Recent Trends in Lateral Flow Immunoassays with Optical Nanoparticles.

Rapid, accurate, and convenient diagnosis is essential for effective disease management. Various detection methods, such as enzyme-linked immunosorbent assay, have been extensively used, with lateral flow immunoassay (LFIA) recently emerging as a major diagnostic tool. Nanoparticles (NPs) with characteristic optical properties are used as probes for LFIA, and researchers have presented various types of optical NPs with modified optical properties. Herein, we review the literature on LFIA with optical NPs for the detection of specific targets in the context of diagnostics.

In Vitro Tracking of Human Umbilical Vein Endothelial Cells Using Ultra-Sensitive Quantum Dot-Embedded Silica Nanoparticles.

The nanoscale spatiotemporal resolution of single-particle tracking (SPT) renders it a powerful method for exploring single-molecule dynamics in living cells or tissues, despite the disadvantages of using traditional organic fluorescence probes, such as the weak fluorescent signal against the strong cellular autofluorescence background coupled with a fast-photobleaching rate. Quantum dots (QDs), which enable tracking targets in multiple colors, have been proposed as an alternative to traditional organic fluorescence dyes; however, they are not ideally suitable for applying SPT due to their hydrophobicity, cytotoxicity, and blinking problems. This study reports an improved SPT method using silica-coated QD-embedded silica nanoparticles (QD2), which represent brighter fluorescence and are less toxic than single QDs. After treatment of QD2 in 10 µg/mL, the label was retained for 96 h with 83.76% of labeling efficiency, without impaired cell function such as angiogenesis. The improved stability of QD2 facilitates the visualization of in situ endothelial vessel formation without real-time staining. Cells retain QD2 fluorescence signal for 15 days at 4 °C without significant photobleaching, indicating that QD2 has overcome the limitations of SPT enabling long-term intracellular tracking. These results proved that QD2 could be used for SPT as a substitute for traditional organic fluorophores or single quantum dots, with its photostability, biocompatibility, and superior brightness.

Hyaluronic Acid Nanoparticles as a Topical Agent for Treating Psoriasis.

Although conventional topical approaches for treating psoriasis have been offered as an alternative, there are still unmet medical needs such as low skin-penetrating efficacy and off-target adverse effects. A hyaluronic acid nanoparticle (HA-NP) formed by self-assembly of HA-hydrophobic moiety conjugates has been broadly studied as a nanocarrier for long-term and target-specific delivery of drugs, owing to their excellent physicochemical and biological characteristics. Here, we identify HA-NPs as topical therapeutics for treating psoriasis using in vivo skin penetration studies and psoriasis animal models. Transcutaneously administered HA-NPs were found to be accumulated and associated with pro-inflammatory macrophages in the inflamed dermis of a psoriasis mouse model. Importantly, HA-NP exerted potent therapeutic efficacy against psoriasis-like skin dermatitis in a size-dependent manner by suppressing innate immune responses and restoring skin barrier function without overt toxicity signs. The therapeutic efficacy of HA-NPs on psoriasis-like skin dermatitis was due to the outermost hydrophilic HA shell layer of HA-NPs, independent of the molecular weight of HA and hydrophobic moiety, and comparable with that of other conventional psoriasis therapeutics widely used in the clinical settings. Overall, HA-NPs have the potential as a topical nanomedicine for treating psoriasis effectively and safely.

Highly Bright Silica-Coated InP/ZnS Quantum Dot-Embedded Silica Nanoparticles as Biocompatible Nanoprobes.

