Dynamic light scattering on bioconjugated laser generated gold nanoparticles

Exploring Intensity Distributions and Sampling in SERS-Based Immunoassays

Surface-enhanced Raman scattering (SERS) is a sensitive, widely used spectroscopic technique. However, SERS is perceived as poorly reproducible and insufficiently robust for standard applications in analytical chemistry. Here, we demonstrated that reliable SERS immunoassay quantification at low concentrations (pM range) can be achieved by careful experimental design and appropriate data analysis statistics. A SERS-based immunoassay for IgG in human serum (3.1-50.0 ng mL^-1 or 20.6-333 pM) was developed as a proof of concept. Calibration curves were created using the population median of the band area, centered at 592 cm^-1, of a SERS reporter (Nile Blue A). Histograms of 7200 SERS spectra show lognormal distributions. SEM images of the sensor platform confirm a correlation between the number of SERS probes (ERLs) at the surface and the SERS intensity response. The IgG immunosensor reported here presented a limit of detection of 1.11 ng mL^-1 or 7.39 pM and a limit of quantification of 9.04 ng mL^-1 or 60.30 pM, within a 95% confidence level. The % error of the predicted versus the actual response of a quality control (QC) sample was 0.13%. The percent error of the QC sample decreases exponentially with the number of measurements. Randomly selected spatially separated measurements provided lower QC % error than a larger number of measurements that were closely spaced. We propose that it is necessary to describe the measured populations using an appropriate sample size for good statistics and consider the interrogation of sufficiently large and well-separated areas of the sensor surface to achieve a reliable sampling.

Valence State Tuning of Gold Nanoparticles in the Dewetting Process: An X-ray Photoelectron Spectroscopy Study

Gold nanoparticles (AuNPs) are commonly synthesized using the citrate reduction method, reducing Au³⁺ into Au¹⁺ ions and facilitating the disproportionation of aurous species to Au atoms (Au⁰). This method results on citrate-capped AuNPs with valence single states Au⁰. Here, we report a methodology that allows obtaining AuNPs by the dewetting process with three different valence states (Au³⁺, Au¹⁺, and Au⁰), which can be fine-tuned with ion bombardment. The chemical surface changes and binding state of the NPs were investigated using core-level X-ray photoelectron spectroscopy (XPS). This is achieved by recording high-resolution Au 4f XPS spectra as a function of ion dose exposure. The results obtained show a time-dependent tuning effect on the Au valence states using low-energy 200 V acceleration voltage Ar⁺ ion bombardment, and the valence state conversion kinetics involves the reduction from Au³⁺ and Au¹⁺ to Au⁰. Proper control of the reduction in the valence states is critical in surface engineering for controlling catalytic reactions.

The anticancer activity of doxorubicin-loaded levan-functionalized gold nanoparticles synthesized by laser ablation

Here, gold nanoparticles (AuNPs) were synthesized upon exposure to nano-pulsed Nd-YAG laser irradiation in de-ionized water (PLAL) and functionalized with levan polysaccharide for assessing the anticancer efficacy of doxorubicin (DOX)-conjugated levan-capped AuNPs complexes to MCF-7 breast cancer cells. According to the physicochemical test results, the increments in levan amount enhanced the colloidal stability and the drug encapsulation efficiency (DEE) significantly. For the 10L-AuNP group having the highest levan amount (10 mg/mL levan), DEE was calculated as 92.21 ± 0.56%. The lean levan, uncapped AuNPs, and 10L-AuNP were found non-cytotoxic (>80% cell viability) in the studied concentrations with 48 h MTT assays. At higher DOX loadings (25, 50, and 100 μg/mL) of 10L-AuNP, the cell viability reduced significantly compared to free DOX. Overall, these nanoparticle complexes could be proposed as potent drug delivery vehicles for cancer drugs such as DOX, as well as other drugs in the prospective studies.

