Mechanistic insights of the reduction of gold salts in the Turkevich protocol

Peptide Stereochemistry Effects from pKa-Shift to Gold Nanoparticle Templating in a Supramolecular Hydrogel

The divergent supramolecular behavior of a series of tripeptide stereoisomers was elucidated through spectroscopic, microscopic, crystallographic, and computational techniques. Only two epimers were able to effectively self-organize into amphipathic structures, leading to supramolecular hydrogels or crystals, respectively. Despite the similarity between the two peptides' turn conformations, stereoconfiguration led to different abilities to engage in intramolecular hydrogen bonding. Self-assembly further shifted the pKa value of the C-terminal side chain. As a result, across the pH range 4-6, only one epimer predominated sufficiently as a zwitterion to reach the critical molar fraction, allowing gelation. By contrast, the differing pKa values and higher dipole moment of the other epimer favored crystallization. The four stereoisomers were further tested for gold nanoparticle (AuNP) formation, with the supramolecular hydrogel being the key to control and stabilize AuNPs, yielding a nanocomposite that catalyzed the photodegradation of a dye. Importantly, the AuNP formation occurred without the use of reductants other than the peptide, and the redox chemistry was investigated by LC-MS, NMR, and infrared scattering-type near field optical microscopy (IR s-SNOM). This study provides important insights for the rational design of simple peptides as minimalistic and green building blocks for functional nanocomposites.

A novel dual-function SERS-based identification strategy for preliminary screening and accurate diagnosis of circulating tumor cells

Non-specific adsorption of bioprobes based on surface-enhanced Raman spectroscopy (SERS) technology inevitably endows white blood cells (WBC) in the peripheral blood with Raman signals, which greatly interfere the identification accuracy of circulating tumor cells (CTCs). In this study, an innovative strategy was proposed to effectively identify CTCs by using SERS technology assisted by a receiver operating characteristic (ROC) curve. Firstly, a magnetic Fe3O4-Au complex SERS bioprobe was developed, which could effectively capture the triple negative breast cancer (TNBC) cells and endow the tumor cells with distinct SERS signals. Then, the ROC curve obtained based on the comparison of SERS intensity of TNBC cells and WBC was used to construct a tumor cell identification model. The merit of the model was that the detection sensitivity and specificity could be intelligently switched according to different identification purposes such as accurate diagnosis or preliminary screening of tumor cells. Finally, the difunctional recognition ability of the model for accurate diagnosis and preliminary screening of tumor cells was further validated by using the healthy human blood added with TNBC cells and blood samples of real tumor patients. This novel difunctional identification strategy provides a new perspective for identification of CTCs based on the SERS technology.

Role of the Deep Eutectic Solvent Reline in the Synthesis of Gold Nanoparticles

This work presents a mechanistic understanding of the synthesis of small (<3 nm) gold nanoparticles in a nontoxic, eco-friendly, and biodegradable eutectic mixture of choline chloride and urea (reline) without the addition of external reducing or stabilization agents. Reline acts as a reducing agent by releasing ammonia (via urea hydrolysis), forming gold nanoparticles even at trace ammonia concentration levels. Reline also affects the speciation of the gold precursor forming gold chloro-complexes, stabilizing Au⁺ species, leading to an easier reduction and avoiding the otherwise fast disproportionation reaction. Such a capability is however lost in the presence of large amounts of water, where water replaces the chloride ligands in the precursor speciation. In addition, reline acts as a weak stabilizing agent, leading to small particles (<3 nm) and narrow distributions although agglomerates quickly form. Such properties are maintained in the presence of water, indicating that it is linked to the urea stabilization rather than the hydrogen-bonding network. This work has important implications in the field of green synthesis of nanoparticles with small sizes, especially for biomedical and health care applications, due to the nontoxic nature of the components of deep eutectic solvents in contrast to the conventional routes.

Rational Truncation of Aptamer for Ultrasensitive Aptasensing of Chloramphenicol: Studies Using Bio-Layer Interferometry

Aptamers are an excellent choice for the selective detection of small molecules. However, the previously reported aptamer for chloramphenicol suffers from low affinity, probably as a result of steric hindrance due to its bulky nature (80 nucleotides) leading to lower sensitivity in analytical assays. The present work was aimed at improving this binding affinity by truncating the aptamer without compromising its stability and three-dimensional folding. Shorter aptamer sequences were designed by systematically removing bases from each or both ends of the original aptamer. Thermodynamic factors were evaluated computationally to provide insight into the stability and folding patterns of the modified aptamers. Binding affinities were evaluated using bio-layer interferometry. Among the eleven sequences generated, one aptamer was selected based on its low dissociation constant, length, and regression of model fitting with association and dissociation curves. The dissociation constant could be lowered by 86.93% by truncating 30 bases from the 3' end of the previously reported aptamer. The selected aptamer was used for the detection of chloramphenicol in honey samples, based on a visible color change upon the aggregation of gold nanospheres caused by aptamer desorption. The detection limit could be reduced 32.87 times (1.673 pg mL^-1) using the modified length aptamer, indicating its improved affinity as well as its suitability in real-sample analysis for the ultrasensitive detection of chloramphenicol.

