ISOTHERMAL TITRATION CALORIMETRY (ITC) AS A PROMISING TOOL IN PHARMACEUTICAL NANOTECHNOLOGY

Isothermal titration calorimetry (ITC) is a technique for evaluating the thermodynamic profiles of connection between two molecules, allowing the experimental design of nanoparticles systems with drugs and/or biological molecules. Taking into account the relevance of ITC, we conducted, therefore, an integrative revision of the literature, from 2000 to 2023, on the main purposes of using this technique in pharmaceutical nanotechnology. The search were carried out in the Pubmed, Sciencedirect, Web of Science, and Scifinder databases using the descriptors "Nanoparticles", "Isothermal Titration Calorimetry", and "ITC". We have observed that the ITC technique has been increasingly used in pharmaceutical nanotechnology, seeking to understand the interaction mechanisms in the formation of nanoparticles. Additionally, to understand the behavior of nanoparticles with biological materials (proteins, DNA, cell membranes, among others), thereby helping to understand the behavior of nanocarriers in vivo studies. As a contribution, we intended to reveal the importance of ITC in the laboratory routine, which is itself a quick and easy technique to obtain relevant results that help to optimize the nanosystems formulation process.

Active control of equilibrium, near-equilibrium, and far-from-equilibrium colloidal systems

The development of top-down active control over bottom-up colloidal assembly processes has the potential to produce materials, surfaces, and objects with applications in a wide range of fields spanning from computing to materials science to biomedical engineering. In this review, we summarize recent progress in the field using a taxonomy based on how active control is used to guide assembly. We find there are three distinct scenarios: (1) navigating kinetic pathways to reach a desirable equilibrium state, (2) the creation of a desirable metastable, kinetically trapped, or kinetically arrested state, and (3) the creation of a desirable far-from-equilibrium state through continuous energy input. We review seminal works within this framework, provide a summary of important application areas, and present a brief introduction to the fundamental concepts of control theory that are necessary for the soft materials community to understand this literature. In addition, we outline current and potential future applications of actively-controlled colloidal systems, and we highlight important open questions and future directions.

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.

Peptide adsorption on silica surfaces: Simulation and experimental insights

The understanding of interactions between proteins with silica surface is crucial for a wide range of different applications: from medical devices, drug delivery and bioelectronics to biotechnology and downstream processing. We show the application of EISM (Effective Implicit Surface Model) for discovering the set of peptide interactions with silica surface. The EISM is employed for a high-speed computational screening of peptides to model the binding affinity of small peptides to silica surfaces. The simulations are complemented with experimental data of peptides with silica nanoparticles from microscale thermophoresis and from infrared spectroscopy. The experimental work shows excellent agreement with computational results and verifies the EISM model for the prediction of peptide-surface interactions. 57 peptides, with amino acids favorable for adsorption on Silica surface, are screened by EISM model for obtaining results, which are worth to be considered as a guidance for future experimental and theoretical works. This model can be used as a broad platform for multiple challenges at surfaces which can be applied for multiple surfaces and biomolecules beyond silica and peptides.

Rapid Colorimetric pH-Responsive Gold Nanocomposite Hydrogels for Sensing Applications

