An ultra-sensitive and colorimetric sensor for copper and iron based on glutathione-functionalized gold nanoclusters

Chitosan-initiated gold nanoparticles with enhanced fluorescence for unique Fe3+/PPi sensing and photothermal therapy

The design of surface ligands is crucial for ligand-protected gold nanoparticles (AuNPs). Herein, following the principle of green synthesis, environmentally friendly gold nanoparticles (AuNPs@His@CC, AuHC) were fabricated based on dual ligands of histidine and carboxylated chitosan. AuHC showed the advantages of low toxicity, good photoluminescent stability and ideal biocompatibility. Compared with single histidine-coated gold nanoclusters (AuNCs@His, AuH), AuHC presented enhanced fluorescence attributed to the addition of chitosan. The blue-emitting AuHC has a unique response to Fe3+ with detection limits as low as 9.51 nM. Interestingly, the quenched fluorescence of AuHC-Fe3+ system could be restored through the introduction of PPi with a detection limit of 10.6 μM. So an "on-off-on" fluorescence sensing platform was achieved. Apart from good optical properties and sensing, the designed AuHC demonstrated outstanding photothermal conversion efficiency (27.8 %), which made it ideal material for thermal ablation of tumor. To be specific, after laser irradiation (660 nm, 0.78 W cm-2, 10 min) of AuHC, the survival rate of HeLa cells as a tumor cell model decreased to 12.7 %, indicating that AuHC has a significant tumor inhibition effect in vitro. Besides, AuHC also could be a befitting candidate for overcoming drug-resistant tumor cells such as MCF-7/ADR cells. Notably, AuHC can markedly ablate solid tumors in 4T1 tumor-bearing mice after laser irradiation (660 nm, 0.78 W cm-2, 10 min). Hence this work provides insight into the design of multifunctional AuNPs platform for simultaneously integrating the ion sensing and photothermal therapy of cancer.

Photoluminescent gold nanoclusters as two-photon excited ratiometric pH sensor and photoactivated peroxidase

A two-photon excited ratiometric fluorescent pH sensor is reported by combining L-cysteine-protected AuNCs (Cys@AuNCs) with fluorescein isothiocyanate (FITC). Cys@AuNCs were synthesized through a one-step self-reduction route and showed pH-responsive photoluminescence at 650 nm. Benefiting from the opposite pH response of Cys@AuNCs and FITC, the fluorescence ratio (F_{515 nm}/F_{650 nm}) of FITC&Cys@AuNCs provided a large dynamic range of 200-fold for pH measurement in the response interval of pH 5.0-8.0. Based on the excellent two-photon absorption coefficient of Cys@AuNCs, the sensor was expected to achieve sensitive quantitation of pH in living cells under two-photon excitation. In addition, colorimetric biosensing based on enzyme-like metal nanoclusters has attracted wide attention due to their low-cost, simplicity, and practicality. It is crucial to develop high catalytic activity nanozyme from the viewpoint of practical application. The synthesized Cys@AuNCs exhibited excellent photoactivated peroxidase-like activity with high substrate affinity and catalytic reaction rate, promising for rapid colorimetric biosensing of field analysis and the control of catalytic reactions by photostimulation.

Differential Sensing of Antibiotics Using Metal Ions and Gold Nanoclusters Based on TMB–H2O2 System

In the water system, antibiotic pollution significantly impacts the human body and the environment. Therefore, it is essential to quickly identify the types of antibiotics in the system and detect their concentration. It has been reported that many metal ions interact with antibiotics, and some of them can also change the enzyme-like catalytic properties of gold clusters (AuNCs). In the experiments, we found significant differences in the experimental results when different antibiotics and metal ions were placed in a TMB-H2O2 system with AuNCs as catalysts. Based on this result, we devised a simple and sensitive colorimetric method for the simultaneous detection of multiple antibiotics using AuNCs-metal ions as the sensor, a multifunctional microplate detector as the detection instrument, and LDA as the analytical method. This method was successfully applied for the identification of antibiotics and the detection of their concentrations in river water.

