Sensitive detection of cyanide using bovine serum albumin-stabilized cerium/gold nanoclusters

From synthesis to applications of biomolecule-protected luminescent gold nanoclusters

Gold nanoclusters (AuNCs) are a class of novel luminescent nanomaterials that exhibit unique properties of ultra-small size, featuring strong anti-photo-bleaching ability, substantial Stokes shift, good biocompatibility, and low toxicity. Various biomolecules have been developed as templates or ligands to protect AuNCs with enhanced stability and luminescent properties for biomedical applications. In this review, the synthesis of AuNCs based on biomolecules including amino acids, peptides, proteins and DNA are summarized. Owing to the advantages of biomolecule-protected AuNCs, they have been employed extensively for diverse applications. The biological applications, particularly in bioimaging, biosensing, disease therapy and biocatalysis have been described in detail herein. Finally, current challenges and future potential prospects of bio-templated AuNCs in biological research are briefly discussed.

Horseradish peroxidase-catalyzed polyacrylamide gels: monitoring their polymerization with BSA-stabilized gold nanoclusters and their functional validation in electrophoresis

Polyacrylamide gel (PAG) is extensively used as a matrix for biomolecular analysis and fractionation. However, the traditional polymerization catalyst system N,N,N',N'-tetramethylethylenediamine (TEMED)/ammonium persulphate (APS) of PAG presents non-negligible toxicity. Herein, we utilized the green and efficient bio-enzyme horseradish peroxidase (HRP) to catalyze the gel polymerization of polyacrylamide. At the same time, the efficacy of this gel system in separating nucleic acids and proteins was confirmed by applying the gel system in electrophoresis. This study aims to explore a higher biosafety polyacrylamide gel polymerization catalytic system which can be applied to electrophoresis technology. Furthermore, in order to differentiate between the bio-enzymatic catalytic system and the traditional toxic catalytic system during polymerization, aggregation-induced luminescence (AIE) of bovine serum albumin-stabilized gold nanoclusters (BSA-Au NCs) was used to monitor the polymerization reaction of the system. The results indicated that the fluorescence intensity of the polymeric system containing BSA-Au NCs increased with the polymerization of the monomers. Subsequently, we assessed whether certain components of nucleic acid electrophoresis and protein electrophoresis such as sodiumdodecylsulfate (SDS) and TBE buffer (Tris-boric acid, EDTA, pH 8.3) would affect the polymerization of the polyacrylamide gels catalyzed by the biological enzymes. The experimental conditions were also optimized to explore the optimal concentration of the ternary system of HRP, H2O2 and ACAC. Our results suggested that the bioenzyme-catalyzed system could be a feasible alternative to the TEMED/APS-catalyzed system, which also could provide new insights into the methods of monitoring the polymerization system.

Optical Sensing of Toxic Cyanide Anions Using Noble Metal Nanomaterials

Water toxicity, one of the major concerns for ecosystems and the health of humanity, is usually attributed to inorganic anions-induced contamination. Particularly, cyanide ions are considered one of the most harmful elements required to be monitored in water. The need for cyanide sensing and monitoring has tempted the development of sensing technologies without highly sophisticated instruments or highly skilled operations for the objective of in-situ monitoring. Recent decades have witnessed the growth of noble metal nanomaterials-based sensors for detecting cyanide ions quantitatively as nanoscience and nanotechnologies advance to allow nanoscale-inherent physicochemical properties to be exploited for sensing performance. Particularly, noble metal nanostructure e-based optical sensors have permitted cyanide ions of nanomolar levels, or even lower, to be detectable. This capability lends itself to analytical application in the quantitative detection of harmful elements in environmental water samples. This review covers the noble metal nanomaterials-based sensors for cyanide ions detection developed in a variety of approaches, such as those based on colorimetry, fluorescence, Rayleigh scattering (RS), and surface-enhanced Raman scattering (SERS). Additionally, major challenges associated with these nano-platforms are also addressed, while future perspectives are given with directions towards resolving these issues.

