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A one-step homogeneous immunoassay for the detection of a prostate cancer biomarker, free-PSA (prostate specific antigen), was developed using gold nanoparticle probes coupled with dynamic light scattering (DLS) measurements. A spherical gold nanoparticle with a core diameter around 37 nm and a gold nanorod with a dimension of 40 by 10 nm were first conjugated with two different primary anti-PSA antibodies and then used as optical probes for the immunoassay. In the presence of antigen f-PSA in solution, the nanoparticles and nanorods aggregate together into pairs and oligomers through the formation of a sandwich type antibody-antigen-antibody linkage. The relative ratio of nanoparticle-nanorod pairs and oligomers versus individual nanoparticles was quantitatively monitored by DLS measurement. A correlation can be established between this relative ratio and the amount of antigen in solution. The light scattering intensity of nanoparticles and nanoparticle oligomers is several orders of magnitude higher than proteins and other typical molecules, making it possible to detect nanoparticle probes in the low picomolar concentration range. f-PSA in the concentration range from 0.1 to 10 ng/mL was detected by this one-step and washing-free homogeneous immunoassay.

Gold nanoparticles (AuNPs) are some of the most extensively studied nanomaterials. Because of their unique optical, chemical, electrical, and catalytic properties, AuNPs have attracted enormous amount of interest for applications in biological and chemical detection and analysis. The purpose of this critical review is to provide the readers with an update on the recent developments in the field of AuNPs for sensing applications based on their optical properties. An overview of the optical properties of AuNPs is presented first, followed by a more detailed literature survey. As the last part of this review, we compare the advantages and disadvantages of each technique, briefly discuss their commercialization status, and some technical issues that remain to be solved in order to move the technique forward (151 references).

Dynamic light scattering (DLS) is an analytical tool used routinely for measuring the hydrodynamic size of nanoparticles and colloids in a liquid environment. Gold nanoparticles (GNPs) are extraordinary light scatterers at or near their surface plasmon resonance wavelength. In this study, we demonstrate that DLS can be used as a very convenient and powerful tool for gold nanoparticle bioconjugation and biomolecular binding studies. The conjugation process between protein A and gold nanoparticles under different experimental conditions and the quality as well as the stability of the prepared conjugates were monitored and characterized systematically by DLS. Furthermore, the specific interactions between protein A-conjugated gold nanoparticles and a target protein, human IgG, can be detected and monitored in situ by measuring the average particle size change of the assay solution. For the first time, we demonstrate that DLS is able to directly and quantitatively measure the binding stoichiometry between a protein-conjugated GNP probe and a target analyte protein in solution.

Despite many of the intriguing and excellent electrical, mechanical and optical properties of carbon nanotubes (CNTs), [1,2] extensive applications of these nanomaterials are still limited. One of the main challenges in carbon nanotube research field is the dispersion and stabilization of CNTs in different solvent media and polymer matrices. The as-synthesized CNTs are often bundled together due to strong van der Waals interactions between the nanotubes. There have been three most widely used methods to disperse CNTs into solvents and polymer matrices: [3] physical blending, [4][5][6] chemical functionalization, [7][8][9][10][11] and dispersants-assisted dispersion. [12][13][14][15][16] Each of these methods has its own advantages and disadvantages. For physical blending using mechanical forces such as sonication, although simple and cost effective, the dispersion quality is often the poorest. The so-dispersed nanotubes will quickly precipitate out again when sonication stops. For chemical modification and functionalization, CNTs are treated with strong oxidizing reagents to form functional groups such as carboxylic acids on the nanotube walls. CNTs can be made water or organic solvent soluble, depending on the modification degree and further molecular moieties attached to the nanotubes. Although most effective as a dispersion method, such treatment inevitably disrupts the long range p conjugation of the nanotube, often leads to decreased electrical conductivity, diminished mechanical strength, and other undesired properties. In the dispersants-assisted dispersion, a third component chemical is mixed with CNTs in solutions. Through sonication, the CNTs are mechanically de-bundled and then stabilized by a dispersant chemical through noncovalent interactions, therefore, avoiding the destruction of the chemical structures, electronic and mechanical properties of the carbon nanotubes. Recently, there are two types of materials that have attracted significant amount of attention as dispersants to assist carbon nanotube dispersion. One of these materials is the conjugated polymers, such as poly(m-phenylene vinylene), [17,18] poly(3-alkylthiophene), [19,20] and poly(arylene ethynylene). [21,22] These polymers stabilize carbon nanotubes by forming strong p-p stack interactions with carbon nanotube walls. However, the thus-formed dispersions have limited solubility and stability because the conjugated polymers themselves face solubility and miscibility issues due to the strong inter-chain p-p interactions. A second family of interesting materials with potential for third componentassisted carbon nanotube dispersion are block copolymers. [23][24][25] In general, the block copolymer is designed in such a way that one block of the polymer will form a close interaction with the carbon nanotube walls, while the other block(s) will provide the solubility to the exfoliated nanotubes by forming a steric barrier or repulsion interaction between polymer-wrapped nanotubes.[26] So far, a wide range of charged and neutral block copoly...

The detection limit is one of the most important performance parameters for bioanalytical techniques. Here we present a generic method to estimate the detection limit of biomolecular assays based on a step-by-step analysis of the assay procedure. Enzyme-linked immunosorbent assay (ELISA) is used here as an example; however, much of the information presented in this article may be applied to other types of biomolecular assays and analytical techniques. A clear understanding of what affects the detection limit can help researchers to evaluate different bio-analytical techniques properly, and to design better strategies to optimize and achieve the best analytical performance.

A one-step homogeneous DNA detection method with high sensitivity was developed using gold nanoparticles (AuNPs) coupled with dynamic light scattering (DLS) measurement. Citrate-protected AuNPs with a diameter of 30 nm were first functionalized with two sets of single-stranded DNA probes and then used as optical probes for DNA detection. In the presence of target DNA, the hybridization between target DNA and the two nanoparticle probes caused the formation of nanoparticle dimers, trimers, and oligomers. As a result, the nanoparticle aggregation increased the average diameter of the whole nanoparticle population, which can be monitored simply by DLS measurement. A quantitative correlation can be established between the average diameter of the nanoparticles and the target DNA concentration. This DLS-based assay is extremely easy to conduct and requires no additional separation and amplification steps. The detection limit is around 1 pM, which is 4 orders of magnitude better than that of light-absorption-based methods. Single base pair mismatched DNAs can be readily discriminated from perfectly matched target DNAs using this assay.

A strategy for label-free oligonucleotide (DNA) analysis has been proposed by measuring the DNA-morpholino hybridization hindered diffusion flux of probe ions Fe(CN)(6)(3-) through nanochannels of a porous anodic alumina (PAA) membrane. The flux of Fe(CN)(6)(3-) passing through the PAA nanochannels is recorded using an Au film electrochemical detector sputtered at the end of the nanochannels. Hybridization of the end-tethered morpholino in the nanochannel with DNA forms a negatively charged DNA-morpholino complex, which hinders the diffusion of Fe(CN)(6)(3-) through the nanochannels and results in a decreased flux. This flux is strongly dependent on ionic strength, nanochannel aperture, and target DNA concentration, which indicates a synergetic effect of steric and electrostatic repulsion effects in the confined nanochannels. Further comparison of the probe flux with different charge passing through the nanochannels confirms that the electrostatic effect between the probe ions and DNA dominates the hindered diffusion process. Under optimal conditions, the present nanochannel array-based DNA biosensor gives a detection limit of 0.1 nM.