Surface functionalization of metallic nanoparticles (NPs) with external groups can be engineered to fabricate sensors that are responsive to various stimuli like temperature, pH, and numerous ions. Herein, we report the synthesis of gold nanoparticles (GNPs) functionalized with 3-mercaptopropionic acid (GNPs-MPA) and the doping of these nanoparticles into hydrogel materials using the breathing-in/breathing-out (BI-BO) method. MPA has a carboxyl group that becomes protonated and, thus, ionized at a pH below its pK_a (4.32); hence, the GNPs-MPA solutions and gels were mostly pH-responsive in the range of 3–5. Optical properties were assessed through ultraviolet-visible (UV-Vis) spectroscopy, namely: transmission and absorption, and the parameters used to quantify the pH changes were the full width at half maximum (FWHM) and position of surface plasmon resonance (SPR). The solutions and gels gradually changed their colors from red to indigo with pH decrementation from 5 to 3, respectively. Furthermore, the solutions’ and doped gels’ highest FWHM sensitivities towards pH variations were 20 nm and 55 nm, respectively, while the SPR’s position sensitivities were 18 nm and 10 nm, respectively. Also, transmission and scanning electron microscopy showed synchronized dispersion and aggregation of NPs with pH change in both solution and gel forms. The gel exhibited excellent repeatability and reversibility properties, and its response time was instantaneous, which makes its deployment as a colorimetric pH-triggered sensor practical. To the best of our knowledge, this is the first study that has incorporated GNPs into hydrogels utilizing the BI-BO method and demonstrated the pH-dependent optical and colorimetric properties of the developed nanocomposites.

Hysteresis in the Thermo-Responsive Assembly of Hexa(ethylene glycol) Derivative-Modified Gold Nanodiscs as an Effect of Shape

Anisotropic gold nanodiscs (AuNDs) possess unique properties, such as large flat surfaces and dipolar plasmon modes, which are ideal constituents for the fabrication of plasmonic assemblies for novel and emergent functions. In this report, we present the thermo-responsive assembly and thermo-dynamic behavior of AuNDs functionalized with methyl-hexa(ethylene glycol) undecane-thiol as a thermo-responsive ligand. Upon heating, the temperature stimulus caused a blue shift of the plasmon peak to form a face-to-face assembly of AuNDs due to the strong hydrophobic and van der Waals interactions between their large flat surfaces. Importantly, AuNDs allowed for the incorporation of the carboxylic acid-terminated ligand while maintaining their thermo-responsive assembly ability. With regard to their reversible assembly/disassembly behavior in the thermal cycling process, significant rate-independent hysteresis, which is related to their thermo-dynamics, was observed and was shown to be dependent on the carboxylic acid content of the surface ligands. As AuNDs have not only unique plasmonic properties but also high potential for attachment due to the fact of their flat surfaces, this study paves the way for the exploitation of AuNDs in the development of novel functional materials with a wide range of applications.

How to Use Localized Surface Plasmon for Monitoring the Adsorption of Thiol Molecules on Gold Nanoparticles?

The functionalization of spherical gold nanoparticles (AuNPs) in solution with thiol molecules is essential for further developing their applications. AuNPs exhibit a clear localized surface plasmon resonance (LSPR) at 520 nm in water for 20 nm size nanoparticles, which is extremely sensitive to the local surface chemistry. In this study, we revisit the use of UV-visible spectroscopy for monitoring the LSPR peak and investigate the progressive reaction of thiol molecules on 22 nm gold nanoparticles. FTIR spectroscopy and TEM are used for confirming the nature of ligands and the nanoparticle diameter. Two thiols are studied: 11-mercaptoundecanoic acid (MUDA) and 16-mercaptohexadecanoic acid (MHDA). Surface saturation is detected after adding 20 nmol of thiols into 1.3 × 10⁻³ nmol of AuNPs, corresponding approximately to 15,000 molecules per AuNPs (which is equivalent to 10.0 molecules per nm²). Saturation corresponds to an LSPR shift of 2.7 nm and 3.9 nm for MUDA and MHDA, respectively. This LSPR shift is analyzed with an easy-to-use analytical model that accurately predicts the wavelength shift. The case of dodecanehtiol (DDT) where the LSPR shift is 15.6 nm is also quickly commented. An insight into the kinetics of the functionalization is obtained by monitoring the reaction for a low thiol concentration, and the reaction appears to be completed in less than one hour.