Synthesis of gold nanoclusters-loaded lysozyme nanoparticles for ratiometric fluorescent detection of cyanide in tap water, cyanogenic glycoside-containing plants, and soils

The modification of protein-stabilized gold nanoclusters with fluorophores has been intensively applied for the ratiometric detection of biomolecules, metal ions, and anions. This study developed a straightforward strategy to prepare lysozyme nanoparticle-encapsulated gold nanoclusters (LysNP-AuNCs) as a dual-emission probe for the ratiometric sensing of cyanide through fluorescence resonance energy transfer (FRET) without the conjugation of additional fluorophores. The reduction of gold ion precursors with lysozyme generated lysozyme-stabilized AuNCs under an alkaline pH, which were demonstrated to self-assemble into nanoaggregates during the formation of AuNCs. The aggregated lysozyme molecules on the AuNCs were treated with glutaraldehyde, triggering the conversion of the aggregated lysozymes into blue-emitting lysozyme nanoparticles. As a result, the AuNCs were well distributed inside a single lysozyme nanoparticle, as demonstrated by transmission electron microscopy. The presence of cyanide triggered the etching of the AuNCs in the LysNP-AuNCs, leading to the suppression of FRET from lysozyme nanoparticle to AuNCs. The LysNP-AuNC probe was implemented for FRET detection of cyanide with a linear range of 3-100 μM. Additionally, the selectivity of the LysNP-AuNC probe for cyanide toward other anions was remarkably high. The practicality of the proposed probe was evaluated by quantifying cyanide in tap water and soils and monitoring the liberation of hydrogen cyanide from cyanogenic glycoside-containing foods.

Nitrogen-rich silicon quantum dots: facile synthesis and application as a fluorescent "on-off-on" probe for sensitive detection of Hg2+ and cyanide ions

The sensitive and reliable detection of Hg²⁺ and CN^- as harsh environmental contaminants are of great importance. In view of this, a novel "on-off-on" fluorescent probe based on nitrogen-rich silicon quantum dots (NR-SiQDs) has been designed for sensitive detecting Hg²⁺ and CN^- ions in aqueous media. NR-SiQDs were synthesized by a facile, one-step, and environment friendly procedure in the presence of 3-aminopropyl trimethoxysilane (APTMS) and ascorbic acid (AA) as precursors, with L-asparagine as a nitrogen source for surface modification. The NR-SiQDs exhibited strong fluorescence emission at 450 nm with 42.34% quantum yield, satisfactory salt tolerance, and superior photo- and pH-stability. The fluorescence emission was effectively quenched by Hg²⁺ (turn off) due to the formation of a non-fluorescent stable NR-SiQDs/Hg²⁺ complex while after the addition of cyanide ions (CN^- ), Hg²⁺ ions can be leached from the surface of the NR-SiQDs and the fluorescence emission intensity of the quenched NR-SiQDs fully recovered (turn on) due to the formation of highly stable [Hg (CN)₄ ]^2- species. After optimizing the response conditions, the obtained limits of detection were found to be 53 nM and 0.46 μM for Hg²⁺ and CN^- , respectively. Finally, the NR-SiQDs based fluorescence probe was utilized to detect Hg²⁺ and CN^- ions in water samples and satisfactory results were obtained, suggesting its potential application for environmental monitoring.

Copper nanoclusters with/without salicylaldehyde-modulation for multifunctional detection of mercury, cobalt, nitrite and cyanide ions in aqueous solution and bioimaging

The sensitive determination of multiple heavy metal ions and toxic anions is important in biological and environmental fields. Here we report a facile strategy to construct a multifunctional chemosensor for the detection of Hg²⁺, [Formula: see text]Co²⁺, and CN^- in aqueous solution based on the fluorescent copper nanoclusters (Cu NCs). It was interesting to find that salicylaldehyde (SA) could effectively modulate the fluorescence property and sensing behavior of Cu NCs. In the absence of SA, Cu NCs showed 'on-off' fluorescence responses at the addition of Hg²⁺ and [Formula: see text] under different quenching mechanisms. Upon the presence of SA, Cu NCs exhibited a strong intramolecular charge transfer emission at 500 nm, accompanied by the decrease of the initial fluorescence of Cu NCs at 430 nm. This fluorescence on-state of Cu NC-SA at 500 nm was found to be exclusively turned off by Co²⁺ and enhanced by CN^-. Spectroscopy results combined with thermodynamic analysis provided sufficient information to deduce the sensing mechanisms. Finally, the Cu NCs showed high biocompatibility and were able to be used for fluorescence bioimaging in living cells. This study provided a novel and simple strategy to construct the multifunctional chemosensors for bioanalytical applications.

