related to foodborne diseases. This paper proposes a rapid-detection biosensor for the sensitive and quantitative analysis of E. coli O157:H7 in biological samples by surface-enhanced Raman scattering (SERS)-based lateral flow immunoassay (LFIA). A novel gold-shell silica-core (SiO2/Au) nanosphere (NP) with monodispersity, good stability, and excellent SERS activity was utilized to prepare high-performance tags for the SERS-based LFIA system. The SiO2/Au SERS tags, which were modified with two layers of Raman reporter molecules and monoclonal antibodies, effectively bind with E. coli O157:H7 and form sandwich immune complexes on the test lines. E. coli O157:H7 was quantitatively detected easily by detecting the Raman intensity of the test lines. Under optimal conditions, the limit of detection (LOD) of the SiO2/Au-based SERS-LIFA strips for the target bacteria was 50 cells/mL in PBS solution, indicating these strips are 2,000 times more sensitive than colloidal Au-based LFIA strips. Moreover, the proposed assay demonstrated high applicability in E. coli O157:H7 detection in biological samples, including tap water, milk, human urine, lettuce extract and beef, with a low LOD of 100 cells/mL. Results indicate that the proposed SERS-based LFIA strip is applicable for the sensitive and quantitative determination of E. coli O157:H7.

Portable and multiplexed lateral flow immunoassay reader based on SERS for highly sensitive point-of-care testing

A portable surface-enhanced Raman scattering (SERS)-based lateral flow immunoassay (LFIA) reader with multiplexed detection was developed using an integrated LFIA reaction column. The proposed LFIA reader was designed to simultaneously detect multiple samples or samples with multiple biomarkers. With the integrated LFIA reaction column, we achieved the specific detection of alpha-fetoprotein (AFP), carcinoembryonic antigen (CEA), and prostate-specific antigen (PSA) with a detection limit of 0.01 ng/mL, which was three orders of magnitude lower than that of the visual signal. We also investigated the uniformity of channels based on an eight-channel integrated LFIA reaction column. The relative standard deviation values of the SERS intensity of the eight-channel for measuring the AFP, CEA, and PSA antigens at 1323 cm^-1 were 13%, 4.8%, and 5%, respectively. We detected 45 clinical serum samples of the three antigens using the proposed portable SERS-based LFIA reader to further confirm its applicability to clinical samples. The SERS signals of the positive sera were higher than those of the negative sera and their thrice standard deviation. This result indicated the practicality of the developed integrated reaction column and the proposed portable and multiplexed Raman reader. This work provides a new high-sensitivity, multiplexed, and automated SERS-based LFIA detector for use in the point-of-care setting.