Mechanically Tunable Lattice-Plasmon Resonances by Templated Self-Assembled Superlattices for Multi-Wavelength Surface-Enhanced Raman Spectroscopy

Lattice plasmons, i.e., diffractively coupled localized surface plasmon resonances, occur in long-range ordered plasmonic nanostructures such as 1D and 2D periodic lattices. Such far-field coupled resonances can be employed for ultrasensitive surface-enhanced Raman spectroscopy (SERS), provided they are spectrally matched to the excitation wavelength. The spectral positions of lattice plasmon modes critically depend on the lattice period and uniformity, owing to their pronounced sensitivity to structural disorder. We report the fabrication of superlattices by templated self-assembly of gold nanoparticles on a flexible support, with tunable lattice-plasmon resonances by means of macroscopic strain. We demonstrate that the highest SERS performance is achieved by matching the lattice plasmon mode to the excitation wavelength, by post-assembly fine-tuning of long-range structural parameters. Both asymmetric and symmetric lattice deformations can be used to adapt a single lattice structure to both red-shifted and blue-shifted excitation lines, as exemplified by lattice expansion and contraction, respectively. This proof-of-principle study represents a basis for alternative designs of adaptive functional nanostructures with mechanically tunable lattice resonances using strain as a macroscopic control parameter.

Chiral gold-PPA nanocomposites with tunable helical sense and morphology

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

Stimuli-responsive self-assembly of nanoparticles

Ligand-protected nanoparticles can serve as attractive building blocks for constructing complex chemical systems.

Nanoparticle-based diagnostic and therapeutic systems for brain tumors

Many theranostic nanoparticles have been tailored for high-efficiency diagnostic or therapeutic agents or applied as carriers and might provide new possibilities for brain tumor diagnosis and treatment.

Synthesis and Characterization of Tunable, pH-Responsive Nanoparticle-Microgel Composites for Surface-Enhanced Raman Scattering Detection

The synthesis of microgels with pH-tunable swelling leads to adjustable and pH-responsive substrates for surface-enhanced Raman scattering (SERS)-active nanoparticles (NPs). Sterically stabilized and cross-linked latexes were synthesized from random copolymers of styrene (S) and 2-vinylpyridine (2VP). The pH-dependent latex-to-microgel transition and swellability were tuned based on their hydrophobic-to-hydrophilic content established by the S/2VP ratio. The electrostatic loading of polystyrene/poly(2-vinylpyridine) microgels [PS _x P2VP _y (M)] with anions such as tetrachloroaurate (AuCl₄ ^-) and borate-capped Ag NPs was quantified. The PS _x P2VP _y (M) can load ∼0.3 equiv of AuCl₄ ^- and the subsequent photoreduction results in Au NP-loaded PS _x P2VP _y (M) with NPs located throughout the structure. Loading PS _x P2VP _y (M) with borate-capped Ag NPs produces PS _x P2VP _y (M) with NPs located on the surface of the microgels, where the Ag content is set by S/2VP. The pH-responsive SERS activity is also reported for these Ag NP-loaded microgels. Analytical enhancement factors for dissolved crystal violet are high (i.e., 10⁹ to 10¹⁰) and are set by S/2VP. The Ag NP-loaded microgels with ∼80 wt % 2VP exhibited the most stable pH dependent response.

The plasmonic properties of gold nanoparticle clusters formed via applying an AC electric field

An external electric AC field with a field strength ranging from 10 V mm-1 to 30 V mm-1 and a frequency ranging from 0.1 kHz to 1 MHz was applied to suspensions of gold nanoparticles (Au NPs) to control their plasmonic properties. Apparent differences in the UV-vis spectra of the Au NPs were observed between the spectra with and without the field application. The characteristic red color of the Au NP suspension darkened; this suggested that the application of the AC field caused the aggregation of the Au NPs. In addition, the sizes of the Au NP clusters in suspension formed by the AC field application depended on the frequency of the applied field. The surface-enhanced Raman scattering (SERS) effects of Au NP clusters were examined by comparing the difference in Raman intensities obtained at 30 V mm-1 and in a frequency range of 0.1 kHz to 1 MHz. The application of a low-frequency field at 0.1 kHz caused a rapid aggregation of the Au NPs, resulting in low Raman intensities of the probe molecules. Conversely, high-frequency applications between 1 kHz and 1 MHz successfully enhanced the Raman intensities of the molecules in suspension. The strong correlation of the optical/sensing properties with the Au NP clustering states reveals that the application of an AC electric field is a powerful tool for control over the plasmonic properties.

