Magnetic gold nanoparticles in SERS-based sandwich immunoassay for antigen detection by well oriented antibodies

Trace detection of tetrabromobisphenol A by SERS with DMAP-modified magnetic gold nanoclusters

Magnetic gold nanoclusters (MGNCs) functionalized with 4-dimethylaminopyridine (DMAP) enables the trace detection of tetrabromobisphenol A (TBBPA), an environmental pollutant, using surface-enhanced Raman scattering (SERS) spectroscopy. The synthesis, cleansing, and functionalization of MGNCs are conducted in aqueous solutions; SERS samples are prepared by magnetic precipitation in the presence of trace analyte. The limit of detection (LOD) for TBBPA is greatly increased by the use of DMAP as a reporter molecule: DMAP-modified MGNCs can detect TBBPA at 10 pM in water, whereas the LOD for TBBPA by unfunctionalized Au is 1 nM. The reproducibility of picomolar TBBPA detection with DMAP-modified MGNCs is confirmed by two-dimensional correlation analysis. The high SERS sensitivity for TBBPA can be attributed to its capacity to modulate the Raman spectrum of adsorbed DMAP. This indirect mode of detection can also be applied toward the detection of other hydrophobic analytes, each identifiable by its characteristic SERS identity.

Phage-AgNPs complex as SERS probe for U937 cell identification

The early diagnosis of malignancy is the most critical factor for patient survival and the treatment of cancer. In particular, leukemic cells are highly heterogeneous, and there is a need to develop new rapid and accurate detection systems for early diagnosis and monitoring of minimal residual disease. This study reports the utilization of molecular networks consisting of entire bacteriophage structure, displaying specific peptides, directly assembled with silver nanoparticles as a new Surface Enhanced Raman Spectroscopy (SERS) probe for U937 cells identification in vitro. A 9-mer pVIII M13 phage display library is screened against U937 to identify peptides that selectively recognize these cells. Then, phage clone is assembled with silver nanoparticles and the resulting network is used to obtain a SERS signal on cell-type specific molecular targets. The proposed strategy could be a very sensitive tool for the design of biosensors for highly specific and selective identification of hematological cancer cells and for detection of minimal residual disease in a significant proportion of human blood malignancy.

Development of a HIV-1 Virus Detection System Based on Nanotechnology

Development of a sensitive and selective detection system for pathogenic viral agents is essential for medical healthcare from diagnostics to therapeutics. However, conventional detection systems are time consuming, resource-intensive and tedious to perform. Hence, the demand for sensitive and selective detection system for virus are highly increasing. To attain this aim, different aspects and techniques have been applied to develop virus sensor with improved sensitivity and selectivity. Here, among those aspects and techniques, this article reviews HIV virus particle detection systems incorporated with nanotechnology to enhance the sensitivity. This review mainly focused on four different detection system including vertically configured electrical detection based on scanning tunneling microscopy (STM), electrochemical detection based on direct electron transfer in virus, optical detection system based on localized surface plasmon resonance (LSPR) and surface enhanced Raman spectroscopy (SERS) using plasmonic nanoparticle.

Development of rolling circle amplification based surface-enhanced Raman spectroscopy method for 35S promoter gene detection

In this study, we developed the genetically modified organism detection method by using the combination of rolling circle amplification (RCA) and surface-enhanced Raman spectroscopy (SERS). An oligonucleotide probe which is specific for 35S DNA promoter target was immobilised onto the gold slide and a RCA reaction was performed. A self-assembled monolayer was formed on gold nanorods using 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and the second probe of the 35S DNA promoter target was immobilised on the activated gold coated slide surfaces. Probes on the nanoparticles were hybridised with the target oligonucleotide. Quantification of the target concentration was performed via SERS spectra of DTNB on the nanorods. SERS spectra of target molecules were enhanced through the RCA reaction and the detection limit was found to be 6.3fM. The sensitivity of the developed RCA-SERS method was compared with another method which had been performed without using RCA reaction, and the detection limit was found to be 0.1pM. The correlation between the target concentration and the SERS signal was found to be linear, within the range of 1pM to 10nM for the traditional assay and 100fM to 100nM for the RCA assay. For the developed RCA-SERS assay, the specificity tests were performed using the 35S promoter of Bt-176 maize gene. It was found out that the developed RCA-SERS sandwich assay method is quite sensitive, selective and specific for target sequences in model and real systems.

