Controlling supramolecular complex formation on the surface of a monolayer-protected gold nanoparticle in water

Exploiting multivalency and cooperativity of gold nanoparticles for binding phosphatidylinositol (3,4,5)-trisphosphate at sub-nanomolar concentrations

Cationic, monolayer-protected gold nanoparticles provide a multivalent charged surface and a hydrophobic monolayer that synergistically contribute to the binding of phosphatidylinositol (3,4,5)-trisphosphate, a relevant biomarker. The observed dissociation constant is in the picomolar region, providing the possibility of using these gold nanoparticles for the selective extraction of this molecule from biological fluids.

Data-informed discovery of hydrolytic nanozymes

Nanozyme is a collection of nanomaterials with enzyme-like activity but higher environmental tolerance and long-term stability than their natural counterparts. Improving the catalytic activity and expanding the category of nanozymes are prerequisites to complement or even supersede enzymes. However, the development of hydrolytic nanozymes is still challenged by diverse hydrolytic substrates and following complicated mechanisms. Here, two strategies are informed by data to screen and predict catalytic active sites of MOF (metal-organic framework) based hydrolytic nanozymes: (1) to increase the intrinsic activity by finely tuned Lewis acidity of the metal clusters; (2) to improve the density of active sites by shortening the length of ligands. Finally, as-obtained Ce-FMA-MOF-based hydrolytic nanozyme is capable of cleaving phosphate bonds, amide bonds, glycosidic bonds, and even their mixture, biofilms. This work provides a rational methodology to design hydrolytic nanozyme, enriches the diversity of nanozymes, and potentially sheds light on future evolution of enzyme engineering.

An asymmetric Schiff base-functionalized gold nanoparticle-based colorimetric sensor for Hg2+ ion determination: experimental and DFT studies

We report a colorimetric sensor for the detection of Hg²⁺ ions utilizing surface-modified gold nanoparticles. Gold nanoparticles (GNPs) were synthesized by direct reduction and were subsequently functionalized using Schiff base ligands. Schiff base ligands as electron transfer agents have been frequently used for the determination of heavy metal ions. From the spectroscopic analysis, it was found that the mechanism could be defined as coordination between azomethine nitrogen and the carbonyl oxygen of the ligand with Hg²⁺ ions. The affinity of Hg²⁺ ions towards the bidentate Schiff base on the GNPs result from their self-aggregation and investigated to be a powerful asset for the development of Hg²⁺ ion-selective sensors, which is accompanied by a visible color change from pink to purple or can be detect by UV-Vis spectroscopy. The optimized structures and binding mechanisms were supported with a high correlation and agreement via spectroscopy and DFT calculations. These simple colorimetric tests can be extended for the rapid pre-screening of a wide variety of heavy metal ions for onsite detection and mitigation.

Probing Multiscale Factors Affecting the Reactivity of Nanoparticle-Bound Molecules

The structures and physicochemical properties of surface-stabilizing molecules play a critical role in defining the properties, interactions, and functionality of hybrid nanomaterials such as monolayer-stabilized nanoparticles. Concurrently, the distinct surface-bound interfacial environment imposes very specific conditions on molecular reactivity and behavior in this setting. Our ability to probe hybrid nanoscale systems experimentally remains limited, yet understanding the consequences of surface confinement on molecular reactivity is crucial for enabling predictive nanoparticle synthon approaches for postsynthesis engineering of nanoparticle surface chemistry and construction of devices and materials from nanoparticle components. Here, we have undertaken an integrated experimental and computational study of the reaction kinetics for nanoparticle-bound hydrazones, which provide a prototypical platform for understanding chemical reactivity in a nanoconfined setting. Systematic variation of just one molecular-scale structural parameter-the distance between reactive site and nanoparticle surface-showed that the surface-bound reactivity is influenced by multiscale effects. Nanoparticle-bound reactions were tracked in situ using ¹⁹F NMR spectroscopy, allowing direct comparison to the reactions of analogous substrates in bulk solution. The surface-confined reactions proceed more slowly than their solution-phase counterparts, and kinetic inhibition becomes more significant for reactive sites positioned closer to the nanoparticle surface. Molecular dynamics simulations allowed us to identify distinct supramolecular architectures and unexpected dynamic features of the surface-bound molecules that underpin the experimentally observed trends in reactivity. This study allows us to draw general conclusions regarding interlinked structural and dynamical features across several length scales that influence interfacial reactivity in monolayer-confined environments.

