Influence of particle size on the SARS-CoV-2 spike protein detection using IgG-capped gold nanoparticles and dynamic light scattering

Due to the unprecedented and ongoing nature of the coronavirus outbreak, the development of rapid immunoassays to detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its highly contagious variants is an important and challenging task. Here, we report the development of polyclonal antibody-functionalized spherical gold nanoparticle biosensors as well as the influence of the nanoparticle sizes on the immunoassay response to detect the SARS-CoV-2 spike protein by dynamic light scattering. By monitoring the increment in the hydrodynamic diameter (ΔD_H) by dynamic light scattering measurements in the antigen–antibody interaction, SARS-CoV-2 S-protein can be detected in only 5 min. The larger the nanoparticles, the larger ΔD_H in the presence of spike protein. From adsorption isotherm, the calculated binding constant (K_D) was 83 nM and the estimated limit of detection was 13 ng/mL (30 pM). The biosensor was stable up to 90 days at 4 °C. Therefore, the biosensor developed in this work could be potentially applied as a fast and sensible immunoassay to detect SARS-CoV-2 infection in patient samples.     

Glucose detection at attomole levels using dynamic light scattering and gold nanoparticles

Glucose is directly related to brain activity and to diabetes.Therefore,developing a rapid and sensitive method for glucose detection is essential.Here,label-free glucose detection at attomole levels was realized by detecting the average diameter change of gold nanoparticles(AuNPs)utilizing dynamic light scattering(DLS).Single-strand DNA(ssDNA)adsorbed into the AuNPs’surfaces and prevented them from aggregating in solution that contained NaCl.However,ssDNA cleaved onto ssDNA fragments upon addition of glucose,and these fragments could not adsorb onto the AuNPs’surfaces.Therefore,in high-salt solution,AuNPs would aggregate and their average diameter would increase.Based on monitoring the average diameter of AuNPs with DLS,glucose could be detected in the range from 15 pmol/L to 2.0 nmol/L,with a detection limit of 8.3 pmol/L.Satisfactory results were also obtained when the proposed method was applied in human serum glucose detection.     

Enhanced resonance light scattering properties of gold nanoparticles due to cooperative binding

The interaction of 11-mercaptoundecanoic acid capped gold nanoparticles (MUA-GNPs) with europium ions and aminoacids has been studied by UV-Vis spectrophotometry, fluorescence, confocal fluorescence microscopy, resonance light scattering and TEM. Results demonstrated that hyper-Rayleigh scattering emission occurs upon the addition of lysine to the MUA-GNPs–Eu(III) system, thus providing an inherently sensitive method for lysine determination. The effects of geometrical factors of the gold nanoparticles (aspect ratio, particle size, cluster formation) and the surrounding medium (pH) on this behavior are discussed. The cooperative binding interactions of Eu³⁺ and lysine with gold nanoparticles permitted the discrimination of lysine from other amino acids. The probable mechanism for the spectral changes and the enhanced resonance light scattering observed is outlined. Figure Gold nanoparticle resonance light scattering plasmon enhancement through cooperative binding with europium and lysine     

A sensitive resonance light scattering spectrometry of trace Hg with sulfur ion modified gold nanoparticles

A new resonance light scattering (RLS) spectrometric method for mercury ions (Hg²⁺) in aqueous solutions with sulfur ion (S²⁻) modified gold nanoparticles (Au-NPs-S) has been developed in this contribution. It was found that S²⁻ at the surface of Au-NPs resulting from the surface modification can interact with Hg²⁺ to form very stable S-Hg-S bonds when Hg²⁺ concentration is lower than that of S²⁻, resulting in the aggregation of Au-NPs-S and causing enhanced RLS signals. The enhanced RLS intensities (ΔI_RLS) characterized at 392 nm were found to be proportional to the concentration of Hg²⁺ in the range of 0.025-0.25 μmol L⁻¹ with a detection limit (3σ) of 0.013 μmol L⁻¹. Our results showed that this approach has excellent selectivity for Hg²⁺ over other substances in aqueous solutions.     