Photoinduced Rapid Transformation from Au Nanoagglomerates to Drug-Conjugated Au Nanovesicles

Gold (Au) agglomerates (AGs) are reassembled using Triton X-100 (T) and doxorubicin (D) dissolved in ethanol under 185 nm photoirradiation to form TAuD nanovesicles (NVs) under ambient gas flow conditions. The positively charged Au particles are then electrostatically conjugated with the anionic chains of TD components via a flowing drop (FD) reaction. Photoirradiation of the droplets in a tubular reactor continues the photophysicochemical reactions, resulting in the reassembly of Au AGs and TD into TAuD NVs. The fabricated NVs are electrostatically collected onto a polished aluminum rod in a single-pass configuration. The dispersion of NVs is employed for bioassays to confirm uptake by cells and accumulation in tumors. The chemo-photothermal activity is determined both in vitro and in vivo. Different combinations of components are also used to fabricate NVs using the FD reaction, and these NVs are suitable for gene delivery as well. This newly designed gaseous single-pass process results in the reassembly of Au AGs for incorporation with TD without the need of batch wet chemical reactions, modifications, separations, or purifications. Thus, this process offers an efficient platform for preparing biofunctional Au nanostructures that requires neither complex physicochemical steps nor special storage techniques.

Recognition of chiral zwitterionic interactions at nanoscale interfaces by chiroplasmonic nanosensors

The ability to detect chiral molecules renders plasmonic nanosensors as promising tools for the study of chirality phenomena in living systems. Using gold nanorod based plasmonic nanosensors, we investigated here typically chiral zwitterionic electrostatic (Zw-Es) and hydrogen-bonding (Hb) interactions occurring via amine and carboxylic groups at nanoscale interfaces in aqueous solutions. Our results reveal that the plasmonic circular dichroism responses of the nanosensors can have both conformational sensitivity and chiral selectivity to the interfacial molecular interactions. Such a dual function of the plasmonic nanosensors enables a new chiroptical way to differentiate between chiral Zw-Es and Hb interactions, to monitor the transformation between these two interaction forces, and particularly to recognize homochiral Zw-Es interactions in solution. Together with the surface enhanced Raman scattering (SERS) technique, this plasmonic CD based biosensing could have important values for the insightful understanding of chirality-dependent molecular recognition in biological and pharmaceutical systems.

Density-matrix evaluation of the enhancement to resonant Raman scattering and fluorescence of molecules confined in metallic nanoparticle dimers

In the present work we study the surface-enhanced resonant Raman scattering (SERRS) and fluorescence (SEF) spectra of a general model molecule confined in metallic dimers consisting of Ag, Au and hybrid AuAg nanoparticles (NPs). The electromagnetic (EM) enhancement factors were simulated by the generalized Mie scatting method and the scattering cross section of the molecules were obtained by density-matrix calculations. The influence of the size of the NPs and the separation between the dimer on the Raman scattering and fluorescence were systematically studied and analyzed in detail. It was found that the SERRS mainly related to EM enhancement and the SEF depended on the competition between EM enhancement and quantum yield, both of which could be controlled by tuning the radius and separation of the metallic dimers. The optimal radius of the NPs for SERRS were found to be around 30 nm for AgNPs, 40 nm for AuNPs and 50 nm for hybrid AuAgNPs. The strongest Raman enhancement as predicted by the theoretical simulations were 6.2 × 10¹⁰, 1.5 × 10⁷ and 5.2 × 10⁸ for the three types of structures, respectively. These results could offer valuable information for the design of metallic substrates for surface enhanced Raman and fluorescence measurements.

