Interparticle interactions in glutathione mediated assembly of gold nanoparticles

Fabrication and application of gold nanoparticles functionalized polymer monolith in spin column for the determination of S-nitrosoglutathione in meat

S-nitrosoglutathione (GSNO) is the most important S-nitrosothiol in vivo, which could affect the quality of meat by participating in calcium release, glucose metabolism, proteolysis and apoptosis, therefore may potentially serve as a marker for meat freshness. In this work, a solid-phase extraction (SPE) monolithic spin column modified with gold nanoparticles was prepared for GSNO extraction. The optimized SPE-LC-MS/MS method for GSNO quantification displays low limit of detection (0.01 nM), good precision (RSD < 15 %) and acceptable recovery (> 77.7 %). Furthermore, this approach has been applied to monitor GSNO levels in beef and pork stored at -20 °C for different days, showing that endogenous GSNO level increases during prolonged storage and could be employed as a marker to evaluate the freshness of ice stored meat. Additionally, the monolithic spin column remains in good quality after a half-year storage, which is promising to develop into commercial enrichment kit for endogenous GSNO analysis.

Highly specific colorimetric detection based on aggregation of l-cysteine functionalized gold nanoparticles for cypermethrin in water samples

A fast, simple, and selective colorimetric assay for quantifying cypermethrin in water samples is proposed using l-cysteine functionalized gold nanoparticles (AuNPs@Cyst). Cypermethrin is hydrolyzed by potassium hydroxide to obtain hydrolyzed cypermethrin in the form of 3-phenoxybenzaldehyde by-product (HCy). The detection strategy is based on the aggregation of AuNPs@Cyst caused by hydrogen-bonding recognition between the aldehyde group of HCy and the amine group of l-cysteine on the surface of AuNPs@Cyst. As a result, in the presence of HCy under optimal pH 7, AuNPs@Cyst aggregates within 7 min, exhibiting a distinct color change from red to blue-gray, which can be evaluated with the naked eye and UV-visible spectrophotometry. From FE-TEM image, the stable and spherical AuNPs@Cyst had an average size of 13.8 ± 1.6 nm, and from zeta potential analysis, the charge of AuNPs@Cyst was -25.04 ± 1.66 mV. The surface plasmon resonance band of dispersed AuNPs@Cyst was red shifted from 525 nm to 634 nm when AuNPs@Cyst was aggregated. The absorbance ratio (A634/A525) was linearly related to cypermethrin concentrations from 0.5 to 13.0 mg L-1. The limit of detection was 0.2 mg L-1 and precision, expressed as relative standard deviations (RSDs), ranged from 1.9 to 7.3%. In the presence of interfering pesticides (carbaryl, ethion, profenofos and abamectin), only cypermethrin produced a significantly different response, confirming the selectivity of AuNPs@Cyst. Finally, AuNPs@Cyst was applied to determine cypermethrin in water samples, achieving very satisfied recoveries (>98.6%) and RSDs lower than 6.1%.

DNA-Condensed Plasmonic Supraballs Transparent to Molecules

DNA nanotechnology offers an unrivaled programmability of plasmonic nanoassemblies based on encodable Watson-Crick basepairing. However, it is very challenging to build rigidified three-dimensional supracolloidal assemblies with strong electromagnetic coupling and a self-confined exterior shape. We herein report an alternative strategy based on a DNA condensation reaction to make such structures. Using DNA-grafted gold nanoparticles as building blocks and metal ions with suitable phosphate affinities as abiological DNA-bonding agents, a seedless growth of spheroidal supraparticles is realized via metal-ion-induced DNA condensation. Some governing rules are disclosed in this process, including kinetic and thermodynamic effects stemming from electrostatic and coordinative forces with different interaction ranges. The supraballs are tailorable by adjusting the volumetric ratio between DNA grafts and gold cores and by overgrowing extra gold layers toward tunable plasmon coupling. Various appealing and highly desirable properties are achieved for the resulting metaballs, including (i) chemical reversibility and fixation ability, (ii) stability against denaturant, salt, and molecular adsorbates, (iii) enriched and continuously tunable plasmonic hotspots, (iv) permeability to small guest molecules and antifoulingness against protein contaminates, and (v) Raman-enhancing and photocatalytic activities. Innovative applications are thus foreseeable for this emerging class of meta-assemblies in contrast to what is achieved by DNA-basepaired ones.

A sensitive dual mode turn-on fluorescence and colorimetric nanosensor for ultrasensitive detection of trace amount of gluten proteins in bread products based on crystalline nano cellulose and gold nanoparticles

In this work, Gold nanoparticles (AuNPs) encapsulated in the surface of crystalline nano cellulose grafted poly citric acid (CNC-g-PCA) and CNC-g-PCA/Au nanocomposite were synthesized successfully that exhibited stable and intense fluorescence property in aqueous buffer. A dual-mode nanosensor is reported with both colorimetric and fluorimetric readout based on citrate-protected AuNPs for discriminative detection of gluten proteins. The proposed sensing system consists of AuNPs and fluorescent CNCs, where CNCs function as a fluorimetric reporter and AuNPs serve a dual function as a colorimetric reporter and fluorescence quencher. The mechanism of the reported dual-mode nanosensor is based on two distance-dependent phenomena, the color change of AuNPs and FRET. The presence of gluten proteins can reverse the process by enlarging the inter-particle distance between AuNPs and CNCs and recovering the fluorescence emission of CNC. The linear range was 0.05 to 0.40 μgmL^-1 for UV-vis spectroscopy and 0.017 to 0.298 μgmL^-1 for fluorescence spectroscopy, The limit of detection was 4.43 ± 0.019 ngmL^-1 for UV-vis spectroscopy and 3.13 ± 0.033 ngmL^-1 for fluorescence spectroscopy (n = 6). The fabricated nanosensor was applied to the gluten analysis in gluten-free bread successfully.