Rapid and sensitive detection of cholera toxin using gold nanoparticle-based simple colorimetric and dynamic light scattering assay

Herein, a rapid and simple gold nanoparticle based colorimetric and dynamic light scattering (DLS) assay for the sensitive detection of cholera toxin has been developed. The developed assay is based on the distance dependent properties of gold nanoparticles which cause aggregation of antibody-conjugated gold nanoparticles in the presence of cholera toxin resulting discernible color change. This aggregation induced color change caused a red shift in the plasmon band of nanoparticles which was measured by UV–Vis spectroscopy. In addition, we employed DLS assay to monitor the extent of aggregation in the presence of different concentration of cholera toxin. Our assay can visually detect as low as 10 nM of cholera toxin which is lower than the previously reported colorimetric methods. The reported assay is very fast and showed an excellent specificity against other diarrhetic toxins. Moreover, we have demonstrated the feasibility of our method for cholera toxin detection in local lake water.     

Electrochemical determination of melamine using oligonucleotides modified gold electrodes

A novel and simple electrochemical method for determination of melamine is developed based on oligonucleotides film modified gold electrodes. The electrochemical probe of ferricyanide was used to investigate the interactions between oligonucleotides and melamine. Results of cyclic voltammetries, differential pulse stripping voltammetries, electrochemical impedance spectrometry and atomic force microscope, proved that melamine might interact with oligonucleotides mainly through electrostatic and hydrogen-bonding interactions. The interactions between oligonucleotides and melamine lead to the increase in the peak currents of ferricyanide, which could be used for electrochemical sensing of melamine. The redox peak currents of ferricyanide were linear with the concentration of melamine in the range from 3.9 × 10⁻⁸ to 3.3 × 10⁻⁶ M with a linear coefficiency of 0.990. The detection limit was 9.6 × 10⁻⁹ M. The proposed electrochemical biosensor is rapid, convenient and low-cost for effective sensing of melamine. Particularly, the proposed method was applied successfully to the determination of melamine in milk products, and the recovery was 95%.     

Homogeneous immunoassay for soy protein determination in food samples using gold nanoparticles as labels and light scattering detection

A homogeneous aggregation immunoassay involving the use of gold nanoparticles (AuNPs) and light scattering detection is described for soy protein determination in food samples. AuNPs act as enhancers of the precipitate that appears when the antigen-antibody complex is formed. The AuNPs-antibody conjugate has been synthesized by physical adsorption of polyclonal anti-soy protein antibodies onto the surface of commercial AuNPs with a nominal diameter of 20 nm. The direct assay is based on the reaction of the conjugate with soy protein, which reaches the equilibrium in about 10 min, and the measurement of the light scattering intensity at 530 nm, which is proportional to the analyte concentration. The dynamic range of the calibration graph is 0.2-20 μg mL⁻¹ and the detection limit value is 65 ng mL⁻¹. The precision, expressed as relative standard deviation, has been assayed at two different concentrations, 0.2 and 1 μg mL⁻¹, giving values ranging from 4.7 to 5.9%. The interference of other proteins has been assayed. The usefulness of this method has been shown by its application to the analysis of fruit juice and “nonmilk yoghourt” samples. The results obtained with the proposed method are similar to those obtained by using a commercial ELISA kit, but the assay time is significantly shorter and the detection limit was about 10 times lower. A recovery study has been also performed, giving values in the range of 84.0-119.3%.     

Selective extraction of melamine using 11-mercaptoundecanoic acid-capped gold nanoparticles followed by capillary electrophoresis

This study describes the use of 11-mercaptoundecanoic acid-capped gold nanoparticles (MUA-AuNPs) for selective extraction of melamine prior to analysis by capillary electrophoresis with UV detection. The highest degree of melamine-induced aggregation of MUA-AuNPs was found to occur at pH 5.0, indicating that the NP aggregation is mainly because of hydrogen bonding between the carboxylate groups of MUA and the amine groups of melamine. Moreover, the degree of melamine-induced NP aggregation gradually increased when the chain length of the mercaptoalkanoic acid was increased from two to 12 carbon atoms. At pH 5.0, the extraction efficiency of melamine was highly dependent on the concentration of MUA-AuNPs, the concentration of dithiothreitol (DTT), the extraction time between MUA-AuNPs and melamine, and the incubation time between melamine-adsorbed AuNPs and DTT. The separation of the extracted melamine and DTT (releasing agent) was accomplished using a solution of 10 mM phosphate (pH 6.0) containing 1.6% (v/v) poly(diallyldimethylammonium chloride). Under the optimum extraction and separation conditions, the limit of detection at a signal-to-noise ratio of 3 was estimated to be 77 pM for melamine, with linear range of 1-1000 nM. The proposed method was successfully applied to the determination of melamine in tap water and in milk.     