A novel dual-emission fluorescent probe for ratiometric and visual detection of Cu2+ ions and Ag+ ions

In this work, the biomolecule glutathione was used to prepare cyan fluorescent carbon dots (GSH@CDs) by a hydrothermal method. The GSH@CDs were adopted as the scaffolds to synthesize fluorescent gold nanoclusters (GSH@CDs-Au NCs) with two independent emission peaks at 430 nm and 700 nm. A fluorescent method for the Cu²⁺ and Ag⁺ ion assay was established based on the fluorescence quenching or enhancement at 700 nm of GSH@CDs-Au NCs. The fluorescent test strips were successfully prepared for visual detection of Cu²⁺ ions and Ag⁺ ions based on GSH@CDs-Au NCs. In addition, GSH@CDs-Au NCs were found to possess well peroxidase-like activity.

Weak Interaction-Tailored Catalytic Interface of Ultrasmall Gold Nanoclusters as Enzyme Mimics for Enhanced Colorimetric Biosensing

Gold nanoclusters (AuNCs) represent an emerging type of engineered nanomaterials with intrinsic enzymatic activity for both chemical and biological applications, but the catalytic activity of most reported AuNCs remains rather limited. Herein, we report a new, efficient strategy of promoting the peroxidase-mimic activity of AuNCs by tailoring their catalytic interfaces via small molecule-mediated weak interactions. Inspired by the presence of imidazole structures in many biocatalytic centers, we screened a series of imidazole-containing small molecules to evaluate their impact on the enzymatic activity of AuNCs. Through monitoring the absorbance change of 3,3',5,5'-tetramethylbenzidine, 1-methyl-2-imidazolecarboxaldehyde (MCA) was identified to possess the most significant effect on enhancing the peroxidase-mimic activity of glutathione-stabilized AuNCs (GSH-AuNCs) among all the examined molecules. Interestingly, the enhancement effect of MCA on the catalytic activity of these AuNCs was found to be highly reversible and can be switched on/off by simply adding MCA/dialysis treatment. Molecular dynamics simulations and further experimental analysis confirmed that these MCA molecules were adsorbed on the surface of GSH-AuNCs through weak non-covalent interactions. The underlying mechanism analysis suggested that the presence of MCA can efficiently promote the production of •OH in the GSH-AuNC system. As a proof of example, we then demonstrated that the presence of MCA can greatly increase the bioanalytical performance of AuNC-based peroxidase mimics, as evidenced by a 65-fold lower LOD for glucose detection of AuNCs@MCA than that using AuNCs only. Finally, the present system has been successfully applied for sensing the blood glucose level of both healthy people and diabetics with promising results.

A spore-based portable kit for on-site detection of fluoride ions

The excess residues of fluoride ions cause serious human health problems, making their detection highly valuable. In this work, a whole-cell-based biosensor was presented for the detection of fluoride ions, which can inhibit the color reaction of 3,3',5,5',-tetramethylbenzidine (TMB) catalyzed by the CotA-laccase of spore surface. This reaction for the detection of fluoride ions could be read out through UV-vis spectrophotometer, smartphone, or standard colorimetric card within 10 min. Under optimum conditions, a linear range of 1-600 μmol L^-1 with a detection limit of 0.12 μmol L^-1 (3σ/k) was achieved for fluoride ions detection by using UV-vis spectrophotometer. The biosensor coupling with smartphone had a good linear response to fluoride ions concentration in the range of 5-600 μmol L^-1 with LOD of 0.90 μmol L^-1 (3σ/k). The standard colorimetric card can be directly used for recognizing the fluoride ions level via naked-eyes. A portable kit based on a colorimetric card and smartphone was developed and has been successfully applied for fluoride ions monitoring in surface waters and groundwater. This developed method has several advantages such as rapid, outstanding selectivity and anti-interference, low-cost, ease of operation and storage, and eco-friendliness, meeting the demands of point-of-care testing of fluoride ions and disease prevention.

