Gold Nanomaterials and their Composites as Electrochemical Sensing Platforms for Nitrite Detection

Nitrite is one of the abundant toxic components existing in the environment and is likely to have a great potential to affect human health badly. For that reason, it has become crucial to build a reliable nitrite detection method. In recent years, several nitrite monitoring systems have been proposed. Compared with traditional analytical strategies, the electrochemical approach has a bunch of advantages, including low cost, rapid response, easy operation, simplicity, etc. In this case, noble metal nanomaterials, especially Au-based nanomaterials, have attracted attention in electrode modification because of higher catalytic activity, facile mass transfer, and broad active area for determining nitrite. This review is based on the state-of-the-art, which includes a variety of nanomaterials that have been coupled with AuNPs for the creation of nanocomposites, and the construction as well as development of electrochemical sensors for nitrite detection over the last few years (2016-2022). A background study on synthesizing different morphological AuNPs and nanocomposites has also been introduced. The fabrication methods and sensing capabilities of modified electrodes are given special consideration.

The Role of Polymer-AuNP Interaction in the Stimuli-Response Properties of PPA-AuNP Nanocomposites

The helical sense control of dynamic helical polymers such as poly(phenylacetylene)s (PPAs) is greatly affected when they are conjugated to AuNPs through a strong thiol-Au connection, which restricts conformational changes at the polymer. Thus, the classical thiol-MNP bonds must be replaced by weaker ones, such as supramolecular amide-Au interactions. A straightforward preparation of the PPA-Au nanocomposite by reduction of a preformed PPA-Au³⁺ complex cannot be used due to a redox reaction between the two components of the complex which degrades the polymer. To avoid the interaction between the PPA and the Au³⁺ ions before the reduction takes place, the metal ions are added to the polymer solution capped as a TOAB complex, which keeps the PPA stable due to the lack of PPA-Au³⁺ interactions. Ulterior reduction of the Au³⁺ ions by NaBH₄ affords the desired nanocomposite, where the AuNPs are stabilized by supramolecular anilide-AuNPs interactions. By using this approach, 3.7 nm gold nanoparticles are generated and aligned along the polymer chain with a regular distance between particles of 6 nm that corresponds to two helical pitches. These nanocomposites show stimuli-responsive properties and are also able to form macroscopically chiral nanospheres with tunable size.

Chiral gold-PPA nanocomposites with tunable helical sense and morphology

A novel type of stimuli-responsive dynamic helical polymer-metal nanoparticle nanocomposite formed by a helical poly(phenylacetylene) (PPA) combined with gold nanoparticles (AuNPs) is described. Thus, several PPA copolymers containing the ethynyl-4-benzamide of (S)-phenylglycine methyl ester (M1) to dictate the helical structure/sense of the copolymer, and the ethynyl-4-benzamide of the 11-((2-(2-(2-aminoethoxy)ethoxy)ethyl)amino)undecane-1-thiol (M2) to link the copolymer to the AuNPs are prepared. Different morphologies of these nanocomposites were obtained by considering the thiol ratio and the self-assembly properties of the PPA, which generates from dispersed AuNPs to fibre-like structures. All these nanocomposites show a dynamic chiral behaviour, it being possible to manipulate their helical sense by the action of external stimuli. Moreover, it is possible to control the aggregation of these nanocomposites into macroscopically chiral nanospheres with low polydispersity by using Ba2+ as a crosslinking agent.

The Renewable and Sustainable Conversion of Chitin into a Chiral Nitrogen-Doped Carbon-Sheath Nanofiber for Enantioselective Adsorption

Well-known hard-template methods for nitrogen (N)-doped chiral carbon nanomaterials require complicated construction and removal of the template, high-temperature pyrolysis, harsh chemical treatments, and additional N-doping processes. If naturally occurring chiral nematic chitin nanostructures [(C₈ H₁₃ NO₅ )_n ] in exoskeletons were wholly transformed into an N-doped carbon, this would be an efficient and sustainable method to obtain a useful chiral nanomaterial. Here, a simple, sacrificial-template-free, and environmentally mild method was developed to produce an N-doped chiral nematic carbon-sheath nanofibril hydrogel with a surface area >300 m²  g^-1 and enantioselective properties from renewable chitin biomass. Calcium-saturated methanol physically exfoliated bulk chitin and produced a chiral nematic nanofibril hydrogel. Hydrothermal treatment of the chiral chitin hydrogel at 190 °C produced an N-doped chiral carbon-sheath nanofibril hydrogel without N-doping. This material preferentially adsorbed d-lactic acid over l-lactic acid and produced 16.3 % enantiomeric excess of l-lactic acid from a racemic mixture.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Cyclodextrin-Based Metal-Organic Nanotube as Fluorescent Probe for Selective Turn-On Detection of Hydrogen Sulfide in Living Cells Based on H2S-Involved Coordination Mechanism

Hydrogen sulfide (H₂S) has been considered as the third biologically gaseous messenger (gasotransmitter) after nitric oxide (NO) and carbon monoxide (CO). Fluorescent detection of H₂S in living cells is very important to human health because it has been found that the abnormal levels of H₂S in human body can cause Alzheimer’s disease, cancers and diabetes. Herein, we develop a cyclodextrin-based metal-organic nanotube, CD-MONT-2, possessing a {Pb₁₄} metallamacrocycle for efficient detection of H₂S. CD-MONT-2′ (the guest-free form of CD-MONT-2) exhibits turn-on detection of H₂S with high selectivity and moderate sensitivity when the material was dissolved in DMSO solution. Significantly, CD-MONT-2′ can act as a fluorescent turn-on probe for highly selective detection of H₂S in living cells. The sensing mechanism in the present work is based on the coordination of H₂S as the auxochromic group to the central Pb(II) ion to enhance the fluorescence intensity, which is studied for the first time.