Observing the dynamic “hot spots” on two-dimensional Au nanoparticles monolayer film

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays.

Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10-100 mW/μm²) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm². Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery.

Scattering based hyperspectral imaging of plasmonic nanoplate clusters towards biomedical applications

A new optical scattering contrast-agent based on polymer-nanoparticle encapsulated silver nanoplates (PESNs) is presented. Silver nanoplates were chosen due to the flexibility of tuning their plasmon frequencies. The polymer coating preserves their physical and optical properties and confers other advantages such as controlled contrast agent delivery. Finite difference time domain (FDTD) simulations model the interaction of light with the nanoplates in different orientations in the cluster. Hyperspectral dark field microscopy (HYDFM) observes the scattering spectra of the PESNs. An unsupervised sequential maximum angle convex cone (SMACC) image analysis resolves spectral endmembers corresponding to different stacking orientations of the nanoplates. The orientation-dependent endmembers qualitatively agree with the FDTD results. For contrast enhancement, the uptake and spatial distribution of PESNs are demonstrated by an HYDFM study of single melanoma cells to result in an enhanced contrast of up to 400%. A supervised spatial mapping of the endmembers obtained by the unsupervised SMACC algorithm reveals spatial distributions of PESNs with various clustering orientations of encapsulated nanoplates. Our study demonstrates tunability in plasmonics properties in clustered metal nanoparticles and its utility for the development of scatter-based imaging contrast agents for a broad range of applications, including studies of single cells and other biomedical systems.

Applications of isothermal titration calorimetry - the research and technical developments from 2011 to 2015

Isothermal titration calorimetry is a widely used biophysical technique for studying the formation or dissociation of molecular complexes. Over the last 5 years, much work has been published on the interpretation of isothermal titration calorimetry (ITC) data for single binding and multiple binding sites. As over 80% of ITC papers are on macromolecules of biological origin, this interpretation is challenging. Some researchers have attempted to link the thermodynamics constants to events at the molecular level. This review highlights work carried out using binding sites characterized using x-ray crystallography techniques that allow speculation about individual bond formation and the displacement of individual water molecules during ligand binding and link these events to the thermodynamic constants for binding. The review also considers research conducted with synthetic binding partners where specific binding events like anion-π and π-π interactions were studied. The revival of assays that enable both thermodynamic and kinetic information to be collected from ITC data is highlighted. Lastly, published criticism of ITC research from a physical chemistry perspective is appraised and practical advice provided for researchers unfamiliar with thermodynamics and its interpretation. Copyright © 2016 John Wiley & Sons, Ltd.

Controlling dynamic SERS hot spots on a monolayer film of Fe3O4@Au nanoparticles by a magnetic field

A large surface-enhanced Raman scattering (SERS) effect is critically dependent on the gap distance of adjacent nanostructures, i.e., "hot spots". However, the fabrication of dynamically controllable hot spots still remains a remarkable challenge. In the present study, we employed an external magnetic field to dynamically control the interparticle spacing of a two-dimensional monolayer film of Fe3O4@Au nanoparticles at a hexane/water interface. SERS measurements were performed to monitor the expansion and shrinkage of the nanoparticles gaps, which produced an obvious effect on SERS activities. The balance between the electrostatic repulsive force, surface tension, and magnetic attractive force allowed observation of the magnetic-field-responsive SERS effect. Upon introduction of an external magnetic field, a very weak SERS signal appeared initially, indicating weak enhancement due to a monolayer film with large interparticle spacing. The SERS intensity reached maximum after 5s and thereafter remained almost unchanged. The results indicated that the observed variations in SERS intensities were fully reversible after removal of the external magnetic field. The reduction of interparticle spacing in response to a magnetic field resulted in about one order of magnitude of SERS enhancement. The combined use of the monolayer film and external magnetic field could be developed as a strategy to construct hot spots both for practical application of SERS and theoretical simulation of enhancement mechanisms.