Quantum dots (QDs) have outstanding optical properties such as strong fluorescence, excellent photostability, broad absorption spectra, and narrow emission bands, which make them useful for bioimaging. However, cadmium (Cd)-based QDs, which have been widely studied, have potential toxicity problems. Cd-free QDs have also been studied, but their weak photoluminescence (PL) intensity makes their practical use in bioimaging challenging. In this study, Cd-free QD nanoprobes for bioimaging were fabricated by densely embedding multiple indium phosphide/zinc sulfide (InP/ZnS) QDs onto silica templates and coating them with a silica shell. The fabricated silica-coated InP/ZnS QD-embedded silica nanoparticles (SiO2@InP QDs@SiO2 NPs) exhibited hydrophilic properties because of the surface silica shell. The quantum yield (QY), maximum emission peak wavelength, and full-width half-maximum (FWHM) of the final fabricated SiO2@InP QDs@SiO2 NPs were 6.61%, 527.01 nm, and 44.62 nm, respectively. Moreover, the brightness of the particles could be easily controlled by adjusting the amount of InP/ZnS QDs in the SiO2@InP QDs@SiO2 NPs. When SiO2@InP QDs@SiO2 NPs were administered to tumor syngeneic mice, the fluorescence signal was prominently detected in the tumor because of the preferential distribution of the SiO2@InP QDs@SiO2 NPs, demonstrating their applicability in bioimaging with NPs. Thus, SiO2@InP QDs@SiO2 NPs have the potential to successfully replace Cd-based QDs as highly bright and biocompatible fluorescent nanoprobes.

High-quantum yield alloy-typed core/shell CdSeZnS/ZnS quantum dots for bio-applications.

BACKGROUND: Quantum dots (QDs) have been used as fluorophores in various imaging fields owing to their strong fluorescent intensity, high quantum yield (QY), and narrow emission bandwidth. However, the application of QDs to bio-imaging is limited because the QY of QDs decreases substantially during the surface modification step for bio-application. RESULTS: In this study, we fabricated alloy-typed core/shell CdSeZnS/ZnS quantum dots (alloy QDs) that showed higher quantum yield and stability during the surface modification for hydrophilization compared with conventional CdSe/CdS/ZnS multilayer quantum dots (MQDs). The structure of the alloy QDs was confirmed using time-of-flight medium-energy ion scattering spectroscopy. The alloy QDs exhibited strong fluorescence and a high QY of 98.0%. After hydrophilic surface modification, the alloy QDs exhibited a QY of 84.7%, which is 1.5 times higher than that of MQDs. The QY was 77.8% after the alloy QDs were conjugated with folic acid (FA). Alloy QDs and MQDs, after conjugation with FA, were successfully used for targeting human KB cells. The alloy QDs exhibited a stronger fluorescence signal than MQD; these signals were retained in the popliteal lymph node area for 24 h. CONCLUSION: The alloy QDs maintained a higher QY in hydrophilization for biological applications than MQDs. And also, alloy QDs showed the potential as nanoprobes for highly sensitive bioimaging analysis.

Optimizing the Aspect Ratio of Nanopatterned Mesoporous TiO2 Thin-Film Layer to Improve Energy Conversion Efficiency of Perovskite Solar Cells.

The energy conversion efficiency (ECE) (η), current density (Jsc), open-circuit voltage (Voc), and fill factor (ff) of perovskite solar cells were studied by using the transmittance of a nanopatterned mesoporous TiO2 (mp-TiO2) thin-film layer. To improve the ECE of perovskite solar cells, a mp-TiO2 thin-film layer was prepared to be used as an electron transport layer (ETL) via the nanoimprinting method for nanopatterning, which was controlled by the aspect ratio. The nanopatterned mp-TiO2 thin-film layer had a uniform and well-designed structure, and the diameter of nanopatterning was 280 nm. The aspect ratio was controlled at the depths of 75, 97, 127, and 167 nm, and the perovskite solar cell was fabricated with different depths. The ECE of the perovskite solar cells with the nanopatterned mp-TiO2 thin-film layer was 14.50%, 15.30%, 15.83%, or 14.24%, which is higher than that of a non-nanopatterned mp-TiO2 thin-film layer (14.07%). The enhancement of ECE was attributed to the transmittance of the nanopatterned mp-TiO2 thin-film layer that is due to the improvement of the electron generation. As a result, better electron generation affected the electron density, and Jsc increased the Voc, and ff of perovskite solar cells.

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.

Multi-Quantum Dots-Embedded Silica-Encapsulated Nanoparticle-Based Lateral Flow Assay for Highly Sensitive Exosome Detection.