Advances in biomedical and pharmaceutical applications of protein-stabilized gold nanoclusters

The interest in using gold nanoclusters (AuNCs) as imaging probes is growing, covering wide ranges of applications. The stabilization of AuNCs with protein ligands enhances their biomedical and pharmaceutical applications. This is due to the biocompatibility, water solubility and bioactivity of proteins. Different factors can control the optical properties of AuNCs such as protein size, amino acids content and conformational structure. Controlling the synthesis conditions can result in tuning the AuNCs excitation, emission, fluorescence intensity and physicochemical properties to fulfill different applications. NIR-emitting protein-stabilized AuNCs are promising as imaging agents for targeting and visualization of cancer in vitro and in vivo. They are promising to be included as an important part of multifunctional theranostic nanosystems, due to their potential dual functions as imaging and photosensitizing agent for photodynamic therapy. Additionally, the protein around AuNCs represents a rich environment of active functional groups that are susceptible for conjugation with various biomolecules. Protein-AuNCs can act as fluorescent probes for rapid and selective analysis of different analytes in solution, cells or biological fluids. In conclusion, the variability of protein-AuNC applications can advance research in different biomedical and pharmaceutical fields.

Ratiometric and sensitive cyanide sensing using dual-emissive gold nanoclusters

The detection of cyanide anion (CN^-), a highly toxic pollutant, has attracted growing attention in the past years. In this work, a nanosensor composed of hyperbranched polyethyleneimine (hPEI)-assisted dual-emissive gold nanoclusters (DE-Au NCs) is proposed for ratiometric detection of CN^- based on surface valence state-driving etch. The ratiometric color change of fluorescence is based on a fact that the red-emissive Au NCs with a high content of surface Au(I) can be easily etched by CN^-, while the blue-emissive Au NCs with nearly neutral character can resist CN^-. Because of the specific gold-CN^- chemistry and electrostatic attraction between the positively charged hPEI protecting layer and the negatively charged CN^-, the DE-Au NC-based nanosensor provides high selectivity toward CN^- over other anions with a limit of detection of 10 nM. Practical application of the proposed DE-Au NC nanosensor is verified by satisfying recoveries of CN^- determination in river water and urine samples. Graphical abstract.

Colorimetric and visual detection of cyanide ions based on the morphological transformation of gold nanobipyramids into gold nanoparticles

In this study, we developed a facile, rapid, selective and sensitive colorimetric method for the detection of cyanide ions (CN⁻) by using gold nanobipyramids (Au NBPs).

The presence of a single-nucleotide mismatch in linker increases the fluorescence of guanine-enhanced DNA-templated Ag nanoclusters and their application for highly sensitive detection of cyanide

Fluorescence of DNA-templated silver nanoclusters can be enhanced by more than 100-fold by placing the nanoclusters in proximity to guanine-rich DNA sequences after hybridization. We found that the fluorescence of the guanine-enhanced silver nanoclusters is not increased with the guanine-rich DNA sequence closer to the silver nanoclusters. By studying the different numbers of mismatches in the linker sequences, we found that the presence of a single-nucleotide mismatch in the linker increases fluorescence more than the complementary nucleotide. Further study indicated the mismatch position of the linker sequence also affects the fluorescence of the hybridized DNA-Ag NCs. The evidence reported here indicated that the mismatch of the linker sequence affects the fluorescence enhancement of guanine-enhanced silver nanoclusters. We also found that DNA-Ag NCs is an excellent fluorescence sensor for cyanide, as cyanide effectively quenches the fluorescence of NCs at a very low concentration with high selectivity. Cyanide in the range from 0.10 μM to 0.35 μM could be linearly detected, with a detection limit of 25.6 nM.

Protein-Localized Bright-Red Fluorescent Gold Nanoclusters as Cyanide-Selective Colorimetric and Fluorometric Nanoprobes

Herein, we describe a bright-red-emitting ovalbumin-protected gold nanoclusters (OVA-AuNCs) that were prepared and applied as a luminescent probe for a simple, rapid, and highly sensitive determination of cyanide ions (CN- ions) based on an emission quenching and colorimetric method. Initially, an intense red-emissive fluorescence of the OVA-AuNCs successfully disappeared upon the addition of CN- ions. The resultant emission-quenching process involved CN- ions etching the OVA-AuNC surface, which produced AuCN2 - complexes in the presence of ambient oxygen. Under optimized experimental conditions, the relative emission intensity is inversely relative to CN- ion concentrations ranging from 5.00 × 10-7 to 75.00 × 10-7 mol/L with a linear correlation coefficient of 0.9932. Furthermore, OVA-AuNC-based optical detection systems on both colorimetric and fluorometric assays were tested, which expose highly sensitive and specific determination of CN- ions, and it is easily visualized by the naked eye (day light and UV light). Because of the distinct Elsner reaction between Au atoms of OVA-AuNCs and CN- ions, the recent nanoprobe offered ultrasensitivity and good selectivity with the lowest limit of detection value of 68.00 × 10-9 mol/L. In addition, this fluorescence "turn-off" CN- ion detection method was executed in real water samples. The demonstrated route of OVA-AuNC preparation is extremely easy and quick, making the proposed selective and sensitive CN- ion sensing assay based on the fluorescence response of the OVA-AuNCs for numerous practical applications.