Determination of ketamine using melamine-modified gold nanoparticles

Ketamine is used in medicine because of its anaesthetic and antidepressant effects at low doses. Unfortunately, due to its narcotic effect when used at high doses, its abuse among young people is increasing. It is also one of the most common drugs used in rape. Therefore, there is a need for fast and inexpensive tests that can be performed on-site. With the advancement of nanotechnology, nanoparticle-based approaches have found their place in selective analyses as in many fields. In the developed method, firstly gold nanoparticles were modified with melamine (AuNPs@Mel). Under optimized conditions, hydrogen bonds formed between ketamine and AuNPs@Mel cause the red colour of AuNPs@Mel to shift to blue-purple (i.e. aggregation-induced surface plasmon absorption shift). The association between absorbance and concentration produced a calibration line (curve) having a linearity correlation coefficient of 0.9981 for ketamine concentrations ranging from 4.76 to 47.6 mg L-1. The detection limit of the proposed method was 1.5 mg L-1 and the RSD (relative standard deviation) values of concentrations were changed ranging from 5.2% to 8.2%. The intra-assay and inter-assay measurements using the suggested method resulted in coefficients of variation (CVs) of 5.7% and 8.5%, respectively. Scan transmission electron microscopy (STEM), UV-vis spectrophotometry and FTIR spectroscopy were used to characterize the synthesized and modified AuNPs. Additionally, the procedure was successfully carried out with some interference materials and a real sample of fetal bovine serum. Lastly, using the Student t-test and F tests, the suggested technique was compared to and confirmed against an LC-MS/MS procedure previously published.

Gold nanoparticles synthesis and immobilization by atmospheric pressure DBD plasma torch method

Herein, we report the impact of plasma on gold nanoparticles synthesis. We used an atmospheric plasma torch fed with an aerosolized tetrachloroauric(iii) acid trihydrate (HAuCl₄·3H₂O) solution. The investigation showed that using pure ethanol as a solvent for the gold precursor enabled a better dispersion compared to a water-containing solution. We demonstrated here that the deposition parameters are easy to control, presenting the influence of solvent concentration and deposition time. The advantage of our method is that no capping agent was used. We assume that plasma creates a carbon-based matrix around the gold nanoparticles preventing them to agglomerate. The XPS results revealed the impact of using plasma. Metallic gold was detected in the plasma-treated sample, whereas the no-plasma sample revealed only Au(i) and Au(iii) contributions originating from the HAuCl₄ precursor. Detailed HRTEM, EDS mapping, and SAED analyses led to more insights into the structure.

Visual colorimetric sensor for nitroguanidine detection based on hydrogen bonding-induced aggregation of uric acid-functionalized gold nanoparticles

A colorimetric assay is proposed for the quantification of nitroguanidine (NQ), based on triggering the aggregation of uric acid-modified gold nanoparticles (AuNPs@UA) by intermolecular hydrogen bonding interaction between uric acid (UA) and NQ. The red-to-purplish blue (lavender) color change of AuNPs@UA with increasing NQ concentrations could be perceived with the naked eye or detected by UV-vis spectrophotometry. The absorbance versus concentration correlation gave a linear calibration curve in the range of 0.6-3.2 mg L^-1 NQ, with a correlation coefficient of 0.9995. The detection limit of the developed method was 0.063 mg L^-1, lower than those of noble metal aggregation methods in the literature. The synthesized and modified AuNPs were characterized using UV-vis spectrophotometry, scanning transmission electron microscopy (STEM), dynamic light scattering (DLS), and Fourier transform infrared spectroscopy (FTIR). Some critical parameters such as modification conditions of AuNPs, UA concentration, solvent environment, pH, and reaction time were optimized for the proposed method. The non-interference of common explosives (i.e., nitroaromatic, nitramine, nitrate ester, insensitive and inorganic explosives), common soil and groundwater ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Cl^-, NO₃^-, SO₄^2-, CO₃^2-, PO₄^3-) and possible interfering compounds (used as camouflage agents for explosives; D-(+)-glucose, sweeteners, acetylsalicylic acid (aspirin), household powder detergents, and paracetamol) on the proposed method was demonstrated, proving that the procedure was fairly selective for NQ, due to special hydrogen bonding interactions between UA-functionalized AuNPs and NQ. Finally, the proposed spectrophotometric method was applied to NQ-contaminated soil, and the obtained results were statistically compared with those of the liquid chromatography-tandem mass spectrometric (LC-MS/MS) method in the literature.