Antibacterial activity of biogenic silver and gold nanoparticles synthesized from Salvia africana-lutea and Sutherlandia frutescens

Nanoparticles (NPs) synthesized using various chemical and physical methods are often cytotoxic which restricts their use in biomedical applications. In contrast, metallic biogenic NPs synthesized using biological systems such as plant extracts are said to be safer and their production more cost effective. NPs synthesized from plants with known medicinal properties can potentially have similar bioactivities as these plants. It has been shown that Salvia africana-lutea (SAL) and Sutherlandia frutescens (SF) have antibacterial activities. This study used water extracts of SAL and SF to produce biogenic silver NPs (AgNPs) and gold NPs (AuNPs). The antibacterial activity of AgNPs and AuNPs was tested against two pathogens (Staphylococcus epidermidis and P. aeruginosa). NP synthesis was optimized by varying the synthesis conditions which include synthesis time and temperature, plant extract concentration, silver nitrate (AgNO₃) concentration and sodium tetrachloroaurate (III) dihydrate (NaAuCl₄ · 2H₂O) concentration. The NPs were characterized using Ultraviolet-visible (UV-vis) spectroscopy, dynamic light scattering, high-resolution transmission electron microscopy (HR-TEM), and Fourier transform infrared (FT-IR) spectroscopy. SAL was able to synthesize both Ag (SAL AgNP) and Au (SAL AuNP) nanoparticles, whilst SF synthesized Ag (SF AgNP) nanoparticles only. The absorbance spectra revealed the characteristic surface plasmon resonance peak between 400-500 nm and 500-600 nm for AgNP and AuNP, respectively. HR-TEM displayed the presence of spherical and polygon shaped nanoparticles with varying sizes whilst the Energy Dispersive x-ray spectra and selected area diffraction pattern confirmed the successful synthesis of the AgNPs and AuNPs by displaying the characteristic crystalline nature, optical adsorption peaks and lattice fringes. FT-IR spectroscopy was employed to identify the functional groups involved in the NP synthesis. The microtitre plate method was employed to determine the minimum inhibitory concentration (MIC) of the NPs and the extracts. The water extracts and SAL AuNP did not have significant antibacterial activity, while SAL AgNP and SF AgNP displayed high antibacterial activity. In conclusion, the data generated suggests that SAL and SF could be used for the efficient synthesis of antibacterial biogenic nanoparticles.

Plasmonic Enhancement of Two-Photon Excitation Fluorescence by Colloidal Assemblies of Very Small AuNPs Templated on M13 Phage

In this study, an engineered M13 bacteriophage was examined as a biological template to create a well-defined spacing between very small gold nanoparticles (AuNPs 3-13 nm). The effect of the AuNP particle size on the enhancement of the nonlinear process of two-photon excitation fluorescence (2PEF) was investigated. Compared to conventional (one-photon) microscopy techniques, such nonlinear processes are less susceptible to scattering given that the density of background-scattered photons is too low to generate a detectable signal. Besides this, the use of very small AuNPs in 2PEF microscopy becomes more advantageous because individual "isolated" AuNPs of this size do not sufficiently enhance 2PEF to produce a detectable signal, resulting in even less background signal. To investigate the 2PEF of the AuNP-M13 assemblies, a variety of sample preparation approaches are tested, and surface-enhanced Raman spectroscopy (SERS) is employed to study the strength of plasmon coupling within the gaps of AuNPs assembled on the M13 template. Results indicate that assemblies prepared with 9-13 nm AuNP were able to clearly label Escherichia coli cells and produce a 2PEF signal that was orders of magnitude higher than the isolated AuNP (below the threshold of detection). This study thus provides a better understanding of the opportunities and limitations relevant to the use of such small AuNPs within colloidal plasmonic assemblies, for applications in biodetection or as imaging contrast agents.

