Gold nanoparticle-catalyzed uranine reduction for signal amplification in fluorescent assays for melamine and aflatoxin B1

Fluorescent Noncovalent Organic Framework for Supporting Gold Nanoparticles as Heterogeneous Catalyst with Merits of Easy Detection and Recycle

A porous noncovalent organic framework with AIE effect is designed and synthesized as the support for gold nanoparticles (AuNPs). The framework is fabricated through the electrostatic complexation between carboxymethyl cellulose and tetraphenylethene-containing ammonium surfactant, which can complex AuNPs via the noncovalent interactions to offer a heterogeneous catalyst. Compared to the covalent modification on cellulose, this noncovalent framework gains superiorities in the catalyst synthesis and the size control of AuNPs. The AIE property and water-insolubility allow such heterogeneous catalysts to be easily detected, separated, and recycled, opening a new pathway for the reduction of nitrobenzene compounds and some dye compounds in aqueous conditions, which present the features of green chemistry. The use of cellulose for developing new heterogeneous metal catalysts, especially in a noncovalent way, would promote the value-added utilization of cellulose. This work provides a design strategy for gaining heterogeneous metal catalysts by taking advantage of natural bioresources.

Highly sensitive colorimetric assay for penconazole using gold nanostar-graphene quantum dot composite

In this study, a simple and sensitive colorimetric method for penconazole sensing is reported. It is based on the etching/anti-etching strategy in which gold nanostar@graphene quantum dot (AuNS@GQD) nanocomposite acts as a sensing nanoprobe and IO₄^- as an etching agent. With adding IO₄^- to the solution of AuNSs@GQDs, the sharp tips of NSs are etched and they are transformed into nearly spherical nanostructures, and the color of the solution changes from green to violet. These changes can be attributed to the oxidation of gold atoms at the sharp corners and tips of NSs with I₂, produced by the reaction of IO₄^- with AuNS@GQD solution. When penconazole is added into the system, the color and shape variations of AuNSs caused by periodate are impressively prevented. It can be assigned to the strong binding affinity between nitrogen atoms of penconazole and gold atoms, which protects them from etching by I₂. This mechanism has been affirmed via transmission electron microscopy (TEM) and UV-Vis spectra. This approach exhibited a good linear relationship (R² = 0.999) between the wavelength shift and penconazole concentration in the range of 5.0-1000.0 nM with a limit of detection (LOD) of 1.5 nM. The sensor is stable and reproducible and was used to measure penconazole in spiked water samples with recoveries over 97%.

One-pot green synthesis of gold nanoparticles using Sarcophyton crassocaule, a marine soft coral: Assessing biological potentialities of antibacterial, antioxidant, anti-diabetic and catalytic degradation of toxic organic pollutants

Marine bio-resources are being extensively researched as a priceless supply of substances with therapeutic potential. This work report the first time attempt made towards the green synthesis of gold nanoparticles (AuNPs) using the aqueous extract of marine soft coral (SCE), Sarcophyton crassocaule. The synthesis was conducted under optimized conditions and the visual coloration of reaction mixture changed from yellowish to ruby red at 540 nm. The electron microscopic (TEM, SEM) studies exhibited spherical and oval shaped SCE-AuNPs in the size ranges of 5–50 nm. The organic compounds present in SCE were primarily responsible for the biological reduction of gold ions validated by FT-IR while the zeta potential confirmed the overall stability of SCE-AuNPs. The synthesized SCE-AuNPs exhibited variety of biological efficacies like antibacterial, antioxidant and anti-diabetic in nature. The biosynthesized SCE-AuNPs demonstrated remarkable bactericidal efficacy against clinically significant bacterial pathogens with inhibition zones of mm. Additionally, SCE-AuNPs exhibited greater antioxidant capacity in terms of DPPH: 85 ± 0.32% and RP: 82 ± 0.41%). The ability of enzyme inhibition assays to inhibit α-amylase (68 ± 0.21%) and α-glucosidase (79 ± 0.2%) was quite high. The study also highlighted the spectroscopic analysis of the biosynthesized SCE-AuNPs' catalytic effectiveness of 91% in the reduction processes of the perilous organic dyes, exhibiting pseudo-first order kinetics.