Surfactant-Free Colloidal Syntheses of Gold-Based Nanomaterials in Alkaline Water and Mono-alcohol Mixtures

Gold nanoparticles (Au NPs) and gold-based nanomaterials combine unique properties relevant for medicine, imaging, optics, sensing, catalysis, and energy conversion. While the Turkevich-Frens and Brust-Schiffrin methods remain the state-of-the-art colloidal syntheses of Au NPs, there is a need for more sustainable and tractable synthetic strategies leading to new model systems. In particular, stabilizers are almost systematically used in colloidal syntheses, but they can be detrimental for fundamental and applied studies. Here, a surfactant-free synthesis of size-controlled colloidal Au NPs stable for months is achieved by the simple reduction of HAuCl₄ at room temperature in alkaline solutions of low-viscosity mono-alcohols such as ethanol or methanol and water, without the need for any other additives. Palladium (Pd) and bimetallic Au _x Pd _y NPs, nanocomposites and multimetallic samples, are also obtained and are readily active (electro)catalysts. The multiple benefits over the state-of-the-art syntheses that this simple synthesis bears for fundamental and applied research are highlighted.

Effects of morphology and size of nanoscale drug carriers on cellular uptake and internalization process: a review

In the field of targeted drug delivery, the effects of size and morphology of drug nanocarriers are of great importance and need to be discussed in depth. To be concise, among all the various shapes of nanocarriers, rods and tubes with a narrow cross-section are the most preferred shapes for the penetration of a cell membrane. In this regard, several studies have focused on methods to produce nanorods and nanotubes with controlled optimized size and aspect ratio (AR). Additionally, a non-spherical orientation could affect the cellular uptake process while a tangent angle of less than 45° is better at penetrating the membrane, and Ω = 90° is beneficial. Moreover, these nanocarriers show different behaviors when confronting diverse cells whose fields should be investigated in future studies. In this survey, a comprehensive classification based on carrier shape is first submitted. Then, the most commonly used methods for control over the size and shape of the carriers are reviewed. Finally, influential factors on the cellular uptake and internalization processes and related analytical methods for evaluating this process are discussed.

Surfactant stabilized gold nanomaterials for environmental sensing applications - A review

Surfactant stabilized Gold (Au) nanomaterials (NMs) have been documented extensively in recent years for numerous sensing applications in the academic literature. Despite the crucial role these surfactants play in the sensing applications, the comprehensive reviews that highlights the fundamentals associated with these assemblies and impact of these surfactants on the properties and sensing mechanisms are still quite scare. This review is an attempt in organizing the vast literature associated with this domain by providing critical insights into the fundamentals, preparation methodologies and sensing mechanisms of these surfactant stabilized Au NMs. For the simplification, the surfactants are divided into the typical and advanced surfactants and the Au NMs are classified into Au nanoparticles (NPs) and Au nanoclusters (NCs) depending upon the complexity in structure and size of the NMs respectively. The preparative methodologies are also elaborated for enhancing the understanding of the readers regarding such assemblies. The case studies regarding surfactant stabilized Au NMs were further divided into colorimetric sensors, surface plasmonic resonance (SPR) based sensors, luminescence-based sensors, and electrochemical/electrical sensors depending upon the property utilized by the sensor for the sensing of an analyte. Future perspectives are also discussed in detail for the researchers looking for further progress in that particular research domain.

Model for Gold Nanoparticle Synthesis: Effect of pH and Reaction Time

The synthesis of gold nanoparticles is dependent on both the concentration of trisodium citrate dihydrate and the time that it interacts with tetrachloroauric acid. A wide range of gold nanoparticles with various sizes and dispersity can be produced based on control variables, such as time of reaction and acid concentration, using a similar approach to that of the Turkevich model. In this model, the pH of the solution decreases slightly throughout the reaction (0.005 unit/min) due to the chemical interactions between trisodium citrate dihydrate and tetrachloroauric acid. Dicarboxy acetone is formed during citrate oxidization, resulting in gold nuclei formation over time. In addition, gold nanoparticle nucleation causes pH fluctuation over time based on gold nanoparticle sizes. An inverse correlation (coefficient of smaller than -0.97) was calculated between the pH and reaction time at different ratios of trisodium citrate dihydrate to tetrachloroauric acid. Regression analysis was used to develop a model for the prediction of the size of gold nanoparticles ranging from 18 to 38 nm based on the concentration of trisodium citrate dihydrate and the reaction time.