Rapid immunoenzyme assay of aflatoxin B1 using magnetic nanoparticles

The main limitations of microplate-based enzyme immunoassays are the prolonged incubations necessary to facilitate heterogeneous interactions, the complex matrix and poorly soluble antigens, and the significant sample dilutions often required because of the presence of organic extractants. This study presents the use of antibody immobilization on the surface of magnetic particles to overcome these limitations in the detection of the mycotoxin, aflatoxin B1. Features of the proposed system are a high degree of nanoparticle dispersion and methodologically simple immobilization of the antibodies by adsorption. Reactions between the immobilized antibodies with native and labeled antigens are conducted in solution, thereby reducing the interaction period to 5 min without impairing the analytical outcome. Adsorption of immunoglobulins on the surface of magnetic nanoparticles increases their stability in aqueous-organic media, thus minimizing the degree of sample dilution required. Testing barley and maize extracts demonstrated a limit of aflatoxin B1 detection equal to 20 pg/mL and total assay duration of 20 min. Using this method, only the 3-fold dilution of the initial methanol/water (60/40) extraction mixture in the microplate wells is necessary. The proposed pseudo-homogeneous approach could be applied toward immunodetection of a wide range of compounds.

Diagnostic applications of gastric carcinoma cell aptamers in vitro and in vivo

Gastric carcinoma is the most malignant tumor. Due to lacking of efficient means to diagnose the cancer at the early stage, it is necessary to develop effective molecular probes for early diagnosis and treatment. We have selected aptamers with high specificity and affinity against SGC7901 cells by cell-SELEX (Systematic Evolution of Ligands by Exponential Enrichment) method, which shown important clinical applications: (1) Specific recognize human gastric tumor tissues compared to the normal tissues. (2)When used to capture cancerous cells, the aptamer-functionalized fluorescent-magnetic nanospheres (FMNS) could specifically capture 93% target cancer cells and about 70% target cells can be released. (3) The aptamer probe displayed a quenched fluorescence in the absence of target cancer cells and went through a conformational transformation upon binding to target cancer cells that induced fluorescence. (4) The aptamer probe could target gastric tumors transplanted into mice with obvious fluorescence. The newly generated aptamers hold great potential in early cancer diagnosis.

Surface modification of HSA containing magnetic PLGA nanoparticles by poloxamer to decrease plasma protein adsorption

Lifetime prolongation for hydrophobic drug carriers has been the focus of interest for many years. Poloxamer (Pluronic F68, PF68) has been employed in this study for modifying the surface of magnetic poly(d,l-lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) loaded with human serum albumin (HSA) model drug. Surface characteristics of untreated and PF68 treated NPs were analyzed by size, zeta potential and electrophoretic mobility studies. UV-vis spectroscopic analysis, isothermal titration calorimetry (ITC) and dynamic light scattering methods were used to investigate serum protein (bovine serum albumin, BSA) adsorption. Results showed the successful surface attachment of PF68. Among different concentrations (0.1-1%, wt/vol) of PF68 studied, 0.5% was found to be the most useful, since a higher concentration can issue in micelle formation. 50% less BSA tended to be adsorbed on the treated NPs in comparison to the untreated ones.

Effect of hydration on plasmonic coupling of bioconjugated gold nanoparticles immobilized on a gold film probed by surface-enhanced Raman spectroscopy

Gold nanoparticle (AuNP)-Au film constructs were prepared using antibody-antigen interactions or a small organic cross-linker to systematically control the gap between the AuNP and Au film. Surface-enhanced Raman spectroscopy (SERS), scanning electron micrsocopy (SEM), and atomic force microscopy (AFM) were used to characterize each construct and elucidate structure-activity relationships. Interestingly, plasmonic coupling and SERS intensity were reversibly modulated with wetting/drying cycles for the protein immobilized AuNP, and this effect was attributed to changes in protein size with hydration state. This work provides insight into fundamental limitations of AuNP-enabled SERS bioassays and will facilitate rational design of novel biospecific ligands that maximize SERS sensitivity.

Biotechnological approaches toward nanoparticle biofunctionalization

Nanomedicine has emerged in the past decade as a promising tool for several therapeutic and diagnostic applications. The development of nanoconjugates containing bioactive ligands specific for targeting cancer cell receptors has become a primary objective of modern nanotechnology. The design of ideal nanoconjugates requires optimization of fundamental parameters including size, shape, ligand shell composition, and reduction in nonspecific protein adsorption. Of great importance is the choice of bioconjugation approach, given that it affects the orientation, accessibility, and bioactivity of the targeting molecule. We provide an overview of recent advances in the immobilization of targeting proteins, focusing on methods to control ligand orientation and density, and highlight criteria for nanoparticle design and development required to achieve enhanced receptor-targeting efficiency.