Reversible assembly of nanoparticles: theory, strategies and computational simulations

The significant advances in synthesis and functionalization have enabled the preparation of high-quality nanoparticles that have found a plethora of successful applications. The unique physicochemical properties of nanoparticles can be manipulated through the control of size, shape, composition, and surface chemistry, but their technological application possibilities can be further expanded by exploiting the properties that emerge from their assembly. The ability to control the assembly of nanoparticles not only is required for many real technological applications, but allows the combination of the intrinsic properties of nanoparticles and opens the way to the exploitation of their complex interplay, giving access to collective properties. Significant advances and knowledge gained over the past few decades on nanoparticle assembly have made it possible to implement a growing number of strategies for reversible assembly of nanoparticles. In addition to being of interest for basic studies, such advances further broaden the range of applications and the possibility of developing innovative devices using nanoparticles. This review focuses on the reversible assembly of nanoparticles and includes the theoretical aspects related to the concept of reversibility, an up-to-date assessment of the experimental approaches applied to this field and the advanced computational schemes that offer key insights into the assembly mechanisms. We aim to provide readers with a comprehensive guide to address the challenges in assembling reversible nanoparticles and promote their applications.

Comparative and Selective Interaction of Amino Acid d-Cysteine with Colloidal Gold Nanoparticles in the Presence of a Fluorescent Probe in Aqueous Medium

In this communication, we report the comparative and selective interaction of amino acid d-cysteine (d-Cys) with citrate caped gold nanoparticles (Au NPs) in the presence of a fluorescent dye, rhodamine B (RhB), in aqueous solution. Au NPs of size 27.5 nm could almost fully quench the steady-state fluorescence emission of RhB at their optimum concentrations in the mixed solution. The interactions of d-Cys, l-Cys, all other relevant d- and l-amino acids, neurotransmitters, and other relevant biological compounds with the Au NPs/RhB mixed solution have been explored by monitoring the fluorescence recovery efficiencies from the almost fully quenched state of RhB fluorescence via a simple steady-state spectrofluorometric method. The higher fluorescence recovery for the interaction of d-Cys with the Au NPs/RhB mixed system is accompanied by a distinct color change (red-wine to bluish-black) of the assay medium after the reaction compared to that of all other interfering compounds considered in this work. The sensitivity of this fluorometric response lies in a broad linear range of concentrations of d-Cys and the limit of detection (LOD) is found to be 4.2 nM, which is low compared to many other methods available in the literature. The different degrees of interaction of d-Cys and l-Cys with the Au NPs/RhB mixed sample have been further explored by circular dichroism (CD) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The selective interaction of d-Cys with the proposed Au NPs/RhB mixed system is also found to be correlated with interparticle cross-linking and aggregations of nanoparticles by the analysis of ζ potential and dynamic light scattering (DLS) study, transmission electron microscopy (TEM), atomic force microscopy (AFM), UV-vis absorption spectroscopy etc. The proposed interaction mechanism is further studied with a normal human urine sample to elucidate that the optimized combination of Au NPs and RhB may be realized as an efficient platform for detection of the amino acid d-Cys in a real biosample via a simple fluorometric approach.

Gold-activated luminol chemiluminescence for the selective determination of cysteine over homocysteine and glutathione

This work reports a chemiluminescence assay for the highly selective determination of cysteine in biological fluids without separation techniques. The method is based on the ability of cysteine to selectively enhance the metal-catalyzed chemiluminescence generated by the oxidation of luminol from gold tetrachloride anions under alkaline conditions. The selectivity of the method stems from the fact that, under strongly alkaline conditions, the formation of the four-membered ring transition state of cysteine is less favorable as compared to the formation of the respective 5- and 9- membered ring transition states of homocysteine and glutathione, respectively. These transition states exert stronger hindrance and hydrophobic interactions repelling the negatively charged luminol dianion and possibly exhibit lower reducing ability for dissolved oxygen, towards the formation of superoxide radicals, thus reducing the oxidation of luminol. Under the optimum experimental conditions, the linear range of the method extended from 0.5 to 20 μΜ while cysteine could be determined at concentrations as low as 0.5 μM, with good reproducibility (<3.5%) and recoveries between 80 and 93% in artificial and real biological fluids.

Reversible Self-Assembly of Gold Nanoparticles Based on Co-Functionalization with Zwitterionic and Cationic Binding Motifs*

We report a pH- and temperature-controlled reversible self-assembly of Au-nanoparticles (AuNPs) in water, based on their surface modification with cationic guanidiniocarbonyl pyrrole (GCP) and zwitterionic guanidiniocarbonyl pyrrole carboxylate (GCPZ) binding motifs. When both binding motifs are installed in a carefully balanced ratio, the resulting functionalized AuNPs self-assemble at pH 1, pH 7 and pH 13, whereas they disassemble at pH 3 and pH 11. Further disassembly can be achieved at elevated temperatures at pH 1 and pH 13. Thus, we were able to prepare functionalized nanoparticles that can be assembled/disassembled in seven alternating regimes, simply controlled by pH and temperature.

