Citrate-reduced Gold Nanoparticles Research Articles

Cellulose Nanofiber Films with Gold Nanoparticles Electrostatically Adsorbed for Facile Surface-Enhanced Raman Scattering Detection.

Cellulose nanofiber (CNF) holds great promise in applications such as surface-enhanced Raman scattering (SERS), catalysis, esthesia, and detection. This study aimed to build novel CNF-based SERS substrates through a facile synthetic method. Citrate-reduced gold nanoparticles (AuNPs) were adsorbed on the cationized CNF surface due to electrostatic interactions, and uniform AuNPs@(2,3-epoxypropyl trimethylammonium chloride)EPTMAC@CNF flexible SERS substrates were prepared by a simple vacuum-assisted filtration method. The probe molecule methylene blue was chosen to assess the performance of the CNF-based SERS substrate with a sensitivity up to 10-9 M, superior signal reproducibility (relative standard deviation (RSD) = 4.67%), and storage stability (more than 30 days). Tensile strength tests indicated that the CNF-based films had good mechanical properties. In addition, CNF-based substrates can easily capture and visually identify microplastics in water. These results demonstrate the potential application of the flexible, self-assembled AuNPs@EPTMAC@CNF flexible SERS substrate for prompt and sensitive detection of trace substances.

An optimization of blocking agents for designing reliable electrochemical biosensors for ovarian cancer

Here, we report development of an electrochemical biosensor for ovarian cancer diagnosis using an optimized blocking agent for reliable performance of the device. The sensitivity, specificity and reproducibility of the device are negatively impacted by non-specific binding of molecules present in sample - proteins, DNA, RNA, small drug molecules, hormones etc, other than the analyte of interest bind to biorecognition molecule on the biosensor surface. Electrochemical biosensor was fabricated by functionalizing carbon screen printed electrodes with citrate-reduced gold nanoparticles followed by immobilization of 5′-amine modified ssDNA probe against miRNA 204 (ovarian can biomarker). To avoid or minimize non-specific binding, blocking reagent was optimized. Blocking reagents and methods are typically chosen in an empirical manner since a single standardized procedure has not been determined suitable for all applications. This paper presents the commonly used blocking buffers with varying molecular weights to minimize the non-specific binding process on the surface of the electrode. Here, three different blocking buffers are used - Bovine serum albumin (BSA; MW: 66430.3 kDa), Gelatin (MW: 40 kDa), and Polyethylene Glycol (PEG MW 4 kDa and 6 kDa). Additionally, different surfactants like tween20, Tritonx100, and HEPES buffer are used for preparation of blocking agents. Further, to evaluate the performance of different blocking agents, the sample of miRNA-204 is spiked in 0.01 M Phosphate buffer saline (PBS) and fetal bovine serum (FBS) were tested using chroamperometry. The difference in the saturation current was recorded for both curves obtained for 0.01 M PBS and FBS. Optimum blocking buffer 1% Gelatin in Tween20 was found to be giving negligible nonspecific binding for DNA based ovarian cancer biosensor.

Gold nanoparticle decorated blu-ray digital versatile disc as a highly reproducible surface-enhanced Raman scattering substrate for detection and analysis of rotavirus RNA in laboratory environment.

Detection and estimation of various biomolecular samples are often required in research and clinical laboratory applications. Present work demonstrates the functioning of a surface-enhanced Raman scattering (SERS) substrate that has been obtained by drop-casting of citrate-reduced gold nanoparticles (AuNPs) of average dimension of 23 nm on a bare blu-ray digital versatile disc (BR-DVD) substrate. The performance of the proposed SERS substrate has been initially evaluated with standard Raman active samples, namely malachite green (MG) and 1,2-bis(4-pyridyl)ethylene (BPE). The designed SERS substrate yields an average enhancement factor of 3.2 × 106 while maintaining reproducibility characteristics as good as 94% over the sensing region of the substrate. The usability of the designed SERS substrate has been demonstrated through the detection and analysis of purified rotavirus double-stranded RNA (dsRNA) samples in the laboratory environment condition. Rotavirus RNA concentrations as low as 10ng/μL could be detected with the proposed sensing scheme.

