Highly sensitive near-infrared SERS nanoprobes for in vivo imaging using gold-assembled silica nanoparticles with controllable nanogaps.

BACKGROUND: To take advantages, such as multiplex capacity, non-photobleaching property, and high sensitivity, of surface-enhanced Raman scattering (SERS)-based in vivo imaging, development of highly enhanced SERS nanoprobes in near-infrared (NIR) region is needed. A well-controlled morphology and biocompatibility are essential features of NIR SERS nanoprobes. Gold (Au)-assembled nanostructures with controllable nanogaps with highly enhanced SERS signals within multiple hotspots could be a breakthrough. RESULTS: Au-assembled silica (SiO2) nanoparticles (NPs) (SiO2@Au@Au NPs) as NIR SERS nanoprobes are synthesized using the seed-mediated growth method. SiO2@Au@Au NPs using six different sizes of Au NPs (SiO2@Au@Au50-SiO2@Au@Au500) were prepared by controlling the concentration of Au precursor in the growth step. The nanogaps between Au NPs on the SiO2 surface could be controlled from 4.16 to 0.98 nm by adjusting the concentration of Au precursor (hence increasing Au NP sizes), which resulted in the formation of effective SERS hotspots. SiO2@Au@Au500 NPs with a 0.98-nm gap showed a high SERS enhancement factor of approximately 3.8 × 106 under 785-nm photoexcitation. SiO2@Au@Au500 nanoprobes showed detectable in vivo SERS signals at a concentration of 16 µg/mL in animal tissue specimen at a depth of 7 mm. SiO2@Au@Au500 NPs with 14 different Raman label compounds exhibited distinct SERS signals upon subcutaneous injection into nude mice. CONCLUSIONS: SiO2@Au@Au NPs showed high potential for in vivo applications as multiplex nanoprobes with high SERS sensitivity in the NIR region.

Silica Shell Thickness-Dependent Fluorescence Properties of SiO2@Ag@SiO2@QDs Nanocomposites.

Silica shell coatings, which constitute important technology for nanoparticle (NP) developments, are utilized in many applications. The silica shell's thickness greatly affects distance-dependent optical properties, such as metal-enhanced fluorescence (MEF) and fluorescence quenching in plasmonic nanocomposites. However, the precise control of silica-shell thicknesses has been mainly conducted on single metal NPs, and rarely on complex nanocomposites. In this study, silica shell-coated Ag nanoparticle-assembled silica nanoparticles (SiO2@Ag@SiO2), with finely controlled silica shell thicknesses (4 nm to 38 nm), were prepared, and quantum dots (QDs) were introduced onto SiO2@Ag@SiO2. The dominant effect between plasmonic quenching and MEF was defined depending on the thickness of the silica shell between Ag and QDs. When the distance between Ag NPs to QDs was less than ~10 nm, SiO2@Ag@SiO2@QDs showed weaker fluorescence intensities than SiO2@QD (without metal) due to the quenching effect. On the other hand, when the distance between Ag NPs to QDs was from 10 nm to 14 nm, the fluorescence intensity of SiO2@Ag@SiO2@QD was stronger than SiO2@QDs due to MEF. The results provide background knowledge for controlling the thickness of silica shells in metal-containing nanocomposites and facilitate the development of potential applications utilizing the optimal plasmonic phenomenon.

Ultra-Fine Control of Silica Shell Thickness on Silver Nanoparticle-Assembled Structures.

To study the distance-dependent electromagnetic field effects related to the enhancement and quenching mechanism of surface-enhanced Raman scattering (SERS) or fluorescence, it is essential to precisely control the distance from the surface of the metal nanoparticle (NP) to the target molecule by using a dielectric layer (e.g., SiO2, TiO2, and Al2O3). However, precisely controlling the thickness of this dielectric layer is challenging. Herein, we present a facile approach to control the thickness of the silica shell on silver nanoparticle-assembled silica nanocomposites, SiO2@Ag NPs, by controlling the number of reacting SiO2@Ag NPs and the silica precursor. Uniform silica shells with thicknesses in the range 5-40 nm were successfully fabricated. The proposed method for creating a homogeneous, precise, and fine silica coating on nanocomposites can potentially contribute to a comprehensive understanding of the distance-dependent electromagnetic field effects and optical properties of metal NPs.