Biothiol modulated growth and aggregation of gold nanoparticles and their determination in biological fluids using digital photometry

This work describes a novel and easy to use method for the determination of biologically important thiols that relies on their ability to inhibit the catalytic enlargement of AuNP seeds in the presence of ACl₄^- ions and trigger their aggregation. UV-vis spectroscopic monitoring of the plasmon resonance bands of the formed AuNPs showed that the spectral and color transitions depend both on the concentration and the structure of biothiols. The colorimetric changes induced by biothiols were quantified in the concentration range from 5 to 300 μM in the RGB color system with digital photometry using a commercially available flatbed scanner as detector. On the basis of these results, the applicability of the method was tested to the determination of glutathione in red blood cells and cysteine in blood plasma with satisfactory recoveries (88.7-96.5%), low detection limits (1.0 μM), good selectivity against major biomolecules under physiologically relevant conditions and satisfactory reproducibility (<8%). The method requires minimum technical expertise, is easy to use and is performed without scientific equipment, holding promise as a simple assay of biothiol testing even by non-experts.

Colorimetric detection of individual biothiols by tailor made reactions with silver nanoprisms

We herein described a rapid, sensitive, and selective colorimetric sensing platform for biothiols in human serum, which relies on the dual functions of biothiols as anti-etching and aggregating agent for silver nanoprisms (AgNPRs). In principle, the target biothiols that bind to the surface of AgNPRs through Ag–S covalent interactions protect the AgNPRs from being etched by chloride ion (Cl⁻) in human serum, thus exhibiting the blue/purple color that is indicative of AgNPRs. On the other hand, the color of AgNPRs turned to yellow in the absence of biothiols or the presence of non-sulfur-containing amino acids, indicating the formation of small silver nanoparticles (AgNPs). Importantly, we found that individual biothiols (Hcy, Cys, and GSH) exert not only the anti-etching effect, but also the aggregating effect on AgNPRs, which can be modulated by simply tuning the pH conditions, and this consequently allows for the discriminative detection of each biothiol. Based on this simple and cost-effective strategy, we successfully determined the Hcy, Cys, and GSH in human serum with high sensitivity and selectivity within 10 min, demonstrating the diagnostic capability and potential in practical applications.

Glutathione Self-Assembles into a Shell of Hydrogen-Bonded Intermolecular Aggregates on "Naked" Silver Nanoparticles

A detailed understanding of the molecular structure in nanoparticle ligand capping layers is crucial for their efficient incorporation into modern scientific and technological applications. Peptide ligands render the nanoparticles as biocompatible materials. Glutathione, a γ-ECG tripeptide, self-assembles into aggregates on the surface of ligand-free silver nanoparticles through intermolecular hydrogen bonding and forms a few nanometer-thick shells. Two-dimensional nonlinear infrared (2DIR) spectroscopy suggests that aggregates adopt a conformation resembling the β-sheet secondary structure. The shell thickness was evaluated with localized surface plasmon resonance spectroscopy and X-ray photoelectron spectroscopy. The amount of glutathione on the surface was obtained with spectrophotometry of a thiol-reactive probe. Our results suggest that the shell consists of ∼15 stacked molecular layers. These values correspond to the inter-sheet distances, which are significantly shorter than those in amyloid fibrils with relatively bulky side chains, but are comparable to glycine-rich silk fibrils, where the side chains are compact. The tight packing of the glutathione layers can be facilitated by hydrogen-bonded carboxylic acid dimers of glycine and the intermolecular salt bridges between the zwitterionic γ-glutamyl groups. The structure of the glutathione aggregates was studied by 2DIR spectroscopy of the amide-I vibrational modes using ¹³C isotope labeling of the cysteine carbonyl. Isotope dilution experiments revealed the coupling of modes forming vibrational excitons along the cysteine chain. The coupling along the γ-glutamyl exciton chain was estimated from these values. The obtained coupling strengths are slightly lower than those of native β-sheets, yet they appear large enough to point onto an ordered conformation of the peptides within the aggregate. Analysis of the excitons' anharmonicities and the strength of the transition dipole moments generally is in agreement with these observations.

Experimental and Computational Study on the Selective Interaction of Functionalized Gold Nanoparticles with Metal Ions: Sensing Prospects

Herein, we have developed citrate-, glutathione-, and ascorbate-functionalized gold nanoparticles (AuNPs) to examine their interactions with diverse heavy metal ions, such as Cd, Mn, Cr, Fe, Co, Pb, Hg, Zn, and Ti. These interactions are crucial in defining the final outcome of AuNP-based sensing/removal of heavy metals. We have evaluated these interactions by analyzing the variations in the color and spectroscopic signals of functionalized AuNPs. Additionally, the obtained results were also compared and validated with the computational studies. It has been observed that citrate-AuNPs and GSH-AuNPs displayed high selectivity toward Cr and Mn with E_force values of -23.4 and -14.0 kJ/mol, respectively. Likewise, the ascorbate-AuNPs displayed sensitivity for multiple ions, for example, Cd, Fe, and Mn, with an E_force value of -19.6 kJ/mol. A detailed analysis focusing on the electrostatic charges, ionic sizes, and interaction energy values has been provided to show specific interactions between functionalized AuNPs and heavy metal ions. The respective mechanisms of interaction between heavy metal ions and functionalized AuNPs have been explored with the help of experimental and computational outcomes.

