Assembling patchy plasmonic nanoparticles with aggregation-dependent antibacterial activity

We realise an antibacterial nanomaterial based on the self-limited assembly of patchy plasmonic colloids, obtained by adsorption of lysozyme to gold nanoparticles. The possibility of selecting the size of the assemblies within several hundred nanometres allows for tuning their optical response in a wide range of frequencies from visible to near infrared. We also demonstrate an aggregation-dependent modulation of the catalytic activity, which results in an enhancement of the antibacterial performances for assemblies of the proper size. The gained overall control on structure, optical properties and biological activity of such nanomaterial paves the way for the development of novel antibacterial nanozymes with promising applications in treating multi drug resistant bacteria

Toward a unified description of the electrostatic assembly of microgels and nanoparticles

The combination of soft responsive particles, such as microgels, with nanoparticles (NPs) yields highly versatile complexes of great potential for applications, from ad-hoc plasmonic sensors to controlled protocols for loading and release. However, the assembly process between these microscale networks and the co-dispersed nano-objects has not been investigated so far at the microscopic level, preempting the possibility of designing such hybrid complexes a priori. In this work, we combine state-of-the-art numerical simulations with experiments, to elucidate the fundamental mechanisms taking place when microgels-NPs assembly is controlled by electrostatic interactions. We find a general behavior where, by increasing the number of interacting NPs, the microgel deswells up to a minimum size, after which a plateau behavior occurs. This occurs either when NPs are mainly adsorbed to the microgel corona via the folding of the more external chains, or when NPs penetrate inside the microgel, thereby inducing a collective reorganization of the polymer network. By varying microgel properties, such as fraction of crosslinkers or charge, as well as NPs size and charge, we further show that the microgel deswelling curves can be rescaled onto a single master curve, for both experiments and simulations, demonstrating that the process is entirely controlled by the charge of the whole microgel-NPs complex. Our results thus have a direct relevance in fundamental materials science and offer novel tools to tailor the nanofabrication of hybrid devices of technological interest

Molecular origin of the two-step mechanism of gellan aggregation

Among hydrocolloids, gellan is one of the most studied polysaccharides due to its ability to form mechanically stable gels. Despite its long-standing use, the gellan aggregation mechanism is still not understood because of the lack of atomistic information. Here, we fill this gap by developing a new gellan force field. Our simulations offer the first microscopic overview of gellan aggregation, detecting the coil to single-helix transition at dilute conditions and the formation of higher-order aggregates at high concentration through a two-step process: first, the formation of double helices and then their assembly into superstructures. For both steps, we also assess the role of monovalent and divalent cations, complementing simulations with rheology and atomic force microscopy experiments and highlighting the leading role of divalent cations. These results pave the way for future use of gellan-based systems in a variety of applications, from food science to art restoration

Performance Assessment in Fingerprinting and Multi Component Quantitative NMR Analyses

An interlaboratory comparison (ILC) was organized with the aim to set up quality control indicators suitable for multicomponent quantitative analysis by nuclear magnetic resonance (NMR) spectroscopy. A total of 36 NMR data sets (corresponding to 1260 NMR spectra) were produced by 30 participants using 34 NMR spectrometers. The calibration line method was chosen for the quantification of a five-component model mixture. Results show that quantitative NMR is a robust quantification tool and that 26 out of 36 data sets resulted in statistically equivalent calibration lines for all considered NMR signals. The performance of each laboratory was assessed by means of a new performance index (named Qp-score) which is related to the difference between the experimental and the consensus values of the slope of the calibration lines. Laboratories endowed with a Qp-score falling within the suitable acceptability range are qualified to produce NMR spectra that can be considered statistically equivalent in terms of relative intensities of the signals. In addition, the specific response of nuclei to the experimental excitation/relaxation conditions was addressed by means of the parameter named NR. NR is related to the difference between the theoretical and the consensus slopes of the calibration lines and is specific for each signal produced by a well-defined set of acquisition parameters

Thermophilic rearrangement of bio-plasmonic aggregates: morphological and plasmonic related evidences

The peculiar interaction of metallic nanoparticles with the electromagnetic radiation paved the way to design novel nanoarchitectures whose optical properties can be tuned by controlling their structure and the features of the surrounding environment. The research of the last few years heads up to the idea of creating hybrid assemblies made up of metallic nanoparticles and biomolecules with promising applications in the field of nano-medicine and nano-biotechnology, providing a new and powerful tool for innovative diagnosis and therapeutical approaches. We recently developed a bio-plasmonic system based on the colloidal aggregation in solution of anionic gold nanoparticles (AuNPs) mediated by lysozyme. The aggregation is driven by patch-charge interactions [6], induced by the adsorption of the positively charged protein on the AuNPs surface. We demonstrated that the optical properties of the system can be tuned through the clusters morphology, acting on several parameters such as the AuNPs size, the Lysozyme-AuNPs relative molar ratio and the pH of the solution. Proceeding from these, here we would consider also the role of the temperature as a further tool to fine tuning the structural morphology together with the plasmonic properties of the aggregates. In this framework, the thermally enhanced diffusion of the NPs within the clusters can affect aggregate stability and shape, and thereby the own plasmonic profiles. On the other hand, the unfolding of the protein, induced by the increasing temperature and its consequent relaxation on the AuNps surface [7], implies a redistribution of the surface charge, together with an increase of the hydrophobic interactions. Lysozyme unfolding can thus be employed to change the nature of the interaction which holds the aggregates, switching from electrostatic to hydrophobic. As a first step in this direction we undertook a combined study of the temperature effects on the localized surface plasmon resonance and on the size of preformed Lysozyme-NPs aggregates. The plasmonic profile and the related inter-particles plasmonic bands which arise due to the NPs aggregation were monitored by UV-Visible Absorption Spectroscopy at varying the temperature from 20°C to 90°C, while information on the aggregates size has been obtained by Dynamic Light Scattering experiments. The combination of these techniques allowed us to disentangle the two abovementioned aspects, which can interplay in the stability of the clusters, leading to their disaggregation or resulting in the cluster reorganization, depending on the Lysozyme-AuNPs relative molar ratio. It is well known that the localised heating can be also induced by the plasmonic absorption, hence our work sets the foundations to realize a “thermo-plasmonic based annealing”