A simple method to estimate the mean number of lipophilic molecules on nanoparticle surfaces by fluorescence measurements

Measurements of fluorescence intensity of the hydrophobic pyridinium salt (DTPSH) remaining in the organic phase after partition experiments in the DCM/H₂O system allowed an approximate method to be developed to estimate the mean number of molecules (N = 942) on the surface of 22.8 nm gold nanoparticles and the separation (1.89 nm) between these organic molecules. This protocol is based on the ability that the organic molecules possess to coat the surface of the nanoparticle, which can migrate from the organic to the aqueous phase as a result of the driving force of the strong binding of sulfur to gold. To validate our estimation, we used a projection of the results obtained by Wales and Ulker to solve the Thomson problem, a mathematicians' challenge, used as a model to calculate the mean distance (1.82 nm) separating particles on the surface, in excellent agreement with the results obtained by our method. The quality of results, the simplicity of calculations, the low fluorescence detection limit, and the inexpensive materials, recommend this procedure for rapid estimates of the mean number of molecules on the surface of nanoparticles.

Nanoparticle-assisted NMR spectroscopy: A chemosensing perspective

Sensing methodologies for the detection of target compounds in mixtures are important in many different contexts, ranging from medical diagnosis to environmental analysis and quality assessment. Ideally, such detection methods should allow for both identification and quantification of the targets, minimizing the possibility of false positives. With very few exceptions, most of the available sensing techniques rely on the selective interaction of the analyte with some detector, which in turn produces a signal as a result of the interaction. This approach hence provides indirect information on the targets, whose identity is generally ensured by comparison with known standards, if available, or by the selectivity of the sensor system itself. Pursuing a different approach, NMR chemosensing aims at generating signals directly from the analytes, in the form of a (complete) NMR spectrum. In this way, not only are the targets unequivocally identified, but it also becomes possible to identify and assign the structures of unknown species. In this review we show how relaxation- and diffusion-based NMR techniques, assisted by appropriate nanoparticles, can be used to edit the ¹H NMR spectrum of a mixture and extract the signals of specific target compounds. Monolayer-protected nanoparticles, in particular those made from gold, are well suited to this task because they provide a versatile, protein-size support to build or incorporate supramolecular receptors. Remarkably, the self-organized and multifunctional nature of the nanoparticle coating allows exploitation of different kinds of non-covalent interactions, to provide tailored binding sites for virtually any class of molecules. From the NMR standpoint, the reduced translational and rotational diffusion rates of bulky nanoparticles offer a way to manipulate the states of the monolayer spins and build a reservoir of magnetization that can be selectively transferred to the interacting analytes. In addition, the low correlation time and the enhanced rigidity of the coating molecules (due to their grafting and crowding on the particle surface) promote efficient spin diffusion, useful in saturation transfer experiments. The optimized combination of NMR experiments and nanoreceptors can ultimately allow the detection of relevant analytes in the micromolar concentration range, paving the way to applications in the diagnostic field and beyond.

High affinity heparin detection by multivalent supramolecular polymers through aggregation induced emission

Supramolecular polymers based on aggregation induced emission active cationic cyanostilbenes provide a highly sensitive “light-up” platform for heparin detection.

Molecular-Dynamics-Simulation-Directed Rational Design of Nanoreceptors with Targeted Affinity

Here, we demonstrate the possibility of rationally designing nanoparticle receptors with targeted affinity and selectivity for specific small molecules. We used atomistic molecular-dynamics (MD) simulations to gradually mutate and optimize the chemical structure of the molecules forming the coating monolayer of gold nanoparticles (1.7 nm gold-core size). The MD-directed design resulted in nanoreceptors with a 10-fold improvement in affinity for the target analyte (salicylate) and a 100-fold decrease of the detection limit by NMR-chemosensing from the millimolar to the micromolar range. We could define the exact binding mode, which features prolonged contacts and deep penetration of the guest into the monolayer, as well as a distinct shape of the effective binding pockets characterized by exposed interacting points.

Detection and identification of designer drugs by nanoparticle-based NMR chemosensing

Properly designed monolayer-protected nanoparticles (2 nm core diameter) can be used as nanoreceptors for selective detection and identification of phenethylamine derivatives (designer drugs) in water. The molecular recognition mechanism is driven by the combination of electrostatic and hydrophobic interactions within the coating monolayer. Each nanoparticle can bind up to 30-40 analyte molecules. The affinity constants range from 105 to 106 M-1 and are modulated by the hydrophobicity of the aromatic moiety in the substrate. Detection of drug candidates (such as amphetamines and methamphetamines) is performed by using magnetization (NOE) or saturation (STD) transfer NMR experiments. In this way, the NMR spectrum of the drug is isolated from that of the mixture, allowing broad-class multianalyte detection and even identification of unknowns. The introduction of a dimethylsilane moiety in the coating monolayer allows performing STD experiments in complex mixtures. In this way, a detection limit of 30 μM is reached with standard instruments.