Highly sensitive determination of Co(II) ions in solutions by using modified silver nanoparticles

In this manuscript, we report the development of a rapid and facile optic sensor for highly sensitive and selective detection of cobalt ions (Co(II)). The detection strategy was based on the specific silver nanoparticle-glutathione interaction and later on secondary interaction of this structure with cysteine and Co(II) ion. The interaction of these structures creates a new absorption band in the UV region. The intensity change of this band can be correlated by Co(II) ion concentration. The addition of cysteine decreases the Surface Plasmon Resonance (SPR) of silver nanoparticles but does not provide quantitative information. The proposed method offers the advantage of improved sensitivity for detection of Co(II) ions in a very short time period. Co(II) ions create a unique absorption peak during the complex formation and this peak provides sensitive determination of this metal ion in existence of other metal ions. The peak is visible only after taking the first derivative of absorption spectra. Under the optimized conditions, the detection limit of the method is around 0.68 μM. In addition, the synthesized silver nanoparticles (AgNPs) were characterized by atomic force microscopy (AFM). The proposed metal ion sensor provides a very facile and convenient way to determine the concentration of Co(II) ions in aqueous system.

Self-assembling two-dimensional nanophotonic arrays for reflectivity-based sensing

We propose a nanoplasmonic platform that can be used for sensing trace levels of heavy metals in solutions via simple optical reflectivity measurements. The considered example is a lead sensor, which relies on the lead-mediated assembly of glutathione-functionalized gold nanoparticles (NPs) at a self-healing water/DCE liquid | liquid interface (LLI). Capillary forces tend to trap each NP at the LLI while the negatively charged ligands prevent the NPs settling too close to each other. In the presence of lead, due to chelation between the lead ion and glutathione ligand, the NPs assemble into a dense quasi-2D interfacial array. Such a dense assembly of plasmonic NPs can generate a remarkable broad-band reflectance signal, which is absent when NPs are adsorbed at the interface far apart from each other. The condensing effect of the LLI and the plasmonic coupling effect among the NP array gives rise to a dramatic enhancement of the reflectivity signals. Importantly, we show that our theory of the optical reflectivity from such an array of NPs works in perfect harmony with the physics and chemistry of the system with the key parameter being the interparticle distance at the interface. As a lead sensor, the system is fast, stable, and can achieve detection limits down to 14 ppb. Future alternative recognizing ligands can be used to build sister platforms for detecting other heavy metals.

We propose a nanoplasmonic platform that can be used for sensing trace levels of heavy metals in solutions via simple optical reflectivity measurements at the liquid–liquid interface.

Dual pH-Induced Reversible Self-Assembly of Gold Nanoparticles by Surface Functionalization with Zwitterionic Ligands

The dual pH-induced reversible self-assembly (PIRSA) of Au-nanoparticles (Au NPs) is reported, based on their decoration with the self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion (GCPZ). The assembly of such functionalized Au NPs is found at neutral pH, based on supramolecular pairing of the GCPZ groups. The resulting self-assembled system can be switched back to the disassembled state by addition of base or acid. Two predominant effects that contribute to the dual-PIRSA of Au NPs are identified, namely the ionic hydrogen bonding between the GCPZ groups, but also a strong hydrophobic effect. The contribution of each interaction is depending on the concentration of GCPZ on NPs, which allows to control the self-assembly state over a wide range of different water/solvent ratios.

Sensitive colorimetric detection of Pb2+ by geometric field amplification and surface plasmon resonance visualization

Pb²⁺ is one of the major environmental pollutants, which can be visually detected by surface plasmon resonance of nanoparticles. Paper based analytical device, as a newly developed microfluidic detection platform, is featured in cost-effective and suitable for on-site analysis. In this paper, a sensitive and portable detection method for Pb²⁺ was proposed, in which Pb²⁺ was electrokinetically stacked on the paper fluidic channel by geometric field amplification effect and visualized online by glutathione-modified silver nanoparticles. Colorimetric quantification of the visualized stacking band was conducted by smart phone camera. To avoid unfavorable influence from pH change on the surface plasmon resonance visualization, field amplification effect was introduced by geometric design of the paper fluidic channel. The enriched Pb²⁺ was clearly visible on the paper substrate, and the stacking band intensity was about four orders of magnitude enhanced, comparing to the intensity without stacking. A linear response to Pb²⁺ was observed in the range of 0.3-7.0 μM (R² = 0.997) with a limit of detection of 86 nM and a limit of quantity of 0.28 μM. The established method was used in the detection of Pb²⁺ from river and lake water samples, and the results were confirmed by atomic absorption spectroscopy method.

Ag2S-Glutathione quantum dots for NIR image guided photothermal therapy

Synthesis of ultrasmall, colloidally stable, biocompatible Ag₂S-gluthatione quantum dots for NIR image guided-long wavelength photothermal therapy agents.

Gold Nanocluster-Mediated Efficient Delivery of Cas9 Protein through pH-Induced Assembly-Disassembly for Inactivation of Virus Oncogenes

The CRISPR/Cas gene editing system has been successfully applied to combating bacteria, cancer, virus, and genetic disorders. While viral vectors have been used for the delivery of the CRISPR/Cas9 system, the time required for insert cloning, and virus packaging and standardization, hinders its efficient use. Additionally, the high molecular weight of the Cas9 endonuclease makes it not easy for packing into the vehicles. Herein we report the self-assembly of gold nanoclusters (AuNCs) with SpCas9 protein (SpCas9-AuNCs) under physiological conditions and the efficient delivery of SpCas9 into the cell nucleus. This assembly process is highly dependent on pH. SpCas9-AuNCs are stable at a higher pH but are disassembled at a lower pH. Significantly, this assembly-disassembly process facilitates the delivery of SpCas9 into cells and the cell nucleus, where the SpCas9 exerts its cleavage function. As a proof-of-concept, the assembled SpCas9-AuNCs nanoparticles are successfully used for efficient knockout of the E6 oncogene, restoring the function of tumor-suppressive protein p53 and inducing apoptosis in cervical cancer cells with little effect on normal human cells. The SpCas9-AuNCs are useful for sgRNA functional validation, sgRNA library screening, and genomic manipulation.