Template-Assisted Plasmonic Nanogap Shells for Highly Enhanced Detection of Cancer Biomarkers.

We present a template-assisted method for synthesizing nanogap shell structures for biomolecular detections based on surface-enhanced Raman scattering. The interior nanogap-containing a silver shell structure, referred to as a silver nanogap shell (Ag NGS), was fabricated on silver nanoparticles (Ag NPs)-coated silica, by adsorbing small aromatic thiol molecules on the Ag NPs. The Ag NGSs showed a high enhancement factor and good signal uniformity, using 785-nm excitation. We performed in vitro immunoassays using a prostate-specific antigen as a model cancer biomarker with a detection limit of 2 pg/mL. To demonstrate the versatility of Ag NGS nanoprobes, extracellular duplex surface-enhanced Raman scattering (SERS) imaging was also performed to evaluate the co-expression of cancer biomarkers, human epidermal growth factor-2 (HER2) and epidermal growth factor receptor (EGFR), in a non-small cell lung cancer cell line (H522). Developing highly sensitive Ag NGS nanoprobes that enable multiplex biomolecular detection and imaging can open up new possibilities for point-of-care diagnostics and provide appropriate treatment options and prognosis.

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 Encapsulation of Hydrophobic Optical NP-Embedded Silica Particles with Trimethoxy(2-Phenylethyl)silane.

Nanoparticles (NP) with optical properties embedded silica particles have been widely used in various fields because of their unique properties. The surfaces of optical NPs have been modified with various organic ligands to maintain their unique optical properties and colloidal stability. Among the surface modification methods, silica encapsulation of optical NPs is widely used to enhance their biocompatibility and stability. However, in the case of NPs with hydrophobic ligands on the surface, the ligands that determine the optical properties of the NPs may detach from the NPs, thereby changing the optical properties during silica encapsulation. Herein, we report a generally applicable silica encapsulation method using trimethoxy(2-phenylethyl)silane (TMPS) for non-hydrophilic optical NPs, such as quantum dots (QDs) and gold NPs. This silica encapsulation method was applied to fabricate multiple silica-encapsulated QD-embedded silica NPs (SiO2@QD@SiO2 NPs; QD2) and multiple silica-encapsulated gold NP-embedded silica NPs labeled with 2-naphthalene thiol (SiO2@Au2-NT@SiO2). The fabricated silica-encapsulated NPs exhibited optical properties without significant changes in the quantum yield or Raman signal intensity.

Movable Layer Device for Rapid Detection of Influenza a H1N1 Virus Using Highly Bright Multi-Quantum Dot-Embedded Particles and Magnetic Beads.

Preventing the rapid spread of viral infectious diseases has become a major concern for global health. In this study, we present a microfluidic platform that performs an immunoassay of viral antigens in a simple, automated, yet highly sensitive manner. The device uses silica particles embedded with highly bright quantum dots (QD2) and performs the immunoassay with a vertically movable top layer and a rotating bottom layer. Through the motion of the layers and the surface tension in the liquids, reagents move from top chambers to bottom chambers and mix homogeneously. A tip in the top layer with a mobile permanent magnet moves the immune complexes comprising the magnetic beads, virus particles, and QD2 between the bottom chambers. In this way, our automated device achieves a highly sensitive magnetic bead-based sandwich immunoassay for the influenza A H1N1 virus within 32.5 min. The detection limit of our method is 5.1 × 10-4 hemagglutination units, which is 2 × 103 times more sensitive than that of the conventional hemagglutination method and is comparable to PCR. Our device is useful for the rapid and sensitive detection of infectious diseases in point-of-care applications and resource-limited environments.

