Tiziana Placido

Ion-directed assembly of gold nanorods: a strategy for mercury detection.

Water-soluble gold nanorods (Au NRs) have been functionalized with an N-alkylaminopyrazole ligand, 1-[2-(octylamino)ethyl]-3,5-diphenylpyrazole (PyL), that has been demonstrated able to coordinate heavy metal ions. The N-alkylaminopyrazole functionalized Au NRs have been characterized by electron microscopy and spectroscopic investigation and tested in optical detection experiments of different ions, namely, Zn(2+), Cd(2+), Hg(2+), Cu(2+), Pb(2+), and As(3+). In particular, the exposure of the functionalized NRs to increasing amounts of Hg(2+) ions has resulted in a gradual red-shift and broadening of the longitudinal plasmon band, up to 900 nm. Interestingly, a significantly different response has been recorded for the other tested ions. In fact, no significant shift in the longitudinal plasmon band has been observed for any of them, while a nearly linear reduction in the plasmon band intensity versus ion concentration in solution has been detected. The very high sensitivity for Hg(2+) with respect to other investigated ions, with a limit of detection of 3 ppt, demonstrates that the functionalization of Au NRs with PyL is a very effective method to be implemented in a reliable colorimetric sensing device, able to push further down the detection limit achieved by applying similar strategies to spherical Au NPs.

Electroactive layer-by-layer plasmonic architectures based on Au nanorods.

Nanostructured films based on Au nanorods (NRs) have been obtained by layer-by-layer (LbL) assembly driven by electrostatic interaction between metal nanoparticles and polyelectrolytes. Multilayer films have been fabricated by using LbL assembly of poly(sodium styrenesulfonate) (PSS) and positively charged Au NRs on a polyelectrolyte-modified substrate. The effect of fabrication parameters, including the nature of the substrate, the polyelectrolyte initial anchoring layer, and the number of layers has been investigated by means of UV-vis absorbance spectroscopy and atomic force microscopy (AFM). The results demonstrated the dependence of morphology and plasmonic features in the multilayered nanostructured architectures from the nature of the anchoring polyelectrolyte on the substrate, the number of layers, and the kind of NR mutual assembly. In addition, a study of the electrochemical activity at the solid/liquid interface has been carried out in order to assess charge transport through the NR multilayer by using two molecular probes in solution, namely, potassium ferricyanide, a common and well-established redox mediator with reversible behavior, and cytochrome C, a robust model redox protein. The presented systematic study of the immobilization of Au NRs opens the venue to several application areas, such as (bio)chemical sensing.

Templating gold nanorods with liquid crystalline DNA

A liquid crystalline, negatively charged, whole-genome DNA is exploited to organize positively charged gold nanorods (GNRs) by means of electrostatic interaction. A mesoscopic alignment of the composite system along a preferred direction is obtained by casting a droplet of the DNA-nanorods solution onto an untreated glass substrate. Gel electrophoresis analysis enables evaluating the effective electric charge of the system, thus minimizing the DNA fragmentation. Polarized optical microscopy, combined with transmission and scanning electron microscopy, shows that, up to 20% in weight of GNR solution, the system exhibits both a long range order, induced by the liquid crystalline phase of the DNA, and a nanoscale organization, due to the DNA self-assembly. These evidences are confirmed by a polarized spectral analysis, which also points out that the optical properties of GNRs strongly depend on the polarization of the impinging probe light. The capability to organize plasmonic nanoparticles by means of DNA material represents a significant advance towards the realization of life science inspired optical materials.

Assembly of Gold Nanorods for Highly Sensitive Detection of Mercury Ions

A simple and cost-effective strategy for mercury ion sensing based on an easy and reliable colorimetric approach is investigated through L-cysteine functionalized gold nanorods (Au NRs). Detection tests are performed on several ions, such as Hg2+, Zn2+, Cd2+, Cu2+, Pb2+, and As2+, and monitored by UV-Vis absorption spectroscopy, transmission electron microscopy (TEM), and infrared spectroscopy (ATR-FTIR). L-cysteine functionalized Au NRs demonstrated a remarkable sensitivity for Hg2+ with limit of detection (LOD) at a few ppt level. A red-shift in the maximum of the typical longitudinal plasmon band of Au NRs is observed and recognized related to aggregation phenomena occurring among functionalized Au NRs and triggered only by the presence of Hg2+ ions in solution. Interestingly, a significantly different response is recorded for the other tested ions. The results highlight that the functionalization of Au NRs with L-cysteine is an excellent route to implement a reliable colorimetric sensing device, able to push further down the LOD recorded for similar strategies based on spherical Au NPs.

Plasmonic Thermometer Based on Thermotropic Liquid Crystals

Localized plasmon resonance (LPR) of noble Metal Nanoparticles (MNPs) opens up a new horizon for nanoscale materials able to convert light into heat, since the strong electric field generated around the MNPs can transform them into original heat nanosources. Thus, investigation of the heat transport mechanism, from the heated MNPs to their surrounding medium, is fundamental for realizing applications in nanotechnology and thermal-based therapies, and a challenge is definitely represented by the possibility of measuring temperature variations at the surface of the MNPs undergoing optical illumination. In this framework, we show that an ingenious combination of characteristics of short pitch liquid crystalline compounds and MNPs has demonstrated effective to provide an advanced tool to monitor nanoscale temperature variations.

