Robust, reproducible, recyclable SERS substrates: monolayers of gold nanostars grafted on glass and coated with a thin silica layer.

We prepared and characterized recyclable surface enhanced Raman spectroscopy (SERS) active glass chips. Gold nanostars were grafted on properly functionalized glasses by means of electrostatic interactions and then they were coated with a silica layer of controllable thickness in the nanometer range. The SERS activity of the obtained substrates were tested in terms of reproducibility and homogeneity intra-samples and inter-samples from different batches using the Raman reporter as the model compound rhodamine 6G. The uncoated substrates were used as reference to evaluate the effect of silica spacers on SERS enhancement factors (EFs). The chemical route to obtain silica-coated SERS chips is described in detail, and the morphology and the optical response of substrates have been characterized. We demonstrate that SERS substrates coated with 1 nm silica conserve a good EF, and that the coating confers to the SERS platform an extreme robustness leading to reusability of the substrates.

Tailored coating of gold nanostars: rational approach to prototype of theranostic device based on SERS and photothermal effects at ultralow irradiance.

The last decade has come across an increasing demand for theranostic biocompatible nanodevices possessing the double ability of diagnosis and therapy. In this work, we report the design, synthesis and step-by-step characterization of rationally coated gold nanostars (GNSs) for the SERS imaging and photothermal therapy of HeLa cancer cells. The nanodevices were realized by synthesizing GNSs with a seed growth approach, coating them with a controlled mixture of thiols composed of a Raman reporter and a polyethylene glycol with a terminal amino group, and then reacting these amino groups with folic acid (FA), in order to impart selectivity towards cancer cells which overexpress folate receptors on their membranes. After a complete characterization, we demonstrate that these FA-functionalized GNSs (FA-GNSs) are able to bind selectively to the membranes of HeLa cells, acting as SERS tags and allowing SERS imaging. Moreover, we demonstrate that once bound to HeLa cell membranes, FA-GNSs exhibit photothermal effect which can be exploited to kill the same cells in vitro using laser irradiation in the NIR at a very low and safe irradiance. We thus demonstrate that the FA-GNSs designed following the described approach are an efficient prototype of theranostic nanodevices.

Silver nanoparticles synthesized and coated with pectin: An ideal compromise for anti-bacterial and anti-biofilm action combined with wound-healing properties.

The synthesis of Ag nanoparticles from Ag+ has been investigated, with pectin acting both as reductant and coating.∼100% Ag+ to Ag(0) one-pot conversion was obtained, yielding p-AgNP, i.e. an aqueous solution of pectin-coated spherical Ag nanoparticles (d=8.0±2.6nm), with a<1ppm concentration of free Ag+ cation. Despite the low free Ag+ concentration and low Ag+ release with time, the nature of the coating allows p-AgNP to exert excellent antibacterial and antibiofilm actions, comparable to those of ionic silver, tested on E. coli (Gram-) and S. epidermidis (Gram+) both on planctonic cells and on pre- and post-biofilm formation conditions. Moreover, p-AgNP were tested on fibroblasts: not only p-AgNP were found to be cytocompatible but also revealed capable of promoting fibroblasts proliferation and to be effective for wound healing on model cultures. The antibacterial activity and the wound healing ability of silver nanoparticles are two apparently irreconcilable properties, as the former usually requires a high sustained Ag+ release while the latter requires low Ag+ concentration. p-AgNP represents an excellent compromise between opposite requirements, candidating as an efficient medication for repairing wounds and/or to treat vulnerable surgical site tissues, including the pre-treatment of implants as an effective prophylaxis in implant surgery.

Tunable coating of gold nanostars: tailoring robust SERS labels for cell imaging.

Surface modification of noble metal nanoparticles with mixed molecular monolayers is one of the most powerful tools in nanotechnology, and is used to impart and tune new complex surface properties. In imaging techniques based on surface enhanced Raman spectroscopy (SERS), precise and controllable surface modifications are needed to carefully design reproducible, robust and adjustable SERS nanoprobes. We report here the attainment of SERS labels based on gold nanostars (GNSs) coated with a mixed monolayer composed of a poly ethylene glycol (PEG) thiol (neutral or negatively charged) that ensure stability in biological environments, and of a signalling unit 7-Mercapto-4-methylcoumarin as a Raman reporter molecule. The composition of the coating mixture is precisely controlled using an original method, allowing the modulation of the SERS intensity and ensuring overall nanoprobe stability. The further addition of a positively charged layer of poly (allylamine hydrocloride) on the surface of negatively charged SERS labels does not change the SERS response, but it promotes the penetration of GNSs in SH-SY5Y neuroblastoma cells. As an example of an application of such an approach, we demonstrate here the internalization of these new labels by means of visualization of cell morphology obtained with SERS mapping.

Synthesis of branched Au nanoparticles with tunable near-infrared LSPR using a zwitterionic surfactant.

Asymmetric branched gold nanoparticles are obtained using for the first time in the seed-growth approach a zwitterionic surfactant, laurylsulfobetaine, whose concentration in the growth solution allows to control both the length to base-width ratio of the branches and the LSPR position, that can be tuned in the 700-1100 nm near infrared range.