Structural and equilibrium effects of the surface passivant on the stability of Au nanorods

Au nanomaterials are well-known for their optical properties, where Au nanorods have demonstrated unique capabilities because of their readily tunable size and shape. Unfortunately, functionalization of the material surface is challenging because of their lack of stability after only a few purification cycles. Here, we demonstrate that enhanced Au-nanorod stability can be achieved by purifying the materials using dilute cetyltrimethylammonium bromide (CTAB) wash solutions. To this end, purifying the materials in such a manner shifts the passivant on/off equilibrium to maintain surfactant adsorption to the metal surface, leading to enhanced stability. Interestingly, from this study, a bimodal distribution of Au nanorods was evident, where one species was prone to bulk aggregation, whereas the second population remained stable in solution. This likely arose from defects within the CTAB bilayer at the nanorod surface, resulting in selective material aggregation. For this, those structures with high numbers of defects aggregated, whereas nanorods with a more pristine bilayer remained stable. Coating of the Au nanorods using polyelectrolytes was also explored for enhanced stability, where the composition of the anionic polymer played an important role in controlling materials stability. Taken together, these results demonstrate that the stability of Au nanorods can be directly tuned by the solvent-exposed surface structure, which could be manipulated to allow for the extensive material functionalization that is required for the generation of nanoplatforms with multiple applications.

Direct detection of orchid viruses using nanorod-based fiber optic particle plasmon resonance immunosensor

A fiber optic particle plasmon resonance (FOPPR) immunosensor is developed for label-free detection of orchid viruses that use gold nanorods (AuNRs) as the sensing material. The AuNRs are employed to create a near-infrared sensing window to solve the color interference problem of sample matrix for direct sensing of target analyte. This work cannot be achieved using gold nanospheres (AuNSs) because the signal of sample color absorption largely overlaps the signal of molecular recognition events in the visible spectrum, making the signal interpretation much more difficult. The AuNRs are immobilized on the unclad fiber core surface, and functionalized by antibodies which can specifically recognize the corresponding Cymbidium mosaic virus (CymMV) or Odontoglossum ringspot virus (ORSV) for rapid viral infection diagnosis. The refractive index resolution of the AuNR-FOPPR sensor is estimated to be 8×10(-6) RIU. The limits of detection (LODs) for CymMV and ORSV in leaf saps are 48 and 42 pg/mL, respectively, which are better than the LODs of 1200 pg/mL for both viruses obtained by enzyme-linked immunosorbent assay (ELISA). Exploiting the AuNR-FOPPR sensing strategy not only solves the color interference problem encountered by using AuNSs, but provides faster analysis, better reproducibility, and lower detection limit than ELISA. The sensor can distinguish between healthy and infected orchids in 10 min, and can further provide the quantitative analysis of infection level. It is potentially applicable to the quality control of orchid cultivation industry, but not limited to this, especially for creating special spectral sensing window for particular samples.

Site-directed antibody immobilization techniques for immunosensors

Immunosensor sensitivity, regenerability, and stability directly depend on the type of antibodies used for the immunosensor design, quantity of immobilized molecules, remaining activity upon immobilization, and proper orientation on the sensing interface. Although sensor surfaces prepared with antibodies immobilized in a random manner yield satisfactory results, site-directed immobilization of the sensing molecules significantly improves the immunosensor sensitivity, especially when planar supports are employed. This review focuses on the three most conventional site-directed antibody immobilization techniques used in immunosensor design. One strategy of immobilizing antibodies on the sensor surface is via affinity interactions with a pre-formed layer of the Fc binding proteins, e.g., protein A, protein G, Fc region specific antibodies or various recombinant proteins. Another immobilization strategy is based on the use of chemically or genetically engineered antibody fragments that can be attached to the sensor surface covered in gold or self-assembled monolayer via the sulfhydryl groups present in the hinge region. The third most common strategy is antibody immobilization via an oxidized oligosaccharide moiety present in the Fc region of the antibody. The principles, advantages, applications, and arising problems of these most often applied immobilization techniques are reviewed.