Visual detection of melamine in milk products by label-free gold nanoparticles

A simple, rapid, field-portable colorimetric method for the detection of melamine based on melamine-induced color change of label-free gold nanoparticles (Au NPs) was developed in this study. Melamine can induced the aggregation of Au NPs and results in the color change from wine-red to purple, which provided a platform for rapid and field-portable colorimetric detection of melamine. The proposed method can be used to detect melamine in liquid milk and infant formula with a detection limit of 1.0 and 4.2 ppm, respectively, within 30 min by naked eyes observation without the aid of any advanced instrument and the need of any complex pretreatment, and detect as low as 0.15 ppm of melamine in liquid milk and 2.5 ppm of melamine in infant formula with UV-vis-spectroscopy. The proposed method is promising for on-site screening of melamine adulterant in milk products.     

Colorimetric recognition of melamine in milk using novel pincer zinc complex stabilized gold nanoparticles

A convenient colorimetric approach for visual detection of melamine in raw milk was realized by using gold nanoparticles (AuNPs) stabilized by an unsymmetrical terpyridyl zinc complex with a thymine fragment at one terminal and a quaternary ammonium salt at the other. Even without pre-addition of melamine or relative additives, obvious color change from red to blue was observed by naked eye in the presence of trace amount of melamine, which was attributed to the alternation of aggregation state of AuNPs caused by the selective binding between the thymine fragment and melamine via triple hydrogen-bonding interactions. Remarkably, the detection limit for melamine was as low as 2.4 ppb, providing a highly sensitive and efficient approach for the visual detection of melamine.     

金纳米微粒作探针共振瑞利散射光谱法测定盐酸异丙嗪

在pH 1.8～3.0的酸性介质中,质子化的盐酸异丙嗪(PMZ)可与带负电荷的金纳米微粒依靠静电和疏水作用相互结合,导致共振瑞利散射(RRS)强度显著增强,其最大散射峰位于368 nm,并在284,440,498 nm处有明显的散射峰,在选定的测量波长下,盐酸异丙嗪在0.04～0.10μg/mL的浓度范围内与RRS强度成正比,该法具有高的灵敏度,其检出限为1.34 ng/mL。考察了体系的RRS光谱特征,研究了适宜的反应条件、影响因素,研究了共存物质的影响,据此建立了金纳米微粒作探针RRS法测定盐酸异丙嗪的新方法。     

Surface-enhanced Raman scattering for perchlorate detection using cystamine-modified gold nanoparticles

Perchlorate (ClO₄⁻) has recently emerged as a widespread environmental contaminant found in groundwater and surface water, and there is a great need for rapid detection and monitoring of this contaminant. This study presents a new technique using cystamine-modified gold nanoparticles as a substrate for surface-enhanced Raman scattering (SERS) detection of perchlorate at low concentrations. A detection limit of 5 × 10⁻⁶ M (0.5 mg/L) has been achieved using this method without sample preconcentration. This result was attributed to a strong plasmon enhancement by gold metal surfaces and the electrostatic attraction of ClO₄⁻ onto positively charged, cystamine-modified gold nanoparticles at a low pH. The methodology also was found to be reproducible, quantitative, and not susceptible to significant interference from the presence of anions such as sulfate, phosphate, nitrate and chloride at concentrations <1 mM, making it potentially suitable for rapid screening and routine analysis of perchlorate in environmental samples.     

Ultrasensitive optical detection of trinitrotoluene by ethylenediamine-capped gold nanoparticles

This study found that 1,2-ethylenediamine (EDA) as a primary amine could be modified onto the surface of citrate-stabilized gold nanoparticles (Au NPs), and the EDA-capped Au NPs were successfully used as an ultrasensitive optical probe for TNT detection. The strong donor–acceptor (D–A) interactions between EDA and trinitrotoluene (TNT) at the Au NP/solution interface induced significant aggregation of the EDA-capped Au NPs, and enabled to easily realize the direct colorimetric detection of ultratrace TNT. The results showed that such a color change was readily seen by the naked eye, and the colorimetric detection could be down to 400 pM level of TNT with excellent discrimination against other nitro compounds. UV–vis absorption spectroscopy was used to examine the TNT-induced changes in local surface plasmon resonance (LSPR) of EDA-capped Au NPs, and a new LSPR band at ca. 630 nm arose along with the addition of TNT, which produced a detection limit of TNT down to ca. 40 pM. Furthermore, dynamic light scattering measurements evidenced the ultratrace TNT-induced small changes in the size of the EDA-capped Au NPs, and realized the quick and accurate detection of TNT in 0.4 pM level. These results demonstrated the ultrahigh sensitivity of this optical probe for TNT detection. Moreover, this optical probe is sample, stable, low-cost, and these excellent properties make it quite promising for infield and rapid detection of TNT.     