Light-induced reversible hydrophobization of cationic gold nanoparticles via electrostatic adsorption of a photoacid

The ability to switch the hydrophilicity/hydrophobicity of nanoparticles promises great potential for applications. Here we report a generic approach that allows hydrophobization of cationic surfaces by light-induced photoacid switching from the unbound zwitterionic form to the electrostatically bound anionic form. Importantly, this allows reversible assembly and disassembly of cationic AuNPs, with disassembly kinetics controlled by temperature. The AuNPs can be repeatedly transferred between aqueous and non-polar solvents using light, showing potential in purification processes. In the macroscopic scale, nontrivially, light triggers the in situ hydrophobization of a flat cationized gold surface. The current approach is generic and opens up a new way to control the surface properties and self-assembly of nanoparticles.

Two-Dimensional Infrared Spectroscopy Reveals Molecular Self-Assembly on the Surface of Silver Nanoparticles

The conformation of molecules, peptides, and proteins, self-assembled into structured monolayers on the surface of metal nanoparticles (NPs), can strongly affect their properties and use in chemical or nanobiomedical applications. Elucidating molecular conformations on the NP surface is highly challenging, and the microscopic details mostly remain elusive. Using polarization-selective third-order two-dimensional ultrafast infrared spectroscopy, we revealed the highly ordered intermolecular structure of γ-tripeptide glutathione on the surface of silver NPs in aqueous solution. Glutathione is an antioxidant thiol abundant in living cells; it is extensively used in NP chemistry and related research. We identified conditions where the interaction of glutathione with the NP surface facilitates formation of a β-sheet-like structure enclosing the NPs. A spectroscopic signature associated with the assembly of β-sheets into an amyloid fibril-like structure was also observed. Remarkably, the interaction with the metal surface promotes formation of a fibril-like structure by a small peptide involving only two amino acids.

Progress in rapid optical assays for heavy metal ions based on the use of nanoparticles and receptor molecules

This review (with 230 refs.) covers recent progress in rapid optical assays for heavy metals (primarily lead and mercury as the most relevant) based on the use of nanoparticles and receptor molecules. An introduction surveys the importance, regulatory demands (such as maximum permissible concentrations) and potential and limitations of various existing methods. This is followed by a general discussion on the use of nanoparticles in optical assays of heavy metals (including properties, basic mechanisms of signal generation). The next sections cover methods for the functionalization of nanoparticles with (a) sulfur-containing compounds (used for modification of nanoparticles or added to the reaction medium), (b) nitrogen-containing compounds (such as amino acids, polypeptides, and heterocyclic molecules), and (c) oxygen-containing species (such as hydroxy and carbonyl compounds). This is continued by a specific description of specific assays based on the use of aptamers as receptors, on the use of deoxyribozymes as synthetic reaction catalysts, of G-quadruplex aptamers, of aptamers in logic gate-type of assays of linear (unstructured) aptamers ("hairpins"), and on the use of aptamers in lateral flow assays. A next section covers assays based on the employment of antibodies as receptors (used in the immunoassay development). The properties of various nanoparticles and their applicability in optical assays are also discussed in some detail. Final sections discuss the selectivity of assays, potential interferences by other cations, methods for their elimination, and also matrix effects and approaches for sample pretreatment. A concluding section discusses current challenges and future trends. Analysis based on enzyme inhibition assay is not treated here but enzyme-like action of some receptor molecules such as DNAzymes is discussed. Graphical abstract Schematic presentation of main principles of application of various nanoparticles with receptor molecules (S-, N-, O-containing, heterocyclic compounds, proteins, antibody, aptamers) for heavy metals ions detection. The included methods cover optical assays with description of mechanisms of interactions and signal generation.

A Colorimetric Sensor for the Visual Detection of Azodicarbonamide in Flour Based on Azodicarbonamide-Induced Anti-Aggregation of Gold Nanoparticles

Azodicarbonamide (ADA) in flour products can be converted into carcinogenic biurea and semicarbazide hydrochloride after baking. Thus, it is mandatory to determine ADA in flour. We herein developed a colorimetric method for the rapid and visual detection of ADA in flour based on glutathione (GSH)-induced gold nanoparticles (AuNPs) aggregation and specific reaction between ADA and GSH. The GSH can react to AuNPs via Au-SH covalent bond to form a network structure, which leads to AuNPs aggregation to produce color change, whereas ADA can specifically react with GSH to lead to the coupling of two GSH molecules, which makes GSH lose a -SH group and thus decreases the aggregation degree of AuNPs induced by GSH. This provided a platform for field-portable colorimetric detection of ADA. The colorimetric sensor can be used to detect as little as 0.33 μM (38.3 ppb) of ADA by naked eye observation and 0.23 μM (26.7 ppb) of ADA by spectrophotometry within 2 h. The method was successfully used to detect ADA in flour with a recovery of 91-104% and a relative standard deviation (RSD) < 6%. The visual detection limit of sensor is lower than the ADA limitation in flour (45 mg/kg), which makes the sensor a potential approach for the instrument-free visual and on-site detection of ADA in flour.

