In the past years, laser ablation synthesis in solution (LASiS) emerged as a reliable alternative to traditional chemical reduction methods for obtaining noble metal nanoparticles (NMNp). LASiS is a "green" technique for the synthesis of stable NMNp in water or in organic solvents, which does not need stabilizing molecules or other chemicals. The so obtained NMNp are highly available for further functionalization or can be used wherever unprotected metal nanoparticles are desired. Surface functionalization of NMNp can be monitored in real time by UV-visible spectroscopy of the plasmon resonance. However LASiS has some limitations in the size control of NMNp, which can be overcome by "chemical free" laser treatments of NMNp. In this paper we provide an overview of LASiS, size manipulation by laser irradiation and functionalization of NMNp, with special care in pointing out some of the main issues about this research area.

Laser ablation synthesis in liquid solution (LASiS) is a "green" technique that gives access to the preparation of a library of nanomaterials. Bare noble metal spherical particles, multiphase core-shell oxides, metal-semiconductor heterostructures, layered organometallic compounds and other complex nanostructures can be obtained with the same experimental set up, just by varying a few synthetic parameters. How to govern such versatility is one of the current challenges of LASiS and requires a thorough understanding of the physical and chemical processes involved in the synthesis. In this perspective, the fundamental mechanisms of laser ablation in liquids are summarized, organized according to their temporal sequence and correlated with relevant examples taken from the library of nanomaterials disclosed by LASiS, in order to show how synthesis parameters influence the composition and the structure of products. The resulting framework suggests that, to date, much attention has been devoted to the physical aspects of laser-matter interaction and to the characterization of the final products of the synthesis. Conversely, the clarification of chemical processes active during LASiS deserves more research efforts and requires the synergy among multiple investigation techniques.

We present a method for the evaluation of the average size of gold nanoparticles based on the fitting of their UV-vis spectra by the Mie model for spheres. The method gives good results using a calibration of the dumping frequency of the surface plasmon resonance and accounting for the presence of nonspherical AuNP in solution by the Gans model for spheroids. It has been successfully applied to free and functionalized gold nanoparticles in various solvents with diameters in the 4-25 nm range. Despite the differences among samples, we found an accuracy of about 6% on the nanoparticles average size with respect to sizes measured by transmission electron microscopy (TEM). Moreover, the fitting model provides other information not available from TEM like the concentration of AuNP in the sample and the fraction of nonspherical nanoparticles, which is particularly useful for measuring aggregation processes. The fitting procedure and models are thoroughly discussed in the text, and the fitting programs are freely accessible on the web.

Single-walled carbon nanotubes (SWNTs) are π-conjugated, quasi-one-dimensional structures consisting of rolled-up graphene sheets that, depending on their chirality, behave as semiconductors or metals 1 ; owing to their unique properties, they enable groundbreaking applications in mechanics, nanoelectronics and photonics 2,3 . In semiconducting SWNTs, medium-sized excitons (3-5 nm) with large binding energy and oscillator strength are the fundamental excitations 4-8 ; exciton wavefunction localization and one-dimensionality give rise to a strong electron-phonon coupling 9-11 , the study of which is crucial for the understanding of their electronic and optical properties. Here we report on the use of resonant sub-10-fs visible pulses 12 to generate and detect, in the time domain, coherent phonons in SWNT ensembles. We observe vibrational wavepackets for the radial breathing mode (RBM) and the G mode, and in particular their anharmonic coupling, resulting in a frequency modulation of the G mode by the RBM. Quantumchemical modelling 13 shows that this effect is due to a corrugation of the SWNT surface on photoexcitation, leading to a coupling between longitudinal and radial vibrations.Electron-phonon coupling in SWNTs is usually studied using Raman spectroscopy; this technique is useful for investigating ground-state vibrations 14 , whereas photoexcited-state vibrational dynamics remain largely unknown because, in the frequency domain, phonon replicas are hardly detectable in the presence of substantial inhomogeneous broadening. Time-domain observation of phonon dynamics has much lower sensitivity with respect to conventional Raman, but it enables direct measurement of excitedstate dynamics, vibrational dephasing and mode coupling in a distinct way 15,16 . Coherent phonon detection allows resolution in time of wavepacket dynamics that is otherwise averaged-out in standard Raman scattering.To detect coherent phonons in SWNTs, we use a standard pump-probe configuration, in which the observed quantity is the modulation depth in the differential transmission 17 ( T /T); details of the experimental setup are provided in the Methods section. Figure 1a shows T /T dynamics of SWNTs grown by the high-pressure carbon monoxide procedure dispersed in polymethylmethacrylate films following excitation with a sub-10-fs visible pulse (1.8-2.4 eV bandwidth), probed at an energy of 2.1 eV. The signal exhibits an initial photobleaching, which quickly turns into photoinduced absorption (PA). The fast photobleaching decay is ascribed to relaxation of the higher-lying exciton (second in an increasing energy scale) to the lower one, taking place with a 40-fs time constant 18 . The PA signal is generated by this lower exciton 4,5 and decays on the ps timescale, in agreement with previous results [19][20][21] . As shown in Fig. 1a, there is a clear oscillation in the T /T amplitude. The Fourier transform (FT) of the oscillatory component (Fig. 2a) shows a strong peak at 252 cm −1 (132-fs period). This frequency can be recognized as the RBM...

The average size of gold nanoparticles (AuNP), obtained by laser ablation in solution, is reduced to a few nanometers or increased to tens of nanometers using laser treatments with different strategies. The techniques do not require any stabilizing molecules and the AuNP surface is free from strongly linked ligands. This allowed one-step, immediate and very efficient functionalization of AuNP with commonly used proteins like bovine serum albumin (BSA), which can be detected, using small nanoparticles, down to 3 picomoles and with a 1 : 10 concentration ratio of BSA : AuNP