Applications of nanotechnology in virus detection, tracking, and infection mechanisms

Viruses are among the most infectious pathogens, responsible for the highest death toll around the world. Lack of effective clinical drug for most of the viruses emphasizes the rapid and accurate diagnosis at early stages of infection to prevent rapid spread of the pathogens. Nanotechnology is an emerging field with applications in various domains, where nano‐biomedical science has many significant contributions such as effective delivery of drugs/therapeutic molecules to specific organs, imaging, sensitive detection of virus, and their accurate tracking in host cells. The nanomaterials reported for virus detection and tracking mainly include magnetic and gold NPs, ZnO/Pt‐Pd, graphene, and quantum dots (QDs). In addition, the single virus tracking technology (SVT) allowed to track the life cycle stages of an individual virus for better understanding of their dynamics within the living cells. Inorganic as well as non‐metallic fluorescent materials share the advantages of high photochemical stability, a wide range of light absorption curves and polychromatic emission. Hence, are considered as potential fluorescent nano‐probes for SVT. However, there are still some challenges: (i) clinical false positive rate of some detection methods is still high; (ii) in the virus tracking process, less adaptability of QDs owing to larger size, flicker, and possible interference with virus function; and (iii) in vivo tracking of a single virus, in real time needs further refinement. In the future, smaller, non‐toxic, and chemically stable nanomaterials are needed to improve the efficiency and accuracy of detection, and monitoring of virus infections to curb the mortalities.

This article is categorized under:

Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease

Biology‐Inspired Nanomaterials > Protein and Virus‐Based Structures

Recent advances in functionalization of plasmonic nanostructures for optical sensing

This review summarizes the progress that has been made in the use of nanostructured SPR-based chemical sensors and biosensors. Following an introduction into the field, a first large section covers principles of nanomaterial-based SPR sensing, mainly on methods using noble metal nanoparticles (spheres, cubes, triangular plates, etc.). The next section covers methods for functionalization of plasmonic nanostructures, with subsections on functionalization using (a) amino acids and proteins; (b) oligonucleotides, (c) organic polymers, and (d) organic compounds. Several tables are presented that give an overview on the wealth of methods and materials published. A concluding section summarizes the current status, addresses current challenges, and gives an outlook on potential future trends. This review is not intended to be a comprehensive compilation of the literature in the field but rather is a systematic overview of the state of the art in surface chemistry of plasmonic nanostructures. The ability of various ligands and receptors for functionalization of nanoparticles as well as their sensing capability is discussed.

MOF-derived porous ZnO-Co3O4 nanocages as peroxidase mimics for colorimetric detection of copper(ii) ions in serum

Sensitive detection of copper ions (Cu²⁺) in biological samples is extremely important since an abnormal level of Cu²⁺ is linked with many diseases. Herein, we demonstrated a novel turn-on colorimetric sensor for selective detection of Cu²⁺ both in buffered solution and serum samples based on porous bimetallic transition metal oxide nanocages (ZnO-Co₃O₄ NCs) as peroxidase mimics. The ZnO-Co₃O₄ NCs were prepared by using ZnCo-zeolitic-imidazolate-framework (ZnCo-ZIF) as precursors via direct calcination. With the high peroxidase-like activity, the obtained ZnO-Co₃O₄ NCs can catalyze the oxidation of tetramethylbenzidine (TMB) in the presence of H₂O₂ to form a blue colored product. The inhibition effect of cysteine (Cys) on the catalytic activity of ZnO-Co₃O₄ NCs and its strong binding ability toward Cu²⁺ enabled the ZnO-Co₃O₄ NCs/Cys system to be utilized for sensitive detection of Cu²⁺, in which the catalytic activity of ZnO-Co₃O₄ NCs/Cys can be recovered by the introduction of Cu²⁺ with an obvious color change of the solution. The linear range for Cu²⁺ determination was 2 to 100 nM with a detection limit of 1.08 nM. More importantly, this colorimetric sensor has been successfully applied to detect Cu²⁺ in serum without pretreatment. Our findings are expected to expand the scope of application of nanozyme and shed light on early disease diagnosis.