Recent progress in nanomaterial-based assay for the detection of phytotoxins in foods

Phytotoxins refers to toxic chemicals derived from plants. They include both secondary metabolites that are dose-dependently toxic and allergens that can cause anaphylactic shock in sensitive individuals. Detecting phytotoxins in foods is increasingly important. Conventional methods for detecting phytotoxins lack sufficient sensitivity and operational convenience. Nanomaterial-based determination assays show great competence in fast and accurate sensing of trace substances. In the present review, representative phytotoxin categories of alkaloids, cyanides, and proteins are discussed. Application of notable nanomaterials, e.g. carbon nanotubes, graphene oxide, magnetic nanoparticles, metal-based nanotools, and quantum dots, in specific sensing strategies to fit the physiochemical properties of the target toxins are summarized. Nanomaterials mainly play four roles in phytotoxin detection: 1) analyte enricher; 2) sensor structure mediator; 3) target recognizer or reactant; 4) signaling agent. Great achievements have been made in the detection of trace plant-derived toxins in food matrices, yet there are still challenges awaiting further investigation.

Shape-Controlled Synthesis of Luminescent Hemoglobin Capped Hollow Porous Platinum Nanoclusters and their Application to Catalytic Oxygen Reduction and Cancer Imaging

Engineering hollow and porous platinum nanostructures using biomolecular templates is currently a significant focus for the enhancement of their facet-dependent optical, electronic, and electrocatalytic properties. However, remains a formidable challenge due to lack of appropriate biomolecules to have a structure-function relationship with nanocrystal facet development. Herein, human hemoglobin found to have facet-binding abilities that can control the morphology and optical properties of the platinum nanoclusters (Pt NCs) by regulation of the growth kinetics in alkaline media. Observations revealed the growth of unusual polyhedra by shape-directed nanocluster attachment along a certain orientation accompanied by Ostwald ripening and, in turn, yield well-dispersed hollow single-crystal nanotetrahedrons, which can easily self-aggregated and crystallized into porous and polycrystalline microspheres. The spontaneous, biobased organization of Pt NCs allow the intrinsic aggregation-induced emission (AIE) features in terms of the platinophilic interactions between Pt(II)-Hb complexes on the Pt(0) cores, thereby controlling the degree of aggregation and the luminescent intensity of Pt(0)@Pt(II)−Hb core−shell NCs. The Hb-Pt NCs exhibited high-performance electrocatalytic oxygen reduction providing a fundamental basis for outstanding catalytic enhancement of Hb-Pt catalysts based on morphology dependent and active site concentration for the four-electron reduction of oxygen. The as-prepared Hb-Pt NCs also exhibited high potential to use in cellular labeling and imaging thanks to the excellent photostability, chemical stability, and low cytotoxicity.

Highly fluorescent gold nanoclusters stabilized by food proteins: From preparation to application in detection of food contaminants and bioactive nutrients

Applications of nanotechnology in food have rapidly increased in the past decades. Ultra-small gold nanoclusters (Au NCs), composed of several to roughly a hundred atoms, represent a kind of novel nanomaterials. The Au NCs directed by food proteins have drawn considerable research attention due to their environment-friendly preparation, strong fluorescence, excellent photo-stability, and favorable biocompatibility. These interesting protein-Au hybrids have opened up a new area at the nano-bio-food interface, not only did they provide the missing link between single metal atoms and plasmonic metal nanoparticles, but also developed the hybrid system between biomacromolecule and inorganic ions. In this review, we highlighted the synthesis strategies and optical properties of the Au NCs stabilized by typical food proteins as well as their applications in detection of food contaminants or bioactive nutrients. In addition, we discussed current challenges and future development in food proteins- directed gold nanoclusters for size-controlled synthesis and multifunctional applications.

Gold nanoparticles as sensitive optical probes

Recent advances in Au NP based optical sensing systems for various analytes based on absorption, fluorescence and SERS are summarized.