Apolipoprotein E3-mediated cellular uptake of reconstituted high-density lipoprotein bearing core 3, 10, or 17 nm hydrophobic gold nanoparticles

We have developed a high-density lipoprotein (HDL)-based platform for transport and delivery of hydrophobic gold nanoparticles (AuNPs). The ability of apolipoprotein E3 (apoE3) to act as a high-affinity ligand for the low-density lipoprotein receptor (LDLr) was exploited to gain entry of HDL with AuNPs into glioblastoma cells. AuNPs of 3, 10, and 17 nm diameter, the latter two synthesized by phase transfer process, were solubilized by integration with phospholipids and apoE3, yielding reconstituted HDL (rHDL) bearing AuNPs. Ultraviolet–visible spectra of rHDL-AuNP indicated the presence of stable particles with surface plasmon band at ~530 nm. Transmission electron microscopy (TEM) of rHDL-AuNP revealed roughly spherical particles with AuNPs embedded in the core. The rHDL-AuNP particles displayed robust binding to the LDLr and were internalized by receptor-mediated endocytosis in glioblastoma cells. Confocal microscopy confirmed cellular uptake of AuNPs in the endosomal–lysosomal compartments, while TEM revealed intracellular aggregated AuNPs. Cell viability assay demonstrated that >85% of cells were viable with rHDL-AuNP treatment of 0.1–100 μg/mL for 24 hours. These findings are significant since they offer an effective means of delivering AuNPs across the cell membrane, which is particularly relevant in tumor cells that overexpress LDLr.

Reversible Self-Assembly of Glutathione-Coated Gold Nanoparticle Clusters via pH-Tunable Interactions

Nanoparticle (NP) clusters with diameters ranging from 20 to 100 nm are reversibly assembled from 5 nm gold (Au) primary particles coated with glutathione (GSH) in aqueous solution as a function of pH in the range of 5.4 to 3.8. As the pH is lowered, the GSH surface ligands become partially zwitterionic and form interparticle hydrogen bonds that drive the self-limited assembly of metastable clusters in <1 min. Whereas clusters up to 20 nm in size are stable against cluster-cluster aggregation for up to 1 day, clusters up to 80 nm in size can be stabilized over this period via the addition of citrate to the solution in equal molarity with GSH molecules. The cluster diameter may be cycled reversibly by tuning pH to manipulate the colloidal interactions; however, modest background cluster-cluster aggregation occurs during cycling. Cluster sizes can be stabilized for at least 1 month via the addition of PEG-thiol as a grafted steric stabilizer, where PEG-grafted clusters dissociate back to starting primary NPs at pH 7 in fewer than 3 days. Whereas the presence of excess citrate has little effect on the initial size of the metastable clusters, it is necessary for both the cycling and dissociation to mediate the GSH-GSH hydrogen bonds. In summary, these metastable clusters exhibit significant characteristics of equilibrium self-limited assembly between primary particles and clusters on time scales where cluster-cluster aggregation is not present.

Transformation of CuO Nanoparticles in the Aquatic Environment: Influence of pH, Electrolytes and Natural Organic Matter

Many studies have shown the effect of solution chemistry on the environmental behavior of metal-based nanoparticles (NPs), except CuO NPs. Here, we investigated the agglomeration, sedimentation, dissolution, and speciation of CuO NPs by varying pH, ionic strength, ionic valence, and natural organic matter (NOM). The results showed that as the pH moved away from 6, the size of CuO agglomerates decreased, along with the enhanced NP suspension stabilization, due to the increase of electrostatic repulsive force. Increasing ionic strength and valence intensified the agglomeration and sedimentation of CuO NPs because of the compression of electrical double layers. The presence of humic acid and citric acid enhanced the dispersion and stabilization of CuO NP suspension, but l-cysteine showed a different impact. Decreasing pH, increasing ionic strength and all NOM improved the dissolution of CuO NPs, but the divalent electrolyte (CaCl₂) inhibited the Cu²⁺ release from CuO NPs compared to the monovalent electrolyte (NaCl). In addition, X-ray absorption near edge structure (XANES) analysis demonstrated that the presence of l-cysteine transformed more than 30% of CuO NPs to Cu(I)-cysteine by coordinating with thiol group. This study can give us an in-depth understanding on the environmental behavior and fate of CuO NPs in the aquatic environment.

Visual Detection of Denatured Glutathione Peptides: A Facile Method to Visibly Detect Heat Stressed Biomolecules

Every year pharmaceutical companies use significant resources to mitigate aggregation of pharmaceutical drug products. Specifically, peptides and proteins that have been denatured or degraded can lead to adverse patient reactions such as undesired immune responses. Current methods to detect aggregation of biological molecules are limited to costly and time consuming processes such as high pressure liquid chromatography, ultrahigh pressure liquid chromatography and SDS-PAGE gels. Aggregation of pharmaceutical drug products can occur during manufacturing, processing, packaging, shipment and storage. Therefore, a facile in solution detection method was evaluated to visually detect denatured glutathione peptides, utilizing gold nanoparticle aggregation via 3-Aminopropyltreithoxysilane. Glutathione was denatured using a 70 °C water bath to create an accelerated heat stressed environment. The peptide, gold nanoparticle and aminosilane solution was then characterized via, UV-Vis spectroscopy, FTIR spectroscopy, dynamic light scattering and scanning electron microscopy. Captured images and resulting absorbance spectra of the gold nanoparticle, glutathione, and aminosilane complex demonstrated visual color changes detectable with the human eye as a function of the denaturation time. This work serves as an extended proof of concept for fast in solution detection methods for glutathione peptides that have experienced heat stress.