Applications of nanomaterials in modern medicine

Nanomaterials represent a class of materials based on nanoscale structures. Nanomaterials are currently used in a wide variety of applications, including, optoelectronics, energy conversion, biology health care and medicine. Among different types of nanomaterials, gold nanoparticles have received considerable attention in disease diagnosis and therapy due to their optical and chemical properties (Liz-Marzan in Mater Today 7:26–31, 2004). This paper reports the main optical and photo-thermal properties of gold nanoparticles. Particularly, we show that gold nanorods embedded in cholesteric liquid crystals demonstrate to control the “selective reflection” of a light beam. Investigation of the optical properties of the obtained material reveals an original and efficient tool to detect temperature variations at the nanoscale useful for photo-thermal based therapies applications. Finally, the concept of ‘nanoparticle-protein corona interaction can be exploited for application ranging from regenerative medicine to theranostics.

Nematic liquid crystals used to control photo-thermal effects in gold nanoparticles

We report on photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in Nematic Liquid Crystals (NLCs). Under a suitable optical radiation, GNPs exhibit a strong light absorption/scattering; the effect depends on the refractive index of the medium surrounding the nanoparticles, which can be electrically or optically tuned. In this way, the system represents an ideal nano-source of heat, remotely controllable by light to adjust the temperature at the nanoscale. Photo-induced temperature variations in GNPs dispersed in NLCs have been investigated by implementing a theoretical model based on the thermal heating equation applied to an anisotropic medium; theoretical predictions have been compared with results of experiments carried out in a NLC medium hosting GNPs. Both theory and experiments represent a step forward to understand the physics of heat production at the nanoscale, with applications that range from photonics to nanomedicine.

Plasmonics Meets Biology through Optics

Plasmonic metallic nanoparticles (NPs) represent a relevant class of nanomaterials, which is able to achieve light localization down to nanoscale by exploiting a phenomenon called Localized Plasmon Resonance. In the last few years, NPs have been proposed to trigger DNA release or enhance ablation of diseased tissues, while minimizing damage to healthy tissues. In view of the therapeutic relevance of such plasmonic NPs; a detailed characterization of the electrostatic interaction between positively charged gold nanorods (GNRs) and a negatively charged whole-genome DNA solution is reported. The preparation of the hybrid biosystem has been investigated as a function of DNA concentration by means of ζ-potential; hydrodynamic diameter and gel electrophoresis analysis. The results have pointed out the specific conditions to achieve the most promising GNRs/DNA complex and its photo-thermal properties have been investigated. The overall study allows to envisage the possibility to ingeniously combine plasmonic and biological materials and, thus, enable design and development of an original non invasive all-optical methodology for monitoring photo-induced temperature variation with high sensitivity.

Photo-Induced Heating in Plasmonic Nanoparticles Trapped in Thermo-Sensitive Liquid Crystals

Thermo-sensitive liquid crystals may result, for some aspects, good host materials for plasmonic nanoparticles. In particular they are suitable to study and measure the temperature variations produced by photo-induced plasmonic joule effect in the metallic nanoparticles. Combining the properties of liquid crystals and metallic nanoparticles, allows to measure temperature variations in different ways by exploiting the optical properties of thermotropic liquid crystals: In a first attempt, by combining nematic liquid crystals and spherical metallic nanoparticles, we have predicted and measured temperature changes, under a suitable (resonant) optical illumination, by measuring the photo-thermal induced birefringence variation. In a different experiment, we have combined cholesteric liquid crystals and gold nanorods: Light-induced variations of structural colorations exhibited by cholesteric liquid crystals has been used as a new methodology to measure nanoscale heat variation with a very high sensitivity (0.03 K).

Gold-chlorophyll a-hybrid nanoparticles and chlorophyll a/cetyltrimethylammonium chloride self-assembled-suprastructures as novel carriers for chlorophyll a delivery in water medium: Photoactivity and photostability

The stability of Chlorophyll a in water during prolonged exposure, at room temperature, to a neon lamp has been investigated by means of UV-vis and fluorescence spectroscopies. In addition, the Chlorophyll a (photo)stability evaluation in presence of suitable carriers has been performed in order to investigate its reactivity under the same conditions, for possible and future applications in Antimicrobial Photodynamic Therapy. Cetyltrimethylammonium chloride was chosen to solubilize Chlorophyll a in water. While, cetyltrimethylammonium chloride-capped gold nanoparticles offer a great opportunity because combine the Chlorophyll a action, used as a photosensitizer in Antimicrobial Photodynamic Therapy, with gold nanoparticles effect used in photothermal therapy. Indeed, the latter ones have exhibited an interesting rise of temperature if irradiated with visible light. Overall, both examined systems, cetyltrimethylammonium chloride/Chlorophyll a and gold nanoparticles/Chlorophyll a, were able to induce the Reactive Oxygen Species formation fundamental for a potential application in Antimicrobial Photodynamic Therapy.