Rapid point-of-need detection of bacteria and their toxins in food using gold nanoparticles

Biosensors need to meet the rising food industry demand for sensitive, selective, safe, and fast food safety quality control. Disposable colorimetric sensors based on gold nanoparticles (AuNPs) and localized surface plasmon resonance are low-cost and easy-to-perform devices intended for rapid point-of-need measurements. Recent studies demonstrate various facile and versatile AuNPs-based analytical platforms for the detection of bacteria and their toxins in milk, meat, and other foods. In this review, we introduce the general characteristics and mechanisms of AuNPs calorimetric biosensors, and highlight optimizations needed to strengthen and improve the quality of devices for their application in food matrices.

A nanofluidic device for parallel single nanoparticle catalysis in solution

Studying single catalyst nanoparticles, during reaction, eliminates averaging effects that are an inherent limitation of ensemble experiments. It enables establishing structure-function correlations beyond averaged properties by including particle-specific descriptors such as defects, chemical heterogeneity and microstructure. Driven by these prospects, several single particle catalysis concepts have been implemented. However, they all have limitations such as low throughput, or that they require very low reactant concentrations and/or reaction rates. In response, we present a nanofluidic device for highly parallelized single nanoparticle catalysis in solution, based on fluorescence microscopy. Our device enables parallel scrutiny of tens of single nanoparticles, each isolated inside its own nanofluidic channel, and at tunable reaction conditions, ranging from the fully mass transport limited regime to the surface reaction limited regime. In a wider perspective, our concept provides a versatile platform for highly parallelized single particle catalysis in solution and constitutes a promising application area for nanofluidics.

Review on Nanomaterial-Based Melamine Detection

Illegal adulteration of milk products by melamine and its analogs has become a threat to the world. In 2008, the misuse of melamine with infant formula caused serious effects on babies of China. Thereafter, the government of China and the US Food and Drug Administration (FDA) limited the use of melamine of 1 mg/kg for infant formula and 2.5 mg/kg for other dairy products. Similarly, the World Health Organization (WHO) has also limited the daily intake of melamine of 0.2 mg/kg body weight per day. Many sensory schemes have been proposed by the scientists for carrying out screening on melamine poisoning. Among them, nanomaterial-based sensing techniques are very promising in terms of real-time applicability. These materials uncover and quantify the melamine by means of diverse mechanisms, such as fluorescence resonance energy transfer (FRET), aggregation, inner filter effect, surface-enhanced Raman scattering (SERS), and self-assembly, etc. Nanomaterials used for the melamine determination include carbon dots, quantum dots, nanocomposites, nanocrystals, nanoclusters, nanoparticles, nanorods, nanowires, and nanotubes. In this review, we summarize and comment on the melamine sensing abilities of these nanomaterials for their suitability and future research directions.

The role of OH- in the formation of highly selective gold nanowires at extreme pH: multi-fold enhancement in the rate of the catalytic reduction reaction by gold nanowires

There is a quest to understand the mechanism governing the morphology and geometry control of the particle growth of nanomaterials for their optical and catalytic applications. In the available literature, the role of OH^- in dictating the size and shape of Au nanowires is unknown. As one of the first examples, herein, we explore how excess OH^- ions in CTAB micelles play a significant role during the highly selective formation of gold nanowires having controlled diameters of ∼20-25 nm and length >1 μm, by reducing Au³⁺ to Au⁰ in a one pot, simple synthesis procedure in the presence of Ag⁺ ions. At pH 4-11, the same procedure does not harvest Au NWs, but Au NPs of diameter 50-70 nm, indicating that excess OH^- is needed for nanowire formation. XRD, TGA, DSC, EDX, FT-IR and fluorescence spectroscopic analysis confirm that both CTAB and curcumin act as capping and stabilizing agents for Au NWs as well as Au NPs - there is no remarkable difference in the curcumin/CTAB content between Au NWs and NPs prepared in different pH environments. However, changing the CTAB micellar media to DPPC liposome media inhibits the formation of nanowires at pH ∼13; the growth of the Au NPs diminishes in DPPC liposomes, offering smaller NPs of diameter ∼25 to 30 nm, suggesting that the role of CTAB is necessary in nanowire formation. The rate of NW formation has been found to be 0.13 h^-1 and the growth mechanism advocates elongation in the [110] facet of Au [110] as opposed to the [100] or [111] facets. Curcumin capped Au nanowires serve as excellent nano-catalysts for the reduction of nitro-compounds and the rate of reduction of 4-nitrophenol, a model compound, by curcumin capped Au NWs is found to be ∼10 fold higher, compared to Au NPs, which signifies that catalytic activities can be dictated by the size and shape of Au NPs.