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.

pH-Responsive assembly of metal nanoparticles and fluorescent dyes by diblock copolymer micelles

Hybrid assemblies consisting of metal nanoparticles (NPs) and fluorophores are quite interesting because the intrinsic properties of fluorophores can be engineered in the assembled structure. In this regard, we utilized the self-segregation properties of block copolymer micelles to organize metal NPs and fluorophores simultaneously in a specific arrangement. From the viewpoint of assembly methods, we first encapsulated Au NPs in the PS cores of polystyrene-block-poly(acrylic acid) (PS-PAA) micelles. Then, positively charged fluorescent dyes of rhodamine 123 (R123) were bound to the negatively charged PAA coronas by electrostatic interactions. Since carboxylic acid in the PAA block is a weak acid, the degree of R123 binding to PS-PAA micelles can be adjusted by varying the pH of the solution. Therefore, by changing the pH, we were able to control the assembly and disassembly of R123 molecules to PS-PAA micelles and the corresponding change in the fluorescence signal.

Ultrasensitive SERS detection of trinitrotoluene through capillarity-constructed reversible hot spots based on ZnO-Ag nanorod hybrids

A simple and efficient self-approach strategy was used to apply ultrasensitivity and self-revive ZnO-Ag hybrid surface-enhanced Raman scattering (SERS) sensors for the highly sensitive and selective detection of explosive TNT in both solution and vapour conditions. The good ultrasensitive sensing performance is a result of the abundant Raman hot spots, which were spontaneously formed in a reversible way by the self-approaching of flexible ZnO-Ag hybrid nanorods driven by the capillary force of solvent evaporation. Moreover, the enhancement effect was repeatedly renewed by the reconstruction of molecular bridges, which could selectively detect TNT with a lower limit of 4 × 10(-14) M. In addition, TNT vapor was also tested under this sensor, whereby once the ZnO-Ag NRs hybrid substrate was dipped in TNT, this substrate could detect the existence of TNT even in 5 detection cycles via a capillarity-constructed reversible hot spots approach. Compared with other pure Ag-based SERS sensors, this ZnO-Ag hybrid SERS sensor could rapidly self-revive SERS-activity by simple UV light irradiation and could retain stable SERS sensitivity for one month when used for TNT detection. This stable and ultrasensitive SERS substrate demonstrates a new route to eliminate the oxidized inactive problem of traditional Ag-based SERS substrates and suggests promising use in the applications of such hybrids as real-time online sensors for explosives detection.

Polymer/gold hybrid nanoparticles: from synthesis to cancer theranostic applications

This minireview highlights the preparation methods for various polymer/AuNP hybrids and the recent progress on their cancer theranostic applications.

Highly stable polymer coated nano-clustered silver plates: a multimodal optical contrast agent for biomedical imaging

Here, we present a new optical contrast agent based on silver nanoplate clusters embedded inside of a polymer nano matrix. Unlike nanosphere clusters, which have been well studied, nanoplate clusters have unique properties due to the different possible orientations of interaction between the individual plates, resulting in a significant broadening of the absorption spectra. These nanoclusters were immobilized inside of a polymer cladding so as to maintain their stability and optical properties under in vivo conditions. The polymer-coated silver nanoplate clusters show a lower toxicity compared to the uncoated nanoparticles. At high nanoparticle concentrations, cell death occurs mostly due to apoptosis. These nanoparticles were used for targeted fluorescence imaging in a rat glioma cell line by incorporating a fluorescent dye into the matrix, followed by conjugation of a tumor targeting an F3 peptide. We further used these nanoparticles as photoacoustic contrast agents in vivo to enhance the contrast of the vasculature structures in a rat ear model. We observed a contrast enhancement of over 90% following the nanoparticle injection. It is also shown that these NPs can serve as efficient contrast agents, with specific targeting abilities for broadband multimodal imaging that are usable for diagnostic applications and that extend into use as therapeutic agents as well.