Characterization of Binding of Magnetic Nanoparticles to Rolling Circle Amplification Products by Turn-On Magnetic Assay

The specific binding of oligonucleotide-tagged 100 nm magnetic nanoparticles (MNPs) to rolling circle products (RCPs) is investigated using our newly developed differential homogenous magnetic assay (DHMA). The DHMA measures ac magnetic susceptibility from a test and a control samples simultaneously and eliminates magnetic background signal. Therefore, the DHMA can reveal details of binding kinetics of magnetic nanoparticles at very low concentrations of RCPs. From the analysis of the imaginary part of the DHMA signal, we find that smaller MNPs in the particle ensemble bind first to the RCPs. When the RCP concentration increases, we observe the formation of agglomerates, which leads to lower number of MNPs per RCP at higher concentrations of RCPs. The results thus indicate that a full frequency range of ac susceptibility observation is necessary to detect low concentrations of target RCPs and a long amplification time is not required as it does not significantly increase the number of MNPs per RCP. The findings are critical for understanding the underlying microscopic binding process for improving the assay performance. They furthermore suggest DHMA is a powerful technique for dynamically characterizing the binding interactions between MNPs and biomolecules in fluid volumes.

Stability and morphology of Ag nanoplatelets probed by depolarized dynamic light scattering

The stability of silver nanoplatelet (NP) suspensions prepared with different concentrations of trisodium citrate (TSC) was studied by depolarized dynamic light scattering (DDLS) and UV-vis spectrometry. The morphology of the nanoparticles, as well as the color and stability of the sols, are tuned by the concentration of the capping agent. The nanoparticles prepared with high TSC concentration (>10^-4 M) are blue triangular NPs showing a slight truncation of the tips with aging. When low TSC concentrations are used, the color of the sols changes from blue to yellow with aging time and a strong modification of the morphology occurs: the nanoparticle shape changes from triangular to spherical. Remarkably, they show a high degree of anisotropy. The aging process was followed by the UV-vis spectra and by measuring the rotational diffusion coefficient by DDLS, providing information on the nanoparticle size and shape evolution. The high intensity of depolarized signal and the high value of rotational diffusion coefficient suggest that the aging process increases the thickness and the roughness of the nanoparticles.

Cross-linking gold nanoparticles aggregation method based on localised surface plasmon resonance for quantitative detection of miR-155

MiR-155 plays a critical role in the formation of cancers and other diseases. In this study, the authors aimed to design and fabricate a biosensor based on cross-linking gold nanoparticles (AuNPs) aggregation for the detection and quantification of miR-155. Also, they intended to compare this method with SYBR Green real-time polymerase chain reaction (PCR). Primers for real-time PCR, and two thiolated capture probes for biosensor, complementary with miR-155, were designed. Citrate capped AuNPs (18.7 ± 3.6 nm) were synthesised and thiolated capture probes immobilised to AuNPs. The various concentrations of synthetic miR-155 were measured by this biosensor and real-time PCR method. Colorimetric changes were studied, and the calibration curves were plotted. Results showed the detection limit of 10 nM for the fabricated biosensor and real-time PCR. Also, eye detection using colour showed the weaker detection limit (1 µM), for this biosensor. MiR-133b as the non-complementary target could not cause a change in both colour and UV-visible spectrum. The increase in hydrodynamic diameter and negative zeta potential of AuNPs after the addition of probes verified the biosensor accurately fabricated. This fabricated biosensor could detect miR-155 simpler and faster than previous methods.

Utilization of unmodified gold nanoparticles for label-free detection of mercury (II): Insight into rational design of mercury-specific oligonucleotides

Colorimetric detection of mercury (II) with the use of DNA oligonucleotides and unmodified gold nanoparticles (AuNPs) as indicators has been extensively studied. This study provides in-depth insights into the rational design of mercury-specific oligonucleotides (MSO) in the biosensing system. The leftover bases of MSO, as a result of the formation of T-Hg²⁺-T base pairs, can adsorb on the AuNPs and hinder their aggregation at concentrations of salt. This phenomenon was directly verified by the changes in particle sizes characterized by dynamic light scattering for the first time. Based on these findings, we proposed a rational design for the MSO with approximately 20-fold improvement in detection sensitivity. The detection limit of the proposed assay decreased to 15nM with a linear working range from 50nM to 300nM for Hg²⁺. The cross-reactivity against eight other metal ions was negligible compared with the response to Hg²⁺. Considering the diverse applications of AuNPs with oligonucleotides, this study can serve as a good reference and provides important implications in sensing and DNA-directed nanoparticle assembly.