Metallic nanoparticles as drug delivery system for the treatment of cancer

INTRODUCTION

The targeted delivery of anticancer agents to tumor is a major challenge because most of the drugs show off-target effect resulting in nonspecific cell death. Multifunctionalized metallic nanoparticles (NPs) are explored as new carrier system in the era of cancer therapeutics. Researchers investigated the potential of metallic NPs to target tumor cells by active and passive mechanisms, thereby reducing off-target effects of anticancer agents. Moreover, photocatalytic activity of upconversion nanoparticles (UCNPs) and the enhanced permeation and retention (EPR) effect have also gained wide potential in cancer treatment. Recent advancement in the field of nanotechnology highlights their potency for cancer therapy.

AREAS COVERED

This review summarizes the types of gold and silver metallic NPs with targeting mechanisms and their potentiality in cancer therapy.

EXPERT OPINION

Recent advances in the field of nanotechnology for cancer therapy offer high specificity and targeting efficiency. Targeting tumor cells through mechanistic pathways using metallic NPs for the disruption/alteration of molecular profile and survival rate of the tumor cells has led to an effective approach for cancer therapeutics. This alteration in the survival rate of the tumor cells might decrease the proliferation thereby resulting in more efficient management in the treatment of cancer.

Experimental and Theoretical Studies on Sustainable Synthesis of Gold Sol Displaying Dichroic Effect

Gold nanoparticles depending on their shape and mixtures of multiple shapes can exhibit peculiar optical properties, including the dichroic effect typical of the Lycurgus cup, which has puzzled scientists for a long time. Such optical properties have been recently exploited in several fields such as paint technology, sensors, dichroic polarizers, display (LCD) devices, laser applications, solar cells and photothermal therapy among others. In this article, we have demonstrated a simple room temperature one-pot synthesis of gold sol displaying a dichroic effect using a slow reduction protocol involving only trisodium citrate as a reducing agent. We found that the dichroic gold sol can be easily formed at room temperature by reducing gold salt by trisodium citrate below a certain critical concentration. The sol displayed an orangish-brown color in scattered/reflected light and violet/blue/indigo/purple/red/pink in transmitted light, depending on the experimental conditions. With minor changes such as the introduction of a third molecule or replacing a small amount of water in the reaction mixture with ethanol, the color of the gold sol under transmitted light changed and a variety of shades of red, pink, cobalt blue, violet, magenta and purple were obtained. The main advantage of the proposed method lies in its simplicity, which involves the identification of the right ratio of the reactants, and simple mixing of reactants at room temperature with no other requirements. TEM micrographs displayed the formation of two main types of particles viz. single crystal gold nanoplates and polycrystalline faceted polyhedron nanoparticles. The mechanism of growth of the nanoplates and faceted polyhedron particles have been described by an enhanced diffusion limited aggregation numerical scheme, where it was assumed that both trisodium citrate and the gold ions in solution undergo a stochastic Brownian motion, and that the evolution of the entire system is regulated by a principle of energy minimization. The predictions of the model matched with the experiments with a good accuracy, indicating that the initial hypothesis is correct.

Poly (Vinyl Alcohol) Assisted Synthesis and Anti-Solvent Precipitation of Gold Nanoparticles

Gold nanoparticles (GNPs) are commonly synthesized using the Turkevich method, but there are limitations on the maximum concentration of gold nanoparticles that can be achieved using this method (often < 1 mM (=0.34 mg/mL) gold precursor loading). Here, we report an inverse Turkevich method which significantly increases the concentration of gold nanoparticles (up to 5-fold) in the aqueous phase by introducing poly (vinyl alcohol) (PVA) to the synthesis system for stabilization. The aim of this study is to understand the effect of PVA and other synthesis parameters, such as trisodium citrate and tetrachloroauric acid concentration, with the goal of maximizing concentration while maintaining gold nanoparticle morphology, stability, and narrow size distribution. The size distribution of GNPs is investigated for a range of parameters by dynamic light scattering and electron microscopy, and ultraviolet-visible (UV–vis) spectroscopy is also utilized to explore the localized surface plasmon resonance (LSPR). Further, the interaction between GNPs and PVA is investigated by Fourier-transform infrared spectroscopy. In addition to increasing the gold loading by varying synthesis parameters, we also develop a novel anti-solvent precipitation method for the PVA-coated GNPs, which enables continuous condensation and purification of GNPs by forming a gold/PVA nanocomposite.