Probing multifunctional azure B conjugated gold nanoparticles with serum protein binding properties for trimodal photothermal, photodynamic, and chemo therapy: Biophysical and photophysical investigations

Multimodal or combination therapy has been considered as a powerful approach for treatment of complex diseases like cancer. The fascinating physicochemical and optoelectronic properties of gold nanoparticles make them potential candidate for cancer therapeutic and diagnostic applications. Herein, we design a multifunctional nanosystem by conjugating a photosensitizer, Azure B (AB) with citrate reduced gold nanoparticles (CI-Au NPs) through non-covalent interactions. The conjugation of AB with CI-Au NPs was confirmed through UV–Visible absorption spectroscopy and Fourier Transform Infra-red (FT-IR) spectroscopy. The morphology, size, and charge of the prepared nano-conjugates (AB@CI-Au NPs) were determined by transmission electron microscopy (TEM), Dynamic light scattering (DLS), and zeta potential measurements. The proficiency of AB@CI-Au NPs for cancer photo-therapies was demonstrated by evaluating their potential for photothermal heating and singlet oxygen generation in solution upon Visible laser (635 nm, 500 mW/cm2) irradiation. The AB@CI-Au NPs display superior heating efficiency than CI-Au NPs alone or free AB, and offer better photostability as well as singlet oxygen generation rate compared to free photosensitizer. The interaction of AB@CI-Au NPs with Calf thymus DNA (Ct-DNA) and transport protein Bovine Serum Albumin (BSA) were studied using various biophysical techniques including Circular dichroism, UV–Visible and fluorescence spectroscopic studies. AB@CI-Au NPs show intercalative binding with Ct-DNA by inducing local perturbations in double helical structure and hence can exert chemotherapeutic action by targeting DNA. AB@CI-Au NPs also display moderate binding with BSA without any adverse effect on BSA structure, which is desirable for significant biodistribution and pharmacokinetics of AB@CI-Au NPs. Also, in vitro cytotoxicity of the AB@CI-Au NPs with and without laser irradiation (635 nm, 500 mW/cm2) was demonstrated using the hepatocellular carcinoma (HepG2) cell lines. Our findings through photophysical and biophysical studies strongly recommend the exploitation of AB@CI-Au NPs nanoconjugates as a multifunctional probe for trimodal anticancer therapy.

Designing a two-stage colorimetric sensing strategy based on citrate reduced gold nanoparticles: Sequential detection of Sanguinarine (anticancer drug) and visual sensing of DNA.

Distance dependent optical properties of colloidal gold nanoparticles offer designing of colorimetric sensing modalities for detection of a variety of analytes. Herein, we report a simple and facile colorimetric detection assay for an anti-cancer drug, Sanguinarine (SNG) and Calf Thymus DNA (Ct-DNA) based on citrate reduced gold nanoparticles (CI-Au NPs). The electrostatic interaction between SNG and CI-Au NPs induce aggregation of Au NPs accompanied with visible colour change of colloidal solution. The assay conditions like salt concentration, pH and reaction time had been adjusted to achieve highly sensitive and fast colorimetric response. Furthermore, the optimized CI-Au NPs/SNG sensing system is used for the detection of Ct-DNA based on the mechanism of anti-aggregation of CI-Au NPs. The simultaneous presence of SNG and Ct-DNA prevent aggregation of Au NPs owing to preferential formation of Ct-DNA-SNG intercalation complex and colour of the Au NPs solution tends to remain red, depending on the concentration of Ct-DNA in solution. The degree of aggregation and anti-aggregation of CI-Au NPs was monitored using Transmission electron microscopic (TEM) measurements and UV-Visible spectrophotometry by analysing the ratio of absorptions for aggregated and dispersed Au NPs. The intercalation mode of binding between SNG and Ct-DNA in CI-Au NPs/SNG sensing system was determined by Fluorescence spectral studies and UV-thermal melting studies. The absorption ratio (A627/A525) of Au NPs exhibited a linear correlation with SNG concentrations in the range from 0 to 0.9μM with detection limit as 0.046μM. This optical method can determine Ct-DNA as low as 0.36μM and the calibration is linear for concentration range 0 to 5μM. The proposed sensing strategy enables detection as well as quantification of SNG & Ct-DNA in real samples with satisfactory results and finds application in drug or DNA monitoring.