Photocatalytic reduction of organic pollutant under visible light by green route synthesized gold nanoparticles

We report a rapid method of green chemistry approach for synthesis of gold nanoparticles (AuNPs) using Lagerstroemia speciosa leaf extract (LSE). L. speciosa plant extract is known for its effective treatment of diabetes and kidney related problems. The green synthesis of AuNPs was complete within 30min at 25°C. The same could also be achieved within 2min at a higher reaction temperature (80°C). Both UV-visible spectroscopy and transmission electron microscopy results suggest that the morphology and size distribution of AuNPs are dependent on the pH of gold solution, gold concentration, volume of LSE, and reaction time and temperature. Comparison between Fourier transform infrared spectroscopy (FT-IR) spectra of LSE and the synthesized AuNPs indicate an active role of polyphenolic functional groups (from gallotannins, lagerstroemin, and corosolic acid) in the green synthesis and capping of AuNPs. The green route synthesized AuNPs show strong photocatalytic activity in the reduction of dyes viz., methylene blue, methyl orange, bromophenol blue and bromocresol green, and 4-nitrophenol under visible light in the presence of NaBH₄. The non-toxic and cost effective LSE mediated AuNPs synthesis proposed in this study is extremely rapid compared to the other reported methods that require hours to days for complete synthesis of AuNPs using various plant extracts. Strong and stable photocatalytic behavior makes AuNPs attractive in environmental applications, particularly in the reduction of organic pollutants in wastewater.

An enzyme-free catalytic DNA circuit for amplified detection of aflatoxin B1 using gold nanoparticles as colorimetric indicators

An enzyme-free biosensor for the amplified detection of aflatoxin B1 has been constructed based on a catalytic DNA circuit. Three biotinylated hairpin DNA probes (H1, H2, and H3) were designed as the assembly components to construct the sensing system (triplex H1-H2-H3 product). Cascaded signal amplification capability was obtained through toehold-mediated strand displacement reactions to open the hairpins and recycle the trigger DNA. By the use of streptavidin-functionalized gold nanoparticles as the signal indicators, the colorimetric readout can be observed by the naked eye. In the presence of a target, the individual nanoparticles (red) aggregate into a cross-linked network of nanoparticles (blue) via biotin-streptavidin coupling. The colorimetric assay is ultrasensitive, enabling the visual detection of trace levels of aflatoxin B1 (AFB1) as low as 10 pM without instrumentation. The calculated limit of detection (LOD) is 2 pM in terms of 3 times standard deviation over the blank response. The sensor is robust and works even when challenged with complex sample matrices such as rice samples. Our sensing platform is simple and convenient in operation, requiring only the mixing of several solutions at room temperature to achieve visible and intuitive results, and holds great promise for the point-of-use monitoring of AFB1 in environmental and food samples.

Nanobiosensors in diagnostics

Medical diagnosis has been greatly improved thanks to the development of new techniques capable of performing very sensitive detection and quantifying certain parameters. These parameters can be correlated with the presence of specific molecules and their quantity. Unfortunately, these techniques are demanding, expensive, and often complicated. On the other side, progress in other fields of science and technology has contributed to the rapid growth of nanotechnology. Although being an emerging discipline, nanotechnology has raised huge interest and expectations. Most of the enthusiasm comes from new possibilities and properties of nanomaterials. Biosensors (simple, robust, sensitive, cost-effective) combined with nanomaterials, also called nanobiosensors, are serving as bridge between advanced detection/diagnostics and daily/routine tests. Here we review some of the latest applications of nanobiosensors in diagnostics field.