Exosomes are attracting attention as new biomarkers for monitoring the diagnosis and prognosis of certain diseases. Colorimetric-based lateral-flow assays have been previously used to detect exosomes, but these have the disadvantage of a high limit of detection. Here, we introduce a new technique to improve exosome detection. In our approach, highly bright multi-quantum dots embedded in silica-encapsulated nanoparticles (M-QD-SNs), which have uniform size and are brighter than single quantum dots, were applied to the lateral flow immunoassay method to sensitively detect exosomes. Anti-CD63 antibodies were introduced on the surface of the M-QD-SNs, and a lateral flow immunoassay with the M-QD-SNs was conducted to detect human foreskin fibroblast (HFF) exosomes. Exosome samples included a wide range of concentrations from 100 to 1000 exosomes/µL, and the detection limit of our newly designed system was 117.94 exosome/µL, which was 11 times lower than the previously reported limits. Additionally, exosomes were selectively detected relative to the negative controls, liposomes, and newborn calf serum, confirming that this method prevented non-specific binding. Thus, our study demonstrates that highly sensitive and quantitative exosome detection can be conducted quickly and accurately by using lateral immunochromatographic analysis with M-QD-SNs.

Au-Ag assembled on silica nanoprobes for visual semiquantitative detection of prostate-specific antigen.

BACKGROUND: Blood prostate-specific antigen (PSA) levels are widely used as diagnostic biomarkers for prostate cancer. Lateral-flow immunoassay (LFIA)-based PSA detection can overcome the limitations associated with other methods. LFIAbased PSA detection in clinical samples enables prognosis and early diagnosis owing to the use of high-performance signal reporters. RESULTS: Here, a semiquantitative LFIA platform for PSA detection in blood was developed using Au-Ag nanoparticles (NPs) assembled on silica NPs (SiO2@Au-Ag NPs) that served as signal reporters. Synthesized SiO2@Au-Ag NPs exhibited a high absorbance at a wide wavelength range (400-800 nm), with a high scattering on nitrocellulose membrane test strips. In LFIA, the color intensity of the test line on the test strip differed depending on the PSA concentration (0.30-10.00 ng/mL), and bands for the test line on the test strip could be used as a standard. When clinical samples were assessed using this LFIA, a visual test line with particular color intensity observed on the test strip enabled the early diagnosis and prognosis of patients with prostate cancer based on PSA detection. In addition, the relative standard deviation of reproducibility was 1.41%, indicating high reproducibility, and the signal reporter showed good stability for 10 days. CONCLUSION: These characteristics of the signal reporter demonstrated the reliability of the LFIA platform for PSA detection, suggesting potential applications in clinical sample analysis.

Lateral Flow Immunoassay with Quantum-Dot-Embedded Silica Nanoparticles for Prostate-Specific Antigen Detection.

Prostate cancer can be detected early by testing the presence of prostate-specific antigen (PSA) in the blood. Lateral flow immunoassay (LFIA) has been used because it is cost effective and easy to use and also has a rapid sample-to-answer process. Quantum dots (QDs) with very bright fluorescence have been previously used to improve the detection sensitivity of LFIAs. In the current study, a highly sensitive LFIA kit was devised using QD-embedded silica nanoparticles. In the present study, only a smartphone and a computer software program, ImageJ, were used, because the developed system had high sensitivity by using very bright nanoprobes. The limit of PSA detection of the developed LFIA system was 0.138 ng/mL. The area under the curve of this system was calculated as 0.852. The system did not show any false-negative result when 47 human serum samples were analyzed; it only detected PSA and did not detect alpha-fetoprotein and newborn calf serum in the samples. Additionally, fluorescence was maintained on the strip for 10 d after the test. With its high sensitivity and convenience, the devised LFIA kit can be used for the diagnosis of prostate cancer.

Facile Histamine Detection by Surface-Enhanced Raman Scattering using SiO2@Au@Ag Alloy Nanoparticles.