Colour tunability in a bimodal fluorescent hybrid nanostructure UCNPs@AuNPs@QDs

In the present work, lysine modified NaY0.78Er0.02Yb0.2F4 upconversion nanoparticles (UCNPs, positively charged) and lysine modified ZnSe:Mn2+ quantum dots (QDs, positively charged) are attached onto the surface of citrate reduced gold nanoparticles (AuNPs, negatively charged). The gold nanoparticles not only entangle the QDs and the UCNPs, through electrostatic interaction, but also tune the optical properties of UCNPs through the effect of surface plasmon resonance. The hybrid nanostructure gives green emission both through photoluminescence (under UV excitation) and through photon upconversion (under IR light excitation) process. The colour tuning is observed through variation in the size of QDs and through plasmonic effect of gold nanoparticles. In both the cases, the colour of emission gradually changes from green to red. The colour tunability and bi-modal photon conversion property of this material could be useful for its application in the field of bio-imaging and solar energy harvesting.

Multiple plasmon resonances in small-sized citrate reduced gold nanoparticles

Metallic nanoparticles exhibit localized surface plasmon resonances resulting from the coherent oscillation of free electrons in response to electric field from incident electromagnetic radiation. In this work, the dependence of peak surface plasmon resonance of gold nanoparticles on the particle size using UV–Vis spectra is shown. Small-sized gold nanoparticles (10–15 nm) were prepared through citrate reduction method. From theoretical modelling based on their resonance peaks and full wave at half maximum, the gold nanoparticles were found to have a narrow size distribution of ∼12 nm optimized over 1.0–1.10 mL of the citrate. A comparison of the experimental data and data from multiple scattering theory revealed that the plasmon resonances were centered at wavelengths between 520 and 518 nm for all samples under citrate volume range from 0.85 to 1.15 mL. Further analysis indicated that, small sized AuNPs exhibited multiple resonances, which could be attributed to the radiative interband recombination between the sp- and d-bands.

Tryptophan Tight Binding to Gold Nanoparticles Induces Drastic Changes in Indole Ring Raman Markers

Experimental conditions favoring the tryptophan tight binding to the surface of citrate-reduced gold nanoparticles (Cit-AuNPs) were explored. For this, the adsorption of three molecular compounds, free amino acid (Trp), tripeptide NH2-Gly-Trp-Gly-CONH2, and lanreotide, a synthetic cyclic somatostatin analogue, on large-size (∼70 nm) Cit-AuNPs was analyzed. UV–visible absorption, transmission electron microscopy, and surface-enhanced Raman scattering (SERS) were jointly used in the present investigation. At low pH (∼3.5), when the repulsive electrostatic interactions between gold particles are sufficiently reduced, both peptides can induce strong NP aggregation, leading particularly to substantial changes in the characteristic tryptophan Raman markers, as well as to the appearance of strong SERS markers at 1228 and 1113 cm–1. In contrast, upon increasing the pH toward a neutral value, the mentioned Trp Raman markers tend to adopt a spectral shape comparable to that observed in solution (bulk). In free amin...

Exploring the DNA damaging potential of chitosan and citrate-reduced gold nanoparticles: Physicochemical approach