Control of Primary Particle Spacing in Gold Nanoparticle Clusters for Both High NIR Extinction and Full Reversibility

Reversible NIR-active nanoparticle clusters with controlled size from 20 to 100 nm were assembled from 5 nm gold nanoparticles (Au NP), with either citrate (CIT) or various binary ligands on the surface, by tuning the electrostatic repulsion and the hydrogen bonding via pH. The nanoclusters were bound together by vdW forces between the cores and the hydrogen bonds between the surface ligands and dissociated to primary nanoparticles over a period of 20 days at pH 5 and at pH 7. When high levels of citrate ligands were used on the primary particle surfaces, the large particle spacings in the nanoclusters led to only modest NIR extinction. However, a NIR extinction (E_1000/525) ratio of up to ∼0.4 was obtained for nanoclusters with binary ligand mixtures composed of citrate and either cysteine (CYS), glutathione (GSH), or thioctic acid zwitterion (TAZ) while maintaining full reversibility to primary particles. The optimum ligand ratio for both an E_1000/525 of ∼0.4 and full reversibility decreased with increasing length of the secondary ligand (1.5/1 for CYS/CIT, 0.75/1 for GSH/CIT, and 0.5/1 for TAZ/CIT) because a longer secondary ligand maintains a sufficient interparticle spacing required for dissociation more effectively. Interestingly, the zeta potential and the first-order rate constant for nanocluster dissociation were similar for all three systems at the optimum ligand ratios. After incubation in 10 mM GSH solution (intracellular concentration), only the TAZ/CIT primary nanoparticles were resistant to protein opsonization in 100% fetal bovine serum, as the bidentate binding and zwitterion tips of TAZ resisted GSH exchange and protein opsonization, respectively.

Nanoparticle-based paper sensor for thiols evaluation in human skin

A new sensitive non-invasive gold nanoparticle-based sensor that enables to detect thiols in the human skin has been developed. The detection procedure implied the assessment of the color change of a paper sensor resulting from aggregation of gold nanoparticles caused by thiols. The ratio of the intensity of the photo image blue channel vs the red one (in units of RGB coloration) served as analytical response. The main thiol in the skin is glutathione, therefore, it was used as model biothiol and spiking substance. The range of linearity for glutathione was 8-75µM, the detection limit was 6.9µM. RSD≤7% is for inter-day determination of 10μM glutathione and RSD≤12% is the intra-day value. The recovery of 5µM and 10µM of glutathione was evaluated by applying solution, containing thiol-spikes, on skin. The results varied in the range 77-138%. A hundred-fold excess of serine, alanine, histidine, threonine, creatinine, urea, and ammonia; a ten-fold excess of glycine, proline, leucine, isoleucine, phenylalanine, asparagine; and a five-fold excess of valine, tryptophan, tyrosine, and uric acid, which can be extracted from the skin and is contained in the test matrix, have no significant effect on 10µM glutathione signal. Thiols level in the skin of volunteers (21-65 years old, men and women) detected with the use of a proposed non-invasive sensor was 11.6-47.5µM.

Self-assembled AuNPs on sulphur-doped graphene: a dual and highly efficient electrochemical sensor for nitrite (NO2−) and nitric oxide (NO)

Gold nanoparticles self-assembled over sulphur-doped graphene as a reusable electrocatalyst for selective and sensitive quantification of NO₂⁻ and NO.

Specific pH effect for selective colorimetric assay of glutathione using anti-aggregation of label-free gold nanoparticles

An operationally simple colorimetric method for measuring glutathione (GSH) concentration was developed using anti-aggregation of gold nanoparticles (AuNPs) in this work.

New Insight into a Cancer Theranostic Probe: Efficient Cell-Specific Delivery of SN-38 Guided by Biotinylated Poly(vinyl alcohol)

An optically modulated "turn-on" theranostic prodrug TP1 has been explored and formulated with biotinylated poly(vinyl alcohol) (biotinPVA) to obtain desired pharmacokinetics. TP1, consisting of the antineoplastic camptothecin analogue SN-38, and the fluorescent dye rhodol green have been covalently conjugated through a disulfide bond. Glutathione triggering the release of drug and fluorophore has been well established by UV-vis measurements through mass spectral analysis in physiological conditions. The biocompatible biotinPVA formulated prodrug (PTP1) showed remarkably higher stability against blood serum and cell-specific activation in contrast to that of TP1. Significantly, PTP1 permits monitoring of the delivery and release of well-known topoisomerase I inhibitor SN-38 by modulating fluorescence signal at λ_em 550 nm within intracellular milieus. Moreover, theranostic probe PTP1 exhibited dose-dependent antiproliferative activity against receptor-positive HeLa cells, whereas it did not show such an effect against receptor-negative NIH3T3 cells. Finally, the cell-specific antiproliferative activity of PTP1 via the apoptotic pathway is an efficient approach in cancer theranostics. Thus, futuristic PTP1 could be a promising agent in which diagnostic and prognostic data will be monitored synergistically.

A colorimetric detection of acrylamide in potato chips based on nucleophile-initiated thiol–ene Michael addition

A visible, highly sensitive colorimetric biosensor based on thiol–ene Michael addition reaction was first reported to detect acrylamide (AA) in potato chips.