Visual detection of melamine in milk samples based on label-free and labeled gold nanoparticles

Melamine that can cause serious damage to the organs of animal or human beings was found to bind to polythymine via hydrogen bonding. With this novel discovery, colorimetric detection of melamine based on label-free and labeled gold nanoparticles was developed, respectively. Both of the methods revealed good selectivity for melamine over other components that may exist in milk and good anti-influence ability. The raw milk samples were pretreated according to the National standard method combined with a solid phase extraction monolithic column. The accurate quantification of melamine as low as 41.7 nM and 46.5 nM was obtained, respectively. It also guarantees fast and reliable readout with naked eyes, making visual detection possible. Further comparison between label-free and labeled based methods was discussed in this paper.     

Resonance Rayleigh scattering spectral method for the determination of raloxifene using gold nanoparticle as a probe

When gold nanoparticles were being prepared by sodium citrate reduction method, citrate anions self-assembled on the surface of gold nanoparticles to form supermolecular complex anions with negative charges, and protonated raloxifene (Ralo) was positively charged and could bind with the complex anions to form larger aggregates through electrostatic force and hydrophobic effects, which could result in the remarkable enhancement of the resonance Rayleigh scattering intensity (RRS), and the appearance of new RRS spectra. At the same time, the second-order scattering (SOS) and frequency-doubling scattering (FDS) intensities were also enhanced. The maximum wavelengths were located near 370 nm for RRS, 520 nm for SOS, and 350 nm for FDS, respectively. Among them, the RRS method had the highest sensitivity and the detection limit was 5.60 ng mL⁻¹ for Ralo, and its linear range was 0.05-2.37 μg mL⁻¹. A new RRS method for the determination of trace Ralo using gold nanoparticles probe was developed. The optimum conditions of the reaction and influencing factors were investigated. In addition, the reaction mechanism and the reasons for the enhancement of RRS were discussed.     

Selective determination of melamine in milk samples using 3-mercapto-1-propanesulfonate-modified gold nanoparticles as colorimetric probe

A novel and sensitive colorimetric method for determination of melamine in milk samples was developed by a 3-mercapto-1-propanesulfonate-modified gold nanoparticles (MPS-GNPs) probe. Melamine molecule has multiple -NH₂ groups. These functional groups can interact with MPS to form strong hydrogen bonding and induce the aggregation of the MPS-GNPs, resulting in a dramatic color change from red to blue. Therefore, the concentration of melamine in milk samples can be quantitatively detected by the naked eyes or a UV-vis spectrometer. Moreover, investigations have revealed that the sensitivity of the detection could be clearly improved by adding NaCl to the modified GNPs solution, which leads to a more rapid color change in the NaCl-optimized GNPs system. It is worth noting that the absorption ratio (A₆₅₀/A₅₂₀) of the modified GNPs in the NaCl-optimized system exhibited a linear correlation with melamine concentration and the limit of detection is 8 nM, well below the safety limit (1 ppm for infant formula in China).     

金纳米微粒共振光散射光谱探针测定维生素B4-水溶性腺嘌呤的研究

建立了一种以金纳米微粒为探针共振光散射(RLS)法测定维生素B4的新方法.在弱酸性介质中(pH 4.2),金纳米微粒在635 nm有一最大共振散射峰.加入微量维生素B4后,金纳米微粒与维生素B4通过静电引力结合.形成了粒径较大的聚集体,导致RLS强度显著增强.研究了体系的共振光散射光谱特征和反应适宜条件,探讨了共振光散射增强的机理.结果表明,维生素B4质量浓度在0.1～5.0μg/mL 时与散射强度(△I)呈线性关系,检出限(3σ)为12.0 ng/mL,相对标准偏差(RSD)为2.2%.该方法已用于片剂中维生素B4的测定.     