Nanomaterials offer a wide range of biomedical applications including gene/drug delivery, biosensing and bioimaging. The cytotoxic and genotoxic potential of nanoparticles need to be thoroughly investigated before their biomedical usage. This study aims to investigate and compare the nanotoxicology of chitosan (CH-Au-Np) and citrate (CI-Au-Np) reduced gold nanoparticles via exploring their interaction with Calf thymus DNA (Ct-DNA) utilizing various physicochemical techniques. Structural characterization of these Nps was done using UV–Visible Spectroscopy and Transmission Electron Microscopy (TEM). Analysis of UV–Visible absorbance spectra indicates that interaction of CH-Au-Np with Ct-DNA causes destabilization of DNA by inducing significant structural and conformational changes in Ct-DNA in a concentration dependent manner, whereas there was negligible interaction between CI-Au-Np and Ct-DNA. These observations were further supported by the results of agarose gel mobility, UV-thermal melting, Circular Dichroism (CD), Dynamic Light Scattering (DLS) and TEM studies. Fluorescence spectral studies using acridine orange (AO) as a fluorescence probe and analysis of thermodynamic parameters reveal that the interactions between Ct-DNA and CH-Au-Np were mainly governed by Van der Waal interactions and Hydrogen bonding. An insightful understanding of genotoxicity induced by CH-Au-Np can be advantageous, as it may provide valuable anticancer approach for cytotoxic drug designing.

Preliminary Quality Criteria of Citrate-Protected Gold Nanoparticles for Medicinal Applications

Gold nanoparticles (GNPs) are one of the most widely studied and utilized materials in nanomedicine, but there is no business or national quality standard for their medicinal applications. This has become the bottleneck in the translation of clinical applications of gold-based nanomaterials. To solve this issue, the most frequently used citrate-reduced GNPs were selected as demonstration in this study to conduct quality standard research. The different molar ratios of chloroauric acid to trisodium citrate in preparation were changed, from 1:1 to 1:5, to adjust the particle size and main ingredients in gold colloids. The compositions of reduced Au0 and residual Au3+ were determined by X-ray photoelectron spectroscopy (XPS), inductively coupled plasma–atomic emission spectroscopy (ICP-AES), and photodynamic catalysis, while the residual citrate and its oxidation products in supernatants were detected by nuclear magnetic resonance (1H NMR) spectroscopy and nonaqueous titration. Cell viability of human normal...

Preparation and Characterization of Ganciclovir Loaded Glutathione Modified Gold Nanoparticles

In this research, ganciclovir-loaded gold nanoparticles were developed via glutathione surface modification. First citrate reduced gold nanoparticles were prepared from chloroauric acid. Further the surface of gold nanoparticles was also modified with glutathione. Then ganciclovir was loaded onto glutathione modified gold nanoparticles through the reaction between carboxyl group of glutathione and amino group of ganciclovir. Gold nanoparticles were characterized by photon correlation spectroscopy, transmission electron microscopy and infrared spectroscopy. The drug loading of 87.27±2.52% was observed for ganciclovir loaded gold nanoparticles. The particle size distribution of gold nanoparticles was ranged from 26.3 to 31 nm. Thus ganciclovir loaded gold nanoparticles were successfully prepared which may further improve the oral bioavailability of ganciclovir.

Gold nanoparticles coated polystyrene/reduced graphite oxide microspheres with improved dispersibility and electrical conductivity for dopamine detection

Gold nanoparticles coated polystyrene/reduced graphite oxide (AuNPs@PS/RGO) microspheres have been successfully prepared via a facile process, and the decorative gold nanoparticles could prevent the aggregation of RGO by electrostatic repulsive interaction, and lead to high dispersibility of the composite. The prepared composite has a highly increased conductivity of 129Sm−1 due to the unique electrical properties of citrate reduced gold nanoparticles. Being employed as an electrochemical sensor for detection of dopamine, the modified electrode exhibits remarkable sensitivity (3.44μA/μM) and lower detection limit (5nM), with linear response in a range of 0.05–20μM. Moreover, valid response to dopamine obtained in present work also indicates the prospective performances of AuNPs@PS/RGO microspheres to other biological molecules, such as nucleic acids, proteins and enzymes.