Histamine intoxication associated with seafood consumption represents a global health problem. The consumption of high concentrations of histamine can cause illnesses ranging from light symptoms, such as a prickling sensation, to death. In this study, gold-silver alloy-embedded silica (SiO2@Au@Ag) nanoparticles were created to detect histamine using surface-enhanced Raman scattering (SERS). The optimal histamine SERS signal was measured following incubation with 125 µg/mL of SiO2@Au@Ag for 2 h, with a material-to-histamine solution volume ratio of 1:5 and a phosphate-buffered saline-Tween 20 (PBS-T) solvent at pH 7. The SERS intensity of the histamine increased proportionally with the increase in histamine concentration in the range 0.1-0.8 mM, with a limit of detection of 3.698 ppm. Our findings demonstrate the applicability of SERS using nanomaterials for histamine detection. In addition, this study demonstrates that nanoalloys could have a broad application in the future.

Silica-Coated Magnetic Iron Oxide Nanoparticles Grafted onto Graphene Oxide for Protein Isolation.

In this study, silica-coated magnetic iron oxide nanoparticles (MNPs@SiO2) were covalently conjugated onto graphene oxide (GO/MNP@SiO2) for protein isolation. First, MNPs were precisely coated with a silica layer on the surface by using the reverse microemulsion method, followed by incubation with 3-glycidyloxypropyltrimethoxysilane (GPTS) to produce the GPTS-functionalized MNPs@SiO2 (GPTS-coated MNPs@SiO2) that display epoxy groups on the surface. The silica shell on the MNPs was optimized at 300 µL of Igepal®CO-520, 5 mg of MNP, 100 µL of TEOS, 100 µL of NH4OH and 3% of 3-glycidyloxypropyltrimethoxysilane (GPTS). Simultaneously, polyethyleneimine (PEI) was covalently conjugated to GO to enhance the stability of GO in aqueous solutions and create the reaction sites with epoxy groups on the surface of GPTS-coated MNP@SiO2. The ratio of PEI grafted GO and GPTS-coated MNP@SiO2 (GO/MNP ratio) was investigated to produce GO/MNPs@SiO2 with highly saturated magnetization without aggregation. As a result, the GO/MNP ratio of 5 was the best condition to produce the GO/MNP@SiO2 with 9.53 emu/g of saturation superparamagnetization at a magnetic field of 2.0 (T). Finally, the GO/MNPs@SiO2 were used to separate bovine serum albumin (BSA) to investigate its protein isolation ability. The quantity of BSA adsorbed onto 1 mg of GO/MNP@SiO2 increased sharply over time to reach 628 ± 9.3 µg/mg after 15 min, which was 3.5-fold-higher than that of GPTS-coated MNP@SiO2. This result suggests that the GO/MNP@SiO2 nanostructure can be used for protein isolation.

4-Mercaptobenzoic Acid Labeled Gold-Silver-Alloy-Embedded Silica Nanoparticles as an Internal Standard Containing Nanostructures for Sensitive Quantitative Thiram Detection.

In this study, SiO2@Au@4-MBA@Ag (4-mercaptobenzoic acid labeled gold-silver-alloy-embedded silica nanoparticles) nanomaterials were investigated for the detection of thiram, a pesticide. First, the presence of Au@4-MBA@Ag alloys on the surface of SiO2 was confirmed by the broad bands of ultraviolet-visible spectra in the range of 320-800 nm. The effect of the 4-MBA (4-mercaptobenzoic acid) concentration on the Ag shell deposition and its intrinsic SERS (surface-enhanced Raman scattering) signal was also studied. Ag shells were well coated on SiO2@Au@4-MBA in the range of 1-1000 µM. The SERS intensity of thiram-incubated SiO2@Au@4-MBA@Ag achieved the highest value by incubation with 500 µL thiram for 30 min, and SERS was measured at 200 µg/mL SiO2@Au@4-MBA@Ag. Finally, the SERS intensity of thiram at 560 cm-1 increased proportionally with the increase in thiram concentration in the range of 240-2400 ppb, with a limit of detection (LOD) of 72 ppb.

Control of Silver Coating on Raman Label Incorporated Gold Nanoparticles Assembled Silica Nanoparticles.