Usefulness of gold nanoparticles as labels for the determination of gliadins by immunoaffinity chromatography with light scattering detection

A simple and fast immunoaffinity method is proposed for the determination of gliadins for the first time using gold nanoparticles (AuNPs) as labels. The tracer used consists in a gliadin-AuNP conjugate prepared by the adsorption of gliadins onto the nanoparticle surface. Two AuNP sizes with diameters of 10 nm and 20 nm were assayed to compare the behaviour of the corresponding tracer in the assay. The method relies on the injection in a commercial Protein G column of a preincubated mixture containing gliadins, polyclonal anti-gliadin antibodies, and the gliadin-AuNP tracer. This approach allows the separation of free and bound tracer fractions without any additional elution step, and the direct measurement of the resonance light scattering intensity of the free tracer through the peak height of the immunochromatogram, which is proportional to the analyte concentration. The immunocolumn can be used up to 25 times without eluting and it can be regenerated for at least 20 times. The dynamic ranges of the calibration graphs and the detection limits are 0.5-15.0 and 1.5-15.0 μg mL⁻¹ gliadins, and 0.2 μg mL⁻¹ and 0.8 μg mL⁻¹ gliadins, using 20-nm and 10-nm Au-NPs as labels, respectively. The precision, expressed as relative standard deviation, ranges between 2.7% and 2.9% using 20-nm AuNPs and 4% and 6.1% for 10-nm AuNPs. The method has been applied to the determination of the prolamin fraction in beer samples, obtaining recovery values in the range 71.2% and 101.7%.     

Asymmetrical flow field-flow fractionation with multi-angle light scattering detection for the analysis of structured nanoparticles

Synthesis and applications of new functional nanoparticles are topics of increasing interest in many fields of nanotechnology. Chemical modifications of inorganic nanoparticles are often necessary to improve their features as spectroscopic tracers or chemical sensors, and to increase water solubility and biocompatibility for applications in nano-biotechnology. Analysis and characterization of structured nanoparticles are then key steps for their synthesis optimization and final quality control. Many properties of structured nanoparticles are size-dependent. Particle size distribution analysis then provides fundamental analytical information. Asymmetrical flow field-flow fractionation (AF4) with multi-angle light scattering (MALS) detection is able to size-separate and to characterize nanosized analytes in dispersion. In this work we focus on the central role of AF4-MALS to analyze and characterize different types of structured nanoparticles that are finding increasing applications in nano-biotechnology and nanomedicine: polymer-coated gold nanoparticles, fluorescent silica nanoparticles, and quantum dots. AF4 not only size-fractionated these nanoparticles and measured their hydrodynamic radius (r_h) distribution but it also separated them from the unbound, relatively low-M_r components of the nanoparticle structures which were still present in the sample solution. On-line MALS detection on real-time gave the gyration radius (r_g) distribution of the fractionated nanoparticles. Additional information on nanoparticle morphology was then obtained from the r_h/r_g index. Stability of the nanoparticle dispersions was finally investigated. Aggregation of the fluorescent silica nanoparticles was found to depend on the concentration at which they were dispersed. Partial release of the polymeric coating from water-soluble QDs was found when shear stress was induced by increasing flowrates during fractionation.     

Ferric nanoparticle-based resonance light scattering determination of DNA at nanogram levels

A novel assay of DNA has been proposed by using ferric nanoparticles as probes coupled with resonance light scattering (RLS) detection. At pH 7.40, the RLS intensity of ferric nanoparticles can be greatly enhanced by the aggregation of positively charged ferric nanoparticles through electrostatic interaction with negatively charged DNA. The enhanced intensity of RLS at 452 nm is proportional to the concentration of DNA in the range of 0.01-0.8 μg ml⁻¹ for calf thymus and salmon sperm DNA and in the range of 0.005-0.3 μg ml⁻¹ for E. coli K12 genomic DNA. Detection limits are 3.6 ng ml⁻¹ for calf thymus DNA, 4.4 ng ml⁻¹ for salmon sperm DNA, and 1.9 ng ml⁻¹ for E. coli K12 genomic DNA, respectively. Compared with the chromophores previously used in RLS assay, the ferric nanoparticles have offered several advantages in easy preparation, good photostability and high sensitivity without being modified or functionalized.