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.

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.

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.

Ultrasensitive NIR-SERRS Probes with Multiplexed Ratiometric Quantification for In Vivo Antibody Leads Validation.

Immunotargeting ability of antibodies may show significant difference between in vitro and in vivo. To select antibody leads with high affinity and specificity, it is necessary to perform in vivo validation of antibody candidates following in vitro antibody screening. Herein, a robust in vivo validation of anti-tetraspanin-8 antibody candidates against human colon cancer using ratiometric quantification method is reported. The validation is performed on a single mouse and analyzed by multiplexed surface-enhanced Raman scattering using ultrasensitive and near infrared (NIR)-active surface-enhanced resonance Raman scattering nanoprobes (NIR-SERRS dots). The NIR-SERRS dots are composed of NIR-active labels and Au/Ag hollow-shell assembled silica nanospheres. A 93% of NIR-SERRS dots is detectable at a single-particle level and signal intensity is 100-fold stronger than that from nonresonant molecule-labeled spherical Au NPs (80 nm). The result of SERRS-based antibody validation is comparable to that of the conventional method using single-photon-emission computed tomography. The NIR-SERRS-based strategy is an alternate validation method which provides cost-effective and accurate multiplexing measurements for antibody-based drug development.

Ag shell-Au satellite hetero-nanostructure for ultra-sensitive, reproducible, and homogeneous NIR SERS activity.

It is critical to create isotropic hot spots in developing a reproducible, homogeneous, and ultrasensitive SERS probe. Here, an Ag shell-Au satellite (Ag-Au SS) nanostructure composed of an Ag shell and surrounding Au nanoparticles was developed as a near-IR active SERS probe. The heterometallic shell-satellite structure based SERS probe produced intense and uniform SERS signals (SERS enhancement factor ∼1.4 × 10(6) with 11% relative standard deviation) with high detectability (100% under current measurement condition) by 785 nm photoexcitation. This signal enhancement was independent of the laser polarizations, which reflects the isotropic feature of the SERS activity of Ag-Au SS from the three-dimensional (3D) distribution of SERS hot spots between the shell and the surrounding satellite particles. The Ag-Au SS nanostructure shows a great potential as a reproducible and quantifiable NIR SERS probe for in vivo targets.

Single-step and rapid growth of silver nanoshells as SERS-active nanostructures for label-free detection of pesticides.

We explored a single-step approach for the rapid growth of Ag nanoshells (Ag NSs) under mild conditions. Without predeposition of seed metals, a uniform and complete layer of Ag shells was rapidly formed on silica core particles within 2 min at 25 °C via single electron transfer from octylamine to Ag(+) ions. The size and thickness of the Ag NSs were effectively tuned by adjusting the concentration of silica nanoparticles (silica NPs) with optimal concentrations of AgNO3 and octylamine. This unusually rapid growth of Ag NSs was attributed to a significant increase in the reduction potential of the Ag(+) ions in ethylene glycol (EG) through the formation of an Ag/EG complex, which in turn led to their facile reduction by octylamine, even at room temperature. A substantial enhancement in the surface-enhanced Raman scattering (SERS) of the prepared Ag NSs was demonstrated. The Ag NSs were also utilized as SERS-active nanostructures for label-free detection of the pesticide thiram. The Ag NS-based SERS approach successfully detected thiram on apple peel down to the level of 38 ng/cm(2) in a label-free manner, which is very promising with respect to its potential use for the on-site detection of residual pesticides.

One-step synthesis of silver nanoshells with bumps for highly sensitive near-IR SERS nanoprobes.

A seedless, one-step synthetic route to uniform bumpy silver nanoshells (AgNSs) as highly NIR sensitive SERS substrates is reported. These substrates can incorporate Raman label compounds and biocompatible polymers on their surface. AgNS based NIR-SERS probes are successfully applied to cell tracking in a live animal using a portable Raman system.