Signal reproducibility in surface-enhanced Raman scattering (SERS) remains a challenge, limiting the scope of the quantitative applications of SERS. This drawback in quantitative SERS sensing can be overcome by incorporating internal standard chemicals between the core and shell structures of metal nanoparticles (NPs). Herein, we prepared a SERS-active core Raman labeling compound (RLC) shell material, based on Au⁻Ag NPs and assembled silica NPs (SiO2@Au@RLC@Ag NPs). Three types of RLCs were used as candidates for internal standards, including 4-mercaptobenzoic acid (4-MBA), 4-aminothiophenol (4-ATP) and 4-methylbenzenethiol (4-MBT), and their effects on the deposition of a silver shell were investigated. The formation of the Ag shell was strongly dependent on the concentration of the silver ion. The negative charge of SiO2@Au@RLCs facilitated the formation of an Ag shell. In various pH solutions, the size of the Ag NPs was larger at a low pH and smaller at a higher pH, due to a decrease in the reduction rate. The results provide a deeper understanding of features in silver deposition, to guide further research and development of a strong and reliable SERS probe based on SiO2@Au@RLC@Ag NPs.

Multilayer Ag-Embedded Silica Nanostructure as a Surface-Enhanced Raman Scattering-Based Chemical Sensor with Dual-Function Internal Standards.

Surface-enhanced Raman scattering (SERS) spectroscopy is attractive in various detection analysis fields. However, the quantitative method using SERS spectroscopy remains as an area to be developed. The key issues in developing quantitative analysis methods by using SERS spectroscopy are the fabrication of reliable SERS-active materials such as nanoparticle-based structures and the acquisition of the SERS signal without any disturbance that may change the SERS signal intensity and frequency. Here, the fabrication of seamless multilayered core-shell nanoparticles with an embedded Raman label compound as an internal standard (MLRLC dots) for quantitative SERS analysis is reported. The embedded Raman label compound in the nanostructure provides a reference value for calibrating the SERS signals. By using the MLRLC dots, it is possible to gain target analyte signals of different concentrations while retaining the Raman signal of the internal standard. The ML4-BBT dots, containing 4-bromobenzenethiol (4-BBT) as an internal standard, are successfully applied in the quantitative analysis of 4-fluorobenzenethiol and thiram, a model pesticide. Additionally, ratiometric analysis was proved practical through normalization of the relative SERS intensity. The ratiometric strategy could be applied to various SERS substrates for quantitative detection of a wide variety of targets.

Assembly of Plasmonic and Magnetic Nanoparticles with Fluorescent Silica Shell Layer for Tri-functional SERS-Magnetic-Fluorescence Probes and Its Bioapplications.

In this study, we report on the fabrication of multilayered tri-functional magnetic-SERS-fluorescence nanoprobes (MF-SERS particles) containing clustered superparamagnetic Fe3O4 nanoparticles (NPs), silver NPs, and a fluorescent silica layer. The MF-SERS particles exhibited strong SERS signals from the silver NPs as well as both superparamagnetism and fluorescence. MF-SERS particles were uptaken by cells, allowing successful separation using an external magnetic field. SERS and fluorescence signals could be detected from the NP-containing cells, and CD44 antibody-conjugated MF-SERS particles selectively targeted MDA-MB-231 cells. Based on these properties, MF-SERS particles proved to be a useful nanoprobe for multiplex detection and separation of cancer cells.

Enzyme-catalyzed Ag Growth on Au Nanoparticle-assembled Structure for Highly Sensitive Colorimetric Immunoassay.