Catalytic signal amplification for the discrimination of ATP and ADP using functionalised gold nanoparticles

Diagnostic assays that incorporate a signal amplification mechanism permit the detection of analytes with enhanced selectivity. Herein, we report a gold nanoparticle-based chemical system able to differentiate ATP from ADP by means of catalytic signal amplification. The discrimination between ATP and ADP is of relevance for the development of universal assays for the detection of enzymes which consume ATP. For example, protein kinases are a class of enzymes critical for the regulation of cellular functions, and act to modulate the activity of other proteins by transphosphorylation, transferring a phosphate group from ATP to give ADP as a byproduct. The system described here exploits the ability of cooperative catalytic head groups on gold nanoparticles to very efficiently catalyze chromogenic reactions such as the transphosphorylation of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNPP). A series of chromogenic substrates have been synthesized and evaluated by means of Michaelis-Menten kinetics (compounds 2, 4-6). 2-Hydroxypropyl-(3-trifluoromethyl-4-nitro)phenyl phosphate (5) was found to display higher reactivity (kcat) and higher binding affinity (KM) when compared to HPNPP. This higher binding affinity allows phosphate 5 to compete with ATP and ADP to different extents for binding on the monolayer surface, thus enabling a catalytically amplified signal only when ATP is absent. Overall, this represents a viable new approach for monitoring the conversion of ATP into ADP with high sensitivity.

Sensor arrays made by self-organized nanoreceptors for detection and discrimination of carboxylate drugs

An array of self-organized nanoreceptors based on monolayer-protected gold nanoparticles in combination with different commercially available fluorescent dyes can detect and discriminate nonsteroidal anti-inflammatory drugs.

Photoswitchable Catalysis by a Nanozyme Mediated by a Light-Sensitive Cofactor

The activity of a gold nanoparticle-based catalyst can be reversibly up- and down-regulated by light. Light is used to switch a small molecule between cis- and trans-isomers, which inhibits the catalytic activity of the nanoparticles to different extent. The system is functional in aqueous buffer, which paves the way for integrating the system in biological networks.

Unsupported Micellar Palladium Nanoparticles for Biphasic Hydrogenation and Isomerization of Hydrophobic Allylic Alcohols in Water

This article presents the evaluation of water-soluble palladium nanoparticles with hydrophobic active sites that are ideal for the biphasic colloidal catalysis of water-insoluble organic substrates in aqueous solution. Palladium nanoparticles stabilized with ω-carboxylate-functionalized alkanethiolate are first synthesized using ω-carboxylate-S-alkylthiosulfate as their ligand precursor. The biphasic catalysis is carried out for the reaction of hydrophobic allylic alcohols without using any additional mixing solvent or surfactant, which results in the complete consumption of substrates under the atmospheric pressure of H₂ gas and at room temperature in less than 24 h. Systematic investigations on the influence of pH and substrate size are also performed to examine the utility of these thiolate-capped palladium nanoparticles as structurally stable and water-soluble micellar catalysts for the biphasic reaction.

Dynamic nanoproteins: self-assembled peptide surfaces on monolayer protected gold nanoparticles

Small peptides self-assemble on gold nanoparticles to form a dynamic multivalent peptide surface.

Transient signal generation in a self-assembled nanosystem fueled by ATP

A fundamental difference exists in the way signal generation is dealt with in natural and synthetic systems. While nature uses the transient activation of signalling pathways to regulate all cellular functions, chemists rely on sensory devices that convert the presence of an analyte into a steady output signal. The development of chemical systems that bear a closer analogy to living ones (that is, require energy for functioning, are transient in nature and operate out-of-equilibrium) requires a paradigm shift in the design of such systems. Here we report a straightforward strategy that enables transient signal generation in a self-assembled system and show that it can be used to mimic key features of natural signalling pathways, which are control over the output signal intensity and decay rate, the concentration-dependent activation of different signalling pathways and the transient downregulation of catalytic activity. Overall, the reported methodology provides temporal control over supramolecular processes.

Natural and synthetic systems have fundamentally different approaches to signal generation. Here, the authors report a strategy that enables transient signal generation in a self-assembled system and show that it can be used to mimic several key features of natural signalling pathways.