Metal nanoparticles and supramolecular macrocycles: a tale of synergy

In this minireview, we summarize current research dealing with the combination of noble-metal nanoparticles and different families of supramolecular macrocycles (cyclodextrins, cucurbit[n]urils, calixarenes, and pillar[n]arenes). We intended to select relevant publications on the synthesis of noble-metal nanoparticles with macrocycles acting as capping agents or/and reducing agents, as well as on the post-synthetic metal-nanoparticle modification with macrocycles. We also discuss strategies in which supramolecular chemistry is applied to direct the self-assembly of nanoparticles and formation of polymer composites. We finally describe the main applications of these materials in various fields.

Controllable and reversible hot spot formation on silver nanorod arrays

Reversible hot spot formation is achieved for free-standing silver nanorod (AgNR) arrays fabricated by oblique angle deposition and modified with a hydrophilic surface coating.

In vitro dosimetry of agglomerates

Agglomeration of nanoparticles in biological fluids is a pervasive phenomenon that leads to difficulty in the interpretation of results from in vitro exposure, primarily due to differing particokinetics of agglomerates to nanoparticles. Therefore, well-defined small agglomerates were designed that possessed different particokinetic profiles, and their cellular uptake was compared to a computational model of dosimetry. The approach used here paves the way for a better understanding of the impact of agglomeration on the nanoparticle-cell interaction.

Detection of hydrogen sulfide through photoluminescence quenching of penicillamine-copper nanocluster aggregates

We have developed a one-pot, inexpensive, simple and rapid method to synthesize photoluminescent copper nanocluster (Cu NC) aggregates from Cu(2+) ions in 65% (v v(-1)) dimethylformamide aqueous solution containing penicillamine (PA) as a capping and reducing agent. As-prepared PA-Cu NC aggregates emit at 580 nm when excited at 326 nm, with a quantum yield of 2.0%. The photoluminescence of PA-Cu NC aggregates originate from ligand-to-metal charge transfer, which is supported by a long lifetime (126.5 ns) and a large Stokes shift (254 nm). As-prepared PA-Cu NC aggregates have different emission wavelengths with the same excitation wavelength in various organic-aqueous solutions. The PA-Cu NC aggregates are highly selective and sensitive to the detection of hydrogen sulfide (H₂S), based on analyte-induced photoluminescence quenching through the formation of CuS nanoparticles. The probe allows the detection of H₂S, with a linear range of 1-100 μM and a limit of detection (signal-to-noise ratio = 3) of 500 nM. The practicality of this probe has been validated through the analysis of hot spring water samples.

Flexible membranes of Ag-nanosheet-grafted polyamide-nanofibers as effective 3D SERS substrates

We report on a synthetic approach to produce self-supported flexible surface-enhanced Raman scattering (SERS) active membranes consisting of polyamide (PA) nanofibers grafted with vertical Ag-nanosheets, via a combinatorial process of electrospinning PA-nanofiber membranes, assembling Au-nanoparticles on the PA-nanofibers as seeds for subsequent growth of Ag-nanosheets, and electrodepositing Ag-nanosheets on the electrospun PA-nanofibers. As a high density of Ag-nanosheets are vertically grown around each PA-nanofiber in the three-dimensional (3D) networked PA-nanofiber membranes, homogeneous nano-scaled gaps between the neighboring Ag-nanosheets are formed, leading to a high density of 3D SERS "hot spots" within the Ag-nanosheet-grafted PA-nanofiber membranes. The Ag-nanosheet-grafted PA-nanofiber membranes demonstrate high SERS activity with excellent Raman signal reproducibility for rhodamine 6G over the whole membrane. For a SERS-based trial analysis of polychlorinated biphenyls (PCBs, a kind of global environmental hazard), the 3D SERS substrate membranes are modified with mono-6-β-cychlodextrin to effectively capture PCB molecules. As a result, not only a low concentration down to 10(-6) M is reached, but also two congeners of PCBs in their mixed solution are identified, showing promising potential in SERS-based rapid detection of trace organic pollutants such as PCBs in the environment.