A simple, quantitative method for spectroscopic detection of metformin using gold nanoclusters

We synthesized bovine serum albumin (BSA)-stabilized gold nanoclusters (BSA-GNCs) and confirmed their ultra-small size using HRTEM (High-resolution Transmission Electron Microscope) and DLS (Dynamic Light Scattering). The fluorescence intensity of BSA-GNCs is "turned off" in the presence of Cu(II) metal ions. The resulting Cu(II)-mediated BSA-GNCs were utilized to detect metformin, a drug used to control diabetes. Metformin binds to and displaces Cu(II) ions from the BSA on the surface of the nanoclusters, which turns on the fluorescence of the nanoclusters. The interactions between the protein-stabilized nanoclusters were investigated in the absence and presence of Cu(II) using circular dichroism (CD) and Fourier-transform infrared spectroscopy (FTIR). Cu(II)-quenched BSA-GNCs had an extremely high sensitivity to detect metformin, with a low limit of detection (LOD) of 0.068 μM and a dynamic range of limit of quantification (LOQ = 10/3 LOD) of 0.22 to 11 μM. The ability of this novel "turn-on" nanosensor to detect metformin in human serum and urine samples was confirmed: the percentage recovery in fluorescence for spiked analyte ranged from 96.00-98.50% and 92.60-96.62% in human serum and urine samples, respectively. Thus, BSA-GNCs provide a valid, sensitive, specific fluorometric methodology for the detection of metformin in biomedical applications.

Novel Synthesis of Thiolated Gold Nanoclusters Induced by Lanthanides for Ultrasensitive and Luminescent Detection of the Potential Anthrax Spores' Biomarker

In this study, we reported a facile, one-pot, and "green" synthesis of glutathione-protected gold nanoclusters (GSH@AuNCs) initiated by samarium (Sm³⁺) lanthanides for the first time. Sm³⁺ lanthanides more efficiently induced the formation of GSH@AuNCs with significantly enhanced luminescence than other lanthanides or heavy metal ions (Cd²⁺, Pb²⁺) did. Using this strategy, a detection for Sm³⁺ was made with a linearity range of (10.0-100.0 μM) and a limit of detection (LOD) of 0.5 μM. The Sm³⁺-based GSH@AuNCs were characterized by eco-friendliness, photostability, and low-cost synthesis with low biological toxicity and had great potential in the application for biosensing and bioimaging. They were successfully employed in the detection of dipicolinic acid (DPA), a well-reported biomarker for sensing potential infection by strongly hazardous anthrax spores. A good linear response was obtained for DPA detection ranging from 1.0 to 120.0 μM with a low LOD of 0.1 μM, which was much lower (600 times) than the infectious dosage of anthrax spores (6 × 10^-5 M). The detection was due to the strong binding affinity and strong chelation capability of DPA to Sm³⁺ lanthanides, which caused the dissociation of the aggregates with an obvious decrease or even a turning-off effect of their luminescence.

Colorimetric detection of serum doxycycline with d-histidine-functionalized gold nanoclusters as nanozymes

An assay for selective detection of DC was developed due to the nanozymatic activity of d-His@AuNCs inhibited by Cu²⁺ and restored by DC.

Functional nanomaterials with unique enzyme-like characteristics for sensing applications

We highlight the recent developments in functional nanomaterials with unique enzyme-like characteristics for sensing applications.