Gold nanoparticles-immobilized, hierarchically ordered, porous TiO2 nanotubes for biosensing of glutathione

Glutathione (GSH) is vital for several functions of our human body such as neutralization of free radicals and reactive oxygen compounds, maintaining the active forms of vitamin C and E, regulation of nitric oxide cycle, iron metabolism, etc. It is also an endogenous antioxidant in most of the biological reactions. Given the importance of GSH, a simple strategy is proposed in this work to develop a biosensor for quantitative detection of GSH. This particular biosensor comprises of gold nanoparticles (Au NPs)-immobilized, hierarchically ordered titanium dioxide (TiO₂) porous nanotubes. Hexagonally arranged, honeycomb-like nanoporous tubular TiO₂ electrodes are prepared by using a simple electrochemical anodization process by applying a constant potential of 30 V for 24 hours using ethylene glycol consisting of ammonium fluoride as an electrolytic medium. Structural morphology and crystalline nature of such TiO₂ nanotubes are analyzed using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Interestingly, nanocomposites of TiO₂ with Au NPs is prepared in an effort to alter the intrinsic properties of TiO₂, especially tuning of its band gap. Au NPs are prepared by a well-known Brust and Schiffrin method and are immobilized onto TiO₂ electrodes which act as a perfect electrochemical sensing platform for GSH detection. Structural characterization and analysis of these modified electrodes are performed using FESEM, XRD, and UV-visible spectroscopic studies. GSH binding events on Au NPs-immobilized porous TiO₂ electrodes are monitored by electrochemical techniques, namely, cyclic voltammetry (CV) and chronoamperometry (CA). Several parameters such as sensitivity, selectivity, stability, limit of detection, etc are investigated. In addition, Au NPs dispersed in aqueous medium are also explored for naked-eye detection of GSH using UV-visible spectroscopy in order to compare the performance of the proposed sensor. Our studies clearly indicate that these materials could potentially be used for GSH sensing applications.

Interaction of gold nanoparticles mediated by captopril and S-nitrosocaptopril: the effect of manganese ions in mild acid medium

We report herein results regarding reactivity and assembly of citrate-capped gold nanoparticles (AuNPs) mediated by captopril (cap) and S-nitrosocaptopril (NOcap), two angiotensin converting enzyme inhibitors and antihypertensive agents. The results were compared with that of cysteine (Cys), a thiol-containing amino acid found in plasma. The interparticle interactions were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method. The original gold nanoparticles were efficiently modified by small amounts of Mn(+2) ions, which are adsorbed onto the surface of 15.4 nm citrate-capped gold nanoparticles, giving rise to manganese-gold nanoparticles (Mn-AuNPs) that, in mild acid medium, have proved to be highly sensitive and a rapid colorimetric detection method for thiols. Depending on the concentration of the Mn(+2) ions the aggregation of AuNPs can be rapidly induced. The kinetics of the assembly process has been studied. Good first-order kinetics has been observed, with the exception of captopril-mediated nanoparticle aggregation at low concentration of either cap or acid. The rate of Cys-mediated assembly of gold nanoparticles in aqueous 10 mM acetic acid is more than 20-times faster than pure AuNPs and concentrations of Cys as low as 34 nM can be detected in less than 40 min under conditions of stable Mn-AuNPs. Similar effects were observed with cap or NOcap. The assembly-disassembly reversibility is shown with cap and NOcap and depends highly on pH.

Stability, cytotoxicity and cell uptake of water-soluble dendron-conjugated gold nanoparticles with 3, 12 and 17 nm cores

This article describes the synthesis of water-soluble dendron-conjugated gold nanoparticles (Den-AuNPs) with various average core sizes and the evaluation of stability, cytotoxicity, cell permeability and uptake of these materials. The characterization of Den-AuNPs using various techniques including transmission electron microscopy (TEM), matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS), 1H NMR, FT-IR, and UV-vis spectroscopy confirms the dendron conjugation to the glutathione-capped gold nanoparticles (AuNPs). The stability of AuNPs and Den-AuNPs in solutions of different pH and salt concentration is determined by monitoring the changes in surface plasmon bands of gold using UV-vis spectroscopy. The stability of Den-AuNPs at different pH remained about the same compared to that of AuNPs. In comparison, the Den-AuNPs are found to be more stable than the precursor AuNPs maintaining their solubility in the aqueous solution with the salt concentration of up to 100 mM. The improved stability of Den-AuNPs suggests that the post-functionalization of thiol-capped gold nanoparticle surfaces with dendrons can further improve the physiological stability and biocompatibility of gold nanoparticle-based materials. Cytotoxicity studies of AuNPs and Den-AuNPs with and without fluorophores are also performed by examining cell viability for 3T3 fibroblasts using a MTT cell proliferation assay. The conjugation of dendrons to the AuNPs with a fluorophore is able to decrease the cytotoxicity brought about by the fluorophore. The successful uptake of Den-AuNPs in mouse fibroblast 3T3 cells shows the physiological viability of the hybrid materials.

Highly sensitive colorimetric determination of amoxicillin in pharmaceutical formulations based on induced aggregation of gold nanoparticles

A novel, simple and highly sensitive colorimetric method is developed for determination of Amoxicillin (AMX). The system is based on aggregation of citrate-capped gold nanoparticles (AuNP) in acetate buffer (pH=4.5) in the presence of the degradation product of Amoxicillin (DPAMX). It was found that the color of gold nanoparticles changed from red to purple and the intensity of surface plasmon resonance (SPR) peak of AuNPs decreased. A new absorption band was appeared in the wavelength range of 600-700nm upon addition of DPAMX. The absorbance ratio at the wavelength of 660 and 525nm (A660/A525) was chosen as the analytical signal indirectly related to AMX concentration. The linearity of the calibration graph was found over the concentration range of 0.3-4.5μM AMX with a correlation coefficient of 0.9967. Under the optimum experimental conditions, the detection limit was found to be 0.15μM. The applicability of the method was successfully demonstrated by analysis of AMX in pharmaceutical formulations including capsules and oral suspensions.

Highly responsive glutathione functionalized green AuNP probe for precise colorimetric detection of Cd2+ contamination in the environment

Precise colorimetric detection of Cd²⁺ using a glutathione functionalized phytosynthesized AuNP probe provides an ecofriendly approach to heavy metal detection.