Emergence of complex chemistry on an organic monolayer

In many origin-of-life scenarios, inorganic materials, such as FeS or mineral clays, play an important role owing to their ability to concentrate and select small organic molecules on their surface and facilitate their chemical transformations into new molecules. However, considering that life is made up of organic matter, at a certain stage during the evolution the role of the inorganic material must have been taken over by organic molecules. How this exactly happened is unclear, and, indeed, a big gap separates the rudimentary level of organization involving inorganic materials and the complex organization of cells, which are the building blocks of life. Over the past years, we have extensively studied the interaction of small molecules with monolayer-protected gold nanoparticles (Au NPs) for the purpose of developing innovative sensing and catalytic systems. During the course of these studies, we realized that the functional role of this system is very similar to that typically attributed to inorganic surfaces in the early stages of life, with the important being difference that the functional properties (molecular recognition, catalysis, signaling, adaptation) originate entirely from the organic monolayer rather than the inorganic support. This led us to the proposition that this system may serve as a model that illustrates how the important role of inorganic surfaces in dictating chemical processes in the early stages of life may have been taken over by organic matter. Here, we reframe our previously obtained results in the context of the origin-of-life question. The following functional roles of Au NPs will be discussed: the ability to concentrate small molecules and create different local populations, the ability to catalyze the chemical transformation of bound molecules, and, finally, the ability to install rudimentary signaling pathways and display primitive adaptive behavior. In particular, we will show that many of the functional properties of the system originate from two features: the presence of metal ions that are complexed in the organic monolayer and the multivalent nature of the system. Complexed metal ions play an important role in determining the affinity and selectivity of the interaction with small molecules, but serve also as regulatory elements for determining how many molecules are bound simultaneously. Importantly, neighboring metal ion complexes also create catalytic pockets in which two metal ions cooperatively catalyze the cleavage of an RNA-model compound. The multivalent nature of the system permits multiple noncovalent interactions with small molecules that enhances the affinity, but is also at the basis of simple signal transduction pathways and adaptive behavior.

Dynamic combinatorial chemistry on a monolayer protected gold nanoparticle

Here, we show that the addition of Hg(2+) or Ag(+) metal ions to a dynamic system composed of monolayer protected gold nanoparticles (Au NPs) and a mixture of four nucleotides (dGMP, dAMP, TMP, and dCMP) leads to the self-selection of TMP or dGMP, respectively, on the monolayer surface.

Dipeptide recognition in water mediated by mixed monolayer protected gold nanoparticles

Mixed monolayer protected gold nanoparticles with three orthogonal binding sites on their surface were shown to bind dipeptides in water better than analogues containing only one binding site or a combination of two.

Label-free fluorescence detection of kinase activity using a gold nanoparticle based indicator displacement assay

A straightforward indicator-displacement assay (IDA) has been developed for the quantitative analysis of ATP→ADP conversion. The IDA relies on the use of gold nanoparticles passivated with a monolayer of thiols terminating with a 1,4,7-triazacyclononane (TACN)·Zn(2+) head group. The analytes ATP and ADP compete to a different extent with a fluorescent probe for binding to the monolayer surface. In the presence of ATP the fluorescent probe is free in solution, whereas in the presence of ADP the fluorescent probe is captured by the nanoparticles and its fluorescence is quenched. The linear response of the fluorescence signal towards different ratios of ATP : ADP permitted the detection of protein kinase activity simply by adding aliquots of the enzyme solution to the assay solution followed by measurement of the fluorescent intensity. The assay poses no restrictions on the target kinase nor does it require labeling of the kinase substrate. The assay was tested on the protein kinases PIM-1 and Src and validated through a direct comparison with the classical radiometric assay using the [γ-(32)P]-labeled ATP.

Light-triggered thiol-exchange on gold nanoparticles at low micromolar concentrations in water

The place-exchange reaction of thiol-containing peptides in a cationic monolayer on gold nanoparticles occurs very rapidly at low micromolar concentrations in water with excellent control over the degree of substitution. The driving force for this process is the binding of anionic peptides to a cationic monolayer surface which causes a strong increase in the local concentration of thiols. The place-exchange reaction can be triggered by light using a photolabile protecting group on the thiol moiety.

Catalysis of transesterification reactions by a self-assembled nanosystem

Histidine-containing peptides self-assemble on the surface of monolayer protected gold nanoparticles to form a catalytic system for transesterification reactions. Self-assembly is a prerequisite for catalysis, since the isolated peptides do not display catalytic activity by themselves. A series of catalytic peptides and substrates are studied in order to understand the structural parameters that are of relevance to the catalytic efficiency of the system. It is shown that the distance between the His-residue and the anionic tail does not affect the catalytic activity. On the other hand, the catalytic His-residue is sensitive to the chemical nature of the flanking amino acid residues. In particular, the presence of polar Ser-residues causes a significant increase in activity. Finally, kinetic studies of a series of substrates reveal that substrates with a hydrophobic component are very suitable for this catalytic system.