We have developed a sensitive colorimetric immunoassay with broad dynamic range using enzyme-catalyzed Ag growth on gold nanoparticle (NP)-assembled silica (SiO2@Au@Ag). To reduce Ag+ ion content and promote Ag growth on the assembled Au NPs, alkaline phosphatase (AP)-based enzymatic amplification was incorporated, which considerably increased the colorimetric read-out. As a model study, sandwich enzyme-linked immunosorbent assay (ELISA) was used to quantify target IgG. The immune complexes capture the Ab-IgG-AP-labeled detection Ab and trigger the enzyme-catalyzed reaction to convert 2-phospho-L-ascorbic acid to ascorbic acid in the presence of the target IgG. Ascorbic acid reduced Ag+ to Ag, which formed Ag shells on the surface of SiO2@Au and enhanced the absorbance of the SiO2@Au@Ag solution. Plasmonic immunoassay showed a significant linear relationship between absorbance and the logarithm of IgG concentration in the range of ca. 7 × 10-13 M to 7 × 10-11 M. The detection limit was at 1.4 × 10-13 M, which is several hundred folds higher than that of any conventional colorimetric immunoassay. Thus, our novel approach of signal-amplification can be used for highly sensitive in vitro diagnostics and detection of target proteins with the naked eye without using any sophisticated instrument.

Polyethylene Glycol-Engrafted Graphene Oxide as Biocompatible Materials for Peptide Nucleic Acid Delivery into Cells.

Graphene oxide (GO) is known to strongly bind single-stranded nucleic acids with fluorescence quenching near the GO surface. However, GO exhibits weak biocompatibility characteristics, such as low dispersibility in cell culture media and significant cytotoxicity. To improve dispersibility in cell culture media and cell viability of GO, we prepared nanosized GO (nGO) constructs and modified the nGO surface using polyethylene glycol (PEG-nGO). Single-stranded peptide nucleic acid (PNA) was adsorbed onto the PEG-nGO and was readily desorbed by adding complementary RNA or under low pH conditions. PNA adsorbed on the PEG-nGO was efficiently delivered into lung cancer cells via endocytosis without affecting cell viability. Furthermore, antisense PNA delivered using PEG-nGO effectively downregulated the expression of the target gene in cancer cells. Our results suggest that PEG-nGO is a biocompatible carrier useful for PNA delivery into cells and serves as a promising gene delivery tool.

Thin silica shell coated Ag assembled nanostructures for expanding generality of SERS analytes.

Surface-enhanced Raman scattering (SERS) provides a unique non-destructive spectroscopic fingerprint for chemical detection. However, intrinsic differences in affinity of analyte molecules to metal surface hinder SERS as a universal quantitative detection tool for various analyte molecules simultaneously. This must be overcome while keeping close proximity of analyte molecules to the metal surface. Moreover, assembled metal nanoparticles (NPs) structures might be beneficial for sensitive and reliable detection of chemicals than single NP structures. For this purpose, here we introduce thin silica-coated and assembled Ag NPs (SiO2@Ag@SiO2 NPs) for simultaneous and quantitative detection of chemicals that have different intrinsic affinities to silver metal. These SiO2@Ag@SiO2 NPs could detect each SERS peak of aniline or 4-aminothiophenol (4-ATP) from the mixture with limits of detection (LOD) of 93 ppm and 54 ppb, respectively. E-field distribution based on interparticle distance was simulated using discrete dipole approximation (DDA) calculation to gain insight into enhanced scattering of these thin silica coated Ag NP assemblies. These NPs were successfully applied to detect aniline in river water and tap water. Results suggest that SiO2@Ag@SiO2 NP-based SERS detection systems can be used as a simple and universal detection tool for environment pollutants and food safety.

Highly Sensitive Magnetic-SERS Dual-Function Silica Nanoprobes for Effective On-Site Organic Chemical Detection.

We report magnetic silver nanoshells (M-AgNSs) that have both magnetic and SERS properties for SERS-based detection. The M-AgNSs are composed of hundreds of Fe3O4 nanoparticles for rapid accumulation and bumpy silver shell for sensitive SERS detection by near-infrared laser excitation. The intensity of the SERS signal from the M-AgNSs was strong enough to provide single particle-level detection. We obtained much stronger SERS signal intensity from the aggregated M-AgNSs than from the non-aggregated AgNSs. 4-Fluorothiophenol was detected at concentrations as low as 1 nM, which corresponds to 0.16 ppb. The limit of detection for tetramethylthiuram disulfide was 10 µM, which corresponds to 3 ppm. The M-AgNSs can be used to detect trace amounts of organic molecules using a portable Raman system.