Synthesis and NIR optical properties of hollow gold nanospheres with LSPR greater than one micrometer

Optical analysis in the near infrared region is of significant biological importance due to better tissue penetration and reduced autofluorescence. In this work, an improved synthesis of hollow gold nanospheres (HGNs), which provides a tunable localized surface plasmon resonance (LSPR) from 610 nm up to 1320 nm, is demonstrated. The scattering properties of these nanoparticles are shown using surface enhanced Raman scattering (SERS) at 1064 nm excitation wavelength and are compared to citrate reduced gold and silver nanoparticles of similar physical sizes and surface properties. After the addition of salts, a strong signal was observed from hollow gold with a LSPR of 650 nm and a weaker, yet observable, signal from HGNs with a LSPR of 775 nm. However, no obvious signals were observed in the case of standard citrate reduced gold, silver or HGNs with a LSPR of 1080 nm. The absorption properties of HGNs were investigated by monitoring their photothermal activity. In this case, different nanoparticle suspensions including citrate reduced gold, silver, and HGNs were illuminated by a continuous laser at 785 nm excitation wavelength and the absorption efficiency of HGNs with a LSPR of 775 nm was calculated to be 0.81% which is more than 5 times higher than the absorption efficiency of citrate reduced gold nanoparticles under similar conditions.

Gelatin–nanogold bioconjugates as effective plasmonic platforms for SERS detection and tagging

It is well known that standard citrate-reduced gold nanoparticles (AuNPs) are unstable at high ionic strength solution, which limits their applications in the biomedical field. In this work we present an environmentally friendly approach for the stabilization of citrate-reduced AuNPs in aqueous solution. Specifically, the stability of the AuNPs against salt-induced aggregation was greatly improved in the presence of gelatin biopolymer and stabilization of individual or small assemblies of nanoparticles can be controlled by the amount of gelatin. Furthermore, the gelatin–nanogold bioconjugates were demonstrated to be operational as highly sensitive surface-enhanced Raman scattering (SERS) active substrate for the detection of Rose Bengal fluorophore in solution at very low concentration. The results suggest that such bioconjugates can be successfully employed not only for detection of analytes, but more interestingly for building SERS-active tags in view of imaging purpose. The stabilization of bioconjugates was analyzed by localized surface plasmon resonance spectroscopy (LSPR), transmission electron microscopy (TEM), dynamic light scattering (DLS) and zeta-potential, and the chemical interaction of gelatin with AuNPs was inferred from Fourier transform infrared spectroscopy (FT-IR).

In vitro toxicity of serum protein-adsorbed citrate-reduced gold nanoparticles in human lung adenocarcinoma cells

We examined the cytotoxicity effect of the serum protein coated gold nanoparticles (AuNPs) in the A549 cells. Negatively charged AuNPs were prepared by chemical reduction using citrate. The dimension and surface charge of AuNPs were characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), and zeta potential measurements. The AuNPs modified by the citrate anion were presumed to adsorb the serum proteins as indicated from the visible absorption spectroscopy, DLS, and quartz crystal microbalance (QCM) data. The QCM results indicated that among the constituents, fetal bovine serum (FBS) should be the major adsorbate species on the AuNPs incubated in the RPMI medium. The internalization of AuNPs into the A549 cells was also monitored using TEM and dark-field microscopy (DFM). Both methylthiazol tetrazolium (MTT) and lactate dehydrogenase (LDH) assays revealed that AuNPs were toxic as determined by their half-maximal inhibitory concentration. A flow cytometric and real-time PCR analysis of apoptotic genes along with the ATP depletion measurements suggested that AuNPs induce cell damages through extrinsic and intrinsic apoptotic pathways.

Size sorting of citrate reduced gold nanoparticles by sedimentation field-flow fractionation

Gold nanoparticles (GNPs) have been synthesized through the citrate reduction method; the citrate/gold(III) ratio was changed from 1:1 up to 10:1 and the size of the resulting nanoparticles was measured by sedimentation field-flow fractionation (SdFFF). Experimental data showed that the GNPs size decreases in the ratio range 1:1–3:1 and then increases from 5:1 to 10:1 passing through a plateau region in between, and is almost independent of the precursor solution concentrations. In the zone of minimum diameters the synthetic process does not produce monodispersed GNPs but often multiple distributions, very close in size, are observed as evidenced by the particle size distributions (PSDs) derived from the SdFFF fractograms. UV–vis spectrophotometry, being the most common technique employed in the optical characterization of nanoparticles suspensions, was used throughout this work. A confirmation of the nucleation–aggregation–fragmentation mechanism was inferred from the cross-correlation between UV–vis and SdFFF results.

