Step-addition Method for Enhancing the Yield of Gold Nanoparticles in Block Copolymer Solution

Triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) has been used to synthesize gold nanoparticles from hydrogen tetrachloroaureate (III) hydrate (HAuCl₄·3H₂O) salt in aqueous solution at room temperature. Measurements were performed using the triblock copolymer Pluronic P85 (EO₂₆PO₃₉EO₂₆) at a fixed concentration (1 wt%) mixed with varying HAuCl₄·3H₂O concentration in the range of 0.001 to 0.1 wt%. The surface plasmon resonance (SPR) band in UV-visible absorption spectra confirmed the formation of the gold nanoparticles. The maximum yield of the nanoparticles was found at 0.005 wt% of the salt solution. Small-angle neutron scattering (SANS) does not show any significant change in the scattering profile in these suspensions of the nanoparticles. A similar behavior was also observed in dynamic light scattering (DLS) experiments where autocorrelation function was found to be independent of the salt concentration. This can be understood since a high-block copolymer-to-gold ion ratio (r ～ 22) is required in the reduction reaction to produce gold particles. As a result, a very small fraction of the block copolymers were associated with the gold nanoparticles, and hence lead to a very low yield. Both SANS and DLS basically see the micelles of most of these block copolymers, which are not associated with nanoparticles. Based on this explanation, a step-addition method was used to enhance the yield of gold nanoparticles by manifold, where the gold salt is added in small steps to maintain higher value of r (>22), and therefore continuous formation of nanoparticles.     

Unravelling Malaria Antigen Binding to Antibody‐Gold Nanoparticle Conjugates

Conjugates formed by antibody adsorption to gold nanoparticles (AuNP) have found extensive utilization in immunoassays due to the high surface area and interesting optical and electronic properties of the nanomaterials. Nevertheless, the mechanism of formation of antibody‐AuNP conjugates and their antigen binding characteristics have not been sufficiently explored in terms of specificity and consequent clinical applicability. Dynamic light scattering and related techniques have been successfully employed to detect antigen binding to antibody‐AuNP complexes. Here, a range of different techniques from the bionanotechnology realm have been applied to obtain a detailed picture of a competitive immunoassay for malaria antigen detection, based on fluorescence‐quenching by AuNPs. Both agarose gel electrophoresis and differential centrifugal sedimentation (DCS) analyses provide binding constants in the same order of magnitude, for antibody binding to AuNP and for antigen binding to antibody‐AuNP conjugates. Both techniques are also able to reveal inhibition of antigen binding in the presence of a major blood plasma protein, transferrin (via competitive binding). DCS is further used to show inhibition of the binding of the antigen in the presence of human plasma, a realistic testing condition, of high relevance to the implementation of immunoassays at the clinical level.     

High–Yield Production of Uniform Gold Nanoparticles with Sizes from 31 to 577 nm via One‐Pot Seeded Growth and Size‐Dependent SERS Property

In this work, uniform, quasi‐spherical gold nanoparticles (Au NPs) with sizes of 31–577 nm are prepared via one‐pot seeded growth with the aid of tris‐base (TB). Distinct from the seeded growth methods available in literature, the present method can be simply implemented by subsequently adding the aqueous dispersion of the 17 nm Au‐NP seeds and the aqueous solution of HAuCl₄ into the boiling aqueous TB solution. It is found that at the optimal pH range, the sizes of the final Au NPs and their concentrations are simply controlled by either the particle number of the Au seed dispersion or the concentration of the HAuCl₄ solution, while the latter enables us to produce large Au NPs at very high concentration. Moreover, as‐prepared Au NPs of various sizes are coated on glass substrates to test their surface‐enhanced Raman scattering (SERS) activities by using 4‐aminothiophenol (4‐ATP) molecules as probes, which exhibit “volcano type” dependence on the Au NP sizes at fixed excitation wavelength. Furthermore, the Au NPs with sizes of ≈97 and 408 nm exhibit the largest SERS enhancement at the excitation wavelength of 633 and 785 nm, respectively.