Photoluminescent AuCu bimetallic nanoclusters as pH sensors and catalysts

A facile and one-pot approach to the preparation of gold (Au) and copper (Cu) bimetallic nanoclusters (NCs) is unveiled. AuCu NCs reveal features of orange photoluminescence (PL), reversible pH-dependent PL properties, and efficient catalytic activity for degradation of methylene blue (MB).

Tuning gold nanoparticles interfaces by specific peptide interaction for surface enhanced Raman spectroscopy (SERS) and separation applications

Surface functionalization and control over nanostructured interfaces represents a key aspect in nanoscience and nanobiotechnology. Nanoplasmonic structures for analyte detection typically require sophisticated nanofabrication techniques, as well as bioactivated nanostructures that need multistep conjugations for chemical ligation. An alternative to such complex processes is to rely on specific biomolecules adsorption for decoration or self-assembly of nanoparticles at solid/liquid interface. In principle, small biomolecules with specific binding properties to nanostructures could control the assembly without modifying the nanoparticle chemistry, pH of the solution or salt concentration. Importantly, such an approach could be direct, robust, and reversible. In this work, we report about the use of a specific peptide for direct and reversible adsorption on gold nanoparticles with tuned interfacial properties just by simply adjusting the ratio between the numbers of peptide molecules to the number of gold nanoparticles. This easy, direct and reversible assembly of gold nanoparticles mediated by the specific peptide makes this platform ideal for small-volume samples and low concentrations detection using surface enhanced Raman Spectroscopy, as well as for the capture or separation of biomolecules in complex mix.

Controllable nitric oxide release in the presence of gold nanoparticles

A major problem associated with nitric oxide (NO) donors is the release of the desired amount of NO at a specific site. A number of platforms have been developed for the regulation of NO dosage. We present the use of citrate-stabilized gold nanoparticles as a platform to regulate NO release. Because of the affinity between gold and thiols, the characteristic -S-NO bond of S-nitrosothiols (RSNOs) breaks in the presence of gold nanoparticles, thereby releasing NO and modifying the gold nanoparticle surface with the corresponding thiol. This system allows for surface-controlled NO release, where the amount of NO released is proportional to the number of thiols bound to the gold nanoparticle surface. Moreover, by employing an amperometric technique to detect the maximum NO release, we were able to estimate the stoichiometry of the reaction, that is, the number of adsorbed RSNO molecules per gold nanoparticle. A kinetic model for NO release and its subsequent decomposition is proposed and used to fit the experimental results. The reaction was found to be zeroth- and first-order with respect to RSNO and gold nanoparticles, respectively.

Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications

In recent years, Surface-Enhanced Raman Spectroscopy (SERS) has experienced a tremendous increase of attention in the scientific community, expanding to a continuously wider range of diverse applications in nanoscience, which can mostly be attributed to significant improvements in nanofabrication techniques that paved the way for the controlled design of reliable and effective SERS nanostructures. In particular, the plasmon coupling properties of interacting nanoparticles are extremely intriguing due to the concentration of enormous electromagnetic enhancements at the interparticle gaps. Recently, great efforts have been devoted to develop new nanoparticle assembly strategies in suspension with improved control over hot-spot architecture and cluster structure, laying the foundation for the full exploitation of their exceptional potential as SERS materials in a wealth of chemical and biological sensing. In this review we summarize in an exhaustive and systematic way the state-of-art of plasmonic nanoparticle assembly in suspension specifically developed for SERS applications in the last 5 years, focusing in particular on those strategies which exploited molecular linkers to engineer interparticle gaps in a controlled manner. Importantly, the novel advances in this rather new field of nanoscience are organized into a coherent overview aimed to rationally describe the different strategies and improvements in the exploitation of colloidal nanoparticle assembly for SERS application to real problems.