High performance bioanode based on direct electron transfer of fructose dehydrogenase at gold nanoparticle-modified electrodes

The direct electron transfer reaction of fructose dehydrogenase (FDH) from Gluconobacter sp. on alkanethiol-modified gold nanoparticles (AuNPs) was examined. AuNP-modified electrodes were simply fabricated by depositing citrate-reduced gold nanoparticles onto a gold electrode and carbon fiber paper and then covering the surface with a self-assembled monolayer of alkanethiols. The immobilization of AuNPs provided a large effective surface area for the adsorption of FDH. Catalytic oxidation currents based on the direct electron transfer reaction of FDH were observed from a potential about −100 mV (vs. Ag/AgCl, 3 M NaCl) in the presence of d-fructose without a mediator. The current density reached as high as 14.3 ± 0.93 mA/cm 2 (at +500 mV), which was achieved in the presence of 200 mM d-fructose by immobilization of FDH on 2-mercaptoethanol-modified AuNP/carbon fiber paper electrodes.

Gold Nanoparticle-Based pH Sensor in Highly Alkaline Region at pH > 11: Surface-Enhanced Raman Scattering Study

A surface plasmon resonance spectroscopy study showed that citrate-reduced gold nanoparticles ( approximately 15 nm diameter, approximately 9 x 10(-9) M concentration, approximately 2 x 10(-2) M ionic strength) were found to be utilized as a colorimetric sensor by exhibiting a distinct color change at a highly alkaline pH > 11.5. Surface-enhanced Raman scattering (SERS) of 4-ethynylpyridine (4-EP) on gold nanoparticle surfaces indicated that the multiple peaks in the v(C identical withC) stretching bands should vary significantly in the highly alkaline region from pH 12 to 14. As the pH value increased, the v(C identical withC) stretching band intensity at approximately 2080 cm(-1) became stronger than that at approximately 2010 cm(-1). The pK(1/2) value was determined to be around 13 by the SERS titration of taking intensity ratios of I(2080) with respect to I(2010). Using SERS enhancements and conspicuous spectral changes, self-assembled monolayers (SAMs) of 4-EP on Au nanoparticles holds potential as a pH sensor for sensitive detection of the hydroxide OH(-) concentration at around pH 13 in an aqueous solution. The pH calibration from SERS titration of 4-EP is expected to have advantages in terms of higher alkaline detection limit and more precise measurements, if compared with the indigo carmine, the pK(1/2) value of which is 12.2.

Fabrication of gold nanocrystal-coated polypyrrole nanotubules

A method of fabricating gold nanocrystals coated polypyrrole (PPy) tubules was developed by coating carboxylic acid–thiol-capped or citrate-reduced gold nanoparticles on polypyrrole tubule templates. Electrostatic interaction between gold nanoparticles and PPy tubules was used as a driving force for this assembly process. The gold nanoparticle coated PPy tubule can be applied as signal-enhancing cuvettes for Raman spectroscopic analysis of trace molecules. The Raman scattering of trace Rhodamine B on the Au nanoparticle-coated tubule was enhanced by a factor of 104–105.

Surface Modification of Citrate-Reduced Colloidal Gold Nanoparticles with 2-Mercaptosuccinic Acid

Surface modification of citrate-reduced gold nanoparticles with 2-mercaptosuccinic acid (MSA) was carried out in the aqueous phase. This provides a way to obtain carboxylic acid functionalized gold nanoparticles with diameter above 10 nm. The influence of the protecting MSA layer on the behavior of the modified gold nanoparticles in comparison with that of the traditional citrate-reduced gold colloid was evaluated by HCl titration, cyanide etching, and seeded growth tests. The modified gold nanoparticles show an improved stability against pH changes and cyanide etching. They do show further growth, which appears to be more homogeneous than for the unmodified particles.