Highly Sensitive and Selective Fluorescent Detection of Gossypol Based on BSA-Stabilized Copper Nanoclusters

In this paper, fluorescent copper nanoclusters (NCs) are used as a novel probe for the sensitive detection of gossypol for the first time. Based on a fluorescence quenching mechanism induced by interactions between bovine serum albumin (BSA) and gossypol, fluorescent BSA-Cu NCs were seen to exhibit a high sensitivity to gossypol in the range of 0.1–100 µM. The detection limit for gossypol is 25 nM at a signal-to-noise ratio of three, which is approximately 35 times lower than the acceptable limit (0.9 µM) defined by the US Food and Drug Administration for cottonseed products. Moreover, the proposed method for gossypol displays excellent selectivity over many common interfering species. We also demonstrate the application of the present method to the measurement of several real samples with satisfactory recoveries, and the results agree well with those obtained using the high-performance liquid chromatography (HPLC) method. The method based on Cu NCs offers the followings advantages: simplicity of design, facile preparation of nanomaterials, and low experimental cost.

Chitosan-Stabilized Self-Assembled Fluorescent Gold Nanoclusters for Cell Imaging and Biodistribution in Vivo

Biocompatible, near-infrared luminescent gold nanoclusters were synthesized in situ using as-prepared chitosan grafted with N-acetyl-l-cysteine (NAC-CS). The fluorescent gold nanoclusters coated with chitosan-N-acetyl-l-cysteine (AuNCs@NAC-CS) were aggregated by multiple ultrasmall gold nanoclusters closing with each other, with strong fluorescence emission at 680 nm upon excitation at 360 nm. AuNCs@NAC-CS did not display any appreciable cytotoxicity on cells even at a concentration of 1.0 mg mL^-1. AuNCs@NAC-CS were more insensitive to H₂O₂ and trypsin compared with fluorescent gold nanoclusters coated with Albumin Bovine V (AuNCs@BSA), which make them have long time imaging in HeLa cells. Furthermore, the obvious fluorescence signal of AuNCs@NAC-CS appeared in the liver and kidney of the normal mice after 6 h injection. And the fluorescence intensity decreased after that because of the highly efficient clearance characteristics of ultrasmall nanoparticles. These findings demonstrated that AuNCs@NAC-CS possessed good fluorescence, low cytotoxicity, and low sensitivity to some content of cells, allowing imaging of the living cells.

Nucleotide-directed syntheses of gold nanohybrid systems with structure-dependent optical features: Selective fluorescence sensing of Fe3+ ions

This study demonstrates a one-step synthesis for the preparation of both adenosine monophosphate (AMP)-stabilized colloidal gold nanoparticles (AMP-Au NPs) and fluorescent gold nanoclusters (AMP-Au NCs). The dominant role of AMP:AuCl₄^- molar ratios in the formation of diverse nanosized Au products was proved. The size, the structure and the unique structure-dependent optical properties of the NPs and NCs were determined based on the results of numerous spectroscopic (UV-vis, fluorescence, infrared, x-ray photoelectron), high resolution electron microscopy (HRTEM) and dynamic light scattering (DLS) techniques. Stabile AMP-Au NPs with diameter of ca. 11nm and ultra-small AMP-Au NCs having blue fluorescence (λ_em=480nm) were identified. In addition, the AMP-Au NCs have been utilized to develop a selective sensor for the detection of Fe³⁺ ions in aqueous medium based on fluorescence quenching. Several essential metal ions and anions have been tested but our results clearly supported that dominant quenching was observed only for Fe³⁺ ions. Based on the determined limit of detection (LOD=2.0μM) our system is capable of detecting Fe³⁺ ions in drinking water. The Stern-Volmer constants (K_SV) and various thermodynamic parameters (ΔG, ΔH°, ΔS°, ΔC_p) of the quenching process have also been determined by the Stern-Volmer fitting of the fluorescence data in order to better understand the quenching mechanism.