SERS active pH-nanosensor with tunable properties

The extraordinary optical properties of gold nanoparticles (AuNPs), together with their rewarding chemical stability and ease of functionalization, make them an invaluable platform to develop ultrasensitive and molecular specific chemical sensors. The huge amplification of the spectroscopic signal of molecules located at the metal interface, arising from the confinement of strong electromagnetic fields on the AuNPs surface, results in a remarkable increase of the sensitivity of vibrational spectroscopies. In particular, Surface Enhanced Raman Spectroscopy (SERS) emerged as a powerful analytical tool with detection limits lowered down to the single molecule recognition. In this framework, we developed and characterized a plasmonic pH-nanosensor by conjugating AuNPs with the pH-sensitive molecular probe 4-mercaptobenzoic acid (4MBA), which shows a SERS signal depending on its protonation degree. pH is indeed a key target parameter in a wide field of applications, ranging from environmental science to industry to biomedicine. Exposing the AuNP-4MBA nanosensors to solutions at varying pH, we identified the dynamic range of sensitivity as a function of the relative intensity of selected pHdependent SERS bands. From the comparison of the obtained calibration curve with a standard acid-base titration curve of the free molecule, we enlightened that the pKa of the molecule shifts to higher values when it is measured by SERS at the interface of the plasmonic nanostructure. In particular, for AuNPs with a diameter of 60 nm, the pKa value results around pH 6, making this system suitable for pH measurements in physiological environment, at the single cell level. Proceeding from this, we will explore the possibility of tuning the 4MBA acidic properties by varying the AuNPs size as well as the core material, with the final aim toreach a modulation of the working point of the nanosensor, depending on the system of interest

Responsivity of Fractal Nanoparticle Assemblies to Multiple Stimuli: Structural Insights on the Modulation of the Optical Properties

Multi-responsive nanomaterials based on the self-limited assembly of plasmonic nanoparticles are of great interest due to their widespread employment in sensing applications. We present a thorough investigation of a hybrid nanomaterial based on the protein-mediated aggregation of gold nanoparticles at varying protein concentration, pH and temperature. By combining Small Angle X-ray Scattering with extinction spectroscopy, we are able to frame the morphological features of the formed fractal aggregates in a theoretical model based on patchy interactions. Based on this, we established the main factors that determine the assembly process and their strong correlation with the optical properties of the assemblies. Moreover, the calibration curves that we obtained for each parameter investigated based on the extinction spectra point out to the notable flexibility of this nanomaterial, enabling the selection of different working ranges with high sensitivity. Our study opens for the rational tuning of the morphology and the optical properties of plasmonic assemblies to design colorimetric sensors with improved performances

Folate conjugated, SERS-active gold nanoparticles for selective targeting of human cancer cells

One of the most interesting applications of Surface Enhanced Raman Scattering (SERS), ever since its discovery, has been the spectroscopic study and detection of biomolecules [1,2]. More recently, the application of SERS-active nanostructures as spectroscopic antennas inside biological cells has proved to be a simple way to perform spectroscopic imaging of both healthy and damaged living tissues [3]: an interesting task for biophysicists is indeed to find ways to distinguish between healthy and cancer cells. Moreover, much interest has lately risen around the concept of “theranostics”, i.e. combining diagnostics with therapy, the latter to perform selectively on cancer cells without damaging the healthy tissue [4]. Plasmonics-based theranostics is often performed combining SERS and photothermal bleaching. We will here present a biocompatible system based on gold nanoparticles (Nps) functionalized with the Raman active bifunctional linker 4-aminothiophenol and further conjugated with folic acid, a biomolecule with an essential role in cell reproduction (see Fig. 1., (a)). Our system can be considered a nanobiovector, as it is capable of targeting a specific kind of cell and locate in its proximity and/or inside it. Folic acid receptors are more frequent in cancer cells, as they reproduce more quickly [5]. For this reason, our nanovector targets cancer cells much more efficiently (Fig. 1., (b)). After presenting the system characterization, we will illustrate the interaction of our nanobiovector with different cell cultures. Proofs of selective targeting and eventual internalization in cells will be shown. The presence of SERS-active Nps allows not only to perform Raman imaging, but also the implementation of a SERS-based screening method that allows for a precise single cell diagnosi